TDP-43 proteinopathies including frontotemporal lobar dementia (FTLD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative disorders characterized by aggregation and mislocalization of the nucleic-acid binding protein TDP-43 and subsequent neuronal dysfunction. Here, we developed an endogenous model of sporadic TDP-43 proteinopathy based on the principle that disease-associated TDP-43 acetylation at lysine 145 (K145) alters TDP-43 conformation, impairs RNA-binding capacity, and induces downstream mis-regulation of target genes. Expression of acetylation-mimic TDP-43K145Q resulted in stress-induced nuclear TDP-43 foci and loss-of-TDP-43-function in primary mouse and human induced pluripotent stem cell (hiPSC)-derived cortical neurons. Mice harboring the TDP-43K145Q mutation recapitulated key hallmarks of FTLD, including progressive TDP-43 phosphorylation and insolubility, TDP-43 mis-localization, transcriptomic and splicing alterations, and cognitive dysfunction. Our study supports a model in which TDP-43 acetylation drives neuronal dysfunction and cognitive decline through aberrant splicing and transcription of critical genes that regulate synaptic plasticity and stress response signaling. The neurodegenerative cascade initiated by TDP-43 acetylation recapitulates many aspects of FTLD and provides a new paradigm to further interrogate TDP-43 proteinopathies.
KeywordsneurodegenerationTDP-43proteinopathymouse modelConflict of Interest
The authors have no conflicts of interest to be disclosed.
Introduction
TDP-43 proteinopathies are characterized by the dysfunction and aggregation of Transactivation response element DNA-binding Protein of 43 kDa (TDP-43), with ~ 95% of all amyotrophic lateral sclerosis (ALS) and ~ 50-60% of all frontotemporal lobar dementia (FTLD-TDP) cases harboring TDP-43 pathology1–4. There is significant neuropathologic and clinical overlap between FTLD and ALS with many individuals developing a mixed phenotype, providing strong evidence for a common FTLD/ALS spectrum of disorders5–9. It is also notable that TDP-43 pathology is abundant in other sporadic neurodegenerative diseases including Alzheimer disease (AD)10,11, Limbic-Predominant Age-related TDP-43 Encephalopathy (LATE)12,13, and Parkinson’s disease (PD)14,15. The clinical and neuropathological overlap suggests that common pathogenic mechanisms may link TDP-43 to neurodegeneration16,17. However, modeling sporadic TDP-43 pathogenesis has been challenging since its expression levels are tightly regulated18,19, which has precluded a clear separation of TDP-43 disease-related dysfunction from general toxicity resulting from TDP-43 over- or under-expression20–23. Current knock-in models of TDP-43 disease-causing mutations24–28 provide valuable insights but may be limited in their application to sporadic disease.
Under normal physiological conditions, TDP-43 resides in the nucleus to control RNA processing (RNA splicing, transport, and stability) and gene transcription29–31. Structurally, nuclear retention is primarily mediated by an N-terminal nuclear localization sequence (NLS) through interactions with α1/β-importins32,33, and association with nucleic acids is mediated by two tandem RNA recognition motifs (RRM1/RRM2)34,35 (Fig. S1A), however there is interplay between nucleic acid binding and nuclear localization36,37. The C-terminal glycine-rich domain (also termed the intrinsically disordered or low complexity domain) mediates protein-protein interactions37–39 and harbors most, but not all, familial TARDBP mutations that are causative for FTLD/ALS40,41. In sporadic or familial TDP-43 proteinopathies, TDP-43 undergoes nuclear depletion and concomitant nuclear or cytoplasmic accumulation and aggregation42–44. Both TDP-43 loss-of-function (e.g., aberrant cryptic splicing) and gain-of-function (e.g., aggregate-induced toxicity) mechanisms have been proposed as drivers of TDP-43 pathogenesis45–48.
How TDP-43 becomes dysfunctional in sporadic disease remains unresolved, however TDP-43 post-translational modifications (PTMs) such as phosphorylation, acetylation, and ubiquitination modulate TDP-43’s biochemical properties leading to conformational changes, modulation of nucleic acid binding affinity, regulation of liquid-liquid phase separation (LLPS), and propensity to form insoluble TDP-43 aggregates49–51. Among the various PTMs, TDP-43 acetylation at lysine residue 145 (K145) within RRM1 has emerged as a regulator of both loss and gain of function toxicity52,53. Acetylated TDP-43 is found within TDP-43 inclusions of sporadic ALS (sALS) spinal cord motor neurons but not age-matched control tissue52. Inclusions in FTLD cortex are largely composed of C-terminal fragmented TDP-43 lacking the K145 residue54,55, precluding an assessment of Ac-K145 in FTLD patients. However, TARDBP mutations that disrupt RNA binding, and thereby may act in a similar manner to TDP-43 acetylation, have been identified in FTLD-TDP patients56–58, supporting a pathogenic role of altered nucleic acid binding in disease. Mimicking TDP-43 acetylation with a lysine-to-glutamine substitution (TDP-43K145Q) is sufficient to neutralize the positive charge, disrupt RNA binding, and induce several hallmarks of TDP-43 pathology in vitro52,53, supporting a model whereby TDP-43 acetylation drives both loss-of-function (e.g., RNA-binding deficiency) and gain-of-function (e.g., aggregation) toxicity.
Here, we used CRISPR/Cas9 genome editing to introduce a K145Q substitution into the endogenous mouse Tardbp gene, thereby generating acetylation-mimic, TDP-43K145Q knock-in mice, which enabled us to investigate the potential pathophysiological impacts of an aberrant TDP-43 PTM without disturbing up- or downstream genomic elements that govern TDP-43 autoregulation. Using mouse cortical neurons, human hiPSC-derived cortical neurons, and aged cohorts of TDP-43K145Q homozygous mice, we found that acetylation-mimic TDP-43K145Q induced nuclear TDP-43 foci and cytoplasmic TDP-43 accumulation, which coincided with several disease-associated and loss-of-function measures including widespread transcriptome and splicing alterations. Finally, we observed prominent FTLD-like cognitive and behavioral deficits in acetylation-mimic TDP-43 mice that correlated with neurodegeneration and TDP-43 biochemical abnormalities in affected brain regions. Our study supports lysine acetylation as a driver of TDP-43 dysfunction and neurodegeneration in sporadic TDP-43 proteinopathies.
ResultsNeurons expressing TDP-43<sup>K145Q</sup> form nuclear TDP-43-positive foci in response to overexpression or oxidative stress
We originally showed that TDP-43 acetylation can promote RNA-binding deficiency, aggregation, and pathology52,53. We sought to expand these findings by exploring the behavior of acetylation-mimic TDP-43 variants in primary murine cortical neurons. We employed lentiviral vectors that encode either wild-type (TDP-43wt), acetylation-deficient (TDP-43K145R), and acetylation-mimic (TDP-43K145Q) variants to overexpress TDP-43 species in neurons, and then examined their subcellular localization by immunofluorescence microscopy. In the absence of acute cellular stress, most neurons overexpressing TDP-43K145Q showed distinct nuclear foci that were not detected with TDP-43wt or TDP-43K145R constructs (Fig. 1A, B). When neurons were exposed to acute an oxidative stress (200 µM sodium arsenite), a sensitizing trigger commonly used to enhance TDP-43 dysfunction59–62, there was a significant increase in TDP-43 foci formation with all variants (Fig. 1A, C). The response in neurons expressing TDP-43K145Q was robust, resulting in the formation of numerous large, bright TDP-43-positive foci, as well as small TDP-43-positive nuclear puncta. These aberrant TDP-43 structures, which like represent conformations recently termed anisosomes63, were absent from cells expressing TDP-43wt or acetylation-null TDP-43K145R, indicating that acetylation-mimic TDP-43K145Q alters TDP-43 conformation within the nucleus, sensitizes neurons to oxidative stress, and drives anisosome formation. By coupling high content wide-field microscopy with quantitative image analysis, we observed a three-fold increase in TDP-43 foci/anisosomes formation in neurons expressing TDP-43K145Q (Fig. 1C).
Overexpression of acetylation-mimic TDP-43<sup>K145Q</sup> enhances TDP-43 stress-induced nuclear foci formation
A Representative images of immunofluorescence evaluation of TDP-43 in DIV14 mouse primary cortical neurons overexpressing TDP-43wt, TDP-43K145Q, or TDP-43K145R after vehicle or 200µM NaAsO2 treatment followed by labeling of TDP-43 (green), NeuN (magenta), and DAPI (blue). Stick arrows highlight nuclei with small TDP-43+ puncta and arrowheads highlight nuclei with large TDP-43+ foci. Scale bar = 20µm. B Quantification of percentage of neurons with TDP-43+ foci in vehicle-treated neurons [One-way ANOVA followed by Tukey’s multiple comparisons testing; main effect of treatment between TDP-43 variants F (3,184) = 66.35, p<0.0001]. C Quantification of the average number of TDP-43+ foci per neuron [Two-way ANOVA followed by Tukey’s multiple comparisons testing; main effect of TDP-43 variant F (3, 343) = 36.23, p<0.0001]. Data shown as mean ± SEM. Each data point represents the average value within neurons in a single field of view with 10-100 neurons per field and n=48 fields per condition across 3 biological replicates. Statistical significance is represented by asterisks ****p<0.0001. DIV= Day in vitro
TDP-43 overexpression can result in general toxicity and altered TDP-43 function, depending on duration and the level of over-expression20,64,65. To avoid potentially confounding non-specific toxicity, we transitioned to a more physiologically relevant model to further elucidate the impact of RNA-binding deficient acetylation-mimic TDP-43. We employed CRISPR-Cas9 mutagenesis to introduce a single amino acid substitution at position 145 (K145Q) in the endogenous mouse Tardbp locus, thereby generating the TDP-43K145Q knock-in line (Fig. S1B). By targeting the native mouse gene, we avoid both TDP-43 overexpression and disruption of autoregulatory genomic elements of the endogenous Tardbp transcript18. A TDP-43K145Q founder line was propagated as heterozygotes and continually re-sequenced to confirm retention and propagation of the K145Q substitution (Fig. S1C, D). Both heterozygous and homozygous TDP-43K145Q mice were born at normal mendelian frequencies and showed no obvious developmental defects.
We first investigated the effects of TDP-43K145Q expression in neurons in vitro by isolating and culturing primary cortical neurons from homozygous TDP-43K145Q, hereafter referred to as TDP-43KQ/KQ mice and compared them to TDP-43wt -derived neurons. Quantitative image analysis of untreated TDP-43KQ/KQ neurons showed mild nuclear TDP-43 depletion and slightly increased levels of cytoplasmic TDP-43 compared to TDP-43wt (Fig. 2A, C). Exposing neurons to acute oxidative stress induced larger and more abundant TDP-43-positive nuclear foci (anisosomes) in acetylation-mimic TDP-43KQ/KQ neurons than in TDP-43wt neurons (Fig. 2A, B). Because TDP-43 anisosome formation is associated with loss-of-function defects63,66,67, we next sought to determine if TDP-43 function is impaired. A TDP-43-dependent CFTR splicing reporter was employed, in which a full-length GFP gene is fused to an mCherry gene that is interrupted by exon 9 of CFTR68. Functional TDP-43 suppresses CFTR exon 9 inclusion69, thereby promoting expression of both GFP and mCherry, while loss of TDP-43 function allows GFP but not mCherry to be expressed (Fig. 2D) The spliced and unspliced constructs differ greatly in size, facilitating analysis by RT-PCR. A neuron-specific synapsin (hSyn) promoter was used to selectively deliver the CFTR reporter via lentiviral transduction of primary neurons. Using RT-PCR, we observed impaired splicing in TDP-43KQ/KQ neurons, as assessed by reduced exclusion of the CFTR exon 9 (Fig. 2E-F). Overall, these results indicate that a single endogenously expressed acetylation-mimic TDP-43K145Q mutation is sufficient to alter TDP-43 localization, induce TDP-43 phase separation, and impair splicing in a murine primary neuron culture model.
An endogenously encoded acetylation-mimic TDP-43<sup>K145Q</sup> mutation causes altered TDP-43 localization and functional impairments in mouse and human cortical neurons
A Representative images of primary cortical neurons derived from TDP-43wt or TDP-43KQ/KQ mice that were treated with vehicle or 200µM NaAsO2 and immunolabeled for endogenous TDP-43 (green), NeuN (magenta), and DAPI (blue). B-C Quantification of the number of TDP-43+ neurons and the cytoplasmic:nuclear ratio of TDP-43 fluorescence intensity in TDP-43KQ/KQ compared to TDP-43wt neurons [Data shown as mean ± SEM, with data point representing the average value per neuron in a single field of view, 10-110 neurons per field, n=72 fields across 3 biological replicates; analysis by two-way ANOVA followed by Tukey’s multiple comparisons testing; B main effect of genotype F (1, 281) = 109.1, p<0.0001; C main effect of genotype F (1, 274) = 34.05, p<0.0001; TDP-43wt vs. TDP-43KQ/KQ +NaAsO2 p=0.0363; TDP-43wt Vehicle vs +NaAsO2 p=0.0158]. D Schematic of TDP-43 dependent CFTR splicing reporter construct68,153. E Gel electrophoresis following RT-PCR shows the splicing patterns of CFTR reporter in TDP-43wt and TDP-43KQ/KQ neurons at DIV14 and DIV28. F Quantification of band intensity in e to yield splicing efficiency values [Data shown as mean ± SD, n=3 biological replicates per condition; analysis by two-way ANOVA followed by Tukey’s multiple comparisons testing; main effect of genotype F (1, 8) = 196.6, p<0.0001; *p=0.0133]. G Representative confocal images of CRISPR-modified human iPSC-derived cortical neurons harboring homozygous TDP-43K145Q (clonal line 12), TDP-43K145R (clonal line 2) knock-in mutations or no TARDBP modifications (wt) as a control. Additional clones displayed in Supplementary figure 4. Differentiated cortical neurons were treated with vehicle or 200µM NaAsO2 and then immunolabeled for endogenous TDP-43 (green), Map2 (magenta), and DAPI (blue). Scale bars = 20µm. Statistical significance is represented by asterisks *p<0.05, ***p<0.001, ****p<0.0001. DIV= Day in vitro
To assess this model’s relevance to human neurons, we used CRISPR/Cas9 to generate a panel of human induced pluripotent stem cell (hiPSC) lines harboring acetylation-mimic TDP-43 (TDP-43K145Q.12 and TDP-43K145Q.18), acetylation-deficient TDP-43 (TDP-43K145R.2 and TDP-43K145R.12), or unmodified TDP-43 (TDP-43wt) (Fig. S2) and differentiated these lines into cortical neurons (Fig. S3). Untreated hiPSC-derived TDP-43K145Q cortical neurons were morphologically identical to TDP-43wt and TDP-43K145R neurons and showed similar patterns of TDP-43 localization (Fig. 2G, Fig. S4). All hiPSC-derived lines showed a granular nuclear TDP-43 localization pattern under normal conditions, consistent with physiologic de-mixing of nuclear TDP-4361. Following acute oxidative stress, TDP-43K145Q neurons showed TDP-43 nuclear clearing and the formation of large, intensely labeled TDP-43-positive anisosomes (Fig. 2G, Fig. S4). In comparison, cortical neurons expressing TDP-43wt or TDP-43K145R maintained nuclear TDP-43 and formed small stippled TDP-43 foci. We note that TDP-43 nuclear clearing and foci formation was more robust in hiPSC-derived neurons compared to mouse neurons, suggesting human neurons may be more sensitive to the effects of RNA-binding deficient TDP-43. Similar to mouse neurons, hiPSC-derived TDP-43K145R neurons were indistinguishable from TDP-43wt neurons, further supporting charge neutralization as a critical driver of TDP-43 loss of function rather than an inherent effect of mutation at the K145 locus.
TDP-43 acetylation-mimic mice develop age-dependent cognitive and behavioral defects
To evaluate neurodegenerative phenotypes in TDP-43 acetylation-mimic mice, we aged homozygous TDP-43KQ/KQ mice and WT littermates and performed an extensive battery of behavioral analysis to assess cognitive and motor function, which reflect impairments that are commonly impacted in the spectrum of TDP-43 proteinopathies6. Since there were no significant differences between males and females in any behavioral analyses described below, we pooled both sexes into either WT or TDP-43KQ/KQ groups. At 12 months old, TDP-43KQ/KQ mice showed significant reduction of body weight compared to WT littermates, and this difference was maintained until end point analysis at 18 months old (Fig. 3A). Evaluation of exploratory activity and locomotion in an open field test demonstrated that TDP-43KQ/KQ mice spend significantly more time in the center region (Fig. 3B), with no differences in total distance traveled (Fig. S5A), indicative of decreased anxiety-like behavior70,71. Acoustic startle testing revealed impaired prepulse inhibition (PPI) at 12 months old (Fig. S6A), indicative of deficits in sensorimotor gating, a form of inhibitory behavioral control, in TDP-43KQ/KQ mice72,73, a phenomenon that can be observed in early dementia74. In 18-month-old animals, evaluation of acoustic startle response and PPI was confounded by hearing impairment (Fig. S6B), however the altered activity reflected by increased time in the center region of an open field test was maintained at this advanced age (Fig. 3B). Thus, consistent patterns of behavioral disinhibition and reduced anxiety-like behavior were apparent in TDP-43KQ/KQ mice over time.
We next performed contextual and cue-dependent fear conditioning as an index of hippocampal and cortical function75–79. Context-dependent fear testing revealed reduced freezing times in TDP-43KQ/KQ mice, with trends observed at 12 months old and more significant deficits at 18 months, suggesting age-dependent impairments in contextual learning (Fig. 3C). Similarly, auditory cue-dependent fear testing revealed significant impairments in associative cue learning in TDP-43KQ/KQ mice at 12 months of age (Fig. 3D), a behavior thought to be mediated by the amygdala and higher-order cortical regions important in inhibitory control80. As mentioned above, general auditory defects in both genotypes at 18 months of age confounded interpretations of any cue-dependent learning deficits at this advanced age (Fig. 3D).
Morris Water Maze testing was used to evaluate swimming ability and spatial learning81, which showed equivalent swim speeds, suggesting no motor impairments in TDP-43KQ/KQ mice at 18 months of age (Fig. S5D). While assessment of spatial learning showed a trend towards impaired acquisition learning (Fig. 3E, F), we observed more prominent defects in reversal learning after moving the location of the platform, as determined by significant delays in escape latency. These findings support deficits in cognitive flexibility in TDP-43KQ/KQ mice compared to WT littermates (Fig. 3G, H)82.
TDP-43<sup>KQ/KQ</sup> mice develop age-dependent cognitive and behavioral defects.
A Body weight of TDP-43wt and TDP-43KQ/KQ mice in males [left panel; F(1,48)=17.52, p=0.0001] and females [right panel; F(1,36)=14.17, p=0.0006] at different ages. B Quantification of time in the center region of an open field in mice at 12 months [left panel; F(1,20)=6.118, p=0.0225] and 18 months [right panel; F(1,17)=9.622, p=0.0065]. C Quantification of time spent frozen (immobile) following context-dependent conditioned fear testing 18-months-old [right panel; F(1,17)=5.402, p=0.0328]. D Quantification of time spent frozen following cue-dependent conditioned fear testing at 12-months-old [left panel; F(1,20)=5.285, p=0.0324] and 18-months-old. Filled bars represent presence of auditory cue (tone). E-F Morris Water Maze (MWM) analysis displaying time to find a hidden platform (escape latency), quantified as daily trial means per animal (E) or average across all days (F). G-H Quantification of escape latencies during MWM reversal learning trials in daily trials (G) [F(1,16)=5.273, p=0.0355] and comparing averages per mouse across all days (H).
Bar and scatter plots shown as mean ± SD. Box and whiskers show line at median, ‘+’ at mean, and whiskers run min to max. a Two-way ANOVA followed by Šídák's multiple comparisons test. B-D,E,G Two-way repeated measures ANOVA followed by Holm-Šídák's multiple comparisons tests; F statistics and p-values in legend represent main effect of genotype. F, H Unpaired student’s t-test. Sample sizes as follows unless otherwise indicated: 12-month TDP-43wt n=15; 12-month TDP-43KQ/KQ n=7; 18-month TDP-43wt n=10; 18-month TDP-43KQ/KQ n=9; one 18-month TDP-43KQ/KQ extreme outlier removed for MWM analysis (E,F). Statistical significance is represented by asterisks *p<0.05, **p<0.01, ***p<0.001; ns=not significant. Further statistical information is located Figure 3 Source Data 1 file.
Finally, we assessed motor function in these cohorts and were surprised to find no overt signs of motor impairment in TDP-43KQ/KQ mice 18 months of age. TDP-43KQ/KQ mice do not differ from WT littermates in motor coordination as assessed by rotarod testing (Fig. S5C-D) or in grip strength as measured using digital force meters (Fig. S5E). Moreover, there were no differences in swim speed or distance traveled in an open field at any age tested (Fig. S5A, B). The preferential deficits in learning and behavioral control support a prominent FTLD- or dementia-like phenotype in TDP-43KQ/KQ at the ages tested. We note that we cannot currently exclude the possibility of more subtle motor phenotypes or the emergence of motor defects in aged mice beyond 18 months old, which will require follow-up studies.
TDP-43<sup>KQ/KQ</sup> mice demonstrate TDP-43 dysfunction and neuron loss in neocortex and hippocampus
Given the robust cognitive phenotype observed in TDP-43KQ/KQ mice, which was most pronounced at 18 months, we focused the remainder of this study on the neocortex and hippocampus of aged animals. We first characterized aspects of TDP-43 pathology including TDP-43 aggregation, mislocalization, and neuronal loss17,43. To do so, we performed immunohistochemical and immunofluorescent labelling, confocal microscopy, and automated quantitative image analysis to assess TDP-43 localization in 18-month-old TDP-43KQ/KQ. Quantification of NeuN-positive cells revealed significantly reduced neuron density in TDP-43KQ/KQ neocortex compared to WT mice (Fig. 4A, B). In contrast, using a Cresyl Violet stain, which labels the rough endoplasmic reticulum and nuclei of all cell types including glia83, we did not observe significant differences in overall cell density in the neocortex (Fig. 4C, D), suggesting preferential neuronal loss in TDP-43KQ/KQ cortex. Intriguingly, we observed no significant astrogliosis, as measured by intensity and area of GFAP fluorescence, or microgliosis, as measured by intensity and area of Iba1 fluorescence intensity, in the neocortex or hippocampus in 18-month-old TDP-43KQ/KQ mice compared to TDP-43wt animals (Fig. S7).
Neuronal loss in aged TDP-43<sup>KQ/KQ</sup> mice with retention of predominantly nuclear TDP-43.
A Representative confocal images of immunofluorescent labelling of NeuN+ neurons in the neocortex of 18-month TDP-43wt or TDP-43KQ/KQ mice. B Quantification of NeuN+ neurons per square millimeter (mm) in TDP-43wt and TDP-43KQ/KQ mice. C Representative widefield images of neocortex sections following Cresyl Violet (CV) labelling of endoplasmic reticulum Nissl substance. E Quantification of total brain cell density within the neocortex. E,H Representative confocal images of neocortex (E) and hippocampus (H) sections from 18-month-old TDP-43wt and TDP-43KQ/KQ mice immunolabeled with TDP-43 (green), NeuN (magenta), and DAPI (blue). F,I Quantification of TDP-43 fluorescence intensity within NeuN+ neurons in the neocortex (F) or hippocampal CA3 region (I). G,J Quantification of the nuclear:cytoplasmic ratio of TDP-43 fluorescence intensity within NeuN+ neurons in the neocortex (G) and hippocampus (J). Scale bars = 100 µm (A), 500 µm (B), 20 µm (E,H). SuperPlots87 show average value per animal in solid color bordered symbol (as mean ± SEM) over top of semi-transparent individual values from each animal. One color represents one animal. Individual datapoints in (B) and (C) represent density in a single field of view, with 10-16 fields across 4 brain sections per animal, n = 3-4 mice per genotype. Individual datapoints in F-G and I-J represent values from a single neuron within one field of view, with 1000-5000 neurons per animal across 12-16 fields from 4 (neocortex) or 2 (hippocampus) brain sections. Genotype comparisons by unpaired student’s t-test. Statistical significance is represented by asterisks, **p<0.01; ns = not significant; unmarked = not significant.
We next examined the tissue for hallmarks of TDP-43 aggregation and did not detect prominent nuclear or cytoplasmic TDP-43 inclusions in the neocortex or hippocampus of TDP-43wt or TDP-43KQ/KQ mice at 18 months of age. Since TDP-43 expression may be altered in FTLD-TDP and other TDP-43 proteinopathies18,84–86, we assessed TDP-43 immunoreactivity and abundance in NeuN-positive neurons in the neocortex and hippocampus, and then visualized the single-cell resolution data using SuperPlots87. We observed trends of elevated TDP-43 protein in the neocortex and the CA3 region of the hippocampus (Fig. 4E-F, H-I) and also a potential subtle increase in the cytoplasmic to nuclear TDP-43 ratio within CA3 neurons (Fig. 4G, J). We note that these effects were not significant using microscopy and quantitative image analysis, which may result from technical challenges in achieving accurate image segmentation at the subcellular level needed to separate nuclear and cytoplasmic TDP-43 intensity within tissue samples. Therefore, to more reliably detect alterations in TDP-43 localization and solubility, we turned to an alternative biochemical approach of sequentially fractionating isolated hippocampus and neocortex tissue to generate soluble (RIPA-extracted) and insoluble (Urea-extracted) protein fractions. TDP-43KQ/KQ mouse neocortex harbored insoluble phosphorylated TDP-43 at the disease-associated Ser409/410 locus88 (p409/410) at 12 months of age, which was even more prominent at 18 months (Fig. 5A, C), a timepoint at which TDP-43KQ/KQ mice show prominent behavioral and cognitive defects (Fig. 4). Notably, p409/410 immunoreactivity was minimal in the hippocampus of 12-month-old TDP-43KQ/KQ mice but increased dramatically by 18 months (Fig. 5D, F), coinciding with the onset of hippocampal-mediated learning deficits (Fig. 3C). Though p409/410 was elevated in TDP-43KQ/KQ mice, we were surprised to find that the total insoluble TDP-43 pool was not prominently altered, suggesting increases in TDP-43 phosphorylation may precede overt conversion towards insoluble TDP-43 accumulation.
TDP-43 is hyper-phosphorylated and mislocalized in the neocortex and hippocampus of aged TDP-43<sup>KQ/KQ</sup> mice
A-F Western blot images comparing soluble and insoluble protein fractions in neocortex (A) and hippocampus (D) from TDP-43wt and TDP-43KQ/KQ mice at 12- and 18-months of age. Quantification of soluble TDP-43 protein levels in neocortex and hippocampus tissue relative to total transferred protein (TTP) [neocortex (B) F (1, 14) = 97.67, p<0.0001; hippocampus (E) main effect of genotype F (1, 13) = 3.699, p=0.0766 and main effect of genotype*age interaction F (1,13) = 10.21, p=0.0070). Quantification of insoluble phosphorylated p(409/410)-TDP-43 levels in neocortex and hippocampus tissue [neocortex (C) F (1, 14) = 18.81, p=0.0007; hippocampus (F) F (1, 13) = 23.07, p=0.0003]. G-I Western blot images following isolation of soluble nuclear and cytoplasmic protein fractions from neocortex (G) and hippocampus (J) of TDP-43wt and TDP-43KQ/KQ mice at 12- and 18-months of age. J-L Quantification of cytoplasmic TDP-43 measured as percent of total TDP-43 in neocortex and hippocampus [neocortex (H) F (1, 8) = 69.38, p<0.0001; hippocampus (K) F (1, 8) = 22.40, p=0.0015] and relative cytoplasmic mislocalization of TDP-43 after normalizing to cytoplasmic/nuclear ratio of the nuclear Sp1 protein in neocortex and hippocampus [neocortex (I) F (1, 8) = 19.79, p=0.0021; hippocampus (L) F (1, 8) = 29.94, p=0.0006]. A-F n=4 TDP-43wt and n=5 TDP-43KQ/KQ at all ages and regions except 18-month hippocampus n=4 TDP-43KQ/KQ. F-L n=4 all ages and regions. Data are presented as mean ± SD. Two-way ANOVA followed by Šídák's multiple comparisons test. F statistics represent the main effect of genotype unless otherwise stated. Statistical significance represented by asterisks, *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001. Further statistical information is located Figure 5 Source Data 1 file.
We also examined soluble TDP-43 levels, as acetylation-induced loss of function may result in autoregulatory feedback that increases production of TDP-4318. The soluble TDP-43 pool was significantly increased in TDP-43KQ/KQ at 12 months in the neocortex and at 18 months in both neocortex (Fig. 5A, B) and hippocampus (Fig. 5D, E), consistent with increases in autoregulated TDP-43 protein levels. To assess soluble TDP-43 localization using a biochemical approach, we performed subcellular fractionation to isolate nuclear and cytoplasmic proteins from the neocortex and hippocampus and found a striking increase in cytoplasmic TDP-43 in TDP-43KQ/KQ mice at both 12 and 18 months of age (Fig. 5G-L). The detection of hippocampal cytoplasmic TDP-43 at 12 months (Fig. 5J-L), which is not yet phosphorylated (Fig. 5D, F), suggests that cytoplasmic mislocalization occurs prior to TDP-43 phosphorylation89,90. Overall, our findings indicate that phosphorylated mislocalized TDP-43 coincides with neurodegeneration in TDP-43KQ/KQ mice, consistent with progressive FTLD-TDP44.
Disease-linked transcriptomic and splicing defects are prevalent in acetylation-mimic TDP-43<sup>KQ/KQ</sup> mice
TDP-43 acetylation drives RNA dissociation and loss of TDP-43 function52, implying that reduced RNA-binding capacity may impact transcriptional regulation and mRNA splicing36,50,91. We performed total RNA sequencing on neocortex and hippocampus tissue from 18-month-old TDP-43wt or TDP-43KQ/KQ mice to determine how TDP-43 acetylation affects RNA profiles in vivo. We identified nearly 400 differentially expressed genes (DEGs) in each brain region in TDP-43KQ/KQ mice compared to TDP-43wt, after correcting for underlying batch effects. As expected by acetylation-induced loss of TDP-43 function and subsequent autoregulation, the Tardbp transcript was increased in both the hippocampus and neocortex (Supplementary Tables S1, S2). Follow-up RT-qPCR analysis confirmed a 2- to 3-fold increase in Tardbp expression in neocortex and hippocampus tissue (Fig. S8), which correlated with increased TDP-43 protein levels in these regions (Fig. 5B, E).
We then clustered the DEGs based on their up- or down-regulation and the brain region affected (Fig. 6), which revealed similar, but distinct, patterns of transcriptional alterations (Fig. 6, Fig. S9, Supplementary Tables S1, S2). To investigate the potential biological implications of the altered transcriptome, we performed Gene Ontology (GO) term enrichment analyses on DEGs identified in each of the six clusters, which revealed that similar pathways were affected in cortical and hippocampal tissues (Supplementary Table S3). In both brain regions, the most highly downregulated genes were involved in developmental processes, including many related to CNS development and maintenance, such as neurogenesis (e.g., Sema5b, Rnd2, Brinp1), gliogenesis (e.g., Tlr2, Olig2, Sox10), and myelination (e.g., Nkx2-2, Nkx6-2, Sox10). Sema5b was the most dramatically reduced transcript in the hippocampus, and the third most in the cortex, (Supplementary Tables S1, S2) with a two-fold reduction in expression in TDP-43KQ/KQ mice. Downregulated genes in both the hippocampus and neocortex were also enriched for terms related to synapse homeostasis and transmembrane signaling, however the dysregulated pathways were distinct. Genes related to GABAergic synapses (e.g., Gad1, Gad2, Abat, Gnb5) were selectively downregulated in the neocortex, while trans-synaptic signaling and ion transport mechanisms (e.g., Homer3, Camk4a,Nsmf, Cnih2) were decreased in the hippocampus.
RNA sequencing of aged TDP-43<sup>KQ/KQ</sup> mouse brain reveals dysregulation of neurodegeneration and FTLD-TDP-associated pathways
Genes differentially expressed between TDP-43KQ/KQ (TDP43-KQ) and TDP-43wt (TDP43-WT) cortex and hippocampus were compared and their intersections were defined as 6 gene groups, as represented by an UpSet plot (left). Normalized gene expression values, centered around the mean of TDP-43wt samples for each brain region, were then hierarchically clustered and plotted as a heatmap (center, red indicates expression higher than that of WT, blue indicates expression lower than WT). Genes that were significantly altered in human FTLD-TDP temporal and/or frontal cortex (Hasan et. al.92, “FTLD-TDP Cortex” bar), or striatal mouse brain TDP-43-knockdown (Polymenidou et al.93, “TDP-43 knockdown” bar) were demarcated with tick marks. Each gene group was assessed for over-enrichment of Gene Ontology terms and these results were summarized as word clouds (right). Neocortex n=5 TDP-43wt, n=6 TDP-43KQ/KQ; hippocampus n=4 TDP-43wt, n=5 TDP-43KQ/KQ.
In contrast, there was significant upregulation of cellular stress response genes (Sesn1, Nrros, Plat, Klf15) and many apoptotic regulators (e.g., Trp53inp1, Pmaip1, Bcl2l1, Plekhf1) in both the hippocampus and neocortex, along with an over-representation of GO terms related to metabolism, localization, cell adhesion (Fig. 6, Supplementary Table S3). Several pathways were uniquely altered, however, suggesting brain region-specific transcriptional effects of acetylation-mimic TDP-43 in TDP-43KQ/KQ mice. For example, coagulation and complement cascades were only upregulated in the hippocampus (e.g., F3, Plat, CD59a). Intriguingly, while genes associated with trans-synaptic signaling were decreased in the hippocampus, another set of genes involved in this same pathway was upregulated in the cortex (e.g.,Syt7, Synpo, Nptx1, Spg11) (Supplementary Table S3).
Given the cognitive defects in TDP-43KQ/KQ mice, we sought to draw parallels between the TDP-43KQ/KQ mouse and human FTLD-TDP transcriptome. Comparison of the DEGs in TDP-43KQ/KQ mice to the mouse orthologs of those found in FTLD-TDP frontal or temporal cortex tissue92 revealed marked overlap between our mouse and their human data sets (Fig. 6, see “FTLD-TDP Cortex” bar), particularly in the hippocampus (Supplementary Table S4; p = 0.0003 "Hippocampus Down" vs Downregulated in FTLD-TDP frontal cortex; p = 0.0014 "Hippocampus Down" vs Downregulated in FTLD-TDP temporal cortex; p = 0.0048 "Hippocampus Up" vs Upregulated in FTLD-TDP frontal cortex). A similar alignment comparing DEGs in mouse striatum following TDP-43 knockdown93 also identified commonly altered genes, particularly in the TDP-43KQ/KQ downregulated gene sets (Fig. 6, see “TDP-43 knockdown” bar; Supplementary Table 4; p = 0.019 "Hippocampus Down" vs Downregulated in TDP-43-KD; p = 0.046 "Cortex Down + Hippocampus Down" vs Upregulated in TDP-43-KD). Together, this data defines distinct functional signatures including altered synaptic gene expression and stress response signaling that are reflective of acetylation-induced TDP-43 dysfunction and the progression of FTLD-TDP.
Alternative splicing defects, particularly impaired repression of cryptic exons, due to TDP-43 dysregulation are implicated in FTLD and ALS pathogenesis94–99. In line with our findings above that TDP-43KQ/KQ primary cortical neurons in vitro show splicing deficits (Fig. 2E-F), we identified widespread splicing alterations in vivo. Analysis of TDP-43KQ/KQ mouse neocortex samples identified 289 differentially spliced genes (DSGs), with 81.7% of loci containing at least one cryptic splice junction, and 29.8% containing two cryptic splice junctions (Supplementary Table S5). In the hippocampus, we found 126 DSGs, 77.0% of which contain a cryptic splice junction and 41.3% that are formed by two cryptic splice sites (Supplementary Table S6). The alternative splicing events were relatively consistent between brain regions, as over 70% of the DSGs identified in the hippocampus were also present in the cortex. Among the most significant DSGs identified were several known TDP-43 splicing targets (e.g., Kcnip2, Pdp1, Poldipp3, Ppfibp1, Dnajc5. Tmem2, Sort1)24,93,95, transcripts associated with particular neurodegenerative diseases (e.g., Mapt, Atxn1, Lrrk2)100–106, and also many robustly altered transcripts that are not well-characterized but have been linked to neurodegeneration (e.g., Nrxn3, Nos1, Arfgef2, Arhgap10, Lrp8, Smarca4, Rims2). Of all identified DSGs, the most substantially altered transcript was Sort1, encoding the Sortilin-1 (SORT1) protein. Exclusion of an exon toward the 3’ end of the Sort1 transcript was reduced by 55.9% in TDP-43KQ/KQ cortex and by 57.0% in the hippocampus (Fig. 7A, B), such that in both tissues the aberrant Sort1 transcript was the predominant variant detected via RNA sequencing.
Dysfunctional splicing regulation of the <italic>Sort1</italic> transcript in TDP-43<sup>KQ/KQ</sup> mice
A Differential splicing analysis of TDP-43wt and TDP-43KQ/KQ neocortex (“Ctx”) and hippocampus (“Hpc”) using LeafCutter180 (visualized with LeafViz181) demonstrates reduction in exclusion of a late cryptic exon within the Sort1 transcript. Diagram in (A) shows full-length Sort1 gene in upper panel and highlights the differentially spliced region in gray. Chromosomal location, intron start and end points, annotation status, and Δ percent spliced in (dPSI) of the intronic region is listed in the lower table. B-C qPCR analysis of additional WT and TDP-43KQ/KQ neocortex (B) and hippocampus (C) samples using primers specific for Sort1 splice variants [neocortex Sort1-ex17b F (1, 23) = 35.64 p<0.0001, Sort1-WT F (1, 24) = 15.22 p=0.0007, Sort1 total F (1, 25) = 0.01856 p=0.8927; hippocampus Sort1-ex17b F (1, 14) = 44.33 p<0.0001, Sort1-WT F (1, 15) = 9.798 p=0.0069, Sort1 total F (1, 16) = 2.086 p=0.1679]. D-G Images of western blots probed for SORT1 protein from neocortex (D) and hippocampus (E) lysates from 12- and 18-month-old mice. The SORT1 protein band intensity is plotted relative to total transferred protein (TTP) and quantified in F-G [neocortex, F(1,14)=9.308, p=0.0086; hippocampus, F(1,13)=6.117, p=0.0280]. Neocortex n=5 TDP-43wt, n=6 TDP-43KQ/KQ; hippocampus n=4 TDP-43wt, n=5 TDP-43KQ/KQ. Data are presented as mean ± SD. Two-way ANOVA followed by Šídák's multiple comparisons test. F statistics represent main effect of genotype, unless otherwise stated. Statistical significance is represented by asterisks, *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001. Further statistical information is located Figure 7 Source Data 1 file.
SORT1 is a highly expressed neurotrophic factor receptor that binds progranulin (PGRN) and regulates endosomal/lysosomal function through a pathway that is genetically linked to FTLD-TDP107–111. Recent studies suggest that TDP-43 depletion results in inappropriate inclusion of the Sort1 exon 17b and production of a soluble and putatively toxic SORT1 variant that is increased in FTLD-TDP patients93,112,113. To confirm that the altered splicing in our sequencing data was Sort1 exon 17b inclusion, we performed qPCR using primers specific for the mouse Sort1+ex17b transcript, the appropriately spliced variant (Sort1-WT), and all Sort1 variants (Sort1 total) on tissues isolated from TDP-43KQ/KQ or WT mice113. This analysis showed a nearly 8-fold increase in the Sort1+ex17b transcript in the cortex (Fig. 7C) and hippocampus (Fig. 7D) of both 12- and 18-month-old animals, which is accompanied by an approximately 25% decrease in Sort1-WT transcripts. Interestingly, the total level of Sort1 transcript varied with age, with slightly increased levels at 12 months and reduced levels at 18 months of age, suggestive of a negative feedback mechanism regulating Sort1 expression.
To determine whether the Sort1+ex17b transcript identified in TDP-43KQ/KQ mice might generate a distinct SORT1 protein, we immunoblotted hippocampus and cortex homogenates and identified a higher molecular weight SORT1 in TDP-43KQ/KQ compared to WT mice (Fig. 7E, F), consistent with Sort1 exon 17b inclusion. Notably, this abnormal SORT1 protein showed decreased stability in TDP-43KQ/KQ mice, as suggested by reduced protein levels in acetylation-mimic animals, compared to controls (Fig. 7G, H). These data indicate failure to repress cryptic exon inclusion and an altered SORT1-PGRN axis, indicating an aberrant splicing pattern in TDP-43KQ/KQ mice that resembles alterations seen in human FTLD-TDP.
Discussion
Here, we show that a TDP-43 acetylation-mimic mutation that disrupts RNA binding (TDP-43K145Q) is sufficient to recapitulate TDP-43 dysfunction using a combination of biochemical, molecular, and behavioral analyses. Our knock-in TDP-43K145Q mice showed impaired learning and cognition, as well as behavioral disinhibition with age, which are representative of an dementia or FTLD-like neurodegenerative phenotype. The progressive age-dependent behavioral defects and parallel increases in insoluble phosphorylated TDP-43 in the neocortex and hippocampus of TDP-43KQ/KQ mice are consistent with age acting as a primary risk factor and putative trigger for the pathogenic effects of aberrant TDP-43. Lastly, we show significant transcriptomic alterations in aged mice expressing aberrant TDP-43, including abnormal splicing activity and upregulation of stress response and neurodegeneration-associated pathways. Our findings indicate that disrupted RNA binding due to TDP-43 acetylation within RRM domains contributes to the pathogenesis of TDP-43 proteinopathies and the evolution of a dementia-like syndrome.
Assessment of neurodegeneration in this acetylation-mimic mouse model revealed that 18-month-old TDP-43KQ/KQ animals have reduced neuronal density in the neocortex without a significant decline in total brain cell density in this region. Intriguingly, this occurs without substantial astro- or microgliosis as assessed by measurements of GFAP or Iba1 fluorescence, respectively. The role of neuroinflammation in this model remain to be determined, as does the composition of the non-neuronal, non-astrocytic, non-microglial cells that seem to reside in aged TDP-43KQ/KQ neocortex. Potential components of this cell population include oligodendrocytes and infiltrating peripheral immune cells, as both have been implicated in the neurodegenerative process and in TDP-43 proteinopathies114–118. Future studies are warranted to better understand the potential neurodegenerative and neuroinflammatory phenotype in the TDP-43KQ/KQ mouse model.
Our prior mass spectrometry analysis identified acetylated lysines K145 and K19252, while a more recent study identified residues K84 (within the NLS) and K136 (within RRM1) as additional lysine residues of interest66. It is likely that acetylation within the RRMs impairs TDP-43 function, as multiple studies have shown that acetylation mimic TDP-43K136Q or TDP-43K145Q variants phenocopy one another, with both showing nuclear foci (or anisosome) formation and reduced RNA binding. This suggests that deficient RNA binding may explain why acetylation, or acetylation-mimic mutations within RRM1, alter TDP-43 dynamics and cause pathology. RNA-binding deficient TDP-43, including variants with RRM mutations or deletions, showed increased TDP-43 phase separation, aggregation, nuclear egress, and neurotoxicity34,58,63,66. Conversely, enhancing RNA binding mitigates these phenotypes67,119,120, likely in large part due to TDP-43 retention within the nucleus36. We hypothesize that the reduced nucleic-acid binding capacity of acetylation-mimic TDP-43K145Q is responsible for the TDP-43 mislocalization, aggregation, and RNA dyshomeostasis observed in our model, and the neurotoxicity and deficits these impart. Thus, impaired RNA binding achieved by K145 acetylation, or other RRM domain modifications, may promote pathogenesis of TDP-43 proteinopathies.
Acetylated TDP-43 (detected with an acetylation-specific ac-K145 antibody) is found within ALS spinal cord inclusions and has not yet been detected in FTLD-TDP cortex52, however pathology in these cases is dominated by C-terminal fragments lacking RRM1 thereby precluding detection of the acetylated K145 site54,121. It is possible that TDP-43 acetylation still occurs in FTLD at one or more lysine residues, thus future mass spectrometry studies will be needed to determine whether TDP-43 acetylation occurs in earlier in the progression of FTLD-TDP. While the precise physiological or cellular role for TDP-43 acetylation remains unclear, given its impact on RNA binding, one can imagine TDP-43 acetylation may fine-tune TDP-43’s normal cellular processes, and in conjunction with other disease risk factors such as aging, drive vulnerable neuronal populations toward degeneration.
Consistent with acetylation-induced loss of function, the TDP-43KQ/KQ mice show striking similarities to other TDP-43 depletion models. For example, the RNA profiles in TDP-43KQ/KQ brain, particularly the hippocampus, were similar to a mouse model of striatal TDP-43 knock down (Fig. 6), including upregulation of immune response genes (Serpine1, Serping1) and altered splicing of TDP-43 targets (Kcnip2, Sort1, Dnajc5, Pdp1, Poldip3)93. Similar splicing deficits were found in conditional CAMKIIa-driven Tardbp knockout mice, accompanied by an FTLD-like phenotype with disinhibitory behavior and spatial learning deficits resembling those seen in TDP-43KQ/KQ mice122,123. Hippocampal knockdown of TDP-43 in adult mice impaired learning and memory, likely due to synaptic loss124. Together, these findings imply that acetylation-induced TDP-43 loss of function imparts neurotoxicity and neurodegeneration. However, unlike TDP-43KQ/KQ mice, complete TDP-43 depletion is not viable23,125, and therefore the K145Q substitution creates a powerful yet viable model that cannot represent a complete loss of function, as these mice do not display developmental defects or ALS-like motor phenotypes (at least up until 18 months old), nor does their expression profile entirely align with striatal TDP-43 knockdown93. This is likely in part because TDP-43K145Q maintains some level of RNA binding capacity, leaving some TDP-43-dependent processes unperturbed. In turn, acetylated TDP-43 may also impart gain-of-function toxicity due to altered RNA binding patterns and conformational instability, leading to nuclear export, enhanced phosphorylation and aggregation, and a dysregulated transcriptome, as we observe in TDP-43KQ/KQ mice. As both loss- and gain-of-function effects have been proposed for familial disease-causing TARDBP mutations45,46,95,126, we hypothesize that TDP-43K145Q acts in a similar manner, which is supported by our present findings.
RNA sequencing revealed profound transcriptomic alterations within aged TDP-43KQ/KQ mouse brains. Many up- and down-regulated transcripts identified in acetylation-mimic mice were also differentially expressed in FTLD-TDP human tissue (Fig. 6), particularly genes associated with cellular stress response, synaptic regulation, apoptotic signaling, and cellular adhesion92,127, representing potential FTLD-TDP pathomechanisms. Disease-related alternative splicing events were also common within TDP-43KQ/KQ cortex and hippocampus. For example, the altered splicing of Sort1 and subsequent generation of Sort1+ex17b we observed in TDP-43 acetylation-mimic mice (Fig. 7), is also found in FTLD-TDP brain samples108,113. Moreover, SORT1 facilitates the endocytosis and lysosomal degradation of PGRN108,110, a protein whose deficiency is causative of 10-30% familial FTLD cases128–130. In humans, SORT1+ex17b contains an additional proteolytic site, resulting in the production of a soluble toxic variant that impairs the function of SORT1, prevents binding to PGRN and impacts neuronal survival107,113. A recent study showed that TDP-43 depletion increased SORT1+ex17b, impaired brain derived neurotrophic factor (BDNF) signaling, and reduced synaptic plasticity in mice112. We speculate that acetylated TDP-43 alters SORT1/PGRN signaling, contributing to the synaptic dysregulation that is so evident in our transcriptome data. Future mechanistic studies are aimed at interrogating PGRN signaling in TDP-43KQ/KQ mice.
Other aberrantly spliced transcripts in TDP-43KQ/KQ mice provide additional insight into novel pathogenic mechanisms. The synaptic regulators Arfgef2131 and Arhgap10132,133 were among the most affected transcripts. Concurrently, Sema5b which regulates synaptic connections and axon guidance134–137, was strongly downregulated in in TDP-43KQ/KQ hippocampus and cortex, as were other genes involved in synapse homeostasis and signaling. Interestingly, Argef2 and Arghap10 are also known to regulate apoptosis131,132, suggesting the observed neuronal loss in TDP-43KQ/KQ cortex may occur via the upregulation of apoptosis-related transcripts. Furthermore, splicing of LRP8 (low-density lipoprotein receptor-related protein 8) was significantly altered in TDP-43KQ/KQ mice, and abnormalities in LRP8 can cause learning and memory defects, likely by disrupting Reelin-mediated synaptic plasticity and long-term potentiation (LTP)138–141. Similarly, Nos1, encoding the neuronal nitric oxide synthase (nNOS) protein that regulates LTP and synaptic plasticity via the production of nitric oxide142,143, showed aberrant splicing patterns in TDP-43KQ/KQ mice. Importantly, all of these synaptic regulators (Nos1, Lrp8, Argef2, Sema5b) and other significantly altered splice variants (e.g., Adipor2, Mapk14, Smarca4, Sort1, Mapt) have all been linked to AD and other neurodegenerative diseases101,141,143–151. Altogether, our transcriptome data strongly suggests that splicing and transcriptional abnormalities due to TDP-43 acetylation impacts synaptic plasticity, neurotransmission, and neuronal survival pathways in the TDP-43KQ/KQ model, which overlaps with abnormalities found in human FTLD-TDP patients.
In conclusion, disrupting RNA-binding with an acetylation mimic TDP-43K145Q mutation results in an age-dependent, dementia-like neurodegenerative phenotype characterized by many signatures of FTLD, including progressive cognitive deficits, TDP-43 mislocalization and phosphorylation, and prominent RNA dysregulation. Interrogation of the affected genes linked TDP-43 acetylation to many known FTLD-associated pathways, including SORT1-PGRN and stress response signaling, while also pinpointing new putative factors as mediators of disease pathogenesis. Thus, by developing new human and mouse models of TDP-43 acetylation, we expanded our understanding of sporadic TDP-43 proteinopathies and provide a valuable model in which to identify and test new avenues for therapeutics that target the TDP-43 proteinopathy pathogenic cascade.
MethodsAnimal Husbandry
Mice were housed in ventilated microbarrier cages on racks providing HEPA filtered air supply to each cage. Animals were kept on a 12hr light-dark cycle with access to food and water ad libitum. All animal husbandry, experiments, and procedures were performed in strict compliance with animal protocols approved by the Institutional Animal Care and Use Committee (IACUC) of the University of North Carolina at Chapel Hill (Protocol #21.257).
Primary neuron cultures
Murine primary cortical neurons were performed using wild-type C57Bl/6 mice (Charles River) or TDP-43wt/KQ breeding pairs. Timed pregnant females at embryonic day 15-16 were lethally anesthetized with isoflurane. The abdominal cavity was opened, and the uterus incised to remove the placentas and embryonic sacs, which were washed briefly in ice-cold 70% ethanol and then placed in cold HEPES-buffered Bank’s balanced salt solution (HBSS). The embryos were transferred into a 10cm dish containing cold HBSS, the embryos removed from the amniotic sac, and the brains were extracted from the cranium. For TDP-43wt/KQ dissections, process was paused for approximately 2 hours to permit genotyping of the fetuses, during which the brains were stored at 4° C protected from light in a Hibernate-E (BrainBits NC9063748) solution supplemented with B27 (Gibco 17504044) and GlutaMAX (ThermoFisher 35050061). After genotyping, if applicable, the cerebral cortices from each brain were isolated under a stereomicroscope, pooled by genotype, minced with forceps, and digested for 30min at 37°C in a filter-sterilized HBSS solution containing 20U/mL papain (Worthington Biochemical LS003126), 1mM EDTA, 0.2mg/mL L-cysteine, and 5U/mL DNAse (Promega M6101). The enzyme solution was removed, and the digested tissue was washed twice with sterile HBSS. Warm plating media [BrainPhys media (Stemcell 05790), 5% fetal bovine serum, 1x penicillin/streptomycin (Gibco 15140122), 1x B27, 1x GlutaMAX] containing 5U/mL DNAse, was added and the tissue was dissociated mechanically using a P1000 pipette. The resulting cell suspension was spun down for 5min at 1.5rcf to pellet the cells, resuspended in plating media, and filtered through a 40 mm cell strainer. Cells were counted using a hemocytometer and plated onto poly-D-lysine (PDL)-coated 12-well tissue culture plates (Corning 356470) at 300K cells/well (for RNA extraction) or 96-well glass bottom black wall plates (Cellvis P96-1.5H-N) at 30K/well (for immunofluorescence and microscopy). 16-24hours after plating, all plating media was removed and replaced with neuronal cell media (BrainPhys, 1x GlutaMAX, 1x B27, 1x penicillin/streptomycin). Cultures were incubated at 37°C, 5% CO2 and 95% humidity with half-media exchanges every 3 days for the duration of all experiments.
Lentivirus preparation and neuron transduction
Lentiviral vectors were constructed in-house using the pUltra vector as a backbone for cloning via restriction enzyme digestion followed by ligation. The pUltra construct was acquired from Addgene (gift from Malcolm Moore; Addgene plasmid # 24129 ; http://n2t.net/addgene:24129 ; RRID:Addgene_24129)152. To generate TDP-43 constructs, TDP-43 variant gene fragments were PCR amplified using TDP-43 F’ and R’ primers (Supplementary Table 7) and a pcDNA5/TO-myc-TDP-43wt, - myc-TDP-43K145Q, or -myc-TDP-43K145R plasmid52 as the template. The pUltra construct was digested with AgeI and BamHI, and then incubated with the appropriate TDP-43 PCR product in the presence of T4 DNA ligase and T4 Polynucleotide Kinase for 1 hour at room temperature. The resulting ligation product was transformed into NEB stable competent cells using standard protocols.
To generate the hSyn-IBB-GFP-mCherry3E construct, we isolated the IBB-GFP-mCherry3E coding sequence from pHBS1389-IBB-GFP-mCherry3E (gift from Rajat Rohatgi; Addgene plasmid # 118803 ; http://n2t.net/addgene:118803 ; RRID:Addgene_118803)68,153 via digest with AgeI and XhoI. In addition, the pUltra Ubc promoter was excised by PacI and AgeI digestion, and the hSyn promoter sequence was inserted via ligation as described above to generate a hSyn-eGFP construct. Finally, the eGFP of hSyn-eGFP was removed with AgeI and SalI digestion and then replaced by incubation with the IBB-GFP-mCherry3E fragment in the presence of T4 DNA ligase and T4 Polynucleotide Kinase for 1 hour at room temperature. The resulting ligation product was transformed into NEB stable competent cells using standard protocol.
Lentiviral production was performed by co-transfecting (CalPhos™ Mammalian Transfection Kit, TakaRa 631312) 37.5 μg lenti-plasmid with 25 μg psPAX2, 12.5 μg VSVG, and 6.25 μg REV for each 15 cm dish of lenti-X 293T cells (Takara 632180) and 3 dishes of cells were used for each lentiviral production. 3 days after transfection, culture media were collected and centrifuged at 2000g for 10 min. Lentiviral particles were purified using a double-sucrose gradient method. Briefly, the supernatants were loaded onto a 70%-60%-30%-20% sucrose gradient and centrifuged at 70,000g for 2 hr at 17°C (Beckman Optima LE-80K Ultracentrifuge, SW 32 Ti Swinging-Bucket Rotor). The 30%-60% fraction containing the viral particles was retrieved, resuspended in PBS, filtered with a 0. 45μm filter flask before loaded onto a 20% sucrose cushion, and centrifuged a second time at 70,000g for 2 hr at 17°C. The supernatants were carefully discarded, and the viral particles present in the pellet were resuspended in PBS, aliquoted and stored at -80°C.
Lentivirus aliquots were evaluated for neuronal transduction ability prior to each experiment and diluted to approximately equal concentrations of effective virus. Lentiviral transductions were performed on DIV10 for a DIV14 harvest or DIV24 for a DIV28 harvest. In brief, lentiviruses were diluted to a 2x concentration in neuronal media, and then added to plates via half-media exchange. 24 hours after viral transduction, all virus-containing media was removed and replaced with 50% fresh neuronal media and 50% conditioned media from plates containing WT non-transduced neurons. Neurons were then cultured to DIV14 or DIV28 as the experiment required.
hiPSC-derived cortical neurons
iPSC maintenance: The HPC26 iPSCs were maintained on Matrigel-coated dishes (Corning 354480) in StemFlex Medium (Thermo FisherScientific A3349401) and passaged every 3-4 days with 0.5 mM EDTA dissociation solution as previously described154.
CRISPR/Cas9 genome editing: We used a control wildtype HPC26 cell line154 to generate two separate point mutations in the TARDBP gene that encodes the TDP-43 protein. Both mutations created single amino acid substitutions to change lysine 145 to glycine or arginine (K145Q and K145R). Benchling was used to design two guide RNAs and the corresponding single-stranded DNA donor oligos. The guide RNAs were purchased from Synthego, and the donor oligos from IDT. Genome editing was performed as described in Battaglia et. al155. Briefly, 3x105 iPSCs were electroporated on the Neon electroporation system (Thermo Fisher Scientific) with recombinant protein complexes made of Cas9 v2 protein (Thermo Fisher Scientific A36498), 900 ng sgRNA-TARDBP and 2700 ng of single-stranded donor oligonucleotide-K145Q or donor oligonucleotide-K145R (Supplementary Table 7). Seventy-two hours after electroporation, cells were plated for single cell screening on a 96-well plate format using the limited dilution method. After two weeks, single cells were expanded, genome DNA collected and the exon 4 of TARDBP amplified using the specific primers Ex4Fr and Ex4Rv (Supplementary Table 7). Screening for single or double allele gene edits was perform by Sanger sequencing (Fig. S2a-c).
Characterization of edited iPSC clones: Stemness edited iPSCs was assessed by immunofluorescent staining of the pluripotency factors OCT4, SOX2, SSEA4 and Tra-1-60 using specific antibodies (Supplementary Table 8) as described154 (Fig. S2d-e). To confirmed stemness and differentiation capabilities of the edited iPSCs we used the Taqman hPSC Scorecard (ThermoFisher A15871) as described in Battaglia et al155. Briefly, iPSCs were differentiated into all three germ layers using STEMdiff Trilineage Differentiation Kit (StemCell Technologies), a mono-layer based protocol to directly differentiate hES cells in parallel into the three germ layers. Non-differentiated and differentiated cells were lysed and total RNA purified using the RNeasy kit (QIAGEN 74004). RNA reversed transcription was performed with the high-capacity cDNA Reverse Transcription kit (ThermoFisher 4368813) following the Taqman Scorecard’s manufacture guidelines. qRT-PCR was carried out using the QuantStudio 7 Flex Real-Time PCR system. The TaqMan PCR assay combines DNA methylation mapping, gene expression profiling, and transcript counting of lineage marker genes156.
iPSC-derived cortical neurons: We adapted, modified and standardized a protocol to generate mature cortical neurons from iPSCs using a dual SMAD inhibition protocol157,158. First, undifferentiated iPSCs were collected with Accutase, counted and 3x105 cells cultured in StemScale PSC medium (A4965001) supplemented with 10 µM Y27632 (PeproTech 1293823) and cultured in suspension on an orbital shaker at 37 C and 5% CO2. After 48 hours, cells were dissociated with Accutase and 3x105 cells differentiated into NPCs as neutrosphere in StemScale PSC medium supplemented with 10 µM Y27632 (PeproTech 1293823) and 1.5 µM CHIR99021 (PeproTech 2520691),10 µM SB431542 (PeproTech 3014193) and 50 nM LDN-193189 (Sigma SML0755). CHIR99021 was removed after 24 hours and cells were cultured for 10 days with daily medium changes and neurospheres dissociated at 1:3 ratio twice a week. Then NPCs were expanded for 7 days in the presence of 20 ng/ml FGF (Peprotech 100-18B) in Neuronal Expansion Medium [1:1 Advanced DMEM/F12 (12634028) and Neural Induction Supplement (A1647801)]. NPCs were plated for maturation on PDL/Laminin coated plates or coverslips in cortical neuron maturation medium [1:1 Advanced DMEM/F12 and Neurobasal; 1X GlutaMAX, 100mM B-mercaptoethanol, 1x B27, 0.5x N2, 1x NEAA and 2.5 mg/ml insulin (Sigma-Aldrich I9278)] supplemented with 10 ng/ml BDNF, 10 ng/ml GDNF, and 10 uM DAPT. Seventy-two hours after plating, cells were treated with 1 µg/ml Mitomycin C for (Sigma M5353) for 1 hour. Cellular identity was assessed specific markers of neuronal progenitor (PAX6, SOX2, and SOX1), and mature neurons (MAP2, TUJ1, CTIP2 and SATB2) (Fig. S3) (Supplementary Table 8). All reagents were purchased from ThermoFisher unless otherwise noted.
Primary mouse neuron and hiPSC-derived cortical neuron arsenite treatments and immunocytochemistry
On DIV14 (mouse primary neurons) or mature hiPSC-derived cortical neurons (~DIV50), neurons were treated for 2 hours with 200µM sodium arsenite (NaAsO2) or vehicle (molecular biology grade water). Following treatment, neurons were fixed with 4% paraformaldehyde (PFA) in 1x phosphate buffered saline (PBS) and washed with 1xPBS. Neurons were permeabilized with 0.3% Triton X-100 in 1xPBS for 15min at RT, blocked in an 8% normal goat serum (NGS) 0.2% Triton X-100 in 1xPBS solution for 1-2 hours at RT, and incubated with primary antibodies (Supplementary Table 8) diluted in 4% NGS, 0.2% Triton X-100, 1xPBS solution overnight at 4°C. The next day, the neurons were washed with 1xPBS, followed by application of fluorescent secondary antibodies (Supplementary Table 8) diluted in 4% NGS, 0.2% Triton X-100, 1xPBS solution for 2 hours at RT, covered. Cells were washed four times with 1xPBS, with the third wash containing 1µg/mL 4′,6-Diamidine-2′-phenylindole dihydrochloride (DAPI). 96-well plates were preserved in an 85% glycerol in 1xPBS solution containing 0.4% sodium azide for microbial prevention. Coverslips were mounted onto pre-cleaned glass slides using ProLongTM Diamond Antifade Mountant (Invitrogen P36961).
Cultured neuron fluorescence microscopy and image analysis
Mouse primary cortical neurons in 96-well plates were visualized via automated fluorescence microscopy using EVOS M7000 Imaging System (ThermoFisher AMF7000) equipped with Olympus 20x/0.75 NA UPlanSApo and Olympus 40x/0.95 NA UPlanSApo objectives, and the following filter cubes: DAPI (357/44 nm Excitation; 447/60 nm Emission), GFP (482/25 nm Excitation; 524/24 nm Emission), Texas Red (585/29 nm Excitation; 628/32 nm Emission), and Cy5.5 (655/40 nm Excitation; 794/160 nm Emission). Images were acquired in an automated fashion, using DAPI fluorescence as the autofocus substrate, taking 16-32 images per well of 2-3 wells per experimental condition. Automated quantitative image analysis was performed using CellProfiler 4.0159,160 to measure TDP-43 fluorescence intensity within neuronal compartments and to count TDP-43-positive foci within neurons. To identify neurons and subcellular compartments, DAPI was used to delineate nuclei and NeuN was used to label neuronal soma. hiPSC-derived cortical neurons were imaged on a Leica SP8X Falcon confocal microscope equipped with a 63x/1.40 NA Plan Apochromatic (oil) objective and hybrid GaAsP detectors, Leica Application Suite X Life Sciences software (Leica, Wetzlar, Germany).
Primary neuron splicing function assays
Primary cortical neurons were cultured and transduced with the hSyn-IBB-GFP-mCherry3E CFTR splicing reporter at DIV10 or DIV24, as described above. For RT-PCR experiments, neurons were washed with 1xPBS and harvested on DIV14 and DIV28, and RNA was extracted using the Qiagen RNeasy mini kit (Qiagen, Inc. 74106) with on-column DNAse digestion per manufacturer’s instructions. RNA concentration was determined using a NanoDrop 2.0 spectrophotometer, 250ng of RNA was used to generate cDNA using Applied Biosystems High-capacity RNA-to-cDNA kit (#4387406) per manufacturer’s instructions. PCR was performed using GoTaq Green Master Mix (Promega M7122) and the Cryptic Exon F’ and Cryptic Exon R’ primers (Supplementary Table 7). PCR products were separated on a 2% agarose Tris-acetate-EDTA gel containing SybrSafe stain (Invitrogen S33102) and visualized on an ImageQuant LAS4000 machine. Band intensities were quantified using LI-COR Image Studio Lite.
Mouse model generation and genotypingCRISPR/Cas9 Reagents
Cas9 guide RNAs targeting the mouse Tardbp K145 codon were identified using Benchling software (Benchling, San Francisco, CA, USA). Three guide RNAs were selected for activity testing. Guide RNAs were cloned into a T7 promoter vector followed by in vitro transcription and spin column purification (RNeasy, Qiagen). Guide RNAs were tested for cleavage activity by in vitro cleavage assay. Each guide RNA was incubated with Cas9 protein (UNC Protein Expression and Purification Core Facility) and PCR-amplified guide RNA target site. The products were run on an agarose gel for analysis of target site cleavage. Based on this assay, the guide RNA selected for genome editing in embryos was Tardbp-g79T (Supplementary Table 7). The donor oligonucleotide for insertion of the K145Q and silent genotyping mutations was Tardbp-K145Q-T (Supplementary Table 7). Two silent mutations were induced to create a unique HinfI restriction enzyme digestion site and permit genotyping by PCR and gel electrophoresis.
C57BL/6J zygotes were microinjected with microinjection buffer (5 mM Tris-HCl pH 7.5, 0.1 mM EDTA) containing 1) 20 ng/ul Cas9 mRNA, 10 ng/ul g79T guide RNA and 50 ng/ul donor oligonucleotide (founder #9) or 2) 20 ng/ul Cas9 mRNA, 400 nM Cas9 protein, 10 ng/ul g79T guide RNA and 50 ng/ul donor oligonucleotide (founders #23, 26, 39, 44, 53). Injected embryos were implanted in recipient pseudopregnant females and resulting pups were screened by PCR and sequencing for the presence of the desired mutant allele. Founder lines were propagated as heterozygotes and regularly sequenced to confirm retention of the K145Q and silent mutations.
We assessed the likelihood of off-target mutations using prediction algorithms to ensure low likelihood of off-target effects. The Benchling MIT off-target score for Tardbp-g79T is 72161, and the CRISPOR-generated MIT off-target score is 86. CRISPOR also gave a Cutting frequency determination score of 86162–164. Moreover, all of the predicted exonic off-target sites from CRISPOR have 4 mismatches, making them unlikely to be mutated. Sequencing of the founder lines and their progeny did not detect any insertions or deletions within a 550bp region surrounding the Tardbp locus.
TDP-43<sup>K145Q</sup> mouse line genotyping
Each mouse was genotyped prior to any experimental use. DNA was extracted from ear punch or toe clip tissue using the HotSHOT method165. PCR amplification of the modified Tardbp locus was performed with ApexRed master mix (Genesee Scientific 42-138B) and TDPKQ F’ and TDPKQ R’ primers (Supplementary Table 7) using the following cycling parameters: 95°C denaturation for 3min; 15 cycles of 95°C for 30sec, 72°C for 30sec then -1°C per cycle, 72°C 60sec; 25 cycles of 95°C for 30sec, 58°C for 30sec, 72°C for 60sec; 72°C extension for 5min. The PCR product was then incubated with HinfI enzyme (New England Biosystems R0155S) in CutSmart buffer (New England Biosystems B7204) for 1hr at 37°C. Digested PCR products were separated on a 2% agarose Tris-acetate-EDTA gel containing SybrSafe stain (Invitrogen S33102) and visualized on an ImageQuant LAS4000 machine.
Mouse Behavior
All testing was performed by experimenters blinded to mouse genotype.
Open field. Exploratory activity in a novel environment was assessed by a one-hour trial in an open field chamber (41 cm x 41 cm x 30 cm) crossed by a grid of photobeams (VersaMax system, AccuScan Instruments). Counts were taken of the number of photobeams broken during the trial in five-minute intervals, with separate measures for locomotor activity (total distance traveled) and vertical rearing movements. Time spent in the center region was used as an index of anxiety-like behavior.
Conditioned fear. Mice were evaluated for conditioned fear using the Near-Infrared image tracking system (MED Associates, Burlington, VT). The procedure had the following phases: training on Day 1, a test for context-dependent learning on Day 2, and a test for cue-dependent learning on Day 3.
Training. On Day 1, mice were placed in the test chambers, contained in sound-attenuating boxes. The mice were allowed to explore the novel chambers for 2 minutes before presentation of a 30-sec tone (80 dB), which co-terminated with a 2-sec scrambled foot shock (0.4 mA). Mice received 2 additional shock-tone pairings, with 80 sec between each pairing, and were removed from the test chambers 80 sec following the third shock.
Context- and cue-dependent learning. On Day 2, mice were placed back into the original conditioning chambers for a test of contextual learning. Levels of freezing (immobility) were determined across a 5-min session. On Day 3, mice were evaluated for associative learning to the auditory cue in another 5-min session. The conditioning chambers were modified using a Plexiglas insert to change the wall and floor surface, and a novel odor (dilute vanilla flavoring) was added to the sound-attenuating box. Mice were placed in the modified chamber and allowed to explore. After 2 min, the acoustic stimulus (an 80 dB tone) was presented for a 3-min period. Levels of freezing before and during the stimulus were obtained by the image tracking system.
Grip strength. Grip strength was evaluated using precision force gauges. Measures were based on paw grasp of a metal grid by a mouse gently pulled by the tail. Digital force meters (Chatillon DFIS-10; Largo, FL) were mounted on an acrylic platform (San Diego Instruments), with two different grids: a left-hand grid for the front paws, and a right-hand grid for all-four paws. Each grid connected to a force transducer, which provided measures of peak force (newtons). Each mouse was given 3 trials, with at least one minute between each trial. Each trial had 2 components: front-paw measures from the left-hand grid, immediately followed by all-four-paw measures from the right-hand grid.
Rotarod. Subjects were tested for motor coordination and motor learning on an accelerating rotarod (Ugo Basile, Stoelting Co., Wood Dale, IL). For the first test, mice were given three trials, with 45 seconds between each trial. Two additional trials were given 48 hours later. Rpm (revolutions per minute) was set at an initial value of 3, with a progressive increase to a maximum of 30 rpm. across five minutes (the maximum trial length). Measures were taken for latency to fall from the top of the rotating barrel.
Morris water maze. The water maze was used to assess spatial and reversal learning, swimming ability, and vision. The water maze consisted of a large circular pool (diameter = 122 cm) partially filled with water (45 cm deep, 24-26o C), located in a room with numerous visual cues. The procedure involved three separate phases: a visible platform test, acquisition in the hidden platform task, and a test for reversal learning (an index of cognitive flexibility).
Visible platform test. Each mouse was given 4 trials per day, across 2 days, to swim to an escape platform cued by a patterned cylinder extending above the surface of the water. For each trial, the mouse was placed in the pool at 1 of 4 possible locations (randomly ordered), and then given 60 sec to find the visible platform. If the mouse found the platform, the trial ended, and the animal was allowed to remain 10 sec on the platform before the next trial began. If the platform was not found, the mouse was placed on the platform for 10 sec, and then given the next trial. Measures were taken of latency to find the platform and swimming speed via an automated tracking system (Noldus Ethovision).
Acquisition and reversal learning in a hidden platform task. Following the visible platform task, mice were tested for their ability to find a submerged, hidden escape platform (diameter = 12 cm). Each mouse was given 4 trials per day, with 1 min per trial, to swim to the hidden platform. The criterion for learning was an average group latency of 15 sec or less to locate the platform. Mice were tested until the group reached criterion, with a maximum of 9 days of testing. When the group reached criterion (on day 5 in the present study), mice were given a 1-min probe trial in the pool with the platform removed. Selective quadrant search was evaluated by measuring the number of crosses over the location where the platform (the target) had been placed during training, versus the corresponding area in the opposite quadrant. Following the acquisition phase, mice were tested for reversal learning, using the same procedure as described above. In this phase, the hidden platform was re-located to the opposite quadrant in the pool. As before, measures were taken of latency to find the platform. On day 5 of testing, the platform was removed from the pool, and the group was given a probe trial to evaluate reversal learning.
Acoustic startle test. This procedure was used to assess auditory function, reactivity to environmental stimuli, and sensorimotor gating. The test was based on the reflexive whole-body flinch, or startle response, that follows exposure to a sudden noise. Measures were taken of startle magnitude and prepulse inhibition, which occurs when a weak prestimulus leads to a reduced startle in response to a subsequent louder noise.
Mice were placed into individual small Plexiglas cylinders within larger, sound-attenuating chambers. Each cylinder was seated upon a piezoelectric transducer, which allowed vibrations to be quantified and displayed on a computer (San Diego Instruments SR-Lab system). The chambers included a ceiling light, fan, and a loudspeaker for the acoustic stimuli. Background sound levels (70 dB) and calibration of the acoustic stimuli were confirmed with a digital sound level meter (San Diego Instruments). Each session began with a 5-min habituation period, followed by 42 trials of 7 different types: no-stimulus (NoS) trials, trials with the acoustic startle stimulus (AS; 40 msec, 120 dB) alone, and trials in which a prepulse stimulus (20 msec; either 74, 78, 82, 86, or 90 dB) occurred 100 msec before the onset of the startle stimulus. Measures were taken of the startle amplitude for each trial across a 65-msec sampling window, and an overall analysis was performed for each subject's data for levels of prepulse inhibition at each prepulse sound level (calculated as 100 - [(response amplitude for prepulse stimulus and startle stimulus together / response amplitude for startle stimulus alone) x 100].
Tissue harvest and preparation
At each end point, mice were anesthetized deeply with isoflurane and euthanized via rapid decapitation. The hippocampus and neocortex were dissected out of the brain on a cold surface using clean surgical tools, placed into cryo-safe nuclease-free microcentrifuge tubes, flash frozen in liquid nitrogen (LN2), and stored at −80C until processing. Frozen tissue was then pulverized in LN2-cooled stainless steel Cryo-Cups using cold stainless-steel pestles and transferred into cold nuclease-free microcentrifuge tubes and immediately stored at −80C until use.
Solubility fractionation, nucleo-cytoplasmic fractionation, and immunoblotting
All steps of protein fractionation were performed on ice unless otherwise indicated. For solubility fractionation, pulverized tissue was suspended in 5µL/mg of ice-cold 1xRIPA buffer (50mM Tris pH 8.0, 150mM NaCl, 1%NP-40, 5mM EDTA, 0.5% sodium deoxycholate, 0.1%SDS) containing a mix of protease, phosphatase, and deacetylase inhibitors [1µg/mL Peptstatin A (Sigma P4265), 1µg/mL Leupeptin (Sigma L2023), 1µg/mL Nα-Tosyl-L-lysine chloromethyl ketone hydrochloride (TPCK) (Sigma T7254), 1µg/mL Trypsin inhibitor (Sigma T9003), 1µg/mL N-p-Tosyl-L-phenylalanine chloromethyl ketone (TLCK) (Sigma T4376), 0.67µg/mL trichostatin A, 10mM nicotidamide, 1mM phenyline thanosulfyl fluoride, 1mM phenylmethylsulfonyl fluoride]. The solution was homogenized by sonication and centrifuged at 4°C for 45min at 18000xrcf. The supernatant was removed and saved as the RIPA-soluble (“soluble”) protein fraction. The pellet was resuspended in RIPA buffer with inhibitor mixture, sonicated, and centrifuged as described above, and the supernatant discarded. The resulting pellet of RIPA-insoluble material was resuspended in approximately 1µL/µg of Urea buffer (7M urea, 2M Thiourea, 4%CHAPS, 30mM Tris, pH 8.5) with the inhibitor mixture (as above), sonicated to homogenize, and centrifuged at room temperature (RT) for 45min at 21000xrcf. The resulting supernatant of RIPA-insoluble, urea-soluble protein fraction was saved as the “insoluble” protein fraction.
Nuclear and cytoplasmic soluble proteins were isolated using the Thermofisher NE-PER Nuclear and Cytoplasmic Extraction Kit (Thermo Scientific 78835) per the manufacturer’s instructions, with the addition of the protease, phosphatase, and deacetylase inhibitors (as above) to each buffer.
All soluble protein fractions were analyzed by BCA assay (Thermo Scientific 23225) to determine protein concentration. Equal quantities of protein per sample were run onto 4-20% Tris-Glycine SDS-PAGE gels (Biorad 5671095) under reducing conditions and then transferred onto nitrocellulose membranes. Total transferred protein (TTP) was assessed using PonceauS (Research Products International Corp P56200) protein stain on nitrocellulose membranes per manufacturer’s instructions. Membranes were washed three times in TBST, once in TBS, and blocked in 2% nonfat milk in 1xTBS for 1-2 hours at RT. Membranes were incubated with primary antibodies (Supplementary Table 8) diluted in 2% milk overnight at 4°C. The primary antibody solution was removed and membranes washed three times in TBST, once in TBS, and incubated with cross adsorbed HRP-conjugated goat secondary antibodies (Supplementary Table 8) diluted in 2% milk at 1:10000 or 1:2000 for soluble or insoluble protein immunoblots, respectively, for 1-2 hours at RT. Blots were then visualized by chemiluminescent imaging using an ImageQuant LAS4000 machine. Densitometry analysis to quantify western blot images was performed in LI-COR Image Studio Lite (Lincoln, NE, USA).
Mouse brain RNA isolation, RNA sequencing, and data analysis
Mouse brain tissue was isolated, flash frozen, and pulverized as described above. Approximately 20mg of pulverized brain tissue per sample was used to isolate RNA. 1mL of TRIzol (Invitrogen 15596018) was added to each nuclease-free eppendorf tube containing pulverized tissue, and tissues were lysed via trituration with a P1000 pipette tip, followed by trituration with a 21G and then a 25G needle on a 1mL syringe. Samples were centrifuged at 4°C for 5min at 10,000rcf to remove tissue debris. The supernatant was removed and added to a new tube containing 200uL of chloroform, which were mixed by inversion and cooled on ice for 5min. Samples were centrifuged at 4°C for 15min at 10,000rcf, and the upper aqueous phase containing RNA was transferred into a new tube, followed by the addition of 100uL of isopropanol to precipitate RNA and overnight incubation at -20°C. The next day, samples were centrifuged at top speed (18,000rcf) for 20min at 4°C to pellet the RNA. The pellets were washed twice with 1mL of ice-cold 70% molecular biology grade ethanol and then air dried for 15min at RT. The RNA pellets were resuspended in 20uL of nuclease-free water. On-column DNAse digestion and RNA clean-up was then performed using the Qiagen RNeasy mini kit (Qiagen, Inc. 74106) per manufacturer’s instructions, followed by elution in nuclease-free water.
RNA concentration was assessed using Qubit® RNA BR Assay Kit (Q10210) and a Qubit® 3.0 Fluorometer. RNA integrity was assessed using an Agilent 4150 TapeStation system and associated RNA screen tape reagents (Agilent 5067-5576). Only samples with an estimated RNA integrity number (RIN) ≥7.0 were sent to the New York Genome Center (NYGC) for bulk total RNA sequencing. Upon receipt at NYGC, RNA samples were re-evaluated for quantification and integrity, using Ribogreen and Fragment Analyzer 5300, respectively. Total RNA libraries were prepped using Kapa Total library prep with Ribo-Erase, in accordance with manufacturer recommendations. Briefly, 500ng of total RNA was used for ribosomal depletion and fragmentation of total RNA. Depleted RNA underwent first and second strand cDNA synthesis. cDNA was then adenylated, ligated to Illumina sequencing adapters, and amplified by PCR (using 9 cycles). The cDNA libraries were quantified using Fragment Analyzer 5300 (Advanced Analytical) kit FA-NGS-HS (Agilent DNF-474-1000) and Spectramax M2 (Molecular Devices) kit Picogreen (Life Technologies P7589). Libraries were sequenced on an Illumina NovaSeq sequencer, using paired end sequencing (2 x 100 bp cycles) to a depth of >75M read pairs per sample.
Raw reads were then trimmed and filtered of adapter sequencing using cutadapt166 and filtered such that at least 90% of bases had a quality score of at least 20. Reads were then aligned to the reference mouse genome (mm10, RefSeq gene annotations) using STAR v2.5.2b167, and transcript abundance was estimated using salmon168. Differential expression between TDP43-KQ and TDP43-WT cortex and hippocampus was then detected using DESeq2 v1.34.0169 in R v4.1.0170, using a design that corrects for both mouse sex and litter effects. These batch effects were also removed from the VST-normalized expression values using limma171. The correlation between Log2 fold change values between differentially expressed genes in the cortex and hippocampus was determined and Pearson correlation coefficient and two-sided p-value were computed by ggpubr stat_cor172,173, and the smoothed linear model was fit using geom_smooth with method="lm".
Differentially expressed genes (padj < 0.05) were then separated into 6 groups based on their intersections between the two brain regions using UpSetR v1.4.0174 and plotted with ComplexUpset175. Normalized expression values were then centered around the mean of TDP43-WT for each respective brain region and plotted with ComplexHeatmap176. Gene Ontology enrichments were then assessed using gprofiler2 v0.2.1177,178 and summarized using simplifyEnrichment v1.7.2179. Previously published differentially expressed genes from Hasan et al92 and Polymenidou et al93 were retrieved from the respective publications; significant over-enrichments as well as human gene symbol mappings to mouse orthologs were performed using gprofiler2 (g:Orth). Differential splicing analyses were performed on splice junctions extracted from genome-aligned BAM files using regtools and LeafCutter180, where tests compared TDP43-KQ to TDP43-WT correcting for sex for each brain region. Results were then summarized and visualized using LeafViz181.
Quantitative RT-PCR
RNA was isolated from mouse brain tissue as described above. cDNA was generated from 500ng of RNA using Applied Biosystems High-capacity RNA-to-cDNA kit (#4387406) per manufacturer’s instructions. Quantitative PCR was performed on a QuantStudio 6 Real Time PCR system (with Thermo Fisher Design & Analysis Software version 2.6.0) using PowerUp SYBR Green master mix (Applied Biosystems A25776). The PCR phase consisted of 40 cycles of 15s at 95°C and 1min at 60°C. Forward and reverse primer sequences for Tardbp, Sort-WT, Sort1-total, Sort1-ex17b and the reference genes β-Actin and Pgk1 are listed in Supplementary Table 7. Relative quantification of transcripts was performed using the Pflaff method182 with both β-Actin and Pgk1 as reference genes. Primers generated for this paper were designed using PrimerBank183 and synthesized by Integrated DNA Technologies (IDT).
Tissue Collection, Staining, and Immunofluorescence
Mice were transcardially perfused with 1xPBS followed by 4% PFA, and brains were removed and post-fixed for 24 hours in 4% PFA. Brains were cryoprotected in 15% sucrose in 1xPBS for 24hrs followed then 30% sucrose in 1xPBS for 48hrs, and then embedded in Fisher Tissue-Plus OCT compound (4585). Forebrain (~Bregma +1.78 mm), midbrain (~Bregma +0.50 mm), and hippocampal (~Bregma -1.94 mm) 10 mm cryosections were collected onto Superfrost Plus charged slides using a Leica CM1950 cryostat.
Luxol Fast Blue and Cresyl Violet staining. Slides were acclimated to room temperature and rinsed in distilled water. They were then incubated in 70% ethanol for ~60 hours followed by 95% ethanol for 30 minutes. Tissues were stained with 0.1% Luxol Fast Blue Solution overnight at 60°C, rinsed in distilled water, and differentiated as needed by 0.05% Lithium Carbonate followed by 70% ethanol. They were then counterstained with 0.1% Cresyl Echt Violet, rinsed in distilled water, and differentiated again by 95% ethanol. Finally, slides were dehydrated in 100% ethanol, cleared in Xylene, and coverslipped with DPX mountant (Millipore-Sigma 44581).
Immunofluorescence. Slides were acclimated to room temperature and rinsed in distilled water. Heat-induced epitope retrieval was performed at 120°C using pH 6.0 buffer (Epredia, TA-135-HBL). The tissues were then blocked in 10% normal goat serum for 1 hour. Primary antibodies were applied overnight at 4°C: Mouse Anti-NeuN (clone A60) conjugated to Alexa Fluor 555 (Millipore, MAB377A5), and Rabbit Anti-TDP-43 (Cell Signaling, 89789) or Rabbit anti-Iba1 (Wako, 019-19741) and mouse anti-GFAP conjugated to Alexa Fluor 488 (Cell Signaling, 3655) (Supplementary Table 8). After rinsing, secondary antibodies were applied for 2 hours at room temperature: Alexa Fluor 488 Goat Anti-Rabbit IgG (Invitrogen, A32731) or Alexa Fluor 680 Goat anti-Rabbit IgG (Invitrogen 32734). All antibodies were diluted using Da Vinci Green Diluent (Biocare Medical, PD900L). Finally, slides were coverslipped using Fluorogel II with DAPI (Electron Microscopy Sciences, 17985-50).
Tissue microscopy and Image Analysis
Brain sections stained with Cresyl Violet (CV) and brain sections labelled with Iba1 and GFAP anatibodies were imaged on a Nikon Eclipse Ti2 widefield microscope equipped with a Nikon DS-Fi3 CMOS color camera (for CV imaging) and a pco.edge 4.2Q High QE sCMOS camera (PCO, Kelheim, Germany) using a 20x/0.5 NA Plan Fluor objective and NIS-Elements software (Nikon, Minato City, Tokyo, Japan). Immunofluorescently labeled brain sections were imaged on a Leica SP8X Falcon confocal microscope equipped with hybrid GaAsP detectors using a 40x/1.30 NA Plan Apochromatic (oil) objective and Leica Application Suite X Life Sciences software (Leica, Wetzlar, Germany). Cell counting (density of CV-positive and NeuN-positive cells), as well as TDP-43 fluorescence intensity, TDP-43 localization measurements, GFAP fluorescence intensity and area, and Iba1 fluorescence intensity and area were performed using CellProfiler159,160. Prior to quantifying TDP-43 immunofluorescence intensity and prior to confocal image segmentation, immunofluorescent images were denoised using NIS-Elements Batch Denoise.ai under default conditions. Prior to quantifying Iba1 and GFAP fluorescence characteristics, images of the necortex and hippocampus were cropped to exclude fluorescent signal from the corpus callosum. Within CellProfiler, the Cellpose 2.0 plugin184,185 was used to perform neuron identification and the subcellular segmentation of nucleus and cytoplasm within neurons. DAPI fluorescence was used to identify nuclei, and NeuN immunoreactivity was used to identify neuronal nuclei and the surrounding soma. Images were pseudocolored and formatted for publication using Fiji ImageJ186.
Statistical Analysis
Statistical analysis of RNA sequencing data was performed as described above. All other data was analyzed in GraphPad Prism Version 9.4.1 for Windows, GraphPad Software, San Diego, California USA, www.graphpad.com. For data sets with n>10, outliers were identified and removed (if any) using the ROUT method187 at Q=1%. Details regarding the statistical test performed, sample sizes, and what the data points and error bars represent can be found in the appropriate figure legends. All statistical tests were two-sided. Statistical significance was determined as p<0.05.
Data availability
The TDP-43K145Q mouse line is now available at the Mutant Mouse Resource and Research Center (MMRRC) at University of North Carolina at Chapel Hill, an NIH-funded strain repository, with the following identifiers (RRID:MMRRC_068119-UNC). Raw and processed RNA-seq data have been deposited to the Gene Expression Omnibus (GEO) under accession GSE216294. All data generated in this work are included in the manuscript and the supporting files. Any plasmids or lentiviruses generated in this study are available upon request.
Supplementary statistical information about data presented in main and supplemental figures can be found in the figure’s accompanying source data file. Figures 3, 5, and 7 have accompanying source data files. Supplementary figures S5, S6, and S8 have accompanying source data files.
Author contributions
JCN designed, executed, and directed most of the experiments in this study, primary neuron experiments, primary neuron and hiPSC immunofluorescence, microscopy, and image analysis, mouse brain tissue analysis, protein and RNA isolation, immunoblotting, and data analysis. YC assisted with plasmid preparation, mouse breeding and genotyping, and mouse tissue harvest and mouse perfusion. XT constructed plasmids and generated all lentiviruses. BAE assisted with assays of splicing function. ABM and ASB created and maintained human induced pluripotent stem cell clones, including CRISPR-mediated genome modification, and differentiated hiPSC lines into cortical neurons. PW and DA assisted with mouse handling, immunoblotting, and early characterization of the mouse line. Behavioral testing and initial data analysis was conducted by VDN, KMH, and SSM. JAE performed histology and immunofluorescent labeling of mouse brain sections. SN assisted with immunoblotting. JMS performed RNAseq data analysis and figure construction. This study was supervised and guided by TJC.
Acknowledgements
We would like to thank Dale Cowley and the UNC Animal Models Core Facility for their creation of the TDP-43K145Q mouse line and the initial sequencing. We also thank Natallia Riddick and the UNC Mouse Breeding and Colony Management Core for their assistance in animal husbandry and breeding. Confocal and Nikon Ti2 widefield microscopy was performed at the UNC Neuroscience Microscopy Core (RRID:SCR_019060), supported, in part, by funding from the NIH-NINDS Neuroscience Center Support Grant P30 NS045892 and the NIH-NICHD Intellectual and Developmental Disabilities Research Center Support Grant P50 HD103573, and core director Michele Itano assisted in image analysis protocol design. We are grateful for the UNC Histology Research Core for their assistance in tissue processing and staining. RNA sequencing was performed by the New York Genome Center. The research reported in this publication was supported by the National Institute on Aging (grant F30AG072786), the National Institute on Neurologic Disorders and Stroke (grants R01NS105981, P30NS045892, F31NS122242), the National Institute of General Medical Sciences (grants 1T32GM133364-01A1, 5T32GM008719-19), and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (grants U54HD079124; P50HD103573) of the National Institutes of Health. We also received support from the Department of Defense (grant AL180038) and the Muscular Dystrophy Association (grant MDA573414 and NINDS (P30NS045892).
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10.7554/eLife.85921.1.sa3eLife AssessmentPfefferSuzanne RReviewing Editorhttps://ror.org/00f54p054Stanford UniversityStanfordUnited States of AmericaConvincingImportant
Necarsulmer et al describe an interesting new mouse model of TDP-43 proteinopathy in which gene editing was used to introduce a K145Q acetylation-mimic mutation previously shown to impair RNA-binding capacity and induce downstream misregulation of target genes. Mice homozygous for this mutation are convincingly shown to display cognitive/behavioral impairment, TDP-43 phosphorylation and insolubility, and changes in gene expression and splicing. This novel mouse model replicates some important hallmarks of human frontotemporal lobar degeneration and will be an important contribution to the field after several points are adequately addressed.
With this work, the authors address a central question regarding the potential consequences of post-translational modifications for the pathogenesis of neurodegenerative diseases. Phosphorylation and mislocalization of the RNA binding protein TDP43 are characteristic of ~50% of frontotemporal lobar degeneration (FTLD), as well as >95% of amyotrophic lateral sclerosis (ALS). To determine if acetylation is a primary, disease-driving event, they generated a TDP-43 mutant harboring an acetylation-mimicking mutation (K145Q). Animals carrying the acetylation-mimic mutation (K145Q) displayed key pathological features of disease, including more cytoplasmic TDP43 and impaired TDP43 splicing activity, together with behavioral phenotypes reminiscent of FTLD.
This is a well-written and well-illustrated manuscript, with clear and convincing findings. The observations are significant and emphasize the importance of post-translational modifications to TDP-43 function and disease phenotypes. In addition, the TDP43(K145Q) mice may prove to be a valuable model for studying TDP-43-related mechanisms of neurodegeneration and therapeutic strategies.
However, as it stands it is challenging to determine if any or all of the phenotypes are a direct consequence of interrupted RNA binding by TDP-43, rather than acetylation per se. Furthermore, all the results are obtained using an acetylation-mimic mutation that may simply be disrupting a key residue involved in RNA binding by TDP43, instead of mirroring acetylation itself, which in theory is a reversible modification. Lastly, it remains unknown why TDP43(K145Q) mice developed features of FTLD, but not ALS, despite the fact that TDP-43 acetylation was found in ALS tissue and not FTLD.
This paper extends prior work demonstrating the importance of K145 acetylation of TDP-43 as a post-translational modification that impacts its RNA-binding capacity and may contribute to pathology in FTLD-ALS. The main strengths of this paper are the generation of a novel mouse model, using CRISPR gene editing, in which an acetylation-mimetic mutation (K to Q) is introduced at position 145. Behavioral, biochemical, and genetic analyses indicate that these mice display phenotypes relevant to TDP-43-associated disease and will be a valuable contribution to the field. While most of the data are rigorous and clearly presented, several weaknesses should be addressed to strengthen the manuscript and further characterize the phenotype of mutant mice.
Numerous experimental models are phenotyped in this manuscript including mouse neurons, iPSC-derived human neurons, knock-in mice, and knock-in iPSCs. Expression of acetylation-mimic or acetylation-null TDP-43 protein is achieved either with overexpression or CRISPR-Cas9-based knock-in. A complex phenotype is observed including loss of TDP-43 function (reduced autoregulation, increased cryptic splicing) and a gain of TDP-43 (increased insoluble TDP-43 protein). These correlate with downstream neurobehavioral changes which are most consistent with a cortical/hippocampal phenotype without a motor phenotype. Post-translational modifications of disease-associated proteins are thought to contribute to neurodegenerative disease pathogenesis, and this study succeeds in demonstrating that TDP-43 acetylation results in downstream molecular and behavioral phenotypes.
There are numerous additional strengths. TDP-43 acetylation is a post-translational modification that is known to be associated with TDP-43 inclusions that are characteristic of human diseases. An important strength is the rigorous use of multiple different experimental models (rodent cells, iPSC-derived neurons, mice, overexpression, knock-in) with overall consistent results. Moreover, multiple orthogonal endpoints are presented including histology/cytology/immunostaining, biochemistry, molecular biology, and neurobehavioral assays. As TDP-43 acetylation is known to block RNA binding, these novel cellular and mouse models represent interesting albeit complex tools to study the functional consequences of a partial loss of function. As TDP-43 regulates its own expression (i.e. autoregulation), the complexity lies at least in part due to the loss of RNA binding leading to a functional loss of TDP-43 function which includes the increased expression of the TARDBP transcript and TDP-43 protein.
Conceptually, there is a disconnect in that the mouse model exhibits primarily a cortical/hippocampal phenotype more akin to frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP), while TDP-43 acetylation is only seen in ALS tissues and not in FTLD-TDP tissues because most of the pathologic protein in the latter is N-terminally truncated (i.e. the acetylation site is not present). That being said, there is no mouse model which completely and faithfully recapitulates the human disease, and this mouse model avoids overt overexpression (increased TDP-43 protein expression stemming from altered autoregulation) and avoids the use of synthetic/artificial mutation (such as mutation of the TDP-43 nuclear localization signal).
In terms of the CNS phenotype, it is difficult to interpret the reduced density of NeuN-positive neurons in the mouse model as a neurodegenerative phenotype. The reduction in NeuN density but not overall cellular density is only suggestive of neurodegeneration (as opposed to, for example, a developmental phenotype) without more rigorous stereological approaches that take into account potential volumetric changes. Indeed, the absence of astrocytosis and microgliosis argues against neurodegeneration.
Some of the loss of function measures (CFTR construct splicing, shift in SORT1 protein) are subtle, although RNA sequencing clearly shows many splicing aberrations including cryptic splicing events which overall supports that a loss of TDP-43 function is observed.
Finally, there are multiple instances where multiple measurements are made on a few biological replicates. ANOVA or t-tests are not appropriate in these instances (lack of independence).