The toxicity of the RNA/DNA binding protein TDP-43, first identified a decade ago by its cytosolic mislocalization in spinal motor neurons of Amyotrophic lateral sclerosis (ALS) patients was subsequently implicated in several other neurodegenerative disorders including Frontotemporal dementia (FTD) as well as Alzheimer's disease (AD) related dementias (ADRD). Studies have also suggested a possible cross-talk of tau and TDP-43 pathology in AD and brain injury-predisposed dementia. However, the role of TDP-43 in dementia development/progression is unclear. A major challenge in the field is the non-availability of an appropriate animal model that mimics complex nature of TDP-43 pathology, which includes both its loss of nuclear functions and gain of toxicity due to its cytosolic aggregation. The hemizygous knock out (KO) does not show consistent neurodegenerative or behavioral phenotype due to autoregulation of Tdp-43 level, whereas the homozygous KO of Tdp-43 causes embryonic lethality. Contrarily, various transgenic models were not appropriate for exploring new therapeutics as they do not represent both loss of function and gain of toxicity of Tdp-43 pathology. We recently identified for the first time, TDP-43's role in genome maintenance in spinal motor neurons, specifically for the repair of DNA double strand breaks (DSBs). Guided by our strong preliminary data in human iPSC-derived motor neurons that revealed that nucleus-specific loss of TDP-43 rather than its total loss or overexpression closely mimics its pathology in ALS patients, we have generated a novel conditional Tdp-43?NLS mouse model using a combination of CRISPR/Cas9 technology and FLEx (Cre-dependent Flip-Excision genetic switch) strategy, with a targeted insertion of loxP-tagged nuclear localization signal (NLS) deleted exon3, in reverse orientation next to wild-type exon3 of the mouse Tardbp gene. This mouse model conditionally expresses the mutant Tdp- 43?NLS protein when crossed with inducible Cre mice, exhibiting dual phenotype of nuclear clearance and cytoplasmic sequestration. Furthermore, our inducible approach makes it highly versatile, to study the impact of Tdp-43's nuclear loss in a specific region of CNS, neuron or astrocytes, separately, by crossing with appropriate Cre-mice. Our initial characterization of heterozygous Tdp-43?NLS-Cre mice showed strong motor and cognitive defects, which also strongly correlated with accumulated DSBs in the brain and spinal cord. The goal of this project is to comprehensively characterize this mouse model for ALS, FTD and AD phenotype, by inducing Tdp-43 NLS deletion in the CNS or specifically in cortex and hippocampus using appropriate Cre mice and to test whether Tdp-43 toxicity-mediated defective DNA damage response (DDR) contributes to motor dysfunction/dementia. This will be the first conditional Tdp-43-ALS knock-in mouse model to represent both loss-of-function and gain-of-toxicity phenotypes, using endogenous Tdp-43 as target and thus, provides a distinct animal model, for exploring therapeutic interventions.
/ Health Relevance Statement RNA/DNA binding protein TDP-43 has been linked to Amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and other Alzheimer's disease related dementia (ADRD) for which no effective treatments are available even to slow the rate of disease progression. However, most of the research focus of the last decade was limited to TDP-43 and ALS, its implications in dementia is largely unexplored. This is primarily due to non- availability of appropriate animal models that closely mimic TDP-43 pathology in these conditions. Now we have generated a conditional TDP-43 nuclear loss of function knock in model that allows for tissue-specific induction of TDP-43 pathology in age-dependent manner. We also discovered a crucial role of TDP-43 in genome maintenance in ALS. Using this novel mouse model, we propose to test the connection between TDP- 43 pathology and genome maintenance defects in development and progression of dementia. These innovative studies thus represent an unexplored area of dementia research and would lead to a major paradigm shift in our understanding of the molecular pathogenesis of TDP-43 pathology not only ALS, but in FTD and ADRD and should lead to mechanism-driven avenues for prevention strategies.
