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.
