O3-04-03: Tau splicing in vivo : Application in mouse models of tauopathy confers functional and pathological changes

Select tauopathies, including Alzheimer's disease, exhibit characteristic deposition of 3R or 4R tau within neurofibrillary tangles despite their physiologically equal expression. Furthermore, certain tau mutations cause exclusive 4R tau expression, leading to neurodegenerative disease. Given this evidence, we hypothesized that reducing 4R tau will mitigate age-related pathological and behavioral deficits observed in a mouse model of 4R tauopathy (TauN279K). To further implicate 4R tau as a disease mediator, we hypothesized that driving greater 4R tau expression in a human tau (hTau)-expressing mouse would worsen functional outcome and increase pathological tau burden. Antisense oligonucleotides (ASOs) designed to either exclude exon 10, thereby promoting 3R tau expression, or include exon 10, for increased 4R tau were continuously administered into the lateral ventricle to young N279K or hTau mice. Following treatment, mice were assessed for functional outcomes and euthanized for mRNA and protein analysis and tau pathology. Both splicing ASO treatment in N279K (4R to 3R) and hTau (3R to 4R) mice afforded a pronounced tau isoform switch, indicative of successful ASO activity in vivo. Tau splicing from 4R to 3R in N279K mice significantly attenuated deficits in both movement behavior and male mice nesting activity compared to saline-treated littermate controls. Additionally, tau accumulation in the frontal cortex and dentate nucleus of N279K mice was mitigated with splicing alteration. When the tau isoform ratio was reversed using 3R to 4R ASOs in hTau mice, functional and pathological outcomes appeared to worsen. hTau mice given splicing ASO exhibited more severe chemically-induced seizures and significantly reduced seizure latencies. Hyperphosphorylated tau within the select brain regions was elevated in splicing ASO-treated hTau mice compared to control treatments, further indicating an exacerbated neurodegenerative phenotype. Overall, tau splicing alteration from 4R to 3R tau afforded both behavioral and pathological improvements in N279K mice, while increasing 4R tau in a separate model resulted in a more severe excitability profile and worsened tau pathology. This study is the first to demonstrate effective tau splicing in vivo, identifying 4R tau as an important target and providing support for tau isoform manipulation by ASOs as a potentially effective therapy for isoform-biased tauopathies.