Emerging roles for RNA binding proteins in the pathogenesis of Alzheimer's disease and frontotemporal dementia
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Abnormal aggregation of microtubule associated protein tau is the defining pathological hallmark of tauopathies, which include Alzheimer’s disease (AD) and related frontotemporal dementias (FTLD-tau). However, the cellular events precipitating tau pathogenesis in disease are unknown. Here, we demonstrate a novel mechanism regulating tau aggregation in tauopathies. We have previously shown that RNA binding proteins (RBPs) associated with stress granules (SGs) progressively accumulate with tau in multiple mouse models of tauopathy, as well as in human AD and FTLD-tau brain tissue. We now present a novel functional role for tau in regulating the biology of SGs in neurons. Tau facilitates the rapid formation of SGs in the soma and dendrites in response to exogenous stress, which functions to transiently reprogram protein synthesis to promote cell survival (also known as the ‘translational stress response’). However, the chronic interaction of tau with SG proteins in disease, such as with the SG nucleating protein T cell intracellular antigen 1 (TIA1), promotes tau misfolding and neurotoxicity, which can be modulated in primary neurons by pharmacological or genetic manipulations that increase (i.e. puromycin, TIA1 overexpression) or decrease (i.e. cycloheximide, TIA1 knockdown or knockout) SG formation, respectively. In order to test whether SGs also mediate the progression of tauopathy in vivo, we crossed PS19 transgenic (P301S) tau mice with Tia1-/- or C57BL/6J (background strain) mice. PS19 mice with heterozygous reduction in TIA1 (P301S TIA1+/-) developed less SGs compared to P301S TIA1+/+ mice, which was associated with marked neuronal protection, improved cognitive function, and prolonged lifespan. The behavioral neuroprotection in P301S TIA1+/- mice was associated with decreased accumulation of soluble tau oligomers, and occurred despite the increased presence of neurofibrillary tangles. Our findings suggest that TIA1 stabilizes tau in its oligomeric state, preventing its further assembly into insoluble fibrils, which are less toxic. More importantly, the studies described in this dissertation identify modulation of RBP aggregation in SGs as a promising therapeutic strategy for the treatment of AD and FTLD-tau.