Role of Tau Cleavage and Phosphorylation in Tauopathy

PROJECT SUMMARY It is widely believed that insoluble aggregates of tau underlie its pathogenicity in tauopathies, but recent data implicate a role for soluble tau as well. We identified a candidate soluble tau cleavage product whose formation results in impaired cognition in transgenic mice expressing human tau with the P301L mutation (tauP301L), which causes FTDP-17. This cleavage product, ?tau314, forms when caspase-2 (Casp2) cleaves tauP301L at Asp-314 (D314). Importantly, mutating the site at which Casp2 cleaves tau (D314E) preserves memory function in mice expressing tauP301L, and lowering Casp2 levels in tauP301L-expressing mice restores memory function. Tau is a new, previously unknown substrate of Casp2, a caspase that is unusual in exhibiting both catalytic and non-catalytic activities. When tauP301L is tagged on its N-terminus with GFP and expressed in cultured neurons, a GFP-tagged form of tau mislocalizes to dendritic spines, and this mislocalization depends on both the cleavage and phosphorylation of tau. Tau mislocalization is associated with alterations in synaptic function (diminished amplitudes of mEPSCs) and reduced levels of AMPA receptors (AMPARs) in dendritic spines. Cells lacking Casp2 or expressing Casp2-resistant tauP301L,D314E do not show tau mislocalization or altered dendritic spines. However, the mechanism by which the reduction of Casp2 improves synaptic and cognitive function remains uncertain. Based on our data, we hypothesize that mutant tauP301L leads to activation of kinases and Casp2, resulting in differential site-specific phosphorylation that is required for Casp2 generation of ?tau314, mislocalization of tau, and reductions in AMPAR number and activity that impair synaptic and cognitive function. Our long-term goal is to develop new therapies to treat tauopathies, and our results provide initial evidence that targeting Casp2 may alleviate cognitive impairment. The overall goal of this application is to obtain more evidence of a role for Casp2 and gain more knowledge about the mechanisms by which Casp2 mediates neural dysfunction in mouse models of FTDP-17. As previously reported, Casp2 is believed not only to catalytically cleave target proteins, but also to regulate autophagy in neuronal tissue independent of its catalytic activity. Thus, while not likely, it is possible that some of the effects we observe could be due to non-catalytic activities of Casp2. Our immediate goals are (1) to evaluate the contribution of the catalytic and non-catalytic activities of Casp2 to tauP301L-mediated cognitive and synaptic deficits, (2) to delineate the functional role of ?tau314 in vivo, and (3) to test the hypothesis that in vivo Casp2-tau interactions represent a common downstream effector mechanism of other FTDP-17 mutations.