DESCRIPTION (provided by applicant): Tau aggregation is a hallmark of a growing list of over 20 neurodegenerative diseases, including Alzheimer's disease (AD). The overall objective of the proposed research is to identify a molecular mechanism underlying tau proteostasis, and to determine in vivo how this mechanism modifies tau aggregation and neurodegeneration. Chaperones are key mediators of proteostasis, and the chaperone protein Hsc70 is known to bind tau directly. However, the fate of Hsc70-bound clients is determined by co-chaperones. In a targeted screen for DnaJ co-chaperones that act on tau in neurons, the PI's lab identified two co-chaperones that affect tau in diametrically opposite directions: CSPα (cysteine string protein-α) increases tau levels and stability, while Hsj1 (Homo sapiens DnaJ1) leads to tau degradation. Each of these co-chaperones bind tau in distinct chaperone- complexes: the CSPα/Hsc70/SGT-foldase complex and the Hsj1/Hsc70/CHIP-degradase complex. This study will test the central hypothesis that tau aggregation and neurodegeneration are ameliorated by the activities of these two chaperone machines: tau is stabilized by the CSPα/Hsc70/SGT-foldase complex, and tau is degraded via the Hsj1/Hsc70/CHIP-degradase complex. To test this hypothesis, three specific aims are proposed: Aim 1. Determine how CSPα affects tau stability and aggregation. Aim 2. Determine how Hsj1 affects tau stability and aggregation. In these two aims, actions of CSPα and Hsj1 on tau will be elucidated in molecular de- tail, using purified proteins and primary neurons. Aim 3. Determine how CSPα and Hsj1 affect tau-dependent neurodegeneration in vivo, in mouse models of tauopathic neurodegeneration. Completion of these studies is expected to identify two molecular processes which reduce tau aggregation, by either chaperoning (via CSPα) or degrading (via Hsj1) the Hsc70-bound tau, as well as to reveal how these proteostatic activities affect tau- driven neurodegeneration in vivo. The proposed study is innovative, because two new functions of Hsj1 and CSPα on their new client tau are proposed, and because of the use of a comprehensive experimental approach including, a) purified proteins to reconstitute molecular interactions, b) lentiviral vectors for knocking down and overexpressing CSPα and Hsj1 in primary neurons, c) stereotactic mouse brain injections of lentiviral vectors to study CSPα and Hsj1 effects on neurodegeneration in vivo, d) using patient brain samples to establish relevance of the findings to human Alzheimer's disease pathology. The proposed study is significant because a) understanding how neurons prevent tau from aggregating will establish therapeutic targets com- mon to multiple tauopathies, including Alzheimer's disease, b) the concept of multifaceted proteostasis, where protein aggregation is reduced by the integrated action of "degradase" and "foldase" mechanisms, can be readily translated to other neurodegeneration-linked pr...