BAG3 and SYNPO (synaptopodin) facilitate phospho-MAPT/Tau degradation via autophagy in neuronal processes.

ABSTRACT

A major cellular catabolic pathway in neurons is macroautophagy/autophagy, through which misfolded or aggregation-prone proteins are sequestered into autophagosomes that fuse with lysosomes, and are degraded. MAPT (microtubule-associated protein tau) is one of the protein clients of autophagy. Given that accumulation of hyperphosphorylated MAPT contributes to the pathogenesis of Alzheimer disease and other tauopathies, decreasing endogenous MAPT levels has been shown to be beneficial to neuronal health in models of these diseases. A previous study demonstrated that the HSPA/HSP70 co-chaperone BAG3 (BCL2-associated athanogene 3) facilitates endogenous MAPT clearance through autophagy. These findings prompted us to further investigate the mechanisms underlying BAG3-mediated autophagy in the degradation of endogenous MAPT. Here we demonstrate for the first time that BAG3 plays an important role in autophagic flux in the neurites of mature neurons (20-24 days in vitro [DIV]) through interaction with the post-synaptic cytoskeleton protein SYNPO (synaptopodin). Loss of either BAG3 or SYNPO impeded the fusion of autophagosomes and lysosomes predominantly in the post-synaptic compartment. A block of autophagy leads to accumulation of the autophagic receptor protein SQSTM1/p62 (sequestosome 1) as well as MAPT phosphorylated at Ser262 (p-Ser262). Furthermore, p-Ser262 appears to accumulate in autophagosomes at post-synaptic densities. Overall these data provide evidence of a novel role for the co-chaperone BAG3 in synapses. In cooperation with SYNPO, it functions as part of a surveillance complex that facilitates the autophagic clearance of MAPT p-Ser262, and possibly other MAPT species at the post-synapse. This appears to be crucial for the maintenance of a healthy, functional synapse.Abbreviations aa amino acids; ACTB actin beta; BafA1 bafilomycin A1; BAG3 BCL2 associated athanogene 3; CQ chloroquine; CTSL cathepsin L; DIV days in vitro; DLG4/PSD95 discs large MAGUK scaffold protein 4; HSPA/HSP70 heat shock protein family A (Hsp70); MAP1LC3B/LC3B microtubule associated protein 1 light chain 3 beta; MAP2 microtubule associated protein 2; MAPT microtubule associated protein tau; p-Ser262 MAPT phosphorylated at serine 262; p-Ser396/404 MAPT phosphorylated at serines 396 and 404; p-Thr231 MAPT phosphorylated at threonine 231; PBS phosphate buffered saline; PK proteinase K; scr scrambled; shRNA short hairpin RNA; SQSTM1/p62 sequestosome 1; SYN1 synapsin I; SYNPO synaptopodin; SYNPO2/myopodin synaptopodin 2; VPS vacuolar protein sorting.