S4‐03‐06: Analysis of tau associated proteins in secreted exosomes: Clues to tau‐mediated neurodegeneration?

The identification of beta amyloid (BA), prion protein (PrP), alpha synuclein (ASN) and (recently) tau as exosomally secreted proteins raise questions of a) whether synergistic pathogenetic interactions between these proteins are due to interactions between these proteins in the exosomal pathway in AD, Parkinson's Disease', Creutzfeld Jacob disease and related neurodegenerative conditions, and b) whether secretion and interneuronal transfer of exosomal Abeta, ASN, PrP and tau are involved in disease pathogenesis. In order to characterize the protein interactions that occur with the misprocessing and secretion of overexpressed tau in human neurons, we performed mass spectroscopic analysis of the proteins that copurified with tau in exosome fractions from M1C cell conditioned media. M1C cells (Ko et. al 2004) were induced to secrete 4R0N human tau using the “TET-off” system (Kim et. al. 2010). Exosomes purified from M1C concentrated media samples and cell lysates were prepared for mass spectroscopic analysis. Identification of proteins that were co-enriched with tau relative to exosomal lysate fractions was achieved by quantitative mass spectrometry and computational analysis and by the use of analysis MASCOT database for protein identification. Co-enriched proteins with tau in exosomal M1C media samples relative to lysate crude and exosomal samples from the same cultures included lipid raft, endosome and exosome associated membrane proteins (flotillin, annexins, srk family kinases, Rab family proteins, Alix) also cytoskeleton associated proteins often found in exosomes (tubulin, dyneins). Of particular note were the presence of 1) tau kinases and phosphastases (PP2A, ERK and Src family kinases) 2) specific proteins known to interact abnormally with tau in AD (Fyn kinase, beta amyloid) and 3) proteins associated with interrelated cellular functions that involve tau and appear to be disrupted in AD, such as microRNA mediated neuronal polarization (TRIM family proteins), chaperone-mediated degradation mechanisms (HSPc70 associated proteins) and both (e2 ubiquitin ligases). We propose that tau-induced neurotoxicity in AD and non-AD tauopathies involving oligomer/aggregate formation and/or tau secretion may involve 1) loss of normal tau localization to axons and 2) abnormal juxtaposition/interaction of tau and tau associated proteins (fyn, HSc70) with elements of exosomal secretory pathways.