Heat Shock Protein 70 Prevents both Tau Aggregation and the Inhibitory Effects of Preexisting Tau Aggregates on Fast Axonal Transport

Tau is a microtubule-associated protein predominantly expressed in axons where it is involved in the maintenance and stabilization of microtubules.1 Under physiological conditions, tau is a soluble protein with limited secondary structure.2 However, in Alzheimer's disease (AD), tau dissociates from microtubules and self-associates to form both fibrillar and prefibrillar oligomeric aggregates.3,4 Aggregated forms of tau are also found in various other tauopathies, including Pick's disease, corticobasal degeneration, and progressive supranuclear palsy.5 Importantly, the identification of mutations in the tau gene that cause hereditary tauopathies demonstrates that tau dysfunction is sufficient to cause neuronal degeneration. Neurofibrillary tangles (NFTs), a pathological hallmark of AD and other tauopathies, are composed of fibrillar tau aggregates and positively correlate with cognitive decline.6 However, recent evidence suggests that prefibrillar oligomeric tau aggregates may represent the main toxic species.7 For instance, neurodegeneration occurs in some tau overexpression animal models that lack overt neurofibrillary pathology.8,9 Another study demonstrated that levels of early multimeric tau aggregates that preceded neurofibrillary pathology correlated better with memory deficits.10 Moreover, suppression of tau expression improved memory function without affecting existing NFTs.11,12 The exact mechanisms underlying tau toxicity remain a matter of debate. However, recent experiments demonstrated that abnormal activation of kinase-based pathways and disruption of fast axonal transport (FAT) represent toxic gains of function associated with aggregated but not soluble tau species.13–15 Specifically, experiments in isolated squid axoplasm revealed that aggregated tau activates a protein phosphatase 1 (PP1) and glycogen synthase kinase 3 (GSK3)-dependent signaling pathway that results in the inhibition of conventional kinesin-dependent anterograde FAT.13 Given that aggregated tau pathology is a common denominator in several neurodegenerative diseases and that tau aggregates are demonstrably toxic, it follows that prevention of tau aggregation represents a reasonable therapeutic objective. Molecular chaperones make up a highly conserved family of related proteins that prevent protein misfolding and aggregation. Chaperone involvement has been implicated in several neurodegenerative diseases, including Parkinson's disease, Huntington's disease, and AD.16–20 In particular, molecular chaperones of the Hsp70 family are upregulated in AD and attenuate toxicity in a variety of neurodegenerative disease models.21 Hsp70 has been found to facilitate microtubule binding of tau and is associated with decreased levels of insoluble tau.22,23 In addition, Hsp70 facilitates the degradation of insoluble tau aggregates via a direct interaction with CHIP (carboxyl terminus of the Hsc70-interacting protein), a ubiquitin ligase,23 or BAG2 (BCL2-associated athanogene 2), a cochaperone.24 Taken together, the available data suggest that Hsp70 attenuates tau toxicity by maintaining tau in a soluble, nonaggregated state and by facilitating the degradation of aggregated tau species. However, the exact tau aggregate species targeted by Hsp70 chaperones remains unknown. Moreover, specific cellular processes protected by Hsp70 chaperones have not been identified, and thus data showing reduction of tau toxicity remain largely correlative. In this study, we demonstrate that Hsp70 directly inhibits tau aggregation by a mechanism involving preferential associations with soluble, monomeric and prefibrillar oligomeric tau species. In addition, Hsp70 prevents the toxic effect of preformed tau aggregates on anterograde FAT. When added to preformed tau aggregates in vitro, Hsp70 did not appreciably dissociate tau filaments. Interestingly, Hsp70 was found to associate preferentially with oligomeric versus fibrillar tau aggregates, suggesting that oligomeric aggregates may represent the main toxic species associated with aggregated tau.