The active Hsc70/tau complex can be exploited to enhance tau turnover without damaging microtubule dynamics

Abstract

The pathological accumulation of abnormally hyperphosphorylated and aggregated tau, a neuronal microtubule (MT)-associated protein that functions to maintain MT stability, is implicated in a number of hereditary and sporadic neurodegenerative diseases including frontotemporal dementia and Alzheimer's disease. Targeting tau for the treatment of these diseases is an area of intense interest and toward that end, modulation of cellular molecular chaperones is a potential therapeutic target. In particular, the constitutive Hsp70 isoform, Hsc70, seems highly interconnected with tau, preserving tau protein levels and synergizing with it to assemble MTs. But the relationship between tau and Hsc70, as well as the impact of this interaction in neurons and its therapeutic implications remain unknown. Using a human dominant negative Hsc70 that resembles isoform selective inhibition of this important chaperone, we found for the first time that Hsc70 activity is required to stimulate MT assembly in cells and brain. However, surprisingly, active Hsc70 also requires active tau to regulate MT assembly in vivo, suggesting that tau acts in some ways as a co-chaperone for Hsc70 to coordinate MT assembly. This was despite tau binding to Hsc70 as substrate, as determined biochemically. Moreover, we show that while chronic Hsc70 inhibition damaged MT dynamics, intermittent treatment with a small molecule Hsp70 inhibitor lowered tau in brain tissue without disrupting MT integrity. Thus, in tauopathies, where MT injury would be detrimental to neurons, the unique relationship of tau with the Hsc70 machinery can be exploited to deplete tau levels without damaging MT networks.

HspB1 and Hsc70 chaperones engage distinct tau species and have different inhibitory effects on amyloid formation

“…NMR studies of Hsc70 binding to tau2N4R show that Hsc70 causes slight broadening in peaks corresponding to residues 375-380 of tau2N4R, though this broadening is not as strong as that of the 275 VQIINK 280 and 306 VQIVYK 311 motifs (22,23). Because we found similar Hsc70 chaperone activity against tau4RD, which does not contain these residues, and tau2N4R, which does, we believe that this interaction does not significantly alter Hsc70's mechanism of action.…”

“…Individuals carrying diseaseassociated mutations in the tau gene that enhance aggregation propensity only develop symptoms later in life (19), suggesting that neuronal cells have the machinery to maintain aggregation-prone tau in a soluble form for years. Interactions between tau and a number of chaperones have been documented, indicating that multiple players within the chaperone network are involved in this process (20)(21)(22)(23)(24)(25). However, the mechanisms by which these chaperones interact with tau and their effects on tau fibril formation remain unclear.…”

“…However, ATP-independent activity has also been reported in Hsp70-family chaperones (33)(34)(35). Hsc70 binds to the two aggregation-prone motifs in tau and has been proposed as a pharmacological target (22,23,36). Its overexpression leads to an increase in the concentration of tau in cell culture models, while an Hsc70 inhibitor causes a decrease in tau concentration in cell cultures and rescues long-term potentiation deficits in tauopathy model mouse brain slices (22,36).…”

Therapeutic strategies for the treatment of tauopathies: Hopes and challenges

“…CHIP-mediated ubiquitination of phosphorylated tau results in decreased cell death [140, 141] and deletion of CHIP results in the accumulation of hyperphosphorylated tau [142, 143]. Interestingly, the Hsp70 family member, Hsc70, interacts with tau with greater affinity than Hsp72 and stabilizes tau [137, 144], and expression of a dominant-negative variant of Hsc70 led to clearance of tau via the proteasome in HEK293T cells, as well as in brain tissue [145]. …”

Inhibition of Both Hsp70 Activity and Tau Aggregation in Vitro Best Predicts Tau Lowering Activity of Small Molecules

“…12 In particular, inhibitors of the heat shock protein 70 kDa (Hsp70) chaperone family are highly effective at lowering tau. 8,14 When the activity of these molecular chaperones is pharmacologically or even genetically inhibited, tau can be degraded. 15–18 Recently, a number of other small molecules with anti-Hsp70 activity have been described.…”