Chemical Modulators of Heat Shock Protein 70 (Hsp70) Validate this Molecular Chaperone as a Target for Tauopathy

Abstract

Misregulation of tau, an intrinsically disordered protein, is implicated in Alzheimer’s disease (AD) and more than 15 related tauopathies. These diseases are characterized by the accumulation of tau aggregates into insoluble neurofibrillary tangles (NFTs), which contribute to neurodegeneration. As discussed in Chapter 1, accelerating the clearance of tau is an emerging treatment strategy. The molecular chaperone heat shock protein 70 (Hsp70) binds tau and facilitates its degradation, making it a particularly attractive target for normalizing tau homeostasis. It was recently discovered that the rhodacyanine MKT-077 and its analogs bind Hsp70 and accelerate the degradation of pathogenic tau in multiple disease models. In Chapter 2, I was interested in using MKT-077 analogs as probes to better understand this mechanism. We found that MKT-077 analogs bind to an allosteric site on Hsp70, stabilizing the ADP-bound conformation and increasing tau binding. These molecules also disrupted protein-protein interactions between Hsp70 and its co-chaperones. These findings suggest that the affinity of Hsp70 for its substrates, like tau, is correlated with degradation. While exciting, it still was not clear which of the Hsp70 paralogs was most important for this process. There are thirteen Hsp70 genes in humans, including those expressed in the mitochondria and endoplasmic reticulum (ER). In Chapter 3, I used CRISPR/cas9 technology to understand the targets of the MKT-077 analogs, showing that some of the compounds are selective for mitochondrial Hsp70, while others require the ER paralog. This selectivity seemed to emerge from differential subcellular partitioning of the analogs, as suggested by fluorescence microscopy. These results are important for better understanding the roles of Hsp70 in tau homeostasis, because we could then select molecules that were relatively paralog-selective and ask how well they reduce tau levels. Using this approach, I found that the ER version of Hsp70 seemed to be an excellent target for accelerating tau degradation, potentially through regulating stress response. Consistent with this finding, I found that over-expression of ER Hsp70 promotes tau clearance. As discussed in Chapter 4, these results have important implications for therapeutic treatment of tauopathies. In addition, these findings provide new insights into mechanisms of Hsp70-mediated tau homeostasis.