Development of Chemical Modulators for Heat Shock Protein 70 (Hsp70): a Potential Therapeutic Target for Tauopathies.

Abstract

Tauopathies are a class of neurodegenerative disorders, which affect millions of people worldwide. These diseases are characterized by the accumulation of protein aggregates composed of abnormally modified variants of the microtubule-binding protein tau. Despite extensive efforts, there is currently no cure for these devastating diseases.

The goal of this study is to understand whether heat shock protein 70 (Hsp70) may be an under-explored target for tauopathies. Hsp70 is a molecular chaperone that plays important roles in protein quality control, including the turnover of misfolded polypeptides. However, the molecular mechanisms of tau quality control were not known and it wasn’t clear how pharmacological inhibition of Hsp70 might impact tau accumulation. In this thesis, we review the literature linking Hsp70 to tau and hypothesize that inhibiting the ATPase activity of Hsp70 might lead to the degradation of aberrant tau by favoring the triage pathway. To test this hypothesis, we developed a new high-throughput screen platform and identified promising inhibitors, including methylene blue (MB). MB inhibited the ATPase activity of Hsp70 and, consistent with the major hypothesis, caused a dramatic, Hsp70-dependent reduction in the accumulation of tau in cellular and animal models of tauopathy. In mechanistic studies, we found that MB inhibits Hsp70 by selectively oxidizing two key cysteine residues in the chaperone. To further explore this process, we developed a parallel chemical series of Hsp70 inhibitors based on the known anti-cancer agent, MKT-077. We found that these compounds inhibit the ATPase activity of Hsp70 and, like MB, they dramatically reduce tau levels. However, the MKT-077 analogs acted by a distinct mechanism: binding to the ADP-bound form of Hsp70 and allosterically blocking nucleotide cycling. Together, these studies suggest that Hsp70 normally protects tau from degradation, while inhibitors lead to its rapid degradation.

In summary, we have used a chemical biology approach to show that Hsp70 plays a critical role in tau stability and inhibitors reduce tau levels. Thus, this work suggests a potential path towards new treatments for tauopathies. In addition, these findings provide molecular insights into how Hsp70 mediates protein quality control.