Characterization of the interactions between molecular chaperones and the amyloid protein Huntingtin throughout its aggregation pathway

Abstract

Protein misfolding and subsequent aggregation characterize most neurodegenerative diseases and are associated with impaired protein homeostasis (proteostasis). As prominent components of the proteostasis network, molecular chaperones have been identified as modulators of protein aggregation. Understanding their mechanisms of action paves the way for their use as a therapeutic strategy against neurodegenerative diseases. Huntington's disease (HD) is an inherited neurodegenerative disease caused by the abnormal expansion of CAG repeats in the first exon of the Huntingtin protein. Aggregation of the pathogenic HTTExon1 is suggested as the primary cause of neuronal deterioration. Previously, it was shown that a trimeric chaperone complex composed of Hsc70, DNAJB1, and Apg2 can inhibit pathogenic HTTExon1 aggregation and disaggregate preformed HTTExon1 fibrils. As a member of the J-domain protein (JDP) family, and a co-chaperone of Hsc70, DNAJB1 is known to recognize protein substrates, to transfer them to Hsc70, and stimulate the ATPase activity of Hsc70. Recently, a binding interface between DNAJB1 and HTTExon1 has been identified and this work has contributed to the analysis of the effect of this binding site in HTTExon1 suppression and disaggregation activities by the trimeric chaperone complex. In vitro analyses showed that this binding site and unique features of DNAJB1 conferred specificity and functionality, distinguishing it from other JDPs. Furthermore, association between the trimeric chaperones and HTTExon1 along its aggregation pathway was investigated. The data revealed that DNAJB1 and Hsc70 associate with aggregating HTTExon1 only in the presence of each other and an intact ATPase cycle, suggesting a cooperative association of these chaperones with HTTExon1 during aggregation. This association was found to be stable to detergent, suggesting that irreversible sequestration of chaperones in HTTExon1 aggregates is associated with the eventual failure of the trimeric chaperone complex in suppressing HTTExon1 aggregation.