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

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.