Suppression of protein aggregation by chaperone modification of high molecular weight complexes

Abstract

Protein misfolding and aggregation are associated with many neurodegenerative diseases, including Huntington’s disease. The cellular machinery for maintaining proteostasis includes molecular chaperones that facilitate protein folding and reduce proteotoxicity. Increasing the protein folding capacity of cells through manipulation of DNAJ chaperones has been shown to suppress aggregation and ameliorate polyglutamine toxicity in cells and flies. However, to date these promising findings have not been translated to mammalian models of disease. To address this issue, we developed transgenic mice that over-express the neuronal chaperone HSJ1a (DNAJB2a) and crossed them with the R6/2 mouse model of Huntington’s disease. Over-expression of HSJ1a significantly reduced mutant huntingtin aggregation and enhanced solubility. Surprisingly, this was mediated through specific association with K63 ubiquitylated, detergent insoluble, higher order mutant huntingtin assemblies that decreased their ability to nucleate further aggregation. This was dependent on HSJ1a client binding ability, ubiquitin interaction and functional co-operation with HSP70. Importantly, these changes in mutant huntingtin solubility and aggregation led to improved neurological performance in R6/2 mice. These data reveal that prevention of further aggregation of detergent insoluble mutant huntingtin is an additional level of quality control for late stage chaperone-mediated neuroprotection. Furthermore, our findings represent an important proof of principle that DNAJ manipulation is a valid therapeutic approach for intervention in Huntington’s disease.

Chaperone-like Activity of High-Mobility Group Box 1 Protein and Its Role in Reducing the Formation of Polyglutamine Aggregates

“…HSP70 increases the level of soluble polyQ proteins by directly associating with polyQ aggregates in a dynamic and transient manner (48,49), and HSP40 (DNAJ) associates with aggregates and suppresses aggregation by chaperone modification of high m.w. complexes (29).…”

“…Chaperones like HSP70 can suppress the formation of mHtt aggregates, alleviating its toxicity in cultured cells and animal models (26)(27)(28). Upregulation of chaperones such as HSP70 and HSP40 decrease the formation of aggregates, and, therefore, chaperone molecules have been considered as one of the therapeutic targets in many aggregate-prone diseases (29).…”

Molecular chaperones and neuronal proteostasis

“…Schematic showing the effect of HSJ1a on Htt, SOD1, parkin and tau in neurons. HSJ1a can bind to ubiquitylated oligomers of Htt in the nucleus blocking the recruitment of more misfolded Htt and further aggregation, leading to increases in soluble Htt and potential autophagic clearance of cytoplasmic Htt oligomers [39]. HSJ1 also facilitates proteasomal degradation of Htt [37].…”

“…Only the cytosolic and nuclear HSJ1a isoform was able to significantly reduce aggregation in a cellular model expressing polyQ huntingtin, highlighting that the localisation of chaperones is critical for their protective function [37]. The overexpression of HSJ1a was also shown to be neuroprotective in a R6/2 mouse model of HD [39]. HSJ1a expression significantly reduced the aggregation of mutant huntingtin in brain, whilst increasing the level of soluble protein.…”

Modulation of Molecular Chaperones in Huntington’s Disease and Other Polyglutamine Disorders

“…While Hsp40 overexpression inhibited mHtt aggregation and/or toxicity in cells [130,133,134], primary neurons [135], Xenopus laevis [136], and mice [137], Hsp70 overexpression was less efficient in reducing mHtt aggregation than Hsp40. Indeed, HEK293 cells overexpressing Hsp40 exhibited fewer mHtt aggregates than those overexpressing Hsp70 [135].…”