Allosteric activation of Hsp70 reduces mutant huntingtin levels, the clustering of N-terminal fragments, and their nuclear accumulation

Abstract

Aims: Huntington's disease (HD) is caused by a mutant huntingtin protein that misfolds, yields toxic N-terminal fragments, aggregates, and disrupts proteostasis. The Hsp70 chaperone is a potential therapeutic target as it prevents proteotoxicity by favouring protein folding, disaggregation, or degradation. We tested the hypothesis that allosteric Hsp70 activation with a pharmacological mimetic of the Hsp70 co-chaperone Hip, YM-1, could modulate huntingtin proteostasis.

Main methods: We used HD cell models expressing either N-terminal or full-length huntingtin. Using single-cell analysis we studied huntingtin aggregation in different cellular compartments by fluorescence microscopy. Protein interaction was evaluated by immunoprecipitation, while protein levels were quantified by immunofluorescence and western-blot.

Key findings: N-terminal huntingtin interacted with Hsp70 and increased its levels. Treatment with YM-1 reduced N-terminal huntingtin clustering and nuclear aggregation. Full-length mutant huntingtin also interacted with Hsp70, and treatment with YM-1 reduced huntingtin levels when combined with Hsp70 induction by heat shock. Mechanistically, YM-1 increases the Hsp70 affinity for substrates, promoting their proteasomal degradation. Consistently, YM-1 reduced the levels of ubiquitinated proteins. Interestingly, YM-1 accumulated in mitochondria, interfered with its Hsp70 isoform involved in protein import, and increased NRF1 levels, a regulator of proteasome genes. We thus suggest that YM-1 may trigger the coordination of mitochondrial and cytosolic proteostasis, enhancing protein degradation.

Significance: Our findings show that the strategy of allosteric Hsp70 activation holds potential for HD. While drug efficacy may be limited to tissues with elevated Hsp70, combined therapies with Hsp70 elevating strategies could harness the full potential of allosteric Hsp70 activators for HD.

Keywords: Hsp70; Huntington's disease; Mitochondria; Neurodegeneration; Proteostasis; Ubiquitin-proteasome system.