Soluble forms of polyQ-expanded huntingtin rather than large aggregates cause endoplasmic reticulum stress

Abstract

In Huntington's disease, as in other neurodegenerative diseases, it was initially thought that insoluble protein aggregates are the toxic species. However, growing evidence implicates soluble oligomeric polyglutamine-expanded huntingtin in cytotoxicity. Here we show that pathogenic huntingtin inhibits endoplasmic reticulum (ER)-associated degradation and induces ER stress before its aggregation into visible inclusions. All three branches of the unfolded protein response are activated. ER stress can be compensated by overexpression of p97/VCP, suggesting its sequestration by pathogenic huntingtin as a main cause. Stress correlates with the presence of huntingtin oligomers and is independent of continual huntingtin synthesis. Stress levels, measured in striatal neurons, are stabilized but only slowly subside on huntingtin aggregation into inclusions. Our results can be explained by the constant conversion of huntingtin monomers to toxic oligomers; large aggregates sequester the former, precluding further conversion, whereas pre-existing toxic oligomers are only gradually depleted.

Protein Quality Control in Health and Disease

“…Visualization of neurons that either contain deposition sites that harbor aggregated, HD-linked polyQ stretches or lack such structures, unveiled that cells that contain deposition sites exhibit higher survival rates compared with their counterparts that do not contain such foci (Arrasate et al 2004). This finding coincides with the finding that soluble polyQ oligomers, rather than large fibrils, activate the UPR ER (Leitman et al 2013). Nevertheless, a…”

Proteostasis impairment in protein-misfolding and -aggregation diseases

“…Inclusions disappear when huntingtin expression is blocked [108]. Multiple lines of evidence suggest that polyQ expanded proteins titrate chaperones away from their clients, leading to proteostasis impairment [44,74,100,109,110]. Conversely, overexpression of various chaperones, such as members of the Hsp70 system, suppresses polyQ toxicity [3, 69,75].…”

“…On the other hand, sustained aggregate stress may result in a disproportionate response. For example, the long-term down-regulation of translation caused by chronic ER stress can be especially detrimental to neuronal cells [99,100] which rely critically on ongoing translation for functionality. It will be crucial to unravel the mechanisms by which protein aggregation deregulates stress response pathways and undermines the cellular defense against toxic protein species.…”

The VCP/p97 and YOD1 Proteins Have Different Substrate-dependent Activities in Endoplasmic Reticulum-associated Degradation (ERAD)

“…Other criteria, such as association of the accumulated material (even when ubiquitylated) to the pellet (microsomal/ER-containing fraction) in cell fractionation experiments and partial resistance to protease digestion, have led to the same conclusions. Moreover, down-regulation or sequestration of p97 induces the unfolded protein response, suggesting that unfolded protein response activation is the consequence of substrate accumula- tion in the ER lumen (31,59). Nevertheless, whether and how p97 acts on substrates by "pulling" molecules already exposed to the cytosol or on subunits of the dislocation complex to trigger ERAD substrate movement to the cytosol is not clear (14).…”