In neurodegenerative diseases, pathogenic proteins tend to misfold and form aggregates that are difficult to remove and able to induce excessive endoplasmic reticulum (ER) stress, leading to neuronal injury and apoptosis. Homocysteine-induced endoplasmic reticulum protein (Herp), an E3 ubiquitin ligase, is an important early marker of ER stress and is involved in the ubiquitination and degradation of many neurodegenerative proteins. However, in Huntington's disease (HD), a typical polyglutamine disease, whether Herp is also involved in the metabolism and degradation of the pathogenic protein, mutant huntingtin, has not been reported. Therefore, we studied the relationship between Herp and N-terminal fragments of huntingtin (HttN-20Q and HttN-160Q). We found that Herp was able to bind to the overexpressed Htt N-terminal, and this interaction was enhanced by expansion of the polyQ fragment. Confocal microscopy demonstrated that Herp was co-localized with the HttN-160Q aggregates in the cytoplasm and tightly surrounded the aggregates. Overexpression of Herp significantly decreased the amount of soluble and insoluble HttN-160Q, promoted its ubiquitination, and inhibited its cytotoxicity. In contrast, knockdown of Herp resulted in more HttN-160Q protein, less ubiquitination, and stronger cytotoxicity. Inhibition of the autophagy-lysosomal pathway (ALP) had no effect on the function of Herp. However, blocking the ubiquitin-proteasome pathway (UPP) inhibited the reduction in soluble HttN-160Q caused by Herp. Interestingly, blocking the UPP did not weaken the ability of Herp to reduce HttN-160Q aggregates. Deletions of the N-terminal of Herp weakened its ability to inhibit HttN-160Q aggregation but did not result in a significant increase in its soluble form. However, loss of the C-terminal led to a significant increase in soluble HttN-160Q, but Herp still maintained the ability to inhibit aggregate formation. We further found that the expression level of Herp was significantly increased in HD animal and cell models. Our findings suggest that Herp is a newly identified huntingtin-interacting protein that is able to reduce the cytotoxicity of mutant huntingtin by inhibiting its aggregation and promoting its degradation. The N-terminal of Herp serves as the molecular chaperone to inhibit protein aggregation, while its C-terminal functions as an E3 ubiquitin ligase to promote the degradation of misfolded proteins through the UPP. Increased expression of Herp in HD models may be a pro-survival mechanism under stress.
Keywords: E3 ubiquitin ligase; Homocysteine-induced endoplasmic reticulum protein; Huntingtin; Huntington’s disease; Molecular chaperone; Protein misfolding; Ubiquitylation.