Herp Promotes Degradation of Mutant Huntingtin: Involvement of the Proteasome and Molecular Chaperones

Abstract

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.

How Do Post-Translational Modifications Influence the Pathomechanistic Landscape of Huntington’s Disease? A Comprehensive Review

“…However, the upregulation of Herp expression can reduce the level of mHTT but is unable to clear it out entirely. Thus, the authors speculate that upregulated Herp can only delay the progression of HD and not prevent it [122].…”

Protein Misfolding and ER Stress in Huntington's Disease

“…In fact, an important interference was attributed to sequestration and depletion of p97/VCP and its cofactors Npl4 and Ufd1 by mHtt, which would not cause global UPS deficiency, but rather a crippling dysfunction of ERAD (Duennwald and Lindquist, 2008; Yang et al, 2010; Leitman et al, 2013). Homocysteine-induced endoplasmic reticulum protein (Herp), an important factor in ERAD, was recently reported to be directly involved in targeting of mHtt for degradation (Luo et al, 2018). The inhibition of ERAD leads to accumulation of unfolded proteins in the ER, ER stress, and UPR induction, which were observed in HD models in yeast and mammalian cells (Duennwald and Lindquist, 2008; Reijonen et al, 2008; Carnemolla et al, 2009; Leitman et al, 2013, 2014), in animal HD models (Carnemolla et al, 2009; Cho et al, 2009; Noh et al, 2009; Vidal et al, 2012), and in post-mortem samples from HD patients (Carnemolla et al, 2009).…”

Global Proteome and Ubiquitinome Changes in the Soluble and Insoluble Fractions of Q175 Huntington Mice Brains

“…Although differences in phosphorylation between wtHtt and mHtt have been shown by various research groups, much less is known about differences in ubiquitination between wtHtt and mHtt. Several studies have reported the effects of ubiquitin ligases and deubiquitinating enzymes (DUBs) on aggregate formation by mHtt (15)(16)(17)(18)(19), indicating that altering the ubiquitination of mHtt improves the turnover of the mutant protein and decreases aggregate formation (20 -22). However, it is unknown whether mHtt is differentially ubiquitinated because of the polyQ expansion.…”