Proteasome impairment does not contribute to pathogenesis in R6/2 Huntington's disease mice: exclusion of proteasome activator REGγ as a therapeutic target

Open Access
Publisher Website
Google Scholar
Abstract
Huntington's disease (HD) is one of a group of neurodegenerative disorders caused by the pathological expansion of a glutamine tract. A hallmark of these so-called polyglutamine diseases is the presence of ubiquitylated inclusion bodies, which sequester various components of the 19S and 20S proteasomes. In addition, the ubiquitin–proteasome system (UPS) has been shown to be severely impaired in vitro in cells overexpressing mutant huntingtin. Thus, because of its fundamental housekeeping function, impairment of the UPS in neurons could contribute to neurotoxicity. We have recently proposed that the proteasome activator REGγ could contribute to UPS impairment in polyglutamine diseases by suppressing the proteasomal catalytic sites responsible for cleaving Gln–Gln bonds. Capping of proteasomes with REGγ could therefore contribute to a potential ‘clogging’ of the proteasome by pathogenic polyglutamines. We show here that genetic reduction of REGγ has no effect on the well-defined neurological phenotype of R6/2 HD mice and does not affect inclusion body formation in the R6/2 brain. Surprisingly, we observe increased proteasomal ‘chymotrypsin-like’ activity in 13-week-old R6/2 brains relative to non-R6/2, irrespective of REGγ levels. However, assays of 26S proteasome activity in mouse brain extracts reveal no difference in proteolytic activity regardless of R6/2 or REGγ genotype. These findings suggest that REGγ is not a viable therapeutic target in polyglutamine disease and that overall proteasome function is not impaired by trapped mutant polyglutamine in R6/2 mice.