The ubiquitin conjugating enzyme Ube2W regulates solubility of the Huntington's disease protein, huntingtin.

Huntington's disease (HD) is caused by a CAG repeat expansion that encodes a polyglutamine (polyQ) expansion in the HD disease protein, huntingtin (HTT). PolyQ expansion promotes misfolding and aggregation of mutant HTT (mHTT) within neurons. The cellular pathways, including ubiquitin-dependent processes, by which mHTT is regulated remain incompletely understood. Ube2W is the only ubiquitin conjugating enzyme (E2) known to ubiquitinate substrates at their amino (N)-termini, likely favoring substrates with disordered N-termini. By virtue of its N-terminal polyQ domain, HTT has an intrinsically disordered amino terminus. In studies employing immortalized cells, primary neurons and a knock-in (KI) mouse model of HD, we tested the effect of Ube2W deficiency on mHTT levels, aggregation and neurotoxicity. In cultured cells, deficiency of Ube2W activity markedly decreases mHTT aggregate formation and increases the level of soluble monomers, while reducing mHTT-induced cytotoxicity. Consistent with this result, the absence of Ube2W in HdhQ200 KI mice significantly increases levels of soluble monomeric mHTT while reducing insoluble oligomeric species. This study sheds light on the potential function of the non-canonical ubiquitin-conjugating enzyme, Ube2W, in this polyQ neurodegenerative disease.

Loss of the Ubiquitin-conjugating Enzyme UBE2W Results in Susceptibility to Early Postnatal Lethality and Defects in Skin, Immune, and Male Reproductive Systems.

UBE2W ubiquitinates N termini of proteins rather than internal lysine residues, showing a preference for substrates with intrinsically disordered N termini. The in vivo functions of this intriguing E2, however, remain unknown. We generated Ube2w germ line KO mice that proved to be susceptible to early postnatal lethality without obvious developmental abnormalities. Although the basis of early death is uncertain, several organ systems manifest changes in Ube2w KO mice. Newborn Ube2w KO mice often show altered epidermal maturation with reduced expression of differentiation markers. Mirroring higher UBE2W expression levels in testis and thymus, Ube2w KO mice showed a disproportionate decrease in weight of these two organs (~50%), suggesting a functional role for UBE2W in the immune and male reproductive systems. Indeed, Ube2w KO mice displayed sustained neutrophilia accompanied by increased G-CSF signaling and testicular vacuolation associated with decreased fertility. Proteomic analysis of a vulnerable organ, presymptomatic testis, showed a preferential accumulation of disordered proteins in the absence of UBE2W, consistent with the view that UBE2W preferentially targets disordered polypeptides. These mice further allowed us to establish that UBE2W is ubiquitously expressed as a single isoform localized to the cytoplasm and that the absence of UBE2W does not alter cell viability in response to various stressors. Our results establish that UBE2W is an important, albeit not essential, protein for early postnatal survival and normal functioning of multiple organ systems.

Differential recruitment of UBQLN2 to nuclear inclusions in the polyglutamine diseases HD and SCA3.

Accumulation of mutant polyglutamine proteins in intraneuronal inclusions is a hallmark of polyglutamine diseases. Impairment of protein clearance systems and sequestration of clearance-related proteins into inclusions occur in many protein folding diseases, including polyglutamine diseases. The ubiquitin-binding and proteasome adaptor protein UBQLN2 participates in protein homeostasis and localizes to inclusions in various neurodegenerative diseases. Employing mouse models and human brain tissue of Huntington's disease (HD) and spinocerebellar ataxia type 3 (SCA3), we show that UBQLN2 is selectively recruited to inclusions in HD but not SCA3. Consistent with this result, in a cell-based system mutant HTT interacts with UBQLN2 through the UBA domain while the SCA3 disease protein ATXN3, a deubiquitinating enzyme, does not interact with UBQLN2. Differential recruitment of UBQLN2 to aggregates in HD and SCA3 underscores the heterogeneity of inclusions in polyglutamine diseases and suggests that components of neuronal protein quality control may be differentially perturbed in distinct polyQ diseases.