USP28 Deficiency is Linked to Impaired Ubiquitin-dependent Proteostasis in Huntington's Disease.

Huntington's disease (HD) is characterized by mutant huntingtin (mHTT) aggregation and impaired proteostasis; however, upstream regulators of ubiquitin system imbalance remain incompletely understood. This study identified the deubiquitinase USP28 as a potential modulator of ubiquitin-dependent proteostasis in HD. Bulk RNA sequencing of R6/2 mouse brain tissues showed reduced USP28 expression compared with wild-type controls. Consistently, USP28 expression decreased in STHdh striatal cells expressing expanded polyQ huntingtin (Q111). HD cells (STHdh-Q111) exhibited increased accumulation of ubiquitinated proteins and altered ubiquitin turnover, consistent with impaired proteostasis. USP28 overexpression attenuated ubiquitinated protein accumulation and reduced mHTT aggregation, whereas a catalytically inactive USP28 mutant showed limited rescue effects. In addition, USP28 depletion was associated with reduced UBR5 levels, while USP28 restoration partially recovered UBR5 expression in a catalytic activity-dependent manner. Modulation of HECT E3 ligase activity further altered ubiquitination dynamics and mHTT aggregation, suggesting that HECT E3 ligase-related pathways may contribute to proteostasis regulation in HD cells. Collectively, these findings identify USP28 as a proteostasis-associated deubiquitinase reduced in HD models and suggest that USP28 deficiency contributes to ubiquitin burden and mHTT aggregation. Changes in UBR5 expression further point to a potential involvement of HECT E3 ligase-linked ubiquitin regulation, although the direct mechanistic relationship between USP28 and UBR5 remains.