USP21-mediated deubiquitylation stimulates NuMA recruitment to the cell cortex to promote mitotic spindle orientation

Proper orientation of the mitotic spindle is critical for a wide range of biological processes. However, the molecular mechanisms that regulate the formation of the spindle orientation machinery remain incompletely understood. In this study, we identify an essential role for ubiquitin-specific peptidase 21 (USP21)-mediated deubiquitylation in the control of spindle orientation. Histological analyses of Usp21 knockout mice revealed multiple epithelial abnormalities, including delayed corneal epithelial stratification, dilated renal tubules, and impaired skin wound healing. These defects were attributable to spindle misorientation in corresponding epithelial tissues. In vitro studies further confirmed that USP21 deficiency disrupts spindle orientation in cultured cells. Mechanistically, USP21 interacts with nuclear mitotic apparatus protein (NuMA) and removes the lysine 63-linked ubiquitylation of NuMA at lysine 1330 and lysine 1335. The deubiquitylation of NuMA triggers its interaction with Leu-Gly-Asn repeat-enriched protein (LGN), thereby recruiting NuMA to the cell cortex to form the spindle orientation machinery. Collectively, these findings uncover a previously unrecognized role for USP21 in spindle orientation and underscore its importance in epithelial development and homeostasis through the deubiquitylation of NuMA.