USP18 regulates skeletal muscle regeneration independently of its deISGylation activity

USP18 is a multifunctional protein that attenuates type I interferon (IFN-I) signalling and regulates ISG15-mediated ISGylation. Although USP18 has been suggested to suppress myogenic differentiation in vitro, its role in muscle regeneration and inflammatory muscle disease remains unclear. This study aimed to define the spatial localization and functional contribution of USP18 and ISG15 in inflamed and regenerating muscle.

USP18 function in myogenesis was examined using inducible knockout and catalytic-inactive mutant muscle cell models. Spatial expression of USP18 and ISG15 was analyzed by quantitative imaging in mouse models of muscular dystrophy, cardiotoxin-induced injury, and healthy muscle, with validation in dermatomyositis patient biopsies.

USP18 depletion enhanced myogenesis and mitochondrial activity in vitro. Importantly, a catalytic-inactive USP18 mutant regulated myogenesis comparably to the wild-type protein, demonstrating that USP18 acts independently of its deISGylation activity. In vivo, USP18 and ISG15 expression diverged in low-inflammatory regions but colocalized in highly inflamed regenerative areas, highlighting context-dependent regulation. Regenerating myofibres exhibited high USP18 expression in the absence of ISG15, whereas fibrotic regions displayed persistent ISG15 with reduced USP18 levels. In vitro, USP18 increased MYOG expression during early differentiation while suppressing mature myosin heavy chain expression, indicating that USP18 promotes early myogenic commitment but limits myofibre maturation.

USP18 acts as a context-dependent regulator of muscle regeneration independent of its deISGylation function. These findings identify USP18 as a dual modulator of myogenesis and highlight its potential as a therapeutic target in inflammatory muscle disease.