USP18 is an essential regulator of muscle cell differentiation and maturation

Abstract Muscle degeneration, a key feature in a wide range of human pathologies, is typified by impaired proteastasis, in which the ubiquitin proteasomal system in particular plays an important role. Yet, little is known about the function of deubiquitinating enzymes (DUBs) in muscle cell biology. We performed a genetic screen to identify DUBs regulators of muscle cell differentiation. Surprisingly, we observed that USP18-depletion induced differentiation and reduced proliferation of muscle cells. USP18 enzymatic function typically attenuates the immune response by removing ISG15, but in muscle cells, we found that USP18 regulates differentiation independent of ISG15 and the IFN-1 pathway. USP18 accumulation in muscle cell nuclei was concomitant with reduced expression of the cell-cycle gene network and altered transcription-regulated gene networks, including myogenic transcription factors. Additionally, USP18-depletion altered calcium channel gene networks, which was in line with reduced calcium flux in myotubes. Reduced sarcomeric genes were linked to reduced contractile force in an engineered muscle model. Our results revealed nuclear USP18 as a critical regulator of differentiation initiation and differentiation maintenance, independent of the IFN-1 pathway.