Leucine-rich repeat containing family 8 (LRRC8) proteins form the volume-regulated anion channel (VRAC). Recently, they were shown to be required for normal differentiation and fusion of C2C12 myoblasts, by promoting membrane hyperpolarization and intracellular Ca2+ signals. However, the mechanism by which they are involved remained obscure. Here, using a FRET-based sensor for VRAC activity, we show temporary activation of VRAC within the first 2 h of myogenic differentiation. During this

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... , we also observed a significant decrease in the intracellular Cl- concentration that was abolished by the VRAC inhibitor carbenoxolone. However, lowering the intracellular Cl(- )concentration by extracellular Cl(- )depletion did not promote differentiation as judged by the percentage of myogenin- positive nuclei or total myogenin levels in C2C12 cells. Instead, it inhibited myosin expression and myotube formation. Together, these data suggest that VRAC is activated and mediates Cl(- )efflux early on during myogenic differentiation, and a moderate intracellular Cl(- )concentration is necessary for myoblast fusion.