Résumé :
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Communication n° 155. During post-natal growth or after muscle injury, skeletal muscle myoblasts differentiate and fuse to form myotubes. We have shown recently that one of the earliest events of the differentiation is the hyperpolarization of human myoblast resting potential due to an up-regulation of Kir2.1 K+ channels. This up-regulation occurs within the first 6 hours of the differentiation process and several hours before the expression of myogenin and MEF2, two major regulators of myoblast differentiation. We proposed that Kir2.1 activity induces an influx of calcium that is necessary for the expression and activity of myogenin and MEF2. We presently explore the molecular mechanisms that lead to Kir2.1 channels up-regulation at the onset of differentiation. We first tested whether protein synthesis or ER-to-Golgi transport was required. Kir2.1 channels activity was measured by electrophysiology in presence of cycloheximide, a protein synthesis inhibitor, or in presence of brefeldin A, an inhibitor of the ER-to-Golgi transport. Neither protein synthesis nor ER-to-Golgi transport seems required as the increase of Kir2.1 channel activity still occurs in presence of these drugs when they are applied simultaneously with the initiation of differentiation. Then, as it is known that Kir2.1 channels can be regulated by direct tyrosine phosphorylation, we investigated whether tyrosine kinase (or phosphatase) inhibition affects Kir2.1 channel activity or myoblast fusion. Preliminary results suggest that Kir2.1 channels are already present at the plasma membrane but silenced when myoblasts proliferate. Indeed, we found that application of genistein, a tyrosine kinase inhibitor, increases Kir2.1 channel activity and accelerates the fusion process, whereas bpV(Phen), a tyrosine phosphatase inhibitor, slows down the fusion process. Together, these results suggest that, in human myoblasts, the initiation of the differentiation process is due to a tyrosine dephosphorylation of Kir2.1 channels.
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