Résumé :
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Our work focuses on the comprehension of the early steps of myoblast differentiation. The model we use consists of a human myoblast culture derived from satellite cells extracted from muscle biopsies. We previously showed that, to differentiate, myoblasts require a cell membrane hyperpolarization driven by the activity of an inward rectifier potassium channel, Kir2.1. This hyperpolarization from -40mV to -70mV triggers a calcium influx essential for the expression and activation of the myogenic transcription factors myogenin and MEF2 leading to the expression of muscle specific protein such as the Myosin (heavy chain). Recently, we found that Kir2.1 channels are regulated by a tyrosine phosphorylation, maintaining them inactive in proliferation condition. Within the 6 first hours of differentiation the channels get dephosphorylated and thus active. Investigating the activity of receptor tyrosine kinases, we identified the EGF receptor as a regulator of myoblast differentiation. We demonstrated that, in proliferation condition, EGFR knock down is followed by an activation of Kir2.1 currents (current density of -2.22pA/pF for the siRNA EGFR vs. -0.27pA/pF for the control; p<0.005) and an increase in the store operated calcium influx (+52% with the siRNA EGFR compared to control; p<0.01). These events led to, at least, a 5-fold increase in the expression of myogenic transcription factors and muscle specific proteins. Taken together, our results show that, surprisingly, the knock down of EGFR in proliferating myoblasts is sufficient to induce the process of differentiation, from the membrane hyperpolarization to the formation of myotubes. Our study defines the physiological down regulation of EGFR as an early and essential trigger of myoblast differentiation through the activation of Kir2.1 currents.
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