Résumé :
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Communication n° 144. We have showed previously that human myoblasts must hyperpolarize to approximately -70 mV before they can proceed through the differentiation process. This hyperpolarization occurs through the expression of Kir2.1 K+ channels. It is known that the process of myoblast differentiation is mainly regulated by myogenic bHLH and MEF2 transcription factors. These factors are controlled by regulatory enzymes such as p38 MAPK, CaMK and Calcineurin. Our goal is to study, in human myoblasts undergoing differentiation, the interactions between ionic channels and the intracellular signaling network. We show presently that functional Kir2.1 channels can be detected several hours before expression of myogenin and MEF2 transcription factors. This result shows that, during myoblast differentiation, hyperpolarization is an event that precedes the expression of an early myogenic factor such as myogenin. Drugs or antisense reducing Kir2.1 current inhibit expression/activity of myogenin and MEF2 as well as the rate of fusion of the myoblasts into myotubes. LY294002, an inhibitor of phosphatidylinositol 3-kinase which inhibits both myogenin/MEF2 expression did not affect Kir2.1 current demonstrating that Kir2.1 current acts upstream of myogenin and MEF2. Taken together, these results suggest that Kir2.1 channel activation is a required key early event that initiates myogenesis by turning on myogenin and MEF2 transcription factors. We hypothesized that the Kir2.1-induced hyperpolarization controls one or several regulatory pathways implicated in the expression of these factors. We found that activation of the calcineurin pathway, which is a calcium-dependant process, is inhibited after the blockade of Kir2.1 potassium currents. In contrast, we observed that activation of a calcium-independant pathway, such as the p38 MAPK regulatory pathway, is not modulated by the differentiation-linked membrane hyperpolarization. The calcium-dependant CaMK pathway is currently under investigation.
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