Résumé :
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Myogenesis process involves the expression of muscle-specific transcription factors such as myogenin and MEF2 (myocyte enhancer factor 2), and is fundamentally regulated by intracellular Ca2+ fluctuations. Both ER Ca2+ release and Ca2+ entry generate Ca2+ signals required for myoblasts to differentiate. Two families of ER Ca2+ channels are expressed in mammalian cells, the inositol-1,2,3 trisphosphate receptors (IP3R) and the ryanodine receptors (RyR). We assessed the role of IP3R family in human primary myoblast differentiation, as RyR are not expressed in proliferating myoblasts. Human myoblasts expressed the three isoforms of IP3R (IP3R1, IP3R2 and IP3R3). Silencing IP3R1 and IP3R3 using siRNAs reduced MEF2 and myogenin expression by 75% and 60%, respectively, while silencing IP3R2 did not impact on myogenesis. Accordingly, IP3R1 overexpression significantly increased the number of MEF2 positive cells, suggesting an acceleration of the differentiation process in these conditions. Moreover, a decreased expression of IP3R1 reduced significantly NFAT activity, in agreement with the Ca2+-dependency of the NFAT signaling pathway. Finally, we could show that IP3R1 expression strongly increased during the first 48h of differentiation, while IP3R3 expression decreased 24-48h after the initiation of differentiation suggesting a role of IP3R1 in mature myotubes. Indeed, KCl-induced Ca2+ elevation was reduced in IP3R1 knockdown myotubes. Together, these data demonstrate that IP3R1 and 3-induced Ca2+ release are required during normal human myoblast differentiation.
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