Résumé :
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Communication n° 573 At the neuromuscular junction (NMJ) not yet well explored is the possible role of neuromuscular activity mediated via an IP3 cascade in perisynaptic Schwann cells, and in the postsynaptic elements of the NMJ. The well documented IP3/calcium cascade in some cells contributes to different calcium signaling patterns (single transients, repetitive oscillations or sustained plateaus) which can encode specific cellular responses. In cultured muscle, calcium release from internal stores may follow more than one set of kinetics and may have multiple functions. The presence of IP3 receptors (IP3Rs) in cultured muscle and in adult skeletal muscle further suggests a role for IP3 signals in both nuclear and cytoplasmic compartments. We show here that IP3Rs are localized in postsynaptic components of the NMJ, at the postsynaptic gutters, surrounding the subsynaptic nuclei of the motor endplate, close to the nicotinic acetylcholine receptors. IP3Rs are found also in perisynaptic Schwann cells and in muscle satellite cells, as determined using confocal laser scanning microscopy and immunocytochemistry. These cells have recently been shown to contribute to nuclei in adult fibers following muscle exercise. Satellite cells were identified by their peripheral position, their size, and the shape of their nucleus, the paucity of apparent cytoplasm, and their immunostaining with specific molecular markers. Calcium imaging experiments, using fluo-3/AM show high K+-induced release of calcium precisely in IP3R rich regions. Interestingly, long-lasting calcium signals were detected in satellite cells in situ, following depolarization with high K+. We suggest that depolarization of these cells stimulates IP3-associated calcium signals that could act in concert with signaling pathways triggered by growth factors and/or hormones. This work was made possible by the ECOS Sud-CONYCIT exchange program (C03S02) and was supported in part by the AFM (grant to J.M.) and FONDAP (grant to E.J).
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