Résumé :
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Caveolins constitute a membrane-associated family of proteins believed to regulate various signaling proteins, including ion channels. Mutations in the CAV3 gene which encodes the muscle specific isoform caveolin-3 (Cav-3) lead to muscle diseases such as limb-girdle muscle dystrophy 1C (LGMD-1C). The molecular events responsible for muscle wasting in LGMD-1C remain however largely unknown. The present study aimed at characterizing the functional and molecular links between Cav-3 and the L-type calcium channel. The consequences of a LGMD 1C-associated Cav-3 mutation (P104L) on membrane excitability and intracellular calcium homeostasis was investigated in mouse adult skeletal muscle fibers. YFP-tagged Cav-3P104L was expressed in vivo in flexor digitorum brevis muscles and YFP-positive isolated fibers were studied under whole-cell “silicone voltage-clamp” conditions. The L-type Ca2+ current density was found considerably reduced in Cav-3P104L expressing fibers, consistent with our previous data on cultured myotubes (Couchoux et al., 2007, J. Physiol. 580:745). Interestingly intramembrane charge movement was unaltered, suggesting that the total number of voltage-sensing dihydropyridine receptors remained unchanged. Also, there was no detectable alteration of intracellular calcium regulation neither in resting conditions nor subsequently to voltage-activation. These results thus suggest that in differentiating as well as in adult muscle cells, acute expression of Cav-3P104L specifically alters the ionic function of the dihydropyridine receptor. We also found that Cav-3 co-fractionnates and co-immunoprecipitates with Cav1.1, the pore forming subunit of the L-type calcium channel, in muscle triadic fractions. Furthermore in-vitro binding assays showed that Cav-3 directly interacts with the I-II interdomain loop of Cav1.1, probably through its alpha interaction domain (AID). Our results thus suggest that the ionic function of the L-type calcium channel is specifically regulated by Cav-3 and that this functional link could be supported by direct molecular interactions. An alteration of these interactions could participate to the physio-pathological mechanisms of skeletal muscle caveolinopathies
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