Résumé :
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MyoD is part of the myogenic regulatory factors (MRFs) family, which are the essential factors controlling the myogenesis during embryonic development or muscular regeneration in the adulthood. CAPN3 is a calcium-dependent cysteine protease mainly expressed in skeletal muscle. Mutations in capn3 gene, have been identified as responsible for the Limb-girdle muscular dystrophy type 2A (LGMD2A), an autosomal recessive muscular dystrophy. This emphasizes a key role for CAPN3 in maintaining the integrity of muscular fibers, however which intracellular pathway is altered and therefore results in muscular dystrophy still remain controversial. We have shown that CAPN3 inhibits the transcriptional activity of MyoD either in myoblastic cells (C2C12) or in fibroblastic ones (C3H10T1/2). On the contrary, no variation in the transcriptional activity of the other members of the MRFs family (Myf5, Myogenin, or MRF4) is observed. We show that CAPN3 affects the transcriptional activity of MyoD by decreasing the quantity of the endogenous protein MyoD (Western-blots, confocal microscopy experiments), without affecting its mRNA level (Q-PCR). Furthermore, the inhibitory effect of CAPN3 on MyoD is independent of the ubiquitine proteasome proteolytic pathway that is known to play a role during MyoD degradation. Indeed, MyoD mutants resistant to proteolytic degradation by the proteasome are sensitive to CAPN3 activity. Interestingly, in C2C12 differentiating cells, the overexpression of CAPN3 induces a delay in myogenic differentiation with a decrease in the total number of myotubes which correlated to a delay in the apparition of differentiation marker. Experiments are in progress to determine whether CAPN3 acts directly or not on MyoD. This study identifies CAPN3 as a new regulator of the myogenic factor MyoD and could help to understand the mechanisms sustaining the development of the LGMD2A muscular dystrophy.
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