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Limb-girdle muscular dystrophy type 2A (LGMD2A) is a recessive genetic disorder caused by mutations in the cysteine protease calpain 3 (CAPN3) that leads to selective muscle wasting. We previously showed that CAPN3 deficiency is associated with a profound perturbation of the NF-κB/IκBα survival pathway. In this study, the consequences of altered NF-κB/IKBα pathway were investigated using biological materials from LGMD2A patients. We first show that the antiapoptotic factor cellular-FLICE inhibitory protein (c-FLIP), which is dependent on the NF-κB pathway in normal muscle cells, is down-regulated in LGMD2A biopsies. In muscle cells isolated from LGMD2A patients, NF-κB is readily activated on cytokine induction as shown by an increase in its DNA binding activity. However, we observed discrepant transcriptional responses depending on the NF-κB target genes. IκBα is expressed following NF-κB activation independent of the CAPN3 status, whereas expression of c-FLIP is obtained only when CAPN3 is present. These data lead us to postulate that CAPN3 intervenes in the regulation of the expression of NF-κB-dependent survival genes to prevent apoptosis in skeletal muscle. Deregulations in the NF-κB pathway could be part of the mechanism responsible for the muscle wasting resulting from CAPN3 deficiency. [ABSTRACT FROM AUTHOR]

Pathological cellular hallmarks of Duchenne muscular dystrophy (DMD) include, among others, abnormal calcium homeostasis. Changes in the expression of specific receptors for extracellular ATP in dystrophic muscle have been recently documented: here, we demonstrate that at the earliest, myoblast stage of developing dystrophic muscle a purinergic dystrophic phenotype arises. In myoblasts of a dystrophin-negative muscle cell line established from the mdx mouse model of DMD but not in normal myoblasts, exposure to extracellular ATP triggered a strong increase in cytoplasmic Ca2+ concentrations. Influx of extracellular Ca2+ was stimulated by ATP and BzATP and inhibited by zinc, Coomassie Brilliant Blue-G, and KN-62, demonstrating activation of P2X7 receptors. Significant expression of P2X4 and P2X7 proteins was immunodetected in dystrophic myoblasts. Therefore, full-length dystrophin appears, surprisingly, to play an important role in myoblasts in controlling responses to ATP. Our results suggest that altered function of P2X receptors may be an important contributor to pathogenic Ca entry in dystrophic mouse muscle and may have implications for the pathogenesis of muscular dystrophies. Treatments aiming at inhibition of specific ATP receptors could be of a potential therapeutic benefit. [ABSTRACT FROM AUTHOR]

Muscle fibers and myoblasts can participate in immune cell interactions and may play an important role in initiating or perpetuating muscle-derived or directed immune responses. Up-regulation of major histocompatibility (MHC) class I and II antigens under pathological conditions is essential for the interaction of muscle with CD8 cytotoxic T cells and CD4 T helper cells, as observed in the idiopathic inflammatory myopathies. Human muscle cells can present antigens to CD4 T cells and are able to modulate T cell responses via accessory molecules. The expression of B7 costimulatory molecules distinct from B7.1/2; (CD80/CD86) has been postulated but lacks molecular definition and characterization of its functional significance. [ABSTRACT FROM AUTHOR]