Résumé :
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Communication n° 262 Spinal muscular atrophy (SMA) is a frequent recessive autosomal neuromuscular disorder characterized by the degeneration of spinal motor neurons (MN) associated with muscle paralysis and atrophy. Mutations of the survival of motor neuron gene (SMN) are responsible for SMA. Cellular and molecular mechanisms involved in SMA physiopathology remain to be elucidated. Recent studies in our laboratory indicated that differentiated muscle fibers (that arise from the fusion of myoblasts) isolated from our severe muscular mutant mouse model (HSA-Cre, Smn?7/F7) display identical length whereas the number of nuclei was reduced when compared to normal muscle fibers. Our goal was to determine whether SMN mutation results in defect of cellular fusion and to identify the signaling pathway. We studied cellular fusion in myoblasts and osteoclasts (OC), two primary multinucleated cellular models. These primary cultures derived from mice carrying the heterozygous deletion of Smn exon 7 in all cell types (Smn?7/+). Our results indicated that primary myogenic cultures from Smn?7/+ mice display a reduced proportion of myotube compared to controls at day 6 (16.2% Æ 1.0 vs 40.2% Æ 4.1, p<0.01). We then studied primary OC that arise from fusion of bone marrow hematopoietic mononuclear cells. Our data indicated a 50% decrease of OC proportion isolated from SMN?7/+ mice when compared to control mice (p<0.05). Consistently, the osteoclastic activity was decreased (50%, p<0.05) in mutant cells compared to wild type cells. Altogether these data strongly suggest a role of SMN in cellular fusion. Several signaling pathways are known to be involved in muscle and OC fusion and are currently investigated in our cell systems. We will next determine whether the signaling pathway involved in cell fusion defect is also involved in MN degeneration using primary cultures of MN deficient in SMN. These approaches should allow a better understanding of the SMA pathogenesis to design targeted therapeutics.
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