Résumé :
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Hereditary skeletal muscle channelopathies are characterized by abnormal muscle excitability. They result from mutations in genes coding for skeletal muscle voltage-gated ion channels involved in the propagation of action potential and excitation-contraction coupling. Related diseases that clinically resemble these channelopathies include Schwartz-Jampel syndrome (SJS, MIM 255800), which is a rare autosomal recessive human disorder. It is characterized by muscle hyperactivity clinically related to myotonia. SJS is due to a lack of perlecan, the major heparan sulfate proteoglycan of basement membranes (BMs). The synaptic BM is rich in laminin isoforms that play a crucial role in the organization of neuromuscular junctions (NMJs). The C-terminus of the laminin beta2 subunit binds directly to the pore forming alpha1 subunit of the presynaptic P/Q type voltage-gated calcium channel (CaV2.1) by a leucine-arginine-glutamate (LRE)-motif. Two LRE-motifs are present in the human perlecan (aa 4149-4151 and 4299-4301), which is highly enriched at the BM of NMJs. This observation led us to study a possible interaction between the BM protein perlecan and CaV2.1 in vivo and in vitro. First, the SJS mouse mutant line was used to analyze the correlation between the CaV2.1 expression and the absence of perlecan. Immunostaining and confocal microscopy of dissected longitudinal and transversal muscle slices revealed an up-regulation of CaV2.1 at the NMJ in the SJS line compared to wildtype mice. Secondly, immunocytochemical experiments using the recombinant HEK293 expression system showed a selective colocalization at the outer membrane of endogenously expressed perlecan and the transfected alpha1 subunits of CaV2.1. These preliminary results suggest an interaction, direct or indirect, between the presynaptic calcium channel and perlecan. We currently investigate this issue further by biochemical analyses.
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