Résumé :
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The myotendinous junction (MTJ) provides a structural link between the muscle cell cytoskeleton and the extracellular matrix (ECM) of tendons. Lack of one of the MTJ components can alter dramatically muscle cell anchoring to the tendon. Still, its morphogenesis remains elusive and has been neglected by biologists. Collagen XXII is a novel component of MTJ (Koch et al, 2004). It belongs to the FACIT (Fibril-associated collagen with interrupted triple helices) subset of the collagen family which are characterized by their capacity to associate with the fibrillar collagens and to mediate protein interactions. We aim at analyzing the role of collagen XXII in MTJ morphogenesis in zebrafish. Visual direct observations of embryos and rapid and efficient protein knock-down unmatched in other animal models make zebrafish a particularly valuable model for exploring muscle/tendon development. We identify the zebrafish ortholog of the human collagen XXII gene (COL22A1) and show that its transcripts are exclusively expressed in myotomal muscle at 24 hpf. As muscle cells differentiate, the signal concentrates at the extremities of muscle fibers close to the MTJ and sandwiches the myosepta, the structure equivalent to mammalian tendons. Confocal double immunofluorescence with antibodies to dystrophin and to zebrafish collagen XXII shows that collagen XXII is deposited at the MTJ. Morpholino-based knock-down of col22a1 causes bending of the tail and important morphofunctional alterations in muscles/tendon formation: myosepta interruptions and presence of giant muscle cells spanning two myotomes at the interruption sites, muscle fiber detachment from the MTJ… Several mutations in ECM genes have been associated to myopathies. The genetic heterogeneity of the disorders suggests that other matrix proteins could account for unexplained cases. Our results suggest that collagen XXII plays a crucial role in muscle homeostasis and thereby represents a good candidate gene for muscular dystrophies.
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