Collagen XV, a novel factor in zebrafish notochord differentiation and muscle development.

Muscle cells are surrounded by extracellular matrix, the components of which play an important role in signalling mechanisms involved in their development. In mice, loss of collagen XV, a component of basement membranes expressed primarily in skeletal muscles, results in a mild skeletal myopathy. We have determined the complete zebrafish collagen XV primary sequence and analysed its expression and function in embryogenesis. During the segmentation period, expression of the Col15a1 gene is mainly found in the notochord and its protein product is deposited exclusively in the peri-notochordal basement membrane. Morpholino mediated knock-down of Col15a1 causes defects in notochord differentiation and in fast and slow muscle formation as shown by persistence of axial mesodermal marker gene expression, disorganization of the peri-notochodal basement membrane and myofibrils, and a U-shape myotome. In addition, the number of medial fast-twitch muscle fibers was substantially increased, suggesting that the signalling by notochord derived Hh proteins is enhanced by loss of collagen XV. Consistent with this, there is a concomitant expansion of patched-1 expression in the myotome of morphant embryos. Together, these results indicate that collagen XV is required for notochord differentiation and muscle development in the zebrafish embryo and that it interplays with Shh signalling.