Integrin α6β1-laminin interactions regulate early myotome formation in the mouse embryo

We addressed the potential role of cell-laminin interactions during epaxial myotome formation in the mouse embryo. Assembly of the myotomal laminin matrix occurs as epaxial myogenic precursor cells enter the myotome. Most Myf5-positive and myogenin-negative myogenic precursor cells localise near assembled laminin, while myogenin-expressing cells are located either away from this matrix or in areas where it is being assembled. In Myf5(nlacZlnlacZ) (Myf5-null) embryos, laminin, collagen type IV and perlecan are present extracellularly near myogenic precursor cells, but do not form a basement membrane and cells are not contained in the myotomal compartment. Unlike wild-type myogenic precursor cells, Myf5-null cells do not express the alpha 6 beta 1 integrin, a laminin receptor, suggesting that integrin alpha 6 beta 1-laminin interactions are required for myotomal laminin matrix assembly. Blocking alpha 6 beta 1-laminin binding in cultured wild-type mouse embryo explants resulted in dispersion of Myf5-positive cells, a phenotype also seen in Myf5(nlacZlnlacZ) embryos. Furthermore, inhibition of alpha 6 beta 1 resulted in an increase in Myf5 protein and ectopic myogenin expression in dermomyotomal cells, suggesting that alpha 6 beta 1-laminin interactions normally repress myogenesis in the dermomyotome. We conclude that Myf5 is required for maintaining alpha 6 beta 1 expression on myogenic precursor cells, and that alpha 6 beta 1 is necessary for myotomal laminin matrix assembly and cell guidance into the myotome. Engagement of laminin by alpha 6 beta 1 also plays a role in maintaining the undifferentiated state of cells in the dermomyotome prior to their entry into the myotome.