Knock-in of integrin β1D affects primary but not secondary myogenesis in mice

Integrins are extracellular matrix receptors composed of (X and beta subunits involved in cell adhesion, migration and signal transduction. The beta1 subunit has two isoforms, beta1A ubiquitously expressed and beta1D restricted to striated muscle. They are not functionally equivalent. Replacement of beta1A by beta1D (beta1D knock-in) in the mouse leads to midgestation lethality on a 50% Ola/50% FVB background [Baudoin, C., Goumans, M. J., Mummery, C. and Sonnenberg, A. (1998). Genes Dev. 12, 1202-1216]. We crossed the beta1D knock-in line into a less penetrant genetic background. This led to an attenuation of the midgestation lethality and revealed a second period of lethality around birth. Midgestation death was apparently not caused by failure in cell migration, but rather by abnormal placentation. The beta1D knock-in embryos that survived midgestation developed until birth, but exhibited severely reduced skeletal muscle mass. Quantification of myotube numbers showed that substitution of beta1A with beta1D impairs primary myogenesis with no direct effect on secondary myogenesis. Furthermore, long-term primary myotube survival was affected in beta1D knock-in embryos. Finally, overexpression of beta1D in C2C12 cells impaired myotube formation while overexpression of beta1A primarily affected myotube maturation. Together these results demonstrate for the first time distinct roles for beta1 integrins in primary versus secondary myogenesis and that the beta1A and beta1D variants are not functionally equivalent in this process.