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

Integrins are extracellular matrix receptors composed of αand β subunits involved in cell adhesion, migration and signal transduction. The β1 subunit has two isoforms, β1A ubiquitously expressed and β1D restricted to striated muscle. They are not functionally equivalent. Replacement of β1A byβ 1D (β1D 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 β1D 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 β1D knock-in embryos that survived midgestation developed until birth, but exhibited severely reduced skeletal muscle mass. Quantification of myotube numbers showed that substitution ofβ 1A with β1D impairs primary myogenesis with no direct effect on secondary myogenesis. Furthermore, long-term primary myotube survival was affected in β1D knock-in embryos. Finally,overexpression of β1D in C2C12 cells impaired myotube formation while overexpression of β1A primarily affected myotube maturation. Together these results demonstrate for the first time distinct roles forβ 1 integrins in primary versus secondary myogenesis and that theβ 1A and β1D variants are not functionally equivalent in this process.