MOUSE MYOBLASTS CAN FUSE AND FORM A NORMAL SARCOMERE IN THE ABSENCE OF BETA-1 INTEGRIN EXPRESSION

Antibody perturbation experiments suggested that migration, terminal d ifferentiation and fusion of myoblasts are dependent on beta 1 integri n expression. In addition, several studies have postulated that beta 1 integrins have a role in the formation of sarcomeres. In the present report we have analysed skeletal myogenesis in wild-type/beta 1-null c himeric mice and beta 1-null embryoid bodies. Trunk and limbs of beta 1-null chimeric mice contained muscle tissue composed of normal and be ta 1-null myoblasts indicating that all myotomic sublineages can form, migrate to their peripheral targets and fuse in the absence of beta 1 integrin expression. Pure populations of beta 1-null myoblasts and sa tellite cells isolated from beta 1-null chimeric embryos and chimeric newborn mice, respectively, were able to differentiate in vitro and to fuse into multinucleated myotubes, Quantitative and qualitative compa risons between normal and beta 1-null myoblasts revealed no apparent d ifference in their capacity to terminally differentiate and fuse, Furt hermore, beta 1-null myotubes developed sarcomeres which were indistin guishable from wild-type controls. When normal and beta 1-null ES cell s were differentiated into embryoid bodies, they contained fully diffe rentiated myotubes with normal sarcomeres and normal deposition of cos tameric components. However, formation of beta 1-null myotubes was del ayed and was less efficient in beta 1-null embryoid bodies than in wil d-type controls. High expression of av integrin subunit at the tips of normal as well as beta 1-null myotubes indicated that the lack of bet a 1 integrins did not result in a significant redistribution of alpha v-containing receptors.