ADAM12 and α9β1Integrin Are Instrumental in Human Myogenic Cell Differentiation

Knowledge on molecular systems involved in myogenic precursor cell (mpc) fusion into myotubes is fragmentary. Previous studies have implicated the a disintegrin and metalloproteinase (ADAM) family in most mammalian cell fusion processes. ADAM12 is likely involved in fusion of murine mpc and human rhabdomyosarcoma cells, but it requires yet unknown molecular partners to launch myogenic cell fusion. ADAM12 was shown able to mediate cell-to-cell attachment through binding α9β1integrin. We report that normal human mpc express both ADAM12 and α9β1integrin during their differentiation. Expression of α9parallels that of ADAM12 and culminates at time of fusion. α9and ADAM12 coimmunoprecipitate and participate to mpc adhesion. Inhibition of ADAM12/α9β1integrin interplay, by either ADAM12 antisense oligonucleotides or blocking antibody to α9β1, inhibited overall mpc fusion by 47–48%, with combination of both strategies increasing inhibition up to 62%. By contrast with blockade of vascular cell adhesion molecule-1/α4β1, which also reduced fusion, exposure to ADAM12 antisense oligonucleotides or anti-α9β1antibody did not induce detachment of mpc from extracellular matrix, suggesting specific involvement of ADAM12–α9β1interaction in the fusion process. Evaluation of the fusion rate with regard to the size of myotubes showed that both ADAM12 antisense oligonucleotides and α9β1blockade inhibited more importantly formation of large (≥5 nuclei) myotubes than that of small (2–4 nuclei) myotubes. We conclude that both ADAM12 and α9β1integrin are expressed during postnatal human myogenic differentiation and that their interaction is mainly operative in nascent myotube growth.