Analysis of Fibronectin and Vitronectin Receptors on Human Fetal Skeletal Muscle Cells upon Differentiation

Abstract

The role of fibronectin (FN) and vitronectin (VN) receptors for cell adhesion and matrix assembly was analyzed during human fetal myogenesis in vivo and in vitro. In human fetal muscle at 10 weeks gestational age FN and laminin are present in the extracellular matrix. Analysis of the integrin repertoire at this developmental stage reveals that the differentiated muscle cells in vivo express α5 and α6 integrins, but not αv, α1, and α3 integrins. However, in vitro cultured myoblasts (G6) isolated from the same gestational age express αv, α1, and α3 integrins in addition to α5 and α6 integrins. A more detailed analysis of FN and VN receptors in vitro shows that the localization of different αv heterodimers into focal contacts is differently regulated. αvβ1 and αvβ3 are present at focal contacts throughout in vitro myogenesis whereas αvβ5 appears to depend on an endogenously produced factor to localize to focal contacts. The αvβ1, αvβ5, and α3β1 heterodimers, often reported not to focalize, did form focal contacts in G6 cells, indicating that these myoblasts possess components facilitating the formation of cytoskeletal linkages containing these integrins. α5β1 colocalized with FN in myoblast cultures, whereas myotubes lacked both FN and α5β1 on the cell surface. In summary, we show that concomitant with in vitro differentiation of G6 cells, FN matrix contacts are abolished, but vitronectin receptors continue to fulfill an anchoring function during the differentiation process in vitro. Further studies are needed to assess the relative importance of the FN and VN binding integrins for the differentiation process in comparison with the laminin-binding integrins α6 and α7, also present on these cells.