ANALYSIS OF FIBRONECTIN AND VITRONECTIN RECEPTORS ON HUMAN FETAL SKELETAL-MUSCLE CELLS UPON DIFFERENTIATION

The role of fibronectin (FN) and vitronectin (VN) receptors for cell a dhesion and matrix assembly was analyzed during human fetal myogenesis in vivo and in vitro. In human fetal muscle at 10 weeks gestational a ge 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 alpha 5 and alpha 6 integr ins, but not alpha v, alpha 1, and alpha 3 integrins. However, in vitr o cultured myoblasts (G6) isolated from the same gestational age expre ss alpha v, alpha 1, and alpha 3 integrins in addition to alpha 5 and alpha 6 integrins. A more detailed analysis of FN and VN receptors in vitro shows that the localization of different alpha v heterodimers in to focal contacts is differently regulated. alpha v beta 1 and alpha v beta 3 are present at focal contacts throughout in vitro myogenesis w hereas alpha v beta 5 appears to depend on an endogenously produced fa ctor to localize to focal contacts. The alpha v beta 1, alpha v beta 5 , and alpha 3 beta 1 heterodimers, often reported not to focalize, did form focal contacts in G6 cells, indicating that these myoblasts poss ess components facilitating the formation of cytoskeletal linkages con taining these integrins. alpha 5 beta 1 colocalized with FN in myoblas t cultures, whereas myotubes lacked both FN and alpha 5 beta 1 on the cell surface. In summary, we show that concomitant with in vitro diffe rentiation of G6 cells, FN matrix contacts are abolished, but vitronec tin receptors continue to fulfill an anchoring function during the dif ferentiation process in vitro. Further studies are needed to assess th e relative importance of the FN and VN binding integrins for the diffe rentiation process in comparison with the laminin-binding integrins al pha 6 and alpha 7, also present on these cells. (C) 1995 Academic Pres s, Inc.