The Integrin-Ligand Interaction Regulates Adhesion and Migration through a Molecular Clutch

Adhesive and migratory behavior can be cell type, integrin, and substrate dependent. We have compared integrin and substrate differences using three integrin receptors: alpha 5 beta 1, alpha 6 beta 1, and alpha L beta 2 expressed in a common cell type, CHO center dot B2 cells, which lack integrin alpha subunits, as well as in different cell types that express one or more of these integrins. We find that CHO center dot B2 cells expressing either alpha 6 beta 1 or alpha L beta 2 integrins migrate and protrude faster and are more directionally persistent on laminin or ICAM-1, respectively, than CHO center dot B2 cells expressing alpha 5 beta 1 on fibronectin. Despite rapid adhesion maturation and the presence of large adhesions in both the alpha 6 beta 1- and alpha L beta 2-expressing cells, they display robust tyrosine phosphorylation. In addition, whereas myosin II regulates adhesion maturation and turnover, protrusion rates, and polarity in cells migrating on fibronectin, surprisingly, it does not have comparable effects in cells expressing alpha 6 beta 1 or alpha L beta 2. This apparent difference in the integration of myosin II activity, adhesion, and migration arises from alterations in the ligand-integrin-actin linkage (molecular clutch). The elongated adhesions in the protrusions of the alpha 6 beta 1-expressing cells on laminin or the alpha L beta 2-expressing cells on ICAM-1 display a novel, rapid retrograde flux of integrin; this was largely absent in the large adhesions in protrusions of alpha 5 beta 1-expressing cells on fibronectin. Furthermore, the force these adhesions exert on the substrate in protrusive regions is reduced compared to similar regions in alpha 5-expressing cells, and the adhesion strength is reduced. This suggests that intracellular forces are not efficiently transferred from actomyosin to the substratum due to altered adhesion strength, that is, avidity, affinity, or the ligand-integrin-actin interaction. Finally, we show that the migration of fast migrating leukocytes on fibronectin or ICAM-1 is also largely independent of myosin II; however, their adhesions are small and do not show retrograde fluxing suggesting other intrinsic factors determine their migration differences.