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Disruption of actin‐myosin interactions results in the inhibition of focal adhesion assembly in Xenopus XR1 glial cells

In the present study we have investigated the role of actin‐myosin interactions in regulating focal adhesion assembly in Xenopus XR1 glial cells. Actin‐myosin interactions, stress fiber formation, and focal adhesion assembly are thought to allow cells to exert tension in the surrounding extracellular matrix, a process essential during morphogenesis and wound healing. Immunocytochemical analysis has revealed that myosin heavy chain‐A (MHC‐A), the predominant isoform in XR1 cells, was distributed in a filamentous pattern in the central region but was more diffuse towards the cell periphery. Myosin heavy chain‐A‐like immunoreactivity (IR) partially colocalized with phalloidin stained F‐actin microfilaments in XR1 cells but not with microtubules. Furthermore, MHC‐A‐IR colocalized with immunoreactivity for β1 integrin receptors and vinculin at focal adhesions located more centrally along the ventral surface of the cells. The partial colocalization of MHC‐A with the F‐actin cytoskeleton, as well as at focal adhesions, provides evidence that actin‐myosin interactions may be involved in regulating focal adhesion assembly and stabilization. To examine this possibility, we have used drugs shown to inhibit cell contractility: the kinase inhibitors H7 and HA100, and 2,3‐butanedione 2‐monoxime (BDM), which inhibits muscle and nonmuscle ATPase activity. Compared to control cultures, those treated with the inhibitors exhibited a dose‐dependent decrease in the percentage of cells that displayed focal adhesions. In addition, these cells also displayed disrupted actin cytoskeletons and a similar disruption in myosin filaments. Taken together, these results provide evidence for an important role of actin‐myosin generated forces during focal adhesion assembly in glial cells. GLIA 26:245–259, 1999. © 1999 Wiley‐Liss, Inc.