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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 beta1 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. |
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