ACTIN AND ACTIN-BINDING PROTEINS IN DIFFERENTIATING ASTROGLIA IN TISSUE-CULTURE

In this paper we have described the organization of F-actin and actin-binding proteins (ABP): alpha-actinin, myosin, tropomyosin, caldesmon, vinculin, talin, and spectrin, in differentiating astroglia in colony cultures. We observed that the microfilament (MF) network arrangements differ at various stages of astroglia development, but the composition of MF bundles and stress fibers is the same at all developmental stages. F-actin is closely colocalized with myosin, tropomyosin, caldesmon, and alpha-actinin. The striated pattern of myosin, tropomyosin, and caldesmon are superimposable. Tropomyosin and caldesmon extend along F-actin but are interrupted for short periods, whereas myosin is interrupted for longer periods. alpha-actinin colocalizes with tropomyosin and caldesmon but not with myosin. In astroglia at different stages of development spectrin is arranged in the form of fine networks spreading through the cell and does not follow the arrangement of MF bundles. Only F-actin, alpha-actinin, and vinculin can be detected at cell-cell junctions. In the areas of the focal contacts, F-actin, alpha-actinin, vinculin, and talin are present. They overlap each other, although talin and vinculin extend toward the cell membrane beyond F-actin and alpha-actinin. Astroglia undergo well-defined states of nonmotility, motility, and nonmotility again during differentiation. The changes in motility are paralled by changes in the organization of F-actin and ABP: as GFAP-containing intermediate filaments increase in differentiating astroglia, the F-actin and ABP are down-regulated, leading to non motility.