Tropomodulin and tropomyosin mediate lens cell actin cytoskeleton reorganization in vitro.

PURPOSE To determine the role of the actin cytoskeleton regulatory proteins tropomyosin and tropomodulin (Tmod) in the reorganization of the actin cytoskeleton during lens epithelial cell differentiation. METHODS Primary cultures of chick lens epithelial cells were allowed to differentiate in vitro to form lentoid bodies. Localization of F-actin, Tmod, and tropomyosin were determined by immunofluorescent staining followed by confocal microscopy. Tropomyosin and Tmod isoform expression was determined by immunoprecipitation and western blot analysis. RESULTS In undifferentiated epithelial cells F-actin was organized in polygonal arrays of stress fibers and was also associated with the adherens belt. In contrast, F-actin in differentiated cells was predominantly associated with membranes in a reticular or fibrillar pattern and was organized in curvilinear fibrils in the cytoplasm. Tmod was not detected in the undifferentiated epithelial cells but was expressed upon cell differentiation and assembled into F-actin and non-F-actin structures. Tmod isoforms expressed in the lens cell cultures were identical with those expressed in the embryonic chick lens fiber cells. Tropomyosin was associated with the polygonal arrays of stress fibers in the undifferentiated epithelial cells and was recruited to cortical F-actin at the cell periphery during differentiation. This occurred coincident with a shift in tropomyosin isoform expression. CONCLUSIONS Expression and sequential assembly of low-molecular-weight tropomyosin and Tmod into the cortical actin cytoskeleton of differentiated lens cells may help to reorganize the actin cytoskeleton during morphogenetic differentiation. Moreover, lens epithelial cell differentiation may include the generation of novel Tmod-containing, non-F-actin cytoskeletal structures.