Intracellular relationship between actin and alpha-actinin in a whole corneal epithelial tissue

Alpha-actinin is an actin crosslinking protein that may be one of the proteins involved in the attachment of the actin cytoskeletal framework to the plasma membrane. We investigated the distribution of alpha-actinin in whole-mount embryonic chick corneal epithelia using confocal laser scanning analysis. The intracellular alpha-actinin distribution was compared with F-actin using phalloidin, or total actin using an anti-actin antibody. Corneal epithelial tissues were isolated with or without the basal lamina (+ or -BL), and fixed immediately. In addition, epithelia isolated -BL were cultured for 2 hours with either control medium, laminin-supplemented medium or laminin and cytochalasin D (CD)-containing medium. The single- and double-labeled epithelia showed that alpha-actinin delineated the cell borders and microvilli of the periderm cells in the most apical optical sections of control and laminin-treated epithelia. At the optical plane through the basal cell nuclei, the alpha-actinin was distributed diffusely throughout the cytoplasm, whereas the actin was sparse, only associated with the lateral cell membranes. Epithelia (-BL) cultured in control medium had cytoplasmic protrusions or blebs on the basal cell surface. The blebs contained both actin and alpha-actinin. In epithelial cultured with laminin, the basal cell surface was flat. The actin cortical mat became reorganized within two hours. Actin and alpha-actinin were colocalized in the re-formed basal cytoskeletal network. In cells cultured with cytochalasin D (CD) and laminin the actin cortical mat was not reorganized. Actin networks from both cell layers were eliminated and replaced by aggregates scattered throughout the cytoplasm. The alpha-actinin remained diffusely distributed in the cytoplasm and failed to colocalize with the actin aggregates. The alpha-actinin appeared closer to the basal cell membrane than the actin in cross-sectional views of the tissue. Results from these double-labeling experiments confirmed the intimate association of alpha-actinin and actin in the laminin-stimulated actin cortical mat reorganization. This study is the first to demonstrate that CD-aggregated F-actin does not capture the alpha-actinin. The alpha-actinin appeared to remain diffuse throughout the cytoplasm and separate from F-actinin; however, there was some overlap with G-actin.