ACTIN FILAMENT BUNDLES IN CORTICAL FIBER CELLS OF THE RAT LENS

The distribution and organization of actin filament bundles were studi ed in cortical fiber cells of rat lenses at various ages (3 days to 2. 5 months old), using thin-section electron microscopy, immunocytochemi stry and immunoblotting. Electron microscopy showed that actin bundles were regularly found along cortical fiber cell membranes of the lens at all ages studied. The actin bundles were commonly arranged in three distinct units, one bundle in each fiber cell, located at the interse ctions where three hexagonal fiber cells meet as seen in cross section s. These actin bundles were approximately 150 nm in diameter and were composed of 7-nm small filaments. They were aligned parallel to the lo ng axis of fiber cells as judged by both cross and longitudinal sectio ns. The outside border of each bundle was always surrounded by a zone of 10-nm intermediate filaments which have the same orientation as tha t of the actin bundles. In longitudinal sections, elongated actin bund les were always parallel to the cell membranes. A number of individual actin bundles sometimes were found to form a chain with periodic shor t intervals. In addition, actin bundles were frequently associated wit h adherens junctions near the intersections and other regions of fiber cell membranes. By immunoelectron microscopy, we demonstrated that th ese filament bundles indeed contained actins. By rhodamine-phalloidin labelling, we found that labeled actin bundles appeared as large, dist inct dots at the corners of hexagonal fiber cells in all ages studied. In addition, nonbundle F-actins were labeled preferentially along the cell membranes of the short sides of hexagonal fiber cells. This resu lted in a unique zigzag pattern of actin labeling commonly seen in the cortical fiber cells of a mature rat lens. Finally, we showed that al pha-actinin was associated with the actin bundles in the fiber cells b y immunofluorescent double labeling and immunoblotting. It is suggeste d that this unique arrangement of actin bundles in fiber cells may pro vide a stabilizing structure for forming a sharp angle at each corner of fiber cells, thereby the hexagonal shape of the cells can be mainta ined. (C) 1997 Academic Press Limited.