Ankyrin of the ocular lens

Functions of the intermediate filament cytoskeleton in the eye lens

Intermediate filaments (IFs) are a key component of the cytoskeleton in virtually all vertebrate cells, including those of the lens of the eye. IFs help integrate individual cells into their respective tissues. This Review focuses on the lens-specific IF proteins beaded filament structural proteins 1 and 2 (BFSP1 and BFSP2) and their role in lens physiology and disease. Evidence generated in studies in both mice and humans suggests a critical role for these proteins and their filamentous polymers in establishing the optical properties of the eye lens and in maintaining its transparency. For instance, mutations in both BFSP1 and BFSP2 cause cataract in humans. We also explore the potential role of BFSP1 and BFSP2 in aging processes in the lens.

ERM proteins of the lens

Ezrin and radixin and protein 4.1 were detected in the lens of the eye. These proteins were mainly present in the young elongating cortical fiber cells and localized to the plasma membranes. Moesin was not detected. Ezrin, radixin, and protein 4.1 provide another means whereby actin is linked to the plasma membrane in addition to the known adherens junctions in the lens.

Targeted ablation of NrCAM or ankyrin-B results in disorganized lens fibers leading to cataract formation

The NgCAM-related cell adhesion molecule (NrCAM) is an immunoglobulin superfamily member of the L1 subgroup that interacts intracellularly with ankyrins. We reveal that the absence of NrCAM causes the formation of mature cataracts in the mouse, whereas significant pathfinding errors of commissural axons at the midline of the spinal cord or of proprioceptive axon collaterals are not detected. Cataracts, the most common cause of visual impairment, are generated in NrCAM-deficient mice by a disorganization of lens fibers, followed by cellular disintegration and accumulation of cellular debris. The disorganization of fiber cells becomes histologically distinct during late embryonic development and includes abnormalities of the cytoskeleton and of connexin50-containing gap junctions. Furthermore, analysis of lenses of ankyrin-B mutant mice also reveals a disorganization of lens fibers at postnatal day 1, indistinguishable from that generated by the absence of NrCAM, indicating that NrCAM and ankyrin-B are required to maintain contact between lens fiber cells. Also, these studies provide genetic evidence of an interaction between NrCAM and ankyrin-B.

Stabilization and remodeling of the membrane skeleton during lens fiber cell differentiation and maturation

Actin filaments are integral components of the plasma membrane-associated cytoskeleton (membrane skeleton) and are believed to play important roles in the determination of cell polarity, shape, and membrane mechanical properties, however the roles of actin regulatory proteins in controlling the assembly, stability, and organization of actin filaments in the membrane skeleton are not well understood. Tropomodulin is a tropomyosin and actin-binding protein that stabilizes tropomyosin-actin filaments by capping their pointed ends and is associated with the spectrin-actin membrane skeleton in erythrocytes, skeletal muscle cells, and lens fiber cells, a specialized epithelial cell type. In this study, we have investigated the role of tropomodulin and other membrane skeleton components in lens fiber cell differentiation and maturation. Our results demonstrate that tropomodulin is expressed concomitantly with lens fiber cell differentiation and assembles onto the plasma membrane only after fiber cells have begun to elongate and form apical-apical contacts with the undifferentiated epithelium. In contrast, other membrane skeleton components, spectrin, actin, and tropomyosin, are constitutively expressed and assembled on the plasma membranes of both undifferentiated and differentiated fiber cells. Tropomodulin, but not other membrane skeleton components, is also enriched at a novel structure at the apical and basal ends of newly elongated fiber cells at the fiber cell-epithelium and fiber cell-capsule interface, respectively. Once assembled, tropomodulin and its binding partners, tropomyosin and actin, remain membrane-associated and are not proteolyzed during fiber cell maturation and aging, despite proteolysis of alpha-spectrin and other cytoskeletal filament systems such as microtubules and intermediate filaments. We propose that actin filament stabilization by tropomodulin, coupled with partial proteolysis of other cytoskeletal components, represents a programmed remodeling of the lens membrane skeleton that may be essential to maintain plasma membrane integrity and transparency of the extremely elongated, long-lived cells of the lens. The unique localization of tropomodulin at fiber cell tips further suggests a new role for tropomodulin at cell-cell and cell-substratum contacts; this may be important for cell migration and/or adhesion during differentiation and morphogenesis.

Properties of purified lens transglutaminase and regulation of its transamidase/crosslinking activity by GTP

On account of its protein crosslinking activity, the Ca2+-dependent transglutaminase of the lens is likely to be involved in the formation of cataracts. We have now purified the rabbit lens enzyme to near homogeneity as judged by SDS-PAGE (Mr approximately 78 kDa), and a key feature of the procedure was the use of a highly selective affinity chromatographic step with a fibronectin fragment as ligand. The catalytic activity of the lens transglutaminase, measured by the incorporation of dansylcadaverine into dimethylcasein, was compared with those of two similar enzymes isolated from human red cells and from guinea pig liver, respectively. All three enzymes were inhibited by GTP, but the lens enzyme was most sensitive to inhibition by the nucleotide. Moreover, GTP was also shown to inhibit the formation of the approximately 55 kDa betacrystallin dimers in the Ca2+-treated rabbit lens homogenate, proving that the nucleotide is a negative regulator for the crosslinking activity of transglutaminase in this tissue.

Immunodetection of membrane skeletal protein 4.2 in bovine and chicken eye lenses and erythrocytes

PURPOSE

Protein 4.2 is a major erythrocyte membrane skeletal protein, playing an important role in maintaining the integrity and stability of the membrane. It is a transglutaminase-like molecule with no enzymatic cross-linking activity. Several protein 4.2-associated proteins (i.e. band 3, ankyrin, and protein 4.1) and transglutaminase activities have been detected in the lens. The purpose of this study is to find out if protein 4.2 is also expressed in lens fiber membranes.

METHODS

Western blot analysis of cell membranes isolated from bovine and chicken lens fibers and erythrocytes, and immunocytochemistry of frozen sections of bovine and chicken lens fibers were carried out using two protein 4.2-specific antibodies. These two peptide antibodies have been used to identify two alternatively spliced protein 4.2 isoforms in human erythrocyte membranes: the short (P4.2S, or hP4.2(691)) and the long (P4.2L, or hP4.2(721)) isoforms.

RESULTS

Western blot analysis using anti-P4.2(L) antibody demonstrated specific immunoreactive polypeptides in bovine and chicken lens fiber membranes and erythrocyte membranes, co-migrating with hP4.2(721). Immunofluorescence staining of bovine and chicken lenses, using anti-P4.2(L) antibody, revealed specific signals along the cell membranes of cortical fibers. The signals exhibited a unique, patchy pattern along the cortical fiber cell membranes in both cross-sectional and longitudinal views. In cross sections, the labeling of anti-P4.2(L) along the entire cell membranes gave an appearance of a hexagonal shape of fiber cells.

CONCLUSIONS

Protein 4.2, or its analogs, is present in the lens fiber membranes. Its specific staining pattern in the lens fibers suggests that it participates in the architecture of the lens fiber cell membranes, and may play a role in the lens mechanics and pathology.