Cytoplasmic filaments in the crystalline lens of various species: functional correlations

Refractive index measurement of the mouse crystalline lens using optical coherence tomography

In recent years, there has been a growing interest for using mouse models in refractive development and myopia research. The crystalline lens is a critical optical component of the mouse eye that occupies greater than 50% of the ocular space, and significant increases in thickness with age. However, changes in refractive index of the mouse crystalline lens are less known. In this study, we examined the changes in thickness and refractive index of the mouse crystalline lens for two different strains, wild-type (WT) and a nyx mutant (nob) over the course of normal visual development or after form deprivation. Refractive index and lens thickness measurements were made on ex vivo lenses using spectral domain optical coherence tomography (SD-OCT). Comparison of refractive index measurements on 5 standard ball lenses using the SD-OCT and their known refractive indices (manufacturer provided) indicated good precision (intra-class correlation coefficient, 0.998 and Bland-Altman coefficient of repeatability, 0.116) of the SD-OCT to calculate mouse lens refractive index ex vivo. During normal visual development, lens thickness increased significantly with age for three different cohorts of mice, aged 4 (average thickness from both eyes; WT: 1.78 ± 0.03, nob: 1.79 ± 0.08 mm), 10 (WT: 2.02 ± 0.05, nob: 2.01 ± 0.04 mm) and 16 weeks (WT: 2.12 ± 0.06, nob: 2.09 ± 0.06 mm, p < 0.001). Lens thickness was not significantly different between the two strains at any age (p = 0.557). For mice with normal vision, refractive index for isolated crystalline lenses in nob mice was significantly greater than WT mice (mean for all ages; WT: 1.42 ± 0.01, nob: 1.44 ± 0.001, p < 0.001). After 4 weeks of form deprivation to the right eye using a skull-mounted goggling apparatus, a thinning of the crystalline lens was observed in both right and left eyes of goggled animals compared to their naïve controls (average from both the right and the left eye) for both strains (p = 0.052). In form deprived mice, lens refractive index was significantly different between the goggled animals and non-goggled naïve controls in nob mice, but not in WT mice (p = 0.009). Both eyes of goggled nob mice had significantly greater lens refractive index (goggled, 1.49 ± 0.01; opposite, 1.47 ± 0.03) compared to their naïve controls (1.45 ± 0.02, p < 0.05). The results presented here suggest that there are genetic differences in the crystalline lens refractive index of the mouse eye, and that the lens refractive index in mice significantly increase with form deprivation. Research applications requiring precise optical measurements of the mouse eye should take these lens refractive indices into account when interpreting SD-OCT data.

Ultrastructural analysis of the human lens fiber cell remodeling zone and the initiation of cellular compaction

The purpose is to determine the nature of the cellular rearrangements occurring through the remodeling zone (RZ) in human donor lenses, identified previously by confocal microscopy to be about 100 μm from the capsule. Human donor lenses were fixed with 10% formalin followed by 4% paraformaldehyde prior to processing for transmission electron microscopy. Of 27 fixed lenses, ages 22, 55 and 92 years were examined in detail. Overview electron micrographs confirmed the loss of cellular organization present in the outer cortex (80 μm thick) as the cells transitioned into the RZ. The transition occurred within a few cell layers and fiber cells in the RZ completely lost their classical hexagonal cross-sectional appearance. Cell interfaces became unusually interdigitated and irregular even though the radial cell columns were retained. Gap junctions appeared to be unaffected. After the RZ (40 μm thick), the cells were still irregular but more recognizable as fiber cells with typical interdigitations and the appearance of undulating membranes. Cell thickness was irregular after the RZ with some cells compacted, while others were not, up to the zone of full compaction in the adult nucleus. Similar dramatic cellular changes were observed within the RZ for each lens regardless of age. Because the cytoskeleton controls cell shape, dramatic cellular rearrangements that occur in the RZ most likely are due to alterations in the associations of crystallins to the lens-specific cytoskeletal beaded intermediate filaments. It is also likely that cytoskeletal attachments to membranes are altered to allow undulating membranes to develop.

A novel terminal web-like structure in cortical lens fibers: architecture and functional assessment

This study describes a novel cytoskeletal array in fiber cells of the ocular lens of the rat and shows its relationship to the classical terminal web of other epithelial tissues. Naive adult Sprague-Dawley rats (n = 28) were utilized. F-actin, fodrin, myosin IIA, and CP49 distribution was assessed in anterior and posterior polar sections. For functional analysis, lenses were cultured with or without cytochalasin-D for 3 hr, then processed for confocal microscopy or assessed by laser scan analysis along sutures. Phalloidin labeling demonstrated a dense mesh of F-actin adjacent to posterior sutural domains to a subcapsular depth of 400 μm. Anterior polar sections revealed a comparable actin structure adjacent to anterior suture branches however, it was not developed in superficial fibers. Fodrin and myosin were localized within the web-like actin apparatus. The data was used to construct a model showing that the cytoskeletal array is located within the blunt, variable-width fiber ends that abut at sutures such that the "terminal web" flanks the suture on either side. Treatment with cytochalasin-D resulted in partial disassembly of the "terminal web" and perturbed cellular organization. Laser scan analysis revealed that cytochalasin-D treated lenses had significantly greater focal variability than control lenses (P = 0.020). We conclude that cortical fibers of rat lenses contain a bipolar structure that is structurally and compositionally analogous to classical terminal webs. The results indicate that the lens "terminal web" functions to stabilize lens fiber ends at sutures thus minimizing structural disorder, which in turn, promotes the establishment and maintenance of lens transparency.

Polygonal arrays of microfilaments in epithelial cells of the intact lens

Polygonal arrays of microfilaments have been discovered to line the inner apical plasma membrane of anterior epithelial cells of the intact rabbit lens. When tangential sections are studied with the electron microscope, the polygonal arrays are seen to consist of central vertices interconnected by rays of filaments. The rays near the cell periphery insert into the lateral plasma membrane. The vertices are spaced about 1 micron apart and appear to be attached to the apical plasma membrane. The polygonal arrays have little depth as judged by stereo-pairs and are incorporated within the dense band of microfilaments seen in cross-section at the epithelio-fiber junction. The diameter of the filaments and their similarity to actin-containing polygonal arrays described by other investigators in cultured cells suggest that these structures contain actin in lens epithelial cells. The function of the polygonal arrays in relation to maintenance of lens shape or to changes in lens shape in accommodation is discussed.

The lens capsule

The lens capsule is a modified basement membrane that completely surrounds the ocular lens. It is known that this extracellular matrix is important for both the structure and biomechanics of the lens in addition to providing informational cues to maintain lens cell phenotype. This review covers the development and structure of the lens capsule, lens diseases associated with mutations in extracellular matrix genes and the role of the capsule in lens function including those proposed for visual accommodation, selective permeability to infectious agents, and cell signaling.

Primary sequence, secondary structure, gene structure, and assembly properties suggests that the lens-specific cytoskeletal protein filensin represents a novel class of intermediate filament protein

The ocular lens fiber cell assembles a novel cytoskeletal element, the Beaded Filament, from CP49 and filensin, two proteins expressed only in the differentiated lens fiber cell. We report the primary sequence, secondary structural analysis, gene structure and Yeast Two Hybrid interaction data for human filensin, and develop a consensus model of filensin from the human and previously reported bovine and chicken filensin sequences. This consensus model, combined with gene structure and Yeast Two Hybrid studies establish that filensin is a member of the Intermediate Filament family of proteins. Specifically, filensin exhibits (1) divergence at amino acid sequence motifs otherwise highly conserved among intermediate filament proteins, (2) a loss of 29 amino acids from the central rod domain which is unique among cytoplasmic intermediate filament proteins, (3) an absence of sequence identity with any existing class of intermediate filament protein, (4) a gene structure unique among intermediate filament family, (5) an inability to dimerize with representatives of Type I, II, and III intermediate filament proteins. Thus, at each level of analysis, we find that filensin is similar to the consensus model of intermediate filament proteins, supporting our conclusion that filensin's relatedness to the IF family is not the consequence of convergent evolution. However, filensin also shows unique or extreme distinctions from the consensus intermediate filament protein at each level of analysis, indicating that filensin constitutes a novel class of IF protein. Some of filensin's unique features are incompatible with current models of IF assembly. Analysis of filensin gene structure suggests that the 29 amino acid reduction in the central rod domain was not the result of a single splice site mutation, the mechanism suggested for the transition between nuclear lamins and cytoplasmic intermediate filament proteins.

Actin filament bundles in cortical fiber cells of the rat lens

The distribution and organization of actin filament bundles were studied in cortical fiber cells of rat lenses at various ages (3 days to 2.5 months old), using thin-section electron microscopy, immunocytochemistry 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 intersections where three hexagonal fiber cells meet as seen in cross sections. These actin bundles were approximately 150 nm in diameter and were composed of 7-nm small filaments. They were aligned parallel to the long axis of fiber cells as judged by both cross and longitudinal sections. The outside border of each bundle was always surrounded by a zone of 10-nm intermediate filaments which have the same orientation as that of the actin bundles. In longitudinal sections, elongated actin bundles were always parallel to the cell membranes. A number of individual actin bundles sometimes were found to form a chain with periodic short intervals. In addition, actin bundles were frequently associated with adherens junctions near the intersections and other regions of fiber cell membranes. By immunoelectron microscopy, we demonstrated that these filament bundles indeed contained actins. By rhodamine-phalloidin labelling, we found that labeled actin bundles appeared as large, distinct dots at the corners of hexagonal fiber cells in all ages studied. In addition, non-bundle F-actins were labeled preferentially along the cell membranes of the short sides of hexagonal fiber cells. This resulted in a unique zigzag pattern of actin labeling commonly seen in the cortical fiber cells of a mature rat lens. Finally, we showed that alpha-actinin was associated with the actin bundles in the fiber cells by immunofluorescent double labeling and immunoblotting. It is suggested that this unique arrangement of actin bundles in fiber cells may provide a stabilizing structure for forming a sharp angle at each corner of fiber cells, thereby the hexagonal shape of the cells can be maintained.

L-alpha-glycerophosphate binding to bovine gamma-crystallin: a potential link between metabolism and supramolecular order

The highly selective nature of protein-ligand interactions provides a sensitive mechanism for the modulation of cellular activity by proteins. In the eye lens the supramolecular order of the lens crystallins, which is expected to be susceptible to protein electrostatic charge, in part defines transparency. The binding of charged ligands to proteins is one way of achieving an alteration in protein electrostatic charge. Evidence is presented that L-alpha-glycerophosphate, a major phosphorus metabolite of eye lens metabolism, binds to the globular protein, gamma-crystallin with moderately high affinity and in a positive cooperative manner. The following binding parameters were obtained from equilibrium measurements: minimum number of binding sites, n = 2; Kassoc = 6.2 +/- 0.5 x 10(3) M-1; cooperativity parameter, alpha H = 1.9 +/- 0.1. Interactive computer graphics display techniques were used to locate putative ligand binding sites, and in turn, to identify the possible molecular interactions responsible for the binding of ligand to protein at one of the sites. One putative binding site was located in the cleft between the two domains of gamma II-crystallin. Arginyl residues 79 and 147 are involved in ligand binding as are the peptide carbonyl oxygens of residues Tyrosyl-50 and Aspartyl-156. Five hydrogen bonds between the ligand and the protein structure are predicted for the binding of L-alpha-glycerophosphate, whereas only 3 occur for the binding of the "unnatural" D-enantiomorph. Modulation of both lens protein supramolecular organization and lens metabolism is predicted to be a consequence of L-alpha-glycerophosphate binding to gamma-crystallin in the lens.

Comparative study of actin filament patterns in lens epithelial cells. Are these determined by the mechanisms of lens accommodation?

Actin filament patterns in lens epithelia from animals of various taxonomic groups were studied using rhodamine phalloidin fluorescence microscopy of epithelial whole mounts and transmission electron microscopy of tangential sections. The results were compared with the accommodative mechanism operating in each case as reported in the literature. Lenses that accommodate by deformation of the anterior surface, in squirrel, chipmunk, rabbit, monkey and human, showed polygonal arrays (PAs) at the apical end of the epithelial cells. Lenses that translate as a whole, in shark, bony fish and frog, showed stress fibers (SFs) at the basal or apical end of the cells. No specialized actin pattern was seen in turtle and bird, which have lenses that are squeezed into an anterior lenticonus; cat, where the lens is translated forward; or rat, cow and most mice, which have no defined accommodation. In exception, certain strains of laboratory mice did show sequestered actin bundles (SABs) and/or PAs. Based on our findings, we conclude that PAs, which resemble geodesic domes, do not take an active part in near-point accommodation; but like SFs, may serve to resist overextension by internal pressure of the fiber mass or by zonular tension.

Adherens junctions in the ocular lens of various species: ultrastructural analysis with an improved fixation

The ultrastructure and distribution of adherens junctions in the intact adult lens of human, chicken, dove, rat, and rainbow trout were studied with thin-section electron microscopy, using an improved fixation containing a mixture of glutaraldehyde, lysine, and tannic acid. The nature of adherens junctions in the fiber-cells of the lens was also verified by immunofluorescence and rhodamine-phalloidin labelings for vinculin and actin. Electron microscopy revealed that adherens junctions of the lens were different ultrastructurally from the desmosomes found only between the lateral epithelial cells of the lens. The adherens junctions had the same structural characteristics as the zonulae adherentes, except that they were macular contacts, not belts. However, cross bridges were evident within the interspace of the junctions. Adherens junctions were located between the fiber-cells, between the epithelial cells and fiber-cells, and between the epithelial cells. They had a characteristic distribution in the "intersections" where three hexagonal fiber-cells met, as seen in cross-sections in all species studied. In addition, adherens junctions and associated actin were found distributed randomly along the entire cell membranes of both wide and narrow sides of cortical fiber-cells in the human, chicken, and dove lenses which have good accomodating capability. However, in the poorly-accomodating lenses of rat and fish, these junctions were seen predominantly on the narrow sides and at the regions of the wide sides that were very close to the "intersections". It is suggested that adherens junctions and associated actin microfilaments are involved in stabilizing the structural integrity of lens cells during accomodation and in preserving a specific lens shape.

The ultrastructure of fiber cells in primate lenses: a model for studying membrane senescence

We have compared the surface morphology of the youngest (cortical) fiber cells with that of the most senescent (nuclear) fiber cells in monkey and baboon crystalline lenses by stereo scanning electron microscopy (SEM) and thick-section stereo transmission electron microscopy (TEM). Both the broad and the narrow faces of the most senescent fiber cells featured distinctive, polygonal areas (domains) of furrowed cell membrane. The domains ranged in size from 2.42 to 8.78 microns2. Stereopair SEM and TEM micrographs demonstrated precisely oriented microvilli measuring approximately 0.14 micron in diameter and ranging in length from 1.27 to 4.65 microns overlying each ridge in the domains. Formation of microvilli on senescent cells has been noted in other types of aging cells but they are imprecisely arranged and their function is unknown. Since every fiber cell remains in a fixed location (relative to other fiber cells) throughout life, the lens provides a unique model to study structure-function relationships of senescent microvilli in situ. The discovery of an age-related elaboration of numerous microvilli on senescent fiber cells of noncataractous lenses invalidates the currently accepted theory that close, parallel apposition of the broad faces of lens fiber cells is necessary for the lens to be transparent.

Immunogold-EM localization of actin and vimentin filaments in relation to polygonal arrays in lens epithelium in situ

An indirect immunogold technique for transmission electron microscopy was used for localizing two cytoskeletal proteins, actin and vimentin, in the epithelium of freshly removed rabbit lens, especially in relation to the polygonal array structures located at the apices of the epithelial cells. Antibody specificity was determined on semi-pure chicken breast muscle actin and bovine lens vimentin using Western blotting of these proteins and extracts of rabbit lens epithelium separated by SDS-PAGE. Whole lenses of rabbits were lightly fixed in glutaraldehyde and embedded in LR White resin. Tangential sections were taken at 70 to 80 nm and at 0.25 micron and used for single-labeling, and double-labeling with antibodies raised in different hosts and treated with appropriate second antibodies conjugated with non-overlapping sizes of gold particles. Routine and stereomicroscopy were used to analyze gold-label patterns. The study shows that the rays of the polygons project deeply into the cell from the vertices lying on the inner apical membrane. Actin is located on the filaments of rays, but vimentin is not associated with the polygons at the level in the cell that we studied. Vimentin filaments are found in deeper regions of the epithelial cell. Stereopairs were useful in differentiating where the gold-label was located and in fact, this technique demonstrated that most of the label is on the surface of sections where the filaments are exposed.

Actin filament patterns in mouse lens epithelium: a study of the effects of aging, injury, and genetics

Using mainly fluorescence microscopy after rhodamine-phalloidin staining, the F-actin distribution in the mouse lens epithelium was studied with regard to the effects of age, genetic strain, and mechanical injury. These studies have revealed that aside from its association with the plasma membrane the structural organization of F-actin in the mouse lens epithelium in situ is characterized by two major configurations: (1) a filamentous arrangement in such patterns as stress fibers, polygonal arrays (PAs), and meshworks, and (2) a highly concentrated structure called a sequestered actin bundle (SAB). The aging study indicated that the SAB is a consistent character in C57BL/6 mice from the age of 5 wk on, but not in CF1 mice. The size and shape of the SAB change gradually with age as inferred from two-dimensional measurements. The genetic study on the SAB character using hybrids and congenic strains showed that it is inherited as a Mendelian dominant, probably multigenic mode. Finally, the injury study revealed a structural modification in cells around the wound, including flattening of cells at the edge and extension of processes into the wound space. In the rest of the epithelium, injury amplified membrane infolding and fluorescence of polygonal arrays but diminished the size and fluorescence intensity of SABs. These changes are thought to be correlated with wound repair involving cell division and migration. These studies illustrate the variability in F-actin expression in situ in lens epithelial cells that can be induced by intrinsic and extrinsic factors.

Square arrays and their role in ridge formation in human lens fibers

Square arrays in human lens fibers were studied with freeze-fracture and thin-section TEM. In superficial fibers a number of patches of square array particles in the P face and pits in the E face are found in the smooth membrane. In the deeper cortex and the nucleus, fiber cells have undulating membranes and many ridges. Numerous patches of the particles (P face) are distributed in the concave regions, and the pits (E face) in the convex areas of the bumpy membrane. In most ridges, patches of the particles occur at regular intervals in the "valley" portion, while the pits are on the "crest" portion of ridges. Also, continuous square arrays having the same "valley" location as the regularly arranged patches are found in areas with extensive ridge patterns. The overlapping of the outer portions of two adjacent square arrays is found on the sides between the "crest" and the "valley" of the ridges. Structurally, square arrays are located in a nonjunctional part of the membrane; in an orthogonal crystalline arrangement; and with a particle size of about 6 nm and center-center spacing about 6.4 nm. They are structurally different from gap junctions found in the lens fibers. Thin-section studies reveal two types of cellular contacts: thin pentalamellar structures (about 12-13 nm in overall thickness) associated with the ridge patterns are believed to be square arrays; thick heptalamellar structures (about 16-17 nm in overall thickness) with a narrow gap in between the two central laminae are believed to be gap junctions. This study strongly suggests that square arrays are specifically involved in ridge formation in human lens fibers.

Actin in polygonal arrays of microfilaments and sequestered actin bundles (SABs) in lens epithelial cells of rabbits and mice

The pattern of localization of actin filaments was compared in whole mounts of lens epithelium of rabbit and mouse using the fluorescently-labeled actin-specific probe, rhodamine-phalloidin. In the adult rabbit lens, fluorescent polygonal arrays consisting of central vertices and interconnecting filaments were present in the apical end of each epithelial cell. Electron microscopy confirmed that these arrays lined the cytoplasmic side of the apical membrane. In the mature adult mouse, polygonal arrays were not seen either with fluorescence or electron microscopy. Instead, the actin was packaged in a single, elongated, often curved bundle near the epithelial cell apex, referred to as a "sequestered actin bundle" or SAB. The SAB often appeared attached to the plasma membrane and to approach the perinuclear basket of microfilaments. The significance of the differences in these two patterns of actin is discussed in terms of differences in the accommodative ability and static lens shape in these two animals.

Formation of a 55 000-weight cross-linked beta crystallin dimer in the Ca2+-treated lens. A model for cataract

Incubation of lens in Ca2+-containing media, considered by several investigators to be a useful model of cataract formation, gave rise to significant alterations in the covalent structures of various proteins. In rabbit lens, when sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used after reduction of disulfides in urea, the most readily observable changes were (i) disappearance of 210K, 95K, and 60K proteins, (ii) modifications of alpha crystallin subunits, (iii) alterations of beta H crystallins, and (iv) de novo production of 55K and higher molecular weight polymers. The addition of leupeptin inhibited the disappearances of 210K, 95K, and 60K proteins and the alteration of alpha crystallins, suggesting that all these were caused by a Ca2+-activated protease. The proteolytically sensitive 60K species was identified as vimentin, a component of intermediate filaments. Formation of the 55K material and of higher molecular weight polymers during Ca2+ treatment of the lens could be prevented by histamine, a compound known to inhibit the transglutaminase-mediated cross-linking of proteins by epsilon-(gamma-glutamyl)lysine peptide bonds in other biological systems. It could also be shown by immunoblotting that an antibody raised against the 55K material reacted selectively with beta crystallins of normal lens. This indicates that the 55K product is in all likelihood an essential intermediate toward higher polymers and that the 55K product is a cross-linked dimer of certain polypeptides of beta crystallin.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the intermediate-filament-associated protein synemin in chicken lens cells

Synemin, a 230-kilodalton polypeptide component of avian muscle and erythrocyte intermediate filaments, is also found in association with the vimentin filaments of lens tissue. In chicken lens cells, synemin is bound to the core vimentin polymer with the same 180-nm periodicity that it exhibits in erythrocytes. Its solubility properties are characteristic of those of intermediate filaments in general and similar to those of synemin in muscle cells and erythrocytes. Synemin appears at an early stage of lens development and undergoes a dramatic accumulation as the epithelial cells elongate and differentiate into fiber cells. In contrast to synemin in cultured skeletal muscle, lens synemin is not confined to postmitotic, terminally differentiating cells but is present in proliferative cells as well. It is lost from the fibers near the center of the lens, as are many other cellular structures including intermediate filaments. These findings provide new information about the occurrence and expression of avian synemin and new insight regarding its presumptive role as a modulator of intermediate-filament function.

Identification of two of the major phosphorylated polypeptides of the bovine lens utilizing a lens cAMP-dependent protein kinase system

Two of the major in vitro phosphorylated polypeptides of the bovine lens have been identified. Analysis by means of two-dimensional gel electrophoresis (IEF) has demonstrated that the lens phosphorylated 57,000 and 43,000 dalton polypeptides correspond in mobility to purified phosphorylated bovine lens vimentin and chicken gizzard actin, respectively. Purified actin and vimentin were phosphorylated by a partially purified cAMP-dependent protein kinase isolated from the outer cortex water soluble fraction. All detectable bovine lens vimentin isoelectric variants were phosphorylated. In both the lens fiber cell and chicken gizzard actin preparations, the phosphorylated actin isoelectric variants did not correspond in mobility to the major actin isoelectric variant, but were more acidic. Phosphorylation in all preparations occurred at serine residues.

Expression of the intermediate-filament-associated protein synemin in chicken lens cells

Synemin, a 230-kilodalton polypeptide component of avian muscle and erythrocyte intermediate filaments, is also found in association with the vimentin filaments of lens tissue. In chicken lens cells, synemin is bound to the core vimentin polymer with the same 180-nm periodicity that it exhibits in erythrocytes. Its solubility properties are characteristic of those of intermediate filaments in general and similar to those of synemin in muscle cells and erythrocytes. Synemin appears at an early stage of lens development and undergoes a dramatic accumulation as the epithelial cells elongate and differentiate into fiber cells. In contrast to synemin in cultured skeletal muscle, lens synemin is not confined to postmitotic, terminally differentiating cells but is present in proliferative cells as well. It is lost from the fibers near the center of the lens, as are many other cellular structures including intermediate filaments. These findings provide new information about the occurrence and expression of avian synemin and new insight regarding its presumptive role as a modulator of intermediate-filament function.

Association of alpha-crystallin with actin in cultured lens cells

The nature of the beaded filaments in the lens fiber cell has been debated for some time. One explanation is that beaded filaments represent an association of alpha-crystallin with actin filaments. By using a double labelling technique that allowed us to view actin filaments and alpha-crystallin in the same cell we have demonstrated that some of the alpha-crystallin in lens cells is indeed associated with actin.

In situ localization of S1-labeled actin filaments in chick lens epithelial cells

The actin filaments in the lens epithelial cells of three-day and eight-day post-hatched chicks have been labeled in situ with myosin subfragment 1 (S1). Labeling was accomplished by injuring the lens transcorneally with an ultramicroneedle 5 min or 24 hr before detergent treatment and incubation in S1. A band of filaments found at the epithelio-fiber junction in normal, uninjured chick lens is labeled in the 5 min and 24 hr injury. A subcapsular labeled band is found only in the 24 hr injury, and may be the result of a healing process.

Degradation of actin and vimentin by calpain II, a Ca2+-dependent cysteine proteinase, in bovine lens

Calpain II, a high Ca2+-requiring form of Ca2+-dependent cysteine proteinase (EC 3.4.22.17), isolated from bovine lens was found to cleave actin and vimentin, two major cytoskeletal elements of the lens. Polyacrylamide gel electrophoresis revealed that actin (Mr 43 000) was broken down through intermediary products of approximate Mr 42 000 and 40 000, while vimentin (Mr 57 000) was rapidly cleaved into several fragments ranging from Mr 44 000 to 20 000. The cleavage was dependent on Ca2+ and could be blocked by calpastatin , a calpain-specific inhibitor. These findings suggest that calpain might play a role in age-related degradation of the lens cytoskeleton.

Chicken embryo lens cultures mimic differentiation in the lens

Embryonic chicken lenses, which had been disrupted by trypsin, were grown in culture. These cultures mimic lens development as it occurred in vivo, forming lens-like structures known as lentoids. Using a variety of techniques including electron microscopic analysis, autoradiography, immunofluorescence, and polyacrylamide gel electrophoresis, it was shown that the lentoid cells had many characteristics in common with the differentiated cells of the intact lens, the elongated fiber cells. These characteristics included a shut off of DNA synthesis, a loss of cell organelles, an increase in cell volume, an increase in delta-crystallin protein, and the development of extensive intercellular junctions. The cultures began as a simple epithelial monolayer but then underwent extensive morphogenesis as they differentiated. This morphogenesis involved three distinctive morphological types which appeared in sequence as an epithelial monolayer of polygonal shaped cells with pavement packing, elongated cells oriented end to end, and the multilayered, multicellular lentoids. These distinct morphological stages of differentiation in culture mimic morphogenesis as it occurs in the lens.

Glial fibrillary acidic protein is localized in the lens epithelium

The epithelium of the mouse lens stains intensely with antisera to glial fibrillary acidic protein (GFAP). A protein co-migrating with GFAP and immunoreactive with antisera to GFAP can be demonstrated in lens epithelium protein extracts by immunoblots. GFAP has previously been considered unique to cells of neural origin, but this study demonstrates that ectodermally derived cells express GFAP or a highly similar protein.

Studies on lens vimentin

Antibody prepared against chick lens vimentin cross-reacts with chick fibroblast vimentin and with vimentin of mammalian, reptilian, amphibian and fish lenses. This protein is localized in the epithelial and cortical fiber cells and is progressively lost from the deeper cortical cells. It is absent from the nuclear cells. Lens vimentin is readily oxidized to form high molecular components.

Lens calcium activated proteinase: degradation of vimentin

The lens has been shown to contain a Ca+2 activated proteinase specific for vimentin. The proteinase is present in the soluble fraction of the cortex but not in the epithelium. It is suggested that this proteinase is expressed during terminal differentiation of the epithelial cells and may be responsible for degradation of the intermediate filaments in the fiber cells. The proteinase is inhibited by EGTA but not by several proteinase inhibitors.

Lens actin: purification and localization

Actin was purified from the chick lens using DEAE-52 column chromatography followed by hydroxylapatite chromatography. The antibody produced against the purified actin cross-reacted specifically with lens actin from other species in addition to smooth and skeletal muscle actin and labelled the stress bundles of cultured fibroblasts. Actin was localized, using immunological methods, primarily to the plasma membrane of the epithelial and fiber cells of the chick and human lens. Actin filaments were also identified by HMM S-1 labeling in bovine cortical fiber cells. Using this procedure, the actin filaments were found throughout the fiber cell but were mainly concentrated near the plasma membrane and in cell processes. They formed a population distinct from the beaded filaments. The initial DEAE-52 column chromatography was also useful in the initial purification of lens fiber cell intermediate filament protein and two species of beta-crystallins.

Two-dimensional gel analysis of chick lens proteins

Two-dimensional analyses of the chick lens water-soluble and water-insoluble proteins were conducted according to the method of O'Farrell (1975). The results define the isoelectric properties of the water-soluble and the urea-soluble polypeptides and demonstrate differences in composition for cortical and nuclear proteins. Chick lens vimentin consists of at least two isoelectric variants, and its breakdown products were identified. Chick lens actin is primarily of the gamma-type. The 47 K polypeptide specific for fiber cells shows considerable charge heterogeneity, and its most acidic component is found primarily in the nuclear fiber cells. This study also shows that apparently single bands resolved by one-dimensional SDS-polyacrylamide gel electrophoresis of the urea-soluble fraction consists of different proteins, and that the composition of such bands may further be altered by ph. This is especially relevant to the composition of the 47 and 50 K bands.

Electron microscopy supports a fibrous substructure for lens intermediate filaments

The substructure of intermediate filaments from bovine lens cortical fiber cells was investigated by electron microscopy. Native filaments and synthetic ones regenerated from the total cytoskeletal extract and from the three purified subunits were examined. The morphologies from these various sources were essentially identical, with the exception that filaments reconstituted from one of the purified polypeptides were much shorter, very contorted and showed strings of aggregated protein. The solid cylindrical, unbranching filaments consisted of a helical arrangement of at least two, 5 nm diameter strands. The evidence indicated that each strand was composed of two, 2 nm diameter protofilaments which were also helically constructed (right-handed) with a periodicity of 11.6 nm. Intermediate filament diameter varied widely (8-14.8 nm, average 11.3 nm) and in a direct, linear manner relative to the apparent progression (helical) angle of the strands across the filaments face. These conclusions were obtained from observations on negatively stained intact filaments and reconstituted 4.4 nm fibrils and on positively stained transverse sections of fixed and embedded filaments.

Is actin in eye lens a possible factor in visual accomodation?

Actin has been purified from varius non-muscle cells and characterized by its molecular weight and ability to polymerize into filaments. Although the occurrence of this protein has been postulated in the mammalian eye lens after observation of actin-like filaments in the electron microscope, definite (bio)chemical proof has been provided only recently. Amino acid analysis, peptide mapping and affinity chromatography revealed the identity of lens actin with the corresponding protein in other tissues. As the filaments could be obtained by co-isolation with highly purified lens plasma membranes, we were interested to know how the actin-containing structures wre located in situ. In the experimental approach reported here, the indirect immunofluorescence technique (IFT) was applied to unfixed cryostat sections of lens tissue. The distribution of actin in calf, rat and pigeon lens is described, and evidence from this for the role of actin in visual accommodation discussed.

Age changes in the skeleton of the human lens

The cytoskeleton of the human lens (newborn, 24 year-old and 80 year-old) was studied by morphological methods. Actin and intermediate (10 nm) filaments were identified in the epithelial cells of all the lenses. In the newborn lens intermediate filaments and chains of protein were found in cortical and nuclear fiber cells. Many intermediate filaments and protein chains in the elongated form were observed in the superficial cortical fiber cells of the 24 year-old lens. However, in the deep cortical cells the number of intermediate filaments decreased and the protein chains became more coiled and aggregated. In the nuclear region the protein chains were markedly aggregated and intermediate filaments were absent. The 80 year-old fiber cells contained only protein aggregates in the fiber cells. While the size of the chain backbone corresponds to actin, the conclusive demonstration that it is actin remains to be shown.