Gap junction structures and distribution patterns of immunoreactive connexins 46 and 50 in lens regrowths of Rhesus monkeys

Role and Posttranslational Regulation of Cx46 Hemichannels and Gap Junction Channels in the Eye Lens

Connexins are a family of proteins that can form two distinct types of channels: hemichannels and gap junction channels. Hemichannels are composed of six connexin subunits and when open allow for exchanges between the cytoplasm and the extracellular milieu. Gap junction channels are formed by head-to-head docking of two hemichannels in series, each one from one of two adjacent cells. These channels allow for exchanges between the cytoplasms of contacting cells. The lens is a transparent structure located in the eye that focuses light on the retina. The transparency of the lens depends on its lack of blood irrigation and the absence of organelles in its cells. To survive such complex metabolic scenario, lens cells express Cx43, Cx46 and Cx50, three connexins isoforms that form hemichannels and gap junction channels that allow for metabolic cooperation between lens cells. This review focuses on the roles of Cx46 hemichannels and gap junction channels in the lens under physiological conditions and in the formation of cataracts, with emphasis on the modulation by posttranslational modifications.

Genome-wide DNA methylation profiles may reveal new possible epigenetic pathogenesis of sporadic congenital cataract

Aim: To investigate the possible epigenetic pathogenesis of sporadic congenital cataract. Materials & methods: We conducted whole genome bisulfite sequencing on peripheral blood from sporadic binocular or monocular congenital cataract patients and cataract-free participants. Results: We found massive differentially methylated regions within the whole genomes between any two groups. Meanwhile, we identified five genes (ACTN4, ACTG1, TUBA1A, TUBA1C, TUBB4B) for the binocular and control groups and TUBA1A for the monocular and control groups as the core differentially methylated region-related genes. The proteins encoded by these core genes are involved in building cytoskeleton and intercellular junctions. Conclusion: Changes in the methylation levels of core genes may disturb the function of cytoskeleton and intercellular junctions, eventually leading to sporadic congenital cataract.

Lens ion homeostasis relies on the assembly and/or stability of large connexin 46 gap junction plaques on the broad sides of differentiating fiber cells

The eye lens consists of layers of tightly packed fiber cells, forming a transparent and avascular organ that is important for focusing light onto the retina. A microcirculation system, facilitated by a network of gap junction channels composed of connexins 46 and 50 (Cx46 and Cx50), is hypothesized to maintain and nourish lens fiber cells. We measured lens impedance in mice lacking tropomodulin 1 (Tmod1, an actin pointed-end capping protein), CP49 (a lens-specific intermediate filament protein), or both Tmod1 and CP49. We were surprised to find that simultaneous loss of Tmod1 and CP49, which disrupts cytoskeletal networks in lens fiber cells, results in increased gap junction coupling resistance, hydrostatic pressure, and sodium concentration. Protein levels of Cx46 and Cx50 in Tmod1(-/-);CP49(-/-) double-knockout (DKO) lenses were unchanged, and electron microscopy revealed normal gap junctions. However, immunostaining and quantitative analysis of three-dimensional confocal images showed that Cx46 gap junction plaques are smaller and more dispersed in DKO differentiating fiber cells. The localization and sizes of Cx50 gap junction plaques in DKO fibers were unaffected, suggesting that Cx46 and Cx50 form homomeric channels. We also demonstrate that gap junction plaques rest in lacunae of the membrane-associated actin-spectrin network, suggesting that disruption of the actin-spectrin network in DKO fibers may interfere with gap junction plaque accretion into micrometer-sized domains or alter the stability of large plaques. This is the first work to reveal that normal gap junction plaque localization and size are associated with normal lens coupling conductance.

Connexin50 couples axon terminals of mouse horizontal cells by homotypic gap junctions

Horizontal cells in the mouse retina are of the axon-bearing B-type and contribute to the gain control of photoreceptors and to the center-surround organization of bipolar cells by providing feedback and feedforward signals to photoreceptors and bipolar cells, respectively. Horizontal cells form two independent networks, coupled by dendro-dendritic and axo-axonal gap junctions composed of connexin57 (Cx57). In Cx57-deficient mice, occasionally the residual tracer coupling of horizontal cell somata was observed. Also, negative feedback from horizontal cells to photoreceptors, potentially mediated by connexin hemichannels, appeared unaffected. These results point to the expression of a second connexin in mouse horizontal cells. We investigated the expression of Cx50, which was recently identified in axonless A-type horizontal cells of the rabbit retina. In the mouse retina, Cx50-immunoreactive puncta were predominantly localized on large axon terminals of horizontal cells. Electron microscopy did not reveal any Cx50-immunolabeling at the membrane of horizontal cell tips invaginating photoreceptor terminals, ruling out the involvement of Cx50 in negative feedback. Moreover, Cx50 colocalized only rarely with Cx57 on horizontal cell processes, indicating that both connexins form homotypic rather than heterotypic or heteromeric gap junctions. To check whether the expression of Cx50 is changed when Cx57 is lacking, we compared the Cx50 expression in wildtype and Cx57-deficient mice. However, Cx50 expression was unaffected in Cx57-deficient mice. In summary, our results indicate that horizontal cell axon terminals form two independent sets of homotypic gap junctions, a feature which might be important for light adaptation in the retina.

Variety of horizontal cell gap junctions in the rabbit retina

In the rabbit retina, there are two types of horizontal cell (HC). The axonless A-type HCs form a coupled network via connexin 50 (Cx50) gap junctions in the outer plexiform layer (OPL). The axon-bearing B-type HCs form two independently coupled networks; the dendritic network via gap junctions consisted of unknown Cx and the axon terminal network via Cx57. The present study was conducted to examine the localization and morphological features of Cx50 and Cx57 gap junctions in rabbit HCs at cellular and subcellular levels. The results showed that each gap junction composed of Cx50 or Cx57 showed distinct features. The larger Cx50 gap junctions were located more proximally than the smaller Cx50 gap junctions. Both Cx50 plaques formed symmetrical homotypic gap junctions, but some small ones had an asymmetrical appearance, suggesting the presence of heterotypic gap junctions or hemichannels. In contrast, Cx57 gap junctions were found in the more distal part of the OPL but never on the axon terminal endings entering the rod spherules, and they were exclusively homotypic. Interestingly, about half of the Cx57 gap junctions appeared to be invaginated. These distinct features of Cx50 and Cx57 gap junctions show the variety of HC gap junctions and may provide insights into the function of different types of HCs.

Biphasic effect of linoleic acid on connexin 46 hemichannels

Connexins form hemichannels at undocked plasma membranes and gap-junction channels (GJCs) at intercellular contacting zones. Under physiological conditions, hemichannels have low open probabilities, but their activation under pathological conditions, such as ischemia, induces and/or accelerates cell death. Connexin 46 (Cx46) is a major connexin of the lens, and mutations of this connexin induce cataracts. Here, we report the effects of linoleic acid (LA) on the electrical properties of Cx46 GJCs and hemichannels expressed in Xenopus laevis oocytes. LA has a biphasic effect, increasing hemichannel current at 0.1 μM and decreasing it at concentrations of 100 μM or higher. The effects of extracellular and microinjected LA conjugated to coenzyme A (LA-CoA) suggest that the current activation site is accessible from the intracellular but not extracellular compartment, whereas the current inhibitory site is either located in a region of the hemichannel pore inaccessible to intracellular LA-CoA, or requires crossing of LA through an organelle membrane. Experiments with other fatty acids demonstrated that the block of hemichannels depends on the presence of a hydrogenated double bond at position 9 and is directly proportional to the number of double bonds. Experiments in paired oocytes expressing Cx46 showed that LA does not affect GJCs. The block by unsaturated fatty acids reported here opens the possibility that increases in the concentration of these lipids in the lens induce cataract formation by blocking Cx46 hemichannels.

Connexins, cell motility, and the cytoskeleton

Connexins (Cx) comprise a family of transmembrane proteins, which form intercellular channels between plasma membranes of two adjoining cells, commonly known as gap junctions. Recent reports revealed that Cx proteins interact with diverse cellular components to form a multiprotein complex, which has been termed "Nexus". Potential interaction partners include proteins such as cytoskeletal proteins, scaffolding proteins, protein kinases and phosphatases. These interactions allow correct subcellular localization of Cxs and functional regulation of gap junction-mediated intercellular communication. Evidence is accruing that Cxs might have channel-independent functions, which potentially include regulation of cell migration, cell polarization and growth control. In the current review, we summarize recent knowledge on Cx interactions with cytoskeletal proteins and highlight some aspects of their role in cellular motility.

Sorting of lens aquaporins and connexins into raft and nonraft bilayers: role of protein homo-oligomerization

Two classes of channel-forming proteins in the eye lens, the water channel aquaporin-0 (AQP-0) and the connexins Cx46 and Cx50, are preferentially located in different regions of lens plasma membranes (1,2). Because these membranes contain high concentrations of cholesterol and sphingomyelin, as well as phospholipids such as phosphatidylcholine with unsaturated hydrocarbon chains, microdomains (rafts) form in these membranes. Here we test the hypothesis that sorting into lipid microdomains can play a role in the disposition of AQP-0 and the connexins in the plane of the membrane. For both crude membrane fractions and proteoliposomes composed of lens proteins in phosphatidylcholine/sphingomyelin/cholesterol lipid bilayers, detergent extraction experiments showed that the connexins were located primarily in detergent soluble membrane (DSM) fractions, whereas AQP-0 was found in both detergent resistant membrane and DSM fractions. Analysis of purified AQP-0 reconstituted in raft-containing bilayers showed that the microdomain location of AQP-0 depended on protein/lipid ratio. AQP-0 was located almost exclusively in DSMs at a 1:1200 AQP-0/lipid ratio, whereas approximately 50% of the protein was sequestered into detergent resistant membranes at a 1:100 ratio, where freeze-fracture experiments show that AQP-0 oligomerizes (3). Consistent with these detergent extraction results, confocal microscopy images showed that AQP-0 was sequestered into raft microdomains in the 1:100 protein/lipid membranes. Taken together these results indicate that AQP-0 and connexins can be segregated in the membrane by protein-lipid interactions as modified by AQP-0 homo-oligomerization.

Establishment of human corneal epithelial cells stably expressing human connexin43

Corneal epithelial cells communicate with each other through gap junctions. Whereas this property is retained in corneal epithelial cells in primary culture, it is often lost in immortalized epithelial cells. However, the life span of primary cultured corneal epithelial cells is short and the availability of human tissue for their preparation is limited. To examine the role of the gap-junction protein connexin43 (Cx43) in human corneal epithelial cells, we set out to establish an immortal human corneal epithelial cell line that stably expresses this protein. An expression vector encoding human Cx43 fused to enhanced green fluorescent protein (EGFP) was constructed and introduced by transfection into SV40-immortalized human corneal epithelial (HCE) cells. Stable transfectants were isolated by selection with the antibiotic G418. The expression and localization of the Cx43-EGFP fusion protein were examined by immunoblot analysis and fluorescence microscopy, respectively, and gap-junctional intercellular communication was monitored on the basis of dye coupling. HCE cells stably expressing Cx43-EGFP manifested intercellular dye transfer, whereas those stably expressing EGFP alone did not. Cx43-EGFP localized to the interfaces of neighboring cells. Stable expression of Cx43-EGFP in HCE cells did not affect the expression of keratins 3 and 12, which is a characteristic of corneal epithelial cells, but it did inhibit cell proliferation. We have established an HCE cell line that stably expresses human Cx43 and forms functional gap junctions. These cells may prove useful for studies of the role of gap junctions in the human corneal epithelium.

Distribution of Connexin50 Channels and Hemichannels in Lens Fibers: A Structural Approach

A detailed understanding of the mechanisms regulating cell-to-cell communication in the lens necessitates information about the distribution and density of Cx46 and Cx50 in their native cellular environment. These isoforms constitute the extensive pathway between the lens surface and the interior, helping to maintain its striking optical properties. To identify Cx50 channels and hemichannels in the plasma membrane and to differentiate between them, immuno-freeze-fracture-labeling (FRIL) with immuno-gold particles in used. In equatorial lens fibers, the Cx50-gold complexes label gap junctions at high densities and non-junctional plasma membranes at lower densities. Small depressions in the non-junctional plasma membrane labeled by the gold-complexes most likely represent points of hemichannel insertion. Measurement of the width of the extra-cellular space separating adjacent plasma membranes indicates that the gold complexes in the gap junctions represent Cx50 channels and those in the non-junctional plasma membrane, Cx50 hemichannels. Estimates of their densities indicate that the channels are at least one order of magnitude more numerous than the hemichannels. Therefore, in lens fibers, Cx50 hemichannels are inserted via exocytosis and are rapidly assembled into channels assembled in gap junction plaques.

Protein Kinase C-γ Activation in the Early Streptozotocin Diabetic Rat Lens

PURPOSE

The purpose of this study is to demonstrate the early activation of the protein kinase C-gamma (PKC-gamma) pathway in the streptozotocin (STZ)-induced diabetic rat lens.

METHODS

Twelve-week-old male and female Sprague-Dawley rats were injected with 80 mg/kg (body weight) of STZ (N-[methylnitrosocarbamoyl]-D-glucosamine) intraperitoneally. Very high glucose (VHG) diabetes was defined as a nonfasting blood glucose level of at least 450 mg/dl, confirmed by daily monitoring with Accu-Check Advantage test strips, and occurred about 2 weeks after STZ administration. All assayed lenses were from VHG or age-matched control rats, harvested within 24 hr of VHG detection. PKC-gamma activation was measured by enzyme activity assay and by Western blotting to show autophosphorylation on Thr514. Cellular insulin-like growth factor-1 (IGF-1), PKC-gamma phosphorylation of Cx43 on Ser368, and activation of phospholipase C-gamma 1 (PLC-gamma 1), extracellular signal-regulated kinase (ERK1/2), and caspase-3 were determined by Western blotting. Endogenous diacylglycerol (DAG) levels were measured with a DAG assay kit. Lens gap junction activity was determined by the microinjection/Lucifer yellow dye transfer assay. Electron microscopy was applied to affirm fiber cell damage in the VHG diabetic lenses.

RESULTS

In the lenses of VHG diabetic rats, PKC-gamma enzyme was activated. PKC-gamma could be further activated by 400 nM phorbol-12-myristate-13-acetate (PMA), but the PKC-gamma protein levels remained constant. No elevation of IGF-1 level was observed. Western blots showed that activation of PKC-gamma may be due to activation of PLC-gamma 1, which synthesized endogenous DAG, a native PKC activator. The level of PKC-gamma -catalyzed phosphorylation of Cx43 on Ser368 and resulting inhibition of lens gap junction dye transfer activity was increased in the VHG diabetic lenses. At this early time period, the diabetic lens showed no activation of either caspase-3 or ERK1/2. Only a single fiber cell layer deep within the cortex (approximately 90 cell layers from capsule surface) showed vacuoles and damaged cell connections.

CONCLUSIONS

Early activation of PLC-gamma 1 and elevated DAG were observed within VHG diabetic lenses. These were correlated with activation of PKC-gamma, phosphorylation of Cx43 on Ser368, and inhibition of dye transfer. Abnormal signaling from PKC-gamma to Cx43 in the epithelial cells/early fiber cells, observed within VHG diabetic lenses, may be responsible for fiber cell damage deeper in the lens cortex.

Diverse gap junctions modulate distinct mechanisms for fiber cell formation during lens development and cataractogenesis

Different mutations of alpha3 connexin (Cx46 or Gja8) and alpha8 connexin (Cx50 or Gja8), subunits of lens gap junction channels, cause a variety of cataracts via unknown mechanisms. We identified a dominant cataractous mouse line (L1), caused by a missense alpha8 connexin mutation that resulted in the expression of alpha8-S50P mutant proteins. Histology studies showed that primary lens fiber cells failed to fully elongate in heterozygous alpha8(S50P/+) embryonic lenses, but not in homozygous alpha8(S50P/S50P), alpha8-/- and alpha3-/- alpha8-/- mutant embryonic lenses. We hypothesized that alpha8-S50P mutant subunits interacted with wild-type alpha3 or alpha8, or with both subunits to affect fiber cell formation. We found that the combination of mutant alpha8-S50P and wild-type alpha8 subunits specifically inhibited the elongation of primary fiber cells, while the combination of alpha8-S50P and wild-type alpha3 subunits disrupted the formation of secondary fiber cells. Thus, this work provides the first in vivo evidence that distinct mechanisms, modulated by diverse gap junctions, control the formation of primary and secondary fiber cells during lens development. This explains why and how different connexin mutations lead to a variety of cataracts. The principle of this explanation can also be applied to mutations of other connexin isoforms that cause different diseases in other organs.

Paralemnin of the lens

Paralemmin was identified in the chicken lens as a protein with mol. wt 65 kDa and a splice variant of 60 kDa, both soluble in Triton X-100. Paralemmin is localized to the plasma membrane of fiber cells, and was not detected in the annular pad cells. Thus in the chick lens it is another feature of fiber cell differentiation. Its localization to the short side of the fiber cell and the sites of fiber cell interlocking suggests that paralemmin may play a role in the development of such interdigitating processes.

Disruption ofGja8(α8 connexin) in mice leads to microphthalmia associated with retardation of lens growth and lens fiber maturation

The development of the vertebrate lens utilizes a sophisticated cell-cell communication network via gap junction channels, which are made up of at least three connexin isoforms, α8 (Cx50), α3 (Cx46) and α1 (Cx43), and which are encoded by three different genes. In a previous study, we reported that, with a disruption of Gja3 (α3 connexin), mice developed nuclear cataracts with a normal sized lens. We show that Gja8tm1 (α8–/–) mice develop microphthalmia with small lenses and nuclear cataracts, while the α8 heterozygous (+/–) mice have relatively normal eyes and lenses. A comparative study of these α3 and α8 knockout mice showed that the protein levels of both α3 and α8 were independently regulated and there was no compensation for either the α3 or α8 protein from the wild-type allele when the other allele was disrupted. More interestingly, western blotting data indicated that the presence of α8 in the lens nucleus is dependent on α3 connexin, but not vice versa. The staining of the knock-in lacZ reporter gene showed the promoter activity of α8 connexin is much higher than that of α3 connexin in embryonic lenses and in adult lens epithelium. More importantly, a delayed denucleation process was observed in the interior fibers of the α8–/– lenses. Therefore, α8 connexin is required for proper fiber cell maturation and control of lens size.

Epithelial organization of the mammalian lens

To understand the structural organization responsible for lens function, we have studied the three-dimensional arrangement of cells in the lens, and the location and molecular composition of specialized junctions controlling the paracellular and transcellular pathways. The lens is formed by a single layer of polarized cells that elongate along their apical-basal axis from the anterior to the posterior pole to form the cortex, and fold inward at the posterior pole to form the nucleus. The basal surfaces of all cells of the cortex (approximately two thirds of all lens cells) are bathed by the aqueous and vitreous humors. Therefore, their metabolism is not limited by diffusion of nutrients into the avascular lens. The apical surfaces of all cortical fibers are directed toward the interior of the lens, where they form two distinct structures here referred to as the 'apical interface' and the 'modiolus'. The apical interface is located at a point close to the anterior pole, and is formed by the association of the apical surface of anterior cortical cells and the apical surface of cortical fibers extending from the posterior pole. The modiolus is located close to the equator at the lateral edge of the apical interface, and is formed by the tapered apical ends of equatorial cortical fibers. The plasma membrane of cortical cells at the anterior pole are connected through 'leaky' tight junctions and small gap junctions. Extensive gap junction plaques composed of connexin43 connect equatorial fibers at the modiolus and posterior cortical fibers at the apical interface. Single cell-to-cell channels composed of connexin46 and connexin50 connect the lateral surfaces of equatorial and posterior cortical fibers. The lateral surfaces of these fibers also contain extensive junctions composed of aquaporin-0. The nucleus is connected to the humors through the paracellular pathway represented by the anterior (apical) and posterior (basal) suture lines. Therefore, the metabolic needs of nuclear fibers cannot be fulfilled by simple diffusion and requires the cell-to-cell pathway formed by specialized junctions. The lateral surfaces of nuclear fibers contain extensive wavy junctions composed of aquaporin-0, probably for the control of the permeability of the paracellular pathway. We propose a simple epithelium model for the lens in which nutrients move into the nucleus through the paracellular pathway represented principally by the suture lines, and the transcellular pathway represented by an extensive network of gap junction plaques composed of connexin43 at the apical surface, and single or small plaques of cell-to-cell channels composed of connexin46 and connexin50 in the lateral surfaces.

Management of monocular congenital cataracts

Monocular congenital cataracts have been treated for the past two decades with early surgery, contact lens correction and patching of the unaffected eye. While an occasional patient has had a good visual outcome with this treatment regiment, the majority end up being legally blind in the aphakic eye. Intraocular lenses (IOLs) are increasingly being used as an alternative means of optically correcting aphakia during infancy. A growing body of literature suggests that an IOL correction is associated with an improved visual outcome and a relatively low incidence of post-operative complications. A monkey model has also been used to study the safety and feasibility of correcting aphakia in neonates with IOLs. These studies have revealed that the visual outcome is as good if not better with an IOL correction. A multicentre clinical trial, the Infant Aphakia Treatment Study (IATS), is being organised in the United States to critically compare an IOL and contact lens correction for infantile aphakia.

Gap junctional coupling in lenses lacking alpha3 connexin

Fiber cells of the lens are interconnected by an extensive network of gap junctions containing alpha3 (Cx46) and alpha8 (Cx50) connexins. A specific role for these connexins in lens homeostasis is not known. To determine the contribution of these connexins to lens function, we used impedance techniques to study cell-to-cell coupling in lenses from homozygous alpha3 knockout (-/-), heterozygous (+/-), and wild-type (+/+) mice. Western blots and immunofluorescence data indicated that alpha8 remained at similar levels in the three classes of lenses, whereas alpha3 was approximately 50% of the normal level in the +/- lenses, and it was absent from the -/- lenses. Moreover, the data from +/+ lenses suggest that a cleavage of connexins occurs abruptly between the peripheral shell of differentiating fibers (DF) and the inner core of mature fibers (MF). The appearance of the cleaved connexins was correlated to a change in the coupling conductance. In -/- lenses the coupling conductance of MF was zero, and these fibers were depolarized by about 30 mV from normal (approximately -65 mV). The DF remained coupled, but the conductance was reduced to 30-35% of normal. However, the gap junctions in the DF of alpha3 -/- lenses remained sensitive to pH. We conclude that alpha3 connexin is necessary for the coupling of central fibers to peripheral cells, and that this coupling is essential for fiber cell homeostasis because uncoupled MF depolarize and subsequently become opaque.

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.

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.