Identification of a 70,000-D protein in lens membrane junctional domains

Macrophage recruitment in immune-privileged lens during capsule repair, necrotic fiber removal, and fibrosis

Emerging evidence challenges the lens as an immune-privileged organ. Here, we provide a direct mechanism supporting a role of macrophages in lens capsule rupture repair. Posterior lens capsule rupture in a connexin 50 and aquaporin 0 double-knockout mouse model resulted in lens tissue extrusion into the vitreous cavity with formation of a “tail-like” tissue containing delayed regressed hyaloid vessels, fibrotic tissue and macrophages at postnatal (P) 15 days. The macrophages declined after P 30 days with M2 macrophages detected inside the lens. By P 90 days, the “tail-like” tissue completely disappeared and the posterior capsule rupture was sealed with thick fibrotic tissue. Colony-stimulating factor 1 (CSF-1) accelerated capsule repair, whereas inhibition of the CSF-1 receptor delayed the repair. Together, these results suggest that lens posterior rupture leads to the recruitment of macrophages delivered by the regression delayed hyaloid vessels. CSF-1-activated M2 macrophages mediate capsule rupture repair and development of fibrosis.

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Lens posterior rupture delays regression of the hyaloid vessels.

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Lens posterior rupture recruits macrophages delivered by the hyaloid vessels.

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Macrophages mediate necrotic fiber cell removal and capsule rupture sealing.

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CSF-1 activated M2 macrophages facilitate capsular rupture sealing by fibrosis.

Connexin 50 and AQP0 are Essential in Maintaining Organization and Integrity of Lens Fibers

Purpose

Connexins and aquaporins play essential roles in maintaining lens homeostasis and transparency and there is a close physical and functional relationship between these two proteins. Aquaporin 0 (AQP0), in addition to its role in water transport in the lens, acts as a cell-cell adhesion molecule. Recently, we showed a new role of connexin (Cx) 50 in mediating cell-cell adhesion. However, the cooperative roles of these two proteins in the lens in vivo have not been reported.

Methods

We generated an AQP0/Cx50 double knockout (dKO) mouse model. Light, fluorescence, transmission thin section, and freeze-fracture electron microscopy, as well as wheat germ agglutinin and phalloidin labeling were used to evaluate lens structure. Mechanical properties of lenses were determined by mechanical compression testing.

Results

DKO mice exhibited small eyes and lenses with severe cataracts, along with lens posterior defects, including posterior capsule rupture. The dKO mouse lenses had severe structural disruption associated with increased spaces between lens fiber cells when compared with wild-type lenses or lenses deficient in either Cx50 or AQP0. DKO mice also exhibited greater reduction in lens size compared with Cx50 KO mice. Gap-junction plaque size was greatly decreased in cortical fiber cells in dKO mice. Moreover, lens stiffness and elasticity were completely diminished, exhibiting a gelatinous texture in adult dKO mice.

Conclusions

This novel mouse model reveals that Cx50 and AQP0 play an important role in mediating cell-cell adhesion function in the lens fiber cells and their deficiency impairs lens fiber organization, integrity, mechanical properties, and lens development.

Crucial motifs and residues in the extracellular loops influence the formation and specificity of connexin docking

Most of the early studies on gap junction (GJ) channel function and docking compatibility were on rodent connexins, while recent research on GJ channels gradually shifted from rodent to human connexins largely due to the fact that mutations in many human connexin genes are found to associate with inherited human diseases. The studies on human connexins have revealed some key differences from those found in rodents, calling for a comprehensive characterization of human GJ channels. Functional studies revealed that docking and formation of functional GJ channels between two hemichannels are possible only between docking-compatible connexins. Two groups of docking-compatible rodent connexins have been identified. Compatibility is believed to be due to their amino acid residue differences at the extracellular loop domains (E1 and E2). Sequence alignment of the E1 and E2 domains of all connexins known to make GJs revealed that they are highly conserved and show high sequence identity with human Cx26, which is the only connexin with near atomic resolution GJ structure. We hypothesize that different connexins have a similar structure as that of Cx26 at the E1 and E2 domains and use the corresponding residues in their E1 and E2 domains for docking. Based on the Cx26 GJ structure and sequence analysis of well-studied connexins, we propose that the E1-E1 docking interactions are staggered with each E1 interacting with two E1s on the docked connexon. The putative E1 docking residues are conserved in both docking-compatible and -incompatible connexins, indicating that E1 does not likely serve a role in docking compatibility. However, in the case of E2-E2 docking interactions, the putative docking residues are only conserved within the docking-compatible connexins, suggesting the E2 is likely to serve the function of docking compatibility. Docking compatibility studies on human connexins have attracted a lot of attention due to the fact that putative docking residues are mutational hotspots for several connexin-linked human diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

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.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

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.

Gap junctions or hemichannel-dependent and independent roles of connexins in cataractogenesis and lens development

In the last decade or so, increasing evidences suggest that the mutations of two connexin genes, GJA3 and GJA8, are directly linked to human congenital cataracts in North and Central America, Europe and Asia. GIA3 and GIA8 genes encode gap junction-forming proteins, connexin (Cx) 46 and Cx50, respectively. These two connexins are predominantly expressed in lens fiber cells. Majority of identified mutations are missense, and the mutated sites are scattered across various domains of connexin molecules. Genetic deletion of either of these two genes leads to the development of cataracts; however, the types of cataracts developed are distinctive. More interestingly, microphthalmia is only developed in Cx50, but not Cx46 deficient mice, suggesting the unique role of Cx50 in lens cell growth and development. Knockin studies with the replacement of Cx46 or Cx50 at their respective gene locus further demonstrate the unique properties of these two connexins. Furthermore, the function of Cx50 in epithelial-fiber differentiation appears to be independent of its conventional role in forming gap junction junction channels. Due to their specific functions in maintaining lens clarity and development, and their malfunctions resulting in lens cataractogenesis and developmental impairment, connexin molecules could be developed as potential drug targets for therapeutic intervention for treatment of cataracts and other eye disorders. Recent advances in basic research of lens connexins and the discoveries of clinical disorders as a result of lens connexin dysfunctions are summarized and discussed here.

Gap junctions are selectively associated with interlocking ball-and-sockets but not protrusions in the lens

PURPOSE

Ball-and-sockets and protrusions are specialized interlocking membrane domains between lens fibers of all species studied. Ball-and-sockets and protrusions are similar in their shape, size, and surface morphology, and are traditionally believed to play a key role in maintaining fiber-to-fiber stability. Here, we evaluate the hypothesis that ball-and-sockets and protrusions possess important structural and functional differences during fiber cell differentiation and maturation.

METHODS

Intact lenses of leghorn chickens (E7 days to P62 weeks old) and rhesus monkeys (1.5-20 years old) were studied with SEM, freeze-fracture TEM, freeze-fracture immunogold labeling (FRIL), and filipin cytochemistry for membrane cholesterol detection.

RESULTS

SEM showed that ball-and-sockets were distributed along the long and short sides of hexagonal fiber cells, whereas protrusions were located along the cell corners, from superficial to deep cortical regions in both chicken and monkey lenses. Importantly, by freeze-fracture TEM, we discovered the selective association of gap junctions with all ball-and-sockets examined, but not with protrusions, in both species. In the embryonic chicken lens (E18), the abundant distribution of ball-and-socket gap junctions was regularly found in an approximate zone extending at least 300 μm deep from the equatorial surface of the superficial cortical fibers. Many ball-and-socket gap junctions often protruded deeply into neighboring cells. However, in the mature fibers of monkey lenses, several ball-and-sockets exhibited only partial occupancy of gap junctions with disorganized connexons, possibly due to degradation of gap junctions during fiber maturation and aging. FRIL analysis confirmed that both connexin46 (Cx46) and connexin50 (Cx50) antibodies specifically labeled ball-and-socket gap junctions, but not protrusions. Furthermore, filipin cytochemistry revealed that the ball-and-socket gap junctions contained different amounts of cholesterol (i.e., cholesterol-rich versus cholesterol-free) as seen with the filipin-cholesterol-complexes (FCC) in different cortical regions during maturation. In contrast, the protrusions contained consistently high cholesterol amounts (i.e., 402 FCCs/μm2 membrane) which were approximately two times greater than that of the cholesterol-rich gap junctions (i.e., 188 FCCs/μm2 membrane) found in ball-and-sockets.

CONCLUSIONS

Gap junctions are regularly associated with all ball-and-sockets examined in metabolically active young cortical fibers, but not with protrusions, in both chicken and monkey lenses. Since these unique gap junctions often protrude deeply into neighboring cells to increase membrane surface areas, they may significantly facilitate cell-to-cell communication between young cortical fiber cells. In particular, the large number of ball-and-socket gap junctions found near the equatorial region may effectively facilitate the flow of outward current toward the equatorial surface for internal circulation of ions in the lens. In contrast, a consistent distribution of high concentrations of cholesterol in protrusions would make the protrusion membrane less deformable and would be more suitable for maintaining fiber-to-fiber stability during visual accommodation. Thus, the ball-and-sockets and protrusions are two structurally and functionally distinct membrane domains in the lens.

Gap junction remodeling associated with cholesterol redistribution during fiber cell maturation in the adult chicken lens

PURPOSE

To investigate the structural remodeling in gap junctions associated with cholesterol redistribution during fiber cell maturation in the adult chicken lens. We also evaluated the hypothesis that the cleavage of the COOH-terminus of chick Cx50 (formerly Cx45.6) during fiber cell maturation would affect the gap junction remodeling.

METHODS

Freeze-fracture TEM and filipin cytochemistry were applied to visualize structural remodeling of gap junction connexons associated with cholesterol redistribution during fiber cell maturation in adult leghorn chickens (42-62 weeks old). Freeze-fracture immunogold labeling (FRIL) was used to label the specific Cx50 COOH-terminus antibody in various structural configurations of gap junctions.

RESULTS

Cortical fiber cells of the adult lenses contained three subtypes of cholesterol-containing gap junctions (i.e., cholesterol-rich, cholesterol-intermediate, and cholesterol-poor or -free) in both outer and inner cortical zones. Quantitative studies showed that approximately 81% of gap junctions in the outer cortex were cholesterol-rich gap junctions whereas approximately 78% of gap junctions in the inner cortex were cholesterol-free ones. Interestingly, all cholesterol-rich gap junctions in the outer cortex displayed loosely-packed connexons whereas cholesterol-free gap junctions in the deep zone exhibited tightly, hexagonal crystalline-arranged connexons. Also, while the percentage of membrane area specialized as gap junctions in the outer cortex was measured approximately 5 times higher than that of the inner cortex, the connexon density of the crystalline-packed gap junctions in the inner cortex was about 2 times higher than that of the loosely-packed ones in the outer cortex. Furthermore, FRIL demonstrated that while the Cx50 COOH-terminus antibody was labeled in all loosely-packed gap junctions examined in the outer cortex, little to no immunogold labeling was seen in the crystalline-packed connexons in the inner cortex.

CONCLUSIONS

Gap junctions undergo significant structural remodeling during fiber cell maturation in the adult chicken lens. The cholesterol-rich gap junctions with loosely-packed connexons in the young outer cortical fibers are transformed into cholesterol-free ones with crystalline-packed connexons in the mature inner fibers. In addition, the loss of the COOH-terminus of Cx50 seems to contribute equally to the transformation of the loosely-packed connexons to the crystalline-packed connexons during fiber cell maturation. This transformation causes a significant increase in the connexon density in crystalline gap junctions. As a result, it compensates considerably for the large decrease in the percentage of membrane area specialized as gap junctions in the mature inner fibers in the adult chicken lens.

Modulation of Cx46 hemichannels by nitric oxide

Gap-junction hemichannels are composed of six protein subunits (connexins). Undocked hemichannels contribute to physiological autocrine/paracrine cell signaling, including release of signaling molecules, cell-volume regulation, and glucose uptake. In addition, hemichannels may be pathologically activated by dephosphorylation and cell-membrane depolarization. Such hemichannel opening may induce and/or accelerate cell death. It has been suggested that connexin43 (Cx43) hemichannels are sensitive to redox potential changes and that one or more intracellular cysteines is/are important for this process. Cx46 is expressed in the lens, and its dysfunction induces cataract formation. It contains six cysteines in the extracellular loops, one in the fourth transmembrane helix, and two in the COOH-terminal domain. The latter may be susceptible to oxidation by nitric oxide (NO), which could be involved in cataract formation through cysteine S-nitrosylation. Here we report studies of the effects of the NO donor S-nitrosoglutathione (GSNO) on the electrical properties and fluorescent-dye permeability of wild-type Cx46 and mutant hemichannels expressed in Xenopus laevis oocytes. GSNO enhanced hemichannel voltage sensitivity, increased tail-current amplitude, and changed activation and closing kinetics in Cx46 and Cx46-CT43 (Cx46 mutant in which the COOH terminus was replaced with that of Cx43), but not in Cx46-C3A (Cx46 in which the intracellular and transmembrane helix 4 cysteines were mutated to alanine). We conclude that Cx46 hemichannels are sensitive to NO and that the NO effects are mediated by modification of one or more intracellular cysteines. However, it is unlikely that NO induces cataract formation due to the hemichannel activation, because at normal resting potential, NO had no major effects on Cx46 hemichannel permeability.

Mutation of the gap junction protein alpha 8 (GJA8) gene causes autosomal recessive cataract

GJA8 encodes connexin-50, a gap junction protein in the eye lens. Mutations in GJA8 have been reported in families with autosomal dominant cataract. The objective of this report was to identify the disease gene in a family with congenital cataract of autosomal recessive inheritance. Eight candidate genes were screened for pathogenic alterations in affected and unaffected family members and in normal unrelated controls. A single base insertion leading to frameshift at codon 203 of connexin 50 was found to co-segregate with disease in the family. These results confirm involvement of GJA8 in autosomal recessive cataract.

The cytoplasmic accumulations of the cataract-associated mutant, Connexin50P88S, are long-lived and form in the endoplasmic reticulum

Mutant connexins have been linked to hereditary congenital cataracts. One such mutant causes a proline-to-serine substitution at position 88 in human connexin 50 (CX50P88S). In transfected cells, CX50P88S does not form gap junctions, but localizes in cytoplasmic multilamellar structures. We studied the dynamics of formation and the stability of these structures in HeLa cells stably transfected with CX50P88S containing a tetracysteine motif appended to its C-terminus (HeLa-CX50P88S(Cys)(4) cells). The tetracysteine motif binds the membrane-permeable biarsenical compounds, FlAsH and ReAsH, which become fluorescent upon binding allowing detection of CX50P88S(Cys)(4) by fluorescence microscopy or by transmission electron microscopy after the ReAsH-driven fluorescent photoconversion of diaminobenzidine. CX50P88S structures were long-lived. Pulse labeling of HeLa-CX50P88S(Cys)(4) cells with FlAsH followed by a chase and ReAsH labeling showed a differential distribution of the labels, with older CX50P88S surrounded by newly synthesized protein. Formation of CX50P88S accumulations was not affected by treatments that block ER-to-Golgi transport. Transmission electron microscopy and tomographic reconstruction revealed that CX50P88S accumulations corresponded to closely apposed circular or semicircular membrane stacks that were sometimes continuous with the rough endoplasmic reticulum. These results suggest that CX50P88S accumulations originate from the rough endoplasmic reticulum and that mutant protein is sequentially added resulting in long-lived cytoplasmic particles. The persistence of these particles in the lens may cause light scattering and the pulverulent cataracts observed in affected individuals.

Mutation of the gap junction protein alpha 8 (GJA8) gene causes autosomal recessive cataract

BACKGROUND

GJA8 encodes connexin-50, a gap junction protein in the eye lens. Mutations in GJA8 have been reported in families with autosomal dominant cataract.

OBJECTIVE

To identify the disease gene in a family with congenital cataract of autosomal recessive inheritance.

METHODS

Eight candidate genes were screened for pathogenic alterations in affected and unaffected family members and in normal unrelated controls.

RESULTS

A single base insertion leading to frameshift at codon 203 of connexin 50 was found to co-segregate with disease in the family.

CONCLUSIONS

These results confirm involvement of GJA8 in autosomal recessive cataract.

Sequence diversity of gap junction proteins

This paper summarizes our understanding of the molecular organization of gap junction proteins. There appear to be overall similarities in the organization of heart and liver junctions in terms of general domains, even though the molecular sizes of the two proteins are quite different. Sequence data on the amino-terminal regions of these two proteins show 43% of the residues to be identical and 25% more to be homologous. The major intrinsic protein of lens (MIP), believed by many to be the lens-fibre junction protein, does not show such sequence homology with the known portions of junction proteins from either heart or liver. Yet the sequence of MIP, which is completely known, suggests a conformation for this molecule quite compatible with a junctional role. It thus appears that molecules potentially involved in junction formation will prove to form a rather diverse family, with special characteristics of organ-specific molecules that may well be related to their function.

The Lens Circulation

The lens is the largest organ in the body that lacks a vasculature. The reason is simple: blood vessels scatter and absorb light while the physiological role of the lens is to be transparent so it can assist the cornea in focusing light on the retina. We hypothesize this lack of blood supply has led the lens to evolve an internal circulation of ions that is coupled to fluid movement, thus creating an internal micro-circulatory system, which makes up for the lack of vasculature. This review covers the membrane transport systems that are believed to generate and direct this internal circulatory system.

Presbyopia: the first stage of nuclear cataract?

Presbyopia, the inability to accommodate, affects almost everyone at middle age. Recently, it has been shown that there is a massive increase in the stiffness(1) of the lens with age and, since the shape of the lens must change during accommodation, this could provide an explanation for presbyopia. In this review, we propose that presbyopia may be the earliest observable symptom of age-related nuclear (ARN) cataract. ARN cataract is a major cause of world blindness. The genesis of ARN cataract can be traced to the onset of a barrier within the lens at middle age. This barrier restricts the ability of small molecules, such as antioxidants, to penetrate into the centre of the lens leaving the proteins in this region susceptible to oxidation and post-translational modification. Major protein oxidation and colouration are the hallmarks of ARN cataract. We postulate that the onset of the barrier, and the hardening of the nucleus, are intimately linked. Specifically, we propose that progressive age-dependent hardening of the lens nucleus may be responsible for both presbyopia and ARN cataract.

Decreased gap junctional communication in neurobiotin microinjected lens epithelial cells after taxol treatment

The aim of the study was to examine gap-junction-mediated intercellular communication after experimentally induced aggregations of microtubules in cultured bovine lens epithelial cells. Intercellular communication between lens cells appears to be crucial for normal lens homeostasis. However, investigations on the maintenance of direct ion and metabolite exchange via gap junctions and its quantified dependency of cytoskeletal microtubules have not been available under conditions leading to bundling of microtubules. Thus, metabolic coupling of neighboring lens epithelial cells was quantified following microinjections of neurobiotin into single cells under various conditions. In controls, intensive gap-junction-mediated intercellular communication could be documented by dye-spreading of microinjected neurobiotin. In contrast, taxol treatment for 1-3 days impaired, but did not completely block gap-junction-mediated intercellular communication. After depletion of taxol, a complete recovery of intercellular communication was achieved. In addition, confocal laser scanning microscopy and rapid-freeze deep-etch electron microscopy revealed a displacement of actin-filaments from the perinuclear cytoplasm, accompanied by an abnormal aggregation of microtubules after taxol treatment, including impeded translocation of connexin 43 from the cytoplasm into the plasma membrane. Incubation of cells with nocodazole destroyed the microtubule network, accompanied by a clear reduction of plasma-membrane-integrated connexin 43 and significant impairment of dye spreading. Thus, in lens epithelial cells intercellular communication at gap junctions made by connexin 43 depends on the integrity of the microtubule network through the translocation of connexins to the plasma membrane.

Connexin disorders of the ear, skin, and lens

Gap junctions provide coupled cells with a direct pathway for sharing ions, nutrients, and small metabolites, thus helping to maintain homeostasis in various tissues. Abnormal function and/or expression of specific connexin genes has been linked to several diseases, including genetic deafness, skin disease, peripheral neuropathies, and cataracts. Research has provided significant insight into the function of gap junction proteins in both in vitro and in vivo models; however, questions regarding the exact mechanisms by which connexin related diseases occur in mammalian systems remain. Here, we discuss the disease states that are related to three human connexin genes, Cx26 (GJB2), Cx46 (GJA3) and Cx50 (GJA8), and recent scientific evidence characterizing those diseases in various experimental models.

Interaction of major intrinsic protein (aquaporin-0) with fiber connexins in lens development

We observed that chick lens-fiber gap-junction-forming proteins, connexin (Cx) 45.6 and Cx56, were associated with an unknown protein, which was then identified as major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), the most abundant membrane protein in lens fibers. A 1063 bp cDNA of chick MIP(AQP0) was identified that encodes a 262 amino acid protein with a predicted molecular weight of 28.1 kDa. Dual immunofluorescence and confocal microscopy of sagittal and coronal sections of the lens tissues showed that MIP(AQP0) consistently localized with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development. Immunoprecipitation combined with immunoblotting analyses revealed that MIP(AQP0) was associated with Cx45.6 and Cx56 at these developmental stages. The specificity of this interaction was further confirmed with the silver staining of the protein components of immunoprecipitates. The pull-down analysis of lens lysates revealed that C-terminus of MIP(AQP0) probably interacted with these two fiber connexins. In late embryonic and adult lenses, however, uniform co-distribution of MIP(AQP0) and fiber connexins was largely disrupted, except for the area surrounding the actively differentiating bow regions, as was revealed by immunofluorescence and immunoprecipitation experiments. The interaction of MIP(AQP0) with lens fiber connexins in differentiating lens cells but not in mature lens fibers suggests a potential role for MIP(AQP0) in the facilitation of fiber connexins for the formation of gap junctions during lens development.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Connexin expression patterns in the rat cornea: molecular evidence for communication compartments

PURPOSE

To identify and localize candidate connexin family members in adult rat cornea that may be important in coordinating corneal cell biology.

METHODS

To identify candidate connexin family members in adult rat cornea, a RT-PCR-based screening approach was initially adopted. Fourteen pairs of connexin isoform-specific primers were used to amplify connexin transcripts from two populations of RNA isolated from either the central cornea or the whole cornea. Immunohistochemistry and confocal microscopy were then used to confirm the presence and localization of connexins.

RESULTS

Eight connexin transcripts (Cxs 26, 30.3, 31, 31.1, 33, 37, 43, 50) are present in central cornea, and the peripheral cornea additionally expresses Cxs 30, 40, 45, and 46. No Cx32 or Cx36 transcripts were amplified. Immunohistochemistry revealed that Cxs 26, 30, 31.1, 37, and 43 are expressed in spatially distinct patterns within the cornea. Cx26 and Cx43 occur in basal cells of the whole corneal epithelium and between endothelial cells. Cx26 also immunolocalizes to the first layer of intermediate epithelial cells, and Cx43 antibody labels stromal keratocytes. Cx30 is expressed in the peripheral corneal epithelium and disappears toward the central cornea. Cx31.1 expression is restricted to superficial corneal epithelial cells, and Cx37 spans the intermediate corneal epithelium.

CONCLUSION

The spatially distinct cellular expression patterns of Cxs 26, 30, 31.1, 37, and 43 in the corneal epithelium imply that gap junctions play important roles in controlling corneal epithelial proliferation and differentiation and overall corneal maintenance.

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.

Surface tongue-and-groove contours on lens MIP facilitate cell-to-cell adherence

The lens major intrinsic protein (MIP, AQP0) is known to function as a water and solute channel. However, MIP has also been reported to occur in close membrane contacts between lens fiber cells, indicating that it has adhesive properties in addition to its channel function. Using atomic force and cryo-electron microscopy we document that crystalline sheets reconstituted from purified ovine lens MIP mostly consisted of two layers. MIP lattices in the apposing membranes were in precise register, and determination of the membrane sidedness demonstrated that MIP molecules bound to each other via their extracellular surfaces. The surface structure of the latter was resolved to 0.61 nm and revealed two protruding domains providing a tight "tongue-and-groove" fit between apposing MIP molecules. Cryo-electron crystallography produced a projection map at 0.69 nm resolution with a mirror symmetry axis at 45 degrees to the lattice which was consistent with the double-layered nature of the reconstituted sheets. These data strongly suggest an adhesive function of MIP, and strengthen the view that MIP serves dual roles in the lens.

Intercellular communication in the eye: clarifying the need for connexin diversity

In the vertebrate eye, virtually every cell type is directly coupled to its neighbors by intercellular channels present in gap junctions. Although these structures share the common property of allowing adjacent cells to directly exchange ions, second messengers and small metabolites, intercellular channels in the eye also play a specific role in distinct functions such as neuronal transmission at electrotonic synapses in the retina, and the maintenance of homeostasis in the avascular lens. The structural proteins comprising these channels, the connexins (Cx), are a multigene family of which many members are expressed in the eye, even in the same cell type. This molecular heterogeneity poses the crucial question of whether and how a diversity in gap junctional structural proteins influences intercellular communication in ocular tissues. This review will focus on two recent advances in the understanding of connexin diversity in regard to the eye. First, connexin knockouts have demonstrated that postnatal development and homeostasis in the lens requires multiple connexin proteins. Secondly, functional characterization of new connexins that are abundantly expressed in the retina has revealed biophysical properties that mimic those recorded from retinal neurons.

Gap junctions containing alpha8-connexin (MP70) in the adult mammalian lens epithelium suggests a re-evaluation of its role in the lens

A missense mutation in one of the three lens connexins, alpha8-connexin, has been recently shown to be the genetic basis of the zonular pulverant lens cataract. This connexin had been considered to be expressed only in lens fibre cells. The present studies show that alpha8-connexin is also expressed in the lens epithelial cell layer. For this study, the distribution of gap junctions in the adult bovine lens has been investigated by confocal immunofluorescence microscopy using antibodies against alpha8-connexin (MP70) and alpha1-connexin (Cx43). In addition to the anticipated localisation of alpha8-connexin to the broad faces of lens fibre cells as reported in other species, alpha8-connexin was also found colocalized with alpha1-connexin at plaques in the lateral epithelial-epithelial plasma membranes of the bovine lens. These data suggest that mixed alpha8-connexin/alpha1-connexin plaques are between epithelial cells at their apico-lateral plasma membranes, rather than between epithelial and fibre cells. Indeed, freeze fracture analyses of the epithelial-fibre cell interface failed to reveal gap junctions connecting the epithelium and the underlying fibre cells. Importantly, microdissection and subsequent immunoblotting of lens epithelium samples confirmed the immunolocalisation results. The data suggest mature mammalian lens epithelial cells could form either heteromeric, heterotypic and/or mixed homomeric-homotypic gap junctional complexes with unique physiological properties, an important point when considering the role of epithelial cell connexins in cataractogenesis.

Endogenous casein kinase I catalyzes the phosphorylation of the lens fiber cell connexin49

The lens fiber cell-specific gap junction protein connexin49 is a substrate for a membrane-associated Ser/Thr protein kinase that can be extracted from lens cell membranes by 0.6 M KCl. However, the identity of this protein kinase has not been defined. In this report, evidence is presented indicating that it is casein kinase I. Thus, connexin49 was shown to be a substrate for purified casein kinase I but not for casein kinase II; the endogenous connexin49 protein kinase activity extracted from lens membranes with KCl was inhibited by the casein kinase I-specific inhibitor, N-(2-aminoethyl)-5-chloroisoquinoline-8-sulfonamide (CKI-7); the connexin49 protein kinase activity in the lens membrane KCl extract, which could be partially purified by gel filtration and affinity purification with a casein-Sepharose 4B column, copurified with casein kinase activity; phosphopeptide analysis showed that casein kinase I and the connexin49 protein kinase activity in the lens membrane KCl extract probably share the same phosphorylation sites in connexin49. Reverse transcription-PCR using total ovine lens RNA and casein kinase I isoform-specific oligonucleotide primers resulted in the amplification of cDNAs encoding casein kinase I-alpha and -gamma, while an in-gel casein kinase assay indicated casein kinase activity in the lens membrane KCl extract was associated with a major 39.2-kDa species, which is consistent with the 36 to 40-kDa size of casein kinase I-alpha in other animal species. These results demonstrate that the protein kinase activity present in the lens membrane 0.6 M KCl extract that catalyzes the phosphorylation of connexin49 is casein kinase I, probably the alpha isoform.

Voltage dependence of macroscopic and unitary currents of gap junction channels formed by mouse connexin50 expressed in rat neuroblastoma cells

1. The macroscopic and single channel gating characteristics of connexin (Cx) 50 gap junction channels between pairs of N2A neuroblastoma cells transfected with mouse Cx50 DNA were investigated using the dual whole-cell voltage clamp technique. 2. The macroscopic junctional current (Ij) of Cx50-transfected cells decayed exponentially with time in response to transjunctional voltage (Vj) steps (time constant (tau) of approximately 4 s at a Vj of 30-40 mV and 100-200 ms at a Vj of 80-100 mV). The steady-state junctional conductance (gj) was well described by a two-state Boltzmann equation. The half-inactivation voltage (V0), the ratio of minimal to maximal gj (gmin/gmax) and the equivalent gating charge were +/- 37 mV, 0.21 and 4, respectively. 3. The conductance of single Cx50 channels measured using patch pipettes containing 130 mM CsCl was 220 +/- 13.1 pS (12 cell pairs). A prominent residual or subconductance state corresponding to 43 +/- 4. 2 pS (10 cell pairs) was also observed at large Vj s. 4. The relationship between channel open probability (Po) and Vj was well described by a Boltzmann relationship with parameters similar to those obtained for macroscopic gj (V0 = 34 mV, gating charge = 4.25, maximum P= 0.98). The ensemble average of single channel currents at Vj = 50 mV declined in a monoexponential manner (tau = 905 ms), a value similar to the decline of the macroscopic Ij of Cx50 channels at the same voltage. 5. Ion substitution experiments indicated that Cx50 channels have a lower permeability to anions than to cations (transjunctional conductance of KCl vs. potassium glutamate (gammaj, KCl/gammaj,KGlut), 1.2; 6 cell pairs). 6. The results have important implications for understanding the role of connexins in tissues where Cx50 is a major gap junction component, including the lens.

Molecular mechanism underlying a Cx50-linked congenital cataract

Mutations in gap junctional channels have been linked to certain forms of inherited congenital cataract (D. Mackay, A. Ionides, V. Berry, A. Moore, S. Bhattacharya, and A. Shiels. Am. J. Hum. Genet. 60: 1474-1478, 1997; A. Shiels, D. Mackay, A. Ionides, V. Berry, A. Moore, and S. Bhattacharya. Am. J. Hum. Genet. 62: 526-532, 1998). We used the Xenopus oocyte pair system to investigate the functional properties of a missense mutation in the human connexin 50 gene (P88S) associated with zonular pulverulent cataract. The associated phenotype for the mutation is transmitted in an autosomal dominant fashion. Xenopus oocytes injected with wild-type connexin 50 cRNA developed gap junctional conductances of approximately 5 microS 4-7 h after pairing. In contrast, the P88S mutant connexin failed to form functional gap junctional channels when paired homotypically. Moreover, the P88S mutant functioned in a dominant negative manner as an inhibitor of human connexin 50 gap junctional channels when coinjected with wild-type connexin 50 cRNA. Cells injected with 1:5 and 1:11 ratios of P88S mutant to wild-type cRNA exhibited gap junctional coupling of approximately 8% and 39% of wild-type coupling, respectively. Based on these findings, we conclude that only one P88S mutant subunit is necessary per gap junctional channel to abolish channel function.

Co-expression of lens fiber connexins modifies hemi-gap-junctional channel behavior

Lens fiber cells contain two gap junction proteins (Cx56 and Cx45.6 in the chicken). Biochemical studies have suggested that these two proteins can form heteromeric connexons. To investigate the biophysical properties of heteromeric lens connexons, Cx56 was co-expressed with Cx45.6 (or its mouse counterpart, Cx50) in Xenopus oocytes. Whole-cell and single-channel currents were measured in single oocytes by conventional two-microelectrode voltage-clamp and patch clamp techniques, respectively. Injection of Cx56 cRNA induced a slowly activating, nonselective cation current that activated on depolarization to potentials higher than -10 mV. In contrast, little or no hemichannel current was induced by injection of Cx50 or Cx45.6 cRNA. Co-expression of Cx56 with Cx45.6 or Cx50 led to a shift in the threshold for activation to -40 or -70 mV, respectively. It also slowed the rate of deactivation of the hemichannel currents. Moreover, an increase in the unitary conductance, steady state probability of hemichannel opening and mean open times at negative potentials, was observed in (Cx56 + Cx45.6) cRNA-injected oocytes compared with Cx56 cRNA-injected oocytes. These results indicate that co-expression of lens fiber connexins gives rise to novel channels that may be explained by the formation of heteromeric hemichannels that contain both connexins.

Connexin diversity and gap junction regulation by pHi

The molecular mechanisms controlling pH-sensitivity of gap junctions formed of two different connexins are yet to be determined. We used a proton-sensitive fluorophore and electrophysiological techniques to correlate changes in intracellular pH (pHi) with electrical coupling between connexin-expressing Xenopus oocytes. The pH sensitivities of alpha 3 (connexin46), alpha 2 (connexin38), and alpha 1 (connexin43) were studied when these proteins were expressed as: 1) nonjunctional hemichannels (for alpha 3 and alpha 2), 2) homotypic gap junctions, and 3) heterotypic gap junctions. We found that alpha 3 hemichannels are sensitive to changes in pHi within a physiological range (pKa = 7.13 +/- 0.03; Hill coefficient = 3.25 +/- 1.73; n = 8; mean +/- SEM); an even more alkaline pKa was obtained for alpha 2 hemichannels (pKa = 7.50 +/- 0.03; Hill coefficient = 3.22 +/- 0.66; n = 13). The pH sensitivity curves of alpha 2 and alpha 3 homotypic junctions were indistinguishable from those recorded from hemichannels of the same connexin. Based on a comparison of pKa values, both alpha 3 and alpha 2 gap junctions were more pHi-dependent than alpha 1. The pH sensitivity of alpha 2-containing heterotypic junctions could not be predicted from the behavior of the two connexons in the pair. When alpha 2 was paired with alpha 3, the pH sensitivity curve was similar to that obtained from alpha 2 homotypic pairs. Yet, pairing alpha 2 with alpha 1 shifted the curve similar to homotypic alpha 1 channels. Pairing alpha 2 with a less pH sensitive mutant of alpha 1 (M257) yielded the same curve as when alpha 1 was used. However, the pH sensitivity curve of alpha 3/alpha 1 channels was similar to alpha 3/alpha 3, while alpha 3/M257 was indistinguishable from alpha 3/alpha 1. Our results could not be consistently predicted by a probabilistic model of two independent gates in series. The data show that dissimilarities in the pH regulation of gap junctions are due to differences in the primary sequence of connexins. Moreover, we found that pH regulation is an intrinsic property of the hemichannels, but pH sensitivity is modified by the interactions between connexons. These interactions should provide a higher level of functional diversity to gap junctions that are formed by more than one connexin.

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.

Targeted ablation of connexin50 in mice results in microphthalmia and zonular pulverulent cataracts

In the ocular lens, gap junctional communication is a key component of homeostatic mechanisms preventing cataract formation. Gap junctions in rodent lens fibers contain two known intercellular channel-forming proteins, connexin50 (Cx50) and Cx46. Since targeted ablation of Cx46 has been shown to cause senile-type nuclear opacities, it appears that Cx50 alone cannot meet homeostatic requirements. To determine if lens pathology arises from a reduction in levels of communication or the loss of a connexin-specific function, we have generated mice with a targeted deletion of the Cx50 gene. Cx50-null mice exhibited microphthalmia and nuclear cataracts. At postnatal day 14 (P14), Cx50-knockout eyes weighed 32% less than controls, whereas lens mass was reduced by 46%. Cx50-knockout lenses also developed zonular pulverulent cataracts, and lens abnormalities were detected by P7. Deletion of Cx50 did not alter the amounts or distributions of Cx46 or Cx43, a component of lens epithelial junctions. In addition, intercellular passage of tracers revealed the persistence of communication between all cell types in the Cx50-knockout lens. These results demonstrate that Cx50 is required not only for maintenance of lens transparency but also for normal eye growth. Furthermore, these data indicate that unique functional properties of both Cx46 and Cx50 are required for proper lens development.

A mutation in the connexin 50 (Cx50) gene is a candidate for the No2 mouse cataract

PURPOSE

The No2 cataractous mouse mutant displays a bilateral, congenital, hereditary nuclear opacity of the ocular lens. The aim of this work was to identify and subsequently screen an optimal candidate gene for a mutation correlated and consistent with the observed phenotype.

METHODS

The No2 cataract was mapped in relation to genes and microsatellite markers by crossing to the wild mouse strain Mus spretus and then backcrossing to the inbred strain C3H/ HeH. The Cx50 (MP70) protein coding region and flanking sequences were amplified from normal parental as well as heterozygous and homozygous mutant genomic DNAs. These PCR products were then sequenced directly. Sequence data was corroborated by restriction analysis of PCR products.

RESULTS

Mapping of the No2 cataract placed it in the vicinity of Gja8, the gene encoding connexin 50 (MP70), a major component of lens fiber gap junctions. Amplification and subsequent sequencing of the Cx50 protein coding regions revealed a single A-->C transversion within codon 47. This sequence change resulted in the creation of an HhaI restriction endonuclease restriction site, allowing for corroboration of the sequence data via restriction analysis using this enzyme. The sequence alteration is also predicted to result in the nonconservative substitution of alanine (Ala) for the normally encoded aspartic acid (Asp) at this position within the polypeptide.

CONCLUSIONS

The identified mutation in Gja8 is both correlated and consistent with the cataract observed in the No2 mouse mutant, making it an ideal candidate for the cataract. This study provides the first evidence that a mutation in a lens connexin can result in congenital hereditary cataract, highlighting the importance of lens connexins in maintaining lens transparency.

Connexin50, a gap junction protein of macrogliaP6n the mammalian retina and visual pathway

Reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunocytochemistry were used to study the expression of gap junction proteins (connexins; Cx) in the rat and rabbit retina. RT-PCR of rabbit total retinal RNA using primers selected for the human Cx50 (alpha 8 Cx) DNA template yielded cDNA fragments of the predicted base pair size. Western blots of rat and rabbit retinal membrane preparations probed with a monoclonal antibody which recognizes Cx50 in the lens of several mammalian species revealed a single band (MW 50 kD), identical to that recognized in lens membrane extracts. In frozen retinal sections of both species, the same monoclonal antibody as well as two polyclonal antisera raised against a synthetic peptide from the C-terminal region of the human Cx50 polypeptide labeled Müller cells and astrocytes. In Müller cells, labeling was strongest in the endfeet and in the filamentous processes ensheathing the photoreceptors. Extending from the neural retina, Cx50-like immuno-reactivity was detected in astrocytes of the optic nerve and along retinal projections within the CNS. Our data indicate that Müller cells and astrocytes of mammalian retinas and throughout the visual pathway are coupled through gap junctions composed of connexin50.

Molecular cloning of sheep connexin49 and its identity with MP70

The nucleotide sequence of the sheep homologue of the lens-specific mouse connexin50, chicken connexin45.6, and human connexin50 has been obtained following screening of a sheep genomic library. This connexin comprises 1323 nucleotides, coding for a protein of 440 amino acid residues and a predicted molecular weight of 49,160 daltons, so by convention is termed sheep connexin49. A connexin49 cDNA probe detected a single major band with a mobility of 6.8 kb in sheep lens RNA, but not in RNA isolated from five other sheep organs. The N-terminal amino acid sequence of sheep connexin49 is identical to that of mouse connexin50 and closely matches that of MP70, indicating the identity of sheep connexin49 with MP70. The nucleotide and translated amino acid sequences of connexin49 have 69-87% and 76%-87% identity respectively with chicken connexin45.6, human connexin50 and mouse connexin50. Like other members of this lens connexin family, sheep connexin49 coding region is completely contained within one exon, and the sequence of the N-terminal region, the four transmembrane domains and the two extracellular loops are highly conserved.

Heteromeric connexons in lens gap junction channels

Gap junction channels are formed by paired oligomeric membrane hemichannels called connexons, which are composed of proteins of the connexin family. Experiments with transfected cell lines and paired Xenopus oocytes have demonstrated that heterotypic intercellular channels which are formed by two connexons, each composed of a different connexin, can selectively occur. Studies by Stauffer [Stauffer, K. A. (1995) J. Biol. Chem. 270, 6768-6772] have shown that recombinant Cx26 and Cx32 coinfected into insect cells may form heteromeric connexons. By solubilizing and subfractionating individual connexons from ovine lenses, we show by immunoprecipitation that connexons can contain two different connexins forming heteromeric assemblies in vivo.

Characterization of the ovine-lens plasma-membrane protein-kinase substrates

The cAMP-dependent protein-kinase-catalyzed phosphorylation of the two major intrinsic lens fiber cell plasma membrane proteins, MP20 and MP26, is likely restricted to the inner cortical and nuclear regions of the lens in vivo. The ovine-lens-specific connexin, MP70, that has been identified as Cx50 in mice and Cx45.6 in the chick, is also a protein kinase substrate although it does not appear to be phosphorylated by a number of protein kinases including cAMP-dependent protein kinase, calmodulin-dependent protein kinase or protein kinase C. Rather, an extrinsic lens membrane fraction was isolated which contained protein kinase activity that catalyzed the phosphorylation of MP70; this protein kinase activity was cAMP-independent, Ca(2+)-independent, Mg(2+)-dependent, phosphorylated MP70 on a serine residue(s) and migrated with a molecular mass of 35 kDa on a gel filtration column. Both MP70 phosphorylation and the endogenous protein kinase activity were restricted to the lens outer cortical region. This membrane-associated protein kinase activity represents the first reported partial characterization of an endogenous lens fiber cell protein kinase activity that catalyzes the phosphorylation of a lens connexin protein. The phosphatase-induced shift in the electrophoretic mobility of MP70 is not reversed by this protein kinase, indicating that MP70 is likely phosphorylated on different residues by two or more protein kinases.

Purification of bovine lens cell-to-cell channels composed of connexin44 and connexin50

Cell-to-cell channels composed of connexin44 and connexin50 were purified from plasma membranes of calf and fetal bovine lenses. The channels were treated with the nonionic detergents octyl-beta-D-glucopyranoside and decyl-beta-D-maltopyranoside, and the channel/detergent complexes purified by ion and gel filtration column chromatography. In negative staining, the channels appeared as annuli 11 +/- 0.6 nm (s.d., n = 105) in diameter and as 16 +/- 0.8 nm (s.d., n = 96) long particles which corresponded to top and side views of ‘complete’ cell-to-cell channels. The purified cell-to-cell channels were composed principally of a protein, called MP70, that appeared as a diffuse 55–75 kDa band in SDS-PAGE. Dephosphorylation with alkaline phosphatase transformed the diffuse 55–75 kDa band into two distinct bands of almost equal intensity. Immunoblotting showed the bands to be connexin44 and connexin50, respectively. The antibodies also recognized weaker bands composed of the unphosphorylated form of both connexins. The connexins appear to be processed independently ‘in vivo’. The unphosphorylated form of connexin50 was present in channels and membranes from fetal, calf and adult bovine lenses, while unphosphorylated connexin44 only in channels purified from fetal lenses. Therefore, lens cell-to-cell channels are composed principally of equal amounts of phosphorylated connexins 44 and 50 that appear to be assembled in the same channel (‘hybrid’).

Purification and oligomeric state of the major lens fiber cell membrane proteins

Purification of the lens fiber cell membrane proteins MP20 and MP26, and the partial co-purification of the lens connexin-related proteins MP70 and connexin 46 has been achieved using anion- and cation-exchange chromatography of lens fiber cell membrane proteins solubilized in n-octyl-beta-D-glucopyranoside (octyl glucoside). The apparent molecular weights of the solubilized protein-detergent complexes were significantly greater than that expected for the monomeric proteins. The purified proteins retained their ability to be phosphorylated by cAMP-dependent protein kinase, and to bind calmodulin in a calcium and magnesium dependent manner. The heterobifunctional covalent chemical crosslinking agent N-5-azido-2-nitro-benzoyloxysuccinimide (ANB-NOS), and the thiol oxidant cupric phenanthroline were used to identify the oligomeric states of these proteins. Crosslinking of either the purified proteins or native lens membranes generated a ladder of crosslinked MP20 or MP26 homo-oligomers. The largest detectable crosslinked homo-oligomer of MP20 was at least a hexamer, while for MP26 the largest crosslinked homo-oligomer was at least a tetramer. The possible oligomeric states of MP70 and connexin 46 could not be determined with the crosslinking reagents used in this study. The procedure described here for the purification of detergent-solubilized major lens proteins should provide a valuable approach in future studies aimed at clarifying the roles of these different lens membrane proteins.

Ocular lens gap junctions: protein expression, assembly, and structure-function analysis

Recent advances in understanding lens fiber gap junction formation are reviewed. These include studies of junctional protein expression in the embryonic lens, and of age related changes affecting gap junction structure and composition in the adult lens. An in vitro assembly system based on detergent solubilized pore complexes and endogenous lipids has been developed to provide information on the molecular interactions involved in gap junction formation and to provide material for structure analysis. Important information on the electrical properties of lens gap junction channels is obtained using electrophysiological techniques including planar lipid bilayer analysis and patch clamping.

Reconstitution of native-type noncrystalline lens fiber gap junctions from isolated hemichannels

Gap junctions contain numerous channels that are clustered in apposed membrane patches of adjacent cells. These cell-to-cell channels are formed by pairing of two hemichannels or connexons, and are also referred to as connexon pairs. We have investigated various detergents for their ability to separately solubilize hemichannels or connexon pairs from isolated ovine lens fiber membranes. The solubilized preparations were reconstituted with lipids with the aim to reassemble native-type gap junctions and to provide a model system for the characterization of the molecular interactions involved in this process. While small gap junction structures were obtained under a variety of conditions, large native-type gap junctions were assembled using a novel two-step procedure: in the first step, hemichannels that had been solubilized with octylpolyoxyethylene formed connexon pairs by dialysis against n-decyl-beta-D-maltopyranoside. In the second step, connexon pairs were reconstituted with phosphatidylcholines by dialysis against buffer containing Mg2+. This way, double-layered gap junctions with diameter < or = 300 nm were obtained. Up to several hundred channels were packed in a noncrystalline arrangement, giving these reconstituted gap junctions an appearance that was indistinguishable from that of the gap junctions in the lens fiber membranes.

Selective interactions among the multiple connexin proteins expressed in the vertebrate lens: the second extracellular domain is a determinant of compatibility between connexins

Gap junctions are collections of intercellular channels composed of structural proteins called connexins (Cx). We have examined the functional interactions of the three rodent connexins present in the lens, Cx43, Cx46, and Cx50, by expressing them in paired Xenopus oocytes. Homotypic channels containing Cx43, Cx46, or Cx50 all developed high conductance. heterotypic channels composed of Cx46 paired with either Cx43 or Cx50 were also well coupled, whereas Cx50 did not form functional channels with Cx43. We also examined the functional response of homotypic and heterotypic channels to transjunctional voltage and cytoplasmic acidification. We show that all lens connexins exhibited sensitivity to cytoplasmic acidification as well as to voltage, and that voltage-dependent closure of heterotypic channels for a given connexin was dramatically influenced by its partner connexins in the adjacent cell. Based on the observation that Cx43 can discriminate between Cx46 and Cx50, we investigated the molecular determinants that specify compatibility by constructing chimeric connexins from portions of Cx46 and Cx50 and testing them for their ability to form channels with Cx43. When the second extracellular (E2) domain in Cx46 was replaced with the E2 of Cx50, the resulting chimera could no longer form heterotypic channels with Cx43. A reciprocal chimera, where the E2 of Cx46 was inserted into Cx50, acquired the ability to functionally interact with Cx43. Together, these results demonstrate that formation of intercellular channels is a selective process dependent on the identity of the connexins expressed in adjacent cells, and that the second extracellular domain is a determinant of heterotypic compatibility between connexins.

Molecular cloning and functional characterization of chick lens fiber connexin 45.6

The avian lens is an ideal system to study gap junctional intercellular communication in development and homeostasis. The lens is experimentally more accessible in the developing chick embryo than in other organisms, and chick lens cells differentiate well in primary cultures. However, only two members of the connexin gene family have been identified in the avian lens, whereas three are known in the mammalian system. We report here the molecular cloning and characterization of the third lens connexin, chick connexin45.6 (ChCx45.6), a protein with a predicted molecular mass of 45.6 kDa. ChCx45.6 was encoded by a single copy gene and was expressed specifically in the lens. There were two mRNA species of 6.4 kilobase (kb) and 9.4 kb in length. ChCx45.6 was a functional connexin protein, because expression in Xenopus oocyte pairs resulted in the development of high levels of conductance with a characteristic voltage sensitivity. Antisera were raised against ChCx45.6 and chick connexin56 (ChCx56), another avian lens-specific connexin, permitting the examination of the distribution of both proteins. Immunofluorescence localization showed that both ChCx45.6 and ChCx56 were abundant in lens fibers. Treatment of lens membranes with alkaline phosphatase resulted in electrophoretic mobility shifts, demonstrating that both ChCx45.6 and ChCx56 were phosphoproteins in vivo.

The human lens fiber-cell intrinsic membrane protein MP19 gene: isolation and sequence analysis

DNA sequence analysis of overlapping shotgun and restriction fragments have revealed the entire sequence of the human lens fiber cell intrinsic membrane protein MP19 gene (also termed MP17, MP18, and MP20). The 8,056 bp MP19 gene contains 5 exons encoding a mature protein of 173 amino acids, which displayed a very high degree of identity (91%) with that of bovine MP19, deduced from a bovine cDNA sequence. The exon range in size from 52 bases (exon 1) to about 340 bases (exon 5). The introns consist of two large segments (introns B and C) of about 4,700 bases and 1,800 bases, respectively, and two small segments (intron A and D) of about 450 and 250 bases each. Seven Alu family DNA repeats are found within the human MP19 gene. The sequenced gene includes 100 bases of 5' flanking sequence.

Multiple structural types of gap junctions in mouse lens

Gap junctions in the epithelium and superficial fiber cells from young mice were examined in lenses prepared by rapid-freezing, and processed for freeze-substitution and freeze-fracture electron microscopy. There appeared to be three structural types of gap junction: one type between epithelial cells and two types between fiber cells. Epithelial gap junctions seen by freeze-substitution were approximately 20 nm thick and consistently associated with layers of dense material lying along both cytoplasmic surfaces. Fiber gap junctions, in contrast, were 15–16 nm (type 1) or 17–18 nm thick (type 2), and had little associated cytoplasmic material. Type 1 fiber gap junctions were extensive in flat expanses of cell membrane and had a thin, discontinuous central lamina, whereas type 2 fiber gap junctions were associated with the ball-and-socket domains and exhibited a dense, continuous central lamina. Both types of fiber gap junction had a diffuse arrangement of junctional intramembrane particles, whereas particles and pits of epithelial gap junctions were in a tight, hexagonal configuration. The type 2 fiber gap junctions, however, had a larger particle size (approximately 9 nm) than the type 1 (approximately 7.5 nm). In addition, a large number of junctional particles typified the E-faces of both fiber types but not the epithelial type of gap junction. Gap junctions between fiber and epithelial cells had structural features of type 1 fiber gap junctions.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical analysis of rat liver using a monoclonal antibody (HAM8) against gap junction

Four monoclonal antibodies were raised against crude gap junction fractions of rat liver to clarify the distribution of gap junctions during animal development and to analyze gap junction expression in vivo and the polarity of hepatocytes in vitro. Among the monoclonal antibodies obtained, HAM8 antibody recognized the 27-kDa rat liver gap junction protein connexin 32. This antibody recognized gap junctions at the contiguous faces of hepatocytes, and the antigen was also observed in exocrine pancreas and salivary gland but not in kidney, heart, esophagus, or thymus. HAM8 did not react with amphibian or fish liver, heart, esophagus, stomach, or intestine as assessed via the immunofluorescence method on frozen sections. A few hepatocytes and many hemopoietic cells were seen in rat fetal liver at 15 days of gestation. HAM8 antigen was expressed on some hepatocytes but not on any hemopoietic cells. As the fetus grew, the number of hepatocytes in the liver increased gradually, together with the amount of HAM8 antigen. The distribution of HAM8 antigen at 25 days after birth was similar to that in adult liver. When the expression of HAM8 antigen was examined in primary cultured hepatocytes using the immunofluorescence method, the antigen was observed clearly between the hepatocytes. However, most of the HAM8 antigen on the free surface of hepatocytes disappeared within 4 hr. HAM8 antigen was not expressed on AH-7974 rat hepatoma cells when they formed small islets in the rat peritoneal cavity or within the liver. When HAM8 IgG antibody was injected intravenously, the HAM8 signal was expressed in the liver.

The permeability of reconstituted liposomes containing the purified lens fiber cell integral membrane proteins MP20, MP26 and MP70

A number of lens fiber cell integral membrane proteins have been localized to junctional regions where they have been proposed to play a role in either mediating or controlling cell-to-cell communication. We have examined the effect of three lens fiber cell membrane proteins, MP20, MP26 and MP70, on the permeability properties of unilamellar phospholipid liposomes. This approach has been previously used to examine the channel-forming properties of MP26. Liposome permeability was determined by measuring the effect of Co2+ on the quenching of the fluorescence of N-4-nitrobenzo-2-oxa-1,3 diazole phosphatidyl ethanolamine (NBD-PE)-containing liposomes as described previously by Scaglione and Rintoul (Invest. Ophthalmol. Vis. Sci. 30:961-966, 1989). The effect of all three proteins on liposome permeability was similar. Permeability was dependent on the protein/phospholipid ratio and was not significantly affected by agents known to modify gap junctional permeability in vivo. Glycophorin A, a non-channel-forming integral membrane protein derived from erythrocytes, was also shown to increase the permeability of unilamellar phospholipid liposomes. The ability of a non-channel membrane protein to increase Co2+ quenching of NBD-PE-containing liposomes (presumably in a nonspecific manner) indicates that reports describing the permeability of lens membrane protein-containing liposomes should be interpreted with caution in terms of their relationship to cell-to-cell communication.