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

Cx46 hemichannels contribute to the sodium leak conductance in lens fiber cells

The lens is proposed to have an internal microcirculation system consisting of continuously circulating ionic fluxes that play an essential role in maintaining lens transparency. One of the key components of this system is the sodium leak conductance. Here we investigate the contribution of Cx46 hemichannels to the basal membrane permeability of peripheral fiber cells isolated from transgenic mouse lenses lacking Cx50 or both Cx50 and Cx46 (dKO) using the whole cell patch-clamp technique. Our results show that Cx46 hemichannels were largely closed at a resting voltage of -60 mV in the presence of millimolar divalent cation concentrations. However, even though the vast majority of these channels were closed at -60 mV, a small, persistent, inward current could still be detected. This current could be mostly blocked by exposure to 1 mM La(3+) and was not observed in fiber cells isolated from dKO mouse lenses suggesting that it was due to Cx46 hemichannels. In addition, Cx50(-/-) fiber cells showed increased open channel noise and a depolarized resting potential compared with dKO fiber cells. Exposure of Cx50(-/-) fiber cells to La(3+) hyperpolarized the resting potential to -58 mV, which is similar to the value of resting potential measured in dKO fiber and significantly reduced the open channel noise. In conclusion, these results suggest that Cx46 hemichannels may contribute to the sodium leak conductance in lens fiber cells.

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.

Functional characterization of an AQP0 missense mutation, R33C, that causes dominant congenital lens cataract, reveals impaired cell-to-cell adhesion

Aquaporin 0 (AQP0) performs dual functions in the lens fiber cells, as a water pore and as a cell-to-cell adhesion molecule. Mutations in AQP0 cause severe lens cataract in both humans and mice. An arginine to cysteine missense mutation at amino acid 33 (R33C) produced congenital autosomal dominant cataract in a Chinese family for five generations. We re-created this mutation in wild type human AQP0 (WT-AQP0) cDNA by site-directed mutagenesis, and cloned and expressed the mutant AQP0 (AQP0-R33C) in heterologous expression systems. Mutant AQP0-R33C showed proper trafficking and membrane localization like WT-AQP0. Functional studies conducted in Xenopus oocytes showed no significant difference (P > 0.05) in water permeability between AQP0-R33C and WT-AQP0. However, the cell-to-cell adhesion property of AQP0-R33C was significantly reduced (P < 0.001) compared to that of WT-AQP0, indicated by cell aggregation and cell-to-cell adhesion assays. Scrape-loading assay using Lucifer Yellow dye showed reduction in cell-to-cell adhesion affecting gap junction coupling (P < 0.001). The data provided suggest that this mutation might not have caused significant alterations in protein folding since there was no obstruction in protein trafficking or water permeation. Reduction in cell-to-cell adhesion and development of cataract suggest that the conserved positive charge of Extracellular Loop A may play an important role in bringing fiber cells closer. The proposed schematic models illustrate that cell-to-cell adhesion elicited by AQP0 is vital for lens transparency and homeostasis.

Functional effects of Cx50 mutations associated with congenital cataracts

Mutations in connexin50 (Cx50) cause dominant cataracts in both humans and mice. The exact mechanisms by which mutations cause these variable phenotypes are poorly understood. We have examined the functional properties of gap junctions made by three Cx50 mutations, V44E, D47N, and V79L, expressed in mammalian cell lines. V44E trafficked to the plasma membrane properly and formed gap junctional plaques. However, the mutant did not form functional gap junctions when expressed alone, or with wild-type (WT) Cx46 and Cx50, indicating that V44E is a dominant negative inhibitor of WT connexin function. In contrast, D47N subunits did not localize to junctional plaques or form functional homotypic gap junctions; however, mixed expression of D47N and WT subunits of either Cx50 or Cx46 resulted in functional intercellular channels, with high levels of coupling. Single-channel studies indicated that D47N formed heteromeric channels with WT Cx46 with unique properties. Unlike either V44E or D47N, V79L formed functional homotypic intercellular channels. However, the mutation caused an alteration in voltage gating and a dramatic reduction in the single-channel open probability, resulting in much lower levels of conductance in cells expressing V79L alone, or together with WT connexin subunits. Thus, each mutation produced distinct changes in the properties of junctional coupling. V44E failed to form intercellular channels in any configuration, D47N formed only heteromeric channels with WT connexins, and V79L formed homotypic and heteromeric channels with altered properties. These results suggest that unique interactions between mutant and wild-type lens connexins might underlie the development of various cataract phenotypes in humans.

A connexin50 mutant, CX50fs, that causes cataracts is unstable, but is rescued by a proteasomal inhibitor

The mechanisms by which mutant connexins lead to disease are diverse, including those of connexin50 (CX50) encoded by the GJA8 gene. We investigated the cellular and functional behavior of CX50fs, a mutant CX50 that has a frameshift after amino acid 255 and causes recessive congenital cataracts. Cellular levels of CX50fs were much lower than those of wild type CX50 in stably transfected HeLa cells. Whereas CX50 localized at distinct gap junction plaques and supported extensive intercellular transfer of Neurobiotin, CX50fs gap junctions were rare, and their support of Neurobiotin transfer was reduced by >90%. After inhibition of new protein synthesis with cycloheximide, CX50fs disappeared much more rapidly than CX50, suggesting increased degradation of the mutant. Treatment of cells with epoxomicin (a proteasomal inhibitor) led to a dramatic increase in CX50fs levels and in the abundance of gap junctions. Epoxomicin treatment also rescued intercellular transfer of Neurobiotin to levels similar to those in cells expressing the wild type protein. Treatment with eeyarestatin I (an inhibitor of p97-dependent protein degradation) resulted in many abundant slowly migrating CX50 and CX50fs bands consistent with polyubiquitination of the proteins. These results demonstrate that the CX50fs mutant is rapidly degraded by endoplasmic reticulum-associated degradation in mammalian cells. This accelerated degradation reduces the abundance of gap junctions and the extent of intercellular communication, potentially explaining the pathogenesis of cataracts linked to this mutant. The efficacy of epoxomicin in restoring function suggests that protease inhibition might have therapeutic value for this and other diseases caused by mutants with similar defects.

Cataracts and microphthalmia caused by a Gja8 mutation in extracellular loop 2

The mouse semi-dominant Nm2249 mutation displays variable cataracts in heterozygous mice and smaller lenses with severe cataracts in homozygous mice. This mutation is caused by a Gja8(R205G) point mutation in the second extracellular loop of the Cx50 (or α8 connexin) protein. Immunohistological data reveal that Cx50-R205G mutant proteins and endogenous wild-type Cx46 (or α3 connexin) proteins form diffuse tiny spots rather than typical punctate signals of normal gap junctions in the lens. The level of phosphorylated Cx46 proteins is decreased in Gja8(R205G/R205G) mutant lenses. Genetic analysis reveals that the Cx50-R205G mutation needs the presence of wild-type Cx46 to disrupt lens peripheral fibers and epithelial cells. Electrophysiological data in Xenopus oocytes reveal that Cx50-R205G mutant proteins block channel function of gap junctions composed of wild-type Cx50, but only affect the gating of wild-type Cx46 channels. Both genetic and electrophysiological results suggest that Cx50-R205G mutant proteins alone are unable to form functional channels. These findings imply that the Gja8(R205G) mutation differentially impairs the functions of Cx50 and Cx46 to cause cataracts, small lenses and microphthalmia. The Gja8(R205G) mutation occurs at the same conserved residue as the human GJA8(R198W) mutation. This work provides molecular insights to understand the cataract and microphthalmia/microcornea phenotype caused by Gja8 mutations in mice and humans.

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

Lens regenerates by means of similar processes and timeline in adults and larvae of the newt Cynops pyrrhogaster

BACKGROUND

It is widely accepted that juvenile animals can regenerate faster than adults. For example, in the case of lens regeneration of the newt Cynops pyrrhogaster, larvae and adults require approximately 30 and 80 days for completion of lens regeneration, respectively. However, when we carefully observed lens regeneration in C. pyrrhogaster at the cellular level using molecular markers in the present study, we found that lens regeneration during the larval stage proceeded at similar speed and by means of similar steps to those in adults.

RESULTS

We could not find any drastic difference between regeneration at these two stages, except that the size of the eyes was very different.

CONCLUSIONS

Our observations suggested that larvae could regenerate a lens of the original size within a shorter time than adults because the larval lens was smaller than the adult lens, but the speed of regeneration was not faster in larvae. In addition, by repeatedly observing the regeneration in one individual transgenic newt that expressed fluorescence specifically in lens fiber cells in vivo and comparing the regeneration process at the embryonic, larval, and postmetamorphosis stages, we confirmed that the regeneration speed was the same at each of these stages in the same individual.

Primary cultures of embryonic chick lens cells as a model system to study lens gap junctions and fiber cell differentiation

A major limitation in lens gap junction research has been the lack of experimentally tractable ex vivo systems to study the formation and regulation of fiber-type gap junctions. Although immortalized lens-derived cell lines are amenable to both gene transfection and siRNA-mediated knockdown, to our knowledge none are capable of undergoing appreciable epithelial-to-fiber differentiation. Lens central epithelial explants have the converse limitation. A key advance in the field was the development of a primary embryonic chick lens cell culture system by Drs. Sue Menko and Ross Johnson. Unlike central epithelial explants, these cultures also include cells from the peripheral (preequatorial and equatorial) epithelium, which is the most physiologically relevant population for the study of fiber-type gap junction formation. We have modified the Menko/Johnson system and refer to our cultures as dissociated cell-derived monolayer cultures (DCDMLs). We culture DCDMLs without serum to mimic the avascular lens environment and on laminin, the major matrix component of the lens capsule. Here, I review the features of the DCDML system and how we have used it to study lens gap junctions and fiber cell differentiation. Our results demonstrate the power of DCDMLs to generate new findings germane to the mammalian lens and how these cultures can be exploited to conduct experiments that would be impossible, prohibitively expensive and/or difficult to interpret using transgenic animals in vivo.

The unfolded protein response is activated in connexin 50 mutant mouse lenses

The unfolded protein response is a set of cell signaling pathways recently recognized to be activated in the lens during both normal development and endoplasmic reticulum stress induced by either unfolded proteins or oxidative damage. While mutations in the gene for connexin 50 are known to cause autosomal dominant cataracts, it has not been previously reported whether mutant connexins can activate the unfolded protein response in the lens. Mice homozygous for the S50P or G22R mutation of connexin 50 have reduced amounts of connexin 50 protein at the cell membrane, with some intracellular staining consistent with retention in the endoplasmic reticulum. Connexin 50 mutants have elevated levels of BiP expression in both lens epithelial and fiber cells from E15.5 with the most robust elevation detected in newborns. While this elevation decreases in magnitude postnatally, BiP expression is still abnormally high in adults, particularly in the perinuclear endoplasmic reticulum of cell nuclei that are inappropriately retained in adult homozygous mutant lenses. Xbp1 splicing was elevated in lenses from both connexin mutants studied, while Atf4 and Atf6 levels were not majorly affected. Overall, these data suggest that UPR may be a contributing factor to the phenotype of connexin 50 mutant lenses even though the relatively modest extent of the response suggests that it is unlikely to be a major driver of the pathology.

The role of Eph receptors in lens function and disease

Cataract is the single largest contributor to blindness in the world, with the disease having a strong genetic component. In recent years the Eph family of receptor tyrosine kinases has been identified as a key regulator in lens clarity. In this review we discuss the roles of the Eph receptors in lens biology and cataract development.

Developmental truncations of connexin 50 by caspases adaptively regulate gap junctions/hemichannels and protect lens cells against ultraviolet radiation

The gap junction-forming connexin (Cx) 50 is truncated gradually during lens development. Premature cleavage of lens connexins is thought to be associated with cataract formation. We have shown previously that Cx50 is likely to be cleaved by caspase-3 like protease during chick lens development. Here, using HPLC-electrospray tandem mass spectrometry, we mapped two cleavage sites at the C terminus of Cx50 after Glu-368 and Asp-379 and identified caspase-3 and caspase-1 as the responsible proteases, respectively. The activity of caspase-1, like caspase-3, was detected in the outer cortex increased during lens development, which coincided with the accumulation of the truncated fragments of Cx50 in the core region of the lens. The truncated Cx50 fragments present in older lenses were reproduced in the younger lens after treatment with UV radiation; this cleavage could be partially blocked by caspase-1/3-specific inhibitors. Interestingly, as compared with full-length Cx50, caspase-truncated Cx50 showed a dramatic decrease in gap junction coupling and a loss of hemichannel function. Furthermore, expression of caspase-truncated Cx50 fragments increased cell viability against UV radiation as compared with full-length Cx50. Together, these results suggest that both caspase-1 and -3 are responsible for the cleavage at the C terminus of Cx50 during lens development. The reduction of gap junction coupling and closure of hemichannels formed by truncated Cx50 are likely to adaptively protect cells against elevated oxidative stress associated with lens aging.

Cx50 requires an intact PDZ-binding motif and ZO-1 for the formation of functional intercellular channels

The three connexins expressed in the ocular lens each contain PDZ domain-binding motifs directing a physical association with the scaffolding protein ZO-1, but the significance of the interaction is unknown. We found that Cx50 with PDZ-binding motif mutations did not form gap junction plaques or induce cell-cell communication in HeLa cells, whereas the addition of a seven-amino acid PDZ-binding motif restored normal function to Cx50 lacking its entire C-terminal cytoplasmic domain. C-Terminal deletion had a similar although weaker effect on Cx46 but little if any effect on targeting and function of Cx43. Furthermore, small interfering RNA knockdown of ZO-1 completely inhibited the formation of gap junctions by wild-type Cx50 in HeLa cells. Thus both a PDZ-binding motif and ZO-1 are necessary for Cx50 intercellular channel formation in HeLa cells. Knock-in mice expressing Cx50 with a PDZ-binding motif mutation phenocopied Cx50 knockouts. Furthermore, differentiating lens fibers in the knock-in displayed extensive intracellular Cx50, whereas plaques in mature fibers contained only Cx46. Thus normal Cx50 function in vivo also requires an intact PDZ domain-binding motif. This is the first demonstration of a connexin-specific requirement for a connexin-interacting protein in gap junction assembly.

The pathologic effect of a novel neomorphic Fgf9(Y162C) allele is restricted to decreased vision and retarded lens growth

Fibroblast growth factor (Fgf) signalling plays a crucial role in many developmental processes. Among the Fgf pathway ligands, Fgf9 (UniProt: P54130) has been demonstrated to participate in maturation of various organs and tissues including skeleton, testes, lung, heart, and eye. Here we establish a novel Fgf9 allele, discovered in a dominant N-ethyl-N-nitrosourea (ENU) screen for eye-size abnormalities using the optical low coherence interferometry technique. The underlying mouse mutant line Aca12 was originally identified because of its significantly reduced lens thickness. Linkage studies located Aca12 to chromosome 14 within a 3.6 Mb spanning interval containing the positional candidate genes Fgf9 (MGI: 104723), Gja3 (MGI: 95714), and Ift88 (MGI: 98715). While no sequence differences were found in Gja3 and Ift88, we identified an A→G missense mutation at cDNA position 770 of the Fgf9 gene leading to an Y162C amino acid exchange. In contrast to previously described Fgf9 mutants, Fgf9^Y162C carriers were fully viable and did not reveal reduced body-size, male-to-female sexual reversal or skeletal malformations. The histological analysis of the retina as well as its basic functional characterization by electroretinography (ERG) did not show any abnormality. However, the analysis of head-tracking response of the Fgf9^Y162C mutants in a virtual drum indicated a gene-dosage dependent vision loss of almost 50%. The smaller lenses in Fgf9^Y162C suggested a role of Fgf9 during lens development. Histological investigations showed that lens growth retardation starts during embryogenesis and continues after birth. Young Fgf9^Y162C lenses remained transparent but developed age-related cataracts. Taken together, Fgf9^Y162C is a novel neomorphic allele that initiates microphakia and reduced vision without effects on organs and tissues outside the eye. Our data point to a role of Fgf9 signalling in primary and secondary lens fiber cell growth. The results underline the importance of allelic series to fully understand multiple functions of a gene.

Investigation of the reciprocal relationship between the expression of two gap junction connexin proteins, connexin46 and connexin43

Connexins are the transmembrane proteins that form gap junctions between adjacent cells. The function of the diverse connexin molecules is related to their tissue-specific expression and highly dynamic turnover. Although multiple connexins have been previously reported to compensate for each other's functions, little is known about how connexins influence their own expression or intracellular regulation. Of the three vertebrate lens connexins, two connexins, connexin43 (Cx43) and connexin46 (Cx46), show reciprocal expression and subsequent function in the lens and in lens cell culture. In this study, we investigate the reciprocal relationship between the expression of Cx43 and Cx46. Forced depletion of Cx43, by tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate, is associated with an up-regulation of Cx46 at both the protein and message level in human lens epithelial cells. An siRNA-mediated down-regulation of Cx43 results in an increase in the level of Cx46 protein, suggesting endogenous Cx43 is involved in the regulation of endogenous Cx46 turnover. Overexpression of Cx46, in turn, induces the depletion of Cx43 in rabbit lens epithelial cells. Cx46-induced Cx43 degradation is likely mediated by the ubiquitin-proteasome pathway, as (i) treatment with proteasome inhibitors restores the Cx43 protein level and (ii) there is an increase in Cx43 ubiquitin conjugation in Cx46-overexpressing cells. We also present data that shows that the C-terminal intracellular tail domain of Cx46 is essential to induce degradation of Cx43. Therefore, our study shows that Cx43 and Cx46 have novel functions in regulating each other's expression and turnover in a reciprocal manner in addition to their conventional roles as gap junction proteins in lens cells.

Structure of the gap junction channel and its implications for its biological functions

Gap junctions consist of arrays of intercellular channels composed of integral membrane proteins called connexin in vertebrates. Gap junction channels regulate the passage of ions and biological molecules between adjacent cells and, therefore, are critically important in many biological activities, including development, differentiation, neural activity, and immune response. Mutations in connexin genes are associated with several human diseases, such as neurodegenerative disease, skin disease, deafness, and developmental abnormalities. The activity of gap junction channels is regulated by the membrane voltage, intracellular microenvironment, interaction with other proteins, and phosphorylation. Each connexin channel has its own property for conductance and molecular permeability. A number of studies have tried to reveal the molecular architecture of the channel pore that should confer the connexin-specific permeability/selectivity properties and molecular basis for the gating and regulation. In this review, we give an overview of structural studies and describe the structural and functional relationship of gap junction channels.

A genome-wide association study identifies a susceptibility locus for refractive errors and myopia at 15q14

Refractive errors are the most common ocular disorders worldwide and may lead to blindness. Although this trait is highly heritable, identification of susceptibility genes has been challenging. We conducted a genome-wide association study for refractive error in 5,328 individuals from a Dutch population-based study with replication in four independent cohorts (combined 10,280 individuals in the replication stage). We identified a significant association at chromosome 15q14 (rs634990, P = 2.21 × 10⁻¹⁴). The odds ratio of myopia compared to hyperopia for the minor allele (minor allele frequency = 0.47) was 1.41 (95% CI 1.16-1.70) for individuals heterozygous for the allele and 1.83 (95% CI 1.42-2.36) for individuals homozygous for the allele. The associated locus is near two genes that are expressed in the retina, GJD2 and ACTC1, and appears to harbor regulatory elements which may influence transcription of these genes. Our data suggest that common variants at 15q14 influence susceptibility for refractive errors in the general population.

Connexin mediated cataract prevention in mice

Cataracts, named for any opacity in the ocular lens, remain the leading cause of vision loss in the world. Non-surgical methods for cataract prevention are still elusive. We have genetically tested whether enhanced lens gap junction communication, provided by increased α3 connexin (Cx46) proteins expressed from α8(Kiα3) knock-in alleles in Gja8^tm1(Gja3)Tww mice, could prevent nuclear cataracts caused by the γB-crystallin S11R mutation in Crygb^S11R/S11R mice. Remarkably, homozygous knock-in α8(Kiα3/Kiα3) mice fully prevented nuclear cataracts, while single knock-in α8(Kiα3/−) allele mice showed variable suppression of nuclear opacities in Crygb^S11R/S11R mutant mice. Cataract prevention was correlated with the suppression of many pathological processes, including crystallin degradation and fiber cell degeneration, as well as preservation of normal calcium levels and stable actin filaments in the lens. This work demonstrates that enhanced intercellular gap junction communication can effectively prevent or delay nuclear cataract formation and suggests that small metabolites transported through gap junction channels protect the stability of crystallin proteins and the cytoskeletal structures in the lens core. Thus, the use of an array of small molecules to promote lens homeostasis may become a feasible non-surgical approach for nuclear cataract prevention in the future.

A novel role of gap junction connexin46 protein to protect breast tumors from hypoxia

Connexin proteins are the principle structural components of the gap junctions. Colocalization and tissue-specific expression of diverse connexin molecules are reported to occur in a variety of organs. Impairment of gap junctional intercellular communication, caused by mutations, gain of function or loss of function of connexins, is involved in a number of diseases including the development of cancer. Here we show that human breast cancer cells, MCF-7 and breast tumor tissues express a novel gap junction protein, connexin46 (Cx46) and it plays a critical role in hypoxia. Previous studies have shown that connexin46 is predominantly expressed in lens and our studies find that Cx46 protects human lens epithelial cells from hypoxia induced death. Interestingly, we find that Cx46 is upregulated in MCF-7 breast cancer cells and human breast cancer tumors. Downregulation of Cx46 by siRNA promotes 40% MCF-7 cell death at 24 hr under hypoxic conditions. Furthermore, direct injection of anti-Cx46 siRNA into xenograft tumors prevents tumor growth in nude mice. This finding will provide an exciting new direction for drug development for breast cancer treatment and suggests that both normal hypoxic tissue (lens) and adaptive hypoxic tissue (breast tumor) utilize the same protein, Cx46, as a protective strategy from hypoxia.

Amino acid residue Val362 plays a critical role in maintaining the structure of C terminus of connexin 50 and in lens epithelial-fiber differentiation

We have previously shown that connexin (Cx) 50, unlike the other two lens connexins, Cx43 and Cx46, promotes chicken lens epithelial-fiber differentiation in a channel-independent manner. Here, we show that deletion of the PEST motif at the C terminus (CT) domain of Cx50 attenuates the stimulatory effect of Cx50 on lens fiber differentiation. Valine 362, a residue located within the PEST domain, is functionally involved. The structure of the Cx50 CT predicted by molecular modeling revealed four alpha-helices and Val(362) was found to be located in the middle of the 3rd helix. Replacement of Val(362) with amino acid residues that disrupt the alpha-helical structure predicted by molecular modeling, such as arginine, glutamate, or phenylalanine, attenuated the stimulatory effects of Cx50 on lens differentiation, whereas replacement with threonine, isoleucine, leucine, or proline, which maintain the structure preserved the function of Cx50. Circular dichroism (CD) studies supported the structural predictions and showed that the substitution with Glu, but not Thr or Pro, disrupted the alpha-helix, which appears to be the structural feature important for lens epithelial-fiber differentiation. Together, our results suggest that Val(362) is important for maintaining the helical structure and is crucial for the role of Cx50 in promoting lens epithelial-fiber differentiation.

Lens gap junctions in growth, differentiation, and homeostasis

The cells of most mammalian organs are connected by groups of cell-to-cell channels called gap junctions. Gap junction channels are made from the connexin (Cx) family of proteins. There are at least 20 isoforms of connexins, and most tissues express more than 1 isoform. The lens is no exception, as it expresses three isoforms: Cx43, Cx46, and Cx50. A common role for all gap junctions, regardless of their Cx composition, is to provide a conduit for ion flow between cells, thus creating a syncytial tissue with regard to intracellular voltage and ion concentrations. Given this rather simple role of gap junctions, a persistent question has been: Why are there so many Cx isoforms and why do tissues express more than one isoform? Recent studies of lens Cx knockout (KO) and knock in (KI) lenses have begun to answer these questions. To understand these roles, one must first understand the physiological requirements of the lens. We therefore first review the development and structure of the lens, its numerous transport systems, how these systems are integrated to generate the lens circulation, the roles of the circulation in lens homeostasis, and finally the roles of lens connexins in growth, development, and the lens circulation.

Phosphorylation and truncation sites of bovine lens connexin 46 and connexin 50

Connexins 46 and 50 combine to form the gap junctions in ocular lens fiber cells. These proteins are known to be modified with fiber cell age; however, limited work has been done to characterize specific lens connexin modifications. In this report, bovine lens membrane proteins were isolated, digested by multiple enzymes, and analyzed by HPLC-tandem mass spectrometry. Automated database searching revealed the locations of both phosphorylation and truncation sites. The results confirmed the full sequence of connexin 46 and 99% of the connexin 50 sequence. Eighteen phosphorylation sites on connexin 50 and nine phosphorylation sites on connexin 46 were identified, all on serine or threonine residues. All but three phosphorylation sites on connexin 50 were located the cytoplasmic C-terminus. All of the truncation sites of connexin 50 were localized in the cytoplasmic C-terminus (region 280-304). Truncation sites in connexin 46 were found in four different regions including: the N-terminus (residue G2), the cytoplasmic loop (residues 121-124), the cytoplasmic C-terminus (residues 251-285), and the distal C-terminus (residues 344-395). In an analysis of dissected lenses some truncation sites were specific to nucleus samples and others were detected in both nucleus and cortex samples.

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.

Connexin57 is expressed in dendro-dendritic and axo-axonal gap junctions of mouse horizontal cells and its distribution is modulated by light

Mouse horizontal cells are coupled by gap junctions composed of connexin57. These gap junctions are regulated by ambient light via multiple neuromodulators including dopamine. In order to analyze the distribution and structure of horizontal cell gap junctions in the mouse retina, and examine the effects of light adaptation on gap junction density, we developed antibodies that detect mouse retinal connexin57. Using immunohistochemistry in retinal slices, flat-mounted retinas, and dissociated retinal cells, we showed that connexin57 is expressed in the dendrites and axon terminal processes of mouse horizontal cells. No staining was found in retinas of connexin57-deficient mice. Significantly more connexin57-positive puncta were found in the distal than in the proximal outer plexiform layer, indicating a higher level of expression in axon terminal processes than in the dendrites. We also examined the gap junctions using immunoelectron microscopy and showed that connexin57 does not form hemichannels in the horizontal cell dendritic tips. Light adaptation resulted in a significant increase in the number of connexin57-immunoreactive plaques in the outer plexiform layer, consistent with previously reported effects of light adaptation on connexin57 expression in the mouse retina. This study shows for the first time the detailed location of connexin57 expression within mouse horizontal cells, and provides the first ultrastructural data on mouse horizontal cell gap junctions.

Interaction between Connexin50 and mitogen-activated protein kinase signaling in lens homeostasis

Both connexins and signal transduction pathways have been independently shown to play critical roles in lens homeostasis, but little is known about potential cooperation between these two intercellular communication systems. To investigate whether growth factor signaling and gap junctional communication interact during the development of lens homeostasis, we examined the effect of mitogen-activated protein kinase (MAPK) signaling on coupling mediated by specific lens connexins by using a combination of in vitro and in vivo assays. Activation of MAPK signaling pathways significantly increased coupling provided by Cx50, but not Cx46, in paired Xenopus laevis oocytes in vitro, as well as between freshly isolated lens cells in vivo. Constitutively active MAPK signaling caused macrophthalmia, cataract, glucose accumulation, vacuole formation in differentiating fibers, and lens rupture in vivo. The specific removal or replacement of Cx50, but not Cx46, ameliorated all five pathological conditions in transgenic mice. These results indicate that MAPK signaling specifically modulates coupling mediated by Cx50 and that gap junctional communication and signal transduction pathways may interact in osmotic regulation during postnatal fiber development.

Oxidative stress, lens gap junctions, and cataracts

The eye lens is constantly subjected to oxidative stress from radiation and other sources. The lens has several mechanisms to protect its components from oxidative stress and to maintain its redox state, including enzymatic pathways and high concentrations of ascorbate and reduced glutathione. With aging, accumulation of oxidized lens components and decreased efficiency of repair mechanisms can contribute to the development of lens opacities or cataracts. Maintenance of transparency and homeostasis of the avascular lens depend on an extensive network of gap junctions. Communication through gap junction channels allows intercellular passage of molecules (up to 1 kDa) including antioxidants. Lens gap junctions and their constituent proteins, connexins (Cx43, Cx46, and Cx50), are also subject to the effects of oxidative stress. These observations suggest that oxidative stress-induced damage to connexins (and consequent altered intercellular communication) may contribute to cataract formation.

Connexin-caused genetic diseases and corresponding mouse models

The human and mouse genomes contain 21 and 20 connexin genes, respectively. During the last 10-year period, genetic research on connexins has been stimulated by two parallel approaches: first, the characterization of genetic diseases that are caused by connexin mutations and, second, the generation and characterization of connexin knockout (null) mutated mice in which the coding region of nearly all connexin genes has been deleted. We summarize the current results of each of these two approaches. More recently, first results have been published in which connexin point mutations in human connexin genes were inserted at the corresponding position of the orthologous mouse gene. Under these conditions, the mutated connexin protein is expressed, in contrast to a connexin null mutation, and its interaction with other connexin isoforms or other connexin-binding proteins can be maintained. In this review, we discuss advantages and problems of such an approach and possible implications regarding the mechanism of the disease. The long-term goal is to understand the biologic function of each connexin isoform and the contribution of these proteins to the physiology of the corresponding organs in health and disease.

The cataract causing Cx50-S50P mutant inhibits Cx43 and intercellular communication in the lens epithelium

Mutations in Connexin50 (Cx50) cause cataracts in both humans and mice. The mechanism(s) behind how mutated connexins lead to a variety of cataracts have yet to be fully elucidated. Here, we tested whether the cataract inducing Cx50-S50P mutant interacts with wild-type Connexin43 (Cx43) to form mixed channels with attenuated function. Using dual whole-cell voltage clamp, immunofluorescent microscopy and in situ dye transfer analysis we identified a unique interaction between the mutant subunit and wild-type Cx43. In paired Xenopus oocytes, co-expression of Cx50-S50P with Cx43 reduced electrical coupling >/=90%, without a reduction in protein expression. In transfected cells, Cx50-S50P did not target to cell-cell interfaces by itself, but co-expression of Cx50-S50P with Cx43 resulted in its localization at areas of cell-cell contact. We used Cx43 conditional knockout, Cx50 knockout and Cx50-S50P mutant mice to examine this interaction in vivo. Mice expressing both Cx43 and Cx50-S50P in the lens epithelium revealed a unique expression pattern for Cx43 and a reduction in Cx43 protein. In situ dye transfer experiments showed that the Cx50-S50P mutant, but not the Cx50, or Cx43 conditional knockout, greatly inhibited epithelial cell gap junctional communication in a manner similar to a double knockout of Cx43 and Cx50. The inhibitory affects of Cx50-S50P lead to diminished electrical coupling in vitro, as well as a discernable reduction in epithelial cell dye permeation. These data suggest that dominant inhibition of Cx43 mediated epithelial cell coupling may play a role in the lens pathophysiology caused by the Cx50-S50P mutation.

Structural function of MIP/aquaporin 0 in the eye lens; genetic defects lead to congenital inherited cataracts

Aquaporin 0 (AQP0) was originally characterized as a membrane intrinsic protein, specifically expressed in the lens fibers of the ocular lens and designated MIP, for major intrinsic protein of the lens. Once the gene was cloned, an internal repeat was identified, encoding for the amino acids Asp-Pro-Ala, the NPA repeat. Shortly, the MIP gene family was emerging, with members being characterized in mammals, insects, and plants. Once Peter Agre's laboratory developed a functional assay for water channels, the MIP family became the aquaporin family and MIP became known as aquaporin 0. Besides functioning as a water channel, aquaporin 0 also plays a structural role, being required for maintaining the transparency and optical accommodation of the ocular lens. Mutations in the AQP0 gene in human and mice result in genetic cataracts; deletion of the MIP/AQP0 gene in mice results in lack of suture formation required for maintenance of the lens fiber architecture, resulting in perturbed accommodation and focus properties of the ocular lens. Crystallography studies support the notion of the double function of aquaporin 0 as a water channel (open configuration) or adhesion molecule (closed configuration) in the ocular lens fibers. The functions of MIP/AQP0, both as a water channel and an adhesive molecule in the lens fibers, contribute to the narrow intercellular space of the lens fibers that is required for lens transparency and accommodation.

Lens fiber connexin turnover and caspase-3-mediated cleavage are regulated alternately by phosphorylation

Lens connexins are phosphorylated in vivo; however, the function and regulation of the phosphorylation remain largely unknown. We have previously identified an in vivo phosphorylation site, Ser(364), at the COOH terminus of lens connexin (Cx) Cx45.6 and phosphorylation appears to regulate connexin protein turnover. To assess the specific mechanism of Ser(364) phosphorylation in Cx45.6, exogenous wild type and Ser(364) mutant Cx45.6 were expressed in primary lens cultures through retroviral infection. Cx45.6 turnover was attenuated primarily by proteasomal inhibitors and to a lesser extent by lysosomal inhibitors. Furthermore, the level of Cx45.6 protein in ubiquitin co-expressed cells was significantly reduced as compared to the cells expressing Cx45.6 alone. Moreover, overexpression of ubiquitin led to a more significant decrease in wild type Cx45.6 than Cx45.6(S364A), a mutant deficient of phosphorylation site at Ser(364), although we did not detect any difference in the levels of ubiquitination between wild type and mutant Cx45.6. Interestingly, the mutant mimicking constitutive phosphorylation, Cx45.6(S364D), partially prevented the cleavage of Cx45.6 by caspase-3. Together, our data suggest that phosphorylation of Cx45.6 at Ser(364) appears to stimulate Cx45.6 turnover primarily through proteasome pathway and this phosphorylation inhibits the cleavage of Cx45.6 by caspase-3. These findings provide further insights into regulatory mechanism of the specific phosphorylation of connexins in the lens.

Gap junction communication influences intercellular protein distribution in the lens

Lens transparency and high refractive index presumably depend on the appropriate arrangement and distribution of lens proteins among lens fiber cells. Intercellular gap junction channels formed by alpha3 and alpha8 connexins are known to transport small molecules, ions and water, but not proteins, in the lens. Mosaic expression of green fluorescent protein (GFP) in the lens is a useful marker for monitoring macromolecule distribution between fiber cells and for constructing three-dimensional images of living lens cells. In alpha3(-/-) alpha8(-/-) double knockout (DKO) lenses, three-dimensional images of GFP-positive cells demonstrate the changes of epithelial cell surfaces and insufficient elongation of inner fiber cells. Uniform distribution of GFP between inner lens fiber cells is observed in both wild-type and alpha3(-/-) lenses. In contrast, uniform GFP distribution is slightly delayed in alpha8(-/-) lenses and is abolished in DKO lenses. Without endogenous wild-type alpha3 and alpha8 connexins, knock-in alpha3 connexin (expressed under the alpha8 gene promoter) restores the uniform distribution of GFP protein in the lens. Thus, the presence of either alpha3 or alpha8 connexins seems sufficient to support the uniform distribution of GFP between differentiated lens fiber cells. Although the mechanism that drives GFP transport between fiber cells remains unknown, this work reveals that gap junction communication plays a novel role in the regulation of intercellular protein distribution in the lens.

Rho GDP dissociation inhibitor-mediated disruption of Rho GTPase activity impairs lens fiber cell migration, elongation and survival

To explore the role of the Rho GTPases in lens morphogenesis, we overexpressed bovine Rho GDP dissociation inhibitor (Rho GDI alpha), which serves as a negative regulator of Rho, Rac and Cdc42 GTPase activity, in a lens-specific manner in transgenic mice. This was achieved using a chimeric promoter of delta-crystallin enhancer and alpha A-crystallin, which is active at embryonic day 12. Several individual transgenic (Tg) lines were obtained, and exhibited ocular specific phenotype comprised of microphthalmic eyes with lens opacity. The overexpression of bovine Rho GDI alpha disrupted membrane translocation of Rho, Rac and Cdc42 GTPases in Tg lenses. Transgenic lenses also revealed abnormalities in the migration pattern, elongation and organization of lens fibers. These changes appeared to be associated with impaired organization of the actin cytoskeleton and cell-cell adhesions. At E14.5, the size of the Rho GDI alpha Tg lenses was larger compared to wild type (WT) and the central lens epithelium and differentiating fibers exhibited an abnormal increase of bromo-deoxy-uridine incorporation. Postnatal Tg eyes, however, were much smaller in size compared to WT eyes, revealing increased apoptosis in the disrupted lens fibers. Taken together, these data demonstrate a critical role for Rho GTPase-dependent signaling pathways in processes underlying morphogenesis, fiber cell migration, elongation and survival in the developing lens.

Intracellular calcium regulation of connexin43

The mechanism by which intracellular Ca(2+) concentration ([Ca(2+)](i)) regulates the permeability of gap junctions composed of connexin43 (Cx43) was investigated in HeLa cells stably transfected with this connexin. Extracellular addition of Ca(2+) in the presence of the Ca(2+) ionophore ionomycin produced a sustained elevation in [Ca(2+)](i) that resulted in an inhibition of the cell-to-cell transfer of the fluorescent dye Alexa fluor 594 (IC(50) of 360 nM Ca(2+)). The Ca(2+) dependency of this inhibition of Cx43 gap junctional permeability is very similar to that described in sheep lens epithelial cell cultures that express the three sheep lens connexins (Cx43, Cx44, and Cx49). The intracellular Ca(2+)-mediated decrease in cell-to-cell dye transfer was prevented by an inhibitor of calmodulin action but not by inhibitors of Ca(2+)/calmodulin-dependent protein kinase II or protein kinase C. In experiments that used HeLa cells transfected with a Cx43 COOH-terminus truncation mutant (Cx43(Delta257)), cell-to-cell coupling was similarly decreased by an elevation of [Ca(2+)](i) (IC(50) of 310 nM Ca(2+)) and similarly prevented by the addition of an inhibitor of calmodulin. These data indicate that physiological concentrations of [Ca(2+)](i) regulate the permeability of Cx43 in a calmodulin-dependent manner that does not require the major portion of the COOH terminus of Cx43.

Connexins in lens development and cataractogenesis

The lens is an avascular organ that transmits and focuses light images onto the retina. Intercellular gap junction channels, formed by at least three different connexin protein subunits, alpha1 (connexin43 or Gja1), alpha3 (connexin46 or Gja3) and alpha8 (connexin50 or Gja8), are utilized to transport metabolites, ions and water in the lens. In combination with physiological and biochemical analyses, recent genetic studies have significantly improved our understanding about the roles of diverse gap junction channels formed by alpha3 and alpha8 connexin subunits during lens development and cataract formation. These studies have demonstrated that alpha3 connexin is essential for lens transparency while alpha8 connexin is important for lens growth and transparency. Diverse gap junction channels formed by alpha3 and alpha8 subunits are important for the differentiation, elongation and maturation of lens fiber cells. Aberrant gap junction communication, caused by alterations of channel assembly, channel gating or channel conductance, can lead to different types of cataracts. These findings provide some molecular insights for essential roles of connexins and gap junctions in lens formation and the establishment and maintenance of lifelong lens transparency.

Gap junctional communication in morphogenesis

Gap junctions permit the direct passage of small molecules from the cytosol of one cell to that of its neighbor, and thus form a system of cell-cell communication that exists alongside familiar secretion/receptor signaling. Because of the rich potential for regulation of junctional conductance, and directional and molecular gating (specificity), gap junctional communication (GJC) plays a crucial role in many aspects of normal tissue physiology. However, the most exciting role for GJC is in the regulation of information flow that takes place during embryonic development, regeneration, and tumor progression. The molecular mechanisms by which GJC establishes local and long-range instructive morphogenetic cues are just beginning to be understood. This review summarizes the current knowledge of the involvement of GJC in the patterning of both vertebrate and invertebrate systems and discusses in detail several morphogenetic systems in which the properties of this signaling have been molecularly characterized. One model consistent with existing data in the fields of vertebrate left-right patterning and anterior-posterior polarity in flatworm regeneration postulates electrophoretically guided movement of small molecule morphogens through long-range GJC paths. The discovery of mechanisms controlling embryonic and regenerative GJC-mediated signaling, and identification of the downstream targets of GJC-permeable molecules, represent exciting next areas of research in this fascinating field.

Absence of alpha3 (Cx46) and alpha8 (Cx50) connexins leads to cataracts by affecting lens inner fiber cells

Lens development and transparency have been hypothesized to depend on intercellular gap junction channels, consisting of alpha3 (Cx46) and alpha8 (Cx50) connexin subunits, to transport metabolites, secondary messages and ions between lens cells. To evaluate this hypothesis, we have generated alpha3(-/-) alpha8(-/-) double knockout mice and characterized their lens phenotypes. Without gap junctions between lens fiber cells, alpha3(-/-) alpha8(-/-) lenses displayed severe cataracts resulting from cell swelling and degeneration of inner fibers while normal peripheral fiber cells continued to form throughout life. Neither an increase of degraded crystallins nor an increase of water-insoluble crystallins was found in alpha3(-/-) alpha8(-/-) lenses. However, a substantial reduction of gamma-crystallin proteins, but not alpha- and beta-crystallins, was detected. These results suggest that gap junction communication is important for maintaining lens homeostasis of inner fiber cells and that a loss of gap junctions leads to cataract formation as well as reductions of gamma-crystallin proteins and transcripts.

Structural and immunocytochemical alterations in eye lens fiber cells from Cx46 and Cx50 knockout mice

In the current study we describe the changes of overall organization of lens fiber cells in connexin 46 (Cx46) and connexin 50 (Cx50) knockout mice. Morphometric analyses and the application of immunocytochemical techniques revealed that in Cx46 knockout lens (Cx46 -/-), where Cx50 is expressed alone, the postnatal differentiation of secondary fiber cells proceeds faster and is characterized by an increased number of smaller fiber cells. Conversely, in Cx50 knockout mice (Cx50 -/-), the lenticular mass is considerably reduced and characterized by a small number of fiber cells added during the postnatal period. The process of terminal differentiation was impaired and generated larger fiber cells still possessing cytoplasmic organelles. Freeze-fracture and fracture labeling revealed that the junctional assembly, packing organization and topographic interactions between connexons and MP26 differed when Cx46 and Cx50 were co-assembled in the wild-type or expressed separately in the two distinct knockout phenotypes. Filipin cytochemistry provided indirect evidence that Cx46 and Cx50 expressed alone are recruited into different lipid environments. Our results represent the structural proof that interaction of connexins and MP26 contributes to the overall organization of the fiber cells.

The gating effect of calmodulin and calcium on the connexin50 hemichannel

Gap junction channels formed by connexin50 (Cx50) are critical for the maintenance of eye lens transparency, which is sensitive to pH and external Ca2+concentration, but the mechanism is still not clear. In this study we performed dye uptake-leakage assays, patch clamping and confocal co-localization experiments to confirm the function of calmodulin (CaM) and Ca2+in the Cx50 hemichannel. Below pH 7.4, lucifer yellow (LY)-preloaded Cx50-HeLa cells allow dye to leak out when washed with Ca2+-free solution or incubated in solution containing 50 μg/ml W7 (CaM inhibitor) first, then washed in solution containing 2 mM Ca2+, whereas little or no dye leakage was observed when LY-preloaded Cx50-HeLa cells were incubated in solution containing 2 mM Ca2+. Moreover, in the absence of Ca2+, polarizing pulses applied to Cx50-HeLa activated outward transmembrane currents, which were inhibited by 2 mM external Ca2+. When Cx50-HeLa cells were incubated with 2 mM Ca2+and 50 μg/ml W7, the transmembrane currents were activated again. This indicates that Ca2+and CaM play a gating role in Cx50 hemichannels. Either the chelation of Ca2+or the inhibition of CaM increased the permeability of Cx50 hemichannels. The same phenomena were observed below pH 6.5. Furthermore, CaM could be localized in gap junctions formed by Cx50 below pH 6.5. Our results demonstrate that CaM and Ca2+can cooperate in the gating control of Cx50 hemichannels.

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.

Effects of iron ions, protons and X rays on human lens cell differentiation

We have investigated molecular changes in cultured differentiating human lens epithelial cells exposed to high-energy accelerated iron-ion beams as well as to protons and X rays. In this paper, we present results on the effects of radiation on gene families that include or are related to DNA damage, cell cycle regulators, cell adhesion molecules, and cell cytoskeletal function. A limited microarray survey with a panel of cell cycle-regulated genes illustrates that irradiation with protons altered the gene expression pattern of human lens epithelial cells. A focus of our work is CDKN1A (p21(CIP1/WAF1)), a protein that we demonstrate here has a role in several pathways functionally related to LET-responsive radiation damage. We quantitatively assessed RNA and protein expression in a time course before and after single 4-Gy radiation doses and demonstrated that transcription and translation of CDKN1A are both temporally regulated after exposure. Furthermore, we show qualitative differences in the distribution of CDKN1A immunofluorescence signals after exposure to X rays, protons or iron ions, suggesting that LET effects likely play a role in the misregulation of gene function in these cells. A model of molecular and cellular events is proposed to account for precataractous changes in the human lens after exposure to low- or high-LET radiations.

Role of intramolecular interaction in connexin50: mediating the Ca2+-dependent binding of calmodulin to gap junction

Gap junction channels formed by connexin50 (Cx50) are critical for maintenance of eye lens transparency. Cleavage of the carboxyl terminus (CT) of Cx50 to produce truncated Cx50 (Cx50trunc) occurred naturally during maturation of lens fiber cells. The mechanism of its altered properties is under confirmation. It has been suggested that calmodulin (CaM) participates in gating some kinds of gap junction. Here, we performed confocal colocalization and co-immunoprecipitation experiments to study the relationships between Cx50 and CaM. Results exhibited that the CaM could colocalize Ca2+ dependently with CT in the linear area of cell-to-cell contact formed by Cx50trunc, while it could not localize in the linear area without expression of CT. Further study indicated that the CT could interact Ca2+ independently with the cytoplasmic loop (CL) of Cx50. These data put forward the importance of Ca2+-independent intramolecular interaction between CT and CL of Cx50, which mediate the Ca2+-dependent binding of CaM to Cx50. These intra- and intermolecular interactions may further improve our understanding of biological significance of the Cx50 in the eye lens.

CONNEXINS AND CELL SIGNALING IN DEVELOPMENT AND DISEASE

Gap junctions contain hydrophilic membrane channels that allow direct communication between neighboring cells through the diffusion of ions, metabolites, and small cell signaling molecules. They are made up of a hexameric array of polypeptides encoded by the connexin multi-gene family. Cell-cell communication mediated by connexins is crucial to various cellular functions, including the regulation of cell growth, differentiation, and development. Mutations in connexin genes have been linked to a variety of human diseases, including cardiovascular anomalies, peripheral neuropathy, deafness, skin disorders, and cataracts. In addition to their coupling function, recent studies suggest that connexin proteins may also mediate signaling. This could involve interactions with other protein partners that may play a role not only in connexin assembly, trafficking, gating and turnover, but also in the coordinate regulation of cell-cell communication with cell adhesion and cell motility. The integration of these cell functions is likely to be important in the role of gap junctions in development and disease.

Expression of connexin48.5, connexin44.1, and connexin43 during zebrafish (Danio rerio) lens development

Connexins (Cx), the protein units of gap junctions, play important roles in lens development and homeostasis. Here, we report the mRNA expression patterns of zebrafish Cx48.5, Cx44.1, Cx43 during lens development. The expression of all three connexins in the adult lens was first confirmed by reverse transcriptase-polymerase chain reaction. By whole-mount in situ hybridization, we detected Cx48.5 expression throughout the lens, except the lateral lens epithelium, at 36 hours postfertilization (hpf). The pattern remained the same at 2 days postfertilization (dpf). By 3 and 4 dpf, Cx48.5 expression was restricted to the differentiating lens fibers in the equatorial and medial regions. Cx44.1 was expressed in a similar manner as Cx48.5 from 36 hpf to 4 dpf. However, Cx44.1 expression was also detected in the lens at 24 hpf. Cx43 expression was detected throughout the lens at 24 and 36 hpf but became restricted to the lateral epithelium at later stages.

Connexin 48.5 is required for normal cardiovascular function and lens development in zebrafish embryos

Gap junctions are composed of connexin (Cx) proteins and mediate intercellular communication required for many developmental and physiological processes. Here we describe the isolation and characterization of Cx48.5, a zebrafish connexin with the highest sequence identity to mammalian Cx46. Expression analysis showed that Cx48.5 is expressed in the adult and embryonic lens and heart, adult testis, and transiently in the embryonic otic vesicles. Injection of Cx48.5 cRNA into Xenopus oocytes elicited intercellular electrical coupling with voltage sensitivity similar to mammalian Cx46. In single oocytes, Cx48.5 also induced large outward currents on depolarization, consistent with gap-junctional hemichannels. Disruption of Cx48.5 expression in embryos with antisense morpholino oligos (morpholinos) revealed that Cx48.5 has an essential role in the maintenance of lens homeostasis. The morpholino-treated embryos also developed small lenses and eyes as well as severe cardiovascular abnormalities.

Reversible pore block of connexin channels by cyclodextrins

Cyclodextrins (CDs), a series of hollow cyclic glucosaccharides, can reversibly block molecular permeation through channels formed by connexin-32 and/or connexin-26 reconstituted into liposomes. The character of the block changes as a function of the size of the CD relative to the connexin pore diameter, suggesting that the block occurs via entry of the CD into the pore lumen and occlusion of the permeability pathway. The block occurs only when the CD is applied to the side of the pore that is normally cytoplasmic and not from the side that is normally extracellular. The block is potentiated when organic analytes are sequestered in the hydrophobic interior of the CDs. CDs may be useful as molecular tools with which to explore the structure of the connexin pore and to alter molecular movement through connexin channels.

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.

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.

A retained selection cassette increases reporter gene expression without affecting tissue distribution in SPI3 knockout/GFP knock-in mice

The human serpin, proteinase inhibitor 6 (PI-6/SERPINB6), is a protease inhibitor expressed in many tissues. It inhibits a large number of proteases, including cathepsin G in granulocytes and monocytes. To determine the temporal and spatial distribution of PI-6, mice were generated in which exon 2 of the PI-6 ortholog SPI3 (Serpinb6) was replaced with a green fluorescent protein (GFP) reporter gene. This placed GFP under the control of the regulatory elements and initiation codon of the SPI3 gene. The neomycin selection cassette was flanked by loxP sites to allow excision from the targeted allele. GFP expression in heterozygous and SPI3-deficient mice accurately reflected the tissue distribution of SPI3 in all organs tested and allowed precise comparisons of expression levels. Interestingly, retention of the neomycin cassette in targeted mice resulted in 2-10-fold increases of GFP in leukocytes, but without affecting tissue-specific expression patterns. This is the first example of selection cassette retention specifically increasing reporter gene expression in targeted mice and reinforces the view that selection cassettes must be removed to avoid confounding effects on reporter gene expression patterns.