Unique and redundant connexin contributions to lens development

Differential regulation of Connexin50 and Connexin46 by PI3K signaling

Gap junction channels can modify their activity in response to cell signaling pathways. Here, we demonstrate that Connexin50 (Cx50) coupling, but not Connexin46 (Cx46), increased when co-expressed with a constitutively active p110α subunit of PI3K in Xenopus oocytes. In addition, inhibition of PI3K signaling by blocking p110α, or Akt, significantly decreased gap junctional conductance in Cx50 transfected HeLa cells, with no effect on Cx46. Alterations in coupling levels were not a result of reduced Cx50 unitary conductance, suggesting that changes in the number of active channels were responsible. These data indicate that Cx50 is specifically regulated by the PI3K signaling pathway.

A stochastic model of eye lens growth

The size and shape of the ocular lens must be controlled with precision if light is to be focused sharply on the retina. The lifelong growth of the lens depends on the production of cells in the anterior epithelium. At the lens equator, epithelial cells differentiate into fiber cells, which are added to the surface of the existing fiber cell mass, increasing its volume and area. We developed a stochastic model relating the rates of cell proliferation and death in various regions of the lens epithelium to deposition of fiber cells and radial lens growth. Epithelial population dynamics were modeled as a branching process with emigration and immigration between proliferative zones. Numerical simulations were in agreement with empirical measurements and demonstrated that, operating within the strict confines of lens geometry, a stochastic growth engine can produce the smooth and precise growth necessary for lens function.

Identification of a novel GJA3 mutation in congenital nuclear cataract

PURPOSE

Congenital cataract is a visual impairment that needs correction as early as possible after birth. This study aimed to identify whether genetic defects exist in a Chinese Han pedigree with congenital nuclear cataract.

METHODS

A family consisting of six members and three patients with nuclear cataract spanning three generations and 100 unrelated ethnically matched normal subjects were recruited in this study. Exome sequencing was performed in the 24-year-old proband, and Sanger sequencing was then conducted in other family members and 100 normal controls.

RESULTS

A novel missense variant, c.428G>A (p.G143E), in the gap junction protein-alpha 3 gene (GJA3) was identified in three patients of the family but unidentified in three family members without lens opacity and 100 normal controls.

CONCLUSIONS

A novel missense mutation, c.428G>A (p.G143E), in the GJA3 gene, localized to the cytoplasmic loop, was suggested to be the genetic cause of congenital nuclear cataract, which further expands the gene mutation spectrum. Our findings suggest that exome sequencing is a powerful and cost-effective tool to discover mutation(s) in disorders with high genetic and clinical heterogeneity. Further functional studies in the GJA3 gene mutations may help uncover pathogenic mechanisms of congenital cataract and therefore provide a possible genetic therapy for this disorder.

Connexin mutant embryonic stem cells and human diseases

Intercellular communication via gap junctions allows cells within multicellular organisms to share small molecules. The effect of such interactions has been elucidated using mouse gene knockout strategies. Although several mutations in human gap junction-encoding connexin (Cx) have been described, Cx mutants in mice do not always recapitulate the human disease. Among the 20 mouse Cxs, Cx26, Cx43, and Cx45 play roles in early cardiac or placental development, and disruption of the genes results in lethality that hampers further analyses. Embryonic stem cells (ESCs) that lack Cx43 or Cx45 have made analysis feasible in both in vitro differentiated cell cultures and in vivo chimeric tissues. The success of mouse ESCs studies is leading to the use of induced pluripotent stem cells to learn more about the pathogenesis of human Cx diseases. This review summarizes the current status of mouse Cx disruption models and ESC differentiation studies, and discusses their implication for understanding human Cx diseases.

Carboxy terminus and pore-forming domain properties specific to Cx37 are necessary for Cx37-mediated suppression of insulinoma cell proliferation

Connexin 37 (Cx37) suppresses cell proliferation when expressed in rat insulinoma (Rin) cells, an effect also manifest in vivo during vascular development and in response to tissue injury. Mutant forms of Cx37 with nonfunctional channels but normally localized, wild-type carboxy termini are not growth suppressive. Here we determined whether the carboxy-terminal (CT) domain is required for Cx37-mediated growth suppression and whether the Cx37 pore-forming domain can be replaced with the Cx43 pore-forming domain and still retain growth-suppressive properties. We show that despite forming functional gap junction channels and hemichannels, Cx37 with residues subsequent to 273 replaced with a V5-epitope tag (Cx37-273tr*V5) had no effect on the proliferation of Rin cells, did not facilitate G1-cell cycle arrest with serum deprivation, and did not prolong cell cycle time comparably to the wild-type protein. The chimera Cx43*CT37, comprising the pore-forming domain of Cx43 and CT of Cx37, also did not suppress proliferation, despite forming functional gap junctions with a permselective profile similar to wild-type Cx37. Differences in channel behavior of both Cx37-273tr*V5 and Cx43*CT37 relative to their wild-type counterparts and failure of the Cx37-CT to interact as the Cx43-CT does with the Cx43 cytoplasmic loop suggest that the Cx37-CT and pore-forming domains are both essential to growth suppression by Cx37.

Connexin50D47A decreases levels of fiber cell connexins and impairs lens fiber cell differentiation

PURPOSE

Substitutions of aspartate-47 (D47) of Connexin50 (Cx50) have been linked to autosomal dominant congenital cataracts in several human pedigrees. To elucidate the lens abnormalities caused by a substitution at this position, we studied No2 mice, which carry the Cx50D47A mutation and parallel the human pathology.

METHODS

Lenses from mice of different ages (neonatal to 4 months) were examined by dark-field and immunofluorescence microscopy. Protein levels were determined by immunoblotting using primary antibodies directed against connexins, other membrane proteins, crystallins, and proteins residing in different organelles.

RESULTS

Lenses of both heterozygous and homozygous Cx50D47A mice had cataracts and were smaller than those of wild-type littermates. Levels of Cx50 were severely reduced in mutant animals as compared with those in wild-type mice (<20% in heterozygotes and ≤3% in homozygotes). Levels of Cx46 and aquaporin0 were also decreased, but to a lesser extent. The immunostaining pattern of lens connexins was altered in mutant animals. The lenses of Cx50D47A mice showed persistence of nuclear remnants in deep regions of the lens and elevated levels of H3 histone and the mitochondrial protein, Tom20. γ-Crystallin levels were decreased in lenses of all mutant mice, and β-crystallins were reduced in homozygotes.

CONCLUSIONS

These data suggest that mice expressing Cx50D47A develop cataracts due to a severe decrease in the abundance of functional connexin channels. They also implicate Cx50 in fiber cell differentiation, since mutant lenses showed impaired degradation of organelles and decreased levels of some crystallins.

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.

Human articular chondrocytes express multiple gap junction proteins: differential expression of connexins in normal and osteoarthritic cartilage

Osteoarthritis (OA) is the most common joint disease and involves progressive degeneration of articular cartilage. The aim of this study was to investigate if chondrocytes from human articular cartilage express gap junction proteins called connexins (Cxs). We show that human chondrocytes in tissue express Cx43, Cx45, Cx32, and Cx46. We also find that primary chondrocytes from adults retain the capacity to form functional voltage-dependent gap junctions. Immunohistochemistry experiments in cartilage from OA patients revealed significantly elevated levels of Cx43 and Cx45 in the superficial zone and down through the next approximately 1000 μm of tissue. These zones corresponded with regions damaged in OA that also had high levels of proliferative cell nuclear antigen. An increased number of Cxs may help explain the increased proliferation of cells in clusters that finally lead to tissue homeostasis loss. Conversely, high levels of Cxs in OA cartilage reflect the increased number of adjacent cells in clusters that are able to interact directly by gap junctions as compared with hemichannels on single cells in normal cartilage. Our data provide strong evidence that OA patients have a loss of the usual ordered distribution of Cxs in the damaged zones and that the reductions in Cx43 levels are accompanied by the loss of correct Cx localization in the nondamaged areas.

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.

Connexins and Cap-independent translation: role of internal ribosome entry sites

Cap-independent translation using an internal ribosome entry site instead of the 5'-Cap structure has been discovered in positive-sense RNA viruses and eukaryotic genomes including a subset of gap junction forming connexins genes. With a growing number of mutations found in human connexin genes and studies on genetically modified mouse models mechanisms highlighting the important role of gap junctional communication in multicellular organism it is obvious that mechanism need to be in place to preserve this critical property even under conditions when Cap-mediated translation is scrutinized. To ensure sustained gap junctional communication, rapid initiation of translation of preexisting connexin mRNAs is one possibility, and the presence of internal ribosome entry sites in gap junction genes comply with such a requirement. In this review, we will summarize past and recent findings to build a case for IRES mediated translation as an alternative regulatory pathway facilitating gap junctional communication. This article is part of a Special Issue entitled Electrical Synapses.

Analysis of intercellular communication by flexible hydrodynamic gating on a microfluidic chip

Intercellular Ca(2+) waves are propagation of Ca(2+) transients among cells that could be initiated by chemical stimulation. Current methods for analyzing intercellular Ca(2+) waves are difficult to realize localized chemical stimulations upon the target cell without interfering with adjacent contacting cells. In this paper, a simple and flexible microfluidic method was developed for investigating the intercellular communication of Ca(2+) signals. A cross-patterned microfluidic chip was designed and fabricated with polydimethylsiloxane as the structural material. Localized chemical stimulation was achieved by a new strategy based on hydrodynamic gating technique. Clusters of target cells were seeded at the location within 300 μm downstream of the intersection of the cross-shaped microchannel. Confined lateral molecular diffusion largely minimized the interference from diffusion-induced stimulation of adjacent cells. Localized stimulation of the target cell with adenosine 5'-triphosphate successfully induced the propagation of intercellular Ca(2+) waves among a population of adjacent contacting cells. Further inhibition studies verified that the propagation of calcium signals among NIH-3 T3 cells was dependent on direct cytosolic transfer via gap junctions. The developed microfluidic method provides a versatile platform for investigating the dynamics of intercellular communications.

The role of Connexin 46 promoter in lens and other hypoxic tissues

Gap junctions are multimeric membrane protein channels that connect the cytoplasm of one cell to another. Much information about connexins regards electrophysiology and channel function but relatively little information is known about non-channel functions of connexins. Lens connexins, Cx43, Cx46 and Cx50, have been extensively studied for their role in lens homeostasis. Connexins allow the movement of small metabolically relevant molecules and ions between cells and this action in the lens prevents cataract formation. Interruption of Cx46 channel function leads to cataract formation due to dysregulation of lens homeostasis. The loss of Cx46 upregulates Cx43 in lens cell culture and suppresses tumor growth in breast and retinoblastoma tumor xenografts. Upregulation of Cx46 in hypoxic tissues has been noted and may be due in part to the effects of hypoxia and HIF activators. Here, we report that the Cx46 promoter is regulated by hypoxia and also offer speculation about the role of Cx46 in lens differentiation and solid tumor growth.

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

Connexin multi-site phosphorylation: mass spectrometry-based proteomics fills the gap

Connexins require an integrated network for protein synthesis, assembly, gating, internalization, degradation and feedback control that are necessary to regulate the biosynthesis, and turnover of gap junction channels. At the most fundamental level, the introduction of sequence-altering, modifications introduces changes in protein conformation, activity, charge, stability and localization. Understanding the sites, patterns and magnitude of protein post-translational modification, including phosphorylation, is absolutely critical. Historically, the examination of connexin phosphorylation has been placed within the context that one or small number of sites of modification strictly corresponds to one molecular function. However, the release of high-profile proteomic datasets appears to challenge this dogma by demonstrating connexins undergo multiple levels of multi-site phosphorylation. With the growing prominence of mass spectrometry in biology and medicine, we are now getting a glimpse of the richness of connexin phosphate signals. Having implications to health and disease, this review provides an overview of technologies in the context of targeted and discovery proteomics, and further discusses how these techniques are being applied to "fill the gaps" in understanding of connexin post-translational control. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Analysis of intercellular calcium signaling using microfluidic adjustable laminar flow for localized chemical stimulation

The propagation of intercellular calcium signals provides a mechanism to coordinate cell population activity, which is essential for regulating cell behavior and organ development. However, existing analytical methods are difficult to realize localized chemical stimulation of a single cell among a population of cells that are in close contact with one another for studying the propagation of calcium wave. In this work, a microfluidic method is presented for the analysis of contact-dependent propagation of intercellular calcium wave induced by extracellular ATP using multiple laminar flows. Adjacent cells were seeded ∼300 μm downstream the intersection of a Y-shaped microchannel with negative pressure pulses. Consequently, the lateral diffusion distance of the chemical at cell locations was limited to ∼26 μm with a total flow rate of 20 μL min(-1), which prevented the interference of diffusion-induced cellular responses. Localized stimulation of the target cell with ATP induced the propagation of intercellular calcium wave among the cell population. In addition, studies on the spread of intercellular calcium wave under octanol inhibition allowed us to characterize the gap junction mediated cell-cell communication. Thus, this novel device will provide a versatile platform for intercellular signal transduction studies and high throughput drug screening.

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.

Functional redundancy and compensation among members of gap junction protein families?

Gap junctions are intercellular conduits for small molecules made up by protein subunits called connexins. A large number of connexin genes were found in mouse and man, and most cell types express several connexins, lending support to the view that redundancy and compensation among family members exist. This review gives an overview of the current knowledge on redundancy and functional compensation - or lack thereof. It takes into account the different properties of connexin subunits which comprise gap junctional intercellular channels, but also the compatibility of connexins in gap junctions. Most insight has been gained by the investigation of mice deficient for one or more connexins and transgenic mice with functional replacement of one connexin gene by another. Most single deficient mice show phenotypical alterations limited to critical developmental time points or to specific organs and tissues, while mice doubly deficient for connexins expressed in the same cell type usually show more severe phenotypical alterations. Replacement of a connexin by another connexin in some cases gave rise to rescue of phenotypical alterations of connexin deficiencies, which were restricted to specific tissues. In many tissues, connexin substitution did not restore phenotypical alterations of connexin deficiencies, indicating that connexins are specialized in function. In some cases, fatal consequences arose from the replacement. The current consensus gained from such studies is that redundancy and compensation among connexins exists at least to a limited extent. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Functional interactions between polydnavirus and host cellular innexins

Polydnaviruses are double-stranded DNA viruses associated with some subfamilies of ichneumonoid parasitoid wasps. Polydnavirus virions are delivered during wasp parasitization of a host, and virus gene expression in the host induces alterations of host physiology. Infection of susceptible host caterpillars by the polydnavirus Campoletis sonorensis ichnovirus (CsIV) leads to expression of virus genes, resulting in immune and developmental disruptions. CsIV carries four homologues of insect gap junction genes (innexins) termed vinnexins, which are expressed in multiple tissues of infected caterpillars. Previously, we demonstrated that two of these, VinnexinD and VinnexinG, form functional gap junctions in paired Xenopus oocytes. Here we show that VinnexinQ1 and VinnexinQ2, likewise, form junctions in this heterologous system. Moreover, we demonstrate that the vinnexins interact differentially with the Innexin2 orthologue of an ichnovirus host, Spodoptera frugiperda. Cell pairs coexpressing a vinnexin and Innexin2 or pairs in which one cell expresses a vinnexin and the neighboring cell Innexin2 assemble functional junctions with properties that differ from those of junctions composed of Innexin2 alone. These data suggest that altered gap junctional intercellular communication may underlie certain cellular pathologies associated with ichnovirus infection of caterpillar hosts.

A functional channel is necessary for growth suppression by Cx37

Connexin 37 (Cx37) profoundly suppresses the proliferation of rat insulinoma (Rin) cells by unknown mechanisms. To determine whether a functional pore domain is necessary for Cx37-mediated growth suppression, we introduced a mutation that converted threonine 154 into alanine (T154A). Like other connexins mutated at the homologous site, Cx37-T154A localized to appositional membrane but failed to form functional channels and exerted a dominant-negative effect on coexpressed wild-type Cx37 or Cx43. Unlike the wild-type protein, Cx37-T154A did not suppress the proliferation of Rin cells and did not, with serum deprivation, result in cell cycle arrest. Furthermore, progression through the cell cycle was unaffected by expression of Cx37-T154A. These results indicate that a pore-forming domain that is able to form functional channels is essential for the anti-proliferative, cell-cycle arrest and serum-sensitivity effects of Cx37, and furthermore that the normally localized C-terminal domain is not sufficient for these effects of Cx37.

PKCγ, role in lens differentiation and gap junction coupling

PURPOSE

To determine the role of PKCγ in the regulation of gap junction coupling in the normal lens, we have compared the properties of coupling in lenses from wild type (WT) and PKC-γ knockout (KO) mice.

METHODS

Western blotting, confocal immunofluorescence microscopy, immunoprecipitation, RT-PCR and quantitative real time PCR were used to study gap junction protein and message expression; gap junction coupling conductance and pH gating were measured in intact lenses using impedance studies.

RESULTS

There were no gross differences in size, clarity, or expression of full-length Cx46 or Cx50 in lenses from WT and PKCγ KO mice. However, total Cx43 protein expression was ~150% higher in the KO lenses. In WT lenses, Cx43 was found only in epithelial cells whereas in KO lenses, its expression continued into the fiber cells. Gap junction coupling conductance in the differentiating fibers (DF) of PKCγ KO lenses was 34% larger than that of WT. In the mature fiber (MF), the effect was much larger with the KO lenses having an 82% increase in coupling over WT. pH gating of the DF fibers was not altered by the absence of PKCγ.

CONCLUSION

PKCγ has a major role in the regulation of gap junction expression and coupling in the normal lens.

Lens intracellular hydrostatic pressure is generated by the circulation of sodium and modulated by gap junction coupling

We recently modeled fluid flow through gap junction channels coupling the pigmented and nonpigmented layers of the ciliary body. The model suggested the channels could transport the secretion of aqueous humor, but flow would be driven by hydrostatic pressure rather than osmosis. The pressure required to drive fluid through a single layer of gap junctions might be just a few mmHg and difficult to measure. In the lens, however, there is a circulation of Na(+) that may be coupled to intracellular fluid flow. Based on this hypothesis, the fluid would cross hundreds of layers of gap junctions, and this might require a large hydrostatic gradient. Therefore, we measured hydrostatic pressure as a function of distance from the center of the lens using an intracellular microelectrode-based pressure-sensing system. In wild-type mouse lenses, intracellular pressure varied from ∼330 mmHg at the center to zero at the surface. We have several knockout/knock-in mouse models with differing levels of expression of gap junction channels coupling lens fiber cells. Intracellular hydrostatic pressure in lenses from these mouse models varied inversely with the number of channels. When the lens' circulation of Na(+) was either blocked or reduced, intracellular hydrostatic pressure in central fiber cells was either eliminated or reduced proportionally. These data are consistent with our hypotheses: fluid circulates through the lens; the intracellular leg of fluid circulation is through gap junction channels and is driven by hydrostatic pressure; and the fluid flow is generated by membrane transport of sodium.

A knock-in mouse model for the R120G mutation of αB-crystallin recapitulates human hereditary myopathy and cataracts

An autosomal dominant missense mutation in αB-crystallin (αB-R120G) causes cataracts and desmin-related myopathy, but the underlying mechanisms are unknown. Here, we report the development of an αB-R120G crystallin knock-in mouse model of these disorders. Knock-in αB-R120G mice were generated and analyzed with slit lamp imaging, gel permeation chromatography, immunofluorescence, immunoprecipitation, histology, and muscle strength assays. Wild-type, age-matched mice were used as controls for all studies. Both heterozygous and homozygous mutant mice developed myopathy. Moreover, homozygous mutant mice were significantly weaker than wild-type control littermates at 6 months of age. Cataract severity increased with age and mutant gene dosage. The total mass, precipitation, and interaction with the intermediate filament protein vimentin, as well as light scattering of αB-crystallin, also increased in mutant lenses. In skeletal muscle, αB-R120G co-aggregated with desmin, became detergent insoluble, and was ubiquitinated in heterozygous and homozygous mutant mice. These data suggest that the cataract and myopathy pathologies in αB-R120G knock-in mice share common mechanisms, including increased insolubility of αB-crystallin and co-aggregation of αB-crystallin with intermediate filament proteins. These knock-in αB-R120G mice are a valuable model of the developmental and molecular biological mechanisms that underlie the pathophysiology of human hereditary cataracts and myopathy.

Zebrafish cx30.3: identification and characterization of a gap junction gene highly expressed in the skin

We have identified and characterized a zebrafish connexin, Cx30.3. Sequence similarity analyses suggested that Cx30.3 was orthologous to both mammalian Cx26 and Cx30, known to play important roles in the skin and inner ear of mammals. Analysis of mRNA expression showed that Cx30.3 was present in early embryos, and was highly abundant in skin, but also detected in other tissues including fins, inner ear, heart, and the retina. Injection of Cx30.3 cRNA into Xenopus oocytes elicited robust intercellular coupling with voltage gating sensitivity similar to mammalian Cx26 and Cx30. The similarities in functional properties and expression patterns suggest that Cx30.3, like mammalian Cx26 and Cx30, may play a significant role in skin development, hearing, and balance in zebrafish. Thus, zebrafish could potentially serve as an excellent model to study disorders of the skin and deafness that result from human connexin mutations.

Properties of connexin 46 hemichannels in dissociated lens fiber cells

PURPOSE

To characterize the properties of connexin 46 hemichannels in differentiating fiber cells isolated from mouse lenses.

METHODS

Differentiating fiber cells were isolated from mouse lenses using collagenase. Cellular localization of connexin 50 (Cx50) and connexin 46 (Cx46) was assessed by immunofluorescence. Membrane currents were recorded using whole cell patch clamping. Dye uptake was measured using time-lapse imaging.

RESULTS

In freshly dissociated fiber cells isolated from knockout Cx50 (KOCx50) mouse lenses, removal of external divalent cations induced a macroscopic current composed of large conductance channels. This current was reduced at a holding potential of -60 mV, activated on depolarization, and had a reversal potential near 0 mV. These properties were very similar to those of Cx46 hemichannel currents recorded in single Xenopus oocytes. If the currents observed in divalent cation-free Ringer's solution were due to Cx46 hemichannel opening, then dye influx by gap junctional/hemichannel permeable dyes should be measurable in the fiber cells. To measure dye influx, the authors used the positively charged dyes, propidium iodide (PrI) and 4'-6-diamidino-2-phenylindole (DAPI). In the absence of external calcium, fiber cells took up both dyes. Furthermore, dye influx could be inhibited by hemichannel blockers. To confirm that this current was due to Cx46 hemichannels, the authors studied fiber cells isolated from the lenses of double knockout (Cx46(-/-); Cx50(-/-)) mice and demonstrated that both the calcium-sensitive conductance and dye influx were absent.

CONCLUSIONS

These results show that Cx46 can form functional hemichannels in the nonjunctional membrane of fiber cells.

Gap junction permeability: selectivity for anionic and cationic probes

Gap junction channels formed by different connexins exhibit specific permeability to a variety of larger solutes including second messengers, polypeptides, and small interfering RNAs. Here, we report the permeability of homotypic connexin26 (Cx26), Cx40, Cx43, and Cx45 gap junction channels stably expressed in HeLa cells to solutes with different size and net charge. Channel permeability was determined using simultaneous measurements of junctional conductance and the cell-cell flux of a fluorescent probe. All four connexins allowed passage of both cationic and anionic probes, but the transfer rates were connexin dependent. The negatively charged probes [Lucifer yellow (LY; median axial diameter 9.9 Å, charge -2), carboxyfluorescein (CF; 8.2 Å; -2), and Alexa Fluor350 (AF350, 5.4 Å; -1)] exhibited the following permeability order: Cx43 > Cx45 > Cx26 > Cx40. In contrast, for the positively charged species permeability, the orders were as follows: Cx26 ≈ Cx43 ≈ Cx40 ≈ Cx45 for N,N,N-trimethyl-2-[methyl-(7-nitro-2,1,3-benzoxadiol-4-yl) amino] ethanaminium (NBD-m-TMA; 5.5 Å, +1) and Cx26 ≥ Cx43 ≈ Cx40 > Cx45 for ethidium bromide (10.3 Å, +1). Comparison of probe permeability relative to K(+) revealed that Cx43 and Cx45 exhibited similar permeability for NBD-m-TMA and AF350, indicating weak charge selectivity. However, lesser transfer of CF and LY through Cx45 relative to Cx43 channels suggests stronger size-dependent discrimination of solute. The permeability of NBD-m-TMA for Cx40 and Cx26 channels was approximately three times higher than to anionic AF350 despite the fact that both have similar minor diameters, suggesting charge selectivity. In conclusion, these results confirm that channels formed from individual connexins can discriminate for solutes based on size and charge, suggesting that channel selectivity may be a key factor in cell signaling.

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.

Neuronal connexin-36 can functionally replace connexin-45 in mouse retina but not in the developing heart

The gap junction protein connexin-45 (Cx45) is expressed in the conduction system of the heart and in certain neurons of the retina and brain. General and cardiomyocyte-directed deficiencies of Cx45 in mice lead to lethality on embryonic day 10.5 as a result of cardiovascular defects. Neuron-directed deletion of Cx45 leads to defects in transmission of visual signals. Connexin-36 (Cx36) is co-expressed with Cx45 in certain types of retinal interneurons. To determine whether these two connexins have similar functions and whether Cx36 can compensate for Cx45, we generated knock-in mice in which DNA encoding Cx45 was replaced with that encoding Cx36. Neuron-directed replacement of Cx45 with Cx36 resulted in viable animals. Electroretinographic and neurotransmitter coupling analyses demonstrated functional compensation in the retina. By contrast, general and cardiomyocyte-directed gene replacement led to lethality on embryonic day 11.5. Mutant embryos displayed defects in cardiac morphogenesis and conduction. Thus, functional compensation of Cx45 by Cx36 did not occur during embryonic heart development. These data suggest that Cx45 and Cx36 have similar functions in the retina, whereas Cx45 fulfills special functions in the developing heart that cannot be compensated by Cx36.

Gap junctions

Gap junctions are aggregates of intercellular channels that permit direct cell-cell transfer of ions and small molecules. Initially described as low-resistance ion pathways joining excitable cells (nerve and muscle), gap junctions are found joining virtually all cells in solid tissues. Their long evolutionary history has permitted adaptation of gap-junctional intercellular communication to a variety of functions, with multiple regulatory mechanisms. Gap-junctional channels are composed of hexamers of medium-sized families of integral proteins: connexins in chordates and innexins in precordates. The functions of gap junctions have been explored by studying mutations in flies, worms, and humans, and targeted gene disruption in mice. These studies have revealed a wide diversity of function in tissue and organ biology.

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.

Gap junctions in inherited human disease

Gap junctions (GJ) provide direct intercellular communication. The structures underlying these cell junctions are membrane-associated channels composed of six integral membrane connexin (Cx) proteins, which can form communicating channels connecting the cytoplasms of adjacent cells. This provides coupled cells with a direct pathway for sharing ions, nutrients, or small metabolites to establish electrical coupling or balancing metabolites in various tissues. Genetic approaches have uncovered a still growing number of mutations in Cxs related to human diseases including deafness, skin disease, peripheral and central neuropathies, cataracts, or cardiovascular dysfunctions. The discovery of a growing number of inherited human disorders provides an unequivocal demonstration that gap junctional communication is crucial for diverse physiological processes.

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.

A mutant connexin50 with enhanced hemichannel function leads to cell death

PURPOSE

To determine the consequences of expression of a novel connexin50 (CX50) mutant identified in a child with congenital total cataracts.

METHODS

The GJA8 gene was directly sequenced. Formation of functional channels was assessed by the two-microelectrode voltage-clamp

METHOD

Connexin protein levels and distribution were assessed by immunoblot analysis and immunofluorescence. The proportion of apoptotic cells was determined by flow cytometry.

RESULTS

Direct sequencing of the GJA8 gene identified a 137 G>T transition that resulted in the replacement of glycine by valine at position 46 of the coding region of CX50 (CX50G46V). Both CX50 and CX50G46V induced gap junctional currents in pairs of Xenopus oocytes. In single Xenopus oocytes, CX50G46V induced connexin hemichannel currents that were activated by removal of external calcium; their magnitudes were much higher than those in oocytes injected with similar amounts of CX50 cRNA. When expressed in HeLa cells under the control of an inducible promoter, both CX50 and CX50G46V formed gap junctional plaques. Induction of CX50G46V expression led to a decrease in the number of cells and an increase in the proportion of apoptotic cells. CX50G46V-induced cell death was prevented by high concentrations of extracellular calcium ions.

CONCLUSIONS

Unlike previously characterized CX50 mutants that exhibit impaired trafficking and/or lack of function, CX50G46V traffics properly to the plasma membrane and forms functional hemichannels and gap junction channels; however, it causes cell death even when expressed at minute levels. The biochemical results indirectly suggest a potential novel mechanism by which connexin mutants could lead to cataracts: cytotoxicity due to enhanced hemichannel function.

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

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

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.

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.

The stratified syncytium of the vertebrate lens

The fusion of cells to generate syncytial tissues is a crucial event in the development of many organisms. In the lens of the vertebrate eye, proteins and other macromolecules diffuse from cell to cell via the large molecule diffusion pathway (LMDP). We used the tamoxifen-induced expression of GFP to investigate the nature and role of the LMDP in living, intact lenses. Our data indicate that the LMPD preferentially connects cells lying within a stratum of the lens cortex and that formation of the LMPD depends on the expression of Lim2, a claudin-like molecule. The conduits for intercellular protein exchange are most likely regions of partial cellular fusion, which are commonly observed in wild-type lenses but rare or absent in Lim2-deficient lenses. The observation that lens tissue constitutes a stratified syncytium has implications for the transparency, refractive function and pathophysiology of the tissue.

Substitution of connexin40 with connexin45 prevents hyperreninemia and attenuates hypertension

Connexins (Cxs) are a family of transmembrane proteins that form gap junctions with unique and redundant biophysical functions. Juxtaglomerular cells express Cx40, which is crucial to the control of renin secretion by blood pressure and angiotensin II, and mice that lack Cx40 have high plasma renin and hypertension. To examine whether normal juxtaglomerular cell function depends on the unique properties of Cx40, we measured renin release in mice where the coding sequence for Cx40 was replaced by that for Cx45, using the knock-in method. We first found that the knock-in strategy indeed resulted in expression of Cx45 but not Cx40 in the juxtaglomerular cells of these mice. The plasma renin concentration of the knock-in mice was similar to that in wild-type mice. The high blood pressure of the Cx40 knockout mice was significantly reduced when Cx45 was knocked into the locus but remained mildly elevated compared to wild-type mice. Blockade of angiotensin II formation by enalapril increased the plasma renin concentration in wild-type and the Cx45 knock-in mice but not in the Cx40 knockout mice. Infusion of angiotensin II into isolated perfused kidneys results in decreased renin release, a phenomenon that was attenuated in the Cx40 knockout mice. However, in the Cx45 knock-in mice, angiotensin II suppressed renin release similar to its effect in wild type mice. Unilateral renal artery stenosis increased the plasma renin concentration and blood pressure in both the wild-type and the Cx45 knock-in mice but not in the Cx40 knockout mice. Since Cx40 can be replaced by Cx45, a connexin with a significantly lower conductivity, we suggest that the regulation of renin release is not dependent on the unique electrical properties of these channel proteins.

Diverse deafness mechanisms of connexin mutations revealed by studies using in vitro approaches and mouse models

Mutations in connexins (Cxs), the constitutive protein subunits of gap junction (GJ) intercellular channels, are one of the most common human genetic defects that cause severe prelingual non-syndromic hearing impairments. Many subtypes of Cxs (e.g., Cxs 26, 29, 30, 31, 43) and pannexins (Panxs) are expressed in the cochlea where they contribute to the formation of a GJ-based intercellular communication network. Cx26 and Cx30 are the predominant cochlear Cxs and they co-assemble in most GJ plaques to form hybrid GJs. The cellular localization of specific Cx subtypes provides a basis for understanding the molecular structure of GJs and hemichannels in the cochlea. Information about the interactions among the various co-assembled Cx partners is critical to appreciate the functional consequences of various types of genetic mutations. In vitro studies of reconstituted GJs in cell lines have yielded surprisingly heterogeneous mechanisms of dysfunction caused by various Cx mutations. Availability of multiple lines of Cx-mutant mouse models has provided some insight into the pathogenesis processes in the cochlea of deaf mice. Here we summarize recent advances in understanding the structure and function of cochlear GJs and give a critical review of current findings obtained from both in vitro studies and mouse models on the mechanisms of Cx mutations that lead to cell death in the cochlea and hearing loss.

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.

Gap junction mediated intercellular metabolite transfer in the cochlea is compromised in connexin30 null mice

Connexin26 (Cx26) and connexin30 (Cx30) are two major protein subunits that co-assemble to form gap junctions (GJs) in the cochlea. Mutations in either one of them are the major cause of non-syndromic prelingual deafness in humans. Because the mechanisms of cochlear pathogenesis caused by Cx mutations are unclear, we investigated effects of Cx30 null mutation on GJ-mediated ionic and metabolic coupling in the cochlea of mice. A novel flattened cochlear preparation was used to directly assess intercellular coupling in the sensory epithelium of the cochlea. Double-electrode patch clamp recordings revealed that the absence of Cx30 did not significantly change GJ conductance among the cochlear supporting cells. The preserved electrical coupling is consistent with immunolabeling data showing extensive Cx26 GJs in the cochlea of the mutant mice. In contrast, dye diffusion assays showed that the rate and extent of intercellular transfer of multiple fluorescent dyes (including a non-metabolizable D-glucose analogue, 2-NBDG) among cochlear supporting cells were severely reduced in Cx30 null mice. Since the sensory epithelium in the cochlea is an avascular organ, GJ-facilitated intercellular transfer of nutrient and signaling molecules may play essential roles in cellular homeostasis. To test this possibility, NBDG was used as a tracer to study the contribution of GJs in transporting glucose into the cochlear sensory epithelium when delivered systemically. NBDG uptake in cochlear supporting cells was significantly reduced in Cx30 null mice. The decrease was also observed with GJ blockers or glucose competition, supporting the specificity of our tests. These data indicate that GJs facilitate efficient uptake of glucose in the supporting cells. This study provides the first direct experimental evidence showing that the transfer of metabolically-important molecules in cochlear supporting cells is dependent on the normal function of GJs, thereby suggesting a novel pathogenesis process in the cochlea for Cx-mutation-linked deafness.

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.

Connexin 37 profoundly slows cell cycle progression in rat insulinoma cells

In addition to providing a pathway for intercellular communication, the gap junction-forming proteins, connexins, can serve a growth-suppressive function that is both connexin and cell-type specific. To assess its potential growth-suppressive function, we stably introduced connexin 37 (Cx37) into connexin-deficient, tumorigenic rat insulinoma (Rin) cells under the control of an inducible promoter. Proliferation of these iRin37 cells, when induced to express Cx37, was profoundly slowed: cell cycle time increased from 2 to 9 days. Proliferation and cell cycle time of Rin cells expressing Cx40 or Cx43 did not differ from Cx-deficient Rin cells. Cx37 suppressed Rin cell proliferation irrespective of cell density at the time of induced expression and without causing apoptosis. All phases of the cell cycle were prolonged by Cx37 expression, and progression through the G(1)/S checkpoint was delayed, resulting in accumulation of cells at this point. Serum deprivation augmented the effect of Cx37 to accumulate cells in late G(1). Cx43 expression also affected cell cycle progression of Rin cells, but its effects were opposite to Cx37, with decreases in G(1) and increases in S-phase cells. These effects of Cx43 were also augmented by serum deprivation. Cx-deficient Rin cells were unaffected by serum deprivation. Our results indicate that Cx37 expression suppresses cell proliferation by significantly increasing cell cycle time by extending all phases of the cell cycle and accumulating cells at the G(1)/S checkpoint.

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.

Elevated amounts of myocilin in the aqueous humor of transgenic mice cause significant changes in ocular gene expression

Myocilin is a 55-57kDa secreted glycoprotein and member of the olfactomedin family, which is mutated in some forms of primary open-angle glaucoma. To assess the effects of elevated amounts of myocilin on aqueous humor outflow dynamics in an in vivo system, transgenic betaB1-crystallin-MYOC mice have been developed that strongly overexpress myocilin in their eyes. The transgenic overexpression of myocilin results in an almost five-fold increase of secreted normal myocilin in the aqueous humor of betaB1-crystallin-MYOC mice. In the present study, we wanted to use betaB1-crystallin-MYOC as a tool to identify the response of ocular tissues to the presence of higher than normal amounts of myocilin, and to identify changes in gene expression that could help to shed light on the functional in vivo properties of myocilin. RNA was isolated from ocular tissues of betaB1-crystallin-MYOC mice and wild-type littermates. Changes in gene expression were determined by hybridization of gene microarrays and confirmed by real time RT-PCR and Western blotting. The expression of genes that had been found to be differentially regulated in betaB1-crystallin-MYOC mice was further analyzed in cultured human trabecular meshwork (HTM) cells treated with recombinant myocilin. Although betaB1-crystallin-MYOC mice do not have an obvious phenotype, a statistically significant up- and downregulation of several distinct genes was found when compared to gene expression in wild-type littermates. Among the genes that were found to be differentially regulated were Wasl, Ceacam1, and Spon2, which are involved in cell adhesion and cell-matrix interactions. Differences in expression were also found for Six1 which encodes for a transcription factor, and for Pftk1 whose gene product is a cdc2-related protein kinase. The expression of these genes was also found to be regulated in vitro in HTM cells treated with recombinant myocilin. Substantially higher amounts in ocular tissues of betaB1-crystallin-MYOC mice were found for connexin 46 and alphaB-crystallin. In addition, several genes that encode for olfactomedin proteins showed distinct changes in expression. Olfml3 was significantly downregulated, while Lphn1, Lphn2, and Lphn3 were significantly upregulated. Our findings support a role for myocilin in modulating cellular adhesion, and suggest functional processes that involve other proteins of the olfactomedin family.