Disruption of alpha3 connexin gene leads to proteolysis and cataractogenesis in mice

Abnormal skeletal and cardiac development, cardiomyopathy, muscle atrophy and cataracts in mice with a targeted disruption of the Nov (Ccn3) gene

BACKGROUND

Signals from the extracellular environment control many aspects of cell behaviour including proliferation, survival, differentiation, adhesion and migration. It is increasingly evident that these signals can be modulated by a group of matricellular proteins called the CCN family. CCN proteins have multiple domains through which they regulate the activities of a variety of signalling molecules including TGFbeta, BMPs and integrins, thereby influencing a wide range of processes in development and disease. Whilst the developmental roles of CCN1 and CCN2 have been elucidated, very little is known about the function of CCN3 (NOV). To investigate this, we have generated mice carrying a targeted mutation in the Nov gene (Novdel3) which reveal for the first time its diverse functions in embryos and adults.

RESULTS

By replacing Nov exon 3 with a TKneomycin cassette, we have generated Novdel3-/- mice which produce no full length NOV protein and express at a barely detectable level a mutant NOV protein that lacks the VWC domain. In Novdel3-/- embryos, and to a lesser extent in Novdel3+/- embryos, development of the appendicular and axial skeleton was affected with enlarged vertebrae, elongated long bones and digits, delayed ossification, increased bone mineralization and severe joint malformations. Primary embryo fibroblasts from Novdel3-/- mutant embryos showed enhanced chondrogenesis and osteogenesis. Cardiac development was also influenced leading to enlargement and abnormal modelling of the endocardial cushions, associated with septal defects and delayed fusion. In adults, cardiomyopathy was apparent, with hypertrophy and calcification of the septum and left ventricle dilation. Muscle atrophy was seen by 5 months of age, associated with transdifferentiation to fat. Premature tissue degeneration was also seen in the lens, with cataracts present from 6 months.

CONCLUSION

We have generated the first mice with a mutation in the Nov gene (Novdel3). Our data demonstrate that NOV is a regulator of skeletal and cardiac development, and implicates NOV in various disease processes including cardiomyopathy, muscle atrophy and cataract formation. Novdel3 mutants represent a valuable resource for studying NOV's role in the modulation and co-ordination of multiple signalling pathways that underpin organogenesis and tissue homeostasis.

Phosphorylation at S365 is a gatekeeper event that changes the structure of Cx43 and prevents down-regulation by PKC

Phosphorylation at unspecified sites is known to regulate the life cycle (assembly, gating, and turnover) of the gap junction protein, Cx43. In this paper, we show that Cx43 is phosphorylated on S365 in cultured cells and heart tissue. Nuclear magnetic resonance structural studies of the C-terminal region of Cx43 with an S365D mutation indicate that it forms a different stable conformation than unphosphorylated wild-type Cx43. Immunolabeling with an antibody specific for Cx43 phosphorylated at S365 shows staining on gap junction structures in heart tissue that is lost upon hypoxia when Cx43 is no longer specifically localized to the intercalated disk. Efficient phosphorylation at S368, an important Cx43 channel regulatory event that increases during ischemia or PKC activation, depends on S365 being unphosphorylated. Thus, phosphorylation at S365 can serve a “gatekeeper” function that may represent a mechanism to protect cells from ischemia and phorbol ester-induced down-regulation of channel conductance.

The cataract-inducing S50P mutation in Cx50 dominantly alters the channel gating of wild-type lens connexins

Mutations within connexin50 (Cx50) have been linked to various cataract phenotypes. To determine the mechanism behind cataract formation we used the paired Xenopus oocyte system in conjunction with transfected HeLa cells and genetically engineered mouse models to examine the functional characteristics of gap junctions in which a cataract-causing mutant of Cx50 (hereafter referred to as Cx50-S50P) is expressed. Channels comprising Cx50-S50P subunits alone failed to induce electrical coupling. However, the mixed expression of Cx50-S50P and wild-type subunits of either Cx50 or Cx46 - to create heteromeric gap junctions - resulted in functional intercellular channels with altered voltage-gating properties compared with homotypic wild-type channels. Additionally, immunofluorescence microscopy showed that channels of Cx50-S50P subunits alone failed to localize to the plasma membrane - unlike channels composed of Cx46 subunits, which concentrated at cell-cell appositions. Cx50-S50P colocalized with wild-type Cx46 in both transfected HeLa cells in vitro and mouse lens sections in vivo. Taken together, these data define the electrophysiological properties and intracellular targeting of gap junctions formed by the heteromeric combination of Cx50 or Cx46 and Cx50-S50P mutant proteins. Additionally, mixed channels displayed significantly altered gating properties, a phenomenon that may contribute to the cataract that is associated with this mutation.

Promotion of lens epithelial-fiber differentiation by the C-terminus of connexin 45.6 – a role independent of gap junction communication

We previously reported that, among the three connexins expressed in chick lens, overexpression of connexin (Cx) 45.6, not Cx43 or Cx56, stimulates lens cell differentiation; however, the underlying mechanism responsible for this effect is unclear. Here, we took advantage of naturally occurring loss-of-gap-junction function mutations of Cx50 (ortholog of chick Cx45.6) and generated the corresponding site mutants in Cx45.6: Cx45.6(D47A) and Cx45.6(P88S). In contrast to wild-type Cx45.6, the mutants failed to form functional gap junctions, and Cx45.6(P88S) and, to a lesser degree, Cx45.6(D47A) functioned in a dominant-negative manner. Interestingly, overexpression of both mutants incapable of forming gap junctions significantly increased epithelial-fiber differentiation to a level comparable to that of wild-type Cx45.6. To map the functional domain of Cx45.6, we generated a C-terminus chimera as well as deletion mutants. Overexpression of Cx56*45.6C, the mutant in which the C-terminus of Cx56 was replaced with that of Cx45.6, had a stimulatory effect on lens cell differentiation similar to that of Cx45.6. However, cells overexpressing Cx45.6*56C, the mutant in which C-terminus of Cx45.6 was replaced with that of Cx56, and Cx45.6(–C), in which the C-terminus was deleted, failed to promote differentiation. Taken together, we conclude that the expression of Cx45.6, but not Cx45.6-dependent gap junction channels, is involved in lens epithelial-fiber cell differentiation, and the C-terminal domain of Cx45.6 plays a predominant role in mediating this process.

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.

Interpreting voltage-sensitivity of gap junctions as a mechanism of cardiac memory

Memory in the nervous system is essentially a network effect, resulting from activity-dependent synaptic modification in a network of neurons. Like the nervous system, the heart is a network of cardiac cells electrically coupled by gap junctions. The heart too has memory, termed cardiac memory, whereby the effect of an external electrical activation persists long after the presentation of stimulus is terminated. We have earlier proposed that adaptation of gap junctions, as a function of membrane voltages of the cells that are coupled by the gap junctions, is related to cardiac memory [V.S. Chakravarthy, J. Ghosh, On Hebbian-like adaption in heart muscle: a proposal for "Cardiac Memory", Biol. Cybern. 76 (1997) 207, J. Krishnan, V.S. Chakravarthy, S. Radhakrishnan, On the role of gap junctions on cardiac memory effect, Comput. Cardiol. 32 (2005) 13]. Using the proposed mechanism, we demonstrate memory effect using computational models of interacting cell pairs. In this paper, we address the biological validity of the proposed mechanism of gap junctional adaptation. It is known from electrophysiology of gap junctions that the conductance of these channels adapts as a function of junctional voltage. At a first sight, this form of voltage dependence seems to be at variance with the form required by our mechanism. But we show, with the help of a theoretical model, that the proposed mechanism of voltage-dependent adaptation of gap junctions, is compatible with the known voltage-sensitivity of gap junctions observed in electrophysiological studies. Our analysis suggests a new significance of the voltage-sensitivity of gap junctions and its possible link to the phenomenon of cardiac memory.

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.

Age-related cataracts in alpha3Cx46-knockout mice are dependent on a calpain 3 isoform

PURPOSE

Previous studies have demonstrated that in 129alpha3Cx46-/- mice, age-related nuclear cataract is formed. In the present study, a more in vivo-relevant model was generated to test the hypothesis that the calpain 3 gene is involved in age-related nuclear cataractogenesis in alpha3Cx46 knockout mice.

METHODS

To test the hypothesis that the calpain 3 gene is involved in age-related nuclear cataractogenesis in alpha3Cx46 knockout mice, 129alpha3Cx46-/- and CAPN3-/- mice were mated to generate homozygous double-knockout (dKO) mice. Lenses from the mice were examined by visual observation, laser scan analysis, and histologic and biochemical methods.

RESULTS

In the absence of the CAPN3 gene, the formation of a cataract was delayed, and its appearance was changed to a more diffuse, pulverulent type. Unlike in the 129alpha3Cx46-/- mouse, cleavage of gamma-crystallin was not detected in the dKO mouse. In both 129alpha3Cx46-/- and dKO mice, total Ca2+ increased.

CONCLUSIONS

The present study shows for the first time that calpain 3 is necessary for the formation of age-dependent nuclear cataracts in alpha3Cx46-/- mice. Evidence that the calpain 3 gene is directly involved in, or part of the pathway that leads to, gamma-crystallin cleavage is presented. These results are consistent with the hypothesis that the loss of alpha3Cx46 leads to increased levels of Ca2+ ions, and this increase activates the CAPN3 isoform, Lp82/85, which results in the formation of a nuclear cataract.

PKCgamma knockout mouse lenses are more susceptible to oxidative stress damage

Cataracts, or lens opacities, are the leading cause of blindness worldwide. Cataracts increase with age and environmental insults, e.g. oxidative stress. Lens homeostasis depends on functional gap junctions. Knockout or missense mutations of lens gap junction proteins, Cx46 or Cx50, result in cataractogenesis in mice. We have previously demonstrated that protein kinase Cgamma (PKCgamma) regulates gap junctions in the lens epithelium and cortex. In the current study, we further determined whether PKCgamma control of gap junctions protects the lens from cataractogenesis induced by oxidative stress in vitro, using PKCgamma knockout and control mice as our models. The results demonstrate that PKCgamma knockout lenses are normal at 2 days post-natal when compared to control. However, cell damage, but not obvious cataract, was observed in the lenses of 6-week-old PKCgamma knockout mice, suggesting that the deletion of PKCgamma causes lenses to be more susceptible to damage. Furthermore, in vitro incubation or lens oxidative stress treatment by H(2)O(2) significantly induced lens opacification (cataract) in the PKCgamma knockout mice when compared to controls. Biochemical and structural results also demonstrated that H(2)O(2) activation of endogenous PKCgamma resulted in phosphorylation of Cx50 and subsequent inhibition of gap junctions in the lenses of control mice, but not in the knockout. Deletion of PKCgamma altered the arrangement of gap junctions on the cortical fiber cell surface, and completely abolished the inhibitory effect of H(2)O(2) on lens gap junctions. Data suggest that activation of PKCgamma is an important mechanism regulating the closure of the communicating pathway mediated by gap junction channels in lens fiber cells. The absence of this regulatory mechanism in the PKCgamma knockout mice may cause those lenses to have increased susceptibility to oxidative damage.

Transgenic overexpression of connexin50 induces cataracts

To examine the effects of increased expression of Cx50 in the mouse lens, transgenic mice were generated using a DNA construct containing the human Cx50 coding region and a C-terminal FLAG epitope driven by the chicken betaB1-crystallin promoter. Expression of this protein in paired Xenopus oocytes induced gap junctional currents of similar magnitude to wild type human Cx50. Three lines of transgenic mice expressing the transgenic protein were analyzed. Lenses from transgenic mice were smaller than those from non-transgenic littermates, and had cataracts that were already visible at postnatal day 1. Expression of the transgene resulted in a 3- to 13-fold increase in Cx50 protein levels above those of non-transgenic animals. Light microscopy revealed alterations in epithelial cell differentiation, fiber cell structure, interactions between fiber cells and areas of liquefaction. Scanning electron microscopy showed fiber cells of varying widths with bulging areas along single fibers. Anti-Cx50 and anti-FLAG immunoreactivities were detected at appositional membranes and in intracellular vesicles in transgenic lenses. N-cadherin, Cx46, ZO-1 and aquaporin 0 localized mainly at the plasma membrane, although some N-cadherin and aquaporin 0 was associated with the intracellular vesicles. The abundance and solubility/integrity of alphaA-, alphaB-, beta- and gamma-crystallin were unaffected. These results demonstrate that transgenic expression of Cx50 in mice leads to cataracts associated with formation of cytoplasmic vesicles containing Cx50 and decreased or slowed epithelial differentiation without major alterations in the distribution of other integral membrane or membrane-associated proteins or the integrity/solubility of crystallins.

The ether lipid-deficient mouse: Tracking down plasmalogen functions

Chemical and physico-chemical properties as well as physiological functions of major mammalian ether-linked glycerolipids, including plasmalogens were reviewed. Their chemical structures were described and their effect on membrane fluidity and membrane fusion discussed. The recent generation of mouse models with ether lipid deficiency offered the possibility to study ether lipid and particularly plasmalogen functions in vivo. Ether lipid-deficient mice revealed severe phenotypic alterations, including arrest of spermatogenesis, development of cataract and defects in central nervous system myelination. In several cell culture systems lack of plasmalogens impaired intracellular cholesterol distribution affecting plasma membrane functions and structural changes of ER and Golgi cisternae. Based on these phenotypic anomalies that were accurately described conclusions were drawn on putative functions of plasmalogens. These functions were related to cell-cell or cell-extracellular matrix interactions, formation of lipid raft microdomains and intracellular cholesterol homeostasis. There are several human disorders, such as Zellweger syndrome, rhizomelic chondrodysplasia punctata, Alzheimer's disease, Down syndrome, and Niemann-Pick type C disease that are distinguished by altered tissue plasmalogen concentrations. The role plasmalogens might play in the pathology of these disorders is discussed.

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.

Analysis of Connexin43 phosphorylated at S325, S328 and S330 in normoxic and ischemic heart

The functional consequences of Connexin43 (Cx43) phosphorylation remain largely unexplored. Using an antibody that specifically recognizes Cx43 phosphorylated at serine residues 325, 328 and/or 330 (pS325/328/330-Cx43), we show that labeling of this form of Cx43 as well as of total Cx43 is restricted to the intercalated disk region of normal ventricular tissue. In ischemic heart, significant relocalization of total Cx43 to the lateral edges of myocytes was evident; however pS325/328/330-Cx43 remained predominately at the intercalated disk. Western blots indicated a eightfold decrease in pS325/328/330-Cx43 in ischemic tissue. Peptide-binding- and competition-experiments indicated that our antibody mainly detected Cx43 phosphorylated at S328 and/or S330 in heart tissue. To evaluate how this change in Cx43 phosphorylation contributes to ischemia-induced downregulation of intercellular communication, we stably transfected Cx43(-/-) cells with a Cx43 construct in which serine residues 325, 328 and 330 had been mutated to alanine (Cx43-TM). Cx43-TM was not efficiently processed to isoforms that have been correlated with gap junction assembly. Nevertheless, Cx43-TM cells were electrically coupled, although development of coupling was delayed. Fully opened channels were only rarely observed in Cx43-TM cells, and Lucifer-Yellow-dye-coupling was significantly reduced compared with wild-type cells. These data suggest that phosphorylation of Cx43 at serine residues 325, 328 and/or 330 influences channel permselectivity and regulates the efficiency of gap junction assembly.

Knock-in of alpha3 connexin prevents severe cataracts caused by an alpha8 point mutation

A G22R point mutation in alpha8 connexin (Cx50) has been previously shown to cause a severe cataract by interacting with endogenous wild-type alpha3 connexin (Cx46) in mouse lenses. Here, we tested whether a knocked-in alpha3 connexin expressed on the locus of the endogenous alpha8 connexin could modulate the severe cataract caused by the alpha8-G22R mutation. We found that the alpha3(-/-) alpha8(G22R/-) mice developed severe cataracts with disrupted inner fibers and posterior rupture while the alpha3(-/-) alpha8(G22R/KIalpha3) lens contained relatively normal inner fibers without lens posterior rupture. The alpha8-G22R mutant proteins produced typical punctate staining of gap junctions between fiber cells of alpha3(-/-) alpha8(G22R/KIalpha3) lenses, but not in those of alpha3(-/-) alpha8(G22R/-) lenses. Thus, we hypothesize that the knocked-in alpha3 connexin subunits interact with the alpha8-G22R connexin subunits to form functional gap junction channels and rescue the lens phenotype. Using an electrical coupling assay consisting of paired Xenopus oocytes, we demonstrated that only co-expression of mutant alpha8-G22R and wild-type alpha3 connexin subunits forms functional gap junction channels with reduced conductance and altered voltage sensitivity compared with the channels formed by alpha3 connexin subunits alone. Thus, knocked-in alpha3 connexin and mutant alpha8-G22R connexin probably form heteromeric gap junction channels that influence lens homeostasis and lens transparency.

Chondrocytes utilize a cholesterol-dependent lipid translocator to externalize phosphatidylserine

During endochondral ossification, growth plate chondrocytes release plasma membrane (PM) derived matrix vesicles (MV), which are the site of initial hydroxyapatite crystal formation. MV constituents which facilitate the mineralization process include the integral membrane ectoenzymes alkaline phosphatase (ALPase) and nucleotide pyrophosphatase phosphodiesterase (NPP1/PC-1), along with a phosphatidylserine- (PS-) rich membrane surface that binds annexins and calcium, resulting in enhanced calcium entry into MV. In this study, we determined that chick growth plate MV were highly enriched in membrane raft microdomains containing high levels of cholesterol, glycophosphatidylinositol- (GPI-) anchored ALPase, and phosphatidylserine (PS) localized to the external leaflet of the bilayer. To determine how such membrane microdomains arise during chondrocyte maturation, we explored the role of PM cholesterol-dependent lipid assemblies in regulating the activities of lipid translocators involved in the externalization of PS. We first isolated and determined the composition of detergent-resistant membranes (DRMs) from chondrocyte PM. DRMs isolated from chondrocyte PM were enhanced in ganglioside 1 (GM1) and cholesterol as well as GPI-anchored ALPase. Furthermore, these membrane domains were enriched in PS (localized to the external leaflet of the bilayer) and had significantly higher ALPase activity than non-cholesterol-enriched domains. To understand the role of cholesterol-dependent lipid assemblies in the externalization of PS, we measured the activities of two lipid transporters involved in PS externalization, aminophospholipid translocase (APLT) and phospholipid scramblase (PLSCR1), during maturation of a murine chondrocytic cell line, N1511. In this report, we provide the first evidence that maturing chondrocytes express PLSCR1 and have scramblase activity. We propose that redistribution of PS is dependent on an increase in phospholipid scramblase activity and a decrease in APLT activity. Lastly, we show that translocator activity is most likely to be modulated by membrane cholesterol levels through a membrane raft microdomain.

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

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

Eye lens proteomics

The eye lens is a fascinating organ as it is in essence living transparent matter. Lenticular transparency is achieved through the peculiarities of lens morphology, a semi-apoptotic process where cells elongate and loose their organelles and the precise molecular arrangement of the bulk of soluble lenticular proteins, the crystallins. The 16 crystallins ubiquitous in mammals and their modifications have been extensively characterized by 2-DE, liquid chromatography, mass spectrometry and other protein analysis techniques. The various solubility dependant fractions as well as subproteomes of lenticular morphological sections have also been explored in detail. Extensive post translational modification of the crystallins is encountered throughout the lens as a result of ageing and disease resulting in a vast number of protein species. Proteomics methodology is therefore ideal to further comprehensive understanding of this organ and the factors involved in cataractogenesis.

Lanosterol synthase mutations cause cholesterol deficiency-associated cataracts in the Shumiya cataract rat

The Shumiya cataract rat (SCR) is a hereditary cataractous strain. It is thought that the continuous occurrence of poorly differentiated epithelial cells at the bow area of the lens forms the pathophysiological basis for cataract formation in SCRs. In this study, we attempted to identify the genes associated with cataract formation in SCRs by positional cloning. Genetic linkage analysis revealed the presence of a major cataract locus on chromosome 20 as well as a locus on chromosome 15 that partially suppressed cataract onset. Hypomorphic mutations were identified in genes for lanosterol synthase (Lss) on chromosome 20 and farnesyl diphosphate farnesyl transferase 1 (Fdft1) on chromosome 15, both of which function in the cholesterol biosynthesis pathway. A null mutation for Lss was also identified. Cataract onset was associated with the specific combination of Lss and Fdft1 mutant alleles that decreased cholesterol levels in cataractous lenses to about 57% of normal. Thus, cholesterol insufficiency may underlie the deficient proliferation of lens epithelial cells in SCRs, which results in the loss of homeostatic epithelial cell control of the underlying fiber cells and eventually leads to cataractogenesis. These findings may have some relevance to other types of cataracts, inborn defects of cholesterol synthesis, and the effects of cholesterol-lowering medication.

A novel GJA8 mutation is associated with autosomal dominant lamellar pulverulent cataract: further evidence for gap junction dysfunction in human cataract

PURPOSE

To identify the gene responsible for autosomal dominant lamellar pulverulent cataract in a four-generation British family and characterise the functional and cellular consequences of the mutation.

METHODS

Linkage analysis was used to identify the disease locus. The GJA8 gene was sequenced directly. Functional behaviour and cellular trafficking of connexins were examined by expression in Xenopus oocytes and HeLa cells.

RESULTS

A 262C>A transition that resulted in the replacement of proline by glutamine (P88Q) in the coding region of connexin50 (Cx50) was identified. hCx50P88Q did not induce intercellular conductance and significantly inhibited gap junctional activity of co-expressed wild type hCx50 RNA in paired Xenopus oocytes. In transfected cells, immunoreactive hCx50P88Q was confined to the cytoplasm but showed a temperature sensitive localisation at gap junctional plaques.

CONCLUSIONS

The pulverulent cataract described in this family is associated with a novel GJA8 mutation and has a different clinical phenotype from previously described GJA8 mutants. The cataract likely results from lack of gap junction function. The lack of function was associated with improper targeting to the plasma membrane, most probably due to protein misfolding.

New genetic model rat for congenital cataracts due to a connexin 46 (Gja3 ) mutation

A rat strain with congenital nuclear cataracts has been established. Segregation analyses indicated that this phenotype had an autosomal recessive mode of inheritance, implying that a loss of function mutation of a single autosomal gene was responsible. The gene was mapped to the D15Rat6 locus on chromosome 15 through a linkage analysis using 93 backcrossed rats. The connexin 46 gene (Gja3) was found to be located close to the locus, and was regarded as a strong candidate because of its pivotal role in the lens fiber cells. Expression of the gene in the lens was comparable between the cataract and control rats when evaluated with immunohistochemistry and reverse transcription-polymerase chain reaction. However, a non-conservative missense mutation, Glu42Lys, was found in the gene of the cataract rats, which was likely to be responsible for the pathogenesis. This strain will be useful in pathophysiological studies on nuclear cataracts.

Gap junctional communication in the male reproductive system

Male fertility is a highly controlled process that allows proliferation, meiosis and differentiation of male germ cells in the testis, final maturation in the epididymis and also requires functional male accessory glands: seminal vesicles, prostate and corpus cavernosum. In addition to classical endocrine and paracrine controls, mainly by gonadotropins LH and FSH and steroids, there is now strong evidence that all these processes are dependent upon the presence of homocellular or heterocellular junctions, including gap junctions and their specific connexins (Cxs), between the different cell types that structure the male reproductive tract. The present review is focused on the identification of Cxs, their distribution in the testis and in different structures of the male genital tract (epididymis, seminal vesicle, prostate, corpus cavernosum), their crucial role in the control of spermatogenesis and their implication in the function of the male accessory glands, including functional smooth muscle tone. Their potential dysfunctions in some testis (spermatogenic arrest, seminoma) and prostate (benign hyperplasia, adenocarcinoma) diseases and in the physiopathology of the human erectile function are also discussed.

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.

Connexin 46 and connexin 50 in selenite cataract

The purpose of this work was to determine if the lens gap junction proteins connexin 46 (Cx46) and connexin 50 (Cx50) were altered with the development of selenite-induced cataract. Cataracts were induced in young Sprague-Dawley rats with a single subcutaneous injection of sodium selenite; age-matched uninjected rats served as controls. Membrane fractions were isolated from homogenates of cortex and nucleus of normal and cataractous lenses by differential and discontinuous sucrose gradient centrifugation. Aliquots of urea-insoluble protein from membrane fractions were analyzed by quantitative densitometry of Western blots probed with antibodies to Cx46 and Cx50. A significant decrease in the more slowly migrating Cx46-reactive band, which represents phosphorylated Cx46, was found in the major membrane fraction of the cortex of cataractous lenses. There was no significant difference in the amounts of either Cx46 or Cx50 associated with selenite cataract in any of the membrane fractions examined. These results suggest that alteration of gap junction function (as evidenced by the change in phosphorylation of Cx46) may be associated with the development of the selenite cataract, but that neither Cx46 nor Cx50 is subject to the well-characterized proteolysis associated with the selenite cataract model.

Temporal regulation of connexin phosphorylation in embryonic and adult tissues

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells in tissues and are important in development, tissue/cellular homeostasis, and carcinogenesis. Genome databases indicate that there are at least 20 connexins in the mouse and human. Connexin phosphorylation has been implicated in connexin assembly into gap junctions, gap junction turnover, and cell signaling events that occur in response to tumor promoters and oncogenes. Connexin43 (Cx43), the most widely expressed and abundant gap junction protein, can be phosphorylated at several different serine and tyrosine residues. Here, we focus on the dynamic regulation of Cx43 phosphorylation in tissue and how these regulatory events are affected during development, wound healing, and carcinogenesis. The activation of several kinases, including protein kinase A, protein kinase C, p34cdc2/cyclin B kinase, casein kinase 1, mitogen-activated protein kinase, and pp60src kinase, can lead to the phosphorylation of different residues in the C-terminal region of Cx43. The use of antibodies specific for phosphorylation at defined residues has allowed the examination of specific phosphorylation events both in tissue culture and in vivo. These new antibody tools and those under development will allow us to correlate specific phosphorylation events with changes in connexin function.

Functional consequences of heterogeneous gap junction channel formation and its influence in health and disease

The capacity of multiple connexins to hetero-oligomerize into functional heterogeneous gap junction channels has been demonstrated in vivo, in vitro, and in nonmammalian expression systems. These heterogeneous channels display gating activity, channel conductances, selectivity and regulatory behaviors that are sometimes not predicted by the behaviors of the corresponding homogeneous channels. Such observations suggest that heteromerization of gap junction proteins offers an efficient cellular strategy for finely regulating cell-to-cell communication. The available evidence strongly indicates that heterogeneous gap junction assembly is important to normal growth and differentiation, and may influence the appearance of several disease states. Definitive evidence that heterogeneous gap junction channels differentially regulate electrical conduction in excitable cells is absent. This review examines the prevalence, regulation, and implications of gap junction channel hetero-oligomerization.

Developmental Regulation of the Direct Interaction between the Intracellular Loop of Connexin 45.6 and the C Terminus of Major Intrinsic Protein (Aquaporin-0)

The eye lens is dependent upon a network of gap junction-mediated intercellular communication to facilitate its homeostasis and development. Three gap junction-forming proteins are expressed in the lens of which two are in lens fibers, namely connexin (Cx) 45.6 and 56. Major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), is the most abundant membrane protein in lens fibers. However, its role in the lens is not clear. Our previous studies show that MIP(AQP0) associates with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development but not in late embryonic and adult lenses. We report here that MIP(AQP0) directly interacts with Cx45.6 but not with Cx56. We further identified the intracellular loop of Cx45.6 as the interacting domain for the MIP(AQP0) C terminus. Surface plasmon resonance experiments indicated that the C-terminal domain of MIP(AQP0) interacts with two binding sites within the intracellular loop region of Cx45.6 with a K(D(app)) of 7.5 and 10.3 microm, respectively. The K(D(app)) for the full-length loop region is 7.7 microm. The cleavage at the intracellular loop of Cx45.6 was observed during lens development, and the C terminus of MIP(AQP0) did not interact with the loop-cleaved form of Cx45.6. Thus, the dissociation between these two proteins that occurs in the mature fibers of late lens development is likely caused by this cleavage. Finally this interaction had no impact on Cx45.6-mediated intercellular communication, suggesting that the Cx45.6-MIP(AQP0) interaction plays a novel unidentified role in lens fibers.

Crystallin {gamma}B-I4F mutant protein binds to {alpha}-crystallin and affects lens transparency

A new mouse mutant line, Clapper, identified from N-ethyl-N-nitrosurea (ENU)-mutagenized mice, develops a dominant lamellar cataract. The cataract blocks the image of retinal fundus and transmits a fuzzy fluorescein image of retinal vasculature during angiography. The cataractous lens opacity decreases as the mice age. The Clapper mutation has been identified to be a missense mutation of the gammaB-crystallin gene that replaces the 4th isoleucine residue with a phenylalanine (gammaB-I4F). Unlike wild type gammaB, the gammaB-I4F mutant protein binds to alpha-crystallin to form high molecular weight complexes in vivo and in vitro. Circular dichroism measurements indicate that gammaB-I4F protein is less stable than wild type gammaB at high temperature. Darkly stained aggregates, enlarged interfiber spaces, and disorganized and smaller inner mature fibers were found in the regions of the cataract in homozygous Clapper mutant lenses. Thus, the lamellar cataract is likely due to the light-scattering effects of the enlarged interfiber spaces and protein aggregates caused by gammaB-I4F mutant proteins interacting with alpha-crystallin in the lens.

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

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

Gap junction channels reconstituted in two closely apposed lipid bilayers

Intercellular communication mediated by gap junction channels plays an important role in many cellular processes. In contrast to other channels, gap junction channels span two plasma membranes resulting in an intracellular location for both ends of the junctional pore and the regulatory sites for channel gating. This configuration presents unique challenges for detailed experimental studies of junctional channel physiology and ligand-activation in situ. Availability of an appropriate model system would significantly facilitate future studies of gap junction channel function and structure. Here we show that the double-membrane channel can be reconstituted in pairs of closely apposed lipid bilayers, as experienced in cells. We have trapped the calcium-sensitive dye, arsenazo III (AIII), partially calcium-saturated (AIII-Ca), in one population of connexin32 reconstituted-liposomes, and EGTA in a second one. In such mixtures, the interaction of EGTA with AIII-Ca was measured by a large color shift from blue to red (decreased absorbance at 652 nm). The exchange of these compounds through gap junctions was proportional to these decrements. Results indicate that these connexon-mediated interliposomal channels are functional and are inhibited by the addition of alpha-glycyrrhetinic acid and by flufenamic acid, two gap junction communication inhibitors. Future use of this model system has the potential to improve our understanding of the permeability and modulation of junctional channels in its native intercellular assembly.

Exchange of gating properties between rat cx46 and chicken cx45.6

Cx46 and Cx50 are coexpressed in lens fiber cells where they form fiber-fiber gap junctions. Recent studies have shown that both proteins play a critical role in maintaining lens transparency. Although both Cx46 and Cx50 (or its chicken ortholog, Cx45.6) show a high degree of sequence homology, they exhibit marked differences in gap junctional channel gating, unitary gap junctional channel conductance, and hemichannel gating. To better understand which regions of the protein are responsible for these functional differences, we have constructed a series of chimeric Cx46-Cx45.6 gap junctional proteins in which a single transmembrane or intracellular domain of Cx45.6 was replaced with the corresponding domain of Cx46, expressed them in Xenopus oocyte pairs or N2A cells, and examined the resulting gap junctional conductances. Our results showed that four out of six of the chimeras induced high levels of gap junctional coupling. Of these chimeras, only Cx45.6-46NT showed significant changes in voltage-dependent gating properties. Exchanging the N-terminus had multiple effects. It slowed the inactivation kinetics of the macroscopic junctional currents so that they resembled those of Cx46, reduced the voltage sensitivity of the steady-state junctional conductance, and decreased the conductance of single gap junctional channels. Additional point mutations identified a uniquely occurring arginine in the N-terminus of Cx46 as the main determinant for the change in voltage-dependent gating.

New insights into the expression and function of neural connexins with transgenic mouse mutants

Gap junctions represent direct intercellular conduits between contacting cells. The subunit proteins of these conduits are called connexins. To date, 20 and 21 connexin genes have been described in the mouse and human genome, respectively, many of them represent sequence-orthologous pairs. Targeted deletion of connexin genes in the mouse genome opened new insights into the biological function of these channel forming proteins, which, in some cases, could be correlated to phenotypic abnormalities in humans, suffering from inherited diseases caused by mutations in the corresponding orthologous connexin gene. Replacing the connexin coding DNA by an appropriate reporter gene has clarified in several cases its cell type specific expression in mouse brain. Various studies demonstrated that connexin36 is mainly expressed in interneurons of retina and brain. Targeted deletion of connexin36 evoked a loss of electrical signal transduction and interferes with synchrony which probably leads to defects in visual transmission and memory. Deletion of connexin43 in astrocytes of mouse brain resulted in increased spreading depression consistent with the notion of altered "spatial buffering" of K(+) ions and glutamate secreted by active neurons. General connexin30-deficiency led to hearing impairment and apoptosis of hair cells, similar to that observed in mice with cochlea specific deletion of connexin26. Reporter gene expression in connexin30-deficient mice indicated that astrocytes in certain brain regions and leptomeningeal as well as ependymal cells are labelled. Reporter gene expression in connexin45- and connexin47-deficient mice was used to reassign connexin45 expression to certain CNS neurons and connexin47 expression to oligodendrocytes.

Ocular manifestations of keratitis– ichthyosis–deafness (KID) syndrome

OBJECTIVE

Keratitis-ichthyosis-deafness (KID) syndrome is a rare congenital ectodermal dysplasia characterized by the association of hyperkeratotic skin lesions, moderate to profound sensorineural hearing loss and vascularizing keratitis. Mutations in the GJB2 gene coding for connexin 26, a component of gap junctions in epithelial cells, have been observed in several KID patients. Variable ocular manifestations of the disease in 3 patients with molecular genetically confirmed KID syndrome are reported.

DESIGN

Retrospective case series.

METHODS

Clinical examination and molecular genetic analysis for mutations in the GJB2 gene were performed in 3 patients with KID syndrome ages 5, 13, and 41 years.

RESULTS

Visual acuity ranged from normal to severe visual loss. The ocular signs included loss of eyebrows and lashes, thickened and keratinized lids, trichiasis, recurrent corneal epithelial defects, superficial and deep corneal stromal vascularization with scarring, keratoconjunctivitis sicca, and, in one patient, presumed limbal insufficiency. Whereas ocular surface integrity could be maintained with artificial tears in one patient, and an epithelial defect healed under conservative treatment in the second patient, multiple surgical procedures including superficial keratectomies, limbal allograft transplantation with systemic immunosuppression, amniotic membrane transplantation, lateral tarsorrhaphies, and lamellar keratoplasty could not preserve useful vision in the third patient.

CONCLUSIONS

KID syndrome may affect the ocular adnexae and surface with variable severity independent of the age of the patient. Lid abnormalities, corneal surface instability, limbal stem cell deficiency with resulting corneal complications, and dry eye are the main ocular manifestations.

Role of gap junctions during early embryo development

Gap junctional communication plays a central role in the maintenance of cellular homeostasis by allowing the passage of small molecules between adjacent cells. Gap junctions are composed of a family of proteins termed connexins. During preimplantation development several connexin proteins are expressed and assembled into gap junctions in the plasma membrane at compaction but the functional significance of connexin diversity remains controversial. Although, many of the connexin genes have been disrupted using homologous recombination in embryonic stem cells to obtain unique phenotypes, none of these studies has demonstrated a specific role for connexins during preimplantation development in the null mutants. This review surveys evidence for the involvement of gap junctional communication during embryo development highlighting discrepancies in the literature. Although some evidence suggests that gap junctions may be dispensable during preimplantation development this is difficult to envisage particularly for the process of cavitation and the maintenance of homeostasis between the differentiated trophectoderm cells and the pluripotent inner cell mass cells of the blastocyst.

Connexins and their environment: effects of lipids composition on ion channels

Intercellular communication is mediated through paired connexons that form an aqueous pore between two adjacent cells. These membrane proteins reside in the plasma membrane of their respective cells and their activity is modulated by the composition of the lipid bilayer. The effects of the bilayer on connexon structure and function may be direct or indirect, and may arise from specific binding events or the physicochemical properties of the bilayer. While the effects of the bilayer and its constituent lipids on gap junction activity have been described in the literature, the underlying mechanisms of the interaction of connexin with its lipidic microenvironment are not as well characterized. Given that the information regarding connexons is limited, in this review, the specific roles of lipids and the properties of the bilayer on membrane protein structure and function are described for other ion channels as well as for connexons.

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.

Connections between connexins, calcium, and cataracts in the lens

There is a good deal of evidence that the lens generates an internal micro circulatory system, which brings metabolites, like glucose, and antioxidants, like ascorbate, into the lens along the extracellular spaces between cells. Calcium also ought to be carried into the lens by this system. If so, the only path for Ca2+ to get out of the lens is to move down its electrochemical gradient into fiber cells, and then move by electrodiffusion from cell to cell through gap junctions to surface cells, where Ca-ATPase activity and Na/Ca exchange can transport it back into the aqueous or vitreous humors. The purpose of the present study was to test this calcium circulation hypothesis by studying calcium homeostasis in connexin (Cx46) knockout and (Cx46 for Cx50) knockin mouse lenses, which have different degrees of gap junction coupling. To measure intracellular calcium, FURA2 was injected into fiber cells, and the gradient in calcium concentration from center to surface was mapped in each type of lens. In wild-type lenses the coupling conductance of the mature fibers was approximately 0.5 S/cm2 of cell to cell contact, and the best fit to the calcium concentration data varied from 700 nM in the center to 300 nM at the surface. In the knockin lenses, the coupling conductance was approximately 1.0 S/cm2 and calcium varied from approximately 500 nM at the center to 300 nM at the surface. Thus, when the coupling conductance doubled, the concentration gradient halved, as predicted by the model. In knockout lenses, the coupling conductance was zero, hence the efflux path was knocked out and calcium accumulated to approximately 2 microM in central fibers. Knockout lenses also had a dense central cataract that extended from the center to about half the radius. Others have previously shown that this cataract involves activation of a calcium-dependent protease, Lp82. We can now expand on this finding to provide a hypothesis on each step that leads to cataract formation: knockout of Cx46 causes loss of coupling of mature fiber cells; the efflux path for calcium is therefore blocked; calcium accumulates in the central cells; at concentrations above approximately 1 microM (from the center to about half way out of a 3-wk-old lens) Lp82 is activated; Lp82 cleaves cytoplasmic proteins (crystallins) in central cells; and the cleaved proteins aggregate and scatter light.

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.

7-Ketocholesterol modulates intercellular communication through gap-junction in bovine lens epithelial cells

Background

Connexin43 (Cx43) is an integral membrane protein that forms intercellular channels called gap junctions. Intercellular communication in the eye lens relies on an extensive network of gap junctions essential for the maintenance of lens transparency. The association of Cx43 with cholesterol enriched lipid raft domains was recently demonstrated. The objective of this study is to assess if products of cholesterol oxidation (oxysterols) affect gap junction intercellular communication (GJIC).

Results

Primary cultures of lens epithelial cells (LEC) were incubated with 7-ketocholesterol (7-Keto), 25-hydroxycholesterol (25-OH) or cholesterol and the subcellular distribution of Cx43 was evaluated by immunofluorescence confocal microscopy. The levels of Cx43 present in gap junction plaques were assessed by its insolubility in Triton X-100 and quantified by western blotting. The stability of Cx43 at the plasma membrane following incubation with oxysterols was evaluated by biotinylation of cell surface proteins. Gap junction intercellular communication was evaluated by transfer of the dye Lucifer yellow. The results obtained showed that 7-keto induces an accumulation of Cx43 at the plasma membrane and an increase in intercellular communication through gap junction. However, incubation with cholesterol or 25-OH did not lead to significant alterations on subcellular distribution of Cx43 nor in intercellular communication. Data further suggests that increased intercellular communication results from increased stability of Cx43 at the plasma membrane, presumably forming functional gap-junctions, as suggested by decreased solubility of Cx43 in 1% Triton X-100. The increased stability of Cx43 at the plasma membrane seems to be specific and not related to disruption of endocytic pathway, as demonstrated by dextran uptake.

Conclusions

Results demonstrate, for the first time, that 7-keto induces an increase in gap junction intercellular communication, that is most likely due to an increased stability of protein at the plasma membrane and to increased abundance of Cx43 assembled in gap junction plaques.

Bfsp2 mutation found in mouse 129 strains causes the loss of CP49' and induces vimentin-dependent changes in the lens fibre cell cytoskeleton

Here we report the first natural mutation in the mouse Bfsp2 gene. Characterisation of mouse Bfsp2 in the 129X1/SvJ revealed a mutation that deleted the acceptor site of exon 2. This results in exon 1 being erroneously spliced to exon 3 causing a frameshift in the reading frame and the introduction of a stop codon at position 2 of exon 3 in the Bfsp2 transcript. RT-PCR studies of lens RNA isolated from 129S1/SvImJ, 129S2/SvPas and 129S4/SvJae strains confirmed the presence of this mutation in these diverse 129 strains and similar mutations were found in both CBA and 101 strains, but not in C3H or C57BL/6J mouse strains. This mutation is predicted to result in a severely truncated protein product called CP49, comprising essentially only exon 1, but polyclonal antibodies to CP49 failed to detect either full length or fragments of CP49 in extracts made from either 129S1/SvImJ or 129S4/SvJae suggesting that these 129 strains lack CP49 protein. Like the knockout of Bfsp2 reported recently, filensin protein levels and its proteolytic processing were altered also in the 129S1/SvImJ and 129S4/SvJae strains compared to C57BL/6J. Electron microscopy of the lens cytoskeleton from 129S2/SvPas revealed similar morphological changes in the cytoskeleton as compared to the CP49 knockout, with beaded and intermediate filaments being apparently replaced by poorly defined filament-like material. Vimentin was a key component of this residual material as shown by immunoelectron microscopy and by the generation of a CP49/vimentin double knockout mouse. This report of a natural mutation in Bfsp2 in the 129 and other mouse strains also has important implications for lens studies that have used the 129X1/SvJ strain in knockout strategies.

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.

Mice deficient for the gap junction protein Connexin32 exhibit increased radiation-induced tumorigenesis associated with elevated mitogen-activated protein kinase (p44/Erk1, p42/Erk2) activation

Loss of connexin expression/gap junction intercellular communication (GJIC) has been correlated with decreased growth control and increased tumorigenesis. Studies utilizing Connexin32 (Cx32)-deficient knockout mice have demonstrated that loss of Cx32 increases susceptibility to chemically induced liver tumorigenesis. Here, in addition to dramatically increased liver tumorigenesis, we show that tumor induction utilizing X-ray radiation resulted in a statistically significant increase in overall tumor burden in Cx32-deficient mice compared with wild-type mice due to tumorigenesis in several other tissues (lung, adrenal, lymph and small intestine) even when excluding prevalent liver tumors. Irradiated Cx32-deficient mice were particularly sensitive to liver tumorigenesis (46% incidence compared with 18% in wild-type mice, P = 0.007) demonstrating that Cx32 functions as a hepatic tumor suppressor in response to radiation-associated mutation events. Cx32-deficient mice also exhibited increased lung tumorigenesis (bronchioloalveolar) with an increased progression to carcinoma when compared with wild-type mice. Two Cx32-deficient mice developed an uncommon, invasive medullary adrenal tumor type (pheochromocytoma) not observed in irradiated wild-type mice. Immunohistochemical analysis revealed increased levels of activated mitogen-activated protein kinase (MAPK) (p44/Erk1, p42/Erk2) in Cx32-deficient mouse liver tumors (P = 0.006), lung tumors (P = 0.056) and adrenal tumors (primary and metastases) compared with wild-type counterparts implicating elevated activation of MAPK-interacting pathways in Cx32-deficient tumorigenesis. Interestingly, lung tumors from Cx32-deficient mice also demonstrated decreased p27Kip1 levels compared with wild-type lung tumors (P = 0.05). This study demonstrates that loss of Cx32/GJIC plays a significant role in radiation-induced tumorigenesis of the liver and importantly that Cx32 may also play a role in tumor suppression and/or tumor progression in other tissue types such as lung and adrenal gland. Additionally, this mouse model suggests that MAPK-related pathways may be preferentially activated or conversely that tumors harboring activated MAPK pathways may selectively progress towards more advanced tumor states in the absence of Cx32-mediated GJIC.

The lens: local transport and global transparency

The perception of the lens changed remarkably during the career of David Maurice. The early view was that it was an inert sack of protein that assisted the cornea in focusing light on the retina. As investigators looked more carefully, more and more complexity was revealed and today we know the lens is a living, dynamic organ that carries out a host of biochemical and physiological processes necessary for homeostasis. We have worked on the lens over this period and have provided a small part of the data on lens physiology. This paper is an overview of our own contributions, in the context of the ever evolving view of the lens. Given this is a brief tribute to the career of David Maurice, there is not enough space nor is it appropriate to provide a complete review of all the work that has contributed to this evolving

Bfsp2 mutation found in mouse 129 strains causes the loss of CP49 and induces vimentin-dependent changes in the lens fibre cell cytoskeleton

Here we report the first natural mutation in the mouse Bfsp2 gene. Characterisation of mouse Bfsp2 in the 129X1/SvJ revealed a mutation that deleted the acceptor site of exon 2. This results in exon 1 being erroneously spliced to exon 3 causing a frameshift in the reading frame and the introduction of a stop codon at position 2 of exon 3 in the Bfsp2 transcript. RT-PCR studies of lens RNA isolated from 129S1/SvImJ, 129S2/SvPas and 129S4/SvJae strains confirmed the presence of this mutation in these diverse 129 strains and similar mutations were found in both CBA and 101 strains, but not in C3H or C57BL/6J mouse strains. This mutation is predicted to result in a severely truncated protein product called CP49, comprising essentially only exon 1, but polyclonal antibodies to CP49 failed to detect either full length or fragments of CP49 in extracts made from either 129S1/SvImJ or 129S4/SvJae suggesting that these 129 strains lack CP49 protein. Like the knockout of Bfsp2 reported recently, filensin protein levels and its proteolytic processing were altered also in the 129S1/SvImJ and 129S4/SvJae strains compared to C57BL/6J. Electron microscopy of the lens cytoskeleton from 129S2/SvPas revealed similar morphological changes in the cytoskeleton as compared to the CP49 knockout, with beaded and intermediate filaments being apparently replaced by poorly defined filament-like material. Vimentin was a key component of this residual material as shown by immunoelectron microscopy and by the generation of a CP49/vimentin double knockout mouse. This report of a natural mutation in Bfsp2 in the 129 and other mouse strains also has important implications for lens studies that have used the 129X1/SvJ strain in knockout strategies.

Gap junctions in the inner ear: comparison of distribution patterns in different vertebrates and assessement of connexin composition in mammals

The distribution and size of gap junctions (GJ) in the sensory epithelia of the inner ear have been examined in a reptile (gecko), birds (chicken and owl), and mammals (mouse, guinea pig, gerbil, and bat), and the connexin composition of GJs in the mammalian inner ear has been assessed. Freeze fracture revealed a common pattern of GJ distribution in auditory and vestibular sensory epithelia in the different vertebrate classes. In all these tissues, GJs are numerous, often occupying more than 25% of the plasma membrane area of supporting cells and sometimes composed of more than 100,000 channels. Screening for 12 members of the connexin family in the mammalian inner ear by RT-PCR, Western blotting, and immunohistochemistry revealed four connexin isotypes, cx26, cx30, cx31, and cx43, in the cochlea and three, cx26, cx30, and cx43, in the vestibular organs. With antibodies characterised for their specificity, cx26 and cx30 colocalised in supporting cells of the organ of Corti, in the basal cell region of the stria vascularis, and in type 1 fibrocytes of the spiral ligament. No other connexin was detected in these regions. Cx31 was localised among type 2 fibrocytes below the spiral prominence, a region where cx30 was not expressed and cx26 expression appeared to be low. Cx43 was detected only in the region of "tension fibrocytes" lining the inner aspect of the otic capsule. This suggests separate functional compartments in the cochlea. In addition to cx26 and cx30, cx43 was detected in supporting cells of the vestibular sensory epithelia. Where cx26 and cx30 were colocalised, double immunogold labelling of thin sections showed both cx26 and cx30 evenly distributed in individual GJ plaques, a pattern consistent with the presence of heteromeric connexons. Coimmunoprecipitation of cochlear membrane proteins solubilised with a procedure that preserves the oligomeric structure of connexons confirmed the presence of heteromeric cx26/cx30 connexons. Heteromeric cx26/cx30 connexons may be unique to the inner ear, which could be one factor underlying the non-syndromic character of the deafness caused by mutations in cx26.

Calmodulin and protein kinase C regulate gap junctional coupling in lens epithelial cells

The mechanisms regulating the permeability of lens epithelial cell gap junctions in response to calcium ionophore or ATP agonist-mediated increases in cytosolic Ca2+ (Cai2+) have been investigated using inhibitors of calmodulin (CaM) and PKC. Cell-to-cell transfer of the fluorescent dye AlexaFluor594 decreased after the rapid and sustained increase in Cai2+ (to micromolar concentrations) observed after the addition of ionophore plus Ca2+ but was prevented by pretreatment with inhibitors of CaM but not PKC. In contrast, the delayed, transient decrease in cell-to-cell coupling observed after the addition of ATP that we have reported previously (Churchill G, Lurtz MM, and Louis CF. Am J Physiol Cell Physiol 281: C972-C981, 2001) could be prevented by either the direct or indirect inhibition of PKC but not by inhibition of CaM. Surprisingly, there was no change in the relative proportion of the different phosphorylated forms of lens connexin43 after this ATP-dependent transient decrease in cell-to-cell coupling. Although BAPTA-loaded cells did not display the ATP-dependent transient increase in Cai2+, the delayed, transient decrease in cell-to-cell dye transfer was still observed, indicating it was Cai2+ independent. Thus CaM-mediated inhibition of lens gap junctions is associated with sustained, micromolar Cai2+ concentrations, whereas PKC-mediated inhibition of lens gap junctions is associated with agonist activation of second messenger pathways that are independent of changes in Cai2+.

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.

The genetic and molecular basis of congenital eye defects

The mature eye is a complex organ that develops through a highly organized process during embryogenesis. Alterations in its genetic programming can lead to severe disorders that become apparent at birth or shortly afterwards; for example, one-half of the cases of blindness in children have a genetic cause. This review outlines the genetic basis of eye development, as determined by mutation analysis in patients and in model organisms. A better understanding of how this intricate organ develops at the genetic and cellular level is central to our understanding of the pathologies that afflict it.