Molecular mechanism underlying a Cx50-linked congenital cataract

Human Cx50 Isoleucine177 prevents heterotypic docking and formation of functional gap junction channels with Cx43

The human lens is an avascular organ, and its transparency is dependent on gap junction (GJ)-mediated microcirculation. Lens GJs are composed of three connexins with Cx46 and Cx50 being expressed in lens fiber cells and Cx43 and Cx50 in the epithelial cells. Impairment of GJ communication by either Cx46 or Cx50 mutations has been shown to be one of the main molecular mechanisms of congenital cataracts in mutant carrier families. The docking compatibility and formation of functional heterotypic GJs for human lens connexins have not been studied. Previous study on rodent lens connexins revealed that Cx46 can form functional heterotypic GJs with Cx50 and Cx43, but Cx50 cannot form heterotypic GJ with Cx43 due to its second extracellular (EL2) domain. To study human lens connexin docking and formation of functional heterotypic GJs, we developed a genetically engineered HEK293 cell line with endogenously expressed Cx43 and Cx45 ablated. The human lens connexins showed docking compatibility identical to those found in the rodent connexins. To reveal the structural mechanisms of the docking incompatibility between Cx50 and Cx43, we designed eight variants based on the differences between the EL2 of Cx50 and Cx46. We found that Cx50I177L is sufficient to establish heterotypic docking with Cx43 with some interesting gating properties. Our structure models indicate this residue is important for interdomain interactions within a single connexin, Cx50 I177L showed an increased interdomain interaction which might alter the docking interface structure to be compatible with Cx43.NEW & NOTEWORTHY The human lens is an avascular organ, and its transparency is partially dependent on gap junction (GJ) network composed of Cx46, Cx50, and Cx43. We found that human Cx46 can dock and form functional heterotypic GJs with Cx50 and Cx43, but Cx50 is unable to form functional heterotypic GJs with Cx43. Through mutagenesis and patch-clamp study of several designed variants, we found that Cx50 I177L was sufficient to form functional heterotypic GJs with Cx43.

Novel and known variants in GJA3 and LIM2 in congenital cataract families from North India

BACKGROUND

To identify the underlying genetic defects in autosomal dominant (ADCC) and autosomal recessive (ARCC) congenital cataract families from North India.

METHODS

Detailed family histories were collected, pedigrees drawn followed by slit-lamp examination and lens photography. Mutation screening was performed using Sanger sequencing in the known candidate genes for crystallins, connexins, and membrane proteins. The pathogenicity of identified variants was assessed bioinformatically.

RESULTS

In two ADCC families (CC-281 and CC-3015) with posterior lenticonus cataract, a novel change c.263C > T (p.P88L) in GJA3 in CC-281 family and a previously reported substitution c.388C > T (p.R130C) in LIM2 in CC-3015 family was observed. In an ARCC family (CC-3005) having central pulverulent cataract, a novel frameshift deletion (c.764delT;p.L255R46fs) in GJA3 was detected. The observed variants segregated completely with phenotypes in the affected members and were neither present in unaffected family members nor in the ethnically matched 150 controls (tested for two novel variants), hence excluding these as polymorphisms.

CONCLUSIONS

Present study identified two novel mutations i.e., c.263C > T;p.P88L and c.764delT;p.L255R46fs in GJA3 in an ADCC and an ARCC family having posterior lenticonus and central pulverulent cataract, respectively. In another ADCC family with posterior lenticonus cataract, a previously reported mutation c.388C > T;p.R130C in LIM2 was observed. R130 may be a mutation hotspot as previously ADCC families from different ethnicities (UK/Czechia, China, Spain, Japan) also harbored the same substitution, however, with different phenotypes i.e., nuclear pulverulent, membranous, nuclear, lamellar, and sutural/lamellar. Findings in present study thus expand the mutation spectrum and phenotypic heterogeneity linked with GJA3 and LIM2.

A novel mutation GJA8 NM_005267.5: c.124G > A, p.(E42K) causing congenital nuclear cataract

Background

To identify the genetic mutation of a four-generation autosomal dominant congenital cataract family in China.

Methods

Targeted region sequencing containing 778 genes associated with ocular diseases was performed to screen for the potential mutation, and Sanger sequencing was used to confirm the mutation. The homology model was constructed to identify the protein structural change, several online software were used to predict the mutation impact. CLUSTALW was used to perform multiple sequence alignment from different species.

Results

A novel heterozygous mutation, GJA8 NM_005267.5: c.124G > A, p.(E42K) was found, which cosegregated with congenital cataract phenotype in this family. Bioinformatics analysis of the mutation showed that the surface potential diagram of proteins changed. Several online programs predicted the mutation was ‘Pathogenic’, ‘Damaging’, ‘Disease causing’ or ‘Deleterious’.

Conclusions

A novel mutation NM_005267.5(GJA8):c.124G > A was identified in our study. Our finding can broaden the mutation spectrum of GJA8, enrich the phenotype-genotype correlation of congenital cataract and help to better understand the genetic background of congenital cataract.

Molecular mechanisms underlying enhanced hemichannel function of a cataract-associated Cx50 mutant

Connexin-50 (Cx50) is among the most frequently mutated genes associated with congenital cataracts. Although most of these disease-linked variants cause loss of function because of misfolding or aberrant trafficking, others directly alter channel properties. The mechanistic bases for such functional defects are mostly unknown. We investigated the functional and structural properties of a cataract-linked mutant, Cx50T39R (T39R), in the Xenopus oocyte system. T39R exhibited greatly enhanced hemichannel currents with altered voltage-gating properties compared to Cx50 and induced cell death. Coexpression of mutant T39R with wild-type Cx50 (to mimic the heterozygous state) resulted in hemichannel currents whose properties were indistinguishable from those induced by T39R alone, suggesting that the mutant had a dominant effect. Furthermore, when T39R was coexpressed with Cx46, it produced hemichannels with increased activity, particularly at negative potentials, which could potentially contribute to its pathogenicity in the lens. In contrast, coexpression of wild-type Cx50 with Cx46 was associated with a marked reduction in hemichannel activity, indicating that it may have a protective effect. All-atom molecular dynamics simulations indicate that the R39 substitution can form multiple electrostatic salt-bridge interactions between neighboring subunits that could stabilize the open-state conformation of the N-terminal (NT) domain while also neutralizing the voltage-sensing residue D3 as well as residue E42, which participates in loop gating. Together, these results suggest T39R acts as a dominant gain-of-function mutation that produces leaky hemichannels that may cause cytotoxicity in the lens and lead to development of cataracts.

Differential Domain Distribution of gnomAD- and Disease-Linked Connexin Missense Variants

Twenty-one human genes encode connexins, a family of homologous proteins making gap junction (GJ) channels, which mediate direct intercellular communication to synchronize tissue/organ activities. Genetic variants in more than half of the connexin genes are associated with dozens of different Mendelian inherited diseases. With rapid advances in DNA sequencing technology, more variants are being identified not only in families and individuals with diseases but also in people in the general population without any apparent linkage to Mendelian inherited diseases. Nevertheless, it remains challenging to classify the pathogenicity of a newly identified connexin variant. Here, we analyzed the disease- and Genome Aggregation Database (gnomAD, as a proxy of the general population)-linked variants in the coding region of the four disease-linked α connexin genes. We found that the most abundant and position-sensitive missense variants showed distinct domain distribution preference between disease- and gnomAD-linked variants. Plotting missense variants on topological and structural models revealed that disease-linked missense variants are highly enriched on the structurally stable/resolved domains, especially the pore-lining domains, while the gnomAD-linked missense variants are highly enriched in the structurally unstable/unresolved domains, especially the carboxyl terminus. In addition, disease-linked variants tend to be on highly conserved residues and those positions show evolutionary co-variation, while the gnomAD-linked missense variants are likely on less conserved residue positions and on positions without co-variation. Collectively, the revealed distribution patterns of disease- and gnomAD-linked missense variants further our understanding of the GJ structure–biological function relationship, which is valuable for classifying the pathogenicity of newly identified connexin variants.

Whole Exome Sequencing Reveals Novel and Recurrent Disease-Causing Variants in Lens Specific Gap Junctional Protein Encoding Genes Causing Congenital Cataract

Pediatric cataract is clinically and genetically heterogeneous and is the most common cause of childhood blindness worldwide. In this study, we aimed to identify disease-causing variants in three large British families and one isolated case with autosomal dominant congenital cataract, using whole exome sequencing. We identified four different heterozygous variants, three in the large families and one in the isolated case. Family A, with a novel missense variant (c.178G>C, p.Gly60Arg) in GJA8 with lamellar cataract; family B, with a recurrent variant in GJA8 (c.262C>T, p.Pro88Ser) associated with nuclear cataract; and family C, with a novel variant in GJA3 (c.771dupC, p.Ser258GlnfsTer68) causing a lamellar phenotype. Individual D had a novel variant in GJA3 (c.82G>T, p.Val28Leu) associated with congenital cataract. Each sequence variant was found to co-segregate with disease. Here, we report three novel and one recurrent disease-causing sequence variant in the gap junctional protein encoding genes causing autosomal dominant congenital cataract. Our study further extends the mutation spectrum of these genes and further facilitates clinical diagnosis. A recurrent p.P88S variant in GJA8 causing isolated nuclear cataract provides evidence of further phenotypic heterogeneity associated with this variant.

A novel missense mutation of GJA8 causes congenital cataract in a large Mauritanian family

OBJECTIVE OF THE STUDY

Inborn lens opacity is the most frequent cause of childhood blindness. In this study, we aimed to define the presumed genetic cause of a congenital cataract present in a Mauritanian family over the last nine generations.

METHODS

A family history of the disease and eye examination were carried out for the family members. Next-generation sequencing using a panel of 116 cataract underlying genes was selectively conducted on the proband's DNA. Nucleotide and amino acid changes and their impact on the phenotype were evaluated using various data analyzing software.

RESULTS

Congenital nuclear cataract, with autosomal dominant mode, was observed in the family. All patients had consequences on their vision in the first 2 years of life. Genetic screening revealed a new mutation c.166A>C (p.Thr56Pro) in GJA8, encoding the Cx50 α-connexin protein. This mutation co-segregated in all patients and was not observed in the unaffected family members and controls. The predicted secondary structure impacted by p.Thr56Pro revealed a localized disruption, in the first extra membrane loop of the wild-type sheet, which is replaced in the mutant protein by a turn then a coil. This conformational change was functionally predicted as probably damaging.

CONCLUSION

A new mutation (c.166A>C) in GJA8 underlying a nuclear congenital cataract was identified in this study. Its segregation with the phenotype might be useful as a predicting marker of the disease.

Connexin hemichannels mediate glutathione transport and protect lens fiber cells from oxidative stress

Elevated oxidized stress contributes to lens cataracts, and gap junctions play important roles in maintaining lens transparency. As well as forming gap junctions, connexin (Cx) proteins also form hemichannels. Here, we report a new mechanism whereby hemichannels mediate transport of reductant glutathione into lens fiber cells and protect cells against oxidative stress. We found that Cx50 (also known as GJA8) hemichannels opened in response to H2O2 in lens fiber cells but that transport through the channels was inhibited by two dominant-negative mutants in Cx50, Cx50P88S, which inhibits transport through both gap junctions and hemichannels, and Cx50H156N, which only inhibits transport through hemichannels and not gap junctions. Treatment with H2O2 increased the number of fiber cells undergoing apoptosis, and this increase was augmented with dominant-negative mutants that disrupted both hemichannels formed from Cx46 (also known as GJA3) and Cx50, while Cx50E48K, which only impairs gap junctions, did not have such an effect. Moreover, hemichannels mediate uptake of glutathione, and this uptake protected lens fiber cells against oxidative stress, while hemichannels with impaired transport had less protective benefit from glutathione. Taken together, these results show that oxidative stress activates connexin hemichannels in the lens fiber cells and that hemichannels likely protect lens cell against oxidative damage through transporting extracellular reductants.

Connexin 50 Functions as an Adhesive Molecule and Promotes Lens Cell Differentiation

Connexins play essential roles in lens homeostasis and development. Here, we identified a new role for Cx50 that mediates cell-cell adhesion function. Cx50 enhanced the adhesive capability of AQP0. Interestingly, the expression of Cx50 alone promoted cell adhesion at a comparable level to AQP0; however, this cell adhesive function was not observed with other lens connexins, Cx43 and Cx46. Moreover, the adhesive property occurred in both homotypic with Cx50 expressed in both pairing cells and heterotypic with Cx50 in only one pairing cell, and this function appears to be unrelated to its role in forming gap junction channels. Cx50 KO lenses exhibited increased intercellular spaces between lens fiber cells. The second extracellular loop domain (E2) is primarily responsible for this adhesive function. Treatment with a fusion protein containing E2 domain inhibited cell adhesion. Furthermore, disruption of cell adhesion by the E2 domains impaired primary lens cell differentiation. Five critical amino acid residues in the E2 domain primarily are involved in cell adhesive function as well as lens epithelial-fiber differentiation. Together, these results suggest that in addition to forming gap junction channels, Cx50 acts as an adhesive molecule that is critical in maintaining lens fiber integrity and epithelial-fiber differentiation.

Heterozygous connexin 50 mutation affects metabolic syndrome attributes in spontaneously hypertensive rat

BACKGROUND

Several members of connexin family of transmembrane proteins were previously implicated in distinct metabolic conditions. In this study we aimed to determine the effects of complete and heterozygous form of connexin50 gene (Gja8) mutation L7Q on metabolic profile and oxidative stress parameters in spontaneously hypertensive inbred rat strain (SHR).

METHODS

Adult, standard chow-fed male rats of SHR, heterozygous SHR-Dca+/- and SHR-Dca-/- coisogenic strains were used. At the age of 4 months, dexamethasone (2.6 μg/ml) was administered in the drinking water for three days. The lipidemic profile (cholesterol and triacylglycerol concentration in 20 lipoprotein fractions, chylomicron, VLDL, LDL and HDL particle sizes) together with 33 cytokines and hormones in serum and several oxidative stress parameters in plasma, liver, kidney and heart were assessed.

RESULTS

SHR and SHR-Dca-/- rats had similar concentrations of triacylglycerols and cholesterol in all major lipoprotein fractions. The heterozygotes reached significantly highest levels of total (SHR-Dca+/-: 51.3 ± 7.2 vs. SHR: 34.5 ± 2.4 and SHR-Dca-/-: 34.4 ± 2.5 mg/dl, p = 0.026), chylomicron and VLDL triacylglycerols. The heterozygotes showed significantly lowest values of HDL cholesterol (40.9 ± 2.3 mg/dl) compared both to SHR (51.8 ± 2.2 mg/dl) and SHR-Dca-/- (48.6 ± 2.7 mg/dl). Total and LDL cholesterol in SHR-Dca+/- was lower compared to SHR. Glucose tolerance was improved and insulin concentrations were lowest in SHR-Dca-/- (1.11 ± 0.20 pg/ml) in comparison with both SHR (2.32 ± 0.49 pg/ml) and SHR-Dca+/- (3.04 ± 0.21 pg/ml). The heterozygous rats showed profile suggestive of increased oxidative stress as well as highest serum concentrations of several pro-inflammatory cytokines including interleukins 6, 12, 17, 18 and tumor necrosis factor alpha.

CONCLUSIONS

Our results demonstrate that connexin50 mutation in heterozygous state affects significantly the lipid profile and the oxidative stress parameters in the spontaneously hypertensive rat strain.

Vascular Effects of Natriuretic Peptides in Healthy Men

Background: Most effects of atrial (ANP) and B-type natriuretic peptide (BNP) result from stimulation of the guanylyl-cyclase type A receptor. Chronic elevation causes hyporesponsiveness to ANP, whereas BNP effects tend to be preserved, implying an additional pathway of action. We, therefore, investigated the hemodynamic effects of co-infusion of ANP, BNP, and, as a positive control acting on type B receptor, C-type natriuretic peptide (CNP). Furthermore, vascular responses to short and prolonged infusions were compared to investigate rapid hyporesponsiveness of guanylyl-cyclase type A receptor.

Methods: In 11 healthy volunteers, arterial response to continuous intra-arterial infusion of ANP (60 pmol/100 mL forearm tissue volume [FAV]/min) was assessed by venous occlusion plethysmography. Then, co-infusion of a similar dose of ANP, BNP, or CNP was administered in randomized order. Each infusion phase was followed by a washout period. Then, ANP was restarted, followed by co-infusion of one of the natriuretic peptides not yet infused. After a further washout period, ANP was restarted, followed by co-infusion of the natriuretic peptide not yet co-infused. In 6 subjects, infusion time was adjusted to plasma half-lives (5 times), and in the other 5 subjects, infusion time was 5 minutes.

Results: ANP alone caused the expected vasodilation from 2.7 ± 0.3 mL/min/100 mL FAV to 6.0 ± 0.9 mL/min/100 mL FAV ( P < .004). This response remained unchanged in the group that received short-term infusions (6.2 ± 0.8 mL/min/100 mL FAV to 6.6 ± 1.1 mL/min/100mL FAV) but was reduced over time in the group receiving longer-term infusions (6.5 ± 1.2 mL/min/100 mL FAV to 4.5 ± 0.7 mL/min/100mL FAV, P < .05; difference between groups P < .05). Co-infusions of ANP, BNP, and CNP caused minor additional vasodilation (mean 0.8 ± 0.2 mL/min/100ml FAV, P < .01), which did not differ between the different co-infused natriuretic peptides.

Conclusion: Our data provide evidence for rapid desensitization of the guanylyl-cyclase type A receptor in humans, but do not support the presence of a BNP-specific receptor.

Identification and functional analysis of two novel connexin 50 mutations associated with autosome dominant congenital cataracts

Autosomal dominant congenital cataracts (ADCC) are clinically and genetically heterogeneous diseases. The present study recruited two Chinese families with bilateral nuclear cataract or zonular pulverulent phenotype. Direct sequencing of candidate genes identified two novel missense mutations of Cx50, Cx50P59A (c.175C > G) and Cx50R76H (c.227G > A), both co-segregated well with all affected individuals. Bioinformatics analysis predicted deleterious for both mutations. Functional and cellular behaviors of wild type and mutant Cx50 examined by stably transfecting recombinant systems revealed similar protein expression levels. Protein distribution pattern by fluorescence microscopy showed that Cx50R76H localized at appositional membranes forming gap junctions with enormous cytoplasmic protein accumulation, whereas the Cx50P59A mutation was found inefficient at forming detectable plaques. Cell growth test by MTT assay showed that induction of Cx50P59A decreased cell viability. Our study constitutes the first report that the Cx50P59A and Cx50R76H mutations are associated with ADCC and expands the mutation spectrum of Cx50 in association with congenital cataracts. The genetic, cellular, and functional data suggest that the altered intercellular communication governed by mutated Cx50 proteins may act as the molecular mechanism underlying ADCC, which further confirms the role of Cx50 in the maintenance of human lens transparency.

Connexinopathies: a structural and functional glimpse

Mutations in human connexin (Cx) genes have been related to diseases, which we termed connexinopathies. Such hereditary disorders include nonsyndromic or syndromic deafness (Cx26, Cx30), Charcot Marie Tooth disease (Cx32), occulodentodigital dysplasia and cardiopathies (Cx43), and cataracts (Cx46, Cx50). Despite the clinical phenotypes of connexinopathies have been well documented, their pathogenic molecular determinants remain elusive. The purpose of this work is to identify common/uncommon patterns in channels function among Cx mutations linked to human diseases. To this end, we compiled and discussed the effect of mutations associated to Cx26, Cx32, Cx43, and Cx50 over gap junction channels and hemichannels, highlighting the function of the structural channel domains in which mutations are located and their possible role affecting oligomerization, gating and perm/selectivity processes.

A novel Cx50 (GJA8) p.H277Y mutation associated with autosomal dominant congenital cataract identified with targeted next-generation sequencing

BACKGROUND

To unravel the molecular genetic background responsible for autosomal dominant congenital pulverulent nuclear cataracts in a four-generation Chinese family.

METHODS

Family history data were collected, ophthalmological examinations were performed, and genomic DNA was extracted from peripheral blood of the family members. The candidate genes were captured and sequenced by targeted next-generation sequencing, and the results were confirmed by Sanger sequencing. The structure modelling of the protein was displayed based on Swiss-Model Server, and its possible changes in the secondary structure were predicted using Antheprot 2000 software. The chemical dissimilarity and possible functional impact of an amino acid substitution were performed with Grantham score, PolyPhen-2, and SIFT predictions. Protein distributions were assessed by confocal microscopy.

RESULTS

A novel heterozygous c.829C > T transition that led to the substitution of a highly conserved histidine by tyrosine at codon 277 (p.H277Y) in the coding region of connexin50 (Cx50, GJA8) was identified. Bioinformatics analysis showed that the mutation likely altered the secondary structure of the protein by replacing the helix of the COOH-terminal portion with a turn. The mutation was predicted to be moderately conservative by Grantham score and to be deleterious by both PolyPhen-2 and SIFT with consistent results. In addition, when expressed in COS1 cells, the mutation led to protein accumulation and caused changes in Cx 50 protein localization pattern.

CONCLUSIONS

This is a novel missense mutation [c.829C > T, (p.H277Y)] identified in exon 2 of Cx50. Our findings expand the spectrum of Cx50 mutations that are associated with autosomal dominant congenital pulverulent nuclear cataract.

The connexin46 mutant, Cx46T19M, causes loss of gap junction function and alters hemi-channel gating

An N-terminal mutant of connexin46 (T19M) alters a highly conserved threonine and has been linked to autosomal dominant cataracts. To study the cellular and functional consequences of substitution of this amino acid, T19M was expressed in Xenopus oocytes and in HeLa cells. Unlike wild-type Cx46, T19M did not induce intercellular conductances in Xenopus oocytes. In transfected HeLa cells, T19M was largely localized within the cytoplasm, with drastically reduced formation of gap junction plaques. Expression of rat T19M was cytotoxic, as evidenced by an almost complete loss of viable cells expressing the mutant protein by 48-72 h following transfection. When incubated in medium containing physiological concentrations of divalent cations, T19M-expressing cells showed increased uptake of DAPI as compared with cells expressing wild-type Cx46, suggesting aberrant connexin hemi-channel activity. Time-lapse and dye uptake studies suggested that T19M hemi-channels had reduced sensitivity to Ca(2+). Whole cell patch clamp studies of single transfected HeLa cells demonstrated that rat T19M formed functional hemi-channels with altered voltage-dependent gating. These data suggest that T19M causes cataracts by loss of gap junctional channel function and abnormally increased hemi-channel activity. Furthermore, they implicate this conserved threonine in both gap junction plaque formation and channel/hemi-channel gating in Cx46.

Degradation of a connexin40 mutant linked to atrial fibrillation is accelerated

Several Cx40 mutants have been identified in patients with atrial fibrillation (AF). We have been working to identify physiological or cell biological abnormalities of several of these human mutants that might explain how they contribute to disease pathogenesis. Wild type (wt) Cx40 or four different mutants (P88S, G38D, V85I, and L229M) were expressed by the transfection of communication-deficient HeLa cells or HL-1 cardiomyocytes. Biophysical channel properties and the sub-cellular localization and protein levels of Cx40 were characterized. Wild type Cx40 and all mutants except P88S formed gap junction plaques and induced significant gap junctional conductances. The functional mutants showed only modest alterations of single channel conductances or gating by trans-junctional voltage as compared to wtCx40. However, immunoblotting indicated that the steady state levels of G38D, V85I, and L229M were reduced relative to wtCx40; most strikingly, G38D was only 20-31% of wild type levels. After the inhibition of protein synthesis with cycloheximide, G38D (and to a lesser extent the other mutants) disappeared much faster than wtCx40. Treatment with the proteasomal inhibitor, epoxomicin, greatly increased levels of G38D and restored the abundance of gap junctions and the extent of intercellular dye transfer. Thus, G38D, V85I, and L229M are functional mutants of Cx40 with small alterations of physiological properties, but accelerated degradation by the proteasome. These findings suggest a novel mechanism (protein instability) for the pathogenesis of AF due to a connexin mutation and a novel approach to therapy (protease inhibition).

Structure-function correlation analysis of connexin50 missense mutations causing congenital cataract: electrostatic potential alteration could determine intracellular trafficking fate of mutants

Connexin50 (Cx50) mutations are reported to cause congenital cataract probably through the disruption of intercellular transport in the lens. Cx50 mutants that undergo mistrafficking have generally been associated with failure to form functional gap junction channels; however, sometimes even properly trafficked mutants were found to undergo similar consequences. We hereby wanted to elucidate any structural bases of the varied functional consequences of Cx50 missense mutations through in silico approach. Computational studies have been done based on a Cx50 homology model to assess conservation, solvent accessibility, and 3-dimensional localization of mutated residues as well as mutation-induced changes in surface electrostatic potential, H-bonding, and steric clash. This was supplemented with meta-analysis of published literature on the functional properties of connexin missense mutations. Analyses revealed that the mutation-induced critical alterations of surface electrostatic potential in Cx50 mutants could determine their fate in intracellular trafficking. A similar pattern was observed in case of mutations involving corresponding conserved residues in other connexins also. Based on these results the trafficking fates of 10 uncharacterized Cx50 mutations have been predicted. Further experimental analyses are needed to validate the observed correlation.

Identification of a novel GJA8 (Cx50) point mutation causes human dominant congenital cataracts

Hereditary cataracts are clinically and genetically heterogeneous lens diseases that cause a significant proportion of visual impairment and blindness in children. Human cataracts have been linked with mutations in two genes, GJA3 and GJA8, respectively. To identify the causative mutation in a family with hereditary cataracts, family members were screened for mutations by PCR for both genes. Sequencing the coding regions of GJA8, coding for connexin 50, revealed a C > A transversion at nucleotide 264, which caused p.P88T mutation. To dissect the molecular consequences of this mutation, plasmids carrying wild-type and mutant mouse ORFs of Gja8 were generated and ectopically expressed in HEK293 cells and human lens epithelial cells, respectively. The recombinant proteins were assessed by confocal microscopy and Western blotting. The results demonstrate that the molecular consequences of the p.P88T mutation in GJA8 include changes in connexin 50 protein localization patterns, accumulation of mutant protein, and increased cell growth.

Connexin hemichannels in the lens

The normal function and survival of cells in the avascular lens is facilitated by intercellular communication through an extensive network of gap junctions formed predominantly by three connexins (Cx43, Cx46, and Cx50). In expression systems, these connexins can all induce hemichannel currents, but other lens proteins (e.g., pannexin1) can also induce similar currents. Hemichannel currents have been detected in isolated lens fiber cells. These hemichannels may make significant contributions to normal lens physiology and pathophysiology. Studies of some connexin mutants linked to congenital cataracts have implicated hemichannels with aberrant voltage-dependent gating or modulation by divalent cations in disease pathogenesis. Hemichannels may also contribute to age- and disease-related cataracts.

Functional effects of Cx50 mutations associated with congenital cataracts

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

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

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

Properties of two cataract-associated mutations located in the NH2 terminus of connexin 46

Mutations in connexin 46 are associated with congenital cataracts. The purpose of this project was to characterize cellular and functional properties of two congenital cataract-associated mutations located in the NH2 terminus of connexin 46: Cx46D3Y and Cx46L11S, which we found localized to gap junctional plaques like wild-type Cx46 in transfected HeLa cells. Dual two-microelectrode-voltage-clamp studies of Xenopus oocyte pairs injected with wild-type or mutant rat Cx46 showed that oocyte pairs injected with D3Y or L11S cRNA failed to induce gap junctional coupling, whereas oocyte pairs injected with Cx46 showed high levels of coupling. D3Y, but not L11S, functionally paired with wild-type Cx46. To determine whether coexpression of D3Y or L11S affected the junctional conductance produced by wild-type lens connexins, we studied pairs of oocytes coinjected with equal amounts of mutant and wild-type connexin cRNA. Expression of D3Y or L11S almost completely abolished gap junctional coupling induced by Cx46. In contrast, expression of D3Y or L11S failed to inhibit junctional conductance induced by Cx50. To examine effects of the D3Y and L11S mutations on hemichannel activity, hemichannel currents were measured in connexin cRNA-injected oocytes. Oocytes expressing D3Y exhibited reduced hemichannel activity as well as alterations in voltage gating and charge selectivity while oocytes expressing L11S showed no hemichannel activity. Moreover, coexpression of mutant with wild-type Cx50 or Cx46 gave rise to hemichannels with distinct electrophysiological properties, suggesting that the mutant connexins were forming heteromeric channels with wild-type connexins. These data suggest D3Y and L11S cause cataracts by similar but not identical mechanisms.

Connexin mutants and cataracts

The lens is a multicellular, but avascular tissue that must stay transparent to allow normal transmission of light and focusing of it on the retina. Damage to lens cells and/or proteins can cause cataracts, opacities that disrupt these processes. The normal survival of the lens is facilitated by an extensive network of gap junctions formed predominantly of connexin46 and connexin50. Mutations of the genes that encode these connexins (GJA3 and GJA8) have been identified and linked to inheritance of cataracts in human families and mouse lines. In vitro expression studies of several of these mutants have shown that they exhibit abnormalities that may lead to disease. Many of the mutants reduce or modify intercellular communication due to channel alterations (including loss of function or altered gating) or due to impaired cellular trafficking which reduces the number of gap junction channels within the plasma membrane. However, the abnormalities detected in studies of other mutants suggest that they cause cataracts through other mechanisms including gain of hemichannel function (leading to cell injury and death) and formation of cytoplasmic accumulations (that may act as light scattering particles). These observations and the anticipated results of ongoing studies should elucidate the mechanisms of cataract development due to mutations of lens connexins and abnormalities of other lens proteins. They may also contribute to our understanding of the mechanisms of disease due to connexin mutations in other tissues.

Biphasic effect of linoleic acid on connexin 46 hemichannels

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

Different consequences of cataract-associated mutations at adjacent positions in the first extracellular boundary of connexin50

Gap junction channels, which are made of connexins, are critical for intercellular communication, a function that may be disrupted in a variety of diseases. We studied the consequences of two cataract-associated mutations at adjacent positions at the first extracellular boundary in human connexin50 (Cx50), W45S and G46V. Both of these mutants formed gap junctional plaques when they were expressed in HeLa cells, suggesting that they trafficked to the plasma membrane properly. However, their functional properties differed. Dual two-microelectrode voltage-clamp studies showed that W45S did not form functional intercellular channels in paired Xenopus oocytes or hemichannel currents in single oocytes. When W45S was coexpressed with wild-type Cx50, the mutant acted as a dominant negative inhibitor of wild-type function. In contrast, G46V formed both functional gap junctional channels and hemichannels. G46V exhibited greatly enhanced currents compared with wild-type Cx50 in the presence of physiological calcium concentrations. This increase in hemichannel activity persisted when G46V was coexpressed with wild-type lens connexins, consistent with a dominant gain of hemichannel function for G46V. These data suggest that although these two mutations are in adjacent amino acids, they have very different effects on connexin function and cause disease by different mechanisms: W45S inhibits gap junctional channel function; G46V reduces cell viability by forming open hemichannels.

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

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

Molecular characteristics of inherited congenital cataracts

Congenital cataracts are a major cause of induced blindness in children, and inherited cataracts are the major cause of congenital cataracts. Inherited congenital cataracts have been associated with mutations in specific genes, including those of crystallins, gap junction proteins, membrane transport and channel proteins, the cytoskeleton, and growth and transcription factors. Locating and identifying the genes and mutations involved in cataractogenesis are essential to gaining an understanding of the molecular defects and pathophysiologic characteristics of inherited congenital cataracts. In this review, we summarize the current research in this field.

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.

Role of spleen in integrated control of splanchnic vascular tone: physiology and pathophysiology

Aside from its established immunologic and hematologic functions, the spleen also plays an important role in cardiovascular regulation. This occurs through changes in intrasplenic microvascular tone, as well as through splenic neurohormonal modulation of the renal and mesenteric vascular beds. Splenic regulation of blood volume occurs predominantly through fluid extravasation from the splenic circulation into lymphatic reservoirs; this is controlled by direct modulation of splenic pre- and postcapillary resistance by established physiologic agents such as atrial natriuretic peptide (ANP), nitric oxide (NO), and adrenomedullin (ADM). In addition to physiologic fluid regulation, splenic extravasation is a key factor in the inability to maintain adequate intravascular volume in septic shock. The spleen also controls renal microvascular tone through reflex activation of the splenic afferent and renal sympathetic nerves. This splenorenal reflex not only contributes to the physiologic regulation of blood pressure, but also contributes to the cardiovascular dysregulation associated with both septic shock and portal hypertension. In septic shock, the splenorenal reflex protectively limits splenic extravasation and potentially promotes renal sodium and water reabsorption and release of the vasoconstrictor angiotensin II; this function is eventually overwhelmed as shock progresses. In portal hypertension, on the other hand, the splenorenal reflex-mediated reduction in renal vascular conductance exacerbates sodium and water retention in the kidneys and may eventually contribute to renal dysfunction. Preliminary evidence suggests that the spleen also may play a role in the hemodynamic complications of portal hypertension via neurohormonal modulation of the mesenteric vascular bed. Lastly, the spleen itself may be a source of a vasoactive factor.

Connexin mutation that causes dominant congenital cataracts inhibits gap junctions, but not hemichannels, in a dominant negative manner

The connexin (Cx) 50, E48K, mutation is associated with a human dominant congenital cataract; however, the underlying molecular mechanism has not been characterized. The glutamate (E) residue at position 48 is highly conserved across animal species and types of connexins. When expressed in paired Xenopus oocytes, human (h) and chicken (ch) Cx50 E48K mutants showed no electrical coupling. In addition, this mutation acts in a dominant negative manner when paired hetero-typically or hetero-merically with wild-type Cx50, but has no such effect on Cx46, the other lens fiber connexin. A similar loss-of-function and dominant negative effect was observed using dye transfer assays in the same system. By using two different dye transfer methods, with two different tracer dyes, we found chCx50 E48K expressed in chicken lens embryonic fibroblast cells by retroviral infection similarly failed to induce dye coupling, and prevented wild-type chCx50 from forming functional gap junctions. In contrast to its effect on gap junctions, the E48K mutation has no effect on hemichannel activity when assayed using electrical conductance in oocytes, and mechanically induced dye uptake in cells. Cx50 is functionally involved in cell differentiation and lens development, and the E48K mutant promotes primary lens cell differentiation indistinguishable from wild-type chCx50, despite its lack of junctional channel function. Together the data show that mutations affecting gap junctions but not hemichannel function of Cx50 can lead to dominant congenital cataracts in humans. This clearly supports the model of intercellular coupling of fiber cells creating a microcirculation of nutrients and metabolites required for lens transparency.

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

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

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.

An intact connexin N-terminus is required for function but not gap junction formation

The cytoplasmic N-termini of connexins have been implicated in protein trafficking, oligomerization and channel gating. To elucidate the role of the N-terminus in connexin37 (CX37), we studied mutant constructs containing partial deletions of its 23 N-terminal amino acids and a construct with a complete N-terminus in which residues 2-8 were replaced with alanines. All mutants containing nine or more N-terminal amino acids form gap junction plaques in transiently transfected HeLa cells, whereas most of the longer deletions do not. Although wild-type CX37 allowed intercellular transfer of microinjected neurobiotin in HeLa cells and formed conducting hemichannels in Xenopus oocytes, none of the mutant constructs tested show evidence of channel function. However, in coexpression experiments, N-terminal mutants that formed gap junction plaques potently inhibit hemichannel conductance of wild-type CX37 suggesting their co-oligomerization. We conclude that as much as half the length of the connexin N-terminus can be deleted without affecting formation of gap junction plaques, but an intact N-terminus is required for hemichannel gating and intercellular communication.

Congenital cataracts and their molecular genetics

Cataract can be defined as any opacity of the crystalline lens. Congenital cataract is particularly serious because it has the potential for inhibiting visual development, resulting in permanent blindness. Inherited cataracts represent a major contribution to congenital cataracts, especially in developed countries. While cataract represents a common end stage of mutations in a potentially large number of genes acting through varied mechanisms in practice most inherited cataracts have been associated with a subgroup of genes encoding proteins of particular importance for the maintenance of lens transparency and homeostasis. The increasing availability of more detailed information about these proteins and their functions and is making it possible to understand the pathophysiology of cataracts and the biology of the lens in general.

Cataracts are caused by alterations of a critical N-terminal positive charge in connexin50

PURPOSE

To elucidate the basis of the autosomal dominant congenital nuclear cataracts caused by the connexin50 mutant, CX50R23T, by determining its cellular distribution and functional behavior and the consequences of substituting other amino acids for arginine-23.

METHODS

Connexin50 (CX50) mutants were generated by PCR and transfected into HeLa or N2a cells. Expressed CX50 protein was detected by immunoblot analysis and localized by immunofluorescence. Intercellular communication was assessed by microinjection of neurobiotin or by double whole-cell patch-clamp recording.

RESULTS

HeLa cells stably transfected with CX50R23T or wild-type CX50 produced immunoreactive CX50 bands of identical electrophoretic mobility. Whereas HeLa cells stably expressing CX50 contained abundant gap junction plaques, CX50R23T localized predominantly in the cytoplasm. HeLa cells expressing wild-type CX50 showed large gap junctional conductances and extensive transfer of neurobiotin, but those expressing CX50R23T did not show significant intercellular communication by either assay. Moreover, CX50R23T inhibited the function of coexpressed wild-type CX50. Three CX50R23 substitution mutants (CX50R23K, CX50R23L, and CX50R23W) formed gap junction plaques, whereas two mutant substitutions with negatively charged residues (CX50R23D, CX50R23E) did not form detectable plaques. Only the mutant with a positive charge substitution (CX50R23K) allowed neurobiotin transfer at levels similar to those of wild-type CX50; none of the other mutants induced transfer.

CONCLUSIONS

These results suggest that replacement of amino acid 23 in CX50 by any residue that is not positively charged would lead to cataract formation.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

These results confirm involvement of GJA8 in autosomal recessive cataract.

A novel connexin50 mutation associated with congenital nuclear pulverulent cataracts

PURPOSE

To screen for mutations of connexin50 (Cx50)/GJA8 in a panel of patients with inherited cataract and to determine the cellular and functional consequences of the identified mutation.

METHODS

All patients in the study underwent a full clinical examination and leucocyte DNA was extracted from venous blood. The GJA8 gene was sequenced directly. Connexin function and cellular trafficking were examined by expression in Xenopus oocytes and HeLa cells.

RESULTS

Screening of the GJA8 gene identified a 139 G to A transition that resulted in the replacement of aspartic acid by asparagine (D47N) in the coding region of Cx50. This change co-segregated with cataract among affected members of a family with autosomal dominant nuclear pulverulent cataracts. While pairs of Xenopus oocytes injected with wild type Cx50 RNA formed functional gap junction channels, pairs of oocytes injected with Cx50D47N showed no detectable intercellular conductance. Co-expression of Cx50D47N did not inhibit gap junctional conductance of wild type Cx50. In transiently transfected HeLa cells, wild type Cx50 localised to appositional membranes and within the perinuclear region, but Cx50D47N showed no immunostaining at appositional membranes with immunoreactivity confined to the cytoplasm. Incubation of HeLa cells transfected with Cx50D47N at 27 degrees C resulted in formation of gap junctional plaques.

CONCLUSIONS

The pulverulent cataracts present in members of this family are associated with a novel GJA8 mutation, Cx50D47N, that acts as a loss-of-function mutation. The consequent decrease in lens intercellular communication and changes associated with intracellular retention of the mutant connexin may contribute to cataract formation.

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.

Differential potency of dominant negative connexin43 mutants in oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a congenital autosomal dominant disorder with phenotypic variability, which has been associated with mutations in the GJA1 gene encoding connexin43 (Cx43). Given that Cx43 mutants are thought to be equally co-expressed with wild-type Cx43 in ODDD patients, it is imperative to examine the consequence of these mutants in model systems that reflect this molar ratio. To that end, we used differential fluorescent protein tagging of mutant and wild-type Cx43 to quantitatively monitor the ratio of mutant/wild-type within the same putative gap junction plaques and co-immunoprecipitation to determine if the mutants interact with wild-type Cx43. Together the fluorescence-based assay was combined with patch clamp analysis to assess the dominant negative potency of Cx43 mutants. Our results revealed that the ODDD-linked Cx43 mutants, G21R and G138R, as well as amino terminus green fluorescent protein-tagged Cx43, were able to co-localize with wild-type Cx43 at the gap junction plaque-like structures and to co-immunoprecipitate with wild-type Cx43. All Cx43 mutants demonstrated dominant negative action on gap junctional conductance of wild-type Cx43 but not that of Cx32. More interestingly, these Cx43 mutants demonstrated different potencies in inhibiting the function of wild-type Cx43 with the G21R mutant being two times more potent than the G138R mutant. The potency difference in the dominant negative properties of ODDD-linked Cx43 mutants may have clinical implications for the various symptoms and disease severity observed in ODDD patients.

Somatic mutations in the connexin 40 gene (GJA5) in atrial fibrillation

BACKGROUND

Atrial fibrillation is the most common type of cardiac arrhythmia and a leading cause of cardiovascular morbidity, particularly stroke. The cardiac gap-junction protein connexin 40 is expressed selectively in atrial myocytes and mediates the coordinated electrical activation of the atria. We hypothesized that idiopathic atrial fibrillation has a genetic basis and that tissue-specific mutations in GJA5, the gene encoding connexin 40, may predispose the atria to fibrillation.

METHODS

We sequenced GJA5 from genomic DNA isolated from resected cardiac tissue and peripheral lymphocytes from 15 patients with idiopathic atrial fibrillation. Identified GJA5 mutations were transfected into a gap-junction-deficient cell line to assess their functional effects on protein transport and intercellular electrical coupling.

RESULTS

Four novel heterozygous missense mutations were identified in 4 of the 15 patients. In three patients, the mutations were found in the cardiac-tissue specimens but not in the lymphocytes, indicating a somatic source of the genetic defects. In the fourth patient, the sequence variant was detected in both cardiac tissue and lymphocytes, suggesting a germ-line origin. Analysis of the expression of mutant proteins revealed impaired intracellular transport or reduced intercellular electrical coupling.

CONCLUSIONS

Mutations in GJA5 may predispose patients to idiopathic atrial fibrillation by impairing gap-junction assembly or electrical coupling. Our data suggest that common diseases traditionally considered to be idiopathic may have a genetic basis, with mutations confined to the diseased tissue.

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.

Connexin43 with a cytoplasmic loop deletion inhibits the function of several connexins

Connexins (Cx) form gap junction channels mediating direct intercellular communication. To study the role of amino acids within the cytoplasmic loop, we produced a recombinant adenovirus containing Cx43 with a deletion of amino acids 130-136 (Cx43del(130-136)). Cx43del(130-136) expressed alone in HeLa cells localized within the cytoplasm and did not allow transfer of ions, neurobiotin or Lucifer yellow. When co-expressed with wild type Cx43, Cx43del(130-136) blocked electrical coupling and transfer of neurobiotin or Lucifer yellow. Cx43del(130-136) and Cx43 co-localized by immunofluorescence and were co-purified from Triton X-100-solubilized cell extracts. Intercellular transfer mediated by Cx37 and Cx45 (but not Cx26 or Cx40) was inhibited when co-expressed with Cx43del(130-136). Cx43del(130-136) co-localized with Cx37, Cx40, or Cx45, but not Cx26. These data suggest that Cx43del(130-136) produces connexin-specific inhibition of intercellular communication through formation of heteromeric connexons that are non-functional and/or retained in the cytoplasm.

Cardioprotective functions of atrial natriuretic peptide and B-type natriuretic peptide: a brief review

1. If one was to design a hormone to protect the heart, it would have a number of features shown by the cardiac natriuretic peptides atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP). These hormones are made in cardiomyocytes and are released into the circulation in response to atrial and ventricular stretch, respectively. Atrial natriuretic peptide and BNP can reduce the preload and after-load in normal and failing hearts. They reduce blood volume over the short term by sequestering plasma and over the longer term by promoting renal salt and water excretion and by antagonizing the renin-angiotensin-aldosterone system at many levels. Each of these actions affords indirect benefit to a volume- or pressure-threatened heart. 2. Recent studies have identified additional modes of action of the natriuretic peptides that may also confer cardioprotective benefits, especially in heart disease. The emerging findings are: (i) that ANP and BNP antagonize the cardiac hypertrophic action of angiotensin II and continue working under conditions where endothelial nitric oxide (NO) function is compromised, such as in the presence of high glucose in diabetes; (ii) they potentiate the bradycardia caused by inhibitory ('autoprotective') cardio-cardiac reflexes; and, furthermore, (iii) BNP can suppress cardiac sympathetic nerve activity in humans, including those with heart failure. Thus, it appears that natriuretic peptides can shift sympathovagal balance in a beneficial direction (away from the sympathetic). The vagal reflex and antihypertrophic actions of the peptides are mediated by particulate guanylyl cyclase (pGC) natriuretic peptide receptors. 3. The multiple synergistic actions of the natriuretic peptides make them and their pGC receptors attractive targets for therapy in heart disease. Encouragingly, exogenous natriuretic peptides remain effective even when endogenous peptide levels are raised, as is the case in heart failure. They also remain effective in disease states where other protective mechanisms, such as the NO system, have become ineffective, offering yet further encouragement for the therapeutic use of the natriuretic peptides.

Novel Connexin 43 (GJA1) mutation causes oculo-dento-digital dysplasia with curly hair

Oculo-dento-digital dysplasia (ODDD) [OMIM 164200] is a rare autosomal dominant pleiotropic disorder comprising ocular, craniofacial, and digital anomalies, caused by mutations in the gap junction alpha-1 gene (GJA1 or Connexin 43 (CX43)) [Paznekas et al., 2003]. In a Danish family affected over five generations, we found a novel mutation, 286G --> A, resulting in Val96Met. We provide an easy method for mutation detection by use of the restriction enzyme Nde1 and discuss possible pathogenetic mechanisms, arguing that loss of function cannot be excluded. This is the second article reporting ODDD mutations.

Genetics of childhood cataract

PURPOSE OF REVIEW

Congenital cataracts, although much less common than their age-related counterparts, account for one-tenth of cases of childhood blindness. Approximately half are inherited, either in isolation or as part of a syndrome of ocular or systemic anomalies. This article reviews recent advances made in understanding the molecular genetic basis of isolated, nonsyndromic inherited cataract.

RECENT FINDINGS

New disease-causing mutations continue to be identified and now encompass genes encoding a wide variety of different lens proteins. More detailed investigations of the functional consequences of each mutation are being reported and suggest that lens opacification results not only from precipitation and amyloid-like accumulation of proteins essential for lens transparency but also from interference with their secondary functions.

SUMMARY

Improved functional characterization of mutations causing childhood cataract will improve understanding of lens development and physiology but will also have implications for the more common age-related cataract. This too has a significant genetic component to its etiology, and genes causing monogenic forms of childhood inherited cataract represent excellent candidate genes for age-related cataract. The identification of the genes conferring increased risk of developing age-related cataract will bring closer the development of a medical treatment to delay the onset of lens opacification and need for surgery.

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 role of connexins in human disease

Connexins are the building blocks of gap junctions. In forming a gap junction, six connexins oligomerize to form a hexameric torus called a connexon. The number of gap junctions in a cell ranges from a few to over 105 and imparts to interconnected cells a uniform phenotype. The crucial role that gap junctions play in normal physiology is reflected by the diverse spectrum of human diseases in which allele variants of different gap junction genes are implicated. In particular, mutations in GJB2 are a major cause of autosomal recessive non-syndromic deafness. This discovery has impacted medical practice and makes it incumbent on clinicians to familiarize themselves with the genetic advances that are rapidly occurring in our field.