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

Different GJA8 missense variants reveal distinct pathogenic mechanisms in congenital cataract

AIM

Congenital cataract, a lenticular opacity diagnosed at birth or early in the postnatal period, often causes visual impairment. The pathogenic mechanisms of various cataract-associated variants are complex and diverse, and current knowledge is insufficient. This study aimed to determine the molecular etiology of congenital nuclear cataract in a Han-Chinese family and to reveal the pathogenic mechanisms of common cataract-associated variants with unclear mechanisms.

METHODS

Genetic analysis including whole exome sequencing and bioinformatics analysis were conducted in the family. Functional analysis was performed to elucidate the changes in protein cellular distribution, degradation, and function induced by the variants.

RESULTS

A heterozygous c.773C>T transition (p.S258F) in the gap junction protein alpha 8 gene (GJA8), encoding connexin 50 (Cx50), was identified in a family with congenital nuclear cataract. Functional analysis of this variant and two other GJA8 variants with unclear pathogenic mechanisms showed that the Cx50V44M mutant correctly trafficked to the plasma membrane, whereas the Cx50R76C mutant and Cx50S258F mutant exhibited trafficking defects resulting from delayed degradation and accelerated degradation, respectively. All three mutants exhibited increased autophagic activity, while only the Cx50V44M mutant and Cx50S258F mutant underwent autophagy-mediated Cx50 degradation. All mutants failed to form functional hemichannels and gap junction channels.

SIGNIFICANCE

This study identified a heterozygous GJA8 missense variant c.773C>T (p.S258F) responsible for congenital nuclear cataract, and revealed three distinct pathogenic mechanisms of three cataract-associated GJA8 variants, particularly emphasizing dysregulated autophagy involving in aberrant Cx50 degradation.

Long-term hybridization in a karst window reveals the genetic basis of eye loss in cavefish

Eye loss is a hallmark trait of animals inhabiting perpetual darkness, such as caves. The Mexican tetra ( Astyanax mexicanus) provides an unparalleled model for studying the genetic basis of eye loss. There are two interfertile morphs of the Mexican tetra, sighted surface fish and multiple independently evolved eyeless, blind cavefish populations. Despite decades of research on eye loss in this species, our understanding of the precise genetic basis remains sparse. Here, we focused on the unique Caballo Moro cave, where there is a karst window collapse that introduced sunlight and coexistence of both eyed and eyeless cavefish of similar genetic background. This unique genetic mosaic allowed us to pinpoint coding mutations in Connexin 50 (Cx50), also known as gap junction protein alpha 8b (Gja8b), as critical in the genetic basis of eye loss. CRISPR based knockouts of Cx50 in surface fish result in small or absent eyes as young as 48 hours post-fertilization. Further, we identified similar mutations in Cx50 that alter predicted protein structure among other cave-dwelling fish and even subterranean mammals, indicating a conserved evolutionary mechanism of Cx50 mutations. We introduced a mutation (Cx50-S89K) in mice, which resulted in cataracts, smaller eyes, and smaller lenses. Mutations in Cx50 mimic those identified in human congenital cataracts. We additionally leveraged phenotypic variation in a hybrid cavefish population to demonstrate that eyes provide fish with a metabolic advantage, providing a mechanism by which loss of eyes could be favored by selection. This unique hybridization event allowed us to identify novel alleles that contribute to the convergent evolution of eye loss, providing profound insights into the genetic underpinnings of one of nature's most fascinating adaptive traits.

Mechano-activated connexin hemichannels and glutathione transport protect lens fiber cells against oxidative insults

Long-lived lens fiber cells require a robust cellular protective function against oxidative insults to maintain their hemostasis and viability; however, the underlying mechanism is largely obscure. In this study, we unveiled a new mechanism that protects lens fiber cells against oxidative stress-induced cell death. We found that mechano-activated connexin (Cx) hemichannels (HCs) mediate the transport of glutathione (GSH) into chick embryonic fibroblasts (CEF) and primary lens fiber cells, resulting in a decrease in the accumulation of intracellular reactive oxygen species induced by both H2O2 and ultraviolet B, providing protection to lens fiber cells against cell apoptosis and necrosis. Furthermore, HCs formed by both homomeric Cx50 or Cx46 and heteromeric Cx50/Cx46 were mechanosensitive and could transport GSH into CEF cells. Notably, mechano-activated Cx50 HCs exhibited a greater capacity to transport GSH than Cx46 HCs. Consistently, the deficiency of Cx50 in single lens fiber cells led to a higher level of oxidative stress. Additionally, outer cortical short lens fiber cells expressing full length Cxs demonstrated greater resistance to oxidative injury compared to central core long lens fibers. Taken together, our results suggest that the activation of Cx HCs by interstitial fluid flow in cultured epithelial cells and isolated fiber cells shows that HCs can serve as a pathway for moving GSH across the cell membrane to offer protection against oxidative stress.

Association of variants in GJA8 with familial acorea-microphthalmia-cataract syndrome

Congenital acorea is a rare disease with the absence of a pupil in the eye. To date, only one family and two isolated cases with congenital acorea have been reported. The gene associated with acorea has not been identified. In this study, we recruited a Chinese family acorea-microphthalmia-cataract syndrome. By analyzing the whole-exome sequencing (WES) data of this Chinese family, we revealed the association of a novel heterozygous variant, NM_005267.5:c.137G>A (p.G46E) in the gap junction protein alpha 8 (GJA8) gene encoding connexin 50 or CX50, with familial acorea-microphthalmia-cataract syndrome. Additionally, another variant, NM_005267.5:c.151G>A (p.D51N) in GJA8, was identified to co-segregate with this syndrome in an unrelated Japanese family. Ectopic expression of p.G46E and p.D51N mutant GJA8 genes in cultured cells caused protein mislocalization, suggesting that the p.G46E and p.D51N mutations in GJA8 impaired the function of the gap junction channels. These results established GJA8 as the first gene associated with familial acorea-microphthalmia-cataract syndrome.

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.

Mutation screening in autosomal dominant congenital cataract families from North India

Congenital cataract an opacity of the eye lens is present at birth and results in visual impairment during early childhood. If left untreated, it can lead to permanent blindness. Its prevalence is ten times higher in developing countries like India. Thus, we aimed to investigate the underlying genetic defects in three autosomal dominant congenital cataract (ADCC) families from North India. Detailed family histories were collected, pedigrees drawn followed by slit-lamp examination and lens photography. Mutation screening was performed in the candidate genes for crystallins, connexins, and membrane proteins by Sanger sequencing. Pathogenicity of novel variant was assessed bioinformatically. In an ADCC (CC-3006) family with bilateral membranous cataract and microcornea, a novel change (c.1114C>T;p.P372S) in GJA3 has been detected. In other two ADCC families affected with subcapsular (CC-286) and shrunken membranous hypermature cataract (CC-3014), a nonsense mutation (c.463C>T;p.Q155X) in CRYβB2 and a frameshift deletion (c.590_591delAG;p.E197VfsX22) in CRYβA1/A3 respectively, are observed. These variants segregated completely with the phenotypes in respective families and were absent in their unaffected family members and unrelated controls (tested for novel variant in GJA3). Earlier p.Q155X (CRYβB2) and p.E197VfsX22 (CRYβA1/A3) are reported with entirely different phenotypes. Thus, findings in present study expand the mutation spectrum and phenotypic heterogeneity linked with GJA3, CRYβB2, and CRYβA1/A3 for congenital cataracts. Identifying underlying genetic defects is essential for disease management and appropriate genetic counseling.

Study of The Molecular Nature of Congenital Cataracts in Patients from The Volga-Ural Region

Hereditary cataracts are characterized by significant clinical and genetic heterogeneity, which can pose challenges for early DNA diagnosis. To comprehensively address this problem, it is essential to investigate the epidemiology of the disease, perform population studies to determine the spectrum and frequencies of mutations in the responsible genes, and examine clinical and genetic correlations. Based on modern concepts, non-syndromic hereditary cataracts are predominantly caused by genetic disease forms associated with mutations in crystallin and connexin genes. Therefore, a comprehensive approach to studying hereditary cataracts is necessary for early diagnosis and improved treatment outcomes. The crystallin (CRYAA, CRYAB, CRYGC, CRYGD, and CRYBA1) and connexin (GJA8, GJA3) genes were analyzed in 45 unrelated families from the Volga-Ural Region (VUR) with hereditary congenital cataracts. Pathogenic and probably pathogenic nucleotide variants were identified in ten unrelated families, nine of which had cataracts in an autosomal dominant pattern of inheritance. Two previously undescribed likely pathogenic missense variants were identified in the CRYAA gene: c.253C > T (p.L85F) in one family and c.291C > G (p.H97Q) in two families. The known mutation c.272_274delGAG (p.G91del) was found in the CRYBA1 gene in one family, while no pathogenic variants were found in the CRYAB, CRYGC, or CRYGD genes in the examined patients. In the GJA8 gene, the known mutation c.68G > C (p.R23T) was found in two families, and previously undescribed variants were identified in two other families: a c.133_142del deletion (p.W45Sfs*72) and a missense variant, c.179G > A (p.G60D). In one patient with a recessive form of cataract, two compound-heterozygous variants were identified-a previously undescribed likely pathogenic missense variant, c.143A > G (p.E48G), and a known variant with uncertain pathogenetic significance, c.741T > G (p.I24M). Additionally, a previously undescribed deletion, c.del1126_1139 (p.D376Qfs*69), was identified in the GJA3 gene in one family. In all families where mutations were identified, cataracts were diagnosed either immediately after birth or during the first year of life. The clinical presentation of the cataracts varied depending on the type of lens opacity, resulting in various clinical forms. This information emphasizes the importance of early diagnosis and genetic testing for hereditary congenital cataracts to guide appropriate management and improve outcomes.

Protein kinase A activation alleviates cataract formation via increased gap junction intercellular communication

Cataract is the leading cause of blindness worldwide. Here, we reported a potential, effective therapeutic mean for cataract prevention and treatment. Gap junction communication, an important mechanism in maintaining lens transparency, is increased by protein kinase A (PKA). We found that PKA activation reduced cataracts induced by oxidative stress, increased gap junctions/hemichannels in connexin (Cx) 50, Cx46 or Cx50 and Cx46 co-expressing cells, and decreased reactive oxygen species (ROS) levels. However, ROS reduction was shown in wild-type, Cx46 and Cx50 knockout, but not in Cx46/Cx50 double KO lens. In addition, PKA activation protects lens fiber cell death induced by oxidative stress via hemichannel-mediated glutathione transport. Connexin deletion increased lens opacity induced by oxidative stress associated with reduction of anti-oxidative stress gene expression. Together, our results suggest that PKA activation through increased connexin channels in lens fiber cell decreases ROS levels and cell death, leading to alleviated cataracts.

The second extracellular domain of connexin 50 is important for in cell adhesion, lens differentiation, and adhesion molecule expression

Connexin (Cx)-forming channels play essential roles in maintaining lens homeostasis and transparency. We showed here channel-independent roles of Cx50 in cell-cell adhesion and confirmed the second extracellular (E2) domain as a critical domain for cell adhesion function. We found that cell adhesion decreased in cells expressing chimeric Cx50 in which the E2 domain was swapped with the E2 domain of either Cx43 or Cx46. In contrast, adhesion increased in cells expressing chimeric Cx43 and Cx46 with the Cx50 (E2) domain. This function is Cx channel-independent and Cx50 E2 domain-dependent cell adhesion acting in both homotypic and heterotypic manners. In addition, we generated eight site mutations of unique residues between Cx50 and the other two lens Cxs and found that mutation of any one of the residues abolished the adhesive function. Moreover, expression of adhesive-impaired mutants decreased adhesion-related proteins, N-cadherin and β-catenin. Expression of the adhesion-impaired Cx50W188P mutant in embryonic chick lens caused enlarged extracellular spaces, distorted fiber organization, delayed nuclear condensation, and cortical cataracts. In summary, the results from both in vitro and in vivo studies demonstrate the importance of the adhesive function of Cx50 in the lens.

Connexin Mutations and Hereditary Diseases

Inherited diseases caused by connexin mutations are found in multiple organs and include hereditary deafness, congenital cataract, congenital heart diseases, hereditary skin diseases, and X-linked Charcot–Marie–Tooth disease (CMT1X). A large number of knockout and knock-in animal models have been used to study the pathology and pathogenesis of diseases of different organs. Because the structures of different connexins are highly homologous and the functions of gap junctions formed by these connexins are similar, connexin-related hereditary diseases may share the same pathogenic mechanism. Here, we analyze the similarities and differences of the pathology and pathogenesis in animal models and find that connexin mutations in gap junction genes expressed in the ear, eye, heart, skin, and peripheral nerves can affect cellular proliferation and differentiation of corresponding organs. Additionally, some dominant mutations (e.g., Cx43 p.Gly60Ser, Cx32 p.Arg75Trp, Cx32 p.Asn175Asp, and Cx32 p.Arg142Trp) are identified as gain-of-function variants in vivo, which may play a vital role in the onset of dominant inherited diseases. Specifically, patients with these dominant mutations receive no benefits from gene therapy. Finally, the complete loss of gap junctional function or altered channel function including permeability (ions, adenosine triphosphate (ATP), Inositol 1,4,5-trisphosphate (IP3), Ca²⁺, glucose, miRNA) and electric activity are also identified in vivo or in vitro.

Cell transdifferentiation in ocular disease: Potential role for connexin channels

Cell transdifferentiation is the conversion of a cell type to another without requiring passage through a pluripotent cell state, and encompasses epithelial- and endothelial-mesenchymal transition (EMT and EndMT). EMT and EndMT are well defined processes characterized by a loss of epithelial/endothelial phenotype and gain in mesenchymal spindle shaped morphology, which results in increased cell migration and decreased apoptosis and cellular senescence. Such cells often develop invasive properties. Physiologically, these processes may occur during embryonic development and can resurface, for example, to promote wound healing in later life. However, they can also be a pathological process. In the eye, EMT, EndMT and cell transdifferentiation have all been implicated in development, homeostasis, and multiple diseases affecting different parts of the eye. Connexins, constituents of connexin hemichannels and intercellular gap junctions, have been implicated in many of these processes. In this review, we firstly provide an overview of the molecular mechanisms induced by transdifferentiation (including EMT and EndMT) and its involvement in eye diseases. We then review the literature for the role of connexins in transdifferentiation in the eye and eye diseases. The evidence presented in this review supports the need for more studies into the therapeutic potential for connexin modulators in prevention and treatment of transdifferentiation related eye diseases, but does indicate that connexin channel modulation may be an upstream and unifying approach for regulating these otherwise complex processes.

Development of a potent embryonic chick lens model for studying congenital cataracts in vivo

Congenital cataracts are associated with gene mutations, yet the underlying mechanism remains largely unknown. Here we reported an embryonic chick lens model that closely recapitulates the process of cataract formation. We adopted dominant-negative site mutations that cause congenital cataracts, connexin, Cx50E48K, aquaporin 0, AQP0R33C, αA-crystallin, CRYAA R12C and R54C. The recombinant retroviruses containing these mutants were microinjected into the occlusive lumen of chick lenses at early embryonic development. Cx50E48K expression developed cataracts associated with disorganized nuclei and enlarged extracellular spaces. Expression of AQP0R33C resulted in cortical cataracts, enlarged extracellular spaces and distorted fiber cell organization. αA crystallin mutations distorted lens light transmission and increased crystalline protein aggregation. Together, retroviral expression of congenital mutant genes in embryonic chick lenses closely mimics characteristics of human congenital cataracts. This model will provide an effective, reliable in vivo system to investigate the development and underlying mechanism of cataracts and other genetic diseases.

Cutaneous and hepatic vascular lesions due to a recurrent somatic GJA4 mutation reveal a pathway for vascular malformation

The term “cavernous hemangioma” has been used to describe vascular anomalies with histology featuring dilated vascular spaces, vessel walls consisting mainly of fibrous stromal bands lined by a layer of flattened endothelial cells, and an irregular outer rim of interrupted smooth muscle cells. Hepatic hemangiomas (HHs) and cutaneous venous malformations (VMs) share this histologic pattern, and we examined lesions in both tissues to identify genetic drivers. Paired whole-exome sequencing (WES) of lesional tissue and normal liver in HH subjects revealed a recurrent GJA4 c.121G>T (p.Gly41Cys) somatic mutation in four of five unrelated individuals, and targeted sequencing in paired tissue from 9 additional HH individuals identified the same mutation in 8. In cutaneous lesions, paired targeted sequencing in 5 VMs and normal epidermis found the same GJA4 c.121G>T (p.Gly41Cys) somatic mutation in three. GJA4 encodes gap junction protein alpha 4, also called connexin 37 (Cx37), and the p.Gly41Cys mutation falls within the first transmembrane domain at a residue highly conserved among vertebrates. We interrogated the impact of the Cx37 mutant via lentiviral transduction of primary human endothelial cells. We found that the mutant induced changes in cell morphology and activated serum/glucocorticoid-regulated kinase 1 (SGK1), a serine/threonine kinase known to regulate cell proliferation and apoptosis, via non-canonical activation. Treatment with spironolactone, an inhibitor of angiogenesis, suppressed mutant SGK1 activation and reversed changes in cell morphology. These findings identify a recurrent somatic GJA4 c.121G>T mutation as a driver of hepatic and cutaneous VMs, revealing a new pathway for vascular anomalies, with spironolactone a potential pathogenesis-based therapy.

Connexin-46/50 in a dynamic lipid environment resolved by CryoEM at 1.9 Å

Gap junctions establish direct pathways for cells to transfer metabolic and electrical messages. The local lipid environment is known to affect the structure, stability and intercellular channel activity of gap junctions; however, the molecular basis for these effects remains unknown. Here, we incorporate native connexin-46/50 (Cx46/50) intercellular channels into a dual lipid nanodisc system, mimicking a native cell-to-cell junction. Structural characterization by CryoEM reveals a lipid-induced stabilization to the channel, resulting in a 3D reconstruction at 1.9 Å resolution. Together with all-atom molecular dynamics simulations, it is shown that Cx46/50 in turn imparts long-range stabilization to the dynamic local lipid environment that is specific to the extracellular lipid leaflet. In addition, ~400 water molecules are resolved in the CryoEM map, localized throughout the intercellular permeation pathway and contributing to the channel architecture. These results illustrate how the aqueous-lipid environment is integrated with the architectural stability, structure and function of gap junction communication channels.

Characterization of a p.R76H mutation in Cx50 identified in a Chinese family with congenital nuclear cataract

BACKGROUND/PURPOSE

A three-generation Chinese family with autosomal dominant congenital nuclear cataract was recruited. This study aimed to identify the disease-causing gene for nuclear cataract with functional dissections of the identified mutant.

METHODS

Detailed clinical data and family history were recorded. Candidate gene sequencing was performed to identify the disease-causing mutation. Recombinant connexin50 (Cx50) wild type and mutant constructs were synthesized. Triton X-100 solubility and subcellular localization of the recombinant Cx50 proteins were analyzed in HeLa cells. Apoptosis was assayed as the percentage of fragmented nuclei in transfected cells.

RESULTS

All affected individuals in the family displayed clear phenotypes of dense nuclear cataracts. A c.227 G > A variation was found in the coding region of Cx50, which arginine residue at position 76 was substituted by histidine (p.R76H). This mutation was co-segregated with the disease in the family, and was not observed in 110 unrelated Chinese controls. No statistically significant differences were found in the Triton X-100 solubility and apoptosis rate between wild type and mutant Cx50 in HeLa cells. However, Cx50 mutant was unable to form gap junctional plaques between adjacent cells as the wild type proteins did.

CONCLUSION

This study identified a novel cataract phenotype caused by the p.R76H mutation in Cx50, providing evidence of further phenotypic heterogeneity associated with this mutation. Functional analysis showed that the mutation affected the formation of gap junction channels and led to opacity in the lens.

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.

A Novel Cx50 Insert Mutation from a Chinese Congenital Cataract Family Impairs Its Cellular Membrane Localization and Function

Mutations in GJA8 are associated with hereditary autosomal dominant and recessive cataract formation. In this study, a novel insert mutation in GJA8 was identified in a Chinese congenital cataract family and cosegregated with the disease in this pedigree. This insert mutation introduces five additional amino acid residues YAVHY after histidine at the 95 site (p.H95_A96insYAVHY) within the second transmembrane (TM2) domain of Cx50 protein (Cx50-insert). Ectopic expression of Cx50-insert protein impairs the hemichannel functions and gap junction activity compared to wild-type Cx50 protein in human lens epithelial cells. Cx50-insert proteins were mislocated from cytoplasmic membrane to endoplasmic reticulum and lysosome. In mouse lens tissue, our results showed that Cx50 predominant expresses in epithelial cells and fiber cells at the transition zone of lens hinting its roles in lens differentiation. Taken together, these data suggest that the novel insert mutation in the TM2 domain of Cx50 protein, which impairs its trafficking to the cell membrane and gap-junction function, is associated with the cataract formation in this Chinese pedigree.

Identification of a novel GJA3 mutation in a large Chinese family with congenital cataract using targeted exome sequencing

Autosomal dominant congenital cataract (ADCC) is a clinically and genetically heterogeneous ocular disease in children that results in serious visual impairments or even blindness. Targeted exome sequencing (TES) is an efficient method used for genetic diagnoses of inherited diseases. In the present study, we used a custom-made TES panel to identify the genetic defect of a four-generation Chinese family with bilateral pulverulent nuclear cataracts. A novel heterozygous missense mutation c.443C>T (p. T148I) in GJA3 was identified. The results of the bioinformatic analysis showed that the mutation was deleterious to the structure and hemichannel function of Cx46 encoded by GJA3. Plasmids expressing wild-type and mutant human Cx46 were constructed and ectopically expressed in human lens epithelial cells (HLECs) or human embryonic kidney (HEK-293) cells. Fluorescent images indicated aggregated signals of mutant protein in the cytoplasm, and a higher protein level was also detected in T148I stable cell lines. In summary, we identified a novel mutation in GJA3 for ADCC, which provided molecular insights into the pathogenic mechanism of ADCC.

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.

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.

Mutant connexin 50 (S276F) inhibits channel and hemichannel functions inducing cataract

This study was designed to detect the expression, detergent resistance, subcellular localization, and channel and hemichannel functions of mutant Cx50 to understand the forming mechanism for inducing congenital cataract by a novel mutation p.S276F in connexin 50 (Cx50) reported previously by us. HeLa and human lens epithelial (HLE) cells were transfected with wild-type Cx50 and mutant Cx50 (S276F). We examined the functional characteristics of mutant Cx50 (S276F) in comparison with those of wild-type Cx50 using immunoblot, confocal fluorescence microscopy, dye transfer analysis and dye uptake assay. The mutant and wild-type Cx50 were expressed in equal levels and could efficiently localize to the plasma membrane without transportation and assembly problems. Scrape loading dye transfer was significantly evident in cells transfected with wild-type Cx50 compared to those in cells transfected with mutant Cx50 and cotransfected with wild-type and mutant Cx50. The dye uptake was found to be significantly lower in cells transfected with mutant Cx50 than in cells transfected with wild- type Cx50 and cells cotransfected with wild-type and mutant Cx50. The transfected HeLa and HLE cell lines showed similar performance in all the experiments. These results indicated that the mutant Cx50 (S276F) might inhibit the function of gap junction channel in a dominant negative manner, but inhibit the hemichannel function in a recessive negative manner.

Connexin Controls Cell-Cycle Exit and Cell Differentiation by Directly Promoting Cytosolic Localization and Degradation of E3 Ligase Skp2

Connexins and connexin channels play important roles in cell growth/differentiation and tumorigenesis. Here, we identified a relationship between a connexin molecule and a critical cell-cycle regulator. Our data show that connexin (Cx) 50 regulated lens cell-cycle progression and differentiation by modulating expression of cyclin-dependent kinase inhibitor p27/p57 and E3 ubiquitin ligase Skp2. Cx50 directly interacted with and retained Skp2 in the cytosol by masking the nuclear targeting domain of Skp2, and this effect was supported by an increased nuclear localization of Skp2, disruption of Skp2 interaction with importin-7, and decreased levels of p27/p57 in mouse lenses lacking Cx50. As a result, Cx50 increased auto-ubiquitination and subsequent degradation of Skp2. A mutation (V362E) on the C terminus of Cx50 disrupted the interaction between Cx50 and Skp2 and completely abolished such effects. Therefore, this study identifies a role for connexins in regulating cell-cycle modulators and, consequently, cell growth and differentiation.

A novel GJA8 mutation (p.V44A) causing autosomal dominant congenital cataract

Purpose

To examine the mechanism by which a novel connexin 50 (Cx50) mutation, Cx50 V44A, in a Chinese family causes suture-sparing autosomal dominant congenital nuclear cataracts.

Methods

Family history and clinical data were recorded and direct gene sequencing was used to identify the disease-causing mutation. The Cx50 gene was cloned from a human lens cDNA library. Connexin protein distributions were assessed by fluorescence microscopy. Hemichannel functions were analyzed by dye uptake assay. Formation of functional channels was assessed by dye transfer experiments.

Results

Direct sequencing of the candidate GJA8 gene revealed a novel c.131T>C transition in exon 2, which cosegregated with the disease in the family and resulted in the substitution of a valine residue with alanine at codon 44 (p. V44A) in the extracellular loop 1 of the Cx50 protein. Both Cx50 and Cx50V44A formed functional gap junctions, as shown by the neurobiotin transfer assay. However, unlike wild-type Cx50, Cx50V44A was unable to form open hemichannels in dye uptake experiments.

Conclusion

This work identified a unique congenital cataract in the Chinese population, caused by the novel mutation Cx50V44A, and it showed that the V44A mutation specifically impairs the gating of the hemichannels but not the gap junction channels. The dysfunctional hemichannels resulted in the development of human congenital cataracts.

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.

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 cataract-causing connexin 50 mutant is mislocalized to the ER due to loss of the fourth transmembrane domain and cytoplasmic domain

Mutations in the eye lens gap junction protein connexin 50 cause cataract. Earlier we identified a frameshift mutant of connexin 50 (c.670insA; p.Thr203AsnfsX47) in a family with autosomal recessive cataract. The mutant protein is smaller and contains 46 aberrant amino acids at the C-terminus after amino acid 202. Here, we have analysed this frameshift mutant and observed that it localized to the endoplasmic reticulum (ER) but not in the plasma membrane. Moreover, overexpression of the mutant resulted in disintegration of the ER-Golgi intermediate compartment (ERGIC), reduction in the level of ERGIC-53 protein and breakdown of the Golgi in many cells. Overexpression of the frameshift mutant partially inhibited the transport of wild type connexin 50 to the plasma membrane. A deletion mutant lacking the aberrant sequence showed predominant localization in the ER and inhibited anterograde protein transport suggesting, therefore, that the aberrant sequence is not responsible for improper localization of the frameshift mutant. Further deletion analysis showed that the fourth transmembrane domain and a membrane proximal region (231-294 amino acids) of the cytoplasmic domain are needed for transport from the ER and localization to the plasma membrane. Our results show that a frameshift mutant of connexin 50 mislocalizes to the ER and causes disintegration of the ERGIC and Golgi. We have also identified a sequence of connexin 50 crucial for transport from the ER and localization to the plasma membrane.

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.

Regulation of connexin hemichannel activity by membrane potential and the extracellular calcium in health and disease

Connexins are thought to solely mediate cell-to-cell communication by forming gap junction channels composed of two membrane-spanning hemichannels positioned end-to-end. However, many if not all connexin isoforms also form functional hemichannels (i.e., the precursors of complete channels) that mediate the rapid exchange of ions, second messengers and metabolites between the cell interior and the interstitial space. Electrical and molecular signaling via connexin hemichannels is now widely recognized to be important in many physiological scenarios and pathological conditions. Indeed, mutations in connexins that alter hemichannel function have been implicated in several diseases. Here, we present a comprehensive overview of how hemichannel activity is tightly regulated by membrane potential and the external calcium concentration. In addition, we discuss the genetic mutations known to alter hemichannel function and their deleterious effects, of which a better understanding is necessary to develop novel therapeutic approaches for diseases caused by hemichannel dysfunction. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

A novel connexin 50 gene (gap junction protein, alpha 8) mutation associated with congenital nuclear and zonular pulverulent cataract

PURPOSE

To characterize the disease-causing mutations in four generations of a Chinese family affected with bilateral congenital nuclear and zonular pulverulent cataract.

METHODS

Detailed family history and clinical data were recorded. The phenotype was documented using slit-lamp photography. Candidate genes were amplified using PCR and screened for mutations using bidirectional sequencing.

RESULTS

Affected individuals had nuclear and zonular pulverulent cataract with Y-sutural opacities. Sequencing of the candidate genes revealed a heterozygous c. 139G>C change in the coding sequence of the connexin 50 gene (gap junction protein, alpha 8 [GJA8]), which results in the substitution of a wild-type aspartic acid with a histidine (D47H). This mutation cosegregated with all affected individuals in the family and was not found in unaffected family members or in 100 unrelated controls.

CONCLUSIONS

Our study has identified a novel connexin 50 gene (GJA8) mutation, resulting in the amino substitution p. D47H in a Chinese family with nuclear and zonular pulverulent congenital cataracts. This mutation is probably the causative lesion for the observed phenotype in this family.

Glial connexin expression and function in the context of Alzheimer's disease

A hallmark of neurodegenerative diseases is the reactive gliosis characterized by a phenotypic change in astrocytes and microglia. This glial response is associated with modifications in the expression and function of connexins (Cxs), the proteins forming gap junction channels and hemichannels. Increased Cx expression is detected in most reactive astrocytes located at amyloid plaques, the histopathological lesions typically present in the brain of Alzheimer's patients and animal models of the disease. The activity of Cx channels analyzed in vivo as well as in vitro after treatment with the amyloid β peptide is also modified and, in particular, hemichannel activation may contribute to neuronal damage. In this review, we summarize and discuss recent data that suggest glial Cx channels participate in the neurodegenerative process of Alzheimer's disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Phosphorylation of connexin 50 by protein kinase A enhances gap junction and hemichannel function

Phosphorylation of connexins is an important mechanism regulating gap junction channels. However, the role(s) of connexin (Cx) phosphorylation in vivo are largely unknown. Here, we showed by mass spectrometry that Ser-395 in the C terminus of chicken Cx50 was phosphorylated in the lens. Ser-395 is located within a PKA consensus site. Analyses of Cx50 phosphorylation by two-dimensional thin layer chromatography tryptic phosphopeptide profiles suggested that Ser-395 was targeted by PKA in vivo. PKA activation increased both gap junction dye coupling and hemichannel dye uptake in a manner not involving increases in total Cx50 expression or relocation to the cell surface or gap junctional plaques. Single channel recordings indicated PKA enhanced transitions between the closed and ∼200-pS open state while simultaneously reducing transitions between this open state and a ∼65-pS subconductance state. The mutation of Ser-395 to alanine significantly attenuated PKA-induced increases in dye coupling and uptake by Cx50. However, channel records indicated that phosphorylation at this site was unnecessary for enhanced transitions between the closed and ∼200-pS conductance state. Together, these results suggest that Cx50 is phosphorylated in vivo by PKA at Ser-395 and that this event, although unnecessary for PKA-induced alterations in channel conductance, promotes increased dye permeability of Cx50 channels, which plays an important role in metabolic coupling and transport in lens fibers.

Mutations in connexin genes and disease

BACKGROUND

Connexins are a family of transmembrane proteins that are widely expressed in the human body. Connexins play an important role in cell-cell communication and homeostasis in various tissues by forming gap junction channels, which enable a direct passage of ions or metabolites from one cell to another. Twenty-one different connexins are expressed in humans, each having distinct expression patterns and regulation properties. Knowledge on this family of proteins can be gained by making an inventory of mutations and associated diseases in human.

DESIGN

PubMed and other relevant databases were searched. In addition, key review articles were screened for relevant original publications. Sections of representative organs were photographed and annotated.

RESULTS

The crucial role of connexins is highlighted by the discovery of mutations in connexin genes which cause a variety of disorders such as myelin-related diseases, skin disorders, hearing loss, congenital cataract, or more complex syndromes such as the oculodendrodigital dysplasia. This review systematically addresses current knowledge on mutations in connexin genes and disease, focusing on the correlation between genetic defects, cellular phenotypes and clinical manifestations.

CONCLUSIONS

The review of diseases caused by mutations in connexin genes highlights the essential nature of connexin function and intercellular communication in tissue homeostasis.

Genetic linkage analyses and Cx50 mutation detection in a large multiplex Chinese family with hereditary nuclear cataract

PURPOSE

The aim of the study was to characterize the underlying mutation in a large multiplex Chinese family with hereditary nuclear cataract.

METHODS

A 6-generation Chinese family having hereditary nuclear cataract was recruited and clinically verified. Blood DNA samples were obtained from 53 available family members. Linkage analyses were performed on the known candidate regions for hereditary cataract with 36 polymorphic microsatellite markers. To identify mutations related to cataract, a direct sequencing approach was applied to a candidate gene residing in our linkage locus.

RESULTS

A linkage locus was identified with a maximum 2-point LOD score of 4.31 (recombination fraction = 0) at marker D1S498 and a maximum multipoint LOD score of 5.7 between markers D1S2344 and D1S498 on chromosome 1q21.1, where the candidate gene Cx50 is located. Direct sequencing of Cx50 showed a 139 G to A transition occurred in all affected family members. This transitional mutation resulted in a replacement of aspartic acid by asparagine at residue 47 (D47N) and led to a loss-of-function of the protein.

CONCLUSIONS

The D47N mutation of Cx50 causes the hereditary nuclear cataract in this family in an autosomal dominant mode of inheritance with incomplete penetrance.

Aquaporin 0 enhances gap junction coupling via its cell adhesion function and interaction with connexin 50

Both connexin 50 (Cx50) and aquaporin 0 (AQP0) have important roles in lens development and homeostasis, and their mutations are associated with human congenital cataracts. We have previously shown that Cx50 directly interacts with AQP0. Here, we demonstrate the importance of the Cx50 intracellular loop (IL) domain in mediating the interaction with AQP0 in the lens in vivo. AQP0 significantly increased (~20-30%) the intercellular coupling and conductance of Cx50 gap junctions. However, this increase was not observed when the IL domain was replaced with those from other lens connexins. The Cx50-AQP0 interaction had no effect on Cx50 hemichannel function. A fusion protein containing three extracellular loop domains of AQP0 efficiently blocked the cell-to-cell adhesion of AQP0 and attenuated the stimulatory effect of AQP0 on Cx50 gap junction conductance. These data suggest that the specific interaction between Cx50 and AQP0 enhances the coupling of Cx50 gap junctions, but not hemichannels, through the cell adhesion function of AQP0. This result establishes a physiological role of AQP0 in the functional regulation of gap junction channels.

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.