Extracellular domains play different roles in gap junction formation and docking compatibility

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.

Biological functions of connexins in the development of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a group of chronic intestinal inflammatory diseases with unknown etiology. Gap junctions composed of connexins (Cxs) have been recently validated as an important factor in the development of IBD. Under IBD-induced inflammatory response in the gut, gap junctions connect multiple signaling pathways involved in the interaction between inflammatory cells with other intestinal cells, which altogether mediate the development of IBD. This paper is a narrative review aiming to comprehensively elucidate the biological function of connexins, especially the ubiquitously and predominantly expressed Cx43, in the pathogenesis of IBD.

Cryo-EM structures of human Cx36/GJD2 neuronal gap junction channel

Connexin 36 (Cx36) is responsible for signal transmission in electrical synapses by forming interneuronal gap junctions. Despite the critical role of Cx36 in normal brain function, the molecular architecture of the Cx36 gap junction channel (GJC) is unknown. Here, we determine cryo-electron microscopy structures of Cx36 GJC at 2.2-3.6 Å resolutions, revealing a dynamic equilibrium between its closed and open states. In the closed state, channel pores are obstructed by lipids, while N-terminal helices (NTHs) are excluded from the pore. In the open state with pore-lining NTHs, the pore is more acidic than those in Cx26 and Cx46/50 GJCs, explaining its strong cation selectivity. The conformational change during channel opening also includes the α-to-π-helix transition of the first transmembrane helix, which weakens the protomer-protomer interaction. Our structural analyses provide high resolution information on the conformational flexibility of Cx36 GJC and suggest a potential role of lipids in the channel gating.

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.

Understanding the Role of ATP Release through Connexins Hemichannels during Neurulation

Neurulation is a crucial process in the formation of the central nervous system (CNS), which begins with the folding and fusion of the neural plate, leading to the generation of the neural tube and subsequent development of the brain and spinal cord. Environmental and genetic factors that interfere with the neurulation process promote neural tube defects (NTDs). Connexins (Cxs) are transmembrane proteins that form gap junctions (GJs) and hemichannels (HCs) in vertebrates, allowing cell-cell (GJ) or paracrine (HCs) communication through the release of ATP, glutamate, and NAD⁺; regulating processes such as cell migration and synaptic transmission. Changes in the state of phosphorylation and/or the intracellular redox potential activate the opening of HCs in different cell types. Cxs such as Cx43 and Cx32 have been associated with proliferation and migration at different stages of CNS development. Here, using molecular and cellular biology techniques (permeability), we demonstrate the expression and functionality of HCs-Cxs, including Cx46 and Cx32, which are associated with the release of ATP during the neurulation process in Xenopus laevis. Furthermore, applications of FGF2 and/or changes in intracellular redox potentials (DTT), well known HCs-Cxs modulators, transiently regulated the ATP release in our model. Importantly, the blockade of HCs-Cxs by carbenoxolone (CBX) and enoxolone (ENX) reduced ATP release with a concomitant formation of NTDs. We propose two possible and highly conserved binding sites (N and E) in Cx46 that may mediate the pharmacological effect of CBX and ENX on the formation of NTDs. In summary, our results highlight the importance of ATP release mediated by HCs-Cxs during neurulation.

Characterization of Pannexin1, Connexin32, and Connexin43 in Spotted Sea Bass (Lateolabrax maculatus): They Are Important Neuro-Related Immune Response Genes Involved in Inflammation-Induced ATP Release

Many immunological diseases can be treated by regulating neurobehavior, in which extracellular ATP is a vital member of endogenous danger-associated molecular pattern signaling molecule that plays a crucial part in innate neuro-related immunity. It is actively released through pannexin (Panx) and connexin (Cx) hemichannels from activated or stressed cells during inflammation, injury, or apoptosis. In addition to participating in ATP release, Panxs and Cxs also have crucial immune functions. In this study, pannexin1, three connexin32 isoforms and connexin43 were identified and characterized in spotted sea bass (Lateolabrax maculatus), which were named LmPanx1, LmCx32.2, LmCx32.3, LmCx32.7, and LmCx43. Their similar topological structures were discovered by sequence analysis: a relatively unconserved C-terminal region and four highly conserved transmembrane (TM) domains, and so on. Each extracellular (ECL) region of Panx1 has two conserved cysteine residues. Unlike Panx1, each ECL region of Cx32 and Cx43 contains three conserved cysteine residues, forming two conserved motifs: CX6CX3C motif in ECL1 and CX4CX5C motif in ECL2. Furthermore, Panx1 and Cx43 share similar genomic organization and synteny with their counterparts in selected vertebrates. Cx32 and CX43 were located in the same locus in fish, but diverged into two loci from amphibian. Moreover, despite varying expression levels, the identified genes were constitutively expressed in all examined tissues. All genes were upregulated by PAMP [lipopolysaccharide and poly(I:C)] stimulation or bacterial infection in vivo and in vitro, but they were downregulated in the brain at 6 or 12 h after stimulation. Especially, the three LmCx32 isoforms and LmCx43 were upregulated by ATP stimulation in primary head kidney leukocytes; however, downregulation of LmCx32.3 and LmCx43 expression were noted at 12 h. Conversely, ATP treatment inhibited the expression of LmPanx1. Importantly, we showed that the spotted sea bass Panx1, Cx43, and Cx32 were localized on the cellular membrane and involved in inflammation-induced ATP release. Taken together, our results demonstrated that Panx1, Cx32, and Cx43 are important neuro-related immune response genes involved in inflammation-induced ATP release.

Aberrant cytoplasmic connexin43 expression as a helpful marker in vascular neoplasms

BACKGROUND

Gap junctions consisting of connexins (Cx) are fundamental in controlling cell proliferation, differentiation, and cell death. Cx43 is the most broadly expressed Cx in humans and is attributed an important role in skin tumor development. Its role in cutaneous vascular neoplasms is yet unknown.

METHODS

Fifteen cases each of cutaneous angiosarcoma (cAS), Kaposi sarcoma (KS), and cherry hemangioma (CH) were assessed by immunohistochemistry for expression of Cx43. Expression pattern, intensity, and percentage of positively stained cells were analyzed. Solid basal cell carcinomas served as positive and healthy skin as negative controls.

RESULTS

Most cases of cAS presented with a strong Cx43 staining of almost all tumor cells, whereas endothelia of KS showed medium expression and CH showed mostly weak expression. In comparison with KS or cAS, the staining intensity of CH was significantly lower (P ≤ 0.001). All tissue sections of both cAS and KS were characterized by a mostly diffuse, cytoplasmic staining pattern of the vascular endothelia. None of those showed nuclear staining.

CONCLUSION

The high-to-intermediate expression of Cx43 observed in all cases of cAS and KS suggests that this Cx may play a role in the development of malignant vascular neoplasms and serve as a helpful diagnostic marker.

A role of connexin 43 on the drug-induced liver, kidney, and gastrointestinal tract toxicity with associated signaling pathways

Drug-induced organ toxicity/injury, especially in the liver, kidney, and gastrointestinal tract, is a systematic disorder that causes oxidative stress formation and inflammation resulting in cell death and organ failure. Current therapies target reactive oxygen species (ROS) scavenging and inhibit inflammatory factors in organ injury to restore the functions and temporary relief. Organ cell function and tissue homeostasis are maintained through gap junction intercellular communication, regulating connexin hemichannels. Mis-regulation of such connexin, especially connexin (Cx) 43, affects a comprehensive process, including cell differentiation, inflammation, and cell death. Aim to describe knowledge about the importance of connexin role and insights therapeutic targeting. Cx43 misregulation has been implicated in recent decades in various diseases. Moreover, in recent years there is increasing evidence that Cx43 is involved in the toxicity process, including hepatic, renal, and gastrointestinal disorders. Cx43 has the potential to initiate the immune system to cause cell death, which has been activated in the acceleration of apoptosis, necroptosis, and autophagy signaling pathway. So far, therapies targeting Cx43 have been under inspection and are subjected to clinical trial phases. This review elucidates the role of Cx43 in drug-induced vital organ injury, and recent reports compromise its function in the major signaling pathways.

The potential antiepileptogenic effect of neuronal Cx36 gap junction channel blockage

Epilepsy is one of the most prevalent neurological disorders and can result in neuronal injury and degeneration. Consequently, research into new antiepileptic drugs capable of providing protection against neuronal injury and degeneration is extremely important. Neuronal Cx36 gap junction channels have been found to play an important role in epilepsy; thus, pharmacological interference using Cx36 gap junction channel blockers may be a promising strategy for disrupting the synchronization of neurons during seizure activity and protecting neurons. Based on these promising findings, several in vivo and in vitro studies are ongoing and the first encouraging results have been published. The results bring hope that neurons can be protected from injury and degeneration in patients with epilepsy, which is currently impossible.

Characterization of three connexin32 genes and their role in inflammation-induced ATP release in the Japanese flounder Paralichthys olivaceus

Extracellular ATP (eATP) is a potent singling molecule in activation of fish innate immunity while the molecular determinants for eATP release in fish were not completely understood. Connexin32 (Cx32) is a member of gap junction protein family that plays important immunological functions in mammals. However, the immune relevance of Cx32 and its role in ATP release in fish has not been investigated. Here, we identified, characterized three Cx32 isoform genes (Cx32.2, Cx32.2x and Cx32.7) from the Japanese flounder Paralichthys olivaceus, and investigated their role in inflammation-induced ATP release in fish. Expression analysis revealed that even though all the three Cx32 genes are constitutively expressed in all examined Japanese flounder tissues, Cx32.2 and Cx32.2x are dominantly expressed in liver, and Cx32.7 is highly expressed in intestine and head kidney macrophages. In addition, we showed that gene expression of all the three Cx32 isoforms was modulated by cAMP stimulation and inflammatory challenges. Furthermore, we revealed that Cx32 expression was upregulated in TNF-alpha overexpressed Japanese flounder FG-9307 cells. Moreover, overexpression of the three Cx32 isoforms significantly reduced the gene expression level of LPS-induced pro-inflammatory cytokine IL-8 and TNF-alpha, indicating that Cx32 is involved in modulating inflammatory response in fish. Finally, we showed that inflammation-induced ATP release was significantly increased in Cx32-overexpressed Japanese flounder FG-9307 cells, and this increased ATP release could be attenuated by pre-incubation with gap junction protein blocker carbenoxolone. Taken together, we for the first time reported the involvement of Cx32 in fish immunity. Our findings suggested that in addition to Cx43 and pannexin1 channels, Cx32 also plays a role in inflammation-induced ATP release in fish.

Expression of connexin 43 by atypical fibroxanthoma

INTRODUCTION

Connexins are transmembrane channel proteins that interconnect adjacent cells and allow the exchange of signaling molecules between cells and the extracellular milieu. They have been investigated in many tumors to obtain information about tumor nature, behavior, and prognosis.

METHODS

Herein, we present a study on the immunohistochemical expression of connexin (Cx) 43 in 16 cases of atypical fibroxanthoma (AFX). For the immunohistochemical staining, a tissue array was obtained from the paraffin-embedded blocks.

RESULTS

The expression was membranous and cytoplasmic in all cases. Thirteen cases (81.25%) showed strong staining. In the other three cases (18.75%), the staining was medium. None of the cases showed nuclear staining. Fifteen out of 16 cases showed a diffuse pattern, and only one case showed a focal pattern.

CONCLUSIONS

Our results suggest that Cx43 may play an important role in the natural behavior of AFX.

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.

Analysis of the dominant mutation N188T of human connexin46 (hCx46) using concatenation and molecular dynamics simulation

Connexins (Cx) are proteins that form cell-to-cell gap junction channels. A mutation at position 188 in the second extracellular loop (E2) domain of hCx46 has been linked to an autosomal dominant zonular pulverulent cataract. As it is dominantly inherited, it is possible that the mutant variant affects the co-expressed wild-type Cx and/or its interaction with other cellular components. Here, we proposed to use concatenated hCx46wt-hCx46N188T and hCx46N188T-hCx46wt to analyze how hCx46N188T affected co-expressed hCx46wt to achieve a dominant inheritance. Heterodimer hCx46wt-hCx46N188T formed fewer gap junction plaques compared to homodimer hCx46wt-hCx46wt, while the hCx46N188T-hCx46N188T homodimer formed almost no gap junction plaques. Dye uptake experiments showed that hemichannels of concatenated variants were similar to hemichannels of monomers. Molecular dynamics simulations revealed that for docking, the N188 of a protomer was engaged in hydrogen bonds (HBs) with R180, N189, and D191 of the counterpart protomer of the adjacent hemichannel. T188 suppressed the formation of HBs between protomers. Molecular dynamics simulations of an equimolar hCx46wt/hCx46N188T gap junction channel revealed a reduced number of HBs between protomers, suggesting reduction of gap junction channels between lens fibers co-expressing the variants.

Connexin and Pannexin (Hemi)Channels: Emerging Targets in the Treatment of Liver Disease

Connexin proteins are the building blocks of hemichannels, which dock further between adjacent cells to form gap junctions. Gap junctions control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions control virtually all aspects of the hepatic life cycle. In the last decade, it has become clear that connexin hemichannels also provide a pathway for cellular communication on their own independent of their role as structural precursors of gap junctions, namely between the cytosol of an individual cell and its extracellular environment. In contrast to gap junctions, connexin hemichannels become particularly active in liver disease by facilitating inflammation and cell death. This equally holds true for cellular channels composed of pannexins, being connexin-like proteins recently identified in the liver that gather in structures reminiscent of hemichannels. This paper gives an overview of the involvement of connexin-based and pannexin-based channels in noncancerous liver disease.

Heterotypic docking compatibility of human connexin37 with other vascular connexins

Human vascular connexins (Cx37, Cx40, Cx43, and Cx45) can form various types of gap junction channels to synchronize vasodilation/constriction to control local circulation. Most of our knowledge on heterotypic gap junctions of these vascular connexins was from studies on rodent connexins. In human vasculature, the same four homolog connexins exist, but whether these human connexins can form heterotypic GJs as those of rodents have not been fully studied. Here we used in vitro expression system to study the coupling status and GJ channel properties of human heterotypic Cx37/Cx40, Cx37/Cx43, and Cx37/Cx45 GJs. Our results showed that Cx37/Cx43 and Cx37/Cx45 GJs, but not Cx37/Cx40 GJs, were functional and each with unique rectifying channel properties. The failure of docking between Cx37 and Cx40 could be rescued by designed Cx40 variants. Characterization of the heterotypic Cx37/Cx43 and Cx37/Cx45 GJs may help us in understanding the intercellular communication at the myoendothelial junction.

The Role of Connexins in Gastrointestinal Diseases

Gap junctions are hexagonal arrays of protein molecules in the plasma membrane and were first described in Mauthner cell synapses of goldfish. They form pathways for coupling between cells, allowing passive, electrotonic spread of ions and also passage of larger molecules such as amino acids and nucleotides. They are expressed in both excitable and non-excitable tissues. Each gap junction is made of two connexons, which are hexameric proteins of the connexin subunit. In this review, the roles that connexins play in gastrointestinal motility, the mechanisms of altered connexin expression leading to inflammatory bowel disease, gastrointestinal infections, and gastrointestinal symptoms in autistic spectrum disorder are discussed in detail.

Variants with increased negative electrostatic potential in the Cx50 gap junction pore increased unitary channel conductance and magnesium modulation

Gap junction (GJ) channels are oligomers of connexins forming channels linking neighboring cells. GJs formed by different connexins show distinct unitary channel conductance (γj), transjunctional voltage-dependent gating (Vj-gating) properties, and modulation by intracellular magnesium ([Mg2+]i). The underlying molecular determinants are not fully clear. Previous experimental evidence indicates that residues in the amino terminal (NT) and initial segment of the first extracellular (E1) domain influence the γj, Vj-gating, and/or [Mg2+]i modulation in several GJs. Increasing negatively charged residues in Cx50 (connexin50) E1 (G46D or G46E) increased γj, while increasing positively charged residue (G46K) reduced the γj Sequence alignment of Cx50 and Cx37 in the NT and E1 domains revealed that in Cx50 G8 and V53, positions are negatively charged residues in Cx37 (E8 and E53, respectively). To evaluate these residues together, we generated a triple variant in Cx50, G8E, G46E, and V53E simultaneously to study its γj, Vj-gating properties, and modulation by [Mg2+]i Our data indicate that the triple variant and individual variants G8E, G46E, and V53E significantly increased Cx50 GJ γj without a significant change in the Vj gating. In addition, elevated [Mg2+]i reduced γj in Cx50 and all the variant GJs. These results and our homology structural models suggest that these NT/E1 residues are likely to be pore-lining and the variants increased the negative electrostatic potentials along the GJ pore to facilitate the γj of this cation-preferring GJ channel. Our results indicate that electrostatic properties of the Cx50 GJ pore are important for the γj and the [Mg2+]i modulation.

Concatenation of Human Connexin26 (hCx26) and Human Connexin46 (hCx46) for the Analysis of Heteromeric Gap Junction Hemichannels and Heterotypic Gap Junction Channels

Gap junction channels and hemichannels formed by concatenated connexins were analyzed. Monomeric (hCx26, hCx46), homodimeric (hCx46-hCx46, hCx26-hCx26), and heterodimeric (hCx26-hCx46, hCx46-hCx26) constructs, coupled to GFP, were expressed in HeLa cells. Confocal microscopy showed that the tandems formed gap junction plaques with a reduced plaque area compared to monomeric hCx26 or hCx46. Dye transfer experiments showed that concatenation allows metabolic transfer. Expressed in Xenopus oocytes, the inside-out patch-clamp configuration showed single channels with a conductance of about 46 pS and 39 pS for hemichannels composed of hCx46 and hCx26 monomers, respectively, when chloride was replaced by gluconate on both membrane sides. The conductance was reduced for hCx46-hCx46 and hCx26-hCx26 homodimers, probably due to the concatenation. Heteromerized hemichannels, depending on the connexin-order, were characterized by substates at 26 pS and 16 pS for hCx46-hCx26 and 31 pS and 20 pS for hCx26-hCx46. Because of the linker between the connexins, the properties of the formed hemichannels and gap junction channels (e.g., single channel conductance) may not represent the properties of hetero-oligomerized channels. However, should the removal of the linker be successful, this method could be used to analyze the electrical and metabolic selectivity of such channels and the physiological consequences for a tissue.

Functional Characterization of Novel Atrial Fibrillation-Linked GJA5 (Cx40) Mutants

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia. Recently, four novel heterozygous Cx40 mutations-K107R, L223M, Q236H, and I257L-were identified in 4 of 310 unrelated AF patients and a followup genetic analysis of the mutant carriers' families showed that the mutants were present in all the affected members. To study possible alterations associated with these Cx40 mutants, including their cellular localization and gap junction (GJ) function, we expressed GFP-tagged and untagged mutants in connexin-deficient model cells. All four Cx40 mutants showed clustered localization at cell-cell junctions similar to that observed of wildtype Cx40. However, cell pairs expressing Cx40 Q236H, but not the other individual mutants, displayed a significantly lower GJ coupling conductance (Gj) than wildtype Cx40. Similarly, co-expression of Cx40 Q236H with Cx43 resulted in a significantly lower Gj. Transjunctional voltage-dependent gating (Vj gating) properties were also altered in the GJs formed by Q236H. Reduced GJ function and altered Vj gating may play a role in promoting the Q236H carriers to AF.

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.

Crucial motifs and residues in the extracellular loops influence the formation and specificity of connexin docking

Most of the early studies on gap junction (GJ) channel function and docking compatibility were on rodent connexins, while recent research on GJ channels gradually shifted from rodent to human connexins largely due to the fact that mutations in many human connexin genes are found to associate with inherited human diseases. The studies on human connexins have revealed some key differences from those found in rodents, calling for a comprehensive characterization of human GJ channels. Functional studies revealed that docking and formation of functional GJ channels between two hemichannels are possible only between docking-compatible connexins. Two groups of docking-compatible rodent connexins have been identified. Compatibility is believed to be due to their amino acid residue differences at the extracellular loop domains (E1 and E2). Sequence alignment of the E1 and E2 domains of all connexins known to make GJs revealed that they are highly conserved and show high sequence identity with human Cx26, which is the only connexin with near atomic resolution GJ structure. We hypothesize that different connexins have a similar structure as that of Cx26 at the E1 and E2 domains and use the corresponding residues in their E1 and E2 domains for docking. Based on the Cx26 GJ structure and sequence analysis of well-studied connexins, we propose that the E1-E1 docking interactions are staggered with each E1 interacting with two E1s on the docked connexon. The putative E1 docking residues are conserved in both docking-compatible and -incompatible connexins, indicating that E1 does not likely serve a role in docking compatibility. However, in the case of E2-E2 docking interactions, the putative docking residues are only conserved within the docking-compatible connexins, suggesting the E2 is likely to serve the function of docking compatibility. Docking compatibility studies on human connexins have attracted a lot of attention due to the fact that putative docking residues are mutational hotspots for several connexin-linked human diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Human diseases associated with connexin mutations

Gap junctions and hemichannels comprised of connexins impact many cellular processes. Significant advances in our understanding of the functional role of these channels have been made by the identification of a host of genetic diseases caused by connexin mutations. Prominent features of connexin disorders are the inability of other connexins expressed in the same cell type to compensate for the mutated one, and the ability of connexin mutants to dominantly influence the activity of other wild-type connexins. Functional studies have begun to identify some of the underlying mechanisms whereby connexin channel mutation contributes to the disease state. Detailed mechanistic understanding of these functional differences will help to facilitate new pathophysiology driven therapies for the diverse array of connexin genetic disorders. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Cardiac Cx43, Cx40 and Cx45 co-assembling: involvement of connexins epitopes in formation of hemichannels and Gap junction channels

Background

This review comes after the International Gap Junction Conference (IGJC 2015) and describes the current knowledge on the function of the specific motifs of connexins in the regulation of the formation of gap junction channels. Moreover the review is complemented by a summarized description of the distinct contribution of gap junction channels in the electrical coupling.

Results

Complementary biochemical and functional characterization on cell models and primary cells have improved our understanding on the oligomerization of connexins and the formation and the electrical properties of gap junction channels. Studies mostly focused cardiac connexins Cx43 and Cx40 expressed in myocytes, while Cx45 and Cx30.2 have been less investigated, for which main findings are reviewed to highlight their critical contribution in the formation of gap junction channels for ensuring the orchestrated electrical impulse propagation and coordination of atrial and ventricular contraction and heart function, whereas connexin dysfunction and remodeling are pro-arrhythmic factors. Common and specific motifs of residues identified in different domain of each type of connexin determine the connexin homo- and hetero-oligomerization and the channels formation, which leads to specific electrical properties.

Conclusions

These motifs and the resulting formation of gap junction channels are keys to ensure the tissue homeostasis and function in each connexin expression pattern in various tissues of multicellular organisms. Altogether, the findings to date have significantly improved our understanding on the function of the different connexin expression patterns in healthy and diseased tissues, and promise further investigations on the contribution in the different types of connexin.

Engineered Cx26 variants established functional heterotypic Cx26/Cx43 and Cx26/Cx40 gap junction channels

Gap junction (GJ) channels mediate direct intercellular communication and are composed of two docked hemichannels (connexin oligomers). It is well documented that the docking and formation of GJs are possible only between compatible hemichannels (or connexins). The mechanisms of heterotypic docking compatibility are not fully clear. We aligned the protein sequences of docking-compatible and -incompatible connexins with that of connexin26 (Cx26). We found that two docking hydrogen bond (HB)-forming residues on the second extracellular domain (E2) of Cx26 and their equivalent residues are well conserved within docking-compatible connexins, but different between docking-incompatible connexins. Replacing one or both of these residues of Cx26 into the corresponding residues in the docking incompatible connexins (K168V, N176H or K168V-N176H) increased the formation of morphological and functional heterotypic GJs with connexin43 (Cx43) or connexin40 (Cx40), indicating that these two residues are important for docking incompatibility between Cx26 and these connexins. Our homology structure models predict that both HBs and hydrophobic interactions at the E2 docking interface are important docking mechanisms in heterotypic Cx26 K168V-N176H/Cx43 GJs and probably other docking compatible connexins. Revealing the key residues and mechanisms of heterotypic docking compatibility will assist us in understanding why these putative docking residues are hotspots of disease-linked mutants.

Structure, Regulation and Function of Gap Junctions in Liver

Gap junctions are a specialized group of cell-to-cell junctions that mediate direct intercellular communication between cells. They arise from the interaction of two hemichannels of adjacent cells, which in turn are composed of six connexin proteins. In liver, gap junctions are predominantly found in hepatocytes and play critical roles in virtually all phases of the hepatic life cycle, including cell growth, differentiation, liver-specific functionality and cell death. Liver gap junctions are directed through a broad variety of mechanisms ranging from epigenetic control of connexin expression to post-translational regulation of gap junction activity. This paper reviews established and novel aspects regarding the architecture, control and functional relevance of liver gap junctions.

Connexins and pannexins in liver damage

Connexins and pannexins are key players in the control of cellular communication and thus in the maintenance of tissue homeostasis. Inherent to this function these proteins are frequently involved in pathological processes. The present paper reviews the role of connexins and pannexins in liver toxicity and disease. As they act both as sensors and effectors in these deleterious events connexins and pannexins could represent a set of novel clinical diagnostic biomarkers and drug targets.

Altered cellular localization and hemichannel activities of KID syndrome associated connexin26 I30N and D50Y mutations

Background

Gap junctions facilitate exchange of small molecules between adjacent cells, serving a crucial function for the maintenance of cellular homeostasis. Mutations in connexins, the basic unit of gap junctions, are associated with several human hereditary disorders. For example, mutations in connexin26 (Cx26) cause both non-syndromic deafness and syndromic deafness associated with skin abnormalities such as keratitis-ichthyosis-deafness (KID) syndrome. These mutations can alter the formation and function of gap junction channels through different mechanisms, and in turn interfere with various cellular processes leading to distinct disorders. The KID associated Cx26 mutations were mostly shown to result in elevated hemichannel activities. However, the effects of these aberrant hemichannels on cellular processes are recently being deciphered. Here, we assessed the effect of two Cx26 mutations associated with KID syndrome, Cx26I30N and D50Y, on protein biosynthesis and channel function in N2A and HeLa cells.

Results

Immunostaining experiments showed that Cx26I30N and D50Y failed to form gap junction plaques at cell-cell contact sites. Further, these mutations resulted in the retention of Cx26 protein in the Golgi apparatus. Examination of hemichannel function by fluorescent dye uptake assays revealed that cells with Cx26I30N and D50Y mutations had increased dye uptake compared to Cx26WT (wild-type) containing cells, indicating abnormal hemichannel activities. Cells with mutant proteins had elevated intracellular calcium levels compared to Cx26WT transfected cells, which were abolished by a hemichannel blocker, carbenoxolone (CBX), as measured by Fluo-3 AM loading and flow cytometry.

Conclusions

Here, we demonstrated that Cx26I30N and D50Y mutations resulted in the formation of aberrant hemichannels that might result in elevated intracellular calcium levels, a process which may contribute to the hyperproliferative epidermal phenotypes of KID syndrome.

Electronic supplementary material

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Connexin channels in congenital skin disorders

Gap junctions and hemichannels comprised of connexins influence epidermal proliferation and differentiation. Significant advances in our understanding of the functional role of connexins in the skin have been made by studying the diseases caused by connexin mutations. Eleven clinically defined cutaneous disorders with an overlapping spectrum of phenotypes are caused by mutations in five different connexin genes, highlighting that disease presentation must be deciphered with an understanding of how connexin functions are affected. Increasing evidence suggests that the skin diseases produced by connexin mutations result from dominant gains of function. In palmoplantar keratoderma with deafness, the connexin 26 mutations transdominantly alter the function of wild-type connexin 43 and create leaky heteromeric hemichannels. In keratitis-ichthyosis-deafness syndrome, different connexin 26 mutations can either form dominant hemichannels with altered calcium regulation or increased calcium permeability, leading to clinical subtypes of this syndrome. It is only with detailed understanding of these subtle functional differences that we can hope to create successful pathophysiology driven therapies for the connexin skin disorders.

An electrostatic mechanism for Ca(2+)-mediated regulation of gap junction channels

Gap junction channels mediate intercellular signalling that is crucial in tissue development, homeostasis and pathologic states such as cardiac arrhythmias, cancer and trauma. To explore the mechanism by which Ca(2+) blocks intercellular communication during tissue injury, we determined the X-ray crystal structures of the human Cx26 gap junction channel with and without bound Ca(2+). The two structures were nearly identical, ruling out both a large-scale structural change and a local steric constriction of the pore. Ca(2+) coordination sites reside at the interfaces between adjacent subunits, near the entrance to the extracellular gap, where local, side chain conformational rearrangements enable Ca(2+)chelation. Computational analysis revealed that Ca(2+)-binding generates a positive electrostatic barrier that substantially inhibits permeation of cations such as K(+) into the pore. Our results provide structural evidence for a unique mechanism of channel regulation: ionic conduction block via an electrostatic barrier rather than steric occlusion of the channel pore.

The cataract related mutation N188T in human connexin46 (hCx46) revealed a critical role for residue N188 in the docking process of gap junction channels

The mutation N188T in human connexin46 (hCx46) correlates with a congenital nuclear pulverulent cataract. This mutation is in the second extracellular loop, a domain involved in docking of gap junction hemichannels. To analyze the functional consequences of this mutation, we expressed hCx46N188T and the wild type (hCx46wt) in Xenopus oocytes and HeLa cells. In Xenopus oocytes, hemichannels formed by hCx46wt and hCx46N188T had similar electrical properties. Additionally, a Ca(2+) and La(3+) sensitive current was observed in HeLa cells expressing eGFP-labeled hCx46wt or eGFP-labeled hCx46N188T. These results suggest that the N188T mutation did not alter apparent expression and the membrane targeting of the protein. Cells expressing hCx46wt-eGFP formed gap junction plaques, but plaques formed by hCx46N188T were extremely rare. A reduced plaque formation was also found in cells cotransfected with hCx46N188T-eGFP and mCherry-labeled hCx46wt as well as in cocultured cells expressing hCx46N188T-eGFP and hCx46wt-mCherry. Dye transfer experiments in cells expressing hCx46N188T revealed a lower transfer rate than cells expressing hCx46wt. We postulate that the N188T mutation affects intercellular connexon docking. This hypothesis is supported by molecular modeling of hCx46 using the crystal structure of hCx26 as a template. The model indicated that N188 is important for hemichannel docking through formation of hydrogen bonds with the residues R180, T189 and D191 of the opposing hCx46. The results suggest that the N188T mutation hinders the docking of the connexons to form gap junction channels. Moreover, the finding that a glutamine substitution (hCx46N188Q) could not rescue the docking emphasizes the specific role of N188.

Engineered Cx40 variants increased docking and function of heterotypic Cx40/Cx43 gap junction channels

Gap junction (GJ) channels provide low resistance passages for rapid action potential propagation in the heart. Both connexin40 (Cx40) and Cx43 are abundantly expressed in and frequently co-localized between atrial myocytes, possibly forming heterotypic GJ channels. However, conflicting results have been obtained on the functional status of heterotypic Cx40/Cx43 GJs. Here we provide experimental evidence that the docking and formation of heterotypic Cx40/Cx43 GJs can be substantially increased by designed Cx40 variants on the extracellular domains (E1 and E2). Specifically, Cx40 D55N and P193Q, substantially increased the probability to form GJ plaque-like structures at the cell-cell interfaces with Cx43 in model cells. More importantly the coupling conductance (Gj) of D55N/Cx43 and P193Q/Cx43 GJ channels are significantly increased from the Gj of Cx40/Cx43 in N2A cells. Our homology models indicate the electrostatic interactions and surface structures at the docking interface are key factors preventing Cx40 from docking to Cx43. Improving heterotypic Gj of these atrial connexins might be potentially useful in improving the coupling and synchronization of atrial myocardium.

Structural analysis of key gap junction domains--Lessons from genome data and disease-linked mutants

A gap junction (GJ) channel is formed by docking of two GJ hemichannels and each of these hemichannels is a hexamer of connexins. All connexin genes have been identified in human, mouse, and rat genomes and their homologous genes in many other vertebrates are available in public databases. The protein sequences of these connexins align well with high sequence identity in the same connexin across different species. Domains in closely related connexins and several residues in all known connexins are also well-conserved. These conserved residues form signatures (also known as sequence logos) in these domains and are likely to play important biological functions. In this review, the sequence logos of individual connexins, groups of connexins with common ancestors, and all connexins are analyzed to visualize natural evolutionary variations and the hot spots for human disease-linked mutations. Several gap junction domains are homologous, likely forming similar structures essential for their function. The availability of a high resolution Cx26 GJ structure and the subsequently-derived homology structure models for other connexin GJ channels elevated our understanding of sequence logos at the three-dimensional GJ structure level, thus facilitating the understanding of how disease-linked connexin mutants might impair GJ structure and function. This knowledge will enable the design of complementary variants to rescue disease-linked mutants.

The First Extracellular Domain Plays an Important Role in Unitary Channel Conductance of Cx50 Gap Junction Channels

Gap junction (GJ) channels provide direct passage for ions and small molecules to be exchanged between neighbouring cells and are crucial for many physiological processes. GJ channels can be gated by transjunctional voltage (known as Vj-gating) and display a wide range of unitary channel conductance (γj), yet the domains responsible for Vj-gating and γj are not fully clear. The first extracellular domain (E1) of several connexins has been shown to line part of their GJ channel pore and play important roles in Vj-gating properties and/or ion permeation selectivity. To test roles of the E1 of Cx50 GJ channels, we generated a chimera, Cx50Cx36E1, where the E1 domain of Cx50 was replaced with that of Cx36, a connexin showing quite distinct Vj-gating and γj from those of Cx50. Detailed characterizations of the chimera and three point mutants in E1 revealed that, although the E1 domain is important in determining γj, the E1 domain of Cx36 is able to effectively function within the context of the Cx50 channel with minor changes in Vj-gating properties, indicating that sequence differences between the E1 domains in Cx36 and Cx50 cannot account for their drastic differences in Vj-gating and γj. Our homology models of the chimera and the E1 mutants revealed that electrostatic properties of the pore-lining residues and their contribution to the electric field in the pore are important factors for the rate of ion permeation of Cx50 and possibly other GJ channels.

Connexin and pannexin signaling in gastrointestinal and liver disease

Gap junctions, which mediate intercellular communication, are key players in digestive homeostasis. They are also frequently involved in gastrointestinal and liver pathology. This equally holds true for connexin (Cx) hemichannels, the structural precursors of gap junctions, and pannexin (Panx) channels, Cx-like proteins assembled in a hemichannel configuration. Both Cx hemichannels and Panx channels facilitate extracellular communication and drive a number of deteriorative processes, such as cell death and inflammation. Cxs, Panxs, and their channels underlie a wide spectrum of gastrointestinal and liver diseases, including gastritis and peptic ulcer disease, inflammatory intestinal conditions, acute liver failure, cholestasis, hepatitis and steatosis, liver fibrosis and cirrhosis, infectious gastrointestinal pathologies, and gastrointestinal and liver cancer. This could open promising perspectives for the characterization of new targets and biomarkers for therapeutic and diagnostic clinical purposes in the area of gastroenterology and hepatology.

Carbohydrate-binding module assisting glycosynthase-catalysed polymerizations

Glycosynthase-catalyzed polymerization is enhanced by the addition of a carbohydrate binding module (CBM), either as an isolated protein or fused to the glycosynthase, which results in an increase of the degree of polymerization of the polysaccharide products.

Roles of connexins and pannexins in digestive homeostasis

Connexin proteins are abundantly present in the digestive system. They primarily form gap junctions, which control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions drive a plethora of gastrointestinal and hepatic functional features, including gastric and gut motility, gastric acid secretion, intestinal innate immune defense, xenobiotic biotransformation, glycogenolysis, bile secretion, ammonia detoxification and plasma protein synthesis. In the last decade, it has become clear that connexin hemichannels, which are the structural precursors of gap junctions, also provide a pathway for cellular communication, namely between the cytosol and the extracellular environment. Although merely pathological functions have been described, some physiological roles have been attributed to connexin hemichannels, in particular in the modulation of colonic motility. This equally holds true for cellular channels composed of pannexins, connexin-like proteins recently identified in the intestine and the liver, which have become acknowledged key players in inflammatory processes and that have been proposed to control colonic motility, secretion and blood flow.

Voltage regulation of connexin channel conductance

Voltage is an important parameter that regulates the conductance of both intercellular and plasma membrane channels (undocked hemichannels) formed by the 21 members of the mammalian connexin gene family. Connexin channels display two forms of voltage-dependence, rectification of ionic currents and voltage-dependent gating. Ionic rectification results either from asymmetries in the distribution of fixed charges due to heterotypic pairing of different hemichannels, or by channel block, arising from differences in the concentrations of divalent cations on opposite sides of the junctional plaque. This rectification likely underpins the electrical rectification observed in some electrical synapses. Both intercellular and undocked hemichannels also display two distinct forms of voltage-dependent gating, termed Vj (fast)-gating and loop (slow)-gating. This review summarizes our current understanding of the molecular determinants and mechanisms underlying these conformational changes derived from experimental, molecular-genetic, structural, and computational approaches.

Motifs in the permeation pathway of connexin channels mediate voltage and Ca (2+) sensing

Connexin channels mediate electrical coupling, intercellular molecular signaling, and extracellular release of signaling molecules. Connexin proteins assemble intracellularly as hexamers to form plasma membrane hemichannels. The docking of two hemichannels in apposed cells forms a gap junction channel that allows direct electrical and selective cytoplasmic communication between adjacent cells. Hemichannels and junctional channels are gated by voltage, but extracellular Ca (2+) also gates unpaired plasma membrane hemichannels. Unlike other ion channels, connexin channels do not contain discrete voltage- or Ca (2+)-sensing modules linked to a separate pore-forming module. All studies to date indicate that voltage and Ca (2+) sensing are predominantly mediated by motifs that lie within or are exposed to the pore lumen. The sensors appear to be integral components of the gates, imposing an obligatory structural linkage between sensing and gating not commonly present in other ion channels, in which the sensors are semi-independent domains distinct from the pore. Because of this, the structural and electrostatic features that define connexin channel gating also define pore permeability properties, and vice versa; analysis/mutagenesis of gating and of permeability properties are linked. This offers unique challenges and opportunities for elucidating mechanisms of ligand and voltage-driven gating.

Mix and match: investigating heteromeric and heterotypic gap junction channels in model systems and native tissues

This review is based in part on a roundtable discussion session: "Physiological roles for heterotypic/heteromeric channels" at the 2013 International Gap Junction Conference (IGJC 2013) in Charleston, South Carolina. It is well recognized that multiple connexins can specifically co-assemble to form mixed gap junction channels with unique properties as a means to regulate intercellular communication. Compatibility determinants for both heteromeric and heterotypic gap junction channel formation have been identified and associated with specific connexin amino acid motifs. Hetero-oligomerization is also a regulated process; differences in connexin quality control and monomer stability are likely to play integral roles to control interactions between compatible connexins. Gap junctions in oligodendrocyte:astrocyte communication and in the cardiovascular system have emerged as key systems where heterotypic and heteromeric channels have unique physiologic roles. There are several methodologies to study heteromeric and heterotypic channels that are best applied to either heterologous expression systems, native tissues or both. There remains a need to use and develop different experimental approaches in order to understand the prevalence and roles for mixed gap junction channels in human physiology.