Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

Cx31.1 can selectively intermix with co-expressed connexins to facilitate its assembly into gap junctions

Connexins are channel-forming proteins that function to facilitate gap junctional intercellular communication. Here, we use dual cell voltage clamp and dye transfer studies to corroborate past findings showing that Cx31.1 (encoded by GJB5) is defective in gap junction channel formation, illustrating that Cx31.1 alone does not form functional gap junction channels in connexin-deficient mammalian cells. Rather Cx31.1 transiently localizes to the secretory pathway with a subpopulation reaching the cell surface, which is rarely seen in puncta reminiscent of gap junctions. Intracellular retained Cx31.1 was subject to degradation as Cx31.1 accumulated in the presence of proteasomal inhibition, had a faster turnover when Cx43 was present and ultimately reached lysosomes. Although intracellularly retained Cx31.1 was found to interact with Cx43, this interaction did not rescue its delivery to the cell surface. Conversely, the co-expression of Cx31 dramatically rescued the assembly of Cx31.1 into gap junctions where gap junction-mediated dye transfer was enhanced. Collectively, our results indicate that the localization and functional status of Cx31.1 is altered through selective interplay with co-expressed connexins, perhaps suggesting Cx31.1 is a key regulator of intercellular signaling in keratinocytes.

Generation of Connexin-Expressing Stable Cell Pools

Stable cell pools have the advantage of providing a definite, consistent, and reproducible transmission of a transgene of interest, compared to transient expression from a plasmid transfection. Stably expressing a transgene of interest in cells under induction is a powerful way to (switch on and) study a gene function in both in vitro and in vivo assays. Taking advantage of the ability of lentivirus (LV) to promote transgene delivery, and genomic integration and expression in both dividing and nondividing cells, a doxycycline-inducible transfer vector expressing a bicistronic transgene was developed to study the function of connexins in HeLa DH cells. Here, delving on connexin 32 (Cx32), we report how to use the backbone of this vector as a tool to generate stable pools to study the function of a gene of interest (GOI), especially with assays involving Ca2+ imaging, employing the GCaMP6s indicator. We describe a step-by-step protocol to produce the LV particle by transient transfection and the direct use of the harvested LV stock to generate stable cell pools. We further present step-by-step immunolabeling protocols to characterize the transgene protein expression by confocal microscopy using an antibody that targets an extracellular domain epitope of Cx32 in living cells, and in fixed permeabilized cells using high affinity anti-Cx32 antibodies. Using common molecular biology laboratory techniques, this protocol can be adapted to generate stable pools expressing any transgene of interest, for both in vitro and in vivo functional assays, including molecular, immune, and optical assays.

Cytomembrane Trafficking Pathways of Connexin 26, 30, and 43

The connexin gene family is the most prevalent gene that contributes to hearing loss. Connexins 26 and 30, encoded by GJB2 and GJB6, respectively, are the most abundantly expressed connexins in the inner ear. Connexin 43, which is encoded by GJA1, appears to be widely expressed in various organs, including the heart, skin, the brain, and the inner ear. The mutations that arise in GJB2, GJB6, and GJA1 can all result in comprehensive or non-comprehensive genetic deafness in newborns. As it is predicted that connexins include at least 20 isoforms in humans, the biosynthesis, structural composition, and degradation of connexins must be precisely regulated so that the gap junctions can properly operate. Certain mutations result in connexins possessing a faulty subcellular localization, failing to transport to the cell membrane and preventing gap junction formation, ultimately leading to connexin dysfunction and hearing loss. In this review, we provide a discussion of the transport models for connexin 43, connexins 30 and 26, mutations affecting trafficking pathways of these connexins, the existing controversies in the trafficking pathways of connexins, and the molecules involved in connexin trafficking and their functions. This review can contribute to a new way of understanding the etiological principles of connexin mutations and finding therapeutic strategies for hereditary deafness.

Connexin 43 trafficking and regulation of gap junctional intercellular communication alters ovarian cancer cell migration and tumorigenesis

Ovarian cancer persists to be the most lethal gynecological malignancy, demanding rigorous treatments involving radio-chemotherapy that trigger toxicity and consequently mortality among patients. An improved understanding of the disease progression may pioneer curative therapies. Mouse epithelial ovarian cancer cell lines, ID8 and ID8-VEGF (overexpressing VEGF) were intraperitoneally injected in C57BL/6 female mice to develop a Syngeneic Ovarian cancer mouse model. It was observed that ID8-VEGF cells were able to induce aggressive tumor growth in mice compared to ID8 cells. Furthermore, results of the current in vitro study comparing ID8 and ID8-VEGF demonstrated that highly tumorigenic ID8-VEGF had reduced gap junctional intercellular communication (GJIC) due to intracellular Connexin 43 (Cx43) expression. Additionally, ID8 cells with reduced tumorigenic capability expressed significant GJIC. Furthermore, loss of GJIC in ID8-VEGF cells induced shorter tunneling nanotube formations, while ID8 cells develops longer tunneling nanotube to maintain cellular crosstalk. The administration of a pharmacological drug 4-phenylbutyrate (4PBA) ensured the restoration of GJIC in both the ovarian cancer cell lines. Additionally, 4PBA treatment significantly inhibited the migration of ovarian cancer cell lines and tumor formation in ovarian cancer mice models. In summary, the 4PBA-mediated restoration of GJIC suppressed migration (in vitro) and tumorigenesis (in vivo) of ovarian cancer cells. The present study suggests that Cx43 assembled GJIC and its supportive signaling pathways are a prospective target for restricting ovarian cancer progression.

GJB4 variants linked to skin disease exhibit a trafficking deficiency en route to gap junction formation that can be restored by co-expression of select connexins

Epidermal keratinocytes are enriched with at least nine connexins that are key regulators of epidermal homeostasis. The role of Cx30.3 in keratinocytes and epidermal health became evident when fourteen autosomal dominant mutations in the Cx30.3-encoding GJB4 gene were linked to a rare and incurable skin disorder called erythrokeratodermia variabilis et progressiva (EKVP). While these variants are linked to EKVP, they remain largely uncharacterized hindering therapeutic options. In this study, we characterize the expression and functional status of three EKVP-linked Cx30.3 mutants (G12D, T85P, and F189Y) in tissue-relevant and differentiation-competent rat epidermal keratinocytes. We found that GFP-tagged Cx30.3 mutants were non-functional likely due to their impaired trafficking and primary entrapment within the endoplasmic reticulum (ER). However, all mutants failed to increase BiP/GRP78 levels suggesting they were not inducing an unfolded protein response. FLAG-tagged Cx30.3 mutants were also trafficking impaired yet occasionally exhibited some capacity to assemble into gap junctions. The pathological impact of these mutants may extend beyond their trafficking deficiencies as keratinocytes expressing FLAG-tagged Cx30.3 mutants exhibited increased propidium iodide uptake in the absence of divalent cations. Attempts to rescue the delivery of trafficking impaired GFP-tagged Cx30.3 mutants into gap junctions by chemical chaperone treatment were ineffective. However, co-expression of wild type Cx30.3 greatly enhanced the assembly of Cx30.3 mutants into gap junctions, although endogenous levels of Cx30.3 do not appear to prevent the skin pathology found in patients harboring these autosomal dominant mutations. In addition, a spectrum of connexin isoforms (Cx26, Cx30, and Cx43) exhibited the differential ability to trans-dominantly rescue the assembly of GFP-tagged Cx30.3 mutants into gap junctions suggesting a broad range of connexins found in keratinocytes may favourably interact with Cx30.3 mutants. We conclude that selective upregulation of compatible wild type connexins in keratinocytes may have potential therapeutic value in rescuing epidermal defects invoked by Cx30.3 EKVP-linked mutants.

Recent insights into gap junction biogenesis in the cochlea

In the cochlea, connexin 26 (Cx26) and connexin 30 (Cx30) co-assemble into two types of homomeric and heteromeric gap junctions between adjacent non-sensory epithelial cells. These channels provide a mechanical coupling between connected cells, and their activity is critical to maintain cochlear homeostasis. Many of the mutations in GJB2 or GJB6, which encode Cx26 and Cx30 in humans, impair the formation of membrane channels and cause autosomal syndromic and non-syndromic hearing loss. Thus, deciphering the connexin trafficking pathways in situ should represent a major step forward in understanding the pathogenic significance of many of these mutations. A growing body of evidence now suggests that Cx26/Cx30 heteromeric and Cx30 homomeric channels display distinct assembly mechanisms. Here, we review the most recent advances that have been made toward unraveling the biogenesis and stability of these gap junctions in the cochlea.

A Quantitative Assay for Ca2+ Uptake through Normal and Pathological Hemichannels

Connexin (Cx) hemichannels (HCs) are large pore hexameric structures that allow the exchange of ions, metabolites and a variety of other molecules between the cell cytoplasm and extracellular milieu. HC inhibitors are attracting growing interest as drug candidates because deregulated fluxes through HCs have been implicated in a plethora of genetic conditions and other diseases. HC activity has been mainly investigated by electrophysiological methods and/or using HC-permeable dye uptake measurements. Here, we present an all-optical assay based on fluorometric measurements of ionized calcium (Ca²⁺) uptake with a Ca²⁺-selective genetically encoded indicator (GCaMP6s) that permits the optical tracking of cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) changes with high sensitivity. We exemplify use of the assay in stable pools of HaCaT cells overexpressing human Cx26, Cx46, or the pathological mutant Cx26G45E, under control of a tetracycline (Tet) responsive element (TRE) promoter (Tet-on). We demonstrate the usefulness of the assay for the characterization of new monoclonal antibodies (mAbs) targeting the extracellular domain of the HCs. Although we developed the assay on a spinning disk confocal fluorescence microscope, the same methodology can be extended seamlessly to high-throughput high-content platforms to screen other kinds of inhibitors and/or to probe HCs expressed in primary cells and microtissues.

Mutant Cx30-A88V mice exhibit hydrocephaly and sex-dependent behavioral abnormalities, implicating a functional role for Cx30 in the brain

Connexin 30 (Cx30; also known as Gjb6 when referring to the mouse gene) is expressed in ependymal cells of the brain ventricles, in leptomeningeal cells and in astrocytes rich in connexin 43 (Cx43), leading us to question whether patients harboring GJB6 mutations exhibit any brain anomalies. Here, we used mice harboring the human disease-associated A88V Cx30 mutation to address this gap in knowledge. Brain Cx30 levels were lower in male and female Cx30^A88V/A88V mice compared with Cx30^A88V/+ and Cx30^+/+ mice, whereas Cx43 levels were lower only in female Cx30 mutant mice. Characterization of brain morphology revealed a disrupted ependymal cell layer, significant hydrocephalus and enlarged ventricles in 3- to 6-month-old adult male and female Cx30^A88V/A88V mice compared with Cx30^A88V/+ or Cx30^+/+ sex-matched littermate mice. To determine the functional significance of these molecular and morphological changes, we investigated a number of behavioral activities in these mice. Interestingly, only female Cx30^A88V/A88V mice exhibited abnormal behavior compared with all other groups. Cx30^A88V/A88V female mice demonstrated increased locomotor and exploratory activity in both the open field and the elevated plus maze. They also exhibited dramatically reduced ability to learn the location of the escape platform during Morris water maze training, although they were able to swim as well as other genotypes. Our findings suggest that the homozygous A88V mutation in Cx30 causes major morphological changes in the brain of aging mice, possibly attributable to an abnormal ependymal cell layer. Remarkably, these changes had a more pronounced consequence for cognitive function in female mice, which is likely to be linked to the dysregulation of both Cx30 and Cx43 levels in the brain.

Palmoplantar keratoderma with deafness phenotypic variability in a patient with an inherited GJB2 frameshift variant and novel missense variant

Background

Variants in the GJB2 gene encoding the gap junction protein connexin‐26 (Cx26) can cause autosomal recessive nonsyndromic hearing loss or a variety of phenotypically variable autosomal dominant disorders that effect skin and hearing, such as palmoplantar keratoderma (PPK) with deafness and keratitis–ichthyosis–deafness (KID) syndrome. Here, we report a patient with chronic mucocutaneous candidiasis, hyperkeratosis with resorption of the finger tips, profound bilateral sensorineural hearing loss, and normal hair and ocular examination. Exome analysis identified a novel missense variant in GJB2 (NM_004004.5:c.101T>A, p.Met34Lys) that was inherited from a mosaic unaffected parent in the setting of a well‐reported GJB2 loss of function variant (NM_004004.5:c.35delG, p.Gly12Valfs*2) on the other allele.

Method

Rat epidermal keratinocytes were transfected with cDNA encoding wildtype Cx26 and/or the Met34Lys mutant of Cx26. Fixed cells were immunolabeled in order to assess the subcellular location of the Cx26 mutant and cell images were captured.

Results

Expression in rat epidermal keratinocytes revealed that the Met34Lys mutant was retained in the endoplasmic reticulum, unlike wildtype Cx26, and failed to reach the plasma membrane to form gap junctions. Additionally, the Met34Lys mutant acted dominantly to wildtype Cx26, restricting its delivery to the cell surface.

Conclusion

Overall, we show the p.Met34Lys variant is a novel dominant acting variant causing PPK with deafness. The presence of a loss a function variant on the other allele creates a more severe clinical phenotype, with some features reminiscent of KID syndrome.

Tricellular adherens junctions provide a cell surface delivery platform for connexin 26/30 oligomers in the cochlea

In the cochlea, connexins 26 (Cx26) and 30 (Cx30) largely co-assemble into heteromeric gap junctions, which connect adjacent non-sensory epithelial cells. These channels are believed to ensure the rapid removal of K⁺ away from the base of sensory hair cells, resulting in K⁺ recycling back to the endolymph to maintain cochlear homeostasis. Many of the mutations in GJB2 and GJB6, which encode CX26 and CX30, impair the formation of membrane channels and cause autosomal hearing loss in humans. Although recent advances have been made, several important questions remain about connexin trafficking and gap junction biogenesis. Here we show that tricellular adherens junctions present at the crossroad between adjacent gap junction plaques, provide an unexpected cell surface delivery platform for Cx26/Cx30 oligomers. Using an in situ proximity ligation assay, we detected the presence of non-junctional Cx26/Cx30 oligomers within lipid raft-enriched tricellular junction sites. In addition, we observed that cadherin homophilic interactions are critically involved in microtubule-mediated trafficking of Cx26/Cx30 oligomers to the cell surface. Overall, our results unveil an unexpected role for tricellular junctions in the trafficking and assembly of membrane channels.

Comparative Analysis of Cx31 and Cx43 in Differentiation-Competent Rodent Keratinocytes

When considering connexin expression and regulation, the epidermis of the skin is one of the most complex tissues found in mammals even though it largely contains a single cell type, the keratinocyte. In the rodent epidermis, up to 9 connexin family members have been detected at the mRNA level. Many of these connexins are temporally and spatially regulated in coordination with keratinocyte progenitor cell differentiation and migration from the stratum basale to form the stratum spinosum and stratum granulosum layers before finally forming the stratum corneum. Cx43 is the principal connexin found in basal keratinocytes and to a lesser degree found in keratinocytes that have begun to differentiate where Cx26, Cx30 and Cx31 become prevalent. Here we show that the CRISPR-Cas9 ablation of Cx43 reduces overall gap junction coupling in monolayer cultures of rat epidermal keratinocytes (REKs) and dysregulates the differentiation of REKs when grown in organotypic cultures. Natively found in differentiated keratinocytes, Cx31 readily assembles into gap junctions when expressed in REKs where it can extensively co-assemble into the same gap junctions with co-expressed Cx30. Time-lapse imaging indicated that many Cx31 gap junctions are mobile within the plasma membrane undergoing both fusion and fission events. Finally, the persistence of pre-existing Cx31 gap junctions in the presence of the protein trafficking blocker, brefeldin A, is longer than that found for Cx43 gap junctions indicating that it has a distinctly different life expectancy in REKs. Collectively, this study highlights the importance of Cx43 in rodent keratinocyte differentiation and suggests that Cx31 acquires life-cycle properties that are distinct from Cx43.

Toxicologic Evaluation for Amorphous Silica Nanoparticles: Genotoxic and Non-Genotoxic Tumor-Promoting Potential

Amorphous silica nanoparticles (SiO₂NPs) have been widely used in medicine including targeted drug/DNA delivery, cancer therapy, and enzyme immobilization. Nevertheless, SiO₂NPs should be used with caution due to safety concerns associated with unique physical and chemical characteristics. The objective of this study was to determine the effects of SiO₂NPs on genotoxic and non-genotoxic mechanisms associated with abnormal gap junctional intercellular communication (GJIC) in multistage carcinogenesis. The SiO₂NPs exhibited negative responses in standard genotoxicity tests including the Ames test, chromosome aberration assay, and micronucleus assay. In contrast, the SiO₂NPs significantly induced DNA breakage in comet assay. Meanwhile, SiO₂NPs inhibited GJIC based on the results of scrape/loading dye transfer assay for the identification of non-genotoxic tumor-promoting potential. The reduction in expression and plasma membrane localization of Cx43 was detected following SiO₂NP treatment. Particularly, SiO₂NP treatment increased Cx43 phosphorylation state, which was significantly attenuated by inhibitors of extracellular signal-regulated kinases 1/2 (ERK1/2) and threonine and tyrosine kinase (MEK), but not by protein kinase C (PKC) inhibitor. Taken together, in addition to a significant increase in DNA breakage, SiO₂NP treatment resulted in GJIC dysregulation involved in Cx43 phosphorylation through the activation of mitogen-activated protein kinase (MAPK) signaling. Overall findings of the genotoxic and non-genotoxic carcinogenic potential of SiO₂NPs provide useful toxicological information for clinical application at an appropriate dose.

A potent antagonist antibody targeting connexin hemichannels alleviates Clouston syndrome symptoms in mutant mice

BACKGROUND

Numerous currently incurable human diseases have been causally linked to mutations in connexin (Cx) genes. In several instances, pathological mutations generate abnormally active Cx hemichannels, referred to also as "leaky" hemichannels. The goal of this study was to assay the in vivo efficacy of a potent antagonist antibody targeting Cx hemichannels.

METHODS

We employed the antibody to treat Cx30A88V/A88V adult mutant mice, the only available animal model of Clouston syndrome, a rare orphan disease caused by Cx30 p.A88V leaky hemichannels. To gain mechanistic insight into antibody action, we also performed patch clamp recordings, Ca2+ imaging and ATP release assay in vitro.

FINDINGS

Two weeks of antibody treatment sufficed to repress cell hyperproliferation in skin and reduce hypertrophic sebaceous glands (SGs) to wild type (wt) levels. These effects were obtained whether mutant mice were treated topically, by application of an antibody cream formulation, or systemically, by intraperitoneal antibody injection. Experiments with mouse primary keratinocytes and HaCaT cells revealed the antibody blocked Ca2+ influx and diminished ATP release through leaky Cx30 p.A88V hemichannels.

INTERPRETATION

Our results show anti-Cx antibody treatment was effective in vivo and sufficient to counteract the effects of pathological connexin expression in Cx30A88V/A88V mice. In vitro experiments suggest antibodies gained control over leaky hemichannels and contributed to restoring epidermal homeostasis. Therefore, regulating cell physiology by antibodies targeting the extracellular domain of Cxs may enforce an entirely new therapeutic strategy. These findings support the further development of antibodies as drugs to address unmet medical needs for Cx-related diseases. FUND: Fondazione Telethon, GGP19148; University of Padova, SID/BIRD187130; Consiglio Nazionale delle Ricerche, DSB.AD008.370.003\TERABIO-IBCN; National Science Foundation of China, 31770776; Science and Technology Commission of Shanghai Municipality, 16DZ1910200.

GJB2 Mutations Linked to Hearing Loss Exhibit Differential Trafficking and Functional Defects as Revealed in Cochlear-Relevant Cells

GJB2 gene (that encodes Cx26) mutations are causal of hearing loss highlighting the importance of Cx26-based channel signaling amongst the supporting cells in the organ of Corti. While the majority of these GJB2 mutations are inherited in an autosomal recessive manner, others are inherited in an autosomal dominant manner and lead to syndromic hearing loss as well as skin diseases. To assess if common or divergent mechanisms are at the root of GJB2-linked hearing loss, we expressed several mutants in cochlear-relevant HEI-OC1 cells derived from the developing organ of Corti. Since supporting cells of the mature mammalian organ of Corti have negligible Cx43, but HEI-OC1 cells are rich in Cx43, we first used CRISPR-Cas9 to ablate endogenous Cx43, thus establishing a connexin-deficient platform for controlled reintroduction of hearing-relevant connexins and Cx26 mutants. We found three distinct outcomes and cellular phenotypes when hearing loss-linked Cx26 mutants were expressed in cochlear-relevant cells. The dominant syndromic Cx26 mutant N54K had trafficking defects and did not fully prevent wild-type Cx26 gap junction plaque formation but surprisingly formed gap junctions when co-expressed with Cx30. In contrast, the dominant syndromic S183F mutant formed gap junctions incapable of transferring dye and, as expected, co-localized in the same gap junctions as wild-type Cx26 and Cx30, but also gained the capacity to intermix with Cx43 within gap junctions. Both recessive non-syndromic Cx26 mutants (R32H and R184P) were retained in intracellular vesicles including early endosomes and did not co-localize with Cx30. As might be predicted, none of the Cx26 mutants prevented Cx43 gap junction plaque formation in Cx43-rich HEI-OC1 cells while Cx43-ablation had little effect on the expression of reference genes linked to auditory cell differentiation. We conclude from our studies in cochlear-relevant cells that the selected Cx26 mutants likely evoke hearing loss via three unique connexin defects that are independent of Cx43 status.

Structural determinants underlying permeant discrimination of the Cx43 hemichannel

Connexin (Cx) gap junction channels comprise two hemichannels in neighboring cells, and their permeability is well-described, but permeabilities of the single Cx hemichannel remain largely unresolved. Moreover, determination of isoform-specific Cx hemichannel permeability is challenging because of concurrent expression of other channels with similar permeability profiles and inhibitor sensitivities. The mammalian Cx hemichannels Cx30 and Cx43 are gated by extracellular divalent cations, removal of which promotes fluorescent dye uptake in both channels but atomic ion conductance only through Cx30. To determine the molecular determinants of this difference, here we employed chimeras and mutagenesis of predicted pore-lining residues in Cx43. We expressed the mutated channels in Xenopus laevis oocytes to avoid background activity of alternative channels. Oocytes expressing a Cx43 hemichannel chimera containing the N terminus or the first extracellular loop from Cx30 displayed ethidium uptake and, unlike WT Cx43, ion conduction, an observation further supported by molecular dynamics simulations. Additional C-terminal truncation of the chimeric Cx43 hemichannel elicited an even greater ion conductance with a magnitude closer to that of Cx30. The inhibitory profile for the connexin hemichannels depended on the permeant, with conventional connexin hemichannel inhibitors having a higher potency toward the ion conductance pathway than toward fluorescent dye uptake. Our results demonstrate a permeant-dependent, isoform-specific inhibition of connexin hemichannels. They further reveal that the outer segments of the pore-lining region, including the N terminus and the first extracellular loop, together with the C terminus preclude ion conductance of the open Cx43 hemichannel.

Inner Ear Connexin Channels: Roles in Development and Maintenance of Cochlear Function

Connexin 26 and connexin 30 are the prevailing isoforms in the epithelial and connective tissue gap junction systems of the developing and mature cochlea. The most frequently encountered variants of the genes that encode these connexins, which are transcriptionally coregulated, determine complete loss of protein function and are the predominant cause of prelingual hereditary deafness. Reducing connexin 26 expression by Cre/loxP recombination in the inner ear of adult mice results in a decreased endocochlear potential, increased hearing thresholds, and loss of >90% of outer hair cells, indicating that this connexin is essential for maintenance of cochlear function. In the developing cochlea, connexins are necessary for intercellular calcium signaling activity. Ribbon synapses and basolateral membrane currents fail to mature in inner hair cells of mice that are born with reduced connexin expression, even though hair cells do not express any connexin. In contrast, pannexin 1, an alternative mediator of intercellular signaling, is dispensable for hearing acquisition and auditory function.

A Human-Derived Monoclonal Antibody Targeting Extracellular Connexin Domain Selectively Modulates Hemichannel Function

Connexin hemichannels, which are plasma membrane hexameric channels (connexons) composed of connexin protein protomers, have been implicated in a host of physiological processes and pathological conditions. A number of single point pathological mutations impart a “leaky” character to the affected hemichannels, i.e., make them more active or hyperactive, suggesting that normal physiological condition could be recovered using selective hemichannel inhibitors. Recently, a human-derived monoclonal antibody named abEC1.1 has been shown to inhibit both wild type and hyperactive hemichannels composed of human (h) connexin 26 (hCx26) subunits. The aims of this work were (1) to characterize further the ability of abEC1.1 to selectively modulate connexin hemichannel function and (2) to assess its in vitro stability in view of future translational applications. In silico analysis of abEC1.1 interaction with the hCx26 hemichannel identified critically important extracellular domain amino acids that are conserved in connexin 30 (hCx30) and connexin 32 (hCx32). Patch clamp experiments performed in HeLa DH cells confirmed the inhibition efficiency of abEC1.1 was comparable for hCx26, hCx30 and hCx32 hemichannels. Of note, even a single amino acid difference in the putative binding region reduced drastically the inhibitory effects of the antibody on all the other tested hemichannels, namely hCx30.2/31.3, hCx30.3, hCx31, hCx31.1, hCx37, hCx43 and hCx45. Plasma membrane channels composed of pannexin 1 were not affected by abEC1.1. Finally, size exclusion chromatography assays showed the antibody does not aggregate appreciably in vitro. Altogether, these results indicate abEC1.1 is a promising tool for further translational studies.

The connexin 30 A88V mutant reduces cochlear gap junction expression and confers long-term protection against hearing loss

Mutations in the genes that encode the gap junction proteins connexin 26 (Cx26, encoded by GJB2) and Cx30 (GJB6) are the leading cause of hereditary hearing loss. That said, the Cx30 p.Ala88Val (A88V) mutant causes Clouston syndrome, but not hearing loss. Here, we report that the Cx30-A88V mutant, despite being toxic to inner ear-derived HEI-OC1 cells, conferred remarkable long-term protection against age-related high frequency hearing loss in Cx30^A88V/A88V mice. During early development, there were no overt structural differences in the cochlea between genotypes, including a normal complement of hair cells; however, the supporting cell Cx30 gap junction plaques in mutant mice were reduced in size. In adulthood, Cx30^A88V/A88V mutant mice had a reduction of cochlear Cx30 mRNA and protein, yet a full complement of hair cells. Conversely, the age-related high frequency hearing loss in Cx30^+/+ and Cx30^+/A88V mice was due to extensive loss of outer hair cells. Our data suggest that the Cx30-A88V mutant confers long-term hearing protection and prevention of hair cell death, possibly via a feedback mechanism that leads to the reduction of total Cx30 gap junction expression in the cochlea.

Cancer Connectors: Connexins, Gap Junctions, and Communication

Despite concerted clinical and research efforts, cancer is a leading cause of death worldwide. Surgery, radiation, and chemotherapy have remained the most common standard-of-care strategies against cancer for decades. However, the side effects of these therapies demonstrate the need to investigate adjuvant novel treatment modalities that minimize the harm caused to healthy cells and tissues. Normal and cancerous cells require communication amongst themselves and with their surroundings to proliferate and drive tumor growth. It is vital to understand how intercellular and external communication impacts tumor cell malignancy. To survive and grow, tumor cells, and their normal counterparts utilize cell junction molecules including gap junctions (GJs), tight junctions, and adherens junctions to provide contact points between neighboring cells and the extracellular matrix. GJs are specialized structures composed of a family of connexin proteins that allow the free diffusion of small molecules and ions directly from the cytoplasm of adjacent cells, without encountering the extracellular milieu, which enables rapid, and coordinated cellular responses to internal and external stimuli. Importantly, connexins perform three main cellular functions. They enable direct gap junction intercellular communication (GJIC) between cells, form hemichannels to allow cell communication with the extracellular environment, and serve as a site for protein-protein interactions to regulate signaling pathways. Connexins themselves have been found to promote tumor cell growth and invasiveness, contributing to the overall tumorigenicity and have emerged as attractive anti-tumor targets due to their functional diversity. However, connexins can also serve as tumor suppressors, and therefore, a complete understanding of the roles of the connexins and GJs in physiological and pathophysiological conditions is needed before connexin targeting strategies are applied. Here, we discuss how the three aspects of connexin function, namely GJIC, hemichannel formation, and connexin-protein interactions, function in normal cells, and contribute to tumor cell growth, proliferation, and death. Finally, we discuss the current state of anti-connexin therapies and speculate which role may be most amenable for the development of targeting strategies.

Roles of aberrant hemichannel activities due to mutant connexin26 in the pathogenesis of KID syndrome

Germline missense mutations in GJB2 encoding connexin (Cx) 26 have been found in keratitis, ichthyosis and deafness (KID) syndrome. We explored the effects of three mouse Cx26 mutants (Cx26-G12R, -G45E and -D50N) corresponding to KID syndrome-causative human mutants on hemichannel activities leading to cell death and the expression of immune response-associated genes. We analyzed the 3D images of cells expressing wild-type (WT) or mutant Cx26 molecules to demonstrate clearly the intracellular localization of Cx26 mutants and hemichannel formation. High extracellular Ca²⁺ conditions lead to the closure of gap junction hemichannels in Cx26-G12R or Cx26-G45E expressing cells, resulting in prohibition of the Cx26 mutant-induced cell death. Fluorescent dye uptake assays revealed that cells with Cx26-D50N had aberrantly high hemichannel activities, which were abolished by a hemichannel blocker, carbenoxolone and 18α-Glycyrrhetinic acid. These results further support the idea that abnormal hemichannel activities play important roles in the pathogenesis of KID syndrome. Furthermore, we revealed that the expressions of IL15, CCL5, IL1A, IL23R and TLR5 are down-regulated in keratinocytes expressing Cx26-D50N, suggesting that immune deficiency in KID syndrome expressing Cx26-D50N might be associated not only with skin barrier defects, but also with the down-regulated expression of immune response-related genes.

Mechanistic effect of the human GJB6 gene and its mutations in HaCaT cell proliferation and apoptosis

We constructed lentiviral vectors containing the human wild-type GJB6 gene and the mutant variants A88V and G11R. The three proteins were stably expressed by the Tet-on system in the HaCaT cell line and used to study the functional effect of the variants. The CCK-8 assay and flow cytometric analyses were used to determine the levels of cell proliferation and apoptosis. Western blot analyses were performed to analyze the relevant clinical indicators of hidrotic ectodermal dysplasia and markers of apoptosis in transfected HaCaT cells. The CCK8 assay and the flow cytometry results showed a significant increase (P<0.05) in the apoptosis of HaCaT cells expressing the A88V and G11R mutants. In addition, we demonstrated that the A88V and G11R mutants induced the apoptosis of transfected HaCaT cells via the activation of caspase-3, -8, -9, and PARA. No change was observed in the activity of BAX compared with the control. This study provides further clarification on the mechanisms underlying the effect of the mutant variants A88V and G11R of the GJB6 gene on the induction of HaCaT cell apoptosis.

A mild phenotype of sensorineural hearing loss and palmoplantar keratoderma caused by a novel GJB2 dominant mutation

Dominant GJB2 mutations are known to cause a syndromic form of sensorineural hearing loss associated with palmo-plantar skin manifestations. We present the genotype/phenotype correlations of a new GJB2 mutation identified in three generations of an Italian family (proband, mother and grandfather) whose members are affected by sensorineural hearing impairment associated with adult-onset palmoplantar keratoderma. In all affected members we identified a new heterozygous GJB2 mutation (c.66G > T, p.Lys22Asn) whose segregation, population frequency and in silico prediction analysis have suggested a pathogenic role. The p.Lys22Asn GJB2 mutation causes a dominant form of hearing loss associated with variable expression of palmoplantar keratoderma, representing a model of full penetrance, with an age-dependent effect on the phenotype.

Induction of cell death and gain-of-function properties of connexin26 mutants predict severity of skin disorders and hearing loss

Connexin26 (Cx26) is a gap junction protein that oligomerizes in the cell to form hexameric transmembrane channels called connexons. Cell surface connexons dock between adjacent cells to allow for gap junctional intercellular communication. Numerous autosomal dominant mutations in the Cx26-encoding GJB2 gene lead to many skin disorders and sensorineural hearing loss. Although some insights have been gained into the pathogenesis of these diseases, it is not fully understood how distinct GJB2 mutations result in hearing loss alone or in skin pathologies with comorbid hearing loss. Here we investigated five autosomal dominant Cx26 mutants (N14K, D50N, N54K, M163V, and S183F) linked to various syndromic or nonsyndromic diseases to uncover the molecular mechanisms underpinning these disease links. We demonstrated that when gap junction-deficient HeLa cells expressed the N14K and D50N mutants, they undergo cell death. The N54K mutant was retained primarily within intracellular compartments and displayed dominant or transdominant properties on wild-type Cx26 and coexpressed Cx30 and Cx43. The S183F mutant formed some gap junction plaques but was largely retained within the cell and exhibited only a mild transdominant reduction in gap junction communication when co-expressed with Cx30. The M163V mutant, which causes only hearing loss, exhibited impaired gap junction function and showed no transdominant interactions. These findings suggest that Cx26 mutants that promote cell death or exert transdominant effects on other connexins in keratinocytes will lead to skin diseases and hearing loss, whereas mutants having reduced channel function but exhibiting no aberrant effects on coexpressed connexins cause only hearing loss. Moreover, cell death-inducing GJB2 mutations lead to more severe syndromic disease.

The unique electrical properties in an extracellular fluid of the mammalian cochlea; their functional roles, homeostatic processes, and pathological significance

The cochlea of the mammalian inner ear contains an endolymph that exhibits an endocochlear potential (EP) of +80 mV with a [K(+)] of 150 mM. This unusual extracellular solution is maintained by the cochlear lateral wall, a double-layered epithelial-like tissue. Acoustic stimuli allow endolymphatic K(+) to enter sensory hair cells and excite them. The positive EP accelerates this K(+) influx, thereby sensitizing hearing. K(+) exits from hair cells and circulates back to the lateral wall, which unidirectionally transports K(+) to the endolymph. In vivo electrophysiological assays demonstrated that the EP stems primarily from two K(+) diffusion potentials yielded by [K(+)] gradients between intracellular and extracellular compartments in the lateral wall. Such gradients seem to be controlled by ion channels and transporters expressed in particular membrane domains of the two layers. Analyses of human deafness genes and genetically modified mice suggested the contribution of these channels and transporters to EP and hearing. A computational model, which reconstitutes unidirectional K(+) transport by incorporating channels and transporters in the lateral wall and connects this transport to hair cell transcellular K(+) fluxes, simulates the circulation current flowing between the endolymph and the perilymph. In this model, modulation of the circulation current profile accounts for the processes leading to EP loss under pathological conditions. This article not only summarizes the unique physiological and molecular mechanisms underlying homeostasis of the EP and their pathological relevance but also describes the interplay between EP and circulation current.

Specific functional pathologies of Cx43 mutations associated with oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a rare genetic disease that affects the development of multiple organs in the human body. More than 70 mutations in the gap junction connexin43 (Cx43) gene, GJA1, are associated with ODDD, most of which are inherited in an autosomal dominant manner. Many patients exhibit similar clinical presentations. However, there is high intrafamilial and interfamilial phenotypic variability. To better understand this variability, we established primary human dermal fibroblast cultures from several ODDD patients and unaffected controls. In the present study, we characterized three fibroblast lines expressing heterozygous p.L7V, p.G138R, and p.G143S Cx43 variants. All ODDD fibroblasts exhibited slower growth, reduced migration, and defective cell polarization, traits common to all ODDD fibroblasts studied so far. However, we found striking differences in overall expression levels, with p.L7V down-regulated at the mRNA and protein level. Although all of the Cx43 variants could traffic to the cell surface, there were stark differences in gap junction plaque formation, gap junctional intercellular communication, Cx43 phosphorylation, and hemichannel activity among Cx43 variants, as well as subtle differences in myofibroblast differentiation. Together these findings enabled us to discover mutation-specific pathologies that may help to predict future clinical outcomes.

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

The online version of this article (doi:10.1186/s12860-016-0081-0) contains supplementary material, which is available to authorized users.

Roles of connexins in testis development and spermatogenesis

The development and differentiation of cells involved in spermatogenesis requires highly regulated and coordinated interactions between cells. Intercellular communication, particularly via connexin43 (Cx43) gap junctions, plays a critical role in the development of germ cells during fetal development and during spermatogenesis in the adult. Loss of Cx43 in the fetus results in a decreased number of germ cells, while the loss of Cx43 in the adult Sertoli cells results in complete inhibition of spermatogenesis. Connexins 26, 32, 33, 36, 45, 46 and 50 have also been localized to specific compartments of the testis in various mammals. Loss of Cx46 is associated with an increase in germ cell apoptosis and loss of the integrity of the blood-testis barrier, while loss of other connexins appears to have more subtle effects within the seminiferous tubule. Outside the seminiferous tubule, the interstitial Leydig cells express connexins 36 and 45 along with Cx43; deletion of the latter connexin did not reveal it to be crucial for steroidogenesis or for the development and differentiation of Leydig cells. In contrast, loss of Cx43 from Sertoli cells results in Leydig cell hyperplasia, suggesting important cross-talk between Sertoli and Leydig cells. In the epididymis connexins 26, 30.3, Cx31.1, 32, and 43 have been identified and differentiation of the epithelium is associated with dramatic changes in their expression. Decreased expression of Cx43 results in decreased sperm motility, a function acquired by spermatozoa during epididymal transit. Clearly, intercellular gap junctional communication within the testis and epididymis represents a critical aspect of male reproductive function and fertility. The implications of this mode of intercellular communication for male fertility remains a poorly understood but important facet of male reproduction.

Isoform-specific phosphorylation-dependent regulation of connexin hemichannels

Connexins form gap junction channels made up of two connexons (hemichannels) from adjacent cells. Unopposed hemichannels may open toward the extracellular space upon stimulation by, e.g., removal of divalent cations from the extracellular solution and allow isoform-specific transmembrane flux of fluorescent dyes and physiologically relevant molecules, such as ATP and ions. Connexin (Cx)43 and Cx30 are the major astrocytic connexins. Protein kinase C (PKC) regulates Cx43 in its cell-cell gap junction configuration and may also act to keep Cx43 hemichannels closed. In contrast, the regulation of Cx30 hemichannels by PKC is unexplored. To determine phosphorylation-dependent regulation of Cx30 and Cx43 hemichannels, these were heterologously expressed in Xenopus laevis oocytes and opened with divalent cation-free solution. Inhibition of PKC activity did not affect hemichannel opening of either connexin. PKC activation had no effect on Cx43-mediated hemichannel activity, whereas both dye uptake and current through Cx30 hemichannels were reduced. We detected no PKC-induced connexin internalization from the plasma membrane, indicating that PKC reduced Cx30 hemichannel activity by channel closure. In an attempt to resolve the PKC phosphorylation site(s) on Cx30, alanine mutations of putative cytoplasmic PKC consensus sites were created to prevent phosphorylation (T5A, T8A, T102A, S222A, S225A, S239A, and S258A). These Cx30 mutants responded to PKC activation, suggesting that Cx30 hemichannels are not regulated by phosphorylation of a single site. In conclusion, Cx30, but not Cx43, hemichannels close upon PKC activation, illustrating that connexin hemichannels display not only isoform-specific permeability profiles but also isoform-specific regulation by PKC.

Connexins and pannexins in the integumentary system: the skin and appendages

The integumentary system comprises the skin and its appendages, which includes hair, nails, feathers, sebaceous and eccrine glands. In this review, we focus on the expression profile of connexins and pannexins throughout the integumentary system in mammals, birds and fish. We provide a picture of the complexity of the connexin/pannexin network illustrating functional importance of these proteins in maintaining the integrity of the epidermal barrier. The differential regulation and expression of connexins and pannexins during skin renewal, together with a number of epidermal, hair and nail abnormalities associated with mutations in connexins, emphasize that the correct balance of connexin and pannexin expression is critical for maintenance of the skin and its appendages with both channel and non-channel functions playing profound roles. Changes in connexin expression during both hair and feather regeneration provide suggestions of specialized communication compartments. Finally, we discuss the potential use of zebrafish as a model for connexin skin biology, where evidence mounts that differential connexin expression is involved in skin patterning and pigmentation.

Diseases associated with leaky hemichannels

Hemichannels (HCs) and gap junction channels (GJCs) formed by protein subunits called connexins (Cxs) are major pathways for intercellular communication. While HCs connect the intracellular compartment with the extracellular milieu, GJCs allow the interchange of molecules between cytoplasm of two contacting cells. Under physiological conditions, HCs are mostly closed, but they can open under certain stimuli allowing the release of autocrine and paracrine molecules. Moreover, some pathological conditions, like ischemia or other inflammation conditions, significantly increase HCs activity. In addition, some mutations in Cx genes associated with human diseases, such as deafness or cataracts, lead to the formation of more active HCs or “leaky HCs.” In this article we will revise cellular and molecular mechanisms underlying the appearance of leaky HCs, and the consequences of their expression in different cellular systems and animal models, in seeking a common pattern or pathological mechanism of disease.

Cellular and Deafness Mechanisms Underlying Connexin Mutation-Induced Hearing Loss - A Common Hereditary Deafness

Hearing loss due to mutations in the connexin gene family, which encodes gap junctional proteins, is a common form of hereditary deafness. In particular, connexin 26 (Cx26, GJB2) mutations are responsible for ~50% of non-syndromic hearing loss, which is the highest incidence of genetic disease. In the clinic, Cx26 mutations cause various auditory phenotypes ranging from profound congenital deafness at birth to mild, progressive hearing loss in late childhood. Recent experiments demonstrate that congenital deafness mainly results from cochlear developmental disorders rather than hair cell degeneration and endocochlear potential reduction, while late-onset hearing loss results from reduction of active cochlear amplification, even though cochlear hair cells have no connexin expression. However, there is no apparent, demonstrable relationship between specific changes in connexin (channel) functions and the phenotypes of mutation-induced hearing loss. Moreover, new experiments further demonstrate that the hypothesized K⁺-recycling disruption is not a principal deafness mechanism for connexin deficiency induced hearing loss. Cx30 (GJB6), Cx29 (GJC3), Cx31 (GJB3), and Cx43 (GJA1) mutations can also cause hearing loss with distinct pathological changes in the cochlea. These new studies provide invaluable information about deafness mechanisms underlying connexin mutation-induced hearing loss and also provide important information for developing new protective and therapeutic strategies for this common deafness. However, the detailed cellular mechanisms underlying these pathological changes remain unclear. Also, little is known about specific mutation-induced pathological changes in vivo and little information is available for humans. Such further studies are urgently required.

Degradation of gap junction connexins is regulated by the interaction with Cx43-interacting protein of 75 kDa (CIP75)

Connexins are a family of transmembrane proteins that form gap junction channels. These proteins undergo both proteasomal and lysosomal degradation, mechanisms that serve to regulate connexin levels. Our previous work described CIP75 [connexin43 (Cx43)-interacting protein of 75 kDa], a protein involved in proteasomal degradation, as a novel Cx43-interacting protein. We have discovered two additional connexins, connexin40 (Cx40) and connexin45 (Cx45), that interact with CIP75. Nuclear magnetic resonance (NMR) analyses identified the direct interaction of the CIP75 UBA domain with the carboxyl-terminal (CT) domains of Cx40 and Cx45. Reduction in CIP75 by shRNA in HeLa cells expressing Cx40 or Cx45 resulted in increased levels of the connexins. Furthermore, treatment with trafficking inhibitors confirmed that both connexins undergo endoplasmic reticulum-associated degradation (ERAD), and that CIP75 preferentially interacts with the connexin proteins bound for proteasomal degradation from the ER. In addition, we have also discovered that CIP75 interacts with ER-localized Cx32 in a process that is likely mediated by Cx32 ubiquitination. Thus, we have identified novel interacting connexin proteins of CIP75, indicating a role for CIP75 in regulating the levels of connexins in general, through proteasomal degradation.

Connexins and skin disease: insights into the role of beta connexins in skin homeostasis

Cell-to-cell communication triggered by connexin channels plays a central role in maintaining epidermal homeostasis. Here, we discuss the role of the beta connexin subgroup, where site-specific mutations in at least 4 of these proteins lead to distinctive non-inflammatory and inflammatory hyperproliferative epidermal disorders. Recent advances in the molecular pathways evoked and correlation with clinical outcome are discussed. The latest data provide increasing evidence that connexins in the epidermis are sensors to environmental stress and that targeting aberrant hemichannel activity holds significant therapeutic potential for inflammatory skin disorders.

Cx30 exhibits unique characteristics including a long half-life when assembled into gap junctions

In the present study we investigated the life-cycle, trafficking, assembly and cell surface dynamics of a poorly characterized connexin family member, connexin 30 (Cx30), which plays a critical role in skin health and hearing. Unexpectedly, Cx30 localization at the cell surface and gap junctional intercellular communication was not affected by prolonged treatments with the ER-Golgi transport inhibitor brefeldin-A or the protein synthesis inhibitor cycloheximide, whereas Cx43 was rapidly cleared. Fluorescent recovery after photobleaching revealed that Cx30 plaques were rebuilt from the outer edges in keeping with older channels residing in the inner core of the plaque. Expression of a dominant-negative form of Sar1 GTPase led to the accumulation of Cx30 within the ER in contrast to a report that Cx30 traffics via a Golgi-independent pathway. Co-expression of Cx30 with Cx43 revealed that these connexins segregate into distinct domains within common gap junction plaques suggesting their assembly is governed by different mechanisms. In summary, Cx30 was found to be an unusually stable, long-lived connexin (half-life &gt;12 hrs), which may underlie its specific role in the epidermis and cochlea.

Connexins: junctional and non-junctional modulators of proliferation

Mounting evidence indicates that dysregulation of gap junctions and their structural subunits-connexins-often occurs in, and sometimes causes, a variety of proliferative disorders, including cancer. Connexin-mediated regulation of cell proliferation is complex and may involve modulation of gap junction intercellular communication (GJIC), hemichannel signalling, or gap junction-independent paths. However, the exact mechanisms linking connexins to proliferation remain poorly defined and a number of contradictory studies report both pro- and anti-proliferative effects, effects that often depend on the cell or tissue type or the microenvironment. The present review covers junctional and non-junctional regulation of proliferation by connexins, with a particular emphasis on their association with cancer.