The potency of the fs260 connexin43 mutant to impair keratinocyte differentiation is distinct from other disease-linked connexin43 mutants

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.

Conformational changes in the human Cx43/GJA1 gap junction channel visualized using cryo-EM

Connexin family proteins assemble into hexameric hemichannels in the cell membrane. The hemichannels dock together between two adjacent membranes to form gap junction intercellular channels (GJIChs). We report the cryo-electron microscopy structures of Cx43 GJICh, revealing the dynamic equilibrium state of various channel conformations in detergents and lipid nanodiscs. We identify three different N-terminal helix conformations of Cx43-gate-covering (GCN), pore-lining (PLN), and flexible intermediate (FIN)-that are randomly distributed in purified GJICh particles. The conformational equilibrium shifts to GCN by cholesteryl hemisuccinates and to PLN by C-terminal truncations and at varying pH. While GJIChs that mainly comprise GCN protomers are occluded by lipids, those containing conformationally heterogeneous protomers show markedly different pore sizes. We observe an α-to-π-helix transition in the first transmembrane helix, which creates a side opening to the membrane in the FIN and PLN conformations. This study provides basic structural information to understand the mechanisms of action and regulation of Cx43 GJICh.

Role of the Connexin C-terminus in skin pattern formation of Zebrafish

Background

Zebrafish display a striped skin pattern on their body; two types of connexins, namely, Connexin39.4 (Cx39.4) and Connexin41.8 (Cx41.8), are involved in stripe pattern formation. Herein, we investigated the role of the C-terminal (CT) domains of Cx39.4 and Cx41.8 in vivo and in vitro.

Methods

To investigate the role of CT domains in vivo, we established transgenic zebrafish lines expressing the CT-domain-modified connexin series in pigmented cells and observed skin patterns in fish. To investigate the role of the CT domains in vitro, we expressed the CT-domain modified connexin series in Neuro-2a (N2a) cells and calculated the plaque formation frequency.

Results

The overexpression of Cx39.4 lacking a CT domain produced skin patterns similar to that produced by full-length Cx39.4 in the cx39.4 ^-/- mutant and in cx39.4 and cx41.8 double-knockout mutant zebrafish. Fluorescence-protein-fused CT-domain-modified Cx39.4 formed gap junction plaques between N2a cells. The overexpression of CT-truncated Cx41.8 rescued the mutant phenotype in the cx41.8 ^-/- mutant but did not function in the double knockout zebrafish. Fluorescence-protein-fused CT-truncated Cx41.8 hardly formed plaques between N2a cells without Cx39.4 but formed gap junction plaques when co-expressed with Cx39.4.

Conclusions

The CT domain of Cx39.4 is not required for protein function, at least in the pigment cells of zebrafish. However, the need for the CT domain of Cx41.8 depends on Cx39.4 expression.

General significance

These results provide evidence for the interactions between Cx39.4 and Cx41.8 in pigment cells of zebrafish and suggest that at least one connexin must have a CT domain.

The dynamic Nexus: gap junctions control protein localization and mobility in distinct and surprising ways

Gap junction (GJ) channels permit molecules, such as ions, metabolites and second messengers, to transfer between cells. Their function is critical for numerous cellular interactions, providing exchange of metabolites, signaling molecules, and ionic currents. GJ channels are composed of Connexin (Cx) hexamers paired across extracellular space and typically form large rafts of clustered channels, called plaques, at cell appositions. Cxs together with molecules that interact with GJ channels make up a supramolecular structure known as the GJ Nexus. While the stability of connexin localization in GJ plaques has been studied, mobility of other Nexus components has yet to be addressed. Colocalization analysis of several nexus components and other membrane proteins reveal that certain molecules are excluded from the GJ plaque (Aquaporin 4, EAAT2b), while others are quite penetrant (lipophilic molecules, Cx30, ZO-1, Occludin). Fluorescence recovery after photobleaching of tagged Nexus-associated proteins showed that mobility in plaque domains is affected by mobility of the Cx proteins. These novel findings indicate that the GJ Nexus is a dynamic membrane organelle, with cytoplasmic and membrane-embedded proteins binding and diffusing according to distinct parameters.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

The connexin 43 C-terminus: A tail of many tales

Connexins are chordate gap junction channel proteins that, by enabling direct communication between the cytosols of adjacent cells, create a unique cell signalling network. Gap junctional intercellular communication (GJIC) has important roles in controlling cell growth and differentiation and in tissue development and homeostasis. Moreover, several non-canonical connexin functions unrelated to GJIC have been discovered. Of the 21 members of the human connexin family, connexin 43 (Cx43) is the most widely expressed and studied. The long cytosolic C-terminus (CT) of Cx43 is subject to extensive post-translational modifications that modulate its intracellular trafficking and gap junction channel gating. Moreover, the Cx43 CT contains multiple domains involved in protein interactions that permit crosstalk between Cx43 and cytoskeletal and regulatory proteins. These domains endow Cx43 with the capacity to affect cell growth and differentiation independently of GJIC. Here, we review the current understanding of the regulation and unique functions of the Cx43 CT, both as an essential component of full-length Cx43 and as an independent signalling hub. We highlight the complex regulatory and signalling networks controlled by the Cx43 CT, including the extensive protein interactome that underlies both gap junction channel-dependent and -independent functions. We discuss these data in relation to the recent discovery of the direct translation of specific truncated forms of Cx43. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

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.

Role of connexins and pannexins during ontogeny, regeneration, and pathologies of bone

Electron micrographs revealed the presence of gap junctions in osteoblastic cells over 40 years ago. These intercellular channels formed from connexins are present in bone forming osteoblasts, bone resorbing osteoclasts, and osteocytes (mature osteoblasts embedded in the mineralized bone matrix). More recently, genetic and pharmacologic studies revealed the role of connexins, and in particular Cx43, in the differentiation and function of all bone types. Furthermore, mutations in the gene encoding Cx43 were found to be causally linked to oculodentodigital dysplasia, a condition that results in an abnormal skeleton. Pannexins, molecules with similar structure and single-membrane channel forming potential as connexins when organized as hemichannels, are also expressed in osteoblastic cells. The function of pannexins in bone and cartilage is beginning to be uncovered, but more research is needed to determine the role of pannexins in bone development, adult bone mass and skeletal homeostasis. We describe here the current knowledge on the role of connexins and pannexins on skeletal health and disease.

Connexinopathies: a structural and functional glimpse

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

Erythrokeratodermia variabilis et progressiva allelic to oculo-dento-digital dysplasia

Erythrokeratodermia variabilis et progressiva (EKVP) is a genodermatosis with clinical and genetic heterogeneity, most often transmitted in an autosomal dominant manner, caused by mutations in GJB3 and GJB4 genes encoding connexins (Cx)31 and 30.3, respectively. In this issue, Boyden et al. (2015) report for the first time de novo dominant mutations in GJA1 encoding the ubiquitous Cx43 in patients with EKVP. These results expand the genetic heterogeneity of EKVP and the human disease phenotypes associated with GJA1 mutations. They disclose that EKVP is allelic to oculo-dento-digital dysplasia, a rare syndrome previously known to be caused by dominant GJA1 mutations.

Mutations in cardiovascular connexin genes

Connexins (Cxs) form a family of transmembrane proteins comprising 21 members in humans. Cxs differ in their expression patterns, biophysical properties and ability to combine into homomeric or heteromeric gap junction channels between neighbouring cells. The permeation of ions and small metabolites through gap junction channels or hemichannels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. Among others, Cx37, Cx40, Cx43, Cx45 and Cx47 are found in heart, blood and lymphatic vessels. Mutations or polymorphisms in the genes coding for these Cxs have not only been implicated in cardiovascular pathologies but also in a variety of other disorders. While mutations in Cx43 are mostly linked to oculodentodigital dysplasia, Cx47 mutations are associated with Pelizaeus-Merzbacher-like disease and lymphoedema. Cx40 mutations are principally linked to atrial fibrillation. Mutations in Cx37 have not yet been described, but polymorphisms in the Cx37 gene have been implicated in the development of arterial disease. This review addresses current knowledge on gene mutations in cardiovascular Cxs systematically and links them to alterations in channel properties and disease.

Myogenic bladder defects in mouse models of human oculodentodigital dysplasia

To date, over 65 mutations in the gene encoding Cx43 (connexin43) have been linked to the autosomal-dominant disease ODDD (oculodentodigital dysplasia). A subset of these patients experience bladder incontinence which could be due to underlying neurogenic deterioration or aberrant myogenic regulation. BSMCs (bladder smooth muscle cells) from wild-type and two Cx43 mutant lines (Cx43(G60S) and Cx43(I130T)) that mimic ODDD exhibit a significant reduction in total Cx43. Dye transfer studies revealed that the G60S mutant was a potent dominant-negative inhibitor of co-expressed Cx43, a property not equally shared by the I130T mutant. BSMCs from both mutant mouse strains were defective in their ability to contract, which is indicative of phenotype changes due to harbouring the Cx43 mutants. Upon stretching, Cx43 levels were significantly elevated in controls and mutants containing BSMCs, but the non-muscle myosin heavy chain A levels were only reduced in cells from control mice. Although the Cx43(G60S) mutant mice showed no difference in voided urine volume or frequency, the Cx43(I130T) mice voided less frequently. Thus, similar to the diversity of morbidities seen in ODDD patients, genetically modified mice also display mutation-specific changes in bladder function. Furthermore, although mutant mice have compromised smooth muscle contraction and response to stretch, overriding bladder defects in Cx43(I130T) mice are likely to be complemented by neurogenic changes.

Syndromic and non-syndromic disease-linked Cx43 mutations

There are now at least 14 distinct diseases linked to germ line mutations in the 21 genes that encode the connexin (Cx) family of gap junction proteins. This review focuses on the links between germ-line mutations in the gene encoding Cx43 (GJA1) and the human disease termed oculodentodigital dysplasia (ODDD). This disease is clinically characterized by soft tissue fusion of the digits, abnormal craniofacial bone development, small eyes and loss of tooth enamel. However, the disease is considerably more complex and somewhat degenerative as patients often suffer from other syndromic effects that include incontinence, glaucoma, skin diseases and neuropathies that become more pronounced during aging. The challenge continues to be understanding how distinct Cx43 gene mutations cause such a diverse range of tissue phenotypes and pathophysiological changes while other Cx43-rich organs are relatively unaffected. This review will provide an overview of many of these studies and distill some themes and outstanding questions that need to be addressed in the coming years.

Gap junction remodeling in skin repair following wounding and disease

In the present review, we provide an overview of connexin expression during skin development and remodeling in wound healing, and reflect on how loss- or gain-of-function connexin mutations may change cellular phenotypes and lead to diseases of the skin. We also consider the therapeutic value of targeting connexins in wound healing.

Cerebral ischemic injury is enhanced in a model of oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a rare autosomal dominant disease that results in visible developmental anomalies of the limbs, face, eyes and teeth. Recently analysis of human connexin43 (Cx43) DNA sequences has revealed a number of different missense, duplication and frame shift mutations resulting in this phenotype. A mouse model of this disorder has been created with a missense point mutation of the glycine amino acid at position 60 to serine (G60S). Heterozygote +/G60S mice exhibit a similar ODDD phenotype as observed in humans. In addition to the malformations listed above, ODDD patients often have neurological findings. In the brain, Cx43 is highly expressed in astrocytes and has been shown to play a role in neuroprotection. We were interested in determining the effect of the +/G60S mutation following stroke. Four days after middle cerebral artery occlusion the volume of infarct was larger in mice with the +/G60S mutation. In astrocyte-neuron co-cultures, exposure to glutamate also resulted in greater cellular death in the +/G60S mutants. Protein levels of Cx43 in the mutant mouse were found to be reduced when compared to the normal tissue. Cx43 protein was observed as a continual line of small punctate aggregates in the plasma membrane with increased intracellular localization, which is distinct from the larger plaques seen in the normal mouse astrocytes. Functionally, primary +/G60S astrocytes exhibited reduced gap junctional coupling and increased hemichannel activity, which may underlie the mechanism of increased damage during stroke. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Autosomal recessive GJA1 (Cx43) gene mutations cause oculodentodigital dysplasia by distinct mechanisms

Oculodentodigital dysplasia (ODDD) is mainly an autosomal dominant human disease caused by mutations in the GJA1 gene, which encodes the gap junction protein connexin43 (Cx43). Surprisingly, there have been two autosomal recessive mutations reported that cause ODDD: a single amino acid substitution (R76H) and a premature truncation mutation (R33X). When expressed in either gap junctional intercellular communication (GJIC)-deficient HeLa cells or Cx43-expressing NRK cells, the R76H mutant trafficked to the plasma membrane to form gap junction-like plaques, whereas the R33X mutant remained diffusely localized throughout the cell, including the nucleus. As expected, the R33X mutant failed to form functional channels. In the case of the R76H mutant, dye transfer studies in HeLa cells and electrical conductance analysis in GJIC-deficient N2a cells revealed that this mutant could form functional gap junction channels, albeit with reduced macroscopic and single channel conductance. Alexa 350 dye transfer studies further revealed that the R76H mutant had no detectable negative effect on the function of co-expressed Cx26, Cx32, Cx37 or Cx40, whereas the R33X mutant exhibited significant dominant or trans-dominant effects on Cx43 and Cx40 as manifested by a reduction in wild-type connexin gap junction plaques. Taken together, our results suggest that the trans-dominant effect of R33X together with its complete inability to form a functional channel may explain why patients harboring this autosomal recessive R33X mutant exhibit greater disease burden than patients harboring the R76H mutant.

Phosphorylation on Ser-279 and Ser-282 of connexin43 regulates endocytosis and gap junction assembly in pancreatic cancer cells

The molecular mechanisms regulating the assembly of connexins (Cxs) into gap junctions are poorly understood. Using human pancreatic tumor cell lines BxPC3 and Capan-1, which express Cx26 and Cx43, we show that, upon arrival at the cell surface, the assembly of Cx43 is impaired. Connexin43 fails to assemble, because it is internalized by clathrin-mediated endocytosis. Assembly is restored upon expressing a sorting-motif mutant of Cx43, which does not interact with the AP2 complex, and by expressing mutants that cannot be phosphorylated on Ser-279 and Ser-282. The mutants restore assembly by preventing clathrin-mediated endocytosis of Cx43. Our results also document that the sorting-motif mutant is assembled into gap junctions in cells in which the expression of endogenous Cx43 has been knocked down. Remarkably, Cx43 mutants that cannot be phosphorylated on Ser-279 or Ser-282 are assembled into gap junctions only when connexons are composed of Cx43 forms that can be phosphorylated on these serines and forms in which phosphorylation on these serines is abolished. Based on the subcellular fate of Cx43 in single and contacting cells, our results document that the endocytic itinerary of Cx43 is altered upon cell-cell contact, which causes Cx43 to traffic by EEA1-negative endosomes en route to lysosomes. Our results further show that gap-junctional plaques formed of a sorting motif-deficient mutant of Cx43, which is unable to be internalized by the clathrin-mediated pathway, are predominantly endocytosed in the form of annular junctions. Thus the differential phosphorylation of Cx43 on Ser-279 and Ser-282 is fine-tuned to control Cx43's endocytosis and assembly into gap junctions.

Structure and functional studies of N-terminal Cx43 mutants linked to oculodentodigital dysplasia

Mutations in the connexin-43 gap junction protein cause the developmental disease known as oculodentodigital dysplasia. Structure and function approaches are used to demonstrate that the nature of the missense mutation in the amino-terminal domain of connexin-43 governs the mechanism that leads to loss of connexin-43 function.

Mutations in the gene encoding connexin-43 (Cx43) cause the human development disorder known as oculodentodigital dysplasia (ODDD). In this study, ODDD-linked Cx43 N-terminal mutants formed nonfunctional gap junction–like plaques and exhibited dominant-negative effects on the coupling conductance of coexpressed endogenous Cx43 in reference cell models. Nuclear magnetic resonance (NMR) protein structure determination of an N-terminal 23–amino acid polypeptide of wild-type Cx43 revealed that it folded in to a kinked α-helical structure. This finding predicted that W4 might be critically important in intramolecular and intermolecular interactions. Thus we engineered and characterized a W4A mutant and found that this mutant formed a regular, nonkinked α-helix but did not form functional gap junctions. Furthermore, a G2V variant peptide of Cx43 showed a kinked helix that now included V2 interactions with W4, resulting in the G2V mutant forming nonfunctional gap junctions. Also predicted from the NMR structures, a G2S mutant was found to relieve these interactions and allowed the protein to form functional gap junctions. Collectively, these studies suggest that the nature of the mutation conveys loss of Cx43 function by distinctly different mechanisms that are rooted in the structure of the N-terminal region.

Human dermal fibroblasts derived from oculodentodigital dysplasia patients suggest that patients may have wound-healing defects

Oculodentodigital dysplasia (ODDD) is primarily an autosomal dominant human disease caused by any one of over 60 mutations in the GJA1 gene encoding the gap junction protein Cx43. In the present study, wound healing was investigated in a G60S ODDD mutant mouse model and by using dermal fibroblasts isolated from two ODDD patients harboring the p.D3N and p.V216L mutants along with dermal fibroblasts isolated from their respective unaffected relatives. Punch biopsies revealed a delay in wound closure in the G60S mutant mice in comparison to wild-type littermates, and this delay appeared to be due to defects in the dermal fibroblasts. Although both the p.D3N and p.V216L mutants reduced gap junctional intercellular communication in human dermal fibroblasts, immunolocalization studies revealed that Cx43 gap junctions were prevalent at the cell surface of p.D3N expressing fibroblasts but greatly reduced in p.V216L expressing fibroblasts. Mutant expressing fibroblasts were further found to have reduced proliferation and migration capabilities. Finally, in response to TGFβ1, mutant expressing fibroblasts expressed significantly less alpha smooth muscle actin suggesting they were inefficient in their ability to differentiate into myofibroblasts. Collectively, our results suggest that ODDD patients may have subclinical defects in wound healing due to impaired function of dermal fibroblasts.

The G60S connexin43 mutant regulates hair growth and hair fiber morphology in a mouse model of human oculodentodigital dysplasia

Patients expressing mutations in the gene encoding the gap junction protein Cx43 suffer from a disease called oculodentodigital dysplasia (ODDD). Patients with ODDD are often reported to develop hair that is dry, dull, sparse, and slow growing. To evaluate the linkage between Cx43 and hair growth, structure, and follicle density we employed a mouse model of ODDD that harbors a Cx43 G60S point mutant. Regionally sparse and overall dull hair were observed in mutant mice compared with their wild-type (WT) littermates. However, histological analysis of overall hair follicle density in mutant and WT mice did not reveal any significant differences. After epilation, mutant mouse hair grew back slower, and hair growth was asynchronous. In addition, ultrastructural scanning electron microscopic imaging of hair fibers taken from mutant mice and two patients harboring the G143S mutation revealed severe cuticle weathering. Nodule formation was also observed in the proximal region of hair fibers taken from mutant mice. These results suggest that the G60S mutant mouse model mimics the hair phenotype found in at least some ODDD patients and suggests an important role for Cx43 in hair regeneration, growth, and cuticle formation.