1.A.24 The Gap Junction-forming Connexin (Connexin) Family
Gap junctions, found in the plasma membranes of vertebrate animal cells, consist of clusters of closely packed pairs of transmembrane channels, the connexons, through which small molecules diffuse between neighboring cells (Zhou and Jiang 2014). The connexons consist of homo- or heterohexameric arrays of connexins (Cxs), and the connexon in one plasma membrane docks end-to-end with a connexon in the membrane of a closely opposed cell. The hemichannel is made of six connexin subunits (Kar et al., 2012). The properties and possible functions of unpaired connexin and pannexin hemichannels and the implications this has for a variety of events, such as cell death, glutamate release, oxidative stress, cortical spreading depression, that occur during an ischemic insult and may affect its outcome, have been reviewed (Bargiotas et al. 2009). The two connexons are docked by interdigitated, anti-parallel beta strands across the extracellular gap. The second extracellular loop guides selectivity in docking between connexons formed by different isoforms (Kovacs et al. 2007). There is considerably more sequence variability of the N-terminal portion of E2; possibly this region dictates connexon coupling. Structure/function relationships for connexins have been reviewed (Beyer and Berthoud 2017). The roles of connexin hemichannels in normal cochlear function and in promoting hearing loss have been reviewed (Verselis 2017). Connexin-mediated cell communication in the kidney presents a potential therapeutic target for intervention of diabetic kidney disease (Price et al. 2020). Both connexins and pannexins contribute to the induction and spreading of orofacial pain (Li et al. 2020). Cxs may play a role in preeclampsia, and ROS and RNS may alter Cxs-formed channels (Rozas-Villanueva et al. 2020). Connexins have been linked to cancers, cardiac and brain disorders, chronic lung and kidney conditions and wound healing processes (Nalewajska et al. 2020). Gap junction liposomes have been used for efficient delivery of chemotherapeutics to solid tumors (Trementozzi et al. 2020). Connexins may play a role in spinal cord injury, and Cx-specific inhibitors that may be useful for treatment are known (Abou-Mrad et al. 2020). Over-activated hemichannels may be targets for drugs treating human diseases (Retamal et al. 2021). In fact, screens for inhibitors of Cx43 hemichannel function have revealed several candidates (Soleilhac et al. 2021). Endothelial cell Cxs regulate physiological and pathological angiogenesis through canonical and noncanonical functions (Haefliger et al. 2022).
Over 15 connexin subunit isoforms are known. They vary in size between about 25 kDa and 60 kDa. They have four putative transmembrane α-helical spanners, and direct experimental evidence favors the α-helical folding of at least two of these TMSs. Connexins are similar in sequence and are designated connexins α1-8 and β1-6. Low resolution structural data are available for a gap junction membrane channel. A dodecameric channel is formed by the end-to-end docking of two hexamers, each displaying 24 TMSs (4 α-helical TMSs per connexin subunit) (Bosco et al., 2011). Gap junctional channels are parts of multiprotein complexes (Hervé et al., 2011). Regulation of cardiovascular connexins have been reviewed (Meens et al. 2013). The proteins interacting with Cx43, the most prevalent connexin (TC# 1.A.24.1.1; the rat and human orthologs are 98 % identical), include: c-Src (TC#1.A.23.1.12; P12931), ZO-1 (8.A.24.1.9; Q07157), drebrin (TC#; DBN1; Q16643), CIP85 (TC# 8.A.87.1.5; Q96HU1) and CCN3 (8.A.87.1.6; P48745), as well as feedback between gap junctions, adherens junctions (N-cadherin and catenins) and the cytoskeleton (microtubules and actin) (Giepmans 2006). Genome-wide characterization of gap junction (connexin and pannexin) genes in turbot (Scophthalmus maximus L.) with respect to evolution and immune response following Vibrio anguillarum infection has been published (Cai et al. 2022). Connexins and pannexins connect the external environment with the cytoplasm of the cell, but only connexins are able to link two cells together, allowing transport from one cell to another (Roterman et al. 2022).
Connexin channels have been reconstituted in unilamellar phospholipid vesicles from purified rat liver connexin 43. The vesicles were shown to be permeable to sucrose and the dye, lucifer yellow, and channel activity was reversibly inhibited by phosphorylation of connexin 43 by mitogen-activated protein (MAP) kinase. Other kinases may also effect inhibition. Gating of connexin 43 channels may therefore be regulated by phosphorylation of the connexin subunit in vivo. However, the cytoplasmic tails differ considerably in the size and amino acid sequence for different connexins and are predicted to be involved in the channel open and closed conformations. A ball and chain model for hemichannel conformational changes has been proposed for some connexins (e.g., Cx43) with large cytoplasmic tails (Liu et al., 2006). The tail folds into a ball or 'gating particle' and binds to the cytoplasmic loop domain, leading to channel closure (Liu et al., 2006). The involvement of lymphatic connexins and pannexins in health and disease has been reviewed (Ehrlich et al. 2021). Ephaptic coupling is a mechanism of conduction reserve during reduced gap junction coupling (Lin et al. 2022). Pannexin1, Connexin32, and Connexin43 in spotted sea bass (Lateolabrax maculatus are important neuro-related immune response genes involved in inflammation-induced ATP release (Sun et al. 2022).
Different connexins may exhibit differing specificities for solutes. For example, adenosine passed about 12-fold better through channels formed by Cx32 while AMP and ADP passed about 8-fold better, and ATP greater than 300-fold better, through channels formed by Cx43. Thus, addition of phosphate to adenosine appears to shift its relative permeability from channels formed by Cx32 to channels formed by Cx43. This may have functional consequence because the energy status of a cell could be controlled via connexin expression and channel formation (Goldberg et al., 2002). The relationship between redox signaling and Cxs has shown that redox signaling molecules (e.g., hydrogen peroxide (H2O2) and nitric oxide (NO)) affect Cxs-based channel function while the opening of Cx channels also triggers the transfer of various redox-related metabolites (e.g., reactive oxygen species, glutathione, nicotinamide adenine dinucleotide and NO). On the basis of this evidence, Zhang et al. 2021 proposed the existence of redox-Cxs crosstalk.
There are about 20 isoforms of connexin proteins, each forming channels with distinct channel properties (Ayad et al., 2006). Moreover, connexins can form both homomeric and heteromeric connexin channels. Two homomeric channels may have different permeability properties that differ from those of the heteromeric channels including both proteins (see 1.A.24.1.3; Ayad et al., 2006). Connexin23 has only 4 conserved cysteines in the extracellular domain, but they still form hemichannels (Iovine et al., 2008) A robust and updated classification of the human 4 TMS protein complement has appeared (Attwood et al. 2016). The connexin gene family is under extensive regulation at the transcriptional and post-transcriptional levels, and they undergoes numerous modifications at the protein level, including phosphorylation, which ultimately affects their trafficking, stability, and function (Aasen et al. 2018). Cx26, Cx32, and Cx43 proteins are present in human labial salivary gland biopsies (hLSGBs) in the duct and acinar cells, as well as in myoepithelial cells (Falleni et al. 2022). Gap juntional proteins, connexins and pannexins, interact with tight junctions, adhesive junctions, and cell adhesions to form a complex network that participates in cell-cell junctional organization, ATP binding, ion channel, and voltage-gated conduction (Liu et al. 2023).
Deletion or mutation of the various connexin isoforms produces distinctive phenotypes and pathologies. This observation reflects (1) the different molecular specificities, (2) the different relative magnitudes of transport rates of various compounds via these channels, and (3) the regulatory properties via these dissimilar channels. Genetic diseases indicate that the normal function of CNS myelin depends on connexin32 (Cx32) and Cx47, gap junction (GJ) proteins expressed by oligodendrocytes. GJs couple oligodendrocytes to themselves (O/O channels), astrocytes to themselves (A/A channels), and oligodendrocytes to astrocytes (O/A channels). Astrocytes and oligodendrocytes express different connexins. Cx47/Cx43 and Cx32/Cx30 efficiently form functional channels, but neither Cx47 nor Cx43 formed channels with Cx30 or Cx32 (Orthoann-Murphy et al., 2007). Cx47/Cx43 and Cx32/Cx30 channels have distinct properties and permeabilities. Cx47 mutants that cause Pelizaeus-Merzbacher-like disease do not efficiently form functional channels with Cx43, indicating that disrupted Cx47/Cx43 channels cause this disease. The mutations in connexins that give rise to disease have been summarized and discussed (Pfenniger et al. 2011). While mutations in Cx43 are mostly linked to oculodentodigital dysplasia, Cx47 mutations are associated with Pelizaeus-Merzbacher-like disease and lymphedema. 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 (Molica et al. 2014).
Maeda et al. (2009) have reported the crystal structure of the gap junction channel formed by human connexin 26 (Cx26, also known as GJB2) at 3.5 Å resolution. The density map showed the two membrane-spanning hemichannels and the arrangement of the four transmembrane helices of the six protomers forming each hemichannel. The hemichannels feature a postively charged cytoplasmic entrance, a funnel, a negatively charged transmembrane pathway, and an extracellular cavity. The pore is narrowed at the funnel, which is formed by the six amino-terminal helices lining the wall of the channel, which thus determines the molecular size restriction at the channel entrance. The structure of the Cx26 gap junction channel also has implications for the gating of the channel by the transjunctional voltage (Nakagawa et al., 2010). The N-terminal half of connexin 46 appears to contain the core elements of the pore and voltage gates (Kronengold et al., 2012).
Research has revealed a multilevel platform via which connexins (Cxs) and pannexins (Panxs) can influence the following cellular functions within a tissue: (1) Cx gap junctional channels (GJCs) enable direct cell-cell communication of small molecules, (2) Cx hemichannels and Panx channels can contribute to autocrine/paracrine signaling pathways, and (3) different structural domains of these proteins allow for channel-independent functions, such as cell-cell adhesion, interactions with the cytoskeleton, and the activation of intracellular signaling pathways. Decrock et al. 2015 discuss their multifaceted contributions to brain development and specific processes in the NGVU, including synaptic transmission and plasticity, glial signaling, vasomotor control, and blood-brain barrier integrity in the mature CNS. Connectosomes, cell-derived lipid vesicles that contain functional gap junction channels and encapsulate molecular cargos, have been used to deliver cargos such as drugs into the cytoplasm of a cell (Gadok et al. 2016).
Connexins (Cx) contain both highly ordered domains (i.e., 4 transmembrane domains) and primarily unstructured regions (i.e., N- and C-terminal domains). The C-terminal domains vary in length and amino acid composition from the shortest on Cx26 to the longest on Cx43. With the exception of Cx26, the C-terminal domains contain multiple sites for posttranslational modification (PTM) including serines (S), threonines (T), and tyrosines (Y) for phosphorylation as well as cysteines (C) for S-nitrosylation. These PTMs are critical for regulating cellular localization, protein-protein interactions, and channel functionality (Lohman et al. 2016).
Fatty acids (FAs) have effects on connexin- and pannexin-based channels. FAs regulate diverse cellular functions, including the activities of connexin (Cx) and Panx channels which form hexameric hemichannels (HCs), which assemble into dodecameric gap junction channels (GJCs). It has been shown that FAs decrease GJC-mediated cell-cell communication. Changes in GJCs mediated by FAs have been associated with post-translational modifications (e.g., phosphorylation), and seem to be directly related to chemical properties of FAs (Puebla et al. 2017).
Connexins participate in the generation of intercellular calcium waves, in which calcium-mediated signaling responses spread to contiguous cells through gap junction to transmit calcium signaling throughout the airway epithelium. Pannexins in the nasal mucosa contribute not only to ciliary beat modulation via ATP release, but also regulation of mucus blanket components via H2O efflux. The synchronized roles of pannexin and connexin may allow effective mucociliary clearance in nasal mucosa (Ohbuchi and Suzuki 2018).
Gadok et al. 2016 have developed 'connectosomes', cell-derived lipid vesicles that contain functional gap junction channels and encapsulate molecular cargos. They showed that these vesicles form gap junctions with cells, opening a direct and efficient route for the delivery of molecular cargo to the cellular cytoplasm. Specifically, they demonstrated that using gap junctions to deliver doxorubicin reduces the therapeutically effective dose of the drug by more than an order of magnitude (Gadok et al. 2016). Single-domain antibodies on connectosomes allows gap junction-mediated drug targetting to specific cell types (Gadok et al. 2018). An overview of connexin biology, including their synthesis and degradation, their regulation and interactions, and their involvement in cardiac pathophysiology, including their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription and signaling regulation have been reviewed (Rodríguez-Sinovas et al. 2021).
Connexins have been implicated in cancer biology for their context-dependent roles that can either promote or suppress cancer cell functions. They are able to modulate many aspects of cellular metabolism including the intercellular coupling of nutrients and signaling molecules (Jones and Bodenstine 2022). During cancer progression, changes to substrate utilization occur to support energy production and biomass accumulation. This results in metabolic plasticity that promotes cell survival and proliferation, and can impact therapeutic resistance (Jones and Bodenstine 2022).
The transport reaction catalyzed by connexin gap junctions is:
Small molecules (cell 1 cytoplasm) Small molecules (cell 2 cytoplasm)
Small molecules include small proteins, cyclic nucleotides, chemotherapeutics and small RNAs.
Connexin 43 (gap junction α-1 protein), CX43 encoded by the GJA1 gene (transports ATP, ADP and AMP better than CX32 does; Goldberg et al., 2002). Hemichannels mediate efflux of glutathione, glutamate and other amino acids as well as ATP (Stridh et al., 2008; Kang et al., 2008). CX43 has a half life of ~3 h due to ubiquitination and lysosomal and proteasomal degradation (Leithe and Rivedal, 2007). Cx43 and Cx46 regulate each other's expression and turnover in a reciprocal manner in addition to their conventional roles as gap junction proteins in lens cells (Banerjee et al., 2011). A mutant form of Connexin 43 causes Oculodentodigital dysplasia (Gabriel et al., 2011). Suppressing the function of Cx43 promotes expression of wound healing-associated genes and hibitits scarring (Tarzemany et al. 2015). Channel conductance and size selectivity are largely determined by pore diameter, whereas charge selectivity results from the amino-terminal domains; transitions between fully open and (multiple) closed states involves global changes in structure of the pore-forming domains (Ek Vitorín et al. 2016). The human Cx43 orthologue is almost identical to the rat protein. It may mediate resistance against the parkinsonian toxin, 1-methyl-4-phenylpyridine (MPP+) which induces apoptosis in neuroblastoma cells by modulating mitochondrial apoptosis (Kim et al. 2016). Dopamine neurons may be the target of MPP+ and play a role in Parkinson's disease. In humans, Cx43 plays roles in the development of the central nervous system and in the progression of glioma (Wang et al. 2017). It interacts with and is regulated by many proteins including NOV (CCN3, IGFBP9; P48745) (Giepmans 2006). Cx43 plays roles in intercellular communication mediated by extracellular vesicles, tunnelling nanotubes and gap junctions (Ribeiro-Rodrigues et al. 2017). Phosphorylation of Cx43 leads to astrocytic coupling and apoptosis, and ultimately, to vascular regeneration in retinal ischemia. Paxillin (Pxn; 591 aas; P49023), a cytoskeletal protein involved in focal adhesion, leads to changes in connexin 43 by direct protein-protein binding, thereby influencing osteocyte gap junction elongation (Zhang et al. 2018). Regulation of Cx43 abundance involves transcriptional/post-transcriptional and translational/post-translational mechanisms that are modulated by an interplay between TGF-beta isoforms and PGE2, IL-1beta, TNF-alpha and IFN-gamma (Cheng et al. 2018). In the developing fetal human kidney, cytoplasmic expression of Cx36 was localized to nephrons in different developmental stages, glomerular vessels and collecting ducts, and of Cx43 was localized to the endothelium of glomerular and peritubular vessels, as well as to the epithelium of the proximal tubules (Ráduly et al. 2019). Mutations in the gap junction protein α1 (GPA1) gene cause oculodentodigital dysplasia (Pace et al. 2019). Expression of connexin 43 is elevated in atypical fibroxanthoma cells (Fernandez-Flores et al. 2020). Astrocytic connexin43 channels are candidate targets in epilepsy treatment (Walrave et al. 2020). Cx43 plays roles in physiological functions such as regulating cell growth, differentiation, and maintaining tissue homeostasis (Sha et al. 2020). Amyloid-beta (TC# 1.C.50) regulates connexin 43 trafficking in cultured primary astrocytes (Maulik et al. 2020). Gap junction protein Cx43 plays a role in regulating cellular function and paracrine effects of smooth muscle progenitor cells (Tien (田婷怡) et al. 2021). A serine residues in the connexin43 carboxyl tail is important for B-cell antigen receptor-mediated spreading of B-lymphocytes (Pournia et al. 2020). Connexin 43 plays an antagonistic role in the development of primary bone tumors as a tumor suppressor and also as a tumor promoter (Talbot et al. 2020). Retinal astrocytes abundantly express Cx43 that forms gap junction (GJ) channels and unopposed hemichannels, and Cx43 is upregulated in retinal injuries. Astrocytic Cx43 plays a role in retinal ganglion cell (RGC) loss associated with injury (Toychiev et al. 2021). Screens for inhibitors of Cx43 hemichannel function have revealed several candidates (Soleilhac et al. 2021). The dodecameric channel is formed by the end-to-end docking of two hexameric connexons, each comprised of 24 transmembrane alpha-helices (Cheng et al. 2019). Cx43 appears to be involved in the tumorigenesis of most pituitary adenomas and have a potential therapeutic value for pituitary tumor therapy (Nunes et al. 2022).
CX43 of Rattus norvegicus
Connexin31, Cx31 of 270 aas and 4 TMSs. Also called the gap junction β-3 protein. Mutation Thr202Asn in TMS4 gives rise to erythrokeratodermia (Sugiura et al. 2015).
Cx31 of Homo sapiens
Gap junction α-1 protein, GJα-1, Cx43, shf, sof, of 281 aas and 4 TMSs. Can function both as a gap junction and a hemichannel and plays critical diverse roles in zebrafish bone growth (Misu et al. 2016).
Cx43 of Danio rerio (Zebrafish) (Brachydanio rerio)
Connexin 29 (Cx29, Gjc3, Gje1) of 269 aas and 4 TMSs. The Cx29E269D mutant has a dominant negative effect on the formation and function of gap junctions, explaining the role Cx29 in the development of hearing loss (Hong et al. 2010). Direct axon-to-myelin linkage by abundant KV1 (TC# 1.A.1.2.10 and 12)/Cx29 channel interactions in rodent axons supports the idea of an electrically active role for myelin in increasing both the saltatory conduction velocity and the maximal propagation frequency in mammalian myelinated axons (Rash et al. 2016).
Cx29 of Mus musculus
Connexin36, connexin delta2, Cxδ2, GJD2, Cx36 of 321 aas and 4 TMSs. In the developing fetal kidney, cytoplasmic expression of Cx36 is localized to nephrons in different developmental stages, glomerular vessels and collecting ducts. Cx43 is localized to the endothelium of glomerular and peritubular vessels, as well as to the epithelium of the proximal tubules (Ráduly et al. 2019). A reciprocal relationship between Cx36 and seizure-associated neuronal hyperactivityhas been obseerved; thus, Cx36 deficiency contributes to region-specific susceptibility to neuronal hyperactivity, while neuronal hyperactivity-induced downregulation of Cx36 may increase the risk of future epileptic events (Brunal et al. 2020).
Cx36 of Homo sapiens
Gap junction protein B4, GJB4, or Cx30.3 of 266 aas and 4 TMSs. Small molecules and ions diffuse from one cell to a neighboring cell via the central pore in these dodecameric channels. Mutation can cause a familial form of hypertrophic cardiomyopathy (HCM) and therefore could be a target for the treatment of cardiac hypertrophy and dysfunction (Okamoto et al. 2020).
GJB4 of Homo sapiens
Gap junction protein alpha 5, GJA5 or CxA5, of 358 aas and 4 TMSs. One gap junction consists of a cluster of closely packed pairs of transmembrane channels, the connexons, through which materials of low MW diffuse from one cell to a neighboring cell. GAJ5 is enriched for the function of ion transmembrane transport regulation and is a key atrial fibrillation (AF)-valvular heart disease (VHD) protein (Zhao et al. 2021).
Connexin-43, Cx43, or Gap Junction α-1 protein, GJA1, of 382 aas and 4 TMSs in a 2 + 2 TMS arrangement. Extracellular vesicles enriched in connexin 43 promote a senescent phenotype in bone and synovial cells, contributing to osteoarthritis progression (Varela-Eirín et al. 2022). It is 98% identical to the rat ortholog (TC# 1.A.24.1.1). Cx43 expression is highly sensitive to oxidative distress, leading to reduced expression (Wahl et al. 2022). This effect can be efficiently prevented by the glutathione peroxidase mimetic ebselen. Cx43 expression is tightly regulated by miR-1, which is activated by tachypacing-induced oxidative distress. In light of the high arrhythmogenic potential of altered Cx43 expression, miR-1 may be a target for pharmacological interventions to prevent the maladaptive remodeling processes during chronic distress in the heart (Wahl et al. 2022). Cx43 hemichannels can reversibly transport NAD+ and cyclic ADP-ribose, the latter which acts on cytoplasmic ryanodine receptors (RyRs) (Astigiano et al. 2022). Connexin 43 hemichannels regulate mitochondrial ATP generation, mobilization, and mitochondrial homeostasis against oxidative stress (Zhang et al. 2022).
Cx43 of Homo sapiens
Connexin 32 (gap junction β1-protein), CX32 (transports adenosine better than CX43 does; Goldberg et al., 2002). The carboxyl tail regulates gap junction assembly (Katoch et al. 2015). The modeled channel pore-facing regions of TMSs 1 and 2 were highly sensitive to tryptophan substitution while lipid-facing regions of TMSs 3 and 4 were variably tolerant. Residues facing a putative intracellular water pocket (the IC pocket) were also sensitive. Interactions important for voltage gating occurred mainly in the mid-region of the channel in TMS 1. TMS 1 of Cx43 was scanned revealing similar but not identical sensitivities (Brennan et al. 2015). Single point mutations in Cx32, which cause Charcot-Marie-Tooth disease, causes failure in membrane integration, transport defects and rapid degradation. Multiple chaperones detect and remedy this aberrant behavior including the ER-membrane complex (EMC) which helps insert low-hydrophobicity TMSs (Coelho et al. 2019). If they fail to integrate, they are recognized by the ER-lumenal chaperone BiP. Ultimately, the E3 ligase gp78 ubiquitinates Cx32, targeting it for degradation. Thus, cells use a coordinated system of chaperones for membrane protein biogenesis. Dileucine-like motifs in the C-terminal tail of connexin32 control its endocytosis and assembly into gap junctions (Ray et al. 2018).
CX32 of Rattus norvegicus
Heteromeric connexin (Cx)32/Cx26; (CxB2, GJβ2, GJB2) (transports cAMP, cGMP and all inositol phosphates with 1-4 esterified phosphate groups (homomeric Cx26(β2) or homomeric Cx32 do not transport the inositol phosphates as well) (Ayad et al., 2006). The GJB2 gene encodes connexin 26, the protein involved in cell-cell attachment in many tissues. GJB2 mutations cause autosomal recessive (DFNB1) and sometimes dominant (DFNA3) non-syndromic sensorineural hearing loss as well as various skin disease phenotypes (Iossa et al., 2011; Tian et al. 2022). TMS1 regulates oligomerization and function (Jara et al., 2012). The carboxyl tail pg Cx32 regulates gap junction assembly (Katoch et al. 2015). In Cx46, neutralization of negative charges or addition of positive charge in the Cx26 equivalent region reduced the slow gate voltage dependence. In Cx50 the addition of a glutamate in the same region decreased the voltage dependence and the neutralization of a negative charge increased it. Thus, the charges at the end of TMS1 are part of the slow gate voltage sensor in Cxs. The fact that Cx42, which has no charge in this region, still presents voltage dependent slow gating suggests that charges still unidentified also contribute to the slow gate voltage sensitivity (Pinto et al. 2016). Syndromic deafness mutations at Asn14 alter the open stability of Cx26 hemichannels (Sanchez et al. 2016). The Leu89Pro substitution in the second TMS of CX32 disrupts the trafficking of the protein, inhibiting the assembly of CX32 gap junctions, which in turn may result in peripheral neuropathy (Da et al. 2016). Cx26 mutants that promote cell death or exert transdominant effects on other connexins in keratinocytes lead to skin diseases and hearing loss, whereas mutants having reduced channel function without aberrant effects on coexpressed connexins cause only hearing loss (Press et al. 2017). When challenged by a field of 0.06 V/nm, the Cx26 hemichannel relaxed toward a novel configuration characterized by a widened pore and an increased bending of the second TMS at the level of the conserved Pro87. A point mutation that inhibited such a transition impeded hemichannel opening in electrophysiology and dye uptake experiments. Thus, the Cx26 hemichannel uses a global degree of freedom to transit between different configuration states, which may be shared among all connexins (Zonta et al. 2018). A group of human mutations within the N-terminal (NT) domain of connexin 26 hemichannels produce aberrant channel activity, which gives rise to deafness and skin disorders, including keratitis-ichthyosis-deafness (KID) syndrome. Structural and functional studies indicate that the NT domain of connexin hemichannels is folded into the pore, where it plays important roles in permeability and gating. The mutation, N14K disrupts cytosolic intersubunit interactions and promotes channel opening (Valdez Capuccino et al. 2018). A missense mutation in the Connexin 26 gene is associated with hereditary autosomal recessive sensorineural deafness (Leshinsky-Silver et al. 2005, Zytsar et al. 2020). Cx26 hemichannels mediate the passage of contents between the cytoplasm and extracellular space. To generate hemichannels, the mutation N176Y was introduced into the second extracellular loop of Cx26. The cryoEM structure of the hexameric hemichannel in lipid bilayer nanodiscs displays an open pore and a 4-helix bundle transmembrane design that is nearly identical to dodecameric GJCs. In contrast to the high resolution of the transmembrane alpha-helices, the extracellular loops are less well resolved. The conformational flexibility of the extracellular loops may be essential to facilitate surveillance of hemichannels in apposed cells to identify compatible Cx isoforms that enable intercellular docking (Khan et al. 2021). A rare variant c.516G>C (p.Trp172Cys) in the GJB2 (connexin 26) gene is associated with nonsyndromic hearing loss (Maslova et al. 2021). Keratitis-ichthyosis-deafness (KID) syndrome is caused by mutations in the GJB2 gene (Asgari et al. 2020). An increase in the partial pressure of carbon dioxide (PCO2) has been shown to cause Cx26 gap junctions to close. Cryo-EM was used to determine the structure of human Cx26 gap junctions under increasing levels of PCO2. Brotherton et al. 2022 showed a correlation between the level of PCO2 and the size of the aperture of the pore, governed by the N-terminal helices that line the pore. Thus, CO2 alone is sufficient to cause conformational changes in the protein. Analysis of the conformational states showed that movements at the N-terminus are linked to both subunit rotation and flexing of the transmembrane helices (Brotherton et al. 2022). Cysteine residues in the C-terminal tail of connexin32 regulate its trafficking (Ray and Mehta 2021).
Cx26/Cx32 of Homo sapiens
Heteromeric (or homomeric) Connexin46/Connexin50 junction (Cx46/Cx50; Cnx46/Cnx50; GJA8/GJA3) protein. Mutations in CX46 or Cx50 cause cataracts, a cause of visual impairment and blindness (Derosa et al., 2007; Wang and Zhu 2012; Ye et al. 2019), and mutations in Cx46 can cause breast cancer (Grek et al. 2016). Cx43 and Cx46 regulate each other's expression and turnover in a reciprocal manner in addition to their conventional roles as gap junction proteins in lens cells (Banerjee et al., 2011). The N-terminal half of connexin 46 appears to contain the core elements of the pore and voltage gates (Kronengold et al. 2012). In Cx46, neutralization of negative charges or addition of positive charge in the Cx26 equivalent region reduced the slow gate voltage dependence. In Cx50 the addition of a glutamate in the same region decreased the voltage dependence, and the neutralization of a negative charge increased it. Thus, the charges at the end of TMS1 are part of the slow gate voltage sensor in Cxs. The fact that Cx42, which has no charge in this region, still presents voltage dependent slow gating, suggests that charges still unidentified also contribute to the slow gate voltage sensitivity (Pinto et al. 2016). Cx43 is regulated by phosphorylation of Ser-373 (Puebla et al. 2016). A connexin50 mutation in the heterozygous state affects the lipid profile and the oxidative stress parameters in a spontaneously hypertensive rat strain (Šeda et al. 2016). Mutations in Cx50 (N220D and V44M) are responsible for congenital cataracts (Kuo et al. 2017; Zhang et al. 2018) Mutations its gene cause defects in early eye development (Ceroni et al. 2019). Cx50 is important for eye lens transparency, and calmoduin and Ca2+ cooperate in the gating control of Cx50 hemichannels (Zhang et al. 2006). Cx46 hemichannels are modulated by nitric oxide, and the fourth TMS cysteine may be involved in cataract formation (Retamal et al. 2019). Gap19 is a Cx43 hemichannel inhibitor that acts as a gating modifier that decreases main state opening while increasing substate gating (Lissoni et al. 2020). Cx46, almost exclusively expressed in the eye lens, is upregulated in human breast cancer, and correlates with tumor growth (Acuña et al. 2020). EphA2 is required for normal Cx50 localization to the cell membrane, and conductance of lens fiber cells requires normal Eph-ephrin signaling and water channel (Aqp0) localization (Cheng et al. 2021).
Cx46/Cx50 of Homo sapiens:
Connexin37 of Homo sapiens (P35212)
Connexin 30 complex (connexin30.2/connexin31.3 (CX30.2/CX31.3)). Also called connexinΥ3/GJC3/GJε1; 279 aas, encoded by the GJB6 (13q12) gene (Cascella et al. 2016)). ATP is released from cells that stably expressed CX30.2 in a medium with low calcium, suggesting a hemichannel-based function. Liang et al. (2011) suggested that it shares functional properties with pannexin hemichannels rather than gap junction channels. Defects cause nonsyndromic hypoacusia (hearing loss) due to partial loss of channel activity (Su et al. 2012; Su et al. 2013; Cascella et al. 2016). Cx30, but not Cx43, hemichannels close upon protein kinase C activation, showing that connexin hemichannels display not only isoform-specific permeability profiles but also isoform-specific regulation by PKC (Alstrom et al. 2015). The W77S mutant has a dominant negative effect on the formation and function of the gap junction and is probably responsible for hearing loss (Wong et al. 2017). Mutations in Cs30 rescue hearing and reveal roles for gap junctions in cochlear amplification (Lukashkina et al. 2017). The cryo-EM structure of the human Cx31.3/GJC3 connexin hemichannel has been solved (Lee et al. 2020). Cx31.3)/GJC3 hemichannels in the presence and absence of calcium ions and with a hearing-loss mutation R15G were solved at 2.3-, 2.5- and 2.6-Å resolutions, respectively. Compared with available structures of GJICh in the open conformation, the Cx31.3 hemichannel shows substantial structural changes of highly conserved regions in the connexin family, including opening of calcium ion-binding tunnels, reorganization of salt-bridge networks, exposure of lipid-binding sites, and collocation of amino-terminal helices at the cytoplasmic entrance. The hemichannel has a pore with a diameter of ~8 Å and selectively transports chloride ions (Lee et al. 2020).
Cx30.2 of Homo sapiens (Q8NFK1)
Connexin40 (Cx40; Gap Junction Protein δ4; GJδ4) of 370 aas and 4 TM (Kopanic et al. 2015).
Cx40 of Homo sapiens
Gap junction epsilon-1 protein, Gjf1 of 205 aas and 4 TMSs. Mutations result in variable small eyes and affect lens development (Puk et al. 2008).
Gjf1 of Mus musculus
Invertebrate cordate Connexin 47 (White et al., 2004).
Connexin 47 of Halocynthia pyriformis (Q6U1M0)
Inverebrate cordate Connexin (Hervé et al., 2005).
Connexin of Oikopleura dioica (E4YIP4)
Connexin45 (Cx45; Cx-45; Gap Junction protein γ1; GJγ1; Gjc1; Gja7; CxG1) of 396 aas and 4 TMSs (Kopanic et al. 2015).
Cx45 of Homo sapiens