1.A.24.1.3
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). The pathogenesis of common Gjb2 mutations are associated with human hereditary deafness (Li et al. 2023). Pan-cancer analysis of the prognostic and immunological role of GJB2 identifies a potential target for survival and immunotherapy (Jia et al. 2023).  The keratitis-ichthyosis-deafness (KID) syndrome is a rare genetic disease caused by pathogenic variants in connexin 26 (gene GJB2), which is a transmembrane channel of the epithelia (López-Sundh et al. 2023).  Consequences of pathogenic variants of the GJB2 gene (Cx26) localized in different Cx26 domains have been evaluated (Posukh et al. 2023). A pore locus in E1 of Cx26 and Cx30 impacts hemichannel functionality (Sanchez et al. 2024). An Ala/Glu difference in E1 of Cx26 and Cx30 contributes to their differential anionic permeabilities (Kraujaliene et al. 2024). Differential regulation of Cx26 hemichannels and gap junction channels by RhoA GTPase and the actin cytoskeleton has been observed (Jara et al. 2024).  GJB2, KCNH6, and KCNN4 are oncogenic, and GJB2 and KCNN4 were upregulated, while KCNH6 was downregulated in high risk group and glioblastoma (GBM) cells. The regulatory network showed that KCNH6 was targeted by more miRNAs and transcription factors and KCNN4 interacted with more drugs (Huang et al. 2024).