Inherited disease-linked arginine76/75 mutants in Cx50 and Cx45 showed impaired homotypic and heterotypic gap junction function, but not Cx43

Connexins form intercellular communication channels, known as gap junctions (GJs), in many tissues/organs. Mutations in connexin genes are found to be linked to various inherited diseases, but the mechanisms are not fully clear. The Arg76 (R76) in Cx50 is fully conserved across the entire connexin family and is a hotspot for five connexin-linked inherited diseases, including Cx50 and Cx46-linked congenital cataract, Cx43-linked oculodentodigital dysplasia, and Cx45-linked cardiac arrhythmias. To better understand the molecular and cellular mechanism of dysfunction caused by R76/75 mutations, we examined the functional status and properties of GJs containing R76 mutations in Cx50 (R76H/C), Cx43 (R76H/S/C), and Cx45 (R75H) with an emphasis on heterotypic GJs in connexin-deficient model cells. All tested mutants showed an impairment of homotypic GJ function reflected by a decreased coupling% and conductance, except for Cx43 R76H/S. These connexin mutants also showed impaired GJ function when paired with a docking-compatible connexin, such as Cx50/Cx46 or Cx45/Cx43, except for all mutants on Cx43 which formed functional heterotypic GJs with Cx45. Localization studies on fluorescent protein tagged connexin mutants revealed that Cx45 R75H and Cx43 R76C showed impaired localization. Our homology structure models indicated that mutations of R76/75 in these GJs led to a loss of intra- and/or inter-connexin non-covalent interactions (salt bridges) at the sidechain of this residue, which could contribute to the observed GJ impairments underlying diseases. It is interesting that unlike those disease-linked variants in Cx50 and Cx45, Cx43 can tolerate some variations at R76.