Gating on the outside

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.

Role of amino terminus in voltage gating and junctional rectification of Shaking B innexins

Rectifying electrical synapses are rare gap junctions that favor transmission of signals in one direction. Such synapses have been identified in neural systems, including those mediating rapid escape responses of arthropods. In the Drosophila giant fiber system, adjacent cells express and contribute different transcript variants of the innexin Shaking B, resulting in heterotypic gap junctions with rectifying properties. When expressed exogenously, variants Shaking B Lethal (ShakBL) and Shaking B neural + 16 (ShakBN16) form heterotypic junctions that gate asymmetrically in response to transjunctional voltage. To determine whether the amino terminus confers properties of gating and rectification, amino-terminal domains were exchanged between ShakBL and ShakBN16, creating chimeric proteins SBL NTN16 and SBN16 NTL. The properties were analyzed in paired Xenopus oocytes. Our results suggest that the amino terminus plays an important role in establishing rectifying properties inherent to heterotypic junctions composed of ShakBL and ShakBN16. ShakBL/SBL NTN16 junctions behaved similarly to ShakBL/ShakBN16 junctions, gating in response to transjunctional voltage of one polarity and inducing a highly asymmetric conductance-voltage relationship. However, the amino terminus did not act independently to confer sensitivity to transjunctional voltage. The complementary pairing ShakBN16/SBN16 NTL displayed little sensitivity to voltage of either polarity, and in homotypic pairings SBL NTN16 was strongly gated by transjunctional voltage. We propose a model in which the amino terminus induces gating only when matched with an accommodating innexin body.

Permeation pathway of homomeric connexin 26 and connexin 30 channels investigated by molecular dynamics

Mutations in the genes GJB2 and GJB6 encoding human connnexin26 (hCx26) and connexin30 (hCx30), respectively, are the leading cause of non-syndromic prelingual deafness in several human populations. In this work, we exploited the high degree (77%) of sequence similarity shared by hCx26 and hCx30 to create atomistic models of homomeric hCx26 and hCx30 connexons starting from the X-ray crystallographic structure of an intercellular channel formed by hCx26 protomers at 3.5-å resolution. The equilibrium dynamics of the two protein complexes was followed for 40 ns each by Molecular Dynamics (MD) simulations. Our results indicate that, in hCx26, positively charged Lys41 residues establish a potential barrier within the fully open channel, hindering ion diffusion in the absence of an electrochemical gradient. A similar role is played, in hCx30, by negatively charged Glu49 residues. The different position and charge of these two ion sieves account for the differences in unitary conductance observed experimentally. Our results are discussed in terms of present models of voltage gating in connexin channels.