Tryptophan Scanning Reveals Dense Packing of Connexin Transmembrane Domains in Gap Junction Channels Composed of Connexin32

Skin disease-associated GJB4 variants differentially influence connexin stability, cell viability and channel function

Here we characterize seven Cx30.3 gene variants (R22H, S26Y, P61R, C86S, E99K, T130M and M190L) clinically associated with the rare skin disorder erythrokeratodermia variabilis et progressiva (EKVP) in tissue-relevant and differentiation-competent rat epidermal keratinocytes (REKs). We found that all variants, when expressed alone or together with wildtype (WT) Cx30.3, had the capacity to traffic and form gap junctions with an efficiency like WT Cx30.3. Cx30.3 was found to have a slower relative turnover than Cx43. However, turnover was more rapid for the R22H and P61R variants relative to Cx30.3. Furthermore, REKs that expressed the P61R variant exhibited reduced viability and were more permeable to fluorescent dyes, indicative of leaky hemichannels and/or the loss of membrane integrity associated with cell death. In connexin-null AD-293 cells, dual patch clamp studies revealed that the variants had either reduced (C86S) or no (S26Y and T130M) gap junction channel function. The remaining variants formed functional gap junction channels with enhanced transjunctional voltage (Vj)-dependent gating. Moreover, WT Cx30.3 and functional variant gap junction channels had similar unitary conductance of ∼34-42 pS, though variant channels appeared to have lower open probability than WT Cx30.3 channels at high Vjs. In conclusion, EKVP-associated Cx30.3 variants each alter one or more Cx30.3 characteristics although the molecular changes identified for E99K were limited to enhanced Vj gating. The breadth of molecular changes identified may all be sufficient to cause EKVP, but this remains to be firmly established as more familial patients are genotyped for these variants. KEY POINTS: Here we characterize seven Cx30.3 variants (R22H, S26Y, P61R, C86S, E99K, T130M and M190L) that have been clinically associated with the rare skin disorder erythrokeratodermia variabilis et progressiva (EKVP). We discovered human Cx30.3 undergoes relatively slow turnover compared with Cx43 and exhibits kinetically slow and limited voltage gating. Wildtype Cx30.3 and all variants localized to intracellular compartments and gap junctions in rat epidermal keratinocytes. Each EKVP-associated Cx30.3 variant altered one or more Cx30.3 characteristics related to protein stability, cell viability and/or channel function. The breadth of molecular changes identified for each Cx30.3 variant may independently be sufficient to cause EKVP, but this remains to be firmly established through additional genetic and molecular analysis.

Connexin channels and hemichannels are modulated differently by charge reversal at residues forming the intracellular pocket

BACKGROUND

Members of the β-subfamily of connexins contain an intracellular pocket surrounded by amino acid residues from the four transmembrane helices. The presence of this pocket has not previously been investigated in members of the α-, γ-, δ-, and ε-subfamilies. We studied connexin50 (Cx50) as a representative of the α-subfamily, because its structure has been determined and mutations of Cx50 are among the most common genetic causes of congenital cataracts.

METHODS

To investigate the presence and function of the intracellular pocket in Cx50 we used molecular dynamics simulation, site-directed mutagenesis, gap junction tracer intercellular transfer, and hemichannel activity detected by electrophysiology and by permeation of charged molecules.

RESULTS

Employing molecular dynamics, we determined the presence of the intracellular pocket in Cx50 hemichannels and identified the amino acids participating in its formation. We utilized site-directed mutagenesis to alter a salt-bridge interaction that supports the intracellular pocket and occurs between two residues highly conserved in the connexin family, R33 and E162. Substitution of opposite charges at either position decreased formation of gap junctional plaques and cell-cell communication and modestly reduced hemichannel currents. Simultaneous charge reversal at these positions produced plaque-forming non-functional gap junction channels with highly active hemichannels.

CONCLUSIONS

These results show that interactions within the intracellular pocket influence both gap junction channel and hemichannel functions. Disruption of these interactions may be responsible for diseases associated with mutations at these positions.

Gap junction gene and protein families: Connexins, innexins, and pannexins

Gap junction channels facilitate the intercellular exchange of ions and small molecules. While this process is critical to all multicellular organisms, the proteins that form gap junction channels are not conserved. Vertebrate gap junctions are formed by connexins, while invertebrate gap junctions are formed by innexins. Interestingly, vertebrates and lower chordates contain innexin homologs, the pannexins, which also form channels, but rarely (if ever) make intercellular channels. While the connexin and the innexin/pannexin polypeptides do not share significant sequence similarity, all three of these protein families share a similar membrane topology and some similarities in quaternary structure. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

A structural and functional comparison of gap junction channels composed of connexins and innexins

Methods such as electron microscopy and electrophysiology led to the understanding that gap junctions were dense arrays of channels connecting the intracellular environments within almost all animal tissues. The characteristics of gap junctions were remarkably similar in preparations from phylogenetically diverse animals such as cnidarians and chordates. Although few studies directly compared them, minor differences were noted between gap junctions of vertebrates and invertebrates. For instance, a slightly wider gap was noted between cells of invertebrates and the spacing between invertebrate channels was generally greater. Connexins were identified as the structural component of vertebrate junctions in the 1980s and innexins as the structural component of pre-chordate junctions in the 1990s. Despite a lack of similarity in gene sequence, connexins and innexins are remarkably similar. Innexins and connexins have the same membrane topology and form intercellular channels that play a variety of tissue- and temporally specific roles. Both protein types oligomerize to form large aqueous channels that allow the passage of ions and small metabolites and are regulated by factors such as pH, calcium, and voltage. Much more is currently known about the structure, function, and structure-function relationships of connexins. However, the innexin field is expanding. Greater knowledge of innexin channels will permit more detailed comparisons with their connexin-based counterparts, and provide insight into the ubiquitous yet specific roles of gap junctions. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 522-547, 2017.

Gap junction structure: unraveled, but not fully revealed

Gap junction channels facilitate the intercellular exchange of ions and small molecules, a process that is critical for the function of many different kinds of cells and tissues. Recent crystal structures of channels formed by one connexin isoform (connexin26) have been determined, and they have been subjected to molecular modeling. These studies have provided high-resolution models to gain insights into the mechanisms of channel conductance, molecular permeability, and gating. The models share similarities, but there are some differences in the conclusions reached by these studies. Many unanswered questions remain to allow an atomic-level understanding of intercellular communication mediated by connexin26. Because some domains of the connexin polypeptides are highly conserved (like the transmembrane regions), it is likely that some features of the connexin26 structure will apply to other members of the family of gap junction proteins. However, determination of high-resolution structures and modeling of other connexin channels will be required to account for the diverse biophysical properties and regulation conferred by the differences in their sequences.