Characterization of the structure and intermolecular interactions between the connexin 32 carboxyl-terminal domain and the protein partners synapse-associated protein 97 and calmodulin

Calcium Role in Gap Junction Channel Gating: Direct Electrostatic or Calmodulin-Mediated?

The chemical gating of gap junction channels is mediated by cytosolic calcium (Ca2+i) at concentrations ([Ca2+]i) ranging from high nanomolar (nM) to low micromolar (µM) range. Since the proteins of gap junctions, connexins/innexins, lack high-affinity Ca2+-binding sites, most likely gating is mediated by a Ca2+-binding protein, calmodulin (CaM) being the best candidate. Indeed, the role of Ca2+-CaM in gating is well supported by studies that have tested CaM blockers, CaM expression inhibition, testing of CaM mutants, co-localization of CaM and connexins, existence of CaM-binding sites in connexins/innexins, and expression of connexins (Cx) mutants, among others. Based on these data, since 2000, we have published a Ca2+-CaM-cork gating model. Despite convincing evidence for the Ca2+-CaM role in gating, a recent study has proposed an alternative gating model that would involve a direct electrostatic Ca2+-connexin interaction. However, this study, which tested the effect of unphysiologically high [Ca2+]i on the structure of isolated junctions, reported that neither changes in the channel's pore diameter nor connexin conformational changes are present, in spite of exposure of isolated gap junctions to [Ca2+]i as high at the 20 mM. In conclusion, data generated in the past four decades by multiple experimental approaches have clearly demonstrated the direct role of Ca2+-CaM in gap junction channel gating.

Dual-regulation by Cx32 in hepatocyte to trigger and worsen liver graft injury

Liver transplantation is the ultimate treatment option for end-stage liver failure. However, liver graft injury remains a challenge. This study aimed to investigate the role of connexin32 (Cx32) in liver graft injury and elucidate its mechanism of action. Through detecting liver graft samples from 6 patients, we observed that changes in the Cx32 level coincided with liver graft injury. Therefore, we established autologous orthotopic liver transplantation (AOLT) models using Cx32-knockout and wild-type mice and hypoxia/reoxygenation (H/R) and lipopolysaccharide (LPS) pretreatment models using alpha mouse liver 12 (AML12) cells, to explore Cx32 mechanisms in liver graft injury. Following in vivo and in vitro Cx32 knockout, oxidative stress and inflammatory response were inhibited through the regulation of PKC-α/NF-κB/NLRP3 and Nrf2/NOX4/ROS signaling pathways, thereby reducing Bak/Bax-related apoptosis and ameliorating liver graft injury. When the Cx32-based gap junction (GJ) was blocked with 2-aminoethoxydiphenyl borate (2-APB), ROS transfer was attenuated between neighboring cells, exacerbated oxidative stress and inflammatory response were prevented, and aggravation of liver graft injury was mitigated. These results highlight the dual regulation mechanism of Cx32 in liver graft injury. Through interaction with PKC-α, Cx32 regulated the NF-κB/NLRP3 and Nrf2/NOX4/ROS signaling pathways, thus directly triggering oxidative stress and inflammatory response. Simultaneously, mass-produced ROS were transferred to neighboring cells through Cx32 channels, for which oxidative stress and the inflammatory response were aggravated indirectly. Finally, Bak/Bax-related apoptosis was activated, thereby worsening liver graft injury. Our findings propose Cx32 as a dual mechanistic factor for oxidative stress and inflammatory signaling pathways in regulating cell apoptosis on liver graft injury, which suggests a promising therapeutic targets for liver graft injury.

The human discs large protein 1 interacts with and maintains connexin 43 at the plasma membrane in keratinocytes

Gap junction channels, composed of connexins, allow direct cell-to-cell communication. Connexin 43 (Cx43; also known as GJA1) is widely expressed in tissues, including the epidermis. In a previous study of human papillomavirus-positive cervical epithelial tumour cells, we identified Cx43 as a binding partner of the human homologue of Drosophila Discs large (Dlg1; also known as SAP97). Dlg1 is a member of the membrane associated-guanylate kinase (MAGUK) scaffolding protein family, which is known to control cell shape and polarity. Here, we show that Cx43 also interacts with Dlg1 in uninfected keratinocytes in vitro and in keratinocytes, dermal cells and adipocytes in normal human epidermis in vivo. Depletion of Dlg1 in keratinocytes did not alter Cx43 transcription but was associated with a reduction in Cx43 protein levels. Reduced Dlg1 levels in keratinocytes resulted in a reduction in Cx43 at the plasma membrane with a concomitant reduction in gap junctional intercellular communication and relocation of Cx43 to the Golgi compartment. Our data suggest a key role for Dlg1 in maintaining Cx43 at the plasma membrane in keratinocytes.

Ca2+-Dependent and -Independent Calmodulin Binding to the Cytoplasmic Loop of Gap Junction Connexins

Ca²⁺/calmodulin (Ca²⁺/CaM) interaction with connexins (Cx) is well-established; however, the mechanistic basis of regulation of gap junction function by Ca²⁺/CaM is not fully understood. Ca²⁺/CaM is predicted to bind to a domain in the C-terminal portion of the intracellular loop (CL2) in the vast majority of Cx isoforms and for a number of Cx-s this prediction has proved correct. In this study, we investigate and characterise both Ca²⁺/CaM and apo-CaM binding to selected representatives of each of the α, β and γ connexin family to develop a better mechanistic understanding of CaM effects on gap junction function. The affinity and kinetics Ca²⁺/CaM and apo-CaM interactions of CL2 peptides of β-Cx32, γ-Cx35, α-Cx43, α-Cx45 and α-Cx57 were investigated. All five Cx CL2 peptides were found to have high affinity for Ca²⁺/CaM with dissociation constants (K_d(+Ca)) from 20 to 150 nM. The limiting rate of binding and the rates of dissociation covered a broad range. In addition, we obtained evidence for high affinity Ca²⁺-independent interaction of all five peptides with CaM, consistent with CaM remaining anchored to gap junctions in resting cells. However, for the α-Cx45 and α-Cx57 CL2 peptides, Ca²⁺-dependent association at resting [Ca²⁺] of 50-100 nM is indicated in these complexes as one of the CaM Ca²⁺ binding sites displays high affinity with K_d of 70 and 30 nM for Ca²⁺, respectively. Furthermore, complex conformational changes were observed in peptide-apo-CaM complexes with the structure of CaM compacted or stretched by the peptide in a concentration dependent manner suggesting that the CL2 domain may undergo helix-to-coil transition and/or forms bundles, which may be relevant in the hexameric gap junction. We demonstrate inhibition of gap junction permeability by Ca²⁺/CaM in a dose dependent manner, further cementing Ca²⁺/CaM as a regulator of gap junction function. The motion of a stretched CaM-CL2 complex compacting upon Ca²⁺ binding may bring about the Ca²⁺/CaM block of the gap junction pore by a push and pull action on the CL2 C-terminal hydrophobic residues of transmembrane domain 3 (TM3) in and out of the membrane.

Morphological and Functional Colonic Defects Caused by a Mutated Thyroid Hormone Receptor α

Background: Mutations of thyroid hormone receptor α (TRα1) result in resistance to thyroid hormone (RTHα), exhibiting symptoms of retarded growth, delayed bone maturation, anemia, and severe constipation. Using a mouse model of RTHα (Thra1PV/+ mouse), we aimed at understanding the molecular basis underlying the severe constipation observed in patients. Methods: The Thra1PV/+ mouse expresses a strong dominant negative mutant, PV, which has lost T3 binding and transcription activity. Thra1PV/+ mouse faithfully reproduces growth abnormalities and anemia as shown in RTHα patients and therefore is a valid model to examine causes of severe constipation in patients. We used histopathological analysis, confocal fluorescence imaging, transmission electron microscopy (TEM), and gene expression profiles to comprehensively analyze the colonic abnormalities of Thra1PV/+ mouse. Results: We found a significant increase in colonic transit time and decrease stool water content in Thra1PV/+ mouse, mimicking constipation as found in patients. Histopathological analysis showed expanded lamina propria filled with interstitium fluid between crypt columns, enlarged muscularis mucosa, and increased content of collagen in expanded submucosa. The TEM analysis revealed shorter muscle fibers with wider gap junctions between muscle cells, fewer caveolae, and hypoplastic interstitial cells of Cajal (ICC) in the rectal smooth muscles of Thra1PV/+ mice. These abnormal histological manifestations suggested defective intercellular transfer of small molecules, electrolytes, and signals for communication among muscles cells, validated by Lucifer Yellow transferring assays. Expression of key smooth muscle contractility regulators, such as calmodulin, myosin light-chain kinase, and phosphorylated myosin light chain, was markedly lower, and c-KIT signaling in ICC was attenuated, resulting in decreased contractility of the rectal smooth muscles of Thra1PV/+ mice. Collectively, these abnormal histopathological alterations and diminished contractility regulators led to the constipation exhibited in patients. Conclusions: This is the first demonstration that TRα1 mutants could act to cause abnormal rectum smooth muscle organization, defects in intercellular exchange of small molecules, and decreased expression of contractility regulators to weaken the contractility of rectal smooth muscles. These findings provide new insights into the molecular basis underlying constipation found in RTHα patients.

GJB1 mutations c.212T>G and c.311A>C induce apoptosis and inwardly rectifying potassium current changes in X-linked Charcot-Marie-Tooth type 1

Gap junction beta 1 (GJB1) is the pathogenic gene of X-linked Charcot-Marie-Tooth type 1 (CMTX1), a rare hereditary sensorimotor neuropathy. However, different mutations of GJB1 result in heterogeneous clinical manifestations with only some mutations leading to central nervous system involvement. We previously reported two GJB1 missense mutations: one novel mutation (c.212T > G) found in a CMTX1 family that only manifested as peripheral neuropathy, and another previously reported mutation GJB1(c.311A > C) leading to involvement of the peripheral nerves and cerebral white matter. However, the mechanism by which GJB1 mutations lead to CMTX1 has not been fully characterized. Here, we generated Schwann cells and primary cultured oligodendrocytes with these two mutations, resulting in the Cx32I71S (GJB1 c.212T > G) and Cx32K104T (GJB1 c.311A > C) mutants, to analyze the pathogenic mechanism using cytology, molecular biology, and electrophysiological methods. Both mutants showed abnormal endoplasmic reticulum aggregation, especially the Cx32K104T mutant, leading to an increase in endoplasmic reticulum stress, resulting in apoptosis. Furthermore, whole-cell patch clamp experiments in oligodendrocytes revealed that the Cx32K104T mutant reduced the cell membrane potential and inwardly rectifying potassium currents, which may be a vital element for central involvement. Therefore, our results may provide a new perspective for understanding the pathogenesis of CMTX1.

Calmodulin-Connexin Partnership in Gap Junction Channel Regulation-Calmodulin-Cork Gating Model

In the past four decades numerous findings have indicated that gap junction channel gating is mediated by intracellular calcium concentrations ([Ca²⁺_i]) in the high nanomolar range via calmodulin (CaM). We have proposed a CaM-based gating model based on evidence for a direct CaM role in gating. This model is based on the following: CaM inhibitors and the inhibition of CaM expression to prevent chemical gating. A CaM mutant with higher Ca²⁺ sensitivity greatly increases gating sensitivity. CaM co-localizes with connexins. Connexins have high-affinity CaM-binding sites. Connexin mutants paired to wild type connexins have a higher gating sensitivity, which is eliminated by the inhibition of CaM expression. Repeated trans-junctional voltage (Vj) pulses progressively close channels by the chemical/slow gate (CaM’s N-lobe). At the single channel level, the gate closes and opens slowly with on-off fluctuations. Internally perfused crayfish axons lose gating competency but recover it by the addition of Ca-CaM to the internal perfusion solution. X-ray diffraction data demonstrate that isolated gap junctions are gated at the cytoplasmic end by a particle of the size of a CaM lobe. We have proposed two types of CaM-driven gating: “Ca-CaM-Cork” and “CaM-Cork”. In the first, the gating involves Ca²⁺-induced CaM activation. In the second, the gating occurs without a [Ca²⁺]_i rise.

The Roles of Calmodulin and CaMKII in Cx36 Plasticity

Anatomical and electrophysiological evidence that gap junctions and electrical coupling occur between neurons was initially confined to invertebrates and nonmammals and was thought to be a primitive form of synaptic transmission. More recent studies revealed that electrical communication is common in the mammalian central nervous system (CNS), often coexisting with chemical synaptic transmission. The subsequent progress indicated that electrical synapses formed by the gap junction protein connexin-36 (Cx36) and its paralogs in nonmammals constitute vital elements in mammalian and fish synaptic circuitry. They govern the collective activity of ensembles of coupled neurons, and Cx36 gap junctions endow them with enormous adaptive plasticity, like that seen at chemical synapses. Moreover, they orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie the fundamental integrative processes, such as memory and learning. Here, we review the available mechanistic evidence and models that argue for the essential roles of calcium, calmodulin, and the Ca2+/calmodulin-dependent protein kinase II in integrating calcium signals to modulate the strength of electrical synapses through interactions with the gap junction protein Cx36.

Calmodulin Directly Interacts with the Cx43 Carboxyl-Terminus and Cytoplasmic Loop Containing Three ODDD-Linked Mutants (M147T, R148Q, and T154A) that Retain α-Helical Structure, but Exhibit Loss-of-Function and Cellular Trafficking Defects

The autosomal-dominant pleiotropic disorder called oculodentodigital dysplasia (ODDD) is caused by mutations in the gap junction protein Cx43. Of the 73 mutations identified to date, over one-third are localized in the cytoplasmic loop (Cx43CL) domain. Here, we determined the mechanism by which three ODDD mutations (M147T, R148Q, and T154A), all of which localize within the predicted 1-5-10 calmodulin-binding motif of the Cx43CL, manifest the disease. Nuclear magnetic resonance (NMR) and circular dichroism revealed that the three ODDD mutations had little-to-no effect on the ability of the Cx43CL to form α-helical structure as well as bind calmodulin. Combination of microscopy and a dye-transfer assay uncovered these mutations increased the intracellular level of Cx43 and those that trafficked to the plasma membrane did not form functional channels. NMR also identify that CaM can directly interact with the Cx43CT domain. The Cx43CT residues involved in the CaM interaction overlap with tyrosines phosphorylated by Pyk2 and Src. In vitro and in cyto data provide evidence that the importance of the CaM interaction with the Cx43CT may lie in restricting Pyk2 and Src phosphorylation, and their subsequent downstream effects.

Calmodulin-Cork Model of Gap Junction Channel Gating-One Molecule, Two Mechanisms

The Calmodulin-Cork gating model is based on evidence for the direct role of calmodulin (CaM) in channel gating. Indeed, chemical gating of cell-to-cell channels is sensitive to nanomolar cytosolic calcium concentrations [Ca2+]i. Calmodulin inhibitors and inhibition of CaM expression prevent chemical gating. CaMCC, a CaM mutant with higher Ca2+-sensitivity greatly increases chemical gating sensitivity (in CaMCC the NH2-terminal EF-hand pair (res. 9-76) is replaced by the COOH-terminal pair (res. 82-148). Calmodulin colocalizes with connexins. Connexins have high-affinity CaM binding sites. Several connexin mutants paired to wild-type connexins have a high gating sensitivity that is eliminated by inhibition of CaM expression. Repeated transjunctional voltage (Vj) pulses slowly and progressively close a large number of channels by the chemical/slow gate (CaM lobe). At the single-channel level, the chemical/slow gate closes and opens slowly with on-off fluctuations. The model proposes two types of CaM-driven gating: "Ca-CaM-Cork" and "CaM-Cork". In the first, gating involves Ca2+-induced CaM-activation. In the second, gating takes place without [Ca2+]i rise. The Ca-CaM-Cork gating is only reversed by a return of [Ca2+]i to resting values, while the CaM-Cork gating is reversed by Vj positive at the gated side.

Calmodulin-Mediated Regulation of Gap Junction Channels

Evidence that neighboring cells uncouple from each other as one dies surfaced in the late 19th century, but it took almost a century for scientists to start understanding the uncoupling mechanism (chemical gating). The role of cytosolic free calcium (Ca2+i) in cell-cell channel gating was first reported in the mid-sixties. In these studies, only micromolar [Ca2+]i were believed to affect gating-concentrations reachable only in cell death, which would discard Ca2+i as a fine modulator of cell coupling. More recently, however, numerous researchers, including us, have reported the effectiveness of nanomolar [Ca2+]i. Since connexins do not have high-affinity calcium sites, the effectiveness of nanomolar [Ca2+]i suggests the role of Ca-modulated proteins, with calmodulin (CaM) being most obvious. Indeed, in 1981 we first reported that a CaM-inhibitor prevents chemical gating. Since then, the CaM role in gating has been confirmed by studies that tested it with a variety of approaches such as treatments with CaM-inhibitors, inhibition of CaM expression, expression of CaM mutants, immunofluorescent co-localization of CaM and gap junctions, and binding of CaM to peptides mimicking connexin domains identified as CaM targets. Our gating model envisions Ca2+-CaM to directly gate the channels by acting as a plug ("Cork" gating model), and probably also by affecting connexin conformation.

Regulation of Connexin32 by ephrin receptors and T-cell protein-tyrosine phosphatase

Gap junctions are intercellular conduits that permit the passage of ions, small metabolites, and signaling molecules between cells. Connexin32 (Cx32) is a major gap junction protein in the liver and brain. Phosphorylation is integral to regulating connexin assembly, degradation, and electrical and metabolic coupling, as well as to interactions with molecular partners. Cx32 contains two intracellular tyrosine residues, and tyrosine phosphorylation of Cx32 has been detected after activation of the epidermal growth factor receptor; however, the specific tyrosine residue and the functional implication of this phosphorylation remain unknown. To address the limited available information on Cx32 regulation by tyrosine kinases, here we used the Cx32 C-terminal (CT) domain in an in vitro kinase-screening assay, which identified ephrin (Eph) receptor family members as tyrosine kinases that phosphorylate Cx32. We found that EphB1 and EphA1 phosphorylate the Cx32CT domain residue Tyr243 Unlike for Cx43, the tyrosine phosphorylation of the Cx32CT increased gap junction intercellular communication. We also demonstrated that T-cell protein-tyrosine phosphatase dephosphorylates pTyr243 The data presented above along with additional examples throughout the literature of gap junction regulation by kinases, indicate that one cannot extrapolate the effect of a kinase on one connexin to another.

Phosphorylation of Cx43 residue Y313 by Src contributes to blocking the interaction with Drebrin and disassembling gap junctions

Phosphorylation regulates connexin43 (Cx43) function from assembly/disassembly to coupling at the plaque. Src is a tyrosine kinase known to both phosphorylate Cx43 (residues Y247 and Y265) and affect gap junction intercellular communication. However, the Cx43 carboxyl-terminal (CT) domain contains additional tyrosine residues and proteomic discovery mass spectrometry data identified Y313 as a potential phosphorylation target. Based upon the study of Lin et al. (2001) J. Cell Biol., which still observed tyrosine phosphorylation by Src when using a Cx43 Y247/Y265F mutant, we addressed the possibility of Y313 phosphorylation (pY313) by Src. In vitro Src phosphorylation of purified Cx43CT followed by mass spectroscopy revealed that Src also phosphorylates Y313. This observation was confirmed by repeating the in vitro phosphorylation using different combinations of Cx43CT Y → F mutants and a general anti-pTyr antibody. Next, a phospho-specific antibody was generated to help characterize the importance of pY313. We established an in cyto experimental system by stably expressing Cx43 WT and mutants (Y247F, Y265F, Y313F, Y247/265F, Y247/313F, Y265/313F, or Y247/265/313F) in Cx43-deficient HeLa cells. Cx43 WT and mutants, in the absence of v-Src, localized to the plasma membrane and formed gap junctions. When v-Src was over-expressed, Cx43 WT localized intracellularly, while all of the single and double mutants remained able to form plaques and transfer dye, albeit variable in number and amount, respectively. Complete Src-resistance was only achieved with the Cx43 Y247/265/313F mutant. Furthermore, Cx43 Y265F inhibited the ability of v-Src to phosphorylate Y247 and Y313 as well as phosphorylation at both Y265 and Y313 was necessary to inhibit the Cx43 interaction with Drebrin. Finally, we observed in diseased cardiac tissue, in which Src is active, an increase in intercalated disc and intracellular localized Cx43 pY313.

Acetylation of C-terminal lysines modulates protein turnover and stability of Connexin-32

BACKGROUND

The gap junction protein, Connexin32 (Cx32), is expressed in various tissues including liver, exocrine pancreas, gastrointestinal epithelium, and the glia of the central and peripheral nervous system. Gap junction-mediated cell-cell communication and channel-independent processes of Cx32 contribute to the regulation of physiological and cellular activities such as glial differentiation, survival, and proliferation; maintenance of the hepatic epithelium; and axonal myelination. Mutations in Cx32 cause X-linked Charcot-Marie-Tooth disease (CMT1X), an inherited peripheral neuropathy. Several CMT1X causing mutations are found in the cytoplasmic domains of Cx32, a region implicated in the regulation of gap junction assembly, turnover and function. Here we investigate the roles of acetylation and ubiquitination in the C-terminus on Cx32 protein function. Cx32 protein turnover, ubiquitination, and response to deacetylase inhibitors were determined for wild-type and C-terminus lysine mutants using transiently transfected Neuro2A (N2a) cells.

RESULTS

We report here that Cx32 is acetylated in transfected N2a cells and that inhibition of the histone deacetylase, HDAC6, results in an accumulation of Cx32. We identified five lysine acetylation targets in the C-terminus. Mutational analysis demonstrates that these lysines are involved in the regulation of Cx32 ubiquitination and turnover. While these lysines are not required for functional Cx32 mediated cell-cell communication, BrdU incorporation studies demonstrate that their relative acetylation state plays a channel-independent role in Cx32-mediated control of cell proliferation.

CONCLUSION

Taken together these results highlight the role of post translational modifications and lysines in the C-terminal tail of Cx32 in the fine-tuning of Cx32 protein stability and channel-independent functions.

Connexin43 Carboxyl-Terminal Domain Directly Interacts with β-Catenin

Activation of Wnt signaling induces Connexin43 (Cx43) expression via the transcriptional activity of β-catenin, and results in the enhanced accumulation of the Cx43 protein and the formation of gap junction channels. In response to Wnt signaling, β-catenin co-localizes with the Cx43 protein itself as part of a complex at the gap junction plaque. Work from several labs have also shown indirect evidence of this interaction via reciprocal co-immunoprecipitation. Our goal for the current study was to identify whether β-catenin directly interacts with Cx43, and if so, the location of that direct interaction. Identifying residues involved in direct protein⁻protein interaction is of importance when they are correlated to the phosphorylation of Cx43, as phosphorylation can modify the binding affinities of Cx43 regulatory protein partners. Therefore, combining the location of a protein partner interaction on Cx43 along with the phosphorylation pattern under different homeostatic and pathological conditions will be crucial information for any potential therapeutic intervention. Here, we identified that β-catenin directly interacts with the Cx43 carboxyl-terminal domain, and that this interaction would be inhibited by the Src phosphorylation of Cx43CT residues Y265 and Y313.

Protein⁻Protein Interactions with Connexin 43: Regulation and Function

Connexins are integral membrane building blocks that form gap junctions, enabling direct cytoplasmic exchange of ions and low-molecular-mass metabolites between adjacent cells. In the heart, gap junctions mediate the propagation of cardiac action potentials and the maintenance of a regular beating rhythm. A number of connexin interacting proteins have been described and are known gap junction regulators either through direct effects (e.g., kinases) or the formation of larger multifunctional complexes (e.g., cytoskeleton scaffold proteins). Most connexin partners can be categorized as either proteins promoting coupling by stimulating forward trafficking and channel opening or inhibiting coupling by inducing channel closure, internalization, and degradation. While some interactions have only been implied through co-localization using immunohistochemistry, others have been confirmed by biophysical methods that allow detection of a direct interaction. Our understanding of these interactions is, by far, most well developed for connexin 43 (Cx43) and the scope of this review is to summarize our current knowledge of their functional and regulatory roles. The significance of these interactions is further exemplified by demonstrating their importance at the intercalated disc, a major hub for Cx43 regulation and Cx43 mediated effects.

Cx32 hemichannel opening by cytosolic Ca2+ is inhibited by the R220X mutation that causes Charcot-Marie-Tooth disease

Mutations of the GJB1 gene encoding connexin 32 (Cx32) cause the X-linked form of Charcot-Marie-Tooth disease (CMTX1), a demyelinating peripheral neuropathy for which there is no cure. A growing body of evidence indicates that ATP release through Cx32 hemichannels in Schwann cells could be critical for nerve myelination, but it is unknown if CMTX1 mutations alter the cytosolic Ca2+-dependent gating mechanism that controls Cx32 hemichannel opening and ATP release. The current study uncovered that loss of the C-terminus in Cx32 (R220X mutation), which causes a severe CMTX1 phenotype, inhibits hemichannel opening during a canonical IP3-mediated increase in cytosolic Ca2+ in HeLa cells. Interestingly, the gating function of R220X hemichannels was completely restored by both the intracellular and extracellular application of a peptide that mimics the Cx32 cytoplasmic loop. All-atom molecular dynamics simulations suggest that loss of the C-terminus in the mutant hemichannel triggers abnormal fluctuations of the cytoplasmic loop which are prevented by binding to the mimetic peptide. Experiments that stimulated R220X hemichannel opening by cell depolarization displayed reduced voltage sensitivity with respect to wild-type hemichannels which was explained by loss of subconductance states at the single channel level. Finally, experiments of intercellular diffusion mediated by wild-type or R220X gap junction channels revealed similar unitary permeabilities to ions, signalling molecules (cAMP) or larger solutes (Lucifer yellow). Taken together, our findings support the hypothesis that paracrine signalling alteration due to Cx32 hemichannel dysfunction underlies CMTX1 pathogenesis and suggest a candidate molecule for novel studies investigating a therapeutic approach.

Chemical shift assignments of the connexin37 carboxyl terminal domain

Connexin37 (Cx37) is a gap junction protein involved in cell-to-cell communication in the vasculature and other tissues. Cx37 suppresses proliferation of vascular cells involved in tissue development and repair in vivo, as well as tumor cells. Global deletion of Cx37 in mice leads to enhanced vasculogenesis in development, as well as collateralgenesis and angiogenesis in response to injury, which together support improved tissue remodeling and recovery following ischemic injury. Here we report the 1H, 15N, and 13C resonance assignments for an important regulatory domain of Cx37, the carboxyl terminus (CT; C233-V333). The predicted secondary structure of the Cx37CT domain based on the chemical shifts is that of an intrinsically disordered protein. In the 1H-15N HSQC, N-terminal residues S254-Y259 displayed a second weaker peak and residues E261-Y266 had significant line broadening. These residues are flanked by prolines (P250, P258, P260, and P268), suggesting proline cis-trans isomerization. Overall, these assignments will be useful for identifying the binding sites for intra- and inter-molecular interactions that affect Cx37 channel activity.

Intramolecular signaling in a cardiac connexin: Role of cytoplasmic domain dimerization

Gap junctions, composed of connexins, mediate electrical coupling and impulse propagation in the working myocardium. In the human heart, the spatio-temporal regulation and distinct functional properties of the three dominant connexins (Cx43, Cx45, and Cx40) suggests non-redundant physiological roles for each isoform. There are substantial differences in gating properties, expression, and trafficking among these isoforms, however, little is known about the determinants of these different phenotypes. To gain insight regarding these determinants, we focused on the carboxyl-terminal (CT) domain because of its importance in channel regulation and large degree of sequence divergence among connexin family members. Using in vitro biophysical experiments, we identified a structural feature unique to Cx45: high affinity (K_D~100nM) dimerization between CT domains. In this study, we sought to determine if this dimerization occurs in cells and to identify the biological significance of the dimerization. Using a bimolecular fluorescence complementation assay, we demonstrate that the CT domains dimerize at the plasma membrane. By inhibiting CT dimerization with a mutant construct, we show that CT dimerization is necessary for proper Cx45 membrane localization, turnover, phosphorylation status, and binding to protein partners. Furthermore, CT dimerization is needed for normal intercellular communication and hemichannel activity. Altogether, our results demonstrate that CT dimerization is a structural feature important for correct Cx45 function. This study is significant because discovery of how interactions mediated by the CT domains can be modulated would open the door to strategies to ameliorate the pathological effects of altered connexin regulation in the failing heart.

Secondary structural analysis of the carboxyl-terminal domain from different connexin isoforms

The connexin carboxyl-terminal (CxCT) domain plays a role in the trafficking, localization, and turnover of gap junction channels, as well as the level of gap junction intercellular communication via numerous post-translational modifications and protein-protein interactions. As a key player in the regulation of gap junctions, the CT presents itself as a target for manipulation intended to modify function. Specific to intrinsically disordered proteins, identifying residues whose secondary structure can be manipulated will be critical toward unlocking the therapeutic potential of the CxCT domain. To accomplish this goal, we used biophysical methods to characterize CxCT domains attached to their fourth transmembrane domain (TM4). Circular dichroism and nuclear magnetic resonance were complementary in demonstrating the connexin isoforms that form the greatest amount of α-helical structure in their CT domain (Cx45 > Cx43 > Cx32 > Cx50 > Cx37 ≈ Cx40 ≈ Cx26). Studies compared the influence of 2,2,2-trifluoroethanol, pH, phosphorylation, and mutations (Cx32, X-linked Charcot-Marie Tooth disease; Cx26, hearing loss) on the TM4-CxCT structure. While pH modestly influences the CT structure, a major structural change was associated with phosphomimetic substitutions. Since most connexin CT domains are phosphorylated throughout their life cycle, studies of phospho-TM4-CxCT isoforms will be critical toward understanding the role that structure plays in regulating gap junction function.

Connexin43 Forms Supramolecular Complexes through Non-Overlapping Binding Sites for Drebrin, Tubulin, and ZO-1

Gap junctions are membrane specialization domains identified in most tissue types where cells abut each other. The connexin channels found in these membrane domains are conduits for direct cell-to-cell transfer of ions and molecules. Connexin43 (Cx43) is the most ubiquitous connexin, with critical roles in heart, skin, and brain. Several studies described the interaction between Cx43 and the cytoskeleton involving the actin binding proteins Zonula occludens (ZO-1) and drebrin, as well as with tubulin. However, a direct interaction has not been identified between drebrin and Cx43. In this study, co-IP and NMR experiments were used to demonstrate that the Cx43-CT directly interacts with the highly conserved N-terminus region of drebrin. Three Cx43-CT areas were found to be involved in drebrin binding, with residues 264–275 being critical for the interaction. Mimicking Src phosphorylation within this region (Y265) significantly disrupted the interaction between the Cx43-CT and drebrin. Immunofluorescence showed colocalization of Cx43, drebrin, and F-actin in astrocytes and Vero cells membrane, indicating that Cx43 forms a submembrane protein complex with cytoskeletal and scaffolding proteins. The co-IP data suggest that Cx43 indirectly interacts with F-actin through drebrin. Along with the known interaction of the Cx43-CT with ZO-1 and tubulin, the data presented here for drebrin indicate non-overlapping and separated binding sites for all three proteins for which simultaneous binding could be important in regulating cytoskeleton rearrangements, especially for neuronal migration during brain development.

Connexinopathies: a structural and functional glimpse

Mutations in human connexin (Cx) genes have been related to diseases, which we termed connexinopathies. Such hereditary disorders include nonsyndromic or syndromic deafness (Cx26, Cx30), Charcot Marie Tooth disease (Cx32), occulodentodigital dysplasia and cardiopathies (Cx43), and cataracts (Cx46, Cx50). Despite the clinical phenotypes of connexinopathies have been well documented, their pathogenic molecular determinants remain elusive. The purpose of this work is to identify common/uncommon patterns in channels function among Cx mutations linked to human diseases. To this end, we compiled and discussed the effect of mutations associated to Cx26, Cx32, Cx43, and Cx50 over gap junction channels and hemichannels, highlighting the function of the structural channel domains in which mutations are located and their possible role affecting oligomerization, gating and perm/selectivity processes.

Death of Neurons following Injury Requires Conductive Neuronal Gap Junction Channels but Not a Specific Connexin

Pharmacological blockade or genetic knockout of neuronal connexin 36 (Cx36)-containing gap junctions reduces neuronal death caused by ischemia, traumatic brain injury and NMDA receptor (NMDAR)-mediated excitotoxicity. However, whether Cx36 gap junctions contribute to neuronal death via channel-dependent or channel-independent mechanism remains an open question. To address this, we manipulated connexin protein expression via lentiviral transduction of mouse neuronal cortical cultures and analyzed neuronal death twenty-four hours following administration of NMDA (a model of NMDAR excitotoxicity) or oxygen-glucose deprivation (a model of ischemic injury). In cultures prepared from wild-type mice, over-expression and knockdown of Cx36-containing gap junctions augmented and prevented, respectively, neuronal death from NMDAR-mediated excitotoxicity and ischemia. In cultures obtained form from Cx36 knockout mice, re-expression of functional gap junction channels, containing either neuronal Cx36 or non-neuronal Cx43 or Cx31, resulted in increased neuronal death following insult. In contrast, the expression of communication-deficient gap junctions (containing mutated connexins) did not have this effect. Finally, the absence of ethidium bromide uptake in non-transduced wild-type neurons two hours following NMDAR excitotoxicity or ischemia suggested the absence of active endogenous hemichannels in those neurons. Taken together, these results suggest a role for neuronal gap junctions in cell death via a connexin type-independent mechanism that likely relies on channel activities of gap junctional complexes among neurons. A possible contribution of gap junction channel-permeable death signals in neuronal death is discussed.

Degradation of gap junction connexins is regulated by the interaction with Cx43-interacting protein of 75 kDa (CIP75)

Connexins are a family of transmembrane proteins that form gap junction channels. These proteins undergo both proteasomal and lysosomal degradation, mechanisms that serve to regulate connexin levels. Our previous work described CIP75 [connexin43 (Cx43)-interacting protein of 75 kDa], a protein involved in proteasomal degradation, as a novel Cx43-interacting protein. We have discovered two additional connexins, connexin40 (Cx40) and connexin45 (Cx45), that interact with CIP75. Nuclear magnetic resonance (NMR) analyses identified the direct interaction of the CIP75 UBA domain with the carboxyl-terminal (CT) domains of Cx40 and Cx45. Reduction in CIP75 by shRNA in HeLa cells expressing Cx40 or Cx45 resulted in increased levels of the connexins. Furthermore, treatment with trafficking inhibitors confirmed that both connexins undergo endoplasmic reticulum-associated degradation (ERAD), and that CIP75 preferentially interacts with the connexin proteins bound for proteasomal degradation from the ER. In addition, we have also discovered that CIP75 interacts with ER-localized Cx32 in a process that is likely mediated by Cx32 ubiquitination. Thus, we have identified novel interacting connexin proteins of CIP75, indicating a role for CIP75 in regulating the levels of connexins in general, through proteasomal degradation.

The carboxyl tail of connexin32 regulates gap junction assembly in human prostate and pancreatic cancer cells

Connexins, the constituent proteins of gap junctions, are transmembrane proteins. A connexin (Cx) traverses the membrane four times and has one intracellular and two extracellular loops with the amino and carboxyl termini facing the cytoplasm. The transmembrane and the extracellular loop domains are highly conserved among different Cxs, whereas the carboxyl termini, often called the cytoplasmic tails, are highly divergent. We have explored the role of the cytoplasmic tail of Cx32, a Cx expressed in polarized and differentiated cells, in regulating gap junction assembly. Our results demonstrate that compared with the full-length Cx32, the cytoplasmic tail-deleted Cx32 is assembled into small gap junctions in human pancreatic and prostatic cancer cells. Our results further document that the expression of the full-length Cx32 in cells, which express the tail-deleted Cx32, increases the size of gap junctions, whereas the expression of the tail-deleted Cx32 in cells, which express the full-length Cx32, has the opposite effect. Moreover, we show that the tail is required for the clustering of cell-cell channels and that in cells expressing the tail-deleted Cx32, the expression of cell surface-targeted cytoplasmic tail alone is sufficient to enhance the size of gap junctions. Our live-cell imaging data further demonstrate that gap junctions formed of the tail-deleted Cx32 are highly mobile compared with those formed of full-length Cx32. Our results suggest that the cytoplasmic tail of Cx32 is not required to initiate the assembly of gap junctions but for their subsequent growth and stability. Our findings suggest that the cytoplasmic tail of Cx32 may be involved in regulating the permeability of gap junctions by regulating their size.

TC-PTP directly interacts with connexin43 to regulate gap junction intercellular communication

Protein kinases have long been reported to regulate connexins; however, little is known about the involvement of phosphatases in the modulation of intercellular communication through gap junctions and the subsequent downstream effects on cellular processes. Here, we identify an interaction between the T-cell protein tyrosine phosphatase (TC-PTP, officially known as PTPN2) and the carboxyl terminus of connexin43 (Cx43, officially known as GJA1). Two cell lines, normal rat kidney (NRK) cells endogenously expressing Cx43 and an NRK-derived cell line expressing v-Src with temperature-sensitive activity, were used to demonstrate that EGF and v-Src stimulation, respectively, induced TC-PTP to colocalize with Cx43 at the plasma membrane. Cell biology experiments using phospho-specific antibodies and biophysical assays demonstrated that the interaction is direct and that TC-PTP dephosphorylates Cx43 residues Y247 and Y265, but does not affect v-Src. Transfection of TC-PTP also indirectly led to the dephosphorylation of Cx43 S368, by inactivating PKCα and PKCδ, with no effect on the phosphorylation of S279 and S282 (MAPK-dependent phosphorylation sites). Dephosphorylation maintained Cx43 gap junctions at the plaque and partially reversed the channel closure caused by v-Src-mediated phosphorylation of Cx43. Understanding dephosphorylation, along with the well-documented roles of Cx43 phosphorylation, might eventually lead to methods to modulate the regulation of gap junction channels, with potential benefits for human health.

Characterization of the connexin45 carboxyl-terminal domain structure and interactions with molecular partners

Mechanisms underlying the initiation and persistence of lethal cardiac rhythms are of significant clinical and scientific interests. Gap junctions are principally involved in forming the electrical connections between myocytes, and changes in distribution, density, and properties are consistent characteristics in arrhythmic heart disease. Therefore, understanding the structure and function of gap junctions during normal and abnormal impulse propagation are essential in the control of arrhythmias. For example, Cx45 is predominately expressed in the specialized myocytes of the impulse generation and conduction system. In both ventricular and atrial human working myocytes, Cx45 is present in very low quantities. However, a reduction in Cx43 coupled with an increased Cx45 protein levels within the ventricles have been observed after myocardial infarction and end-stage heart failure. Cx45 may influence electrical and/or metabolic coupling as a result of pathophysiological overexpression. Our goal was to identify mechanisms that could cause cellular coupling to be different between the cardiac connexins. Based upon the conserved transmembrane and extracellular loop segments, our focus was on identifying features within the divergent cytoplasmic portions. Here, we biophysically characterize the carboxyl-terminal domain of Cx45 (Cx45CT). Purification revealed the possibility of oligomeric species, which was confirmed by analytical ultracentrifugation experiments. Sedimentation equilibrium and circular dichroism studies of different Cx45CT constructs identified one region of α-helical structure (A333-N361) that mediates CT dimerization through hydrophobic contacts. Interestingly, the binding affinity of Cx45CT dimerization is 1000-fold stronger than Cx43CT dimerization. Cx45CT resonance assignments were also used to identify the binding sites and affinities of molecular partners involved in the Cx45 regulation; although none disrupted dimerization, many of these proteins interacted within one intrinsically disordered region (P278-P285). This domain has similarities with other cardiac connexins, and we propose they constitute a master regulatory domain, which contains overlapping molecular partner binding, cis-trans proline isomerization, and phosphorylation sites.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Gap junction regulation by calmodulin

Intracellular Ca(2+) activated calmodulin (CaM) inhibits gap junction channels in the low nanomolar to high micromolar range of [Ca(2+)]i. This regulation plays an essential role in numerous cellular processes that include hearing, lens transparency, and synchronized contractions of the heart. Previous studies have indicated that gap junction mediated cell-to-cell communication was inhibited by CaM antagonists. More recent evidence indicates a direct role of CaM in regulating several members of the connexin family. Since the intracellular loop and carboxyl termini of connexins are largely "invisible" in electron microscopy and X-ray crystallographic structures due to disorder in these domains, peptide models encompassing the putative CaM binding sites of several intracellular domains of connexins have been used to identify the Ca(2+)-dependent CaM binding sites of these proteins. This approach has been used to determine the CaM binding affinities of peptides derived from a number of different connexin-subfamilies.

Role of connexin 32 hemichannels in the release of ATP from peripheral nerves

Extracellular purines elicit strong signals in the nervous system. Adenosine-5'-triphosphate (ATP) does not spontaneously cross the plasma membrane, and nervous cells secrete ATP by exocytosis or through plasma membrane proteins such as connexin hemichannels. Using a combination of imaging, luminescence and electrophysiological techniques, we explored the possibility that Connexin 32 (Cx32), expressed in Schwann cells (SCs) myelinating the peripheral nervous system could be an important source of ATP in peripheral nerves. We triggered the release of ATP in vivo from mice sciatic nerves by electrical stimulation and from cultured SCs by high extracellular potassium concentration-evoked depolarization. No ATP was detected in the extracellular media after treatment of the sciatic nerve with Octanol or Carbenoxolone, and ATP release was significantly inhibited after silencing Cx32 from SCs cultures. We investigated the permeability of Cx32 to ATP by expressing Cx32 hemichannels in Xenopus laevis oocytes. We found that ATP release is coupled to the inward tail current generated after the activation of Cx32 hemichannels by depolarization pulses, and it is sensitive to low extracellular calcium concentrations. Moreover, we found altered ATP release in mutated Cx32 hemichannels related to the X-linked form of Charcot-Marie-Tooth disease, suggesting that purinergic-mediated signaling in peripheral nerves could underlie the physiopathology of this neuropathy.

Novel GJB1 mutation causing adult-onset Charcot-Marie-Tooth disease in a female patient

Charcot-Marie-Tooth disease (CMT), which is the eponym for hereditary motor and sensory neuropathy (HMSN), affects ∼1 in 2500 individuals. The most common subtype of X-linked CMT, CMTX1, is caused by mutations in GJB1, the gene encoding connexin 32, a gap junction protein in myelinated Schwann cells. We report a woman, who presented at the age of 56 years with gait unsteadiness and tingling in her feet. Clinical investigation revealed impaired sensation to pinprick, light touch and vibration in her distal lower limbs. Ankle reflexes were bilaterally absent. Sequencing revealed a novel heterozygous c.712C>T (p.R238C) mutation in the GJB1 gene. This mutation is predicted to result in the loss of disulfide bonds and thus in abnormal protein structure. In this woman, the reported novel GJB1 mutation resulted in sensory abnormalities, slowly progressive loss of distal lower limb strength, and notable loss of balance, with onset of symptoms late in adult age.

The carboxyl terminal residues 220-283 are not required for voltage gating of a chimeric connexin32 hemichannel

Connexin hemichannels display two distinct forms of voltage-dependent gating, corresponding to the operation of Vj- or fast gates and loop- or slow gates. The carboxyl terminus (CT) of connexin 32 has been reported to be required for the operation of the Vj (fast) gates, but this conclusion was inferred from the loss of a fast kinetic component in macroscopic currents of CT-truncated intercellular channels elicited by transjunctional voltage. Such inferences are complicated by presence of both fast and slow gates in each hemichannel and the serial head-to-head arrangement of these gates in the intercellular channel. Examination of voltage gating in undocked hemichannels and Vj gate polarity reversal by a negative charge substitution (N2E) in the amino terminal domain allow unequivocal separation of the two gating processes in a Cx32 chimera (Cx32(∗)43E1). This chimera expresses currents as an undocked hemichannel in Xenopus oocytes and provides a model system to study the molecular determinants and mechanisms of Cx32 voltage gating. Here, we demonstrate that both Vj- and loop gates are operational in a truncation mutation that removes all but the first four CT residues (ACAR(219)) of the Cx32(∗)43E1 hemichannel. We conclude that an operational Cx32 Vj (fast) gate does not require CT residues 220-283, as reported previously by others.

Regulation of connexin- and pannexin-based channels by post-translational modifications

Connexin (Cx) and pannexin (Panx) proteins form large conductance channels, which function as regulators of communication between neighbouring cells via gap junctions and/or hemichannels. Intercellular communication is essential to coordinate cellular responses in tissues and organs, thereby fulfilling an essential role in the spreading of signalling, survival and death processes. The functional properties of gap junctions and hemichannels are modulated by different physiological and pathophysiological stimuli. At the molecular level, Cxs and Panxs function as multi-protein channel complexes, regulating their channel localisation and activity. In addition to this, gap junctional channels and hemichannels are modulated by different post-translational modifications (PTMs), including phosphorylation, glycosylation, proteolysis, N-acetylation, S-nitrosylation, ubiquitination, lipidation, hydroxylation, methylation and deamidation. These PTMs influence almost all aspects of communicating junctional channels in normal cell biology and pathophysiology. In this review, we will provide a systematic overview of PTMs of communicating junction proteins and discuss their effects on Cx and Panx-channel activity and localisation.