Structural Changes in the Carboxyl Terminus of the Gap Junction Protein Connexin43 Indicates Signaling between Binding Domains for c-Src and Zonula Occludens-1

Connexin Gap Junction Channels and Hemichannels: Insights from High-Resolution Structures

Connexins (Cxs) are a family of integral membrane proteins, which function as both hexameric hemichannels (HCs) and dodecameric gap junction channels (GJCs), behaving as conduits for the electrical and molecular communication between cells and between cells and the extracellular environment, respectively. Their proper functioning is crucial for many processes, including development, physiology, and response to disease and trauma. Abnormal GJC and HC communication can lead to numerous pathological states including inflammation, skin diseases, deafness, nervous system disorders, and cardiac arrhythmias. Over the last 15 years, high-resolution X-ray and electron cryomicroscopy (cryoEM) structures for seven Cx isoforms have revealed conservation in the four-helix transmembrane (TM) bundle of each subunit; an αβ fold in the disulfide-bonded extracellular loops and inter-subunit hydrogen bonding across the extracellular gap that mediates end-to-end docking to form a tight seal between hexamers in the GJC. Tissue injury is associated with cellular Ca2+ overload. Surprisingly, the binding of 12 Ca2+ ions in the Cx26 GJC results in a novel electrostatic gating mechanism that blocks cation permeation. In contrast, acidic pH during tissue injury elicits association of the N-terminal (NT) domains that sterically blocks the pore in a "ball-and-chain" fashion. The NT domains under physiologic conditions display multiple conformational states, stabilized by protein-protein and protein-lipid interactions, which may relate to gating mechanisms. The cryoEM maps also revealed putative lipid densities within the pore, intercalated among transmembrane α-helices and between protomers, the functions of which are unknown. For the future, time-resolved cryoEM of isolated Cx channels as well as cryotomography of GJCs and HCs in cells and tissues will yield a deeper insight into the mechanisms for channel regulation. The cytoplasmic loop (CL) and C-terminal (CT) domains are divergent in sequence and length, are likely involved in channel regulation, but are not visualized in the high-resolution X-ray and cryoEM maps presumably due to conformational flexibility. We expect that the integrated use of synergistic physicochemical, spectroscopic, biophysical, and computational methods will reveal conformational dynamics relevant to functional states. We anticipate that such a wealth of results under different pathologic conditions will accelerate drug discovery related to Cx channel modulation.

Mapping the Anti-Cancer Activity of α-Connexin Carboxyl-Terminal (aCT1) Peptide in Resistant HER2+ Breast Cancer

Connexin 43 (Cx43) is a protein encoded by the GJA1 gene and is a component of cell membrane structures called gap junctions, which facilitate intercellular communication. Prior evidence indicates that elevated GJA1 expression in the HER2-positive (HER2+) subtype of breast cancer is associated with poor prognosis. Prior evidence also suggests that HER2+ breast cancers that have become refractory to HER2-targeted agents have a loss of Cx43 gap junction intercellular communication (GJIC). In this study, a Cx43-targeted agent called alpha-connexin carboxyl-terminal peptide (aCT1) is examined to determine whether GJIC can be rescued in refractory HER2+ breast cancer cells. A proposed mechanism of action for aCT1 is binding to the tight junction protein Zonal Occludens-1 (ZO-1). However, the true scope of activity for aCT1 has not been explored. In this study, mass spectrometry proteomic analysis is used to determine the breadth of aCT1-interacting proteins. The NanoString nCounter Breast Cancer 360 panel is also used to examine the effect of aCT1 on cancer signaling in HER2+ breast cancer cells. Findings from this study show a dynamic range of binding partners for aCT1, many of which regulate gene expression and RNA biology. nCounter analysis shows that a number of pathways are significantly impacted by aCT1, including upregulation of apoptotic factors, leading to the prediction and demonstration that aCT1 can boost the cell death effects of cisplatin and lapatinib in HER2+ breast cancer cells that have become resistant to HER2-targeted agents.

Gap Junctions or Hemichannel-Dependent and Independent Roles of Connexins in Fibrosis, Epithelial-Mesenchymal Transitions, and Wound Healing

Fibrosis initially appears as a normal response to damage, where activated fibroblasts produce large amounts of the extracellular matrix (ECM) during the wound healing process to assist in the repair of injured tissue. However, the excessive accumulation of the ECM, unresolved by remodeling mechanisms, leads to organ dysfunction. Connexins, a family of transmembrane channel proteins, are widely recognized for their major roles in fibrosis, the epithelial-mesenchymal transition (EMT), and wound healing. Efforts have been made in recent years to identify novel mediators and targets for this regulation. Connexins form gap junctions and hemichannels, mediating communications between neighboring cells and inside and outside of cells, respectively. Recent evidence suggests that connexins, beyond forming channels, possess channel-independent functions in fibrosis, the EMT, and wound healing. One crucial channel-independent function is their role as the primary functional component for cell adhesion. Other channel-independent functions of connexins involve their roles in mitochondria and exosomes. This review summarizes the latest advances in the channel-dependent and independent roles of connexins in fibrosis, the EMT, and wound healing, with a particular focus on eye diseases, emphasizing their potential as novel, promising therapeutic targets.

Inhibition of Pyk2 Improves Cx43 Intercalated Disc Localization, Infarct Size, and Cardiac Function in Rats With Heart Failure

BACKGROUND

Heart failure causes changes in Cx43 (Connexin43) regulation that are associated with arrhythmic heart disease. Pyk2 (proline-rich tyrosine kinase 2) is activated in cardiomyopathies and phosphorylates Cx43 to decrease intercellular communication. This study was designed to determine if Pyk2 inhibition improves cardiac function in a myocardial infarction (MI)-induced heart failure model in rats.

METHODS

MI (ligation of left anterior descending artery) rats were treated with the Pyk2 inhibitor PF4618433. Hemodynamic and structural parameters were monitored in Sham (n=5), MI-vehicle (n=5), and MI-PF4618433 (n=8) groups. Heart tissues were collected after 6 weeks to assess Pyk2 and Cx43 protein level and localization.

RESULTS

PF4618433 produced no observed adverse effects and inhibited ventricular Pyk2. PF4618433 reduced the MI infarct size from 34% to 17% (P=0.007). PF4618433 improved stroke volume (P=0.031) and cardiac output (P=0.009) in comparison to MI-vehicle with values similar to the Sham group. PF4618433 also led to an increase in the ejection fraction (P=0.002) and fractional shortening (P=0.006) when compared with the MI-vehicle (32% and 35% improvement, respectively) yet were lower in comparison with the Sham group. Pyk2 inhibition decreased Cx43 tyrosine phosphorylation (P=0.043) and maintained Cx43 at the intercalated disc in the distal ventricle 6 weeks post-MI.

CONCLUSIONS

Unlike other attempts to decrease Cx43 remodeling after MI-induced heart failure, inhibition of Pyk2 activity maintained Cx43 at the intercalated disc. This may have aided in the reduced infarct size (acute time frame) and improved cardiac function (chronic time frame). Additionally, we provide evidence that Pyk2 is activated following MI in human left ventricle, implicating a novel potential target for therapy in patients with heart failure.

The intercalated disc: a unique organelle for electromechanical synchrony in cardiomyocytes

The intercalated disc (ID) is a highly specialized structure that connects cardiomyocytes via mechanical and electrical junctions. Although described in some detail by light microscopy in the 19th century, it was in 1966 that electron microscopy images showed that the ID represented apposing cell borders and provided detailed insight into the complex ID nanostructure. Since then, much has been learned about the ID and its molecular composition, and it has become evident that a large number of proteins, not all of them involved in direct cell-to-cell coupling via mechanical or gap junctions, reside at the ID. Furthermore, an increasing number of functional interactions between ID components are emerging, leading to the concept that the ID is not the sum of isolated molecular silos but an interacting molecular complex, an "organelle" where components work in concert to bring about electrical and mechanical synchrony. The aim of the present review is to give a short historical account of the ID's discovery and an updated overview of its composition and organization, followed by a discussion of the physiological implications of the ID architecture and the local intermolecular interactions. The latter will focus on both the importance of normal conduction of cardiac action potentials as well as the impact on the pathophysiology of arrhythmias.

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.

Mitochondrial Connexins and Mitochondrial Contact Sites with Gap Junction Structure

Mitochondria contain connexins, a family of proteins that is known to form gap junction channels. Connexins are synthesized in the endoplasmic reticulum and oligomerized in the Golgi to form hemichannels. Hemichannels from adjacent cells dock with one another to form gap junction channels that aggregate into plaques and allow cell-cell communication. Cell-cell communication was once thought to be the only function of connexins and their gap junction channels. In the mitochondria, however, connexins have been identified as monomers and assembled into hemichannels, thus questioning their role solely as cell-cell communication channels. Accordingly, mitochondrial connexins have been suggested to play critical roles in the regulation of mitochondrial functions, including potassium fluxes and respiration. However, while much is known about plasma membrane gap junction channel connexins, the presence and function of mitochondrial connexins remain poorly understood. In this review, the presence and role of mitochondrial connexins and mitochondrial/connexin-containing structure contact sites will be discussed. An understanding of the significance of mitochondrial connexins and their connexin contact sites is essential to our knowledge of connexins' functions in normal and pathological conditions, and this information may aid in the development of therapeutic interventions in diseases linked to mitochondria.

Src: coordinating metabolism in cancer

Metabolism must be tightly regulated to fulfil the dynamic requirements of cancer cells during proliferation, migration, stemness and differentiation. Src is a node of several signals involved in many of these biological processes, and it is also an important regulator of cell metabolism. Glucose uptake, glycolysis, the pentose-phosphate pathway and oxidative phosphorylation are among the metabolic pathways that can be regulated by Src. Therefore, this oncoprotein is in an excellent position to coordinate and finely tune cell metabolism to fuel the different cancer cell activities. Here, we provide an up-to-date summary of recent progress made in determining the role of Src in glucose metabolism as well as the link of this role with cancer cell metabolic plasticity and tumour progression. We also discuss the opportunities and challenges facing this field.

Tumor Necrosis Factor-Alpha Disrupts Cx43-Mediated Corneal Endothelial Gap Junction Intercellular Communication

Connexin43 (Cx43)-mediated gap junctions are vital in maintaining corneal endothelium homeostasis. Tumor necrosis factor-alpha (TNF-α) is among the most important inflammatory factors which cause corneal endothelial dysfunction in various eye diseases. However, the effect of TNF-α on Cx43-mediated gap junctions of the corneal endothelium remains undefined. In the current research, we determined the effect of TNF-α on gap junction intercellular communication (GJIC) in rabbit corneal endothelium. To evaluate alterations of GJIC, if any, we treated ex vivo cultured rabbit corneal endothelium with different concentrations of TNF-α (2-20 ng/ml). The localization of Cx43 was analyzed by immunostaining, while RT-qPCR and western blot were used to profile the expression of Cx43 and zonula occludens-1 (ZO-1). The association between ZO-1 and Cx43 was evaluated using immunoprecipitation and double staining. GJIC activity was determined by the scrap loading and dye transfer assay (SLDT). Our data demonstrated that a high concentration of TNF-α (10 ng/ml and 20 ng/ml) disrupts the Cx43 mediated gap junction distribution in rabbit corneal endothelium and suppresses the expression of Cx43 protein. Furthermore, rabbit corneal endothelial GJIC was inhibited due to the decreased association between the ZO-1 and Cx43 proteins. Current results demonstrate that TNF-α inhibits corneal endothelial GJIC via decreasing the association between ZO-1 and Cx43, disrupting the distribution of Cx43, and downregulating the expression of Cx43 protein. This study offers a new theoretical foundation for diagnosing and treating corneal endothelial cell decompensation induced by elevated TNF-α in various eye diseases.

The Role of Connexin in Ophthalmic Neovascularization and the Interaction between Connexin and Proangiogenic Factors

The formation of new blood vessels is an important physiological process that occurs during development. When the body is injured, new blood vessel formation helps the body recuperate by supplying more oxygen and nutrients. However, this mechanism can have a negative effect. In ophthalmologic diseases, such as corneal new blood vessels, neonatal vascular glaucoma, and diabetes retinopathy, the formation of new blood vessels has become a critical component in patient survival. Connexin is a protein that regulates the cellular and molecular material carried by cells. It has been demonstrated that it is widely expressed in vascular endothelial cells, where it forms a slit connection between adjacent cells to promote cell-cell communication via hemichannels, as well as substance exchange into intracellular environments. Numerous studies have demonstrated that connexin in vascular endothelial cells plays an important role in angiogenesis and vascular leakage. The purpose of this study was to investigate the effect between the angiogenesis-associated factor and the connexin. It also reveals the effect of connexin on ophthalmic neovascularization.

Hypoglycemia-Exacerbated Mitochondrial Connexin 43 Accumulation Aggravates Cardiac Dysfunction in Diabetic Cardiomyopathy

Background

Diabetic cardiomyopathy (DCM) is a complex multifaceted disease responsible for elevated heart failure (HF) morbidity and mortality in patients with diabetes mellitus (DM). Patients with DCM exhibit subclinical diastolic dysfunction, progression toward systolic impairment, and abnormal electrophysiology. Hypoglycemia events that occur spontaneously or due to excess insulin administration threaten the lives of patients with DM—with the increased risk of sudden death. However, the molecular underpinnings of this fatal disease remain to be elucidated.

Methods and Results

Here, we used the established streptozotocin-induced DCM murine model to investigate how hypoglycemia aggravates DCM progression. We confirmed connexin 43 (Cx43) dissociation from cell–cell interaction and accumulation at mitochondrial inner membrane both in the cardiomyocytes of patients with DM and DCM murine. Here, we observed that cardiac diastolic function, induced by chronic hyperglycemia, was further aggravated upon hypoglycemia challenge. Similar contractile defects were recapitulated using neonatal mouse ventricular myocytes (NMVMs) under glucose fluctuation challenges. Using immunoprecipitation mass spectrometry, we identified and validated that hypoglycemia challenge activates the mitogen-activated protein kinase kinase (MAPK kinase) (MEK)/extracellular regulated protein kinase (ERK) and inhibits phosphoinositide 3-kinase (PI3K)/Akt pathways, which results in Cx43 phosphorylation by Src protein and translocation to mitochondria in cardiomyocytes. To determine causality, we overexpressed a mitochondrial targeting Cx43 (mtCx43) using adeno-associated virus serotype 2 (AAV2)/9. At normal blood glucose levels, mtCx43 overexpression recapitulated cardiac diastolic dysfunction as well as aberrant electrophysiology in vivo. Our findings give support for therapeutic targeting of MEK/ERK/Src and PI3K/Akt/Src pathways to prevent mtCx43-driven DCM.

Conclusion

DCM presents compensatory adaptation of mild mtCx43 accumulation, yet acute hypoglycemia challenges result in further accumulation of mtCx43 through the MEK/ERK/Src and PI3K/Akt/Src pathways. We provide evidence that Cx43 mislocalization is present in hearts of patients with DM hearts, STZ-induced DCM murine model, and glucose fluctuation challenged NMVMs. Mechanistically, we demonstrated that mtCx43 is responsible for inducing aberrant contraction and disrupts electrophysiology in cardiomyocytes and our results support targeting of mtCx43 in treating DCM.

The Cx43 Carboxyl-Terminal Mimetic Peptide αCT1 Protects Endothelial Barrier Function in a ZO1 Binding-Competent Manner

The Cx43 carboxyl-terminus (CT) mimetic peptide, αCT1, originally designed to bind to Zonula Occludens 1 (ZO1) and thereby inhibit Cx43/ZO1 interaction, was used as a tool to probe the role of Cx43/ZO1 association in regulation of epithelial/endothelial barrier function. Using both in vitro and ex vivo methods of barrier function measurement, including Electric Cell-Substrate Impedance Sensing (ECIS), a TRITC-dextran Transwell permeability assay, and a FITC-dextran cardiovascular leakage protocol involving Langendorff-perfused mouse hearts, αCT1 was found to protect the endothelium from thrombin-induced breakdown in cell-cell contacts. Barrier protection was accompanied by significant remodeling of the F-actin cytoskeleton, characterized by a redistribution of F-actin away from the cytoplasmic and nuclear regions of the cell, towards the endothelial cell periphery, in association with alterations in cellular chiral orientation distribution. In line with observations of increased cortical F-actin, αCT1 upregulated cell-cell border localization of endothelial VE-cadherin, the tight junction protein Zonula Occludens 1 (ZO1), and the Gap Junction Protein (GJ) Connexin43 (Cx43). A ZO1 binding-incompetent variant of αCT1, αCT1-I, indicated that these effects on barrier function and barrier-associated proteins, were likely associated with Cx43 CT sequences retaining ability to interact with ZO1. These results implicate the Cx43 CT and its interaction with ZO1, in the regulation of endothelial barrier function, while revealing the therapeutic potential of αCT1 in the treatment of vascular edema.

The mutual interplay of redox signaling and connexins

Connexins (Cxs) are ubiquitous transmembrane proteins that possess both channel function (e.g., formations of gap junction and hemichannel) and non-channel properties (e.g., gene transcription and protein-protein interaction). Several factors have been identified to play a role in the regulation of Cxs, which include those acting intracellularly, as redox potential, pH, intramolecular interactions, and post-translational modifications (e.g., phosphorylation, S-nitrosylation) as well as those acting extracellularly, such as Ca²⁺ and Mg²⁺. The relationship between redox signaling and Cxs attracts considerable attention in recent years. There is ample evidence showing that redox signaling molecules (e.g., hydrogen peroxide (H₂O₂), nitric oxide (NO)) affect Cxs-based channel function while the opening of Cx channels also triggers the transfer of various redox-related metabolites (e.g., reactive oxygen species, glutathione, nicotinamide adenine dinucleotide, and NO). On the basis of these evidences, we propose the existence of redox-Cxs crosstalk. In this review, we briefly discuss the interaction between redox signaling and Cxs and the implications of the intersection in disease pathology and future therapeutic interventions.

An Update on Connexin Gap Junction and Hemichannels in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the main causes of vision loss in the working age population. It is characterized by a progressive deterioration of the retinal microvasculature, caused by long-term metabolic alterations inherent to diabetes, leading to a progressive loss of retinal integrity and function. The mammalian retina presents an orderly layered structure that executes initial but complex visual processing and analysis. Gap junction channels (GJC) forming electrical synapses are present in each retinal layer and contribute to the communication between different cell types. In addition, connexin hemichannels (HCs) have emerged as relevant players that influence diverse physiological and pathological processes in the retina. This article highlights the impact of diabetic conditions on GJC and HCs physiology and their involvement in DR pathogenesis. Microvascular damage and concomitant loss of endothelial cells and pericytes are related to alterations in gap junction intercellular communication (GJIC) and decreased connexin 43 (Cx43) expression. On the other hand, it has been shown that the expression and activity of HCs are upregulated in DR, becoming a key element in the establishment of proinflammatory conditions that emerge during hyperglycemia. Hence, novel connexin HCs blockers or drugs to enhance GJIC are promising tools for the development of pharmacological interventions for diabetic retinopathy, and initial in vitro and in vivo studies have shown favorable results in this regard.

Src Regulation of Cx43 Phosphorylation and Gap Junction Turnover

The gap junction protein Connexin43 (Cx43) is highly regulated by phosphorylation at over a dozen sites by probably at least as many kinases. This Cx43 “kinome” plays an important role in gap junction assembly and turnover. We sought to gain a better understanding of the interrelationship of these phosphorylation events particularly related to src activation and Cx43 turnover. Using state-of-the-art live imaging methods, specific inhibitors and many phosphorylation-status specific antibodies, we found phospho-specific domains in gap junction plaques and show evidence that multiple pathways of disassembly exist and can be regulated at the cellular and subcellular level. We found Src activation promotes formation of connexisomes (internalized gap junctions) in a process involving ERK-mediated phosphorylation of S279/282. Proteasome inhibition dramatically and rapidly restored gap junctions in the presence of Src and led to dramatic changes in the Cx43 phospho-profile including to increased Y247, Y265, S279/282, S365, and S373 phosphorylation. Lysosomal inhibition, on the other hand, nearly eliminated phosphorylation on Y247 and Y265 and reduced S368 and S373 while increasing S279/282 phosphorylation levels. We present a model of gap junction disassembly where multiple modes of disassembly are regulated by phosphorylation and can have differential effects on cellular signaling.

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.

The dynamic Nexus: gap junctions control protein localization and mobility in distinct and surprising ways

Gap junction (GJ) channels permit molecules, such as ions, metabolites and second messengers, to transfer between cells. Their function is critical for numerous cellular interactions, providing exchange of metabolites, signaling molecules, and ionic currents. GJ channels are composed of Connexin (Cx) hexamers paired across extracellular space and typically form large rafts of clustered channels, called plaques, at cell appositions. Cxs together with molecules that interact with GJ channels make up a supramolecular structure known as the GJ Nexus. While the stability of connexin localization in GJ plaques has been studied, mobility of other Nexus components has yet to be addressed. Colocalization analysis of several nexus components and other membrane proteins reveal that certain molecules are excluded from the GJ plaque (Aquaporin 4, EAAT2b), while others are quite penetrant (lipophilic molecules, Cx30, ZO-1, Occludin). Fluorescence recovery after photobleaching of tagged Nexus-associated proteins showed that mobility in plaque domains is affected by mobility of the Cx proteins. These novel findings indicate that the GJ Nexus is a dynamic membrane organelle, with cytoplasmic and membrane-embedded proteins binding and diffusing according to distinct parameters.

Inhibition of Pyk2 and Src activity improves Cx43 gap junction intercellular communication

Identification of proteins that interact with Cx43 has been instrumental in the understanding of gap junction (GJ) regulation. An in vitro phosphorylation screen identified that Protein tyrosine kinase 2 beta (Pyk2) phosphorylated purified Cx43CT and this led us to characterize the impact of this phosphorylation on Cx43 function. Mass spectrometry identified Pyk2 phosphorylates Cx43 residues Y247, Y265, Y267, and Y313. Western blot and immunofluorescence staining using HeLa^Cx43 cells, HEK 293 T cells, and neonatal rat ventricular myocytes (NRVMs) revealed Pyk2 can be activated by Src and active Pyk2 interacts with Cx43 at the plasma membrane. Overexpression of Pyk2 increases Cx43 phosphorylation and knock-down of Pyk2 decreases Cx43 phosphorylation, without affecting the level of active Src. In HeLa^Cx43 cells treated with PMA to activate Pyk2, a decrease in Cx43 GJ intercellular communication (GJIC) was observed when assayed by dye transfer. Moreover, PMA activation of Pyk2 could be inhibited by the small molecule PF4618433. This partially restored GJIC, and when paired with a Src inhibitor, returned GJIC to the no PMA control-level. The ability of Pyk2 and Src inhibitors to restore Cx43 function in the presence of PMA was also observed in NRVMs. Additionally, an animal model of myocardial infarction induced heart failure showed a higher level of active Pyk2 activity and increased interaction with Cx43 in ventricular myocytes. Src inhibitors have been used to reverse Cx43 remodeling and improve heart function after myocardial infarction; however, they alone could not fully restore proper Cx43 function. Our data suggest that Pyk2 may need to be inhibited, in addition to Src, to further (if not completely) reverse Cx43 remodeling and improve intercellular communication.

Molecular mechanisms of dendritic cell migration in immunity and cancer

Dendritic cells (DCs) are a heterogeneous population of antigen-presenting cells that act to bridge innate and adaptive immunity. DCs are critical in mounting effective immune responses to tissue damage, pathogens and cancer. Immature DCs continuously sample tissues and engulf antigens via endocytic pathways such as phagocytosis or macropinocytosis, which result in DC activation. Activated DCs undergo a maturation process by downregulating endocytosis and upregulating surface proteins controlling migration to lymphoid tissues where DC-mediated antigen presentation initiates adaptive immune responses. To traffic to lymphoid tissues, DCs must adapt their motility mechanisms to migrate within a wide variety of tissue types and cross barriers to enter lymphatics. All steps of DC migration involve cell-cell or cell-substrate interactions. This review discusses DC migration mechanisms in immunity and cancer with a focus on the role of cytoskeletal processes and cell surface proteins, including integrins, lectins and tetraspanins. Understanding the adapting molecular mechanisms controlling DC migration in immunity provides the basis for therapeutic interventions to dampen immune activation in autoimmunity, or to improve anti-tumour immune responses.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Protective effect of Dioscorea zingiberensis ethanol extract on the disruption of blood-testes barrier in high-fat diet/streptozotocin-induced diabetic mice by upregulating ZO-1 and Nrf2

Testicular injury is the primary pathogenesis of diabetes-induced male infertility. Dioscorea zingiberensis (DZ), a traditional Chinese medicine (TCM) including saponins, flavonoids and cellulose, is used to treat diseases in the reproductive system. But the protective effects of DZ on diabetes-induced testicular injury remain poorly understood. In this study, the therapeutic effects of chronic oral DZ treatment on testis impairment in a diabetic mouse model were explored by assessing sperm morphology, blood-testes barrier (BTB) integrity and testicular histological examination. Our results showed that DZ significantly reversed BTB disruption, testicular tissue injury and abnormal sperm morphology in diabetic mice. Interestingly, diabetes-induced disruption of the BTB was associated with a decrease in the tight junction (TJ) protein zonula occludens-1 (ZO-1). Dioscorea zingiberensis effectively increased ZO-1 expression in testis tissue to restore the integrity of the BTB. Moreover, DZ treatment significantly reduced hyperglycaemia-induced increases in malondialdehyde (MDA) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels. Further mechanistic studies revealed that DZ substantially enhanced the expression of Nrf2, NOQ1 and HO-1, which indicated that DZ exerts potential antioxidant effects against testicular tissue damage via the activation of Nrf2. In conclusion, the protective effects of DZ rely on repairing the integrity of the BTB and on reducing oxidative stress damage by mediating ZO-1 and Nrf2. The study contributes to discovering the DZ possible mechanism of action.

Involvement of the Gap Junction Protein, Connexin43, in the Formation and Function of Invadopodia in the Human U251 Glioblastoma Cell Line

The resistance of glioblastomas to treatments is mainly the consequence of their invasive capacities. Therefore, in order to better treat these tumors, it is important to understand the molecular mechanisms which are responsible for this behavior. Previous work suggested that gap junction proteins, the connexins, facilitate the aggressive nature of glioma cells. Here, we show that one of them—connexin43 (Cx43)—is implicated in the formation and function of invadopodia responsible for invasion capacity of U251 human glioblastoma cells. Immunofluorescent approaches—combined with confocal analyses—revealed that Cx43 was detected in all the formation stages of invadopodia exhibiting proteolytic activity. Clearly, Cx43 appeared to be localized in invadopodia at low cell density and less associated with the establishment of gap junctions. Accordingly, lower extracellular matrix degradation correlated with less mature invadopodia and MMP2 activity when Cx43 expression was decreased by shRNA strategies. Moreover, the kinetics of invadopodia formation could be dependent on Cx43 dynamic interactions with partners including Src and cortactin. Interestingly, it also appeared that invadopodia formation and MMP2 activity are dependent on Cx43 hemichannel activity. In conclusion, these results reveal that Cx43 might be involved in the formation and function of the invadopodia of U251 glioblastoma cells.

RyR2 regulates Cx43 hemichannel intracellular Ca2+-dependent activation in cardiomyocytes

AIMS

Connexin-based gap junctions are crucial for electrical communication in the heart; they are each composed of two docked hemichannels (HCs), supplied as unpaired channels via the sarcolemma. When open, an unpaired HC forms a large pore, high-conductance and Ca2+-permeable membrane shunt pathway that may disturb cardiomyocyte function. HCs composed of connexin 43 (Cx43), a major cardiac connexin, can be opened by electrical stimulation but only by very positive membrane potentials. Here, we investigated the activation of Cx43 HCs in murine ventricular cardiomyocytes voltage-clamped at -70 mV.

METHODS AND RESULTS

Using whole-cell patch-clamp, co-immunoprecipitation, western blot analysis, immunocytochemistry, proximity ligation assays, and protein docking studies, we found that stimulation of ryanodine receptors (RyRs) triggered unitary currents with a single-channel conductance of ∼220 pS, which were strongly reduced by Cx43 knockdown. Recordings under Ca2+-clamp conditions showed that both RyR activation and intracellular Ca2+ elevation were necessary for HC opening. Proximity ligation studies indicated close Cx43-RyR2 apposition (<40 nm), and both proteins co-immunoprecipitated indicating physical interaction. Molecular modelling suggested a strongly conserved RyR-mimicking peptide sequence (RyRHCIp), which inhibited RyR/Ca2+ HC activation but not voltage-triggered activation. The peptide also slowed down action potential repolarization. Interestingly, alterations in the concerned RyR sequence are known to be associated with primary familial hypertrophic cardiomyopathy.

CONCLUSION

Our results demonstrate that Cx43 HCs are intimately linked to RyRs, allowing them to open at negative diastolic membrane potential in response to RyR activation.

Targeting different domains of gap junction protein to control malignant glioma

A rational treatment strategy for glioma, the most common primary central nervous system tumor, should focus on early invasive growth and resistance to current therapeutics. Connexin 43 (Cx43), a gap junction protein, plays important roles not only in the development of the central nervous system and but also in the progression of glioma. The different structural domains of Cx43, including extracellular loops, transmembrane domains, and an intracellular carboxyl terminal, have distinct functions in the invasion and proliferation of gliomas. Targeting these domains of Cx43, which is expressed in distinct patterns in the heterogeneous glioma cell population, can inhibit tumor cell invasion and new tumor formation. Thus, this review summarizes the structural characteristics of Cx43, the effects of regulating different Cx43 domains on the biological characteristics of glioma cells, intervention strategies targeting different domains of Cx43, and future research directions.

The lipidated connexin mimetic peptide SRPTEKT-Hdc is a potent inhibitor of Cx43 channels with specificity for the pS368 phospho-isoform

Connexin (Cx) mimetic peptides derived from extracellular loop II sequences (e.g., Gap27: SRPTEKTIFII; Peptide5: VDCFLSRPTEKT) have been used as reversible, Cx-specific blockers of hemichannel (HCh) and gap junction channel (GJCh) function. These blockers typically require high concentrations (~5 µM, <1 h for HCh; ~100 µM, >1 h for GJCh) to achieve inhibition. We have shown that addition of a hexadecyl (Hdc) lipid tail to the conserved SRPTEKT peptide sequence (SRPTEKT-Hdc) results in a novel, highly efficacious, and potent inhibitor of mechanically induced Ca2+-wave propagation (IC50 64.8 pM) and HCh-mediated dye uptake (IC50 45.0 pM) in Madin-Darby canine kidney cells expressing rat Cx43 (MDCK43). The lack of similar effect on dye coupling (NBD-MTMA) suggested channel conformation-specific inhibition. Here we report that SRPTEKT-Hdc inhibition of Ca2+-wave propagation, dye coupling, and dye uptake depended on the functional configuration of Cx43 as determined by phosphorylation at serine 368 (S368). Ca2+-wave propagation was enhanced in MDCK cells expressing single-site mutants of Cx43 that mimicked (MDCK43-S368D) or favored (MDCK43-S365A) phosphorylation at S368. Furthermore, SRPTEKT-Hdc potently inhibited GJCh-mediated Ca2+-wave propagation (IC50 230.4 pM), dye coupling, and HCh-mediated dye uptake in MDCK43-S368D and -S365A cells. In contrast, Ca2+-wave propagation, dye coupling, and dye uptake were largely unaffected (IC50 12.3 μM) by SRPTEKT-Hdc in MDCK43-S368A and -S365D cells, mutations that mimic or favor dephosphorylation at S368. Together, these data indicate that SRPTEKT-Hdc is a potent inhibitor of physiological Ca2+-wave signaling mediated specifically by the pS368 phosphorylated form of Cx43.

Interaction of α Carboxyl Terminus 1 Peptide With the Connexin 43 Carboxyl Terminus Preserves Left Ventricular Function After Ischemia-Reperfusion Injury

Background

α Carboxyl terminus 1 (αCT1) is a 25–amino acid therapeutic peptide incorporating the zonula occludens‐1 (ZO‐1)–binding domain of connexin 43 (Cx43) that is currently in phase 3 clinical testing on chronic wounds. In mice, we reported that αCT1 reduced arrhythmias after cardiac injury, accompanied by increases in protein kinase Cε phosphorylation of Cx43 at serine 368. Herein, we characterize detailed molecular mode of action of αCT1 in mitigating cardiac ischemia‐reperfusion injury.

Methods and Results

To study αCT1‐mediated increases in phosphorylation of Cx43 at serine 368, we undertook mass spectrometry of protein kinase Cε phosphorylation assay reactants. This indicated potential interaction between negatively charged residues in the αCT1 Asp‐Asp‐Leu‐Glu‐Iso sequence and lysines (Lys345, Lys346) in an α‐helical sequence (helix 2) within the Cx43‐CT. In silico modeling provided further support for this interaction, indicating that αCT1 may interact with both Cx43 and ZO‐1. Using surface plasmon resonance, thermal shift, and phosphorylation assays, we characterized a series of αCT1 variants, identifying peptides that interacted with either ZO‐1–postsynaptic density‐95/disks large/zonula occludens‐1 2 or Cx43‐CT, but with limited or no ability to bind both molecules. Only peptides competent to interact with Cx43‐CT, but not ZO‐1–postsynaptic density‐95/disks large/zonula occludens‐1 2 alone, prompted increased pS368 phosphorylation. Moreover, in an ex vivo mouse model of ischemia‐reperfusion injury, preischemic infusion only with those peptides competent to bind Cx43 preserved ventricular function after ischemia‐reperfusion. Interestingly, a short 9–amino acid variant of αCT1 (αCT11) demonstrated potent cardioprotective effects when infused either before or after ischemic injury.

Conclusions

Interaction of αCT1 with the Cx43, but not ZO‐1, is correlated with cardioprotection. Pharmacophores targeting Cx43‐CT could provide a translational approach to preserving heart function after ischemic injury.

[SRC kinase inhibitor PP2 inhibits invasion and metastasis of lung cancer A549 cells by upregulating connexin43 expression]

OBJECTIVE

To investigate the effect of SRC kinase inhibitor PP2 on the invasion and metastasis of lung cancer A549 cells and explore its molecular mechanism.

METHODS

MTT assay was used to evaluate the inhibitory effect of PP2 on the proliferation of A549 cells. Cell scratch and Transwell assays were performed to assess the invasion and metastatic capacity of A549 cells after treatment with 1, 2, 4, 8, and 16 μmol/L PP2 for 24 h. Western blotting was used to detect the expressions of connexin43 (Cx43) and MMP-2 in the cells after small interfering RNA (siRNA)-mediated silencing or overexpression of Cx43; the changes in the cell invasion and metastasis in response to PP2 treatment after Cx43 silencing or overexpression were investigated.

RESULTS

MTT assay showed that treatment with PP2 at 2, 4, 8, 16, and 32 μmol/L significantly inhibited the proliferation of A549 cells in a concentration-dependent manner. Treatments with PP2 at 1, 2, 4, 8, and 16 μmol/L for 24 h also concentration-dependently lowered the invasion and metastatic abilities of the cells (P < 0.05). At 4 and 8 μmol/L, PP2 significantly increased the expression level of Cx43 protein and decreased the expression level of MMP-2 protein. Overexpression of Cx43 significantly enhanced the inhibitory effect of PP2 on the cell invasion and metastasis, and Cx43 silencing significantly attenuated the inhibitory effect of PP2 (P < 0.05).

CONCLUSIONS

PP2 treatment can suppress the invasion and metastasis of A549 cells in vitro possibly by modulating the expression of Cx43.

A Logistic Regression Model for Detecting the Presence of Malignant Progression in Atypical Meningiomas

OBJECTIVE

To develop a method to distinguish atypical meningiomas (AMs) with malignant progression (MP) from primary AMs without a clinical history.

METHODS

The clinical, radiologic, and pathologic data of 33 previously Simpson grade I resected (if any) as well as no radiotherapy treated intracranial AMs between January 2008 and December 2015 were reviewed. Immunohistochemical staining for connexin 43 (Cx43) and Ki-67 was performed. Descriptive analysis and univariate and multivariate logistic regression analyses were used to explore independent predictors of MP. A multivariable logistic model was developed to estimate the risk of MP, and its diagnostic value was determined from a receiver operating characteristic curve.

RESULTS

There were 11 AMs (33.3%) with histopathologically confirmed MP from benign meningiomas. The other 22 (66.7%) were initially diagnosed AMs with no histopathologically confirmed MP during a median 60.5 months (range, 42-126 months) of follow-up. Univariate and multivariate logistic analyses showed that irregular tumor shape (P = 0.010) and low Cx43 expression (P = 0.010) were independent predictors of the presence of MP, and the predicted probability was calculated by the following formula: P = 1/[1+exp.{1.218-(3.202×Shape)+(3.814×Cx43)}]. P > 0.5 for an irregularly shaped (score 1) AM with low Cx43 expression (score 0) indicated a high probability of MP. The sensitivity, specificity, positive predictive value, negative predictive value, and overall predictive accuracy were 63.6, 95.6, 87.5, 84.0, and 84.8%, respectively.

CONCLUSIONS

Low Cx43 expression and irregular tumor shape were independent predictors of the presence of MP. The relevant logistic regression model was found to be effective in distinguishing MP-AMs from primary AMs.

Potential of cryo-EM for high-resolution structural analysis of gap junction channels

Gap junction family proteins form conduits connecting the cytoplasm of adjacent cells, thereby enabling electrical and chemical coupling to maintain physiological homeostasis. Gap junction proteins comprise two gene families, connexins in chordates and innexins in pre-chordates. Their channel structures have been analyzed by electron or X-ray crystallography, but only a few atomic structures have been reported. Recent advances in single-particle cryo-electron microscopy (cryo-EM) will help to elucidate these structures further. Here the structural biology of gap junction channels utilizing crystallography and single-particle cryo-EM is overviewed to shed light on the functional mechanisms of cell-cell communication that are essential for multicellular organisms.

Phosphorylation-Dependent Intra-Domain Interaction of the Cx37 Carboxyl-Terminus Controls Cell Survival

Differential phosphorylation of the carboxyl-terminus of connexin 37 (Cx37-CT) regulates phenotypic switching between cell growth phenotypes (cell death, cell cycle arrest, proliferation). The specific phosphorylation events in the Cx37-CT that are necessary for these growth regulatory effects are currently unknown. Through the combined use of deletion and site specific (de)phospho-mimetic Cx37-CT mutants, our data suggest a phosphorylation-dependent interaction between the mid-tail (aa 273⁻317) and end-tail (aa 318⁻333) portions of the Cx37-CT that regulates cell survival. As detected by mass spectrometry, Cx37 was phosphorylated at serines 275, 321, and 328; phosphomimetic mutations of these sites resulted in cell death when expressed in rat insulinoma cells. Alanine substitution at S328, but not at S275 or S321, also triggered cell death. Cx37-S275D uniquely induced the death of only low density, non-contact forming cells, but neither hemichannel open probability nor channel conductance distinguished death-inducing mutants. As channel function is necessary for cell death, together the data suggest that the phosphorylation state of the Cx37-CT controls an intra-domain interaction within the CT that modifies channel function and induces cell death.

Concatenation of Human Connexin26 (hCx26) and Human Connexin46 (hCx46) for the Analysis of Heteromeric Gap Junction Hemichannels and Heterotypic Gap Junction Channels

Gap junction channels and hemichannels formed by concatenated connexins were analyzed. Monomeric (hCx26, hCx46), homodimeric (hCx46-hCx46, hCx26-hCx26), and heterodimeric (hCx26-hCx46, hCx46-hCx26) constructs, coupled to GFP, were expressed in HeLa cells. Confocal microscopy showed that the tandems formed gap junction plaques with a reduced plaque area compared to monomeric hCx26 or hCx46. Dye transfer experiments showed that concatenation allows metabolic transfer. Expressed in Xenopus oocytes, the inside-out patch-clamp configuration showed single channels with a conductance of about 46 pS and 39 pS for hemichannels composed of hCx46 and hCx26 monomers, respectively, when chloride was replaced by gluconate on both membrane sides. The conductance was reduced for hCx46-hCx46 and hCx26-hCx26 homodimers, probably due to the concatenation. Heteromerized hemichannels, depending on the connexin-order, were characterized by substates at 26 pS and 16 pS for hCx46-hCx26 and 31 pS and 20 pS for hCx26-hCx46. Because of the linker between the connexins, the properties of the formed hemichannels and gap junction channels (e.g., single channel conductance) may not represent the properties of hetero-oligomerized channels. However, should the removal of the linker be successful, this method could be used to analyze the electrical and metabolic selectivity of such channels and the physiological consequences for a tissue.

The SH3-binding domain of Cx43 participates in loop/tail interactions critical for Cx43-hemichannel activity

Connexin 43 (Cx43) hemichannels establish local signaling networks via the release of ATP and other molecules, but their excessive opening may result in cell death. Hence, the activity of Cx43-hemichannels ought to be critically controlled. This involves interactions between the C-terminal tail (CT) and the cytoplasmic loop (CL), more particularly the L2 domain within CL. Previous work revealed an important role for the last nine amino acids of the Cx43 CT by targeting the L2 domain, as these nine amino acids were sufficient to restore the activity of CT-truncated Cx43-hemichannels. However, we discovered that deletion of the last 19 amino acids of the CT only partially lowered the binding to the L2 domain, indicating that a second L2-binding region is present in the CT. We here provide evidence that the SH3-binding domain is another CT region that targets the L2 domain. At the functional level, the SH3-binding domain was able to restore the activity of CT-truncated Cx43-hemichannels and alleviate the inhibition of full-length Cx43-hemichannels by high intracellular Ca2+ concentration ([Ca2+]i) as demonstrated by various approaches including patch clamp studies of unitary Cx43-hemichannel activity. Finally, we show that in full-length Cx43-hemichannels, deletion of either the SH3-binding domain or the CT9 region suppresses the hemichannel activity, while deletion of both domains completely annihilates the hemichannel activity. These results demonstrate that the Cx43 SH3-binding domain, in addition to the CT9 region, critically controls hemichannel activity at high [Ca2+]i, which may be involved in pathological hemichannel opening.

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.

Connexins: Synthesis, Post-Translational Modifications, and Trafficking in Health and Disease

Connexins are tetraspan transmembrane proteins that form gap junctions and facilitate direct intercellular communication, a critical feature for the development, function, and homeostasis of tissues and organs. In addition, a growing number of gap junction-independent functions are being ascribed to these proteins. The connexin gene family is under extensive regulation at the transcriptional and post-transcriptional level, and undergoes numerous modifications at the protein level, including phosphorylation, which ultimately affects their trafficking, stability, and function. Here, we summarize these key regulatory events, with emphasis on how these affect connexin multifunctionality in health and disease.

Dendritic Cell Migration Toward CCL21 Gradient Requires Functional Cx43

Dendritic cells (DCs) travel through lymphatic vessels to transport antigens and present them to T cells in lymph nodes. DCs move directionally toward lymphatics by virtue of their CCR7 and a CCL21 chemotactic gradient. We evaluated in vivo and in bone marrow-derived dendritic cells (BMDCs) whether the gap junction protein Cx43 contributes to CCL21/CCR7-dependent DC migration in wild-type (WT) mice, heterozygous (Cx43^+/−) mice and mice expressing a truncated form of Cx43 lacking its regulatory C-terminus (Cx43^K258/−). In a model of flank skin inflammation, we found that the recruitment of myeloid DCs (mDCs) to skin draining lymph nodes was reduced in Cx43^K258/− mice as compared to WT and Cx43^+/− mice. In addition, the migration of Cx43^K258/− BMDCs toward CCL21 was abolished in an in vitro chemotactic assay while it was only reduced in Cx43^+/− cells. Both mutant genotypes showed defects in the directionality of BMDC migration as compared to WT BMDCs. No difference was found between the three populations of BMDCs in terms of expression of surface markers (CD11c, CD86, CD80, CD40, MHC-II, and CCR7) after differentiation and TLR activation. Finally, examination of the CCR7-induced signaling pathways in BMDCs revealed normal receptor-induced mobilization of intracellular Ca²⁺. These results demonstrate that full expression of an intact Cx43 is critical to the directionality and rate of DC migration, which may be amenable to regulation of the immune response.

Connexin43 recruits PTEN and Csk to inhibit c-Src activity in glioma cells and astrocytes

Connexin43 (Cx43), the major protein forming gap junctions in astrocytes, is reduced in high-grade gliomas, where its ectopic expression exerts important effects, including the inhibition of the proto-oncogene tyrosine-protein kinase Src (c-Src). In this work we aimed to investigate the mechanism responsible for this effect. The inhibition of c-Src requires phosphorylation at tyrosine 527 mediated by C-terminal Src kinase (Csk) and dephosphorylation at tyrosine 416 mediated by phosphatases, such as phosphatase and tensin homolog (PTEN). Our results showed that the antiproliferative effect of Cx43 is reduced when Csk and PTEN are silenced in glioma cells, suggesting the involvement of both enzymes. Confocal microscopy and immunoprecipitation assays confirmed that Cx43, in addition to c-Src, binds to PTEN and Csk in glioma cells transfected with Cx43 and in astrocytes. Pull-down assays showed that region 266–283 in Cx43 is sufficient to recruit c-Src, PTEN and Csk and to inhibit the oncogenic activity of c-Src. As a result of c-Src inhibition, PTEN was increased with subsequent inactivation of Akt and reduction of proliferation of human glioblastoma stem cells. We conclude that the recruitment of Csk and PTEN to the region between residues 266 and 283 within the C-terminus of Cx43 leads to c-Src inhibition.

Comparative analysis of membrane protein structure databases

BACKGROUND

Membrane proteins play important roles in cell survival and cell communication, as they function as transporters, receptors, anchors and enzymes. They are also potential targets for drugs that block receptors or inhibit enzymes related to diseases. Although the number of known structures of membrane proteins is still small relative to the size of the proteome as a whole, many new membrane protein structures have been determined recently.

SCOPE OF THE ARTICLE

We compared and analyzed the widely used membrane protein databases, mpstruc, Orientations of Proteins in Membranes (OPM), and PDBTM, as well as the extended dataset of mpstruc based on sequence similarity, the PDB structures whose classification field indicates that they are "membrane proteins" and the proteins with Structural Classification of Proteins (SCOP) class-f domains. We evaluated the relationships between these databases or datasets based on the overlap in their contents and the degree of consistency in the structural, topological, and functional classifications and in the transmembrane domain assignment.

MAJOR CONCLUSIONS

The membrane databases differ from each other in their coverage, and in the criteria that they use for annotation and classification. To ensure the efficient use of these databases, it is important to understand their differences and similarities. The establishment of more detailed and consistent annotations for the sequence, structure, membrane association, and function of membrane proteins is still required.

GENERAL SIGNIFICANCE

Considering the recent growth of experimentally determined structures, a broad survey and cumulative analysis of the sum of knowledge as presented in the membrane protein structure databases can be helpful to elucidate structures and functions of membrane proteins. We also aim to provide a framework for future research and classification of membrane proteins.

Biotinylated Cell-penetrating Peptides to Study Intracellular Protein-protein Interactions

Here we present a protocol to study intracellular protein-protein interactions that is based on the widely used biotin-avidin pull-down system. The modification presented includes the combination of this technique with cell-penetrating sequences. We propose to design cell-penetrating baits that can be incubated with living cells instead of cell lysates and therefore the interactions found will reflect those that occur within the intracellular context. Connexin43 (Cx43), a protein that forms gap junction channels and hemichannels is down-regulated in high-grade gliomas. The Cx43 region comprising amino acids 266-283 is responsible for the inhibition of the oncogenic activity of c-Src in glioma cells. Here we use TAT as the cell-penetrating sequence, biotin as the pull-down tag and the region of Cx43 comprised between amino acids 266-283 as the target to find intracellular interactions in the hard-to-transfect human glioma stem cells. One of the limitations of the proposed method is that the molecule used as bait could fail to fold properly and, consequently, the interactions found could not be associated with the effect. However, this method can be especially interesting for the interactions involved in signal transduction pathways because they are usually carried out by intrinsically disordered regions and, therefore, they do not require an ordered folding. In addition, one of the advantages of the proposed method is that the relevance of each residue on the interaction can be easily studied. This is a modular system; therefore, other cell-penetrating sequences, other tags, and other intracellular targets can be employed. Finally, the scope of this protocol is far beyond protein-protein interaction because this system can be applied to other bioactive cargoes such as RNA sequences, nanoparticles, viruses or any molecule that can be transduced with cell-penetrating sequences and fused to pull-down tags to study their intracellular mechanism of action.

A c-Src Inhibitor Peptide Based on Connexin43 Exerts Neuroprotective Effects through the Inhibition of Glial Hemichannel Activity

The non-receptor tyrosine kinase c-Src is an important mediator in several signaling pathways related to neuroinflammation. Our previous study showed that cortical injection of kainic acid (KA) promoted a transient increase in c-Src activity in reactive astrocytes surrounding the neuronal lesion. As a cell-penetrating peptide based on connexin43 (Cx43), specifically TAT-Cx43_266–283, inhibits Src activity, we investigated the effect of TAT-Cx43_266–283 on neuronal death promoted by cortical KA injections in adult mice. As expected, KA promoted neuronal death, estimated by the reduction in NeuN-positive cells and reactive gliosis, characterized by the increase in glial fibrillary acidic protein (GFAP) expression. Interestingly, TAT-Cx43_266–283 injected with KA diminished neuronal death and reactive gliosis compared to KA or KA+TAT injections. In order to gain insight into the neuroprotective mechanism, we used in vitro models. In primary cultured neurons, TAT-Cx43_266–283 did not prevent neuronal death promoted by KA, but when neurons were grown on top of astrocytes, TAT-Cx43_266–283 prevented neuronal death promoted by KA. These observations demonstrate the participation of astrocytes in the neuroprotective effect of TAT-Cx43_266–283. Furthermore, the neuroprotective effect was also present in non-contact co-cultures, suggesting the contribution of soluble factors released by astrocytes. As glial hemichannel activity is associated with the release of several factors, such as ATP and glutamate, that cause neuronal death, we explored the participation of these channels on the neuroprotective effect of TAT-Cx43_266–283. Our results confirmed that inhibitors of ATP and NMDA receptors prevented neuronal death in co-cultures treated with KA, suggesting the participation of astrocyte hemichannels in neurotoxicity. Furthermore, TAT-Cx43_266–283 reduced hemichannel activity promoted by KA in neuron-astrocyte co-cultures as assessed by ethidium bromide (EtBr) uptake assay. In fact, TAT-Cx43_266–283 and dasatinib, a potent c-Src inhibitor, strongly reduced the activation of astrocyte hemichannels. In conclusion, our results suggest that TAT-Cx43_266–283 exerts a neuroprotective effect through the reduction of hemichannel activity likely mediated by c-Src in astrocytes. These data unveil a new role of c-Src in the regulation of Cx43-hemichannel activity that could be part of the mechanism by which astroglial c-Src participates in neuroinflammation.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

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.

Cysteine residues in the cytoplasmic carboxy terminus of connexins dictate gap junction plaque stability

Cysteine residues within the cytoplasmic carboxyl-terminus of gap junction–forming proteins are required to stabilize gap junction plaque organization. The stability of gap junction plaque organization can be modified. Gap junction stability may provide a stable supramolecular platform for modulation of gap junction functions.

Gap junctions are cellular contact sites composed of clustered connexin transmembrane proteins that act in dual capacities as channels for direct intercellular exchange of small molecules and as structural adhesion complexes known as gap junction nexuses. Depending on the connexin isoform, the cluster of channels (the gap junction plaque) can be stably or fluidly arranged. Here we used confocal microscopy and mutational analysis to identify the residues within the connexin proteins that determine gap junction plaque stability. We found that stability is altered by changing redox balance using a reducing agent—indicating gap junction nexus stability is modifiable. Stability of the arrangement of connexins is thought to regulate intercellular communication by establishing an ordered supramolecular platform. By identifying the residues that establish plaque stability, these studies lay the groundwork for exploration of mechanisms by which gap junction nexus stability modulates intercellular communication.

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.

Connexin channel and its role in diabetic retinopathy

Diabetic retinopathy is the leading cause of blindness in the working age population. Unfortunately, there is no cure for this devastating ocular complication. The early stage of diabetic retinopathy is characterized by the loss of various cell types in the retina, namely endothelial cells and pericytes. As the disease progresses, vascular leakage, a clinical hallmark of diabetic retinopathy, becomes evident and may eventually lead to diabetic macular edema, the most common cause of vision loss in diabetic retinopathy. Substantial evidence indicates that the disruption of connexin-mediated cellular communication plays a critical role in the pathogenesis of diabetic retinopathy. Yet, it is unclear how altered communication via connexin channel mediated cell-to-cell and cell-to-extracellular microenvironment is linked to the development of diabetic retinopathy. Recent observations suggest the possibility that connexin hemichannels may play a role in the pathogenesis of diabetic retinopathy by allowing communication between cells and the microenvironment. Interestingly, recent studies suggest that connexin channels may be involved in regulating retinal vascular permeability. These cellular events are coordinated at least in part via connexin-mediated intercellular communication and the maintenance of retinal vascular homeostasis. This review highlights the effect of high glucose and diabetic condition on connexin channels and their impact on the development of diabetic retinopathy.

The connexin 43 C-terminus: A tail of many tales

Connexins are chordate gap junction channel proteins that, by enabling direct communication between the cytosols of adjacent cells, create a unique cell signalling network. Gap junctional intercellular communication (GJIC) has important roles in controlling cell growth and differentiation and in tissue development and homeostasis. Moreover, several non-canonical connexin functions unrelated to GJIC have been discovered. Of the 21 members of the human connexin family, connexin 43 (Cx43) is the most widely expressed and studied. The long cytosolic C-terminus (CT) of Cx43 is subject to extensive post-translational modifications that modulate its intracellular trafficking and gap junction channel gating. Moreover, the Cx43 CT contains multiple domains involved in protein interactions that permit crosstalk between Cx43 and cytoskeletal and regulatory proteins. These domains endow Cx43 with the capacity to affect cell growth and differentiation independently of GJIC. Here, we review the current understanding of the regulation and unique functions of the Cx43 CT, both as an essential component of full-length Cx43 and as an independent signalling hub. We highlight the complex regulatory and signalling networks controlled by the Cx43 CT, including the extensive protein interactome that underlies both gap junction channel-dependent and -independent functions. We discuss these data in relation to the recent discovery of the direct translation of specific truncated forms of Cx43. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

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.

Spatio-temporal regulation of connexin43 phosphorylation and gap junction dynamics

Gap junctions are specialized membrane domains containing tens to thousands of intercellular channels. These channels permit exchange of small molecules (<1000Da) including ions, amino acids, nucleotides, metabolites and secondary messengers (e.g., calcium, glucose, cAMP, cGMP, IP₃) between cells. The common reductionist view of these structures is that they are composed entirely of integral membrane proteins encoded by the 21 member connexin human gene family. However, it is clear that the normal physiological function of this structure requires interaction and regulation by a variety of proteins, especially kinases. Phosphorylation is capable of directly modulating connexin channel function but the most dramatic effects on gap junction activity occur via the organization of the gap junction structures themselves. This is a direct result of the short half-life of the primary gap junction protein, connexin, which requires them to be constantly assembled, remodeled and turned over. The biological consequences of this remodeling are well illustrated during cardiac ischemia, a process wherein gap junctions are disassembled and remodeled resulting in arrhythmia and ultimately heart failure. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Recruitment of RNA molecules by connexin RNA-binding motifs: Implication in RNA and DNA transport through microvesicles and exosomes

Connexins (Cxs) are integral membrane proteins that form high-conductance plasma membrane channels, allowing communication from cell to cell (via gap junctions) and from cells to the extracellular environment (via hemichannels). Initially described for their role in joining excitable cells (nerve and muscle), gap junctions (GJs) are found between virtually all cells in solid tissues and are essential for functional coordination by enabling the direct transfer of small signalling molecules, metabolites, ions, and electrical signals from cell to cell. Several studies have revealed diverse channel-independent functions of Cxs, which include the control of cell growth and tumourigenicity. Connexin43 (Cx43) is the most widespread Cx in the human body. The myriad roles of Cx43 and its implication in the development of disorders such as cancer, inflammation, osteoarthritis and Alzheimer's disease have given rise to many novel questions. Several RNA- and DNA-binding motifs were predicted in the Cx43 and Cx26 sequences using different computational methods. This review provides insights into new, ground-breaking functions of Cxs, highlighting important areas for future work such as transfer of genetic information through extracellular vesicles. We discuss the implication of potential RNA- and DNA-binding domains in the Cx43 and Cx26 sequences in the cellular communication and control of signalling pathways.

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.