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

Impairment of α-tubulin and F-actin interactions of GJB3 induces aneuploidy in urothelial cells and promotes bladder cancer cell invasion

BACKGROUND

We have previously identified an unsuspected role for GJB3 showing that the deficiency of this connexin protein induces aneuploidy in human and murine cells and accelerates cell transformation as well as tumor formation in xenograft models. The molecular mechanisms by which loss of GJB3 leads to aneuploidy and cancer initiation and progression remain unsolved.

METHODS

GJB3 expression levels were determined by RT-qPCR and Western blot. The consequences of GJB3 knockdown on genome instability were assessed by metaphase chromosome counting, multinucleation of cells, by micronuclei formation and by the determination of spindle orientation. Interactions of GJB3 with α-tubulin and F-actin was analyzed by immunoprecipitation and immunocytochemistry. Consequences of GJB3 deficiency on microtubule and actin dynamics were measured by live cell imaging and fluorescence recovery after photobleaching experiments, respectively. Immunohistochemistry was used to determine GJB3 levels on human and murine bladder cancer tissue sections. Bladder cancer in mice was chemically induced by BBN-treatment.

RESULTS

We find that GJB3 is highly expressed in the ureter and bladder epithelium, but it is downregulated in invasive bladder cancer cell lines and during tumor progression in both human and mouse bladder cancer. Downregulation of GJB3 expression leads to aneuploidy and genomic instability in karyotypically stable urothelial cells and experimental modulation of GJB3 levels alters the migration and invasive capacity of bladder cancer cell lines. Importantly, GJB3 interacts both with α-tubulin and F-actin. The impairment of these interactions alters the dynamics of these cytoskeletal components and leads to defective spindle orientation.

CONCLUSION

We conclude that deregulated microtubule and actin dynamics have an impact on proper chromosome separation and tumor cell invasion and migration. Consequently, these observations indicate a possible role for GJB3 in the onset and spreading of bladder cancer and demonstrate a molecular link between enhanced aneuploidy and invasive capacity cancer cells during tumor cell dissemination.

Simulation of gap junction formation reveals critical role of Cys disulfide redox state in connexin hemichannel docking

Video Abstract.

Diversity of Intercellular Communication Modes: A Cancer Biology Perspective

From the moment a cell is on the path to malignant transformation, its interaction with other cells from the microenvironment becomes altered. The flow of molecular information is at the heart of the cellular and systemic fate in tumors, and various processes participate in conveying key molecular information from or to certain cancer cells. For instance, the loss of tight junction molecules is part of the signal sent to cancer cells so that they are no longer bound to the primary tumors and are thus free to travel and metastasize. Upon the targeting of a single cell by a therapeutic drug, gap junctions are able to communicate death information to by-standing cells. The discovery of the importance of novel modes of cell-cell communication such as different types of extracellular vesicles or tunneling nanotubes is changing the way scientists look at these processes. However, are they all actively involved in different contexts at the same time or are they recruited to fulfill specific tasks? What does the multiplicity of modes mean for the overall progression of the disease? Here, we extend an open invitation to think about the overall significance of these questions, rather than engage in an elusive attempt at a systematic repertory of the mechanisms at play.

Evaluation of Tyrosine Kinase Inhibitors Loaded Injectable Hydrogels for Improving Connexin43 Gap Junction Intercellular Communication

Myocardial infarction (MI) is one of the leading causes of death in the developed world, and the loss of cardiomyocytes plays a critical role in the pathogenesis of heart failure. Implicated in this process is a decrease in gap junction intercellular communication due to remodeling of Connexin43 (Cx43). We previously identified that intraperitoneal injection of the Pyk2 inhibitor PF4618433 reduced infarct size, maintained Cx43 at the intercalated disc in left ventricle hypertrophic myocytes, and improved cardiac function in an MI animal model of heart failure. With the emergence of injectable hydrogels as a therapeutic toward the regeneration of cardiac tissue after MI, here, we provide proof of concept that the release of tyrosine kinase inhibitors from hydrogels could have beneficial effects on cardiomyocytes. We developed an injectable hydrogel consisting of thiolated hyaluronic acid and P123-maleimide micelles that can incorporate PF4618433 as well as the Src inhibitor Saracatinib and achieved sustained release (of note, Src activates Pyk2). Using neonatal rat ventricular myocytes in the presence of a phorbol ester, endothelin-1, or phenylephrine to stimulate cardiac hypertrophy, the release of PF4618433 from the hydrogel had the same ability to decrease Cx43 tyrosine phosphorylation and maintain Cx43 localization at the plasma membrane as when directly added to the growth media. Additional beneficial effects included decreases in apoptosis, the hypertrophic marker atrial natriuretic peptide (ANP), and serine kinases upregulated in hypertrophy. Finally, the presence of both PF4618433 and Saracatinib further decreased the level of ANP and apoptosis than each inhibitor alone, suggesting that a combinatorial approach may be most beneficial. These findings provide the groundwork to test if tyrosine kinase inhibitor release from hydrogels will have a beneficial effect in an animal model of MI-induced heart failure.

Mapping Molecular Interaction Interface Between Diaphanous Formin-2 and Neuron-Specific Drebrin A

Actin cytoskeleton is critical for neuronal shape and function. Drebrin and formins are key regulators of neuronal actin networks. Neuron-specific drebrin A is highly enriched in dendritic spines (postsynaptic terminals) of mature excitatory neurons. Decreased levels of drebrin in dendritic spines is a hallmark of Alzheimer's disease, epilepsy, and other complex disorders, which calls for better understanding of its regulatory functions. Drebrin A was previously shown to inhibit actin nucleation and bundling by the diaphanous formin-2 (mDia2) - an actin nucleator that is involved in the initiation of dendritic spines. Characterization of the molecular binding interface between mDia2 and drebrin is necessary to better understand the functional consequences of this interaction and its biological relevance. Prior work suggested a multi-pronged interface between mDia2 and drebrin, which involves both N-terminal and C-terminal regions of the drebrin molecule. Here we used mass spectrometry analysis, deletion mutagenesis, and an array of synthetic peptides of neuronal drebrin A to map its formin-binding interface. The mDia2-interacting interface on drebrin was narrowed down to three highly conserved 9-16 residue sequences that were used to identify some of the key residues involved in this interaction. Deletion of the C-terminal region of drebrin greatly reduces its binding to mDia2 and the extent of its inhibition of formin-driven actin assembly. Moreover, our experiments with formins from different subfamilies showed that drebrin is a specific rather than general inhibitor of these proteins. This work contributes to a molecular level understanding of the formin-drebrin interaction and will help to unravel its biological significance.

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.

Cytomembrane Trafficking Pathways of Connexin 26, 30, and 43

The connexin gene family is the most prevalent gene that contributes to hearing loss. Connexins 26 and 30, encoded by GJB2 and GJB6, respectively, are the most abundantly expressed connexins in the inner ear. Connexin 43, which is encoded by GJA1, appears to be widely expressed in various organs, including the heart, skin, the brain, and the inner ear. The mutations that arise in GJB2, GJB6, and GJA1 can all result in comprehensive or non-comprehensive genetic deafness in newborns. As it is predicted that connexins include at least 20 isoforms in humans, the biosynthesis, structural composition, and degradation of connexins must be precisely regulated so that the gap junctions can properly operate. Certain mutations result in connexins possessing a faulty subcellular localization, failing to transport to the cell membrane and preventing gap junction formation, ultimately leading to connexin dysfunction and hearing loss. In this review, we provide a discussion of the transport models for connexin 43, connexins 30 and 26, mutations affecting trafficking pathways of these connexins, the existing controversies in the trafficking pathways of connexins, and the molecules involved in connexin trafficking and their functions. This review can contribute to a new way of understanding the etiological principles of connexin mutations and finding therapeutic strategies for hereditary deafness.

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.

Regulation of Cx43 Gap Junction Intercellular Communication by Bruton's Tyrosine Kinase and Interleukin-2-Inducible T-Cell Kinase

T and B cell receptor signaling involves the activation of Akt, MAPKs, and PKC as well as an increase in intracellular Ca²⁺ and calmodulin activation. While these coordinate the rapid turnover of gap junctions, also implicated in this process is Src, which is not activated as part of T and B cell receptor signaling. An in vitro kinase screen identified that Bruton's tyrosine kinase (BTK) and interleukin-2-inducible T-cell kinase (ITK) phosphorylate Cx43. Mass spectroscopy revealed that BTK and ITK phosphorylate Cx43 residues Y247, Y265, and Y313, which are identical to the residues phosphorylated by Src. Overexpression of BTK or ITK in the HEK-293T cells led to increased Cx43 tyrosine phosphorylation as well as decreased gap junction intercellular communication (GJIC) and Cx43 membrane localization. In the lymphocytes, activation of the B cell receptor (Daudi cells) or T cell receptor (Jurkat cells) increased the BTK and ITK activity, respectively. While this led to increased tyrosine phosphorylation of Cx43 and decreased GJIC, the cellular localization of Cx43 changed little. We have previously identified that Pyk2 and Tyk2 also phosphorylate Cx43 at residues Y247, Y265, and Y313 with a similar cellular fate to that of Src. With phosphorylation critical to Cx43 assembly and turnover, and kinase expression varying between different cell types, there would be a need for different kinases to achieve the same regulation of Cx43. The work presented herein suggests that in the immune system, ITK and BTK have the capacity for the tyrosine phosphorylation of Cx43 to alter the gap junction function in a similar manner as Pyk2, Tyk2, and Src.

Rat Hepatocytes Mitigate Cadmium Toxicity by Forming Annular Gap Junctions and Degrading Them via Endosome-Lysosome Pathway

Gap junction protein connexin 43 (Cx43) plays a critical role in gap junction communication in rat hepatocytes. However, those located between hepatocytes are easily internalized following exposure to poisons. Herein, we investigated the potential of buffalo rat liver 3A (BRL 3A) cells to generate annular gap junctions (AGJs) proficient at alleviating cadmium (Cd) cytotoxic injury through degradation via an endosome-lysosome pathway. Our results showed that Cd-induced damage of liver microtubules promoted Cx43 internalization and increased Cx43 phosphorylation at Ser373 site. Furthermore, we established that Cd induced AGJs generation in BRL 3A cells, and AGJs were subsequently degraded through the endosome-lysosome pathway. Overall, our results suggested that Cx43 internalization and the generation of AGJs were cellular protective mechanisms to alleviate Cd toxicity in rat hepatocytes.

Simple and automatic monitoring of cancer cell invasion into an epithelial monolayer using label-free holographic microscopy

The invasiveness of cancer cells describes the metastasizing capability of a primary tumor. The straightforward detection and quantification of cancer cell invasion are important to predict the survival rate of a cancer patient and to test how anti-cancer compounds influence cancer progression. Digital holographic microscopy based M4 Holomonitor (HM) is a technique that allows the label-free monitoring of cell morphological and kinetical parameters in real-time. Here, a fully confluent epithelial monolayer derived from the African green monkey kidney (Vero) on a gelatin-coated surface was established, then HeLa cells were seeded on top of the monolayer, and their behavior was monitored for 24 h using HM. Several cancer cells showing invasiveness were detected during this period, while other HeLa cells did not show any signs of aggressivity. It was demonstrated that the invasion of single cancer cells is soundly observable and also quantifiable through monitoring parameters such as phase shift, optical volume, area, and motility, which parameters can easily be obtained and processed automatically. Based on the experimental data, the invasion speed of cancer cells entering the epithelial layer can be defined as the shrinking of detected single-cell volume per unit time. The invasion speed and its correlation with cell migration parameters were analyzed in depth. A clear linear relationship between migration and invasion speed was found, cancer cells with stronger migration have slower invasion speed. These results not only describe the effect of how cancer cells invade the underlying monolayer in contrast to non-invasive HeLa cells, but could help in future research to optimize drugs affecting cell invasibility in a fully automated, label-free and high-throughput manner.

Cx43 upregulation in HUVECs under stretch via TGF-β1 and cytoskeletal network

Many physiological and pathophysiological processes in cells or tissues are involved in mechanical stretch, which induces the gap junction gene expression and cytokine TGF beta changes. However, the underlying mechanisms of the gap junction gene expression remain unknown. Here, we showed that the mRNA and protein levels of Cx43 in human umbilical vein endothelial cells (HUVECs) were significantly increased after 24 h stretch stimulation, and TGF beta1 (not TGF beta2) expression was also upregulated. Administration of TGF beta1 into HUVECs without stretch also induced upregulation of Cx43 expression. However, SB431542, a specific inhibitor of the TGF beta1 receptor, blocked the Cx43 protein upregulation caused by TGF beta1. Further, the increase of Cx43 protein expression under the stretch condition was partially blocked by SB431542; it was also partially blocked by simultaneous administration of anti-TGF beta1 monoclonal neutralization antibody. Importantly, the upregulation of Cx43 induced by stretch was blocked by the administration of actin and microtubule inhibitors, while NEDD4, a key element in mediating Cx43 protein ubiquitination and degradation, was not changed under the stretch condition. In conclusion, upregulation of Cx43 expression under the 24 h stretch condition is mediated via TGF beta1 receptor signaling pathway, and it also involves the actin and microtubule cytoskeletal network.

miR-485 inhibits histone deacetylase HDAC5, HIF1α and PFKFB3 expression to alleviate epilepsy in cellular and rodent models

We investigated the role of microRNA (miR)-485 and its downstream signaling molecules on mediating epilepsy in cellular and rat models. We established a cellular epilepsy model by exposing hippocampal neurons to magnesium and a rat model by treating ICR mice with lithium chloride (127 mg/kg) and pilocarpine (30 mg/kg). We confirmed that miR-485 could bind and inhibit histone deacetylase 5 (HDAC5) and then measured expression of miR-485 and in mice and cells. Cells were transfected with overexpression or knockdown of miR-485, HDAC5, hypoxia-inducible factor-1alpha (HIF1α), or 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 enzyme (PFKFB3) to verify their roles in apoptosis, oxidative stress, and inflammation in epileptic hippocampal neurons. Binding relationship between miR-485, HDAC5, HIF1α, and PFKFB3 was verified. Oxidative stress and inflammation marker levels in epilepsy model mice were assessed. miR-485 was downregulated and HDAC5 was upregulated in cell and animal model of epilepsy. Seizure, neuronal apoptosis, oxidative stress (increased SOD and GSH-Px expression and decreased MDA and 8-OHdG expression) and inflammation (reduced IL-1β, TNF-α, and IL-6 expression) were reduced by miR-485 in epileptic cells. HIF1α and PFKFB3 expression was reduced by HDAC5 knockdown in cells, which was recapitulated in vivo. Thus, miR-485 alleviates neuronal damage and epilepsy by inhibiting HDAC5, HIF1α, and PFKFB3.

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

TMAO Aggregates Neurological Damage Following Ischemic Stroke by Promoting Reactive Astrocytosis and Glial Scar Formation via the Smurf2/ALK5 Axis

Ischemic stroke has been reported to cause significant changes to memory, thinking, and behavior. Intriguingly, recently reported studies have indicated the association of Trimethylamine N-oxide (TMAO) with the acute phase of ischemic stroke. However, the comprehensive underlying mechanism remained unknown. The objective of the present study was to investigate the association between TMAO and recovery of neurological function after ischemic stroke. For this purpose, a middle cerebral artery occlusion/reperfusion (MCAO/R) rat model was established and treated with TMAO or/and sh-ALK5, followed by the neurological function evaluation. Behaviors of rats were observed through staircase and cylinder tests. Moreover, the expression of Smurf2 and ALK5 was detected by immunohistochemistry while expression of GFAP, Neurocan, and Phosphacan in brain tissues was determined by immunofluorescence. Thereafter, gain- and loss-of-function assays in astrocytes, the proliferation, viability, and migration were evaluated by the EdU, CCK-8, and Transwell assays. Besides, Smurf2 mRNA expression was determined by the RT-qPCR, whereas, Smurf2, ALK5, GFAP, Neurocan, and Phosphacan expression was evaluated by the Western blotting. Finally, the interaction of Smurf2 with ALK5 and ALK5 ubiquitination was assessed by the co-immunoprecipitation. Notably, our results showed that TMAO promoted the proliferation of reactive astrocyte and formation of glial scar in MCAO/R rats. However, this effect was abolished by the Smurf2 overexpression or ALK5 silencing. We further found that TMAO upregulated the ALK5 expression by inhibiting the ubiquitination role of Smurf2. Overexpression of ALK5 reversed the inhibitory effect of Smurf2 on astrocyte proliferation, migration, and viability. Collectively, our work identifies the evolutionarily TMAO/Smurf2/ALK5 signaling as a major genetic factor in the control of reactive astrocyte proliferation and glial scar formation in ischemic stroke, thus laying a theoretical foundation for the identification of ischemic stroke.

Drebrin regulates cytoskeleton dynamics in migrating neurons through interaction with CXCR4

Stromal cell-derived factor-1 (SDF-1) and chemokine receptor type 4 (CXCR4) are regulators of neuronal migration (e.g., GnRH neurons, cortical neurons, and hippocampal granule cells). However, how SDF-1/CXCR4 alters cytoskeletal components remains unclear. Developmentally regulated brain protein (drebrin) stabilizes actin polymerization, interacts with microtubule plus ends, and has been proposed to directly interact with CXCR4 in T cells. The current study examined, in mice, whether CXCR4 under SDF-1 stimulation interacts with drebrin to facilitate neuronal migration. Bioinformatic prediction of protein-protein interaction highlighted binding sites between drebrin and crystallized CXCR4. In migrating GnRH neurons, drebrin, CXCR4, and the microtubule plus-end binding protein EB1 were localized close to the cell membrane. Coimmunoprecipitation (co-IP) confirmed a direct interaction between drebrin and CXCR4 using wild-type E14.5 whole head and a GnRH cell line. Analysis of drebrin knockout (DBN1 KO) mice showed delayed migration of GnRH cells into the brain. A decrease in hippocampal granule cells was also detected, and co-IP confirmed a direct interaction between drebrin and CXCR4 in PN4 hippocampi. Migration assays on primary neurons established that inhibiting drebrin (either pharmacologically or using cells from DBN1 KO mice) prevented the effects of SDF-1 on neuronal movement. Bioinformatic prediction then identified binding sites between drebrin and the microtubule plus end protein, EB1, and super-resolution microscopy revealed decreased EB1 and drebrin coexpression after drebrin inhibition. Together, these data show a mechanism by which a chemokine, via a membrane receptor, communicates with the intracellular cytoskeleton in migrating neurons during central nervous system development.

Betulinic acid attenuated bleomycin-induced pulmonary fibrosis by effectively intervening Wnt/β-catenin signaling

BACKGROUND

Idiopathic pulmonary fibrosis (IPF) is a fatal and progressive fibrotic lung disease lacking a validated and effective therapy. Aberrant activation of the Wnt/β-catenin signaling cascade plays the key role in the pathogenesis of IPF. Betulinic acid is a natural pentacyclic triterpenoid molecule that has excellent antitumor and antiviral activities.

HYPOTHESIS

We hypothesized that BA has an anti-pulmonary fibrosis effect mediated by the suppression of the Wnt/β-catenin pathway. Study design Pulmonary fibrosis markers were detected in vitro and in vivo to confirm the antifibrotic effect of BA. The Wnt/β-catenin pathway-related proteins were overexpressed to determine the effect of BA on Wnt signaling.

METHODS AND RESULTS

BA dose-dependently inhibited Wnt3a-induced fibroblast activation in vitro. Moreover, BA decreased Wnt3a- and LiCl-induced transcriptional activity, as assessed by the TOPFlash assay in fibroblasts, and repressed the expression of the Wnt target genes cyclin D1, axin 2, and S100A4. Further investigation indicated that BA restrained the nuclear accumulation of β-catenin, mainly by increasing the phospho-β-catenin ratio (S33/S37/T41 and S45), inhibited the phosphorylation of DVL2 and LRP, and decreased the levels of Wnt3a and LRP6. In agreement with the results of the in vitro assays, the in vivo experiments indicated that BA significantly decreased bleomycin-induced pulmonary fibrosis in mice and suppressed myofibroblast activation by inhibiting Wnt/β-catenin signaling.

CONCLUSION

BA may directly interfere with the Wnt/β-catenin pathway to subsequently repress myofibroblast activation and pulmonary fibrosis.

Role of the Connexin C-terminus in skin pattern formation of Zebrafish

Background

Zebrafish display a striped skin pattern on their body; two types of connexins, namely, Connexin39.4 (Cx39.4) and Connexin41.8 (Cx41.8), are involved in stripe pattern formation. Herein, we investigated the role of the C-terminal (CT) domains of Cx39.4 and Cx41.8 in vivo and in vitro.

Methods

To investigate the role of CT domains in vivo, we established transgenic zebrafish lines expressing the CT-domain-modified connexin series in pigmented cells and observed skin patterns in fish. To investigate the role of the CT domains in vitro, we expressed the CT-domain modified connexin series in Neuro-2a (N2a) cells and calculated the plaque formation frequency.

Results

The overexpression of Cx39.4 lacking a CT domain produced skin patterns similar to that produced by full-length Cx39.4 in the cx39.4 ^-/- mutant and in cx39.4 and cx41.8 double-knockout mutant zebrafish. Fluorescence-protein-fused CT-domain-modified Cx39.4 formed gap junction plaques between N2a cells. The overexpression of CT-truncated Cx41.8 rescued the mutant phenotype in the cx41.8 ^-/- mutant but did not function in the double knockout zebrafish. Fluorescence-protein-fused CT-truncated Cx41.8 hardly formed plaques between N2a cells without Cx39.4 but formed gap junction plaques when co-expressed with Cx39.4.

Conclusions

The CT domain of Cx39.4 is not required for protein function, at least in the pigment cells of zebrafish. However, the need for the CT domain of Cx41.8 depends on Cx39.4 expression.

General significance

These results provide evidence for the interactions between Cx39.4 and Cx41.8 in pigment cells of zebrafish and suggest that at least one connexin must have a CT domain.

Cx43 and the Actin Cytoskeleton: Novel Roles and Implications for Cell-Cell Junction-Based Barrier Function Regulation

Barrier function is a vital homeostatic mechanism employed by epithelial and endothelial tissue. Diseases across a wide range of tissue types involve dynamic changes in transcellular junctional complexes and the actin cytoskeleton in the regulation of substance exchange across tissue compartments. In this review, we focus on the contribution of the gap junction protein, Cx43, to the biophysical and biochemical regulation of barrier function. First, we introduce the structure and canonical channel-dependent functions of Cx43. Second, we define barrier function and examine the key molecular structures fundamental to its regulation. Third, we survey the literature on the channel-dependent roles of connexins in barrier function, with an emphasis on the role of Cx43 and the actin cytoskeleton. Lastly, we discuss findings on the channel-independent roles of Cx43 in its associations with the actin cytoskeleton and focal adhesion structures highlighted by PI3K signaling, in the potential modulation of cellular barriers. Mounting evidence of crosstalk between connexins, the cytoskeleton, focal adhesion complexes, and junctional structures has led to a growing appreciation of how barrier-modulating mechanisms may work together to effect solute and cellular flux across tissue boundaries. This new understanding could translate into improved therapeutic outcomes in the treatment of barrier-associated diseases.

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.

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.

Connexins: Key Players in the Control of Vascular Plasticity and Function

Of the 21 members of the connexin family, 4 (Cx37, Cx40, Cx43, and Cx45) are expressed in the endothelium and/or smooth muscle of intact blood vessels to a variable and dynamically regulated degree. Full-length connexins oligomerize and form channel structures connecting the cytosol of adjacent cells (gap junctions) or the cytosol with the extracellular space (hemichannels). The different connexins vary mainly with regard to length and sequence of their cytosolic COOH-terminal tails. These COOH-terminal parts, which in the case of Cx43 are also translated as independent short isoforms, are involved in various cellular signaling cascades and regulate cell functions. This review focuses on channel-dependent and -independent effects of connexins in vascular cells. Channels play an essential role in coordinating and synchronizing endothelial and smooth muscle activity and in their interplay, in the control of vasomotor actions of blood vessels including endothelial cell reactivity to agonist stimulation, nitric oxide-dependent dilation, and endothelial-derived hyperpolarizing factor-type responses. Further channel-dependent and -independent roles of connexins in blood vessel function range from basic processes of vascular remodeling and angiogenesis to vascular permeability and interactions with leukocytes with the vessel wall. Together, these connexin functions constitute an often underestimated basis for the enormous plasticity of vascular morphology and function enabling the required dynamic adaptation of the vascular system to varying tissue demands.

Identification of serine residues in the connexin43 carboxyl tail important for BCR-mediated spreading of B-lymphocytes

B-lymphocytes recognize antigen via B-cell antigen receptors (BCRs). This binding induces signaling, leading to B-cell activation, proliferation and differentiation. Early events of BCR signaling include reorganization of actin and membrane spreading, which facilitates increased antigen gathering. We have previously shown that the gap junction protein connexin43 (Cx43; also known as GJA1) is phosphorylated upon BCR signaling, and its carboxyl tail (CT) is important for BCR-mediated spreading. Here, specific serine residues in the Cx43 CT that are phosphorylated following BCR stimulation were identified. A chimeric protein containing the extracellular and transmembrane domains of CD8 fused to the Cx43 CT was sufficient to support cell spreading. Cx43 CT truncations showed that the region between amino acids 246-307 is necessary for B-cell spreading. Site-specific serine-to-alanine mutations (S255A, S262A, S279A and S282A) resulted in differential effects on both BCR signaling and BCR-mediated spreading. These serine residues can serve as potential binding sites for actin remodeling mediators and/or BCR signaling effectors; therefore, our results may reflect unique roles for each of these serines in terms of linking the Cx43 CT to actin remodeling.

Connexins in Astrocyte Migration

Astrocytes have long been considered the supportive cells of the central nervous system, but during the last decades, they have gained much more attention because of their active participation in the modulation of neuronal function. For example, after brain damage, astrocytes become reactive and undergo characteristic morphological and molecular changes, such as hypertrophy and increase in the expression of glial fibrillary acidic protein (GFAP), in a process known as astrogliosis. After severe damage, astrocytes migrate to the lesion site and proliferate, which leads to the formation of a glial scar. At this scar-forming stage, astrocytes secrete many factors, such as extracellular matrix proteins, cytokines, growth factors and chondroitin sulfate proteoglycans, stop migrating, and the process is irreversible. Although reactive gliosis is a normal physiological response that can protect brain cells from further damage, it also has detrimental effects on neuronal survival, by creating a hostile and non-permissive environment for axonal repair. The transformation of astrocytes from reactive to scar-forming astrocytes highlights migration as a relevant regulator of glial scar formation, and further emphasizes the importance of efficient communication between astrocytes in order to orchestrate cell migration. The coordination between astrocytes occurs mainly through Connexin (Cx) channels, in the form of direct cell-cell contact (gap junctions, GJs) or contact between the extracellular matrix and the astrocytes (hemichannels, HCs). Reactive astrocytes increase the expression levels of several proteins involved in astrocyte migration, such as α_vβ₃ Integrin, Syndecan-4 proteoglycan, the purinergic receptor P2X7, Pannexin1, and Cx43 HCs. Evidence has indicated that Cx43 HCs play a role in regulating astrocyte migration through the release of small molecules to the extracellular space, which then activate receptors in the same or adjacent cells to continue the signaling cascades required for astrocyte migration. In this review, we describe the communication of astrocytes through Cxs, the role of Cxs in inflammation and astrocyte migration, and discuss the molecular mechanisms that regulate Cx43 HCs, which may provide a therapeutic window of opportunity to control astrogliosis and the progression of neurodegenerative diseases.

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.

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.

Connexins in the control of vasomotor function

Vascular endothelial cells, as well as smooth muscle cells, show heterogeneity with regard to their receptor expression and reactivity. For the vascular wall to act as a functional unit, the various cells' responses require integration. Such an integration is not only required for a homogeneous response of the vascular wall, but also for the vasomotor behaviour of consecutive segments of the microvascular arteriolar tree. As flow resistances of individual sections are connected in series, sections require synchronization and coordination to allow effective changes of conductivity and blood flow. A prerequisite for the local coordination of individual vascular cells and different sections of an arteriolar tree is intercellular communication. Connexins are involved in a dual manner in this coordination. (i) By forming gap junctions between cells, they allow an intercellular exchange of signalling molecules and electrical currents. In particular, the spread of electrical currents allows for coordination of cell responses over longer distances. (ii) Connexins are able to interact with other proteins to form signalling complexes. In this way, they can modulate and integrate individual cells' responses also in a channel-independent manner. This review outlines mechanisms allowing the vascular connexins to exert their coordinating function and to regulate the vasomotor reactions of blood vessels both locally, and in vascular networks. Wherever possible, we focus on the vasomotor behaviour of small vessels and arterioles which are the main vessels determining vascular resistance, blood pressure and local blood flow.

Cancer Connectors: Connexins, Gap Junctions, and Communication

Despite concerted clinical and research efforts, cancer is a leading cause of death worldwide. Surgery, radiation, and chemotherapy have remained the most common standard-of-care strategies against cancer for decades. However, the side effects of these therapies demonstrate the need to investigate adjuvant novel treatment modalities that minimize the harm caused to healthy cells and tissues. Normal and cancerous cells require communication amongst themselves and with their surroundings to proliferate and drive tumor growth. It is vital to understand how intercellular and external communication impacts tumor cell malignancy. To survive and grow, tumor cells, and their normal counterparts utilize cell junction molecules including gap junctions (GJs), tight junctions, and adherens junctions to provide contact points between neighboring cells and the extracellular matrix. GJs are specialized structures composed of a family of connexin proteins that allow the free diffusion of small molecules and ions directly from the cytoplasm of adjacent cells, without encountering the extracellular milieu, which enables rapid, and coordinated cellular responses to internal and external stimuli. Importantly, connexins perform three main cellular functions. They enable direct gap junction intercellular communication (GJIC) between cells, form hemichannels to allow cell communication with the extracellular environment, and serve as a site for protein-protein interactions to regulate signaling pathways. Connexins themselves have been found to promote tumor cell growth and invasiveness, contributing to the overall tumorigenicity and have emerged as attractive anti-tumor targets due to their functional diversity. However, connexins can also serve as tumor suppressors, and therefore, a complete understanding of the roles of the connexins and GJs in physiological and pathophysiological conditions is needed before connexin targeting strategies are applied. Here, we discuss how the three aspects of connexin function, namely GJIC, hemichannel formation, and connexin-protein interactions, function in normal cells, and contribute to tumor cell growth, proliferation, and death. Finally, we discuss the current state of anti-connexin therapies and speculate which role may be most amenable for the development of targeting strategies.

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.

Tubulin-Na+ , K + -ATPase interaction: Involvement in enzymatic regulation and cellular function

A new function for tubulin was described by our laboratory: acetylated tubulin forms a complex with Na⁺ ,K ⁺ -ATPase (NKA) and inhibits its activity. This process was shown to be a regulatory factor of physiological importance in cultured cells, human erythrocytes, and several rat tissues. Formation of the acetylated tubulin-NKA complex is reversible. We demonstrated that in cultured cells, high concentrations of glucose induce translocation of acetylated tubulin from cytoplasm to plasma membrane with a consequent inhibition of NKA activity. This effect is reversed by adding glutamate, which is coctransported to the cell with Na ⁺ . Another posttranslational modification of tubulin, detyrosinated tubulin, is also involved in the regulation of NKA activity: it enhances the NKA inhibition induced by acetylated tubulin. Manipulation of the content of these modifications of tubulin could work as a new strategy to maintain homeostasis of Na ⁺ and K ⁺ , and to regulate a variety of functions in which NKA is involved, such as osmotic fragility and deformability of human erythrocytes. The results summarized in this review show that the interaction between tubulin and NKA plays an important role in cellular physiology, both in the regulation of Na ⁺ /K ⁺ homeostasis and in the rheological properties of the cells, which is mechanically different from other roles reported up to now.

Structural and Intermolecular Associations Between Connexin36 and Protein Components of the Adherens Junction-Neuronal Gap Junction Complex

Intimate structural and functional relationships between gap junctions and adherens junctions have been demonstrated in peripheral tissues, but have not been thoroughly examined in the central nervous system, where adherens junctions are often found in close proximity to neuronal gap junctions. Here, we used immunofluorescence approaches to document the localization of various protein components of adherens junctions in relation to those that we have previously reported to occur at electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36). The adherens junction constituents N-cadherin and nectin-1 were frequently found to localize near or overlap with Cx36-containing gap junctions in several brain regions examined. This was also true of the adherens junction-associated proteins α-catenin and β-catenin, as well as the proteins zonula occludens-1 and AF6 (aka, afadin) that were reported constituents of both adherens junctions and gap junctions. The deployment of the protein constituents of these junctions was especially striking at somatic contacts between primary afferent neurons in the mesencephalic trigeminal nucleus (MesV), where the structural components of adherens junctions appeared to be maintained in connexin36 null mice. These results support emerging views concerning the multi-molecular composition of electrical synapses and raise possibilities for various structural and functional protein-protein interactions at what now can be considered the adherens junction-neuronal gap junction complex. Further, the results point to intracellular signaling pathways that could potentially contribute to the assembly, maintenance and turnover of this complex, as well as to the dynamic nature of neuronal communication at electrical synapses.

Connexin 43 C-terminus directly inhibits the hyperphosphorylation of Akt/ERK through protein-protein interactions in glioblastoma

Although the deregulation of epidermal growth factor receptor (EGFR) is one of the most common molecular mechanisms of glioblastoma (GBM) pathogenesis, the efficacy of anti-EGFR therapy is limited. Additionally, response to anti-EGFR therapy is not solely dependent on EGFR expression and is more promising in patients with reduced activity of EGFR downstream signaling pathways. Thus, there is considerable interest in identifying the compensatory regulatory factors of the EGFR signaling pathway to improve the efficacy of anti-EGFR therapies for GBM. In this study, we confirmed the low efficacy of EGFR inhibitors in GBM patients by meta-analysis. We then identified a negative correlation between connexin 43 (Cx43) expression and Akt/ERK activation, which was caused by the direct interactions between Akt/ERK and Cx43. By comparing the interactions between Akt/ERK and Cx43 using a series of truncated and mutated Cx43 variants, we revealed that the residues T286/A305/Q308/Y313 and S272/S273 at the carboxy terminus of Cx43 are critical for its binding with Akt and ERK, respectively. In addition, Kaplan-Meier survival analysis using data from The Cancer Genome Atlas datasets indicated that the expression of Cx43 significantly improved the prognosis of GBM patients who express EGFR. Together, our results suggested that Cx43 acts as an inhibitory regulator of the activation of growth factor receptor downstream signaling pathways, indicating the potential of Cx43 as a marker for predicting the efficacy of EGFR inhibitor treatments for GBM. Targeting the interaction between the carboxy terminus of Cx43 and Akt/ERK could be an effective therapeutic strategy against GBM.

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.

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.

Gap junctions contribute to anchorage-independent clustering of breast cancer cells

BACKGROUND

Cancer cell aggregation is a key process involved in the formation of clusters of circulating tumor cells. We previously reported that cell-cell adhesion proteins, such as E-cadherin, and desmosomal proteins are involved in cell aggregation to form clusters independently of cell migration or matrix adhesion. Here, we investigated the involvement of gap junction intercellular communication (GJIC) during anchorage-independent clustering of MCF7 breast adenocarcinoma cells.

METHODS

We used live cell image acquisition and analysis to monitor the kinetics of MCF7 cell clustering in the presence/absence of GJIC pharmacological inhibitors and to screen a LOPAC® bioactive compound library. We also used a calcein transfer assay and flow cytometry to evaluate GJIC involvement in cancer cell clustering.

RESULTS

We first demonstrated that functional GJIC are established in the early phase of cancer cell aggregation. We then showed that pharmacological inhibition of GJIC using tonabersat and meclofenamate delayed MCF7 cell clustering and reduced calcein transfer. We also found that brefeldin A, an inhibitor of vesicular trafficking, which we identified by screening a small compound library, and latrunculin A, an actin cytoskeleton-disrupting agent, both impaired MCF7 cell clustering and calcein transfer.

CONCLUSIONS

Our results demonstrate that GJIC are involved from the earliest stages of anchorage-independent cancer cell aggregation. They also give insights into the regulatory mechanisms that could modulate the formation of clusters of circulating tumor cells.

Perineurium-like sheath derived from long-term surviving mesenchymal stem cells confers nerve protection to the injured spinal cord

The functional multipotency enables mesenchymal stem cells (MSCs) promising translational potentials in treating spinal cord injury (SCI). Yet the fate of MSCs grafted into the injured spinal cord has not been fully elucidated even in preclinical studies, rendering concerns of their safety and genuine efficacy. Here we used a rat spinal cord transection model to evaluate the cell fate of allograft bone marrow derived MSCs. With the application of immunosuppressant, donor cells, delivered by biocompatible scaffold, survived up to 8 weeks post-grafting. Discernible tubes formed by MSCs were observed beginning 2 weeks after transplantation and they dominated the morphological features of implanted MSCs at 8 weeks post-grafting. The results of immunocytochemistry and transmission electron microscopy displayed the formation of perineurium-like sheath by donor cells, which, in a manner comparable to the perineurium in peripheral nerve, enwrapped host myelins and axons. The MSC-derived perineurium-like sheath secreted a group of trophic factors and permissive extracellular matrix, and served as a physical and chemical barrier to insulate the inner nerve fibers from ambient oxidative insults by the secretion of soluble antioxidant, superoxide dismutase-3 (SOD3). As a result, many intact regenerating axons were preserved in the injury/graft site following the forming of perineurium-like sheath. A parallel study utilizing a good manufacturing practice (GMP) grade human umbilical cord-derived MSCs or allogenic MSCs in an acute contusive/compressive SCI model exhibited a similar perineurium-like sheath formed by surviving donor cells in rat spinal cord at 3 weeks post-grafting. The present study for the first time provides an unambiguous morphological evidence of perineurium-like sheath formed by transplanted MSCs and a novel therapeutic mechanism of MSCs in treating SCI.

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.

GJA1-20k Arranges Actin to Guide Cx43 Delivery to Cardiac Intercalated Discs

RATIONALE

Delivery of Cx43 (connexin 43) to the intercalated disc is a continuous and rapid process critical for intercellular coupling. By a pathway of targeted delivery involving microtubule highways, vesicles of Cx43 hemichannels are efficiently trafficked to adherens junctions at intercalated discs. It has also been identified that actin provides rest stops for Cx43 forward trafficking and that Cx43 has a 20 kDa internally translated small C terminus isoform, GJA1-20k (Gap Junction Protein Alpha 1- 20 kDa), which is required for full-length Cx43 trafficking, but by an unknown mechanism.

OBJECTIVE

We explored the mechanism by which the GJA1-20k isoform is required for full-length Cx43 forward trafficking to intercalated discs.

METHODS AND RESULTS

Using an in vivo Adeno-associated virus serotype 9-mediated gene transfer system, we confirmed in whole animal that GJA1-20k markedly increases endogenous myocardial Cx43 gap junction plaque size at the intercalated discs. In micropatterned cell pairing systems, we found that exogenous GJA1-20k expression stabilizes filamentous actin without affecting actin protein expression and that GJA1-20k complexes with both actin and tubulin. We also found that filamentous actin regulates microtubule organization as inhibition of actin polymerization with a low dose of latrunculin A disrupts the targeting of microtubules to cell-cell junctions. GJA1-20k protects actin filament from latrunculin A disruption, preserving microtubule trajectory to the cell-cell border. For therapeutic implications, we found that prior in vivo Adeno-associated virus serotype 9-mediated gene delivery of GJA1-20k to the heart protects Cx43 localization to the intercalated discs against acute ischemic injury.

CONCLUSIONS

The internally translated GJA1-20k isoform stabilizes actin filaments, which guides growth trajectories of the Cx43 microtubule trafficking machinery, increasing delivery of Cx43 hemichannels to cardiac intercalated discs. Exogenous GJA1-20k helps to maintain cell-cell coupling in instances of anticipated myocardial ischemia.

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.

A Short Region of Connexin43 Reduces Human Glioma Stem Cell Migration, Invasion, and Survival through Src, PTEN, and FAK

Connexin43 (CX43), a protein that forms gap junction channels and hemichannels in astrocytes, is downregulated in high-grade gliomas. Its relevance for glioma therapy has been thoroughly explored; however, its positive effects on proliferation are counterbalanced by its effects on migration and invasion. Here, we show that a cell-penetrating peptide based on CX43 (TAT-Cx43_266-283) inhibited c-Src and focal adhesion kinase (FAK) and upregulated phosphatase and tensin homolog in glioma stem cells (GSCs) derived from patients. Consequently, TAT-Cx43_266-283 reduced GSC motility, as analyzed by time-lapse microscopy, and strongly reduced their invasive ability. Interestingly, we investigated the effects of TAT-Cx43_266-283 on freshly removed surgical specimens as undissociated glioblastoma blocks, which revealed a dramatic reduction in the growth, migration, and survival of these cells. In conclusion, a region of CX43 (amino acids 266–283) exerts an important anti-tumor effect in patient-derived glioblastoma models that includes impairment of GSC migration and invasion.

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TAT-Cx43_266-283 exerts anti-tumor effects in patient-derived glioblastoma models

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TAT-Cx43_266-283 targets Src, PTEN, and FAK

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TAT-Cx43_266-283 inhibits glioma stem cell migration and invasion

Sorting nexin 27 interactome in T-lymphocytes identifies zona occludens-2 dynamic redistribution at the immune synapse

T Lymphocyte recognition of antigens leads to the formation of a highly organized structure termed immune synapse (IS) by analogy with the neuronals synapse. Sorting nexin 27 (SNX27) controls the endosomal traffic of PSD95, Dlg1, ZO-1 (PDZ) domain-interacting proteins, and its alteration is associated with impaired synaptic function and neurological diseases. In T-lymphocytes, SNX27-positive vesicles polarize to the IS, the identity of SNX27 interactors in these conditions nonetheless remains unknown. Here we used proteomics to analyze the SNX27 interactome purified from IS-forming T cells, and confirmed the conserved nature of the SNX27/WASH/retromer association in hematopoietic cells. Furthermore, our comparative interactome analysis of SNX27 wild-type and a mutant-deficient for PDZ cargo recognition identified the epithelial cell-cell junction protein zona occludens-2 (ZO-2) as an IS component. Biochemistry and microscopy approaches in T cells confirmed SNX27/ZO-2 PDZ-dependent interaction, and demonstrated its role controlling the dynamic localization of ZO-2 at the IS. This study broadens our knowledge of SNX27 function in T lymphocytes, and suggests that pathways that delimit polarized structures in nervous and epithelial systems also participate in IS regulation.

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.

Down-Regulated Drebrin Aggravates Cognitive Impairments in a Mouse Model of Alzheimer's Disease

The developmentally regulated brain protein drebrin (Dbn) is a functional protein involved with long-term memory formation and is widely distributed in brain neurons, especially in the dendritic spines. A noticeable decline of this protein has been found in the hippocampus and cortex of patients with Alzheimer's disease (AD), yet the relationship between Dbn and AD has not been fully understood. In the present study, we examined how down-regulation of Dbn impacts the progression of AD in experimental animals. Accordingly, we injected Dbn interference vector (rAAV-mDbn1 ShRNA) into the hippocampus of three-month old APP(swe)/PS1(ΔE9) mice (APP/PS1 mice) and then successfully down-regulated Dbn expression in this brain region. Behavioral tests, including the Morris water maze test, the open field test, and the novel object test were conducted when the animals were nine months old. Subsequently, MicroPET/CT imaging to monitor glucose metabolism was done. We then investigated Aβ, GFAP, PSD-95, MAP2, vimentin, Cox43, and Syn1 expressions in the brain of the experimental animals via immunohistochemical or immunofluorescence methods. We found that AD mice with a low expression of Dbn performed poorly in the behavioral tests and showed decreased glucose utilization. In the brains of these animals, we detected a slight increase of Aβ, GFAP and vimentin and a significant decline of PSD-95. Altogether our data warrant further studies to elucidate the effect of Dbn on the development and progression of AD.

Thrombin Induces Inositol Trisphosphate-Mediated Spatially Extensive Responses in Lung Microvessels

Activation of plasma membrane receptors initiates compartmentalized second messenger signaling. Whether this compartmentalization facilitates the preferential intercellular diffusion of specific second messengers is unclear. Toward this, the receptor-mediated agonist, thrombin, was instilled into microvessels in a restricted region of isolated blood-perfused mouse lungs. Subsequently, the thrombin-induced increase in endothelial F-actin was determined using confocal fluorescence microscopy. Increased F-actin was evident in microvessels directly treated with thrombin and in those located in adjoining thrombin-free regions. This increase was abrogated by inhibiting inositol trisphosphate-mediated calcium release with Xestospongin C (XeC). XeC also inhibited the thrombin-induced increase in the amplitude of endothelial cytosolic Ca²⁺ oscillations. Instillation of thrombin and XeC into adjacent restricted regions increased F-actin in microvessels in the thrombin-treated and adjacent regions but not in those in the XeC-treated region. Thus, inositol trisphosphate, and not calcium, diffused interendothelially to the spatially remote thrombin-free microvessels. Thus, activation of plasma membrane receptors increased the ambit of inflammatory responses via a second messenger different from that used by stimuli that induce cell-wide increases in second messengers. Thrombin however failed to induce the spatially extensive response in microvessels of mice lacking endothelial connexin43, suggesting a role for connexin43 gap junctions. Compartmental second messenger signaling and interendothelial communication define the specific second messenger involved in exacerbating proinflammatory responses to receptor-mediated agonists.