Connexins and pannexins in the immune system and lymphatic organs

The role of ion channels in the regulation of dendritic cell function

Ion channels, membrane proteins that facilitate the transport of various inorganic ions across hydrophobic cellular lipid membranes, are ubiquitous in a wide variety of cell and tissue types. They are involved in establishing the cell membrane potential and play a role in various physiological activities by regulating ion concentrations within the cell. Dendritic cells (DCs) are specialised antigen-presenting cells found mainly on the surface of the body (skin and mucous membranes), in the mesenchyme of most organs, in the T-cell compartment of the spleen and in lymph nodes. DCs exert an important influence on the regulation of inflammation by activating T cells and producing cytokines. Studies have shown that ion channels expressed in DCs contribute to the regulation of the immune response, making them a key component of the immune system. This review summarises the major scientific advances in understanding the functional impact of ion channels (calcium channels, sodium channels and aquaporin) in DCs, including the regulation of inflammatory responses, antigen presentation, maturation, migration and cytokine production, to inform ongoing studies of ion channel function in DCs.

Mechanism of connexin channel inhibition by mefloquine and 2-aminoethoxydiphenyl borate

Gap junction intercellular communication (GJIC) between two adjacent cells involves direct exchange of cytosolic ions and small molecules via connexin gap junction channels (GJCs). Connexin GJCs have emerged as drug targets, with small molecule connexin inhibitors considered a viable therapeutic strategy in several diseases. The molecular mechanisms of GJC inhibition by known small molecule connexin inhibitors remain unknown, preventing the development of more potent and connexin-specific therapeutics. Here we show that two GJC inhibitors, mefloquine (MFQ) and 2-aminoethoxydiphenyl borate (2APB) bind to Cx32 and block dye permeation across Cx32 hemichannels (HCs) and GJCs. Cryo-EM analysis shows that 2APB binds to "site A", close to the N-terminal gating helix of Cx32 GJC, restricting the entrance to the channel pore. In contrast, MFQ binds to a distinct "site M", deeply buried within the pore. MFQ binding to this site modifies the electrostatic properties of Cx32 pore. Mutagenesis of V37, a key residue located in the site M, renders Cx32 HCs and GJCs insensitive to MFQ-mediated inhibition. Moreover, our cryo-EM analysis, mutagenesis and activity assays show that MFQ targets the M site in Cx43 GJC similarly to Cx32. Taken together, our results point to a conserved inhibitor binding site in connexin channels, opening a new route for development of specific drugs targeting connexins.

Insights into the role of connexins and specialized intercellular communication pathways in breast cancer: Mechanisms and applications

Gap junctions, membrane-based channels comprised of connexin proteins (Cxs), facilitate direct communication among neighbouring cells and between cells and the extracellular space through their hemichannels. The normal human breast expresses various Cxs family proteins, such as Cx43, Cx30, Cx32, Cx46, and Cx26, crucial for proper tissue development and function. These proteins play a significant role in breast cancer development, progression, and therapy response. In primary tumours, there is often a reduction and cytoplasmic mislocalization of Cx43 and Cx26, while metastatic lesions show an upregulation of these and other Cxs. Although existing research predominantly supports the tumour-suppressing role of Cxs in primary carcinomas through channel-dependent and independent functions, controversies persist regarding their involvement in the metastatic process. This review aims to provide an updated perspective on Cxs in human breast cancer, with a specific focus on intrinsic subtypes due to the heterogeneous nature of this disease. Additionally, the manuscript will explore the role of Cxs in immune interactions and novel forms of intercellular communication, such as tunneling nanotubes and extracellular vesicles, within the breast tumour context and tumour microenvironment. Recent findings suggest that Cxs hold potential as therapeutic targets for mitigating metastasis and drug resistance. Furthermore, they may serve as novel biomarkers for cancer prognosis, offering promising avenues for future research and clinical applications.

Estradiol and 3β-diol protect female cortical astrocytes by regulating connexin 43 Gap Junctions

While estrogens have been described to protect or preserve neuronal function in the face of insults such as oxidative stress, the prevailing mechanistic model would suggest that these steroids exert direct effects on the neurons. However, there is growing evidence that glial cells, such as astrocytes, are key cellular mediators of protection. Noting that connexin 43 (Cx43), a protein highly expressed in astrocytes, plays a key role in mediating inter-cellular communication, we hypothesized that Cx43 is a target of estradiol (E2), and the estrogenic metabolite of DHT, 3β-diol. Additionally, we sought to determine if either or both of these hormones attenuate oxidative stress-induced cytotoxicity by eliciting a reduction in Cx43 expression or inhibition of Cx43 channel permeability. Using primary cortical astrocytes, we found that E2 and 3β-diol were each protective against the mixed metabolic/oxidative insult, iodoacetic acid (IAA). Moreover, these effects were blocked by estrogen receptor antagonists. However, E2 and 3β-diol did not alter Cx43 mRNA levels in astrocytes but did inhibit IAA-induced Cx43 gap junction opening/permeability. Taken together, these data implicate astrocyte Cx43 gap junction as an understudied mediator of the cytoprotective effects of estrogens in the brain. Given the wide breadth of disease states associated with Cx43 function/dysfunction, further understanding the relationship between gonadal steroids and Cx43 channels may contribute to a better understanding of the biological basis for sex differences in various diseases.

Cellular Interactions in Cell Sheets Enhance Mesenchymal Stromal Cell Immunomodulatory Properties

Immune-related applications of mesenchymal stromal cells (MSCs) in cell therapy seek to exploit immunomodulatory paracrine signaling pathways to reduce inflammation. A key MSC therapeutic challenge is reducing patient outcome variabilities attributed to insufficient engraftment/retention of injected heterogenous MSCs. To address this, we propose directly transplantable human single-cell-derived clonal bone marrow MSC (hcBMSC) sheets. Cell sheet technology is a scaffold-free tissue engineering strategy enabling scalable production of highly engraftable cell constructs retaining endogenous cell-cell and cell-matrix interactions, important to cell function. cBMSCs, as unique MSC subset populations, facilitate rational selection of therapeutically relevant MSC clones from donors. Here, we combine human cBMSCs with cell sheet technology, demonstrating cell sheet fabrication as a method to significantly upregulate expression of immunomodulatory molecules interleukin (IL)-10, indoleamine 2,3-dioxygenase (IDO-1), and prostaglandin E synthase 2 (PTGES2) across GMP-grade hcBMSC lines and whole human bone marrow-derived MSCs compared to respective conventional cell suspensions. When treated with carbenoxolone, a gap junction inhibitor, cell sheets downregulate IL-10 and IDO-1 expression, implicating functional roles for intercellular sheet interactions. Beyond producing directly transferable multicellular hcBMSC constructs, cell sheet technology amplifies hcBMSC expression of immunomodulatory factors important to therapeutic action. In addition, this work demonstrates the importance of cell-cell interactions as a tissue engineering design criterion to enhance consistent MSC functions.

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.

Cross-Activation of Hemichannels/Gap Junctions and Immunoglobulin-Like Domains in Innate–Adaptive Immune Responses

Hemichannels (HCs)/gap junctions (GJs) and immunoglobulin (Ig)-like domain-containing proteins (IGLDCPs) are involved in the innate–adaptive immune response independently. Despite of available evidence demonstrating the importance of HCs/GJs and IGLDCPs in initiating, implementing, and terminating the entire immune response, our understanding of their mutual interactions in immunological function remains rudimentary. IGLDCPs include immune checkpoint molecules of the immunoglobulin family expressed in T and B lymphocytes, most of which are cluster of differentiation (CD) antigens. They also constitute the principal components of the immunological synapse (IS), which is formed on the cell surface, including the phagocytic synapse, T cell synapse, B cell synapse, and astrocytes–neuronal synapse. During the three stages of the immune response, namely innate immunity, innate–adaptive immunity, and adaptive immunity, HCs/GJs and IGLDCPs are cross-activated during the entire process. The present review summarizes the current understanding of HC-released immune signaling factors that influence IGLDCPs in regulating innate–adaptive immunity. ATP-induced “eat me” signals released by HCs, as well as CD31, CD47, and CD46 “don’t eat me” signaling molecules, trigger initiation of innate immunity, which serves to regulate phagocytosis. Additionally, HC-mediated trogocytosis promotes antigen presentation and amplification. Importantly, HC-mediated CD4+ T lymphocyte activation is critical in the transition of the innate immune response to adaptive immunity. HCs also mediate non-specific transcytosis of antibodies produced by mature B lymphocytes, for instance, IgA transcytosis in ovarian cancer cells, which triggers innate immunity. Further understanding of the interplay between HCs/GJs and IGLDCPs would aid in identifying therapeutic targets that regulate the HC–Ig-like domain immune response, thereby providing a viable treatment strategy for immunological diseases. The present review delineates the clinical immunology-related applications of HC–Ig-like domain cross-activation, which would greatly benefit medical professionals and immunological researchers alike. HCs/GJs and IGLDCPs mediate phagocytosis via ATP; “eat me and don’t eat me” signals trigger innate immunity; HC-mediated trogocytosis promotes antigen presentation and amplification in innate–adaptive immunity; HCs also mediate non-specific transcytosis of antibodies produced by mature B lymphocytes in adaptive immunity.

Characterization of Pannexin1, Connexin32, and Connexin43 in Spotted Sea Bass (Lateolabrax maculatus): They Are Important Neuro-Related Immune Response Genes Involved in Inflammation-Induced ATP Release

Many immunological diseases can be treated by regulating neurobehavior, in which extracellular ATP is a vital member of endogenous danger-associated molecular pattern signaling molecule that plays a crucial part in innate neuro-related immunity. It is actively released through pannexin (Panx) and connexin (Cx) hemichannels from activated or stressed cells during inflammation, injury, or apoptosis. In addition to participating in ATP release, Panxs and Cxs also have crucial immune functions. In this study, pannexin1, three connexin32 isoforms and connexin43 were identified and characterized in spotted sea bass (Lateolabrax maculatus), which were named LmPanx1, LmCx32.2, LmCx32.3, LmCx32.7, and LmCx43. Their similar topological structures were discovered by sequence analysis: a relatively unconserved C-terminal region and four highly conserved transmembrane (TM) domains, and so on. Each extracellular (ECL) region of Panx1 has two conserved cysteine residues. Unlike Panx1, each ECL region of Cx32 and Cx43 contains three conserved cysteine residues, forming two conserved motifs: CX6CX3C motif in ECL1 and CX4CX5C motif in ECL2. Furthermore, Panx1 and Cx43 share similar genomic organization and synteny with their counterparts in selected vertebrates. Cx32 and CX43 were located in the same locus in fish, but diverged into two loci from amphibian. Moreover, despite varying expression levels, the identified genes were constitutively expressed in all examined tissues. All genes were upregulated by PAMP [lipopolysaccharide and poly(I:C)] stimulation or bacterial infection in vivo and in vitro, but they were downregulated in the brain at 6 or 12 h after stimulation. Especially, the three LmCx32 isoforms and LmCx43 were upregulated by ATP stimulation in primary head kidney leukocytes; however, downregulation of LmCx32.3 and LmCx43 expression were noted at 12 h. Conversely, ATP treatment inhibited the expression of LmPanx1. Importantly, we showed that the spotted sea bass Panx1, Cx43, and Cx32 were localized on the cellular membrane and involved in inflammation-induced ATP release. Taken together, our results demonstrated that Panx1, Cx32, and Cx43 are important neuro-related immune response genes involved in inflammation-induced ATP release.

Connexin channels modulation in pathophysiology and treatment of immune and inflammatory disorders

Connexin-mediated intercellular communication mechanisms include bidirectional cell-to-cell coupling by gap junctions and release/influx of molecules by hemichannels. These intercellular communications have relevant roles in numerous immune system activities. Here, we review the current knowledge about the function of connexin channels, mainly those formed by connexin-43, on immunity and inflammation. Focusing on those evidence that support the design and development of therapeutic tools to modulate connexin expression and/or channel activities with treatment potential for infections, wounds, cancer, and other inflammatory conditions.

A Systematic Study of the Mechanism of Acacetin Against Sepsis Based on Network Pharmacology and Experimental Validation

Sepsis is a dysregulated systemic response to infection, and no effective treatment options are available. Acacetin is a natural flavonoid found in various plants, including Sparganii rhizoma, Sargentodoxa cuneata and Patrinia scabiosifolia. Studies have revealed that acacetin potentially exerts anti-inflammatory and antioxidative effects on sepsis. In this study, we investigated the potential protective effect of acacetin on sepsis and revealed the underlying mechanisms using a network pharmacology approach coupled with experimental validation and molecular docking. First, we found that acacetin significantly suppressed pathological damage and pro-inflammatory cytokine expression in mice with LPS-induced fulminant hepatic failure and acute lung injury, and in vitro experiments further confirmed that acacetin attenuated LPS-induced M1 polarization. Then, network pharmacology screening revealed EGFR, PTGS2, SRC and ESR1 as the top four overlapping targets in a PPI network, and GO and KEGG analyses revealed the top 20 enriched biological processes and signalling pathways associated with the therapeutic effects of acacetin on sepsis. Further network pharmacological analysis indicated that gap junctions may be highly involved in the protective effects of acacetin on sepsis. Finally, molecular docking verified that acacetin bound to the active sites of the four targets predicted by network pharmacology, and in vitro experiments further confirmed that acacetin significantly inhibited the upregulation of p-src induced by LPS and attenuated LPS-induced M1 polarization through gap junctions. Taken together, our results indicate that acacetin may protect against sepsis via a mechanism involving multiple targets and pathways and that gap junctions may be highly involved in this process.

Connexin and gap junctions: perspectives from biology to nanotechnology based therapeutics

The concept of gap junctions and their role in intercellular communication has been known for around 50 years. Considerable progress has been made in understanding the fundamental biology of connexins in mediating gap junction intercellular communication (GJIC) and their role in various cellular processes including pathological conditions. However, this understanding has not led to development of advanced therapeutics utilizing GJIC. Inadequacies in strategies that target specific connexin protein in the affected tissue, with minimal or no collateral damage, are the primary reason for the lack of development of efficient therapeutic models. Herein, nanotechnology has a role to play, giving plenty of scope to circumvent these problems and develop more efficient connexin based therapeutics. AsODN, antisense oligodeoxynucleotides; BMPs, bone morphogenetic proteins; BMSCs, bone marrow stem cells; BG, bioglass; Cx, Connexin; CxRE, connexin-responsive elements; CoCr NPs, cobalt-chromium nanoparticles; cGAMP, cyclic guanosine monophosphate-adenosine monophosphate; cAMP, cyclic adenosine monophosphate; ERK1/2, extracellular signal-regulated kinase 1/2; EMT, epithelial-mesenchymal transition; EPA, eicosapentaenoic acids; FGFR1, fibroblast growth factor receptor 1; FRAP, fluorescence recovery after photobleaching; 5-FU, 5-fluorouracil; GJ, gap junction; GJIC, gap junctional intercellular communication; HGPRTase, hypoxanthine phosphoribosyltransferase; HSV-TK, herpes virus thymidine kinase; HSA, human serum albumin; HA, hyaluronic acid; HDAC, histone deacetylase; IRI, ischemia reperfusion injury; IL-6, interleukin-6; IL-8, interleukin-8; IONPs, iron-oxide nanoparticles; JNK, c-Jun N-terminal kinase; LAMP, local activation of molecular fluorescent probe; MSCs, mesenchymal stem cells; MMP, matrix metalloproteinase; MI, myocardial infarction; MAPK, mitogen-activated protein kinase; NF-κB, nuclear factor kappa B; NO, nitric oxide; PKC, protein kinase C; QDs, quantum dots; ROI, region of interest; RGO, reduced graphene oxide; siRNA, small interfering RNA; TGF-β1, transforming growth factor-β1; TNF-α, tumor necrosis factor-α; UCN, upconversion nanoparticles; VEGF, vascular endothelial growth factor. In this review, we discuss briefly the role of connexins and gap junctions in various physiological and pathological processes, with special emphasis on cancer. We further discuss the application of nanotechnology and tissue engineering in developing treatments for various connexin based disorders.

Anti-parasitic drugs modulate the non-selective channels formed by connexins or pannexins

The proteins connexins, innexins, and pannexins are the subunits of non-selective channels present in the cell membrane in vertebrates (connexins and pannexins) and invertebrates (innexins). These channels allow the transfer of ions and molecules across the cell membrane or, and in many cases, between the cytoplasm of neighboring cells. These channels participate in various physiological processes, particularly under pathophysiological conditions, such as bacterial, viral, and parasitic infections. Interestingly, some anti-parasitic drugs also block connexin- or pannexin-formed channels. Their effects on host channels permeable to molecules that favor parasitic infection can further explain the anti-parasitic effects of some of these compounds. In this review, the effects of drugs with known anti-parasitic activity that modulate non-selective channels formed by connexins or pannexins are discussed. Previous studies that have reported the presence of these proteins in worms, ectoparasites, and protozoa that cause parasitic infections have also been reviewed.

Brain gray matter astroglia-specific connexin 43 ablation attenuates spinal cord inflammatory demyelination

Background

Brain astroglia are activated preceding the onset of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). We characterized the effects of brain astroglia on spinal cord inflammation, focusing on astroglial connexin (Cx)43, because we recently reported that Cx43 has a critical role in regulating neuroinflammation.

Methods

Because glutamate aspartate transporter (GLAST)⁺ astroglia are enriched in the brain gray matter, we generated Cx43^fl/fl;GLAST-CreER^T2/+ mice that were brain gray matter astroglia-specific Cx43 conditional knockouts (Cx43 icKO). EAE was induced by immunization with myelin oligodendroglia glycoprotein (MOG) _35–55 peptide 10 days after tamoxifen injection. Cx43^fl/fl mice were used as controls.

Results

Acute and chronic EAE signs were significantly milder in Cx43 icKO mice than in controls whereas splenocyte MOG-specific responses were unaltered. Histologically, Cx43 icKO mice showed significantly less demyelination and fewer CD45⁺ infiltrating immunocytes, including F4/80⁺ macrophages, and Iba1⁺ microglia in the spinal cord than controls. Microarray analysis of the whole cerebellum revealed marked upregulation of anti-inflammatory A2-specific astroglia gene sets in the pre-immunized phase and decreased proinflammatory A1-specific and pan-reactive astroglial gene expression in the onset phase in Cx43 icKO mice compared with controls. Astroglia expressing C3, a representative A1 marker, were significantly decreased in the cerebrum, cerebellum, and spinal cord of Cx43 icKO mice compared with controls in the peak phase. Isolated Cx43 icKO spinal microglia showed more anti-inflammatory and less proinflammatory gene expression than control microglia in the pre-immunized phase. In particular, microglial expression of Ccl2, Ccl5, Ccl7, and Ccl8 in the pre-immunized phase and of Cxcl9 at the peak phase was lower in Cx43 icKO than in controls. Spinal microglia circularity was significantly lower in Cx43 icKO than in controls in the peak phase. Significantly lower interleukin (IL)-6, interferon-γ, and IL-10 levels were present in cerebrospinal fluid from Cx43 icKO mice in the onset phase compared with controls.

Conclusions

The ablation of Cx43 in brain gray matter astroglia attenuates EAE by promoting astroglia toward an anti-inflammatory phenotype and suppressing proinflammatory activation of spinal microglia partly through depressed cerebrospinal fluid proinflammatory cytokine/chemokine levels. Brain astroglial Cx43 might be a novel therapeutic target for MS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02176-1.

Lymphatic Connexins and Pannexins in Health and Disease

This review highlights current knowledge on the expression and function of connexins and pannexins, transmembrane channel proteins that play an important role in intercellular communication, in both the developing and mature lymphatic vasculature. A particular focus is given to the involvement of these proteins in functions of the healthy lymphatic system. We describe their influence on the maintenance of extracellular fluid homeostasis, immune cell trafficking to draining lymph nodes and dietary nutrient absorption by intestinal villi. Moreover, new insights into connexin mutations in primary and secondary lymphedema as well as on the implication of lymphatic connexins and pannexins in acquired cardiovascular diseases are discussed, allowing for a better understanding of the role of these proteins in pathologies linked to dysfunctions in the lymphatic system.

Connexins in Cancer: Jekyll or Hyde?

The expression, localization, and function of connexins, the protein subunits that comprise gap junctions, are often altered in cancer. In addition to cell-cell coupling through gap junction channels, connexins also form hemichannels that allow communication between the cell and the extracellular space and perform non-junctional intracellular activities. Historically, connexins have been considered tumor suppressors; however, they can also serve tumor-promoting functions in some contexts. Here, we review the literature surrounding connexins in cancer cells in terms of specific connexin functions and propose that connexins function upstream of most, if not all, of the hallmarks of cancer. The development of advanced connexin targeting approaches remains an opportunity for the field to further interrogate the role of connexins in cancer phenotypes, particularly through the use of in vivo models. More specific modulators of connexin function will both help elucidate the functions of connexins in cancer and advance connexin-specific therapies in the clinic.

The Role of Connexin 43 and Pannexin 1 During Acute Inflammation

During acute inflammation, the recruitment of leukocytes from the blood stream into the inflamed tissue is a well-described mechanism encompassing the interaction of endothelial cells with leukocytes allowing leukocytes to reach the site of tissue injury or infection where they can fulfill their function such as phagocytosis. This process requires a fine-tuned regulation of a plethora of signaling cascades, which are still incompletely understood. Here, connexin 43 (Cx43) and pannexin 1 (Panx1) are known to be pivotal for the correct communication of endothelial cells with leukocytes. Pharmacological as well as genetic approaches provide evidence that endothelial Cx43-hemichannels and Panx1-channels release signaling molecules including ATP and thereby regulate vessel function and permeability as well as the recruitment of leukocytes during acute inflammation. Furthermore, Cx43 hemichannels and Panx1-channels in leukocytes release signaling molecules and can mediate the activation and function of leukocytes in an autocrine manner. The focus of the present review is to summarize the current knowledge of the role of Cx43 and Panx1 in endothelial cells and leukocytes in the vasculature during acute inflammation and to discuss relevant molecular mechanisms regulating Cx43 and Panx1 function.

Formation of functional innexin hemichannels, as well as gap junctional channels, in an insect cell line, NIAs-AeAl-2, derived from Asian tiger mosquito Aedes albopictus (Diptera: Culicidae): A partial but significant contribution of innexin 2

In vertebrates, gap junctions and hemichannels consisting of connexins are important cell surface structures for communication with neighboring cells and for the regulation of various cell functions. To date, various gap-junction-related proteins have been found, including innexins in invertebrates and pannexins in vertebrates. Significant contributions of gap junctions by innexins and (hemi-)channels by pannexins to numerous functions have been reported. Verification of the presence and functional significance of innexin hemichannels, however, remains a gap in our knowledge in innexin physiology. In this study, we revealed the localization of an innexin protein (innexin 2) on the cell surface in mosquito tissues and cultured cells. Furthermore, we demonstrated the presence of functional hemichannels, as well as gap junctions, in mosquito cells using dye transfer assays. The inward uptake of fluorescent dye was inhibited by anti-innexin 2 antibody. These results suggest that innexin hemichannels are formed to function in cultured mosquito cells, in at least a partially innexin 2-dependent manner. Although only a few studies on insect hemichannels have been published, innexin-based hemichannels, as well as innexin gap junctions, could also significantly contribute to insect intercellular signal transduction.

Angiotensin II induces RAW264.7 macrophage polarization to the M1‑type through the connexin 43/NF‑κB pathway

Angiotensin II (AngII) serves an important inflammatory role in cardiovascular disease; it can induce macrophages to differentiate into the M1-type, produce inflammatory cytokines and resist pathogen invasion, and can cause a certain degree of damage to the body. Previous studies have reported that connexin 43 (Cx43) and NF-κB (p65) are involved in the AngII-induced inflammatory pathways of macrophages; however, the mechanisms underlying the effects of Cx43 and NF-κB (p65) on AngII-induced macrophage polarization have not been determined. Thus, the present study aimed to investigate the effects of Cx43 and NF-κB (p65) on the polarization process of AngII-induced macrophages. The macrophage polarization-related proteins and mRNAs were examined by flow cytometry, western blotting, immunofluorescence, ELISA and reverse transcription-quantitative PCR analyses. RAW264.7 macrophages were treated with AngII to simulate chronic inflammation and it was subsequently found that AngII promoted RAW 264.7 macrophage polarization towards the M1-type by such effects as the release of inducible nitric oxide synthase (iNOS), tumour necrosis factor (TNF)-α, IL-1β, the secretion of IL-6, and the expression of M1-type indicators, such as CD86. Simultaneously, compared with the control group, the protein expression levels of Cx43 and phosphorylated (p)-p65 were significantly increased following AngII treatment. The M1-related phenotypic indicators, iNOS, TNF-α, IL-1β, IL-6 and CD86, were inhibited by the NF-κB (p65) signalling pathway inhibitor BAY117082. Similarly, the Cx43 inhibitors, Gap26 and Gap19, also inhibited the expression of M1-related factors, and the protein expression levels of p-p65 in the Gap26/Gap19 groups were significantly decreased compared with the AngII group. Altogether, these findings suggested that AngII may induce the polarization of RAW264.7 macrophages to the M1-type through the Cx43/NF-κB (p65) signalling pathway.

Connexin43 Is Required for the Effective Activation of Spleen Cells and Immunoglobulin Production

Gap junctions (Gjs), formed by specific protein termed connexins (Cxs), regulate many important cellular processes in cellular immunity. However, little is known about their effects on humoral immunity. Here we tested whether and how Gj protein connexin43 (Cx43) affected antibody production in spleen cells. Detection of IgG in mouse tissues and serum revealed that wild-type (Cx43^+/+) mouse had a significantly higher level of IgG than Cx43 heterozygous (Cx43^+/−) mouse. Consistently, spleen cells from Cx43^+/+ mouse produced more IgG under both basal and lipopolysaccharide (LPS)-stimulated conditions. Further analysis showed that LPS induced a more dramatic activation of ERK and cell proliferation in Cx43^+/+ spleen cells, which was associated with a higher pro-oxidative state, as indicated by the increased NADPH oxidase 2 (NOX2), TXNIP, p38 activation and protein carbonylation. In support of a role of the oxidative state in the control of lymphocyte activation, exposure of spleen cells to exogenous superoxide induced Cx43 expression, p38 activation and IgG production. On the contrary, inhibition of NOX attenuated the effects of LPS. Collectively, our study characterized Cx43 as a novel molecule involved in the control of spleen cell activation and IgG production. Targeting Cx43 could be developed to treat certain antibody-related immune diseases.

Connexins in cancer: bridging the gap to the clinic

Gap junctions comprise arrays of intercellular channels formed by connexin proteins and provide for the direct communication between adjacent cells. This type of intercellular communication permits the coordination of cellular activities and plays key roles in the control of cell growth and differentiation and in the maintenance of tissue homoeostasis. After more than 50 years, deciphering the links among connexins, gap junctions and cancer, researchers are now beginning to translate this knowledge to the clinic. The emergence of new strategies for connexin targeting, combined with an improved understanding of the molecular bases underlying the dysregulation of connexins during cancer development, offers novel opportunities for clinical applications. However, different connexin isoforms have diverse channel-dependent and -independent functions that are tissue and stage specific. This can elicit both pro- and anti-tumorigenic effects that engender significant challenges in the path towards personalised medicine. Here, we review the current understanding of the role of connexins and gap junctions in cancer, with particular focus on the recent progress made in determining their prognostic and therapeutic potential.

Enhancement of connexin30.3 expression in mouse embryonic stem cell line EB3 in response to cell-cell contacts

To clarify the potential role of gap junction in cell-cell contact response, the expression of connexin30.3 gene (Cx30.3), a specifically expressed isoform in undifferentiated state of mouse embryonic stem (ES) cell line EB3 was investigated under different cell-cell contact conditions. ES cells were cultured by hanging drop culture method to increase cell-cell contact frequency. As control, a single cell culture was conducted. After culture for 12 h, the Cx30.3 expression level in hanging drop culture reached 1.73-fold that of the control (p < 0.001). By contrast, connexin43 gene (Cx43), a ubiquitously expressed gene, showed no difference between both cultures. The experiment of E-cadherin inhibition and β-catenin knockdown suggested the action of E-cadherin upstream of the Cx30.3 regulating pathway. The cell-cell contacts with different cell lines such as HeLa cells and B16/BL6 caused no effect on the Cx30.3 in ES cells. These suggest a potential role of Cx30.3 as a cell-cell contact signal mediator partially regulated by E-cadherin signaling.

The Role of Connexin-43 in the Inflammatory Process: A New Potential Therapy to Influence Keratitis

The studies outlined in this review highlight the relationship between inflammatory signaling molecules and connexin-43 (Cx43). Gap junction (GJ) channels and hemichannels (HCs) participate in the metabolic activity between intra- and extracellular space. Some ions and small molecules are exchanged from cell to cell or cell to extracellular space to affect the process of inflammation via GJ. We analyzed the effects of signaling molecules, such as innate immunity messengers, transcription factors, LPS, cytokine, inflammatory chemokines, and MMPs, on Cx43 expression during the inflammatory process. At the same time, we found that these signaling molecules play a critical role in the pathogenesis of keratitis. Thus, we assessed the function of Cx43 during inflammatory corneal disease. Corneal healing plays an essential role in the late stage of keratitis. We found that Cx43 is involved in wound healing. Studies have shown that the decrease of Cx43 can decrease the time of healing. We also report several Cx43 mimic peptides which can inhibit the activity of Cx43 Hc to mediate the releasing of adenosine triphosphate (ATP), which may in turn influence the inflammatory process.

Adrenomedullin Induces Cardiac Lymphangiogenesis After Myocardial Infarction and Regulates Cardiac Edema Via Connexin 43

RATIONALE

Cardiac lymphangiogenesis contributes to the reparative process post-myocardial infarction, but the factors and mechanisms regulating it are not well understood.

OBJECTIVE

To determine if epicardial-secreted factor AM (adrenomedullin; Adm=gene) improves cardiac lymphangiogenesis post-myocardial infarction via lateralization of Cx43 (connexin 43) in cardiac lymphatic vasculature.

METHODS AND RESULTS

Firstly, we identified sex-dependent differences in cardiac lymphatic numbers in uninjured mice using light-sheet microscopy. Using a mouse model of Adm hi/hi ( Adm overexpression) and permanent left anterior descending ligation to induce myocardial infarction, we investigated cardiac lymphatic structure, growth, and function in injured murine hearts. Overexpression of Adm increased lymphangiogenesis and cardiac function post-myocardial infarction while suppressing cardiac edema and correlated with changes in Cx43 localization. Lymphatic function in response to AM treatment was attenuated in mice with a lymphatic-specific Cx43 deletion. In vitro experiments in cultured human lymphatic endothelial cells identified a novel mechanism to improve gap junction coupling by pharmaceutically targeting Cx43 with verapamil. Finally, we show that connexin protein expression in cardiac lymphatics is conserved between mouse and human.

CONCLUSIONS

AM is an endogenous, epicardial-derived factor that drives reparative cardiac lymphangiogenesis and function via Cx43, and this represents a new therapeutic pathway for improving myocardial edema after injury.

Expression analysis of Pannexin1 channel gene in response to immune challenges and its role in infection-induced ATP release in tilapia (Oreochromis niloticus)

ATP released from immune cells plays an important role in activation of host innate immunity. However, the molecular mechanisms for pathogen infection-induced ATP release in fish remains unclear. Pannexin1 (Panx1) is a recently identified ATP release channel important for controlling immune responses. The immune relevance of Panx1 in fish, however, is still poorly understood. In this study, we characterized a Panx1 gene homologue (termed tPanx1) from Nile tilapia (Oreochromis niloticus) and analyzed its expression in response to different immune challenges. We also investigated the role of tPanx1 channel in bacterial infection-induced ATP release. Real-time quantitative PCR analysis revealed that tPanx1 gene is expressed in all tested tissues with predominant expression in intestine. Immune challenges with lipopolysaccharide, polyinosinic-polycytidylic acid and zymosan led to increased gene expression of tPanx1 in tilapia head kidney cells and peripheral blood leucocytes. In addition, tPanx1 gene was up-regulated in hepatopancreas, muscle, spleen, gill, head kidney and blood after Aeromonas hydrophila infection. Furthermore, pharmacological inhibition of tPanx1 channel activity with Panx1 channel inhibitor, carbenoxolone, significantly attenuated A. hydrophila infection-induced ATP release in tilapia head kidney cells. Taken together, our findings suggested that tPanx1 is an important immune response gene involved in bacterial infection-induced ATP release in tilapia O. niloticus.

Increased expression and functionality of the gap junction in peripheral blood lymphocytes is associated with hypertension-mediated inflammation in spontaneously hypertensive rats

BACKGROUND

Imbalances in circulating T lymphocytes play critical roles in the pathogenesis of hypertension-mediated inflammation. Connexins (Cxs) in immune cells are involved in the maintenance of homeostasis of T lymphocytes. However, the association between Cxs in peripheral blood T lymphocytes and hypertension-mediated inflammation remains unknown. This study was designed to investigate the role of Cxs in T lymphocytes in hypertension-mediated inflammation in spontaneously hypertensive rats (SHRs).

METHODS

The systolic blood pressure (SBP) in Wistar-Kyoto (WKY) rats and SHRs was monitored using the tail-cuff method. The serum cytokine level was determined using ELISA. The proportions of different T-lymphocyte subtypes in the peripheral blood, the expressions of Cx40/Cx43 in the T-cell subtypes, and the gap junctional intracellular communication (GJIC) of peripheral blood lymphocytes were measured using flow cytometry (FC). The accumulations of Cx40/Cx43 at the plasma membrane and/or in the cytoplasm were determined using immunofluorescence staining. The in vitro mRNA levels of cytokines and GJIC in the peripheral blood lymphocytes were respectively examined using real-time PCR and FC after treatment with Gap27 and/or concanavalin A (Con A).

RESULTS

The percentage of CD4+ T cells and the CD4+/CD8+ ratio were high, and the accumulation or expressions of Cx40/Cx43 in the peripheral blood lymphocytes in SHRs were higher than in those of WKY rats. The percentage of CD8+ and CD4+CD25+ T cells was lower in SHRs. The serum levels of IL-2, IL-4 and IL-6 from SHRs were higher than those from WKY rats, and the serum levels of IL-2 and IL-6 positively correlated with the expression of Cx40/Cx43 in the peripheral blood T lymphocytes from SHRs. The peripheral blood lymphocytes of SHRs exhibited enhanced GJIC. Cx43-based channel inhibition, which was mediated by Gap27, remarkably reduced GJIC in lymphocytes, and suppressed IL-2 and IL-6 mRNA expressions in Con A stimulated peripheral blood lymphocytes.

CONCLUSIONS

Our data suggest that Cxs may be involved in the regulation of T-lymphocyte homeostasis and the production of cytokines. A clear association was found between alterations in Cxs expression or in Cx43-based GJIC and hypertension-mediated inflammation.

Characterization of UDP-Activated Purinergic Receptor P2Y₆ Involved in Japanese Flounder Paralichthys olivaceus Innate Immunity

Uridine 5’-diphosphate (UDP)-activated purinergic receptor P2Y₆ is a member of a G-protein-coupled purinergic receptor family that plays an important role in mammalian innate immunity. However, the role of the P2Y₆ receptor (P2Y₆R) in fish immunity has not been investigated. In this report, we characterized a P2Y₆R gene from Japanese flounder (Paralichthys olivaceus) and examined its role in fish innate immunity. Sequence analysis reveals that the Japanese flounder P2Y₆R protein is conserved and possesses four potential glycosylation sites. Quantitative real-time RT-PCR analysis shows that P2Y₆R is broadly distributed in all examined Japanese flounder tissues with dominant expression in the liver. In addition, P2Y₆R gene expression was up-regulated in head kidney macrophages (HKMs) upon lipopolysaccharides (LPS) and poly(I:C) stimulations but down-regulated by LPS challenge in peripheral blood leukocytes (PBLs). Furthermore, pharmacological inhibition of the endogenous P2Y₆ receptor activity by the potently selective P2Y₆R antagonist, MRS 2578, greatly up-regulated pro-inflammatory cytokine interleukin (IL)-1β, IL-6 and TNF-α gene expression in PBL cells treated with UDP. Moreover, LPS- and poly(I:C)-induced gene expression of IL-1β and TNF-α in Japanese flounder PBL cells was attenuated significantly by inhibition of P2Y₆R activity with antagonist MRS 2578. Collectively, we, for the first time, showed the involvement of functional purinergic P2Y₆R in fish innate immunity.

Sustained Connexin43 Mimetic Peptide Release From Loaded Nanoparticles Reduces Retinal and Choroidal Photodamage

Purpose

To evaluate the long-term effect on inflammation and inflammasome activation of intravitreally delivered connexin43 mimetic peptide (Cx43MP) in saline or incorporated within nanoparticles (NPs) for the treatment of the light-damaged rat eye.

Methods

Light-induced damage to the retina was created by exposure of adult albino Sprague-Dawley rats to intense light for 24 hours. A single dose of Cx43MP, Cx43MP-NPs, or saline was injected intravitreally at 2 hours after onset of light damage. Fluorescein isothiocyanate (FITC)-labelled Cx43MP-NPs were intravitreally injected to confirm delivery into the retina. Electroretinogram (ERG) recordings were performed at 24 hours, 1 week, and 2 weeks post cessation of light damage. The retinal and choroidal layers were analyzed in vivo using optical coherence tomography (OCT) and immunohistochemistry was performed on harvested tissues using glial fibrillary acidic protein (GFAP), leukocyte common antigen (CD45), and Cx43 antibodies.

Results

FITC was visualized 30 minutes after injection in the ganglion cell layer and in the choroid. Cx43MP and Cx43MP-NP treatments improved a-wave and b-wave function of the ERG compared with saline-injected eyes at 1 week and 2 weeks post treatment, and prevented photoreceptor loss by 2 weeks post treatment. Inflammation was also reduced and this was in parallel with downregulation of Cx43 expression.

Conclusions

The slow release of Cx43MP incorporated into NPs is more effective at treating retinal injury than a single dose of native Cx43MP in solution by reducing inflammation and maintaining both retinal structure and function. This NP preparation has clinical relevance as it reduces possible ocular complications associated with repeated intravitreal injections.

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.

A minimal RNA ligand for potent RIG-I activation in living mice

We have developed highly potent synthetic activators of the vertebrate immune system that specifically target the RIG-I receptor. When introduced into mice, a family of short, triphosphorylated stem-loop RNAs (SLRs) induces a potent interferon response and the activation of specific genes essential for antiviral defense. Using RNA sequencing, we provide the first in vivo genome-wide view of the expression networks that are initiated upon RIG-I activation. We observe that SLRs specifically induce type I interferons, subsets of interferon-stimulated genes (ISGs), and cellular remodeling factors. By contrast, polyinosinic:polycytidylic acid [poly(I:C)], which binds and activates multiple RNA sensors, induces type III interferons and several unique ISGs. The short length (10 to 14 base pairs) and robust function of SLRs in mice demonstrate that RIG-I forms active signaling complexes without oligomerizing on RNA. These findings demonstrate that SLRs are potent therapeutic and investigative tools for targeted modulation of the innate immune system.

Mast Cells Interact with Endothelial Cells to Accelerate In Vitro Angiogenesis

Angiogenesis is a complex process that involves interactions between endothelial cells and various other cell types as well as the tissue microenvironment. Several previous studies have demonstrated that mast cells accumulate at angiogenic sites. In spite of the evidence suggesting a relationship between mast cells and angiogenesis, the association of mast cells and endothelial cells remains poorly understood. The present study aims to investigate the relationship between mast cells and endothelial cells during in vitro angiogenesis. When endothelial cells were co-cultured with mast cells, angiogenesis was stimulated. Furthermore, there was direct intercellular communication via gap junctions between the two cell types. In addition, the presence of mast cells stimulated endothelial cells to release angiogenic factors. Moreover, conditioned medium from the co-cultures also stimulated in vitro angiogenesis. The results from this investigation demonstrate that mast cells have both direct and indirect proangiogenic effects and provide new insights into the role of mast cells in angiogenesis.

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

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.

Mind the Gaps in Tumor Immunity: Impact of Connexin-Mediated Intercellular Connections

Gap junctions (GJs)-mediated intercellular communications (GJICs) are connexin (Cx)-formed plasma membrane channels that allow for the passage of small molecules between adjacent cells, and are involved in several physiopathological processes, including immune responses against cancer. In general, tumor cells are poorly coupled through GJs, mainly due to low Cx expression or reduced channel activity, suggesting that Cxs may have tumor suppressor roles. However, more recent data indicate that Cxs and/or GJICs may also in some cases promote tumor progression. This dual role of Cx channels in tumor outcome may be due, at least partially, to the fact that GJs not only interconnect cells from the same type, such as cancer cells, but also promote the intercellular communication of tumor cells with different types of cells from their microenvironment, and such diverse intercellular interactions have distinctive impact on tumor development. For example, whereas GJ-mediated interactions among tumor cells and microglia have been implicated in promotion of tumor growth, tumor cells delivery to dendritic cells of antigenic peptides through GJs have been associated with enhanced immune-mediated tumor elimination. In this review, we provide an updated overview on the role of GJICs in tumor immunity, focusing on the pro-tumor and antitumor effect of GJs occurring among tumor and immune cells. Accumulated data suggest that GJICs may act as tumor suppressors or enhancers depending on whether tumor cells interact predominantly with antitumor immune cells or with stromal cells. The complex modulation of immune-tumor cell GJICs should be taken into consideration in order to potentiate current cancer immunotherapies.

The role of connexin and pannexin containing channels in the innate and acquired immune response

Connexin (Cx) and pannexin (Panx) containing channels - gap junctions (GJs) and hemichannels (HCs) - are present in virtually all cells and tissues. Currently, the role of these channels under physiological conditions is well defined. However, their role in the immune response and pathological conditions has only recently been explored. Data from several laboratories demonstrates that infectious agents, including HIV, have evolved to take advantage of GJs and HCs to improve viral/bacterial replication, enhance inflammation, and help spread toxicity into neighboring areas. In the current review, we discuss the role of Cx and Panx containing channels in immune activation and the pathogenesis of several infectious diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Pannexins in vision, hearing, olfaction and taste

In mammals, the pannexin gene family consists of three members (Panx1, 2, 3), which represent a class of integral membrane channel proteins sharing some structural features with chordate gap junction proteins, the connexins. Since their discovery in the early 21st century, pannexin expression has been detected throughout the vertebrate body including eye, ear, nose and tongue, making the investigation of the roles of this new class of channel protein in health and disease very appealing. The localization in sensory organs, coupled with unique channel properties and associations with major signaling pathways make Panx1, and its relative's, significant contributors for fundamental functions in sensory perception. Until recently, cell-based studies were at the forefront of pannexin research. Lately, the availability of mice with genetic ablation of pannexins opened new avenues for testing pannexin functions and behavioural phenotyping. Although we are only at the beginning of understanding the roles of pannexins in health and disease, this review summarizes recent advances in elucidating the various emerging roles pannexins play in sensory systems, with an emphasis on unresolved conflicts.

Connexin30.3 is expressed in mouse embryonic stem cells and is responsive to leukemia inhibitory factor

The expression of 19 connexin (Cx) isoforms was observed in the mouse embryonic stem (ES) cell line, EB3. Their expression patterns could be classified into either pluripotent state-specific, differentiating stage-specific, or non-specific Cxs. We focused on Cx30.3 as typical of the first category. Cx30.3 was pluripotent state-specific and upregulated by leukemia inhibitory factor (LIF), a specific cytokine that maintains the pluripotent state of ES cell, via a Jak signaling pathway. Cx30.3 protein was localized to both the cell membrane and cytosol. The dynamic movement of Cx30.3 in the cell membrane was suggested by the imaging analysis by means of overexpressed Cx30.3-EGFP fusion protein. The cytosolic portion was postulated to be a ready-to-use Cx pool. The Cx30.3 expression level in ES cell colonies dramatically decreased immediately after their separation into single cells. It was suggested that mRNA for Cx30.3 and Cx30.3 protein might be decomposed more rapidly than mRNA for Cx43 and Cx43 protein, respectively. These indicate possible involvement of Cx30.3 in the rapid formation and/or decomposition of gap junctions; implying a functional relay between Cx30.3 and other systems such as adhesion proteins.

Connexin-based signaling and drug-induced hepatotoxicity

Being critical mediators of liver homeostasis, connexins and their channels are frequently involved in liver toxicity. In the current paper, specific attention is paid to actions of hepatotoxic drugs on these communicative structures. In a first part, an overview is provided on the structural, regulatory and functional properties of connexin-based channels in the liver. In the second part, documented effects of acetaminophen, hypolipidemic drugs, phenobarbital and methapyriline on connexin signaling are discussed. Furthermore, the relevance of this subject for the fields of clinical and in vitro toxicology is demonstrated.

Relevance for patients: The role of connexin signaling in drug-induced hepatotoxicity may be of high clinical relevance, as it offers perspectives for the therapeutic treatment of such insults by interfering with connexin channel opening.

Defective lymphatic valve development and chylothorax in mice with a lymphatic-specific deletion of Connexin43

Lymphatic valves (LVs) are cusped luminal structures that permit the movement of lymph in only one direction and are therefore critical for proper lymphatic vessel function. Congenital valve aplasia or agenesis can, in some cases, be a direct cause of lymphatic disease. Knowledge about the molecular mechanisms operating during the development and maintenance of LVs may thus aid in the establishment of novel therapeutic approaches to treat lymphatic disorders. In this study, we examined the role of Connexin43 (Cx43), a gap junction protein expressed in lymphatic endothelial cells (LECs), during valve development. Mouse embryos with a null mutation in Cx43 (Gja1) were previously shown to completely lack mesenteric LVs at embryonic day 18. However, interpreting the phenotype of Cx43^-/- mice was complicated by the fact that global deletion of Cx43 causes perinatal death due to heart defects during embryogenesis. We have now generated a mouse model (Cx43^∆LEC) with a lymphatic-specific ablation of Cx43 and show that the absence of Cx43 in LECs causes a delay (rather than a complete block) in LV initiation, an increase in immature valves with incomplete leaflet elongation, a reduction in the total number of valves, and altered lymphatic capillary patterning. The physiological consequences of these lymphatic changes were leaky valves, insufficient lymph transport and reflux, and a high incidence of lethal chylothorax. These results demonstrate that the expression of Cx43 is specifically required in LECs for normal development of LVs.

Expression and role of gap junction protein connexin43 in immune challenge-induced extracellular ATP release in Japanese flounder (Paralichthys olivaceus)

Connexin43 (Cx43) is the best characterized gap junction protein that allows the direct exchange of signaling molecules during cell-to-cell communications. The immunological functions and ATP permeable properties of Cx43 have been insensitively examined in mammals. The similar biological significance of Cx43 in lower vertebrates, however, is not yet understood. In the present study we identified and characterized a Cx43 ortholog (termed PoCx43) from Japanese flounder (Paralichthys olivaceus) and investigated its role in immune challenge-induced extracellular ATP release. PoCx43 mRNA transcripts are widely distributed in all tested normal tissues and cells with predominant expression in the brain, and are significantly up-regulated by LPS, poly(I:C) and zymosan challenges and Edwardsiella tarda infections as well, suggesting that PoCx43 expression was modulated by the inflammatory stresses. In addition, cyclic AMP (cAMP), an essential second messenger, also plays an important role in regulating PoCx43 gene expression, by which the PoCx43-mediated gap junctional communication may be regulated. Furthermore, overexpression of PoCx43 in Japanese flounder FG-9307 cells significantly potentiates the LPS- and poly(I:C)-induced extracellular ATP release and this enhanced ATP release was attenuated by pre-incubation with Cx43 inhibitor carbenoxolone. In a complementary experiment, down-regulation of PoCx43 endogenous expression in FG-9307 cells with small interfering RNA also significantly reduced the PAMP-induced extracellular ATP release, suggesting that PoCx43 is an important ATP release conduit under the immune challenge conditions. Finally, we showed that extracellular ATP stimulation led to an increased PoCx43 expression which probably provides a feedback mechanism in regulating PoCx43 expression at the transcriptional level. These findings suggest that PoCx43 is an inducible immune response gene and an important conduit for immune challenge-induced extracellular ATP release in fish.

Connexins and their channels in inflammation

Inflammation may be caused by a variety of factors and is a hallmark of a plethora of acute and chronic diseases. The purpose of inflammation is to eliminate the initial cell injury trigger, to clear out dead cells from damaged tissue and to initiate tissue regeneration. Despite the wealth of knowledge regarding the involvement of cellular communication in inflammation, studies on the role of connexin-based channels in this process have only begun to emerge in the last few years. In this paper, a state-of-the-art overview of the effects of inflammation on connexin signaling is provided. Vice versa, the involvement of connexins and their channels in inflammation will be discussed by relying on studies that use a variety of experimental tools, such as genetically modified animals, small interfering RNA and connexin-based channel blockers. A better understanding of the importance of connexin signaling in inflammation may open up towards clinical perspectives.

Phagocytosis in Teleosts. Implications of the New Cells Involved

Phagocytosis is the process by which cells engulf some solid particles to form internal vesicles known as phagosomes. Phagocytosis is in fact a specific form of endocytosis involving the vesicular interiorization of particles. Phagocytosis is essentially a defensive reaction against infection and invasion of the body by foreign substances and, in the immune system, phagocytosis is a major mechanism used to remove pathogens and/or cell debris. For these reasons, phagocytosis in vertebrates has been recognized as a critical component of the innate and adaptive immune responses to pathogens. Furthermore, more recent studies have revealed that phagocytosis is also crucial for tissue homeostasis and remodeling. Professional phagocytes in teleosts are monocyte/macrophages, granulocytes and dendritic cells. Nevertheless, in recent years phagocytic properties have also been attributed to teleost lymphocytes and thrombocytes. The possible implications of such cells on this important biological process, new factors affecting phagocytosis, evasion of phagocytosis or new forms of phagocytosis will be considered and discussed.