Cardiac mitochondrial connexin 43 regulates apoptosis

Connexin-43 in Cancer: Above and Beyond Gap Junctions!

Connexin-43 (Cx43) is the most characterized gap junction protein, primarily involved in the Gap Junctional Intercellular Communication (GJIC) between adjacent cells to facilitate molecule exchange and the formation of a signaling network. It is increasingly evident that the importance of Cx43 is not only limited to its GJIC function, but rather includes its role in connecting the intracellular and extracellular environment by forming membrane hemichannels, as well as its intracellular signaling function mediated by its C-terminal tail (Cx43-CT). Notably, Cx43 has been implicated in a variety of cancers, with earlier notions suggesting a tumor-suppressor function, whereas new studies shed light on its pro-tumorigenic role. Moreover, apart from GJIC-based activities, the relevance of the non-canonical functions of Cx43 in tumor progression is being actively studied. This review provides an analysis of the current research on the pro-tumorigenic roles of Cx43, with a focus on Cx43-CT interactions and the function of hemichannels in cancer progression. A better understanding of the multifaceted functions of Cx43 in cancer biology could foster its recognition as a pivotal target for the development of innovative therapeutic strategies.

Epigenetics of Homocystinuria, Hydrogen Sulfide, and Circadian Clock Ablation in Cardiovascular-Renal Disease

Morning-time heart attacks are associated with an ablation in the sleep-time dip in blood pressure, the mechanism of which is unknown. The epigenetic changes are the hallmark of sleep and circadian clock disruption and homocystinuria (HHcy). The homocystinuria causes ablation in the dip in blood pressure during sleep. Interestingly, HHcy is generated during the epigenetic gene turning off and turning on (i.e., imprinting) by methylation of the DNA promoter. The mitochondrial sulfur metabolism by 3-mercaptopyruvate sulfur transferase (3MST), ATP citrate lyase (ACYL), and epigenetic rhythmic methylation are regulated by folate 1-carbon metabolism (FOCM), i.e., the methionine (M)-SAM-SAH-Hcy, adenosine, and uric acid cycle. Epigenetic gene writer (DNMT), gene eraser (TET/FTO), and editor de-aminase (ADAR) regulate the rhythmic, i.e., reversible methylation/demethylation of H3K4, H3K9, H4K20, m6A, and m5C. The mitochondrial ATP citrate cycle and creatine kinase (CK) regulate chromatin transcription, maturation, and accessibility as well as muscle function. The transcription is regulated by methylation. The maturation and accessibility are controlled by acetylation. However, it is unclear whether a high fat dysbiotic diet (HFD) causes dysrhythmic expression of the gene writer, eraser, and editor, creating hyperuricemia and cardiac and renal dysfunction. We hypothesized that an HFD increases the gene writer (DNMT1) and editor (ADAR), decreases the eraser (TET/FTO), and increases uric acid to cause chronic diseases. This increases the levels of H3K4, H3K9, H4K20, m6A, and m5C. Interestingly, the DNMT1KO mitigates. Further, the DNMT1KO and ADAR inhibition attenuate HFD-induced NGAL/FGF23/TMPRSS2/MMP2, 9, 13, and uric acid levels and improve cardiac and renal remodeling. Although the novel role of nerve endings by the Piezo channels (i.e., the combination of ENaC, VDAC, TRPV, K+, and Mg2+ channels) in the interoception is suggested, interestingly, we and others have shown mechanisms independent of the nerve, by interoception, such as the cargo of the exosome in denervation models of heart failure. If proper and appropriate levels of these enzymes are available to covert homocysteine to hydrogen sulfide (H2S) during homocystinuria, then the H2S can potentially serve as a newer form of treatment for morning heart attacks and renal sulfur transsulfuration transport diseases.

Diversity in connexin biology

Over 35 years ago the cell biology community was introduced to connexins as the subunit employed to assemble semicrystalline clusters of intercellular channels that had been well described morphologically as gap junctions. The decade that followed would see knowledge of the unexpectedly large 21-member human connexin family grow to reflect unique and overlapping expression patterns in all organ systems. While connexin biology initially focused on their role in constructing highly regulated intercellular channels, this was destined to change as discoveries revealed that connexin hemichannels at the cell surface had novel roles in many cell types, especially when considering connexin pathologies. Acceptance of connexins as having bifunctional channel properties was initially met with some resistance, which has given way in recent years to the premise that connexins have multifunctional properties. Depending on the connexin isoform and cell of origin, connexins have wide-ranging half-lives that vary from a couple of hours to the life expectancy of the cell. Diversity in connexin channel characteristics and molecular properties were further revealed by X-ray crystallography and single-particle cryo-EM. New avenues have seen connexins or connexin fragments playing roles in cell adhesion, tunneling nanotubes, extracellular vesicles, mitochondrial membranes, transcription regulation, and in other emerging cellular functions. These discoveries were largely linked to Cx43, which is prominent in most human organs. Here, we will review the evolution of knowledge on connexin expression in human adults and more recent evidence linking connexins to a highly diverse array of cellular functions.

Mitochondrial Connexins and Mitochondrial Contact Sites with Gap Junction Structure

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

The Implications of Connexin 43 Deficiency during the Early Stages of Chemically Induced Mouse Colon Carcinogenesis

Colorectal cancer (CRC), associated with an increased intake of processed red meats, saturated fats, and simple carbohydrates accompanied by low dietary fiber, fruits, and vegetables consumption, presents a high epidemiological burden. Connexin43 (Cx43) protein, which forms gap junctions or hemichannels, has tumor suppressor or oncogenic activities in a cancer type- and stage-dependent manner. Cx43 expression varies during colon carcinogenesis, and its functional role is not fully understood. Thus, we evaluated the implications of Cx43 heterologous deletion (Cx43^+/-) during the early stages of a chemically induced model of colon carcinogenesis. Female C57BL/6J mice (wild-type or Cx43^+/-) were submitted to a colon carcinogenesis model induced by 1,2 dimethylhydrazine (DMH). Mice were euthanized eight hours (week 7) or 30 weeks (week 37) after the last DMH administration to evaluate subacute colon toxicity outcomes or the burden of (pre)neoplastic lesions, respectively. At week 7, Cx43 deficiency inferred no alterations in the DMH-induced increase in systemic (peripheral blood), in situ (colonocytes) DNA damage, and apoptosis in the colonocytes. At week 30, Cx43^+/- mice presented an increase in preneoplastic aberrant crypt foci (ACF) multiplicity, while no alterations were observed in colorectal adenoma (CRA) occurrence, multiplicity, volume, proliferation, growth, and β-catenin immunoexpression. Similarly, an in silico analysis of human CRA showed decreased mRNA expression of Cx43 with no correlation with proliferation, apoptosis, and β-catenin markers. These findings indicate the discrete role of Cx43 in the early stages of chemically induced mouse colon carcinogenesis.

Connexins and Glucose Metabolism in Cancer

Connexins are a family of transmembrane proteins that regulate diverse cellular functions. Originally characterized for their ability to mediate direct intercellular communication through the formation of highly regulated membrane channels, their functions have been extended to the exchange of molecules with the extracellular environment, and the ability to modulate numerous channel-independent effects on processes such as motility and survival. Notably, connexins have been implicated in cancer biology for their context-dependent roles that can both promote or suppress cancer cell function. Moreover, connexins are able to mediate many aspects of cellular metabolism including the intercellular coupling of nutrients and signaling molecules. During cancer progression, changes to substrate utilization occur to support energy production and biomass accumulation. This results in metabolic plasticity that promotes cell survival and proliferation, and can impact therapeutic resistance. Significant progress has been made in our understanding of connexin and cancer biology, however, delineating the roles these multi-faceted proteins play in metabolic adaptation of cancer cells is just beginning. Glucose represents a major carbon substrate for energy production, nucleotide synthesis, carbohydrate modifications and generation of biosynthetic intermediates. While cancer cells often exhibit a dependence on glycolytic metabolism for survival, cellular reprogramming of metabolic pathways is common when blood perfusion is limited in growing tumors. These metabolic changes drive aggressive phenotypes through the acquisition of functional traits. Connections between glucose metabolism and connexin function in cancer cells and the surrounding stroma are now apparent, however much remains to be discovered regarding these relationships. This review discusses the existing evidence in this area and highlights directions for continued investigation.

Connexin 43 in Mitochondria: What Do We Really Know About Its Function?

Connexins are known for their ability to mediate cell-cell communication via gap junctions and also form hemichannels that pass ions and molecules over the plasma membrane when open. Connexins have also been detected within mitochondria, with mitochondrial connexin 43 (Cx43) being the best studied to date. In this review, we discuss evidence for Cx43 presence in mitochondria of cell lines, primary cells and organs and summarize data on its localization, import and phosphorylation status. We further highlight the influence of Cx43 on mitochondrial function in terms of respiration, opening of the mitochondrial permeability transition pore and formation of reactive oxygen species, and also address the presence of a truncated form of Cx43 termed Gja1-20k. Finally, the role of mitochondrial Cx43 in pathological conditions, particularly in the heart, is discussed.

High Glucose-Induced Apoptosis Is Linked to Mitochondrial Connexin 43 Level in RRECs: Implications for Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the most common causes of vision loss and blindness among the working-age population. High glucose (HG)-induced decrease in mitochondrial connexin 43 (mtCx43) level is known to promote mitochondrial fragmentation, cytochrome c release, and apoptosis in retinal endothelial cells associated with DR. In this study, we investigated whether counteracting HG-induced decrease in mtCx43 level would preserve mitochondrial integrity and prevent apoptosis. Rat retinal endothelial cells (RRECs) were grown in normal (N; 5 mM glucose) or HG (30 mM glucose) medium for 7 days. In parallel, cells grown in HG were transfected with Cx43 plasmid, or empty vector (EV), as control. Western blot (WB) analysis showed a significant decrease in mtCx43 level concomitant with increased cleaved caspase-3, Bax, cleaved PARP, and mitochondrial fragmentation in cells grown in HG condition compared to those grown in N medium. When cells grown in HG were transfected with Cx43 plasmid, mtCx43 level was significantly increased and resulted in reduced cleaved caspase-3, Bax, cleaved PARP and preservation of mitochondrial morphology with a significant decrease in the number of TUNEL-positive cells compared to those grown in HG alone. Findings from the study indicate a novel role for mtCx43 in regulating apoptosis and that maintenance of mtCx43 level could be useful in preventing HG-induced apoptosis by reducing mitochondrial fragmentation associated with retinal vascular cell loss in DR.

Majonoside-R2 Postconditioning Protects Cardiomyocytes Against Hypoxia/Reoxygenation Injury by Attenuating the Expression of HIF1α and Activating RISK Pathway

Reoxygenation of hypoxic cardiac myocytes can paradoxically induce myocardial injury and affect the recovery processes. Pharmacological postconditioning is an efficient strategy used in clinical practice that protects cardiomyocytes from hypoxia/reoxygenation (HR) injury. Natural products or foods have been known to possess effective cardioprotective properties. Majonoside-R2 (MR2) is a dominant saponin component of Vietnamese ginseng that has several biological effects. In this study, we evaluated the protective effect of MR2 on HR-stimulated cardiomyocytes and investigated the related molecular mechanisms. H9C2 cardiomyocytes were exposed to HR conditions with or without MR2 supplementation. Samples from experimental groups were used to analyze the expression of apoptosis- and activating reperfusion injury salvage kinase (RISK)-related factors in response to HR injury by using enzyme-linked immunosorbent assay, real-time polymerase chain reaction, and Western blotting. Post-treatment, MR2 enhanced cell viability under HR conditions. We found that MR2 suppressed the expression of hypoxia-inducible factor 1-alpha (HIF1α) and transforming growth factor beta 1 (TGFβ1), modulated Akt/GSK3ß/cAMP response element-binding signaling, and regulated gene expression related to apoptosis (B cell lymphoma-extra-large [Bcl-xl], Bcl-2 homologous killer [Bak], Bcl-2 associated X [Bax], and connexin 43 [Cnx43]). Thus, the present findings demonstrate that MR2 protects cardiomyocytes against HR injury by suppressing the expression of HIF1α and activating the RISK pathway.

Cellular crosstalk in cardioprotection: Where and when do reactive oxygen species play a role?

A well-balanced intercellular communication between the different cells within the heart is vital for the maintenance of cardiac homeostasis and function. Despite remarkable advances on disease management and treatment, acute myocardial infarction remains the major cause of morbidity and mortality worldwide. Gold standard reperfusion strategies, namely primary percutaneous coronary intervention, are crucial to preserve heart function. However, reestablishment of blood flow and oxygen levels to the infarcted area are also associated with an accumulation of reactive oxygen species (ROS), leading to oxidative damage and cardiomyocyte death, a phenomenon termed myocardial reperfusion injury. In addition, ROS signaling has been demonstrated to regulate multiple biological pathways, including cell differentiation and intercellular communication. Given the importance of cell-cell crosstalk in the coordinated response after cell injury, in this review, we will discuss the impact of ROS in the different forms of inter- and intracellular communication, as well as the role of gap junctions, tunneling nanotubes and extracellular vesicles in the propagation of oxidative damage in cardiac diseases, particularly in the context of ischemia/reperfusion injury.

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.

Cardiac Connexin-43 Hemichannels and Pannexin1 Channels: Provocative Antiarrhythmic Targets

Cardiac connexin-43 (Cx43) creates gap junction channels (GJCs) at intercellular contacts and hemi-channels (HCs) at the peri-junctional plasma membrane and sarcolemmal caveolae/rafts compartments. GJCs are fundamental for the direct cardiac cell-to-cell transmission of electrical and molecular signals which ensures synchronous myocardial contraction. The HCs and structurally similar pannexin1 (Panx1) channels are active in stressful conditions. These channels are essential for paracrine and autocrine communication through the release of ions and signaling molecules to the extracellular environment, or for uptake from it. The HCs and Panx1 channel-opening profoundly affects intracellular ionic homeostasis and redox status and facilitates via purinergic signaling pro-inflammatory and pro-fibrotic processes. These conditions promote cardiac arrhythmogenesis due to the impairment of the GJCs and selective ion channel function. Crosstalk between GJCs and HCs/Panx1 channels could be crucial in the development of arrhythmogenic substrates, including fibrosis. Despite the knowledge gap in the regulation of these channels, current evidence indicates that HCs and Panx1 channel activation can enhance the risk of cardiac arrhythmias. It is extremely challenging to target HCs and Panx1 channels by inhibitory agents to hamper development of cardiac rhythm disorders. Progress in this field may contribute to novel therapeutic approaches for patients prone to develop atrial or ventricular fibrillation.

Gap Junction Intercellular Communication Negatively Regulates Cadmium-Induced Autophagy and Inhibition of Autophagic Flux in Buffalo Rat Liver 3A Cells

Cadmium is an important environmental pollutant that poses a serious threat to the health of humans and animals. A large number of studies have shown that the liver is one of the important target organs of cadmium. Stimulation of cells can lead to rapid changes in gap junction intercellular communication (GJIC) and autophagy. Previous studies have shown that cadmium can inhibit GJIC and induce autophagy. In order to understand the dynamic changes of GJIC and autophagy in the process of cadmium-induced hepatotoxic injury and the effects of GJIC on autophagy, a time-gradient model of cadmium cytotoxicity was established. The results showed that within 24 h of cadmium exposure, 5 μmol/L cadmium inhibited GJIC by down regulating the expression levels of connexin 43 (Cx43) and disturbing the localization of Cx43 in Buffalo rat liver 3A (BRL 3A) cells. In addition, cadmium induced autophagy and then inhibited autophagic flux in the later stage. During this process, inhibiting of GJIC could exacerbate the cytotoxic damage of cadmium and induce autophagy, but further blocked autophagic flux, promoting GJIC in order to obtain the opposite results.

Mitochondrial ion channels as targets for cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often result remain the leading causes of death and disability worldwide. As such, new therapeutic targets need to be discovered to protect the myocardium against acute ischaemia/reperfusion (I/R) injury in order to reduce myocardial infarct (MI) size, preserve left ventricular function and prevent the onset of HF. Mitochondrial dysfunction during acute I/R injury is a critical determinant of cell death following AMI, and therefore, ion channels in the inner mitochondrial membrane, which are known to influence cell death and survival, provide potential therapeutic targets for cardioprotection. In this article, we review the role of mitochondrial ion channels, which are known to modulate susceptibility to acute myocardial I/R injury, and we explore their potential roles as therapeutic targets for reducing MI size and preventing HF following AMI.

Connexin/Innexin Channels in Cytoplasmic Organelles. Are There Intracellular Gap Junctions? A Hypothesis!

This paper proposes the hypothesis that cytoplasmic organelles directly interact with each other and with gap junctions forming intracellular junctions. This hypothesis originated over four decades ago based on the observation that vesicles lining gap junctions of crayfish giant axons contain electron-opaque particles, similar in size to junctional innexons that often appear to directly interact with junctional innexons; similar particles were seen also in the outer membrane of crayfish mitochondria. Indeed, vertebrate connexins assembled into hexameric connexons are present not only in the membranes of the Golgi apparatus but also in those of the mitochondria and endoplasmic reticulum. It seems possible, therefore, that cytoplasmic organelles may be able to exchange small molecules with each other as well as with organelles of coupled cells via gap junctions.

Inhibition of connexin 43 attenuates oxidative stress and apoptosis in human umbilical vein endothelial cells

BACKGROUND

Previous studies demonstrated an important role for connexin 43 (Cx43) in the regulation of apoptosis by influencing mitochondrial functions. This study aimed to investigate the relationship between Cx43 and lipopolysaccharide (LPS)-induced oxidative stress and apoptosis in human umbilical vein endothelial cells (HUVECs).

METHODS

Western blot was performed to determine mitochondrial Cx43 (MtCx43) protein level and phosphorylation (p-MtCx43). Gap19, a selective Cx43 inhibitor, was used to examine the effects of Cx43 on LPS-induced oxidative stress and apoptosis in HUVECs. Expression of regulatory genes associated with oxidative stress was examined by quantitative polymerase chain reaction (qPCR) and Western blot. Apoptosis was assessed by flow cytometry.

RESULTS

LPS stimulation resulted in increased levels of MtCx43 and p-MtCx43. Interestingly, Gap19 antagonized the upregulation of glutathione S-transferase Zeta 1 (GSTZ1) and cytochrome b alpha beta (CYBB), and the downregulation of antioxidant 1 (ATOX1), glutathione synthetase (GSS) and heme oxygenase 1 (HMOX1) induced by LPS or Cx43 overexpression. Moreover, the increased production of reactive oxygen species (ROS) and apoptosis elicited by LPS or Cx43 overexpression were reduced following treatment with Gap19.

CONCLUSIONS

Selective inhibition of Cx43 hemichannels protects HUVECs from LPS-induced apoptosis and this may be via a reduction in oxidative stress production.

A role of AMPK and connexin 43 in the suppression of CoCl2-induced apoptosis of spiral modiolar artery smooth muscle cells by adiponectin

AIMS

Adiponectin (APN) is a protein hormone secreted mainly by adipose tissue that exhibits biological functions such as anti-inflammatory, anti-atherosclerotic, anti-apoptotic, hearing-protective and microcirculation-regulating functions. In this study, we explored whether APN could attenuate damage caused by CoCl₂-induced hypoxic conditions in smooth muscle cells (SMCs) of the spiral modiolar artery (SMA).

MAIN METHODS

We first cultured and identified primary SMCs of the SMA. Afterward, the SMCs were pre-treated with APN and then stimulated with CoCl₂.

KEY FINDINGS

Compared with the control group, the group treated with CoCl₂ for 24 h exhibited significantly decreased cell viability, significantly increased apoptosis rates and Malondialdehyde (MDA) levels, and decreased Superoxide Dismutase (SOD) activity. In addition, the expression levels of Bax and cleaved caspase-3 were upregulated, while those of Bcl2 were downregulated evidently. Compared with the CoCl₂ group, the group pre-treated with APN before receiving CoCl₂ treatment had increased cell viability and SOD activity but decreased MDA levels and apoptosis rates. The expression levels of Bcl2, p-AMPKα and Cx43 were evidently increased, while those of Bax and cleaved caspase-3 were decreased, in the group pre-treated with APN compared to the CoCl₂ group. The protective effect of APN was blocked by the AMPK inhibitor Compound C and the Cx43 inhibitor Gap19.

SIGNIFICANCE

Our study demonstrated that APN protected SMCs against CoCl₂-induced hypoxic injury via the AMPK signalling pathway and regulated the expression of Cx43 in cells. Therefore, APN might be a promising treatment for diseases related to circulation disturbances of the inner ear.

Mitochondrial Structural Changes in the Pathogenesis of Diabetic Retinopathy

At the core of proper mitochondrial functionality is the maintenance of its structure and morphology. Physical changes in mitochondrial structure alter metabolic pathways inside mitochondria, affect mitochondrial turnover, disturb mitochondrial dynamics, and promote mitochondrial fragmentation, ultimately triggering apoptosis. In high glucose condition, increased mitochondrial fragmentation contributes to apoptotic death in retinal vascular and Müller cells. Although alterations in mitochondrial morphology have been detected in several diabetic tissues, it remains to be established in the vascular cells of the diabetic retina. From a mechanistic standpoint, our current work supports the notion that increased expression of fission genes and decreased expression of fusion genes are involved in promoting excessive mitochondrial fragmentation. While mechanistic insights are only beginning to reveal how high glucose alters mitochondrial morphology, the consequences are clearly seen as release of cytochrome c from fragmented mitochondria triggers apoptosis. Current findings raise the prospect of targeting excessive mitochondrial fragmentation as a potential therapeutic strategy for treatment of diabetic retinopathy. While biochemical and epigenetic changes have been reported to be associated with mitochondrial dysfunction, this review focuses on alterations in mitochondrial morphology, and their impact on mitochondrial function and pathogenesis of diabetic retinopathy.

The activation of retinal HCA2 receptors by systemic beta-hydroxybutyrate inhibits diabetic retinal damage through reduction of endoplasmic reticulum stress and the NLRP3 inflammasome

OBJECTIVE

The role of the hydroxycarboxylic acid receptor 2 (HCA2) in the retinal damage induced by diabetes has never been explored. In this context, the present study highlights an upregulation of retinal HCA2 receptors in diabetic C57BL6J mice. Moreover, we illustrate that HCA2 receptors exert an anti-inflammatory effect on the retinal damage induced by diabetes when activated by the endogenous ligand β-hydroxybutyrate.

METHODOLOGY

Seven-to-10-week-old C57BL6J mice were rendered diabetic by a single intraperitoneal injection of streptozotocin (75 mg/kg of body weight) and monitored intermittently over a 10-week period extending from the initial diabetes assessment. Mice with a fasting blood glucose level higher than 250 mg/dl for 2 consecutive weeks after streptozotocin injection were treated twice a week with intraperitoneal injections of 25-50-100 mg/kg β-hydroxybutyrate.

RESULTS

Interestingly, while the retinal endoplasmic reticulum stress markers (pPERK, pIRE1, ATF-6α) were elevated in diabetic C57BL6J mice, their levels were significantly reduced by the systemic intraperitoneal treatment with 50 mg/kg and 100 mg/kg β-hydroxybutyrate. These mice also exhibited high NLRP3 inflammasome activity and proinflammatory cytokine levels. In fact, the elevated levels of retinal NLRP3 inflammasome activation markers (NLRP3, ASC, caspase-1) and of the relative proinflammatory cytokines (IL-1β, IL-18) were significantly reduced by 50 mg/kg and 100 mg/kg β-hydroxybutyrate treatment. These doses also reduced the high apoptotic cell number exhibited by the diabetic mice in the retinal outer nuclear layer (ONL) and increased the ONL low connexin 43 expression, leading to an improvement in retinal permeability and homeostasis.

CONCLUSIONS

These data suggest that the systemic treatment of diabetic C57BL6J mice with BHB activates retinal HCA2 and inhibits local damage.

Visualization of Annular Gap Junction Vesicle Processing: The Interplay Between Annular Gap Junctions and Mitochondria

It is becoming clear that in addition to gap junctions playing a role in cell⁻cell communication, gap junction proteins (connexins) located in cytoplasmic compartments may have other important functions. Mitochondrial connexin 43 (Cx43) is increased after ischemic preconditioning and has been suggested to play a protective role in the heart. How Cx43 traffics to the mitochondria and the interactions of mitochondria with other Cx43-containing structures are unclear. In this study, immunocytochemical, super-resolution, and transmission electron microscopy were used to detect cytoplasmic Cx43-containing structures and to demonstrate their interactions with other cytoplasmic organelles. The most prominent cytoplasmic Cx43-containing structures-annular gap junctions-were demonstrated to form intimate associations with lysosomes as well as with mitochondria. Surprisingly, the frequency of associations between mitochondria and annular gap junctions was greater than that between lysosomes and annular gap junctions. The benefits of annular gap junction/mitochondrial associations are not known. However, it is tempting to suggest, among other possibilities, that the contact between annular gap junction vesicles and mitochondria facilitates Cx43 delivery to the mitochondria. Furthermore, it points to the need for investigating annular gap junctions as more than only vesicles destined for degradation.

Hemichannel-mediated volume regulation contributes to IPC-induced cardiomyocyte protection

Cx43 has been documented to be involved in ischemic preconditioning (IPC). However, the participation of Cx43-formed hemichannels in IPC and the potential underlying mechanisms remain unclear. The present study focused on cardiomyocytes' volume regulation during IPC to investigate the role of hemichannels in the IPC-induced cardioprotection. In the study, mice cardiomyocytes were respectively treated with a hemichannel blocker, octanol or 18a-Glycyrrhizic acid (18a-GA), and a Cx43-silenced lentivirus. They were subsequently cultured in hypotonic solution to simulate ischemic reperfusion (SIR) and systemic ischemic preconditioning (SIP). Cell morphology and volumetric (area) change were detected by inverted microscopy at 30 min following the addition of hypotonic solution. Cardiomyocyte mortality was assessed by trypan blue stain assay. The analyses revealed that regardless of the treatments, hypotonic solution aggravated cell edema: Compared with the initial condition (the moment before the solution addition, 0 min), the volumetric area increased significantly 30 min later (for hypotonic+DMSO, 5,050±1,511 vs. 3,464±723 µm²; for hypotonic+scramble lentiviral vector, 5,517±1,128 vs. 2,331±536 µm²; P<0.05, respectively). Either treatment alleviated the edematous condition when a comparison was made between 30 min after the hypotonic addition and 0 min (for hypotonic+octanol, 2,990±765 vs. 2,821±773 µm²; for hypotonic+18a-GA, 4,817±1,306 vs. 4,762±1,271 µm²; for hypotonic+Cx43-silenced, 3,627±688 vs. 3,419±814 µm²; P>0.05 for all). Notably, results indicated that the SIP group had lower mortality rates compared with its SIR counterpart; the hypotonic+octanol, hypotonic+18a-GA, and hypotonic+Cx43-silenced group showed markedly-declined mortality when compared with their respective control groups (respectively, 35.70±1.02, 30.76±2.20 vs. 53.58±2.14%; 30.89±2.37 vs. 54.12±2.55%; P<0.05 for all). The results suggest that ischemic preconditioning may provide cardioprotection by blocking the opening of the hemichannels and further mediating the volume regulation of cardiomyocytes.

The role of connexins during early embryonic development: pluripotent stem cells, gene editing, and artificial embryonic tissues as tools to close the knowledge gap

Since almost 4 decades, connexins have been discussed as important regulators of embryogenesis. Several different members of the gene family can be detected in the preimplantation embryo and during gastrulation. However, genetically engineered mice deficient for every connexin expressed during early development are available and even double-deficient mice were generated. Interestingly, all of these mice complete gastrulation without any abnormalities. This raises the question if the role of connexins has been overrated or if other gene family members compensate and mask their importance. To answer this question, embryos completely devoid of any gap junctional communication need to be investigated. This is challenging because a variety of connexin genes are co-expressed and some null mutations lead to a lethal phenotype. In addition, maternal connexin transcripts were described to persist until the blastocyst stage. In this review, we summarize the current knowledge about the role of connexins during preimplantation development and in embryonic stem cells. We propose that the use of pluripotent stem cells, trophoblast stem cells, as well as artificial embryo-like structures and organoid cultures in combination with multiplex CRISPR/Cas9-based genome editing provides a powerful platform to comprehensively readdress this issue and decipher the role of connexins during lineage decision, differentiation, and morphogenesis in a cell culture model for mouse and human development.

A novel mechanism of depression: role for connexins

Major depressive disorder (MDD) is a chronic and debilitating illness that affects over 350 million people worldwide; however, current treatments have failed to cure or prevent the progress of depression. Increasing evidence suggests a crucial role for connexins in MDD. In this review, we have summarised recent accomplishments regarding the role of connexins, gap junctions, and hemichannels in the aetiology of MDD, and discussed the limitations of current research. A blockage of gap junctions or hemichannels induces depressive behaviour. Possible underlying mechanisms include the regulation of neurosecretory functions and synaptic activity by gap junctions and hemichannels. Gap junctions are functionally inhibited under stress conditions. Conversely, hemichannel permeability is increased. Antidepressants inhibit hemichannel permeability; however, they have contrasting effects on the function of gap junctions under normal conditions and can protect them against stress. In conclusion, the blockage of hemichannels concurrent with improvements in gap junction functionality might be potential targets for depression treatment.

Protective effects of kenpaullone on cardiomyocytes following H2O2-induced oxidative stress are attributed to inhibition of connexin 43 degradation by SGSM3

A previous study showed that small G protein signaling modulator 3 (SGSM3) was highly correlated with Cx43 in heart functions and that high levels of SGSM3 may induce Cx43 turnover through lysosomal degradation in infarcted rat hearts. Here, we investigated the protective effects of kenpaullone on cardiomyocytes following H₂O₂-induced oxidative stress mediated by the interaction of SGSM3 with Cx43. We found that the gap junction protein Cx43 was significantly down-regulated in an H₂O₂ concentration-dependent manner, whereas expression of SGSM3 was up-regulated upon H₂O₂ exposure in H9c2 cells. The effect of kenpaullone pretreatment on H₂O₂-induced cytotoxicity was evaluated in H9c2 cells. H₂O₂ markedly increased the release of lactate dehydrogenase (LDH), while kenpaullone pretreatment suppressed LDH release in H9c2 cells. Moreover, kenpaullone pretreatment significantly reduced ROS fluorescence intensity and significantly down-regulated the level of apoptosis-activating genes (cleaved caspase-3, cleaved caspase-9 and cytochrome C), autophagy markers (LC3A/B), and the Cx43-interacting partner SGSM3. These results suggest that kenpaullone plays a role in protecting cardiomyocytes from oxidative stress and that the turnover of Cx43 through SGSM3-induced lysosomal degradation underlies the anti-apoptotic effect of kenpaullone.

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.

Mitochondrial Cx43, an important component of cardiac preconditioning

Connexin 43 (Cx43) forms gap junction channels that are essential for the propagation of electrical depolarization in cardiomyocytes, but also with important roles in the pathophysiology of reperfusion injury. However, more recent studies have shown that Cx43 has also important functions independent from intercellular communication between adjacent cardiomyocytes. Some of these actions have been related to the presence of Cx43 in the mitochondria of these cells (mitoCx43). The functions of mitoCx43 have not been completely elucidated, but there is strong evidence indicating that mitoCx43 modulates mitochondrial respiration at respiratory complex I, production of radical oxygen species and ATP synthesis. These functions of mitoCx43 modulate mitochondrial and cellular tolerance to reperfusion after prolonged ischemia and are necessary for the cardioprotective effect of ischemic preconditioning. In the present review article we discuss available knowledge on these functions of mitoCx43 in relation to reperfusion injury, the molecular mechanisms involved and explore the possibility that mitoCx43 may constitute a new pharmacological target in patients with ST-segment elevation myocardial infarction (STEMI). This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Multiple and complex influences of connexins and pannexins on cell death

Cell death is a fundamental process for organogenesis, immunity and cell renewal. During the last decades a broad range of molecular tools were identified as important players for several different cell death pathways (apoptosis, pyroptosis, necrosis, autosis…). Aside from these direct regulators of cell death programs, several lines of evidence proposed connexins and pannexins as potent effectors of cell death. In the present review we discussed the potential roles played by connexins, pannexins and innexins in the different cell death programs at different scales from gap junction intercellular communication to protein-protein interactions. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Connexin channel and its role in diabetic retinopathy

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

Mitochondrial connexin40 regulates mitochondrial calcium uptake in coronary endothelial cells

Connexins (Cxs) are a group of integral membrane proteins that can form gap junctions between adjacent cells. Recently, it was reported that Cx43 is expressed not only in the plasma membrane but also in the inner mitochondrial membrane and that it regulates mitochondrial functions. Cx40 is predominantly expressed in vascular endothelial cells (ECs) and plays an important role in the electrical propagation between ECs and endothelial/smooth muscle cells. However, it is unknown whether Cx40 is expressed in the mitochondria and what the role of mitochondrial Cx40 is in endothelial functions. We observed in coronary ECs that Cx40 protein was expressed in the mitochondria, as determined by Western blot and immunofluorescence studies. We found that mouse coronary ECs (MCECs) isolated from Cx40 knockout (Cx40 KO) mice exhibited significantly lower resting mitochondrial calcium concentration ([Ca2+]mito) than MCECs from wild-type (WT) mice. After increase in cytosolic Ca2+ concentration ([Ca2+]cyto) with cyclopiazonic acid, calcium uptake into the mitochondria was significantly attenuated in MCECs from Cx40 KO mice compared with WT MCECs. There was no difference in resting [Ca2+]cyto and store-operated calcium entry in MCECs from WT and Cx40 KO mice. We also detected a significant decrease in the concentration of mitochondrial reactive oxygen species (ROS) in Cx40 KO MCECs. Cx40 overexpression in ECs significantly increased resting [Ca2+]mito level and calcium uptake by mitochondria in response to increased [Ca2+]cyto and augmented mitochondrial ROS production. These data suggest that mitochondrial Cx40 contributes to the regulation of mitochondrial calcium homeostasis.

Gap junctions and hemichannels: communicating cell death in neurodevelopment and disease

Gap junctions are unique membrane channels that play a significant role in intercellular communication in the developing and mature central nervous system (CNS). These channels are composed of connexin proteins that oligomerize into hexamers to form connexons or hemichannels. Many different connexins are expressed in the CNS, with some specificity with regard to the cell types in which distinct connexins are found, as well as the timepoints when they are expressed in the developing and mature CNS. Both the main neuronal Cx36 and glial Cx43 play critical roles in neurodevelopment. These connexins also mediate distinct aspects of the CNS response to pathological conditions. An imbalance in the expression, translation, trafficking and turnover of connexins, as well as mutations of connexins, can impact their function in the context of cell death in neurodevelopment and disease. With the ever-increasing understanding of connexins in the brain, therapeutic strategies could be developed to target these membrane channels in various neurological disorders.

Connexin 43 and Mitochondria in Cardiovascular Health and Disease

Connexin 43 (Cx43) is the major connexin protein in ventricular cardiomyocytes. Six Cx43 proteins assemble into so-called hemichannels at the sarcolemma and opposing hemichannels form gap junctions, which allow the passage of small molecules and electrical current flow between adjacent cells. Apart from its localization at the plasma membrane, Cx43 is also present in cardiomyocyte mitochondria, where it is important for mitochondrial function in terms of oxygen consumption and potassium fluxes. The expression of gap junctional and mitochondrial Cx43 is altered under several pathophysiological conditions among them are hypertension, hypertrophy, hypercholesterolemia, ischemia/reperfusion injury, post-infarction remodeling, and heart failure. The present review will focus on the role of Cx43 in cardiovascular diseases and will highlight the importance of mitochondrial Cx43 in cardioprotection.

Gap junctions and cancer: communicating for 50 years

Fifty years ago, tumour cells were found to lack electrical coupling, leading to the hypothesis that loss of direct intercellular communication is commonly associated with cancer onset and progression. Subsequent studies linked this phenomenon to gap junctions composed of connexin proteins. Although many studies support the notion that connexins are tumour suppressors, recent evidence suggests that, in some tumour types, they may facilitate specific stages of tumour progression through both junctional and non-junctional signalling pathways. This Timeline article highlights the milestones connecting gap junctions to cancer, and underscores important unanswered questions, controversies and therapeutic opportunities in the field.

Doxorubicin-Mediated Cardiotoxicity: Role of Mitochondrial Connexin 43

Doxorubicin is the highly effective anthracycline, but its clinical use is limited by cardiotoxicity and consequent dysfunction. It has been proposed that the etiology of this is related to mitochondrial dysfunction. Connexin 43 (Cx43), the principal protein building block of cardiac gap junctions and hemichannels, plays an important role in cardioprotection. Recent reports confirmed the presence of Cx43 in the mitochondria as well. In this study, the role of mitochondrial Cx43 was evaluated 3 or 6 h after Doxorubicin administration to the rat heart cell line H9c2. Pharmacological inhibition of Hsp90 demonstrated that the mitochondrial Cx43 conferred cardioprotection by reducing cytosolic and mitochondrial reactive oxygen species production, mitochondrial calcium overload and mitochondrial membrane depolarization and cytochrome c release. In conclusion, our study demonstrates that Cx43 plays an important role in the protection of cardiac cells from Doxorubicin-induced toxicity.

Structure, Regulation and Function of Gap Junctions in Liver

Gap junctions are a specialized group of cell-to-cell junctions that mediate direct intercellular communication between cells. They arise from the interaction of two hemichannels of adjacent cells, which in turn are composed of six connexin proteins. In liver, gap junctions are predominantly found in hepatocytes and play critical roles in virtually all phases of the hepatic life cycle, including cell growth, differentiation, liver-specific functionality and cell death. Liver gap junctions are directed through a broad variety of mechanisms ranging from epigenetic control of connexin expression to post-translational regulation of gap junction activity. This paper reviews established and novel aspects regarding the architecture, control and functional relevance of liver gap junctions.

Role of connexin 43 in cardiovascular diseases

Gap junctions (GJs) channels provide the basis for intercellular communication in the cardiovascular system for maintenance of the normal cardiac rhythm, regulation of vascular tone and endothelial function as well as metabolic interchange between the cells. They allow the transfer of small molecules and may enable slow calcium wave spreading, transfer of "death" or of "survival" signals. In the cardiomyocytes the most abundant isoform is Connexin 43 (Cx43). Alterations in Cx43 expression and distribution were observed in myocardium disease; i.e. in hypertrophic cardiomyopathy, heart failure and ischemia. Recent reports suggest the presence of Cx43 in the mitochondria as well, at least in the inner mitochondrial membrane, where it plays a central role in ischemic preconditioning. In this review, the current knowledge on the relationship between the remodeling of cardiac gap junctions and cardiac diseases are summarized.

Helicobacter pylori VacA induces apoptosis by accumulation of connexin 43 in autophagic vesicles via a Rac1/ERK-dependent pathway

Helicobacter pylori (H. pylori) produces vacuolating cytotoxin (VacA), a potent protein toxin, which is associated with gastric inflammation and ulceration. Recent studies demonstrated that connexins (Cxs), which are responsible for intracellular communication at gap junctions (GJs) as well as cell homeostasis, participate in VacA-induced cell death. We now demonstrate in AZ-521 cells that VacA increased cytoplasmic Cx43, accompanied by LC3-II generation in a time- and dose-dependent manner without induction of Cx43 mRNA expression. Inhibition of VacA-induced Rac1 activity prevented ERK phosphorylation and the increase in Cx43. Suppression of ERK activity and addition of N-acetyl-cysteine inhibited VacA-dependent increase in Cx43 and LC3-II. DIDS, an anion-selective inhibitor, suppressed VacA-dependent increase in Cx43, suggesting that VacA channel activity was involved in this pathway. By confocal microscopy, Cx43 increased by VacA was predominately localized in cholesterol-rich, detergent-resistant membranes including GJs, and a fraction of Cx43 was incorporated in endocytotic vesicles and autophagolysosomes. Accumulation of Cx43 was also observed in gastric mucosa from H. pylori-infected patients compared with healthy controls, suggesting that the pathogen caused a similar effect in vivo. Our findings show that VacA-mediated effects on autophagy inhibits turnover of Cx43, resulting in increased levels in the cytoplasm, leading eventually to apoptotic cell death.

Roles of connexins and pannexins in digestive homeostasis

Connexin proteins are abundantly present in the digestive system. They primarily form gap junctions, which control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions drive a plethora of gastrointestinal and hepatic functional features, including gastric and gut motility, gastric acid secretion, intestinal innate immune defense, xenobiotic biotransformation, glycogenolysis, bile secretion, ammonia detoxification and plasma protein synthesis. In the last decade, it has become clear that connexin hemichannels, which are the structural precursors of gap junctions, also provide a pathway for cellular communication, namely between the cytosol and the extracellular environment. Although merely pathological functions have been described, some physiological roles have been attributed to connexin hemichannels, in particular in the modulation of colonic motility. This equally holds true for cellular channels composed of pannexins, connexin-like proteins recently identified in the intestine and the liver, which have become acknowledged key players in inflammatory processes and that have been proposed to control colonic motility, secretion and blood flow.

Connexin 43 is an emerging therapeutic target in ischemia/reperfusion injury, cardioprotection and neuroprotection

Connexins are widely distributed proteins in the body that are crucially important for heart and brain functions. Six connexin subunits form a connexon or hemichannel in the plasma membrane. Interactions between two hemichannels in a head-to-head arrangement result in the formation of a gap junction channel. Gap junctions are necessary to coordinate cell function by passing electrical current flow between heart and nerve cells or by allowing exchange of chemical signals and energy substrates. Apart from its localization at the sarcolemma of cardiomyocytes and brain cells, connexins are also found in the mitochondria where they are involved in the regulation of mitochondrial matrix ion fluxes and respiration. Connexin expression is affected by age and gender as well as several pathophysiological alterations such as hypertension, hypertrophy, diabetes, hypercholesterolemia, ischemia, post-myocardial infarction remodeling or heart failure, and post-translationally connexins are modified by phosphorylation/de-phosphorylation and nitros(yl)ation which can modulate channel activity. Using knockout/knockin technology as well as pharmacological approaches, one of the connexins, namely connexin 43, has been identified to be important for cardiac and brain ischemia/reperfusion injuries as well as protection from it. Therefore, the current review will focus on the importance of connexin 43 for irreversible injury of heart and brain tissues following ischemia/reperfusion and will highlight the importance of connexin 43 as an emerging therapeutic target in cardio- and neuroprotection.

Topical administration of a connexin43-based peptide augments healing of chronic neuropathic diabetic foot ulcers: A multicenter, randomized trial

Nonhealing neuropathic foot ulcers remain a significant problem in individuals with diabetes. The gap-junctional protein connexin43 (Cx43) has roles in dermal wound healing and targeting Cx43 signalling accelerates wound reepithelialization. In a prospective, randomized, multicenter clinical trial we evaluated the efficacy and safety of a peptide mimetic of the C-terminus of Cx43, alpha connexin carboxy-terminal (ACT1), in accelerating the healing of chronic diabetic foot ulcers (DFUs) when incorporated into standard of care (SOC) protocols. Adults with DFUs of at least four weeks duration were randomized to receive SOC with or without topical application of ACT1. Primary outcome was mean percent ulcer reepithelialization and safety variables included incidence of treatment related adverse events (AEs) and detection of ACT1 immunogenicity. ACT1 treatment was associated with a significantly greater reduction in mean percent ulcer area from baseline to 12 weeks (72.1% vs. 57.1%; p = 0.03). Analysis of incidence and median time-to-complete-ulcer closure revealed that ACT1 treatment was associated with a greater percentage of participants that reached 100% ulcer reepitheliazation and a reduced median time-to-complete-ulcer closure. No AEs reported were treatment related, and ACT1 was not immunogenic. Treatment protocols that incorporate ACT1 may present a therapeutic strategy that safely augments the reepithelialization of chronic DFUs.

Neuroprotection in the treatment of glaucoma--A focus on connexin43 gap junction channel blockers

Glaucoma is a form of optic neuropathy and a common cause of blindness, affecting over 60 million people worldwide with an expected rise to 80 million by 2020. Successful treatment is challenging due to the various causes of glaucoma, undetectable symptoms at an early stage and inefficient delivery of drugs to the back of the eye. Conventional glaucoma treatments focus on the reduction of elevated intraocular pressure (IOP) using topical eye drops. However, their efficacy is limited to patients who suffer from high IOP glaucoma and do not address the underlying susceptibility of retinal ganglion cells (RGC) to degeneration. Glaucoma is known as a neurodegenerative disease which starts with RGC death and eventually results in damage of the optic nerve. Neuroprotective strategies therefore offer a novel treatment option for glaucoma by not only preventing neuronal loss but also disease progression. This review firstly gives an overview of the pathophysiology of glaucoma as well as current treatment options including conventional and novel delivery strategies. It then summarizes the rational for neuroprotection as a novel therapy for glaucomatous neuropathies and reviews current potential neuroprotective strategies to preserve RGC, with a focus on connexin43 (Cx43) gap junction channel blockers.

Suppression of PKCε-mediated mitochondrial connexin 43 phosphorylation at serine 368 is involved in myocardial mitochondrial dysfunction in a rat model of dilated cardiomyopathy

Mitochondrial connexin 43 (Cx43) is important in cardioprotection by ischemic preconditioning; however, whether mitochondrial Cx43 is involved in mitochondrial dysfunction in the pathogenesis of dilated cardiomyopathy (DCM) remains to be elucidated. The present study was performed to investigate the changes in expression and the phosphorylation state of mitochondrial Cx43 in a rat model of DCM, and to determine whether the altered phosphorylation state of mitochondrial Cx43 was involved in mitochondrial dysfunction. A rat model of DCM was generated by daily oral administration of furazolidone (FZD) for 30 weeks. Reverse transcription polymerase chain reaction and western blot analysis revealed a decrease in the overall expression of Cx43, accompanied by reduced levels of serine 368‑phosphorylated‑Cx43 immunoreactivity in the myocardium and myocardial mitochondria. In addition, the mitochondrial membrane potential and the activities of cytochrome c oxidase, succinate dehydrogenase and protein kinase C (PKC) ε were all significantly reduced compared with those of the control group. Phorbol‑12‑myristate‑13‑acetate (PMA), a specific PKC activator, partially reversed the FZD‑induced mitochondrial Cx43 dephosphorylation at serine 368 and mitochondrial dysfunction in the cardiomyocytes. However, pretreatment with 18β‑glycerrhetinic acid, a connexin channel inhibitor, eliminated the mitochondrial protective effect of PMA in the cardiomyocytes sparsely plated without cell to cell contact. These results suggested that dephosphorylation of mitochondrial Cx43 at serine 368, due to the suppression of PKCε activity, may be a novel mechanism for mitochondrial dysfunction in the pathogenesis of DCM.

Preparation of synaptic plasma membrane and postsynaptic density proteins using a discontinuous sucrose gradient

Neuronal subcellular fractionation techniques allow the quantification of proteins that are trafficked to and from the synapse. As originally described in the late 1960's, proteins associated with the synaptic plasma membrane can be isolated by ultracentrifugation on a sucrose density gradient. Once synaptic membranes are isolated, the macromolecular complex known as the post-synaptic density can be subsequently isolated due to its detergent insolubility. The techniques used to isolate synaptic plasma membranes and post-synaptic density proteins remain essentially the same after 40 years, and are widely used in current neuroscience research. This article details the fractionation of proteins associated with the synaptic plasma membrane and post-synaptic density using a discontinuous sucrose gradient. Resulting protein preparations are suitable for western blotting or 2D DIGE analysis.

Different impacts of α- and β-blockers in neurogenic hypertension produced by brainstem lesions in rat

BACKGROUND

Bilateral lesions of nucleus tractus solitarii in rat result in acute hypertension, pulmonary edema, and death within hours. The hypertension results from excessive catecholamine release. Catecholamine can activate connexin43 to regulate cell death. There is no study investigating the cardiopulmonary impacts of different adrenergic blockers and apoptosis mechanism in rat model.

METHODS

The authors microinjected 6-hydroxydopamine into nucleus tractus solitarii of the rat (n = 8 per group) and evaluated the cardiopulmonary changes after treatment with different concentrations of α1-blockers, α2-blockers, β-blockers, and α-agonists.

RESULTS

In the rat model, the authors found that prazosin (0.15 mg/kg) treatment could preserve cardiac output and reverse neutrophil infiltrations in lungs and lead to prevent pulmonary hemorrhagic edema. The time-dependent increases in connexin43 and terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells induced by 6-hydroxydopamine lesions were decreased after prazosin treatment (terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells at 6 h: 64.01 ± 2.41% vs. 24.47 ± 3.10%; mean ± SD, P < 0.001, in heart, and 80.83 ± 2.52% vs. 2.60 ± 1.03%, P < 0.001, in lung). However, propranolol caused further compromise of the already impaired cardiac output with consequence of rapid death. Phenylephrine enhanced the phenotype in the link between connexin43 expressions and terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells but not yohimbine. Connexin43 expressions and terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells were more decreased with prazosin (0.15 and 0.3 mg/kg) than that with prazosin (0.05 mg/kg) treatment.

CONCLUSIONS

α1-Receptors are the keystones of the phenotype. In some brainstem encephalitis and brain injury with nucleus tractus solitarii involvement, early α1-receptor blockade treatment may prevent acute death from tissue apoptosis. α-Blockers can also decrease cerebral perfusion pressure, and further studies are needed in translation to brain injury with increased intracranial pressure.

Multiple roles of connexins in atherosclerosis- and restenosis-induced vascular remodelling

Endothelial dysfunction is the initial step in atherosclerotic plaque development in large- and medium-sized arteries. This progressive disease, which starts during childhood, is characterized by the accumulation of lipids, macrophages, neutrophils, T lymphocytes and smooth muscle cells in the intima of the vessels. Erosion and rupture of the atherosclerotic plaque may induce myocardial infarction and cerebrovascular accidents, which are responsible for a large percentage of sudden deaths. The most common treatment for atherosclerosis is angioplasty and stent implantation, but these surgical interventions favour a vascular reaction called restenosis and the associated de-endothelialization increases the risk of thrombosis. This review provides an overview of the role of connexins, a large family of transmembrane proteins, in vascular remodelling associated with atherosclerosis and restenosis. The connexins expressed in the vascular wall are Cx37, Cx40, Cx43 and Cx45; their expressions vary with vascular territory and species. Connexins form hemichannels or gap junction channels, allowing the exchange of ions and small metabolites between the cytosol and extracellular space or between neighbouring cells, respectively. Connexins have important roles in vascular physiology; they support radial and longitudinal cell-to-cell communication in the vascular wall, and significant changes in their expression patterns have been described during atherosclerosis and restenosis.

Dysregulation of Mfn2 and Drp-1 proteins in heart failure

Therapeutic approaches for cardiac regenerative mechanisms have been explored over the past decade to target various cardiovascular diseases (CVD). Structural and functional aberrations of mitochondria have been observed in CVD. The significance of mitochondrial maturation and function in cardiomyocytes is distinguished by their attribution to embryonic stem cell differentiation into adult cardiomyocytes. An abnormal fission process has been implicated in heart failure, and treatment with mitochondrial division inhibitor 1 (Mdivi-1), a specific inhibitor of dynamin related protein-1 (Drp-1), has been shown to improve cardiac function. We recently observed that the ratio of mitofusin 2 (Mfn2; a fusion protein) and Drp-1 (a fission protein) was decreased during heart failure, suggesting increased mitophagy. Treatment with Mdivi-1 improved cardiac function by normalizing this ratio. Aberrant mitophagy and enhanced oxidative stress in the mitochondria contribute to abnormal activation of MMP-9, leading to degradation of the important gap junction protein connexin-43 (Cx-43) in the ventricular myocardium. Reduced Cx-43 levels were associated with increased fibrosis and ventricular dysfunction in heart failure. Treatment with Mdivi-1 restored MMP-9 and Cx-43 expression towards normal. In this review, we discuss mitochondrial dynamics, its relation to MMP-9 and Cx-43, and the therapeutic role of fission inhibition in heart failure.

Aliskiren-attenuated myocardium apoptosis via regulation of autophagy and connexin-43 in aged spontaneously hypertensive rats

There are controversies about the mechanism of myocardium apoptosis in hypertensive heart disease. The aim of this study was to investigate the relationship among autophagy, Cx43 and apoptosis in aged spontaneously hypertensive rats (SHRs) and establish whether Aliskiren is effective or not for the treatment of myocardium apoptosis. Twenty-one SHRs aged 52 weeks were randomly divided into three groups, the first two receiving Aliskiren at a dose of 10 and 25 mg/kg/day respectively; the third, placebo for comparison with seven Wistar-Kyoto (WKY) as controls. After a 2-month treatment, systolic blood pressure (SBP), heart to bw ratios (HW/BW%) and angiotensin II (AngII) concentration were significantly enhanced in SHRs respectively. Apoptotic cardiomyocytes detected with TUNEL and immunofluorescent labelling for active caspase-3 increased nearly fourfolds in SHRs, with a decline in the expression of survivin and AKT activation, and an increase in caspase-3 activation and the ratio of Bax/Bcl-2. Myocardium autophagy, detected with immunofluorescent labelling for LC3-II, increased nearly threefolds in SHRs, with the up-regulation of Atg5, Atg16L1, Beclin-1 and LC3-II. The expression of Cx43 plaque was found to be down-regulated in SHRs. Aliskiren significantly reduced SBP, HW/BW%, AngII concentration and the expression of AT(1)R. Thus, Aliskiren protects myocardium against apoptosis by decreasing autophagy, up-regulating Cx43. These effects showed a dose-dependent tendency, but no significance. In conclusion, the myocardium apoptosis developed during the hypertensive end-stage of SHRs could be ameliorated by Aliskiren via the regulation of myocardium autophagy and maladaptive remodelling of Cx43.

Cardiac to cancer: connecting connexins to clinical opportunity

Gap junctions and their connexin components are indispensable in mediating the cellular coordination required for tissue and organ homeostasis. The critical nature of their existence mandates a connection to disease while at the same time offering therapeutic potential. Therapeutic intervention may be offered through the pharmacological and molecular disruption of the pathways involved in connexin biosynthesis, gap junction assembly, stabilization, or degradation. Chemical inhibitors aimed at closing connexin channels, peptide mimetics corresponding to short connexin sequences, and gene therapy approaches have been incredibly useful molecular tools in deciphering the complexities associated with connexin biology. Recently, therapeutic potential in targeting connexins has evolved from basic research in cell-based models to clinical opportunity in the form of human trials. Clinical promise is particularly evident with regards to targeting connexin43 in the context of wound healing. The following review is aimed at highlighting novel advances where the pharmacological manipulation of connexin biology has proven beneficial in animals or humans.