Protein kinase Cepsilon mediates salutary effects on electrical coupling induced by ischemic preconditioning

Guidelines for assessment of cardiac electrophysiology and arrhythmias in small animals

Cardiac arrhythmias are a major cause of morbidity and mortality worldwide. Although recent advances in cell-based models, including human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM), are contributing to our understanding of electrophysiology and arrhythmia mechanisms, preclinical animal studies of cardiovascular disease remain a mainstay. Over the past several decades, animal models of cardiovascular disease have advanced our understanding of pathological remodeling, arrhythmia mechanisms, and drug effects and have led to major improvements in pacing and defibrillation therapies. There exist a variety of methodological approaches for the assessment of cardiac electrophysiology and a plethora of parameters may be assessed with each approach. This guidelines article will provide an overview of the strengths and limitations of several common techniques used to assess electrophysiology and arrhythmia mechanisms at the whole animal, whole heart, and tissue level with a focus on small animal models. We also define key electrophysiological parameters that should be assessed, along with their physiological underpinnings, and the best methods with which to assess these parameters.

Acute Glycogen Synthase Kinase-3 Inhibition Modulates Human Cardiac Conduction

Glycogen synthase kinase 3 (GSK-3) inhibition has emerged as a potential therapeutic target for several diseases, including cancer. However, the role for GSK-3 regulation of human cardiac electrophysiology remains ill-defined. We demonstrate that SB216763, a GSK-3 inhibitor, can acutely reduce conduction velocity in human cardiac slices. Combined computational modeling and experimental approaches provided mechanistic insight into GSK-3 inhibition-mediated changes, revealing that decreased sodium-channel conductance and tissue conductivity may underlie the observed phenotypes. Our study demonstrates that GSK-3 inhibition in human myocardium alters electrophysiology and may predispose to an arrhythmogenic substrate; therefore, monitoring for adverse arrhythmogenic events could be considered.

Reactive oxygen species in cardiovascular diseases: an update

Cardiovascular diseases are among the leading causes of death worldwide, imposing major health threats. Reactive oxygen species (ROS) are one of the most important products from the process of redox reactions. In the onset and progression of cardiovascular diseases, ROS are believed to heavily influence homeostasis of lipids, proteins, DNA, mitochondria, and energy metabolism. As ROS production increases, the heart is damaged, leading to further production of ROS. The vicious cycle continues on as additional ROS are generated. For example, recent evidence indicated that connexin 43 (Cx43) deficiency and pyruvate kinase M2 (PKM2) activation led to a loss of protection in cardiomyocytes. In this context, a better understanding of the mechanisms behind ROS production is vital in determining effective treatment and management strategies for cardiovascular diseases.

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.

Canonical and Non-Canonical Roles of Connexin43 in Cardioprotection

Since the mid-20th century, ischemic heart disease has been the world’s leading cause of death. Developing effective clinical cardioprotection strategies would make a significant impact in improving both quality of life and longevity in the worldwide population. Both ex vivo and in vivo animal models of cardiac ischemia/reperfusion (I/R) injury are robustly used in research. Connexin43 (Cx43), the predominant gap junction channel-forming protein in cardiomyocytes, has emerged as a cardioprotective target. Cx43 posttranslational modifications as well as cellular distribution are altered during cardiac reperfusion injury, inducing phosphorylation states and localization detrimental to maintaining intercellular communication and cardiac conduction. Pre- (before ischemia) and post- (after ischemia but before reperfusion) conditioning can abrogate this injury process, preserving Cx43 and reducing cell death. Pre-/post-conditioning has been shown to largely rely on the presence of Cx43, including mitochondrial Cx43, which is implicated to play a major role in pre-conditioning. Posttranslational modifications of Cx43 after injury alter the protein interactome, inducing negative protein cascades and altering protein trafficking, which then causes further damage post-I/R injury. Recently, several peptides based on the Cx43 sequence have been found to successfully diminish cardiac injury in pre-clinical studies.

Intercalated discs: cellular adhesion and signaling in heart health and diseases

Intercalated discs (ICDs) are highly orchestrated structures that connect neighboring cardiomyocytes in the heart. Three major complexes are distinguished in ICD: desmosome, adherens junction (AJ), and gap junction (GJ). Desmosomes are major cell adhesion junctions that anchor cell membrane to the intermediate filament network; AJs connect the actin cytoskeleton of adjacent cells; and gap junctions metabolically and electrically connect the cytoplasm of adjacent cardiomyocytes. All these complexes work as a single unit, the so-called area composita, interdependently rather than individually. Mutation or altered expression of ICD proteins results in various cardiac diseases, such as ARVC (arrhythmogenic right ventricular cardiomyopathy), dilated cardiomyopathy, and hypotrophy cardiomyopathy, eventually leading to heart failure. In this article, we first review the recent findings on the structural organization of ICD and their functions and then focus on the recent advances in molecular pathogenesis of the ICD-related heart diseases, which include two major areas: i) the ICD gene mutations in cardiac diseases, and ii) the involvement of ICD proteins in signal transduction pathways leading to myocardium remodeling and eventual heart failure. These major ICD-related signaling pathways include Wnt/β-catenin pathway, p38 MAPK cascade, Rho-dependent serum response factor (SRF) signaling, calcineurin/NFAT signaling, Hippo kinase cascade, etc., which are differentially regulated in pathological conditions.

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

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

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

Background

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

Methods and Results

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

Conclusions

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

Cx43 phosphorylation-mediated effects on ERK and Akt protect against ischemia reperfusion injury and alter the stability of the stress-inducible protein NDRG1

Gap junctions contain intercellular channels that enable intercellular communication of small molecules while also serving as a signaling scaffold. Connexins, the proteins that form gap junctions in vertebrates, are highly regulated and typically have short (<2 h) half-lives. Connexin43 (Cx43), the predominate connexin in the myocardium and epithelial tissues, is phosphorylated on more than a dozen serine residues and interacts with a variety of protein kinases. These interactions regulate Cx43 and gap junction formation and stability. Casein kinase 1 (CK1)-mediated phosphorylation of Cx43 promotes gap junction assembly. Using murine knock-in technology and quantitative PCR, immunoblotting, and immunoprecipitation assays, we show here that mutation of the CK1 phosphorylation sites in Cx43 reduces the levels of total Cx43 in the myocardium and increases Cx43 phosphorylation on sites phosphorylated by extracellular signal-regulated kinase (ERK). In aged myocardium, we found that, compared with WT Cx43, mutant Cx43 expression increases ERK activation, phosphorylation of Akt substrates, and protection from ischemia-induced injury. Our findings also uncovered that Cx43 interacts with the hypoxia-inducible protein N-Myc downstream-regulated gene 1 protein (NDRG1) and that Cx43 phosphorylation status controls this interaction and dramatically affects NDRG1 stability. We propose that, in addition to altering gap junction stability, Cx43 phosphorylation directly and dynamically regulates cellular signaling through ERK and Akt in response to ischemic injury. We conclude that gap junction-dependent NDRG1 regulation might explain some cellular responses to hypoxia.

PKC and PKN in heart disease

The protein kinase C (PKC) and closely related protein kinase N (PKN) families of serine/threonine protein kinases play crucial cellular roles. Both kinases belong to the AGC subfamily of protein kinases that also include the cAMP dependent protein kinase (PKA), protein kinase B (PKB/AKT), protein kinase G (PKG) and the ribosomal protein S6 kinase (S6K). Involvement of PKC family members in heart disease has been well documented over the years, as their activity and levels are mis-regulated in several pathological heart conditions, such as ischemia, diabetic cardiomyopathy, as well as hypertrophic or dilated cardiomyopathy. This review focuses on the regulation of PKCs and PKNs in different pathological heart conditions and on the influences that PKC/PKN activation has on several physiological processes. In addition, we discuss mechanisms by which PKCs and the closely related PKNs are activated and turned-off in hearts, how they regulate cardiac specific downstream targets and pathways, and how their inhibition by small molecules is explored as new therapeutic target to treat cardiomyopathies and heart failure.

Oxygen-Glucose Deprivation in Mouse Astrocytes is Associated with Ultrastructural Changes in Connexin 43 Gap Junctions

In the intact brain, astrocytes play an important role in a number of physiological functions like spatial buffering of potassium, maintenance of calcium homeostasis, neurotransmitter release, regulation of the cerebral blood flow, and many more. As pathophysiological events upon hypoxic-ischemic brain injury include excitotoxicity by glutamate release as well as oxidative stress, astrocytes and their gap junction-based syncytium are of major relevance for regulating the extent of resulting brain damage. The gap junction protein Connexin (Cx) 43 contributes mainly to the astrocytic intercellular communication. As little is known about the ultrastructural assemblage of Cx43 and its changes in response to hypoxic events, we chose temporary oxygen and glucose deprivation with subsequent reoxygenation (OGD-R) as a metabolic inhibition model of hypoxia in primary murine astrocytes. Gap junction morphology and assembly/disintegration were analyzed at the ultrastructural level using freeze-fracture replica immunolabeling. The exposure of cultured astrocytes to short-term OGD-R resulted in the activation of ERK1/2 (p44/p42), downregulation of Cx43 protein expression, and the rearrangement of Cx43 particles within the cell membrane and within gap junctions. These changes in gap junction morphology were associated with phosphorylation of Cx43 at Serine 368. Analysis of the nearest-neighbor distance within gap junction plaques revealed the loosening of Cx43 particle clusters. Together with the observation of additional connexons being present in the vicinity of gap junction plaques after OGD-R treatment, our study indicates that changes in gap junction assembly are associated with the early phase of hypoxic cell damage.

Nitric oxide, PKC-ε, and connexin43 are crucial for ischemic preconditioning-induced chemical gap junction uncoupling

Ischemic preconditioning (IPC) maintains connexin43 (Cx43) phosphorylation and reduces chemical gap junction (GJ) coupling in cardiomyocytes to protect against ischemic damage. However, the signal transduction pathways underlying these effects are not fully understood. Here, we investigated whether nitric oxide (NO) and protein kinase C-ε (PKC-ε) contribute to IPC-induced cardioprotection by maintaining Cx43 phosphorylation and inhibiting chemical GJ coupling. IPC reduced ischemia-induced myocardial infarction and increased cardiomyocyte survival; phosphorylated Cx43, eNOS, and PKC-ε levels; and chemical GJ uncoupling. Administration of the NO donor SNAP mimicked the effects of IPC both in vivo and in vitro, maintaining Cx43 phosphorylation, promoting chemical GJ uncoupling, and reducing myocardial infarction. Preincubation with the NO synthase inhibitor L-NAME or PKC-ε translocation inhibitory peptide (PKC-ε-TIP) abolished these effects of IPC. Additionally, by inducing NO production, IPC induced translocation of PKC-ε, but not PKC-δ, from the cytosolic to the membrane fraction in primary cardiac myocytes. IPC-induced cardioprotection thus involves increased NO production, PKC-ε translocation, Cx43 phosphorylation, and chemical GJ uncoupling.

Potential Therapeutics for Vascular Cognitive Impairment and Dementia

BACKGROUND

As the human lifespan increases, the number of people affected by agerelated dementia is growing at an epidemic pace. Vascular pathology dramatically affects cognitive profiles, resulting in dementia and cognitive impairment. While vascular dementia itself constitutes a medical challenge, hypo-perfusion/vascular risk factors enhance amyloid toxicity and other memory- damaging factors and hasten Alzheimer's disease (AD) and other memory disorders' progression, as well as negatively affect treatment outcome.

METHODS

Research and online content related to vascular cognitive impairment and dementia is reviewed, specifically focusing on the potential treatment of the disorder.

RESULTS

Few therapeutic options are currently available to improve the prognosis of patients with vascular dementia and cognitive impairment, mixed AD dementia with vascular pathology, or other memory disorders. Emerging evidence, however, indicates that, like AD and other memory disorders, synaptic impairment underlies much of the memory impairment in the cognitive decline of vascular cognitive impairment and vascular dementia.

CONCLUSION

Effective rescues of the memory functions might be achieved through synaptic and memory therapeutics, targeting distinct molecular signaling pathways that support the formation of new synapses and maintaining their connections. Potential therapeutic agents include: 1) memory therapeutic agents that rescue synaptic and memory functions after the brain insults; 2) antipathologic therapeutics and an effective management of vascular risk factors; and 3) preventative therapeutic agents that achieve memory therapy through functional enhancement. These therapeutic agents are also likely to benefit patients with AD and/or other types of memory disorders.

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.

MicroRNA-143 promotes cardiac ischemia-mediated mitochondrial impairment by the inhibition of protein kinase Cepsilon

The cardioprotection of protein kinase Cepsilon (PKCε) against myocardial infarction (MI) mediated by its anti-apoptotic property and underlying mechanism of targeted regulation by microRNA (miRNA) are not established. MI-induced injury, PKCε expression, and targeted regulation of miRNA-143 (miR-143) to PKCε have been evaluated using animal MI and cellular hypoxic models conjugated with series of state-of-art molecular techniques. The results demonstrated that PKCε significantly downregulated along with increased infarcted area and apoptotic and necrotic damage in MI model, and the targeted relationship and potential binding profile were established between miR-143 and PKCε. Both in vivo and in vitro ischemic tests showed that miR-143 induced apoptosis and necrosis, which was reversed by antagomiR-143 or AMO-143. The upregulation of miR-143 by transfection of miR-143 in vitro also induced cell loss, and this effect of miR-143 was completely reversed by co-transfection of miR-143 with AMO-143. The identically deleterious action of miR-143 on mitochondrial membrane potential and ATP synthesis was also observed in both animal MI and cellular hypoxic models, as well as miR-143 overexpressed models and converted by either antagomiR or AMO. Importantly, overexpression of miR-143 downregulated PKCε in all tested models and this downregulation was reversed in the presence of antagomiR or AMO. The direct targeted regulation of miR-143 on PKCε was confirmed by luciferase reporter and miRNA-masking tests. In conclusion, MI-mediated upregulation of miR-143 inhibits PKCε expression and consequently interference with the cardioprotection of PKCε to mitochondrial, and leads to mitochondrial membrane potential dissipation and myocardial death eventually.

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.

Interaction between nitric oxide signaling and gap junctions during ischemic preconditioning: Importance of S-nitrosylation vs. protein kinase G activation

Much effort has been dedicated to exploring the mechanisms of IPC, and the GJ is one of the proposed targets of IPC. Several lines of evidence have indicated that NO affects GJ permeability regulation and expression of connexin isoforms. NO-induced stimulation of the sGC-cGMP pathway and the subsequent PKG activation could lead directly to connexin phosphorylation and GJ coupling modification. Additionally, because NO-induced cardioprotection against I/R injury beyond the cGMP/PKG-dependent pathway has been reported in isolated cardiomyocytes, it has been posited that NO-mediated GJ coupling might be independent from the activation of the NO-induced cGMP/PKG pathway during IPC. S-nitrosylation by NO exerts a major influence in IPC-induced cardioprotection. It has been suggested that NO-mediated cardioprotection during IPC was not dependent on sGC/cGMP/PKG but on SNO signaling. We need more researches to prove that which signaling pathway (S-nitrosylation or protein kinase G activation) is the major one modulating GJ coupling during IPC. The aim of review article is to discuss the possible signaling pathways of NO in regulating GJ during IPC.

Spatio-temporal regulation of connexin43 phosphorylation and gap junction dynamics

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

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.

A new flavonoid glycoside (APG) isolated from Clematis tangutica attenuates myocardial ischemia/reperfusion injury via activating PKCε signaling

Clematis tangutica has been shown to be beneficial for the heart; however, the mechanism of this effectremains unknown. Apigenin-7-O-β-D-(-6″-p-coumaroyl)-glucopyranoside (APG) is a new flavonoid glycoside isolated from Clematis tangutica. This study investigates the effects of APG on myocardial ischemia/reperfusion (IR) injury (IRI). An IRI model of primary myocardial cells and mice was used in this study. Compared with the IR group, APG preconditioning is protective against IRI in primary myocardial cells and in mice hearts in a dose-dependent manner. The cardioprotective mechanisms of APG may involve a significant PKCε translocation into the mitochondria and an activation of the Nrf2/HO-1 pathway, which respectively suppressesmitochondrial oxidative stress and inhibits apoptosis. In addition, PKCε-targeted siRNA and a PKCε specialized inhibitor (ε-V1-2) were used to inhibit PKCε expression and activity. The inhibition of PKCε reversed the cardioprotective effect of APG, with an inhibition of Nrf2/HO-1 activation and increased mitochondrial oxidative stress and cardiomyocyte apoptosis. In conclusion, PKCε activation plays an important role in the cardioprotective effects of APG. PKCε activation induced by APG preconditioning reduces mitochondrial oxidative stress and promotes Nrf2/HO-1-mediated anti-apoptosis signaling.

Heart Rate and Extracellular Sodium and Potassium Modulation of Gap Junction Mediated Conduction in Guinea Pigs

Background: Recent studies suggested that cardiac conduction in murine hearts with narrow perinexi and 50% reduced connexin43 (Cx43) expression is more sensitive to relatively physiological changes of extracellular potassium ([K⁺]_o) and sodium ([Na⁺]_o).

Purpose: Determine whether similar [K⁺]_o and [Na⁺]_o changes alter conduction velocity (CV) sensitivity to pharmacologic gap junction (GJ) uncoupling in guinea pigs.

Methods: [K⁺]_o and [Na⁺]_o were varied in Langendorff perfused guinea pig ventricles (Solution A: [K⁺]_o = 4.56 and [Na⁺]_o = 153.3 mM. Solution B: [K⁺]_o = 6.95 and [Na⁺]_o = 145.5 mM). Gap junctions were inhibited with carbenoxolone (CBX) (15 and 30 μM). Epicardial CV was quantified by optical mapping. Perinexal width was measured with transmission electron microscopy. Total and phosphorylated Cx43 were evaluated by western blotting.

Results: Solution composition did not alter CV under control conditions or with 15μM CBX. Decreasing the basic cycle length (BCL) of pacing from 300 to 160 ms decreased CV uniformly with both solutions. At 30 μM CBX, a change in solution did not alter CV either longitudinally or transversely at BCL = 300 ms. However, reducing BCL to 160 ms caused CV to decrease more in hearts perfused with Solution B than A. Solution composition did not alter perinexal width, nor did it change total or phosphorylated serine 368 Cx43 expression. These data suggest that the solution dependent CV changes were independent of altered perinexal width or GJ coupling. Action potential duration was always shorter in hearts perfused with Solution B than A, independent of pacing rate and/or CBX concentration.

Conclusions: Increased heart rate and GJ uncoupling can unmask small CV differences caused by changing [K⁺]_o and [Na⁺]_o. These data suggest that modulating extracellular ionic composition may be a novel anti-arrhythmic target in diseases with abnormal GJ coupling, particularly when heart rate cannot be controlled.

Phosphorylation at Connexin43 Serine-368 Is Necessary for Myocardial Conduction During Metabolic Stress

Connexin43 (Cx43) phosphorylation alters gap junction localization and function. In particular, phosphorylation at serine-368 (S368) has been suggested to alter gap junctional conductance, but previous reports have shown inconsistent results for both timing and functional effects of S368 phosphorylation. The objective of this study was to determine the functional effects of isolated S368 phosphorylation. We evaluated wild-type Cx43 (AdCx43) and mutations simulating permanent phosphorylation (Ad368E) or preventing phosphorylation (Ad368A) at S368. Function was assessed by optical mapping of electrical conduction in patterned cultures of neonatal rat ventricular myocytes, under baseline and metabolic stress (MS) conditions. Baseline conduction velocity (CV) was similar for all groups. In the AdCx43 and Ad368E groups, MS moderately decreased CV. Ad368A caused complete conduction block during MS. Triton-X solubility assessment showed no change in Cx43 location during conduction impairment. Western blot analysis showed that Cx43-S368 phosphorylation was present at baseline, and that it decreased during MS. Our data indicate that phosphorylation at S368 does not affect CV under baseline conditions, and that preventing S368 phosphorylation makes Cx43 hypersensitive to MS. These results show the critical role of S368 phosphorylation during stress conditions.

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.

Protein kinase C (PKC) mediated interaction between conexin43 (Cx43) and K(+)(ATP) channel subunit (Kir6.1) in cardiomyocyte mitochondria: Implications in cytoprotection against hypoxia induced cell apoptosis

PURPOSE OF RESEARCH

We have recently shown that adenosine-triphosphate-sensitive potassium [K(+)(ATP)] channel protein subunit, Kir6.1 is a phosphospecific interaction partner of the gap-junction protein connexin43 (Cx43). Since, both Cx43 and K(+)(ATP) are known to be involved in cell survival during hypoxia, we addressed the question, whether the interaction between Cx43 and K(+)(ATP) has a role in protecting cell against hypoxia-induced cell death.

PRINCIPLE RESULTS

We report here that the Kir6.1 protein interacts, in a phosphospecific manner with Cx43 in the mitochondria of cardiomyocytic cell line H9C2. The hypoxia for 12-h resulted in the appreciable increase in the phosphorylation at the serine 262 (S262) of the Cx43 with the concomitant increase in the Cx43 and Kir6.1 interaction. Moreover, the increased interaction was mediated by a signaling pathway involving PKC and more specifically by PKC epsilon. Functional implications of the association between the Cx43 and Kir6.1 were found to prevent mitochondria mediated hypoxia induced cell apoptosis.

MAJOR CONCLUSIONS

Our results demonstrate that PKC epsilon regulates the interaction between Cx43 and Kir6.1 in the cardiomyocyte mitochondria and this interaction prevents hypoxia induced cell death. Our results provide an interesting lead in developing effective strategies to protect cardiomyocytes from hypoxia/ischemia induced cell death.

Specific Cx43 phosphorylation events regulate gap junction turnover in vivo

Gap junctions, composed of proteins from the connexin gene family, are highly dynamic structures that are regulated by kinase-mediated signaling pathways and interactions with other proteins. Phosphorylation of Connexin43 (Cx43) at different sites controls gap junction assembly, gap junction size and gap junction turnover. Here we present a model describing how Akt, mitogen activated protein kinase (MAPK) and src kinase coordinate to regulate rapid turnover of gap junctions. Specifically, Akt phosphorylates Cx43 at S373 eliminating interaction with zona occludens-1 (ZO-1) allowing gap junctions to enlarge. Then MAPK and src phosphorylate Cx43 to initiate turnover. We integrate published data with new data to test and refine this model. Finally, we propose that differential coordination of kinase activation and Cx43 phosphorylation controls the specific routes of disassembly, e.g., annular junction formation or gap junctions can potentially "unzip" and be internalized/endocytosed into the cell that produced each connexin.

β(IV)-Spectrin regulates TREK-1 membrane targeting in the heart

AIMS

Cardiac function depends on the highly regulated and co-ordinate activity of a large ensemble of potassium channels that control myocyte repolarization. While voltage-gated K(+) channels have been well characterized in the heart, much less is known about regulation and/or targeting of two-pore K(+) channel (K(2P)) family members, despite their potential importance in modulation of heart function.

METHODS AND RESULTS

Here, we report a novel molecular pathway for membrane targeting of TREK-1, a mechano-sensitive K(2P) channel regulated by environmental and physical factors including membrane stretch, pH, and polyunsaturated fatty acids (e.g. arachidonic acid). We demonstrate that β(IV)-spectrin, an actin-associated protein, is co-localized with TREK-1 at the myocyte intercalated disc, associates with TREK-1 in the heart, and is required for TREK-1 membrane targeting. Mice expressing β(IV)-spectrin lacking TREK-1 binding (qv(4J)) display aberrant TREK-1 membrane localization, decreased TREK-1 activity, delayed action potential repolarization, and arrhythmia without apparent defects in localization/function of other cardiac potassium channel subunits. Finally, we report abnormal β(IV)-spectrin levels in human heart failure.

CONCLUSIONS

These data provide new insight into membrane targeting of TREK-1 in the heart and establish a broader role for β(IV)-spectrin in organizing functional membrane domains critical for normal heart function.

Roles of 3,4-methylenedioxymethamphetamine (MDMA)-induced alteration of connexin43 and intracellular Ca(2+) oscillation in its cardiotoxicity

Although it is well known that 3,4-methylenedioxymethamphetamine (MDMA) can cause various cardiovascular abnormalities and even sudden death from cardiac arrhythmia, whether it has any effect on myocardial gap junctions, which might be one of the targets mediating MDMA-induced cardiotoxicity, remains unclear.

OBJECTIVE

To test the hypothesis that MDMA may affect the myocardial gap junction protein connexin43 (Cx43) and induce cardiac dysrhythmia.

METHOD

(1) In vivo study: adult rats were treated with a single dose MDMA administration (20mg/kg, i.p.). Electrocardiogram detection and immunohistochemical analysis were performed to evaluate cardiac function and expression of Cx43, respectively; (2) in vitro study: cultured ventricular myocytes of neonatal rats were treated with MDMA (10, 100, 1000μmol/L) for 1h. Western blotting and real-time quantitative polymerase chain reaction (RT-qPCR) were performed to investigate the total Cx43 mRNA expression. Immunofluorescent analysis was used to evaluate the amount of junctional Cx43. The phosphorylation status of Cx43 at site Ser368 and intracellular Ca(2+) oscillation were also studied.

RESULTS

Obvious changes in electrocardiographic patterns were found in rats following MDMA administration. They were characterized by prolonged QRS duration associated with increased amplitude of QRS complex. The heart rates in treated rats were significantly decreased compared to the rats in the control group. The immunohistochemical findings revealed a significant decrease in Cx43 expression. The in vitro study also showed a marked decline in total Cx43 protein associated with reduction of Cx43 mRNA, whereas the phosphorylated Cx43 at Ser368 was increased. Decrease of junctional Cx43 was found correlated with reduction in N-cadherin induced by high concentration of MDMA. Additionally, confocal microscopy findings revealed alteration of intracellular calcium oscillation patterns characterized by high frequency and increasing influx Ca(2+).

CONCLUSIONS

MDMA reduces expression of cardiac gap junction protein Cx43. The increase of phosphorylation status of Cx43 at Ser368 induced by MDMA is attributed, at least in part, to the Ca(2+)-dependent regulation of protein kinase C (PKC) activity. Our findings provide first evidence of MDMA-mediated changes in those cardiac gap junctions that may underlie MDMA-induced cardiac arrhythmia.

A new approach for resolution of complex tissue impedance spectra in hearts

This study was designed to test the feasibility of using sinusoidal approximation in combination with a new instrumentation approach to resolve complex impedance (uCI) spectra from heart preparations. To assess that feasibility, we applied stimuli in the 10-4000 Hz range and recorded potential differences (uPDs) in a four-electrode configuration that allowed identification of probe constants (Kp) during calibration that were in turn used to measure total tissue resistivity ρt from rabbit ventricular epicardium. Simultaneous acquisition of a signal proportional to the supplied current (Vstim) with uPD allowed identification of the V- I ratio needed for ρt measurement, as well as the phase shift from Vstim to uPD needed for uCI spectra resolution. Performance with components integrated to reduce noise in cardiac electrophysiologic experiments, in particular, and provide accurate electrometer-based measurements, in general, was first characterized in tests using passive loads. Load tests showed accurate uCI recovery with mean uPD SNRs between 10 (1) and 10 (3) measured with supplied currents as low as 10 nA. Comparable performance characteristics were identified during calibration of nine arrays built with 250 μm Ag/AgCl electrodes, with uCIs that matched analytic predictions and no apparent effect of frequency ( F = 0.12, P = 0.99). The potential ability of parasitic capacitance in the presence of the electrode-electrolyte interface associated with the small sensors to influence the uCI spectra was therefore limited by the instrumentation. Resolution of uCI spectra in rabbit ventricle allowed measurement of ρt = 134 ± 53 Ω· cm. The rapid identification available with this strategy provides an opportunity for new interpretations of the uCI spectra to improve quantification of disease-, region-, tissue-, and species-dependent intercellular uncoupling in hearts.

Up-regulation of myocardial connexin-43 in spontaneously hypertensive rats fed red palm oil is most likely implicated in its anti-arrhythmic effects

The purpose of this study was to test our hypothesis that red palm oil (RPO) intake may affect abnormalities of myocardial connexin-43 (Cx43) and protein kinase Cε (PKCε) signaling, and consequently the propensity of the spontaneously hypertensive rat heart (SHR) heart to arrhythmias. SHR and Wistar-Kyoto (WKY) rats fed a standard rat chow plus red palm oil (200 µL/day) for 5 weeks were compared with untreated rats. Cytosolic but not particulate PKCε expression as well as Cx43-mRNA, total Cx43 proteins, and its phoshorylated forms were increased, and disordered localization of Cx43 was attenuated in the left ventricle of RPO-fed SHR compared with untreated rats. These alterations were associated with suppression of early post-ischemic-reperfusion-related ventricular tachycardia and electrically inducible ventricular fibrillation. However, the treatment dose of RPO caused down-regulation of myocardial Cx43, but did not alter its cell membrane distribution or overall PKCε expression in WKY rats. It was, however, associated with poor arrhythmia protection, suggesting overdosing. Results indicate that SHR benefit from RPO intake, particularly because of its apparent anti-arrhythmic effects. This protection can be, in part, attributed to the preservation of cell-to-cell communication via up-regulation of myocardial Cx43, but not with PKCε activation.

Connexin43 phosphorylation in brain, cardiac, endothelial and epithelial tissues

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells in essentially all tissues. There are 21 connexin genes in the human genome and different tissues express different connexin genes. Most connexins are known to be phosphoproteins. Phosphorylation can regulate connexin assembly into gap junctions, gap junction turnover and channel gating. Given the importance of gap junctions in development, proliferation and carcinogenesis, regulation of gap junction phosphorylation in response to wounding, hypoxia and other tissue insults is proving to be critical for cellular response and return to homeostasis. Connexin43 (Cx43) is the most widely and highly expressed gap junction protein, both in cell culture models and in humans, thus more research has been done on it and more reagents to it are available. In particular, antibodies that can report Cx43 phosphorylation status have been created allowing temporal examination of specific phosphorylation events in vivo. This review is focused on the use of these antibodies in tissue in situ, predominantly looking at Cx43 phosphorylation in brain, heart, endothelium and epithelium with reference to other connexins where data is available. These data allow us to begin to correlate specific phosphorylation events with changes in cell and tissue function. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexin43 phosphorylation and cytoprotection in the heart

The fundamental role played by connexins including connexin43 (Cx43) in forming intercellular communication channels (gap junctions), ensuring electrical and metabolic coupling between cells, has long been recognized and extensively investigated. There is also increasing recognition that Cx43, and other connexins, have additional roles, such as the ability to regulate cell proliferation, migration, and cytoprotection. Multiple phosphorylation sites, targets of different signaling pathways, are present at the regulatory, C-terminal domain of Cx43, and contribute to constitutive as well as transient phosphorylation Cx43 patterns, responding to ever-changing environmental stimuli and corresponding cellular needs. The present paper will focus on Cx43 in the heart, and provide an overview of the emerging recognition of a relationship between Cx43, its phosphorylation pattern, and development of resistance to injury. We will also review our recent work regarding the role of an enhanced phosphorylation state of Cx43 in cardioprotection. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Identification of CaMKII phosphorylation sites in Connexin43 by high-resolution mass spectrometry

Connexin43 (Cx43) is a major cardiac gap junction channel protein required for normal electrical and contractile activity. Gap junction channel assembly, function, and turnover are regulated by phosphorylation under both normal and disease conditions. The carboxyl terminus (CT) of Cx43 contains numerous amino acid residues that are phosphorylated by protein kinases. However, our knowledge of the specific residues and kinases involved is incomplete. The objective of this study was to identify amino acid residues in the Cx43-CT that are targets of the multifunctional protein kinase, Ca(2+)/calmodulin protein kinase II (CaMKII), an enzyme known to play critical roles in Ca(2+) homeostasis, transcription, apoptosis, and ischemic heart disease. We subjected fusion protein containing the Cx43-CT to phosphorylation by CaMKII in vitro, digestion with Lys-C and trypsin followed by enrichment for phosphorylated peptides using TiO(2), and analysis in an LTQ XL Orbitrap with collision-induced dissociation and electron transfer dissociation. We deduced the sites of modification by interpreting tandem spectra from these "orthogonal" methods of gas phase peptide fragmentation. We have identified 15 serine residues, including one novel site, in the Cx43-CT that are phosphorylated by CaMKII, the activity of which may be important in regulating Cx43 in normal and diseased hearts.

Epac stimulation induces rapid increases in connexin43 phosphorylation and function without preconditioning effect

It has been recently shown that beta-adrenergic receptors are able to activate phospholipase C via the cyclic adenosine monophosphate-binding protein Epac. This new interconnection may participate in isoproterenol (Iso)-induced preconditioning. We evaluated here whether Epac could induce PKCepsilon activation and could play a role in ischemic preconditioning through the phosphorylation of connexin43 (Cx43) and changes in gap junctional intercellular communication (GJIC). In cultured rat neonatal cardiomyocytes, we showed that in response to Iso and 8-CPT, a specific Epac activator, PKCepsilon content was increased in particulate fractions of cell lysates independently of protein kinase A (PKA). This was associated with an increased Cx43 phosphorylation. Both Iso and 8-CPT induced an increase in GJIC that was blocked by the PKC inhibitor bisindolylmaleimide. Interestingly, inhibition of PKA partly suppressed both Iso-induced increases in Cx43 phosphorylation and in GJIC. The same PKCepsilon-dependent Cx43 phosphorylation by beta-adrenergic stimulation via Epac was found in adult rat hearts. However, in contrast with Iso that induced a preconditioning effect, perfusion of isolated hearts with 8-CPT prior to ischemia failed to improve the post-ischemia functional recovery. In conclusion, Epac stimulation induces PKCepsilon activation and Cx43 phosphorylation with an increase in GJIC, but Epac activation does not induce preconditioning to ischemia in contrast with beta-adrenergic stimulation.

Connexin 43 mimetic peptide Gap26 confers protection to intact heart against myocardial ischemia injury

Unapposed connexin 43 hemichannels (Cx43Hc) are present on sarcolemma of cardiomyocytes. Whereas Cx43Hc remain closed during physiological conditions, their opening under ischemic stress contributes to irreversible tissue injury and cell death. To date, conventional blockers of connexin channels act unselectively on both gap junction channels and unapposed hemichannels. Here, we test the hypothesis that Gap26, a synthetic structural mimetic peptide deriving from the first extracellular loop of Cx43 and a presumed selective blocker of Cx43Hc, confers resistance to intact rat heart against ischemia injury. Langendorff-perfused intact rat hearts were utilized. Regional ischemia was induced by 40-min occlusion of the left anterior descendent coronary and followed by 180 min of reperfusion. Gap26 was applied either 10 min before or 30 min after the initiation of ischemia. Interestingly, myocardial infarct size was reduced by 48% and 55% in hearts treated with Gap26 before or during ischemia, respectively, compared to untreated hearts. Additionally, myocardial perfusate flow was increased in both groups during reperfusion by 37% and 32%, respectively. Application of Gap26 increased survival of isolated cardiomyocytes after simulated ischemia-reperfusion by nearly twofold compared to untreated cells. On the other hand, superfusion of tsA201 cells transiently expressing Cx43 with Gap26 caused 61% inhibition of Cx43Hc-mediated currents recorded using the patch clamp technique. In summary, we demonstrate for the first time that Cx43 mimetic peptide Gap26 confers protection to intact heart against ischemia-reperfusion injury whether administered before or after the occurrence of ischemia. In addition, we provide unequivocal evidence for the inhibitory effect of Gap26 on genuine Cx43Hc.

Role of the gap junction in ischemic preconditioning in the heart

The gap junction plays roles not only in electrical coupling of cardiomyocytes but also in intercellular transport of biologically active substances. Furthermore, the gap junction participates in decision making on cell survival versus cell death in various types of cells, and a part of reperfusion injury in the heart has been indicated to be gap junction mediated. The contribution of gap junction communication (GJC) and/or mitochondrial “hemichannels” to protective signaling during the trigger phase of ischemic preconditioning (IPC) is suggested by observations that IPC failed to protect the heart when GJC was blocked during IPC. Although ischemia suppresses both electrical and chemical GJC, chemical GJC persists for a considerable time after electrical GJC is lost. IPC facilitates the ischemia-induced suppression of chemical GJC, whereas IPC delays the reduction of electrical GJC after ischemia. The inhibition of GJC during sustained ischemia and reperfusion by GJC blockers mimics the effect of IPC on myocardial necrosis. IPC induces distinct effects on the interaction of connexin-43 with protein kinases, and the phosphorylation of connexin-43 at Ser368 by PKCε is a primary mechanism of inhibition of chemical GJC by IPC. Several lines of evidence support the notion that the modulation of GJC is a part of the mechanism of IPC-induced protection against myocardial necrosis and arrhythmias, though what percentage of IPC protection is attributable to the inhibition of GJC during ischemia-reperfusion still remains unclear.

Phosphorylation of extrajunctional Cx43 in ischemic-preconditioned rat hearts

Ischemic preconditioning (IP) has been found to intensify ischemia-induced spreading of connexin 43 (Cx43) into the extrajunctional plasma membrane of the rat heart. Since ischemia is known to induce connexin dephosphorylation and plasmalemmal permeabilization, we considered whether IP might delay dephosphorylation of also extrajunctional connexin and thus impede myocyte permeabilization. Regional myocardial ischemia of both 15 and 45 min duration with and without IP, respectively, was induced in anesthetized rats. Sections of the hearts were immunostained for phosphorylated and dephosphorylated Cx43. The ratios of immunofluorescence in gap junctions and extrajunctional membranes, respectively, versus myocyte interiors were determined as relative fluorescence units (RFU). IP, however, was not found to reduce gap junctional dephosphorylated Cx43 (normal perfusion: 1.23 +/- 0.06 RFU; 15 min ischemia without and with IP: 1.80 +/- 0.09 and 2.10 +/- 0.24 RFU; 45 min ischemia without and with IP: 3.12 +/- 0.55 and 2.57 +/- 0.33 RFU, respectively). Extrajunctionally, only dephosphorylated Cx43 was found. Its occurrence was not prevented by IP, it was rather intensified by IP (normal perfusion: 1.02 +/- 0.01 RFU, 15 min ischemia without and with IP: 1.06 +/- 0.03 and 1.15 +/- 0.05 RFU (P < 0.02), and 45 min ischemia without and with IP: 1.30 +/- 0.07 and 1.27 +/- 0.07 RFU, respectively). Although under ischemia, junctional and extrajunctional Cx43 predominated in the dephosphorylated state, only a small fraction of myocytes were found permeabilized to propidium iodide. In conclusion, IP did not prevent ischemia-induced Cx43 dephosphorylation in gap junctions and extrajunctional plasma membranes. It is thus more likely that changes in non-channel functions or other localizations of Cx43 are involved in IP-induced myocardial protection.

Changes in connexin expression and the atrial fibrillation substrate in congestive heart failure

RATIONALE

Although connexin changes are important for the ventricular arrhythmic substrate in congestive heart failure (CHF), connexin alterations during CHF-related atrial arrhythmogenic remodeling have received limited attention.

OBJECTIVE

To analyze connexin changes and their potential contribution to the atrial fibrillation (AF) substrate during the development and reversal of CHF.

METHODS AND RESULTS

Three groups of dogs were studied: CHF induced by 2-week ventricular tachypacing (240 bpm, n=15); CHF dogs allowed a 4-week nonpaced recovery interval after 2-week tachypacing (n=16); and nonpaced sham controls (n=19). Left ventricular (LV) end-diastolic pressure and atrial refractory periods increased with CHF and normalized on CHF recovery. CHF caused abnormalities in atrial conduction indexes and increased the duration of burst pacing-induced AF (DAF, from 22+/-7 seconds in control to 1100+/-171 seconds, P<0.001). CHF did not significantly alter overall atrial connexin (Cx)40 and Cx43 mRNA and protein expression levels, but produced Cx43 dephosphorylation, increased Cx40/Cx43 protein expression ratio and caused Cx43 redistribution toward transverse cell-boundaries. All of the connexin-alterations reversed on CHF recovery, but CHF-induced conduction abnormalities and increased DAF (884+/-220 seconds, P<0.001 versus control) remained. The atrial fibrous tissue content increased from 3.6+/-0.7% in control to 14.7+/-1.5% and 13.3+/-2.3% in CHF and CHF recovery, respectively (both P<0.01 versus control), with transversely running zones of fibrosis physically separating longitudinally directed muscle bundles. In an ionically based action potential/tissue model, fibrosis was able to account for conduction abnormalities associated with CHF and recovery.

CONCLUSIONS

CHF causes atrial connexin changes, but these are not essential for CHF-related conduction disturbances and AF promotion, which are rather related primarily to fibrotic interruption of muscle bundle continuity.

Sex-dependent impairment of cardiac action potential conduction in type 1 diabetic rats

The incidence of diabetes mellitus is increasing. Cardiac dysfunction often develops, resulting in diverse arrhythmias. These arise from ion channel remodeling or from altered speed and pattern of impulse propagation. Few studies have investigated impulse propagation in the diabetic heart. We previously showed a reduced conduction reserve in the diabetic heart, with associated changes in intercellular gap junctions. The present study investigated whether these effects are sex specific. Hearts from control and streptozotocin-diabetic male and female rats were used. Optical mapping was performed with the voltage-sensitive dye di-4-ANEPPS, using Langendorff-perfused hearts. Isolated ventricular cells and tissue sections were used for immunofluorescent labeling of the gap junction protein connexin43 (Cx43). The gap junction uncoupler heptanol (0.75 mM) or elevated K+ (9 mM, to reduce cell excitability) produced significantly greater slowing of propagation in diabetic males than females. In ovariectomized diabetic females, 9 mM K+ slowed conduction significantly more than in nonovariectomized females. The subcellular redistribution (lateralization) of the gap junction protein Cx43 was smaller in diabetic females. Pretreatment of diabetic males with the angiotensin-converting enzyme inhibitor quinapril reduced Cx43 lateralization and the effects of 9 mM K+ on propagation. In conclusion, the slowing of cardiac impulse propagation in type 1 diabetes is smaller in female rats, partly due to the presence of female sex hormones. This difference is (partly) mediated by sex differences in activation of the cardiac renin-angiotensin system.

Connexin43 phosphorylation: structural changes and biological effects

Vertebrate gap junctions, composed of proteins from the connexin gene family, play critical roles in embryonic development, co-ordinated contraction of excitable cells, tissue homoeostasis, normal cell growth and differentiation. Phosphorylation of connexin43, the most abundant and ubiquitously expressed connexin, has been implicated in the regulation of gap junctional communication at several stages of the connexin 'life cycle', including hemichannel oligomerization, export of the protein to the plasma membrane, hemichannel activity, gap junction assembly, gap junction channel gating and connexin degradation. Consistent with a short (1-5 h) protein half-life, connexin43 phosphorylation is dynamic and changes in response to activation of many different kinases. The present review assesses our current understanding of the effects of phosphorylation on connexin43 structure and function that in turn regulate gap junction biology, with an emphasis on events occurring in heart and skin.

Ethanol for cardiac ischemia: the role of protein kinase c

The physiological effects of ethanol are dependent upon the amount and duration of consumption. Chronic excessive consumption can lead to diseases such as liver cirrhosis, and cardiac arrhythmias, while chronic moderate consumption can have therapeutic effects on the cardiovascular system. Recently, it has also been observed that acute administration of ethanol to animals prior to an ischemic event provides significant protection to the heart. This review focuses on the different modalities of chronic vs. acute ethanol consumption and discusses recent evidence for a protective effect of acute ethanol exposure and the possible use of ethanol as a therapeutic agent.

Hypoxia-regulated activity of PKCepsilon in the lens

PURPOSE

To show that hypoxia is necessary to prevent opacification of the lens. Protein kinase C (PKC)-epsilon serves a role that is distinct from PKC-gamma when both PKC isoforms are expressed in the lens. PKCepsilon serves a very important role in hypoxic conditions, helping to prevent opacification of the lens.

METHODS

Digital image analysis, confocal microscopy, dye transfer assay, coimmunoprecipitation, Western blot analysis, and enzyme activity assays were used, respectively, to study opacification of the lens, intercellular communications, cellular localization of connexin-43 (Cx43), and the interactions between PKCepsilon, PKCgamma, and Cx43 in the lens epithelial cells.

RESULTS

Hypoxic conditions (1%-5% of oxygen) were very important in maintaining clarity of the lenses of wild-type (WT) mice. Normoxic conditions induced opacification of the WT lens. Lenses from the PKCepsilon-knockout mice underwent rapid opacification, even in hypoxic conditions. Hypoxia did not induce apoptosis in the lens epithelial cells, judging by the absence of active caspase-3, and it did not change intercellular communication and did not affect the number and localization of junctional Cx43 plaques in the lens epithelial cell culture. Hypoxia activated PKCepsilon, whereas phorbol ester (TPA), oxidation (H(2)O(2)), and insulin-like growth factor-1 (IGF-1) activated PKCgamma and decreased the activity of PKCepsilon. Hypoxia did not induce the phosphorylation of the Cx43.

CONCLUSIONS

Hypoxia-induced activation of PKCepsilon is very important in surviving hypoxia and maintaining the clarity of the lens. However, PKCgamma is utilized in the control of Cx43 gap junctions.

No impact of protein phosphatases on connexin 43 phosphorylation in ischemic preconditioning

Cardiac connexin 43 (Cx43) is involved in infarct propagation, and the uncoupling of Cx43-formed channels reduces infarct size. Cx43-formed channels open upon Cx43 dephosphorylation, and ischemic preconditioning (IP) prevents the ischemia-induced Cx43 dephosphorylation. In addition to the sarcolemma, Cx43 is also present in the cardiomyocyte mitochondria. We now examined the interaction of Cx43 with protein phosphatases PP1alpha, PP2Aalpha, and PP2Balpha and the role of such interaction for Cx43 phosphorylation in preconditioned myocardium. Infarct size (in %area at risk) in left ventricular anterior myocardium of Göttinger minipigs subjected to 90 min of low-flow ischemia and 120 min of reperfusion was 23.1 +/- 2.7 [n = 7, nonpreconditioned (NIP) group] and was reduced by IP to 10.0 +/- 3.2 (n = 6, P < 0.05). Mitochondrial and gap junctional Cx43 dephosphorylation increased after 85 min of ischemia in NIP myocardium, whereas Cx43 phosphorylation was preserved with IP. PP2Aalpha and PP1alpha, but not PP2Balpha, were detected by Western blot analysis in the left ventricular myocardium. Cx43 coprecipitated with PP2Aalpha but not with PP1alpha. Although the total PP2Aalpha immunoreactivity (confocal laser scan) was increased to 154 +/- 24% and 194 +/- 13% of baseline (P < 0.05) after 85 min of ischemia in NIP and IP myocardium, respectively, the PP2A activities were similar between the groups. The amount of PP2Aalpha coimmunoprecipitated with Cx43 remained unchanged. Only PP2Aalpha coprecipitates with Cx43 in pig myocardium. This interaction is not affected by IP, suggesting that PP2Aalpha is not involved in the prevention of the ischemia-induced Cx43 dephosphorylation by IP.