Remodeling of mechanical junctions and of microtubule-associated proteins accompany cardiac connexin43 lateralization

Downregulation of EB1 impedes Cx43 localization and cardiac conduction after hypothermic ischemia-reperfusion in rats

Background

Hypothermic ischemia-reperfusion arrhythmia is a common complication after cardiopulmonary bypass heart surgery, which can lead to hemodynamic disorders and even sudden cardiac death and is still not effectively prevented. This study aims to investigate the role and mechanisms of EB1 in hypothermic ischemia-reperfusion arrhythmia.

Methods

4-6 week old male Sprague-Dawley (SD) rats were randomly assigned to four groups with a control group receiving no treatment. In the treatment groups, the rats received an injection of a negative control adenovirus (AAV9-CON) or an adenoviral vector containing Mapre1 gene (AAV9-EB1) or an equal volume of saline via the tail vein. After 4 weeks, untreated rat hearts underwent continuous isolated heart perfusion for 5 min, while the treatment groups were subjected to Langendorff isolated heart ischemia-reperfusion. The multi-electrode array (MEA) technique was used to measure the conduction heterogeneity of rat heart, evaluating the protective effects of EB1 overexpression against reperfusion arrhythmias. Additionally, histological staining and western blotting were used to explore the potential pathways by which EB1 exerts its anti-arrhythmic effects, potentially through promoting the localization of connexin 43 (Cx43) to the intercalated discs (IDs). Furthermore, western blot analysis was conducted to assess microtubule stability and evaluate the possible mechanism by which EB1 facilitates the localization of Cx43 to the IDs.

Results

Following ischemia-reperfusion, EB1 expression was downregulated, accompanied by a reduction in Cx43. Overexpression of myocardial EB1 reduced the incidence of reperfusion arrhythmias and shortened their duration, which was associated with improved myocardial conduction. Male SD rats injected with AAV overexpressing EB1 had significantly higher levels of both total myocardial Cx43 and gap junction Cx43 after ischemia-reperfusion compared to the non-overexpression groups. Histological staining revealed lateralization of Cx43 in ischemia-reperfusion myocardium, which was corrected by EB1 overexpression. Additionally, EB1 overexpression increased the distribution of Cx43 at the IDs, overall reducing Cx43 remodeling. Moreover, EB1 overexpression can also alleviate microtubule damage caused by ischemia-reperfusion, which may be an important mechanism for the transport of Cx43 to the IDs.

Conclusions

EB1 downregulation following hypothermic ischemia-reperfusion was accompanied by a reduction in gap junction Cx43. EB1 overexpression improved cardiac conduction and reduced reperfusion arrhythmias by promoting Cx43 localization to IDs, facilitating gap junctions (GJs) formation. These findings contribute to the development of new therapeutic targets for reperfusion arrhythmias.

Overexpression of Cx43: Is It an Effective Approach for the Treatment of Cardiovascular Diseases?

In the heart, Connexin 43 (Cx43) is involved in intercellular communication through gap junctions and exosomes. In addition, Cx43-formed hemichannels at the plasma membrane are important for ion homeostasis and cellular volume regulation. Through its localization within nuclei and mitochondria, Cx43 influences the function of the respective organelles. Several cardiovascular diseases such as heart failure, ischemia/reperfusion injury, hypertrophic cardiomyopathy and arrhythmias are characterized by Cx43 downregulation and a dysregulated Cx43 function. Accordingly, a putative therapeutic approach of these diseases would include the induction of Cx43 expression in the damaged heart, albeit such induction may have both beneficial and detrimental effects. In this review we discuss the consequences of increasing cardiac Cx43 expression, and discuss this manipulation as a strategy for the treatment of cardiovascular diseases.

Chronic angiotensin-converting enzyme inhibition attenuates frailty and protects against atrial fibrillation in aging mice

BACKGROUND

Aging is a major risk factor for atrial fibrillation (AF); however, not all individuals age at the same rate. Frailty, which is a measure of susceptibility to adverse health outcomes, can be quantified with a frailty index (FI).

OBJECTIVE

This study aimed to determine the effects of angiotensin-converting enzyme (ACE) inhibition on AF and atrial remodeling in aging and frail mice.

METHODS

Aging mice were treated with the ACE inhibitor enalapril for 6 months beginning at 16.5 months of age and frailty was quantified. AF susceptibility and atrial structure and function were assessed by intracardiac electrophysiology in anesthetized mice, high-resolution optical mapping in intact atrial preparations, patch clamping in isolated atrial myocytes, and histology and molecular biology in atrial tissues.

RESULTS

Enalapril attenuated frailty in aging mice with larger effects in females. AF susceptibility was increased in aging mice but attenuated by enalapril. AF susceptibility and duration also increased as a function of FI score. P-wave duration was increased and atrial conduction velocity was reduced in aging mice and improved after enalapril treatment. Furthermore, P-wave duration and atrial conduction velocity were strongly correlated with FI score. Atrial action potential upstroke velocity (Vmax) and Na+ current (INa) were reduced whereas atrial fibrosis was increased in aging mice. Action potential Vmax, INa, and fibrosis were improved by enalapril and also correlated with FI scores.

CONCLUSION

ACE inhibition with enalapril attenuates frailty and reduces AF susceptibility in aging mice by preventing atrial electrical and structural remodeling.

Ion channel traffic jams: the significance of trafficking deficiency in long QT syndrome

A well-balanced ion channel trafficking machinery is paramount for the normal electromechanical function of the heart. Ion channel variants and many drugs can alter the cardiac action potential and lead to arrhythmias by interfering with mechanisms like ion channel synthesis, trafficking, gating, permeation, and recycling. A case in point is the Long QT syndrome (LQTS), a highly arrhythmogenic disease characterized by an abnormally prolonged QT interval on ECG produced by variants and drugs that interfere with the action potential. Disruption of ion channel trafficking is one of the main sources of LQTS. We review some molecular pathways and mechanisms involved in cardiac ion channel trafficking. We highlight the importance of channelosomes and other macromolecular complexes in helping to maintain normal cardiac electrical function, and the defects that prolong the QT interval as a consequence of variants or the effect of drugs. We examine the concept of "interactome mapping" and illustrate by example the multiple protein-protein interactions an ion channel may undergo throughout its lifetime. We also comment on how mapping the interactomes of the different cardiac ion channels may help advance research into LQTS and other cardiac diseases. Finally, we discuss how using human induced pluripotent stem cell technology to model ion channel trafficking and its defects may help accelerate drug discovery toward preventing life-threatening arrhythmias. Advancements in understanding ion channel trafficking and channelosome complexities are needed to find novel therapeutic targets, predict drug interactions, and enhance the overall management and treatment of LQTS patients.

Relaxin suppresses atrial fibrillation, reverses fibrosis and reduces inflammation in aged hearts

Healthy aging results in cardiac structural and electrical remodeling that increase susceptibility to cardiovascular diseases. Relaxin has shown broad cardioprotective effects including anti-fibrotic, anti-arrhythmic and anti-inflammatory outcomes in multiple models. This paper focuses on the cardioprotective effects of Relaxin in a rat model of aging. Sustained atrial or ventricular fibrillation are readily induced in the hearts of aged but not young control animals. Treatment with Relaxin suppressed this arrhythmogenic response by increasing conduction velocity, decreasing fibrosis and promoting substantial cardiac remodeling. Relaxin treatment resulted in a significant increase in the levels of: Nav1.5, Cx43, βcatenin and Wnt1 in rat hearts. In isolated cardiomyocytes, Relaxin increased Nav1.5 expression. These effects were mimicked by CHIR 99021, a pharmacological activator of canonical Wnt signaling, but blocked by the canonical Wnt inhibitor Dickkopf1. Relaxin prevented TGF-β-dependent differentiation of cardiac fibroblasts into myofibroblasts while increasing the expression of Wnt1; the effects of Relaxin on cardiac fibroblast differentiation were blocked by Dickkopf1. RNASeq studies demonstrated reduced expression of pro-inflammatory cytokines and an increase in the expression of α- and β-globin in Relaxin-treated aged males. Relaxin reduces arrhythmogenicity in the hearts of aged rats by reduction of fibrosis and increased conduction velocity. These changes are accompanied by substantial remodeling of the cardiac tissue and appear to be mediated by increased canonical Wnt signaling. Relaxin also exerts significant anti-inflammatory and anti-oxidant effects in the hearts of aged rodents. The mechanisms by which Relaxin increases the expression of Wnt ligands, promotes Wnt signaling and reprograms gene expression remain to be determined.

Acute Adenoviral Infection Elicits an Arrhythmogenic Substrate Prior to Myocarditis

BACKGROUND

Viral cardiac infection represents a significant clinical challenge encompassing several etiological agents, disease stages, complex presentation, and a resulting lack of mechanistic understanding. Myocarditis is a major cause of sudden cardiac death in young adults, where current knowledge in the field is dominated by later disease phases and pathological immune responses. However, little is known regarding how infection can acutely induce an arrhythmogenic substrate before significant immune responses. Adenovirus is a leading cause of myocarditis, but due to species specificity, models of infection are lacking, and it is not understood how adenoviral infection may underlie sudden cardiac arrest. Mouse adenovirus type-3 was previously reported as cardiotropic, yet it has not been utilized to understand the mechanisms of cardiac infection and pathology.

METHODS

We have developed mouse adenovirus type-3 infection as a model to investigate acute cardiac infection and molecular alterations to the infected heart before an appreciable immune response or gross cardiomyopathy.

RESULTS

Optical mapping of infected hearts exposes decreases in conduction velocity concomitant with increased Cx43Ser368 phosphorylation, a residue known to regulate gap junction function. Hearts from animals harboring a phospho-null mutation at Cx43Ser368 are protected against mouse adenovirus type-3-induced conduction velocity slowing. Additional to gap junction alterations, patch clamping of mouse adenovirus type-3-infected adult mouse ventricular cardiomyocytes reveals prolonged action potential duration as a result of decreased IK1 and IKs current density. Turning to human systems, we find human adenovirus type-5 increases phosphorylation of Cx43Ser368 and disrupts synchrony in human induced pluripotent stem cell-derived cardiomyocytes, indicating common mechanisms with our mouse whole heart and adult cardiomyocyte data.

CONCLUSIONS

Together, these findings demonstrate that adenoviral infection creates an arrhythmogenic substrate through direct targeting of gap junction and ion channel function in the heart. Such alterations are known to precipitate arrhythmias and likely contribute to sudden cardiac death in acutely infected patients.

Desmosomal Junctions and Connexin-43 Remodeling in High-Pacing-Induced Heart Failure Dogs

BACKGROUND

While desmosomal junctions and gap junction remodeling are among the arrhythmogenic substrates, the fate of desmosomal and gap junctions in high-pacing-induced heart failure remains unclear. This aim of this study was to determine the fate of desmosomal junctions in high-pacing-induced heart failure.

METHODS

Dogs were randomly divided into 2 equal groups, a high-pacing-induced heart failure model group (heart failure group, n = 6) and a sham operation group (control group, n = 6). Echocardiography and cardiac electrophysiological examination were performed. Cardiac tissue was analyzed by immunofluorescence and transmission electron microscopy. The expression of desmoplakin and desmoglein-2 proteins was detected by western blot.

RESULTS

A significant decrease in ejection fraction, significant cardiac dilatation, diastolic and systolic dysfunction, and ventricular thinning occurred after 4 weeks in high-pacing-induced dog model of heart failure. Effective refractory period action potential duration at 90% repolarization was prolonged in the heart failure group. Immunofluorescence analysis and transmission electron microscopy demonstrated connexin-43 lateralization accompanies desmoglein-2 and desmoplakin remodeling in the heart failure group. Western blotting showed that the expression of desmoplakin and desmoglein-2 proteins was higher in heart failure than in normal tissue.

CONCLUSION

Desmosome (desmoglein-2 and desmoplakin) redistribution and desmosome (desmoglein-2) overexpression accompanying connexin-43 lateralization were parts of a complex remodeling in high-pacing-induced heart failure.

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

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

Nesprin-1: novel regulator of striated muscle nuclear positioning and mechanotransduction

Nesprins (nuclear envelope spectrin repeat proteins) are multi-isomeric scaffolding proteins. Giant nesprin-1 and -2 localise to the outer nuclear membrane, interact with SUN (Sad1p/UNC-84) domain-containing proteins at the inner nuclear membrane to form the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex, which, in association with lamin A/C and emerin, mechanically couples the nucleus to the cytoskeleton. Despite ubiquitous expression of nesprin giant isoforms, pathogenic mutations in nesprin-1 and -2 are associated with tissue-specific disorders, particularly related to striated muscle such as dilated cardiomyopathy and Emery-Dreifuss muscular dystrophy. Recent evidence suggests this muscle-specificity might be attributable in part, to the small muscle specific isoform, nesprin-1α2, which has a novel role in striated muscle function. Our current understanding of muscle-specific functions of nesprin-1 and its isoforms will be summarised in this review to provide insight into potential pathological mechanisms of nesprin-related muscle disease and may inform potential targets of therapeutic modulation.

Hypertension Induces Pro-arrhythmic Cardiac Connexome Disorders: Protective Effects of Treatment

Prolonged population aging and unhealthy lifestyles contribute to the progressive prevalence of arterial hypertension. This is accompanied by low-grade inflammation and over time results in heart dysfunction and failure. Hypertension-induced myocardial structural and ion channel remodeling facilitates the development of both atrial and ventricular fibrillation, and these increase the risk of stroke and sudden death. Herein, we elucidate hypertension-induced impairment of "connexome" cardiomyocyte junctions. This complex ensures cell-to-cell adhesion and coupling for electrical and molecular signal propagation. Connexome dysfunction can be a key factor in promoting the occurrence of both cardiac arrhythmias and heart failure. However, the available literature indicates that arterial hypertension treatment can hamper myocardial structural remodeling, hypertrophy and/or fibrosis, and preserve connexome function. This suggests the pleiotropic effects of antihypertensive agents, including anti-inflammatory. Therefore, further research is required to identify specific molecular targets and pathways that will protect connexomes, and it is also necessary to develop new approaches to maintain heart function in patients suffering from primary or pulmonary arterial hypertension.

A Multi-omic and Multi-Species Analysis of Right Ventricular Failure

Right ventricular failure (RVF) is a leading cause of morbidity and mortality in multiple cardiovascular diseases, but there are no approved treatments for RVF as therapeutic targets are not clearly defined. Contemporary transcriptomic/proteomic evaluations of RVF are predominately conducted in small animal studies, and data from large animal models are sparse. Moreover, a comparison of the molecular mediators of RVF across species is lacking. Here, we used transcriptomics and proteomics analyses to define the molecular pathways associated with cardiac MRI-derived values of RV hypertrophy, dilation, and dysfunction in pulmonary artery banded (PAB) piglets. Publicly available data from rat monocrotaline-induced RVF and pulmonary arterial hypertension patients with preserved or impaired RV function were used to compare the three species. Transcriptomic and proteomic analyses identified multiple pathways that were associated with RV dysfunction and remodeling in PAB pigs. Surprisingly, disruptions in fatty acid oxidation (FAO) and electron transport chain (ETC) proteins were different across the three species. FAO and ETC proteins and transcripts were mostly downregulated in rats, but were predominately upregulated in PAB pigs, which more closely matched the human data. Thus, the pig PAB metabolic molecular signature was more similar to human RVF than rodents. These data suggest there may be divergent molecular responses of RVF across species, and that pigs more accurately recapitulate the metabolic aspects of human RVF.

Actin-microtubule cytoskeletal interplay mediated by MRTF-A/SRF signaling promotes dilated cardiomyopathy caused by LMNA mutations

Mutations in the lamin A/C gene (LMNA) cause dilated cardiomyopathy associated with increased activity of ERK1/2 in the heart. We recently showed that ERK1/2 phosphorylates cofilin-1 on threonine 25 (phospho(T25)-cofilin-1) that in turn disassembles the actin cytoskeleton. Here, we show that in muscle cells carrying a cardiomyopathy-causing LMNA mutation, phospho(T25)-cofilin-1 binds to myocardin-related transcription factor A (MRTF-A) in the cytoplasm, thus preventing the stimulation of serum response factor (SRF) in the nucleus. Inhibiting the MRTF-A/SRF axis leads to decreased α-tubulin acetylation by reducing the expression of ATAT1 gene encoding α-tubulin acetyltransferase 1. Hence, tubulin acetylation is decreased in cardiomyocytes derived from male patients with LMNA mutations and in heart and isolated cardiomyocytes from Lmna^{p.H222P/H222P} male mice. In Atat1 knockout mice, deficient for acetylated α-tubulin, we observe left ventricular dilation and mislocalization of Connexin 43 (Cx43) in heart. Increasing α-tubulin acetylation levels in Lmna^{p.H222P/H222P} mice with tubastatin A treatment restores the proper localization of Cx43 and improves cardiac function. In summary, we show for the first time an actin-microtubule cytoskeletal interplay mediated by cofilin-1 and MRTF-A/SRF, promoting the dilated cardiomyopathy caused by LMNA mutations. Our findings suggest that modulating α-tubulin acetylation levels is a feasible strategy for improving cardiac function.

A microtubule-connexin-43 regulatory link suppresses arrhythmias and cardiac fibrosis in Duchenne muscular dystrophy mice

Dilated cardiomyopathy is the leading cause of death in Duchenne muscular dystrophy (DMD), an inherited degenerative disease of the cardiac and skeletal muscle caused by absence of the protein dystrophin. We showed one hallmark of DMD cardiomyopathy is the dysregulation of cardiac gap junction channel protein connexin-43 (Cx43). Proper Cx43 localization and function at the cardiac intercalated disc (ID) is regulated by post-translational phosphorylation of Cx43-carboxy-terminus residues S325/S328/S330 (pS-Cx43). Concurrently, Cx43 traffics along microtubules (MTs) for targeted delivery to the ID. In DMD hearts, absence of dystrophin results in a hyperdensified and disorganized MT cytoskeleton, yet the link with pS-Cx43 remains unaddressed. To gain insight into the relationship between MTs and pS-Cx43, DMD mice (mdx) and pS-Cx43-deficient (mdxS3A) mice were treated with an inhibitor of MT polymerization, colchicine (Colch). Colch treatment protected mdx, not mdxS3A mice, against Cx43 remodeling, improved MT directionality, and enhanced pS-Cx43/tubulin interaction. Likewise, severe arrhythmias were prevented in isoproterenol-stressed mdx, not mdxS3A mice. Furthermore, MT directionality was improved in pS-Cx43-mimicking mdx (mdxS3E). Mdxutr+/- and mdxutr+/-S3A mice, lacking one copy of dystrophin homolog utrophin, displayed enhanced cardiac fibrosis and reduced lifespan compared with mdxutr+/-S3E; and Colch treatment corrected cardiac fibrosis in mdxutr+/- but not mdxutr+/-S3A. Collectively, the data suggest that improved MT directionality reduces Cx43 remodeling and that pS-Cx43 is necessary and sufficient to regulate MT organization, which plays crucial role in correcting cardiac dysfunction in DMD mice. Thus, identification of novel organizational mechanisms acting on pS-Cx43-MT will help develop novel cardioprotective therapies for DMD cardiomyopathy.NEW & NOTEWORTHY We found that colchicine administration to Cx43-phospho-deficient dystrophic mice fails to protect against Cx43 remodeling. Conversely, Cx43-phospho-mimic dystrophic mice display a normalized MT network. We envision a bidirectional regulation whereby correction of the dystrophic MTs leads to correction of Cx43 remodeling, which in turn leads to further correction of the MTs. Our findings suggest a link between phospho-Cx43 and MTs that provides strong foundations for novel therapeutics in DMD cardiomyopathy.

Remodeling of Ion Channel Trafficking and Cardiac Arrhythmias

Both inherited and acquired cardiac arrhythmias are often associated with the abnormal functional expression of ion channels at the cellular level. The complex machinery that continuously traffics, anchors, organizes, and recycles ion channels at the plasma membrane of a cardiomyocyte appears to be a major source of channel dysfunction during cardiac arrhythmias. This has been well established with the discovery of mutations in the genes encoding several ion channels and ion channel partners during inherited cardiac arrhythmias. Fibrosis, altered myocyte contacts, and post-transcriptional protein changes are common factors that disorganize normal channel trafficking during acquired cardiac arrhythmias. Channel availability, described notably for hERG and K_V1.5 channels, could be another potent arrhythmogenic mechanism. From this molecular knowledge on cardiac arrhythmias will emerge novel antiarrhythmic strategies.

Targeting the Microtubule EB1-CLASP2 Complex Modulates NaV1.5 at Intercalated Discs

[Figure: see text].

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.

Importance of Cx43 for Right Ventricular Function

In the heart, connexins form gap junctions, hemichannels, and are also present within mitochondria, with connexin 43 (Cx43) being the most prominent connexin in the ventricles. Whereas the role of Cx43 is well established for the healthy and diseased left ventricle, less is known about the importance of Cx43 for the development of right ventricular (RV) dysfunction. The present article focusses on the importance of Cx43 for the developing heart. Furthermore, we discuss the expression and localization of Cx43 in the diseased RV, i.e., in the tetralogy of Fallot and in pulmonary hypertension, in which the RV is affected, and RV hypertrophy and failure occur. We will also introduce other Cx molecules that are expressed in RV and surrounding tissues and have been reported to be involved in RV pathophysiology. Finally, we highlight therapeutic strategies aiming to improve RV function in pulmonary hypertension that are associated with alterations of Cx43 expression and function.

Prevention of connexin-43 remodeling protects against Duchenne muscular dystrophy cardiomyopathy

Aberrant expression of the cardiac gap junction protein connexin-43 (Cx43) has been suggested as playing a role in the development of cardiac disease in the mdx mouse model of Duchenne muscular dystrophy (DMD); however, a mechanistic understanding of this association is lacking. Here, we identified a reduction of phosphorylation of Cx43 serines S325/S328/S330 in human and mouse DMD hearts. We hypothesized that hypophosphorylation of Cx43 serine-triplet triggers pathological Cx43 redistribution to the lateral sides of cardiomyocytes (remodeling). Therefore, we generated knockin mdx mice in which the Cx43 serine-triplet was replaced with either phospho-mimicking glutamic acids (mdxS3E) or nonphosphorylatable alanines (mdxS3A). The mdxS3E, but not mdxS3A, mice were resistant to Cx43 remodeling, with a corresponding reduction of Cx43 hemichannel activity. MdxS3E cardiomyocytes displayed improved intracellular Ca2+ signaling and a reduction of NADPH oxidase 2 (NOX2)/ROS production. Furthermore, mdxS3E mice were protected against inducible arrhythmias, related lethality, and the development of cardiomyopathy. Inhibition of microtubule polymerization by colchicine reduced both NOX2/ROS and oxidized CaMKII, increased S325/S328/S330 phosphorylation, and prevented Cx43 remodeling in mdx hearts. Together, these results demonstrate a mechanism of dystrophic Cx43 remodeling and suggest that targeting Cx43 may be a therapeutic strategy for preventing heart dysfunction and arrhythmias in DMD patients.

β-adrenergic-mediated dynamic augmentation of sarcolemmal CaV 1.2 clustering and co-operativity in ventricular myocytes

Key points

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

Abstract

Voltage‐dependent L‐type Ca_V1.2 channels play an indispensable role in cardiac excitation–contraction coupling. Activation of the β‐adrenergic receptor (βAR)/cAMP/protein kinase A (PKA) signalling pathway leads to enhanced Ca_V1.2 activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. Ca_V1.2 channels exhibit a clustered distribution along the T‐tubule sarcolemma of ventricular myocytes where nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by dynamic, physical, allosteric interactions between adjacent channel C‐terminal tails. This amplifies Ca²⁺ influx and augments myocyte Ca²⁺ transient and contraction amplitudes. We investigated whether βAR signalling could alter Ca_V1.2 channel clustering to facilitate co‐operative channel interactions and elevate Ca²⁺ influx in ventricular myocytes. Bimolecular fluorescence complementation experiments reveal that the βAR agonist, isoproterenol (ISO), promotes enhanced Ca_V1.2–Ca_V1.2 physical interactions. Super‐resolution nanoscopy and dynamic channel tracking indicate that these interactions are expedited by enhanced spatial proximity between channels, resulting in the appearance of Ca_V1.2 ‘super‐clusters’ along the z‐lines of ISO‐stimulated cardiomyocytes. The mechanism that leads to super‐cluster formation involves rapid, dynamic augmentation of sarcolemmal Ca_V1.2 channel abundance after ISO application. Optical and electrophysiological single channel recordings confirm that these newly inserted channels are functional and contribute to overt co‐operative gating behaviour of Ca_V1.2 channels in ISO stimulated myocytes. The results of the present study reveal a new facet of βAR‐mediated regulation of Ca_V1.2 channels in the heart and support the novel concept that a pre‐synthesized pool of sub‐sarcolemmal Ca_V1.2 channel‐containing vesicles/endosomes resides in cardiomyocytes and can be mobilized to the sarcolemma to tune excitation–contraction coupling to meet metabolic and/or haemodynamic demands.

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

Antiarrhythmic effect of sevoflurane as an additive to HTK solution on reperfusion arrhythmias induced by hypothermia and ischaemia is associated with the phosphorylation of connexin 43 at serine 368

Background

Reperfusion ventricular arrhythmia (RA) associated with hypothermic ischaemic storage is increasingly recognized as a substantial contributor to adverse consequences after heart transplantation. Ischemia- or hypothermia-induced gap junction (GJ) remodelling is closely linked to RA. Reducing GJ remodelling contributes to RA attenuation and is important in heart transplantation. However, sevoflurane has an antiarrhythmic effect associated with the connexin 43 (Cx43) protein that has not yet been fully established.

Methods

Hearts were divided into two groups according to a random number table: all hearts were arrested by an infusion of histidine-tryptophan-ketoglutarate (HTK) solution (4 °C) followed by (1) storage in HTK solution (4 °C) alone for 6 h (n = 8, Control group) or (2) storage in HTK solution supplemented with sevoflurane (2.5%) (4 °C) for 6 h (n = 8, Sevo-HTK group). First, the total Cx43 level and the phosphorylation of Cx43 at Ser368 (Cx43-pS368) were assessed by Western blotting, and the distribution of Cx43 was assessed by immunohistochemistry. Second, programmed electrical stimulation (PES) and monophasic action potential (MAP) recording were used to analyse the MAP duration (MAPD), conduction velocity (CV) and transmural repolarization dispersion (TDR). In addition, haematoxylin and eosin (HE) and terminal deoxynucleotidyl transferase-dUTP nick end labelling (TUNEL) staining were individually used to investigate the degree of myocardial pathological damage and cell apoptosis. Finally, bipolar electrograms were used to record the graft re-beating time and monitor RA during reperfusion for 15 to 30 min.

Results

Sevo-HTK solution relatively increased the total Cx43 (P < 0.01) and Cx43-pS368 (P < 0.01) levels and prevented Cx43 redistribution (P < 0.05) and CV slowing (P < 0.001) but did not change TDR (P > 0.05). Additionally, the Cx43-pS368/total Cx43 ratio (P>0.05) was similar in the two groups. However, with Sevo-HTK solution, the graft re-beating times were shortened, myocardial pathological damage was ameliorated, and the number of apoptotic cells was markedly decreased.

Conclusion

The reduction in hypothermia and ischaemia-induced reperfusion arrhythmias by the addition of sevoflurane to HTK solution may be related to the phosphorylation of Cx43 at serine 368.

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

RATIONALE

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

OBJECTIVE

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Novel Mechanistic Roles for Ankyrin-G in Cardiac Remodeling and Heart Failure

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The pathogenesis of human heart failure is complex, and the creation of new therapeutic strategies for human heart failure is critical.

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Identifying the molecular pathways underlying heart failure is important to define potential new therapeutic targets.

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Ankyrin polypeptides serve to target and stabilize membrane proteins in cardiomyocytes.

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Ankyrin-G levels are altered in humans and mice with heart failure, and mice lacking ankyrin-G in cardiomyocytes develop cardiomyopathy and systolic dysfunction.

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Mechanistically, ankyrin-G is necessary for the expression and localization of critical myocyte proteins essential for regulating cardiac structural and electrical activity.

Ankyrin polypeptides are intracellular proteins responsible for targeting cardiac membrane proteins. Here, the authors demonstrate that ankyrin-G plays an unexpected role in normal compensatory physiological remodeling in response to myocardial stress and aging; the authors implicate disruption of ankyrin-G in human heart failure. Mechanistically, the authors illustrate that ankyrin-G serves as a key nodal protein required for cardiac myofilament integration with the intercalated disc. Their data define novel in vivo mechanistic roles for ankyrin-G, implicate ankyrin-G as necessary for compensatory cardiac physiological remodeling under stress, and implicate disruption of ankyrin-G in the development and progression of human heart failure.

Mechanosensitive Gene Regulation by Myocardin-Related Transcription Factors Is Required for Cardiomyocyte Integrity in Load-Induced Ventricular Hypertrophy

BACKGROUND

Hypertrophic cardiomyocyte growth and dysfunction accompany various forms of heart disease. The mechanisms responsible for transcriptional changes that affect cardiac physiology and the transition to heart failure are not well understood. The intercalated disc (ID) is a specialized intercellular junction coupling cardiomyocyte force transmission and propagation of electrical activity. The ID is gaining attention as a mechanosensitive signaling hub and hotspot for causative mutations in cardiomyopathy.

METHODS

Transmission electron microscopy, confocal microscopy, and single-molecule localization microscopy were used to examine changes in ID structure and protein localization in the murine and human heart. We conducted detailed cardiac functional assessment and transcriptional profiling of mice lacking myocardin-related transcription factor (MRTF)-A and MRTF-B specifically in adult cardiomyocytes to evaluate the role of mechanosensitive regulation of gene expression in load-induced ventricular remodeling.

RESULTS

We found that MRTFs localize to IDs in the healthy human heart and accumulate in the nucleus in heart failure. Although mice lacking MRTFs in adult cardiomyocytes display normal cardiac physiology at baseline, pressure overload leads to rapid heart failure characterized by sarcomere disarray, ID disintegration, chamber dilation and wall thinning, cardiac functional decline, and partially penetrant acute lethality. Transcriptional profiling reveals a program of actin cytoskeleton and cardiomyocyte adhesion genes driven by MRTFs during pressure overload. Indeed, conspicuous remodeling of gap junctions at IDs identified by single-molecule localization microscopy may partially stem from a reduction in Mapre1 expression, which we show is a direct mechanosensitive MRTF target.

CONCLUSIONS

Our study describes a novel paradigm in which MRTFs control an acute mechanosensitive signaling circuit that coordinates cross-talk between the actin and microtubule cytoskeleton and maintains ID integrity and cardiomyocyte homeostasis in heart disease.

The Cardiac Sodium Channel and Its Protein Partners

Activation of the electrical signal and its transmission as a depolarizing wave in the whole heart requires highly organized myocyte architecture and cell-cell contacts. In addition, complex trafficking and anchoring intracellular machineries regulate the proper surface expression of channels and their targeting to distinct membrane domains. An increasing list of proteins, lipids, and second messengers can contribute to the normal targeting of ion channels in cardiac myocytes. However, their precise roles in the electrophysiology of the heart are far from been extensively understood. Nowadays, much effort in the field focuses on understanding the mechanisms that regulate ion channel targeting to sarcolemma microdomains and their organization into macromolecular complexes. The purpose of the present section is to provide an overview of the characterized partners of the main cardiac sodium channel, Na_V1.5, involved in regulating the functional expression of this channel both in terms of trafficking and targeting into microdomains.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

A nanodomain is a collection of proteins localized within a specialized, nanoscale structural environment, which can serve as the functional unit of macroscopic physiologic processes. We are beginning to recognize the key roles of cardiomyocyte nanodomains in essential processes of cardiac physiology such as electrical impulse propagation and excitation–contraction coupling (ECC). There is growing appreciation of nanodomain dysfunction, i.e., nanopathy, as a mechanistic driver of life-threatening arrhythmias in a variety of pathologies. Here, we offer an overview of current research on the role of nanodomains in cardiac physiology with particular emphasis on: (1) sodium channel-rich nanodomains within the intercalated disk that participate in cell-to-cell electrical coupling and (2) dyadic nanodomains located along transverse tubules that participate in ECC. The beat to beat function of cardiomyocytes involves three phases: the action potential, the calcium transient, and mechanical contraction/relaxation. In all these phases, cell-wide function results from the aggregation of the stochastic function of individual proteins. While it has long been known that proteins that exist in close proximity influence each other’s function, it is increasingly appreciated that there exist nanoscale structures that act as functional units of cardiac biophysical phenomena. Termed nanodomains, these structures are collections of proteins, localized within specialized nanoscale structural environments. The nano-environments enable the generation of localized electrical and/or chemical gradients, thereby conferring unique functional properties to these units. Thus, the function of a nanodomain is determined by its protein constituents as well as their local structural environment, adding an additional layer of complexity to cardiac biology and biophysics. However, with the emergence of experimental techniques that allow direct investigation of structure and function at the nanoscale, our understanding of cardiac physiology and pathophysiology at these scales is rapidly advancing. Here, we will discuss the structure and functions of multiple cardiomyocyte nanodomains, and novel strategies that target them for the treatment of cardiac arrhythmias.

Connexins and Disease

Inherited or acquired alterations in the structure and function of connexin proteins have long been associated with disease. In the present work, we review current knowledge on the role of connexins in diseases associated with the heart, nervous system, cochlea, and skin, as well as cancer and pleiotropic syndromes such as oculodentodigital dysplasia (ODDD). Although incomplete by virtue of space and the extent of the topic, this review emphasizes the fact that connexin function is not only associated with gap junction channel formation. As such, both canonical and noncanonical functions of connexins are fundamental components in the pathophysiology of multiple connexin related disorders, many of them highly debilitating and life threatening. Improved understanding of connexin biology has the potential to advance our understanding of mechanisms, diagnosis, and treatment of disease.

A Cell Junctional Protein Network Associated with Connexin-26

GJB2 mutations are the leading cause of non-syndromic inherited hearing loss. GJB2 encodes connexin-26 (CX26), which is a connexin (CX) family protein expressed in cochlea, skin, liver, and brain, displaying short cytoplasmic N-termini and C-termini. We searched for CX26 C-terminus binding partners by affinity capture and identified 12 unique proteins associated with cell junctions or cytoskeleton (CGN, DAAM1, FLNB, GAPDH, HOMER2, MAP7, MAPRE2 (EB2), JUP, PTK2B, RAI14, TJP1, and VCL) by using mass spectrometry. We show that, similar to other CX family members, CX26 co-fractionates with TJP1, VCL, and EB2 (EB1 paralogue) as well as the membrane-associated protein ASS1. The adaptor protein CGN (cingulin) co-immuno-precipitates with CX26, ASS1, and TJP1. In addition, CGN co-immunoprecipitation with CX30, CX31, and CX43 indicates that CX association is independent on the CX C-terminus length or sequence. CX26, CGN, FLNB, and DAMM1 were shown to distribute to the organ of Corti and hepatocyte plasma membrane. In the mouse liver, CX26 and TJP1 co-localized at the plasma membrane. In conclusion, CX26 associates with components of other membrane junctions that integrate with the cytoskeleton.

Improving electrical properties of iPSC-cardiomyocytes by enhancing Cx43 expression

The therapeutic potential of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) is limited by immature functional features including low impulse propagation and reduced cell excitability. Key players regulating electrical activity are voltage-gated Na⁺ channels (Na_v1.5) and gap junctions built from connexin-43 (Cx43). Here we tested the hypothesis that enhanced Cx43 expression increases intercellular coupling and influences excitability by modulating Na_v1.5. Using transgenic approaches, Cx43 and Na_v1.5 localization and cell coupling were studied by confocal imaging. Na_v1.5 currents and action potentials (APs) were measured using the patch-clamp technique. Enhanced sarcolemmal Cx43 expression significantly improved intercellular coupling and accelerated dye transfer kinetics. Furthermore, Cx43 modulated Na_v1.5 function leading to significantly higher current and enhanced AP upstroke velocities, thereby improving electrical activity as measured by microelectrode arrays. These findings suggest a mechanistic link between cell coupling and excitability controlled by Cx43 expression in iPSC-CMs. Therefore, we propose Cx43 as novel molecular target for improving electrical properties of iPSC-CMs to match the functional properties of native myocytes.

Three-dimensional reconstruction of the intercalated disc including the intercellular junctions by applying volume scanning electron microscopy

The intercalated disc (ID) contains different kinds of intercellular junctions: gap junctions (GJs), desmosomes and areae compositae, essential for adhesion and communication between adjacent cardiomyocytes. The junctions can be identified based on their morphology when imaged using transmission electron microscopy (TEM), however, only with very limited information in the z-dimension. The application of volume EM techniques can give insight into the three-dimensional (3-D) organization of complex biological structures. In this study, we generated 3-D datasets using serial block-face scanning electron microscopy (SBF-SEM) and focused ion beam SEM (FIB-SEM), the latter resulting in datasets with 5 nm isotropic voxels. We visualized cardiomyocytes in murine ventricular heart tissue and, for the first time, we could three-dimensionally reconstruct the ID including desmosomes and GJs with 5 nm precision in a large volume. Results show in three dimensions a highly folded structure of the ID, with the presence of GJs and desmosomes in both plicae and interplicae regions. We observed close contact of GJs with mitochondria and a variable spatial distribution of the junctions. Based on measurements of the shape of the intercellular junctions in 3-D, it is seen that GJs and desmosomes vary in size, depending on the region within the ID. This demonstrates that volume EM is essential to visualize morphological changes and its potential to quantitatively determine structural changes between normal and pathological conditions, e.g., cardiomyopathies.

Apelin Ameliorates High Glucose-Induced Downregulation of Connexin 43 via AMPK-Dependent Pathway in Neonatal Rat Cardiomyocytes

Diabetes Mellitus is a common disorder, with increasing risk of cardiac arrhythmias. Studies have shown that altered connexin expression and gap junction remodeling under hyperglycemia contribute to the high prevalence of cardiac arrhythmias and even sudden death. Connexin 43 (Cx43), a major protein that assembles to form cardiac gap junctions, has been found to be downregulated under high glucose conditions, along with inhibition of gap junctional intercellular communication (GJIC). While, apelin, a beneficial adipokine, increases Cx43 protein expression in mouse and human embryonic stem cells during cardiac differentiation. However, it remains unknown whether apelin influences GJIC capacity in cardiomyocytes. Here, using Western blotting and dye transfer assays, we found that Cx43 protein expression was reduced and GJIC was impaired after treatment with high glucose, which, however, could be abrogated after apelin treatment for 48 h. We also found that apelin increased Cx43 expression under normal glucose. Real-time PCR showed that the Cx43 mRNA was not significantly affected under high glucose conditions in the presence of apelin or high glucose and apelin. High glucose decreased the phosphorylation of AMPKα; however, apelin activated AMPKα. Interestingly, we found that Cx43 expression was increased after treatment with AICAR, an activator of AMPK signaling. AMPKα inhibition mediated with transfection of siRNA-AMPKα1 and siRNA-AMPKα2 abolished the protective effect of apelin on Cx43 expression. Our data suggest that apelin attenuates high glucose-induced Cx43 downregulation and improves the loss of functional gap junctions partly through the AMPK pathway.

Newly Identified NO-Sensor Guanylyl Cyclase/Connexin 43 Association Is Involved in Cardiac Electrical Function

BACKGROUND

Guanylyl cyclase, a heme-containing α1β1 heterodimer (GC1), produces cGMP in response to Nitric oxide (NO) stimulation. The NO-GC1-cGMP pathway negatively regulates cardiomyocyte contractility and protects against cardiac hypertrophy-related remodeling. We recently reported that the β1 subunit of GC1 is detected at the intercalated disc with connexin 43 (Cx43). Cx43 forms gap junctions (GJs) at the intercalated disc that are responsible for electrical propagation. We sought to determine whether there is a functional association between GC1 and Cx43 and its role in cardiac homeostasis.

METHODS AND RESULTS

GC1 and Cx43 immunostaining at the intercalated disc and coimmunoprecipitation from membrane fraction indicate that GC1 and Cx43 are associated. Mice lacking the α subunit of GC1 (GCα1 knockout mice) displayed a significant decrease in GJ function (dye-spread assay) and Cx43 membrane lateralization. In a cardiac-hypertrophic model, angiotensin II treatment disrupted the GC1-Cx43 association and induced significant Cx43 membrane lateralization, which was exacerbated in GCα1 knockout mice. Cx43 lateralization correlated with decreased Cx43-containing GJs at the intercalated disc, predictors of electrical dysfunction. Accordingly, an ECG revealed that angiotensin II-treated GCα1 knockout mice had impaired ventricular electrical propagation. The phosphorylation level of Cx43 at serine 365, a protein-kinase A upregulated site involved in trafficking/assembly of GJs, was decreased in these models.

CONCLUSIONS

GC1 modulates ventricular Cx43 location, hence GJ function, and partially protects from electrical dysfunction in an angiotensin II hypertrophy model. Disruption of the NO-cGMP pathway is associated with cardiac electrical disturbance and abnormal Cx43 phosphorylation. This previously unknown NO/Cx43 signaling could be a protective mechanism against stress-induced arrhythmia.

Dose-related ethanol intake, Cx43 and Nav1.5 remodeling: Exploring insights of altered ventricular conduction and QRS fragmentation in excessive alcohol users

BACKGROUND

Chronic, excessive ethanol intake has been linked with various electrical instabilities, conduction disturbances, and even sudden cardiac death, but the underlying cause for the latter is insufficiently delineated.

METHODS

We studied surface electrocardiography (ECG) in a community-dwelling cohort with moderate-to-heavy daily alcohol intake (grouped as >90g/day, ≤90g/day, and nonintake).

RESULTS

Compared with nonintake, heavier alcohol users showed markedly widened QRS duration and higher prevalence of QRS fragmentation (64.3%, 50.9%, and 33.7%, respectively, χ² 12.0, both p<0.05) on surface ECG across the 3 groups. These findings were successfully recapitulated in 14-week-old C57BL/6 mice that were chronically given a 4% or 6% alcohol diet and showed dose-related slower action potential upstroke, reduced resting membrane potential, and disorganized or decreased intraventricular conduction (all p<0.05). Immunodetection further revealed increased ventricular collagen I depots with Cx43 downregulation and remodeling, together with clustered and diminished membrane Nav1.5 distribution. Administration of Cx43 blocker (heptanol) and Nav1.5 inhibitor (tetrodotoxin) in the mice each attenuated the suppression ventricular conduction compared with nonintake mice (p<0.05).

CONCLUSIONS

Chronic excessive alcohol ingestion is associated with dose-related phenotypic intraventricular conduction disturbances and QRS fragmentation that can be recapitulated in mice. The mechanisms may involve suppressed gap junction and sodium channel functions, together with enhanced cardiac fibrosis that may contribute to arrhythmogenesis.

Mechanism of action of the anti-inflammatory connexin43 mimetic peptide JM2

Connexin-based therapeutics have shown the potential for therapeutic efficacy in improving wound healing. Our previous work demonstrated that the connexin43 (Cx43) mimetic peptide juxtamembrane 2 (JM2) reduced the acute inflammatory response to a submuscular implant model by inhibiting purinergic signaling. Given the prospective application in improving tissue-engineered construct tolerance that these results indicated, we sought to determine the mechanism of action for JM2 in the present study. Using confocal microscopy, a gap-FRAP cell communication assay, and an ethidium bromide uptake assay of hemichannel function we found that the peptide reduced cell surface Cx43 levels, Cx43 gap junction (GJ) size, GJ communication, and hemichannel activity. JM2 is based on the sequence of the Cx43 microtubule binding domain, and microtubules have a confirmed role in intracellular trafficking of Cx43 vesicles. Therefore, we tested the effect of JM2 on Cx43-microtubule interaction and microtubule polymerization. We found that JM2 enhanced Cx43-microtubule interaction and that microtubule polymerization was significantly enhanced. Taken together, these data suggest that JM2 inhibits trafficking of Cx43 to the cell surface by promoting irrelevant microtubule polymerization and thereby reduces the number of hemichannels in the plasma membrane available to participate in proinflammatory purinergic signaling. Importantly, this work indicates that JM2 may have therapeutic value in the treatment of proliferative diseases such as cancer. We conclude that the targeted action of JM2 on Cx43 channels may improve the tolerance of implanted tissue-engineered constructs against the innate inflammatory response.

Intracellular trafficking pathways of Cx43 gap junction channels

Gap Junction (GJ) channels, including the most common Connexin 43 (Cx43), have fundamental roles in excitable tissues by facilitating rapid transmission of action potentials between adjacent cells. For instance, synchronization during each heartbeat is regulated by these ion channels at the cardiomyocyte cell-cell border. Cx43 protein has a short half-life, and rapid synthesis and timely delivery of those proteins to particular subdomains are crucial for the cellular organization of gap junctions and maintenance of intracellular coupling. Impairment in gap junction trafficking contributes to dangerous complications in diseased hearts such as the arrhythmias of sudden cardiac death. Of recent interest are the protein-protein interactions with the Cx43 carboxy-terminus. These interactions have significant impact on the full length Cx43 lifecycle and also contribute to trafficking of Cx43 as well as possibly other functions. We are learning that many of the known non-canonical roles of Cx43 can be attributed to the recently identified six endogenous Cx43 truncated isoforms which are produced by internal translation. In general, alternative translation is a new leading edge for proteome expansion and therapeutic drug development. This review highlights recent mechanisms identified in the trafficking of gap junction channels, involvement of other proteins contributing to the delivery of channels to the cell-cell border, and understanding of possible roles of the newly discovered alternatively translated isoforms in Cx43 biology. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Desmosomal junctions are necessary for adult sinus node function

AIMS

Current mechanisms driving cardiac pacemaker function have focused on ion channel and gap junction channel function, which are essential for action potential generation and propagation between pacemaker cells. However, pacemaker cells also harbour desmosomes that structurally anchor pacemaker cells to each other in tissue, but their role in pacemaker function remains unknown.

METHODS AND RESULTS

To determine the role of desmosomes in pacemaker function, we generated a novel mouse model harbouring cardiac conduction-specific ablation (csKO) of the central desmosomal protein, desmoplakin (DSP) using the Hcn4-Cre-ERT2 mouse line. Hcn4-Cre targets cells of the adult mouse sinoatrial node (SAN) and can ablate DSP expression in the adult DSP csKO SAN resulting in specific loss of desmosomal proteins and structures. Dysregulation of DSP via loss-of-function (adult DSP csKO mice) and mutation (clinical case of a patient harbouring a pathogenic DSP variant) in mice and man, respectively, revealed that desmosomal dysregulation is associated with a primary phenotype of increased sinus pauses/dysfunction in the absence of cardiomyopathy. Underlying defects in beat-to-beat regulation were also observed in DSP csKO mice in vivo and intact atria ex vivo. DSP csKO SAN exhibited migrating lead pacemaker sites associated with connexin 45 loss. In vitro studies exploiting ventricular cardiomyocytes that harbour DSP loss and concurrent early connexin loss phenocopied the loss of beat-to-beat regulation observed in DSP csKO mice and atria, extending the importance of DSP-associated mechanisms in driving beat-to-beat regulation of working cardiomyocytes.

CONCLUSION

We provide evidence of a mechanism that implicates an essential role for desmosomes in cardiac pacemaker function, which has broad implications in better understanding mechanisms underlying beat-to-beat regulation as well as sinus node disease and dysfunction.

The "tail" of Connexin43: An unexpected journey from alternative translation to trafficking

With each heartbeat, Connexin43 (Cx43) cell-cell communication gap junctions are needed to rapidly spread and coordinate excitation signals for an effective heart contraction. The correct formation and delivery of channels to their respective membrane subdomain is referred to as protein trafficking. Altered Cx43 trafficking is a dangerous complication of diseased myocardium which contributes to the arrhythmias of sudden cardiac death. Cx43 has also been found to regulate many other cellular processes that cannot be explained by cell-cell communication. We recently identified the existence of up to six endogenous internally translated Cx43 N-terminal truncated isoforms from the same full-length mRNA molecule. This is the first evidence that alternative translation is possible for human ion channels and in human heart. Interestingly, we found that these internally translated isoforms, more specifically the 20 kDa isoform (GJA1-20k), is important for delivery of Cx43 to its respective membrane subdomain. This review covers recent advances in Cx43 trafficking and potential importance of alternatively translated Cx43 truncated isoforms. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

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.

Desmosome regulation and signaling in disease

Desmosomes are cell-cell adhesive organelles with a well-known role in forming strong intercellular adhesion during embryogenesis and in adult tissues subject to mechanical stress, such as the heart and skin. More recently, desmosome components have also emerged as cell signaling regulators. Loss of expression or interference with the function of desmosome molecules results in diseases of the heart and skin and contributes to cancer progression. However, the underlying molecular mechanisms that result in inherited and acquired disorders remain poorly understood. To address this question, researchers are directing their studies towards determining the functions that occur inside and outside of the junctions and the extent to which functions are adhesion-dependent or independent. This review focuses on recent discoveries that provide insights into the role of desmosomes and desmosome components in cell signaling and disease; wherever possible, we address molecular functions within and outside of the adhesive structure.

Desmosomal hotspots, microtubule delivery, and cardiac arrhythmogenesis

Microtubules can target proteins such as Connexin43 to plasma membrane subdomains. Patel et al. (2014) now show that the structural desmosome complex participates in targeted trafficking of membrane components through interactions between the microtubule network and the N terminus of desmoplakin, a region that is a pathogenic mutation hotspot.

Cardiomyocyte protein trafficking: Relevance to heart disease and opportunities for therapeutic intervention

Cardiomyocytes, the individual contractile units of heart muscle, are long-lived and robust. Given the longevity of these cells, it can be easy to overlook their dynamic intracellular environment that contain rapid protein movements and frequent protein turnover. Critical gene transcription and protein translation occur continuously, as well as trafficking and localization of proteins to specific functional zones of cell membrane. As heart failure becomes an increasingly important clinical entity, growing numbers of investigative teams are examining the cell biology of healthy and diseased cardiomyocytes. In this review, we introduce the major architectural structures and types of protein movements within cardiac cells, and then review recent studies that explore the regulation of such movements. We conclude by introducing current translational directions of the basic studies with a focus on novel areas of therapeutic development.

Disease mutations in desmoplakin inhibit Cx43 membrane targeting mediated by desmoplakin-EB1 interactions

Mechanisms by which microtubule plus ends interact with regions of cell-cell contact during tissue development and morphogenesis are not fully understood. We characterize a previously unreported interaction between the microtubule binding protein end-binding 1 (EB1) and the desmosomal protein desmoplakin (DP), and demonstrate that DP-EB1 interactions enable DP to modify microtubule organization and dynamics near sites of cell-cell contact. EB1 interacts with a region of the DP N terminus containing a hotspot for pathogenic mutations associated with arrhythmogenic cardiomyopathy (AC). We show that a subset of AC mutations, in addition to a mutation associated with skin fragility/woolly hair syndrome, impair gap junction localization and function by misregulating DP-EB1 interactions and altering microtubule dynamics. This work identifies a novel function for a desmosomal protein in regulating microtubules that affect membrane targeting of gap junction components, and elucidates a mechanism by which DP mutations may contribute to the development of cardiac and cutaneous diseases.

Nicorandil improves electrical remodelling, leading to the prevention of electrically induced ventricular tachyarrhythmia in a mouse model of desmin-related cardiomyopathy

1. Transgenic (TG) mice overexpressing an arg120gly missense mutation in heat shock protein B5 (HSPB5; i.e. R120G TG mice) exhibit desmin-related cardiomyopathy. Recently, the cardioprotective effect of nicorandil has been shown to prolong the survival of R120G TG mice. However, whether the TG mice exhibit ventricular arrhythmias and whether nicorandil can inhibit these arrhythmias remain unknown. In the present study we examined the effects of chronic nicorandil administration on ventricular electrical remodelling and arrhythmias in R120G TG mice. 2. Mice were administered nicorandil (15 mg/kg per day) or vehicle (water) orally from 5 to 30 weeks of age. Electrocardiograms (ECG) and optical action potentials were recorded from R120G TG mouse hearts. In addition, the expression of ventricular connexin 43 and the cardiac Na(+) channel Nav1.5 was examined in TG mice. 3. All ECG parameters tested were prolonged in R120G TG compared with non-transgenic (NTG) mice. Nicorandil improved the prolonged P, PQ and QRS intervals in R120G TG mice. Interestingly, impulse conduction slowing and increases in the expression of total and phosphorylated connexin 43 and Nav1.5 were observed in ventricles from R120G TG compared with NTG mice. Nicorandil improved ventricular impulse conduction slowing and normalized the increased protein expression levels of total and phosphorylated connexin 43, but not of Nav1.5, in R120G TG mouse hearts. Electrical rapid pacing at the ventricle induced ventricular tachyarrhythmias (VT) in six of eight R120G TG mouse hearts, but not in any of the eight nicorandil-treated R120G TG mouse hearts (P < 0.05). 4. These findings demonstrate that nicorandil inhibits cardiac electrical remodelling and that the prevention of VT by nicorandil is associated with normalization of connexin 43 expression in this model.

Arrhythmogenic cardiomyopathy and Brugada syndrome: diseases of the connexome

This review summarizes data in support of the notion that the cardiac intercalated disc is the host of a protein interacting network, called "the connexome", where molecules classically defined as belonging to one particular structure (e.g., desmosomes, gap junctions, sodium channel complex) actually interact with others, and together, control excitability, electrical coupling and intercellular adhesion in the heart. The concept of the connexome is then translated into the understanding of the mechanisms leading to two inherited arrhythmia diseases: arrhythmogenic cardiomyopathy, and Brugada syndrome. The cross-over points in these two diseases are addressed to then suggest that, though separate identifiable clinical entities, they represent "bookends" of a spectrum of manifestations that vary depending on the effect that a particular mutation has on the connexome as a whole.

Missense mutations in plakophilin-2 cause sodium current deficit and associate with a Brugada syndrome phenotype

BACKGROUND

Brugada syndrome (BrS) primarily associates with the loss of sodium channel function. Previous studies showed features consistent with sodium current (INa) deficit in patients carrying desmosomal mutations, diagnosed with arrhythmogenic cardiomyopathy (or arrhythmogenic right ventricular cardiomyopathy). Experimental models showed correlation between the loss of expression of desmosomal protein plakophilin-2 (PKP2) and reduced INa. We hypothesized that PKP2 variants that reduce INa could yield a BrS phenotype, even without overt structural features characteristic of arrhythmogenic right ventricular cardiomyopathy.

METHODS AND RESULTS

We searched for PKP2 variants in the genomic DNA of 200 patients with a BrS diagnosis, no signs of arrhythmogenic cardiomyopathy, and no mutations in BrS-related genes SCN5A, CACNa1c, GPD1L, and MOG1. We identified 5 cases of single amino acid substitutions. Mutations were tested in HL-1-derived cells endogenously expressing NaV1.5 but made deficient in PKP2 (PKP2-KD). Loss of PKP2 caused decreased INa and NaV1.5 at the site of cell contact. These deficits were restored by the transfection of wild-type PKP2, but not of BrS-related PKP2 mutants. Human induced pluripotent stem cell cardiomyocytes from a patient with a PKP2 deficit showed drastically reduced INa. The deficit was restored by transfection of wild type, but not BrS-related PKP2. Super-resolution microscopy in murine PKP2-deficient cardiomyocytes related INa deficiency to the reduced number of channels at the intercalated disc and increased separation of microtubules from the cell end.

CONCLUSIONS

This is the first systematic retrospective analysis of a patient group to define the coexistence of sodium channelopathy and genetic PKP2 variations. PKP2 mutations may be a molecular substrate leading to the diagnosis of BrS.

Inhibitors of SCF-Skp2/Cks1 E3 ligase block estrogen-induced growth stimulation and degradation of nuclear p27kip1: therapeutic potential for endometrial cancer

In many human cancers, the tumor suppressor, p27(kip1) (p27), a cyclin-dependent kinase inhibitor critical to cell cycle arrest, undergoes perpetual ubiquitin-mediated proteasomal degradation by the E3 ligase complex SCF-Skp2/Cks1 and/or cytoplasmic mislocalization. Lack of nuclear p27 causes aberrant cell cycle progression, and cytoplasmic p27 mediates cell migration/metastasis. We previously showed that mitogenic 17-β-estradiol (E2) induces degradation of p27 by the E3 ligase Skp1-Cullin1-F-Box- S phase kinase-associated protein2/cyclin dependent kinase regulatory subunit 1 in primary endometrial epithelial cells and endometrial carcinoma (ECA) cell lines, suggesting a pathogenic mechanism for type I ECA, an E2-induced cancer. The current studies show that treatment of endometrial carcinoma cells-1 (ECC-1) with small molecule inhibitors of Skp2/Cks1 E3 ligase activity (Skp2E3LIs) stabilizes p27 in the nucleus, decreases p27 in the cytoplasm, and prevents E2-induced proliferation and degradation of p27 in endometrial carcinoma cells-1 and primary ECA cells. Furthermore, Skp2E3LIs increase p27 half-life by 6 hours, inhibit cell proliferation (IC50, 14.3μM), block retinoblastoma protein (pRB) phosphorylation, induce G1 phase block, and are not cytotoxic. Similarly, using super resolution fluorescence localization microscopy and quantification, Skp2E3LIs increase p27 protein in the nucleus by 1.8-fold. In vivo, injection of Skp2E3LIs significantly increases nuclear p27 and reduces proliferation of endometrial epithelial cells by 42%-62% in ovariectomized E2-primed mice. Skp2E3LIs are specific inhibitors of proteolytic degradation that pharmacologically target the binding interaction between the E3 ligase, SCF-Skp2/Cks1, and p27 to stabilize nuclear p27 and prevent cell cycle progression. These targeted inhibitors have the potential to be an important therapeutic advance over general proteasome inhibitors for cancers characterized by SCF-Skp2/Cks1-mediated destruction of nuclear p27.

Super-resolution fluorescence microscopy of the cardiac connexome reveals plakophilin-2 inside the connexin43 plaque

AIMS

Cell function requires formation of molecular clusters localized to discrete subdomains. The composition of these interactomes, and their spatial organization, cannot be discerned by conventional microscopy given the resolution constraints imposed by the diffraction limit of light (∼200-300 nm). Our aims were (i) Implement single-molecule imaging and analysis tools to resolve the nano-scale architecture of cardiac myocytes. (ii) Using these tools, to map two molecules classically defined as components 'of the desmosome' and 'of the gap junction', and defined their spatial organization.

METHODS AND RESULTS

We built a set-up on a conventional inverted microscope using commercially available optics. Laser illumination, reducing, and oxygen scavenging conditions were used to manipulate the blinking behaviour of individual fluorescent reporters. Movies of blinking fluorophores were reconstructed to generate subdiffraction images at ∼20 nm resolution. With this method, we characterized clusters of connexin43 (Cx43) and of 'the desmosomal protein' plakophilin-2 (PKP2). In about half of Cx43 clusters, we observed overlay of Cx43 and PKP2 at the Cx43 plaque edge. SiRNA-mediated loss of Ankyrin-G expression yielded larger Cx43 clusters, of less regular shape, and larger Cx43-PKP2 subdomains. The Cx43-PKP2 subdomain was validated by a proximity ligation assay (PLA) and by Monte-Carlo simulations indicating an attraction between PKP2 and Cx43.

CONCLUSIONS

(i) Super-resolution fluorescence microscopy, complemented with Monte-Carlo simulations and PLAs, allows the study of the nanoscale organization of an interactome in cardiomyocytes. (ii) PKP2 and Cx43 share a common hub that permits direct physical interaction. Its relevance to excitability, electrical coupling, and arrhythmogenic right ventricular cardiomyopathy, is discussed.

Regulation of cardiovascular connexins by mechanical forces and junctions

Connexins form a family of transmembrane proteins that consists of 20 members in humans and 21 members in mice. Six connexins assemble into a connexon that can function as a hemichannel or connexon that can dock to a connexon expressed by a neighbouring cell, thereby forming a gap junction channel. Such intercellular channels synchronize responses in multicellular organisms through direct exchange of ions, small metabolites, and other second messenger molecules between the cytoplasms of adjacent cells. Multiple connexins are expressed in the cardiovascular system. These connexins not only experience the different biomechanical forces within this system, but may also act as effector proteins in co-ordinating responses within groups of cells towards these forces. This review discusses recent insights regarding regulation of cardiovascular connexins by mechanical forces and junctions. It specifically addresses effects of (i) shear stress on endothelial connexins, (ii) hypertension on vascular connexins, and (iii) changes in afterload and the composition of myocardial mechanical junctions on cardiac connexins.

Deletion of the last five C-terminal amino acid residues of connexin43 leads to lethal ventricular arrhythmias in mice without affecting coupling via gap junction channels

The cardiac intercalated disc harbors mechanical and electrical junctions as well as ion channel complexes mediating propagation of electrical impulses. Cardiac connexin43 (Cx43) co-localizes and interacts with several of the proteins located at intercalated discs in the ventricular myocardium. We have generated conditional Cx43D378stop mice lacking the last five C-terminal amino acid residues, representing a binding motif for zonula occludens protein-1 (ZO-1), and investigated the functional consequences of this mutation on cardiac physiology and morphology. Newborn and adult homozygous Cx43D378stop mice displayed markedly impaired and heterogeneous cardiac electrical activation properties and died from severe ventricular arrhythmias. Cx43 and ZO-1 were co-localized at intercalated discs in Cx43D378stop hearts, and the Cx43D378stop gap junction channels showed normal coupling properties. Patch clamp analyses of isolated adult Cx43D378stop cardiomyocytes revealed a significant decrease in sodium and potassium current densities. Furthermore, we also observed a significant loss of Nav1.5 protein from intercalated discs in Cx43D378stop hearts. The phenotypic lethality of the Cx43D378stop mutation was very similar to the one previously reported for adult Cx43 deficient (Cx43KO) mice. Yet, in contrast to Cx43KO mice, the Cx43 gap junction channel was still functional in the Cx43D378stop mutant. We conclude that the lethality of Cx43D378stop mice is independent of the loss of gap junctional intercellular communication, but most likely results from impaired cardiac sodium and potassium currents. The Cx43D378stop mice reveal for the first time that Cx43 dependent arrhythmias can develop by mechanisms other than impairment of gap junction channel function.

Sarcolemmal localisation of Na+/H+ exchange and Na+-HCO3- co-transport influences the spatial regulation of intracellular pH in rat ventricular myocytes

Membrane acid extrusion by Na(+)/H(+) exchange (NHE1) and Na(+)-HCO3(-) co-transport (NBC) is essential for maintaining a low cytoplasmic [H(+)] (∼60 nm, equivalent to an intracellular pH (pHi) of 7.2). This protects myocardial function from the high chemical reactivity of H(+) ions, universal end-products of metabolism. We show here that, in rat ventricular myocytes, fluorescent antibodies map the NBC isoforms NBCe1 and NBCn1 to lateral sarcolemma, intercalated discs and transverse tubules (t-tubules), while NHE1 is absent from t-tubules. This unexpected difference matches functional measurements of pHi regulation (using AM-loaded SNARF-1, a pH fluorophore). Thus, myocyte detubulation (by transient exposure to 1.5 m formamide) reduces global acid extrusion on NBC by 40%, without affecting NHE1. Similarly, confocal pHi imaging reveals that NBC stimulation induces spatially uniform pHi recovery from acidosis, whereas NHE1 stimulation induces pHi non-uniformity during recovery (of ∼0.1 units, for 2-3 min), particularly at the ends of the cell where intercalated discs are commonly located, and where NHE1 immunostaining is prominent. Mathematical modelling shows that this induction of local pHi microdomains is favoured by low cytoplasmic H(+) mobility and long H(+) diffusion distances, particularly to surface NHE1 transporters mediating high membrane flux. Our results provide the first evidence for a spatial localisation of [H(+)]i regulation in ventricular myocytes, suggesting that, by guarding pHi, NHE1 preferentially protects gap junctional communication at intercalated discs, while NBC locally protects t-tubular excitation-contraction coupling.