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

Differential Regulation of Hemichannels and Gap Junction Channels by RhoA GTPase and Actin Cytoskeleton: A Comparative Analysis of Cx43 and Cx26

Connexins (Cxs) are transmembrane proteins that assemble into gap junction channels (GJCs) and hemichannels (HCs). Previous researches support the involvement of Rho GTPases and actin microfilaments in the trafficking of Cxs, formation of GJCs plaques, and regulation of channel activity. Nonetheless, it remains uncertain whether distinct types of Cxs HCs and GJCs respond differently to Rho GTPases or changes in actin polymerization/depolymerization dynamics. Our investigation revealed that inhibiting RhoA, a small GTPase that controls actin polymerization, or disrupting actin microfilaments with cytochalasin B (Cyto-B), resulted in reduced GJCs plaque size at appositional membranes and increased transport of HCs to non-appositional plasma membrane regions. Notably, these effects were consistent across different Cx types, since Cx26 and Cx43 exhibited similar responses, despite having distinct trafficking routes to the plasma membrane. Functional assessments showed that RhoA inhibition and actin depolymerization decreased the activity of Cx43 GJCs while significantly increasing HC activity. However, the functional status of GJCs and HCs composed of Cx26 remained unaffected. These results support the hypothesis that RhoA, through its control of the actin cytoskeleton, facilitates the transport of HCs to appositional cell membranes for GJCs formation while simultaneously limiting the positioning of free HCs at non-appositional cell membranes, independently of Cx type. This dynamic regulation promotes intercellular communications and reduces non-selective plasma membrane permeability through a Cx-type dependent mechanism, whereby the activity of Cx43 HCs and GJCs are differentially affected but Cx26 channels remain unchanged.

Low-Dose Colchicine Ameliorates Doxorubicin Cardiotoxicity Via Promoting Autolysosome Degradation

BACKGROUND

The only clinically approved drug that reduces doxorubicin cardiotoxicity is dexrazoxane, but its application is limited due to the risk of secondary malignancies. So, exploring alternative effective molecules to attenuate its cardiotoxicity is crucial. Colchicine is a safe and well-tolerated drug that helps reduce the production of reactive oxygen species. High doses of colchicine have been reported to block the fusion of autophagosomes and lysosomes in cancer cells. However, the impact of colchicine on the autophagy activity within cardiomyocytes remains inadequately elucidated. Recent studies have highlighted the beneficial effects of colchicine on patients with pericarditis, postprocedural atrial fibrillation, and coronary artery disease. It remains ambiguous how colchicine regulates autophagic flux in doxorubicin-induced heart failure.

METHODS AND RESULTS

Doxorubicin was administered to establish models of heart failure both in vivo and in vitro. Prior studies have reported that doxorubicin impeded the breakdown of autophagic vacuoles, resulting in damaged mitochondria and the accumulation of reactive oxygen species. Following the administration of a low dose of colchicine (0.1 mg/kg, daily), significant improvements were observed in heart function (left ventricular ejection fraction: doxorubicin group versus treatment group=43.75%±3.614% versus 57.07%±2.968%, P=0.0373). In terms of mechanism, a low dose of colchicine facilitated the degradation of autolysosomes, thereby mitigating doxorubicin-induced cardiotoxicity.

CONCLUSIONS

Our research has shown that a low dose of colchicine is pivotal in restoring the autophagy activity, thereby attenuating the cardiotoxicity induced by doxorubicin. Consequently, colchicine emerges as a promising therapeutic candidate to improve doxorubicin cardiotoxicity.

KLF5 promotes the ossification process of ligamentum flavum by transcriptionally activating CX43

BACKGROUND

Ossification of ligamentum flavum (OLF) is a prevalent degenerative spinal disease, typically causing severe neurological dysfunction. Kruppel-like factor 5 (KLF5) plays an essential role in the regulation of skeletal development. However, the mechanism KLF5 plays in OLF remains unclear, necessitating further investigative studies.

METHODS

qRT-PCR, immunofluorescent staining and western blot were used to measure the expression of KLF5. Alkaline Phosphatase (ALP) staining, Alizarin red staining (ARS), and the expression of Runt-related transcription factor 2 (RUNX2), osteopontin (OPN), and osteocalcin (OCN) were used to evaluate the osteogenic differentiation. Luciferase activity assay and ChIP-PCR were performed to investigate the molecular mechanisms.

RESULTS

KLF5 was significantly upregulated in OLF fibroblasts in contrast to normal ligamentum flavum (LF) fibroblasts. Silencing KLF5 diminished osteogenic markers and mineralized nodules, while its overexpression had the opposite effect, confirming KLF5's role in promoting ossification. Moreover, KLF5 promotes the ossification of LF by activating the transcription of Connexin 43 (CX43), and overexpressing CX43 could reverse the suppressive impact of KLF5 knockdown on OLF fibroblasts' osteogenesis.

CONCLUSION

KLF5 promotes the OLF by transcriptionally activating CX43. This finding contributes significantly to our understanding of OLF and may provide new therapeutic targets.

Colchicine Inhibits NETs and Alleviates Cardiac Remodeling after Acute Myocardial Infarction

PURPOSE

Colchicine, a multipotent anti-inflammatory drug, has been reported to alleviate cardiac remodeling and improve cardiac function after acute myocardial infarction (AMI). However, the underlying mechanism remains incompletely understood. Because neutrophils extracellular traps (NETs) enhance inflammation and participate in myocardial ischemia injury, and colchicine can inhibit NETosis, we thus aimed to determine whether colchicine exerts cardioprotective effects on AMI via suppressing NETs.

METHODS

Adult C57BL/6 mice were subjected to permanent ligation of the left anterior descending coronary artery and treated with colchicine (0.1 mg/kg/day) or Cl-amidine (10 mg/kg/day) for 7 or 28 days after AMI. Cardiac function was evaluated by echocardiography, and NETs detected by immunofluorescence. ROS production was detected using 2',7'-dichlorodihydrofluorescein diacetates (DCFH-DA) fluorometry. Intracellular Ca2+ concentration was assessed by a fluorometric ratio technique.

RESULTS

We found that colchicine treatment significantly increased mice survival (89.8% in the colchicine group versus 67.9% in control, n = 32 per group; log-rank test, p < 0.05) and improved cardiac function at day 7 (left ventricular ejection fraction (LVEF): 28.0 ± 9.2% versus 12.6 ± 3.9%, n = 8 per group; p < 0.001) and at day 28 (LVEF: 26.2 ± 7.2% versus 14.8 ± 6.7%, n = 8 per group; p < 0.001) post-AMI. In addition, the administration of colchicine inhibited NETs formation and inflammation. Furthermore, colchicine inhibited NETs formation by reducing NOX2/ROS production and Ca2+ influx. Moreover, prevention of NETs formation with Cl-amidine significantly alleviated AMI-induced cardiac remodeling.

CONCLUSIONS

Colchicine inhibited NETs and cardiac inflammation, and alleviated cardiac remodeling after acute myocardial infarction.

Casein Kinase 1 Phosphomimetic Mutations Negatively Impact Connexin-43 Gap Junctions in Human Pluripotent Stem Cell-Derived Cardiomyocytes

The transplantation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) has shown promise in preclinical models of myocardial infarction, but graft myocardium exhibits incomplete host-graft electromechanical integration and a propensity for pro-arrhythmic behavior. Perhaps contributing to this situation, hPSC-CM grafts show low expression of connexin 43 (Cx43), the major gap junction (GJ) protein, in ventricular myocardia. We hypothesized that Cx43 expression and function could be rescued by engineering Cx43 in hPSC-CMs with a series of phosphatase-resistant mutations at three casein kinase 1 phosphorylation sites (Cx43-S3E) that have been previously reported to stabilize Cx43 GJs and reduce arrhythmias in transgenic mice. However, contrary to our predictions, transgenic Cx43-S3E hPSC-CMs exhibited reduced Cx43 expression relative to wild-type cells, both at baseline and following ischemic challenge. Cx43-S3E hPSC-CMs showed correspondingly slower conduction velocities, increased automaticity, and differential expression of other connexin isoforms and various genes involved in cardiac excitation-contraction coupling. Cx43-S3E hPSC-CMs also had phosphorylation marks associated with Cx43 GJ internalization, a finding that may account for their impaired GJ localization. Taken collectively, our data indicate that the Cx43-S3E mutation behaves differently in hPSC-CMs than in adult mouse ventricular myocytes and that multiple biological factors likely need to be addressed synchronously to ensure proper Cx43 expression, localization, and function.

Remodeled connexin 43 hemichannels alter cardiac excitability and promote arrhythmias

Connexin-43 (Cx43) is the most abundant protein forming gap junction channels (GJCs) in cardiac ventricles. In multiple cardiac pathologies, including hypertrophy and heart failure, Cx43 is found remodeled at the lateral side of the intercalated discs of ventricular cardiomyocytes. Remodeling of Cx43 has been long linked to spontaneous ventricular arrhythmia, yet the mechanisms by which arrhythmias develop are still debated. Using a model of dystrophic cardiomyopathy, we previously showed that remodeled Cx43 function as aberrant hemichannels (non-forming GJCs) that alter cardiomyocyte excitability and, consequently, promote arrhythmias. Here, we aim to evaluate if opening of remodeled Cx43 can serve as a general mechanism to alter cardiac excitability independent of cellular dysfunction associated with a particular cardiomyopathy. To address this issue, we used a genetically modified Cx43 knock-in mouse (S3A) that promotes cardiac remodeling of Cx43 protein without apparent cardiac dysfunction. Importantly, when S3A mice were subjected to cardiac stress using the β-adrenergic agonist isoproterenol (Iso), they displayed acute and severe arrhythmias, which were not observed in WT mice. Pretreatment of S3A mice with the Cx43 hemichannel blocker, Gap19, prevented Iso-induced abnormal electrocardiographic behavior. At the cellular level, when compared with WT, Iso-treated S3A cardiomyocytes showed increased membrane permeability, greater plasma membrane depolarization, and Ca2+ overload, which likely caused prolonged action potentials, delayed after depolarizations, and triggered activity. All these cellular dysfunctions were also prevented by Cx43 hemichannel blockers. Our results support the notion that opening of remodeled Cx43 hemichannels, regardless of the type of cardiomyopathy, is sufficient to mediate cardiac-stress-induced arrhythmogenicity.

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.

Connexin 43 Phosphorylation: Implications in Multiple Diseases

Connexin 43 (Cx43) is most widely distributed in mammals, especially in the cardiovascular and nervous systems. Its phosphorylation state has been found to be regulated by the action of more than ten kinases and phosphatases, including mitogen-activated protein kinase/extracellular signaling and regulating kinase signaling. In addition, the phosphorylation status of different phosphorylation sites affects its own synthesis and assembly and the function of the gap junctions (GJs) to varying degrees. The phosphorylation of Cx43 can affect the permeability, electrical conductivity, and gating properties of GJs, thereby having various effects on intercellular communication and affecting physiological or pathological processes in vitro and in vivo. Therefore, clarifying the relationship between Cx43 phosphorylation and specific disease processes will help us better understand the disease. Based on the above clinical and preclinical findings, we present in this review the functional significance of Cx43 phosphorylation in multiple diseases and discuss the potential of Cx43 as a drug target in Cx43-related disease pathophysiology, with an emphasis on the importance of connexin 43 as an emerging therapeutic target in cardiac and neuroprotection.

Molecular chaperones HSP40, HSP70, STIP1, and HSP90 are involved in stabilization of Cx43

To investigate the involvement of stress induced phosphoprotein 1 (STIP1), heat shock protein (HSP) 70, and HSP90 in ubiquitination of connexin 43 (Cx43) in rat H9c2 cardiomyocytes. Co-immunoprecipitation was used to detect protein-protein interactions and Cx43 ubiquitination. Immunofluorescence was used for protein co-localization. The protein binding, Cx43 protein expression, and Cx43 ubiquitination were reanalyzed in H9c2 cells with modified STIP1 and/or HSP90 expression. STIP1 bound to HSP70 and HSP90, and Cx43 bound to HSP40, HSP70, and HSP90 in normal H9c2 cardiomyocytes. Overexpression of STIP1 promoted the transition of Cx43-HSP70 to Cx43-HSP90 and inhibited Cx43 ubiquitination; knockdown of STIP1 resulted in the opposite effects. Inhibition of HSP90 counteracted the inhibitory effect of STIP1 overexpression on Cx43 ubiquitination. STIP1 suppresses Cx43 ubiquitination in H9c2 cardiomyocytes by promoting the transition of Cx43-HSP70 to Cx43-HSP90.

Fibroblast-specific inflammasome activation predisposes to atrial fibrillation

Background

Recent work has shown that the NLR-family-pyrin-domain-containing 3 (NLRP3) inflammasome is expressed in cardiomyocytes and when specifically activated causes atrial electrical remodeling and arrhythmogenicity. Whether the NLRP3-inflammasome system is functionally important in cardiac fibroblasts (FBs) remains controversial. In this study, we sought to uncover the potential contribution of FB NLRP3-inflammasome signaling to the control of cardiac function and arrhythmogenesis.

Methods

Digital-PCR was performed to determine the expression of NLRP3-pathway components in FBs isolated from human biopsy samples of AF and sinus rhythm patients. NLRP3-system protein expression was determined by immunoblotting in atria of canines with electrically maintained AF. Using the inducible, resident fibroblast (FB)-specific Tcf21-promoter-Cre system (Tcf21iCre as control), we established a FB-specific knockin (FB-KI) mouse model with FB-restricted expression of constitutively active NLRP3. Cardiac function and arrhythmia susceptibility in mice were assessed by echocardiography, programmed electrical stimulation, and optical mapping studies.

Results

NLRP3 and IL1B were upregulated in atrial FBs of patients with persistent AF. Protein levels of NLRP3, ASC, and pro-Interleukin-1β were increased in atrial FBs of a canine AF model. Compared with the control mice, FB-KI mice exhibited enlarged left atria (LA) and reduced LA contractility, a common determinant of AF. The FBs from FB-KI mice were more transdifferentiated, migratory, and proliferative compared to the FBs from control mice. FB-KI mice showed increased cardiac fibrosis, atrial gap junction remodeling, and reduced conduction velocity, along with increased AF susceptibility. These phenotypic changes were supported by single nuclei (sn)RNA-seq analysis, which revealed enhanced extracellular matrix remodeling, impaired communication among cardiomyocytes, and altered metabolic pathways across multiple cell types.

Conclusions

Our results show that the FB-restricted activation of the NLRP3-inflammasome system leads to fibrosis, atrial cardiomyopathy, and AF. Activation of NLRP3-inflammasome in resident FBs exhibits cell-autonomous function by increasing the activity of cardiac FBs, fibrosis, and connexin remodeling. This study establishes the NLRP3-inflammasome as a novel FB-signaling pathway contributing to AF pathogenesis.

Connexin hemichannels as candidate targets for cardioprotective and anti-arrhythmic treatments

Connexins are crucial cardiac proteins that form hemichannels and gap junctions. Gap junctions are responsible for the propagation of electrical and chemical signals between myocardial cells and cells of the specialized conduction system in order to synchronize the cardiac cycle and steer cardiac pump function. Gap junctions are normally open, while hemichannels are closed, but pathological circumstances may close gap junctions and open hemichannels, thereby perturbing cardiac function and homeostasis. Current evidence demonstrates an emerging role of hemichannels in myocardial ischemia and arrhythmia, and tools are now available to selectively inhibit hemichannels without inhibiting gap junctions as well as to stimulate hemichannel incorporation into gap junctions. We review available experimental evidence for hemichannel contributions to cellular pro-arrhythmic events in ventricular and atrial cardiomyocytes, and link these to insights at the level of molecular control of connexin-43-based hemichannel opening. We conclude that a double-edged approach of both preventing hemichannel opening and preserving gap junctional function will be key for further research and development of new connexin-based experimental approaches for treating heart disease.

Role of Connexin 43 phosphorylation on Serine-368 by PKC in cardiac function and disease

Intercellular communication mediated by gap junction channels and hemichannels composed of Connexin 43 (Cx43) is vital for the propagation of electrical impulses through cardiomyocytes. The carboxyl terminal tail of Cx43 undergoes various post-translational modifications including phosphorylation of its Serine-368 (S368) residue. Protein Kinase C isozymes directly phosphorylate S368 to alter Cx43 function and stability through inducing conformational changes affecting channel permeability or promoting internalization and degradation to reduce intercellular communication between cardiomyocytes. Recent studies have implicated this PKC/Cx43-pS368 circuit in several cardiac-associated diseases. In this review, we describe the molecular and cellular basis of PKC-mediated Cx43 phosphorylation and discuss the implications of Cx43 S368 phosphorylation in the context of various cardiac diseases, such as cardiomyopathy, as well as the therapeutic potential of targeting this pathway.

CYLD deubiquitinates plakoglobin to promote Cx43 membrane targeting and gap junction assembly in the heart

During heart maturation, gap junctions assemble into hemichannels and polarize to the intercalated disc at cell borders to mediate electrical impulse conduction. However, the molecular mechanism underpinning cardiac gap junction assembly remains elusive. Herein, we demonstrate an important role for the deubiquitinating enzyme cylindromatosis (CYLD) in this process. Depletion of CYLD in mice impairs the formation of cardiac gap junctions, accelerates cardiac fibrosis, and increases heart failure. Mechanistically, CYLD interacts with plakoglobin and removes lysine 63-linked polyubiquitin chains from plakoglobin. The deubiquitination of plakoglobin enhances its interaction with the desmoplakin/end-binding protein 1 complex localized at the microtubule plus end, thereby promoting microtubule-dependent transport of connexin 43 (Cx43), a key component of gap junctions, to the cell membrane. These findings establish CYLD as a critical player in regulating gap junction assembly and have important implications in heart development and diseases.

Integrin α7 Mutations Are Associated With Adult-Onset Cardiac Dysfunction in Humans and Mice

Background Integrin α7β1 is a major laminin receptor in skeletal and cardiac muscle. In skeletal muscle, integrin α7β1 plays an important role during muscle development and has been described as an important modifier of skeletal muscle diseases. The integrin α7β1 is also highly expressed in the heart, but its precise role in cardiac function is unknown. Mutations in the integrin α7 gene (ITGA7) have been reported in children with congenital myopathy. Methods and Results In this study, we described skeletal and cardiac muscle pathology in Itga7-/- mice and 5 patients from 2 unrelated families with ITGA7 mutations. Proband in family 1 presented a homozygous c.806_818del [p.S269fs] variant, and proband in family 2 was identified with 2 intron variants in the ITGA7 gene. The complete absence of the integrin α7 protein in muscle supports the ITGA7 mutations are pathogenic. We performed electrocardiography, echocardiography, or cardiac magnetic resonance imaging, and histological biopsy analyses in patients with ITGA7 deficiency and Itga7-/- mice. The patients exhibited cardiac dysrhythmia and dysfunction from the third decade of life and late-onset respiratory insufficiency, but with relatively mild limb muscle involvement. Mice demonstrated corresponding abnormalities in cardiac conduction and contraction as well as diaphragm muscle fibrosis. Conclusions Our data suggest that loss of integrin α7 causes a novel form of adult-onset cardiac dysfunction indicating a critical role for the integrin α7β1 in normal cardiac function and highlights the need for long-term cardiac monitoring in patients with ITGA7-related congenital myopathy.

A Drosophila model for Meniere's disease: Dystrobrevin is required for support cell function in hearing and proprioception

Meniere's disease (MD) is an inner ear disorder characterised by recurrent vertigo attacks associated with sensorineural hearing loss and tinnitus. Evidence from epidemiology and Whole Exome Sequencing (WES) suggests a genetic susceptibility involving multiple genes, including α-Dystrobrevin (DTNA). Here we investigate a Drosophila model. We show that mutation, or knockdown, of the DTNA orthologue in Drosophila, Dystrobrevin (Dyb), results in defective proprioception and impaired function of Johnston's Organ (JO), the fly's equivalent of the inner ear. Dyb and another component of the dystrophin-glycoprotein complex (DGC), Dystrophin (Dys), are expressed in support cells within JO. Their specific locations suggest that they form part of support cell contacts, thereby helping to maintain the integrity of the hemolymph-neuron diffusion barrier, which is equivalent to a blood-brain barrier. These results have important implications for the human condition, and notably, we note that DTNA is expressed in equivalent cells of the mammalian inner ear.

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.

Tumor Necrosis Factor-Alpha Disrupts Cx43-Mediated Corneal Endothelial Gap Junction Intercellular Communication

Connexin43 (Cx43)-mediated gap junctions are vital in maintaining corneal endothelium homeostasis. Tumor necrosis factor-alpha (TNF-α) is among the most important inflammatory factors which cause corneal endothelial dysfunction in various eye diseases. However, the effect of TNF-α on Cx43-mediated gap junctions of the corneal endothelium remains undefined. In the current research, we determined the effect of TNF-α on gap junction intercellular communication (GJIC) in rabbit corneal endothelium. To evaluate alterations of GJIC, if any, we treated ex vivo cultured rabbit corneal endothelium with different concentrations of TNF-α (2-20 ng/ml). The localization of Cx43 was analyzed by immunostaining, while RT-qPCR and western blot were used to profile the expression of Cx43 and zonula occludens-1 (ZO-1). The association between ZO-1 and Cx43 was evaluated using immunoprecipitation and double staining. GJIC activity was determined by the scrap loading and dye transfer assay (SLDT). Our data demonstrated that a high concentration of TNF-α (10 ng/ml and 20 ng/ml) disrupts the Cx43 mediated gap junction distribution in rabbit corneal endothelium and suppresses the expression of Cx43 protein. Furthermore, rabbit corneal endothelial GJIC was inhibited due to the decreased association between the ZO-1 and Cx43 proteins. Current results demonstrate that TNF-α inhibits corneal endothelial GJIC via decreasing the association between ZO-1 and Cx43, disrupting the distribution of Cx43, and downregulating the expression of Cx43 protein. This study offers a new theoretical foundation for diagnosing and treating corneal endothelial cell decompensation induced by elevated TNF-α in various eye diseases.

Arrhythmogenic Cardiomyopathy: Exercise Pitfalls, Role of Connexin-43, and Moving beyond Antiarrhythmics

Arrhythmogenic Cardiomyopathy (ACM), a Mendelian disorder that can affect both left and right ventricles, is most often associated with pathogenic desmosomal variants that can lead to fibrofatty replacement of the myocardium, a pathological hallmark of this disease. Current therapies are aimed to prevent the worsening of disease phenotypes and sudden cardiac death (SCD). Despite the use of implantable cardioverter defibrillators (ICDs) there is no present therapy that would mitigate the loss in electrical signal and propagation by these fibrofatty barriers. Recent studies have shown the influence of forced vs. voluntary exercise in a variety of healthy and diseased mice; more specifically, that exercised mice show increased Connexin-43 (Cx43) expression levels. Fascinatingly, increased Cx43 expression ameliorated the abnormal electrical signal conduction in the myocardium of diseased mice. These findings point to a major translational pitfall in current therapeutics for ACM patients, who are advised to completely cease exercising and already demonstrate reduced Cx43 levels at the myocyte intercalated disc. Considering cardiac dysfunction in ACM arises from the loss of cardiomyocytes and electrical signal conduction abnormalities, an increase in Cx43 expression-promoted by low to moderate intensity exercise and/or gene therapy-could very well improve cardiac function in ACM patients.

Connexin 43 hyper-phosphorylation at serine 282 triggers apoptosis in rat cardiomyocytes via activation of mitochondrial apoptotic pathway

Cx43 is the major connexin in ventricular gap junctions, and plays a pivotal role in control of electrical and metabolic communication among adjacent cardiomyocytes. We previously found that Cx43 dephosphorylation at serine 282 (pS282) caused cardiomyocyte apoptosis, which is involved in cardiac ischemia/reperfusion injury. In this study we investigated whether Cx43-S282 hyper-phosphorylation could protect cardiomyocytes against apoptosis. Adenovirus carrying rat full length Cx43 gene (Cx43-wt) or a mutant gene at S282 substituted with aspartic acid (S282D) were transfected into neonatal rat ventricular myocytes (NRVMs) or injected into rat ventricular wall. Rat abdominal aorta constriction model (AAC) was used to assess Cx43-S282 phosphorylation status. We showed that Cx43 phosphorylation at S282 was increased over 2-times compared to Cx43-wt cells at 24 h after transfection, while pS262 and pS368 were unaltered. S282D-transfected cells displayed enhanced gap junctional communication, and increased basal intracellular Ca²⁺ concentration and spontaneous Ca²⁺ transients compared to Cx43-wt cells. However, spontaneous apoptosis appeared in NRVMs transfected with S282D for 34 h. Rat ventricular myocardium transfected with S282D in vivo also exhibited apoptotic responses, including increased Bax/Bcl-xL ratio, cytochrome c release as well as caspase-3 and caspase-9 activities, while factor-associated suicide (Fas)/Fas-associated death domain expression and caspase-8 activity remained unaltered. In addition, AAC-induced hypertrophic ventricles had apoptotic injury with Cx43-S282 hyper-phosphorylation compared with Sham ventricles. In conclusion, Cx43 hyper-phosphorylation at S282, as dephosphorylation, also triggers cardiomyocyte apoptosis, but through activation of mitochondrial apoptosis pathway, providing a fine-tuned Cx43-S282 phosphorylation range required for the maintenance of cardiomyocyte function and survival.

Dapagliflozin Improves Diabetic Cardiomyopathy by Modulating the Akt/mTOR Signaling Pathway

Background

Dapagliflozin can significantly improve heart failure, and Cx43 is one of the molecular mechanisms of heart failure. This study investigated the effect of dapagliflozin on Cx43 and Akt/mTOR signaling pathway in ventricular myocytes.

Methods

A rat model of type 2 diabetes mellitus was established by high-fat diet combined with streptozotocin, and the animals were treated randomly with dapagliflozin. The morphological changes of the myocardium were observed by hematoxylin eosin staining, and the expression and distribution of Cx43 in ventricular myocytes were detected by immunohistochemistry. And Western blot determined the expressions of Cx43, Akt, mTOR, p62, and LC3 proteins in rat myocardium.

Results

Compared with the normal control group, the heart rate of diabetic rats decreased significantly (p < 0.05), QRS wave of ECG widened, and QT interval prolonged (p < 0.05). Dapagliflozin treatment in diabetic rats resulted in improvements in these ECG indexes (p < 0.05) with early administration group obtaining greater efficacy than the late administration group (p < 0.05). In the normal control group, the cardiomyocytes were arranged orderly, and the expression of Cx43 was dense, uniform, and regular, which was higher than that in the intercalated disc. In the diabetic control model group, the cardiomyocytes were enlarged and presented disorderly with detection of Cx43 in the cytoplasm. Early use of dapagliflozin better improved these myocardial tissue lesions. Of note, as diabetic rats exhibited decreased expression of Cx43, Akt, and mTOR (p < 0.05), increased p62 expression (p < 0.05), and decreased LC3-II/I ratio (p < 0.05), administration of dapagliflozin partially reversed the expression of the above proteins (p < 0.05) with greater improvement in the early administration group compared with the late administration group (p < 0.05).

Conclusions

Dapagliflozin increases the expression of Cx43 in cardiomyocytes of diabetic rats and thereby alleviates heart failure partly through regulating the Akt/mTOR signaling pathway.

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

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

Connexin and Pannexin Large-Pore Channels in Microcirculation and Neurovascular Coupling Function

Microcirculation homeostasis depends on several channels permeable to ions and/or small molecules that facilitate the regulation of the vasomotor tone, hyperpermeability, the blood–brain barrier, and the neurovascular coupling function. Connexin (Cxs) and Pannexin (Panxs) large-pore channel proteins are implicated in several aspects of vascular physiology. The permeation of ions (i.e., Ca²⁺) and key metabolites (ATP, prostaglandins, D-serine, etc.) through Cxs (i.e., gap junction channels or hemichannels) and Panxs proteins plays a vital role in intercellular communication and maintaining vascular homeostasis. Therefore, dysregulation or genetic pathologies associated with these channels promote deleterious tissue consequences. This review provides an overview of current knowledge concerning the physiological role of these large-pore molecule channels in microcirculation (arterioles, capillaries, venules) and in the neurovascular coupling function.

Colchicine Ameliorates Dilated Cardiomyopathy Via SIRT2-Mediated Suppression of NLRP3 Inflammasome Activation

Background Dilated cardiomyopathy remains a leading cause of heart failure worldwide. Immune inflammation response is recognized as a significant player in the progression of heart failure; however, immunomodulatory strategies remain a long-term challenge. Colchicine, a potent anti-inflammatory drug, has many benefits in ischemic cardiovascular events, but its role in nonischemic heart failure remains unclear. Methods and Results Doxorubicin administration was used to establish a murine dilated cardiomyopathy model, and colchicine or saline was orally given. At the end point, cardiac function and fibrosis were measured to investigate the effects of colchicine. Inflammatory cytokine levels, neutrophil recruitment, and NLRP3 (NOD-like receptor protein 3) inflammasome activation were detected to evaluate the inflammatory response. Furthermore, to examine the downstream target of colchicine, SIRT2 (Sirtuin 2) was pharmacologically inhibited in vitro; thus, changes in the NLRP3 inflammasome were detected by immunoblotting. These results showed that murine cardiac function was significantly improved and fibrosis was significantly alleviated after colchicine treatment. Moreover, the infiltration of neutrophils and the levels of inflammatory cytokines in the failing myocardium were both decreased by colchicine treatment. Mechanistically, colchicine upregulated the expression of SIRT2, leading to the inactivation of the NLRP3 inflammasome in an NLRP3 deacetylated manner. Conversely, the inhibition of SIRT2 attenuated the suppressive effect of colchicine on NLRP3 inflammasome activation. Conclusions This study indicated that colchicine could be a promising therapeutic candidate for dilated cardiomyopathy and other nonischemic heart failure associated with the inflammatory response.

Src-homology domain 2 containing protein tyrosine phosphatase-1 (SHP-1) directly binds to proto-oncogene tyrosine-protein kinase Src (c-Src) and promotes the transcriptional activation of connexin 43 (Cx43)

The prevalence of atrial fibrillation (AF), which is one of the common arrhythmias in clinics, is increasing sharply and has affected millions of patients, which is expected to triple by 2050. The purpose of the study was to explore the regulatory relationship between Src-homology domain 2 containing protein tyrosine phosphatase-1 (SHP-1) and proto-oncogene tyrosine-protein kinase Src (c-Src) and the regulation of Connexins 43 (Cx43), and its effect on AF was also studied. Mouse atrial myocyte line (HL-1 cell line) was used as the research object. After overexpression of SHP-1, the expressions of p-c-Src, Cx43, and SHP-1 were detected by Western blot and cellular immunofluorescence, respectively. The location and interaction of SHP-1 and c-Src in the cells were detected by immunofluorescence co-localization and co-immunoprecipitation (Co-IP). The regulation of c-Src and Cx43 was detected by DNA pull down, chromatin co-immunoprecipitation (CHIP), and dual-luciferase reporter system. The results revealed that overexpression of SHP-1 could inhibit the phosphorylation and activation of c-Src and increase the expression of Cx43. Moreover, there was a direct binding between SHP-1 and c-Src, and c-Src could bind to the promoter region of Cx43 and inhibit the transcription of Cx43. In conclusion, SHP-1 could bind to c-Src and inhibit the activity of c-Src, thus enhancing the transcriptional activation of Cx43 and improving the function of gap junction.

Cardiovascular Disease in Duchenne Muscular Dystrophy

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Cardiomyopathy is the leading cause of death in patients with DMD.

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DMD has no cure, and there is no current consensus for treatment of DMD cardiomyopathy.

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This review discusses therapeutic strategies to potentially reduce or prevent cardiac dysfunction in DMD patients.

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Additional studies are needed to firmly establish optimal treatment modalities for DMD cardiomyopathy.

Duchenne muscular dystrophy (DMD) is a devastating disease affecting approximately 1 in every 3,500 male births worldwide. Multiple mutations in the dystrophin gene have been implicated as underlying causes of DMD. However, there remains no cure for patients with DMD, and cardiomyopathy has become the most common cause of death in the affected population. Extensive research is under way investigating molecular mechanisms that highlight potential therapeutic targets for the development of pharmacotherapy for DMD cardiomyopathy. In this paper, the authors perform a literature review reporting on recent ongoing efforts to identify novel therapeutic strategies to reduce, prevent, or reverse progression of cardiac dysfunction in DMD.

The microtubule cytoskeleton in cardiac mechanics and heart failure

The microtubule network of cardiac muscle cells has unique architectural and biophysical features to accommodate the demands of the working heart. Advances in live-cell imaging and in deciphering the 'tubulin code' have shone new light on this cytoskeletal network and its role in heart failure. Microtubule-based transport orchestrates the growth and maintenance of the contractile apparatus through spatiotemporal control of translation, while also organizing the specialized membrane systems required for excitation-contraction coupling. To withstand the high mechanical loads of the working heart, microtubules are post-translationally modified and physically reinforced. In response to stress to the myocardium, the microtubule network remodels, typically through densification, post-translational modification and stabilization. Under these conditions, physically reinforced microtubules resist the motion of the cardiomyocyte and increase myocardial stiffness. Accordingly, modified microtubules have emerged as a therapeutic target for reducing stiffness in heart failure. In this Review, we discuss the latest evidence on the contribution of microtubules to cardiac mechanics, the drivers of microtubule network remodelling in cardiac pathologies and the therapeutic potential of targeting cardiac microtubules in acquired heart diseases.

MiR-206 improves intervertebral disk degeneration by targeting GJA1

Background

A large amount of evidence suggested that miRNA was involved in the progression of intervertebral disk degeneration (IDD). The purpose of our study was to explore the function and potential mechanism of miR-206/GJA1 axis in IDD.

Methods

IDD nucleus pulposus (NP) cell model was established through treatment of LPS. IDD rat model was established by annulus fibrosus puncture. The expression of miR-206 and GJA1 was detected by RT-PCR, apoptosis was evaluated by flow cytometry or TUNEL, inflammatory factors were tested by ELISA, extracellular matrix related protein expression was detected by western blot, and HE and safranin-O staining were used to assess the pathological changes of IDD.

Results

GJA1 was found to be highly expressed in IDD tissues and LPS-induced NP cells. Down regulation of GJA1 reduced inflammatory factors, inhibited apoptosis and enhanced extracellular matrix in LPS-induced NP cells. MiR-206 was downregulated in IDD tissues and directly targeted GJA1, and the expression of miR-206 was negatively correlated with the expression of GJA1 in IDD tissues. Further, it was demonstrated that overexpression of miR-206 could attenuate LPS-induced NP cell injury by targeting GJA1. In vivo, the upregulation of miR-206 improved IDD and reduced NP cell apoptosis.

Conclusion

Our study showed that miR-206 reduced the level of inflammatory factors, restrained NP cell apoptosis and increases extracellular matrix by targeting GJA1. These data suggested that miR-206/GJA1 might be potential therapeutic targets for IDD.

1,8-cineole Ameliorates Right Ventricle Dysfunction Associated With Pulmonary Arterial Hypertension by Restoring Connexin 43 and Mitochondrial Homeostasis

For the first time, the present study unravels a cardiospecific therapeutic approach for Pulmonary Arterial Hypertension (PAH), a disease with a very poor prognosis and high mortality rates due to right ventricle dysfunction. We first established a new in vitro model of high-pressure-induced hypertrophy that closely resembles heart defects associated with PAH and validated our in vitro findings on a preclinical in vivo model of monocrotaline (MCT)-induced PAH. Our results showed the in vitro antihypertrophic effect of 1,8-cineole, a monoterpene widely found in several essential oils. Also, a decrease in RV hypertrophy and fibrosis, and an improvement in heart function in vivo was observed, when 1,8-cineole was applied topically. Furthermore, 1,8-cineole restored gap junction protein connexin43 distribution at the intercalated discs and mitochondrial functionality, suggesting it may act by preserving cardiac cell-to-cell communication and bioenergetics. Overall, our results point out a promising therapeutic compound that can be easily applied topically, thus paving the way for the development of effective cardiac-specific therapies to greatly improve PAH outcomes.

Oxidative Stress, Inflammation and Connexin Hemichannels in Muscular Dystrophies

Muscular dystrophies (MDs) are a heterogeneous group of congenital neuromuscular disorders whose clinical signs include myalgia, skeletal muscle weakness, hypotonia, and atrophy that leads to progressive muscle disability and loss of ambulation. MDs can also affect cardiac and respiratory muscles, impairing life-expectancy. MDs in clude Duchenne muscular dystrophy, Emery-Dreifuss muscular dystrophy, facioscapulohumeral muscular dystrophy and limb-girdle muscular dystrophy. These and other MDs are caused by mutations in genes that encode proteins responsible for the structure and function of skeletal muscles, such as components of the dystrophin-glycoprotein-complex that connect the sarcomeric-actin with the extracellular matrix, allowing contractile force transmission and providing stability during muscle contraction. Consequently, in dystrophic conditions in which such proteins are affected, muscle integrity is disrupted, leading to local inflammatory responses, oxidative stress, Ca²⁺-dyshomeostasis and muscle degeneration. In this scenario, dysregulation of connexin hemichannels seem to be an early disruptor of the homeostasis that further plays a relevant role in these processes. The interaction between all these elements constitutes a positive feedback loop that contributes to the worsening of the diseases. Thus, we discuss here the interplay between inflammation, oxidative stress and connexin hemichannels in the progression of MDs and their potential as therapeutic targets.

The Role of Junctophilin Proteins in Cellular Function

Junctophilins (JPHs) comprise a family of structural proteins that connect the plasma membrane to intracellular organelles such as the endo/sarcoplasmic reticulum. Tethering of these membrane structures results in the formation of highly organized subcellular junctions that play important signaling roles in all excitable cell types. There are four JPH isoforms, expressed primarily in muscle and neuronal cell types. Each JPH protein consists of 6 'membrane occupation and recognition nexus' (MORN) motifs, a joining region connecting these to another set of 2 MORN motifs, a putative alpha-helical region, a divergent region exhibiting low homology between JPH isoforms, and a carboxy-terminal transmembrane region anchoring into the ER/SR membrane. JPH isoforms play essential roles in developing and maintaining subcellular membrane junctions. Conversely, inherited mutations in JPH2 cause hypertrophic or dilated cardiomyopathy, while trinucleotide expansions in the JPH3 gene cause Huntington Disease-Like 2. Loss of JPH1 protein levels can cause skeletal myopathy, while loss of cardiac JPH2 levels causes heart failure and atrial fibrillation, among other disease. This review will provide a comprehensive overview of the JPH gene family, phylogeny, and evolutionary analysis of JPH genes and other MORN domain proteins. JPH biogenesis, membrane tethering, and binding partners will be discussed, as well as functional roles of JPH isoforms in excitable cells. Finally, potential roles of JPH isoform deficits in human disease pathogenesis will be reviewed.

A scalable, clinically severe pig model for Duchenne muscular dystrophy

Large animal models for Duchenne muscular dystrophy (DMD) are crucial for evaluation of diagnostic procedures and treatment strategies. Pigs cloned from male cells lacking DMD exon 52 (DMDΔ52) resemble molecular, clinical and pathological hallmarks of DMD, but die before sexual maturity and cannot be propagated by breeding. Therefore, we generated female DMD+/- carriers. A single founder animal had 11 litters with 29 DMDY/-, 34 DMD+/- as well as 36 male and 29 female wild-type offspring. Breeding with F1 and F2 DMD+/- carriers resulted in additional 114 DMDY/- piglets. With intensive neonatal management, the majority survived for 3-4 months, providing statistically relevant cohorts for experimental studies. Pathological investigations and proteome studies of skeletal muscles and myocardium confirmed the resemblance of human disease mechanisms. Importantly, DMDY/- pigs reveal progressive myocardial fibrosis and increased expression of connexin-43, associated with significantly reduced left ventricular ejection fraction already at age 3 months. Furthermore, behavioral tests provided evidence for impaired cognitive ability. Our breeding cohort of DMDΔ52 pigs and standardized tissue repositories provide important resources for studying DMD disease mechanisms and for testing novel treatment strategies.

Serine-threonine protein phosphatase regulation of Cx43 dephosphorylation in arrhythmogenic disorders

Regulation of cell-to-cell communication in the heart by the gap junction protein Connexin43 (Cx43) involves modulation of Cx43 phosphorylation state by protein kinases, and dephosphorylation by protein phosphatases. Dephosphorylation of Cx43 has been associated with impaired intercellular coupling and enhanced arrhythmogenesis in various pathologic states. While there has been extensive study of the protein kinases acting on Cx43, there has been limited studies of the protein phosphatases that may underlie Cx43 dephosphorylation. The focus of this review is to introduce serine-threonine protein phosphatase regulation of Cx43 phosphorylation state and cell-to-cell communication, and its impact on arrhythmogenesis in the setting of chronic heart failure and myocardial ischemia, as well as on atrial fibrillation. We also discuss the therapeutic potential of modulating protein phosphatases to treat arrhythmias in these clinical settings.

Remodeling of Cardiac Gap Junctional Cell-Cell Coupling

The heart works as a functional syncytium, which is realized via cell-cell coupling maintained by gap junction channels. These channels connect two adjacent cells, so that action potentials can be transferred. Each cell contributes a hexameric hemichannel (=connexon), formed by protein subuntis named connexins. These hemichannels dock to each other and form the gap junction channel. This channel works as a low ohmic resistor also allowing the passage of small molecules up to 1000 Dalton. Connexins are a protein family comprising of 21 isoforms in humans. In the heart, the main isoforms are Cx43 (the 43 kDa connexin; ubiquitous), Cx40 (mostly in atrium and specific conduction system), and Cx45 (in early developmental states, in the conduction system, and between fibroblasts and cardiomyocytes). These gap junction channels are mainly located at the polar region of the cardiomyocytes and thus contribute to the anisotropic pattern of cardiac electrical conductivity. While in the beginning the cell–cell coupling was considered to be static, similar to an anatomically defined structure, we have learned in the past decades that gap junctions are also subject to cardiac remodeling processes in cardiac disease such as atrial fibrillation, myocardial infarction, or cardiomyopathy. The underlying remodeling processes include the modulation of connexin expression by e.g., angiotensin, endothelin, or catecholamines, as well as the modulation of the localization of the gap junctions e.g., by the direction and strength of local mechanical forces. A reduction in connexin expression can result in a reduced conduction velocity. The alteration of gap junction localization has been shown to result in altered pathways of conduction and altered anisotropy. In particular, it can produce or contribute to non-uniformity of anisotropy, and thereby can pre-form an arrhythmogenic substrate. Interestingly, these remodeling processes seem to be susceptible to certain pharmacological treatment.

The oxidation-resistant CaMKII-MM281/282VV mutation does not prevent arrhythmias in CPVT1

Catecholaminergic polymorphic ventricular tachycardia type 1 (CPVT1) is an inherited arrhythmogenic disorder caused by missense mutations in the cardiac ryanodine receptors (RyR2), that result in increased β-adrenoceptor stimulation-induced diastolic Ca2+ leak. We have previously shown that exercise training prevents arrhythmias in CPVT1, potentially by reducing the oxidation of Ca2+ /calmodulin-dependent protein kinase type II (CaMKII). Therefore, we tested whether an oxidation-resistant form of CaMKII protects mice carrying the CPVT1-causative mutation RyR2-R2474S (RyR2-RS) against arrhythmias. Antioxidant treatment (N-acetyl-L-cysteine) reduced the frequency of β-adrenoceptor stimulation-induced arrhythmogenic Ca2+ waves in isolated cardiomyocytes from RyR2-RS mice. To test whether the prevention of CaMKII oxidation exerts an antiarrhythmic effect, mice expressing the oxidation-resistant CaMKII-MM281/282VV variant (MMVV) were crossed with RyR2-RS mice to create a double transgenic model (RyR2-RS/MMVV). Wild-type mice served as controls. Telemetric ECG surveillance revealed an increased incidence of ventricular tachycardia and an increased arrhythmia score in both RyR2-RS and RyR2-RS/MMVV compared to wild-type mice, both following a β-adrenoceptor challenge (isoprenaline i.p.), and following treadmill exercise combined with a β-adrenoceptor challenge. There were no differences in the incidence of arrhythmias between RyR2-RS and RyR2-RS/MMVV mice. Furthermore, no differences were observed in β-adrenoceptor stimulation-induced Ca2+ waves in RyR2-RS/MMVV compared to RyR2-RS. In conclusion, antioxidant treatment reduces β-adrenoceptor stimulation-induced Ca2+ waves in RyR2-RS cardiomyocytes. However, oxidation-resistant CaMKII-MM281/282VV does not protect RyR2-RS mice from β-adrenoceptor stimulation-induced Ca2+ waves or arrhythmias. Hence, alternative oxidation-sensitive targets need to be considered to explain the beneficial effect of antioxidant treatment on Ca2+ waves in cardiomyocytes from RyR2-RS mice.

Peptidic Connexin43 Therapeutics in Cardiac Reparative Medicine

Connexin (Cx43)-formed channels have been linked to cardiac arrhythmias and diseases of the heart associated with myocardial tissue loss and fibrosis. These pathologies include ischemic heart disease, ischemia-reperfusion injury, heart failure, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, and Duchenne muscular dystrophy. A number of Cx43 mimetic peptides have been reported as therapeutic candidates for targeting disease processes linked to Cx43, including some that have advanced to clinical testing in humans. These peptides include Cx43 sequences based on the extracellular loop domains (e.g., Gap26, Gap 27, and Peptide5), cytoplasmic-loop domain (Gap19 and L2), and cytoplasmic carboxyl-terminal domain (e.g., JM2, Cx43tat, CycliCX, and the alphaCT family of peptides) of this transmembrane protein. Additionally, RYYN peptides binding to the Cx43 carboxyl-terminus have been described. In this review, we survey preclinical and clinical data available on short mimetic peptides based on, or directly targeting, Cx43, with focus on their potential for treating heart disease. We also discuss problems that have caused reluctance within the pharmaceutical industry to translate peptidic therapeutics to the clinic, even when supporting preclinical data is strong. These issues include those associated with the administration, stability in vivo, and tissue penetration of peptide-based therapeutics. Finally, we discuss novel drug delivery technologies including nanoparticles, exosomes, and other nanovesicular carriers that could transform the clinical and commercial viability of Cx43-targeting peptides in treatment of heart disease, stroke, cancer, and other indications requiring oral or parenteral administration. Some of these newly emerging approaches to drug delivery may provide a path to overcoming pitfalls associated with the drugging of peptide therapeutics.

Regulatory mechanism of calcium/calmodulin-dependent protein kinase II in the occurrence and development of ventricular arrhythmia (Review)

Ventricular arrhythmia (VA) is a highly fatal arrhythmia that involves multiple ion channels. Of all sudden cardiac death events, ~85% result from VAs, including ventricular tachycardia and ventricular fibrillation. Calcium/calmodulin-dependent pro-tein kinase II (CaMKII) is an important ion channel regulator that participates in the excitation-contraction coupling of the heart, and as such is important for regulating its electrophysiological function. CaMKII can be activated in a Ca²⁺/calmodulin (CaM)-dependent or Ca²⁺/CaM-independent manner, serving a key role in the occurrence and development of VA. The present review aimed to determine whether activated CaMKII induces early afterdepolarizations and delayed afterdepolarizations that result in VA by regulating sodium, potassium and calcium ions. Assessing VA mechanisms based on the CaMKII pathway is of great significance to the clinical treatment of VA and the de-velopment of effective drugs for use in clinical practice.

Abnormal Calcium Handling in Duchenne Muscular Dystrophy: Mechanisms and Potential Therapies

Duchenne muscular dystrophy (DMD) is an X-linked muscle-wasting disease caused by the loss of dystrophin. DMD is associated with muscle degeneration, necrosis, inflammation, fatty replacement, and fibrosis, resulting in muscle weakness, respiratory and cardiac failure, and premature death. There is no curative treatment. Investigations on disease-causing mechanisms offer an opportunity to identify new therapeutic targets to treat DMD. An abnormal elevation of the intracellular calcium (Cai2+) concentration in the dystrophin-deficient muscle is a major secondary event, which contributes to disease progression in DMD. Emerging studies have suggested that targeting Ca²⁺-handling proteins and/or mechanisms could be a promising therapeutic strategy for DMD. Here, we provide an updated overview of the mechanistic roles the sarcolemma, sarcoplasmic/endoplasmic reticulum, and mitochondria play in the abnormal and sustained elevation of Cai2+ levels and their involvement in DMD pathogenesis. We also discuss current approaches aimed at restoring Ca²⁺ homeostasis as potential therapies for DMD.

Role of Cdkn2a in the Emery-Dreifuss Muscular Dystrophy Cardiac Phenotype

The Cdkn2a locus is one of the most studied tumor suppressor loci in the context of several cancer types. However, in the last years, its expression has also been linked to terminal differentiation and the activation of the senescence program in different cellular subtypes. Knock-out (KO) of the entire locus enhances the capability of stem cells to proliferate in some tissues and respond to severe physiological and non-physiological damages in different organs, including the heart. Emery-Dreifuss muscular dystrophy (EDMD) is characterized by severe contractures and muscle loss at the level of skeletal muscles of the elbows, ankles and neck, and by dilated cardiomyopathy. We have recently demonstrated, using the LMNA Δ8-11 murine model of Emery-Dreifuss muscular dystrophy (EDMD), that dystrophic muscle stem cells prematurely express non-lineage-specific genes early on during postnatal growth, leading to rapid exhaustion of the muscle stem cell pool. Knock-out of the Cdkn2a locus in EDMD dystrophic mice partially restores muscle stem cell properties. In the present study, we describe the cardiac phenotype of the LMNA Δ8-11 mouse model and functionally characterize the effects of KO of the Cdkn2a locus on heart functions and life expectancy.

Cx43 hemichannel microdomain signaling at the intercalated disc enhances cardiac excitability

Cx43, a major cardiac connexin, forms precursor hemichannels that accrue at the intercalated disc to assemble as gap junctions. While gap junctions are crucial for electrical conduction in the heart, little is known about the potential roles of hemichannels. Recent evidence suggests that inhibiting Cx43 hemichannel opening with Gap19 has antiarrhythmic effects. Here, we used multiple electrophysiology, imaging, and super-resolution techniques to understand and define the conditions underlying Cx43 hemichannel activation in ventricular cardiomyocytes, their contribution to diastolic Ca2+ release from the sarcoplasmic reticulum, and their impact on electrical stability. We showed that Cx43 hemichannels were activated during diastolic Ca2+ release in single ventricular cardiomyocytes and cardiomyocyte cell pairs from mice and pigs. This activation involved Cx43 hemichannel Ca2+ entry and coupling to Ca2+ release microdomains at the intercalated disc, resulting in enhanced Ca2+ dynamics. Hemichannel opening furthermore contributed to delayed afterdepolarizations and triggered action potentials. In single cardiomyocytes, cardiomyocyte cell pairs, and arterially perfused tissue wedges from failing human hearts, increased hemichannel activity contributed to electrical instability compared with nonfailing rejected donor hearts. We conclude that microdomain coupling between Cx43 hemichannels and Ca2+ release is a potentially novel, targetable mechanism of cardiac arrhythmogenesis in heart failure.

Connexin 43 phosphorylation by casein kinase 1 is essential for the cardioprotection by ischemic preconditioning

Myocardial connexin 43 (Cx43) forms gap junctions and hemichannels, and is also present within subsarcolemmal mitochondria. The protein is phosphorylated by several kinases including mitogen-activated protein kinase (MAPK), protein kinase C (PKC), and casein kinase 1 (CK1). A reduction in Cx43 content abrogates myocardial infarct size reduction by ischemic preconditioning (IPC). The present study characterizes the contribution of Cx43 phosphorylation towards mitochondrial function, hemichannel activity, and the cardioprotection by IPC in wild-type (WT) mice and in mice in which Cx43-phosphorylation sites targeted by above kinases are mutated to non-phosphorylatable residues (Cx43MAPKmut, Cx43PKCmut, and Cx43CK1mut mice). The amount of Cx43 in the left ventricle and in mitochondria was reduced in all mutant strains compared to WT mice and Cx43 phosphorylation was altered at residues not directly targeted by the mutations. Whereas complex 1 respiration was reduced in all strains, complex 2 respiration was decreased in Cx43CK1mut mice only. In Cx43 epitope-mutated mice, formation of reactive oxygen species and opening of the mitochondrial permeability transition pore were not affected. The hemichannel open probability was reduced in Cx43PKCmut and Cx43CK1mut but not in Cx43MAPKmut cardiomyocytes. Infarct size in isolated saline-perfused hearts after ischemia/reperfusion (45 min/120 min) was comparable between genotypes and was significantly reduced by IPC (3 × 3 min ischemia/5 min reperfusion) in WT, Cx43MAPKmut, and Cx43PKCmut, but not in Cx43CK1mut mice, an effect independent from the amount of Cx43 and the probability of hemichannel opening. Taken together, our study shows that alterations of Cx43 phosphorylation affect specific cellular functions and highlights the importance of Cx43 phosphorylation by CK1 for IPC's cardioprotection.

Melatonin to Rescue the Aged Heart: Antiarrhythmic and Antioxidant Benefits

Aging comes with gradual loss of functions that increase the vulnerability to disease, senescence, and death. The mechanisms underlying these processes are linked to a prolonged imbalance between damage and repair. Damaging mechanisms include oxidative stress, mitochondrial dysfunction, chronodisruption, inflammation, and telomere attrition, as well as genetic and epigenetic alterations. Several endogenous tissue repairing mechanisms also decrease. These alterations associated with aging affect the entire organism. The most devastating manifestations involve the cardiovascular system and may lead to lethal cardiac arrhythmias. Together with structural remodeling, electrophysiological and intercellular communication alterations during aging predispose to arrhythmic events. Despite the knowledge on repairing mechanisms in the cardiovascular system, effective antiaging strategies able to reduce the risk of arrhythmias are still missing. Melatonin is a promising therapeutic candidate due to its pleiotropic actions. This indoleamine regulates chronobiology and endocrine physiology. Of relevance, melatonin is an antiaging, antioxidant, antiapoptotic, antiarrhythmic, immunomodulatory, and antiproliferative molecule. This review focuses on the protective effects of melatonin on age-induced cardiac functional and structural alterations, potentially becoming a new fountain of youth for the heart.

Intercellular Communication in the Heart: Therapeutic Opportunities for Cardiac Ischemia

The maintenance of tissue, organ, and organism homeostasis relies on an intricate network of players and mechanisms that assist in the different forms of cell-cell communication. Myocardial infarction, following heart ischemia and reperfusion, is associated with profound changes in key processes of intercellular communication, involving gap junctions, extracellular vesicles, and tunneling nanotubes, some of which have been implicated in communication defects associated with cardiac injury, namely arrhythmogenesis and progression into heart failure. Therefore, intercellular communication players have emerged as attractive powerful therapeutic targets aimed at preserving a fine-tuned crosstalk between the different cardiac cells in order to prevent or repair some of harmful consequences of heart ischemia and reperfusion, re-establishing myocardial function.

Essential roles of the dystrophin-glycoprotein complex in different cardiac pathologies

The dystrophin-glycoprotein complex (DGC), situated at the sarcolemma dynamically remodels during cardiac disease. This review examines DGC remodeling as a common denominator in diseases affecting heart function and health. Dystrophin and the DGC serve as broad cytoskeletal integrators that are critical for maintaining stability of muscle membranes. The presence of pathogenic variants in genes encoding proteins of the DGC can cause absence of the protein and/or alterations in other complex members leading to muscular dystrophies. Targeted studies have allowed the individual functions of affected proteins to be defined. The DGC has demonstrated its dynamic function, remodeling under a number of conditions that stress the heart. Beyond genetic causes, pathogenic processes also impinge on the DGC, causing alterations in the abundance of dystrophin and associated proteins during cardiac insult such as ischemia-reperfusion injury, mechanical unloading, and myocarditis. When considering new therapeutic strategies, it is important to assess DGC remodeling as a common factor in various heart diseases. The DGC connects the internal F-actin-based cytoskeleton to laminin-211 of the extracellular space, playing an important role in the transmission of mechanical force to the extracellular matrix. The essential functions of dystrophin and the DGC have been long recognized. DGC based therapeutic approaches have been primarily focused on muscular dystrophies, however it may be a beneficial target in a number of disorders that affect the heart. This review provides an account of what we now know, and discusses how this knowledge can benefit persistent health conditions in the clinic.

The role of connexin proteins and their channels in radiation-induced atherosclerosis

Radiotherapy is an effective treatment for breast cancer and other thoracic tumors. However, while high-energy radiotherapy treatment successfully kills cancer cells, radiation exposure of the heart and large arteries cannot always be avoided, resulting in secondary cardiovascular disease in cancer survivors. Radiation-induced changes in the cardiac vasculature may thereby lead to coronary artery atherosclerosis, which is a major cardiovascular complication nowadays in thoracic radiotherapy-treated patients. The underlying biological and molecular mechanisms of radiation-induced atherosclerosis are complex and still not fully understood, resulting in potentially improper radiation protection. Ionizing radiation (IR) exposure may damage the vascular endothelium by inducing DNA damage, oxidative stress, premature cellular senescence, cell death and inflammation, which act to promote the atherosclerotic process. Intercellular communication mediated by connexin (Cx)-based gap junctions and hemichannels may modulate IR-induced responses and thereby the atherosclerotic process. However, the role of endothelial Cxs and their channels in atherosclerotic development after IR exposure is still poorly defined. A better understanding of the underlying biological pathways involved in secondary cardiovascular toxicity after radiotherapy would facilitate the development of effective strategies that prevent or mitigate these adverse effects. Here, we review the possible roles of intercellular Cx driven signaling and communication in radiation-induced atherosclerosis.

A role for connexin-43 in Duchenne muscular dystrophy cardiomyopathy

The cardiomyopathy of Duchenne muscular dystrophy (DMD) is an important cause of morbidity and mortality in affected males with this dreaded muscle disease. Previous studies have implicated changes in expression and subcellular localization of connexin-43 (Cx43), the major ventricular gap junction protein, in DMD cardiomyopathy. In this issue of the JCI, Himelman et al. explore how hypophosphorylation of Cx43 at a triplet of serine residues (S325/S328/S330) in the regulatory C-terminus contributes to multiple features of the cardiomyopathy phenotype. Using a mouse model of DMD cardiomyopathy in which phosphomimetic glutamic acids are substituted for serines at these residues in Cx43, Himelman et al. observed reduced gap junction remodeling and lateralization of Cx43 immunosignals, protection against isoproterenol-induced arrhythmias, and improved Ca2+ homeostasis. This study contributes to the understanding of pathologic Cx43 remodeling and encourages further research into developing strategic interventions to mitigate cardiac dysfunction and arrhythmias in DMD patients.

Gap19, a Cx43 Hemichannel Inhibitor, Acts as a Gating Modifier That Decreases Main State Opening While Increasing Substate Gating

Cx43 hemichannels (HCs) are electrically and chemically gated transmembrane pores with low open probability and multiple conductance states, which makes kinetic studies of channel gating in large datasets challenging. Here, we developed open access software, named HemiGUI, to analyze HC gating transitions and investigated voltage-induced HC opening based on up to ≈4000 events recorded in HeLa-Cx43-overexpressing cells. We performed a detailed characterization of Cx43 HC gating profiles and specifically focused on the role of the C-terminal tail (CT) domain by recording the impact of adding an EGFP tag to the Cx43 CT end (Cx43-EGFP) or by supplying the Cx43 HC-inhibiting peptide Gap19 that interferes with CT interaction with the cytoplasmic loop (CL). We found that Gap19 not only decreased HC opening activity to the open state (≈217 pS) but also increased the propensity of subconductance (≈80 pS) transitions that additionally became slower as compared to the control. The work demonstrates that large sample transition analysis allows detailed investigations on Cx43 HC gating and shows that Gap19 acts as a HC gating modifier by interacting with the CT that forms a crucial gating element.

Apocynin Treatment Prevents Cardiac Connexin 43 Hemichannels Hyperactivity by Reducing Nitroso-Redox Stress in Mdx Mice

Duchenne muscular dystrophy (DMD) is a fatal disease that causes cardiomyopathy and is associated with oxidative stress. In the heart, oxidative stress interferes with the location of connexin 43 (Cx43) to the intercalated discs causing its lateralization to the plasma membrane where Cx43 forms hemichannels. We tested the hypothesis that in DMD cardiomyopathy, increased oxidative stress is associated with the formation and activation of Cx43 hemichannels. For this, we used mdx mice as a DMD model and evaluated cardiac function, nitroso-redox changes and Cx43 hemichannels permeability. Mdx hearts presented increased NADPH oxidase-derived oxidative stress and increased Cx43 S-nitrosylation compared to controls. These redox changes were associated with increased Cx43 lateralization, decreased cardiac contractility and increased arrhythmic events. Pharmacological inhibition of NADPH oxidase using apocynin (one month) reduced systemic oxidative stress and reversed the aforementioned changes towards normal, except Cx43 lateralization. Opening of Cx43 hemichannels was blocked by apocynin treatment and by acute hemichannel blockade with carbenoxolone. NADPH oxidase inhibition also prevented the occurrence of apoptosis in mdx hearts and reversed the ventricular remodeling. These results show that NADPH oxidase activity in DMD is associated with S-nitrosylation and opening of Cx43 hemichannels. These changes lead to apoptosis and cardiac dysfunction and were prevented by NADPH oxidase inhibition.

Connexin-43 reduction prevents muscle defects in a mouse model of manifesting Duchenne muscular dystrophy female carriers

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular disorder that affects males. However, 8% of female carriers are symptomatic and underrepresented in research due to the lack of animal models. We generated a symptomatic mouse model of DMD carriers via injection of mdx (murine DMD) embryonic stem cells (ESCs) into wild-type (WT) blastocysts (mdx/WT chimera). mdx/WT chimeras developed cardiomyopathic features and dystrophic skeletal muscle phenotypes including elevated mononuclear invasion, central nucleation, fibrosis and declined forelimb grip strength. The disease was accompanied by connexin-43 (Cx43) aberrantly enhanced in both cardiac and skeletal muscles and remodeled in the heart. Genetic reduction of Cx43-copy number in mdx/WT-Cx43(+/-) chimeras protected them from both cardiac and skeletal muscle fiber damage. In dystrophic skeletal muscle, Cx43 expression was not seen in the fibers but in adjacent F4/80+ mononuclear cells. Ethidium Bromide uptake in purified F4/80+/CD11b+ mdx macrophages revealed functional activity of Cx43, which was inhibited by administration of Gap19 peptide mimetic, a Cx43 hemichannel-specific inhibitor. Thus, we suggest that Cx43 reduction in symptomatic DMD carrier mice leads to prevention of Cx43 remodeling in the heart and prevention of aberrant Cx43 hemichannel activity in the skeletal muscle macrophages neighboring Cx43 non-expressing fibers.