Effects of diminished expression of connexin43 on gap junction number and size in ventricular myocardium

Three-dimensional co-culturing of stem cell-derived cardiomyocytes and cardiac fibroblasts reveals a role for both cell types in Marfan-related cardiomyopathy

Pathogenic variants in the FBN1 gene, which encodes the extracellular matrix protein fibrillin-1, cause Marfan syndrome (MFS), which affects multiple organ systems, including the cardiovascular system. Myocardial dysfunction has been observed in a subset of patients with MFS and in several MFS mouse models. However, there is limited understanding of the intrinsic consequences of FBN1 variants on cardiomyocytes (CMs). To elucidate the CM-specific contribution in Marfan's cardiomyopathy, cardiosphere cultures of CMs and cardiac fibroblasts (CFs) are used. CMs and CFs were derived by human induced pluripotent stem cell (iPSC) differentiation from MFS iPSCs with a pathogenic variant in FBN1 (c.3725G>A; p.Cys1242Tyr) and the corresponding CRISPR-corrected iPSC line (Cor). Cardiospheres containing MFS CMs show decreased FBN1, COL1A2 and GJA1 expression. MFS CMs cultured in cardiospheres have fewer binucleated CMs in comparison with Cor CMs. 13% of MFS CMs in cardiospheres are binucleated and 15% and 16% in cardiospheres that contain co-cultures with respectively MFS CFs and Cor CFs, compared to Cor CMs, that revealed up to 23% binucleation when co-cultured with CFs. The sarcomere length of CMs, as a marker of development, is significantly increased in MFS CMs interacting with Cor CF or MFS CF, as compared to monocultured MFS CMs. Nuclear blebbing was significantly more frequent in MFS CFs, which correlated with increased stiffness of the nuclear area compared to Cor CFs. Our cardiosphere model for Marfan-related cardiomyopathy identified a contribution of CFs in Marfan-related cardiomyopathy and suggests that abnormal early development of CMs may play a role in the disease mechanism.

ERGIC2 and ERGIC3 regulate the ER-to-Golgi transport of gap junction proteins in metazoans

The extremely dynamic life cycle of gap junction connections requires highly efficient intracellular trafficking system especially designed for gap junction proteins, but the underlying mechanisms are largely unknown. Here, we identified that the COPII-associated proteins ERGIC2 (ER-Golgi intermediate compartment) and ERGIC3 are specifically required for the efficient intracellular transport of gap junction proteins in both C. elegans and mice. In the absence of Ergic2 or Ergic3, gap junction proteins accumulate in the ER and Golgi apparatus and the size of endogenous gap junction plaques is reduced. Knocking out the Ergic2 or Ergic3 in mice results in heart enlargement and cardiac malfunction accompanied by reduced number and size of connexin 43 (Cx43) gap junctions. Invertebrates' gap junction protein innexins share no sequence similarity with vertebrates' connexins. However, ERGIC2 and ERGIC3 could bind to gap junction proteins in both worms and mice. Characterization of the highly specialized roles of ERGIC2 and ERGIC3 in metazoans reveals how the early secretory pathway could be adapted to facilitate the efficient transport for gap junction proteins in vivo. This article is protected by copyright. All rights reserved.

Prospective Evaluation of Clinico-Pathological Predictors of Postoperative Atrial Fibrillation: An Ancillary Study From the OPERA Trial

BACKGROUND

Postoperative atrial fibrillation (POAF) occurs in 30% to 50% of patients undergoing cardiac surgery and is associated with increased morbidity and mortality. Prospective identification of structural/molecular changes in atrial myocardium that correlate with myocardial injury and precede and predict risk of POAF may identify new molecular pathways and targets for prevention of this common morbid complication.

METHODS

Right atrial appendage samples were prospectively collected during cardiac surgery from 239 patients enrolled in the OPERA trial (Omega-3 Fatty Acids for Prevention of Post-Operative Atrial Fibrillation), fixed in 10% buffered formalin, and embedded in paraffin for histology. We assessed general tissue morphology, cardiomyocyte diameters, myocytolysis (perinuclear myofibril loss), accumulation of perinuclear glycogen, interstitial fibrosis, and myocardial gap junction distribution. We also assayed NT-proBNP (N-terminal pro-B-type natriuretic peptide), hs-cTnT, CRP (C-reactive protein), and circulating oxidative stress biomarkers (F2-isoprostanes, F3-isoprostanes, isofurans) in plasma collected before, during, and 48 hours after surgery. POAF was defined as occurrence of postcardiac surgery atrial fibrillation or flutter of at least 30 seconds duration confirmed by rhythm strip or 12-lead ECG. The follow-up period for all arrhythmias was from surgery until hospital discharge or postoperative day 10.

RESULTS

Thirty-five percent of patients experienced POAF. Compared with the non-POAF group, they were slightly older and more likely to have chronic obstructive pulmonary disease or heart failure. They also had a higher European System for Cardiac Operative Risk Evaluation and more often underwent valve surgery. No differences in left atrial size were observed between patients with POAF and patients without POAF. The extent of atrial interstitial fibrosis, cardiomyocyte myocytolysis, cardiomyocyte diameter, glycogen score or Cx43 distribution at the time of surgery was not significantly associated with incidence of POAF. None of these histopathologic abnormalities were correlated with levels of NT-proBNP, hs-cTnT, CRP, or oxidative stress biomarkers.

CONCLUSIONS

In sinus rhythm patients undergoing cardiac surgery, histopathologic changes in the right atrial appendage do not predict POAF. They also do not correlate with biomarkers of cardiac function, inflammation, and oxidative stress. Graphic Abstract: A graphic abstract is available for this article.

Isoform Specific Effects of Mef2C during Direct Cardiac Reprogramming

Direct conversion of cardiac fibroblasts into induced cardiomyocytes (iCMs) by forced expression of defined factors holds great potential for regenerative medicine by offering an alternative strategy for treatment of heart disease. Successful iCM conversion can be achieved by minimally using three transcription factors, Mef2c (M), Gata4(G), and Tbx5 (T). Despite increasing interest in iCM mechanistic studies using MGT(polycistronic construct with optimal expression of M,G and T), the reprogramming efficiency varies among different laboratories. Two main Mef2c isoforms (isoform2, Mi2 and isoform4, Mi4) are present in heart and are used separately by different labs, for iCM reprogramming. It is currently unknown if differently spliced isoform of Mef2c contributes to varied reprogramming efficiency. Here, we used Mi2 and Mi4 together with Gata4 and Tbx5 in separate vectors or polycistronic vector, to convert fibroblasts to iCMs. We found that Mi2 can induce higher reprogramming efficiency than Mi4 in MEFs. Addition of Hand2 to MGT retroviral cocktail or polycistronic Mi2-GT retroviruses further enhanced the iCM conversion. Overall, this study demonstrated the isoform specific effects of Mef2c, during iCM reprogramming, clarified some discrepancy about varied efficiency among labs and might lead to future research into the role of alternative splicing and the consequent variants in cell fate determination.

The role of histone modification and a regulatory single-nucleotide polymorphism (rs2071166) in the Cx43 promoter in patients with TOF

Abnormal level of Cx43 expression could result in CHD. Epigenetic modification and disease-associated, non-coding SNPs might influence gene transcription and expression. Our study aimed to determine the role of histone modification and an rSNP (rs2071166) in the Cx43 promoter in patients with TOF. Our results indicate that H3K18ac bind to Cx43 promoter and that their levels are reduced in TOF patients relative to controls. The relationship between the non-coding SNP in the Cx43 gene and TOF patients was evaluated in 158 patients and 300 controls. The C allele of rs2071166 was confirmed to result in an increased risk of TOF (OR = 1.586, 95%CI 1.149–2.189). Individuals with the CC genotype at rs2071166 also showed a significant susceptibility to TOF (OR = 2.961, 95%CI 1.452–6.038). The mRNA level in TOF who were CC genotype was lower than that in patients with the AA/AC genotype. Functional analysis in cells and transgenic zebrafish models showed that rs2071166 decreased the activity of the promoter and could block the interaction between RXRα and RARE. This is the first study to illustrate that epigenetic modification and an rSNP in the Cx43 promoter region play a critical role in TOF by impacting the transcriptional activity and expression level of Cx43.

Development of dilated cardiomyopathy and impaired calcium homeostasis with cardiac-specific deletion of ESRRβ

Mechanisms underlying the development of idiopathic dilated cardiomyopathy (DCM) remain poorly understood. Using transcription factor expression profiling, we identified estrogen-related receptor-β (ESRRβ), a member of the nuclear receptor family of transcription factors, as highly expressed in murine hearts and other highly oxidative striated muscle beds. Mice bearing cardiac-specific deletion of ESRRβ (MHC-ERRB KO) develop DCM and sudden death at ~10 mo of age. Isolated adult cardiomyocytes from the MHC-ERRB KO mice showed an increase in calcium sensitivity and impaired cardiomyocyte contractility, which preceded echocardiographic cardiac remodeling and dysfunction by several months. Histological analyses of myocardial biopsies from patients with various cardiomyopathies revealed that ESRRβ protein is absent from the nucleus of cardiomyocytes from patients with DCM but not other forms of cardiomyopathy (ischemic, hypertrophic, and arrhythmogenic right ventricular cardiomyopathy). Taken together these observations suggest that ESRRβ is a critical component in the onset of DCM by affecting contractility and calcium balance.NEW & NOTEWORTHY Estrogen-related receptor-β (ESRRβ) is highly expressed in the heart and cardiac-specific deletion results in the development of a dilated cardiomyopathy (DCM). ESRRβ is mislocalized in human myocardium samples with DCM, suggesting a possible role for ESRRβ in the pathogenesis of DCM in humans.

Extracellular sodium dependence of the conduction velocity-calcium relationship: evidence of ephaptic self-attenuation

Our laboratory previously demonstrated that perfusate sodium and potassium concentrations can modulate cardiac conduction velocity (CV) consistent with theoretical predictions of ephaptic coupling (EpC). EpC depends on the ionic currents and intercellular separation in sodium channel rich intercalated disk microdomains like the perinexus. We suggested that perinexal width (WP) correlates with changes in extracellular calcium ([Ca(2+)]o). Here, we test the hypothesis that increasing [Ca(2+)]o reduces WP and increases CV. Mathematical models of EpC also predict that reducing WP can reduce sodium driving force and CV by self-attenuation. Therefore, we further hypothesized that reducing WP and extracellular sodium ([Na(+)]o) will reduce CV consistent with ephaptic self-attenuation. Transmission electron microscopy revealed that increasing [Ca(2+)]o (1 to 3.4 mM) significantly decreased WP Optically mapping wild-type (WT) (100% Cx43) mouse hearts demonstrated that increasing [Ca(2+)]o increases transverse CV during normonatremia (147.3 mM), but slows transverse CV during hyponatremia (120 mM). Additionally, CV in heterozygous (∼50% Cx43) hearts was more sensitive to changes in [Ca(2+)]o relative to WT during normonatremia. During hyponatremia, CV slowed in both WT and heterozygous hearts to the same extent. Importantly, neither [Ca(2+)]o nor [Na(+)]o altered Cx43 expression or phosphorylation determined by Western blotting, or gap junctional resistance determined by electrical impedance spectroscopy. Narrowing WP, by increasing [Ca(2+)]o, increases CV consistent with enhanced EpC between myocytes. Interestingly, during hyponatremia, reducing WP slowed CV, consistent with theoretical predictions of ephaptic self-attenuation. This study suggests that serum ion concentrations may be an important determinant of cardiac disease expression.

TMEM43 mutation p.S358L alters intercalated disc protein expression and reduces conduction velocity in arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a myocardial disease characterized by fibro-fatty replacement of myocardium in the right ventricular free wall and frequently results in life-threatening ventricular arrhythmias and sudden cardiac death. A heterozygous missense mutation in the transmembrane protein 43 (TMEM43) gene, p.S358L, has been genetically identified to cause autosomal dominant ARVC type 5 in a founder population from the island of Newfoundland, Canada. Little is known about the function of the TMEM43 protein or how it leads to the pathogenesis of ARVC. We sought to determine the distribution of TMEM43 and the effect of the p.S358L mutation on the expression and distribution of various intercalated (IC) disc proteins as well as functional effects on IC disc gap junction dye transfer and conduction velocity in cell culture. Through Western blot analysis, transmission electron microscopy (TEM), immunofluorescence (IF), and electrophysiological analysis, our results showed that the stable expression of p.S358L mutation in the HL-1 cardiac cell line resulted in decreased Zonula Occludens (ZO-1) expression and the loss of ZO-1 localization to cell-cell junctions. Junctional Plakoglobin (JUP) and α-catenin proteins were redistributed to the cytoplasm with decreased localization to cell-cell junctions. Connexin-43 (Cx43) phosphorylation was altered, and there was reduced gap junction dye transfer and conduction velocity in mutant TMEM43-transfected cells. These observations suggest that expression of the p.S358L mutant of TMEM43 found in ARVC type 5 may affect localization of proteins involved in conduction, alter gap junction function and reduce conduction velocity in cardiac tissue.

Arrhythmogenic right ventricular cardiomyopathy mutations alter shear response without changes in cell-cell adhesion

AIMS

The majority of patients diagnosed with arrhythmogenic right ventricular cardiomyopathy (ARVC) have mutations in genes encoding desmosomal proteins, raising the possibility that abnormal intercellular adhesion plays an important role in disease pathogenesis. We characterize cell mechanical properties and molecular responses to oscillatory shear stress in cardiac myocytes expressing mutant forms of the desmosomal proteins, plakoglobin and plakophilin, which are linked to ARVC in patients.

METHODS AND RESULTS

Cells expressing mutant plakoglobin or plakophilin showed no differences in cell-cell adhesion relative to controls, while knocking down these proteins weakened cell-cell adhesion. However, cells expressing mutant plakoglobin failed to increase the amount of immunoreactive signal for plakoglobin or N-cadherin at cell-cell junctions in response to shear stress, as seen in control cells. Cells expressing mutant plakophilin exhibited a similar attenuation in the shear-induced increase in junctional plakoglobin immunoreactive signal in response to shear stress, suggesting that the phenotype is independent of the type of mutant protein being expressed. Cells expressing mutant plakoglobin also showed greater myocyte apoptosis compared with controls. Apoptosis rates increased greatly in response to shear stress in cells expressing mutant plakoglobin, but not in controls. Abnormal responses to shear stress in cells expressing either mutant plakoglobin or plakophilin could be reversed by SB216763, a GSK3β inhibitor.

CONCLUSIONS

Desmosomal mutations linked to ARVC do not significantly affect cell mechanical properties, but cause myocytes to respond abnormally to mechanical stress through a mechanism involving GSK3β. These results may help explain why patients with ARVC experience disease exacerbations following strenuous exercise.

Heptanol decreases the incidence of ischemia-induced ventricular arrhythmias through altering electrophysiological properties and connexin 43 in rat hearts

Heptanol is a type of gap junction inhibitor that decreases electrical conduction velocity. However, little is known regarding the effects of heptanol on the arrhythmias induced by regional myocardial ischemia. This study aimed to investigate the effects of heptanol on ventricular arrhythmias and the underlying mechanisms. On the Langendorff apparatus, isolated hearts of Sprague-Dawley rats underwent 30 min of ischemia, with or without pretreatment with heptanol (0.1, 0.3 or 0.5 mM), 15 min prior to the induction of regional ischemia through ligation of the left anterior descending coronary artery. The incidence of ventricular tachycardia (VT) and ventricular fibrillation (VF) were recorded after ligation. Heptanol decreased the incidence of ventricular arrhythmias (45% in the control group vs. 10% in the 0.1 mM group, 0% in the 0.3 mM group and 0% in the 0.5 mM group, P<0.05), whereas it prolonged the PR interval, QT interval and monophasic action potential duration at 90% repolarization (MAPD90). As evaluated with immunofluorescence microscopy, heptanol was able to partly reverse the downregulation of connexin 43 (Cx43) induced by ischemia. The results of the reverse transcription-polymerase chain reaction were consistent with those of immunofluorescence. In conclusion, heptanol significantly decreased the incidence of VT and VF induced by regional ischemia and prolonged the PR interval, QT interval and MAPD90. Heptanol also partly reversed the downregulation of Cx43 induced by ischemia.

Atrial fibrillation from the pathologist's perspective

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia encountered in clinical practice, is associated with increased morbidity and mortality. Electrophysiologically, it is characterized by a high rate of asynchronous atrial cell depolarization causing a loss of atrial contractile function and irregular ventricular rates. For a long time, AF was considered as a pure functional disorder without any structural background. Only in recent years, have new mapping and imaging techniques identified atrial locations, which are very often involved in the initiation and maintenance of this supraventricular arrhythmia (i.e. the distal portion of the pulmonary veins and the surrounding atrial myocardium). Morphological analysis of these myocardial sites has demonstrated significant structural remodeling as well as paved the way for further knowledge of AF natural history, pathogenesis, and treatment. This architectural myocardial disarrangement is induced by the arrhythmia itself and the very frequently associated cardiovascular disorders. At the same time, the structural remodeling is also capable of sustaining AF, thereby creating a sort of pathogenetic vicious circle. This review focuses on current understanding about the structural and genetic bases of AF with reference to their classification, pathogenesis, and clinical implications.

Effect of beta-carotene on oxidative stress and expression of cardiac connexin 43

BACKGROUND

Intervention studies have shown an increased mortality in patients who received beta-carotene. However, the mechanisms involved in this phenomenon are still unknown.

OBJECTIVE

Evaluate the influence of beta-carotene on oxidative stress and the expression of connexin 43 in rat hearts.

METHODS

Wistar rats, weighing approximately 100 g, were allocated in two groups: CONTROL GROUP (n=30), that received the diet routinely used in our laboratory, and Beta-Carotene Group (n = 28), which received beta-carotene (in crystal form, added and mixed to the diet) at a dose of 500 mg of beta-carotene/kg of diet. The animals received the treatment until they reached 200-250 g, when they were sacrificed. Samples of blood, liver and heart were collected to perform Western blotting and immunohistochemistry for connexin 43; morphometric studies, dosages of beta-carotene by high-performance liquid chromatography as well as reduced glutathione, oxidized glutathione and lipids hydroperoxides were performed by biochemical analysis.

RESULTS

Beta-carotene was detected only in the liver of Beta-Carotene Group animals (288 ± 94.7 µg/kg). Levels of reduced/oxidized glutathione were higher in the liver and heart of Beta-Carotene Group animals (liver -

CONTROL GROUP

42.60 ± 1.62; liver - Beta-Carotene Group: 57.40 ± 5.90; p = 0.04; heart: -

CONTROL GROUP

117.40 ± 1.01; heart - Beta-Carotene Group: 121.81 ± 1.32 nmol/mg protein; p = 0.03). The content of total connexin 43 was larger in Beta-Carotene Group.

CONCLUSION

Beta-carotene demonstrated a positive effect, characterized by the increase of intercellular communication and improvement of anti-oxidizing defense system. In this model, mechanism does not explain the increased mortality rate observed with the beta-carotene supplementation in clinical studies.

Evolving form to fit function: cardiomyocyte intercalated disc and transverse-tubule membranes

The vertebrate cardiac myocyte has evolved a highly organized cellular membrane architecture and cell-cell contacts in order to effectively transmit precisely timed and homogeneous depolarizing waves without failure (>2 billion times/human life span). Two unique specialized membrane domains, the intercalated disc and the transverse tubule (T-tubule), function to ensure the rapid and coordinated propagation of the action potential throughout the heart. Based on their critical roles in structure, signaling, and electric inter- and intracellular communication, it is not surprising that dysfunction in these membrane structures is associated with aberrant vertebrate physiology, resulting in potentially fatal congenital and acquired disease. This chapter will review the fundamental components of cardiomyocyte intercalated disc and transverse-tubule membranes with a focus on linking dysfunction in these membranes with human cardiovascular disease.

Reduced connexin40 protein expression in the right atrial appendage of patients bearing the minor connexin40 allele (-44 G --> A)

AIMS

The occurrence of connexin40 (Cx40) minor polymorphism (-44 G → A) was increased in patients with idiopathic atrial fibrillation (AF), although its effect on atrial Cx40 protein expression is unknown. We aimed to evaluate whether alterations in Cx40 are directly linked to the development of AF, we studied the effect of this polymorphism on Cx40 expression and distribution in patients without any history of AF and in patients who developed post-operative AF.

METHODS AND RESULTS

Hundred and eight patients (mean age 67 ± 9 years), without a history of AF or conditions that predispose to AF, were included. During heart surgery, 10 cc blood was collected for DNA genotyping and the right atrial appendage was partly excised. Ten patients (9%) were homozygous for the minor allele (AA, Group 1), 30 (28%) were heterozygous (AG, Group 2), and 68 (63%) were non-carriers (GG, Group 3). Ten age- and sex-matched tissue samples per group were analysed for Cx40 expression by: (i) real-time quantitative polymerase chain reaction (Q-PCR), (ii) western blotting, and (iii) immunohistochemistry on cryosections. Real-time quantitative polymerase chain reaction showed no significant differences of Cx40 mRNA among the groups. Western blot analysis, however, revealed a reduction in Cx40 protein in Groups 1 (-36.4%) and 2 (-39.5%) as compared with Group 3. Immunohistochemistry confirmed this reduction but indicated an unaltered subcellular distribution of the remaining Cx40. Incidence of post-operative AF (28%) was age-dependent but unrelated to the presence of the polymorphism or fibrosis.

CONCLUSION

Presence of the Cx40 minor allele (-44 G → A) results in a uniform down-regulation of right atrial appendage Cx40 protein which was not significantly related to development of post-operative AF.

Heart repair by reprogramming non-myocytes with cardiac transcription factors

The adult mammalian heart possesses little regenerative potential following injury. Fibrosis due to activation of cardiac fibroblasts impedes cardiac regeneration and contributes to loss of contractile function, pathological remodelling and susceptibility to arrhythmias. Cardiac fibroblasts account for a majority of cells in the heart and represent a potential cellular source for restoration of cardiac function following injury through phenotypic reprogramming to a myocardial cell fate. Here we show that four transcription factors, GATA4, HAND2, MEF2C and TBX5, can cooperatively reprogram adult mouse tail-tip and cardiac fibroblasts into beating cardiac-like myocytes in vitro. Forced expression of these factors in dividing non-cardiomyocytes in mice reprograms these cells into functional cardiac-like myocytes, improves cardiac function and reduces adverse ventricular remodelling following myocardial infarction. Our results suggest a strategy for cardiac repair through reprogramming fibroblasts resident in the heart with cardiogenic transcription factors or other molecules.

Electrical coupling and propagation in engineered ventricular myocardium with heterogeneous expression of connexin43

RATIONALE

Spatial heterogeneity in connexin (Cx) expression has been implicated in arrhythmogenesis.

OBJECTIVE

This study was performed to quantify the relation between the degree of heterogeneity in Cx43 expression and disturbances in electric propagation.

METHODS AND RESULTS

Cell pairs and strands composed of mixtures of Cx43(-/-) (Cx43KO) or GFP-expressing Cx43(+/+) (WT(GFP)) murine ventricular myocytes were patterned using microlithographic techniques. At the interface between pairs of WT(GFP) and Cx43KO cells, dual-voltage clamp showed a marked decrease in electric coupling (approximately 5% of WT) and voltage gating suggested the presence of mixed Cx43/Cx45 channels. Cx43 and Cx45 immunofluorescence signals were not detectable at this interface, probably because of markedly reduced gap junction size. Macroscopic propagation velocity, measured by multisite high-resolution optical mapping of transmembrane potential in strands of cells of mixed Cx43 genotype, decreased with an increasing proportion of Cx43KO cells in the strand. A marked decrease in conduction velocity was observed in strands composed of <50% WT cells. Propagation at the microscopic scale showed a high degree of dissociation between WT(GFP) and Cx43KO cells, but consistent excitation without development of propagation block.

CONCLUSIONS

Heterogeneous ablation of Cx43 leads to a marked decrease in propagation velocity in tissue strands composed of <50% cells with WT Cx43 expression and marked dissociation of excitation at the cellular level. However, the small residual electric conductance between Cx43 and WT(GFP) myocytes assures excitation of Cx43(-/-) cells. This explains the previously reported undisturbed contractility in tissues with spatially heterogeneous downregulation of Cx43 expression.

Loss of cadherin-binding proteins β-catenin and plakoglobin in the heart leads to gap junction remodeling and arrhythmogenesis

Arrhythmic right ventricular cardiomyopathy (ARVC) is a hereditary heart muscle disease that causes sudden cardiac death (SCD) in young people. Almost half of ARVC patients have a mutation in genes encoding cell adhesion proteins of the desmosome, including plakoglobin (JUP). We previously reported that cardiac tissue-specific plakoglobin (PG) knockout (PG CKO) mice have no apparent conduction abnormality and survive longer than expected. Importantly, the PG homolog, β-catenin (CTNNB1), showed increased association with the gap junction protein connexin43 (Cx43) in PG CKO hearts. To determine whether β-catenin is required to maintain cardiac conduction in the absence of PG, we generated mice lacking both PG and β-catenin specifically in the heart (i.e., double knockout [DKO]). The DKO mice exhibited cardiomyopathy, fibrous tissue replacement, and conduction abnormalities resulting in SCD. Loss of the cadherin linker proteins resulted in dissolution of the intercalated disc (ICD) structure. Moreover, Cx43-containing gap junction plaques were reduced at the ICD, consistent with the arrhythmogenicity of the DKO hearts. Finally, ambulatory electrocardiogram monitoring captured the abrupt onset of spontaneous lethal ventricular arrhythmia in the DKO mice. In conclusion, these studies demonstrate that the N-cadherin-binding partners, PG and β-catenin, are indispensable for maintaining mechanoelectrical coupling in the heart.

Cell-to-cell coupling in engineered pairs of rat ventricular cardiomyocytes: relation between Cx43 immunofluorescence and intercellular electrical conductance

Gap junctions are composed of connexin (Cx) proteins, which mediate intercellular communication. Cx43 is the dominant Cx in ventricular myocardium, and Cx45 is present in trace amounts. Cx43 immunosignal has been associated with cell-to-cell coupling and electrical propagation, but no studies have directly correlated Cx43 immunosignal to electrical cell-to-cell conductance, g(j), in ventricular cardiomyocyte pairs. To assess the correlation between Cx43 immunosignal and g(j), we developed a method to determine both parameters from the same cell pair. Neonatal rat ventricular cardiomyocytes were seeded on micropatterned islands of fibronectin. This allowed formation of cell pairs with reproducible shapes and facilitated tracking of cell pair locations. Moreover, cell spreading was limited by the fibronectin pattern, which allowed us to increase cell height by reducing the surface area of the pattern. Whole cell dual voltage clamp was used to record g(j) of cell pairs after 3-5 days in culture. Fixation of cell pairs before removal of patch electrodes enabled preservation of cell morphology and offline identification of patched pairs. Subsequently, pairs were immunostained, and the volume of junctional Cx43 was quantified using confocal microscopy, image deconvolution, and three-dimensional reconstruction. Our results show a linear correlation between g(j) and Cx43 immunosignal within a range of 8-50 nS.

Molecular changes in the heart of a severe case of arrhythmogenic right ventricular cardiomyopathy caused by a desmoglein-2 null allele

INTRODUCTION

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a genetic disorder caused by mutations in desmosomal genes. It is often associated with life-threatening arrhythmias. Some affected individuals develop progressive heart failure and may require cardiac transplantation.

METHODS

The explanted heart of a young adult with end-stage heart failure due to a null allele in desmoglein-2 was studied at macroscopic, microscopic, and molecular level. Myocardial samples were probed for junctional localization of desmosomal components and the gap junction protein connexin43 by immunohistochemical staining. In addition, the protein content of desmosomal and adherens junction markers as well as connexin43 was assessed by Western blotting.

RESULTS

Histological analysis confirmed ARVC. Despite the loss of specific immunoreactive signal for desmosomal components at the cardiac intercalated disks (shown for plakoglobin, desmoplakin, and plakophilin-2), these proteins could be detected by Western blotting. Only for desmoglein-2, desmocollin-2, and plakoglobin were reduced protein levels observed. Adherens junction proteins were not affected. Lower phosphorylation levels were observed for connexin43; however, localization of the gap junction protein displayed regional differences. At the molecular level, disease progression was more severe in the right ventricle compared to the left ventricle.

CONCLUSION

Our data suggest that, in the ARVC heart, plakoglobin is mainly redistributed from the junctions to other cellular pools and that protein degradation only plays a secondary role. Homogenous changes in the phosphorylation status of connexin43 were observed in multiple ARVC samples, suggesting that this might be a general feature of the disease.

S-diclofenac protects against doxorubicin-induced cardiomyopathy in mice via ameliorating cardiac gap junction remodeling

Hydrogen sulfide (H₂S), as a novel gaseous mediator, plays important roles in mammalian cardiovascular tissues. In the present study, we investigated the cardioprotective effect of S-diclofenac (2-[(2,6-dichlorophenyl)amino] benzeneacetic acid 4-(3H-1,2,dithiol-3-thione-5-yl)phenyl ester), a novel H₂S-releasing derivative of diclofenac, in a murine model of doxorubicin-induced cardiomyopathy. After a single dose injection of doxorubicin (15 mg/kg, i.p.), male C57BL/6J mice were given daily treatment of S-diclofenac (25 and 50 µmol/kg, i.p.), diclofenac (25 and 50 µmol/kg, i.p.), NaHS (50 µmol/kg, i.p.), or same volume of vehicle. The cardioprotective effect of S-diclofenac was observed after 14 days. It showed that S-diclofenac, but not diclofenac, dose-dependently inhibited the doxorubicin-induced downregulation of cardiac gap junction proteins (connexin 43 and connexin 45) and thus reversed the remodeling of gap junctions in hearts. It also dose-dependently suppressed doxorubicin-induced activation of JNK in hearts. Furthermore, S-diclofenac produced a dose-dependent anti-inflammatory and anti-oxidative effect in this model. As a result, S-diclofenac significantly attenuated doxorubicin-related cardiac injury and cardiac dysfunction, and improved the survival rate of mice with doxorubicin-induced cardiomyopathy. These effects of S-diclofenac were mimicked in large part by NaHS. Therefore, we propose that H₂S released from S-diclofenac in vivo contributes to the protective effect in doxorubicin-induced cardiomyopathy. These data also provide evidence for a critical role of H₂S in the pathogenesis of doxorubicin-induced cardiomyopathy.

GATA6 reporter gene reveals myocardial phenotypic heterogeneity that is related to variations in gap junction coupling

This study examined transgenic mice whose expression of a β-galactosidase (lacZ) reporter is driven by a GATA6 gene enhancer. Previous investigations established that transcription of the transgene was associated with precardiac mesoderm and primary heart tube myocardium, which decreased progressively, so that its expression was no longer observed within ventricular myocardium by midgestation. Expression of this reporter in the adult was investigated for insights into myocyte homeostasis and cardiovascular biology. Morphometric analysis determined that <1% of myocytes, often found in small clusters, express this GATA6-associated reporter in the adult heart. LacZ expression was also found in the ascending aorta. Myocardial expression of the transgene was not associated with a proliferative phenotype or new myocyte formation, as lacZ-positive myocytes neither labeled with cell division markers nor following 5-bromodeoxyuridine pulse-chase experimentation. Despite exhibiting normal adherens junctions, these myocytes appeared to exhibit decreased connexin 43 gap junctions. Treatment with the gap junctional blocker heptanol both in vivo and in culture elevated myocardial β-galactosidase activity, suggesting that deficient gap junctional communication underlies expression of the transgenic reporter. LacZ expression within the myocardium was also enhanced in response to cryoinjury and isoproterenol-induced hypertrophy. These results reveal a previously uncharacterized phenotypic heterogeneity in the myocardium and suggest that decreased gap junctional coupling leads to induction of a signaling pathway that utilizes a unique GATA6 enhancer. Upregulation of lacZ reporter gene expression following cardiac injury indicates this transgenic mouse may serve as a model for examining the transition of the heart from healthy to pathological states.

Altered desmosomal proteins in granulomatous myocarditis and potential pathogenic links to arrhythmogenic right ventricular cardiomyopathy

BACKGROUND

Immunoreactive signal for the desmosomal protein plakoglobin (γ-catenin) is reduced at cardiac intercalated disks in patients with arrhythmogenic right ventricular cardiomyopathy (ARVC), a highly arrhythmogenic condition caused by mutations in genes encoding desmosomal proteins. Previously, we observed a false-positive case in which plakoglobin signal was reduced in a patient initially believed to have ARVC but who actually had cardiac sarcoidosis. Sarcoidosis can masquerade clinically as ARVC but has not been previously associated with altered desmosomal proteins.

METHODS AND RESULTS

We observed marked reduction in immunoreactive signal for plakoglobin at cardiac myocyte junctions in patients with sarcoidosis and giant cell myocarditis, both highly arrhythmogenic forms of myocarditis associated with granulomatous inflammation. In contrast, plakoglobin signal was not depressed in lymphocytic (nongranulomatous) myocarditis. To determine whether cytokines might promote dislocation of plakoglobin from desmosomes, we incubated cultures of neonatal rat ventricular myocytes with selected inflammatory mediators. Brief exposure to low concentrations of interleukin (IL)-17, tumor necrosis factor-α (TNF-α), and IL-6 (cytokines implicated in granulomatous myocarditis) caused translocation of plakoglobin from cell-cell junctions to intracellular sites, whereas other potent cytokines implicated in nongranulomatous myocarditis had no effect, even at much higher concentrations. We also observed myocardial expression of IL-17 and TNF-α and elevated levels of serum inflammatory mediators, including IL-6R, IL-8, monocyte chemoattractant protein 1, and macrophage inflammatory protein 1β, in patients with ARVC (all P<0.0001 compared with controls).

CONCLUSIONS

The results suggest novel disease mechanisms involving desmosomal proteins in granulomatous myocarditis and implicate cytokines, perhaps derived in part from the myocardium, in disruption of desmosomal proteins and arrhythmogenesis in ARVC.

The connexin43 carboxyl terminus and cardiac gap junction organization

The precise spatial order of gap junctions at intercalated disks in adult ventricular myocardium is thought vital for maintaining cardiac synchrony. Breakdown or remodeling of this order is a hallmark of arrhythmic disease of the heart. The principal component of gap junction channels between ventricular cardiomyocytes is connexin43 (Cx43). Protein-protein interactions and modifications of the carboxyl-terminus of Cx43 are key determinants of gap junction function, size, distribution and organization during normal development and in disease processes. Here, we review data on the role of proteins interacting with the Cx43 carboxyl-terminus in the regulation of cardiac gap junction organization, with particular emphasis on Zonula Occludens-1. The rapid progress in this area suggests that in coming years we are likely to develop a fuller understanding of the molecular mechanisms causing pathologic remodeling of gap junctions. With these advances come the promise of novel approach to the treatment of arrhythmia and the prevention of sudden cardiac death. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Effects of ischemic preconditioning on ischemia/reperfusion-induced arrhythmias by upregulatation of connexin 43 expression

BACKGROUND

The susceptibility of hypertrophied myocardium to ischemia-reperfusion injury is associated with increased risk of postoperative arrhythmias. We investigate the effects of ischemic preconditioning (IP) on post-ischemic reperfusion arrhythmias in hypertrophic rabbit hearts.

METHODS

Thirty-three rabbit models of myocardial hypertrophy were randomly divided into three groups of 11 each: non-ischemia-reperfusion group (group A), ischemia-reperfusion group (group B), and ischemic preconditioning group (group C). Another ten healthy rabbits with normal myocardium served as the healthy control group. Rabbit models of myocardial hypertrophy were induced by abdominal aortic banding. Surface electrocardiogram (ECG) was recorded and Curtis-Ravingerova score was used for arrhythmia quantification. Connexin 43 (Cx43) expression was assessed by immunohistochemistry.

RESULTS

Ratios of heart weight to body weight and left ventricular weight to body weight increase significantly in the three groups compared with the healthy control group (p < 0.05). Arrhythmia incidence in group C is significantly lower than group B (p < 0.05). Curtis-Ravingerova score in group C is lower than group B (p < 0.05). Cx43 expression area in group A is smaller by comparison with the healthy control group (p < 0.05). Cx43 expression area and fluorescence intensity in group B are reduced by 60.9% and 23.9%, respectively, compared with group A (p < 0.05). In group C, Cx43 expression area increases by 32.5% compared with group B (p < 0.05), and decreases by 54.8% compared with group A (p < 0.05).

CONCLUSIONS

The incidence of ischemia/reperfusion-induced arrhythmias in hypertrophic rabbit hearts decreases after IP, which plays an important protecting role on the electrophysiology of hypertrophied myocardium by up-regulating the expression of Cx43.

The pathobiology of arrhythmogenic cardiomyopathy

This review highlights current knowledge about arrhythmogenic cardiomyopathy and considers clinical, pathological, genetic, biomechanical, and pathophysiological aspects of disease pathogenesis. Although relatively uncommon, arrhythmogenic cardiomyopathy is of particular interest as a model system for study. It is caused in at least half of all cases by single-gene mutations that provide direct entry points into studies designed to elucidate mechanisms of disease. These mutations involve proteins that form desmosomes, directly implicating altered cellular biomechanical properties in disease pathogenesis and providing opportunities to investigate more broadly the ways in which abnormal cell and tissue biomechanics induce cardiac myocyte injury and alter cell biology. The highly arrhythmogenic phenotype is a cardinal feature of the disease. A more complete understanding of the pathogenesis of this aspect of arrhythmogenic cardiomyopathy may shed light onto the basic mechanisms underlying lethal ventricular arrhythmias and sudden cardiac death in more common forms of heart disease.

Mechanistic insights into arrhythmogenic right ventricular cardiomyopathy caused by desmocollin-2 mutations

Aims

Recent immunohistochemical studies observed the loss of plakoglobin (PG) from the intercalated disc (ID) as a hallmark of arrhythmogenic right ventricular cardiomyopathy (ARVC), suggesting a final common pathway for this disease. However, the underlying molecular processes are poorly understood.

Methods and results

We have identified novel mutations in the desmosomal cadherin desmocollin 2 (DSC2 R203C, L229X, T275M, and G371fsX378). The two missense mutations (DSC2 R203C and T275M) have been functionally characterized, together with a previously reported frameshift variant (DSC2 A897fsX900), to examine their pathogenic potential towards PG's functions at the ID. The three mutant proteins were transiently expressed in various cellular systems and assayed for expression, processing, localization, and binding to other desmosomal components in comparison to wild-type DSC2a protein. The two missense mutations showed defects in proteolytic cleavage, a process which is required for the functional activation of mature cadherins. In both cases, this is thought to cause a reduction of functional DSC2 at the desmosomes in cardiac cells. In contrast, the frameshift variant was incorporated into cardiac desmosomes; however, it showed reduced binding to PG.

Conclusion

Despite different modes of action, for all three variants, the reduced ability to provide a ligand for PG at the desmosomes was observed. This is in agreement with the reduced intensity of PG at these structures observed in ARVC patients.

Inhibiting N-cadherin-mediated adhesion affects gap junction communication in isolated rat hearts

Cadherin-mediated adherens junctions is impaired concomitant with a decrease in connexin 43 (Cx43) in diseases or pathological processes. We have investigated the acute effects of adherens junction impairment in isolated rat hearts by introducing Ala-His-Ala-Val-Asp-NH(2) (AHAVD, a synthetic peptide) as a specific inhibitor of N-cadherin. Effect of AHAVD on N-cadherin mediated adhension was analyzed by Cardiomy-ocyte aggregation assay. Laser confocal microscopy showed disrupted cell-cell contacts in cultured neonatal cardiomyocytes co-incubated with 0.2 mM AHAVD. In isolated adult rat hearts, Cx43 was redistributed along the bilateral of cardiomyocytes from the intercalated discs and significant dephosphorylation of Cx43 on serine368 occurred concomitantly with decreased gap junction (GJ) function in dose dependent manner after 1 h perfusion with AHAVD. These results indicate that impairing cad-herin-mediated adhesion by AHAVD rapidly results in Cx43 redistribution and dephosphorylation of serine368, thereby impairing GJ communication function.

Arrhythmogenic properties of dismantling cadherin-mediated adhesion in murine hearts

Objective

To evaluate the arrhythmogenic effects of dismantling cadherin-mediated adhesion by recombinant mouse aminopeptidase N (rmAPN) in murine hearts.

Methods

rmAPN was incubated with cultured neonatal rat cardiomyocytes as well as being infused in adult mice. The cell-cell connections were immunolabelled and observed by laser confocal microscopy. Disruption of the N-terminal of N-cadherin (N-cad) was detected by western blot and quantitative immunofluorescence. The risk of inducible ventricular tachyarrhythmia was evaluated in mice by an electrophysiological study.

Results

Disrupted cell-cell contact was observed in cultured neonatal rat cardiomyocytes in response to 30-40 ng/µL rmAPN. Loss of the N-terminal in N-cad and altered distribution of connexin 43 (Cx43) were observed in hearts from rmAPN-infused mice. In addition, a reduction of phosphorylated Cx43 was also detected concomitant with redistribution of Cx43. Electrophysiological studies of rmAPN-infused mice showed prolonged QRS duration and increased inducibility of ventricular tachycardias.

Conclusion

Disruption of N-cad by rmAPN contributes to gap junction remodeling and may elicit arrhythmogenic effects. The disorder of adherent junctions by proteolytic enzymes may play an important role in arrhythmogenic mechanisms in correlated diseases.

Oxidized phospholipid species promote in vivo differential cx43 phosphorylation and vascular smooth muscle cell proliferation

Regulation of both the expression and function of connexins in the vascular wall is important during atherosclerosis. Progression of the disease state is marked by vascular smooth muscle cell (VSMC) proliferation, which coincides with the reduced expression levels of connexin 43 (Cx43). However, nothing is currently known about the factors that regulate post-translational modifications of Cx43 in atherogenesis, which could be of particular importance, due to the association between site-specific Cx43 phosphorylation and cellular proliferation. We compared the effects of direct carotid applications of two oxidized phospholipid derivatives, 1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphorylcholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphorylcholine (PGPC), on Cx43 expression and phosphorylation, and on cell proliferation. Since both POVPC and PGPC have been shown to act through different intracellular pathways, we hypothesized that each oxidized phospholipid species could induce differential Cx43 phosphorylation events in the cytoplasmically located carboxyl-terminal region of the protein, which could potentially enhance cell proliferation. Application of POVPC caused a reduction in VSMC Cx43 levels, enhanced its phosphorylation at serine (pS) 279/282, and increased VSMC proliferation both in vivo and in vitro. Treatment with PGPC enhanced VSMC pS368 levels with no associated change in proliferation. These oxidized phospholipid-induced Cx43 post-translational changes in VSMCs were consistent with those identified in ApoE(-/-) mice. Taken together, these results demonstrate that post-translational phosphorylation of Cx43 could be a key factor in the pathogenesis of atherosclerosis.

Disparate effects of different mutations in plakoglobin on cell mechanical behavior

Mutations in genes encoding desmosomal proteins have been implicated in the pathogenesis of heart and skin diseases. This has led to the hypothesis that defective cell-cell adhesion is the underlying cause of injury in tissues that repeatedly bear high mechanical loads. In this study, we examined the effects of two different mutations in plakoglobin on cell migration, stiffness, and adhesion. One is a C-terminal mutation causing Naxos disease, a recessive syndrome of arrhythmogenic right ventricular cardiomyopathy (ARVC) and abnormal skin and hair. The other is an N-terminal mutation causing dominant inheritance of ARVC without cutaneous abnormalities. To assess the effects of plakoglobin mutations on a broad range of cell mechanical behavior, we characterized a model system consisting of stably transfected HEK cells which are particularly well suited for analyses of cell migration and adhesion. Both mutations increased the speed of wound healing which appeared to be related to increased cell motility rather than increased cell proliferation. However, the C-terminal mutation led to dramatically decreased cell-cell adhesion, whereas the N-terminal mutation caused a decrease in cell stiffness. These results indicate that different mutations in plakoglobin have markedly disparate effects on cell mechanical behavior, suggesting complex biomechanical roles for this protein.

In calcineurin-induced cardiac hypertrophy expression of Nav1.5, Cx40 and Cx43 is reduced by different mechanisms

Alterations in expression levels of Na(v)1.5, Cx43 and Cx40 have been frequently reported in cardiac disease and are associated with the development of arrhythmias, but little is known about the underlying molecular mechanisms. In this study we investigated electrical conduction and expression of Na(v)1.5, Cx43 and Cx40 in hearts of transgenic mice overexpressing a constitutively active form of calcineurin (MHC-CnA). ECG recordings showed that atrial, atrioventricular and ventricular activation were significantly prolonged in MHC-CnA hearts as compared to wildtype (WT) littermates. Epicardial activation and arrhythmia susceptibility analysis revealed increased ventricular activation thresholds and arrhythmia vulnerability. Moreover, epicardial ventricular activation patterns in MHC-CnA mice were highly discontinuous with multiple areas of block. These impaired conduction properties were associated with severe reductions in Na(v)1.5, Cx43 and Cx40 protein expression in MHC-CnA hearts as visualized by immunohistochemistry and immunoblotting. Real-time RT-PCR demonstrated that the decreased protein levels for Na(v)1.5 and Cx40, but not for Cx43, were accompanied by corresponding reductions at the RNA level. Cx43 RNA isoform analysis indicated that the reduction in Cx43 protein expression is caused by a post-transcriptional mechanism rather than by RNA isoform switching. In contrast, RNA isoform analysis for Cx40 and Na(v)1.5 provided additional evidence that in calcineurin-induced hypertrophy the downregulation of these proteins originates at the transcriptional level. These results provide the molecular rationale for Na(v)1.5, Cx43 and Cx40 downregulation in this model of hypertrophy and failure and the development of the pro-arrhythmic substrate.

C-Terminal Truncation of Connexin43 Changes Number, Size, and Localization of Cardiac Gap Junction Plaques

Haplodeficient mice expressing carboxyl-terminally truncated Cx43 (K258stop/KO), instead of the wild-type Cx43 isoform, reach adulthood and reveal no abnormalities in heart morphology. Here, we have analyzed the expression of K258stop protein and the morphology of gap junctions in adult hearts of these mice. Coimmunofluorescence analysis revealed reduced juxtaposition of K258stop with other junctional proteins at the intercalated disc. Immunoprecipitation studies documented changes in the interaction with previously described Cx43 binding proteins. Quantitative transmission electron and confocal microscopy confirmed the localization of K258stop gap junctions to the periphery of the intercalated disc and further revealed an increase in the size of K258stop gap junction plaques and a reduction in their number. Dual whole cell patch clamp analysis confirmed that K258stop gap junctions were functional, with single channel properties similar to those described in exogenous systems. We conclude that the carboxyl-terminal domain of Cx43 (Cx43CT) is involved in regulating the localization, number and size of Cx43 plaques in vivo. Conversely, protein interactions or posttranslational modifications taking place within the Cx43CT are not required for the assembly of functional gap junctions in the intercalated disc.

N-cadherin haploinsufficiency affects cardiac gap junctions and arrhythmic susceptibility

Cardiac-specific deletion of the murine gene (Cdh2) encoding the cell adhesion molecule, N-cadherin, results in disassembly of the intercalated disc (ICD) structure and sudden arrhythmic death. Connexin 43 (Cx43)-containing gap junctions are significantly reduced in the heart after depleting N-cadherin, therefore we hypothesized that animals expressing half the normal levels of N-cadherin would exhibit an intermediate phenotype. We examined the effect of N-cadherin haploinsufficiency on Cx43 expression and susceptibility to induced arrhythmias in mice either wild-type or heterozygous for the Cx43 (Gja1)-null allele. An increase in hypophosphorylated Cx43 accompanied by a modest decrease in total Cx43 protein levels was observed in the N-cadherin heterozygous mice. Consistent with these findings N-cadherin heterozygotes exhibited increased susceptibility to ventricular arrhythmias compared to wild-type mice. Quantitative immunofluorescence microscopy revealed a reduction in size of large Cx43-containing plaques in the N-cadherin heterozygous animals compared to wild-type. Gap junctions were further decreased in number and size in the N-cad/Cx43 compound heterozygous mice with increased arrhythmic susceptibility compared to the single mutants. The scaffold protein, ZO-1, was reduced at the ICD in N-cadherin heterozygous cardiomyocytes providing a possible explanation for the reduction in Cx43 plaque size. These data provide further support for the intimate relationship between N-cadherin and Cx43 in the heart, and suggest that germline mutations in the human N-cadherin (Cdh2) gene may predispose patients to increased risk of cardiac arrhythmias.

A novel dominant mutation in plakoglobin causes arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disorder associated with arrhythmias and sudden death. A recessive mutation in the gene encoding plakoglobin has been shown to cause Naxos disease, a cardiocutaneous syndrome characterized by ARVC and abnormalities of hair and skin. Here, we report, for the first time, a dominant mutation in the gene encoding plakoglobin in a German family with ARVC but no cutaneous abnormalities. The mutation (S39_K40insS) is predicted to insert an extra serine residue at position 39 in the N-terminus of plakoglobin. Analysis of a biopsy sample of the right ventricle from the proband showed markedly decreased localization of plakoglobin, desmoplakin, and connexin43 at intercalated discs in cardiac myocytes. A yeast-two-hybrid screen revealed that the mutant protein established novel interactions with histidine-rich calcium-binding protein and TGF beta induced apoptosis protein 2. Immunoblotting and confocal microscopy in human embryonic kidney 293 (HEK293) cell lines transfected to stably express either wild-type or mutant plakoglobin protein showed that the mutant protein was apparently ubiquitylated and was preferentially located in the cytoplasm, suggesting that the S39_K40insS mutation may increase plakoglobin turnover via proteasomal degradation. HEK293 cells expressing mutant plakoglobin also showed higher rates of proliferation and lower rates of apoptosis than did cells expressing the wild-type protein. Electron microscopy showed smaller and fewer desmosomes in cells expressing mutant plakoglobin. Taken together, these observations suggest that the S39_K40insS mutation affects the structure and distribution of mechanical and electrical cell junctions and could interfere with regulatory mechanisms mediated by Wnt-signaling pathways. These results implicate novel molecular mechanisms in the pathogenesis of ARVC.

Loss of mXinalpha, an intercalated disk protein, results in cardiac hypertrophy and cardiomyopathy with conduction defects

The intercalated disk protein Xin was originally discovered in chicken striated muscle and implicated in cardiac morphogenesis. In the mouse, there are two homologous genes, mXinalpha and mXinbeta. The human homolog of mXinalpha, Cmya1, maps to chromosomal region 3p21.2-21.3, near a dilated cardiomyopathy with conduction defect-2 locus. Here we report that mXinalpha-null mouse hearts are hypertrophied and exhibit fibrosis, indicative of cardiomyopathy. A significant upregulation of mXinbeta likely provides partial compensation and accounts for the viability of the mXinalpha-null mice. Ultrastructural studies of mXinalpha-null mouse hearts reveal intercalated disk disruption and myofilament disarray. In mXinalpha-null mice, there is a significant decrease in the expression level of p120-catenin, beta-catenin, N-cadherin, and desmoplakin, which could compromise the integrity of the intercalated disks and functionally weaken adhesion, leading to cardiac defects. Additionally, altered localization and decreased expression of connexin 43 are observed in the mXinalpha-null mouse heart, which, together with previously observed abnormal electrophysiological properties of mXinalpha-deficient mouse ventricular myocytes, could potentially lead to conduction defects. Indeed, ECG recordings on isolated, perfused hearts (Langendorff preparations) show a significantly prolonged QT interval in mXinalpha-deficient hearts. Thus mXinalpha functions in regulating the hypertrophic response and maintaining the structural integrity of the intercalated disk in normal mice, likely through its association with adherens junctional components and actin cytoskeleton. The mXinalpha-knockout mouse line provides a novel model of cardiac hypertrophy and cardiomyopathy with conduction defects.

Aconitine alters connexin43 phosphorylation status and [Ca2+] oscillation patterns in cultured ventricular myocytes of neonatal rats

Aconitine, a highly poisonous type of alkaloid, has a widespread effect in stimulating the membranes of cardiomyocyte. However, other effects of aconitine on cardiomyocyte are unknown. In this study, we investigated whether aconitine also affects the phosphorylation status of connexin43 (Cx43) and intracellular [Ca(2+)] oscillation patterns in cultured ventricular myocytes of neonatal rats. As determined by Western blot analysis, a decreased percentage (47.68+/-2.29%) of phosphorylated Cx43 (P-Cx43) and a concomitant increased percentage (52.32+/-2.29%) of nonphosphorylated Cx43 (NP-Cx43) were found in aconitine-treated cultures, compared to the controls (82.77+/-2.04% for P-Cx43 and 17.23+/-2.04% for NP-Cx43). Quantitative immunofluorescent microscopy revealed similar changes in phosphorylation status occurring in Cx43 containing gap junctions in the cultures under the same treatment conditions. Real-time laser scanning microscopy indicated that intracellular [Ca(2+)] oscillations were relatively stable in control cultures, with occasional calcium sparks; after being treated with aconitine, high frequency [Ca(2+)] oscillations emerged, whereas typical calcium sparks disappeared. Furthermore, Western blot analysis revealed that, after aconitine treatment, the amount of phosphorylated PKCalpha decreased significantly. These observations suggest that aconitine not only induces dephosphorylation of Cx43 and PKCalpha, but also alters intracellular [Ca(2+)] oscillation patterns in cultured cardiomyocytes.

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

BACKGROUND

Ischemic preconditioning delays the onset of electrical uncoupling and prevents loss of the primary ventricular gap junction protein connexin 43 (Cx43) from gap junctions during subsequent ischemia.

OBJECTIVE

To test the hypothesis that these effects are mediated by protein kinase C epsilon (PKCepsilon), we studied isolated Langendorff-perfused hearts from mice with homozygous germline deletion of PKCepsilon (PKCepsilon-KO).

METHODS

Cx43 phosphorylation and distribution were measured by quantitative immunoblotting and confocal microscopy. Changes in electrical coupling were monitored using the 4-electrode technique to measure whole-tissue resistivity.

RESULTS

The amount of Cx43 located in gap junctions, measured by confocal microscopy under basal conditions, was significantly greater in PKCepsilon-KO hearts compared with wild-type, but total Cx43 content measured by immunoblotting was not different. These unanticipated results indicate that PKCepsilon regulates subcellular distribution of Cx43 under normal conditions. Preconditioning prevented loss of Cx43 from gap junctions during ischemia in wild-type but not PKCepsilon-KO hearts. Specific activation of PKCepsilon, but not PKCdelta, also prevented ischemia-induced loss of Cx43 from gap junctions. Preconditioning delayed the onset of uncoupling in wild-type but hastened uncoupling in PKCepsilon-KO hearts. Cx43 phosphorylation at the PKC site Ser368 increased 5-fold after ischemia in wild-type hearts, and surprisingly, by nearly 10-fold in PKCepsilon-KO hearts. Preconditioning prevented phosphorylation of Cx43 in gap junction plaques at Ser368 in wild-type but not PKCepsilon-KO hearts.

CONCLUSION

Taken together, these results indicate that PKCepsilon plays a critical role in preconditioning to preserve Cx43 signal in gap junctions and delay electrical uncoupling during ischemia.

Propagation of the cardiac impulse in the diabetic rat heart: reduced conduction reserve

Diabetes mellitus is a growing epidemic with severe cardiovascular complications. Although much is known about mechanical and electrical cardiac dysfunction in diabetes, few studies have investigated propagation of the electrical signal in the diabetic heart and the associated changes in intercellular gap junctions. This study was designed to investigate these issues, using hearts from control and diabetic rats. Diabetic conditions were induced by streptozotocin (STZ), given i.v. 7-14 days before experiments. Optical mapping with the voltage-sensitive dye di-4-ANEPPS, using hearts perfused on a Langendorff apparatus, showed little change in baseline conduction velocity in diabetic hearts, reflecting the large reserve of function. However, both the gap junction uncoupler heptanol (0.5-1 mM) and elevated potassium (9 mM, to reduce cell excitability) produced a significantly greater slowing of impulse propagation in diabetic hearts than in controls. The maximal action potential upstroke velocity (an index of the sodium current) and resting potential was similar in single ventricular myocytes from control and diabetic rats, suggesting similar electrical excitability. Immunoblotting of connexin 43 (Cx43), a major gap junction component, showed no change in total expression. However, immunofluorescence labelling of Cx43 showed a significant redistribution, apparent as enhanced Cx43 lateralization. This was quantified and found to be significantly larger than in control myocytes. Labelling of two other gap junction proteins, N-cadherin and beta-catenin, showed a (partial) loss of co-localization with Cx43, indicating that enhancement of lateralized Cx43 is associated with non-functional gap junctions. In conclusion, conduction reserve is smaller in the diabetic heart, priming it for impaired conduction upon further challenges. This can desynchronize contraction and contribute to arrhythmogenesis.

Relative contributions of connexins 40 and 43 to atrial impulse propagation in synthetic strands of neonatal and fetal murine cardiomyocytes

Atrial tissue expresses both connexin 40 (Cx40) and 43 (Cx43) proteins. To assess the relative roles of Cx40 and Cx43 in atrial electrical propagation, we synthesized cultured strands of atrial myocytes derived from mice with genetic deficiency in Cx40 or Cx43 expression and measured propagation velocity (PV) by high-resolution optical mapping of voltage-sensitive dye fluorescence. The amount of Cx40 and/or Cx43 in gap junctions was measured by immunohistochemistry and total or sarcolemmal Cx43 or Cx40 protein by immunoblotting. Progressive genetic reduction in Cx43 expression decreased PV from 34+/-6 cm/sec in Cx43(+/+) to 30+/-8 cm/sec in Cx43(+/-) and 19+/-11 cm/sec in Cx43(-/-) cultures. Concomitantly, the cell area occupied by Cx40 immunosignal in gap junctions decreased from 2.0+/-1.6% in Cx43(+/+) to 1.7+/-0.5% in Cx43(+/-) and 1.0+/-0.2% in Cx43(-/-) strands. In contrast, progressive genetic reduction in Cx40 expression increased PV from 30+/-2 cm/sec in Cx40(+/+) to 40+/-7 cm/sec in Cx40(+/-) and 45+/-10 cm/sec in Cx40(-/-) cultures. Concomitantly, the cell area occupied by Cx43 immunosignal in gap junctions increased from 1.2+/-0.9% in Cx40(+/+) to 2.8+/-1.4% in Cx40(+/-) and 3.1+/-0.6% in Cx40(-/-) cultures. In accordance with the immunostaining results, immunoblots of the Triton X-100-insoluble fraction revealed an increase of Cx43 in gap junctions in extracts from Cx40-ablated atria, whereas total cellular Cx43 remained unchanged. Our results suggest that the relative abundance of Cx43 and Cx40 is an important determinant of atrial impulse propagation in neonatal hearts, whereby dominance of Cx40 decreases and dominance of Cx43 increases local propagation velocity.

Knockout of the neural and heart expressed gene HF-1b results in apical deficits of ventricular structure and activation

OBJECTIVE

Knockout of the neural and cardiac expressed transcription factor HF-1b causes electrophysiological abnormalities including fatal ventricular arrhythmias that occur with increasing frequency around the 4th week of postnatal life. This study addresses factors that may contribute to conduction disturbance in the ventricle of the HF-1b knockout mouse. Disruptions to gap junctional connexin40 (Cx40) have been reported in distal (i.e., apically located), but not proximal His-Purkinje conduction tissues of the HF-1b knockout mouse. This abnormality in myocardial Cx40 led us to address whether 4-week-old HF-1b knockout postnates display other disruptions to ventricular structure and function.

METHODS

Western blotting and immunoconfocal quantification of Cx43 and coronary arteriole density and function were undertaken in the ventricle. Electrical activation was described by optical mapping.

RESULTS

Western blotting and immunoconfocal microscopy indicated that overall levels of Cx43 (p<0.001) and percent of Cx43 localized in intercalated disks (p<0.001) were significantly decreased in the ventricular myocardium of knockouts relative to wildtype littermate controls. Analysis of the reduction in Cx43 level by basal and apical territories revealed that the decrease was most pronounced in the lower, apical half of the ventricle of knockouts relative to controls (p<0.001). Myocyte size also showed a significant decrease in the knockout, that was more marked within the apical half of the ventricle (p<0.05). Optical recordings of ventricular activation indicated apically localized sectors of slowed conduction in knockout ventricles not occurring in controls that could be correlated directly to tissues showing reduced Cx43. These discrete sectors of abnormal conduction in the knockout heart were resolved following point stimulation of the ventricular epicardium and thus were not explained by dysfunction of the His-Purkinje system. To further probe base-to-apex abnormalities in the HF-1b knockout ventricle, we analyzed coronary arterial structure and function. These analyses indicated that relative to controls, the apical ventricular territory of the HF-1b knockout had reductions in the density of small resistance vessels (p<0.01) and deficits in arterial function as assayed by bead perfusion (p<0.01).

CONCLUSION

The HF-1b knockout ventricle displays abnormalities in Cx43 level, myocyte size, activation spread and coronary arterial structure and function. These abnormalities tend to be more pronounced in the apical territory of the ventricle and seem likely to be factors contributing to the pathological disturbance of cardiac conduction that characterizes the heart of the HF-1b knockout mouse.

Overexpression of cardiac connexin45 increases susceptibility to ventricular tachyarrhythmias in vivo

Electrophysiological remodeling involving gap junctions has been demonstrated in failing hearts and may contribute to intercellular uncoupling, delayed conduction, enhanced arrhythmias, and vulnerability to sudden death in patients with heart failure. Recently, we showed that failing human hearts exhibit marked increases in connexin45 (Cx45) expression in addition to previously documented decreases in connexin43 (Cx43) expression. Each of these changes results in reduced gap junction coupling. The objective of the present study was to examine functional consequences of increased Cx45 in cardiac gap junctions. Transgenic mice with cardiac-selective overexpression of the developmentally downregulated cardiac connexin, connexin45 (Cx45OE mice) were subjected to in vivo electrophysiology studies in which an intracardiac catheter was used to induce ventricular arrhythmias in anesthetized mice, and in which ambulatory ECG monitoring was used to detect spontaneous arrhythmias in unanesthetized mice. Hearts were analyzed by TaqMan RT-PCR, immunostaining, immunoblotting, and echocardiography. Lucifer yellow and neurobiotin dye transfer was used to assess coupling in transgenic and control myocyte cultures. Cx45 mRNA was two orders of magnitude greater in Cx45OE mice. Cx45-immunoreactive signal at gap junctions increased twofold and total Cx45 protein by immunoblotting increased 25% in Cx45OE mice compared with nontransgenic littermate controls. Functionally, Cx45OE mice exhibited more inducible ventricular tachycardia than controls but did not exhibit any other functional or structural derangements as assessed by echocardiography. Ventricular myocytes isolated from Cx45OE mice exhibited diminished intercellular transfer of Lucifer yellow dye and increased transfer of neurobiotin, consistent with altered cell-to-cell communication. Thus increased myocardial expression of Cx45 results in remodeling of intercellular coupling and greater susceptibility to ventricular arrhythmias in vivo.