Focal gap junction uncoupling and spontaneous ventricular ectopy

Evaluation of the Cardioprotective Effect of Granulocyte Colony Stimulating Factor in Patients with Carbon Monoxide Poisoning

BACKGROUND

Carbon monoxide (CO), which is well known as silent killer, has many toxic effects on organs with high rate of metabolism such as heart and brain. CO-induced cardiotoxicity resulted in a wide range of disabilities including electrocardiogram (ECG) abnormalities, elevation in level of cardiac enzymes, arrhythmias, impairment of left ventricular and myocardial infarction (MI). Cardio-protective effects of Granulocyte colony-stimulating factor (G-CSF) on infarcted heart was proved previously in various reports.

OBJECTIVE

In this study, possible effect of G-CSF on cardiac function of patients with moderate to severe acute CO poisoning was investigated.

METHODS

Cardioprotective effects of G-CSF in CO-poisoned patients was evaluated through ECG, Holter monitoring, echocardiography, and biochemical studies. Continuous intravenous infusion of G-CSF (90 μg/kg) and normal saline were administered respectively to treatment and placebo groups.

RESULTS

The results demonstrated that in moderate to severe CO poisoning, myocardial injury is common. ECG changes (e.g., ST-segment and T-wave changes, QTC), cardiac arrhythmias (e.g., heart blocks and ventricular arrhythmias), serum level of Troponin I, left ventricular ejection fraction were determined after G-CSF administration. Frequencies of ST depression, inversion or flatting of T wave and QTC in ECG were significantly reduced after G-CSF treatment. In addition, incidence of cardiac arrhythmias due to CO poisoning were reduced after G-CSF treatment. However, G-CSF did not exert protective effects on TPI level and function of left ventricular in CO-poisoned patients.

CONCLUSION

GCSF could probably reduce CO-induced cardiac ischemia in patients with acute CO poisoning.

CLINICAL TRIAL REGISTRATION

The trial protocol was registered in the Iranian Registry of Clinical Trials (http://www.irct.ir) registry (Irct ID: IRCT201607232083N7).

MicroRNA-206 Downregulates Connexin43 in Cardiomyocytes to Induce Cardiac Arrhythmias in a Transgenic Mouse Model

BACKGROUND

MicroRNAs (miRNAs) are critical modulators of various physiological and pathological processes, but their role in cardiac arrhythmias remains yet to be completely understood. Connexin43 (Cx43) is an important cardiac gap junction protein and a potential target of miR-206, and downregulation of Cx43 induces ventricular tachyarrhythmias.

METHODS

We investigated the effects of miR-206 overexpression on the adult mouse heart and in cardiac arrhythmias. Luciferase activity assay was employed to validate Cx43 as a direct target of miR-206. Expression of Cx43 was measured in cardiac muscle cell line HL-1 securely expressing miR-206. An inducible miR-206 overexpression mouse model was established to evaluate the in vivo effect of miR-206 on Cx43 expression and cardiac rhythm.

RESULTS

MiR-206 directly recognised 3'-untranslated region of Cx43 mRNA to inhibit its expression in HL-1 cells. Induction of miR-206 in the adult mouse heart suppressed Cx43 expression, particularly in the atria and ventricle. Importantly, miR-206 overexpression also induced abnormal heart-rate and PR interval, and shortened life-span in the experimental mice.

CONCLUSIONS

In cardiomyocytes, miR-206 is a upstream regulator of Cx43, and its overexpression downregulates Cx43 to induce abnormal heart-rate and PR interval.

Disordered Myocardial Ca2+ Homeostasis Results in Substructural Alterations That May Promote Occurrence of Malignant Arrhythmias

We aimed to determine the impact of Ca2+-related disorders induced in intact animal hearts on ultrastructure of the cardiomyocytes prior to occurrence of severe arrhythmias. Three types of acute experiments were performed that are known to be accompanied by disturbances in Ca2+ handling. Langedorff-perfused rat or guinea pig hearts subjected to K+-deficient perfusion to induce ventricular fibrillation (VF), burst atrial pacing to induce atrial fibrillation (AF) and open chest pig heart exposed to intramyocardial noradrenaline infusion to induce ventricular tachycardia (VT). Tissue samples for electron microscopic examination were taken during basal condition, prior and during occurrence of malignant arrhythmias. Cardiomyocyte alterations preceding occurrence of arrhythmias consisted of non-uniform sarcomere shortening, disruption of myofilaments and injury of mitochondria that most likely reflected cytosolic Ca2+ disturbances and Ca2+ overload. These disorders were linked with non-uniform pattern of neighboring cardiomyocytes and dissociation of adhesive junctions suggesting defects in cardiac cell-to-cell coupling. Our findings identified heterogeneously distributed high [Ca2+]i-induced subcellular injury of the cardiomyocytes and their junctions as a common feature prior occurrence of VT, VF or AF. In conclusion, there is a link between Ca2+-related disorders in contractility and coupling of the cardiomyocytes pointing out a novel paradigm implicated in development of severe arrhythmias.

Microinjection Technique for Assessment of Gap Junction Function

Gap junctions are essential for the proper function of many native mammalian tissues including neurons, cardiomyocytes, embryonic tissues, and muscle. Assessing these channels is therefore fundamental to understanding disease pathophysiology, developing therapies for a multitude of acquired and genetic conditions, and providing novel approaches to drug delivery and cellular communication. Microinjection is a robust, albeit difficult, technique, which provides considerable information that is superior to many of the simpler techniques due to its ability to isolate cells, quantify kinetics, and allow cross-comparison of multiple cell lines. Despite its user-dependent nature, the strengths of the technique are considerable and with the advent of new, automation technologies may improve further. This text describes the basic technique of microinjection and briefly discusses modern automation advances that can improve the success rates of this technique.

Can we protect from malignant arrhythmias by modulation of cardiac cell-to-cell coupling?

Defects in intercellular coupling in the heart play a key role in the initiation and persistence of malignant arrhythmias. Such disorders result from abnormal expression and distribution of connexins, the major constituents of cardiac gap junction channels. The alterations of myocardial connexin are well established as a consistent feature of both human and animal heart disease and aging. Following these facts, the modulation of connexin mediated intercellular coupling is suggested as a new antiarrhythmic approach. This review provides recent data supporting this concept. It can be challenging for the development of new antiarrhythmic drugs. Moreover, findings point out the implication of some endogenous compounds in protection from life-threatening arrhythmias via preservation of myocardial connexin.

Connexin mutant embryonic stem cells and human diseases

Intercellular communication via gap junctions allows cells within multicellular organisms to share small molecules. The effect of such interactions has been elucidated using mouse gene knockout strategies. Although several mutations in human gap junction-encoding connexin (Cx) have been described, Cx mutants in mice do not always recapitulate the human disease. Among the 20 mouse Cxs, Cx26, Cx43, and Cx45 play roles in early cardiac or placental development, and disruption of the genes results in lethality that hampers further analyses. Embryonic stem cells (ESCs) that lack Cx43 or Cx45 have made analysis feasible in both in vitro differentiated cell cultures and in vivo chimeric tissues. The success of mouse ESCs studies is leading to the use of induced pluripotent stem cells to learn more about the pathogenesis of human Cx diseases. This review summarizes the current status of mouse Cx disruption models and ESC differentiation studies, and discusses their implication for understanding human Cx diseases.

Dietary omega-3 fatty acids attenuate myocardial arrhythmogenic factors and propensity of the heart to lethal arrhythmias in a rodent model of human essential hypertension

OBJECTIVE

Hypertension-induced myocardial remodeling is known to be associated with increased risk for malignant arrhythmias and alterations in electrical coupling protein, connexin-43 (Cx43), may be involved. We investigated whether omega-3 fatty acids intake affects abnormalities of Cx43 as well as protein kinase C (PKC) signaling and myosin heavy chain (MyHC) profile at the early and late stage of hypertension in the context of the heart's susceptibility to ventricular fibrillation and ability to restore sinus rhythm.

METHODS

Untreated young and old male spontaneously hypertensive rats (SHRs) and age-matched normotensive rats were compared with animals supplemented by omega-3 (eicosapentaneoic acid + docosahexaneoic acid, 200 mg/kg body weight/day) for 2 months. Left ventricular tissues were taken for examination of subcellular integrity of gap junctions, Cx43 mRNA and protein expression, PKCε and PKCδ as well as MyHC determination. Electrically inducible ventricular fibrillation and sinus rhythm restoration (SRR) were examined on Langedorff-perfused heart preparation.

RESULTS

Omega-3 intake significantly reduced cardiovascular risk factors, suppressed inducible ventricular fibrillation, and facilitated SRR in hypertensive rats. Supplementation attenuated lateralization and internalization of Cx43, suppressed elevated Cx43 mRNA, enhanced total Cx43 protein expression and/or expression of its functional phosphorylated forms as well as the expression of cardioprotective PKC-ε and suppressed pro-apoptotic PKC-δ isoform. Moreover, the omega-3 diet normalized MyHC profiles in SHR at early stage of disease and old nonhypertensive rats, but failed to do so in old SHR at late stage of disease.

CONCLUSION

Findings suggest that amelioration of myocardial Cx43-related abnormalities, positive modulation of PKC pathways, and normalization of MyHC can significantly contribute to the antiarrhythmic effects of omega-3 in rat model mimicking human essential hypertension. Our results support the prophylactic use of omega-3 to minimize cardiovascular risk and sudden arrhythmic death.

Downregulation of connexin43 by microRNA-130a in cardiomyocytes results in cardiac arrhythmias

MicroRNAs (miRNAs) are now recognized as critical regulators of diverse physiological and pathological processes; however, studies of miRNAs and arrhythmogenesis remain sparse. Connexin43 (Cx43), a major cardiac gap junction protein, has elicited great interest in its role in arrhythmias. Additionally, Cx43 was a potential target for miR-130a as predicted by several computational algorithms. This study investigates the effect of miR-130a overexpression in the adult heart and its effect on cardiac rhythm. Using a cardiac-specific inducible system, transgenic mice demonstrated both atrial and ventricular arrhythmias. We performed ventricular-programmed electrical stimulation and found that the αMHC-miR130a mice developed sustained ventricular tachycardia beginning 6weeks after overexpression. Western blot analysis demonstrated a steady decline in Cx43 after 2weeks of overexpression with over a 90% reduction in Cx43 levels by 10weeks. Immunofluorescent staining confirmed a near complete loss of Cx43 throughout the heart. To validate Cx43 as a direct target of miR-130a, we performed in vitro target assays in 3T3 fibroblasts and HL-1 cardiomyocytes, both known to endogenously express miR-130a. Using a luciferase reporter fused to the 3'UTR of Cx43, we found a 52.9% reduction in luciferase activity in 3T3 cells (p<0.0001) and a 47.6% reduction in HL-1 cells (p=0.0056) compared to controls. Addition of an antisense miR-130a inhibitor resulted in a loss of inhibitory activity of the Cx43 3'UTR reporter. We have identified an unappreciated role for miR-130a as a direct regulator of Cx43. Overexpression of miR-130a may contribute importantly to gap junction remodeling and to the pathogenesis of atrial and ventricular arrhythmias.

High-throughput measurement of gap junctional intercellular communication

Gap junctional intercellular communication (GJIC) is a critical part of cellular activities and is necessary for electrical propagation among contacting cells. Disorders of gap junctions are a major cause for cardiac arrhythmias. Dye transfer through microinjection is a conventional technique for measuring GJIC. To overcome the limitations of manual microinjection and perform high-throughput GJIC measurement, here we present a new robotic microinjection system that is capable of injecting a large number of cells at a high speed. The highly automated system enables large-scale cell injection (thousands of cells vs. a few cells) without major operator training. GJIC of three cell lines of differing gap junction density, i.e., HeLa, HEK293, and HL-1, was evaluated. The effect of a GJIC inhibitor (18-α-glycyrrhetinic acid) was also quantified in the three cell lines. System operation speed, success rate, and cell viability rate were quantitatively evaluated based on robotic microinjection of over 4,000 cells. Injection speed was 22.7 cells per min, with 95% success for cell injection and >90% survival. Dye transfer cell counts and dye transfer distance correlated with the expected connexin expression of each cell type, and inhibition of dye transfer correlated with the concentration of GJIC inhibitor. Additionally, real-time monitoring of dye transfer enables the calculation of coefficients of molecular diffusion through gap junctions. This robotic microinjection dye transfer technique permits rapid assessment of gap junction function in confluent cell cultures.

A microstructural model of reentry arising from focal breakthrough at sites of source-load mismatch in a central region of slow conduction

Regions of cardiac tissue that have a combination of focal activity and poor, heterogeneous gap junction coupling are often considered to be arrhythmogenic; however, the relationship between the properties of the cardiac microstructure and patterns of abnormal propagation is not well understood. The objective of this study was to investigate the effect of microstructure on the initiation of reentry from focal stimulation inside a poorly coupled region embedded in more well-coupled tissue. Two-dimensional discrete computer models of ventricular monolayers (1 × 1 cm) were randomly generated to represent heterogeneity in the cardiac microstructure. A small, central poorly coupled patch (0.40 × 0.40 cm) was introduced to represent the site of focal activity. Simulated unipolar electrogram recordings were computed at various points in the tissue. As the gap conductance of the patch decreased, conduction slowed and became increasingly complex, marked by fractionated electrograms with reduced amplitude. Near the limit of conduction block, isolated breakthrough sites occurred at single cells along the patch boundary and were marked by long cell-to-cell delays and negative deflections on electrogram recordings. The strongest determinant of the site of wavefront breakthrough was the connectivity of the brick wall architecture, which enabled current flow through small regions of overlapping cells to drive propagation into the well-coupled zone. In conclusion, breakthroughs at the size scale of a single cell can occur at the boundary of source-load mismatch allowing focal activations from slow conducting regions to produce reentry. These breakthrough regions, identifiable by distinct asymmetric, reduced amplitude electrograms, are sensitive to tissue architecture and may be targets for ablation.

Connexin diversity in the heart: insights from transgenic mouse models

Cardiac conduction is mediated by gap junction channels that are formed by connexin (Cx) protein subunits. The connexin family of proteins consists of more than 20 members varying in their biophysical properties and ability to combine with other connexins into heteromeric gap junction channels. The mammalian heart shows regional differences both in connexin expression profile and in degree of electrical coupling. The latter reflects functional requirements for conduction velocity which needs to be low in the sinoatrial and atrioventricular nodes and high in the ventricular conduction system. Over the past 20 years knowledge of the biology of gap junction channels and their role in the genesis of cardiac arrhythmias has increased enormously. This review focuses on the insights gained from transgenic mouse models. The mouse heart expresses Cx30, 30.2, 37, 40, 43, 45, and 46. For these connexins a variety of knock-outs, heart-specific knock-outs, conditional knock-outs, double knock-outs, knock-ins and overexpressors has been studied. We discuss the cardiac phenotype in these models and compare Cx expression between mice and men. Mouse models have enhanced our understanding of (patho)-physiological implications of Cx diversity in the heart. In principle connexin-specific modulation of electrical coupling in the heart represents an interesting treatment strategy for cardiac arrhythmias and conduction disorders.

Designer gap junctions that prevent cardiac arrhythmias

Cardiac gap junctions are specialized membrane structures comprised of arrays of intercellular channels responsible for propagation of the cardiac impulse. These channels are formed by oligomerization of individual protein subunits known as connexins. In response to a broad array of pathologic stressors, gap junction expression is disturbed, resulting in aberrant cardiac conduction and increased propensity for rhythm disturbances. In this article, we review some of the recently identified molecular regulators of connexin assembly, membrane targeting, and degradation, focusing on the role of post-translational phosphorylation of connexin 43, the major gap junctional protein expressed in ventricular myocardium. We also describe efforts to engineer "designer" gap junctions that are resistant to pathologic remodeling.

Cardiomyocyte-specific expression of lamin a improves cardiac function in Lmna-/- mice

Lmna(-/-) mice display multiple tissue defects and die by 6-8 weeks of age reportedly from dilated cardiomyopathy with associated conduction defects. We sought to determine whether restoration of lamin A in cardiomyocytes improves cardiac function and extends the survival of Lmna(-/-) mice. We observed increased total desmin protein levels and disorganization of the cytoplasmic desmin network in ~20% of Lmna(-/-) ventricular myocytes, rescued in a cell-autonomous manner in Lmna(-/-) mice expressing a cardiac-specific lamin A transgene (Lmna(-/-); Tg). Lmna(-/-); Tg mice displayed significantly increased contractility and preservation of myocardial performance compared to Lmna(-/-) mice. Lmna(-/-); Tg mice attenuated ERK1/2 phosphorylation relative to Lmna(-/-) mice, potentially underlying the improved localization of connexin43 to the intercalated disc. Electrocardiographic recordings from Lmna(-/-) mice revealed arrhythmic events and increased frequency of PR interval prolongation, which is partially rescued in Lmna(-/-); Tg mice. These findings support our observation that Lmna(-/-); Tg mice have a 12% median extension in lifespan compared to Lmna(-/-) mice. While significant, Lmna(-/-); Tg mice only have modest improvement in cardiac function and survival likely stemming from the observation that only 40% of Lmna(-/-); Tg cardiomyocytes have detectable lamin A expression. Cardiomyocyte-specific restoration of lamin A in Lmna(-/-) mice improves heart-specific pathology and extends lifespan, demonstrating that the cardiac pathology of Lmna(-/-) mice limits survival. The expression of lamin A is sufficient to rescue certain cellular defects associated with loss of A-type lamins in cardiomyocytes in a cell-autonomous fashion.

Connexin43 mutation causes heterogeneous gap junction loss and sudden infant death

BACKGROUND

An estimated 10% to 15% of sudden infant death syndrome (SIDS) cases may stem from channelopathy-mediated lethal arrhythmias. Loss of the GJA1-encoded gap junction channel protein connexin43 is known to underlie formation of lethal arrhythmias. GJA1 mutations have been associated with cardiac diseases, including atrial fibrillation. Therefore, GJA1 is a plausible candidate gene for premature sudden death.

METHODS AND RESULTS

GJA1 open reading frame mutational analysis was performed with polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing on DNA from 292 SIDS cases. Immunofluorescence and dual whole-cell patch-clamp studies were performed to determine the functionality of mutant gap junctions. Immunostaining for gap junction proteins was performed on SIDS-associated paraffin-embedded cardiac tissue. Two rare, novel missense mutations, E42K and S272P, were detected in 2 of 292 SIDS cases, a 2-month-old white boy and a 3-month-old white girl, respectively. Analysis of the E42K victim's parental DNA demonstrated a de novo mutation. Both mutations involved highly conserved residues and were absent in >1000 ethnically matched reference alleles. Immunofluorescence demonstrated no trafficking abnormalities for either mutation, and S272P demonstrated wild-type junctional conductance. However, junctional conductance measurements for the E42K mutation demonstrated a loss of function not rescued by wild type. Moreover, the E42K victim's cardiac tissue demonstrated a mosaic immunostaining pattern for connexin43 protein.

CONCLUSIONS

This study provides the first molecular and functional evidence implicating a GJA1 mutation as a novel pathogenic substrate for SIDS. E42K-connexin43 demonstrated a trafficking-independent reduction in junctional coupling in vitro and a mosaic pattern of mutational DNA distribution in deceased cardiac tissue, suggesting a novel mechanism of connexin43-associated sudden death.

Myocardial performance is reduced immediately prior to ventricular ectopy

BACKGROUND

We recently demonstrated local voltage potentials indicating conduction impairment and block in the sinus beats preceding ventricular premature beats (VPBs) originating in the ventricular outflow tracts.

OBJECTIVE

The purpose of this study was to test the hypothesis that impairment of impulse conduction would also lead to changes in the contractile performance of sinus beats preceding ventricular ectopy using Tissue Doppler echocardiography.

METHODS

Twenty-three consecutive patients with VPBs were examined in the apical 4-chamber view with a frame rate of 150 Hz (GE VIVID VII). Eleven patients had no structural heart disease, 5 had dilated cardiomyopathy, 4 had ischemic heart disease, 2 had arrhythmogenic right ventricular dysplasia, and 1 had aortic stenosis. The ectopy originated in the ventricular outflow tracts in 15 patients and in the left ventricle 8. Eleven of the patients underwent radiofrequency ablation of the VPBs.

RESULTS

Tissue Doppler imaging demonstrated a highly statistically significant decrease in myocardial performance in the last sinus beat before the VPB compared to earlier sinus beats. Thus, ejection time (time to peak end-systolic contraction) and peak systolic velocity shortened significantly (P <.001 for both) with a subsequent reduction in systolic shortening (end-systolic displacement; P <.001).

CONCLUSION

Ventricular ectopy is preceded by a significant decrease in myocardial performance in the last sinus beat preceding VPBs as observed in consecutive patients with a broad variety of heart conditions pointing to a mutual underlying electrical mechanism (ie, localized conduction block confined to an area surrounding the ectopic pacemaker).

Connexin43 mutation causes heterogeneous gap junction loss and sudden infant death

BACKGROUND

An estimated 10% to 15% of sudden infant death syndrome (SIDS) cases may stem from channelopathy-mediated lethal arrhythmias. Loss of the GJA1-encoded gap junction channel protein connexin43 is known to underlie formation of lethal arrhythmias. GJA1 mutations have been associated with cardiac diseases, including atrial fibrillation. Therefore, GJA1 is a plausible candidate gene for premature sudden death.

METHODS AND RESULTS

GJA1 open reading frame mutational analysis was performed with polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing on DNA from 292 SIDS cases. Immunofluorescence and dual whole-cell patch-clamp studies were performed to determine the functionality of mutant gap junctions. Immunostaining for gap junction proteins was performed on SIDS-associated paraffin-embedded cardiac tissue. Two rare, novel missense mutations, E42K and S272P, were detected in 2 of 292 SIDS cases, a 2-month-old white boy and a 3-month-old white girl, respectively. Analysis of the E42K victim's parental DNA demonstrated a de novo mutation. Both mutations involved highly conserved residues and were absent in >1000 ethnically matched reference alleles. Immunofluorescence demonstrated no trafficking abnormalities for either mutation, and S272P demonstrated wild-type junctional conductance. However, junctional conductance measurements for the E42K mutation demonstrated a loss of function not rescued by wild type. Moreover, the E42K victim's cardiac tissue demonstrated a mosaic immunostaining pattern for connexin43 protein.

CONCLUSIONS

This study provides the first molecular and functional evidence implicating a GJA1 mutation as a novel pathogenic substrate for SIDS. E42K-connexin43 demonstrated a trafficking-independent reduction in junctional coupling in vitro and a mosaic pattern of mutational DNA distribution in deceased cardiac tissue, suggesting a novel mechanism of connexin43-associated sudden death.

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.

Increased Cell-Cell Coupling Increases Infarct Size and Does not Decrease Incidence of Ventricular Tachycardia in Mice

Increasing connexin43 (Cx43) gap junctional conductance as a means to improve cardiac conduction has been proposed as a novel antiarrhythmic modality. Yet, transmission of molecules via gap junctions may be associated with increased infarct size. To determine whether maintaining open gap junction channels impacts on infarct size and induction of ventricular tachycardia (VT) following coronary occlusion, we expressed the pH- and voltage-independent connexin isoform connexin32 (Cx32) in ventricle and confirmed Cx32 expression. Wild-type (WT) mice injected with adenovirus-Cx32 (Cx32inj) were examined following coronary occlusion to determine infarct size and inducibility of VT. There was an increased infarct size in Cx32inj hearts as compared to WT (WT 22.9 ± 4%; Cx32inj 44.3 ± 5%; p < 0.05). Programmed electrical stimulation showed no difference in VT inducibility in WT and Cx32inj mice (VT was reproducibly inducible in 55% of shams and 50% of Cx32inj mice (p > 0.05). Following coronary occlusion, improving cell–cell communication increased infarct size, and conferred no antiarrhythmic benefit.

Connexin40 and connexin43 determine gating properties of atrial gap junction channels

While ventricular gap junctions contain only Cx43, atrial gap junctions contain both Cx40 and Cx43; yet the functional consequences of this co-expression remain poorly understood. We quantitated the expression of Cx40 and Cx43 and their contributions to atrial gap junctional conductance (g(j)). Neonatal murine atrial myocytes showed similar abundances of Cx40 and Cx43 proteins, while ventricular myocytes contained at least 20 times more Cx43 than Cx40. Since Cx40 gap junction channels are blocked by 2 mM spermine while Cx43 channels are unaffected, we used spermine block as a functional dual whole cell patch clamp assay to determine Cx40 contributions to cardiac g(j). Slightly more than half of atrial g(j) and <or=20% of ventricular g(j) were inhibited. In myocytes from Cx40 null mice, the inhibition of ventricular g(j) was completely abolished, and the block of atrial g(j) was reduced to <20%. Compared to ventricular gap junctions, the transjunctional voltage (V(j))-dependent inactivation of atrial g(j) was reduced and kinetically slowed, while the V(j)-dependence of fast and slow inactivation was unchanged. We conclude that Cx40 and Cx43 are equally abundant in atrium and make similar contributions to atrial g(j). Co-expression of Cx40 accounts for most, but not all, of the differences in the V(j)-dependent gating properties between atrium and ventricle that may play a role in the genesis of slow myocardial conduction and arrhythmias.

Decreased connexin43 expression in the mouse heart potentiates pacing-induced remodeling of repolarizing currents

Gap junction redistribution and reduced expression, a phenomenon termed gap junction remodeling (GJR), is often seen in diseased hearts and may predispose toward arrhythmias. We have recently shown that short-term pacing in the mouse is associated with changes in connexin43 (Cx43) expression and localization but not with increased inducibility into sustained arrhythmias. We hypothesized that short-term pacing, if imposed on murine hearts with decreased Cx43 abundance, could serve as a model for evaluating the electrophysiological effects of GJR. We paced wild-type (normal Cx43 abundance) and heterozygous Cx43 knockout (Cx43^+/−; 66% mean reduction in Cx43) mice for 6 h at 10–15% above their average sinus rate. We investigated the electrophysiological effects of pacing on the whole animal using programmed electrical stimulation and in isolated ventricular myocytes with patch-clamp studies. Cx43^+/− myocytes had significantly shorter action potential durations (APD) and increased steady-state (I_ss) and inward rectifier (I_K1) potassium currents compared with those of wild-type littermate cells. In Cx43^+/− hearts, pacing resulted in a significant prolongation of ventricular effective refractory period and APD and significant diminution of I_ss compared with unpaced Cx43^+/− hearts. However, these changes were not seen in paced wild-type mice. These data suggest that Cx43 abundance plays a critical role in regulating currents involved in myocardial repolarization and their response to pacing. Our study may aid in understanding how dyssynchronous activation of diseased, Cx43-deficient myocardial tissue can lead to electrophysiological changes, which may contribute to the worsened prognosis often associated with pacing in the failing heart.

Enhancement of ventricular gap-junction coupling by rotigaptide

AIMS

Rotigaptide is proposed to exert its anti-arrhythmic effects by improving myocardial gap-junction communication. To directly investigate the mechanisms of rotigaptide action, we treated cultured neonatal murine ventricular cardiomyocytes with clinical pharmacological doses of rotigaptide and directly determined its effects on gap-junctional currents.

METHODS AND RESULTS

Neonatal murine ventricular cardiomyocytes were enzymatically isolated and cultured for 1-4 days. Primary culture cell pairs were subjected to dual whole cell patch-clamp procedures to directly measure gap-junctional currents (I(j)) and voltage (V(j)). Rotigaptide (0-350 nM) was applied overnight or acutely perfused into 35 mm culture dishes. Rotigaptide (35-100 nM) acutely and chronically increased the resting gap-junction conductance (g(j)), and normalized steady-state minimum g(j) (G(min)) by 5-20%. Higher concentrations produced a diminishing response, which mimics the observed therapeutic efficacy of the drug. The inactivation kinetics was similarly slowed in a therapeutic concentration-dependent manner without affecting the V(j) dependence of inactivation or recovery. The effects of 0-100 nM rotigaptide on ventricular g(j) during cardiac action potential propagation were accurately modelled by computer simulations which demonstrate that clinically effective concentrations of rotigaptide can partially reverse conduction slowing due to decreases in g(j) and inactivation.

CONCLUSION

These results demonstrate that therapeutic concentrations of rotigaptide increase the resting gap-junction conductance and reduce the magnitude and kinetics of steady-state inactivation in a concentration-dependent manner. Rotigaptide may be effective in treating re-entrant forms of cardiac arrhythmias by improving conduction and preventing the formation of re-entrant circuits in partially uncoupled myocardium.

Targeted deletion of kcne2 impairs ventricular repolarization via disruption of I(K,slow1) and I(to,f)

Mutations in human KCNE2, which encodes the MiRP1 potassium channel ancillary subunit, associate with long QT syndrome (LQTS), a defect in ventricular repolarization. The precise cardiac role of MiRP1 remains controversial, in part, because it has marked functional promiscuity in vitro. Here, we disrupted the murine kcne2 gene to define the role of MiRP1 in murine ventricles. kcne2 disruption prolonged ventricular action potential duration (APD), suggestive of reduced repolarization capacity. Accordingly, kcne2 (-/-) ventricles exhibited a 50% reduction in I(K,slow1), generated by Kv1.5--a previously unknown partner for MiRP1. I(to,f), generated by Kv4 alpha subunits, was also diminished, by approximately 25%. Ventricular MiRP1 protein coimmunoprecipitated with native Kv1.5 and Kv4.2 but not Kv1.4 or Kv4.3. Unexpectedly, kcne2 (-/-) ventricular membrane fractions exhibited 50% less mature Kv1.5 protein than wild type, and disruption of Kv1.5 trafficking to the intercalated discs. Consistent with the reduction in ventricular K(+) currents and prolonged ventricular APD, kcne2 deletion lengthened the QT(c) under sevoflurane anesthesia. Thus, targeted disruption of kcne2 has revealed a novel cardiac partner for MiRP1, a novel role for MiRPs in alpha subunit targeting in vivo, and a role for MiRP1 in murine ventricular repolarization with parallels to that proposed for the human heart.

Short-term pacing in the mouse alters cardiac expression of connexin43

Background

Cardiac insults such as ischemia, infarction, hypertrophy and dilatation are often accompanied by altered abundance and/or localization of the connexin43 gap junction protein, which may predispose towards arrhythmic complications. Models of chronic dyssynchronous cardiac activation have also been shown to result in redistribution of connexin43 in cardiomyocytes. We hypothesized that alterations in connexin43 expression and localization in the mouse heart might be induced by ventricular pacing over a short period of time.

Results

The subdiaphragmatic approach was used to pace a series of wild type mice for six hours before the hearts were removed for analysis. Mice were paced at 10–15% above their average anesthetized sinus rate and monitored to ensure 1:1 capture. Short-term pacing resulted in a significant reduction in connexin43 mRNA abundance, a partial redistribution of connexin43 from the sarcolemma to a non-sarcolemmal fraction, and accumulation of ubiquitinated connexin43 without a significant change in overall connexin43 protein levels. These early pacing-induced changes in connexin43 expression were not accompanied by decreased cardiac function, prolonged refractoriness or increased inducibility into sustained arrhythmias.

Conclusion

Our data suggest that short-term pacing is associated with incipient changes in the expression of the connexin43 gap junction, possibly including decreased production and a slowed rate of degradation. This murine model may facilitate the study of early molecular changes induced by pacing and may ultimately assist in the development of strategies to prevent gap junction remodeling and the associated arrhythmic complications of cardiac disease.

Electrical remodeling contributes to complex tachyarrhythmias in connexin43-deficient mouse hearts

Loss of connexin43 (Cx43) gap junction channels in the heart results in a marked increase in the incidence of spontaneous and inducible polymorphic ventricular tachyarrhythmias (PVTs). The mechanisms resulting in this phenotype remain unclear. We hypothesized that uncoupling promotes regional ion channel remodeling, thereby increasing electrical heterogeneity and facilitating the development of PVT. In isolated-perfused control hearts, programmed electrical stimulation elicited infrequent monomorphic ventricular tachyarrhythmias (MVT), and dominant frequencies (DFs) during MVT were similar in the right ventricle (RV) and left ventricle (LV). Moreover, conduction properties, action potential durations (APDs), and repolarizing current densities were similar in RV and LV myocytes. In contrast, PVT was common in Cx43 conditional knockout (OCKO) hearts, and arrhythmias were characterized by significantly higher DFs in the RV compared to the LV. APDs in OCKO myocytes were significantly shorter than those from chamber-matched controls, with RV OCKO myocytes being most affected. APD shortening was associated with higher levels of sustained current in myocytes from both chambers as well as higher levels of the inward rectifier current only in RV myocytes. Thus, alterations in cell-cell coupling lead to regional changes in potassium current expression, which in this case facilitates the development of reentrant arrhythmias. We propose a new mechanistic link between electrical uncoupling and ion channel remodeling. These findings may be relevant not only in cardiac tissue but also to other organ systems where gap junction remodeling is known to occur.

Abnormal conduction and morphology in the atrioventricular node of mice with atrioventricular canal targeted deletion of Alk3/Bmpr1a receptor

BACKGROUND

The atrioventricular (AV) node is essential for the sequential excitation and optimized contraction of the adult multichambered heart; however, relatively little is known about its formation from the embryonic AV canal. A recent study demonstrated that signaling by Alk3, the type 1a receptor for bone morphogenetic proteins, in the myocardium of the AV canal was required for the development of both the AV valves and annulus fibrosus. To test the hypothesis that bone morphogenetic protein signaling also plays a role in AV node formation, we investigated conduction system function and AV node morphology in adult mice with conditional deletion of Alk3 in the AV canal.

METHODS AND RESULTS

High-resolution optical mapping with correlative histological analysis of 28 mutant hearts revealed 4 basic phenotypic classes based on electrical activation patterns and volume-conducted ECGs. The frequency of AV node conduction and morphological abnormalities increased from no detectable anomalies (class I) to severe defects (class IV), which included the presence of bypass tracts, abnormal ventricular activation patterns, fibrosis of the AV node, and twin AV nodes.

CONCLUSIONS

The present findings demonstrate that bone morphogenetic protein signaling is required in the myocardium of the AV canal for proper AV junction development, including the AV node.

Stem cells and their potential relevance to paediatric cardiology

Basic scientists, as well as cardiologists, are caught by the idea of curing ischaemic heart disease with cardiac progenitor or stem cells. This short review provides an overview of our current knowledge on the potential use of stem cells for cardiac disease. Since, in infants and children, aetiologies and pathomechanisms of critical cardiac disease are fundamentally different from those in adults, we will also address the question as to whether such young patients could be a therapeutic target at all, and in which respect it may be necessary to view treatment with stem cells from a different stance in the developing organism.

Dysregulation of cell adhesion proteins and cardiac arrhythmogenesis

Proper mechanical and electrical coupling of cardiomyocytes is crucial for normal propagation of the electrical impulse throughout the working myocardium. Various proteins on the surface of cardiomyocytes are responsible for the integration of structural information and cell-cell communication. Increasing evidence from diseased myocardium and animal models indicates that alteration in electrical coupling via gap junctions is a critical determinant in the development of an arrhythmogenic substrate. What is less clear is how gap junctions are maintained and regulated in the working myocardium. In this review, we present data from human disease and animal models that support the idea that cell adhesion proteins regulate the stability of the gap junction protein, connexin.