Slow conduction and enhanced anisotropy increase the propensity for ventricular tachyarrhythmias in adult mice with induced deletion of connexin43

Strain-controlled electrophysiological wave propagation alters in silico scar-based substrate for ventricular tachycardia

Introduction: Assessing a patient's risk of scar-based ventricular tachycardia (VT) after myocardial infarction is a challenging task. It can take months to years after infarction for VT to occur. Also, if selected for ablation therapy, success rates are low. Methods: Computational ventricular models have been presented previously to support VT risk assessment and to provide ablation guidance. In this study, an extension to such virtual-heart models is proposed to phenomenologically incorporate tissue remodeling driven by mechanical load. Strain amplitudes in the heart muscle are obtained from simulations of mechanics and are used to adjust the electrical conductivity. Results: The mechanics-driven adaptation of electrophysiology resulted in a more heterogeneous distribution of propagation velocities than that of standard models, which adapt electrophysiology in the structural substrate from medical images only. Moreover, conduction slowing was not only present in such a structural substrate, but extended in the adjacent functional border zone with impaired mechanics. This enlarged the volumes with high repolarization time gradients (≥10 ms/mm). However, maximum gradient values were not significantly affected. The enlarged volumes were localized along the structural substrate border, which lengthened the line of conduction block. The prolonged reentry pathways together with conduction slowing in functional regions increased VT cycle time, such that VT was easier to induce, and the number of recommended ablation sites increased from 3 to 5 locations. Discussion: Sensitivity testing showed an accurate model of strain-dependency to be critical for low ranges of conductivity. The model extension with mechanics-driven tissue remodeling is a potential approach to capture the evolution of the functional substrate and may offer insight into the progression of VT risk over time.

Parishin treatment alleviates cardiac aging in naturally aged mice

Background

Cardiac aging progressively decreases physiological function and drives chronic/degenerative aging-related heart diseases. Therefore, it is crucial to postpone the aging process of heart and create products that combat aging.

Aims & methods

The objective of this study is to examine the effects of parishin, a phenolic glucoside isolated from traditional Chinese medicine Gastrodia elata, on anti-aging and its underlying mechanism. To assess the senescent biomarkers, cardiac function, cardiac weight/body weight ratio, cardiac transcriptomic changes, and cardiac histopathological features, heart tissue samples were obtained from young mice (12 weeks), aged mice (19 months) treated with parishin, and aged mice that were not treated.

Results

Parishin treatment improved cardiac function, ameliorated aging-induced cardiac injury, hypertrophy, and fibrosis, decreased cardiac senescence biomarkers p16Ink4a, p21Cip1, and IL-6, and increased the "longevity factor" SIRT1 expression in heart tissue. Furthermore, the transcriptomic analysis demonstrated that parishin treatment alleviated the cardiac aging-related Gja1 downregulation and Cyp2e1, Ccna2, Cdca3, and Fgf12 upregulation in the heart tissues. The correlation analysis suggested a strong connection between the anti-aging effect of parishin and its regulation of gut microbiota and metabolism in the aged intestine.

Conclusion

The present study demonstrates the protective role and underlying mechanism of parishin against cardiac aging in naturally aged mice.

Interindividual Age-Independent Differences in Human CX43 Impact Ventricular Arrhythmic Risk

Connexin 43 (CX43) is one of the major components of gap junctions, the structures responsible for the intercellular communication and transmission of the electrical impulse in the left ventricle. There is limited information on the histological changes of CX43 with age and their effect on electrophysiology, especially in humans. Here, we analyzed left ventricular biopsies from living donors starting at midlife to characterize age-related CX43 remodeling. We assessed its quantity, degree of lateralization, and spatial heterogeneity together with fibrotic deposition. We observed no significant age-related remodeling of CX43. Only spatial heterogeneity increased slightly with age, and this increase was better explained by biological age than by chronological age. Importantly, we found that CX43 features varied considerably among individuals in our population with no relevant relationship to age or fibrosis content, in contrast to animal species. We used our experimental results to feed computational models of human ventricular electrophysiology and to assess the effects of interindividual differences in specific features of CX43 and fibrosis on conduction velocity, action potential duration, and arrhythmogenicity. We found that larger amounts of fibrosis were associated with the highest arrhythmic risk, with this risk being increased when fibrosis deposition was combined with a reduction in CX43 amount and/or with an increase in CX43 spatial heterogeneity. These mechanisms underlying high arrhythmic risk in some individuals were not associated with age in our study population. In conclusion, our data rule out CX43 remodeling as an age-related arrhythmic substrate in the population beyond midlife, but highlight its potential as a proarrhythmic factor at the individual level, especially when combined with increased fibrosis.

Cooling Down Inflammation in the Cardiovascular System via the Nicotinic Acetylcholine Receptor

ABSTRACT

Inflammation is a major player in many cardiovascular diseases including hypertension, atherosclerosis, myocardial infarction, and heart failure. In many individuals, these conditions coexist and mutually exacerbate each other's progression. The pathophysiology of these diseases entails the active involvement of both innate and adaptive immune cells. Immune cells that possess the α7 subunit of the nicotinic acetylcholine receptor on their surface have the potential to be targeted through both pharmacological and electrical stimulation of the cholinergic system. The cholinergic system regulates the inflammatory response to various stressors in different organ systems by systematically suppressing spleen-derived monocytes and chemokines and locally improving immune cell function. Research on the cardiovascular system has demonstrated the potential for atheroma plaque stabilization and regression as favorable outcomes. Smaller infarct size and reduced fibrosis have been associated with improved cardiac function and a decrease in adverse cardiac remodeling. Furthermore, enhanced electrical stability of the myocardium can lead to a reduction in the incidence of ventricular tachyarrhythmia. In addition, improving mitochondrial dysfunction and decreasing oxidative stress can result in less myocardial tissue damage caused by reperfusion injury. Restoring baroreflex activity and reduction in renal damage can promote blood pressure regulation and help counteract hypertension. Thus, the present review highlights the potential of nicotinic acetylcholine receptor activation as a natural approach to alleviate the adverse consequences of inflammation in the cardiovascular system.

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

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

Age-related increases in cardiac excitability, refractoriness and impulse conduction favor arrhythmogenesis in male rats

The effects of excitability, refractoriness, and impulse conduction have been independently related to enhanced arrhythmias in the aged myocardium in experimental and clinical studies. However, their combined arrhythmic effects in the elderly are not yet completely understood. Hence, the aim of the present work is to relate relevant cardiac electrophysiological parameters to enhanced arrhythmia vulnerability in the in vivo senescent heart. We used multiple-lead epicardial potential mapping in control (9-month-old) and aged (24-month-old) rat hearts. Cardiac excitability and refractoriness were evaluated at numerous epicardial test sites by means of the strength-duration curve and effective refractory period, respectively. During sinus rhythm, durations of electrogram intervals and waves were prolonged in the senescent heart, compared with control, demonstrating a latency in tissue activation and recovery. During ventricular pacing, cardiac excitability, effective refractory period, and dispersion of refractoriness increased in the aged animal. This scenario was accompanied by impairment of impulse propagation. Moreover, both spontaneous and induced arrhythmias were increased in senescent cardiac tissue. Histopathological evaluation of aged heart specimens revealed connective tissue deposition and perinuclear myocytolysis in the atria, while scattered microfoci of interstitial fibrosis were mostly present in the ventricular subendocardium. This work suggests that enhanced arrhythmogenesis in the elderly is a multifactorial process due to the joint increase in excitability and dispersion of refractoriness in association with enhanced conduction inhomogeneity. The knowledge of these electrophysiological changes will possibly contribute to improved prevention of the age-associated increase in cardiac arrhythmias.

Redifferentiated cardiomyocytes retain residual dedifferentiation signatures and are protected against ischemic injury

Cardiomyocyte proliferation and dedifferentiation have fueled the field of regenerative cardiology in recent years, whereas the reverse process of redifferentiation remains largely unexplored. Redifferentiation is characterized by the restoration of function lost during dedifferentiation. Previously, we showed that ERBB2-mediated heart regeneration has these two distinct phases: transient dedifferentiation and redifferentiation. Here we survey the temporal transcriptomic and proteomic landscape of dedifferentiation-redifferentiation in adult mouse hearts and reveal that well-characterized dedifferentiation features largely return to normal, although elements of residual dedifferentiation remain, even after the contractile function is restored. These hearts appear rejuvenated and show robust resistance to ischemic injury, even 5 months after redifferentiation initiation. Cardiomyocyte redifferentiation is driven by negative feedback signaling and requires LATS1/2 Hippo pathway activity. Our data reveal the importance of cardiomyocyte redifferentiation in functional restoration during regeneration but also protection against future insult, in what could lead to a potential prophylactic treatment against ischemic heart disease for at-risk patients.

The IgSF Cell Adhesion Protein CLMP and Congenital Short Bowel Syndrome (CSBS)

The immunoglobulin-like cell adhesion molecule CLMP is a member of the CAR family of cell adhesion proteins and is implicated in human congenital short-bowel syndrome (CSBS). CSBS is a rare but very severe disease for which no cure is currently available. In this review, we compare data from human CSBS patients and a mouse knockout model. These data indicate that CSBS is characterized by a defect in intestinal elongation during embryonic development and impaired peristalsis. The latter is driven by uncoordinated calcium signaling via gap junctions, which is linked to a reduction in connexin43 and 45 levels in the circumferential smooth muscle layer of the intestine. Furthermore, we discuss how mutations in the CLMP gene affect other organs and tissues, including the ureter. Here, the absence of CLMP produces a severe bilateral hydronephrosis—also caused by a reduced level of connexin43 and associated uncoordinated calcium signaling via gap junctions.

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

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

Overlapping and Distinct Features of Cardiac Pathology in Inherited Human and Murine Ether Lipid Deficiency

Inherited deficiency in ether lipids, a subgroup of glycerophospholipids with unique biochemical and biophysical properties, evokes severe symptoms in humans resulting in a multi-organ syndrome. Mouse models with defects in ether lipid biosynthesis have widely been used to understand the pathophysiology of human disease and to study the roles of ether lipids in various cell types and tissues. However, little is known about the function of these lipids in cardiac tissue. Previous studies included case reports of cardiac defects in ether-lipid-deficient patients, but a systematic analysis of the impact of ether lipid deficiency on the mammalian heart is still missing. Here, we utilize a mouse model of complete ether lipid deficiency (Gnpat KO) to accomplish this task. Similar to a subgroup of human patients with rhizomelic chondrodysplasia punctata (RCDP), a fraction of Gnpat KO fetuses present with defects in ventricular septation, presumably evoked by a developmental delay. We did not detect any signs of cardiomyopathy but identified increased left ventricular end-systolic and end-diastolic pressure in middle-aged ether-lipid-deficient mice. By comprehensive electrocardiographic characterization, we consistently found reduced ventricular conduction velocity, as indicated by a prolonged QRS complex, as well as increased QRS and QT dispersion in the Gnpat KO group. Furthermore, a shift of the Wenckebach point to longer cycle lengths indicated depressed atrioventricular nodal function. To complement our findings in mice, we analyzed medical records and performed electrocardiography in ether-lipid-deficient human patients, which, in contrast to the murine phenotype, indicated a trend towards shortened QT intervals. Taken together, our findings demonstrate that the cardiac phenotype upon ether lipid deficiency is highly heterogeneous, and although the manifestations in the mouse model only partially match the abnormalities in human patients, the results add to our understanding of the physiological role of ether lipids and emphasize their importance for proper cardiac development and function.

Cardiac-Specific Overexpression of Caveolin-1 in Rats With Ischemic Cardiomyopathy Improves Arrhythmogenicity and Cardiac Remodelling

BACKGROUND

Ischemic cardiomyopathy (ICM) is associated with electrical and structural remodelling, leading to arrhythmias. Caveolin-1 (Cav1) is a membrane protein involved in the pathogenesis of ischemic injury. Cav1 deficiency has been associated with arrhythmogenicity. The current study aimed to determine how Cav1 overexpression inhibits arrhythmias and cardiac remodelling in ICM.

METHODS

ICM was modelled using left anterior descending (LAD) artery ligation for 4 weeks. Cardiac-specific Cav1 overexpression in ICM on arrhythmias, excitation-contraction coupling, and cardiac remodelling were investigated using the intramyocardial injection of an adeno-associated virus serotype 9 (AAV-9) system, carrying a specific sequence expressing Cav1 (AAV^Cav1) under the cardiac troponin T (cTnT) promoter.

RESULTS

Cav1 overexpression decreased susceptibility to arrhythmias by upregulating gap junction connexin 43 (CX43) and reducing spontaneous irregular proarrhythmogenic Ca²⁺ waves in ventricular cardiomyocytes. It also alleviated ischemic injury-induced contractility weakness by improving Ca²⁺ cycling through normalizing Ca²⁺-handling protein levels and improving Ca²⁺ homeostasis. Masson stain and immunoblotting revealed that the deposition of excessive fibrosis was attenuated by Cav1 overexpression, inhibiting the transforming growth factor-β (TGF-β)/Smad2 signalling pathway. Coimmunoprecipitation assays demonstrated that the interaction between Cav1 and cSrc modulated CX43 expression and Ca²⁺-handling protein levels.

CONCLUSIONS

Cardiac-specific overexpression of Cav1 attenuated ventricular arrhythmia, improved Ca²⁺ cycling, and attenuated cardiac remodelling. These effects were attributed to modulation of CX43, normalized Ca²⁺-handling protein levels, improved Ca²⁺ homeostasis, and attenuated cardiac fibrosis.

The IgCAM CAR Regulates Gap Junction-Mediated Coupling on Embryonic Cardiomyocytes and Affects Their Beating Frequency

The IgCAM coxsackie-adenovirus receptor (CAR) is essential for embryonic heart development and electrical conduction in the mature heart. However, it is not well-understood how CAR exerts these effects at the cellular level. To address this question, we analyzed the spontaneous beating of cultured embryonic hearts and cardiomyocytes from wild type and CAR knockout (KO) embryos. Surprisingly, in the absence of the CAR, cultured cardiomyocytes showed increased frequencies of beating and calcium cycling. Increased beatings of heart organ cultures were also induced by the application of reagents that bind to the extracellular region of the CAR, such as the adenovirus fiber knob. However, the calcium cycling machinery, including calcium extrusion via SERCA2 and NCX, was not disrupted in CAR KO cells. In contrast, CAR KO cardiomyocytes displayed size increases but decreased in the total numbers of membrane-localized Cx43 clusters. This was accompanied by improved cell-cell coupling between CAR KO cells, as demonstrated by increased intercellular dye diffusion. Our data indicate that the CAR may modulate the localization and oligomerization of Cx43 at the plasma membrane, which could in turn influence electrical propagation between cardiomyocytes via gap junctions.

The Primary Alteration of Ventricular Myocardium Conduction: The Significant Determinant of Left Bundle Branch Block Pattern

Intraventricular conduction disturbances (IVCD) are currently generally accepted as ECG diagnostic categories. They are characterized by defined QRS complex patterns that reflect the abnormalities in the intraventricular sequence of activation that can be caused by pathology in the His-Purkinje conduction system (HP) or ventricular myocardium. However, the current understanding of the IVCD's underlying mechanism is mostly attributed to HP structural or functional alterations. The involvement of the working ventricular myocardium is only marginally mentioned or not considered. This opinion paper is focused on the alterations of the ventricular working myocardium leading to the most frequent IVCD pattern-the left bundle branch block pattern (LBBB). Recognizing the underlying mechanisms of the LBBB patterns and the involvement of the ventricular working myocardium is of utmost clinical importance, considering a patient's prognosis and indication for cardiac resynchronization therapy.

Expression of connexin 43 protein in cardiomyocytes of heart failure mouse model

Heart failure (HF) is the end stage of various cardiovascular diseases, with high morbidity and mortality, and is associated with a poor prognosis. One of the primary causes of HF is aortic valve disease, manifested by progressive aortic valve stenosis (AVS), resulting in increased left ventricular load, ventricular hypertrophy, ultimately ventricular dysfunction, and HF. Early assessment of the degree of cardiomyopathy and timely intervention is expected to improve patients’ cardiac function and delay or even avoid the occurrence of HF. The Wnt signaling pathway is mainly involved in regulating myocardial insufficiency after valve stenosis. Connexin 43 protein (Cx43) is an essential target of Wnt signaling pathway that forms gap junction (GJ) structures and is widely distributed in various organs and tissues, especially in the heart. The distribution and transformation of Cx43 among cardiac cells are crucial for the development of HF. To specifically label Cx43 in vivo, we established a new Cx43-BFP-GFP mouse model with two loxp sites on both sides of the tag BFP-polyA box, which can be removed by Cre recombination. This double-reporter line endowed us with a powerful genetic tool for determining the area, spatial distribution, and functional status of Cx43. It also indicated changes in electrical conduction between cells in a steady or diseased state.

The caspase-1 inhibitor VX765 upregulates connexin 43 expression and improves cell-cell communication after myocardial infarction via suppressing the IL-1β/p38 MAPK pathway

Connexin 43 (Cx43) is the most important protein in the gap junction channel between cardiomyocytes. Abnormalities of Cx43 change the conduction velocity and direction of cardiomyocytes, leading to reentry and conduction block of the myocardium, thereby causing arrhythmia. It has been shown that IL-1β reduces the expression of Cx43 in astrocytes and cardiomyocytes in vitro. However, whether caspase-1 and IL-1β affect connexin 43 after myocardial infarction (MI) is uncertain. In this study we investigated the effects of VX765, a caspase-1 inhibitor, on the expression of Cx43 and cell-to-cell communication after MI. Rats were treated with VX765 (16 mg/kg, i.v.) 1 h before the left anterior descending artery (LAD) ligation, and then once daily for 7 days. The ischemic heart was collected for histochemical analysis and Western blot analysis. We showed that VX765 treatment significantly decreased the infarct area, and alleviated cardiac dysfunction and remodeling by suppressing the NLRP3 inflammasome/caspase-1/IL-1β expression in the heart after MI. In addition, VX765 treatment markedly raised Cx43 levels in the heart after MI. In vitro experiments were conducted in rat cardiac myocytes (RCMs) stimulated with the supernatant from LPS/ATP-treated rat cardiac fibroblasts (RCFs). Pretreatment of the RCFs with VX765 (25 μM) reversed the downregulation of Cx43 expression in RCMs and significantly improved intercellular communication detected using a scrape-loading/dye transfer assay. We revealed that VX765 suppressed the activation of p38 MAPK signaling in the heart tissue after MI as well as in RCMs stimulated with the supernatant from LPS/ATP-treated RCFs. Taken together, these data show that the caspase-1 inhibitor VX765 upregulates Cx43 expression and improves cell-to-cell communication in rat heart after MI via suppressing the IL-1β/p38 MAPK pathway.

Pro-Arrhythmic Effects of Discontinuous Conduction at the Purkinje Fiber-Ventricle Junction Arising From Heart Failure-Induced Ionic Remodeling - Insights From Computational Modelling

Heart failure is associated with electrical remodeling of the electrical properties and kinetics of the ion channels and transporters that are responsible for cardiac action potentials. However, it is still unclear whether heart failure-induced ionic remodeling can affect the conduction of excitation waves at the Purkinje fiber-ventricle junction contributing to pro-arrhythmic effects of heart failure, as the complexity of the heart impedes a detailed experimental analysis. The aim of this study was to employ computational models to investigate the pro-arrhythmic effects of heart failure-induced ionic remodeling on the cardiac action potentials and excitation wave conduction at the Purkinje fiber-ventricle junction. Single cell models of canine Purkinje fiber and ventricular myocytes were developed for control and heart failure. These single cell models were then incorporated into one-dimensional strand and three-dimensional wedge models to investigate the effects of heart failure-induced remodeling on propagation of action potentials in Purkinje fiber and ventricular tissue and at the Purkinje fiber-ventricle junction. This revealed that heart failure-induced ionic remodeling of Purkinje fiber and ventricular tissue reduced conduction safety and increased tissue vulnerability to the genesis of the unidirectional conduction block. This was marked at the Purkinje fiber-ventricle junction, forming a potential substrate for the genesis of conduction failure that led to re-entry. This study provides new insights into proarrhythmic consequences of heart failure-induced ionic remodeling.

Generation and Characterization of an Inducible Cx43 Overexpression System in Mouse Embryonic Stem Cells

Connexins (Cx) are a large family of membrane proteins that can form intercellular connections, so-called gap junctions between adjacent cells. Cx43 is widely expressed in mammals and has a variety of different functions, such as the propagation of electrical conduction in the cardiac ventricle. Despite Cx43 knockout models, many questions regarding the biology of Cx43 in health and disease remain unanswered. Herein we report the establishment of a Cre-inducible Cx43 overexpression system in murine embryonic stem (ES) cells. This enables the investigation of the impact of Cx43 overexpression in somatic cells. We utilized a double reporter system to label Cx43-overexpressing cells via mCherry fluorescence and exogenous Cx43 via fusion with P2A peptide to visualize its distribution pattern. We proved the functionality of our systems in ES cells, HeLa cells, and 3T3-fibroblasts and demonstrated the formation of functional gap junctions based on dye diffusion and FRAP experiments. In addition, Cx43-overexpressing ES cells could be differentiated into viable cardiomyocytes, as shown by the formation of cross striation and spontaneous beating. Analysis revealed faster and more rhythmic beating of Cx43-overexpressing cell clusters. Thus, our Cx43 overexpression systems enable the investigation of Cx43 biology and function in cardiomyocytes and other somatic cells.

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.

Decrease of Pdzrn3 is required for heart maturation and protects against heart failure

Heart failure is the final common stage of most cardiopathies. Cardiomyocytes (CM) connect with others via their extremities by intercalated disk protein complexes. This planar and directional organization of myocytes is crucial for mechanical coupling and anisotropic conduction of the electric signal in the heart. One of the hallmarks of heart failure is alterations in the contact sites between CM. Yet no factor on its own is known to coordinate CM polarized organization. We have previously shown that PDZRN3, an ubiquitine ligase E3 expressed in various tissues including the heart, mediates a branch of the Planar cell polarity (PCP) signaling involved in tissue patterning, instructing cell polarity and cell polar organization within a tissue. PDZRN3 is expressed in the embryonic mouse heart then its expression dropped significantly postnatally corresponding with heart maturation and CM polarized elongation. A moderate CM overexpression of Pdzrn3 (Pdzrn3 OE) during the first week of life, induced a severe eccentric hypertrophic phenotype with heart failure. In models of pressure-overload stress heart failure, CM-specific Pdzrn3 knockout showed complete protection against degradation of heart function. We reported that Pdzrn3 signaling induced PKC ζ expression, c-Jun nuclear translocation and a reduced nuclear ß catenin level, consistent markers of the planar non-canonical Wnt signaling in CM. We then show that subcellular localization (intercalated disk) of junction proteins as Cx43, ZO1 and Desmoglein 2 was altered in Pdzrn3 OE mice, which provides a molecular explanation for impaired CM polarization in these mice. Our results reveal a novel signaling pathway that controls a genetic program essential for heart maturation and maintenance of overall geometry, as well as the contractile function of CM, and implicates PDZRN3 as a potential therapeutic target for the prevention of human heart failure.

Changes in ion channel expression and function associated with cardiac arrhythmogenic remodeling by Sorbs2

The Sorbin and SH3 domain-containing protein 2 (Sorbs2) is an important component of cardiomyocyte sarcomere. It has been recently reported that loss of Sorbs2 is causally associated with arrhythmogenic cardiomyopathy in human. However, the ionic mechanisms leading to cardiac arrhythmogenesis by Sorbs2 deficiency are unknown. In this study, we hypothesized that Sorbs2 plays an important role in regulating cardiac ion channel expression and function. Using electrophysiological and molecular biological approaches, we found that the Sorbs2 knockout (KO) mice progressively developed cardiac structural and electrical remodeling as early as 1 to 2 months of age and died prematurely at 5 to 7 months of age. Electrocardiographic recordings showed that Sorbs2 KO mice had conduction delays, spontaneous ventricular extrasystoles and polymorphic ventricular tachyarrhythmia. Intracellular recordings revealed abnormal action potentials with depolarized resting potential, reduced upstroke velocity, prolonged repolarization, and effective refractory period in the ventricular preparations of Sorbs2 KO mice. Patch clamp experiments demonstrated that Sorbs2 KO mice displayed distinct abnormalities in the expression and function of cardiac ion channels, including those of the voltage-gated Na⁺ channels, L-type Ca²⁺ channels, the voltage-gated K⁺ channels and the inward-rectifier K⁺ channels. Moreover, Sorbs2 physically interacted with the RNAs and/or proteins of important cardiac ion channels and directly regulated their expression in vitro. Our results indicate that Sorbs2 plays a pivotal role in the regulation of cardiac channel physiology. Loss of Sorbs2 promotes cardiac ion channelopathies and life-threatening arrhythmias.

Abrogation of CC Chemokine Receptor 9 Ameliorates Ventricular Electrical Remodeling in Mice After Myocardial Infarction

Introduction: Myocardial infarction (MI) triggers structural and electrical remodeling. CC chemokine receptor 9 (CCR9) mediates chemotaxis of inflammatory cells in MI. In our previous study, CCR9 knockout has been found to improve structural remodeling after MI. Here, we further investigate the potential influence of CCR9 on electrical remodeling following MI in order to explore potential new measures to improve the prognosis of MI.

Methods and Results: Mice was used and divided into four groups: CCR9^+/+/Sham, CCR9^−/−/Sham, CCR9^+/+/MI, CCR9^−/−/MI. Animals were used at 1 week after MI surgery. Cardiomyocytes in the infracted border zone were acutely dissociated and the whole-cell patch clamp was used to record action potential duration (APD), L-type calcium current (I_Ca,L) and transient outward potassium current (I_to). Calcium transient and sarcoplasmic reticulum (SR) calcium content under stimulation of Caffeine were measured in isolated cardiomyocytes by confocal microscopy. Multielectrode array (MEA) was used to measure the conduction of the left ventricle. The western-blot was performed for the expression level of connexin 43. We observed prolonged APD₉₀, increased I_Ca,L and decreased I_to following MI, while CCR9 knockout attenuated these changes (APD₉₀: 50.57 ± 6.51 ms in CCR9^−/−/MI vs. 76.53 ± 5.98 ms in CCR9^+/+/MI, p < 0.05; I_Ca,L: −13.15 ± 0.86 pA/pF in CCR9^−/−/MI group vs. −17.05 ± 1.11 pA/pF in CCR9^+/+/MI, p < 0.05; I_to: 4.01 ± 0.17 pA/pF in CCR9^−/−/MI group vs. 2.71 ± 0.16 pA/pF in CCR9^+/+/MI, p < 0.05). The confocal microscopy results revealed CCR9 knockout reversed the calcium transient and calcium content reduction in sarcoplasmic reticulum following MI. MEA measurements showed improved conduction velocity in CCR9^−/−/MI mice (290.1 ± 34.47 cm/s in CCR9^−/−/MI group vs. 113.2 ± 14.4 cm/s in CCR9^+/+/MI group, p < 0.05). Western-blot results suggested connexin 43 expression was lowered after MI while CCR9 knockout improved its expression.

Conclusion: This study shows CCR9 knockout prevents the electrical remodeling by normalizing ion currents, the calcium homeostasis, and the gap junction to maintain APD and the conduction function. It suggests CCR9 is a promising therapeutic target for MI-induced arrhythmia, which warrants further investigation.

Cardiac Conduction Velocity, Remodeling and Arrhythmogenesis

Cardiac electrophysiological disorders, in particular arrhythmias, are a key cause of morbidity and mortality throughout the world. There are two basic requirements for arrhythmogenesis: an underlying substrate and a trigger. Altered conduction velocity (CV) provides a key substrate for arrhythmogenesis, with slowed CV increasing the probability of re-entrant arrhythmias by reducing the length scale over which re-entry can occur. In this review, we examine methods to measure cardiac CV in vivo and ex vivo, discuss underlying determinants of CV, and address how pathological variations alter CV, potentially increasing arrhythmogenic risk. Finally, we will highlight future directions both for methodologies to measure CV and for possible treatments to restore normal CV.

Ventricular arrhythmias in mouse models of diabetic kidney disease

Chronic kidney disease (CKD) affects more than 20 million people in the US, and it is associated with a significantly increased risk of sudden cardiac death (SCD). Despite the significance, the mechanistic relationship between SCD and CKD is not clear and there are few effective therapies. Using optical mapping techniques, we tested the hypothesis that mouse models of progressive diabetic kidney disease (DKD) exhibit enhanced ventricular arrhythmia incidence and underlying arrhythmia substrates. Compared to wild-type mice, both Lepr^db/db eNOS^−/− (2KO) and high fat diet plus low dose streptozotocin (HFD + STZ) mouse models of DKD experienced sudden death and greater arrhythmia inducibility, which was more common with isoproterenol than programmed electrical stimulation. 2KO mice demonstrated slowed conduction velocity, prolonged action potential duration (APD), and myocardial fibrosis; both 2KO and HFD + STZ mice exhibited arrhythmias and calcium dysregulation with isoproterenol challenge. Finally, circulating concentrations of the uremic toxin asymmetric dimethylarginine (ADMA) were elevated in 2KO mice. Incubation of human cardiac myocytes with ADMA prolonged APD, as also observed in 2KO mice hearts ex vivo. The present study elucidates an arrhythmia-associated mechanism of sudden death associated with DKD, which may lead to more effective treatments in the vulnerable DKD patient population.

Electrical Ventricular Remodeling in Dilated Cardiomyopathy

Ventricular arrhythmias contribute significantly to morbidity and mortality in patients with heart failure (HF). Pathomechanisms underlying arrhythmogenicity in patients with structural heart disease and impaired cardiac function include myocardial fibrosis and the remodeling of ion channels, affecting electrophysiologic properties of ventricular cardiomyocytes. The dysregulation of ion channel expression has been associated with cardiomyopathy and with the development of arrhythmias. However, the underlying molecular signaling pathways are increasingly recognized. This review summarizes clinical and cellular electrophysiologic characteristics observed in dilated cardiomyopathy (DCM) with ionic and structural alterations at the ventricular level. Furthermore, potential translational strategies and therapeutic options are highlighted.

GJA1 Expression and Left Atrial Remodeling in the Incidence of Atrial Fibrillation in Patients with Obstructive Sleep Apnea Syndrome

Obstructive sleep apnea syndrome (OSAS) is an important risk factor for atrial fibrillation (AF). GJA1 gene encoding connexin43, a major protein in cardiac gap junctions, plays a crucial role in the synchronized contraction of the heart and in cardiac arrhythmia. However, little is known regarding the role of GJA1 expression in the incidence of AF in patients with OSAS. All prospectively enrolled OSAS patients underwent polysomnography, electrocardiography, a 24-h Holter test, and echocardiography. Moderate-to-severe OSAS was defined as an apnea-hypopnea index (AHI) ≥ 15. Exosomes were purified from the plasma of all OSAS patients and incubated in HL-1 cells to investigate the effect of exosomes from patients with and without AF on GJA1 expression. A total of 129 patients were recruited for this study; 26 were excluded due to an AHI < 15. Of the 103 enrolled patients, 21 had AF, and 82 did not. Multivariate analysis showed diabetes mellitus, lower sleep efficiency, lower left ventricular ejection fraction, and larger left atrial (LA) size were independent predictors of AF occurrence in OSAS patients. The area under the receiver operating characteristic curve for LA with a size ≥38.5 mm for predicting AF occurrence in OSAS patients was 0.795 (95% confidence interval [0.666, 0.925]); p < 0.001). GJA1 expression in HL-1 cells incubated with exosomes from OSAS patients with AF was lower than that with exosomes from patients without AF after controlling for age and sex and was negatively correlated with the AHI and oxygen desaturation index (ODI), especially during the non-rapid eye movement period (NREM) of OSAS patients with AF (all p < 0.05). LA size was an independent predictor of AF occurrence in OSAS patients. The AHI and ODI in the NREM period of OSAS patients with AF were negatively correlated with GJA1 expression in HL-1 cells, which offers a hint that GJA1 may play a crucial role in the development of AF in patients with OSAS.

A 3-D human model of complex cardiac arrhythmias

Cardiac arrhythmias impact over 12 million people globally, with an increasing incidence of acquired arrhythmias. Although animal models have shed light onto fundamental arrhythmic mechanisms, species-specific differences and ethical concerns remain. Current human models using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) either lack the higher order tissue organization of the heart or implement unreliable arrhythmia induction techniques. Our goal was to develop a robust model of acquired arrhythmia by disrupting cardiomyocyte cell-cell signaling - one of the hallmarks of complex arrhythmias. Human 3D microtissues were generated by seeding hydrogel-embedded hiPSC-CMs and cardiac fibroblasts into an established microwell system designed to enable active and passive force assessment. Cell-cell signaling was disrupted using methyl-beta cyclodextrin (MBCD), previously shown to disassemble cardiac gap junctions. We demonstrate that arrhythmias were progressive and present in all microtissues within 5 days of treatment. Arrhythmic tissues exhibited reduced conduction velocity, an increased number of distinct action potentials, and reduced action potential cycle length. Arrhythmic tissues also showed significant reduction in contractile force generation, increased beating frequency, and increased passive tension and collagen deposition, in line with fibrosis. A subset of tissues with more complex arrhythmias exhibited 3D spatial differences in action potential propagation. Pharmacological and electrical defibrillation was successful. Transcriptomic data indicated an enrichment of genes consistent with cardiac arrhythmias. MBCD removal reversed the arrhythmic phenotype, resulting in synchronicity despite not resolving fibrosis. This innovative & reliable human-relevant 3D acquired arrhythmia model shows potential for improving our understanding of arrhythmic action potential conduction and furthering therapeutic development. STATEMENT OF SIGNIFICANCE: This work describes a 3D human model of cardiac arrhythmia-on-a-chip with high reproducibility, fidelity, and extensive functional applicability. To mimic in vivo conditions, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and cardiac fibroblasts from healthy controls were combined in a biocompatible fibrin hydrogel and seeded between two deflectable polymeric rods. Using the innate functional properties of this 3D model as well as advanced optical imaging techniques we demonstrated dramatic changes in contraction rate, synchronicity, and electrophysiological conduction in arrhythmic tissues relative to controls. Taken together, these data demonstrate the distinctive potential of this new model for pathophysiological studies, and for arrhythmia drug testing applications.

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).

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

Cx43 hemichannel microdomain signaling at the intercalated disc enhances cardiac excitability

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

Relaxin abrogates genomic remodeling of the aged heart

"Healthy" aging drives structural and functional changes in the heart including maladaptive electrical remodeling, fibrosis and inflammation, which lower the threshold for cardiovascular diseases such as heart failure (HF) and atrial fibrillation (AF). Despite mixed results in clinical trials, Relaxin-therapy for 2-days reduced mortality by 37% at 180-days post-treatment, in patients with acute decompensated HF. Relaxin's short lifespan (2-3h) but long-lasting protective actions suggested that relaxin acts at a genomic level to reverse maladaptive remodeling in AF, HF and aging. Our recent studies showed that a 2-week treatment with Relaxin (0.4mg/kg/day) of aged (24months old F-344 rats) increases the expression of voltage-gated Na⁺ channels (mRNA, Nav1.5 and I_Na), connexin-43, abrogates inflammatory and immune responses and reverses myocardial fibrosis and cellular hypertrophy of the aged hearts. Relaxin acts directly at a wide range of cell types in the cardiovascular system that express its cognate GPCR receptor, RXFP1. RNA-seq analysis of young and aged hearts with and without Relaxin treatment revealed that "normal" aging altered the expression of ~10% of genes expressed in the ventricles, including: ion channels, components of fibrosis, hemodynamic biomarkers, immune and inflammatory responses which were reversed by Relaxin. The extensive cardiovascular remodeling caused by Relaxin was mediated through the activation of the Wnt/β-catenin signaling pathway which was otherwise suppressed by in adult cardiomyocytes intracellular by cytosolic Dickkopf1 (Dkk1). Wnt/β-catenin signaling is a mechanism that can explain the pleiotropic actions of Relaxin and the marked reversal of genomic changes that occur in aged hearts.

Nanoscale Study of Calcium Handling Remodeling in Right Ventricular Cardiomyocytes Following Pulmonary Hypertension

Pulmonary hypertension is a complex disorder characterized by pulmonary vascular remodeling and right ventricular hypertrophy, leading to right heart failure. The mechanisms underlying this process are not well understood. We hypothesize that the structural remodeling occurring in the cardiomyocytes of the right ventricle affects the cytosolic Ca²⁺ handling leading to arrhythmias. After 12 days of monocrotaline-induced pulmonary hypertension in rats, epicardial mapping showed electrical remodeling in both ventricles. In myocytes isolated from the hypertensive rats, a combination of high-speed camera and confocal line-scan documented a prolongation of Ca²⁺ transients along with a higher local Ca²⁺-release activity. These Ca²⁺ transients were less synchronous than in controls, likely due to disorganized transverse-axial tubular system. In fact, following pulmonary hypertension, hypertrophied right ventricular myocytes showed significantly reduced number of transverse tubules and increased number of axial tubules; however, Stimulation Emission Depletion microscopy demonstrated that the colocalization of L-type Ca²⁺ channels and RyR2 (ryanodine receptor 2) remained unchanged. Finally, Stimulation Emission Depletion microscopy and super-resolution scanning patch-clamp analysis uncovered a decrease in the density of active L-type Ca²⁺ channels in right ventricular myocytes with an elevated open probability of the T-tubule anchored channels. This may represent a general mechanism of how nanoscale structural changes at the early stage of pulmonary hypertension impact on the development of the end stage failing phenotype in the right ventricle.

Anisotropic Cardiac Conduction

Anisotropy is the property of directional dependence. In cardiac tissue, conduction velocity is anisotropic and its orientation is determined by myocyte direction. Cell shape and size, excitability, myocardial fibrosis, gap junction distribution and function are all considered to contribute to anisotropic conduction. In disease states, anisotropic conduction may be enhanced, and is implicated, in the genesis of pathological arrhythmias. The principal mechanism responsible for enhanced anisotropy in disease remains uncertain. Possible contributors include changes in cellular excitability, changes in gap junction distribution or function and cellular uncoupling through interstitial fibrosis. It has recently been demonstrated that myocyte orientation may be identified using diffusion tensor magnetic resonance imaging in explanted hearts, and multisite pacing protocols have been proposed to estimate myocyte orientation and anisotropic conduction in vivo. These tools have the potential to contribute to the understanding of the role of myocyte disarray and anisotropic conduction in arrhythmic states.

A Working Hypothesis Regarding Identical Pathomechanisms between Clinical Efficacy and Adverse Reaction of Clozapine via the Activation of Connexin43

Clozapine (CLZ) is an approved antipsychotic agent for the medication of treatment-resistant schizophrenia but is also well known as one of the most toxic antipsychotics. Recently, the World Health Organization’s (WHO) global database (VigiBase) reported the relative lethality of severe adverse reactions of CLZ. Agranulocytosis is the most famous adverse CLZ reaction but is of lesser lethality compared with the other adverse drug reactions of CLZ. Unexpectedly, VigiBase indicated that the prevalence and relative lethality of pneumonia, cardiotoxicity, and seizures associated with CLZ were more serious than that of agranulocytosis. Therefore, haematological monitoring in CLZ patients monitoring system provided success in the prevention of lethal adverse events from CLZ-induced agranulocytosis. Hereafter, psychiatrists must amend the CLZ patients monitoring system to protect patients with treatment-resistant schizophrenia from severe adverse CLZ reactions, such as pneumonia, cardiotoxicity, and seizures, according to the clinical evidence and pathophysiology. In this review, we discuss the mechanisms of clinical efficacy and the adverse reactions of CLZ based on the accumulating pharmacodynamic findings of CLZ, including tripartite synaptic transmission, and we propose suggestions for amending the monitoring and medication of adverse CLZ reactions associated with pneumonia, cardiotoxicity, and seizures.

Knockout of SORBS2 Protein Disrupts the Structural Integrity of Intercalated Disc and Manifests Features of Arrhythmogenic Cardiomyopathy

Background Sorbs2b (sorbin and SH3 domain-containing 2b) was recently identified as a cardiomyopathy gene from a zebrafish mutagenesis screen. However, cardiac functions of its mammalian ortholog remain elusive. Methods and Results We conducted a detailed expression and subcellular localization analysis of Sorbs2 ortholog in mice and a phenotypic characterization in Sorbs2 knockout mice. Sorbs2 is highly expressed in the mouse heart and encodes an adhesion junction/desmosome protein that is mainly localized to the intercalated disc. A mutation with near complete depletion of the Sorbs2 protein in mice results in phenotypes characteristic of human arrhythmogenic cardiomyopathy (ACM), including right ventricular dilation, right ventricular dysfunction, spontaneous ventricular tachycardia, and premature death. Sorbs2 is required to maintain the structural integrity of intercalated disc. Its absence resulted in profound cardiac electrical remodeling with impaired impulse conduction and action potential derangements. Targeted sequencing of human patients with ACM identified 2 rare splicing variants classified as likely pathogenic were in 2 unrelated individuals with ACM from a cohort of 59 patients with ACM. Conclusions The Sorbs2 knockout mouse manifests several key features reminiscent of human ACM. Although the candidacy of SORBS2 as a new ACM-susceptibility gene is supported by preliminary human genetics study, future validation in larger cohorts with ACM is needed.

Shensong Yangxin Protects Against Metabolic Syndrome-Induced Ventricular Arrhythmias by Inhibiting Electrical Remodeling

Shensong Yangxin (SSYX) is a traditional Chinese medicine, which has been proven to improve the clinical symptoms of arrhythmia. However, the role of SSYX in metabolic syndrome (MetS)-induced electrical remodeling remains to be fully elucidated. Here, we sought to clarify whether SSYX can alter the electrophysiological remodeling of cardiac myocytes from MetS rats by regulating transient outward potassium current (I_to) and calcium current (I_Ca-L). Male Wistar rats were subjected to 16 weeks of high-carbohydrate, high-fat to produce a MetS model group. SSYX (0.4 g/kg) was administrated by daily gavage 8 weeks following high-carbohydrate, high-fat for 8 weeks. In vivo electrophysiological study was performed to evaluated ventricular arrhythmias (VA) vulnerability and electrophysiological properties. The potential electrical mechanisms were estimated by whole-cell patch-clamp and molecular analysis. The H9C2 cells were used to verify the protective role of SSYX in vitro. After 16-week high-carbohydrate, high-fat feeding, MetS model rats showed increased body weight (BW), blood pressure (BP), blood sugar (BS), heart rate (HR) and heart weights to tibia length (HW/TL) ratio. Furthermore, MetS rats depicted increased VA inducibility, shortened effective refractory period (ERP) and prolonged action potential duration (APD). Lower I_Ca-L and I_to current densities were observed in MetS rats than CTL rats. Additionally, MetS rats exhibited significantly increased cardiac fibrosis, decreased Cx43 expression and protein levels of Cav1.2, Kv4.2, Kv4.3 than CTL group. As expected, these MetS-induced effects above were reversed when SSYX was administrated. Mechanistically, SSYX administrated significantly down-regulated the TLR4/MyD88/CaMKII signaling pathway both in vivo and in vitro. Collectively, our data indicated that the electrical remodeling induced by MetS contributed to the increased VA susceptibility. SSYX protects against MetS-induced VA by inhibiting electrical remodeling through TLR4/MyD88/CaMKII signaling pathway.

Liraglutide Effect on Ventricular Transient Outward K + Channel and Connexin-43 Protein Expression

BACKGROUND

Human glucagon-like peptide-1 analogue, Liraglutide, has shown cardioprotective effects in animal and clinical studies of type 2 diabetes mellitus. This study was conducted to assess the effect of Liraglutide on diabetes-induced myocardial electrical remodeling.

MATERIALS AND METHODS

A rat model of type 2 diabetes mellitus was induced by high-fat diet and low dose Streptozotocin (35 mg/kg). Diabetic rats were randomized into 4 subgroups (n=6-7): diabetic-untreated, diabetics treated with Liraglutide, diabetics treated with Ramipril, and diabetics treated with Metformin in addition to a control group. Changes in serum glucose, insulin, lipid profile and revised quantitative insulin sensitivity check index (QUICKI index) were assessed. QT and QTc intervals were measured and the degree of cardiac interstitial and perivascular fibrosis was examined. The expression of myocardial I_to channel α subunits, gap junction protein; Kv 4.2/4.3 and connexin 43 (Cx43) respectively, were assessed by western blotting and immunohistochemistry.

RESULTS

Similar to Ramipril, both Liraglutide and Metformin effectively inhibited the diabetes-induced myocardial hypertrophy and fibrosis. However, Liraglutide treatment significantly improved Kv 4.2/4.3 and Cx43 expression/distribution and prevented diabetes-related QTc interval prolongation.

CONCLUSIONS

We have shown that pathological alterations in myocardial Cx43 expression and distribution, in addition to reduced I_to channel expression, may underlie the QTc interval prolongation in high-fat diet/STZ rat model of type 2 diabetes mellitus. The beneficial effects of Liraglutide, as those of Ramipril, on cardiac electrophysiology could be at least attributed to its direct ability to normalize expression and distribution of Cx43 and I_to channels in the diabetic rat heart.

Gap-134, a Connexin43 activator, prevents age-related development of ventricular fibrosis in Scn5a+/- mice

Down-regulation of Connexin43 (Cx43) has often been associated with the development of cardiac fibrosis. We showed previously that Scn5a heterozygous knockout mice (Scn5a^+/-), which mimic familial progressive cardiac conduction defect, exhibit an age-dependent decrease of Cx43 expression and phosphorylation concomitantly with activation of TGF-β pathway and fibrosis development in the myocardium between 45 and 60 weeks of age. The aim of this study was to investigate whether Gap-134 prevents Cx43 down-regulation with age and fibrosis development in Scn5a^+/- mice. We observed in 60-week-old Scn5a^+/- mouse heart a Cx43 expression and localization remodeling correlated with fibrosis. Chronic administration of a potent and selective gap junction modifier, Gap-134 (danegaptide), between 45 and 60 weeks, increased Cx43 expression and phosphorylation on serine 368 and prevented Cx43 delocalization. Furthermore, we found that Gap-134 prevented fibrosis despite the persistence of the conduction defects and the TGF-β canonical pathway activation. In conclusion, the present study demonstrates that the age-dependent decrease of Cx43 expression is involved in the ventricular fibrotic process occurring in Scn5a^+/- mice. Finally, our study suggests that gap junction modifier, such as Gap-134, could be an effective anti-fibrotic agent in the context of age-dependent fibrosis in progressive cardiac conduction disease.

Connexin 43 Deficiency Is Associated with Reduced Myocardial Scar Size and Attenuated TGFβ1 Signaling after Transient Coronary Occlusion in Conditional Knock-Out Mice

Previous studies demonstrated a reduction in myocardial scar size in heterozygous Cx43^+/- mice subjected to permanent coronary occlusion. However, patients presenting with ST segment elevation myocardial infarction often undergo rapid coronary revascularization leading to prompt restoration of coronary flow. Therefore, we aimed to assess changes in scar size and left ventricular remodeling following transient myocardial ischemia (45 min) followed by 14 days of reperfusion using Cx43^fl/fl (controls) and Cx43^Cre-ER(T)/fl inducible knock-out (Cx43 content: 50%) mice treated with vehicle or 4-hydroxytamoxifen (4-OHT) to induce a Cre-ER(T)-mediated global deletion of the Cx43 floxed allele. The scar area (picrosirius red), measured 14 days after transient coronary occlusion, was similarly reduced in both vehicle and 4-OHT-treated Cx43^Cre-ER(T)/fl mice, compared to Cx43^fl/fl animals, having normal Cx43 levels (15.78% ± 3.42% and 16.54% ± 2.31% vs. 25.40% ± 3.14% and 22.43% ± 3.88% in vehicle and 4-OHT-treated mice, respectively, p = 0.027). Left ventricular dilatation was significantly attenuated in both Cx43-deficient groups (p = 0.037 for left ventricular end-diastolic diameter). These protective effects were correlated with an attenuated enhancement in pro-transforming growth factor beta 1 (TGFβ1) expression after reperfusion. In conclusion, our data demonstrate that Cx43 deficiency induces a protective effect on scar formation after transient coronary occlusion in mice, an effect associated with reduced left ventricular remodeling and attenuated enhancement in pro-TGFβ1 expression.

Dexmedetomidine Exerted Anti-arrhythmic Effects in Rat With Ischemic Cardiomyopathy via Upregulation of Connexin 43 and Reduction of Fibrosis and Inflammation

Background

Persistent myocardial ischemia post-myocardial infarction can lead to fatal ventricular arrhythmias such as ventricular tachycardia and fibrillation, both of which carry high mortality rates. Dexmedetomidine (Dex) is a highly selective α2-agonist used in surgery for congenital cardiac disease because of its antiarrhythmic properties. Dex has previously been reported to prevent or terminate various arrhythmias. The purpose of the present study was to determine the anti-arrhythmic properties of Dex in the context of ischemic cardiomyopathy (ICM) after myocardial infarction.

Methods and Results

We randomly allocated 48 rats with ICM, created by persistent ligation of the left anterior descending artery for 4 weeks, into six groups: Sham (n = 8), Sham + BML (n = 8), ICM (n = 8), ICM + BML (n = 8), ICM + Dex (n = 8), and ICM + Dex + BML (n = 8). Treatments started after ICM was confirmed (the day after echocardiographic measurement) and continued for 4 weeks (inject intraperitoneally, daily). Dex inhibited the generation of collagens, cytokines, and other inflammatory mediators in rats with ICM via the suppression of NF-κB activation and increased the distribution of connexin 43 (Cx43) via phosphorylation of adenosine 5′-monophosphate-activated protein kinase (AMPK). Dex reduced the occurrence of spontaneous ventricular arrhythmias (ventricular premature beat or ventricular tachycardia), decreased the inducibility quotient of ventricular arrhythmias induced by PES, and partly improved cardiac contraction. The AMPK antagonist BML-275 dihydrochloride (BML) partly weakened the cardioprotective effect of Dex.

Conclusion

Dex conferred anti-arrhythmic effects in the context of ICM via upregulation of Cx43 and suppression of inflammation and fibrosis. The anti-arrhythmic and anti-inflammatory properties of Dex may be mediated by phosphorylation of AMPK and subsequent suppression of NF-κB activation.

The proarrhythmic features of pathological cardiac hypertrophy in neonatal rat ventricular cardiomyocyte cultures

Different factors may trigger arrhythmias in diseased hearts, including fibrosis, cardiomyocyte hypertrophy, hypoxia, and inflammation. This makes it difficult to establish the relative contribution of each of them to the occurrence of arrhythmias. Accordingly, in this study, we used an in vitro model of pathological cardiac hypertrophy (PCH) to investigate its proarrhythmic features and the underlying mechanisms independent of fibrosis or other PCH-related processes. Neonatal rat ventricular cardiomyocyte (nr-vCMC) monolayers were treated with phorbol 12-myristate 13-acetate (PMA) to create an in vitro model of PCH. The electrophysiological properties of PMA-treated and control monolayers were analyzed by optical mapping at day 9 of culture. PMA treatment led to a significant increase in cell size and total protein content. It also caused a reduction in sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2 level (32%) and an increase in natriuretic peptide A (42%) and α1-skeletal muscle actin (34%) levels, indicating that the hypertrophic response induced by PMA was, indeed, pathological in nature. PMA-treated monolayers showed increases in action potential duration (APD) and APD dispersion, and a decrease in conduction velocity (CV; APD₃₀ of 306 ± 39 vs. 148 ± 18 ms, APD₃₀ dispersion of 85 ± 19 vs. 22 ± 7 and CV of 10 ± 4 vs. 21 ± 2 cm/s in controls). Upon local 1-Hz stimulation, 53.6% of the PMA-treated cultures showed focal tachyarrhythmias based on triggered activity (n = 82), while the control group showed 4.3% tachyarrhythmias (n = 70). PMA-treated nr-vCMC cultures may, thus, represent a well-controllable in vitro model for testing new therapeutic interventions targeting specific aspects of hypertrophy-associated arrhythmias.NEW & NOTEWORTHY Phorbol 12-myristate 13-acetate (PMA) treatment of neonatal rat ventricular cardiomyocytes (nr-vCMCs) led to induction of many significant features of pathological cardiac hypertrophy (PCH), including action potential duration prolongation and dispersion, which provided enough time and depolarizing force for formation of early afterdepolarization (EAD)-induced focal tachyarrhythmias. PMA-treated nr-vCMCs represent a well-controllable in vitro model, which mostly resembles to moderate left ventricular hypertrophy (LVH) rather than severe LVH, in which generation of a reentry is the putative mechanism of its arrhythmias.

Missing Link between Molecular Aspects of Ventricular Arrhythmias and QRS Complex Morphology in Left Ventricular Hypertrophy

The aim of this opinion paper is to point out the knowledge gap between evidence on the molecular level and clinical diagnostic possibilities in left ventricular hypertrophy (LVH) regarding the prediction of ventricular arrhythmias and monitoring the effect of therapy. LVH is defined as an increase in left ventricular size and is associated with increased occurrence of ventricular arrhythmia. Hypertrophic rebuilding of myocardium comprises interrelated processes on molecular, subcellular, cellular, tissue, and organ levels affecting electrogenesis, creating a substrate for triggering and maintaining arrhythmias. The knowledge of these processes serves as a basis for developing targeted therapy to prevent and treat arrhythmias. In the clinical practice, the method for recording electrical phenomena of the heart is electrocardiography. The recognized clinical electrocardiogram (ECG) predictors of ventricular arrhythmias are related to alterations in electrical impulse propagation, such as QRS complex duration, QT interval, early repolarization, late potentials, and fragmented QRS, and they are not specific for LVH. However, the simulation studies have shown that the QRS complex patterns documented in patients with LVH are also conditioned remarkably by the alterations in impulse propagation. These QRS complex patterns in LVH could be potentially recognized for predicting ventricular arrhythmia and for monitoring the effect of therapy.

Opposite Effects of Moderate and Extreme Cx43 Deficiency in Conditional Cx43-Deficient Mice on Angiotensin II-Induced Cardiac Fibrosis

Connexin 43 (Cx43) is essential for cardiac electrical coupling, but its effects on myocardial fibrosis is controversial. Here, we analyzed the role of Cx43 in myocardial fibrosis caused by angiotensin II (AngII) using Cx43^fl/fl and Cx43^Cre-ER(T)/fl inducible knock-out (Cx43 content: 50%) mice treated with vehicle or 4-hydroxytamoxifen (4-OHT) to induce a Cre-ER(T)-mediated global deletion of the Cx43 floxed allele. Myocardial collagen content was enhanced by AngII in all groups (n = 8–10/group, p < 0.05). However, animals with partial Cx43 deficiency (vehicle-treated Cx43^Cre-ER(T)/fl) had a significantly higher AngII-induced collagen accumulation that reverted when treated with 4-OHT, which abolished Cx43 expression. The exaggerated fibrotic response to AngII in partially deficient Cx43^Cre-ER(T)/fl mice was associated with enhanced p38 MAPK activation and was not evident in Cx43 heterozygous (Cx43^+/-) mice. In contrast, normalization of interstitial collagen in 4-OHT-treated Cx43^Cre-ER(T)/fl animals correlated with enhanced MMP-9 activity, IL-6 and NOX2 mRNA expression, and macrophage content, and with reduced α-SMA and SM22α in isolated fibroblasts. In conclusion, our data demonstrates an exaggerated, p38 MAPK-dependent, fibrotic response to AngII in partially deficient Cx43^Cre-ER(T)/fl mice, and a paradoxical normalization of collagen deposition in animals with an almost complete Cx43 ablation, an effect associated with increased MMP-9 activity and inflammatory response and reduced fibroblasts differentiation.

Sensory contact to the stressor prevents recovery from structural and functional heart damage following psychosocial trauma

Cardiovascular disorders (CVD) and posttraumatic stress disorder (PTSD) are highly comorbid, but the underlying mechanisms are not fully understood. Chronic psychosocial stress was induced in male mice by chronic subordinate colony housing (CSC), a pre-clinically validated mouse model for PTSD. Cardiac structure and function were assessed on day 20 of the CSC paradigm. Following CSC, mice were kept in different sensory contact modalities to the last aggressor for 30 days, and development of cardiac function and behavioral aspects were determined. Here we show that psychosocial trauma affects heart structure by disturbing cell-to-cell integrity of cardiomyocytes, causes tachycardia, disturbance of diurnal heart rate rhythmicity and behavioral deficits in a mouse model for PTSD. Structural and functional alterations were also found in cardiomyocytes upon in vitro treatment with pro-inflammatory cytokines typically increased after psychosocial trauma. Interestingly, sensory contact to the aggressor subsequent to psychosocial trauma prohibits functional and structural heart recovery, while isolation was beneficial for cardiac but detrimental for mental health. These findings contribute to our understanding of potential mechanisms underlying the high comorbidity of CVD and PTSD.

Properties of cardiac conduction in a cell-based computational model

The conduction of electrical signals through cardiac tissue is essential for maintaining the function of the heart, and conduction abnormalities are known to potentially lead to life-threatening arrhythmias. The properties of cardiac conduction have therefore been the topic of intense study for decades, but a number of questions related to the mechanisms of conduction still remain unresolved. In this paper, we demonstrate how the so-called EMI model may be used to study some of these open questions. In the EMI model, the extracellular space, the cell membrane, the intracellular space and the cell connections are all represented as separate parts of the computational domain, and the model therefore allows for study of local properties that are hard to represent in the classical homogenized bidomain or monodomain models commonly used to study cardiac conduction. We conclude that a non-uniform sodium channel distribution increases the conduction velocity and decreases the time delays over gap junctions of reduced coupling in the EMI model simulations. We also present a theoretical optimal cell length with respect to conduction velocity and consider the possibility of ephaptic coupling (i.e. cell-to-cell coupling through the extracellular potential) acting as an alternative or supporting mechanism to gap junction coupling. We conclude that for a non-uniform distribution of sodium channels and a sufficiently small intercellular distance, ephaptic coupling can influence the dynamics of the sodium channels and potentially provide cell-to-cell coupling when the gap junction connection is absent.

The electrochemical wave traversing the heart during every beat is essential for cardiac pumping function and supply of blood to the body. Understanding the stability of this wave is crucial to understanding how lethal arrhythmias are generated. Despite this importance, our knowledge of the physical determinants of wave propagation are still evolving. One particular challenge has been the lack of accurate mathematical models of conduction at the cellular level. Because cardiac muscle is an electrical syncytium, in which direct charge transfer between cells drives wave propagation, classical bidomain and monodomain tissue models employ a homogenized approximation of this process. This approximation is not valid at the length scale of single cells, and prevents any analysis of how cellular structures impact cardiac conduction. Instead, so-called microdomain models must be used for these questions. Here we utilize a recently developed modelling framework that is well suited to represent small collections of cells. By applying this framework, we show that concentration of sodium channels at the longitudinal borders of myocytes accelerates cardiac conduction. We also demonstrate that when juxtaposed cells are sufficiently close, this non-uniform distribution induces large ephaptic currents, which contribute to intercellular coupling.

Electrocardiographic conduction and repolarization markers associated with sudden cardiac death: moving along the electrocardiography waveform

The QT interval along with its heart rate corrected form (QTc) are well-established ECG markers that have been found to be associated with malignant ventricular arrhythmogenesis. However, extensive preclinical and clinical investigations over the years have allowed for novel clinical ECG markers to be generated as predictors of arrhythmogenesis and sudden cardiac death. Repolarization markers include the older QTc, QT dispersion and newer Tpeak - Tend intervals, (Tpeak - Tend) / QT ratios, T-wave alternans (TWA), microvolt TWA and T-wave area dispersion. Meanwhile, conduction markers dissecting the QRS complex, such as QRS dispersion (QRSD) and fragmented QRS, were also found to correlate conduction velocity and unidirectional block with re-entrant substrates in various cardiac conditions. Both repolarization and conduction parameters can be combined into the excitation wavelength (λ). A surrogate marker for λ is the index of Cardiac Electrophysiological Balance (iCEB: QT / QRSd). Other markers based on conduction-repolarization are [QRSD x (Tpeak-Tend) / QRSd] and [QRSD x (Tpeak-Tend) / (QRSd x QT)]. Advancement in technology permitted sophisticated electrophysiological analyses such as principal component analysis and periodic repolarization dynamics to further improve risk stratification. This was closely followed by other novel indices including ventricular ectopic QRS interval, the f99 index and EntropyXQT, which integrates mathematical and physical calculations for determining the risk markers. Though proven to be effective in limited patient cohorts, more clinical studies across different cardiac pathologies are required to confirm their validity. As such, this review seeks to encapsulate the development of old and new ECG markers along with their associated utility and shortcomings in clinical practice.

Connexin43 dephosphorylation at serine 282 is associated with connexin43-mediated cardiomyocyte apoptosis

Gap junction protein connexin 43 (Cx43) plays an important role in regulating cardiomyocyte survival in addition to regulating electrical coordination. Cx43 dephosphorylation, found in severe cardiac pathologies, is thought to contribute to myocardial injury. However, the mechanisms underlying Cx43 mediation of cell survival and myocardial lesions remain unknown. Here, we found that transfecting an adenovirus carrying a mutant gene of Cx43-serine 282 substituted with alanine (S282A) into neonatal rat ventricular myocytes (NRVMs) induced cell apoptosis and Ca²⁺ transient desynchronization, whereas using gap junction inhibitor or knocking down Cx43 expression with Cx43-miRNA caused uncoupled Ca²⁺ signaling without cell death. Similarly, while Cx43-S282A^+/+ failed in generation, Cx43-S282A^+/- mice exhibited cardiomyocyte apoptosis and ventricular arrhythmias dependent on S282 dephosphorylation. Further, Cx43 dephosphorylation at S282 activated p38 mitogen-activated protein kinase (p38 MAPK), factor-associated suicide and the caspase-8 apoptotic pathway by physically interacting with p38 MAPK. These findings uncovered a specific Cx43 phosphorylation residue involved in regulating cardiomyocyte homeostasis. S282 phosphorylation deficiency acts as a trigger inducing cardiomyocyte apoptosis and cardiac arrhythmias, providing a potential mechanism for Cx43-mediated myocardial injury in severe cardiac diseases.

ADAMTS13 Deficiency Shortens the Life Span of Mice With Experimental Diabetes

In patients with diabetes, impaired activity of ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13), the plasma metalloprotease that cleaves highly thrombogenic von Willebrand factor multimers, is a major risk factor of cardiovascular events. Here, using Adamts13^-/- mice made diabetic by streptozotocin, we investigated the impact of the lack of ADAMTS13 on the development of diabetes-associated end-organ complications. Adamts13^-/- mice experienced a shorter life span than their diabetic wild-type littermates. It was surprising that animal death was not related to the occurrence of detectable thrombotic events. The lack of ADAMTS13 drastically increased the propensity for ventricular arrhythmias during dobutamine-induced stress in diabetic mice. Cardiomyocytes of diabetic Adamts13^-/- mice exhibited an aberrant distribution of the ventricular gap junction connexin 43 and increased phosphorylation of Ca²⁺/calmodulin-dependent kinase II (CaMKII), and with the consequent CaMKII-induced disturbance in Ca²⁺ handling, which underlie propensity for arrhythmia. In vitro, thrombospondin 1 (TSP1) promoted, in a paracrine manner, CaMKII phosphorylation in murine HL-1 cardiomyocytes, and ADAMTS13 acted to inhibit TSP1-induced CaMKII activation. In conclusion, the deficiency of ADAMTS13 may underlie the onset of lethal arrhythmias in diabetes through increased CaMKII phosphorylation in cardiomyocytes. Our findings disclose a novel function for ADAMTS13 beyond its antithrombotic activity.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

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

Role of ion channels in heart failure and channelopathies

Heart failure (HF) is a complication of multiple cardiac diseases and is characterized by impaired contractile and electric function. Patients with HF are not only limited by reduced contractile function but are also prone to life-threatening ventricular arrhythmias. HF itself leads to remodeling of ion channels, gap junctions, and intracellular calcium handling abnormalities that in combination with structural remodeling, e.g., fibrosis, produce a substrate for an arrhythmogenic disorders. Not only ventricular life-threatening arrhythmias contribute to increased morbidity and mortality but also atrial arrhythmias, especially atrial fibrillation (AF), are common in HF patients and contribute to morbidity and mortality. The distinct ion channel remodeling processes in HF and in channelopathies associated with HF will be discussed. Further basic research and clinical studies are needed to identify underlying molecular pathways of HF pathophysiology to provide the basis for improved patient care and individualized therapy based on individualized ion channel composition and remodeling.