The molecular basis of anisotropy: role of gap junctions

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

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

Molecular remodeling of Cx43, but not structural remodeling, promotes arrhythmias in an arrhythmogenic canine model of nonischemic heart failure

BACKGROUND

Both gap junctional remodeling and interstitial fibrosis have been linked to impaired electrical conduction velocity (CV) and fatal ventricular arrhythmias in nonischemic heart failure (HF). However, the arrhythmogenic role of the ventricular gap junctional Cx43 in nonischemic HF remains in debate. Here, we assessed this in a newly developed arrhythmogenic canine model of nonischemic HF.

METHODS AND RESULTS

Nonischemic HF was induced in canines by combined aortic valve insufficiency and aortic constriction. Left ventricular (LV) myocardium from HF dogs showed similar pathological changes to that of humans. HF dogs had reduced LV function, widened QRS complexes, and spontaneous nonsustained ventricular tachycardia. CV was measured in intact LV epicardium with high-density grid mapping. Total (Cx43-T) and nonphosphorylated Cx43 (Cx43-NP) and histological interstitial fibrosis were assessed from these mapped LV tissues. Longitudinal CV, which was slowed in HF (49 ± 1 vs. 65 ± 2 cm/s in Ctl), was positively correlated with reduced total junctional Cx43 and negatively correlated with markedly increased junctional Cx43-NP (2-fold) in HF. Cx43 dephosphorylation in HF was associated with enhanced colocalization of PP2A at the level of Cx43. Unchanged action potential upstroke and transverse CV were associated with unaltered Cx43 lateralization and interstitial fibrosis in the nonischemic HF canine LV.

CONCLUSION

Our unique arrhythmogenic canine model of HF resembles human nonischemic HF (prior to the end stage). Cx43 remodeling occurs prior to the structural remodeling (with lack of fibrosis) in HF and it is crucial in slowed CV and ventricular arrhythmia development. Our findings suggest that altered Cx43 alone is arrhythmogenic and modulation of Cx43 has the anti-arrhythmic therapeutic potential for HF patients.

Multi-scale approaches for the simulation of cardiac electrophysiology: II - Tissue-level structure and function

Computational models of the heart, from cell-level models, through one-, two- and three-dimensional tissue-level simplifications, to biophysically-detailed three-dimensional models of the ventricles, atria or whole heart, allow the simulation of excitation and propagation of this excitation, and have provided remarkable insight into the normal and pathological functioning of the heart. In this article we present equations for modelling cellular excitation (i.e. the cell action potential) from both a phenomenological and a biophysical perspective. Hodgkin-Huxley formalism is discussed, along with the current generation of biophysically-detailed cardiac cell models. Alternative Markovian formulations for modelling ionic currents are also presented. Equations describing propagation of this cellular excitation, through one-, two- and three-dimensional idealised or realistic tissues, are then presented. For all types of model, from cell to tissue, methods for discretisation and integration of the underlying equations are discussed. The article finishes with a discussion of two tissue-level experimental imaging techniques - diffusion tensor magnetic resonance imaging and optical imaging - that can be used to provide data for parameterisation and validation of cell- and tissue-level cardiac models.

The stress kinase JNK regulates gap junction Cx43 gene expression and promotes atrial fibrillation in the aged heart

BACKGROUND

The stress kinase c-jun N-terminal kinase (JNK) is critical in the pathogenesis of cardiac diseases associated with an increased incidence of atrial fibrillation (AF), the most common arrhythmia in the elderly. We recently discovered that JNK activation is linked to the loss of gap junction connexin43 (Cx43) and enhanced atrial arrhythmogenicity. However, direct evidence for JNK-mediated impairment of intercellular coupling (cell-cell communication) in the intact aged atrium is lacking, as is evidence for whether and how JNK suppresses Cx43 in the aged human atrium.

METHODS AND RESULTS

JNK activity in human atrial samples is correlated with both reduced Cx43 expression and increasing age. Using a unique technique of optical mapping space constant measurement, we found that impaired intercellular coupling and reduced Cx43 were linked to enhanced activation of JNK in intact aged rabbit atria. These JNK-associated alterations were further confirmed in naturally JNK activated aged mice and in cardiac-specific inducible MKK7D (JNK upstream activator) young mice. Moreover, JNK inhibition, using either JNK specific inhibitors in aged wild-type (WT) mice and JNK activator anisomycin-treated young WT mice or JNK1/2 dominant-negative mice with genetically inhibited cardiac JNK activity, completely eliminated these functional abnormalities. Furthermore, we discovered for the first time that long-term JNK activation downregulates Cx43 expression via c-jun suppressed transcriptional activity of the Cx43 gene promoter.

CONCLUSION

Our results demonstrate that JNK is a critical regulator of Cx43 expression, and that augmented JNK activation in aged atria downregulates Cx43 to impair cell-cell communication and promote the development of AF. JNK inhibition may represent a promising therapeutic approach to prevent or treat AF in the elderly.

Regulation of cardiac gap junctions by protein phosphatases

Sufficient connexin-mediated intercellular coupling is critical to maintain gap junctional communication for proper cardiac function. Alterations in connexin phosphorylation state, particularly dephosphorylation of connexin 43 (Cx43), may impact cell coupling and conduction in disease states. Cx43 dephosphorylation may be carried out by protein phosphatase activity. Here, we present an overview of the key phosphatases known to interact with Cx43 or modulators of Cx43, as well as some possible therapeutic targets to regulate phosphatase activity in the heart.

Localisation Microscopy of Breast Epithelial ErbB-2 Receptors and Gap Junctions: Trafficking after γ-Irradiation, Neuregulin-1β, and Trastuzumab Application

In cancer, vulnerable breast epithelium malignance tendency correlates with number and activation of ErbB receptor tyrosine kinases. In the presented work, we observe ErbB receptors activated by irradiation-induced DNA injury or neuregulin-1β application, or alternatively, attenuated by a therapeutic antibody using high resolution fluorescence localization microscopy. The gap junction turnover coinciding with ErbB receptor activation and co-transport is simultaneously recorded. DNA injury caused by 4 Gray of 6 MeV photon γ-irradiation or alternatively neuregulin-1β application mobilized ErbB receptors in a nucleograde fashion—a process attenuated by trastuzumab antibody application. This was accompanied by increased receptor density, indicating packing into transport units. Factors mobilizing ErbB receptors also mobilized plasma membrane resident gap junction channels. The time course of ErbB receptor activation and gap junction mobilization recapitulates the time course of non-homologous end-joining DNA repair. We explain our findings under terms of DNA injury-induced membrane receptor tyrosine kinase activation and retrograde trafficking. In addition, we interpret the phenomenon of retrograde co-trafficking of gap junction connexons stimulated by ErbB receptor activation.

Reconstruction of Cell Surface Densities of Ion Pumps, Exchangers, and Channels from mRNA Expression, Conductance Kinetics, Whole-Cell Calcium, and Current-Clamp Voltage Recordings, with an Application to Human Uterine Smooth Muscle Cells

Uterine smooth muscle cells remain quiescent throughout most of gestation, only generating spontaneous action potentials immediately prior to, and during, labor. This study presents a method that combines transcriptomics with biophysical recordings to characterise the conductance repertoire of these cells, the ‘conductance repertoire’ being the total complement of ion channels and transporters expressed by an electrically active cell. Transcriptomic analysis provides a set of potential electrogenic entities, of which the conductance repertoire is a subset. Each entity within the conductance repertoire was modeled independently and its gating parameter values were fixed using the available biophysical data. The only remaining free parameters were the surface densities for each entity. We characterise the space of combinations of surface densities (density vectors) consistent with experimentally observed membrane potential and calcium waveforms. This yields insights on the functional redundancy of the system as well as its behavioral versatility. Our approach couples high-throughput transcriptomic data with physiological behaviors in health and disease, and provides a formal method to link genotype to phenotype in excitable systems. We accurately predict current densities and chart functional redundancy. For example, we find that to evoke the observed voltage waveform, the BK channel is functionally redundant whereas hERG is essential. Furthermore, our analysis suggests that activation of calcium-activated chloride conductances by intracellular calcium release is the key factor underlying spontaneous depolarisations.

A well-known problem in electrophysiologal modeling is that the parameters of the gating kinetics of the ion channels cannot be uniquely determined from observed behavior at the cellular level. One solution is to employ simplified “macroscopic” currents that mimic the behavior of aggregates of distinct entities at the protein level. The gating parameters of each channel or pump can be determined by studying it in isolation, leaving the general problem of finding the densities at which the channels occur in the plasma membrane. We propose an approach, which we apply to uterine smooth muscle cells, whereby we constrain the list of possible entities by means of transcriptomics and chart the indeterminacy of the problem in terms of the kernel of the corresponding linear transformation. A graphical representation of this kernel visualises the functional redundancy of the system. We show that the role of certain conductances can be fulfilled, or compensated for, by suitable combinations of other conductances; this is not always the case, and such “non-substitutable” conductances can be regarded as functionally non-redundant. Electrogenic entities belonging to the latter category are suitable putative clinical targets.

Tissue-Mimicking Geometrical Constraints Stimulate Tissue-Like Constitution and Activity of Mouse Neonatal and Human-Induced Pluripotent Stem Cell-Derived Cardiac Myocytes

The present work addresses the question of to what extent a geometrical support acts as a physiological determining template in the setup of artificial cardiac tissue. Surface patterns with alternating concave to convex transitions of cell size dimensions were used to organize and orientate human-induced pluripotent stem cell (hIPSC)-derived cardiac myocytes and mouse neonatal cardiac myocytes. The shape of the cells, as well as the organization of the contractile apparatus recapitulates the anisotropic line pattern geometry being derived from tissue geometry motives. The intracellular organization of the contractile apparatus and the cell coupling via gap junctions of cell assemblies growing in a random or organized pattern were examined. Cell spatial and temporal coordinated excitation and contraction has been compared on plain and patterned substrates. While the α-actinin cytoskeletal organization is comparable to terminally-developed native ventricular tissue, connexin-43 expression does not recapitulate gap junction distribution of heart muscle tissue. However, coordinated contractions could be observed. The results of tissue-like cell ensemble organization open new insights into geometry-dependent cell organization, the cultivation of artificial heart tissue from stem cells and the anisotropy-dependent activity of therapeutic compounds.

Alterations in connexin 43 during diabetic cardiomyopathy: competition of tyrosine nitration versus phosphorylation

BACKGROUND

Cardiac conduction abnormalities are observed early in the progression of type 1 diabetes (T1D), but the mechanism(s) involved are undefined. Connexin 43, a critical component of ventricular gap junctions, depends on tyrosine phosphorylation status to modulate channel conductance; changes in connexin 43 content, distribution, and/or phosphorylation status may be involved in cardiac rhythm disturbances. We tested the hypothesis that cardiac content and/or distribution of connexin 43 is altered in a rat model of T1D cardiomyopathy, investigating a mechanistic role for tyrosine.

METHODS

Electrocardiographic analyses were conducted during the progression of diabetic cardiomyopathy in rats dosed with streptozotocin (STZ; 65 mg/kg) 3, 7, and 35 days after the induction of diabetes. Following functional analyses, we conducted immunohistochemical and immunoprecipitation studies to assess alterations in connexin 43.

RESULTS

There was significant evidence of ventricular conduction abnormalities (QRS complex, Q-T interval) as early as 7 days after STZ, persisting throughout the study. Connexin 43 levels were increased 7 days after STZ and remained elevated throughout the study. Connexin 40 content was unchanged relative to controls throughout the study. Changes in connexin 43 distribution were also observed: connexin 43 staining was dispersed from myocyte short axis junctions. Connexin 43 tyrosine phosphorylation declined during the progression of diabetes, with concurrent increases in tyrosine nitration.

CONCLUSIONS

The data suggest that changes in connexin 43 content and distribution occur during experimental diabetes and likely contribute to alterations in cardiac function, and that oxidative modification of tyrosine-mediated signaling may play a mechanistic role.

Novel methods of automated quantification of gap junction distribution and interstitial collagen quantity from animal and human atrial tissue sections

BACKGROUND

Gap junctions (GJs) are the principal membrane structures that conduct electrical impulses between cardiac myocytes while interstitial collagen (IC) can physically separate adjacent myocytes and limit cell-cell communication. Emerging evidence suggests that both GJ and interstitial structural remodeling are linked to cardiac arrhythmia development. However, automated quantitative identification of GJ distribution and IC deposition from microscopic histological images has proven to be challenging. Such quantification is required to improve the understanding of functional consequences of GJ and structural remodeling in cardiac electrophysiology studies.

METHODS AND RESULTS

Separate approaches were employed for GJ and IC identification in images from histologically stained tissue sections obtained from rabbit and human atria. For GJ identification, we recognized N-Cadherin (N-Cad) as part of the gap junction connexin 43 (Cx43) molecular complex. Because N-Cad anchors Cx43 on intercalated discs (ID) to form functional GJ channels on cell membranes, we computationally dilated N-Cad pixels to create N-Cad units that covered all ID-associated Cx43 pixels on Cx43/N-Cad double immunostained confocal images. This approach allowed segmentation between ID-associated and non-ID-associated Cx43. Additionally, use of N-Cad as a unique internal reference with Z-stack layer-by-layer confocal images potentially limits sample processing related artifacts in Cx43 quantification. For IC quantification, color map thresholding of Masson's Trichrome blue stained sections allowed straightforward and automated segmentation of collagen from non-collagen pixels. Our results strongly demonstrate that the two novel image-processing approaches can minimize potential overestimation or underestimation of gap junction and structural remodeling in healthy and pathological hearts. The results of using the two novel methods will significantly improve our understanding of the molecular and structural remodeling associated functional changes in cardiac arrhythmia development in aged and diseased hearts.

Fibroblast-myocyte electrotonic coupling: does it occur in native cardiac tissue?

Heterocellular electrotonic coupling between cardiac myocytes and non-excitable connective tissue cells has been a long-established and well-researched fact in vitro. Whether or not such coupling exists in vivo has been a matter of considerable debate. This paper reviews the development of experimental insight and conceptual views on this topic, describes evidence in favour of and against the presence of such coupling in native myocardium, and identifies directions for further study needed to resolve the riddle, perhaps less so in terms of principal presence which has been demonstrated, but undoubtedly in terms of extent, regulation, patho-physiological context, and actual relevance of cardiac myocyte–non-myocyte coupling in vivo. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

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Electrical coupling of cardiomyocytes and fibroblasts is well-established in vitro

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Whether such hetero-cellular coupling exists in vivo has been a matter of debate

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We review the development of experimental and conceptual insight into the topic

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Conclusion 1: hetero-cellular coupling in heart tissue has been shown in principle

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Conclusion 2: extent, regulation, context, and relevance remain to be established

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Regulation of gap junctions by nitric oxide influences the generation of arrhythmias resulting from acute ischemia and reperfusion in vivo

Myocardial ischemia resulting from sudden occlusion of a coronary artery is one of the major causes in the appearance of severe, often life-threatening ventricular arrhythmias. Although the underlying mechanisms of these acute arrhythmias are many and varied, there is no doubt that uncoupling of gap junctions (GJs) play an important role especially in arrhythmias that are generated during phase Ib, and often terminate in sudden cardiac death. In the past decades considerable efforts have been made to explore mechanisms which regulate the function of GJs, and to find new approaches for protection against arrhythmias through the modulation of GJs. These investigations led to the development of GJ openers and inhibitors. The pharmacological modulation of GJs, however, resulted in conflicting results. It is still not clear whether opening or closing of GJs would be advantageous for the ischemic myocardium. Both maneuvers can result in protection, depending on the models, endpoints and the time of opening and closing of GJs. Furthermore, although there is substantial evidence that preconditioning decreases or delays the uncoupling of GJs, the precise mechanisms by which this attains have not yet been elucidated. In our own studies in anesthetized dogs preconditioning suppressed the ischemia and reperfusion-induced ventricular arrhythmias, and this protection was associated with the preservation of GJ function, manifested in less marked changes in electrical impedance, as well as in the maintenance of GJ permeability and phosphorylation of connexin43. Since we have substantial previous evidence that nitric oxide (NO) is an important trigger and mediator of the preconditioning-induced antiarrhythmic protection, we hypothesized that NO, among its several effects, may lead to this protection by influencing cardiac GJs. The hypotheses and theories relating to the pharmacological modulation of GJs will be discussed with particular attention to the role of NO.

c-Jun N-terminal kinase activation contributes to reduced connexin43 and development of atrial arrhythmias

AIMS

c-Jun N-terminal kinase (JNK) activation is implicated in cardiovascular diseases and ageing, which are linked to enhanced propensity to atrial fibrillation (AF). However, the contribution of JNK to AF remains unknown. Thus, we assessed the role of JNK in remodelling of gap junction connexin43 (Cx43) and development of AF.

METHODS AND RESULTS

AF induction, optical mapping, and biochemical assays were performed in young and aged New Zealand white rabbit left atria (LA) and cultured HL-1 atrial cells. In aged rabbit LA, pacing-induced atrial arrhythmias were dramatically increased and conduction velocity (CV) was significantly slower compared with young controls. Aged rabbit LA contained 120% more activated JNK and 54% less Cx43 than young LA. Young rabbits treated with JNK activator anisomycin also exhibited increased pacing-induced atrial arrhythmias and reduced Cx43 (by 34%), similar to that found in aged LA. In HL-1 cell cultures, anisomycin treatment for 16 h led to 42% reduction in Cx43, 24% reduction in CV, and an increased incidence of irregular rapid spontaneous activities. These effects were prevented by a specific JNK inhibitor, SP600125. Moreover, a 63% reduction in Cx43 after anisomycin treatment for 24 h led to further slowed CV (by 41%) along with dramatically increased irregular rapid spontaneous activity and highly discontinuous conduction. These JNK-induced functional abnormalities were completely reversed by overexpressed exogenous wild-type Cx43, but not by inactive Cx43.

CONCLUSION

JNK activation contributes to Cx43 reductions that promote development of AF. Modulation of JNK may be a potential novel therapeutic approach to prevent and treat AF.

Slow conduction and gap junction remodeling in murine ventricle after chronic alcohol ingestion

BACKGROUND

Long-term heavy alcohol drinkers are prone to the development of cardiac arrhythmia. To understand the mechanisms, we evaluated the cardiac structural and electrophysiological changes in mice chronically drinking excessive alcohol.

RESULTS

Male C57BL/6J mice were given 36% alcohol in the drinking water. Those given blank water were used as control. Twelve weeks later, the phenotypic characteristics of the heart, including gap junctions and electrical properties were examined. In the alcohol group the ventricles contained a smaller size of cardiomyocytes and a higher density of capillary networks, compared to the control. Western blots showed that, after drinking alcohol, the content of connexin43 (Cx43) protein in the left ventricle was increased by 18% (p < 0.05). Consistently, immunoconfocal microscopy demonstrated that Cx43 gap junctions were up-regulated in the alcohol group with a disorganized distribution, compared to the control. Optical mapping showed that the alcohol group had a reduced conduction velocity (40 ± 18 vs 60 ± 7 cm/sec, p < 0.05) and a higher incidence of ventricular tachyarrhythmia (62% vs 30%, p < 0.05).

CONCLUSION

Long-term excessive alcohol intake resulted in extensive cardiac remodeling, including changes in expression and distribution of gap junctions, growth of capillary network, reduction of cardiomyocyte size, and decrease of myocardial conduction.

The connexin43 carboxyl terminus and cardiac gap junction organization

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

Enhanced activation of p21-activated kinase 1 in heart failure contributes to dephosphorylation of connexin 43

AIMS

We previously showed decreased cellular coupling and dephosphorylation of the gap junctional protein connexin 43 (Cx43) in left ventricular (LV) myocytes from an arrhythmogenic rabbit model of non-ischaemic heart failure (HF) that was associated with a 2.5-fold increase in the amount of protein phosphatase type 2A (PP2A) co-localized with Cx43. Here, we further explore the molecular mechanisms of enhanced dephosphorylation of Cx43 in HF. p21-activated kinase 1 (PAK1) is a serine-threonine protein kinase that has been shown to activate PP2A.

METHODS AND RESULTS

We found that total PAK1 and activated PAK1 (PAK1-P(Thr423)) were both increased in HF rabbit LV (vs. controls). PAK1 co-immunoprecipitated (co-IP'd) with Cx43 protein and, with HF, co-IP'd PAK1 and PAK1-P(Thr423) were increased. With failing human LV, PAK1 total protein and PAK1-P(Thr423) were also increased globally and locally (co-IP'd with Cx43). To further explore the role of PAK1 in modulating Cx43 dephosphorylation and intercellular coupling, we overexpressed active PAK1 in isolated LV myocytes from control rabbits and in HEK293 cells with genetically modified overexpression of Cx43 (HEK293-Cx43). PAK1 overexpression in both rabbit myocytes and HEK293-Cx43 cells significantly increased PP2A activity (globally and at the level of Cx43), increased dephosphorylated Cx43, and markedly reduced intercellular dye coupling. These effects were attenuated with PP2A inhibition using okadaic acid (10 nM).

CONCLUSIONS

PAK1 and PP2A are integral components of a macromolecular complex with cardiac Cx43, and increased activation of associated PAK1 can contribute to enhanced Cx43 dephosphorylation and impaired intercellular coupling that may underlie slow conduction in HF.

Changes in connexin expression and the atrial fibrillation substrate in congestive heart failure

RATIONALE

Although connexin changes are important for the ventricular arrhythmic substrate in congestive heart failure (CHF), connexin alterations during CHF-related atrial arrhythmogenic remodeling have received limited attention.

OBJECTIVE

To analyze connexin changes and their potential contribution to the atrial fibrillation (AF) substrate during the development and reversal of CHF.

METHODS AND RESULTS

Three groups of dogs were studied: CHF induced by 2-week ventricular tachypacing (240 bpm, n=15); CHF dogs allowed a 4-week nonpaced recovery interval after 2-week tachypacing (n=16); and nonpaced sham controls (n=19). Left ventricular (LV) end-diastolic pressure and atrial refractory periods increased with CHF and normalized on CHF recovery. CHF caused abnormalities in atrial conduction indexes and increased the duration of burst pacing-induced AF (DAF, from 22+/-7 seconds in control to 1100+/-171 seconds, P<0.001). CHF did not significantly alter overall atrial connexin (Cx)40 and Cx43 mRNA and protein expression levels, but produced Cx43 dephosphorylation, increased Cx40/Cx43 protein expression ratio and caused Cx43 redistribution toward transverse cell-boundaries. All of the connexin-alterations reversed on CHF recovery, but CHF-induced conduction abnormalities and increased DAF (884+/-220 seconds, P<0.001 versus control) remained. The atrial fibrous tissue content increased from 3.6+/-0.7% in control to 14.7+/-1.5% and 13.3+/-2.3% in CHF and CHF recovery, respectively (both P<0.01 versus control), with transversely running zones of fibrosis physically separating longitudinally directed muscle bundles. In an ionically based action potential/tissue model, fibrosis was able to account for conduction abnormalities associated with CHF and recovery.

CONCLUSIONS

CHF causes atrial connexin changes, but these are not essential for CHF-related conduction disturbances and AF promotion, which are rather related primarily to fibrotic interruption of muscle bundle continuity.

Pathological remodeling of cardiac gap junction connexin 43-With special reference to arrhythmogenesis

A dysfunction of the cardiac gap junction, which contributes to electrical cell-to-cell coupling is one of essential factors known to generate arrhythmias. The function of the gap junction depends on the regulation of connexin which composes the gap junction channel. A dysfunction of the gap junction is possibly caused by the down-regulation of connexin. In this review, the relationship between pathological remodeling of connexin 43 (Cx43) and susceptibility of the heart to the ventricular fibrillation, which is a lethal ventricular tachyarrhythmia, is addressed. A suppression of the PKA-mediated phosphorylation or an augmentation of the PKC-mediated phosphorylation of Cx43 induces the downward remodeling of Cx43. Factors regarding downward remodeling of Cx43, such as hypoxia (including intracellular Ca overload and intracellular acidosis), angiotensin II or an activation of PKCvarepsilon make the heart more susceptible to the ventricular fibrillation, while factors regarding upward remodeling of Cx43, such as cyclic AMP or an activation of PKA, lower susceptibility. As a result, from a clinical point of view, angiotensin II antagonists (synthesis inhibitors or receptor blockades), PKC inhibitors or PKA activators are thus considered to provide a therapeutic approach for the treatment of the initiation or advancement of the ventricular fibrillation.

Pharmacological approach to the treatment of long and short QT syndromes

Inherited channelopathies have received increasing attention in recent years. The past decade has witnessed impressive progress in our understanding of the molecular and cellular basis of arrhythmogenesis associated with inherited channelopathies. An imbalance in ionic forces induced by these channelopathies affects the duration of ventricular repolarization and amplifies the intrinsic electrical heterogeneity of the myocardium, creating an arrhythmogenic milieu. Today, many of the channelopathies have been linked to mutations in specific genes encoding either components of ion channels or membrane or regulatory proteins. Many of the channelopathies are genetically heterogeneous with a variable degree of expression of the disease. Defining the molecular basis of channelopathies can have a profound impact on patient management, particularly in cases in which genotype-specific pharmacotherapy is available. The long QT syndrome (LQTS) is one of the first identified and most studied channelopathies where abnormal prolongation of ventricular repolarization predisposes an individual to life threatening ventricular arrhythmia called Torsade de Pointes. On the other hand of the spectrum, molecular defects favoring premature repolarization lead to Short QT syndrome (SQTS), a recently described inherited channelopathy. Both of these channelopathies are associated with a high risk of sudden cardiac death due to malignant ventricular arrhythmia. Whereas pharmacological therapy is first line treatment for LQTS, defibrillators are considered as primary treatment for SQTS. This review provides a comprehensive review of the molecular genetics, clinical features, genotype-phenotype correlations and genotype-specific approach to pharmacotherapy of these two mirror-image channelopathies, SQTS and LQTS.

Effect of nonuniform interstitial space properties on impulse propagation: a discrete multidomain model

This work presents a discrete multidomain model that describes ionic diffusion pathways between connected cells and within the interstitium. Unlike classical models of impulse propagation, the intracellular and extracellular spaces are represented as spatially distinct volumes with dynamic/static boundary conditions that electrically couple neighboring spaces. The model is used to investigate the impact of nonuniform geometrical and electrical properties of the interstitial space surrounding a fiber on conduction velocity and action potential waveshape. Comparison of the multidomain and bidomain models shows that although the conduction velocity is relatively insensitive to cases that confine 50% of the membrane surface by narrow extracellular depths (> or =2 nm), the action potential morphology varies greatly around the fiber perimeter, resulting in changes in the magnitude of extracellular potential in the tight spaces. Results also show that when the conductivity of the tight spaces is sufficiently reduced, the membrane adjacent to the tight space is eliminated from participating in propagation, and the conduction velocity increases. Owing to its ability to describe the spatial discontinuity of cardiac microstructure, the discrete multidomain can be used to determine appropriate tissue properties for use in classical macroscopic models such as the bidomain during normal and pathophysiological conditions.

Pharmacological modulation of gap junction function with the novel compound rotigaptide: a promising new principle for prevention of arrhythmias

Existing anti-arrhythmic therapy is hampered by lack of efficacy and unacceptable side effects. Thus, ventricular tachycardia and fibrillation remains the strongest predictor of in-hospital mortality in patients with myocardial infarction. In atrial fibrillation, rhythm control with conventional ion channel blockers provide no therapeutic benefit relative to rate control. Several lines of research indicate that impaired gap junctional cell-to-cell coupling between neighbouring cardiomyocytes is critical for the development of cardiac re-entry arrhythmias. Rotigaptide is the first drug that has been developed to prevent arrhythmias by re-establishing gap junctional intercellular communication. During conditions with acute cardiac ischaemia, rotigaptide effectively prevents induction of both ventricular and atrial tachyarrhythmia. Moreover, rotigaptide effectively prevents ischaemia reperfusion arrhythmias. At the cellular level, rotigaptide inhibits ischaemia-induced dephosphorylation of Ser297 and Ser368, which is considered important for the gating of connexin43 gap junction channels. No drug-related toxicity has been demonstrated at plasma concentrations 77,000 times above therapeutic concentrations. In rats and dogs, rotigaptide reduces infarct size following myocardial infarction. A series of phase I trials has been completed in which rotigaptide has been administered intravenously to ~200 healthy persons. No drug-related side effects have been demonstrated in healthy human beings. Clinical safety, tolerability and efficacy in patients with heart disease are being evaluated in ongoing clinical trials. Rotigaptide represents a pioneering pharmacological principle with a highly favourable preclinical and clinical safety profile, which makes this molecule a promising drug candidate for the prevention of cardiac arrhythmias.

Increasing gap junction coupling reduces transmural dispersion of repolarization and prevents torsade de pointes in rabbit LQT3 model

INTRODUCTION

Increased transmural dispersion of repolarization (TDR) contributes importantly to the development of torsades de pointes (TdP) in long QT syndrome (LQTS). Intercellular electrical coupling via gap junctions plays an important role in maintaining TDR in both normal and diseased hearts. This study examined the effects of antiarrhythmic peptide AAP10, a gap junction enhancer, on TDR and induction of TdP in a rabbit LQT3 model.

METHODS AND RESULTS

An arterially perfused rabbit left ventricular preparation and sea anemone toxin II (ATX-II, 20 nM) were used to establish a LQT3 model. Transmural ECG as well as action potentials from both endocardium and epicardium were simultaneously recorded. Changes in nonphosphorylated connexin43 (Cx43) were measured by immunoblotting. Compared with the control group, the QT interval, TDR, early afterdepolariztion (EAD), R-on-T extrasystole, and TdP increased sharply with augmented nonphosphorylated Cx43 in the LQT3 group (P < 0.001 for both). Interestingly, compared with the LQT3 group, 500 nM AAP10 reduced QT interval, TDR (P < 0.001 for both), and prevented EAD, R-on-T extrasystole, and TdP (P = 0.003, P = 0.001, P = 0.02) with a parallel decrease in nonphosphorylated Cx43 in the presence of ATX-II (P < 0.001).

CONCLUSION

Gap junction enhancer AAP10 is capable of abbreviating the QT interval, reducing TDR, and suppressing TdP in a rabbit LQT3 model probably via its effect by preventing dephosphorylation of Cx43. These data suggest that increasing intercellular coupling may reduce TDR and, therefore, prevent TdP in LQTS.

Left ventricular form and function revisited: applied translational science to cardiovascular ultrasound imaging

Doppler tissue imaging (DTI) and DTI-derived strain imaging are robust physiologic tools used for the noninvasive assessment of regional myocardial function. As a result of high temporal and spatial resolution, regional function can be assessed for each phase of the cardiac cycle and within the transmural layers of the myocardial wall. Newer techniques that measure myocardial motion by speckle tracking in gray-scale images have overcome the angle dependence of DTI strain, allowing for measurement of 2-dimensional strain and cardiac rotation. DTI, DTI strain, and speckle tracking may provide unique information that deciphers the deformation sequence of complexly oriented myofibers in the left ventricular wall. The data are, however, limited. This review examines the structure and function of the left ventricle relative to the potential clinical application of DTI and speckle tracking in assessing the global mechanical sequence of the left ventricle in vivo.

Development of the cardiac conduction system

The cardiac conduction system (CCS) is a specialized tissue network that initiates and maintains a rhythmic heartbeat. The CCS consists of several functional subcomponents responsible for producing a pacemaking impulse and distributing action potentials across the heart in a coordinated manner. The formation of the distinct subcomponents of the CCS occurs within a precise temporal and spatial framework; thereby assuring that as the system matures from a tubular to a complex chambered organ, a rhythmic heartbeat is always maintained. Therefore, a defect in differentiation of any CCS component would lead to severe rhythm disturbances. Recent molecular, cell biological and physiological approaches have provided fresh and unexpected perspectives of the relationships between cell fate, gene expression and differentiation of specialized function within the developing myocardium. In particular, biomechanical forces created by the heartbeat itself have important roles in the inductive patterning and functional integration of the developing conduction system. This new understanding of the cellular origin and molecular induction of CCS tissues during embryogenesis may provide the foundation for tissue engineering, replacement and repair of these essential cardiac tissues in the future.

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

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

Molecular basis of isolated cardiac conduction disease

Cardiac conduction disorders are among the most common rhythm disturbances causing disability in millions of people worldwide and necessitating pacemaker implantation. Isolated cardiac conduction disease (ICCD) can affect various regions within the heart, and therefore the clinical features also vary from case to case. Typically, it is characterized by progressive alteration of cardiac conduction through the atrioventricular node, His-Purkinje system, with right or left bundle branch block and QRS widening. In some instances, the disorder may progress to complete atrioventricular block, with syncope and even death. While the role of genetic factors in conduction disease has been suggested as early as the 1970s, it was only recently that specific genetic loci have been reported. Multiple mutations in the gene encoding for the cardiac voltage-gated sodium channel (SCN5A), which plays a fundamental role in the initiation, propagation, and maintenance of normal cardiac rhythm, have been linked to conduction disease, allowing for genotype-phenotype correlation. The electrophysiological characterization of heterologously expressed mutant Na+ channels has revealed gating defects that consistently lead to a loss of channel function. However, studies have also revealed significant overlap between aberrant rhythm phenotypes, and single mutations have been identified that evoke multiple distinct rhythm disorders with common gating lesions. These new insights highlight the complexities involved in linking single mutations, ion-channel behavior, and cardiac rhythm but suggest that interplay between multiple factors could underlie the manifestation of the disease phenotype.

Effect of fiber orientation on propagation: electrical mapping of genetically altered mouse hearts

BACKGROUND

Epicardial potentials reveal the strong effects of fiber anisotropy, rotation, imbrication, and coupling on propagation in the intact heart. From the patterns of the surface potentials, we can obtain information about the local fiber orientation, anisotropy, the transmural fiber rotation, and which direction the wave front is traveling through the wall. In this study, lessons learned from epicardial potential mapping of large hearts were applied to studies conducted in genetically altered mouse hearts.

METHODS

An inducible model of the overexpression of a gain-of-function alpha5 integrin (cytoplasmic domain truncation) was created in mouse. After 3 days of administration of doxycycline, the animals exhibited an altered electrical phenotype of markedly reduced amplitude of the QRS complex on the surface electrocardiogram. Epicardial potentials were recorded from Langendorff-perfused mouse hearts with alpha5 integrin gain-of-function mutations and from wild-type (WT) control hearts. A cylindrical electrode array consisting of 184 sites with 1-mm uniform interelectrode spacing was placed around the heart, and unipolar electrograms were recorded during atrial and ventricular stimulation at different basic cycle lengths.

RESULTS

The total ventricular activation time for the transgenic animals was greater than that of the WT hearts for atrial and ventricular pacing locations. The isopotential maps from the mutated hearts showed a loss of anisotropy, as revealed by the more rounded and less elliptically shaped wave fronts seen immediately after epicardial point stimulation when compared with WT hearts. The weaker potential maxima in the mutated hearts did not exhibit the normal expansion and rotation associated with an advancing wave front in a normal heart, suggesting abnormalities in myocyte coupling in these hearts. Isopotential maps provided additional information about fiber architecture from the electric field that was not obtained from optical recordings alone. These findings provided a phenotypic characterization and specific insights into the mechanisms of the electrical abnormalities associated with altered integrin signaling in cardiac myocytes.

Nature, significance, and mechanisms of electrical heterogeneities in ventricle

Previously, dispersion of repolarization (DOR) has been extensively linked to the development of arrhythmias and sudden cardiac death. The electrical heterogeneities that cause DOR between transmural myocyte layers have been reported in a wide variety of animals and humans. The underlying causes of transmural electrical heterogeneities are in part due to heterogeneous functional expression of proteins responsible for ion handling. Recently, we found that electrophysiologic heterogeneities between subepicardial and midmyocardial cells can form a substrate for reentrant ventricular arrhythmias. However, cell-to-cell coupling through gap junctions is expected to attenuate transmural heterogeneities between cell types spanning the ventricular wall. In this article we review a hypothesis that regional uncoupling resulting from expression patterns of gap junctions across the ventricular wall underlies DOR, and DOR can be amplified under disease conditions which remodel gap junctions. We find the principle gap junction protein, connexin43 (Cx43), is selectively reduced in the subepicardium (by 24%) compared to deeper layers of normal canine left ventricle. Additionally, the greatest DOR occurs within the subepicardial-midmyocardial interface, precisely where Cx43 expression is reduced. The present data suggests that ion channel and gap junction heterogeneities act in conjunction to form and maintain transmural DOR. Importantly, both ion channel and gap junction remodeling occurs during many disease states such as heart failure. Importantly, in the absence of ion channel remodeling, pharmacological uncoupling increases transmural DOR, particularly within the epicardial-midmyocardial interface, to values observed in heart failure. Therefore, these data suggest that heterogeneous Cx43 expression produces functionally significant electrophysiologic heterogeneities across the ventricular wall and may be a mechanism for promoting DOR which underlie arrhythmias in heart failure.

Defibrillation Depends on Conductivity Fluctuations and the Degree of Disorganization in Reentry Patterns

Defibrillation depends on conductivity and disorganization.

INTRODUCTION

Cardiac fibrillation is the deterioration of the heart's normally well-organized activity into one or more meandering spiral waves, which subsequently break up into many meandering wave fronts. Delivery of an electric shock (defibrillation) is the only effective way of restoring the normal rhythm. This study focuses on examining whether higher degrees of disorganization requires higher shock strengths to defibrillate and whether microscopic conductivity fluctuations favor shock success.

METHODS AND RESULTS

We developed a three-dimensional computer bidomain model of a block of cardiac tissue with straight fibers immersed in a conductive bath. The membrane behavior was described by the Courtemanche human atrial action potential model incorporating electroporation and an acetylcholine- (ACh) dependent potassium current. Intracellular conductivities were varied stochastically around nominal values with variations of up to 50%. A single rotor reentry was initiated and, by adjusting the spatial ACh variation, the level of organization could be controlled. The single rotor could be stabilized or spiral wave breakup could be provoked leading to fibrillatory-like activity. For each level of organization, multiple shock timings and strengths were applied to compute the probability of shock success as a function of shock strength.

CONCLUSIONS

Our results suggest that the level of the small-scale conductivity fluctuations is a very important factor in defibrillation. A higher variation significantly lowers the required shock strength. Further, we demonstrated that success also heavily depends on the level of organization of the fibrillatory episode. In general, higher levels of disorganization require higher shock strengths to defibrillate.

Connexin 43 Downregulation and Dephosphorylation in Nonischemic Heart Failure Is Associated With Enhanced Colocalized Protein Phosphatase Type 2A

In nonischemic heart failure (HF), ventricular tachycardia initiates by a nonreentrant mechanism, but there is altered conduction (that could lead to re-entry) that could arise from changes in gap junctional proteins, especially connexin43 (Cx43). We studied Cx43 expression and phosphorylation state in the left ventricle (LV) from an arrhythmogenic rabbit model of nonischemic HF and from patients with HF attributable to idiopathic dilated cardiomyopathy. We also investigated the role of protein phosphatases that dephosphorylate Cx43—PP1 and PP2A. In HF rabbit LV, Cx43 mRNA and total protein were decreased by 29% and 34%, respectively ( P <0.05 and P <0.001). In controls, Cx43 was primarily in the phosphorylated state, but with HF there was a 64% increase in nonphosphorylated Cx43 (Cx43-NP, normalized to total Cx43; P <0.05). Similar results were noted in HF rabbit myocytes ( P <0.05) and in human idiopathic dilated cardiomyopathy LV ( P <0.05). We found that PP1 and PP2A colocalized with Cx43 in rabbit LV. With HF, the level of colocalized PP2A increased >2.5-fold ( P <0.002), whereas colocalized PP1 was unchanged. We also found intercellular coupling (assessed by Lucifer Yellow dye transfer) was markedly reduced in HF. However, okadaic acid (10 nmol/L) reduced the amount of Cx43-NP and significantly improved cell coupling in HF. Thus, in nonischemic HF in rabbits and humans, there is a decrease in both Cx43 expression and phosphorylation that contributes to uncoupling. Increased levels of PP2A that colocalize with Cx43 can underlie enhanced levels of Cx43-NP in HF. Modulation of Cx43 phosphorylation may be a potential therapeutic target to improve conduction in HF.

Optical imaging of the heart

Optical techniques have revolutionized the investigation of cardiac cellular physiology and advanced our understanding of basic mechanisms of electrical activity, calcium homeostasis, and metabolism. Although optical methods are widely accepted and have been at the forefront of scientific discoveries, they have been primarily applied at cellular and subcellular levels and considerably less to whole heart organ physiology. Numerous technical difficulties had to be overcome to dynamically map physiological processes in intact hearts by optical methods. Problems of contraction artifacts, cellular heterogeneities, spatial and temporal resolution, limitations of surface images, depth-of-field, and need for large fields of view (ranging from 2x2 mm2 to 3x3 cm2) have all led to the development of new devices and optical probes to monitor physiological parameters in intact hearts. This review aims to provide a critical overview of current approaches, their contributions to the field of cardiac electrophysiology, and future directions of various optical imaging modalities as applied to cardiac physiology at organ and tissue levels.

Mechanism of oleic acid-induced gap junctional disassembly in rat cardiomyocytes

This study investigated the mechanism of oleic acid (OA) on gap junctions and identified the protein kinase C (PKC) isoforms involved in OA-mediated gap junction disassembly in cardiomyocytes. Control cardiomyocytes showed continuous staining of the plasma membrane at cell-cell contact areas using antibodies reacting with connexin 43 (Cx43). The spontaneous contraction rate of cultured cardiomyocytes was reduced in a time-dependent manner by OA. In addition, Cx43 expression at cell-cell junction decreased, suggesting the disassembly of gap junction. Staining for PKC and PKCalpha, which were shown to colocalize with Cx43, also decreased with increased duration of OA treatment. The effects of OA on these distributional changes at cell junctions were reversed by 24 h incubation in fresh culture medium devoid of OA. Immunoprecipitation assays confirmed the biochemical binding between Cx43 and PKC/PKCalpha, and this protein interaction was not affected by OA. This may provide the basis for simultaneous detachment of Cx and PKC/PKCalpha from the cell-cell junction to the cytosol upon OA stimulation. Western blot analysis showed that OA-induced Cx43 Ser368 phosphorylation, and that this effect could be blocked by cotreatment with the general PKC inhibitor, calphostin C, the PKC inhibitor, eV1-2, or the Src kinase inhibitor, PP1, but not by the PKCalpha inhibitor, Gö6976. eV1-2 also prevented the OA-induced disassembly of gap junctions. Taken together, these data suggest that OA-induced Cx43 Ser368 phosphorylation is mediated by activation of PKC and Src kinase and might be responsible for OA-induced gap junctional disassembly.

Electrical propagation in synthetic ventricular myocyte strands from germline connexin43 knockout mice

To characterize the role of connexin43 (Cx43) as a determinant of cardiac propagation, we synthesized strands and pairs of ventricular myocytes from germline Cx43-/- mice. The amount of Cx43, Cx45, and Cx40 in gap junctions was analyzed by immunohistochemistry and confocal microscopy. Intercellular electrical conductance, gj, was measured by the dual-voltage clamp technique (DVC), and electrical propagation was assessed by multisite optical mapping of transmembrane potential using a voltage-sensitive dye. Compared with wild-type (Cx43+/+) strands, immunoreactive signal for Cx43 was reduced by 46% in Cx43+/- strands and was absent in Cx43-/- strands. Cx45 signal was reduced by 46% in Cx43+/- strands and to the limit of detection in Cx43-/- strands, but total Cx45 protein levels measured in immunoblots of whole cell homogenates were equivalent in all genotypes. Cx40 was detected in 2% of myocytes. Intercellular conductance, gj, was reduced by 32% in Cx43+/- cell pairs and by 96% in Cx43-/- cell pairs. The symmetrical dependence of gj on transjunctional voltage and properties of single-channel recordings indicated that Cx45 was the only remaining connexin in Cx43-/- cells. Propagation in Cx43-/- strands was very slow (2.1 cm/s versus 52 cm/s in Cx43+/+) and highly discontinuous, with simultaneous excitation within and long conduction delays (2 to 3 ms) between individual cells. Propagation was abolished by 1 mmol/L heptanol, indicating residual junctional coupling. In summary, knockout of Cx43 in ventricular myocytes leads to very slow conduction dependent on the presence of Cx45. Electrical field effect transmission does not contribute to propagation in synthetic strands.

Effects of mechanical forces and mediators of hypertrophy on remodeling of gap junctions in the heart

This review article focuses on remodeling of gap junctions in response to chemical mediators of ventricular hypertrophy, mechanical forces, and alterations in cell-to-cell adhesion. Signaling mediated by mechanical forces is likely to be involved in the upregulation of cardiac gap junctions during the early phase of cardiac hypertrophy and the subsequent downregulation in cardiac failure. Several signaling pathways involving cAMP, angiotensin II, transforming growth factor-beta, vascular endothelial growth factor, and integrin-mediated regulators have been shown to affect expression of gap junction proteins. However, a comprehensive view of regulation of gap junction trafficking, synthesis, and degradation is still lacking. In addition to gap junction regulation by extracellular mechanical forces, there is a close relation between gap junctions and adhesion junctions and their linkage to the cytoskeleton. This can be inferred from experiments on neoformation of cell-to-cell coupling, concomitant upregulation of adherens and gap junctions after mechanical stretch, and human cardiomyopathies caused by genetic defects in cell-cell adhesion junction proteins. The molecular mechanisms responsible for the interaction between mechanical and functional cell-to-cell coupling remain to be elucidated.

Basic mechanisms of cardiac impulse propagation and associated arrhythmias

Propagation of excitation in the heart involves action potential (AP) generation by cardiac cells and its propagation in the multicellular tissue. AP conduction is the outcome of complex interactions between cellular electrical activity, electrical cell-to-cell communication, and the cardiac tissue structure. As shown in this review, strong interactions occur among these determinants of electrical impulse propagation. A special form of conduction that underlies many cardiac arrhythmias involves circulating excitation. In this situation, the curvature of the propagating excitation wavefront and the interaction of the wavefront with the repolarization tail of the preceding wave are additional important determinants of impulse propagation. This review attempts to synthesize results from computer simulations and experimental preparations to define mechanisms and biophysical principles that govern normal and abnormal conduction in the heart.

Impulse propagation in synthetic strands of neonatal cardiac myocytes with genetically reduced levels of connexin43

Connexin43 (Cx43) is a major determinant of the electrical properties of the myocardium. Closure of gap junctions causes rapid slowing of propagation velocity (theta), but the precise effect of a reduction in Cx43 levels due to genetic manipulation has only partially been clarified. In this study, morphological and electrical properties of synthetic strands of cultured neonatal ventricular myocytes from Cx43+/+ (wild type, WT) and Cx+/- (heterozygote, HZ) mice were compared. Quantitative immunofluorescence analysis of Cx43 demonstrated a 43% reduction of Cx43 expression in the HZ versus WT mice. Cell dimensions, connectivity, and alignment were independent of genotype. Measurement of electrical properties by microelectrodes and optical mapping showed no differences in action potential amplitude or minimum diastolic potential between WT and HZ. However, maximal upstroke velocity of the transmembrane action potential, dV/dtmax, was increased and action potential duration was reduced in HZ versus WT. theta was similar in the two genotypes. Computer simulation of propagation and dV/dtmax showed a relatively small dependence of theta on gap junction coupling, thus explaining the lack of observed differences in theta between WT and HZ. Importantly, the simulations suggested that the difference in dV/dtmax is due to an upregulation of INa in HZ versus WT. Thus, heterozygote-null mutation of Cx43 produces a complex electrical phenotype in synthetic strands that is characterized by both changes in ion channel function and cell-to-cell coupling. The lack of changes in theta in this tissue is explained by the dominating role of myoplasmic resistance and the compensatory increase of dV/dtmax.

Remodeling of gap junctional channel function in epicardial border zone of healing canine infarcts

The epicardial border zone (EBZ) of canine infarcts has increased anisotropy because of transverse conduction slowing. It remains unknown whether changes in gap junctional conductance (Gj) accompany the increased anisotropy. Ventricular cell pairs were isolated from EBZ and normal hearts (NZ). Dual patch clamp was used to quantify Gj. At a transjunctional voltage (Vj) of +10 mV, side-to-side Gj of EBZ pairs (9.2+/-3.4 nS, n=16) was reduced compared with NZ side-to-side Gj (109.4+/-23.6 nS, n=14, P<0.001). Gj of end-to-end coupled cells was not reduced in EBZ. Steady-state Gj of both NZ and EBZ showed voltage dependence, described by a two-way Boltzmann function. Half-maximal activation voltage in EBZ was shifted to higher Vj in positive and negative directions. Immunoconfocal planimetry and quantification showed no change in connexin43 per unit cell volume or surface area in EBZ. Decreased side-to-side coupling occurs in EBZ myocytes, independent of reduced connexin43 expression, and is hypothesized to contribute to increased anisotropy and reentrant arrhythmias.

Differential expression of connexin43 gap junctions in cardiomyocytes isolated from canine thoracic veins

We investigated the phenotypic features of cardiomyocytes, including the gap junctions, in the myocardial sleeve of thoracic veins. Single cardiomyocytes, isolated from the canine pulmonary veins (PV) and superior vena cava (SVC) using digestive enzymes, were examined by immunoconfocal microscopy using antisera against connexin43 (Cx43), Cx40, and other cell markers. The results showed that isolated cardiomyocytes displayed rod shapes of various sizes, ranging from <50 microm to >200 microm in length, and all the cells expressed alpha-actinin and vinculin. Gap junctions made of various amounts of Cx43 and Cx40 were found at the cell borders. These two connexins were extensively co-localized. Comparison between the thoracic veins showed that cells of the SVC contained more Cx43 gap junctions (total Cx43 gap junctions area per cell surface area, 4.0 +/- 0.2% vs 1.5 +/- 0.2%; p<0.01). In addition, for single-nucleus cells, those from the PV were longer (103.7 +/- 3.6 vs 85.0 +/- 3.1 microm; p<0.01) but narrower (14.4 +/- 0.5 vs 16.9 +/- 0.9 microm; p<0.01). In conclusion, canine thoracic veins contain cardiomyocytes with differences in shape and gap junctions, suggesting that the electrical conduction properties may be different between the thoracic veins.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

Heterogeneous expression of Gap junction channels in the heart leads to conduction defects and ventricular dysfunction

BACKGROUND

- Heterogeneous remodeling of gap junctions is observed in many forms of heart disease. The consequent loss of synchronous ventricular activation has been hypothesized to result in diminished cardiac performance. To directly test this hypothesis, we designed a murine model of heterogeneous gap junction channel expression. Methods and Results-- We generated chimeric mice formed from connexin43 (Cx43)-deficient embryonic stem cells and wild-type or genetically marked ROSA26 recipient blastocysts. Chimeric mice developed normally, without histological evidence of myocardial fibrosis or hypertrophy. Heterogeneous Cx43 expression resulted in conduction defects, however, as well as markedly depressed contractile function. Optical mapping of chimeric hearts by use of voltage-sensitive dyes revealed highly irregular epicardial conduction patterns, quantified as significantly greater negative curvature of the activation wave front (-1.86+/-0.40 mm in chimeric mice versus -0.86+/-0.098 mm in controls; P<0.01; n=6 for each group). Echocardiographic studies demonstrated significantly reduced fractional shortening in chimeric mice (26.6+/-2.3% versus 36.5+/-1.6% in age-matched 129/SvxC57BL/6F1 wild-type controls; P<0.05).

CONCLUSIONS

- These data suggest that heterogeneous Cx43 expression, by perturbing the normal pattern of coordinated myocardial excitation, may directly depress cardiac performance.

Distributed computing for membrane-based modeling of action potential propagation

Action potential propagation simulations with physiologic membrane currents and macroscopic tissue dimensions are computationally expensive. We, therefore, analyzed distributed computing schemes to reduce execution time in workstation clusters by parallelizing solutions with message passing. Four schemes were considered in two-dimensional monodomain simulations with the Beeler-Reuter membrane equations. Parallel speedups measured with each scheme were compared to theoretical speedups, recognizing the relationship between speedup and code portions that executed serially. A data decomposition scheme based on total ionic current provided the best performance. Analysis of communication latencies in that scheme led to a load-balancing algorithm in which measured speedups at 89 +/- 2% and 75 +/- 8% of theoretical speedups were achieved in homogeneous and heterogeneous clusters of workstations. Speedups in this scheme with the Luo-Rudy dynamic membrane equations exceeded 3.0 with eight distributed workstations. Cluster speedups were comparable to those measured during parallel execution on a shared memory machine.

Wnt-1 regulation of connexin43 in cardiac myocytes

Gap junction channels composed of connexin43 (Cx43) are essential for normal heart formation and function. We studied the potential role of the Wnt family of secreted polypeptides as regulators of Cx43 expression and gap junction channel function in dissociated myocytes and intact hearts. Neonatal rat cardiomyocytes responded to Li(+), which mimics Wnt signaling, by accumulating the effector protein beta-catenin and by inducing Cx43 mRNA and protein markedly. Induction of Cx43 expression was also observed in cardiomyocytes cocultured with Rat-2 fibroblasts or N2A neuroblastoma cells programmed to secrete bioactive Wnt-1. By transfecting a Cx43 promoter-reporter gene construct into cardiomyocytes, we demonstrated that the inductive effect of Wnt signaling was transcriptionally mediated. Enhanced expression of Cx43 increased cardiomyocyte cell coupling, as determined by Lucifer Yellow dye transfer and by calcium wave propagation. Conversely, in a transgenic cardiomyopathic mouse model that exhibits ventricular arrhythmias and gap junctional remodeling, beta-catenin and Cx43 expression were downregulated concordantly. In response to Wnt signaling, the accumulating Cx43 colocalized with beta-catenin in the junctional membrane; moreover, forced expression of Cx43 in cardiomyocytes reduced the transactivation potential of beta-catenin. These findings demonstrate that Wnt signaling is an important modulator of Cx43-dependent intercellular coupling in the heart, and they support the hypothesis that dysregulated signaling contributes to altered impulse propagation and arrhythmia in the myopathic heart.