Heart-directed expression of a human cardiac isoform of cAMP-response element modulator in transgenic mice

Cardiac-specific overexpression of CREM-IbΔC-X via CRISPR/Cas9 in mice presents a new model of atrial cardiomyopathy with spontaneous atrial fibrillation

Atrial cardiomyopathy (ACM) forms the substrate for atrial fibrillation (AF) and underlies the potential for atrial thrombus formation and subsequent stroke. However, generating stable animal models that accurately replicate the entire progression of atrial lesions, particularly the onset of AF, presents significant challenges. In the present study, we found that the isoform of CRE-binding protein modulator (CREM-IbΔC-X), which is involved in the regulation of cardiac development and atrial rhythm, was highly expressed in atrial biopsies from patients with AF. Building upon this finding, we employed CRISPR/Cas9 technology to create a mouse model with cardiac-specific overexpression of CREM-IbΔC-X (referred to as CS-CREM mice). This animal model effectively illustrated the development of ACM through electrophysiological and structural remodelings over time. Proteomics and Chip-qPCR analysis of atrial samples revealed significant upregulation of cell-matrix adhesion and extracellular matrix structural components, alongside significant downregulation of genes related to atrial functions in the CS-CREM mice. Furthermore, the corresponding responses to anti-arrhythmia drugs, i.e., amiodarone and propafenone, suggested that CS-CREM mice could serve as an ideal in vivo model for drug testing. Our study introduced a novel ACM model with spontaneous AF by cardiac-specifically overexpressing CREM-IbΔC-X in mice, providing valuable insights into the mechanisms and therapeutic targets of ACM.

Atrial proteomic profiling reveals a switch towards profibrotic gene expression program in CREM-IbΔC-X mice with persistent atrial fibrillation

BACKGROUND

Overexpression of the CREM (cAMP response element-binding modulator) isoform CREM-IbΔC-X in transgenic mice (CREM-Tg) causes the age-dependent development of spontaneous AF.

PURPOSE

To identify key proteome signatures and biological processes accompanying the development of persistent AF through integrated proteomics and bioinformatics analysis.

METHODS

Atrial tissue samples from three CREM-Tg mice and three wild-type littermates were subjected to unbiased mass spectrometry-based quantitative proteomics, differential expression and pathway enrichment analysis, and protein-protein interaction (PPI) network analysis.

RESULTS

A total of 98 differentially expressed proteins were identified. Gene ontology analysis revealed enrichment for biological processes regulating actin cytoskeleton organization and extracellular matrix (ECM) dynamics. Changes in ITGAV, FBLN5, and LCP1 were identified as being relevant to atrial fibrosis and structural based on expression changes, co-expression patterns, and PPI network analysis. Comparative analysis with previously published datasets revealed a shift in protein expression patterns from ion-channel and metabolic regulators in young CREM-Tg mice to profibrotic remodeling factors in older CREM-Tg mice. Furthermore, older CREM-Tg mice exhibited protein expression patterns reminiscent of those seen in humans with persistent AF.

CONCLUSIONS

This study uncovered distinct temporal changes in atrial protein expression patterns with age in CREM-Tg mice consistent with the progressive evolution of AF. Future studies into the role of the key differentially abundant proteins identified in this study in AF progression may open new therapeutic avenues to control atrial fibrosis and substrate development in AF.

Atrial Proteomic Profiling Reveals a Switch Towards Profibrotic Gene Expression Program in CREM-IbΔC-X Mice with Persistent Atrial Fibrillation

Background

Overexpression of the CREM (cAMP response element-binding modulator) isoform CREM-IbΔC-X in transgenic mice (CREM-Tg) causes the age-dependent development of spontaneous AF.

Purpose

To identify key proteome signatures and biological processes accompanying the development of persistent AF through integrated proteomics and bioinformatics analysis.

Methods

Atrial tissue samples from three CREM-Tg mice and three wild-type littermates were subjected to unbiased mass spectrometry-based quantitative proteomics, differential expression and pathway enrichment analysis, and protein-protein interaction (PPI) network analysis.

Results

A total of 98 differentially expressed proteins were identified. Gene ontology analysis revealed enrichment for biological processes regulating actin cytoskeleton organization and extracellular matrix (ECM) dynamics. Changes in ITGAV, FBLN5, and LCP1 were identified as being relevant to atrial fibrosis and remodeling based on expression changes, co-expression patterns, and PPI network analysis. Comparative analysis with previously published datasets revealed a shift in protein expression patterns from ion-channel and metabolic regulators in young CREM-Tg mice to profibrotic remodeling factors in older CREM-Tg mice. Furthermore, older CREM-Tg mice exhibited protein expression patterns that resembled those of humans with persistent AF.

Conclusions

This study uncovered distinct temporal changes in atrial protein expression patterns with age in CREM-Tg mice consistent with the progressive evolution of AF. Future studies into the role of the key differentially abundant proteins identified in this study in AF progression may open new therapeutic avenues to control atrial fibrosis and substrate development in AF.

New drug discovery of cardiac anti-arrhythmic drugs: insights in animal models

Cardiac rhythm regulated by micro-macroscopic structures of heart. Pacemaker abnormalities or disruptions in electrical conduction, lead to arrhythmic disorders may be benign, typical, threatening, ultimately fatal, occurs in clinical practice, patients on digitalis, anaesthesia or acute myocardial infarction. Both traditional and genetic animal models are: In-vitro: Isolated ventricular Myocytes, Guinea pig papillary muscles, Patch-Clamp Experiments, Porcine Atrial Myocytes, Guinea pig ventricular myocytes, Guinea pig papillary muscle: action potential and refractory period, Langendorff technique, Arrhythmia by acetylcholine or potassium. Acquired arrhythmia disorders: Transverse Aortic Constriction, Myocardial Ischemia, Complete Heart Block and AV Node Ablation, Chronic Tachypacing, Inflammation, Metabolic and Drug-Induced Arrhythmia. In-Vivo: Chemically induced arrhythmia: Aconitine antagonism, Digoxin-induced arrhythmia, Strophanthin/ouabain-induced arrhythmia, Adrenaline-induced arrhythmia, and Calcium-induced arrhythmia. Electrically induced arrhythmia: Ventricular fibrillation electrical threshold, Arrhythmia through programmed electrical stimulation, sudden coronary death in dogs, Exercise ventricular fibrillation. Genetic Arrhythmia: Channelopathies, Calcium Release Deficiency Syndrome, Long QT Syndrome, Short QT Syndrome, Brugada Syndrome. Genetic with Structural Heart Disease: Arrhythmogenic Right Ventricular Cardiomyopathy/Dysplasia, Dilated Cardiomyopathy, Hypertrophic Cardiomyopathy, Atrial Fibrillation, Sick Sinus Syndrome, Atrioventricular Block, Preexcitation Syndrome. Arrhythmia in Pluripotent Stem Cell Cardiomyocytes. Conclusion: Both traditional and genetic, experimental models of cardiac arrhythmias' characteristics and significance help in development of new antiarrhythmic drugs.

Towards Improved Human In Vitro Models for Cardiac Arrhythmia: Disease Mechanisms, Treatment, and Models of Atrial Fibrillation

Heart rhythm disorders, arrhythmias, place a huge economic burden on society and have a large impact on the quality of life of a vast number of people. Arrhythmias can have genetic causes but primarily arise from heart tissue remodeling during aging or heart disease. As current therapies do not address the causes of arrhythmias but only manage the symptoms, it is of paramount importance to generate innovative test models and platforms for gaining knowledge about the underlying disease mechanisms which are compatible with drug screening. In this review, we outline the most important features of atrial fibrillation (AFib), the most common cardiac arrhythmia. We will discuss the epidemiology, risk factors, underlying causes, and present therapies of AFib, as well as the shortcomings and opportunities of current models for cardiac arrhythmia, including animal models, in silico and in vitro models utilizing human pluripotent stem cell (hPSC)-derived cardiomyocytes.

An improved reporter identifies ruxolitinib as a potent and cardioprotective CaMKII inhibitor

Ca2+/calmodulin-dependent protein kinase II (CaMKII) hyperactivity causes cardiac arrhythmias, a major source of morbidity and mortality worldwide. Despite proven benefits of CaMKII inhibition in numerous preclinical models of heart disease, translation of CaMKII antagonists into humans has been stymied by low potency, toxicity, and an enduring concern for adverse effects on cognition due to an established role of CaMKII in learning and memory. To address these challenges, we asked whether any clinically approved drugs, developed for other purposes, were potent CaMKII inhibitors. For this, we engineered an improved fluorescent reporter, CaMKAR (CaMKII activity reporter), which features superior sensitivity, kinetics, and tractability for high-throughput screening. Using this tool, we carried out a drug repurposing screen (4475 compounds in clinical use) in human cells expressing constitutively active CaMKII. This yielded five previously unrecognized CaMKII inhibitors with clinically relevant potency: ruxolitinib, baricitinib, silmitasertib, crenolanib, and abemaciclib. We found that ruxolitinib, an orally bioavailable and U.S. Food and Drug Administration-approved medication, inhibited CaMKII in cultured cardiomyocytes and in mice. Ruxolitinib abolished arrhythmogenesis in mouse and patient-derived models of CaMKII-driven arrhythmias. A 10-min pretreatment in vivo was sufficient to prevent catecholaminergic polymorphic ventricular tachycardia, a congenital source of pediatric cardiac arrest, and rescue atrial fibrillation, the most common clinical arrhythmia. At cardioprotective doses, ruxolitinib-treated mice did not show any adverse effects in established cognitive assays. Our results support further clinical investigation of ruxolitinib as a potential treatment for cardiac indications.

Mouse models of spontaneous atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in adults, with a prevalence increasing with age. Current clinical management of AF is focused on tertiary prevention (i.e., treating the symptoms and sequelae) rather than addressing the underlying molecular pathophysiology. Robust animal models of AF, particularly those that do not require supraphysiologic stimuli to induce AF (i.e., showing spontaneous AF), enable studies that can uncover the underlying mechanisms of AF. Several mouse models of AF have been described to exhibit spontaneous AF, but pathophysiologic drivers of AF differ among models. Here, we describe relevant AF mechanisms and provide an overview of large and small animal models of AF. We then provide an in-depth review of the spontaneous mouse models of AF, highlighting the relevant AF mechanisms for each model.

Role of Dopamine in the Heart in Health and Disease

Dopamine has effects on the mammalian heart. These effects can include an increase in the force of contraction, and an elevation of the beating rate and the constriction of coronary arteries. Depending on the species studied, positive inotropic effects were strong, very modest, or absent, or even negative inotropic effects occurred. We can discern five dopamine receptors. In addition, the signal transduction by dopamine receptors and the regulation of the expression of cardiac dopamine receptors will be of interest to us, because this might be a tempting area of drug development. Dopamine acts in a species-dependent fashion on these cardiac dopamine receptors, but also on cardiac adrenergic receptors. We will discuss the utility of drugs that are currently available as tools to understand cardiac dopamine receptors. The molecule dopamine itself is present in the mammalian heart. Therefore, cardiac dopamine might act as an autocrine or paracrine compound in the mammalian heart. Dopamine itself might cause cardiac diseases. Moreover, the cardiac function of dopamine and the expression of dopamine receptors in the heart can be altered in diseases such as sepsis. Various drugs for cardiac and non-cardiac diseases are currently in the clinic that are, at least in part, agonists or antagonists at dopamine receptors. We define the research needs in order to understand dopamine receptors in the heart better. All in all, an update on the role of dopamine receptors in the human heart appears to be clinically relevant, and is thus presented here.

PI3K(p110α) as a determinant and gene therapy for atrial enlargement in atrial fibrillation

Atrial fibrillation (AF) is an irregular heart rhythm, characterised by chaotic atrial activation, which is promoted by remodelling. Once initiated, AF can also propagate the progression of itself in the so-called ''AF begets AF''. Several lines of investigation have shown that signalling molecules, including reactive oxygen species, angiotensin II, and phosphoinositide 3-kinases (PI3Ks), in presence or absence of cardiovascular disease risk factors, stabilise and promote AF maintenance. In particular, reduced cardiac-specific PI3K activity that is not associated with oncology is cardiotoxic and increases susceptibility to AF. Atrial-specific PI3K(p110α) transgene can cause pathological atrial enlargement. Highlighting the crucial importance of the p110α protein in a clinical problem that currently challenges the professional health care practice, in over forty (40) transgenic mouse models of AF (Table1), currently existing, of which some of the models are models of human genetic disorders, including PI3K(p110α) transgenic mouse model, over 70% of them reporting atrial size showed enlarged, greater atrial size. Individuals with minimal to severely dilated atria develop AF more likely. Left atrial diameter and volume stratification are an assessment for follow-up surveillance to detect AF. Gene therapy to reduce atrial size will be associated with a reduction in AF burden. In this overview, PI3K(p110α), a master regulator of organ size, was investigated in atrial enlargement and in physiological determinants that promote AF.

Animal Models to Study Cardiac Arrhythmias

Cardiac arrhythmias are a significant cause of morbidity and mortality worldwide, accounting for 10% to 15% of all deaths. Although most arrhythmias are due to acquired heart disease, inherited channelopathies and cardiomyopathies disproportionately affect children and young adults. Arrhythmogenesis is complex, involving anatomic structure, ion channels and regulatory proteins, and the interplay between cells in the conduction system, cardiomyocytes, fibroblasts, and the immune system. Animal models of arrhythmia are powerful tools for studying not only molecular and cellular mechanism of arrhythmogenesis but also more complex mechanisms at the whole heart level, and for testing therapeutic interventions. This review summarizes basic and clinical arrhythmia mechanisms followed by an in-depth review of published animal models of genetic and acquired arrhythmia disorders.

Schisandrin A protects against isoproterenol‑induced chronic heart failure via miR‑155

Schisandrin A (Sch A) has a protective effect on cardiomyocytes. Circulating miR-155 levels are related to chronic heart failure (CHF). The present study aimed to clarify the role and the molecular mechanism of Sch A in CHF. C57BL/6JGpt mice were used for an isoproterenol (ISO)-induced CHF model to collect heart samples. Echocardiography was employed to detect heartbeat indicators. The degree of myocardial hypertrophy was evaluated based on the measurement of heart weight (HW), body weight (BW) and tibia length (TL) and the observation using hematoxylin-eosin staining. Sprague-Dawley rats were purchased for the separation of neonatal rat ventricular myocytes (NRVMs), which were treated with ISO for 24 h. Transfection regulated the level of miR-155. The viability of NRVMs was detected via MTT assay. The mRNA and protein levels were measured via reverse transcription-quantitative PCR and western blotting and immunofluorescence was used to detect the content of α-smooth muscle actin (α-SMA). Treatment with ISO resulted in rising left ventricular posterior wall thickness, intra-ventricular septum diastole, left ventricular end diastolic diameter, left ventricular end systolic diameter, HW/BW, HW/TL and falling ejection fraction and fractional shortening, the trend of which could be reversed by Sch A. Sch A ameliorated myocardial hypertrophy in CHF mice. In addition, Sch A inhibited ISO-induced upregulated expressions of atrial natriuretic peptide, B-type natriuretic peptide, B-myosin heavy chain and miR-155 in myocardial tissue. Based on the results in vitro, Sch A had no significant effect on the viability of NRVMs when its concentration was <24 µmol/l. Sch A inhibited the levels of miR-155, α-SMA and the phosphorylation levels of AKT and cyclic AMP response-element binding protein (CREB) in ISO-induced NRVMs, which was reversed by the upregulation of miR-155. Schisandrin A mediated the AKT/CREB signaling pathway to prevent CHF by regulating the expression of miR-155, which may shed light on a possible therapeutic target for CHF.

Genetic inhibition of Nuclear Factor of Activated T-cell c2 (NFATc2) prevents atrial fibrillation in CREM transgenic mice

AIMS

Abnormal intracellular calcium handling contributes to the progressive nature of atrial fibrillation (AF), the most common sustained cardiac arrhythmia. Evidence in mouse models suggests that activation of the nuclear factor of activated T-cell (NFAT) signaling pathway contributes to atrial remodeling. Our aim was to determine the role of NFATc2 in AF in humans and mouse models.

METHODS AND RESULTS

Expression levels of NFATc1-c4 isoforms were assessed by quantitative reverse transcription-polymerase chain reaction in right atrial appendages from patients with chronic AF. NFATc1 and NFATc2 mRNA levels were elevated in chronic AF (cAF) patients compared with those in sinus rhythm (SR). Western blotting revealed increased cytosolic and nuclear levels of NFATc2 in AF patients. Similar findings were obtained in CREM-IbΔC-X transgenic (CREM) mice, a model of progressive AF. Telemetry ECG recordings revealed age-dependent spontaneous AF in CREM mice, which was prevented by NFATc2 knockout in CREM: NFATc2-/- mice. Programmed electrical stimulation revealed that CREM: NFATc2-/- mice lacked an AF substrate. Morphometric analysis and histology revealed increased atrial weight and atrial fibrosis in CREM mice compared with WT controls, which was reversed in CREM: NFATc2-/- mice. Confocal microscopy showed an increased Ca2+ spark frequency despite a reduced sarcoplasmic reticulum (SR) Ca2+ load in CREM mice compared with controls, whereas these abnormalities were normalized in CREM: NFATc2-/- mice. Western blotting revealed that genetic inhibition of Ca2+/calmodulin-dependent protein kinase II-mediated phosphorylation of S2814 on RyR2 in CREM: RyR2-S2814A mice suppressed NFATc2 activation observed in CREM mice, suggesting that NFATc2 is activated by excessive SR Ca2+ leak via RyR2. Finally, chromatin immunoprecipitation sequencing from AF patients identified Ras And EF-Hand Domain-Containing Protein (RASEF) as a direct target of NFATc2 mediated transcription.

CONCLUSION

Our findings reveal activation of the NFAT signaling pathway in patients of Chinese and European descent. NFATc2 knockout prevents the progression of AF in the CREM mouse model.

TRANSLATIONAL PERSPECTIVE

Atrial fibrillation (AF) is a progressive disease characterized by electrical and structural remodeling which promotes atrial arrhythmias. This study provides evidence for increased 'nuclear factor of activated T-cell' (NFAT) signaling in patients with chronic AF. Studies in the CREM transgenic model of progressive AF revealed that the NFATc2 isoform mediates atrial remodeling associated with AF substrate development. Chromatin immunoprecipitation sequencing of atrial biopsies from AF patients identified 'Ras And EF-Hand Domain-Containing Protein' (RASEF) as a downstream target of NFATc2-mediated transcription, suggesting that targeting these factors might be beneficial for curtailing AF progression.

Impact of High Altitude on Cardiovascular Health: Current Perspectives

Globally, about 400 million people reside at terrestrial altitudes above 1500 m, and more than 100 million lowlanders visit mountainous areas above 2500 m annually. The interactions between the low barometric pressure and partial pressure of O2, climate, individual genetic, lifestyle and socio-economic factors, as well as adaptation and acclimatization processes at high elevations are extremely complex. It is challenging to decipher the effects of these myriad factors on the cardiovascular health in high altitude residents, and even more so in those ascending to high altitudes with or without preexisting diseases. This review aims to interpret epidemiological observations in high-altitude populations; present and discuss cardiovascular responses to acute and subacute high-altitude exposure in general and more specifically in people with preexisting cardiovascular diseases; the relations between cardiovascular pathologies and neurodegenerative diseases at altitude; the effects of high-altitude exercise; and the putative cardioprotective mechanisms of hypobaric hypoxia.

Transcriptional factors in calcium mishandling and atrial fibrillation development

Healthy cardiac conduction relies on the coordinated electrical activity of distinct populations of cardiomyocytes. Disruption of cell-cell conduction results in cardiac arrhythmias, a leading cause of morbidity and mortality worldwide. Recent genetic studies have highlighted a major heritable component and identified numerous loci associated with risk of atrial fibrillation, including transcription factor genes, particularly those important in cardiac development, microRNAs, and long noncoding RNAs. Identification of such genetic factors has prompted the search to understand the mechanisms that underlie the genetic component of AF. Recent studies have found several mechanisms by which genetic alterations can result in AF formation via disruption of calcium handling. Loss of developmental transcription factors in adult cardiomyocytes can result in disruption of SR calcium ATPase, sodium calcium exchanger, calcium channels, among other ion channels, which underlie action potential abnormalities and triggered activity that can contribute to AF. This review aims to summarize the complex network of transcription factors and their roles in calcium handling.

JDP2, a Novel Molecular Key in Heart Failure and Atrial Fibrillation?

Heart failure (HF) and atrial fibrillation (AF) are two major life-threatening diseases worldwide. Causes and mechanisms are incompletely understood, yet current therapies are unable to stop disease progression. In this review, we focus on the contribution of the transcriptional modulator, Jun dimerization protein 2 (JDP2), and on HF and AF development. In recent years, JDP2 has been identified as a potential prognostic marker for HF development after myocardial infarction. This close correlation to the disease development suggests that JDP2 may be involved in initiation and progression of HF as well as in cardiac dysfunction. Although no studies have been done in humans yet, studies on genetically modified mice impressively show involvement of JDP2 in HF and AF, making it an interesting therapeutic target.

Structural, Pro-Inflammatory and Calcium Handling Remodeling Underlies Spontaneous Onset of Paroxysmal Atrial Fibrillation in JDP2-Overexpressing Mice

BACKGROUND

Cardiac-specific JDP2 overexpression provokes ventricular dysfunction and atrial dilatation in mice. We performed in vivo studies on JDP2-overexpressing mice to investigate the impact of JDP2 on the predisposition to spontaneous atrial fibrillation (AF).

METHODS

JDP2-overexpression was started by withdrawal of a doxycycline diet in 4-week-old mice. The spontaneous onset of AF was documented by ECG within 4 to 5 weeks of JDP2 overexpression. Gene expression was analyzed by real-time RT-PCR and Western blots.

RESULTS

In atrial tissue of JDP2 mice, besides the 3.6-fold increase of JDP2 mRNA, no changes could be detected within one week of JDP2 overexpression. Atrial dilatation and hypertrophy, combined with elongated cardiomyocytes and fibrosis, became evident after 5 weeks of JDP2 overexpression. Electrocardiogram (ECG) recordings revealed prolonged PQ-intervals and broadened P-waves and QRS-complexes, as well as AV-blocks and paroxysmal AF. Furthermore, reductions were found in the atrial mRNA and protein level of the calcium-handling proteins NCX, Cav1.2 and RyR2, as well as of connexin40 mRNA. mRNA of the hypertrophic marker gene ANP, pro-inflammatory MCP1, as well as markers of immune cell infiltration (CD68, CD20) were increased in JDP2 mice.

CONCLUSION

JDP2 is an important regulator of atrial calcium and immune homeostasis and is involved in the development of atrial conduction defects and arrhythmogenic substrates preceding paroxysmal AF.

Inward Rectifier K+ Currents Contribute to the Proarrhythmic Electrical Phenotype of Atria Overexpressing Cyclic Adenosine Monophosphate Response Element Modulator Isoform CREM-IbΔC-X

BACKGROUND Transgenic mice (TG) with heart-directed overexpresion of the isoform of the transcription factor cyclic adenosine monophosphate response element modulator (CREM), CREM-IbΔC-X, display spontaneous atrial fibrillation (AF) and action potential prolongation. The remodeling of the underlying ionic currents remains unknown. Here, we investigated the regulatory role of CREM-IbΔC-X on the expression of K⁺ channel subunits and the corresponding K⁺ currents in relation to AF onset in TG atrial myocytes. METHODS AND RESULTS ECG recordings documented the absence or presence of AF in 6-week-old (before AF onset) and 12-week-old TG (after AF onset) and wild-type littermate mice before atria removal to perform patch clamp, contractility, and biochemical experiments. In TG atrial myocytes, we found reduced repolarization reserve K⁺ currents attributed to a decrease of transiently outward current and inward rectifier K⁺ current with phenotype progression, and of acetylcholine-activated K⁺ current, age independent. The molecular determinants of these changes were lower mRNA levels of Kcnd2/3, Kcnip2, Kcnj2/4, and Kcnj3/5 and decreased protein levels of K⁺ channel interacting protein 2 (KChIP2 ), Kir2.1/3, and Kir3.1/4, respectively. After AF onset, inward rectifier K⁺ current contributed less to action potential repolarization, in line with the absence of outward current component, whereas the acetylcholine-induced action potential shortening before AF onset (6-week-old TG mice) was smaller than in wild-type and 12-week-old TG mice. Atrial force of contraction measured under combined vagal-sympathetic stimulation revealed increased sensitivity to isoprenaline irrespective of AF onset in TG. Moreover, we identified Kcnd2, Kcnd3, Kcnj3, and Kcnh2 as novel CREM-target genes. CONCLUSIONS Our study links the activation of cyclic adenosine monophosphate response element-mediated transcription to the proarrhythmogenic electrical remodeling of atrial inward rectifier K⁺ currents with a role in action potential duration, resting membrane stability, and vagal control of the electrical activity.

Induction of ICER is superseded by smICER, challenging the impact of ICER under chronic beta-adrenergic stimulation

ICER (inducible cAMP early repressor) isoforms are transcriptional repressors encoded by the Crem (cAMP responsive element modulator) gene. They were linked to the regulation of a multitude of cellular processes and pathophysiological mechanisms. Here, we show for the first time that two independent induction patterns for CREM repressor isoforms exist in the heart, namely for ICER and smICER (small ICER), which are induced in response to β-adrenergic stimulation in a transient- and saturation-like manner, respectively. This time-shifted induction pattern, driven by two internal promoters in the Crem gene, leads to the predominant transcription of smIcer after prolonged β-adrenergic stimulation. Using an ICER knockout mouse model with preserved smICER induction, we show that the transient-like induction of Icer itself has minor effects on gene regulation, cardiac hypertrophy or contractile function in the heart. We conclude that the functions previously linked to ICER may be rather attributed to smICER, also beyond the cardiac background.

Animal Models of Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia encountered in humans and is a significant source of morbidity and mortality. Despite its prevalence, our mechanistic understanding is incomplete, the therapeutic options have limited efficacy, and are often fraught with risks. A better biological understanding of AF is needed to spearhead novel therapeutic avenues. Although "natural" AF is nearly nonexistent in most species, animal models have contributed significantly to our understanding of AF and some therapeutic options. However, the impediments of animal models are also apparent and stem largely from the differences in basic physiology as well as the complexities underlying human AF; these preclude the creation of a "perfect" animal model and have obviated the translation of animal findings. Herein, we review the vast array of AF models available, spanning the mouse heart (weighing 1/1000th of a human heart) to the horse heart (10× heavier than the human heart). We attempt to highlight the features of each model that bring value to our understanding of AF but also the shortcomings and pitfalls. Finally, we borrowed the concept of a SWOT analysis from the business community (which stands for strengths, weaknesses, opportunities, and threats) and applied this introspective type of analysis to animal models for AF. We identify unmet needs and stress that is in the context of rapidly advancing technologies, these present opportunities for the future use of animal models.

HDAC (Histone Deacetylase) Inhibitor Valproic Acid Attenuates Atrial Remodeling and Delays the Onset of Atrial Fibrillation in Mice

Supplemental Digital Content is available in the text.

Background:

A structural, electrical and metabolic atrial remodeling is central in the development of atrial fibrillation (AF) contributing to its initiation and perpetuation. In the heart, HDACs (histone deacetylases) control remodeling associated processes like hypertrophy, fibrosis, and energy metabolism. Here, we analyzed, whether the HDAC class I/IIa inhibitor valproic acid (VPA) is able to attenuate atrial remodeling in CREM-IbΔC-X (cAMP responsive element modulator isoform IbΔC-X) transgenic mice, a mouse model of extensive atrial remodeling with age-dependent progression from spontaneous atrial ectopy to paroxysmal and finally long-lasting AF.

Methods:

VPA was administered for 7 or 25 weeks to transgenic and control mice. Atria were analyzed macroscopically and using widefield and electron microscopy. Action potentials were recorded from atrial cardiomyocytes using patch-clamp technique. ECG recordings documented the onset of AF. A proteome analysis with consecutive pathway mapping identified VPA-mediated proteomic changes and related pathways.

Results:

VPA attenuated many components of atrial remodeling that are present in transgenic mice, animal AF models, and human AF. VPA significantly (P<0.05) reduced atrial dilatation, cardiomyocyte enlargement, atrial fibrosis, and the disorganization of myocyte’s ultrastructure. It significantly reduced the occurrence of atrial thrombi, reversed action potential alterations, and finally delayed the onset of AF by 4 to 8 weeks. Increased histone H4-acetylation in atria from VPA-treated transgenic mice verified effective in vivo HDAC inhibition. Cardiomyocyte-specific genetic inactivation of HDAC2 in transgenic mice attenuated the ultrastructural disorganization of myocytes comparable to VPA. Finally, VPA restrained dysregulation of proteins in transgenic mice that are involved in a multitude of AF relevant pathways like oxidative phosphorylation or RhoA (Ras homolog gene family, member A) signaling and disease functions like cardiac fibrosis and apoptosis of muscle cells.

Conclusions:

Our results suggest that VPA, clinically available, well-tolerated, and prescribed to many patients for years, has the therapeutic potential to delay the development of atrial remodeling and the onset of AF in patients at risk.

Cardioembolic Ischemic Stroke Gene Expression Fingerprint in Blood: a Systematic Review and Verification Analysis

An accurate etiological classification is key to optimize secondary prevention after ischemic stroke, but the cause remains undetermined in one third of patients. Several studies pointed out the usefulness of circulating gene expression markers to discriminate cardioembolic (CE) strokes, mainly due to atrial fibrillation (AF), while only exploring them in small cohorts. A systematic review of studies analyzing high-throughput gene expression in blood samples to discriminate CE strokes was performed. Significantly dysregulated genes were considered as candidates, and a selection of them was validated by RT-qPCR in 100 patients with defined CE or atherothrombotic (LAA) stroke etiology. Longitudinal performance was evaluated in 12 patients at three time points. Their usefulness as biomarkers for AF was tested in 120 cryptogenic strokes and 100 individuals at high-risk for stroke. Three published studies plus three unpublished datasets were considered for candidate selection. Sixty-seven genes were found dysregulated in CE strokes. CREM, PELI1, and ZAK were verified to be up-regulated in CE vs LAA (p = 0.010, p = 0.003, p < 0.001, respectively), without changes in their expression within the first 24 h after stroke onset. The combined up-regulation of these three biomarkers increased the probability of suffering from CE stroke by 23-fold. In cryptogenic strokes with subsequent AF detection, PELI1 and CREM showed overexpression (p = 0.017, p = 0.059, respectively), whereas in high-risk asymptomatic populations, all three genes showed potential to detect AF (p = 0.007, p = 0.007, p = 0.015). The proved discriminatory capacity of these gene expression markers to detect cardioembolism even in cryptogenic strokes and asymptomatic high-risk populations might bring up their use as biomarkers.

Enhanced Cardiomyocyte NLRP3 Inflammasome Signaling Promotes Atrial Fibrillation

BACKGROUND

Atrial fibrillation (AF) is frequently associated with enhanced inflammatory response. The NLRP3 (NACHT, LRR, and PYD domain containing protein 3) inflammasome mediates caspase-1 activation and interleukin-1β release in immune cells but is not known to play a role in cardiomyocytes (CMs). Here, we assessed the role of CM NLRP3 inflammasome in AF.

METHODS

NLRP3 inflammasome activation was assessed by immunoblot in atrial whole-tissue lysates and CMs from patients with paroxysmal AF or long-standing persistent (chronic) AF. To determine whether CM-specific activation of NLPR3 is sufficient to promote AF, a CM-specific knockin mouse model expressing constitutively active NLRP3 (CM-KI) was established. In vivo electrophysiology was used to assess atrial arrhythmia vulnerability. To evaluate the mechanism of AF, electric activation pattern, Ca2+ spark frequency, atrial effective refractory period, and morphology of atria were evaluated in CM-KI mice and wild-type littermates.

RESULTS

NLRP3 inflammasome activity was increased in the atrial CMs of patients with paroxysmal AF and chronic AF. CM-KI mice developed spontaneous premature atrial contractions and inducible AF, which was attenuated by a specific NLRP3 inflammasome inhibitor, MCC950. CM-KI mice exhibited ectopic activity, abnormal sarcoplasmic reticulum Ca2+ release, atrial effective refractory period shortening, and atrial hypertrophy. Adeno-associated virus subtype-9-mediated CM-specific knockdown of Nlrp3 suppressed AF development in CM-KI mice. Finally, genetic inhibition of Nlrp3 prevented AF development in CREM transgenic mice, a well-characterized mouse model of spontaneous AF.

CONCLUSIONS

Our study establishes a novel pathophysiological role for CM NLRP3 inflammasome signaling, with a mechanistic link to the pathogenesis of AF, and establishes the inhibition of NLRP3 as a potential novel AF therapy approach.

Homozygous CREM-IbΔC-X Overexpressing Mice Are a Reliable and Effective Disease Model for Atrial Fibrillation

Background: Atrial fibrillation (AF) is a significant cause of morbidity and mortality with foreseeably increasing prevalence. While large animal models of the disease are well established but resource intensive, transgenic AF mouse models are not yet widely used to develop or validate novel therapeutics for AF. Hemizygous mice with a cardiomyocyte-specific overexpression of the human cAMP response element modulator (CREM) isoform IbΔC-X spontaneously develop AF on grounds of an arrhythmogenic substrate consisting of alterations in structure, conduction, and calcium handling.

Objective: We investigated if homozygous expression of the CREM-IbΔC-X transgene in mice alters the time course of AF development, and if homozygous CREM-IbΔC-X transgenics could be suitable as a disease model of AF.

Methods: Southern Blot, quantitative real-time PCR, and immunoblotting were used to identify and verify homozygous transgenics. Cardiac gravimetry, quantitative real-time RT-PCR, histology, survival analysis, and repeated ECG recordings allowed assessment of phenotypic development and effects of antiarrhythmic drugs.

Results: Homozygous animals could be identified by Southern blot and quantitative PCR, showing a strong trend to increased transgenic protein expression. In homozygous animals, atrial hypertrophy appeared earlier and more pronounced than in hemizygous animals, going along with an earlier onset of spontaneous AF, while no increased early mortality was observed. Application of a rate-controlling drug (esmolol) led to the expected result of a decreased heart rate. Application of a rhythm-controlling drug (flecainide) showed effects on heart rate variability, but did not lead to a definitive conversion to sinus rhythm.

Conclusion: We suggest homozygous CREM-IbΔC-X overexpressing mice as a reliable model of early onset, rapidly progressive AF.

Atrial thrombogenesis in atrial fibrillation : Results from atrial fibrillation models and AF-patients

Atrial fibrillation (AF) is the most common cause of thromboembolic complications. The risk of suffering a thromboembolic complication depends on the accompanying cardiac risk factors and the patient's age. For patients who have an increased risk, which is now classified using the CHA2DS2-VASc score, initiation of long-term oral anticoagulation is the first-line treatment. In AF, thrombi arise in the left atrial appendage. The present review will summarize the basic pathophysiology of thrombogenesis in AF and will provide the molecular basis of a process called prothrombotic endocardial remodeling. Despite oral anticoagulation being a central component of therapy, the present results can be used to support concomitant therapy with statins, angiotensin II blockers, etc. to inhibit atrial thromogenesis.

Regulation of heterotrimeric G-protein signaling by NDPK/NME proteins and caveolins: an update

Heterotrimeric G proteins are pivotal mediators of cellular signal transduction in eukaryotic cells and abnormal G-protein signaling plays an important role in numerous diseases. During the last two decades it has become evident that the activation status of heterotrimeric G proteins is both highly localized and strongly regulated by a number of factors, including a receptor-independent activation pathway of heterotrimeric G proteins that does not involve the classical GDP/GTP exchange and relies on nucleoside diphosphate kinases (NDPKs). NDPKs are NTP/NDP transphosphorylases encoded by the nme/nm23 genes that are involved in a variety of cellular events such as proliferation, migration, and apoptosis. They therefore contribute, for example, to tumor metastasis, angiogenesis, retinopathy, and heart failure. Interestingly, NDPKs are translocated and/or upregulated in human heart failure. Here we describe recent advances in the current understanding of NDPK functions and how they have an impact on local regulation of G-protein signaling.

Characterization of the Genetic Program Linked to the Development of Atrial Fibrillation in CREM-IbΔC-X Mice

Background—

Reduced expression of genes regulated by the transcription factors CREB/CREM (cAMP response element-binding protein/modulator) is linked to atrial fibrillation (AF) susceptibility in patients. Cardiomyocyte-directed expression of the inhibitory CREM isoform CREM-IbΔC-X in transgenic mice (TG) leads to spontaneous-onset AF preceded by atrial dilatation and conduction abnormalities. Here, we characterized the altered gene program linked to atrial remodeling and development of AF in CREM-TG mice.

Methods and Results—

Atria of young (TGy, before AF onset) and old (TGo, after AF onset) TG mice were investigated by mRNA microarray profiling in comparison with age-matched wild-type controls (WTy/WTo). Proteomic alterations were profiled in young mice (8 TGy versus 8 WTy). Annotation of differentially expressed genes revealed distinct differences in biological functions and pathways before and after onset of AF. Alterations in metabolic pathways, some linked to altered peroxisome proliferator–activated receptor signaling, muscle contraction, and ion transport were already present in TGy. Electron microscopy revealed significant loss of sarcomeres and mitochondria and increased collagen and glycogen deposition in TG mice. Alterations in electrophysiological pathways became prominent in TGo, concomitant with altered gene expression of K + -channel subunits and ion channel modulators, relevant in human AF.

Conclusions—

The most prominent alterations of the gene program linked to CREM-induced atrial remodeling were identified in the expression of genes related to structure, metabolism, contractility, and electric activity regulation, suggesting that CREM transgenic mice are a valuable experimental model for human AF pathophysiology.

CREM-transgene mice: An animal model of atrial fibrillation and thrombogenesis

BACKGROUND

The molecular pathomechanisms underlying atrial thrombogenesis are multifactorial and still require detailed investigations. Transgenic mice with cardiomyocyte-directed expression of the transcriptional repressor CREM-IbΔC-X (CREM-TG) represent an experimental model of atrial fibrillation (AF) that shows a gradual, age-dependent progression from atrial ectopy to persistent AF. Importantly, this model develops biatrial thrombi. The molecular characteristics related to the thrombogenesis in CREM-TG mice have not been studied, yet.

METHODS

The inflammatory and prothrombotic state was evaluated at the transcriptional (qRT-PCR) and protein level in the left (LA) and right atria (RA) from CREM-TG mice at the age of 20weeks and compared to wild-type controls. Moreover, histological analyses of atrial thrombi were performed.

RESULTS

The endocardial dysfunction was mirrored by diminished levels of eNOS-mRNA in both atria (RA: 0.79±0.04, LA: 0.72±0.06; each P<0.05). Moreover, the PAI-1/t-PA mRNA ratio was significantly increased in both atria (RA: 3.6±0.6; P<0.01, LA: 4.0±1.0; P<0.05) indicating a high risk of thrombus formation. However, the inflammatory phenotype was more pronounced in the RA and was reflected by a significant increase in the mRNA levels encoding adhesion molecules ICAM-1 (2.1±0.2; P<0.01), VCAM-1 (2.3±0.5; P<0.05), and selectin P (3.6±0.5: P<0.05).

CONCLUSIONS

CREM-TG mice represent a valuable model for studying atrial thrombogenesis and assessing therapeutic approaches preventing embolic events in the systemic and pulmonary circulation.

Transcription Factor CREM Mediates High Glucose Response in Cardiomyocytes and in a Male Mouse Model of Prolonged Hyperglycemia

This study aims at investigating the epigenetic landscape of cardiomyocytes exposed to elevated glucose levels. High glucose (30 mM) for 72 hours determined some epigenetic changes in mouse HL-1 and rat differentiated H9C2 cardiomyocytes including upregulation of class I and III histone deacetylase protein levels and activity, inhibition of histone acetylase p300 activity, increase in histone H3 lysine 27 trimethylation, and reduction in H3 lysine 9 acetylation. Gene expression analysis focused on cardiotoxicity revealed that high glucose induced markers associated with tissue damage, fibrosis, and cardiac remodeling such as Nexilin (NEXN), versican, cyclic adenosine 5'-monophosphate-responsive element modulator (CREM), and adrenoceptor α2A (ADRA2). Notably, the transcription factor CREM was found to be important in the regulation of cardiotoxicity-associated genes as assessed by specific small interfering RNA and chromatin immunoprecipitation experiments. In CD1 mice, made hyperglycemic by streptozotoicin (STZ) injection, cardiac structural alterations were evident at 6 months after STZ treatment and were associated with a significant increase of H3 lysine 27 trimethylation and reduction of H3 lysine 9 acetylation. Consistently, NEXN, CREM, and ADRA2 expression was significantly induced at the RNA and protein levels. Confocal microscopy analysis of NEXN localization showed this protein irregularly distributed along the sarcomeres in the heart of hyperglycemic mice. This evidence suggested a structural alteration of cardiac Z-disk with potential consequences on contractility. In conclusion, high glucose may alter the epigenetic landscape of cardiac cells. Sildenafil, restoring guanosine 3', 5'-cyclic monophosphate levels, counteracted the increase of CREM and NEXN, providing a protective effect in the presence of hyperglycemia.

Calcium Signaling and Cardiac Arrhythmias

There has been a significant progress in our understanding of the molecular mechanisms by which calcium (Ca2+) ions mediate various types of cardiac arrhythmias. A growing list of inherited gene defects can cause potentially lethal cardiac arrhythmia syndromes, including catecholaminergic polymorphic ventricular tachycardia, congenital long QT syndrome, and hypertrophic cardiomyopathy. In addition, acquired deficits of multiple Ca2+-handling proteins can contribute to the pathogenesis of arrhythmias in patients with various types of heart disease. In this review article, we will first review the key role of Ca2+ in normal cardiac function-in particular, excitation-contraction coupling and normal electric rhythms. The functional involvement of Ca2+ in distinct arrhythmia mechanisms will be discussed, followed by various inherited arrhythmia syndromes caused by mutations in Ca2+-handling proteins. Finally, we will discuss how changes in the expression of regulation of Ca2+ channels and transporters can cause acquired arrhythmias, and how these mechanisms might be targeted for therapeutic purposes.

Transforming Growth Factor-β Receptor III is a Potential Regulator of Ischemia-Induced Cardiomyocyte Apoptosis

Background

Myocardial infarction (MI) is often accompanied by cardiomyocyte apoptosis, which decreases heart function and leads to an increased risk of heart failure. The aim of this study was to examine the effects of transforming growth factor‐β receptor III (TGFβR3) on cardiomyocyte apoptosis during MI.

Methods and Results

An MI mouse model was established by left anterior descending coronary artery ligation. Cell viability, apoptosis, TGFβR3, and mitogen‐activated protein kinase signaling were assessed by methylthiazolyldiphenyl‐tetrazolium bromide assay, terminal deoxynucleotidyl transferase‐mediated dUTP nick end labeling assay, immunofluorescence, electron microscopy, and Western blotting. Our results demonstrated that TGFβR3 expression in the border region of the heart was dynamically changed during MI. After stimulation with H₂O₂, TGFβR3 overexpression in cardiomyocytes led to increased cell apoptosis and activation of p38 signaling, whereas TGFβR3 knockdown had the opposite effect. ERK1/2 and JNK1/2 signaling was not altered by TGFβR3 modulation, and p38 inhibitor (SB203580) reduced the effect of TGFβR3 on apoptosis, suggesting that p38 has a nonredundant function in activating apoptosis. Consistent with the in vitro observations, cardiac TGFβR3 transgenic mice showed augmented cardiomyocyte apoptosis, enlarged infarct size, increased injury, and enhanced p38 signaling upon MI. Conversely, cardiac loss of function of TGFβR3 by adeno‐associated viral vector serotype 9–TGFβR3 short hairpin RNA attenuated the effects of MI in mice.

Conclusions

TGFβR3 promotes apoptosis of cardiomyocytes via a p38 pathway–associated mechanism, and loss of TGFβR3 reduces MI injury, which suggests that TGFβR3 may serve as a novel therapeutic target for MI.

Stretch-activated two-pore-domain (K2P) potassium channels in the heart: Focus on atrial fibrillation and heart failure

Two-pore-domain potassium (K_2P) channels modulate cellular excitability. The significance of stretch-activated cardiac K_2P channels (K_2P2.1, TREK-1, KCNK2; K_2P4.1, TRAAK, KCNK4; K_2P10.1, TREK-2, KCNK10) in heart disease has not been elucidated in detail. The aim of this work was to assess expression and remodeling of mechanosensitive K_2P channels in atrial fibrillation (AF) and heart failure (HF) patients in comparison to murine models. Cardiac K_2P channel levels were quantified in atrial (A) and ventricular (V) tissue obtained from patients undergoing open heart surgery. In addition, control mice and mouse models of AF (cAMP-response element modulator (CREM)-IbΔC-X transgenic animals) or HF (cardiac dysfunction induced by transverse aortic constriction, TAC) were employed. Human and murine KCNK2 displayed highest mRNA abundance among mechanosensitive members of the K_2P channel family (V > A). Disease-associated K_2P2.1 remodeling was studied in detail. In patients with impaired left ventricular function, atrial KCNK2 (K_2P2.1) mRNA and protein expression was significantly reduced. In AF subjects, downregulation of atrial and ventricular KCNK2 (K_2P2.1) mRNA and protein levels was observed. AF-associated suppression of atrial Kcnk2 (K_2P2.1) mRNA and protein was recapitulated in CREM-transgenic mice. Ventricular Kcnk2 expression was not significantly altered in mouse models of disease. In conclusion, mechanosensitive K_2P2.1 and K_2P10.1 K⁺ channels are expressed throughout the heart. HF- and AF-associated downregulation of KCNK2 (K_2P2.1) mRNA and protein levels suggest a mechanistic contribution to cardiac arrhythmogenesis.

Nucleoside Diphosphate Kinase-C Suppresses cAMP Formation in Human Heart Failure

BACKGROUND

Chronic heart failure (HF) is associated with altered signal transduction via β-adrenoceptors and G proteins and with reduced cAMP formation. Nucleoside diphosphate kinases (NDPKs) are enriched at the plasma membrane of patients with end-stage HF, but the functional consequences of this are largely unknown, particularly for NDPK-C. Here, we investigated the potential role of NDPK-C in cardiac cAMP formation and contractility.

METHODS

Real-time polymerase chain reaction, (far) Western blot, immunoprecipitation, and immunocytochemistry were used to study the expression, interaction with G proteins, and localization of NDPKs. cAMP levels were determined with immunoassays or fluorescent resonance energy transfer, and contractility was determined in cardiomyocytes (cell shortening) and in vivo (fractional shortening).

RESULTS

NDPK-C was essential for the formation of an NDPK-B/G protein complex. Protein and mRNA levels of NDPK-C were upregulated in end-stage human HF, in rats after long-term isoprenaline stimulation through osmotic minipumps, and after incubation of rat neonatal cardiomyocytes with isoprenaline. Isoprenaline also promoted translocation of NDPK-C to the plasma membrane. Overexpression of NDPK-C in cardiomyocytes increased cAMP levels and sensitized cardiomyocytes to isoprenaline-induced augmentation of contractility, whereas NDPK-C knockdown decreased cAMP levels. In vivo, depletion of NDPK-C in zebrafish embryos caused cardiac edema and ventricular dysfunction. NDPK-B knockout mice had unaltered NDPK-C expression but showed contractile dysfunction and exacerbated cardiac remodeling during long-term isoprenaline stimulation. In human end-stage HF, the complex formation between NDPK-C and Gα_i2 was increased whereas the NDPK-C/Gα_s interaction was decreased, producing a switch that may contribute to an NDPK-C-dependent cAMP reduction in HF.

CONCLUSIONS

Our findings identify NDPK-C as an essential requirement for both the interaction between NDPK isoforms and between NDPK isoforms and G proteins. NDPK-C is a novel critical regulator of β-adrenoceptor/cAMP signaling and cardiac contractility. By switching from Gα_s to Gα_i2 activation, NDPK-C may contribute to lower cAMP levels and the related contractile dysfunction in HF.

Electrophysiological and molecular mechanisms of paroxysmal atrial fibrillation

Atrial fibrillation (AF) is an extremely prevalent arrhythmia that presents a wide range of therapeutic challenges. AF usually begins in a self-terminating paroxysmal form (pAF). With time, the AF pattern often evolves to become persistent (nonterminating within 7 days). Important differences exist between pAF and persistent AF in terms of clinical features, in particular the responsiveness to antiarrhythmic drugs and ablation therapy. AF mechanisms have been extensively reviewed, but few or no Reviews focus specifically on the pathophysiology of pAF. Accordingly, in this Review, we examine the available data on the electrophysiological basis for pAF occurrence and maintenance, as well as the molecular mechanisms forming the underlying substrate. We first consider the mechanistic insights that have been obtained from clinical studies in the electrophysiology laboratory, noninvasive observations, and genetic studies. We then discuss the information about underlying molecular mechanisms that has been obtained from experimental studies on animal models and patient samples. Finally, we discuss the data available from animal models with spontaneous AF presentation, their relationship to clinical findings, and their relevance to understanding the mechanisms underlying pAF. Our analysis then turns to potential factors governing cases of progression from pAF to persistent AF and the clinical implications of the basic mechanisms we review. We conclude by identifying and discussing questions that we consider particularly important to address through future research in this area.

Cardiac expression of the CREM repressor isoform CREM-IbΔC-X in mice leads to arrhythmogenic alterations in ventricular cardiomyocytes

Chronic β-adrenergic stimulation is regarded as a pivotal step in the progression of heart failure which is associated with a high risk for arrhythmia. The cAMP-dependent transcription factors cAMP-responsive element binding protein (CREB) and cAMP-responsive element modulator (CREM) mediate transcriptional regulation in response to β-adrenergic stimulation and CREM repressor isoforms are induced after stimulation of the β-adrenoceptor. Here, we investigate whether CREM repressors contribute to the arrhythmogenic remodeling in the heart by analyzing arrhythmogenic alterations in ventricular cardiomyocytes (VCMs) from mice with transgenic expression of the CREM repressor isoform CREM-IbΔC-X (TG). Patch clamp analyses, calcium imaging, immunoblotting and real-time quantitative RT-PCR were conducted to study proarrhythmic alterations in TG VCMs vs. wild-type controls. The percentage of VCMs displaying spontaneous supra-threshold transient-like Ca(2+) releases was increased in TG accompanied by an enhanced transduction rate of sub-threshold Ca(2+) waves into these supra-threshold events. As a likely cause we discovered enhanced NCX-mediated Ca(2+) transport and NCX1 protein level in TG. An increase in I NCX and decrease in I to and its accessory channel subunit KChIP2 was associated with action potential prolongation and an increased proportion of TG VCMs showing early afterdepolarizations. Finally, ventricular extrasystoles were augmented in TG mice underlining the in vivo relevance of our findings. Transgenic expression of CREM-IbΔC-X in mouse VCMs leads to distinct arrhythmogenic alterations. Since CREM repressors are inducible by chronic β-adrenergic stimulation our results suggest that the inhibition of CRE-dependent transcription contributes to the formation of an arrhythmogenic substrate in chronic heart disease.

Atrial fibrillation: mechanisms, therapeutics, and future directions

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia, affecting 1% to 2% of the general population. It is characterized by rapid and disorganized atrial activation leading to impaired atrial function, which can be diagnosed on an EKG by lack of a P-wave and irregular QRS complexes. AF is associated with increased morbidity and mortality and is a risk factor for embolic stroke and worsening heart failure. Current research on AF support and explore the hypothesis that initiation and maintenance of AF require pathophysiological remodeling of the atria, either specifically as in lone AF or secondary to other heart disease as in heart failure-associated AF. Remodeling in AF can be grouped into three categories that include: (i) electrical remodeling, which includes modulation of L-type Ca(2+) current, various K(+) currents and gap junction function; (ii) structural remodeling, which includes changes in tissues properties, size, and ultrastructure; and (iii) autonomic remodeling, including altered sympathovagal activity and hyperinnervation. Electrical, structural, and autonomic remodeling all contribute to creating an AF-prone substrate which is able to produce AF-associated electrical phenomena including a rapidly firing focus, complex multiple reentrant circuit or rotors. Although various remodeling events occur in AF, current AF therapies focus on ventricular rate and rhythm control strategies using pharmacotherapy and surgical interventions. Recent progress in the field has started to focus on the underlying substrate that drives and maintains AF (termed upstream therapies); however, much work is needed in this area. Here, we review current knowledge of AF mechanisms, therapies, and new areas of investigation.

Norepinephrine-Induced Adrenergic Activation Strikingly Increased the Atrial Fibrillation Duration through β1- and α1-Adrenergic Receptor-Mediated Signaling in Mice

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmias among old people. It causes serious long-term health problems affecting the quality of life. It has been suggested that the autonomic nervous system is involved in the onset and maintenance of AF in human. However, investigation of its pathogenesis and potential treatment has been hampered by the lack of suitable AF models in experimental animals.

OBJECTIVES

Our aim was to establish a long-lasting AF model in mice. We also investigated the role of adrenergic receptor (AR) subtypes, which may be involved in the onset and duration of AF.

METHODS AND RESULTS

Trans-esophageal atrial burst pacing in mice could induce AF, as previously shown, but with only a short duration (29.0 ± 8.1 sec). We found that adrenergic activation by intraperitoneal norepinephrine (NE) injection strikingly increased the AF duration. It increased the duration to more than 10 minutes, i.e., by more than 20-fold (656.2 ± 104.8 sec; P<0.001). In this model, a prior injection of a specific β1-AR blocker metoprolol and an α1-AR blocker prazosin both significantly attenuated NE-induced elongation of AF. To further explore the mechanisms underlying these receptors' effects on AF, we assessed the SR Ca(2+) leak, a major trigger of AF, and consequent spontaneous SR Ca(2+) release (SCR) in atrial myocytes. Consistent with the results of our in-vivo experiments, both metoprolol and prazosin significantly inhibited the NE-induced SR Ca(2+) leak and SCR. These findings suggest that both β1-AR and α1-AR may play important roles in the development of AF.

CONCLUSIONS

We have established a long-lasting AF model in mice induced by adrenergic activation, which will be valuable in future AF study using experimental animals, such as transgenic mice. We also revealed the important role of β1- and α1-AR-mediated signaling in the development of AF through in-vivo and in-vitro experiments.

Long-term methionine-diet induced mild hyperhomocysteinemia associated cardiac metabolic dysfunction in multiparous rats

Mild hyperhomocysteinemia (HHcy, clinically defined as less than 30 μmol/L) is an independent cardiovascular disease (CVD) risk factor, and is associated with many complications during pregnancy, such as preeclampsia (PE). The aim of this study was to assess the effect of long-term mild HHcy on cardiac metabolic function of multiparous rats. Female rats were mated 3 to 4 times and were fed with methionine in drinking water to increase plasma Hcy (2.9 ± 0.3 to 10.5 ± 2.3 μmol/L) until termination. This caused significant increase of heart weight/body weight (0.24 ± 0.01 to 0.27 ± 0.01 g/100 g) and left ventricle weight (0.69 ± 0.03 to 0.78 ± 0.01 g). Superoxide production was increased by 2.5-fold in HHcy hearts using lucigenin chemiluminescence. The ability of bradykinin and carbachol to regulate myocardial oxygen consumption (MVO₂) in vitro was impaired by 59% and 66% in HHcy heart, and it was restored by ascorbic acid (AA), tempol, or apocynin (Apo). Protein expression of p22^phox subunit of NAD(P)H oxidase was increased by 2.6-fold, but there were no changes in other NAD(P)H oxidase subunits, NOSs or SODs. Microarray revealed 1518 genes to be differentially regulated (P < 0.05). The mRNA level of NAD(P)H oxidase subunits, NOSs or SODs remained unchanged. In conclusion, long-term mild HHcy increases cardiac superoxide mainly through regulation of p22^phox component of the NAD(P)H oxidase and impairs the ability of NO to regulate MVO₂ in heart of multiparous mothers.

Feedback mechanisms for cardiac-specific microRNAs and cAMP signaling in electrical remodeling

BACKGROUND

Loss of transient outward K(+) current (Ito) is well documented in cardiac hypertrophy and failure both in animal models and in humans. Electrical remodeling contributes to prolonged action potential duration and increased incidence of arrhythmias. Furthermore, there is a growing body of evidence linking microRNA (miR) dysregulation to the progression of both conditions. In this study, we examined the mechanistic basis underlying miR dysregulation in electrical remodeling and revealed a novel interaction with the adrenergic signaling pathway.

METHODS AND RESULTS

We first used a tissue-specific knockout model of Dicer1 in cardiomyocytes to reveal the overall regulatory effect of miRs on the ionic currents and action potentials. We then validated the inducible cAMP early repressor as a target of miR-1 and took advantage of a clinically relevant model of post myocardial infarction and miR delivery to probe the mechanistic basis of miR dysregulation in electrical remodeling. These experiments revealed the role of inducible cAMP early repressor as a repressor of miR-1 and Ito, leading to prolonged action potential duration post myocardial infarction. In addition, delivery of miR-1 and miR-133a suppressed inducible cAMP early repressor expression and prevented both electrical remodeling and hypertrophy.

CONCLUSIONS

Taken together, our results illuminate the mechanistic links between miRs, adrenergic signaling, and electrical remodeling. They also serve as a proof-of-concept for the therapeutic potential of miR delivery post myocardial infarction.

Left atrial transcriptional changes associated with atrial fibrillation susceptibility and persistence

BACKGROUND

Prior transcriptional studies of atrial fibrillation (AF) have been limited to specific transcripts, animal models, chronic AF, right atria, or small samples. We sought to characterize the left atrial transcriptome in human AF to distinguish changes related to AF susceptibility and persistence.

METHODS AND RESULTS

Left atrial appendages from 239 patients stratified by coronary artery disease, valve disease, and AF history (no history of AF, AF history in sinus rhythm at surgery, and AF history in AF at surgery) were selected for genome-wide mRNA microarray profiling. Transcripts were examined for differential expression with AF phenotype group. Enrichment in differentially expressed genes was examined in 3 gene set collections: a transcription factor collection, defined by shared conserved cis-regulatory motifs, a miRNA collection, defined by shared 3' untranslated region motifs, and a molecular function collection, defined by shared Gene Ontology molecular function. AF susceptibility was associated with decreased expression of the targets of CREB/ATF family, heat-shock factor 1, ATF6, SRF, and E2F1 transcription factors. Persistent AF activity was associated with decreased expression in genes and gene sets related to ion channel function consistent with reported functional changes.

CONCLUSIONS

AF susceptibility was associated with decreased expression of targets of several transcription factors related to inflammation, oxidation, and cellular stress responses. In contrast, changes in ion channel expression were associated with AF activity but were limited in AF susceptibility. Our results suggest that significant transcriptional remodeling marks susceptibility to AF, whereas remodeling of ion channel expression occurs later in the progression or as a consequence of AF.

A compartmentalized mathematical model of the β1-adrenergic signaling system in mouse ventricular myocytes

The β₁-adrenergic signaling system plays an important role in the functioning of cardiac cells. Experimental data shows that the activation of this system produces inotropy, lusitropy, and chronotropy in the heart, such as increased magnitude and relaxation rates of [Ca²⁺]_i transients and contraction force, and increased heart rhythm. However, excessive stimulation of β₁-adrenergic receptors leads to heart dysfunction and heart failure. In this paper, a comprehensive, experimentally based mathematical model of the β₁-adrenergic signaling system for mouse ventricular myocytes is developed, which includes major subcellular functional compartments (caveolae, extracaveolae, and cytosol). The model describes biochemical reactions that occur during stimulation of β₁-adrenoceptors, changes in ionic currents, and modifications of Ca²⁺ handling system. Simulations describe the dynamics of major signaling molecules, such as cyclic AMP and protein kinase A, in different subcellular compartments; the effects of inhibition of phosphodiesterases on cAMP production; kinetics and magnitudes of phosphorylation of ion channels, transporters, and Ca²⁺ handling proteins; modifications of action potential shape and duration; magnitudes and relaxation rates of [Ca²⁺]_i transients; changes in intracellular and transmembrane Ca²⁺ fluxes; and [Na⁺]_i fluxes and dynamics. The model elucidates complex interactions of ionic currents upon activation of β₁-adrenoceptors at different stimulation frequencies, which ultimately lead to a relatively modest increase in action potential duration and significant increase in [Ca²⁺]_i transients. In particular, the model includes two subpopulations of the L-type Ca²⁺ channels, in caveolae and extracaveolae compartments, and their effects on the action potential and [Ca²⁺]_i transients are investigated. The presented model can be used by researchers for the interpretation of experimental data and for the developments of mathematical models for other species or for pathological conditions.

Correlated mRNAs and miRNAs from co-expression and regulatory networks affect porcine muscle and finally meat properties

Background

Physiological processes aiding the conversion of muscle to meat involve many genes associated with muscle structure and metabolic processes. MicroRNAs regulate networks of genes to orchestrate cellular functions, in turn regulating phenotypes.

Results

We applied weighted gene co-expression network analysis to identify co-expression modules that correlated to meat quality phenotypes and were highly enriched for genes involved in glucose metabolism, response to wounding, mitochondrial ribosome, mitochondrion, and extracellular matrix. Negative correlation of miRNA with mRNA and target prediction were used to select transcripts out of the modules of trait-associated mRNAs to further identify those genes that are correlated with post mortem traits.

Conclusions

Porcine muscle co-expression transcript networks that correlated to post mortem traits were identified. The integration of miRNA and mRNA expression analyses, as well as network analysis, enabled us to interpret the differentially-regulated genes from a systems perspective. Linking co-expression networks of transcripts and hierarchically organized pairs of miRNAs and mRNAs to meat properties yields new insight into several biological pathways underlying phenotype differences. These pathways may also be diagnostic for many myopathies, which are accompanied by deficient nutrient and oxygen supply of muscle fibers.

cAMP responsive element modulator: a critical regulator of cytokine production

T lymphocytes from patients with systemic lupus erythematosus (SLE) display a complex array of cellular, molecular, and signaling anomalies, many of which have been attributed to increased expression of the transcriptional regulator cAMP responsive element modulator α (CREMα). Recent evidence indicates that CREMα, in addition to its regulatory functions on gene promoters in T lymphocytes, alters the epigenetic conformation of cytokine genes by interacting with enzymes that control histone methylation and acetylation as well as cytosine-phosphate-guanosine (CpG) DNA methylation. This review summarizes the most recent findings on CREM protein expression in various cell types, in particular its effects on T lymphocyte biology in the context of both health and SLE. We emphasize CREMα as a key molecule that drives autoimmunity.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

Calmodulin-dependent protein kinase II: linking heart failure and arrhythmias

Understanding relationships between heart failure and arrhythmias, important causes of suffering and sudden death, remains an unmet goal for biomedical researchers and physicians. Evidence assembled over the past decade supports a view that activation of the multifunctional Ca(2+) and calmodulin-dependent protein kinase II (CaMKII) favors myocardial dysfunction and cell membrane electrical instability. CaMKII activation follows increases in intracellular Ca(2+) or oxidation, upstream signals with the capacity to transition CaMKII into a Ca(2+) and calmodulin-independent constitutively active enzyme. Constitutively active CaMKII appears poised to participate in disease pathways by catalyzing the phosphorylation of classes of protein targets important for excitation-contraction coupling and cell survival, including ion channels and Ca(2+) homeostatic proteins, and transcription factors that drive hypertrophic and inflammatory gene expression. This rich diversity of downstream targets helps to explain the potential for CaMKII to simultaneously affect mechanical and electrical properties of heart muscle cells. Proof-of-concept studies from a growing number of investigators show that CaMKII inhibition is beneficial for improving myocardial performance and for reducing arrhythmias. We review the molecular physiology of CaMKII and discuss CaMKII actions at key cellular targets and results of animal models of myocardial hypertrophy, dysfunction, and arrhythmias that suggest CaMKII inhibition may benefit myocardial function while reducing arrhythmias.

An introduction to murine models of atrial fibrillation

Understanding the mechanism of re-entrant arrhythmias in the past 30 years has allowed the development of almost curative therapies for many rhythm disturbances. The complex, polymorphic arrhythmias of atrial fibrillation (AF) and sudden death are, unfortunately, not yet well understood, and hence still in need of adequate therapy. AF contributes markedly to morbidity and mortality in aging Western populations. In the past decade, many genetically altered murine models have been described and characterized. Here, we review genetically altered murine models of AF; powerful tools that will enable a better understanding of the mechanisms of AF and the assessment of novel therapeutic interventions.

A Novel Transgenic Mouse Model of Cardiac Hypertrophy and Atrial Fibrillation

Cardiac hypertrophy is a major risk factor for the development of atrial fibrillation (AF). However, there are few animal models of AF associated with cardiac hypertrophy. In this study, we describe the in vivo electrophysiological characteristics and histopathology of a mouse model of cardiac hypertrophy that develops AF. Myostatin is a well-known negative regulator of skeletal muscle growth that was recently found to additionally regulate cardiac muscle growth. Using cardiac-specific expression of the inhibitory myostatin pro-peptide, we generated transgenic (TG) mice with dominant-negative regulation of MSTN (DN-MSTN). One line (DN-MSTN TG13) displayed ventricular hypertrophy, as well as spontaneous AF on the surface electrocardiogram (ECG), and was further evaluated. DN-MSTN TG13 had normal systolic function, but displayed atrial enlargement on cardiac MRI, as well as atrial fibrosis histologically. Baseline ECG revealed an increased P wave duration and QRS interval compared with wild-type littermate (WT) mice. Seven of 19 DN-MSTN TG13 mice had spontaneous or inducible AF, while none of the WT mice had atrial arrhythmias (p<0.05). Connexin40 (Cx40) was decreased in DN-MSTN TG13 mice, even in the absence of AF or significant atrial fibrosis, raising the possibility that MSTN signaling may play a role in Cx40 down-regulation and the development of AF in this mouse model. In conclusion, DN-MSTN TG13 mice represent a novel model of AF, in which molecular changes including an initial loss of Cx40 are noted prior to fibrosis and the development of atrial arrhythmias.

Overexpression of cAMP-response element modulator causes abnormal growth and development of the atrial myocardium resulting in a substrate for sustained atrial fibrillation in mice

BACKGROUND AND METHODS

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice. The substrate of AF is composed of a complex interplay between structural and functional changes of the atrial myocardium often preceding the occurrence of persistent AF. However, there are only few animal models reproducing the slow progression of the AF substrate to the spontaneous occurrence of the arrhythmia. Transgenic mice (TG) with cardiomyocyte-directed expression of CREM-IbΔC-X, an isoform of transcription factor CREM, develop atrial dilatation and spontaneous-onset AF. Here we tested the hypothesis that TG mice develop an arrhythmogenic substrate preceding AF using physiological and biochemical techniques.

RESULTS

Overexpression of CREM-IbΔC-X in young TG mice (<8weeks) led to atrial dilatation combined with distension of myocardium, elongated myocytes, little fibrosis, down-regulation of connexin 40, loss of excitability with a number of depolarized myocytes, atrial ectopies and inducibility of AF. These abnormalities continuously progressed with age resulting in interatrial conduction block, increased atrial conduction heterogeneity, leaky sarcoplasmic reticulum calcium stores and the spontaneous occurrence of paroxysmal and later persistent AF. This distinct atrial remodelling was associated with a pattern of non-regulated and up-regulated marker genes of myocardial hypertrophy and fibrosis.

CONCLUSIONS

Expression of CREM-IbΔC-X in TG hearts evokes abnormal growth and development of the atria preceding conduction abnormalities and altered calcium homeostasis and the development of spontaneous and persistent AF. We conclude that transcription factor CREM is an important regulator of atrial growth implicated in the development of an arrhythmogenic substrate in TG mice.