Cardiomyocytes hypertrophic status after myocardial infarction determines distinct types of arrhythmia: role of the ryanodine receptor

RyR2 inhibition with dantrolene is antiarrhythmic, prevents further pathological remodeling, and improves cardiac function in chronic ischemic heart disease

Diastolic Ca2+ leak due to cardiac ryanodine receptor (RyR2) hyperactivity has been widely documented in chronic ischemic heart disease (CIHD) and may contribute to ventricular tachycardia (VT) risk and progressive left-ventricular (LV) remodeling. Here we test the hypothesis that targeting RyR2 hyperactivity can suppress VT inducibility and progressive heart failure in CIHD by the RyR2 inhibitor dantrolene. METHODS AND RESULTS: CIHD was induced in C57BL/6 J mice by left coronary artery ligation. Four weeks later, mice were randomized to either acute or chronic (6 weeks via implanted osmotic pump) treatment with dantrolene or vehicle. VT inducibility was assessed by programmed stimulation in vivo and in isolated hearts. Electrical substrate remodeling was assessed by optical mapping. Ca2+ sparks and spontaneous Ca2+ releases were measured in isolated cardiomyocytes. Cardiac remodeling was quantified by histology and qRT-PCR. Cardiac function and contractility were measured using echocardiography. Compared to vehicle, acute dantrolene treatment reduced VT inducibility. Optical mapping demonstrated reentrant VT prevention by dantrolene, which normalized the shortened refractory period (VERP) and prolonged action potential duration (APD), preventing APD alternans. In single CIHD cardiomyocytes, dantrolene normalized RyR2 hyperactivity and prevented spontaneous intracellular Ca2+ release. Chronic dantrolene treatment not only reduced VT inducibility but also reduced peri-infarct fibrosis and prevented further progression of LV dysfunction in CIHD mice. CONCLUSIONS: RyR2 hyperactivity plays a mechanistic role for VT risk, post-infarct remodeling, and contractile dysfunction in CIHD mice. Our data provide proof of concept for the anti-arrhythmic and anti-remodeling efficacy of dantrolene in CIHD.

New role of TRPM4 channel in the cardiac excitation-contraction coupling in response to physiological and pathological hypertrophy in mouse

The transient receptor potential Melastatin 4 (TRPM4) channel is a calcium-activated non-selective cation channel expressed widely. In the heart, using a knock-out mouse model, the TRPM4 channel has been shown to be involved in multiple processes, including β-adrenergic regulation, cardiac conduction, action potential duration and hypertrophic adaptations. This channel was recently shown to be involved in stress-induced cardiac arrhythmias in a mouse model overexpressing TRPM4 in ventricular cardiomyocytes. However, the link between TRPM4 channel expression in ventricular cardiomyocytes, the hypertrophic response to stress and/or cellular arrhythmias has yet to be elucidated. In this present study, we induced pathological hypertrophy in response to myocardial infarction using a mouse model of Trpm4 gene invalidation, and demonstrate that TRPM4 is essential for survival. We also demonstrate that the TRPM4 is required to activate both the Akt and Calcineurin pathways. Finally, using two hypertrophy models, either a physiological response to endurance training or a pathological response to myocardial infarction, we show that TRPM4 plays a role in regulating transient calcium amplitudes and leads to the development of cellular arrhythmias potentially in cooperation with the Sodium-calcium exchange (NCX). Here, we report two functions of the TRPM4 channel: first its role in adaptive hypertrophy, and second its association with NCX could mediate transient calcium amplitudes which trigger cellular arrhythmias.

Concomitant systolic and diastolic alterations during chronic hypertension in pig

The mechanical and cellular relationships between systole and diastole during left ventricular (LV) dysfunction remain to be established. LV contraction-relaxation coupling was examined during LV hypertrophy induced by chronic hypertension. Chronically instrumented pigs received angiotensin II infusion for4weeks to induce chronic hypertension (133 ± 7 mmHg vs 98 ± 5 mmHg for mean arterial pressure at Day 28 vs 0, respectively) and LV hypertrophy. LV function was investigated with the instrumentation and echocardiography for LV twist-untwist assessment before and after dobutamine infusion. The cellular mechanisms were investigated by exploring the intracellular Ca²⁺ handling. At Day 28, pigs exhibited LV hypertrophy with LV diastolic dysfunction (impaired LV isovolumic relaxation, increased LV end-diastolic pressure, decreased and delayed LV untwisting rate) and LV systolic dysfunction (impaired LV isovolumic contraction and twist) although LV ejection fraction was preserved. Isolated cardiomyocytes exhibited altered shortening and lengthening. Interestingly, contraction-relaxation coupling remained preserved both in vivo and in vitro during LV hypertrophy. LV systolic and diastolic dysfunctions were associated to post-translational remodeling and dysfunction of the type 2 cardiac ryanodine receptor/Ca²⁺ release channel (RyR2), i.e., PKA hyperphosphorylation of RyR2, depletion of calstabin 2 (FKBP12.6), RyR2 leak and hypersensitivity of RyR2 to cytosolic Ca²⁺ during both contraction and relaxation phases. In conclusion, LV contraction-relaxation coupling remained preserved during chronic hypertension despite LV systolic and diastolic dysfunctions. This implies that LV diastolic dysfunction is accompanied by LV systolic dysfunction. At the cellular level, this is linked to sarcoplasmic reticulum Ca²⁺ leak through PKA-mediated RyR2 hyperphosphorylation and depletion of its stabilizing partner.

Heat-shock transcription factor 1 is critically involved in the ischaemia-induced cardiac hypertrophy via JAK2/STAT3 pathway

Cardiac hypertrophy after myocardial infarction (MI) is an independent risk factor for heart failure. Regression of cardiac hypertrophy has emerged as a promising strategy in the treatment of MI patients. Here, we have been suggested that heat-shock transcription factor 1 (HSF1) is a novel repressor of ischaemia-induced cardiac hypertrophy. Ligation of left anterior descending coronary was used to produce MI in HSF1-deficient heterozygote (KO), HSF1 transgenic (TG) mice and their wild-type (WT) littermates, respectively. Neonatal rat cardiomyocytes (NRCMs) were treated by hypoxia to mimic MI in vitro. The HSF1 phosphorylation was significantly reduced in the infarct border zone of mouse left ventricles (LVs) 1 week after MI and in the hypoxia-treated NRCMs. HSF1 KO mice showed more significant maladaptive cardiac hypertrophy and deteriorated cardiac dysfunction 1 week after MI compared to WT MI mice. Deficiency of HSF1 by siRNA transfection notably increased the hypoxia-induced myocardial hypertrophy in NRCMs. Mechanistically, Janus kinase 2 (JAK2) and its effector, signal transducer and activator of transcription 3 (STAT3) were found to be significantly increased in the LV infarct border zone of WT mice after MI as well as the NRCMs treated by hypoxia. These alterations were more significant in HSF1 KO mice and NRCMs transfected with HSF1 SiRNA. Inversely, HSF1 TG mice showed significantly ameliorated cardiac hypertrophy and heart failure 1 week after LAD ligation compared to their WT littermates. Our data collectively demonstrated that HSF1 is critically involved in the pathological cardiac hypertrophy after MI via modulating JAK2/STAT3 signalling and may constitute a potential therapeutic target for MI patients.

Biodegradable Polymeric Nanocapsules Prevent Cardiotoxicity of Anti-Trypanosomal Lychnopholide

Chagas disease is a neglected parasitic disease caused by the protozoan Trypanosoma cruzi. New antitrypanosomal options are desirable to prevent complications, including a high rate of cardiomyopathy. Recently, a natural substance, lychnopholide, has shown therapeutic potential, especially when encapsulated in biodegradable polymeric nanocapsules. However, little is known regarding possible adverse effects of lychnopholide. Here we show that repeated-dose intravenous administration of free lychnopholide (2.0 mg/kg/day) for 20 days caused cardiopathy and mortality in healthy C57BL/6 mice. Echocardiography revealed concentric left ventricular hypertrophy with preserved ejection fraction, diastolic dysfunction and chamber dilatation at end-stage. Single cardiomyocytes presented altered contractility and Ca²⁺ handling, with spontaneous Ca²⁺ waves in diastole. Acute in vitro lychnopholide application on cardiomyocytes from healthy mice also induced Ca²⁺ handling alterations with abnormal RyR2-mediated diastolic Ca²⁺ release. Strikingly, the encapsulation of lychnopholide prevented the cardiac alterations induced in vivo by the free form repeated doses. Nanocapsules alone had no adverse cardiac effects. Altogether, our data establish lychnopholide presented in nanocapsule form more firmly as a promising new drug candidate to cure Chagas disease with minimal cardiotoxicity. Our study also highlights the potential of nanotechnology not only to improve the efficacy of a drug but also to protect against its adverse effects.

Trpm4 gene invalidation leads to cardiac hypertrophy and electrophysiological alterations

RATIONALE

TRPM4 is a non-selective Ca2+-activated cation channel expressed in the heart, particularly in the atria or conduction tissue. Mutations in the Trpm4 gene were recently associated with several human conduction disorders such as Brugada syndrome. TRPM4 channel has also been implicated at the ventricular level, in inotropism or in arrhythmia genesis due to stresses such as ß-adrenergic stimulation, ischemia-reperfusion, and hypoxia re-oxygenation. However, the physiological role of the TRPM4 channel in the healthy heart remains unclear.

OBJECTIVES

We aimed to investigate the role of the TRPM4 channel on whole cardiac function with a Trpm4 gene knock-out mouse (Trpm4-/-) model.

METHODS AND RESULTS

Morpho-functional analysis revealed left ventricular (LV) eccentric hypertrophy in Trpm4-/- mice, with an increase in both wall thickness and chamber size in the adult mouse (aged 32 weeks) when compared to Trpm4+/+ littermate controls. Immunofluorescence on frozen heart cryosections and qPCR analysis showed no fibrosis or cellular hypertrophy. Instead, cardiomyocytes in Trpm4-/- mice were smaller than Trpm4+/+with a higher density. Immunofluorescent labeling for phospho-histone H3, a mitosis marker, showed that the number of mitotic myocytes was increased 3-fold in the Trpm4-/-neonatal stage, suggesting hyperplasia. Adult Trpm4-/- mice presented multilevel conduction blocks, as attested by PR and QRS lengthening in surface ECGs and confirmed by intracardiac exploration. Trpm4-/-mice also exhibited Luciani-Wenckebach atrioventricular blocks, which were reduced following atropine infusion, suggesting paroxysmal parasympathetic overdrive. In addition, Trpm4-/- mice exhibited shorter action potentials in atrial cells. This shortening was unrelated to modifications of the voltage-gated Ca2+ or K+ currents involved in the repolarizing phase.

CONCLUSIONS

TRPM4 has pleiotropic roles in the heart, including the regulation of conduction and cellular electrical activity which impact heart development.

Nitric oxide synthase regulation of cardiac excitation-contraction coupling in health and disease

Significant advances in our understanding of the ability of nitric oxide synthases (NOS) to modulate cardiac function have provided key insights into the role NOS play in the regulation of excitation-contraction (EC) coupling in health and disease. Through both cGMP-dependent and cGMP-independent (e.g. S-nitrosylation) mechanisms, NOS have the ability to alter intracellular Ca(2+) handling and the myofilament response to Ca(2+), thereby impacting the systolic and diastolic performance of the myocardium. Findings from experiments using nitric oxide (NO) donors and NOS inhibition or gene deletion clearly implicate dysfunctional NOS as a critical contributor to many cardiovascular disease states. However, studies to date have only partially addressed NOS isoform-specific effects and, more importantly, how subcellular localization of NOS influences ion channels involved in myocardial EC coupling and excitability. In this review, we focus on the contribution of each NOS isoform to cardiac dysfunction and on the role of uncoupled NOS activity in common cardiac disease states, including heart failure, diabetic cardiomyopathy, ischemia/reperfusion injury and atrial fibrillation. We also review evidence that clearly indicates the importance of NO in cardioprotection. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Myofilament calcium de-sensitization and contractile uncoupling prevent pause-triggered ventricular tachycardia in mouse hearts with chronic myocardial infarction

Myocardial infarction (MI) is a major risk for ventricular arrhythmia. Pause-triggered ventricular arrhythmia can be caused by increased myofilament Ca binding due to sarcomeric mutations or Ca-sensitizing compounds. Myofilament Ca sensitivity is also increased after MI. Here we hypothesize that MI increases risk for pause-triggered ventricular arrhythmias, which can be prevented by myofilament Ca-desensitization and contractile uncoupling. To test this hypothesis, we generated a murine chronic MI model using male B6SJLF1/J mice (n=40) that underwent permanent ligation of the left anterior descending coronary artery. 4 weeks post MI, cardiac structure, function and myofilament Ca sensitivity were evaluated. Pause-dependent arrhythmia susceptibility was quantified in isolated hearts with pacing trains of increasing frequency, followed by a pause and an extra stimulus. Coronary ligation resulted in a mean infarct size of 39.6±5.7% LV and fractional shortening on echocardiography was reduced by 40% compared to non-infarcted controls. Myofilament Ca sensitivity was significantly increased in post MI hearts (pCa50: Control=5.66±0.03; MI=5.84±0.05; P<0.01). Exposure to the Ca desensitizer/contractile uncoupler blebbistatin (BLEB, 3 μM) reduced myofilament Ca sensitivity of MI hearts to that of control hearts and selectively reduced the frequency of post-pause ectopic beats (MI 0.12±0.04 vs MI+BLEB 0.01±0.005 PVC/pause; P=0.02). BLEB also reduced the incidence of ventricular tachycardia in chronic MI hearts from 59% to 10% (P<0.05). We conclude that chronic MI hearts exhibit increased myofilament Ca sensitivity and pause-triggered ventricular arrhythmias, which can be prevented by blebbistatin. Decreasing myofilament Ca sensitivity may be a strategy to reduce arrhythmia burden after MI.

M3 subtype of muscarinic acetylcholine receptor promotes cardioprotection via the suppression of miR-376b-5p

The M₃ subtype of muscarinic acetylcholine receptors (M₃-mAChR) plays a protective role in myocardial ischemia and microRNAs (miRNAs) participate in many cardiac pathophysiological processes, including ischemia-induced cardiac injury. However, the role of miRNAs in M₃-mAChR mediated cardioprotection remains unexplored. The present study was designed to identify miRNAs that are involved in cardioprotective effects of M₃-mAChR against myocardial ischemia and elucidate the underlying mechanisms. We established rat model of myocardial ischemia and performed miRNA microarray analysis to identify miRNAs involved in the cardioprotection of M₃-mAChR. In H9c2 cells, the viability, intracellular free Ca²⁺ concentration ([Ca²⁺]i), intracellular reactive oxygen species (ROS), miR-376b-5p expression level, brain derived neurophic factor (BDNF) and nuclear factor kappa-B (NF-κB) levels were measured. Our results demonstrated that M₃-mAChR protected myocardial ischemia injury. Microarray analysis and qRT-PCR revealed that miR-376b-5p was significantly up-regulated in ischemic heart tissue and the M₃-mAChRs agonist choline reversed its up-regulation. In vitro, miR-376b-5p promoted H₂O₂-induced H9c2 cell injuries measured by cells viability, [Ca²⁺]i and ROS. Western blot and luciferase assay identified BDNF as a direct target of miR-376b-5p. M₃-mAChR activated NF-κB and thereby inhibited miR-376b-5p expression. Our data show that a novel M₃-mAChR/NF-κB/miR-376b-5p/BDNF axis plays an important role in modulating cardioprotection. MiR-376b-5p promotes myocardial ischemia injury possibly by inhibiting BDNF expression and M₃-mAChR provides cardioprotection at least partially mediated by the downregulation of miR-376b-5p through NF-κB. These findings provide new insight into the potential mechanism by which M₃-mAChR provides cardioprotection against myocardial ischemia injury.