A Titin Missense Variant Causes Atrial Fibrillation.

Rare and common genetic variants contribute to the risk of atrial fibrillation (AF). Although ion channels were among the first AF candidate genes identified, rare loss-of-function variants in structural genes such as have also been implicated in AF pathogenesis partly by the development of an atrial myopathy, but the underlying mechanisms are poorly understood. While truncating variants (tvs) have been causally linked to arrhythmia and cardiomyopathy syndromes, the role of missense variants (mvs) remains unclear.

Modulation of NOX2 causes obesity-mediated atrial fibrillation.

Obesity is linked to an increased risk of atrial fibrillation (AF) via increased oxidative stress. While NADPH oxidase 2 (NOX2), a major source of oxidative stress and reactive oxygen species (ROS) in the heart, predisposes to AF, the underlying mechanisms remain unclear. Here, we studied NOX2-mediated ROS production in obesity-mediated AF using Nox2-knockout mice and mature human induced pluripotent stem cell-derived atrial cardiomyocytes (hiPSC-aCMs).

Isolated Cardiac Ryanodine Receptor Function Varies Between Mammals.

Concerted robust opening of cardiac ryanodine receptors' (RyR2) Ca release 1oplasmic reticulum (SR) is fundamental for normal systolic cardiac function. During diastole, infrequent spontaneous RyR2 openings mediate the SR Ca leak that normally constrains SR Ca load. Abnormal large diastolic RyR2-mediated Ca leak events can cause delayed after depolarizations (DADs) and arrhythmias.

Inositol 1,4,5-trisphosphate receptor - reactive oxygen signaling domain regulates excitation-contraction coupling in atrial myocytes.

The inositol 1,4,5-trisphosphate receptor (InsPR) is up-regulated in patients with atrial fibrillation (AF) and InsP-induced Ca release (IICR) is linked to pro-arrhythmic spontaneous Ca release events. Nevertheless, knowledge of the physiological relevance and regulation of InsPRs in atrial muscle is still limited. We hypothesize that InsPR and NADPH oxidase 2 (NOX2) form a functional signaling domain where NOX2 derived reactive oxygen species (ROS) regulate InsPR agonist affinity and thereby Ca release.

JNK2, a Newly-Identified SERCA2 Enhancer, Augments an Arrhythmic [Ca] Leak-Load Relationship.

Rationale: We recently discovered pivotal contributions of stress kinase JNK2 (c-Jun N-terminal kinase isoform 2) in increased risk of atrial fibrillation through enhanced diastolic sarcoplasmic reticulum (SR) calcium (Ca) leak via RyR2 (ryanodine receptor isoform 2). However, the role of JNK2 in the function of the SERCA2 (SR Ca-ATPase), essential in maintaining SR Ca content cycling during each heartbeat, is completely unknown.

Objective: To test the hypothesis that JNK2 increases SERCA2 activity SR Ca content and exacerbates an arrhythmic SR Ca content leak-load relationship.

Loss of p21-activated kinase 1 (Pak1) promotes atrial arrhythmic activity.

Background: Atrial fibrillation (AF) is initiated through arrhythmic atrial excitation from outside the sinus node or remodeling of atrial tissue that allows reentry of excitation. Angiotensin II (AngII) has been implicated in the initiation and maintenance of AF through changes in Ca handling and production of reactive oxygen species (ROS).

Objective: We aimed to determine the role of p21-activated kinase 1 (Pak1), a downstream target in the AngII signaling cascade, in atrial electrophysiology and arrhythmia.

Dyssynchronous calcium removal in heart failure-induced atrial remodeling.

We tested the hypothesis that in atrial myocytes from a rabbit left ventricular heart failure (HF) model, spatial inhomogeneity and temporal dyssynchrony of Ca removal during excitation-contraction coupling together with increased Na/Ca exchange (NCX) activity generate a substrate for proarrhythmic Ca release. Ca removal occurs via Ca reuptake into the sarcoplasmic reticulum and extrusion via NCX exclusively in the cell periphery since rabbit atrial myocytes lack transverse tubules. Ca removal kinetics were assessed by the time constant τ of decay of local peripheral subsarcolemmal (SS) and central (CT) action potential (AP)-induced Ca transients (CaTs) recorded in confocal line scan mode (using Fluo-4).

p21-Activated kinase1 (Pak1) is a negative regulator of NADPH-oxidase 2 in ventricular myocytes.

Ischemic conditions reduce the activity of the p21-activated kinase (Pak1) resulting in increased arrhythmic activity. Triggered arrhythmic activity during ischemia is based on changes in cellular ionic balance and the cells Ca(2+) handling properties. In the current study we used isolated mouse ventricular myocytes (VMs) deficient for the expression of Pak1 (Pak1(-/-)) to determine the mechanism by which Pak1 influences the generation of arrhythmic activity during simulated ischemia.

The C-terminus of the long AKAP13 isoform (AKAP-Lbc) is critical for development of compensatory cardiac hypertrophy.

The objective of this study was to determine the role of A-Kinase Anchoring Protein (AKAP)-Lbc in the development of heart failure, by investigating AKAP-Lbc-protein kinase D1 (PKD1) signaling in vivo in cardiac hypertrophy. Using a gene-trap mouse expressing a truncated version of AKAP-Lbc (due to disruption of the endogenous AKAP-Lbc gene), that abolishes PKD1 interaction with AKAP-Lbc (AKAP-Lbc-ΔPKD), we studied two mouse models of pathological hypertrophy: i) angiotensin (AT-II) and phenylephrine (PE) infusion and ii) transverse aortic constriction (TAC)-induced pressure overload. Our results indicate that AKAP-Lbc-ΔPKD mice exhibit an accelerated progression to cardiac dysfunction in response to AT-II/PE treatment and TAC.

Ischemia/Reperfusion injury protection by mesenchymal stem cell derived antioxidant capacity.

Mesenchymal stem cell (MSC) transplantation after ischemia/reperfusion (I/R) injury reduces infarct size and improves cardiac function. We used mouse ventricular myocytes (VMs) in an in vitro model of I/R to determine the mechanism by which MSCs prevent reperfusion injury by paracrine signaling. Exposure of mouse VMs to an ischemic challenge depolarized their mitochondrial membrane potential (Ψmito), increased their diastolic Ca(2+), and significantly attenuated cell shortening.

Functional integrity of the T-tubular system in cardiomyocytes depends on p21-activated kinase 1.

p21-activated kinase (Pak1), a serine-threonine protein kinase, regulates cytoskeletal dynamics and cell motility. Recent experiments further demonstrate that loss of Pak1 results in exaggerated hypertrophic growth in response to pathophysiological stimuli. Calcium (Ca) signaling plays an important role in the regulation of transcription factors involved in hypertrophic remodeling.

While systolic cardiomyocyte function is preserved, diastolic myocyte function and recovery from acidosis are impaired in CaMKIIδ-KO mice.

Objective: CaMKII contributes to impaired contractility in heart failure by inducing SR Ca(2+)-leak. CaMKII-inhibition in the heart was suggested to be a novel therapeutic principle. Different CaMKII isoforms exist.

Excitation-contraction coupling in ventricular myocytes is enhanced by paracrine signaling from mesenchymal stem cells.

In clinical trials mesenchymal stem cells (MSCs) are transplanted into cardiac ischemic regions to decrease infarct size and improve contractility. However, the mechanism and time course of MSC-mediated cardioprotection are incompletely understood. We tested the hypothesis that paracrine signaling by MSCs promotes changes in cardiac excitation-contraction (EC) coupling that protects myocytes from cell death and enhances contractility.

SR-targeted CaMKII inhibition improves SR Ca²+ handling, but accelerates cardiac remodeling in mice overexpressing CaMKIIδC.

Cardiac myocyte overexpression of CaMKIIδ(C) leads to cardiac hypertrophy and heart failure (HF) possibly caused by altered myocyte Ca(2+) handling. A central defect might be the marked CaMKII-induced increase in diastolic sarcoplasmic reticulum (SR) Ca(2+) leak which decreases SR Ca(2+) load and Ca(2+) transient amplitude. We hypothesized that inhibition of CaMKII near the SR membrane would decrease the leak, improve Ca(2+) handling and prevent the development of contractile dysfunction and HF.

The IP3 receptor regulates cardiac hypertrophy in response to select stimuli.

Rationale: Inositol 1,4,5-trisphosphate (IP(3)) is a second messenger that regulates intracellular Ca(2+) release through IP(3) receptors located in the sarco(endo)plasmic reticulum of cardiac myocytes. Many prohypertrophic G protein-coupled receptor (GPCR) signaling events lead to IP(3) liberation, although its importance in transducing the hypertrophic response has not been established in vivo.

Objective: Here, we generated conditional, heart-specific transgenic mice with both gain- and loss-of-function for IP(3) receptor signaling to examine its hypertrophic growth effects following pathological and physiological stimulation.

Impaired contractile function and calcium handling in hearts of cardiac-specific calcineurin b1-deficient mice.

To define the necessity of calcineurin (Cn) signaling for cardiac maturation and function, the postnatal phenotype of mice with cardiac-specific targeted ablation of the Cn B1 regulatory subunit (Ppp3r1) gene (csCnb1(-/-) mice) was characterized. csCnb1(-/-) mice develop a lethal cardiomyopathy, characterized by impaired postnatal growth of the heart and combined systolic and diastolic relaxation abnormalities, despite a lack of structural derangements. Notably, the csCnb1(-/-) hearts did not exhibit diastolic dilatation, despite the severe functional phenotype.

Arrhythmogenic effects of beta2-adrenergic stimulation in the failing heart are attributable to enhanced sarcoplasmic reticulum Ca load.

Ventricular tachycardia in heart failure (HF) can initiate by nonreentrant mechanisms such as delayed afterdepolarizations. In an arrhythmogenic rabbit model of HF, we have shown that isoproterenol induces ventricular tachycardia in vivo and aftercontractions and transient inward currents in HF myocytes. To determine whether beta(2)-adrenergic receptor (beta(2)-AR) stimulation contributes, we performed in vivo drug infusion, in vitro myocyte and biochemical studies.

Ca2+ /H+ exchange via the plasma membrane Ca2+ ATPase in skeletal muscle.

The aims of this work were to determine: 1) whether Ca2+ exit via the plasmalemmal Ca2+ ATPase (PMCA) is coupled to H+ entry via a Ca2+/H+ exchange; 2) whether operation of PMCA has an absolute requirement on external H+ (Ho); and 3) the stoichiometry and voltage-dependence of the Ca2+/H+ exchange. Barnacle muscle cells were used because of the ease with which they can be internally-perfused (e.g.

Cardiac alternans do not rely on diastolic sarcoplasmic reticulum calcium content fluctuations.

Cardiac alternans are thought to be a precursor to life-threatening arrhythmias. Previous studies suggested that alterations in sarcoplasmic reticulum (SR) Ca2+ content are either causative or not associated with myocyte Ca2+ alternans. However, those studies used indirect measures of SR Ca2+.

The inotropic effect of cardioactive glycosides in ventricular myocytes requires Na+-Ca2+ exchanger function.

Glycoside-induced cardiac inotropy has traditionally been attributed to direct Na(+)-K(+)-ATPase inhibition, causing increased intracellular [Na(+)] and consequent Ca(2+) gain via the Na(+)-Ca(2+) exchanger (NCX). However, recent studies suggested alternative mechanisms of glycoside-induced inotropy: (1) direct activation of sarcoplasmic reticulum Ca(2+) release channels (ryanodine receptors; RyRs); (2) increased Ca(2+) selectivity of Na(+) channels (slip-mode conductance); and (3) other signal transduction pathways. None of these proposed mechanisms requires NCX or an altered [Na(+)] gradient.

Targeted inhibition of sarcoplasmic reticulum CaMKII activity results in alterations of Ca2+ homeostasis and cardiac contractility.

Transgenic (TG) mice expressing a Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitory peptide targeted to the cardiac myocyte longitudinal sarcoplasmic reticulum (LSR) display reduced phospholamban phosphorylation at Thr17 and develop dilated myopathy when stressed by gestation and parturition (Ji Y, Li B, Reed TD, Lorenz JN, Kaetzel MA, and Dedman JR. J Biol Chem 278: 25063-25071, 2003). In the present study, these animals (TG) are evaluated for the effect of inhibition of sarcoplasmic reticulum (SR) CaMKII activity on the contractile characteristics and Ca2+ cycling of myocytes.

Phospholamban is required for CaMKII-dependent recovery of Ca transients and SR Ca reuptake during acidosis in cardiac myocytes.

Initially during acidosis, Ca transient amplitude (Delta[Ca]i) and the rate constant of [Ca]i decline (k(Ca)) are decreased, but later during acidosis Delta[Ca]i and k(Ca) partially recover. This recovery in rat myocytes could be inhibited by KN-93 suggesting that CaMKII-dependent protein phosphorylation (and enhanced SR Ca uptake) may be responsible. To test whether phospholamban (PLB) is required for the Delta[Ca]i and k(Ca) recovery during acidosis, we used isolated myocytes from PLB knockout (PLB-KO) vs.

Transgenic CaMKIIdeltaC overexpression uniquely alters cardiac myocyte Ca2+ handling: reduced SR Ca2+ load and activated SR Ca2+ release.

Ca2+/calmodulin-dependent protein kinase II (CaMKII) delta is the predominant cardiac isoform, and the deltaC splice variant is cytoplasmic. We overexpressed CaMKIIdeltaC in mouse heart and observed dilated heart failure and altered myocyte Ca2+ regulation in 3-month-old CaMKIIdeltaC transgenic mice (TG) versus wild-type littermates (WT). Heart/body weight ratio and cardiomyocyte size were increased about 2-fold in TG versus WT.