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Background: Advances in congestive heart failure (CHF) management depend on biomarkers for monitoring disease progression and therapeutic response. During systole, intracellular Ca 2+ is released from the sarcoplasmic reticulum into the cytoplasm through type-2 ryanodine receptor/Ca 2+ release channels. In CHF, chronically elevated circulating catecholamine levels cause pathological remodeling of type-2 ryanodine receptor/Ca 2+ release channels resulting in diastolic sarcoplasmic reticulum Ca 2+ leak and decreased myocardial contractility. Similarly, skeletal muscle contraction requires sarcoplasmic reticulum Ca 2+ release through type-1 ryanodine receptors (RyR1), and chronically elevated catecholamine levels in CHF cause RyR1-mediated sarcoplasmic reticulum Ca 2+ leak, contributing to myopathy and weakness. Circulating B-lymphocytes express RyR1 and catecholamine-responsive signaling cascades, making them a potential surrogate for defects in intracellular Ca 2+ handling because of leaky RyR channels in CHF. Methods: Whole blood was collected from patients with CHF, CHF following left-ventricular assist device implant, and controls. Blood was also collected from mice with ischemic CHF, ischemic CHF+S107 (a drug that specifically reduces RyR channel Ca 2+ leak), and wild-type controls. Channel macromolecular complex was assessed by immunostaining RyR1 immunoprecipitated from lymphocyte-enriched preparations. RyR1 Ca 2+ leak was assessed using flow cytometry to measure Ca 2+ fluorescence in B-lymphocytes in the absence and presence of RyR1 agonists that empty RyR1 Ca 2+ stores within the endoplasmic reticulum. Results: Circulating B-lymphocytes from humans and mice with CHF exhibited remodeled RyR1 and decreased endoplasmic reticulum Ca 2+ stores, consistent with chronic intracellular Ca 2+ leak. This Ca 2+ leak correlated with circulating catecholamine levels. The intracellular Ca 2+ leak was significantly reduced in mice treated with the Rycal S107. Patients with CHF treated with left-ventricular assist devices exhibited a heterogeneous response. Conclusions: In CHF, B-lymphocytes exhibit remodeled leaky RyR1 channels and decreased endoplasmic reticulum Ca 2+ stores consistent with chronic intracellular Ca 2+ leak. RyR1-mediated Ca 2+ leak in B-lymphocytes assessed using flow cytometry provides a surrogate measure of intracellular Ca 2+ handling and systemic sympathetic burden, presenting a novel biomarker for monitoring response to pharmacological and mechanical CHF therapy.

Rationale: Atrial fibrillation (AF) is the most common cardiac arrhythmia, however the mechanism(s) causing AF remain poorly understood and therapy is suboptimal. The ryanodine receptor (RyR2) is the major calcium (Ca 2+ ) release channel on the sarcoplasmic reticulum (SR) required for excitation-contraction coupling in cardiac muscle. Objective: In the present study, we sought to determine whether intracellular diastolic SR Ca 2+ leak via RyR2 plays a role in triggering AF and whether inhibiting this leak can prevent AF. Methods and Results: We generated 3 knock-in mice with mutations introduced into RyR2 that result in leaky channels and cause exercise induced polymorphic ventricular tachycardia in humans [catecholaminergic polymorphic ventricular tachycardia (CPVT)]. We examined AF susceptibility in these three CPVT mouse models harboring RyR2 mutations to explore the role of diastolic SR Ca 2+ leak in AF. AF was stimulated with an intra-esophageal burst pacing protocol in the 3 CPVT mouse models (RyR2-R2474S +/− , 70%; RyR2-N2386I +/− , 60%; RyR2-L433P +/− , 35.71%) but not in wild-type (WT) mice ( P 2+ leak in atrial myocytes isolated from the CPVT mouse models. Calstabin2 (FKBP12.6) is an RyR2 subunit that stabilizes the closed state of RyR2 and prevents a Ca 2+ leak through the channel. Atrial RyR2 from RyR2-R2474S +/− mice were oxidized, and the RyR2 macromolecular complex was depleted of calstabin2. The Rycal drug S107 stabilizes the closed state of RyR2 by inhibiting the oxidation/phosphorylation induced dissociation of calstabin2 from the channel. S107 reduced the diastolic SR Ca 2+ leak in atrial myocytes and decreased burst pacing–induced AF in vivo. S107 did not reduce the increased prevalence of burst pacing–induced AF in calstabin2-deficient mice, confirming that calstabin2 is required for the mechanism of action of the drug. Conclusions: The present study demonstrates that RyR2-mediated diastolic SR Ca 2+ leak in atrial myocytes is associated with AF in CPVT mice. Moreover, the Rycal S107 inhibited diastolic SR Ca 2+ leak through RyR2 and pacing-induced AF associated with CPVT mutations.

The cardiac ryanodine receptor (RyR2)/Ca2+ release channel on the sarcoplasmic reticulum (SR) is regulated by evolutionarily highly conserved signaling pathways that control excitation-contraction (EC) coupling in the heart. Phosphorylation of RyR2 by cAMP-dependent protein kinase (PKA) plays a key role in regulating the channel in response to stress via activation of the sympathetic nervous system (the “fight-or-flight response”).1 Maladaptive PKA hyperphosphorylation of RyR2 in failing hearts alters channel function, which may cause depletion of SR Ca2+ and diastolic release of SR Ca2+. This can initiate delayed afterdepolarizations that trigger ventricular arrhythmias.1 Mutations in RyR2 recently have been identified in patients with catecholaminergic induced sudden cardiac death (SCD).2–4⇓⇓ There may be a direct link between the PKA hyperphosphorylation of RyR2 that occurs during the progression of heart failure and fatal cardiac arrhythmias. Regulation of cardiac EC coupling by the release of Ca2+ from the SR via RyR2 in cardiomyocytes, known as Ca2+-induced Ca2+ release (CICR), has been appreciated for more than a decade.5,6⇓ Furthermore, it is well known that the amplitude of the Ca2+ transient generated by SR Ca2+ release determines contractile force in cardiomyocytes. The systems that regulate SR Ca2+ release include: (1) the triggers (predominantly Ca2+ influx through the voltage-gated Ca2+ channel on the plasma membrane); (2) the SR Ca2+ release channel or type 2 RyR2; and (3) the SR Ca2+ reuptake pump (SERCA2a) and its regulator phospholamban. These systems (trigger, release, and reuptake) are modulated by signaling pathways, including the β-adrenergic receptor (β-AR) signaling pathway (ie, phosphorylation by PKA). Activation of the sympathetic nervous system in response to stress results in elevation of cAMP levels and activation of PKA. Phosphorylation of RyR2 may not correlate directly …

β-Adrenergic receptor (βAR) signaling, which elevates intracellular cAMP and enhances cardiac contractility, is severely impaired in the failing heart. Protein kinase A (PKA) is activated by cAMP, but the long-term physiological effect of PKA activation on cardiac function is unclear. To investigate the consequences of chronic cardiac PKA activation in the absence of upstream events associated with βAR signaling, we generated transgenic mice that expressed the catalytic subunit of PKA in the heart. These mice developed dilated cardiomyopathy with reduced cardiac contractility, arrhythmias, and susceptibility to sudden death. As seen in human heart failure, these abnormalities correlated with PKA-mediated hyperphosphorylation of the cardiac ryanodine receptor/Ca 2+ -release channel, which enhances Ca 2+ release from the sarcoplasmic reticulum, and phospholamban, which regulates the sarcoplasmic reticulum Ca 2+ -ATPase. These findings demonstrate a specific role for PKA in the pathogenesis of heart failure, independent of more proximal events in βAR signaling, and support the notion that PKA activity is involved in the adverse effects of chronic βAR signaling.

Background —Left ventricular (LV) assist devices (LVADs) can improve contractile strength and normalize characteristics of the Ca 2+ transient in myocytes isolated from failing human hearts. The purpose of the present study was to determine whether LVAD support also improves contractile strength at different frequencies of contraction (the force-frequency relationship [FFR]) of intact myocardium and alters the expression of genes encoding for proteins involved in Ca 2+ handling. Methods and Results —The isometric FFRs of LV trabeculae isolated from 15 patients with end-stage heart failure were compared with those of 7 LVAD-supported patients and demonstrated improved contractile force at 1-Hz stimulation, with reversal of a negative FFR after LVAD implantation. In 20 failing hearts, Northern blot analysis for sarcoplasmic endoreticular Ca 2+ -ATPase subtype 2a (SERCA2a), the ryanodine receptor, and the sarcolemmal Na + -Ca 2+ exchanger was performed on LV tissue obtained before and after LVAD implantation. These paired data demonstrated an upregulation of all 3 genes after LVAD support. In tissue obtained from subsets of these patients, Western blot analysis was performed, and oxalate-supported Ca 2+ uptake by isolated sarcoplasmic reticular membranes was determined. Despite higher mRNA for all genes after LVAD support, only SERCA2a protein was increased. Functional significance of increased SERCA2a was confirmed by augmented Ca 2+ uptake by sarcoplasmic reticular membranes isolated from LVAD-supported hearts. Conclusions —LVAD support can improve contractile strength of intact myocardium and reverse the negative FFR associated with end-stage heart failure. The expression of genes encoding for proteins involved in Ca 2+ cycling is upregulated (reverse molecular remodeling), but only the protein content of SERCA2a is increased.

The cardiac ryanodine receptor (RyR2)/calcium release channel on the sarcoplasmic reticulum is required for muscle excitation-contraction coupling. Using site-directed mutagenesis, we identified the specific Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) phosphorylation site on recombinant RyR2, distinct from the site for protein kinase A (PKA) that mediates the “fight-or-flight” stress response. CaMKII phosphorylation increased RyR2 Ca 2+ sensitivity and open probability. CaMKII was activated at increased heart rates, which may contribute to enhanced Ca 2+ -induced Ca 2+ release. Moreover, rate-dependent CaMKII phosphorylation of RyR2 was defective in heart failure. CaMKII-mediated phosphorylation of RyR2 may contribute to the enhanced contractility observed at higher heart rates. The full text of this article is available online at http://circres.ahajournals.org .