Chronic SR Ca2+-ATPase inhibition causes adaptive changes in cellular Ca2+ transport

Istaroxime Metabolite PST3093 Selectively Stimulates SERCA2a and Reverses Disease-Induced Changes in Cardiac Function

Heart failure (HF) therapeutic toolkit would strongly benefit from the availability of ino-lusitropic agents with a favorable pharmacodynamics and safety profile. Istaroxime is a promising agent, which combines Na+/K+ pump inhibition with sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) stimulation; however, it has a very short half-life and extensive metabolism to a molecule named PST3093. The present work aims to investigate whether PST3093 still retains the pharmacodynamic and pharmacokinetic properties of its parent compound. We studied PST3093 for its effects on SERCA2a and Na+/K+ ATPase activities, Ca2+ dynamics in isolated myocytes, and hemodynamic effects in an in vivo rat model of diabetic [streptozotocin (STZ)-induced] cardiomyopathy. Istaroxime infusion in HF patients led to accumulation of PST3093 in the plasma; clearance was substantially slower for PST3093 than for istaroxime. In cardiac rat preparations, PST3093 did not inhibit the Na+/K+ ATPase activity but retained SERCA2a stimulatory activity. In in vivo echocardiographic assessment, PST3093 improved overall cardiac performance and reversed most STZ-induced abnormalities. PST3093 intravenous toxicity was considerably lower than that of istaroxime, and it failed to significantly interact with 50 off-targets. Overall, PST3093 is a "selective" SERCA2a activator, the prototype of a novel pharmacodynamic category with a potential in the ino-lusitropic approach to HF with prevailing diastolic dysfunction. Its pharmacodynamics are peculiar, and its pharmacokinetics are suitable to prolong the cardiac beneficial effect of istaroxime infusion. SIGNIFICANCE STATEMENT: Heart failure (HF) treatment would benefit from the availability of ino-lusitropic agents with favourable profiles. PST3093 is the main metabolite of istaroxime, a promising agent combining Na+/K+ pump inhibition and sarcoplasmic reticulum Ca2+ ATPase2a (SERCA2a) stimulation. PST3093 shows a longer half-life in human circulation compared to istaroxime, selectively activates SERCA2a, and improves cardiac performance in a model of diabetic cardiomyopathy. Overall, PST3093 as a selective SERCA2a activator can be considered the prototype of a novel pharmacodynamic category for HF treatment.

Impact of RyR2 potentiation on myocardial function

This perspective attempts to shed light on an old and not yet solved controversy in cardiac physiology, i.e., the impact of increasing ryanodine receptor (RyR)2 open probability on myocardial function. Based on an already proven myocyte model, it was shown that increasing RyR2 open probability results in a purely short-lived increase in Ca²⁺ transient amplitude, and, therefore, it does not increase cardiac contractility. However, potentiation of RyR2 activity permanently enhances fractional Ca²⁺ release, shifting the intracellular Ca²⁺ transient versus sarcoplasmic reticulum (SR) Ca²⁺ content curve to a new state of higher efficiency. This would allow the heart to maintain a given contractility despite a decrease in SR Ca²⁺ content, to enhance contractility if SR Ca²⁺ content is simultaneously preserved or to successfully counteract the effects of a negative inotropic intervention.NEW & NOTEWORTHY Increasing ryanodine receptor (RyR)2 open probability does not increase cardiac contractility. However, RyR2 potentiation shifts the intracellular Ca²⁺ transient-sarcoplasmic reticulum (SR) Ca²⁺ content relationship toward an enhanced efficiency state, which may contribute to a positive inotropic effect, preserve contractility despite decreased SR Ca²⁺ content, or successfully counteract the effects of a negative inotropic action.

Phospholamban phosphorylation, mutation, and structural dynamics: a biophysical approach to understanding and treating cardiomyopathy

We review the recent development of novel biochemical and spectroscopic methods to determine the site-specific phosphorylation, expression, mutation, and structural dynamics of phospholamban (PLB), in relation to its function (inhibition of the cardiac calcium pump, SERCA2a), with specific focus on cardiac physiology, pathology, and therapy. In the cardiomyocyte, SERCA2a actively transports Ca²⁺ into the sarcoplasmic reticulum (SR) during relaxation (diastole) to create the concentration gradient that drives the passive efflux of Ca²⁺ required for cardiac contraction (systole). Unphosphorylated PLB (U-PLB) inhibits SERCA2a, but phosphorylation at S16 and/or T17 (producing P-PLB) changes the structure of PLB to relieve SERCA2a inhibition. Because insufficient SERCA2a activity is a hallmark of heart failure, SERCA2a activation, by gene therapy (Andino et al. 2008; Fish et al. 2013; Hoshijima et al. 2002; Jessup et al. 2011) or drug therapy (Ferrandi et al. 2013; Huang 2013; Khan et al. 2009; Rocchetti et al. 2008; Zhang et al. 2012), is a widely sought goal for treatment of heart failure. This review describes rational approaches to this goal. Novel biophysical assays, using site-directed labeling and high-resolution spectroscopy, have been developed to resolve the structural states of SERCA2a-PLB complexes in vitro and in living cells. Novel biochemical assays, using synthetic standards and multidimensional immunofluorescence, have been developed to quantitate PLB expression and phosphorylation states in cells and human tissues. The biochemical and biophysical properties of U-PLB, P-PLB, and mutant PLB will ultimately resolve the mechanisms of loss of inhibition and gain of inhibition to guide therapeutic development. These assays will be powerful tools for investigating human tissue samples from the Sydney Heart Bank, for the purpose of analyzing and diagnosing specific disorders.

Balanced changes in Ca buffering by SERCA and troponin contribute to Ca handling during β-adrenergic stimulation in cardiac myocytes

AIMS

During activation of cardiac myocytes, less than 1% of cytosolic Ca is free; the rest is bound to buffers, largely SERCA, and troponin C. Signalling by phosphorylation, as occurs during β-adrenergic stimulation, changes the Ca-binding affinity of these proteins and may affect the systolic Ca transient. Our aim was to determine the effects of β-adrenergic stimulation on Ca buffering and to differentiate between the roles of SERCA and troponin.

METHODS AND RESULTS

Ca buffering was studied in cardiac myocytes from mice: wild-type (WT), phospholamban-knockout (PLN-KO), and mice expressing slow skeletal troponin I (ssTnI) that is not protein kinase A phosphorylatable. WT cells showed no change in Ca buffering in response to the β-adrenoceptor agonist isoproterenol (ISO). However, ISO decreased Ca buffering in PLN-KO myocytes, presumably unmasking the role of troponin. This effect was confirmed in WT cells in which SERCA activity was blocked with the application of thapsigargin. In contrast, ISO increased Ca buffering in ssTnI cells, presumably revealing the effect of an increase in Ca binding to SERCA.

CONCLUSIONS

These data indicate the individual roles played by SERCA and troponin in Ca buffering during β-adrenergic stimulation and that these two buffers effectively counterbalance each other so that Ca buffering remains constant during β-adrenergic stimulation, a factor which may be physiologically important. This study also emphasizes the importance of taking into account Ca buffering, particularly in disease states where Ca binding to myofilaments or SERCA may be altered.

Role of RyR2 phosphorylation in heart failure and arrhythmias: protein kinase A-mediated hyperphosphorylation of the ryanodine receptor at serine 2808 does not alter cardiac contractility or cause heart failure and arrhythmias

This Controversies in Research article discusses the hypothesis that protein kinase A (PKA)-mediated phosphorylation of the Ryanodine Receptor (RyR) at a single serine (RyRS2808) is essential for normal sympathetic regulation of cardiac myocyte contractility and is responsible for the disturbed Ca(2+) regulation that underlies depressed contractility in heart failure. Studies supporting this hypothesis have associated hyperphosphorylation of RyRS2808 and heart failure progression in animals and humans and have shown that a phosphorylation defective RyR mutant mouse (RyRS2808A) does not respond normally to sympathetic agonists and does not exhibit heart failure symptoms after myocardial infarction. Studies to confirm and extend these ideas have failed to support the original data. Experiments from many different laboratories have convincingly shown that PKA-mediated RyRS2808 phosphorylation does not play any significant role in the normal sympathetic regulation of sarcoplasmic reticulum Ca2+ release or cardiac contractility. Hearts and myocytes from RyRS2808A mice have been shown to respond normally to sympathetic agonists, and to increase Ca(2+) influx, Ca(2+) transients, and Ca(2+) efflux. Although the RyR is involved in heart failure-related Ca(2+) disturbances, this results from Ca(2+)-calmodulin kinase II and reactive oxygen species-mediated regulation rather than by RyR2808 phosphorylation. Also, a new study has shown that RyRS2808A mice are not protected from myocardial infarction. Collectively, there is now a clear consensus in the published literature showing that dysregulated RyRs contribute to the altered Ca(2+) regulatory phenotype of the failing heart, but PKA-mediated phosphorylation of RyRS2808 has little or no role in these alterations.

Calcium leak through ryanodine receptors leads to atrial fibrillation in 3 mouse models of catecholaminergic polymorphic ventricular tachycardia

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 (Ca2+) 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 Ca2+ 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 Ca2+ 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<0.05). Consistent with these in vivo results, there was a significant diastolic SR Ca2+ 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 Ca2+ 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 Ca2+ 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 Ca2+ leak in atrial myocytes is associated with AF in CPVT mice. Moreover, the Rycal S107 inhibited diastolic SR Ca2+ leak through RyR2 and pacing-induced AF associated with CPVT mutations.

The human phospholamban Arg14-deletion mutant localizes to plasma membrane and interacts with the Na/K-ATPase

Depressed Ca-handling in cardiomyocytes is frequently attributed to impaired sarcoplasmic reticulum (SR) function in human and experimental heart failure. Phospholamban (PLN) is a key regulator of SR and cardiac function, and PLN mutations in humans have been associated with dilated cardiomyopathy (DCM). We previously reported the deletion of the highly conserved amino acid residue arginine 14 (nucleic acids 39, 40 and 41) in DCM patients. This basic amino acid is important in maintaining the upstream consensus sequence for PKA phosphorylation of Ser 16 in PLN. To assess the function of this mutant PLN, we introduced the PLN-R14Del in cardiac myocytes of the PLN null mouse. Transgenic lines expressing mutant PLN-R14Del at similar protein levels to wild types exhibited no inhibition of the initial rates of oxalate-facilitated SR Ca uptake compared to PLN-knockouts (PLN-KO). The contractile parameters and Ca-kinetics also remained highly stimulated in PLN-R14Del cardiomyocytes, similar to PLN-KO, and isoproterenol did not further stimulate these hyper-contractile basal parameters. Consistent with the lack of inhibition on SR Ca-transport and contractility, confocal microscopy indicated that the PLN-R14Del failed to co-localize with SERCA2a. Moreover, PLN-R14Del did not co-immunoprecipitate with SERCA2a (as did WT-PLN), but rather co-immunoprecipitated with the sarcolemmal Na/K-ATPase (NKA) and stimulated NKA activity. In addition, studies in HEK cells indicated significant fluorescence resonance energy transfer between PLN-R14Del-YFP and NKAα1-CFP, but not with the NKA regulator phospholemman. Despite the enhanced cardiac function in PLN-R14Del hearts (as in PLN-knockouts), there was cardiac hypertrophy (unlike PLN-KO) coupled with activation of Akt and the MAPK pathways. Thus, human PLN-R14Del is misrouted to the sarcolemma, in the absence of endogenous PLN, and alters NKA activity, leading to cardiac remodeling.

Calcium-calmodulin kinase II mediates digitalis-induced arrhythmias

BACKGROUND

Digitalis-induced Na(+) accumulation results in an increase in Ca(2+)(i) via the Na(+)/Ca(2+) exchanger, leading to enhanced sarcoplasmic reticulum (SR) Ca(2+) load, responsible for the positive inotropic and toxic arrhythmogenic effects of glycosides. A digitalis-induced increase in Ca(2+)(i) could also activate calcium-calmodulin kinase II (CaMKII), which has been shown to have proarrhythmic effects. Here, we investigate whether CaMKII underlies digitalis-induced arrhythmias and the subcellular mechanisms involved.

METHODS AND RESULTS

In paced rat ventricular myocytes (0.5 Hz), 50 μmol/L ouabain increased contraction amplitude by 160 ± 5%. In the absence of electric stimulation, ouabain promoted spontaneous contractile activity and Ca(2+) waves. Ouabain activated CaMKII (p-CaMKII), which phosphorylated its downstream targets, phospholamban (PLN) (Thr17) and ryanodine receptor (RyR) (Ser2814). Ouabain-induced spontaneous activity was prevented by inhibiting CaMKII with 2.5 μmol/L KN93 but not by 2.5 μmol/L of the inactive analog, KN92. Similar results were obtained using the CaMKII inhibitor, autocamtide-2 related inhibitory peptide (AIP) (1 to 2.5 μmol/L), and in myocytes from transgenic mice expressing SR-targeted AIP. Consistently, CaMKII overexpression exacerbated ouabain-induced spontaneous contractile activity. Ouabain was associated with an increase in SR Ca(2+) content and Ca(2+) spark frequency, indicative of enhanced SR Ca(2+) leak. KN93 suppressed the ouabain-induced increase in Ca(2+) spark frequency without affecting SR Ca(2+) content. Similar results were obtained with digoxin. In vivo, ouabain-induced arrhythmias were prevented by KN93 and absent in SR-AIP mice.

CONCLUSIONS

These results show for the first time that CaMKII mediates ouabain-induced arrhythmic/toxic effects. We suggest that CaMKII-dependent phosphorylation of the RyR, resulting in Ca(2+) leak from the SR, is the underlying mechanism involved.

Calcium-calmodulin dependent protein kinase II (CaMKII): a main signal responsible for early reperfusion arrhythmias

To explore whether CaMKII-dependent phosphorylation events mediate reperfusion arrhythmias, Langendorff perfused hearts were submitted to global ischemia/reperfusion. Epicardial monophasic or transmembrane action potentials and contractility were recorded. In rat hearts, reperfusion significantly increased the number of premature beats (PBs) relative to pre-ischemic values. This arrhythmic pattern was associated with a significant increase in CaMKII-dependent phosphorylation of Ser2814 on Ca(2+)-release channels (RyR2) and Thr17 on phospholamban (PLN) at the sarcoplasmic reticulum (SR). These phenomena could be prevented by the CaMKII-inhibitor KN-93. In transgenic mice with targeted inhibition of CaMKII at the SR membranes (SR-AIP), PBs were significantly decreased from 31±6 to 5±1 beats/3min with a virtually complete disappearance of early-afterdepolarizations (EADs). In mice with genetic mutation of the CaMKII phosphorylation site on RyR2 (RyR2-S2814A), PBs decreased by 51.0±14.7%. In contrast, the number of PBs upon reperfusion did not change in transgenic mice with ablation of both PLN phosphorylation sites (PLN-DM). The experiments in SR-AIP mice, in which the CaMKII inhibitor peptide is anchored in the SR membrane but also inhibits CaMKII regulation of L-type Ca(2+) channels, indicated a critical role of CaMKII-dependent phosphorylation of SR proteins and/or L-type Ca(2+) channels in reperfusion arrhythmias. The experiments in RyR2-S2814A further indicate that up to 60% of PBs related to CaMKII are dependent on the phosphorylation of RyR2-Ser2814 site and could be ascribed to delayed-afterdepolarizations (DADs). Moreover, phosphorylation of PLN-Thr17 and L-type Ca(2+) channels might contribute to reperfusion-induced PBs, by increasing SR Ca(2+) content and Ca(2+) influx.

Effects of increased systolic Ca²⁺ and phospholamban phosphorylation during β-adrenergic stimulation on Ca²⁺ transient kinetics in cardiac myocytes

Previous studies demonstrated higher systolic intracellular Ca(2+) concentration ([Ca(2+)](i)) amplitudes result in faster [Ca(2+)](i) decline rates, as does β-adrenergic (β-AR) stimulation. The purpose of this study is to determine the major factor responsible for the faster [Ca(2+)](i) decline rate with β-AR stimulation, the increased systolic Ca(2+) concentration levels, or phosphorylation of phospholamban. Mouse myocytes were perfused under basal conditions [1 mM extracellular Ca(2+) concentration ([Ca(2+)](o))], followed by high extracellular Ca(2+) (3 mM [Ca(2+)](o)), washout with 1 mM [Ca(2+)](o), followed by 1 μM isoproterenol (ISO) with 1 mM [Ca(2+)](o). ISO increased Ser(16) phosphorylation compared with 3 mM [Ca(2+)](o), whereas Thr(17) phosphorylation was similar. Ca(2+) transient (CaT) (fluo 4) data were obtained from matched CaT amplitudes with 3 mM [Ca(2+)](o) and ISO. [Ca(2+)](i) decline was significantly faster with ISO compared with 3 mM [Ca(2+)](o). Interestingly, the faster decline with ISO was only seen during the first 50% of the decline. CaT time to peak was significantly faster with ISO compared with 3 mM [Ca(2+)](o). A Ca(2+)/calmodulin-dependent protein kinase (CAMKII) inhibitor (KN-93) did not affect the CaT decline rates with 3 mM [Ca(2+)](o) or ISO but normalized ISO's time to peak with 3 mM [Ca(2+)](o). Thus, during β-AR stimulation, the major factor for the faster CaT decline is due to Ser(16) phosphorylation, and faster time to peak is due to CAMKII activation.

Small heat shock protein 20 interacts with protein phosphatase-1 and enhances sarcoplasmic reticulum calcium cycling

BACKGROUND

Heat shock proteins (Hsp) are known to enhance cell survival under various stress conditions. In the heart, the small Hsp20 has emerged as a key mediator of protection against apoptosis, remodeling, and ischemia/reperfusion injury. Moreover, Hsp20 has been implicated in modulation of cardiac contractility ex vivo. The objective of this study was to determine the in vivo role of Hsp20 in the heart and the mechanisms underlying its regulatory effects in calcium (Ca) cycling.

METHODS AND RESULTS

Hsp20 overexpression in intact animals resulted in significant enhancement of cardiac function, coupled with augmented Ca cycling and sarcoplasmic reticulum Ca load in isolated cardiomyocytes. This was associated with specific increases in phosphorylation of phospholamban (PLN) at both Ser16 and Thr17, relieving its inhibition of the apparent Ca affinity of SERCA2a. Accordingly, the inotropic effects of Hsp20 were abrogated in cardiomyocytes expressing nonphosphorylatable PLN (S16A/T17A). Interestingly, the activity of type 1 protein phosphatase (PP1), a known regulator of PLN signaling, was significantly reduced by Hsp20 overexpression, suggesting that the Hsp20 stimulatory effects are partially mediated through the PP1-PLN axis. This hypothesis was supported by cell fractionation, coimmunoprecipitation, and coimmunolocalization studies, which revealed an association between Hsp20, PP1, and PLN. Furthermore, recombinant protein studies confirmed a physical interaction between AA 73 to 160 in Hsp20 and AA 163 to 330 in PP1.

CONCLUSIONS

Hsp20 is a novel regulator of sarcoplasmic reticulum Ca cycling by targeting the PP1-PLN axis. These findings, coupled with the well-recognized cardioprotective role of Hsp20, suggest a dual benefit of targeting Hsp20 in heart disease.

Factors controlling the activity of the SERCA2a pump in the normal and failing heart

Heart failure is the leading cause of death in western countries and is often associated with impaired Ca(2+) handling in the cardiomyocyte. In fact, cardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (ER/SR) Ca(2+) pump SERCA2a, pumping Ca(2+) from the cytosol into the lumen of the ER/SR. This review addresses three important facets that control the SERCA2 activity in the heart. First, we focus on the alternative splicing of the SERCA2 messenger, which is strictly regulated in the developing heart. This splicing controls the formation of three SERCA2 splice variants with different enzymatic properties. Second, we will discuss the role and regulation of SERCA2a activity in the normal and failing heart. The two well-studied Ca(2+) affinity modulators phospholamban and sarcolipin control the activity of SERCA2a within a narrow window. An aberrantly high or low Ca(2+) affinity is often observed in and may even trigger cardiac failure. Correcting SERCA2a activity might therefore constitute a therapeutic approach to improve the contractility of the failing heart. Finally, we address the controversies and unanswered questions of other putative regulators of the cardiac Ca(2+) pump, such as sarcalumenin, HRC, S100A1, Bcl-2, HAX-1, calreticulin, calnexin, ERp57, IRS-1, and -2.

Regulation of excitation-contraction coupling in mouse cardiac myocytes: integrative analysis with mathematical modelling

Background

The cardiomyocyte is a prime example of inherently complex biological system with inter- and cross-connected feedback loops in signalling, forming the basic properties of intracellular homeostasis. Functional properties of cells and tissues have been studied e.g. with powerful tools of genetic engineering, combined with extensive experimentation. While this approach provides accurate information about the physiology at the endpoint, complementary methods, such as mathematical modelling, can provide more detailed information about the processes that have lead to the endpoint phenotype.

Results

In order to gain novel mechanistic information of the excitation-contraction coupling in normal myocytes and to analyze sophisticated genetically engineered heart models, we have built a mathematical model of a mouse ventricular myocyte. In addition to the fundamental components of membrane excitation, calcium signalling and contraction, our integrated model includes the calcium-calmodulin-dependent enzyme cascade and the regulation it imposes on the proteins involved in excitation-contraction coupling. With the model, we investigate the effects of three genetic modifications that interfere with calcium signalling: 1) ablation of phospholamban, 2) disruption of the regulation of L-type calcium channels by calcium-calmodulin-dependent kinase II (CaMK) and 3) overexpression of CaMK. We show that the key features of the experimental phenotypes involve physiological compensatory and autoregulatory mechanisms that bring the system to a state closer to the original wild-type phenotype in all transgenic models. A drastic phenotype was found when the genetic modification disrupts the regulatory signalling system itself, i.e. the CaMK overexpression model.

Conclusion

The novel features of the presented cardiomyocyte model enable accurate description of excitation-contraction coupling. The model is thus an applicable tool for further studies of both normal and defective cellular physiology. We propose that integrative modelling as in the present work is a valuable complement to experiments in understanding the causality within complex biological systems such as cardiac myocytes.

EC-coupling in normal and failing hearts

Systolic heart failure may be due to too few cardiomyocytes, or to reduced contractile function of the heart cells. In the latter situation the myocardial function is impaired and this condition is called myocardial failure. The pathophysiological mechanism behind this cellular defect is not known, but Ca2+ handling is altered. Although the most important trigger of sarcoplasmatic reticulum (SR) Ca2+ release, the L-type Ca2+ current, seems to be unaltered, SR Ca2+ load is reduced in human heart failure. This could explain the reduced contractility observed in failing hearts. Three possible mechanisms have been suggested to explain the reduction in SR Ca2+ load. They are leak through the SR Ca2+ release channel (RyR), impaired SR Ca2+ ATPase (SERCA) function and increased Na+/Ca2+-exchanger (NCX) function. Leak through RyR is not consistently found. Increased NCX function is probably secondary to a change in Ca2+ handling, and thus not a primary mechanism, but blockade of the NCX might have therapeutic potential. Reduced SERCA function is probably a primary mechanism for the observed systolic dysfunction, and further insight is to be gained through studies in genetically modified models.

Diminished sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) expression contributes to airway remodelling in bronchial asthma

Phenotypic modulation of airway smooth muscle (ASM) is an important feature of airway remodeling in asthma that is characterized by enhanced proliferation and secretion of pro-inflammatory chemokines. These activities are regulated by the concentration of free Ca(2+) in the cytosol ([Ca(2+)](i)). A rise in [Ca(2+)](i) is normalized by rapid reuptake of Ca(2+) into sarcoplasmic reticulum (SR) stores by the sarco/endoplasmic reticulum Ca(2+) (SERCA) pump. We examined whether increased proliferative and secretory responses of ASM from asthmatics result from reduced SERCA expression. ASM cells were cultured from subjects with and without asthma. SERCA expression was evaluated by western blot, immunohistochemistry and real-time PCR. Changes in [Ca(2+)](i), cell spreading, cellular proliferation, and eotaxin-1 release were measured. Compared with control cells from healthy subjects, SERCA2 mRNA and protein expression was reduced in ASM cells from subjects with moderately severe asthma. SERCA2 expression was similarly reduced in ASM in vivo in subjects with moderate/severe asthma. Rises in [Ca(2+)](i) following cell surface receptor-induced SR activation, or inhibition of SERCA-mediated Ca(2+) re-uptake, were attenuated in ASM cells from asthmatics. Likewise, the return to baseline of [Ca](i) after stimulation by bradykinin was delayed by approximately 50% in ASM cells from asthmatics. siRNA-mediated knockdown of SERCA2 in ASM from healthy subjects increased cell spreading, eotaxin-1 release and proliferation. Our findings implicate a deficiency in SERCA2 in ASM in asthma that contributes to its secretory and hyperproliferative phenotype in asthma, and which may play a key role in mechanisms of airway remodeling.

Temporal and mutation-specific alterations in Ca2+ homeostasis differentially determine the progression of cTnT-related cardiomyopathies in murine models

Naturally occurring mutations in cardiac troponin T (cTnT) result in a clinical subset of familial hypertrophic cardiomyopathy. To determine the mechanistic links between thin-filament mutations and cardiovascular phenotypes, we have generated and characterized several transgenic mouse models carrying cTnT mutations. We address two central questions regarding the previously observed changes in myocellular mechanics and Ca(2+) homeostasis: 1) are they characteristic of all severe cTnT mutations, and 2) are they primary (early) or secondary (late) components of the myocellular response? Adult left ventricular myocytes were isolated from 2- and 6-mo-old transgenic mice carrying missense mutations at residue 92, flanking the TNT1 NH(2)-terminal tail domain. Results from R92L and R92W myocytes showed mutation-specific alterations in contraction and relaxation indexes at 2 mo with improvements by 6 mo. Alterations in Ca(2+) kinetics remained consistent with mechanical data in which R92L and R92W exhibited severe diastolic impairments at the early time point that improved with increasing age. A normal regulation of Ca(2+) kinetics in the context of an altered baseline cTnI phosphorylation suggested a pathogenic mechanism at the myofilament level taking precedence for R92L. The quantitation of Ca(2+)-handling proteins in R92W mice revealed a synergistic compensatory mechanism involving an increased Ser16 and Thr17 phosphorylation of phospholamban, contributing to the temporal onset of improved cellular mechanics and Ca(2+) homeostasis. Therefore, independent cTnT mutations in the TNT1 domain result in primary mutation-specific effects and a differential temporal onset of altered myocellular mechanics, Ca(2+) kinetics, and Ca(2+) homeostasis, complex mechanisms which may contribute to the clinical variability in cTnT-related familial hypertrophic cardiomyopathy mutations.

Moderate heart dysfunction in mice with inducible cardiomyocyte-specific excision of the Serca2 gene

The sarco(endo)plasmic reticulum calcium ATPase 2 (SERCA2) transports Ca(2+) from cytosol into the sarcoplasmic reticulum (SR) of cardiomyocytes, thereby maintaining the store of releasable Ca(2+) necessary for contraction. Reduced SERCA function has been linked to heart failure, and loss of SERCA2 in the adult mammalian heart would be expected to cause immediate severe myocardial contractile dysfunction and death. We investigated heart function in adult mice with an inducible cardiomyocyte-specific excision of the Atp2a2 (Serca2) gene (SERCA2 KO). Seven weeks after induction of Serca2 gene excision, the mice displayed a substantial reduction in diastolic function with a 5-fold increase in the time constant of isovolumetric pressure decay (tau). However, already at 4 weeks following gene excision less than 5% SERCA2 protein was found in myocardial tissue. Surprisingly, heart function was only moderately impaired at this time point. Tissue Doppler imaging showed slightly reduced peak systolic tissue velocity and a less than 2-fold increase in tau was observed. The SR Ca(2+) content was dramatically reduced in cardiomyocytes from 4-week SERCA2 KO mice, and Ca(2+) transients were predominantly generated by enhanced Ca(2+) flux through L-type Ca(2+) channels and the Na(+)-Ca(2+) exchanger. Moreover, equivalent increases in cytosolic [Ca(2+)] in control and SERCA2 KO myocytes induced greater cell shortening in SERCA2 KO, suggesting enhanced myofilament responsiveness. Our data demonstrate that SR-independent Ca(2+) transport mechanisms temporarily can prevent major cardiac dysfunction despite a major reduction of SERCA2 in cardiomyocytes.

Intracellular devastation in heart failure

End-stage heart failure is characterized by a number of abnormalities at the cellular level, which include changes in excitation-contraction coupling, alterations in contractile proteins and activation/deactivation of signaling pathways. Even though many of these changes are adaptive to the high workload and stress in heart failure, a significant number of these alterations are deeply deleterious to the cardiac cell. In this article, we will review the changes in calcium cycling that occur in myopathic hearts and how they can be effectively targeted. We will also focus on protein misfolding in the setting of cardiac dysfunction.

Molecular basis of diastolic dysfunction

Diastolic dysfunction is characterized by prolonged relaxation, increased filling pressure, decreased contraction velocity, and reduced cardiac output. Phenotypical features of diastolic dysfunction can be observed at the level of the isolated myocyte. This article reviews the cellular mechanisms that control relaxation at the level of the myocyte in the healthy situation and discusses the alterations that can affect physiologic function during disease. It focuses specifically on the mechanisms that regulate intracellular calcium handling, and the response of the myofilaments to calcium, including the changes in these components that can contribute to diastolic dysfunction.

The Ca2+ ATPase of cardiac sarcoplasmic reticulum: Physiological role and relevance to diseases

The Ca(2+) ATPase of sarcoplasmic reticulum has a prominent role in excitation/contraction coupling of cardiac muscle, as it induces relaxation by sequestering Ca(2+) from the cytoplasm. The stored Ca(2+) is in turn released to trigger contraction. We review here experiments demonstrating that in cardiac myocytes Ca(2+) signaling and contractile activation are strongly altered by pharmacological inhibition or transcriptional down-regulation of SERCA. On the other hand, kinetics, and intensity of Ca(2+) signaling are improved by SERCA overexpression following delivery of exogenous cDNA by adenovirus vectors. Experiments on adrenergic hypertrophy demonstrate SERCA down-regulation, consistent with its pathogenetic involvement in cardiac hypertrophy and failure, as also shown in other experimental models and clinical studies. Compensation by alternate Ca(2+) signaling proteins, including functional activation and increased expression of Na(+)/Ca(2+) exchanger and TRPC proteins has been observed. These compensatory mechanisms, including calcineurin activation, remain to be clarified and are a most important subject of current studies.

Reduced phospholamban phosphorylation is associated with impaired relaxation in left ventricular myocytes from neuronal NO synthase-deficient mice

Stimulation of nitric oxide (NO) release from the coronary endothelium facilitates myocardial relaxation via a cGMP-dependent reduction in myofilament Ca2+ sensitivity. Recent evidence suggests that NO released by a neuronal NO synthase (nNOS) in the myocardium can also hasten left ventricular relaxation; however, the mechanism underlying these findings is uncertain. Here we show that both relaxation (TR50) and the rate of [Ca2+]i transient decay (tau) are significantly prolonged in field-stimulated or voltage-clamped left ventricular myocytes from nNOS-/- mice and in wild-type myocytes (nNOS+/+) after acute nNOS inhibition. Disabling the sarcoplasmic reticulum abolished the differences in TR50 and tau, suggesting that impaired sarcoplasmic reticulum Ca2+ reuptake may account for the slower relaxation in nNOS-/- mice. In line with these findings, disruption of nNOS (but not of endothelial NOS) decreased phospholamban phosphorylation (P-Ser16 PLN), whereas nNOS inhibition had no effect on TR50 or tau in PLN-/- myocytes. Inhibition of cGMP signaling had no effect on relaxation in either group whereas protein kinase A inhibition abolished the difference in relaxation and PLN phosphorylation by decreasing P-Ser16 PLN and prolonging TR50 in nNOS+/+ myocytes. Conversely, inhibition of type 1 or 2A protein phosphatases shortened TR50 and increased P-Ser16 PLN in nNOS-/- but not in nNOS+/+ myocytes, in agreement with data showing increased protein phosphatase activity in nNOS-/- hearts. Taken together, our findings identify a novel mechanism by which myocardial nNOS promotes left ventricular relaxation by regulating the protein kinase A-mediated phosphorylation of PLN and the rate of sarcoplasmic reticulum Ca2+ reuptake via a cGMP-independent effect on protein phosphatase activity.

A simple technique for isolating healthy heart cells from mouse models

Single heart cells of mouse models provide powerful tools for heart research. However, their isolation is not easy, and it imposes a significant bottleneck on their use in cellular studies of the heart. Aiming to overcome this problem, this report introduces a novel technique that reproducibly isolates healthy heart cells from mouse models. Using simple devices that ensure easy handling and the rapid aortic cannulation of a small mouse heart, cell isolation was done under physiological conditions without using the "KB" medium or 2,3-butanedione monoxime (BDM). The isolated cells consistently had a healthy appearance and a high viability of 75 +/- 5% (mean +/- SD) in Tyrode solution containing 1.8 mM Ca2+. After 8 h of storage at 37 degrees C, they still had a viability of 45 +/- 12%. The cells showed normal contraction properties when field-stimulated, and they generated normal action potentials and membrane currents under the whole-cell clamp condition. The beta-adrenergic signal transduction of the cells was also normal when it was examined with the isoproterenol enhancement of the L-type Ca2+ current.

Superior logistic model for decay of Ca2+ transient and isometric relaxation force curve in rabbit and mouse papillary muscles

A decrease in myocardial intracellular calcium concentration ([Ca(2+)](i)) precedes relaxation, and a monoexponential function is typically used for fitting the decay of the Ca(2+) transient. However, a logistic function has been shown to be a better fit for the relaxation force curve, compared to the conventional monoexponential function. In the present study, we compared the logistic and monoexponential functions for fitting the [Ca(2+)](i) declines, which were measured using the aequorin method, and isometric relaxation force curves at 4 different onsets: the minimum time-derivative of [Ca(2+)](i) (d[Ca(2+)](i)/dt (min)) and force (dF/dt(min)), and the 10%, 20% and 30% lower [Ca(2+)](i) levels and forces over the data-sampling period in 7 isolated rabbit right ventricular and 15 isolated mouse left ventricular papillary muscles. Logistic functions were significantly superior for fitting the [Ca(2+)] (i) declines and relaxation force curves, compared to monoexponential functions. Changes in the normalized logistic [Ca(2+)] (i) decline and relaxation force time constants at the delayed onsets relative to their 100% values at d[Ca(2+)] (i)/dt(min) and dF/dt(min) were significantly smaller than the changes in the normalized monoexponential time constants. The ratio of the logistic relaxation force time constant relative to the logistic [Ca(2+)](i) decline time constant was significantly smaller in mouse than in rabbit. We conclude that the logistic function more reliably characterizes the [Ca(2+)](i) decline and relaxation force curve at any onset, irrespective of animal species. Simultaneous analyses using the logistic model for decay of the Ca(2+) transient and myocardial lusitropism might be a useful strategy for analysis of species-specific myocardial calcium handling.

Na:Ca Stoichiometry and Cytosolic Ca-Dependent Activation of NCX in Intact Cardiomyocytes

We are studying both Na:Ca exchange stoichiometry and cytosolic [Ca] ([Ca]i)-dependent regulation of Na-Ca exchange (NCX) in intact rabbit ventricular myocytes. Analysis of NCX fluxes in subcellular systems strongly supports a dominant 3Na:1Ca exchange, and our measurements in intact cells confirm this. However, in intact native cells, local ion gradients and other factors complicate the process of inferring stoichiometry. From a functional viewpoint, NCX stoichiometry is near 3:1 but is affected by ion accumulation/depletion as well as non-NCX fluxes. We and others have viewed [Ca]i-dependent NCX regulation as a static process (dependent on instantaneous local [Ca]i). However, evidence from subcellular and expression systems shows the process to be dynamic, and our observations confirm this to be the case in intact cardiac cells as well.

The emerging role of neuronal nitric oxide synthase in the regulation of myocardial function

The recent discovery of a NOS1 gene product (i.e. a neuronal-like isoform of nitric oxide synthase or nNOS) in the mammalian left ventricular (LV) myocardium has provided a new key for the interpretation of the complex experimental evidence supporting a role for myocardial constitutive nitric oxide (NO) production in the regulation of basal and beta-badrenergic cardiac function. Importantly, nNOS gene deletion has been associated with more severe LV remodelling and functional deterioration in murine models of myocardial infarction, suggesting that nNOS-derived NO may also be involved in the myocardial response to injury. To date, the mechanisms by which nNOS influences myocardial pathophysiology remain incompletely understood. In particular, it seems over simplistic to assume that all aspects of the myocardial phenotype of nNOS knockout (nNOS(-/-)) mice are a direct consequence of lack of NO production from this source. Emerging data showing co-localisation of xanthine oxidoreductase (XOR) and nNOS in the sarcoplasmic reticulum of rodents, and increased XOR activity in the nNOS(-/-) myocardium, suggest that nNOS gene deletion may have wider implications on the myocardial redox state. Similarly, the mechanisms regulating the targeting of myocardial nNOS to different subcellular compartments and the functional consequences of intracellular nNOS trafficking have not been fully established. Whether this information could be translated into a better understanding and management of human heart failure remains the most important challenge for future investigations.

Computer model of action potential of mouse ventricular myocytes

We have developed a mathematical model of the mouse ventricular myocyte action potential (AP) from voltage-clamp data of the underlying currents and Ca2+ transients. Wherever possible, we used Markov models to represent the molecular structure and function of ion channels. The model includes detailed intracellular Ca2+ dynamics, with simulations of localized events such as sarcoplasmic Ca2+ release into a small intracellular volume bounded by the sarcolemma and sarcoplasmic reticulum. Transporter-mediated Ca2+ fluxes from the bulk cytosol are closely matched to the experimentally reported values and predict stimulation rate-dependent changes in Ca2+ transients. Our model reproduces the properties of cardiac myocytes from two different regions of the heart: the apex and the septum. The septum has a relatively prolonged AP, which reflects a relatively small contribution from the rapid transient outward K+ current in the septum. The attribution of putative molecular bases for several of the component currents enables our mouse model to be used to simulate the behavior of genetically modified transgenic mice.

Modulation of excitation-contraction coupling by isoproterenol in cardiomyocytes with controlled SR Ca2+ load and Ca2+ current trigger

Cardiac Ca(2+) transients are enhanced by cAMP-dependent protein kinase (PKA). However, PKA-dependent modulation of ryanodine receptor (RyR) function in intact cells is difficult to measure, because PKA simultaneously increases Ca(2+) current (I(Ca)), SR Ca(2+) uptake and SR Ca(2+) loading (which independently increase SR Ca(2+) release). We measured I(Ca) and SR Ca(2+) release +/- 1 microm isoproterenol (ISO; isoprenaline) in voltage-clamped ventricular myocytes of rabbits and transgenic mice (expressing only non-phosphorylatable phospholamban). This mouse model helps control for any effect of ISO-enhanced SR uptake on observed release, but the two species produced essentially identical results. SR Ca(2+) load and I(Ca) were adjusted by conditioning. We thus evaluated PKA effects on SR Ca(2+) release at constant SR Ca(2+) load and I(Ca) trigger (with constant unitary I(Ca)). The amount of SR Ca(2+) release increased as a function of either I(Ca) or SR Ca(2+) load, but ISO did not alter the relationships (measured as gain or fractional release). This was true over a wide range of SR Ca(2+) load and I(Ca). However, the maximal rate of SR Ca(2+) release was approximately 50% faster with ISO (at most loads and I(Ca) levels). We conclude that the isolated effect of PKA on SR Ca(2+) release is an increase in maximal rate of release and faster turn-off of release (such that integrated SR Ca(2+) release is unchanged). The increased amount of SR Ca(2+) release normally seen with ISO depends primarily on increased I(Ca) trigger and SR Ca(2+) load, whereas faster release kinetics may be the main result of RyR phosphorylation.