Modulation of sarcoplasmic reticulum function by PST2744 [istaroxime; (E,Z)-3-((2-aminoethoxy)imino) androstane-6,17-dione hydrochloride)] in a pressure-overload heart failure model

Selective SERCA2a activator as a candidate for chronic heart failure therapy

BACKGROUND

The sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA2a) depression substantially contributes to diastolic dysfunction in heart failure (HF), suggesting that SERCA2a stimulation may be a mechanism-based HF therapy. Istaroxime is a drug endowed with both a SERCA2a stimulatory activity and a Na+/K+ pump inhibitory activity for acute HF treatment. Its main metabolite PST3093 shows a more favorable therapeutic profile as compared to the parent drug, but it is still unsuitable for chronic usage. Novel PST3093 derivatives have been recently developed for oral (chronic) HF treatment; compound 8 was selected among them and here characterized.

METHODS

Effects of compound 8 were evaluated in a context of SERCA2a depression, by using streptozotocin-treated rats, a well-known model of diastolic dysfunction. The impact of SERCA2a stimulation by compound 8 was assessed at the cellular level ad in vivo, following i.v. infusion (acute effects) or oral administration (chronic effects).

RESULTS

As expected from SERCA2a stimulation, compound 8 induced SR Ca2+ compartmentalization in STZ myocytes. In-vivo echocardiographic analysis during i.v. infusion and after repeated oral administration of compound 8, detected a significant improvement of diastolic function. Moreover, compound 8 did not affect electrical activity of healthy guinea-pig myocytes, in line with the absence of off-target effects. Finally, compound 8 was well tolerated in mice with no evidence of acute toxicity.

CONCLUSIONS

The pharmacological evaluation of compound 8 indicates that it may be a safe and selective drug for a mechanism-based treatment of chronic HF by restoring SERCA2a activity.

Mechanisms for cardiac calcium pump activation by its substrate and a synthetic allosteric modulator using fluorescence lifetime imaging

The discovery of allosteric modulators is an emerging paradigm in drug discovery, and signal transduction is a subtle and dynamic process that is challenging to characterize. We developed a time-correlated single photon-counting imaging approach to investigate the structural mechanisms for small-molecule activation of the cardiac sarcoplasmic reticulum Ca2+-ATPase, a pharmacologically important pump that transports Ca2+ at the expense of adenosine triphosphate (ATP) hydrolysis. We first tested whether the dissociation of sarcoplasmic reticulum Ca2+-ATPase from its regulatory protein phospholamban is required for small-molecule activation. We found that CDN1163, a validated sarcoplasmic reticulum Ca2+-ATPase activator, does not have significant effects on the stability of the sarcoplasmic reticulum Ca2+-ATPase-phospholamban complex. Time-correlated single photon-counting imaging experiments using the nonhydrolyzable ATP analog β,γ-Methyleneadenosine 5'-triphosphate (AMP-PCP) showed ATP is an allosteric modulator of sarcoplasmic reticulum Ca2+-ATPase, increasing the fraction of catalytically competent structures at physiologically relevant Ca2+ concentrations. Unlike ATP, CDN1163 alone has no significant effects on the Ca2+-dependent shifts in the structural populations of sarcoplasmic reticulum Ca2+-ATPase, and it does not increase the pump's affinity for Ca2+ ions. However, we found that CDN1163 enhances the ATP-mediated modulatory effects to increase the population of catalytically competent sarcoplasmic reticulum Ca2+-ATPase structures. Importantly, this structural shift occurs within the physiological window of Ca2+ concentrations at which sarcoplasmic reticulum Ca2+-ATPase operates. We demonstrated that ATP is both a substrate and modulator of sarcoplasmic reticulum Ca2+-ATPase and showed that CDN1163 and ATP act synergistically to populate sarcoplasmic reticulum Ca2+-ATPase structures that are primed for phosphorylation. This study provides novel insights into the structural mechanisms for sarcoplasmic reticulum Ca2+-ATPase activation by its substrate and a synthetic allosteric modulator.

Fluorescence lifetime FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

We have used FRET-based biosensors in live cells, in a robust high-throughput screening (HTS) platform, to identify small-molecules that alter the structure and activity of the cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA2a). Our primary aim is to discover drug-like small-molecule activators that improve SERCA's function for the treatment of heart failure. We have previously demonstrated the use of an intramolecular FRET biosensor, based on human SERCA2a, by screening two different small validation libraries using novel microplate readers that detect the fluorescence lifetime or emission spectrum with high speed, precision, and resolution. Here we report results from FRET-HTS of 50,000 compounds using the same biosensor, with hit compounds functionally evaluated using assays for Ca2+-ATPase activity and Ca2+-transport. We focused on 18 hit compounds, from which we identified eight structurally unique scaffolds and four scaffold classes as SERCA modulators, approximately half of which are activators and half are inhibitors. Five of these compounds were identified as promising SERCA activators, one of which activates Ca2+-transport even more than Ca2+-ATPase activity thus improving SERCA efficiency. While both activators and inhibitors have therapeutic potential, the activators establish the basis for future testing in heart disease models and lead development, toward pharmaceutical therapy for heart failure.

SERCA2a Agonist Effects on Cardiac Performance During Exercise in Heart Failure With Preserved Ejection Fraction

BACKGROUND

Impaired ventricular relaxation influences left ventricular pressures during exercise in heart failure with preserved ejection fraction (HFpEF). Sarco/endoplasmic reticulum calcium-adenosine triphosphatase (SERCA2a) facilitates myocardial relaxation by increasing calcium reuptake and is impaired in HFpEF.

OBJECTIVES

This study sought to investigate the effects of istaroxime, a SERCA2 agonist, on lusitropic and hemodynamic function during exercise in patients with HFpEF and control subjects.

METHODS

Eleven control subjects (7 male, 4 female) and 15 patients with HFpEF (8 male, 7 female) performed upright cycle exercise with right-sided heart catheterization. Participants received istaroxime (0.5 mg/kg/min) or saline placebo (single-blind, crossover design). Cardiac output, pulmonary capillary wedge pressure (PCWP), and diastolic function were measured at rest and during submaximal exercise. In an exploratory analysis (Hedges' g), 7 patients with HFpEF received higher-dose istaroxime (1.0 mg/kg/min). End-systolic elastance (Ees) was calculated by dividing systolic blood pressure (SBP) × 0.9 by end-systolic volume (ESV; on 3-dimensional echocardiography).

RESULTS

Patients with HFpEF had higher PCWP (25 ± 10 mm Hg vs 12 ± 5 mm Hg; P < 0.001) and lower tissue Doppler velocities during exercise. Istaroxime (0.5 mg/kg/min) had no effect on resting or exercise measures in patients with HFpEF or control subjects. Control subjects had a larger increase in Ees (Δ 1.55 ± 0.99 mm Hg/mL vs D 0.86 ± 1.31 mm Hg/mL; P = 0.03), driven by lower ESV. Comparing placebo and istaroxime 1.0 mg /kg/min during exercise, PCWP during the 1.0 mg /kg/min istaroxime dose was slightly lower (Δ 2.2 mm Hg; Hedges' g = 0.30). There were no effects on diastolic function, but there were increases in SBP and s', suggesting a mild inotropic effect.

CONCLUSIONS

Low-dose istaroxime had no effect on cardiac filling pressure or parameters of relaxation in patients with HFpEF during exercise. Higher doses of istaroxime may have been more effective in reducing exercise PCWP in patients with HFpEF. (Hemodynamic Response to Exercise in HFpEF Patients After Upregulation of SERCA2a; NCT02772068).

Insights from an electro-mechanical heart failure cell model: Role of SERCA enhancement on arrhythmogenesis and myocyte contraction

BACKGROUND AND OBJECTIVE

Structural and electrical remodeling in heart failure predisposes the heart to ventricular arrhythmias. Computer modeling approaches, used to complement experimental results, can provide a more mechanistic knowledge of the biophysical phenomena underlying cardiac pathologies. Indeed, previous in-silico studies have improved the understanding of the electrical correlates of heart failure involved in arrhythmogenesis; however, information on the crosstalk between electrical activity, intracellular Ca²⁺ and contraction is still incomplete. This study aims to investigate the electro-mechanical behavior of virtual failing human ventricular myocytes to help in the development of therapies, which should ideally target pump failure and arrhythmias at the same time.

METHODS

We implemented characteristic remodeling of heart failure with reduced ejection fraction by including reported changes in ionic conductances, sarcomere function and cell structure (e.g. T-tubules disarray). Model parametrization was based on published experimental data and the outcome of simulations was validated against experimentally observed patterns. We focused on two aspects of myocardial dysfunction central in heart failure: altered force-frequency relationship and susceptibility to arrhythmogenic early afterdepolarizations. Because biological variability is a major problem in the generalization of in-silico findings based on a unique set of model parameters, we generated and evaluated a population of models.

RESULTS

The population-based approach is crucial in robust identification of parameters at the core of abnormalities and in generalizing the outcome of their correction. As compared to non-failing ones, failing myocytes had prolonged repolarization, a higher incidence of early afterdepolarizations, reduced contraction and a shallower force-frequency relationship, all features peculiar of heart failure. Component analysis applied to the model population identified reduced SERCA function as a relevant contributor to most of these derangements, which were largely reverted or diminished by restoration of SERCA function alone.

CONCLUSIONS

These simulated results encourage the development of strategies comprising SERCA stimulation and highlight the need to evaluate both electrical and mechanical outcomes.

FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

We have used FRET-based biosensors in live cells, in a robust high-throughput screening (HTS) platform, to identify small-molecules that alter the structure and activity of the cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA2a). Our primary aim is to discover drug-like small-molecule activators that improve SERCA’s function for the treatment of heart failure. We have previously demonstrated the use of an intramolecular FRET biosensor, based on human SERCA2a, by screening a small validation library using novel microplate readers that can detect the fluorescence lifetime or emission spectrum with high speed, precision, and resolution. Here we report results from a 50,000-compound screen using the same biosensor, with hit compounds functionally evaluated using Ca 2+ -ATPase and Ca 2+ -transport assays. We focused on 18 hit compounds, from which we identified eight structurally unique compounds and four compound classes as SERCA modulators, approximately half of which are activators and half are inhibitors. While both activators and inhibitors have therapeutic potential, the activators establish the basis for future testing in heart disease models and lead development, toward pharmaceutical therapy for heart failure.

FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

We have used FRET-based biosensors in live cells, in a robust high-throughput screening (HTS) platform, to identify small-molecules that alter the structure and activity of the cardiac sarco/endoplasmic reticulum calcium ATPase (SERCA2a). Our primary aim is to discover drug-like small-molecule activators that improve SERCA’s function for the treatment of heart failure. We have previously demonstrated the use of an intramolecular FRET biosensor, based on human SERCA2a, by screening a small validation library using novel microplate readers that can detect the fluorescence lifetime or emission spectrum with high speed, precision, and resolution. Here we report results from a 50,000-compound screen using the same biosensor, with hit compounds functionally evaluated using Ca 2+ -ATPase and Ca 2+ -transport assays. We focused on 18 hit compounds, from which we identified eight structurally unique compounds and four compound classes as SERCA modulators, approximately half of which are activators and half are inhibitors. While both activators and inhibitors have therapeutic potential, the activators establish the basis for future testing in heart disease models and lead development, toward pharmaceutical therapy for heart failure.

Efficacy of the New Inotropic Agent Istaroxime in Acute Heart Failure

Current therapeutic strategies for acute heart failure (AHF) are based on traditional inotropic agents that are often associated with untoward effects; therefore, finding new effective approaches with a safer profile is dramatically needed. Istaroxime is a novel compound, chemically unrelated to cardiac glycosides, that is currently being studied for the treatment of AHF. Its effects are essentially related to its inotropic and lusitropic positive properties exerted through a dual mechanism of action: activation of the sarcoplasmic reticulum Ca2+ ATPase isoform 2a (SERCA2a) and inhibition of the Na+/K+-ATPase (NKA) activity. The advantages of istaroxime over the available inotropic agents include its lower arrhythmogenic action combined with its capability of increasing systolic blood pressure without augmenting heart rate. However, it has a limited half-life (1 hour) and is associated with adverse effects including pain at the injection site and gastrointestinal issues. Herein, we describe the main mechanism of action of istaroxime and we present a systematic overview of both clinical and preclinical trials testing this drug, underlining the latest insights regarding its adoption in clinical practice for AHF.

SERCA2a stimulation by istaroxime improves intracellular Ca2+ handling and diastolic dysfunction in a model of diabetic cardiomyopathy

Aims 

Diabetic cardiomyopathy is a multifactorial disease characterized by an early onset of diastolic dysfunction (DD) that precedes the development of systolic impairment. Mechanisms that can restore cardiac relaxation improving intracellular Ca²⁺ dynamics represent a promising therapeutic approach for cardiovascular diseases associated to DD. Istaroxime has the dual properties to accelerate Ca²⁺ uptake into sarcoplasmic reticulum (SR) through the SR Ca²⁺ pump (SERCA2a) stimulation and to inhibit Na⁺/K⁺ ATPase (NKA). This project aims to characterize istaroxime effects at a concentration (100 nmol/L) marginally affecting NKA, in order to highlight its effects dependent on the stimulation of SERCA2a in an animal model of mild diabetes.

Methods and results 

Streptozotocin (STZ) treated diabetic rats were studied at 9  weeks after STZ injection in comparison to controls (CTR). Istaroxime effects were evaluated in vivo and in left ventricular (LV) preparations. STZ animals showed (i) marked DD not associated to cardiac fibrosis, (ii) LV mass reduction associated to reduced LV cell dimension and T-tubules loss, (iii) reduced LV SERCA2 protein level and activity and (iv) slower SR Ca²⁺ uptake rate, (v) LV action potential (AP) prolongation and increased short-term variability (STV) of AP duration, (vi) increased diastolic Ca²⁺, and (vii) unaltered SR Ca²⁺ content and stability in intact cells. Acute istaroxime infusion (0.11 mg/kg/min for 15 min) reduced DD in STZ rats. Accordingly, in STZ myocytes istaroxime (100 nmol/L) stimulated SERCA2a activity and blunted STZ-induced abnormalities in LV Ca²⁺ dynamics. In CTR myocytes, istaroxime increased diastolic Ca²⁺ level due to NKA blockade albeit minimal, while its effects on SERCA2a were almost absent.

Conclusions 

SERCA2a stimulation by istaroxime improved STZ-induced DD and intracellular Ca²⁺ handling anomalies. Thus, SERCA2a stimulation can be considered a promising therapeutic approach for DD treatment.

Mechanisms underlying pathological Ca2+ handling in diseases of the heart

Cardiomyocyte contraction relies on precisely regulated intracellular Ca²⁺ signaling through various Ca²⁺ channels and transporters. In this article, we will review the physiological regulation of Ca²⁺ handling and its role in maintaining normal cardiac rhythm and contractility. We discuss how inherited variants or acquired defects in Ca²⁺ channel subunits contribute to the development or progression of diseases of the heart. Moreover, we highlight recent insights into the role of protein phosphatase subunits and striated muscle preferentially expressed protein kinase (SPEG) in atrial fibrillation, heart failure, and cardiomyopathies. Finally, this review summarizes current drug therapies and new advances in genome editing as therapeutic strategies for the cardiac diseases caused by aberrant intracellular Ca²⁺ signaling.

The role of β-adrenergic system remodeling in human heart failure: A mechanistic investigation

β-adrenergic receptor antagonists (β-blockers) are extensively used to improve cardiac performance in heart failure (HF), but the electrical improvements with these clinical treatments are not fully understood. The aim of this study was to analyze the electrophysiological effects of β-adrenergic system remodeling in heart failure with reduced ejection fraction and the underlying mechanisms. We used a combined mathematical model that integrated β-adrenergic signaling with electrophysiology and calcium cycling in human ventricular myocytes. HF remodeling, both in the electrophysiological and signaling systems, was introduced to quantitatively analyze changes in electrophysiological properties due to the stimulation of β-adrenergic receptors in failing myocytes. We found that the inotropic effect of β-adrenergic stimulation was reduced in HF due to the altered Ca²⁺ dynamics resulting from the combination of structural, electrophysiological and signaling remodeling. Isolated cells showed proarrhythmic risk after sympathetic stimulation because early afterdepolarizations appeared, and the vulnerability was greater in failing myocytes. When analyzing coupled cells, β-adrenergic stimulation reduced transmural repolarization gradients between endocardium and epicardium in normal tissue, but was less effective at reducing these gradients after HF remodeling. The comparison of the selective activation of β-adrenergic isoforms revealed that the response to β₂-adrenergic receptors stimulation was blunted in HF while β₁-adrenergic receptors downstream effectors regulated most of the changes observed after sympathetic stimulation. In conclusion, this study was able to reproduce an altered β-adrenergic activity on failing myocytes and to explain the mechanisms involved. The derived predictions could help in the treatment of HF and guide in the design of future experiments.

Why has positive inotropy failed in chronic heart failure? Lessons from prior inotrope trials

Current pharmacological therapies for heart failure with reduced ejection fraction are largely either repurposed anti‐hypertensives that blunt overactivation of the neurohormonal system or diuretics that decrease congestion. However, they do not address the symptoms of heart failure that result from reductions in cardiac output and reserve. Over the last few decades, numerous attempts have been made to develop and test positive cardiac inotropes that improve cardiac haemodynamics. However, definitive clinical trials have failed to show a survival benefit. As a result, no positive inotrope is currently approved for long‐term use in heart failure. The focus of this state‐of‐the‐art review is to revisit prior clinical trials and to understand the causes for their findings. Using the learnings from those experiences, we propose a framework for future trials of such agents that maximizes their potential for success. This includes enriching the trials with patients who are most likely to derive benefit, using biomarkers and imaging in trial design and execution, evaluating efficacy based on a wider range of intermediate phenotypes, and collecting detailed data on functional status and quality of life. With a rapidly growing population of patients with advanced heart failure, the epidemiologic insignificance of heart transplantation as a therapeutic intervention, and both the cost and morbidity associated with ventricular assist devices, there is an enormous potential for positive inotropic therapies to impact the outcomes that matter most to patients.

Ca2+ Cycling Impairment in Heart Failure Is Exacerbated by Fibrosis: Insights Gained From Mechanistic Simulations

Heart failure (HF) is characterized by altered Ca²⁺ cycling, resulting in cardiac contractile dysfunction. Failing myocytes undergo electrophysiological remodeling, which is known to be the main cause of abnormal Ca²⁺ homeostasis. However, structural remodeling, specifically proliferating fibroblasts coupled to myocytes in the failing heart, could also contribute to Ca²⁺ cycling impairment. The goal of the present study was to systematically analyze the mechanisms by which myocyte–fibroblast coupling could affect Ca²⁺ dynamics in normal conditions and in HF. Simulations of healthy and failing human myocytes were performed using established mathematical models, and cells were either isolated or coupled to fibroblasts. Univariate and multivariate sensitivity analyses were performed to quantify effects of ion transport pathways on biomarkers computed from intracellular [Ca²⁺] waveforms. Variability in ion channels and pumps was imposed and populations of models were analyzed to determine effects on Ca²⁺ dynamics. Our results suggest that both univariate and multivariate sensitivity analyses are valuable methodologies to shed light into the ionic mechanisms underlying Ca²⁺ impairment in HF, although differences between the two methodologies are observed at high parameter variability. These can result from either the fact that multivariate analyses take into account ion channels or non-linear effects of ion transport pathways on Ca²⁺ dynamics. Coupling either healthy or failing myocytes to fibroblasts decreased Ca²⁺ transients due to an indirect sink effect on action potential (AP) and thus on Ca²⁺ related currents. Simulations that investigated restoration of normal physiology in failing myocytes showed that Ca²⁺ cycling can be normalized by increasing SERCA and L-type Ca²⁺ current activity while decreasing Na⁺–Ca²⁺ exchange and SR Ca²⁺ leak. Changes required to normalize APs in failing myocytes depended on whether myocytes were coupled to fibroblasts. In conclusion, univariate and multivariate sensitivity analyses are helpful tools to understand how Ca²⁺ cycling is impaired in HF and how this can be exacerbated by coupling of myocytes to fibroblasts. The design of pharmacological actions to restore normal activity should take into account the degree of fibrosis in the failing heart.

Istaroxime, a positive inotropic agent devoid of proarrhythmic properties in sensitive chronic atrioventricular block dogs

Current inotropic agents in heart failure therapy associate with low benefit and significant adverse effects, including ventricular arrhythmias. Istaroxime, a novel Na⁺/K⁺-transporting ATPase inhibitor, also stimulates SERCA2a activity, which would confer improved inotropic and lusitropic properties with less proarrhythmic effects. We investigated hemodynamic, electrophysiological and potential proarrhythmic and antiarrhythmic effects of istaroxime in control and chronic atrioventricular block (CAVB) dogs sensitive to drug-induced Torsades de Pointes arrhythmias (TdP). In isolated normal canine ventricular cardiomyocytes, istaroxime (0.3-10 μM) evoked no afterdepolarizations and significantly shortened action potential duration (APD) at 3 and 10 μM. Istaroxime at 3 μg/kg/min significantly increased left ventricular (LV) contractility (dP/dt+) and relaxation (dP/dt-) respectively by 81 and 94% in anesthetized control dogs (n = 6) and by 61 and 49% in anesthetized CAVB dogs (n = 7) sensitive to dofetilide-induced TdP. While istaroxime induced no ventricular arrhythmias in control conditions, only single ectopic beats occurred in 2/7 CAVB dogs, which were preceded by increase of short-term variability of repolarization (STV) and T wave alternans in LV unipolar electrograms. Istaroxime pre-treatment (3 μg/kg/min for 60 min) did not alleviate dofetilide-induced increase in repolarization and STV, and mildly reduced incidence of TdP from 6/6 to 4/6 CAVB dogs. In six CAVB dogs with dofetilide-induced TdP, administration of istaroxime (90 μg/kg/5 min) suppressed arrhythmic episodes in two animals. Taken together, inotropic and lusitropic properties of istaroxime in CAVB dogs were devoid of significant proarrhythmic effects in sensitive CAVB dogs, and istaroxime provides a moderate antiarrhythmic efficacy in prevention and suppression of dofetilide-induced TdP.

Drug Interactions With the Ca2+-ATPase From Sarco(Endo)Plasmic Reticulum (SERCA)

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) is an intracellular membrane transporter that utilizes the free energy provided by ATP hydrolysis for active transport of Ca²⁺ ions from the cytoplasm to the lumen of sarco(endo)plasmic reticulum. SERCA plays a fundamental role for cell calcium homeostasis and signaling in muscle cells and also in cells of other tissues. Because of its prominent role in many physiological processes, SERCA dysfunction is associated to diseases displaying various degrees of severity. SERCA transport activity can be inhibited by a variety of compounds with different chemical structures. Specific SERCA inhibitors were identified which have been instrumental in studies of the SERCA catalytic and transport mechanism. It has been proposed that SERCA inhibition may represent a novel therapeutic strategy to cure certain diseases by targeting SERCA activity in pathogens, parasites and cancer cells. Recently, novel small molecules have been developed that are able to stimulate SERCA activity. Such SERCA activators may also offer an innovative and promising therapeutic approach to treat diseases, such as heart failure, diabetes and metabolic disorders. In the present review the effects of pharmacologically relevant compounds on SERCA transport activity are presented. In particular, we will discuss the interaction of SERCA with specific inhibitors and activators that are potential therapeutic agents for different diseases.

Sensitivity analysis revealing the effect of modulating ionic mechanisms on calcium dynamics in simulated human heart failure

Abnormal intracellular Ca²⁺ handling is the major contributor to the depressed cardiac contractility observed in heart failure. The electrophysiological remodeling associated with this pathology alters both the action potential and the Ca²⁺ dynamics, leading to a defective excitation-contraction coupling that ends in mechanical dysfunction. The importance of maintaining a correct intracellular Ca²⁺ concentration requires a better understanding of its regulation by ionic mechanisms. To study the electrical activity and ionic homeostasis of failing myocytes, a modified version of the O’Hara et al. human action potential model was used, including electrophysiological remodeling. The impact of the main ionic transport mechanisms was analyzed using single-parameter sensitivity analyses, the first of which explored the modulation of electrophysiological characteristics related to Ca²⁺ exerted by the remodeled parameters. The second sensitivity analysis compared the potential consequences of modulating individual channel conductivities, as one of the main effects of potential drugs, on Ca²⁺ dynamic properties under both normal conditions and in heart failure. The first analysis revealed the important contribution of the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) dysfunction to the altered Ca²⁺ homeostasis, with the Na⁺/Ca²⁺ exchanger (NCX) and other Ca²⁺ cycling proteins also playing a significant role. Our results highlight the importance of improving the SR uptake function to increase Ca²⁺ content and restore Ca²⁺ homeostasis and contractility. The second sensitivity analysis highlights the different response of the failing myocyte versus the healthy myocyte to potential pharmacological actions on single channels. The result of modifying the conductances of the remodeled proteins such as SERCA and NCX in heart failure has less impact on Ca²⁺ modulation. These differences should be taken into account when designing drug therapies.

Elucidating arrhythmogenic mechanisms of long-QT syndrome CALM1-F142L mutation in patient-specific induced pluripotent stem cell-derived cardiomyocytes

Aims

Calmodulin (CaM) is a small protein, encoded by three genes (CALM1-3), exerting multiple Ca2+-dependent modulatory roles. A mutation (F142L) affecting only one of the six CALM alleles is associated with long QT syndrome (LQTS) characterized by recurrent cardiac arrests. This phenotypic severity is unexpected from the predicted allelic balance. In this work, the effects of heterozygous CALM1-F142L have been investigated in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) obtained from a LQTS patient carrying the F142L mutation, i.e. in the context of native allelic ratio and potential gene modifiers.

Methods and Results

Skin fibroblasts of the mutation carrier and two unrelated healthy subjects (controls) were reprogrammed to hiPSC and differentiated into hiPSC-CMs. Scanty IK1 expression, an hiPSC-CMs feature potentially biasing repolarization, was corrected by addition of simulated IK1 (Dynamic-Clamp). Abnormalities in repolarization rate-dependency (in single cells and cell aggregates), membrane currents and intracellular Ca2+ dynamics were evaluated as putative arrhythmogenic factors. CALM1-F142L prolonged repolarization, altered its rate-dependency and its response to isoproterenol. This was associated with severe impairment of Ca2+-dependent inactivation (CDI) of ICaL, resulting in augmented inward current during the plateau phase. As a result, the repolarization of mutant cells failed to adapt to high pacing rates, a finding well reproduced by using a recent hiPSC-CM action potential model. The mutation failed to affect IKs and INaL and changed If only marginally. Intracellular Ca2+ dynamics and Ca2+ store stability were not significantly modified. Mutation-induced repolarization abnormalities were reversed by verapamil.

Conclusion

The main functional derangement in CALM1-F142L was prolonged repolarization with altered rate-dependency and sensitivity to β-adrenergic stimulation. Impaired CDI of ICaL underlined the electrical abnormality, which was sensitive to ICaL blockade. High mutation penetrance was confirmed in the presence of the native genotype, implying strong dominance of effects.

Heart failure: novel therapeutic approaches

Heart failure (HF) is a complex clinical syndrome that can result from any structural or functional cardiac disorders that impairs the ability of the ventricle to fill with or eject blood. Despite effective medical interventions, mortality and morbidity remain substantial. There have been significant advances in the therapy of HF in recent decades, such as the introduction of beta-blockers and antagonists of the renin-angiotensin system but still there is a major unmet need for better therapies for HF. In the present era, pathophysiology of HF has been explored. Various novel pathways, molecular sites have been identified, which contribute to the progression of the disease. By targeting these sites, newer pharmacological agents have been developed, which can play a promising role in the treatment of HF. This article focuses on recent advancements in pharmacotherapy of HF, which include agents targeting myocardial contractility, cytokines and inflammation, fibrosis and remodeling, myocardial metabolism, oxidative stress, and other newly defined pathways.

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.

Istaroxime stimulates SERCA2a and accelerates calcium cycling in heart failure by relieving phospholamban inhibition

BACKGROUND AND PURPOSE

Calcium handling is known to be deranged in heart failure. Interventions aimed at improving cell Ca(2) (+) cycling may represent a promising approach to heart failure therapy. Istaroxime is a new luso-inotropic compound that stimulates cardiac contractility and relaxation in healthy and failing animal models and in patients with acute heart failure (AHF) syndrome. Istaroxime is a Na-K ATPase inhibitor with the unique property of increasing sarcoplasmic reticulum (SR) SERCA2a activity as shown in heart microsomes from humans and guinea pigs. The present study addressed the molecular mechanism by which istaroxime increases SERCA2a activity.

EXPERIMENTAL APPROACH

To study the effect of istaroxime on SERCA2a-phospholamban (PLB) complex, we applied different methodologies in native dog healthy and failing heart preparations and heterologous canine SERCA2a/PLB co-expressed in Spodoptera frugiperda (Sf21) insect cells.

KEY RESULTS

We showed that istaroxime enhances SERCA2a activity, Ca(2) (+) uptake and the Ca(2) (+) -dependent charge movements into dog healthy and failing cardiac SR vesicles. Although not directly demonstrated, the most probable explanation of these activities is the displacement of PLB from SERCA2a.E2 conformation, independently from cAMP/PKA. We propose that this displacement may favour the SERCA2a conformational transition from E2 to E1, thus resulting in the acceleration of Ca(2) (+) cycling.

CONCLUSIONS AND IMPLICATIONS

Istaroxime represents the first example of a small molecule that exerts a luso-inotropic effect in the failing human heart through the stimulation of SERCA2a ATPase activity and the enhancement of Ca(2) (+) uptake into the SR by relieving the PLB inhibitory effect on SERCA2a in a cAMP/PKA independent way.

Chronic istaroxime improves cardiac function and heart rate variability in cardiomyopathic hamsters

PURPOSE

Istaroxime is a new luso-inotropic compound. It exerts inotropic action by reducing Na+/K+-ATPase activity, and simultaneously it stimulates sarcoplasmic reticulum Ca(2+)-ATPase function, thus also inducing lusitropic action. The aim of present study is to assess the effect of chronic istaroxime treatment on cardiac function and heart rate variability in Bio TO.2 Syrian hamster model of progressive heart failure.

METHODS

Bio TO.2 hamsters were daily treated, from 12 to 28 weeks of age, with 30 mg/kg/day oral istaroxime. Age-matched Bio TO.2 and Bio F1B hamsters were treated with vehicle and used as diseased and healthy controls. At the end of treatment, hearts function and autonomic cardiac control were evaluated.

RESULTS

Hearts from vehicle-treated Bio TO.2 when compared with hearts from Bio F1B showed higher heart/body weight ratio, and lower left ventricular systolic pressure (LVSP), positive and negative derivative of LV pressure (dP/dT), coronary flow rate (CFR). Hearts from istaroxime-treated when compared with those of vehicle-treated hamsters, showed the reduction of heart/body weight ratio, and the increase of LVSP, of both positive and negative dP/dT, and of CFR. Autonomic cardiac control, evaluated by HRV analysis, indicated in vehicle-treated Bio TO.2 hamsters, when compared to healthy, a shift towards increased sympathetic and decreased parasympathetic activities. Istaroxime-treatment preserved parasympathetic activity.

CONCLUSIONS

Chronic istaroxime improves cardiac function and heart rate variability in Bio TO.2 Syrian hamster model of progressive heart failure.

Role and mechanism of subcellular Ca2+ distribution in the action of two inotropic agents with different toxicity

Pro-arrhythmic risk strongly limits the therapeutic value of current inotropic interventions. Istaroxime (previously PST2744) is a novel inotropic agent, significantly less pro-arrhythmic than digoxin that, in addition to block Na(+)/K(+) pump, stimulates sarcoplasmic reticulum (SR) Ca(2+) ATPase (SERCA2). Here we compare istaroxime and digoxin effects to further address the role of SR modulation in reducing the toxicity associated with Na(+)/K(+) pump blockade. In murine ventricular myocytes both compounds increased cell twitch (inotropy) in a concentration-dependent fashion. At high concentrations digoxin, but not istaroxime, induced unstimulated contractions, a sign of pro-arrhythmic toxicity. To evaluate the mechanism of this difference, we compared the two drugs at concentrations exerting equal inotropy but different toxicity. At these concentrations: (1) the two drugs equally inhibited the Na(+)/K(+) pump; (2) digoxin induced larger increases in resting Ca(2+) and in diastolic Ca(2+) during pacing; (3) neither drug affected the relationship between RyR-mediated SR Ca(2+) leak and Ca(2+) content; (4) istaroxime, but not digoxin, enhanced SR Ca(2+) reuptake rate. In conclusion, digoxin toxicity was associated to larger accumulation of cytosolic Ca(2+), which did not result from RyR facilitation, but which might ultimately induce it to promote unstimulated Ca(2+) release. The lower toxicity of Na(+)/K(+) pump blockade by istaroxime may thus reflect improved Ca(2+) confinement within the SR, likely to result from concomitant SERCA2 stimulation.

Sarcoplasmic reticulum Ca(2+) ATPase as a therapeutic target for heart failure

The cardiac isoform of the sarco/endoplasmic reticulum Ca(2+)ATPase (SERCA2a) plays a major role in controlling excitation/contraction coupling. In both experimental and clinical heart failure, SERCA2a expression is significantly reduced which leads to abnormal Ca(2+) handling and deficient contractility. A large number of studies in isolated cardiac myocytes and in small and large animal models of heart failure showed that restoring SERCA2a expression by gene transfer corrects the contractile abnormalities and improves energetics and electrical remodeling. Following a long line of investigation, a clinical trial is underway to restore SERCA2a expression in patients with heart failure using adeno-associated virus type 1. This review addresses the following issues regarding heart failure gene therapy: i) new insights on calcium regulation by SERCA2a; ii) SERCA2a as a gene therapy target in animal models of heart failure; iii) advances in the development of viral vectors and gene delivery; and iv) clinical trials on heart failure using SERCA2a. This review focuses on the new advances in SERCA2a- targeted gene therapy made in the last three years. In conclusion, SERCA2a is an important therapeutic target in various cardiovascular disorders. Ongoing clinical gene therapy trials will provide answers on its safety and applicability.