Regulation of Ca2+ and Na+ in normal and failing cardiac myocytes

Piscidin-1 regulates lipopolysaccharide-induced intracellular calcium, sodium dysregulation, and oxidative stress in atrial cardiomyocytes

Lipopolysaccharide (LPS) triggers an inflammatory response, causing impairment of cardiomyocyte Ca2+ and Na + regulation. This study aimed to determine whether piscidin-1 (PCD-1), an antimicrobial peptide, improves intracellular Ca2+ and Na + regulation in LPS-challenged atrial cardiomyocytes. Rabbit atrial cardiomyocytes were enzymatically isolated from the left atria. Patch-clamp ionic current recording, intracellular Ca2+ monitoring using Fluo-3, and detection of cytosolic reactive oxygen species production were conducted in control, LPS-challenged, and LPS + PCD-1-treated atrial cardiomyocytes. LPS-challenged cardiomyocytes showed shortened durations of action potential at their 50% and 90% repolarizations, which was reversed by PCD-1 treatment. LPS-challenged cardiomyocytes showed decreased L-type Ca2+ channel currents and larger Na+/Ca2+ exchange currents compared to controls. While LPS did not affect the sodium current, an enhanced late sodium current with increased cytosolic Na+ levels was observed in LPS-challenged cardiomyocytes. These LPS-induced alterations in the ionic current were ameliorated by PCD-1 treatment. LPS-challenged cardiomyocytes displayed lowered Ca2+ transient amplitudes and decreased Ca2+ stores and greater Ca2+ leakage in the sarcoplasmic reticulum compared to the control. Exposure to PCD-1 attenuated LPS-induced alterations in Ca2+ regulation. The elevated reactive oxygen species levels observed in LPS-challenged myocytes were suppressed after PCD-1 treatment. The protein levels of NF-κB and IL-6 increased following LPS treatment. Decreased sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2a protein levels were observed in LPS-challenged cardiomyocytes. PCD-1 modulates LPS-induced alterations in inflammatory and Ca2+ regulatory protein levels. Our results suggest that PCD-1 modulates LPS-induced alterations in intracellular Ca2+ and Na + homeostasis, reactive oxygen species production, and the NF-κB inflammatory pathway in atrial cardiomyocytes.

Ion channel trafficking implications in heart failure

Heart failure (HF) is recognized as an epidemic in the contemporary world, impacting around 1%-2% of the adult population and affecting around 6 million Americans. HF remains a major cause of mortality, morbidity, and poor quality of life. Several therapies are used to treat HF and improve the survival of patients; however, despite these substantial improvements in treating HF, the incidence of HF is increasing rapidly, posing a significant burden to human health. The total cost of care for HF is USD 69.8 billion in 2023, warranting a better understanding of the mechanisms involved in HF. Among the most serious manifestations associated with HF is arrhythmia due to the electrophysiological changes within the cardiomyocyte. Among these electrophysiological changes, disruptions in sodium and potassium currents' function and trafficking, as well as calcium handling, all of which impact arrhythmia in HF. The mechanisms responsible for the trafficking, anchoring, organization, and recycling of ion channels at the plasma membrane seem to be significant contributors to ion channels dysfunction in HF. Variants, microtubule alterations, or disturbances of anchoring proteins lead to ion channel trafficking defects and the alteration of the cardiomyocyte's electrophysiology. Understanding the mechanisms of ion channels trafficking could provide new therapeutic approaches for the treatment of HF. This review provides an overview of the recent advances in ion channel trafficking in HF.

A dual-targeted drug inhibits cardiac ryanodine receptor Ca2+ leak but activates SERCA2a Ca2+ uptake

In the heart, genetic or acquired mishandling of diastolic [Ca2+] by ryanodine receptor type 2 (RyR2) overactivity correlates with risks of arrhythmia and sudden cardiac death. Strategies to avoid these risks include decrease of Ca2+ release by drugs modulating RyR2 activity or increase in Ca2+ uptake by drugs modulating SR Ca2+ ATPase (SERCA2a) activity. Here, we combine these strategies by developing experimental compounds that act simultaneously on both processes. Our screening efforts identified the new 1,4-benzothiazepine derivative GM1869 as a promising compound. Consequently, we comparatively studied the effects of the known RyR2 modulators Dantrolene and S36 together with GM1869 on RyR2 and SERCA2a activity in cardiomyocytes from wild type and arrhythmia-susceptible RyR2R2474S/+ mice by confocal live-cell imaging. All drugs reduced RyR2-mediated Ca2+ spark frequency but only GM1869 accelerated SERCA2a-mediated decay of Ca2+ transients in murine and human cardiomyocytes. Our data indicate that S36 and GM1869 are more suitable than dantrolene to directly modulate RyR2 activity, especially in RyR2R2474S/+ mice. Remarkably, GM1869 may represent a new dual-acting lead compound for maintenance of diastolic [Ca2+].

Sodium channel subpopulations with distinct biophysical properties and subcellular localization enhance cardiac conduction

Sodium (Na+) current is responsible for the rapid depolarization of cardiac myocytes that triggers the cardiac action potential upstroke. Recent studies have illustrated the presence of multiple pools of Na+ channels with distinct biophysical properties and subcellular localization, including clustering of channels at the intercalated disk and along the lateral membrane. Computational studies predict that Na+ channel clusters at the intercalated disk can regulate cardiac conduction via modulation of the narrow intercellular cleft between electrically coupled myocytes. However, these studies have primarily focused on the redistribution of Na+ channels between intercalated disk and lateral membranes and have not considered the distinct biophysical properties of the Na+ channel subpopulations. In this study, we use computational modeling to simulate computational models of single cardiac cells and one-dimensional cardiac tissues and predict the function of distinct Na+ channel subpopulations. Single-cell simulations predict that a subpopulation of Na+ channels with shifted steady-state activation and inactivation voltage dependency promotes an earlier action potential upstroke. In cardiac tissues that account for distinct subcellular spatial localization, simulations predict that shifted Na+ channels contribute to faster and more robust conduction in response to changes in tissue structure (i.e., cleft width), gap junctional coupling, and rapid pacing rates. Simulations predict that the intercalated disk-localized shifted Na+ channels contribute proportionally more to total Na+ charge than lateral membrane-localized Na+ channels. Importantly, our work supports the hypothesis that Na+ channel redistribution may be a critical mechanism by which cells can respond to perturbations to support fast and robust conduction.

N-terminal truncated cardiac troponin I enhances Frank-Starling response by increasing myofilament sensitivity to resting tension

Cardiac troponin I (cTnI) of higher vertebrates has evolved with an N-terminal extension, of which deletion via restrictive proteolysis occurs as a compensatory adaptation in chronic heart failure to increase ventricular relaxation and stroke volume. Here, we demonstrate in a transgenic mouse model expressing solely N-terminal truncated cTnI (cTnI-ND) in the heart with deletion of the endogenous cTnI gene. Functional studies using ex vivo working hearts showed an extended Frank-Starling response to preload with reduced left ventricular end diastolic pressure. The enhanced Frank-Starling response effectively increases systolic ventricular pressure development and stroke volume. A novel finding is that cTnI-ND increases left ventricular relaxation velocity and stroke volume without increasing the end diastolic volume. Consistently, the optimal resting sarcomere length (SL) for maximum force development in cTnI-ND cardiac muscle was not different from wild-type (WT) control. Despite the removal of the protein kinase A (PKA) phosphorylation sites in cTnI, β-adrenergic stimulation remains effective on augmenting the enhanced Frank-Starling response of cTnI-ND hearts. Force-pCa relationship studies using skinned preparations found that while cTnI-ND cardiac muscle shows a resting SL-resting tension relationship similar to WT control, cTnI-ND significantly increases myofibril Ca2+ sensitivity to resting tension. The results demonstrate that restrictive N-terminal deletion of cTnI enhances Frank-Starling response by increasing myofilament sensitivity to resting tension rather than directly depending on SL. This novel function of cTnI regulation suggests a myofilament approach to utilizing Frank-Starling mechanism for the treatment of heart failure, especially diastolic failure where ventricular filling is limited.

Transient Receptor Potential Channels, Natriuretic Peptides, and Angiotensin Receptor-Neprilysin Inhibitors in Patients With Heart Failure

Heart failure (HF) remains the leading cause of death, morbidity, and medical expenses worldwide. Treatments for HF with reduced ejection fraction have progressed in recent years; however, acute decompensated heart failure remains difficult to treat. The transient receptor potential (TRP) channel family plays roles in various cardiovascular diseases, responding to neurohormonal and mechanical load stimulation. Thus, TRP channels are promising targets for drug discovery, and many studies have evaluated the roles of TRP channels expressed on pain neurons. The natriuretic peptide (NP) family of proteins regulates blood volume, natriuresis, and vasodilation and can antagonize the renin-angiotensin-aldosterone system and participate in the pathogenesis of major cardiovascular diseases, such as HF, coronary atherosclerotic heart disease, and left ventricular hypertrophy. NPs are degraded by neprilysin, and the blood level of NPs has predictive value in the diagnosis and prognostic stratification of HF. In this review, we discuss the relationships between typical TRP family channels (e.g., transient receptor potential cation channel subfamily V member 1 andTRPV1, transient receptor potential cation channel subfamily C member 6) and the NP system (e.g., atrial NP, B-type NP, and C-type NP) and their respective roles in HF. We also discuss novel drugs introduced for the treatment of HF.

Highly Selective SERCA2a Activators: Preclinical Development of a Congeneric Group of First-in-Class Drug Leads against Heart Failure

The stimulation of sarcoplasmic reticulum calcium ATPase SERCA2a emerged as a novel therapeutic strategy to efficiently improve overall cardiac function in heart failure (HF) with reduced arrhythmogenic risk. Istaroxime is a clinical-phase IIb compound with a double mechanism of action, Na⁺/K⁺ ATPase inhibition and SERCA2a stimulation. Starting from the observation that istaroxime metabolite PST3093 does not inhibit Na⁺/K⁺ ATPase while stimulates SERCA2a, we synthesized a series of bioisosteric PST3093 analogues devoid of Na⁺/K⁺ ATPase inhibitory activity. Most of them retained SERCA2a stimulatory action with nanomolar potency in cardiac preparations from healthy guinea pigs and streptozotocin (STZ)-treated rats. One compound was further characterized in isolated cardiomyocytes, confirming SERCA2a stimulation and in vivo showing a safety profile and improvement of cardiac performance following acute infusion in STZ rats. We identified a new class of selective SERCA2a activators as first-in-class drug candidates for HF treatment.

Metabolic Remodeling and Implicated Calcium and Signal Transduction Pathways in the Pathogenesis of Heart Failure

The heart is an organ with high-energy demands in which the mitochondria are most abundant. They are considered the powerhouse of the cell and occupy a central role in cellular metabolism. The intermyofibrillar mitochondria constitute the majority of the three-mitochondrial subpopulations in the heart. They are also considered to be the most important in terms of their ability to participate in calcium and cellular signaling, which are critical for the regulation of mitochondrial function and adenosine triphosphate (ATP) production. This is because they are located in very close proximity with the endoplasmic reticulum (ER), and for the presence of tethering complexes enabling interorganelle crosstalk via calcium signaling. Calcium is an important second messenger that regulates mitochondrial function. It promotes ATP production and cellular survival under physiological changes in cardiac energetic demand. This is accomplished in concert with signaling pathways that regulate both calcium cycling and mitochondrial function. Perturbations in mitochondrial homeostasis and metabolic remodeling occupy a central role in the pathogenesis of heart failure. In this review we will discuss perturbations in ER-mitochondrial crosstalk and touch on important signaling pathways and molecular mechanisms involved in the dysregulation of calcium homeostasis and mitochondrial function in heart failure.

Structural basis for allosteric control of the SERCA-Phospholamban membrane complex by Ca2+ and phosphorylation

Phospholamban (PLN) is a mini-membrane protein that directly controls the cardiac Ca²⁺-transport response to β-adrenergic stimulation, thus modulating cardiac output during the fight-or-flight response. In the sarcoplasmic reticulum membrane, PLN binds to the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), keeping this enzyme's function within a narrow physiological window. PLN phosphorylation by cAMP-dependent protein kinase A or increase in Ca²⁺ concentration reverses the inhibitory effects through an unknown mechanism. Using oriented-sample solid-state NMR spectroscopy and replica-averaged NMR-restrained structural refinement, we reveal that phosphorylation of PLN's cytoplasmic regulatory domain signals the disruption of several inhibitory contacts at the transmembrane binding interface of the SERCA-PLN complex that are propagated to the enzyme's active site, augmenting Ca²⁺ transport. Our findings address long-standing questions about SERCA regulation, epitomizing a signal transduction mechanism operated by posttranslationally modified bitopic membrane proteins.

SAR340835, a Novel Selective Na+/Ca2+ Exchanger Inhibitor, Improves Cardiac Function and Restores Sympathovagal Balance in Heart Failure

In failing hearts, Na⁺/Ca²⁺ exchanger (NCX) overactivity contributes to Ca²⁺ depletion, leading to contractile dysfunction. Inhibition of NCX is expected to normalize Ca²⁺ mishandling, to limit afterdepolarization-related arrhythmias, and to improve cardiac function in heart failure (HF). SAR340835/SAR296968 is a selective NCX inhibitor for all NCX isoforms across species, including human, with no effect on the native voltage-dependent calcium and sodium currents in vitro. Additionally, it showed in vitro and in vivo antiarrhythmic properties in several models of early and delayed afterdepolarization-related arrhythmias. Its effect on cardiac function was studied under intravenous infusion at 250,750 or 1500 µg/kg per hour in dogs, which were either normal or submitted to chronic ventricular pacing at 240 bpm (HF dogs). HF dogs were infused with the reference inotrope dobutamine (10 µg/kg per minute, i.v.). In normal dogs, NCX inhibitor increased cardiac contractility (dP/dt_max) and stroke volume (SV) and tended to reduce heart rate (HR). In HF dogs, NCX inhibitor significantly and dose-dependently increased SV from the first dose (+28.5%, +48.8%, and +62% at 250, 750, and 1500 µg/kg per hour, respectively) while significantly increasing dP/dt_max only at 1500 (+33%). Furthermore, NCX inhibitor significantly restored sympathovagal balance and spontaneous baroreflex sensitivity (BRS) from the first dose and reduced HR at the highest dose. In HF dogs, dobutamine significantly increased dP/dt_max and SV (+68.8%) but did not change HR, sympathovagal balance, or BRS. Overall, SAR340835, a selective potent NCX inhibitor, displayed a unique therapeutic profile, combining antiarrhythmic properties, capacity to restore systolic function, sympathovagal balance, and BRS in HF dogs. NCX inhibitors may offer new therapeutic options for acute HF treatment. SIGNIFICANCE STATEMENT: HF is facing growing health and economic burden. Moreover, patients hospitalized for acute heart failure are at high risk of decompensation recurrence, and no current acute decompensated HF therapy definitively improved outcomes. A new potent, Na⁺/Ca²⁺ exchanger inhibitor SAR340835 with antiarrhythmic properties improved systolic function of failing hearts without creating hypotension, while reducing heart rate and restoring sympathovagal balance. SAR340835 may offer a unique and attractive pharmacological profile for patients with acute heart failure as compared with current inotrope, such as dobutamine.

Oxysterols Modulate the Acute Effects of Ethanol on Hippocampal N-Methyl-d-Aspartate Receptors, Long-Term Potentiation, and Learning

Ethanol is a noncompetitive inhibitor of N-methyl-d-aspartate receptors (NMDARs) and acutely disrupts hippocampal synaptic plasticity and learning. In the present study, we examined the effects of oxysterol positive allosteric modulators (PAMs) of NMDARs on ethanol-mediated inhibition of NMDARs, block of long-term potentiation (LTP) and long-term depression (LTD) in rat hippocampal slices, and defects in one-trial learning in vivo. We found that 24S-hydroxycholesterol and a synthetic oxysterol analog, SGE-301, overcame effects of ethanol on NMDAR-mediated synaptic responses in the CA1 region but did not alter acute effects of ethanol on LTD; the synthetic oxysterol, however, overcame acute inhibition of LTP. In addition, both oxysterols overcame persistent effects of ethanol on LTP in vitro, and the synthetic analog reversed defects in one-trial inhibitory avoidance learning in vivo. These results indicate that effects of ethanol on both LTP and LTD arise by complex mechanisms beyond NMDAR antagonism and that oxysterol NMDAR PAMS may represent a novel approach for preventing and reversing acute ethanol-mediated changes in cognition.

Heart Rate-Induced Myocardial Ca2+ Retention and Left Ventricular Volume Loss in Patients With Heart Failure With Preserved Ejection Fraction

Background

Increases in heart rate are thought to result in incomplete left ventricular (LV) relaxation and elevated filling pressures in patients with heart failure with preserved ejection fraction (HFpEF). Experimental studies in isolated human myocardium have suggested that incomplete relaxation is a result of cellular Ca²⁺ overload caused by increased myocardial Na⁺ levels. We tested these heart rate paradigms in patients with HFpEF and referent controls without hypertension.

Methods and Results

In 22 fully sedated and instrumented patients (12 controls and 10 patients with HFpEF) in sinus rhythm with a preserved ejection fraction (≥50%) we assessed left‐sided filling pressures and volumes in sinus rhythm and with atrial pacing (95 beats per minute and 125 beats per minute) before atrial fibrillation ablation. Coronary sinus blood samples and flow measurements were also obtained. Seven women and 15 men were studied (aged 59±10 years, ejection fraction 61%±4%). Patients with HFpEF had a history of hypertension, dyspnea on exertion, concentric LV remodeling and a dilated left atrium, whereas controls did not. Pacing at 125 beats per minute lowered the mean LV end‐diastolic pressure in both groups (controls −4.3±4.1 mm Hg versus patients with HFpEF −8.5±6.0 mm Hg, P=0.08). Pacing also reduced LV end‐diastolic volumes. The volume loss was about twice as much in the HFpEF group (controls −15%±14% versus patients with HFpEF −32%±11%, P=0.009). Coronary venous [Ca²⁺] increased after pacing at 125 beats per minute in patients with HFpEF but not in controls. [Na⁺] did not change.

Conclusions

Higher resting heart rates are associated with lower filling pressures in patients with and without HFpEF. Incomplete relaxation and LV filling at high heart rates lead to a reduction in LV volumes that is more pronounced in patients with HFpEF and may be associated with myocardial Ca²⁺ retention.

The effect of Sirt1 deficiency on Ca2+ and Na+ regulation in mouse ventricular myocytes

This study addressed the hypothesis that cardiac Sirtuin 1 (Sirt1) deficiency alters cardiomyocyte Ca²⁺ and Na⁺ regulation, leading to cardiac dysfunction and arrhythmogenesis. We used mice with cardiac‐specific Sirt1 knockout (Sirt1^−/−). Sirt1^flox/flox mice were served as control. Sirt1^−/− mice showed impaired cardiac ejection fraction with increased ventricular spontaneous activity and burst firing compared with those in control mice. The arrhythmic events were suppressed by KN93 and ranolazine. Reduction in Ca²⁺ transient amplitudes and sarcoplasmic reticulum (SR) Ca²⁺ stores, and increased SR Ca²⁺ leak were shown in the Sirt1^−/− mice. Electrophysiological measurements were performed using patch‐clamp method. While L‐type Ca²⁺ current (I_{Ca, L}) was smaller in Sirt1^−/− myocytes, reverse‐mode Na⁺/Ca²⁺ exchanger (NCX) current was larger compared with those in control myocytes. Late Na⁺ current (I_{Na, L}) was enhanced in the Sirt1^−/− mice, alongside with elevated cytosolic Na⁺ level. Increased cytosolic and mitochondrial reactive oxygen species (ROS) were shown in Sirt1^−/− mice. Sirt1^−/− cardiomyocytes showed down‐regulation of L‐type Ca²⁺ channel α1c subunit (Cav1.2) and sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2a (SERCA2a), but up‐regulation of Ca²⁺/calmodulin‐dependent protein kinase II and NCX. In conclusions, these findings suggest that deficiency of Sirt1 impairs the regulation of intracellular Ca²⁺ and Na⁺ in cardiomyocytes, thereby provoking cardiac dysfunction and arrhythmogenesis.

Roles of Sorcin in Drug Resistance in Cancer: One Protein, Many Mechanisms, for a Novel Potential Anticancer Drug Target

The development of drug resistance is one of the main causes of failure in anti-cancer treatments. Tumor cells adopt many strategies to counteract the action of chemotherapeutic agents, e.g., enhanced DNA damage repair, inactivation of apoptotic pathways, alteration of drug targets, drug inactivation, and overexpression of ABC (Adenosine triphosphate-binding cassette, or ATP-binding cassette) transporters. These are broad substrate-specificity ATP-dependent efflux pumps able to export toxins or drugs out of cells; for instance, ABCB1 (MDR1, or P-glycoprotein 1), overexpressed in most cancer cells, confers them multidrug resistance (MDR). The gene coding for sorcin (SOluble Resistance-related Calcium-binding proteIN) is highly conserved among mammals and is located in the same chromosomal locus and amplicon as the ABC transporters ABCB1 and ABCB4, both in human and rodent genomes (two variants of ABCB1, i.e., ABCB1a and ABCB1b, are in rodent amplicon). Sorcin was initially characterized as a soluble protein overexpressed in multidrug (MD) resistant cells and named "resistance-related" because of its co-amplification with ABCB1. Although for years sorcin overexpression was thought to be only a by-product of the co-amplification with ABC transporter genes, many papers have recently demonstrated that sorcin plays an important part in MDR, indicating a possible role of sorcin as an oncoprotein. The present review illustrates sorcin roles in the generation of MDR via many mechanisms and points to sorcin as a novel potential target of different anticancer molecules.

MitoQ regulates redox-related noncoding RNAs to preserve mitochondrial network integrity in pressure-overload heart failure

Evidence suggests that mitochondrial network integrity is impaired in cardiomyocytes from failing hearts. While oxidative stress has been implicated in heart failure (HF)-associated mitochondrial remodeling, the effect of mitochondrial-targeted antioxidants, such as mitoquinone (MitoQ), on the mitochondrial network in a model of HF (e.g., pressure overload) has not been demonstrated. Furthermore, the mechanism of this regulation is not completely understood with an emerging role for posttranscriptional regulation via long noncoding RNAs (lncRNAs). We hypothesized that MitoQ preserves mitochondrial fusion proteins (i.e., mitofusin), likely through redox-sensitive lncRNAs, leading to improved mitochondrial network integrity in failing hearts. To test this hypothesis, 8-wk-old C57BL/6J mice were subjected to ascending aortic constriction (AAC), which caused substantial left ventricular (LV) chamber remodeling and remarkable contractile dysfunction in 1 wk. Transmission electron microscopy and immunostaining revealed defective intermitochondrial and mitochondrial-sarcoplasmic reticulum ultrastructure in AAC mice compared with sham-operated animals, which was accompanied by elevated oxidative stress and suppressed mitofusin (i.e., Mfn1 and Mfn2) expression. MitoQ (1.36 mg·day-1·mouse-1, 7 consecutive days) significantly ameliorated LV dysfunction, attenuated Mfn2 downregulation, improved interorganellar contact, and increased metabolism-related gene expression. Moreover, our data revealed that MitoQ alleviated the dysregulation of an Mfn2-associated lncRNA (i.e., Plscr4). In summary, the present study supports a unique mechanism by which MitoQ improves myocardial intermitochondrial and mitochondrial-sarcoplasmic reticulum (SR) ultrastructural remodeling in HF by maintaining Mfn2 expression via regulation by an lncRNA. These findings underscore the important role of lncRNAs in the pathogenesis of HF and the potential of targeting them for effective HF treatment.NEW & NOTEWORTHY We have shown that MitoQ improves cardiac mitochondrial network integrity and mitochondrial-SR alignment in a pressure-overload mouse heart-failure model. This may be occurring partly through preventing the dysregulation of a redox-sensitive lncRNA-microRNA pair (i.e., Plscr4-miR-214) that results in an increase in mitofusin-2 expression.

MCUB Regulates the Molecular Composition of the Mitochondrial Calcium Uniporter Channel to Limit Mitochondrial Calcium Overload During Stress

BACKGROUND

The mitochondrial calcium uniporter (mtCU) is an ≈700-kD multisubunit channel residing in the inner mitochondrial membrane required for mitochondrial Ca²⁺ (_mCa²⁺) uptake. Here, we detail the contribution of MCUB, a paralog of the pore-forming subunit MCU, in mtCU regulation and function and for the first time investigate the relevance of MCUB to cardiac physiology.

METHODS

We created a stable MCUB knockout cell line (MCUB^-/-) using CRISPR-Cas9n technology and generated a cardiac-specific, tamoxifen-inducible MCUB mutant mouse (CAG-CAT-MCUB x MCM; MCUB-Tg) for in vivo assessment of cardiac physiology and response to ischemia/reperfusion injury. Live-cell imaging and high-resolution spectrofluorometery were used to determine intracellular Ca²⁺ exchange and size-exclusion chromatography; blue native page and immunoprecipitation studies were used to determine the molecular function and impact of MCUB on the high-molecular-weight mtCU complex.

RESULTS

Using genetic gain- and loss-of-function approaches, we show that MCUB expression displaces MCU from the functional mtCU complex and thereby decreases the association of mitochondrial calcium uptake 1 and 2 (MICU1/2) to alter channel gating. These molecular changes decrease MICU1/2-dependent cooperative activation of the mtCU, thereby decreasing _mCa²⁺ uptake. Furthermore, we show that MCUB incorporation into the mtCU is a stress-responsive mechanism to limit _mCa²⁺ overload during cardiac injury. Indeed, overexpression of MCUB is sufficient to decrease infarct size after ischemia/reperfusion injury. However, MCUB incorporation into the mtCU does come at a cost; acute decreases in _mCa²⁺ uptake impair mitochondrial energetics and contractile function.

CONCLUSIONS

We detail a new regulatory mechanism to modulate mtCU function and _mCa²⁺ uptake. Our results suggest that MCUB-dependent changes in mtCU stoichiometry are a prominent regulatory mechanism to modulate _mCa²⁺ uptake and cellular physiology.

Calcium-handling abnormalities underlying atrial arrhythmogenesis in a Fontan operation canine model

BACKGROUND

Atrial tachyarrhythmia (AT) is a common complication in patients who have undergone a Fontan operation. In this study, we investigated whether abnormal Ca²⁺ handling contributes to the Fontan operation-related atrial arrhythmogenic substrate.

METHODS

Mongrel dogs were randomly assigned to sham and Fontan groups. The Fontan operation model was developed by performing an atriopulmonary anastomosis. After 14 days, an electrophysiological study was performed to evaluate the AT vulnerability. Ca²⁺ handling properties were measured by loading atrial cardiomyocytes (CMs) with fura-2 AM. The L-type Ca²⁺ (I_Ca-L) and Na⁺-Ca²⁺ exchanger (I_NCX) currents of the CMs were recorded by the whole-cell patch-clamp technique. The key Ca²⁺ handling proteins expression was assessed by Western blotting.

RESULTS

The AT inducibility was higher in the Fontan group than in the sham group (85.71 vs. 14.29%, P < 0.05). The Fontan operation resulted in decreased Ca²⁺ transient (CaT) amplitude and sarcoplasmic reticulum (SR) Ca²⁺ content, but in enhanced diastolic intracellular Ca²⁺ concentration and SR Ca²⁺ leak in the atrial CMs. The spontaneous CaT events, triggered ectopic activity and I_NCX density were increased, but I_Ca-L density was reduced in CMs from the Fontan atria (all P < 0.05). Additionally, the Fontan operation resulted in decreased SR Ca²⁺ ATPase expression and Cav1.2 expression, but in increased NCX1 and Ser2814-phosphorylated ryanodine receptor 2. The calmodulin-dependent protein kinase II expression and function were markedly enhanced in the Fontan atria.

CONCLUSION

The Fontan operation caused atrial CM Ca²⁺ handling abnormalities that produced arrhythmogenic-triggered activity and increased vulnerability to AT in experimental Fontan dogs.

A privileged role for neuronal Na+ channels in regulating ventricular [Ca2+] and arrhythmias

Jones et al. provide commentary on the intricate crosstalk between ion transporters that goes awry in long QT arrhythmia.

Increased passive stiffness promotes diastolic dysfunction despite improved Ca2+ handling during left ventricular concentric hypertrophy

Aims

Concentric hypertrophy following pressure-overload is linked to preserved systolic function but impaired diastolic function, and is an important substrate for heart failure with preserved ejection fraction. While increased passive stiffness of the myocardium is a suggested mechanism underlying diastolic dysfunction in these hearts, the contribution of active diastolic Ca2+ cycling in cardiomyocytes remains unclear. In this study, we sought to dissect contributions of passive and active mechanisms to diastolic dysfunction in the concentrically hypertrophied heart following pressure-overload.

Methods and results

Rats were subjected to aortic banding (AB), and experiments were performed 6 weeks after surgery using sham-operated rats as controls. In vivo ejection fraction and fractional shortening were normal, confirming preservation of systolic function. Left ventricular concentric hypertrophy and diastolic dysfunction following AB were indicated by thickening of the ventricular wall, reduced peak early diastolic tissue velocity, and higher E/e' values. Slowed relaxation was also observed in left ventricular muscle strips isolated from AB hearts, during both isometric and isotonic stimulation, and accompanied by increases in passive tension, viscosity, and extracellular collagen. An altered titin phosphorylation profile was observed with hypophosphorylation of the phosphosites S4080 and S3991 sites within the N2Bus, and S12884 within the PEVK region. Increased titin-based stiffness was confirmed by salt-extraction experiments. In contrast, isolated, unloaded cardiomyocytes exhibited accelerated relaxation in AB compared to sham, and less contracture at high pacing frequencies. Parallel enhancement of diastolic Ca2+ handling was observed, with augmented NCX and SERCA2 activity and lowered resting cytosolic [Ca2+].

Conclusion

In the hypertrophied heart with preserved systolic function, in vivo diastolic dysfunction develops as cardiac fibrosis and alterations in titin phosphorylation compromise left ventricular compliance, and despite compensatory changes in cardiomyocyte Ca2+ homeostasis.

Structural heterogeneity of the rat pulmonary vein myocardium: consequences on intracellular calcium dynamics and arrhythmogenic potential

Mechanisms underlying ectopic activity in the pulmonary vein (PV) which triggers paroxysmal atrial fibrillation are unknown. Although several studies have suggested that calcium signalling might be involved in these arrhythmias, little is known about calcium cycling in PV cardiomyocytes (CM). We found that individual PV CM showed a wide range of transverse tubular incidence and organization, going from their virtual absence, as described in atrial CM, to well transversally organised tubular systems, like in ventricular CM. These different types of CM were found in groups scattered throughout the tissue. The variability of the tubular system was associated with cell to cell heterogeneity of calcium channel (Ca_v1.2) localisation and, thereby, of Ca_v1.2-Ryanodine receptor coupling. This was responsible for multiple forms of PV CM calcium transient. Spontaneous calcium sparks and waves were not only more abundant in PV CM than in LA CM but also associated with a higher depolarising current. In conclusion, compared with either the atrium or the ventricle, PV myocardium presents marked structural and functional heterogeneity.

A Sarcoplasmic Reticulum Localized Protein Phosphatase Regulates Phospholamban Phosphorylation and Promotes Ischemia Reperfusion Injury in the Heart

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PP2Ce is Ser-Thr phosphatase specifically localized on SR and expressed in cardiomyocytes.

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PP2Ce has specific phosphatase activity to dephosphorylate Thr-17 site of phospholamban.

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PP2Ce expression is induced upon pathological stress, including beta-AR stimulation and ROS.

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PP2Ce induction suppresses cardiomyocyte calcium cycling, reduces beta-AR-induced contractility, and promotes oxidative ischemia/reperfusion injury.

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PP2Ce is a new molecular component of stress-mediated cardiomyocyte calcium regulation.

Phospholamban (PLN) is a key regulator of sarcolemma calcium uptake in cardiomyocyte; its inhibitory activity to sarcolemma-endoplasmic reticulum calcium ATPase is regulated by phosphorylation. PLN hypophosphorylation is a common molecular feature in the failing heart. The current study provided evidence at the molecular, cellular, and whole-heart levels to implicate a sarcolemma membrane-targeted protein phosphatase, PP2Ce, as a specific and potent PLN phosphatase. PP2Ce expression was elevated in failing human heart and induced acutely at protein level by β-adrenergic stimulation or oxidative stress in cardiomyocytes. PP2Ce expression in mouse heart blunted β-adrenergic response and exacerbated ischemia/reperfusion injury. Therefore, PP2Ce is a new regulator for cardiac function and pathogenesis.

Species-Dependent Mechanisms of Cardiac Arrhythmia: A Cellular Focus

Although ventricular arrhythmia remains a leading cause of morbidity and mortality, available antiarrhythmic drugs have limited efficacy. Disappointing progress in the development of novel, clinically relevant antiarrhythmic agents may partly be attributed to discrepancies between humans and animal models used in preclinical testing. However, such differences are at present difficult to predict, requiring improved understanding of arrhythmia mechanisms across species. To this end, we presently review interspecies similarities and differences in fundamental cardiomyocyte electrophysiology and current understanding of the mechanisms underlying the generation of afterdepolarizations and reentry. We specifically highlight patent shortcomings in small rodents to reproduce cellular and tissue-level arrhythmia substrate believed to be critical in human ventricle. Despite greater ease of translation from larger animal models, discrepancies remain and interpretation can be complicated by incomplete knowledge of human ventricular physiology due to low availability of explanted tissue. We therefore point to the benefits of mathematical modeling as a translational bridge to understanding and treating human arrhythmia.

The Effect of a Novel Highly Selective Inhibitor of the Sodium/Calcium Exchanger (NCX) on Cardiac Arrhythmias in In Vitro and In Vivo Experiments

Background

In this study the effects of a new, highly selective sodium-calcium exchanger (NCX) inhibitor, ORM-10962 were investigated on cardiac NCX current, Ca²⁺ transients, cell shortening and in experimental arrhythmias. The level of selectivity of the novel inhibitor on several major transmembrane ion currents (L-type Ca²⁺ current, major repolarizing K⁺ currents, late Na⁺ current, Na⁺/K⁺ pump current) was also determined.

Methods

Ion currents in single dog ventricular cells (cardiac myocytes; CM), and action potentials in dog cardiac multicellular preparations were recorded utilizing the whole-cell patch clamp and standard microelectrode techniques, respectively. Ca²⁺ transients and cell shortening were measured in fluorescent dye loaded isolated dog myocytes. Antiarrhythmic effects of ORM-10962 were studied in anesthetized ouabain (10 μg/kg/min i.v.) pretreated guinea pigs and in ischemia-reperfusion models (I/R) of anesthetized coronary artery occluded rats and Langendorff perfused guinea pigs hearts.

Results

ORM-10962 significantly reduced the inward/outward NCX currents with estimated EC50 values of 55/67 nM, respectively. The compound, even at a high concentration of 1 μM, did not modify significantly the magnitude of I_CaL in CMs, neither had any apparent influence on the inward rectifier, transient outward, the rapid and slow components of the delayed rectifier potassium currents, the late and peak sodium and Na⁺/K⁺ pump currents. NCX inhibition exerted moderate positive inotropic effect under normal condition, negative inotropy when reverse, and further positive inotropic effect when forward mode was facilitated. In dog Purkinje fibres 1 μM ORM-10962 decreased the amplitude of digoxin induced delayed afterdepolarizations (DADs). Pre-treatment with 0.3 mg/kg ORM-10962 (i.v.) 10 min before starting ouabain infusion significantly delayed the development and recurrence of ventricular extrasystoles (by about 50%) or ventricular tachycardia (by about 30%) in anesthetized guinea pigs. On the contrary, ORM-10962 pre-treatment had no apparent influence on the time of onset or the severity of I/R induced arrhythmias in anesthetized rats and in Langendorff perfused guinea-pig hearts.

Conclusions

The present study provides strong evidence for a high efficacy and selectivity of the NCX-inhibitory effect of ORM-10962. Selective NCX inhibition can exert positive as well as negative inotropic effect depending on the actual operation mode of NCX. Selective NCX blockade may contribute to the prevention of DAD based arrhythmogenesis, in vivo, however, its effect on I/R induced arrhythmias is still uncertain.

Reduced response to IKr blockade and altered hERG1a/1b stoichiometry in human heart failure

Heart failure (HF) claims 250,000 lives per year in the US, and nearly half of these deaths are sudden and presumably due to ventricular tachyarrhythmias. QT interval and action potential (AP) prolongation are hallmark proarrhythmic changes in the failing myocardium, which potentially result from alterations in repolarizing potassium currents. Thus, we aimed to examine whether decreased expression of the rapid delayed rectifier potassium current, IKr, contributes to repolarization abnormalities in human HF. To map functional IKr expression across the left ventricle (LV), we optically imaged coronary-perfused LV free wall from donor and end-stage failing human hearts. The LV wedge preparation was used to examine transmural AP durations at 80% repolarization (APD80), and treatment with the IKr-blocking drug, E-4031, was utilized to interrogate functional expression. We assessed the percent change in APD80 post-IKr blockade relative to baseline APD80 (∆APD80) and found that ∆APD80s are reduced in failing versus donor hearts in each transmural region, with 0.35-, 0.43-, and 0.41-fold reductions in endo-, mid-, and epicardium, respectively (p=0.008, 0.037, and 0.022). We then assessed hERG1 isoform gene and protein expression levels using qPCR and Western blot. While we did not observe differences in hERG1a or hERG1b gene expression between donor and failing hearts, we found a shift in the hERG1a:hERG1b isoform stoichiometry at the protein level. Computer simulations were then conducted to assess IKr block under E-4031 influence in failing and nonfailing conditions. Our results confirmed the experimental observations and E-4031-induced relative APD80 prolongation was greater in normal conditions than in failing conditions, provided that the cellular model of HF included a significant downregulation of IKr. In human HF, the response to IKr blockade is reduced, suggesting decreased functional IKr expression. This attenuated functional response is associated with altered hERG1a:hERG1b protein stoichiometry in the failing human LV, and failing cardiomyoctye simulations support the experimental findings. Thus, of IKr protein and functional expression may be important determinants of repolarization remodeling in the failing human LV.

Exercise training reverses myocardial dysfunction induced by CaMKIIδC overexpression by restoring Ca2+ homeostasis

Several conditions of heart disease, including heart failure and diabetic cardiomyopathy, are associated with upregulation of cytosolic Ca(2+)/calmodulin-dependent protein kinase II (CaMKIIδC) activity. In the heart, CaMKIIδC isoform targets several proteins involved in intracellular Ca(2+) homeostasis. We hypothesized that high-intensity endurance training activates mechanisms that enable a rescue of dysfunctional cardiomyocyte Ca(2+) handling and thereby ameliorate cardiac dysfunction despite continuous and chronic elevated levels of CaMKIIδC CaMKIIδC transgenic (TG) and wild-type (WT) mice performed aerobic interval exercise training over 6 wk. Cardiac function was measured by echocardiography in vivo, and cardiomyocyte shortening and intracellular Ca(2+) handling were measured in vitro. TG mice had reduced global cardiac function, cardiomyocyte shortening (47% reduced compared with WT, P < 0.01), and impaired Ca(2+) homeostasis. Despite no change in the chronic elevated levels of CaMKIIδC, exercise improved global cardiac function, restored cardiomyocyte shortening, and reestablished Ca(2+) homeostasis to values not different from WT. The key features to explain restored Ca(2+) homeostasis after exercise training were increased L-type Ca(2+) current density and flux by 79 and 85%, respectively (P < 0.01), increased sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) function by 50% (P < 0.01), and reduced diastolic SR Ca(2+) leak by 73% (P < 0.01), compared with sedentary TG mice. In conclusion, exercise training improves global cardiac function as well as cardiomyocyte function in the presence of a maintained high CaMKII activity. The main mechanisms of exercise-induced improvements in TG CaMKIIδC mice are mediated via increased L-type Ca(2+) channel currents and improved SR Ca(2+) handling by restoration of SERCA2a function in addition to reduced diastolic SR Ca(2+) leak.

Enzyme localization, crowding, and buffers collectively modulate diffusion-influenced signal transduction: Insights from continuum diffusion modeling

Biochemical reaction networks consisting of coupled enzymes connect substrate signaling events with biological function. Substrates involved in these reactions can be strongly influenced by diffusion "barriers" arising from impenetrable cellular structures and macromolecules, as well as interactions with biomolecules, especially within crowded environments. For diffusion-influenced reactions, the spatial organization of diffusion barriers arising from intracellular structures, non-specific crowders, and specific-binders (buffers) strongly controls the temporal and spatial reaction kinetics. In this study, we use two prototypical biochemical reactions, a Goodwin oscillator, and a reaction with a periodic source/sink term to examine how a diffusion barrier that partitions substrates controls reaction behavior. Namely, we examine how conditions representative of a densely packed cytosol, including reduced accessible volume fraction, non-specific interactions, and buffers, impede diffusion over nanometer length-scales. We find that diffusion barriers can modulate the frequencies and amplitudes of coupled diffusion-influenced reaction networks, as well as give rise to "compartments" of decoupled reactant populations. These effects appear to be intensified in the presence of buffers localized to the diffusion barrier. These findings have strong implications for the role of the cellular environment in tuning the dynamics of signaling pathways.

Cardiac fatty acid oxidation in heart failure associated with obesity and diabetes

Obesity and diabetes are major public health problems, and are linked to the development of heart failure. Emerging data highlight the importance of alterations in cardiac energy metabolism as a major contributor to cardiac dysfunction related to obesity and diabetes. Increased rates of fatty acid oxidation and decreased rates of glucose utilization are two prominent changes in cardiac energy metabolism that occur in obesity and diabetes. This metabolic profile is probably both a cause and consequence of a prominent cardiac insulin resistance, which is accompanied by a decrease in both cardiac function and efficiency, and by the accumulation of potentially toxic lipid metabolites in the heart that can further exaggerate insulin resistance and cardiac dysfunction. The high cardiac fatty acid oxidation rates seen in obesity and diabetes are attributable to several factors, including: 1) increased fatty acid supply and uptake into the cardiomyocyte, 2) increased transcription of fatty acid metabolic enzymes, 3) decreased allosteric control of mitochondrial fatty acid uptake and fatty acid oxidation, and 4) increased post-translational acetylation control of various fatty acid oxidative enzymes. Emerging evidence suggests that therapeutic approaches aimed at switching the balance of cardiac energy substrate preference from fatty acid oxidation to glucose use can prevent cardiac dysfunction associated with obesity and diabetes. Modulating acetylation control of fatty acid oxidative enzymes is also a potentially attractive strategy, although presently this is limited to precursors of nicotinamide adenine or nonspecific activators of deacetylation such as resveratrol. This review will focus on the metabolic alterations in the heart that occur in obesity and diabetes, as well as on the molecular mechanisms controlling these metabolic changes. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Lessons learned from multi-scale modeling of the failing heart

Heart failure constitutes a major public health problem worldwide. Affected patients experience a number of changes in the electrical function of the heart that predispose to potentially lethal cardiac arrhythmias. Due to the multitude of electrophysiological changes that may occur during heart failure, the scientific literature is complex and sometimes ambiguous, perhaps because these findings are highly dependent on the etiology, the stage of heart failure, and the experimental model used to study these changes. Nevertheless, a number of common features of failing hearts have been documented. Prolongation of the action potential (AP) involving ion channel remodeling and alterations in calcium handling have been established as the hallmark characteristics of myocytes isolated from failing hearts. Intercellular uncoupling and fibrosis are identified as major arrhythmogenic factors. Multi-scale computational simulations are a powerful tool that complements experimental and clinical research. The development of biophysically detailed computer models of single myocytes and cardiac tissues has contributed greatly to our understanding of processes underlying excitation and repolarization in the heart. The electrical, structural, and metabolic remodeling that arises in cardiac tissues during heart failure has been addressed from different computational perspectives to further understand the arrhythmogenic substrate. This review summarizes the contributions from computational modeling and simulation to predict the underlying mechanisms of heart failure phenotypes and their implications for arrhythmogenesis, ranging from the cellular level to whole-heart simulations. The main aspects of heart failure are presented in several related sections. An overview of the main electrophysiological and structural changes that have been observed experimentally in failing hearts is followed by the description and discussion of the simulation work in this field at the cellular level, and then in 2D and 3D cardiac structures. The implications for arrhythmogenesis in heart failure are also discussed including therapeutic measures, such as drug effects and cardiac resynchronization therapy. Finally, the future challenges in heart failure modeling and simulation will be discussed.

Bendavia restores mitochondrial energy metabolism gene expression and suppresses cardiac fibrosis in the border zone of the infarcted heart

AIMS

We have observed that Bendavia, a mitochondrial-targeting peptide that binds the phospholipid cardiolipin and stabilizes the components of electron transport and ATP generation, improves cardiac function and prevents left ventricular remodeling in a 6week rat myocardial infarction (MI) model. We hypothesized that Bendavia restores mitochondrial biogenesis and gene expression, suppresses cardiac fibrosis, and preserves sarco/endoplasmic reticulum (SERCA2a) level in the noninfarcted border zone of infarcted hearts.

MAIN METHODS

Starting 2h after left coronary artery ligation, rats were randomized to receive Bendavia (3mg/kg/day), water or sham operation. At 6weeks, PCR array and qRT-PCR was performed to detect gene expression. Picrosirius red staining was used to analyze collagen deposition.

KEY FINDINGS

There was decreased expression of 70 out of 84 genes related to mitochondrial energy metabolism in the border zone of untreated hearts. This down-regulation was largely reversed by Bendavia treatment. Downregulated mitochondrial biogenesis and glucose & fatty acid (FA) oxidation related genes were restored by administration of Bendavia. Matrix metalloproteinase (MMP9) and tissue inhibitor of metalloproteinase (TIMP1) gene expression were significantly increased in the border zone of untreated hearts. Bendavia completely prevented up-regulation of MMP9, but maintained TIMP1 gene expression. Picrosirius red staining demonstrated that Bendavia suppressed collagen deposition within border zone. In addition, Bendavia showed a trend toward restoring SERCA2a expression.

SIGNIFICANCE

Bendavia restored expression of mitochondrial energy metabolism related genes, prevented myocardial matrix remodeling and preserved SERCA2a expression in the noninfarcted border, which may have contributed to the preservation of cardiac structure and function.

Targeting cardiomyocyte Ca2+ homeostasis in heart failure

Improved treatments for heart failure patients will require the development of novel therapeutic strategies that target basal disease mechanisms. Disrupted cardiomyocyte Ca²⁺ homeostasis is recognized as a major contributor to the heart failure phenotype, as it plays a key role in systolic and diastolic dysfunction, arrhythmogenesis, and hypertrophy and apoptosis signaling. In this review, we outline existing knowledge of the involvement of Ca²⁺ homeostasis in these deficits, and identify four promising targets for therapeutic intervention: the sarcoplasmic reticulum Ca²⁺ ATPase, the Na⁺-Ca²⁺ exchanger, the ryanodine receptor, and t-tubule structure. We discuss experimental data indicating the applicability of these targets that has led to recent and ongoing clinical trials, and suggest future therapeutic approaches.

Short-term beat-to-beat variability of the QT interval is increased and correlates with parameters of left ventricular hypertrophy in patients with hypertrophic cardiomyopathy

Stratification models for the prediction of sudden cardiac death (SCD) are inappropriate in patients with hypertrophic cardiomyopathy (HCM). We investigated conventional electrocardiogram (ECG) repolarization parameters and the beat-to-beat short-term QT interval variability (QT-STV), a new parameter of proarrhythmic risk, in 37 patients with HCM (21 males, average age 48 ± 15 years). Resting ECGs were recorded for 5 min and the frequency corrected QT interval (QTc), QT dispersion (QTd), beat-to-beat short-term variability of QT interval (QT-STV), and the duration of terminal part of T waves (Tpeak–Tend) were calculated. While all repolarization parameters were significantly increased in patients with HCM compared with the controls (QTc, 488 ± 61 vs. 434 ± 23 ms, p < 0.0001; QT-STV, 4.5 ± 2 vs. 3.2 ± 1 ms, p = 0.0002; Tpeak–Tend duration, 107 ± 27 vs. 91 ± 10 ms, p = 0.0015; QTd, 47 ± 17 vs. 34 ± 9 ms, p = 0.0002), QT-STV had the highest relative increase (+41%). QT-STV also showed the best correlation with indices of left ventricular (LV) hypertrophy, i.e., maximal LV wall thickness normalized for body surface area (BSA; r = 0.461, p = 0.004) or LV mass (determined by cardiac magnetic resonance imaging) normalized for BSA (r = 0.455, p = 0.015). In summary, beat-to-beat QT-STV showed the most marked increase in patients with HCM and may represent a novel marker that merits further testing for increased SCD risk in HCM.

Intracellular calcium attenuates late current conducted by mutant human cardiac sodium channels

BACKGROUND

Mutations of the cardiac voltage-gated sodium channel (SCN5A gene encoding voltage-gated sodium channel [NaV1.5]) cause congenital long-QT syndrome type 3 (LQT3). Most NaV1.5 mutations associated with LQT3 promote a mode of sodium channel gating in which some channels fail to inactivate, contributing to increased late sodium current (INaL), which is directly responsible for delayed repolarization and prolongation of the QT interval. LQT3 patients have highest risk of arrhythmia during sleep or during periods of slow heart rate. During exercise (high heart rate), there is elevated steady-state intracellular free calcium (Ca(2+)) concentration. We hypothesized that higher levels of intracellular Ca(2+) may lower arrhythmia risk in LQT3 subjects through effects on INaL.

METHODS AND RESULTS

We tested this idea by examining the effects of varying intracellular Ca(2+) concentrations on the level of INaL in cells expressing a typical LQT3 mutation, delKPQ, and another SCN5A mutation, R225P. We found that elevated intracellular Ca(2+) concentration significantly reduced INaL conducted by mutant channels but not wild-type channels. This attenuation of INaL in delKPQ expressing cells by Ca(2+) was not affected by the CaM kinase II inhibitor KN-93 but was partially attenuated by truncating the C-terminus of the channel.

CONCLUSIONS

We conclude that intracellular Ca(2+) contributes to the regulation of INaL conducted by NaV1.5 mutants and propose that, during excitation-contraction coupling, elevated intracellular Ca(2+) suppresses mutant channel INaL and protects cells from delayed repolarization. These findings offer a plausible explanation for the lower arrhythmia risk in LQT3 subjects during fast heart rates.

αB-Crystallin R120G variant causes cardiac arrhythmias and alterations in the expression of Ca(2+) -handling proteins and endoplasmic reticulum stress in mice

Mutations of αB-crystallin (CryαB), a small heat shock protein abundantly expressed in cardiac and skeletal muscles, are known to cause desmin-related myopathies. The CryαB R120G allele has been linked to a familial desminopathy and, in transgenic mice, causes a sudden death at about 28 weeks of age. To investigate the mechanisms of the sudden cardiac arrest of CryαB R120G transgenic mice, we prepared protein samples from left ventricular tissues of two different age groups (10 and 28 weeks) and examined Ca(2+) -handling proteins. Expression of sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) 2, phospholamban, ryanodine receptor 2 and calsequestrin 2 was significantly decreased in 28- versus 10-week-old CryαB R120G transgenic mice. In addition, low heart rate variability, including heart rate, total power and low frequency, was observed and continuous electrocardiogram monitoring revealed cardiac arrhythmias, such as ventricular tachycardia, atrioventricular block and atrial flutter, in 28-week-old CryαB R120G transgenic mice. In contrast, expression of endoplasmic reticulum (ER) degradation enhancing α-mannosidase-like protein, inositol requirement 1 and X-box binding protein 1 were increased significantly in 28- versus 10-week-old CryαBR120G transgenic mice, suggesting that the CryαBR120G transgenic mice exhibit increased ER stress compared with wild-type mice. Together, the data suggest that the CryαB R120G dominant variant induces ER stress and impairs Ca(2+) regulation, leading to ageing-related cardiac dysfunction, arrhythmias and decreased autonomic tone with shortened lifespan.

Depolarization of cardiac membrane potential synchronizes calcium sparks and waves in tissue

The diastolic membrane potential (Vm) can be hyperpolarized or depolarized by various factors such as hyperkalemia or hypokalemia in the long term, or by delayed afterdepolarizations in the short term. In this study, we investigate how Vm affects Ca sparks and waves. We use a physiologically detailed mathematical model to investigate individual factors that affect Ca spark generation and wave propagation. We focus on the voltage range of -90 ∼ -70 mV, which is just below the Vm for sodium channel activation. We find that Vm depolarization promotes Ca wave propagation and hyperpolarization prevents it. This finding is directly validated in voltage clamp experiments with Ca waves using isolated rat ventricular myocytes. Ca transport by the sodium-calcium exchanger (NCX) is determined by Vm as well as Na and Ca concentrations. Depolarized Vm reduces NCX-mediated efflux, elevating [Ca]i, and thus promoting Ca wave propagation. Moreover, depolarized Vm promotes spontaneous Ca releases that can cause initiation of multiple Ca waves. This indicates that during delayed afterdepolarizations, Ca release units (CRUs) interact with not just the immediately adjacent CRUs via Ca diffusion, but also further CRUs via fast (∼0.1 ms) changes in Vm mediated by the voltage and Ca-sensitive NCX. This may contribute significantly to synchronization of Ca waves among multiple cells in tissue.

Upregulation of SERCA2a following short-term ACE inhibition (by enalaprilat) alters contractile performance and arrhythmogenicity of healthy myocardium in rat

Chronic angiotensin-converting enzyme inhibitor (ACEIs) treatment can suppress arrhythmogenesis. To examine whether the effect is more immediate and independent of suppression of pathological remodelling, we tested the antiarrhythmic effect of short-term ACE inhibition in healthy normotensive rats. Wistar rats were administered with enalaprilat (ENA, i.p., 5 mg/kg every 12 h) or vehicle (CON) for 2 weeks. Intraarterial blood pressure in situ was measured in A. carotis. Cellular shortening was measured in isolated, electrically paced cardiomyocytes. Standard 12-lead electrocardiography was performed, and hearts of anaesthetized open-chest rats were subjected to 6-min ischemia followed by 10-min reperfusion to examine susceptibility to ventricular arrhythmias. Expressions of calcium-regulating proteins (SERCA2a, cardiac sarco/endoplasmic reticulum Ca(2+)-ATPase; CSQ, calsequestrin; TRD, triadin; PLB, phospholamban; Thr(17)-PLB-phosphorylated PLB at threonine-17, FKBP12.6, FK506-binding protein, Cav1.2-voltage-dependent L-type calcium channel alpha 1C subunit) were measured by Western blot; mRNA levels of L-type calcium channel (Cacna1c), ryanodine receptor (Ryr2) and potassium channels Kcnh2 and Kcnq1 were measured by qRT-PCR. ENA decreased intraarterial systolic as well as diastolic blood pressure (by 20%, and by 31%, respectively, for both P < 0.05) but enhanced shortening of cardiomyocytes at basal conditions (by 34%, P < 0.05) and under beta-adrenergic stimulation (by 73%, P < 0.05). Enalaprilat shortened QTc interval duration (CON 78 ± 1 ms vs. ENA 72 ± 2 ms; P < 0.05) and significantly decreased the total duration of ventricular fibrillations (VF) and the number of VF episodes (P < 0.05). Reduction in arrhythmogenesis was associated with a pronounced upregulation of SERCA2a (CON 100 ± 20 vs. ENA 304 ± 13; P < 0.05) and complete absence of basal Ca(2+)/calmodulin-dependent phosphorylation of PLB at Thr(17). Short-term ACEI treatment can provide protection against I/R injury-induced ventricular arrhythmias in healthy myocardium, and this effect is associated with increased SERCA2a expression.

Ca(2+) cycling properties are conserved despite bradycardic effects of heart failure in sinoatrial node cells

BACKGROUND

In animal models of heart failure (HF), heart rate decreases due to an increase in intrinsic cycle length (CL) of the sinoatrial node (SAN). Pacemaker activity of SAN cells is complex and modulated by the membrane clock, i.e., the ensemble of voltage gated ion channels and electrogenic pumps and exchangers, and the Ca(2+) clock, i.e., the ensemble of intracellular Ca(2+) ([Ca(2+)]i) dependent processes. HF in SAN cells results in remodeling of the membrane clock, but few studies have examined its effects on [Ca(2+)]i homeostasis.

METHODS

SAN cells were isolated from control rabbits and rabbits with volume and pressure overload-induced HF. [Ca(2+)]i concentrations, and action potentials (APs) and Na(+)-Ca(2+) exchange current (INCX) were measured using indo-1 and patch-clamp methodology, respectively.

RESULTS

The frequency of spontaneous [Ca(2+)]i transients was significantly lower in HF SAN cells (3.0 ± 0.1 (n = 40) vs. 3.4 ± 0.1 Hz (n = 45); mean ± SEM), indicating that intrinsic CL was prolonged. HF slowed the [Ca(2+)]i transient decay, which could be explained by the slower frequency and reduced sarcoplasmic reticulum (SR) dependent rate of Ca(2+) uptake. Other [Ca(2+)]i transient parameters, SR Ca(2+) content, INCX density, and INCX-[Ca(2+)]i relationship were all unaffected by HF. Combined AP and [Ca(2+)]i recordings demonstrated that the slower [Ca(2+)]i transient decay in HF SAN cells may result in increased INCX during the diastolic depolarization, but that this effect is likely counteracted by the HF-induced increase in intracellular Na(+). β-adrenergic and muscarinic stimulation were not changed in HF SAN cells, except that late diastolic [Ca(2+)]i rise, a prominent feature of the Ca(2+) clock, is lower during β-adrenergic stimulation.

CONCLUSIONS

HF SAN cells have a slower [Ca(2+)]i transient decay with limited effects on pacemaker activity. Reduced late diastolic [Ca(2+)]i rise during β-adrenergic stimulation may contribute to an impaired increase in intrinsic frequency in HF SAN cells.

Succinate causes pathological cardiomyocyte hypertrophy through GPR91 activation

Background

Succinate is an intermediate of the citric acid cycle as well as an extracellular circulating molecule, whose receptor, G protein-coupled receptor-91 (GPR91), was recently identified and characterized in several tissues, including heart. Because some pathological conditions such as ischemia increase succinate blood levels, we investigated the role of this metabolite during a heart ischemic event, using human and rodent models.

Results

We found that succinate causes cardiac hypertrophy in a GPR91 dependent manner. GPR91 activation triggers the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), the expression of calcium/calmodulin dependent protein kinase IIδ (CaMKIIδ) and the translocation of histone deacetylase 5 (HDAC5) into the cytoplasm, which are hypertrophic-signaling events. Furthermore, we found that serum levels of succinate are increased in patients with cardiac hypertrophy associated with acute and chronic ischemic diseases.

Conclusions

These results show for the first time that succinate plays an important role in cardiomyocyte hypertrophy through GPR91 activation, and extend our understanding of how ischemia can induce hypertrophic cardiomyopathy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-014-0078-2) contains supplementary material, which is available to authorized users.

Contribution of sodium channel neuronal isoform Nav1.1 to late sodium current in ventricular myocytes from failing hearts

KEY POINTS

Late Na(+) current (INaL) contributes to action potential remodelling and Ca(2+)/Na(+) changes in heart failure. The molecular identity of INaL remains unclear. The contributions of different Na(+) channel isoforms, apart from the cardiac isoform, remain unknown. We discovered and characterized a substantial contribution of neuronal isoform Nav1.1 to INaL. This new component is physiologically relevant to the control of action potential shape and duration, as well as to cell Ca(2+) dynamics, especially in heart failure.

ABSTRACT

Late Na(+) current (INaL) contributes to action potential (AP) duration and Ca(2+) handling in cardiac cells. Augmented INaL was implicated in delayed repolarization and impaired Ca(2+) handling in heart failure (HF). We tested if Na(+) channel (Nav) neuronal isoforms contribute to INaL and Ca(2+) cycling defects in HF in 17 dogs in which HF was achieved via sequential coronary artery embolizations. Six normal dogs served as control. Transient Na(+) current (INaT ) and INaL in left ventricular cardiomyocytes (VCMs) were recorded by patch clamp while Ca(2+) dynamics was monitored using Fluo-4. Virally delivered short interfering RNA (siRNA) ensured Nav1.1 and Nav1.5 post-transcriptional silencing. The expression of six Navs was observed in failing VCMs as follows: Nav1.5 (57.3%) > Nav1.2 (15.3%) > Nav1.1 (11.6%) > Nav2.1 (10.7%) > Nav1.3 (4.6%) > Nav1.6 (0.5%). Failing VCMs showed up-regulation of Nav1.1 expression, but reduction of Nav1.6 mRNA. A similar Nav expression pattern was found in samples from human hearts with ischaemic HF. VCMs with silenced Nav1.5 exhibited residual INaT and INaL (∼30% of control) with rightwardly shifted steady-state activation and inactivation. These currents were tetrodotoxin sensitive but resistant to MTSEA, a specific Nav1.5 blocker. The amplitude of the tetrodotoxin-sensitive INaL was 0.1709 ± 0.0299 pA pF(-1) (n = 7 cells) and the decay time constant was τ = 790 ± 76 ms (n = 5). This INaL component was lacking in VCMs with a silenced Nav1.1 gene, indicating that, among neuronal isoforms, Nav1.1 provides the largest contribution to INaL. At -10 mV this contribution is ∼60% of total INaL. Our further experimental and in silico examinations showed that this new Nav1.1 INaL component contributes to Ca(2+) accumulation in failing VCMs and modulates AP shape and duration. In conclusion, we have discovered an Nav1.1-originated INaL component in dog heart ventricular cells. This component is physiologically relevant to controlling AP shape and duration, as well as to cell Ca(2+) dynamics.

Basal and β-adrenergic regulation of the cardiac calcium channel CaV1.2 requires phosphorylation of serine 1700

L-type calcium (Ca(2+)) currents conducted by voltage-gated Ca(2+) channel CaV1.2 initiate excitation-contraction coupling in cardiomyocytes. Upon activation of β-adrenergic receptors, phosphorylation of CaV1.2 channels by cAMP-dependent protein kinase (PKA) increases channel activity, thereby allowing more Ca(2+) entry into the cell, which leads to more forceful contraction. In vitro reconstitution studies and in vivo proteomics analysis have revealed that Ser-1700 is a key site of phosphorylation mediating this effect, but the functional role of this amino acid residue in regulation in vivo has remained uncertain. Here we have studied the regulation of calcium current and cell contraction of cardiomyocytes in vitro and cardiac function and homeostasis in vivo in a mouse line expressing the mutation Ser-1700-Ala in the CaV1.2 channel. We found that preventing phosphorylation at this site decreased the basal L-type CaV1.2 current in both neonatal and adult cardiomyocytes. In addition, the incremental increase elicited by isoproterenol was abolished in neonatal cardiomyocytes and was substantially reduced in young adult myocytes. In contrast, cellular contractility was only moderately reduced compared with wild type, suggesting a greater reserve of contractile function and/or recruitment of compensatory mechanisms. Mutant mice develop cardiac hypertrophy by the age of 3-4 mo, and maximal stress-induced exercise tolerance is reduced, indicating impaired physiological regulation in the fight-or-flight response. Our results demonstrate that phosphorylation at Ser-1700 alone is essential to maintain basal Ca(2+) current and regulation by β-adrenergic activation. As a consequence, blocking PKA phosphorylation at this site impairs cardiovascular physiology in vivo, leading to reduced exercise capacity in the fight-or-flight response and development of cardiac hypertrophy.

The late sodium current in heart failure: pathophysiology and clinical relevance

Large and growing body of data suggest that an increased late sodium current (I_Na,late ) can have a significant pathophysiological role in heart failure and other heart diseases. The first goal of this article is to describe how I_Na,late functions under physiological circumstances. The second goal is to show the wide range of cellular mechanisms that can increase I_Na,late in cardiac disease, and also to describe how the up-regulated I_Na,late contributes to the pathophysiology of heart failure. The final section of the article discusses the possible use of I_Na,late -modifying drugs in heart failure, on the basis of experimental and preclinical data.

The effect of SN-6, a novel sodium-calcium exchange inhibitor, on contractility and calcium handling in isolated failing rat ventricular myocytes

BACKGROUND AND PURPOSE

Specific Na(+) /Ca(2+) exchanger (NCX) inhibition is a potential strategy to correct reduced contractility and depleted sarcoplasmic reticulum (SR) Ca(2+) content in heart failure (HF). SN-6, a benzyloxyphenyl derivative and proposed selective NCX inhibitor, could be used for this purpose. This study aimed to evaluate the effects of SN-6 on contractility and Ca(2+) handling in normal and failing rat cardiomyocytes.

EXPERIMENTAL APPROACH

HF was induced in rats by coronary artery ligation. Left ventricular myocytes were isolated and superfused with increasing concentrations of SN-6.

KEY RESULTS

Sarcomere shortening, induced by field-stimulation, was reduced in amplitude with increasing concentrations of SN-6 compared with control solution. This effect was greater in failing cells. Kinetics of contractility (time to 90% peak and time to 50% relaxation) were significantly faster. Despite this, intracellular Ca(2+) transients demonstrated no change in the peak amplitude at low concentrations of SN-6, suggesting that SN-6 may affect myofilament sensitivity to Ca(2+) . Ten micro molar SN-6 significantly reduced peak Ca(2+) amplitude by 61.57% and 64.73% in normal and failing cells, respectively. Diastolic Ca(2+) was significantly increased at 1 μM SN-6. SR Ca(2+) content, assessed by rapid application of caffeine, was reduced in failing cells with 1 μM SN-6. Peak ICa , measured by whole-cell patch clamping, was significantly reduced in normal and failing myocytes at 1 μM SN-6.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that SN-6 is not a selective inhibitor of NCX and impairs contractility and Ca(2+) handling. Its use, together with similar putative NCX blockers, in correcting the contractile abnormalities of heart failure requires further studies.

Electrophysiological and structural remodeling in heart failure modulate arrhythmogenesis. 1D simulation study

Background

Heart failure is a final common pathway or descriptor for various cardiac pathologies. It is associated with sudden cardiac death, which is frequently caused by ventricular arrhythmias. Electrophysiological remodeling, intercellular uncoupling, fibrosis and autonomic imbalance have been identified as major arrhythmogenic factors in heart failure etiology and progression.

Objective

In this study we investigate in silico the role of electrophysiological and structural heart failure remodeling on the modulation of key elements of the arrhythmogenic substrate, i.e., electrophysiological gradients and abnormal impulse propagation.

Methods

Two different mathematical models of the human ventricular action potential were used to formulate models of the failing ventricular myocyte. This provided the basis for simulations of the electrical activity within a transmural ventricular strand. Our main goal was to elucidate the roles of electrophysiological and structural remodeling in setting the stage for malignant life-threatening arrhythmias.

Results

Simulation results illustrate how the presence of M cells and heterogeneous electrophysiological remodeling in the human failing ventricle modulate the dispersion of action potential duration and repolarization time. Specifically, selective heterogeneous remodeling of expression levels for the Na⁺/Ca²⁺ exchanger and SERCA pump decrease these heterogeneities. In contrast, fibroblast proliferation and cellular uncoupling both strongly increase repolarization heterogeneities. Conduction velocity and the safety factor for conduction are also reduced by the progressive structural remodeling during heart failure.

Conclusion

An extensive literature now establishes that in human ventricle, as heart failure progresses, gradients for repolarization are changed significantly by protein specific electrophysiological remodeling (either homogeneous or heterogeneous). Our simulations illustrate and provide new insights into this. Furthermore, enhanced fibrosis in failing hearts, as well as reduced intercellular coupling, combine to increase electrophysiological gradients and reduce electrical propagation. In combination these changes set the stage for arrhythmias.

Sorcin, a calcium binding protein involved in the multidrug resistance mechanisms in cancer cells

Sorcin is a penta-EF hand calcium binding protein, which participates in the regulation of calcium homeostasis in cells. Sorcin regulates calcium channels and exchangers located at the plasma membrane and at the endo/sarcoplasmic reticulum (ER/SR), and allows high levels of calcium in the ER to be maintained, preventing ER stress and possibly, the unfolded protein response. Sorcin is highly expressed in the heart and in the brain, and overexpressed in many cancer cells. Sorcin gene is in the same amplicon as other genes involved in the resistance to chemotherapeutics in cancer cells (multi-drug resistance, MDR) such as ABCB4 and ABCB1; its overexpression results in increased drug resistance to a number of chemotherapeutic agents, and inhibition of sorcin expression by sorcin-targeting RNA interference leads to reversal of drug resistance. Sorcin is increasingly considered a useful marker of MDR and may represent a therapeutic target for reversing tumor multidrug resistance.

Cardiac hypertrophy in mice expressing unphosphorylatable phospholemman

AIMS

Elevation of intracellular Na in the failing myocardium contributes to contractile dysfunction, the negative force-frequency relationship, and arrhythmias. Although phospholemman (PLM) is recognized to form the link between signalling pathways and Na/K pump activity, the possibility that defects in its regulation contribute to elevation of intracellular Na has not been investigated. Our aim was to test the hypothesis that the prevention of PLM phosphorylation in a PLM(3SA) knock-in mouse (in which PLM has been rendered unphosphorylatable) will exacerbate cardiac hypertrophy and cellular Na overload. Testing this hypothesis should determine whether changes in PLM phosphorylation are simply bystander effects or are causally involved in disease progression.

METHODS AND RESULTS

In wild-type (WT) mice, aortic constriction resulted in hypophosphorylation of PLM with no change in Na/K pump expression. This under-phosphorylation of PLM occurred at 3 days post-banding and was associated with a progressive decline in Na/K pump current and elevation of [Na]i. Echocardiography, morphometry, and pressure-volume (PV) catheterization confirmed remodelling, dilation, and contractile dysfunction, respectively. In PLM(3SA) mice, expression of Na/K ATPase was increased and PLM decreased such that net Na/K pump current under quiescent conditions was unchanged (cf. WT myocytes); [Na(+)]i was increased and forward-mode Na/Ca exchanger was reduced in paced PLM(3SA) myocytes. Cardiac hypertrophy and Na/K pump inhibition were significantly exacerbated in banded PLM(3SA) mice compared with banded WT.

CONCLUSIONS

Decreased phosphorylation of PLM reduces Na/K pump activity and exacerbates Na overload, contractile dysfunction, and adverse remodelling following aortic constriction in mice. This suggests a novel therapeutic target for the treatment of heart failure.

Tight coupling of Na+/K+-ATPase with glycolysis demonstrated in permeabilized rat cardiomyocytes

The effective integrated organization of processes in cardiac cells is achieved, in part, by the functional compartmentation of energy transfer processes. Earlier, using permeabilized cardiomyocytes, we demonstrated the existence of tight coupling between some of cardiomyocyte ATPases and glycolysis in rat. In this work, we studied contribution of two membrane ATPases and whether they are coupled to glycolysis--sarcoplasmic reticulum Ca2+ ATPase (SERCA) and plasmalemma Na+/K+-ATPase (NKA). While SERCA activity was minor in this preparation in the absence of calcium, major role of NKA was revealed accounting to ∼30% of the total ATPase activity which demonstrates that permeabilized cell preparation can be used to study this pump. To elucidate the contribution of NKA in the pool of ATPases, a series of kinetic measurements was performed in cells where NKA had been inhibited by 2 mM ouabain. In these cells, we recorded: ADP- and ATP-kinetics of respiration, competition for ADP between mitochondria and pyruvate kinase (PK), ADP-kinetics of endogenous PK, and ATP-kinetics of total ATPases. The experimental data was analyzed using a series of mathematical models with varying compartmentation levels. The results show that NKA is tightly coupled to glycolysis with undetectable flux of ATP between mitochondria and NKA. Such tight coupling of NKA to PK is in line with its increased importance in the pathological states of the heart when the substrate preference shifts to glucose.