Induced overexpression of Na+/Ca2+ exchanger transgene: altered myocyte contractility, [Ca2+]i transients, SR Ca2+ contents, and action potential duration

Cardiac function is regulated by the sodium-dependent inhibition of the sodium-calcium exchanger NCX1

The Na+-Ca2+ exchanger (NCX1) is the dominant Ca2+ extrusion mechanism in cardiac myocytes. NCX1 activity is inhibited by intracellular Na+ via a process known as Na+-dependent inactivation. A central question is whether this inactivation plays a physiological role in heart function. Using CRISPR/Cas9, we inserted the K229Q mutation in the gene (Slc8a1) encoding for NCX1. This mutation removes the Na+-dependent inactivation while preserving transport properties and other allosteric regulations. NCX1 mRNA levels, protein expression, and protein localization are unchanged in K229Q male mice. However, they exhibit reduced left ventricular ejection fraction and fractional shortening, while displaying a prolonged QT interval. K229Q ventricular myocytes show enhanced NCX1 activity, resulting in action potential prolongation, higher incidence of aberrant action potentials, a faster decline of Ca2+ transients, and depressed cell shortening. The results demonstrate that NCX1 Na+-dependent inactivation plays an essential role in heart function by affecting both cardiac excitability and contractility.

ZBTB20 Regulates SERCA2a Activity and Myocardial Contractility Through Phospholamban

BACKGROUND

Intracellular Ca2+ cycling determines myocardial contraction and relaxation in response to physiological demands. SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a) is responsible for the sequestration of cytosolic Ca2+ into intracellular stores during cardiac relaxation, and its activity is reversibly inhibited by PLN (phospholamban). However, the regulatory hierarchy of SERCA2a activity remains unclear.

METHODS

Cardiomyocyte-specific ZBTB20 knockout mice were generated by crossing ZBTB20flox mice with Myh6-Cre mice. Echocardiography, blood pressure measurements, Langendorff perfusion, histological analysis and immunohistochemistry, quantitative reverse transcription-PCR, Western blot analysis, electrophysiological measurements, and chromatin immunoprecipitation assay were performed to clarify the phenotype and elucidate the molecular mechanisms.

RESULTS

Specific ablation of ZBTB20 in cardiomyocyte led to a significant increase in basal myocardial contractile parameters both in vivo and in vitro, accompanied by an impairment in cardiac reserve and exercise capacity. Moreover, the cardiomyocytes lacking ZBTB20 showed an increase in sarcoplasmic reticular Ca2+ content and exhibited a remarkable enhancement in both SERCA2a activity and electrically stimulated contraction. Mechanistically, PLN expression was dramatically reduced in cardiomyocytes at the mRNA and protein levels by ZBTB20 deletion or silencing, and PLN overexpression could largely restore the basal contractility in ZBTB20-deficient cardiomyocytes.

CONCLUSIONS

These data point to ZBTB20 as a fine-tuning modulator of PLN expression and SERCA2a activity, thereby offering new perspective on the regulation of basal contractility in the mammalian heart.

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

Ca²⁺ homeostasis is essential for cell function and survival. As such, the cytosolic Ca²⁺ concentration is tightly controlled by a wide number of specialized Ca²⁺ handling proteins. One among them is the Na⁺ -Ca²⁺ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na⁺ to drive Ca²⁺ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca²⁺ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

New understanding of electrical activity brought by surface potential of cardiomyocytes

Aiming at the problem encountered in the previous research: during the electrical activity of cardiomyocytes, the influent ions do not seem to be directly derived from the extracellular fluid. We chose to cut in from the colloidal properties of the cells, follow the basic principles of physical chemistry, and establish hypotheses along the derivation of the structural characteristics of cardiomyocytes. Through the surface ion adsorption experiment and patch clamp experiment of living cells, under the condition of sequentially reducing the concentration of Na⁺ in the extracellular fluid, we observed the exchange and diffusion of adsorbed ions on the cell surface; the changes of inflow I_Na, I_Ca-L and action potential; and correlation between results. The results showed that the hypothesis is true. The observed parameter changes were consistent with the fact that during depolarization of cardiomyocytes, the ions of influx were derived from the inference of adsorbed ions on the cell surface; at the same time, it also provided an objective and realistic explanation for the generation of electrocardiogram.

Novel BAG3 Variants in African American Patients With Cardiomyopathy: Reduced β-Adrenergic Responsiveness in Excitation-Contraction

BACKGROUND

We reported 3 novel nonsynonymous single nucleotide variants of Bcl2-associated athanogene 3 (BAG3) in African Americans with heart failure (HF) that are associated with a 2-fold increase in cardiac events (HF hospitalization, heart transplantation, or death).

METHODS AND RESULTS

We expressed BAG3 variants (P63A, P380S, and A479V) via adenovirus-mediated gene transfer in adult left ventricular myocytes isolated from either wild-type (WT) or cardiac-specific BAG3 haploinsufficient (cBAG3^+/-) mice: the latter to simulate the clinical situation in which BAG3 variants are only found on 1 allele. Compared with WT myocytes, cBAG3^+/- myocytes expressed approximately 50% of endogenous BAG3 levels and exhibited decreased [Ca²⁺]_i and contraction amplitudes after isoproterenol owing to decreased L-type Ca²⁺ current. BAG3 repletion with WT BAG3 but not P380S, A479V, or P63A/P380S variants restored contraction amplitudes in cBAG3^+/- myocytes to those measured in WT myocytes, suggesting excitation-contraction abnormalities partly account for HF in patients harboring these mutants. Because P63A is near the WW domain (residues 21-55) and A479V is in the BAG domain (residues 420-499), we expressed BAG3 deletion mutants (Δ1-61 and Δ421-575) in WT myocytes and demonstrated that the BAG but not the WW domain was involved in enhancement of excitation-contraction by isoproterenol.

CONCLUSIONS

The BAG3 variants contribute to HF in African American patients partly by decreasing myocyte excitation-contraction under stress, and that both the BAG and PXXP domains are involved in mediating β-adrenergic responsiveness in myocytes.

Ca2+ signaling of human pluripotent stem cells-derived cardiomyocytes as compared to adult mammalian cardiomyocytes

Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) have been extensively used for in vitro modeling of human cardiovascular disease, drug screening and pharmacotherapy, but little rigorous studies have been reported on their biophysical or Ca2+ signaling properties. There is also considerable concern as to the level of their maturity and whether they can serve as reliable models for adult human cardiac myocytes. Ultrastructural difference such as lack of t-tubular network, their polygonal shapes, disorganized sarcomeric myofilament, and their rhythmic automaticity, among others, have been cited as evidence for immaturity of hiPSC-CMs. In this review, we will deal with Ca2+ signaling, its regulation, and its stage of maturity as compared to the mammalian adult cardiomyocytes. We shall summarize the data on functional aspects of Ca2+signaling and its parameters that include: L-type calcium channel (Cav1.2), ICa-induced Ca2+release, CICR, and its parameters, cardiac Na/Ca exchanger (NCX1), the ryanodine receptors (RyR2), sarco-reticular Ca2+pump, SERCA2a/PLB, and the contribution of mitochondrial Ca2+ to hiPSC-CMs excitation-contraction (EC)-coupling as compared with adult mammalian cardiomyocytes. The comparative studies suggest that qualitatively hiPSC-CMs have similar Ca2+signaling properties as those of adult cardiomyocytes, but quantitative differences do exist. This review, we hope, will allow the readers to judge for themselves to what extent Ca2+signaling of hiPSC-CMs represents the adult form of this signaling pathway, and whether these cells can be used as good models of human cardiomyocytes.

Short-Period Temporal Dispersion Repolarization Markers Predict 30-Days Mortality in Decompensated Heart Failure

BACKGROUND AND OBJECTIVES

Electrocardiographic (ECG) markers of the temporal dispersion of the myocardial repolarization phase have been shown able to identify chronic heart failure (CHF) patients at high mortality risk. The present prospective single-center study sought to investigate in a well-characterized cohort of decompensated heart failure (HF) patients the ability of short-term myocardial temporal dispersion ECG variables in predicting the 30-day mortality, as well as their relationship with N-terminal Pro Brain Natriuretic Peptide (NT-proBNP) plasmatic values.

METHOD

One hundred and thirteen subjects (male: 59, 67.8%) with decompensated CHF underwent 5 min of ECG recording, via a mobile phone. We obtained QT end (QTe), QT peak (QTp) and T peak to T end (Te) and calculated the mean, standard deviation (SD), and normalized index (VN).

RESULTS

Death occurred for 27 subjects (24%) within 30 days after admission. Most of the repolarization indexes (QTe mean (p < 0.05), QTe_SD (p < 0.01), QTp_SD (p < 0.05), mean Te (p < 0.05), Te_SD (p < 0.001) QTeVN (p < 0.05) and TeVN (p < 0.01)) were significantly higher in those CHF patients with the highest NT-proBNP (>75th percentile). In all the ECG data, only Te_SD was significantly and positively related to the NT-proBNP levels (r: 0.471; p < 0.001). In the receiver operating characteristic (ROC) analysis, the highest accuracy for 30-day mortality was found for QTe_SD (area under curve, AUC: 0.705, p < 0.01) and mean Te (AUC: 0.680, p < 0.01), whereas for the NT-proBNP values higher than the 75th percentile, the highest accuracy was found for Te_SD (AUC: 0.736, p < 0.001) and QTe_SD (AUC: 0.696, p < 0.01).

CONCLUSION

Both mean Te and Te_SD could be considered as reliable markers of worsening HF and of 30-day mortality. Although larger and possibly interventional studies are needed to confirm our preliminary finding, these non-invasive and transmissible ECG parameters could be helpful in the remote monitoring of advanced HF patients and, possibly, in their clinical management. (ClinicalTrials.gov number, NCT04127162).

Chronic Sympathetic Hyperactivity Triggers Electrophysiological Remodeling and Disrupts Excitation-Contraction Coupling in Heart

The sympathetic nervous system is essential for maintenance of cardiac function via activation of post-junctional adrenergic receptors. Prolonged adrenergic receptor activation, however, has deleterious long-term effects leading to hypertrophy and the development of heart failure. Here we investigate the effect of chronic adrenergic receptors activation on excitation-contraction coupling (ECC) in ventricular cardiomyocytes from a previously characterized mouse model of chronic sympathetic hyperactivity, which are genetically deficient in the adrenoceptor α2A and α2C genes (ARDKO). When compared to wild-type (WT) cardiomyocytes, ARDKO displayed reduced fractional shortening (~33%) and slower relaxation (~20%). Furthermore, ARDKO cells exhibited several electrophysiological changes such as action potential (AP) prolongation (~50%), reduced L-type calcium channel (LCC) current (~33%), reduced outward potassium (K+) currents (~30%), and increased sodium/calcium exchanger (NCX) activity (~52%). Consistent with reduced contractility and calcium (Ca2+) currents, the cytosolic Ca2+ ([Ca2+]i) transient from ARDKO animals was smaller and decayed slower. Importantly, no changes were observed in membrane resting potential, AP amplitude, or the inward K+ current. Finally, we modified our existing cardiac ECC computational model to account for changes in the ARDKO heart. Simulations suggest that cellular changes in the ARDKO heart resulted in variable and dyssynchronous Ca2+-induced Ca2+ release therefore altering [Ca2+]i transient dynamics and reducing force generation. In conclusion, chronic sympathetic hyperactivity impairs ECC by changing the density of several ionic currents (and thus AP repolarization) causing altered Ca2+ dynamics and contractile activity. This demonstrates the important role of ECC remodeling in the cardiac dysfunction secondary to chronic sympathetic activity.

Antidotal effects of methylene blue against cyanide neurological toxicity: in vivo and in vitro studies

The aim of the present study was to determine whether methylene blue (MB) could directly oppose the neurological toxicity of a lethal cyanide (CN) intoxication. KCN, infused at the rate of 0.375 mg/kg/min intravenously, produced 100% lethality within 15 min in unanaesthetized rats (n = 12). MB at 10 (n = 5) or 20 mg/kg (n = 5), administered 3 min into CN infusion, allowed all animals to survive with no sequelae. No apnea and gasping were observed at 20 mg/kg MB (P < 0.001). The onset of coma was also significantly delayed and recovery from coma was shortened in a dose-dependent manner (median of 359 and 737 seconds, respectively, at 20 and 10 mg/kg). At 4 mg/kg MB (n = 5), all animals presented faster onset of coma and apnea and a longer period of recovery than at the highest doses (median 1344 seconds, P < 0.001). MB reversed NaCN-induced resting membrane potential depolarization and action potential depression in primary cultures of human fetal neurons intoxicated with CN. MB restored calcium homeostasis in the CN-intoxicated human SH-SY5Y neuroblastoma cell line. We conclude that MB mitigates the neuronal toxicity of CN in a dose-dependent manner, preventing the lethal depression of respiratory medullary neurons and fatal outcome.

Genetically modified mice to unravel physiological and pathophysiological roles played by NCX isoforms

Since the discovery of the three isoforms of the Na⁺/Ca²⁺ exchanger, NCX1, NCX2 and NCX3 in 1990s, many studies have been devoted to identifying their specific roles in different tissues under several physiological or pathophysiological conditions. In particular, several seminal experimental works laid the foundation for better understanding gene and protein structures, tissue distribution, and regulatory functions of each antiporter isoform. On the other hand, despite the efforts in the development of specific compounds selectively targeting NCX1, NCX2 or NCX3 to test their physiological or pathophysiological roles, several drawbacks hampered the achievement of these goals. In fact, at present no isoform-specific compounds have been yet identified. Moreover, these compounds, despite their potency, possess some nonspecific actions against other ion antiporters, ion channels, and channel receptors. As a result, it is difficult to discriminate direct effects of inhibition/activation of NCX isoforms from the inhibitory or stimulatory effects exerted on other antiporters, channels, receptors, or enzymes. To overcome these difficulties, some research groups used transgenic, knock-out and knock-in mice for NCX isoforms as the most straightforward and fruitful strategy to characterize the biological role exerted by each antiporter isoform. The present review will survey the techniques used to study the roles of NCXs and the current knowledge obtained from these genetic modified mice focusing on the advantages obtained with these strategies in understanding the contribution exerted by each isoform.

Trpm2 enhances physiological bioenergetics and protects against pathological oxidative cardiac injury: Role of Pyk2 phosphorylation

The mechanisms by which Trpm2 channels enhance mitochondrial bioenergetics and protect against oxidative stress-induced cardiac injury remain unclear. Here, the role of proline-rich tyrosine kinase 2 (Pyk2) in Trpm2 signaling is explored. Activation of Trpm2 in adult myocytes with H₂ O₂ resulted in 10- to 21-fold increases in Pyk2 phosphorylation in wild-type (WT) myocytes which was significantly lower (~40%) in Trpm2 knockout (KO) myocytes. Pyk2 phosphorylation was inhibited (~54%) by the Trpm2 blocker clotrimazole. Buffering Trpm2-mediated Ca²⁺ increase with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) resulted in significantly reduced pPyk2 in WT but not in KO myocytes, indicating Ca²⁺ influx through activated Trpm2 channels phosphorylated Pyk2. Part of phosphorylated Pyk2 translocated from cytosol to mitochondria which has been previously shown to augment mitochondrial Ca²⁺ uptake and enhance adenosine triphosphate generation. Although Trpm2-mediated Ca²⁺ influx phosphorylated Ca²⁺ -calmodulin kinase II (CaMKII), the CaMKII inhibitor KN93 did not significantly affect Pyk2 phosphorylation in H₂ O₂ -treated WT myocytes. After ischemia/reperfusion (I/R), Pyk2 phosphorylation and its downstream prosurvival signaling molecules (pERK1/2 and pAkt) were significantly lower in KO-I/R when compared with WT-I/R hearts. After hypoxia/reoxygenation, mitochondrial membrane potential was lower and superoxide level was higher in KO myocytes, and were restored to WT values by the mitochondria-targeted superoxide scavenger MitoTempo. Our results suggested that Ca²⁺ influx via tonically activated Trpm2 phosphorylated Pyk2, part of which translocated to mitochondria, resulting in better mitochondrial bioenergetics to maintain cardiac health. After I/R, Pyk2 activated prosurvival signaling molecules and prevented excessive increases in reactive oxygen species, thereby affording protection from I/R injury.

The central role of protein kinase C epsilon in cyanide cardiotoxicity and its treatment

In adult mouse myocytes, brief exposure to sodium cyanide (CN) in the presence of glucose does not decrease ATP levels, yet produces profound reduction in contractility, intracellular Ca2+ concentration ([Ca2+]i) transient and L-type Ca2+ current (ICa) amplitudes. We analyzed proteomes from myocytes exposed to CN, focusing on ionic currents associated with excitation-contraction coupling. CN induced phosphorylation of α1c subunit of L-type Ca2+ channel and α2 subunit of Na+-K+-ATPase. Methylene blue (MB), a CN antidote that we previously reported to ameliorate CN-induced reduction in contraction, [Ca2+]i transient and ICa amplitudes, was able to reverse this phosphorylation. CN decreased Na+-K+-ATPase current contributed by α2 but not α1 subunit, an effect that was also counteracted by MB. Peptide consensus sequences suggested CN-induced phosphorylation was mediated by protein kinase C epsilon (PKCε). Indeed, CN stimulated PKC kinase activity and induced PKCε membrane translocation, effects that were prevented by MB. Pre-treatment with myristoylated PKCε translocation activator or inhibitor peptides mimicked and inhibited the effects of CN on ICa and myocyte contraction, respectively. We conclude that CN activates PKCε, which phosphorylates L-type Ca2+ channel and Na+-K+-ATPase, resulting in depressed cardiac contractility. We hypothesize that this inhibition of ion fluxes represents a novel mechanism by which the cardiomyocyte reduces its ATP demand (decreased ion fluxes and contractility), diminishes ATP turnover and preserves cell viability. However, this cellular protective effect translates into life-threatening cardiogenic shock in vivo, thereby creating a profound disconnect between survival mechanisms at the cardiomyocyte level from those at the level of the whole organism.

Methylene Blue Counteracts H2S-Induced Cardiac Ion Channel Dysfunction and ATP Reduction

We have previously demonstrated that methylene blue (MB) counteracts the effects of hydrogen sulfide (H₂S) cardiotoxicity by improving cardiomyocyte contractility and intracellular Ca²⁺ homeostasis disrupted by H₂S poisoning. In vivo, MB restores cardiac contractility severely depressed by sulfide and protects against arrhythmias, ranging from bundle branch block to ventricular tachycardia or fibrillation. To dissect the cellular mechanisms by which MB reduces arrhythmogenesis and improves bioenergetics in myocytes intoxicated with H₂S, we evaluated the effects of H₂S on resting membrane potential (E_m), action potential (AP), Na⁺/Ca²⁺ exchange current (I_NaCa), depolarization-activated K⁺ currents and ATP levels in adult mouse cardiac myocytes and determined whether MB could counteract the toxic effects of H₂S on myocyte electrophysiology and ATP. Exposure to toxic concentrations of H₂S (100 µM) significantly depolarized E_m, reduced AP amplitude, prolonged AP duration at 90% repolarization (APD₉₀), suppressed I_NaCa and depolarization-activated K⁺ currents, and reduced ATP levels in adult mouse cardiac myocytes. Treating cardiomyocytes with MB (20 µg/ml) 3 min after H₂S exposure restored E_m, APD₉₀, I_NaCa, depolarization-activated K⁺ currents, and ATP levels toward normal. MB improved mitochondrial membrane potential (∆ψ_m) and oxygen consumption rate in myocytes in which Complex I was blocked by rotenone. We conclude that MB ameliorated H₂S-induced cardiomyocyte toxicity at multiple levels: (1) reversing excitation-contraction coupling defects (Ca²⁺ homeostasis and L-type Ca²⁺ channels); (2) reducing risks of arrhythmias (E_m, APD, I_NaCa and depolarization-activated K⁺ currents); and (3) improving cellular bioenergetics (ATP, ∆ψ_m).

Methylene blue counteracts cyanide cardiotoxicity: cellular mechanisms

In adult left ventricular mouse myocytes, exposure to sodium cyanide (NaCN) in the presence of glucose dose-dependently reduced contraction amplitude, with ~80% of maximal inhibitory effect attained at 100 µM. NaCN (100 µM) exposure for 10 min significantly decreased contraction and intracellular Ca2+ concentration ([Ca2+]i) transient amplitudes, systolic but not diastolic [Ca2+]i, and maximal L-type Ca2+ current ( ICa) amplitude, indicating acute alteration of [Ca2+]i homeostasis largely accounted for the observed excitation-contraction abnormalities. In addition, NaCN depolarized resting membrane potential ( Em), reduced action potential (AP) amplitude, prolonged AP duration at 50% (APD50) and 90% repolarization (APD90), and suppressed depolarization-activated K+ currents but had no effect on Na+-Ca2+ exchange current ( INaCa). NaCN did not affect cellular adenosine triphosphate levels but depolarized mitochondrial membrane potential (ΔΨm) and increased superoxide (O2·-) levels. Methylene blue (MB; 20 µg/ml) added 3 min after NaCN restored contraction and [Ca2+]i transient amplitudes, systolic [Ca2+]i, Em, AP amplitude, APD50, APD90, ICa, depolarization-activated K+ currents, ΔΨm, and O2·- levels toward normal. We conclude that MB reversed NaCN-induced cardiotoxicity by preserving intracellular Ca2+ homeostasis and excitation-contraction coupling ( ICa), minimizing risks of arrhythmias ( Em, AP configuration, and depolarization-activated K+ currents), and reducing O2·- levels. NEW & NOTEWORTHY Cyanide poisoning due to industrial exposure, smoke inhalation, and bioterrorism manifests as cardiogenic shock and requires rapidly effective antidote. In the early stage of cyanide exposure, adenosine triphosphate levels are normal but myocyte contractility is reduced, largely due to alterations in Ca2+ homeostasis because of changes in oxidation-reduction environment of ion channels. Methylene blue, a drug approved by the U.S. Food and Drug Administration, ameliorates cyanide toxicity by normalizing oxidation-reduction state and Ca2+ channel function.

Haplo-insufficiency of Bcl2-associated athanogene 3 in mice results in progressive left ventricular dysfunction, β-adrenergic insensitivity, and increased apoptosis

Bcl2-associated athanogene 3 (BAG3) is a 575 amino acid protein that is found predominantly in the heart, skeletal muscle, and many cancers. Deletions and truncations in BAG3 that result in haplo-insufficiency have been associated with the development of dilated cardiomyopathy. To study the cellular and molecular events attributable to BAG3 haplo-insufficiency we generated a mouse in which one allele of BAG3 was flanked by loxP recombination sites (BAG3^fl/+ ). Mice were crossed with α-MHC-Cre mice in order to generate mice with cardiac-specific haplo-insufficiency (cBAG3^+/-) and underwent bi-weekly echocardiography to assess their cardiac phenotype. By 10 weeks of age, cBAG3^+/- mice demonstrated increased heart size and diminished left ventricular ejection fraction when compared with non-transgenic littermates (Cre^-/- BAG3^fl/+ ). Contractility in adult myocytes isolated from cBAG3^+/- mice were similar to those isolated from control mice at baseline, but showed a significantly decreased response to adrenergic stimulation. Intracellular calcium ([Ca²⁺ ]_i ) transient amplitudes in myocytes isolated from cBAG3^+/- mice were also similar to myocytes isolated from control mice at baseline but were significantly lower than myocytes from control mice in their response to isoproterenol. BAG3 haplo-insufficiency was also associated with decreased autophagy flux and increased apoptosis. Taken together, these results suggest that mice in which BAG3 has been deleted from a single allele provide a model that mirrors the biology seen in patients with heart failure and BAG3 haplo-insufficiency.

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.

MG53 is dispensable for T-tubule maturation but critical for maintaining T-tubule integrity following cardiac stress

The cardiac transverse (T)-tubule membrane system is the safeguard for cardiac function and undergoes dramatic remodeling in response to cardiac stress. However, the mechanism by which cardiomyocytes repair damaged T-tubule network remains unclear. In the present study, we tested the hypothesis that MG53, a muscle-specific membrane repair protein, antagonizes T-tubule damage to protect against maladaptive remodeling and thereby loss of excitation-contraction coupling and cardiac function. Using MG53-knockout (MG53-KO) mice, we first established that deficiency of MG53 had no impact on maturation of the T-tubule network in developing hearts. Additionally, MG53 ablation did not influence T-tubule integrity in unstressed adult hearts as late as 10months of age. Following left ventricular pressure overload-induced cardiac stress, MG53 protein levels were increased by approximately three-fold in wild-type mice, indicating that pathological stress induces a significant upregulation of MG53. MG53-deficient mice had worsened T-tubule disruption and pronounced dysregulation of Ca²⁺ handling properties, including decreased Ca²⁺ transient amplitude and prolonged time to peak and decay. Moreover, MG53 deficiency exacerbated cardiac hypertrophy and dysfunction and decreased survival following cardiac stress. Our data suggest MG53 is not required for T-tubule development and maintenance in normal physiology. However, MG53 is essential to preserve T-tubule integrity and thereby Ca²⁺ handling properties and cardiac function under pathological cardiac stress.

Adeno-associated Virus Serotype 9 - Driven Expression of BAG3 Improves Left Ventricular Function in Murine Hearts with Left Ventricular Dysfunction Secondary to a Myocardial Infarction

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BAG3 is a highly conserved protein having pleiotropic effects that is expressed at high levels in the heart, skeletal muscles, and many cancers.

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BAG3 levels are reduced in many forms of LV dysfunction including mice after ligation of the left coronary artery.

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Retro-orbital injection of mice with an adeno-associated virus coupled to murine BAG3 under the control of a CMV promoter (rAAV9-BAG3) increased myocardial levels of BAG3 by 7 days post-injection.

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Retro-orbital injection of rAAV9-BAG3 in mice post-myocardial infarction improved LV function, whereas rAAV9-BAG3 had no effect on LV function in the absence of an MI.

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BAG3 may prove to be a new therapeutic target in the treatment of heart failure.

Mutations in Bcl-2–associated athanogene 3 (BAG3) were associated with skeletal muscle dysfunction and dilated cardiomyopathy. Retro-orbital injection of an adeno-associated virus serotype 9 expressing BAG3 (rAAV9-BAG3) significantly (p < 0.0001) improved left ventricular ejection fraction, fractional shortening, and stroke volume 9 days post-injection in mice with cardiac dysfunction secondary to a myocardial infarction. Furthermore, myocytes isolated from mice 3 weeks after injection showed improved cell shortening, enhanced systolic [Ca²⁺]_i and increased [Ca²⁺]_i transient amplitudes, and increased maximal L-type Ca²⁺ current amplitude. These results suggest that BAG3 gene therapy may provide a novel therapeutic option for the treatment of heart failure.

Induced NCX1 overexpression attenuates pressure overload-induced pathological cardiac remodelling

AIMS

Although increased Na(+)/Ca(2+) exchanger 1 (NCX1) expression is observed during heart failure (HF), the pathological role of NCX1 during the progression of HF remains unclear. We examined alterations of NCX1 expression and activity in hearts after transverse aortic constriction (TAC) surgery and explored whether NCX1 influences pressure overload-induced pathological cardiac remodelling.

METHODS AND RESULTS

We generated novel transgenic mice in which NCX1 expression is controlled by a cardiac-specific, doxycycline (DOX)-dependent promoter. In the absence of DOX, TAC surgery caused substantial chamber dilation with a gradual decrease in contractility by 16 weeks. Cardiomyocytes showed a decline in contractility with abnormal Ca(2+) handling during excitation-contraction (E-C) coupling. Reduced NCX1 activity was observed 8 weeks after TAC and was still apparent at 17 weeks. Induced NCX1 overexpression by DOX treatment starting 8 weeks after TAC returned NCX1 activity to pre-TAC levels and prevented chamber dilation with cardiac dysfunction. DOX treatment not only upregulated NCX1 expression in TAC-operated hearts but also returned L-type Ca(2+) channel and sarcoplasmic reticulum (SR) Ca(2+) ATPase expression levels to those in sham-operated hearts. In DOX-treated myocytes, contractility, T-tubule integrity, synchrony of Ca(2+) release from the SR, and Ca(2+) handling during E-C coupling was preserved 16 weeks after TAC surgery. In addition, DOX treatment attenuated the down-regulation of survival signalling and up-regulation of apoptosis signalling 16 weeks after TAC surgery.

CONCLUSION

Induced overexpression of NCX1 attenuated pressure overload-induced pathological cardiac remodelling. Thus, maintaining NCX1 activity may be a potential therapeutic strategy for preventing the progression of HF.

Methylene blue counteracts H2S toxicity-induced cardiac depression by restoring L-type Ca channel activity

We have previously reported that methylene blue (MB) can counteract hydrogen sulfide (H2S) intoxication-induced circulatory failure. Because of the multifarious effects of high concentrations of H2S on cardiac function, as well as the numerous properties of MB, the nature of this interaction, if any, remains uncertain. The aim of this study was to clarify 1) the effects of MB on H2S-induced cardiac toxicity and 2) whether L-type Ca(2+) channels, one of the targets of H2S, could transduce some of the counteracting effects of MB. In sedated rats, H2S infused at a rate that would be lethal within 5 min (24 μM·kg(-1)·min(-1)), produced a rapid fall in left ventricle ejection fraction, determined by echocardiography, leading to a pulseless electrical activity. Blood concentrations of gaseous H2S reached 7.09 ± 3.53 μM when cardiac contractility started to decrease. Two to three injections of MB (4 mg/kg) transiently restored cardiac contractility, blood pressure, and V̇o2, allowing the animals to stay alive until the end of H2S infusion. MB also delayed PEA by several minutes following H2S-induced coma and shock in unsedated rats. Applying a solution containing lethal levels of H2S (100 μM) on isolated mouse cardiomyocytes significantly reduced cell contractility, intracellular calcium concentration ([Ca(2+)]i) transient amplitudes, and L-type Ca(2+) currents (ICa) within 3 min of exposure. MB (20 mg/l) restored the cardiomyocyte function, ([Ca(2+)]i) transient, and ICa The present results offer a new approach for counteracting H2S toxicity and potentially other conditions associated with acute inhibition of L-type Ca(2+) channels.

BAG3 regulates contractility and Ca(2+) homeostasis in adult mouse ventricular myocytes

Bcl2-associated athanogene 3 (BAG3) is a 575 amino acid anti-apoptotic protein that is constitutively expressed in the heart. BAG3 mutations, including mutations leading to loss of protein, are associated with familial cardiomyopathy. Furthermore, BAG3 levels have been found to be reduced in end-stage non-familial failing myocardium. In contrast to neonatal myocytes in which BAG3 is found in the cytoplasm and involved in protein quality control and apoptosis, in adult mouse left ventricular (LV) myocytes BAG3 co-localized with Na(+)-K(+)-ATPase and L-type Ca(2+) channels in the sarcolemma and t-tubules. BAG3 co-immunoprecipitated with β1-adrenergic receptor, L-type Ca(2+) channels and phospholemman. To simulate decreased BAG3 protein levels observed in human heart failure, we targeted BAG3 by shRNA (shBAG3) in adult LV myocytes. Reducing BAG3 by 55% resulted in reduced contraction and [Ca(2+)]i transient amplitudes in LV myocytes stimulated with isoproterenol. L-type Ca(2+) current (ICa) and sarcoplasmic reticulum (SR) Ca(2+) content but not Na(+)/Ca(2+) exchange current (INaCa) or SR Ca(2+) uptake were reduced in isoproterenol-treated shBAG3 myocytes. Forskolin or dibutyryl cAMP restored ICa amplitude in shBAG3 myocytes to that observed in WT myocytes, consistent with BAG3 having effects upstream and at the level of the receptor. Resting membrane potential and action potential amplitude were unaffected but APD50 and APD90 were prolonged in shBAG3 myocytes. Protein levels of Ca(2+) entry molecules and other important excitation-contraction proteins were unchanged in myocytes with lower BAG3. Our findings that BAG3 is localized at the sarcolemma and t-tubules while modulating myocyte contraction and action potential duration through specific interaction with the β1-adrenergic receptor and L-type Ca(2+) channel provide novel insight into the role of BAG3 in cardiomyopathies and increased arrhythmia risks in heart failure.

Cardiac Dysfunction in HIV-1 Transgenic Mouse: Role of Stress and BAG3

Since highly active antiretroviral therapy improved long-term survival of acquired immunodeficiency syndrome (AIDS) patients, AIDS cardiomyopathy has become an increasingly relevant clinical problem. We used human immunodeficiency virus (HIV)-1 transgenic (Tg26) mouse to explore molecular mechanisms of AIDS cardiomyopathy. Tg26 mice had significantly lower left ventricular (LV) mass and smaller end-diastolic and end-systolic LV volumes. Under basal conditions, cardiac contractility and relaxation and single myocyte contraction dynamics were not different between wild-type (WT) and Tg26 mice. Ten days after open heart surgery, contractility and relaxation remained significantly depressed in Tg26 hearts, suggesting that Tg26 mice did not tolerate surgical stress well. To simulate heart failure in which expression of Bcl2-associated athanogene 3 (BAG3) is reduced, we down-regulated BAG3 by small hairpin ribonucleic acid in WT and Tg26 hearts. BAG3 down-regulation significantly reduced contractility in Tg26 hearts. BAG3 overexpression rescued contractile abnormalities in myocytes expressing the HIV-1 protein Tat. We conclude: (i) Tg26 mice exhibit normal contractile function at baseline; (ii) Tg26 mice do not tolerate surgical stress well; (iii) BAG3 down-regulation exacerbated cardiac dysfunction in Tg26 mice; (iv) BAG3 overexpression rescued contractile abnormalities in myocytes expressing HIV-1 protein Tat; and (v) BAG3 may occupy a role in pathogenesis of AIDS cardiomyopathy.

Regulation of L-type calcium channel by phospholemman in cardiac myocytes

We evaluated whether phospholemman (PLM) regulates L-type Ca(2+) current (ICa) in mouse ventricular myocytes. Expression of α1-subunit of L-type Ca(2+) channels between wild-type (WT) and PLM knockout (KO) hearts was similar. Compared to WT myocytes, peak ICa (at -10 mV) from KO myocytes was ~41% larger, the inactivation time constant (τ(inact)) of ICa was ~39% longer, but deactivation time constant (τ(deact)) was similar. In the presence of isoproterenol (1 μM), peak ICa was ~48% larger and τ(inact) was ~144% higher in KO myocytes. With Ba(2+) as the permeant ion, PLM enhanced voltage-dependent inactivation but had no effect on τ(deact). To dissect the molecular determinants by which PLM regulated ICa, we expressed PLM mutants by adenovirus-mediated gene transfer in cultured KO myocytes. After 24h in culture, KO myocytes expressing green fluorescent protein (GFP) had significantly larger peak ICa and longer τ(inact) than KO myocytes expressing WT PLM; thereby independently confirming the observations in freshly isolated myocytes. Compared to KO myocytes expressing GFP, KO myocytes expressing the cytoplasmic domain truncation mutant (TM43), the non-phosphorylatable S68A mutant, the phosphomimetic S68E mutant, and the signature PFXYD to alanine (ALL5) mutant all resulted in lower peak ICa. Expressing PLM mutants did not alter expression of α1-subunit of L-type Ca(2+) channels in cultured KO myocytes. Our results suggested that both the extracellular PFXYD motif and the transmembrane domain of PLM but not the cytoplasmic tail were necessary for regulation of peak ICa amplitude. We conclude that PLM limits Ca(2+) influx in cardiac myocytes by reducing maximal ICa and accelerating voltage-dependent inactivation.

Overexpression of junctophilin-2 does not enhance baseline function but attenuates heart failure development after cardiac stress

Heart failure is accompanied by a loss of the orderly disposition of transverse (T)-tubules and a decrease of their associations with the junctional sarcoplasmic reticulum (jSR). Junctophilin-2 (JP2) is a structural protein responsible for jSR/T-tubule docking. Animal models of cardiac stresses demonstrate that down-regulation of JP2 contributes to T-tubule disorganization, loss of excitation-contraction coupling, and heart failure development. Our objective was to determine whether JP2 overexpression attenuates stress-induced T-tubule disorganization and protects against heart failure progression. We therefore generated transgenic mice with cardiac-specific JP2 overexpression (JP2-OE). Baseline cardiac function and Ca(2+) handling properties were similar between JP2-OE and control mice. However, JP2-OE mice displayed a significant increase in the junctional coupling area between T-tubules and the SR and an elevated expression of the Na(+)/Ca(2+) exchanger, although other excitation-contraction coupling protein levels were not significantly changed. Despite similar cardiac function at baseline, overexpression of JP2 provided significantly protective benefits after pressure overload. This was accompanied by a decreased percentage of surviving mice that developed heart failure, as well as preservation of T-tubule network integrity in both the left and right ventricles. Taken together, these data suggest that strategies to maintain JP2 levels can prevent the progression from hypertrophy to heart failure.

Intracellular calcium and the mechanism of the dip in the anodal strength-interval curve in cardiac tissue

BACKGROUND

The strength-interval (SI) curve is an important measure of refractoriness in cardiac tissue. The anodal SI curve contains a "dip" in which the S2 threshold increases with interval. Two explanations exist for this dip: (1) electrotonic interaction between regions of depolarization and hyperpolarization; and (2) the sodium-calcium exchange (NCX) current. The goal of this study is to use mathematical modeling to determine which explanation is correct.

METHODS AND RESULTS

The bidomain model represents cardiac tissue and the Luo-Rudy model describes the active membrane. The SI curve is determined by applying a threshold stimulus at different time intervals after a previous action potential. During space-clamped and equal-anisotropy-ratios simulations, anodal excitation does not occur. During unequal-anisotropy-ratios simulations, electrotonic currents, not membrane currents, are present during the few milliseconds before excitation. The dip disappears with no NCX current, but is present with 50% or 75% reduction of it. The calcium-induced-calcium-release (CICR) current has little effect on the dip.

CONCLUSIONS

These results indicate that neither the NCX nor the CICR current is responsible for the dip in the anodal SI curve. It is caused by the electrotonic interaction between regions of depolarization and hyperpolarization following the S2 stimulus.

TRPM2 channels protect against cardiac ischemia-reperfusion injury: role of mitochondria

Cardiac TRPM2 channels were activated by intracellular adenosine diphosphate-ribose and blocked by flufenamic acid. In adult cardiac myocytes the ratio of GCa to GNa of TRPM2 channels was 0.56 ± 0.02. To explore the cellular mechanisms by which TRPM2 channels protect against cardiac ischemia/reperfusion (I/R) injury, we analyzed proteomes from WT and TRPM2 KO hearts subjected to I/R. The canonical pathways that exhibited the largest difference between WT-I/R and KO-I/R hearts were mitochondrial dysfunction and the tricarboxylic acid cycle. Complexes I, III, and IV were down-regulated, whereas complexes II and V were up-regulated in KO-I/R compared with WT-I/R hearts. Western blots confirmed reduced expression of the Complex I subunit and other mitochondria-associated proteins in KO-I/R hearts. Bioenergetic analyses revealed that KO myocytes had a lower mitochondrial membrane potential, mitochondrial Ca(2+) uptake, ATP levels, and O2 consumption but higher mitochondrial superoxide levels. Additionally, mitochondrial Ca(2+) uniporter (MCU) currents were lower in KO myocytes, indicating reduced mitochondrial Ca(2+) uptake was likely due to both lower ψm and MCU activity. Similar to isolated myocytes, O2 consumption and ATP levels were also reduced in KO hearts. Under a simulated I/R model, aberrant mitochondrial bioenergetics was exacerbated in KO myocytes. Reactive oxygen species levels were also significantly higher in KO-I/R compared with WT-I/R heart slices, consistent with mitochondrial dysfunction in KO-I/R hearts. We conclude that TRPM2 channels protect the heart from I/R injury by ameliorating mitochondrial dysfunction and reducing reactive oxygen species levels.

Induced overexpression of phospholemman S68E mutant improves cardiac contractility and mortality after ischemia-reperfusion

Phospholemman (PLM), when phosphorylated at Ser(68), inhibits cardiac Na+ / Ca2+ exchanger 1 (NCX1) and relieves its inhibition on Na+ -K+ -ATPase. We have engineered mice in which expression of the phosphomimetic PLM S68E mutant was induced when dietary doxycycline was removed at 5 wk. At 8-10 wk, compared with noninduced or wild-type hearts, S68E expression in induced hearts was ∼35-75% that of endogenous PLM, but protein levels of sarco(endo)plasmic reticulum Ca2+ -ATPase, α1- and α2-subunits of Na+ -K+ -ATPase, α1c-subunit of L-type Ca2+ channel, and phosphorylated ryanodine receptor were unchanged. The NCX1 protein level was increased by ∼47% but the NCX1 current was depressed by ∼34% in induced hearts. Isoproterenol had no effect on NCX1 currents but stimulated Na+ -K+ -ATPase currents equally in induced and noninduced myocytes. At baseline, systolic intracellular Ca2+ concentrations ([Ca2+]i), sarcoplasmic reticulum Ca2+ contents, and [Ca(2+)]i transient and contraction amplitudes were similar between induced and noninduced myocytes. Isoproterenol stimulation resulted in much higher systolic [Ca2+]i, sarcoplasmic reticulum Ca2+ content, and [Ca2+]i transient and contraction amplitudes in induced myocytes. Echocardiography and in vivo close-chest catheterization demonstrated similar baseline myocardial function, but isoproterenol induced a significantly higher +dP/dt in induced compared with noninduced hearts. In contrast to the 50% mortality observed in mice constitutively overexpressing the S68E mutant, induced mice had similar survival as wild-type and noninduced mice. After ischemia-reperfusion, despite similar areas at risk and left ventricular infarct sizes, induced mice had significantly higher +dP/dt and -dP/dt and lower perioperative mortality compared with noninduced mice. We propose that phosphorylated PLM may be a novel therapeutic target in ischemic heart disease.

Na/Ca exchange and contraction of the heart

Sodium-calcium exchange (NCX) is the major calcium (Ca) efflux mechanism of ventricular cardiomyocytes. Consequently the exchanger plays a critical role in the regulation of cellular Ca content and hence contractility. Reductions in Ca efflux by the exchanger, such as those produced by elevated intracellular sodium (Na) in response to cardiac glycosides, raise sarcoplasmic reticulum (SR) Ca stores. The result is an increased Ca transient and cardiac contractility. Enhanced Ca efflux activity by the exchanger, for example during heart failure, may reduce diadic cleft Ca and excitation-contraction (EC) coupling gain. This aggravates the impaired contractility associated with SR Ca ATPase dysfunction and reduced SR Ca load in failing heart muscle. Recent data from our laboratories indicate that NCX can also impact the efficiency of EC coupling and contractility independent of SR Ca load through diadic cleft priming with Ca during the upstroke of the action potential. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Induced overexpression of Na(+)/Ca(2+) exchanger does not aggravate myocardial dysfunction induced by transverse aortic constriction

BACKGROUND

Alterations in expression and activity of cardiac Na(+)/Ca(2+) exchanger (NCX1) have been implicated in the pathogenesis of heart failure.

METHODS AND RESULTS

Using transgenic mice in which expression of rat NCX1 was induced at 5 weeks of age, we performed transverse aortic constriction (TAC) at 8 weeks and examined cardiac and myocyte function at 15-18 weeks after TAC (age 23-26 weeks). TAC induced left ventricular (LV) and myocyte hypertrophy and increased myocardial fibrosis in both wild-type (WT) and NCX1-overexpressed mice. NCX1 and phosphorylated ryanodine receptor expression was increased by TAC, whereas sarco(endo)plasmic reticulum Ca(2+)-ATPase levels were decreased by TAC. Action potential duration was prolonged by TAC, but to a greater extent in NCX1 myocytes. Na(+)/Ca(2+) exchange current was similar between WT-TAC and WT-sham myocytes, but was higher in NCX1-TAC myocytes. Both myocyte contraction and [Ca(2+)](i) transient amplitudes were reduced in WT-TAC myocytes, but restored to WT-sham levels in NCX1-TAC myocytes. Despite improvement in single myocyte contractility and Ca(2+) dynamics, induced NCX1 overexpression in TAC animals did not ameliorate LV hypertrophy, increase ejection fraction, or enhance inotropic (maximal first derivative of LV pressure rise, +dP/dt) responses to isoproterenol.

CONCLUSIONS

In pressure-overload hypertrophy, induced overexpression of NCX1 corrected myocyte contractile and [Ca(2+)](i) transient abnormalities but did not aggravate or improve myocardial dysfunction.

Overactive bladder mediated by accelerated Ca2+ influx mode of Na+/Ca2+ exchanger in smooth muscle

The Na(+)/Ca(2+) exchanger (NCX) is thought to be a key molecule in the regulation of cytosolic Ca(2+) dynamics. The relative importance of the two Ca(2+) transport modes of NCX activity leading to Ca(2+) efflux (forward) and influx (reverse) in smooth muscle, however, remains unclear. Unexpectedly, spontaneous contractions of urinary bladder smooth muscle (UBSM) were enhanced in transgenic mice overexpressing NCX1.3 (NCX1.3(tg/tg)). The enhanced activity was attenuated by KB-R7943 or SN-6. Whole cell outward NCX current sensitive to KB-R7943 or Ni(2+) was readily detected in UBSM cells from NCX1.3(tg/tg) but not wild-type mice. Spontaneous Ca(2+) transients in myocytes of NCX1.3(tg/tg) were larger and frequently resulted in propagating events and global elevations in cytosolic Ca(2+) concentration. Significantly, NCX1.3(tg/tg) mice exhibited a pattern of more frequent urination of smaller volumes and this phenotype was reversed by oral administration of KB-R7943. On the other hand, KB-R7943 did not improve it in KB-R7943-insensitive (G833C-)NCX1.3(tg/tg) mice. We conclude that NCX1.3 overexpression is associated with abnormal urination owing to enhanced Ca(2+) influx via reverse mode NCX leading to prolonged, propagating spontaneous Ca(2+) release events and a potentiation of spontaneous UBSM contraction. These findings suggest the possibility that NCX is a candidate molecular target for overactive bladder therapy.

The second member of transient receptor potential-melastatin channel family protects hearts from ischemia-reperfusion injury

The second member of the transient receptor potential-melastatin channel family (TRPM2) is expressed in the heart and vasculature. TRPM2 channels were expressed in the sarcolemma and transverse tubules of adult left ventricular (LV) myocytes. Cardiac TRPM2 channels were functional since activation with H2O2 resulted in Ca(2+) influx that was dependent on extracellular Ca(2+), was significantly higher in wild-type (WT) myocytes compared with TRPM2 knockout (KO) myocytes, and inhibited by clotrimazole in WT myocytes. At rest, there were no differences in LV mass, heart rate, fractional shortening, and +dP/dt between WT and KO hearts. At 2-3 days after ischemia-reperfusion (I/R), despite similar areas at risk and infarct sizes, KO hearts had lower fractional shortening and +dP/dt compared with WT hearts. Compared with WT I/R myocytes, expression of the Na(+)/Ca(2+) exchanger (NCX1) and NCX1 current were increased, expression of the α1-subunit of Na(+)-K(+)-ATPase and Na(+) pump current were decreased, and action potential duration was prolonged in KO I/R myocytes. Post-I/R, intracellular Ca(2+) concentration transients and contraction amplitudes were equally depressed in WT and KO myocytes. After 2 h of hypoxia followed by 30 min of reoxygenation, levels of ROS were significantly higher in KO compared with WT LV myocytes. Compared with WT I/R hearts, oxygen radical scavenging enzymes (SODs) and their upstream regulators (forkhead box transcription factors and hypoxia-inducible factor) were lower, whereas NADPH oxidase was higher, in KO I/R hearts. We conclude that TRPM2 channels protected hearts from I/R injury by decreasing generation and enhancing scavenging of ROS, thereby reducing I/R-induced oxidative stress.

Role of the Na(+)/Ca(2+) exchanger on the development of diabetes mellitus and its chronic complications

Diabetes mellitus (DM) is a serious metabolic disorder with micro- and macrovascular complications that results in significant morbidity and mortality. It is well established that cytosolic Ca(2+) play an important role in controlling insulin secretion in pancreatic β-cells. The Na(+)/Ca(2+) exchanger (NCX), an ion transport protein, is expressed in the plasma membrane of virtually all animal cells. NCX is a reversible carrier that can mediate the transport of Ca(2+) across the plasma membrane in both directions. Therefore, great efforts have been made to identify NCX associated with DM. NCX is expressed in several tissues, and acts in the protection against intracellular calcium overload; in the regulation of insulin secretion by beta cells, and in improving vascular endothelium-dependent relaxation. All these mechanisms are associated with DM pathogenesis and its chronic complications. Therefore, NCX is a candidate protein for the development of these disorders. Only a few studies investigated NCX in relation to chronic complications of diabetes, with inconclusive results.

Na+/Ca2+ exchanger-1 protects against systolic failure in the Akitains2 model of diabetic cardiomyopathy via a CXCR4/NF-κB pathway

Diabetic cardiomyopathy is characterized, in part, by calcium handling imbalances associated with ventricular dysfunction. The cardiac Na(+)/Ca(2+) exchanger 1 (NCX1) has been implicated as a compensatory mechanism in response to reduced contractility in the heart; however, its role in diabetic cardiomyopathy remains unknown. We aimed to fully characterize the Akita(ins2) murine model of type 1 diabetes through assessing cardiac function and NCX1 regulation. The CXCL12/CXCR4 chemokine axis is well described in its cardioprotective effects via progenitor cell recruitment postacute myocardial infarction; however, it also functions in regulating calcium dependent processes in the cardiac myocyte. We therefore investigated the potential impact of CXCR4 in diabetic cardiomyopathy. Cardiac performance in the Akita(ins2) mouse was monitored using echocardiography and in vivo pressure-volume analysis. The Akita(ins2) mouse is protected against ventricular systolic failure evident at both 5 and 12 mo of age. However, the preserved contractility was associated with a decreased sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a)/phospholamban ratio and increased NCX1 content. Direct myocardial injection of adenovirus encoding anti-sense NCX1 significantly decreased NCX1 expression and induced systolic failure in the Akita(ins2) mouse. CXCL12 and CXCR4 were both upregulated in the Akita(ins2) heart, along with an increase in IκB-α and NF-κB p65 phosphorylation. We demonstrated that CXCR4 activation upregulates NCX1 expression through a NF-κB-dependent signaling pathway in the cardiac myocyte. In conclusion, the Akita(ins2) type 1 diabetic model is protected against systolic failure due to increased NCX1 expression. In addition, our studies reveal a novel role of CXCR4 in the diabetic heart by regulating NCX1 expression via a NF-κB-dependent mechanism.

MicroRNA-214 protects the mouse heart from ischemic injury by controlling Ca²⁺ overload and cell death

Early reperfusion of ischemic cardiac tissue remains the most effective intervention for improving clinical outcome following myocardial infarction. However, abnormal increases in intracellular Ca²⁺ during myocardial reperfusion can cause cardiomyocyte death and consequent loss of cardiac function, referred to as ischemia/reperfusion (IR) injury. Therapeutic modulation of Ca²⁺ handling provides some cardioprotection against the paradoxical effects of restoring blood flow to the heart, highlighting the significance of Ca²⁺ overload to IR injury. Cardiac IR is also accompanied by dynamic changes in the expression of microRNAs (miRNAs); for example, miR-214 is upregulated during ischemic injury and heart failure, but its potential role in these processes is unknown. Here, we show that genetic deletion of miR-214 in mice causes loss of cardiac contractility, increased apoptosis, and excessive fibrosis in response to IR injury. The cardioprotective roles of miR-214 during IR injury were attributed to repression of the mRNA encoding sodium/calcium exchanger 1 (Ncx1), a key regulator of Ca²⁺ influx; and to repression of several downstream effectors of Ca²⁺ signaling that mediate cell death. These findings reveal a pivotal role for miR-214 as a regulator of cardiomyocyte Ca²⁺ homeostasis and survival during cardiac injury.

Constitutive overexpression of phosphomimetic phospholemman S68E mutant results in arrhythmias, early mortality, and heart failure: potential involvement of Na+/Ca2+ exchanger

Expression and activity of cardiac Na(+)/Ca(2+) exchanger (NCX1) are altered in many disease states. We engineered mice in which the phosphomimetic phospholemman S68E mutant (inhibits NCX1 but not Na(+)-K(+)-ATPase) was constitutively overexpressed in a cardiac-specific manner (conS68E). At 4-6 wk, conS68E mice exhibited severe bradycardia, ventricular arrhythmias, increased left ventricular (LV) mass, decreased cardiac output (CO), and ∼50% mortality compared with wild-type (WT) littermates. Protein levels of NCX1, calsequestrin, ryanodine receptor, and α(1)- and α(2)-subunits of Na(+)-K(+)-ATPase were similar, but sarco(endo)plasmic reticulum Ca(2+)-ATPase was lower, whereas L-type Ca(2+) channels were higher in conS68E hearts. Resting membrane potential and action potential amplitude were similar, but action potential duration was dramatically prolonged in conS68E myocytes. Diastolic intracellular Ca(2+) ([Ca(2+)](i)) was higher, [Ca(2+)](i) transient and maximal contraction amplitudes were lower, and half-time of [Ca(2+)](i) transient decline was longer in conS68E myocytes. Intracellular Na(+) reached maximum within 3 min after isoproterenol addition, followed by decline in WT but not in conS68E myocytes. Na(+)/Ca(2+) exchange, L-type Ca(2+), Na(+)-K(+)-ATPase, and depolarization-activated K(+) currents were decreased in conS68E myocytes. At 22 wk, bradycardia and increased LV mass persisted in conS68E survivors. Despite comparable baseline CO, conS68E survivors at 22 wk exhibited decreased chronotropic, inotropic, and lusitropic responses to isoproterenol. We conclude that constitutive overexpression of S68E mutant was detrimental, both in terms of depressed cardiac function and increased arrhythmogenesis.

Regulation of in vivo cardiac contractility by phospholemman: role of Na+/Ca2+ exchange

Phospholemman (PLM), when phosphorylated at serine 68, relieves its inhibition on Na(+)-K(+)-ATPase but inhibits Na(+)/Ca(2+) exchanger 1 (NCX1) in cardiac myocytes. Under stress when catecholamine levels are high, enhanced Na(+)-K(+)-ATPase activity by phosphorylated PLM attenuates intracellular Na(+) concentration ([Na(+)](i)) overload. To evaluate the effects of PLM on NCX1 on in vivo cardiac contractility, we injected recombinant adeno-associated virus (serotype 9) expressing either the phosphomimetic PLM S68E mutant or green fluorescent protein (GFP) directly into left ventricles (LVs) of PLM-knockout (KO) mice. Five weeks after virus injection, ∼40% of isolated LV myocytes exhibited GFP fluorescence. Expression of S68E mutant was confirmed with PLM antibody. There were no differences in protein levels of α(1)- and α(2)-subunits of Na(+)-K(+)-ATPase, NCX1, and sarco(endo)plasmic reticulum Ca(2+)-ATPase between KO-GFP and KO-S68E LV homogenates. Compared with KO-GFP myocytes, Na(+)/Ca(2+) exchange current was suppressed, but resting [Na(+)](i), Na(+)-K(+)-ATPase current, and action potential amplitudes were similar in KO-S68E myocytes. Resting membrane potential was slightly lower and action potential duration at 90% repolarization (APD(90)) was shortened in KO-S68E myocytes. Isoproterenol (Iso; 1 μM) increased APD(90) in both groups of myocytes. After Iso, [Na(+)](i) increased monotonically in paced (2 Hz) KO-GFP but reached a plateau in KO-S68E myocytes. Both systolic and diastolic [Ca(2+)](i) were higher in Iso-stimulated KO-S68E myocytes paced at 2 Hz. Echocardiography demonstrated similar resting heart rate, ejection fraction, and LV mass between KO-GFP and KO-S68E mice. In vivo closed-chest catheterization demonstrated enhanced contractility in KO-S68E compared with KO-GFP hearts stimulated with Iso. We conclude that under catecholamine stress when [Na(+)](i) is high, PLM minimizes [Na(+)](i) overload by relieving its inhibition of Na(+)-K(+)-ATPase and preserves inotropy by simultaneously inhibiting Na(+)/Ca(2+) exchanger.

Phospholemman and beta-adrenergic stimulation in the heart

Phosphorylation at serine 68 of phospholemman (PLM) in response to beta-adrenergic stimulation results in simultaneous inhibition of cardiac Na(+)/Ca(2+) exchanger NCX1 and relief of inhibition of Na(+)-K(+)-ATPase. The role of PLM in mediating beta-adrenergic effects on in vivo cardiac function was investigated with congenic PLM-knockout (KO) mice. Echocardiography showed similar ejection fraction between wild-type (WT) and PLM-KO hearts. Cardiac catheterization demonstrated higher baseline contractility (+dP/dt) but similar relaxation (-dP/dt) in PLM-KO mice. In response to isoproterenol (Iso), maximal +dP/dt was similar but maximal -dP/dt was reduced in PLM-KO mice. Dose-response curves to Iso (0.5-25 ng) for WT and PLM-KO hearts were superimposable. Maximal +dP/dt was reached 1-2 min after Iso addition and declined with time in WT but not PLM-KO hearts. In isolated myocytes paced at 2 Hz. contraction and intracellular Ca(2+) concentration ([Ca(2+)](i)) transient amplitudes and [Na(+)](i) reached maximum 2-4 min after Iso addition, followed by decline in WT but not PLM-KO myocytes. Reducing pacing frequency to 0.5 Hz resulted in much smaller increases in [Na(+)](i) and no decline in contraction and [Ca(2+)](i) transient amplitudes with time in Iso-stimulated WT and PLM-KO myocytes. Although baseline Na(+)-K(+)-ATPase current was 41% higher in PLM-KO myocytes because of increased alpha(1)- but not alpha(2)-subunit activity, resting [Na(+)](i) was similar between quiescent WT and PLM-KO myocytes. Iso increased alpha(1)-subunit current (I(alpha1)) by 73% in WT but had no effect in PLM-KO myocytes. Iso did not affect alpha(2)-subunit current (I(alpha2)) in WT and PLM-KO myocytes. In both WT and NCX1-KO hearts, PLM coimmunoprecipitated with Na(+)-K(+)-ATPase alpha(1)- and alpha(2)-subunits, indicating that association of PLM with Na(+)-K(+)-ATPase did not require NCX1. We conclude that under stressful conditions in which [Na(+)](i) was high, beta-adrenergic agonist-mediated phosphorylation of PLM resulted in time-dependent reduction in inotropy due to relief of inhibition of Na(+)-K(+)-ATPase.