Myocardial function with reduced expression of the sodium-calcium exchanger

Increased in vivo perpetuation of whole-heart ventricular arrhythmia in heterozygous Na+/Ca2+ exchanger knockout mice

Aims

Na⁺/Ca²⁺ exchanger (NCX) upregulation in cardiac diseases like heart failure promotes as an independent proarrhythmic factor early and delayed afterdepolarizations (EADs/DADs) on the single cell level. Consequently, NCX inhibition protects against EADs and DADs in isolated cardiomyocytes. We here investigate, whether these promising cellular in vitro findings likewise apply to an in vivo setup.

Methods/Results

Programmed ventricular stimulation (PVS) and isoproterenol were applied to a murine heterozygous NCX-knockout model (KO) to investigate ventricular arrhythmia initiation and perpetuation compared to wild-type (WT). KO displayed a reduced susceptibility towards isoproterenol-induced premature ventricular complexes. During PVS, initiation of single or double ectopic beats was similar between KO and WT. But strikingly, perpetuation of ventricular tachycardia (VT) was significantly increased in KO (animals with VT - KO: 82 %; WT: 47 %; p = 0.0122 / median number of VTs - KO: 4.5 (1.0, 6.25); WT: 0.0 (0.0, 4.0); p = 0.0039). The median VT duration was prolonged in KO (in s; KO: 0.38 (0.19, 0.96); WT: 0.0 (0.0, 0.60); p = 0.0239). The ventricular refractory period (VRP) was shortened in KO (in ms; KO: 15.1 ± 0.7; WT: 18.7 ± 0.7; p = 0.0013).

Conclusions

Not the initiation, but the perpetuation of provoked whole-heart in vivo ventricular arrhythmia was increased in KO. As a potential mechanism, we found a significantly reduced VRP, which may promote perpetuation of reentrant ventricular arrhythmia. On a translational perspective, the antiarrhythmic concept of therapeutic NCX inhibition seems to be ambivalent by protecting from initiating afterdepolarizations but favoring arrhythmia perpetuation in vivo at least in a murine model.

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.

Acute Genetic Ablation of Cardiac Sodium/Calcium Exchange in Adult Mice: Implications for Cardiomyocyte Calcium Regulation, Cardioprotection, and Arrhythmia

Background

Sodium‐calcium (Ca²⁺) exchanger isoform 1 (NCX1) is the dominant Ca²⁺ efflux mechanism in cardiomyocytes and is critical to maintaining Ca²⁺ homeostasis during excitation‐contraction coupling. NCX1 activity has been implicated in the pathogenesis of cardiovascular diseases, but a lack of specific NCX1 blockers complicates experimental interpretation. Our aim was to develop a tamoxifen‐inducible NCX1 knockout (KO) mouse to investigate compensatory adaptations of acute ablation of NCX1 on excitation‐contraction coupling and intracellular Ca²⁺ regulation, and to examine whether acute KO of NCX1 confers resistance to triggered arrhythmia and ischemia/reperfusion injury.

Methods and Results

We used the α‐myosin heavy chain promoter (Myh6)‐MerCreMer promoter to create a tamoxifen‐inducible cardiac‐specific NCX1 KO mouse. Within 1 week of tamoxifen injection, NCX1 protein expression and current were dramatically reduced. Diastolic Ca²⁺ increased despite adaptive reductions in Ca²⁺ current and action potential duration and compensatory increases in excitation‐contraction coupling gain, sarcoplasmic reticulum Ca²⁺ ATPase 2 and plasma membrane Ca2+ ATPase. As these adaptations progressed over 4 weeks, diastolic Ca²⁺ normalized and SR Ca²⁺ load increased. Left ventricular function remained normal, but mild fibrosis and hypertrophy developed. Transcriptomics revealed modification of cardiovascular‐related gene networks including cell growth and fibrosis. NCX1 KO reduced spontaneous action potentials triggered by delayed afterdepolarizations and reduced scar size in response to ischemia/reperfusion.

Conclusions

Tamoxifen‐inducible NCX1 KO mice adapt to acute genetic ablation of NCX1 by reducing Ca²⁺ influx, increasing alternative Ca²⁺ efflux pathways, and increasing excitation‐contraction coupling gain to maintain contractility at the cost of mild Ca²⁺‐activated hypertrophy and fibrosis and decreased survival. Nevertheless, KO myocytes are protected against spontaneous action potentials and ischemia/reperfusion injury.

Effect of C reactive protein on the sodium-calcium exchanger 1 in cardiomyocytes

Numerous previous studies have found that C-reactive protein (CRP) is associated with cardiac arrhythmia and cardiac remodeling. However, the underlying mechanisms of this association remain unclear. Sodium-calcium exchanger 1 (NCX1) serves an important role in the regulation of intracellular calcium concentration, which is closely related with cardiac arrhythmia and cardiac remodeling. The present study aimed to evaluate the effects of CRP on NCX1 and intracellular calcium concentration in cardiomyocytes. Primary neonatal mouse ventricular cardiomyocytes were cultured and treated with varying concentrations of CRP (0, 5, 10, 20 and 40 µg/ml). The cardiomyocytes were also treated with NF-κB-specific inhibitor PTDC and a specific inhibitor of the reverse NCX1 KB-R7943 before their intracellular calcium concentrations were measured. mRNA and protein expression levels of NCX1 were detected by reverse transcription-quantitative PCR and western blotting, respectively and intracellular calcium concentration was evaluated by flow cytometry. CRP treatment significantly increased mRNA and protein expression levels of NCX1 in myocytes (P=0.024), as well as intracellular calcium concentration (P=0.01). These results were significantly attenuated by the NF-κB-specific inhibitor PDTC and a specific inhibitor of the reverse NCX1, KB-R7943. CRP significantly upregulated NCX1 expression and increased intracellular calcium concentration in cardiomyocytes via the NF-κB pathway, suggesting that CRP may serve a pro-arrhythmia role via direct influence on the calcium homeostasis of cardiomyocytes.

Role of Circular RNAs in the Pathogenesis of Cardiovascular Disease

Circular RNAs (circRNAs) are single-strand covalently closed circular noncoding RNAs that are endogenous transcripts generated from linear precursor mRNA through a backsplicing mechanism. With the development of high-throughput sequencing technology, a number of circRNAs have been identified and proved to play key roles in various pathophysiological processes, such as metabolic diseases, cancers, and cardiovascular diseases. An increasing number of studies have shown that circRNAs are widely expressed in cardiac tissues and play important roles in the development of multiple cardiovascular diseases. Here, we review the current understanding of circRNA biogenesis and functions and the roles of circRNAs in cardiovascular diseases. We also highlight the molecular mechanisms underlying the role of circRNAs in the pathogenesis of cardiovascular diseases. A better understanding of the biological function of circRNAs in cardiovascular diseases will be helpful for the development of effective biomarkers for the diagnosis and treatment of these diseases.

Na/Ca exchange in the atrium: Role in sinoatrial node pacemaking and excitation-contraction coupling

Na/Ca exchange is the dominant calcium (Ca) efflux mechanism in cardiac myocytes. Although our knowledge of exchanger function (NCX1 in the heart) was originally established using biochemical and electrophysiological tools such as cardiac sarcolemmal vesicles and the giant patch technique [1-4], many advances in our understanding of the physiological/pathophysiological roles of NCX1 in the heart have been obtained using a suite of genetically modified mice. Early mouse studies focused on modification of expression levels of NCX1 in the ventricles, with transgenic overexpressors, global NCX1 knockout (KO) mice (which were embryonic lethal if homozygous), and finally ventricular-specific NCX1 KO [5-12]. We found, to our surprise, that ventricular cardiomyocytes lacking NCX1 can survive and function by engaging a clever set of adaptations to minimize Ca entry, while maintaining contractile function through an increase in excitation-contraction (EC) coupling gain [5,6,13]. Having studied ventricular NCX1 ablation in detail, we more recently focused on elucidating the role of NCX1 in the atria through altering NCX1 expression. Using a novel atrial-specific NCX1 KO mouse, we found unexpected changes in atrial cell morphology and calcium handling, together with dramatic alterations in the function of sinoatrial node (SAN) pacemaker activity. In this review, we will discuss these findings and their implications for cardiac disease.

Effects of ticagrelor on the sodium/calcium exchanger 1 (NCX1) in cardiac derived H9c2 cells

Ticagrelor is a direct acting and reversibly binding P2Y₁₂ antagonist approved for the prevention of thromboembolic events. Clinical effects of ticagrelor cannot be simply accounted for by pure platelet inhibition, and off-target mechanisms can potentially play a role. In particular, recent evidence suggests that ticagrelor may also influence heart function and improve the evolution of myocardial ischemic injury by more direct effects on myocytes. The cardiac sodium/calcium exchanger 1 (NCX1) is a critical player in the generation and control of calcium (Ca²⁺) signals, which orchestrate multiple myocyte activities in health and disease. Altered expression and/or activity of NCX1 can have profound consequences for the function and fate of myocytes. Whether ticagrelor affects cardiac NCX1 has not been investigated yet. To explore this hypothesis, we analyzed the expression, localization and activity of NCX1 in the heart derived H9c2-NCX1 cells following ticagrelor exposure. We found that ticagrelor concentration- and time-dependently reduced the activity of the cardiac NCX1 in H9c2 cells. In particular, the inhibitory effect of ticagrelor on the Ca²⁺-influx mode of NCX1 was evident within 1 h and further developed after 24 h, when NCX1 activity was suppressed by about 55% in cells treated with 1 μM ticagrelor. Ticagrelor-induced inhibition of exchanger activity was reached at clinically relevant concentrations, without affecting the expression levels and subcellular distribution of NCX1. Collectively, these findings suggest that cardiac NCX1 is a new downstream target of ticagrelor, which may contribute to the therapeutic profile of ticagrelor in clinical practice.

Beyond Intracellular Signaling: The Ins and Outs of Second Messengers Microdomains

A typical characteristic of eukaryotic cells compared to prokaryotes is represented by the spatial heterogeneity of the different structural and functional components: for example, most of the genetic material is surrounded by a highly specific membrane structure (the nuclear membrane), continuous with, yet largely different from, the endoplasmic reticulum (ER); oxidative phosphorylation is carried out by organelles enclosed by a double membrane, the mitochondria; in addition, distinct domains, enriched in specific proteins, are present in the plasma membrane (PM) of most cells. Less obvious, but now generally accepted, is the notion that even the concentration of small molecules such as second messengers (Ca²⁺ and cAMP in particular) can be highly heterogeneous within cells. In the case of most organelles, the differences in the luminal levels of second messengers depend either on the existence on their membrane of proteins that allow the accumulation/release of the second messenger (e.g., in the case of Ca²⁺, pumps, exchangers or channels), or on the synthesis and degradation of the specific molecule within the lumen (the autonomous intramitochondrial cAMP system). It needs stressing that the existence of a surrounding membrane does not necessarily imply the existence of a gradient between the cytosol and the organelle lumen. For example, the nuclear membrane is highly permeable to both Ca²⁺ and cAMP (nuclear pores are permeable to solutes up to 50 kDa) and differences in [Ca²⁺] or [cAMP] between cytoplasm and nucleoplasm are not seen in steady state and only very transiently during cell activation. A similar situation has been observed, as far as Ca²⁺ is concerned, in peroxisomes.

Circular noncoding RNAs as potential therapies and circulating biomarkers for cardiovascular diseases

Recent advancements in genome-wide analyses and RNA-sequencing technologies led to the discovery of small noncoding RNAs, such as microRNAs (miRs), as well as both linear long noncoding RNAs (lncRNAs) and circular long noncoding RNAs (circRNAs). The importance of miRs and lncRNAs in the treatment, prognosis and diagnosis of cardiovascular diseases (CVDs) has been extensively reported. We also previously reviewed their implications in therapies and as biomarkers for CVDs. More recently, circRNAs have also emerged as important regulators in CVDs. CircRNAs are circular genome products that are generated by back splicing of specific regions of pre-messenger RNAs (pre-mRNAs). Growing interest in circRNAs led to the discovery of a wide array of their pathophysiological functions. CircRNAs have been shown to be key regulators of CVDs such as myocardial infarction, atherosclerosis, cardiomyopathy and cardiac fibrosis. Accordingly, circRNAs have been recently proposed as potential therapeutic targets and biomarkers for CVDs. In this review, we summarize the current state of the literature on circRNAs, starting with their biogenesis and global mechanisms of actions. We then provide a synopsis of their involvement in various CVDs. Lastly, we emphasize the great potential of circRNAs as biomarkers for the early detection of CVDs, and discuss several patents and recent papers that highlight the utilization of circRNAs as promising biomarkers.

The Effects of SEA0400 on Ca2+ Transient Amplitude and Proarrhythmia Depend on the Na+/Ca2+ Exchanger Expression Level in Murine Models

Background/Objective: The cardiac Na⁺/Ca²⁺ exchanger (NCX) has been identified as a promising target to counter arrhythmia in previous studies investigating the benefit of NCX inhibition. However, the consequences of NCX inhibition have not been investigated in the setting of altered NCX expression and function, which is essential, since major cardiac diseases (heart failure/atrial fibrillation) exhibit NCX upregulation. Thus, we here investigated the effects of the NCX inhibitor SEA0400 on the Ca²⁺ transient amplitude and on proarrhythmia in homozygous NCX overexpressor (OE) and heterozygous NCX knockout (hetKO) mice compared to corresponding wild-types (WT_OE/WT_hetKO). Methods/Results: Ca²⁺ transients of field-stimulated isolated ventricular cardiomyocytes were recorded with fluo-4-AM or indo-1-AM. SEA0400 (1 μM) significantly reduced NCX forward mode function in all mouse lines. SEA0400 (1 μM) significantly increased the amplitude of field-stimulated Ca²⁺ transients in WT_OE, WT_hetKO, and hetKO, but not in OE (% of basal; OE = 98.7 ± 5.0; WT_OE = 137.8 ± 5.2^*; WT_hetKO = 126.3 ± 6.0^*; hetKO = 140.6 ± 12.8^*; ^*p < 0.05 vs. basal). SEA0400 (1 μM) significantly reduced the number of proarrhythmic spontaneous Ca²⁺ transients (sCR) in OE, but increased it in WT_OE, WT_hetKO and hetKO (sCR per cell; basal/+SEA0400; OE = 12.5/3.7; WT_OE = 0.2/2.4; WT_hetKO = 1.3/8.8; hetKO = 0.2/5.5) and induced Ca²⁺ overload with subsequent cell death in hetKO. Conclusion: The effects of SEA0400 on Ca²⁺ transient amplitude and the occurrence of spontaneous Ca²⁺ transients as a proxy measure for inotropy and cellular proarrhythmia depend on the NCX expression level. The antiarrhythmic effect of SEA0400 in conditions of increased NCX expression promotes the therapeutic concept of NCX inhibition in heart failure/atrial fibrillation. Conversely, in conditions of reduced NCX expression, SEA0400 suppressed the NCX function below a critical level leading to adverse Ca²⁺ accumulation as reflected by an increase in Ca²⁺ transient amplitude, proarrhythmia and cell death. Thus, the remaining NCX function under inhibition may be a critical factor determining the inotropic and antiarrhythmic efficacy of SEA0400.

Circular RNAs: analysis, expression and potential functions

Just a few years ago, it had been assumed that the dominant RNA isoforms produced from eukaryotic genes were variants of messenger RNA, functioning as intermediates in gene expression. In early 2012, however, a surprising discovery was made: circular RNA (circRNA) was shown to be a transcriptional product in thousands of human and mouse genes and in hundreds of cases constituted the dominant RNA isoform. Subsequent studies revealed that the expression of circRNAs is developmentally regulated, tissue and cell-type specific, and shared across the eukaryotic tree of life. These features suggest important functions for these molecules. Here, we describe major advances in the field of circRNA biology, focusing on the regulation of and functional roles played by these molecules.

Biogenesis of circular RNAs and their roles in cardiovascular development and pathology

Circular RNAs (circRNAs) are a newly discovered type of RNA generated by back-splicing of precursor mRNA and found in many species. They are, expressed in a tissue-specific manner and fulfill regulatory activities in many biological processes. Recent research has revealed that circRNAs play critical roles in the development and pathologies of the cardiovascular system. Some of these circRNAs show aberrant expression and regulatory activities during heart disease including heart failure and cardiac infarction and hypertrophy. These findings suggest that circRNAs might be a suitable target for the treatment and prevention of heart disease. In this review, we summarize the latest research on the biogenesis and functions of circRNAs with emphasis on the regulatory roles of circRNAs in the development and pathologies of the cardiovascular system.

Triggered activity in atrial myocytes is influenced by Na+/Ca2+ exchanger activity in genetically altered mice

AIMS

In atrial fibrillation, increased function of the Na⁺/Ca²⁺-exchanger (NCX) is one among several electrical remodeling mechanisms.

METHODS/RESULTS

Using the patch-clamp- and Ca²⁺ imaging-methods, we investigated atrial myocytes from NCX-homozygous-overexpressor (OE)- and heterozygous-knockout (KO)-mice and their corresponding wildtypes (WT_OE; WT_KO). NCX mediated Ca²⁺ extrusion capacity was reduced in KO and increased in OE. There was no evidence for structural or molecular remodeling. During a proarrhythmic pacing-protocol, the number of low amplitude delayed afterdepolarizations (DADs) was unaltered in OE vs. WT_OE and KO vs. WT_KO. However, DADs triggered full spontaneous action potentials (sAP) significantly more often in OE vs. WT_OE (ratio sAP/DAD: OE:0.18±0.05; WT_OE:0.02±0.02; p<0.001). Using the same protocol, a DAD triggered an sAP by tendency less often in KO vs. WT_KO (p=0.06) and significantly less often under a more aggressive proarrhythmic protocol (ratio sAP/DAD: KO:0.01±0.003; WT KO: 0.12±0.05; p=0.007). The DAD amplitude was increased in OE vs. WT_OE and decreased in KO vs. WT_KO. There were no differences in SR-Ca²⁺-load, the number of spontaneous Ca²⁺-release-events or IKACh/IK1.

CONCLUSIONS

Atrial myocytes with increased NCX expression exhibited increased vulnerability towards sAPs while atriomyocytes with reduced NCX expression were protected. The underlying mechanism consists of a modification of the DAD-amplitude by the level of NCX-activity. Thus, although the number of spontaneous Ca²⁺-releases and therefore DADs is unaltered, the higher DAD-amplitude in OE made a transgression of the voltage-threshold of an sAP more likely. These findings indicate that the level of NCX activity could influence triggered activity in atrial myocytes independent of possible remodeling processes.

Muscleblind-like 3 deficit results in a spectrum of age-associated pathologies observed in myotonic dystrophy

Myotonic dystrophy type I (DM1) exhibits distinctive disease specific phenotypes and the accelerated onset of a spectrum of age-associated pathologies. In DM1, dominant effects of expanded CUG repeats result in part from the inactivation of the muscleblind-like (MBNL) proteins. To test the role of MBNL3, we deleted Mbnl3 exon 2 (Mbnl3(ΔE2)) in mice and examined the onset of age-associated diseases over 4 to 13 months of age. Accelerated onset of glucose intolerance with elevated insulin levels, cardiac systole deficits, left ventricle hypertrophy, a predictor of a later onset of heart failure and the development of subcapsular and cortical cataracts is observed in Mbnl3(ΔE2) mice. Retention of embryonic splice isoforms in adult organs, a prominent defect in DM1, is not observed in multiple RNAs including the Insulin Receptor (Insr), Cardiac Troponin T (Tnnt2), Lim Domain Binding 3 (Ldb3) RNAs in Mbnl3(ΔE2) mice. Although rare DM1-like splice errors underlying the observed phenotypes cannot be excluded, our data in conjunction with the reported absence of alternative splice errors in embryonic muscles of a similar Mbnl3(ΔE2) mouse by RNA-seq studies, suggest that mechanisms distinct from the adult retention of embryonic splice patterns may make important contributions to the onset of age-associated pathologies in DM1.

Pivotal role of α2 Na+ pumps and their high affinity ouabain binding site in cardiovascular health and disease

Reduced smooth muscle (SM)-specific α2 Na⁺ pump expression elevates basal blood pressure (BP) and increases BP sensitivity to angiotensin II (Ang II) and dietary NaCl, whilst SM-α2 overexpression lowers basal BP and decreases Ang II/salt sensitivity. Prolonged ouabain infusion induces hypertension in rodents, and ouabain-resistant mutation of the α2 ouabain binding site (α2^R/R mice) confers resistance to several forms of hypertension. Pressure overload-induced heart hypertrophy and failure are attenuated in cardio-specific α2 knockout, cardio-specific α2 overexpression and α2^R/R mice. We propose a unifying hypothesis that reconciles these apparently disparate findings: brain mechanisms, activated by Ang II and high NaCl, regulate sympathetic drive and a novel neurohumoral pathway mediated by both brain and circulating endogenous ouabain (EO). Circulating EO modulates ouabain-sensitive α2 Na⁺ pump activity and Ca²⁺ transporter expression and, via Na⁺ /Ca²⁺ exchange, Ca²⁺ homeostasis. This regulates sensitivity to sympathetic activity, Ca²⁺ signalling and arterial and cardiac contraction.

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.

Myoscape controls cardiac calcium cycling and contractility via regulation of L-type calcium channel surface expression

Calcium signalling plays a critical role in the pathogenesis of heart failure. Here we describe a cardiac protein named Myoscape/FAM40B/STRIP2, which directly interacts with the L-type calcium channel. Knockdown of Myoscape in cardiomyocytes decreases calcium transients associated with smaller Ca(2+) amplitudes and a lower diastolic Ca(2+) content. Likewise, L-type calcium channel currents are significantly diminished on Myoscape ablation, and downregulation of Myoscape significantly reduces contractility of cardiomyocytes. Conversely, overexpression of Myoscape increases global Ca(2+) transients and enhances L-type Ca(2+) channel currents, and is sufficient to restore decreased currents in failing cardiomyocytes. In vivo, both Myoscape-depleted morphant zebrafish and Myoscape knockout (KO) mice display impairment of cardiac function progressing to advanced heart failure. Mechanistically, Myoscape-deficient mice show reduced L-type Ca(2+)currents, cell capacity and calcium current densities as a result of diminished LTCC surface expression. Finally, Myoscape expression is reduced in hearts from patients suffering of terminal heart failure, implying a role in human disease.

Myoscape controls cardiac calcium cycling and contractility via regulation of L-type calcium channel surface expression

Calcium signalling plays a critical role in the pathogenesis of heart failure. Here we describe a cardiac protein named Myoscape/FAM40B/STRIP2, which directly interacts with the L-type calcium channel. Knockdown of Myoscape in cardiomyocytes decreases calcium transients associated with smaller Ca²⁺ amplitudes and a lower diastolic Ca²⁺ content. Likewise, L-type calcium channel currents are significantly diminished on Myoscape ablation, and downregulation of Myoscape significantly reduces contractility of cardiomyocytes. Conversely, overexpression of Myoscape increases global Ca²⁺ transients and enhances L-type Ca²⁺ channel currents, and is sufficient to restore decreased currents in failing cardiomyocytes. In vivo, both Myoscape-depleted morphant zebrafish and Myoscape knockout (KO) mice display impairment of cardiac function progressing to advanced heart failure. Mechanistically, Myoscape-deficient mice show reduced L-type Ca²⁺currents, cell capacity and calcium current densities as a result of diminished LTCC surface expression. Finally, Myoscape expression is reduced in hearts from patients suffering of terminal heart failure, implying a role in human disease.

Heart failure is a major public health issue but due to our poor disease understanding the current therapies are symptomatic. Here the authors identify Myoscape as a novel cardiac protein regulating membrane localization of the L-type calcium channel and heart's contractile force, thus promising new therapeutic avenues for heart failure.

Sarcospan Regulates Cardiac Isoproterenol Response and Prevents Duchenne Muscular Dystrophy-Associated Cardiomyopathy

Background

Duchenne muscular dystrophy is a fatal cardiac and skeletal muscle disease resulting from mutations in the dystrophin gene. We have previously demonstrated that a dystrophin‐associated protein, sarcospan (SSPN), ameliorated Duchenne muscular dystrophy skeletal muscle degeneration by activating compensatory pathways that regulate muscle cell adhesion (laminin‐binding) to the extracellular matrix. Conversely, loss of SSPN destabilized skeletal muscle adhesion, hampered muscle regeneration, and reduced force properties. Given the importance of SSPN to skeletal muscle, we investigated the consequences of SSPN ablation in cardiac muscle and determined whether overexpression of SSPN into mdx mice ameliorates cardiac disease symptoms associated with Duchenne muscular dystrophy cardiomyopathy.

Methods and Results

SSPN‐null mice exhibited cardiac enlargement, exacerbated cardiomyocyte hypertrophy, and increased fibrosis in response to β‐adrenergic challenge (isoproterenol; 0.8 mg/day per 2 weeks). Biochemical analysis of SSPN‐null cardiac muscle revealed reduced sarcolemma localization of many proteins with a known role in cardiomyopathy pathogenesis: dystrophin, the sarcoglycans (α‐, δ‐, and γ‐subunits), and β1D integrin. Transgenic overexpression of SSPN in Duchenne muscular dystrophy mice (mdx^TG) improved cardiomyofiber cell adhesion, sarcolemma integrity, cardiac functional parameters, as well as increased expression of compensatory transmembrane proteins that mediate attachment to the extracellular matrix.

Conclusions

SSPN regulates sarcolemmal expression of laminin‐binding complexes that are critical to cardiac muscle function and protects against transient and chronic injury, including inherited cardiomyopathy.

Suppression of Early and Late Afterdepolarizations by Heterozygous Knockout of the Na+/Ca2+ Exchanger in a Murine Model

BACKGROUND

The Na(+)/Ca(2+) exchanger (NCX) has been implied to cause arrhythmias. To date, information on the role of NCX in arrhythmogenesis derived from models with increased NCX expression, hypertrophy, and heart failure. Furthermore, the exact mechanism by which NCX exerts its potentially proarrhythmic effect, ie, by promoting early afterdepolarization (EAD) or delayed afterdepolarization (DAD) or both, is unknown.

METHODS AND RESULTS

We investigated isolated cardiomyocytes from a murine model with heterozygous knockout of NCX (hetKO) using the patch clamp and Ca(2+) imaging techniques. Action potential duration was shorter in hetKO with IKtot not being increased. The rate of spontaneous Ca(2+) release events and the rate of DADs were unaltered; however, DADs had lower amplitude in hetKO. A DAD triggered a spontaneous action potential significantly less often in hetKO when compared with wild-type. The occurrence of EADs was also drastically reduced in hetKO. ICa activity was reduced in hetKO, an effect that was abolished in the presence of the Ca(2+) buffer BAPTA.

CONCLUSIONS

Genetic suppression of NCX reduces both EADs and DADs. The following molecular mechanisms apply: (1) Although the absolute number of DADs is unaffected, an impaired translation of DADs into spontaneous action potentials results from a reduced DAD amplitude. (2) EADs are reduced in absolute number of occurrence, which is presumably a consequence of shortened action potential duration because of reduced NCX activity but also reduced ICa the latter possibly being caused by a direct modulation of Ca(2+)-dependent ICa inhibition by reduced NCX activity. This is the first study to demonstrate that genetic inhibition of NCX protects against afterdepolarizations and to investigate the underlying mechanisms.

Statistically based splicing detection reveals neural enrichment and tissue-specific induction of circular RNA during human fetal development

Background

The pervasive expression of circular RNA is a recently discovered feature of gene expression in highly diverged eukaryotes, but the functions of most circular RNAs are still unknown. Computational methods to discover and quantify circular RNA are essential. Moreover, discovering biological contexts where circular RNAs are regulated will shed light on potential functional roles they may play.

Results

We present a new algorithm that increases the sensitivity and specificity of circular RNA detection by discovering and quantifying circular and linear RNA splicing events at both annotated and un-annotated exon boundaries, including intergenic regions of the genome, with high statistical confidence. Unlike approaches that rely on read count and exon homology to determine confidence in prediction of circular RNA expression, our algorithm uses a statistical approach. Using our algorithm, we unveiled striking induction of general and tissue-specific circular RNAs, including in the heart and lung, during human fetal development. We discover regions of the human fetal brain, such as the frontal cortex, with marked enrichment for genes where circular RNA isoforms are dominant.

Conclusions

The vast majority of circular RNA production occurs at major spliceosome splice sites; however, we find the first examples of developmentally induced circular RNAs processed by the minor spliceosome, and an enriched propensity of minor spliceosome donors to splice into circular RNA at un-annotated, rather than annotated, exons. Together, these results suggest a potentially significant role for circular RNA in human development.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0690-5) contains supplementary material, which is available to authorized users.

Loss of muscleblind-like 1 results in cardiac pathology and persistence of embryonic splice isoforms

Cardiac dysfunction is a prominent cause of mortality in myotonic dystrophy I (DM1), a disease where expanded CUG repeats bind and disable the muscleblind-like family of splice regulators. Deletion of muscleblind-like 1 (Mbnl1^ΔE2/ΔE2) in 129 sv mice results in QRS, QTc widening, bundle block and STc narrowing at 2–4 months of age. With time, cardiac function deteriorates further and at 6 months, decreased R wave amplitudes, sinus node dysfunction, cardiac hypertrophy, interstitial fibrosis, multi-focal myocardial fiber death and calcification manifest. Sudden death, where no end point illness is overt, is observed at a median age of 6.5 and 4.8 months in ~67% and ~86% of male and female Mbnl1^ΔE2/ΔE2 mice, respectively. Mbnl1 depletion results in the persistence of embryonic splice isoforms in a network of cardiac RNAs, some of which have been previously implicated in DM1, regulating sodium and calcium currents, Scn5a, Junctin, Junctate, Atp2a1, Atp11a, Cacna1s, Ryr2, intra and inter cellular transport, Clta, Stx2, Tjp1, cell survival, Capn3, Sirt2, Csda, sarcomere and cytoskeleton organization and function, Trim55, Mapt, Pdlim3, Pdlim5, Sorbs1, Sorbs2, Fhod1, Spag9 and structural components of the sarcomere, Myom1, Tnnt2, Zasp. Thus this study supports a key role for Mbnl1 loss in the initiation of DM1 cardiac disease.

Long-term administration of the TNF blocking drug Remicade (cV1q) to mdx mice reduces skeletal and cardiac muscle fibrosis, but negatively impacts cardiac function

Duchenne muscular dystrophy (DMD) is a degenerative skeletal muscle disease caused by mutations in the gene encoding dystrophin (DYS). Tumor necrosis factor (TNF) has been implicated in the pathogenesis since short-term treatment of mdx mice with TNF blocking drugs proved beneficial; however, it is not clear whether long-term treatment will also improve long-term outcomes of fibrosis and cardiac health. In this investigation, short and long-term dosing studies were carried out using the TNF blocking drug Remicade and a variety of outcome measures were assessed. Here we show no demonstrable benefit to muscle strength or morphology with 10mg/kg or 20mg/kg Remicade; however, 3mg/kg produced positive strength benefits. Remicade treatment correlated with reductions in myostatin mRNA in the heart, and concomitant reductions in cardiac and skeletal fibrosis. Surprisingly, although Remicade treated mdx hearts were less fibrotic, reductions in LV mass and ejection fraction were also observed, and these changes coincided with reductions in AKT phosphorylation on threonine 308. Thus, TNF blockade benefits mdx skeletal muscle strength and fibrosis, but negatively impacts AKT activation, leading to deleterious changes to dystrophic heart function. These studies uncover a previously unknown relationship between TNF blockade and alteration of muscle growth signaling pathways.

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.

Complete atrial-specific knockout of sodium-calcium exchange eliminates sinoatrial node pacemaker activity

The origin of sinoatrial node (SAN) pacemaker activity in the heart is controversial. The leading candidates are diastolic depolarization by "funny" current (If) through HCN4 channels (the "Membrane Clock" hypothesis), depolarization by cardiac Na-Ca exchange (NCX1) in response to intracellular Ca cycling (the "Calcium Clock" hypothesis), and a combination of the two ("Coupled Clock"). To address this controversy, we used Cre/loxP technology to generate atrial-specific NCX1 KO mice. NCX1 protein was undetectable in KO atrial tissue, including the SAN. Surface ECG and intracardiac electrograms showed no atrial depolarization and a slow junctional escape rhythm in KO that responded appropriately to β-adrenergic and muscarinic stimulation. Although KO atria were quiescent they could be stimulated by external pacing suggesting that electrical coupling between cells remained intact. Despite normal electrophysiological properties of If in isolated patch clamped KO SAN cells, pacemaker activity was absent. Recurring Ca sparks were present in all KO SAN cells, suggesting that Ca cycling persists but is uncoupled from the sarcolemma. We conclude that NCX1 is required for normal pacemaker activity in murine SAN.

Overexpression of the Na+/K+ ATPase α2 but not α1 isoform attenuates pathological cardiac hypertrophy and remodeling

RATIONALE

The Na+ / K+ ATPase (NKA) directly regulates intracellular Na+ levels, which in turn indirectly regulates Ca2+ levels by proximally controlling flux through the Na+ / Ca2+ exchanger (NCX1). Elevated Na+ levels have been reported during heart failure, which permits some degree of reverse-mode Ca2+ entry through NCX1, as well as less efficient Ca2+ clearance.

OBJECTIVE

To determine whether maintaining lower intracellular Na+ levels by NKA overexpression in the heart would enhance forward-mode Ca2+ clearance and prevent reverse-mode Ca2+ entry through NCX1 to protect the heart.

METHODS AND RESULTS

Cardiac-specific transgenic mice overexpressing either NKA-α1 or NKA-α2 were generated and subjected to pressure overload hypertrophic stimulation. We found that although increased expression of NKA-α1 had no protective effect, overexpression of NKA-α2 significantly decreased cardiac hypertrophy after pressure overload in mice at 2, 10, and 16 weeks of stimulation. Remarkably, total NKA protein expression and activity were not altered in either of these 2 transgenic models because increased expression of one isoform led to a concomitant decrease in the other endogenous isoform. NKA-α2 overexpression but not NKA-α1 led to significantly faster removal of bulk Ca2+ from the cytosol in a manner requiring NCX1 activity. Mechanistically, overexpressed NKA-α2 showed greater affinity for Na+ compared with NKA-α1, leading to more efficient clearance of this ion. Furthermore, overexpression of NKA-α2 but not NKA-α1 was coupled to a decrease in phospholemman expression and phosphorylation, which would favor greater NKA activity, NCX1 activity, and Ca2+ removal.

CONCLUSIONS

Our results suggest that the protective effect produced by increased expression of NKA-α2 on the heart after pressure overload is due to more efficient Ca2+ clearance because this isoform of NKA preferentially enhances NCX1 activity compared with NKA-α1.

Impaired baroreflex function in mice overexpressing alpha-synuclein

Cardiovascular autonomic dysfunction, such as orthostatic hypotension consequent to baroreflex failure and cardiac sympathetic denervation, is frequently observed in the synucleinopathy Parkinson’s disease (PD). In the present study, the baroreceptor reflex was assessed in mice overexpressing human wildtype alpha-synuclein (Thy1-aSyn), a genetic mouse model of synucleinopathy. The beat-to-beat change in heart rate (HR), computed from R–R interval, in relation to blood pressure was measured in anesthetized and conscious mice equipped with arterial blood pressure telemetry transducers during transient bouts of hypertension and hypotension. Compared to wildtype, tachycardia following nitroprusside-induced hypotension was significantly reduced in Thy1-aSyn mice. Thy1-aSyn mice also showed an abnormal cardiovascular response (i.e., diminished tachycardia) to muscarinic blockade with atropine. We conclude that Thy1-aSyn mice have impaired basal and dynamic range of sympathetic and parasympathetic-mediated changes in HR and will be a useful model for long-term study of cardiovascular autonomic dysfunction associated with PD.

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.

An SRF/miR-1 axis regulates NCX1 and annexin A5 protein levels in the normal and failing heart

AIMS

The expression of the sodium/calcium exchanger NCX1 increases during cardiac hypertrophy and heart failure, playing an important role in Ca(2+) extrusion. This increase is presumed to result from stress signalling induced changes in the interplay between transcriptional and post-transcriptional regulations. We aimed to determine the impact of the SRF transcription factor known to regulate the NCX1 promoter and microRNA genes, on the expression of NCX1 mRNA and protein and annexin A5 (AnxA5), a Ca(2+)-binding protein interacting with NCX1 and increased during HF.

METHODS AND RESULTS

NCX1 mRNA was decreased while the protein was increased in the failing heart of the cardiomyocyte-restricted SRF knock-out mice (SRF(HKO)). The induction of NCX1 mRNA by the pro-hypertrophic drug phenylephrine observed in control mice was abolished in the SRF(HKO) though the protein was strongly increased. AnxA5 protein expression profile paralleled the expression of NCX1 protein in the SRF(HKO). MiR-1, a microRNA regulated by SRF, was decreased in the SRF(HKO) and repressed by phenylephrine. In vitro and in vivo manipulation of miR-1 levels and site-directed mutagenesis showed that NCX1 and AnxA5 mRNAs are targets of miR-1. AnxA5 overexpression slowed down Ca(2+) extrusion during caffeine application in adult rat cardiomyocytes.

CONCLUSION

Our study reveals the existence of a complex regulatory loop where SRF regulates the transcription of NCX1 and miR-1, which in turn functions as a rheostat limiting the translation of NCX1 and AnxA5 proteins. The decrease of miR-1 and increase of AnxA5 appear as important modulators of NCX1 expression and activity during heart failure.

Mechanical unloading reverses transverse tubule remodelling and normalizes local Ca(2+)-induced Ca(2+)release in a rodent model of heart failure

AIMS

Ca(2+)-induced Ca(2+) release (CICR) is critical for contraction in cardiomyocytes. The transverse (t)-tubule system guarantees the proximity of the triggers for Ca(2+) release [L-type Ca(2+) channel, dihydropyridine receptors (DHPRs)] and the sarcoplasmic reticulum Ca(2+) release channels [ryanodine receptors (RyRs)]. Transverse tubule disruption occurs early in heart failure (HF). Clinical studies of left ventricular assist devices in HF indicate that mechanical unloading induces reverse remodelling. We hypothesize that unloading of failing hearts normalizes t-tubule structure and improves CICR.

METHODS AND RESULTS

Heart failure was induced in Lewis rats by left coronary artery ligation for 12 weeks; sham-operated animals were used as controls. Failing hearts were mechanically unloaded for 4 weeks by heterotopic abdominal heart transplantation (HF-UN). HF reduced the t-tubule density measured by di-8-ANEPPS staining in isolated left ventricular myocytes, and this was reversed by unloading. The deterioration in the regularity of the t-tubule system in HF was also reversed in HF-UN. Scanning ion conductance microscopy showed the reappearance of normal surface striations in HF-UN. Electron microscopy revealed recovery of normal t-tubule microarchitecture in HF-UN. L-type Ca(2+) current density, measured using whole-cell patch clamping, was reduced in HF but unaffected by unloading. The variance of the time-to-peak of the Ca(2+) transient, an index of CICR dyssynchrony, was increased in HF and normalized by unloading. The increased Ca(2+) spark frequency observed in HF was reduced in HF-UN. These results could be explained by the recoupling of orphaned RyRs in HF, as indicated by immunofluorescence.

CONCLUSIONS

Our data show that mechanical unloading of the failing heart reverses the pathological remodelling of the t-tubule system and improves CICR.

Decreased cardiac L-type Ca²⁺ channel activity induces hypertrophy and heart failure in mice

Antagonists of L-type Ca²⁺ channels (LTCCs) have been used to treat human cardiovascular diseases for decades. However, these inhibitors can have untoward effects in patients with heart failure, and their overall therapeutic profile remains nebulous given differential effects in the vasculature when compared with those in cardiomyocytes. To investigate this issue, we examined mice heterozygous for the gene encoding the pore-forming subunit of LTCC (calcium channel, voltage-dependent, L type, α1C subunit [Cacna1c mice; referred to herein as α1C⁻/⁺ mice]) and mice in which this gene was loxP targeted to achieve graded heart-specific gene deletion (termed herein α1C-loxP mice). Adult cardiomyocytes from the hearts of α1C⁻/⁺ mice at 10 weeks of age showed a decrease in LTCC current and a modest decrease in cardiac function, which we initially hypothesized would be cardioprotective. However, α1C⁻/⁺ mice subjected to pressure overload stimulation, isoproterenol infusion, and swimming showed greater cardiac hypertrophy, greater reductions in ventricular performance, and greater ventricular dilation than α1C⁺/⁺ controls. The same detrimental effects were observed in α1C-loxP animals with a cardiomyocyte-specific deletion of one allele. More severe reductions in α1C protein levels with combinatorial deleted alleles produced spontaneous cardiac hypertrophy before 3 months of age, with early adulthood lethality. Mechanistically, our data suggest that a reduction in LTCC current leads to neuroendocrine stress, with sensitized and leaky sarcoplasmic reticulum Ca²⁺ release as a compensatory mechanism to preserve contractility. This state results in calcineurin/nuclear factor of activated T cells signaling that promotes hypertrophy and disease.

Baroreceptor reflex dysfunction in the BACHD mouse model of Huntington's disease

Huntington’s disease is a progressive, neurodegenerative disorder that presents with a triad of clinical symptoms, which include movement abnormalities, emotional disturbance and cognitive impairment. Recent studies reported dysfunction of the autonomic nervous system in Huntington’s disease patients, which may contribute to the increased incidence of cardiovascular events in this patient population that often leads to death. We measured the baroreceptor reflex, a process dependent on proper autonomic function, in the BACHD mouse model of Huntington’s disease. We found a blunted response of the baroreceptor reflex as well as significantly higher daytime blood pressure in BACHD mice compared to WT controls, which are both indications of autonomic dysfunction. BACHD mice had increased heart weight to tibia length ratios at 7 and 12 mo of age suggesting hypertrophic changes of the heart, which we speculate is a response to the increased blood pressure and aberrant baroreceptor reflex. Despite these structural changes, the hearts of BACHD mice continue to function normally as assessed by echocardiographic analysis. Studies of autonomic and cardiovascular function in BACHD mice may help elucidate the pathophysiology of Huntington’s disease and aid in the development of clinical strategies to offset the incidence of fatal cardiovascular events in the Huntington’s disease patient population.

Unraveling the secrets of a double life: contractile versus signaling Ca2+ in a cardiac myocyte

No other inorganic molecule known in biology is considered as versatile as Ca(2+). In a vast majority of cell types, Ca(2+) acts as a universal second messenger underlying critical cellular processes varying from gene transcription to cell death. Although the role of Ca(2+) in myocyte contraction has been known for over a century, it was only more recently that this divalent cation has been implicated in mediating reactive signal transduction to promote cardiac hypertrophy. However, it remains unclear how Ca(2+)-dependent signaling pathways are regulated/activated in a cardiac myocyte given the prevailing conditions throughout the cytosol where Ca(2+) concentration oscillates between 100 nM and upwards of 1-2 μM during each contractile cycle. In this review we will examine three hypotheses put forward to explain how Ca(2+) might still function as a hypertrophic signaling molecule in cardiac myocytes and discuss the current literature that supports each of these views. This article is part of a special issue entitled "Local Signaling in Myocytes."

Triple threat: the Na+/Ca2+ exchanger in the pathophysiology of cardiac arrhythmia, ischemia and heart failure

The Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) extrusion mechanism of the cardiac myocyte and thus is crucial for maintaining Ca(2+) homeostasis. It is involved in the regulation of several parameters of cardiac excitation contraction coupling, such as cytosolic Ca(2+) concentration, repolarization and contractility. Increased NCX activity has been identified as a mechanism promoting heart failure, cardiac ischemia and arrhythmia. Transgenic mice as well as pharmacological interventions have been used to support the idea of using NCX inhibition as a future pharmacological strategy to treat cardiovascular disease.