Overexpression of the Na(+)-Ca2+ exchanger leads to enhanced inotropic responsiveness to Na(+)-channel agonist without sarcoplasmic reticulum protein changes in transgenic mice

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.

Hypertrophy and heart failure in mice overexpressing the cardiac sodium-calcium exchanger

BACKGROUND

The cardiac sodium-calcium exchanger (NCX1) is a key sarcolemmal protein for the maintenance of calcium homeostasis in the heart. Because heart failure is associated with increased expression of NCX1, heterozygous (HET) and homozygous (HOM) transgenic mice overexpressing NCX1 were developed and evaluated.

METHODS AND RESULTS

The NCX1 transgenic mice display 2.3-fold (HET) and 3.1-fold (HOM) increases in exchanger activity from wild-type (WT) mice. Functional information was obtained by echocardiography and catheterizations before and after hemodynamic stress from pregnancy, treadmill exercise or transaortic constriction (TAC). HET and HOM mice exhibited hypertrophy and blunted responses with beta-adrenergic stimulation. Postpartum mice from all groups were hypertrophied, but only the HOM mice exhibited premature death from heart failure. HOM mice became exercise intolerant after 6 weeks of daily treadmill running. After 21 days TAC, HET, and HOM mice exhibited significant contractile dysfunction and 15% to 40% mortality with clinical evidence of heart failure.

CONCLUSIONS

Hemodynamic stress results in a compensated hypertrophy in WT mice, but NCX1 transgenic mice exhibit decreased contractile function and heart failure in proportion to their level of NCX1 expression. Thus exchanger overexpression in mice leads to abnormal calcium handling and a decompensatory transition to heart failure with stress.

Functional alterations after cardiac sodium-calcium exchanger overexpression in heart failure

The sodium-calcium exchanger (NCX) is discussed as one of the key proteins involved in heart failure. However, the causal role and the extent to which NCX contributes to contractile dysfunction during heart failure are poorly understood. NCX overexpression was induced by infection with an adenovirus coding for NCX, which coexpressed green fluorescence protein (GFP) (AdNCX) by ex vivo gene transfer to nonfailing and failing rabbit cardiomyocytes. Myocardial gene transfer in rabbits in vivo was achieved by adenoviral delivery via aortic cross-clamping. Peak cell shortening of cardiomyocytes was determined photo-optically. Hemodynamic parameters in vivo were determined by echocardiography (fractional shortening) and tip catheter [maximal first derivative of left ventricular (LV) pressure (dP/d tmax); maximal negative derivative of LV pressure (−dP/d tmax)]. Peak cell shortening was depressed after NCX gene delivery in isolated nonfailing and in failing cardiomyocytes. In nonfailing rabbits in vivo, basal systolic contractility (fractional shortening and dP/d tmax) and maximum rate of LV relaxation (−dP/d tmax) in vivo were largely unaffected after NCX overexpression. However, during heart failure, long-term NCX overexpression over 2 wk significantly improved fractional shortening and dP/d tmax compared with AdGFP-infected rabbits, both without inotropic stimulation and after β-adrenergic stimulation with isoproterenol. −dP/d tmax was also improved after NCX overexpression in the failing rabbits group. These results indicate that short-term effects of NCX overexpression impair contractility of isolated failing and nonfailing rabbit cardiomyocytes. NCX overexpression over 2 wk in vivo does not seem to affect myocardial contractility in nonfailing rabbits. Interestingly, in vivo overexpression of NCX decreased the progression of systolic and diastolic contractile dysfunction and improved β-adrenoceptor-mediated contractile reserve in heart failure in rabbits in vivo.

Genetic manipulation of cardiac Na+/Ca2+ exchange expression

The Na+/Ca2+ exchanger (NCX) is the primary Ca2+ extrusion mechanism in cardiomyocytes. To further investigate the role of NCX in excitation-contraction coupling and Ca2+ homeostasis, we created murine models with altered expression levels of NCX. Homozygous overexpression of NCX resulted in mild cardiac hypertrophy. Decline of the Ca2+ transient and relaxation of contraction were increased and the reverse mode of NCX was augmented. Overexpression also led to a higher susceptibility to ischemia-reperfusion injury and to a greater ability of NCX to trigger Ca2+-induced Ca2+ release. Furthermore, an increase in peak L-type Ca2+ current was observed suggesting a direct influence of NCX on L-type Ca2+ current. Whereas global knockout of NCX led to prenatal death, a recently generated cardiac-specific NCX knockout mouse was viable with surprisingly normal contractile properties. Expression levels of other Ca2+-handling proteins were not altered. Ca2+ influx in these animals is limited by a decrease of peak L-type Ca2+ current. An alternative Ca2+ efflux mechanism, presumably the plasma membrane Ca2+-ATPase, is sufficient to maintain Ca2+-homeostasis in the NCX knockout mice.

Ouabain treatment is associated with upregulation of phosphatase inhibitor-1 and Na+/Ca2+-exchanger and β-adrenergic sensitization in rat hearts

Cardiac glycosides are widely used in the treatment of congestive heart failure. While the mechanism of the positive inotropic effect after acute application of cardiac glycosides is explained by blockade of the Na+/K+-pump, little is known about consequences of a prolonged therapy. Here male Wistar rats were treated for 4 days with continuous infusions of ouabain (6.5 mg/kg/day) or 0.9% NaCl (control) via osmotic minipumps. Electrically driven (1 Hz, 35 degrees C) papillary muscles from ouabain-treated rats exhibited shorter relaxation time (-15%) and a twofold increase in the sensitivity for the positive inotropic effect of isoprenaline. The density and affinity of beta1- and beta2-adrenoceptors as well as mRNA and protein levels of stimulatory (G(s)alpha) and inhibitory (G(i)alpha-2, G(i)alpha-3) G-proteins were unaffected by ouabain. Similarly, SR-Ca2+-ATPase 2A, phospholamban, ryanodine-receptor expression as well as the oxalate-stimulated 45Ca-uptake of membrane vesicles remained unchanged. However, mRNA abundance of the protein phosphatase inhibitor-1 (I-1) and the Na+/Ca2+-exchanger (NCX) were increased by 52% and 26%, respectively. I-1 plays an amplifier role in cardiac signaling. Downregulation of I-1 in human heart failure is associated with desensitization of the beta-adrenergic signaling pathway. The present data suggest that the ouabain-induced increase in I-1 expression might be at least partly responsible for the increased isoprenaline sensitivity and increased expression of NCX for the accelerated relaxation after chronic ouabain in this model.

Learning about Cardiac Calcium Signaling from Genetic Engineering

Genetic engineering has already provided critical data on the Ca-induced Ca(2+) release (CICR) hypothesis issues and promises even greater future insights. The two approaches employed thus far are (1) the construction of transgenic animal models with deletion or overexpression of Ca(2+) signaling proteins, and (2) direct structure-function studies of these proteins in artificial systems. In our laboratory both approaches have provided some insight into molecular modulation of CICR and the pathophysiology arising from the deletion or overactivity of these proteins. Probing the cytoplasmic segments of the carboxyl c-terminal tail of Ca(2+) channel, we identified two calcium sensing and calmodulin binding domains (LA and K) that have been implicated in Ca(2+)-induced inactivation of Ca(2+) channels. Introducing these peptides into atrial myocytes, where a large fraction of Ca(2+) release sites are unassociated with the dihydropyridine receptors (DHPRs) (no t-tubules), suggests that LA, but not K motif, increases the sensitivity of RyRs to Ca(2+), is responsible for the higher frequency of Ca(2+) sparks in the peripheral sites, and provides for the voltage dependence of CICR. Genetic overexpression or deletion of the primary proteins of the Ca(2+) signaling cascade also provides supportive evidence for the Ca(2+) current (I(Ca))-gated CICR mechanism, generates some novel and unexpected cardiac phenotypes in transgenic mice, and suggests that Ca(2+) signaling defects can trigger compensatory molecular mechanisms that underlie the observed cardiac phenotype and pathophysiology.

Effects of Na(+)/Ca(2+)-exchanger overexpression on excitation-contraction coupling in adult rabbit ventricular myocytes

The Na(+)/Ca(2+)-exchanger (NCX) is the main mechanism by which Ca(2+) is transported out of the ventricular myocyte. NCX levels are raised in failing human heart, and the consequences of this for excitation-contraction coupling are still debated. We have increased NCX levels in adult rabbit myocytes by adenovirally-mediated gene transfer and examined the effects on excitation-contraction coupling after 24 and 48 h. Infected myocytes were identified through expression of green fluorescent protein (GFP), transfected under a separate promoter on the same viral construct. Control experiments were done with both non-infected myocytes and those infected with adenovirus expressing GFP only. Contraction amplitude was markedly reduced in NCX-overexpressing myocytes at either time point, and neither increasing frequency nor raising extracellular Ca(2+) could reverse this depression. Resting membrane potential and action potential duration were largely unaffected by NCX overexpression, as was peak Ca(2+) entry via the L-type Ca(2+) channel. Systolic and diastolic Ca(2+) levels were significantly reduced, with peak systolic Ca(2+) in NCX-overexpressing myocytes lower than diastolic levels in control cells at 2 m m extracellular Ca(2+). Both cell relengthening and the decay of the Ca(2+) transient were significantly slowed. Sarcoplasmic reticulum (SR) Ca(2+) stores were completely depleted in a majority of myocytes, and remained so despite increasingly vigorous loading protocols. Depressed contractility following NCX overexpression is therefore related to decreased SR Ca(2+) stores and low diastolic Ca(2+) levels rather than reduced Ca(2+) entry.

Chronic alpha-adrenergic receptor stimulation modulates the contractile phenotype of cardiac myocytes in vitro

BACKGROUND

Heart failure is characterized by contractile dysfunction of the myocardium and elevated sympathetic activity. We tested the hypothesis that chronic alpha-adrenergic (alpha-ADR) stimulation modifies the molecular and contractile phenotype of cardiac myocytes.

METHODS AND RESULTS

Adult rat ventricular myocytes in culture were exposed to alpha-ADR stimulation (norepinephrine + propranolol) for 48 hours. alpha-ADR stimulation decreased the mRNAs for sarcoplasmic reticulum Ca(2+)-ATPase and Ca(2+) release channel by 56% and 52%, respectively, and increased mRNA and protein for the Na(+)-Ca(2+) exchanger by 70% and 39%, respectively. After washout of the alpha-ADR agonist, simultaneous measurement of [Ca(2+)](i) transients with fura 2 and myocyte shortening by video edge-detection showed that [Ca(2+)](i) amplitude and myocyte shortening were decreased in alpha-ADR-treated myocytes, and the time to peak and time from peak to 80% decline of both [Ca(2+)](i) and myocyte shortening were increased. The concentration-response curve for myocyte shortening by the Na(+) channel activator veratridine was shifted leftward in alpha-ADR-stimulated myocytes (EC(50), 21.6+/-4.6 versus 105.8+/-10.5 nmol/L, P:<0.001).

CONCLUSIONS

Chronic alpha-ADR stimulation of cardiac myocytes causes decreases in the expression of sarcoplasmic reticulum Ca(2+)-ATPase and the Ca(2+) release channel that are associated with decreases in [Ca(2+)](i) and contractility. alpha-ADR stimulation simultaneously increases Na(+)-Ca(2+) exchanger expression, thereby increasing sensitivity to intracellular Na(+).

Genetically engineered models with alterations in cardiac membrane calcium-handling proteins

Regulation of intracellular Ca2+ provides a means by which the strength and duration of cardiac muscle contraction is altered on a beat-to-beat basis. Ca2+ homeostasis is maintained by proteins of the outer cell membrane or sarcolemma and the sarcoplasmic reticulum, which is the major intracellular Ca2+ storage organelle. Recently, genetic engineering techniques designed to induce specific mutations, manipulate expression levels, or change a particular isoform of various membrane Ca(2+)-handling proteins have provided novel approaches in elucidating the physiological role of these gene products in the mammalian heart. This review summarizes findings in murine genetic models with alterations in the expression levels of the sarcolemmal Ca(2+)-ATPase and Na+/Ca2+ exchanger, which move Ca2+ across the cell membrane, and the sarcoplasmic reticulum proteins, which are involved in Ca2+ sequestration (Ca(2+)-ATPase and its regulator, phospholamban), Ca2+ storage (calsequestrin), and Ca2+ release (ryanodine receptor, FK506-binding protein and junctin) during excitation-contraction coupling. Advances in genetic technology, coupled with the development of miniaturized technology to assess cardiac function at multiple levels in the mouse, have added a wealth of new information to our understanding of the functional role of each of these membrane Ca(2+)-handling proteins in cardiac physiology and pathophysiology. Furthermore, these genetic models have provided valuable insights into the compensatory cross-talk mechanisms between the major membrane Ca(2+)-handling proteins in the mammalian heart.

Overexpression of the Na(+)/K(+)/Cl(-) cotransporter gene induces cell proliferation and phenotypic transformation in mouse fibroblasts

Na(+)/K(+)/Cl(-) cotransporter activity is stimulated in early G(1) phase of the cell cycle and this stimulation was shown to be an essential event in fibroblast cell proliferation. In order to elucidate further the role of the Na(+)/K(+)/Cl(-) cotransporter in cell proliferation, we overexpressed the gene encoding the Na(+)/K(+)/Cl(-) cotransporter in mouse fibroblasts, and analyzed cellular phenotypic changes. Mouse Balb/c 3T3 cells were stably transfected with the cDNA of the shark rectal gland Na(+)/K(+)/Cl(-) cotransporter gene (NKCC1), and expressed in a mammalian vector under the cytomegalovirus promoter (Balb/c-NKCC1 cells). The transfected cells exhibited up to 10-fold greater bumetanide-sensitive Rb(+) influx compared to the control cells. The Balb/c-NKCC1 cells have acquired a typical transformation phenotype indicated by: (1) Loss of contact inhibition exhibited by growth to a higher cell density in confluent cultures, and formation of cell foci; (2) proliferation in low serum concentrations; and (3) formation of cell colonies in soft agar. The control cells transfected with the NKCC1 gene inserted in the opposite orientation in the vector retained their normal phenotype. Furthermore, the two specific inhibitors of the Na(+)/K(+)/Cl(-) cotransporter activity; bumetanide and furosemide inhibited the clonogenic efficiency in the NKCC1 transfected cells. These control experiments indicate that the apparent transformation phenotype acquired by the Balb/c-NKCC1 cells was not merely associated with the process of transfection and selecting for the neomycin-resistant clones, but rather with the overexpression of the Na(+)/K(+)/Cl(-) cotransporter gene. In order to ascertain that the regulated and normal expression of the Na(+)/K(+)/Cl(-) cotransporter control cell proliferation, the effect of bumetanide a specific inhibitor of the cotransporter, was tested on Balb/c 3T3 cell proliferation, induced by fibroblasts growth factor (FGF) and fetal calf serum (FCS). Bumetanide inhibited synchronized Balb/c 3T3 cell exit from the G(0)/G(1) arrest and entering S-phase. The inhibition was reversible, as removal of bumetanide completely released cell proliferation. Taken together, these results propose that the NKCC1 gene is involved in the control of normal cell proliferation, while its overexpression results in apparent cell transformation, in a manner similar to some protooncogenes.

Functional role of ionic regulation of Na+/Ca2+ exchange assessed in transgenic mouse hearts

Na+/Ca2+ exchange is the primary mechanism mediating Ca2+ efflux from cardiac myocytes during diastole and, thus, can prominently influence contractile force. In addition to transporting Na+ and Ca2+, the exchanger is also regulated by these ions. Although structure-function studies have identified protein regions of the exchanger subserving these regulatory processes, their physiological importance is unknown. In this study, we examined the electrophysiological and mechanical consequences of cardiospecific overexpression of the canine cardiac exchanger NCX1.1 and a deletion mutant of NCX1.1 (Delta680-685), devoid of intracellular Na+ (Na+i)- and Ca2+ (Ca2+i)- dependent regulatory properties, in transgenic mice. Using the giant excised patch-clamp technique, normal ionic regulation was observed in membrane patches from cardiomyocytes isolated from control and transgenic mice overexpressing NCX1.1. In contrast, ionic regulation was nearly abolished in mice overexpressing Delta680-685, indicating that the native regulatory processes could be overwhelmed by expression of the transgene. To address the physiological consequences of ionic regulation of the Na+/Ca2+ exchanger, we examined postrest force development in papillary muscles from NCX1.1 and Delta680-685 transgenic mice. Postrest potentiation was found to be substantially greater in Delta680-685 than in NCX1.1 transgenic mice, supporting the notion that ionic regulation of Na+/Ca2+ exchange plays a significant functional role in cardiac contractile properties.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Effects of antisense oligonucleotides to the cardiac Na+/Ca2+ exchanger on calcium dynamics in cultured cardiac myocytes

The present study was designed to explore the role of the Na+/Ca2+ exchanger on spontaneous beating of cultured cardiac myocytes. Antisense oligonucleotides (AS) based on the sequence of the cardiac Na+/Ca2+ exchanger were used to decrease expression of this Ca2+ transporting protein in cardiac myocytes. An application of AS (10 microM) caused an increase in beating rate of myocytes within 6-24 h. After 24 h of exposure, AS increased the beating rate from an average rate of 77 beats/min in control and sense-treated myocytes to 103 beats/min. Moreover, myocytes treated for 24 h with 10 microM AS exhibited an increase in diastolic [Ca2+]i levels. The antisense treatment also led to a approximately 20% decrease in expression of Na+/Ca2+ exchanger proteins within 6-24 h. Changes in mRNA levels following AS treatment could not be detected within 3- to 24-h periods. The results of these studies suggest that the Na+/Ca2+ exchanger plays a potentiating role in spontaneous the beating process by regulating [Ca2+]i dynamics and that even a small reduction in the levels of the exchanger protein has marked effects on the handling of [Ca2+]i during the cardiac cycle.