A distant upstream region of the rat multipartite Na(+)-Ca(2+) exchanger NCX1 gene promoter is sufficient to confer cardiac-specific expression

Resveratrol ameliorates cyprodinil-induced zebrafish cardiac developmental defects as an aryl hydrocarbon receptor antagonist

Cyprodinil, a globally utilized broad-spectrum pyrimidine amine fungicide, has been observed to elicit cardiac abnormality. Resveratrol (RSV), a naturally occurring polyphenolic compound, showcases remarkable defensive properties in nurturing cardiac development. To investigate whether RSV could protect against cyprodinil-induced cardiac defects, we exposed zebrafish embryos to cyprodinil (500 μg/L) in the presence or absence of RSV (1 μM). Our results showed that RSV significantly mitigated the decrease of survival rate and embryo movement and the hatching delay induced by cyprodinil. In addition, RSV also improved cyprodinil-induced zebrafish cardiac developmental toxicity, including pericardial edema and cardiac function impairment. In mechanism, RSV attenuated the cyprodinil-induced changes in mRNA expression involved in cardiac development, such as myh6, myl7, tbx5, and gata4, and calcium ion channels, such as ncx1h, slc8a4a, and atp2a2b. We further showed that RSV might inhibit the activity of aryl hydrocarbon receptor (AhR) signaling pathways induced by cyprodinil. In summary, our findings establish that the protective effects of RSV against the cardiac developmental toxicity are induced by cyprodinil due to its remarkable ability to inhibit AhR activity. Our findings not only shed light on a new avenue for regulating and ensuring the safe utilization of cyprodinil but also presents a novel concept to promote its responsible use.

The Senescent Heart-"Age Doth Wither Its Infinite Variety"

Cardiovascular diseases are a leading cause of morbidity and mortality world-wide. While many factors like smoking, hypertension, diabetes, dyslipidaemia, a sedentary lifestyle, and genetic factors can predispose to cardiovascular diseases, the natural process of aging is by itself a major determinant of the risk. Cardiac aging is marked by a conglomerate of cellular and molecular changes, exacerbated by age-driven decline in cardiac regeneration capacity. Although the phenotypes of cardiac aging are well characterised, the underlying molecular mechanisms are far less explored. Recent advances unequivocally link cardiovascular aging to the dysregulation of critical signalling pathways in cardiac fibroblasts, which compromises the critical role of these cells in maintaining the structural and functional integrity of the myocardium. Clearly, the identification of cardiac fibroblast-specific factors and mechanisms that regulate cardiac fibroblast function in the senescent myocardium is of immense importance. In this regard, recent studies show that Discoidin domain receptor 2 (DDR2), a collagen-activated receptor tyrosine kinase predominantly located in cardiac fibroblasts, has an obligate role in cardiac fibroblast function and cardiovascular fibrosis. Incisive studies on the molecular basis of cardiovascular aging and dysregulated fibroblast function in the senescent heart would pave the way for effective strategies to mitigate cardiovascular diseases in a rapidly growing elderly population.

Stroke Causes DNA Methylation at Ncx1 Heart Promoter in the Brain Via DNMT1/MeCP2/REST Epigenetic Complex

BACKGROUND

REST (Repressor-Element 1 [RE1]-silencing transcription factor) inhibits Na+/Ca2+exchanger-1 (Ncx1) transcription in neurons through the binding of RE1 site on brain promoter (Br) after stroke. We identified a new putative RE1 site in Ncx1 heart promoter (Ht) sequence (Ht-RE1) that participates in neuronal Ncx1 transcription. Because REST recruits DNA-methyltransferase-1 (DNMT1) and MeCP2 (methyl-CpG binding protein 2) on different neuronal genes, we investigated the role of this complex in Ncx1 transcriptional regulation after stroke.

METHODS AND RESULTS

Luciferase experiments performed in SH-SY5Y cells demonstrated that Br activity was selectively decreased by REST, whereas Ht activity was reduced by DNMT1, MeCP2, and REST. Notably, site-direct mutagenesis of Ht-RE1 prevented REST-dependent downregulation of Ncx1. Furthermore, in temporoparietal cortex of 8-week-old male wild-type mice (C57BL/6) subjected to transient middle cerebral artery occlusion, DNMT1, MeCP2, and REST binding to Ht promoter was increased, with a consequent DNA promoter hypermethylation. Intracerebroventricular injection of siREST prevented DNMT1/MeCP2 binding to Ht and Ncx1 downregulation, thus causing a reduction in stroke-induced damage. Consistently, in cortical neurons subjected to oxygen and glucose deprivation plus reoxygenation Ncx1 knockdown counteracted neuronal protection induced by the demethylating agent 5-azacytidine. For comparisons between 2 experimental groups, Student's t test was used, whereas for more than 2 experimental groups, 1-way ANOVA was used, followed by Tukey or Newman Keuls. Statistical significance was set at P<0.05.

CONCLUSIONS

If the results of this study are confirmed in humans, it could be asserted that DNMT1/MeCP2/REST complex disruption could be a new pharmacological strategy to reduce DNA methylation of Ht in the brain, ameliorating stroke damage.

Cardiac Na+-Ca2+ exchanger 1 (ncx1h) is critical for the ventricular cardiomyocyte formation via regulating the expression levels of gata4 and hand2 in zebrafish

Ca²⁺ signaling is critical for heart development; however, the precise roles and regulatory pathways of Ca²⁺ transport proteins in cardiogenesis remain largely unknown. Sodium-calcium exchanger 1 (Ncx1) is responsible for Ca²⁺ efflux in cardiomyocytes. It is involved in cardiogenesis, while the mechanism is unclear. Here, using the forward genetic screening in zebrafish, we identified a novel mutation at a highly-conserved leucine residue in ncx1 gene (mutant^LDD353/ncx1h^L154P) that led to smaller hearts with reduced heart rate and weak contraction. Mechanistically, the number of ventricular but not atrial cardiomyocytes was reduced in ncx1h^L154P zebrafish. These defects were mimicked by knockdown or knockout of ncx1h. Moreover, ncx1h^L154P had cytosolic and mitochondrial Ca²⁺ overloading and Ca²⁺ transient suppression in cardiomyocytes. Furthermore, ncx1h^L154P and ncx1h morphants downregulated cardiac transcription factors hand2 and gata4 in the cardiac regions, while overexpression of hand2 and gata4 partially rescued cardiac defects including the number of ventricular myocytes. These findings demonstrate an essential role of the novel 154th leucine residue in the maintenance of Ncx1 function in zebrafish, and reveal previous unrecognized critical roles of the 154th leucine residue and Ncx1 in the formation of ventricular cardiomyocytes by at least partially regulating the expression levels of gata4 and hand2.

A Novel Fluorescent Reporter System Identifies Laminin-511/521 as Potent Regulators of Cardiomyocyte Maturation

Pluripotent stem cell-derived cardiomyocytes (PSC-CMs) hold great promise for disease modeling and drug discovery. However, PSC-CMs exhibit immature phenotypes in culture, and the lack of maturity limits their broad applications. While physical and functional analyses are generally used to determine the status of cardiomyocyte maturation, they could be time-consuming and often present challenges in comparing maturation-enhancing strategies. Therefore, there is a demand for a method to assess cardiomyocyte maturation rapidly and reproducibly. In this study, we found that Myomesin-2 (Myom2), encoding M-protein, is upregulated postnatally, and based on this, we targeted TagRFP to the Myom2 locus in mouse embryonic stem cells. Myom2-RFP⁺ PSC-CMs exhibited more mature phenotypes than RFP^- cells in morphology, function and transcriptionally, conductive to sarcomere shortening assays. Using this system, we screened extracellular matrices (ECMs) and identified laminin-511/521 as potent enhancers of cardiomyocyte maturation. Together, we developed and characterized a novel fluorescent reporter system for the assessment of cardiomyocyte maturation and identified potent maturation-enhancing ECMs through this simple and rapid assay. This system is expected to facilitate use of PSC-CMs in a variety of scientific and medical investigations.

Sodium-Calcium Exchangers of the SLC8 Family in Oligodendrocytes: Functional Properties in Health and Disease

The solute carrier 8 (SLC8) family of sodium-calcium exchangers (NCXs) functions as an essential regulatory system that couples opposite fluxes of sodium and calcium ions across plasmalemmal membranes. NCXs, thereby, play key roles in maintaining an ion homeostasis that preserves cellular integrity. Hence, alterations in NCX expression and regulation have been found to lead to ionic imbalances that are often associated with intracellular calcium overload and cell death. On the other hand, intracellular calcium has been identified as a key driver for a multitude of downstream signaling events that are crucial for proper functioning of biological systems, thus highlighting the need for a tightly controlled balance. In the CNS, NCXs have been primarily characterized in the context of synaptic transmission and ischemic brain damage. However, a much broader picture is emerging. NCXs are expressed by virtually all cells of the CNS including oligodendrocytes (OLGs), the cells that generate the myelin sheath. With a growing appreciation of dynamic calcium signals in OLGs, NCXs are becoming increasingly recognized for their crucial roles in shaping OLG function under both physiological and pathophysiological conditions. In order to provide a current update, this review focuses on the importance of NCXs in cells of the OLG lineage. More specifically, it provides a brief introduction into plasmalemmal NCXs and their modes of activity, and it discusses the roles of OLG expressed NCXs in regulating CNS myelination and in contributing to CNS pathologies associated with detrimental effects on OLG lineage cells.

Mapping metabolic changes by noninvasive, multiparametric, high-resolution imaging using endogenous contrast

Monitoring subcellular functional and structural changes associated with metabolism is essential for understanding healthy tissue development and the progression of numerous diseases, including cancer, diabetes, and cardiovascular and neurodegenerative disorders. Unfortunately, established methods for this purpose either are destructive or require the use of exogenous agents. Recent work has highlighted the potential of endogenous two-photon excited fluorescence (TPEF) as a method to monitor subtle metabolic changes; however, mechanistic understanding of the connections between the detected optical signal and the underlying metabolic pathways has been lacking. We present a quantitative approach to detecting both functional and structural metabolic biomarkers noninvasively, relying on endogenous TPEF from two coenzymes, NADH (reduced form of nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). We perform multiparametric analysis of three optical biomarkers within intact, living cells and three-dimensional tissues: cellular redox state, NADH fluorescence lifetime, and mitochondrial clustering. We monitor the biomarkers in cells and tissues subjected to metabolic perturbations that trigger changes in distinct metabolic processes, including glycolysis and glutaminolysis, extrinsic and intrinsic mitochondrial uncoupling, and fatty acid oxidation and synthesis. We demonstrate that these optical biomarkers provide complementary insights into the underlying biological mechanisms. Thus, when used in combination, these biomarkers can serve as a valuable tool for sensitive, label-free identification of changes in specific metabolic pathways and characterization of the heterogeneity of the elicited responses with single-cell resolution.

Ascorbic acid promotes cardiomyogenesis through SMAD1 signaling in differentiating mouse embryonic stem cells

Numerous groups have documented that Ascorbic Acid (AA) promotes cardiomyocyte differentiation from both mouse and human ESCs and iPSCs. AA is now considered indispensable for the routine production of hPSC-cardiomyocytes (CMs) using defined media; however, the mechanisms involved with the inductive process are poorly understood. Using a genetically modified mouse embryonic stem cell (mESC) line containing a dsRED transgene driven by the cardiac-restricted portion of the ncx1 promoter, we show that AA promoted differentiation of mESCs to CMs in a dose- and time-dependent manner. Treatment of mPSCs with AA did not modulate total SMAD content; however, the phosphorylated/active forms of SMAD2 and SMAD1/5/8 were significantly elevated. Co-administration of the SMAD2/3 activator Activin A with AA had no significant effect, but the addition of the nodal co-receptor TDGF1 (Cripto) antagonized AA’s cardiomyogenic-promoting ability. AA could also reverse some of the inhibitory effects on cardiomyogenesis of ALK/SMAD2 inhibition by SB431542, a TGFβ pathway inhibitor. Treatment with BMP2 and AA strongly amplified the positive cardiomyogenic effects of SMAD1/5/8 in a dose-dependent manner. AA could not, however, rescue dorsomorphin-mediated inhibition of ALK/SMAD1 activity. Using an inducible model system, we found that SMAD1, but not SMAD2, was essential for AA to promote the formation of TNNT2⁺-CMs. These data firmly demonstrate that BMP receptor-activated SMADs, preferential to TGFβ receptor-activated SMADs, are necessary to promote AA stimulated cardiomyogenesis. AA-enhanced cardiomyogenesis thus relies on the ability of AA to modulate the ratio of SMAD signaling among the TGFβ-superfamily receptor signaling pathways.

Characterization of the zebrafish cx36.7 gene promoter: Its regulation of cardiac-specific expression and skeletal muscle-specific repression

Zebrafish connexin 36.7 (cx36.7/ecx) has been identified as a key molecule in the early stages of heart development in this species. A defect in cx36.7 causes severe heart malformation due to the downregulation of nkx2.5 expression, a result which resembles congenital heart disease in humans. It has been shown that cx36.7 is expressed specifically in early developing heart cardiomyocytes. However, the regulatory mechanism for the cardiac-restricted expression of cx36.7 remains to be elucidated. In this study we isolated the 5'-flanking promoter region of the cx36.7 gene and characterized its promoter activity in zebrafish embryos. Deletion analysis showed that a 316-bp upstream region is essential for cardiac-restricted expression. This region contains four GATA elements, the proximal two of which are responsible for promoter activation in the embryonic heart and serve as binding sites for gata4. When gata4, gata5 and gata6 were simultaneously knocked down, the promoter activity was significantly decreased. Moreover, the deletion of the region between -316 and -133bp led to EGFP expression in the embryonic trunk muscle. The distal two GATA and A/T-rich elements in this region act as repressors of promoter activity in skeletal muscle. These results suggest that cx36.7 expression is directed by cardiac promoter activation via the two proximal GATA elements as well as by skeletal muscle-specific promoter repression via the two distal GATA elements.

Isolation of contractile cardiomyocytes from human pluripotent stem-cell-derived cardiomyogenic cultures using a human NCX1-EGFP reporter

The prospective isolation of defined contractile human pluripotent stem cell (hPSC)-derived cardiomyocytes is advantageous for regenerative medicine and drug screening applications. Currently, enrichment of cardiomyocyte populations from such cultures can be achieved by combinations of cell surface markers or the labor-intensive genetic modification of cardiac developmental genes, such as NKX2.5 or MYH6, with fluorescent reporters. To create a facile, portable method for the isolation of contractile cardiomyocytes from cardiomyogenic hPSC cultures, we employed a highly conserved cardiac enhancer sequence in the SLC8A1 (NCX1) gene to generate a lentivirally deliverable, antibiotic-selectable NCX1cp-EGFP reporter. We show that human embryonic stem cells (and induced pluripotent stem cells) transduced with the NCX1cp-EGFP reporter cassette exhibit enhanced green fluorescent protein (EGFP) expression in cardiac progenitors from 5 days into the directed cardiac hPSC differentiation protocol, with all reporter-positive cells transitioning to spontaneously contracting foci 3 days later. In subsequent stages of cardiomyocyte maturation, NCX1cp-EGFP expression was exclusively limited to contractile cells expressing high levels of cardiac troponin T (CTNT), MLC2a/v, and α-actinin proteins, and was not present in CD90/THY1(+) cardiac stromal cells or CD31/PECAM(+) endothelial cells. Flow-assisted cytometrically sorted EGFP(+) fractions of differentiated cultures were highly enriched in both early (NKX2.5 and TBX5) and late (CTNT/TNNI2, MYH6, MYH7, NPPA, and MYL2) cardiomyocyte markers, with a significant proportion of cells displaying a ventricular-like action potential pattern in patch-clamp recordings. We conclude that the use of the cardiac-specific promoter of the human SLC8A1(NCX1) gene is an effective strategy to isolate contractile cardiac cells and their progenitors from hPSC-derived cardiomyogenic cultures.

GATA-dependent transcriptional and epigenetic control of cardiac lineage specification and differentiation

Heart progenitor cells differentiate into various cell types including pacemaker and working cardiomyocytes. Cell-type specific gene expression is achieved by combinatorial interactions between tissue-specific transcription factors (TFs), co-factors, and chromatin remodelers and DNA binding elements in regulatory regions. Dysfunction of these transcriptional networks may result in congenital heart defects. Functional analysis of the regulatory DNA sequences has contributed substantially to the identification of the transcriptional network components and combinatorial interactions regulating the tissue-specific gene programs. GATA TFs have been identified as central players in these networks. In particular, GATA binding elements have emerged as a platform to recruit broadly active histone modification enzymes and cell-type-specific co-factors to drive cell-type-specific gene programs. Here, we discuss the role of GATA factors in cell fate decisions and differentiation in the developing heart.

Molecular mechanisms of cardiomyocyte aging

Western societies are rapidly aging, and cardiovascular diseases are the leading cause of death. In fact, age and cardiovascular diseases are positively correlated, and disease syndromes affecting the heart reach epidemic proportions in the very old. Genetic variations and molecular adaptations are the primary contributors to the onset of cardiovascular disease; however, molecular links between age and heart syndromes are complex and involve much more than the passage of time. Changes in CM (cardiomyocyte) structure and function occur with age and precede anatomical and functional changes in the heart. Concomitant with or preceding some of these cellular changes are alterations in gene expression often linked to signalling cascades that may lead to a loss of CMs or reduced function. An understanding of the intrinsic molecular mechanisms underlying these cascading events has been instrumental in forming our current understanding of how CMs adapt with age. In the present review, we describe the molecular mechanisms underlying CM aging and how these changes may contribute to the development of cardiovascular diseases.

Embryonic stem cell-derived cardiomyocyte heterogeneity and the isolation of immature and committed cells for cardiac remodeling and regeneration

Pluripotent stem cells represent one promising source for cell replacement therapy in heart, but differentiating embryonic stem cell-derived cardiomyocytes (ESC-CMs) are highly heterogeneous and show a variety of maturation states. In this study, we employed an ESC clonal line that contains a cardiac-restricted ncx1 promoter-driven puromycin resistance cassette together with a mass culture system to isolate ESC-CMs that display traits characteristic of very immature CMs. The cells display properties of proliferation, CM-restricted markers, reduced mitochondrial mass, and hypoxia-resistance. Following transplantation into rodent hearts, bioluminescence imaging revealed that immature cells, but not more mature CMs, survived for at least one month following injection. These data and comparisons with more mature cells lead us to conclude that immature hypoxia resistant ESC-CMs can be isolated in mass in vitro and, following injection into heart, form grafts that may mediate long-term recovery of global and regional myocardial contractile function following infarction.

The mouse C2C12 myoblast cell surface N-linked glycoproteome: identification, glycosite occupancy, and membrane orientation

Endogenous regeneration and repair mechanisms are responsible for replacing dead and damaged cells to maintain or enhance tissue and organ function, and one of the best examples of endogenous repair mechanisms involves skeletal muscle. Although the molecular mechanisms that regulate the differentiation of satellite cells and myoblasts toward myofibers are not fully understood, cell surface proteins that sense and respond to their environment play an important role. The cell surface capturing technology was used here to uncover the cell surface N-linked glycoprotein subproteome of myoblasts and to identify potential markers of myoblast differentiation. 128 bona fide cell surface-exposed N-linked glycoproteins, including 117 transmembrane, four glycosylphosphatidylinositol-anchored, five extracellular matrix, and two membrane-associated proteins were identified from mouse C2C12 myoblasts. The data set revealed 36 cluster of differentiation-annotated proteins and confirmed the occupancy for 235 N-linked glycosylation sites. The identification of the N-glycosylation sites on the extracellular domain of the proteins allowed for the determination of the orientation of the identified proteins within the plasma membrane. One glycoprotein transmembrane orientation was found to be inconsistent with Swiss-Prot annotations, whereas ambiguous annotations for 14 other proteins were resolved. Several of the identified N-linked glycoproteins, including aquaporin-1 and beta-sarcoglycan, were found in validation experiments to change in overall abundance as the myoblasts differentiate toward myotubes. Therefore, the strategy and data presented shed new light on the complexity of the myoblast cell surface subproteome and reveal new targets for the clinically important characterization of cell intermediates during myoblast differentiation into myotubes.

Enhanced proliferation of monolayer cultures of embryonic stem (ES) cell-derived cardiomyocytes following acute loss of retinoblastoma

Background

Cardiomyocyte (CM) cell cycle analysis has been impeded because of a reliance on primary neonatal cultures of poorly proliferating cells or chronic transgenic animal models with innate compensatory mechanisms.

Methodology/Principal Findings

We describe an in vitro model consisting of monolayer cultures of highly proliferative embryonic stem (ES) cell-derived CM. Following induction with ascorbate and selection with puromycin, early CM cultures are >98% pure, and at least 85% of the cells actively proliferate. During the proliferative stage, cells express high levels of E2F3a, B-Myb and phosphorylated forms of retinoblastoma (Rb), but with continued cultivation, cells stop dividing and mature functionally. This developmental transition is characterized by a switch from slow skeletal to cardiac TnI, an increase in binucleation, cardiac calsequestrin and hypophosphorylated Rb, a decrease in E2F3, B-Myb and atrial natriuretic factor, and the establishment of a more negative resting membrane potential. Although previous publications suggested that Rb was not necessary for cell cycle control in heart, we find following acute knockdown of Rb that this factor actively regulates progression through the G1 checkpoint and that its loss promotes proliferation at the expense of CM maturation.

Conclusions/Significance

We have established a unique model system for studying cardiac cell cycle progression, and show in contrast to previous reports that Rb actively regulates both cell cycle progression through the G1 checkpoint and maturation of heart cells. We conclude that this in vitro model will facilitate the analysis of cell cycle control mechanisms of CMs.

Pluripotency of embryonic stem cells

Embryonic stem (ES) cells derived from pre-implantation embryos have the potential to differentiate into any cell type derived from the three germ layers of ectoderm (epidermal tissues and nerves), mesoderm (muscle, bone, blood), and endoderm (liver, pancreas, gastrointestinal tract, lungs), including fetal and adult cells. Alone, these cells do not develop into a viable fetus or adult animal because they do not retain the potential to contribute to extraembryonic tissue, and in vitro, they lack spatial and temporal signaling cues essential to normal in vivo development. The basis of pluripotentiality resides in conserved regulatory networks composed of numerous transcription factors and multiple signaling cascades. Together, these regulatory networks maintain ES cells in a pluripotent and undifferentiated form; however, alterations in the stoichiometry of these signals promote differentiation. By taking advantage of this differentiation capacity in vitro, ES cells have clearly been shown to possess the potential to generate multipotent stem and progenitor cells capable of differentiating into a limited number of cell fates. These latter types of cells may prove to be therapeutically viable, but perhaps more importantly, the studies of these cells have led to a greater understanding of mammalian development.

Evidence of intercellular coupling between co-cultured adult rabbit ventricular myocytes and myofibroblasts

Intercellular coupling between ventricular myocytes and myofibroblasts was studied by co-culturing adult rabbit ventricular myocytes with previously prepared layers of cardiac myofibroblasts. Intercellular coupling was examined by: (i) tracking the movement of the fluorescent dye calcein; (ii) immunostaining for connexin 43 (Cx43); and (iii) measurement of intracellular [Ca2+] ([Ca2+]i). The effects of stimulating ventricular myocytes on the underlying myofibroblasts was examined by confocal measurements of [Ca2+]i using fluo-3. When ventricular myocytes were preloaded with calcein and co-cultured with myofibroblasts for 24 h, calcein fluorescence was detected in 52+/-4% (n=8 co-cultures) of surrounding myofibroblasts. Treatment with the gap junction uncoupler heptanol significantly reduced the movement of calcein (12+/-3%, n=6 co-cultures). Immunostaining showed expression of Cx43 in co-cultured myofibroblasts and myocytes. Field stimulation of ventricular myocytes co-cultured with myofibroblasts increased myofibroblast [Ca2+]i, no response was observed after treatment with heptanol or stimulation of fibroblasts in the absence of ventricular myocytes. Action potential parameters of ventricular myocytes in co-culture were similar to control values. However, application of the hormone sphingosine-1-phosphate (S-1-P) to the co-culture caused a depolarization of ventricular myocytes to approximately -20 mV. Sphingosine-1-phosphate had no effect on ventricular myocytes alone. Voltage-clamp measurements of isolated myofibroblasts indicated that S-1-P activated a significant quasi-linear current with a reversal potential of approximately -40 mV. In conclusion, this study shows that stimulation of the ventricular myocyte influences the intracellular Ca2+ of the linked myofibroblast via connexons. These intercellular links also allow the myofibroblasts to influence the electrical activity of the myocyte. This work indicates the nature of the gap junction-mediated bi-directional interactions that occur between ventricular myocyte and myofibroblast.

Calcium cycling protein density and functional importance to automaticity of isolated sinoatrial nodal cells are independent of cell size

Spontaneous, localized, rhythmic ryanodine receptor (RyRs) Ca(2+) releases occur beneath the cell membrane during late diastolic depolarization in cardiac sinoatrial nodal cells (SANCs). These activate the Na(+)/Ca(2+) exchanger (NCX1) to generate inward current and membrane excitation that drives normal spontaneous beating. The morphological background for the proposed functional of RyR and NCX crosstalk, however, has not been demonstrated. Here we show that the average isolated SANC whole cell labeling density of RyRs and SERCA2 is similar to atrial and ventricle myocytes, and is similar among SANCs of all sizes. Labeling of NCX1 is also similar among SANCs of all sizes and exceeds that in atrial and ventricle myocytes. Submembrane colocalization of NCX1 and cardiac RyR (cRyR) in all SANCs exceeds that in the other cell types. Further, the Cx43 negative primary pacemaker area of the intact rabbit sinoatrial node (SAN) exhibits robust positive labeling for cRyR, NCX1, and SERCA2. Functional studies in isolated SANCs show that neither the average action potential (AP) characteristics, nor those of intracellular Ca(2+) releases, nor the spontaneous cycle length vary with cell size. Chelation of intracellular [Ca(2+)], or disabling RyRs or NCX1, markedly attenuates or abolishes spontaneous SANC beating in all SANCs. Thus, there is dense labeling of SERCA2, RyRs, and NCX1 in small-sized SANCs, thought to reside within the SAN center, the site of impulse initiation. Because normal automaticity of these cells requires intact Ca(2+) cycling, interactions of SERCA, RyR2 and NCX molecules are implicated in the initiation of the SAN impulse.

Regulation of Ncx1 gene expression in the normal and hypertrophic heart

The Na+/Ca2+ exchanger (NCX1) is crucial in the regulation of [Ca2+]i in the cardiac myocyte. The exchanger is upregulated in cardiac hypertrophy, ischemia, and failure. This upregulation can have an effect on Ca2+ transients and possibly contribute to diastolic dysfunction and an increased risk of arrhythmias. Studies from both in vivo and in vitro model systems have provided an initial skeleton of the potential signaling pathways that regulate the exchanger during development, growth, and hypertrophy. The Ncx1 gene is upregulated in response to alpha-adrenergic stimulation. We have shown that this is via p38alpha activation of transcription factors binding to the Ncx1 promotor at the -80 CArG element. Interestingly, most of the elements, including the CArG element, which we have demonstrated to be important for regulation of Ncx1 expression are in the proximal 184 bp of the promotor. Using a transgenic mouse, we have shown that the proximal 184 bp is sufficient for expression of reporter genes in adult cardiomyocytes and for the correct spatiotemporal pattern of Ncx1 expression in development but not for upregulation in response to pressure overload.

Molecular structure and developmental expression of three muscle-type troponin T genes in zebrafish

Troponin T (Tnnt), a troponin component, interacts with tropomyosin and is crucial to the regulation of striated muscle contraction. To gain insight into the molecular evolution and developmental regulation of Tnnt gene (Tnnt) in lower vertebrates, zebrafish Tnnt1 (slow Tnnt), Tnnt2 (cardiac Tnnt), and Tnnt3b (fast Tnnt isoform b) were characterized. The polypeptides of zebrafish Tnnt1, Tnnt2, and Tnnt3b were conserved in the central tropomyosin- and C-terminal troponin I-binding domains. However, the N-terminal hypervariable regions were highly extended and rich in glutamic acid in polypeptides of Tnnt1 and Tnnt2, but not Tnnt3b. The Tnnt2 and Tnnt3b contain introns, whereas Tnnt1 is intron-free. During development, large to small, alternatively spliced variants were detected in Tnnt2, but not in Tnnt1 or Tnnt3. Whole-mount in situ hybridization showed zebrafish Tnnt1 and Tnnt2 are activated during early somitogenesis (10 hr postfertilization, hpf) and cardiogenesis (14 hpf), respectively, but Tnnt3b is not activated until middle somitogenesis (18 hpf). Tnnt2 and Tnnt3b expression was cardiac- and fast-muscle specific, but Tnnt1 was expressed in both slow and fast muscles. We propose that three, distinct, muscle-type Tnnt evolved after the divergence of fish and deuterostome invertebrates. In zebrafish, the developmental regulation of Tnnt during somitogenesis and cardiogenesis is more restricted and simpler than in tetrapods. These new findings may provide insight into the developmental regulation and molecular evolution of vertebrate Tnnt.

Cyclic variation of intracellular calcium: a critical factor for cardiac pacemaker cell dominance

While a diversity of cell types and distribution within the sinoatrial node and cell-cell interactions add complexity to a complete elucidation of the heart's pacemaker function, it has become clear that cyclic variation of submembrane [Ca2+] and activation of the Na+-Ca2+ exchanger during diastolic depolarization (DD) act in concert with ion channels to confer on sinoatrial node cells (SANCs) their status of dominance with respect to pacemaker function. Studies using confocal microscopy indicate that subsarcolemmal Ca2+ release via ryanodine receptors occurs not only in response to the action potential (AP) upstroke, but also during the DD, and this is augmented by beta-adrenergic receptor (beta-AR) stimulation. Spontaneous APs simulated by mathematical SANC models beat at a faster rate when this subsarcolemmal Ca2+ waveform measured under beta-AR stimulation is introduced into the modeling scheme. Thus, in future investigation of pacemaker functioning in health, disease, and disease therapies the "bar ought to be raised" to embrace the impact of cyclic variation in submembrane [Ca2+] on pacemaker function. The full text of this article is available at http://www.circresaha.org.

Role of sodium-calcium exchanger (Ncx1) in embryonic heart development: a transgenic rescue?

Na(+)/Ca(2+) exchanger (Ncx-1) is highly expressed in cardiomyocytes, is thought to be required to maintain a low intracellular Ca(2+) concentration, and may play a role in excitation-contraction coupling. Significantly, targeted deletion of Ncx-1 results in Ncx1-null embryos that do not have a spontaneously beating heart and die in utero. Ultrastructural analysis revealed gross anomalies in the Ncx1-null contractile apparatus, but physiologic analysis showed normal field-stimulated Ca(2+) transients, suggesting that Ncx-1 function may not be critical for Ca(2+) extrusion from the cytosol as previously thought. Using caffeine to empty the intracellular Ca(2+) stores, we show that the sarcoplasmic reticulum is not fully functional within the 9.5-dpc mouse heart, indicating that the sarcoplasmic reticulum is unlikely to account for the unexpected maintenance of intracellular Ca(2+) homeostasis. Using the Ncx1-lacZ reporter, our data indicate restricted expression patterns of Ncx1 and that Ncx1 is highly expressed within the conduction system, suggesting Ncx1 may be required for spontaneous pacemaking activity. To test this hypothesis, we used transgenic mice overexpressing one of the two known adult Ncx1 isoforms under the control of the cardiac-specific a-myosin heavy chain promoter to restore Ncx1 expression within the Ncx1-null hearts. Results indicate that the transgenic re-expression of one Ncx1 isoform was unable to rescue the lethal null mutant phenotype. Furthermore, our in situ results indicate that both known adult Ncx1 isoforms are coexpressed within the embryonic heart, suggesting that effective transgenic rescue may require the presence of both isoforms within the developing heart.