The role of GATA, CArG, E-box, and a novel element in the regulation of cardiac expression of the Na+-Ca2+ exchanger gene

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.

Mechano-sensitive regulation of gene expression during the embryonic development

Cell movements during embryogenesis produce mechanical tensions that shape the embryo and can also regulate gene expression, thereby affecting cell differentiation. Increasing evidence indicates that mechanosensitive regulation of gene expression plays important roles during embryogenesis by coupling the processes of morphogenesis and differentiation. However, the molecular mechanisms of this phenomenon remain poorly understood. This review focuses on the molecular mechanisms that "translate" mechanical stimuli into gene expression.

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.

Swimming training promotes cardiac remodeling and alters the expression of mRNA and protein levels involved in calcium handling in hypertensive rats

AIM

The aim of this study was to identify the effects of swimming training on the mRNA expression and protein levels of the calcium handling proteins in the hearts of renovascular hypertensive rats submitted to swimming protocol during 6 weeks.

MAIN METHODS

Fischer rats with renovascular hypertension 2-kidney 1-clip (2K1C) and SHAM groups were divided among sedentary and exercised groups. The exercise protocol lasted for 6 weeks (1 h/day, 5×/week), and the mean arterial pressure, cardiomyocytes hypertrophy parameters, mRNA expression and protein levels of some calcium handling proteins in the left ventricle were evaluated.

KEY FINDINGS

Swimming training was able to reduce the levels of mean arterial pressure in the hypertensive group compared to 2K1C SED, and to promote cardiac hypertrophy in SHAM EX and 2K1C EX groups in comparison to the respective control groups. The mRNA levels of B-type natriuretic peptide were reduced in the 2K1C EX when compared to 2K1C SED. The mRNA and protein levels of the sarcoplasmic reticulum Ca2 +-ATPase increased after the swimming training in SHAM and 2K1C groups. The mRNA and protein levels of phospholamban, displayed an increase in their levels in the exercised SHAM and in hypertensive rats in comparison to their respective controls; while mRNA levels of Na+/Ca2 + exchanger was reduced in the left ventricle comparing to the sedentary hypertensive rats.

SIGNIFICANCE

Taken altogether, we provide evidence that the aerobic training may lead to cardiac remodeling, and modulate the calcium handling proteins expression in the heart of hypertensive rats.

Calcium handling proteins: structure, function, and modulation by exercise

Heart failure is a serious public health issue with a growing prevalence, and it is related with the aging of the population. Hypertension is identified as the main precursor of left ventricular hypertrophy and therefore can lead to diastolic dysfunction and heart failure. Scientific studies have confirmed the beneficial effects of the physical exercise by reducing the blood pressure and improving the functional status of the heart in hypertension. Several proteins are involved in the mobilization of calcium during the coupling excitation-contraction process in the heart among those are sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban, calsequestrin, sodium-calcium exchanger, L-type calcium's channel, and ryanodine receptors. Our goal is to address the beneficial effects of exercise on the calcium handling proteins in a heart with hypertension.

Recent structural and functional insights into the family of sodium calcium exchangers

Maintenance of calcium homeostasis is necessary for the development and survival of all animals. Calcium ions modulate excitability and bind effectors capable of initiating many processes such as muscular contraction and neurotransmission. However, excessive amounts of calcium in the cytosol or within intracellular calcium stores can trigger apoptotic pathways in cells that have been implicated in cardiac and neuronal pathologies. Accordingly, it is critical for cells to rapidly and effectively regulate calcium levels. The Na(+) /Ca(2+) exchangers (NCX), Na(+) /Ca(2+) /K(+) exchangers (NCKX), and Ca(2+) /Cation exchangers (CCX) are the three classes of sodium calcium antiporters found in animals. These exchanger proteins utilize an electrochemical gradient to extrude calcium. Although they have been studied for decades, much is still unknown about these proteins. In this review, we examine current knowledge about the structure, function, and physiology and also discuss their implication in various developmental disorders. Finally, we highlight recent data characterizing the family of sodium calcium exchangers in the model system, Caenorhabditis elegans, and propose that C. elegans may be an ideal model to complement other systems and help fill gaps in our knowledge of sodium calcium exchange biology.

Genetic isolation of stem cell-derived pacemaker-nodal cardiac myocytes

Dysfunction of the cardiac pacemaker tissues due to genetic defects, acquired diseases, or aging results in arrhythmias. When arrhythmias occur, artificial pacemaker implants are used for treatment. However, the numerous limitations of electronic implants have prompted studies of biological pacemakers that can integrate into the myocardium providing a permanent cure. Embryonic stem (ES) cells cultured as three-dimensional (3D) spheroid aggregates termed embryoid bodies possess the ability to generate all cardiac myocyte subtypes. Here, we report the use of a SHOX2 promoter and a Cx30.2 enhancer to genetically identify and isolate ES cell-derived sinoatrial node (SAN) and atrioventricular node (AVN) cells, respectively. The ES cell-derived Shox2 and Cx30.2 cardiac myocytes exhibit a spider cell morphology and high intracellular calcium loading characteristic of pacemaker-nodal myocytes. These cells express abundant levels of pacemaker genes such as endogenous HCN4, Cx45, Cx30.2, Tbx2, and Tbx3. These cells were passaged, frozen, and thawed multiple times while maintaining their pacemaker-nodal phenotype. When cultured as 3D aggregates in an attempt to create a critical mass that simulates in vivo architecture, these cell lines exhibited an increase in the expression level of key regulators of cardiovascular development, such as GATA4 and GATA6 transcription factors. In addition, the aggregate culture system resulted in an increase in the expression level of several ion channels that play a major role in the spontaneous diastolic depolarization characteristic of pacemaker cells. We have isolated pure populations of SAN and AVN cells that will be useful tools for generating biological pacemakers.

Transcriptional pathways and potential therapeutic targets in the regulation of Ncx1 expression in cardiac hypertrophy and failure

Changes in cardiac gene expression contribute to the progression of heart failure by affecting cardiomyocyte growth, function, and survival. The Na(+)-Ca(2+) exchanger gene (Ncx1) is upregulated in hypertrophy and is often found elevated in end-stage heart failure. Studies have shown that the change in its expression contributes to contractile dysfunction. Several transcriptional pathways mediate Ncx1 expression in pathological cardiac remodeling. Both α-adrenergic receptor (α-AR) and β-adrenergic receptor (β-AR) signaling can play a role in the regulation of calcium homeostasis in the cardiomyocyte, but chronic activation in periods of cardiac stress contributes to heart failure by mechanisms which include Ncx1 upregulation. Our studies have even demonstrated that NCX1 can directly act as a regulator of "activity-dependent signal transduction" mediating changes in its own expression. Finally, we present evidence that histone deacetylases (HDACs) and histone acetyltransferases (HATs) act as master regulators of Ncx1 expression. We show that many of the transcription factors regulating Ncx1 expression are important in cardiac development and also in the regulation of many other genes in the so-called fetal gene program, which are activated by pathological stimuli. Importantly, studies have revealed that the transcriptional network regulating Ncx1 expression is also mediating many of the other changes in genetic remodeling contributing to the development of cardiac dysfunction and revealed potential therapeutic targets for the treatment of hypertrophy and failure.

Transcriptional regulation of ncx1 gene in the brain

The ubiquitous sodium-calcium exchanger isoform 1 (NCX1) is a -bidirectional transporter that plays a relevant role under physiological and pathophysiological conditions including brain ischemia by regulating intraneuronal Ca(2+) and Na(+) homeostasis. Although changes in ncx1 protein and transcript expression have been detected during stroke, its transcriptional regulation is still largely unexplored. Here, we reviewed our recent findings on several transcription factors including cAMP response element-binding protein (CREB), nuclear factor kappa B (NF-κB), and hypoxia-inducible factor-1 (HIF-1) in the control of the ncx1 gene expression in neuronal cells.

SERCA2a gene therapy restores microRNA-1 expression in heart failure via an Akt/FoxO3A-dependent pathway

AIMS

Impaired myocardial sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) activity is a hallmark of failing hearts, and SERCA2a gene therapy improves cardiac function in animals and patients with heart failure (HF). Deregulation of microRNAs has been demonstrated in HF pathophysiology. We studied the effects of therapeutic AAV9.SERCA2a gene therapy on cardiac miRNome expression and focused on regulation, expression, and function of miR-1 in reverse remodelled failing hearts.

METHODS AND RESULTS

We studied a chronic post-myocardial infarction HF model treated with AAV9.SERCA2a gene therapy. Heart failure resulted in a strong deregulation of the cardiac miRNome. miR-1 expression was decreased in failing hearts, but normalized in reverse remodelled hearts after AAV9.SERCA2a gene delivery. Increased Akt activation in cultured cardiomyocytes led to phosphorylation of FoxO3A and subsequent exclusion from the nucleus, resulting in miR-1 gene silencing. In vitro SERCA2a expression also rescued miR-1 in failing cardiomyocytes, whereas SERCA2a inhibition reduced miR-1 levels. In vivo, Akt and FoxO3A were highly phosphorylated in failing hearts, but reversed to normal by AAV9.SERCA2a, leading to cardiac miR-1 restoration. Likewise, enhanced sodium-calcium exchanger 1 (NCX1) expression during HF was normalized by SERCA2a gene therapy. Validation experiments identified NCX1 as a novel functional miR-1 target.

CONCLUSION

SERCA2a gene therapy of failing hearts restores miR-1 expression by an Akt/FoxO3A-dependent pathway, which is associated with normalized NCX1 expression and improved cardiac function.

CaMKIIδB mediates aberrant NCX1 expression and the imbalance of NCX1/SERCA in transverse aortic constriction-induced failing heart

Ca²⁺/calmodulin-dependent protein kinase II δB (CaMKIIδB) is one of the predominant isoforms of CaMKII in the heart. The precise role of CaMKIIδB in the transcriptional cross-talk of Ca²⁺-handling proteins during heart failure remains unclear. In this work, we aim to determine the mechanism of CaMKIIδB in modulating the expression of sarcolemmal Na⁺–Ca²⁺ exchange (NCX1). We also aim to address the potential effects of calmodulin antagonism on the imbalance of NCX1 and sarcoendoplasmic reticulum Ca²⁺ ATPase (SERCA) during heart failure. Eight weeks after transverse aortic constriction (TAC)-induced heart failure in mice, we found that the heart weight/tibia length (HW/TL) ratio and the lung weight/body weight (LW/BW) ratio increased by 59% and 133%, respectively. We further found that the left ventricle-shortening fraction decreased by 40% compared with the sham-operated controls. Immunoblotting revealed that the phosphorylation of CaMKIIδB significantly increased 8 weeks after TAC-induced heart failure. NCX1 protein levels were also elevated, whereas SERCA2 protein levels decreased in the same animal model. Moreover, transfection of active CaMKIIδB significantly increased NCX1 protein levels in adult mouse cardiomyocytes via class IIa histone deacetylase (HDAC)/myocyte enhancer factor-2 (MEF2)-dependent signaling. In addition, pharmacological inhibition of calmodulin/CaMKIIδB activity improved cardiac function in TAC mice, which partially normalized the imbalance between NCX1 and SERCA2. These data identify NCX1 as a cellular target for CaMKIIδB. We also suggest that the CaMKIIδB-induced imbalance between NCX1 and SERCA2 is partially responsible for the disturbance of intracellular Ca²⁺ homeostasis and the pathological process of heart failure.

Cell signaling pathways for the regulation of GATA4 transcription factor: Implications for cell growth and apoptosis

GATA4 is a member of the GATA family of zinc finger transcription factor, which regulates gene transcription by binding to GATA elements. GATA4 was originally discovered as a regulator of cardiac development and subsequently identified as a major regulator of adult cardiac hypertrophy. GATA4 regulates gene expression of various genes, which are involved in cardiac development and cardiac hypertrophy and heart failure. In addition to the heart, GATA4 plays important roles in the reproductive system, gastrointestinal system, respiratory system and cancer. Positive and negative regulations of GATA4 therefore are important components of biologic functions. The activation of GATA4 occurs via various cell signaling events. Earlier studies have identified protein-protein interactions of GATA4 with other factors. The discovery of interactions of GATA4 with nuclear factor for activated T cells (NFAT) revealed the importance of calcium signaling in the activation of GATA4. GATA4 can also be phosphorylated by mitogen activated protein kinases and protein kinase A. Lysine modifications also occur on the GATA4 molecule including acetylation and sumoylation. Both reactive oxygen-dependent and -independent antioxidant-sensitive pathways for GATA4 activation have also been demonstrated. The GATA4 activity is also regulated by modulating the level of GATA4 expression via transcriptional as well as translational mechanisms. This work summarizes the current understanding of regulatory mechanisms for modulating GATA4 activity.

Serum response factor expression is enriched in pancreatic β cells and regulates insulin gene expression

Serum response factor (SRF) is an essential regulator of myogenic and neurogenic genes and the ubiquitously expressed immediate-early genes. The purpose of this study is to determine SRF expression pattern in murine pancreas and examine the role of SRF in pancreatic gene expression. Immunohistochemical analysis of wild-type pancreas and LacZ staining of pancreas from SRF LacZ knock-in animals showed that SRF expression is restricted to β cells. SRF bound to the rat insulin promoter II (RIP II) serum response element, an element conserved in both rat I and murine I and II insulin promoters. SRF activated RIP II, and SRF binding to RIP II and the exon 5-encoded 64-aa subdomain of SRF was required for this activation. Transient or stable knockdown of SRF leads to down-regulation of insulin gene expression, suggesting that SRF is required for insulin gene expression. Further, SRF physically interacted with the pancreas and duodenum homeobox-1 (Pdx-1) and synergistically activated RIP II. Elevated glucose concentration down-regulated SRF binding to RIP II SRE, and this down-regulation was associated with decreased RIP II activity and increased SRF phosphorylation on serine 103. Together, our results demonstrate that SRF is a glucose concentration-sensitive regulator of insulin gene expression.

Identification of distal cis-regulatory elements at mouse mitoferrin loci using zebrafish transgenesis

Mitoferrin 1 (Mfrn1; Slc25a37) and mitoferrin 2 (Mfrn2; Slc25a28) function as essential mitochondrial iron importers for heme and Fe/S cluster biogenesis. A genetic deficiency of Mfrn1 results in a profound hypochromic anemia in vertebrate species. To map the cis-regulatory modules (CRMs) that control expression of the Mfrn genes, we utilized genome-wide chromatin immunoprecipitation (ChIP) datasets for the major erythroid transcription factor GATA-1. We identified the CRMs that faithfully drive the expression of Mfrn1 during blood and heart development and Mfrn2 ubiquitously. Through in vivo analyses of the Mfrn-CRMs in zebrafish and mouse, we demonstrate their functional and evolutionary conservation. Using knockdowns with morpholinos and cell sorting analysis in transgenic zebrafish embryos, we show that GATA-1 directly regulates the expression of Mfrn1. Mutagenesis of individual GATA-1 binding cis elements (GBE) demonstrated that at least two of the three GBE within this CRM are functionally required for GATA-mediated transcription of Mfrn1. Furthermore, ChIP assays demonstrate switching from GATA-2 to GATA-1 at these elements during erythroid maturation. Our results provide new insights into the genetic regulation of mitochondrial function and iron homeostasis and, more generally, illustrate the utility of genome-wide ChIP analysis combined with zebrafish transgenesis for identifying long-range transcriptional enhancers that regulate tissue development.

SMYD1, the myogenic activator, is a direct target of serum response factor and myogenin

SMYD1 is a heart and muscle specific SET-MYND domain containing protein, which functions as a histone methyltransferase and regulates downstream gene transcription. We demonstrated that the expression of SMYD1 is restricted in the heart and skeletal muscle tissues in human. To reveal the regulatory mechanisms of SMYD1 expression during myogenesis and cardiogenesis, we cloned and characterized the human SMYD1 promoter, which contains highly conserved serum response factor (SRF) and myogenin binding sites. Overexpression of SRF and myogenin significantly increased the endogenous expression level of Smyd1 in C2C12 cells, respectively. Deletion of Srf in the heart of mouse embryos dramatically decreased the expression level of Smyd1 mRNA and the expression of Smyd1 can be rescued by exogenous SRF introduction in SRF null ES cells during differentiation. Furthermore, we demonstrated that SRF binds to the CArG site and myogenin binds to the E-box element on Smyd1 promoter region using EMSA and ChIP assays. Moreover, forced expression of SMYD1 accelerates myoblast differentiation and myotube formation in C2C12 cells. Taken together, these studies demonstrated that SMYD1 is a key regulator of myogenic differentiation and acts as a downstream target of muscle regulatory factors, SRF and myogenin.

Chronic administration of KB-R7943 induces up-regulation of cardiac NCX1

The NCX1 (sodium-calcium exchanger) is up-regulated in human heart failure and in many animal models of heart failure. The potential benefits and risks of therapeutically blocking NCX1 in heart failure and during ischemia-reperfusion are being actively investigated. In this study, we demonstrate that prolonged administration of the NCX1 inhibitor KB-R7943 resulted in the up-regulation of Ncx1 gene expression in both isolated adult cardiomyocytes and intact mouse hearts. Ncx1 up-regulation is mediated by the activation of p38. Importantly, p38 is not activated by KB-R7943 treatment in heart tubes from Ncx1(-/-) mice at 9.5 days postcoitum but is activated in heart tubes from Ncx1(+/+) mice. p38 activation does not appear to be in response to changes in cytosolic calcium concentration, [Ca(2+)](i). Interestingly, chronic KB-R7943 treatment in mice leads to the formation of an NCX1-p38 complex. Our study demonstrates for the first time that the electrogenic sarcolemma membrane cardiac NCX1 can act as a regulator of "activity-dependent signal transduction" leading to changes in gene expression.

Histone deacetylases facilitate sodium/calcium exchanger up-regulation in adult cardiomyocytes

It is becoming increasingly evident that histone deacetylases (HDACs) have a prominent role in the alteration of gene expression during the growth remodeling process of cardiac hypertrophy. HDACs are generally viewed as corepressors of gene expression. However, we demonstrate that class I and class II HDACs play an important role in the basal expression and up-regulation of the sodium calcium exchanger (Ncx1) gene in adult cardiomyocytes. Treatment with the HDAC inhibitor trichostatin A (TSA) prevented the pressure-overload-stimulated up-regulation of Ncx1 expression. Overexpression of HDAC5 resulted in the dose-dependent up-regulation of basal and alpha-adrenergic stimulated Ncx1 expression. We show that Nkx2.5 recruits HDAC5 to the Ncx1 promoter, where HDAC5 complexes with HDAC1. Nkx2.5 also interacts with transcriptional activator p300, which is recruited to the Ncx1 promoter. We demonstrate that when Nkx2.5 is acetylated, it is found associated with HDAC5, whereas deacetylated Nkx2.5 is in complex with p300. Notably, TSA treatment prevents p300 from being recruited to the endogenous Ncx1 promoter, resulting in the repression of Ncx1 expression. We propose a novel model for Ncx1 regulation in which deacetylation of Nkx2.5 is required for the recruitment of p300 and results in up-regulation of exchanger expression.

Role of the 5'-untranslated region in regulating translational efficiency of specific mRNAs in adult cardiocytes

It has been hypothesized that translational efficiency is determined by the amount of secondary structure in the 5'-untranslated region (5'-UTR) of mRNA. Here, we examined whether specific 5'-UTRs with excessive secondary structure selectively regulate translational efficiency in adult cardiocytes. Recombinant adenoviruses were generated to express reporter mRNAs consisting of the 5'-UTR derived from c-jun or ornithine decarboxylase (ODC) fused to beta-galactosidase (betaGal) coding sequence. Each adenovirus expressed GFP mRNA as a control for 5'-UTRs with minimal secondary structure. Subsequently, cardiocytes were electrically stimulated to contract at 1 Hz to accelerate protein synthesis as compared to quiescent controls. Translational efficiency was calculated by measuring protein expression as a function of mRNA levels. Translational efficiency of c-jun/betaGal mRNA increased significantly by 3.7-fold in contracting vs. quiescent cardiocytes, but ODC/betaGal mRNA was unchanged. Contraction increased c-jun/betaGal mRNA levels in polyribosomes by 2.3-fold, which indicates that translational efficiency was enhanced by mobilization. A short, unstructured 5'-UTR was sufficient for efficient translation of betaGal mRNA in quiescent and contracting cardiocytes. GFP mRNA produced similar results. These studies demonstrate that the 5'-UTR functions as a determinant of translational efficiency of specific mRNAs, such as c-jun, that regulate growth of the adult cardiocyte.

Cardiac contractility modulation electrical signals normalize activity, expression, and phosphorylation of the Na+-Ca2+ exchanger in heart failure

BACKGROUND

Expression and phosphorylation of the cardiac Na(+)-Ca(2+) exchanger-1 (NCX-1) are up-regulated in heart failure (HF). We examined the effects of chronic cardiac contractility modulation (CCM) therapy on the expression and phosphorylation of NCX-1 and its regulators GATA-4 and FOG-2 in HF dogs.

METHODS AND RESULTS

Studies were performed in LV tissue from 7 CCM-treated HF dogs, 7 untreated HF dogs, and 6 normal (NL) dogs. mRNA expression of NCX-1, GATA-4, and FOG-2 was measured using reverse transcriptase polymerase chain reaction, and protein level was determined by Western blotting. Phosphorylated NCX-1 (P-NCX) was determined using a phosphoprotein enrichment kit. Compared with NL dogs, NCX-1 mRNA and protein expression and GATA-4 mRNA and protein expression increased in untreated HF dogs, whereas FOG-2 expression decreased. Compared with NL dogs, the level of P-NCX-1 normalized to total NCX-1 increased in untreated HF dogs (0.80+/-0.10 vs 0.37+/-0.04; P < .05). CCM therapy normalized NCX-1 expression, GATA-4, and FOG-2 expression, and the ratio of P-NCX-1 to total NCX-1 (0.62+/-0.10).

CONCLUSION

Chronic monotherapy with CCM restores expression and phosphorylation of NCX-1. These findings are consistent with previous observations of improved LV function and normalized sarcoplasmic reticulum calcium cycling in the left ventricles of HF dogs treated with CCM therapy.

Modulation of expression of Na+/Ca2+ exchanger in heart of rat and mouse under stress

AIM

The Na(+)/Ca(2+) exchanger (NCX) is a major Ca(2+) extrusion system in the plasma membrane of cardiomyocytes and an important component participating on the excitation-contraction coupling process in muscle cells. NCX1 isoform is the most abundant in the heart and is known to be changed after development of ischaemia or myocardial infarction. Objective of this study was to investigate the effect of stress factors (immobilization, cold and short-term hypoxia) on the expression of NCX1, in vivo, in the heart of rat and mouse.

METHODS

We compared gene expression and protein levels of control and stressed animals. The activity of NCX was measured by the whole cell configuration using the patch clamp. We also measured physiological parameters of the heart in physiological conditions and under ischaemia-reperfusion to compare response of control and stressed hearts.

RESULTS

We have found that only strong stress stimulus (hypoxia, immobilization) applied repeatedly for several days elevated the NCX1 mRNA level. Cold, which is a weaker stressor that activates mainly sympathoneural, and only marginally adrenomedullary system did not affect the gene expression of NCX1. Thus, from these results it appears that hormones produced by the adrenal medulla (mainly adrenaline) might be involved in this process. To study possible mechanism of the NCX1 regulation by stress, we focused on the possible role of the hypothalamo-pituitary-adrenocortical pathway in the activation of catecholamine synthesis in the adrenal medulla. We have already published that cortisol affects activity, but not the gene expression of NCX1. In this work, we used corticotropin-releasing hormone (CRH) knockout mice, where secretion of corticosterone and subsequently adrenaline is significantly suppressed. As no increase in NCX1 mRNA was observed in CRH knockout mice due to immobilization stress, we proposed that adrenaline (probably regulated via corticosterone) is involved in the regulation of NCX1 gene expression during stress.

CONCLUSIONS

The gene expression and protein levels of the NCX1 are increased by the strong stress stimuli, e.g. hypoxia, or immobilization stress. The activity of NCX1 is decreased. Based on these results, we assume that the gene expression of NCX is increased as a consequence of suppressed activity of this transport system.

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.

Maintaining serum response factor activity in the older heart equal to that of the young adult is associated with better cardiac response to isoproterenol stress

To understand the effect of transcription regulation in modulating cardiac aging, we sought to study the role of serum response factor (SRF), a key transcription factor in the heart that is normally increased with senescence and also in congestive heart failure. A Tet-Off gene expression system was used for cardiac-specific over-expression of a mutant SRF protein. In these binary transgenic mice, there is no age-related increase in SRF protein expression; in fact, there appeared to be a mild reduction of SRF protein (Mild-R SRF Tg). The older, middle-aged (15 mo) Mild-R SRF Tg mice appeared healthier and were better able to maintain their left ventricular systolic pressure (LVSP) in response to moderate â-adrenergic stimulation compared with age-matched Non-Tg mice, which demonstrated a negative ionotropic response. The Mild-R SRF Tg hearts had lower mRNA expression of BNP (p < 0.05), and the sodium calcium exchanger (p < 0.05), compared to Non-Tg. Mild-R SRF Tg had higher mRNA levels of SERCA2 (p < 0.05) and ryanodine receptor 2 (p < 0.05) compared to Non-Tg hearts. These findings suggest that preventing the age-associated increase in SRF is associated with better preserved intracellular calcium handling and functional response to stress; it might be advantageous for the older adult heart. This mouse model could be helpful in elucidating the molecular mechanisms underlying certain age-related changes in cardiac reserve capacity and response to stress.

Regulation of Ncx1 expression. Identification of regulatory elements mediating cardiac-specific expression and up-regulation

The Na+-Ca2+ exchanger (NCX1) is up-regulated in hypertrophy and is often found up-regulated in end-stage heart failure. Studies have shown that the change in its expression contributes to contractile dysfunction. We have previously shown that the 1831-bp Ncx1 H1 (1831Ncx1) promoter directs cardiac-specific expression of the exchanger in both development and in the adult, and is sufficient for the up-regulation of Ncx1 in response to pressure overload. Here, we utilized adenoviral mediated gene transfer and transgenics to identify minimal regions and response elements that mediate Ncx1 expression in the heart. We demonstrate that the proximal 184 bp of the Ncx1 H1 (184Ncx1) promoter is sufficient for expression of reporter genes in adult cardiomyocytes and for the correct spatiotemporal pattern of Ncx1 expression in development but not for up-regulation in response to pressure overload. Mutational analysis revealed that both the -80 CArG and the -50 GATA elements were required for expression in isolated adult cardiomyocytes. Chromatin immunoprecipitation assays in adult cardiocytes demonstrate that SRF and GATA4 are associated with the proximal region of the endogenous Ncx1 promoter. Transgenic lines were established for the 1831Ncx1 promoter-luciferase containing mutations in the -80 CArG or -50 GATA element. No luciferase activity was detected during development, in the adult, or after pressure overload in any of the -80 CArG transgenic lines. The Ncx1 -50 GATA mutant promoter was sufficient for driving the normal spatiotemporal pattern of Ncx1 expression in development and for up-regulation in response to pressure overload but importantly, expression was no longer cardiac restricted. This work is the first in vivo study that demonstrates which cis elements are important for Ncx1 regulation.

Conditional inactivation of the murine serum response factor in the pancreas leads to severe pancreatitis

The Serum Response Factor (SRF) is widely expressed transcription factor acting at the confluence of multiple signaling pathways and has been implicated in the control of differentiation, growth, and cell death. In the present study, we found that SRF is expressed in the developing and adult pancreas. To explore the possible role of SRF in this organ, we have generated mutant mice with conditional disruption of the Srf gene. Such mutants presented normal development of both the exocrine and endocrine pancreas indicating that SRF is dispensable for pancreas ontogenesis. However, after weaning, these mice developed profound morphological alterations of the exocrine pancreas, which were reminiscent of severe pancreatitis. In these mice, massive acinar injury, Nuclear Factor Kappa B activation and proinflammatory cytokines release led to complete destruction of the exocrine pancreas and its replacement by adipose tissue. Despite these changes, the organization and function of the endocrine islets of Langerhans remained well-preserved. This new animal model of spontaneous pancreatitis could prove a valuable tool to gain further insight into the physiopathology of this disease.

Nitric oxide stimulates cyclooxygenase-2 in cultured cTAL cells through a p38-dependent pathway

To examine the interaction of nitric oxide (NO) and cyclooxygenase (COX-2) and the signaling pathway involved, primary cultured rabbit cortical thick ascending limb (cTAL) were used. In these cells, immunoreactive COX-2 and vasodilatory prostaglandins were increased by a NO donor, S-nitros- N-acetylpenicillamine (SNAP; 2.5 ± 0.3-fold control, n = 6, P < 0.01). SNAP increased expression of phosphorylated p38 (pp38; 2.4 ± 0.3-fold control; n = 5; P < 0.01), which was inhibited by the p38 inhibitor SB-203580 (1.3 ± 0.1-fold control, n = 5, P < 0.01). SB-203580 inhibited SNAP-induced COX-2 expression [1.4 ± 0.2-fold control, n = 6, not significant (NS) vs. control] and levels of PGE2significantly. In cTAL cells transfected with a luciferase reporter driven by the wild-type mouse COX-2 promoter, SNAP stimulated luciferase activity, which was reversed by SB-203580 (control vs. SNAP vs. SNAP + SB-203580: 1.4 ± 0.2-, 8.3 ± 1.4-, and 0.4 ± 0.1-fold control, respectively, n = 4, P < 0.01). Electrophoretic mobility shift assay indicated that SNAP stimulated nuclear factor (NF)-κB binding activity in cTAL that was also inhibited by the p38 inhibitor. SNAP was not able to stimulate a mutant COX-2 promoter construct that is not activated by NF-κB (0.9 ± 0.1, 1.2 ± 0.1, and 1.0 ± 0.2 respectively, n = 4, NS). Low chloride increased COX-2 expression (2.7 ± 0.4-fold control, n = 6, P < 0.01) and pp38 expression (2.8 ± 0.3-fold; n = 5, P < 0.01), which were reversed by the specific NO synthase (NOS) inhibitor 7-nitroindazole. Administration of a low-salt diet increased immunoreactive COX-2 and neuronal NOS (nNOS) in the macula densa and surrounding cTAL of kidneys of wild-type mice but did not significantly elevate COX-2 expression in nNOS−/−mice. In summary, these studies indicate that, in cTAL, NO can increase COX-2 expression in cTAL and macula densa through p38-dependent signaling pathways via activation of NF-κB.

GATA-4 Gene Organization and Analysis of Its Promoter

The mouse GATA-4 gene is separated by six introns, and this gene organization is conserved in rodents and man. The transcriptional start site of the GATA-4 gene is essentially the same in rat heart, stomach and testis, and in cultured cells expressing GATA-4 such as TM3, TM4, I-10 and P19.CL6 cells. The 5'-upstream of the GATA-4 gene is also conserved in rodents and man. We examined its promoter activity by means of luciferase reporter gene assay using testis-derived TM3 and TM4 cells. The GC-boxes and E-box located in the several tens of base pairs upstream of the transcriptional start sites of the GATA-4 gene were found to be critical for its promoter activity in these cells, consistent with the mode of transcription characteristics of the TATA-less promoter. P19.CL6 cells differentiate into beating cardiomyocytes upon induction by DMSO, accompanied by stimulation of the transcription of heart-specific genes including GATA-4. Interestingly, they exhibit increased luciferase reporter gene activity upon induction by DMSO. Both proximal tandem GC-boxes and the E-box are also contributed to the reporter gene activity in P19.CL6 cells.

Role of the serum response factor in regulating contractile apparatus gene expression and sarcomeric integrity in cardiomyocytes

The serum response factor (SRF) is a transcriptional regulator required for mesodermal development, including heart formation and function. Previous studies have described the role of SRF in controlling expression of structural genes involved in conferring the myogenic phenotype. Recent studies by us and others have demonstrated embryonic lethal cardiovascular phenotypes in SRF-null animals, but have not directly addressed the mechanistic role of SRF in controlling broad regulatory programs in cardiac cells. In this study, we used a loss-of-function approach to delineate the role of SRF in cardiomyocyte gene expression and function. In SRF-null neonatal cardiomyocytes, we observed severe defects in the contractile apparatus, including Z-disc and stress fiber formation, as well as mislocalization and/or attenuation of sarcomeric proteins. Consistent with this, gene array and reverse transcription-PCR analyses showed down-regulation of genes encoding key cardiac transcriptional regulatory factors and proteins required for the maintenance of sarcomeric structure, function, and regulation. Chromatin immunoprecipitation analysis revealed that at least a subset of these proteins are likely regulated directly by SRF. The results presented here indicate that SRF is an essential coordinator of cardiomyocyte function due to its ability to regulate expression of numerous genes (some previously identified and at least 28 targets newly identified in this study) that are involved in multiple and disparate levels of sarcomeric function and assembly.

Stem cells and their derivatives can bypass the requirement of myocardin for smooth muscle gene expression

The Serum Response Factor (SRF) coactivator myocardin stimulates the transcription of multiple muscle genes during cardiac and smooth muscle development. Mouse embryos lacking myocardin die during the earliest stages of smooth muscle development and fail to express multiple smooth muscle marker genes in the embryonic dorsal aorta and other vascular structures. In this study, we used mutant embryonic stem cell lines to further define the role of myocardin in smooth muscle differentiation and vascular development. Misexpression of myocardin in undifferentiated muscle stem cells resulted in efficient activation of smooth muscle genes, and weaker activation of genes involved in cardiac and skeletal muscle differentiation. Remarkably, myocardin(-/-) embryonic stem cell lines differentiated into smooth muscle cells in vitro, although these cells expressed significantly decreased levels of smooth muscle contractile genes. Moreover, genetically labeled myocardin(-/-) ES cells were able to contribute to smooth muscle lineages in vivo. These results indicate that while myocardin function is sufficient for activation of SRF-dependent muscle gene expression in multiple cell types, myocardin-independent mechanism(s) can suffice for expression in some smooth muscle lineages.

Myocardin is sufficient and necessary for cardiac gene expression in Xenopus

Myocardin is a cardiac- and smooth muscle-specific cofactor for the ubiquitous transcription factor serum response factor (SRF). Using gain-of-function approaches in the Xenopus embryo, we show that myocardin is sufficient to activate transcription of a wide range of cardiac and smooth muscle differentiation markers in non-muscle cell types. We also demonstrate that, for the myosin light chain 2 gene (MLC2), myocardin cooperates with the zinc-finger transcription factor Gata4 to activate expression. Inhibition of myocardin activity in Xenopus embryos using morpholino knockdown methods results in inhibition of cardiac development and the absence of expression of cardiac differentiation markers and severe disruption of cardiac morphological processes. We conclude that myocardin is an essential component of the regulatory pathway for myocardial differentiation.

Transcriptional regulation by cAMP and Ca2+ links the Na+/Ca2+ exchanger 3 to memory and sensory pathways

The signaling cascades triggered by neurotrophins such as BDNF and by several neurotransmitters and hormones lead to the rapid induction of gene transcription by increasing the intracellular concentration of cAMP and Ca2+. This review examines the mechanisms by which these second messengers control transcriptional initiation at CRE promoters via transcription factor CREB, as well as at DRE sites via transcriptional repressor DREAM. The regulation of the SLC8A3 gene encoding the Na+/Ca2+ exchanger 3 (NCX3) is taken as an example to illustrate both mechanisms since it includes a CRE site in the promoter and several DRE sites in the exon 1 sequence. The upregulation of the NCX3 by Ca2+ signals may be specifically required to establish the Ca2+ balance that regulates several physiological and pathological processes in neurons. The regulatory features and the expression pattern of SLC8A3 gene suggest that NCX3 activity could be crucial in neuronal functions such as memory formation and sensory processing.

Na+/Ca2+ exchanger plays a key role in inducing apoptosis after hypoxia in cultured guinea pig ventricular myocytes

Altered Na(+)/Ca(2+) exchanger (NCX) protein expression or activity is thought to contribute to various aspects of cardiac pathology. In guinea pig ventricular myocytes, NCX-mediated Ca(2+) entry is almost entirely responsible for Ca(2+) overload during hypoxia-reoxygenation. Because Ca(2+) overload is a common initiator of apoptosis, the purpose of this study was to test the hypotheses that NCX activity is critically involved in initiating apoptosis after hypoxia-reoxygenation and that hypoxia-reoxygenation-induced apoptosis can be modulated by changes in NCX protein expression or activity. An NCX antisense oligonucleotide was used to reduce NCX protein expression in cultured adult guinea pig ventricular myocytes. Caspase-3 activation and cytochrome c release were used as markers of apoptosis. Hypoxia-reoxygenation-induced apoptosis was significantly decreased in antisense-treated myocytes compared with untreated control or nonsense-treated myocytes. Pretreatment of cultured myocytes for 24 h with either endothelin-1 or phenylephrine was found to increase both NCX protein expression and evoked NCX activity as well as enhance hypoxia-reoxygenation-induced apoptosis. Control experiments demonstrated that endothelin-1 and phenylephrine did not induce apoptosis on their own nor did they enhance the apoptotic response in a model of Ca(2+)-dependent, NCX-independent apoptosis. Additional control experiments demonstrated that the NCX antisense oligonucleotide did not alter the apoptotic response of myocytes to either H(2)O(2) or isoproterenol. Taken together, these data suggest that the NCX has a critical and specific role in the initiation of apoptosis after hypoxia-reoxygenation in guinea pig myocytes and that hypoxia-reoxygenation-induced apoptosis is quite sensitive to changes in NCX activity.

Regulation of protein synthesis by eIF4E phosphorylation in adult cardiocytes: the consequence of secondary structure in the 5'-untranslated region of mRNA

In adult cardiocytes, eIF4E (eukaryotic initiation factor 4E) activity and protein synthesis are increased concomitantly in response to stimuli that induce hypertrophic growth. We tested the hypothesis that increases in eIF4E activity selectively improve the translational efficiency of mRNAs that have an excessive amount of secondary structure in the 5'-UTR (5'-untranslated region). The activity of eIF4E was modified in primary cultures of adult cardiocytes using adenoviral gene transfer to increase either the amount of eIF4E or the extent of endogenous eIF4E phosphorylation. Subsequently, the effects of eIF4E on translational efficiency were assayed following adenoviral-mediated expression of luciferase reporter mRNAs that were either 'stronger' (less structure in the 5'-UTR) or 'weaker' (more structure in the 5'-UTR) with respect to translational efficiency. The insertion of G+C-rich repeats into the 5'-UTR doubled the predicted amount of secondary structure and was sufficient to reduce translational efficiency of the reporter mRNA by 48+/-13%. Translational efficiency of the weaker reporter mRNA was not significantly improved by overexpression of wild-type eIF4E when compared with the stronger reporter mRNA. In contrast, overexpression of the eIF4E kinase Mnk1 [MAP (mitogen-activated protein) kinase signal-integrating kinase 1] was sufficient to increase the translational efficiency of either reporter mRNA, independent of the amount of secondary structure in their respective 5'-UTRs. The increases in translational efficiency produced by Mnk1 occurred in association with corresponding decreases in mRNA levels. These findings indicate that the positive effect of eIF4E phosphorylation on translational efficiency in adult cardiocytes is coupled with the stability of mRNA.

Sodium/calcium exchanger (NCX1) macromolecular complex

The sodium-calcium exchanger, NCX1, is a ubiquitously expressed membrane protein essential in calcium homeostasis for many cells including those in mammalian heart and brain. The function of NCX1 depends on subcellular ("local") factors, the phosphorylation state of NCX1, and the subcellular location of NCX1 within the cell. Here we investigate the molecular organization of NCX1 within the cardiac myocyte. We show that NCX1 is dynamically phosphorylated by protein kinase A (PKA)-dependent phosphorylation in vitro. We also provide evidence that the regulation of this phosphorylation is attributed to the existence of an NCX1 macromolecular complex. Specifically, we show that the macromolecular complex includes both the catalytic and regulatory subunits of PKA. However, only the RI regulatory subunit is found in this macromolecular complex, not RII. Other critical regulatory enzymes are also associated with NCX1, including protein kinase C (PKC) and two serine/threonine protein phosphatases, PP1 and PP2A. Importantly, the protein kinase A-anchoring protein, mAKAP, is found and its presence in the macromolecular complex suggests that these regulatory enzymes are coordinately positioned to regulate NCX1 as has been found in diverse cells for a number of channel proteins. Dual immunocytochemical staining showed the colocalization of NCX1 protein with mAKAP and PKA-RI proteins in cardiomyocytes. Finally, leucine/isoleucine zipper motifs have been identified as possible sites of interaction. Our finding of an NCX1 macromolecular complex in heart suggests how NCX1 regulation is achieved in heart and other cells. The existence of the NCX1 macromolecular complex may also provide an explanation for recent controversial findings.

Calcium/calmodulin-dependent protein kinase activates serum response factor transcription activity by its dissociation from histone deacetylase, HDAC4. Implications in cardiac muscle gene regulation during hypertrophy

Serum response factor (SRF) plays a pivotal role in cardiac myocyte development, muscle gene transcription, and hypertrophy. Previously, elevation of intracellular levels of Ca2+ was shown to activate SRF function without involving the Ets family of tertiary complex factors through an unknown regulatory mechanism. Here, we tested the hypothesis that the chromatin remodeling enzymes of class II histone deacetylases (HDAC4) regulate SRF activity in a Ca2+-sensitive manner. Expression of HDAC4 profoundly repressed SRF-mediated transcription in both muscle and nonmuscle cells. Protein interaction studies demonstrated physical association of HDAC4 with SRF in living cells. The SRF/HDAC4 co-association was disrupted by treatment of cells with hypertrophic agonists such as angiotensin-II and a Ca2+ ionophore, ionomycin. Furthermore, activation of Ca2+/calmodulin-dependent protein kinase (CaMK)-IV prevented SRF/HDAC4 interaction and derepressed SRF-dependent transcription activity. The SRF.HDAC4 complex was localized to the cell nucleus, and the activated CaMK-IV disrupted HDAC4/SRF association, leading to export of HDAC4 from the nucleus and stimulation of SRF transcription activity. Thus, these results identify SRF as a functional interacting target of HDAC4 and define a novel tertiary complex factor-independent mechanism for SRF activation by Ca2+/CaMK-mediated signaling.

Roles of cardiac transcription factors in cardiac hypertrophy

Different cell types, equipped with unique structure and function, synthesize different sets of proteins on the basis of different patterns of gene expression, even though their genomes are identical. Cardiac transcription factors have been reported to control a cardiac gene program and thus to play a crucial role in transcriptional regulation during embryogenesis. Recently, postnatal roles of cardiac transcription factors have been extensively investigated. Consistent with the direct transactivation of numerous cardiac genes reactivated in response to hypertrophic stimulation, cardiac transcription factors are profoundly involved in the generation of cardiac hypertrophy or in cardioprotection from cytotoxic stress in the adult heart. In this review, the regulation of a cardiac gene program by cardiac transcription factors is summarized, with an emphasis on their potential role in the generation of cardiac hypertrophy.

Upstream stimulatory factor represses the induction of carnitine palmitoyltransferase-Ibeta expression by PGC-1

Transcriptional regulation of carnitine palmitoyltransferase-1beta (CPT-1beta) is coordinated with contractile gene expression through cardiac-enriched transcription factors, GATA4 and SRF. Metabolic modulation of CPT-1beta promoter activity has been described with the stimulation of gene expression by oleate that is mediated through the peroxisome proliferator-activated receptor (PPAR) pathway. The coactivator, peroxisomal proliferator-activated receptor gamma coactivator (PGC-1), enhances gene expression through interactions with nuclear hormone receptors and the myocyte enhancer factor 2 (MEF2) family. PGC-1 and MEF2A synergistically activate CPT-1beta promoter activity. This stimulation is enhanced by mutation of the E-box sequences that flank the MEF2A binding site. These elements bind the upstream stimulatory factors (USF1 and USF2), which activate transcription in CV-1 fibroblasts. However, overexpression of the USF proteins in myocytes depresses CPT-1beta activity and significantly reduces MEF2A and PGC-1 synergy. Co-immunoprecipitation studies demonstrate that PGC-1 and USF2 proteins can physically interact. Our studies demonstrate that PGC-1 stimulates CPT-1beta gene expression through MEF2A. USF proteins have a novel role in repressing the expression of the CPT-1beta gene and modulating the induction by the coactivator, PGC-1.

GATA-4 and GATA-6 transcription factors: expression, immunohistochemical localization, and possible function in the porcine ovary

The expression and localization of GATA-4 and GATA-6 mRNAs and proteins were assessed in porcine ovaries at different stages of the estrous cycle. Reverse transcription polymerase chain reaction and Western blot analyses revealed that GATA-4 and GATA-6 transcripts and proteins were strongly expressed in granulosa cells isolated from antral follicles, intact antral follicles, corpora hemorrhagica (CH), and midluteal phase corpora lutea (CL). Immunoblot analyses showed two predominant proteins with molecular masses of approximately 53 and 55 kDa for GATA-4 and one 55-kDa protein for GATA-6. Immunohistochemical studies revealed GATA-4 and GATA-6 nuclear staining in granulosa cells of healthy primordial and primary antral follicles and antral follicle of various sizes. The percentage of immunopositive thecal cell nuclei increased with follicular development. In CH and CL, luteal cells displayed nuclear immunoreactivity for both transcription factors. Regressing CL showed a decrease in GATA-immunopositive cells. Immunoreactivity for GATA-4 and GATA-6 was present in most blood vessels. In electrophoretic mobility shift assays, nuclear protein extracts isolated from granulosa cells and CL exhibited both GATA-4 and GATA-6 binding to a GATA consensus oligonucleotide, with GATA-4 the predominant binding protein. GATA-4 and GATA-6 DNA binding was elevated in granulosa cell nuclear extracts from preovulatory (8-10 mm) follicles. Cotransfection of primary cultures of luteinizing granulosa cells with GATA-4 or GATA-6 expression vectors increased the activity of the porcine steroidogenic acute regulatory protein gene promoter significantly but did not significantly activate the inhibin alpha gene promoter. The detection of GATA-4 and GATA-6 mRNAs and proteins in porcine ovaries and the identification of at least one possible target gene may help to establish roles for these GATA factors in follicular development and luteal function.

Cyclooxygenase-2 expression in cultured cortical thick ascending limb of Henle increases in response to decreased extracellular ionic content by both transcriptional and post-transcriptional mechanisms. Role of p38-mediated pathways

We showed previously that decreased extracellular salt or chloride up-regulates the cortical thick ascending limb of Henle (cTALH) COX-2 expression via a p38-dependent pathway. The present studies determined that low salt medium increased COX-2 mRNA expression 3.9-fold control by 6 h in cultured cTALH, which was blocked by actinomycin D pretreatment, suggesting transcriptional regulation. Luciferase activity (normalized to beta-galactosidase activity) of the full-length (-3400) COX-2 promoter in cTALH increased from 1.8 +/- 0.3 in control media to 5.8 +/- 0.7 in low salt (n = 9; p < 0.01). Low chloride medium had similar effects as low salt has on COX-2 promoter activity. Deletion constructs -815, -512, and -410 were similarly stimulated, but -385 could not be stimulated significantly by low salt (1.8 +/- 0.3 versus 2.4 +/- 0.5, n = 10). This suggested involvement of an NF-kappaB cis-element located in this region, which was confirmed by utilizing a construct with a point mutation of this NF-kappaB-binding site that was not stimulated by low salt medium. Co-incubation of the specific p38 inhibitor, SB203580 or PD169316, inhibited a low salt-induced increase in luciferase activity of the intact COX-2 promoter (5.8 +/- 0.7 versus 1.1 +/- 0.2, n = 8 and 1.4 +/- 0.4, n = 4 respectively, p < 0.01). Mobility shift assays indicated that the low salt medium stimulated NF-kappaB binding activity, and this stimulation was inhibited by p38 inhibitors. To test whether p38 also increased COX-2 expression by increasing mRNA stability, cTALH were incubated in low salt for 2 h, and actinomycin was then added with or without SB203580. p38 inhibition led to a decreased half-life of COX-2 mRNA (from 68 to 18 min, n = 4-7, p < 0.05). Therefore, these studies indicate that p38 stimulates COX-2 expression in cTALH and macula densa by transcriptional regulation predominantly via a NF-kappaB-dependent pathway and by post-transcriptional increases in mRNA stability.

Molecular embryogenesis of the heart

Development of the heart is a complex process involving primary and secondary heart fields that are set aside to generate myocardial and endocardial cell lineages. The molecular inductions that occur in the primary heart field appear to be recapitulated in induction and myocardial differentiation of the secondary heart field, which adds the conotruncal segments to the primary heart tube. While much is now known about the initial steps and factors involved in induction of myocardial differentiation, little is known about induction of endocardial development. Many of the genes expressed by nascent myocardial cells, which then become committed to a specific heart segment, have been identified and studied. In addition to the heart fields, several other "extracardiac" cell populations contribute to the fully functional mature heart. Less is known about the genetic programs of extracardiac cells as they enter the heart and take part in cardiogenesis. The molecular/genetic basis of many congenital cardiac defects has been elucidated in recent years as a result of new insights into the molecular control of developmental events.

Cyclosporin A regulates sodium-calcium exchanger (NCX1) gene expression in vitro and cardiac hypertrophy in NCX1 transgenic mice

The cardiac-specific sodium-calcium exchanger (NCX1) is a GATA-4 dependent gene that is upregulated during cardiac hypertrophy and heart failure. To date, lack of an appropriate inhibitor of NCX1 and embryonic lethality of NCX1 knockout mice have slowed investigation of the relation between NCX1 upregulation and cardiac hypertrophy. Recently, in vitro studies have shown that cyclosporin A (CSA), a calcineurin inhibitor, significantly downregulated expression of the hypertrophic genes atrial natriuretic factor and beta-myosin heavy chain and protected against cardiac hypertrophy and heart failure in calcineurin overexpressing mice. This suggested that CSA might play an important role in the treatment of hypertrophy and heart failure. In an in vitro model of cardiac hypertrophy, we showed that CSA is a potent inhibitor of NCX1 basal expression and NCX1 promoter activity. Female homozygous transgenic mice that overexpress NCX1 develop heart failure and die prematurely after two or more pregnancies. Others have demonstrated that pressure overloaded wild-type mice treated with CSA do not develop cardiac hypertrophy and downregulate expression of NCX1. We investigated the effect of CSA on NCX1 expression and transverse aortic constriction-induced cardiac hypertrophy in NCX1 overexpressing mice. We found that CSA blunted these responses.

Pathways regulating Na+/Ca2+ exchanger expression in the heart

The Na(+)/Ca(2+) exchanger (NCX1) is regulated at the transcriptional level in cardiac hypertrophy, ischemia, and failure. Following pressure overload, activation of MAPKs coincides with the kinetics of NCX1 gene upregulation in adult cardiocytes. Using adenoviral gene delivery, we begin to identify the molecular pathways responsible for upregulation of the exchanger gene. Inhibition of ERK with the MEK inhibitor UO126, the ERK protein phosphatase MKP-3, inhibited ERK activation, but only inhibited alpha-adrenergic-induced NCX1 upregulation by 30%. Overexpression of DN-JNK lowered basal NCX1 expression. Overexpression of activated MKK-3 was sufficient for alpha-adrenergic-stimulated upregulation of the reporter gene. Together, this data indicates that (1) JNK mediates basal cardiac expression of the NCX1 gene, (2) ERK and p38 play a role in alpha-adrenergic-stimulated NCX1 upregulation, and (3) p38 activation alone is sufficient for NCX1 upregulation.

Ontogeny of excitation-contraction coupling in the mammalian heart

The neonate mammalian heart is phenotypically different from the adult heart in many respects. Understanding these phenotypic differences are a fundamental component of understanding the mechanisms of congenital heart disease and its treatment. Differences in excitation-contraction (E-C) coupling of the neonatal heart from that of the adult include less reliance on intercellular sources of Ca(2+) such as that from sarcoplasmic reticulum (SR). Electron micrographs indicate that these immature cardiomyocytes lack transverse tubules and the SR is sparse. This paper focuses on the changes in the phenotype of E-C coupling during ontogeny in the mammalian heart and the molecular mechanisms underlying these changes.

The transcription factors GATA4 and dHAND physically interact to synergistically activate cardiac gene expression through a p300-dependent mechanism

An intricate array of heterogeneous transcription factors participate in programming tissue-specific gene expression through combinatorial interactions that are unique to a given cell-type. The zinc finger-containing transcription factor GATA4, which is widely expressed in mesodermal and endodermal derived tissues, is thought to regulate cardiac myocyte-specific gene expression through combinatorial interactions with other semi-restricted transcription factors such as myocyte enhancer factor 2, nuclear factor of activated T-cells, serum response factor, and Nkx2.5. Here we determined that GATA4 also interacts with the cardiac-expressed basic helix-loop-helix transcription factor dHAND (also known as HAND2). GATA4 and dHAND synergistically activated expression of cardiac-specific promoters from the atrial natriuretic factor gene, the b-type natriuretic peptide gene, and the alpha-myosin heavy chain gene. Using artificial reporter constructs this functional synergy was shown to be GATA site-dependent, but E-box site-independent. A mechanism for the transcriptional synergy was suggested by the observation that the bHLH domain of dHAND physically interacted with the C-terminal zinc finger domain of GATA4 forming a higher order complex. This transcriptional synergy observed between GATA4 and dHAND was associated with p300 recruitment, but not with alterations in DNA binding activity of either factor. Moreover, the bHLH domain of dHAND directly interacted with the CH3 domain of p300 suggesting the existence of a higher order complex between GATA4, dHAND, and p300. Taken together with previous observations, these results suggest the existence of an enhanceosome complex comprised of p300 and multiple semi-restricted transcription factors that together specify tissue-specific gene expression in the heart.

Differential regulation of the cardiac sodium calcium exchanger promoter in adult and neonatal cardiomyocytes by Nkx2.5 and serum response factor

Nkx2.5 and serum response factor (SRF) are critically important transcription factors in cardiac morphogenesis. They are also widely expressed in adult cardiomyocytes, but there is little data to indicate their possible role in adult cardiac cells. In this paper we demonstrate that the interaction of Nkx2.5 and SRF in cardiac-specific gene regulation is different between neonatal and adult cardiomyocytes. Our experimental model utilizes transient transfection and adenovirus mediated gene transfer of the proximal promoter fragment of the cardiac isoform of the sodium-calcium exchanger gene (NCX1). This promoter construct (NCX184) contains a single Nkx2.5-response element (NKE) and a single serum response element (CArG). In rat neonatal cardiomyocytes NCX184 activity is substantially induced with Nkx2.5 or SRF and additively with both. Mutagenesis of these NKE and CArG elements demonstrated the specificity of the interactions, which was confirmed with gel retardation analysis of cardiac ventricular tissue. In contrast, in adult cardiomyocytes, co-infection of Nkx2.5 and SRF adenovirus vectors showed Nkx2.5 induction but SRF did not have additive effects on NCX1 promoter regulation. As opposed to NCX1, the proximal atrial natriuretic factor (ANF) promoter was regulated identically in response to SRF and Nkx2.5 in both adult and neonatal cardiomyocytes. These results show that Nkx2.5-SRF interactions are capable of producing different transcriptional responses in adult versus neonatal cardiomyocytes, implying important differences in NCX1 promoter tertiary complex formation dependent on developmental stage.

Glycogen synthase kinase-3beta: a novel regulator of cardiac hypertrophy and development

Glycogen synthase kinase-3beta (GSK-3beta) is a ubiquitously expressed constitutively active serine/threonine kinase that phosphorylates cellular substrates and thereby regulates a wide variety of cellular functions, including development, metabolism, gene transcription, protein translation, cytoskeletal organization, cell cycle regulation, and apoptosis. The activity of GSK-3beta is negatively regulated by protein kinase B/Akt and by the Wnt signaling pathway. Increasing lines of evidence show that GSK-3beta is an essential negative regulator of cardiac hypertrophy and that the inhibition of GSK-3beta by hypertrophic stimuli is an important mechanism contributing to the development of cardiac hypertrophy. GSK-3beta also plays an important role in regulating cardiac development. In this review, the role of GSK-3beta in cardiac hypertrophy and development and the potential underlying mechanisms are discussed.