Characterization of the murine A1 adenosine receptor promoter, potent regulation by GATA-4 and Nkx2.5

Pre- and Post-Conditioning of the Heart: An Overview of Cardioprotective Signaling Pathways

Since the discovery of ischemic pre- and post-conditioning, more than 30 years ago, the knowledge about the mechanisms and signaling pathways involved in these processes has significantly increased. In clinical practice, on the other hand, such advancement has yet to be seen. This article provides an overview of ischemic pre-, post-, remote, and pharmacological conditioning related to the heart. In addition, we reviewed the cardioprotective signaling pathways and therapeutic agents involved in the above-mentioned processes, aiming to provide a comprehensive evaluation of the advancements in the field. The advancements made over the last decades cannot be ignored and with the exponential growth in techniques and applications. The future of pre- and post-conditioning is promising.

Transcription factor-induced activation of cardiac gene expression in human c-kit+ cardiac progenitor cells

Although transplantation of c-kit+ cardiac progenitor cells (CPCs) significantly alleviates post-myocardial infarction left ventricular dysfunction, generation of cardiomyocytes by exogenous CPCs in the recipient heart has often been limited. Inducing robust differentiation would be necessary for improving the efficacy of the regenerative cardiac cell therapy. We assessed the hypothesis that differentiation of human c-kit+ CPCs can be enhanced by priming them with cardiac transcription factors (TFs). We introduced five different TFs (Gata4, MEF2C, NKX2.5, TBX5, and BAF60C) into CPCs, either alone or in combination, and then examined the expression of marker genes associated with the major cardiac cell types using quantitative RT-PCR. When introduced individually, Gata4 and TBX5 induced a subset of myocyte markers. Moreover, Gata4 alone significantly induced smooth muscle cell and fibroblast markers. Interestingly, these gene expression changes brought by Gata4 were also accompanied by morphological changes. In contrast, MEF2C and NKX2.5 were largely ineffective in initiating cardiac gene expression in CPCs. Surprisingly, introduction of multiple TFs in different combinations mostly failed to act synergistically. Likewise, addition of BAF60C to Gata4 and/or TBX5 did not further potentiate their effects on cardiac gene expression. Based on our results, it appears that GATA4 is able to potentiate gene expression programs associated with multiple cardiovascular lineages in CPCs, suggesting that GATA4 may be effective in priming CPCs for enhanced differentiation in the setting of stem cell therapy.

Ion Fluxes through KCa2 (SK) and Cav1 (L-type) Channels Contribute to Chronoselectivity of Adenosine A1 Receptor-Mediated Actions in Spontaneously Beating Rat Atria

Impulse generation in supraventricular tissue is inhibited by adenosine and acetylcholine via the activation of A₁ and M₂ receptors coupled to inwardly rectifying GIRK/K_IR3.1/3.4 channels, respectively. Unlike M₂ receptors, bradycardia produced by A₁ receptors activation predominates over negative inotropy. Such difference suggests that other ion currents may contribute to adenosine chronoselectivity. In isolated spontaneously beating rat atria, blockade of K_Ca2/SK channels with apamin and Ca_v1 (L-type) channels with nifedipine or verapamil, sensitized atria to the negative inotropic action of the A₁ agonist, R-PIA, without affecting the nucleoside negative chronotropy. Patch-clamp experiments in the whole-cell configuration mode demonstrate that adenosine, via A₁ receptors, activates the inwardly-rectifying GIRK/K_IR3.1/K_IR3.4 current resulting in hyperpolarization of atrial cardiomyocytes, which may slow down heart rate. Conversely, the nucleoside inactivates a small conductance Ca²⁺-activated K_Ca2/SK outward current, which eventually reduces the repolarizing force and thereby prolong action potentials duration and Ca²⁺ influx into cardiomyocytes. Immunolocalization studies showed that differences in A₁ receptors distribution between the sinoatrial node and surrounding cardiomyocytes do not afford a rationale for adenosine chronoselectivity. Immunolabelling of K_IR3.1, K_Ca2.2, K_Ca2.3, and Ca_v1 was also observed throughout the right atrium. Functional data indicate that while both A₁ and M₂ receptors favor the opening of GIRK/K_IR3.1/3.4 channels modulating atrial chronotropy, A₁ receptors may additionally restrain K_Ca2/SK activation thereby compensating atrial inotropic depression by increasing the time available for Ca²⁺ influx through Ca_v1 (L-type) channels.

Nuclear Receptor-Like Structure and Interaction of Congenital Heart Disease-Associated Factors GATA4 and NKX2-5

AIMS

Transcription factor GATA4 is a dosage sensitive regulator of heart development and alterations in its level or activity lead to congenital heart disease (CHD). GATA4 has also been implicated in cardiac regeneration and repair. GATA4 action involves combinatorial interaction with other cofactors such as NKX2-5, another critical cardiac regulator whose mutations also cause CHD. Despite its critical importance to the heart and its evolutionary conservation across species, the structural basis of the GATA4-NKX2-5 interaction remains incompletely understood.

METHODS AND RESULTS

A homology model was constructed and used to identify surface amino acids important for the interaction of GATA4 and NKX2-5. These residues were subjected to site-directed mutagenesis, and the mutant proteins were characterized for their ability to bind DNA and to physically and functionally interact with NKX2-5. The studies identify 5 highly conserved amino acids in the second zinc finger (N272, R283, Q274, K299) and its C-terminal extension (R319) that are critical for physical and functional interaction with the third alpha helix of NKX2-5 homeodomain. Integration of the experimental data with computational modeling suggests that the structural arrangement of the zinc finger-homeodomain resembles the architecture of the conserved DNA binding domain of nuclear receptors.

CONCLUSIONS

The results provide novel insight into the structural basis for protein-protein interactions between two important classes of transcription factors. The model proposed will help to elucidate the molecular basis for disease causing mutations in GATA4 and NKX2-5 and may be relevant to other members of the GATA and NK classes of transcription factors.

Cross sectional PET study of cerebral adenosine A₁ receptors in premanifest and manifest Huntington's disease

PURPOSE

To study cerebral adenosine receptors (AR) in premanifest and manifest stages of Huntington's disease (HD).

METHODS

We quantified the cerebral binding potential (BP ND) of the A₁AR in carriers of the HD CAG trinucleotide repeat expansion using the radioligand [(18) F]CPFPX and PET. Four groups were investigated: (i) premanifest individuals far (preHD-A; n = 7) or (ii) near (preHD-B; n = 6) to the predicted symptom onset, (iii) manifest HD patients (n = 8), and (iv) controls (n = 36).

RESULTS

Cerebral A₁AR values of preHD-A subjects were generally higher than those of controls (by up to 31%, p < .01, in the thalamus on average). Across stages a successive reduction of A₁AR BPND was observed to the levels of controls in preHD-B and undercutting controls in manifest HD by down to 25%, p < .01, in the caudatus and amygdala. There was a strong correlation between A₁AR BP ND and years to onset. Before onset of HD, the assumed annual rates of change of A₁AR density were -1.2% in the caudatus, -1.7% in the thalamus and -3.4% in the amygdala, while the corresponding volume losses amounted to 0.6%, 0.1% and 0.2%, respectively.

CONCLUSIONS

Adenosine receptors switch from supra to subnormal levels during phenoconversion of HD. This differential regulation may play a role in the pathophysiology of altered energy metabolism.

Lrrc10 is a novel cardiac-specific target gene of Nkx2-5 and GATA4

Cardiac gene expression is precisely regulated and its perturbation causes developmental defects and heart disease. Leucine-rich repeat containing 10 (Lrrc10) is a cardiac-specific factor that is crucial for proper cardiac development and deletion of Lrrc10 in mice results in dilated cardiomyopathy. However, the mechanisms regulating Lrrc10 expression in cardiomyocytes remain unknown. Therefore, we set out to determine trans-acting factors and cis-elements critical for mediating Lrrc10 expression. We identify Lrrc10 as a transcriptional target of Nkx2-5 and GATA4. The Lrrc10 promoter region contains two highly conserved cardiac regulatory elements, which are functional in cardiomyocytes but not in fibroblasts. In vivo, Nkx2-5 and GATA4 endogenously occupy the proximal and distal cardiac regulatory elements of Lrrc10 in the heart. Moreover, embryonic hearts of Nkx2-5 knockout mice have dramatically reduced expression of Lrrc10. These data demonstrate the importance of Nkx2-5 and GATA4 in regulation of Lrrc10 expression in vivo. The proximal cardiac regulatory element located at around -200bp is synergistically activated by Nkx2-5 and GATA4 while the distal cardiac regulatory element present around -3kb requires SRF in addition to Nkx2-5 and GATA4 for synergistic activation. Mutational analyses identify a pair of adjacent Nkx2-5 and GATA binding sites within the proximal cardiac regulatory element that are necessary to induce expression of Lrrc10. In contrast, only the GATA site is functional in the distal regulatory element. Taken together, our data demonstrate that the transcription factors Nkx2-5 and GATA4 cooperatively regulate cardiac-specific expression of Lrrc10.

Regulation of cardiovascular development by adenosine and adenosine-mediated embryo protection

Few signaling molecules have as much potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (A1AR, A2aAR, A2bAR, and A3AR). We examined how adenosine acts via A1ARs to influence embryo development. Transgenic mice were studied along with embryo cultures. Embryos lacking A1ARs were markedly growth retarded following intrauterine hypoxia exposure. Studies of mice selectively lacking A1AR in the heart identify the heart as a key site of adenosine's embryo-protective effects. Studies of isolated embryos showed that adenosine plays a key role in modulating embryo cardiac function, especially in the setting of hypoxia. When pregnant mice were treated during embryogenesis with the adenosine antagonist caffeine, adult mice had abnormal heart function. Adenosine acts via A1ARs to play an essential role in protecting the embryo against intrauterine stress, and adenosine antagonists, including caffeine, may be an unwelcome exposure for the embryo.

Adverse and protective influences of adenosine on the newborn and embryo: implications for preterm white matter injury and embryo protection

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (ARs). In the postnatal period, A1AR activation may contribute to white matter injury in the preterm infant by altering oligodendrocyte (OL) development. In models of perinatal brain injury, caffeine is neuroprotective against periventricular white matter injury (PWMI) and hypoxic-ischemic encephalopathy (HIE). Supporting the notion that blockade of adenosine action is of benefit in the premature infant, caffeine reduces the incidence of bronchopulmonary dysplasia and CP in clinical studies. In comparison with the adverse effects on the postnatal brain, adenosine acts via A1ARs to play an essential role in protecting the embryo from hypoxia. Embryo protective effects are blocked by caffeine, and caffeine intake during early pregnancy increases the risk of miscarriage and fetal growth retardation. Adenosine and adenosine antagonists play important modulatory roles during mammalian development. The protective and deleterious effects of adenosine depend on the time of exposure and target sites of action.

Selective deletion of the A1 adenosine receptor abolishes heart-rate slowing effects of intravascular adenosine in vivo

Objective

Intravenous adenosine induces temporary bradycardia. This is due to the activation of extracellular adenosine receptors (ARs). While adenosine can signal through any of four ARs (A1AR, A2AAR, A2BAR, A3AR), previous ex vivo studies implicated the A1AR in the heart-rate slowing effects. Here, we used comparative genetic in vivo studies to address the contribution of individual ARs to the heart-rate slowing effects of intravascular adenosine.

Methods and Results

We studied gene-targeted mice for individual ARs to define their in vivo contribution to the heart-rate slowing effects of adenosine. Anesthetized mice were treated with a bolus of intravascular adenosine, followed by measurements of heart-rate and blood pressure via a carotid artery catheter. These studies demonstrated dose-dependent slowing of the heart rate with adenosine treatment in wild-type, A2AAR^−/−, A2BAR^−/−, or A3AR^−/− mice. In contrast, adenosine-dependent slowing of the heart-rate was completely abolished in A1AR^−/− mice. Moreover, pre-treatment with a specific A1AR antagonist (DPCPX) attenuated the heart-rate slowing effects of adenosine in wild-type, A2AAR^−/−, or A2BAR^−/− mice, but did not alter hemodynamic responses of A1AR^−/− mice.

Conclusions

The present studies combine pharmacological and genetic in vivo evidence for a selective role of the A1AR in slowing the heart rate during adenosine bolus injection.

Embryonic caffeine exposure induces adverse effects in adulthood

The purpose of this study was to determine both the short-term effects on cardiac development and embryo growth and the long-term effects on cardiac function and body composition of in utero caffeine exposure. Pregnant mice (C57BL/6) were exposed to hypoxia (10% O(2)) or room air from embryonic days (E) 8.5-10.5, and treated with caffeine (20 mg/kg, i.p.) or vehicle (normal saline, 0.9% NaCl). This caffeine dose results in a circulating level that is equivalent to 2 cups of coffee in humans. Hypoxic exposure acutely reduced embryonic growth by 30%. Exposure to a single dose of caffeine inhibited cardiac ventricular development by 53% in hypoxia and 37% in room air. Caffeine exposure resulted in inhibition of hypoxia-induced HIF1alpha protein expression in embryos by 40%. When offspring from dams treated with a single dose of caffeine were studied in adulthood, we observed that caffeine treatment alone resulted in a decrease in cardiac function of 38%, as assessed by echocardiography. We also observed a 20% increase in body fat with male mice exposed to caffeine. Caffeine was dissolved in normal saline, so it was used as a control. Room air controls were used to compare to the hypoxic mice. Exposure to a single dose of caffeine during embryogenesis results in both short-term effects on cardiac development and long-term effects on cardiac function.

Mechanisms of induction of adenosine receptor genes and its functional significance

Adenosine is a metabolite generated and released from cells, particularly under injury or stress. It elicits protective or damaging responses via signaling through the adenosine receptors, including the adenylyl cyclase inhibitory A(1) and A(3), and the adenylyl cyclase stimulatory A(2A) and A(2B). Multiple adenosine receptor types, including stimulatory and inhibitory, can be found in the same cell, suggesting that a careful balance of adenosine receptor expression in a particular cell is necessary for a specific adenosine-induced response. This balance could be controlled by differential expression of the adenosine receptor genes under different stimuli. Here, we have reviewed an array of studies that have characterized basal or induced expression of the adenosine receptors and common as well as distinct mechanisms of effect, in hopes that ongoing studies on this topic will further elucidate detailed mechanisms of adenosine receptor regulation, leading to potential therapeutic applications.

Glucocorticoids regulate innate immunity in a model of multiple sclerosis: reciprocal interactions between the A1 adenosine receptor and beta-arrestin-1 in monocytoid cells

Desensitization of seven transmembrane receptors (7TMRs), which are modulated by the beta-arrestins, leads to altered G protein activation. The A1 adenosine receptor (A1AR) is an antiinflammatory 7TMR exhibiting reduced expression and activity in both multiple sclerosis (MS) and the murine MS model, experimental autoimmune encephalomyelitis (EAE) in monocytoid cells. Herein, we report that beta-arrestin-1 expression was increased in brains of MS patients relative to non-MS brains, whereas A1AR expression was concomitantly reduced. This inverse relationship between beta-arrestin-1 and A1AR was confirmed in cultured monocytoid cells as beta-arrestin-1 overexpression resulted in a down-regulation of A1AR together with the internalization of the surface receptor. Moreover, a physical interaction between beta-arrestin-1 and A1AR was demonstrated in monocytoid cells. Proinflammatory cytokines regulated the A1AR/beta-arrestin-1 interactions, while A1AR activation also modulated proinflammatory cytokines expression. During EAE, beta-arrestin-1 and A1AR expression in the spinal cord displayed a similar pattern compared to that observed in MS brains. EAE-induced neuroinflammation and neurobehavioral deficits were suppressed by glucocorticoid treatments, accompanied by concurrent reduced beta-arrestin-1 and enhanced A1AR expression. Thus, the interplay between beta-arrestin-1 and A1AR in the central nervous system during neuroinflammation represents a reciprocal regulatory mechanism through which neuroprotective therapeutic strategies for neuroinflammatory diseases might be further developed.

Aspects of the general biology of adenosine A2A signaling

Many of our current hopes of finding better ways to treat Parkinson's disease or to stop its progression rely on studies of adenosine A2A receptors in the brain. Yet any drug targeting central receptors will also potentially affect receptors in other sites. Furthermore, several fundamental aspects of adenosine receptor biology must be taken into account. For these reasons the "Targeting adenosine A2A receptors in Parkinson's disease and other CNS disorders" meeting in Boston included selected aspects of the general biology of adenosine A2A receptor signaling. Some of the presentations from this part of the meeting are summarized in this first chapter. As will be apparent to the reader, these different parts do not form an integrated whole, but they do indicate areas the organizers felt might illuminate remaining questions regarding the roles of adenosine A2A receptors. The contributors to this part of the meeting have summarized some of the key questions below.

A1 adenosine receptors play an essential role in protecting the embryo against hypoxia

Embryos can be exposed to environmental factors that induce hypoxia. Currently, our understanding of the effects of hypoxia on early mammalian development is modest. Potential mediators of hypoxia action include the nucleoside adenosine, which acts through A(1) adenosine receptors (A(1)ARs) and mediates adverse effects of hypoxia on the neonatal brain. We hypothesized that A(1)ARs may also play a role in mediating effects of hypoxia on the embryo. When pregnant dams were exposed to hypoxia (10% O(2)) beginning at embryonic day (E) 7.5 or 8.5 and continued for 24-96 h, A(1)AR+/+ embryos manifested growth inhibition and a disproportionate reduction in heart size, including thinner ventricular walls. Yet, when dams were exposed to hypoxia, embryos lacking A(1)ARs (A(1)AR-/-) had much more severe growth retardation than A(1)AR+/+ or +/- embryos. When levels of hypoxia-inducible factor 1alpha (HIF1alpha) were examined, A(1)AR-/- embryos had less stabilized HIF1alpha protein than A(1)AR+/- littermates. Normal patterns of cardiac gene expression were also disturbed in A(1)AR-/- embryos exposed to hypoxia. These results show that short periods of hypoxia during early embryogenesis can result in intrauterine growth retardation. We identify adenosine and A(1)ARs as playing an essential role in protecting the embryo from hypoxia.

Sex differences in mouse heart rate and body temperature and in their regulation by adenosine A1 receptors

AIM

To examine cardiac function, body temperature and locomotor behaviour in the awake adenosine A(1) receptor knock out mouse of both sexes.

METHODS

Male and female A(1)R (+/+) and (-/-) mice, instrumented with telemetric devices, were recorded during basal conditions and after drug administration.

RESULTS

Female mice had higher heart rate, body temperature and locomotion, both during daytime and during the night. Awake A(1)R (-/-) mice had a slightly elevated heart rate, and this was more clear-cut in males. Heart rate was also higher in Langendorff-perfused denervated A(1)R (-/-) hearts. Body temperature was higher in A(1)R (-/-) males and females; locomotor activity was higher in A(1)R (-/-) females, but not in males. The adenosine receptor agonist R-PIA (0.2 mg kg(-1)) decreased heart rate and body temperature, but less in A(1)R (-/-) animals than in A(1)R (+/+) mice (P < 0.001 in both parameters). The unselective adenosine receptor antagonist caffeine had a minor stimulatory effect on heart rate in lower doses, but depressed it at a dose of 75 mg kg(-1). Body temperature was increased after a low dose (7.5 mg kg(-1)) of caffeine in both sexes and genotypes, and markedly reduced after a high dose (75 mg kg(-1)) of caffeine. An intermediary dose of caffeine 30 mg kg(-1) increased or decreased body temperature depending on genotype and sex. Locomotor responses to caffeine were variable depending both on genotype and sex.

CONCLUSION

Thus, the adenosine A(1) receptor is involved in the regulation of heart rate, body temperature and locomotor activity, but the magnitude of the involvement is different in males and females.

Transcription factors and congenital heart defects

Although there have been important advances in diagnostic modalities and therapeutic strategies for congenital heart defects (CHD), these malformations still lead to significant morbidity and mortality in the human population. Over the past 10 years, characterization of the genetic causes of CHD has begun to elucidate some of the molecular causes of these defects. Linkage analysis and candidate-gene approaches have been used to identify gene mutations that are associated with both familial and sporadic cases of CHD. Complementation of the human studies with developmental studies in mouse models provides information for the roles of these genes in normal development as well as indications for disease pathogenesis. Biochemical analysis of these gene mutations has provided further insight into the molecular effects of these genetic mutations. Here we review genetic, developmental, and biochemical studies of six cardiac transcription factors that have been identified as genetic causes for CHD in humans.

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.

Cardiac transcription factor Csx/Nkx2-5: Its role in cardiac development and diseases

During the past decade, an emerging body of evidence has accumulated that cardiac transcription factors control a cardiac gene program and play a critical role in transcriptional regulation during cardiogenesis and during the adaptive process in adult hearts. Especially, an evolutionally conserved homeobox transcription factor Csx/Nkx2-5 has been in the forefront in the field of cardiac biology, providing molecular insights into the mechanisms of cardiac development and diseases. Csx/Nkx2-5 is indispensable for normal cardiac development, and mutations of the gene are associated with human congenital heart diseases (CHD). In the present review, the regulation of a cardiac gene program by Csx/Nkx2-5 is summarized, with an emphasis on its role in the cardiac development and diseases.

The Zinc Finger-Containing Transcription Factor Gata-4 Is Expressed in the Developing Endocrine Pancreas and Activates Glucagon Gene Expression

Gene inactivation studies have shown that members of the Gata family of transcription factors are critical for endoderm development throughout evolution. We show here that Gata-4 and/or Gata-6 are not only expressed in the adult exocrine pancreas but also in glucagonoma and insulinoma cell lines, whereas Gata-5 is restricted to the exocrine pancreas. During pancreas development, Gata-4 is expressed already at embryonic d 10.5 and colocalizes with early glucagon+ cells at embryonic d 12.5. Gata-4 was able to transactivate the glucagon gene both in heterologous BHK-21 (nonislet Syrian baby hamster kidney) and in glucagon-producing InR1G9 cells. Using gel-mobility shift assays, we identified a complex formed with nuclear extracts from InR1G9 cells on the G5 control element (−140 to −169) of the glucagon gene promoter as Gata-4. Mutation of the GATA binding site on G5 abrogated the transcriptional activation mediated by Gata-4 and reduced basal glucagon gene promoter activity in glucagon-producing cells by 55%. Furthermore, Gata-4 acted more than additively with Forkhead box A (hepatic nuclear factor-3) to trans-activate the glucagon gene promoter. We conclude that, besides its role in endoderm differentiation, Gata-4 might be implicated in the regulation of glucagon gene expression in the fetal pancreas and that Gata activity itself may be modulated by interactions with different cofactors.

Transcriptional regulation of the murine Connexin40 promoter by cardiac factors Nkx2-5, GATA4 and Tbx5

OBJECTIVE

Connexin40 (Cx40) is a gap junction protein expressed specifically in developing and mature atrial myocytes and cells of the conduction system. In this report, we identify cis-acting elements within the mouse Cx40 promoter and unravel part of the complex pathways involved in the cardiac expression of this gene.

METHODS

To identify the factors involved in the cardiac expression of Cx40, we used transient transfections in mammalian cells coupled with electrophoretic mobility shift assays (EMSA) and RT-PCR.

RESULTS

Within the promoter region, we identified the minimal elements required for transcriptional activity within 150 base pairs (bp) upstream of the transcriptional start site. Several putative regulatory sites for transcription factors were predicted within this region by computer analysis, and we demonstrated that the nuclear factors Sp1, Nkx2-5, GATA4 and Tbx5 could interact specifically with elements present in the minimal promoter region of the Cx40. Furthermore, co-transfection experiments showed the ability of Nkx2-5 and GATA4 to transactivate the minimal Cx40 promoter while Tbx5 repressed Nkx2-5/GATA4-mediated activation. Mutagenesis of the Nkx2-5 core site in the Cx40 promoter led to significantly decreased activity in rat smooth muscle cell line A7r5. Consistent with this, mouse embryos lacking Nkx2-5 showed a marked decrease in Cx40 expression.

CONCLUSION

In this work, we cloned the promoter region of the Cx40 and demonstrated that the core promoter was modulated by cardiac transcriptional factors Nkx2-5, Tbx5 and GATA4 acting together with ubiquitous Sp1.

Characterization of the rat A2A adenosine receptor gene: a 4.8-kb promoter-proximal DNA fragment confers selective expression in the central nervous system

We isolated and characterized a 4.8-kb 5' flanking region of the rat A2A adenosine receptor (A2A-R) gene in the present study. Promoter activity was observed with this DNA fragment in PC12 cells and C6 cells which contain endogenous A2A-Rs. A fusion fragment consisting of the 4.8-kb promoter-proximal DNA fragment of the A2A-R gene, and the coding region of lacZ was utilized to produce mice harbouring the fusion gene. In three independent founder lines, proteins and transcripts of the transgene were found in many areas of the central nervous system (CNS), but not in three peripheral tissues examined. Double immunohistochemical analyses revealed that the transgene was coexpressed with endogenous A2A-R and proper neuronal markers in the brain. Specifically, the transgene in the striatum was found in the enkephalin-containing GABAergic neurons and in the cholinergic neurons as was found for the endogenous A2A-R. However, a selectively enriched striatal expression of the transgene was not found as was observed for the endogenous A2A-R. Collectively, the 4.8-kb promoter-proximal DNA fragment of the rat A2A-R gene contains important element(s) to direct its expression in the CNS where functional A2A-R are found, but were not sufficient to confer the highly concentrated expression of the striatal A2A-R. Furthermore, expressions of A2A-R and the transgene were found in both neurons and astrocytes, suggesting that adenosine might mediate its function through A2A-R in both cell types.

The different cardiac expression of the type 2 iodothyronine deiodinase gene between human and rat is related to the differential response of the Dio2 genes to Nkx-2.5 and GATA-4 transcription factors

By producing T3 from T4, type 2 iodothyronine deiodinase (D2) catalyzes the first step in the cascade underlying the effect exerted by thyroid hormone. Type 2 iodothyronine deiodinase mRNA is expressed at high levels in human heart but is barely detectable in the corresponding rodent tissue. Although the heart is a major target of thyroid hormone, the role of cardiac D2 and the factors that regulate its expression are unknown. Here we report that the human Dio2 promoter is very sensitive to the cardiac transcription factors Nkx-2.5 and GATA-4. Nkx-2.5 transactivates a 6.5-kb human (h)Dio2-chloramphenicol acetyltransferase construct, with maximal induction reached with a 633-bp proximal promoter region. Interestingly, despite 73% identity with the corresponding human region, the rat Dio2 promoter is much less responsive to Nkx-2.5 induction. Using EMSA, we found that two sites in the human promoter (C and D) specifically bind Nkx-2.5. In coexpression studies, GATA-4 alone was a poor inducer of the hDio2 promoter; however in synergy with Nkx-2.5, it activated D2 reporter gene expression in the human, but not the rat promoter. Functional analysis showed that both C and D sites are required for the complete Nkx-2.5 response and for the Nkx-2.5/GATA-4 synergistic effect. In neonatal rat primary myocardiocytes, most of the hDio2-chloramphenicol acetyltransferase activity was suppressed by mutation of the Nkx-2.5 binding sites. Finally, a mutant Nkx-2.5 protein (N188K), which causes, in heterozygosity, congenital heart diseases, did not transactivate the Dio2 promoter and interfered with its activity in cardiomyocytes, possibly by titrating endogenous Nkx-2.5 protein away from the promoter. In conclusion, this study shows that Nkx-2.5 and GATA-4 play prime roles in Dio2 gene regulation in the human heart and suggests that it is their synergistic action in humans that causes the differential expression of the cardiac Dio2 gene between humans and rats.

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.

Identification of nuclear factor 1 (NF1) as a transcriptional modulator of rat A(2A) adenosine receptor

By a combination of PCR and DNA walking technique, we isolated a 4.8-kb DNA fragment containing a 4.3 kb 5'-flanking region and a 0.5-kb 5'-untranslated region of the rat A(2A) adenosine receptor (A(2A)-R) gene. Various lengths of the 5'-flanking region of the A(2A)-R gene were inserted into an expression vector and transfected into several different cell lines for promoter analysis. Our results reveal that a consensus NF1 element (designated as A(2A)-R/NF1), located between bases -2846 and -2827 of the A(2A)-R gene, functions as a repressor for A(2A)-R promoters in the rat brain-derived type-2 astrocyte cell line (RBA2), which expresses no A(2A)-R. Electrophoretic gel mobility shift assay (EMSA) revealed that two A(2A)-R/NF1-protein complexes of RBA2 nuclear extract were formed. Supershift experiments using an anti-NF1 antibody suggest that NF1 proteins exist in both A(2A)-R/NF1-protein complexes. Furthermore, mutations in the conserved NF1 binding site of this A(2A)-R/NF1 element disturbed DNA-protein formation. Thus, NF1 proteins appear to mediate this cell line-specific suppression of A(2A)-R promoters in RBA2 cells. The importance of NF1 proteins in regulating A(2A)-R promoters was further confirmed in another cell line (Siha) which expresses no endogenous A(2A)-R. Moreover, addition of the A(2A)-R/NF1element upstream of an irrelevant thymidine kinase (TK) promoter suppressed its promoter activity in Siha cells, but not in RBA2 cells. Thus, the NF1-mediated inhibition of the A(2A)-R promoter was promoter- and cell line-specific. In summary, we have defined a distal negative element (A(2A)-R/NF1) that plays a functional role in modulating the expression of A(2A)-R.

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.

Influences of adenosine on the fetus and newborn

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. In contrast to most neurotransmitters, adenosine is released by all cells and is present in all tissues. The adenosinergic system is therefore not dependent on the presence of mature synaptic structures or an intact autonomic nervous system for its release. However, similar to other signaling molecules, adenosine levels are dynamically regulated and increase with increased tissue activity, hypoxia, or stress. Local adenosine concentrations thus provide a "humoral barometer" of acute changes in cellular physiology. The receptors that transduce adenosine action include A1, A2a, A2b, and A3 adenosine receptors. These receptors differ in their affinities for adenosine and in patterns of tissues expression. During development A1 adenosine receptors (A1ARs) are especially important, and A1ARs are among the earliest receptors expressed in the embryonic brain and heart. In the developing heart, the adenosinergic system is the dominant regulator of fetal cardiac function and A1AR activation inhibits cardiac cell division leading to cardiac hypoplasia. In the forming central nervous system, A1AR activation potently inhibits the development of axons and can lead to leukomalacia. These recent data suggest that adenosine is an important modulator of mammalian development.

The small muscle-specific protein Csl modifies cell shape and promotes myocyte fusion in an insulin-like growth factor 1-dependent manner

We have isolated a murine cDNA encoding a 9-kD protein, Chisel (Csl), in a screen for transcriptional targets of the cardiac homeodomain factor Nkx2-5. Csl transcripts were detected in atria and ventricles of the heart and in all skeletal muscles and smooth muscles of the stomach and pulmonary veins. Csl protein was distributed throughout the cytoplasm in fetal muscles, although costameric and M-line localization to the muscle cytoskeleton became obvious after further maturation. Targeted disruption of Csl showed no overt muscle phenotype. However, ectopic expression in C2C12 myoblasts induced formation of lamellipodia in which Csl protein became tethered to membrane ruffles. Migration of these cells was retarded in a monolayer wound repair assay. Csl-expressing myoblasts differentiated and fused normally, although in the presence of insulin-like growth factor (IGF)-1 they showed dramatically enhanced fusion, leading to formation of large dysmorphogenic "myosacs." The activities of transcription factors nuclear factor of activated T cells (NFAT) and myocyte enhancer-binding factor (MEF)2, were also enhanced in an IGF-1 signaling-dependent manner. The dynamic cytoskeletal localization of Csl and its dominant effects on cell shape and behavior and transcription factor activity suggest that Csl plays a role in the regulatory network through which muscle cells coordinate their structural and functional states during growth, adaptation, and repair.

Cloning of the mouse Pax4 gene promoter and identification of a pancreatic beta cell specific enhancer

Pax4 encodes a paired-box transcription factor and is essential for the differentiation of islet cells since the Pax4 homozygous mutant mice lack mature beta and delta cells. However, little is known about the transcriptional regulation of the Pax4 gene. We isolated and sequenced a 2.4-kb mouse genomic DNA fragment containing the 5' flanking sequence of Pax4 and identified a previously unrecognized intron. Primer extension revealed that this TATA-less promoter had only one transcription start site. The promoter activity of this fragment with various deletion mutants when tested in beta and non-beta cell lines indicated the presence of a beta-cell specific enhancer in the region, -1858 to -1954 bp. DNase 1 footprinting and gel retardation assays indicated that nuclear proteins from betaHC3 cells interacted with two sequences which contained putative CdxA/Nkx.2 and GATA-1,-2 binding sites. Site-directed mutagenesis indicated that both of these regions were necessary for beta-cell specific enhancer activity.

Functional analyses of three Csx/Nkx-2.5 mutations that cause human congenital heart disease

A homeodomain-containing transcription factor Csx/Nkx-2.5 is an important regulator of cardiogenesis in mammals. Three different mutants, Gln170ter (designated A) and Thr178Met (designated B) in the helix 2 of the homeodomain and Gln198ter mutation (designated C) just after homeodomain, have been reported to cause atrial septal defect with atrial ventricular block. We here examined the functions of these three mutants of Csx/Nkx-2.5. The atrial natriuretic peptide (ANP) promoter was activated by wild type Csx/Nkx-2.5 (WT, approximately 8-fold), B ( approximately 2-fold), and C ( approximately 6-fold) but not by A. When A, B, or C was cotransfected into COS-7 cells with the same amount of WT, WT-induced activation of the ANP promoter was attenuated by A and B (A > B), whereas C further enhanced the activation. Immunocytochemical analysis using anti-Myc tag antibody indicated that transfected Myc-tagged WT, B, and C were localized in the nucleus of both COS-7 cells and cardiomyocytes of neonatal rats, whereas A was distributed diffusely in the cytoplasm and nucleus in COS-7 cells. Electrophoretic mobility shift assay showed that Csx/Nkx-2.5-binding sequences were bound strongly by WT and C, weakly by B, but not by A. Immunoprecipitation and GST pull-down assay revealed that WT and all mutants interacted with GATA-4. The synergistic activation of the ANP promoter by WT and GATA-4 was further enhanced by C but was inhibited by A and B. In the cultured cardiomyocytes, overexpression of C but not WT, A, or B, induced apoptosis. These results suggest that although the three mutants induce the same cardiac phenotype, transactivation ability and DNA binding ability are different among the three mutants and that apoptosis may be a cause for C-induced cardiac defect.