The lncRNA Sweetheart regulates compensatory cardiac hypertrophy after myocardial injury in murine males

After myocardial infarction in the adult heart the remaining, non-infarcted tissue adapts to compensate the loss of functional tissue. This adaptation requires changes in gene expression networks, which are mostly controlled by transcription regulating proteins. Long non-coding transcripts (lncRNAs) are taking part in fine-tuning such gene programs. We describe and characterize the cardiomyocyte specific lncRNA Sweetheart RNA (Swhtr), an approximately 10 kb long transcript divergently expressed from the cardiac core transcription factor coding gene Nkx2-5. We show that Swhtr is dispensable for normal heart development and function but becomes essential for the tissue adaptation process after myocardial infarction in murine males. Re-expressing Swhtr from an exogenous locus rescues the Swhtr null phenotype. Genes that depend on Swhtr after cardiac stress are significantly occupied and therefore most likely regulated by NKX2-5. The Swhtr transcript interacts with NKX2-5 and disperses upon hypoxic stress in cardiomyocytes, indicating an auxiliary role of Swhtr for NKX2-5 function in tissue adaptation after myocardial injury.

Assessment of the Cardiac Noncoding Transcriptome by Single-Cell RNA Sequencing Identifies FIXER, a Conserved Profibrogenic Long Noncoding RNA

BACKGROUND

Cardiac fibroblasts have crucial roles in the heart. In particular, fibroblasts differentiate into myofibroblasts in the damaged myocardium, contributing to scar formation and interstitial fibrosis. Fibrosis is associated with heart dysfunction and failure. Myofibroblasts therefore represent attractive therapeutic targets. However, the lack of myofibroblast-specific markers has precluded the development of targeted therapies. In this context, most of the noncoding genome is transcribed into long noncoding RNAs (lncRNAs). A number of lncRNAs have pivotal functions in the cardiovascular system. lncRNAs are globally more cell-specific than protein-coding genes, supporting their importance as key determinants of cell identity.

METHODS

In this study, we evaluated the value of the lncRNA transcriptome in very deep single-cell RNA sequencing. We profiled the lncRNA transcriptome in cardiac nonmyocyte cells after infarction and probed heterogeneity in the fibroblast and myofibroblast populations. In addition, we searched for subpopulation-specific markers that can constitute novel targets in therapy for heart disease.

RESULTS

We demonstrated that cardiac cell identity can be defined by the sole expression of lncRNAs in single-cell experiments. In this analysis, we identified lncRNAs enriched in relevant myofibroblast subpopulations. Selecting 1 candidate we named FIXER (fibrogenic LOX-locus enhancer RNA), we showed that its silencing limits fibrosis and improves heart function after infarction. Mechanitically, FIXER interacts with CBX4, an E3 SUMO protein ligase and transcription factor, guiding CBX4 to the promoter of the transcription factor RUNX1 to control its expression and, consequently, the expression of a fibrogenic gene program.. FIXER is conserved in humans, supporting its translational value.

CONCLUSIONS

Our results demonstrated that lncRNA expression is sufficient to identify the various cell types composing the mammalian heart. Focusing on cardiac fibroblasts and their derivatives, we identified lncRNAs uniquely expressed in myofibroblasts. In particular, the lncRNA FIXER represents a novel therapeutic target for cardiac fibrosis.

Resveratrol Reverses Endothelial Colony-Forming Cell Dysfunction in Adulthood in a Rat Model of Intrauterine Growth Restriction

Individuals born after intrauterine growth restriction (IUGR) are at risk of developing cardiovascular diseases (CVDs). Endothelial dysfunction plays a role in the pathogenesis of CVDs; and endothelial colony-forming cells (ECFCs) have been identified as key factors in endothelial repair. In a rat model of IUGR induced by a maternal low-protein diet, we observed an altered functionality of ECFCs in 6-month-old males, which was associated with arterial hypertension related to oxidative stress and stress-induced premature senescence (SIPS). Resveratrol (R), a polyphenol compound, was found to improve cardiovascular function. In this study, we investigated whether resveratrol could reverse ECFC dysfunctions in the IUGR group. ECFCs were isolated from IUGR and control (CTRL) males and were treated with R (1 μM) or dimethylsulfoxide (DMSO) for 48 h. In the IUGR-ECFCs, R increased proliferation (5'-bromo-2'-deoxyuridine (BrdU) incorporation, p < 0.001) and improved capillary-like outgrowth sprout formation (in Matrigel), nitric oxide (NO) production (fluorescent dye, p < 0.01), and endothelial nitric oxide synthase (eNOS) expression (immunofluorescence, p < 0.001). In addition, R decreased oxidative stress with reduced superoxide anion production (fluorescent dye, p < 0.001); increased Cu/Zn superoxide dismutase expression (Western blot, p < 0.05); and reversed SIPS with decreased beta-galactosidase activity (p < 0.001), and decreased p16^ink4a (p < 0.05) and increased Sirtuin-1 (p < 0.05) expressions (Western blot). No effects of R were observed in the CTRL-ECFCs. These results suggest that R reverses long-term ECFC dysfunctions related to IUGR.

Inhibition of the NOTCH1 Pathway in the Stressed Heart Limits Fibrosis and Promotes Recruitment of Non-Myocyte Cells into the Cardiomyocyte Fate

Cardiac pathologies lead to an acute or gradual loss of cardiomyocytes. Because of the limited regenerative capacity of the mammalian heart, cardiomyocytes are only replaced by fibrotic tissue. Excessive fibrosis contributes to the deterioration of cardiac function and the transition to heart failure, which is the leading cause of morbidity and mortality worldwide. Currently, no treatments can promote replenishment of the injured heart with newly formed cardiomyocytes. In this context, regenerative strategies explore the possibility to promote recovery through induction of cardiomyocyte production from pre-existing cardiomyocytes. On the other hand, cardiac non-myocyte cells can be directly reprogrammed into induced cardiac precursor cells and cardiomyocytes, suggesting that these cells could be exploited to produce cardiomyocytes in vivo. Here, we provide evidence that the sequential activation and inhibition of the NOTCH1 signaling pathway in the stressed heart decreases fibrosis and improves cardiac function in the stressed heart. This is accompanied by the emergence of new cardiomyocytes from non-myocyte origin. Overall, our data show how a developmental pathway such as the NOTCH pathway can be manipulated to provide therapeutic benefit in the damaged heart.

Inhibition of G-protein signalling in cardiac dysfunction of intellectual developmental disorder with cardiac arrhythmia (IDDCA) syndrome

BACKGROUND

Pathogenic variants of GNB5 encoding the β5 subunit of the guanine nucleotide-binding protein cause IDDCA syndrome, an autosomal recessive neurodevelopmental disorder associated with cognitive disability and cardiac arrhythmia, particularly severe bradycardia.

METHODS

We used echocardiography and telemetric ECG recordings to investigate consequences of Gnb5 loss in mouse.

RESULTS

We delineated a key role of Gnb5 in heart sinus conduction and showed that Gnb5-inhibitory signalling is essential for parasympathetic control of heart rate (HR) and maintenance of the sympathovagal balance. Gnb5-/- mice were smaller and had a smaller heart than Gnb5+/+ and Gnb5+/- , but exhibited better cardiac function. Lower autonomic nervous system modulation through diminished parasympathetic control and greater sympathetic regulation resulted in a higher baseline HR in Gnb5-/- mice. In contrast, Gnb5-/- mice exhibited profound bradycardia on treatment with carbachol, while sympathetic modulation of the cardiac stimulation was not altered. Concordantly, transcriptome study pinpointed altered expression of genes involved in cardiac muscle contractility in atria and ventricles of knocked-out mice. Homozygous Gnb5 loss resulted in significantly higher frequencies of sinus arrhythmias. Moreover, we described 13 affected individuals, increasing the IDDCA cohort to 44 patients.

CONCLUSIONS

Our data demonstrate that loss of negative regulation of the inhibitory G-protein signalling causes HR perturbations in Gnb5-/- mice, an effect mainly driven by impaired parasympathetic activity. We anticipate that unravelling the mechanism of Gnb5 signalling in the autonomic control of the heart will pave the way for future drug screening.

Diverging effects of enalapril or eplerenone in primary prevention against doxorubicin-induced cardiotoxicity

Aims

Clinical studies suggest beneficial effects of renin-angiotensin system blockade for prevention of left ventricular (LV) dysfunction after chemotherapy. However, the efficacy of this strategy as primary prevention has been poorly studied. This study aimed at identifying the pathophysiological mechanisms by which mineralocorticoid receptor antagonism (MRA) or angiotensin converting enzyme inhibition (ACEi) provide protection against doxorubicin-induced cardiotoxicity (DIC) in mouse models of acute and chronic toxicity.

Methods and results

Acute DIC was induced by a single injection of Dox at 15 mg/kg and chronic DIC applied 5 injections of Dox at 4 mg/kg/week. MRA was achieved using eplerenone or cardiomyocyte-specific ablation of the MR gene in transgenic mice and ACEi using enalapril. Drugs were provided with the first dose of Dox and applied until the end of the study. In both model of DIC, Dox induced cardiac atrophy with decreased LV volume, reduced cardiomyocyte cell size, and cardiac dysfunction. In the acute model, neither MRA nor ACEi protected against these manifestations of DIC. In the chronic model, concomitant treatment with eplerenone did not protect against DIC and drastically increased plasma aldosterone levels and cardiac levels of angiotensin II type 1 receptor and of connective tissue growth factor (CTGF), as observed in acute DIC. Enalapril treatment in the chronic model, however, protected against cardiac dysfunction and cardiomyocyte atrophy and was associated with increased activation of the PI3K/AKT/mTOR pathway along with normal levels of CTGF.

Conclusion

Enalapril and eplerenone disparately impact on cellular signalling in DIC. Eplerenone, on top of Dox treatment was not protective and associated with increased levels of plasma aldosterone and of cardiac CTGF. In contrast, we show that primary prevention with enalapril preserves LV morphology and function in a clinically relevant model of chronic DIC, with increased stimulation of the PI3K/AKT/mTOR axis and normal CTGF levels suggesting potential therapeutic implications.

Inhibition of the Hypoxia-Inducible Factor 1α-Induced Cardiospecific HERNA1 Enhance-Templated RNA Protects From Heart Disease

Supplemental Digital Content is available in the text.

Background:

Enhancers are genomic regulatory elements conferring spatiotemporal and signal-dependent control of gene expression. Recent evidence suggests that enhancers can generate noncoding enhancer RNAs, but their (patho)biological functions remain largely elusive.

Methods:

We performed chromatin immunoprecipitation–coupled sequencing of histone marks combined with RNA sequencing of left ventricular biopsies from experimental and genetic mouse models of human cardiac hypertrophy to identify transcripts revealing enhancer localization, conservation with the human genome, and hypoxia-inducible factor 1α dependence. The most promising candidate, hypoxia-inducible enhancer RNA (HERNA)1, was further examined by investigating its capacity to modulate neighboring coding gene expression by binding to their gene promoters by using chromatin isolation by RNA purification and λN–BoxB tethering–based reporter assays. The role of HERNA1 and its neighboring genes for pathological stress–induced growth and contractile dysfunction, and the therapeutic potential of HERNA1 inhibition was studied in gapmer-mediated loss-of-function studies in vitro using human induced pluripotent stem cell–derived cardiomyocytes and various in vivo models of human pathological cardiac hypertrophy.

Results:

HERNA1 is robustly induced on pathological stress. Production of HERNA1 is initiated by direct hypoxia-inducible factor 1α binding to a hypoxia-response element in the histoneH3-lysine27acetylation marks–enriched promoter of the enhancer and confers hypoxia responsiveness to nearby genes including synaptotagmin XVII, a member of the family of membrane-trafficking and Ca²⁺-sensing proteins and SMG1, encoding a phosphatidylinositol 3-kinase–related kinase. Consequently, a substrate of SMG1, ATP-dependent RNA helicase upframeshift 1, is hyperphoshorylated in a HERNA1- and SMG1-dependent manner. In vitro and in vivo inactivation of SMG1 and SYT17 revealed overlapping and distinct roles in modulating cardiac hypertrophy. Finally, in vivo administration of antisense oligonucleotides targeting HERNA1 protected mice from stress-induced pathological hypertrophy. The inhibition of HERNA1 postdisease development reversed left ventricular growth and dysfunction, resulting in increased overall survival.

Conclusions:

HERNA1 is a novel heart-specific noncoding RNA with key regulatory functions in modulating the growth, metabolic, and contractile gene program in disease, and reveals a molecular target amenable to therapeutic exploitation.

The long noncoding RNA Wisper controls cardiac fibrosis and remodeling

Long noncoding RNAs (lncRNAs) are emerging as powerful regulators of cardiac development and disease. However, our understanding of the importance of these molecules in cardiac fibrosis is limited. Using an integrated genomic screen, we identified Wisper (Wisp2 super-enhancer-associated RNA) as a cardiac fibroblast-enriched lncRNA that regulates cardiac fibrosis after injury. Wisper expression was correlated with cardiac fibrosis both in a murine model of myocardial infarction (MI) and in heart tissue from human patients suffering from aortic stenosis. Loss-of-function approaches in vitro using modified antisense oligonucleotides (ASOs) demonstrated that Wisper is a specific regulator of cardiac fibroblast proliferation, migration, and survival. Accordingly, ASO-mediated silencing of Wisper in vivo attenuated MI-induced fibrosis and cardiac dysfunction. Functionally, Wisper regulates cardiac fibroblast gene expression programs critical for cell identity, extracellular matrix deposition, proliferation, and survival. In addition, its association with TIA1-related protein allows it to control the expression of a profibrotic form of lysyl hydroxylase 2, implicated in collagen cross-linking and stabilization of the matrix. Together, our findings identify Wisper as a cardiac fibroblast-enriched super-enhancer-associated lncRNA that represents an attractive therapeutic target to reduce the pathological development of cardiac fibrosis in response to MI and prevent adverse remodeling in the damaged heart.

Comparative transcriptome profiling of the injured zebrafish and mouse hearts identifies miRNA-dependent repair pathways

Aims

The adult mammalian heart has poor regenerative capacity. In contrast, the zebrafish heart retains a robust capacity for regeneration into adulthood. These distinct responses are consequences of a differential utilization of evolutionary-conserved gene regulatory networks in the damaged heart. To systematically identify miRNA-dependent networks controlling cardiac repair following injury, we performed comparative gene and miRNA profiling of the cardiac transcriptome in adult mice and zebrafish.

Methods and results

Using an integrated approach, we show that 45 miRNA-dependent networks, involved in critical biological pathways, are differentially modulated in the injured zebrafish vs. mouse hearts. We study, more particularly, the miR-26a-dependent response. Therefore, miR-26a is down-regulated in the fish heart after injury, whereas its expression remains constant in the mouse heart. Targets of miR-26a involve activators of the cell cycle and Ezh2, a component of the polycomb repressive complex 2 (PRC2). Importantly, PRC2 exerts repressive functions on negative regulators of the cell cycle. In cultured neonatal cardiomyocytes, inhibition of miR-26a stimulates, therefore, cardiomyocyte proliferation. Accordingly, miR-26a knockdown prolongs the proliferative window of cardiomyocytes in the post-natal mouse heart.

Conclusions

This novel strategy identifies a series of miRNAs and associated pathways, in particular miR-26a, which represent attractive therapeutic targets for inducing repair in the injured heart.

Jagged1 intracellular domain-mediated inhibition of Notch1 signalling regulates cardiac homeostasis in the postnatal heart

Aims

Notch1 signalling in the heart is mainly activated via expression of Jagged1 on the surface of cardiomyocytes. Notch controls cardiomyocyte proliferation and differentiation in the developing heart and regulates cardiac remodelling in the stressed adult heart. Besides canonical Notch receptor activation in signal-receiving cells, Notch ligands can also activate Notch receptor-independent responses in signal-sending cells via release of their intracellular domain. We evaluated therefore the importance of Jagged1 (J1) intracellular domain (ICD)-mediated pathways in the postnatal heart.

Methods and results

In cardiomyocytes, Jagged1 releases J1ICD, which then translocates into the nucleus and down-regulates Notch transcriptional activity. To study the importance of J1ICD in cardiac homeostasis, we generated transgenic mice expressing a tamoxifen-inducible form of J1ICD, specifically in cardiomyocytes. Using this model, we demonstrate that J1ICD-mediated Notch inhibition diminishes proliferation in the neonatal cardiomyocyte population and promotes maturation. In the neonatal heart, a response via Wnt and Akt pathway activation is elicited as an attempt to compensate for the deficit in cardiomyocyte number resulting from J1ICD activation. In the stressed adult heart, J1ICD activation results in a dramatic reduction of the number of Notch signalling cardiomyocytes, blunts the hypertrophic response, and reduces the number of apoptotic cardiomyocytes. Consistently, this occurs concomitantly with a significant down-regulation of the phosphorylation of the Akt effectors ribosomal S6 protein (S6) and eukaryotic initiation factor 4E binding protein1 (4EBP1) controlling protein synthesis.

Conclusions

Altogether, these data demonstrate the importance of J1ICD in the modulation of physiological and pathological hypertrophy, and reveal the existence of a novel pathway regulating cardiac homeostasis.

Genome-wide profiling of the cardiac transcriptome after myocardial infarction identifies novel heart-specific long non-coding RNAs

AIM

Heart disease is recognized as a consequence of dysregulation of cardiac gene regulatory networks. Previously, unappreciated components of such networks are the long non-coding RNAs (lncRNAs). Their roles in the heart remain to be elucidated. Thus, this study aimed to systematically characterize the cardiac long non-coding transcriptome post-myocardial infarction and to elucidate their potential roles in cardiac homoeostasis.

METHODS AND RESULTS

We annotated the mouse transcriptome after myocardial infarction via RNA sequencing and ab initio transcript reconstruction, and integrated genome-wide approaches to associate specific lncRNAs with developmental processes and physiological parameters. Expression of specific lncRNAs strongly correlated with defined parameters of cardiac dimensions and function. Using chromatin maps to infer lncRNA function, we identified many with potential roles in cardiogenesis and pathological remodelling. The vast majority was associated with active cardiac-specific enhancers. Importantly, oligonucleotide-mediated knockdown implicated novel lncRNAs in controlling expression of key regulatory proteins involved in cardiogenesis. Finally, we identified hundreds of human orthologues and demonstrate that particular candidates were differentially modulated in human heart disease.

CONCLUSION

These findings reveal hundreds of novel heart-specific lncRNAs with unique regulatory and functional characteristics relevant to maladaptive remodelling, cardiac function and possibly cardiac regeneration. This new class of molecules represents potential therapeutic targets for cardiac disease. Furthermore, their exquisite correlation with cardiac physiology renders them attractive candidate biomarkers to be used in the clinic.

Functional importance of cardiac enhancer-associated noncoding RNAs in heart development and disease

The key information processing units within gene regulatory networks are enhancers. Enhancer activity is associated with the production of tissue-specific noncoding RNAs, yet the existence of such transcripts during cardiac development has not been established. Using an integrated genomic approach, we demonstrate that fetal cardiac enhancers generate long noncoding RNAs (lncRNAs) during cardiac differentiation and morphogenesis. Enhancer expression correlates with the emergence of active enhancer chromatin states, the initiation of RNA polymerase II at enhancer loci and expression of target genes. Orthologous human sequences are also transcribed in fetal human hearts and cardiac progenitor cells. Through a systematic bioinformatic analysis, we identified and characterized, for the first time, a catalog of lncRNAs that are expressed during embryonic stem cell differentiation into cardiomyocytes and associated with active cardiac enhancer sequences. RNA-sequencing demonstrates that many of these transcripts are polyadenylated, multi-exonic long noncoding RNAs. Moreover, knockdown of two enhancer-associated lncRNAs resulted in the specific downregulation of their predicted target genes. Interestingly, the reactivation of the fetal gene program, a hallmark of the stress response in the adult heart, is accompanied by increased expression of fetal cardiac enhancer transcripts. Altogether, these findings demonstrate that the activity of cardiac enhancers and expression of their target genes are associated with the production of enhancer-derived lncRNAs.

Targeting cancer stem-like cells as an approach to defeating cellular heterogeneity in Ewing sarcoma

Plasticity in cancer stem-like cells (CSC) may provide a key basis for cancer heterogeneity and therapeutic response. In this study, we assessed the effect of combining a drug that abrogates CSC properties with standard-of-care therapy in a Ewing sarcoma family tumor (ESFT). Emergence of CSC in this setting has been shown to arise from a defect in TARBP2-dependent microRNA maturation, which can be corrected by exposure to the fluoroquinolone enoxacin. In the present work, primary ESFT from four patients containing CD133(+) CSC subpopulations ranging from 3% to 17% of total tumor cells were subjected to treatment with enoxacin, doxorubicin, or both drugs. Primary ESFT CSC and bulk tumor cells displayed divergent responses to standard-of-care chemotherapy and enoxacin. Doxorubicin, which targets the tumor bulk, displayed toxicity toward primary adherent ESFT cells in culture but not to CSC-enriched ESFT spheres. Conversely, enoxacin, which enhances miRNA maturation by stimulating TARBP2 function, induced apoptosis but only in ESFT spheres. In combination, the two drugs markedly depleted CSCs and strongly reduced primary ESFTs in xenograft assays. Our results identify a potentially attractive therapeutic strategy for ESFT that combines mechanism-based targeting of CSC using a low-toxicity antibiotic with a standard-of-care cytotoxic drug, offering immediate applications for clinical evaluation.

The Notch pathway controls fibrotic and regenerative repair in the adult heart

Aims

In the adult heart, Notch signalling regulates the response to injury. Notch inhibition leads to increased cardiomyocyte apoptosis, and exacerbates the development of cardiac hypertrophy and fibrosis. The role of Notch in the mesenchymal stromal cell fraction, which contains cardiac fibroblasts and cardiac precursor cells, is, however, largely unknown. In the present study, we evaluate, therefore, whether forced activation of the Notch pathway in mesenchymal stromal cells regulates pathological cardiac remodelling.

Methods and results

We generated transgenic mice overexpressing the Notch ligand Jagged1 on the surface of cardiomyocytes to activate Notch signalling in adjacent myocyte and non-myocyte cells. In neonatal transgenic mice, activated Notch sustained cardiac precursor and myocyte proliferation after birth, and led to increased numbers of cardiac myocytes in adult mice. In the adult heart under pressure overload, Notch inhibited the development of cardiomyocyte hypertrophy and transforming growth factor-β/connective tissue growth factor-mediated cardiac fibrosis. Most importantly, Notch activation in the stressed adult heart reduced the proliferation of myofibroblasts and stimulated the expansion of stem cell antigen-1-positive cells, and in particular of Nkx2.5-positive cardiac precursor cells.

Conclusions

We conclude that Notch is pivotal in the healing process of the injured heart. Specifically, Notch regulates key cellular mechanisms in the mesenchymal stromal cell population, and thereby controls the balance between fibrotic and regenerative repair in the adult heart. Altogether, these findings indicate that Notch represents a unique therapeutic target for inducing regeneration in the adult heart via mobilization of cardiac precursor cells.

Reactive oxygen species are produced at low glucose and contribute to the activation of AMPK in insulin-secreting cells

Excess reactive oxygen species (ROS) production is thought to play a key role in the loss of pancreatic β-cell number and/or function, in response to high glucose and/or fatty acids. However, contradictory findings have been reported showing that in pancreatic β cells or insulin-secreting cell lines, ROS are produced under conditions of either high or low glucose. Superoxide production was measured in attached INS1E cells as a function of glucose concentration, by following in real time the oxidation of dihydroethidine. Minimal values of superoxide production were measured at glucose concentrations of 5-20 mM, whereas superoxide generation was maximal at 0-1 mM glucose. Superoxide generation started rapidly (15-30 min) after exposure to low glucose and was suppressed by its addition within minutes. Superoxide was totally suppressed by rotenone, but not myxothiazol, suggesting a role for complex I in this process. Indirect evidence for mitochondrial ROS generation was also provided by a decrease in aconitase activity. Activation of AMPK, a cellular metabolic sensor, and its downstream target ACC by low glucose concentration was largely inhibited by addition of MnTBAP, a MnSOD and catalase mimetic that also totally suppressed superoxide production. Taken together, the data show that low glucose activates AMPK in a superoxide-dependent, AMP-independent way.

Differential contribution of mitochondria, NADPH oxidases, and glycolysis to region-specific oxidant stress in the anoxic-reoxygenated embryonic heart

The ability of the developing myocardium to tolerate oxidative stress during early gestation is an important issue with regard to possible detrimental consequences for the fetus. In the embryonic heart, antioxidant defences are low, whereas glycolytic flux is high. The pro- and antioxidant mechanisms and their dependency on glucose metabolism remain to be explored. Isolated hearts of 4-day-old chick embryos were exposed to normoxia (30 min), anoxia (30 min), and hyperoxic reoxygenation (60 min). The time course of ROS production in the whole heart and in the atria, ventricle, and outflow tract was established using lucigenin-enhanced chemiluminescence. Cardiac rhythm, conduction, and arrhythmias were determined. The activity of superoxide dismutase, catalase, gutathione reductase, and glutathione peroxidase as well as the content of reduced and oxidized glutathione were measured. The relative contribution of the ROS-generating systems was assessed by inhibition of mitochondrial complexes I and III (rotenone and myxothiazol), NADPH oxidases (diphenylene iodonium and apocynine), and nitric oxide synthases ( N-monomethyl-l-arginine and N-iminoethyl-l-ornithine). The effects of glycolysis inhibition (iodoacetate), glucose deprivation, glycogen depletion, and lactate accumulation were also investigated. In untreated hearts, ROS production peaked at 10.8 ± 3.3, 9 ± 0.8, and 4.8 ± 0.4 min (means ± SD; n = 4) of reoxygenation in the atria, ventricle, and outflow tract, respectively, and was associated with arrhythmias. Functional recovery was complete after 30–40 min. At reoxygenation, 1) the respiratory chain and NADPH oxidases were the main sources of ROS in the atria and outflow tract, respectively; 2) glucose deprivation decreased, whereas glycogen depletion increased, oxidative stress; 3) lactate worsened oxidant stress via NADPH oxidase activation; 4) glycolysis blockade enhanced ROS production; 5) no nitrosative stress was detectable; and 6) the glutathione redox cycle appeared to be a major antioxidant system. Thus, the glycolytic pathway plays a predominant role in reoxygenation-induced oxidative stress during early cardiogenesis. The relative contribution of mitochondria and extramitochondrial systems to ROS generation varies from one region to another and throughout reoxygenation.

Specific inhibition of HCN channels slows rhythm differently in atria, ventricle and outflow tract and stabilizes conduction in the anoxic-reoxygenated embryonic heart model

The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed in pacemaker cells very early during cardiogenesis. This work aimed at determining to what extent these channels are implicated in the electromechanical disturbances induced by a transient oxygen lack which may occur in utero. Spontaneously beating hearts or isolated ventricles and outflow tracts dissected from 4-day-old chick embryos were exposed to a selective inhibitor of HCN channels (ivabradine 0.1-10microM) to establish a dose-response relationship. The effects of ivabradine on electrocardiogram, excitation-contraction coupling and contractility of hearts submitted to anoxia (30min) and reoxygenation (60min) were also determined. The distribution of the predominant channel isoform, HCN4, was established in atria, ventricle and outflow tract by immunoblotting. Intrinsic beating rate of atria, ventricle and outflow tract was 164+/-22 (n=10), 78+/-24 (n=8) and 40+/-12bpm (n=23, mean+/-SD), respectively. In the whole heart, ivabradine (0.3microM) slowed the firing rate of atria by 16% and stabilized PR interval. These effects persisted throughout anoxia-reoxygenation, whereas the variations of QT duration, excitation-contraction coupling and contractility, as well as the types and duration of arrhythmias were not altered. Ivabradine (10microM) reduced the intrinsic rate of atria and isolated ventricle by 27% and 52%, respectively, whereas it abolished activity of the isolated outflow tract. Protein expression of HCN4 channels was higher in atria and ventricle than in the outflow tract. Thus, HCN channels are specifically distributed and control finely atrial, ventricular and outflow tract pacemakers as well as conduction in the embryonic heart under normoxia and throughout anoxia-reoxygenation.

Fatty acids do not activate UCP2 in pancreatic beta cells: comparison with UCP1

UCP2 is expressed in pancreatic beta cells where its postulated uncoupling activity will modulate glucose-induced changes in ATP/ADP ratio and insulin secretion. The consequences of UCP2 over/underexpression on beta-cell function has mainly been studied in the basal state; however, a UCP has no uncoupling activity unless stimulated by fatty acids and/or reactive oxygen species. Here, UCP2 was overexpressed in INS-1 cells and parameters reflecting mitochondrial coupling measured in the basal state and after stimulation by fatty acids. For comparison, UCP1 was expressed to similar levels and the same parameters measured. Neither UCP1 expression nor UCP2 overexpression modified basal or glucose-stimulated metabolic changes. Upon addition of fatty acids, UCP1-expressing cells displayed the expected mitochondrial uncoupling effect, while UCP2 did not elicit any measurable change in mitochondrial function. Taken together, our data demonstrate that, in pancreatic beta-cells, UCP2 has no uncoupling activity in the basal state or after fatty acid stimulation.

The L-Type Ca+ and KATP channels may contribute to pacing-induced protection against anoxia-reoxygenation in the embryonic heart model

L-Type Ca(2+) and K(ATP) Channels in Pacing-Induced Cardioprotection.

AIMS

The L-type Ca(2+) channel, the sarcolemmal (sarcK(ATP)), and mitochondrial K(ATP) (mitoK(ATP)) channels are involved in myocardial preconditioning. We aimed at determining to what extent these channels can also participate in pacing-induced cardioprotection.

METHODS

Hearts of 4-day-old chick embryos were paced in ovo during 12 hour using asynchronous intermittent ventricular stimulation at 110% of the intrinsic rate. Sham operated and paced hearts were then submitted in vitro to anoxia (30 minutes) and reoxygenation (60 minutes). These hearts were exposed to L-type Ca(2+) channel agonist Bay-K-8644 (BAY-K) or blocker verapamil, nonselective K(ATP) channel antagonist glibenclamide (GLIB), mitoK(ATP) channel agonist diazoxide (DIAZO), or antagonist 5-hydroxydecanoate. Electrocardiogram, electromechanical delay (EMD) reflecting excitation-contraction (E-C) coupling, and contractility were determined.

RESULTS

Under normoxia, heart rate, QT duration, conduction, EMD, and ventricular shortening were similar in sham and paced hearts. During reoxygenation, arrhythmias ceased earlier and ventricular EMD recovered faster in paced hearts than in sham hearts. In sham hearts, BAY-K (but not verapamil), DIAZO (but not 5-hydroxydecanoate) or GLIB accelerated recovery of ventricular EMD, reproducing the pacing-induced protection. By contrast, none of these agents further ameliorated recovery of the paced hearts.

CONCLUSION

The protective effect of chronic asynchronous pacing at near physiological rate on ventricular E-C coupling appears to be associated with subtle activation of L-type Ca(2+) channel, inhibition of sarcK(ATP) channel, and/or opening of mitoK(ATP) channel.

Arrhythmogenesis in the Developing Heart During Anoxia-Reoxygenation and Hypothermia-Rewarming: An In Vitro Model

INTRODUCTION

The spatio-temporal pattern of arrhythmias in the embryonic/fetal heart subjected to a transient hypoxic or hypothermic stress remains to be established.

METHODS AND RESULTS

Spontaneously beating hearts or isolated atria, ventricles, and conotruncus from 4-day-old chick embryos were subjected in vitro to 30-minute anoxia and 60-minute reoxygenation. Hearts were also submitted to 30-minute hypothermia (0-4 degrees C) and 60-minute rewarming. ECG disturbances and alterations of atrial and ventricular electromechanical delay (EMD) were systematically investigated. Baseline functional parameters were stable during at least 2 hours. Anoxia induced tachycardia, followed by bradycardia, atrial ectopy, first-, second-, and third-degree atrio-ventricular blocks and, finally, transient electromechanical arrest after 6.8 minutes, interquartile ranges (IQR) 3.1-16.2 (n = 8). Reoxygenation triggered also Wenckebach phenomenon and ventricular escape beats. At the onset of reoxygenation QT, PR, and ventricular EMD increased by 68%, 70%, and 250%, respectively, whereas atrial EMD was not altered. No fibrillations, no ventricular ectopic beats, and no electromechanical dissociation were observed. Arrhythmic activity of the isolated atria persisted throughout anoxia and upon reoxygenation, whereas activity of the isolated ventricles abruptly ceased after 5 minutes of anoxia and resumed after 5 minutes of reoxygenation. During hypothermia-rewarming, cardiac activity stopped at 17.9 degrees C, IQR 16.2-20.6 (n = 4) and resumed at the same temperature with no arrhythmias. All preparations fully recovered after 40 minutes of reoxygenation or rewarming.

CONCLUSION

In the embryonic heart, arrhythmias mainly originated in the sinoatrial tissue and resembled those observed in the adult heart. Furthermore, oxygen readmission was by far more arrhythmogenic than rewarming and the chronotropic, dromotropic, and inotropic effects were fully reversible.

Physiopathology of the embryonic heart (with special emphasis on hypoxia and reoxygenation)

The adaptative response of the developing heart to adverse intrauterine environment such as reduced O2 delivery can result in alteration of gene expression with short- and long-term consequences including adult cardiovascular diseases. The tolerance of the developing heart of acute or chronic oxygen deprivation, its capacity to recover during reperfusion and the mechanisms involved in reoxygenation injury are still under debate. Indeed, the pattern of response of the immature myocardium to hypoxia-reoxygenation differs from that of the adult. This review deals with the structural and metabolic characteristics of the embryonic heart and the functional consequences of hypoxia and reoxygenation. The relative contribution of calcium and sodium overload, pH disturbances and oxidant stress to the hypoxia-induced cardiac dysfunction is examined, as well as various cellular signaling pathways (e.g. MAP kinases) involved in cell survival or death. In the context of the recent advances in developmental cardiology and fetal cardiac surgery, a better understanding of the physiopathology of the stressed developing heart is required.

mitoKATP channel activation in the postanoxic developing heart protects E-C coupling via NO-, ROS-, and PKC-dependent pathways

Whereas previous studies have shown that opening of the mitochondrial ATP-sensitive K+ (mitoKATP) channel protects the adult heart against ischemia-reperfusion injury, it remains to be established whether this mechanism also operates in the developing heart. Isolated spontaneously beating hearts from 4-day-old chick embryos were subjected to 30 min of anoxia followed by 60 min of reoxygenation. The chrono-, dromo-, and inotropic disturbances, as well as alterations of the electromechanical delay (EMD), reflecting excitation-contraction (E-C) coupling, were investigated. Production of reactive oxygen species (ROS) in the ventricle was determined using the intracellular fluorescent probe 2′,7′-dichlorofluorescin (DCFH). Effects of the specific mitoKATP channel opener diazoxide (Diazo, 50 μM) or the blocker 5-hydroxydecanoate (5-HD, 500 μM), the nitric oxide synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME, 50 μM), the antioxidant N-(2-mercaptopropionyl)glycine (MPG, 1 mM), and the PKC inhibitor chelerythrine (Chel, 5 μM) on oxidative stress and postanoxic functional recovery were determined. Under normoxia, the baseline parameters were not altered by any of these pharmacological agents, alone or in combination. During the first 20 min of postanoxic reoxygenation, Diazo doubled the peak of ROS production and, interestingly, accelerated recovery of ventricular EMD and the PR interval. Diazo-induced ROS production was suppressed by 5-HD, MPG, or l-NAME, but not by Chel. Protection of ventricular EMD by Diazo was abolished by 5-HD, MPG, l-NAME, or Chel, whereas protection of the PR interval was abolished by l-NAME exclusively. Thus pharmacological opening of the mitoKATP channel selectively improves postanoxic recovery of cell-to-cell communication and ventricular E-C coupling. Although the NO-, ROS-, and PKC-dependent pathways also seem to be involved in this cardioprotection, their interrelation in the developing heart can differ markedly from that in the adult myocardium.

Ventricular but not atrial electro-mechanical delay of the embryonic heart is altered by anoxia-reoxygenation and improved by nitric oxide

BACKGROUND/AIM

Excitation-contraction coupling is modulated by nitric oxide (NO) which otherwise has either beneficial or detrimental effects on myocardial function during hypoxia-reoxygenation. This work aimed at characterizing the variations of electromechanical delay (EMD) induced by anoxia-reoxygenation within the developing heart and determining whether atrial and ventricular EMD are modulated by NO to the same extent.

METHODS

Hearts of 4 or 4.5-day-old chick embryos were excised and submitted in vitro to normoxia (45 min), anoxia (30 min) and reoxygenation (60 min). Electrocardiogram and atrial and ventricular contractions were simultaneously recorded throughout experiment. Anoxia-reoxygenation-induced chrono-, dromo-and inotropic disturbances and changes in EMD in atrium (EMDa) and ventricle (EMDv) were investigated in control hearts and in hearts exposed to 0.1, 1, 10, 50 and 100 microM of DETA-NONOate (a NO donating agent) or to 50 microM of L-NAME (a NOS inhibitor).

RESULTS

Under normoxia, heart rate, PR interval, ventricular shortening velocity, EMDa and EMDv were similar in control, L-NAME-treated and DETA-NONOate-treated hearts. Under anoxia, cardiac activity became markedly erratic within less than 10 min in all groups. At the onset of reoxygenation, EMDv was increased by about 300% with respect to the preanoxic value while EMDa did not vary significatively. Compared to control conditions, L-NAME or DETA-NONOate had no influence on the negative chrono-, dromo- and inotropic effects induced by anoxia-reoxygenation. However, L-NAME prolonged EMDv during anoxia and delayed EMDv recovery during reoxygenation while 100 microM DETA-NONOate had the opposite effects. EMDa was neither affected by NOS inhibitor nor NO donor. At the end of reoxygenation, all the investigated parameters returned to their basal values.

CONCLUSION

This work provides evidence that a NO-dependent pathway is involved in regulation of the ventricular excitation-contraction coupling in the anoxic-reoxygenated developing heart.

Involvement of the serotonin 5-HT2B receptor in cardiac hypertrophy linked to sympathetic stimulation: control of interleukin-6, interleukin-1beta, and tumor necrosis factor-alpha cytokine production by ventricular fibroblasts

BACKGROUND

The serotonergic 5-HT2B receptor regulates cardiomyocyte development and growth. A putative contribution of this receptor to fibroblast-dependent cardiac function has not been identified.

METHODS AND RESULTS

By mimicking sympathetic stimulation with chronic isoproterenol perfusion in vivo, we found that mice developed a cardiac hypertrophy, which was prevented by exposure to the 5-HT2B receptor antagonists SB206553 or SB215505 or in 5-HT2B receptor-knockout mice. The isoproterenol-induced hypertrophy was associated with an increase in the plasma levels of interleukin-1beta and tumor necrosis factor-alpha but not interleukin-6. In contrast, the plasma isoproterenol-induced cytokine increase was not observed in either 5-HT2B receptor-mutant or wild-type mice perfused with isoproterenol+SB206553. We demonstrated that stimulation of wild-type cardiac fibroblasts by isoproterenol markedly increased the production of the interleukin-6, interleukin-1beta, and tumor necrosis factor-alpha cytokines. Strikingly, we found that this isoproterenol-induced cytokine production was abolished by SB206553 or in 5-HT2B receptor-knockout fibroblasts. Serotonin also stimulated production of the 3 cytokines in wild-type fibroblasts, which was effectively reduced in 5-HT2B receptor-knockout fibroblasts.

CONCLUSIONS

Our results demonstrate for the first time that 5-HT2B receptors are essential for isoproterenol-induced cardiac hypertrophy, which involves the regulation of interleukin-6, interleukin-1beta, and tumor necrosis factor-alpha cytokine production by cardiac fibroblasts.

Functional and morphological evidence for a ventricular conduction system in zebrafish and Xenopus hearts

Zebrafish and Xenopus have become popular model organisms for studying vertebrate development of many organ systems, including the heart. However, it is not clear whether the single ventricular hearts of these species possess any equivalent of the specialized ventricular conduction system found in higher vertebrates. Isolated hearts of adult zebrafish (Danio rerio) and African toads (Xenopus laevis) were stained with voltage-sensitive dye and optically mapped in spontaneous and paced rhythms followed by histological examination focusing on myocardial continuity between the atrium and the ventricle. Spread of the excitation wave through the atria was uniform with average activation times of 20 +/- 2 and 50 +/- 2 ms for zebrafish and Xenopus toads, respectively. After a delay of 47 +/- 8 and 414 +/- 16 ms, the ventricle became activated first in the apical region. Ectopic ventricular activation was propagated significantly more slowly (total ventricular activation times: 24 +/- 3 vs. 14 +/- 2 ms in zebrafish and 74 +/- 14 vs. 35 +/- 9 ms in Xenopus). Although we did not observe any histologically defined tracts of specialized conduction cells within the ventricle, there were trabecular bands with prominent polysialic acid-neural cell adhesion molecule staining forming direct myocardial continuity between the atrioventricular canal and the apex of the ventricle; i.e., the site of the epicardial breakthrough. We thus conclude that these hearts are able to achieve the apex-to-base ventricular activation pattern observed in higher vertebrates in the apparent absence of differentiated conduction fascicles, suggesting that the ventricular trabeculae serve as a functional equivalent of the His-Purkinje system.