Presence of functional sarcoplasmic reticulum in the developing heart and its confinement to chamber myocardium

Exploring the role non-coding RNAs during myocardial cell fate

Myocardial cell fate specification takes place during the early stages of heart development as the precardiac mesoderm is configured into two symmetrical sets of bilateral precursor cells. Molecular cues of the surrounding tissues specify and subsequently determine the early cardiomyocytes, that finally matured as the heart is completed at early postnatal stages. Over the last decade, we have greatly enhanced our understanding of the transcriptional regulation of cardiac development and thus of myocardial cell fate. The recent discovery of a novel layer of gene regulation by non-coding RNAs has flourished their implication in epigenetic, transcriptional and post-transcriptional regulation of cardiac development. In this review, we revised the current state-of-the-art knowledge on the functional role of non-coding RNAs during myocardial cell fate.

Genetic and Molecular Mechanisms in Brugada Syndrome

Brugada syndrome is a rare hereditary arrhythmia disorder characterized by a distinctive electrocardiogram pattern and an elevated risk of ventricular arrhythmias and sudden cardiac death in young adults. Despite recent advances, it remains a complex condition, encompassing mechanisms, genetics, diagnosis, arrhythmia risk stratification, and management. The underlying electrophysiological mechanism of Brugada syndrome requires further investigation, with current theories focusing on abnormalities in repolarization, depolarization, and current-load match. The genetic basis of the syndrome is strong, with mutations found in genes encoding subunits of cardiac sodium, potassium, and calcium channels, as well as genes involved in channel trafficking and regulation. While the initial discovery of mutations in the SCN5A gene provided valuable insights, Brugada syndrome is now recognized as a multifactorial disease influenced by several loci and environmental factors, challenging the traditional autosomal dominant inheritance model. This comprehensive review aims to provide a current understanding of Brugada syndrome, focusing on its pathophysiology, genetic mechanisms, and novel models of risk stratification. Advancements in these areas hold the potential to facilitate earlier diagnosis, improve risk assessments, and enable more targeted therapeutic interventions.

Equal force generation potential of trabecular and compact wall ventricular cardiomyocytes

Trabecular myocardium makes up most of the ventricular wall of the human embryo. A process of compaction in the fetal period presumably changes ventricular wall morphology by converting ostensibly weaker trabecular myocardium into stronger compact myocardium. Using developmental series of embryonic and fetal humans, mice and chickens, we show ventricular morphogenesis is driven by differential rates of growth of trabecular and compact layers rather than a process of compaction. In mouse, fetal cardiomyocytes are relatively weak but adult cardiomyocytes from the trabecular and compact layer show an equally large force generating capacity. In fetal and adult humans, trabecular and compact myocardium are not different in abundance of immunohistochemically detected vascular, mitochondrial and sarcomeric proteins. Similar findings are made in human excessive trabeculation, a congenital malformation. In conclusion, trabecular and compact myocardium is equally equipped for force production and their proportions are determined by differential growth rates rather than by compaction.

The Role of Epigenetics in Congenital Heart Disease

Congenital heart disease (CHD) is the most common birth defect among newborns worldwide and contributes to significant infant morbidity and mortality. Owing to major advances in medical and surgical management, as well as improved prenatal diagnosis, the outcomes for these children with CHD have improved tremendously so much so that there are now more adults living with CHD than children. Advances in genomic technologies have discovered the genetic causes of a significant fraction of CHD, while at the same time pointing to remarkable complexity in CHD genetics. For this reason, the complex process of cardiogenesis, which is governed by multiple interlinked and dose-dependent pathways, is a well investigated process. In addition to the sequence of the genome, the contribution of epigenetics to cardiogenesis is increasingly recognized. Significant progress has been made dissecting the epigenome of the heart and identified associations with cardiovascular diseases. The role of epigenetic regulation in cardiac development/cardiogenesis, using tissue and animal models, has been well reviewed. Here, we curate the current literature based on studies in humans, which have revealed associated and/or causative epigenetic factors implicated in CHD. We sought to summarize the current knowledge on the functional role of epigenetics in cardiogenesis as well as in distinct CHDs, with an aim to provide scientists and clinicians an overview of the abnormal cardiogenic pathways affected by epigenetic mechanisms, for a better understanding of their impact on the developing fetal heart, particularly for readers interested in CHD research.

Cardiac Morphogenesis: Specification of the Four-Chambered Heart

Early heart morphogenesis involves a process in which embryonic precursor cells are instructed to form a cyclic contracting muscle tube connected to blood vessels, pumping fluid. Subsequently, the heart becomes structurally complex and its size increases several orders of magnitude to functionally keep up with the demands of the growing organism. Programmed transcriptional regulatory networks control the early steps of cardiac development. However, already during the early stages of its assembly, the heart tube starts to produce electrochemical potentials, contractions, and flow, which are transduced into signals that feed back into the process of morphogenesis itself. Heart morphogenesis, thus, involves the interplay between progressively changing genetic networks, function, and shape. Morphogenesis is evolutionarily conserved, but species-specific differences occur and in mouse, for instance, distinct phases of development become overlapping and compounded in an extremely fast gestation. Here, we review the early morphogenesis of the chambered heart that maintains a circulation supporting development of an organism rapidly growing in size and requirements.

Development of the human heart

In 2014, an extensive review discussing the major steps of cardiac development focusing on growth, formation of primary and chamber myocardium and the development of the cardiac electrical system, was published. Molecular genetic lineage analyses have since furthered our insight in the developmental origin of the various component parts of the heart, which currently can be unambiguously identified by their unique molecular phenotype. Moreover, genetic, molecular and cell biological analyses have driven insights into the mechanisms underlying the development of the different cardiac components. Here, we build on our previous review and provide an insight into the molecular mechanistic revelations that have forwarded the field of cardiac development. Despite the enormous advances in our knowledge over the last decade, the development of congenital cardiac malformations remains poorly understood. The challenge for the next decade will be to evaluate the different developmental processes using newly developed molecular genetic techniques to further unveil the gene regulatory networks operational during normal and abnormal cardiac development.

The human phrenic nerve serves as a morphological conduit for autonomic nerves and innervates the caval body of the diaphragm

Communicating fibres between the phrenic nerve and sympathetic nervous system may exist, but have not been characterized histologically and immunohistochemically, even though increased sympathetic activity due to phrenic nerve stimulation for central sleep apnoea may entail morbidity and mortality. We, therefore, conducted a histological study of the phrenic nerve to establish the presence of catecholaminergic fibres throughout their course. The entire phrenic nerves of 35 formalin-fixed human cadavers were analysed morphometrically and immunohistochemically. Furthermore, the right abdominal phrenic nerve was serially sectioned and reconstructed. The phrenic nerve contained 3 ± 2 fascicles in the neck that merged to form a single fascicle in the thorax and split again into 3 ± 3 fascicles above the diaphragm. All phrenic nerves contained catecholaminergic fibres, which were distributed homogenously or present as distinct areas within a fascicle or as separate fascicles. The phrenicoabdominal branch of the right phrenic nerve is a branch of the celiac plexus and, therefore, better termed the “phrenic branch of the celiac plexus”. The wall of the inferior caval vein in the diaphragm contained longitudinal strands of myocardium and atrial natriuretic peptide-positive paraganglia (“caval bodies”) that where innervated by the right phrenic nerve.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Genetic deletion of Rnd3/RhoE results in mouse heart calcium leakage through upregulation of protein kinase A signaling

RATIONALE

Rnd3, a small Rho GTPase, is involved in the regulation of cell actin cytoskeleton dynamics, cell migration, and proliferation. The biological function of Rnd3 in the heart remains unexplored.

OBJECTIVE

To define the functional role of the Rnd3 gene in the animal heart and investigate the associated molecular mechanism.

METHODS AND RESULTS

By loss-of-function approaches, we discovered that Rnd3 is involved in calcium regulation in cardiomyocytes. Rnd3-null mice died at the embryonic stage with fetal arrhythmias. The deletion of Rnd3 resulted in severe Ca(2+) leakage through destabilized ryanodine receptor type 2 Ca(2+) release channels. We further found that downregulation of Rnd3 attenuated β2-adrenergic receptor lysosomal targeting and ubiquitination, which in turn resulted in the elevation of β2-adrenergic receptor protein levels leading to the hyperactivation of protein kinase A (PKA) signaling. The PKA activation destabilized ryanodine receptor type 2 channels. This irregular spontaneous Ca(2+) release can be curtailed by PKA inhibitor treatment. Increases in the PKA activity along with elevated cAMP levels were detected in Rnd3-null embryos, in neonatal rat cardiomyocytes, and noncardiac cell lines with Rnd3 knockdown, suggesting a general mechanism for Rnd3-mediated PKA signaling activation. β2-Adrenergic receptor blocker treatment reduced arrhythmia and improved cardiac function.

CONCLUSIONS

Rnd3 is a novel factor involved in intracellular Ca(2+) homeostasis regulation in the heart. Deficiency of the protein induces ryanodine receptor type 2 dysfunction by a mechanism that attenuates Rnd3-mediated β2-adrenergic receptor ubiquitination, which leads to the activation of PKA signaling. Increased PKA signaling in turn promotes ryanodine receptor type 2 hyperphosphorylation, which contributes to arrhythmogenesis and heart failure.

Lessons from the heart: mirroring electrophysiological characteristics during cardiac development to in vitro differentiation of stem cell derived cardiomyocytes

The ability of human pluripotent stem cells (hPSCs) to differentiate into any cell type of the three germ layers makes them a very promising cell source for multiple purposes, including regenerative medicine, drug discovery, and as a model to study disease mechanisms and progression. One of the first specialized cell types to be generated from hPSC was cardiomyocytes (CM), and differentiation protocols have evolved over the years and now allow for robust and large-scale production of hPSC-CM. Still, scientists are struggling to achieve the same, mainly ventricular, phenotype of the hPSC-CM in vitro as their adult counterpart in vivo. In vitro generated cardiomyocytes are generally described as fetal-like rather than adult. In this review, we compare the in vivo development of cardiomyocytes to the in vitro differentiation of hPSC into CM with focus on electrophysiology, structure and contractility. Furthermore, known epigenetic changes underlying the differences between adult human CM and CM differentiated from pluripotent stem cells are described. This should provide the reader with an extensive overview of the current status of human stem cell-derived cardiomyocyte phenotype and function. Additionally, the reader will gain insight into the underlying signaling pathways and mechanisms responsible for cardiomyocyte development.

Evolution and development of the building plan of the vertebrate heart

Early cardiac development involves the formation of a heart tube, looping of the tube and formation of chambers. These processes are highly similar among all vertebrates, which suggest the existence of evolutionary conservation of the building plan of the heart. From the jawless lampreys to man, T-box transcription factors like Tbx5 and Tbx20 are fundamental for heart formation, whereas Tbx2 and Tbx3 repress chamber formation on the sinu-atrial and atrioventricular borders. Also, electrocardiograms from different vertebrates are alike, even though the fish heart only has two chambers whereas the mammalian heart has four chambers divided by septa and in addition has much higher heart rates. We conclude that most features of the high-performance hearts of mammals and birds can be traced back to less developed traits in the hearts of ectothermic vertebrates. This article is part of a Special Issue entitled: Cardiomyocyte biology: Cardiac pathways of differentiation, metabolism and contraction.

Modulation of conductive elements by Pitx2 and their impact on atrial arrhythmogenesis

The development of the heart is a complex process during which different cell types progressively contribute to shape a four-chambered pumping organ. Over the last decades, our understanding of the specification and transcriptional regulation of cardiac development has been greatly augmented as has our understanding of the functional bases of cardiac electrophysiology during embryogenesis. The nascent heart gradually acquires distinct cellular and functional characteristics, such as the formation of contractile structures, the development of conductive capabilities, and soon thereafter the co-ordinated conduction of the electrical impulse, in order to fulfil its functional properties. Over the last decade, we have learnt about the consequences of impairing cardiac morphogenesis, which in many cases leads to congenital heart defects; however, we are not yet aware of the consequences of impairing electrical function during cardiogenesis. The most prevalent cardiac arrhythmia is atrial fibrillation (AF), although its genetic aetiology remains rather elusive. Recent genome-wide association studies have identified several genetic variants highly associated with AF. Among them are genetic variants located on chromosome 4q25 adjacent to PITX2, a transcription factor known to play a critical role in left-right asymmetry and cardiogenesis. Here, we review new insights into the cellular and molecular links between PITX2 and AF.

Developmental aspects of cardiac arrhythmogenesis

The transcriptional regulation orchestrating the development of the heart is increasingly recognized to play an essential role in the regulation of ion channel and gap junction gene expression and consequently the proper generation and conduction of the cardiac electrical impulse. This has led to the realization that in some instances, abnormal cardiac electrical function and arrhythmias in the postnatal heart may stem from a developmental abnormality causing maintained (epigenetic) changes in gene regulation. The role of developmental genes in the regulation of cardiac electrical function is further underscored by recent genome-wide association studies that provide strong evidence that common genetic variation, at loci harbouring these genes, modulates electrocardiographic indices of conduction and repolarization and susceptibility to arrhythmia. Here we discuss recent findings and provide background insight into these complex mechanisms.

The beginning of the calcium transient in rat embryonic heart

Although many researchers have tried to observe the beginning of the heartbeat, no study has shown the beginning of the calcium transient. Here, we evaluate the beginning of the calcium transient in the Wistar rat heart. We first tried to reveal when the heart of the Wistar rat begins to contract because no previous study has evaluated the beginning of the heartbeat in Wistar rats. Observation of embryos transferred to a small incubator mounted on a microscope revealed that the heart primordium, the so-called cardiac crescent, began to contract at embryonic day 9.99-10.13. Observation of embryos loaded with fluo-3 AM revealed that the beginning of the calcium transient precedes the initiation of contraction which precedes the appearance of the linear heart tube. Nifedipine (1 μM), but not ryanodine (1 μM), abolished the calcium transients. These results indicate that calcium transients in the early embryonic period involve exclusively calcium entry through L-type calcium channels in contrast to the situation in mature hearts. This study provides the first demonstration of the relationship between morphological changes in the heart primordium and the beginning of the calcium transient and contraction.

Spatial and temporal heterogeneities are localized to the right ventricular outflow tract in a heterozygotic Scn5a mouse model

Ventricular tachycardia (VT) in Brugada Syndrome patients often originates in the right ventricular outflow tract (RVOT). We explore the physiological basis for this observation using murine whole heart preparations. Ventricular bipolar electrograms and monophasic action potentials were recorded from seven epicardial positions in Langendorff-perfused wild-type and Scn5a+/− hearts. VT first appeared in the RVOT, implicating it as an arrhythmogenic focus in Scn5a+/− hearts. RVOTs showed the greatest heterogeneity in refractory periods, response latencies, and action potential durations, and the most fractionated electrograms. However, incidences of concordant alternans in dynamic pacing protocol recordings were unaffected by the Scn5a+/− mutation or pharmacological intervention. Conversely, particularly at the RVOT, Scn5a+/− hearts showed earlier and more frequent transitions into discordant alternans. This was accentuated by flecainide, but reduced by quinidine, in parallel with their respective pro- and anti-arrhythmic effects. Discordant alternans preceded all episodes of VT. The RVOT of Scn5a+/− hearts also showed steeper restitution curves, with the diastolic interval at which the gradient equaled one strongly correlating with the diastolic interval at which discordant alternans commenced. We attribute the arrhythmic tendency within the RVOT to the greater spatial heterogeneities in baseline electrophysiological properties. These, in turn, give rise to a tendency to drive concordant alternans phenomena into an arrhythmogenic discordant alternans. Our findings may contribute to future work investigating possible pharmacological treatments for a disease in which the current mainstay of treatment is implantable cardioverter defibrillator implantation.

Fetal and neonatal development of Ca2+ transients and functional sarcoplasmic reticulum in beating mouse hearts

BACKGROUND

It is generally accepted that Ca(2+)-induced Ca(2+) release is not the predominant mechanism during embryonic stages. Most studies have been conducted either on primary cultures or acutely isolated cells, in which an apparent reduction of ryanodine receptor density and alterations in the cell shape have been reported. The aim of the present study was to investigate developmental changes in Ca(2+) transients using whole hearts of mouse embryos and neonates.

METHODS AND RESULTS

Fluo-3 fluorescence signals from stimulated whole hearts were detected using a photomultiplier and stored as Ca(2+) transients. The upstroke and decay of Ca(2+) transients became more rapid from the late embryonic stages to the neonatal stage. After thapsigargin application (an inhibitor of the sarcoplasmic Ca(2+)-ATPase [SERCA]), time to 50% relaxation (T(50)) of Ca(2+) transients was significantly prolonged. There were no significant changes in T(50) after Ru360 application (an inhibitor of mitochondrial Ca(2+) uniporter). The rate of increase in the amplitude of Ca(2+) transients after caffeine application became larger during developmental stages.

CONCLUSIONS

Ca(2+) homeostasis developmentally changes from a slow rise and decay of Ca(2+) transients to rapid kinetics after the mid-embryonic stage. SERCA began to contribute significantly to Ca(2+) homeostasis at early embryonic stages and sarcoplasmic reticulum Ca(2+) contents increased from embryonic to neonatal stages, whereas mitochondrial Ca(2+) uptake did not contribute to Ca(2+) transients on a beat-to-beat basis.

Comparison of contractile behavior of native murine ventricular tissue and cardiomyocytes derived from embryonic or induced pluripotent stem cells

Cardiomyocytes generated from embryonic stem cells (ESCs) and induced pluripotent stem (iPS) cells are suggested for repopulation of destroyed myocardium. Because contractile properties are crucial for functional regeneration, we compared cardiomyocytes differentiated from ES cells (ESC-CMs) and iPS cells (iPS-CMs). Native myocardium served as control. Murine ESCs or iPS cells were differentiated 11 d in vitro and cocultured 5-7 d with irreversibly injured myocardial tissue slices. Vital embryonic ventricular tissue slices of similar age served for comparison. Force-frequency relationship (FFR), effects of Ca(2+), Ni(2+), nifedipine, ryanodine, beta-adrenergic, and muscarinic modulation were studied during loaded contractions. FFR was negative for ESC-CMs and iPS-CMs. FFR was positive for embryonic tissue and turned negative after treatment with ryanodine. In all groups, force of contraction and relaxation time increased with the concentration of Ca(2+) and decreased with nifedipine. Force was reduced by Ni(2+). Isoproterenol (1 microM) increased the force most pronounced in embryonic tissue (207+/-31%, n=7; ESC-CMs: 123+/-5%, n=4; iPS-CMs: 120+/-4%, n=8). EC(50) values were similar. Contractile properties of iPS-CMs and ESC-CMs were similar, but they were significantly different from ventricular tissue of comparable age. The results indicate immaturity of the sarcoplasmic reticulum and the beta-adrenergic response of iPS-CMs and ESC-CMs.

Ca2+removal mechanisms in mouse embryonic stem cell-derived cardiomyocytes

In mammalian adult cardiomyocytes, sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) plays a major role in controlling the decline of cytosolic free Ca2+concentration ([Ca2+]i) in comparison with sarcolemmal Na+/Ca2+exchanger (NCX). However, the functional importance of SERCA and NCX in cytosolic Ca2+removal during early cardiomyogenesis is still debated. In this study, the functional contributions of Ca2+transporters to [Ca2+]idecline in mouse embryonic stem cell-derived cardiomyocytes (mESCMs), a suitable model for investigation of early cardiogenesis, at various differentiation stages were investigated. We estimated that even at early differentiation stages of mESCMs, SERCA was responsible for ∼76% of total Ca2+removal, while NCX was responsible for ∼21%. The contributions of SERCA and NCX to cytosolic Ca2+clearance were increased to ∼88% and decreased to ∼10%, respectively, at the late differentiation stage. Dynamical analysis of the transient decay phases in normal and Na+-free solutions suggests that the contribution of NCX to [Ca2+]idecline is more apparent in the terminal slow decay phase than that in the initial fast phase. When SR function was suppressed in type 2 ryanodine receptor-null mESCMs or with ryanodine receptor and SERCA inhibitors (ryanodine and thapsigargin), NCX acted as the main pathway for [Ca2+]idecline. We conclude that the rapid [Ca2+]idecline is mainly achieved by the SR uptake even at the early differentiation stage of mESCMs, while NCX acts as the main Ca2+remover when SR function is suppressed. These findings suggest a critical role of SR in the regulation of [Ca2+]ihomeostasis even in differentiating cardiomyocytes.

Concepts of cardiac development in retrospect

Recent research, enabled by powerful molecular techniques, has revolutionized our concepts of cardiac development. It was firmly established that the early heart tube gives rise to the left ventricle only, and that the remainder of the myocardium is recruited from surrounding mesoderm during subsequent development. Also, the cardiac chambers were shown not to be derived from the entire looping heart tube, but only from the myocardium at its outer curvatures. Intriguingly, many years ago, classic experimental embryological studies reached very similar conclusions. However, with the current scientific emphasis on molecular mechanisms, old morphological insights became underexposed. Since cardiac development occurs in an architecturally complex and dynamic fashion, molecular insights can only fully be exploited when placed in a proper morphological context. In this communication we present excerpts of important embryological studies of the pioneers of experimental cardiac embryology of the previous century, to relate insights from the past to current observations.

Can recent insights into cardiac development improve our understanding of congenitally malformed hearts?

Congenital cardiac malformations account for one-quarter of all human congenital abnormalities. They are caused by environmental and genetic factors. Despite increasing efforts in fundamental research, as yet, the morphogenesis of only a limited number of malformations has been elucidated. Over the last decades, new genetic modifications have made it possible to manipulate the mammalian embryo. Evidence provided using these transgenic techniques has, over the past decade, necessitated re-evaluation of several developmental processes, important in the understanding of normal as opposed to abnormal cardiac development. In this review, we discuss current understanding of the patterning of the initial heart tube, new insights into formation of the atrial and ventricular chambers, and novel information on the origin of the cells that are added to the heart after formation of the initial tube. All of these advances modify our appreciation of malformations involving the venous and arterial poles. As we demonstrate, this new information sheds light not only on normal cardiac development, but also explains the structure of several previously controversial lesions seen in malformed human hearts.

Excitation-contraction coupling of the mouse embryonic cardiomyocyte

In the mammalian embryo, the primitive tubular heart starts beating during the first trimester of gestation. These early heartbeats originate from calcium-induced contractions of the developing heart muscle cells. To explain the initiation of this activity, two ideas have been presented. One hypothesis supports the role of spontaneously activated voltage-gated calcium channels, whereas the other emphasizes the role of Ca(2+) release from intracellular stores initiating spontaneous intracellular calcium oscillations. We show with experiments that both of these mechanisms coexist and operate in mouse cardiomyocytes during embryonic days 9-11. Further, we characterize how inositol-3-phosphate receptors regulate the frequency of the sarcoplasmic reticulum calcium oscillations and thus the heartbeats. This study provides a novel view of the regulation of embryonic cardiomyocyte activity, explaining the functional versatility of developing cardiomyocytes and the origin and regulation of the embryonic heartbeat.

Trichloroethylene disrupts cardiac gene expression and calcium homeostasis in rat myocytes

We have been investigating the molecular mechanisms by which trichloroethylene (TCE) might induce cardiac malformations in the embryonic heart. Previous results indicated that TCE disrupted expression of genes encoding proteins involved in regulation of intracellular Ca2+, [Ca2+](i), in cardiac cells, including ryanodine receptor isoform 2 (Ryr2), and sarcoendoplasmatic reticulum Ca2+ ATPase, Serca2a. These observations are important in light of the notion that altered cardiac contractility can produce morphological defects. The hypothesis tested in this study is that the TCE-induced changes in gene expression of Ca2+-associated proteins resulted in altered Ca2+ flux regulation. We used real-time PCR and digital imaging microscopy to characterize effects of various doses of TCE on gene expression and Ca2+ response to vasopressin (VP) in rat cardiac H9c2 myocytes. We observed a reduction in Serca2a and Ryr2 expression at 12 and 48 h after exposure to TCE. In addition, we found significant differences in Ca2+ response to VP in cells treated with TCE doses as low as 10 parts per billion. Taken all together, our data strongly indicate that exposure to TCE disrupts the ability of myocytes to regulate cellular Ca2+ fluxes. Perturbation of calcium signaling alters cardiac cell physiology and signal transduction and may hint to morphogenetic consequences in the context of heart development. These results point to a novel area of TCE biology and, if confirmed in vivo, may help to explain the apparent cardio-specific toxicity of TCE exposure in the rodent embryo.

SR Ca2+refilling upon depletion and SR Ca2+uptake rates during development in rabbit ventricular myocytes

While it has been reported that a sparse sarcoplasmic reticulum (SR) and a low SR Ca2+pump density exist at birth, we and others have recently shown that significant amounts of Ca2+are stored in the neonatal rabbit heart SR. Here we try to determine developmental changes in SR Ca2+loading mechanisms and Ca2+pump efficacy in rabbit ventricular myocytes. SR Ca2+loading (loadSR) and k0.5(Ca2+concentration at half-maximal SR Ca2+uptake) were higher and lower, respectively, in younger age groups. Inhibition of the L-type calcium current ( ICa) with 15 μM nifedipine dramatically reduced loadSRin older but not in younger age groups. In contrast, subsequent inhibition of the Na+/Ca2+exchanger (NCX) with 10 μM KB-R7943 strongly reduced loadSRin the younger but not the older age groups. Accordingly, the time integral of the inward NCX current (tail INCX) elicited on repolarization was highly sensitive to nifedipine in the older groups and sensitive to KB-R7943 in the younger groups. Interestingly, slow SR loading took place in the presence of both nifedipine and KB-R7943 in all age groups, although it was less prominent in the older groups. We conclude that the SR loading capacity at the earliest postnatal stages is at least as large as that of adult myocytes. However, reverse-mode NCX plays a prominent role in SR Ca2+loading at early postnatal stages while ICais the main source of SR Ca2+loading at late postnatal and adult stages.

Cardiac alternans in embryonic mouse ventricles

T-wave alternans, an important arrhythmogenic factor, has recently been described in human fetuses. Here we sought to determine whether alternans can be induced in the embryonic mouse hearts, despite its underdeveloped sarcoplasmic reticulum (SR) and, if so, to analyze the response to pharmacological and autonomic interventions. Immunohistochemistry confirmed minimal sarcoplasmic-endoplasmic reticulum Ca-ATPase 2a expression in embryonic mouse hearts at embryonic day (E) 10.5 to E12.5, compared with neonatal or adult mouse hearts. We optically mapped voltage and/or intracellular Ca (Ca(i)) in 99 embryonic mouse hearts (dual mapping in 64 hearts) at these ages. Under control conditions, ventricular action potential duration (APD) and Ca(i) transient alternans occurred during rapid pacing at an average cycle length of 212 +/- 34 ms in 57% (n = 15/26) of E10.5-E12.5 hearts. Maximum APD restitution slope was steeper in hearts developing alternans than those that did not (2.2 +/- 0.6 vs. 0.8 +/- 0.4; P < 0.001). Disabling SR Ca(i) cycling with thapsigargin plus ryanodine did not significantly reduce alternans incidence (44%, n = 8/18, P = 0.5), whereas isoproterenol (n = 14) increased the incidence to 100% (P < 0.05), coincident with steepening APD restitution slope. Verapamil abolished Ca(i) transients (n = 9). Thapsigargin plus ryanodine had no major effects on Ca(i)-transient amplitude or its half time of recovery in E10.5 hearts, but significantly depressed Ca(i)-transient amplitude (by 47 +/- 8%) and prolonged its half time of recovery (by 18 +/- 3%) in E11.5 and older hearts. Embryonic mouse ventricles can develop cardiac alternans, which generally is well correlated with APD restitution slope and does not depend on fully functional SR Ca(i) cycling.

TRIC channels are essential for Ca2+ handling in intracellular stores

Cell signalling requires efficient Ca2+ mobilization from intracellular stores through Ca2+ release channels, as well as predicted counter-movement of ions across the sarcoplasmic/endoplasmic reticulum membrane to balance the transient negative potential generated by Ca2+ release. Ca2+ release channels were cloned more than 15 years ago, whereas the molecular identity of putative counter-ion channels remains unknown. Here we report two TRIC (trimeric intracellular cation) channel subtypes that are differentially expressed on intracellular stores in animal cell types. TRIC subtypes contain three proposed transmembrane segments, and form homo-trimers with a bullet-like structure. Electrophysiological measurements with purified TRIC preparations identify a monovalent cation-selective channel. In TRIC-knockout mice suffering embryonic cardiac failure, mutant cardiac myocytes show severe dysfunction in intracellular Ca2+ handling. The TRIC-deficient skeletal muscle sarcoplasmic reticulum shows reduced K+ permeability, as well as altered Ca2+ 'spark' signalling and voltage-induced Ca2+ release. Therefore, TRIC channels are likely to act as counter-ion channels that function in synchronization with Ca2+ release from intracellular stores.

Calcium handling in embryonic stem cell-derived cardiac myocytes: of mice and men

Excitation-contraction (EC) coupling is fundamental to the function of cardiac myocytes (CMs). In mature myocytes plasma membrane (PM) L-type Ca(2+) channels function in close juxtaposition to ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR) membrane. Action potentials (APs) cause the opening of PM L-type Ca(2+) channels, which in turn provide trigger Ca(2+) for a larger RyR-mediated SR Ca(2+) release. In contrast, developing myocytes have a less well-developed SR. This incomplete development is observed in early stage and mid-maturation stages of murine embryonic stem cell-derived cardiac myocytes (ESC-CMs). Despite the absence of a well-developed t-tubule system, murine ESC-CMs use internal Ca(2+) stores for EC coupling. Direct measures of Ca(2+) handling, including pharmacological studies and investigation of genetically modified mouse ESC-CMs, established an important contribution of RyR-mediated internal Ca(2+) store to cell function. Similarly, early-stage human ESC-CMs use internal Ca(2+) store and partially share Ca(2+) handling characteristics with murine ESC-CMs. For example, elementary Ca(2+) release events are present in both murine and human ESC-CMs, and it is likely that Ca(2+) handling contributes to automatic rhythm generation in these cells. However, in human ESC-CMs, a unique voltage-gated Na(+) channel window current is critical for spontaneous, rhythmic depolarization. The advent of the murine and human ES cardiomyocyte differentiating systems has provided initial insights into the early steps of development of excitability and electromechanical coupling in the mammalian heart, including patterns of gene expression, myofibrillogenesis, ion channel development and function, and Ca(2+) handling. Here we discuss the information gained from these models to describe the nexus of voltage-gated channel currents and Ca(2+) handling on rhythmic activity.

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.

Regionalized sequence of myocardial cell growth and proliferation characterizes early chamber formation

Increase in cell size and proliferation of myocytes are key processes in cardiac morphogenesis, yet their regionalization during development of the heart has been described only anecdotally. We have made quantitative reconstructions of embryonic chicken hearts ranging in stage from the fusion of the heart-forming fields to early formation of the chambers. These reconstructions reveal that the early heart tube is recruited from a pool of rapidly proliferating cardiac precursor cells. The proliferation of these small precursor cells ceases as they differentiate into overt cardiomyocytes, producing a slowly proliferating straight heart tube composed of cells increasing in size. The largest cells were found at the ventral side of the heart tube, which corresponds to the site of the forming ventricle, as well as the site where proliferation is reinitiated. The significance of these observations is 2-fold. First, they support a model of early cardiac morphogenesis in 2 stages. Second, they demonstrate that regional increase in size of myocytes contributes significantly to chamber formation.

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.

Functional properties of human embryonic stem cell-derived cardiomyocytes: intracellular Ca2+ handling and the role of sarcoplasmic reticulum in the contraction

Since cardiac transplantation is limited by the small availability of donor organs, regeneration of the diseased myocardium by cell transplantation is an attractive therapeutic modality. To determine the compatibility of human embryonic stem cell-derived cardiomyocytes (hESC-CMs) (7 to 55 days old) with the myocardium, we investigated their functional properties regarding intracellular Ca2+ handling and the role of the sarcoplasmic reticulum in the contraction. The functional properties of hESC-CMs were investigated by recording simultaneously [Ca2+]i transients and contractions. Additionally, we performed Western blot analysis of the Ca2+-handling proteins SERCA2, calsequestrin, phospholamban, and Na+/Ca2+ exchanger (NCX). Our major findings are, first, that hESC-CMs displayed temporally related [Ca2+]i transients and contractions, negative force-frequency relations, and lack of post-rest potentiation. Second, ryanodine, thapsigargin, and caffeine did not affect the [Ca2+]i transient and contraction, indicating that at this developmental stage, contraction depends on transsarcolemmal Ca2+ influx rather than on sarcoplasmic reticulum Ca2+ release. Third, in agreement with the notion that a voltage-dependent Ca2+ current is present in hESC-CMs and contributes to the mechanical function, verapamil completely blocked contraction. Fourth, whereas hESC-CMs expressed SERCA2 and NCX at levels comparable to those of the adult porcine myocardium, calsequestrin and phospholamban were not expressed. Our study shows for the first time that functional properties related to intracellular Ca2+ handling of hESC-CMs differ markedly from the adult myocardium, probably due to immature sarcoplasmic reticulum capacity.

Crucial role of the sarcoplasmic reticulum in the developmental regulation of Ca2+ transients and contraction in cardiomyocytes derived from embryonic stem cells

In adult myocardium, excitation-contraction coupling is critically regulated by sarcoplasmic reticulum (SR) Ca2+ release via type 2 ryanodine receptor (RyR2), but generally, it is believed that SR-function is rudimentary in the fetal heart and in embryonic stem (ES) cell-derived cardiomyocytes (ESCMs), a possible source for cell replacement therapies. This study used wild-type (RyR2+/+) and RyR2 null (RyR2-/-) ESCMs as an in vitro model of cardiomyogenesis, together with pharmacological approaches and expression profiles of genes relevant for SR function, to elucidate the functional importance of RyR2 and SR on the regulation of Ca2+ transients and contraction during early cardiomyocyte development. During differentiation of RyR2+/+ ESCMs, SR function developed progressively with increased basal cytosolic free Ca2+ concentration ([Ca2+]i), enhanced frequency and amplitude, and decreased duration of Ca2+ transients that were inhibited by ryanodine and thapsigargin. These functional traits correlated with SR Ca2+ load and the expression of RyR2, SERCA2a, and phospholamban. RyR2-/- ESCMs, comparatively, demonstrated a significantly prolonged time-to-peak and reduced frequency of Ca2+ transients and contractions. Beta-adrenergic stimulation of RyR2+/+ ESCMs increased the frequency and amplitude of Ca2+ transients with differentiation but was much weaker in RyR2-/- ESCMs. We conclude that functional SR and control of RyR2-mediated SR Ca2+ release directly contribute to the spontaneous and beta-adrenergic receptor-stimulated contraction of ESCMs, even at very immature stages of development.

The MLC1v gene provides a transgenic marker of myocardium formation within developing chambers of the Xenopus heart

Many details of cardiac chamber morphogenesis could be revealed if muscle fiber development could be visualized directly within the hearts of living vertebrate embryos. To achieve this end, we have used the active promoter of the MLC1v gene to drive expression of green fluorescent protein (GFP) in the developing tadpole heart. By using a line of Xenopus laevis frogs transgenic for the MLC1v-EGFP reporter, we have observed regionalized patterns of muscle formation within the ventricular chamber and maturation of the atrial chambers, from the onset of chamber formation through to the adult frog. In f1 generation MLC1v-EGFP animals, promoter activity is first detected within the looping heart tube and delineates the forming ventricular chamber and proximal outflow tract throughout their development. The 8-kb MLC1v promoter faithfully reproduces the embryonic expression of the endogenous MLC1v mRNA. At later larval stages, weak patches of EGFP fluorescence are found on the atrial side of the atrioventricular boundary. Subsequently, an extensive lattice of MLC1v-expressing fibers extend across the mature atrial chambers of adult frog hearts and the transgene reveals the differing arrangement of muscle fibers in chamber versus outflow myocardium. The complete activity of the promoter resides within the proximal 4.5 kb of the MLC1v DNA fragment, whereas key elements regulating chamber-specific expression are present in the proximal-most 1.5 kb. Finally, we demonstrate how cardiac and craniofacial muscle expression of the MLC1v promoter can be used to diagnose mutant phenotypes in living embryos, using the injection of RNA encoding a Tbx1-engrailed repressor-fusion protein as an example.

BOP, a regulator of right ventricular heart development, is a direct transcriptional target of MEF2C in the developing heart

The vertebrate heart is assembled during embryogenesis in a modular manner from different populations of precursor cells. The right ventricular chamber and outflow tract are derived primarily from a population of progenitors known as the anterior heart field. These regions of the heart are severely hypoplastic in mutant mice lacking the myocyte enhancer factor 2C (MEF2C) and BOP transcription factors, suggesting that these cardiogenic regulatory factors may act in a common pathway for development of the anterior heart field and its derivatives. We show that Bop expression in the developing heart depends on the direct binding of MEF2C to a MEF2-response element in the Bop promoter that is necessary and sufficient to recapitulate endogenous Bop expression in the anterior heart field and its cardiac derivatives during mouse development. The Bop promoter also directs transcription in the skeletal muscle lineage, but only cardiac expression is dependent on MEF2. These findings identify Bop as an essential downstream effector gene of MEF2C in the developing heart, and reveal a transcriptional cascade involved in development of the anterior heart field and its derivatives.

The Hand1 and Hand2 transcription factors regulate expansion of the embryonic cardiac ventricles in a gene dosage-dependent manner

The basic helix-loop-helix transcription factors Hand1 and Hand2 display dynamic and spatially restricted expression patterns in the developing heart. Mice that lack Hand2 die at embryonic day 10.5 from right ventricular hypoplasia and vascular defects, whereas mice that lack Hand1 die at embryonic day 8.5 from placental and extra-embryonic abnormalities that preclude analysis of its potential role in later stages of heart development. To determine the cardiac functions of Hand1, we generated mice harboring a conditional Hand1-null allele and excised the gene by cardiac-specific expression of Cre recombinase. Embryos homozygous for the cardiac Hand1 gene deletion displayed defects in the left ventricle and endocardial cushions, and exhibited dysregulated ventricular gene expression. However, these embryos survived until the perinatal period when they died from a spectrum of cardiac abnormalities. Creation of Hand1/2 double mutant mice revealed gene dose-sensitive functions of Hand transcription factors in the control of cardiac morphogenesis and ventricular gene expression. These findings demonstrate that Hand factors play pivotal and partially redundant roles in cardiac morphogenesis, cardiomyocyte differentiation and cardiac-specific transcription.

Prenatal exposure to thyroid hormone is necessary for normal postnatal development of murine heart and lungs

Maternal hypothyroxinemia during early pregnancy poses an increased risk for poor neuropsychological development of the fetus. We tested the hypothesis that maternal hypothyroidism before the onset of fetal thyroid function also affects postnatal development of heart and lungs. This question was addressed in transgenic mice that express herpes simplex virus thymidine kinase in their thyroidal follicle cells. Treatment with ganciclovir rendered these mice severely hypothyroid because viral thymidine kinase converts ganciclovir into a cytotoxic nucleoside analog. Since ganciclovir crosses the placenta, it also destroyed the thyroid of transgenic embryos while leaving the thyroids of nontransgenic littermates unaffected. Hypothyroidism of both mother and fetus did not affect prenatal heart and lung development. However, the postnatal switch from beta- to alpha-myosin heavy chain (beta- and alpha-MHC, respectively) gene expression and the increase of SERCA-2a mRNA expression did not occur in the ventricular myocardium of either the transgenic (thyroid destroyed) or nontransgenic (intact thyroid) offspring of hypothyroid mothers. Similarly, postnatal animals of the latter two groups retained elevated surfactant protein (SP) A, B, and C mRNA levels in their alveolar epithelium. In hypothyroid pups from hypothyroid mothers, these changes were accompanied by decreased alveolar septation. Our study shows that these effects of maternal hypothyroidism become manifest after birth and are aggravated by the concomitant existence of neonatal hypothyroidism.

Regional expression of L-type calcium channel subunits during cardiac development

The contraction of cardiomyocytes is initiated by the entrance of extracellular calcium through specific calcium channels. Within the myocardium, L-type calcium channels are most abundant. In the heart, the main pore-forming subunit is the alpha1C, although there is a larger heterogeneity on auxiliary beta subunits. We have analyzed the distribution pattern of different alpha1C and beta subunits during cardiac development by immunohistochemistry. We observed homogeneous expression of alpha1C and beta subunits within the early tubular heart, whereas regional differences are observed during the late embryogenesis. beta2 and beta4 show differential expression within the embryonic myocardium. alpha1CD1 displays only a transient enhanced expression in the ventricular conduction system. In adult heart, the expression of the different calcium channel subunits analyzed is homogeneous along the entire myocardium except for alpha1CD1 that is practically undetectable. These findings suggest that beta subunits might play a major role in conferring calcium handling heterogeneity within the developing embryonic myocardium, while alpha1C subunits might contribute just transiently.

Cardiac chamber formation: development, genes, and evolution

Concepts of cardiac development have greatly influenced the description of the formation of the four-chambered vertebrate heart. Traditionally, the embryonic tubular heart is considered to be a composite of serially arranged segments representing adult cardiac compartments. Conversion of such a serial arrangement into the parallel arrangement of the mammalian heart is difficult to understand. Logical integration of the development of the cardiac conduction system into the serial concept has remained puzzling as well. Therefore, the current description needed reconsideration, and we decided to evaluate the essentialities of cardiac design, its evolutionary and embryonic development, and the molecular pathways recruited to make the four-chambered mammalian heart. The three principal notions taken into consideration are as follows. 1) Both the ancestor chordate heart and the embryonic tubular heart of higher vertebrates consist of poorly developed and poorly coupled "pacemaker-like" cardiac muscle cells with the highest pacemaker activity at the venous pole, causing unidirectional peristaltic contraction waves. 2) From this heart tube, ventricular chambers differentiate ventrally and atrial chambers dorsally. The developing chambers display high proliferative activity and consist of structurally well-developed and well-coupled muscle cells with low pacemaker activity, which permits fast conduction of the impulse and efficacious contraction. The forming chambers remain flanked by slowly proliferating pacemaker-like myocardium that is temporally prevented from differentiating into chamber myocardium. 3) The trabecular myocardium proliferates slowly, consists of structurally poorly developed, but well-coupled, cells and contributes to the ventricular conduction system. The atrial and ventricular chambers of the formed heart are activated and interconnected by derivatives of embryonic myocardium. The topographical arrangement of the distinct cardiac muscle cells in the forming heart explains the embryonic electrocardiogram (ECG), does not require the invention of nodes, and allows a logical transition from a peristaltic tubular heart to a synchronously contracting four-chambered heart. This view on the development of cardiac design unfolds fascinating possibilities for future research.

Pulmonary surfactant protein A, B, and C mRNA and protein expression in the nitrofen-induced congenital diaphragmatic hernia rat model

Neonates with congenital diaphragmatic hernia (CDH) suffer from a diaphragmatic defect, lung hypoplasia, and pulmonary hypertension, with poor lung function forming the major clinical challenge. Despite prenatal diagnosis and advanced postnatal treatment strategies, the mortality rate of CDH is still high. CDH has been subject of extensive research over the past decades, but its etiology remains unknown. A major problem with CDH is the failure to predict the individual response to treatment modalities like high-frequency ventilation, inhaled nitric oxide, and extracorporeal membrane oxygenation. In this study, we tested the possibility that CDH lungs are surfactant protein deficient, which could explain the respiratory failure and difficulties in treating CDH infants. We investigated this hypothesis in the nitrofen-induced CDH rat model and assessed the cellular concentrations of surfactant protein (SP)-A, -B, and -C mRNA with a quantitative radioactive in situ hybridization technique. No differences were observed between control and CDH lungs for SP mRNA expression patterns. The cellular concentration (mean OD) of SP-A and SP-B mRNA was similar at all stages whereas the mean OD of SP-C mRNA and the volume fraction of cells (% Area) expressing SP mRNA was higher in CDH lungs at term. Immunohistochemical analysis revealed no differences between control and CDH lungs for SP protein expression. No differences in the mean OD or % Area for the SP mRNAs were found between the ipsi- and contralateral side of CDH lungs. We conclude that there is no primary deficiency of surfactant proteins in the nitrofen-induced CDH rat model.

Ectopic pacing at physiological rate improves postanoxic recovery of the developing heart

Recently, rapid and transient cardiac pacing was shown to induce preconditioning in animal models. Whether the electrical stimulation per se or the concomitant myocardial ischemia affords such a protection remains unknown. We tested the hypothesis that chronic pacing of a cardiac preparation maintained in a normoxic condition can induce protection. Hearts of 4-day-old chick embryos were electrically paced in ovo over a 12-h period using asynchronous and intermittent ventricular stimulation (5 min on-10 min off) at 110% of the intrinsic rate. Sham (n = 6) and paced hearts (n = 6) were then excised, mounted in vitro, and subjected successively to 30 min of normoxia (20% O(2)), 30 min of anoxia (0% O(2)), and 60 min of reoxygenation (20% O(2)). Electrocardiogram and atrial and ventricular contractions were simultaneously recorded throughout the experiment. Reoxygenation-induced chrono-, dromo-, and inotropic disturbances, incidence of arrhythmias, and changes in electromechanical delay (EMD) in atria and ventricle were systematically investigated in sham and paced hearts. Under normoxia, the isolated heart beat spontaneously and regularly, and all baseline functional parameters were similar in sham and paced groups (means +/- SD): heart rate (190 +/- 36 beats/min), P-R interval (104 +/- 25 ms), mechanical atrioventricular propagation (20 +/- 4 mm/s), ventricular shortening velocity (1.7 +/- 1 mm/s), atrial EMD (17 +/- 4 ms), and ventricular EMD (16 +/- 2 ms). Under anoxia, cardiac function progressively collapsed, and sinoatrial activity finally stopped after approximately 9 min in both groups. During reoxygenation, paced hearts showed 1) a lower incidence of arrhythmias than sham hearts, 2) an increased rate of recovery of ventricular contractility compared with sham hearts, and 3) a faster return of ventricular EMD to basal value than sham hearts. However, recovery of heart rate, atrioventricular conduction, and atrial EMD was not improved by pacing. Activity of all hearts was fully restored at the end of reoxygenation. These findings suggest that chronic electrical stimulation of the ventricle at a near-physiological rate selectively alters some cellular functions within the heart and constitutes a nonischemic means to increase myocardial tolerance to a subsequent hypoxia-reoxygenation.

Three-dimensional reconstruction of gene expression patterns during cardiac development

The study of the genetic regulation of embryonic development requires the three-dimensional (3D) mapping of gene expression at the microscopic level. Despite the recent burst in the number of methods focusing on 3D reconstruction of embryonic specimens, an adequate and accessible 3D reconstruction protocol for the visualization of patterns of gene expression is lacking. In this communication we describe a protocol that was developed for the 3D visualization of patterns of gene expression determined by in situ hybridization (ISH) on serial sections. The method still requires tissue sectioning, due to penetration limits of the specific staining agents into whole embryo preparations. With regard to expenditure of resources, i.e., hardware, software, and time, the protocol is relatively undemanding. Because the variation between specimens requires the visualization of multiple specimens per stage, it was decided to "do more, less well." The current protocol, therefore, results in reconstructions of sufficient, but not the highest, quality. The use of the protocol is demonstrated on a series of serially sectioned mouse hearts, ranging from embryonic day 8.5 to 14.5. The myocardium of the hearts was identified by ISH using a mixture of specific mRNA probes and reconstructed.

Recruitment of intra- and extracardiac cells into the myocardial lineage during mouse development

The tubular heart differentiates from the bilateral cardiac fields in the splanchnic mesoderm. The expression of smooth muscle proteins has been shown to accompany the early phases of cardiac muscle formation. In this study we show that during elongation of the arterial pole of the mouse linear heart tube, alpha-smooth muscle actin (alpha-Sma) expression extends in the area that has been shown to become recruited into the myocardial lineage, but does not yet express myocardial markers. These data suggest that alpha-Sma identifies mesodermal cells that during subsequent development will be recruited into the myocardial lineage. Myocardium formation is not only observed at the arterial pole, but also at the venous pole and in the intracardiac mesenchyme. This results in the formation of the caval and pulmonary myocardium, the smooth-walled atrial myocardium, the myocardial atrioventricular septum, and the myocardial outlet septum. To determine whether recruitment into the myocardial lineage also takes place in these regions, the spatiotemporal pattern of expression of alpha-Sma and of the myocardial markers sarcoplasmatic reticulum calcium ATPase (Serca2a), alpha-myosin heavy chain (Mhc), and beta-Mhc were examined. We show that prior to the expression of myocardial markers, alpha-Sma is expressed in these regions, which suggests that these mesodermal cells become recruited into the cardiac lineage after formation of the linear heart tube.

TBX5 overexpression stimulates differentiation of chamber myocardium in P19C16 embryonic carcinoma cells

In vitro differentiation of pluripotent embryonic cells is becoming a model system to study factors and genes involved in early developmental processes including cardiogenesis. An additional application involves the development of donor cells for treatment of diseases among which cardiac infarction. For this purpose differentiated cells should meet the functional characteristics of chamber myocardium, a requirement not convincingly reached as yet. The T-box transcription factor Tbx5 has been demonstrated to be crucial for heart formation. Using stably transfected clones of the P19C16 embryonic carcinoma cell line, reported to differentiate efficiently into the cardiac lineage, we investigated whether Tbx5 is sufficient to enhance cardiogenesis and differentiation of chamber myocardium. TBX5-transfected clones started to beat earlier, however, a relation between transgenic TBX5 mRNA levels and the number of beating foci or levels of Serca2a mRNA, a myocardial marker, could not be observed. However, TBX5-transfected clones displayed significantly higher levels of atrial natriuretic factor (Anf) and Connexin (Cx)40 mRNAs, which are associated with the formation of chamber myocardium. This indicates that Tbx5 enhances cardiac maturation within this system rather than cardiogenesis.

Non-radioactive in situ detection of mRNA in ES cell-derived cardiomyocytes and in the developing heart

Non-radioactive in situ hybridisation is an excellent method to visualise mRNA molecules within their topographical context. Recently we have reported a new non-radioactive in situ hybridisation procedure on tissue sections that is essentially based on the whole mount in situ hybridisation procedure. This method is superior in spatial resolution and sensitivity compared to the radioactive in situ hybridisation procedure. Generally, low levels of gene expression, such as found with the developmental onset of gene expression and in differentiating embryonic stem cells, are difficult to detect by in situ hybridisation. Here an application of the protocol is presented which is based on tyramide signal amplification, which enables the detection of very low abundant mRNAs. The significance of this method is two-fold: (1) the molecular phenotype of embryonic stem cell-derived cardiomyocytes can be examined at the cellular level with high sensitivity, and (2) the number of cells that express the gene of interest can be assessed.

Developmental changes of Ca(2+) handling in mouse ventricular cells from early embryo to adulthood

Transplant of immature cardiomyocytes is recently attracting a great deal of interest as a new experimental strategy for the treatment of failing hearts. Full understanding of normal cardiomyogenesis is essential to make this regenerative therapy feasible. We analyzed the molecular and functional changes of Ca(2+) handling proteins during development of the mouse heart from early embryo at 9.5 days postcoitum (dpc) through adulthood. From the early to the late (18 dpc) embryonic stage, mRNAs estimated by the real time PCR for ryanodine receptor (type 2, RyR2), sarcoplasmic reticulum (SR) Ca(2+) pump (type 2, SERCA2) and phospholamban (PLB) increased by 3-15 fold in the values normalized to GAPDH mRNA, although Na(+)/Ca(2+) exchanger (type 1, NCX1) mRNA was unchanged. After birth, there was a further increase in the mRNAs for RyR2, SERCA2 and PLB by 18-33 fold, but a 50% decrease in NCX1 mRNA. The protein levels of RyR2, SERCA2, PLB and NCX1, which were normalized to total protein, showed qualitatively parallel developmental changes. L-type Ca(2+) channel currents (I(Ca-L)) were increased during the development (1.3-fold at 18 dpc, 2.2-fold at adult stage, vs. 9.5 dpc). At 9.5 dpc, the Ca(2+) transient was, unlike adulthood, unaffected by the SR blockers, ryanodine (5 microM) and thapsigargin (2 microM), and also by a blocker of the Ca(2+) entry via Na(+)/Ca(2+) exchanger, KB-R 7943 (1 microM). The Ca(2+) transient was abolished after application of nisoldipine (5 microM). These results indicate that activator Ca(2+) for contraction in the early embryonic stage depends almost entirely on I(Ca-L).

Developmental changes in cardiac recovery from anoxia-reoxygenation

The developing cardiovascular system is known to operate normally in a hypoxic environment. However, the functional and ultrastructural recovery of embryonic/fetal hearts subjected to anoxia lasting as long as hypoxia/ischemia performed in adult animal models remains to be investigated. Isolated spontaneously beating hearts from Hamburger-Hamilton developmental stages 14 (14HH), 20HH, 24HH, and 27HH chick embryos were subjected in vitro to 30 or 60 min of anoxia followed by 60 min of reoxygenation. Morphological alterations and apoptosis were assessed histologically and by transmission electron microscopy. Anoxia provoked an initial tachycardia followed by bradycardia leading to complete cardiac arrest, except for in the youngest heart, which kept beating. Complete atrioventricular block appeared after 9.4 +/- 1.1, 1.7 +/- 0.2, and 1.6 +/- 0.3 min at stages 20HH, 24HH, and 27HH, respectively. At reoxygenation, sinoatrial activity resumed first in the form of irregular bursts, and one-to-one atrioventricular conduction resumed after 8, 17, and 35 min at stages 20HH, 24HH, and 27HH, respectively. Ventricular shortening recovered within 30 min except at stage 27HH. After 60 min of anoxia, stage 27HH hearts did not retrieve their baseline activity. Whatever the stage and anoxia duration, nuclear and mitochondrial swelling observed at the end of anoxia were reversible with no apoptosis. Thus the embryonic heart is able to fully recover from anoxia/reoxygenation although its anoxic tolerance declines with age. Changes in cellular homeostatic mechanisms rather than in energy metabolism may account for these developmental variations.

The ryanodine receptor modulates the spontaneous beating rate of cardiomyocytes during development

In adult myocardium, the heartbeat originates from the sequential activation of ionic currents in pacemaker cells of the sinoatrial node. Ca(2+) release via the ryanodine receptor (RyR) modulates the rate at which these cells beat. In contrast, the mechanisms that regulate heart rate during early cardiac development are poorly understood. Embryonic stem (ES) cells can differentiate into spontaneously contracting myocytes whose beating rate increases with differentiation time. These cells thus offer an opportunity to determine the mechanisms that regulate heart rate during development. Here we show that the increase in heart rate with differentiation is markedly depressed in ES cell-derived cardiomyocytes with a functional knockout (KO) of the cardiac ryanodine receptor (RyR2). KO myocytes show a slowing of the rate of spontaneous diastolic depolarization and an absence of calcium sparks. The depressed rate of pacemaker potential can be mimicked in wild-type myocytes by ryanodine, and rescued in KO myocytes with herpes simplex virus (HSV)-1 amplicons containing full-length RyR2. We conclude that a functional RyR2 is crucial to the progressive increase in heart rate during differentiation of ES cell-derived cardiomyocytes, consistent with a mechanism that couples Ca(2+) release via RyR before an action potential with activation of an inward current that accelerates membrane depolarization.

Patterning the vertebrate heart

The mammalian heart is crafted from a few progenitor cells that are subject to rapidly changing sets of instructions from their environment and from within. These instructions cause them to migrate, expand and diversify in lineage, and acquire form and function. Molecular information from various model systems, combined with increasingly detailed morphogenetic data, has provided insights into some of these key events. Many congenital heart abnormalities might arise from defects in the early stages of heart development, therefore it is important to understand the molecular pathways that underlie the lineage specification and patterning processes that shape this organ.