Sox4 mediates Tbx3 transcriptional regulation of the gap junction protein Cx43

Tbx3, a T-box transcription factor, regulates key steps in development of the heart and other organ systems. Here, we identify Sox4 as an interacting partner of Tbx3. Pull-down and nuclear retention assays verify this interaction and in situ hybridization reveals Tbx3 and Sox4 to co-localize extensively in the embryo including the atrioventricular and outflow tract cushion mesenchyme and a small area of interventricular myocardium. Tbx3, SOX4, and SOX2 ChIP data, identify a region in intron 1 of Gja1 bound by all tree proteins and subsequent ChIP experiments verify that this sequence is bound, in vivo, in the developing heart. In a luciferase reporter assay, this element displays a synergistic antagonistic response to co-transfection of Tbx3 and Sox4 and in vivo, in zebrafish, drives expression of a reporter in the heart, confirming its function as a cardiac enhancer. Mechanistically, we postulate that Sox4 is a mediator of Tbx3 transcriptional activity.

From developmental biology to heart repair

Advances in our understanding of cardiac development have fuelled research into cellular approaches to myocardial repair of the damaged heart. In this collection of reviews we present recent advances into the basic mechanisms of heart development and the resident and non-resident progenitor cell populations that are currently being investigated as potential mediators of cardiac repair. Together these reviews illustrate that despite our current knowledge about how the heart is constructed, caution and much more research in this exciting field is essential. The current momentum to evaluate the potential for cardiac repair will in turn accelerate research into fundamental aspects of myocardial biology.

[Stem cells: biology and possible application to myocardial infarct]

If the heart fails to recover sufficient functionality following an infarct, then heart failure develops, an important cause of death in the western world. One obvious therapy is to create more muscle tissue to supplement the damaged myocardium with new functional contractile cells, together with neovasculogenesis. Stem cells repair recipient tissue by differentiating into tissue-specific cells or by creating an environment that stimulates the process of repair by the body's own cells at the site. In animal studies the heart function stabilised following an injection of stem cells in the infarcted area. In 3 non-randomised trials in humans, bone marrow stem cells were injected via the infarcted artery or round the infarcted area; the results indicated an improved heart function. There is currently still insufficient fundamental knowledge about the behaviour of multipotent cells, about the effects of using them for treatment, and about their long-term risk for these cells to be employed in the treatment of patients with a heart infarct.

Formation of myocardium after the initial development of the linear heart tube

Well after formation of the primary linear heart tube, the mesenchymal cardiac septa become largely myocardial, and myocardial sleeves are formed along the caval and pulmonary veins. This second wave of myocardium formation can be envisioned to be the result of recruitment of cardiomyocytes by differentiation from flanking mesenchyme and/or by migration from existing myocardium (myocardialization). As a first step to elucidate the underlying mechanism, we studied in chicken heart development the formation of myocardial cells within intra- and extracardiac mesenchymal structures. We show that the second wave of myocardium formation proceeds in a caudal-to-cranial gradient in vivo. At the venous pole, loosely arranged networks of cardiomyocytes are observed in the dorsal mesocardium from H/H19 onward, in the atrioventricular cushion region from H/H26 onward, and in the proximal outflow tract (conus) from H/H29 onward. The process is completed at H/H stage 43. Subsequently, we determined the potential of the different cardiac compartments to form myocardial networks in a 3D in vitro culture assay. This analysis showed that the competency to form myocardial networks in vitro is a characteristic of the myocardium that is flanked by intra- or extracardiac mesenchyme, i.e., the inflow tract, atrioventricular canal, and outflow tract. These cardiac compartments can be induced to form myocardial networks by a temporally released or secreted signal that is similar throughout the entire heart. Atrial and ventricular compartments are not competent and do not produce the inducer. Moreover, cardiac cushion mesenchyme was found to be able to (trans-)differentiate into cardiomyocytes in the in vitro culture assay. The combined observations suggest that a common mechanism and molecular regulatory pathway underlies the recruitment of mesodermal cells into the cardiogenic lineage during this second wave of myocardium formation through the entire heart.

Divergent expression of delayed rectifier K(+) channel subunits during mouse heart development

The repolarization phase of the cardiac action potential is dependent on transmembrane K(+) currents. The slow (I(Ks)) and fast (I(Kr)) components of the delayed-rectifier cardiac K(+) current are generated by pore-forming alpha subunits KCNQ1 and KCNH2, respectively, in association with regulatory beta-subunit KCNE1, KCNE2 and perphaps KCNE3. In the present study we have investigated the distribution of transcripts encoding these five potassium channel-forming subunits during mouse heart development as well as the protein distribution of KCNQ1 and KCNH2. KCNQ1 and KCNH2 mRNAs (and protein) are first expressed at embryonic day (E) 9.5, showing comparable levels of expression within the atrial and ventricular myocardium during the embryonic and fetal stages. In contrast, the beta-subunits display a more dynamic pattern of expression during development. KCNE1 expression is first observed at E9.5 throughout the entire myocardium and progressively is confined to the ventricular myocardium. With further development (E16.5), KCNE1 expression is mainly confined to the compact ventricular myocardium. KCNE2 is first expressed at E9.5 and it is restricted already to the atrial myocardium. KCNE3 is first expressed at E8.5 throughout the myocardium and with further development, it becomes restricted to the atrial myocardium. The fact that alpha subunits are homogeneously distributed within the myocardium, whereas the beta subunits display a regionalized expression profile during cardiac development, suggest that differences in the slow and fast component of the delayed-rectifier cardiac K(+) currents between the atrial and the ventricular cardiomyocytes are mainly determined by differential beta-subunit distribution.

Gene and cluster-specific expression of the Iroquois family members during mouse development

Mammalian homologues of the Drosophila Iroquois homeobox gene complex, involved in patterning and regionalization of differentiation, have recently been identified (Mech. Dev., 69 (1997) 169; Dev. Biol., 217 (2000) 266; Dev. Dyn., 218 (2000) 160; Mech. Dev., 91 (2000) 317; Dev. Biol., 224 (2000) 263; Genome Res., 10 (2000) 1453; Mech. Dev., 103 (2001) 193). The six members of the murine family were found to be organized in two cognate clusters of three genes each, Irx1, -2, -4 and Irx3, -5, -6, respectively (Peters et al., 2000). As a basis for further study of their regulation and function we performed a comparative analysis of the genomic organization and of the expression patterns of all six Irx genes. The genes are expressed in highly specific and regionalized patterns of ectoderm, mesoderm and endoderm derived tissues. In most tissues the pattern of expression of the clustered genes, especially of Irx1 and -2 and of Irx3 and -5, respectively, closely resembled each other while those of Irx4 and -6 were very divergent. Interestingly, the expression of cognate genes was found to be mutually exclusive in adjacent and interacting tissues of limb, heart and the laryncho-pharyncheal region. The results indicate that the Irx genes are coordinately regulated at the level of the cluster.

Interleukin-15 expression in atherosclerotic plaques: an alternative pathway for T-cell activation in atherosclerosis?

T-cell activation in atherosclerotic plaques is thought to be initiated by plaque-derived antigens, such as oxidized LDL (oxLDL). An alternative pathway of T-cell activation independent of antigen stimulation, mediated by the cytokine interleukin (IL)-15, was recently described. We investigated IL-15 expression in atherosclerotic plaques in relation to plaque morphology, inflammatory cells, T-cell activation, and oxidation-specific epitopes by use of immunohistochemistry. In situ hybridization was used to evaluate IL-15 mRNA expression. We also studied the proliferative response of plaque-derived T-cell lines to IL-15 in vitro using [(3)H]thymidine incorporation. Fresh-frozen specimens were classified as fibrous (n=9), fibrolipid (n=8), and lipid-rich (n=14) plaques; normal vessels (n=4) served as reference. Expression of IL-15 mRNA and protein was found almost solely in fibrolipid and lipid-rich plaques, associated with oxLDL-positive macrophages. Sequential immunostains revealed colocalization between IL-15- and CD40L-positive T cells. Moreover, plaque-derived T-cell lines were highly responsive to IL-15. Hence, IL-15 could provide a pathway for antigen-independent T-cell activation.

Transgenic mice overexpressing human KvLQT1 dominant-negative isoform. Part I: Phenotypic characterisation

OBJECTIVES

The KCNQ1 gene encodes the KvLQT1 potassium channel, which generates in the human heart the slow component of the cardiac delayed rectifier current, I(Ks). Mutations in KCNQ1 are the most frequent cause of the congenital long QT syndrome. We have previously cloned a cardiac KCNQ1 human isoform, which exerts a strong dominant-negative effect on KvLQT1 channels. We took advantage of this dominant-negative isoform to engineer an in vivo model of KvLQT1 disruption, obtained by overexpressing the dominant-negative subunit under the control of the alpha-myosin heavy chain promoter.

RESULTS

Three different transgenic lines demonstrated a phenotype with increasing severity. Functional suppression of KvLQT1 in transgenic mice led to a markedly prolonged QT interval associated with sinus node dysfunction. Transgenic mice also demonstrated atrio-ventricular block leading to occasional Wenckebach phenomenon. The atrio-ventricular block was associated with prolonged AH but normal HV interval in His recordings. Prolonged QT interval correlated with prolonged action potential duration and with reduced K(+) current density in patch-clamp experiments. RNase protection assay revealed remodeling of K(+) channel expression in transgenic mice.

CONCLUSIONS

Our transgenic mouse model suggests a role for KvLQT1 channels not only in the mouse cardiac repolarisation but also in the sinus node automaticity and in the propagation of the impulse through the AV node.

Development of the myocardium of the atrioventricular canal and the vestibular spine in the human heart

To establish the morphogenetic mechanisms underlying formation and separation of the atrioventricular connections, we studied the remodeling of the myocardium of the atrioventricular canal and the extracardiac mesenchymal tissue of the vestibular spine in human embryonic hearts from 4.5 to 10 weeks of development. Septation of the atrioventricular junction is brought about by downgrowth of the primary atrial septum, fusion of the endocardial cushions, and forward expansion of the vestibular spine between atrial septum and cushions. The vestibular spine subsequently myocardializes to form the ventral rim of the oval fossa. The connection of the atrioventricular canal with the atria expands evenly. In contrast, the expression patterns of creatine kinase M and GlN2, markers for the atrioventricular and interventricular junctions, respectively, show that the junction of the canal with the right ventricle forms by local growth in the inner curvature of the heart. Growth of the caudal portion of the muscular ventricular septum to make contact with the inferior endocardial cushion occurs only after the canal has expanded rightward. The atrioventricular node develops from that part of the canal myocardium that retains its continuity with the ventricular myocardium.

Pitx2 expression defines a left cardiac lineage of cells: evidence for atrial and ventricular molecular isomerism in the iv/iv mice

The homeobox gene Pitx2 has been characterized as a mediator of left-right signaling in heart, gut, and lung morphogenesis. However, the relationship between the developmental role of Pitx2 and its expression pattern at the organ level has not been explored. In this study we focus on the role of Pitx2 in heart morphogenesis. Chicken Pitx2 transcripts are present in the left portion of the cardiac crescent and in the left side of the heart tube. Through looping Pitx2 is present in the left atrium, in the ventral portion of the ventricles and in the left-ventral part of the outflow tract. Mouse Pitx2 shows a similar developmental profile of expression. To test whether Pitx2 represents a lineage marker we have tagged the left portion of the chicken cardiac tube with fluorescent DiD. Labeled cells were found at HH16 in the left atrium and in the ventral region of the ventricles and the outflow tract. In the iv/iv mouse model of cardiac heterotaxia Pitx2 was abnormally expressed in the atrial and in the ventricular chambers. Furthermore, altered Pitx2 expression correlated with the occurrence of DORV. Our data reveal the existence of molecular isomerism not only in the atrial, but also in the ventricular compartment of the heart.

Differential expression of KvLQT1 and its regulator IsK in mouse epithelia

KCNQ1 is the human gene responsible in most cases for the long QT syndrome, a genetic disorder characterized by anomalies in cardiac repolarization leading to arrhythmias and sudden death. KCNQ1 encodes a pore-forming K+ channel subunit termed KvLQT1 which, in association with its regulatory beta-subunit IsK (also called minK), produces the slow component of the delayed-rectifier cardiac K+ current. We used in situ hybridization to localize KvLQT1 and IsK mRNAs in various tissues from adult mice. We showed that KvLQT1 mRNA expression is widely distributed in epithelial tissues, in the absence (small intestine, lung, liver, thymus) or presence (kidney, stomach, exocrine pancreas) of its regulator IsK. In the kidney and the stomach, however, the expression patterns of KvLQT1 and IsK do not coincide. In many tissues, in situ data obtained with the IsK probe coincide with beta-galactosidase expression in IsK-deficient mice in which the bacterial lacZ gene has been substituted for the IsK coding region. Because expression of KvLQT1 in the presence or absence of its regulator generates a K+ current with different biophysical characteristics, the role of KvLQT1 in epithelial cells may vary depending on the expression of its regulator IsK. The high level of KvLQT1 expression in epithelial tissues is consistent with its potential role in K+ secretion and recycling, in maintaining the resting potential, and in regulating Cl- secretion and/or Na+ absorption.

MLC3F transgene expression iniv mutant mice reveals the importance of left-right signalling pathways for the acquisition of left and right atrial but not ventricular compartment identity

Abstract Transcriptional differences between left and right cardiac chambers are revealed by an nlacZ reporter transgene controlled by regulatory sequences of the MLC3F gene, which is expressed in the left ventricle (LV), atrioventricular canal (AVC), and right atrium (RA). To examine the role of left-right signalling in the acquisition of left and right chamber identity, we have investigated MLC3F transgene expression in iv mutant mice. iv/iv mice exhibit randomised direction of heart looping and an elevated frequency of associated laterality defects, including atrial isomerism. At fetal stages, 3F-nlacZ-2E transgene expression remains confined to the morphological LV, AVC, and RA in L-loop hearts, although these appear on the opposite side of the body. In cases of morphologically distinguishable right atrial appendage isomerism, both atrial appendages show strong transgene expression. Conversely, specimens with morphological left atrial appendage isomerism show only weak expression in both atrial appendages. The earliest left-right atrial differences in the expression of the 3F-nlacZ-2E transgene are observed at E8.5. DiI labelling experiments confirmed that transcriptional regionalisation of the 3F-nlacZ-2E transgene at this stage reflects future atrial chamber identity. In some iv/iv embryos at E8.5, the asymmetry of 3F-nlacZ-2E expression was lost, suggesting atrial isomerism at the transcriptional level prior to chamber formation. These data suggest that molecular specification of left and right atrial but not ventricular chambers is dependent on left-right axial cues.

Sensitive nonradioactive detection of mRNA in tissue sections: novel application of the whole-mount in situ hybridization protocol

The relative insensitivity of nonradioactive mRNA detection in tissue sections compared to the sensitive nonradioactive detection of single-copy DNA sequences in chromosome spreads, or of mRNA sequences in whole-mount samples, has remained a puzzling issue. Because of the biological significance of sensitive in situ mRNA detection in conjunction with high spatial resolution, we developed a nonradioactive in situ hybridization (ISH) protocol for detection of mRNA sequences in sections. The procedure is essentially based on the whole-mount ISH procedure and is at least equally sensitive. Increase of the hybridization temperature to 70C while maintaining stringency of hybridization by adaptation of the salt concentration significantly improved the sensitivity and made the procedure more sensitive than the conventional radioactive procedure. Thicker sections, which were no improvement using conventional radioactive ISH protocols, further enhanced signal. Higher hybridization temperatures apparently permit better tissue penetration of the probe. Application of this highly reliable protocol permitted the identification and localization of the cells in the developing heart that express low-abundance mRNAs of different members of the Iroquois homeobox gene family that are supposedly involved in cardiac patterning. The radioactive ISH procedure scarcely permitted detection of these sequences, underscoring the value of this novel method.

A single regulatory module of the carbamoylphosphate synthetase I gene executes its hepatic program of expression

A 469-base pair (bp) upstream regulatory fragment (URF) and the proximal promoter of the carbamoylphosphate synthetase I (CPS) gene were analyzed for their role in the regulation of spatial, developmental, and hormone-induced expression in vivo. The URF is essential and sufficient for hepatocyte-specific expression, periportal localization, perinatal activation and induction by glucocorticoids, and cAMP in transgenic mice. Before birth, the transgene is silent but can be induced by cAMP and glucocorticoids, indicating that these compounds are responsible for the activation of expression at birth. A 102-bp glucocorticoid response unit within the URF, containing binding sites for HNF3, C/EBP, and the glucocorticoid receptor, is the main determinant of the hepatocyte-specific and hormone-controlled activity. Additional sequences are required for a productive interaction between this minimal response unit and the core CPS promoter. These results show that the 469-bp URF, and probably only the 102-bp glucocorticoid response unit, functions as a regulatory module, in that it autonomously executes a correct spatial, developmental and hormonal program of CPS expression in the liver.

Multiple transcriptional domains, with distinct left and right components, in the atrial chambers of the developing heart

During heart development, 2 fast-conducting regions of working myocardium balloon out from the slow-conducting primary myocardium of the tubular heart. Three regions of primary myocardium persist: the outflow tract, atrioventricular canal, and inflow tract, which are contiguous throughout the inner curvature of the heart. The contribution of the inflow tract to the definitive atrial chambers has remained enigmatic largely because of the lack of molecular markers that permit unambiguous identification of this myocardial domain. We now report that the genes encoding atrial natriuretic factor, myosin light chain (MLC) 3F, MLC2V, and Pitx-2, and transgenic mouse lines expressing nlacZ under the control of regulatory sequences of the mouse MLC1F/3F gene, display regionalized patterns of expression in the atrial component of the developing mouse heart. These data distinguish 4 broad transcriptional domains in the atrial myocardium: (1) the atrioventricular canal that will form the smooth-walled lower atrial rim proximal to the ventricles; (2) the atrial appendages; (3) the caval vein myocardium (systemic inlet); and (4) the mediastinal myocardium (pulmonary inlet), including the atrial septa. The pattern of expression of Pitx-2 reveals that each of these transcriptional domains has a distinct left and right component. This study reveals for the first time differential gene expression in the systemic and pulmonary inlets, which is not shared by the contiguous atrial appendages and provides evidence for multiple molecular compartments within the atrial chambers. Furthermore, this work will allow the contribution of each of these myocardial components to be studied in congenitally malformed hearts, such as those with abnormal venous return.

Patterning the embryonic heart: identification of five mouse Iroquois homeobox genes in the developing heart

We isolated cDNAs of mouse Iroquois-related homeobox genes Irx1, -2, -3, -4, and -5 and characterized their patterns of expression in the developing heart. Irx1 and Irx2 were found to be expressed specifically in the ventricular septum from the onset of its formation onward. In fetal stages, the expression of both genes appeared to gradually become confined to the myocardium of the atrioventricular bundle and bundle branches of the forming ventricular conduction system. Irx3 was found to be expressed specifically in the trabeculated myocardium of the ventricles. Irx4 expression was observed in a segment of the linear heart tube and the atrioventricular canal and ventricular myocardium including the inner curvature after looping, resembling the pattern of MLC2V. Transcripts for Irx5 were detected specifically in the endocardium lining the ventricular and atrial working myocardium that also expressed von Willebrand factor, but were absent from the endocardium of the endocardial cushions, i.e., the atrioventricular canal, inner curvature, and outflow tract. The spatiodevelopmental pattern of Irx5 matched that of ANF, a marker for the forming working myocardium of the chambers. Taken together, all members of the Irx gene family were found to be expressed in highly specific patterns in the developing mouse heart, suggesting a critical role in the specification of the distinct components of the four-chambered heart.

An atrioventricular canal domain defined by cardiac troponin I transgene expression in the embryonic myocardium

During early cardiac development the atrial myocardium is continuous with the ventricular myocardium throughout the atrioventricular canal. The atrioventricular canal undergoes complex remodelling involving septation, formation of atrioventricular valves and insulation between atria and ventricles except at the level of the atrioventricular node. Understanding of these processes has been hampered by the lack of markers specific for this heart region. We have generated transgenic mice expressing beta-galactosidase under the control of the cardiac troponin I gene that show transgene expression mainly confined to the atrioventricular canal myocardium during early embryonic development. With further development beta-galactosidase positive cells are observed in the atrioventricular node and in the lower rim of both right and left atria, supporting the view that atrioventricular canal myocardium contributes to the atrioventricular node and is in part incorporated into the lower rim of the atria. These results identify the atrioventricular canal myocardium as a distinct transcriptional domain.

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

During development fast-contracting atrial and ventricular chambers develop from a peristaltic-contracting heart tube. This study addresses the question of whether chamber formation is paralleled by a matching expression of the sarcoplasmic reticulum (SR) Ca(2+) pump. We studied indo-1 Ca(2+) transients elicited by field stimulation of linear heart tube stages and of explants from atria and outflow tracts of the prototypical preseptational E13 rat heart. Ca(2+) transients of H/H 11+ chicken hearts, which constitute the prototypic linear heart tube stage, were sensitive to verapamil only, indicating a minor contribution of Ca(2+)-triggered SR Ca(2+) release. Outflow tract transients displayed sensitivity to the inhibitors similar to that of the linear heart tube stages. Atrial Ca(2+) transients disappeared upon addition of ryanodine, tetracaine, or verapamil, indicating the presence of Ca(2+)-triggered SR Ca(2+) release. Quantitative radioactive in situ hybridization on sections of E13 rat hearts showed approximately 10-fold higher SERCA2a mRNA levels in the atria compared to nonmyocardial tissue and approximately 5-fold higher expression in compact ventricular myocardium. The myocardium of atrioventricular canal, outflow tract, inner curvature, and ventricular trabecules displayed weak expression. Immunohistochemistry on sections of rat and human embryos showed a similar pattern. The significance of these findings is threefold. (i) A functional SR is present long before birth. (ii) SR development is concomitant with cardiac chamber development, explaining regional differences in cardiac function. (iii) The pattern of SERCA2a expression underscores a manner of chamber development by differentiation at the outer curvature, rather than by segmentation of the linear heart tube.

Chamber formation and morphogenesis in the developing mammalian heart

In this study we challenge the generally accepted view that cardiac chambers form from an array of segmental primordia arranged along the anteroposterior axis of the linear and looping heart tube. We traced the spatial pattern of expression of genes encoding atrial natriuretic factor, sarcoplasmic reticulum calcium ATPase, Chisel, Irx5, Irx4, myosin light chain 2v, and beta-myosin heavy chain and related these to morphogenesis. Based on the patterns we propose a two-step model for chamber formation in the embryonic heart. First, a linear heart forms, which is composed of "primary" myocardium that nonetheless shows polarity in phenotype and gene expression along its anteroposterior and dorsoventral axes. Second, specialized ventricular chamber myocardium is specified at the ventral surface of the linear heart tube, while distinct left and right atrial myocardium forms more caudally on laterodorsal surfaces. The process of looping aligns these primordial chambers such that they face the outer curvature. Myocardium of the inner curvature, as well as that of inflow tract, atrioventricular canal, and outflow tract, retains the molecular signature originally found in linear heart tube myocardium. Evidence for distinct transcriptional programs which govern compartmentalization in the forming heart is seen in the patterns of expression of Hand1 for the dorsoventral axis, Irx4 and Tbx5 for the anteroposterior axis, and Irx5 for the distinction between primary and chamber myocardium.