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Function of FK506 binding protein (FKBP) in chick embryonic cardiac development.

FK506 binding protein (BP) 12, an immunophilin of FK506-binding proteins, is involved in intra-cellular signal transduction through the calcineurin-nuclear factor pathway. FKBP12 is reported to be associated with the ryanodine-receptor and IP3 Ca2+ channels, and to regulate cell proliferation via binding transforming growth factor (TGF)-beta receptor and cyclin dependent kinase (CDK). To elucidate the function of FKBP12 in cardiac development, we analyzed the temporal profile and regulation of FKBP12 expression in chick heart and in cultured cardiomyocytes. FKBP12 is expressed in embryos as early as day 4 and is predominantly associated with cardiomyocytes and osteo-chondrocytes. Tissue FKBP level in the heart increases with development. Immunohistochemically, the distribution and levels of FKBP12 appear to be related to sarco-endoplasmic reticulum Ca-ATPase 2 (SERCA2) but not to sarcomeric proteins. In proliferating cells, FKBP12 expression correlates with cellular mitosis, but not with DNA synthesis. In earlier embryos (< day 8), suppressing the activity of FKBP by FK506 administration is lethal, and induces cardiomegaly at later stages. In cultured cardiomyocytes, FK506 reduces the level of contractile proteins and inhibits cell proliferation. These results show that FKBP12 is enriched in cell types involved in dynamic Ca handling, and is likely an important molecule for cardiac development. FKBP12 most likely functions by affecting cellular Ca handling, since its effects are modified by modulators of Ca handling by sarcoplasmic reticulum.

Animals↗

Calreticulin is essential for cardiac development.

Calreticulin is a ubiquitous Ca2+ binding protein, located in the endoplasmic reticulum lumen, which has been implicated in many diverse functions including: regulation of intracellular Ca2+ homeostasis, chaperone activity, steroid-mediated gene regulation, and cell adhesion. To understand the physiological function of calreticulin we used gene targeting to create a knockout mouse for calreticulin. Mice homozygous for the calreticulin gene disruption developed omphalocele (failure of absorption of the umbilical hernia) and showed a marked decrease in ventricular wall thickness and deep intertrabecular recesses in the ventricular walls. Transgenic mice expressing a green fluorescent protein reporter gene under the control of the calreticulin promoter were used to show that the calreticulin gene is highly activated in the cardiovascular system during the early stages of cardiac development. Calreticulin protein is also highly expressed in the developing heart, but it is only a minor component of the mature heart. Bradykinin-induced Ca2+ release by the InsP3-dependent pathway was inhibited in crt-/- cells, suggesting that calreticulin plays a role in Ca2+ homeostasis. Calreticulin-deficient cells also exhibited impaired nuclear import of nuclear factor of activated T cell (NF-AT3) transcription factor indicating that calreticulin plays a role in cardiac development as a component of the Ca2+/calcineurin/NF-AT/GATA-4 transcription pathway.

Animals↗

The effects of two strains of influenza virus on cardiac development in the chick embryo.

To investigate the teratogenic effects of influenza virus on cardiac development two strains of influenza virus (Ty/Calif/5142/66 and Ty/Calif/64/M. Meleagrium) were injected into the yolk sac of fertile white Leghorn eggs during stage 23. Eggs injected with normal saline served as controls. The number of live embryos was significantly reduced in eggs injected with the 5142 strain, whereas the Meleagrium strain did not affect the number of live embryos in a significant manner. Serial sections of the live embryo hearts, extracted during stage 35, showed no cardiac anomalies in control groups. However, cardiac malformations were increased significantly in the embryos infected with the influenza viruses. A comparison of the two viral strains showed that the 5142 strain produced significantly more abnormal hearts than the Meleagrium strain. We conclude that (1) influenza infection during critical stages of embryonic development can lead to the cardiac anomalies, and (2) the virulence of a viral strain has a significant bearing on its teratogenic potential, at least in chick embryo hearts.

Animals↗

Aryl hydrocarbon receptor null mice develop cardiac hypertrophy and increased hypoxia-inducible factor-1alpha in the absence of cardiac hypoxia.

The aryl hydrocarbon receptor (AhR) is a member of the basic helix loop helix PAS (Per-ARNT-SIM) transcription family, which also includes hypoxiainducible factor-1alpha (HIF-1alpha) and its common dimerization partner AhR nuclear translocator (ARNT). Following ligand activation or hypoxia, AhR or HIF-1alpha, respectively, translocate into the nucleus, dimerize with ARNT, and regulate gene expression. Mice lacking the AhR have been shown previously to develop cardiac enlargement. In cardiac hypertrophy, it has been suggested that the myocardium becomes hypoxic, increasing HIF-1alpha stabilization and inducing coronary neovascularization, however, this mechanism has not been demonstrated in vivo. The purpose of this study was to investigate the cardiac enlargement reported in AhR(-/-) mice and to determine if it was associated with myocardial hypoxia and subsequent activation of the HIF-1alpha pathway. We found that AhR(-/-) mice develop significant cardiac hypertrophy at 5 mo. However, this cardiac hypertrophy was not associated with myocardial hypoxia. Despite this finding, cardiac hypertrophy in AhR(-/-) mice was associated with increased cardiac HIF-1alpha protein expression and increased mRNA expression of the neovascularization factor vascular endothelial growth factor (VEGF). These data demonstrate that the development of cardiac hypertrophy in AhR(-/-) mice not associated with myocardial hypoxia, but is correlated with increased cardiac HIF-1alpha protein and VEGF mRNA expression.

Age Factors↗

Endothelial-cardiomyocyte interactions in cardiac development and repair.

Communication between endothelial cells and cardiomyocytes regulates not only early cardiac development but also adult cardiomyocyte function, including the contractile state. In the normal mammalian myocardium, each cardiomyocyte is surrounded by an intricate network of capillaries and is next to endothelial cells. Cardiomyocytes depend on endothelial cells not only for oxygenated blood supply but also for local protective signals that promote cardiomyocyte organization and survival. While endothelial cells direct cardiomyocytes, cardiomyocytes reciprocally secrete factors that impact endothelial cell function. Understanding how endothelial cells communicate with cardiomyocytes will be critical for cardiac regeneration, in which the ultimate goal is not simply to improve systolic function transiently but to establish new myocardium that is both structurally and functionally normal in the long term.

Animals↗

Role of smad- and wnt-dependent pathways in embryonic cardiac development.

The development of the heart is essential for embryogenesis and precedes development of other organs. However, the mechanisms involved in embryonic cardiac development are ill-defined. Recent evidence suggests that Smad and Wnt signaling pathways are important in stem cell fate determination and their commitment to cardiovascular differentiation. We have previously reported that bone morphogenetic proteins (BMP)-2, -5, and -7 and fibroblast growth factors (FGF)-2 and -4 secreted from the adjoining endodermal cells favor cardiac differentiation in murine embryonic stem (ES) cells. Here, we demonstrate that BMP-2, -5, and -7 stimulate receptor-activated Smad1, 5, and 8, which in turn causes oligomerization of Smad4 in the nucleus. We further delineate the role of Wnt signaling pathway as evidenced by induction of Wnt3 and Wnt8b, stimulation of FRP-1, inhibition of GSK-B, accumulation of cytosolic beta-catenin, and transcription of target genes, including c-myc and cyclin-D1. We also ascertained the specificity of BMP- and Wnt-evoked activation of signaling cascades. Our data are consistent with the hypothesis that BMP-dependent activation of transcription factors including GATA-4, Nkx2.5, and MEF-2C augments cardiac differentiation mediated by cooperative control of Smad and Wnt signaling pathways. Our results provide a solid foundation for further study of the biochemistry of cardiac differentiation from stem cells.

Animals↗

The sensitivity of developing cardiac myofibrils to cytochalasin-B (electron microscopy-polarized light-Z-bands-heartbeat).

Developing cardiac muscle cells of 11- to 13-somite chick embryos are sensitive to cytochalasin-B. In cultured chick embryos, ranging in development from 11 to 13 somites, hearts stop beating in the presence of this agent. Both polarized light and electron microscopic examination show that cytochalasin-B disrupts existing myofibrils and inhibits the formation of new ones. Discrete Z-bands are not present in treated heart cells and thick, presumably myosin, filaments are found in disarray. These effects are reversible; after cytochalasin-B is removed from the medium, heartbeat recovers and myofibrils with discrete Z-bands reappear. Fibrillar sensitivity appears to be a function of age since fibrils in hearts of embryos having from 22 to 28 pairs of somites are more resistant.

Animals↗

Well-defined growth factors promote cardiac development in axolotl mesodermal explants.

The effect of growth factors on the formation of cardiac mesoderm in the urodele, Ambystoma mexicanum (axolotl), has been examined using an in vitro explant system. It has previously been shown that cardiac mesoderm is induced by pharyngeal endoderm during neurula stages in urodeles. In this study, explants of prospective cardiac mesoderm from early neurula stage embryos rarely formed beating cardiac tissue in culture. When transforming growth factor beta-1 (TGF-beta 1) or platelet-derived growth factor BB (PDGF) was added to such explants, the frequency of heart tissue formation increased markedly. The addition of other growth factors to these explants did not enhance cardiac mesoderm formation. The addition of basic fibroblast growth factor (bFGF) to prospective heart mesoderm derived from later stage embryos resulted in a decreased tendency to form cardiac tissue. These results suggest that growth factors analogous to TGF-beta 1, PDGF, and bFGF may regulate the initial stages of vertebrate cardiac development in vivo.

Ambystoma↗

Use of embryonic heart grafted in oculo to assess neurohumoral controls of cardiac development.

Culture of embryonic heart in the anterior eye chamber allows neurohumoral and genetic controls of cardiac development to be separated from the influence of hemodynamic load. Hearts from 12-day gestation rat embryos grafted into the anterior eye chamber of an adult host rat attach to the iris and become vascularized and innervated by collaterals from the host iris. The spontaneous beating of grafts is pacemaker-driven and under functional neural control. Grafts do not beat against a pressure load, allowing the influence of neurohumoral factors to be separated from altered hemodynamic load. In oculo, embryonic heart differentiates into mature myocardium by most morphologic and biochemical criteria. Mature intercalated disks and myofibrils with well-defined Z-lines and M-lines are observed. Mature grafts express the high levels of alpha-myosin heavy chain characteristic of young adult myocardium. Surgical sympathetic denervation of the anterior eye chamber prior to grafting of embryonic hearts compromises growth and increases the intrinsic pacemaker rate. Since the grafts are perfused by the host circulation, the hormonal milieu of the graft can be altered by treatment of the host. Thus, the interaction between hormones and innervation of grafts can be studied using the in oculo model system.

Animals↗

Adrenergic modulation of cardiac development in the rat: effects of prenatal exposure to propranolol via continuous maternal infusion.

During early postnatal development, catecholamines are thought to modulate cardiac cell replication and differentiation, and to program future beta-adrenergic sensitivity. To determine if the sensitive period for these events extends to prenatal ages, pregnant rats were infused with propranolol continuously via osmotic minipumps from gestational day 7 through parturition and the offspring were examined for markers of cardiac cellular development (basal ornithine decarboxylase activity and levels of DNA and protein) and for reactivity to acute beta-adrenergic challenge (heart rate responses and stimulation of ornithine decarboxylase). During the propranolol infusion, fetal cardiac responses to terbutaline, a beta-adrenergic agonist, were completely blocked; after discontinuation of beta-blockade at birth, responses became normal and remained unaffected into young adulthood. Biochemical markers indicated a delay in cellular development caused by propranolol: basal ornithine decarboxylase activity was elevated in the fetus and DNA was subnormal for the first week after birth. Cardiac growth was maintained in the face of DNA deficits by cell enlargement (elevated protein/DNA) which persisted through weaning. By young adulthood, all markers were within normal limits. These data suggest that fetal catecholamines, acting on beta-receptors, do play an initial role in cardiac cellular development, but that the critical period for programming of beta-adrenergic responsiveness occurs later in maturation.

Analysis of Variance↗

Cardiac development in zebrafish: coordination of form and function.

Organogenesis is a dynamic process involving multiple phases of pattern formation and morphogenesis. For example, heart formation involves the specification and differentiation of cardiac precursors, the integration of precursors into a tube, and the remodeling of the embryonic tube to create a fully functional organ. Recently, the zebrafish has emerged as a powerful model organism for the analysis of cardiac development. In particular, zebrafish mutations have revealed specific genetic requirements for cardiac fate determination, migration, fusion, tube assembly, looping, and remodeling. These processes ensure proper cardiac function; likewise, cardiac function may influence aspects of cardiac morphogenesis.

Animals↗

A genetic blueprint for cardiac development.

Congenital heart disease is the leading non-infectious cause of death in children. It is becoming increasingly clear that many cardiac abnormalities once thought to have multifactorial aetiologies are attributable to mutations in developmental control genes. The consequences of these mutations can be manifest at birth as life-threatening cardiac malformations or later as more subtle cardiac abnormalities. Understanding the genetic underpinnings of cardiac development has important implications not only for understanding congenital heart disease, but also for the possibility of cardiac repair through genetic reprogramming of non-cardiac cells to a cardiogenic fate.

Animals↗

Maternal ethanol consumption induces transient compensatory hyperplasia of developing cardiac tissue in the neonatal rat.

The effect of continuous exposure to ethanol in utero and postpartum on growth and cell division in developing cardiac tissue was studied in neonatal Fischer rats. Pregnant and lactating females were maintained on three dietary regimens; a control group fed rat chow ad libitum, an experimental group receiving an ethanol-containing (6% by volume) liquid diet, and a pair-fed control group, which received an isocaloric amount of control liquid diet. At days 1, 5, and 10 postpartum, five litters of pups from each control and experimental group were sacrificed and the body weights, heart weights, heart-to-body weight ratios, and mitotic frequency of the ventricular myocardium were measured. When compared to either group of controls, pups continuously exposed to dietary ethanol expressed significantly (P < 0.01) lower body weights. Pups maintained by the pair-fed females had significantly (P < 0.01) lower body weights at days 5 and 10 than pups maintained by the chow-fed females, indicating a pair-fed effect of suboptimal nutrition of the model. As the pups developed, the heart weights of pups maintained by the chow-fed females became progressively greater (P < 0.01) than the heart weights of pups maintained by the pair-fed and ethanol-fed females, which expressed no weight difference. The reduction of heart weight present in the ethanol-fed and pair-fed pups represents a pair-fed effect of suboptimal nutrition and not an obvious effect of exposure to dietary ethanol. The ratio of heart weight to body weight and mitotic frequency were significantly greater (P < 0.01) in 1- to 5-day-old pups exposed to ethanol. Following day 5, these parameters decreased and approached the control values. This indicates that growth of cardiac tissue is not suppressed in the 1- to 5-day-old rat pups exposed continuously to dietary ethanol. These observations further suggest the presence of a mechanism intrinsic to the heart which can provide stage-dependent protection from the adverse effects of ethanol during early development. The decline in heart weight to body weight ratios and mitotic frequency in pups of ethanol-fed females also suggests that ethanol may initiate suppression of the growth of cardiac tissue or may incur stage-dependent injury during the later stages of development. The possible mechanism of this stage-dependent protection during early neonatal development is an increased mitotic activity of the cardiac myocytes.

Animals↗

The c-myc proto-oncogene regulates cardiac development in transgenic mice.

During the maturation of the cardiac myocyte, a transition occurs from hyperplastic to hypertrophic growth. The factors that control this transition in the developing heart are unknown. Proto-oncogenes such as c-myc have been implicated in the regulation of cellular proliferation and differentiation, and in the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc can influence myocyte proliferation or differentiation, we examined the in vivo effect of increasing c-myc expression during embryogenesis and of preventing the decrease in c-myc mRNA expression that normally occurs during cardiac development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. In these transgenic mice, increased c-myc mRNA expression was found to be associated with both atrial and ventricular enlargement. This increase in cardiac mass was secondary to myocyte hyperplasia, with the transgenic hearts containing more than twice as many myocytes as did nontransgenic hearts. The results suggest that in the transgenic animals there is additional hyperplastic growth during fetal development. However, this additional proliferative growth is not reflected in abnormal myocyte maturation, as assessed by the expression of the cardiac and skeletal isoforms of alpha-actin. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth and suggest a regulatory role for this proto-oncogene in cardiac myogenesis.

Aging↗

Diversity of Ca2+ signaling in developing cardiac cells.

During embryonic and postnatal development, the mammalian heart undergoes rapid morphological changes with cellular differentiation that at the ultrastructural level encompasses altered expression and organization of the proteins and organelles associated with Ca(2+) signaling. Here the development and roles of the releasable Ca(2+) stores located within the sarco/endoplasmic reticulum and possibly within the nuclear envelopes are addressed. Confocal Ca(2+) imaging experiments were carried out on (i) neonatal rat cardiomyocytes, (ii) pluripotent P19 stem cells, differentiated to a cardiac phenotype by culturing with 1% dimethylsulfoxide (DMSO) in hanging droplets, and (iii) mouse embryonic cardiomyocytes isolated for short-time culture at embryonic day 9-18. The Ca(2+) release channels in neonatal and "cardiac" P19 cell were activated versus inhibited by targeting ryanodine (Ry) receptors with caffeine versus Ry and IP(3) receptors with adenosine 5'-triphosphate (ATP) or histamine versus U-73122, a phospholipase c (PLC) inhibitor. The neonatal cells displayed four recognizable phenotypes, of which two had specialized Ca(2+) stores releasable via either Ry or IP(3) receptors, and two had both types of receptors, either controlling functionally separate stores or with some degree of overlap, so that caffeine could deplete the stores releasable by ATP. The P19 cells showed variable presence of IP(3)-mediated Ca(2+) stores, and caffeine releasable stores that gained prominence in the "cardiac" phenotype, but were absent in a "neuronal" phenotype. The different roles of Ca(2+) stores were seen clearly in the mouse embryonic cells. Some cells from early stages of development (E 9-10) had Ca(2+) waves that increased in intensity during the diastolic interval and could trigger synchronous electrical excitation (via Na-Ca exchanger [NCX] and excitatory Ca(2+) and Na(+) channels). At later stages of development (E 18) we observed diastolic Ca(2+) sparks that appeared to originate from the nuclear envelope, while the Ca(2+) signals during excitation were faster and stronger in the nuclear region than in the surrounding cytoplasmic regions. However, we also found cells where the nuclear Ca(2+) signals were weaker and showed afterglow compared to the cytosolic Ca(2+) transients. We conclude that the Ca(2+) stores in cardiac cells during embryogenesis and postnatal development, that is, before the maturation of the t-tubular system and in stem cells with cardiac phenotype, show considerable diversity with respect to the pharmacology of the release channels and that regional differences in Ca(2+) signaling are observed centered in, at, and around the nucleus. We suggest that the causal relationship excitation and subcellular Ca(2+) signals in developing cardiac cells is different from that of adult cells and that the developing cardiomyocytes show a diversity that in later stages of development may be reflected in the different properties of atrial, ventricular, and pacemaker cells.

Animals↗

Roles of JUMONJI in mouse embryonic development.

Cardiac development is a complex biological process requiring the integration of cell specification, differentiation, migration, proliferation, and morphogenesis. Although significant progress has been made recently in understanding the molecular basis of cardiac development, mechanisms of transcriptional control of cardiac development remain largely unknown. In search for the developmentally important genes, the jumonji gene (jmj) was identified by gene trap technology and characterized as a critical nuclear factor for mouse embryonic development. Jmj has been shown to play important roles in cardiovascular development, neural tube fusion process, hematopoiesis, and liver development in mouse embryos. The amino acid sequence of the JUMONJI protein (JMJ) reveals that JMJ belongs to the AT-rich interaction domain transcription factor family and more recently has been described as a member of the JMJ transcription factor family. Here, we review the roles of jmj in multiple organ development with a focus on cardiovascular development in mice.

Animals↗

Role of sympathetic nerves during developing cardiac hypertrophy in Grollman hypertensive rats.

Peak left ventricular (LV) function, during rapid volume expansion, and cardiocyte structure were studied in rats with developing cardiac hypertrophy in response to Grollman hypertension (1 kidney, 1 figure 8) after chemical sympathectomy with 6-hydroxydopamine. This form of renovascular hypertension led to the same magnitude of hypertrophy in rats with or without sympathectomy. Indices of peak LV function, measured during acute volume expansion, tended to be normal or slightly higher in hypertensive rats than in controls. Sympathectomy in rats with hypertension significantly improved cardiac and stroke indices while decreasing total peripheral resistance at peak cardiac output. Despite similar magnitudes of LV hypertrophy (LVH) in the two hypertensive groups, cardiocytes in sympathectomized rats had higher mitochondrial volume densities and slightly lower myofibrillar volume densities. After regional sympathectomy of the anterior portion of the LV with phenol, mitochondrial volume density increased by 21% in hypertensive rats with LVH. These data indicate that, during the development of LVH in response to renovascular hypertension, sympathetic nerves do not contribute to the magnitude of LVH but may limit improvement in peak LV performance in response to increased preload. However, sympathetic nerves do play a role in the regulation of mitochondrial and myofibril growth.

Animals↗

Identification and detection of the periostin gene in cardiac development.

Periostin, a member of the fasciclin gene family, acts as a cell adhesion molecule through binding to cell surface integrins. Periostin expression has previously been shown to increase substantially following transforming growth factor beta (TGF-beta) and bone morphogenetic protein stimulation. As these molecules are indispensable for cardiac development, we sought to clone the chicken ortholog of periostin and evaluate its spatiotemporal expression pattern during heart morphogenesis. We show by Northern analysis, whole mount and section in situ hybridization experiments that periostin is predominantly expressed in the developing endothelium of the ventricular trabeculae as well as in the endothelium and mesenchyme of the outflow tract and atrioventricular endocardial cushions. Cardiac expression continues into fetal development where periostin is seen predominantly in the valve leaflets and supporting chordae tendinae.

Amino Acid Sequence↗