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Differential regulation of Hand1 homodimer and Hand1-E12 heterodimer activity by the cofactor FHL2.

The basic helix-loop-helix (bHLH) factor Hand1 plays an essential role in cardiac morphogenesis, and yet its precise function remains unknown. Protein-protein interactions involving Hand1 provide a means of determining how Hand1-induced gene expression in the developing heart might be regulated. Hand1 is known to form either heterodimers with near-ubiquitous E-factors and other lineage-restricted class B bHLH proteins or homodimers with itself in vitro. To date, there have been no reported Hand1 protein interactions involving non-bHLH proteins. Heterodimer-versus-homodimer choice is mediated by the phosphorylation status of Hand1; however, little is known about the in vivo function of these dimers or, importantly, how they are regulated. In an effort to understand how Hand1 activity in the heart might be regulated postdimerization, we have investigated tertiary Hand1-protein interactions with non-bHLH factors. We describe a novel interaction of Hand1 with the LIM domain protein FHL2, a known transcriptional coactivator and corepressor expressed in the developing cardiovascular system. FHL2 interacts with Hand1 via the bHLH domain and is able to repress Hand1/E12 heterodimer-induced transcription but has no effect on Hand1/Hand1 homodimer activity. This effect of FHL2 is not mediated either at the level of dimerization or via an effect of Hand1/E12 DNA binding. In summary, our data describe a novel differential regulation of Hand1 heterodimers versus homodimers by association of the cofactor FHL2 and provide insight into the potential for a tertiary level of control of Hand1 activity in the developing heart.

Animals↗

Cardiac specific differentiation of mouse embryonic stem cells.

Embryonic stem (ES) cells may represent an alternative source of functionally intact cardiomyocytes for the causal treatment of cardiovascular diseases. However, this requires cardiac-specific differentiation of stem cells and the selection of pure lineages consisting of early embryonic cardiomyocytes. Therefore, an understanding of the basic mechanisms of heart development is essential for selective differentiation of embryonic stem cells into cardiac cells. The development of cardiac cells from embryonic stem cells is regulated by several soluble factors and signalling molecules together with cardiac specific transcription factors such as the zinc-finger GATA proteins and Nkx-2.5. GATA-4 and Nkx-2.5 seem to be essential for heart development. The use of enhanced green fluorescent protein (EGFP) under the control of cardiac-specific promoters in combination with the ES cell system has allowed for the functional characterisation of cardiac precursor cells. Embryonic stem cell-derived cardiomyocytes developmentally express similar cardiac-specific proteins, ion channels and signalling molecules to that of adult cardiomyocytes. Furthermore, identification of growth factors and signalling molecules under cell culture conditions is crucial for the selective cardiac differentiation of embryonic stem cells. Therefore, serum-free culture conditions have to be established in order to examine the influence of different growth factors and signalling molecules on cardiac development and/or formation from ES cells. Although significant progress has been made in generating cardiac cell lineage by the combination of genetically manipulative methods with selective culture conditions for cell transplantation therapy, one of the remaining future challenges for transplantation in humans is the immunological rejection of the engrafted cardiomyocytes.

Animals↗

Induction of antibodies reactive to cardiac myosin and development of heart alterations in cruzipain-immunized mice and their offspring.

Human and murine infection with Trypanosoma cruzi parasite is usually accompanied by strong humoral and cellular immune response to cruzipain, a parasite immunodominant antigen. In the present study we report that the immunization of mice with cruzipain devoid of enzymatic activity, was able to induce antibodies which bind to a 223-kDa antigen from a mouse heart extract. We identified this protein as the mouse cardiac myosin heavy chain by sequencing analysis. The study of IgG isotype profile revealed the occurrence of all IgG isotypes against cruzipain and myosin. IgG1 showed the strongest reactivity against cruzipain, whereas IgG2a was the main isotype against myosin. Anti-cruzipain antibodies purified by immunoabsorption recognized the cardiac myosin heavy chain, suggesting cross-reactive epitopes between cruzipain and myosin. Autoimmune response in mice immunized with cruzipain was associated to heart conduction disturbances. In addition, ultrastructural findings revealed severe alterations of cardiomyocytes and IgG deposit on heart tissue of immunized mice. We investigated whether antibodies induced by cruzipain transferred from immunized mothers to their offsprings could alter the heart function in the pups. All IgG isotypes against cruzipain derived from transplacental crossing were detected in pups' sera. Electrocardiographic studies performed in the offsprings born to immunized mothers revealed conduction abnormalities. These results provide strong evidence for a pathogenic role of autoimmune response induced by a purified T. cruzi antigen in the development of experimental Chagas' disease.

Amino Acid Sequence↗

[The relationship between HCY-2 gene and congenital heart teratogenesis in early chick embryos].

OBJECTIVE: To explore the relationship between novel gene (HCY-2), homocysteine and congenital heart defects, and to study their biological mechanisms. METHODS: The teratogenic test of chick embryos which were treated with D.L-homocysteine (0.16 micromol/embryo) was used to observe the developmental toxicity of homocysteine per se. The eukaryotic expressing vector containing whole-length HCY-2 cDNA (2 014 bp) was microinjected into fertile eggs with lipofect AMINE(TM) reagent; then the effects of HCY-2 gene on cardiovascular damages of embryos, the expression and distribution of HCY-2 gene coding product were investigated with the techniques of Western blot, immunohistochemistry, light and electron microscopy. RESULTS: Apparently teratogenic action of homocysteine on developing heart was observed in the critical stage of organogenesis and the damages were concentration dependent (P < 0.05). 10 approximately 20 microgram HCY-2 gene could cause dysmorphogenesis of the cardiovascular system. The congenital heart defect rates were 16.7% and 24.0% respectively. The abnormal forms were ectopic cardis extrathoracic heart and hypoplastic heart. Western blot and immunohistochemical staining showed that HCY-2 protein was largely expressed in the abnormal embryos, with stronger staining at the sites of embryonic heart and brain as compared with controls. Under light and electron microscope, it was seen that HCY-2 gene resulted in structure disturbance, endocardial cushion defect and hypoplasia of heart. CONCLUSION: Homocysteine/HCY-2 gene may be a new heart-developing cytotoxic/genotoxic factor, and HCY-2 gene may play a very important role in the mechanisms of congenital heart defects of chick, and probably humans as well.

Animals↗

Cardiac looping--an uneasy deal with laterality.

Cardiac looping is a key morphogenetic event in vertebrate heart development. In a complex progression, the linear heart tube adopts a sweeping rightward curvature, establishing an architecture for the multichambered organ it soon becomes. Looping is an asymmetric event, its rightward direction being determined by an embryonic left-right axial pathway. The dextral loop has been a vehicle for development of heart complexity during evolution, to the extent that in higher vertebrates and humans, heart structure and function is extremely susceptible to perturbations of laterality, which may be common. The first clues to genetic control of laterality and looping are now emerging. Key issues to resolve include the origin of laterality information in the embryo, the exact nature of the effector molecules that link laterality to looping and the intrinsic mechanisms that drive the looping process itself.

Animals↗

A role for fibronectin in the migration of avian precardiac cells. I. Dose-dependent effects of fibronectin antibody.

An anterior-posterior concentration difference of fibronectin associated with the endoderm in early chick embryos has been implicated in the directional migration of precardiac mesoderm cells. We have examined the effect of increasing concentrations of an antibody to fibronectin (FN) to test the essentiality of FN to precardiac cell migration. For controls embryos were incubated in the presence of antibodies produced against several other extracellular components, such as laminin and anti-collagen types I and IV, as well as against integrin, a cell surface FN receptor. Embryos were also incubated in the presence of a high concentration of exogenous FN, as well as in the presence of an RGD-containing synthetic pentapeptide that is recognized by the FN receptor. After incubation of chick embryos in various concentrations of anti-FN (5 to 80 micrograms/ml), a dose-dependent effect of anti-fibronectin was observed, whereby heart development was arrested at high concentrations of anti-FN. Early developmental stages were more susceptible to lower antibody concentrations than later stages. Incubation in the presence of the RGD-containing synthetic peptide resulted in partial cardiabifida. None of the antibodies serving as controls affected cell migration or early heart development. These results support the hypothesis that FN is a major component in the migratory pathway and plays a role in the directional migration of precardiac cells to the embryonic midline.

Animals↗

Inwardly rectifying potassium current in rat fetal and neonatal ventricular cardiomyocytes.

Whole cell and single-channel inwardly rectifying potassium currents (IK1) of freshly isolated single fetal (12 and 18 days) and neonatal (1, 5, and 10 days) rat ventricular myocytes were recorded using patch-clamp techniques. Whole cell IK1 was elicited by hyperpolarizing test pulses from a holding potential of -40 mV. IK1 densities increased markedly during heart development between fetal day 12 and until neonatal day 5; there was no further increase on neonatal day 10. Cell-attached patch recordings of single IK1 channels were employed with 150 mM K+ both in the pipette and bath solutions. Large-conductance (31 pS) K+ channels with short openings were observed both in fetal day 18 and neonatal day 5 but not in fetal day 12 cells. Only small-conductance (11 pS) channels with long openings were observed in approximately 10% of the cells on fetal day 12. The open probability of the large-conductance channels was 2.6-fold greater on neonatal day 5 than on fetal day 18. In conclusion, the increase in the IK1 density during observed heart development may be primarily due to an increase in the open probability of the IK1 channels, rather than to an increase in their number. We suggest that the small-conductance channel infrequently observed on fetal day 12 could be an immature type of IK1 channel, which disappears during development.

Animals↗

Retinoic acid influences Phox2 expression of cardiac ganglionic cells in the developing rat heart.

We have studied the expression of the homeodomain transcriptional neuronal regulators Phox2a, Phox2b and the non-neuronal Schwann cell response using the marker S-100 in the differentiating phase of cardiac ganglionic cells in rat embryos following exogenous retinoic acid (RA) treatment of pregnant dams. In control embryos, the expression of Phox2b (E11) preceded that of Phox2a, which, along with the terminal neuronal differentiation marker PGP9.5, was expressed from E12 onwards. Phox2b expression remained unchanged in the differentiated phase of cardiac ganglionic cell development after RA treatment, whereas the population of cells expressing Phox2a, PGP9.5 and S-100 was diminished. These results suggest that RA disrupts the differentiation of cardiac neural crest cells into ganglionic cells destined to contribute to the parasympathetic innervation of the heart, by regulating the expression of Phox2a and Phox2b.

Animals↗

Monitoring of teratogenic effects in vitro by analysing a selected gene expression pattern.

The development of in vitro methods for regulatory embryotoxicity testing is challenging since the understanding of chemical effects on the mammalian development is still poor. The aim of the project is to identify marker genes during in vitro cell differentiation of murine embryonic stem cells, in order to predict chemical effects on cell differentiation of specific target tissues. The present study is focusing on the expression pattern by using semi-quantitative reverse transcriptase (RT)-PCR of key genes involved in cardiomyocytes development; i.e. Oct-4, Brachyury, Nkx2.5 and alpha myosin heavy chain (alpha-MHC). Two reference chemicals with well-known in vivo data have been analysed by using this approach: retinoic acid and lithium chloride. Retinoic acid has been selected as a teratogen affecting several target tissues, whereas lithium chloride has been described to affect the development of the cardiovascular system. We demonstrate that retinoic acid already affects in the early stage of germ layer formation, which was demonstrated by a change of Oct-4 and Brachyury gene expression. As we expected, the expression of cardiac specific genes (Nkx2.5, alpha-MHC) has been also modified. In contrary, the Oct-4 and Brachyury expression was not changed by lithium treatment. In this case, we observed a modification in the normal gene expression pattern, for alpha-MHC and Nkx2.5, demonstrating that lithium chloride affects the later stage of heart development. These data suggest that the inclusion of selective target organ genes in an established embryotoxicity test allows to predict effects of chemicals and drugs to the heart development.

Animals↗

Immunochemical analysis of myosin heavy chains in the developing chicken heart.

Monoclonal antibodies (McAb) against myosin from the pectoralis muscle of the adult chicken have been generated and shown to react specifically with the myosin heavy chain (MHC). The reactivities of two such McAbs with myosin from adult chicken atrial and ventricular myocardium were further analysed by immunoautoradiography, radioimmunoassay, and immunofluorescence microscopy. Monoclonal antibody MF 20 was found to bind both atrial and ventricular MHC and stain all striated muscle cells of the adult chicken heart. In contrast, McAb B1 bound specifically to atrial myocytes in immunofluorescence studies, while immunoautoradiography and radioimmunoassay demonstrated the specificity of this antibody for the atrial MHC. Upon reacting these McAbs with myosin isolated from embryonic hearts where definitive atria and ventricles were present, the same specificity of antibody binding was observed. Immunofluorescence studies demonstrated that all striated muscle cells of the embryonic heart contained MHCs recognized by MF 20, while only atrial muscle cells were bound by B1. When extracts of presumptive atrial and ventricular tissue were reacted with MF 20 and B1, significant reactivity of MF 20 was first observed at stage 10 in the presumptive ventricle and thereafter this McAb reacted with all regions of the developing myocardium. Binding of B1 was detected approximately 1 day later at stage 15 and was confined to atrial-forming tissues. These data demonstrate antigenic similarity between adult and embryonic MHC isolated from atrial myocardium and suggest the expression of an atrial-specific MHC early in the regional differentiation of the heart.

Animals↗

Anatomic distribution of autonomic neural tissue in the developing dog heart: II. Nonadrenergic noncholinergic innervation by calcitonin gene-related peptide-immunoreactive tissue.

We used immunocytochemical localization of calcitonin gene-related peptide (CGRP) to trace the ontogenesis and anatomic distribution of this component of nonadrenergic noncholinergic (NANC) innervation in fetal, neonatal, and mature canine hearts and autonomic ganglia which control cardiac function. Rare varicose CGRP-immunoreactive nerve processes were present in the heart during late gestation. Abundant CGRP-immunoreactive neural tissue in the neonate suggested a burst of NANC innervation around birth. Neonatal, 1-, and 2-month-old animals all had many varicose individual nerve processes in addition to processes within bundles; however, the density of all CGRP-immunoreactive tissue appeared to decrease during this stage of development. Similarly, there were relatively more varicose stained nerve processes in the epicardial ganglia and numerous CGRP-immunoreactive cells and smooth nerve processes in the stellate ganglia of the neonate, as compared with older animals. In the mature animal CGRP-immunoreactive neural tissue in the heart was more sparse and largely confined to heterogeneous nerve bundles in the epicardium. The extramural coronary arteries were virtually the only site of innervation by individual nerve processes; CGRP-immunoreactive neural tissue was not found adjacent to working cardiac muscle fibers. At all developmental stages, the area of the sinoatrial node was the primary focus of CGRP innervation, although the atrioventricular nodal region was also preferentially innervated. In general, the atria contained more CGRP-immunoreactive tissue than the ventricles, which were only sparsely innervated. The perinatal peak in density of CGRP-immunoreactive neural tissue with subsequent decline to reach the adult pattern suggests a developmental role for NANC innervation in the dog heart.

Aging↗

Developmentally regulated switching of titin size alters myofibrillar stiffness in the perinatal heart.

Before birth, the compliance of the heart is limited predominantly by extracardiac constraint. Reduction of this constraint at birth requires that myocardial compliance be determined mainly by the heart's own constituents. Because titin is a principal contributor to ventricular passive tension (PT), we studied the expression and mechanics of cardiac-titin isoforms during perinatal rat heart development. Gel electrophoresis and immunoblotting revealed a single, 3.7-MDa, N2BA isoform present 6 days before birth and an additional, also previously unknown, N2BA isoform of 3.5 to 3.6 MDa expressed in the near-term fetus. These large isoforms rapidly disappear after birth and are replaced by a small N2B isoform (3.0 MDa) predominating in 1-week-old and adult rats. In addition, neonatal pig hearts showed large N2BA-titin isoforms distinct from those present in the adult porcine myocardium. By quantitative reverse transcriptase-polymerase chain reaction, developmentally expressed titin-mRNA species were detected in rat heart. Titin-based PT was much lower (approximately 15 times) in fetal than adult rat cardiomyocytes, and measured PT levels were readily predictable with a model of worm-like chain titin elasticity. Immunofluorescence microscopy showed the extensibility of the differentially spliced molecular spring regions of fetal/neonatal titin isoforms in isolated rat cardiomyofibrils. Whereas the titin-isoform shift by 700 kDa ensures high passive stiffness of the postnatal cardiac myofibrils, the expression of specific fetal/neonatal cardiac-titin isoforms may also have important functions for contractile properties, myofibril assembly or turnover, and myocardial signaling during perinatal heart development.

Alternative Splicing↗

Regulation and function of tinman during dorsal mesoderm induction and heart specification in Drosophila.

The homeobox gene tinman plays a key role in the specification of Drosophila heart progenitors and the visceral mesoderm of the midgut, both of which arise at defined positions within dorsal areas of the mesoderm. Here, we show that in addition to the heart and midgut visceral mesoderm, tinman is also required for the specification of all dorsal body wall muscles. Thus it appears that the precursors of the heart, visceral musculature, and dorsal somatic muscles are all specified within the same broad domain of dorsal mesodermal tinman expression. Locally restricted activities of tinman are also observed during its early, general mesodermal expression, where tinman is required for the activation of the homeobox gene buttonless in precursors of the "dorsal median" (DM) glial cells along the ventral midline. These observations, together with others showing only mild effects of ectopic tinman expression on heart development, indicate that tinman function is obligatory, but not sufficient to determine individual tissues within the mesoderm. Therefore, we propose that tinman has a role in integrating positional information that is provided by intersecting domains of additional regulators and signals, which may include Wingless, Sloppy Paired, and Hedgehog in the dorsal mesoderm and EGF-signaling at the ventral midline. Previous studies have shown that Dpp acts as an inductive signal from dorsal ectodermal cells to induce tinman expression in the dorsal mesoderm, which, in turn, is needed for heart and visceral mesoderm formation. In the present report, we show that Thickveins, a type I receptor of Dpp, is essential for the transmission of Dpp signals into the mesoderm. Constitutive activity of Tkv in the entire mesoderm induces ectopic tinman expression in the ventral mesoderm, and this results in the ectopic formation of heart precursors in a defined area of the ventrolateral mesoderm. We further show that Screw, a second BMP2/4-related gene product, Tolloid, a BMP1-related protein, and the zinc finger-containing protein Schnurri, are required to allow full levels of tinman induction during this process. It is likely that some of these functional and regulatory properties of tinman are shared by tinman-related genes from vertebrates that have similarly important roles in embryonic heart development.

Animals↗

[Preliminary study of ALK3 downstream genes related to ventricular septum defect].

To investigate the function of ALK3 gene, the gene regulation and the signaling pathway related to ventricular septum defect during heart development. The model mice with ALK3 gene knock-out via alpha-MHC-Cre/lox P system were bred. The mRNA expression level of control group was compared with that of experiment group and ALK3 downstream genes were screened using PCR-select cDNA subtraction microarray. The mRNA of control group was extracted from E11.5 normal mouse hearts, and that of experiment group, from E11.5 hearts of mice with alpha-MHC Cre(+/-) ALK3(F/+) genotype. It was found that the mice with ALK3 gene knock-out produced heart defects involving the interventricular septum. The platelet-activating factors acetylhydrolase and the transcription factor Pax-8 and so on, were down-regulated. However, the Protein Tyrosine Kinase (PTK) of Focal Adhesion Kinase (FAK) subfamily and beta subtype protein 14-3-3 were up-regulated in the alpha-MHC Cre(+/-) ALK3(F/-) mice. These data provide support that ALK3 gene played an important role during heart development. The platelet-activating factors acetylhydrolase and Pax-8 genes could be important ALK3 downstream genes in the BMP signaling pathway during interventricular septum development. PTK and beta subtype protein 14-3-3 might be regulatory factors in this pathway.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

The complex pattern of SMAD signaling in the cardiovascular system.

The SMAD (small mother against decapentaplegic) family comprises transcription factors that function as signal transducers of TGFbeta (transforming growth factor) superfamily members. The number of studies showing expression, activation or involvement of both SMAD and TGFbeta family members in cardiovascular diseases is constantly rising. In this context, the position of SMADs in the diseased heart is particularly interesting because, besides their well-known fibrotic effects, increasing evidence demonstrates direct action of SMADs on cardiomyocytes as well as on the vascular system. In these systems, SMAD proteins are described to have effects on heart development, cell proliferation, cell growth, and apoptosis. As will be discussed in this review, these different consequences of SMAD activation are dependent on different SMAD isoforms, interaction of SMAD with other transcription factors in the particular situation, and modulation of SMAD activity by various kinases. As a result of all these influences, it turns out that activation of SMAD by members of the BMP (bone morphogenetic protein) family, which is a subfamily of the TGFbeta superfamily, is necessary for correct heart development. On the other hand, activation of SMADs by TGFbeta family members results in fibrotic, apoptotic, and anti-hypertrophic processes that are related to a detrimental cardiac remodeling and progression to heart failure.

Animals↗