Sachiko Miyagawa-Tomita

Mesp1 Expression Is the Earliest Sign of Cardiovascular Development

Understanding the molecular mechanism leading to formation of the heart and vasculature during embryogenesis is critically important because malformation of the cardiovascular system is the most frequently occurring type of birth defect. While the hearts of all vertebrates are derived from bilateral paired fields of primary mesodermal cells that are specified to the cardiac lineage during gastrulation, the mechanism for lineage restriction, and the origin of the myocardium and endocardium have not been defined. Recently, we found that a transcription factor, Mesp1, is expressed in almost all precursors of the cardiovascular system and plays an essential role in cardiac morphogenesis. Mesp1 may play a key role in the early specification for cardiac precursor cells.

Active Remodeling of the Coronary Arterial Lesions in the Late Phase of Kawasaki Disease Immunohistochemical Study

BACKGROUND Remodeling of the coronary artery lesions in Kawasaki disease has been observed in longitudinal angiographic studies. However, mechanisms of such remodeling have not yet been elucidated. METHODS AND RESULTS We examined formalin-fixed specimens of the coronary arteries immunohistochemically by using antibodies against vascular growth factors (GFs) and their receptors in 7 children with Kawasaki disease, 9 children with no coronary disease, and 3 adults with atherosclerosis. In the thickened intima at stenotic sites and at recanalized vessels with Kawasaki disease, extensive expression of vascular GFs, such as transforming GF-beta(1), platelet-derived GF-A, and basic fibroblast GF, was observed both within and surrounding smooth muscle cells. Vascular endothelial GF was observed within smooth muscle cells. Furthermore, all of these GFs were strongly expressed in the newly formed microvessels within the intima. In the thinned media, these GFs were focally and weakly expressed. In contrast, these GFs were expressed only in the media in the control children. In cases of adult atherosclerosis, GFs were expressed diffusely in the media but focally and weakly if at all in the intima. CONCLUSIONS Active remodeling of the coronary artery lesions in Kawasaki disease continues in the form of luxuriant intimal proliferation and neoangiogenesis for several years after the onset of the disease. This process is distinct from adult-onset atherosclerosis.

An FGF autocrine loop initiated in second heart field mesoderm regulates morphogenesis at the arterial pole of the heart

In order to understand how secreted signals regulate complex morphogenetic events, it is crucial to identify their cellular targets. By conditional inactivation of Fgfr1 and Fgfr2 and overexpression of the FGF antagonist sprouty 2 in different cell types, we have dissected the role of FGF signaling during heart outflow tract development in mouse. Contrary to expectation, cardiac neural crest and endothelial cells are not primary paracrine targets. FGF signaling within second heart field mesoderm is required for remodeling of the outflow tract: when disrupted, outflow myocardium fails to produce extracellular matrix and TGFβ and BMP signals essential for endothelial cell transformation and invasion of cardiac neural crest. We conclude that an autocrine regulatory loop, initiated by the reception of FGF signals by the mesoderm, regulates correct morphogenesis at the arterial pole of the heart. These findings provide new insight into how FGF signaling regulates context-dependent cellular responses during development.

Activation of Notch1 signaling in cardiogenic mesoderm induces abnormal heart morphogenesis in mouse

Notch signaling is implicated in many developmental processes. In our current study, we have employed a transgenic strategy to investigate the role of Notch signaling during cardiac development in the mouse. Cre recombinase-mediated Notch1 (NICD1) activation in the mesodermal cell lineage leads to abnormal heart morphogenesis, which is characterized by deformities of the ventricles and atrioventricular (AV) canal. The major defects observed include impaired ventricular myocardial differentiation, the ectopic appearance of cell masses in the AV cushion, the right-shifted interventricular septum (IVS) and impaired myocardium of the AV canal. However, the fates of the endocardium and myocardium were not disrupted in NICD1-activated hearts. One of the Notch target genes, Hesr1, was found to be strongly induced in both the ventricle and the AV canal of NICD1-activated hearts. However, a knockout of the Hesr1 gene from NICD-activated hearts rescues only the abnormality of the AV myocardium. We searched for additional possible targets of NICD1 activation by GeneChip analysis and found that Wnt2, Bmp6, jagged 1 and Tnni2 are strongly upregulated in NICD1-activated hearts, and that the activation of these genes was also observed in the absence of Hesr1. Our present study thus indicates that the Notch1 signaling pathway plays a suppressive role both in AV myocardial differentiation and the maturation of the ventricular myocardium.

Targeted Disruption of hesr2 Results in Atrioventricular Valve Anomalies That Lead to Heart Dysfunction

Genes involved in the Notch signaling pathway have been shown to be critical regulators of cardiovascular development. In vitro studies have revealed that the Notch signaling pathway directly regulates transcription of hairy and enhancer of split-related (hesr) genes, encoding basic helix-loop-helix transcription factors. To assess the functional role of hesr genes in cardiovascular development, we generated mice with a targeted disruption of the hesr2 gene and used echocardiography to analyze heart function of the mutant mice. In the early postnatal period, a majority of hesr2 homozygous mice die as a result of congestive heart failure accompanied by pronounced heart enlargement. Transthoracic echocardiography on 5-day-old homozygous mice revealed tricuspid and mitral valve regurgitation and a dilated left ventricular chamber with markedly diminished fractional shortening of the left ventricle. The hemodynamic anomalies were accompanied by morphological changes, such as dysplastic atrioventricular (AV) valves, a perimembranous ventricular septal defect, and a secundum atrial septal defect. AV valve regurgitations attributable to dysplasia of the AV valves were most likely responsible for the heart dysfunction in hesr2 homozygous mice. These observations indicate that the Notch signaling target hesr2 plays an important role in the formation and function of the AV valves. In addition, hesr2 activity may be important for proper development of cardiomyocytes, thereby assuring normal left ventricular contractility. Because of the unique spectrum of cardiac anomalies expressed by hesr2-null mice, they represent a useful model system for elucidating the genetic basis of heart dysfunction.

Role of Mesodermal FGF8 and FGF10 Overlaps in the Development of the Arterial Pole of the Heart and Pharyngeal Arch Arteries

Rationale: The genes encoding fibroblast growth factor (FGF) 8 and 10 are expressed in the anterior part of the second heart field that constitutes a population of cardiac progenitor cells contributing to the arterial pole of the heart. Previous studies of hypomorphic and conditional Fgf8 mutants show disrupted outflow tract (OFT) and right ventricle (RV) development, whereas Fgf10 mutants do not have detectable OFT defects. Objectives: Our aim was to investigate functional overlap between Fgf8 and Fgf10 during formation of the arterial pole. Methods and Results: We generated mesodermal Fgf8; Fgf10 compound mutants with MesP1Cre. The OFT/RV morphology in these mutants was affected with variable penetrance; however, the incidence of embryos with severely affected OFT/RV morphology was significantly increased in response to decreasing Fgf8 and Fgf10 gene dosage. Fgf8 expression in the pharyngeal arch ectoderm is important for development of the pharyngeal arch arteries and their derivatives. We now show that Fgf8 deletion in the mesoderm alone leads to pharyngeal arch artery phenotypes and that these vascular phenotypes are exacerbated by loss of Fgf10 function in the mesodermal core of the arches. Conclusions: These results show functional overlap of FGF8 and FGF10 signaling from second heart field mesoderm during development of the OFT/RV, and from pharyngeal arch mesoderm during pharyngeal arch artery formation, highlighting the sensitivity of these key aspects of cardiovascular development to FGF dosage.

Morphological Observations on the Pathogenetic Process of Transposition of the Great Arteries Induced by Retinoic Acid in Mice

BACKGROUND The pathogenesis of complete transposition of the great arteries (TGA) is still controversial because useful animal models have not been established. We previously reported that all-trans retinoic acid induced complete TGA at a high proportion in mice. The aim of the present study was to clarify the morphogenesis of the cardiac outflow tract in the retinoic acid-treated embryos destined to develop TGA. METHODS AND RESULTS We first examined the morphology of TGA in mouse fetuses treated with retinoic acid to establish an animal model of TGA (experiment 1) and then examined the retinoic acid-treated embryonic hearts by means of ink injection and histology (experiment 2). All mouse fetuses and embryos showed visceroatrial situs solitus and d-ventricular loop. In experiment 1, among 45 embryos treated with retinoic acid 70 mg/kg at day 8.5 of gestation, 35 (78%) had TGA and 3 (6.7%) had a double-outlet right ventricle with a subpulmonary ventricular septal defect. In experiment 2, all hearts already exhibited d-loop at gestation day 8.5. At gestation day 9.5, conus swellings, composed of acellular cardiac jelly, where hypoplastic, and the conotruncal cavity was nonspiral or tubular. At gestation day 11.0, aberrant conus swellings located anteroposteriorly to give a straight orientation to the conotruncal cavity. At gestation day 12.0, side-by-side great arteries were transposed in that the aorta arose from the right ventricle and the pulmonary artery arose above the interventricular foramen. CONCLUSIONS These results suggest that a reproducible animal model of TGA can be produced in mice by treatment with retinoic acid; that there was no loop anomaly, such as an A-loop or L-loop, in our model; and that hypoplasia of the conus swellings appears to be the primary event leading to TGA.

Immunohistochemical study of apparently intact coronary artery in a child after Kawasaki disease

Abstract Background : Coronary arterial lesions (CAL) due to Kawasaki disease (KD) often show progressive intimal hyperplasia even many years after the disease. However, most patients have no CAL after the acute phase, and it is an important issue whether or not coronary arteries without CAL have significant intimal hyperplasia, and whether or not there is the potential for this to progress to stenosis and/or atherosclerosis.

Mesp1-nonexpressing cells contribute to the ventricular cardiac conduction system

Previous fate mapping analysis, using Cre recombinase driven by the Mesp1 locus, revealed that Mesp1 is expressed in almost all of the precursors of the cardiovascular system, including the endothelium, endocardium, myocardium, and epicardium. Mesp1‐nonexpressing cells were found to be restricted to the outflow tract cushion and along the interventricular septum (IVS), which is a location that is suggestive of specialized cardiac conduction system (CCS). In our current study, we examined the identity of these IVS cells by using the pattern of β‐galactosidase activity in CCS‐lacZ mice. In addition, by crossing Mesp1‐Cre and floxed GFP reporter mice with CCS‐lacZ mice, we have calculated that approximately 20% of the ventricular CCS within the IVS corresponds to Mesp1‐nonexpressing cells. These data suggest that the ventricular CCS is of heterocellular origin. Furthermore, we indicate a possibility that a population of the cells that contribute to the ventricular CCS might be distinguished at an early stage of development. Developmental Dynamics 235:395–402, 2006.

Development of cranial nerves in the chick embryo with special reference to the alterations of cardiac branches after ablation of the cardiac neural crest.

SummaryDevelopment of cranial nerve branches in the cardiac region was observed in whole-mount specimens which were stained with a monoclonal antibody, E/C8, after the ablation of the cardiac neural crest. In early embryos, nerve trunks of IX and X were lacking or only poorly developed, while the early development of pharyngeal branch primordia was normal. In day 5 embryos, the nerve trunks of IX–X were present in all the embryos, however; extensive communication was observed between X and XII. On day 6 and later, the spiral pattern of superior cardiac branches was disturbed, as were the blood vessels. Furthermore, the distal branches of XII passed within the superficial layer of cardiac outflow mesenchyme. Vagal branches passed within the deeper layer. There was no apparent change in the development of the sinal branch. Using quail — chick chimeras, it was found that the cardiac neural crest cells formed the Schwann cells of XII, and that they were also associated with the hypobranchial muscle primordium, suggesting that the absence of the cardiac neural crest not only disturbs the development of the cardiac outflow septation, but also affects the normal morphogenesis of the hypobranchial musculature and its innervation. Embryologically, the tongue is located close to the cardiac outflow tract, which is the migration pathway of the cardiac neural crest-derived cells.