A field of myocardial-endocardial NFAT signaling underlies heart valve morphogenesis

Moving domain computational fluid dynamics to interface with an embryonic model of cardiac morphogenesis

Peristaltic contraction of the embryonic heart tube produces time- and spatial-varying wall shear stress (WSS) and pressure gradients (∇P) across the atrioventricular (AV) canal. Zebrafish (Danio rerio) are a genetically tractable system to investigate cardiac morphogenesis. The use of Tg(fli1a:EGFP) (y1) transgenic embryos allowed for delineation and two-dimensional reconstruction of the endocardium. This time-varying wall motion was then prescribed in a two-dimensional moving domain computational fluid dynamics (CFD) model, providing new insights into spatial and temporal variations in WSS and ∇P during cardiac development. The CFD simulations were validated with particle image velocimetry (PIV) across the atrioventricular (AV) canal, revealing an increase in both velocities and heart rates, but a decrease in the duration of atrial systole from early to later stages. At 20-30 hours post fertilization (hpf), simulation results revealed bidirectional WSS across the AV canal in the heart tube in response to peristaltic motion of the wall. At 40-50 hpf, the tube structure undergoes cardiac looping, accompanied by a nearly 3-fold increase in WSS magnitude. At 110-120 hpf, distinct AV valve, atrium, ventricle, and bulbus arteriosus form, accompanied by incremental increases in both WSS magnitude and ∇P, but a decrease in bi-directional flow. Laminar flow develops across the AV canal at 20-30 hpf, and persists at 110-120 hpf. Reynolds numbers at the AV canal increase from 0.07±0.03 at 20-30 hpf to 0.23±0.07 at 110-120 hpf (p< 0.05, n=6), whereas Womersley numbers remain relatively unchanged from 0.11 to 0.13. Our moving domain simulations highlights hemodynamic changes in relation to cardiac morphogenesis; thereby, providing a 2-D quantitative approach to complement imaging analysis.

Epicardial calcineurin-NFAT signals through Smad2 to direct coronary smooth muscle cell and arterial wall development

AIMS

Congenital coronary artery anomalies produce serious events that include syncope, arrhythmias, myocardial infarction, or sudden death. Studying the mechanism of coronary development will contribute to the understanding of the disease and help design new diagnostic or therapeutic strategies. Here, we characterized a new calcineurin-NFAT signalling which specifically functions in the epicardium to regulate the development of smooth muscle wall of the coronary arteries.

METHODS AND RESULTS

Using tissue-specific gene deletion, we found that calcineurin-NFAT signals in the embryonic epicardium to direct coronary smooth muscle cell development. The smooth muscle wall of coronary arteries fails to mature in mice with epicardial deletion of calcineurin B1 (Cnb1), and accordingly these mutant mice develop cardiac dysfunction with reduced exercise capacity. Inhibition of calcineurin at various developmental windows shows that calcineurin-NFAT signals within a narrow time window at embryonic Day 12.5-13.5 to regulate coronary smooth muscle cell development. Within the epicardium, NFAT transcriptionally activates the expression of Smad2, whose gene product is critical for transducing transforming growth factor β (TGFβ)-Alk5 signalling to control coronary development.

CONCLUSION

Our findings demonstrate new spatiotemporal and molecular actions of calcineurin-NFAT that dictate coronary arterial wall development and a new mechanism by which calcineurin-NFAT integrates with TGFβ signalling during embryonic development.

Accelerated coronary angiogenesis by vegfr1-knockout endocardial cells

During mouse heart development, ventricular endocardial cells give rise to the coronary arteries by angiogenesis. Myocardially-derived vascular endothelial growth factor-a (Vegfa) regulates embryonic coronary angiogenesis through vascular endothelial growth factor-receptor 2 (Vegfr2) expressed in the endocardium. In this study, we investigated the role of endocardially-produced soluble Vegfr1 (sVegfr1) in the coronary angiogenesis. We deleted sVegfr1 in the endocardium of the developing mouse heart and found that this deletion resulted in a precocious formation of coronary plexuses. Using an ex vivo coronary angiogenesis assay, we showed that the Vegfr1-null ventricular endocardial cells underwent excessive angiogenesis and generated extensive endothelial tubular networks. We also revealed by qPCR analysis that expression of genes involved in the Vegf-Notch pathway was augmented in the Vegfr1-null hearts. We further showed that inhibition of Notch signaling blocked the formation of coronary plexuses by the ventricular endocardial cells. These results establish that Vegfr1 produced in the endocardium negatively regulates embryonic coronary angiogenesis, possibly by limiting the Vegf-Notch signaling.

Bmp signaling represses Vegfa to promote outflow tract cushion development

Congenital heart disease (CHD) is a devastating anomaly that affects ∼1% of live births. Defects of the outflow tract (OFT) make up a large percentage of human CHD. We investigated Bmp signaling in mouse OFT development by conditionally deleting both Bmp4 and Bmp7 in the second heart field (SHF). SHF Bmp4/7 deficiency resulted in defective epithelial to mesenchymal transition (EMT) and reduced cardiac neural crest ingress, with resultant persistent truncus arteriosus. Using a candidate gene approach, we found that Vegfa was upregulated in the Bmp4/7 mutant hearts. To determine if Vegfa is a downstream Bmp effector during EMT, we examined whether Vegfa is transcriptionally regulated by the Bmp receptor-regulated Smad. Our findings indicate that Smad directly binds to Vegfa chromatin and represses Vegfa transcriptional activity. We also found that Vegfa is a direct target for the miR-17-92 cluster, which is also regulated by Bmp signaling in the SHF. Deletion of miR-17-92 reveals similar phenotypes to Bmp4/7 SHF deletion. To directly address the function of Vegfa repression in Bmp-mediated EMT, we performed ex vivo explant cultures from Bmp4/7 and miR-17-92 mutant hearts. EMT was defective in explants from the Bmp4/7 double conditional knockout (dCKO; Mef2c-Cre;Bmp4/7(f/f)) and miR-17-92 null. By antagonizing Vegfa activity in explants, EMT was rescued in Bmp4/7 dCKO and miR-17-92 null culture. Moreover, overexpression of miR-17-92 partially suppressed the EMT defect in Bmp4/7 mutant embryos. Our study reveals that Vegfa levels in the OFT are tightly controlled by Smad- and microRNA-dependent pathways to modulate OFT development.

Tbx20 acts upstream of Wnt signaling to regulate endocardial cushion formation and valve remodeling during mouse cardiogenesis

Cardiac valves are essential to direct forward blood flow through the cardiac chambers efficiently. Congenital valvular defects are prevalent among newborns and can cause an immediate threat to survival as well as long-term morbidity. Valve leaflet formation is a rigorously programmed process consisting of endocardial epithelial-mesenchymal transformation (EMT), mesenchymal cell proliferation, valve elongation and remodeling. Currently, little is known about the coordination of the diverse signals that regulate endocardial cushion development and valve elongation. Here, we report that the T-box transcription factor Tbx20 is expressed in the developing endocardial cushions and valves throughout heart development. Ablation of Tbx20 in endocardial cells causes severe valve elongation defects and impaired cardiac function in mice. Our study reveals that endocardial Tbx20 is crucial for valve endocardial cell proliferation and extracellular matrix development, but is not required for initiation of EMT. Elimination of Tbx20 also causes aberrant Wnt/β-catenin signaling in the endocardial cushions. In addition, Tbx20 regulates Lef1, a key transcriptional mediator for Wnt/β-catenin signaling, in this developmental process. Our study suggests a model in which Tbx20 regulates the Wnt pathway to direct endocardial cushion maturation and valve elongation, and provides new insights into the etiology of valve defects in humans.

Nfatc1 directs the endocardial progenitor cells to make heart valve primordium

Heart valves arise from the cardiac endocardial cushions located at the atrioventricular canal (AVC) and cardiac outflow tract (OFT) during development. A subpopulation of cushion endocardial cells undergoes endocardial to mesenchymal transformation (EMT) and generates the cushion mesenchyme, which is then remodeled into the interstitial tissue of the mature valves. The cushion endocardial cells that do not undertake EMT proliferate to elongate valve leaflets. During EMT and the post-EMT valve remodeling, endocardial cells at the cushions highly express nuclear factor in activated T cell, cytoplasmic 1 (Nfatc1), a transcription factor required for valve formation in mice. In this review, we present the current knowledge of Nfatc1 roles in the ontogeny of heart valves with a focus on the fate decision of the endocardial cells in the processes of EMT and valve remodeling.

Brg1 governs a positive feedback circuit in the hair follicle for tissue regeneration and repair

Hair follicle stem cells (bulge cells) are essential for hair regeneration and early epidermal repair after wounding. Here we show that Brg1, a key enzyme in the chromatin-remodeling machinery, is dynamically expressed in bulge cells to control tissue regeneration and repair. In mice, sonic hedgehog (Shh) signals Gli to activate Brg1 in bulge cells to begin hair regeneration, whereas Brg1 recruits NF-κB to activate Shh in matrix cells to sustain hair growth. Such reciprocal Brg1-Shh interaction is essential for hair regeneration. Moreover, Brg1 is indispensable for maintaining the bulge cell reservoir. Without Brg1, bulge cells are depleted over time, partly through the ectopic expression of the cell-cycle inhibitor p27(Kip1). Also, bulge Brg1 is activated by skin injury to facilitate early epidermal repair. Our studies demonstrate a molecular circuit that integrates chromatin remodeling (Brg1), transcriptional regulation (NF-κB, Gli), and intercellular signaling (Shh) to control bulge stem cells during tissue regeneration.

miR-21 represses Pdcd4 during cardiac valvulogenesis

The discovery of small non-coding microRNAs has revealed novel mechanisms of post-translational regulation of gene expression, the implications of which are still incompletely understood. We focused on microRNA 21 (miR-21), which is expressed in cardiac valve endothelium during development, in order to better understand its mechanistic role in cardiac valve development. Using a combination of in vivo gene knockdown in zebrafish and in vitro assays in human cells, we show that miR-21 is necessary for proper development of the atrioventricular valve (AV). We identify pdcd4b as a relevant in vivo target of miR-21 and show that protection of pdcd4b from miR-21 binding results in failure of AV development. In vitro experiments using human pulmonic valve endothelial cells demonstrate that miR-21 overexpression augments endothelial cell migration. PDCD4 knockdown alone was sufficient to enhance endothelial cell migration. These results demonstrate that miR-21 plays a necessary role in cardiac valvulogenesis, in large part due to an obligatory downregulation of PDCD4.

The Wilms' tumor suppressor Wt1 regulates Coronin 1B expression in the epicardium

Coronin 1B has been shown to be critical for cell motility and various actin-dependent processes. To understand its role more extensively, the expression and transcriptional regulation of Coro1b gene during mouse development were explored. Coronin 1B is ubiquitously expressed in the whole embryo but nevertheless shows distinct expression pattern in developing heart. In addition to the localization in endocardium, Coronin 1B is specifically expressed in the endocardial cushion and epicardium where cardiac EMT processes take place as the heart develops. Promoter deletion analysis identified the positions between -1038 and -681 is important for Coro1b basal promoter activity. In addition to a correlation of Coronin 1B localization with Wt1 expression in the epicardium, we also identified putative Wt1 binding sequences within Coro1b promoter. Direct binding of Wt1 to GC-rich sequences within the Coro1b promoter is required for the regulation of Coro1b gene expression. In accordance with the motility defect found in Coronin 1B-knockdown cells, a modest decrease in expression of Coronin 1B in the remaining epicardium of Wt1(EGFPCre/EGFPCre) mutant embryos was observed. These findings seem to shed light on the role of Wt1 during cell migration and suggest that, at least in part, this involves transcriptional control of Coro1b gene expression.

Endocardial to myocardial notch-wnt-bmp axis regulates early heart valve development

Endocardial to mesenchymal transformation (EMT) is a fundamental cellular process required for heart valve formation. Notch, Wnt and Bmp pathways are known to regulate this process. To further address how these pathways coordinate in the process, we specifically disrupted Notch1 or Jagged1 in the endocardium of mouse embryonic hearts and showed that Jagged1-Notch1 signaling in the endocardium is essential for EMT and early valvular cushion formation. qPCR and RNA in situ hybridization assays reveal that endocardial Jagged1-Notch1 signaling regulates Wnt4 expression in the atrioventricular canal (AVC) endocardium and Bmp2 in the AVC myocardium. Whole embryo cultures treated with Wnt4 or Wnt inhibitory factor 1 (Wif1) show that Bmp2 expression in the AVC myocardium is dependent on Wnt activity; Wnt4 also reinstates Bmp2 expression in the AVC myocardium of endocardial Notch1 null embryos. Furthermore, while both Wnt4 and Bmp2 rescue the defective EMT resulting from Notch inhibition, Wnt4 requires Bmp for its action. These results demonstrate that Jagged1-Notch1 signaling in endocardial cells induces the expression of Wnt4, which subsequently acts as a paracrine factor to upregulate Bmp2 expression in the adjacent AVC myocardium to signal EMT.

Cardiogenic differentiation of mesenchymal stem cells on elastomeric poly (glycerol sebacate)/collagen core/shell fibers

AIM: To facilitate engineering of suitable biomaterials to meet the challenges associated with myocardial infarction.

METHODS: Poly (glycerol sebacate)/collagen (PGS/collagen) core/shell fibers were fabricated by core/shell electrospinning technique, with core as PGS and shell as collagen polymer; and the scaffolds were characterized by scanning electron microscope (SEM), fourier transform infrared spectroscopy (FTIR), contact angle and tensile testing for cardiac tissue engineering. Collagen nanofibers were also fabricated by electrospinning for comparison with core/shell fibers. Studies on cell-scaffold interaction were carried out using cardiac cells and mesenchymal stem cells (MSCs) co-culture system with cardiac cells and MSCs separately serving as positive and negative controls respectively. The co-culture system was characterized for cell proliferation and differentiation of MSCs into cardiomyogenic lineage in the co-culture environment using dual immunocytochemistry. The co-culture cells were stained with cardiac specific marker proteins like actinin and troponin and MSC specific marker protein CD 105 for proving the cardiogenic differentiation of MSCs. Further the morphology of cells was analyzed using SEM.

RESULTS: PGS/collagen core/shell fibers, core is PGS polymer having an elastic modulus related to that of cardiac fibers and shell as collagen, providing natural environment for cellular activities like cell adhesion, proliferation and differentiation. SEM micrographs of electrospun fibrous scaffolds revealed porous, beadless, uniform fibers with a fiber diameter in the range of 380 ± 77 nm and 1192 ± 277 nm for collagen fibers and PGS/collagen core/shell fibers respectively. The obtained PGS/collagen core/shell fibrous scaffolds were hydrophilic having a water contact angle of 17.9 ± 4.6° compared to collagen nanofibers which had a contact angle value of 30 ± 3.2°. The PGS/collagen core/shell fibers had mechanical properties comparable to that of native heart muscle with a young’s modulus of 4.24 ± 0.7 MPa, while that of collagen nanofibers was comparatively higher around 30.11 ± 1.68 MPa. FTIR spectrum was performed to confirm the functional groups present in the electrospun scaffolds. Amide I and amide II of collagen were detected at 1638.95 cm^-1 and 1551.64 cm^-1 in the electrospun collagen fibers and at 1646.22 cm^-1 and 1540.73 cm^-1 for PGS/collagen core/shell fibers respectively. Cell culture studies performed using MSCs and cardiac cells co-culture environment, indicated that the cell proliferation significantly increased on PGS/collagen core/shell scaffolds compared to collagen fibers and the cardiac marker proteins actinin and troponin were expressed more on PGS/collagen core/shell scaffolds compared to collagen fibers alone. Dual immunofluorescent staining was performed to further confirm the cardiogenic differentiation of MSCs by employing MSC specific marker protein, CD 105 and cardiac specific marker protein, actinin. SEM observations of cardiac cells showed normal morphology on PGS/collagen fibers and providing adequate tensile strength for the regeneration of myocardial infarction.

CONCLUSION: Combination of PGS/collagen fibers and cardiac cells/MSCs co-culture system providing natural microenvironments to improve cell survival and differentiation, could bring cardiac tissue engineering to clinical application.

Calcium influx through L-type CaV1.2 Ca2+ channels regulates mandibular development

The identification of a gain-of-function mutation in CACNA1C as the cause of Timothy Syndrome (TS), a rare disorder characterized by cardiac arrhythmias and syndactyly, highlighted unexpected roles for the L-type voltage-gated Ca2+ channel CaV1.2 in nonexcitable cells. How abnormal Ca2+ influx through CaV1.2 underlies phenotypes such as the accompanying syndactyly or craniofacial abnormalities in the majority of affected individuals is not readily explained by established CaV1.2 roles. Here, we show that CaV1.2 is expressed in the first and second pharyngeal arches within the subset of cells that give rise to jaw primordia. Gain-of-function and loss-of-function studies in mouse, in concert with knockdown/rescue and pharmacological approaches in zebrafish, demonstrated that Ca2+ influx through CaV1.2 regulates jaw development. Cranial neural crest migration was unaffected by CaV1.2 knockdown, suggesting a role for CaV1.2 later in development. Focusing on the mandible, we observed that cellular hypertrophy and hyperplasia depended upon Ca2+ signals through CaV1.2, including those that activated the calcineurin signaling pathway. Together, these results provide new insights into the role of voltage-gated Ca2+ channels in nonexcitable cells during development.

Signaling and transcriptional networks in heart development and regeneration

The mammalian heart is the first functional organ, the first indicator of life. Its normal formation and function are essential for fetal life. Defects in heart formation lead to congenital heart defects, underscoring the finesse with which the heart is assembled. Understanding the regulatory networks controlling heart development have led to significant insights into its lineage origins and morphogenesis and illuminated important aspects of mammalian embryology, while providing insights into human congenital heart disease. The mammalian heart has very little regenerative potential, and thus, any damage to the heart is life threatening and permanent. Knowledge of the developing heart is important for effective strategies of cardiac regeneration, providing new hope for future treatments for heart disease. Although we still have an incomplete picture of the mechanisms controlling development of the mammalian heart, our current knowledge has important implications for embryology and better understanding of human heart disease.

Genetic interaction between pku300 and fbn2b controls endocardial cell proliferation and valve development in zebrafish

Abnormal cardiac valve morphogenesis is a common cause of human congenital heart disease. The molecular mechanisms regulating endocardial cell proliferation and differentiation into cardiac valves remain largely unknown, although great progress has been made on the endocardial contribution to the atrioventricular cushion and valve formation. We found that scotch tape(te382) (sco(te382)) encodes a novel transmembrane protein that is crucial for endocardial cell proliferation and heart valve development. The zebrafish sco(te382) mutant showed diminished endocardial cell proliferation, lack of heart valve leaflets and abnormal common cardinal and caudal veins. Positional cloning revealed a C946T nonsense mutation of a novel gene pku300 in the sco(te382) locus, which encoded a 540-amino-acid protein on cell membranes with one putative transmembrane domain and three IgG domains. A known G3935T missense mutation of fbn2b was also found ∼570 kb away from pku300 in sco(te382) mutants. The genetic mutant sco(pku300), derived from sco(te382), only had the C946T mutation of pku300 and showed reduced numbers of atrial endocardial cells and an abnormal common cardinal vein. Morpholino knockdown of fbn2b led to fewer atrial endocardial cells and an abnormal caudal vein. Knockdown of both pku300 and fbn2b phenocopied these phenotypes in sco(te382) genetic mutants. pku300 transgenic expression in endocardial and endothelial cells, but not myocardial cells, partially rescued the atrial endocardial defects in sco(te382) mutants. Mechanistically, pku300 and fbn2b were required for endocardial cell proliferation, endocardial Notch signaling and the proper formation of endocardial cell adhesion and tight junctions, all of which are crucial for cardiac valve development. We conclude that pku300 and fbn2b represent the few genes capable of regulating endocardial cell proliferation and signaling in zebrafish cardiac valve development.

Endocardial cells form the coronary arteries by angiogenesis through myocardial-endocardial VEGF signaling

The origins and developmental mechanisms of coronary arteries are incompletely understood. We show here by fate mapping, clonal analysis, and immunohistochemistry that endocardial cells generate the endothelium of coronary arteries. Dye tracking, live imaging, and tissue transplantation also revealed that ventricular endocardial cells are not terminally differentiated; instead, they are angiogenic and form coronary endothelial networks. Myocardial Vegf-a or endocardial Vegfr-2 deletion inhibited coronary angiogenesis and arterial formation by ventricular endocardial cells. In contrast, lineage and knockout studies showed that endocardial cells make a small contribution to the coronary veins, the formation of which is independent of myocardial-to-endocardial Vegf signaling. Thus, contrary to the current view of a common source for the coronary vessels, our findings indicate that the coronary arteries and veins have distinct origins and are formed by different mechanisms. This information may help develop better cell therapies for coronary artery disease.

Brg1 governs distinct pathways to direct multiple aspects of mammalian neural crest cell development

Development of the cerebral vessels, pharyngeal arch arteries (PAAs). and cardiac outflow tract (OFT) requires multipotent neural crest cells (NCCs) that migrate from the neural tube to target tissue destinations. Little is known about how mammalian NCC development is orchestrated by gene programming at the chromatin level, however. Here we show that Brahma-related gene 1 (Brg1), an ATPase subunit of the Brg1/Brahma-associated factor (BAF) chromatin-remodeling complex, is required in NCCs to direct cardiovascular development. Mouse embryos lacking Brg1 in NCCs display immature cerebral vessels, aberrant PAA patterning, and shortened OFT. Brg1 suppresses an apoptosis factor, Apoptosis signal-regulating kinase 1 (Ask1), and a cell cycle inhibitor, p21(cip1), to inhibit apoptosis and promote proliferation of NCCs, thereby maintaining a multipotent cell reservoir at the neural crest. Brg1 also supports Myosin heavy chain 11 (Myh11) expression to allow NCCs to develop into mature vascular smooth muscle cells of cerebral vessels. Within NCCs, Brg1 partners with chromatin remodeler Chromodomain-helicase-DNA-binding protein 7 (Chd7) on the PlexinA2 promoter to activate PlexinA2, which encodes a receptor for semaphorin to guide NCCs into the OFT. Our findings reveal an important role for Brg1 and its downstream pathways in the survival, differentiation, and migration of the multipotent NCCs critical for mammalian cardiovascular development.

Two heterozygous mutations in NFATC1 in a patient with Tricuspid Atresia

Tricuspid Atresia (TA) is a rare form of congenital heart disease (CHD) with usually poor prognosis in humans. It presents as a complete absence of the right atrio-ventricular connection secured normally by the tricuspid valve. Defects in the tricuspid valve are so far not associated with any genetic locus, although mutations in numerous genes were linked to multiple forms of congenital heart disease. In the last decade, Knock-out mice have offered models for cardiologists and geneticists to study the causes of congenital disease. One such model was the Nfatc1(-/-) mice embryos which die at mid-gestation stage due to a complete absence of the valves. NFATC1 belongs to the Rel family of transcription factors members of which were shown to be implicated in gene activation, cell differentiation, and organogenesis. We have previously shown that a tandem repeat in the intronic region of NFATC1 is associated with ventricular septal defects. In this report, we unravel for the first time a potential link between a mutation in NFATC1 and TA. Two heterozygous missense mutations were found in the NFATC1 gene in one indexed-case out of 19 patients with TA. The two amino-acids changes were not found neither in other patients with CHDs, nor in the control healthy population. Moreover, we showed that these mutations alter dramatically the normal function of the protein at the cellular localization, DNA binding and transcriptional levels suggesting they are disease-causing.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

Pharmacological manipulation of blood and lymphatic vascularization in ex vivo-cultured mouse embryos

Formation of new blood and lymphatic vessels is involved in many physiological and pathological processes, including organ and tumor growth, cancer cell metastasis, fluid drainage and lymphedema. Therefore, the ability to manipulate vascularization in a mammalian system is of particular interest to researchers. Here we describe a method for pharmacological manipulation of de novo and sprouting blood and lymphatic vascular development in ex vivo-cultured mouse embryos. The described protocol can also be used to evaluate the properties of pharmacological agents in growing mammalian tissues and to manipulate other developmental processes. The whole procedure, from embryo isolation to image quantification, takes 3-5 d, depending on the analysis and age of the embryos.

Endocardial cushion morphogenesis and coronary vessel development require chicken ovalbumin upstream promoter-transcription factor II

OBJECTIVE

Septal defects and coronary vessel anomalies are common congenital heart defects, yet their ontogeny and the underlying genetic mechanisms are not well understood. Here, we investigated the role of chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII, NR2F2) in cardiac organogenesis.

METHODS AND RESULTS

We analyzed embryos deficient in COUP-TFII and observed a spectrum of cardiac defects, including atrioventricular septal defect, thin-walled myocardium, and abnormal coronary morphogenesis. We show by expression analysis that COUP-TFII is expressed in the endocardium and the epicardium but not in the myocardium of the ventricle. Using endothelial-specific COUP-TFII mutants and molecular approaches, we show that COUP-TFII deficiency resulted in endocardial cushion hypoplasia. This was attributed to the reduced growth and survival of atrioventricular cushion mesenchymal cells and defective epithelial-mesenchymal transformation (EMT) in the underlying endocardium. In addition, the endocardial EMT defect was accompanied by downregulation of Snai1, one of the master regulators of EMT, and upregulation of vascular endothelial-cadherin. Furthermore, we show that although COUP-TFII does not play a major role in the formation of epicardial cell cysts, it is critically important for the formation of epicardium. Ablation of COUP-TFII impairs epicardial EMT and coronary plexus formation.

CONCLUSIONS

Our results reveal that COUP-TFII plays cell-autonomous roles in the endocardium and the epicardium for endocardial and epicardial EMT, which are required for proper valve and coronary vessel formation during heart development.

Uncovering the molecular and cellular mechanisms of heart development using the zebrafish

Over the past 20 years, the zebrafish has emerged as a powerful model organism for studying cardiac development. Its ability to survive without an active circulation and amenability to forward genetics has led to the identification of numerous mutants whose study has helped elucidate new mechanisms in cardiac development. Furthermore, its transparent, externally developing embryos have allowed detailed cellular analyses of heart development. In this review, we discuss the molecular and cellular processes involved in zebrafish heart development from progenitor specification to development of the valve and the conduction system. We focus on imaging studies that have uncovered the cellular bases of heart development and on zebrafish mutants with cardiac abnormalities whose study has revealed novel molecular pathways in cardiac cell specification and tissue morphogenesis.

Flow control in our vessels: vascular valves make sure there is no way back

The efficient transport of blood and lymph relies on competent intraluminal valves that ensure unidirectional fluid flow through the vessels. In the lymphatic vessels, lack of luminal valves causes reflux of lymph and can lead to lymphedema, while dysfunction of venous valves is associated with venous hypertension, varicose veins, and thrombosis that can lead to edema and ulcerations. Despite their clinical importance, the mechanisms that regulate valve formation are poorly understood and have only recently begun to be characterized. Here, we discuss new findings regarding the development of venous and lymphatic valves that indicate the involvement of common molecular mechanisms in regulating valve formation in different vascular beds.

Endocardial and epicardial epithelial to mesenchymal transitions in heart development and disease

Epithelial to mesenchymal transition (EMT) converts epithelial cells to mobile and developmentally plastic mesenchymal cells. All cells in the heart arise from one or more EMTs. Endocardial and epicardial EMTs produce most of the noncardiomyocyte lineages of the mature heart. Endocardial EMT generates valve progenitor cells and is necessary for formation of the cardiac valves and for complete cardiac septation. Epicardial EMT is required for myocardial growth and coronary vessel formation, and it generates cardiac fibroblasts, vascular smooth muscle cells, a subset of coronary endothelial cells, and possibly a subset of cardiomyocytes. Emerging studies suggest that these developmental mechanisms are redeployed in adult heart valve disease, in cardiac fibrosis, and in myocardial responses to ischemic injury. Redirection and amplification of disease-related EMTs offer potential new therapeutic strategies and approaches for treatment of heart disease. Here, we review the role and molecular regulation of endocardial and epicardial EMT in fetal heart development, and we summarize key literature implicating reactivation of endocardial and epicardial EMT in adult heart disease.

Zebrafish models in cardiac development and congenital heart birth defects

The zebrafish has become an ideal vertebrate animal system for investigating cardiac development due to its genetic tractability, external fertilization, early optical clarity and ability to survive without a functional cardiovascular system during development. In particular, recent advances in imaging techniques and the creation of zebrafish transgenics now permit the in vivo analysis of the dynamic cellular events that transpire during cardiac morphogenesis. As a result, the combination of these salient features provides detailed insight as to how specific genes may influence cardiac development at the cellular level. In this review, we will highlight how the zebrafish has been utilized to elucidate not only the underlying mechanisms of cardiac development and human congenital heart diseases (CHDs), but also potential pathways that may modulate cardiac regeneration. Thus, we have organized this review based on the major categories of CHDs-structural heart, functional heart, and vascular/great vessel defects, and will conclude with how the zebrafish may be further used to contribute to our understanding of specific human CHDs in the future.

Cellular decisions in cardiac outflow tract and coronary development: an act by VEGF and NOTCH

Congenital cardiac abnormalities are, due to their relatively high frequency and severe impact on quality of life, an important focus in cardiovascular research. Recently, various human studies have revealed a high coincidence of VEGF and NOTCH polymorphisms with cardiovascular outflow tract anomalies, such as bicuspid aortic valves and Tetralogy of Fallot, next to predisposition for cardiovascular pathologies, including atherosclerosis and aortic valve calcification. This genetic association between VEGF/NOTCH mutations and congenital cardiovascular defects in humans has been supported by substantial proof from animal models, revealing interaction of both pathways in cellular processes that are crucial for cardiac development. This review focuses on the role of VEGF and NOTCH signaling and their interplay in cardiogenesis with special interest to coronary and outflow tract development. An overview of the association between congenital malformations and VEGF/NOTCH polymorphisms in humans will be discussed along with their potential mechanisms and processes as revealed by transgenic mouse models. The molecular and cellular interaction of VEGF and subsequent Notch-signaling in these processes will be highlighted.

Unveiling combinatorial regulation through the combination of ChIP information and in silico cis-regulatory module detection

Computationally retrieving biologically relevant cis-regulatory modules (CRMs) is not straightforward. Because of the large number of candidates and the imperfection of the screening methods, many spurious CRMs are detected that are as high scoring as the biologically true ones. Using ChIP-information allows not only to reduce the regions in which the binding sites of the assayed transcription factor (TF) should be located, but also allows restricting the valid CRMs to those that contain the assayed TF (here referred to as applying CRM detection in a query-based mode). In this study, we show that exploiting ChIP-information in a query-based way makes in silico CRM detection a much more feasible endeavor. To be able to handle the large datasets, the query-based setting and other specificities proper to CRM detection on ChIP-Seq based data, we developed a novel powerful CRM detection method 'CPModule'. By applying it on a well-studied ChIP-Seq data set involved in self-renewal of mouse embryonic stem cells, we demonstrate how our tool can recover combinatorial regulation of five known TFs that are key in the self-renewal of mouse embryonic stem cells. Additionally, we make a number of new predictions on combinatorial regulation of these five key TFs with other TFs documented in TRANSFAC.

Mechanotransduction, PROX1, and FOXC2 cooperate to control connexin37 and calcineurin during lymphatic-valve formation

Lymphatic valves are essential for efficient lymphatic transport, but the mechanisms of early lymphatic-valve morphogenesis and the role of biomechanical forces are not well understood. We found that the transcription factors PROX1 and FOXC2, highly expressed from the onset of valve formation, mediate segregation of lymphatic-valve-forming cells and cell mechanosensory responses to shear stress in vitro. Mechanistically, PROX1, FOXC2, and flow coordinately control expression of the gap junction protein connexin37 and activation of calcineurin/NFAT signaling. Connexin37 and calcineurin are required for the assembly and delimitation of lymphatic valve territory during development and for its postnatal maintenance. We propose a model in which regionally increased levels/activation states of transcription factors cooperate with mechanotransduction to induce a discrete cell-signaling pattern and morphogenetic event, such as formation of lymphatic valves. Our results also provide molecular insights into the role of endothelial cell identity in the regulation of vascular mechanotransduction.

Human gene copy number spectra analysis in congenital heart malformations

The clinical significance of copy number variants (CNVs) in congenital heart disease (CHD) continues to be a challenge. Although CNVs including genes can confer disease risk, relationships between gene dosage and phenotype are still being defined. Our goal was to perform a quantitative analysis of CNVs involving 100 well-defined CHD risk genes identified through previously published human association studies in subjects with anatomically defined cardiac malformations. A novel analytical approach permitting CNV gene frequency "spectra" to be computed over prespecified regions to determine phenotype-gene dosage relationships was employed. CNVs in subjects with CHD (n = 945), subphenotyped into 40 groups and verified in accordance with the European Paediatric Cardiac Code, were compared with two control groups, a disease-free cohort (n = 2,026) and a population with coronary artery disease (n = 880). Gains (≥200 kb) and losses (≥100 kb) were determined over 100 CHD risk genes and compared using a Barnard exact test. Six subphenotypes showed significant enrichment (P ≤ 0.05), including aortic stenosis (valvar), atrioventricular canal (partial), atrioventricular septal defect with tetralogy of Fallot, subaortic stenosis, tetralogy of Fallot, and truncus arteriosus. Furthermore, CNV gene frequency spectra were enriched (P ≤ 0.05) for losses at: FKBP6, ELN, GTF2IRD1, GATA4, CRKL, TBX1, ATRX, GPC3, BCOR, ZIC3, FLNA and MID1; and gains at: PRKAB2, FMO5, CHD1L, BCL9, ACP6, GJA5, HRAS, GATA6 and RUNX1. Of CHD subjects, 14% had causal chromosomal abnormalities, and 4.3% had likely causal (significantly enriched), large, rare CNVs. CNV frequency spectra combined with precision phenotyping may lead to increased molecular understanding of etiologic pathways.

The secondary heart field is a new site of calcineurin/Nfatc1 signaling for semilunar valve development

Semilunar valve malformations are common human congenital heart defects. Bicuspid aortic valves occur in 2-3% of the population, and pulmonic valve stenosis constitutes 10% of all congenital heart disease in adults (Brickner et al., 2000) [1]. Semilunar valve defects cause valve regurgitation, stenosis, or calcification, leading to endocarditis or congestive heart failure. These complications often require prolonged medical treatment or surgical intervention. Despite the medical importance of valve disease, the regulatory pathways governing semilunar valve development are not entirely clear. In this report we investigated the spatiotemporal role of calcineurin/Nfatc1 signaling in semilunar valve development. We generated conditional knockout mice with calcineurin gene disrupted in various tissues during semilunar valve development. Our studies showed that calcineurin/Nfatc1 pathway signals in the secondary heart field (SHF) but not in the outflow tract myocardium or neural crest cells to regulate semilunar valve morphogenesis. Without SHF calcineurin/Nfatc1 signaling, the conal endocardial cushions-the site of prospective semilunar valve formation--first develop and then regress due to apoptosis, resulting in a striking phenotype with complete absence of the aortic and pulmonic valves, severe valve regurgitation, and perinatal lethality. This role of calcineurin/Nfatc1 signaling in the SHF is different from the requirement of calcineurin/Nfatc1 in the endocardium for semilunar valve formation (Chang et al., 2004) [2], indicating that calcineurin/Nfatc1 signals in multiple tissues to organize semilunar valve development. Also, our studies suggest distinct mechanisms of calcineurin/Nfat signaling for semilunar and atrioventricular valve morphogenesis. Therefore, we demonstrate a novel developmental mechanism in which calcineurin signals through Nfatc1 in the secondary heart field to promote semilunar valve morphogenesis, revealing a new supportive role of the secondary heart field for semilunar valve formation.

Analysis of the endocardial-to-mesenchymal transformation of heart valve development by collagen gel culture assay

Malformations of heart valves are one of the most common serious congenital defects. Heart valves are developed from endocardial cushions of the heart. The endocardial cushion in early heart development consists of two cell layers: an outer myocardial cell layer and an inner endocardial cell layer with abundant extracellular matrix (cardiac jelly) in between. Endocardial cells of the cushion, triggered by signals from myocardial cells, delaminate from the surface of the endocardial cushion and undergo transdifferentiation into mesenchymal cells. This process of endocardial-to-mesenchymal transformation (EMT) begins in the atrioventricular canal at embryonic day 9 (E9) and in the cardiac outflow tract at E10 of mouse development. Once formed by the EMT, the mesenchymal cells invade the cardiac jelly, proliferate, and populate the endocardial cushion. The cellularized endocardial cushion then undergoes morphological remodeling; it lengthens and matures into a thin elongated valve leaflet. Here we describe a method to culture endocardial cushions and measure EMT ex vivo. EMT can thus be analyzed independent of other concurrent developmental defects in mice. This culture method also enables ex vivo manipulations of signaling or gene function during EMT to delineate molecular pathways essential for heart valve development.

Use of whole embryo culture for studying heart development

Congenital heart defects occur in approximately 1% of newborns and are a major cause of morbidity and mortality in infants and children. Many adult cardiac diseases also have developmental basis, such as heart valve malformations, among others. Therefore, dissecting the developmental and molecular mechanisms underlying such defects in embryos is of great importance in prevention and developing therapeutics for heart diseases that manifest in infants or later in adults. Whole embryo culture is a valuable tool to study cardiac development in midgestation embryos, in which ventricular chambers are specified and expand, and the myocardium and endocardium interact to form various cardiac structures including heart valves and trabecular myocardium (Cell 118: 649-663, 2004; Dev Cell 14: 298-311, 2008). This technique is essentially growing a midgestation embryo ex utero in a test tube. One of the strengths of embryo culture is that it allows an investigator to easily manipulate or add drugs/chemicals directly to the embryos to test specific hypotheses in situations that are otherwise very difficult to perform for embryos in utero. For instance, embryo culture permits pharmacological rescue experiments to be performed in place of genetic rescue experiments which may require generation of specific mouse strains and crosses. Furthermore, because embryos are grown externally, drugs are directly acting on the cultured embryos rather than being degraded through maternal circulation or excluded from the embryos by the placenta. Drug dosage and kinetics are therefore easier to control with embryo culture. Conversely, drugs that compromise the placental function and are thus unusable for in utero experiments are applicable in cultured embryos since placental function is not required in whole embryo culture. The applications of whole embryo culture in the studies of molecular pathways involved in heart valve formation, myocardial growth, differentiation, and morphogenesis are demonstrated previously (Cell 118: 649-663, 2004; Dev Cell 14: 298-311, 2008; Nature 446: 62-67, 2010). Here we describe a method of embryo culture in a common laboratory setting without using special equipments.

Absence of association between length variation of an intronic region in the NFATC1 gene and congenital heart defects in a Han Chinese population

Congenital heart defects are complicated birth defects due to the interaction of genetic and environmental factors. Previous research indicated the importance of transcription factors in heart development, which suggested that mutations of transcription factor genes could be genetic determinants of congenital heart defects. Recently, the length variation of an intronic region in the NFATC1 gene was linked to ventricular septal defect (VSD). In this study, we detected the length variation of the region in a Han Chinese population of patients with nonsyndromic VSD, atrial septal defect, patent ductus arteriosus, and control individuals. We found a new allele of the length variation with four repeats of a 44-bp region. At the same time, all the alleles were found in both patient and control groups and there were no significant differences in genotype distribution between the patients and controls. The results suggested no association of the length variation of the intronic region in NFATC1 gene with VSD, atrial septal defect, and patent ductus arteriosus.

Patterning and development of the atrioventricular canal in zebrafish

Proper atrioventricular canal (AVC) patterning and subsequent valvulogenesis is a complex process, and defects can result in disease or early death. The zebrafish Danio rerio has become a useful model system for studying AVC development, and much progress has been made in dissecting out the critical steps. Here, we review the recent advances in the field and highlight the cellular and molecular changes observed during zebrafish AVC development.

MicroRNA-23 Restricts Cardiac Valve Formation by Inhibiting Has2 and Extracellular Hyaluronic Acid Production

Rationale:

Since their discovery almost 20 years ago, microRNAs have been shown to perform essential roles during tissue development and disease. Although roles for microRNAs in the myocardium during embryo development and cardiac disease have been demonstrated, very little is know about their role in the endocardium or during cardiac valve formation.

Objective:

To study the role of microRNAs in cardiac valve formation.

Methods and Results:

We show that zebrafish dicer mutant embryos, lacking mature miRNAs, form excessive endocardial cushions. By screening miRNAs expressed in the heart, we found that miR-23 is both necessary and sufficient for restricting the number of endocardial cells that differentiate into endocardial cushion cells. In addition, in mouse endothelial cells, miR-23 inhibited a transforming growth factor-β–induced endothelial-to-mesenchymal transition. By in silico screening of expression data with predicted miR-23 target sites combined with in vivo testing, we identified hyaluronic acid synthase 2 ( Has2) , Icat , and Tmem2 as novel direct targets of miR-23. Finally, we demonstrate that the upregulation of Has2 , an extracellular remodeling enzyme required for endocardial cushion and valve formation, is responsible for the excessive endocardial cushion cell differentiation in dicer mutants.

Conclusions:

MiR-23 in the embryonic heart is required to restrict endocardial cushion formation by inhibiting Has2 expression and extracellular hyaluronic acid production.

The impact of bioactive lipids on cardiovascular development

Lysophospholipids comprise a group of bioactive molecules with multiple biological functions. The cardinal members of this signalling molecule group are sphingosylphosphorylcholine (SPC), lysophosphatidic acid (LPA), and sphingosine 1-phosphate (S1P) which are, at least in part, homologous to each other. Bioactive lipids usually act via G-protein coupled receptors (GPCRs), but can also function as direct intracellular messengers. Recently, it became evident that bioactive lipids play a role during cellular differentiation development. SPC induces mesodermal differentiation of mouse ES cells and differentiation of promyelocytic leukemia cells, by a mechanism being critically dependent on MEK-ERK signalling. LPA stimulates the clonal expansion of neurospheres from neural stem/progenitor cells and induces c-fos via activation of mitogen- and stress-activated protein kinase 1 (MSK1) in ES cells. S1P acts on hematopoietic progenitor cells as a chemotactic factor and has also been found to be critical for cardiac and skeletal muscle regeneration. Furthermore, S1P promotes cardiogenesis and similarly activates Erk signalling in mouse ES cells. Interestingly, S1P may also act to maintain human stem cell pluripotency. Both LPA and S1P positively regulate the proliferative capacity of murine ES cells. In this paper we will focus on the differential and developmental impact of lysophospholipids on cardiovascular development.

High extracellular calcium-induced NFATc3 regulates the expression of receptor activator of NF-κB ligand in osteoblasts

Nuclear factor of activated T cell (NFAT) is a key transcription factor for receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation. However, it is unclear whether NFAT plays a role in the expression of RANKL in osteoblasts. High extracellular calcium ([Ca(2+)](o)) increases intracellular calcium, enhances RANKL expression in osteoblasts/stromal cells, and induces osteoclastogenesis in a coculture of osteoblasts and hematopoietic bone marrow cells. Because intracellular calcium signaling activates the calcineurin/NFAT pathway, we examined the role of NFAT activation on high [Ca(2+)](o)-induced RANKL expression in MC3T3-E1 subclone 4 (MC4) cells. Among the family of NFAT transcription factors, expression of NFATc1 and NFATc3, but not NFATc2, NFATc4 or NFAT5, was observed in MC4 cells. High [Ca(2+)](o) increased the expression levels of NFATc1, NFATc3 and RANKL. Cyclosporin A and FK506, inhibitors of calcineurin phosphatase, blocked high [Ca(2+)](o)-induced expression of NFAT and RANKL. Knockdown of NFATc1 and NFATc3 by siRNA prevented high [Ca(2+)](o)-induced RANKL expression, whereas overexpression of NFATc1 and NFATc3 induced RANKL expression. Furthermore, overexpressed NFATc1 upregulated NFATc3 expression, but NFATc1 knockdown decreased NFATc3 expression. Chromatin immunoprecipitation and reporter assay results showed that NFATc3, but not NFATc1, directly binds to the RANKL promoter and stimulates RANKL expression. In summary, these results demonstrate that high [Ca(2+)](o) increases expression of RANKL via activation of the calcineurin/NFAT pathway in osteoblasts. In addition, high [Ca(2+)](o) induces the activation and expression of NFATc1; NFATc3 expression and activity are subsequently increased; and NFATc3 directly binds to the RANKL promoter to increase its expression.

Genes regulating lymphangiogenesis control venous valve formation and maintenance in mice

Chronic venous disease and venous hypertension are common consequences of valve insufficiency, yet the molecular mechanisms regulating the formation and maintenance of venous valves have not been studied. Here, we provide what we believe to be the first description of venous valve morphogenesis and identify signaling pathways required for the process. The initial stages of valve development were found to involve induction of ephrin-B2, a key marker of arterial identity, by venous endothelial cells. Intriguingly, developing and mature venous valves also expressed a repertoire of proteins, including prospero-related homeobox 1 (Prox1), Vegfr3, and integrin-α9, previously characterized as specific and critical regulators of lymphangiogenesis. Using global and venous valve-selective knockout mice, we further demonstrate the requirement of ephrin-B2 and integrin-α9 signaling for the development and maintenance of venous valves. Our findings therefore identified molecular regulators of venous valve development and maintenance and highlighted the involvement of common morphogenetic processes and signaling pathways in controlling valve formation in veins and lymphatic vessels. Unexpectedly, we found that venous valve endothelial cells closely resemble lymphatic (valve) endothelia at the molecular level, suggesting plasticity in the ability of a terminally differentiated endothelial cell to take on a different phenotypic identity.

Protein kinase D2 controls cardiac valve formation in zebrafish by regulating histone deacetylase 5 activity

BACKGROUND

The molecular mechanisms that guide heart valve formation are not well understood. However, elucidation of the genetic basis of congenital heart disease is one of the prerequisites for the development of tissue-engineered heart valves.

METHODS AND RESULTS

We isolated here a mutation in zebrafish, bungee (bng(jh177)), which selectively perturbs valve formation in the embryonic heart by abrogating endocardial Notch signaling in cardiac cushions. We found by positional cloning that the bng phenotype is caused by a missense mutation (Y849N) in zebrafish protein kinase D2 (pkd2). The bng mutation selectively impairs PKD2 kinase activity and hence Histone deacetylase 5 phosphorylation, nuclear export, and inactivation. As a result, the expression of Histone deacetylase 5 target genes Krüppel-like factor 2a and 4a, transcription factors known to be pivotal for heart valve formation and to act upstream of Notch signaling, is severely downregulated in bungee (bng) mutant embryos. Accordingly, the expression of Notch target genes, such as Hey1, Hey2, and HeyL, is severely decreased in bng mutant embryos. Remarkably, downregulation of Histone deacetylase 5 activity in homozygous bng mutant embryos can rescue the mutant phenotype and reconstitutes notch1b expression in atrioventricular endocardial cells.

CONCLUSIONS

We demonstrate for the first time that proper heart valve formation critically depends on Protein kinase D2-Histone deacetylase 5-Krüppel-like factor signaling.

The Sphingosine-1-phospate receptor 1 mediates S1P action during cardiac development

BACKGROUND

Sphingosine-1-phosophate (S1P) is a biologically active sphingolipid metabolite that influences cellular events including differentiation, proliferation, and migration. S1P acts through five distinct cell surface receptors designated S1P1-5R, with S1P1R having the highest expression level in the developing heart. S1P1R is critical for vascular maturation, with its loss leading to embryonic death by E14.5; however, its function during early cardiac development is not well known. Our previous studies demonstrated that altered S1P levels adversely affects atrioventricular (AV) canal development in vitro, with reduced levels leading to cell death and elevated levels inhibiting cell migration and endothelial to mesenchymal cell transformation (EMT).

RESULTS

We determined, by real-time PCR analysis, that S1P1R was expressed at least 10-fold higher than other S1P receptors in the developing heart. Immunohistochemical analysis revealed S1P1R protein expression in both endothelial and myocardial cells in the developing atrium and ventricle. Using AV canal cultures, we observed that treatment with either FTY720 (an S1P1,3,4,5R agonist) or KRP203 (an S1P1R-specific agonist) caused similar effects on AV canal cultures as S1P treatment, including induction of cell rounding, inhibition of cell migration, and inhibition of EMT. In vivo, morphological analysis of embryonic hearts at E10.5 revealed that S1P1R-/- hearts were malformed with reduced myocardial tissue. In addition to reduced myocardial tissue, E12.5 S1P1R-/- hearts had disrupted morphology of the heart wall and trabeculae, with thickened and disorganized outer compact layer and reduced fibronectin (FN) deposition compared to S1P1R+/+ littermates. The reduced myocardium was accompanied by a decrease in cell proliferation but not an increase in apoptosis.

CONCLUSIONS

These data indicate that S1P1R is the primary mediator of S1P action in AV canal cultures and that loss of S1P1R expression in vivo leads to malformed embryonic hearts, in part due to reduced fibronectin expression and reduced cell proliferation.

Zebrafish as a model to study cardiac development and human cardiac disease

Over the last decade, the zebrafish has entered the field of cardiovascular research as a new model organism. This is largely due to a number of highly successful small- and large-scale forward genetic screens, which have led to the identification of zebrafish mutants with cardiovascular defects. Genetic mapping and identification of the affected genes have resulted in novel insights into the molecular regulation of vertebrate cardiac development. More recently, the zebrafish has become an attractive model to study the effect of genetic variations identified in patients with cardiovascular defects by candidate gene or whole-genome-association studies. Thanks to an almost entirely sequenced genome and high conservation of gene function compared with humans, the zebrafish has proved highly informative to express and study human disease-related gene variants, providing novel insights into human cardiovascular disease mechanisms, and highlighting the suitability of the zebrafish as an excellent model to study human cardiovascular diseases. In this review, I discuss recent discoveries in the field of cardiac development and specific cases in which the zebrafish has been used to model human congenital and acquired cardiac diseases.

Nfatc1 coordinates valve endocardial cell lineage development required for heart valve formation

RATIONALE

Formation of heart valves requires early endocardial to mesenchymal transformation (EMT) to generate valve mesenchyme and subsequent endocardial cell proliferation to elongate valve leaflets. Nfatc1 (nuclear factor of activated T cells, cytoplasmic 1) is highly expressed in valve endocardial cells and is required for normal valve formation, but its role in the fate of valve endocardial cells during valve development is unknown.

OBJECTIVE

Our aim was to investigate the function of Nfatc1 in cell-fate decision making by valve endocardial cells during EMT and early valve elongation.

METHODS AND RESULTS

Nfatc1 transcription enhancer was used to generate a novel valve endocardial cell-specific Cre mouse line for fate-mapping analyses of valve endocardial cells. The results demonstrate that a subpopulation of valve endocardial cells marked by the Nfatc1 enhancer do not undergo EMT. Instead, these cells remain within the endocardium as a proliferative population to support valve leaflet extension. In contrast, loss of Nfatc1 function leads to enhanced EMT and decreased proliferation of valve endocardium and mesenchyme. The results of blastocyst complementation assays show that Nfatc1 inhibits EMT in a cell-autonomous manner. We further reveal by gene expression studies that Nfatc1 suppresses transcription of Snail1 and Snail2, the key transcriptional factors for initiation of EMT.

CONCLUSIONS

These results show that Nfatc1 regulates the cell-fate decision making of valve endocardial cells during valve development and coordinates EMT and valve elongation by allocating endocardial cells to the 2 morphological events essential for valve development.

NFATC1 promotes epicardium-derived cell invasion into myocardium

Epicardium-derived cells (EPDCs) contribute to formation of coronary vessels and fibrous matrix of the mature heart. Nuclear factor of activated T-cells cytoplasmic 1 (NFATC1) is expressed in cells of the proepicardium (PE), epicardium and EPDCs in mouse and chick embryos. Conditional loss of NFATC1 expression in EPDCs in mice causes embryonic death by E18.5 with reduced coronary vessel and fibrous matrix penetration into myocardium. In osteoclasts, calcineurin-mediated activation of NFATC1 by receptor activator of NFκB ligand (RANKL) signaling induces cathepsin K (CTSK) expression for extracellular matrix degradation and cell invasion. RANKL/NFATC1 pathway components also are expressed in EPDCs, and loss of NFATC1 in EPDCs causes loss of CTSK expression in the myocardial interstitium in vivo. Likewise, RANKL treatment induces Ctsk expression in PE-derived cell cultures via a calcineurin-dependent mechanism. In chicken embryo hearts, RANKL treatment increases the distance of EPDC invasion into myocardium, and this response is calcineurin dependent. Together, these data demonstrate a crucial role for the RANKL/NFATC1 signaling pathway in promoting invasion of EPDCs into the myocardium by induction of extracellular matrix-degrading enzyme gene expression.

An endocardial pathway involving Tbx5, Gata4, and Nos3 required for atrial septum formation

In humans, septal defects are among the most prevalent congenital heart diseases, but their cellular and molecular origins are not fully understood. We report that transcription factor Tbx5 is present in a subpopulation of endocardial cells and that its deletion therein results in fully penetrant, dose-dependent atrial septal defects in mice. Increased apoptosis of endocardial cells lacking Tbx5, as well as neighboring TBX5-positive myocardial cells of the atrial septum through activation of endocardial NOS (Nos3), is the underlying mechanism of disease. Compound Tbx5 and Nos3 haploinsufficiency in mice worsens the cardiac phenotype. The data identify a pathway for endocardial cell survival and unravel a cell-autonomous role for Tbx5 therein. The finding that Nos3, a gene regulated by many congenital heart disease risk factors including stress and diabetes, interacts genetically with Tbx5 provides a molecular framework to understand gene-environment interaction in the setting of human birth defects.

VEGF signaling has distinct spatiotemporal roles during heart valve development

Heart valve malformations are one of the most common types of birth defects, illustrating the complex nature of valve development. Vascular endothelial growth factor (VEGF) signaling is one pathway implicated in valve formation, however its specific spatial and temporal roles remain poorly defined. To decipher these contributions, we use two inducible dominant negative approaches in mice to disrupt VEGF signaling at different stages of embryogenesis. At an early step in valve development, VEGF signals are required for the full transformation of endocardial cells to mesenchymal cells (EMT) at the outflow tract (OFT) but not atrioventricular canal (AVC) endocardial cushions. This role likely involves signaling mediated by VEGF receptor 1 (VEGFR1), which is highly expressed in early cushion endocardium before becoming downregulated after EMT. In contrast, VEGFR2 does not exhibit robust cushion endocardium expression until after EMT is complete. At this point, VEGF signaling acts through VEGFR2 to direct the morphogenesis of the AVC cushions into mature, elongated valve leaflets. This latter role of VEGF requires the VEGF-modulating microRNA, miR-126. Thus, VEGF roles in the developing valves are dynamic, transitioning from a differentiation role directed by VEGFR1 in the OFT to a morphogenetic role through VEGFR2 primarily in the AVC-derived valves.