Structural development of endocardial cushions

The functional diversity of essential genes required for mammalian cardiac development

Genes required for an organism to develop to maturity (for which no other gene can compensate) are considered essential. The continuing functional annotation of the mouse genome has enabled the identification of many essential genes required for specific developmental processes including cardiac development. Patterns are now emerging regarding the functional nature of genes required at specific points throughout gestation. Essential genes required for development beyond cardiac progenitor cell migration and induction include a small and functionally homogenous group encoding transcription factors, ligands and receptors. Actions of core cardiogenic transcription factors from the Gata, Nkx, Mef, Hand, and Tbx families trigger a marked expansion in the functional diversity of essential genes from midgestation onwards. As the embryo grows in size and complexity, genes required to maintain a functional heartbeat and to provide muscular strength and regulate blood flow are well represented. These essential genes regulate further specialization and polarization of cell types along with proliferative, migratory, adhesive, contractile, and structural processes. The identification of patterns regarding the functional nature of essential genes across numerous developmental systems may aid prediction of further essential genes and those important to development and/or progression of disease. genesis 52:713–737, 2014.

Constitutively active Notch1 signaling promotes endothelial‑mesenchymal transition in a conditional transgenic mouse model

Endothelial-mesenchymal transition (EndoMT) is a process in which endothelial cells lose their cell-type-specific characteristics and gain a mesenchymal cell phenotype. The Notch signaling pathway is crucial in the regulation of EndoMT; however, its roles have not been fully studied in vivo. In a previous study, we reported the generation of transgenic mice with a floxed β-geo/stop signal between a CMV promoter and the constitutively active intracellular domain of Notch1 (IC-Notch1) linked with a human placental alkaline phosphatase (hPLAP) reporter (ZAP-IC-Notch1). In this study, we examined the results of activating IC-Notch1 in endothelial cells. ZAP-IC-Notch1 mice were crossed with Tie2-Cre mice to activate IC-Notch1 expression specifically in endothelial cells. The ZAP-IC-Notch1/Tie2-Cre double transgenic embryos died at E9.5–10.5 with disruption of vasculature and enlargement of myocardium. VE-cadherin expression was decreased and EphrinB2 expression was increased in the heart of these embryos. Mesenchymal cell marker α-smooth muscle actin (SMA) was expressed in IC-Notch1-expressing endothelial cells. In addition, upregulation of Snail, the key effector in mediating EndoMT, was identified in the cardiac cushion of the double transgenic murine embryo heart. The results of the present study demonstrate that constitutively active Notch signaling promotes EndoMT and differentially regulates endothelial/mesenchymal cell markers during cardiac development.

Functional analysis of candidate genes in 2q13 deletion syndrome implicates FBLN7 and TMEM87B deficiency in congenital heart defects and FBLN7 in craniofacial malformations

Recurrent 2q13 deletion syndrome is associated with incompletely penetrant severe cardiac defects and craniofacial anomalies. We used an atypical, overlapping 1.34 Mb 2q13 deletion in a patient with pathogenically similar congenital heart defects (CHD) to narrow the putative critical region for CHD to 474 kb containing six genes. To determine which of these genes is responsible for severe cardiac and craniofacial defects noted in the patients with the deletions, we used zebrafish morpholino knockdown to test the function of each orthologue during zebrafish development. Morpholino-antisense-mediated depletion of fibulin-7B, a zebrafish orthologue of fibulin-7 (FBLN7), resulted in cardiac hypoplasia, deficient craniofacial cartilage deposition and impaired branchial arch development. TMEM87B depletion likewise resulted in cardiac hypoplasia but with preserved branchial arch development. Depletion of both fibulin-7B and TMEM87B resulted in more severe defects of cardiac development, suggesting that their concurrent loss may enhance the risk of a severe cardiac defect. We postulate that heterozygous loss of FBLN7 and TMEM87B account for some of the clinical features, including cardiac defects and craniofacial abnormalities associated with 2q13 deletion syndrome.

Thymosin beta4 regulates cardiac valve formation via endothelial-mesenchymal transformation in zebrafish embryos

Thymosin beta4 (TB4) has multiple functions in cellular response in processes as diverse as embryonic organ development and the pathogeneses of disease, especially those associated with cardiac coronary vessels. However, the specific roles played by TB4 during heart valve development in vertebrates are largely unknown. Here, we identified a novel function of TB4 in endothelialmesenchymal transformation (EMT) in cardiac valve endocardial cushions in zebrafish. The expressions of thymosin family members in developing zebrafish embryos were determined by whole mount in situ hybridization. Of the thymosin family members only zTB4 was expressed in the developing heart region. Cardiac valve development at 48 h post fertilization was defected in zebrafish TB4 (zTB4) morpholino-injected embryos (morphants). In zTB4 morphants, abnormal linear heart tube development was observed. The expressions of bone morphogenetic protein (BMP) 4, notch1b, and hyaluronic acid synthase (HAS) 2 genes were also markedly reduced in atrio-ventricular canal (AVC). Endocardial cells in the AVC region were stained with anti-Zn5 antibody reactive against Dm-grasp (an EMT marker) to observe EMT in developing cardiac valves in zTB4 morphants. EMT marker expression in valve endothelial cells was confirmed after transfection with TB4 siRNA in the presence of transforming growth factor β (TGFβ) by RT-PCR and immunofluorescent assay. Zn5-positive endocardial AVC cells were not observed in zTB4 morphants, and knockdown of TB4 suppressed TGF-β-induced EMT in ovine valve endothelial cells. Taken together, our results demonstrate that TB4 plays a pivotal role in cardiac valve formation by increasing EMT.1.

Cardiac fibroblast in development and wound healing

Cardiac fibroblasts are the most abundant cell type in the mammalian heart and comprise approximately two-thirds of the total number of cardiac cell types. During development, epicardial cells undergo epithelial-mesenchymal-transition to generate cardiac fibroblasts that subsequently migrate into the developing myocardium to become resident cardiac fibroblasts. Fibroblasts form a structural scaffold for the attachment of cardiac cell types during development, express growth factors and cytokines and regulate proliferation of embryonic cardiomyocytes. In post natal life, cardiac fibroblasts play a critical role in orchestrating an injury response. Fibroblast activation and proliferation early after cardiac injury are critical for maintaining cardiac integrity and function, while the persistence of fibroblasts long after injury leads to chronic scarring and adverse ventricular remodeling. In this review, we discuss the physiologic function of the fibroblast during cardiac development and wound healing, molecular mediators of activation that could be possible targets for drug development for fibrosis and finally the use of reprogramming technologies for reversing scar. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

The Epicardium and the Development of the Atrioventricular Junction in the Murine Heart

Insight into the role of the epicardium in cardiac development and regeneration has significantly improved over the past ten years. This is mainly due to the increasing availability of new mouse models for the study of the epicardial lineage. Here we focus on the growing understanding of the significance of the epicardium and epicardially-derived cells in the formation of the atrioventricular (AV) junction. First, through the process of epicardial epithelial-to-mesenchymal transformation (epiEMT), the subepicardial AV mesenchyme is formed. Subsequently, the AV-epicardium and epicardially-derived cells (EPDCs) form the annulus fibrosus, a structure important for the electrical separation of atrial and ventricular myocardium. Finally, the AV-EPDCs preferentially migrate into the parietal AV valve leaflets, largely replacing the endocardially-derived cell population. In this review, we provide an overview of what is currently known about the regulation of the events involved in this process.

The hLAMP-1-positive particulate matrix involved in cardiac mesenchyme formation in the chick does not include BMP-2

Early heart development involves the transformation of endocardial cells in the atrioventricular canal and outflow tract regions into mesenchymal cells, a process called endocardial mesenchymal transformation (EMT). This process is initiated by factors, termed the particulate matrix, that are secreted by the myocardium. The particulate matrix causes a subset of endocardial cells to hypertrophy, lose their cell-cell contacts, form migratory processes, transform into mesenchymal cells, and migrate into the underlying endocardial cushions. The particulate matrix can be extracted using EDTA and the EDTA extract can initiate the EMT process. Earlier reports from our laboratory have shown that the particulate matrix can be detected with the hLAMP-1 antibody in immunostaining and Western blot analysis. In addition, similar proteins have been isolated from the growth media of stage 15-16 chick embryo myocardial cultures (MyoCM). Since other investigators have identified a possible role for bone morphogenetic protein (BMP)-2 during the EMT process in the heart, we asked whether BMP-2 is a part of the chick hLAMP-1-positive particulate matrix. To answer this question, we double stained stage 15-16 chick embryo sections with hLAMP-1 and BMP-2 antibodies. We found that BMP-2 signals do not colocalize with hLAMP-1-stained particles. In addition, using immunoprecipitation-Western blot analysis, we demonstrated no association of BMP-2 with the hLAMP-1-bound fraction of the EDTA extract or MyoCM. Our results indicate that BMP-2 is not a component of the hLAMP-1-positive particulate matrix in the chick.

Cadherin-11 expression patterns in heart valves associate with key functions during embryonic cushion formation, valve maturation and calcification

Proper fibroblast cell migration and differentiation are critical for valve formation and homeostasis, but uncontrolled myofibroblastic activation may precede osteogenic differentiation and calcification. Cadherin-11 (cad-11) is a cell-cell adhesion protein classically expressed at mesenchymal-osteoblast interfaces that participates in mesenchymal differentiation to osteochondral lineages. This suggests cad-11 may have an important role in heart valve development and pathogenesis, but its expression patterns in valves are largely unknown. In this study, we profiled the spatial and temporal expression patterns of cad-11 in embryonic chick and mouse heart development. We determined that cad-11 is expressed in both endocardial and mesenchymal cells of the atrioventricular and outflow tract cushions (pre-HH30/E14), but becomes restricted to the valve endocardial/endothelial cells during late fetal remodeling and throughout postnatal life. We then investigated changes in cad-11 expression in a murine aortic valve disease model (the ApoE(-/-)). Unlike wild-type mice, cad-11 becomes dramatically re-expressed in the interstitium. Similarly, in calcified human aortic valve leaflets, cad-11 loses endothelial confinement and becomes significantly re-expressed in the valve interstitium. Double labeling identified that 91% of myofibroblastic and 96% of osteoblastic cells in calcified aortic valves were also cad-11 positive. Collectively, our results suggest that cad-11 is important for proper embryonic cushion formation and remodeling, but may also participate in aortic valve pathogenesis if re-expressed in adulthood.

The chick embryo as an expanding experimental model for cancer and cardiovascular research

A long and productive history in biomedical research defines the chick as a model for human biology. Fundamental discoveries, including the description of directional circulation propelled by the heart and the link between oncogenes and the formation of cancer, indicate its utility in cardiac biology and cancer. Despite the more recent arrival of several vertebrate and invertebrate animal models during the last century, the chick embryo remains a commonly used model for vertebrate biology and provides a tractable biological template. With new molecular and genetic tools applied to the avian genome, the chick embryo is accelerating the discovery of normal development and elusive disease processes. Moreover, progress in imaging and chick culture technologies is advancing real-time visualization of dynamic biological events, such as tissue morphogenesis, angiogenesis, and cancer metastasis. A rich background of information, coupled with new technologies and relative ease of maintenance, suggest an expanding utility for the chick embryo in cardiac biology and cancer research.

Loss of the cytoskeletal protein Pdlim7 predisposes mice to heart defects and hemostatic dysfunction

The actin-associated protein Pdlim7 is essential for heart and fin development in zebrafish; however, the expression and function of this PDZ-LIM family member in the mammal has remained unclear. Here, we show that Pdlim7 predominantly localizes to actin-rich structures in mice including the heart, vascular smooth muscle, and platelets. To test the requirement for Pdlim7 in mammalian development and function, we analyzed a mouse strain with global genetic inactivation of Pdlim7. We demonstrate that Pdlim7 loss-of-function leads to significant postnatal mortality. Inactivation of Pdlim7 does not disrupt cardiac development, but causes mild cardiac dysfunction in adult mice. Adult Pdlim7^-/- mice displayed increased mitral and tricuspid valve annulus to body weight ratios. These structural aberrations in Pdlim7^-/- mice were supported by three-dimensional reconstructions of adult cardiac valves, which revealed increased surface area to volume ratios for the mitral and tricuspid valve leaflets. Unexpectedly, we found that loss of Pdlim7 triggers systemic venous and arterial thrombosis, leading to significant mortality shortly after birth in Pdlim7^+/- (11/60) and Pdlim7^-/- (19/35) mice. In line with a prothrombotic phenotype, adult Pdlim7^-/- mice exhibit dramatically decreased tail bleed times compared to controls. These findings reveal a novel and unexpected function for Pdlim7 in maintaining proper hemostasis in neonatal and adult mice.

Histone deacetylase 3 unconventional splicing mediates endothelial-to-mesenchymal transition through transforming growth factor β2

Histone deacetylase 3 (HDAC3) plays a critical role in the maintenance of endothelial integrity and other physiological processes. In this study, we demonstrated that HDAC3 undergoes unconventional splicing during stem cell differentiation. Four different splicing variants have been identified, designated as HD3α, -β, -γ, and -δ, respectively. HD3α was confirmed in stem cell differentiation by specific antibody against the sequences from intron 12. Immunofluorescence staining indicated that the HD3α isoform co-localized with CD31-positive or α-smooth muscle actin-positive cells at different developmental stages of mouse embryos. Overexpression of HD3α reprogrammed human aortic endothelial cells into mesenchymal cells featuring an endothelial-to-mesenchymal transition (EndMT) phenotype. HD3α directly interacts with HDAC3 and Akt1 and selectively activates transforming growth factor β2 (TGFβ2) secretion and cleavage. TGFβ2 functioned as an autocrine and/or paracrine EndMT factor. The HD3α-induced EndMT was both PI3K/Akt- and TGFβ2-dependent. This study provides the first evidence of the role of HDAC3 splicing in the maintenance of endothelial integrity.

Co-ordinating Notch, BMP, and TGF-β signaling during heart valve development

Congenital heart defects affect approximately 1-5 % of human newborns each year, and of these cardiac defects 20-30 % are due to heart valve abnormalities. Recent literature indicates that the key factors and pathways that regulate valve development are also implicated in congenital heart defects and valve disease. Currently, there are limited options for treatment of valve disease, and therefore having a better understanding of valve development can contribute critical insight into congenital valve defects and disease. There are three major signaling pathways required for early specification and initiation of endothelial-to-mesenchymal transformation (EMT) in the cardiac cushions: BMP, TGF-β, and Notch signaling. BMPs secreted from the myocardium set up the environment for the overlying endocardium to become activated; Notch signaling initiates EMT; and both BMP and TGF-β signaling synergize with Notch to promote the transition of endothelia to mesenchyme and the mesenchymal cell invasiveness. Together, these three essential signaling pathways help form the cardiac cushions and populate them with mesenchyme and, consequently, set off the cascade of events required to develop mature heart valves. Furthermore, integration and cross-talk between these pathways generate highly stratified and delicate valve leaflets and septa of the heart. Here, we discuss BMP, TGF-β, and Notch signaling pathways during mouse cardiac cushion formation and how they together produce a coordinated EMT response in the developing mouse valves.

Activin-A in diabetes-induced cardiac malformations in embryos

BACKGROUND

Heart defects are the most common abnormalities in infants of diabetic mothers. Cardiac malformation is associated with altered expression of the genes in the transforming growth factor β system, including inhibin βA, which forms activin-A as a homodimer and functions through its effectors, Smad2 and Smad3. This study aimed to investigate the role of activin-A in diabetes-induced cardiac malformations.

METHODS

Diabetes mellitus in female mice (C57BL/6J) was induced via intravenous injection of streptozotocin. The expression of inhibin βA protein and phosphorylation of Smad2 and Smad3 in the embryonic hearts were examined using immunohistochemical, in situ proximity ligation, and immunoblot assays. Embryos and endocardial cushions of nondiabetic mice were cultured in a high concentration of glucose and treated with activin-A. Mitosis was examined using BrdU incorporation assay and immunohistochemistry of phosphorylated histone H3. Migration of the endocardial cells was assessed using a collagen-based cell migration assay.

RESULTS

The levels of inhibin βA expression and Smad2 and Smad3 activation were significantly reduced by maternal diabetes. Treatment with activin-A significantly increased cell proliferation in the myocardium and migration of endocardial cells, compared with those in vehicle-treated high glucose group, to the level in the euglycemic control group.

CONCLUSIONS

Maternal diabetes suppresses the expression of inhibin βA protein, as well as the activation of Smad2 and Smad3. Activin-A rescues cell proliferation in the myocardium and migration of the endocardial cells suppressed by hyperglycemia. The activin-Smad2/3 signaling system appears to play a role in cardiac malformation in diabetic embryopathy.

Double-chambered right ventricle: a review

A double-chambered right ventricle is a rare heart defect in which the right ventricle is separated into a high-pressure proximal and low-pressure distal chamber. This defect is considered to be congenital and typically presents in infancy or childhood but has been reported to present rarely in adults. It can be caused by the presence of anomalous muscle tissue, hypertrophy of the endogenous trabecular bands, or an aberrant moderator band; all of which will typically result in progressive obstruction of the outflow tract. In this paper, we will discuss the general anatomy of the right ventricle, the relevant embryology of the heart, and the presentation, diagnosis, and treatment of a double-chambered right ventricle.

Discovery of endothelium and mesenchymal properties of primo vessels in the mesentery

Recent evidences demonstrated that endothelial-to-mesenchymal transition (EndMT) has a crucial role in cancer and is recognized as a unique source of cancer-associated fibroblasts (CAFs). Primo vascular system (PVS) is a new circulatory system which may play an important role in cancer metastasis and regeneration. In the current study, we applied previously established time-saving method to identify primo vessels and further investigated the immunocytochemical properties of primo vessels. Both primo vessels and primary primo vessel cells in the mesentery expressed endothelial markers and fibroblast markers. Double-labeling experiments demonstrated that endothelial and fibroblast markers are coexpressed in primo vessels. In addition, under the stimulation of TGF-β1 in vitro, primary primo vessel cells differentiated into fibroblasts. Therefore, we found that primo vessels in the mesentery had a transitional structure between endothelium and mesenchymal. This is a new finding of EndMT in normal postnatal animals.

FGF2-induced Ras/Erk MAPK signalling maintains lymphatic endothelial cell identity by up-regulating endothelial cell-specific gene expression and suppressing TGFβ signalling via Smad2

The lymphatic endothelial cell (LEC) fate decision program during development has been revealed. However, the mechanism underlying the maintenance of differentiated LEC identity remains largely unknown. Here, we show that fibroblast growth factor 2 (FGF2) plays a fundamental role in maintaining a differentiated LEC trait. In addition to demonstrating the appearance of alpha-smooth muscle actin (αSMA) expressing LECs in mouse lymphedematous skin in vivo, we found that mouse-immortalized LECs lose their characteristics and undergo endothelial-to-mesenchymal transition (EndMT) when cultured in FGF2-depleted medium. FGF2 depletion acted synergistically with transforming growth factor (TGF) β to induce EndMT. We also found that H-Ras-overexpressing LECs were resistant to EndMT. Ras activation not only upregulated FGF2-induced Erk MAPK activation, but also suppressed TGFβ-induced activation of Smad2 by modulating Smad2 phosphorylation via Erk MAPKs. These results suggest that FGF2 may regulate LEC-specific gene expression and suppress TGFβ signalling in LECs via Smad2 in a Ras/Erk MAP kinase-dependent manner. Taken together, our findings provide a new insight into the FGF2/Ras/Erk MAPK-dependent mechanism that maintains and modulates the LEC trait.

Olfactomedin-1 activity identifies a cell invasion checkpoint during epithelial-mesenchymal transition in the chick embryonic heart

Endothelia in the atrioventricular (AV) canal of the developing heart undergo a prototypical epithelial mesenchymal transition (EMT) to begin heart valve formation. Using an in vitro invasion assay, an extracellular matrix protein, Olfactomedin-1 (OLFM1), was found to increase mesenchymal cell numbers in AV canals from embryonic chick hearts. Treatment with both anti-OLFM1 antibody and siRNA targeting OLFM1 inhibits mesenchymal cell formation. OLFM1 does not alter cell proliferation, migration or apoptosis. Dispersion, but lack of invasion in the presence of inhibiting antibody, identifies a specific role for OLFM1 in cell invasion during EMT. This role is conserved in other epithelia, as OLFM1 similarly enhances invasion by MDCK epithelial cells in a transwell assay. Synergy is observed when TGFβ2 and OLFM1 are added to MDCK cell cultures, indicating that OLFM-1 activity is cooperative with TGFβ. Inhibition of both OLFM1 and TGFβ in heart invasion assays shows a similar cooperative role during development. To explore OLFM1 activity during EMT, representative EMT markers were examined. Effects of OLFM1 protein and anti-OLFM1 on transcripts of cell-cell adhesion molecules and the transcription factors Snail-1, Snail-2, Twist1 and Sox-9 argue that OLFM1 does not initiate EMT. Rather, regulation of transcripts of Zeb1 and Zeb2, secreted proteases and mesenchymal cell markers by both OLFM1 and anti-OLFM1 is consistent with regulation of the cell invasion step of EMT. We conclude that OLFM1 is present and necessary during EMT in the embryonic chick heart. Its role in cell invasion and mesenchymal cell gene regulation suggests an invasion checkpoint in EMT where OLFM1 acts to promote cell invasion into the three-dimensional matrix.

The zebrafish as a model system to study cardiovascular development

The zebrafish, Brachydanio rerio, is rapidly becoming a system of choice for vertebrate developmental biologists. It presents unique embryological attributes and is amenable to saturation style mutagenesis, a powerful approach that, in invertebrates, has already led to the identification of a large number of key developmental genes. Since fertilization is external, the zebrafish embryo develops in the dish and is thus accessible for continued observation and manipulation at all stages of development. Furthermore, because the embryo is transparent, the developing heart and vessels can be resolved at the single-cell level. A large number of mutations that affect the development of cardiovascular form and function have recently been isolated from large-scale genetic screens for zygotic embryonic lethals. Our further understanding of the development of the cardiovascular system is important not only because of the high incidence, and familial inheritance, of congenital abnormalities, but also because it should lead to novel, differentiation-based strategies for the analysis and therapy of the diseased state.

Biomechanics of the chick embryonic heart outflow tract at HH18 using 4D optical coherence tomography imaging and computational modeling

During developmental stages, biomechanical stimuli on cardiac cells modulate genetic programs, and deviations from normal stimuli can lead to cardiac defects. Therefore, it is important to characterize normal cardiac biomechanical stimuli during early developmental stages. Using the chicken embryo model of cardiac development, we focused on characterizing biomechanical stimuli on the Hamburger–Hamilton (HH) 18 chick cardiac outflow tract (OFT), the distal portion of the heart from which a large portion of defects observed in humans originate. To characterize biomechanical stimuli in the OFT, we used a combination of in vivo optical coherence tomography (OCT) imaging, physiological measurements and computational fluid dynamics (CFD) modeling. We found that, at HH18, the proximal portion of the OFT wall undergoes larger circumferential strains than its distal portion, while the distal portion of the OFT wall undergoes larger wall stresses. Maximal wall shear stresses were generally found on the surface of endocardial cushions, which are protrusions of extracellular matrix onto the OFT lumen that later during development give rise to cardiac septa and valves. The non-uniform spatial and temporal distributions of stresses and strains in the OFT walls provide biomechanical cues to cardiac cells that likely aid in the extensive differential growth and remodeling patterns observed during normal development.

The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

The role of endothelial-mesenchymal transition in development and pathological process

Epithelial-mesenchymal transition is an important developmental process, participates in tumor's formation, invasion, and metastasis and has been extensively studied. Recently, endothelial-mesenchymal transition (EndMT), a newly recognized type of cellular transdifferentiation, has been demonstrated to participate in a number of diseases by causing morphology changes and pathological processes. Previous studies showed that EndMT was a critical process of embryonic cardiac development. Not only that recent advances also suggested that EndMT occurred postnatally in cancer and cardiac fibrosis and emerged as a possible source of cancer-associated fibroblasts (CAFs). CAFs were found to acquire properties that promoted tumor development and metastasis formation. Resident endothelial cells undergoing EndMT lose their endothelial markers, acquire a mesenchymal or myofibroblastic phenotype, express mesenchymal cell products such as α-smooth muscle actin and type I collagen and develop invasive and migratory abilities. EndMT-derived cells are believed to function as fibroblasts in damaged tissue and may therefore have an important role in pathological process. However, little is known about the signaling mechanisms that cause endothelial cells to transform into mesenchymal cells. Transforming growth factor-β, Notch, or other signaling pathways could direct or interact to mediate EndMT. Therefore, to explore the signaling mechanisms of EndMT may provide novel therapeutic strategies for treating cancer.

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.

IL-1β and TGFβ2 synergistically induce endothelial to mesenchymal transition in an NFκB-dependent manner

Endothelial to mesenchymal transition (EndMT) contributes to fibrotic diseases. The main inducer of EndMT is TGFβ signaling. TGFβ2 is the dominant isoform in the physiological embryonic EndMT, but its role in the pathological EndMT in the context of inflammatory co-stimulation is not known. The aim of this study was to investigate TGFβ2-induced EndMT in the context of inflammatory IL-1β signaling. Co-stimulation with IL-1β and TGFβ2, but not TGFβ1, caused synergistic induction of EndMT. Also, TGFβ2 was the only TGFβ isoform that was progressively upregulated during EndMT. External IL-1β stimulation was dispensable once EndMT was induced. The inflammatory transcription factor NFκB was upregulated in an additive manner by IL-1β and TGFβ2 co-stimulation. Co-stimulation also led to the nuclear translocation of NFκB which was sustained over long-term treatment. Activation of NFκB was indispensable for the co-induction of EndMT. Our data suggest that the microenvironment at the verge between inflammation (IL-1β) and tissue remodeling (TGFβ2) can strongly promote the process of EndMT. Therefore our findings provide new insights into the mechanisms of pathological EndMT.

Genetic insights into bicuspid aortic valve formation

Bicuspid aortic valve (BAV) is the most common congenital heart defect, affecting 1-2% of the population. It is generally diagnosed late in adulthood when deterioration of the abnormal leaflet becomes clinically evident. BAV patients have an increased risk of developing serious complications, including stenosis, regurgitation, endocarditis, dilation of the aorta, aortic dissection, and aneurysm. BAV is a heritable trait, but the genetic basis underlying this cardiac malformation remains poorly understood. In the last decade, thanks to studies in animal models as well as genetic and biochemical approaches, a large number of genes that play important roles in heart development have been identified. These discoveries provided valuable insight into cardiac morphogenesis and uncovered congenital-heart-disease-causing genes. This paper will summarize the current knowledge of valve morphogenesis as well as our current understanding of the genetic pathways involved in BAV formation. The impact of these advances on human health including diagnosis of BAV and prevention of cardiovascular complications in individuals with BAV or with a family history of BAV is also discussed.

Collagen gel analysis of epithelial-mesenchymal transition in the embryo heart: an in vitro model system for the analysis of tissue interaction, signal transduction, and environmental effects

The cellular process of epithelial-mesenchymal cell transition (EMT) is a critical event in development that is reiterated in adult pathologies of metastasis and organ fibrosis. An initial understanding of the cellular and molecular events of this process emerged from an in vitro examination of heart valve development. Explants of the chick atrioventricular valve-forming region were placed on collagen gels and removed to show that EMT was regulated by a tissue interaction. Subsequent studies showed that specific TGFβ isoforms and receptors were required and steps of activation and invasion could be distinguished. The assay was modified for mouse hearts and has been used to explore signal transduction and gene expression in both species. The principle advantages of the system are a defined temporal window, when EMT takes place and the ability to isolate cells at various stages of the EMT process. These advantages are largely unavailable in other developmental or adult models. As the mesenchymal cells produced by EMT in the heart are involved in defects found in congenital heart disease, there is also a direct relevance of cardiac EMT to human birth defects. This relationship has been explored in relation to environmental exposures and in a number of genetic models. This review provides both an overview of the findings developed from the assay and protocols to enable the use of the assay by other laboratories. The assay provides a versatile platform to explore roles of specific gene products, drugs, and environmental agents on a critical cellular process.

Atrioventricular valve development: new perspectives on an old theme

Atrioventricular valve development commences with an EMT event whereby endocardial cells transform into mesenchyme. The molecular events that induce this phenotypic change are well understood and include many growth factors, signaling components, and transcription factors. Besides their clear importance in valve development, the role of these transformed mesenchyme and the function they serve in the developing prevalve leaflets is less understood. Indeed, we know that these cells migrate, but how and why do they migrate? We also know that they undergo a transition to a mature, committed cell, largely defined as an interstitial fibroblast due to their ability to secrete various matrix components including collagen type I. However, we have yet to uncover mechanisms by which the matrix is synthesized, how it is secreted, and how it is organized. As valve disease is largely characterized by altered cell number, cell activation, and matrix disorganization, answering questions of how the valves are built will likely provide us with information of real clinical relevance. Although expression profiling and descriptive or correlative analyses are insightful, to advance the field, we must now move past the simplicity of these assays and ask fundamental, mechanistic based questions aimed at understanding how valves are "built". Herein we review current understandings of atrioventricular valve development and present what is known and what isn't known. In most cases, basic, biological questions and hypotheses that were presented decades ago on valve development still are yet to be answered but likely hold keys to uncovering new discoveries with relevance to both embryonic development and the developmental basis of adult heart valve diseases. Thus, the goal of this review is to remind us of these questions and provide new perspectives on an old theme of valve development.

Epicardially derived fibroblasts preferentially contribute to the parietal leaflets of the atrioventricular valves in the murine heart

The importance of the epicardium for myocardial and valvuloseptal development has been well established; perturbation of epicardial development results in cardiac abnormalities, including thinning of the ventricular myocardial wall and malformations of the atrioventricular valvuloseptal complex. To determine the spatiotemporal contribution of epicardially derived cells to the developing fibroblast population in the heart, we have used a mWt1/IRES/GFP-Cre mouse to trace the fate of EPDCs from embryonic day (ED)10 until birth. EPDCs begin to populate the compact ventricular myocardium around ED12. The migration of epicardially derived fibroblasts toward the interface between compact and trabecular myocardium is completed around ED14. Remarkably, epicardially derived fibroblasts do not migrate into the trabecular myocardium until after ED17. Migration of EPDCs into the atrioventricular cushion mesenchyme commences around ED12. As development progresses, the number of EPDCs increases significantly, specifically in the leaflets which derive from the lateral atrioventricular cushions. In these developing leaflets the epicardially derived fibroblasts eventually largely replace the endocardially derived cells. Importantly, the contribution of EPDCs to the leaflets derived from the major AV cushions is very limited. The differential contribution of EPDCs to the various leaflets of the atrioventricular valves provides a new paradigm in valve development and could lead to new insights into the pathogenesis of abnormalities that preferentially affect individual components of this region of the heart. The notion that there is a significant difference in the contribution of epicardially and endocardially derived cells to the individual leaflets of the atrioventricular valves has also important pragmatic consequences for the use of endocardial and epicardial cre-mouse models in studies of heart development.

Three-dimensional and molecular analysis of the arterial pole of the developing human heart

Labeling experiments in chicken and mouse embryos have revealed important roles for different cell lineages in the development of the cardiac arterial pole. These data can only fully be exploited when integrated into the continuously changing morphological context and compared with the patterns of gene expression. As yet, studies on the formation of separate ventricular outlets and arterial trunks in the human heart are exclusively based on histologically stained sections. So as to expand these studies, we performed immunohistochemical analyses of serially sectioned human embryos, along with three-dimensional reconstructions. The development of the cardiac arterial pole involves several parallel and independent processes of formation and fusion of outflow tract cushions, remodeling of the aortic sac and closure of an initial aortopulmonary foramen through formation of a transient aortopulmonary septum. Expression patterns of the transcription factors ISL1, SOX9 and AP2α show that, in addition to fusion of the SOX9-positive endocardial cushions, intrapericardial protrusion of pharyngeal mesenchyme derived from the neural crest contributes to the separation of the developing ascending aorta from the pulmonary trunk. The non-adjacent walls of the intrapericardial arterial trunks are formed through addition of ISL1-positive cells to the distal outflow tract, while the facing parts of the walls form from the protruding mesenchyme. The morphogenetic steps, along with the gene expression patterns reported in this study, are comparable to those observed in the mouse. They confirm the involvement of mesenchymal tissues derived from endocardium, mesoderm and migrating neural crest cells in the process of initial septation of the distal part of the outflow tract, and its subsequent separation into discrete intrapericardial arterial trunks.

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.

The cycle of form and function in cardiac valvulogenesis

The formation and remodeling of the embryonic valves is a complex and dynamic process that occurs within a constantly changing hemodynamic environment. Defects in embryonic and fetal valve remodeling are the leading cause of congenital heart defects, yet very little is known about how fibrous leaflet tissue is created from amorphous gelatinous masses called cushions. Microenvironmental cues such as mechanical forces and extracellular matrix composition play major roles in cell differentiation, but almost all research efforts in valvulogenesis center around genetics and molecular approaches. This review summarizes what is known about the dynamic mechanical and extracellular matrix microenvironment of the atrioventricular and semilunar valves during embryonic development and their possible guidance roles. A variety of new computational tools and sophisticated experimental techniques are progressing that enable precise microenvironmental alterations that are critical to complement genetic gain and loss of function approaches. Studies at the interface of mechanical and genetic signaling in embryonic valvulogenesis will likely pay significant dividends, not only in terms of increasing our mechanistic understanding, but also lead to the development of novel therapeutic strategies for patients with congenital valve abnormalities.

Expression of the Tgfβ2 gene during chick embryogenesis

We performed a comprehensive analysis of the expression of transforming growth factor (TGF) β2 during chick embryogenesis from stage 6 to 30 (Hamburger and Hamilton, J Morphol 1951;88:49-92) using in situ hybridization. During cardiogenesis, Tgfβ2 was expressed in the endothelial/mesenchymal cells of the valvulo-septal endocardial cushion tissue and in the epicardium until the end of embryogenesis. During the formation of major arteries, Tgfβ2 was localized in smooth muscle progenitors but not in the vascular endothelium. During limb development, Tgfβ2 was expressed in the mesenchymal cells in the presumptive limb regions at stage 16, and thereafter it was localized in the skeletal muscle progenitors. In addition, strong Tgfβ2 expression was seen in the mesenchymal cells in the pharyngeal arches. Tgfβ2 mRNA was also detected in other mesoderm-derived tissues, such as the developing bone and pleura. During ectoderm development, Tgfβ2 was expressed in the floor plate of the neural tube, lens, optic nerve, and otic vesicle. In addition, Tgfβ2 was expressed in the developing gut epithelium. Our results suggest that TGFβ2 plays an important role not only in epithelial-mesenchymal interactions but also in cell differentiation and migration and cell death during chick embryogenesis. We also found that chick and mouse Tgfβ2 RNA show very similar patterns of expression during embryogenesis. Chick embryos can serve as a useful model to increase our understanding in the roles of TGFβ2 in cell-cell interactions, cell differentiation, and proliferation during organogenesis.

Roles of TGF-β signals in endothelial-mesenchymal transition during cardiac fibrosis

Most cardiac diseases caused by inflammation are associated with fibrosis in the heart. Fibrosis is characterized by the accumulation of fibroblasts and excess deposition of extracellular matrix (ECM), which results in the distorted organ architecture and function. Recent studies revealed that cardiac fibroblasts are heterogeneous with multiple origins. Endothelial-mesenchymal transition (EndMT) plays important roles in the formation of cardiac fibroblasts during pathological settings. EndMT is regulated by signaling pathways mediated by cytokines including transforming growth factor (TGF)-β. Better understanding of the mechanisms of the formation of cardiac fibroblasts via EndMT may provide an opportunity to develop therapeutic strategies to cure heart diseases.

TGF-β-induced mesenchymal transition of MS-1 endothelial cells requires Smad-dependent cooperative activation of Rho signals and MRTF-A

Endothelial-mesenchymal transition (EndMT) plays important roles in various physiological and pathological processes. While signals mediated by transforming growth factor (TGF)-β have been implicated in EndMT, the molecular mechanisms underlying it remain to be fully elucidated. Here, we examined the effects of TGF-β signals on the EndMT of mouse pancreatic microvascular endothelial cells (MS-1). By addition of TGF-β2, MS-1 cells underwent mesenchymal transition characterized by re-organization of actin stress fibre and increased expression of various mesenchymal markers such as α-smooth muscle actin (α-SMA) through activation of Rho signals. Whereas activation of Rho signals via TGF-β-induced non-Smad signals has been implicated in epithelial-mesenchymal transition (EMT), we found that Arhgef5, a guanine nucleotide exchange factor, is induced by Smad signals and contributes to the TGF-β2-induced α-SMA expression in MS-1 cells. We also found that TGF-β2 induces the expression of myocardin-related transcription factor-A (MRTF-A) in a Smad-dependent fashion and its nuclear accumulation in MS-1 cells and that MRTF-A is required and sufficient for TGF-β2-induced α-SMA expression. These results indicate that activation of Smad signals by TGF-β2 have dual effects on the activation of Rho signals and MRTF-A leading to the mesenchymal transition of MS-1 endothelial cells.

Endoplasmic reticulum stress in maternal diabetes-induced cardiac malformations during critical cardiogenesis period

BACKGROUND

Cardiac abnormalities, including atrioventricular (AV) septal defects (AVSDs), are the most common birth defects in diabetic embryopathy. The AV septum is derived from the endocardial cushions, which undergo development and remodeling during septation. The impact of maternal diabetes on these processes needs to be identified. Maternal diabetes disturbs the function of the endoplasmic reticulum (ER). The role of ER stress in cardiac malformation remains to be delineated to gain information for developing therapy.

METHODS

Female mice were induced diabetic via intravenous injection of streptozotocin. Pregnant mice were made hyperglycemic at desired embryonic (E) days. AVSDs were examined histologically at E15.5. ER stress-associated factors were examined and quantified using immunohistochemical and immunoblot assays at E10.5. The role of ER stress in endocardial cell migration was investigated by treating endocardial cushion explants that were cultured in high glucose with an organic chaperone molecule, sodium 4-phenylbutyrate.

RESULTS

The rate of AVSDs in the embryos that were exposed to maternal hyperglycemia during the period of endocardial cushion development was significantly higher than that in those during endocardial cushion remodeling. ER stress was increased in the hearts. Amelioration of ER stress restored endocardial cell migration under hyperglycemic conditions.

CONCLUSIONS

The development, rather than remodeling, of the endocardial cushions is the cardiomorphogenic process that is susceptible to the insult of maternal hyperglycemia in the formation of AVSDs. Maternal diabetes increases ER stress in the developing heart. ER stress plays an essential role in mediating the effect of hyperglycemia on endocardial cell migration.

Progenitor cells confer plasticity to cardiac valve endothelium

The endothelium covering the aortic, pulmonary, mitral, and tricuspid valves looks much like the endothelium throughout the vasculature, in terms of general morphology and expression of many endothelial markers. Closer examination, however, reveals important differences and hints of a unique phenotype that reflects the valvular endothelium's embryonic history, and potentially, its ability to maintain integrity and function over a life span of dynamic mechanical stress. A well-studied property that sets the cardiac valvular endothelium apart is the ability to transition from an endothelial to a mesenchymal phenotype-an event known as epithelial to mesenchymal transition (EMT). EMT is a critical step during embryonic valvulogenesis, it can occur in post-natal valves and has recently been implicated in the adaptive response of mitral valve leaflets exposed to a controlled in vivo setting designed to mimic the leaflet tethering that occurs in ischemic mitral regurgitation. In this review, we will discuss what is known about valvular endothelial cells, with a particular focus on post-natal, adult valves. We will put forth the idea that at subset of valvular endothelial cells are progenitor cells, which may serve to replenish valvular cells during normal cellular turnover and in response to injury and disease.

Inflammatory regulation of valvular remodeling: the good(?), the bad, and the ugly

Heart valve disease is unique in that it affects both the very young and very old, and does not discriminate by financial affluence, social stratus, or global location. Research over the past decade has transformed our understanding of heart valve cell biology, yet still more remains unclear regarding how these cells respond and adapt to their local microenvironment. Recent studies have identified inflammatory signaling at nearly every point in the life cycle of heart valves, yet its role at each stage is unclear. While the vast majority of evidence points to inflammation as mediating pathological valve remodeling and eventual destruction, some studies suggest inflammation may provide key signals guiding transient adaptive remodeling. Though the mechanisms are far from clear, inflammatory signaling may be a previously unrecognized ally in the quest for controlled rapid tissue remodeling, a key requirement for regenerative medicine approaches for heart valve disease. This paper summarizes the current state of knowledge regarding inflammatory mediation of heart valve remodeling and suggests key questions moving forward.

The F-box protein Ppa is a common regulator of core EMT factors Twist, Snail, Slug, and Sip1

The core EMT regulatory factors Twist, Snail, Slug, and Sip1, while structurally diverse, are coordinately regulated by a common E3 ubiquiting ligase, Ppa.

A small group of core transcription factors, including Twist, Snail, Slug, and Sip1, control epithelial–mesenchymal transitions (EMTs) during both embryonic development and tumor metastasis. However, little is known about how these factors are coordinately regulated to mediate the requisite behavioral and fate changes. It was recently shown that a key mechanism for regulating Snail proteins is by modulating their stability. In this paper, we report that the stability of Twist is also regulated by the ubiquitin–proteasome system. We found that the same E3 ubiquitin ligase known to regulate Snail family proteins, Partner of paired (Ppa), also controlled Twist stability and did so in a manner dependent on the Twist WR-rich domain. Surprisingly, Ppa could also target the third core EMT regulatory factor Sip1 for proteasomal degradation. Together, these results indicate that despite the structural diversity of the core transcriptional regulatory factors implicated in EMT, a common mechanism has evolved for controlling their stability and therefore their function.

Extracellular matrix and heart development

The extracellular matrix (ECM) of the developing heart contains numerous molecules that form a dynamic environment that plays an active and crucial role in the regulation of cellular events. ECM molecules found in the heart include hyaluronan, fibronectin, fibrillin, proteoglycans, and collagens. Tight regulation of the spatiotemporal expression, and the proteolytic processing of ECM components by proteases including members of the ADAMTS family, is essential for normal cardiac development. Perturbation of the expression of genes involved in matrix composition and remodeling can interfere with a myriad of events involved in the formation of the four-chambered heart and result in prenatal lethality or cardiac malformations as seen in humans with congenital heart disease. In this review, we summarize what is known about the specific importance of some of the components of the ECM in relation to the cardiovascular development.

Tgfβ/Alk5 signaling is required for shear stress induced klf2 expression in embryonic endothelial cells

Endothelial cells (EC) translate biomechanical forces into functional and phenotypic responses that play important roles in cardiac development. Specifically, EC in areas of high shear stress, i.e., in the cardiac outflow tract and atrioventricular canal, are characterized by high expression of Krüppel-like factor 2 (Klf2) and by transforming growth factor-beta (Tgfβ)-driven endothelial-to-mesenchymal transition. Extraembryonic venous obstruction (venous clip model) results in congenital heart malformations, and venous clip-induced alterations in shear stress-related gene expression are suggestive for an increase in cardiac shear stress. Here, we study the effects of shear stress on Klf2 expression and Tgfβ-associated signaling in embryonic EC in vivo using the venous clip model and in vitro by subjecting cultured EC to fluid flow. Cellular responses were assessed by analysis of Klf2, Tgfβ ligands, and their downstream signaling targets. Results show that, in embryonic EC, shear stress activates Tgfβ/Alk5 signaling and that induction of Klf2 is an Alk5 dependent process.

The role of cell plasticity in progression and reversal of renal fibrosis

The need for novel insights into the mechanisms of progression of renal disease has become urgent during the last several years because of the increasing incidence of chronic renal disease worldwide. Independent of the underlying disease, the subsequent progression of renal fibrosis is characterized mainly by both an exaggerated synthesis and abnormal accumulation of extracellular matrix proteins produced by mesenchymal cells within the kidney. These cells are mainly myofibroblasts deriving from a variety of renal cells such as vascular smooth muscle, mesangial, resident stem, tubular epithelial, vascular endothelial cells or pericytes. The appearance of myofibroblasts is a reversible process, as suggested by studies in experimental models showing regression of renal fibrosis during therapy with antagonists and/or blockers of the renin-angiotensin system. An additional factor that can also affect the mechanisms of progression/regression of fibrosis is the plasticity of podocytes controlling glomerular filtration.

Review: Endothelial-myofibroblast transition, a new player in diabetic renal fibrosis

Diabetic nephropathy (DN) is the most common cause of chronic kidney failure and end-stage renal disease in the Western world. Studies from diabetic animal models and clinical trials have shown that inhibition of the renin-angiotensin system delays the progression of advanced DN. However, a recent large-scale clinical trial has revealed that inhibition of renin-angiotensin system in early phases of DN does not slow the decline of renal function or the development of morphological lesions, suggesting that different mechanism(s) may be involved in the different stages of DN. The role of epithelial-mesenchymal transition in renal fibrosis has been intensively investigated. Recently, endothelial-mesenchymal transition, or endothelial-myofibroblast transition (EndoMT) has emerged as another mechanism involved in both developmental and pathological processes. The essential role of EndoMT in cardiac development has been thoroughly studied. EndoMT also exists and contributes to the development and progression of cardiac fibrosis, lung fibrosis, liver fibrosis and corneal fibrosis. EndoMT is a specific form of epithelial-mesenchymal transition. During EndoMT, endothelial cells lose endothelial markers and obtain mesenchymal markers. Recent evidence from our laboratory and others suggests that EndoMT plays an important role in the development of renal fibrosis in several pathological settings, including experimental DN. This review considers the evidence supporting the occurrence of EndoMT in normal development and in pathology, as well as the latest findings suggesting EndoMT contributes to fibrosis in DN. Whether experimental findings of EndoMT will be reproduced in human studies remains to be determined.

Expression of the familial cardiac valvular dystrophy gene, filamin-A, during heart morphogenesis

Myxoid degeneration of the cardiac valves is a common feature in a heterogeneous group of disorders that includes Marfan syndrome and isolated valvular diseases. Mitral valve prolapse is the most common outcome of these and remains one of the most common indications for valvular surgery. While the etiology of the disease is unknown, recent genetic studies have demonstrated that an X-linked form of familial cardiac valvular dystrophy can be attributed to mutations in the Filamin-A gene. Since these inheritable mutations are present from conception, we hypothesize that filamin-A mutations present at the time of valve morphogenesis lead to dysfunction that progresses postnatally to clinically relevant disease. Therefore, by carefully evaluating genetic factors (such as filamin-A) that play a substantial role in MVP, we can elucidate relevant developmental pathways that contribute to its pathogenesis. In order to understand how developmental expression of a mutant protein can lead to valve disease, the spatio-temporal distribution of filamin-A during cardiac morphogenesis must first be characterized. Although previously thought of as a ubiquitously expressed gene, we demonstrate that filamin-A is robustly expressed in non-myocyte cells throughout cardiac morphogenesis including epicardial and endocardial cells, and mesenchymal cells derived by EMT from these two epithelia, as well as mesenchyme of neural crest origin. In postnatal hearts, expression of filamin-A is significantly decreased in the atrioventricular and outflow tract valve leaflets and their suspensory apparatus. Characterization of the temporal and spatial expression pattern of filamin-A during cardiac morphogenesis is a crucial first step in our understanding of how mutations in filamin-A result in clinically relevant valve disease.

Integration of a Notch-dependent mesenchymal gene program and Bmp2-driven cell invasiveness regulates murine cardiac valve formation

Cardiac valve formation is crucial for embryonic and adult heart function. Valve malformations constitute the most common congenital cardiac defect, but little is known about the molecular mechanisms regulating valve formation and homeostasis. Here, we show that endocardial Notch1 and myocardial Bmp2 signal integration establish a valve-forming field between 2 chamber developmental domains. Patterning occurs through the activation of endocardial epithelial-to-mesenchymal transition (EMT) exclusively in prospective valve territories. Mice with constitutive endocardial Notch1 activity ectopically express Hey1 and Heyl. They also display an activated mesenchymal gene program in ventricles and a partial (noninvasive) EMT in vitro that becomes invasive upon BMP2 treatment. Snail1, TGF-β2, or Notch1 inhibition reduces BMP2-induced ventricular transformation and invasion, whereas BMP2 treatment inhibits endothelial Gsk3β, stabilizing Snail1 and promoting invasiveness. Integration of Notch and Bmp2 signals is consistent with Notch1 signaling being attenuated after myocardial Bmp2 deletion. Notch1 activation in myocardium extends Hey1 expression to nonchamber myocardium, represses Bmp2, and impairs EMT. In contrast, Notch deletion abrogates endocardial Hey gene transcription and extends Bmp2 expression to the ventricular endocardium. This embryonic Notch1-Bmp2-Snail1 relationship may be relevant in adult valve disease, in which decreased NOTCH signaling causes valve mesenchyme cell formation, fibrosis, and calcification.

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.