NOX2 Is Critical to Endocardial to Mesenchymal Transition and Heart Development.

NADPH oxidases (NOX) are a major source of reactive oxygen species (ROS) production in the heart. ROS signaling regulates gene expression, cell proliferation, apoptosis, and migration. However, the role of NOX2 in embryonic heart development remains elusive. We hypothesized that deficiency of Nox2 disrupts endocardial to mesenchymal transition (EndMT) and results in congenital septal and valvular defects. Our data show that 34% of Nox2-/- neonatal mice had various congenital heart defects (CHDs) including atrial septal defects (ASD), ventricular septal defects (VSD), atrioventricular canal defects (AVCD), and malformation of atrioventricular and aortic valves. Notably, Nox2-/- embryonic hearts show abnormal development of the endocardial cushion as evidenced by decreased cell proliferation and an increased rate of apoptosis. Additionally, Nox2 deficiency disrupted EndMT of atrioventricular cushion explants ex vivo. Furthermore, treatment with N-acetylcysteine (NAC) to reduce ROS levels in the wild-type endocardial cushion explants decreased the number of cells undergoing EndMT. Importantly, deficiency of Nox2 was associated with reduced expression of Gata4, Tgfß2, Bmp2, Bmp4, and Snail1, which are critical to endocardial cushion and valvoseptal development. We conclude that NOX2 is critical to EndMT, endocardial cushion cell proliferation, and normal embryonic heart development.

FRS2α-dependent cell fate transition during endocardial cushion morphogenesis.

Atrioventricular valve development requires endothelial-to-mesenchymal transition (EndMT) that induces cushion endocardial cells to give rise to mesenchymal cells crucial to valve formation. In the adult endothelium, deletion of the docking protein FRS2α induces EndMT by activating TGFß signaling in a miRNA let-7-dependent manner. To study the role of endothelial FRS2α during embryonic development, we generated mice with an inducible endothelial-specific deletion of Frs2α (FRS2αiECKO). Analysis of the FRS2αiECKO embryos uncovered a combination of impaired EndMT in AV cushions and defective maturation of AV valves leading to development of thickened, abnormal valves when Frs2α was deleted early (E7.5) in development. At the same time, no AV valve developmental abnormalities were observed after late (E10.5) deletion. These observations identify FRS2α as a pivotal controller of cell fate transition during both EndMT and post-EndMT valvulogenesis.

RERE deficiency leads to decreased expression of GATA4 and the development of ventricular septal defects.

Deletions of chromosome 1p36 are associated with a high incidence of congenital heart defects (CHDs). The arginine-glutamic acid dipeptide repeats gene (RERE) is located in a critical region for CHD on chromosome 1p36 and encodes a cardiac-expressed nuclear receptor co-regulator. Mutations affecting RERE cause atrial and ventricular septal defects (VSDs) in humans, and RERE-deficient mice also develop VSDs. During cardiac development, mesenchymal cells destined to form part of the atrioventricular (AV) septum are generated when endocardial cells in the AV canal undergo epithelial-to-mesenchymal transition (EMT) and migrate into the space between the endocardium and the myocardium. These newly generated mesenchymal cells then proliferate to fill the developing AV endocardial cushions. Here, we demonstrate that RERE-deficient mouse embryos have reduced numbers of mesenchymal cells in their AV endocardial cushions owing to decreased levels of EMT and mesenchymal cell proliferation. In the endocardium, RERE colocalizes with GATA4, a transcription factor required for normal levels of EMT and mesenchymal cell proliferation. Using a combination of in vivo and in vitro studies, we show that Rere and Gata4 interact genetically in the development of CHDs, RERE positively regulates transcription from the Gata4 promoter and GATA4 levels are reduced in the AV canals of RERE-deficient embryos. Tissue-specific ablation of Rere in the endocardium leads to hypocellularity of the AV endocardial cushions, defective EMT and VSDs, but does not result in decreased GATA4 expression. We conclude that RERE functions in the AV canal to positively regulate the expression of GATA4, and that deficiency of RERE leads to the development of VSDs through its effects on EMT and mesenchymal cell proliferation. However, the cell-autonomous role of RERE in promoting EMT in the endocardium must be mediated by its effects on the expression of proteins other than GATA4.This article has an associated First Person interview with the first author of the paper.

Synergistic regulation of p53 by Mdm2 and Mdm4 is critical in cardiac endocardial cushion morphogenesis during heart development.

Congenital heart defects (CHDs) are the most prevalent human birth defects. More than 85% of CHDs are thought to result from a combination of genetic susceptibilities and environmental stress. However, the stress-related signalling pathways involved remain largely unknown. The p53 transcription factor is a key tumour suppressor and a central regulator of the cellular stress responses. p53 activities are tightly regulated by its inhibitors Mdm2 and Mdm4 at the post-translational level. Here we used the Cre-loxP system to delete Mdm2 (Tie2Cre;Mdm2(FM/FM) ) or one copy of both Mdm2 and Mdm4 (Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) ) in endothelial/endocardial cells and their derivatives in mice to examine the regulation of the p53/Mdm2-Mdm4 pathway during vascular and cardiovascular development. The Tie2Cre;Mdm2(FM/FM) mice died before embryonic day 10.5 (E10.5) and displayed severe vascular defects. On the other hand, the Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) mice displayed atrial and ventricular septal defects (ASD, VSD) of the heart, leading to severe heart dysfunction and postnatal death. During cardiac endocardial cushion morphogenesis, p53 activation was associated with defects in both the epithelial-mesenchymal transition (EMT) of the endocardial cells and the post-EMT proliferation of the mesenchymal cells, and the valvuloseptal phenotypes of the Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) mice were fully rescued by deletion of one copy of p53. Strikingly, maternal exposure to low-dose X-rays in C57BL/6 mice mimicked the congenital heart malformations seen in the Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) model, which was also dependent on p53 status, establishing a link between maternal exposures and CHD susceptibility through the p53 pathway. These data revealed a new regulatory mechanism in cardiac endocardial cushion morphogenesis and suggested a possible cause of CHDs due to environmental stress.

Versican/PG-M is essential for ventricular septal formation subsequent to cardiac atrioventricular cushion development.

Versican (Vcan)/proteoglycan (PG)-M is a large chondroitin sulfate proteoglycan which forms a proteoglycan/hyaluronan (HA) aggregate in the extracellular matrix (ECM). We tried to generate the Vcan knockout mice by a conventional method, which resulted in mutant mice Vcan(Δ3/Δ3) whose Vcan lacks the A subdomain of the G1 domain. The Vcan knockout embryos died during the early development stage due to heart defects, but some Vcan(Δ3/Δ3) embryos survived through to the neonatal period. The hearts in Vcan(Δ3/Δ3) newborn mice showed normal cardiac looping, but had ventricular septal defects. Their atrioventricular canal (AVC) cushion was much smaller than those of wild-type (WT) embryos, and the extracellular space for cardiac jelly was narrow. The Vcan deposition in the Vcan(Δ3/Δ3) AVC cushion had decreased, whereas the HA deposition was maintained and condensed. In the tip of ventricular septa, both Vcan and HA had decreased. The cell proliferation based on the number of Ki67-positive cells had remarkably increased in both the AVC cushion and ventricular septa, compared with that of WT embryos. Vcan(Δ3/Δ3) seemed to have endocardial and mesenchymal mixed characteristics. When the ex vivo explant culture of these regions was performed on the collagen gel, hardly any migration to make sufficient space for the ECM construction was apparent. Our results suggest that the proteoglycan aggregates are necessary in both the AVC cushion and ventricular septa to fuse interventricular septa, and the Vcan A subdomain plays an essential role for the interventricular septal formation by constituting the proteoglycan aggregates.

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.

Conditional ablation of Ezh2 in murine hearts reveals its essential roles in endocardial cushion formation, cardiomyocyte proliferation and survival.

Ezh2 is a histone trimethyltransferase that silences genes mainly via catalyzing trimethylation of histone 3 lysine 27 (H3K27Me3). The role of Ezh2 as a regulator of gene silencing and cell proliferation in cancer development has been extensively investigated; however, its function in heart development during embryonic cardiogenesis has not been well studied. In the present study, we used a genetically modified mouse system in which Ezh2 was specifically ablated in the mouse heart. We identified a wide spectrum of cardiovascular malformations in the Ezh2 mutant mice, which collectively led to perinatal death. In the Ezh2 mutant heart, the endocardial cushions (ECs) were hypoplastic and the endothelial-to-mesenchymal transition (EMT) process was impaired. The hearts of Ezh2 mutant mice also exhibited decreased cardiomyocyte proliferation and increased apoptosis. We further identified that the Hey2 gene, which is important for cardiomyocyte proliferation and cardiac morphogenesis, is a downstream target of Ezh2. The regulation of Hey2 expression by Ezh2 may be independent of Notch signaling activity. Our work defines an indispensible role of the chromatin remodeling factor Ezh2 in normal cardiovascular development.

The FGF-BMP signaling axis regulates outflow tract valve primordium formation by promoting cushion neural crest cell differentiation.

RATIONALE: Heart valves develop from precursor structures called cardiac cushions, an endothelial-lined cardiac jelly that resides in the inner side of the heart tube. The cushions are then invaded by cells from different sources, undergo a series of complicated and poorly understood remodeling processes, and give rise to valves. Disruption of the fibroblast growth factor (FGF) signaling axis impairs morphogenesis of the outflow tract (OFT). Yet, whether FGF signaling regulates OFT valve formation is unknown. OBJECTIVE: To study how OFT valve formation is regulated and how aberrant cell signaling causes valve defects. METHODS AND RESULTS: By using mouse genetic manipulation, cell lineage tracing, ex vivo heart culture, and molecular biology approaches, we demonstrated that FGF signaling in the OFT myocardium upregulated Bmp4 expression, which then enhanced smooth muscle differentiation of neural crest cells (NCCs) in the cushion. FGF signaling also promoted OFT myocardial cell invasion to the cushion. Disrupting FGF signaling interrupted cushion remodeling with reduced NCCs differentiation into smooth muscle and less cardiomyocyte invasion and resulted in malformed OFT valves. CONCLUSIONS: The results demonstrate a novel mechanism by which the FGF-BMP signaling axis regulates formation of OFT valve primordia by controlling smooth muscle differentiation of cushion NCCs.

Noonan syndrome cardiac defects are caused by PTPN11 acting in endocardium to enhance endocardial-mesenchymal transformation.

Noonan syndrome (NS), the most common single-gene cause of congenital heart disease, is an autosomal dominant disorder that also features proportionate short stature, facial abnormalities, and an increased risk of myeloproliferative disease. Germline-activating mutations in PTPN11, which encodes the protein tyrosine phosphatase SHP2, cause about half of NS cases; other causative alleles include KRAS, SOS1, and RAF1 mutants. We showed previously that knock-in mice bearing the NS mutant Ptpn11(D61G) on a mixed 129S4/SvJae X C57BL6/J background exhibit all major NS features, including a variety of cardiac defects, with variable penetrance. However, the cellular and molecular mechanisms underlying NS cardiac defects and whether genetic background and/or the specific NS mutation contribute to the NS phenotype remained unclear. Here, using an inducible knock-in approach, we show that all cardiac defects in NS result from mutant Shp2 expression in the endocardium, not in the myocardium or neural crest. Furthermore, the penetrance of NS defects is affected by genetic background and the specific Ptpn11 allele. Finally, ex vivo assays and pharmacological approaches show that NS mutants cause cardiac valve defects by increasing Erk MAPK activation, probably downstream of ErbB family receptor tyrosine kinases, extending the interval during which cardiac endocardial cells undergo endocardial-mesenchymal transformation. Our data provide a mechanistic underpinning for the cardiac defects in this disorder.

Endothelial expression of bone morphogenetic protein receptor type 1a is required for atrioventricular valve formation.

BACKGROUND: Atrioventricular canal defects account for 4% of all congenital heart anomalies. They arise from failure of endocardial cushion formation, a process dependent on transition of endothelial cells into clustered mesenchymal cells in the mid-atrioventricular septum. To date, the genetic signals necessary for atrioventricular canal defects are poorly understood. We hypothesized that bone morphogenetic protein signaling in cardiac endothelial cells may be crucial to this process. METHODS: To study the role of bone morphogenetic protein receptors (Bmpr) in the developing heart, we created knockout mice with inactivation of Bmpr1a selectively in endocardium. Two strains of null mice were created: one with constitutive endothelial-specific knockout of Bmpr1a and one with time-inducible, endothelial-specific knockout of Bmpr1a. Embryos and animals were analyzed by microscopy, RNA in situ hybridization, and microangiography. RESULTS: Animals with null mutation of Bmpr1a in endothelium were embryonic lethal at E11.5 to 12.0 and demonstrated absence of endocardial cushion formation. Embryos failed to form atrioventricular valves and adjacent septa. Endocardial knockout of Bmpr1a did not affect development of the outflow tract or aortic arches. Using time-inducible, cell-specific knockout mice, we show that Bmpr1a has two functions in the developing atrioventricular canal: to induce endocardial endothelial-mesenchymal transition, and to pattern the septal mesenchyme into endocardial cushions. We demonstrate that these processes are temporally linked to expression of the transcription factors Id1 and Id3. CONCLUSIONS: Endocardial cushion formation is dependent on cell-specific expression of Bmpr1a. Our results suggest that Bmpr1a-mediated signaling is a crucial pathway involved in pathogenesis of atrioventricular septal and valve malformations, which are among the most common congenital heart defects in humans.