The origin of the mammalian lymphatic vasculature has been debated for more than 100 years. Whether lymphatic endothelial cells have a single or dual, venous or mesenchymal origin remains controversial. To resolve this debate, we performed Cre/loxP-based lineage-tracing studies using mouse strains expressing Cre recombinase under the control of the Tie2, Runx1, or Prox1 promoter elements. These studies, together with the analysis of Runx1-mutant embryos lacking definitive hematopoiesis, conclusively determined that from venous-derived lymph sacs, lymphatic endothelial cells sprouted, proliferated, and migrated to give rise to the entire lymphatic vasculature, and that hematopoietic cells did not contribute to the developing lymph sacs. We conclude that the mammalian lymphatic system has a solely venous origin.

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) belong to the steroid/thyroid hormone receptor superfamily. Cloning of their cDNAs demonstrated the existence of two distinct but related genes: COUP-TFI (EAR-3, NR2F1) and COUP-TFII (ARP-1, NR2F2). They are referred to as orphan receptors because ligands for COUP-TFs have yet to be identified. Since 1998, extensive studies have demonstrated their physiological importance in cell-fate specification, organogenesis, angiogenesis, and metabolism, as well as a variety of diseases. In this article, we will comprehensively review the biological functions of COUP-TFII and its underlying mechanism in various developmental processes and diseases. In addition, we will briefly summarize some of the current findings of COUP-TFI.

The lymphatic system plays a key role in tissue fluid homeostasis. Lymphatic dysfunction contributes to the pathogenesis of many human diseases, including lymphedema and tumor metastasis. However, the mechanisms regulating lymphangiogenesis remain largely unknown. Here, we show that COUP-TFII (also known as Nr2f2), an orphan member of the nuclear receptor superfamily, mediates both developmental and pathological lymphangiogenesis in mice. Conditional ablation of COUP-TFII at an early embryonic stage resulted in failed formation of pre-lymphatic ECs (pre-LECs) and lymphatic vessels. COUP-TFII deficiency at a late developmental stage resulted in loss of LEC identity, gain of blood EC fate, and impaired lymphatic vessel sprouting. siRNA-mediated downregulation of COUP-TFII in cultured primary human LECs demonstrated that the maintenance of lymphatic identity and VEGF-C–induced lymphangiogenic activity, including cell proliferation and migration, are COUP-TFII–dependent and cell-autonomous processes. COUP-TFII enhanced the pro-lymphangiogenic actions of VEGF-C, at least in part by directly stimulating expression of neuropilin-2, a coreceptor for VEGF-C. In addition, COUP-TFII inactivation in a mammary gland mouse tumor model resulted in inhibition of tumor lymphangiogenesis, suggesting that COUP-TFII also regulates neo-lymphangiogenesis in the adult. Thus, COUP-TFII is a critical factor that controls lymphangiogenesis in embryonic development and tumorigenesis in adults.

Mouse Ikbkap gene encodes IKAP—one of the core subunits of Elongator—and is thought to be involved in transcription. However, the biological function of IKAP, particularly within the context of an animal model, remains poorly characterized. We used a loss-of-function approach in mice to demonstrate that Ikbkap is essential for meiosis during spermatogenesis. Absence of Ikbkap results in defects in synapsis and meiotic recombination, both of which result in increased apoptosis and complete arrest of gametogenesis. In Ikbkap-mutant testes, a few meiotic genes are down-regulated, suggesting IKAP's role in transcriptional regulation. In addition, Ikbkap-mutant testes exhibit defects in wobble uridine tRNA modification, supporting a conserved tRNA modification function from yeast to mammals. Thus, our study not only reveals a novel function of IKAP in meiosis, but also suggests that IKAP contributes to this process partly by exerting its effect on transcription and tRNA modification. The process of meiosis is responsible for gamete formation and ensures that offspring will inherit a complete set of chromosomes from each parent. Errors arising during this process generally result in spontaneous abortions, birth defects, or infertility. Many genes that are essential in regulating meiosis have also been implicated in DNA repair. Importantly, defects in DNA repair are common causes of cancers. Therefore, identification of genes important for normal meiosis contributes not only to the field of reproduction but also to the field of cancer biology. We studied the effects of deleting mouse Ikbkap, a gene that encodes one of the subunit of the Elongator complex initially described as an RNA polymerase II–associated transcription elongation factor. We demonstrate that Ikbkap mutant mice exhibit infertility and defects in meiotic progression. Specifically, homologous and sex chromosomes fail to synapse (become associated), DNA double-strand breaks are inefficiently repaired, and DNA crossovers are significantly decreased in Ikbkap males. We also demonstrate that the requirement for Elongator in tRNA modification, which has been shown in lower eukaryotes, is conserved in mammals. Our findings suggest novel roles for Ikbkap in meiosis progression and tRNA modification, which have not been reported previously.

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. Conclusion—: 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.