Implantation is a key stage during pregnancy, as the fate of the embryo is often decided upon its first contact with the maternal endometrium. Around this time, DCs accumulate in the uterus; however, their role in pregnancy and, more specifically, implantation, remains unknown. We investigated the function of uterine DCs (uDCs) during implantation using a transgenic mouse model that allows conditional ablation of uDCs in a spatially and temporally regulated manner. Depletion of uDCs resulted in a severe impairment of the implantation process, leading to embryo resorption. Depletion of uDCs also caused embryo resorption in syngeneic and T cell-deficient pregnancies, which argues against a failure to establish immunological tolerance during implantation. Moreover, even in the absence of embryos, experimentally induced deciduae failed to adequately form. Implantation failure was associated with impaired decidual proliferation and differentiation. Dynamic contrast-enhanced MRI revealed perturbed angiogenesis characterized by reduced vascular expansion and attenuated maturation. We suggest therefore that uDCs directly fine-tune decidual angiogenesis by providing two critical factors, sFlt1 and TGF-β1, that promote coordinated blood vessel maturation. Collectively, uDCs appear to govern uterine receptivity, independent of their predicted role in immunological tolerance, by regulating tissue remodeling and angiogenesis. Importantly, our results may aid in understanding the limited implantation success of embryos transferred following in vitro fertilization. IntroductionImplantation is a critical stage in the establishment of pregnancy. Failure of the embryo to implant is clinically relevant to recurrent pregnancy loss and low success of in vitro fertilization. Despite some differences, the general principles of implantation are well conserved among mammalian species (1). Murine implantation starts with blastocyst apposition to the endometrium and attachment (E4-E4.5, vaginal plug is E0.5) that triggers the uterine stroma to proliferate and differentiate into the decidua, a spongy cell mass surrounding the blastocyst. Implantation then continues with the erosion of the epithelium that separates the blastocyst from the stroma, and embryonic trophoblasts invade the decidua and inner myometrium to reach maternal vessels. Decidualization is tightly associated with the spatial and temporal regulation of angiogenesis, i.e., the development of new capillaries from preexisting vessels (2). Angiogenesis at the implantation site (IS) is characterized by localized uterine vascular permeability along with the development of maternal vessels. With time, the latter dramatically increase in number and diameter in order to supply the fetal growing needs for oxygen and metabolites. The decidua supports the pregnancy, sustaining the embryo until the placenta is developed by E10. The decidua basalis, a remnant of the decidua at the implantation chamber will eventually be part of the outer side of the placenta, as it contacts the myometriu...

Rationale:The vascular endothelial growth factor (VEGF) decoy receptor soluble VEGF-R1 (sVEGF-R1) is thought to protect the cells that produce it from adverse VEGF signaling. To accomplish this role, a mechanism for pericellular retention of sVEGF-R1 is required. Local retention may also prevent the accumulation of high circulating levels of sVEGF-R1 and resulting interference with homeostatic VEGF functions in remote organs.Objective: To reveal natural storage depots of sVEGF-R1 and determine mechanisms underlying its pericellular retention. To uncover natural mechanisms regulating its systemic release. Methods and Results:We show that both the canonical and human-specific isoforms of sVEGF-R1 are strongly bound to heparin. sVEGF-R1 produced by vascular smooth muscle cells is stored in the vessel wall and can be displaced from isolated mouse aorta by heparin. Another major reservoir of sVEGF-R1 is the placenta. Heparin increases the level of sVEGF-R1 released by cultured human placental villi, and pregnant women treated with low molecular weight heparin showed markedly elevated levels of sVEGF-R1 in the circulation. Heparanase is expressed in human placenta at the same locales as sVEGF-R1, and its transgenic overexpression in mice resulted in a marked increase in the levels of circulating sVEGF-R1. Conversely, heparanase inhibition, by either a neutralizing antibody or by inhibition of its maturation, reduced the amounts of sVEGF-R1 released from human placental villi, indicating a natural role of heparanase in sVEGF-R1 release.Conclusions: Together, the findings uncover a new level of regulation governing sVEGF-R1 retention versus release and suggest that manipulations of the heparin/heparanase system could be harnessed for reducing unwarranted release of sVEGF-R1 in pathologies such as preeclampsia. (Circ Res.

Abstract-A human-specific splicing variant of vascular endothelial growth factor (VEGF) receptor 1 (Flt1) was discovered, producing a soluble receptor (designated sFlt1-14) that is qualitatively different from the previously described soluble receptor (sFlt1) and functioning as a potent VEGF inhibitor. sFlt1-14 is generated in a cell type-specific fashion, primarily in nonendothelial cells. Notably, in vascular smooth muscle cells, all Flt1 messenger RNA is converted to sFlt1-14, whereas endothelial cells of the same human vessel express sFlt1. sFlt1-14 expression by vascular smooth muscle cells is dynamically regulated as evidenced by its upregulation on coculture with endothelial cells or by direct exposure to VEGF. Increased production of soluble VEGF receptors during pregnancy is entirely attributable to induced expression of placental sFlt1-14 starting by the end of the first trimester. Expression is dramatically elevated in the placenta of women with preeclampsia, specifically induced in abnormal clusters of degenerative syncytiotrophoblasts known as syncytial knots, where it may undergo further messenger RNA editing. sFlt1-14 is the predominant VEGF-inhibiting protein produced by the preeclamptic placenta, accumulates in the circulation, and hence is capable of neutralizing VEGF in distant organs affected in preeclampsia. Together, these findings revealed a new natural VEGF inhibitor that has evolved in humans, possibly to protect nonendothelial cells from adverse VEGF signaling. Furthermore, the study uncovered the identity of a VEGF-blocking protein implicated in preeclampsia. Key Words: VEGF Ⅲ soluble VEGF receptor Ⅲ splicing Ⅲ preeclampsia Ⅲ vascular smooth muscle cell V ascular endothelial growth factor (VEGF) is the key factor promoting vasculogenesis and angiogenesis in the embryo and the factor orchestrating most, if not all, processes of adult neovascularization. In addition, VEGF performs nonvascular developmental functions and plays a number of homeostatic roles in the adult. The latter includes maintenance of endothelial fenestrations, a role in vasodilatation, and neurogenic and neurotrophic activities, among others. 1,2 Not surprisingly, therefore, VEGF is under a tight spatial and temporal regulation, and even a moderate deviation from its exquisite dosage control or perturbation of its precise morphogenetic gradients is detrimental for proper development and organ homeostasis. 3,4 Among the multiple layers of VEGF control, natural VEGF inhibitors, primarily soluble VEGF receptors, are likely to be important. Indeed, VEGF receptor-1 (Flt1) messenger RNA (mRNA) may undergo alternative splicing in a way that the encoded protein retains the ligand-binding domain but is devoid of the membrane-spanning and intracellular kinase domains, hence functioning as a VEGF-trapping soluble receptor. This secreted protein (designated sFlt1) is a potent inhibitor of both VEGF-A, VEGF-B, and placental growth factor. 2 It is widely used as a research tool for VEGF inhibition and, more recently, also harnessed...