Investigating the roles of VEGFR1 in angiogenesis of the allantois and placenta - Project Summary The placenta develops to support fetal growth during pregnancy by facilitating the exchange of gases and nutrients between fetal and maternal circulations. This efficient exchange is enabled by the close apposition of maternal and fetal vessels within the placenta. Fetal placental vessels develop from the allantois, an extraembryonic tissue that itself becomes vascularized before fusing with the chorion to initiate formation of the chorioallantoic placenta in mice and humans. Improper development of the allantois and placental vasculature can lead to early first trimester miscarriage, intrauterine growth restriction, late fetal demise, and preterm birth. Despite the importance of these two tissues for a durable pregnancy, the molecular players involved in angiogenesis of the allantois and placenta remain obscure. Two known players at other studied sites of angiogenesis are VEGFR1 (also known as Flt1) and VEGFR2 (also known as Kdr). VEGFR2 is considered an agonist of vessel growth, while VEGFR1 is considered an antagonist role by binding VEGF ligand without utilizing its tyrosine kinase domain to propagate signal. This proposal will employ genetic and pharmacologic approaches in mouse models to explore the roles of VEGFR1 in the understudied allantois and placental vascular beds. I have deleted VEGFR1 in the allantois using Hoxa13Cre and found partial lethality around the time of chorioallantoic fusion. Mutant allantoides at E8.5 appear rounded and are unable to contact the chorion like their wildtype counterparts. Unexpectedly, VEGFR2 deletion in the allantois and resulting placental endothelial cells does not result in lethality. However, deleting one copy of VEGFR1 in a Hoxa13Cre; Kdr fl/- background placenta does result in lethality before E13.5. I hypothesize that VEGFR1 is required for both allantois and placental angiogenesis, but it serves a different role in each tissue. I propose that VEGFR1 acts as an antagonist of angiogenesis to prevent vascular overgrowth in the allantois but can assume an agonist role in the placenta to support angiogenesis when VEGFR2 is diminished. In Aim 1, I will identify the role of VEGFR1 in the allantois by characterizing allantois vasculature using wholemount imaging approaches when VEGFR1 is deleted genetically from the allantois or when wildtype allantois explants are challenged with VEGFR1 inhibitors pharmacologically. In Aim 2, I will determine the role of VEGFR1 in the fetal placental vasculature by quantifying placental vascular density and intrauterine growth restriction of Hoxa13Cre; Flt1 fl/+; Kdr fl/- mutants compared to Hoxa13Cre; Kdr fl/- controls. I will also test whether VEGFR1’s pro-angiogenic capabilities are due to its tyrosine kinase domain specifically by incorporating a VEGFR1 allele that lacks the tyrosine kinase domain. Overall, this proposal studies the vascular development of two tissues essential for fetal development but also uncovers a novel role for a well-studied receptor with implications for the treatment of vascular diseases.
