*Article* **Endothelial Jagged1 Antagonizes Dll4**/**Notch Signaling in Decidual Angiogenesis during Early Mouse Pregnancy**

**Nicole M. Marchetto <sup>1</sup> , Salma Begum <sup>1</sup> , Tracy Wu <sup>1</sup> , Valerie O'Besso <sup>2</sup> , Christina C. Yarborough <sup>3</sup> , Nuriban Valero-Pacheco 4,5 , Aimee M. Beaulieu 4,5 , Jan K. Kitajewski <sup>6</sup> , Carrie J. Shawber <sup>7</sup> and Nataki C. Douglas 1,5,\***


Received: 30 June 2020; Accepted: 31 August 2020; Published: 5 September 2020

**Abstract:** Maternal spiral arteries and newly formed decidual capillaries support embryonic development prior to placentation. Previous studies demonstrated that Notch signaling is active in endothelial cells of both decidual capillaries and spiral arteries, however the role of Notch signaling in physiologic decidual angiogenesis and maintenance of the decidual vasculature in early mouse pregnancy has not yet been fully elucidated. We used the *Cdh5-CreERT2; Jagged1(Jag1)flox*/*flox* (*Jag1*∆*EC*) mouse model to delete Notch ligand, *Jag1*, in maternal endothelial cells during post-implantation, pre-placentation mouse pregnancy. Loss of endothelial *Jag1* leads to increased expression of Notch effectors, *Hey2* and *Nrarp,* and increased endothelial Notch signaling activity in areas of the decidua with remodeling angiogenesis. This correlated with an increase in Dll4 expression in capillary endothelial cells, but not spiral artery endothelial cells. Consistent with increased Dll4/Notch signaling, we observed decreased VEGFR2 expression and endothelial cell proliferation in angiogenic decidual capillaries. Despite aberrant Dll4 expression and Notch activation in *Jag1*∆*EC* mutants, pregnancies were maintained and the decidual vasculature was not altered up to embryonic day 7.5. Thus, Jag1 functions in the newly formed decidual capillaries as an antagonist of endothelial Dll4/Notch signaling during angiogenesis, but Jag1 signaling is not necessary for early uterine angiogenesis.

**Keywords:** Notch; Jag1; Dll4; endothelial cells; decidua; angiogenesis; capillaries; spiral arteries
