Research Summary The blood, lymphatic, and sinusoidal vasculatures are molecularly and functionally distinct vascular networks whose normal growth and development are essential to support embryogenesis. While there has been notable effort to understand mechanisms of blood vessel development, relatively little is known about the role of vascular endothelial growth factor (VEGF) receptor 3 (VEGFR3) and its main ligand VEGF-C in sinusoidal development. Despite decades of work on VEGFR2 and VEGFR3 signaling, insights into the field have been complicated by use of various genetic and pharmacologic models with divergent phenotypes. Furthermore, VEGF-C can weakly bind and activate VEGFR2 in addition to inducing VEGFR2-VEGFR3 heterodimers. As a result, the role of VEGF-C in vascular development remains incompletely understood. Therefore, our goal is to determine the role of VEGF-C/VEGFR3 in vascular development and identify novel signaling mechanisms through which it functions through a combination of in vivo mouse and in vitro cell culture studies. We and others have previously identified VEGF-C as an important regulator of fetal liver hematopoiesis. This was primarily attributed to defects in erythroid maturation, however the sinusoids are the only VEGFR3-positive cells in the fetal liver. Strikingly, endothelial deletion of VEGFR3 and vascular endothelial (VE-)cadherin gain- of-function mice both phenocopy VEGF-C knockout mice, suggesting that diminished fetal liver hematopoiesis is in fact due to sinusoidal defects. These mice also have decreased bone marrow (BM) sinusoids. BM defects due to endothelial loss of VEGFR3 are rescued by VE-cadherin haploinsufficiency, demonstrating that VEGFR3 negatively regulates VE-cadherin to promote sinusoidal growth. Whether this process depends on VEGF-C and the signaling mechanisms through which VEGFR3 regulates VE-cadherin remain unclear. Therefore, I hypothesize that VEGF-C activates VEGFR3 to directly and negatively regulate VE-cadherin during sinusoidal angiogenesis and lymphangiogenesis. Aim 1 will test whether haploinsufficiency of VE- cadherin can rescue fetal liver and lymphatic growth defects, as I have demonstrated to be true in the bone marrow sinusoids. Aim 2 will test whether VEGFR3 functions in a VEGF-C-dependent or independent manner in the skin, liver, and bone marrow during sinusoidal angiogenesis and lymphangiogenesis. Finally, Aim 3 will determine the mechanism by which VEGFR3 negatively regulates VE-cadherin through in vitro studies using human lymphatic endothelial cells. Together, these studies will bring novel insights into the mechanism by which VEGF-C/VEGFR3 may regulate VE-cadherin across various vascular beds in multiple organs, which could have important implications for liver regeneration, BM reconstitution, and lymphatic disorders.