The vascular system is the first organ system to form during embryonic development. Its proper establishment is required for embryonic growth and survival as well as tissue homeostasis. Blood vessels are comprised of endothelial cells which must coordinate multiple behaviors such as migration, adhesion, sprouting, and proliferation to properly assemble into cord-like structures and then undergo further remodeling and branching to form a patent vascular network. Disruptions in these processes result in a number of human disease states including tumorigenesis, stroke, and atherosclerosis necessitating a better understanding of the factors and pathways that regulate these key developmental steps. Despite the large number of identified growth factors and proteins that are critical for vascular development, the field still lacks a thorough understanding of how endothelial gene programs are regulated. CASTOR (CASZ1) is an evolutionarily conserved transcription factor expressed in the cardiovascular system. Mutations in the human Casz1 locus have been recently genetically associated with cardiovascular disease risk factors including hypertension and high blood pressure although the mechanisms by which CASZ1 functions in vascular biology are unknown. Here we demonstrate that CASZ1 is critical for proper vascular assembly and morphogenesis during embryonic development. Mechanistically, we have determined that CASZ1 directly binds to and regulates the expression of Epidermal growth factor-like domain 7 (Egfl7), an extracellular matrix (ECM) protein previously implicated in vessel sprouting and vascular lumen formation. We further showed that the CASZ1/Egfl7 transcriptional pathway is required to promote the activation of the RhoA GTPase signaling pathway to directly modulate endothelial cell adhesion, proliferation, and cell shape in order to support proper cellular behavior for vessel development. Moreover, for the first time we have uncovered the molecular and cellular basis for vascular lumen formation in Xenopus and show that EGFL7 is required for the proper cell shape changes and resolution of cell-cell junctions necessary to facilitate the transition of cord-like structures into functional vascular tubes. Collectively, this work provides insight into how transcription factors, ECM proteins, and intracellular signaling pathways are coordinated to achieve timely and appropriate assembly of a hierarchically branched and perfusable vascular network.Doctor of Philosoph
