A conserved role of the VEGF pathway in angiogenesis of an ectodermally-derived vasculature.

Angiogenesis, the growth and remodeling of a vascular network, is an essential process during development, growth and disease. Here we studied the role of the vascular endothelial growth factor receptor (VEGFR) in experimentally-induced angiogenesis in the colonial ascidian Botryllus schlosseri (Tunicata, Ascidiacea). The circulatory system of B. schlosseri is composed of two distinct, but interconnected regions: a plot of sinuses and lacunae which line the body, and a transparent, macroscopic extracorporeal vascular network. The vessels of the extracorporeal vasculature are morphologically inverted in comparison to the vasculature in vertebrates: they consist of a single layer of ectodermally-derived cells with the basal lamina lining the lumen of the vessel. We found that when the peripheral circulatory system of a colony is surgically removed, it can completely regenerate within 24 to 48 h and this regeneration is dependent on proper function of the VEGF pathway: siRNA-mediated knockdown of the VEGFR blocked vascular regeneration, and interfered with vascular homeostasis. In addition, a small molecule, the VEGFR kinase inhibitor PTK787/ZK222584, phenocopied the siRNA knockdown in a reversible manner. Despite the disparate germ layer origins and morphology of the vasculature, the developmental program of branching morphogenesis during angiogenesis is controlled by similar molecular mechanisms, suggesting that the function of the VEGF pathway may be co-opted during the regeneration of an ectoderm-derived tubular structure.