Endothelial cell differentiation during angiogenesis

The functional shift of quiescent endothelial cells (ECs) to cells that proliferate and migrate is a key event during angiogenesis. Over 50 human disease states have found ways to trigger this shift, resulting in pathological angiogenesis. Angiogenesis-related ocular diseases such as age-related macular degeneration and diabetic retinopathy are the leading causes of blindness and have a high socioeconomic impact in western countries. Taken together, the costs of medical treatment, loss of income, and need for assistance in daily living make the societal cost of pathological angiogenesis in the eye immense.

The vascular network is formed during early stages of development, and its correct and early function is absolutely critical for survival of the embryo. All blood vessels are lined by ECs, which form the interface between circulating blood in the lumen and the rest of the vessel wall. Under normal conditions, ECs are a remarkably quiescent cell type, undergoing division approximately once every 1000 days. The balance between closely interacting angiogenic and angiostatic factors ultimately eventually determines if, where and when the "angiogenic switch" is turned on with angiogenesis as the result.

Sprouting angiogenesis requires selection of ECs from an existing blood vessel in order to form the new vessel, while at the same time their surrounding ECs must remain quiescent. From recent studies a model has emerged in which ECs differentiate into 3 specialized cell types with distinct phenotypes during angiogenesis. First, a single ‘tip cell’ develops. This EC breaks down the basal lamina, emerges from its parent blood vessel and becomes the leading cell of the sprouting vessel. The tip cell migrates into the extracellular matrix and senses micro-environmental attractive and repulsive signals for guidance. Secondly, following directly behind the migrating tip cell, other ECs differentiate under the influence of the adjacent tip cell into ‘stalk cells’ that proliferate and bridge the gap between the tip cell and the parent vasculature. Stalk cells generate the blood vessel lumen through the formation of intra-cellular vacuoles, a process called ‘lumenogenesis’. Thirdly, ECs behind the stalk cells differentiate into ‘phalanx cells’, and align in a smooth cobblestone monolayer, becoming the most inner cell layer in the new blood vessel. Phalanx cells no longer proliferate, express tight junctions and make contact with mural cells.

This project aims to identify molecular mechanisms that regulate these different EC phenotypes, and to investigate the role of tip cell specific genes in regulating ocular angiogenic processes. Identification of such key molecules involved in EC differentiation is a crucial step forward in finding new and more efficient potential targets for anti-angiogenic therapies. This enables the design and application of targeted therapies aimed at only those ECs that are activated during angiogenesis, without interfering physiological pathways that are also necessary for quiescent ECs, like the VEGF pathway.