Integrative modeling of sprout formation in angiogenesis: coupling the VEGFA-Notch signaling in a dynamic stalk-tip cell selection

During angiogenesis, new blood vessels headed by a migrating endothelial tip cell sprout from preexisting ones. This process is known to be regulated by two signaling pathways concurrently, vascular endothelial growth factor A (VEGFA) and Notch-Delta. Extracellular VEGFA activates the intracellular Notch-Delta pathway in nearby endothelial cells which results in endothelial (stalk, tip) differentiation. Retinal astrocytes appear to play a crucial role in polarizing new sprouts by secreting VEGFA. In vivo retinal angiogenesis experiments in neonatal mouse generated quantitative data on daily cell counts and morphological data of vascular network expanding over fibronectin-rich matrix. Based on this set of data and other existing ones, we developed a cell-based, multiscale mathematical model using the cellular Potts model framework to investigate the sprout evolution by integrating the VEGFA and Notch-Delta signaling pathways. The model incorporates three levels of description: 1) intracellular: each endothelial cell incorporates the VEGFA-activated Notch-Delta signaling pathway, 2) intercellular: tip and stalk endothelial phenotypes along the sprout are dynamically (depending on cell growth, cell-cell adhesion) interchangeable based on their Delta level, and 3) extracellular: the astrocyte-derived VEGFA acts as a chemoattractant and, together with the extracellular matrix (ECM) network, guides cell migration leading to sprout evolution and morphology. Starting with a single astrocyte embedded in a fibronectinrich matrix, we use the model to assess different scenarios regarding VEGFA levels and its interaction with matrix proteins. Simulation results suggest that astrocyte-derived VEGFA gradients along with heterogeneous ECM reproduces sprouting morphology, and the extension speed is in agreement with experimental data in 7 days postnatal mouse retina. Results also reproduce empirical observations in sprouting angiogenesis, including anastomosis, dynamic tip cell competition, and sprout regression as a result of Notch blockade.