Multicellular Agent-Based Models of Angiogenesis Evaluate Endothelial Cell Signaling and the Role of Pericytes in Vascular Network Patterning 656 views

Angiogenesis – the formation of new capillary vessels from a preexisting microvascular network – is a concerted cellular process driven by endothelial cell proliferation and migration. Chemotactic cues through the vascular endothelial growth factor (VEGF) pathway combined with intercellular signaling via Notch1-DLL4 are a canonical driving force behind this patterning process. Despite having identified these two fundamental signaling pathways in angiogenesis, we do not yet understand how these signals propagate thorough multicellular networks and ultimately give rise to geometric pattern diversity. Further, even less is known about the interplay between pericytes, the support cells that enwrap all capillaries, and endothelial cells during angiogenesis. Despite playing a pivotal role in capillary health, we still do not know the extent to which pericytes modulate endothelial cell behaviors or interface with VEGF and Notch1 signaling in vascular networks. This body of work addresses these questions sequentially through a bottom-up computational modeling approach in combination with in vitro and in vivo experimental assays. Using agent-based models (ABMs), I have demonstrated that the Notch1-DLL4 signaling axis combined with VEGF receptor binding is sufficient to accurately predict the frequency and location of angiogenic endothelial sprouting events. This computational modeling approach is novel in its use of time course imaging to inform initial simulated endothelial cell positions and locations of subsequent sprouting events. By comparing experimental outcomes with predictions made by the ABM, I was able to directly and independently determine the accuracy of the ABM in predicting the number of capillary sprouting events and their locations during angiogenesis in an embryoid body tissue culture environment. As an extension of this work, I developed a second ABM of retinal angiogenesis that incorporates pericytes while maintaining endothelial intercellular signaling through Notch1-DLL4 and chemotactic cuing through VEGF receptor binding. Through quantitative analysis of geometric network properties compared to those of the mouse retinal vasculature, I demonstrated that simulated retinal vascular networks with pericytes produced more physiologically accurate geometries than those generated with endothelial cells alone. Further, these simulations suggested that pericytes act as buffers to endothelial signaling and, as a result, significantly affect vascular network geometries in the developing retina, a novel mechanistic hypothesis that warrants additional experimental investigation. Through the combined use of ABM and experimental assays of angiogenesis, I have explored how intracellular signaling and multi-cellular interactions integrate to produce emergent patterning of geometrically-heterogeneous and dynamic networks of blood vessels. In so doing, my work has generated a novel biological hypothesis and contributed new computational approaches for multi-cell modeling and model validation.

PHD (Doctor of Philosophy)

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2015-04-17