Chalmers Conferences, 9th European Conference on Mathematical and Theoretical Biology

A parameter study of a computational angiogenesis model suggests that limited Apelin sensitivity enables tip cells to lead sprouts
Margriet Palm, Marchien Dallinga, Erik van Dijk, Ingeborg Klaassen, Reinier Schlingemann, Roeland Merks

Last modified: 2014-03-28


New blood vessels are formed by endothelial cells (ECs) that detach from the vessel wall and form sprouts. Two types of ECs are observed in these sprouts: tip cells and stalk cells. Tip cells are highly migratory cells that lead the sprouts. Stalk cells are proliferative cells that follow the tip cell and form the stalk of the sprout. Although the behavior of tip and stalk cells during angiogenic sprouting has been well characterized, from a mechanistic point of view it is not well understood why two types of ECs are involved in angiogenesis.

To develop new hypotheses on the role of tip and stalk cell differentiation during angiogenesis, we extend a cell-based model of angiogenesis with differentiated tip and stalk cells. In this ECs form networks due to chemotaxis towards a chemoattractant that all ECs secrete. Tip and stalk cell fate is regulated by Dll4-Notch signaling, which causes lateral inhibition of tip cell fate. At what time scale tip and stalk cells can cross-differentiate is topic of debate. Therefore, we consider both a ''pre-determined'' model in which ECs are stably differentiated into tip or stalk cells, and a ''lateral inhibition'' model in which tip and stalk cells can rapidly cross-differentiate via Dll4-Notch signaling. With the ''pre-determined'' model we systematically search for cell behavior for which tip cells affect network formation and lead sprouts by varying the parameters that control tip cell behavior. Then, with the ''lateral inhibition'' model we further study the mechanisms by which such tip cells affect angiogenesis.

By performing a large-scale parameter screen, we found that in this model cells that are less sensitive to the chemoattractant than the other cells tend to mimic the behavior of tip cells: they move to the sprout tips and change the network. To see if this behavior corresponds with actual tip cells, we searched the literature for chemoattractants that could correspond our model. One chemoattractant, Apelin, stood out because it is secreted by all ECs and and its receptor, APJ, is only detected in stalk cells, suggesting that only stalk cells can respond to Apelin. To validate the proposed role of Apelin, we performed spheroid sprouting assays with either a wild-type population of tip and stalk cells, or a population with only stalk cells. Blocking Apelin signaling, either with an siRNA for Apelin or APJ, reduced sprouting in the wild-type spheroids but did not affect sprouting in the stalk cell spheroids. These results suggest that Apelin is a source of the differences between tip and stalk cells. Thus the high throughput screening employed in this study, combined with an analysis of published, top-down gene expression studies, helped to identify a molecule possibly mediating the interaction between tip and stalk cells during angiogenesis.