Computational Screening of Tip and Stalk Cell Behavior Proposes a Role for Apelin Signaling in Sprout Progression

Angiogenesis involves the formation of new blood vessels by sprouting or splitting of existing blood vessels. During sprouting, a highly motile type of endothelial cell, called the tip cell, migrates from the blood vessels followed by stalk cells, an endothelial cell type that forms the body of the sprout. To get more insight into how tip cells contribute to angiogenesis, we extended an existing computational model of vascular network formation based on the cellular Potts model with tip and stalk differentiation, without making a priori assumptions about the differences between tip cells and stalk cells. To predict potential differences, we looked for parameter values that make tip cells (a) move to the sprout tip, and (b) change the morphology of the angiogenic networks. The screening predicted that if tip cells respond less effectively to an endothelial chemoattractant than stalk cells, they move to the tips of the sprouts, which impacts the morphology of the networks. A comparison of this model prediction with genes expressed differentially in tip and stalk cells revealed that the endothelial chemoattractant Apelin and its receptor APJ may match the model prediction. To test the model prediction we inhibited Apelin signaling in our model and in an in vitro model of angiogenic sprouting, and found that in both cases inhibition of Apelin or of its receptor APJ reduces sprouting. Based on the prediction of the computational model, we propose that the differential expression of Apelin and APJ yields a “self-generated” gradient mechanisms that accelerates the extension of the sprout.血管生成（Angiogenesis）指通过现有血管出芽或分裂生成新生血管的生物学过程。在血管出芽阶段，一类具备高迁移能力的内皮细胞（endothelial cell）——即尖端细胞（tip cell）——会从原有血管中迁出，随后由构成出芽结构主体的柄细胞（stalk cell，一类内皮细胞亚型）接续延伸。为深入解析尖端细胞参与血管生成的具体机制，我们拓展了一项已有的基于细胞Potts模型（cellular Potts model）的血管网络形成计算模型，该模型支持尖端细胞与柄细胞的分化过程，且未对两类细胞间的固有差异做出先验假设。为预测二者的潜在功能差异，我们筛选可使尖端细胞（a）迁移至出芽尖端，且（b）改变血管生成网络形态特征的参数组合。筛选结果显示，若尖端细胞相较于柄细胞，对内皮趋化因子的响应效率更低，则其会迁移至出芽尖端，进而对血管网络的形态产生影响。将该模型预测结果与尖端细胞和柄细胞的差异表达基因进行比对后发现，内皮趋化因子阿朴脂蛋白样肽（Apelin）及其受体APJ（APJ）可能契合该模型的预测结论。为验证该模型预测，我们分别在本计算模型与血管出芽的体外（in vitro）模型中抑制阿朴脂蛋白信号通路，结果显示，无论是抑制阿朴脂蛋白还是其受体APJ，均可削弱血管出芽过程。基于该计算模型的预测结果，我们提出：阿朴脂蛋白与APJ的差异表达可形成一种"自主生成"的浓度梯度机制，从而加速血管出芽的延伸进程。