The Eps8/IRSp53/VASP Network Differentially Controls Actin Capping and Bundling in Filopodia Formation

There is a body of literature that describes the geometry and the physics of filopodia using either stochastic models or partial differential equations and elasticity and coarse-grained theory. Comparatively, there is a paucity of models focusing on the regulation of the network of proteins that control the formation of different actin structures. Using a combination of in-vivo and in-vitro experiments together with a system of ordinary differential equations, we focused on a small number of well-characterized, interacting molecules involved in actin-dependent filopodia formation: the actin remodeler Eps8, whose capping and bundling activities are a function of its ligands, Abi-1 and IRSp53, respectively; VASP and Capping Protein (CP), which exert antagonistic functions in controlling filament elongation. The model emphasizes the essential role of complexes that contain the membrane deforming protein IRSp53, in the process of filopodia initiation. This model accurately accounted for all observations, including a seemingly paradoxical result whereby genetic removal of Eps8 reduced filopodia in HeLa, but increased them in hippocampal neurons, and generated quantitative predictions, which were experimentally verified. The model further permitted us to explain how filopodia are generated in different cellular contexts, depending on the dynamic interaction established by Eps8, IRSp53 and VASP with actin filaments, thus revealing an unexpected plasticity of the signaling network that governs the multifunctional activities of its components in the formation of filopodia.已有大量文献采用随机模型、偏微分方程、弹性力学及粗粒化理论，对丝状伪足（filopodia）的几何结构与物理特性展开描述。相较而言，目前针对调控肌动蛋白（actin）不同结构形成的蛋白质网络的建模研究仍较为匮乏。本研究结合体内（in vivo）与体外（in vitro）实验，辅以常微分方程组建模，聚焦于少数经过充分表征、参与肌动蛋白依赖型丝状伪足形成的相互作用分子：包括肌动蛋白重塑因子Eps8，其盖帽活性与成束活性分别依赖于配体Abi-1与IRSp53；以及VASP与盖帽蛋白（Capping Protein, CP），二者在调控肌动蛋白丝伸长过程中发挥拮抗作用。本模型着重揭示了包含膜形变蛋白IRSp53的复合物在丝状伪足起始过程中的核心作用。该模型能够精准契合所有实验观测结果，其中包括一项看似矛盾的现象：敲除Eps8基因会使HeLa细胞中的丝状伪足数量减少，却会提升海马神经元内的丝状伪足数目；同时模型还生成了可通过实验验证的定量预测结果。此外，本模型还帮助我们阐释了在不同细胞环境中，丝状伪足如何通过Eps8、IRSp53与VASP和肌动蛋白丝之间的动态相互作用得以形成，进而揭示了调控丝状伪足形成过程中各组分多功能活性的信号网络具备此前未被认知的可塑性。