Modeling the formation of in vitro filopodia

TL;DR

This paper presents a kinetic model and simulations of in vitro filopodia formation, revealing bundle size distributions and the effects of capping proteins on dendritic region size, aligning with experimental observations.

Contribution

The study introduces a kinetic aggregation model and 2D simulations that accurately reproduce experimental filopodia formation and bundle size distributions.

Findings

Bundle size distribution is broad below a characteristic size and exponential above.

Simulations agree qualitatively with mean field predictions.

Radial extent of dendritic region varies nonmonotonically with capping protein concentration.

Abstract

Filopodia are bundles of actin filaments that extend out ahead of the leading edge of a crawling cell to probe its upcoming environment. {\it In vitro} experiments [D. Vignjevic {\it et al.}, J. Cell Biol. {\bf 160}, 951 (2003)] have determined the minimal ingredients required for the formation of filopodia from the dendritic-like morphology of the leading edge. We model these experiments using kinetic aggregation equations for the density of growing bundle tips. In mean field, we determine the bundle size distribution to be broad for bundle sizes smaller than a characteristic bundle size above which the distribution decays exponentially. Two-dimensional simulations incorporating both bundling and cross-linking measure a bundle size distribution that agrees qualitatively with mean field. The simulations also demonstrate a nonmonotonicity in the radial extent of the dendritic region as a function of capping protein concentration, as was observed in experiments, due to the interplay between percolation and the ratcheting of growing filaments off a spherical obstacle.