The Mechanosensitivity of Actin Bundles

Abstract

Cells’ ability to sense their environment is essential for many cellular processes including cell division, migration and morphogenesis. The actin cytoskeleton, which has been shown to be mechanosensitive, is organized into different architectures that carry out various functions within the cell. Filopodia, which are finger-like structures consisting of actin filaments bundled in parallel, emerge at the cell front and orient the cell in response to its mechanical environment. These actin filaments are elongated at their barbed ends by formins and Ena/VASP and cross-linked by the bundling protein fascin. These two machineries are thought to collaborate to design a unique type of actin network that governs filopodium dynamics, yet the exact mechanism by which these two key proteins interact, and how mechanosensing is achieved in filopodia, are not well understood. Core questions such as: how actin filaments self-assemble in a bundle; how forces are transmitted along filopodia; how fascin and formin synergize to control the growth of actin filament in filopodia remain to be addressed. To tackle these questions, we use a microfluidics-based approach to reconstitute, in vitro, a minimal system to recapitulate the mechanosensitivy of actin bundles : from single filaments to bundles of several filaments, using the actin binding proteins formin and fascin. On single filaments, the activity of formins and fascins in a large range of biochemical conditions will be probed: first separately, then when present together on the same filament. We will scale up to bundles of several filaments mimicking bundles in filopodia. Again, microfluidics will be used to apply physiologically relevant pulling and pushing forces to actin bundles. This bottom-up approach will allow us to understand actin bundles mechanosensitivity and bring us closer to obtaining a comprehensive description of force generation and transmission in formin and fascin generated bundles.