Conversion of a Signal into Forces for Axon Outgrowth through Pak1-Mediated Shootin1 Phosphorylation

Soluble guidance cues can direct cellular protrusion and migration by modulating adhesion and cytoskeletal dynamics. Actin filaments (F-actins) polymerize at the leading edge of motile cells and depolymerize proximally [1, 2]; this, together with myosin II activity, induces retrograde flow of F-actins [3-5]. It has been proposed that the traction forces underlying cellular motility may be regulated by the modulation of coupling efficiency between F-actin flow and the extracellular substrate via "clutch" molecules [6-10]. However, how cell signaling controls the coupling efficiency remains unknown. Shootin1 functions as a linker molecule that couples F-actin retrograde flow and the substrate at neuronal growth cones to promote axon outgrowth [11]. Here we show that shootin1 is located at a critical interface, transducing a chemical signal into traction forces for axon outgrowth. We found that a chemoattractant, netrin-1, positively regulates traction forces at axonal growth cones via Pak1-mediated shootin1 phosphorylation. This phosphorylation enhanced the interaction between shootin1 and F-actin retrograde flow, thereby promoting F-actin-substrate coupling, force generation, and concomitant filopodium extension and axon outgrowth. These results suggest that dynamic actin-substrate coupling can transduce chemical signals into mechanical forces to control cellular motility and provide a molecular-level description of how this transduction may occur.