Cofilin Regulates Filopodial Structure and Flexibility in Neuronal Growth Cones

Cofilin is best known for its ability to sever actin filaments, and facilitate cytoskeletal recycling inside of cells. At higher concentrations, in vitro , cofilin stabilizes a more flexible, hyper-twisted state of actin known as “cofilactin”, but a structural role for cofilactin, in situ , has not been observed. Combining cryo-electron tomography and live-cell imaging in neuronal growth cones, we show that filopodial actin bundles can switch between a fascin-linked and a cofilin-decorated state, composed of hyper-twisted cofilactin filaments. These cofilactin bundles contribute to the flexibility of filopodial actin networks, thus regulating growth cone searching dynamics. Our results provide mechanistic insight into the processes underlying proper brain development, as well as fundamentals of cytoskeletal mechanics inside confined cellular spaces.