Novel Roles of Formin mDia2 in Lamellipodia and Filopodia Formation in Motile Cells

Abstract

Actin polymerization-driven protrusion of the leading edge is a key element of cell motility. The important actin nucleators formins and the Arp2/3 complex are believed to have nonoverlapping functions in inducing actin filament bundles in filopodia and dendritic networks in lamellipodia, respectively. We tested this idea by investigating the role of mDia2 formin in leading-edge protrusion by loss-of-function and gain-of-function approaches. Unexpectedly, mDia2 depletion by short interfering RNA (siRNA) severely inhibited lamellipodia. Structural analysis of the actin network in the few remaining lamellipodia suggested an mDia2 role in generation of long filaments. Consistently, constitutively active mDia2 (ΔGBD-mDia2) induced accumulation of long actin filaments in lamellipodia and increased persistence of lamellipodial protrusion. Depletion of mDia2 also inhibited filopodia, whereas expression of ΔGBD-mDia2 promoted their formation. Correlative light and electron microscopy showed that ΔGBD-mDia2–induced filopodia were formed from lamellipodial network through gradual convergence of long lamellipodial filaments into bundles. Efficient filopodia induction required mDia2 targeting to the membrane, likely through a scaffolding protein Abi1. Furthermore, mDia2 and Abi1 interacted through the N-terminal regulatory sequences of mDia2 and the SH3-containing Abi1 sequences. We propose that mDia2 plays an important role in formation of lamellipodia by nucleating and/or protecting from capping lamellipodial actin filaments, which subsequently exhibit high tendency to converge into filopodia. Cell motility is a cyclic process, with the protrusion of the leading edge followed by retraction of the rear. Protrusion is driven by polymerization of actin filaments, with the spatial organization of these filaments determining the shape of the protrusions. For example, the spike-like filopodia contain bundles of long actin filaments, whereas the sheet-like lamellipodia contain branched actin networks. In biochemical assays, two stimulators of actin polymerization, Arp2/3 complex and formins, induce branched or individual filaments, respectively. In cells, Arp2/3 complex and formins also appear to be implicated in the formation of lamellipodia and filopodia, respectively. However, when we investigated the role of mDia2 formin by functional approaches, we unexpectedly found that it is essential, not only for filopodia, but also for lamellipodia. Moreover, functions of mDia2 in lamellipodia and filopodia appeared intimately linked. We recorded behavior of cells by light microscopy and then used electron microscopy to study actin architecture in the same cells. We found that an activated form of mDia2 was first recruited to lamellipodia, where it induced many long, unbranched filaments, and from there, drove formation of filopodia through gradual convergence of these lamellipodial filaments into bundles. These data demonstrate a strong relationship between structurally different actin filament arrays and molecular machineries involved in their formation.