• FMNL formins boost lamellipodial force generation.

      Kage, Frieda; Winterhoff, Moritz; Dimchev, Vanessa; Mueller, Jan; Thalheim, Tobias; Freise, Anika; Brühmann, Stefan; Kollasser, Jana; Block, Jennifer; Dimchev, Georgi; et al. (2017-03-22)
      Migration frequently involves Rac-mediated protrusion of lamellipodia, formed by Arp2/3 complex-dependent branching thought to be crucial for force generation and stability of these networks. The formins FMNL2 and FMNL3 are Cdc42 effectors targeting to the lamellipodium tip and shown here to nucleate and elongate actin filaments with complementary activities in vitro. In migrating B16-F1 melanoma cells, both formins contribute to the velocity of lamellipodium protrusion. Loss of FMNL2/3 function in melanoma cells and fibroblasts reduces lamellipodial width, actin filament density and -bundling, without changing patterns of Arp2/3 complex incorporation. Strikingly, in melanoma cells, FMNL2/3 gene inactivation almost completely abolishes protrusion forces exerted by lamellipodia and modifies their ultrastructural organization. Consistently, CRISPR/Cas-mediated depletion of FMNL2/3 in fibroblasts reduces both migration and capability of cells to move against viscous media. Together, we conclude that force generation in lamellipodia strongly depends on FMNL formin activity, operating in addition to Arp2/3 complex-dependent filament branching.
    • FMNL2 and -3 regulate Golgi architecture and anterograde transport downstream of Cdc42.

      Kage, Frieda; Steffen, Anika; Ellinger, Adolf; Ranftler, Carmen; Gehre, Christian; Brakebusch, Cord; Pavelka, Margit; Stradal, Theresia; Rottner, Klemens; Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr.7, 38124 Braunschweig, Germany. (2017-08-29)
      The Rho-family small GTPase Cdc42 localizes at plasma membrane and Golgi complex and aside from protrusion and migration operates in vesicle trafficking, endo- and exocytosis as well as establishment and/or maintenance of cell polarity. The formin family members FMNL2 and -3 are actin assembly factors established to regulate cell edge protrusion during migration and invasion. Here we report these formins to additionally accumulate and function at the Golgi apparatus. As opposed to lamellipodia, Golgi targeting of these proteins required both their N-terminal myristoylation and the interaction with Cdc42. Moreover, Golgi association of FMNL2 or -3 induced a phalloidin-detectable actin meshwork around the Golgi. Importantly, functional interference with FMNL2/3 formins by RNAi or CRISPR/Cas9-mediated gene deletion invariably induced Golgi fragmentation in different cell lines. Furthermore, absence of these proteins led to enlargement of endosomes as well as defective maturation and/or sorting into late endosomes and lysosomes. In line with Cdc42 - recently established to regulate anterograde transport through the Golgi by cargo sorting and carrier formation - FMNL2/3 depletion also affected anterograde trafficking of VSV-G from the Golgi to the plasma membrane. Our data thus link FMNL2/3 formins to actin assembly-dependent functions of Cdc42 in anterograde transport through the Golgi apparatus.