Functional integrity of the contractile actin cortex is safeguarded by multiple Diaphanous-related formins

Significance The actin-rich cell cortex is a viscoelastic structure participating in a variety of cellular processes. However, the complete inventory of actin assembly factors driving its formation and knowledge about their specific contributions is still incomplete. We show here that functional integrity of the cell cortex in Dictyostelium and mammalian cells is backed up by multiple Diaphanous-related formins (DRFs) that are regulated by Rho-subfamily GTPases. These DRFs contribute to the generation of long actin filaments of the contractile actin cortex and are required for cell mechanics. Of note, these factors are excluded from Arp2/3 complex-nucleated networks, implying diversification of the cortex into functional subcompartments to segregate cortical actomyosin contraction in the rear or cleavage furrow ingression from actin-based protrusion in the front. The contractile actin cortex is a thin layer of filamentous actin, myosin motors, and regulatory proteins beneath the plasma membrane crucial to cytokinesis, morphogenesis, and cell migration. However, the factors regulating actin assembly in this compartment are not well understood. Using the Dictyostelium model system, we show that the three Diaphanous-related formins (DRFs) ForA, ForE, and ForH are regulated by the RhoA-like GTPase RacE and synergize in the assembly of filaments in the actin cortex. Single or double formin-null mutants displayed only moderate defects in cortex function whereas the concurrent elimination of all three formins or of RacE caused massive defects in cortical rigidity and architecture as assessed by aspiration assays and electron microscopy. Consistently, the triple formin and RacE mutants encompassed large peripheral patches devoid of cortical F-actin and exhibited severe defects in cytokinesis and multicellular development. Unexpectedly, many forA−/E−/H− and racE− mutants protruded efficiently, formed multiple exaggerated fronts, and migrated with morphologies reminiscent of rapidly moving fish keratocytes. In 2D-confinement, however, these mutants failed to properly polarize and recruit myosin II to the cell rear essential for migration. Cells arrested in these conditions displayed dramatically amplified flow of cortical actin filaments, as revealed by total internal reflection fluorescence (TIRF) imaging and iterative particle image velocimetry (PIV). Consistently, individual and combined, CRISPR/Cas9-mediated disruption of genes encoding mDia1 and -3 formins in B16-F1 mouse melanoma cells revealed enhanced frequency of cells displaying multiple fronts, again accompanied by defects in cell polarization and migration. These results suggest evolutionarily conserved functions for formin-mediated actin assembly in actin cortex mechanics.