Diversified actin protrusions promote environmental exploration but are dispensable for locomotion of leukocytes

Abstract

Most migrating cells extrude their front by the force of actin polymerization. Polymerization requires an initial nucleation step, which is mediated by factors establishing either parallel filaments in the case of filopodia or branched filaments that form the branched lamellipodial network. Branches are considered essential for regular cell motility and are initiated by the Arp2/3 complex, which in turn is activated by nucleation-promoting factors of the WASP and WAVE families. Here we employed rapid amoeboid crawling leukocytes and found that deletion of the WAVE complex eliminated actin branching and thus lamellipodia formation. The cells were left with parallel filaments at the leading edge, which translated, depending on the differentiation status of the cell, into a unipolar pointed cell shape or cells with multiple filopodia. Remarkably, unipolar cells migrated with increased speed and enormous directional persistence, while they were unable to turn towards chemotactic gradients. Cells with multiple filopodia retained chemotactic activity but their migration was progressively impaired with increasing geometrical complexity of the extracellular environment. These findings establish that diversified leading edge protrusions serve as explorative structures while they slow down actual locomotion.

Arp2/3 Complex Is Required for Macrophage Integrin Functions but Is Dispensable for FcR Phagocytosis and In Vivo Motility

“…Another alternative is that NPFs like WASP and WAVE2 have significant Arp2/3 complex-independent functions (as has been proposed for N-WASP (Kovacs et al, 2011) that are required for chemotactic sensing. The emerging consensus that the Arp2/3 complex is dispensable for chemotaxis (Asokan et al, 2014; Collins et al, 2015; Vargas et al, 2015; Wu et al, 2012) meshes well with studies demonstrating that the critical Arp2/3 complex activators WAVE2 and Rac1/2 are not required for leukocyte chemotaxis either, but do lead to morphological changes and migratory behaviors similar to our Arpc2−/− macrophages (Leithner et al, 2016; Wheeler et al, 2006). This body of work can be viewed alongside previous studies on NPF function in macrophages, and the apparent discrepancy should stimulate further investigation.…”

FMNL formins boost lamellipodial force generation

“…It is well established that the Rac/WAVE/Arp2/3 complex pathway is essential for generation and maintenance of lamellipodia681011405153545556. Much less is known about how actin filament elongators such as formins and Ena/VASP family proteins contribute to protrusion and thus lamellipodia-dependent migration121457.…”

Ultrastructure of the actin cytoskeleton

“…Generation of filopodial bundles from branched networks often occurs via elongation of Arp2/3-nucleated branched filaments, which can be either an obligatory or optional pathway [19,20]. However, formin-nucleated filaments become essential for filopodial formation when Arp2/3-mediated nucleation is experimentally downregulated [21*–23]. Formation of filopodia in such cases can be even increased due to a competition between Arp2/3 complex and formins for actin monomers (reviewed in [19,24]).…”

“…Actin filaments in such protrusions are bundled and oriented with barbed ends to the tip of the bundle [21*], which is a shared feature of filopodia and stress fibers. However, the presence of NMII [23] and overall geometry of these bundles [25*] makes them more similar to stress fibers.…”