Tissue-resident dendritic cells patrol for foreign antigens while undergoing slow mesenchymal migration. Using actomyosin-based structures called podosomes, dendritic cells probe and remodel extracellular matrix topographical cues. Podosomes comprise an actin-rich protrusive core surrounded by an adhesive ring of integrins, cytoskeletal adaptor proteins and actin network filaments. Here we reveal how the integrity and dynamics of protrusive cores and adhesive rings are coordinated by the actomyosin apparatus. Core growth by actin polymerization induces podosome protrusion and provides tension within the actin network filaments. The tension transmitted to the ring recruits vinculin and zyxin and preserves overall podosome integrity. Conversely, myosin IIA contracts the actin network filaments and applies tension to the vinculin molecules bound, counterbalancing core growth and eventually reducing podosome size and protrusion. We demonstrate a previously unrecognized interplay between actin and myosin IIA in podosomes, providing novel mechanistic insights into how actomyosin-based structures allow dendritic cells to sense the extracellular environment.

Various cellular processes (including cell motility) are driven by the regulated, polarized assembly of actin filaments into distinct force-producing arrays of defined size and architecture. Branched, linear, contractile and cytosolic arrays coexist in vivo, and cells intricately control the number, length and assembly rate of filaments in these arrays. Recent in vitro and in vivo studies have revealed novel molecular mechanisms that regulate the number of filament barbed and pointed ends and their respective assembly and disassembly rates, thus defining classes of dynamically different filaments, which coexist in the same cell. We propose that a global treadmilling process, in which a steady-state amount of polymerizable actin monomers is established by the dynamics of each network, is responsible for defining the size and turnover of coexisting actin networks. Furthermore, signal-induced changes in the partitioning of actin to distinct arrays (mediated by RHO GTPases) result in the establishment of various steady-state concentrations of polymerizable monomers, thereby globally influencing the growth rate of actin filaments.

Proteins targeting actin filament barbed ends play a pivotal role in motile processes. While formins enhance filament assembly, capping protein (CP) blocks polymerization. On their own, they both bind barbed ends with high affinity and very slow dissociation. Their barbed-end binding is thought to be mutually exclusive. CP has recently been shown to be present in filopodia and controls their morphology and dynamics. Here we explore how CP and formins may functionally coregulate filament barbed-end assembly. We show, using kinetic analysis of individual filaments by microfluidics-assisted fluorescence microscopy, that CP and mDia1 formin are able to simultaneously bind barbed ends. This is further confirmed using single-molecule imaging. Their mutually weakened binding enables rapid displacement of one by the other. We show that formin FMNL2 behaves similarly, thus suggesting that this is a general property of formins. Implications in filopodia regulation and barbed-end structural regulation are discussed.