Lamellipodium tip actin barbed ends serve as a force sensor

Abstract

Cells change direction of migration by sensing rigidity of environment and traction force, yet its underlying mechanism is unclear. Here, we show that tip actin barbed ends serve as an active “force sensor” at the leading edge. We established a method to visualize intracellular single‐molecule fluorescent actin through an elastic culture substrate. We found that immediately after cell edge stretch, actin assembly increased specifically at the lamellipodium tip. The rate of actin assembly increased with increasing stretch speed. Furthermore, tip actin polymerization remained elevated at the subsequent hold step, which was accompanied by a decrease in the load on the tip barbed ends. Stretch‐induced tip actin polymerization was still observed without either the WAVE complex or Ena/VASP proteins. The observed relationships between forces and tip actin polymerization are consistent with a force–velocity relationship as predicted by the Brownian ratchet mechanism. Stretch caused extra membrane protrusion with respect to the stretched substrate and increased local tip polymerization by >5% of total cellular actin in 30 s. Our data reveal that augmentation of lamellipodium tip actin assembly is directly coupled to the load decrease, which may serve as a force sensor for directed cell protrusion.

A mechanism with severing near barbed ends and annealing explains structure and dynamics of dendritic actin networks

“…2D). The latter histogram is not very different from experimentally-measured distributions (Koseki et al, 2019;Maly and Borisy, 2002;Vinzenz et al, 2012).…”

“…This dendritic lamellipodia network structure, evident in electron micrographs of keratocytes (Svitkina et al, 1997) has been quantified by more recent electron tomograms near the leading edge, revealing the number of barbed ends, branches and filaments (Mueller et al, 2017;Vinzenz et al, 2012). Its characteristic pattern with filaments orientated primarily at ±35 • with respect to the protrusion axis (Koseki et al, 2019;Maly and Borisy, 2002;Mueller et al, 2017;Schaub et al, 2007) has been interpreted by two dimensional dendritic network models (Atilgan et al, 2005;Holz and Vavylonis, 2018;Maly and Borisy, 2002;Schaus et al, 2007;Weichsel and Schwarz, 2010).…”

Discrete mechanical model of lamellipodial actin network implements molecular clutch mechanism and generates arcs and microspikes

“…filament insertion) rate while in reality a reduction of polymerization with force is expected [49,55]. Whether the relationship between the external force at the leading edge the actin network extension speed should be concave or convex for actin networks has been debated [40,41,48,57]. Our simulations result in a convex curve for the retrograde flow speed as a function of the pushing force on the actin network.…”

“…We implement polymerization at the leading edge by adding individual filaments as 1 µm-long segments (a typical filament length in lamellipodia [12,15,47]), with their pointed ends at y = 0.5 µm and orientations along the xy plane between -70°and 70°, a distribution that approximately corresponds to the experimental distribution in fibroblast cells, however without peaks at -35°and 35° [15,48]. In this way we leave out most of the complexity of force generation by polymerization close to the leading edge, including filament attachment to the cell membrane and filament orientation [36,[49][50][51].…”

“…Having established a steady-state model at the filament level allows for future improvements and refinement to account for important regulatory mechanisms closer to the molecular level. This includes a more detailed description of force-dependent polymerization at the leading edge as has been done in several other models [36,39,41,48,49,55]. Motors could also be explicitly modeled as in prior works [66,82] allowing for significantly more informative force profiles and network morphology changes due to pulling than in our current model.…”