Fig 1.
Computational model of integrin-based adhesion assembly in the presence of an actin bundle.
A. Schematics of the 2D computational model. The domain consists of a grid of ideal springs (gray particles) with stiffness k. Integrins diffuse (green particles) with diffusion coefficient D and establish harmonic interactions with the substrate springs (magenta particles). If integrins are in the region of the actin fiber (pink area) a force, Fmyo is also added. Fmyo build tension on the integrin-ligand bond. This tension determines the bond lifetime, τ, which is the reciprocal of the rupture rate, koff, as τ = koff. B. The bond lifetime versus force relation for integrin-ligand bonds follows catch-slip bond kinetics.
Fig 2.
Experimental image showing adhesion elongation and stabilization under actin fibers.
COS7 cell, transiently transfected to co-express mScarlet-paxillin and EGFP-F-tractin. A. mScarlet-paxillin imaged by Total Internal Reflection Fluorescence Microscopy (TIRFM) and B. EGFP-F-tractin imaged by epifluorescence. Edge motion is shown in 80s intervals. Nascent adhesions (circles) assemble (orange to red) and disassemble (red to orange) as the leading edge moves forward. Stabilized adhesions under bundled actin fibers (blue arrows) do not disassemble.
Fig 3.
Actomyosin force combined with substrate stiffness result in load-and-fail of adhesions.
A. A heatmap of the average percentage of ligand bound integrins varying substrate stiffness, k, between 0.1–0.7 pN/nm, and actomyosin force, Fmyo, between 0–40 pN. The reported values are averages between 100–300 s of simulations. B. A heatmap of the total ligated time of integrins during 300 s of simulations, varying k between 0.1–0.7 pN/nm and Fmyo between 0–40 pN. C. The average percentage of integrins undergoing unbinding every second of simulations, varying Fmyo between 0–40 pN, using k = 0.1, 0.4, and 0.7 pN/nm. Data are extracted between 100–300 s of simulations.
Fig 4.
Bundling of actin filaments stabilizes the assembly of nascent adhesions.
A. Snapshots of ligand-bound integrins (magenta) at 200 s of simulations, using no fiber (left) and a fiber (right; the pink rectangle indicates the location where the actin fiber overlaps with the adhesion). B. The average percentage of ligated integrins along the fiber short axis in the absence and presence of the fiber. C. Average percentage of ligated integrins using Pbundling = 0 (no fiber) and Pbundling = 1 (fiber). Errorbars indicate standard deviation from the mean. D. Average angle of the adhesion, relative to the direction of the actin fiber, using Pbundling = 0 (no fiber) and Pbundling = 1 (fiber). The angle is calculated from the direction of the first principal component of the 2D positions of ligated integrins, computed at each second of simulations between 100–500 s. Errorbars indicate standard deviation from the mean. E. Frequency of binding events using Pbundling = 0 (no fiber) and Pbundling = 1 (fiber). F. Distribution of total time spent by integrins in the ligated state, calculated as sum of the ligand-bound lifetimes of for each integrin over the course of 500 s of simulations, using Pbundling = 0 (no fiber) and Pbundling = 1 (fiber). All data are computed in the absence of actomyosin contractility, from 3 independent runs using k = 0.6 pN/nm.
Fig 5.
With high actin bundling, actomyosin tension has no effects on adhesion stabilization.
A. The average fraction of ligand-bound integrins varying Pbundling between 0–1, and Fmyo between 0–30 pN and using k = 0.6 pN/nm. B. The average total ligand-bound time for integrins, varying Pbundling between 0–1, and Fmyo between 0–30 pN and using k = 0.6 pN/nm. Data are computed as averages from three independent simulations. C. The average fraction of ligand-bound integrins for probabilities of actin filaments bundling Pbundling = 0, 0.5, and 1, varying Fmyo between 0–30 pN and k between 0.2–0.6 pN/nm. D. Distribution of the final tension on the integrin-ligand bonds before failure varying Fmyo between 0–30 pN, using Pbundling = 0 and 1, and fixed substrate stiffness at k = 0.6 pN/nm. All data are computed between 100–300 s of simulations, from three independent runs. F. The average angle of integrin adhesions relative to the fiber axis using Pbundling = 0 and 1, using Fmyo = 0 and k between 0.2–0.6 pN/nm. The angle is calculated from the direction of the first principal component considering the 2D positions of ligated integrins, at each second of simulation.