Fig 1.
Both β-1 and β-3 integrins localize at the cell periphery, where nascent adhesions assemble.
(A) Cartoon representation of active, fully extended αIIBβ3 integrin, which is closely related to αvβ3 [82]. (B) Representative images of human foreskin fibroblasts (HFF) fixed after 60 minutes of spreading on fibronectin coated glass coverslips and immunostained for actin and either β-1 or β-3 integrins.
Fig 2.
Schematic illustration of the model system.
(A) Side view of the computational domain, where single-point, two-state integrin particles diffuse and assemble laterally. Upon activation (from blue to red), integrins can establish interactions (black spring) with ligands (green particles) and other active integrins (red springs). (B) Schematics of the system with actin flow: a force mimicking actin flow is exerted on ligand-bound integrins, parallel to the substrate, and builds tension on the integrin/ligand bond. (C) Lifetime versus tension curves of catch bonds used to mimic β-1 (black) and β-3 (red) integrins. (D) Schematic illustration of the positive feedback between actin filament binding and integrin activation. Once an integrin particle is bound to a ligand, it can establish interactions with actin. Upon binding actin, its activation rate increases. Upon deactivating and unbinding, its propensity to become active again increases, leading to increased probability of binding new ligands and actin filaments. This results in a positive feedback between ligand binding and engaging the actin cytoskeleton.
Fig 3.
Integrin affinity and avidity determine clustering and ligand binding.
(A) Configuration of clustered integrins (black circles) and ligand-bound integrins (blue circles) at a time point between 80–100 s of simulations, using EII = 7 kBT and EIL = 1 kBT. (B) Configuration of clustered integrins (black circles) and ligand-bound integrins (blue circles) at a time point between 80–100 s of simulations, using EII = 1 kBT and EIL = 7 kBT. (C) Configuration of clustered integrins (black circles) and ligand-bound integrins (blue circles) at a time point between 80–100 s of simulations, using EII = 7 kBT and EIL = 7 kBT. (D) Average percentage of clustered integrins relative to total integrins, by varying EIL and EII. (E) Average percentage of ligand-bound integrins relative to total integrins, by varying EIL and EII. (F) Fraction between clustered and ligand-bound integrins, by varying EIL and EII. This indicates the amount of clustered integrins per ligand-bound integrin. All data are computed between 100–130 s of simulations, from four independent runs.
Fig 4.
Integrin activation rate enhances clustering and ligand binding.
(A) Average percentage of clustered integrins as a function of activation rate, ka, by varying ligand binding affinity, EIL, and keeping EII = 5 kBT. (B) Corresponding average percentage of ligand-bound integrins. Data are computed between 10–200 s of simulations from three independent runs. Error bars indicate standard deviation from the mean.
Fig 5.
Amounts of β-1 and β-3 integrins determine ligand binding, traction stress, and adhesion stability.
(A) Percentage of ligand-bound integrins by varying fraction of β-1 in a system of β-3 integrins, as a function of actin flow speed. (B) Average tension per integrin varying actin flow speed and percentage of simulated β-1 in a system of β-3 integrins. (C) Average distance between nearest ligand-bound integrins, without distinction between β-1 and β-3 integrins. Represented are regions of unstable and stable adhesions, depending upon the minimum spatial separation between any integrin type. Data are computed as averages between 1–120 s of simulations.
Fig 6.
Actin architecture does not impact integrin clustering and ligand binding but changes the physical distribution of integrins in adhesions.
(A-C) Snapshots from the simulations: random, crisscrossed and bundled actin networks above a layer of integrins. Filled circles indicate clustered integrins; empty circles indicate ligand-bound integrins. (D) Average percentage of ligand-bound integrins with respect to increasing amount of β-1 integrins (using EIL = EII = 9 kBT) at varying actin architectures. (E) Corresponding percentage of clustered integrins. (F) Distribution of average nearest neighbor distances between ligand-bound integrins using 50% β-1 integrins at varying actin architectures. Results are computed as averages between 20–30 s of simulation.