A Computational Model of Cell-Generated Traction Forces and Fibronectin Assembly

The extracellular matrix (ECM) is an assembly of proteins that surround cells, and serves as the cell substrate in vivo. A primary component of newly synthesized ECM is the fibronectin (FN). Despite many years of research, the mechanism of FN assembly is still not completely understood. While it is recognized that FN assembly requires application of traction force to expose a buried FN-FN binding site, such a site has never been elucidated. We hypothesize that assembly of fibronectin (FN) fibrils is a complex event: each of the 15 Type III domains in FN is made up of a sandwich of 7 beta strands; when relaxed, the Type III domains are folded such that the beta strands are twisted, blocking the non-specific binding of other proteins. Application of force straightens these beta-strands, allowing for binding of other FNs via a beta-strand addition mechanism. This suggests that all 15 domains are capable of binding FN molecules in a growing fibril. To investigate this hypothesis, we present a mechanistic computational model of cell/FN/substrate biomechanical interactions, which accounts for the unique, nonlinear mechanical properties of each domain and the stochastic binding between molecular clutches and the moving actin bundle. Monte Carlo simulations predict that increasing substrate stiffness leads to longer and thicker fibrils. Additionally, the model demonstrates complex time-dependent dynamics governing the size of the growing fibril, the domains’ stiffness, and traction forces; that is, at low force, a small subset of domains open, allowing for minimal fibril assembly, while large forces unfold a considerable fraction of domains and create large, thick fibrils. Simulation outputs are compared to experimental data in which traction force, FN assembly, and domain opening are quantified using microfabricated pillar arrays and cysteine-labeling of recombinant FNs with introduced buried cysteines.