Quantitative analysis of the mechanics of fibrillar fribronectin

Abstract

The information exchange between cells and their environment is a key mediator of cell behavior that will result in disease or dysfunction if disrupted. A thorough understanding of the in vivo cell environment is critical to relating cell behaviors observed in vitro to cell behaviors in pathogenesis and homeostasis. In addition to neighboring cells, the extracellular matrix (ECM) defines the local cell environment in the body. The protein fibronectin (Fn) is a prominent component of the ECM and a key cell adhesive ligand. Fn is assembled by cells into an extremely extensible, fibrous network through which cells migrate. Fn is also an integral part of the signaling machinery that instructs cell behavior. Cells may bind to Fn through a large number of receptors, in addition Fn binds and presents growth factors to cells, regulating their proliferative and migratory behavior. Stretch, applied to Fn fibers has been demonstrated to alter properties like binding site availability and fiber stiffness. In order to understand how molecular conformations and mechanical stretch regulate these cell instructive properties of Fn fibers, one must build a quantitative understanding of the intermolecular architecture of Fn fibers. In this study we have characterized the physical characteristics of fibronectin, its density, stiffness, extensibility, and viscoelasticity with respect to the extension of the Fn fiber. We have quantified conformational changes within the molecule that regulate both its mechanical properties and the availability of binding sites. In addition, we determined that the configuration of the molecular crosslinks strongly influences the fiber's physical properties. By taking measurements of the Fn fibers under constant tension we have shown that fibronectin is a highly viscoelastic material with extremely slow response times, indicating that in the slow pulling regime of cell tractions Fn material properties may deviate significantly from measurements made at higher pulling rates. A strong quantitative understanding of fibronectin's properties opens the door to new insights into disease and new approaches to creating engineered tissue constructs.