Focal contact assembly through cytoskeletal polymerization: steady state analysis

For many cell types, initial receptor-mediated attachment to a ligand-coated surface is followed by the formation of focal contacts - strong, specialized, discrete adhesive connections between cell and substrate in which receptors are clustered and simultaneously linked to extracellular ligand and cytoskeletal proteins. Since adhesion affects many aspects of cellular physiology including growth, differentiation, and motility, understanding the biochemical factors which regulate focal contact assembly should enhance our understanding of these phenomena. In this paper, we present a mathematical model to examine how receptor-ligand, receptor-cytoskeleton, and cytoskeleton-cytoskeleton interactions affect the formation of receptor clusters which serve as precursors to mature focal contacts. Receptor clustering is presumed to occur through self-recognition of cytoskeletal elements which induce the polymerization of ligand-receptor-cytoskeleton complexes. Polymerization only occurs when the ligand density is above a critical value and a decrease in the receptor-ligand affinity shifts the critical ligand density to higher values. While cytoskeletal protein expression and receptor-cytoskeleton affinity influence the concentration of monomeric complexes, the formation of polymeric ligand-receptor-cytoskeleton aggregates is most sensitive to changes in the self-association affinity between cytoskeletal proteins. We find that a 100-fold enhancement in the affinity between cytoskeletal elements can produce a substantial increase in the total fraction of adhesion receptors associated with focal contact precursors (from 5% to over 90%). Our results suggest that under physiological conditions, cellular control of focal contact assembly most likely occurs through modulation of specific cytoskeletal proteins to solidify cytoskeleton-cytoskeleton connections within precursor focal contact structures.