Mechanobiology of cell-substrate interactions

Abstract

Cell adhesion to substrates is a complex process facilitated by focal adhesion (FA) complexes that help them perform vital cellular functions like migration, growth, and division. Cells probe their surroundings through contractile stresses generated via cross-bridge cycling between actin and myosin. These stresses induce exquisite feedback between the underlying substrate and actomyosin stress fibers, resulting in the remodeling of the cytoskeleton and FA. A repertoire of signaling molecules, including calcium and a mechanosensitive protein, talin, facilitates these interactions in FA. Do cells remodel under dynamic mechanical loads in tissues such as arteries? How do individual components of the FA regulate cell adhesions and tractions? I use numerical methods to address these questions on cell-substrate interaction.

I quantified the cell tractions using micro-pillar array detectors (mPAD) created using soft lithography as a first study. Our study shows that mPAD topography resulted in persistent migration of fibroblasts. I used image analysis to quantify the micropillar deflection and calculated tractions through the neo-Hookean model to report traction variation along the cell length. I next developed a multiscale cell model, incorporating SF, calcium signaling, and FA dynamics. Using the model, I investigated the effects of cyclic stretch and substrate stiffness on cell-substrate interactions.

I used the modified Hill model and reaction-diffusion equations to model SF contractility in the presence of calcium. Furthermore, the Gillespie algorithm was used to simulate the stochastic adaptor protein engagements at FAs. The model shows that cell adhesions and tractions vary along their length under static and cyclic stretch conditions; the maxima occurred behind the cell edge. Cell tractions and adhesion increased initially with substrate stiffness and ligand density but decreased beyond an optimum substrate stiffness. Cyclic stretch enhanced tractions and integrin recruitment on compliant substrates; in contrast, it reduced them on stiff substrates.

Talin orchestrates FA formation and aids in force transfer to cytoskeletal actin in the presence of vinculin. I simulated the force response of talin using a composite worm-like chain model. I show that the talin-vinculin assembly is mechanosensitive to substrate stiffness and extension rate. Talin extension on stiffer substrates resulted in higher tension and vinculin recruitment at a low extension rate. In contrast, a high extension rate lowered vinculin recruitment and abolished talin sensitivity to stiffness. These studies show the importance of adaptor protein mechanics in substrate sensing during cell adhesion.