Publication date: 25th July 2016
Cell spreading on native polymer gels (e.g., collagen and fibrin gels) exhibiting non-linear stiffening is distinct from that on widely used polyacrylamide (PA) hydrogels, which are linearly elastic. However, how the balance of actomyosin forces and substrate adhesions are established on native biopolymers is not fully clear. Here, we address this question by comparing fibroblast cell spreading on soft gelatin methacrylate (GelMA) gels with that of GelMA functionalized PA gels (PA-GelMA) of varying stiffness. Though pore size of GelMA gels polymerized by UV exposure for 1 min reduced with increase in ligand density (5, 10, 15% GelMA), bulk stiffness (assessed with AFM) remained constant at 1 kPa. Cell spreading on these gels was significantly greater than that on the stiffest 13 kPa PA-GelMA gels, and was associated with formation of robust stress fibers and prominent focal adhesions. Interestingly, contractile mechanics of cells revealed constant but low baseline contractility on GelMA gels comparable to that on the softest PA-GelMA gels. Instead, trypsin de-adhesion experiments suggest that adhesive drag exhibits a bi-phasic response with maximal drag on the intermediate (10%) GelMA gels. Consistent with constant contractility, cortical stiffness remained unchanged on GelMA gels. Remarkably, perturbation with the contractile agonist blebbistatin and the contractile activist nocodazole did not appreciably alter cell contractility; instead, adhesions and adhesive drag dropped. Our results suggest that on soft adhesion-rich biopolymer gels, adhesivity dominates cell contractility in regulating cell mechanics.