Abstract
A variety of cell types exhibit phenotype changes in response to the mechanical stiffness of the substrate. Many cells excluding neurons display an increase in the spread area, actin stress fiber formation and larger focal adhesion complexes as substrate stiffness increases in a sparsely populated culture. Cell proliferation is also known to directly correlate with these phenotype changes/changes in substrate stiffness. Augmented spreading and proliferation on stiffer substrates require nuclear transcriptional regulator YAP (Yes associated protein) localization in the cell nucleus and is tightly coupled to larger traction force generation. In this study, we show that different types of fibroblasts can exhibit spread morphology, well defined actin stress fibers, and larger focal adhesions even on very soft collagen gels (modulus in hundreds of Pascals) as if they are on hard glass substrates (modulus in GPa, several orders of magnitude higher). Strikingly, we show, for the first time, that augmented spreading and other hard substrate cytoskeleton architectures on soft collagen gels are not correlated with the cell proliferation pattern and do not require YAP localization in the cell nucleus. Finally, we examine the response of human colon carcinoma (HCT-8) cells on soft collagen gels. Recent studies show that human colon carcinoma (HCT-8) cells form multicellular clusters by 2-3 days when cultured on soft polyacrylamide (PA) gels with a wide range of stiffness (0.5-50 kPa) and coated with an extracellular matrix, ECM (collagen monomer/fibronectin). These clusters show limited spreading/wetting on PA gels, form 3D structures at the edges, and eventually display a remarkable, dissociative metastasis like phenotype (MLP), i.e., epithelial to rounded morphological transition after a week of culture on PA gels only, but not on collagen monomer coated stiff polystyrene/glass where they exhibit enhanced wetting and form confluent monolayers. Here, we show that HCT-8 cell clusters also show augmented spreading/wetting on soft collagen gels and eventually form confluent monolayers as on rigid glass substrates and MLP is completely inhibited on soft collagen gels. Overall, these results suggest that cell-material interactions (soft collagen gels in this case) can induce cellular phenotype and cytoskeleton organization in a remarkably distinct manner compared to a classical synthetic polyacrylamide (PA) hydrogel cell culture model and may contribute in designing new functional biomaterials.