UC San Diego

The ability of cells to interact with their surrounding extracellular matrix plays an important role in the regulation of cell functions. Receptors on cell surfaces allow for cells to adhere to adhesive matrix ligands such as fibronectin or collagens. These interactions, critical to many physiological processes including differentiation and migration, are impacted by biological cues such as ligand composition and ligand density and physical cues such as stiffness, fiber diameter, and pore size, all of which change constantly during normal physiological processes as well as during pathological repair. This dissertation involves investigating these interactions in vitro on a 2D polyacrylamide hydrogel platform in order to closely examine how cells interact and deform substrates of physiologically-relevant stiffness to generate tractions required for migration and stem cell differentiation. Although stiffness has been shown to be a key regulator of stem cell differentiation, and stem cells have the tendency to undergo durotaxis, or migrate towards regions of increasing stiffness, the role of cell adhesion to specific matrix ligands plays in these processes has not been fully resolved. Here, we examine the interplay between extracellular matrix cues such as fibrous protein tethering, porosity, and adhesive ligand density in driving mesenchymal stem cell differentiation and migration. Stiffness has long been shown to determine stem cell fate. In this dissertation, we create substrates of similar stiffness with varying degree of fibrous protein tethering and show that although cells potentially "feel" differences in tethering to a 2D pliable substrate, the degree of tethering has no effect on stem cell differentiation, and does not change how cells deform the underlying hydrogel substrate. We find that mesenchymal stem cells are insensitive to both fibrous protein tethering and adhesive ligand density. However, reducing cell-substrate adhesion via blocking a fraction of surface integrin receptors can increase sensitivity to adhesive ligand density; stem cells that have been treated with integrin-specific peptides will undergo haptotaxis, and migrate towards regions of increasing adhesive ligand density. By contrast, untreated fibroblasts have the innate ability to respond to ligand gradients, indicating that given similar microenvironments, compared to stem cells, fibroblasts may be inherently more sensitive to matrix ligand density. This work reaffirms the importance of substrate mechanics in regulating the signaling pathways that guide stem cell fate and provides further insight into how cells interact and deform the adhesive ligand coating and the underlying polyacrylamide hydrogel, the two major components of a traditional 2D cell culture platform used to study the effect of linearly elastic mechanical properties on cell behavior