Substrate stress relaxation regulates cell migration
Keywords: Chemical Engineering.

Cell migration plays an important role during many biological processes including embryogenesis, wound healing, immune trafficking and response to inflammation, and cancer metastasis. Previous cell migration studies have implicated lamellipodia formation as a key requirement for 2D cell migration. In addition, 3D cell migration has been found to be limited by matrix pore size or the ability of a cell to proteolytically degrade its environment. However, many of these 2D studies utilize substrates that are very rigid or purely elastic. Furthermore, some 3D studies use synthetic matrices with enzymatically cleavable sequences. Other studies utilize natural matrices, like pure type 1 collagen, to study cell migration. Nevertheless, these studies have important limitations. First, mounting evidence suggests that many biological tissues are not purely elastic, but viscoelastic, exhibiting a decrease in stress in response to an applied deformation. Second, matrix such as pure type 1 collagen does not have independently tunable properties. Thus, it is difficult to understand the unique contribution of specific matrix properties to cell migration. Consequently, there is a need to develop matrices with physiologically relevant mechanical properties that can be independently tuned. Here, we demonstrated that the mechanical properties of matrices comprised of alginate and reconstituted basement membrane (rBM) regulate 3D cancer migration. We find that cancer cells can migrate through matrices with sufficient mechanical plasticity without requiring protease activity. Our findings show that in highly plastic matrices, MDA-MB 231 cancer cells develop mature invadopodia that are used to mechanically remodel the nanoporous matrix, generating channels wide enough for the cell to migrate through. Next, we demonstrated that enhanced substrate stress relaxation in alginate-rBM matrices promotes filopodia mediated cell migration. Computer simulations utilizing the motor-clutch model of cell migration reproduce key experimental findings on the impact of substrate stress relaxation on cell migration. In addition, we find that faster substrate stress relaxation increases both filopodia length and filopodia lifetime. Combined experiments and simulations reveal additional molecular insights. We then investigated diffusive migratory behaviors on viscoelastic substrates. These studies reveal that slow-relaxing and fast-relaxing matrices can potentiate transition from sub-diffusive to super-diffusive migration phenotypes. We find that trap time, duration of pauses in-between migration phases, mediates migration phenotype with cells on slow-relaxing substrates exhibiting much longer trap times. Finally, we developed alginate-collagen matrices with independently tunable properties. The stiffness of these matrix was increased from 1kPa to 2.5kPa without collagen fiber architecture. Moreover, the loss tangent was modulated from ~0.06 to ~0.12 without changing fiber architecture. We find that increased matrix stiffness and loss tangent independently enhance 3D migration of human monocytes. These studies show that actin polymerization and talin-1 play a role in human monocyte migration. in addition, we demonstrate that activated monocytes are less migratory than naïve monocytes. Together, these studies demonstrate that matrix viscoelasticity plays an important role in both 2D and 3D cell migration. Our findings implicate tissue viscoelasticity as an important property that could regulate migration of cancer cells in vivo as well as recruitment of immune cells to tumors