Multiscale regulation of cellular mechanical properties

Thesis (Ph.D.)--Boston UniversityIn vivo, cells routinely experience mechanical stresses and strains in the form of circulatory pressure and flow, peristalsis ofthe gut, and airway inflation and deflation. Even on the microscale, all adherent cells apply contractile force to the extracellular matrix and to neighboring cells. Cells respond to these external forces both passively and actively. Passively, cells need to deform in a way that is tissue and function appropriate. Actively, cells use local mechanoreceptors present on their surface to trigger changes in global cell behavior. Dysregulation of cell responses to force are hallmarks of diseases such as atherosclerosis, asthma and cancer. Given the pluripotent role of cell mechanics in both normal cell behavior and disease, cell regulation of mechanical properties has become a major area of focus in biology. In this dissertation, we explore passive mechanical properties and active mechanical responses of cells on the subcellular, single cell and multicellular length scales. In Aim 1, we developed a new tool, called cell biomechanical imaging, for mapping intracellular stiffness and prestress. We have demonstrated a linear relationship between these two quantities, both at the whole cell and subcellular levels, which suggests prestress may be a unifying mechanism by which cells and tissues tune their mechanical properties. In Aim 2, we investigated how coordinated changes in cytoskeletal tension lead to cell reorientation. Previous research has shown that in response to strain applied through focal adhesions, the actin cytoskeleton promptly fluidizes and then slowly resolidifies. Using both experiments and a mathematical model, we found that repeated interplay ofthese phenomena was a driving force behind cytoskeletal reorganization during cell reorientation. It was previously hypothesized that the purpose of cytoskeletal remodeling in response to strain was to minimize changes in intracellular mechanical tension and maintain it at a preferred level. This feedback control mechanism, which balances forces between the cell and its microenvironment, is termed "tensional homeostasis." The dominant paradigm in vascular biology is that tensional homeostasis exists across multiple length and time scales. However, our results from Aim 2 challenged this idea; reoriented cells did not maintain steady levels of contractile force. In Aim 3, we investigated tensional homeostasis and its existence at multiple length scales. We found that cells do not have a preferred level of tension at the subcellular or single cell levels. However, in a cluster of confluent cells, contractile tension is maintained, the more so as cluster size increases. Together, the results of this dissertation emphasize the importance ofa multiscale approach to mechanobiology. Cells and tissue are hierarchically ordered systems that use mechanical stress (prestress) to tune their mechanical properties and responses across lengthscales. Thus, it is important to consider not just the behavior of separate components of each of these systems, but the behaviors that emerge when they interact with one another