Investigating the dynamics and biomechanics of actin stress fiber remodeling

Cells encounter mechanical cues from the environment to which they sense and respond. The actin cytoskeleton is the main network that can not only sense mechanical changes, but can also reorganize in response. Actin stress fibers are predominant in cultured fibroblast cells and are load-bearing structures of the cell. Here, in collaboration with others, I have investigated the mechanisms of stress fiber strain response and remodeling using fluorescently-labeled cytoskeletal proteins and live cell microscopy, traction force microscopy, and genetic manipulation to assess these mechanisms. High resolution image acquisition and analysis have provided novel insight into the mechanosensitivity of actin stress fibers. Specifically, the actin-associated protein zyxin has been implicated in an actin repair mechanism with mechanical consequences. We discovered a novel zyxin-mediated actin repair mechanism that restored structural and mechanical integrity to stress fibers following a hyperleongation event in a single stress fiber sarcomere. We also discovered that while these spontaneously occurring hyperelongation events impact single sarcomeres along a stress fiber, they coincide with compensatory shortening in the near-by regions of stress fiber sarcomeres, suggesting there is active remodeling that occurs in actin stress fibers in order to maintain the structure and mechanical homeostasis in live cells. Lastly, we designed a computational model to test whether actin and myosin-based mechanical changes drive some of these dynamic changes in stress fibers. We discovered that variable differences in actin stiffness and myosin contractility may be the main factors in spontaneous changes in iv stress fiber sarcomere length. The findings presented in this dissertation have made exciting contributions to the field of actin cytoskeletal dynamics, and will provide groundwork to future studies dissecting the role of actin-associated proteins in structural and mechanical homeostasis in stress fibers.