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Yang, Qixin
Losert, Wolfgang
Cells can sense various external cues, such as chemoattractants, electric fields (EFs), and the physical properties of their surroundings. This ability is essential to maintain normal physiological processes, and malfunctions may lead to severe diseases, such as cancer and autoimmune disease. Recent studies have observed that waves of colocalized actin, associated with its upstream signaling molecules, drive cell directional migration. Furthermore, a coupled signal transduction excitable system – cytoskeleton excitable system (STEN-CEN) has been revealed to regulate wave formation in non-neural cell types. The system display hallmarks of excitability, such as refractory periods, all-or-none type responses, and wave behavior. It is traditionally believed that different cells employ different migrational strategies. However, recent studies find that changing the states of STEN-CEN leads to the transition of migrational modes within a single cell type. This dissertation uses molecular experiments, computer-vision quantification techniques, and modeling to understand how cells sense different external cues. Numerous studies have shown that there exist multiple, parallel signal pathways that sense certain external stimuli, which suggests that the external signal is not sensed by a single molecule. Here I investigate the possibility that STEN-CEN waves act as a sensing unit for external cues. To overcome the challenge that wave dynamics are coupled with cell motion in normal-sized cells, I electro-fused tens of Dictyostelium discoideum (D. d) cells together to form giant cells. In giant cells, waves are no longer localized at the cell perimeter. The larger basal membrane area provides the opportunity to study subcellular dynamics. I recorded the dynamics of F-actin and phosphatidylinositol (3,4,5)-triphosphate (PIP3), which are indicative of CEN and STEN, respectively. To decouple STEN and CEN further, I applied a variety of chemical perturbations to suppress/activate STEN and CEN separately. Because the wave properties characterize the stage of STEN-CEN, I developed a series of quantification tools to measure wave area, duration, and speed. I collaborated with theorists to create a reaction-diffusion system model that recreates the experimental results. The dissertation focuses on studying the role of STEN-CEN waves in sensing nanotopography and EF. CEN is found to sense nanotopographical cues directly, forming long-lasting F-actin puncta at ridges in the absence of STEN. At the same time, STEN is essential for long-range wave behaviors for macro-domain nanoridge sensing. STEN and CEN cooperate in the sensing of EF signals. According to quantitative studies of wave properties, nanotopography changes the dimensionality and lifetime of waves, whereas EF can alter the activation thresholds of the intracellular systems. In summary, this thesis shows that the excitable biomechanical and biochemical wave systems act as the sensing units for the physical properties of the extracellular environment. As a result, cells can dynamically sense their surroundings and coordinate intracellular processes to migrate under guidance from external cues.