Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item MODULATION OF INTRACELLULAR EXCITABLE SYSTEMS THROUGH PHYSICAL MICROENVIRONMENTS(2022) Yang, Qixin; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item Directed Cell Migration: From Single Cells to Collectively Moving Cell Groups(2014) Guven, Can; Losert, Wolfgang; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unlike molecules, which are driven thermally by Brownian motion, eukaryotic cells move in a particular direction to accomplish designated tasks that are involved in diverse biological processes such as organ development and tumor progression. In this dissertation, I present experiments, analysis, and modeling of directed individual and collective cell migration. At subcellular scale, the migration of cells can be guided via the interaction of the cell cytoskeleton with the surrounding nanotopographic elements. I show that mechanical waves of actin polymerization are involved in this guidance–known as contact guidance–as dynamic sensors of surface nanotopography. The dynamics of guided actin waves were measured to build and test predictive models of contact guidance. The distributions of actin-wave propagation speed and direction were obtained from experimental observations of cell migration on nanotopographic surfaces as a function of the spacing between adjacent features (varying between 0.8 and 5 microns). I show that actin polymerization is preferentially localized to nanoscale features for a range of spacings. Additionally, the velocity of actin polymerization waves moving parallel to the direction of nanoridges depends on the nanoridge spacing. A model of actin polymerization dynamics in which nanoridges modify the distribution of the nucleation promoting factors captures these key observations. For individual cells, the question is how the intracellular processes result in directed migration of cells. I introduce a coarse-grained model for cell migration to connect contact guidance to intrinsic cellular oscillations. The guidance of collective cell migration can be dictated via intercellular communication, which is facilitated by biochemical signals. I present a coarse-grained stochastic model for the influence of signal relay on the collective behavior of migrating Dictyostelium discoideum cells. In the experiment cells display a range of collective migration patterns including uncorrelated motion, formation of partially localized streams, and clumping, depending on the type of cell and the strength of the external concentration gradient of the signaling molecule cyclic adenosine monophosphate (cAMP). The collective migration model shows that the pattern of migration can be quantitatively described by considering the competition of two processes, the secretion of cAMP by the cells and the degradation of cAMP in the gradient chamber. With degradation, the model secreting cells form streams and efficiently traverse the gradient, but without degradation the model secreting cells form clumps without streaming. This observation indicates that streaming requires not only signal relay but also degradation of the signal. In addition, I show how this model can be extended to other eukaryotic systems that exhibit more complex cell-cell communication, in which the impact on collective migration is more subtle.