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Date of Award
Doctor of Philosophy (PhD)
Cell clusters reside in complex extracellular matrices (ECMs) of varying mechanical properties. Epithelial cells sense and translate the mechanical cues presented by the surrounding ECM into biochemical signals through a process called ‘mechanotransduction’, which controls fundamental aspects of disease and development. During the course of metastasis, mechanical changes in the tumor microenvironment can lead to declustering of epithelial cells through a process called epithelial-to-mesenchymal transition (EMT). Throughout different steps of metastasis, escaped epithelial clusters encounter heterogeneous tissues of varying mechanical properties that ultimately influence their behavior in distant locations within the body. This dissertation investigates the mechanobiology of epithelial clusters inside mechanically heterogeneous tissue microenvironments. Chapter 1 provides an introduction for the mechanobiology of epithelial clusters and describes how mechanical properties of the microenvironment mediates behavior of epithelial cells. Chapter 2 addresses the mechano-regulated epithelial to mesenchymal transition inside matrices of varying stiffness and confinement. Growing evidence suggests that high extracellular matrix (ECM) stiffness induces EMT. Yet, very little is known about how various geometrical parameters of the ECM might influence EMT. To this end, we develop polyacrylamide (PA)-microchannels based matrix platform to culture mammary epithelial cell clusters in ECMs of tunable stiffness and confinement. Our results demonstrate that ECM confinement alone is able to induce EMT in epithelial clusters surrounded by a soft matrix, which otherwise protects against EMT in unconfined environments. Also, we demonstrate that stiffness- and confinement-induced EMT work through cell-matrix adhesions and cytoskeletal polarization, respectively. In chapter 3, we examined the combinatorial effect of phenotypic heterogeneity and matrix heterogeneity in determining the overall migration of the migrating clusters and the motion of individual cells in the cluster. These findings may provide insights into the effect of cellular heterogeneity on the epithelial dynamics during the early stage of cancer progression. In chapter 4, we examined the collective migration of epithelial cells across physically diverse matrices. Although the influence of matrix stiffness on cell migration is well-recognized, it remains unknown whether these matrix-dependent cellular features persist even after cells move to a new microenvironment. We have discovered that epithelial cells primed on a stiff matrix migrate faster, generate higher actomyosin expression, and retain nuclear YAP even after arriving on a soft matrix, as compared to their control behavior on a homogeneously soft matrix. Our results have uncovered a mechanical memory of past matrix stiffness in collective migration of normal and cancer epithelial cells. The depletion of YAP dramatically reduces this memory-dependent migration. These revelations imply that, during metastasis, changes in tumor microenvironment stiffness may influence the future invasion of escaping tumor cells.
Philip Bayly, Elliot Elson, Spencer Lake, Lori Setton,
Available for download on Friday, April 19, 2019