Integration and Competition in Immune Cell Models

Abstract

Cell motility plays an integral role in most biological processes. One principle of motility is the protrusions and retractions of cellular membranes called pseudopods. The physical force behind pseudopod formation is actin polymerization. As the physical driver, actin polymerization integrates the cells’ upstream biochemical signaling cascades and turns the signals into action as the combined output and readout of the state of the cell. Actin polymerization not only occurs within pseudopods but propagates throughout a cell in waves that can be understood and modeled as excitable media.This dissertation focuses on actin wave dynamics in the context of directed cell mi- gration using both experimental and numerical techniques. The directional guidance of cell migration is essential in physiological processes including embryonic development, cancer metastasis, and wound healing. In this thesis, I analyze how immune-like cells are guided by external stimuli that are common in wound environments: electric fields, chemical gradients, and surface texture. A focus of this work is on the emergent excitable wave behavior of actin, and the pseudopods they generate, in simplified, in vitro environments. Actin waves have been previously shown to respond to and be guided by the topography of an underlying substrate. Additionally, static quantifications of actin filaments during or after electric field stimulation have shown that filaments become asymmetrically distributed within the cytoplasm. In neutrophils, the combination of cues leads to higher control of cell motility by guiding the internal actin waves. Using an optical flow algorithm, I quantify the actin waves on multiple length scales to ascertain the role of each guidance cue in affecting cell motion. I find that the waves preferentially polymerize near and travel along the nanoridges. Actin waves nucleate preferentially on the cathode side and reorient the cell’s axis of polarity (i.e., the position of the dominant pseudopod). The second example of competing guidance cues involves studying the collective motion of cells in response to cell-cell signal relay in competition with surface topography. I use Dictyostelium discoideum cells as a model system for this work, as they migrate collectively due to signal relay. The signal relay of these cells is similar to many immune cell species. Using a combination of image analysis tools and a coarse-grained stochastic model, I find that guidance by nanoridges overrides the chemical signal relay and forces cells to migrate individually, suppressing streaming behavior. I model both the secretion and propagation of chemical signals using an excitable systems framework. This work highlights that bidirectional signals can be effective at suppressing cell-cell attraction and streaming motion. The response of immune cells to external stimuli in the wound environment is not universal. Macrophages, one of the largest immune cells, are observed to migrate away from the wound upon wound-induced electric field generation. In the third example, I study actin dynamics of M0 (resting) macrophage cells to elucidate how these cells interact with external electric fields. This cell type exhibits oscillatory actin waves at rest. With electric field stimulation, the oscillatory actin waves start to generate protrusions. Often, the protrusions begin with actin-depleted regions, indicating that contractile ele- ments are involved in conjunction with overall cell volume conservation. This thesis highlights the different methods in which actin waves integrate external cues, specifically electric fields, into cell responses that are cell-type specific.