Individual and collective dynamics of chemotaxing cells

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The study of the dynamics of interacting self-propelled entities is a growing area of physics research. This dissertation investigates individual and collective motion of the eukaryote Dictyostelium discoideum, a system amenable to signal manipulation, mathematical modeling, and quantitative analysis. In the wild, Dictyostelium survive adverse conditions through collective behaviors caused by secreting and responding to chemical signals. We explore this collective behavior on size scales ranging from subcellular biochemistry up to dynamics of thousands of communicating cells.

To study how individual cells respond to multiple signals, we perform stability analysis on a previously-developed computational model of signal sensing.  Polarized cells are linearly stable to perturbations, with a least stable region at about 60 degrees off the polarization axis.  This finding is confirmed through simulations of the model response to additional chemical signals.  The off-axis sensitivity suggests a mechanism for previously observed zig-zag motion of real cells randomly migrating or chemotaxing in a linear gradient.   

Moving up in scale, we experimentally investigate the rules of cell motion and interaction in the context of thousands of cells.  Migrating Dictyostelium discoideum cells communicate by sensing and secreting directional signals, and we find that this process leads to an initial signal having an increased spatial range of an order of magnitude.   While this process steers cells, measurements indicate that intrinsic cell motility remains unaffected.  Additionally, migration of individual cells is unaffected by changing cell-surface adhesion energy by nine orders of magnitude, showing that individual motility is a robust process.  In contrast, we find that collective dynamics depend on cell-surface adhesion, with greater adhesion causing cells to form smaller collective structures.  

Overall, this work suggests that the underlying migration ability of individual Dictyostelium cells operates largely independent of environmental conditions.  Our gradient-sensing model shows that polarized cells are stable to small perturbations, and our experiments demonstrate that the motility apparatus is robust to considerable changes in cell-surface adhesion or complex signaling fields.  However, we find that environmental factors can dramatically affect the collective behavior of cells, emphasizing that the laws governing cell-cell interaction can change migration patterns without altering intrinsic cell motility.