Connecting the Biophysics of Active Matter to Collective Migration

Abstract

Dynamic, biologic systems are often driven by organized forces produced by small constituent molecules or cells. For example, the motion of a neutrophil tracking down a pathogen is powered by directed actin polymerization and myosin contraction. Likewise, the movements of single cells are marshalled together during morphogenesis to properly form an organism. It has become common to call such systems active matter. Because the internal components of an active system produce force, the system as a whole can spontaneously produce persistent motion without external driving forces. This behavior is present in sheets of epithelial cells as well as dense suspensions of swimming bacteria. The work presented here seeks to quantify and model active matter in two archetypical systems, wound healing of crawling eukaryotic cells and dense suspensions of swimming bacteria. In both of these systems, the cells that power the motions are at low Reynolds number, move in a directed fashion, and exert dipole distributed forces. Can similar physics describe the resulting dynamics? Here we address this question using experiments to probe how collective motion of these cells responds to altered or confined environments and compare the results to mathematical models that predict the dynamics of interacting, moving dipole force producers. We begin by determining how to accurately and efficiently quantify flow in these systems. The standard method for measuring cell-scale flows and/or displacements has been particle image velocimetry (PIV); however, alternative methods exist, such as optical flow constraint. We tested these methods head-to-head in the context of cellular biophysics and found that a relatively simple implementation of optical flow outperforms PIV. Next, we studied how biophysical activity of an isolated cell impacts collective dynamics in epithelial layers. Although many reports suggest that wound closure rates depend on isolated cell speed and/or leader cells, we find that these correlations are not universally true. Our experimental data, when coupled with a mathematical model for collective migration, shows that intracellular contractile stress, isolated cell speed, and adhesion all play a substantial role in influencing epithelial dynamics, and that alterations in contraction and/or substrate adhesion can cause confluent epithelial monolayers to exhibit an increase in motility, a feature reminiscent of cancer metastasis. Finally, we turn to dense suspensions of swimming rod-shaped bacteria, where the forces that drive single-cell movements coordinate to generate large-scale fluid flows that lead to complex, and sometimes turbulent, motions. We found that confined suspensions of E. coli form large-scale vortices that undergo periodic reversals. The active nature of this system produces density asymmetries that drive these reversals, and modest changes in cell length inhibit them.