Collective Gradient Sensing: Fundamental Bounds, Cluster Mechanics, and Cell-to-Cell Variability
Professor Brian Camley
Biophysics, Johns Hopkins Univ.
Many eukaryotic cells chemotax, sensing and following chemical gradients. However, experiments find that even under conditions when single cells do not chemotax, small clusters may still follow a gradient. How can cell clusters sense a gradient that individual cells ignore? I will discuss possible "collective guidance” mechanisms underlying this motion, where individual cells measure the mean value of the attractant, but need not measure its gradient to give rise to directional motility for a cell cluster. I show that the collective guidance hypothesis can be directly tested by looking for strong orientational effects in pairs of cells chemotaxing. Collective gradient sensing also has a new wrinkle in comparison to single-cell chemotaxis: to accurately determine a gradient direction, a cluster must integrate information from cells with highly variable properties. When is cell-to-cell variation a limiting factor in sensing accuracy? I provide some initial answers, and discuss how cell clusters can sense gradients in a way that is robust to this variation. Interestingly, these strategies may depend on the cluster's mechanics: there is a fundamental bound that links the cluster's chemotactic accuracy and its rheology. This suggests that in some circumstances,
mechanical changes like fluidization can influence a cluster's sensing ability. Because of this effect, increasing the noise in a single cell's motion can actually increase the accuracy of cluster chemotaxis!