Excitable motility

We’ve talked before about the puzzle of how cells like neutrophils figure out how to follow a shallow gradient of attractive chemicals.  In a recent paper (Xiong et al, 2010.  Cells navigate with a local-excitation, global-inhibition excitable network.  PNAS, PMID 20864631) the Devreotes and Iglesias labs describe a new model of how chemotaxis might work.  It’s a very pretty model for a couple of reasons: it embodies a brand new shiny idea about how ameboid cells manage to be so efficient about moving in the direction of a gradient; it rationalizes a whole series of observations about what mutations in different pathways do to cellular motility and direction-finding; and it points the way to future experiments.

The problem of how ameboid cells (such as neutrophils, or Dictyostelium) do such a good job of identifying and responding to shallow concentration gradients of certain chemicals (chemoattractants) has puzzled biologists for decades.  Mathematicians and modelers have more recently joined the ranks of the puzzled.  A huge amount of information has been amassed about which proteins are involved in the response, and lots of movies have been taken of cells responding to gradients.  One of the observations that is key for this paper is that cells do move around even in the absence of a gradient.  A cell sitting in a uniform environment sends out exploratory projections (pseudopodia) in random directions, which cause the cell to move in a kind of random walk.  When a gradient is added, the orientation of the pseudopodia becomes biased, orienting the cell “up” the gradient.  The pseudopodia look similar in both cases, what changes is just the direction in which they’re pointing.  So the process that makes the cell move seems to be separate from, though influenced by, the process that senses the direction of the gradient.