Moving Forward: Mechanisms of Chemoattractant Gradient Sensing

Cells use an internal compass to sense the direction of chemoattractant gradients. This is used to bias pseudopod extension at the front of the cell and to orient cell polarization. Recent studies have highlighted the important roles played by phosphoinositide-3,4,5-triphosphate and small G proteins, but many questions remain.

Chemoattractant-induced phosphatidylinositol 3,4,5-trisphosphate accumulation is spatially amplified and adapts, independent of the actin cytoskeleton

Experiments in amoebae and neutrophils have shown that local accumulations of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] mediate the ability of cells to migrate during gradient sensing. To define the nature of this response, we subjected Dictyostelium discoideum cells to measurable temporal and spatial chemotactic inputs and analyzed the accumulation of PI(3,4,5)P(3) on the membrane, as well as the recruitment of the enzymes phosphoinositide 3-kinase and PTEN. In latrunculin-treated cells, spatial gradients elicited a PI(3,4,5)P(3) response only on the front portion of the cell where the response increased more steeply than the gradient and did not depend on its absolute concentration. Phosphoinositide 3-kinase bound to the membrane only at the front, although it was less sharply localized than PI(3,4,5)P(3). Membrane-bound PTEN was highest at the rear and varied inversely with receptor occupancy. The localization of PI(3,4,5)P(3) was enhanced further in untreated polarized cells containing an intact cytoskeleton. Interestingly, the treated cells could respond to two independent gradients simultaneously, demonstrating that a response at the front does not necessarily inhibit the back. Combinations of temporal and spatial stimuli provided evidence of an inhibitory process and showed that a gradient generates a persistent steady-state response independent of a previous history of exposure to chemoattractant. These results support a local excitation/global inhibition model and argue against other schemes proposed to explain directional sensing.

Local Excitation, Global Inhibition Mechanism for Gradient Sensing: An Interactive Applet

We have proposed a model in which cells detect gradients of chemoattractant by balancing a fast local excitation and a slower global inhibition. To illustrate this general mechanism, we have developed an interactive applet that mimics laboratory experiments in which either spatially homogeneous or heterogeneous stimuli of chemoattractant are applied.