Parallel signaling pathways regulate excitable dynamics differently to mediate pseudopod formation during eukaryotic chemotaxis.

In eukaryotic chemotaxis, parallel signaling pathways regulate the spatiotemporal pseudopod dynamics at the leading edge of a motile cell through the characteristic dynamics of an excitable system; however, differences in the excitability and the physiological roles of individual pathways remain to be elucidated. Here, we found that two different pathways, mediated by soluble guanylyl cyclase (sGC) and phosphoinositide 3-kinase (PI3K), caused similar all-or-none responses for sGC localization and phosphatidylinositol 3,4,5-trisphosphate production but with different refractory periods, by undertaking simultaneous observations of the excitable properties of the two pathways in Dictyostelium cells. Owing to the shorter refractory period, sGC signaling responded more frequently to chemoattractants, leading to pseudopod formation with higher frequency. sGC excitability was regulated negatively by its product cGMP and by cGMP-binding protein C (GbpC) through the suppression of F-actin polymerization, providing the underlying delayed negative-feedback mechanism for the cyclical pseudopod formation. These results suggest that parallel pathways respond to environmental cues on different timescales in order to mediate chemotactic motility in a manner based on their intrinsic excitability.