Chemotaxis is crucial for many physiological processes including the recruitment of leukocytes to sites of infection, trafficking of lymphocytes in the human body, and metastasis of cancer cells. A family of small proteins, chemokines, serves as the signals, and a family of G-protein coupled receptors (GPCRs) detects chemokines and direct cell migration. One of the basic questions in chemotaxis of eukaryotes is how a GPCR transduces signals to control the assembly of the actin network that generates directional force for cell migration. Over the past decade, a variety of signaling components have been implicated to transduce the GPCR signaling to the actin cytoskeleton. Studies in a lower eukaryotic organism, Dictyostelium discoideum, have allowed us to discover evolutionary conversed components involved in the GPCR-controlled actin network during chemotaxis. However, complete pathways linking GPCR to the actin network are still far from clear. Here we first summarize the previous studies on these components, and then update with our finding showing a new pathway, consisting of a GPCR, Gβγ, Elmo/Dock, Rac and Arp2/3 and actin. We suggest that this pathway serves as a direct linkage between the GPCR/G-protein, the chemoattractant sensing machinery, and the actin cytoskeleton, the machinery of cell movement during chemotaxis of eukaryotic cells.

The Elmo protein family members are important mediators of small G protein activity, regulating actin-mediated processes such as chemotaxis and engulfment. Until recently,1 Elmo function has not been explored in professional phagocytes such as Dictyostelium discoideum. We discuss the significance of this family with respect to pathways that regulate Rac signaling, we present a comparison of Elmo proteins between representative taxa, and discuss our findings on ElmoA, one of six Elmo proteins found in D. discoideum.

Gprotein–coupled receptor (GPCR) signaling mediates a balance of excitatory and inhibitory activities that regulate Dictyostelium chemosensing to cAMP. The molecular nature and kinetics of these inhibitors are unknown. We report that transient cAMP stimulations induce PIP3 responses without a refractory period, suggesting that GPCR-mediated inhibition accumulates and decays slowly. Moreover, exposure to cAMP gradients leads to asymmetric distribution of the inhibitory components. The gradients induce a stable accumulation of the PIP3 reporter PHCrac-GFP in the front of cells near the cAMP source. Rapid withdrawal of the gradient led to the reassociation of G protein subunits, and the return of the PIP3 phosphatase PTEN and PHCrac-GFP to their pre-stimulus distribution. Reapplication of cAMP stimulation produces a clear PHCrac-GFP translocation to the back but not to the front, indicating that a stronger inhibition is maintained in the front of a polarized cell. Our study demonstrates a novel spatiotemporal feature of currently unknown inhibitory mechanisms acting locally on the PI3K activation pathway.