Signaling networks that regulate cell migration

Abstract

Stimuli that promote cell migration, such as chemokines, cytokines, and growth factors in metazoans and cyclic AMP in Dictyostelium, activate signaling pathways that control organization of the actin cytoskeleton and adhesion complexes. The Rho-family GTPases are a key convergence point of these pathways. Their effectors include actin regulators such as formins, members of the WASP/WAVE family and the Arp2/3 complex, and the myosin II motor protein. Pathways that link to the Rho GTPases include Ras GTPases, TorC2, and PI3K. Many of the molecules involved form gradients within cells, which define the front and rear of migrating cells, and are also established in related cellular behaviors such as neuronal growth cone extension and cytokinesis. The signaling molecules that regulate migration can be integrated to provide a model of network function. The network displays biochemical excitability seen as spontaneous waves of activation that propagate along the cell cortex. These events coordinate cell movement and can be biased by external cues to bring about directed migration.

Figure 1.

Regulation of actin dynamics by formins and Arp2/3 in cellular protrusions. The Rho GTPases Rac, RhoA, and Cdc42 regulate actin dynamics at the leading edge via their effects on the activities of formins (mDia), Arp2/3 complex, and LIM kinase (LIMK). Arp2/3 nucleates actin branches that are seen in broad protrusions. Its activity is regulated by Cdc42 and Rac1, which act on WASP/WAVE-containing protein complexes. Rac and Cdc42 also act on PAK, which phosphorylates LIM kinase, which in turn regulates cofilin, a severing protein. Finally, RhoA acts on mDia1 and Cdc42 acts on mDia2 to promote actin polymerization using a processive capping mechanism. RhoA also activates profilin, which binds to actin monomers and increases the rate of polymerization. These GTPases are activated in a clear temporal sequence near the leading edge (Machacek et al. 2009). AID, autoinhibitory domain; FH, formin homology domains; RBD, Rho-GTPase-binding domain.

Figure 2.

Adhesions serve as contact points and signaling centers. Integrin-based adhesions are large, complex assemblies that link the substratum to actin and generate signals that regulate Rho GTPases and cell migration. The structural linkage to actin is thought to be mediated by talin, vinculin, and perhaps α-actinin. The signaling is mediated by adhesion-associated complexes. The paxillin/FAK module and its link to some Rac GEFs and Rho GEFs is shown as an example. The Arp2/3 complex and myosin II, whose activity is regulated by Rho and Rac, are also shown.

Figure 3.

Asymmetric accumulation of PIP₃ is a feature of a spectrum of cell morphological changes. Panels show snapshots of the dynamic distribution of PIP₃ in cells undergoing various morphological changes. (A) Human neutrophils expressing a biosensor for PIP₃ (PHakt-GFP). The cells have been exposed to a gradient formed by a micropipette filled with the chemoattractant C5a (position indicted by *). The arrow shows recruitment of PHakt-GFP to the membrane, indicating an elevated level of PIP₃. (B) A dividing Dictyostelium cell expressing PHCrac-GFP as a biosensor for PIP₃. Arrows point to the accumulation of PIP₃ at the poles of the dividing cell. (C) Prostate epithelial cells expressing PHakt-GFP. (Image courtesy of Tamara Lotan.) Arrows point to the accumulation of PIP₃ on the basal–lateral membranes. (D) Dictyostelium cell expressing PHCrac-GFP phagocytizing latex beads. The arrow indicates accumulation of PIP₃ around two beads; arrowheads point to PIP₃-labeled pseudopods in the same cell. (Image courtesy of Margaret Clarke.) (E) The growth cone of rat dorsal root ganglion expressing PHakt-GFP. Arrows indicate the accumulation of PIP₃ at the leading edge. (Image courtesy of Britta Eickholt.)

Figure 4.

A portion of the Dictyostelium migration signaling network involving PIP₃ and TorC2. Colored blocks delineate modules. The overlapping of blocks indicates that some components belong to several modules. CARE, cystic AMP receptors.

Figure 5.

Responses to uniform increases and gradients of chemoattractants in a LEGI model. (A) Micrographs show translocation of a biosensor for PIP₃ (PHCrac-GFP) to the membrane. PIP₃ levels rise transiently during persistent stimulation with a uniform chemoattractant. The schematic depicts the response of a “front” marker such as PIP₃ to uniform stimulation. A LEGI model assumes that the level of a response regulator is controlled by the difference between rapid excitatory and slower inhibitory processes. The response regulator (RR, blue line) rises when excitation (green line) is higher than inhibition (red line) and then falls as inhibition catches up. (B) The micrograph shows that the steady-state accumulation of PIP₃ forms a crescent facing the high side of the gradient produced by a micropipette releasing chemoattractant. The schematic depicts the behavior of a “front” marker such as PIP₃ in response to a gradient of chemoattractant. In the LEGI model, the response regulator (blue line) rises when excitation (green line) is higher than inhibition (red line) and then falls to a new steady state. Because inhibition is more global than the excitation the differences generate a response regulator that has a higher concentration than basal at the front and a lower concentration than basal at the back.