Rac-GTPases and Rac-GEFs in neutrophil adhesion, migration and recruitment

Abstract

Rac-GTPases and their Rac-GEF activators play important roles in the recruitment and host defence functions of neutrophils. These proteins control the activation of adhesion molecules and the cytoskeletal dynamics that enable the adhesion, migration and tissue recruitment of neutrophils. They also regulate the effector functions that allow neutrophils to kill bacterial and fungal pathogens, and to clear debris. This review focuses on the roles of Rac-GTPases and Rac-GEFs in neutrophil adhesion, migration and recruitment.

Keywords: P-Rex1; Rho-GTPases; Vav; guanine-nucleotide exchange factors; neutrophils; small G proteins.

Figure 1

Neutrophil recruitment. Neutrophil recruitment from the blood stream into inflamed and infected tissues begins with the upregulation of selectins on the surface of vascular endothelial cells and circulating platelets. Selectin‐dependent interactions of neutrophils with platelets and vascular endothelial cells enable the tethering and rolling of neutrophils along the vascular endothelium. GPCR signalling activates integrins, which bring about neutrophil arrest on the vascular wall, followed by firm adhesion, spreading and polarization, intravascular crawling and transendothelial migration. Neutrophils use amoeboid chemotaxis to follow chemokine gradients within the interstitium towards the source of the inflammation

Figure 2

Rac in neutrophil adhesion and migration. Rac‐GTPases control actomyosin cytoskeletal dynamics in several ways, including the IRSp53, WAVE, Arp2/3 pathway, which induces the polymerization of branched actin filaments at the cell periphery to enable firm adhesion and spreading. Localized Rac activity induces leading edge formation and polarization. In addition, Rac blocks actin depolymerization through the Limk pathway and stimulates myosin contractility at the uropod through MLCK. Combined, these pathways stabilize polarity and enable migration, as well as other processes such as degranulation and phagocytosis. In addition, active Rac is an integral subunit of the NADPH oxidate complex, which produces ROS and thus controls the ROS‐dependent production of NETs. Finally, Rac also controls gene expression through Jnk

Figure 3

Regulation of Rac activity. Rac‐GTPases cycle between their GTP‐bound active and GDP‐bound inactive form. They are activated by guanine‐nucleotide exchange factors (GEFs), which remove GDP, thus enabling excess cellular GTP to bind. The binding of active Rac to downstream effector proteins elicits cell responses. GTPase‐activating proteins (GAPs), which increase the GTPase activity of Rac, are the off‐switch. Inactive Rac is sequestered in the cytosol by guanine‐nucleotide dissociation inhibitors (GDIs)

Figure 4

Neutrophil Rac‐GEFs. Neutrophils express several types of Dbl‐type and DOCK‐type Rac‐GEFs. The Dbl‐type Rac‐GEFs, which all feature the typical catalytic DH domain and tandem PH domain, include P‐Rex1, the 3 Vav family Rac‐GEFs (Vav1, Vav2, Vav3) and Tiam2. The Rac‐ and Cdc42‐GEF PIXα is also expressed, but to date, only its Cdc42‐GEF activity has been observed directly. The DOCK‐type Rac‐GEFs, which signal through a DHR2 catalytic domain, include DOCK2 and DOCK5. The precise structure of their catalytic domain determines which Rac isoform the GEFs can activate. The multidomain structure unique to each type of Rac‐GEF couples these proteins to distinct sets of regulators and effectors, ensuring the activation of GEFs within specific signalling networks and thus enabling the activation of selected subsets of Rac responses. The GEF domains that confer this specificity include protein‐binding domains, such as SH3 and PDZ, and lipid‐binding domains such as extra PH domains outside of the catalytic DH/PH tandem

Figure 5

Signalling pathways of neutrophil Rac‐GEFs. The Rac‐GEF P‐Rex1, which mediates signalling through GPCRs, E‐selectin and TLR4 (not all shown here for simplicity), is activated by the lipid second messenger PIP ₃ and by the Gβγ subunits of heterotrimeric G proteins. The Vav family Rac‐GEFs, which are activated by tyrosine phosphorylation, are important in integrin and FcR signalling, but they also couple to TLR4 and GPCRs. It is currently unknown, which mechanisms control Tiam2 in neutrophils, except that this GEF controls chemoattractant‐induced responses. DOCK2 also signals upon GPCR stimulation. It is activated by RhoG and recruited to the plasma membrane by PIP ₃ and phosphatidic acid (PA). The preferred Rac substrate of each Rac‐GEF is shown here, but usually, the GEFs can activate both Rac1 and Rac2 to some extent. P‐Rex1 can also activate RhoG and may thus signal in sequence with DOCK2 in some pathways. The list of cell responses regulated by each neutrophil Rac‐GEF is likely to grow with further study

Figure 6

Rac‐GEFs in neutrophil recruitment. Prex1 and Vav Rac‐GEFs control distinct steps of neutrophil recruitment in mice. Neutrophil‐intrinsic roles comprise Prex1‐dependent rolling, Vav1/3‐mediated firm adhesion and spreading, Prex1‐ and Vav1‐dependent intravascular crawling, and Vav1/2/3 interstitial migration. Platelet Prex1/Vav1 controls the binding of platelets to neutrophils in the inflamed circulation and the platelet‐dependent neutrophil adhesion to the airway microvasculature. Endothelial P‐Rex1 controls E‐cadherin‐dependent vascular permeability and neutrophil extravasation during acute lung inflammation