Trends in Cell Biology
Regulation of innate immunity by Rho GTPases
Introduction
Innate immune responses are dependent on the ability of phagocytic leukocytes to sense and to respond to invading pathogens through various cell-surface receptors. Toll-like receptors (TLRs) recognize conserved motifs on pathogens that are not found in higher eukaryotes and initiate innate immune responses [1]. Subsequently, specific receptors recognizing chemoattractant molecules mobilize phagocytic leukocytes and induce their migration to inflammatory sites (Figure 1) [2]. There, the leukocytes encounter the invading microorganisms and ingest them through the activation of phagocytic receptors that mediate the uptake process [3]. The ingested microbe can then be killed by the release of microbicidal and degradative enzymes contained in leukocyte granules into the phagosome, as well as by the formation of ROS by a specialized membrane nicotinamide adenine dinucleotide phosphate (NADPH) oxidase [4].
All of these activities must be tightly regulated and coordinated for an effective immune response that avoids inflammatory damage to the surrounding tissues. The Rho GTPases, including members of the Rho, Rac and Cdc42 subfamilies, are important components of such regulation. Rho GTPases use the binding and hydrolysis of GTP as a ‘molecular switch’ to regulate numerous basic cell functions (Box 1), including cytoskeletal dynamics and motility, transcription, vesicular trafficking, cell growth and apoptosis.
In leukocytes, Rho GTPases have evolved roles as crucial regulators of the signaling pathways and specialized cellular responses that enable these cells to perform their innate immune functions. Indeed, leukocytes are unique in that they contain two closely related GTPase isoforms, Rac1 and Rac2, that have markedly different biological activities, as shown by recent studies in genetically targeted mice. In combination with sophisticated biochemical investigations and newly developed microscopic imaging techniques, these genetic studies are providing substantial mechanistic information on the basis of immune cell regulation by these ‘molecular switches’. In this review, we discuss these recent studies on the regulation of innate immune responsiveness by Rho GTPases.
Section snippets
Rho GTPases in TLR signaling
The activity of Rho GTPases is stimulated by many receptors involved in leukocyte activation, including receptors for chemoattractants, cytokines, phagocytic particle uptake, and growth factors [5]. Similarly, TLRs have crucial roles in inducing the leukocyte innate immune response to pathogens [1], and Rho GTPases have been recently shown to be important factors in TLR signaling 6, 7, 8, 9. For example, stimulation of TLR2 in the human monocytic THP-1 cell line initiates a signaling pathway
Leukocyte chemotaxis and motility
Neutrophil recruitment from the blood is a central event in the innate immune response to invading pathogens. Indeed, neutrophils are the prototypic chemotactic cells, responding to chemoattractant signals with speeds of up to 30 μm/min. As regulators and coordinators of actin and microtubule cytoskeletal dynamics and adhesion, Rho GTPases naturally have essential, and perhaps unique, roles in the motile responses of leukocytes (Figure 2). These activities are mediated through a continually
Rho GTPases in phagocytosis
The uptake of microbial targets by phagocytosis is an integral part of the innate immune function of neutrophils, macrophages and other phagocytic leukocytes. Phagocytosis requires the highly ordered and localized assembly of actin polymers underneath the phagocytic target 33, 34. Rho GTPases have integral roles in regulating the actin rearrangements that mediate particle uptake by phagocytes; indeed, targeting of an active form of Rac to the plasma membrane via the rapamycin system is
The phagocyte NADPH oxidase
An important component of the innate immune response of phagocytes is the ability to generate superoxide and derived ROS via a membrane-associated NADPH oxidase (Figure 4) [4]. This multicomponent enzyme uses electrons derived from intracellular NADPH to generate superoxide anions, which subsequently dismutate to hydrogen peroxide and other ROS that are used for host defense against pathogens. The importance of ROS to host defense is evident in chronic granulomatous disease, in which genetic
Basis for the distinct biological activities of Rac1 and Rac2
From the above discussion, it is clear that Rac1 and Rac2 have distinct regulatory activities in the innate immune functions of leukocytes (Box 2). This is surprising, given that these proteins share 92% identity and have an identical effector domain (switch I; residues 26–45) that provides a crucial site of interaction with both GEFs and downstream protein targets. Most of the sequence differences (15 out of 192 residues overall) are concentrated in the carboxy (C)-terminal tail (residues
Concluding remarks
The recent development of murine models in which Rac1 and Rac2 have been selectively knocked out has greatly advanced our understanding of the specific roles of these GTPases in innate immune cell function and regulation. The selective deletion of Rac1 and Rac2 has established that these closely related isoforms have markedly different roles in a physiological context. Such differences are not evident in studies based on the expression of constitutively active or dominant-negative Rac mutants,
Acknowledgements
I thank C.M. DerMardirossian, B. Diebold, K. Pestonjamasp, and U.G. Knaus for help with preparing figures and for scientific comments. The editorial assistance of Judith Preston is greatly appreciated. The author's laboratory is supported by grants from the National Institutes of Health. This is publication number 16897-IMM from The Scripps Research Institute.
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