Deficiency of the hematopoietic cell-specific Rho family GTPase Rac2 is characterized by abnormalities in neutrophil function and host defense

Hyperinflammation in chronic granulomatous disease and anti-inflammatory role of the phagocyte NADPH oxidase

Chronic granulomatous disease (CGD) is an immunodeficiency caused by the lack of the superoxide-producing phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. However, CGD patients not only suffer from recurrent infections, but also present with inflammatory, non-infectious conditions. Among the latter, granulomas figure prominently, which gave the name to the disease, and colitis, which is frequent and leads to a substantial morbidity. In this paper, we systematically review the inflammatory lesions in different organs of CGD patients and compare them to observations in CGD mouse models. In addition to the more classical inflammatory lesions, CGD patients and their relatives have increased frequency of autoimmune diseases, and CGD mice are arthritis-prone. Possible mechanisms involved in CGD hyperinflammation include decreased degradation of phagocytosed material, redox-dependent termination of proinflammatory mediators and/or signaling, as well as redox-dependent cross-talk between phagocytes and lymphocytes (e.g. defective tryptophan catabolism). As a conclusion from this review, we propose the existence of ROS high and ROS low inflammatory responses, which are triggered as a function of the level of reactive oxygen species and have specific characteristics in terms of physiology and pathophysiology.

Rho GTPases in cancer cell biology

Rho GTPases contribute to multiple cellular processes that could affect cancer progression, including cytoskeletal dynamics, cell cycle progression, transcriptional regulation, cell survival and vesicle trafficking. In vitro several Rho GTPases have oncogenic activity and/or can promote cancer cell invasion, and this correlates with increased expression and activity in a variety of cancers. Conversely, other family members appear to act as tumour suppressors and are deleted, mutated or downregulated in some cancers. Genetic models are starting to provide new information on how Rho GTPases affect cancer development and progression. Here, we discuss how Rho GTPases could contribute to different steps of cancer progression, including proliferation, survival, invasion and metastasis.

Neutrophil-mediated oxidative burst and host defense are controlled by a Vav-PLCgamma2 signaling axis in mice

Oxidative burst, a critical antimicrobial mechanism of neutrophils, involves the rapid generation and release of reactive oxygen intermediates (ROIs) by the NADPH oxidase complex. Genetic mutations in an NADPH oxidase subunit, gp91 (also referred to as NOX2), are associated with chronic granulomatous disease (CGD), which is characterized by recurrent and life-threatening microbial infections. To combat such infections, ROIs are produced by neutrophils after stimulation by integrin-dependent adhesion to the ECM in conjunction with stimulation from inflammatory mediators, or microbial components containing pathogen-associated molecular patterns. In this report, we provide genetic evidence that both the Vav family of Rho GTPase guanine nucleotide exchange factors (GEFs) and phospholipase C-gamma2 (PLC-gamma2) are critical mediators of adhesion-dependent ROI production by neutrophils in mice. We also demonstrated that Vav was critically required for neutrophil-dependent host defense against systemic infection by Staphylococcus aureus and Pseudomonas aeruginosa, 2 common pathogens associated with fatal cases of hospital-acquired pneumonia. We identified a molecular pathway in which Vav GEFs linked integrin-mediated signaling with PLC-gamma2 activation, release of intracellular Ca2+ cations, and generation of diacylglycerol to control assembly of the NADPH oxidase complex and ROI production by neutrophils. Taken together, our data indicate that integrin-dependent signals generated during neutrophil adhesion contribute to the activation of NADPH oxidase by a variety of distinct effector pathways, all of which require Vav.

SOD1 mutations disrupt redox-sensitive Rac regulation of NADPH oxidase in a familial ALS model

Neurodegeneration in familial amyotrophic lateral sclerosis (ALS) is associated with enhanced redox stress caused by dominant mutations in superoxide dismutase-1 (SOD1). SOD1 is a cytosolic enzyme that facilitates the conversion of superoxide (O(2)(*-)) to H(2)O(2). Here we demonstrate that SOD1 is not just a catabolic enzyme, but can also directly regulate NADPH oxidase-dependent (Nox-dependent) O(2)(*-) production by binding Rac1 and inhibiting its GTPase activity. Oxidation of Rac1 by H(2)O(2) uncoupled SOD1 binding in a reversible fashion, producing a self-regulating redox sensor for Nox-derived O(2)(*-) production. This process of redox-sensitive uncoupling of SOD1 from Rac1 was defective in SOD1 ALS mutants, leading to enhanced Rac1/Nox activation in transgenic mouse tissues and cell lines expressing ALS SOD1 mutants. Glial cell toxicity associated with expression of SOD1 mutants in culture was significantly attenuated by treatment with the Nox inhibitor apocynin. Treatment of ALS mice with apocynin also significantly increased their average life span. This redox sensor mechanism may explain the gain-of-function seen with certain SOD1 mutations associated with ALS and defines new therapeutic targets.

Identification of a conserved Rac-binding site on NADPH oxidases supports a direct GTPase regulatory mechanism

The NADPH oxidases (Noxs) are a family of superoxide-generating enzymes implicated in a variety of biological processes. Full activity of Nox1, -2, and -3 requires the action of a Rac GTPase. A direct regulatory interaction of Rac with Nox2 has been proposed as part of a two-step mechanism for regulating electron transfer during superoxide formation. Using truncation analysis of Rac binding to the cytoplasmic tail of Nox2, along with peptides derived from this region in cell-free assays, we identify a Rac interaction site within amino acids 419-430 of Nox2. This region is required for binding Rac2 but not p47(phox) or p67(phox) cytosolic regulatory factors. A cell-permeant version of the peptide encompassing amino acids 419-430 specifically inhibits NADPH oxidase activation in intact human neutrophils. Mutational analysis of the putative Rac-binding site revealed specific residues, particularly Lys-421, Tyr-425, and Lys-426, individually required for Rac-dependent NADPH oxidase activity that are conserved in the Rac-regulated Nox1, Nox2, and Nox3 enzymes but not in Nox4 or Nox5. Mutation of the conserved residues in the Rac-binding site of Nox1 also result in the loss of Rac-dependent activity. Our data identify a functional Rac interaction site conserved in Rac-dependent Noxs and support a direct regulatory interaction of Rac GTPases to promote activation of these NADPH oxidases.

Identifying the relative contributions of Rac1 and Rac2 to osteoclastogenesis

Rac small GTPases may play an important regulatory role in osteoclastogenesis. Our in vitro and in vivo results show that both Rac1 and Rac2 are required for optimal osteoclast differentiation, but Rac1 is more critical. Rac1 is the key Rac isoform responsible for regulating ROS generation and the actin cytoskeleton during the multiple stages of osteoclast differentiation.

INTRODUCTION

Recent evidence suggests that the Rac small GTPases may play an important regulatory role in osteoclastogenesis. This finding is important because bisphosphonates may regulate their antiresorptive/antiosteoclast effects through the modification of Rho family of small GTPases.

MATERIALS AND METHODS

To elucidate the specific roles of the Rac1 and Rac2 isoforms during osteoclastogenesis, we used mice deficient in Rac1, Rac2, or both Rac1 and Rac2 in monocyte/osteoclast precursors. Macrophage-colony stimulating factor (M-CSF)- and RANKL-mediated osteoclastogenesis in vitro was studied by using bone marrow-derived mononucleated preosteoclast precursors (MOPs). The expression of osteoclast-specific markers was examined using quantitative real-time PCR and Western blot analysis. Free actin barbed ends in bone marrow MOPs after M-CSF stimulation was determined. The ability of MOPs to migrate toward M-CSF was assayed using Boyden chambers. Margin spreading on heparin sulfate-coated glass and RANKL-induced reactive oxygen species generation were also performed. Functional assays of in vitro-generated osteoclasts were ascertained using dentine sections from narwal tusks. Osteoclast levels in vivo were counted in TRACP and immunohistochemically stained distal tibial sections. In vivo microarchitexture of lumbar vertebrate was examined using microCT 3D imaging and analysis.

RESULTS

We show here that, although both Rac isoforms are required for normal osteoclast differentiation, Rac1 deletion results in a more profound reduction in osteoclast formation in vitro because of its regulatory role in pre-osteoclast M-CSF-mediated chemotaxis and actin assembly and RANKL-mediated reactive oxygen species generation. This Rac1 cellular defect also manifests at the tissue level with increased trabecular bone volume and trabeculae number compared with wildtype and Rac2-null mice. This unique mouse model has shown for the first time that Rac1 and Rac2 play different and nonoverlapping roles during osteoclastogenesis and will be useful for identifying the key roles played by these two proteins during the multiple stages of osteoclast differentiation.

CONCLUSIONS

Rac1 and Rac2 play different and nonoverlapping roles during osteoclastogenesis. This model showed that Rac1 is the key Rac isoform responsible for regulating ROS generation and the actin cytoskeleton during the multiple stages of osteoclast differentiation.

An essential role for Rac1 in endothelial cell function and vascular development

Numerous cell surface receptors, including tyrosine kinase and G protein-coupled receptors, play critical roles in endothelial cell function and blood vessel development. These receptors share the ability of stimulating an intricate network of intracellular signaling pathways, including the activation of members of the Ras and Rho family of small GTPases. However, the contribution of these signaling molecules to the numerous biological activities performed by endothelial cells is still not fully understood. Here, we have used a conditional Cre/Flox approach, enabling the deletion of the Rac1 gene in endothelial cells, to examine the role of the Rho-related GTPase Rac1 in endothelial cell function and vascular development. Rac1 excision in primary endothelial cells in vitro revealed that Rac1 plays a central role in endothelial cell migration, tubulogenesis, adhesion, and permeability in response to vascular endothelial growth factor (VEGF) and sphingosine-1-phosphate (S1P), which is likely due to the inability of Rac1-deficient endothelial cells to form lamellipodial structures and focal adhesions, and to remodel their cell-cell contacts. Importantly, endothelial-specific excision of Rac1 results in embryonic lethality in midgestation (around E9.5), and defective development of major vessels and complete lack of small branched vessels was readily observed in these endothelial Rac1-deficient embryos and their yolk sacs. These findings provide direct evidence that the activity of Rac1 in endothelial cells is essential for vascular development and suggest that Rac1 and its downstream targets may represent promising therapeutic targets for the treatment of numerous human diseases that involve aberrant neovascularization.

Reduced activation of protein kinase B, Rac, and F-actin polymerization contributes to an impairment of stromal cell–derived factor-1–induced migration of CD34+ cells from patients with myelodysplasia

Patients with myelodysplasia (MDS) show a differentiation defect in the multipotent stem-cell compartment. An important factor in stem-cell differentiation is their proper localization within the bone marrow microenvironment, which is regulated by stromal cell–derived factor (SDF-1). We now show that SDF-1–induced migration of CD34+ progenitor cells from MDS patients is severely impaired. In addition, these cells show a reduced capacity to polymerize F-actin in response to SDF-1. We demonstrate a major role for Rac and phosphatidylinositol 3-kinase (PI3K) and a minor role for the extracellular signal-regulated kinase (ERK)1/2 signaling pathway in SDF-1–induced migration of normal CD34+ cells. Furthermore, SDF-1–stimulated activation of Rac and the PI3K target protein kinase B is impaired in CD34+ cells from MDS patients. Lentiviral transduction of MDS CD34+ cells with constitutive active Rac1V12 results in a partial restoration of F-actin polymerization in response to SDF-1. In addition, expression of constitutive active Rac increases the motility of MDS CD34+ cells in the absence of SDF-1, although the directional migration of cells toward this chemoattractant is not affected. Taken together, our results show a reduced migration of MDS CD34+ cells toward SDF-1, as a result of impaired activation of the PI3K and Rac pathways and a decreased F-actin polymerization.

Rho GTPases and regulation of hematopoietic stem cell localization

Bone marrow engraftment in the context of hematopoietic stem cell and progenitor (HSC/P) transplantation is based on the ability of intravenously administered cells to lodge in the medullary cavity and be retained in the appropriate marrow space, a process referred to as homing. It is likely that homing is a multistep process, encompassing a sequence of highly regulated events that mimic the migration of leukocytes to inflammatory sites. In leukocyte biology, this process includes an initial phase of tethering and rolling of cells to the endothelium via E- and P-selectins, firm adhesion to the vessel wall via integrins that appear to be activated in an "inside-out" fashion, transendothelial migration, and chemotaxis through the extracellular matrix (ECM) to the inflammatory nidus. For HSC/P, the cells appear to migrate to the endosteal space of the bone marrow. A second phase of engraftment involves the subsequent interaction of specific HSC/P surface receptors, such as alpha(4)beta(1) integrin receptors with vascular cell-cell adhesion molecule-1 and fibronectin in the ECM, and interactions with growth factors that are soluble, membrane, or matrix bound. We have utilized knockout and conditional knockout mouse lines generated by gene targeting to study the role of Rac1 and Rac2 in blood cell development and function. We have determined that Rac is activated via stimulation of CXCR4 by SDF-1, by adhesion via beta(1) integrins, and via stimulation of c-kit by the stem cell factor-all of which involved in stem cell engraftment. Thus Rac proteins are key molecular switches of HSC/P engraftment and marrow retention. We have defined Rac proteins as key regulators of HSC/P cell function and delineated key unique and overlapping functions of these two highly related GTPases in a variety of primary hematopoietic cell lineages in vitro and in vivo. Further, we have begun to define the mechanisms by which each GTPase leads to specific functions in these cells. These studies have led to important new understanding of stem cell bone marrow retention and trafficking in the peripheral circulation and to the development of a novel small molecule inhibitor that can modulate stem cell functions, including adhesion, mobilization, and proliferation. This chapter describes the biochemical footprint of stem cell engraftment and marrow retention related to Rho GTPases. In addition, it reviews abnormalities of Rho GTPases implicated in human immunohematopoietic diseases and in leukemia/lymphoma.

Characterization of the roles of Rac1 and Rac2 GTPases in lymphocyte development

This chapter describes methods for the analysis of B and T lymphocyte development in mice deficient in Rac1 and/or Rac2 GTPases. The development of both B and T cells is critically dependent on transition through checkpoints monitoring for correct rearrangement of antigen receptor genes. Progression through these checkpoints depends on signaling from the antigen receptors. In addition, signals from cytokine, chemokine, Notch, and death receptors play important roles in the survival, proliferation, and migration of developing lymphocytes. Analysis of these processes in mice deficient in these GTPases can illuminate their roles in transducing signals from these different receptors.

Activation of the small GTPase Rac2 via the B cell receptor regulates B cell adhesion and immunological-synapse formation

The integrin leukocyte function-associated antigen-1 (LFA-1) is important in the promotion of B cell adhesion, thereby facilitating immunological synapse (IS) formation and B cell activation. Despite this significance, the associated signaling mechanisms regulating LFA-1 activation remain elusive. Here, we show that both isoforms of the small GTPase Rac expressed by primary B cells, Rac1 and Rac2, were activated rapidly downstream of Src-family kinases, guanine-nucleotide exchange factors Vav1 and Vav2, and phosphoinositide-3 kinase (PI3K) after BCR engagement. We identify Rac2, but not Rac1, as critical for B cell adhesion to intercellular adhesion molecule-1 (ICAM-1) and IS formation. Furthermore, B cells expressing constitutively active Rac2 are highly adhesive. We observe that Rac2-deficient B cells exhibit lower amounts of Rap1-GTP and severe actin polymerization defects, identifying a potential mechanism underlying their behavior. We postulate that this critical role for Rac2 in mediating B cell adhesion and IS formation might apply in all lymphocytes.

Rap1a null mice have altered myeloid cell functions suggesting distinct roles for the closely related Rap1a and 1b proteins

The Ras-related GTPases Rap1a and 1b have been implicated in multiple biological events including cell adhesion, free radical production, and cancer. To gain a better understanding of Rap1 function in mammalian physiology, we deleted the Rap1a gene. Although loss of Rap1a expression did not initially affect mouse size or viability, upon backcross into C57BL/6J mice some Rap1a-/- embryos died in utero. T cell, B cell, or myeloid cell development was not disrupted in Rap1a-/- mice. However, macrophages from Rap1a null mice exhibited increased haptotaxis on fibronectin and vitronectin matrices that correlated with decreased adhesion. Chemotaxis of lymphoid and myeloid cells in response to CXCL12 or CCL21 was significantly reduced. In contrast, an increase in FcR-mediated phagocytosis was observed. Because Rap1a was previously copurified with the human neutrophil NADPH oxidase, we addressed whether GTPase loss affected superoxide production. Neutrophils from Rap1a-/- mice had reduced fMLP-stimulated superoxide production as well as a weaker initial response to phorbol ester. These results suggest that, despite 95% amino acid sequence identity, similar intracellular distribution, and broad tissue distribution, Rap1a and 1b are not functionally redundant but rather differentially regulate certain cellular events.

Cutting edge: Rac GTPases sensitize activated T cells to die via Fas

In activated CD4(+) T cells, TCR restimulation triggers apoptosis that depends on interactions between the death receptor Fas and its ligand, FasL. This process, termed restimulation-induced cell death (RICD), is a mechanism of peripheral immune tolerance. TCR signaling sensitizes activated T cells to Fas-mediated apoptosis, but what pathways mediate this process are not known. In this study we identify the Rho GTPases Rac1 and Rac2 as essential components in restimulation-induced cell death. RNA interference-mediated knockdown of Rac GTPases greatly reduced Fas-dependent, TCR-induced apoptosis. The ability of Rac1 to sensitize T cells to Fas-induced apoptosis correlated with Rac-mediated cytoskeletal reorganization, dephosphorylation of the ERM (ezrin/radixin/moesin) family of cytoskeletal linker proteins, and the translocation of Fas to lipid raft microdomains. In primary activated CD4(+) T cells, Rac1 and Rac2 were independently required for maximal TCR-induced apoptosis. Activating Rac signaling may be a novel way to sensitize chronically stimulated lymphocytes to Fas-induced apoptosis, an important goal in the treatment of autoimmune diseases.

Rac guanosine triphosphatases represent integrating molecular therapeutic targets for BCR-ABL-induced myeloproliferative disease

Chronic myelogenous leukemia (CML) is a clonal myeloproliferative disease (MPD) initiated by expression of the p210-BCR-ABL fusion protein. We demonstrate in a murine model of p210-BCR-ABL-induced MPD that gene targeting of Rac1 and Rac2 significantly delays or abrogates disease development. Attenuation of the disease phenotype is associated with severely diminished p210-BCR-ABL-induced downstream signaling in primary hematopoietic cells. We utilize NSC23766, a small molecule antagonist of Rac activation, to validate biochemically and functionally Rac as a molecular target in both a relevant animal model and in primary human CML cells in vitro and in a xenograft model in vivo, including in Imatinib-resistant p210-BCR-ABL disease. These data demonstrate that Rac is an additional therapeutic target in p210-BCR-ABL-mediated MPD.

Rac1 and Rac2 differentially regulate actin free barbed end formation downstream of the fMLP receptor

Actin assembly at the leading edge of migrating cells depends on the availability of high-affinity free barbed ends (FBE) that drive actin filament elongation and subsequent membrane protrusion. We investigated the specific mechanisms through which the Rac1 and Rac2 small guanosine triphosphatases (GTPases) generate free barbed ends in neutrophils. Using neutrophils lacking either Rac1 or Rac2 and a neutrophil permeabilization model that maintains receptor signaling to the actin cytoskeleton, we assessed the mechanisms through which these two small GTPases mediate FBE generation downstream of the formyl-methionyl-leucyl-phenylalanine receptor. We demonstrate here that uncapping of existing barbed ends is mediated through Rac1, whereas cofilin- and ARP2/3-mediated FBE generation are regulated through Rac2. This unique combination of experimental tools has allowed us to identify the relative roles of uncapping (15%), cofilin severing (10%), and ARP2/3 de novo nucleation (75%) in FBE generation and the respective roles played by Rac1 and Rac2 in mediating actin dynamics.

Rheumatoid arthritis: the role of reactive oxygen species in disease development and therapeutic strategies

Autoimmune diseases such as rheumatoid arthritis (RA) are chronic diseases that cannot be prevented or cured If the pathologic basis of such disease would be known, it might be easier to develop new drugs interfering with critical pathway. Genetic analysis of animal models for autoimmune diseases can result in discovery of proteins and pathways that play key function in pathogenesis, which may provide rationales for new therapeutic strategies. Currently, only the MHC class II is clearly associated with human RA and animal models for RA. However, recent data from rats and mice with a polymorphism in Ncf1, a member of the NADPH oxidase complex, indicate a role for oxidative burst in protection from arthritis. Oxidative burst-activating substances can treat and prevent arthritis in rats, as efficiently as clinically applied drugs, suggesting a novel pathway to a therapeutic target in human RA. Here, the authors discuss the role of oxygen radicals in regulating the immune system and autoimmune disease. It is proposed that reactive oxygen species set the threshold for T cell activation and thereby regulate chronic autoimmune inflammatory diseases like RA. In the light of this new hypothesis, new possibilities for preventive and therapeutic treatment of chronic inflammatory diseases are discussed.

The major outer sheath protein of Treponema denticola selectively inhibits Rac1 activation in murine neutrophils

Treponema denticola major outer sheath protein (Msp) inhibits neutrophil chemotaxis in vitro, but key regulatory mechanisms have not been identified. Because the Rac small GTPases regulate directional migration in response to chemoattractants, the objective was to analyse the effects of Msp on formyl-methionyl-leucyl-phenylalanine (fMLP)-mediated neutrophil polarization and Rac activation in murine neutrophils. Msp pretreatment of neutrophils inhibited both polarization and chemotactic migration in response to fMLP. Activation of small GTPases was measured by p21 binding domain (PBD) pulldown assays, followed by Western analysis, using monoclonal anti-Rac1, anti-Rac2, anti-cdc42 and anti-RhoA antibodies. Enriched native Msp selectively inhibited fMLP-stimulated Rac1 activation in a concentration-dependent manner, but did not affect Rac2, cdc42 or RhoA activation. Murine neutrophils transfected with vectors expressing fluorescent probes PAK-PBD-YFP and PH-AKT-RFP were used to determine the effects of Msp on the localization of activated Rac and PI3 kinase products. Real-time confocal images showed that Msp inhibited the polarized accumulation of activated Rac and PI3-kinase products upon exposure to fMLP. The findings indicate that T. denticola Msp inhibition of neutrophil polarity may be due to the selective suppression of the Rac1 pathway.

L-type voltage-operated Ca(+2) channels modulate transient Ca(+2) influx triggered by activation of Sertoli cell surface L-selectin

Near the base of mammalian seminiferous epithelium, Sertoli cells are joined by tight junctions, which constitute the blood-testis barrier. Differentiating germ cells are completely enveloped by Sertoli cells and must traverse the tight junctions during spermatogenic cycle. Following the specific ligand activation of L-selectin, the up-regulated Rho family small G-proteins have been implicated as important modulators of tight junctional dynamics. Although the activation of L-selectin transmits subsequent intracellular signals in a Ca(+2)-dependent fashion in various cell types, little is understood regarding the signaling pathways utilized by L-selectin in Sertoli cells. Therefore, we have examined the possible resultant calcium influx triggered by specific ligand-activation of cell surface L-selectin receptors or by cross-linking of L-selectin with anti-L-selectin. Spectrofluorimetric studies demonstrate increase of intracellular Ca(+2) levels immediately after the treatment of the L-selectin ligands, fucoidan and sialyl Lewis-a, or after treatment with anti-L-selectin antibody. We then determined the mechanism of Ca(+2) influx by investigating L- and T-type voltage-operated Ca(+2) channels, which have been suggested to present in the membranes of Sertoli cells. Data demonstrate that Sertoli cells treated with L-type voltage-operated Ca(+2) channel antagonists, nifedipine, diltiazem, or verapamil, lead to dose-dependent blockage of L-selectin-induced Ca(+2) influx. Cells treated with mibedradil, a T-type voltage-operated Ca(+2) channel antagonist, results in little or no blocking effect. Therefore, we conclude that activation of Sertoli cell L-selectin induces Ca(+2) influx, which is at least partially regulated by L-type voltage-operated Ca(+2) channels.

Genetic and pharmacologic evidence implicating the p85 alpha, but not p85 beta, regulatory subunit of PI3K and Rac2 GTPase in regulating oncogenic KIT-induced transformation in acute myeloid leukemia and systemic mastocytosis

Oncogenic activation loop KIT mutations are observed in acute myeloid leukemia (AML) and systemic mastocytosis (SM); however, unlike the KIT juxtamembrane mutants, the activation loop mutants are insensitive to imatinib mesylate. Furthermore, as prior studies primarily used heterologous cell lines, the molecular mechanism(s) underlying oncogenic KIT-induced transformation in primary cells is poorly understood. We demonstrate that expression of KITD814V in primary hematopoietic stem/progenitor cells (HSC/Ps) and mast cell progenitors (MCps) induces constitutive KIT autophosphorylation, supports ligand-independent hyperproliferation, and promotes promiscuous cooperation with multiple cytokines. Genetic disruption of p85 alpha, the regulatory subunit of class IA lipid kinase phosphoinositol-3-kinase (PI3K), but not of p85 beta, or genetic disruption of the hematopoietic cell-specific Rho GTPase, Rac2, normalizes KITD814V-induced ligand-independent hyperproliferation. Additionally, deficiency of p85 alpha or Rac2 corrects the promiscuous hyperproliferation observed in response to multiple cytokines in both KITD814V-expressing HSC/Ps and MCps. Treatment of KITD814V-expressing HSC/Ps with a Rac inhibitor (NC23766) or with rapamycin showed a dose-dependent suppression in ligand-independent growth. Taken together, our results identify p85 alpha and Rac2 as potential novel therapeutic targets for the treatment of KITD814V-bearing AML and SM.

A new perspective on the role of CuZn superoxide dismutase (SOD1)

The CuZn superoxide dismutase (SOD1), a member of a group of isoenzymes involved in the scavenger of superoxide anions, is a dimeric carbohydrate free protein, mainly localized in the cytosol. The reactive oxygen species (ROS) are involved in many pathophysiological events correlated with mutagenesis, cancer, degenerative processes and aging. In the first part of this mini-review the well known role of SOD1 and ROS are briefly summarized. Following, a potential novel biological action that SOD1 could exert is described, based on the recent researches demonstrating the secretion of this enzyme in many cellular lines. Moreover, the role of impaired mutant SOD1 secretion, associated with cytoplasmic toxic inclusion, which occurs in familial amyotrophic lateral sclerosis (ALS), is summarized. In addition, a depolarization-dependent release of SOD1 in pituitary GH3 cells and in rat synaptosomes through a calcium and SNARE-dependent mechanism is reported.

Regulation of cyclooxygenase-2 expression by small GTPase Rac2 in bone marrow macrophages

Cyclooxygenase 2 (COX-2) is induced by microbial products, proinflammatory cytokines, growth factors, and oncogenes. The Rho family includes RhoA, Rac1, Rac2, Rac3, and Cdc42 and is involved in regulation of the actin cytoskeleton organization, cell growth, vesicular cell trafficking, and transcriptional regulation. Rac2 binds to NADPH oxidase protein complex, and Rac2 null neutrophils are known to have poor phagocytic activity. We examined whether Rac2, the predominant small GTPase in hematopoietic cells, influences COX-2 expression in bone marrow-derived macrophages (BMDM). We showed that BMDM from Rac2(-/-) null mice have reduced COX-2 expression in response to treatment with endotoxin. Despite a compensatory increase in Rac1, BMDM from Rac2(-/-) null mice have less biologically active GTP-bound Rac in response to LPS stimulation. Signaling molecules (downstream of Rac2 and Toll-like receptor 4) such as p42/44, p38, and pAKT were also affected in BMDM from Rac2(-/-) null mouse macrophages. We also observed that BMDM from Rac2(-/-) null failed to degrade IkappaBalpha significantly and had less immunoreactive PU.1. We show that both NF-kappaB pathway and PU.1 are involved in normal macrophage function and play a role in macrophage COX-2 expression. In summary, these data indicate that Rac2 regulates COX-2 expression in BMDM.

Regulation of Nox and Duox enzymatic activity and expression

In recent years, it has become clear that reactive oxygen species (ROS, which include superoxide, hydrogen peroxide, and other metabolites) are produced in biological systems. Rather than being simply a by-product of aerobic metabolism, it is now recognized that specific enzymes--the Nox (NADPH oxidase) and Duox (Dual oxidase) enzymes--seem to have the sole function of generating ROS in a carefully regulated manner, and key roles in signal transduction, immune function, hormone biosynthesis, and other normal biological functions are being uncovered. The prototypical Nox is the respiratory burst oxidase or phagocyte oxidase, which generates large amounts of superoxide and other reactive species in the phagosomes of neutrophils and macrophages, playing a central role in innate immunity by killing microbes. This enzyme system has been extensively studied over the past two decades, and provides a basis for comparison with the more recently described Nox and Duox enzymes, which generate ROS in a variety of cells and tissues. This review first considers the structure and regulation of the respiratory burst oxidase, and then reviews recent studies relating to the regulation of the activity of the novel Nox/Duox enzymes. The regulation of Nox and Duox expression in tissues and by specific stimuli is also considered here. An accompanying review considers biological and pathological roles of the Nox family of enzymes.

Genetic and pharmacologic evidence that Rac1 GTPase is involved in regulation of platelet secretion and aggregation

BACKGROUND

Rac1 GTPase, a member of the Ras-related Rho GTPase family, is the major Rac isoform present in platelets and has been shown to be involved in cell actin cytoskeleton reorganization and adhesion. Agonists that induce platelet secretion and aggregation also activate Rac1 GTPase, raising the possibility that Rac1 GTPase may be involved in regulation of platelet function.

OBJECTIVES

To rigorously define the role of Rac1 in platelet regulation.

METHODS

We have used a dual approach of gene targeting in mice and pharmacologic inhibition of Rac1 by NSC23766, a rationally designed specific small molecule inhibitor, to study the role of Rac1 in platelet function.

RESULTS

Platelets from mice as well as human platelets treated with NSC23766 exhibited a significant decrease in: (i) active Rac1 species and phosphorylation of the Rac effector, p21-activated kinase; (ii) expression of P-selectin and secretion of adenosine triphosphate induced by thrombin or U46619; and (iii) aggregation induced by adenosine 5'-diphosphate, collagen, thrombin and U46619, a stable analog of thromboxane A(2). NSC23766 did not alter the cAMP or cGMP levels in platelets. Consistent with the requirement of Rac1 for normal platelet function, the bleeding times in Rac1(-/-) mice or mice given NSC23766 were significantly prolonged.

CONCLUSIONS

Our data show that deficiency or inhibition of Rac1 GTPase blocks platelet secretion. The inhibition of secretion, at least in part, is responsible for diminished platelet aggregation and prolonged bleeding times observed in Rac1 knockout or Rac1 inhibitor-treated mice.

Vav proteins control MyD88-dependent oxidative burst

The importance of reactive oxygen intermediate (ROI) production in antimicrobial responses is demonstrated in human patients who suffer from chronic granulomatous disease (CGD) due to defective NADPH oxidase function. Exactly how bacterial products activating Toll-like receptors (TLRs) induce oxidative burst is unknown. Here, we identify the Vav family of Rho guanine nucleotide exchange factors (GEFs) as critical mediators of LPS-induced MyD88-dependent activation of Rac2, NADPH oxidase, and ROI production using mice deficient in Vav1, Vav2, and Vav3. Vav proteins are also required for p38 MAPK activation and for normal regulation of proinflammatory cytokine production, but not for other MyD88-controlled effector pathways such as those involving JNK, COX2, or iNOS and the production of reactive nitrogen intermediates (RNIs). Thus, our data indicate that Vav specifically transduces a subset of signals emanating from MyD88.

Impaired epidermal wound healing in vivo upon inhibition or deletion of Rac1

To address the functions of Rac1 in keratinocytes of the basal epidermal layer and in the outer root sheath of hair follicles, we generated transgenic mice expressing a dominant inhibitory mutant of Rac, N17Rac1, under the control of the keratin 14 promoter. These mice do not exhibit an overt skin phenotype but show protracted skin wound re-epithelialization. Investigation into the underlying mechanisms revealed that in vivo both proliferation of wound-edge keratinocytes and centripetal migration of the neo-epidermis were impaired. Similar results were obtained in mice with an epidermis-specific deletion of Rac1. Primary epidermal keratinocytes that expressed the N17Rac1 transgene were less proliferative than control cells and showed reduced ERK1/2 phosphorylation upon growth factor stimulation. Adhesion, spreading, random migration and closure of scratch wounds in vitro were significantly inhibited on collagen I and, to a lesser extent, on fibronectin. Stroboscopic analysis of cell dynamics (SACED) of N17Rac1 transgenic and control keratinocytes identified decreased lamella-protrusion persistence in connection with increased ruffle frequency as a probable mechanism for the observed impairment of keratinocyte adhesion and migration. We conclude that Rac1 is functionally required for normal epidermal wound healing and, in this context, exerts a dual function - namely the regulation of keratinocyte proliferation and migration.

The Src family kinases Hck and Fgr regulate neutrophil responses to N-formyl-methionyl-leucyl-phenylalanine

The chemotactic peptide formyl-methionyl-leucyl-phenilalanine (fMLP) triggers intracellular protein tyrosine phosphorylation leading to neutrophil activation. Deficiency of the Src family kinases Hck and Fgr have previously been found to regulate fMLP-induced degranulation. In this study, we further investigate fMLP signaling in hck-/-fgr-/- neutrophils and find that they fail to activate a respiratory burst and display reduced F-actin polymerization in response to fMLP. Additionally, albeit migration of both hck-/-fgr-/-mouse neutrophils and human neutrophils incubated with the Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) through 3-microm pore size Transwells was normal, deficiency, or inhibition, of Src kinases resulted in a failure of neutrophils to migrate through 1-microm pore size Transwells. Among MAPKs, phosphorylation of ERK1/2 was not different, phosphorylation of p38 was only partially affected, and phosphorylation of JNK was markedly decreased in fMLP-stimulated hck-/-fgr-/- neutrophils and in human neutrophils incubated with PP2. An increase in intracellular Ca(2+) concentration and phosphorylation of Akt/PKB occurred normally in fMLP-stimulated hck-/-fgr-/- neutrophils, indicating that activation of both phosphoinositide-specific phospholipase C and PI3K is independent of Hck and Fgr. In contrast, phosphorylation of the Rho/Rac guanine nucleotide exchange factor Vav1 and the Rac target p21-activated kinases were markedly reduced in both hck-/-fgr-/- neutrophils and human neutrophils incubated with a PP2. Consistent with these findings, PP2 inhibited Rac2 activation in human neutrophils. We suggest that Hck and Fgr act within a signaling pathway triggered by fMLP receptors that involves Vav1 and p21-activated kinases, leading to respiratory burst and F-actin polymerization.

Rac1 and a GTPase-activating protein, MgcRacGAP, are required for nuclear translocation of STAT transcription factors

STAT transcription factors are tyrosine phosphorylated upon cytokine stimulation and enter the nucleus to activate target genes. We show that Rac1 and a GTPase-activating protein, MgcRacGAP, bind directly to p-STAT5A and are required to promote its nuclear translocation. Using permeabilized cells, we find that nuclear translocation of purified p-STAT5A is dependent on the addition of GTP-bound Rac1, MgcRacGAP, importin α, and importin β. p-STAT3 also enters the nucleus via this transport machinery, and mutant STATs lacking the MgcRacGAP binding site do not enter the nucleus even after phosphorylation. We conclude that GTP-bound Rac1 and MgcRacGAP function as a nuclear transport chaperone for activated STATs.

Cytokine expression and AIF-1-mediated activation of Rac2 in vascular smooth muscle cells: a role for Rac2 in VSMC activation

Allograft inflammatory factor-1 (AIF-1) is a cytoplasmic, calcium-binding, inflammation-responsive scaffold protein involved in vascular smooth muscle cell (VSMC) migration and proliferation. The objective of this study is to characterize AIF-1 functional protein interactions that may regulate VSMC activation. Through use of a bacterial two-hybrid screen, we identified a molecular interaction between AIF-1 and the small GTPase, Rac2, which was verified by pull-down and colocalization experiments. This was unexpected in that Rac2 expression had been considered to be restricted to hematopoietic cells. The Rac2/AIF-1 interaction is functional, in that a loss-of-function, point-mutated AIF-1 does not interact with Rac2; Rac2 colocalizes with AIF-1 in the cytoplasm of VSMC and cotranslocates to lamellopodia upon platelet-derived growth factor stimulation; and AIF-1 expression in VSMC leads to Rac2 activation. Because Rac2 function in VSMC had not been described, we focused on characterization of its function in these cells. Rac2 protein expression in VSMC is inducible by inflammatory cytokines, and Rac2 activation in VSMC is also responsive to inflammatory cytokines. Rac2 expression and activation patterns differ from the ubiquitously expressed Rac1. We hypothesized that Rac2 participates in VSMC activation. Retroviral overexpression of Rac2 in primary VSMC leads to increased migration, activation of the NADPH oxidation cascade, and increased activation of the Rac2 effector protein Pak1 and its proximal effectors, ERK1/2, and p38 (P < 0.05 for all). The major points of this study indicate a functional interaction between AIF-1 and Rac2 in VSMC leading to Rac2 activation and a potential function for Rac2 in inflammation-driven VSMC response to injury.

A regulated adaptor function of p40phox: distinct p67phox membrane targeting by p40phox and by p47phox

In the phagocytic cell, NADPH oxidase (Nox2) system, cytoplasmic regulators (p47(phox), p67(phox), p40(phox), and Rac) translocate and associate with the membrane-spanning flavocytochrome b(558), leading to activation of superoxide production. We examined membrane targeting of phox proteins and explored conformational changes in p40(phox) that regulate its translocation to membranes upon stimulation. GFP-p40(phox) translocates to early endosomes, whereas GFP-p47(phox) translocates to the plasma membrane in response to arachidonic acid. In contrast, GFP-p67(phox) does not translocate to membranes when expressed alone, but it is dependent on p40(phox) and p47(phox) for its translocation to early endosomes or the plasma membrane, respectively. Translocation of GFP-p40(phox) or GFP-p47(phox) to their respective membrane-targeting sites is abolished by mutations in their phox (PX) domains that disrupt their interactions with their cognate phospholipid ligands. Furthermore, GFP-p67(phox) translocation to either membrane is abolished by mutations that disrupt its interaction with p40(phox) or p47(phox). Finally, we detected a head-to-tail (PX-Phox and Bem1 [PB1] domain) intramolecular interaction within p40(phox) in its resting state by deletion mutagenesis, cell localization, and binding experiments, suggesting that its PX domain is inaccessible to interact with phosphatidylinositol 3-phosphate without cell stimulation. Thus, both p40(phox) and p47(phox) function as diverse p67(phox) "carrier proteins" regulated by the unmasking of membrane-targeting domains in distinct mechanisms.

Pollen NAD(P)H Oxidases and Their Contribution to Allergic Inflammation

This article provides an overview of NADPH oxidase and its role in allergic inflammation. A background and historical perspectives of NADPH oxidase are first provided, followed by a detailed overview of mammalian NADPH oxidase subunits and their functional organization. Plant NADPH oxidase, the authors' discovery of NADPH oxidase in pollens, and their contribution to allergic inflammation are then discussed, concluding with a discussion of future directions and outstanding questions that require attention.

Modulation of DNA binding properties of CCAAT/enhancer binding protein epsilon by heterodimer formation and interactions with NFkappaB pathway

C/EBP epsilon is a transcription factor involved in myeloid cell differentiation. Along with C/EBP-alpha, -beta, -gamma, -delta, and -zeta, C/EBP-epsilon belongs to the family of CCAAT/enhancer binding proteins that are implicated in control of growth and differentiation of several cell lineages in inflammation and stress response. We have previously shown that C/EBP-epsilon preferentially binds DNA as a heterodimer with other C/EBP family members such as C/EBP-delta, CHOP (C/EBP-zeta), and the b-zip family protein ATF4. In this study, we define the consensus binding sites for C/EBP-epsilon dimers and C/EBP-epsilon-ATF4 heterodimers. We show that the activated NFkappaB pathway promotes interaction of the C/EBP-epsilon subunit with its cognate DNA binding site via interaction with RelA. RelA-C/EBP interaction is enhanced by phosphorylation of threonine at amino acid 75 and results in increased DNA binding compared with the wild-type nonphosphorylated C/EBP both in vitro and in vivo. We suggest that interaction of the activated NFkappaB pathway and C/EBP-epsilon may be important in selective activation of a subset of C/EBP-epsilon-responsive genes.

Big roles for small GTPases in the control of directed cell movement

Small GTPases are involved in the control of diverse cellular behaviours, including cellular growth, differentiation and motility. In addition, recent studies have revealed new roles for small GTPases in the regulation of eukaryotic chemotaxis. Efficient chemotaxis results from co-ordinated chemoattractant gradient sensing, cell polarization and cellular motility, and accumulating data suggest that small GTPase signalling plays a central role in each of these processes as well as in signal relay. The present review summarizes these recent findings, which shed light on the molecular mechanisms by which small GTPases control directed cell migration.

Defective chemokine-directed lymphocyte migration and development in the absence of Rho guanosine diphosphate-dissociation inhibitors alpha and beta

Rho family small GTP-binding proteins, including Rho, Rac, and Cdc42, are key determinants of cell movement and actin-dependent cytoskeletal morphogenesis. Rho GDP-dissociation inhibitor (GDI) alpha and Rho GDIbeta (or D4/Ly-GDI), closely related regulators for Rho proteins, are both expressed in hemopoietic cell lineages. Nevertheless, the functional contributions of Rho GDIs remain poorly understood in vivo. In this study, we report that combined disruption of both the Rho GDIalpha and Rho GDIbeta genes in mice resulted in reduction of marginal zone B cells in the spleen, retention of mature T cells in the thymic medulla, and a marked increase in eosinophil numbers. Furthermore, these mice showed lower CD3 expression and impaired CD3-mediated proliferation of T cells. While B cells showed slightly enhanced chemotactic migration in response to CXCL12, peripheral T cells showed markedly reduced chemotactic migration in response to CCL21 and CCL19 associated with decreased receptor levels of CCR7. Overall, Rho protein levels were reduced in the bone marrow, spleen, and thymus but sustained activation of the residual part of RhoA, Rac1, and Cdc42 was detected mainly in the bone marrow and spleen. Rho GDIalpha and Rho GDIbeta thus play synergistic roles in lymphocyte migration and development by modulating activation cycle of the Rho proteins in a lymphoid organ-specific manner.

Tumor suppressor PTEN is a physiologic suppressor of chemoattractant-mediated neutrophil functions

The recruitment and activation of neutrophils at infected tissues is essential for host defense against invading microorganisms. However, excessive neutrophil recruitment or activation can also damage the surrounding tissues and cause unwanted inflammation. Hence, the responsiveness of neutrophils needs to be tightly regulated. In this study, we have investigated the functional role of tumor suppressor PTEN in neutrophils by using a mouse line in which PTEN is disrupted only in myeloid-derived cells. Chemoattractant-stimulated PTEN(-/-) neutrophils displayed significantly higher Akt phosphorylation and actin polymerization. A larger fraction of these neutrophils displayed membrane ruffles in response to chemoattractant stimulation. In addition, chemoattractant-induced transwell migration and superoxide production were also augmented. Single-cell chemotaxis assays showed that PTEN(-/-) neutrophils have a small (yet statistically significant) defect in directionality. However, these neutrophils also showed an increase in cell speed. As a result, overall chemotaxis, which depends on speed and directionality, was not affected. Consistent with the increased responsiveness of PTEN(-/-) neutrophils, the in vivo recruitment of these cells to the inflamed peritoneal cavity was significantly enhanced. Thus, as a physiologic-negative regulator, PTEN should be a promising therapeutic target for modulating neutrophil functions in various infectious and inflammatory diseases.

Integrin Regulation of Lymphocyte Trafficking: Lessons from Structural and Signaling Studies

High trafficking capability of lymphocytes is crucial in immune surveillance and antigen responses. Central to this regulatory process is a dynamic control of lymphocyte adhesion behavior regulated by chemokines and adhesion receptors such as integrins. Modulation of lymphocyte adhesive responses occurs in a wide range of time window from less than a second to hours, enabling rolling lymphocyte to attach to and migrate through endothelium and interact with antigen-presenting cells. While there has been a rapid progress in the understanding of integrin structure, elucidation of signaling events to relay extracellular signaling to integrins in physiological contexts has recently emerged from studies using gene-targeting and gene-silencing technique. Regulatory molecules critical for integrin activity control distribution of integrins, polarized cell morphology and motility, suggesting a signaling network that coordinates integrin function with lymphocyte migration. Here, I review recent studies of integrin structural changes and intracellular signal molecules that trigger integrin activation (inside-out signals), and discuss molecular mechanisms that control lymphocyte integrins and how inside-out signals coordinately modulate adhesive reactions and cell shape and migration.

Vav proteins in neutrophils are required for FcgammaR-mediated signaling to Rac GTPases and nicotinamide adenine dinucleotide phosphate oxidase component p40(phox)

Phagocytes generate reactive oxygen species, the regulation of which is important in eliminating ingested microbes while limiting tissue damage. Clustering of FcgammaRs results in the activation of Vav proteins, Rho/Rac guanine nucleotide exchange factors, and results in robust superoxide generation through the NADPH oxidase. In this study, studies in neutrophils isolated from mice deficient in Vav or Rac isoforms demonstrate a critical role for Vav3 in Rac2-dependent activation of the NADPH oxidase following FcgammaR clustering. However, studies in cytokine-primed cells revealed a strict requirement for Vav1 and Vav3 and Rac1 and Rac2 in the FcgammaR-mediated oxidative burst. In comparison, Vav was not essential for PMA or G protein-coupled receptor-mediated superoxide generation. The FcgammaR-mediated oxidative burst defect in Vav-deficient cells was linked to aberrant Rac activation as well as Rac- and actin-polymerization-independent, but PI3K-dependent, phosphorylation of the NADPH oxidase component p40(phox). In macrophages, Vav regulation of Rac GTPases was required specifically in FcgammaR-mediated activation of the oxidative burst, but not in phagocytosis. Thus, Vav proteins specifically couple FcgammaR signaling to NADPH oxidase function through a Rac-dependent as well as an unexpected Rac-independent signal that is proximal to NADPH oxidase activation and does not require actin polymerization.

Cell type-specific functions of Rho GTPases revealed by gene targeting in mice

Mammalian Rho family GTPases are intracellular signal transducers known to regulate multiple signaling pathways involved in actin organization and cell proliferation. However, previous knowledge of their cellular functions came mostly from studies using a dominant-negative or constitutively active mutant expression approach in various clonal cell lines. Such an approach has increasingly been recognized to impose experimental limitations related to specificity, dosage and/or clonal variation. Recent progress in mammalian Rho GTPase cell biology by gene targeting individual Rho GTPases in mice has provided more convincing evidence of their physiological roles and signaling pathways in diverse primary cells. Although adaptive compensation by related Rho GTPase members remains a potential concern in the gene targeting approach, in many cases these studies enable an elucidation of the unique functions of individual Rho GTPases in different cell types in vivo.

Mice deficient in the ALS2 gene exhibit lymphopenia and abnormal hematopietic function

One form of juvenile onset autosomal recessive amyotrophic lateral sclerosis (ALS2) has been linked to the dysfunction of the ALS2 gene. The ALS2 gene is expressed in lymphoblasts, however, whether ALS2-deficiency affects periphery blood is unclear. Here we report that ALS2 knockout (ALS2(-/-)) mice developed peripheral lymphopenia but had higher proportions of hematopoietic stem and progenitor cells in which the stem cell factor-induced cell proliferation was up-regulated. Our findings reveal a novel function of the ALS2 gene in the lymphopoiesis and hematopoiesis, suggesting that the immune system is involved in the pathogenesis of ALS2.

Rac GTPases regulate the morphology and deformability of the erythrocyte cytoskeleton

Actin oligomers are a significant structural component of the erythrocyte cytoskeleton. Rac1 and Rac2 GTPases regulate actin structures and have multiple overlapping as well as distinct roles in hematopoietic cells; therefore, we studied their role in red blood cells (RBCs). Conditional gene targeting with a loxP-flanked Rac1 gene allowed Crerecombinase-induced deletion of Rac1 on a Rac2 null genetic background. The Rac1(-/-);Rac2(-/-) mice developed microcytic anemia with a hemoglobin drop of about 20% and significant anisocytosis and poikilocytosis. Reticulocytes increased more than 2-fold. Rac1(-/-);Rac2(-/-) RBCs stained with rhodamine-phalloidin demonstrated F-actin meshwork gaps and aggregates under confocal microscopy. Transmission electron microscopy of the cytoskeleton demonstrated junctional aggregates and pronounced irregularity of the hexagonal spectrin scaffold. Ektacytometry confirmed that these cytoskeletal changes in Rac1(-/-);Rac2(-/-) erythrocytes were associated with significantly decreased cellular deformability. The composition of the cytoskeletal proteins was altered with an increased actin-to-spectrin ratio and increased phosphorylation (Ser724) of adducin, an F-actin capping protein. Actin and phosphorylated adducin of Rac1(-/-);Rac2(-/-) erythrocytes were more easily extractable by Triton X-100, indicating weaker association to the cytoskeleton. Thus, deficiency of Rac1 and Rac2 GTPases in mice alters actin assembly in RBCs and causes microcytic anemia with reticulocytosis, implicating Rac GTPases as dynamic regulators of the erythrocyte cytoskeleton organization.

The Rac effector p67phox regulates phagocyte NADPH oxidase by stimulating Vav1 guanine nucleotide exchange activity

The phagocyte NADPH oxidase catalyzes the reduction of molecular oxygen to superoxide and is essential for microbial defense. Electron transport through the oxidase flavocytochrome is activated by the Rac effector p67(phox). Previous studies suggest that Vav1 regulates NADPH oxidase activity elicited by the chemoattractant formyl-Met-Leu-Phe (fMLP). We show that Vav1 associates with p67(phox) and Rac2, but not Rac1, in fMLP-stimulated human neutrophils, correlating with superoxide production. The interaction of p67(phox) with Vav1 is direct and activates nucleotide exchange on Rac, which enhances the interaction between p67(phox) and Vav1. This provides new molecular insights into regulation of the neutrophil NADPH oxidase, suggesting that chemoattractant-stimulated superoxide production can be amplified by a positive feedback loop in which p67(phox) targets Vav1-mediated Rac activation.

Rac1 links leading edge and uropod events through Rho and myosin activation during chemotaxis

Chemotactic responsiveness is crucial to neutrophil recruitment to sites of infection. During chemotaxis, highly divergent cytoskeletal programs are executed at the leading and trailing edge of motile neutrophils. The Rho family of small GTPases plays a critical role in cell migration, and recent work has focused on elucidating the specific roles played by Rac1, Rac2, Cdc42, and Rho during cellular chemotaxis. Rac GTPases regulate actin polymerization and extension of the leading edge, whereas Rho GTPases control myosin-based contraction of the trailing edge. Rac and Rho signaling are thought to crosstalk with one another, and previous research has focused on mutual inhibition of Rac and Rho signaling during chemotaxis. Indeed, polarization of neutrophils has been proposed to involve the activity of a negative feedback system where Rac activation at the front of the cell inhibits local Rho activation, and vice versa. Using primary human neutrophils and neutrophils derived from a Rac1/Rac2-null transgenic mouse model, we demonstrate here that Rac1 (and not Rac2) is essential for Rho and myosin activation at the trailing edge to regulate uropod function. We conclude that Rac plays both positive and negative roles in the organization of the Rhomyosin "backness" program, thereby promoting stable polarity in chemotaxing neutrophils.

Rho GTPase Rac1 is critical for neutrophil migration into the lung

Neutrophils are critical in the inflammatory process by moving rapidly to tissue sites of inflammation. Members of the small Rho GTPase family, Rac1, Rac2, CDC42, and RhoA, are central regulators of cell migration by cytoskeleton rearrangement. The role of Rac1 in neutrophil migration related to inflammatory processes has remained elusive and has yet to be determined in physiologic in vivo models. We previously demonstrated a role for Rac1 in tail retraction. Here, we present evidence that Rac1-mediated uropod formation may be due to crosstalk with a related Rho GTPase RhoA. To assess the physiologic relevance of these findings, we used adoptive transfer of Rac1flox/flox bone marrow cells which allows postengraftment in vivo deletion of Rac1 only in blood cells. We examined the specific role of Rac1 in neutrophil migration into the lung during the inflammatory process induced by formyl-methionyl-leucyl-phenylalanine exposure. The loss of Rac1 activity in neutrophils is associated with a significant decreased neutrophil recruitment into lung alveolar and attenuation of emphysematous lesions. Overall, this study suggests that Rac1 is a physiologic integrator of signals for neutrophil recruitment into lung tissue during an inflammatory response.

Mechanisms of degranulation in neutrophils

Neutrophils are critical inflammatory cells that cause tissue damage in a range of diseases and disorders. Being bone marrow-derived white blood cells, they migrate from the bloodstream to sites of tissue inflammation in response to chemotactic signals and induce inflammation by undergoing receptor-mediated respiratory burst and degranulation. Degranulation from neutrophils has been implicated as a major causative factor in pulmonary disorders, including severe asphyxic episodes of asthma. However, the mechanisms that control neutrophil degranulation are not well understood. Recent observations indicate that granule release from neutrophils depends on activation of intracellular signalling pathways, including β-arrestins, the Rho guanosine triphosphatase Rac2, soluble NSF attachment protein (SNAP) receptors, the src family of tyrosine kinases, and the tyrosine phosphatase MEG2. Some of these observations suggest that degranulation from neutrophils is selective and depends on nonredundant signalling pathways. This review focuses on new findings from the literature on the mechanisms that control the release of granule-derived mediators from neutrophils.

Whole genome expression profiling of glucose-dependent insulinotropic peptide (GIP)- and adrenocorticotropin-dependent adrenal hyperplasias reveals novel targets for the study of GIP-dependent Cushing's syndrome

CONTEXT

The mechanisms responsible for the ectopic adrenal expression of glucose-dependent insulinotropic peptide (GIP) receptor (GIPR) in GIP-dependent Cushing's syndrome (CS) are unknown. Chronic adrenal stimulation by ACTH in Cushing's disease or GIP in GIP-dependent ACTH-independent macronodular adrenal hyperplasia both lead to the induction of genes implicated in adrenal proliferation and steroidogenesis.

OBJECTIVE

The objective of the study was to identify genes differentially expressed specifically in GIP-dependent CS that could be implicated in the ectopic expression of GIPR.

METHODS

We used the Affymetrix U133 plus 2.0 microarray oligochips to compare the whole genome expression profile of adrenal tissues from five cases of GIP-dependent bilateral ACTH-independent macronodular adrenal hyperplasia with CS, one case of GIP-dependent unilateral adenoma with CS, five cases of ACTH-dependent hyperplasias, and a pool of adrenals from 62 normal individuals.

RESULTS

After data normalization and statistical filtering, 723 genes with differential expression were identified, including 461 genes or sequences with a known functional implication, classified in eight dominant functional classes. Specific findings include repression of perilipin, the overexpression of 13 G protein-coupled receptors, and the potential involvement of Rho-GTPases. We also isolated 94 probe sets potentially linked to the formation of GIP-dependent nodules adjacent to the diffuse hyperplasia. These included probe sets related to the linker histone H1 and repression of RXRa and CCND2. The expression profiles for eight genes were confirmed by real-time RT-PCR.

CONCLUSION

This study identified an extensive series of potentially novel target candidate genes that could be implicated in the molecular mechanisms of ectopic expression of the GIPR as well as in the multistep progression of GIP-dependent CS.

Regulation of the phagocyte NADPH oxidase by Rac GTPase

Phagocytic leukocytes generate reactive oxygen species important for the killing of invading microorganisms. The source of these oxidants is the NADPH oxidase, a tightly controlled multicomponent enzyme made up of a membrane-associated catalytic moiety and cytosolic regulatory components that must assemble to form the active oxidase. The phagocyte NADPH oxidase was the first mammalian system shown to be directly regulated by a Rac GTPase. We review here our understanding of NADPH oxidase regulation by Rac, as well as the regulation of Rac itself, in phagocytic leukocytes. Rather than viewing Rac as a "cog" in the NADPH oxidase machinery, we argue for a view of Rac GTPases as critical "molecular switches" regulating the formation of ROS by phagocytic leukocytes under physiologic and pathologic conditions.

Role of Nox family NADPH oxidases in host defense

The phagocytic NADPH oxidase is recognized as a critical component of innate immunity, responsible for generation of microbicidal reactive oxygen species (ROS). This enzyme is one representative of the Nox family of oxidases (Nox1-Nox5, Duox1, and Duox2) that exhibit diverse expression patterns and appear to serve a variety of functions related to ROS generation. Mounting evidence now suggests that several of these novel oxidases also serve in host defense, particularly those showing high expression along epithelial surfaces exposed to the external environment. Within these sites, Nox enzymes tend to be located on apical cell surfaces and release ROS into extracellular environments, where they can be used by known antimicrobial peroxidases. Moreover, microbial factors were shown in several cases to cause higher ROS production, either by direct oxidase activation or by inducing higher oxidase expression. Several oxidases are also induced by immune cytokines, including interferon-gamma, interleukin (IL)-4, and IL-13. Although most of the evidence supporting host defense roles for mammalian nonphagocytic oxidases remains circumstantial, recent evidence indicates that Drosophila Duox plays a role in host resistance to infection. Finally, oxidative defense against invading pathogens appears to be an ancient protective mechanism, because related oxidases are known to participate in disease resistance in plants.

Taurine chloramine inhibits PMA-stimulated superoxide production in human neutrophils perhaps by inhibiting phosphorylation and translocation of p47phox

Neutrophils produce microbicidal oxidants to destroy the invading pathogens using nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, a membrane-associated enzyme complex that generates superoxide anion (O(2)(-)). Upon stimulation, the cytosolic components of NADPH oxidase, p47(phox) and p67(phox) and the small GTPase Rac move to phagosomal and plasma membranes where they become associated with the membrane components of NADPH oxidase, gp91(phox) and p22(phox) and express enzyme activity. We previously showed that taurine chloramine (Tau-Cl) inhibits O(2)(-) production in mouse peritoneal neutrophils (Kim, 1996). In the present study, we investigated the mechanisms underlying Tau-Cl-derived inhibition on O(2)(-) production using a human myeloid leukemia cell line, PLB-985 cell, which has been differentiated into neutrophil-like cell. Tau-Cl inhibited the phorbol myristate acetate (PMA)-elicited O(2)(-) production as previously observed in murine peritoneal neutrophils. Translocation of p47(phox), p67(phox) and Rac was increased in response to PMA, and Tau-Cl inhibited the PMA-stimulated translocation of p47(phox) and p67(phox) to plasma membrane without affecting the translocation of Rac. In addition, Tau-Cl inhibited the PMA-derived phosphorylation of p47(phox), a requirement for the translocation of cytosolic NADPH oxidase component to the plasma membrane. These results suggest that Tau-Cl inhibits PMA-elicited O(2)(-) production in PLB-985 granulocytes by inhibiting phosphorylation of p47(phox) and translocation of p47(phox) and p67(phox), eventually blocking the assembly of NADPH oxidase complex.