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

P-Rex1 and Vav1 cooperate in the regulation of formyl-methionyl-leucyl-phenylalanine-dependent neutrophil responses

G protein-coupled receptor (GPCR) activation elicits neutrophil responses such as chemotaxis and reactive oxygen species (ROS) formation, which depend on the small G protein Rac and are essential for host defense. P-Rex and Vav are two families of guanine-nucleotide exchange factors (GEFs) for Rac, which are activated through distinct mechanisms but can both control GPCR-dependent neutrophil responses. It is currently unknown whether they play specific roles or whether they can compensate for each other in controlling these responses. In this study, we have assessed the function of neutrophils from mice deficient in P-Rex and/or Vav family GEFs. We found that both the P-Rex and the Vav family are important for LPS priming of ROS formation, whereas particle-induced ROS responses and cell spreading are controlled by the Vav family alone. Surprisingly, fMLF-stimulated ROS formation, adhesion, and chemotaxis were synergistically controlled by P-Rex1 and Vav1. These responses were more severely impaired in neutrophils lacking both P-Rex1 and Vav1 than those lacking the entire P-Rex family, the entire Vav family, or both P-Rex1 and Vav3. P-Rex1/Vav1 (P1V1) double-deficient cells also showed the strongest reduction in fMLF-stimulated activation of Rac1 and Rac2. This reduction in Rac activity may be sufficient to cause the defects observed in fMLF-stimulated P1V1 neutrophil responses. Additionally, Mac-1 surface expression was reduced in P1V1 cells, which might contribute further to defects in responses involving integrins, such as GPCR-stimulated adhesion and chemotaxis. We conclude that P-Rex1 and Vav1 together are the major fMLFR-dependent Dbl family Rac-GEFs in neutrophils and cooperate in the control of fMLF-stimulated neutrophil responses.

The perplexing case of the geranylgeranyl transferase-deficient mouse

Proteins that end with a CAAX sequence are targeted to cellular membranes by a series of posttranslational modifications that include prenylation, proteolysis, and carboxyl methylation. Two prenyltransferases modify CAAX proteins: farnesyltransferase and geranylgeranyltransferase type I (GGTase-I). Rho family GTPases that control the actin cytoskeleton and are therefore critical to inflammatory cell function are substrates for GGTase-I. In this issue of the JCI, Khan et al. examined mice in which GGTase-I was conditionally deleted in macrophages. Rather than obtunded cells, the authors found activated Rho proteins in fully functional macrophages that hypersecreted inflammatory cytokines and induced an erosive, inflammatory arthritis. This surprising result calls into question the role of protein geranylgeranylation in inflammatory cell signaling.

Control of hepatic nuclear superoxide production by glucose 6-phosphate dehydrogenase and NADPH oxidase-4

Redox-regulated signal transduction is coordinated by spatially controlled production of reactive oxygen species within subcellular compartments. The nucleus has long been known to produce superoxide (O(2)(·-)); however, the mechanisms that control this function remain largely unknown. We have characterized molecular features of a nuclear superoxide-producing system in the mouse liver. Using electron paramagnetic resonance, we investigated whether several NADPH oxidases (NOX1, 2, and 4) and known activators of NOX (Rac1, Rac2, p22(phox), and p47(phox)) contribute to nuclear O(2)(·-) production in isolated hepatic nuclei. Our findings demonstrate that NOX4 most significantly contributes to hepatic nuclear O(2)(·-) production that utilizes NADPH as an electron donor. Although NOX4 protein immunolocalized to both nuclear membranes and intranuclear inclusions, fluorescent detection of NADPH-dependent nuclear O(2)(·-) predominantly localized to the perinuclear space. Interestingly, NADP(+) and G6P also induced nuclear O(2)(·-) production, suggesting that intranuclear glucose-6-phosphate dehydrogenase (G6PD) can control NOX4 activity through nuclear NADPH production. Using G6PD mutant mice and G6PD shRNA, we confirmed that reductions in nuclear G6PD enzyme decrease the ability of hepatic nuclei to generate O(2)(·-) in response to NADP(+) and G6P. NOX4 and G6PD protein were also observed in overlapping microdomains within the nucleus. These findings provide new insights on the metabolic pathways for substrate regulation of nuclear O(2)(·-) production by NOX4.

Osteoclasts lacking Rac2 have defective chemotaxis and resorptive activity

The role of the small Rho GTPase Rac2 in mature osteoclasts has not been extensively studied. Rac2(-/-) mice are of normal size and have normal tooth eruption. However, femoral cortical thickness was significantly greater in Rac2(-/-) compared to wild-type mice, while percent cortical porosity was lower. As assessed by histomorphometry, trabecular bone mass was significantly higher in male Rac2(-/-) than wild-type animals, although trabecular bone mass was similar when data from male and female animals were combined. There were no significant differences in the number of osteoblasts per bone surface; however, the number of osteoclasts per total bone area tended to be higher in Rac2(-/-) mice and was significantly higher in male Rac2(-/-) mice. In the aggregate, these data suggested a defect in osteoclast function and, consistent with that, rates of bone resorption were significantly reduced in Rac2(-/-) osteoclasts. In addition, Rac2(-/-) osteoclasts had a significantly delayed spreading response to treatment with CSF1 for 15 min. Phalloidin staining showed areas of abnormal actin accumulation and impaired actin ring formation in Rac2(-/-) osteoclasts. Finally, Rac2(-/-) osteoclasts showed a marked defect in chemotaxis toward a point source of CSF1, with a dramatic reduction in migratory rate. Together, these findings indicate an important role for Rac2 in mature osteoclasts.

Critical roles for Rac GTPases in T-cell migration to and within lymph nodes

Naive T cells continuously recirculate between secondary lymphoid tissue via the blood and lymphatic systems, a process that maximizes the chances of an encounter between a T cell and its cognate antigen. This recirculation depends on signals from chemokine receptors, integrins, and the sphingosine-1-phosphate receptor. The authors of previous studies in other cell types have shown that Rac GTPases transduce signals leading to cell migration and adhesion; however, their roles in T cells are unknown. By using both 3-dimensional intravital and in vitro approaches, we show that Rac1- and Rac2-deficient T cells have multiple defects in this recirculation process. Rac-deficient T cells home very inefficiently to lymph nodes and the white pulp of the spleen, show reduced interstitial migration within lymph node parenchyma, and are defective in egress from lymph nodes. These mutant T cells show defective chemokine-induced chemotaxis, chemokinesis, and adhesion to integrin ligands. They have reduced lateral motility on endothelial cells and transmigrate in-efficiently. These multiple defects stem from critical roles for Rac1 and Rac2 in transducing chemokine and sphingosine-1-phosphate receptor 1 signals leading to motility and adhesion.

Hem-1: putting the "WAVE" into actin polymerization during an immune response

Most active processes by immune cells including adhesion, migration, and phagocytosis require the coordinated polymerization and depolymerization of filamentous actin (F-actin), which is an essential component of the actin cytoskeleton. This review focuses on a newly characterized hematopoietic cell-specific actin regulatory protein called hematopoietic protein-1 [Hem-1, also known as Nck-associated protein 1-like (Nckap1l or Nap1l)]. Hem-1 is a component of the "WAVE [WASP (Wiskott-Aldrich syndrome protein)-family verprolin homologous protein]" complex, which signals downstream of activated Rac to stimulate F-actin polymerization in response to immuno-receptor signaling. Genetic studies in cell lines and in mice suggest that Hem-1 regulates F-actin polymerization in hematopoietic cells, and may be essential for most active processes dependent on reorganization of the actin cytoskeleton in immune cells.

PtdIns3P and Rac direct the assembly of the NADPH oxidase on a novel, pre-phagosomal compartment during FcR-mediated phagocytosis in primary mouse neutrophils

The generation of reactive oxygen species (ROS) by the nicotinamide adenine dinucleotide phosphate oxidase is an important mechanism by which neutrophils kill pathogens. The oxidase is composed of a membrane-bound cytochrome and 4 soluble proteins (p67(phox), p40(phox), p47(phox), and GTP-Rac). These components form an active complex at the correct time and subcellular location through a series of incompletely understood mutual interactions, regulated, in part, by GTP/GDP exchange on Rac, protein phosphorylation, and binding to lipid messengers. We have used a variety of assays to follow the spatiotemporal assembly of the oxidase in genetically engineered primary mouse neutrophils, during phagocytosis of both serum- and immunoglobulin G-opsonized targets. The oxidase assembles directly on serum-Staphylococcus aureus-containing phagosomes within seconds of phagosome formation; this process is only partially dependent (∼ 30%) on PtdIns3P binding to p40(phox), but totally dependent on Rac1/2 binding to p67(phox). In contrast, in response to immunoglobulin G-targets, the oxidase first assembles on a tubulovesicular compartment that develops at sites of granule fusion to the base of the emerging phagosome; oxidase assembly and activation is highly dependent on both PtdIns3P-p40(phox) and Rac2-p67(phox) interactions and delivery to the phagosome is regulated by Rab27a. These results define a novel pathway for oxidase assembly downstream of FcR-activation.

NADPH oxidase deficiency regulates Th lineage commitment and modulates autoimmunity

Reactive oxygen species are used by the immune system to eliminate infections; however, they may also serve as signaling intermediates to coordinate the efforts of the innate and adaptive immune systems. In this study, we show that by eliminating macrophage and T cell superoxide production through the NADPH oxidase (NOX), T cell polarization was altered. After stimulation with immobilized anti-CD3 and anti-CD28 or priming recall, T cells from NOX-deficient mice exhibited a skewed Th17 phenotype, whereas NOX-intact cells produced cytokines indicative of a Th1 response. These findings were corroborated in vivo by studying two different autoimmune diseases mediated by Th17 or Th1 pathogenic T cell responses. NOX-deficient NOD mice were Th17 prone with a concomitant susceptibility to experimental allergic encephalomyelitis and significant protection against type 1 diabetes. These data validate the role of superoxide in shaping Th responses and as a signaling intermediate to modulate Th17 and Th1 T cell responses.

Rac and Rho GTPases in cancer cell motility control

Rho GTPases represent a family of small GTP-binding proteins involved in cell cytoskeleton organization, migration, transcription, and proliferation. A common theme of these processes is a dynamic reorganization of actin cytoskeleton which has now emerged as a major switch control mainly carried out by Rho and Rac GTPase subfamilies, playing an acknowledged role in adaptation of cell motility to the microenvironment. Cells exhibit three distinct modes of migration when invading the 3 D environment. Collective motility leads to movement of cohorts of cells which maintain the adherens junctions and move by photolytic degradation of matrix barriers. Single cell mesenchymal-type movement is characterized by an elongated cellular shape and again requires extracellular proteolysis and integrin engagement. In addition it depends on Rac1-mediated cell polarization and lamellipodia formation. Conversely, in amoeboid movement cells have a rounded morphology, the movement is independent from proteases but requires high Rho GTPase to drive elevated levels of actomyosin contractility. These two modes of cell movement are interconvertible and several moving cells, including tumor cells, show an high degree of plasticity in motility styles shifting ad hoc between mesenchymal or amoeboid movements. This review will focus on the role of Rac and Rho small GTPases in cell motility and in the complex relationship driving the reciprocal control between Rac and Rho granting for the opportunistic motile behaviour of aggressive cancer cells. In addition we analyse the role of these GTPases in cancer progression and metastatic dissemination.

Neutrophil functions and autoimmune arthritis in the absence of p190RhoGAP: generation and analysis of a novel null mutation in mice

Beta(2) integrins of neutrophils play a critical role in innate immune defense, but they also participate in tissue destruction during autoimmune inflammation. p190RhoGAP (ArhGAP35), a regulator of Rho family small GTPases, is required for integrin signal transduction in fibroblasts. Prior studies have also suggested a role for p190RhoGAP in beta(2) integrin signaling in neutrophils. To directly test that possibility, we have generated a novel targeted mutation completely disrupting the p190RhoGAP-encoding gene in mice. p190RhoGAP deficiency led to perinatal lethality and defective neural development, precluding the analysis of neutrophil functions in adult p190RhoGAP(-/-) animals. This was overcome by transplantation of fetal liver cells from p190RhoGAP(-/-) fetuses into lethally irradiated wild-type recipients. Neutrophils from such p190RhoGAP(-/-) bone marrow chimeras developed normally and expressed normal levels of various cell surface receptors. Although p190RhoGAP(-/-) neutrophils showed moderate reduction of beta(2) integrin-mediated adherent activation, they showed mostly normal migration in beta(2) integrin-dependent in vitro and in vivo assays and normal beta(2) integrin-mediated killing of serum-opsonized Staphylococcus aureus and Escherichia coli. A neutrophil- and beta(2) integrin-dependent transgenic model of the effector phase of autoimmune arthritis also proceeded normally in p190RhoGAP(-/-) bone marrow chimeras. In contrast, all the above responses were completely blocked in CD18(-/-) neutrophils or CD18(-/-) bone marrow chimeras. These results suggest that p190RhoGAP likely does not play a major indispensable role in beta(2) integrin-mediated in vitro and in vivo neutrophil functions or the effector phase of experimental autoimmune arthritis.

Rac GTPases in erythroid biology

Rac1 and Rac2 GTPases, members of the Rho GTPases family, control actin organization and play distinct and overlapping roles in hematopoietic and mature blood cells of all lineages. Here, we review our findings on the role of Rac GTPases in erythroid cells, by using conditional gene-targeting in mice. Rac1 and Rac2 deficiency causes anemia with reticulocytosis, indicating decreased red blood cell (RBC) survival, altered actin assembly in the erythrocyte membrane skeleton and decreased RBC deformability. On the other hand, Rac1(-/-); Rac2(-/-) megakaryocyte-erythrocyte progenitors demonstrate decreased proliferation in the bone marrow, but increased survival and proliferation in the spleen, indicating that stress erythropoiesis circumvents Rac GTPases deficiency. Further elucidation of the signaling pathways controlled by Rac GTPases in erythroid cells may reveal potential therapeutic targets for diseases characterized by hemolytic anemia and erythropoiesis disorders.

Coordinated RhoA signaling at the leading edge and uropod is required for T cell transendothelial migration

Transendothelial migration (TEM) is a tightly regulated process whereby leukocytes migrate from the vasculature into tissues. Rho guanosine triphosphatases (GTPases) are implicated in TEM, but the contributions of individual Rho family members are not known. In this study, we use an RNA interference screen to identify which Rho GTPases affect T cell TEM and demonstrate that RhoA is critical for this process. RhoA depletion leads to loss of migratory polarity; cells lack both leading edge and uropod structures and, instead, have stable narrow protrusions with delocalized protrusions and contractions. By imaging a RhoA activity biosensor in transmigrating T cells, we find that RhoA is locally and dynamically activated at the leading edge, where its activation precedes both extension and retraction events, and in the uropod, where it is associated with ROCK-mediated contraction. The Rho guanine nucleotide exchange factor (GEF) GEF-H1 contributes to uropod contraction but does not affect the leading edge. Our data indicate that RhoA activity is dynamically regulated at the front and back of T cells to coordinate TEM.

Rac2 GTPase deficiency depletes BCR-ABL+ leukemic stem cells and progenitors in vivo

Chronic myelogenous leukemia (CML) is a clonal myeloproliferative disease (MPD) initiated by p210-BCR-ABL-mediated transformation of hematopoietic stem cells (HSCs). Inhibition of the ABL kinase alone is not sufficient to eradicate leukemic stem cells (LSCs). We have previously shown that the deficiency of Rac2 GTPase signaling, but not Rac1, in p210-BCR-ABL-transduced hematopoietic cells prolonged survival of mice with MPD. Here we demonstrate that absence of Rac2 GTPase prolongs survival of HSC-initiated, inducible Scl/p210-BCR-ABL (Scl/p210) binary transgenic mice, it induces apoptosis, and, unlike in normal HSC and progenitor (HSC/P), impairs LSC and progenitor (LSC/P) proliferation in vivo. As a result, Rac2 deficiency causes functional exhaustion of the LSC pool in vivo. This defect is not due to impaired interaction with the hematopoietic microenvironment as reflected by its unaltered adhesion, migration, and homing to recipient organs. In summary, Rac2 deficiency exhausts the LSC pool in vivo through impairment of oncogene-induced proliferation and survival signals.

Rac1-mediated mitochondrial H2O2 generation regulates MMP-9 gene expression in macrophages via inhibition of SP-1 and AP-1

Aberrant matrix deposition is a hallmark of pulmonary fibrosis and is characterized by an imbalance between matrix deposition and degradation. We have previously shown that mice harboring a conditional deletion of the GTP-binding protein, Rac1, in macrophages are protected from asbestos-induced pulmonary fibrosis. To investigate the contribution of aberrant matrix degradation, we addressed the role of Rac1 in regulating expression of macrophage-specific MMP-9 (matrix metalloproteinase-9). We found that MMP-9 gene transcription was significantly greater in Rac1 null macrophages. Deletion and mutational analysis of the MMP-9 promoter revealed that both SP-1 and AP-1 are essential for MMP-9 transcription. Overexpression of constitutive active Rac1 (V12) revealed that H(2)O(2) was derived from the mitochondria. Rac1-induced H(2)O(2) generation down-regulated MMP-9 gene transcription, whereas catalase overexpression in WT cells enhanced MMP-9 expression. SP-1 interacted directly with both c-Jun and c-Fos, and H(2)O(2) decreased this binding, suggesting that SP-1 and AP-1 function cooperatively to regulate MMP-9 transcription. Rac1-mediated H(2)O(2) inhibited the ERK MAPK, which was essential for activation of SP-1 and AP-1. ERK activation and MMP-9 expression were recovered by overexpressing catalase or transfecting siRNA for the mitochondrial iron-sulfur protein, Rieske. These observations were recapitulated in vivo. MMP-9 mRNA was higher in alveolar macrophages isolated from Rac1 null mice and wild type mice given catalase. Rac1 regulates MMP-9 transcription via mitochondrial H(2)O(2) generation, providing a potential mechanism by which Rac1 null mice fail to develop pulmonary fibrosis.

Activation of the A(3) adenosine receptor inhibits fMLP-induced Rac activation in mouse bone marrow neutrophils

Adenosine is released from injured or hypoxic tissues where it exerts numerous anti-inflammatory effects including suppression of neutrophil functions. Although most previous work has implicated the A(2A)AR, we have recently shown that selective activation of the abundantly expressed A(3)AR inhibits neutrophil superoxide production and chemotaxis providing a potential mechanistic explanation for the efficacy of A(3)AR agonists in experimental animal models of inflammation. In this study, we hypothesized that the A(3)AR suppresses neutrophil functions by inhibiting the monomeric GTPase Rac, a central regulator of chemokine-directed neutrophil migration and superoxide production. We found that pre-treating neutrophils with the highly selective A(3)AR agonist CP-532,903 reduced fMLP-induced Rac activation using an ELISA-based assay that detects all three Rac isoforms. CP-532,903 also inhibited fMLP-induced F-actin formation, a downstream effector function of Rac relevant to neutrophil migration, but not activation of ERK1/2 or p38. Pre-treating neutrophils with CP-532,903 did not stimulate cAMP production or alter fMLP-induced calcium transients, implicating that A(3)AR stimulation does not inhibit Rac activation or neutrophil activities by suppressing Ca(2+) signaling, elevating the intracellular concentration of cAMP, or by cross-desensitizing fMLP receptors. Our results suggest that activation of the A(3)AR signals to suppress neutrophil functions by interfering with the monomeric GTPase Rac, thus contributing to the ant-inflammatory actions of adenosine.

A novel Rac-dependent checkpoint in B cell development controls entry into the splenic white pulp and cell survival

Rac1 and Rac2 GTPases transduce signals from multiple receptors leading to cell migration, adhesion, proliferation, and survival. In the absence of Rac1 and Rac2, B cell development is arrested at an IgD- transitional B cell stage that we term transitional type 0 (T0). We show that T0 cells cannot enter the white pulp of the spleen until they mature into the T1 and T2 stages, and that this entry into the white pulp requires integrin and chemokine receptor signaling and is required for cell survival. In the absence of Rac1 and Rac2, transitional B cells are unable to migrate in response to chemokines and cannot enter the splenic white pulp. We propose that loss of Rac1 and Rac2 causes arrest at the T0 stage at least in part because transitional B cells need to migrate into the white pulp to receive survival signals. Finally, we show that in the absence of Syk, a kinase that transduces B cell antigen receptor signals required for positive selection, development is arrested at the same T0 stage, with transitional B cells excluded from the white pulp. Thus, these studies identify a novel developmental checkpoint that coincides with B cell positive selection.

Pivotal Advance: Phospholipids determine net membrane surface charge resulting in differential localization of active Rac1 and Rac2

In this investigation, we used primary murine neutrophils to demonstrate that local changes in membrane phospholipid composition alter the net cytoplasmic membrane surface charge, which results in selective recruitment of Rac1 or Rac2 based on the net charge of their respective C-terminal domains. Murine neutrophils undergoing chemotaxis or carrying out phagocytosis were transfected with K-ras4B-derived membrane charge biosensors and lipid markers, which allowed us to simultaneously monitor the levels of PIP(2), PIP(3), and PS and net membrane charge of the newly developing phagosome membrane and plasma membrane. Our results indicate that the combination of PIP(2), PIP(3), and PS generates a high negative charge (-8) at the plasma membrane of actin-rich pseudopods, where active Rac1 preferentially localizes during phagosome formation. The lipid metabolism that occurs during phagosome maturation results in the localized depletion of PIP(2), PIP(3), and partial decrease in PS. This creates a moderately negative net charge that correlates with the localization of active Rac2. Conversely, the accumulation of PIP(3) at the leading-edge membrane during chemotaxis generates a polarized accumulation of negative charges that recruits Rac1. These results provide evidence that alterations in membrane lipid composition and inner-membrane surface charge are important elements for the recruitment of differentially charged proteins and localization of signaling pathways during phagocytosis and chemotaxis in neutrophils.

A congenital activating mutant of WASp causes altered plasma membrane topography and adhesion under flow in lymphocytes

Leukocytes rely on dynamic actin-dependent changes in cell shape to pass through blood vessels, which is fundamental to immune surveillance. Wiskott-Aldrich Syndrome protein (WASp) is a hematopoietic cell-restricted cytoskeletal regulator important for modulating cell shape through Arp2/3-mediated actin polymerization. A recently identified WASp(I294T) mutation was shown to render WASp constitutively active in vivo, causing increased filamentous (F)-actin polymerization, high podosome turnover in macrophages, and myelodysplasia. The aim of this study was to determine the effect of WASp(I294T) expression in lymphocytes. Here, we report that lymphocytes isolated from a patient with WASp(I294T), and in a cellular model of WASp(I294T), displayed abnormal microvillar architecture, associated with an increase in total cellular F-actin. Microvillus function was additionally altered as lymphocytes bearing the WASp(I294T) mutation failed to roll normally on L-selectin ligand under flow. This was not because of defects in L-selectin expression, shedding, cytoskeletal anchorage, or membranal positioning; however, under static conditions of adhesion, WASp(I294T)-expressing lymphocytes exhibited altered dynamic interaction with L-selectin ligand, with a significantly reduced rate of adhesion turnover. Together, our results demonstrate that WASp(I294T) significantly affects lymphocyte membrane topography and L-selectin-dependent adhesion, which may be linked to defective hematopoiesis and leukocyte function in affected patients.

Characterization of P-Rex1 for its role in fMet-Leu-Phe-induced superoxide production in reconstituted COS(phox) cells

P-Rex1 (phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1) is a Rac-specific guanine nucleotide exchange factor activated by Gbetagamma subunits and by PtdIns((3,4,5))P(3). Recent studies indicate that P-Rex1 plays an important role in signaling downstream of neutrophil chemoattractant receptors. Here we report that heterologous expression of P-Rex1, but not Vav1, reconstitutes formyl peptide receptor 1 (FPR1)-mediated NADPH oxidase activation in the transgenic COS(phox) cells expressing gp91(phox), p22(phox), p67(phox) and p47(phox). A successful reconstitution requires the expression of a full-length P-Rex1 with intact DH and PH domains, and is accompanied by P-Rex1 membrane localization as well as Rac1 activation. P-Rex1-dependent superoxide generation in the reconstituted COS(phox) cells was further enhanced by expression of the novel PKC isoform PKCdelta and by overexpression of Akt. Heterologous expression of P-Rex1 in COS(phox) cells potentiated fMet-Leu-Phe-induced Akt phosphorylation, whereas expression of a constitutively active form of Akt enhanced Rac1 activation. In contrast, a dominant negative Akt mutant reduced the fMet-Leu-Phe stimulated superoxide generation as well as Rac1 activation. These results demonstrate that in COS(phox) cells, P-Rex1 is a critical component for FPR1-mediated signaling leading to NADPH oxidase activation, and there is a crosstalk between the P-Rex1-Rac pathway and Akt in superoxide generation.

ROCK1 functions as a suppressor of inflammatory cell migration by regulating PTEN phosphorylation and stability

Rho kinases belong to a family of serine/threonine kinases whose role in recruitment and migration of inflammatory cells is poorly understood. We show that deficiency of ROCK1 results in increased recruitment and migration of macrophages and neutrophils in vitro and in vivo. Enhanced migration resulting from ROCK1 deficiency is observed despite normal expression of ROCK2 and a significant reduction in overall ROCK activity. ROCK1 directly binds PTEN in response to receptor activation and is essential for PTEN phosphorylation and stability. In the absence of ROCK1, PTEN phosphorylation, stability, and its activity are significantly impaired. Consequently, increased activation of downstream targets of PTEN, including PIP3, AKT, GSK-3beta, and cyclin D1, is observed. Our results reveal ROCK1 as a physiologic regulator of PTEN whose function is to repress excessive recruitment of macrophages and neutrophils during acute inflammation.

Regulation of inflammation by Rac2 in immune complex-mediated acute lung injury

Acute lung injury (ALI) is an inflammatory disorder associated with recruitment and activation of neutrophils in lungs. Rac2, a member of the Rho GTPase subfamily, is an essential regulator of neutrophil degranulation, superoxide release, and chemotaxis. Here, we hypothesized that Rac2 is important in mediating lung injury. Using a model of IgG immune complex-mediated ALI, we showed that injury was attenuated in rac2(-/-) mice compared with wild-type (WT) mice undergoing ALI, with significant decreases in alveolar leukocyte numbers, vascular leakage, and the inflammatory mediators, myeloperoxidase (MPO) and matrix metalloproteinases (MMPs). Reduced injury in rac2(-/-) mice was not associated with diminished cytokine and chemokine production, since bronchoalveolar lavage (BAL) levels of IL-17, TNF, CCL3, CXCL1, and CXCL2 were similarly increased in WT and rac2(-/-) mice with ALI compared with sham-treated mice (no ALI). BAL levels of MMP-2 and MMP-9 were significantly decreased in the airways of rac2(-/-) mice with ALI. Immunohistochemical analysis revealed that MMP-2 and MMP-9 expression was evident in alveolar macrophages and interstitial neutrophils in WT ALI. In contrast, MMP-positive cells were less prominent in rac2(-/-) mice with ALI. Chimeric mice showed that Rac2-mediated lung injury was dependent on hematopoietic cells derived from bone marrow. We propose that lung injury in response to immune complex deposition is dependent on Rac2 in alveolar macrophages and neutrophils.

Rho GTPases in hematopoiesis and hemopathies

Rho family GTPases are intracellular signaling proteins regulating multiple pathways involved in cell actomyosin organization, adhesion, and proliferation. Our knowledge of their cellular functions comes mostly from previous biochemical studies that used mutant overexpression approaches in various clonal cell lines. Recent progress in understanding Rho GTPase functions in blood cell development and regulation by gene targeting of individual Rho GTPases in mice has allowed a genetic understanding of their physiologic roles in hematopoietic progenitors and mature lineages. In particular, mouse gene-targeting studies have provided convincing evidence that individual members of the Rho GTPase family are essential regulators of cell type-specific functions and stimuli-specific pathways in regulating hematopoietic stem cell interaction with bone marrow niche, erythropoiesis, and red blood cell actin dynamics, phagocyte migration and killing, and T- and B-cell maturation. In addition, deregulation of Rho GTPase family members has been associated with multiple human hematologic diseases such as neutrophil dysfunction, leukemia, and Fanconi anemia, raising the possibility that Rho GTPases and downstream signaling pathways are of therapeutic value. In this review we discuss recent genetic studies of Rho GTPases in hematopoiesis and several blood lineages and the implications of Rho GTPase signaling in hematologic malignancies, immune pathology. and anemia.

The small Rho GTPase Cdc42 regulates neutrophil polarity via CD11b integrin signaling

Neutrophil migration to sites of infection is the first line of cellular defense. A key event of migration is the maintenance of a polarized morphology, which is characterized by a single leading edge of filamentous actin and a contractile uropod devoid of filamentous actin protrusions. Using a mouse model of high Cdc42 activity, we previously demonstrated the importance of Cdc42 activity in neutrophil migration. However, the specific functions of Cdc42 in this process remain to be understood. Using neutrophils genetically deficient in Cdc42, we show that Cdc42 regulates directed migration by maintaining neutrophil polarity. Although it is known to be activated at the front, Cdc42 suppresses protrusions at the uropod. Interestingly, Cdc42 makes use of the integrin CD11b during this process. Cdc42 determines the redistribution of CD11b at the uropod. In turn, using CD11b-null cells and CD11b crosslinking experiments, we show that CD11b modulates myosin light chain phosphorylation to suppress lateral protrusions. Our results uncover a new mechanism in which Cdc42 regulates the uropod through CD11b signaling to maintain polarity in migrating neutrophils. It also reveals new functions for CD11b in neutrophil polarity.

Specific roles of Rac1 and Rac2 in motile functions of HT1080 fibrosarcoma cells

Rho family proteins are constitutively activated in the highly invasive human fibrosarcoma HT1080 cells. We now investigated the specific roles of Rac1 and Rac2 in regulating morphology, F-actin organization, adhesion, migration, and chemotaxis of HT1080 cells. Downregulation of Rac1 using specific siRNA probes resulted in cell rounding, markedly decreased spreading, adhesion, and chemotaxis of HT1080 cells. 2D migration on laminin-coated surfaces in contrast was not markedly affected. Selective Rac2 depletion did not affect cell morphology, cell adhesion, and 2D migration, but significantly reduced chemotaxis. Downregulation of both Rac1 and Rac2 resulted in an even more marked reduction, but not complete abolishment, of chemotaxis indicating distinct as well as overlapping roles of both proteins in chemotaxis. Rac1 thus is selectively required for HT1080 cell spreading and adhesion whereas Rac1 and Rac2 are both required for efficient chemotaxis.

Rac1 and Rac2 in osteoclastogenesis: a cell immortalization model

Cell lines generated from primary cells with a particular gene deletion are useful for examining the function of the specific deleted genes and provide the opportunity to genetically rescue the lost genes using standard gene transfection techniques. In the present study, bone marrow monocytes from wild-type (WT), Rac1 null, and Rac2 null mice were primed with macrophage colony-stimulating factor and soluble receptor activator of NF-kappaB ligand to generate preosteoclasts. This was followed by transduction of a retrovirus containing simian virus 40 large T-antigen and a neomycin-resistant cassette. Seven to 19 immortalized cell lines from each genotype were established. Among them, WT2, Rac1 null-D9, and Rac2 null-A2 were characterized to verify that osteoclastogenesis and osteoclast functions were identical to the parental primary cells. Results showed that immortalized WT2 cells were able to differentiate into mature, multinucleated, functional, tartrate-resistant acid phosphatase-positive osteoclasts. Immortal Rac1 null cells, as with their primary cell counterparts, displayed a severe defect in osteoclastogenesis and function. Transfection of the Rac1 gene into Rac1 null cells was sufficient to rescue osteoclastogenesis. We believe this method of generating immortalized preosteoclasts will provide a key tool for studying the signaling mechanisms involved in osteoclastogenesis.

Human neutrophils coordinate chemotaxis by differential activation of Rac1 and Rac2

Rac1 and Rac2, members of the small Rho GTPase family, play essential roles in coordinating directional migration and superoxide production during neutrophil responses to chemoattractants. Although earlier studies in Rac1 and Rac2 knockout mice have demonstrated unique roles for each Rac isoform in chemotaxis and NADPH oxidase activation, it is still unclear how human neutrophils use Rac1 and Rac2 to achieve their immunological responses to foreign agent stimulation. In the current study, we used TAT dominant-negative Rac1-T17N and Rac2-T17N fusion proteins to acutely alter the activity of Rac1 and Rac2 individually in human neutrophils. We demonstrate distinct activation kinetics and different roles for Rac1 and Rac2 in response to low vs high concentrations of fMLP. These observations were verified using neutrophils from mice in which Rac1 or Rac2 was genetically absent. Based on these results, we propose a model to explain how human neutrophils kill invading microbes while limiting oxidative damage to the adjacent surrounding healthy tissue through the differential activation of Rac1 and Rac2 in response to different concentrations of chemoattractant.

Modulation of reactive oxygen species by Rac1 or catalase prevents asbestos-induced pulmonary fibrosis

The release of reactive oxygen species (ROS) and cytokines by alveolar macrophages has been demonstrated in asbestos-induced pulmonary fibrosis, but the mechanism linking alveolar macrophages to the pathogenesis is not known. The GTPase Rac1 is a second messenger that plays an important role in host defense. In this study, we demonstrate that Rac1 null mice are protected from asbestos-induced pulmonary fibrosis, as determined by histological and biochemical analysis. We hypothesized that Rac1 induced pulmonary fibrosis via generation of ROS. Asbestos increased TNF-alpha and ROS in a Rac1-dependent manner. TNF-alpha was elevated only 1 day after exposure, whereas ROS generation progressively increased in bronchoalveolar lavage cells obtained from wild-type (WT) mice. To determine whether ROS generation contributed to pulmonary fibrosis, we overexpressed catalase in WT monocytes and observed a decrease in ROS generation in vitro. More importantly, administration of catalase to WT mice attenuated the development of fibrosis in vivo. For the first time, these results demonstrate that Rac1 plays a crucial role in asbestos-induced pulmonary fibrosis. Moreover, it suggests that a simple intervention may be useful to prevent progression of the disease.

Rho GTPases in hematopoietic stem cell functions

PURPOSE OF REVIEW

Rho GTPases are key molecular switches controlling the transduction of external signals to cytoplasmic and nuclear effectors. In the last few years, the development of genetic and pharmacological tools has allowed a more precise definition of the specific roles of Rho GTPases in hematopoietic stem cells (HSCs) and progeny of these cells. Rho GTPases are now known to be crucial in HSCs response to hematopoietic microenvironment cues. This article will review the known HSC functions, which are regulated by Rho GTPases.

RECENT FINDINGS

This review analyzes the latest data on how different Rho GTPases control adhesion, migration, retention, proliferation, survival, senescence and oncogenic transformation of HSCs and relates these new findings to the physiological functions of these cells.

SUMMARY

The development of small molecule inhibitors with ability to interfere Rho GTPase activation by guanine nucleotide exchange factors offers new therapeutic strategies to manipulate the function of HSCs.

International Union of Basic and Clinical Pharmacology. LXXIII. Nomenclature for the formyl peptide receptor (FPR) family

Formyl peptide receptors (FPRs) are a small group of seven-transmembrane domain, G protein-coupled receptors that are expressed mainly by mammalian phagocytic leukocytes and are known to be important in host defense and inflammation. The three human FPRs (FPR1, FPR2/ALX, and FPR3) share significant sequence homology and are encoded by clustered genes. Collectively, these receptors bind an extraordinarily numerous and structurally diverse group of agonistic ligands, including N-formyl and nonformyl peptides of different composition, that chemoattract and activate phagocytes. N-formyl peptides, which are encoded in nature only by bacterial and mitochondrial genes and result from obligatory initiation of bacterial and mitochondrial protein synthesis with N-formylmethionine, is the only ligand class common to all three human receptors. Surprisingly, the endogenous anti-inflammatory peptide annexin 1 and its N-terminal fragments also bind human FPR1 and FPR2/ALX, and the anti-inflammatory eicosanoid lipoxin A4 is an agonist at FPR2/ALX. In comparison, fewer agonists have been identified for FPR3, the third member in this receptor family. Structural and functional studies of the FPRs have produced important information for understanding the general pharmacological principles governing all leukocyte chemoattractant receptors. This article aims to provide an overview of the discovery and pharmacological characterization of FPRs, to introduce an International Union of Basic and Clinical Pharmacology (IUPHAR)-recommended nomenclature, and to discuss unmet challenges, including the mechanisms used by these receptors to bind diverse ligands and mediate different biological functions.

RhoH/TTF negatively regulates leukotriene production in neutrophils

Leukotriene B(4) (LTB(4)) is an important proinflammatory lipid mediator generated by neutrophils upon activation. GM-CSF stimulation is known to enhance agonist-mediated LTB(4) production of neutrophils within minutes, a process called "priming". In this study, we demonstrate that GM-CSF also limits the production of LTB(4) by neutrophils via a transcriptional mechanism at later time points. We identified hemopoietic-specific Ras homologous (RhoH)/translocation three four (TTF), which was induced following GM-CSF stimulation in neutrophils, as a key regulator in this process. Neutrophils derived from RhoH/TTF-deficient (Rhoh(-/-)) mice demonstrated increased LTB(4) production upon activation compared with normal mouse neutrophils. Moreover, neutrophils from cystic fibrosis patients expressed enhanced levels of RhoH/TTF and generated less LTB(4) upon activation compared with normal human neutrophils. Taken together, these data suggest that RhoH/TTF represents an inducible feedback inhibitor in neutrophils that is involved in the limitation of innate immune responses.

Adaptor protein SLAT modulates Fcgamma receptor-mediated phagocytosis in murine macrophages

SLAT (SWAP-70-like adaptor protein of T cells) is an adaptor protein expressed in cells of the hematopoietic system. SLAT interacts with and alters the function of small GTPase Rac1 in fibroblasts. In these nonhematopoietic models, the SLAT-Rac interaction leads to changes in F-actin and causes cytoskeletal reorganization. In T cells, SLAT expression regulates the development of T helper cells through Cdc42- and Rac1-mediated activation of the NF-AT transcription factor. Here we show that SLAT is expressed in macrophages. Overexpression of SLAT in a macrophage cell line inhibits the IgG Fcgamma receptor-mediated phagocytic ability of THP1 cells. In bone marrow-derived macrophages, SLAT protein is recruited to the early phagosomes formed via Fcgamma receptor engagement. SLAT recruitment to the phagosome was most efficient when the macrophages express at least one isoform of Rac (Rac1 or Rac2), because SLAT recruitment was reduced in macrophages of Rac-deficient mice. Macrophages derived from animals lacking SLAT show an elevation in the rate of Fcgamma receptor-mediated phagocytosis. The absence of SLAT is associated with an increase in the amount of F-actin formed around these phagosomes as well as an increase in the amount of Rac1 protein recruited to the phagosome. Our results suggest that SLAT acts as a gatekeeper for the amount of Rac recruited to the phagosomes formed by Fcgamma receptor engagement and thus is able to regulate F-actin re-organization and consequently phagocytosis.

Rac GTPases play critical roles in early T-cell development

The Rac1 and Rac2 GTPases play important roles in many processes including cytoskeletal reorganization, proliferation, and survival, and are required for B-cell development. Previous studies had shown that deficiency in Rac2 did not affect T-cell development, whereas the function of Rac1 in this process has not been investigated. We now show that simultaneous absence of both GTPases resulted in a very strong developmental block at the pre-TCR checkpoint and in defective positive selection. Unexpectedly, deficiency of Rac1 and Rac2 also resulted in the aberrant survival of thymocytes lacking expression of TCR beta, showing hallmarks of hyperactive Notch signaling. Furthermore, we found a similar novel phenotype in the absence of Vav1, Vav2, and Vav3, which function as guanine nucleotide exchange factors for Rac1 and Rac2. These results show that a pathway containing Vav and Rac proteins may negatively regulate Notch signaling during early thymic development.

The small GTPase Rac2 controls phagosomal alkalinization and antigen crosspresentation selectively in CD8(+) dendritic cells

A unique subpopulation of spleen dendritic cells (DCs) that express the CD8 surface marker efficiently present phagocytosed antigens to CD8(+) T lymphocytes in a process called "crosspresentation," which initiates cytotoxic immune responses. We now show that the small GTPase Rac2 plays a critical role in antigen crosspresentation selectively in this DC subpopulation. In CD8(+) DCs, Rac2 determines the subcellular assembly of the NADPH oxidase complex (NOX2) to phagosomes, whereas in CD8(-) DCs, Rac1 mediates the assembly of NOX2 at the plasma membrane. In the absence of Rac2, the production of reactive oxygen species (ROS) in DC-phagosomes was abolished, the phagosomal pH dropped, and the efficiency of antigen crosspresentation was reduced. We conclude that the activity of Rac1 and 2 control crosspresentation in DC subpopulations through the regulation of phagosomal oxidation and pH.

The protective role of ROS in autoimmune disease

For a long time, reactive oxygen species (ROS) produced by the phagocyte NADPH oxidase (NOX2) complex have been considered harmful mediators of inflammation owing to their highly reactive nature. However, there are an increasing number of findings suggesting that ROS produced by the NOX2 complex are anti-inflammatory and prevent autoimmune responses, thus challenging existing dogma. ROS might not only be produced as a mechanism to eradicate invading pathogens, but rather as a means by which to fine-tune the inflammatory response, depending on when, where and at what amounts they are produced. In this review, we aim to describe the current findings highlighting ROS as regulators of autoimmune inflammation, focusing on autoimmune arthritis.

Suppressed hindlimb perfusion in Rac2-/- and Nox2-/- mice does not result from impaired collateral growth

While tissue perfusion and angiogenesis subsequent to acute femoral artery occlusion are suppressed in NADPH oxidase 2 (Nox2)-null (Nox2(-/-)) mice, studies have not established the role of Nox2 in collateral artery enlargement. Rac2 is a small GTPase that binds Nox2 and activates Nox2-based NAD(P)H oxidase but, unlike Nox2, is primarily restricted to bone marrow-derived cells. In this study, we used Rac2-null (Rac2(-/-)) and Nox2(-/-) mice with a novel method of identifying primary hindlimb collaterals to investigate the hypothesis that collateral growth requires these molecules. When initial experiments performed with femoral ligation demonstrated similar perfusion and collateral growth in Rac2(-/-) and wild-type C57BL/6J (BL6) mice, subsequent experiments were performed with a more severe ischemia model, femoral artery excision. After femoral excision, tissue perfusion was suppressed in Rac2(-/-) mice relative to BL6 mice. Histological assessment of ischemic injury including necrotic and regenerated muscle fibers and lipid and collagen deposition demonstrated greater injury in Rac2(-/-) mice. The diameters of primary collaterals identified during Microfil injection with intravital microscopy were enlarged to a similar extent in BL6 and Rac2(-/-) mice. Intimal cells in collateral cross sections were increased in number in both strains and were CD31 positive and CD45 negative. Circulating leukocytes and CD11b(+) cells were increased more in Rac2(-/-) than BL6 animals. Experiments performed in Nox2(-/-) mice to verify that the unexpected results related to collateral growth were not unique to Rac2(-/-) mice gave equivalent results. The data demonstrate that, subsequent to acute femoral artery excision, perfusion recovery is impaired in Rac2(-/-) and Nox2(-/-) mice but that collateral luminal expansion and intimal cell recruitment/proliferation are normal. These novel results indicate that collateral luminal expansion and intimal cell recruitment/proliferation are not mediated by Rac2 and Nox2.

Expression of RAC2 in endothelial cells is required for the postnatal neovascular response

Herein, we describe an obligate role for the hematopoietic specific GTPase, RAC2 in endothelial integrin signaling and the postnatal neovascularization response in vivo. Using a Rac2 knockout mouse model, we discovered that despite the presence of both RAC1 and RAC2 protein in endothelial cells, RAC2 is obligately required for the postnatal neovascular response and alphavbeta3/ alpha4beta1/alpha5beta1 integrin-directed migration on vitronectin, H296 and CH271, fibronectin fragments, respectively. The molecular basis for RAC2 specificity was explored. A genetic analysis of Syk -/+ or Syk-/+;Rac2 -/+ mice revealed that SYK kinase is required for the integrin induced activation of RAC2. The analysis of endothelial cells from Rac2-/+ versus Syk-/+;Rac2-/+ mice provided genetic evidence that SYK-RAC2 signaling axis regulates integrin (alphavbeta3, alpha4beta1 and alpha5beta1) dependent migration. Our results provide evidence that a specific region of the nonreceptor protein tyrosine kinase, SYK, the B linker region containing Y342 and Y346 is required for SYK's regulation of RAC2 and integrin dependent migration. Moreover, the capacity of mice to vascularize the ischemic hindlimb following femoral artery ligation or matrigel plugs was markedly reduced in mice homozygous deficient for the Rac2 gene. These findings identify a novel signaling axis for the induction and potential modulation of postnatal angiogenesis.

A point mutation in the murine Hem1 gene reveals an essential role for Hematopoietic protein 1 in lymphopoiesis and innate immunity

Hem1 (Hematopoietic protein 1) is a hematopoietic cell-specific member of the Hem family of cytoplasmic adaptor proteins. Orthologues of Hem1 in Dictyostelium discoideum, Drosophila melanogaster, and Caenorhabditis elegans are essential for cytoskeletal reorganization, embryonic cell migration, and morphogenesis. However, the in vivo functions of mammalian Hem1 are not known. Using a chemical mutagenesis strategy in mice to identify novel genes involved in immune cell functions, we positionally cloned a nonsense mutation in the Hem1 gene. Hem1 deficiency results in defective F-actin polymerization and actin capping in lymphocytes and neutrophils caused by loss of the Rac-controlled actin-regulatory WAVE protein complex. T cell development is disrupted in Hem1-deficient mice at the CD4(-)CD8(-) (double negative) to CD4(+)CD8(+) (double positive) cell stages, whereas T cell activation and adhesion are impaired. Hem1-deficient neutrophils fail to migrate in response to chemotactic agents and are deficient in their ability to phagocytose bacteria. Remarkably, some Rac-dependent functions, such as Th1 differentiation and nuclear factor kappaB (NF-kappaB)-dependent transcription of proinflammatory cytokines proceed normally in Hem1-deficient mice, whereas the production of Th17 cells are enhanced. These results demonstrate that Hem1 is essential for hematopoietic cell development, function, and homeostasis by controlling a distinct pathway leading to cytoskeletal reorganization, whereas NF-kappaB-dependent transcription proceeds independently of Hem1 and F-actin polymerization.

Activation of the small GTPase Rac1 by a specific guanine-nucleotide-exchange factor suffices to induce glucose uptake into skeletal-muscle cells

BACKGROUND INFORMATION

Insulin-stimulated glucose uptake into skeletal muscle is crucial for glucose homoeostasis, and depends on the recruitment of GLUT4 (glucose transporter 4) to the plasma membrane. Mechanisms underlying insulin-dependent GLUT4 translocation, particularly the role of Rho family GTPases, remain controversial.

RESULTS

In the present study, we show that constitutively active Rac1, but not other Rho family GTPases tested, induced GLUT4 translocation in the absence of insulin, suggesting that Rac1 activation is sufficient for GLUT4 translocation in muscle cells. Rac1 activation occurred in dorsal membrane ruffles of insulin-stimulated cells as revealed by a novel method to visualize activated Rac1 in situ. We further identified FLJ00068 as a GEF (guanine-nucleotide-exchange factor) responsible for this Rac1 activation. Indeed, constitutively active FLJ00068 caused Rac1 activation in dorsal membrane ruffles and GLUT4 translocation without insulin stimulation. Down-regulation of Rac1 or FLJ00068 by RNA interference, on the other hand, abrogated insulin-induced GLUT4 translocation. Basal, but not insulin-stimulated, activity of the serine/threonine kinase Akt was required for the induction of GLUT4 translocation by constitutively active Rac1 or FLJ00068.

CONCLUSION

Collectively, Rac1 activation specifically in membrane ruffles by the GEF FLJ00068 is sufficient for insulin induction of glucose uptake into skeletal-muscle cells.

Regulation of macrophage adhesion and migration by Rho GTP-binding proteins

The Rho family proteins Rac and Rho are believed to be key regulators of cell migration through their effects on the cytoskeleton and cell adhesion. However, recent studies in macrophages indicate that they are not always essential for migration, although they do affect cell shape and adhesion.

PAK1-mediated activation of ERK1/2 regulates lamellipodial dynamics

PAK1 is a member of the p21-activated kinase (PAK) family of serine/threonine kinases that are activated by the Rho GTPases Rac and Cdc42, and are implicated in regulating morphological polarity, cell migration and adhesion. Here we investigate the function of PAK1 in cell motility using macrophages derived from PAK1-null mice. We show that CSF1, a macrophage chemoattractant, transiently stimulates PAK1 and MAPK activation, and that MAPK activation is reduced in PAK1-/- macrophages. PAK1 regulates the dynamics of lamellipodium extension as cells spread in response to adhesion but is not essential for macrophage migration or chemotaxis towards CSF1. Following adhesion, PAK1-/- macrophages spread more rapidly and have more lamellipodia than wild-type cells; however, these lamellipodia were less stable than those in wild-type macrophages. ERK1/2 activity was reduced in PAK1-/- macrophages during adhesion, and inhibition of ERK1/2 activation in wild-type macrophages was sufficient to increase the spread area and mimic the lamellipodial dynamics of PAK1-/- macrophages. Together, these data indicate that PAK1 signals via ERK1/2 to regulate lamellipodial stability.

Parallel phosphatidylinositol 3-kinase (PI3K)-dependent and Src-dependent pathways lead to CXCL8-mediated Rac2 activation and chemotaxis

The requirement for phosphatidylinositol 3-kinase (PI3K) in the establishment of cell polarity and motility in a number of cell types has recently come into question. In this study, we demonstrate that inhibition of PI3K by wortmannin in neutrophil-like differentiated HL60 cells expressing CXCR2 resulted in reduced cell motility but normal chemotaxis in response to a gradient of CXCL8. However, wortmannin inhibition of PI3K did impair the ability of cells to re-orient their polarity and respond quickly to a change in the direction of the CXCL8 gradient. We hypothesized that Src-regulated ELMO-Dock2-Rac2 activation mediates chemotaxis in the absence of PI3K activity. Inhibition of Src with the small molecule inhibitor, PP2, or inhibition of Dock2 by shRNA knockdown confirmed the functional role of Src and Dock2 in regulating chemotaxis when PI3K was inhibited. Moreover, neutrophils isolated from bone marrow of hck(-/-)fgr(-/-)lyn(-/-) mice exhibited much more severe inhibition of chemotaxis when PI3K was blocked with wortmannin as compared with neutrophils isolated from bone marrow of wild-type mice. Thus, PI3K and Src-ELMO-Dock2 pathways work in parallel to activate Rac2 and modulate chemotaxis in response to a CXCL8 gradient in neutrophils.

Mammalian Rho GTPases: new insights into their functions from in vivo studies

Rho GTPases are key regulators of cytoskeletal dynamics and affect many cellular processes, including cell polarity, migration, vesicle trafficking and cytokinesis. These proteins are conserved from plants and yeast to mammals, and function by interacting with and stimulating various downstream targets, including actin nucleators, protein kinases and phospholipases. The roles of Rho GTPases have been extensively studied in different mammalian cell types using mainly dominant negative and constitutively active mutants. The recent availability of knockout mice for several members of the Rho family reveals new information about their roles in signalling to the cytoskeleton and in development.

Rac GTPase isoforms Rac1 and Rac2 play a redundant and crucial role in T-cell development

Rac GTPases have been implicated in the regulation of diverse functions in various blood cell lineages, but their role in T-cell development is not well understood. We have carried out conditional gene targeting to achieve hematopoietic stem cell (HSC)- or T-cell lineage-specific deletion of Rac1 or Rac1/Rac2 by crossbreeding the Mx-Cre or Lck-Cre transgenic mice with Rac1(loxp/loxp) or Rac1(loxp/loxp);Rac2(-/-) mice. We found that (1) HSC deletion of both Rac1 and Rac2 inhibited production of common lymphoid progenitors (CLPs) in bone marrow and suppressed T-cell development in thymus and peripheral organs, whereas deletion of Rac1 moderately affected CLP production and T-cell development. (2) T cell-specific deletion of Rac1 did not affect T-cell development, whereas deletion of both Rac1 and Rac2 reduced immature CD4(+)CD8(+) and mature CD4(+) populations in thymus as well as CD4(+) and CD8(+) populations in spleen. (3) The developmental defects of Rac1/Rac2 knockout T cells were associated with proliferation, survival, adhesion, and migration defects. (4) Rac1/Rac2 deletion suppressed T-cell receptor-mediated proliferation, IL-2 production, and Akt activation in thymocytes. Thus, Rac1 and Rac2 have unique roles in CLP production and share a redundant but essential role in later stages of T-cell development by regulating survival and proliferation signals.

The role of Rac2 in regulating neutrophil production in the bone marrow and circulating neutrophil counts

Circulating neutrophils are persistently higher in mice deficient in the small GTPase Rac2 than in wild-type (WT) mice. Therefore, we examined the mechanisms through which the small GTPase Rac2 regulates neutrophil production and release. Lethally irradiated WT mice reconstituted with a 50:50 mixture of WT and Rac2(-/-) fetal liver cells were protected from neutrophilia, suggesting that neutrophilia is primarily because of extrinsic defects that can be corrected by WT leukocytes. However, the differential counts and numbers of leukocyte subtypes differed between Rac2(-/-) and WT cells, suggesting that Rac2 modulates leukocyte lineage distribution. Kinetic studies suggest Rac2 modulates the release of neutrophils into the circulation and does not prolong their circulating half life. The percentage of bone marrow cells that expressed the neutrophil marker Gr-1 in lethally irradiated WT or Rac2(-/-) recipients of Rac2(-/-) stem cells was greater than in recipients of WT stem cells; however, circulating neutrophil counts were higher only in Rac2(-/-) recipients of Rac2(-/-) stem cells. Rac2 mRNA was expressed in the bone marrow of WT recipients of Rac2(-/-) stem cells and in human mesenchymal stem cells. The data presented here suggest that Rac2 in hematopoietic cells regulates leukocyte lineage distribution and Rac2 in nonhematopoietic cells might contribute to regulating circulating neutrophil counts.

Sequential binding of cytosolic Phox complex to phagosomes through regulated adaptor proteins: evaluation using the novel monomeric Kusabira-Green System and live imaging of phagocytosis

We engineered a method for detecting intramolecular and intermolecular phox protein interactions in cells by fluorescence microscopy using fusion proteins of complementary fragments of a coral fluorescent reporter protein (monomeric Kusabira-Green). We confirmed the efficacy of the monomeric Kusabira-Green system by showing that the PX and PB1 domains of p40phox interact in intact cells, which we suggested maintains this protein in an inactive closed conformation. Using this system, we also explored intramolecular interactions within p47phox and showed that the PX domain interacts with the autoinhibited tandem Src homology 3 domains maintained in contact with the autoinhibitory region, along with residues 341-360. Furthermore, we demonstrated sequential interactions of p67phox with phagosomes involving adaptor proteins, p47phox and p40phox, during FcgammaR-mediated phagocytosis. Although p67phox is not targeted to phagosomes by itself, p47phox functions as an adaptor for the ternary complex (p47phox-p67phox-p40phox) in early stages of phagocytosis before phagosome closure, while p40phox functions in later stages after phagosomal closure. Interestingly, a mutated "open" form of p40phox linked p47phox to closed phagosomes and prolonged p47phox and p67phox retention on phagosomes. These results indicate that binding of the ternary complex to phagosomes can be temporally regulated by switching between adaptor proteins that have PX domains with distinct lipid-binding specificities.

Glutamate is the chemotaxis-inducing factor in placental extracts

Human placental extracts are known to help wound healing. Rapid migration of neutrophils to the wound site is a prerequisite to the wound healing process. Gel filtration analysis of heat-treated placental extract gave the initial cue to the small nature of the migration promoting factor of the extract. HPLC analysis of the extract revealed glutamate to be the predominant free amino acid. Our studies show that glutamate at an optimum concentration of 8 muM induced phenotypic neutrophil chemotaxis, as seen in the time lapse- and transwell assays. Glutamate was also found to induce chemokinesis of the neutrophil, though the stimulation of chemotaxis was more pronounced. The glutamate induced chemotaxis was accompanied by polarization of the actin cytoskeleton, and by polymerization of F-actin. These data indicate that glutamate has a strong chemotactic functionality in the neutrophil, which could be of interest both therapeutically and in further investigation of the molecular basis of chemotaxis.