Requirements for Vav guanine nucleotide exchange factors and Rho GTPases in FcgammaR- and complement-mediated phagocytosis

Interaction of two phagocytic host defense systems: Fcγ receptors and complement receptor 3

Phagocytosis of foreign pathogens by cells of the immune system is a vitally important function of innate immunity. The phagocytic response is initiated when ligands on the surface of invading microorganisms come in contact with receptors on the surface of phagocytic cells such as neutrophils, monocytes/macrophages, and dendritic cells. The complement receptor CR3 (CD11b/CD18, Mac-1) mediates the phagocytosis of complement protein (C3bi)-coated particles. Fcγ receptors (FcγRs) bind IgG-opsonized particles and provide a mechanism for immune clearance and phagocytosis of IgG-coated particles. We have observed that stimulation of FcγRs modulates CR3-mediated phagocytosis and that FcγRIIA and FcγRI exert opposite (stimulatory and inhibitory) effects. We have also determined that an intact FcγR immunoreceptor tyrosine-based activation motif is required for these effects, and we have investigated the involvement of downstream effectors. The ability to up-regulate or down-regulate CR3 signaling has important implications for therapeutics in disorders involving the host defense system.

Bi-modal regulation of a formin by srGAP2

The maintenance of rapid and efficient actin dynamics in vivo requires coordination of filament assembly and disassembly. This regulation requires temporal and spatial integration of signaling pathways by protein complexes. However, it remains unclear how these complexes form and then regulate the actin cytoskeleton. Here, we identify a srGAP2 and formin-like 1 (FMNL1, also known as FRL1 or FRLα) complex whose assembly is regulated by Rac signaling. Our data suggest srGAP2 regulates FMNL1 in two ways; 1) Rac-mediated activation of FMNL1 leads to the recruitment of srGAP2, which contains a Rac-specific GAP domain; 2) the SH3 domain of srGAP2 binds the formin homology 1 domain of FMNL1 to inhibit FMNL1-mediated actin severing. Thus, srGAP2 can efficiently terminate the upstream activating Rac signal while also opposing an important functional output of FMNL1, namely actin severing. We also show that FMNL1 and srGAP2 localize to the actin-rich phagocytic cup of macrophage-derived cells, suggesting the complex may regulate this Rac- and actin-driven process in vivo. We propose that after Rac-dependent activation of FMNL1, srGAP2 mediates a potent mechanism to limit the duration of Rac action and inhibit formin activity during rapid actin dynamics.

Guanine nucleotide exchange factors for RhoGTPases: good therapeutic targets for cancer therapy?

Rho guanosine triphosphatases (GTPases) are a family of small proteins which function as molecular switches in a variety of signaling pathways following stimulation of cell surface receptors. RhoGTPases regulate numerous cellular processes including cytoskeleton organization, gene transcription, cell proliferation, migration, growth and cell survival. Because of their central role in regulating processes that are dysregulated in cancer, it seems reasonable that defects in the RhoGTPase pathway may be involved in the development of cancer. RhoGTPase activity is regulated by a number of protein families: guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) and guanine nucleotide-dissociation inhibitors (GDIs). This review discusses the participation of RhoGTPases and their regulators, especially GEFs in human cancers. In particular, we focus on the involvement of the RhoGTPase GEF, Vav1, a hematopoietic specific signal transducer which is involved in human neuroblastoma, pancreatic ductal carcinoma and lung cancer. Finally, we summarize recent advances in the design and application of a number of molecules that specifically target individual RhoGTPases or their regulators or effectors, and discuss their potential for cancer therapy.

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.

Vav3 is involved in GABAergic axon guidance events important for the proper function of brainstem neurons controlling cardiovascular, respiratory, and renal parameters

Vav3 is a phosphorylation-dependent activator of Rho/Rac GTPases that has been implicated in hematopoietic, bone, cerebellar, and cardiovascular roles. Consistent with the latter function, Vav3-deficient mice develop hypertension, tachycardia, and renocardiovascular dysfunctions. The cause of those defects remains unknown as yet. Here, we show that Vav3 is expressed in GABAegic neurons of the ventrolateral medulla (VLM), a brainstem area that modulates respiratory rates and, via sympathetic efferents, a large number of physiological circuits controlling blood pressure. On Vav3 loss, GABAergic cells of the caudal VLM cannot innervate properly their postsynaptic targets in the rostral VLM, leading to reduced GABAergic transmission between these two areas. This results in an abnormal regulation of catecholamine blood levels and in improper control of blood pressure and respiration rates to GABAergic signals. By contrast, the reaction of the rostral VLM to excitatory signals is not impaired. Consistent with those observations, we also demonstrate that Vav3 plays important roles in axon branching and growth cone morphology in primary GABAergic cells. Our study discloses an essential and nonredundant role for this Vav family member in axon guidance events in brainstem neurons that control blood pressure and respiratory rates.

Defined particle ligands trigger specific defense mechanisms of macrophages

Phagocytosis is a receptor-mediated process for sequestration and inactivation of infectious microbes. It can be triggered by microbial surface compounds or particle-attached host proteins. We monitored the effector functions of murine bone marrow-derived macrophages (BMMs) in response to polystyrene-streptavidin beads coated with the defined ligands IgG1, β-glucan, mannan, complement factors C1q or iC3b, or fibronectin (FN). Cell-autonomous effector mechanisms (uptake, phagosome maturation, cytokine responses and killing activity) were differentially triggered. All particle-ligand complexes stimulated the release of nitric oxide, but only beads coated with IgG, complement factors or FN caused production of superoxide. Beads coated with C1q, iC3b or FN strongly stimulated the secretion of pro-inflammatory TNF-α, IL-6, and IL-1β and also of anti-inflammatory IL-10. Escherichia coli coated with C1q, iC3b or FN was killed much less efficiently than with any of the other ligands, depending on the presence of IL-10 activity. This indicated an important role of IL-10 as regulator of cell-autonomous immune functions of macrophages. Our data show that defined ligands on microbial surfaces are interesting candidates to activate innate defense mechanisms selectively and specifically.

Podosome regulation by Rho GTPases in myeloid cells

Myeloid cells form a first line of defense against infections. They migrate from the circulation to the infected tissues by adhering to and subsequently crossing the vascular wall. This process requires precise control and proper regulation of these interactions with the environment is therefore crucial. Podosomes are the most prominent adhesion structures in myeloid cells. Podosomes control both the adhesive and migratory properties of myeloid cells and the regulation of podosomes is key to the proper functioning of these cells. Here we discuss the regulation of podosomes by Rho GTPases, well known regulators of adhesion and migration, focusing on myeloid cells. In addition, the regulation of podosomes by GTPase regulators such as GEFs and GAPs, as well as the effects of some Rho GTPase effector pathways, will be discussed.

Phagocytosis of retinal rod and cone photoreceptors

Photoreceptor cells maintain a roughly constant length by continuously generating new outer segments from their base while simultaneously releasing mature outer segments engulfed by the retinal pigment epithelium (RPE). Thus postmitotic RPE cells phagocytose an immense amount of material over a lifetime, disposing of photoreceptor cell waste while retaining useful content. This review focuses on current knowledge of outer segment phagocytosis, discussing the steps involved along with their critical participants as well as how various perturbations in outer segment (OS) disposal can lead to retinopathies.

Structural and energetic mechanisms of cooperative autoinhibition and activation of Vav1

Vav proteins are guanine nucleotide exchange factors (GEFs) for Rho family GTPases. They control processes including T cell activation, phagocytosis, and migration of normal and transformed cells. We report the structure and biophysical and cellular analyses of the five-domain autoinhibitory element of Vav1. The catalytic Dbl homology (DH) domain of Vav1 is controlled by two energetically coupled processes. The DH active site is directly, but weakly, inhibited by a helix from the adjacent Acidic domain. This core interaction is strengthened 10-fold by contacts of the calponin homology (CH) domain with the Acidic, pleckstrin homology, and DH domains. This construction enables efficient, stepwise relief of autoinhibition: initial phosphorylation events disrupt the modulatory CH contacts, facilitating phosphorylation of the inhibitory helix and consequent GEF activation. Our findings illustrate how the opposing requirements of strong suppression of activity and rapid kinetics of activation can be achieved in multidomain systems.

Cytoskeleton plays a dual role of activation and inhibition in myelin and zymosan phagocytosis by microglia

A major innate immune function of microglia in the central nervous system is receptor-mediated phagocytosis of tissue debris and pathogens. We studied how phagocytosis of degenerated myelin (i.e., tissue debris) and zymosan (i.e., yeast pathogen) is regulated by the cytoskeleton through myosin light chain kinase (MLCK) and the small GTPase Rho and its effector Rho-kinase (ROCK) in primary mouse microglia. Our observations suggest a dual role of activation and inhibition of phagocytosis by MLCK and Rho/ROCK signaling. MLCK activated, whereas Rho/ROCK down-regulated complement receptor-3 (CR3) mediated, phagocytosis of C3bi-opsonized and nonopsonized myelin. These opposing roles of MLCK and Rho/ROCK depended on the preferential spatial localization of their distinctive functions. MLCK further activated, and Rho/ROCK down-regulated, phagocytosis of nonopsonized zymosan by nonopsonic receptors (e.g., Dectin-1). In contrast, MLCK down-regulated, but Rho/ROCK activated, CR3-mediated phagocytosis of C3bi-opsonized zymosan. Thus MLCK and Rho/ROCK can each activate or inhibit phagocytosis but always act in opposition. Whether activation or inhibition occurs depends on the nature of the phagocytosed particle (C3bi-opsonized or nonopsonized myelin or zymosan) and the receptors mediating each phagocytosis.

Vav3-deficient mice exhibit a transient delay in cerebellar development

Vav3 is a guanosine diphosphate/guanosine triphosphate exchange factor for Rho/Rac GTPases that has been involved in functions related to the hematopoietic system, bone formation, cardiovascular regulation, angiogenesis, and axon guidance. We report here that Vav3 is expressed at high levels in Purkinje and granule cells, suggesting additional roles for this protein in the cerebellum. Consistent with this hypothesis, we demonstrate using Vav3-deficient mice that this protein contributes to Purkinje cell dendritogenesis, the survival of granule cells of the internal granular layer, the timely migration of granule cells of the external granular layer, and to the formation of the cerebellar intercrural fissure. With the exception of the latter defect, the dysfunctions found in Vav3(-/-) mice only occur at well-defined postnatal developmental stages and disappear, or become ameliorated, in older animals. Vav2-deficient mice do not show any of those defects. Using primary neuronal cultures, we show that Vav3 is important for dendrite branching, but not for primary dendritogenesis, in Purkinje and granule cells. Vav3 function in the cerebellum is functionally relevant, because Vav3(-/-) mice show marked motor coordination and gaiting deficiencies in the postnatal period. These results indicate that Vav3 function contributes to the timely developmental progression of the cerebellum.

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.

Vav GEFs regulate macrophage morphology and adhesion-induced Rac and Rho activation

The Vav family of proteins have the potential to act as both signalling adapters and GEFs for Rho GTPases. They have therefore been proposed as regulators of the cytoskeleton in various cell types. We have used macrophages from mice deficient in all three Vav isoforms to determine how their function affects cell morphology and migration. Macrophages lacking Vav proteins adopt an elongated morphology and have enhanced migratory persistence in culture. To investigate the pathways through which Vav proteins exert their effects we analysed the responses of macrophages to the chemoattractant CSF-1 and to adhesion. We found that morphological and signalling responses of macrophages to CSF-1 did not require Vav proteins. In contrast, adhesion-induced cell spreading, RhoA and Rac1 activation and cell signalling were all dependent on Vav proteins. We propose that Vav proteins affect macrophage morphology and motile behaviour by coupling adhesion receptors to Rac1 and RhoA activity and regulating adhesion signalling events such as paxillin and ERK1/2 phosphorylation by acting as adapters.

SLP-76-ADAP adaptor module regulates LFA-1 mediated costimulation and T cell motility

Although adaptor ADAP (FYB) and its binding to SLP-76 has been implicated in TcR-induced "inside-out" signaling for LFA-1 activation in T cells, little is known regarding its role in LFA-1-mediated "outside-in" signaling. In this study, we demonstrate that ADAP and SLP-76-ADAP binding are coupled to LFA-1 costimulation of IL-2 production, F-actin clustering, cell polarization, and T cell motility. LFA-1 enhancement of anti-CD3-induced IL-2 production was completely dependent on SLP-76-ADAP binding. Further, anti-CD3 was found to require CD11a ligation by antibody or ICAM1 to cause T cell polarization. ADAP augmented this polarization induced by anti-CD3/CD11a, but not by anti-CD3 alone. ADAP expression with LFA-1 ligation alone was sufficient to polarize T cells directly and to increase T cell motility whereas the loss of ADAP in ADAP-/- primary T cells reduced motility. A mutant lacking SLP-76-binding sites (M12) blocked LFA-1 costimulation of IL-2 production, polarization, and motility. LFA-1-ADAP polarization was also dependent on src kinases, Rho GTPases, phospholipase C, and phosphoinositol 3-kinase. Our findings provide evidence of an obligatory role for the SLP-76-ADAP module in LFA-1-mediated costimulation in T cells.

Heterogeneity in macrophage phagocytosis of Staphylococcus aureus strains: high-throughput scanning cytometry-based analysis

Alveolar macrophages (AMs) can phagocytose unopsonized pathogens such as S. aureus via innate immune receptors, such as scavenger receptors (SRs). Cytoskeletal events and signaling pathways involved in phagocytosis of unopsonized bacteria likely govern the fate of ingested pathogens, but are poorly characterized. We have developed a high-throughput scanning cytometry-based assay to quantify phagocytosis of S. aureus by adherent human blood-derived AM-like macrophages in a 96-well microplate format. Differential fluorescent labeling of internalized vs. bound bacteria or beads allowed automated image analysis of collapsed confocal stack images acquired by scanning cytometry, and quantification of total particles bound and percent of particles internalized. We compared the effects of the classic SR blocker polyinosinic acid, the cytoskeletal inhibitors cytochalasin D and nocodazole, and the signaling inhibitors staurosporine, Gö 6976, JNK Inhibitor I and KN-93, on phagocytosis of a panel of live unopsonized S. aureus strains, (Wood, Seattle 1945 (ATCC 25923), and RN6390), as well as a commercial killed Wood strain, heat-killed Wood strain and latex beads. Our results revealed failure of the SR inhibitor polyinosinic acid to block binding of any live S. aureus strains, suggesting that SR-mediated uptake of a commercial killed fluorescent bacterial particle does not accurately model interaction with viable bacteria. We also observed heterogeneity in the effects of cytoskeletal and signaling inhibitors on internalization of different S. aureus strains. The data suggest that uptake of unopsonized live S. aureus by human macrophages is not mediated by SRs, and that the cellular mechanical and signaling processes that mediate S. aureus phagocytosis vary. The findings also demonstrate the potential utility of high-throughput scanning cytometry techniques to study phagocytosis of S. aureus and other organisms in greater detail.

Phagocytosis in macrophages lacking Cbl reveals an unsuspected role for Fc gamma receptor signaling and actin assembly in target binding

Fc gamma receptor (Fc gammaR)-mediated phagocytosis is known to require tyrosine kinases (TKs). We identified c-Cbl and Cbl-b as proteins that undergo tyrosine phosphorylation during phagocytosis. Cbl-deficient macrophages displayed enhanced Fc gammaR-mediated signaling and phagocytosis. Surprisingly, binding of IgG-coated targets (EIgG) was also enhanced. c-Cbl-deficient macrophages expressed less Fc gammaRIIb, the inhibitory Fc gamma receptor; however, this did not account for enhanced target binding. We isolated the function of one Fc receptor isoform, Fc gammaRI, using IgG2a-coated targets (EIgG2a). Cbl-deficient macrophages demonstrated a disproportionate increase in binding EIgG2a, suggesting that signal strength regulates binding efficiency toward opsonized targets. In resting cells, Fc gammaRI colocalized with the Src family TK Hck in F-actin-rich structures, which was enhanced in Cbl-deficient macrophages. Target binding was sensitive to TK inhibitors, profoundly inhibited following depletion of cholesterol, and ablated at 4 degrees C or in the presence of inhibitors of actin polymerization. Sensitivity of EIgG binding to cytoskeletal disruption was inversely proportional to opsonin density. These findings challenge the view that Fc gammaR-mediated binding is a passive event. They suggest that dynamic engagement of TKs and the cytoskeleton enables macrophages to serve as cellular "Venus fly traps", with the capacity to capture phagocytic targets under conditions of limiting opsonin density.

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.

Antimicrobial mechanisms of phagocytes and bacterial evasion strategies

Professional phagocytes have a vast and sophisticated arsenal of microbicidal features. They are capable of ingesting and destroying invading organisms, and can present microbial antigens on their surface, eliciting acquired immune responses. To survive this hostile response, certain bacterial species have developed evasive strategies that often involve the secretion of effectors to co-opt the cellular machinery of the host. In this Review, we present an overview of the antimicrobial defences of the host cell, with emphasis on macrophages, for which phagocytosis has been studied most extensively. In addition, using Mycobacterium tuberculosis, Listeria monocytogenes, Legionella pneumophila and Coxiella burnetii as examples, we describe some of the evasive strategies used by bacteria.

Essential and unique roles of PIP5K-gamma and -alpha in Fcgamma receptor-mediated phagocytosis

The actin cytoskeleton is dynamically remodeled during Fcγ receptor (FcγR)-mediated phagocytosis in a phosphatidylinositol (4,5)-bisphosphate (PIP₂)-dependent manner. We investigated the role of type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K) γ and α isoforms, which synthesize PIP₂, during phagocytosis. PIP5K-γ−/− bone marrow–derived macrophages (BMM) have a highly polymerized actin cytoskeleton and are defective in attachment to IgG-opsonized particles and FcγR clustering. Delivery of exogenous PIP₂ rescued these defects. PIP5K-γ knockout BMM also have more RhoA and less Rac1 activation, and pharmacological manipulations establish that they contribute to the abnormal phenotype. Likewise, depletion of PIP5K-γ by RNA interference inhibits particle attachment. In contrast, PIP5K-α knockout or silencing has no effect on attachment but inhibits ingestion by decreasing Wiskott-Aldrich syndrome protein activation, and hence actin polymerization, in the nascent phagocytic cup. In addition, PIP5K-γ but not PIP5K-α is transiently activated by spleen tyrosine kinase–mediated phosphorylation. We propose that PIP5K-γ acts upstream of Rac/Rho and that the differential regulation of PIP5K-γ and -α allows them to work in tandem to modulate the actin cytoskeleton during the attachment and ingestion phases of phagocytosis.

Wound healing defect of Vav3-/- mice due to impaired {beta}2-integrin-dependent macrophage phagocytosis of apoptotic neutrophils

Vav proteins are guanine-nucleotide exchange factors implicated in leukocyte functions by relaying signals from immune response receptors and integrins to Rho-GTPases. We here provide first evidence for a role of Vav3 for beta(2)-integrins-mediated macrophage functions during wound healing. Vav3(-/-) and Vav1(-/-)/Vav3(-/-) mice revealed significantly delayed healing of full-thickness excisional wounds. Furthermore, Vav3(-/-) bone marrow chimeras showed an identical healing defect, suggesting that Vav3 deficiency in leukocytes, but not in other cells, is causal for the impaired wound healing. Vav3 was required for the phagocytotic cup formation preceding macrophage phagocytosis of apoptotic neutrophils. Immunoprecipitation and confocal microscopy revealed Vav3 activation and colocalization with beta(2)-integrins at the macrophage membrane upon adhesion to ICAM-1. Moreover, local injection of Vav3(-/-) or beta(2)-integrin(CD18)(-/-) macrophages into wound margins failed to restore the healing defect of Vav3(-/-) mice, suggesting Vav3 to control the beta(2)-integrin-dependent formation of a functional phagocytic synapse. Impaired phagocytosis of apoptotic neutrophils by Vav3(-/-) macrophages was causal for their reduced release of active transforming growth factor (TGF)-beta(1), for decreased myofibroblasts differentiation and myofibroblast-driven wound contraction. TGF-beta(1) deficiency in Vav3(-/-) macrophages was causally responsible for the healing defect, as local injection of either Vav3-competent macrophages or recombinant TGF-beta(1) into wounds of Vav3(-/-) mice fully rescued the delayed wound healing.

Overexpression of Tiam1 in hepatocellular carcinomas predicts poor prognosis of HCC patients

Little research has been done to test the usefulness of T-lymphoma invasion and metastasis 1 (Tiam1) as a prognostic marker for hepatocellular carcinoma (HCC). In this study, we investigated Tiam1 expression and its prognostic value for HCC. HCC surgical tissue samples were taken from 152 HCC patients who had been followed up for 5 years. Overexpression of Tiam1 (Tiam1 2+ to 3+) was shown in 63.8% of the cases. The Tiam1 expression level did not correlate with any clinicopathological parameters. However, survival analysis showed that the Tiam1 overexpression group had a significantly shorter overall survival time than the Tiam1 downexpression group (p=0.008). Multivariate analysis showed that Tiam1 expression was a significant and independent prognostic parameter (p=0.042) for HCC patients. Tiam1 expression may be a novel and independent predictor for the prognosis of HCC patients.

Rap1 activation in collagen phagocytosis is dependent on nonmuscle myosin II-A

Rap1 enhances integrin-mediated adhesion but the link between Rap1 activation and integrin function in collagen phagocytosis is not defined. Mass spectrometry of Rap1 immunoprecipitates showed that the association of Rap1 with nonmuscle myosin heavy-chain II-A (NMHC II-A) was enhanced by cell attachment to collagen beads. Rap1 colocalized with NM II-A at collagen bead-binding sites. There was a transient increase in myosin light-chain phosphorylation after collagen-bead binding that was dependent on myosin light-chain kinase but not Rho kinase. Inhibition of myosin light-chain phosphorylation, but not myosin II-A motor activity inhibited collagen-bead binding and Rap activation. In vitro binding assays demonstrated binding of Rap1A to filamentous myosin rods, and in situ staining of permeabilized cells showed that NM II-A filaments colocalized with F-actin at collagen bead sites. Knockdown of NM II-A did not affect talin, actin, or beta1-integrin targeting to collagen beads but targeting of Rap1 and vinculin to collagen was inhibited. Conversely, knockdown of Rap1 did not affect localization of NM II-A to beads. We conclude that MLC phosphorylation in response to initial collagen-bead binding promotes NM II-A filament assembly; binding of Rap1 to myosin filaments enables Rap1-dependent integrin activation and enhanced collagen phagocytosis.

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.

Integrin-dependent phagocytosis: spreading from microadhesion to new concepts

By linking actin dynamics to extracellular components, integrins are involved in a wide range of cellular processes that are associated with or require cytoskeletal remodelling and cell-shape changes. One such function is integrin-dependent phagocytosis, a process that several integrins are capable of mediating and that allows the binding and clearance of particles. Integrin-dependent phagocytosis is involved in a wide range of physiological processes, from the clearance of microorganisms and apoptotic-cell removal to extracellular-matrix remodelling. Integrin signalling is also exploited by microbial pathogens for entry into host cells. Far from being a particular property of specific integrins and specialised cells, integrin-dependent uptake is emerging as a general, intrinsic ability of most integrins that is associated with their capacity to signal to the actin cytoskeleton. Integrin-mediated phagocytosis can therefore be used as a robust model in which to study integrin regulation and signalling.

Membrane ruffles capture C3bi-opsonized particles in activated macrophages

A widespread belief in phagocyte biology is that FcgammaR-mediated phagocytosis utilizes membrane pseudopods, whereas Mac-1-mediated phagocytosis does not involve elaborate plasma membrane extensions. Here we report that dynamic membrane ruffles in activated macrophages promote binding of C3bi-opsonized particles. We identify these ruffles as components of the macropinocytosis machinery in both PMA- and LPS-stimulated macrophages. C3bi-particle capture is facilitated by enrichment of high-affinity Mac-1 and the integrin-regulating protein talin in membrane ruffles. Membrane ruffle formation and C3bi-particle binding are cytoskeleton dependent events, having a strong requirement for F-actin and microtubules (MTs). MT disruption blunts ruffle formation and PMA- and LPS-induced up-regulation of surface Mac-1 expression. Furthermore, the MT motor, kinesin participates in ruffle formation implicating a requirement for intracellular membrane delivery to active membrane regions during Mac-1-mediated phagocytosis. We observed colocalization of Rab11-positive vesicles with CLIP-170, a MT plus-end binding protein, at sites of particle adherence using TIRF imaging. Rab11 has been implicated in recycling endosome dynamics and mutant Rab11 expression inhibits both membrane ruffle formation and C3bi-sRBC adherence to macrophages. Collectively these findings represent a novel membrane ruffle "capture" mechanism for C3bi-particle binding during Mac-1-mediated phagocytosis. Importantly, this work also demonstrates a strong functional link between integrin activation, macropinocytosis and phagocytosis in macrophages.

Signaling pathways required for macrophage scavenger receptor-mediated phagocytosis: analysis by scanning cytometry

BACKGROUND

Scavenger receptors are important components of the innate immune system in the lung, allowing alveolar macrophages to bind and phagocytose numerous unopsonized targets. Mice with genetic deletions of scavenger receptors, such as SR-A and MARCO, are susceptible to infection or inflammation from inhaled pathogens or dusts. However, the signaling pathways required for scavenger receptor-mediated phagocytosis of unopsonized particles have not been characterized.

METHODS

We developed a scanning cytometry-based high-throughput assay of macrophage phagocytosis that quantitates bound and internalized unopsonized latex beads. This assay allowed the testing of a panel of signaling inhibitors which have previously been shown to target opsonin-dependent phagocytosis for their effect on unopsonized bead uptake by human in vitro-derived alveolar macrophage-like cells. The non-selective scavenger receptor inhibitor poly(I) and the actin destabilizer cytochalasin D were used to validate the assay and caused near complete abrogation of bead binding and internalization, respectively.

RESULTS

Microtubule destabilization using nocodazole dramatically inhibited bead internalization. Internalization was also significantly reduced by inhibitors of tyrosine kinases (genistein and herbimycin A), protein kinase C (staurosporine, chelerythrine chloride and Gö 6976), phosphoinositide-3 kinase (LY294002 and wortmannin), and the JNK and ERK pathways. In contrast, inhibition of phospholipase C by U-73122 had no effect.

CONCLUSION

These data indicate the utility of scanning cytometry for the analysis of phagocytosis and that phagocytosis of unopsonized particles has both shared and distinct features when compared to opsonin-mediated phagocytosis.

Shaping cups into phagosomes and macropinosomes

The ingestion of particles or cells by phagocytosis and of fluids by macropinocytosis requires the formation of large endocytic vacuolar compartments inside cells by the organized movements of membranes and the actin cytoskeleton. Fc-receptor-mediated phagocytosis is guided by the zipper-like progression of local, receptor-initiated responses that conform to particle geometry. By contrast, macropinosomes and some phagosomes form with little or no guidance from receptors. The common organizing structure is a cup-shaped invagination of the plasma membrane that becomes the phagosome or macropinosome. Recent studies, focusing on the physical properties of forming cups, indicate that a feedback mechanism regulates the signal transduction of phagocytosis and macropinocytosis.

Collagen phagocytosis is regulated by the guanine nucleotide exchange factor Vav2

Collagen phagocytosis is a crucial alpha2beta1-integrin-dependent process that mediates extracellular matrix remodeling by fibroblasts. We showed previously that after initial contact with collagen, activated Rac1 accelerates collagen phagocytosis but the Rac guanine nucleotide exchange factors (GEFs) that regulate Rac are not defined. We examined here the GEFs that regulate collagen phagocytosis in mouse fibroblasts. Collagen binding enhanced Rac1 activity (5-20 min) but not Cdc42 or RhoA activity. Analysis of collagen bead-associated proteins showed enrichment with Vav2, which correlated temporally with increased Rac1 activity. Knockdown of Vav2 prevented Rac activation, recruitment of Rac1 to collagen bead binding sites, and collagen bead binding, but knockdown of Sos-1 or beta-Pix had no effect on Rac activation or collagen binding. Vav2 was associated with the nucleotide-free Rac1 mutant (G15ARac1) after collagen binding. Collagen bead binding promoted phosphorylation of Vav2, which temporally correlated with Rac1 activation and which required Src kinase activity. Blockage of Src activity prevented collagen bead-induced Rac activation and collagen bead binding. Collectively these data indicate that Vav2 regulates the Rac1 activity associated with the binding step of collagen phagocytosis.

Integrin beta 1 regulates phagosome maturation in macrophages through Rac expression

Phagocytosis and subsequent phagosome maturation by professional phagocytes are essential in the clearance of infectious microbial pathogens. The molecular regulation of phagosome maturation is largely unknown. We show that integrin beta(1) plays critical roles in the phagocytosis of microbial pathogens and phagosome maturation. Macrophages lacking integrin beta(1) expression exhibit reduced phagocytosis of bacteria, including group B streptococcus and Staphylococcus aureus. Furthermore, phagosomes from macrophages lacking integrin beta(1) show lowered maturation rate, defective acquisition of lysosome membrane markers, and reduced F-actin accumulation in the periphagosomal region. Integrin beta(1)-deficient macrophages exhibit impaired bactericidal activity. We found that the expression of the Rho family GTPases Rac1, Rac2, and Cdc42 was reduced in integrin beta(1)-deficient macrophages. Ectopic expression of Rac1, but not Cdc42, in integrin beta(1)-deficient macrophages restored defective phagosome maturation and F-actin accumulation in the periphagosomal region. Importantly, macrophages lacking Rac1/2 also exhibit defective maturation of phagosomes derived from opsonized Escherichia coli or IgG beads. Taken together, these results suggest that integrin beta(1) regulates phagosome maturation in macrophages through Rac expression.

Rho GTPases in alveolar macrophage phagocytosis

Alveolar macrophages are "professional phagocytes" and critical effector cells that protect the lungs from a broad array of microbes that can cause severe respiratory tract infections such as pneumonia. The molecular mechanisms that mediate microbial phagocytosis in alveolar macrophages are not fully known, and the specific role of small Rho GTPases has not been established. Most studies of Rho GTPase and phagocytosis focus on cell lines and transfected cells, and results may not accurately represent mechanisms operant in the lungs of humans. The use of clinically relevant primary human lung macrophages to examine phagocytosis in the context of host defense function may provide data that translate more readily to human conditions in health and disease. This chapter provides a description of methods and techniques for isolating, culturing, and assaying human alveolar macrophages for studies of small Rho GTPases and phagocytosis in the context of host defense. Data support the concept that different macrophage phagocytic receptors may exhibit distinct molecular mechanisms of small Rho GTPase activation that mediate phagocytosis.

Stimulus-dependent regulation of the phagocyte NADPH oxidase by a VAV1, Rac1, and PAK1 signaling axis

The p21-activated kinase-1 (PAK1) is best known for its role in the regulation of cytoskeletal and transcriptional signaling pathways. We show here in the microglia cell line Ra2 that PAK1 regulates NADPH oxidase (NOX-2) activity in a stimulus-specific manner. Thus, conditional expression of PAK1 dominant-positive mutants enhanced, whereas dominant-negative mutants inhibited, NADPH oxidase-mediated superoxide generation following formyl-methionyl-leucylphenylalanine or phorbol 12-myristate 13-acetate stimulation. Both Rac1 and the GTP exchange factor VAV1 were required as upstream signaling proteins in the formyl-methionyl-leucyl-phenylalanine-induced activation of endogenous PAK1. In contrast, PAK1 mutants had no effect on superoxide generation downstream of FcgammaR signaling during phagocytosis of IgG-immune complexes. We further present evidence that the effect of PAK1 on the respiratory burst is mediated through phosphorylation of p47(Phox), and we show that expression of a p47(Phox) (S303D/S304D/S320D) mutant, which mimics phosphorylation by PAK1, induced basal superoxide generation in vivo. In contrast PAK1 substrates LIMK-1 or RhoGDI are not likely to contribute to the PAK1 effect on NADPH oxidase activation. Collectively, our findings define a VAV1-Rac1-PAK1 signaling axis in mononuclear phagocytes regulating superoxide production in a stimulus-dependent manner.

Regulation of FcγR-stimulated phagocytosis by the 72-kDa inositol polyphosphate 5-phosphatase: SHIP1, but not the 72-kDa 5-phosphatase, regulates complement receptor 3–mediated phagocytosis by differential recruitment of these 5-phosphatases to the phagocytic cup

Macrophages phagocytose particles to resolve infections and remove apoptotic cells. Phosphoinositide 3-kinase generates phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3] is restricted to the phagocytic cup, promoting phagocytosis. The PtdIns(3,4,5)P3 5-phosphatase (5-ptase) Src homology 2 (SH2) domain-containing inositol-5-phosphatase 1 (SHIP1) inhibits phagocytosis. We report here that another PtdIns(3,4,5)P3-5-ptase, the 72-kDa-5-phosphatase (72-5ptase), inhibits Fcγ receptor (FcγR)– but not complement receptor 3 (CR3)–mediated phagocytosis, affecting pseudopod extension and phagosome closure. In contrast, SHIP1 inhibited FcγR and CR3 phagocytosis with greater effects on CR3-stimulated phagocytosis. The 72-5ptase and SHIP1 were both dynamically recruited to FcγR-stimulated phagocytic cups, but only SHIP1 was recruited to CR3-stimulated phagocytic cups. To determine whether 5-ptases focally degrade PtdIns(3,4,5)P3 at the phagocytic cup after specific stimuli, time-lapse imaging of specific biosensors was performed. Transfection of dominant-negative 72-5ptase or 72-5ptase small interfering RNA (siRNA) resulted in amplified and prolonged PtdIns(3,4,5)P3 at the phagocytic cup in response to FcγR- but not CR3-stimulation. In contrast, macrophages from Ship1−/−/AktPH-GFP transgenic mice exhibited increased and sustained PtdIns(3,4,5)P3 at the cup in response to CR3 activation, with minimal changes to FcγR activation. Therefore, 72-5ptase and SHIP1 exhibit specificity in regulating FcγR- versus CR3-stimulated phagocytosis by controlling the amplitude and duration of PtdIns(3,4,5)P3 at the phagocytic cup.

An ITAM-signaling pathway controls cross-presentation of particulate but not soluble antigens in dendritic cells

Dendritic cells (DC) possess a unique capacity for presenting exogenous antigen on major histocompatibility class I, a process that is referred to as cross-presentation, which serves a critical role in microbial and tumor immunity. During cross-presentation, antigens derived from pathogen-infected or tumor cells are internalized and processed by DCs for presentation to cytotoxic T lymphocytes (CTLs). We demonstrate that a signaling pathway initiated by the immunoreceptor tyrosine–based activation motif (ITAM)–containing adaptors DAP12 and FcRγ utilizes the Vav family of Rho guanine nucleotide exchange factors (GEFs) for processing and cross-presentation of particulate, but not soluble, antigens by DCs. Notably, this novel pathway is crucial for processing and presentation of particulate antigens, such as those associated with Listeria monocytogenes bacteria, yet it is not required for antigen uptake. Mechanistically, we provide evidence that in DCs, Vav GEFs are essential to link ITAM-dependent receptors with the activation of the NOX2 complex and production of reactive oxygen species (ROS), which regulate phagosomal pH and processing of particulate antigens for cross-presentation. Importantly, we show that genetic disruption of the DAP12/FcRγ–Vav pathway leads to antigen presentation defects that are more profound than in DCs lacking NOX2, suggesting that ITAM signaling also controls cross-presentation in a ROS-independent manner.

Immunoreceptor-like signaling by beta 2 and beta 3 integrins

Although adhesion to extracellular structures is one of the most fundamental cell biological processes, the intracellular signals triggered by integrins, the most important receptors involved, are incompletely understood. Several recent reports indicate that signaling by beta(2) and beta(3) integrins in various cell types (neutrophils, macrophages, osteoclasts and platelets) use components of the signal transduction machinery of lymphocyte antigen receptors. Central to this immunoreceptor-like signaling is the phosphorylation of immunoreceptor tyrosine-based activation motif (ITAM)-containing adapters (such as DAP12 and the Fc receptor gamma-chain) by Src-family kinases and the concomitant recruitment of the Syk tyrosine kinase through its dual SH2 domains. These and other reports reveal an unexpected similarity between the signal-transduction mechanisms used by integrins and immune recognition receptors.

Small rho GTPases mediate tumor-induced inhibition of endocytic activity of dendritic cells

The generation, maturation, and function of dendritic cells (DC) have been shown to be markedly compromised in the tumor microenvironment in animals and humans. However, the molecular mechanisms and intracellular pathways involved in the regulation of the DC system in cancer are not yet fully understood. Recently, we have reported on the role of the small Rho GTPase family members Cdc42, Rac1, and RhoA in regulating DC adherence, motility, and Ag presentation. To investigate involvement of small Rho GTPases in dysregulation of DC function by tumors, we next evaluated how Cdc42, Rac1, and RhoA regulated endocytic activity of DC in the tumor microenvironment. We revealed a decreased uptake of dextran 40 and polystyrene beads by DC generated in the presence of different tumor cell lines, including RM1 prostate, MC38 colon, 3LL lung, and B7E3 oral squamous cell carcinomas in vitro and by DC prepared from tumor-bearing mice ex vivo. Impaired endocytic activity of DC cocultured with tumor cells was associated with decreased levels of active Cdc42 and Rac1. Transduction of DC with the dominant negative Cdc42 and Rac1 genes also led to reduced phagocytosis and receptor-mediated endocytosis. Furthermore, transduction of DC with the constitutively active Cdc42 and Rac1 genes restored endocytic activity of DC that was inhibited by the tumors. Thus, our results suggest that tumor-induced dysregulation of endocytic activity of DC is mediated by reduced activity of several members of the small Rho GTPase family, which might serve as new targets for improving the efficacy of DC vaccines.

The amastigote forms of Leishmania are experts at exploiting host cell processes to establish infection and persist

Leishmania are dimorphic protozoan parasites that live as flagellated forms in the gut of their sandfly vector and as aflagellated forms in their mammalian hosts. Although both parasite forms can infect macrophages and dendritic cells, they elicit distinct responses from mammalian cells. Amastigotes are the parasites forms that persist in the infected host; they infect cells recruited to lesions and disseminate the infection to secondary sites. In this review I discuss studies that have investigated the mechanisms that Leishmania amastigotes employ to harness the host cell's response to infection. It should be acknowledged that our understanding of the mechanisms deployed by Leishmania amastigotes to modulate the host cell's response to infection is still rudimentary. Nonetheless, the results show that amastigote interactions with mammalian cells promote the production of anti-inflammatory cytokines such as IL-10 and TGF-beta while suppressing the production of IL-12, superoxide and nitric oxide. An underlying issue that is considered is how these parasites that reside in sequestered vacuolar compartments target host cell processes in the cytosol or the nucleus; does this occur through the release of parasite molecules from parasitophorous vacuoles or by engaging and sustaining signalling pathways throughout the course of infection?

Bcl10 controls TCR- and FcgammaR-induced actin polymerization

Bcl10 plays an essential role in the adaptive immune response, because Bcl10-deficient lymphocytes show impaired Ag receptor-induced NF-kappaB activation and cytokine production. Bcl10 is a phosphoprotein, but the physiological relevance of this posttranslational modification remains poorly defined. In this study, we report that Bcl10 is rapidly phosphorylated upon activation of human T cells by PMA/ionomycin- or anti-CD3 treatment, and identify Ser(138) as a key residue necessary for Bcl10 phosphorylation. We also show that a phosphorylation-deficient Ser(138)/Ala mutant specifically inhibits TCR-induced actin polymerization yet does not affect NF-kappaB activation. Moreover, silencing of Bcl10, but not of caspase recruitment domain-containing MAGUK protein-1 (Carma1) induces a clear defect in TCR-induced F-actin formation, cell spreading, and conjugate formation. Remarkably, Bcl10 silencing also impairs FcgammaR-induced actin polymerization and phagocytosis in human monocytes. These results point to a key role of Bcl10 in F-actin-dependent immune responses of T cells and monocytes/macrophages.

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.

Role of CrkII in Fcgamma receptor-mediated phagocytosis

Phagocytosis of IgG-opsonized pathogens by Fcgamma receptors requires extensive remodeling of the actin cytoskeleton, a process regulated by the small GTPase Rac. Vav was thought to be the guanine nucleotide exchange factor responsible for the activation of Rac, but recent evidence indicates that Fcgamma receptor-mediated phagocytosis is unaffected in macrophages lacking all three isoforms of Vav. We therefore tested whether another GEF, DOCK180, participates in Fcgamma receptor-initiated phagocytosis. DOCK180 associates with the adaptor protein Crk, which mediates recruitment of the GEF to sites of tyrosine phosphorylation. CrkII and DOCK180 were found to accumulate at the phagocytic cup. Knockdown of Crk or DOCK180 in murine macrophages using small interfering RNA inhibited phagocytosis of IgG-opsonized particles. Moreover, transfection of dominant negative CrkII prevented both recruitment of DOCK180 and the activation of Rac at the phagocytic cup. This is the first report of a role for either Crk or DOCK180 in Fcgamma receptor-mediated phagocytosis. The Crk-DOCK180 complex is involved in the clearance of apoptotic cells, which unlike the ingestion of IgG-opsonized particles, is an anti-inflammatory process. The finding that CrkII-DOCK180 is also responsible, at least in part, for the effects of Fcgamma receptors implies that additional, parallel pathways must account for the associated pro-inflammatory effect.

Fcgamma receptor signaling in phagocytes

Fcgamma receptors are among the best-studied phagocytic receptors. The key features of Fcgamma receptor-mediated phagocytosis include phagocytic cup formation by extensive actin cytoskeletal rearrangements, particle engulfment, and the release of proinflammatory mediators such as cytokines and reactive oxygen species. These events are elegantly regulated by the simultaneous engagement of activating and inhibitory Fcgamma receptors and by intracellular signaling molecules. Extensive studies in the past several years have defined the molecular mechanisms of the phagocytic process. The purpose of this review is to revisit some of the well-established signaling pathways as well as to summarize the new findings in this field.

Integrin signaling in neutrophils and macrophages uses adaptors containing immunoreceptor tyrosine-based activation motifs

At sites of inflammation, ligation of leukocyte integrins is critical for the activation of cellular effector functions required for host defense. However, the signaling pathways linking integrin ligation to cellular responses are poorly understood. Here we show that integrin signaling in neutrophils and macrophages requires adaptors containing immunoreceptor tyrosine-based activation motifs (ITAMs). Neutrophils and macrophages lacking two ITAM-containing adaptor proteins, DAP12 and FcRgamma, were defective in integrin-mediated responses. Activation of the tyrosine kinase Syk by integrins required that DAP12 and FcRgamma were first phosphorylated by Src family kinases. Retroviral transduction of neutrophils and macrophages with wild-type and mutant Syk or DAP12 demonstrated that the Src homology 2 domains of Syk and the ITAM of DAP12 were required for integrin signaling. Our data show that integrin signaling for the activation of cellular responses in neutrophils and macrophages proceeds by an immunoreceptor-like mechanism.

Abr and Bcr, two homologous Rac GTPase-activating proteins, control multiple cellular functions of murine macrophages

Small GTPases of the Rho family are key regulators of phagocytic leukocyte function. Abr and Bcr are homologous, multidomain proteins. Their C-terminal domain has GTPase-activating protein (GAP) activity that, in vitro, is specific for Rac and Cdc42. To address the in vivo relevance of these entire proteins, of which little is known, the current study examined the effect of the genetic ablation of Abr and Bcr in murine macrophages. The concomitant loss of Abr and Bcr induced multiple alterations of macrophage cellular behavior known to be under the control of Rac. Macrophages lacking both Abr and Bcr exhibited an atypical, elongated morphology that was reproduced by the ectopic expression of GAP domain mutant Abr and Bcr in a macrophage cell line and of constitutively active Rac in primary macrophages. A robust increase in colony-stimulating factor 1 (CSF-1)-directed motility was observed in macrophages deficient for both proteins and, in response to CSF-1 stimulation, Abr and Bcr transiently translocated to the plasma membrane. Phagocytosis of opsonized particles was also increased in macrophages lacking both proteins and correlated with sustained Rac activation. Bcr and Abr GAP mutant proteins localized around phagosomes and induced distinct phagocytic cup formation. These results identify Abr and Bcr as the only GAPs to date that specifically negatively regulate Rac function in vivo in primary macrophages.

Rho GTPase CDC42 regulates directionality and random movement via distinct MAPK pathways in neutrophils

Neutrophil transmigration into tissue is a multiple-step process that results from a coordinated rearrangement of the cytoskeleton and adhesion complexes. Assembly and disassembly of actin and adhesion structures dictate motility behavior, while polarity and gradient sensing provide directionality to the cell movement. Here, using mice deficient in the CDC42 regulator CDC42 GTPase-activating protein (CDC42GAP), we demonstrate that CDC42 activity separately regulates neutrophil motility and directionality. CDC42GAP-/- neutrophils showed increased motility, while directed migration was defective. Podosome-like structures present at the leading edge in wild-type neutrophils were significantly reduced in CDC42GAP-/- cells. CDC42GAP-/- neutrophils also showed increased lateral and tail filopodia-like formation, and excess membrane protrusions. We further suggest that CDC42GAP-mediated extracellular signal-regulated kinase (ERK) activity regulates motility associated with podosome-like structures at the cell leading edge, while CDC42GAP-induced p38(MAPK) phosphorylation regulates directed migration by antagonizing filopodia assembly. Overall, this study reveals that CDC42 activity regulates both motility and directionality in neutrophils, but via distinct mitogen-activated protein kinase (MAPK) pathways.

Rewiring phagocytic signal transduction

In this issue of Immunity, primary macrophages from Vav1(-/-)/Vav3(-/-) and Rac1(-/-)/Rac2(-/-) mice to produce a new picture of Fcgamma receptor and complement receptor 3 intracellular signaling have been examined.