Vav1 and Vav2 play different roles in macrophage migration and cytoskeletal organization

Causal effects of gut microbiota on sepsis and sepsis-related death: insights from genome-wide Mendelian randomization, single-cell RNA, bulk RNA sequencing, and network pharmacology

BACKGROUND

Gut microbiota alterations have been implicated in sepsis and related infectious diseases, but the causal relationship and underlying mechanisms remain unclear.

METHODS

We evaluated the association between gut microbiota composition and sepsis using two-sample Mendelian randomization (MR) analysis based on published genome-wide association study (GWAS) summary statistics. Sensitivity analyses were conducted to validate the robustness of the results. Reverse MR analysis and integration of GWAS and expression quantitative trait loci (eQTL) data were performed to identify potential genes and therapeutic targets.

RESULTS

Our analysis identified 11 causal bacterial taxa associated with sepsis, with increased abundance of six taxa showing positive causal relationships. Ten taxa had causal effects on the 28-day survival outcome of septic patients, with increased abundance of six taxa showing positive associations. Sensitivity analyses confirmed the robustness of these associations. Reverse MR analysis did not provide evidence of reverse causality. Integration of GWAS and eQTL data revealed 76 genes passing the summary data-based Mendelian randomization (SMR) test. Differential expression of these genes was observed between sepsis patients and healthy individuals. These genes represent potential therapeutic targets for sepsis. Molecular docking analysis predicted potential drug-target interactions, further supporting their therapeutic potential.

CONCLUSION

Our study provides insights for the development of personalized treatment strategies for sepsis and offers preliminary candidate targets and drugs for future drug development.

Genetic deletion of the Rho GEF Net1 impairs mouse macrophage motility and actin cytoskeletal organization

Macrophages are innate immune cells that constantly patrol an organism to fulfill protective and homeostatic roles. Previous studies have shown that Rho GTPase activity is required for macrophage mobility, yet the roles of upstream regulatory proteins controlling Rho GTPase function in these cells are not well defined. Previously we have shown that the RhoA GEF Net1 is required for human breast cancer cell motility and extracellular matrix invasion. To assess the role of Net1 in macrophage motility, we isolated bone marrow macrophage (BMM) precursors from wild type and Net1 knockout mice. Loss of Net1 did not affect the ability of BMM precursors to differentiate into mature macrophages in vitro, as measured by CD68 and F4/80 staining. However, Net1 deletion significantly reduced RhoA activation, F-actin accumulation, adhesion, and motility in these cells. Nevertheless, similar to RhoA/RhoB double knockout macrophages, Net1 deletion did not impair macrophage recruitment to the peritoneum in a mouse model of sterile inflammation. These data demonstrate that Net1 is an important regulator of RhoA signaling and motility in mouse macrophages in vitro, but that its function may be dispensable for macrophage recruitment to inflammatory sites in vivo.

Involvement of Cbl-b-mediated macrophage inactivation in insulin resistance

Aging and overnutrition cause obesity in rodents and humans. It is well-known that obesity causes various diseases by producing insulin resistance (IR). Macrophages infiltrate the adipose tissue (AT) of obese individuals and cause chronic low-level inflammation associated with IR. Macrophage infiltration is regulated by the chemokines that are released from hypertrophied adipocytes and the immune cells in AT. Saturated fatty acids are recognized by toll-like receptor 4 (TLR4) and induce inflammatory responses in AT macrophages (ATMs). The inflammatory cytokines that are released from activated ATMs promote IR in peripheral organs, such as the liver, skeletal muscle and AT. Therefore, ATM activation is a therapeutic target for IR in obesity. The ubiquitin ligase Casitas b-lineage lymphoma-b (Cbl-b) appears to potently suppress macrophage migration and activation. Cbl-b is highly expressed in leukocytes and negatively regulates signals associated with migration and activation. Cbl-b deficiency enhances ATM accumulation and IR in aging- and diet-induced obese mice. Cbl-b inhibits migration-related signals and SFA-induced TLR4 signaling in ATMs. Thus, targeting Cbl-b may be a potential therapeutic strategy to reduce the IR induced by ATM activation. In this review, we summarize the regulatory functions of Cbl-b in ATMs.

Vav1: A Dr. Jekyll and Mr. Hyde protein--good for the hematopoietic system, bad for cancer

Many deregulated signal transducer proteins are involved in various cancers at numerous stages of tumor development. One of these, Vav1, is normally expressed exclusively in the hematopoietic system, where it functions as a specific GDP/GTP nucleotide exchange factor (GEF), strictly regulated by tyrosine phosphorylation. Vav was first identified in an NIH3T3 screen for oncogenes. Although the oncogenic form of Vav1 identified in the screen has not been detected in clinical human tumors, its wild-type form has recently been implicated in mammalian malignancies, including neuroblastoma, melanoma, pancreatic, lung and breast cancers, and B-cell chronic lymphocytic leukemia. In addition, it was recently identified as a mutated gene in human cancers of various origins. However, the activity and contribution to cancer of these Vav1 mutants is still unclear. This review addresses the physiological function of wild-type Vav1 and its activity as an oncogene in human cancer. It also discusses the novel mutations identified in Vav1 in various cancers and their potential contribution to cancer development as oncogenes or tumor suppressor genes.

Altered function of monocytes/macrophages in patients with autoimmune hepatitis

The pathogenesis of autoimmune hepatitis (AIH) involves the intervention of the innate and adaptive immune responses. In the current study, the alterations in monocytes/Kupffer cells (KCs) were investigated in patients with AIH. A total of 21 patients with AIH at different stages of the disease, and 7 controls with non-alcoholic fatty liver disease were selected. The abundance of VAV1 and p21-activated kinase 1 (PAK1) in the liver and KCs was analyzed. In addition, the expression levels of HLA-DR and CD80 in the peripheral blood monocytes (PBMs) were measured, and phagocytosis of PBMs was assessed. KCs of AIH patients exhibited higher expression levels of VAV1 and PAK1. This upregulated expression was associated with disease progression. A reduced expression of HLA-DR and CD80, and reduced capacity of E. coli phagocytosis in PBMs was observed for patients with AIH. This downregulated expression was associated with disease progression. The results of the current study indicated that defective function of KCs and PBMs may be involved in the pathogenesis of AIH.

Expression of a Chimeric Antigen Receptor in Multiple Leukocyte Lineages in Transgenic Mice

Genetically modified CD8+ T lymphocytes have shown significant anti-tumor effects in the adoptive immunotherapy of cancer, with recent studies highlighting a potential role for a combination of other immune subsets to enhance these results. However, limitations in present genetic modification techniques impose difficulties in our ability to fully explore the potential of various T cell subsets and assess the potential of other leukocytes armed with chimeric antigen receptors (CARs). To address this issue, we generated a transgenic mouse model using a pan-hematopoietic promoter (vav) to drive the expression of a CAR specific for a tumor antigen. Here we present a characterization of the immune cell compartment in two unique vav-CAR transgenic mice models, Founder 9 (F9) and Founder 38 (F38). We demonstrate the vav promoter is indeed capable of driving the expression of a CAR in cells from both myeloid and lymphoid lineage, however the highest level of expression was observed in T lymphocytes from F38 mice. Lymphoid organs in vav-CAR mice were smaller and had reduced cell numbers compared to the wild type (WT) controls. Furthermore, the immune composition of F9 mice differed greatly with a significant reduction in lymphocytes found in the thymus, lymph node and spleen of these mice. To gain insight into the altered immune phenotype of F9 mice, we determined the chromosomal integration site of the transgene in both mouse strains using whole genome sequencing (WGS). We demonstrated that compared to the 7 copies found in F38 mice, F9 mice harbored almost 270 copies. These novel vav-CAR models provide a ready source of CAR expressing myeloid and lymphoid cells and will aid in facilitating future experiments to delineate the role for other leukocytes for adoptive immunotherapy against cancer.

Cross-species epigenetics identifies a critical role for VAV1 in SHH subgroup medulloblastoma maintenance

The identification of key tumorigenic events in Sonic Hedgehog subgroup medulloblastomas (MB_SHH) will be essential for the development of individualized therapies and improved outcomes. However, beyond confirmation of characteristic SHH-pathway mutations, recent genome-wide sequencing studies have not revealed commonly-mutated genes with widespread relevance as potential therapeutic targets. We therefore examined any role for epigenetic DNA methylation events in MB_SHH using a cross-species approach to candidate identification, prioritization and validation. MB_SHH–associated DNA methylation events were first identified in 216 subgrouped human medulloblastomas (50 MB_SHH, 28 WNT_, 44 Group 3, 94 Group 4) and their conservation then assessed in tumors arising from four independent murine models of Shh medulloblastoma, alongside any role in tumorigenesis using functional assessments in mouse and human models. This strategy identified widespread regional CpG hypo-methylation of VAV1, leading to its elevated expression, as a conserved aberrant epigenetic event which characterizes the majority of MB_SHH tumors in both species, and is associated with a poor outcome in MB_SHH patients. Moreover, direct modulation of VAV1 in mouse and human models revealed a critical role in tumor maintenance, and its abrogation markedly reduced medulloblastoma growth. Further, Vav1 activity regulated granule neuron precursor (GNP) germinal zone exit and migration initiation in an ex vivo model of early post-natal cerebellar development. These findings establish VAV1 as a critical epigenetically-regulated oncogene with a key role in MB_SHH maintenance, and highlight its potential as a validated therapeutic target and prognostic biomarker for the improved therapy of medulloblastoma.

CSF-1 receptor signaling in myeloid cells

The CSF-1 receptor (CSF-1R) is activated by the homodimeric growth factors colony-stimulating factor-1 (CSF-1) and interleukin-34 (IL-34). It plays important roles in development and in innate immunity by regulating the development of most tissue macrophages and osteoclasts, of Langerhans cells of the skin, of Paneth cells of the small intestine, and of brain microglia. It also regulates the differentiation of neural progenitor cells and controls functions of oocytes and trophoblastic cells in the female reproductive tract. Owing to this broad tissue expression pattern, it plays a central role in neoplastic, inflammatory, and neurological diseases. In this review we summarize the evolution, structure, and regulation of expression of the CSF-1R gene. We discuss the structures of CSF-1, IL-34, and the CSF-1R and the mechanism of ligand binding to and activation of the receptor. We further describe the pathways regulating macrophage survival, proliferation, differentiation, and chemotaxis downstream from the CSF-1R.

Ubiquitin ligase Cbl-b and obesity-induced insulin resistance

Obesity causes type 2 diabetes, atherosclerosis and cardiovascular diseases by inducing systemic insulin resistance. It is now recognized that obesity is related to chronic low-grade inflammation in adipose tissue. Specifically, activated immune cells infiltrate adipose tissue and cause inflammation. There is increasing evidence that activated macrophages accumulate in the hypertrophied adipose tissue of rodents and humans and induce systemic insulin resistance by secreting inflammatory cytokines. Accordingly, a better understanding of the molecular mechanisms underlying macrophage activation in adipose tissue will facilitate the development of new therapeutic strategies. Currently, little is known about the regulation of macrophage activation, although E3 ubiquitin ligase Casitas B-lineage lymphoma (Cbl)-b was identified recently as a novel negative regulator of macrophage activation in adipose tissue. Cbl-b, which is a suppressor of T- and B-cell activation, inhibits intracellular signal transduction by targeting some tyrosine kinases. Notably, preventing Cbl-b-mediated macrophage activation improves obesity-induced insulin resistance in mice. c-Cbl is another member of the Cbl family that is associated with insulin resistance in obesity. These reports suggest that Cbl-b and c-Cbl are potential therapeutic targets for treating obesity-induced insulin resistance. In this review, we focus on the importance of Cbl-b in macrophage activation in aging-induced and high-fat diet-induced obesity.

Vav Guanine nucleotide exchange factors regulate atherosclerotic lesion development in mice

OBJECTIVE

Atherosclerosis requires migration of monocytes to the arterial intima, with subsequent differentiation into foam cells. We showed previously that the scavenger receptor CD36 contributes to the activation of Vav family guanine nucleotide exchange factors (Vavs) in aortae from hyperlipidemic apoE-null mice and that oxidatively modified low-density lipoprotein induced CD36-dependent activation of macrophage Vavs in vitro. We also discovered that CD36-dependent uptake of oxidized low-density lipoprotein and foam cell formation were reduced in Vav-deficient macrophages. We now tested the hypothesis that Vavs play a role in atherosclerotic lesion development.

APPROACH AND RESULTS

We showed that apoE/vav1 double-null mice fed a Western diet had significant reduction in total aortic lesion area (by en face analysis) compared with apoE-null mice, with no significant differences in body weight or plasma lipid profiles. Histological analysis of aortic sinus lesions showed fewer macrophages and foam cells in double-null mice compared with apoE-null mice, indicating impaired foam cell generation and homing of macrophages to atherosclerotic lesions. An intravital video microscopy-based adhesion assay with fluorescent (Qtracker655)-labeled monocytes showed reduced adhesion of vav1-null monocytes to hyperlipidemic carotid arteries compared with wild-type monocytes. Furthermore, fewer fluorescently labeled vav1-null monocytes accumulated in aortic sinus lesions in hyperlipidemic apoE-null mice. We also found that activation of RhoGTPase Rac and mitogen-activated protein kinase c-Jun N-terminal kinase-2 by CD36-specific oxidized phospholipids was dependent on Vavs.

CONCLUSIONS

These results for the first time link Vavs to atherosclerotic lesion development and suggest that Vavs act as critical molecular links coupling hyperlipidemia with proatherogenic monocyte/macrophage responses.

Vav1 regulates MHCII expression in murine resting and activated B cells

Vav1 is a guanine nucleotide exchange factor (GEF) for Rho GTPases, which is exclusively expressed in cells of the hematopoietic system. In addition to its well-documented GEF activity, it was suggested to have other functions due to the presence of multiple domains and nuclear localization signals in its protein structure. Although GEF-dependent and GEF-independent functions of vav have been implicated in T-cell development and T-cell receptor signaling, the role of vav1 in antigen-presenting cells is poorly understood. We found that vav1 is an important regulator of MHCII expression and transport. Microarray analysis of unstimulated bone marrow-derived macrophages revealed a novel role of vav1 in transcriptional regulation of the MHCII locus, possibly by indirect means. Primary immune cells from vav1-deficient mice had a significantly lower constitutive surface expression of MHCII with the strongest impact observed on splenic and peritoneal B cells. Impaired MHCII expression resulted in a diminished capacity for T-cell activation. Using 6-thio-GTP, a specific inhibitor of the GEF function of vav1, we were able to show that the GEF activity is required for MHCII upregulation in B cells after stimulation with LPS. Furthermore, our data show that vav1 not only affects transcription of the MHCII locus but also is an important regulator of MHCII protein transport to the cell surface.

Oxidized LDL/CD36 interaction induces loss of cell polarity and inhibits macrophage locomotion

Cell polarization is essential for migration and the exploratory function of leukocytes. However, the mechanism by which cells maintain polarity or how cells revert to the immobilized state by gaining cellular symmetry is not clear. Previously we showed that interaction between oxidized low-density lipoprotein (oxLDL) and CD36 inhibits macrophage migration; in the current study we tested the hypothesis that oxLDL/CD36-induced inhibition of migration is the result of intracellular signals that regulate cell polarity. Live cell imaging of macrophages showed that oxLDL actuated retraction of macrophage front end lamellipodia and induced loss of cell polarity. Cd36 null and macrophages null for Vav, a guanine nucleotide exchange factor (GEF), did not show this effect. These findings were caused by Rac-mediated inhibition of nonmuscle myosin II, a cell polarity determinant. OxLDL induced dephosphorylation of myosin regulatory light chain (MRLC) by increasing the activity of Rac. Six-thioguanine triphosphate (6-thio-GTP), which inhibits Vav-mediated activation of Rac, abrogated the effect of oxLDL. Activation of the Vav-Rac-myosin II pathway by oxidant stress may induce trapping of macrophages at sites of chronic inflammation such as atherosclerotic plaque.

Fractal properties of macrophage membrane studied by AFM

Complexity of cell membrane poses difficulties to quantify corresponding morphology changes during cell proliferation and damage. We suggest using fractal dimension of the cell membrane to quantify its complexity and track changes produced by various treatments. Glutaraldehyde fixed mouse RAW 264.7 macrophage membranes were chosen as model system and imaged in PeakForce QNM (quantitative nanomechanics) mode of AFM (atomic force microscope). The morphology of the membranes was characterized by fractal dimension. The parameter was calculated for set of AFM images by three different methods. The same calculations were done for the AFM images of macrophages treated with colchicine, an inhibitor of the microtubule polymerization, and microtubule stabilizing agent taxol. We conclude that fractal dimension can be additional and useful parameter to characterize the cell membrane complexity and track the morphology changes produced by different treatments.

Vav1 in differentiation of tumoral promyelocytes

The multidomain protein Vav1, in addition to promote the acquisition of maturation related properties by normal hematopoietic cells, is a key player in the ATRA- and PMA-induced completion of the differentiation program of tumoral myeloid precursors derived from APL. This review is focussed on the role of Vav1 in differentiating promyelocytes, as part of interconnected networks of functionally related proteins ended to regulate different aspects of myeloid maturation. The role of Vav1 in determining actin cytoskeleton reorganization alternative to the best known function as a GEF for small G proteins is discussed, as well as the binding of Vav1 with cytoplasmic and nuclear signaling molecules which provides a new perspective in the modulation of nuclear architecture and activity. In particular, new hints are provided on the ability of Vav1 to determine the nuclear amount of proteins implicated in modulating mRNA production and stability and in regulating the ATRA-dependent protein expression also by direct interaction with transcription factors known to drive the ATRA-induced maturation of myeloid cells. The reviewed findings summarize the major advances in the understanding of additional, non conventional functions connected with the vast interactive potential of Vav1.

Vav protein guanine nucleotide exchange factor regulates CD36 protein-mediated macrophage foam cell formation via calcium and dynamin-dependent processes

Atherosclerosis, a chronic inflammatory disease, results in part from the accumulation of modified lipoproteins in the arterial wall and formation of lipid-laden macrophages, known as "foam cells." Recently, we reported that CD36, a scavenger receptor, contributes to activation of Vav-family guanine nucleotide exchange factors by oxidatively modified LDL in macrophages. We also discovered that CD36-dependent uptake of oxidized LDL (oxLDL) in vitro and foam cell formation in vitro and in vivo was significantly reduced in macrophages deficient of Vav proteins. The goal of the present study was to identify the mechanisms by which Vav proteins regulate CD36-dependent foam cell formation. We now show that a Vav-dynamin signaling axis plays a critical role in generating calcium signals in mouse macrophages exposed to CD36-specific oxidized phospholipid ligands. Chelation of intracellular Ca(2+) or inhibition of phospholipase C-γ (PLC-γ) inhibited Vav activation (85 and 70%, respectively, compared with vehicle control) and reduced foam cell formation (approximately 75%). Knockdown of expression by siRNA or inhibition of GTPase activity of dynamin 2, a Vav-interacting protein involved in endocytic vesicle fission, significantly blocked oxLDL uptake and inhibited foam cell formation. Immunofluorescence microscopy studies showed that Vav1 and dynamin 2 colocalized with internalized oxLDL in macrophages and that activation and mobilization of dynamin 2 by oxLDL was impaired in vav null cells. These studies identified previously unknown components of the CD36 signaling pathway, demonstrating that Vav proteins regulate oxLDL uptake and foam cell formation via calcium- and dynamin 2-dependent processes and thus represent novel therapeutic targets for atherosclerosis.

Vav1 is a crucial molecule in monocytic/macrophagic differentiation of myeloid leukemia-derived cells

Vav1 is a critical signal transducer for both the development and function of normal hematopoietic cells, in which it regulates the acquisition of maturation-related properties, including adhesion, motility, and phagocytosis. Vav1 is also important for the agonist-induced maturation of acute promyelocytic leukemia (APL)-derived promyelocytes, in which it promotes the acquisition of a mature phenotype by playing multiple functions at both cytoplasmic and nuclear levels. We investigated the possible role of Vav1 in the differentiation of leukemic precursors to monocytes/macrophages. Tumoral promyelocytes in which Vav1 was negatively modulated were induced to differentiate into monocytes/macrophages with phorbol-12-myristate-13-acetate (PMA) and monitored for their maturation-related properties. We found that Vav1 was crucial for the phenotypical differentiation of tumoral myeloid precursors to monocytes/macrophages, in terms of CD11b expression, adhesion capability and cell morphology. Confocal analysis revealed that Vav1 may synergize with actin in modulating nuclear morphology of PMA-treated adherent cells. Our data indicate that, in tumoral promyelocytes, Vav1 is a component of lineage-specific transduction machineries that can be recruited by various differentiating agents. Since Vav1 plays a central role in the completion of the differentiation program of leukemic promyelocytes along diverse hematopoietic lineages, it can be considered a common target for developing new therapeutic strategies for the various subtypes of myeloid leukemias.

Vav family Rho guanine nucleotide exchange factors regulate CD36-mediated macrophage foam cell formation

Lipid-laden macrophages or "foam cells" are the primary components of the fatty streak, the earliest atherosclerotic lesion. Although Vav family guanine nucleotide exchange factors impact processes highly relevant to atherogenesis and are involved in pathways common to scavenger receptor CD36 signaling, their role in CD36-dependent macrophage foam cell formation remains unknown. The goal of the present study was to determine the contribution of Vav proteins to CD36-dependent foam cell formation and to identify the mechanisms by which Vavs participate in the process. We found that CD36 contributes to activation of Vav-1, -2, and -3 in aortae from hyperlipidemic mice and that oxidatively modified LDL (oxLDL) induces activation of macrophage Vav in vitro in a CD36 and Src family kinase-dependent manner. CD36-dependent uptake of oxLDL in vitro and foam cell formation in vitro and in vivo was significantly reduced in Vav null macrophages. These studies for the first time link CD36 and Vavs in a signaling pathway required for macrophage foam cell formation.

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.

Molecular regulators of leucocyte chemotaxis during inflammation

A fundamental feature of any immune response is the movement of leucocytes from one site in the body to another to provide effector functions. Therefore, elucidating the molecular mechanisms underlying the migration of leucocytes from the blood to tissues is critical to our understanding of immune function during inflammation. The classic steps of leucocyte trafficking involve leucocyte tethering and rolling on vessel walls of the vasculature, followed by firm adhesion to the endothelium. Recent evidence suggests that upon adhering, leucocytes crawl within the vessels before transmigrating across vessel walls and crawling into targeted tissues. The directed nature of the crawling events is orchestrated by a complex array of soluble factors and molecular regulators in combination with the local intravascular and extracellular environment. In fact, this process is known as chemotaxis and orientates cell movement in relation to the ligand gradient. Several signalling pathways have been proposed to be involved in this gradient-sensing and amplification process, but the best studied, discussed in detail here, is the phosphatidylinositol 3-kinase pathway. Substantial progress has been made in understanding how cells roll and adhere in blood vessels; however, how cells crawl in blood vessels, emigrate, and then crawl in tissues has received much less attention. Therefore, the focus of this review is to provide recent insights into molecular mechanisms and cellular processes that mediate leucocyte crawling in blood vessels and tissues during the inflammatory response.

Mammary tumors with diverse immunological phenotypes show differing sensitivity to adoptively transferred CD8+ T cells lacking the Cbl-b gene

We tested the efficacy of CD8+ T cells lacking the Cbl-b gene against a panel of mammary tumor lines with different intrinsic sensitivities to T cells. Mice bearing established tumors expressing an ovalbumin-tagged version of HER-2/neu underwent adoptive transfer with Cbl-b-replete or -null CD8+ T cells from OT-I T cell receptor transgenic donor mice. In general, Cbl-b-null OT-I cells showed enhanced expansion, persistence, and capacity for tumor infiltration. This resulted in markedly enhanced efficacy against two tumor lines that normally demonstrate complete (NOP21) or partial (NOP23) regression. Moreover, a third tumor line (NOP6) that normally demonstrates progressive disease underwent complete regression in response to Cbl-b-null OT-I cells. However, a fourth tumor line (NOP18) was resistant to Cbl-b-null OT-I cells owing to a profound barrier to lymphocyte infiltration. Thus, Cbl-b-null CD8+ T cells are generally more efficacious but are nonetheless unable to mediate curative responses against all tumor phenotypes.

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.

Vav1 regulates the migration and adhesion of dendritic cells

Dendritic cells (DCs) are the most potent APCs for activating naive T cells, a process facilitated by the ability of immature DCs to mature and home to lymph nodes after encountering an inflammatory stimulus. Proteins involved in cytoskeletal rearrangement play an important role in regulating the adherence and motility of DCs. Vav1, a guanine nucleotide exchange factor for Rho family GTPases, mediates cytoskeletal rearrangement in hematopoietic cells following integrin ligation. We show that Vav1 is not required for the normal maturation of DCs in vitro; however, it is critical for DC binding to fibronectin and regulates the distribution but not the formation of podosomes. We also found that DC Vav1 was an important component of a signaling pathway involving focal adhesion kinase, phospholipase C-gamma2, and ERK1/2 following integrin ligation. Surprisingly, Vav1(-/-) DCs had increased rates of migration in vivo compared with wild-type control DCs. In vitro findings show that the presence of adhesive substrates such as fibronectin resulted in inhibition of migration. However, there was less inhibition in the absence of Vav1. These findings suggest that DC migration is negatively regulated by adhesion and integrin-mediated signaling and that Vav1 has a central role in this process.

Vav1 is essential for mechanotactic crawling and migration of neutrophils out of the inflamed microvasculature

Mac-1-dependent crawling is a new step in the leukocyte recruitment cascade that follows LFA-1-dependent adhesion and precedes emigration. Neutrophil adhesion via LFA-1 has been shown to induce cytoskeletal reorganization through Vav1-dependent signaling, and the current study investigates the role of Vav1 in the leukocyte recruitment process in vivo with particular attention to the events immediately downstream of LFA-1-dependent adhesion. Intravital and spinning-disk-confocal microscopy was used to investigate intravascular crawling in relation to endothelial junctions in vivo in wild-type and Vav1(-/-) mice. Adherent wild-type neutrophils almost immediately began crawling perpendicular to blood flow via Mac-1 until they reached an endothelial junction where they often changed direction. This pattern of perpendicular, mechanotactic crawling was recapitulated in vitro when shear was applied. In sharp contrast, the movement of Vav1(-/-) neutrophils was always in the direction of flow and appeared more passive as if the cells were dragged in the direction of flow in vivo and in vitro. More than 80% of Vav1(-/-) neutrophils moved independent of Mac-1 and could be detached with LFA-1 Abs. An inability to release the uropod was frequently noted for Vav1(-/-) neutrophils, leading to greatly elongated tails. The Vav1(-/-) neutrophils failed to stop or follow junctions and ultimately detached, leading to fewer emigrated neutrophils. The Vav1(-/-) phenotype resulted in fewer neutrophils recruited in a relevant model of infectious peritonitis. Clearly, Vav1 is critical for the complex interplay between LFA-1 and Mac-1 that underlies the programmed intravascular crawling of neutrophils.

A Pyk2-Vav1 complex is recruited to beta3-adhesion sites to initiate Rho activation

Integrin alphavbeta3-mediated adhesion of haemopoietic cells to vitronectin results in beta3 tyrosine phosphorylation and Rho activation which is necessary for adhesion. Previously, we have shown that the RhoGEF (Rho guanine-nucleotide-exchange factor) Vav1 could associate indirectly with alphavbeta3 during leucocyte adhesion to vitronectin. In the present study, we have identified the non-receptor tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2) as the adaptor protein that links Vav1 with alphavbeta3. The association of Pyk2 and Vav1 with beta3 relies on the presence of Tyr747 in beta3, the primary site of beta3 phosphorylation. However, association of Pyk2 with Vav1 is independent of beta3 tyrosine phosphorylation. Formation of a Pyk2-Vav1 complex occurs upon cell adhesion and Pro717 of Pyk2 plays a key role in Pyk2 interaction with Vav1. Utilizing purified recombinant proteins, we confirmed the direct interaction between Pyk2 and Vav1 In vitro. Cells transfected with GFP (green fluorescent protein)-Pyk2-P717A demonstrated severely suppressed cytoskeletal reorganization, impaired Vav1 recruitment, decreased Rho GTPase activation and loss of cell adhesion. Using siRNA (small interfering RNA) to specifically reduce Pyk2 levels in cells resulted in disrupted association between Vav1 and beta3 and impaired cell adhesion. These results indicate that Pyk2 is a critical signalling molecule downstream of beta3 integrin tyrosine phosphorylation and mediates Vav1 recruitment to accomplish actin reorganization necessary for adhesion.

SRC-dependent signalling regulates actin ruffle formation induced by glycerophosphoinositol 4-phosphate

The glycerophosphoinositols are diffusible phosphoinositide metabolites reported to modulate actin dynamics and tumour cell spreading. In particular, the membrane permeant glycerophosphoinositol 4-phosphate (GroPIns4P) has been shown to act at the level of the small GTPase Rac1, to induce the rapid formation of membrane ruffles. Here, we have investigated the signalling cascade involved in this process, and show that it is initiated by the activation of Src kinase. In NIH3T3 cells, exogenous addition of GroPIns4P induces activation and translocation of Rac1 and its exchange factor TIAM1 to the plasma membrane; in addition, in in-vitro assays, GroPIns4P favours the formation of a protein complex that includes Rac1 and TIAM1. Neither of these processes involves direct actions of GroPIns4P on these proteins. Thus, through the use of specific inhibitors of tyrosine kinases and phospholipase C (and by direct evaluation of kinase activities and inositol 1,4,5-trisphosphate production), we show that GroPIns4P activates Src, and as a consequence, phospholipase Cgamma and Ca(2+)/calmodulin kinase II, the last of which directly phosphorylates TIAM1 and leads to TIAM1/Rac1-dependent ruffle formation.

iPLA2beta: front and center in human monocyte chemotaxis to MCP-1

Monocyte chemoattractant protein-1 (MCP-1) directs migration of blood monocytes to inflamed tissues. Despite the central role of chemotaxis in immune responses, the regulation of chemotaxis by signal transduction pathways and their in vivo significance remain to be thoroughly deciphered. In this study, we examined the intracellular location and functions of two recently identified regulators of chemotaxis, Ca²⁺-independent phospholipase (iPLA₂β) and cytosolic phospholipase (cPLA₂α), and substantiate their in vivo importance. These enzymes are cytoplasmic in unstimulated monocytes. Upon MCP-1 stimulation, iPLA₂β is recruited to the membrane-enriched pseudopod. In contrast, cPLA₂α is recruited to the endoplasmic reticulum. Although iPLA₂β or cPLA₂α antisense oligodeoxyribonucleotide (ODN)–treated monocytes display reduced speed, iPLA₂β also regulates directionality and actin polymerization. iPLA₂β or cPLA₂α antisense ODN–treated adoptively transferred mouse monocytes display a profound defect in migration to the peritoneum in vivo. These converging observations reveal that iPLA₂β and cPLA₂α regulate monocyte migration from different intracellular locations, with iPLA₂β acting as a critical regulator of the cellular compass, and identify them as potential targets for antiinflammatory strategies.

Deficiency of Cbl-b gene enhances infiltration and activation of macrophages in adipose tissue and causes peripheral insulin resistance in mice

OBJECTIVE

c-Cbl plays an important role in whole-body fuel homeostasis by regulating insulin action. In the present study, we examined the role of Cbl-b, another member of the Cbl family, in insulin action.

RESEARCH DESIGN AND METHODS

C57BL/6 (Cbl-b(+/+)) or Cbl-b-deficient (Cbl-b(-/-)) mice were subjected to insulin and glucose tolerance tests and a hyperinsulinemic-euglycemic clamp test. Infiltration of macrophages into white adipose tissue (WAT) was assessed by immunohistochemistry and flow cytometry. We examined macrophage activation using co-cultures of 3T3-L1 adipocytes and peritoneal macrophages.

RESULTS

Elderly Cbl-b(-/-) mice developed glucose intolerance and peripheral insulin resistance; serum insulin concentrations after a glucose challenge were always higher in elderly Cbl-b(-/-) mice than age-matched Cbl-b(+/+) mice. Deficiency of the Cbl-b gene significantly decreased the uptake of 2-deoxyglucose into WAT and glucose infusion rate, whereas fatty liver was apparent in elderly Cbl-b(-/-) mice. Cbl-b deficiency was associated with infiltration of macrophages into the WAT and expression of cytokines, such as tumor necrosis factor-alpha, interleukin-6, and monocyte chemoattractant protein (MCP)-1. Co-culture of Cbl-b(-/-) macrophages with 3T3-L1 adipocytes induced leptin expression and dephosphorylation of insulin receptor substrate 1, leading to impaired glucose uptake in adipocytes. Furthermore, Vav1, a key factor in macrophage activation, was highly phosphorylated in peritoneal Cbl-b(-/-) macrophages compared with Cbl-b(+/+) macrophages. Treatment with a neutralizing anti-MCP-1 antibody improved peripheral insulin resistance and macrophage infiltration into WAT in elderly Cbl-b(-/-) mice.

CONCLUSIONS

Cbl-b is a negative regulator of macrophage infiltration and activation, and macrophage activation by Cbl-b deficiency contributes to the peripheral insulin resistance and glucose intolerance via cytokines secreted from macrophages.

Strongly reduced alloreactivity and long-term survival times of cardiac allografts in Vav1- and Vav1/Vav2-knockout mice

Vav proteins mediate T- and B-cell activation by functioning as GTP/GDP exchange factors for small GTPases. We have studied the role of Vav1 and Vav2 in allogeneic T-cell activation, antibody responses and allograft rejection. Alloantigen-induced proliferation of T cells from Vav1- and Vav1/Vav2-knockout (ko) mice was decreased by >90% in a mixed lymphocyte reaction. In whole-blood cultures, Vav deficiency led to markedly impaired T- and B-cell activation. Expansion of Vav1- or Vav1/Vav2-ko T cells (C57BL/6) was reduced after transfer into severe combined immune deficiency/beige recipient mice (BALB/c). After priming with 2,4-dinitrophenyl (DNP)-keyhole limpet hemocyanin, T-cell-dependent anti-DNP IgM and IgG antibody levels were normal in Vav1-ko mice but undetectable in Vav1/Vav2-ko mice. The median survival time of BALB/c cardiac allografts transplanted into C57BL/6 Vav1-ko mice (n = 13) or Vav1/Vav2-ko mice (n = 5) was >100 and >77 days, compared with 8-9 days in the corresponding wild-type mice. Vav1/Vav2-ko mice with <100 days graft survival developed bacterial skin infections and were prematurely killed with beating cardiac allograft. Long-term surviving transplants of single and double ko mice showed mild cellular interstitial rejection and mild to severe vascular remodeling. In conclusion, our studies show for the first time that the absence of Vav1 and Vav1/Vav2 in ko mice strongly reduces alloreactivity and results in long-term allograft survival, whereas antibody responses were only affected in Vav1/Vav2 ko mice.

Transcriptional signature of epidermal keratinocytes subjected to in vitro scratch wounding reveals selective roles for ERK1/2, p38, and phosphatidylinositol 3-kinase signaling pathways

Covering denuded dermal surfaces after injury requires migration, proliferation, and differentiation of skin keratinocytes. To clarify the major traits controlling these intermingled biological events, we surveyed the genomic modifications occurring during the course of a scratch wound closure of cultured human keratinocytes. Using a DNA microarray approach, we report the identification of 161 new markers of epidermal repair. Expression data, combined with functional analysis performed with specific inhibitors of ERK, p38(MAPK) and phosphatidylinositol 3-kinase (PI3K), demonstrate that kinase pathways exert very selective functions by precisely controlling the expression of specific genes. Inhibition of the ERK pathway totally blocks the wound closure and inactivates many early transcription factors and EGF-type growth factors. p38(MAPK) inhibition only delays "healing," probably in line with the control of genes involved in the propagation of injury-initiated signaling. In contrast, PI3K inhibition accelerates the scratch closure and potentiates the scratch-dependent stimulation of three genes related to epithelial cell transformation, namely HAS3, HBEGF, and ETS1. Our results define in vitro human keratinocyte wound closure as a repair process resulting from a fine balance between positive signals controlled by ERK and p38(MAPK) and negative ones triggered by PI3K. The perturbation of any of these pathways might lead to dysfunction in the healing process, similar to those observed in pathological wounding phenotypes, such as hypertrophic scars or keloids.

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.