Vav: function and regulation in hematopoietic cell signaling

Discovery of an unconventional lamprey lymphocyte lineage highlights divergent features in vertebrate adaptive immune system evolution

Lymphocyte receptors independently evolved in both jawed and jawless vertebrates with similar adaptive immune responses. However, the diversity of functional subtypes and molecular architecture in jawless vertebrate lymphocytes, comparable to jawed species, is not well defined. Here, we profile the gills, intestines, and blood of the lamprey, Lampetra morii, with single-cell RNA sequencing, using a full-length transcriptome as a reference. Our findings reveal higher tissue-specific heterogeneity among T-like cells in contrast to B-like cells. Notably, we identify a unique T-like cell subtype expressing a homolog of the nonlymphoid hematopoietic growth factor receptor, MPL-like (MPL-L). These MPL-L+ T-like cells exhibit features distinct from T cells of jawed vertebrates, particularly in their elevated expression of hematopoietic genes. We further discovered that MPL-L+ VLRA+ T-like cells are widely present in the typhlosole, gill, liver, kidney, and skin of lamprey and they proliferate in response to both a T cell mitogen and recombinant human thrombopoietin. These findings provide new insights into the adaptive immune response in jawless vertebrates, shedding new light on the evolution of adaptive immunity.

Conventional natural killer cells control vascular remodeling in the uterus during pregnancy by acidifying the extracellular matrix with a2V

Vascular remodeling within the uterus immediately before and during early pregnancy increases blood flow in the fetus and prevents the development of gestational hypertension. Tissue-resident natural killer (trNK) cells secrete pro-angiogenic growth factors but are insufficient for uterine artery (UtA) remodeling in the absence of conventional natural killer (cNK) cells. Matrix metalloproteinase-9 (MMP9) is activated in acidic environments to promote UtA remodeling. We have previously shown that ATPase a2V plays a role in regulating the function of cNK cells during pregnancy. We studied the effect of a2V deletion on uterine cNK cell populations and pregnancy outcomes in VavCrea2Vfl/fl mice, where a2V is conditionally deleted in hematopoietic stem cells. Conventional NKcells were reduced but trNK cells were retained in implantation sites at gestational day 9.5, and UtA remodeling was inhibited despite no differences in concentrations of pro-angiogenic growth factors. The ratio of pro-MMP9 to total was significantly elevated in VavCrea2Vfl/fl mice, and MMP9 activity was significantly reduced. The pH of implantation sites was significantly elevated in VavCrea2Vfl/fl mice. We concluded that the role of cNK cells in the uterus is to acidify the extracellular matrix (ECM) using a2V, which activates MMP9 to degrade the ECM, release bound pro-angiogenic growth factors, and contribute to UtA remodeling. Our results are significant for the understanding of the development of gestational hypertension.

RhoA G17E/Vav1 Signaling Induces Cancer Invasion via Matrix Metalloproteinase-9 in Gastric Cancer

BACKGROUND

RAS homolog family member A (RhoA), a member of the Rho family of small GTPases, and Vav1, a guanine nucleotide exchange factor for Rho family GTPases, have been reported to activate pathways related to the actin cytoskeleton and regulation of cell shape, attachment, and motility. The interaction between these molecules in lymphoma is involved in malignant signaling, but its function in epithelial malignancy is unknown. Here, we investigated the malignant signal of mutant RhoA in gastric cancer and demonstrated the potential of RhoA G17E/Vav1 as a therapeutic target for diffuse gastric cancer.

METHODS

The RhoA mutants R5W, G17E, and Y42C were retrovirally transduced into the gastric cancer cell line MKN74. The stably transduced cells were used for morphology, proliferation, and migration/invasion assays in vitro. MKN74 cells stably transduced with ectopic wild-type RhoA and mutant RhoA (G17E) were used in a peritoneal xenograft assay.

RESULTS

The RhoA mutations G17E and Y42C induced morphological changes in MKN74. G17E induced Vav1 expression at the mRNA and protein levels and promoted the migration and invasion of MKN74. An RNA interference assay of Vav1 revealed that RhoA G17E enhanced cancer cell invasion via Vav1. Furthermore, immunoprecipitation revealed that Vav1 and RhoA G17E specifically bind and function together through matrix metalloproteinase -9. In a peritoneal xenograft model of nude mice, RhoA G17E promoted peritoneal dissemination, whereas Vav1 knockdown suppressed it.

CONCLUSION

Overall, our findings indicate that RhoA G17E is associated with Vav1 and promoted cancer invasion via matrix metalloproteinase -9 in gastric cancer cells. Thus, RhoA G17E/Vav1 signaling in diffuse gastric cancer may be a useful therapeutic target.

Regulation of eIF4E guides a unique translational program to control erythroid maturation

Translation control is essential in balancing hematopoietic precursors and differentiation; however, the mechanisms underlying this program are poorly understood. We found that the activity of the major cap-binding protein eIF4E is unexpectedly regulated in a dynamic manner throughout erythropoiesis that is uncoupled from global protein synthesis rates. Moreover, eIF4E activity directs erythroid maturation, and increased eIF4E expression maintains cells in an early erythroid state associated with a translation program driving the expression of PTPN6 and Igf2bp1. A cytosine-enriched motif in the 5' untranslated region is important for eIF4E-mediated translation specificity. Therefore, selective translation of key target genes necessary for the maintenance of early erythroid states by eIF4E highlights a unique mechanism used by hematopoietic precursors to rapidly elicit erythropoietic maturation upon need.

Vav3 Mediates Pseudomonas aeruginosa Adhesion to the Cystic Fibrosis Airway Epithelium

Pseudomonas aeruginosa (Pa) represents the leading cause of airway infection in cystic fibrosis (CF). Early airways colonization can be explained by enhanced adhesion of Pa to the respiratory epithelium. RNA sequencing (RNA-seq) on fully differentiated primary cultures of airway epithelial cells from CF and non-CF donors predict that VAV3, β1 INTEGRIN, and FIBRONECTIN genes are significantly enriched in CF. Indeed, Vav3 is apically overexpressed in CF, associates with active β1 integrin luminally exposed, and increases fibronectin deposition. These luminal microdomains, rich in fibronectin and β1 integrin and regulated by Vav3, mediate the increased Pa adhesion to the CF epithelium. Interestingly, Vav3 inhibition normalizes the CF-dependent fibronectin and β1-integrin ectopic expression, improves the CF epithelial integrity, and prevents the enhanced Pa trapping to the CF epithelium. Through its capacity to promote a luminal complex with active β1 integrin and fibronectin that favors bacteria trapping, Vav3 may represent a new target in CF.

Reactive oxygen species production in the phagosome: impact on antigen presentation in dendritic cells

SIGNIFICANCE

The NADPH oxidase 2 (NOX2) is known to play a major role in innate immunity for several decades. Phagocytic cells provide host defense by ingesting microbes and destroy them by different mechanisms, including the generation of reactive oxygen species (ROS) by NOX2, a process known as oxidative burst. The phagocytic pathway of dendritic cells (DCs), highly adapted to antigen processing, has been shown to display remarkable differences compared to other phagocytes. Contrary to macrophages and neutrophils, the main function of DC phagosomes is antigen presentation rather than pathogen killing or clearance of cell debris.

RECENT ADVANCES

In the last few years, it became clear that NOX2 is also involved in the establishment of adaptive immunity. Several studies support the idea of a relationship between antigen presentation and the level of antigen degradation, the latter one being regulated by the pH and ROS within phagosomes.

CRITICAL ISSUES

The regulation of phagosomal pH exerted by NOX2, and thereby of the efficacy of antigen cross-presentation in DCs, represents a clear illustration of how NOX2 can influence CD8(+) T lymphocyte responses. In this review, we want to put emphasis on the relationship between ROS generation and antigen processing and presentation, since there is growing evidence that the low levels of ROS generated by DCs play an important role in these processes.

FUTURE DIRECTIONS

In the next years, it will be interesting to unravel possible mechanisms involved and to find other possible connections between NOX family members and adaptive immune responses.

Rac signaling in breast cancer: a tale of GEFs and GAPs

Rac GTPases, small G-proteins widely implicated in tumorigenesis and metastasis, transduce signals from tyrosine-kinase, G-protein-coupled receptors (GPCRs), and integrins, and control a number of essential cellular functions including motility, adhesion, and proliferation. Deregulation of Rac signaling in cancer is generally a consequence of enhanced upstream inputs from tyrosine-kinase receptors, PI3K or Guanine nucleotide Exchange Factors (GEFs), or reduced Rac inactivation by GTPase Activating Proteins (GAPs). In breast cancer cells Rac1 is a downstream effector of ErbB receptors and mediates migratory responses by ErbB1/EGFR ligands such as EGF or TGFα and ErbB3 ligands such as heregulins. Recent advances in the field led to the identification of the Rac-GEF P-Rex1 as an essential mediator of Rac1 responses in breast cancer cells. P-Rex1 is activated by the PI3K product PIP3 and Gβγ subunits, and integrates signals from ErbB receptors and GPCRs. Most notably, P-Rex1 is highly overexpressed in human luminal breast tumors, particularly those expressing ErbB2 and estrogen receptor (ER). The P-Rex1/Rac signaling pathway may represent an attractive target for breast cancer therapy.

Pleiotropic contributions of phospholipase C-gamma1 (PLC-gamma1) to T-cell antigen receptor-mediated signaling: reconstitution studies of a PLC-gamma1-deficient Jurkat T-cell line

Phospholipase C-gamma1 (PLC-gamma1) plays a crucial role in the coupling of T-cell antigen receptor (TCR) ligation to interleukin-2 (IL-2) gene expression in activated T lymphocytes. In this study, we have isolated and characterized two novel, PLC-gamma1-deficient sublines derived from the Jurkat T-leukemic cell line. The P98 subline displays a >90% reduction in PLC-gamma1 expression, while the J.gamma1 subline contains no detectable PLC-gamma1 protein. The lack of PLC-gamma1 expression in J.gamma1 cells caused profound defects in TCR-dependent Ca(2+) mobilization and NFAT activation. In contrast, both of these responses occurred at normal levels in PLC-gamma1-deficient P98 cells. Unexpectedly, the P98 cells displayed significant and selective defects in the activation of both the composite CD28 response element (RE/AP) and the full-length IL-2 promoter following costimulation with anti-TCR antibodies and phorbol ester. These transcriptional defects were reversed by transfection of P98 cells with a wild-type PLC-gamma1 expression vector but not by expression of mutated PLC-gamma1 constructs that lacked a functional, carboxyl-terminal SH2 [SH2(C)] domain or the major Tyr(783) phosphorylation site. On the other hand, the amino-terminal SH2 [SH2(N)] domain was not essential for reconstitution of RE/AP- or IL-2 promoter-dependent transcription but was required for the association of PLC-gamma1 with LAT, as well as the tyrosine phosphorylation of PLC-gamma1 itself, in activated P98 cells. These studies demonstrate that the PLC-gamma1 SH2(N) and SH2(C) domains play functionally distinct roles during TCR-mediated signaling and identify a non-Ca(2+)-related signaling function linked to the SH2(C) domain, which couples TCR plus phorbol ester-CD28 costimulation to the activation of the IL-2 promoter in T lymphocytes.

Proliferative involvement of ENX-1, a putative human polycomb group gene, in haematopoietic cells

Homeobox genes have important roles in haematopoiesis and are regulated in an activated state by the trithorax group (trxG) of genes. In a repressed state, they are regulated by the Polycomb group (PcG) of genes. ENX-1, a putative human PcG gene product, interacts with the proto-oncogene product Vav. We report an investigation of the role of ENX-1 in human haematopoiesis. CD34+ cells mobilized to peripheral blood strongly expressed ENX-1. When stimulated to proliferate, both T and B lymphocytes rapidly up-regulated ENX-1. ENX-1 was expressed in all cell lines of the various lineages examined. When HL-60 cells were differentiated to mature granulocytes with all-trans retinoic acid, ENX-1 was down-regulated. Moreover, ENX-1 antisense oligodeoxynucleotide suppressed DNA synthesis in HL-60 cells. Our data indicate that ENX-1 is involved in the proliferation of both normal and malignant haematopoietic cells.

Oligonucleotide therapeutics for hematologic disorders

During the last decade, the catalogue of known genes responsible for cell growth, development, and neoplastic transformation has expanded dramatically. Attempts to translate this information into new therapeutic strategies for both hematologic and non-hematologic diseases have accelerated at a rapid pace as well. Inserting genes into cells which either replace, or counter the effects of disease causing genes has been one of the primary ways in which scientists have tried to exploit this new knowledge. Strategies to directly downregulate gene expression have developed in parallel with this approach. The latter include triple helix forming oligonucleotides (ODN) and 'antisense' ODN. The latter have already entered clinical trials for a variety of disorders. In this monograph, we review the use of these materials in the treatment of hematologic diseases, particularly myelogenous leukemias. Problems and possible solutions associated with the use of ODN will be discussed as well.

Age-sensitive and -insensitive pathways leading to JNK activation in mouse CD4(+) T-cells

The c-Jun N-terminal kinase (JNK) can be activated in T-cells either by the combination of TCR and CD28 costimulation or by a variety of stress-related stimuli including UV light, H(2)O(2), and hyperosmolar sorbitol solutions. In T-lymphocytes, TCR/CD28 stimulation of JNK leads to induction of new gene expression via c-Jun, ATF-2, and Elk-1. Phosphorylation of c-Jun in CD4(+) T-cells stimulated by CD3/CD4/CD28 cross-linking declines with age, due to diminished activation of JNK. Here we show that the age-related decline in TCR/CD28 activation of JNK reflects two effects of age: the accumulation of memory cells (in which JNK stimulation is poor regardless of donor age) and age-dependent declines in JNK activation within the naive subset. Cyclosporin A inhibits induction of JNK function by TCR/CD28, PMA/ionomycin, ceramide, or H(2)O(2), but not induction by UV light or hyperosmolar sorbitol. Although aging impairs JNK induction by UV light, it has no effect on JNK activation by ceramide, H(2)O(2), or sorbitol. The data as a whole indicate that there are at least four pathways that activate JNK in CD4(+) T-cells, of which two are age-sensitive and two others unaffected by aging. Two of the pathways (UV and hyperosmolar sorbitol) are insensitive to cyclosporin inhibition. Finally, we show that the alterations in JNK function are not due to changes in the expression of MKK4, an upstream activator of JNK, and that another JNK kinase, MKK7, is not expressed in splenic T-cells.

Integrin-dependent tyrosine phosphorylation and growth regulation by Vav

The proto-oncogene product p95Vav (Vav) undergoes rapid phosphorylation on tyrosine following stimulation of the T or B cell antigen receptor, and in response to a variety of other cell surface stimuli. Vav contains, among other, a guanine nucleotide exchange factor domain with homology to the Rho/Rac/CDC42 exchange protein Db1. It has been recently shown that Vav is functionally linked to small GTPases of the Rho family, suggesting that it is an activator of Rho GTPases and may participate in regulation of cytoskeletal organization. The present study shows that cell adhesion to fibronectin triggers rapid phosphorylation of Vav on tyrosine in Vav-transfected CHO cells and in Jurkat T cells. Vav phosphorylation is strongly dependent on adhesion and is mediated by beta 1 integrins. Furthermore, Vav overexpression enhances the adhesion-dependent increase in the rate and extent of phosphorylation on focal adhesion kinase and paxillin, and the formation of stress fibers and lamellipodia. In addition, there is a marked increase in the amount of Vav localized to the triton-insoluble fraction following 1 h of incubation on FN. Finally, Vav increases the growth rate of the cells in an adhesion-dependent manner. Our results strongly implicate Vav as a mediator of integrin signal transduction.

Age-related impairment of p56lck and ZAP-70 activities in human T lymphocytes activated through the TcR/CD3 complex

Cellular immune responses decrease with aging. Lymphocytes of aged individuals do not perform as well as cells from young subjects in a number of in vitro assays including cell proliferation, cytokine production, and protection against apoptosis. Here, we have tested the hypothesis that a decrease in T cell responses in tymphocytes from elderly subjects could parallel a decrease in the activity of protein tyrosine kinases (PTK) associated with signal transduction in T lymphocytes. We report that anti-CD3-triggered T lymphocyte proliferation was significantly decreased in T lymphocytes from elderly subjects, but the decrease was not due to an alteration of the percentage or mean fluorescence intensities of CD3, CD4, and CD45. Of significance, the activities of p56lck and ZAP-70 in vitro were significantly decreased in T lymphocytes from elderly subjects compared to young individuals. However, the level of expression of the two kinases did not change with aging. The activity of p59fyn did not show changes with aging, suggesting that p59fyn did not compensate for the decreased activity of p56lck. We also found that the extent of tyrosine phosphorylation of the adaptor protein p95vav was similar in activated T lymphocytes from elderly and young subjects. Our results suggest that the altered cellular immune responses observed in T lymphocytes with aging may be the result, at least in part, of an alteration in early events associated with signal transduction through the TcR/CD3 complex that translates into decreased activities of p56lck and ZAP-70. Impairment in the activities of these twokey components of T cell signaling may contribute to reduced immune functions associated with aging.

Role for the Rac1 Exchange Factor Vav in the Signaling Pathways Leading to NK Cell Cytotoxicity

Here we investigate the activation of and a possible role for the hematopoietic Rac1 exchange factor, Vav, in the signaling mechanisms leading to NK cell-mediated cytotoxicity. Our data show that direct contact of NK cells with a panel of sensitive tumor targets leads to a rapid and transient tyrosine phosphorylation of Vav and to its association with tyrosine-phosphorylated Syk. Vav tyrosine phosphorylation is also observed following the activation of NK cells through the low-affinity Fc receptor for IgG (FcγRIII). In addition, we demonstrate that both direct and Ab-mediated NK cell binding to target cells result in the activation of nucleotide exchange on endogenous Rac1. Furthermore, Vav antisense oligodeoxynucleotide treatment leads to an impairment of NK cytotoxicity, with FcγRIII-mediated killing being more sensitive to the abrogation of Vav expression. These results provide new insight into the signaling pathways leading to cytotoxic effector function and define a role for Vav in the activation of NK cell-mediated killing.

Monitoring antisense oligodeoxynucleotide activity in hematopoietic cells

Traditionally, methods designed to impair translation through direct interactions with target messenger RNA (mRNA) have been designated as "antisense" strategies because of their reliance on the formation of reverse complementary (antisense) Watson-Crick base pairs between the targeting oligodeoxynucleotide (ODN) and the mRNA whose function is to be disrupted. Proof of putative "antisense effects," and other mechanistic studies, would be greatly facilitated by the ability to directly demonstrate hybridization between an antisense (AS) ODN and its mRNA target in vivo. In addition, evidence of AS activity by demonstrating reduced levels of RNA or protein or by showing cleaved target molecules would lend proof of the concept. In this article we discuss how AS ODN may be used to down-regulate target gene expression with an emphasis on those targets chosen for our investigations, and we summarize the methods employed for this type of study.

Fyn and ZAP-70 are required for Vav phosphorylation in T cells stimulated by antigen-presenting cells

In T cells, triggering of the T cell antigen receptor or of the co-stimulatory receptor CD28 can direct tyrosine phosphorylation of the signaling protein Vav. We investigated the role played by the protein tyrosine kinases Fyn, Lck, and ZAP-70 in these processes in a T cell hybridoma after physiological stimulation of the T cell receptor (TCR) and CD28. A dominant-negative mutant approach based on overexpression of catalytically inactive alleles of these kinases showed that CD28-induced Vav phosphorylation preferentially requires Fyn, whereas ZAP-70 had no role. Consistently, Vav was strongly phosphorylated in Lck-deficient JCAM-1 cells after CD28 ligation. In contrast, ZAP-70 appeared to control TCR-directed Vav phosphorylation. However, overexpression of ZAP-70 carrying a mutated Tyr315, contained within a motif previously suggested to be a Vav Src homology 2 domain binding site, had little or no effect. Immunoprecipitation assays showed that phosphorylated Vav associated with Fyn after CD28 triggering and that this interaction, likely to involve binding of Fyn Src homology 2 domain to Vav, was more strongly detectable after concomitant CD28 and TCR stimulation. These data suggest that Fyn plays a major role in controlling Vav phosphorylation upon T cell activation and that the mechanism implicating ZAP-70 in this process may be more complex than previously anticipated.

Transcriptional Regulation of vav, a Gene Expressed Throughout the Hematopoietic Compartment

The vav gene is expressed in all hematopoietic but few other cell types. To explore its unusual compartment-wide regulation, we cloned the murine gene, sequenced its promoter region, identified DNase I hypersensitive (HS) sites in the chromatin, and tested their promoter activity with a β-galactosidase (β-gal) reporter gene in cell lines and transgenic mice. Whereas fibroblasts had no HS sites, a myeloid and an erythroid cell line contained five, located 0.2 kb (HS1), 1.9 kb (HS2), and 3.6 kb (HS3) upstream from the transcription start and 0.6 kb (HS4) and 10 kb (HS5) downstream. A vav DNA fragment including HS1 promoted β-gal expression in a myeloid but not a fibroblast line. Expression in leukocytes of transgenic mice also required HS2 and HS5. Only hematopoietic organs contained β-gal, but virtually all β-gal+ cells were B or T lymphocytes. Expression was always variegated (mosaic), and the proportion of β-gal+ cells declined with lymphoid maturation and animal age. Thus, these vav regulatory elements promoted hematopoietic-specific expression in vivo, at least in lymphocytes, but the transgene was sporadically silenced. Maintaining pan-hematopoietic expression may require additional vavelements or an alternative reporter.

Transcriptional Regulation of vav, a Gene Expressed Throughout the Hematopoietic Compartment

The vav gene is expressed in all hematopoietic but few other cell types. To explore its unusual compartment-wide regulation, we cloned the murine gene, sequenced its promoter region, identified DNase I hypersensitive (HS) sites in the chromatin, and tested their promoter activity with a β-galactosidase (β-gal) reporter gene in cell lines and transgenic mice. Whereas fibroblasts had no HS sites, a myeloid and an erythroid cell line contained five, located 0.2 kb (HS1), 1.9 kb (HS2), and 3.6 kb (HS3) upstream from the transcription start and 0.6 kb (HS4) and 10 kb (HS5) downstream. A vav DNA fragment including HS1 promoted β-gal expression in a myeloid but not a fibroblast line. Expression in leukocytes of transgenic mice also required HS2 and HS5. Only hematopoietic organs contained β-gal, but virtually all β-gal+ cells were B or T lymphocytes. Expression was always variegated (mosaic), and the proportion of β-gal+ cells declined with lymphoid maturation and animal age. Thus, these vav regulatory elements promoted hematopoietic-specific expression in vivo, at least in lymphocytes, but the transgene was sporadically silenced. Maintaining pan-hematopoietic expression may require additional vavelements or an alternative reporter.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.