Activation and function of the Rap1 GTPase in B lymphocytes

Altered expression of metabolic pathways in CLL detected by unlabelled quantitative mass spectrometry analysis

Chronic lymphocytic leukaemia (CLL) remains the most common incurable malignancy of B cells in the western world. Patient outcomes are heterogeneous and can be difficult to predict with current prognostic markers. Here, we used a quantitative label-free proteomic technique to ascertain differences in the B-cell proteome from healthy donors and CLL patients with either mutated (M-CLL) or unmutated (UM-CLL) IGHV to identify new prognostic markers. In peripheral B-CLL cells, 349 (22%) proteins were differentially expressed between normal B cells and B-CLL cells and 189 (12%) were differentially expressed between M-CLL and UM-CLL. We also examined the proteome of proliferating CLL cells in the lymph nodes, and identified 76 (~8%) differentially expressed proteins between healthy and CLL lymph nodes. B-CLL cells show over-expression of proteins involved in lipid and cholesterol metabolism. A comprehensive lipidomic analysis highlighted large differences in glycolipids and sphingolipids. A shift was observed from the pro-apoptotic lipid ceramide towards the anti-apoptotic/chemoresistant lipid, glucosylceramide, which was more evident in patients with aggressive disease (UM-CLL). This study details a novel quantitative proteomic technique applied for the first time to primary patient samples in CLL and highlights that primary CLL lymphocytes display markers of a metabolic shift towards lipid synthesis and breakdown.

Short-term exposure to fine particulate air pollution and genome-wide DNA methylation: A randomized, double-blind, crossover trial

BACKGROUND

Previous studies have associated fine particulate (PM_2.5) exposure with changes in gene-specific DNA methylation. However, the evidence was still limited and inconsistent in genome-wide DNA methylation.

OBJECTIVE

To examine the impact of short-term PM_2.5 exposure on genome-wide DNA methylation.

METHODS

We designed a randomized, double-blind, crossover trial among 36 healthy young adults in Shanghai, China. A two-stage intervention with alternative use of real and sham air purifiers in dormitory rooms for consecutive 9 days were conducted to create natural low and high exposure scenarios of PM_2.5. Blood genome-wide DNA methylation was analyzed using the Illumina Infinium Human Methylation EPIC BeadChip (850k). Mixed-effect models were used to evaluate the impacts of changes in PM_2.5 levels on genome-wide DNA methylation.

RESULTS

There was a drastic contrast for PM_2.5 exposure levels in the two scenarios (24-h averages: 53.1 and 24.3 μg/m³). Between the high and low exposure groups, methylation levels were changed significantly with a false discovery rate < 0.01 at 49 CpG loci, of which 31 CpG sites were annotated to the specific genes. DNA methylation of these annotated genes were elevated in response to increased PM_2.5 exposure, which were implicated in insulin resistance, glucose and lipid metabolism, inflammation, oxidative stress, platelet activation, and cell survival and apoptosis.

CONCLUSIONS

Our results provided novel biological pathways linking ambient PM_2.5 exposure to systemic adverse response through variations in DNA methylation and reinforced the hypothesized role of epigenetics in the development of cardiometabolic diseases induced by PM_2.5 exposure.

Calcium-RasGRP2-Rap1 signaling mediates CD38-induced migration of chronic lymphocytic leukemia cells

CD38 is a transmembrane exoenzyme that is associated with poor prognosis in chronic lymphocytic leukemia (CLL). High CD38 levels in CLL cells are linked to increased cell migration, but the molecular basis is unknown. CD38 produces nicotinic acid adenine dinucleotide phosphate and adenosine 5'-diphosphate-ribose, both of which can act to increase intracellular Ca2+ levels. Here we show that CD38 expression increases basal intracellular Ca2+ levels and stimulates CLL cell migration both with and without chemokine stimulation. We find that CD38 acts via intracellular Ca2+ to increase the activity of the Ras family GTPase Rap1, which is in turn regulated by the Ca2+-sensitive Rap1 guanine-nucleotide exchange factor RasGRP2. Both Rap1 and RasGRP2 are required for CLL cell migration, and RasGRP2 is polarized in primary CLL cells with high CD38 levels. These results indicate that CD38 promotes RasGRP2/Rap1-mediated CLL cell adhesion and migration by increasing intracellular Ca2+ levels.

HMGB1 promotes cellular chemokine synthesis and potentiates mesenchymal stromal cell migration via Rap1 activation

The migration of mesenchymal stem cells (MSCs) and osteogenic differentiation occupy an important role in fracture healing. High mobility group box 1 (HMGB1), a widely distributed inflammatory factor in fractures, has been confirmed to act as a chemoattractant to MSCs. To investigate the effect of HMGB1 on MSC migration and the underlying mechanism, the synthesis of MSC chemokines, and the consequent activation of signaling pathways following HMGB1 stimulation, were evaluated. A Quantibody® array was performed to determine which chemokines were secreted from MSCs with or without treatment with HMGB1. The results indicated differential chemokine synthesis by MSCs following treatment with HMGB1, including that of CCL4 and CCL13. In addition, the Ras‑associated protein‑1 (Rap1) signaling pathway was markedly activated in the HMGB1‑treated groups, suggesting that HMGB1 may enhance the migrational ability of MSCs via Rap1 activation. Furthermore, HMGB1 was able to promote the secretion of various chemokines derived from MSCs, which would, in turn, increase the mobility of MSCs. Taken together, these results provide a mechanistic basis for developing novel approaches to promote fracture healing.

Rho and Rap guanosine triphosphatase signaling in B cells and chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) cells proliferate predominantly in niches in the lymph nodes, where signaling from the B cell receptor (BCR) and the surrounding microenvironment are critical for disease progression. In addition, leukemic cells traffic constantly from the bloodstream into the lymph nodes, migrate within lymphatic tissues and egress back to the bloodstream. These processes are driven by chemokines and their receptors, and depend on changes in cell migration and integrin-mediated adhesion. Here we describe how Rho and Rap guanosine triphosphatases (GTPases) contribute to both BCR signaling and chemokine receptor signaling, particularly by regulating cytoskeletal dynamics and integrin activity. We propose that new inhibitors of BCR-activated kinases are likely to affect CLL cell trafficking via Rho and Rap GTPases, and that upstream regulators or downstream effectors could be good targets for therapeutic intervention in CLL.

Gap junction proteins on the move: connexins, the cytoskeleton and migration

Connexin43 (Cx43) has roles in cell-cell communication as well as channel independent roles in regulating motility and migration. Loss of function approaches to decrease Cx43 protein levels in neural cells result in reduced migration of neurons during cortical development in mice and impaired glioma tumor cell migration. In other cell types, correlations between Cx43 expression and cell morphology, adhesion, motility and migration have been noted. In this review we will discuss the common themes that have been revealed by a detailed comparison of the published results of neuronal cells with that of other cell types. In brief, these comparisons clearly show differences in the stability and directionality of protrusions, polarity of movement, and migration, depending on whether a) residual Cx43 levels remain after siRNA or shRNA knockdown, b) Cx43 protein levels are not detectable as in cells from Cx43(-/-) knockout mice or in cells that normally have no endogenous Cx43 expression, c) gain-of-function approaches are used to express Cx43 in cells that have no endogenous Cx43 and, d) Cx43 is over-expressed in cells that already have low endogenous Cx43 protein levels. What is clear from our comparisons is that Cx43 expression influences the adhesiveness of cells and the directionality of cellular processes. These observations are discussed in light of the ability of cells to rearrange their cytoskeleton and move in an organized manner. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

GIMAP Proteins in T-Lymphocytes

(GIMAPs) GTPase of the immunity associated protein family are a novel protein family of putative small GTPases. GIMAPs are mainly expressed in the cells of the immune system and have been associated with immunological functions, such as thymocyte development, apoptosis of peripheral lymphocytes and T helper cell differentiation. GIMAPs have also been linked to immunological diseases, such as T cell lymphopenia, leukemia and autoimmune diseases. In this review we examine the role of GIMAP proteins in T-lymphocyte biology.

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

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

Interaction of Bcr/Abl with C3G, an exchange factor for the small GTPase Rap1, through the adapter protein Crkl

The Bcr/Abl oncoprotein is directly responsible for the development of chronic myelogenous leukemia and Philadelphia chromosome-positive acute lymphoblastic leukemia in humans. The adapter protein Crkl is one of the most prominently tyrosine-phosphorylated substrates of Bcr/Abl in cells and tissues isolated from such patients. The guanine nucleotide exchange factor for the small GTPase Rap1, C3G, binds constitutively to Crkl. Here, we report that Crkl mediates the formation of protein complexes that include C3G and Bcr/Abl. These complexes contain highly elevated levels of tyrosine-phosphorylated C3G and P130Cas, a scaffolding protein. Moreover, the presence of Rap1 further promoted tyrosine phosphorylation of C3G and Cas. Co-expression of Crkl and C3G with Bcr/Abl generated increased levels of activated Rap1. In addition, lysates from leukemic cells of P190 BCR/ABL transgenic mice and of the myelogenous leukemia cell line K562 contained tyrosine-phosphorylated C3G and activated Rap1. These data suggest a role for C3G-mediated Rap1 activation in Bcr/Abl-induced leukemia development.

Intracellular signalling controlling integrin activation in lymphocytes

Since the discovery that integrins at the surface of lymphocytes undergo dynamic changes in their adhesive activity after stimulation through the T-cell receptor or stimulation with chemokines, intensive research has been carried out in an attempt to clarify the signalling events that lead to the activation of integrins. Whereas structural studies have provided us with a vivid picture of the conformational flexibility of integrins, the signalling pathways that regulate these conformational changes (known as inside-out signalling) have been elusive. However, as I discuss here, recent studies have provided new insight into the pathways that control the regulation of integrin activity and the coordination of complex cellular functions, such as the homing of lymphocytes and the formation of an immunological synapse.

Protein kinase C and beyond

Protein kinase C molecules regulate both positive and negative signal transduction pathways essential for the initiation and homeostasis of immune responses. There are multiple isoforms of protein kinase C that are activated differently by calcium and diacylglycerol, and these are activated mainly by antigen receptors in T cells, B cells and mast cells. Additionally, mammals express several other diacylglycerol binding proteins that are linked to a network of key signal transduction pathways that control lymphocyte biology. Diacylglycerol and protein kinase C regulate a broad range of gene transcription programs but also modulate integrins, chemokine responses and antigen receptors, thereby regulating lymphocyte adhesion, migration, differentiation and proliferation.

Rap1-mediated Lymphocyte Function-associated Antigen-1 Activation by the T Cell Antigen Receptor Is Dependent on Phospholipase C-γ1

The small GTPase, Rap1, is a potent activator of leukocyte integrins and enhances the adhesive activity of lymphocyte function-associated antigen-1 (LFA-1) when stimulated by the T cell receptor (TCR) or chemokines. However, the mechanism by which Rap1 is activated remains unclear. Here, we demonstrate that phospholipase C (PLC)-gamma1 plays a critical role in the signaling pathway leading to Rap1 activation triggered by the TCR. In Jurkat T cells, TCR cross-linking triggered persistent Rap1 activation, and SDF-1 (CXCL12) activated Rap1 transiently. A phospholipase C inhibitor, U73122, abrogated Rap1 activation triggered by both the TCR and SDF-1 (CXCL12). PLC-gamma1-deficient Jurkat T cells showed a marked reduction of TCR-triggered Rap1 activation and adhesion to intercellular adhesion molecule-1 (ICAM-1) mediated by LFA-1. In contrast, SDF-1-triggered Rap1 activation and adhesion were not affected in these cells. Transfection of these cells with an expression plasmid encoding PLC-gamma1 restored Rap1 activation by the TCR and the ability to adhere to ICAM-1, accompanied by polarized LFA-1 surface clustering colocalized with regulator of adhesion and polarization enriched in lymphoid tissues (RAPL). Furthermore, when expressed in Jurkat cells, CalDAG-GEFI, a calcium and diacylglycerol-responsive Rap1 exchange factor, associated with Rap1, and resulted in enhanced Rap1 activation and adhesion triggered by the TCR. Our results demonstrate that TCR activation of Rap1 depends on PLC-gamma1. This activity is likely to be mediated by CalDAG-GEFI, which is required to activate LFA-1.

Differential regulation of NFAT and SRF by the B cell receptor via a PLCgamma-Ca(2+)-dependent pathway

NFAT and SRF are important in the regulation of proliferation and cytokine production in lymphocytes. NFAT activation by the B cell receptor (BCR) occurs via the PLCgamma-Ca(2+)-calcineurin pathway, however how the BCR activates SRF is unclear. We show here that like NFAT, BCR regulation of SRF occurs via an Src-Syk-Tec-PLCgamma-Ca(2+) (Lyn-Syk-Btk-PLCgamma-Ca(2+)) pathway. However, SRF responds to lower Ca(2+) and is less dependent on IP(3)R expression than NFAT. Ca(2+)-regulated calcineurin plays a partial role in SRF activation, in combination with diacylglycerol (DAG), while is fully required for NFAT activation. Signals from the DAG effectors protein kinase C, Ras and Rap1, and the downstream MEK-ERK pathway are required for both SRF and NFAT; however, NFAT but not SRF is dependent on JNK signals. Both SRF and NFAT were also dependent on Rac, Rho, CDC42 and actin. Finally, we show that Ca(2+) is not required for ERK activation, but instead for its association with nuclear areas of the cell. These data suggest that combinatorial assembly of signaling pathways emanating from the BCR differentially regulate NFAT and SRF, to activate gene expression.

Synergistic promotion of c-Src activation and cell migration by Cas and AND-34/BCAR3

The adapter molecule p130Cas (Cas) plays a role in cellular processes such as proliferation, survival, cell adhesion, and migration. The ability of Cas to promote migration has been shown to be dependent upon its carboxyl terminus, which contains a bipartite binding site for the protein tyrosine kinase c-Src (Src). The association between Src and Cas enhances Src kinase activity, and like Cas, Src plays an important role in cell proliferation and migration. In this study, we show that Src and Cas function cooperatively to promote cell migration in a manner that depends upon kinase-active Src. Another carboxyl-terminal binding partner of Cas, AND-34/BCAR3 (AND-34), functions synergistically with Cas to enhance Src activation and cell migration. The carboxyl-terminal guanine nucleotide exchange factor domain of AND-34, as well as the activity of its putative target Rap1, contribute to these events. A mechanism through which AND-34 may regulate Cas-dependent cell migration is suggested by the finding that Cas becomes redistributed from focal adhesions to lamellipodia located at the leading edge of AND-34 overexpressing cells. These data thus provide insight into how Cas and AND-34 may function together to stimulate Src signaling pathways and promote cell migration.

GTPases and T cell activation

Guanine nucleotide binding proteins rapidly cycle between a guanosine diphosphate (GDP)-bound and guanosine triphosphate (GTP)-bound state, and they operate as binary switches that control cell activation in response to environmental cues. GTPases adopt different conformations when binding GTP vs. GDP. The GTP-bound state is generally considered to be the active conformation that allows GTPases to interact with downstream effectors and thereby initiate downstream signaling pathways, which regulate many important biological processes. Many members of the Ras family of GTPases, notably Ras and Rap1A, and the Rho family GTPases, Cdc42Hs, Rac1, Rac2 and RhoA, are important components of signal transduction pathways used by antigen receptors, costimulatory, cytokine and chemokine receptors to regulate the immune response. This review discusses current knowledge and ideas about the regulation and function of these GTPases in lymphocytes.

The Rap GTPases regulate B cell migration toward the chemokine stromal cell-derived factor-1 (CXCL12): potential role for Rap2 in promoting B cell migration

Stromal cell-derived factor-1 (SDF-1) is a potent chemoattractant for B cells and B cell progenitors. Although the binding of SDF-1 to its receptor, CXCR4, activates multiple signaling pathways, the mechanism by which SDF-1 regulates cell migration is not completely understood. In this report we show that activation of the Rap GTPases is important for B cells to migrate toward SDF-1. We found that treating B cells with SDF-1 resulted in the rapid activation of both Rap1 and Rap2. Moreover, blocking the activation of Rap1 and Rap2 via the expression of a Rap-specific GTPase-activating protein significantly reduced the ability of B cells to migrate toward SDF-1. Conversely, expressing a constitutively active form of Rap2 increased SDF-1-induced B cell migration. Thus, the Rap GTPases control cellular processes that are important for B cells to migrate toward SDF-1.

B cell signaling. Introduction

The B cell antigen receptor (BCR) is composed of the membrane form of the immunoglobulin (Ig) and the Ig-alpha/Ig-beta heterodimer, which function as the antigen recognition component and the signaling component, respectively. A signal transmitted by BCR modulates gene expression, adhesion or survival, thereby determining the fate of antigen-encountered B cells. BCR proximal signaling occurs within cholesterol- and sphingolipid-rich plasma membrane microdomains termed lipid rafts, and involves tyrosine kinases such as Lyn, Syk and Btk and the adapter molecule SLP65/BLNK. Although the distal signaling cascades via BCR are not yet fully elucidated, various components are already identified, such as lipid kinases and small G-proteins. BCR signaling is regulated by various membrane molecules termed co-receptors such as CD19 and CD22. The BCR co-receptors appear to be required for normal immune functions. Viral proteins such as LMP2 also regulate BCR signaling to maintain viral latency. Various aspects of BCR signaling and its regulatory mechanisms are discussed in this issue.

To make antibodies or not: signaling by the B-cell antigen receptor

Antibodies produced by B cells play an essential role in protecting against disease-causing pathogens. B cells detect the presence of pathogens via B-cell antigen receptors (BCRs), which consist of a transmembrane form of an antibody that is associated with a signaling subunit. Signaling by BCRs not only initiates antibody production but also regulates B-cell development, B-cell survival and the elimination of B cells that recognize components of one's own body. Identifying the intracellular signals generated by BCRs and determining how these signals specify such diverse responses is the key to understanding how the immune system functions normally and how defects in BCR signaling can lead to either immunodeficiency diseases or autoimmune diseases.