Phospholipase D1 regulates lymphocyte adhesion via upregulation of Rap1 at the plasma membrane

Inhibition of phospholipase D1 induces immunogenic cell death and potentiates cancer immunotherapy in colorectal cancer

Phospholipase D (PLD) is a potential therapeutic target against cancer. However, the contribution of PLD inhibition to the antitumor response remains unknown. We developed a potent and selective PLD1 inhibitor based on computer-aided drug design. The inhibitor enhanced apoptosis in colorectal cancer (CRC) cells but not in normal colonic cells, and in vitro cardiotoxicity was not observed. The inhibitor downregulated the Wnt/β-catenin signaling pathway and reduced the migration, invasion, and self-renewal capacity of CRC cells. In cancer, therapeutic engagement of immunogenic cell death (ICD) leads to more effective responses by eliciting the antitumor immunity of T cells. The CRC cells treated with the inhibitor showed hallmarks of ICD, including downregulation of “do not eat-me” signals (CD24, CD47, programmed cell death ligand 1 [PD-L1]), upregulation of “eat-me” signal (calreticulin), release of high-mobility group Box 1, and ATP. PLD1 inhibition subsequently enhanced the phagocytosis of cancer cells by macrophages through the surface expression of costimulatory molecules; as a result, the cancer cells were more susceptible to cytotoxic T-cell-mediated killing. Moreover, PLD1 inhibition attenuated colitis-associated CRC and orthotopically injected tumors, probably by controlling multiple pathways, including Wnt signaling, phagocytosis checkpoints, and immune signaling. Furthermore, combination therapy with a PLD1 inhibitor and an anti-PD-L1 antibody further enhanced tumor regression via immune activation in the tumor environment. Collectively, in this study, PLD1 was identified as a critical regulator of the tumor microenvironment in colorectal cancer, suggesting the potential of PLD1 inhibitors for cancer immunotherapy based on ICD and immune activation. PLD1 inhibitors may act as promising immune modulators in antitumor treatment via ICD.

A novel drug that can inhibit an enzyme involved in colorectal cancer progression shows promise in trials on mouse models. The phospholipase D1 (PLD1) enzyme reinforces a critical signaling pathway that promotes cancer progression and drug resistance. Using computer-aided drug design, South Korean researchers led by Do Sik Min and Gyoonhee Han at Yonsei University in Incheon and Seoul, respectively, have developed a drug that specifically binds to and inhibits PLD1. In trials, the researchers observed downregulation of PLD1’s associated signaling pathway, and reductions in the ability of colorectal cancer cells to migrate, invade and replicate. The drug suppressed the cancer cells’ “don’t-eat-me” signals and upregulated “eat-me” signals, triggering improved responses from the immune system. The drug was even more effective when used in combination with an immunotherapy agent.

BCALM (AC099524.1) Is a Human B Lymphocyte-Specific Long Noncoding RNA That Modulates B Cell Receptor-Mediated Calcium Signaling

Of the thousands of long noncoding RNAs (lncRNA) identified in lymphocytes, very few have defined functions. In this study, we report the discovery and functional elucidation of a human B cell-specific lncRNA with high levels of expression in three types of B cell cancer and normal B cells. The AC099524.1 gene is upstream of the gene encoding the B cell-specific phospholipase C γ 2 (PLCG2), a B cell-specific enzyme that stimulates intracellular Ca2+ signaling in response to BCR activation. AC099524.1 (B cell-associated lncRNA modulator of BCR-mediated Ca+ signaling [BCALM]) transcripts are localized in the cytoplasm and, as expected, CRISPR/Cas9 knockout of AC099524.1 did not affect PLCG2 mRNA or protein expression. lncRNA interactome, RNA immunoprecipitation, and coimmunoprecipitation studies identified BCALM-interacting proteins in B cells, including phospholipase D 1 (PLD1), and kinase adaptor proteins AKAP9 (AKAP450) and AKAP13 (AKAP-Lbc). These two AKAP proteins form signaling complexes containing protein kinases A and C, which phosphorylate and activate PLD1 to produce phosphatidic acid (PA). BCR stimulation of BCALM-deficient B cells resulted in decreased PLD1 phosphorylation and increased intracellular Ca+ flux relative to wild-type cells. These results suggest that BCALM promotes negative feedback that downmodulates BCR-mediated Ca+ signaling by promoting phosphorylation of PLD1 by AKAP-associated kinases, enhancing production of PA. PA activates SHP-1, which negatively regulates BCR signaling. We propose the name BCALM for B-Cell Associated LncRNA Modulator of BCR-mediated Ca+ signaling. Our findings suggest a new, to our knowledge, paradigm for lncRNA-mediated modulation of lymphocyte activation and signaling, with implications for B cell immune response and BCR-dependent cancers.

Roles for Phospholipase D1 in the Tumor Microenvironment

The lipid-modifying signal transduction enzyme phospholipase D (PLD) has been proposed to have roles in oncogenic processes for well-on 30 years, with most of the early literature focused on potential functions for PLD in the biology of the tumor cells themselves. While such roles remain under investigation, evidence has also now been generated to support additional roles for PLD, in particular PLD1, in the tumor microenvironment, including effects on neoangiogenesis, the supply of nutrients, interactions of platelets with circulating cancer cells, the response of the immune system, and exosome biology. Here, we review these lines of investigation, accompanied by a discussion of the limitations of the existing studies and some cautionary notes regarding the study and interpretation of PLD function using model systems.

Beyond migration-Chemokines in lymphocyte priming, differentiation, and modulating effector functions

Chemokines and their receptors coordinate the positioning of leukocytes, and lymphocytes in particular, in space and time. Discrete lymphocyte subsets, depending on their activation and differentiation status, express various sets of chemokine receptors to be recruited to distinct tissues. Thus, the network of chemokines and their receptors ensures the correct localization of specialized lymphocyte subsets within the appropriate microenvironment enabling them to search for cognate antigens, to become activated, and to fulfill their effector functions. The chemokine system therefore is vital for the initiation as well as the regulation of immune responses to protect the body from pathogens while maintaining tolerance towards self. Besides the well investigated function of orchestrating directed cell migration, chemokines additionally act on lymphocytes in multiple ways to shape immune responses. In this review, we highlight and discuss the role of chemokines and chemokine receptors in controlling cell-to-cell contacts required for lymphocyte arrest on endothelial cells and immunological synapse formation, in lymphocyte priming and differentiation, survival, as well as in modulating effector functions.

The tyrosine phosphatase SHP-1 promotes T cell adhesion by activating the adaptor protein CrkII in the immunological synapse

The adaptor protein CrkII regulates T cell adhesion by recruiting the guanine nucleotide exchange factor C3G, an activator of Rap1. Subsequently, Rap1 stimulates the integrin LFA-1, which leads to T cell adhesion and interaction with antigen-presenting cells (APCs). The adhesion of T cells to APCs is critical for their proper function and education. The interface between the T cell and the APC is known as the immunological synapse. It is characterized by the specific organization of proteins that can be divided into central supramolecular activation clusters (c-SMACs) and peripheral SMACs (p-SMACs). Through total internal reflection fluorescence (TIRF) microscopy and experiments with supported lipid bilayers, we determined that activated Rap1 was recruited to the immunological synapse and localized to the p-SMAC. C3G and the active (dephosphorylated) form of CrkII also localized to the same compartment. In contrast, inactive (phosphorylated) CrkII was confined to the c-SMAC. Activation of CrkII and its subsequent movement from the c-SMAC to the p-SMAC depended on the phosphatase SHP-1, which acted downstream of the T cell receptor. In the p-SMAC, CrkII recruited C3G, which led to Rap1 activation and LFA-1-mediated adhesion of T cells to APCs. Functionally, SHP-1 was necessary for both the adhesion and migration of T cells. Together, these data highlight a signaling pathway in which SHP-1 acts through CrkII to reshape the pattern of Rap1 activation in the immunological synapse.

PLCε1 regulates SDF-1α-induced lymphocyte adhesion and migration to sites of inflammation

Regulation of integrins is critical for lymphocyte adhesion to endothelium and migration throughout the body. Inside-out signaling to integrins is mediated by the small GTPase Ras-proximate-1 (Rap1). Using an RNA-mediated interference screen, we identified phospholipase Cε 1 (PLCε1) as a crucial regulator of stromal cell-derived factor 1 alpha (SDF-1α)-induced Rap1 activation. We have shown that SDF-1α-induced activation of Rap1 is transient in comparison with the sustained level following cross-linking of the antigen receptor. We identified that PLCε1 was necessary for SDF-1α-induced adhesion using shear stress, cell morphology alterations, and crawling on intercellular adhesion molecule 1 (ICAM-1)-expressing cells. Structure-function experiments to separate the dual-enzymatic function of PLCε1 uncover necessary contributions of the CDC25, Pleckstrin homology, and Ras-associating domains, but not phospholipase activity, to this pathway. In the mouse model of delayed type hypersensitivity, we have shown an essential role for PLCε1 in T-cell migration to inflamed skin, but not for cytokine secretion and proliferation in regional lymph nodes. Our results reveal a signaling pathway where SDF-1α induces T-cell adhesion through activation of PLCε1, suggesting that PLCε1 is a specific potential target in treating conditions involving migration of T cells to inflamed organs.

The Rap1-RIAM-talin axis of integrin activation and blood cell function

Integrin adhesion receptors mediate the adhesion of blood cells, such as leukocytes, to other cells, such as endothelial cells. Integrins also are critical for anchorage of hematopoietic precursors to the extracellular matrix. Blood cells can dynamically regulate the affinities of integrins for their ligands ("activation"), an event central to their functions. Here we review recent progress in understanding the mechanisms of integrin activation with a focus on the functions of blood cells. We discuss how talin binding to the integrin β cytoplasmic domain, in conjunction with the plasma membrane, induces long-range allosteric rearrangements that lead to integrin activation. Second, we review our understanding of how signaling events, particularly those involving Rap1 small guanosine triphosphate (GTP)hydrolases, can regulate the talin-integrin interaction and resulting activation. Third, we review recent findings that highlight the role of the Rap1-GTP-interacting adapter molecule (RIAM), encoded by the APBB1IP gene, in leukocyte integrin activation and consequently in leukocyte trafficking.

Analysis of Programmed Death-1 in Patients with Psoriatic Arthritis

Programmed death-1 (PD-1) is an inhibitory co-receptor that is highly expressed in T lymphocytes that has been shown to downregulate inflammatory responses in several inflammatory diseases including systemic lupus erythematosus and rheumatoid arthritis. Yet, the role of PD-1 in psoriatic arthritis (PsA) has not been studied. In order to fill this gap, we measured the expression levels of PD-1 in peripheral T cells from patients with active disease. Twenty patients and fifteen age-matched healthy control subjects were recruited. The percentage of CD3(+)PD-1(+) T cells was measured by flow cytometry. Despite normal concentration of peripheral T cells, the expression levels of PD-1 were significantly higher in patients compared to healthy controls. Interestingly, among the patients, the expression levels inversely correlated with disease activity measured by disease activity scores (DAS28). PD-1 expression levels strongly correlated with the number of tender and swollen joints, but not with C-reactive protein (CRP) levels or psoriasis area and severity index (PASI). Functionally, in vitro ligation of PD-1 receptor in PsA T cells inhibited interleukin-2 (IL-2) secretion, Akt phosphorylation, and Rap1 activation. These findings suggest that PD-1 might serve as a biomarker for disease activity in PsA and highlight the need for additional studies in order to establish the role of PD-1 in PsA pathogenesis.

R-Ras Regulates Murine T Cell Migration and Intercellular Adhesion Molecule-1 Binding

The trafficking of T-lymphocytes to peripheral draining lymph nodes is crucial for mounting an adaptive immune response. The role of chemokines in the activation of integrins via Ras-related small GTPases has been well established. R-Ras is a member of the Ras-subfamily of small guanosine-5’-triphosphate-binding proteins and its role in T cell trafficking has been investigated in R-Ras null mice (Rras^−/−). An examination of the lymphoid organs of Rras^−/− mice revealed a 40% reduction in the cellularity of the peripheral lymph nodes. Morphologically, the high endothelial venules of Rras^−/− mice were more disorganized and less mature than those of wild-type mice. Furthermore, CD4⁺ and CD8⁺ T cells from Rras^−/− mice had approximately 42% lower surface expression of L-selectin/CD62L. These aberrant peripheral lymph node phenotypes were associated with proliferative and trafficking defects in Rras^−/− T cells. Furthermore, R-Ras could be activated by the chemokine, CCL21. Indeed, Rras^−/− T cells had approximately 14.5% attenuation in binding to intercellular adhesion molecule 1 upon CCL21 stimulation. Finally, in a graft-versus host disease model, recipient mice that were transfused with Rras^−/− T cells showed a significant reduction in disease severity when compared with mice transplanted with wild-type T cells. These findings implicate a role for R-Ras in T cell trafficking in the high endothelial venules during an effective immune response.

T Lymphocyte Migration: An Action Movie Starring the Actin and Associated Actors

The actin cytoskeleton is composed of a dynamic filament meshwork that builds the architecture of the cell to sustain its fundamental properties. This physical structure is characterized by a continuous remodeling, which allows cells to accomplish complex motility steps such as directed migration, crossing of biological barriers, and interaction with other cells. T lymphocytes excel in these motility steps to ensure their immune surveillance duties. In particular, actin cytoskeleton remodeling is a key to facilitate the journey of T lymphocytes through distinct tissue environments and to tune their stop and go behavior during the scanning of antigen-presenting cells. The molecular mechanisms controlling actin cytoskeleton remodeling during T lymphocyte motility have been only partially unraveled, since the function of many actin regulators has not yet been assessed in these cells. Our review aims to integrate the current knowledge into a comprehensive picture of how the actin cytoskeleton drives T lymphocyte migration. We will present the molecular actors that control actin cytoskeleton remodeling, as well as their role in the different T lymphocyte motile steps. We will also highlight which challenges remain to be addressed experimentally and which approaches appear promising to tackle them.

Phospholipase D1 Couples CD4+ T Cell Activation to c-Myc-Dependent Deoxyribonucleotide Pool Expansion and HIV-1 Replication

Quiescent CD4+ T cells restrict human immunodeficiency virus type 1 (HIV-1) infection at early steps of virus replication. Low levels of both deoxyribonucleotide triphosphates (dNTPs) and the biosynthetic enzymes required for their de novo synthesis provide one barrier to infection. CD4+ T cell activation induces metabolic reprogramming that reverses this block and facilitates HIV-1 replication. Here, we show that phospholipase D1 (PLD1) links T cell activation signals to increased HIV-1 permissivity by triggering a c-Myc-dependent transcriptional program that coordinates glucose uptake and nucleotide biosynthesis. Decreasing PLD1 activity pharmacologically or by RNA interference diminished c-Myc-dependent expression during T cell activation at the RNA and protein levels. PLD1 inhibition of HIV-1 infection was partially rescued by adding exogenous deoxyribonucleosides that bypass the need for de novo dNTP synthesis. Moreover, the data indicate that low dNTP levels that impact HIV-1 restriction involve decreased synthesis, and not only increased catabolism of these nucleotides. These findings uncover a unique mechanism of action for PLD1 inhibitors and support their further development as part of a therapeutic combination for HIV-1 and other viral infections dependent on host nucleotide biosynthesis.

The phospholipase D superfamily as therapeutic targets

The phospholipase D (PLD) lipid-signaling enzyme superfamily has long been studied for its roles in cell communication and a wide range of cell biological processes. With the advent of loss-of-function genetic mouse models that have revealed that PLD1 and PLD2 ablation is overtly tolerable, small-molecule PLD1/2 inhibitors that do not cause unacceptable clinical toxicity, a PLD2 polymorphism that has been linked to altered physiology, and growing delineation of processes that are subtly altered in mice lacking PLD1/2 activity, the stage is being set for assessment of PLD1/2 inhibition for therapeutic purposes. Based on findings to date, PLD1/2 inhibition may be of more utility in acute rather than chronic settings, although this generalization will depend on the specific risks and benefits in each disease setting.

CD28 inhibits T cell adhesion by recruiting CAPRI to the plasma membrane

CD28 is a coreceptor expressed on T lymphocytes. Signaling downstream of CD28 promotes multiple T cell functions such as proliferation, survival, and cytokine secretion. Adhesion to APCs is another function of T cells; however, little is known with regard to the role of CD28 in this process. Our previous studies have shown that CD28 inhibits T cell adhesion, but the underlying mechanism that mediates this process remains unknown. In the present study we discovered that signaling downstream of CD28 resulted in inhibition of Rap1 activity and decreased LFA-1-mediated adhesion. We showed that this was regulated by the recruitment of calcium-promoted Ras inactivator (CAPRI), a GTPase-activating protein, to the plasma membrane downstream of CD28 signaling. CAPRI trafficking to the plasma membrane was secondary to calcium influx and was mediated by its C2A and C2B domains. We conclude that CD28 inhibits Rap1-mediated adhesion by recruiting the protein CAPRI to the plasma membrane.

Cellular and physiological roles for phospholipase D1 in cancer

Phospholipase D enzymes have long been proposed to play multiple cell biological roles in cancer. With the generation of phospholipase D1 (PLD1)-deficient mice and the development of small molecule PLD-specific inhibitors, in vivo roles for PLD1 in cancer are now being defined, both in the tumor cells and in the tumor environment. We review here tools now used to explore in vivo roles for PLD1 in cancer and summarize recent findings regarding functions in angiogenesis and metastasis.

Static adhesion assay for the study of integrin activation in T lymphocytes

T lymphocyte adhesion is required for multiple T cell functions, including migration to sites of inflammation and formation of immunological synapses with antigen presenting cells. T cells accomplish regulated adhesion by controlling the adhesive properties of integrins, a class of cell adhesion molecules consisting of heterodimeric pairs of transmembrane proteins that interact with target molecules on partner cells or extracellular matrix. The most prominent T cell integrin is lymphocyte function associated antigen (LFA)-1, composed of subunits αL and β2, whose target is the intracellular adhesion molecule (ICAM)-1. The ability of a T cell to control adhesion derives from the ability to regulate the affinity states of individual integrins. Inside-out signaling describes the process whereby signals inside a cell cause the external domains of integrins to assume an activated state. Much of our knowledge of these complex phenomena is based on mechanistic studies performed in simplified in vitro model systems. The T lymphocyte adhesion assay described here is an excellent tool that allows T cells to adhere to target molecules, under static conditions, and then utilizes a fluorescent plate reader to quantify adhesiveness. This assay has been useful in defining adhesion-stimulatory or inhibitory substances that act on lymphocytes, as well as characterizing the signaling events involved. Although described here for LFA-1 - ICAM-1 mediated adhesion; this assay can be readily adapted to allow for the study of other adhesive interactions (e.g. VLA-4 - fibronectin).

JAK tyrosine kinases promote hierarchical activation of Rho and Rap modules of integrin activation

Lymphocyte recruitment is regulated by signaling modules based on the activity of Rho and Rap small guanosine triphosphatases that control integrin activation by chemokines. We show that Janus kinase (JAK) protein tyrosine kinases control chemokine-induced LFA-1- and VLA-4-mediated adhesion as well as human T lymphocyte homing to secondary lymphoid organs. JAK2 and JAK3 isoforms, but not JAK1, mediate CXCL12-induced LFA-1 triggering to a high affinity state. Signal transduction analysis showed that chemokine-induced activation of the Rho module of LFA-1 affinity triggering is dependent on JAK activity, with VAV1 mediating Rho activation by JAKs in a Gαi-independent manner. Furthermore, activation of Rap1A by chemokines is also dependent on JAK2 and JAK3 activity. Importantly, activation of Rap1A by JAKs is mediated by RhoA and PLD1, thus establishing Rap1A as a downstream effector of the Rho module. Thus, JAK tyrosine kinases control integrin activation and dependent lymphocyte trafficking by bridging chemokine receptors to the concurrent and hierarchical activation of the Rho and Rap modules of integrin activation.

Cytotoxic-T-lymphocyte antigen 4 receptor signaling for lymphocyte adhesion is mediated by C3G and Rap1

T-lymphocyte adhesion plays a critical role in both inflammatory and autoimmune responses. The small GTPase Rap1 is the key coordinator mediating T-cell adhesion to endothelial cells, antigen-presenting cells, and virus-infected cells. We describe a signaling pathway, downstream of the cytotoxic T-lymphocyte antigen 4 (CTLA-4) receptor, leading to Rap1-mediated adhesion. We identified a role for the Rap1 guanine nucleotide exchange factor C3G in the regulation of T-cell adhesion and showed that this factor is required for both T-cell receptor (TCR)-mediated and CTLA-4-mediated T-cell adhesion. Our data indicated that C3G translocates to the plasma membrane downstream of TCR signaling, where it regulates activation of Rap1. We also showed that CTLA-4 receptor signaling mediates tyrosine phosphorylation in the C3G protein, and that this is required for augmented activation of Rap1 and increased adhesion mediated by leukocyte function-associated antigen type 1 (LFA-1). Zap70 is required for C3G translocation to the plasma membrane, whereas the Src family member Hck facilitates C3G phosphorylation. These findings point to C3G and Hck as promising potential therapeutic targets for the treatment of T-cell-dependent autoimmune disorders.

Temporal production of the signaling lipid phosphatidic acid by phospholipase D2 determines the output of extracellular signal-regulated kinase signaling in cancer cells

The Ras-extracellular signal-regulated kinase (ERK) cascade is an important signaling module in cells. One regulator of the Ras-ERK cascade is phosphatidic acid (PA) generated by phospholipase D (PLD) and diacylglycerol kinase (DGK). Using a newly developed PA biosensor, PASS (phosphatidic acid biosensor with superior sensitivity), we found that PA was generated sequentially by PLD and DGK in epidermal growth factor (EGF)-stimulated HCC1806 breast cancer cells. Inhibition of PLD2, one of the two PLD members, was sufficient to eliminate most of the PA production, whereas inhibition of DGK decreased PA production only at the later stages of EGF stimulation, suggesting that PLD2 precedes DGK activation. The temporal production of PA by PLD2 is important for the nuclear activation of ERK. While inhibition of both PLD and DGK had no effect on the overall ERK activity, inhibition of PLD2 but not PLD1 or DGK blocked the nuclear ERK activity in several cancer cell lines. The decrease of active ERK in the nucleus inhibited the activation of Elk1, c-fos, and Fra1, the ERK nuclear targets, leading to decreased proliferation of HCC1806 cells. Together, these findings reveal that PA production by PLD2 determines the output of ERK in cancer cell growth factor signaling.

Chemokine unresponsiveness of chronic lymphocytic leukemia cells results from impaired endosomal recycling of Rap1 and is associated with a distinctive type of immunological anergy

Trafficking of malignant lymphocytes is fundamental to the biology of chronic lymphocytic leukemia (CLL). Transendothelial migration (TEM) of normal lymphocytes into lymph nodes requires the chemokine-induced activation of Rap1 and αLβ2 integrin. However, in most cases of CLL, Rap1 is refractory to chemokine stimulation, resulting in failed αLβ2 activation and TEM unless α4β1 is coexpressed. In this study, we show that the inability of CXCL12 to induce Rap1 GTP loading in CLL cells results from failure of Rap1-containing endosomes to translocate to the plasma membrane. Furthermore, failure of chemokine-induced Rap1 translocation/GTP loading was associated with a specific pattern of cellular IgD distribution resembling that observed in normal B cells anergized by DNA-based Ags. Anergic features and chemokine unresponsiveness could be simultaneously reversed by culturing CLL cells ex vivo, suggesting that these two features are coupled and driven by stimuli present in the in vivo microenvironment. Finally, we show that failure of Rap1 translocation/GTP loading is linked to defective activation of phospholipase D1 and its upstream activator Arf1. Taken together, our findings indicate that chemokine unresponsiveness in CLL lymphocytes results from failure of Arf1/phospholipase D1-mediated translocation of Rap1 to the plasma membrane for GTP loading and may be a specific feature of anergy induced by DNA Ags.

Spatial control of Epac2 activity by cAMP and Ca2+-mediated activation of Ras in pancreatic β cells

The cAMP (adenosine 3',5'-monophosphate)-activated guanine nucleotide exchange factor (GEF) Epac2 is an important mediator of cAMP-dependent processes in multiple cell types. We used real-time confocal and total internal reflection fluorescence microscopy to examine the spatiotemporal regulation of Epac2, which is a GEF for the guanosine triphosphatase (GTPase) Rap. We demonstrated that increases in the concentration of cAMP triggered the translocation of Epac2 from the cytoplasm to the plasma membrane in insulin-secreting β cells. Glucose-induced oscillations of the submembrane concentration of cAMP were associated with cyclic translocation of Epac2, and this translocation could be amplified by increases in the cytoplasmic Ca(2+) concentration. Analyses of Epac2 mutants identified the high-affinity cAMP-binding and the Ras association domains as crucial for the translocation. Expression of a dominant-negative Ras mutant reduced Epac2 translocation, and Ca(2+)-dependent oscillations in Ras activity synchronized with Epac2 translocation in single β cells. The cyclic translocation of Epac2 was accompanied by oscillations of Rap GTPase activity at the plasma membrane, and expression of an inactive Rap1B mutant decreased insulin secretion. Thus, Epac2 localization is dynamically controlled by cAMP as well as by Ca(2+)-mediated activation of Ras. These results help to explain how oscillating signals can produce pulses of insulin release from pancreatic β cells.

Reciprocal Negative Regulation between the Guanine Nucleotide Exchange Factor C3G and β-Catenin

The guanine nucleotide exchange factor C3G (RAPGEF1) regulates proliferation, migration, and differentiation of cells and is essential for mammalian embryonic development. The molecular effectors of C3G dependent functions are poorly understood. Here we report that C3G functions as a negative regulator of β-catenin, a major player in pathways known to be deregulated in human cancers. In mammalian cells, C3G is present in a complex with cellular β-catenin. The proline rich Crk binding region of C3G and residues 90-525 of β-catenin are sufficient for the interaction. Knockdown of cellular C3G stimulated, and its overexpression repressed, β-catenin/TCF transcription activity. C3G acts by destabilizing β-catenin protein and inhibiting its nuclear accumulation. Nuclear extracts of C3G overexpressing cells showed reduced binding to TCF consensus oligos. C3G exerts its effects independent of its function as an exchange factor. It also inhibits stability and activity of an N-terminal deletion construct of β-catenin that is not subject to GSK3β dependent phosphorylation, suggesting that C3G exerts its effect independent of GSK3β. β-catenin repression by C3G was not significantly altered in the presence of proteasome inhibitors, MG132 or lactacystin, suggesting that alternate mechanisms are engaged by C3G to cause β-catenin turnover. C3G expression represses β-catenin target gene expression, and stable clones of MCF-7 breast cancer cells expressing C3G showed reduced migration. Activation of cellular β-catenin or expression of constitutively active β-catenin resulted in reduced C3G expression, indicating that C3G gene expression is negatively regulated by β-catenin. Our results identify a novel property of C3G in functioning as a negative regulator of β-catenin signaling by promoting its degradation. In addition, we show that β-catenin inhibits C3G expression, forming a feedback loop.

A role for Rap2 in recycling the extended conformation of LFA-1 during T cell migration

T lymphocytes make use of their major integrin LFA-1 to migrate on surfaces that express ICAM-1 such as blood vessels and inflamed tissue sites. How the adhesions are turned over in order to supply traction for this migration has not been extensively investigated. By following the fate of biotinylated membrane LFA-1 on T lymphocytes, we show in this study that LFA-1 internalization and re-exposure on the plasma membrane are linked to migration. Previously we demonstrated the GTPase Rap2 to be a regulator of LFA-1-mediated migration. SiRNA knockdown of this GTPase inhibits both LFA-1 internalization and also its ability to be re-exposed, indicating that Rap2 participates in recycling of LFA-1 and influences its complete endocytosis-exocytosis cycle. Confocal microscopy images reveal that the intracellular distribution of Rap2 overlaps with endosomal recycling vesicles. Although the homologous GTPase Rap1 is also found on intracellular vesicles and associated with LFA-1 activation, these two homologous GTPases do not co-localize. Little is known about the conformation of the LFA-1 that is recycled. We show that the extended form of LFA-1 is internalized and in Rap2 siRNA-treated T lymphocytes the trafficking of this LFA-1 conformation is disrupted resulting in its intracellular accumulation. Thus LFA-1-mediated migration of T lymphocytes requires Rap2-expressing vesicles to recycle the extended form of LFA-1 that we have previously found to control migration at the leading edge.

Rap1-interacting adapter molecule (RIAM) associates with the plasma membrane via a proximity detector

The Ras association and PH domains of RIAM function as a proximity detector for activated Rap1 and PI(4,5)P₂.

Adaptive immunity depends on lymphocyte adhesion that is mediated by the integrin lymphocyte functional antigen 1 (LFA-1). The small guanosine triphosphatase Rap1 regulates LFA-1 adhesiveness through one of its effectors, Rap1-interacting adapter molecule (RIAM). We show that RIAM was recruited to the lymphocyte plasma membrane (PM) through its Ras association (RA) and pleckstrin homology (PH) domains, both of which were required for lymphocyte adhesion. The N terminus of RIAM inhibited membrane translocation. In vitro, the RA domain bound both Rap1 and H-Ras with equal but relatively low affinity, whereas in vivo only Rap1 was required for PM association. The PH domain bound phosphoinositol 4,5-bisphosphate (PI(4,5)P₂) and was responsible for the spatial distribution of RIAM only at the PM of activated T cells. We determined the crystal structure of the RA and PH domains and found that, despite an intervening linker of 50 aa, the two domains were integrated into a single structural unit, which was critical for proper localization to the PM. Thus, the RA-PH domains of RIAM function as a proximity detector for activated Rap1 and PI(4,5)P₂.

The B subunits of Shiga-like toxins induce regulated VWF secretion in a phospholipase D1-dependent manner

Shiga toxin (Stx) causes diarrhea-associated hemolytic uremic syndrome by damaging renal microvascular endothelium. The pentameric B subunits of Stx types 1 and 2 (Stx1B and Stx2B) are sufficient to stimulate acute VWF secretion from endothelial cells, but Stx1B and Stx2B exert distinct effects on Ca(2+) and cAMP pathways. Therefore, we investigated other signaling components in StxB-induced VWF exocytosis. Incubation of HUVECs with StxB transiently increased phospholipase D (PLD) activity. Inhibition of PLD activity or shRNA-mediated PLD1 knockdown abolished StxB-induced VWF secretion. In addition, treatment with StxB triggered actin polymerization, enhanced endothelial monolayer permeability, and activated RhoA. PLD activation and VWF secretion induced by Stx1B were abolished on protein kinase Cα (PKCα) inhibition or gene silencing but were only moderately reduced by Rho or Rho kinase inhibitors. Conversely, PLD activation and VWF exocytosis induced by Stx2B were reduced by Rho/Rho kinase inhibitors and dominant-negative RhoA, whereas attenuation of PKCα did not affect either process. Another PLD1 activator, ADP-ribosylation factor 6, was involved in VWF secretion induced by Stx1B or Stx2B, but not histamine. These data indicate that Stx1B and Stx2B induce acute VWF secretion in a PLD1-dependent manner but do so by differentially modulating PKCα, RhoA, and ADP-ribosylation factor 6.

The Gαq/11 proteins contribute to T lymphocyte migration by promoting turnover of integrin LFA-1 through recycling

The role of Gαi proteins coupled to chemokine receptors in directed migration of immune cells is well understood. In this study we show that the separate class of Gαq/11 proteins is required for the underlying ability of T cells to migrate both randomly and in a directed chemokine-dependent manner. Interfering with Gαq or Gα11 using dominant negative cDNA constructs or siRNA for Gαq causes accumulation of LFA-1 adhesions and stalled migration. Gαq/11 has an impact on LFA-1 expression at plasma membrane level and also on its internalization. Additionally Gαq co-localizes with LFA-1- and EEA1-expressing intracellular vesicles and partially with Rap1- but not Rab11-expressing vesicles. However the influence of Gαq is not confined to the vesicles that express it, as its reduction alters intracellular trafficking of other vesicles involved in recycling. In summary vesicle-associated Gαq/11 is required for the turnover of LFA-1 adhesion that is necessary for migration. These G proteins participate directly in the initial phase of recycling and this has an impact on later stages of the endo-exocytic pathway.

Chemokines and the signaling modules regulating integrin affinity

Integrin-mediated adhesion is a general concept referring to a series of adhesive phenomena including tethering–rolling, affinity, valency, and binding stabilization altogether controlling cell avidity (adhesiveness) for the substrate. Arrest chemokines modulate each aspect of integrin activation, although integrin affinity regulation has been recognized as the prominent event in rapid leukocyte arrest induced by chemokines. A variety of inside-out and outside-in signaling mechanisms have been related to the process of integrin-mediated adhesion in different cellular models, but only few of them have been clearly contextualized to rapid integrin affinity modulation by arrest chemokines in primary leukocytes. Complex signaling processes triggered by arrest chemokines and controlling leukocyte integrin activation have been described for ras-related rap and for rho-related small GTPases. We summarize the role of rap and rho small GTPases in the regulation of rapid integrin affinity in primary leukocytes and provide a modular view of these pro-adhesive signaling events. A potential, albeit still speculative, mechanism of rho-mediated regulation of cytoskeletal proteins controlling the last step of integrin activation is also discussed. We also discuss data suggesting a functional integration between the rho- and rap-modules of integrin activation. Finally we examine the universality of signaling mechanisms regulating integrin triggering by arrest chemokines.

Distinct roles for talin-1 and kindlin-3 in LFA-1 extension and affinity regulation

In inflammation, neutrophils and other leukocytes roll along the microvascular endothelium before arresting and transmigrating into inflamed tissues. Arrest requires conformational activation of the integrin lymphocyte function-associated antigen-1 (LFA-1). Mutations of the FERMT3 gene encoding kindlin-3 underlie the human immune deficiency known as leukocyte adhesion deficiency-III. Both kindlin-3 and talin-1, another FERM domain-containing cytoskeletal protein, are required for integrin activation, but their individual roles in the induction of specific integrin conformers are unclear. Here, we induce differential LFA-1 activation in neutrophils through engagement of the selectin ligand P-selectin glycoprotein ligand-1 or the chemokine receptor CXCR2. We find that talin-1 is required for inducing LFA-1 extension, which corresponds to intermediate affinity and induces neutrophil slow rolling, whereas both talin-1 and kindlin-3 are required for induction of the high-affinity conformation of LFA-1 with an open headpiece, which results in neutrophil arrest. In vivo, both slow rolling and arrest are defective in talin-1-deficient neutrophils, whereas only arrest is defective in kindlin-3-deficient neutrophils. We conclude that talin-1 and kindlin-3 serve distinct functions in LFA-1 activation.

Overview of integrin signaling in the immune system

It has been well established that integrins mediate cell-cell and cell-matrix adhesion and play crucial roles in the immune system such as leukocyte-endothelium interactions, immune synapse formation, and effector functions. Since the discovery that integrins undergo dynamic changes of adhesive activities in response to external stimuli, intensive studies have been conducted to elucidate the signaling events that control the activation of integrins (inside-out signaling) and signaling events from the induced integrin-dependent adhesion (outside-in signaling). The molecular characterization of these signaling pathways highlights the importance of integrins as bidirectional signaling receptors. The characteristics of integrin signaling are best exemplified in the immune system. This chapter highlights the recent studies of intracellular signaling pathways that regulate integrins in immunological contexts.

Insights into the PX (phox-homology) domain and SNX (sorting nexin) protein families: structures, functions and roles in disease

The mammalian genome encodes 49 proteins that possess a PX (phox-homology) domain, responsible for membrane attachment to organelles of the secretory and endocytic system via binding of phosphoinositide lipids. The PX domain proteins, most of which are classified as SNXs (sorting nexins), constitute an extremely diverse family of molecules that play varied roles in membrane trafficking, cell signalling, membrane remodelling and organelle motility. In the present review, we present an overview of the family, incorporating recent functional and structural insights, and propose an updated classification of the proteins into distinct subfamilies on the basis of these insights. Almost all PX domain proteins bind PtdIns3P and are recruited to early endosomal membranes. Although other specificities and localizations have been reported for a select few family members, the molecular basis for binding to other lipids is still not clear. The PX domain is also emerging as an important protein–protein interaction domain, binding endocytic and exocytic machinery, transmembrane proteins and many other molecules. A comprehensive survey of the molecular interactions governed by PX proteins highlights the functional diversity of the family as trafficking cargo adaptors and membrane-associated scaffolds regulating cell signalling. Finally, we examine the mounting evidence linking PX proteins to different disorders, in particular focusing on their emerging importance in both pathogen invasion and amyloid production in Alzheimer's disease.

The exquisite regulation of PLD2 by a wealth of interacting proteins: S6K, Grb2, Sos, WASp and Rac2 (and a surprise discovery: PLD2 is a GEF)

Phospholipase D (PLD) catalyzes the conversion of the membrane phospholipid phosphatidylcholine to choline and phosphatidic acid (PA). PLD's mission in the cell is two-fold: phospholipid turnover with maintenance of the structural integrity of cellular/intracellular membranes and cell signaling through PA and its metabolites. Precisely, through its product of the reaction, PA, PLD has been implicated in a variety of physiological cellular functions, such as intracellular protein trafficking, cytoskeletal dynamics, chemotaxis of leukocytes and cell proliferation. The catalytic (HKD) and regulatory (PH and PX) domains were studied in detail in the PLD1 isoform, but PLD2 was traditionally studied in lesser detail and much less was known about its regulation. Our laboratory has been focusing on the study of PLD2 regulation in mammalian cells. Over the past few years, we have reported, in regards to the catalytic action of PLD, that PA is a chemoattractant agent that binds to and signals inside the cell through the ribosomal S6 kinases (S6K). Regarding the regulatory domains of PLD2, we have reported the discovery of the PLD2 interaction with Grb2 via Y169 in the PX domain, and further association to Sos, which results in an increase of de novo DNA synthesis and an interaction (also with Grb2) via the adjacent residue Y179, leading to the regulation of cell ruffling, chemotaxis and phagocytosis of leukocytes. We also present the complex regulation by tyrosine phosphorylation by epidermal growth factor receptor (EGF-R), Janus Kinase 3 (JAK3) and Src and the role of phosphatases. Recently, there is evidence supporting a new level of regulation of PLD2 at the PH domain, by the discovery of CRIB domains and a Rac2-PLD2 interaction that leads to a dual (positive and negative) effect on its enzymatic activity. Lastly, we review the surprising finding of PLD2 acting as a GEF. A phospholipase such as PLD that exists already in the cell membrane that acts directly on Rac allows a quick response of the cell without intermediary signaling molecules. This provides only the latest level of PLD2 regulation in a field that promises newer and exciting advances in the next few years.

Inhibition of contact sensitivity by farnesylthiosalicylic acid-amide, a potential Rap1 inhibitor

We hypothesized that Ras proximate 1 (Rap1) functions as an additional target for farnesylthiosalicylic acid (FTS) or its derivatives, and that the inhibition of Rap1 in lymphocytes by these agents may represent a method for treating inflammatory disorders. Indeed, we found that FTS-amide (FTS-A) was able to inhibit the elicitation phase of delayed cutaneous hypersensitivity in vivo. This effect was associated with the inhibition of Rap1 more than with the inhibition of Harvey rat sarcoma viral oncogene (Ras). Moreover, FTS-A inhibited Rap1 and contact sensitivity far better than FTS. We suggest that FTS-A may serve as a possible therapeutic tool in contact sensitivity in particular and T-cell-mediated inflammation in general.

Cytosolic Ras supports eye development in Drosophila

Ras proteins associate with cellular membranes as a consequence of a series of posttranslational modifications of a C-terminal CAAX sequence that include prenylation and are thought to be required for biological activity. In Drosophila melanogaster, Ras1 is required for eye development. We found that Drosophila Ras1 is inefficiently prenylated as a consequence of a lysine in the A(1) position of its CAAX sequence such that a significant pool remains soluble in the cytosol. We used mosaic analysis with a repressible cell marker (MARCM) to assess if various Ras1 transgenes could restore photoreceptor fate to eye disc cells that are null for Ras1. Surprisingly, we found that whereas Ras1 with an enhanced efficiency of membrane targeting could not rescue the Ras1 null phenotype, Ras1 that was not at all membrane targeted by virtue of a mutation of the CAAX cysteine was able to fully rescue eye development. In addition, constitutively active Ras1(12V,C186S) not targeted to membranes produced a hypermorphic phenotype and stimulated mitogen-activated protein kinase (MAPK) signaling in S2 cells. We conclude that the membrane association of Drosophila Ras1 is not required for eye development.

Dependence of phospholipase D1 multi-monoubiquitination on its enzymatic activity and palmitoylation

Phospholipase D (PLD) is an important lipase in many cellular processes, including vesicular trafficking, cell survival, and cell migration. In the present study, we show that PLD1, but not PLD2, is posttranslationally modified by multi-monoubiquitination. Intriguingly, suppression of lipase activity either by mutation of the HKD motif (PLD1 H896R, K898R, or D903A) or the phosphatidylinositol 4,5-bisphosphate binding motif (PLD1 R691G,R695G) or through use of PLD-selective inhibitors impaired the ubiquitination of PLD1, although stimulation of lipase activity by phorbol 12-myristate 13-acetate did not enhance its ubiquitination. A palmitoylation-deficient mutant PLD1 allele, which exhibits altered patterns of vesicular trafficking, had significantly lower levels of monoubiquitination. In addition, the expression of ubiquitin-fused PLD1 induced aberrantly enlarged vesicles partially co-localized with the Golgi complex but not with early endosomes. The altered localization was reduced by the K898R mutation, suggesting a role of multi-monoubiquitination in PLD1 subcellular localization. Surprisingly, the degradation of PLD1, but not of PLD1 K898R or PLD2, was blocked by inhibitors of proteasomes but not by inhibitors of lysosomes or other proteases, suggesting a role of the ubiquitination in proteasomal degradation of PLD1. In summary, our studies show that PLD1, but not PLD2, is multi-monoubiquitinated. The ubiquitination modification might represent a novel regulatory mechanism in PLD1 functioning, particularly in the context of subcellular trafficking between different membrane compartments.