GRK2-dependent S1PR1 desensitization is required for lymphocytes to overcome their attraction to blood

The integration of T cell migration, differentiation and function

T cells function locally. Accordingly, T cells' recognition of antigen, their subsequent activation and differentiation, and their role in the processes of infection control, tumour eradication, autoimmunity, allergy and alloreactivity are intrinsically coupled with migration. Recent discoveries revise our understanding of the regulation and patterns of T cell trafficking and reveal limitations in current paradigms. Here, we review classic and emerging concepts, highlight the challenge of integrating new observations with existing T cell classification schemes and summarize the heuristic framework provided by viewing T cell differentiation and function first through the prism of migration.

Sphingosine-1-phosphate receptor 1 in classical Hodgkin lymphoma: assessment of expression and role in cell migration

Classical Hodgkin lymphoma (CHL), a neoplasm of abnormal B lymphocytes (Hodgkin-Reed-Sternberg (HRS) cells), has been described to have a typical pattern of clinical presentation and dissemination often involving functionally contiguous lymph nodes. Despite the progress made in understanding CHL pathophysiology, the factors that regulate the spread of lymphoma cells in CHL are poorly understood. Sphingosine-1-phosphate (S1P), a bioactive sphingolipid present at high concentrations in the plasma and lymphatic fluid, is known to have a critical role in regulating lymphocyte trafficking mainly through sphingosine-1-phosphate receptor 1 (S1PR1). In this study, we explore the role of the S1P-S1PR1 axis in Hodgkin lymphoma cell migration and the expression of S1PR1 in CHL cell lines and clinical cases. We found that S1PR1 is present in the KM-H2 and SUP-HD1 Hodgkin lymphoma cell lines at the mRNA and protein level. In addition, functionally, S1P potently stimulated migration of both cell lines. S1P-induced migration was inhibited by the S1PR1 antagonist, VPC44116, and the S1PR1 functional antagonist, FTY720-P, but was potentiated by the S1PR2-specific antagonist, JTE013. We also determined that S1PR1 induced migration in the KM-H2 and SUP-HD1 cells via the heterotrimeric G-protein Gi and the phosphatidylinositol-3-kinase pathway. Immunohistochemical assessment of the tissue from CHL samples revealed that a subset of cases (7/57; 12%) show strong, membranous staining for S1PR1 in HRS cells. Altogether, our data indicate that S1PR1 is a functional receptor on HRS cells, which governs tumor cell migration and is expressed in a subset of CHL cases. Given the availability of S1PR1 antagonists, some of which are used clinically for modulation of the immune system, these results suggest that S1PR1 could be a future therapeutic target in the treatment of those cases of S1PR1-positive, refractory/recurrent CHL.

G-protein-coupled receptor kinase 2 and hypertension: molecular insights and pathophysiological mechanisms

Numerous factors partake in the fine-tuning of arterial blood pressure. The heptahelical G-protein-coupled receptors (GPCRs) represent one of the largest classes of cell-surface receptors. Further, ligands directed at GPCRs account for nearly 30 % of current clinical pharmaceutical agents available. Given the wide variety of GPCRs involved in blood pressure control, it is reasonable to speculate for a potential role of established intermediaries involved in the GPCR desensitization process, like the G-protein-coupled receptor kinases (GRKs), in the regulation of vascular tone. Of the seven mammalian GRKs, GRK2 seems to be the most relevant isoform at the cardiovascular level. This review attempts to assemble the currently available information concerning GRK2 and hypertension, opening new potential fields of translational investigation to treat this vexing disease.

Sphingosine-1-phosphate and its receptors: structure, signaling, and influence

The sphingosine-1-phosphate (S1P) receptor signaling system has biological and medical importance and is the first lipid G protein-coupled receptor (GPCR) structure to be solved to 2.8-Å resolution. S1P binds to five high-affinity GPCRs generating multiple downstream signals that play essential roles in vascular development and endothelial integrity, control of cardiac rhythm, and routine oral treatment of multiple sclerosis. Genetics, chemistry, and now structural biology have advanced this integrated biochemical system. The S1P receptors have a novel N-terminal fold that occludes access to the binding pocket from the extracellular environment as well as orthosteric and bitopic ligands with very different physicochemical properties. S1P receptors and metabolizing enzymes have been deleted, inducibly deleted, and knocked in as tagged or altered receptors in mice. An array of genetic models allows analysis of integrated receptor function in vivo. We can now directly understand causal relationships among protein expression, signal, and control points in physiology and pathology.

Phospholipids: "greasing the wheels" of humoral immunity

Phospholipids are major structural components of all cellular membranes. In addition, certain phospholipids execute regulatory activities that affect cell behavior, function and fate in critically important physiological settings. The influence of phospholipids is especially obvious in the adaptive immune system, where these macromolecules mediate both intrinsic and extrinsic effects on B and T lymphocytes. This review article highlights the action of lysophospholipid sphingosine-1-phosphate as a lymphocyte chemoattractant, the function of phosphatidylinositol phosphates as signaling conduits in lymphocytes and the role of phospholipids as raw materials for membrane assembly and organelle biogenesis in activated B lymphocytes. Special emphasis is placed on the means by which these three processes push humoral immune responses forward. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Impaired T-cell responses to sphingosine-1-phosphate in HIV-1 infected lymph nodes

The determinants of HIV-1-associated lymphadenopathy are poorly understood. We hypothesized that lymphocytes could be sequestered in the HIV-1+ lymph node (LN) through impairments in sphingosine-1-phosphate (S1P) responsiveness. To test this hypothesis, we developed novel assays for S1P-induced Akt phosphorylation and actin polymerization. In the HIV-1+ LN, naïve CD4 T cells and central memory CD4 and CD8 T cells had impaired Akt phosphorylation in response to S1P, whereas actin polymerization responses to S1P were impaired dramatically in all LN maturation subsets. These defects were improved with antiretroviral therapy. LN T cells expressing CD69 were unable to respond to S1P in either assay, yet impaired S1P responses were also seen in HIV-1+ LN T cells lacking CD69 expression. Microbial elements, HIV-1, and interferon α - putative drivers of HIV-1 associated immune activation all tended to increase CD69 expression and reduce T-cell responses to S1P in vitro. Impairment in T-cell egress from lymph nodes through decreased S1P responsiveness may contribute to HIV-1-associated LN enlargement and to immune dysregulation in a key organ of immune homeostasis.

Visualization of splenic marginal zone B-cell shuttling and follicular B-cell egress

The splenic marginal zone is a unique microenvironment where resident immune cells are exposed to the open blood circulation. Even though it has an important role in responses against blood-borne antigens, lymphocyte migration in the marginal zone has not been intravitally visualized due to challenges associated with achieving adequate imaging depth in this abdominal organ. Here we develop a two-photon microscopy procedure to study marginal zone and follicular B-cell movement in the live mouse spleen. We show that marginal zone B cells are highly motile and exhibit long membrane extensions. Marginal zone B cells shuttle between the marginal zone and follicles with at least one-fifth of the cells exchanging between compartments per hour, a behaviour that explains their ability to deliver antigens rapidly from the open blood circulation to the secluded follicles. Follicular B cells also transit from follicles to the marginal zone, but unlike marginal zone B cells, they fail to undergo integrin-mediated adhesion, become caught in fluid flow and are carried into the red pulp. Follicular B-cell egress via the marginal zone is sphingosine-1-phosphate receptor-1 (S1PR1)-dependent. This study shows that marginal zone B cells migrate continually between marginal zone and follicles and establishes the marginal zone as a site of S1PR1-dependent B-cell exit from follicles. The results also show how adhesive differences of similar cells critically influence their behaviour in the same microenvironment.

Primary human endothelial cells secrete agents that reduce responsiveness to lysophosphatidic acid (LPA)

The plasma level of LPA (lysophosphatidic acid) (200–600 nM) is well within the range that promotes proliferation and migration of vascular ECs (endothelial cells), yet vessels are quiescent and stable. In this report, we considered one explanation for this paradox: that ECs secrete agents that attenuate responsiveness to LPA. Indeed, we observed that CM (conditioned medium) from confluent, quiescent cultures of primary HUVECs (human umbilical vein ECs) contained an agent that inhibited LPA-mediated signalling events and cellular responses. The putative inhibitor, which we tentatively call ILMR (inhibitor of LPA-mediated responsiveness) seemed to act on cells (instead of at the level of LPA) by suppressing the ability of LPA receptor 1 to signal. The amount and/or activity of ILMR was regulated by growth factors; exposing HUVECs to VEGF-A (vascular endothelial growth factor A), but not bFGF (basic fibroblast growth factor), reduced the amount and/or activity of ILMR in CM. We conclude that in addition to promoting angiogenesis directly, VEGF-A can also act indirectly by modulating the bioactivity of angiomodulators such as LPA.

The interplay between G protein-coupled receptor kinase 2 (GRK2) and histone deacetylase 6 (HDAC6) at the crossroads of epithelial cell motility

G protein-coupled receptor kinase 2 (GRK2) is emerging as a key integrative node in cell migration control. In addition to its canonical role in the desensitization of G protein-coupled receptors involved in chemotaxis, novel recently identified GRK2 substrates and interacting partners appear to mediate the GRK2-dependent modulation of diverse molecular processes involved in motility, such as gradient sensing, cell polarity or cytoskeletal reorganization. We have recently identified an interaction between GRK2 and histone deacetylase 6 (HDAC6), a major cytoplasmic α-tubulin deacetylase involved in cell motility and adhesion. GRK2 dynamically associates with and phosphorylates HDAC6 to stimulate its α-tubulin deacetylase activity at specific cellular localizations such as the leading edge of migrating cells, thus promoting local tubulin deacetylation and enhanced motility. This GRK2-HDAC6 functional interaction may have important implications in pathological contexts related to aberrant epithelial cell migration.

The role of sphingosine 1-phosphate in immunity and sepsis

Sphingosine 1-phosphate (S1P) is a lipid metabolite with intra- and extracellular signalling properties. It activates five G protein-coupled cell surface receptors designated S1P-receptors type 1-5 (S1P1-5) that transmit extracellular signals into cells, and it modulates intracellular signalling as a cofactor. The analysis of sphingosine kinases (SphK) type 1 and 2, the key enzymes for S1P production, in different infection models point to an important role for the activation of different immune cells like macrophages, mast cells, and dendritic cells. S1P additionally influences local and systemic lymphocyte circulation and positioning, the vascular tone, and blood pressure. Modulation of S1P-mediated signalling pathways therefore results either in local immune cell activation or systemic immune suppression, or both. Pharmacological approaches that modulate certain S1P-mediated signalling pathways while leaving others untouched appear to be promising new avenues for next generation pharmaceuticals. This review summarizes current strategies to modulate S1P signalling for immune intervention with the clear focus on the specificity of the different principles applied. Known local and systemic effects of S1P on immunity are discussed as potential pharmaceutical targets to combat immune and autoimmune diseases and sepsis.

Sphingosine kinase and sphingosine-1-phosphate receptors: novel therapeutic targets of rheumatoid arthritis?

Rheumatoid arthritis (RA) is a chronic, destructive, autoimmune joint disease characterized by elevated levels of proinflammatory cytokine production. Sphingosine kinase (SphK) phosphorylates sphingosine into sphingosine-1-phosphate. Synovial fluid of RA patients exhibits significantly higher levels of S1P than their non-inflammatory osteoarthritis counterparts. SphK blockade suppresses cytokines and MMP-9 release in RA peripheral blood mononuclear cells. In addition, downregulation of SphK1 either through a specific siRNA approach or transgenic human TNF-α SphK1-deficient mice (hTNF-α/SphK1(-/-)) exhibit significantly less synovial inflammation and joint pathology. By contrast, SphK2 modulation leads to disease exacerbation. These results clearly demonstrate that such anti- and proinflammatory potential of SphK1/2 modulation may alter the outcome in RA synovitis and raises the possibility that drugs that specifically target SphK1 activity may play a beneficial role in the treatment of RA and other autoimmune rheumatic diseases.

Chemokine-triggered leukocyte arrest: force-regulated bi-directional integrin activation in quantal adhesive contacts

The arrest of rolling leukocytes on target vascular beds is mediated by specialized leukocyte integrins and their endothelial ligands. In the circulation, these integrins are generally maintained as inactive 'clasped' heterodimers. Encounter by leukocytes of specialized endothelial-presented chemoattractants termed arrest chemokines drive these integrins to undergo force-regulated biochemical conformational changes in response to signals from chemokine-stimulated Gi-protein coupled receptors (GPCRs) and actin remodeling Rho GTPases. To arrest rolling leukocytes, integrin:ligand bonds must undergo stabilization by several orders of magnitude within quantal submicron contacts that consist of discrete integrin:ligand bonds. We present a unifying three step model for rapid integrin activation by chemokines in the quantal arrest unit, the smallest firm adhesive contact formed by a rolling or a captured leukocyte: integrin extension triggered by talin, integrin headpiece opening driven by surface-immobilized ligand and stabilized by low force, and full heterodimer unclasping requiring integrin tail associations with actin-connected talin and Kindlin-3. Specialized GPCRs and their Gi-protein signaling assemblies drive these and other adaptors to specifically bind integrin cytoplasmic tails possibly in conjunction with de novo actin remodeling, thereby optimizing bi-directional activation of ligand-occupied integrins.

GRK2: multiple roles beyond G protein-coupled receptor desensitization

G protein-coupled receptor kinases (GRKs) regulate numerous G protein-coupled receptors (GPCRs) by phosphorylating the intracellular domain of the active receptor, resulting in receptor desensitization and internalization. GRKs also regulate GPCR trafficking in a phosphorylation-independent manner via direct protein-protein interactions. Emerging evidence suggests that GRK2, the most widely studied member of this family of kinases, modulates multiple cellular responses in various physiological contexts by either phosphorylating non-receptor substrates or interacting directly with signaling molecules. In this review, we discuss traditional and newly discovered roles of GRK2 in receptor internalization and signaling as well as its impact on non-receptor substrates. We also discuss novel exciting roles of GRK2 in the regulation of dopamine receptor signaling and in the activation and trafficking of the atypical GPCR, Smoothened (Smo).

Sphingosine-1-phosphate and lymphocyte egress from lymphoid organs

Much has been learned about how cells enter lymphoid tissues. But how do they leave? Sphingosine-1-phosphate (S1P) has emerged over the past decade as a central mediator of lymphocyte egress. In this review, we summarize the current understanding of how S1P promotes exit from the secondary lymphoid organs and thymus. We review what is known about additional requirements for emigration and summarize the mostly distinct requirements for exit from the bone marrow. Egress from lymphoid organs is limited during immune responses, and we examine how this regulation works. There is accumulating evidence for roles of S1P in directing immune cell behavior within lymphoid tissues. How such actions can fit together with the egress-promoting role of S1P is discussed. Finally, we examine current understanding of how FTY720, a drug that targets S1P receptors and is approved for the treatment of multiple sclerosis, causes immune suppression.

Setting the clock for recirculating lymphocytes

In their search for antigens, lymphocytes continuously shuttle among blood vessels, lymph vessels, and lymphatic tissues. Chemokines mediate entry of lymphocytes into lymphatic tissues, and sphingosine 1-phosphate (S1P) promotes localization of lymphocytes to the vasculature. Both signals are sensed through G protein-coupled receptors (GPCRs). Most GPCRs undergo ligand-dependent homologous receptor desensitization, a process that decreases their signaling output after previous exposure to high ligand concentration. Such desensitization can explain why lymphocytes do not take an intermediate position between two signals but rather oscillate between them. The desensitization of S1P receptor 1 (S1PR1) is mediated by GPCR kinase 2 (GRK2). Deletion of GRK2 in lymphocytes compromises desensitization by high vascular S1P concentrations, thereby reducing responsiveness to the chemokine signal and trapping the cells in the vascular compartment. The desensitization kinetics of S1PR1 allows lymphocytes to dynamically shuttle between vasculature and lymphatic tissue, although the positional information in both compartments is static.