Succinate acts as an extracellular mediator signaling through the G protein-coupled receptor GPR91. Here we show that dendritic cells had high expression of GPR91. In these cells, succinate triggered intracellular calcium mobilization, induced migratory responses and acted in synergy with Toll-like receptor ligands for the production of proinflammatory cytokines. Succinate also enhanced antigen-specific activation of human and mouse helper T cells. GPR91-deficient mice had less migration of Langerhans cells to draining lymph nodes and impaired tetanus toxoid-specific recall T cell responses. Furthermore, GPR91-deficient allografts elicited weaker transplant rejection than did the corresponding grafts from wild-type mice. Our results suggest that the succinate receptor GPR91 is involved in sensing immunological danger, which establishes a link between immunity and a metabolite of cellular respiration.

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In addition to immune and anti-inflammatory actions, IL-10 redirects macrophage cholesterol handling towards reverse cholesterol transport, which contributes to its anti-atherosclerotic action.

Objectives-Sphingosine 1-phosphate (S1P) is a bioactive phospholipid acting both as a ligand for the G protein-coupled receptors S1P 1-5 and as a second messenger. Because S1P 1 knockout is lethal in the transgenic mouse, an alternative approach to study the function of S1P 1 in endothelial cells is needed. Methods and Results-All human endothelial cells analyzed expressed abundant S1P 1 transcripts. We permanently silenced (by RNA interference) the expression of S1P 1 in the human endothelial cell lines AS-M.5 and ISO-HAS.1. The S1P 1 knock-down cells manifested a distinct morphology and showed neither actin ruffles in response to S1P nor an angiogenic reaction. In addition, these cells were more sensitive to oxidant stress-mediated injury. New S1P 1 -dependent gene targets were identified in human endothelial cells. S1P 1 silencing decreased the expression of platelet-endothelial cell adhesion molecule-1 and VE-cadherin and abolished the induction of E-selectin after cell stimulation with lipopolysaccharide or tumor necrosis factor-␣. Microarray analysis revealed downregulation of further endothelial specific transcripts after S1P 1 silencing. Conclusions-Long-term silencing of S1P 1 enabled us for the first time to demonstrate the involvement of S1P 1 in key functions of endothelial cells and to identify new S1P 1 -dependent gene targets. Key Words: S1P 1 Ⅲ functional analysis Ⅲ siRNA Ⅲ permanent inhibition Ⅲ endothelial cells T he bioactive sphingolipid metabolite, sphingosine 1-phosphate (S1P), is an important component of serum that is released primarily from activated platelets. S1P is a multifunctional physiologic mediator implicated in the regulation of a broad spectrum of biologic processes, including proliferation, survival, regulation of cytoskeletal reorganization, motility, and differentiation in many cell types. [1][2][3][4][5] The response of cells to S1P has been shown to be mediated predominantly by G protein-coupled receptors. Five receptors encoded by the endothelial differentiation gene family that bind S1P with high affinity have been described. 6 -10 The receptors couple to multiple G proteins that activate different intracellular signaling pathways. Several of these receptors are simultaneously coexpressed on the same cell.In addition to acting as a ligand of cell surface receptors, S1P can function as a second messenger. 3,4,11 A variety of external stimuli, particularly growth factors and chemoattractants, as well as lysophosphatidic acid and S1P, have been reported to strongly stimulate S1P kinase (SphK) to generate intracellular S1P, which can mobilize calcium from internal stores, 12 regulate cell survival by activating the transcription factor nuclear factor-B, 13 and control cell proliferation by mediating Ras and extracellular signal-regulated kinase 1/2 activation in cells stimulated with vascular endothelial growth factor (VEGF). 14 Stimulation of the S1P 1 receptor (previously known as endothelial differentiation gene-1, edg-1) activates a G ilinked pathway leading to cell gro...

Age-related macular degeneration (AMD) is a prominent cause of blindness in the Western world. To date, its molecular pathogenesis as well as the sequence of events leading to retinal degeneration remain largely ill-defined. While the invasion of choroidal neovasculature in the retina is the primary mechanism that precipitates loss of sight, an earlier dry form may accompany it. Here we provide the first evidence for the protective role of the Retinal Pigment Epithelium (RPE)-resident metabolite receptor, succinate receptor 1 (SUCNR1; G-Protein coupled Receptor-91 (GPR91), in preventing dry AMD-like lesions of the outer retina. Genetic analysis of 925 patients with geographic atrophy and 1199 AMD-free peers revealed an increased risk of developing geographic atrophy associated with intronic variants in the SUCNR1 gene. In mice, outer retinal expression of SUCNR1 is observed in the RPE as well as microglial cells and decreases progressively with age. Accordingly, Sucnr1−/− mice show signs of premature sub-retinal dystrophy with accumulation of oxidized-LDL, abnormal thickening of Bruch's membrane and a buildup of subretinal microglia. The accumulation of microglia in Sucnr1-deficient mice is likely triggered by the inefficient clearance of oxidized lipids by the RPE as bone marrow transfer of wild-type microglia into Sucnr1−/− mice did not salvage the patho-phenotype and systemic lipolysis was equivalent between wild-type and control mice. Our findings suggest that deficiency in SUCNR1 is a possible contributing factor to the pathogenesis of dry AMD and thus broaden our understanding of this clinically unmet need.

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