Succinate/IL-1β Signaling Axis Promotes the Inflammatory Progression of Endothelial and Exacerbates Atherosclerosis

Inflammation is an important driver of atherosclerosis. Succinate is a new extracellular inflammatory alarm released by activated macrophages. Succinate is sensed by succinate receptor 1 (Sucnr1) and then transferred to effector cells. It is worth exploring whether succinate is capable of facilitating the inflammatory response in atherosclerosis. In this study, we firstly found that arterial serum of Coronary Heart Disease (CHD) patients contained significantly higher succinate and interleukin (IL)-1β than Health control (HC) subjects, and succinate was positively correlated with IL-1β. As demonstrated by the in vitro study, succinate/hypoxia-inducible factor 1α (Hif)-1α/IL-1β signal axis existed and significantly facilitated the inflammatory program in human umbilical vein endothelial cells (HUVECs). Under the coculture, activated macrophages released succinate, which would be transferred to HUVECs via Sucnr1 and then activate Hif-1α to produce a greater amount of IL-1β. Likewise, the aortic sinus’s inflammatory phenotype was found to be more significant within Apoe^-/- mice that were injected with succinate. Furthermore, Sucnr1 inhibitor (NF-56-EJ40) could significantly interrupt succinate/IL-1β signal in HUVECs and macrophages. As revealed by this study, glycolytic metabolism following the release of succinate could be found in atherosclerotic pathology, and succinate would drive succinate/IL-1β signal dependent on Sucnr1 and then exacerbate inflammatory responses. Sucnr1 might be a novel target for cutting off the transduction of succinate signal to prevent the inflammation of atherosclerosis.

Succinate receptor deficiency attenuates arthritis by reducing dendritic cell traffic and expansion of Th17 cells in the lymph nodes

Rheumatoid arthritis is a chronic inflammatory disease that leads to significant changes in metabolic activity. Succinate, an intermediate of the tricarboxylic acid cycle, has emerged as a metabolic mediator of the innate immune response. However, the involvement of succinate in the generation of the adaptive immune response and establishment of autoimmune response has not been addressed thus far. Here we demonstrated that the succinate-sensing receptor (Sucnr1/GPR91) plays a critical role in the development of immune-mediated arthritis. We found that Sucnr1 acts as a chemotactic gradient sensor that guides dendritic cells (DCs) into the lymph nodes, orchestrating the expansion of the T helper (Th)17-cell population and the development of experimental antigen-induced arthritis. Sucnr1-/- mice show reduced articular hyperalgesia, neutrophil infiltration and inflammatory cytokines in the joint, and reduced frequency of Th17 cells in draining lymph nodes. Adoptive transfer of wild-type (WT) DCs into Sucnr1-/- mice restored the development of arthritis. Moreover, DC-depleted mice transferred with Sucnr1-/- DCs developed less arthritis than mice transferred with WT DCs. In contrast, succinate given together with the immunization boosted the recruitment of DCs and the frequency of Th17 cells in draining lymph nodes, increasing arthritis severity. Therefore, the blockade of Sucnr1 may represent a novel therapeutic target of arthritis.-Saraiva, A. L., Veras, F. P., Peres, R. S., Talbot, J., de Lima, K. A., Luiz, J. P., Carballido, J. M., Cunha, T. M., Cunha, F. Q., Ryffel, B., Alves-Filho, J. C. Succinate receptor deficiency attenuates arthritis by reducing dendritic cell traffic and expansion of Th17 cells in the lymph nodes.

Activation of intestinal tuft cell-expressed Sucnr1 triggers type 2 immunity in the mouse small intestine

The hallmark features of type 2 mucosal immunity include intestinal tuft and goblet cell expansion initiated by tuft cell activation. How infectious agents that induce type 2 mucosal immunity are detected by tuft cells is unknown. Published microarray analysis suggested that succinate receptor 1 (Sucnr1) is specifically expressed in tuft cells. Thus, we hypothesized that the succinate-Sucnr1 axis may be utilized by tuft cells to detect certain infectious agents. Here we confirmed that Sucnr1 is specifically expressed in intestinal tuft cells but not in other types of intestinal epithelial cells, and demonstrated that dietary succinate induces tuft and goblet cell hyperplasia via Sucnr1 and the tuft cell-expressed chemosensory signaling elements gustducin and Trpm5. Conventional mice with a genetic Sucnr1 deficiency (Sucnr1^-/-) showed diminished immune responses to treatment with polyethylene glycol and streptomycin, which are known to enhance microbiota-derived succinate, but responded normally to inoculation with the parasitic worm Nippostrongylus brasiliensis that also produces succinate. Thus, Sucnr1 is required for microbiota-induced but not for a generalized worm-induced type 2 immunity.