Mas-related G protein-coupled receptor D exacerbates inflammatory hyperalgesia by promoting NF-κB activation in dorsal root ganglia

Abstract

Background Mas-related G protein–coupled receptor D (MrgprD) is mainly expressed in small-diameter sensory neurons of the dorsal root ganglion (DRG). Results from previous studies suggest that MrgprD participates in mechanical allodynia and nerve injury-induced neuropathic pain. However, it remains elusive whether and how MrgprD is involved in inflammatory pain. Methods we used a mouse model of inflammatory hyperalgesia established by intraperitoneal administration of lipopolysaccharide (LPS). The generation of neuroinflammation and inflammatory hyperalgesia were determined using animal behavioral tests, Western blotting, quantitative real-time PCR and ELISA assay. The expression levels of MrgprD were assessed by Western blotting. The involvements of MrgprD in the LPS-triggered neuroinflammation and inflammation-related signaling pathways were assessed by animal behavioral tests, Western blotting, immunohistochemistry, ELISA, and co-immunoprecipitation assays. Results The LPS injection induced an evident peripheral neuroinflammation and mechanical allodynia in the mice and increased MrgprD expression in the DRG. The LPS administration also augmented the proportion of MrgprD-expressing neurons in the lumbar 4 DRG. The LPS-induced hypersensitivities to mechanical and cold stimuli, but not to a heat stimulus, were substantially attenuated in Mrgprd- knockout mice compared with wildtype littermates. Mrgprd deletion suppressed the LPS-triggered activation of the NF-κB signaling pathway and attenuated LPS-induced up-regulation of proinflammatory cytokines. Moreover, ectopic overexpression of MrgprD in HEK293 cells stably expressing mouse toll-like receptor 4 (TLR4) markedly promoted the LPS-induced NF-κB activation and enhanced NF-κB’s DNA-binding activity. Furthermore, MrgprD physically interacted with TGF-β-activated kinase 1 (TAK1) and I-kappa-B-kinase (IKK) complexes in DRGs and macrophage-like RAW 264.7 cells, and MrgprD agonist treatment was sufficient to elicit the activation of NF-κB signaling, but not of mitogen-activated protein kinases (MAPKs) in RAW 264.7 cells. Conclusions Our findings demonstrate that MrgprD exacerbates LPS-induced inflammatory hyperalgesia by promoting canonical NF-κB activation, indicating that MrgprD could be a potential therapeutic target for managing NF-κB-mediated inflammation and inflammatory pain.