Endosomal GPR65 signaling in fibroblast-like synoviocytes promotes inflammatory cytokine release and nociceptive neuron sensitization

GPR65 is a proton-sensing G protein-coupled receptor implicated in inflammatory pain. In fibroblast-like synoviocytes (FLS), GPR65 activation promotes the release of proinflammatory cytokines capable of sensitizing sensory neurons. Following stimulation by protons, the synthetic agonist BTB09089, and the glycosphingolipid psychosine GPR65 undergoes internalization; however, the contribution of this trafficking to downstream signaling remains unclear. Using heterologous cell systems, the molecular mechanisms governing GPR65 internalization were first defined. Pharmacological and genetic inhibition of internalization revealed that intracellular trafficking is required for activation of extracellular-signal-related kinase (ERK) in the nucleus and transcriptional responses, indicating a spatially restricted signaling program originating from endosomes. The physiological relevance of this pathway was then examined in primary mouse FLS. Inhibition of endogenous GPR65 internalization reduced the ability of the conditioned media from BTB09089 stimulated FLS to sensitize dorsal root ganglia sensory neurons, thus linking receptor trafficking to pro-nociceptive function. Together these findings identify receptor internalization as a key determinant of nuclear ERK signaling and transcription downstream of GPR65 and demonstrate that endosomal signaling is required for pro-nociceptive activity of GPR65 in FLS.