Prostaglandin E2/EP4 signalling facilitates EP4 receptor externalization in primary sensory neurons in vitro and in vivo

Inflammatory pain severely affects the quality of life of millions of individuals worldwide. Prostaglandin E2 (PGE2), a pain mediator enriched in inflamed tissues, plays a pivotal role in nociceptor sensitization and in the genesis of inflammatory pain. Its EP4 receptor mainly mediates its role in inflammatory pain. However, the underlying mechanisms are poorly understood. Here we found that PGE2/EP4 signalling-induced EP4 externalization in dorsal root ganglion (DRG) neurons contributes to nociceptor sensitization and inflammatory pain. In cultured DRG neurons, PGE2 and the EP4 agonist concentration- and time-dependently stimulated EP4 externalization. The inhibitors of anterograde secretory pathway, protein synthesis, or recycling pathway suppressed PGE2-induced EP4 externalization, suggesting that EP4 retained in Golgi apparatus and in recycling endosomes, as well as newly synthesized, are mobilized in this event. Interestingly, the intracellular cAMP levels of cultured DRG explants following 2 sequential treatments with the EP4 agonist were significantly higher than a single treatment, suggesting that the first treatment of agonist likely induces EP4 export to sensitize DRG neurons. Intraplantar injection of complete Freud's adjuvant increases both total and cell-surface EP4 levels of L4-6 DRG neurons, an event suppressed by a cyclooxygenase-2 inhibitor or a selective EP4 antagonist, suggesting that PGE2/EP4 signalling in inflamed paw contributes to EP4 synthesis and export in DRG neurons, thus sensitizing nociceptors during inflammation. We conclude that PGE2/EP4 signalling-induced EP4 externalization in DRG neuron is a novel mechanism underlying nociceptor sensitization and inflammatory pain. Blocking EP4 externalization could open a novel therapeutic avenue to treat inflammatory pain.