Endogenous Mas-related G-protein-coupled receptor X1 activates and sensitizes TRPA1 in a human model of peripheral nerves

Mas-related G-protein-coupled receptor X1 (MrgprX1) is a human-specific Mrgpr and its expression is restricted to primary sensory neurons. However, its role in nociception and pain signaling pathways is largely unknown. This study aims to investigate a role for MrgprX1 in nociception via interaction with the pain receptor, Transient Receptor Potential Ankyrin 1 (TRPA1), using in-vitro and in-vivo human neuronal models. MrgprX1 protein expression in human trigeminal nociceptors was investigated by the immunolabeling of the dental pulp and cultured peripheral neuronal equivalent (PNE) cells. MrgprX1 receptor signaling was monitored by Fura-2-based Ca2+ imaging using PNEs and membrane potential responses were measured using FluoVoltTM. Immunofluorescent staining revealed MrgprX1 expression in-vivo in dental afferents, which was more intense in inflamed compared to healthy dental pulps. Endogenous MrgprX1 protein expression was confirmed in the in-vitro human PNE model. MrgprX1 receptor signaling and the mechanisms through which it couples to TRPA1 were studied by Ca2+ imaging. Results showed that MrgprX1 activates TRPA1 and induces membrane depolarization in a TRPA1 dependent manner. In addition, MrgprX1 sensitizes TRPA1 to agonist stimulation via Protein Kinase C (PKC). The activation and sensitization of TRPA1 by MrgprX1 in a model of human nerves suggests an important role for this receptor in the modulation of nociception.