Nonpeptidergic MrgprD-expressing neurons maintain cutaneous homeostasis via glutamate-mediated mast cell suppression

Cutaneous mast cells (MC) mediate numerous skin inflammatory processes and have anatomical and functional associations with sensory afferent neurons. We found that Langerhans cell (LC)-deficient mice have reduced numbers of MrgprD-expressing epidermal nerve endings and manifest enhanced irritant dermatitis due to exaggerated MC degranulation. Ablation of LC or MrgprD-expressing neurons increased expression of a MC gene module including the activating receptor, Mrgprb2, resulting in increased MC degranulation and cutaneous inflammation in multiple models. ß-alanine agonism of MrgprD-expressing neurons reduced expression of MC module genes and suppressed MC responses. MrgprD-expressing neurons released glutamate which was increased by MrgprD agonism and decreased in LC-deficient mice. Inhibiting glutamate release or glutamate receptor binding yielded hyperresponsive MC and a genomic state similar to that in mice lacking MrgprD-expressing neurons. These data demonstrate that MrgprD-expressing neurons suppress MC hyperresponsiveness and skin inflammation via glutamate release thereby revealing an unexpected neuro-immune mechanism maintaining cutaneous immune homeostasis. Overall design: For whole skin tissue RNAseq, mRNA was isolated from whole ear of DT treated MrgprdDTR mice, DT treated control MrgprdCre mice and WT mice treated with 50 mM ß-ala or vehicle. For purified mast cell RNAseq, mRNA was extracted from sorted mast cells of DT treated MrgprdDTR and control MrgprdCre mice, or from peritoneal-derived mast cells cultured with 4 mM glutamate or 50 uM glutamate receptor anatognist NS102.