Elucidating the underlying Rab network of MRGPRX2-mediated activation of mast cells

While best known for their role in IgE-mediated allergy, mast cells (MCs) are likewise important in IgE-independent innate immunity, which is mediated by a large group of positively charged ligands, including neuropeptides, toxins, and FDA-approved drugs. These ligands stimulate the release by exocytosis of secretory granule (SG) containing inflammatory mediators, by binding to Mas-related G protein coupled receptors (Mrgprs). Rab GTPases are master regulators of cellular trafficking. Identification of the Rab network that governs exocytosis is therefore instrumental for unveiling the intermediate steps and participating proteins along this process. We have previously identified 30 Rabs as regulators of IgE-mediated exocytosis. Here we aimed to identify the Rab network that controls the IgE-independent mode of MC activation. By screening 44 Rabs for their functional and phenotypic impacts on exocytosis triggered by compound 48/80 (c48/80), the gold standard of this pathway, and by the endogenous neuropeptide substance P (SP), we identified Rabs that affect both IgE-dependent and IgE-independent secretion as opposed to a Rab network that selectively affected the responses of MRGPRX2, the human member of the Mrgpr family. Some Rabs selectively affected the SG size in MRGPRX2-activated cells. Moreover, the large SGs contained LC3, suggesting their fusion with autophagosomes, a process we have recently shown to be regulated by SP. Taken together, our results demonstrate that distinct pathways are involved in IgE- dependent versus IgE-independent exocytosis, that likely reflect the distinct modes of receptor internalization, as well as the involvement of the autophagic machinery in MRGPRX2, mediated responses.