Citation

Mast cells (MCs) are critical effector cells of cutaneous hypersensitivity reactions and inflammatory dermatoses alike. Apart from immunologic activation by aggregation of the high-affinity IgE receptor (FcεRI), MCs can be stimulated via a pseudo-allergic/neurogenic route. Antigen independent MC activation occurs mainly by ligation of the MAS-related G protein-coupled receptor-X2 (MRGPRX2). The recent discovery of MRGPRX2 explains multiple findings which could not be explained beforehand. Using MCs isolated directly from human skin, the major producer cells of MRGPRX2, the present study aims to fill in some gaps by exploring MRGPRX2-mediated activation, and to conduct a comparison with FceRI-triggered events. It was found that both receptor systems degranulate skin MCs and elicit similar cytokine patterns. Signaling intermediates provoked by both pathways included ERK1/2, p38 and AKT, while JNK- phosphorylation was weaker. In the kinetic resolution, phosphorylation events triggered by MRGPRX2 were swift, whereas FceRI triggered signals were delayed. ERK was the most influential kinase involved in cytokine synthesis with some contribution also from other kinases, while PI3K and ERK were both implicated in the degranulation response triggered by both pathways. MRGPRX2-mediated degranulation could be inhibited by pertussis toxin (PTx), YM-254890 and 2-Aminoethyl diphenylborinate (2-APB), which indicated Gi and Gq coupling as well as involvement of inositol- (1,4,5)-trisphosphate receptors (IP3R). Trying to connect the early events with downstream outcomes, it was found that p38, JNK and ERK1/2 depended on Gi, Gq, and IP3R, while AKT was only Gq-dependent. In addition to G protein activation, the b-arrestin pathway was also triggered by c48/80 leading to receptor internalization. By using RNA interference, b-arrestin-1 was detected as the dominant isoform in this scenario. MC modulating factors SCF, IL-4 and IL-33 were selected to study the impact from the micromilieu on MC functionality. SCF and IL-4 reversely regulated MRGPRX2 and FcεRI receptor expression and MC activation, whereby the MRGPRX2 pathway was dampened and the FcεRI pathway increased. IL-33’s modulation was time dependent but consistent between the two pathways: Long-term IL-33 decreased expression of both receptors and degranulation mediated by their ligands, while short-term IL-33 priming led to enhanced degranulation. Collectively, MRGPRX2- and FceRI-mediated routes are independent in terms of kinetics and regulation by extracellular stimuli, yet they also share similarities regarding functional outputs and the signaling intermediates underlying these outputs. Research regarding MRGPRX2 is still at its beginning, and a more comprehensive understanding of MRGPRX2 biology will provide a renewed understanding of MCs’ role in skin pathophysiology.