MRGPRX2-Mediated Degranulation of Human Skin Mast Cells Requires the Operation of G(alpha i), G(alpha q), Ca++ Channels, ERK1/2 and PI3K-Interconnection between Early and Late Signaling

The recent discovery of MRGPRX2 explains mast cell (MC)-dependent symptoms independently of Fc epsilon RI-activation. Because of its novelty, signaling cascades triggered by MRGPRX2 are rudimentarily understood, especially in cutaneous MCs, by which MRGPRX2 is chiefly expressed. Here, MCs purified from human skin were used following preculture or ex vivo and stimulated by Fc epsilon RI-aggregation or MRGPRX2 agonists (compound 48/80, Substance P) in the presence/absence of inhibitors. Degranulation was assessed by beta-hexosaminidase or histamine release. Phosphorylation events were studied by immunoblotting. As a G protein-coupled receptor, MRGPRX2 signals by activating G proteins; however, their nature has remained controversial. In skin MCs, G(alpha i) and G(alpha q) were required for degranulation, but G(alpha i )was clearly more relevant. Ca++ channels were likewise crucial. Downstream, PI3K was essential for granule discharge initiated by MRGPRX2 or Fc epsilon RI. ERK1/2 and JNK were additional participants, especially in the allergic route. Addressing possible points of intersection between early and later events, pERK1/2 and pAKT were found to depend on G(alpha i), further highlighting its significance. G(alpha q) and Ca++ channels made some contributions to the phosphorylation of ERK. Ca++ differentially affected PI3K activation in Fc epsilon RI- vis-a-vis MRGPRX2-signaling, as channel inhibition increased pAKT only when triggered via Fc epsilon RI. Collectively, our study significantly extends our understanding of the molecular framework behind granule secretion from skin MCs.