Background: Human mast cells (MCs) from anatomical sites show functional heterogeneity due to expression of various G-protein-coupled receptors (GPCRs), and since the function of human MCs is modulated by various GPCR ligands, the GPCRs expressed on MCs may play an important role in allergic diseases and anaphylaxis. However, the heterogeneity of GPCR expression has not been well investigated.
Methods: Human MCs from lung and skin were cultured and microarray analysis of GPCR expression was performed. To investigate difference of expression levels of GPCR between allergic patients and normal controls, immunohistochemical analysis was performed. The expression of platelet activating factor receptor (PAFR) was investigated by RT-PCR and Western-blot analysis. Cell signaling pathways in MCs in response to PAF were investigated by analyzing the effect of various inhibitors and the silencing of phospholipase (PL) C mRNA on PAF-mediated histamine release. Mas-related G protein-coupled receptor family member X2 (MrgX2) expression in human MCs was reduced using a lentiviral shRNA silencing technique. Ca2+ influx was measured in CHO cells transfected with MrgX2 in response to eosinophil granule proteins.
Results: A total of 68 GPCR genes were expressed in lung and/or skin MCs. Both lung and skin MCs expressed 54 GPCRs including C3aR1, C5aR1, CYSLTR1, and EDG1 (endothelial differentiation gene 1; sphingosine-1-phosphate receptor 1). Six GPCRs were skin MC-specific including GPRC5B, EBI2 (Epstein-barr virus-induced gene 2), and MRGX2. Eight GPCRs were lung MC-specific including CRHR1, and PAFR. Because PAF and substance P are implicated in the pathophysiology of anaphylaxis and urticaria, respectively, we investigated further the expression and function of PAFR and MrgX2 on human MCs. PAF induces histamine release from human lung MCs but not skin MCs. Activation of PAF receptor-coupled Gαi leads to degranulation via PLCg1 and PLCb2 activation in MCs. Substance P, major basic protein (MBP), and eosinophil peroxidase (EPO), but not eosinophil derived neurotoxin, induced histamine release from human skin MCs through MrgX2. The number of MrgX2-positive (+) skin MCs and the percentage of MrgX2+ MCs in all the MCs from severe chronic urticaria patients were significantly higher than those from normal control subjects.
Conclusions: Human MCs from different anatomical sites show different GPCR expression. In allergic diseases, the expression levels of GPCRs might be affected. PAF may mediate an amplification loop for lung MC activation in the generation of anaphylaxis. MrgX2 may be a new target molecule for the treatment of wheal reactions in severe chronic urticaria patients.