(A) transmembrane adaptor protein, LIME, regulates IgE receptor-mediated mast cell activation

Abstract

Mast cells are primary effector cells in immune system to regulate the allergic reaction. Aggregation of high-affinity IgE receptor (FcεR I ) on mast cell membranes triggers the activation signaling events leading to degranulation of mast cells and release of mediators including histamine, β-hexosaminidase and other inflammatory mediators. Many signaling molecules were previously shown to be associated with IgE receptor- mediated signaling pathway in mast cells including protein tyrosine kinase, Lyn, Syk and Fyn, and adaptor proteins LAT, Grb2, PLCy and Fyb. Upon ligation of receptor with IgE antibody and proper antigen, tyrosine kinases such as Lyn and Syk are activated and subsequently phosphorylate a transmembrane adaptor protein LAT. Phosphorylated LAT serves as platform for assembly of signaling complex including Grb2, PLCy. In this study, we demonstrate that another transmembrane adaptor protein, LIME (Lck-interacting-transmembrane protein) participates in the IgE receptor-mediated signaling pathways in mast cells. Upon Western analysis, LIME protein was expressed in mast cell line RBL-2H3 cells and IL-3-induced Bone Marrow-derived Mast cells (BMMCs). Upon fractionation by sucrose density gradient, LIME localized in mainly in non-lipid raft fraction of membranes and, upon FccR I stimulation, a fraction of LIME was found in lipid raft fraction in mast cells. This localization in mast cells is different from the exclusive lipid raft localization of LIME observed in T cells. Confocal immunofluorescence microscopy showed that a fraction of LIME CO-localized with Lyn in the membrane upon FcεR I stimulation. Furthermore, LIME co-localized with filamentous actin at adhesion site of the cell and in adhesion plaques upon FcεR I stimulation. Moreover, overexpression of enhanced degranulation of RBL-2H3 cells upon FcεR I activation. Thus, our results show that LIME act as an adaptor protein regulating IgE receptor- mediated signaling pathway in mast cell in association with Lyn and actin.