Involvement of filamentous actin in setting the threshold for degranulation in mast cells

Previous studies using cytochalasins and latrunculin B, inhibitors of actin polymerization, showed that filamentous (F)-actin had a negative regulatory role in Fcepsilon receptor I (FcepsilonRI) signaling. How F-actin is involved in regulating the activation of mast cells is unknown. In this study we investigated the role of F-actin in mast cell activation induced by aggregation of the glycosylphosphatidylinositol (GPI)-anchored proteins Thy-1 and TEC-21, and compared it to activation via FcepsilonRI. Pretreatment of rat basophilic leukemia cells with latrunculin B inhibited the Thy-1-induced actin polymerization and elevated the Thy-1-mediated secretory and calcium responses. Inhibition of actin polymerization followed by Thy-1 aggregation resulted in an increased tyrosine phosphorylation of Syk, phospholipase Cgamma (PLCgamma), Gab2 and linker for activation of T cells (LAT) adapters, and some other signaling molecules. Enzymatic activities of phosphatidylinositol 3-kinase, PLCgamma, and phosphatase SHP-2 were also upregulated, but tyrosine phosphorylation of ezrin was inhibited. Similar changes were observed in FcepsilonRI-activated cells. Significant changes in intracellular distribution, tyrosine phosphorylation, and/or enzymatic activities of signaling molecules occurred in latrunculin-pretreated cells before cell triggering. The combined data suggest that actin polymerization is critical for setting the thresholds for mast cell signaling via aggregation of both FcepsilonRI and GPI-anchored proteins.