FcεRI-mediated mast cell degranulation requires calcium-independent microtubule-dependent translocation of granules to the plasma membrane

The aggregation of high affinity IgE receptors (Fcε receptor I [FcεRI]) on mast cells is potent stimulus for the release of inflammatory and allergic mediators from cytoplasmic granules. However, the molecular mechanism of degranulation has not yet been established. It is still unclear how FcεRI-mediated signal transduction ultimately regulates the reorganization of the cytoskeleton and how these events lead to degranulation. Here, we show that FcεRI stimulation triggers the formation of microtubules in a manner independent of calcium. Drugs affecting microtubule dynamics effectively suppressed the FcεRI-mediated translocation of granules to the plasma membrane and degranulation. Furthermore, the translocation of granules to the plasma membrane occurred in a calcium-independent manner, but the release of mediators and granule–plasma membrane fusion were completely dependent on calcium. Thus, the degranulation process can be dissected into two events: the calcium-independent microtubule-dependent translocation of granules to the plasma membrane and calcium-dependent membrane fusion and exocytosis. Finally, we show that the Fyn/Gab2/RhoA (but not Lyn/SLP-76) signaling pathway plays a critical role in the calcium-independent microtubule-dependent pathway.