Activated Cdc42/Rac reconstitutes FcεRI-mediated Ca2+ mobilization and degranulation in mutant RBL mast cells

Antigen stimulation of mast cells via Fc epsilon RI, the high-affinity receptor for IgE, triggers a signaling cascade that requires Ca2+ mobilization for exocytosis of secretory granules during an allergic response. This study investigates critical signaling components by using mutant RBL mast cells that are defective in antigen-stimulated phospholipase C gamma (PLC gamma) activation, as well as other signaling activities downstream of stimulated tyrosine phosphorylation. We show that the expression of activated versions of the Cdc42 or Rac1 GTPase restores antigen-stimulated Ca2+ mobilization necessary for degranulation in these mutant cells. Wild-type Cdc42 and Rac1, as well as activated Cdc42 containing effector domain mutations, all fail to restore antigen-stimulated signaling leading to exocytosis, Expression of oncogenic Dbl, a guanine nucleotide exchange factor for Cdc42 and Rac1, partially restores sustained Ca2+ mobilization and degranulation, suggesting that activation of endogenous Cdc42 and/or Rac1 is impaired in the mutant cells. Overexpression of PLC gamma1 with either activated Cdc42 or Rad synergistically stimulates degranulation, consistent with a critical defect in PLC gamma activation in these cells. Thus, our results point to activation of Cdc42 and/or Rad playing an essential role in antigen stimulation of early events that culminate in mast cell degranulation.