Signaling assemblies formed in mast cells activated via Fcε receptor I dimers

Although aggregation of the FC epsilon receptor I (Fc epsilon RI) is necessary for Ag-mediated mast cell triggering, the relationship between the extent of the Fc epsilon RI aggregation and subsequent biochemical and topographical events is incompletely understood. In this study, we analyzed the activation events induced by Fc epsilon RI climers, elicited by binding of anti-Fc epsilon RI mAb to rat basophilic leukemia cells. We found that, in contrast to extensively aggregated Fc epsilon RI, receptor climers (1) induced a less extensive association of Fc epsilon RI with detergent-resistant membranes, (2) delayed the tyrosine phosphorylation and membrane recruitment of several signaling molecules, (3) triggered a slower but more sustained increase in concentration of free cytoplasmic calcium, (4) induced degranulation which was not inhibited at higher concentrations of the cross-linking mAb, and (5) failed to produce clusters of Fc epsilon RI, Syk kinase and Grb2 adapter in osmiophilic membranes, as detected by immunogold electron microscopy on membrane sheets. Despite striking differences in the topography of Fc epsilon RI climers and multimers, biochemical differences were less pronounced. The combined data suggest that Fc epsilon RI-activated mast cells propagate signals from small signaling domains formed around dimerized/oligomerized Fc epsilon RI; formation of large Fc epsilon RI aggregates in osmiophilic membranes seems to promote both strong receptor triggering and rapid termination of the signaling responses.