THE SRC HOMOLOGY 2-CONTAINING INOSITOL PHOSPHATASE (SHIP) IS THE GATEKEEPER OF MAST-CELL DEGRANULATION

To clarify the role that the src homology 2-containing inositol phosph atase (SHIP) plays in mast cell degranulation, the gene for SHIP was d isrupted by homologous recombination in embryonic stem cells, Bone-mar row-derived mast cells from SHIP+/+, +/-, and -/- F-2 litter-mates wer e compared. SHIP-/- mast cells were found to be far more prone to degr anulation, after the crosslinking of IgE preloaded cells, than SHIP+/- or +/+ cells. Intriguingly, IgE alone also stimulated massive degranu lation in SHIP-/- but not in +/+ mast cells. This degranulation with I gE alone, which may be due to low levels of IgE aggregates, correlated with a higher and more sustained intracellular calcium level than tha t observed with SHIP+/+ cells and was dependent upon the entry of extr acellular calcium. Immunoprecipitation studies revealed that the addit ion of IgE alone to normal mast cells stimulates multiple cascades, wh ich are prevented from progressing to degranulation by SHIP. PI 3-kina se inhibitor studies suggested that IgE-induced activation of PI 3-kin ase is upstream of the entry of extracellular calcium and that SHIP re stricts this entry by hydrolyzing phosphatidylinositol 3,4,5-trisphosp hate. These results show the critical role that SHIP plays in setting the threshold for degranulation and that SHIP directly modulates a ''p ositive-acting'' receptor.