The - and -isoforms of type I PIP5K regulate distinct stages of Ca²⁺ signaling in mast cells

Crosslinking of IgE receptors by antigen initiates Ca²⁺ mobilization in mast cells by activating phospholipase-C-mediated hydrolysis of phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P₂]. The resulting inositol 1,4,5-trisphosphate-mediated Ca²⁺ release from the endoplasmic reticulum (ER) activates store-operated Ca²⁺ entry, which is necessary for exocytotic release of inflammatory mediators. To investigate roles for PtdIns(4,5)P₂-synthesizing isozymes of the type I phosphatidylinositol 4-phosphate 5-kinase family (PIP5K-I) in mast cell signaling, we compared the ectopic expression of wild-type and catalytically inactive PIP5K-I in RBL-2H3 mast cells. Surprisingly, both antigen and thapsigargin-stimulated Ca²⁺ influx were reduced by overexpression of active PIP5K-I, whereas antigen-stimulated Ca²⁺ release from ER stores was unaffected. Consistent with these results, Ca²⁺ entry stimulated by antigen or thapsigargin was enhanced by expression of a plasma-membrane-associated inositol polyphosphate 5'-phosphatase, whereas antigen-stimulated Ca²⁺ release from stores was reduced. To investigate the role of PIP5K-I in antigen-stimulated Ca²⁺ mobilization, we used bone-marrow-derived mast cells from PIP5K-I^-/- mice. Antigen-stimulated Ca²⁺ release from ER stores was substantially reduced in the absence of PIP5K-I, but thapsigargin-mediated Ca²⁺ entry was unaffected. In summary, PIP5K-I positively regulates antigen-stimulated Ca²⁺ release from ER stores, whereas PIP5K-I negatively regulates store-operated Ca²⁺ entry, suggesting that these different PIP5K-I isoforms synthesize functionally distinct pools of PtdIns(4,5)P₂ at the plasma membrane.

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