Beyond IP3: roles for higher order inositol phosphates in immune cell signaling.

Nearly 25 years ago the first function of an inositol phosphate, namely Ins(1,4,5)P3, was reported to act as a "second messenger" to mobilize calcium from the endoplasmic reticulum (ER). Since this discovery, many other inositol phosphates and the kinases and phosphatases that generate these inositol phosphates have subsequently been discovered. However, the function of these "higher order" inositol phosphates in biological processes, if any, has remained a mystery. Interest in higher order inositol phosphates, such as Ins(1,3,4,5)P4, was renewed this year following reports of novel roles for these molecules in distinct processes within the immune system ranging from T cell development, B cell development and tolerance induction, as well as neutrophil and mast cell function. In this review, we will touch upon recent advances in inositol phosphate function in mammalian cells. More specifically, we will highlight new studies that have identified novel functions for specific higher order inositol phosphates, such as Ins(1,3,4,5)P4, in the immune system.