[Advances in methods for analyzing IP 3 signaling and understanding of coupled Ca 2+ and IP 3 oscillations].

Inositol 1,4,5-trisphosphate (IP ₃ ) is an important intracellular messenger produced by phospholipase C via the activation of G-protein-coupled receptor- or receptor-tyrosine-kinase-mediated pathways, and is involved in numerous responses to hormones, neurotransmitters, and growth factors through the releases of Ca ²⁺ from intracellular stores via IP ₃ receptors. IP ₃ -mediated Ca ²⁺ signals often exhibit complex spatial and temporal organizations, such as Ca ²⁺ oscillations. Recently, new methods have become available to measure IP ₃ concentration ([IP ₃ ]) using AlphaScreen technology, fluorescence polarization, and competitive ligand binding assay (CFLA). These methods are useful for the high throughput screening in drug discovery. Calcium ions generate versatile intracellular signals such as Ca ²⁺ oscillations and waves. Fluorescent sensors molecules to monitor changes in [IP ₃ ] in single living cells are crucial to study the mechanism for the spatially and temporally regulated Ca ²⁺ signals. In particular, FRET-based IP ₃ sensors are useful for the quantitative monitoring intracellular [IP ₃ ], and allowed to uncovered the oscillatory IP ₃ dynamics in association with Ca ²⁺ oscillations. A mathematical model of coupled Ca ²⁺ and IP ₃ oscillations predicts that Ca ²⁺ oscillations are the result of modulation of the IP ₃ receptor by intracellular Ca ²⁺ , and that the period is modulated by the accompanying IP ₃ oscillations. These model predictions have also been confirmed experimentally. At present, however, usefulness of FRET-based IP ₃ sensors are limited by their relatively small change in fluorescence. Development of novel IP ₃ sensors with improve dynamic range would be important for understanding the regulatory mechanism of Ca ²⁺ signaling and for in vivo IP ₃ imaging.