Phosphoinositide profiling in complex lipid mixtures using electrospray ionization mass spectrometry

Phosphoinositides (phosphorylated derivatives of phosphatidylinositol, PI) are versatile intracellular signaling lipids whose occurrence in low concentrations complicates direct mass measurements. Here we present a sensitive method to detect, identify and quantify phosphatidylinositol phosphate (PIP) and phosphatidylinositol bisphosphate (PIP2) with different fatty acid compositions (phosphoinositide profiles) in total lipid extracts by electrospray ionization mass spectrometry (ESI-MS). Using this method, we detected elevated concentrations of PIP₂ in human fibroblasts from patients with Lowe syndrome, a genetic disorder that affects phosphoinositide metabolism. Saccharomyces cerevisiae cells deficient in enzymes involved in PIP metabolism - Sac1p, a phosphoinositide phosphatase, and Vps34p and Pik1p, a PI 3-kinase and PI 4-kinase, respectively - showed not only different PIP concentrations but also differential changes in PIP profiles indicating metabolic and/or subcellular pooling. Mass spectrometric analysis of phosphoinositides offers unique advantages over existing approaches and may represent a powerful diagnostic tool for human diseases that involve defective phosphoinositide metabolism.