In situ MALDI-TOF MS regional analysis of neutral phospholipids in lens tissue

Phospholipids (PLs) are important sources of lipid second messengers that participate in cell signaling pathways. Consequently, their analysis in biological tissues has received increased attention. Current approaches for PL analysis include an extraction step and subsequent identification of the main PL classes by either 31P NMR spectroscopy or chromatographic separation followed by mass spectrometric detection. Previous in vitro studies revealed regional changes in the PL composition of mammalian lenses at different growth stages. In this report, we demonstrate the feasibility of direct in situ analysis of two relevant PL classes, phosphatidylcholines (PCs) and sphingomyelins (SMs), in slices of fresh or fixed (2.5% formaldehyde) mammalian lenses. The chosen matrix was p-nitroaniline, as it generated superior sensitivity when compared to 2,5-dihydroxybenzoic acid, the compound most commonly used for in vitro analysis of PLs by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). Regional differences in the relative amounts of PCs and SMs were in agreement with trends demonstrated by previous in vitro studies. Fresh and fixed tissue of the same lens gave comparable relative levels of PCs and SMs. In situ analysis of PLs by MALDI-TOF MS offers a rapid and sensitive tool for the mapping of PLs in biological tissues.