Analysis of <i>Drosophila</i> Lipids by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometric Imaging

<i>Drosophila melanogaster</i> is a major model organism for numerous lipid-related diseases. While comprehensive lipidomic profiles have been generated for <i>D. melanogaster</i>, little information is available on the localization of individual lipid classes and species. Here, we show the use of matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI) to profile lipids in <i>D. melanogaster</i> tissue sections. The preparation of intact cryosections from whole insects presents a challenge due to the brittle hydrophobic cuticle surrounding the body and heterogeneous tissue types beneath the cuticle. However, the introduction of a novel sucrose infiltration step and gelatin as an embedding media greatly improved the quality of tissue sections. We generated MS image profiles of six major lipid classes: phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol, phosphatidylserine, and triacylglycerides. In addition, signals corresponding to two male-specific sex pheromones were detected in the ejaculatory bulb, a specialized site of pheromone production. MSI performed with 35 μm lateral resolution provided high sensitivity detection of at least 92 different lipid species, based on exact mass. In contrast, MSI with 10 μm lateral resolution enabled the detection of 36 lipid species but allowed lipid profiling of individual organs. The ability to localize lipid classes in intact sections from whole <i>Drosophila</i> provides a powerful tool for characterizing the effects of diet, age, stress, and environment on lipid production and distribution

MALDI Mass Spectrometry Imaging of Bioactive Lipids in Mouse Brain with a Synapt G2‑S Mass Spectrometer Operated at Elevated Pressure: Improving the Analytical Sensitivity and the Lateral Resolution to Ten Micrometers

Mass spectrometers from the Synapt-G1/G2 family (Waters) are widely employed for matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). A lateral resolution of about 50 μm is typically achieved with these instruments, that is, however, below the often desired cellular resolution. Here, we show the first MALDI-MSI examples demonstrating a lateral resolution of about ten micrometers obtained with a Synapt G2-S HDMS mass spectrometer without oversampling. This improvement became possible by laser beam shaping using a 4:1 beam expander and a circular aperture for spatial mode filtering and by replacement of the default focusing lens. We used dithranol as an effective matrix for imaging of acidic lipids such as sulfatides, gangliosides, and phosphatidylinositols in the negative ion mode. At the same time, the matrix enables MS imaging of more basic lipids in the positive ion mode. Uniform matrix coatings with crystals having average dimensions between 0.5 and 3 μm were obtained upon spraying a chloroform/methanol matrix solution. Increasing the cooling gas pressure in the MALDI ion source after adding an additional gas line was furthermore found to increase the ion abundances of labile lipids such as gangliosides. The combined characteristics are demonstrated with the MALDI-MSI analysis of fine structures in coronal mouse brain slices

On-Tissue Phospholipase C Digestion for Enhanced MALDI-MS Imaging of Neutral Glycosphingolipids

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) can be used to simultaneously visualize the lateral distribution of different lipid classes in tissue sections, but the applicability of the method to real-life samples is often limited by ion suppression effects. In particular, the presence of abundant phosphatidylcholines (PCs) can reduce the ion yields for all other lipid species in positive ion mode measurements. Here, we used on-tissue treatment with buffer-free phospholipase C (PLC) to near-quantitatively degrade PCs in fresh-frozen tissue sections. The ion signal intensities of mono-, di-, and oligohexosylceramides were enhanced by up to 10-fold. In addition, visualization of Shiga toxin receptor globotriaosylceramide (Gb3Cer) in the kidneys of wild-type and α-galactosidase A-knockout (Fabry) mice was possible at about ten micrometer resolution. Importantly, the PLC treatment did not decrease the high lateral resolution of the MS imaging analysis