High-resolution MALDI mass spectrometric imaging of lipids in the mammalian retina

Matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI) is emerging as a powerful tool for the analysis of molecular distributions in biological samples in situ. When compared to classical histology, the major benefit of this method is the ability to identify and localize many molecules in a single tissue sample. MALDI-MSI spatial resolution currently falls short of traditional microscopic methods as it is limited by instrumentation and sample preparation. Tissue preparation steps, such as matrix deposition, are critical when considering strategies to further enhance the spatial resolution. The mammalian retina was selected as the tissue of choice for method development; its stratified anatomy renders it an ideal tissue to test high-resolution MALDI-MSI as the different layers correspond to specific neuronal classes and cellular structures. We compared alcohol-fixed, paraffin-embedded retina to fresh-frozen samples and matrix that had been deposited by spray or by sublimation. We present a lipid imaging method based on MALDI-MSI of frozen retinal sections with sublimated 2,5-dihydroxybenzoic acid matrix, which results in a highly advanced resolution compared to previous established methods. Hierarchical clustering of the primary data allows robust detection and differentiation of molecular distributions at a spatial resolution between 10 and 20 μm, thus approaching single-cell resolution.