Available online at www.sciencedirect.com Micrometric molecular histology of lipids by mass spectrometry imaging David Touboul 1 , Olivier Lapre ´ vote 1,2 and Alain Brunelle 1 Time-Of-Flight Secondary Ion Mass Spectrometry is compared to other mass spectrometry imaging techniques, and recent improvements of the experimental methods, driven by biological and biomedical applications, are described and discussed. This review shows that this method that can be considered as a micrometric molecular histology is particularly efficient for obtaining images of various lipid species at the surface of a tissue sample, without sample preparation, and with a routine spatial resolution of 1 mm or less. Addresses 1 Centre de recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, avenue de la Terrasse, 91198 Gif-sur-Yvette, France 2 Chimie Toxicologie Analytique et Cellulaire, EA 4463, Faculte ´ des Sciences Pharmaceutiques et Biologiques, Universite ´ Paris Descartes, 4, avenue de l’Observatoire, 75006 Paris, France Corresponding author: Brunelle, Alain (Alain.Brunelle@icsn.cnrs-gif.fr) Current Opinion in Chemical Biology 2011, 15:725–732 This review comes from a themed issue on Analytical Techniques Edited by Morgan Alexander and Ian Glimore Available online 24th May 2011 1367-5931/$ – see front matter # 2011 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cbpa.2011.04.017 Introduction Various ex vivo biological imaging methods, such as immunohistochemistry, staining or autoradiography, have been developed to obtain information on the chemical composition at the surface of tissue sections. Among them, mass spectrometry imaging (MSI) is the only one allowing to locate and identify various chemical compounds without selection a priori of a chemical class or compound [1  ,2]. Since this method does not require targeting compounds before the analysis, it makes possible to draw anatomical images of any ion detected in the mass spectra in one single experiment. Lipids, which can be defined as fat-soluble molecules, have been for a long time considered only as energy storage and as major constituents of the cell membranes. It is now agreed that these classes of compounds are important cell signaling molecules, neurotransmitters and precursors in the regulation of various cellular functions [3]. The import- ance of lipids in biological sciences is illustrated by the recognition of ‘lipidomics’ as an emerging field among the ‘omics’. The eight lipid categories as defined by the Lipid Maps initiative [4], fatty acyls, glycerolipids, glyceropho- spholipids, sphingolipids, saccharolipids and polyketides, sterol lipids and prenol lipids (see www.lipidmaps.org), can be analyzed with gas/liquid chromatography coupled to modern mass spectrometers, providing precise identifi- cation and quantification. Before their analysis, lipids have to be extracted from the sample, leading to the loss of the spatial information whereas MSI offers a direct sampling over the tissue surface [3,5]. Comparison with other mass spectrometry imaging methods Basically, in MSI, a focused beam of photons or energetic ions is used to scan over the surface of a tissue section. The size of this beam defines the size of the pixel that corresponds to each point on which a mass spectrum is recorded. The data acquisition consists in the creation of a volume whose dimensions are x and y, the two geometric dimensions of the sample, and m/z the mass-to-charge ratio of the secondary ions. Any slice of this volume along a given m/z value is an ion density map, the so-called ion image [6]. All mass spectrometry methods based on localized ionization processes at the sample surface can be theoretically used for mass spectrometry imaging. Table 1 summarizes the main characteristics of the MSI techniques, listed from left to right with increasing spatial resolution. At both ends are Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Nano-Secondary Ion Mass Spectrometry (Nano-SIMS), which are not applicable to molecular ion localization. These two desorption/ionization methods are too ener- getic to provide signals of intact lipid ions. Nevertheless, the first one is considered as a method of choice for the quantitative localization of trace metals with a resolution of 120 mm [7,8], while the latter is the best known method for submicrometer localization of elements and/or small fragment ions with a precision reaching 100 nm or less [9]. Desorption electrospray ionization (DESI) uses a pneumatically assisted electrospray source to desorb the analytes from the sample surface. DESI is very sensitive for low molecular weight compounds like lipids. It works on samples at atmospheric pressure, but is limited with the current state-of-the art to a spatial resolution well above 100 mm [10]. Matrix-Assisted Laser Desorption Ionization (MALDI) is by far the most pop- ular mass spectrometry imaging method [2,11  ]. The sample requires being preliminary coated with a matrix, and depending on the proper choice of the latter, any class www.sciencedirect.com Current Opinion in Chemical Biology 2011, 15:725–732