MALDI imaging mass spectrometry: spatial molecular analysis to enable a new age of discovery

Abstract

Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) combines the sensitivity and selectivity of mass spectrometry with spatial analysis to provide a new dimension for histological analyses to provide unbiased visualization of the arrangement of biomolecules in tissue. As such, MALDI IMS has the capability to become a powerful new molecular technology for the biological and clinical sciences. In this review, we briefly describe several applications of MALDI IMS covering a range of molecular weights, from drugs to proteins. Current limitations and challenges are discussed along with recent developments to address these issues. This article is part of a Special Issue entitled: 20years of Proteomics in memory of Viatliano Pallini. Guest Editors: Luca Bini, Juan J. Calvete, Natacha Turck, Denis Hochstrasser and Jean-Charles Sanchez.

Keywords: MALDI imaging mass spectrometry.

Figure 1

Overview of the MALDI IMS workflow on a section of a human kidney biopsy. (A) Fresh frozen tissue is cut and mounted on a conductive surface. Matrix is applied by a robotic sprayer or is sublimated, and the section is irradiated by the laser in a raster array. (C) Mass spectra are acquired for each x,y coordinate. (D) Selected ions may be mapped on the tissue surface to create images.

Figure 2

Publications reporting the development and application of IMS through 2013. Data was collected from a PubMed search of the following key words: (imaging mass spectrometry OR mass spectrometry imaging)) AND (“1980”[Date - Publication]: “2013”[Date - Publication])).

Figure 3

Images of the membrane protein myelin proteolipid protein (PLP, 30 kDa). Special sample preparation protocols were developed to enable imaging of membrane proteins [10]. The protein was imaged within cerebrum (left) and cerebellum/medulla (right). Exclusive localization to white matter regions of brain, particularly to the corpus callosum (Cc), caudate-putamen (Cp), and septal nucleus (Sn) of the cerebrum and the arbor vitae (Arb) of the cerebellum and the entire medulla (Med), is apparent.

Figure 4

Target MALDI IMS analysis of synaptophysin (m/z 323) and somatostatin (m/z 532) in single islet cells. Images were obtained using a transmission geometry ion source for high-resolution imaging. (a) Optical image of islet cells. (b) MALDI IMS of an immunoreactive islet cell for synaptophysin. (c) Targeted IMS of a delta cell for synaptophysin (green) and somatostatin (red), showing different localization for each molecule within different cellular structures. [16]

Figure 5

Images of separated and identified isobaric ions of tubulin and ubiquitin tryptic peptide fragments (1039 m/z) in rat brain were imaged using ion mobility spectrometry coupled with imaging mass spectrometry. For these images, two different drift times have been selected: 100–127 and 133–148 bins to reconstruct the respective images. Without any ion mobility, one image is obtained corresponding to the superposition of these two images. [34]

Figure 6

Comparison of imaging results of mouse brain serial sections with matrix pre- and post-coated targets at different spatial resolutions. Ion images were collected in negative ion mode. Tentative identification was based on previous MS/MS analyses of lipids at these masses in mouse brain. Below: H&E optical scanned images from a serial section are shown at the bottom of the figure.

Figure 7

Quantitative analysis of a rat liver section dosed with the drug nevirapine (m/z 267.124) using a mimetic tissue model. (A) Optical scan of sections from the nevirapine-dosed rat liver (left) and the mimetic tissue model (right), before matrix application. (B) Ion image for nevirapine, m/z 267.124, at 75% maximum intensity threshold. (C) Comparison of nevirapine quantification results from the MALDI IMS and LC-MS analyses.