MALDI-ion mobility-TOFMS imaging of lipids in rat brain tissue

Abstract

While maintaining anatomical integrity, matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) has allowed researchers to directly probe tissue, map the distribution of analytes and elucidate molecular structure with minimal preparation. MALDI-ion mobility (IM)-orthogonal time-of-flight mass spectrometry (oTOFMS) provides an advantage by initially separating different classes of biomolecules such as lipids, peptides, and nucleotides by their IM drift times prior to mass analysis. In the present work the distribution of phosphatidlycholine and cerebroside species was mapped from 16 microm thick coronal rat brain sections using MALDI-IM-oTOFMS. Furthermore, the use of gold nanoparticles as a matrix enables detection of cerebrosides, which although highly concentrated in brain tissue, are not easily observed as positive ions because of intense signals from lipids such as phosphatidlycholines and sphingomyelins.

Figure 1

MALDI-IM 2D plot of a rat brain tissue section with 2,5-dihydroxy benzoic acid (DHB) matrix in positive ion mode. Many of the peaks in the trend line identified as ‘matrix’ can be assigned to DHB clusters or DHB clusters + potassium. The rectangular region of interest, 16 ms wide (mobility drift time) and 100 amu long (centered at 800 m/z), was defined in software for subsequent use during acquisition of the data in Fig. 2.

Figure 2

MALDI-IM images using DHB matrix in positive ion mode. (a) 1D mass spectrum obtained from section. (b) Image of total ion count in Fig. 2(a). (c) Image of mass peak at m/z 798.6. (d) Image of mass peak at m/z 826.6. (e) Image of mass peak at m/z 844.6. Images are 51 × 60 pixels. Abbreviations: Cx – cerebral cortex; CC – corpus callosum; CPu – caudate putamen; f – fornix.

Figure 3

MALDI-IM images using 5.5 nm Au particles in positive ion mode. (a) 1D mass spectrum recorded from the tissue section. (b) Image of sodiated cerebroside 24 : 0 OH at m/z 850.7. The image is a 40 × 40 pixel. The adjacent frame is a photograph of the rat brain section. Abbreviations: Cx – cortex; fmi – forceps minor of the corpus callosum; Cpu – caudate putamen (striatum); Acb – nucleus accumbens; ac – anterior commissure; lo – lateral olfactory tract.