Label free analysis and spatial distribution of molecular lipids in brain tissue and cells using mass spectrometry imaging

University dissertation from Chalmers University of Technology

Abstract: Lipids are important inherent components in brain and cell membranes, which play significant biological roles in cell function. Identification and localization of specific biomolecules such as peptides and lipids within cellular membranes is currently a major challenge in metabolomics and biological studies. With recent technological and methodological improvements, imaging mass spectrometry methods including secondary ion mass spectrometry (SIMS) and matrix-assisted laser desorption ionization (MALDI) have become promising techniques to identify and map different lipid species in tissue sections and cells. In biological investigations SIMS and MALDI often provide complementary information. SIMS provides detection of small molecules at high spatial resolution, whereas MALDI is capable of ionizing larger molecules in a sample, albeit at reduced spatial resolution. I present a sample preparation technique with gold and silver nanoparticles (NPs) compatible with both SIMS and MALDI, which provides potential complementary detection in lipid analysis by both NP-laser desorption ionization and SIMS techniques. Although these MS techniques have associated abilities there are some limitations with each method. In the technical approaches of this thesis for mass spectrometry imaging methods, I have investigated the limitations, improvements and technical developments in MALDI and SIMS, which can maximize the information available for biomolecular imaging in brain tissue sections and cells. In MALDI, analyte detection is sometimes limited due to the mass interferences from the organic matrix and the spatial resolution can also be limited by the wet organic matrix deposition on the sample surface. I show images of delocalized lipid standards (as a limitation in wet matrix deposition method in MALDI) by developing a test device. In this work, Au nanoparticles deposited (in nanoparticle-LDI) as an alternative to organic matrix have been shown to provide advantages in different lipid analysis and imaging. I investigate the lipid information that can be obtained by NP-LDI compared to two different organic matrices deposited by sublimation (as a dry matrix application method) in which each technique can be used to detect a variety of lipid species.

As biological samples such as brain tissue are not favorable for a vacuum environment, the techniques for developing of sample preparation in SIMS imaging have been investigated to overcome these sample preparation challenges. Newly developed high-energy gas cluster ion beams (GCIBs) have been investigated as a primary ion source in SIMS. This offers a potent imaging technique to obtain maximum information on intact lipid species and extend the useful mass range for SIMS imaging in biological samples compared to NP-LDI and MALDI. In this work, high energy Ar GCIBs show potential abilities for ionizing the molecular ions of lipids. The second main objective is to apply high energy GCIB SIMS imaging as a promising technique to investigate changes in lipids in the cell membrane induced by application of Cisplatin, a chemotherapeutic agent that also can affect cognition. This work is aimed at gaining a better understanding of the biochemical mechanisms understanding the role of lipids in cognition.

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