Spatial analysis of lipids in tissue samples applying mass spectrometry imaging
Doktorsavhandling, 2018

Lipids are important naturally occurring components in all living cellular organisms. They serve as the main building blocks of cellular membranes, participate in many signaling pathways and are also stored as an energy source. Due to the extreme complex cellular chemistry and structure of lipids, there is a real need to have a label-free technique with high chemical specificity, high accuracy and high sensitivity for study of lipids within the cell membrane. Mass spectrometry imaging (MSI) is capable of providing information on the chemical composition and spatial distribution of complex biological molecules. MSI is a powerful label-free tool for lipid analysis across biological materials. Both matrix-assisted laser desorption/ionization (MALDI) and secondary ion mass spectrometry (SIMS), the two most common MSI techniques, have recently undergone many developments to improve spatial resolution and provide high sensitivity, mainly for higher mass species. These two techniques offer different capabilities in the analysis of a biological system. The main differences are that larger molecules can be ionized and detected using MALDI, whereas SIMS is capable of detecting mainly small molecules but at higher spatial resolution compared to MALDI. This thesis mainly focuses on two scopes of investigation with different sample modifications and also on the overall applicability of MSI for analysis of tissue samples. In recent years, some surface modifications have been developed to enhance the yield of intact molecular species in SIMS. One of them is matrix enhancement secondary ion mass spectrometry (ME-SIMS), which is the combination of the protocol for MALDI sample preparation and normal SIMS. In paper I, the possible mechanism of the signal enhancement in ME-SIMS was studied. Here, sublimation was used to deposit a thin layer of an organic matrix on the surface of a brain tissue slice analyzed with SIMS. In this work, I showed that sublimation could successfully provide enhancement in ion yields for a multitude of lipid species in SIMS. The mechanism of this enhancement could be due to a lower ion suppression followed by removal of the cholesterol crystals from the surface of sample allowing detection for less abundant species. It is also possible that the extraction of some specific lipids into the deposited matrix directly leads to an increase of higher mass lipid ion yield. In paper II, two different surface modifications, including matrix sublimation and nanoparticle deposition were applied on Drosophila brain samples and lipid information obtained from MALDI analysis were compared. Here, it was shown that each technique can be used in a complementary approach to detect a variety of lipid species. In paper III, SIMS imaging was employed to investigate the effect of specially processed cereals, as a specific diet on the alteration of lipid composition across the rodent intestine tissue. In paper IV, I continued the study of changes in lipid content, this time on brain samples of animals exposed to the same diet. Intake of such cereals increases active antisecretory factor (AF) in plasma, an endogenous protein with proven regulatory function on inflammation and fluid secretion. Although, the exact mechanism for the activation process of AF at the cellular level remains unclear. The results show changes in lipid content of cell membrane in response to this cereals intake suggesting a relation to activating AF. In paper V, the techniques for developing of sample preparation in SIMS imaging were investigated to improve the signal intensity of intact molecules at higher resolution.

Lipid

Mass spectrometry imaging

ME-SIMS

Sample preparation

MALDI

SIMS

Kollektorn-salen, Kemivägen 9, Göteborg
Opponent: Professor Arnaud Delcorte, Université catholique de Louvain, Brussels, Belgium

Författare

Masoumeh Dowlatshahi Pour

Kemi och kemiteknik, Kemi och biokemi, Analytisk kemi

Food-induced changes of lipids in rat neuronal tissue visualized by ToF-SIMS imaging

Scientific Reports,; Vol. 6(2016)

Artikel i vetenskaplig tidskrift

An investigation on the mechanism of sublimed DHB matrix on molecular ion yields in SIMS imaging of brain tissue

Analytical and Bioanalytical Chemistry,; Vol. 408(2016)p. 3071-3081

Artikel i vetenskaplig tidskrift

Our brain is the most important and complex organ in our entire body. It makes us a human and who we are. The brain as the centre of the nervous system is a communication structure to control and coordinate all actions and reactions in our body. The brain is involved in many functions including movement, touch sensing, vision, hearing, smelling and also it enables us to have a great number of abilities such as learning, memory, concentration, planning, problem-solving, recognition, parts of speech, judgment and many more. However, the mechanism for most of these brain functions is still unclear. The brain has a complex composition and is mainly composed of water (77 to 78 %), fatty substances called lipids (10 to 12 %), proteins (8%), sugars called carbohydrates (1%), inorganic salts (1%) and other organic substances (2%). Therefore, 50-60% of the brain’s dry weight is made of lipid compounds. In fact, the brain is the second fattiest organ after the liver in the body. Therefore, change in the brain composition of these fats can have a big effect on its function in the body. If we aim to investigate the possible changes in the chemistry of the brain caused by some diseases, drugs or diets, the study of lipid compositions and their distributions in the brain would be critical. Lipids are important naturally occurring molecules in all living cellular organisms. They structurally have a polar head group with non-polar hydrocarbon chains allowing them to have both hydrophilic and hydrophobic properties, which aid in forming the cellular bilayer membrane that surrounds all cells. In fact, they are the main component of the cell membrane and play key roles in many cell functions. In order to analyze the lipid compositions of the brain sample, I used a mass spectrometry imaging (MSI) technique that provides great advantages for biological studies. The MSI technique is a label-free method that is able to detect several hundred different biomolecule species and to illustrate their distribution map at the different regions of brain tissue with the micrometer resolution. This is like taking a high-resolution chemical photograph. In the MSI technique, a beam of charged molecules is used to hit the surface of the sample and knock off chemical compounds from a very small area at the surface called a pixel. The ionized chemicals are then transferred to another instrument called a mass analyzer where they are sorted by their size and counted. This results in a mass spectrum that allows identification of the substances detected. The analyzing beam is used to scan an entire sample to make an image or chemical photograph. In this thesis, I developed new ways to use this imaging method, new ways to prepare samples for imaging, and used the methods to measure the lipid compositions and their localization in brain tissue samples. This enabled me to better understand the effects of diet on the chemical composition of the brain. We are indeed what we eat. This chemical imaging method can be also used to study the mechanism of brain functions and diseases.

Ämneskategorier

Analytisk kemi

Annan biologi

Infrastruktur

Chalmers infrastruktur för masspektrometri

Infrastruktur för kemisk avbildning

ISBN

978-91-7597-690-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4371

Utgivare

Chalmers tekniska högskola

Kollektorn-salen, Kemivägen 9, Göteborg

Opponent: Professor Arnaud Delcorte, Université catholique de Louvain, Brussels, Belgium