Visualizing and profiling tissue lipids by ToF-SIMS imaging mass spectrometry

Abstract

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is an emerging surface analytical method for chemical microanalysis of biological samples. While much work has been done with artificial materials and some with cultured cells and animal tissue, very little ToF-SIMS work has been done so far with clinically relevant tissue samples. The ability of ToF-SIMS to detect and visualize lipids in tissue was therefore evaluated in this project.

Study I: Patients with chronic kidney disease (CKD) display a variety of metabolic and nutritional disturbances, which contribute to various clinical complications and high mortality. The evaluation of plasma constituents in these patients reveals extensive lipoprotein and fatty acid composition abnormalities, but these changes may not necessarily reflect abnormalities in tissue lipid composition which conceivably could contribute to various alterations in the function of cells and tissues. In this work ToFSIMS was used for detecting lipids in human adipose tissue from CKD patients. The results showed systematic variations in the lipid distribution between samples from different individuals, i.e. signal intensities from the saturated lipids relative to unsaturated lipids. These observations suggest that there exists a correlation between CKD and degree of saturation of the lipids.

Study II: Osmium tetroxide has long been used as a stain for unsaturated lipids but there has been disagreement about its site of binding in tissue. The spatial distributions of different lipids and osmium oxides in mouse adipose tissue were therefore analyzed by ToF-SIMS. Comparison of the lipid distributions before and after staining suggests that one type of osmium oxide binds specifically to glyceride lipids containing unsaturated C18 fatty acids. Another osmium oxide was co-localized with protein fragments. Chemical analysis, imaging and compatibility with tissue sections similar or identical to those in histology make ToF-SIMS highly useful for elucidating the chemical background of histological stains.

Study III: ToF-SIMS was used to analyze health human skin. The composition of the different layers of the skin was studied and compared by monitoring the localization of different inorganic substances and lipids. Although the signal at masses correspondingto intact lipids was less intense than from inorganic substances, ion images could be generated for several lipids, which showed a preferential localization close to the skin surface. Some of these lipids such as sapienic acid and lauric acid are considered to be antimicrobial and first line of defense against microbial colonization and infection.

These studies show that good quality ToF-SIMS spectra and ion images can be obtained from tissue sections similar or identical to those used in clinical histopathology. Many of the detected signals correspond to lipids and include a range of unsaturated lipids. ToF-SIMS will therefore be a useful method in the study of chronic diseases associated with local alterations in unsaturated lipids.