Imaging of lipids and proteins in Alzheimer's disease using Time-of-Flight Secondary Ion Mass Spectrometry
Alzheimer’s disease (AD) is a neurodegenerative disease characterized by the formation of senile plaques. These plaques, which consist of aggregations of a peptide called amyloid-β, are deposited in-between the nerve cells in the brain, where they disrupt the signaling processes. The reason for the generation of these plaques is not completely known, but one has found that the regulation of lipids, such as cholesterol, in the cell membrane is one of many factors involved in the process.
One method used to study the generation of AD is imaging of brain tissue samples with fluorescence microscopy. To be able to study individual types of molecules in the tissue, immunohistochemistry is often applied, in which antibodies are used to target the molecule of interest. In this way, several different proteins can be visualized simultaneously, while lipids often remain unseen. Another method that can be used to image molecules in tissue samples, and especially lipids, is time-of-flight secondary ion mass spectrometry (ToF-SIMS). However, this method cannot detect intact molecules over ~2 kDa, thereby excluding most peptides and proteins from being identified.
In this work, the capability of ToF-SIMS to detect lipids is utilized for targeting proteins in tissue samples using antibody-coupled lipid vesicles, so called liposomes. The antibody-coupled liposomes were specifically bound to amyloid-β deposits in transgenic AD mouse brains, enabling ToF-SIMS imaging of both amyloid-β and, at the same time, surrounding lipids, such as cholesterol, in the tissue. The specificity of the liposome binding was investigated by analyzing their interaction with a model surface using quartz crystal microbalance with dissipation monitoring (QCM-D). Furthermore, the binding of the antibody-coupled liposomes to amyloid-β deposits in tissue sections was analyzed with fluorescence microscopy, confirming specific binding. To unravel possible artifacts in the tissue sample due to the demanding sample preparation required for ToF-SIMS imaging, the effects of the tissue preparation protocol were using ToF-SIMS and scanning electron microcopy (SEM), revealing no severe spatial redistribution of the native lipids or any major disruption of the surface morphology.
This method may thus provide an important complement to traditional tissue imaging approaches for the investigation of the interaction between lipids and proteins, which may result in important clues about the generation of different diseases, such as AD.