Distribution of Lipids in the Human Brain and their Differential Expression in Alzheimer's Disease: A Matrix-Assisted Laser Desorption/Ionisation-Imaging Mass Spectrometry (MALDI-IMS) Study

Abstract

Alzheimer’s disease (AD), the leading cause of dementia, is pathologically characterised by β-amyloid plaques and tau tangles. However, there is also evidence of lipid-dyshomeostasis-mediated AD pathology. Given the structural diversity of lipids, mass spectrometry (MS) is a useful tool for studying lipid changes in AD. The use of matrix-assisted laser desorption/ionization (MALDI)-imaging mass spectrometry (IMS) circumvents the limitation of traditional MS, allowing users to visualise the distribution of lipids. Thus, I optimised MALDI-IMS to image the distribution of lipids in the postmortem human middle temporal gyrus (MTG) and hippocampus, and analyse its differential expression in AD. In order to study a large number of cases, compared to previously published MALDI-IMS papers, I developed an analysis workflow to efficiently evaluate large, heterogeneous datasets and accurately detect lipids that were differentially expressed in AD. I hypothesised that the MTG would show similar lipid differences to those previously reported in other cortical regions. Further, given that each hippocampal anatomical sub-field has its own function, I postulated that there would be lipid differences unique to each sub-field. Both positively- and negatively-charged lipid ion species were abundantly detected in the control and AD cohorts. Grey matter and white matter had unique lipid profiles. However, there were variations in the distribution of lipids even within the same region, especially in the grey matter in the MTG and the CA1 region in the hippocampus. In AD, while the distribution patterns of lipids were comparable to the control cohort, some lipids were expressed at different levels. For example, the expression of some phosphatidylethanolamine (PE) lipids was decreased in the MTG. The majority of lipids that were differentially expressed in the hippocampus were found in the CA1 region. Further, there were differences in eight lipids that were specific to the dentate gyrus (DG) region. High-resolution MALDI IMS revealed that these lipids showed a heterogeneous distribution amongst the three DG layers. Finally, I quantified PE lipids with MALDI-IMS, using a lipid-spiked tissue homogenate approach. This is the first time that this approach has been successfully used to quantify lipids the human MTG and the DG. The concentration of PE did not change in the DG in AD; however, the concentrations of four PE species, namely PE 38:4, PE 39:5, and PE 40:6, were reduced in the grey matter in the MTG. Thus, the MALDI-IMS technique, the analysis workflow, and the lipid quantification approach, provided a novel method to investigate specific lipid differences in the postmortem human brain in AD. This work extends the understanding of the lipid composition of the human brain and how it differs in AD. Future work will focus on elucidating if these lipid differences are a driver, or consequence, of AD pathogenesis.