Revealing Sex and Alzheimer's Disease-related Changes in the Spatial Localization of Brain Lipids in Mice using Mass Spectrometry Imaging.

Lipids are essential biomolecules in the brain, as they have important roles in signaling and structural integrity. Of note, the central nervous system has a high concentration of lipids, second only to adipose tissue. Recently, it has been shown that abnormalities in lipid levels play a critical role in the onset of neurodegenerative diseases including Alzheimer's disease (AD). AD is a progressive and severe neurodegenerative disease and the leading cause for dementia, affecting millions worldwide. However, the spatial localization of individual lipid molecules in normal brain tissues, as well as region-specific lipid alterations during AD are yet to be explored. Additionally, current understanding of potential sexual dimorphisms in lipid localization in brain is incomplete. With these in mind, we employed high-resolution matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) coupled to ion mobility mass spectrometry to characterize region-specific lipid distribution in brain obtained from age- and sex-matched wild type and AD mice (18-month-old female C57BL/6J and 3xTg-AD mice, which exhibits both amyloid and tau pathology, n=3 per group). Additionally, the spatial distributions of lipids were investigated in male and female mice within two different age groups (4 and 12-week-old, n=3 per group). From these experiments, we were able to detect more than one thousand ions in brain tissue sections. Following detection, we annotated an approximate 100 lipid molecules with high confidence. Tissue imaging results revealed region-specific distributions of specific lipids representing a range of classes including phosphatidylcholines, lysophosphatidylcholines, phosphatidylethanolamines, ceramide phosphates, sphingomyelins, sulfated hexosyl ceramides, and ceramide phosphoinositols across male and female mice. Interestingly, ceramide phosphate (30:7), lysophosphatidylcholine (16:0), and phosphatidylcholine (36:5) molecules exhibited sex-specific localization patterns restricted to the cerebellum in 4-week-old male mice. Further, ceramide phosphoinositol (42:6) was localized to the cerebellum region of 12-week-old male and female mice whereas 4-week-old male and female mice showed relative homogeneous spatial distribution. Notably, we observed region-specific alterations in the spatial localization of several lipid molecules including phosphatidylcholine [(40:6), and (40:7)] and sulfated hexosyl ceramide [(16:1;2O/22:2;O), (24:2;2O/12:1;O), and (19:0;2O/15:0;O)] in AD mice as compared to age- and sex-matched controls. These observed changes occurred in the hippocampus region and were further confirmed using receiver operating characteristic analysis. Taken together, the above results reveal highly localized patterns of alterations in lipid molecules across male and female mice, as well as during AD.