Lipid Profiling of Alzheimer's Disease Brain Highlights Enrichment in Glycerol(phospho)lipid, and Sphingolipid Metabolism

Abstract

Alzheimer's disease (AD) is reported to be closely linked with abnormal lipid metabolism. To gain a more comprehensive understanding of what causes AD and its subsequent development, we profiled the lipidome of postmortem (PM) human brains (neocortex) of people with a range of AD pathology (Braak 0-6). Using high-resolution mass spectrometry, we employed a semi-targeted, fully quantitative lipidomics profiling method (Lipidyzer) to compare the biochemical profiles of brain tissues from persons with mild AD (n = 15) and severe AD (AD; n = 16), and compared them with age-matched, cognitively normal controls (n = 16). Univariate analysis revealed that the concentrations of 420 lipid metabolites significantly (p < 0.05; q < 0.05) differed between AD and controls. A total of 49 lipid metabolites differed between mild AD and controls, and 439 differed between severe AD and mild AD. Interestingly, 13 different subclasses of lipids were significantly perturbed, including neutral lipids, glycerolipids, glycerophospholipids, and sphingolipids. Diacylglycerol (DAG) (14:0/14:0), triacylglycerol (TAG) (58:10/FA20:5), and TAG (48:4/FA18:3) were the most notably altered lipids when AD and control brains were compared (p < 0.05). When we compare mild AD and control brains, phosphatidylethanolamine (PE) (p-18:0/18:1), phosphatidylserine (PS) (18:1/18:2), and PS (14:0/22:6) differed the most (p < 0.05). PE (p-18:0/18:1), DAG (14:0/14:0), and PS (18:1/20:4) were identified as the most significantly perturbed lipids when AD and mild AD brains were compared (p < 0.05). Our analysis provides the most extensive lipid profiling yet undertaken in AD brain tissue and reveals the cumulative perturbation of several lipid pathways with progressive disease pathology. Lipidomics has considerable potential for studying AD etiology and identifying early diagnostic biomarkers.

Keywords: Alzheimer’s disease; brain; lipidomics; metabolomics; pathogenesis.

Figure 1

Principal component analysis (PCA) of the imputed and transformed data for outlier detection in control, mild AD, and AD groups was performed to check for any intrinsic variation. The following steps were followed to handle the data: standardization of the data; computing the covariance matrix; calculating the eigenvectors and eigenvalues; computing the principal components; and reducing the dimensions of the dataset.

Figure 2

Violin plots comparing the median concentration of different total species of complex lipids in the brain among AD (n = 16), mild AD (n = 15), and control (n = 16) groups. Data was analyzed by t-test, where: ns, p > 5 × 10⁻²; * p < = 5 × 10⁻²; ** p < = 1 × 10⁻²; *** p < = 1 × 10⁻³; **** p < = 1 × 10⁻⁴ Abbreviations: CE, cholesteryl esters; CER, ceramides; DAG, diacylglycerols; DCER, dihydroceramides; HCER, hexosylceramides; LCER, lactosylceramides; LPG, lysyl-phosphatidylglycerol; MAD, Mild AD (Mild Alzheimer’s disease); PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PI, phosphatidylinositol; PS, phosphatidylserine; SM, sphingomyelins; TAG, triacylglycerols.

Figure 3

Proposed feedback modulation of Aβ synthesized from APP by the enzyme γ-secretase. Two main lipid groups, cholesterol and sphingomyelin, are closely related to Aβ production in AD. Cholesterol increases γ-secretase activity, which leads to elevated Aβ production. Conversely, Aβ downregulates cholesterol de novo synthesis from acetyl-CoA by inhibiting enzyme HMG-CoA reductase. Sphingomyelin inhibits γ-secretase activity, and in return reduced γ-secretase activity depresses sphingomyelin synthase activity. (Figure modified from the manuscript Zinser E.G. et al. BBA 2007;1768:1991–2001.) The related lipid species we measured are listed in the parentheses. Acetyl-CoA synthesized from fatty acids is released from diacylglycerol and triacylglycerols. Abbreviations: APP, amyloid precursor protein; CE, cholesteryl esters; CER, ceramides; DAG, diacylglycerols; DCER, dihydroceramides; HCER, hexosylceramides; LCER, lactosylceramides; SM, sphingomyelins; TAG, triacylglycerols.