Targeted Lipidomics of Fontal Cortex and Plasma Diacylglycerols (DAG) in Mild Cognitive Impairment and Alzheimer's Disease: Validation of DAG Accumulation Early in the Pathophysiology of Alzheimer's Disease

Abstract

Previous studies have demonstrated augmented levels of diacylglycerols (DAG) in the frontal cortex and plasma of Alzheimer's disease (AD) patients. We extended these findings from non-targeted lipidomics studies to design a lipidomics platform to interrogate DAGs and monoacylglycerols (MAG) in the frontal cortex and plasma of MCI subjects. Control subjects included both aged normal controls and controls with normal cognition, but AD pathology at autopsy, individuals termed non-demented AD neuropathology. DAGs with saturated, unsaturated, and polyunsaturated fatty acid substituents were found to be elevated in MCI frontal cortex and plasma. Tandem mass spectrometry of the DAGs did not reveal any differences in the distributions of the fatty acid substitutions between MCI and control subjects. While triacylglycerols were not altered in MCI subjects there were increases in MAG levels both in the frontal cortex and plasma. In toto, increased levels of DAGs and MAGs appear to occur early in AD pathophysiology and require both further validation in a larger patient cohort and elucidation of the lipidomics alteration(s) that lead to the accumulation of DAGs in MCI subjects.

Keywords: Alzheimer’s disease; diacylglycerols; mild cognitive impairment; monoacylglycerols.

Figure 1

Levels of diacylglycerols (DAG) in the frontal cortex gray matter of subjects previously diagnosed as mild cognitive impairment (MCI; > 85 yr.; N=19). The control groups included young controls (YC; < 85 yr.; N=20) with no neurological or cognitive impairment and old controls (OC; >85 yr; N=8.) with no cognitive impairment but significant AD neuropathology at autopsy. Y axis is the ratio of the peak area of the endogenous lipid to the peak area of a stable isotope internal standard, corrected for tissue wet weight. The data are presented as mean ± SEM. All DAG levels in the MCI group were significantly (p < 0.05) increased above YC values while OC were not different from YC.

Figure 2

Levels of diacylglycerols (DAG) in the plasma of control subjects (MMSE 25–30), MCI subjects (MMSE 19–24), moderate dementia patients (MMSE 10–18), and severe dementia patients (MMSE 4–9). Y axis is the ratio of the peak area of the endogenous lipid to the peak area of a stable isotope internal standard, in the upper graph but is presented as nM in the lower graph. The data are presented as mean ± SEM. All DAG levels in the MCI, moderate dementia, and severe dementia groups were significantly (p < 0.05) increased above young controls (YC) values.

Figure 3

Levels of monoacylglycerols (MAG) in plasma (Upper panel) and frontal cortex gray matter (Lower panel). Tissue samples were from subjects previously diagnosed as mild cognitive impairment (MCI; > 85 yr.; N=19). The tissue controls included young controls (YC; < 85 yr.; N=20) with no neurological or cognitive impairment and old controls (OC; >85 yr; N=8.) with no cognitive impairment but significant AD pathology at autopsy. The plasma samples were obtained from control subjects (MMSE 25–30), MCI subjects (MMSE 19–24), moderate dementia patients (MMSE 10–18), and severe dementia patients (MMSE 4–9). Y axis is the ratio of the peak area of the endogenous lipid to the peak area of a stable isotope internal standard and were corrected for wet weight in the case of tissues. The data are presented as mean ± SEM. *, p < 0.05 vs. controls.

Figure 4

Presentation of DAG synthesis and metabolism. Figure A presents a broad overview of the interrelationships of DAGs with a number of other lipid pools while Figure B focusses on pathways involving glycerophospholipid metabolism and the associated enzymes reported to be altered in AD (see Discussion). AGPAT, acylglycerol-3-phosphate acyltransferase; ATX, autotaxin (lysophospholipase D); CDPPT, CDP-diacylglycerol-inositol/glycerol phosphatidyltransferase; CEPT, choline and ethanolamine phosphotransferases; CL, cardiolipin; DAG, diacylglycerol; DAGL, diacylglycerol lipase; DGAT, diacylglycerol acyltransferase; DHAP, dihydroxyacetone phosphate; DGK, diacylglycerol kinase; FA, fatty acid; G-3-P, glycerol-3-phosphate; GPAT, glycerol-3-phosphate acyltransferase; GPL, glycerophospholipid; HSL, hormone-sensitive lipase; LPA, lysophosphatidic acid; LPAT, lysophospholipid acyltransferase; LPL, lipoprotein lipase; MAG, monoacylglycerol; MGL, monoacylglycerol lipase; PA, phosphatidic acid; PAP, phosphatidic acid phosphatase; PC, phosphatidylcholines; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PI, phosphatidylinositol; PLA2, phospholipase A2; PLC, phospholipase C; PLD, phospholipase D; PCT, phosphatidic acid cytidyltransferase; PS, phosphatidylserine; TAG, triacylglycerol; TAGL triacylglycerol lipase. Enzymes that have been reported to be elevated in AD are highlighted with an up arrow while enzymes that are decreased are highlighted with a down arrow.