Targeted and untargeted mass spectrometry reveals impact of high fat diet on peripheral amino acid regulation in a mouse model of Alzheimer's disease

Abstract

Background Mass spectrometry‐based metabolomics analyses were performed to examine metabolic changes under diet‐induced obesity in Alzheimer's Disease (AD) and assess whether these changes are reversible with diet modification. Specifically, amino acid metabolism was investigated because amino acid levels are related to obesity/diabetes and elevated levels have been shown to induce many of the pathophysiological hallmarks of AD. Method Targeted hydrophilic interaction liquid chromatography‐triple quadrupole mass spectrometry (HILIC‐MS/MS) and untargeted reversed‐phase liquid chromatography‐high resolution tandem mass spectrometry (RPLC‐HRMS/MS) assays were developed to analyze the metabolic changes that occur in AD and obesity. Frozen liver samples were obtained from a previously defined study, in which APPSwe/PS1ΔE9 (APP/PSEN1) transgenic mice (to represent familial or early‐onset AD) and wild‐type litter mater controls were fed either a high‐fat diet (HFD, 60% kcal from lard), low‐fat diet (LFD, 10% kcal from lard), or reversal diet (REV, high‐fat for 7.5 months followed by low‐fat for 2.5 months). Liver samples were collected after sacrifice and were prepared through homogenization and an established protein precipitation protocol. Result Multiple amino acids (including alanine, glutamic acid, leucine, isoleucine, and phenylalanine), carnitines, and members of the fatty acid oxidation pathway were significantly increased in APP/PSEN1 mice on HFD compared to LFD. More substantial effects and changes were observed in the APP/PSEN1 mice than WT mice, suggesting that they were more sensitive to a HFD. These dysregulated peripheral pathways include numerous amino acid pathways and fatty acid beta oxidation and suggest that obesity combined with AD further enhances cognitive impairment. These dysregulated peripheral pathways include pathways directly linked to the TCA cycle and mitochondrial dysfunction, which suggest that the HFD may contribute to AD pathogenesis by further contributing to this mitochondrial dysfunction. Furthermore, partial reversibility of many altered pathways was observed, which highlights that diet change can mitigate metabolic effects of AD. The same trends in individual amino acids were observed in both strategies, highlighting the biological validity of the results. Conclusion Our targeted and untargeted metabolomics results suggest that numerous peripheral pathways, specifically amino acid metabolism and fatty acid metabolism, were significantly affected in a combinatorial fashion by AD genotype and diet.