Metabolite profiling for the identification of altered metabolic pathways in Alzheimer's disease

Abstract

Gas chromatography coupled to mass spectrometry is the most frequent tool for metabolomic profiling of low molecular weight metabolites. Its suitability in health survey is beyond doubt, given that primary metabolites involved in central pathways of metabolism are usually altered in diseases. The objective of this work is to investigate metabolic differences in serum between Alzheimer's disease patients and healthy controls in order to elucidate pathological mechanisms underlying to disease. Alterations in levels of 23 metabolites were detected, including increased lactic acid, α-ketoglutarate, isocitric acid, glucose, oleic acid, adenosine and cholesterol, as well as decreased urea, valine, aspartic acid, pyroglutamate, glutamine, phenylalanine, asparagine, ornithine, pipecolic acid, histidine, tyrosine, palmitic and uric acid, tryptophan, stearic acid and cystine. Metabolic pathway analysis revealed the involvement of multiple affected pathways, such as energy deficiencies, oxidative stress, hyperammonemia, and others. Moreover, it is noteworthy that some of these compounds have not been previously described in AD research, such as α-ketoglutarate, isocitrate pipecolic acid, pyroglutamate and adenosine, confirming the potential of this metabolomic approach in the search of novel potential markers for early detection of Alzheimer's disease.

Application of metabolomics based on direct mass spectrometry analysis for the elucidation of altered metabolic pathways in serum from the APP/PS1 transgenic model of Alzheimer's disease

“…In this sense we also observed a significant decrease of pyroglutamic acid levels, an essential compound for the regulation of the entry of amino acids into the brain via the ␥-glutamyl cycle, whose dyshomeostasis is indicative of aberrant functioning of this metabolic process. Thereby, previous studies already reported lower levels of pyroglutamate in brain [7], CSF [31] and blood [32,33] from AD patients, corroborating results presented here. On the other hand, reduced serum histidine could point to impaired synthesis of carnosine and/or histamine, important biomolecules associated with oxidative stress [34] and failures in neurotransmission [35], respectively, whose reduction has been previously related to AD pathogenesis [18,19,33,36].…”

“…Thereby, previous studies already reported lower levels of pyroglutamate in brain [7], CSF [31] and blood [32,33] from AD patients, corroborating results presented here. On the other hand, reduced serum histidine could point to impaired synthesis of carnosine and/or histamine, important biomolecules associated with oxidative stress [34] and failures in neurotransmission [35], respectively, whose reduction has been previously related to AD pathogenesis [18,19,33,36]. Moreover the reduction of tryptophan and tyrosine levels, precursors of serotonin and catecholamines, respectively, might denote a severe disturbance in monoaminergic neurotransmission systems, which confirm previous studies in AD subjects [37,38].…”

“…Decreased urea content (Table 1) supports a disturbed homeostasis of ammonia via the urea cycle, which may elicit deleterious effects on the central nervous system and has been closely related to AD pathology [39]. Thereby, an abnormal content of urea and related amino acids has been previously found in serum [19,20,33] and brain [8,9] of AD, accompanied by altered levels of expression in different enzymes and the corresponding genes [40]. Other studies reported an increased concentration of inosine in response to an accelerated degradation of nucleotides in brain from human patients [41] and APP/PS1 mice [9], which is finally reflected in peripheral blood as revealed our metabolomic fingerprinting platform.…”

Metabolic Biomarkers and Neurodegeneration: A Pathway Enrichment Analysis of Alzheimer's Disease, Parkinson's Disease, and Amyotrophic Lateral Sclerosis

“…As for AD, the lipids, sphinganine-1-phosphate and lysophosphatidylcholine (2-lysolecitine), together with pipecolic acid and adenosine were determined in serum of patients (González-Domínguez et al, 2015;Liang et al, 2016;Voyle et al, 2016).…”

Metabolomic Fingerprinting of Blood Samples by Direct Infusion Mass Spectrometry: Application in Alzheimer’s Disease Research

“…These studies revealed significant disturbances in the homeostasis of various lipid classes (e.g., phospholipids, fatty acids, glycerolipids), energy-related metabolites, neurotransmitters, and other metabolites, thus evidencing the utility of simple and easily available serum samples in clinical research. Furthermore, these findings were then validated using orthogonal hyphenated approaches, such as UHPLC-MS [36,37], GC-MS [37,38], and CE-MS [39], demonstrating the reliability of direct MS-based metabolomics. This DIMS platform was latterly adapted for the analysis of other biological matrices, such as urine [40], brain [41], peripheral organs (i.e., liver, kidney, thymus, spleen) [42], as well as muscle, hepatopancreas, and antennal gland [43], providing very valuable results to accomplish a preliminary metabolomic screening prior to the application of other complementary techniques.…”