Serum High-Performance Liquid Chromatography and metabolomics identify disrupted homeostasis of D- and L-amino acids as biochemical signature in Parkinson’s disease and frailty syndrome

Recent studies showed increased concentrations of the glutamatergic NMDA receptor co-agonist D- serine and its precursor L-serine in the striatum of Parkinson’s disease (PD) animal models, the caudate-putamen of human PD brains and the cerebrospinal fluid of de novo PD patients. Moreover, a systemic dysregulation of amino acids metabolism has been described in frailty, a common clinical syndrome of the elderly associated with high risk of disability, mortality and increased incidence of neurodegenerative diseases. Here, I investigated the serum amino acids profile in two independent cohorts composed by (i) PD patients and age-matched healthy subjects (PD cohort); (ii) elderly subjects without neurodegenerative diseases including frail, pre-frail and non-frail individuals (frailty cohort). Frailty was assessed with the Edmonton Frail scale (EFS), a multidimensional tool including nine health items, and with Fried criteria, as a measure of physical frailty. Two complementary analytical approaches were adopted: (i) High-Performance Liquid Chromatography (HPLC) focused on the amino acids acting on glutamatergic transmission (L- glutamate, L-aspartate, glycine, D-serine, and their precursors L-glutamine, L-asparagine, and L- serine); (ii) untargeted 1H-Nuclear Magnetic Resonance (1H-NMR) metabolomics, to obtain an unbiased overview on the serum metabolite profile in PD and frailty. An additional targeted ultraperformance liquid chromatography-mass spectrometry (UPLC/MS) metabolomic analysis was performed in the PD cohort. The HPLC study showed that D-serine and D-/Total serine ratio positively correlated with age and age at disease onset in PD patients, while there were no correlations with age in healthy controls (HC). Moreover, I found that higher Levodopa Equivalent Daily Dose (LEDD) correlated with lower levels of D-serine and the other excitatory amino acids. Following these results, the addition of LEDD as covariate in the analyses disclosed a selective significant increase of D-serine in PD compared to HC (Δ≈38%). Stratification by sex revealed that this effect was specific of female patients. Untargeted serum metabolomics revealed higher glutamine, serine, pyruvate and lower α-ketoglutarate levels in PD compared to HC, while UPLC/MS analysis showed (i) lower glutamate, tryptophan and kynurenine levels and (ii) higher cystathionine, glycine and threonine levels in PD than HC. In the frailty cohort, HPLC analyses showed higher serum D-serine and D-/Total serine ratio as independent predictors of worse EFS score, but not of physical frailty. Furthermore, higher glycine levels and D-/Total serine correlated with worse cognition and depressive symptoms in the frail (but not in the non-frail) group. Untargeted metabolomics revealed distinct alterations of amino acids, lipids and energy metabolism in frail and pre-frail subjects. Notably, upregulated levels of the atypical amino acid betaine emerged as a biochemical fingerprint of pre-frail subjects. Overall, these findings suggest that (1) serum D-serine and D-/Total serine may represent a valuable biochemical signature of PD; (2) higher serum D-serine levels correlate with worse multidimensional frailty severity; (3) increased serum glycine and D-/Total serine ratio could be associated with cognitive decline and depression in frail older populations; (iii) serum metabolomics represents a promising source of novel amino acids signatures in PD and frailty. Future longitudinal studies evaluating whether serum D- and L-amino acids may help in identifying novel biological subtypes of PD and frailty are warranted.