Altered tear fluid protein expression in persons with mild Alzheimer's disease in proteins involved in oxidative stress, protein synthesis, and energy metabolism

BackgroundTear fluid (TF) is a protein-rich solution that reflects pathophysiological changes in Alzheimer's disease (AD).ObjectiveIn this study, we examined whether TF proteins were differently expressed in persons with mild AD dementia compared to cognitively healthy controls (CO).MethodsWe analyzed data from 53 study participants including 34 CO (mean age, 71 years; Mini-Mental State Examination [MMSE] score, 28.9 ± 1.4), and 19 patients with AD (Clinical Dementia Rating, 0.5-1; mean age, 72 years; MMSE score, 23.8 ± 2.8). All participants underwent cognitive testing, as well as neurological and ophthalmological examinations. TF was collected using Schirmer strips, and TF protein content was evaluated using mass spectrometry-based proteomics and label-free quantification.ResultsWe found that 16 proteins exhibited significantly upregulated expression in the AD group compared to the CO group (<i>p </i>≤ 0.05). These proteins were NP1L4, BBOX1, CYTC, RNAS4, PCD, RNT2, AL1A3, SYSC, TPIS, CLH1, PGAM1, EIF3L, 5NTC, HNRNPA2B1, PYGL, and ERO1α. No proteins were significantly downregulated in the AD group compared to the CO group.ConclusionsOur results support the hypothesis that TF is a potential source of biomarkers for AD. Part of those proteins with altered expression have previously linked to increased oxidative stress, changed protein synthesis, and disturbed regulation of energy metabolism related to AD or neurodegenerative disease. The present results indicate the value of continued investigation of TF proteins in AD.