RESUMO
BACKGROUND: Studies suggest that cerebrospinal fluid (CSF) levels of amyloid-ß (Aß)42 and Aß40 present a circadian rhythm. However sustained sampling of large volumes of CSF with indwelling intrathecal catheters used in most of these studies might have affected CSF dynamics and thereby confounded the observed fluctuations in the biomarker levels. METHODS: We included 38 individuals with either normal (N = 20) or abnormal (N = 18) CSF Aß42/Aß40 levels at baseline. CSF and plasma were collected at two visits separated by an average of 53 days with lumbar punctures and venipunctures performed either in the morning or evening. At the first visit, sample collection was performed in the morning for 17 participants and the order was reversed for the remaining 21 participants. CSF and plasma samples were analyzed for Alzheimer' disease (AD) biomarkers, including Aß42, Aß40, GFAP, NfL p-tau181, p-tau217, p-tau231 and t-tau. CSF samples were also tested using mass spectrometry for 22 synaptic and endo-lysosomal proteins. RESULTS: CSF Aß42 (mean difference [MD], 0.21 ng/mL; p = 0.038), CSF Aß40 (MD, 1.85 ng/mL; p < 0.001), plasma Aß42 (MD, 1.65 pg/mL; p = 0.002) and plasma Aß40 (MD, 0.01 ng/mL, p = 0.002) were increased by 4.2-17.0% in evening compared with morning samples. Further, CSF levels of 14 synaptic and endo-lysosomal proteins, including neurogranin and neuronal pentraxin-1, were increased by 4.5-13.3% in the evening samples (MDrange, 0.02-0.56 fmol/µl; p < 0.042). However, no significant differences were found between morning and evening levels for the Aß42/Aß40 ratio, different p-tau variants, GFAP and NfL. There were no significant interaction between sampling time and Aß status for any of the biomarkers, except that CSF t-tau was increased (by 5.74%) in the evening samples compared to the morning samples in Aß-positive (MD, 16.46 ng/ml; p = 0.009) but not Aß-negative participants (MD, 1.89 ng/ml; p = 0.47). There were no significant interactions between sampling time and order in which samples were obtained. DISCUSSION: Our findings provide evidence for diurnal fluctuations in Aß peptide levels, both in CSF and plasma, while CSF and plasma p-tau, GFAP and NfL were unaffected. Importantly, Aß42/Aß40 ratio remained unaltered, suggesting that it is more suitable for implementation in clinical workup than individual Aß peptides. Additionally, we show that CSF levels of many synaptic and endo-lysosomal proteins presented a diurnal rhythm, implying a build-up of neuronal activity markers during the day. These results will guide the development of unified sample collection procedures to avoid effects of diurnal variation for future implementation of AD biomarkers in clinical practice and drug trials.
