Cerebrospinal fluid biomarkers in relation to signature Alzheimer’s disease pathology and cognitive functions

Abstract

The focus has shifted in recent years from clinical towards a biological definition of Alzheimer’s disease (AD), as pathophysiological changes precede the clinical symptoms by decades. In vivo brain imaging and cerebrospinal fluid (CSF) biomarkers have been at the center of this change. The core biomarkers are amyloid beta (Aβ), phosphorylated tau (P-tau) and total tau (T-tau), reflecting signature aspects of AD pathology (Aβ plaques, neurofibrillary tangles and neurodegeneration). Although the diagnostic accuracies of these markers are satisfactory, their levels reach a plateau at later stages of the disease and do not associate well with the progression of cognitive impairment. The discovery of novel biomarkers is therefore of importance for a better understanding of different biological processes driving the pathology and clinical manifestation of the disease. The overall objective of the project was to evaluate the relationships of selected CSF biomarkers with signature AD pathology and cognitive functions among individuals at the symptomatic pre- or early stages of dementia.

Recent findings suggest that dysfunction in neuroinflammation, lipid metabolism and cholinergic neurons play a critical part in the pathogenesis of AD. In this study, we measured novel biomarkers reflecting these different biological processes. Subjects were recruited from The Icelandic MCI study, which included individuals referred to the Landspitali University Hospital (LUH) Memory Clinic. The criteria for inclusion were a score between 24-30 on the Mini-Mental State Examination (MMSE) and a score of 4.0 or less on the Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE) at the first visit. For this project, only subjects who underwent lumbar puncture were recruited from the cohort. All subjects were categorized as having a CSF AD profile reflecting signature AD pathology or non-AD profile, based on a cut-off point (0.52) between the ratio of T-tau and Aβ42 values in CSF.

The aim of Paper I was to evaluate the association of CSF biomarkers reflecting neurodegeneration (neurofilament light [NFL]) and inflammation (chitinase-3-like protein 1 [YKL-40], S100 calcium-binding protein B [S100B], glial fibrillary acidic protein [GFAP]) with CSF AD profile and cognitive decline. Our results showed that although the markers did not accurately differentiate between the two CSF profiles, they associated in different ways with certain cognitive domains. These relationships were mainly observed among subjects with a CSF AD profile, where higher levels of NFL associated with deficits in verbal episodic memory and higher levels of GFAP with deficits in processing speed.

The aim of Paper II was to identify lipid species best distinguishing between CSF profiles (AD and non-AD), and to examine their relationships with measures reflecting AD-related processes (neurodegeneration, inflammation, impairment in verbal episodic memory). Untargeted CSF lipidomic analysis was performed for the detection of mass-to-charge ratio (m/z) features, by the application of ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS). Out of 1008 features detected, eight were selected as best differentiating between the two CSF profiles. One out of the eight features was assigned to a lipid species (C18 ceramide), as it was the only one to be confirmed with high confidence. Lower levels of Aβ42 and higher levels of T-tau related to higher levels of C18 ceramide. Additionally, a positive relationship was found beween levels of inflammatory marker S100B and C18 ceramide.

The aim of Paper III was to assess the association between the activity of cholinesterase enzymes acetylcholine (AChE) and butyrylcholinesterase (BuChE) in CSF and measures reflecting AD-related processes (amyloidosis, neurodegeneration, inflammation, impairment in verbal episodic memory). CSF AChE and BuChE activity did not associate with amyloid status in the brain. Higher levels of T-tau and P-tau related to higher activity of AChE, and to a lesser extent, higher activity of BuChE. Higher levels of inflammatory markers S100B and YKL-40 also related to higher levels of both AChE and BuChE activity.

Overall, these results indicate the potential value of novel CSF biomarkers regarding diagnosis, progression and treatment of AD at the early stages of the disease. Higher levels of C18 ceramide related to AD pathology and inflammatory processes, indicating a potential as a therapeutic target and an additional diagnostic marker. The activity of ACh-degrading cholinergic enzymes related to inflammatory and neurodegenerative processes. Proteins NFL and GFAP, measured in CSF, are promising markers for cognitive decline and progression among individuals with signature AD pathology.