The neuroinflammation signatures in different stages of the Alzheimer’s disease continuum

Abstract

AbstractBackgroundThe natural history of Alzheimer’s disease (AD) comprises a long preclinical stage characterized by pathological changes that start decades before symptoms arise. Despite that AD is defined based on the presence of amyloid‐β (Aβ) and tau pathology, increasing evidence supports neuroinflammation as one of the earliest pathomechanistic alterations throughout the AD continuum. Microglia and astrocytes are key cellular drivers and regulators of neuroinflammation. However, it remains unclear if there are patterns of spatial and temporal susceptibility to neuroinflammatory processes in the brain that may synergize with Aβ and tau accumulation, which drives neurodegeneration in a self‐reinforcing manner.MethodThis was a cross‐sectional study examining a total number of 283 subjects from the TRIAD cohort at McGill University Research Centre for Studies in Aging, Canada. Cerebral amyloid and tau neurofibrillary tangles were assessed using positron emission tomography (PET) radiopharmaceuticals [18F]AZD4694 ([18F]NAV4694) and [18F]MK6240 respectively. Cerebrospinal fluid (CSF) biomarkers including Aβ42, Aβ40, phosphorylated tau (p‐tau), total tau (t‐tau), neurofilament light (NfL), soluble triggering receptor expressed on myeloid cells 2 (sTREM2), YKL40 and glial fibrillary acidic protein (GFAP) were also measured. Voxelwise analyses were performed to evaluate the relationships between cerebral amyloid load, tau burden and neuroinflammation biomarkers.ResultWe modelled biomarker changes as a function of amyloid PET standardized uptake value ratio (SUVR) and tau PET SUVR as proxies of disease progression. The earliest changes observed in the AD continuum were the decrease in the Aβ42/40 ratio and the increases in astrocytic biomarkers CSF YKL40 and CSF GFAP. This is followed by a steep increase in CSF pTau231 and, to a lesser extent, CSF pTau217 and CSF pTau181. Voxelwise analyses revealed that YKL40 and GFAP are associated with amyloid and tau load in the brain, after accounting for age, sex, education and pathological status.ConclusionNeuroinflammation involving astrocytic activation is altered very early in the Alzheimer’s continuum and could be targeted as a promising biomarker.