Importance: The specific pattern/trajectory of β-amyloid (Aβ) pathology spreading in Alzheimer's disease (AD), from default mode network (DMN) regions to sensory-motor areas, is well known, but poorly understood. Objective: To determine if resting-state global brain activity is linked to early Aβ deposition in the DMN. Design: This is a retrospect analysis of multi-modal and longitudinal data from the Alzheimer's disease Neuroimaging Initiative (ADNI) cohort. Setting: The ADNI was a multicenter project involving 63 research centers. Participants: The study included 144 participants (72.6 ± 7.5 years; 73 females) of whom 28 were controls, 21 had significant memory concerns, 72 had cognitive impairment (N=72), and 23 had AD. There were both baseline and 2-year follow-up data for Aβ-PET for 112 of the subjects. They were classified into following stages based on the CSF Aβ42 (CSF+: <192 ng/L) and cortical Aβ (PET+: >0.872 SUVR) levels: non-Aβ-accumulators (CSF-/PET-); early-Aβ-accumulators (CSF+/PET-); and late-Aβ-accumulators (CSF+/PET+). Exposure: Resting-state brain activity was assessed by functional magnetic resonance imaging (rsfMRI), whereas glymphatic function was estimated by the coupling between fMRI blood-oxygen-level-dependent (BOLD) signals and CSF movements. Main Outcomes and Measures: Cortical Aβ accumulation measured by 18F-AV45 amyloid-positron emission tomography (PET), CSF Aβ42, and total and phosphorylated tau protein levels in all participants. Results: Glymphatic function assessed by fMRI was strongly (ρ > 0.43, P < 0.042) associated with various markers of protein aggregation in early Aβ accumulators in whom Aβ just begins to accumulate cortically in the DMN. Among these early accumulators, the preferential Aβ accumulation in the DMN regions in the subsequent two years was correlated with lower gBOLD signal (ρ = 0.51, P = 0.027) and lower local glymphatic function (ρ = 0.48, P = 0.041) in the same regions at baseline. Conclusions and Relevance: Resting-state global brain activity and related glymphatic function are linked to Aβ pathology, particularly its preferential deposition in the DMN at the earliest AD stages. This suggests potential novel early therapeutic directions that might provide disease modification.

The glymphatic system that clears out brain wastes, such as amyloid-beta (Abeta) and tau, through cerebrospinal fluid (CSF) flow may play an important role in aging and dementias. However, a lack of non-invasive tools to assess the glymphatic function in humans hindered the understanding of the glymphatic changes in healthy aging. The global infra-slow (<0.1 Hz) brain activity measured by the global mean resting-state fMRI signal (gBOLD) was recently found to be coupled by large CSF movements. This coupling has been used to measure the glymphatic process and found to correlate with various pathologies of Alzheimer's disease (AD), including Abeta pathology. Using resting-state fMRI data from a large group of 719 healthy aging participants, we examined the sex-specific changes of the gBOLD-CSF coupling, as a measure of glymphatic function, over a wide age range between 36-100 years old. We found that this coupling index remains stable before around age 55 and then starts to decline afterward, particularly in females. Menopause may contribute to the accelerated decline in females.