1. Introduction

Introduction: Blood-brain barrier (BBB) dysfunction is potentially one of the earliest microvascular changes in Alzheimer’s disease (AD) and related dementias¹. Techniques utilizing gadolinium-based contrast agents (GBCA) to measure BBB permeability are not only invasive and costly but also potentially toxic. Given that GBCAs possess higher molecular weights, they are less sensitive to early, subtle BBB changes, such as disruption of aquaporin-4 channels, which have been linked to aging and AD in both animal models² and humans³. An emerging technique to image the time of exchange (Tex) of water across the BBB is multi-echo^4,5 arterial spin labeling (ASL)⁴, which obviates the need for exogenous contrast and has been shown to provide reproducible values of BBB integrity in healthy volunteers⁴. Here, we investigate the associations of Tex values with amyloid positivity and cognitive status. As cerebral blood flow (CBF) can be seen as an established ASL biomarker, we repeated all analyses for CBF for comparison.
Methods: Data from 116 participants older than 50 years were selected from the Center for Lifespan Changes in Brain and Cognition (LCBC) and the Dementia Disease Initiation (DDI) cohorts. LCBC comprises a population-based cohort including only cognitively normal (CN) participants (n=77, CN_LCBC), while DDI is a clinical outpatient cohort including CN and subjective cognitive decline patients (n=24, merged here into a single cohort CN_DDI) and mild cognitive impairment (n=15, MCI_DDI)⁶ patients. Amyloid status was defined as positive (A+) or negative (A-) from the CSF amyloid-beta 42/40 ratio (cut-off ≤ 0.077) or amyloid-PET by visual read, when available. All cohorts were scanned on the same 3T Siemens Prisma scanner with a 32-channel head coil. Two recently developed multi-post-labeling delay (PLD) Hadamard-encoded (HAD) 3D GRASE PCASL sequences were used to estimate Tex and CBF: 1) HAD-8 with a labeling duration (LD) 400 ms, PLD [600:400:3400] ms, and single echo time (TE) 12.5 ms; 2) multi-TE HAD-4 with LD 1000 ms, PLD [1500:1000:3500] ms, and 8 TEs [14.4:28.9:217.2] ms. Data were analyzed with ExploreASL 1.11.0beta⁷, and gray matter (GM) CBF and Tex were quantified with FSL-FABBER⁸. Tex and CBF associations with amyloid and cognitive status were assessed using linear regression adjusted for age and sex.
Results: Of the 116 participants, in total, 77 were from the LCBC (64.6±8.4 years, 64% female) and 39 from the DDI (67.7±7.9 years, 51% female) cohorts. DDI included 15 MCIs and 28 A+, of which 12 were both MCI and A+. Across the whole population, GM Tex was negatively correlated with age (r = -0.38, p < 0.001, Figure 1A), whereas for GM CBF, this correlation was not statistically significant (r = -0.26, p = 0.069, Figure 1B). Figures 2 and 3 show, respectively, whole-brain group average Tex and CBF maps from CN A- controls, MCI patients, and A+ subjects. Histograms next to these maps show the GM mean and standard deviation values of the three groups. It can be observed that Tex and CBF values are both higher in the NC A- group than in the MCI or A+ maps.Tex was 15% lower in A+ compared to A- (t=2.75, p=0.01, Figure 4A). CBF was 5% higher in the A+ group than the A- group but did borderline not reach statistical significance (t=-1.94, p=0.06, Figure 4B)^9,10. The linear regression analysis showed that amyloid status was associated with BBB water permeability (given by Tex), with higher permeability in A+ compared with A- groups when correcting for age, sex, and CBF (β = -35 s, p < 0.001) (Figure 4C). Moreover, cognitive staging was related to Tex (Figure 5A), even when correcting for age and sex (β_MCI = -31.3 s, p < 0.01) (Figure 5C). A similar relationship was not found for CBF (Figure 5B, C).
Discussion: Interestingly, both amyloid positivity and cognitive status were associated with increased BBB water permeability, even when correcting for age and sex. In agreement with previous studies, BBB water permeability was shown to increase with age. These permeability increases might be explained by a normal aging process of increased brain clearance or by BBB dysfunction promoting leakage of toxins to cross from the capillary site to the brain parenchyma. Whether BBB dysfunction is the cause or effect of amyloid deposition cannot be differentiated from these data. These findings encourage the use of BBB-ASL to non-invasively investigate BBB integrity in the early stages of dementia.

Acknowledgements

The DEBBIE project (Developing a non-invasive biomarker for early BBB breakdown in Alzheimer’s disease) is an EU Joint Programme -Neurodegenerative Disease Research (JPND) project. For this DEBBIE substudy, we received funding through the following funding organisations under the aegis of JPND -www.jpnd.eu (BMBF in Germany, NFR in Norway, and ZonMw and Alzheimer Nederland in The Netherlands). The project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 825664.