DKK2 secretion by activated microglia in Alzheimer’s disease contributes to blood-brain barrier dysfunction

Around 80% of Alzheimer’s disease (AD) cases show cerebral amyloid angiopathy (CAA) with amyloid-β (Aβ) deposition around blood vessels. Although blood-brain barrier (BBB) impairment in AD has been reported, detailed information on changes of gene expression in cell types of the neuro-vascular unit (NVU) is missing.

Compared to age-matched healthy controls, AD patients are affected by altered brain vascular morphology, distorted neurovascular coupling, and shared risk factors with cardiovascular disease patients. These vascular abnormalities appear before the classical hallmarks of AD, speaking in favour of an early pathogenic role of the BBB. Along with the vasculature, the immune system has become progressively recognized as a modulator of AD. Microglia (MG) contribute to Aβ deposits, become reactive and initiate an immune response.

The Wnt pathway is a recognized molecular mechanism that maintains the properties of the BBB and its dysregulation has been linked with important brain pathologies such as tumours, stroke, and neurodegenerative diseases [1]. However, little is known about the cellular or molecular mechanisms by which the Wnt pathway modulates vascular function in AD, and its contribution to disease progression.

To investigate AD- and CAA-related effects on the BBB and on pathology progression, we used a transgenic AD mouse model that inherited the known human APP mutations Swedish, Dutch and Iowa under the neuronal Thy1 promoter (Thy1:Tg-SwDI; thereafter as AD^SwDI), showing onset of CAA and AD symptoms by 3 months of age [2]. By RNA-Seq of mouse brain microvessels (MBMVs) and FACS-sorted NVU cells we show a downregulation of the Wnt/β-catenin pathway in ECs and an upregulation of the soluble Wnt pathway inhibitor Dkk2 in microvessel-associated MGs, in young and old AD^SwDI mice compared to controls. These findings went along with pericyte drop off, increased vascular permeability for 4KD-FITC dextran, as well as behavioural deficits. In a primary murine BBB in vitro model, Dkk2 reduced endothelial barrier properties in Transwell®-based measurements of the transendothelial electrical resistance (TEER), suggesting that MG-derived Dkk2 at CAA plaques lead to barrier impairment.

Although MG-derived DKK2 expression in AD mice and patients has previously been shown, a specific role in human AD has been questioned [3,4].

We now show a correlation of DKK2 protein expression and accumulation specifically in CAA plaques in brain biopsy samples of human AD patients, suggesting that the small subset of Dkk2-secreting MGs specifically act at the vascular niche.

Finally, we hypothesized that therapeutic activation of the Wnt/β-catenin pathway may counteract BBB impairment in AD. To this end we crossed the AD^SwDI mouse model with the Cdh5(PAC)-CreERT2:Ctnnb1(Ex3)^fl/fl inducible, endothelial-specific, β-catenin gain-of-function mice [5,6,7]. We observed that early induction of β-catenin signalling between 1-2 month-of-age ameliorated cognitive decline and memory loss, evidenced by Y-maze behavioural tests. Our data suggest that microglia-derived Dkk2 modulates Wnt/β-catenin and BBB properties in AD. We hypothesise that the Wnt/β-catenin pathway at the BBB might be a potential therapeutic target, which might be particularly promising when applied at an early point.

References

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