Microglia prevent beta-amyloid plaque formation in the early stage of an Alzheimer's disease mouse model with suppression of glymphatic clearance

Background: Soluble beta-amyloid (A beta) can be cleared from the brain through various mechanisms including enzymatic degradation, glial cell phagocytosis, transport across the blood-brain barrier, and glymphatic clearance. However, the relative contribution of each clearance system and their compensatory effects in delaying the pathological process of Alzheimer's disease (AD) are currently unknown. Methods: Fluorescent trace, immunofluorescence, and Western blot analyses were performed to compare glymphatic clearance ability and A beta accumulation among 3-month-old APP695/PS1-dE9 transgenic (APP/PS1) mice, wild-type mice, aquaporin 4 knock out (AQP4(-/-)) mice, and AQP4(-/-)/APP/PS1 mice. The consequence of selectively eliminating microglial cells, or downregulating apolipoprotein E (apoE) expression, on A beta burden, was also investigated in the frontal cortex of AQP4(-/-)/APP/PS1 mice and APP/PS1 mice. Results: AQP4 deletion in APP/PS1 mice significantly exaggerated glymphatic clearance dysfunction, and intraneuronal accumulation of A beta and apoE, although it did not lead to A beta plaque deposition. Notably, microglia, but not astrocytes, increased activation and phagocytosis of A beta in the cerebral cortex of AQP4(-/-)/APP/PS1 mice, compared with APP/PS1 mice. Selectively eliminating microglia in the frontal cortex via local injection of clodronate liposomes resulted in deposition of A beta plaques in AQP4(-/-)/APP/PS1 mice, but not APP/PS1 mice. Moreover, knockdown of apoE reduced intraneuronal A beta levels in both APP/PS1 mice and AQP4(-/-)/APP/PS1 mice, indicating an inhibitory effect of apoE on A beta clearance. Conclusion: The above results suggest that the glymphatic system mediated A beta and apoE clearance and microglia mediated A beta degradation synergistically prevent A beta plague formation in the early stages of the AD mouse model. Protecting one or both of them might be beneficial to delaying the onset of AD.