Lymphatic drainage at the meningeal-brain interface in aging and Alzheimer’s disease

Background: Alzheimer’s disease (AD) is the most prevalent form of dementia. We have recently shown that proper meningeal lymphatic drainage is important for brain fluid flow, learning and memory in aging and for brain amyloid beta (Aβ) clearance in mouse models of AD. Interestingly, genetic factors that increased the risk for AD are also linked to altered meningeal lymphatic function. Method: Meningeal lymphatic function was evaluated in mouse models of AD at different ages by RNA sequencing of sorted meningeal lymphatic endothelial cells (LECs) and by assessing of lymphatic vessel morphology and drainage capacity. Meningeal lymphatic dysfunction was induced by photoablation, whereas enhancement of meningeal lymphatic drainage was achieved by increasing the levels of VEGF-C. The transcriptional signatures of brain blood endothelial cells and microglia were determined by single-cell RNA sequencing upon manipulation of meningeal lymphatic drainage. Changes in Aβ plaque clearance by monoclonal antibodies were assessed upon prolonged manipulation of meningeal lymphatic drainage. Integrated analysis was performed using murine and human cell RNA-seq data, as well as AD summary statistics. Result: We show that a progressive deposition of meningeal Aβ with aging leads to gene expression changes in meningeal lymphatic endothelial cells and an accelerated reduction in meningeal lymphatic vessel coverage in the 5xFAD mouse model. Interestingly, modulating meningeal lymphatic function in AD transgenic mice affects the clearance of Aβ by monoclonal antibodies, a feature that is closely associated with the activation signatures in brain blood endothelial cells and microglia. Furthermore, genes that are highly expressed in lymphatic vasculature are associated with increased risk for AD and altered levels of Aβ in the CSF of AD patients. Integrated analysis of murine and human microglia transcriptomes revealed an association between the signatures of activated microglia from mice with defective meningeal lymphatic function and activated microglia isolated from the AD brain. We are now turning our attention to the interaction between the main genetic risk factor for AD, expression of the apolipoprotein E4, and changes in brain drainage by the meningeal lymphatic vascular system. Conclusion: Collectively, our data underline the notion that a dysfunctional meningeal lymphatic vasculature may impact on AD pathophysiology and therapeutic outcome.