Sophia M. Shi

Abstract The blood–brain barrier (BBB) is highly specialized to protect the brain from harmful circulating factors in the blood and maintain brain homeostasis 1,2 . The brain endothelial glycocalyx layer, a carbohydrate-rich meshwork composed primarily of proteoglycans, glycoproteins and glycolipids that coats the BBB lumen, is a key structural component of the BBB 3,4 . This layer forms the first interface between the blood and brain vasculature, yet little is known about its composition and roles in supporting BBB function in homeostatic and diseased states. Here we find that the brain endothelial glycocalyx is highly dysregulated during ageing and neurodegenerative disease. We identify significant perturbation in an underexplored class of densely O-glycosylated proteins known as mucin-domain glycoproteins. We demonstrate that ageing- and disease-associated aberrations in brain endothelial mucin-domain glycoproteins lead to dysregulated BBB function and, in severe cases, brain haemorrhaging in mice. Finally, we demonstrate that we can improve BBB function and reduce neuroinflammation and cognitive deficits in aged mice by restoring core 1 mucin-type O-glycans to the brain endothelium using adeno-associated viruses. Cumulatively, our findings provide a detailed compositional and structural mapping of the ageing brain endothelial glycocalyx layer and reveal important consequences of ageing- and disease-associated glycocalyx dysregulation on BBB integrity and brain health.

Summary Microglia, the brain’s immune cells, are highly responsive to their local environment. Given that circulatory proteins can enter the brain, we asked whether microglia are responsive to such proteins. Here, we identify a stable population of microglia specialized to take up circulatory proteins in a region-specific manner under physiological conditions; human hematopoietic stem cell-derived microglia replacing endogenous microglia in chimeric mice show similar regional specialization. Plasma-positive microglia are characterized by prominent expression of genes related to innate immunity and antigen presentation and exhibit high metabolic and phagocytic activity. This activity is dependent, in part, on microglial uptake and accumulation of circulatory Apolipoprotein AI (ApoA-I). Our findings thus identify a new model of communication between brain and periphery through specialized microglia.

Neurodegenerative diseases affect 1 in 12 people globally and remain incurable. Central to their pathogenesis is a loss of neuronal protein maintenance and the accumulation of protein aggregates with ageing1,2. Here we engineered bioorthogonal tools3 that enabled us to tag the nascent neuronal proteome and study its turnover with ageing, its propensity to aggregate and its interaction with microglia. We show that neuronal protein half-life approximately doubles on average between 4-month-old and 24-month-old mice, with the stability of individual proteins differing among brain regions. Furthermore, we describe the aged neuronal ‘aggregome’, which encompasses 1,726 proteins, nearly half of which show reduced degradation with age. The aggregome includes well-known proteins linked to diseases and numerous proteins previously not associated with neurodegeneration. Notably, we demonstrate that neuronal proteins accumulate in aged microglia, with 54% also displaying reduced degradation and/or aggregation with age. Among these proteins, synaptic proteins are highly enriched, which suggests that there is a cascade of events that emerge from impaired synaptic protein turnover and aggregation to the disposal of these proteins, possibly through microglial engulfment of synapses. These findings reveal the substantial loss of neuronal proteome maintenance with ageing, which could be causal for age-related synapse loss and cognitive decline. Newly developed mouse models that enable cell-specific analyses of proteostasis dynamics across the lifespan of the mice reveal key aspects of neuronal proteostasis with ageing.

Abstract The gut microbiome generates diverse metabolites that can enter the bloodstream and alter host biology, including brain function. Hundreds of physiologically relevant, gut-brain signaling molecules likely exist; however, there has been no systematic, high-throughput effort to identify and validate them. Here, we integrate computational, in vitro , and in vivo approaches to pinpoint microbiome-derived metabolites whose blood levels change during aging, and that induce corresponding changes in the mouse brain. First, we mine large-scale metabolomics datasets from human cohorts (each n ≥ 1200) to identify 30 microbiome-associated metabolites whose blood levels change with age. We then screen this panel in an in vitro transcriptomic assay to identify metabolites that perturb genes linked to age-related neurodegeneration. We then test four metabolites in an acute-exposure mouse model, and use multi-omic approaches to evaluate their impact on cellular functions in the brain. We confirm the known neurodegeneration-promoting effects of trimethylamine N-oxide (TMAO), including mitochondrial dysfunction, and further discover its disruptive impact on the pathways of glycolysis, GABAergic signaling, and RNA splicing. Additionally, we identify glycodeoxycholic acid (GDCA), a microbiome-derived secondary bile acid, as a potent regulator of chromatin accessibility and suppressor of genes that protect the brain from age-related, neurodegeneration-promoting insults. GDCA also acutely reduces mobility. In summary, we present a scalable framework for linking microbiome metabolites to host pathologies, and apply it to identify microbial metabolites that induce molecular changes related to neurodegeneration.

Proper brain function depends on the integrity of the blood-brain barrier (BBB), which is formed by a specialized network of microvessels in the brain. Reliable isolation of these microvessels is crucial for studying BBB composition and function in both health and disease. Here, we describe a protocol for the mechanical dissociation and density-based separation of microvessels from fresh or frozen human and murine brain tissue. The isolated microvessels retain their molecular integrity and are compatible with downstream applications, including fluorescence imaging and biochemical analyses. This method enables direct comparisons across species and disease states using the same workflow, facilitating translational research on BBB biology. Key features • The protocol employs mechanical dissociation and density-based separation to isolate microvessels from brain tissues. • The protocol was used to study molecular changes in brain microvessels in neurodegeneration and aging. • Validated downstream applications of this method include fluorescence imaging, RNA sequencing, proteomics, western blotting, and ELISA. • The protocol can be applied to fresh and frozen human and murine brain samples.