Disruption of the cerebrospinal fluid–plasma protein balance in cognitive impairment and aging

The brain barrier system, including the choroid plexus, meninges and brain vasculature, regulates substrate transport and maintains differential protein concentrations between blood and cerebrospinal fluid (CSF). Aging and neurodegeneration disrupt brain barrier function, but proteomic studies of the effects on blood-CSF protein balance are limited. Here we used SomaScan proteomics to characterize paired CSF and plasma samples from 2,171 healthy or cognitively impaired older individuals from multiple cohorts, including the Global Neurodegeneration Proteomics Consortium. We identified proteins with correlated CSF and plasma levels that are produced primarily outside the brain and are enriched for structural domains that may enable their transport across brain barriers. CSF to plasma ratios of 848 proteins increased with aging in healthy control individuals, including complement and coagulation proteins, chemokines and proteins linked to neurodegeneration, whereas 64 protein ratios decreased with age, suggesting substrate-specific barrier regulation. Notably, elevated CSF to plasma ratios of peripherally derived or vascular-associated proteins, including DCUN1D1, MFGE8 and VEGFA, were associated with preserved cognitive function. Genome-wide association studies identified genetic loci associated with CSF to plasma ratios of 241 proteins, many of which have known disease associations, including FCN2, the collagen-like domain of which may facilitate blood-CSF transport. Overall, this work provides molecular insight into the human brain barrier system and its disruption with age and disease, with implications for the development of brain-permeable therapeutics.