Extracellular vesicular lipids as biomarkers for the diagnosis of Alzheimer’s disease

Abstract Background An increasing number of studies have revealed that dysregulated lipid homeostasis is associated with the pathological processes that lead to Alzheimer?s disease (AD). If changes in key lipid species could be detected in the periphery, it would advance our understanding of the disease and facilitate biomarker discovery. Global lipidomic profiling of sera/blood however has proved challenging with limited disease or tissue specificity. Small extracellular vesicles (EV) in the central nervous system, can pass the blood-brain barrier and enter the periphery, carrying a subset of lipids that could reflect lipid homeostasis in brain. This makes EVs uniquely suited for peripheral biomarker exploration. Method To determine the potential of EV lipids as biomarkers, we isolated brain-derived EV (BDEV) from frontal cortex tissue (Vella, et al. JEV. 2017) and performed quantitative mass spectrometry based lipidome analysis (Rustam and Reid. Anal. Chem. 2018). We developed and optimised methods to isolate pure EVs from plasma while minimising co-isolation of lipoproteins. To confirm the identity of EVs, we are using various techniques including western blot, nanoparticle tracking analysis and transmission electron microscopy. We are mining the mass spectrometric data for brain-specific signals. Result Western blot analysis showed successful isolation and enrichment of BDEV from brain tissue. TEM revealed the typical round and cup-shaped EV morphology with vesicle size being 50-150nm. A total of 692 lipids were identified and quantified. Enrichment of glycerophosphatidylserine (PS) lipids, especially the ether PS lipids, alkyl- and alkenyl-, were observed in BDEV. Remodeling in phosphatidylethanoamine (PE) lipid class and polyunsaturated fatty acyl containing lipids (PUFA-) was observed in AD BDEV. The preliminary experiments demonstrate successful isolation of pure EVs from plasma. Lipidomic and proteomic analysis suggests that plasma EVs contain brain-specific signals. Conclusion We, for the first time, characterized the lipid composition of EVs in human frontal cortex. BDEVs offered improved detection of dysregulated lipids in AD and these lipids have previously been reported to play key roles in AD pathogenesis, suggesting BDEV provide a readout of lipid dysregulation in AD and highlighting the potential use of these lipids as disease biomarkers in the periphery.