Extracellular Vesicles in Peripheral Blood: Potential Biomarkers for Preclinical Alzheimer Disease

Kenneth A. Livingston

Introduction: Disease-modifying interventions for Alzheimer Disease (AD) are suspected to be most efficacious in the earliest and asymptomatic, or preclinical, phase of the disease.1 To date, methods for reliably detecting pathologic changes of AD – mainly β-amyloid deposition in the brain – are invasive and expensive, requiring measurement of β-amyloid in cerebrospinal fluid or positron-emission-tomography imaging.2 Developing a cost-effective, blood-based method for detecting biomarkers associated with preclinical AD is desirable. Here, plasma extracellular vesicles (EVs) of neuronal origin are presented as candidates for a sensitive and effective method to detect preclinical AD.  Methods: Isolating EVs of neuronal origin. Neural cell adhesion molecules (LCAM1) were detected via immunoprecipitation following a high-throughput particle precipitation (Exoquick).4 Further support was offered via Imuno-electron microscopy which used biotinylated antibodies to detect LCAM1+ EVs in peripheral blood. Transgenic mice expressing green fluorescent protein on nestin, a protein specific for the nervous system (CNS), and western blot analysis revealed a higher concentration of markers specific for the CNS.4  Results: L1CAM+ EVs from peripheral blood contain sufficient levels of brain-derived proteins, such as p-T181-tau, brain-derived neurotrophic factor (BDNP), and pro-BDNF to serve as potential biomarkers of pre-clinical AD. First, immunoprecipitation showed a 2.35-fold difference (p<0.0001) for neuronal enolase in L1CAM+ EVs compared to CD81+ EVs. Second, L1CAM+ EVs contained significantly higher levels of p-T181-tau, BDNF, and pro-BDNF relative to serum or plasma alone.4 Third, detection levels (~450pg/ml) for p-T181-tau in L1CAM+ EVs were significantly higher compared to previous studies which used exosomal protein levels of p-T181-tau (85.7±3.75 pg/ml) to successfully detect preclinical AD (10 years prior to clinical diagnosis).4,5 Together, these findings support L1CAM+ EV’s use as precise biomarkers which reflect metabolic processes occurring in the brain. Conclusion: Isolating EVs of neuronal origin allows for unique insight regarding the pathophysiological processes of the brain. As blood-based biomarkers, EVs of neuronal origin offer an inexpensive and non-invasive method for screening patients. They offer increased sensitivity and lower detection threshold (due to the higher concentration of proteins in L1CAM+ EVs) for biomarkers related to AD.4 Due to their abundance of mechanism-specific molecules, EVs of neural origin may even serve as biomarkers for therapeutic response to intervention.

 

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