Evolving Diagnostic Approaches

All cells shed several types of membranous vesicles to the extracellular space, as part of their normal function. Exosomes are extracellular vesicles with a 30-150 nm range of diameters that originate from the endosomal/multivesicular body/lysosomal system, express various surface markers and contain RNA and peptides reflecting their cell of origin. Exosomes participate in inter-cellular signaling and, perhaps, clearance of misfolded proteins. Exosomes circulate in various body fluids, including blood. We developed a methodology for enriching exosomes isolated from blood for neuronal origin based on their expression of neuronal cell adhesion molecules, such as NCAM and L1CAM. We performed a series of case-control studies to examine whether a priori selected proteins differ in enriched exosomes in patients with Alzheimer’s disease (AD) vs. controls. We found that patients have much higher exosomal p-T181-tau, p-S396-tau, Abeta42, that are implicated in the tangle and plaque pathology of AD. In addition, we found that neuronal exosomes from AD patients contain lower signaling forms of phosphorylated insulin receptor substrate-1 compared to both normal and diabetes controls. This finding constitutes the first in vivo evidence of brain insulin resistance in AD. Finally, we found that exosomes from AD patients have altered contents of endosomal-lysosomal enzymes, which may relate to their inability to clear misfolded proteins. Most of the differences were so large that they effectively discriminated between patients and controls with near perfect accuracy. Therefore, a profile of exosomal peptides may serve as a blood test for AD, dramatically improving diagnostic and prognostic accuracy in dementia clinical practice. Moreover, some of the alterations were already present up to 10 years prior to AD diagnosis raising the possibility of preclinical diagnosis for enrollment in secondary prevention clinical trials. A final possibility is that these peptides may respond to experimental treatments and be used as surrogate markers in clinical trials. Exosomal biomarkers have the potential of revolutionizing AD clinical practice and research.