Endosomal dysfunction, a new source of biomarkers for Alzheimer's disease.

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Endosomal dysfunction is a well-accepted cytopathological feature in Alzheimer’s disease (AD). However, biomarkers reflecting endosomal traffic defects are still lacking. Current imaging and cerebrospinal fluid (CSF) AD biomarkers focus primarily on the histopathology of the disease—that is, biomarkers that are linked to neurofibrillary tangles and amyloid plaques. The current proposal is therefore designed to expand this focus to develop biomarkers of the ‘cell biology’ of AD. Such biomarkers could potentially accelerate drug discovery, as therapeutic interventions are currently being developed targeting the AD endosomal trafficking pathway. Genetic and cell biology studies have previously linked retromer -- a multi-modular protein assembly that functions in sorting and trafficking of cargo out of the endosome -- to AD pathology. Most notable are deficiencies and rare mutations in retromer’s core protein, VPS35, and retromer’s receptor, SORL1. Depletion of either VPS35 or SORL1 mimics the core cytopathology in AD, enlarged endosomes in neurons. In an effort to characterize defects in the endocytic pathway resulting from retromer dysfunction, and potentially identify biomarkers for AD’s endosomal trafficking defects, we performed a proteomic screen of cerebrospinal fluid (CSF) of VPS35 knock-out (KO) mice and control littermates. Among the proteins found elevated in the CSF of VPS35 deficient mice were well established β-secretase BACE1 substrates including, Amyloid Precursor Protein (APP); Amyloid Beta Precursor Like Proteins 1 and 2 (APLP1 and APLP2); and Neural cell adhesion molecule L1-like protein (CHL1). Two of these proteins --APLP1 and CHL1-- were further validated in mice, and human CSF from cognitively healthy participants and prodromal AD patients. Collectively, our mouse-to-human preliminary results suggest that BACE1 substrates can potentially act as biomarkers of endosomal dysfunction. Relying on these exciting findings, and moving towards the development of a less invasive, more accessible and less costly biomarker; we will explore peripheral exosomal APLP1 as a biomarker of endosomal dysfunction; one of the earliest cytopathological features of AD. Completion of this study will provide initial evidence that an exosome-based plasma test could be diagnostic for the earliest stages of AD.