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Diminished levels of cellular protective factors present in neurally enriched exosomes in preclinical Alzheimer’s disease

      Background

      Certain transcription factors protect neurons from stresses including misfolded protein accumulation. Recently, we showed that proteins implicated in Alzheimer’s disease (AD) pathogenesis in neurally derived plasma exosomes are increased in clinical and pre-clinical AD. In this study, we assess neutrally-derived exosome content of transcription factors mediating cellular resistance to stress (HSF1, REST, LRP6).

      Methods

      Blood exosomes were obtained from patients with AD (AD = 24) and controls (AC =24), and from 20 other patients at two time-points when diagnosed with AD and 1 to 10 years earlier when cognitively normal. Exosomes were precipitated from 0.5 milliliter of human plasma using Exoquick®, followed by enrichment for neuronal origin by immunoabsorption with anti-L1CAM antibody. Neural origin enrichment was supported by higher levels of neuron-specific enolase and NCAM-1 in enriched than non-enriched exosomes at similar levels of the exosome marker CD81. Transcription factors were quantified by human-specific ELISAs for heat shock factor 1 (HSF1), repressor element 1-silencing transcription factor (REST), and low density lipoprotein receptor-related protein 6 (LRP6). CD81 content, that reflects the amount of exosomes, was used to normalize all protein levels. We examined their performance in diagnostic and prognostic classification of AD using Discriminant Classification and Receiver Operating Characteristic (ROC) analyses.

      Results

      Plasma neural exosomal levels of HSF1, REST and LRP6, that represent their respective contents in brain tissues, were much lower in AD patients than in controls (for HSF1, X vs. Y pg/ml; for REST, X vs. Y pg/ml; for LRP6, x vs. y pg/ml; all p<0.001) (Fig 1). ROC analyses correctly classified 79% of AC subjects and 92% of AD patients. Longitudinally, preclinical (AP) and clinical (AD) levels of HSF1, REST and LRP6 were indistinguishable and different vs. controls (all p<0.001).

      Conclusions

      Neural exosomes contain low levels of transcription factors that protect neurons from diverse stresses similar to human brain tissue from patients with AD at autopsy and mouse models. These findings confirm in living AD patients early deficiencies in the cellular response to stress during AD pathogenesis. Considered as biomarkers these factors predict AD diagnosis and have potential value for early diagnosis and therapeutic monitoring of AD.
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