O2–08–06: Analysis of the synapse‐rich brain proteome reveals potential markers of asymptomatic Alzheimer's disease

It is established that the neuropathological changes of Alzheimer's disease (AD) begin years prior to the onset of symptoms. Advancements in neuroimaging and cerebral spinal fluid analysis have confirmed that approximately one-third of clinically asymptomatic older individuals demonstrate accumulation of beta-amyloid peptide up to a decade before the development of even subtle signs of cognitive impairment. Currently, our understanding of these early pathological events and the molecular transition from this asymptomatic phase to clinically evident disease is extremely limited. Analyzing the brain proteome in this preclinical stage will not only enhance our understanding of early AD progression, but also facilitate more accurate classification of at-risk asymptomatic individuals. By means of an unbiased proteomics approach, this study aimed to identify proteins differentially expressed in asymptomatic AD that could better classify disease progression. Using quantitative liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) we comparatively analyzed the synapse-rich proteomes of post-mortem frontal cortex samples of 18 individuals separated into the following cohorts (n=6): i) cognitively normal individuals without AD pathology, ii) cognitively normal individuals with moderate to severe AD pathology (AsymAD), and iii) cognitively impaired individuals with moderate to severe AD pathology. In total we identified and quantified over 40,000 peptides from 3,230 proteins. Hierarchical cluster analysis based on the overall pattern of protein expression was able to stratify certain AsymAD cases that had attributes similar to symptomatic AD cases. Pathway analysis of these AsymAD cases revealed a severe loss of synaptic proteins and an increase in proteins directly involved in oxidative stress. Interestingly, all AsymAD cases that clustered with AD cases carried at least one APOE-ε4 allele providing strong preliminary evidence of a genotype specific effect on the synaptic proteome. These results support an emerging hypothesis that AsymAD is characterized by differential protein signatures representing specific functional mechanisms which involve synaptic loss and oxidative stress, which are likely to involve known AD risk factors.