The Role of Extremely Long-Lived Proteins in Aging and Alzheimer’s Disease

Loss of proteostasis is a hallmark of aging, and aging is the major risk factor for Alzheimer's disease. Deterioration in function and accumulation of damage to the proteome are largely repaired by protein turnover. These turnover mechanisms are particularly important in long-lived postmitotic neurons, which cannot dilute toxic proteins by cell division. We aimed to identify extremely long-lived proteins (ELLPS) that persist for several months or longer across the aging continuum in wild-type mice and in genetic AD mouse models. We hypothesize that these ELLPs represent key points of vulnerability to the decline of the aging proteome and critical substrates of amyloid pathology. To identify ELLPs, we used whole-animal metabolic stable isotope labeling of wild-type and AD model mice combined with bottom-up proteomic analysis using liquid chromatograph-tandem mass spectrometry. The results were verified and followed up by biochemical, molecular and electrophysiological methods. In wild-type mice, we found that the brain proteome uniquely undergoes dynamic global turnover fluctuations during aging compared to heart and liver tissue. Parallel analyses of the insoluble fraction revealed that several protein sub-complexes experience impaired turnover, in part due to misfolding. Finally, we discovered that age-associated fluctuations in the activity of the ubiquitin proteasome system are linked to the turnover of the catalytic core subunits. In Alzheimer's mouse models, presynaptic protein turnover is selectively impaired during the early stages of Abeta accumulation. Synaptic vesicle (SV)-associated proteins have elevated levels, misfold in a plaque-independent manner, and interact with APP and Abeta. We also found an enlargement of the SV pool and an increase in presynaptic potentiation. Thus, the identification of ELLPs provides a rare opportunity to uncover Achilles heals of aging and initial substrates of amyloid pathology.