Impaired Vasoreactivity, Sleep Degradation, and Impaired Clearance in the APOE4 Brain

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PROJECT SUMMARY/ABSTRACT The ε4 variant of apolipoprotein E (APOE4) is the strongest and most common genetic risk factor for sporadic Alzheimer’s disease (AD), which makes up more than 95% of AD cases. APOE4 has been demonstrated to contribute to a number of molecular processes implicated as promoting AD pathology, including accelerated aggregation of amyloid-β and tau proteins, increased immunoreactivity, altered lipid metabolism, and impaired cerebral blood flow, but an over-arching mechanism of the conveyed AD risk remains unclear. As a leading risk factor for atherosclerosis as well as AD, APOE4 has profound effects on vasculature. Previous studies have found dysregulated neurovascular coupling, high levels of the vasodilator nitric oxide, and impaired flow of cerebrospinal fluid around arteries in the brain, suggesting decreased glymphatic clearance, a process that occurs most dramatically during sleep. Consequently, physiological sleep patterns are also known to be disrupted in AD, suggesting a common theme in many of the identified APOE4-dysregulated processes. We propose that altered neurovascular coupling and vasodilation will alter sleep stage patterns and disrupt sleep, which will impair the clearance of cellular waste products from the brain. Many of these waste products, such as protein aggregates, inflammatory cytokines, and metabolites can be neurotoxic, and can create a positive feedback cycle as they in turn affect neural activity and vascular integrity and function, further disrupting sleep. To test this idea, we will combine the neurobiological and technical expertise of three research groups studying distinct levels of brain function to perform an integrative, multi-scale study of brain physiology and function in knock-in mice carrying humanized APOE4 and APOE3. First, we will use custom-developed hardware for chronic, long-term EEG monitoring of APOE4 and APOE3 mice and specialized machine learning for real-time characterization of sleep stages to identify shifts in sleep patterns and quality over the course of aging (3 months to 15 months). Second, we will quantify impairment of cortical vasodilation and vasoreactivity by conducting 2-photon imaging to measure the diameter of cortical arteries in behaving APOE4 and APOE3 mice, both awake and in natural sleep, at different time points over the course of aging. Third, we will perform multiplexing and -omics assays of brain tissue from APOE4 and APOE3 mice during day (light) and night (dark) over the course of aging to quantify the levels of inflammatory cytokines and metabolites. Finally, we will further test our hypothesis and attempt to ameliorate AD-promoting pathology by introducing exercise as a potent vasodilator and stimulus of cerebrovascular health. Upon completion of the proposed work, we will have determined the trajectories of altered sleep stages and quality, impaired vasodilation, and dysregulated production and clearance of immune and metabolic products in APOE4, and tested the relationship between these pathologies by introducing exercise from young age to ameliorate vascular dysfunction.