PROJECT SUMMARY/ABSTRACT Rare forms of familial Alzheimer disease (fAD) are known to be caused by life-long, genetically determined perturbations in the production of the amyloid ß-protein (Aß), but the cause(s) of the vast majority of so-called sporadic AD (sAD) cases remains remarkably poorly defined. This proposal will use state-of-the-art mouse models of sAD together with several highly innovative approaches to address key temporal and spatial aspects of sAD pathogenesis for the first time, with critical implications for the development of effective therapies. Whereas fAD is attributable to chronic perturbations in the production of Aß, we hypothesize that sAD is triggered by impairments in the clearance of Aß—specifically by transient impairments in Aß clearance. This hypothesis is consistent with evidence showing that several established risk factors for sAD, such as brain trauma, stress, or poor sleep, lead to short-lived or episodic increases in cerebral Aß levels due to reduced Aß clearance. To model this novel mechanistic hypothesis, we employ innovative methods to inhibit Aß clearance transiently and reversibly by blocking either blood-brain barrier transport of Aß or its proteolytic degradation. Because we aim to define the triggers for sAD, we require an animal model that does not develop AD-type pathology on its own, as most AD mouse models do. To this end, we will use an innovative new sAD mouse model, the APPNL-F/hAß mice, which expresses wild-type human Aß only, under the control of the endogenous murine App promoter, with the minimal genetic mutations needed to model sAD. As is true for normal humans, this sAD mouse model develops diffuse deposits of human Aß in an age-dependent manner, and forms very minimal dense-core plaques only at very advanced ages. Accordingly, these mice are ideal for investigating the pathophysiological mechanisms responsible for triggering the conversion of “normal” Aß deposition to the pathological type in sAD. We hypothesize further that the Aß-dependent pathological mechanisms most relevant to sAD occur much earlier than the ages studied in clinical trials, with clinical symptoms emerging only much later, specifically in the context of aging. Accordingly, we will define the temporal window most relevant to the emergence of AD by increasing Aß levels in APPNL-F/hAß mice transiently at various ages, then evaluating the consequences for the development of AD-type pathology longitudinally, up to and including old age. Finally, we will test the novel hypothesize that spatially distinct pools of Aß (e.g., extra- vs. intracellular) impact the pathogenesis of AD in qualitatively different ways. Specifically, we postulate that intracellular Aß is more relevant than extracellular Aß to the neurodegeneration and memory loss that characterize AD. To test this, we will selectively increase extra- vs. intracellular pools of Aß by reversibly downregulating Aß-degrading proteases that, as our preliminary results sh...