Project 3 Abstract Intriguing evidence implicates reactive astrogliosis as impairing brain fluid dynamics (BFD) via multiple mechanisms, including 1) altering expression of the aquaporin-4 (AQP-4) water channel along the astrocytic end feet which line perivascular spaces and 2) modifying cerebrovascular vasomotion, a likely driver of perivascular fluid flow. In turn, impaired BFD may lead to reduced clearance of β-amyloid (Aβ), resulting in aggregation and Aβ plaque deposition. However, the temporal order of reactive astrogliosis, altered AQP-4 expression, impaired BFD, and Aβ deposition along the aging and Alzheimer’s disease (AD) pathophysiological spectrum is not well understood. These relationships and mechanisms, particularly the pathways from reactive astrogliosis to impaired BFD and from impaired BFD to Aβ deposition, remain largely unexamined in vivo in humans. Our preliminary data examining PET ventricular CSF efflux suggest that BFD are impaired early in the pathophysiological spectrum, prior to cognitive decline, and are associated with Aβ deposition. These findings together with growing evidence from animal model and postmortem studies lead us to believe that in vivo neuroimaging markers of impaired BFD and reactive astrogliosis can predict future Aβ deposition. In order to test this overall hypothesis and achieve the proposed aims, we will examine (1) brain fluid pulsatility over the whole head at cardiac, respiratory, and vasomotion frequencies using a novel 7T MRI fast echo-planar imaging (EPI) sequence; (2) ventricular CSF efflux using the Aβ PET tracer 11C-PiB; (3) enlarged perivascular space morphology, potentially indicative of impaired BFD, using 7T MRI T1, T2, and FLAIR sequences; (4) reactive astrogliosis using the novel monoamine oxidase B (MAO-B) PET tracer 18F-SMBT-1; and (5) Aβ deposition using 11C-PiB in a cohort of n=300 older adults (age 55 and older; 10% AD). With this data, we will perform a highly innovative and comprehensive multimodal characterization of in vivo BFD in relation to reactive astrogliosis and Aβ deposition. Cross-sectionally, we will examine the mediating relationship of BFD between reactive astrogliosis and Aβ deposition. Longitudinally, we will test the hypothesis that baseline markers of reactive astrogliosis and impaired BFD can predict follow-up Aβ deposition. In collaboration with Project 4, we will also examine the role AQP-4 expression plays in regulating BFD. In addition, we will develop a novel non-contrast blood-brain barrier (BBB) permeability MRI sequence at 7T and examine its relationships with existing BFD neuroimaging markers. At the conclusion of this proposed study, we will have potentially identified BFD and reactive astrogliosis neuroimaging markers as predictors of Aβ deposition. We will therefore have potentially identified pathways and mechanisms for targeted therapies to prevent Aβ deposition and the onset of AD.