Project Summary Cerebral blood flow (CBF) is reduced in Alzheimer’s disease (AD) patients and mouse models by ~20%, but there remains a limited understanding of the mechanisms causing this hypoperfusion or the potential therapeutic benefit of rescuing CBF deficits. Under the previous award, chronic in vivo two-photon excited fluorescence microscopy was used to study CBF in mouse models of AD. While no blood flow disruption in cortical arterioles or venules was observed, blood flow was found to be stalled in ~2% of cortical capillaries in mouse models of AD, as compared to ~0.4% in wild type controls. These capillary stalls appeared early in disease progression, were caused by arrested neutrophils, and had outsized impacts on CBF because they decreased flow speed in up- and down-stream vessels. Antibodies against the neutrophil surface protein Ly6G were serendipitously found to reduce the incidence of capillary stalls immediately, leading to a rescue of two-thirds of the CBF deficit, and, remarkably, to improved memory function within hours. Preliminary data further link this capillary stalling to cellular damage from reactive oxygen species (ROS). In this competitive renewal, the mechanisms underlying neutrophil arrest in capillaries in mouse models of AD and the consequences of improving CBF on AD-related pathology are explored. First, three different hypotheses about the mechanism of neutrophil arrest in capillary segments are tested: a focal constriction of the capillary by a pericyte that prevents neutrophil passage; binding of the neutrophil to increased inflammatory adhesion molecules on endothelial cells; or binding of the neutrophil to basement membrane and adhesion molecules exposed at widened gaps between endothelial cells. Second, the molecular and cellular origin of the ROS that leads to neutrophil arrest is determined using cell type-specific knockouts of ROS producing enzymes. Third, the impact of long-term CBF rescue on the deposition of amyloid- beta (Aβ), a driver of AD pathology, and on neuropathology will be quantified. Critical for this study are recently- developed knock-in mouse models of AD that may better capture the feedback of CBF reductions on expression of amyloid precursor protein (APP), which is cleaved to produce Aβ. Finally, cutting-edge three-photon excited fluorescence microscopy is used to enable imaging of the hippocampus to determine the role of capillary stalling in CBF deficits in one of the first regions of the brain that exhibits AD pathology. The hypothesis that brain hypoperfusion in AD is due to neutrophil arrest in capillaries is both novel and strongly supported by the findings under the previous award. The work proposed in this competitive renewal would uncover the mechanisms underlying that neutrophil arrest, which could suggest therapeutic targets to improve CBF that would be complementary to anti-amyloid and other treatment approaches for AD.