O2‐12‐04: STALLED BLOOD FLOW IN BRAIN CAPILLARIES IS RESPONSIBLE FOR REDUCED CORTICAL PERFUSION AND IMPACTS COGNITIVE FUNCTION IN MOUSE MODELS OF ALZHEIMER'S DISEASE (2018)

A reduction of ∼30% in cerebral blood flow is observed in patients and animal models of Alzheimer's disease (AD). Although this hypoperfusion likely contributes to cognitive impairment and disease progression, no physiological explanation for this phenomenon has emerged. We observed that leukocytes often occlude capillaries in the brains of AD mouse models, thereby stopping blood flow in the smallest brain blood vessels. Here, we tested the hypothesis that these leukocytes stalls contribute to the blood flow reduction and cognitive impairment. Using in vivo two-photon excited fluorescence imaging in transgenic AD mice (APP/PS1), we quantified the number of flowing and stalled cortical capillaries, and measured blood flow speed in penetrating arterioles as well as performance on memory tasks before and after treatment with antibodies that reduce the incidence of capillary plugging. About 1.5% of cortical capillaries were transiently stalled in AD mice, leading to an overall reduction in cerebral blood flow (CBF). These stalls were released using an antibody against the neutrophil cell surface protein Ly6G, resulting in an immediate ∼25% increase in CBF and an immediate improvement in performance on short-term memory tasks. Mice treated with an isotype control antibody showed no improvement in CBF or memory performance. In a second set of experiments we deterimend that reducing capillary stalls and increasing CBF led to improved cognitive performance in mice with relatively advanced AD pathology (17 month old APP/PS1 mice, first cognitive impacts detectible at 8 months). In this study we uncovered leukocyte adhesion in brain capillaries as a mechanism contributing to reduced CBF in AD mouse models and showed that blocking this adhesion leads to immediate cognitive benefits even in advanced stages of disease development . Stalled capillaries occured at increased incidence in mouse models of Alzheimer's disease, were caused by adhered neutrophils, and contributed to blood flow and cognitive deficits, (a) Individual brain capillaries were scored as flowing or stalled based on the motion of unlabeled blood cells (black) within the fluorescently labeled blood plasma (red), (b) Fraction of capillaries with stalled blood flow in APP/PS1 and wt mice as a function of age. One outlier not shown: APP/PS1, 42 weeks, 4.4% stalled. Shaded regions indicate 95% confidence intervals, (c) 2PEF images of capillaries stalled by a leukocyte (LEU), red blood cells (RBCs), and platelet aggregates (PLT), or found empty of blood cells (EMT), distinguished by fluorescent labels (red: Texas Red-labeled blood plasma; yellow: rhodamine 6G-labeled leukocytes and platelets; green: Hoechst-labeled leukocyte nuclei).The percentage of stalled capillaries due to each cause in the APP/PS1 mice is indicated in each image, (d) Number of capillaries with stalled blood flow ∼1 hr after administration of antibodies against Ly6G (α-Ly6G) or isotype control antibodies (Iso-Ctr) shown as a fraction of the number of stalled capillaries at baseline, (e) Projection of 2PEF image stack of brain surface vasculature, with surface (red lines) and penetrating (red dots) arterioles identified. For each penetrating arteriole, volumetric blood flow is indicated at baseline (left) and after α-Ly6G administration (right), along with the percentage of baseline flow, (f) Volumetric blood flow in penetrating arterioles measured 60-90 min after α-Ly6G or Iso-Ctr antibody administration in young and old APP/PS1 mice and wt control animals shown as a fraction of baseline arteriole flow, (g) Preference score in object replacement task for APP/PS1 and wt mice at baseline and at 3 hr after a single administration of α-Ly6G or Iso-Ctr antibodies.