Neutrophil adhesion in brain capillaries reduces cortical blood flow and impairs memory function in Alzheimer’s disease mouse models

Abstract

Cerebral blood flow (CBF) reductions in Alzheimer’s disease (AD) patients and related mouse models have been recognized for decades, but the underlying mechanisms and resulting consequences on AD pathogenesis remain poorly understood. In APP/PS1 and 5xFAD mice we found that an increased number of cortical capillaries had stalled blood flow as compared to wildtype animals, largely due to neutrophils that adhered in capillary segments and blocked blood flow. Administration of antibodies against the neutrophil marker Ly6G reduced the number of stalled capillaries, leading to an immediate increase in CBF and to rapidly improved performance in spatial and working memory tasks. This study identified a novel cellular mechanism that explains the majority of the CBF reduction seen in two mouse models of AD and demonstrated that improving CBF rapidly improved short-term memory function. Restoring cerebral perfusion by preventing neutrophil adhesion may provide a novel strategy for improving cognition in AD patients.

Neutrophil-mediated dynamic capillary stalls in ischemic penumbra: persistent traffic jams after reperfusion contribute to injury

“…Since anti-Ly6G treatment can lead to a depletion of neutrophils over 24 hours (47,49), we also took blood samples to evaluate leukocyte and differential neutrophil counts before and after antibody administration. The leukocyte and differential neutrophil counts at 2 hours after reperfusion and anti-Ly6G treatment was not different from the controls (Fig 4/a), which is also in line with the previous study on Alzheimer's models with the same monoclonal antibody (43). At the 24 th hour however, we saw a prominent reduction in neutrophil count compared to the control antibody treated group as expected, confirming the specific effect of the anti-Ly6G treatment on neutrophils (Fig 4/a).…”

“…Anti-Ly6G antibodies are traditionally used for in vivo depletion of neutrophils in mice, an effect usually established within 24 hours (47,49). We decided to use the neutrophil-targeting approach based on our in vivo TPM observations that leukocytes were dynamically plugging the ischemic capillaries in penumbra getting stuck in ischemic microcirculations (Supplementary Movie 3, 4 and 5), to see if we could improve capillary flow patterns along with tissue survival, in line with the recent observations on a mouse model with Alzheimer's disease (87). Modification of dynamic capillary stalls was indeed possible even with this treatment being applied after recanalization, one hour after ischemia onset and without the requirement for absolute reduction of neutrophil count.…”

“…Therefore, we chose to apply the antibody intravenously (2mg/kg) immediately after recanalization of the MCA (n=11). We used that specific dose of antibody because it was found effective for improving leukocyte-dependent stall in the previous study (43). For controls, the same dose and volume of isotype control antibodies were injected (n=13).…”

Increasing cerebral blood flow improves cognition into late stages in Alzheimer’s disease mice

“…For the first two time points (10-11 and 12-13 months of age), cognitive testing was performed both before and one day after antibody administration, while for the subsequent four time points cognitive testing was done only one day after antibody administration. Previous work by us (Cruz Hernández et al, 2019) and others (Daley et al, 2008) showed that circulating neutrophil counts are sharply reduced one day after treatment with this dose of α-Ly6G.…”

The neutrophil enzyme myeloperoxidase directly modulates neuronal response after subarachnoid hemorrhage, a sterile injury model

“…For example, neutrophil infiltration into the substantia nigra causes neuronal loss in mice 1 . Neutrophil adherence to cerebral capillary beds is linked to cognitive decline in a mouse model of Alzheimer's disease 2 , and increased neutrophil infiltration in the brain after middle cerebral artery occlusion coincides with increased neuronal loss 3 . However, the cellular and molecular mechanism of action of neutrophils in the CNS is poorly understood.…”