15 Feb 2019

One of the characteristic features of Alzheimer’s disease is reduced blood flow in the brain. What brings it down? A paper in the February 11 Nature Neuroscience suggests that white blood cells can clog up the works. Scientists led by Chris Schaffer, Cornell University, New York, discovered that neutrophils—leukocytes that are some of the first responders in the innate immune system—get stuck to vessel walls on their way through capillaries in mouse models of AD. Since cells have to squeeze through, single-file, in these tiny blood vessels, this completely blocks blood flow until the cells work their way free. Even though less than 2 percent of capillaries are affected, their interconnectedness with other vessels means blockages can reduce overall blood flow in the brain by 30 percent. However, antibodies that interfere with the binding of neutrophils to vessel walls restored blood flow and memory quickly, even at advanced stages of disease. The results suggest a possible contributing mechanism to Alzheimer’s.

Cortical blood flow stalls in AD mouse models.

Neutrophils wedge themselves in brain capillaries, clogging them up.

Blocking neutrophil receptors or depleting the cells restores blood flow and memory.

“The small vessels in the brain are often overlooked in the context of AD,” said Susanne van Veluw, Massachusetts General Hospital, Boston. She emphasized how even the smallest changes in capillaries had a significant impact on cerebral blood flow in the mouse brains. “The authors point to a potentially relevant and overlooked mechanism that could contribute to this well-known phenomenon of reduced cerebral blood flow in Alzheimer’s disease.”

Schaffer’s group wanted to study the effects of microvascular injuries on AD pathology. They used transcranial imaging to visualize the vasculature of the mouse brain. They injected fluorescent dye that labeled just the plasma (not red blood cells) of the mice and allowed them to track blood flow after they induced injuries. However, in the process of studying AD mouse controls, they noticed that about 2 percent of capillaries were already blocked, four times more than in wild-type mice. This same obstructed capillary flow has recently been reported in a tau mouse model (Bennett et al., 2018). What was causing this?

No Flow. In a mouse model of AD, red blood cells moving through capillaries in the brain occlude fluorescence from the plasma (red), appearing as dark spots that move over time (top). In a few percent of vessels, the dark spots hold their position, indicating that blood flow is at a standstill (bottom). [Courtesy of Hernández et al., 2019. Nat Neurosci.]

To examine the phenomenon more closely, first author Jean Cruz Hernández and colleagues first confirmed it happened in several different mouse models of AD. About 1.8 percent of capillaries stalled in APP/PS1 mice as young as 12 weeks old, before plaques had developed. These tiny vessels also clogged in five- to six-month-old 5xFAD mice, and in 10- to 13-month-old TgCRND8 mice. Most plugs lasted less than five minutes, but a third held out for more than 15. The same small subset of capillaries stalled again and again. To identify what caused the obstructions, Hernández tested a series of antibodies to different blood cells and proteins. An antibody to Ly6G, a neutrophil cell-surface marker, labelled almost all the stalled capillaries.

To the authors’ surprise, high doses of the anti-Ly6G label abolished plugs completely within minutes. For both seven- to nine-month-old APP/PS1 animals and five- to six-month-old 5xFAD mice, this boosted blood flow by 30 percent and overall brain perfusion by 20 percent, about two-thirds of the way to wild-type levels. Three hours after treatment, transgenic mice were better able to recognize the location of a new object and remember which arm of a Y-maze they had last explored, demonstrating improvements in spatial and working memory. Though the effects only lasted a couple of days due to neutrophil turnover, repeated doses of anti-Ly6G carried the benefits on for a month. Schaffer has unpublished data suggesting the strategy works for 15- to 16-month-old mice, though not 17- to 20-month-old animals, implying that even at relatively advanced disease stages, improvements in blood flow can improve learning and memory.

What caused the neutrophils to lodge themselves in capillaries? The data aren’t conclusive, but Schaffer strongly suspects that inflammation in the brain and its vasculature driven by Aβ aggregates increases the expression of ICAM1 and VCAM1 on vessel walls (Park et al., 2008). These proteins bind integrin receptors on neutrophils to help them adhere. Basically, the blood vessels become stickier. A previous study reported that anti-Ly6G, through an unknown mechanism, alters the shape of integrins on neutrophils and interferes with adhesion to vessel walls, making the cells themselves less likely to stick (Wang et al., 2012). That may enable them to slide right through the vessel as usual, Schaffer said.

The ties between AD and vascular disease run deep, with countless studies linking poor cardiovascular and cerebrovascular health with greater risk for the disease, and marking it as one of the earliest features of Alzheimer’s (Jun 2014 news; Feb 2012 news; Jul 2016 news). Several reasons have been proposed for the reduced blood flow in the Alzheimer’s brain, including vasoconstriction and loss of vascular density, but scientists don’t fully understand why it occurs (Niwa et al., 2001; Farkas et al., 2001). Could neutrophil stalling be involved? Some drugs that reduce the activation, migration, and adhesion of neutrophils and that are FDA-approved or in clinical trials for autoimmune diseases could help answer this question. However, Schaffer said interfering with neutrophils in Alzheimer’s disease may be unwise because it could compromise the immune system. That said, he is screening some of these drugs to see if they decrease capillary blockages in mice. If one works, a brief clinical trial could help test whether neutrophils reduce cortical blood flow in people. If so, a longer-term strategy might be to target some upstream molecular pathways that lead to the increased adhesion, he speculated.

The study of neutrophils in AD has gained momentum in recent years, noted Gabriela Constantin, University of Verona, Italy. She previously reported that depleting neutrophils improves memory in mouse models of AD (Aug 2015 news). A different study found that fast-declining AD patients have more activated neutrophils than slow decliners or controls (Dong et al., 2018). “This new paper connects neutrophils to capillary stalling, adding a new mechanism by which neutrophils could be involved in disease,” she told Alzforum.

Van Veluw noted that this phenomenon crops up in mice before amyloid pathology appears. “If this is something that happens early in disease and it translates to humans, it could be an interesting target for early intervention,” she said.—Gwyneth Dickey Zakaib