The accumulation of phagocytic microglia in the brain could be good or bad for AD patients. Some evidence suggests the microglia can eat up Aβ and clear plaques, but their activation can also lead to harmful neuroinflammation (see ARF related news story). A new report from Joseph El Khoury and colleagues of Harvard Medical School, published this week in the Journal of Neuroscience, finds a way to reconcile these two opposing functions. By tracking gene expression over time in microglia freshly isolated from AD mouse models, El Khoury and coauthors Suzanne Hickman and Elizabeth Allison report that the cells seem to start out by battling the buildup of amyloid. However, over time, the microglia lose their ability to take up and degrade Aβ, and instead show increased expression of inflammatory cytokines. The Alzforum previously reported on some of the data after they were presented last fall in Boston (see ARF related news story).
“We have found in the past that microglia can be both protective and detrimental, and I think this paper shows that at some stage of the disease, the microglia are good and can function to come in and clear amyloid, but either because they get overwhelmed or because of the inflammatory response itself, later they fail,” El Khoury told Alzforum.
The results offer a nuanced view of microglia action in AD, and suggest that anti-inflammatory therapy for the disease should take both sides of their action into account.
The work is based on a technique El Khoury and colleagues developed for rapidly isolating fresh microglia from adult mouse brain using antibodies to the cell surface marker CD11b. To study changes in the microglia as plaque accumulation progresses and microgliosis sets in, the investigators isolated cells from brains of PS1-APP mice at 1.5, three, eight, and 14 months of age, and measured expression of Aβ receptors, Aβ degrading enzymes, and inflammatory markers by quantitative PCR.
By the time they were eight months old, the PS1-APP mice showed significantly reduced expression of the Aβ receptors scavenger receptor A (SRA), CD36, and RAGE compared to their non-transgenic littermates. Wild-type mice also showed reductions in the Aβ receptors, but to a lesser extent, and except for a decrease in CD36, the changes were not statistically significant. In parallel to the decrease in Aβ scavenger receptors, the microglia also showed decreased mRNA for the Aβ-degrading enzymes insulysin, neprilysin, and matrix metalloproteinase 9. In 14-month-old mice, the levels were reduced by 50-80 percent compared to non-transgenic mice. Together, these data could indicate a decreased capacity of the microglia to take up and degrade Aβ, the authors suggest.
In parallel with the apparent decrease in Aβ handling proteins, the microglia from eight- and 14-month-old mice showed increased expression of mRNA for the inflammatory cytokines IL-1β and TNFα. To test the idea that the production of cytokines in a chronic inflammatory response might control expression of Aβ receptors and degrading enzymes, the researchers treated cultured microglia with TNFα and IL-1β. They found that TNFα, but not IL-1β treatment resulted in a significant decrease in SRA and CD36 mRNA and surface expression, as well as Aβ uptake by the cells. On the other hand, neither neprilysin nor insulysin expression was affected by TNFα.
“Our data provide evidence to support the paradigm that the inflammatory response in AD is a ‘double-edged sword,’” the authors write. Initially, newly recruited microglia act to clear Aβ, but with time, they lose their ability to take up and degrade the peptide, while continuing to produce inflammatory cytokines. Not only do the cytokines have the potential to suppress Aβ clearance, but they may also stimulate production of Aβ via their previously documented effects on β-secretase expression and γ-secretase activity.
The reduction in Aβ receptors and degrading enzymes in microglia was initially surprising, El Khoury said. “We were expecting to see an upregulation of genes, because we think that in inflammation you see an increase in a lot of things. But if you look in the literature, people have described in patients as well as in animal models, a reduction in amyloid degrading enzymes where there is more amyloid in the brain, but this was not attributed to microglia. There have also been reports showing that in peripheral macrophages from the blood of Alzheimer’s patients, there may be a reduction in the ability to phagocytose Aβ [see ARF related news story].” These previous results gave hints that the microglia might be losing their ability to combat Aβ buildup, consistent with the new results, El Khoury said.
The results raise the possibility that microglia might be a moving target for anti-inflammatory action. One possibility for the future will be to identify drugs that will “target specific cytokines or receptors without causing an across-the-board downregulation of microglial phagocytosis and degradation of Aβ,” El Khoury said. “Alternatively, therapy could be designed to upregulate microglial clearing ability without inducing inflammatory cytokines.” Work last year from the El Khoury lab (see ARF related news story), and another recent report (see ARF related news story) have both indicated that microglia recruitment to the brain can protect from amyloid buildup in AD mice.
Going forward, El Khoury reports his lab is analyzing expression of many other genes in freshly isolated microglia, cells that were not readily accessible before. They are especially interested in looking at what happens during normal aging, as well as in AD. “There were some studies in the past but they mostly involved culturing the microglia, which doesn’t really give a good snapshot,” El Khoury said. “One advantage of what we are doing is it gives a peek at what we think might be happening in vivo.”—Pat McCaffrey