The first rule of triage is do no harm, but try telling that to microglia. Though these cellular paramedics may be invaluable first responders to pathological damage in the brain, including accumulation of amyloid-β (Aβ), there is increasing evidence their overzealous attempts to dispose of unwanted debris is accompanied by a torrent of dangerous proinflammatory signals and toxins, such as TNFα and reactive oxygen species. Perhaps these concepts are not mutually exclusive. As demonstrated by two recent papers, the beneficial and detrimental properties of microglia may depend on when and why they are recruited.
Writing in the March 11 Nature Medicine online, Joseph El Khoury and colleagues at Harvard Medical School and the Okayama University Medical School, Japan, report that microglia protect transgenic mice expressing amyloidogenic human Aβ precursor protein (APP). In contrast, William Van Nostrand and colleagues at Stony Brook University, New York, report in the March 21 Journal of Neuroscience that blocking microglial activation in a model of cerebral amyloid angiopathy rescues behavioral deficits. Together, the two papers demonstrate how the relationship between microglia and pathology may be far from simple.
El Khoury and colleagues studied mice lacking chemokine receptor 2 (Ccr2), a microglial cell-surface receptor that mediates chemotaxis and recruitment of microglia to sites of injury. “We figured one way to test whether microglia are toxic is to prevent them from coming into the brain,” El Khoury told ARF. There is good evidence to suggest that microglia infiltrate the AD brain from the blood. Chemotactic cytokines, including monocyte chemotactic protein 1 (Mcp-1), are upregulated in AD brain (see Ishizuka et al., 1997) and are produced by microglia and astrocytes after challenge with Aβ (see Smits et al., 2002 and El Khoury et al., 2003). Recent studies have also identified bone marrow-derived microglia in the brain of APP transgenic mice (see ARF related news story).
To test the role of migratory microglia, El Khoury and colleagues bred transgenic APP mice (Tg2576) with Ccr2-/- animals, generating APP mice both heterozygous and homozygous-negative for Ccr2. The researchers found that the loss of the chemokine receptor had a dramatic effect on survival in the APP transgene background. Only about 15 percent of mice lacking Ccr2 survived to 130 days, at which point about 70 percent of APP animals were still alive. Mice with one copy of Ccr2 faired a bit better—about 40 percent were still alive at day 130. The poor survival was directly related to APP pathology because lack of Ccr2 had no effect on survival in a wild-type background. To investigate this more deeply, El Khoury and colleagues examined pathological changes in the brain. They found that the Ccr2-negative animals had about 1.5- and twofold increases in levels of Aβ40 and Aβ42, respectively. Levels of β-secretase, presenilin 1, and insulin degrading enzyme were unchanged, suggesting that the elevated Aβ was not due to increased production or decreased enzymatic degradation. Aβ deposits were clearly visible in 65-day-old APP Ccr2-/- animals, but not in age-matched APP or wild-type animals.
The pathological changes in these mice seem related to loss of brain microglia. In the absence of Ccr2, the number of microglia in the brain of 65-day-old APP transgenic mice was reduced by sixfold, as judged by staining for the microglial marker Cd11b. Furthermore, using flow cytometry to distinguish resident brain microglia (Cd11 positive and low expression of Cd45) from migratory cells from the blood (Cd11 positive and Cd45 high expression), the authors found that the APP mice had about fivefold more migratory microglia in the brain than APP, Ccr2 +/- animals. In addition, while stereotactic injection of Aβ into the brain of wild-type mice led to robust infiltration of microglia, it had a minimal effect on microglial recruitment in Ccr2-negative animals. The data indicates that migratory microglia may play a crucial role in ridding the brain of toxic amyloid.
The paper from Van Nostrand and colleagues paints microglia in a slightly different light. First author Rong Fan and colleagues used the anti-inflammatory drug minocycline in Tg-SwDI transgenic mice, which express human APP with Swedish, Dutch, and Iowa mutations. These animals accumulate abundant Aβ fibrils in the microvasculature of the brain. Because minocycline readily crosses the blood-brain barrier and suppresses activation of microglia by Aβ (see Familian et al., 2006 and Seabrook et al., 2006), the authors used it determine how tempering microglial activation might affect the transgenic mice.
Fan and colleagues gave minocycline every other day for 28 days to 12-month-old mice. They found that this short treatment had no effect on Aβ accumulation, but it did reduce activation of microglia, particularly in the hippocampus where the treatment reduced the numbers of activated cells by about one half, as judged by the number of MHCII- and collagen type IV-positive cells. The treatment also reduced levels of the inflammatory cytokine interleukin 6 (IL6) by about 25 percent. The number of reactive astrocytes, however, was unchanged by the drug.
If microglia are protective in this model, then minocycline might be expected to exacerbate the disease phenotype. Instead, Fan and colleagues found that treatment with the anti-inflammatory led to significant improvements in performance in the Barnes maze test, where the animals must correctly identify one of eight holes, equally spaced around the circumference of a circular platform, that leads to an escape box. Over a 5-day testing period, transgenic mice that received the drug showed steady improvement in time taken to find refuge, and by the end of testing they were performing almost on par with wild-type animals. Untreated APP mice, however, performed progressively worse. By the end of testing they took on average 100 seconds to find the escape hole, as opposed to about 40 seconds for treated animals. “These data further support the hypothesis that microglial activation is involved in promoting cognitive impairment in this transgenic mouse model of cerebral microvascular amyloid,” write the authors.
Curiously, when El Khoury and colleagues examined the Ccr2-negative animals, they noted Aβ accumulated primarily around blood vessels and that several animals had signs of intracranial hemorrhage on autopsy. These findings suggest that microglia are needed to prevent CAA. So how does one reconcile the two studies? The crucial aspect might be timing, El Khoury suggested. “Microglia in AD may be behaving like monocytes in atherosclerosis. They are initially recruited to clear and get rid of Aβ, but as the disease progresses (and the mice grow older), microglial accumulation and their subsequent activation becomes detrimental.” (See also El Khoury’s recent ARF comment.) Van Nostrand echoed that sentiment in a comment to ARF. Early on, activation of microglia may be helpful, “but if Aβ is not cleared and microglia remain activated this may lead to the chronic neuroinflammation and behavioral deficits that we observed in our model,” he wrote (see full text of his comment below). In short, while their triage attempts may at first prove successful, over the long term these cells may end up doing more harm than good, in which case, a dose of anti-inflammatory medicine may prove beneficial.—Tom Fagan
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- Seabrook TJ, Jiang L, Maier M, Lemere CA. Minocycline affects microglia activation, Abeta deposition, and behavior in APP-tg mice. Glia. 2006 May;53(7):776-82. PubMed.
- El Khoury J, Toft M, Hickman SE, Means TK, Terada K, Geula C, Luster AD. Ccr2 deficiency impairs microglial accumulation and accelerates progression of Alzheimer-like disease. Nat Med. 2007 Apr;13(4):432-8. PubMed.
- Fan R, Xu F, Previti ML, Davis J, Grande AM, Robinson JK, Van Nostrand WE. Minocycline reduces microglial activation and improves behavioral deficits in a transgenic model of cerebral microvascular amyloid. J Neurosci. 2007 Mar 21;27(12):3057-63. PubMed.