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NEUROLOGY Majority of Alzheimer's Plaques Cleared from Brains of Living MiceExperiment Provides Critical Proof-of-Principle, Not Near-Term Cure Lab mice bred to develop the notorious plaques of Alz-heimer's disease had a majority of their plaques disappear three to eight days after treatment with anti-plaque antibodies. Harvard Medical School researchers, working with scientists at Elan Pharmaceuticals, cleared 70 percent of plaques by applying the antibodies directly to the mouse brains through tiny holes in their skulls. Their findings appear in the March Nature Medicine.
Using stained blood vessels (yellow) to return to the same spot in the brain, Brian Bacskai and colleagues observed a dramatic clearing of plaques (green) three days after treatment with antiplaque antibodies. Adapted from originals from the Brad Hyman laboratory A year and a half ago, Elan scientists showed that they could prevent plaque formation in the Alzheimer's-prone mice by vaccinating them with a protein found in the plaques, amyloid-beta. But this is the first time that anyone has been able to clear pre-existing plaques from the brains of living animals. "No one has ever demonstrated directly the clearance of amyloid-beta deposits," said Brian Bacskai, HMS instructor in neurology at Massachusetts General Hospital and lead author of the study. "It was especially surprising because it was so rapid. It really took only a few days for what looks to be almost complete clearance of amyloid-beta deposits." The experiments signify another achievement: this is the first time that anyone has observed plaques in a living animal. Too minute to be imaged by conventional noninvasive methods such as magnetic resonance imaging (MRI) and computed tomography (CT), plaques have only been observed by microscopic examination of postmortem brain samples. Bacskai and his colleagues made their observations on the living animals using a newly invented device, a multiphoton microscope. "With our ability to image the plaques we could tell what happens to plaques before and after treatment with anti–amyloid-beta antibodies," said Bacskai. Miles to GoGiven that the treatment required drilling holes in the skull and applying antibody topically to the brain, it is unlikely to provide a prototype for a new Alzheimer's therapy. "In our experiments, we have a very artificial environment," said Brad Hyman, HMS professor of neurology at MGH and senior author on the paper. "We put a huge amount of antibody directly on the cortex. And it's a mouse."
"The idea that plaques can be cleared is a surprise in the field. Plaques were thought to be rather permanent features," said Brad Hyman (far right) shown with Brian Bacskai and Megan McLellan. Photo by Graham Ramsay Instead, the MGH scientists see their work as providing two proofs-of-principle: first, that Alzheimer's plaques can be reversed. And second, that they can be reversed by the external application of antibodies rather than by internally mustering the immune army of T cells and B cells, as is done through a vaccination. "We've demonstrated that we do not need a T cell response. Direct application of the antibody is effective," said Bacskai. He and his colleagues are looking at other antibodies—directed against a host of plaque proteins—to see whether they, too, can remove the neuron-choking deposits, and by what mechanism. By fully understanding the process of plaque degradation, it may be possible to find ways, other than by topical application of antibodies to the brain, to clear plaques in an Alzheimer's patient. "We feel that these results will have clinical application down the road in terms of understanding how the clearing works," said Bacskai. Keener VisionIt was a suspicion that plaques were degraded naturally in the brain, even in people unaffected by Alzheimer's, that inspired Bacskai and Hyman to undertake their experiment in the first place. "We thought that plaques would be deposited and resorbed as a natural life event," Hyman said.To explore the hypothesis, the researchers turned to the recently developed multiphoton microscope, which can image deeper and more precisely inside tissue than conventional microscopes. The microscope works by training an intense beam of near-infrared light on fluorescent tracers inside tissue. In preparation for their experiment, the researchers gave Alzheimer's-like mice bred by Elan pharmaceuticals a trio of fluorescent agents. (The mice carry a gene for a mutant form of APP, a precursor of the amyloid protein.) They applied the first of the fluorescent tracers, thioflavine S, a well-known marker of plaques, directly to the brains of mice through holes in their skulls. Next, they applied plaque-fighting amyloid-beta antibodies—also labeled with a fluorescent tag—to the same regions. Both fluorescent substances bound to the plaques. Finally, they injected a fluorescent tracer into the bloodstream, which provided a blood vessel map of the plaque's location. Using this road map, they returned to the same locations three to eight days later and stained once again for the plaques. "We saw a vast majority of the amyloid deposits were gone," said Bacskai. Seventy percent of the plaques had disappeared. When they went back and looked at control animals who did not receive the amyloid-beta antibody, they found that 20 percent of their deposits had vanished. Hyman explains: "We account for the 20 percent difference in the control experiment as being the technical limitation of our method," he said. "Another way of saying this is that the best we can do is find 80 percent of the plaques again." Intriguingly, Bacskai and his colleagues observed a flurry of microglia—agents of the immune system—surrounding the plaques that were not yet cleared. If the anti–amyloid-beta antibodies are working through these microglial middlemen, there may be other ways to prod the microglia to carry out their plaque-fighting mission. "One can think of a variety of therapeutic targets that are upstream or downstream of that antibody–microglia interaction," said Hyman. It is not clear what happened to the neurons near the vanished plaques, but the researchers plan to find out. "Once we can see where a plaque is," Hyman said, "we can also inquire of those neurons, using a variety of techniques, whether they're healthy or not." —Misia Landau |
