Lab mice bred to develop the notorious plaques of Alzheimer’s disease had a majority of their plaques disappear 3 to 8 days after treatment with anti-plaque antibodies. Massachusetts General Hospital (MGH) 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.
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, called amyloid-beta. But this is the first time that anyone has been able to clear pre-existing plaques in a living animal points out Bradley Hyman, MD, PhD, an MGH neurologist and senior author of the study.
“No one has ever directly demonstrated the clearance of amyloid-beta deposits,” says Brian Bacskai, PhD, an assistant in neurology at MGH and lead author of the study. “This was an especially surprising process because it was so rapid.
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 non-invasive methods such as magnetic resonance imaging (MRI) and computed tomography (CT), plaques have only been observed by microscopic examination of post mortem brain samples. The MGH team made their observations on the living animals using a newly invented device, a multi-photon microscope.
“With our ability to image the plaques, we could clearly determine what happens to plaques before and after treatment with anti-amyloid- beta antibodies,” says Bacskai.
Although it is still too early to know whether or not this result will one day apply to treatment in patients, the MGH scientists see it as an important pair of proof-of-principles. First, Alzheimer’s plaques can be reversed. Second, the plaques can be reversed by the external application of antibodies rather than by internally mustering up the immune army of T cells and B cells, as is done through a vaccination.
It was a suspicion that plaques were degraded naturally in the brain, even in people unaffected by Alzheimer’s, that inspired the team to undertake their experiment in the first place. “We thought that plaques would be deposited and resorbed as a natural life event,” Hyman says.
To explore the hypothesis, the researchers turned to the recently developed multi-photon microscope, which can image deeper and more precisely inside tissue than conventional microscopes. The multi-photon microscope works by training an intense beam of near infrared light on fluorescent tracers inside tissue.
In preparation for their experiment the MGH 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 that makes up plaques.)
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 capillaries in the vicinity of the plaques, which provided a blood vessel map of the plaque’s location.
Using this road map, they returned to the same locations 3 to 8 days later and stained once again for the plaques. In control animals they were able to relocate 80 percent of the plaques that they had seen the first time; after application of the antibody, seventy percent of the plaques had disappeared.
Microglia—agents of the immune system—surrounded the plaques that were not yet cleared. If the anti-amyloid-beta antibodies are working through these microglia middlemen, there may be other ways to prod the microglia to carry out their plaque-fighting mission.
It is not yet clear what will happen to neurons near the vanished plaques but the researchers plan to find out. “Once we can see where a plaque is, we can also ask whether neurons in their area are healthy or not; the real key is to understand how to improve the function of the brain.” says Hyman.