More than a century has passed since Alois Alzheimer first identified abnormal plaques and tangles in the brain of a woman with dementia. Though much has been learned since then, treatments for Alzheimer’s disease remain largely ineffective and diagnostics inadequate. New work from a collaboration led by HMS professor Dennis Selkoe and his lab, however, may help change that.
Using extracts directly from human brain tissue, Selkoe, the Vincent and Stella Coates professor of neurologic diseases in the Department of Neurology at Brigham and Women’s Hospital, and his colleagues found that dimers of human amyloid-beta protein (Abeta), the smallest possible assembly of the peptide, can induce the synapse dysfunction and loss that are hallmarks of early-stage Alzheimer’s. The study, published online in Nature Medicine on June 22, provides insight into how the disease begins in the human brain and how it might be treated.
This work is a refinement of the hypothesis that excess Abeta initiates Alzheimer’s disease, said Selkoe, who helped propose it in 1991. “All forms of Abeta are bad news. You don’t want it to build up. But smaller assemblies”—the very first assemblies to form when the body produces excess Abeta—“are worse.”
The results are compelling because over the past decade much of the evidence supporting this hypothesis has relied on model experimental systems that use synthesized forms of Abeta or Abeta produced from cell cultures. “That’s fine. It’s useful. It’s easy,” said Selkoe, whose lab first reported that cultured cells produce Abeta, which can be used for research purposes. “But why don’t we go right to the source?”
Selkoe and first author Ganesh Shankar, now a third-year MD student at HMS, did just that by starting with donated brain specimens from recently deceased patients with a variety of types of dementia. Shankar extracted material from these brains and found substantial amounts of soluble Abeta in the Alzheimer’s brains and little in the others.
Soluble Abeta oligomers may float freely inside the brain’s extracellular fluid spaces. Over time, these soluble complexes clump together into insoluble fibrils that are deposited as amyloid plaques. Since previous research had shown that soluble Abeta levels correlate most strongly with cognitive symptoms in Alzheimer’s disease, Shankar, Selkoe, and collaborators focused their efforts on it first.
Shankar and Shaomin Li, HMS instructor in neurology at BWH, found that soluble Abeta extracted directly from human cortex potent-ly inhibited long-term potentiation, a key neural mechanism for forming new memories, in the hippocampus of normal mice. Soluble Abeta also facilitated long-term synaptic depression, essentially priming hippocampal neurons to tune some signals out.
To test the physiological effects of soluble human Abeta on the synapse, Shankar and Selkoe connected with Bernardo Sabatini, HMS associate professor of neurobiology. In 2007, Sabatini had worked with Shankar, then an HMS PhD candidate, on a Journal of Neuroscience paper showing that soluble Abeta secreted by cultured cells disrupts synaptic spines. In the new work, Shankar and Sabatini repeated those experiments, this time using soluble Abeta oligomers from Alzheimer’s brains. Again, they observed synapse loss, which is the strongest neuropathological correlate of Alzheimer-type dementia.
Their analysis implicated the same mechanism observed in their 2007 work. Though still incompletely understood, the human Abeta oligomers appear to be “pathologically activating a normal pathway, biasing it toward this synapse loss,” said Sabatini.
The team then collaborated with researchers at University College Dublin to test the human peptide’s effect on behavior. The scientists injected into the brains of rats trained to avoid a dark chamber either a sample containing soluble human Abeta or a sample from which the Abeta had been removed. The soluble form “made the rats forget and go back into the dark chamber as if they had never been trained before,” said Shankar.
The group then turned from free-floating Abeta assemblies in the brain to the largely insoluble amyloid plaques, also extracted from human brains with Alzheimer’s disease. “A major conundrum in the Alzheimer field has been whether amyloid plaques are bad or good,” said Selkoe. “The answer, almost certainly, is both.”
On the good side, he said, “We found that the isolated plaques were largely without biological activity” on synapses. They seem to act as reservoirs that collect smaller amyloid assemblies and sequester them, thereby keeping toxic Abeta oligomers out of circulation. This suggests that plaques, though they may have some pathologic effects, are not principally involved in initiating the early synaptic impairments in Alzheimer’s disease.
However, Selkoe believes that the plaques appear to have a “maximum capacity.” Once that capacity is reached, excess free-floating assemblies, including toxic soluble dimers, have nowhere to go, leaving them free to “diffuse into synaptic clefts and cause injury,” he explained.
With this evidence confirming that soluble human Abeta is the culprit behind the earliest Alzheimer’s injury, the team wanted to understand more about its biochemistry. Shankar and Selkoe “did a complex and elegant chemical isolation of different Abeta isoforms from the brains of these patients,” said co-author Sabatini. “What they found is that Abeta exists in many different-sized forms.”
They isolated Abeta monomers and oligomers using a form of size exclusion chromatography and retested them. The soluble human Abeta dimer inhibited long-term potentiation while monomers and other higher order assemblies did not affect this form of synaptic plasticity.
“This work shows that just a dimer has the potential to be toxic in the human brain,” said David Holtzman, neurology chair at the Washington University School of Medicine, “but do you need to target dimers therapeutically or will targeting different forms of Abeta suffice?”
This question gets at the heart of what remains to be explored. For instance, asked neurologist Lennart Mucke of the University of California, San Francisco, Medical School, “Are dimers the most important Abeta species in live human beings with early Alzheimer’s?”
The answer is not yet clear, but the results of Shankar’s tests of several potentially therapeutic Abeta antibodies suggest that targeting dimers may be a successful strategy. Antibodies directed to the peptide’s N-terminus most effectively pulled Abeta dimers from soluble brain extracts and neutralized their adverse effects. Early unpublished results of a phase II trial of a humanized version of the same antibody, reported jointly in June by Elan Corporation and Wyeth Pharmaceuticals, also showed positive clinical benefits in certain Alzheimer’s patients in the mild to moderate stages of disease.
Conflict Disclosure: Dennis Selkoe is a founding scientist of Athena Neurosciences (now part of Elan) and a consultant to Elan Corporation.
Funding Sources: The National Institute on Aging (part of the National Institutes of Health), the Science Foundation Ireland, and the Wellcome Trust
