ARTICLE IN BRIEF
Researchers reported at the Society for Neuroscience annual meeting that they have developed a strategy to prevent the formation of amyloid-beta with a compound that blocks the dimerization of the amyloid precursor protein.
CHICAGO—Researchers have developed a strategy to prevent the formation of amyloid-beta (Abeta) with a compound that blocks the dimerization of the amyloid precursor protein (APP), according to a study presented here in October at the annual meeting of the Society for Neuroscience.
Previous research has shown that inducing dimerization — the biochemical reaction that joins two molecules into a single dimer — increases Abeta. Carmela R. Abraham, PhD, a professor of biochemistry and pharmacology at Boston University School of Medicine, and her colleagues wanted to determine what would happen if she and colleages were able to inhibit dimerization.
The findings, which build on research first published in 2012, could provide a new therapeutic target, said Dr. Abraham, whose colleague, Ella Zeldich, PhD, a postdoctoral fellow at the university, presented the results at the meeting.
“There are a lot of companies working on inhibiting beta-secretase and gamma-secretase, the two enzymes that carve Abeta from its precursor, APP, or they are trying to clear the brain of Abeta using immunotherapy,” said Dr. Abraham. “I think it is important to stop Abeta from being made in the first place. If Abeta is already present, as detected by neuroimaging, our compounds could be used in combination with immunotherapy.”
She and her colleagues conducted a high-throughput screening of 77,140 small molecules — using 200 plates with 384 wells on each one — to determine if any of them could inhibit the dimerization of APP. They genetically engineered the APP molecules with the firefly luciferase gene. When APP bound to another APP in the well, the two terminals of the firefly luciferase gene combined and became luminescent. Then, they added tens of thousands of different small molecules to each of the wells and waited to see whether cells emitted less light in the presence of the compounds.
Only one substance in the panel, which they called Y, was able to stop the formation of a dimer between two APP molecules.
The researchers looked at the structure of the molecule and could see that the substance was similar to other kinase inhibitors. When they tested the compound against select kinases, they found that it only inhibited a tyrosine kinase called cKit. And when they inhibited cKit, they observed an increase in the phosphorylation of APP and a decrease in Abeta.
Dr. Abraham said that they believe that blocking dimerization is preventing the cleavage by beta-secretase in the amyloidogenic processing of APP.
“This is a whole pathway that has never been discovered that leads to the formation of Abeta,” said Dr. Abraham. “We don't think that cKit directly induces the phosphorylation of APP, but the pathway definitely has a role in the cleavage of APP to generate Abeta.”
The researchers went on to show that cKit was acting as part of a larger cell signaling complex. Dr. Abraham said she hopes that more research will find the enzyme that acts directly on APP. “That would be the target,” she said.
The research team is now trying to make more efficient compounds that are similar to Y and is looking for other compounds that would have the same effect.
“If you want to treat brain disease, you need small molecules,” said Dr. Abraham. “If you use large molecules like antibodies, only a very small percentage get into the brain, and they can have effects on the rest of the body.”
Completed clinical trials of antibodies that clear Abeta have not been effective so far, Dr. Abraham noted, adding that there are still many unanswered questions about what needs to happen first, dimerization or phosphorylation.
“We are not sure yet,” said Dr. Abraham. “But right now what matters is that there is much less Abeta forming.”
Commenting on the study, Dennis J. Selkoe, MD, FAAN, the Vincent and Stella Coates professor of neurologic diseases at Harvard Medical School and Brigham and Women's Hospital, said that the findings “are quite interesting. It is a novel mechanism that focuses on the dimerization of APP. It is not known what percentage of APP forms dimers. But the key is to lower amyloid-beta, which they have shown it does.”
Samuel Gandy, MD, PhD, a professor of neurology and psychiatry and Mount Sinai Chair in Alzheimer's Disease Research and associate director of the Mount Sinai Alzheimer's Disease Research Center, said he thinks that “Abeta reduction may well work if given early enough, but I think that using a positive amyloid scan to time intervention may be way too late.
“As with beta-secretase inhibitors and gamma-secretase modulators, the limitation will be how many other important substrates for that enzyme will be perturbed by the drug. Protein kinase C modulators and cdk5 modulators are also being studied for Abeta reduction. The limitations of all these substances lie in specificity and side effects, neither of which can be guessed from cell experiments. The strategies are all worthy enough to test in animal models, which will give some clue about specificity and toxicity.”
“Scientists now know how to get rid of amyloid-beta using antibody approaches. The field has been looking beyond Abeta for new ways to treat Alzheimer's,” added Benjamin Wolozin, MD, PhD, a professor of pharmacology at Boston University. “Still, people are not using phosphorylation to reduce Abeta. This definitely is a new approach that might impact the disease process.”
David Harris, MD, PhD, a professor and chair in the department of biochemistry at Boston University School of Medicine, is very familiar with the new study by Dr. Abraham and her colleagues. “It's definitely promising,” he said. “What I found most intriguing is that they identified a kinase inhibitor, which affects a known biochemical process. They can therefore use their compound to define a new biological pathway in AD, and potentially target this pathway for therapeutic purposes. It is in the very early stages, and we will have to see whether these small molecules can have a positive impact on AD in animal models and in patients.”
EXPERTS: ON A STRATEGY TO PREVENT ABETA FORMATION
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