Science is now poised to answer an important and longstanding question about the origins of Alzheimer’s disease: Do Alzheimer’s patients have high levels of a brain protein because they make too much of it or because they can’t clear it from their brains quickly enough?
Researchers from the Alzheimer’s Disease Research Center (ADRC) at Washington University School of Medicine in St. Louis have developed the first safe and sensitive way to monitor the production and clearance rates of amyloid beta peptide (a-beta) in the human central nervous system. According to the authors, the new testing process opens a valuable window into the genesis of Alzheimer’s disease that, in addition to helping scientists better understand the origins of the condition, will likely help them improve its diagnosis and treatment.
The scientists’ results will be published online on June 25 by Nature Medicine.
High levels of a-beta in the brain are a hallmark of Alzheimer’s disease and believed to be a pivotal cause of the condition. Tests that measure a-beta levels in the cerebrospinal fluid have been available for some time. However, those fixed assessments of a-beta gave no indication of whether the flood of a-beta in patient’s brains came from an increase in the mechanisms that make the protein or a reduction in the processes that regularly clear it from the brain.
Because Alzheimer’s symptoms take many years to develop, some researchers had assumed that the creation and clearance rates for a-beta were very slow. But the initial test of the new technique, applied to six healthy volunteers, suggests the opposite.
“A-beta has the second-fastest production rate of any protein whose production rate has been measured so far,” says lead author Randall Bateman, M.D., assistant professor of neurology. “In a time span of about six or seven hours, you make half the amyloid beta found in your central nervous system.”
Ideally, the production and clearance rates stay balanced, causing the overall amount of a-beta in the central nervous system to remain constant. In the healthy volunteers who were the first test subjects, Bateman found the production and clearance rates were the same. He is now applying the technique to individuals with Alzheimer’s disease.
Researchers are developing Alzheimer’s drugs that either decrease a-beta production or increase its clearance, Bateman notes, and the new test could be very important in determining which approach is most effective.
Prior to the new test, the only way to assess the effectiveness of a new Alzheimer’s drug was to follow the mental performance of patients receiving the treatment over many months or years.
“This new test could let us directly monitor patients in clinical trials to see if the drug is really doing what we want it to do in terms of a-beta metabolism,” Bateman says. “If further study confirms the validity of our test, it could be very valuable for determining which drugs go forward in clinical trials and at what doses.”
The test also may be useful in diagnosis of Alzheimer’s prior to the onset of clinical symptoms, which occurs after Alzheimer’s has inflicted widespread and largely irreversible damage to the brain.
“We hope to study whether we can develop ways to identify potential Alzheimer’s patients on the basis of a metabolic imbalance between a-beta synthesis and clearance rates,” Bateman says.
The test combines technologies that have been available for some time but only through recent technical and procedural advances have become sufficiently sensitive. Via an intravenous drip, scientists give test subjects a form of the amino acid leucine that has been very slightly altered to label it. Inside the leucine are carbon atoms with 13 neutrons and protons in their nucleus instead of the more common 12 neutrons and protons—in scientific parlance, carbon 13 instead of carbon 12.
“Normally only about 1.1 percent of the carbon atoms in our bodies are carbon 13—the vast majority is carbon 12,” Bateman notes. “Physiologically and biochemically, carbon 13 acts just like carbon 12, meaning it won’t alter the normal a-beta production and clearance processes and is very safe to use.”
Over the course of hours, cells in the brain pick up the labeled leucine and incorporate it into the new copies they make of a-beta and other proteins. Scientists take periodic samples of the subjects’ cerebrospinal fluid through a lumbar catheter, purify the a-beta from the samples and then use a device known as a mass spectrometer to determine how much of the a-beta includes carbon-13-labeled leucine.
Tracking the rise of the percentage of a-beta with labeled leucine over time gives scientists the subject’s a-beta production rate. When the percentage of a-beta containing labeled leucine plateaus, scientists remove the IV drip supplying the labeled leucine. Periodic sampling of the patients’ CSF continues, allowing scientists to get a measurement of how quickly the nervous system clears out the labeled a-beta. In the first test subjects, the test procedure lasted for 36 hours.
Other research groups have expressed an interest in applying the new test to Alzheimer’s research and to other neurological disorders such as Huntington’s disease.
This study was performed in the laboratories of David M. Holtzman, M.D., the Andrew and Gretchen Jones Professor and chair of Neurology, and Kevin E. Yarasheski, Ph.D., associate professor of medicine and assistant director of the Washington University Biomedical Mass Spectrometry Resource. It was also supported by the ADRC, directed by John C. Morris, M.D., the Friedman Distinguished Professor of Neurology.
Bateman RJ, Munsell LY, Morris JC, Swarm R, Yarasheski KE, Holtzman DM. Human amyloid-b synthesis and clearance rates as measured in cerebrospinal fluid in vivo. Nature Medicine, June 25, 2006.
Funding from the American Academy of Neurology and the National Institutes of Health supported this research.
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