St. Louis, Feb. 29, 2000 – Scientists have known since 1993 that people who make a certain form of apoE, a lipoprotein, are at risk for developing Alzheimer's disease earlier than people who make other forms of apoE. Now researchers have discovered why.
They showed, for the first time, that apoE directly promotes the development of neuritic plaques in the brain. These plaques, which contain damaged nerve cell connections, are a hallmark of the disease. The researchers also discovered that apoE4 - the high-risk version of apoE - does far more damage than apoE3.
These findings, made with a mouse model of Alzheimer's disease that expresses human proteins, are reported in the March 14 Proceedings of the National Academy of Sciences. David M. Holtzman, M.D., an associate professor of neurology and of molecular biology and pharmacology at Washington University School of Medicine in St. Louis, is lead author of the paper. Holtzman's group collaborated with Steven M. Paul, M.D., and Kelly R. Bales at the Lilly Research Laboratories of Eli Lilly and Company.
The brains of Alzheimer patients are dotted with deposits of amyloid-beta, a protein fragment of unknown function. Amyloid-beta normally is found at low levels in bodily fluids. In the mouse studies, the researchers found that amyloid deposits by themselves didn't damage the brain. But when apoE was present, the amyloid formed into hair-shaped fibrils, and neuritic plaques containing the "arms" of damaged nerve cells appeared. Moreover, mice that made both human amyloid-beta and human apoE4 produced more amyloid-beta and had 10 times as many fibrillar amyloid-beta deposits in a key part of the brain as those that made human amyloid-beta and human apoE3. The amyloid fibrils always were associated with neuritic plaques.
"To me, these results strongly suggest that the main reason that apoE4 is a risk factor for Alzheimer's disease is because it interacts with amyloid-beta protein and enables it to become fibrillar, which in turn promotes neurodegeneration," Holtzman says.
Paul and Bales previously had studied mice that overproduce the human protein that is cleaved into amyloid-beta. They found that apoE played an important role in both the deposition of amyloid-beta and its conversion to a fibrillar form. In the present study, Holtzman's and Paul's groups showed that mice that produce apoE develop not only fibrillar amyloid but also numerous neuritic plaques in the hippocampus and neocortex of the brain by the time they are 12 months old. The hippocampus is involved in memory, and the neocortex performs many cognitive functions.
By using a chemical that stains only fibrillar amyloid, the researchers determined that brain cells were damaged only where the amyloid took the form of fibrils.
They then studied mice that made human amyloid-beta but not mouse apoE. These animals deposited significant amounts of amyloid-beta in the hippocampus, but they developed few or no neuritic plaques, even by 15 months of age. "So apoE must be really important in allowing amyloid-beta to convert from a nonfibrillar to a fibrillar form," Holtzman says. "When that happens, nerve cell connections begin to degenerate."
The investigators also inserted the gene for human apoE3 or apoE4 into the mice that made human amyloid but not mouse apoE. By 15 months of age, the hippocampi of many of these animals had amyloid-beta deposits and neuritic plaques that stained for amyloid fibrils. But the researchers found the deposits in 89 percent of the apoE4 mice compared with only 33 percent of the apoE3 mice. Moreover, several apoE4 mice Ð but no apoE3 mice Ð also had amyloid-beta deposits in the neocortex.
Staining for fibrillar amyloid revealed an even more striking difference. The apoE4 animals had more than 10 times as many fibrillar deposits in one part of the hippocampus as the apoE3 animals. All of this fibrillar amyloid was associated with damaged arms of nerve cells.
"The fact that apoE4 can increase both the amount of amyloid-beta and the formation of amyloid fibrils seems likely to explain why this version of the lipoprotein is a genetic risk factor for Alzheimer's," says Holtzman.
ApoE is found in the brain in a high-density lipoprotein particle that differs from the HDL particles in the bloodstream. Holtzman now is studying interactions between amyloid-beta and these particles, which are made by brain cells called astrocytes. He speculates that the particles might actually isolate amyloid-beta from the brain as well as help convert it to the harmful fibrillar form. "We believe it may be possible to alter the levels of brain apoE with drug therapy," says Paul, who is group vice president of discovery research and clinical investigation at Eli Lilly and Company. "Such drugs should inhibit the deposition of amyloid-beta, prevent fibril formation and promote amyloid removal - which would have the potential to slow down or even prevent Alzheimer's disease."
Grants from the National Institute on Aging, the Alzheimer's Association and the Ruth K. Broad Foundation supported Holtzman's study. Eli Lilly and Company also supported the research.
Holtzman is a member of the Alzheimer's Disease Research Center at Washington University School of Medicine in St. Louis.
Holtzman DM, Bales KR, Tenkova T, Fagan AM, Parsadanian M, Sartorius LJ, Mackey B, Olney J, McKeel D, Wozniak D, Paul SM. Apolipoprotein E isoform-dependent amyloid deposition and neuritic degeneration in a mouse model of Alzheimer's disease. Proceedings of the National Academy of Sciences USA, March 14, 2000.
Copies of the paper are available from the PNAS news office (202) 334-2138. Or e-mail pnasnews@nas.edu
Journal
Proceedings of the National Academy of Sciences