Have you heard of a mutant laboratory rodent called "toxic-milk mouse"?
This ill-named animal model is the living stock-in-trade of molecular biologist David Westaway. He is an associate professor at the University of Toronto's Center for Research in Neurodegenerative Diseases. Westaway is senior author of a paper in the Proceedings of the National Academy of Sciences (PNAS), released online Nov. 14, 2003. Its title: "In vivo reduction of amyloid-b by a mutant copper transporter." The same PNAS issue carries a similar article bearing the title "Dietary Cu [copper] stabilizes brain superoxide dismutase-1 activity and reduces amyloid Ab production in APP23 transgenic mice." The second paper's corresponding author is neurobiologist Thomas Bayer at the University of Saarland Medical Center in Hamburg, Germany.
"Many of their findings," Westaway observed, "are very similar to ours. They used a different type of animal model of Alzheimer's disease [AD], and a different type of intervention of dietary dosing to change copper levels.
"In our own findings," Westaway told BioWorld Today, "there are three messages. No. 1 concerns the idea that copper is a causative risk factor in Alzheimer's disease. That hypothesis is called into question.
"No. 2: Our experiments, as well as the experiments from Dr. Bayer, do show that there is a connection between copper physiology and the Alzheimer's precursor protein [APP].
"No. 3: Our experiment shows there's a negative correlation between levels of copper ions and some traits related to AD. The way this may work mechanistically might have to do with how levels of the amyloid-beta peptide are regulated in the periphery of the body - outside the central nervous system. That's a mechanism we don't understand yet, but it may be very useful because that is the way we could intervene to control the level of the beta-amyloid peptide. That figures in neuritic neuronal plaques - the hallmark of AD.
"In the prior general research of amyloid-beta peptides, the inference people made from in vitro studies was that copper is propathogenic. Together with the amyloid-beta peptide, copper ions were thought to be destructive generators of free radicals. Our finding is getting the opposite effect.
"The idea that copper is a bad influence as deduced from test-tube experiments - from analysis of postmortem tissues - may be incorrect. Conversely, our hypothesis indicates that there's a positive payoff, in a beneficial way, as regards the indicators of disease progression."
Four Of Six In Vivo Experiments Win Day
"We performed six types of analysis," Westaway went on. "Two experimental parts of analysis where there was no correlation between copper levels and the amyloid-beta peptide. And in the other four parts of analysis there were copper effects that would be in the direction of improving the status of the animals.
"In the mid-1990s the copper connection started off with reports that amyloid-beta peptide binds to copper. The idea is that when the amyloid-beta peptide is a soluble monomer, it is benign. When it starts assembling to higher-order multimers it becomes toxic.
"The other line of connection between copper and AD is a copper-binding site between copper ions found in the amyloid precursor protein, which is APP. That's a molecule of at least 695-amino-acid residues. It's expressed in many tissues of the body, but at high levels in the brain. It is a precursor, as its name implies, to amyloid-beta peptide.
"In 1996," Westaway continued, "our German co-researchers discovered that there is an interesting copper-binding site within the ectodomain that projects away from the cell. This correlates between levels of copper ions and traits related to AD. Thus there are two components of that connection. One idea is that there is a copper-binding site within the amyloid-beta peptide, and that the second strand there also is a copper-binding site within the ectodomain, but that is in a different part of the APP molecule."
Enter Experimental Mutant Toxic-Milk' Mice
" Toxic milk' is the name of a special strain of mutant mice that have too much copper," Westaway explained. "It's a well-known spontaneous mutation. Eventually those animals get a liver disease. The toxic-milk mice are meant to be quite a reasonable facsimile of a human disorder called Wilson's disease, which is a disease of excess copper. Obviously, the old theory decried the interaction of copper with some of those diseases. What we have done here is to use the toxic molecules to test the old theory that copper contributes to AD. Our paper reports that the supposed culprit can be beneficial. The idea of copper as a culprit in AD may need some rethinking. There could be some bonuses to figuring out how it works.
"We're doing more experiments to see how the copper effect works. We are finding that when the copper interacts with this biology of the AD peptide, the main effect may not be happening in the brain, but in the peripheral organs. So our plan is to define exactly where and how it happens. We're doing some experiments now in vivo. We use live mice to do this. Our preference is not to do in vitro studies, but analyze living animals."
Controlling Copper May Lead To Therapeutics
"The eventual human application would be that copper levels or copper-sensitive proteins be manipulated in patients to halt or cure AD. Seeing whether APP has something to do with controlling copper levels in brain cells is one objective. The other part of the question involves the question, how does the little sub-fragment of APP get involved with copper?' When we elevate copper levels in mice, degradation of this peptide may be increased. So that's what we're interested in now. From the viewpoint of a biotech person," Westaway suggested, "understanding the interface between copper biology and APP biology might be very important. We may be able to generate new, perhaps improved, animal models of Alzheimer's disease. And," he concluded, "we might acquire or design new targets for therapeutic intervention."