New Scientist. 26/8/00
METAL HEADS
Did you know that your
brain is full of metal?
It's absolutely vital, says
Jonathan Knight, but
handled the wrong way
it can be devastating.
An Ore prospector might think he was nearing mother lode. For buried within the grey and white matter in your head, stashed safely inside nuggets of protein, is a surprising quantity of metal. In fact, the human brain contains about 6 milligrams of copper, enough to print a small circuit board. Zinc, iron and manganese are just as plentiful.
While we've known about metals in the body and
brain for decades, it's only recently that scientists have
stopped viewing them as just "trace elements" and
started to ask what on earth they're doing there. "Calling
copper or zinc in the brain trace metals is quite silly,"
says Ashley Bush, a research psychiatrist at the Genetics and
Aging Unit of Massachusetts General Hospital in Charlestown.
"The brain concentrates metals better than any other tissue
in the body." But to what end?
For starters, our brain doesn't work properly without metals.
Many neurons release zinc, copper or iron to help transmit
signals across synapses. In fact, cyanide kills by soaking up
copper at the synapses. And young rats fed diets low in iron
learn poorly, while elderly people with zinc deficiencies seem to
be at greater risk of developing senile dementia. Metals are also
integral to the way brain cells defend themselves. When bound to
anti-oxidant proteins, copper and iron neutralise dangerous free
radicals by mopping up electrons. And cells sometimes release
zinc to help fight infections.
But in the past year, metals have revealed a
more sinister side - a Yin to their antioxidant Yang. A growing
number of researchers, Bush among them, now suspect that
mishandling metal in the brain is responsible for neurological
disorders such asAlzheirner's, Parkinson's and prion diseases. If
he's right, it will completely alter the way scientists think
about these diseases and how they go abouttrying to cure them.
"I think there will be a sea change," says Bush. But it
won't be easy to prove.
It all began early in Bush's career, when he was studying one
very visible problem with the Alzheimer's brain senile plaques.
These clumps of protein accumulate outside diseased neurons inthe
parts of the brain that control higher cognitive functions such
as judgement and memory. Hehappened to spot that the primary
constituent of plaque, a small protein called Ab, binds
to zinc and copper, and that the brains of patients who had died
of Alzheimer's contained three to four times as much copper, zinc
and iron as normal, mostly concentrated in the plaques.
To find out whether metal had anything to do with plaque formation, Bush and his colleagues mixed Ab protein with metal ions in the lab, and left it to form clumps. Then they added a chelator, a chemical that soaks up metals.---The stuff dissolved almost immediately, as fast as we could measure it," Bush says. They tried the same experiment on mashed post-mortern brain from an Alzheimer's patient. Again the plaques vanished like sugar in warm water.
For years, plaques had seemed the most logical cause of Alzheimer's dementia. Dozens of labs showed that Ab was toxic to neurons, and people genetically predisposed to get Alzheirrier's, such as those who carry the ApoE4 gene, develop more plaques at a younger age. Mice engineered to produce large amounts of human Ab developed most of the symptoms of Alzheimer's-including plaques. And now Bush's work suggested that metals might help the plaques form. But it soon became clear to him that this wasn't the whole story. In some cases, patients with the greatest dementia turned out to have few plaques.
Back in 1986, Colin Masters, a pathologist at the University of Melbourne, proposed that Alzheimer's might be caused instead by oxidative stress - surplus electrons knocking about and damaging cells. Six years later, Miguel Pappolla's group at the University of South Alabama in Mobile found abnormal amounts of antioxidant proteins in the brains of Alzheimer's patients, particularly around plaques. Now Masters, Pappolla and others see plaques more as a monument to the brain's battles with oxidative stress than as a source of damage. "The plaque is like a tombstone," says Pappolla.
Finally, in December, an international team of researchers including Bush and Masters reported how the two lines of research might dovetail. They found that when the AP protein binds to copper it can create oxidative havoc (journal of Biological Chemistry, vol 274, p 37111) producing lots of hydrogen peroxide and killing cells.
This immediately begs a question. Our cells produce Ab all the time, and copper is readily available too. So if this tiny protein is so risky, why do we have it at all-and why doesn't everybody get Alzheirner's? Bush believes the answer is that it only becomes dangerous when chemical conditions in the cell allow it to grab too much copper. Normally Ab would be safe, or even beneficial.
That might sound strange, but five years ago, mutations in a protein called SOD1 were linked to familial amyotrophic lateral sclerosis-an inherited motor neuron disease. SOD1 is one of a family of powerful antioxidants. SOD1 binds copper and zinc, and uses the metals to mop up electrons to disarm superoxide (03), a dangerously reactive relative of oxygen. But it wasn't immediately clear how the mutant proteins could cause the disease. In lab tests some were just as good at mopping up superoxide as the normal form.
Then last year, a team of biochemists led by Joseph Beckman of the University of Alabama, Birmingham, found that normal SOD1 binds up to 50 times more zinc than any of the five mutant versions they tested. And somehow, when zinc was missing, the copper bound to SOD1 stole electrons from other chemicals in the cell 3000 times as fast as normal SODI does. It then handed over the extra electrons to make more superoxide. Beckman thinks this sequence of events could cause disease because extra superoxide generates peroxynitrite, the same poison used by damaged or superfluous motor neurons to commit suicide.
Bush thinks that Ab could be just like SOD1: a protective antioxidant when properly loaded with copper and zinc, but a dangerous pro-oxidant when zinc is lacking. He's found many striking parallels. Ab breaks down superoxide, and like SODI it normally binds more easily to copper than zinc. But Bush found that adding extra zinc, mimicking its release by cells under threat, blocks the oxidative havoc that Ab causes. The metal entombs the protein in clumps that do no further damage. Clumps which are very reminiscent of plaques.
All this fits nicely with the discovery of a second, more toxic form of Ab some years ago. Most Ab in the body is 40 amino acids long (Ab40), but brains of Alzheirner's patients are brimming with a longer form, Ab42. This is more toxic to neurons. And as it turns out, it also binds copper much more tightly than Ab40 and is much more efficient at making hydrogen peroxide.
But there is an ironic twist. With the proper zinc balance, Ab42 is also a better antioxidant. So, says Bush, the enzyme that makes Ab42 may have evolved to provide better protection from oxidation damage.
But the same protein that defends our brains through the reproductive years sometimes bashes them up later on in life.
The picture of Alzheimer's emerging from this is a snowball effect. We have Ab all over our bodies our entire lives, but at some stage it begins to accumulate in the brain. Perhaps some of it fails to load zinc properly and does some oxidative damage. In response, neurons make more antioxidants, including Ab. These, especially the long form, spread more oxidative injury (see Diagram). "I think there is a vicious cycle," Bush says. "Once Ab moves from being an antioxidant to a pro-oxidant, it can then initiate its own generation."
But what starts the snowball rolling? One possibility is acid. Oxygen starvation can produce a condition called acidosis, as anaerobic metabolism kicks in. The same phenomenon causes sore muscles when you exercise hard. As a person ages, events that can cause mild acidosis in the brain become more frequent-mild strokes, temporary drops in heart output, infections and even head injuries. And when their PH drops, zinc hardly binds to Ab at all. "This is our number one hypothesis," says Bush.
Genetic predispositions to Alzheirner's could fit into this picture well. ApoE, for example, is a copper-binding protein. Little is known about the metal-binding role, except that it seems to help protect cells from oxidative damage by excess copper. ApoE4, the form associated with familial Alzheirner's, is the worst at binding copper.
The Jekyll and Hyde protein scenario is now reverberating throughout the field of neurodegenerative disease. For example, in prion diseases such as BSE, scrapie and CJD, an insoluble form of the prion protein (PrP) forms clumps in the brain. What, many researchers assume, but no one knows for sure, is that these kill neurons.
Then last year, David Brown, a biochemist at Cambridge University, showed that PrP can act as an antioxidant when copper is bound. Cells containing PrP are known to resist oxidative stress more easily. These results suggest a different picture of mad cow disease than the standard cruddy brain theory, Brown says. 1 think what leads to the death of neurons in prion disease is the loss of antioxidant activity."
Early this year, Brown found that PrP binds not only to copper, but to manganese. And when it does, it becomes inactive. Enzymes that chew up normal PrP can't digest the manganese version, a hallmark of the disease form of PrP (The Embo journal, vol 19, p 1180). So it looks as though metals could have something to do with prion disease too.
This is welcome news to Mark Purcley, a Somerset farmer with a background in biochemistry, who over the past 10 years has analysed soil, water and vegetation for more than a dozen metals at sites of prion disease clusters around the world, including Iceland, Slovakia and Colorado. Only one metal was consistently more abundant in disease-ridden areas: manganese. 1 was really blown out by Brown's work," says Purdey. "After 1 had done my soil tests, 1 read all his papers and thought, 'Wow, the two together explain what is going on."
Purdey is testing the soil in Leicestershire near the town of Queniborough, where a cluster of vCJD cases was recently reported (New Scientist, 22 July, p 3). The tests are not complete, but he has already found a possible source of metal in a nearby rocky outcrop that he says contains manganese oxide in Precambrian strata. Is it possible that chronic exposure to manganese could raise the risk of prion diseases? "It's not absolutely conclusive," says Brown. "There is CJD everywhere in the world. But if there is some factor to do with pipes or water, it could be very focal."
What's to blame?
A SUSPECTED link between aluminium in the diet and Alzheimer's disease prompted some people in the late 1980s to discard their aluminium pans and have their water checked. And worries about an increase in neurological disease in Camelford in Cornwall after the 1988 release of aluminium sulphate into the water supply still reverberate. But the link remains inconclusive. "We know one thing for sure, aluminium is toxic," says Stephen Bondy, of the University of California at Irvine. Aluminium seems to depend on iron for its toxic effects, Bondy says. Aluminium forms particles in the brain, which provoke the brain's immune defences. Cells release iron in an attempt to oxidise and disarm the particles, but they persist and the cells end up self -destructing. Whether this has anything to do with Alzheimer's is the unanswered question. "We are exposed to aluminum our whole lives," Bondy says. 'It's not likely to be the sole factor in Alzheimer's disease, but 1 believe it may be one of several"
All this is bound to make people wonder if they should get rid of their copper pots and avoid foods that are high in manganese. And it's sure to revitalise the debate about aluminium and dementia (see "What's to blame?"). But that's not necessary, Bush says. Experiments on lab animals show that a meal high in iron or zinc has no effect on the amount of these metals in the brain, despite high levels in the blood. The blood-brain barrier keeps them out.
What might make a difference, Bush and Brown agree, is long-term exposure to high levels of metal, but there is currently little evidence for or against this. If it's true, the risk is slight. Even in the regions of Slovakia where people were continually exposed to high levels of manganese, the rate of disease was still only one in a thousand.
But the theory does not mean that preventing and treating neurological diseases might be as easy as soaking up metals. A copper chelator made by Prana Biotechnology in Melbourne is now in the first phase of clinical trials, after promising results with mice. Antioxidants such as vitamin E are already thought to slow the onset of Alzheimer's, but chelators go to the source of the disease. They have potential in amyotrophic lateral sclerosis and vCJD as well.
Evidence linking metals to several other diseases is also beginning to trickle in. Iron deposits have been found in the nigral cells that die in Parkinson's disease. And iron seems to cause the Parkinson's protein a-synuclei to aggregate. The protein mutated in Friedrich's ataxia fails to export iron from mitochondria, the powerhouses of the cell, leaving neurons short of energy. And cataracts form when the lens protein alpha-crystallin becomes crosslinked, possibly by copper-generated superoxide.
Ironically, if Bush is right, approaches to Alzheimer's treatment based on the idea that plaques cause the disease won't work. For example, the antibody against AP made by Elan Pharmaceuticals near San Francisco makes plaques in mouse brains vanish. But if they're just a tombstone that may not matter. Still, Bush realises his work isn't going to change these strategies overnight. "I hope Elan's approach works," he says. 1t's logical, whereas the idea that an antioxidant can also be bad for you is counterintuitive."
Further Reading: "Metals and neuroscience" by Ashley Bush in Current Opinion in Chemical Biology, vol 4, p 184 (2000)
"Oxidative stress and Alzheirner disease" by Yves Christen in American Journal of Clinical Nutrition, vol 71, p 621 S (2000)
©Jonathan Knight 2000.
