A small molecule probe discriminating between Aß amyloid oligomers and fibrils

One of the conceptual shifts that the study of Alzheimer's disease has seen in the past few years is the realization that the long-studied insoluble Aß (1-42) amyloid fibrils may not be the real culprits in the disease. Instead the dubious distinction may belong to soluble Aß oligomers whose morphology differs from that of mature fibrils. Thus instead of focusing on one kind of Aß species, researchers are focusing on a broad range of oligomers and fully formed fibrils that differ in their architecture. Some differences between these species are clear; for instance the oligomers seem to permeate the plasma membrane in cells much better than the fibrils. The factors that dictate the exact morphology of these species sometimes might be subtle (such as pH shifts), and I myself have worked a little on such subtle changes leading to drastically different morphologies (see here for instance).

However, a study of different amyloid morphologies will greatly benefit from probes that allow us to selectively target and isolate certain amyloid architectures. The principal method of doing this until now has been to raise species-specific antibodies that target either oligomers or fully formed fibrils. In this regard small molecules have not been very promising as they tend to indiscriminately bind to all amyloid species; for instance Congo Red which is the archetypal amyloid labeling dye does not discriminate between different amyloid species.

Now a group at the University of Michigan has discovered some very simple probes that seem to target only spherical oligomers and not fibrils. The probes consist of tryptophan and some rather simple and well known biological molecules containing tryptophan which exhibit fluorescence when bound to proteins, and especially hydrophobic regions of proteins. The team started by screening about 70 simple organic molecules that exhibit fluorescence. Most of these molecules did fluoresce, but when bound to both forms of amyloid, thus precluding discrimination between the two species. Some did not fluoresce at all. But about 10 of them showed differential fluorescent quenching; the fluorescence was quenched much more when bound to oligomers compared to fibrils, thus allowing their selective labeling and visualization. Antibody labeling and radiography confirmed that it was indeed the oligomers that were getting labeled.

Intriguingly these probes consist of tryptophan itself as well as some very simple naturally occurring molecules containing the Trp moiety, such as melatonin, tryptamine and serotonin. In my mind this raises a very interesting possibility; could these molecules also be interacting selectively with amyloid and somehow modulating its behavior inside living systems?

In any case, what is even more valuable is that the probe seems to selectively label the oligomers in the presence of the fibrils and this can be determined from fluorescence studies. This opens up some potentially very interesting applications; for instance if there was any way at all to use such probes in vivo, we could gain extremely valuable knowledge on the ratios of oligomers to fully formed fibrils. Recalling that amyloid is astonishingly a deformed version of a naturally occurring protein, insights gained from such studies could be critical in shedding light on the natural and unnatural roles of amyloid in living systems.

Reinke, A., Seh, H., & Gestwicki, J. (2009). A chemical screening approach reveals that indole fluorescence is quenched by pre-fibrillar but not fibrillar amyloid-β Bioorganic & Medicinal Chemistry Letters, 19 (17), 4952-4957 DOI: 10.1016/j.bmcl.2009.07.082

Targeting early AD oligomers with a simple dipeptide

A couple of days ago, an American man in Switzerland became the first human being to have his assisted suicide filmed and broadcasted on television. Craig Ewert was a 59 year old university professor who had been diagnosed with motor neuron disease in 2006. He had been given 3-5 years to live, but the disease took its terrible toll in just a few months. By the end of 2006 Ewert decided he had had enough. He contacted Dignitas, a Swiss organization started by a lawyer that has helped literally hundreds of people to peacefully end their lives. On a quiet, sunny morning, Ewert sipped a lethal cocktail of barbiturates completely of his own volition, and then bit on a switch that stopped his ventilator, again of his own volition. The film crew was allowed to film the entire operation except for after he passed away, when his wife wanted a few minutes of silence to herself.

Assisted suicide is an immensely controversial topic laden with moral issues. I have thought about it and discussed it with my friends many times, and I usually find myself ending up in favour of it at least in some cases where death is inevitable and the quality of life is going to assuredly become worse. Motor neuron disease is one such ailment. Alzheimer's disease is another. I personally don't think there's any other disease as cruel as AD, with loved ones trying to grasp at a person's identity as it slips away all too slowly and painfully. To me AD seems to be one of those unambiguous cases where a person who is still coherent should be able to exercise his right to die a dignified death. As horrible as it is to watch a loved one decide to willingly end his or her life, it is usually infinitely worse to watch them fade away into a wilderness of silence.

But the insidious pain of AD also makes the search for AD therapies a particularly compelling and desperate one. 15 million people are afflicted with AD, and in 20 years that burden is supposed to double. Part of the trouble for targeting AD is a still incomplete understanding of its molecular basis. While the initial amyloid hypothesis that posits the formation of insoluble amyloid protein fibrils is still very much relevant, much less is known about its exact relation to the disease, whether as a cause or symptom. The most important recent finding with respect to this hypothesis is that it's not insoluble aggregates but soluble oligomers that are the toxic species.

A recent overview in Nature (The Plaque Plan, Nature, November 13 2008) talked about the disappointments and ambiguities facing researchers in AD. Amyloid burden does not always correlate with cognitive impairment. In addition, two big clinical trials designed to target the formation of amyloid have both ended in failure and confusion. It's back to the drawing board for many embattled scientists. A major problem inherent in testing AD therapies is the late stage at which they inevitably have to be initiated in the absence of an early test for diagnosis. Thus disease progression is already advanced and perhaps that is the reason the therapies don't work as planned. The future should be as much focused on early brain and cerebrospinal fluid imaging as on new therapies. And of course, we are still in the rudimentary stages of achieving that supreme goal of understanding how memories are formed and stored.

At any rate, the midnight oil keeps burning and the search goes one, and Ehud Gazit from Tel Aviv University has published a paper in Angewandte Chemie that features a simple dipeptide for targeting and disassembling early soluble amyloid fibrils. The dipeptide is D-Trp-Aib, consisting of D-tryptophan and the unnatural amino acid amino isobutyric acid. This amino acid is rather unique and has been extensively studied because it seems to lend pronounced alpha helicity to peptides. The running hypothesis in its inclusion in the inhibitor is that it is a ß-breaker, an amino acid that helps to break the ß sheets that are a signature of amyloid. The work also focuses on the central region of the Aß (1-42) amyloid peptide that forms its aromatic core. The peptide sequence here is KLVFFAE, and the central Phe residues are thought to strongly influence the aggregation of the peptide. Thus the use of the D-Trp; apparently it would help to interfere in aromatic assembly. In addition it would also escape degradation by ubiquitous proteases.

Being a simple uncapped dipeptide, the blood-brain barrier permeability of this molecule is admittedly lousy (It has a depressing logP value of -0.8). Strategies could probably be worked up for enhancing delivery by chemical modification. However, other tests that the authors used attest to its efficacy as a amyloid-interfering agent. The compound inhibits the oligomerization of the amyloid oligomers as shown in a SDS-PAGE gel; it inhibited oligomers that can be stabilized using SDS. Interestingly its inhibition ability decreases at intermediate concentration and then goes up again. The compound also dissociated amyloid fibrils as shown in fluorescence assays with thioflavin-T. Lastly and more importantly, it seems to demonstrate long-term potentiation (LTP) in mice that may lead to an increase in cognitive retention. The molecule also showed good bioavailability and low toxicity.

In addition the researchers also did a NMR study on the interaction of D-Trp-Aib with the core of amyloid Aß (1-42). Using TOCSY and other spectra they detected differences in the chemical shifts of alpha protons of some key amino acids. Using NOE information they have also proposed a family of solution structures representing the bound conformation of the dipeptide with the sequence. I am more skeptical about this result. Small sequences of peptides rapidly interconvert in solution and give averaged NMR signals. The binding of the peptide to the sequence is probably an on-off event in which it preferentially stabilizes a particular conformation. A more detailed structural study combined with kinetics experiments would shed light on the key binding events.

How far such therapies would take us in tackling AD in human beings remains to be seen; in any case, maybe the trials should begin soon.

Other AD-related posts: water in amyloid, amyloid dimers as possible culprits, "seminal" truths about amyloid, and amyloid as a possible window into historical pathogen wars

References:
Anat Frydman-Marom, Meirav Rechter, Irit Shefler, Yaron Bram, Deborah E. Shalev, Ehud Gazit (2008). Cognitive-Performance Recovery of Alzheimer's Disease Model Mice by Modulation of Early Soluble Amyloidal Assemblies Angewandte Chemie International Edition DOI: 10.1002/anie.200802123

Thinking about Alzheimer's Disease as Historians

Head slumped forward, eyes closed, she could be dozing — or knocked out by the pharmacological cocktails that dull her physical and psychic pains.

I approach, singing “Let Me Call You Sweetheart,” off key. Not a move or a flutter. Up close, I caress one freckled cheek, plant a kiss on the other. Still flutterless.

More kisses. I press my forehead to hers. “Pretty nice, huh?” Eyelids do not flicker, no soft smile, nothing.

She inhales. Her lips part. Then one word: “Beautiful.”

My skin prickles, my breath catches.

It is a clear, finely formed “beautiful,” the “t” a taut “tuh,” the first multisyllable word in months, a word that falls perfectly on the moment.

Then it is gone. The flash of synaptic lightning passes. That night, awake, I wonder, Did Pat choose “beautiful?” Or did “beautiful” choose Pat? Does she know?

This heartbreaking and sad account by a husband of his wife's early slide into Alzheimer's Disease (AD) reminds us of how much we need to do to fight this. I personally think that of the myriad diseases afflicting humankind, AD is probably the cruelest of all. Pancreatic cancer might kill you in three months and cause a lot of pain but at least you are in touch with your loved ones till the end. But this is human suffering on a totally different level.

The search for the causes of Alzheimer's disease goes on, and I have recently been thinking in a wild and woolly way about it from an evolutionary standpoint. While my thoughts have not been well-formed, I want to present a cursory outline here.

The thinking was inspired by two books- one book which has been discounted by many, and another which has been praised by many. The lauded book is Paul Ewald's "Plague Time" which puts forth the revolutionary hypothesis that the cause of most chronic diseases is ultimately pathogenic. The other book "Survival of the Sickest" by Sharon Moalem puts forth the potentially equally revolutionary hypothesis that most diseases arose as favourable adaptations to pathogenic onslaughts. Unfortunately the author goes off on a tangent making too many speculative and unfounded suggestions, leading some to consider his writings rather unscientific. As far as I am concerned, the one thing that the book does offer is provocative questions.

On the face of it both these hypotheses make sense. The really interesting question about any chronic disease is; why have the genes responsible for that disease endured even after so many millennia if the disease kills you? Why hasn't evolution weeded out such a harmful genotype? There are two potential answers. One is that evolution simply has not had the time to do this. The other hypothesis, more provocative, is that these diseases have actually been beneficial adaptations against something in our history. That adaptation was so beneficial that its benefits outweighed the obvious harm that it caused. While that something probably does not presently prevail, it was significant in the past. What factor could possibly have existed that needed such a radical adaptation to fight it?

Well, if we think about what it has been that we humans have been fighting the most desperately and constantly ever since we first stepped foot on the planet, it's got to be a foe that was much older than us and more exquisitely adapted than we ever were- bacteria. The history of disease is largely a history of a fierce competition that humans and bacteria have engaged in. This competition plays by the rules of natural selection, and is relentless and ruthless. For most of our history we have been fighting all kinds of astonishingly adaptable bacteria and there have been millions of martyrs in this fight, both bacterial and human. Only recently have we somewhat eroded their malign influence with antibiotics, but hardly so. They still keep evolving and developing resistance (MRSA killed 18,000 in the US in 2005), and some think that it's only a matter of time before we enter a new and terrifying age of infectious diseases.

So from an evolutionary standpoint, it's not unreasonable to assume that at least a few genetic adaptations would have developed in us to fight bacteria, since that fight more than anything else has been keeping our immune system busy and our mortality high since the very beginning. But instead of thinking about genes, why don't we think about phenotypes? Hence arose the hypotheses that many of the age-old chronic diseases that are currently the scourge of humanity may sometime have been genetic adaptations against bacterial infection. While the harm that is done by these diseases is obvious, maybe their benefits outweighed that harm sometime in the past.

When we think of chronic diseases, a few immediately come to mind, most notably heart disease, diabetes, Alzheimer's and cancer. But one of the best cases in point that illustrates this adaptive tendency is hemochromatosis which is an excess of iron absorption and storage, and it was this disease that made me think about AD. A rather fascinating evolutionary explanation has been provided for hemochromatosis. Apparently when certain types of bacteria attack our system, one of the first nutrients they need for survival is iron. By locking down stores of iron the body can protect itself from these bacteria. It turns out that one of the species of bacteria that especially needs iron is Yersinia pestis, the causative agent of the black plague. Now when Yersinia attacks the human body, macrophages rally to the body's defense to swallow it. Yersinia exploits iron resources in macrophages. If the body keeps iron stores from macrophages, it will keep iron from Yersinia, which however will lead to a buildup of iron in the body; hence hemochromatosis. The evidence for this hypothesis is supposed to come from the Black Plague which swept Europe in the Middle Ages and killed almost half the population. Support for the idea comes from the fact that the gene for hemochromatosis has a surprisingly higher frequency among Europeans compared to others. Could it have been passed on because it protected the citizens of that continent from the plague epidemic? It's a tantalizing hypothesis and there is some good correlation. Whether it's true or not in this case, I believe the general hypothesis about looking for past pathogenic causes that may have triggered chronic disease symptoms as adaptations is basically a sound one, and in theory testable. Such hypothesis have been formed for other diseases and are documented in the books.

But I want to hazard such a guess for the causes of AD. I started thinking along the same lines as for hemochromatosis. Apart from the two books, my thinking was also inspired by recent research that suggests that amyloid peptide- a ubiquitous signature in AD- binds to copper, zinc, and possibly iron to generate free radicals that cause oxidative damage to neurons. Oxidative damage they may cause, but we have to note that oxidative damage is also extremely harmful to bacteria. Could amyloid have evolved to generate free radicals that would kill pathogens? Consider that in this case it's also serving a further valuable function akin to that in hemochromatosis- keeping essential metals from the bacteria by binding to them. This would serve a double whammy; denying bacteria their essential nutrients, and bombarding them with deadly free radicals. The damage that neurons suffer would possibly be a small price to pay if the benefit was the death of lethal microorganisms.

For testing this hypothesis, I need to know a couple of things:

1. Are there in fact bacteria which are extremely sensitive to copper or iron deficiency? Well, Yersinia is certainly one and in fact most bacteria are to varying extents. But since AD affects the brain, I am thinking about bacterial infections that affect the brain. How about meningitis caused by Neisseria, one of the deadliest bacterial diseases even now which is almost certainly a death sentence if not treated? Apart from this, many other diseases affect the brain if left untreated; the horrible dementia seen in the last stages of syphilis comes to mind. Potentially the brain would benefit against any of these deadly species by locking its stores of metal nutrients and generating free radicals to kill them, a dual function that amyloid could serve. I have not been able to say which one of these bacteria amyloid and AD might have evolved against. Maybe it could have been against a single species, maybe it could have been a general response to many. I am still exploring this aspect of the idea.

2. More importantly, I need epidemiology information about various epidemics that swept the world in the last thousand years or so. In the case of hemochromatosis, the causative genetic stimulus was pinned down to Yersinia because both the disease etiology and the pandemic are documented in detail. I cannot easily find such detailed information about meningitis or syphilis or other outbreaks.

3. In addition, while risk factors have been suggested for AD (for instance the ApoE epsilon4 gene allele), no specific genes have been suggested as causal factors for the disease. There is a clear problem with correlation and causation in this case. Also, the important role played by environmental factors such as stress and diet is becoming clear now; it's certainly not an exclusively genetic disease, and probably not even predominantly so.

4. Most importantly, I think it is impossible to find instances of AD clusters in history for a simple reason-the disease was simply unknown before 1906 when Alöis Alzheimer first described it. Even today it is not easy to make an assessment of it. All cases of Alzheimer's before a hundred years back would have been dismissed as cases of dementia causes by old age and senility. Thus, while the causative hypothesis is testable, the effects are hard to historically investigate.

The fact that AD is a disease of age might provide some credence to this hypothesis. Two things happen in old age. Firstly, the body's immune defenses start faltering, and this might need the body to marshall extra help to fight pathogens. Amyloid might do this. Secondly, as age progresses evolution is less worried about the tradeoff between beneficial and harmful effects because the reproductive age has already passed. So the devastating effect of AD would be less worrisome for evolution. Thus, the same AD that today is thought to reduce longevity would have ironically increased it in an age where infection would have reduced it even further.

However, if AD is an adaptation especially for old age, then it begs a crucial question; why would it exist in the first place? Evolution is geared toward increasing reproductive success, not toward increasing longevity. There is no use as such for a rather meticulously developed evolutionary adaptation that kicks in after reproductive age has passed. I think the answer may lie in the fact that while AD and amyloid do affect old people, they don't suddenly materialize in old age. What we do know now is that amyloid Aß is a natural component of our body's biochemistry and is regularly synthesized and cleared. Apparently in AD something goes wrong and it starts to suddenly agglomerate and cause harm. But if AD was truly an adaptation in the past, then it should have possibly manifested itself in younger age, perhaps not a much younger age but an age where reproduction was still possible. Consider that some dementia is much preferred to not being able to bear offspring, and so AD at a younger reproductive age would make evolutionary sense even with its vile symptoms. If this were true, then it means that the average age at which AD manifests itself has simply been increasing for the past thousand years. It would mean that AD is not per se a disease of the old; it's just become a disease of the old in recent times.

So after all the convoluted rambling and long-winded thought, here's the hypothesis:
Alzheimer's disease and especially Aß amyloid is an evolutionary adaptation that has evolved to kill pathogens by binding to key metals and generating free radicals

There are several details to unravel here. The precise relationship between metals, amyloid and oxidative damage is yet to be established although support is emerging. Which of the metals really matter? What do they exactly do? The exact role that amyloid plays in AD is of course under much scrutiny these days. And what, if anything, is the relationship between bacterial infection and amyloid Aß load and function in the body?

In the end, I suggest a simple test that could validate at least part of the hypothesis; take a test-tube filled with fresh amyloid Aß, throw in metal ions, and then throw in bacteria that were thought to be responsible for major epidemics throughout history. What do you see? It may not even work in vitro- I wonder if it could be tried in vivo- but it would be worth a shot.

Now I will wait for people to shoot this idea down because we all know that science progresses through mistakes. At least I do.

Aß Dimers- The Long-Sought Minimal Culprit in Alzheimer's Disease?

Following on the heels of the headline-making Nature publication that demonstrated that NSAIDs (Non-steroidal AntiInflammatory Drugs) uniquely targeted a substrate (APP) rather than an active site of the gamma-secretase complex involved in plague formation in Alzheimer's (see Discount Thoughts for a great summary) comes a paper that may turn out to be one of the important papers in the history of Alzheimer's disease (AD) research.

Since 1905 when Alois Alzheimer first detected the symptoms of what we today call AD and identified the characteristic plaques that form in the brains of AD patients, the "amyloid hypothesis" has become almost synonymous with AD. For decades now, insoluble amyloid plaques, later found to consist of 40 (Aß 1-40) and 42 (Aß 1-42) residue oligopeptides, have been thought to be the hallmark of AD. Indeed, amyloid has become the poster boy for diseases caused by protein misfolding. Say "protein misfolding", and college students will pipe up and say "Alzheimer's"

However, the truth as usual has been complicated. In the last few years, attention has been shifting from the insoluble Aß to soluble forms of the peptide that are apparently in equilibrium with the aggegated beasts. Many oligomers have been isolated through antibody labeling and their toxicity has been demonstrated to various extents under various conditions. The "amyloid hypothesis" has become much more complex than before, and one of the original questions- whether these insoluble plaques are really the cause or just a manifestation of AD- has raised its head even more.

Recently exciting progress has been made in the field, with everything from metals to free radicals being implicated in the dementia and neuronal death that AD causes. On a wall in my room I have a Sigma Aldrich poster displaying a huge schematic of the principal species and pathways involved in AD, and one look at the poster clearly indicates how convoluted the whole scenario is. One of the continuing main reasons for slow progress has been the lack of structural information, with amyloid itself not being crystallizable and soluble species by definition being hard to structurally nail down.

But in light of the connection to soluble oligomers unearthed for AD, one lingering question has been foremost on everyone's minds- What is the minimal soluble species responsible for the symptoms of AD? Now it seems that a paper might go a long way in answering this question.

The short answer is "dimers dimers dimers". For the long answer, read the Nature Medicine paper. Charles Selkoe, Ganesh Shankar and others at Harvard separated different Aß species, insoluble and soluble, from the brains of AD patients. They then performed detailed characterization through immunoprecipitation, Western blots and other techniques, and then injected these fractions into rats, documenting which species can be identified as being the minimal as well as dominant contributors to the pathophysiology of AD.

I am no neurologist (paging Retread) but the researchers seem to have focused on three indicators of "brain damage"- an adverse effect on LTP (long-term potentiation); Wikipedia defines this as "the persistent increase in synaptic strength following high-frequency stimulation of a chemical synapse" which seems to indicate the fidelity of synaptic communication and a contributor to memory, LTD (long-term depression) which is the weakening of a synapse, and a decrease in dendritic spine density.

The researchers clearly find that dimers displaying a mass band of 8kD (confirmed by mass spectrometry) provide the greatest effect on these three parameters. Monomers and other soluble oligomers were not just less toxic but inactive. They also performed the interesting experiment of treating insoluble Aß cores with formic acid, this causing some of it to dissociate into dimers. This concoction proved deadly for rat brains, while the original untreated assembly did not prove as toxic. To make sure that the dimers were pure, they also used synthetic Aß dimers and obtained the same results. These set of results are pretty conclusive in demonstrating the toxicity of dimers.

As an interesting sidepoint, the authors also demonstrate the role of the metabotropic glutamate and NMDA receptors in facilitating the symptoms.

The significance of these results are clear. The authors themselves say "Our findings fulfill an essential requirement for establishing disease causation in Alzheimer’s disease". Many questions still remain though. We still don't know the molecular mechanism through which these dimers finally lead to neuronal death. Do they exert their effects by binding to metals like copper or iron? Do they slide into neuronal membranes and cause them to disintegrate? What other species do they actually go through before they cause harm? All these effects have been suggested as part of the list of effects responsible for neuronal damage. Which effects do Aß dimers fit into?

But all this is later. For now it's a significant achievement that we seem to have a handle on the minimal species responsible for AD. It's a staggeringly simple (or not...) structure involved in the progression of a set of maddeningly complex events. This finding seems to open a whole new window of experiments, conjectures and principles related to Aß dimers and AD in general.

My compliments to the team.

1. Shankar, G.M., Li, S., Mehta, T.H., Garcia-Munoz, A., Shepardson, N.E., Smith, I., Brett, F.M., Farrell, M.A., Rowan, M.J., Lemere, C.A., Regan, C.M., Walsh, D.M., Sabatini, B.L., Selkoe, D.J. (2008). Amyloid-ß protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory. Nature Medicine DOI: 10.1038/nm1782

2. Halliwell, B. (2006). Oxidative stress and neurodegeneration: where are we now?. Journal of Neurochemistry, 97(6), 1634-1658. DOI: 10.1111/j.1471-4159.2006.03907.x

3. Bush, A.I. (2003). Copper, ß -amyloid, and Alzheimer's disease: Tapping a sensitive connection. Proceedings of the National Academy of Sciences, 100(20), 11193-11194. DOI: 10.1073/pnas.2135061100

4. Kukar, T.L., Ladd, T.B., Bann, M.A., Fraering, P.C., Narlawar, R., Maharvi, G.M., Healy, B., Chapman, R., Welzel, A.T., Price, R.W., Moore, B., Rangachari, V., Cusack, B., Eriksen, J., Jansen-West, K., Verbeeck, C., Yager, D., Eckman, C., Ye, W., Sagi, S., Cottrell, B.A., Torpey, J., Rosenberry, T.L., Fauq, A., Wolfe, M.S., Schmidt, B., Walsh, D.M., Koo, E.H., Golde, T.E. (2008). Substrate-targeting gamma-secretase modulators. Nature, 453(7197), 925-929. DOI: 10.1038/nature07055

AD, metals, and evolution

There always seems to be something new emerging in Alzheimer's disease research. In the last few years, at least one widely accepted hypothesis, the amyloid hypothesis, has come under scrutiny. In AD, it was thought that peptides aggregate into beta-amyloid fibrils (Aß) which causes toxicity to neurons.

However, many reports in the last two years are suggesting that it's not the aggregated beta-amyloid, but its soluble precursors that seem to be more toxic. There were some indications that this was true before too. For example, neuronal damage and memory impairment did not always correlate with Aß levels.

Another big hypothesis that has been widely prevelant in the AD community is that metals such as copper, aluminium, iron, and zinc, might correlate with Aß aggregation. New reports also suggest that these metals may in fact bind to the soluble and toxic forms of Aß, and help them to aggregate. One very interesting aspect of metabolism emerging from this research seems to be that it's not necessarily a larger amount of these metals from our environment or diet that's causing the problems, but an age-related breakdown in metal metabolism in the brain, which is an especially capacious reservoir of metals. Thus, perhaps we need not be especially worried about getting more copper or zinc from our diets.

One more thing that the metals might be doing is to help in free-radical generation, that might damage neurons. Again, there is still a correlation vs causation problem with this hypothesis, but free radicals are being increasingly found in vitro, when metals are added to Aß fibrils. While aluminium seems to have been largely a scare idea, zinc and copper are emerging as more plausible agents for free radical production and neuronal damage.

Based on evolutionary concepts, I cannot help but think that there may actually have been some protective function of amyloid in ancient human civilizations. Maybe it sequestered copper and provided a ready source when there was a lack of copper (or iron for that matter) in the diet. Or maybe it recruited copper to fight microorganisms and especially bacteria, at a time when there were no defenses against them except for natural ones. In this way, it could have dealt a double blow; denying essentials metals to the bacteria, and recruiting them to produce free radicals which would kill the little beasties.

In any case, AD research at the molecular level seems to be advancing rapidly, and we can expect some interesting discoveries in the years to follow.

References:
1. A nice review on AD and amyloid, free radicals, and oxidative damage
2. A recent Angewandte Chemie paper on the influence of copper concentrations on Aß geometry, morphology, and aggregation.