Clinical studies completed in 2008 caused scientists to raise provocative questions about the “amyloid cascade hypothesis,” which has guided a generation of researchers in their quest to cure Alzheimer’s. Though most current research still follows the path charted by this theory, efforts toward an effective treatment will require new navigation.
The Early Days of a Hypothesis
The amyloid hypothesis began to take shape in 1986 when scientists discovered a gene on chromosome 21 that produces amyloid precursor protein (APP), a substance of uncertain function found mostly in the space around neurons and produced abundantly in the healthy brain.1 The APP gene contains the sequence for the peptide amyloid, which is concentrated in the plaques used to diagnose Alzheimer’s brain pathology. People who inherit a form of early-onset Alzheimer’s have a mutation on chromosome 21 in the APP gene that results in overproduction of the amyloid peptide. People with Down syndrome, who invariably develop Alzheimer’s in middle age, have an extra copy of chromosome 21 containing the gene for APP, causing them to produce excess amounts of the protein as well.2
A variety of enzymes in the brain normally clip APP into harmless fragments that float freely between neurons, possibly contributing to the ability of neurons to form new connections with each other—a brain function that is vital to memory. However, specific “beta” and “gamma” enzymes—the presence of which is predicted by the mutations in the APP gene—clip APP so as to yield amyloid peptide. For reasons unknown, amyloid aggregates to form toxic strings known as oligomers, and it is hypothesized that these disrupt the transmission of signals at the synapse—the gap where signals jump from one neuron to another with the help of chemical neurotransmitters.3
According to the amyloid hypothesis, the toxic oligomers eventually accumulate into immobile clumps of protein known as betaamyloid, or “senile plaques.” Alois Alzheimer found these in the brain of a profoundly demented woman who died in 1906.
One hypothesis suggests that plaques act like magnets that attract and immobilize toxic oligomers, preventing them at least temporarily from committing mischief. However, the plaques themselves trigger damaging inflammation that contributes to the dysfunction and death of nearby neurons.
A genetic mutation predicts the presence of certain enzymes—shown here as scissors—that clip strands of APP into fragments that clump together in betaamyloid plaques. (NIH National Institute on Aging)
Researchers have devised a variety of ways to clear toxic oligomers from mice, but in human tests such treatments have failed to slow the memory loss, confusion, and other cognitive problems that afflict people with the disease. Though researchers and patients hope for new treatments from this line of study, they may be wrong to assume that significantly slowing down Alzheimer’s progression is even possible in patients who already display the symptoms necessary to qualify as a trial patient.
“FDA guidelines generally suggest that one conduct trials first in mild-to-moderate Alzheimer’s patients,” said Dennis Selkoe of Harvard University, a pioneer in the development of antibodies against toxic oligomers. “But by that time plaques, tangles, gliosis, and neuritic dystrophy are relatively advanced.” (Gliosis is the accumulation of glial cells, which clear debris left by neurons when they die. Neuritic dystrophy refers to deformed neurons.)
The free-floating toxic oligomers appear to disrupt synaptic function years if not decades before symptoms of Alzheimer’s appear. Selkoe reported in a 2008 paper that toxic oligomers taken from the brains of Alzheimer’s patients and injected into rodents profoundly disrupted synapses and impaired memory.4 More distressing, toxic oligomers, though they are certainly part of the problem, may not be the right target for treatment at all—plaques may be forming in response to something else entirely.
Conflicting theories and disappointing results prompted two veteran Alzheimer’s researchers, Peter H. St. George-Hyslop of the University of Toronto and John C. Morris of the Washington University School of Medicine in St. Louis, to ask recently in the journal Lancet if the past two decades of anti-amyloid research “were spent barking up the wrong tree.”5
Such pessimism is premature, other scientists argue. Some, such as Selkoe, believe that anti-amyloid therapies would be far more effective if started earlier, before the toxic oligomers have had time to damage synapses and kill neurons. Others have been intensifying the search for subtle indicators in blood or cerebrospinal fluid, or perhaps on magnetic resonance imaging scans, that would indicate the earliest signs of pathology and perhaps allow for the prevention of the accumulation of toxic oligomers.
Anti-amyloid Drug Disappoints
Without such biomarkers, treatment will be limited to people with overt symptoms of Alzheimer’s, and judging from clinical trial results reported in 2008, such people do not respond very well to efforts to remove toxic beta-amyloid from their brain. Myriad Genetics, for example, tested an anti-amyloid drug called tarenflurbil (Flurizan) in patients with Alzheimer’s. After investing $200 million to develop the drug, the company announced in 2008 that it was suspending all further research because an eighteen-month study involving 1,684 patients—the largest Alzheimer's treatment trial ever—showed that it did not produce significant improvement in memory, cognitive functioning, or the ability to perform activities of daily living such as dressing and bathing.6 The trial did not include tests to detect how much beta-amyloid, if any, was removed from participants.
Elsewhere, follow-up studies have been conducted in patients who participated in clinical trials to test the effectiveness of AN-1792, a vaccine against Alzheimer’s developed nearly a decade ago by Elan Pharmaceuticals in cooperation with Wyeth Pharmaceuticals. After conducting one follow-up, Clive Holmes, of the Memory Assessment and Research Centre in England, concluded that “progressive neurodegeneration can occur in Alzheimer’s disease despite removal of plaque.”7 But the data from a larger follow-up study included tantalizing hints of benefit in those patients who responded to the vaccine by producing antibodies against beta-amyloid. “Patients who still had antibodies in their system at the time of the follow-up did significantly better on activities of daily living, and on a measure of dependency,” said Dale Schenk, executive vice president and chief scientific officer for Elan.
Work on AN-1792 was abandoned because it produced serious cerebral inflammation, but Elan and Wyeth have since developed an antibody that attacks beta-amyloid. The eagerly awaited results of a clinical trial of the drug, which bears the unwieldy name of bapineuzumab, were announced at the International Conference on Alzheimer’s Disease meeting in July. The antibody reduced brain atrophy and produced some improvement in mental functioning, primarily among patients who did not possess the gene for ApoE4, the strongest genetic risk factor for Alzheimer’s disease. (About 25 percent of humans carry one or two copies of the gene for ApoE4, but more than half of those with Alzheimer’s carry the gene.) Researchers did not expect the Phase II trial, designed to test for and define safe dose ranges for the antibody, to reveal efficacy in any subgroups. In this respect the study was a success.
However, those in the study who possessed the gene for ApoE4 were barely helped at all. “Perhaps there’s a biological difference between carriers and noncarriers (of the gene),” said Sid Gilman, the neurologist who served as the chair of the independent safety monitoring committee for bapineuzumab, as he announced the findings. “Or perhaps carriers have a greater density of beta-amyloid.”
Or maybe, according to a growing number of researchers, the amyloid hypothesis needs further refinement. “There are several chinks in the armor of the amyloid hypothesis,” said David Morgan, director of the Alzheimer’s Research Laboratory at the University of South Florida. “But the question isn’t whether the amyloid hypothesis is correct. Every gene mutation we know of that causes Alzheimer’s in a dominant fashion modifies the production of amyloid, and there are 100 genes that do this. That seems to provide pretty compelling evidence that amyloid plays a role. The question is whether targeting amyloid will be efficacious.”
What Is ‘Normal Aging’?
The original amyloid hypothesis, which emphasized the presence of amyloid plaques, contained one glaring inconsistency: the number of plaques found in elderly brains does not correlate very well with cognitive difficulties. A much stronger indicator is the other hallmark of Alzheimer’s, the tangles of tau protein found within neurons.
This has led to debate between “tauists,” who believe that tau protein causes Alzheimer’s, and “BAPtists,” who blame the betaamyloid protein found in brain plaques. The tauists have always had a compelling case. Tau is a crucial brain protein found in the microtubules that act like railroad tracks for transporting neurotransmitters from the cell body to the synapse, where the neurotransmitters are released. Any dysfunction involving tau is catastrophic for the brain, as several “tauopathies,” including Alzheimer’s, vividly demonstrate.
Today, researchers generally agree that toxic beta-amyloid initiates the dysfunction that leads to tau degeneration, but the most potent trigger by far is age. The aging process plays such a large role in Alzheimer’s that Peter Whitehouse, the geriatric neurologist who founded Case Western Reserve University’s Memory and Aging Center, published a book in 2008 titled The Myth of Alzheimer’s, in which he argues that the disease is nothing but normal brain aging that takes place faster in some people than in others.8 Everyone, he believes, would get Alzheimer’s if they lived long enough.
But strong evidence contradicts this assertion. For example, Juan Troncoso, codirector of the Alzheimer’s Disease Research Center at Johns Hopkins University, has demonstrated that one region of the hippocampus known as CA1, which is crucial for the formation of short-term memories, remains stable in old age among those who do not have Alzheimer's, but degenerates drastically in people with Alzheimer’s.9
In addition, some people live to extreme old age with only minor loss of mental acuity. An article published in 2008 about the autopsy performed on the world’s oldest woman, who died at the age of 115, reported that her brain was virtually free of signs of Alzheimer’s.10 Clearly Alzheimer’s is a disease, not normal aging. Yet the incidence of Alzheimer’s disease increases in lockstep with aging, producing at least subtle symptoms in about half of all people by the age of 85. Somehow, the aging process must contribute to Alzheimer’s.
Combining New and Old Theories
One idea gaining ground involves synaptic exhaustion. Neurons communicate with each other by releasing neurotransmitters from the synapse. Receiving fibers known as dendrites, which branch like tree boughs from nearby cells, are stimulated by neurotransmitters and propagate the impulse, which travels to the cell body and then down that cell’s axon. Neurons create new synaptic connections among themselves constantly, and this process is energy-intensive.
Few regions of the brain work harder at this than the hippocampus, where short-term memories form—and where Alzheimer’s begins. In 2008, Randy Buckner of Harvard and two colleagues published a paper in which they observed an uncanny similarity between hippocampal changes and those in another area affected by early Alzheimer’s: the brain’s “default network”—regions at the front and rear of the brain connected by long fibers.11 The default network becomes active when the mind wanders and slips into what William James, the founder of modern psychology, dubbed the “stream of consciousness.” Since the mind wanders whenever it is not busy, the default network is one of the busiest areas of the brain. In people with Alzheimer’s, glucose metabolism in the default network drops significantly, suggesting that synaptic transmissions are becoming sluggish. This drop in the brain’s use of glucose continues as the disease progresses, and it correlates with the severity of dementia. In addition, people who possess the gene for ApoE4 show lower glucose metabolism in these areas much earlier in life, suggesting that dysfunction begins years or perhaps decades before the first symptoms appear.
This “metabolism hypothesis,” as Buckner calls it, corresponds with a conception of Alzheimer’s disease long promoted by Marcel Mesulam, director of the Cognitive Neurology and Alzheimer’s Disease Center at Northwestern University Medical School in Chicago. Mesulam, who presented a spirited explanation of his idea at the International Conference on Alzheimer’s Disease meeting in July, believes that Alzheimer’s evolves from the breakdown of neuroplasticity, the process by which synapses form new connections with other neurons. The rapid breakdown and buildup of connections demands a vigorous repair process, and that, Mesulam believes, is what slows down and eventually produces the “cascade” of degeneration that leads to Alzheimer’s disease. Every cause of Alzheimer’s disease ever proposed—including head trauma, the ApoE4 gene, cardiovascular disease, inflammation, stroke, and aging itself—interferes in some way with neuroplasticity, he said. Mesulam first proposed his hypothesis nearly a decade ago in an effort to solve what he calls the “central puzzle” of Alzheimer’s—the genetics of the disease point to betaamyloid as the cause, but the symptoms coincide more closely with the number of tau protein tangles found within neurons.
Neurons communicate by transmitting chemicals at the synapse, a process that becomes sluggish in brains with Alzheimer’s because of a drop in glucose metabolism. (NIH National Institute on Aging)
A revised version of the amyloid cascade hypothesis links these two phenomena by accusing toxic beta-amyloid oligomers of disrupting activity at the synapse, creating stress that leads to the breakdown of the tau protein “tracks” that guide neurotransmitters. In 2006, two researchers at the University of Virginia, Michelle E. King and George S. Bloom, found that beta-amyloid triggers the disassembly of tau microtubules.12 They are preparing to publish more-detailed research on the biochemistry behind this synaptic dysfunction. “We think the breakdown of microtubules in axons caused by the interaction between amyloid and tau simply slows down or halts the replenishment of the proteins involved in making neurotransmitters,” Bloom said. “If these proteins are not replaced, the synapse can’t function properly. Mesulam’s paper was very prophetic. Nobody was thinking at the time he wrote it that amyloid and tau might be conspiring in a way that leads to microtubule disassembly.”
Clutter in the Brain
Another approach to preserving the vigor of synapses focuses on the cell’s ability to break down and dispose of protein debris, a process known as autophagy. Neurons, with their high metabolism, produce a lot of waste and must rely on autophagy to get rid of it. The failure of autophagy results in the accumulation of those toxic protein fragments found in Alzheimer’s disease and other neurodegenerative disorders such as Parkinson’s and amyotrophic lateral sclerosis (ALS), according to Ralph Nixon of the New York University School of Medicine. “We know that this type of dysfunction develops as part of the normal aging process,” he said. “We also have found that genes that promote Alzheimer’s disease add another layer of impairment to this age-related impairment.”
The accumulation of protein debris within the cell body plays a key role in Alzheimer’s, as Nixon and colleagues outlined in a 2008 paper in the journal Autophagy.13 Stimulating autophagy in the elderly presumably would slow or halt the degeneration farther upstream before amyloid plaques, tau protein tangles, and other downstream debris appear, according to Fen Jin-A Lee and Fen-Biao Gao, two researchers at the Gladstone Institute of Neurological Disease in San Francisco.14
Further evidence that autophagy is involved in Alzheimer’s comes from Tony Wyss-Coray of Stanford University, who believes that beclin 1, a key regulator of the autophagy pathway, is reduced in certain brain areas of Alzheimer’s patients.15 When beclin 1 is reduced, neurons produce more APP, setting the stage for Alzheimer’s pathology. “And beclin 1 is reduced by 60–70 percent in Alzheimer’s disease,” said Wyss-Coray. “Autophagy is involved in neurodegeneration in general.”
Latest Avenues of Treatment
Despite these tantalizing hints that the most effective treatment for Alzheimer’s would involve prevention, most treatments in the pipeline in 2008 involved removing the toxic oligomers. The antihistamine Dimebon, for example, was sold for two decades in Russia before neurologists noticed that it seemed to help people with Alzheimer’s. In 2008 the conclusion of an eighteen-month study of Dimebon (dimebolin hydrochloride) showed that the drug improved memory and cognition in Alzheimer’s patients somewhat, possibly by stimulating the function of mitochondria, the power source of cells.16
Another drug known as methylene blue, or methylthioninium chloride (MTC), has been found to inhibit the production in the brain of tau protein tangles, according to Claude M. Wischik, chairman of TauRx Therapeutics, which is marketing the compound for Alzheimer’s disease under the name rember.17 Before World War II and the widespread availability of antibiotics, MTC was sold as Urolene Blue, a treatment for urinary tract infections. A clinical trial completed in 2008 by TauRx found that the compound slowed the decline of Alzheimer’s patients by 81 percent compared with patients taking a placebo.
Elsewhere, Prana Biotechnology is developing a compound known as PBT2, which interrupts the aggregation of beta-amyloid in the brain of Alzheimer’s patients by inhibiting the action of zinc and copper. “The drug will keep metals away from beta-amyloid but make them bioavailable to enzymes that need them,” said Rudolph Tanzi, the Harvard researcher who founded Prana in his laboratory in 1997. A clinical trial completed in 2008 showed that PBT2 reduced levels of A-beta 42—one fragment of beta-amyloid believed to be toxic to the brain—and produced some improvement in cognition.18
Improved function of mitochondria (the small, pill-shaped structures shown with a healthy neuron, above, and with a neuron affected by Alzheimer’s, below), as stimulated by experimental drug dimebolin hydrochloride, may inhibit cell death in brains with Alzheimer’s disease. (Rachelle S. Doody, M.D., Ph.D. / Baylor College of Medicine)
Norman R. Relkin of Weill Medical College of Cornell University is leading an effort to develop a new form of immunotherapy known as intravenous immunoglobulin, or IVIg. IVIg contains antibodies from human blood that attack beta-amyloid, but instead of recognizing the protein’s chemical makeup, the antibodies recognize its misfolded, aggregated shape, and they leave healthy molecules alone. A clinical trial completed in 2008 showed IVIg capable of reducing beta-amyloid and improving cognition, opening the way for a Phase III trial.19
Such varied approaches underscore the complexity of Alzheimer’s—and of the aging process itself. “A greater understanding of the normal aging brain may be necessary before we can fully understand the causes of pathological aging and cognitive decline,” said Harvard’s Bruce Yankner, who has been studying brain aging for many years.
The current amyloid cascade hypothesis leaves room for hope that the disease might be held at bay, perhaps indefinitely, by preventing the aggregation of beta-amyloid fragments cleaved from APP throughout life. But scientists do not yet understand why the appearance of plaques does not correlate with symptoms of the disease.
Researchers will not find a “magic bullet” that cures Alzheimer’s anytime soon. Toxic oligomer research may synthesize the various leading theories. Amyloid toxicity is particularly consequential when it triggers tau formation, a fact that links the amyloid cascade hypothesis to the possible pathogenic role of tau. However, scientists will need to know much more before they can conclude that such a synthesis is possible. Until then, the various hypotheses of Alzheimer’s disease are still just that: hypotheses.
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19. Relkin NR. Natural human antibodies targeting amyloid aggregates in intravenous immunoglobulin. Presentation at ICAD, July 27, 2008. Also, interview with Relkin.