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Improving Our Basic Understanding of AD


From the beginning, studies at the cellular and molecular levels have focused on understanding the wide range of processes that interfere with, or enhance, the function and survival of neurons and their connections. The aim is to identify targets that can be translated and developed into AD therapies. Such therapies may avoid or reduce the cell dysfunction and cell death that occur as the disease progresses and also may keep memory intact.

Interest in mechanisms at the basic level is ongoing, and the potential roles of different forms of beta-amyloid and abnormal tau in neuronal toxicity continue to be a source of intense investigation.

Beta-amyloid
Scientists now know a fair amount about the metabolism of amyloid precursor protein (APP), a large protein associated with the cell membrane that is the starting point for the beta-amyloid that forms plaques. Scientists also know the basic steps of beta-amyloid and plaque formation. They know that three different enzymes—alpha-secretase, beta-secretase, and gamma-secretase—are involved in cleaving APP into discrete fragments, the functions of which are still not completely understood. Depending on which enzymes are involved and where the cleaving occurs, APP processing can follow one of two pathways—a pathway that is helpful to neurons or one that is harmful because it leads to the formation of beta-amyloid and plaques.

Studies in this area have evolved to the point that investigators have begun initial testing in humans of potential therapies aimed at halting the synthesis of beta-amyloid, reducing its levels, or degrading early aggregates before harmful complexes have formed. At the same time, basic science investigators are still probing the mysteries of plaque formation and seeking to understand the potentially toxic effects that beta-amyloid exerts on neurons.

  • Because gamma-secretase is involved in the production of harmful beta-amyloid, scientists have hypothesized that it could be a useful therapeutic target. However, gamma-secretase also is involved in the helpful APP processing pathway and in the cleavage of other developmentally important proteins, so actions to strongly inhibit its activity could have negative side effects. Johns Hopkins University School of Medicine researchers supported by the National Institute of Neurological Disorders and Stroke (NINDS) and NIA demonstrated that genetically reducing gamma-secretase activity by as little as about 30 percent in mice is enough to reduce beta-amyloid formation but leave sufficiently high gamma-secretase levels for the enzyme’s other essential reactions (Li et al., 2007). These findings may have identified a possible anti-amyloid therapeutic strategy—gamma-secretase inhibitors—as well as a way to preclude potentially harmful effects from the inhibitors.

A key focus of beta-amyloid research has always been to understand how this protein peptide actually damages neurons. Recent research suggests that early, small, and soluble aggregates of amyloid, called beta-amyloid-derived diffusible ligands (ADDLs), or oligomers, may be the main culprits in harming neurons. Much evidence also suggests that synapses, the tiny gaps between neurons that are essential for neuronal communication, are oligomers’ prime targets. Several recent studies have examined the pathways that lead from beta-amyloid to eventual synaptic dysfunction or neuron death and studied how beta-amyloid oligomers target specific synaptic connections between neurons, causing them to deteriorate.

  • A research group at Northwestern University examined the ability of ADDLs to affect the composition, structure, and abundance of synapses (Lacor et al., 2007). In this test tube study of neurons situated in the hippocampus, the researchers found that ADDLs bound to synapses of a specific subset of hippocampal neurons, promoting a detrimental change in the composition, structure, and abundance of those synapses. Continued exposure to ADDLs also damaged the neurons’ dendritic spines (the structures that receive messages from other neurons), affecting their ability to function properly.
  • A study in mice by scientists at the Salk Institute for Biological Studies in La Jolla, California, provides evidence that the APP cleavage at a site known as D664 may be necessary for the synaptic dysfunction characteristic of AD to occur (Saganich et al., 2006). The research team, supported by NINDS and NIA, found that synaptic loss and behavioral abnormalities were completely prevented by a mutation at D664, even in mice that had high levels of beta-amyloid and many plaques. Uncovering the mechanism by which the D664 cleavage contributes to dysfunction may ultimately help researchers understand synaptic loss in AD and develop treatment strategies.

Another continuing line of research focuses on the possibility of harnessing an immunization response in people with AD that involves antibodies to beta-amyloid. Immunizing people against disease has been a cornerstone of medical practice for decades, and investigators have pursued the idea that it might be possible to immunize people against AD by injecting them with a beta-amyloid-related immunogen (a substance designed to elicit an immune response). This kind of injection would cause a person’s immune system to make antibodies that, in turn, would lower the levels of brain amyloid.

This technique, called active immunization, has been tested in AD transgenic mice that were actively immunized with a beta-amyloid immunogen. (Transgenic animals are those that have been specially bred to develop AD-like features, such as beta-amyloid plaques.) The mice had fewer plaques and improved performance on memory tests. This finding led to a clinical trial in humans to test the safety and effectiveness of active immunization with the beta-amyloid immunogen. However, about 6 percent of participants in the trial developed brain inflammation in response to the treatment, so the trial was stopped. Despite this setback, interest in developing an AD vaccine remains high.

Research into new ways of shaping the antibody response continues in the laboratory, and more refined antibody approaches are being tested in clinical trials.

  • Using several strains of mice, including transgenic and normal mice, Harvard Medical School investigators tested four different partial fragments of beta-amyloid as potential immunogens (Maier et al., 2006). The researchers found that the immunogens evoked the desired immune response in both sets of mice, reducing plaque levels in their brains without an accompanying inflammatory reaction. The transgenic mice also showed slight improvements in memory tests.

In a second approach to protecting against AD, called passive immunization, antibodies are produced or manufactured outside the body. For example, humanized antibodies to beta-amyloid have been made in cell cultures using recombinant DNA techniques. The antibodies can then be isolated and administered to subjects. Scientists presume that passive immunotherapy produces less of an inflammatory response than active immunotherapy, and a number of investigators are pursuing this approach.

  • Cerebral amyloid angiopathy (CAA) is the accumulation of beta-amyloid in the walls of arteries in the brain. Because CAA is commonly found in AD, many scientists are interested in how beta-amyloid deposits in blood vessels and neurons may generate human disease and whether they can be treated by immunotherapy. Researchers at Massachusetts General Hospital, supported by NIA and the National Institute of Biomedical Imaging and Bioengineering, used microscopy at different time intervals to monitor CAA in a mouse model of AD to evaluate the effects of anti-beta-amyloid passive immunotherapy (Prada et al., 2007). The investigators saw clearance of CAA deposits within 1 week after a single administration of antibody directly to the brain, but the effect was short-lived. Chronic administration of antibody over 2 weeks led to better clearance without evidence of hemorrhage or other destructive changes. This imaging study directly demonstrated that CAA in a transgenic mouse model can be cleared with an enhanced immunotherapy regimen.

Additional insights about beta-amyloid have come from studies of neuronal networks. These studies show how beta-amyloid may damage normal electrical activity of hippocampal neurons, thereby diminishing the cells’ ability to communicate with each other.

  • A research group at the Gladstone Institute of Neurological Research in San Francisco discovered that, compared with normal mice, transgenic mice show anatomical and biochemical alterations in certain brain regions as well as abnormal excitatory electrical activity (Palop et al., 2007). The study focused on the hippocampus (a region of the brain that is key to learning and memory). These findings are important because they suggest another damaging effect of beta-amyloid—namely, that beta-amyloid presumably triggers abnormal electrical activity throughout the brain. This abnormal activity in turn triggers compensatory inhibitory activity, perhaps contributing to AD-related network dysfunction.

Other aspects of beta-amyloid also are yielding their secrets to AD researchers.

  • Scientists at the Buck Institute for Age Research in Novato, California, bred transgenic mice to develop features of AD and to overproduce neuroglobin, a protein expressed predominantly in neurons that is closely related to hemoglobin, the oxygen-carrying protein in the blood. Neuroglobin is abundant in the brains of vertebrates. The function of this globin family protein is largely unknown, but the expression of neuroglobin can be induced when oxygen levels in the brain are lowered, as in a stroke. The transgenic mice performed significantly better on memory tasks and had fewer beta-amyloid plaques than did transgenic mice that only developed AD features (Khan et al., 2007). The researchers, supported by NINDS, speculate that increasing neuroglobin levels may merit additional research as a therapeutic target, not only for cerebrovascular disease but also for beta-amyloid toxicity.

Tau
Tau is a leading player in AD pathology and is generating new excitement as an area of study. The focus on tau was spurred by the finding that a mutant form of this protein is responsible for frontotemporal dementia and parkinsonism linked to chromosome 17, another neurodegenerative disorder that shares some features with AD. That finding indicated that abnormalities in tau can cause dementia.

Recent research has provided other new insights. For example, some studies have suggested that tau’s influence on cell death may have more to do with its interference in the normal cell cycle process than with its involvement in the formation of neurofibrillary tangles. Other studies suggest that, like beta-amyloid, early soluble forms of abnormal tau (not the final neurofibrillary tangle) may be the trigger for cell death.

Transgenic mouse models have played a big role in pushing forward tau research because they can be studied methodically for clues to human diseases. For example, the “triple transgenic” mouse forms plaques and tangles similar to those in human AD over time in brain regions. Another new transgenic mouse model, which contains only human tau, forms clumps of damaging tau filaments in a region-specific fashion similar to that seen in humans with AD.

  • A research group from the Gladstone Institute of Neurological Research supported by NIA and NINDS explored the possibility that a treatment aimed at tau could block the cognitive impairments that result from beta-amyloid accumulation (Roberson et al., 2007). In this study with AD transgenic mice that normally are cognitively impaired, the scientists eliminated the animals’ tau gene. The resulting lower levels of tau produced by the mice prevented behavioral problems that usually occur when too much beta-amyloid is produced, even though beta-amyloid levels remained high. This surprising result suggested that reducing tau levels may present another target for future AD treatments.

AD and Other Neurodegenerative Diseases
Studies of brain abnormalities resulting from common mechanisms in a number of neurodegenerative diseases are providing important insights into AD. Diseases such as AD, Lewy body disease, Huntington’s disease, frontotemporal dementia, amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig’s disease), Parkinson’s disease, and Creutzfeldt-Jacob disease have similar clinical symptoms, including memory loss, movement problems, and sleep-wake disorders.

People with any of these disorders also exhibit the same pathological hallmark—misfolded and mutant proteins in the brain. Under normal conditions, most misfolded proteins are either repaired or degraded, but when too many of these proteins are produced over a long period of time, the body’s repair and clearance process may be overwhelmed. The toxic misfolded proteins accumulate and lead to the age-related neurodegenerative disorder.

In 2007, several studies provided valuable insights into how this process may occur and why some neurodegenerative diseases have overlapping features.

  • Scientists at the University of Pennsylvania School of Medicine identified, for the first time, a protein called TDP-43 as a component of the protein aggregates that form in ALS and in some forms of frontotemporal dementia (Neumann et al., 2006). Finding the same molecular signature in the two diseases suggests that they may represent different facets of the same neurodegenerative disorder. It may be that a number of neurodegenerative diseases that affect different groups of neurons have similar disease processes. If that is true, then developing therapies for these disorders and other similar diseases may be simplified.

  • Another research team, working at Northwestern University, explored how the expression of a single protein that is prone to abnormal aggregation can lead to the disruption of many cellular pathways. These researchers also examined whether one general mechanism might explain the many common features of protein misfolding diseases (Gidalevitz et al., 2006). The investigators used the worm C. elegans as a model to test whether expression of a pathogenic protein known as a “polyQ protein” (similar to the abnormal huntingtin protein that causes Huntington’s disease) could affect the folding or degradation of other proteins.

    Worms with the polyQ protein were crossed with worms that expressed other mutant proteins in either muscle or brain cells. The researchers found that the offspring of those worms showed chronic expression of the aggregation-prone polyQ protein, which caused the other proteins to become toxic under conditions where they were normally innocuous. Moreover, the toxic action was reciprocal in that the other mutant proteins, which had no adverse effect under normal physiological conditions, could enhance the aggregation of polyQ proteins. These clues about the interactions between abnormal proteins and their effects on neurons may provide a valuable boost to efforts to target potential treatments for various age-related neurodegenerative diseases.

  • Researchers at the University of Texas Southwestern Medical Center used transgenic mice to explore relationships between AD and Parkinson’s disease. This study, supported by the National Institute of Mental Health (NIMH), showed that in the spinal cords of mice made to overexpress human normal and mutant alpha-synuclein (a protein implicated in Parkinson’s disease) a change occurred in the ubiquitin/proteasome system. This is a cellular system that degrades misfolded proteins (Gallardo et al., 2008). Curiously, the mice also exhibited a fourfold increase in levels of the ApoE protein (ApoE is a genetic risk factor implicated in AD). This overexpression produced marked increases in aggregates of alpha-synuclein and insoluble beta-amyloid. Deleting the APOE gene, which makes ApoE, had a number of positive effects in the transgenic mice—alpha-synuclein-induced neurodegeneration was delayed, survival increased, accumulation of alpha-synuclein aggregates decreased, and accumulation of beta-amyloid was suppressed. These findings suggest that ApoE is involved in the response to alpha-synuclein toxicity, and that AD and Parkinson’s disease may share a molecular link through the ubiquitin/proteasome system. This insight may have important implications for preventing and treating these devastating diseases.

AD and Aging
Another set of insights about AD derives from an apparent risk factor common to a number of neurodegenerative diseases: aging itself. Age-related changes, such as inflammation, changes in expression of certain proteins, and the generation of free radicals, may precede, follow, or exacerbate the neuronal damage that occurs in AD. In addition, age-related changes in one or more of the hundreds of varieties of proteins could result in inefficient functioning of certain synapses, predisposing neurons to failed communication and death. Scientists are investigating all of these possibilities.

  • In a study using the C. elegans worm, scientists at the Salk Institute for Biological Studies introduced a gene that increased the lifespan of the worms and a gene that made beta-amyloid (Cohen et al., 2006). The investigators found that the worms containing the increased-lifespan gene also suppressed the aggregation-related toxicity of beta-amyloid. These findings suggest that aging itself plays a role in the rate at which beta-amyloid aggregates, and investigators identified two genes essential for this process.
  • An intramural research group at NIA identified disease-specific changes in gene expression in different regions of brain tissue from people with AD, people with other types of dementia, and cognitively healthy people (Weeraratna et al., 2007). The analysis revealed that genes that differed in expression the most between the groups were related to nervous system development and function and neurological disease, followed by genes involved in inflammation and immunological signaling. A specific group of genes associated with beta-amyloid accumulation and clearance was found to be significantly altered in the AD group. The most significantly down-regulated gene in this dataset was one containing the genetic information necessary to make an enzyme implicated in beta-amyloid clearance. Together, these findings open up new avenues of investigation and possible therapeutic strategies targeting inflammation and enzymes associated with amyloid clearance in AD patients.
  • Scientists at the University of North Dakota School of Medicine and Health Sciences supported by the National Center for Research Resources (NCRR) have been investigating cytokines, substances produced by immune system cells. These substances are secreted by cells during the body’s response to inflammation. The investigators found that a cytokine called tumor necrosis factor alpha, which is present in the brain during an inflammatory response, can begin a process in nerve cells that ultimately leads to cell death (Jara et al., 2007). This finding may help explain one mechanism leading to cell death in AD and related diseases.
  • Free radicals—oxygen or nitrogen molecules that combine easily with other molecules—are important in the aging process and may be important in AD as well. Free radicals can help cells in some ways, but overproduction of these highly reactive molecules can damage neurons in a process called oxidative stress. A substance called 4-hydroxy nonenal (4-HNE), formed as a result of oxidative stress, is increased in AD. 4-HNE also is found in AD plaques. Scripps Research Institute scientists, supported by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), found that 4-HNE modifies beta-amyloid fragments, triggering the formation of toxic beta-amyloid oligomers (Siegel et al., 2007). These findings provide impetus for additional research on whether, or to what extent, oxidative stress is a risk factor or a consequence of AD.
New Insights into New Neurons

Graphic of neuronsUntil recently, scientists believed that neurons in mammals were formed only during the fetal period and for a short time after birth. The belief was that once a mammal had reached a certain level of maturity early in life, neurons could only be lost. The notion that new neurons could develop later in life was radical. Finding that this was true, in fact, the case was revolutionary.

This shift in thinking over the past few years was based on studies showing that neurogenesis (the formation of new neurons) takes place in the adult brain, at least in a limited number of brain regions, such as the hippocampus. Neurogenesis declines during aging but can be stimulated by environmental influences, including physical activity and learning tasks.

This evidence raises a big question: Do these new neurons actually help brain regions function or are they merely a reservoir to replace dying neurons? Several recent studies, conducted by scientists from Johns Hopkins University, the Chicago Medical School of the Rosalind Franklin University of Medicine and Science, and the University of Arizona, have helped answer this question.

In studies with mice and rats, investigators showed that new neurons produced in the adult hippocampus are better able to adapt (an attribute called “plasticity”) and to mature (Ge et al., 2007). In fact, they found that because of their enhanced “excitability,” new neurons may help process information and form memories in the hippocampus (Ramirez-Amaya et al., 2006). The researchers also found that certain types of stress disrupted neurogenesis by decreasing the survival of the new neurons (Thomas et al., 2007).

These and other similar findings may help researchers develop future interventions that maintain or enhance the formation of new neurons, thereby helping to slow age-related cognitive decline.

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Page last updated Jan 06, 2009

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