Although Alzheimer's disease is a distinct disease defined by its characteristic clinical course and pathology, it is a heterogeneous condition with varied manifestations. The rate of cognitive impairment, for example, differs markedly among individuals. The characteristic features of Alzheimer's disease brain pathology also differ sharply among people. Though the onset, course, and sequence of events may vary widely, it seems likely nonetheless that the destructive forces involved ultimately converge to cause nerve cell (neuron) dysfunction, loss of connections between nerve cells, and death of some nerve cells. The quest for mechanisms by which neurons lose their ability to communicate with each other and the reasons for selective neuronal death are at the heart of the worldwide scientific effort to discover the cause — or causes — of Alzheimer's disease.

It has been known for some time that the survival of nerve cells in the brain depends on the proper functioning of many interrelated systems. These systems can be characterized by the three aspects of neuronal activity they modulate: communication, metabolism, and repair. The communication system used by most neurons relies on a vast array of chemicals to carry information between and within nerve cells as well as to cells outside the nervous system. Through this complex chemical signal transduction system, the brain functions as a master control center for the whole body. Depletion or absence of any of these chemicals disrupts cell-to-cell communication and interferes with normal brain function.

The metabolic activity of neurons depends on the blood circulation provided by a complex system of both large and extremely small blood vessels in the brain. The supply of oxygen, glucose, and other nutrients to nerve cells is critical to the health, survival, and normal functioning of the brain. A sustained reduction in the supply of oxygen can lead to cell death. The third system involves the repair and cleanup functions of the neuron. Unlike other types of cells in the body, nerve cells do not replicate after birth. Instead, they constantly degrade or digest old, worn-out parts of themselves and synthesize new proteins for replacement parts. This system of continuous protein synthesis and degradation is finely regulated, and any disruption can have disastrous consequences for nerve cell function.

These three interrelated systems normally work in synchrony. Sometimes, however, internal (endogenous) factors, such as changes in an individual’s nutritional, immune, or neuroendocrine status, interfere with the normal functioning of one of these systems, thus disrupting the delicate balance. Alternatively, external (exogenous) factors such as toxins, trauma, or infectious agents might disrupt the equilibrium. There is evidence that the pathology seen in Alzheimer's disease is associated with changes in all three systems.

A.CELL-TO-CELL COMMUNICATION

Changes in the brain’s communication systems ultimately affect the individual’s behavior. The behavioral effects of alcohol consumption, for example, are mediated through changes in the signal transduction pathways of large numbers of nerve cells. In the case of specific behaviors seen in Alzheimer's disease patients — namely, cognitive impairment and performance decline — the most immediate precipitating events in the brain are alterations in the chemical communication pathways within and among neurons.

A vast repertoire of chemicals — including neurotransmitters, neuroendocrine peptides, growth-promoting factors, metal ions, and many others — is used by each neuron for different kinds of communication, much as multilingual individuals in a cosmopolitan community may use different languages for different conversations. In the mid-1970s, Alzheimer's disease researchers found that an enzyme necessary for the synthesis of one such chemical, the neurotransmitter acetylcholine, was deficient in the brains of Alzheimer's disease patients. This was an important discovery in that it provided the first link between Alzheimer's disease and specific biochemical defect in the brain.

The scientific community was particularly ready to accept the challenge of Alzheimer's disease at this time because, in the preceding few years, research in neurotransmitter chemistry had revealed that acetylcholine-containing neurons (i.e., cholinergic neurons) play an important role in memory. Since the initial discovery of a cholinergic deficit in Alzheimer's disease, it has been shown that the disease also involves abnormalities in other neurotransmitters as well as in other chemical signals that modulate neuronal activity. However, over the years, animal studies and analysis of human brain tissue obtained at autopsy have consistently confirmed the relationship between cholinergic deficits and memory impairments.

In the early 1980s, the “cholinergic hypothesis” of Alzheimer's disease engendered great optimism that the cholinergic deficits could be corrected — and the disease cured — through pharmacological manipulations. The confidence that many scientists placed in this approach was based on the apparent similarity between Alzheimer's disease and another neurodengerative disorder, Parkinson’s disease, in which neurotransmitter deficits can be ameliorated by an increase in the supply of the deficient chemical. In subsequent years a number of strategies were tried for correcting the cholinergic deficits in Alzheimer's disease, all designed to maintain or improve the availability of acetylcholine by increasing its synthesis, facilitating its release at the synapse (the contact point between nerve cells), or slowing the rate of its breakdown. Generally, these approaches have not fulfilled their initial promise, in spite of modest successes in some patients for short periods.

1.Promise and Problems of Tacrine

One of these experimental efforts involved a multicenter clinical trial of tetrahydroaminoacridine (THA). This substance was approved as a treatment for Alzheimer's disease by the U.S. Food and Drug Administration in September 1993 under the name of tacrine (Cognex), despite controversy about its effectiveness. Tacrine is a cholinesterase inhibitor, one of a class of compounds that slow the degradation, or breakdown of acetylcholine, thus allowing the small amounts of neurotransmitter that are released to remain at the synapse a bit longer than usual. The rationale was that prolonging acetylcholine availability at the synapse would effectively facilitate the transmission of information from one cell to the other. A number of studies with various cholinesterase inhibitors have shown that in some Alzheimer's disease patients these compounds do slightly slow the rate of decline in performance on some neuropsychological tests. Tacrine is not without drawbacks, however. It does not appear to help all patients, and it can cause liver toxicity, although this can be controlled. Studies are being conducted continually to improve the optimal dosage of tacrine and to identify subgroups of patients who stand to benefit most from the drug. More important, however, the search continues for more effective treatments with fewer side effects that would help more Alzheimer's disease patients for longer periods. At present many laboratories at university medical centers as well as pharmaceutical companies are searching for a more effective and longer lasting agent that would correct this neurotransmitter defect. Some of these compounds are at various stages of testing for approval by the FDA.

There are several possible reasons why treatment strategies directed at correcting cholinergic deficits have not been as successful as expected. One is that the most effective compound for correcting the chemical deficiency or the appropriate molecular target has not yet been found. Another potential reason is that the cholinergic neurons are selectively vulnerable in Alzheimer's disease and are dying; therefore, therapy to increase available acetylcholine is too little too late. Still another distinct possibility is that some other biochemical abnormalities may occur first, placing the cholingergic system at risk; this antecedent event, therefore, should be the target of treatment. Finally, it is highly likely that an effective treatment for Alzheimer's disease would need to select multiple targets since it is known that the disease affects many biochemical systems, all of which influence the neuronal signal transduction pathway.

2.Multiple Therapeutic Targets

Many groups of researchers at major universities and at biotechnology and pharmaceutical companies around the world have become interested in the problems of Alzheimer's disease and committed to developing active agents to intervene at various stages of the signal transduction process. Neuroscientists have made significant advances in discovering the details of molecular mechanisms in cell-to-cell communication and the intricate signal transduction pathways within a neuron. This knowledge now provides a vast array of molecular targets for intervention. To promote and facilitate drug-discovery efforts at academic institutions and to accelerate the testing of promising compounds, the Alzheimer’s Disease Cooperative Study Unit (Alzheimer's diseaseCSU), a 33-site U.S. consortium, was established. It represents a major national resource for developing improved technologies for clinical trials, for conducting clinical trials, and for testing new diagnostic procedures. At present, many potential treatments aimed at enhancing neuronal communications are in various stages of planning for testing.

B.METABOLIC STRESS: CAUSES AND EFFECTS

The second system that is important for neuronal survival consists of the structural and functional elements that regulate nerve cell metabolism. Scientists have known for some time that, without an adequate supply of oxygen and glucose, neurons will die. Moreover, these cells are extremely demanding and fussy about the metabolic fuel they consume; they need an abundant supply of pure glucose, and any sustained depravation, as occurs in asphyxiation or stroke, has disastrous consequences. A number of studies have demonstrated that vascular changes in the brain are intrinsic to the pathology of Alzheimer's disease. Profound structural and biochemical alterations in tiny blood vessels in the brain can lead to chronic deprivation of blood flow, resulting in a progressive decline in neuronal function in selected brain areas. Pathological changes in the capillaries of the brain imply that the function of the blood-brain barrier (BBB) is altered in Alzheimer's disease. The BBB allows oxygen, glucose, and other essential nutrients and chemicals to pass from the capillary circulation into brain tissue while at the same time preventing the passage of undesirable compounds such as environmental toxins, pathogens, and drugs. The association of severe head trauma with an increased risk for Alzheimer's disease is probably related to damage to the brain microvessel system and possible failure of the BBB.

In recent years the application of sophisticated imaging techniques to the study of the brains of Alzheimer's disease patients has yielded insight into metabolic abnormalities. The Alzheimer's disease patients certain parts of the brain involved in cognitive functioning are unable to utilize glucose properly. Scientists are not certain whether these deficits are due to microvessel pathology or dysfunction in other parts of the metabolic cascade, such as a defect in the protein that transports glucose. The end result, however, is that in Alzheimer's disease certain parts of the brain are under a condition of chronic metabolic stress. Continuous malnutrition of neurons, for whatever reason, could have several important implications for understanding of the pathologies associated with Alzheimer's disease. The synthesis of acetylcholine, the key neurotransmitter for memory, is highly dependent on glucose metabolism in the brain. Thus, selective vulnerability of cholinergic neurons might actually be a consequence of inadequate blood circulation to those parts of the brain, resulting in a gradual starvation of these cells. The cholinergic deficits and associated cognitive decline could be the result of metabolic abnormalities that may have proceeded unnoticed for a long period before the onset of obvious, disabling cognitive changes.

Another consequence of chronic glucose insufficiency in the brain is the conversion of a harmless and essential neurotransmitter, glutamate, into a potent killer of neurons. Glutamate is an excitatory amino acid; in appropriate amounts it is essential for development and normal functioning of neurons but, as with other excitatory amino acids, in excessive amounts it can become toxic to the very neurons it normally stimulates. Glutamate becomes neurotoxic when too much of it is present at a synapse or when, in normal amounts, it stimulates a glucose-deprived neuron. Glutamate toxicity is mediated by the influx of calcium into the cell, and it is the excessive internal concentration of calcium that eventually kills the cell.

In recent years scientists have become especially interested in the biochemical mechanisms of neurotoxicity for two reasons. First, it has been shown that a wide variety of toxic compounds, some present in the environment (exogenous toxins), such as aluminum, and others naturally present in the body (endogenous toxins), such as glutamate, can lead to selective neuronal dysfunction and death. Second, neurotoxins have become an important analytical tool, allowing neuroscientists to study different characteristics of nerve cells as reflected by their selective vulnerability.

Among the many potentially neurotoxic compounds in the environment, aluminum has captured the most attention. Aluminum is a ubiquitous element. While autopsy analyses of the brains of Alzheimer's disease patients have produced conflicting results depending on methods used, there appears to be a modest accumulation of aluminum in the brain lesions — the neuritic plaques and neurofibrillary tangles — that are characteristic of the disease.

C.SYNTHESIS AND DEGRADATION: A DELICATE BALANCE

The third essential system for maintaining the health of a neuron is its ability to control and balance two opposing biochemical events, one involving the mechanisms of protein and membrane synthesis, the other involving the processes that degrade or digest proteins. It is through this complex balancing act that neurons repair and renew themselves and drive their unique ability for selfmodification in response to stimuli, experiences, or injuries.

Most nerve cells, once fully developed, are designed to provide a lifetime of service. A neuron, to function properly, must renew between 50,000 and 100,000 different types of proteins. A mistake in the synthesis of any one of these proteins could interfere with an essential cellular function and lead to a failure in a neuron’s ability to communicate vital information. Such errors could result in too much or too little of a protein or one with the wrong sequence of amino acids (the building blocks of protein), something like a string of words with spelling and grammatical errors. Errors in amino acid sequence, in turn, could influence the three-dimensional structure of the protein, thus affecting how well it does the job. What might appear to be a minor change in the position of one or two amino acids could become the cause of a disease such as Alzheimer's disease. Errors in protein degradation can have equally disastrous consequences. Proteins that are not properly digested or broken down could accumulate, forming new, harmful aggregates.

As mentioned above, the neuropathological hallmarks of Alzheimer's disease are two kinds of microscopic lesions, called neuritic (or senile) plaques and neurofibrillary tangles, which are found in the brains of Alzheimer's disease patients at autopsy. Both are consequences of abnormalities in the processing of different types of proteins. The major constituent of the tangles is a protein called tau, which is present in normal brain tissue. Tangles apparently form as a result of abnormal phosphorylation (the addition of phosphate molecules) of tau, a process that interferes with the protein’s role in the construction of vital intracellular transport structures known as microtubules.

The other major lesion associated with Alzheimer's disease, the neuritic plaque, has as its principal constituent beta-amyloid protein. Amyloid is derived from a larger protein, called the amyloid precursor protein (APP), which is normally found partially embedded in the membrane of the neuron. The exact function of APP and how it is related to the clinical signs of dementia is not known. It is believed that it may play an important role in stabilizing synaptic contact points. It is very likely that APP is critical to the plasticity of the nervous system, thus being of great importance for understanding the neurobiology of cognitive functioning. It must also play other, undiscovered roles in the normal functioning of neurons because there are many different forms of APP, each with a slightly different amino acid sequence; they have been found in all kinds of animals, from fruit flies to humans. Amyloid protein has the unusual characteristics of being highly insoluble and resistant to degradation, thus readily accumulating within the nervous system. How it interferes with cell functioning is not totally clear, but there are some suggestions that aggregations of beta-amyloid become highly toxic in neurons in a way similar to glutamate. In fact, both of these substances may inflict their damage by disrupting the internal homeostasis of calcium ions. Recent discoveries concerning the nature of amyloid protein, how it is formed and processed, what it does to a cell, and the genes that determine its structure have created tremendous excitement among neuroscientists. Many believe that pursuit of this clue will lead to discovery of the specific causes of Alzheimer's disease.

D.GENETIC KEYS?

The scientific enthusiasm about the possible role of amyloid protein in the pathology of Alzheimer's disease has been further fueled by the results of molecular genetics studies that have identified genes associated with familial (inherited) Alzheimer's disease on chromosomes 21, 14, 1, and 19. The first specific gene linked with familial Alzheimer's disease was the APP gene on chromosome 21, which is responsible for producing amyloid protein. After the initial report, several other mutations were found in the region of the APP gene in members of families that had a history of Alzheimer's disease onset at a relatively young age. How these mutations alter the behavior of APP and their significance to normal cell functioning are not known, but are being actively studied. Subsequently, a region on chromosome 14 was also linked to an early-onset form of the disease. Recently the exact locus of this gene was pinpointed. The exact function of this gene is still unknown, but it is only a matter of time when this will be no longer be a mystery. Within a short period following the discovery of the of the locus of chromosome 14, a locus on chromosome 1 was linked to a family with an unusually high incidence of Alzheimer's disease, known as the Volga-German families.

The fourth and perhaps the most important recently discovered gene linked to Alzheimer's disease is the Apolipoprotein E (ApoE) gene on chromosome 19, which has been associated with many lateonset familial cases of Alzheimer's disease as well as sporadic cases in the over-60 age group. (Sporadic cases are those occurring in individuals who have no strong family history of the disease.) The ApoE gene directs the synthesis of a cholesterol-transporting blood protein. The gene occurs in three different forms: apoE2, apoE3, and apoE4. One of these, apoE4, is found in 14% of control populations but is present in 30 to 40% of the late-onset sporadic cases of Alzheimer's disease before age 85 and rises to 90% for individuals who are homozygous for apoE4, which means they have inherited this form of the gene from both parents. These people have a 5:1 odds of developing Alzheimer's disease, compared with the 15:1 odds in individuals who have a single ApoE4 gene. It has been estimated that between 25 and 40% of Alzheimer's disease cases can be attributed to the presence of this form of the gene.

Not only does the ApoE gene have a strong and consistent relationship with the disease, but, within a few months after it was identified, researchers postulated a plausible biological explanation for its role in the pathological processes of Alzheimer's disease. It has been shown that the protein encoded by the ApoE gene has a high affinity for and binds with beta-amyloid in the plaques. Among Alzheimer’s patients, those who have the gene for apoE4 have larger plaques than those who lack the gene. It appears that ApoE4 acts as a chaperone to APP and, in some unknown way, promotes the formation of neuritic plaques. It has also been postulated that it plays an important role in the formation of neurofibrillary tangles. In the brain ApoE proteins are taken up by neurons in large quantities after neuronal injury and appear to play an important role in various recuperative processes and in neuronal plasticity.

The excitement and optimism generated by the discovery of a relationship between Alzheimer's disease and ApoE is well warranted; epidemiological studies of Alzheimer's disease will now have a biological marker for sorting patients into homogenous groups and studying them with the hope of finding other contributing factors. At the same time, this research has begun to provide new opportunities for developing alternative treatment strategies.