NEW EVIDENCE SHOWS EARLY LIFE STRESS CAN CONTRIBUTE TO ONSET OF ALZHEIMER'S
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NEW EVIDENCE SHOWS EARLY LIFE STRESS CAN CONTRIBUTE TO ONSET OF ALZHEIMER'S
ATLANTA, October 15, 2006 - New findings on the effects of stress, how damaging proteins act in the brain, and the early events that lead to Alzheimer's disease (AD) are increasing science's understanding of the devastating disorder and prospects for treatment.
Specifically, the new research shows:
- A mutant gene introduced in the mouse genome leads to beta amyloid deposits, and in some cases, memory deficits.
- Stress during life may have effects in the brain that put us at risk for diseases such as Alzheimer's many years later.
- Amyloid beta protein actually binds to its parent, amyloid precursor protein (APP), pointing the way to a target for the development of novel therapeutics for AD.
- The identity of an enzyme that plays a key role in the early development of AD when combined with stress due to low energy metabolism.
- Mice lacking two amyloid precursor proteins develop brain defects prior to birth.
AD is a degenerative disease of the brain that affects over four million Americans. It is currently estimated that about 8 percent to 10 percent of individuals over the age of 65, and roughly 40 percent of individuals over the age of 85, have probable AD.
There is no effective treatment currently. Most of the focus on the underlying mechanism involved in AD has focused on amyloid beta peptide, a short protein -- typically 40 or 42 amino acids -- that is derived from APP, a longer precursor. "Improving our understanding of the disease process should help us to design new, potentially improved therapies," says Edward Koo, MD, at the University of California, San Diego. Koo will chair a minsymposium on the functions of APP at Neuroscience 2006 in Atlanta.
Recent research has found that introducing a mutant gene into the mouse genome leads to beta amyloid deposits in the brain and in some cases memory deficits.
"These findings will have a significant impact on the development of rational therapeutic strategies that alleviate, if not prevent, the devastating clinical and pathological features of AD," says Sangram Sisodia, PhD, at the University of Chicago.
A rare group of patients with AD show symptoms between ages 20 and 50, and many of these individuals carry mutant genes that cause the disease in extended families. Hence the term familial Alzheimer's disease (FAD).
Although these gene variants are rare, a variety of cell biological, molecular and transgenic mouse models have been developed that help examine the consequences of these mutant genes on the metabolism of APP and amyloid beta peptides, together with pathological and physiological outcomes. The symptoms and laboratory test features of these individuals are indistinguishable from the larger AD population whose disease begins after the age 60.
The function of APP in the nervous system is unclear, says Sisodia. But experiments with mice in which the APP gene and genes encoding APP-like proteins have been deleted has revealed a role for these proteins in synapse structure and function.
Presenilins are membrane proteins that play critical roles in determining specific cell fates during embryonic development, and in the breakdown of a variety of membrane proteins, including APP and Notch, a receptor critical for developmental signaling events. A series of genetic and biochemical studies have revealed that presenilins are a component of a larger structure composed of several membrane proteins.
The breakdown of APP is responsible for the generation of 40 to 42 amino acid amyloid beta peptides (amyloid beta 40 and amyloid beta 42), Sisodia says. "The amyloid beta 42 peptide has a high propensity to self-aggregate and form fibrils, and it appears that small aggregates of amyloid beta 42 are both neurotoxic and affect memory-related processes in the brain."
Studies in cultured cells and transgenic animals reveal that one mechanism by which FAD-linked mutant presenilin cause disease is by decreasing production of soluble amyloid beta 40 peptides that prevent the aggregation of amyloid beta 42 peptides. Increasing the ratio of amyloid beta 42/40 peptides leads to accelerated amyloid deposition, he says.
"This is not the sole mechanism by which mutant forms of presenilins cause disease," says Sisodia. Studies in transgenic mouse models have provided important information about the impact of FAD-linked mutant presenilins on the transport of membrane proteins. These membrane proteins are important in synaptic function and plasticity within axons, the intrinsic vulnerability of neurons, the birth and survival of neurons in the adult hippocampus, and gene expression, he says.
The researchers are now testing several approaches to treat the disease, including antibody strategies, a variety of compounds, and the effect of enriched environments on reducing beta amyloid levels. Eventually, these efforts are expected to lead to new drugs for humans.
In a recent study, researchers found that environmental stress rapidly results in high levels of the amyloid beta protein in the brain of a mouse model of AD. "This change occurs well before this is actually buildup of this protein in the brain," says David Holtzman, MD, at Washington University. "This suggests that stress may enhance risk for Alzheimer's disease by affecting a process fundamental to the disease, and that these changes may occur many years before dementia begins."
Holtzman's studies suggest that acute stressors that occur during life may be having effects in the brain that put us at risk for diseases such as Alzheimer's many years later, Holtzman says. "In addition, they suggest that treatments, either pharmacological or behavioral, that alter stress, may be useful to decrease one's risk for Alzheimer's disease," he adds.
These findings assess changes due to stress on the protein amyloid beta in a region of the brain important for memory, the hippocampus, that occur over hours. They also support prior work in this area where stress appears to increase the buildup of amyloid in the brain that occurs over a much longer period of time.
Holtzman's study used a mouse model of AD subjected to isolation stress that results in higher stress hormones and altered behavioral responses. He and colleagues monitored the level of the amyloid beta protein up to every 30 minutes in the brain of these mice prior to the buildup of this protein in the brain. "In three months or three days of isolation stress, there were elevated amyloid-beta levels," says Holtzman. "Interestingly, another acute stressor called restraint stress, elevated amyloid-beta over hours."
In exploring the reason for the elevation of amyloid beta levels, part of the abnormality is due to stress causing a decrease in the ability of the brain to clear amyloid, Holtzman says. He and colleagues found that the half-life of amyloid beta was longer in animals subjected to three months of stress vs. controls.
"We have shown that neuronal activity is also involved in contributing to the levels of amyloid-beta in the brain in the setting of stress," he says, "and that certain proteins in the brain that are elevated due to stress may be contributing to the acute effects of amyloid-beta that occur over hours."
Some of the next steps in his work are to determine which molecules and networks in the brain are responsible for these effects. "We would like to determine whether pharmacological and environmental modulation of stress can prevent these changes, as well as the pathology of Alzheimer's disease, which occurs over a longer period of time," he says. "This may lead to a useful way to prevent or delay the occurrence of cognitive decline, the hallmark of Alzheimer's disease."
In Alzhiemer's, amyloid beta peptide is derived from APP by being cleaved at two points by "molecular scissors" called proteases. However, it remains unclear how the accumulation of amyloid beta peptide actually causes AD.
New research has found evidence that amyloid beta actually binds to its parent, APP, and causes two or more APP molecules to come together, triggering a signal that causes the part of APP that is inside the cell, the "intracytoplasmic domain," to bind to other proteins and be cleaved. This produces a distinct peptide that is damaging to the brain, says Dale Bredesen, MD, at the Buck Institute for Age Research in Novato, Calif. This damaging peptide is called APP-C31, because it consists of the last (C-terminal) 31 amino acids of APP.
"If this is indeed important in Alzheimer's disease, then we should be able to block the formation of APP-C31," says Bredesen. "And, even though amyloid beta will still accumulate, it should not trigger Alzheimer's disease if indeed APP-C31 is an important downstream signal mediating Alzheimer's disease."
To evaluate this possibility, Bredesen created transgenic mice that have "Mouzheimer's disease" due to a human gene for AD derived from a mutation that is associated with AD that runs in families. When the researchers prevented APP-C31 generation by making a simple substitution at the site of cleavage of APP to produce APP-C31, they did not see any effect on the production of amyloid beta peptide or its accumulation into the senile plaques that characterize AD.
In other words, the mouse brains looked like typical AD brains. Surprisingly, however, the mice were completely free of AD symptoms: their memories were normal, they did not have any loss of brain cell connections that characterize AD, they did not show the characteristic brain shrinkage or scar formation, and they did not show the disturbance in brain stem cells.
These results suggest that the cleavage of APP at the APP-C31 site plays a critical role in the generation of Alzheimer-related pathophysiological and behavioral changes in human APP transgenic mice. This makes this site an attractive target for the development of new therapies for AD, says Bredesen. "Our follow-up studies suggest that we can indeed identify potential therapeutics that prevent the cleavage of APP at the APP-C31 site, and determining their effects on Alzheimer's disease will be of great interest," Bredesen says.
Another line of research has identified an enzyme that plays a key role in the early development of AD when combined with stress due to low energy metabolism. This could uncover novel drug treatments or prevention strategies.
Although the cause of AD is still a matter of intense debate, a large and growing body of evidence suggests that amyloid beta may play a pivotal role in the early stages to trigger a pathological cascade that eventually leads to this devastating degenerative disorder, says Robert Vassar, PhD, of Northwestern University Medical School.
Several years ago, Vassar identified an enzyme called BACE1 and proved that it initiates the formation of amyloid beta. Since then, BACE1 has become a very promising drug target for lowering amyloid beta levels in the brains of AD patients. Moreover, factors that increase the level and activity of BACE1 in the brain may elevate the production of amyloid beta and trigger AD.
Because age is the primary risk factor for AD, Vassar investigated whether changes in the aging brain may cause BACE1 levels to increase and drive amyloid beta production. Studies dating back to the early 1980's had shown that blood flow in the brain decreases with age, especially in AD patients, indicating a chronic reduction in the supply of oxygen, nutrients, and energy to the brain.
Vassar and his team speculated that stress due to low energy metabolism may elevate BACE1 levels and cause amyloid beta levels to increase in the brain, thus promoting AD. He tested this in genetically modified mice that overproduce amyloid beta and develop amyloid plaques in the brain. To simulate low energy metabolism in the brains of these mice, researchers treated the mice with a low dose of drugs that block the production of energy in the brain, thus producing mild energy stress.
"Remarkably, we found that after only a single treatment, BACE1 and amyloid beta levels were elevated and stayed high for at least a week,' says Vassar. "In this initial study, published in the Journal of Neuroscience last year, we did not investigate the effects of treatment on amyloid plaque formation. However, we have recently performed a chronic treatment study in which we have treated mice with energy-blocking drugs for three months and then analyzed their brains for BACE1, amyloid beta, and amyloid plaques."
As expected, BACE1 and amyloid beta levels were increased by drug treatment as before, says Vassar. Importantly, amyloid plaques were also increased in number in the brains of chronically treated mice. "Although the amyloid beta and plaque increases that we observed in mice were modest, evidence suggests that only a small elevation in amyloid beta is necessary to cause a large increase in amyloid pathology over the decades that are required to develop Alzheimer's disease in humans," says Vassar.
Their results support the notion that impaired energy production or energy delivery in the brain during aging may trigger AD by elevating BACE1 level and activity, which in turn would lead to the overproduction of amyloid beta. "This process may represent one of the earliest events in the pathological cascade leading to Alzheimer's disease," he says.
"We anticipate that understanding the mechanism of the BACE1 increase at a molecular level may uncover novel drug targets for the treatment of Alzheimer's disease, or may even suggest strategies to prevent its occurrence," says Vassar. "Because there is as yet no cure available for Alzheimer's disease, it is hoped that the knowledge gained from these studies will help advance therapies for the millions Alzheimer's sufferers and their caregivers."
In other studies, scientists have found that mice lacking two amyloid precursor protein-binding proteins develop brain defects that arise prior to birth, while the brain is being formed, which are not observed in control mice or in mice lacking either of the proteins.
One of these defects can be seen as stockpiles of neurons on the outer surface of the brain, where neurons are not normally found in control mice, says Suzanne Guenette, PhD, at the MassGeneral Institute for Neurodegenerative Disease. In addition, neuronal projections are observed in the wrong location and some projections fail to grow to the same lengths as in control mice. These defects are found in the cortex and hippocampus, regions of the brain important for learning and memory that are affected in AD.
"The appearance of mispositioned neurons on the surface of the brain in our mice is particularly interesting because it bears remarkable resemblance to a defect seen in mice lacking all three APP family members, APP, APLP1 and APLP2," says Guenette.
This defect is similar to one observed in a human disease called Type II lissencephaly, which causes mental retardation in children. In both the mouse models and in Type II lissencephaly, the aberrant positioning of neurons occurs on the surface of the cortex, the brain region responsible for higher learning. Understanding the cellular and molecular mechanisms for the development of this brain defect may help understand normal brain development and the events that lead to brain malformations, says Guenette.
Toward that end, Guenette and collaborators have examined cells from a layer on the surface of the cortex that release several proteins, which form an extracellular matrix unique to this region of the brain. This matrix is recognized by newly arrived neurons as a signal that they have reached their final destination on the surface of the brain.
"When we examined cells from this layer, we found that one of the matrix proteins it produces, laminin, does not have a normal distribution," says Guenette. "Mice lacking laminin molecules have brain developmental abnormalities similar to the defect seen in our mice. Our data suggest that interactions between amyloid precursor protein-binding proteins and APP contribute to the production and/or secretion of proteins important for brain development."
APP is a protein of great interest to researchers studying AD because it is the precursor for beta-amyloid, the predominant protein component of senile plaques, which accumulate excessively as deposits in brains of AD patients. "Results from several labs show that increasing or decreasing the levels of the amyloid precursor protein-binding proteins affects the amount of beta-amyloid generated at the cellular level," Guenette says.
To determine whether loss of these proteins affects the levels of brain beta-amyloid, she measured beta-amyloid levels in adult mouse brains of control mice, mice lacking either one of two amyloid precursor protein-binding proteins, FE65 or FE65L1, and those lacking both proteins. "We found a reduction in levels of the longer form of beta-amyloid in male, but not in female mouse brains for mice lacking both FE65 and FE65L1. These data do not support a disease-related role for the APP/FE65 protein interactions, at least for brain beta-amyloid production since beta-amyloid accumulation in AD is not restricted to men."
Collectively, says Guenette, these data show that the FE65/APP protein interactions are crucial for normal brain development and suggest that the FE65 proteins relay an APP signal important to brain development. "Our current research is aimed at further defining the cellular and molecular mechanisms that produce the developmental defects observed in mice lacking the FE65 proteins and to continue searching for parallels between these events and those occurring in AD."