Defects in Crucial Brain Protein Implicated in Memory Loss
DURHAM, N.C. -- The ability to recognize familiar objects
and companions is lost when levels of a protein crucial for
recycling a chemical messenger in the brain are reduced,
mimicking some of the symptoms of Alzheimer's disease, an
international team led by Duke University Medical Center
scientists has discovered.
Mice genetically engineered to have modest defects in this
recycling protein display symptoms that resemble those in
Alzheimer's, such as the inability to remember familiar faces,
according to the researchers. The crucial protein recycles a
chemical called acetylcholine that carries messages between
nerve cells, the scientists said.
"By using these genetically engineered mice as models of
Alzheimer's, we can learn more about the neuronal circuitry of
the brain, and perhaps even discover new ways to alleviate the
symptoms of this devastating disease," said senior study
investigator Marc G. Caron, Ph.D., James B. Duke professor of
The team reports its findings in the Sept. 7, 2006, issue of
the journal Neuron. The research was supported by the National
Institutes of Health and the American Health Assistance
Acetylcholine is a neurotransmitter that carries a number of
vital signals from one nerve cell, or neuron, to another.
Normally, when a signal needs to travel through the brain,
neurons release acetylcholine to transport the signal across
the gap, or synapse, between neurons. Acetylcholine is stored
in tiny hollow spheres, called vesicles, that bud off of the
end of the neurons. A kind of protein pump, called a
transporter, located in each neuron controls the storage and
release of acetylcholine from these vesicles, recycling the
neurotransmitter back to the nerve cell vesicles in preparation
for the next burst of signal.
It is this acetylcholine transporter protein that the
researchers targeted by disrupting the gene that controls its
"Acetylcholine is important for every function in the body
-- breathing, eating, walking, practically everything," Duke's
Caron said. "If we knocked out the function of the protein
completely, then the mice would die. So instead, we just
knocked its function down to a low level."
In the study, the researchers took advantage of a built-in
trait of their animal models -- that is, the fact that mice are
innately curious and tend to explore new objects and companions
extensively by sniffing and touching. The scientists ran mutant
mice, in which the acetylcholine transporter gene was
defective, through a series of tests to evaluate their
performance in behavioral tasks. They ran normal mice through
the same tests to serve as a control group.
The first test assessed the mice's ability to discriminate
unfamiliar objects. The researchers gave the mutant and normal
mice two objects to explore, and then took the objects away. A
short time later, the scientists gave the mice back one of the
objects, along with a nonfamiliar object. Both the normal and
mutant mice initially explored the two objects to the same
degree, but after the break the mutants had trouble remembering
the familiar object, said lead study investigator Vania F.
Prado, Ph.D., an associate professor of biochemistry at
Universidade Federal de Minas Gerais, in Belo Horizonte,
The second test of memory was similar to the first, but
instead of giving the mice objects to explore, the scientists
introduced a new mouse into the cage of their test subjects.
The normal mice extensively explored the "intruder" mouse, but
over time they showed less and less interest, the researchers
said. This behavior, they said, demonstrated that the normal
mice had become familiar with the intruder. In contrast, the
mutant mice failed to recognize the intruder even after several
meetings, thus displaying a defect in what the researchers
called "social memory."
These findings suggested that the decreased levels of
acetylcholine in the mutant mice resulted in their trouble with
social memory, the scientists said.
To determine if this theory was true, the researchers set
out to correct the behavioral defect by treating the mice with
drugs that increase levels of acetylcholine in the brain.
Called cholinesterase inhibitors, the drugs block a brain
enzyme that typically breaks down acetylcholine, thus leaving
more of the neurotransmitter available for sending the signals
involved in learning and memory. Physicians give cholinesterase
inhibitors to people with Alzheimer's to slow their memory loss
and enable them to perform daily tasks, lessening the symptoms
of the disease.
Mutant mice treated with the drugs, when run through the
same tests, recognized intruder mice after several meetings,
the researchers said, adding that this observed improvement in
the performance of social recognition confirmed that the
defects stemmed from the reduced amounts of acetylcholine.
"Now we can use our animal model to screen for similar drugs
that can improve the function of acetylcholine in the brain,"
said Marco A. M. Prado, Ph.D., an associate professor of
pharmacology at Universidade Federal de Minas Gerais and senior
investigator of the study. "This is of importance because
decreases in acetylcholine are thought to be relevant for the
diminishing cognitive function found in aging and also are
believed to be associated with some of the behavioral and
cognitive symptoms in Alzheimer's disease."
Approximately 4 million Americans have Alzheimer's disease,
and 100,000 people die from the disease each year.
Other researchers participating in the study included Amy J.
Ramsey, Tatyana D. Sotnikova, Hyung-Gun Kim, Hui Quan, William
C. Wetsel, and Raul R. Gainetdinov of Duke University Medical
Center; Cristina Martins-Silva, Braulio M. de Castro, Ricardo
F. Lima, Vinicius R. Cota, Marcio F. D. Moraes, Marcus V.
Gomez, Christopher Kushmerick, Grace S. Pereira, Ernani Amarai,
Janaina Koenen and Cristina Guatimosim of Universidade Federal
de Minas Gerais; and Daniela M. Barros, Maria R. Ramirez,
Janine L. Rossato, and Ivan Izquierdo of Pontificia
Universidade Catolica de Rio Grande do Sul, in Porto Alegre,