Amyloid
plaques are one of the two brain abnormalities that define
Alzheimer's disease
(AD). The other hallmark is neurofibrillary
tangles. Technically, an individual may display all the
behavioral and cognitive symptoms of AD, but if the brain
does not contain the hallmark plaques and tangles, there is
no diagnosis of AD. The appearance of amyloid plaques in the
brain can proceed the behavioral symptoms by years.
Amyloid plaques are sticky buildup which
accumulates outside nerve cells, or neurons. Amyloid is a
protein that is normally found throughout the body. For reasons
as yet unknown, in AD, the protein divides improperly, creating
a form called beta amyloid which is toxic to neurons in the
brain. No one really knows why beta amyloid is formed or why
it causes cell death. One possibility is that it may break
into fragments, releasing substances called free radicals
that attack neurons. Another possibility is that beta amyloid
forms tiny channels (holes) in neuron membranes, allowing
influx of unlimited amounts of calcium. Although regulated
amounts of calcium are necessary for normal neuronal function,
too much can kill a neuron. However beta amyloid does its
work, the result is that neurons begin to die. Plaques begin
to form that consist of these degenerating neurons and clumps
of the amyloid protein itself. The body cannot break these
clumps down and dispose of them, so they accumulate in the
brain.
The apoE4 gene, a genetic abnormality which
has been implicated in AD, may be involved in the production
of amyloid plaques. The gene may produce a protein that latches
on to the toxic beta amyloid and makes it impossible for the
body to dissolve. As a result, the beta amyloid accumulates
as plaques in the brain. Molecules called free radicals may
also play a role. Normally, free radicals play important roles
in the body, such as helping the immune system fight off disease.
However, too many free radicals can start to upset the delicate
balance within a neuron. Nerve cells producing beta amyloid
seem also to produce more free radicals. It may be the case
that free radicals thus boost beta amyloid production.
by Catherine E. Myers. Copyright © 2006 Memory Loss and the Brain
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