January 22, 2001
Yerkes' Smith hunts for Huntington's disease clues
By Poul Olson
A team of Yerkes neuroscientists led by Yoland Smith are mapping brain receptors suspected of playing a role in the death of neurons associated with Huntingtons disease. The research, presented in November at the Society for Neurosciences annual conference in New Orleans, could shed new light on how the debilitating neurodegenerative disease develops.
A genetically inherited illness, Huntingtons is characterized by motor and cognitive impairments.
The disease first becomes evident in middle age and develops over 15
to 20 years, killing 95 percent of the neurons in the striatum, an area
of the brain that channels information from the cerebral cortex to the
basal ganglia. The resulting interference in communication between the
cerebral cortex and the basal ganglia disrupts cognitive and motor functions.
In addition to developing psychiatric problems, many people with Huntingtons
experience choreaa motor impairment characterized by jerky, uncontrollable
and often violent movements of the limbs.
How this neuronal death actually occurs has long eluded researchers.
New clues emerged in 1997 following the discovery of a mutation in the
gene that encodes for the brains receptors for kainate, a powerful
neurotoxin. This deviation occurs among a special subset of people with
In their research, Smith and his team have used electron microscopy to
locate the receptors in the striatum of rhesus macaques. Most recently,
they made an intriguing discovery of kainate receptors in cortical terminals
that use glutamate as a neurotransmitter.
Its very unusual for kainate receptors to be located there,
said Smith. This suggests a role for the receptors in modulating
the release of glutamate into the striatum and ultimately mediating the
death of striatal neurons.
If Huntingtons causes kainate receptors to be hyperactivated, they
could release too much glutamate (an excitatory neurotransmitter) into
the striatum, Smith believes. At excess levels, glutamate kills neurons.
While mapping the location of the kainate receptors, Smith and his team,
in collaboration with other colleagues, intend to study their function
using a new class of drugs that selectively bind to these receptors.
We dont know where these receptors are or what they do,
Huntingtons disease belongs to the class of neurodegenerative illnesses
that includes Alzheimers and Parkinsons diseases. In 1993,
the discovery of a mutated gene that causes Huntingtons was heralded
as a breakthrough that would soon lead to a cure.
Scientists believed drugs could be developed to preferentially target
the Huntingtin protein in striatal neurons that degenerate. These hopes
were dashed when researchers discovered that the protein occurs abundantly
throughout the central nervous system.
Huntingtons disease typically strikes between age 40 and 50 and
can take as long as two decades to progress fully. Genetic tests can identify
those at risk for Huntingtons, but having the Huntingtons
gene does not necessarily mean a person will develop the disease.
The complexity of Huntingtons symptoms makes treating the disease
difficult. Smith explained that medications designed to ameliorate motor
dysfunction, for instance, often have adverse side effects on cognition.
Although there are no new treatments on the horizon, Smith hopes that
understanding the role of kainate receptors in the Huntingtons process
eventually could yield new targets for medications.
A good compound that selectively targets kainate receptors could
control the release of glutamate into the striatum, Smith said.
This disease could potentially be thwarted before it has a chance
to start on its path of destruction.
The Huntingtons disease research program is funded by a four-year, $600,000 grant from the U.S. Army Medical Research and Materiel Council.