December 08, 2004.
The Daily Californian.
A group of UC Berkeley scientists has discovered a way to stimulate nerve cells with light, which could one day lead to the restoration of sight for people with certain forms of blindness. UC Berkeley neurobiology professor Richard Kramer and chemistry professor Dirk Trauner have collaborated to create a photo-switch, or light-activated switch, for nerve cells that usually do not react to light.
When exposed to certain wavelengths of light, these ordinary nerve cells can start acting like rods and cones-the light-sensitive cells that are essential for vision-that have been destroyed by degenerative eye diseases.
One such disease is age-related macular degeneration (AMD), the leading cause of vision loss among Americans over 60 years old. With this disease, rods and cones in the middle of the visual system gradually die off, causing patients to lose their central vision. Individuals with AMD eventually lose their ability to read, drive or sew.
A less common disease, retinitis pigmentosa, causes individuals to lose their rods and cones in the periphery of the retina, a light-sensing structure in the eye, causing "tunnel vision." This genetic disease can develop as early as early adulthood.
Restoring sight for these patients may be a future possibility with Kramer and Trauner's special photo-switch.
To build this switch, the team had to first modify a potassium channel. They genetically-engineered this ion channel-a passageway that allows cells to produce electrical signals-to react to a chemical created in Trauner's lab.
"The chemical, when placed onto this channel or basically attached onto the channel, acts as a gate for the channel," Kramer said.
When exposed to ultraviolet light, the channel opens and the cell turns "off." When exposed to green light, the channel closes and the cell turns "on."
The combination of chemical and channel-which the team calls a SPARK (synthetic photoisomerizable azobenzene-regulated potassium) channel-is the key to making cells previously insensitive to light responsive. These stimulated neurons would then be able to send information to the brain, enabling a person to simulate half of the visual apparatus in the brain.
Creating the special ion channel required certain genetic material to be delivered into the cell so the chemical could target that channel.
The group has been able to introduce genetic material into cells on a petri dish, but they are now looking into the next method: using a virus to carry genes into the nerve cells.
These viruses would be safe-it would not reproduce itself, just deliver the genes to the neurons. This method would most likely be used to introduce genes into the eye.
Other medical approaches to blindness include preventing damage to the retina or using stem cells to restore light-sensitive neurons.
Scientists are also using inserted electrodes-electronic prosthetic devices that stimulate nerve cells-into the eye. These devices would overlay what is left of the retina and help the patient see patterns of activity.
But because there are billions of closely-packed neurons, making electrodes small enough to match up and control each neuron is difficult.
The UC Berkeley team hopes to provide an alternative to mechanical or electrical implantations.
"All of these things have potential, but if our (method) works down the road I think it could be less invasive and more global than these electronic prosthetic devices," Kramer said.
But there is still a long way to go. Sending electrical signals to nerve cells in other parts of the body has been proven to be successful-for example, deafness has been ameliorated through ear implants that use this method. Using the same approach for the retina, however, could be difficult because of its complex structure.
Also, modifying a potassium channel would not simulate all of the visual apparatus in the brain. Triggering both "on" and "off" cells are necessary for full sight.
Right now the UC Berkeley team is working on creating a second channel, a sodium channel and the potassium channel's counterpart, for another type of nerve cell that turns "on" with light exposure.
Kramer imagines a device that looks similar to an eyepiece worn by a blind character, Geordi La Forge, in "Star Trek: The Next Generation" for individuals with AMD or retinitis pigmentosa. The eyepiece would use a laser scanner to scan back and forth across the retina, stimulating nerve cells.
"Perhaps . we can combine an apparatus like that with our method of making nerve cells directly light-sensitive to enable you to see images, even if you had no rods and cones anymore," Kramer said.
Science fiction may not just be fantasy any longer.
Contact Andrea V. Hernandez at firstname.lastname@example.org.
Source URL: http://www.dailycal.org/article.php?id=17204.
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