Key protein may help researchers restore light sensitivity in damaged retinas
By Susan Reiss
Millions of light-sensitive cells called photoreceptors fill the delicate tissue in the eye known as the retina. These cells include rods that provide night vision and cones that detect color. During the visual process, light passes through the eye and hits the photoreceptors, activating a chemical process through which light is translated into electrical signals. Retinal nerve cells known as bipolar and ganglion cells then carry these signals to the optic nerve, which leads to the brain.
But what if rods and cones have been destroyed by disease or genetic glitches? Can other cells compensate for the lost light sensitivity?
Indeed, scientists supported by the National Eye Institute (NEI), part of the National Institutes of Health, have identified several methods for jump-starting a damaged visual system. Early research efforts have focused on hardware that transmits electrical signals to the brain, eliminating the need for photoreceptors.
One such device, known as the artificial retina, was developed by NEI grantees Mark Humayun, M.D., Ph.D., and Robert Greenberg, M.D., Ph.D. This engineered visual system includes an array of electrodes that is surgically implanted on the retina. Specialized glasses and microcomputers generate electrical signals which are transmitted to the implanted electrodes. These signals are received by the remaining viable retina cells, which in turn pass the signal to the brain through the optic nerve.
The artificial retina is currently being tested in clinical trials for people who have retinitis pigmentosa, a group of genetic eye diseases that lead to vision loss. While the device has been successful, its image resolution is limited by the number of electrodes in the implanted array, and implantation requires an invasive surgical procedure.
More recently, researchers have been exploring less-invasive genetic approaches that stimulate light sensitivity in an unconventional way, using bipolar and ganglion cells. Like second stringers on a basketball team, these cells could step in and play a crucial role in the visual process when rods and cones can no longer function. By engineering bipolar and ganglion cells to serve as vision’s starting lineup, researchers hope to restore some sight in eyes compromised by disease.
A crucial discovery in algae
The story of these genetic therapies begins with a well-known form of green algae, called chlamydomonas, which is found around the globe, from deep within the ocean to atop snow-covered mountains. Chlamydomonas’s movements are directed by a primitive visual system that includes an organelle known as the eyespot. Nearly a decade ago, researchers showed that a protein called channelrhodopsin-2 (ChR2) was responsible for light sensitivity in the eyespot of one form of the algae. ChR2 is similar to rhodopsin, which is a light-sensitive pigment found in human eyes.
Following this discovery, researchers led by Ernst Bamberg, Ph.D., of the Max Planck Institute for Biophysics in Frankfurt, Germany, cloned and began experimenting with the gene for ChR2. Dr. Bamberg and his team found that when light shined on a cell containing ChR2, pores or channels in the cell membrane opened, allowing certain positively charged molecules, such as sodium and potassium, to pass through. If the light turned off, the pores closed. Thus, ChR2 was the first known channel in a cell to be controlled by the presence or absence of light.
“We were extremely lucky to find this protein in the algae,” said Dr. Bamberg, who has spent his 30-year career exploring how these types of light-sensitive cellular pumps function. “It was a big surprise.”
Rousing the second string
As Dr. Bamberg worked on his ChR2 project, two NEI-supported researchers searched for a new strategy for restoring vision after rods and cones die. Zhuo-Hua Pan, Ph.D., of Wayne State University School of Medicine in Detroit, Mich., and Alexander Dizhoor, Ph.D., now a professor at Pennsylvania College of Optometry in Elkins Park, Penn., looked for ways to genetically convert the retina’s second- and third-order cells, including ganglion cells, to become light sensitive so they would mimic the function of rods and cones.
They first needed to find a suitable light sensor that could be easily inserted into surviving retinal cells. Dr. Pan said he searched through scientific journals for work involving such a compound and found a paper published by Dr. Bamberg’s group in the November 2003 Proceedings of the National Academy of Sciences.
“I was so excited when I read the paper because ChR2 appeared to be an ideal candidate for our purpose,” Dr. Pan said.
So he and his colleagues used a viral vector, or a virus modified to carry a gene as payload, to deliver the ChR2 gene to ganglion cells in rats and mice. They found that the gene did, in fact, trigger the production of ChR2 protein in these cells. The ganglion cells then acted like rods and cones by transmitting electrical signals to the brain when activated by light.
This was the first group to demonstrate that it was possible to restore light sensitivity in the retina after the death of rods and cones. Since publishing these results in 2006, Dr. Pan’s team has been studying the long-term effects of ChR2 by observing treated mice under different light conditions. They now report that the ganglion cells have remained viable thus far.
A collaborative effort
With the significance of ChR2 identified, researchers must now develop and refine the appropriate genetic material, transport mechanism and device engineering necessary to move toward research in humans. To accomplish this, Dr. Bamberg recently joined with Constance Cepko, Ph.D., of Harvard Medical School, and Jean Bennett, M.D., Ph.D., of the University of Pennsylvania, both NEI-supported researchers. In addition, the group includes several international collaborators: Botond Roska, Ph.D., of the Friedrich Miescher Institute for Biomedical Research in Basel, Switzerland; Georg Nagel, Ph.D., of the University of Wurtzburg in Germany; and Jose-Alain Sahel, M.D., and Serge Picaud, Ph.D., both of the Vision Institute in Paris.
“Our goal is to bring this tool to blind patients,” Dr. Roska said. He noted that when the time comes for clinical trials, the group will concentrate on treating legally blind people who have retinitis pigmentosa.
This international consortium has also been experimenting with different forms of ChR2, including an altered type that is 1,000 times more sensitive to illumination than the naturally occurring form, so ganglion cells require less light for activation. However, a safe and effective transport vehicle is necessary to deliver the more efficient form of ChR2 to cells, and a set of goggles is needed to control the levels of light delivered to the retina.
“This work opens a broad window, but there are still hurdles such as safety that need to be addressed,” Dr. Sahel said.
Dr. Cepko agreed. “These kinds of things can’t be hurried along,” she said, estimating that it may be many years before a treatment could be used in humans. And when that point is reached, the retina’s second-string team of ganglion and bipolar cells may never perform at the peak level of the eye’s starting lineup of rods and cones, even with the use of light-activated molecules such as ChR2.
“This is not likely to restore normal vision,” Dr. Bennett said of this genetic therapy. “However, it could restore enough vision to provide independence to individuals by improving their abilities to navigate, identify shapes and patterns, and move. This would be a significant improvement to patients who are severely visually impaired.”
So while the results may be limited, ChR2 provides, at the very least, a starting point for retinal therapy on a cellular level. It may move research another step closer to bringing the vision of those who have retinal degeneration diseases back into the light.
About the artificial retina
Bi A, Cui J, Ma Y, Olshevskay E, Pu M, Dizhoor AM, Pan Z. Ectopic expression of a microbial-type rhodopsin restores visual response in mice with photoreceptor degeneration. Neuron 50, 23-33, April 6, 2006.
Lagali PS, Balya D, Awatramani GB, Munch TA, Kim DS, Busskamp V, Cepko CL, Roska B. Light-activated channels targeted to ON bipolar cells restore visual function in retinal degeneration. Nature Neuroscience, 11:6 667-675, June 2008.