ORNL Review Vol. 34, No. 2, 2001

New Hope for the Blind from a Spinach Protein

Spinach may make Popeye the Sailor Man strong, but a protein from spinach may someday strengthen the vision of people who can barely see. Researchers at ORNL and the University of Southern California (USC) are investigating whether this chlorophyll-containing protein might be useful in restoring sight to the legally blind. The protein could replace a key, light-receiving part of the human eye that has lost its ability to function. People who suffer from age-related macular degeneration (AMD) or retinitis pigmentosa (RP), diseases that are the leading causes of blindness worldwide, may find hope in this research.

Although the neural wiring from eye to brain is intact in patients with these diseases, their eyes lack photoreceptor activity. Eli Greenbaum and his colleagues in ORNL's Chemical Technology Division (CTD) propose replacing these inactive photoreceptors with a spinach protein that gives off a small electrical voltage after capturing the energy of incoming photons of light. Called Photosystem I, or PSI (pronounced PS One), the main function of this "photosynthetic reaction center" protein is to perform photosynthesis, using the energy of the sun to make plant tissue.

Greenbaum made this proposal after meeting with Mark Humayun of the Doheny Retina Institute at USC. Humayun and his research team showed that if retinal tissue is stimulated electrically using pinhead-sized electrodes implanted in the eyes of legally blind patients, many of these people can perceive image patterns that mimic the effects of stimulation by light. Greenbaum suggested that it might be possible to use PSI proteins to restore photoreceptor activity. ORNL experiments showed that PSI proteins can capture photon energy and generate electric voltages (about 1 volt). The question is, can these voltages trigger neural events, allowing the brain to interpret images?

Currently, in the United States, degeneration of the retina—the light-sensitive layer of tissue at the back of the eye—has left 20,000 people totally blind and 500,000 people visually impaired. RP is an inherited condition of the retina in which specific photoreceptor cells, called rods, degenerate. The loss of function of these rod cells diminishes a person's ability to see in dim light and gradually can reduce peripheral vision, as well.

AMD is a disease that affects the center of vision; people rarely go blind from the disease but may have great difficulty reading, driving, and performing other activities that require fine, sharp, straight-ahead vision. AMD affects the macula, the center of the retina. When light is focused onto the macula, millions of cells change the light into an electrical current for the benefit of the neural wiring that tells the brain what the eye is seeing.

Using internal funds from the Laboratory Directed Research and Development Program at ORNL, Greenbaum, Tanya Kuritz, and James W. Lee, all of CTD; Frank W. Larimer of ORNL's Life Sciences Division; Ida Lee and Barry D. Bruce, both of the University of Tennessee; and Humayun and his team at the Doheny Retina Institute at USC are seeking a cure for AMD and RP. "We have assembled an outstanding interdisciplinary team of scientists, vitreo-retinal surgeons, ophthalmologists, and biomedical engineers, to attack this important problem," Greenbaum says.

This project, he adds, is based on recent original discoveries in CTD. "Using the technique of Kelvin force microscopy, we have performed the first measurements of voltages induced by photons of light from single photosynthetic reaction centers. This work was published in 2000 in an issue of the Journal of Physical Chemistry B. The measured photovoltage values, typically 1 volt or more, are sufficiently large to trigger a neural response.

"We are proposing the insertion of purified PSI reaction centers into retinal cells to determine whether they will restore photoreceptor function in persons who have AMD or RP. Once we demonstrate this is possible, USC researchers will test the technique in the laboratory, and if feasible, later in humans in clinical trials."

In recent research, the collaborators showed that PSI reaction centers could be incorporated into a liposome, an artificial membrane made of lipids that mimics the composition of a membrane of a living cell. They also demonstrated that the PSI can be functional inside a liposome—that is, it produces the experimental equivalent of a voltage when light is shone on it. A liposome will likely be used to deliver PSI to a retinal cell.

Liposomes are tiny spheres that consist of a bilayer membrane and an inner aqueous compartment. Photosystem I (PSI) reaction centers have been inserted into the bilayer membrane to create PSI-proteoliposomes. The liposomes will be used as delivery vehicles for insertion of the PSI reaction centers into retina membranes, illustrated here.

The PSI-proteoliposomes will fuse with deficient neural cells of the eye. If functional PSI reaction centers are inserted into a neural cell, sodium and potassium ions may be set in motion by the voltage generated by PSI reaction centers when stimulated by light. This ion motion can lead to the propagation of a voltage impulse along a neuron that transmits information to the brain. Figure by Tanya Kuritz.

Also, in work recently published in Photochemistry and Photobiology (June 2001), the collaborators have shown that isolated PSI reaction centers can photo-evolve hydrogen, indicating that PSI maintains its voltage-generating properties under conditions of current flow.

Greenbaum has long envisioned that his group's research in photosynthesis could have important impacts on humans in terms of energy production and biomolecular electronics. Now, he is especially excited that it also could lead to restoration of vision to the blind.

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