Spinach Protein Could Offer New Hope For The Blind

"Although the neural wiring from the eye to brain is intact in people with these diseases, their eyes lack photoreceptor activity," said Eli Greenbaum of ORNL’s Chemical Technology Division.

Greenbaum and colleagues propose replacing these non-functioning photoreceptors with a spinach protein that gives off a small electrical voltage after capturing the energy of incoming photons. The main function of Photosystem I, a photosynthetic reaction center protein, is to perform photosynthesis in leaves using the energy of the sun to make plant tissue.

Greenbaum’s collaborator is Mark Humayun, a professor in the University of Southern California’s Doheny Eye Institute. 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 can see image patterns that mimic the effects of stimulation by light.

Greenbaum believes that it might be possible to use Photosystem I protein to restore photoreceptor activity. Experiments by Greenbaum’s team showed that Photosystem I protein can capture photon energy and generate electric voltages of up to 1 volt.

"What we need to find out is whether these voltages can trigger neural events and allow the brain to interpret the images," Greenbaum said.

In recent research, the team showed that Photosystem I reaction centers protein could be incorporated into the membrane of an artificial liposome, a tiny spherical particle formed by a fatty (lipid) membrane enclosing a watery compartment. The artificial membrane mimics that of a living cell.

Greenbaum’s team also showed that the Photosystem I reaction centers can work inside a liposome, which means it produces the experimental equivalent of a voltage when it comes into contact with light. A liposome will likely be used to deliver Photosystem I reaction centers protein to a retinal cell.

In the United States, degeneration of the retina has left 20,000 people blind and 500,000 people visually impaired. Retinitis pigmentosa 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.

Age-related macular degeneration is a disease that affects the center of vision. It rarely leads to blindness but people with the disease have difficulty reading, driving and performing other activities that require fine, sharp straight-ahead vision. The disease affects the macula, the center of the retina.

The project also builds upon work using the technique of Kelvin force microscopy, in which Greenbaum and colleagues performed the first measurements of voltages induced by photons of light from single photosynthetic reaction centers. The work was published in 2000 in an issue of the Journal of Physical Chemistry B.

Other ORNL researchers involved in the project are Tanya Kuritz and James Lee of the Chemical Technology Division, Frank Larimer of the Life Sciences Division, and Ida Lee and Barry Bruce of the University of Tennessee.

"We have assembled an outstanding interdisciplinary team of scientists, vitreo-retinal surgeons, ophthalmologists and biomedical engineers to attack this important problem," Greenbaum said.

Greenbaum has long envisioned that his group’s research in photosynthesis could have an impact on people in terms of energy production and biomolecular electronics. Now, he’s especially excited that it could also restore vision to some blind people.

This research is funded by ORNL’s Laboratory Directed Research and Development program, which is provided by DOE. ORNL is a multiprogram research facility managed by UT-Battelle.