Spinach eyes aren't seeing green

Transplants of spinach proteins into the eye could restore sight � to a point

By Chris Williams

The latest science breakthrough is definitely illuminating. According to US scientists, spinach may soon enable the blind to see again. But if Popeye's spinach guzzling finally seems to make sense, think again. This sight-restoring property isn't achieved through diet but through transplantation.

Researchers at Oak Ridge National Laboratory, an institution more famous for atom bombs than medical advances, propose using photosynthesizing proteins from spinach to restore some function to the retinas of people struck by retinitis pigmentosa or macular degeneration.

These diseases affect the photoreceptors at the back of the retina, but the rods and cones, the nerve cells in front of them � which then lead back to the brain � are generally left intact. In a healthy eye, light passes through this layer of nerve cells before falling on the rods and cones, which then transmit an electrical signal forward through a sequence of nerve cells. The signal then doubles back to a central processing area.

Direct electrical stimulation of these nerve cells can produce an image in the mind of a blind person. In fact, efforts to build artificial photoreceptor implants to trigger these cells are quite advanced. There's a limit to the resolution that such artificial implants can provide, however, because humanity's ability to jam photovoltaic cells together on a circuit board is still some way behind the concentrations achieved by nature.

Oak Ridge's biological system relies on photosystem I (PS1) reaction centres, each of which consists of a few spinach proteins bound together in a liposome. When struck by light, an electrical potential is generated on the surface of the liposome. Tests showed the signal would be strong enough to fire an adjacent nerve cell.

Last month, at the Biosensors Conference in Spain, the team presented their latest results and successfully persuaded real eye cells to react to the signal. However, the system comes with some kinks � it can only substitute for rod cells, which provide resolution, but not for cones, which provide colour. So any restored vision would be strictly black and white.

A potentially more serious handicap is that the process bypasses a string of nerve cells that normally perform a good deal of signal processing before the signal is passed on to the main optic nerve. Nobody knows if a useful signal can be generated without them.

Finally, there's the issue of transplantation. The implants could die, or they could cause an immune reaction, or they could damage the nerve cells they're connected to. Vegetable transplants in people don't exactly have a lot of precedent.