In some forms of retinal degenerative disease, including age-related macular degeneration, the death of retinal cells can be due to defects in the underlying retinal pigment epithelial (RPE) cells. In animal models of these disorders, such as the Royal College of Surgeons (RCS) rat, progressive photoreceptor loss can be limited by transplantation of RPE cells into the subretinal space. A new study by Coffey et al., published in Nature Neuroscience, shows that such treatment can also lead to long-term preservation of cortical visual processing in these rats.

The photoreceptors of RCS rats die during the first three weeks after birth. Coffey and colleagues implanted cells from an immortalized human RPE cell line into the three-week-old rats, then monitored their visual ability and the responses of neurons in their visual cortex, to assess the effects of the gradual degeneration of photoreceptors and implantation of the RPE cells on central processing mechanisms.

Several weeks after implantation, the treated rats were as good as control rats at tracking moving stripes and distinguishing between stripe patterns with different spatial frequencies, whereas untreated RCS rats were severely impaired. When the authors recorded from neurons in the primary visual cortex of the treated rats at seven months of age — by which time the corresponding neurons in untreated RCS rats are completely unresponsive to visual input — they found that the neurons responded almost as well as those of control rats with no visual impairment, with some neurons in treated rats showing physiological properties within the normal range.

When the eyes of the grafted animals were examined, they had around half as many photoreceptors as normal rats. It is interesting that these rats were able to perform at similar levels to normal rats despite having only partial protection against photoreceptor loss, indicating that the visual system can adapt to this kind of damage. These results are encouraging for the possible future use of implanted RPE cells in humans with degenerative retinal diseases, particularly as the use of immortalized cell lines would mean that a constant supply of cells for implantation could be maintained, avoiding the logistical and ethical difficulties associated with the use of fetal tissue for transplantation.