Transplants of photoreceptor cells offer hope for treating retinal disease. But getting the cells to make the right connections with the brain has been problematic. It seems the developmental stage of the cells may be the key.
Each year many people lose their sight through disease of the retina. Millions are affected by macular degeneration and retinitis pigmentosa alone. In these diseases, the cone and rod photoreceptors — the cells that convert light into neural signals — degenerate. And once these cells are lost, they are not replaced in the mammalian retina. Treatments are being developed that may delay or prevent the loss of retinal neurons in these disorders. But for those who have already lost their sight, the possibility of retinal transplantation may be the best prospect for the restoration of vision through cell-replacement therapy. Despite many attempts, such transplants have yet to produce better vision in mammals because the transplanted cells do not wire up to the brain properly. In this issue, MacLaren et al. (page 203)1 show in mice that the trick may be to use cells at a particular stage in their development.
Retinal transplantation has a long history. Classic studies in the 1920s showed that transplantation of the eyes of normal salamanders could give vision to blind, cave-dwelling salamanders2. This work allowed developmental biologists to explore the mechanisms that enable the correct connections to form between the eyes and the nervous system. The first successful transplant of a mammalian retina dates back to 1959, when Royo and Quay3 transplanted fetal rat retinas into the eyes of adults of the same strain. Although the transplanted retinas did not seem to connect with the host retinas, they survived for months.
Since then, intact retinal sheets from embryonic mice and rats have been transplanted to the sub-retinal space. There they develop many characteristics of a normal retina4,5,6, and grow to form a second retinal layer underneath the host retina. Also, transplants of 'microaggregates' — clumps of a few retinal neurons — from newborn mice developed most characteristics of rod photoreceptors, including the expression of the rhodopsin protein and even the characteristic structures called outer segments7,8. Unfortunately, both the intact embryonic retinal sheets and the microaggregates of photoreceptors keep to themselves, without interacting or integrating very effectively with the host retinal neurons.
Integration with the host retina is much better when transplanting retinal progenitor cells — the immature cells that are responsible for producing all the retinal cells during embryonic development. These progenitors (also called retinal stem cells by some) are derived from fetal or newborn mice or rats, or from human fetuses. They can be maintained in cell culture and continue to proliferate and generate new neurons and specialized retinal support cells called Müller glia9,10,11. When these cells are transplanted into either normal or degenerated (dystrophic) retinas of rats and mice, they can migrate into all retinal layers and develop morphological characteristics of various retinal cell types12,13,14,15. However, the cells do not seem to integrate efficiently into the outer nuclear layer, where the rods and cones reside, and, for the most part, evidence showing expression of photoreceptor-specific genes in the transplanted cells has been lacking (but see ref. 15).
So, MacLaren et al.1 aimed to combine the migration and integration potential of the progenitor cells with the photoreceptor differentiation properties of the retinal sheets. In the first set of experiments, they tested whether retinal progenitor cells were in fact better at migrating into the host retina than mature, differentiated neurons. Using cells taken from the retina of embryonic mice or newborn mice at different postnatal ages, they found, surprisingly, that the cells that integrated into the outer nuclear layer most effectively were the postnatal cells. This was despite the fact that the embryonic cells included the highest percentage of progenitor cells.
The transplanted rod cells developed morphology identical to normal host rods, even making clear outer segments. On further analysis, the authors found that the best ages for the donor cells were from postnatal day 3 to postnatal day 5, roughly corresponding to two or three days following the peak of rod photoreceptor production in the mouse retina. This suggested that, contrary to expectations, newly born rods, rather than the progenitor cells, were the best for transplanting. The authors further corroborated this idea by harvesting just newly born rods immediately after they are generated by the progenitors and using them for transplants.
In addition to the striking morphological evidence for integration of the transplanted rod photoreceptors into the normal outer nuclear layer, MacLaren et al.1 provide evidence that the transplanted photoreceptors connect up with the host retinal neurons, and can partly restore the response to low light levels in mice that are blind because they are deficient in the rod protein rhodopsin.
These results provide the best evidence so far that cell-replacement therapy may be possible. But there's a catch. If this scenario were to be applied to humans, one would have to obtain newly generated rods from the stage of development comparable to postnatal days 3–7 in the mouse. This is likely to be in the second trimester and is clearly not feasible. However, a recent report from our lab shows that cells expressing the protein Nrl — a marker of newly born rods — can be obtained from human embryonic stem-cell lines, given certain culture conditions16. Further studies will of course be needed to determine whether the stem-cell-derived Nrl+ cells behave like the Nrl+ mouse rod photoreceptors before these can be considered for cell-replacement therapy.
Although these results provide hope for the cell-based treatments of retinal disease, they also have implications for transplantation strategies in other areas of the central nervous system. Transplantation of neural tissue is a potential treatment for several degenerative neurological diseases and traumatic injury. The studies attempting this approach used various ages of developing neural progenitors and stem cells. But it may be that the specific time at which the particular cell is harvested will make all the difference in its potential for integration and functional differentiation following transplantation. Sometimes, timing is everything.