Stem cells for treating retinal degeneration.

The mammalian retina lacks regenerative potency to replace damaged or degenerated cells. Therefore, traumatic or genetic insults that lead to the degeneration of retinal neurons or retinal pigment epithelium (RPE) cells alter visual perception and ultimately can lead to blindness. The advent of human stem cells and their exploitation for vision restoration approaches has boosted the field. Traditionally, animal models - mostly rodents - have been generated and used to mimic certain monogenetic hereditary diseases. Of note, some models were extremely useful to develop specific gene therapies, for example for Retinitis Pigmentosa, Leber congenital amaurosis and achromatopsia. However, complex multifactorial diseases are not well recapitulated in rodent models such as age-related macular degeneration (AMD) as rodents lack a macula. Here, human stem cells are extremely valuable to advance the development of therapies. Particularly, cell replacement therapy is of enormous importance to treat retinal degenerative diseases. Moreover, different retinal degenerative disorders require the transplantation of unique cell types. The most advanced one is to substitute the RPE cells, which stabilize the light-sensitive photoreceptors. Some diseases require also the transplantation of photoreceptors. Depending on the disease pattern, both approaches can also be combined. Within this article, I briefly feature the underlying principle of cell replacement therapies, demonstrate some successes and discuss certain shortcomings of these approaches for clinical application.