Enhanced retinal pigment epithelium regeneration after injury in MRL/MpJ mice

Regenerative medicine holds the promise of restoring cells and tissues that are destroyed in human disease, including degenerative eye disorders. However, development of this approach in the eye has been limited by a lack of animal models that show robust regeneration of ocular tissue. Here, we test whether MRL/MpJ mice, which exhibit enhanced wound healing, can efficiently regenerate the retinal pigment epithelium (RPE) after an injury that mimics the loss of this tissue in age-related macular degeneration. The RPE of MRL/MpJ and control AKR/J mice was injured by retro-orbital injection of sodium iodate at 20 mg/kg body weight, which titration studies indicated was optimal for highlighting strain differences in the response to injury. Five days after sodium iodate injection at this dose, electroretinography of both strains revealed equivalent retinal responses that were significantly reduced compared to untreated mice. At one and two months post-injection, retinal responses were restored in MRL/MpJ but not AKR/J mice. Bright field and fluorescence microscopy of eyecup cryosections indicated an initial central loss of RPE cells and RPE65 immunostaining in MRL/MpJ and AKR/J mice, with preservation of peripheral RPE. Phalloidin staining of posterior eye whole mounts confirmed this pattern of RPE loss, and revealed a transition region characterized by RPE cell shedding and restructuring in both strains, suggesting a similar initial response to injury. At one month post-injection, central RPE cells, RPE65 immunostaining and phalloidin staining were restored in MRL/MpJ but not AKR/J mice. BrdU incorporation was observed throughout the RPE of MRL/MpJ but not AKR/J mice after one month of administration following sodium iodate treatment, consistent with RPE proliferation. These findings provide evidence for a dramatic regeneration of the RPE after injury in MRL/MpJ mice that supports full recovery of retinal function, which has not been observed previously in mammalian eyes. This model should prove useful for understanding molecular mechanisms that underlie regeneration, and for identifying factors that promote RPE regeneration in age-related macular degeneration and related diseases.