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

Abstract

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. Brightfield 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 wholemounts 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.

Regeneration of the zebrafish retinal pigment epithelium after widespread genetic ablation

“…Mammals largely fail to regenerate a functional RPE monolayer following injury (20,21). One exception to this is in "super healer" MRL/MpJ mice, which regenerate RPE within ~30 days after administration of mild doses of sodium iodate that cause central RPE degeneration (61). Beyond this, the mammalian RPE is incapable of regenerating after severe injuries (e.g.…”

“…Several lines of evidence suggest the latter possibility, and highlight the important role played by peripheral RPE: in mouse, a subpopulation of mature RPE cells in the periphery remain in the cell cycle and respond to microscopic photocoagulation injuries by proliferating at a higher rate than central RPE cells (27,70), while experiments in pig have shown that peripheral RPE cells respond to debridement of central RPE by proliferating (71). Indeed, preservation of the peripheral RPE is a prerequisite for successful RPE regeneration in the MRL/MpJ mouse model, which fails to regenerate RPE when high doses of sodium iodate cause degeneration of central and peripheral RPE (61,72). Finally, the discovery of a subpopulation of RPE stem cells (30) suggests that an endogenous regeneration-capable population of RPE cells could exist in the human eye, and these might be analogous to injury responsive cells in zebrafish.…”

Retinal pigment epithelium development, plasticity, and tissue homeostasis

“…Interestingly, the regenerative response of the RPE to sodium iodate injury is enhanced in the Murphy Roths Large mouse strain (MRL) or “healer mouse”, and is correlated with elevated proliferation in the damaged region (Xia et al, 2011). Improved regenerative responses have been observed in several different tissues in the MRL mouse (Heber-Katz et al, 2004), but it is not clear whether the enhanced regenerative potential is due to alterations in systemic responses to injury or to a cell-intrinsic advantage within the regenerative cells themselves.…”

The immune response is a critical regulator of zebrafish retinal pigment epithelium regeneration

“…It is known that mammalian RPE can repair small lesions, but that larger-scale restoration is not possible or leads to over-proliferation and pathology (19). Some insight into the proliferative capacity of mammalian RPE has been gleaned from studies in mice (20,21) and cultured human RPE cells (22), while studies in regeneration-capable non-mammalian systems have focused largely on RPE-to-retina transdifferentiation within the context of understanding retinal regeneration (23,24).…”