Regeneration from three cellular sources and ectopic mini-retina formation upon neurotoxic retinal degeneration in Xenopus

Regenerative abilities are not evenly distributed across the animal kingdom. Interestingly, the underlying modalities are also highly variable, even among closely related species. In fish or amphibians, retinal repair can involve the mobilization of different cellular sources, including stem cells of the ciliary marginal zone (CMZ), retinal pigmented epithelial (RPE) cells, or Muller glia. The mechanisms that trigger the recruitment of one cell type over another remain elusive. To investigate whether the magnitude of retinal damage might influence the regeneration modality of the Xenopus retina, we developed a model based on cobalt chloride (CoCl2) intraocular injection, allowing for a dose-dependent control of cell death extent. Analyses in Xenopus laevis revealed that limited CoCl2-mediated neurotoxicity only triggers cone cell loss and results in a few Muller glia cells reentering the cell cycle, without affecting CMZ cell activity or recruiting RPE cells. Conversely, we found that severe CoCl2-induced retinal degeneration not only potentializes the proliferative response of Muller cells, but also enhances CMZ cell proliferation and, unexpectedly triggers an RPE reprogramming event. Although Muller glia could not regenerate cones under these conditions, both CMZ and RPE-derived proliferative cells could. Strikingly, RPE reprogrammed cells self-organized into an ectopic layered mini retina-like structure laid on top of the original retina. It is thus likely that the injury paradigm determines the awakening of different stem-like cell populations exhibiting distinct neurogenic capacities. Besides, we surprisingly found that Xenopus tropicalis also has the ability to recruit Muller cells and reprogram its RPE following CoCl2-induced damage, whereas only CMZ cell proliferation was reported in previously examined degenerative models. Altogether, these findings highlight the critical role of the injury paradigm and reveal that three cellular sources can be reactivated in the very same degenerative model.