Retinal regeneration in the Xenopus laevis tadpole: a new model system

PURPOSE Retinal regeneration research holds potential for providing new avenues for the treatment of degenerative diseases of the retina. Various animal models have been used to study retinal regeneration over the years, providing insights into different aspects of this process. However the mechanisms that drive this important phenomenon remain to be fully elucidated. In the present study, we introduce and characterize a new model system for retinal regeneration research that uses the tadpole of the African clawed frog, Xenopus laevis. METHODS The neural retina was surgically removed from Xenopus laevis tadpoles at stages 51-54, and a heparin-coated bead soaked in fibroblast growth factor 2 (FGF-2) was introduced in the eyes to induce regeneration. Histological and immunohistochemical analyses as well as DiI tracing were performed to characterize the regenerate. A similar surgical approach but with concomitant removal of the anterior portion of the eye was used to assess the capacity of the retinal pigmented epithelium (RPE) to regenerate a retina. Immunohistochemistry for FGF receptors 1 and 2 and phosphorylated extracellular signal-regulated protein kinase (pERK) was performed to start elucidating the intracellular mechanisms involved in this process. The role of the mitogen activated protein kinase (MAPK) pathway was confirmed through a pharmacological approach using the MAPK kinase (MEK) inhibitor U0126. RESULTS We observed that Xenopus laevis tadpoles were able to regenerate a neural retina upon induction with FGF-2 in vivo. The regenerated tissue has the characteristics of a differentiated retina, as assessed by the presence and distribution of different retinal cell markers, and DiI tracing indicated that it is able to form an optic nerve. We also showed that retinal regeneration in this system could take place independently of the presence of the anterior eye tissues. Finally, we demonstrated that FGF-2 treatment induces ERK phosphorylation in the pigmented epithelia 10 days after retinectomy, and that inhibition of the MAPK pathway significantly decreases the amount of retina regenerated at 30 days post-operation. CONCLUSIONS Regeneration of a complete neural retina can be achieved in larval Xenopus laevis through activation of the MAPK signaling pathway by administering exogenous FGF-2. This mechanism is conserved in other animal models, which can regenerate their retina via pigmented epithelium transdifferentiation. Our results provide an alternative approach to retinal regeneration studies, capitalizing on the advantages of the Xenopus laevis tadpole as a model system.