Summary The onset mechanism of proliferation in mitotically quiescent retinal pigment epithelium (RPE) cells is still obscure in humans and newts, although it can be a clinical target for manipulating both retinal diseases and regeneration. To address this issue, we investigated factors or signaling pathways involved in the first cell‐cycle entry of RPE cells upon retinal injury using a newt retina‐less eye‐cup culture system in which the cells around the wound edge of the RPE exclusively enter the cell cycle. We found that MEK–ERK signaling is necessary for their cell‐cycle entry, and signaling pathways whose activities can be modulated by heparin, such as Wnt‐, Shh‐, and thrombin‐mediated pathways, are capable of regulating the cell‐cycle entry. Furthermore, we found that the cells inside the RPE have low proliferation competence even in the presence of serum, suggesting inversely that a loss of cell‐to‐cell contact would allow the cells to enter the cell cycle.

No abstract

The newt is an indispensable model animal, of particular utility for regeneration studies. Recently, a high-throughput transgenic protocol was established for the Japanese common newt, Cynops pyrrhogaster. For studies of regeneration, metamorphosed animals may be favorable; however, for this species, there is no efficient protocol for maintaining juveniles after metamorphosis in the laboratory. In these animals, survival drops drastically after metamorphosis as their foraging behaviour changes to adapt to a terrestrial habitat, making feeding in the laboratory with live or moving foods more difficult. To elevate the efficiency of laboratory rearing of this species, we examined metamorphosis inhibition (Ml) protocols to bypass the period (four months to two years after hatching) in which the animal feeds exclusively on moving foods. We found that approximately 30% of animals survived after 2-year Ml, and that the survivors continuously grew, only with static food while maintaining their larval form and foraging behaviour in 0.02% thiourea (TU) aqueous solution, then metamorphosed when returned to a standard rearing solution even after 2-year-MI. The morphology and foraging behavior (feeding on static foods in water) of these metamorphosed newts resembled that of normally developed adult newts. Furthermore, they were able to fully regenerate amputated limbs, suggesting regenerative capacity is preserved in these animals. Thus, controlling metamorphosis with TU allows newts to be reared with the same static food under aqueous conditions, providing an alternative rearing protocol that offers the advantage of bypassing the critical period and obtaining animals that have grown sufficiently for use in regeneration studies.