Pax6 in Regeneration of the Retina in Newts
A fair number of researchers are mining the biochemistry of highly regenerative species in search of the differences that enable regrowth of damaged organs. There is the hope that these differences are in at least some cases small enough that human tissues could be adjusted to perform far greater feats of regeneration. At this point it is far too early to say how likely this is, how long the mapping process will take, or how difficult it will be to build therapies when the relevant differences are identified. There have been a few interesting discoveries in recent years, and this is the latest of these:
In contrast to other vertebrates including humans, the newt can regenerate, even as an adult, an entire retina from retinal pigment epithelium (RPE) cells. In adult vertebrate eyes, the RPE is a highly differentiated monolayer-cell-sheet laminating the back of the neural retina (NR) and functions as a partner of the NR for vision. Mature RPE cells, as a rule, do not proliferate under physiological conditions. In the adult newt, RPE cells in the intact eye are also mitotically inactive, but when the NR is removed from the eye by surgery, RPE cells lose their epithelial characteristics and detach from each other as well as from the basement membrane (Bruch's membrane), giving rise to the aggregates of mesenchymal-like cells with multipotency, named RPE stem cells (RPESCs), in the vitreous cavity. RPESCs are subsequently divided into two cell populations which undergo proliferation, so that they can differentiate into two epithelial layers of progenitor cells (named pro-NR and pro-RPE layers) that eventually regenerate new functional NR and RPE, respectively.
It remains unknown how such a sophisticated mechanism for retinal regeneration evolved in the newt. It would be difficult to understand this mechanism solely by the mechanisms underlying RPE-to-NR transdifferentiation which can be induced in a restricted time frame during embryonic development. In these cases, the RPE does not lose its epithelial characteristics, but directly switches into the neuroepithelium, giving rise to the NR while losing the RPE. On the other hand, our recent studies revealed a similarity in early behaviour of RPE cells between adult newt retinal regeneration and human retinal disorders such as proliferative vitreoretinopathy (PVR). In PVR, when the NR suffers a wound from a traumatic injury, RPE cells - as in the newt - start to lose their epithelial characteristics while acquiring the ability to migrate and proliferate. However, unlike in the newt, these cells eventually transform into myofibroblasts, a major component of both the epi- and sub-retinal membranes which close the wound of the NR, but finally withdraw the NR by contraction, leading to a loss of vision. In this process of transformation (classified as the epithelial-mesenchymal transition, or EMT), it has been suggested that RPE cells pass through a multipotent state. Such multipotent RPE cells in humans, which were also named RPESCs, are regarded as the cells corresponding to the newt RPESCs.
Perhaps, in the newt, something may have happened in early processes of retinal disorders like PVR during evolution, so that the fate of RPESCs was directed toward retinal regeneration. If this is the case, when retinal regeneration in the newt is impaired in early processes, symptoms of PVR would become apparent. In this study, we examined this hypothesis. For this, we created for the first time a transgenic newt enabling RPE-targeted gene regulation and successfully hindered retinal regeneration by knocking down the expression of Pax6 in RPESCs.
It would be difficult to understand by comparison with eye development how normal reprogramming by Pax6 prevents RPE cells from transforming into myofibroblast-like cells. This issue would be of mature RPE cells. Pax6 may function even as a key factor that revises a default program which is booted in mature RPE cells after retinal injury and leads them to EMT. It must be noted here that Pax6 may be expressed in human RPESCs which give rise to myofibroblast cells in PVR. In the newt, something might have happened in RPE cells during evolution so that Pax6 can work properly for retinal regeneration while inhibiting EMT. Further understanding of how Pax6 works in reprogramming RPE cells in the adult newt in comparison with the homologous system in humans is necessary not only to uncover the changes that occurred in the newt during evolution but also to unlock the potency of in vivo retinal regeneration from RPE cells in humans. These findings would lead, in the future, to a novel clinical treatment of RPE-mediated retinal disorders that inhibits the EMT of RPE cells while promoting retinal regeneration in the eyes of patients.