A decay of gap junctions associated with ganglion cell differentiation during retinal regeneration of the adult newt

MEK-ERK and heparin-susceptible signaling pathways are involved in cell-cycle entry of the wound edge retinal pigment epithelium cells in the adult newt

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.

MEK mediates in vitro neural transdifferentiation of the adult newt retinal pigment epithelium cells: Is FGF2 an induction factor?

Adult newts can regenerate their entire retinas through transdifferentiation of the retinal pigment epithelium (RPE) cells. As yet, however, underlying molecular mechanisms remain virtually unknown. On the other hand, in embryonic/larval vertebrates, an MEK [mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase] pathway activated by fibroblast growth factor-2 (FGF2) is suggested to be involved in the induction of transdifferentiation of the RPE into a neural retina. Therefore, we examined using culture systems whether the FGF2/MEK pathway is also involved in the adult newt RPE transdifferentiation. Here we show that the adult newt RPE cells can switch to neural cells expressing pan-retinal-neuron (PRN) markers such as acetylated tubulin, and that an MEK pathway is essential for the induction of this process, whereas FGF2 seems an unlikely primary induction factor. In addition, we show by immunohistochemistry that the PRN markers are not expressed until the 1-3 cells thick regenerating retina, which contains retinal progenitor cells, appears. Our current results suggest that the activation of an MEK pathway in RPE cells might be involved in the induction process of retinal regeneration in the adult newt, however if this is the case, we must assume complementary mechanisms that repress the MEK-mediated misexpression of PRN markers in the initial process of transdifferentiation.

Visual cycle protein RPE65 persists in new retinal cells during retinal regeneration of adult newt

Adult newts can regenerate their entire retina through transdifferentiation of the retinal pigment epithelium (RPE). The objective of this study was to redescribe the retina regeneration process by means of modern biological techniques. We report two different antibodies (RPE-No.112 and MAB5428) that recognize the newt homolog of RPE65, which is involved in the visual cycle and exclusively label the RPE cell-layer in the adult newt eye. We analyzed the process of retinal regeneration by immunohistochemistry and immunoblotting and propose that this process should be divided into nine stages. We found that the RPE65 protein is present in the RPE-derived new retinal rudiment at 14 days postoperative (po) and in the regenerating retinas at the 3-4 cell stage (19 days po). These observations suggest that certain characteristics of RPE cells overlap with those of retinal stem/progenitor cells during the period of transdifferentiation. However, RPE65 protein was not detected in either retinal stem/progenitor cells in the ciliary marginal zone (CMZ) of adult eyes or in neuroepithelium present during retina development, where it was first detected in differentiated RPE. Moreover, the gene expression of RPE65 was drastically downregulated in the early phase of transdifferentiation (by 10 days po), while those of Connexin43 and Pax-6, both expressed in regenerating retinas, were differently upregulated. These observations suggest that the RPE65 protein in the RPE-derived retinal rudiment may represent the remainder after protein degradation or discharge rather than newly synthesized protein.

Changes in somatic sodium currents of ganglion cells during retinal regeneration in the adult newt

Adult newts can regenerate their entire retinas following a complete removal of the original tissues. During retinal regeneration, ganglion cells differentiate first from the progenitor cells, and develop their capability of spike firing. In the present study, to understand the process of functional differentiation of ganglion cells, we investigated alterations of their voltage-gated sodium currents during retinal regeneration by a whole-cell patch-clamp technique. To minimize space clamp errors, sodium currents were recorded from neurite-free somata of presumptive ganglion cells that were mechanically isolated from living slices of regenerating retinas at different morphological stages. During retinal regeneration, the somatic sodium current density was increased 2.6-fold (48 to 123 pF/pA) and the half-activating voltage was shifted slightly to more hyperpolarizing membrane potentials (-10 to -13 mV), while steady-state inactivation was not changed obviously. Curve fitting analysis of currents revealed that the sodium current consists of two components with different inactivation time constants. During retinal regeneration, the ratio of slow to fast inactivating current component was increased 2.6-fold (0.11 to 0.29). These results suggest that the somatic sodium currents of ganglion cells may undergo modifications of their voltage dependence and kinetic properties during retinal regeneration. A small number of the presumptive ganglion cells in regenerating retinas with a segregating inner plexiform layer exhibited sodium currents comparable to those in the normal retina. This might suggest that maturational regulation of sodium channel function starts during a period of synaptic layer formation within the retina.

Intraocular implantation of DNA-transfected retinal pigment epithelium cells: a new approach for analyzing molecular functions in the newt retinal regeneration

Adult newts can regenerate their entire retinas, even after surgical removal of the neural retina (retinectomy), through transdifferentiation of the retinal pigment epithelium (RPE) cells. To develop a new experimental system for analyzing molecular functions during retinal regeneration of adult newts, we attempted to deliver a foreign gene into RPE cells of retina-less eye-cups in vitro. Here we used pCS2mt-GFP as a reporter construct, and selected Polyfect as a transfection reagent. DNA-transfection appeared to be restricted to the RPE cells of retina-less eye-cups and its efficiency was 0.1-0.2%. We tried to implant RPE-choroid tissue containing DNA-transfected RPE cells into the eye of a host animal. The tissue was placed into the posterior eye-chamber immediately after retinectomy so that the implanted RPE tissue was facing the cornea (i.e., normal orientation). The implant and host RPE regenerated one continuous hybrid neural retina. Ocular sections after 60 days of implantation showed that a small number of cells in the regenerating retina were intensely stained with an anti-GFP antibody. Some of those cells were believed to be retinal cells such as ganglion cells, amacrine cells and photoreceptors. The GFP-positive cells in the hybrid regenerating retina could represent clones derived from a single RPE cell. These results indicate that this experimental system could become useful in the study of adult newt retinal regeneration.