Spatial and temporal patterns of distribution of the gap junctional protein connexin43 during retinal regeneration of adult newt

Activation of Wnt signaling reduces ipsilaterally projecting retinal ganglion cells in pigmented retina

In mammalian albinism, disrupted melanogenesis in the retinal pigment epithelium (RPE) is associated with fewer retinal ganglion cells (RGCs) projecting ipsilaterally to the brain, resulting in numerous abnormalities in the retina and visual pathway, especially binocular vision. To further understand the molecular link between disrupted RPE and a reduced ipsilateral RGC projection in albinism, we compared gene expression in the embryonic albino and pigmented mouse RPE. We found that the Wnt pathway, which directs peripheral retinal differentiation and, generally, cell proliferation, is dysregulated in the albino RPE. Wnt2b expression is expanded in the albino RPE compared with the pigmented RPE, and the expanded region adjoins the site of ipsilateral RGC neurogenesis and settling. Pharmacological activation of Wnt signaling in pigmented mice by lithium (Li⁺) treatment in vivo reduces the number of Zic2-positive RGCs, which are normally fated to project ipsilaterally, to numbers observed in the albino retina. These results implicate Wnt signaling from the RPE to neural retina as a potential factor in the regulation of ipsilateral RGC production, and thus the albino phenotype.

Gap junctional signaling in pattern regulation: Physiological network connectivity instructs growth and form

Gap junctions (GJs) are aqueous channels that allow cells to communicate via physiological signals directly. The role of gap junctional connectivity in determining single-cell functions has long been recognized. However, GJs have another important role: the regulation of large-scale anatomical pattern. GJs are not only versatile computational elements that allow cells to control which small molecule signals they receive and emit, but also establish connectivity patterns within large groups of cells. By dynamically regulating the topology of bioelectric networks in vivo, GJs underlie the ability of many tissues to implement complex morphogenesis. Here, a review of recent data on patterning roles of GJs in growth of the zebrafish fin, the establishment of left-right patterning, the developmental dysregulation known as cancer, and the control of large-scale head-tail polarity, and head shape in planarian regeneration has been reported. A perspective in which GJs are not only molecular features functioning in single cells, but also enable global neural-like dynamics in non-neural somatic tissues has been proposed. This view suggests a rich program of future work which capitalizes on the rapid advances in the biophysics of GJs to exploit GJ-mediated global dynamics for applications in birth defects, regenerative medicine, and morphogenetic bioengineering. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 643-673, 2017.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Physiological controls of large-scale patterning in planarian regeneration: a molecular and computational perspective on growth and form

Planaria are complex metazoans that repair damage to their bodies and cease remodeling when a correct anatomy has been achieved. This model system offers a unique opportunity to understand how large-scale anatomical homeostasis emerges from the activities of individual cells. Much progress has been made on the molecular genetics of stem cell activity in planaria. However, recent data also indicate that the global pattern is regulated by physiological circuits composed of ionic and neurotransmitter signaling. Here, we overview the multi-scale problem of understanding pattern regulation in planaria, with specific focus on bioelectric signaling via ion channels and gap junctions (electrical synapses), and computational efforts to extract explanatory models from functional and molecular data on regeneration. We present a perspective that interprets results in this fascinating field using concepts from dynamical systems theory and computational neuroscience. Serving as a tractable nexus between genetic, physiological, and computational approaches to pattern regulation, planarian pattern homeostasis harbors many deep insights for regenerative medicine, evolutionary biology, and engineering.

Hyperthermia induces upregulation of connexin43 in the golden hamster neural tube

BACKGROUND

During early embryonic development, maternal exposure to hyperthermia induces neural tube defects (NTDs). Connexins are essential for the formation of gap junctions and Connexin43 (Cx43) is crucially involved in neural tube development. This study was designed to explore the potential role of Cx43 in NTDs induced by hyperthermia.

METHODS

Using PCR, the Cx43 cDNA was screened from the cDNA library of the neural tube from golden hamsters treated with hyperthermia. By Northern blot, the expression of Cx43 in heat-treated and control groups of the golden hamsters at day 8.5 after mating was detected. Finally, by in situ hybridization and RT-PCR, the expression of Cx43 was examined in the neural tube at different time points after heat treatment.

RESULTS

Cx43 was stably expressed in heat-treated and control groups of the golden hamsters, whereas the expression was evidently higher in the heat-treated group. Cx43 expression in the neural tube at different time points after heat treatment was significantly higher than in control groups (p < 0.01). Hyperthermia did not induce any mutations in Cx43 cDNA.

CONCLUSIONS

Our data provide the first evidence that hyperthermia induces upregulation of Cx43 in the golden hamster neural tube. NTDs caused by hyperthermia may be intimately related with the overexpression of Cx43.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

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.

Regeneration of the amphibian retina: role of tissue interaction and related signaling molecules on RPE transdifferentiation

Regeneration of eye tissue is one of the classic subjects in developmental biology and it is now being vigorously studied to reveal the cellular and molecular mechanisms involved. Although many experimental animal models have been studied, there may be a common basic mechanism that governs retinal regeneration. This can also control ocular development, suggesting the existence of a common principle between the development and regeneration of eye tissues. This notion is now becoming more widely accepted by recent studies on the genetic regulation of ocular development. Retinal regeneration can take place in a variety of vertebrates including fish, amphibians and birds. The newt, however, has been considered to be the sole animal that can regenerate the whole retina after the complete removal of the retina. We recently discovered that the anuran amphibian also retains a similar ability in the mature stage, suggesting the possibility that such a potential could be found in other animal species. In the present review article, retinal regeneration of amphibians (the newt and Xenopus laevis) and avian embryos are described, with a particular focus on transdifferentiation of retinal pigmented epithelium. One of the recent progresses in this field is the availability of tissue culture methods to analyze the initial process of transdifferentiation, and this enables us to compare the proliferation and neural differentiation of retinal pigmented epithelial cells from various animal species under the same conditions. It was revealed that tissue interactions between the retinal pigmented epithelium and underlying connective tissues (the choroid) play a substantial role in transdifferentiation and that this is mediated by a diffusible signal such as fibroblast growth factor 2. We propose that tissue interaction, particularly mesenchyme-neuroepithelial interaction, is considered to play a fundamental role both in retinal development and regeneration.

Evidence for Notch signaling involvement in retinal regeneration of adult newt

Involvement of Notch signaling in retinal regeneration by transdifferentiation of pigment epithelium cells was investigated using the adult newt Cynops pyrrhogaster. During retinal regeneration, cells expressing Notch-1 first appeared in the regenerating retina one to two cells thick (stage E-3) originated from the retinal pigment epithelium (RPE) cells, and increased in number as the regenerating retina increased in thickness. Notch-1 expression was decreased in the central retina in association with cell differentiation and became restricted to the peripheral retina. Administration of a Notch signaling blocker DAPT resulted in the appearance of a cluster of neurons, earlier than in normal regeneration, along the regenerating retina 1-3 cells thick (stage E-3 to I-1). Immunoblot analysis suggested that DAPT could perturb the processing of Notch-1. Similar results were obtained in the newt embryonic retinal development. These results suggest that the Notch-1 signaling system may be reset to regulate neurogenesis during retinal regeneration. However, PCR analysis revealed that the adult newt RPE cells express Hes-1, neurogenin1 and sometimes Delta-1 Hes-1, neurogenin1 and sometimes Delta-1 all of which are differently regulated in association with retinal regeneration, implying that Notch signaling might also be involved early in the process of transdifferentiation.

Cellular pattern formation during retinal regeneration: a role for homotypic control of cell fate acquisition

A dominant mechanism of cellular patterning in the growing fish retina is control of cell fate acquisition by negative feedback signals arising from differentiated cells. We tested the ability of a computational model of this pattern formation mechanism to simulate cellular patterns in regenerated goldfish retina. The model successfully simulated quantitative features of in vivo regenerated patterns, indicating that regenerating retina has access to and utilizes patterning mechanisms that are operational during normal growth. The atypical patterns of regenerated retina could arise in part from regenerative progenitors that, compared to normal growth progenitors, are less responsive to the feedback patterning signals.

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.

Characterization of innexin gene expression and functional roles of gap-junctional communication in planarian regeneration

Planaria possess remarkable powers of regeneration. After bisection, one blastema regenerates a head, while the other forms a tail. The ability of previously-adjacent cells to adopt radically different fates could be due to long-range signaling allowing determination of position relative to, and the identity of, remaining tissue. However, this process is not understood at the molecular level. Following the hypothesis that gap-junctional communication (GJC) may underlie this signaling, we cloned and characterized the expression of the Innexin gene family during planarian regeneration. Planarian innexins fall into 3 groups according to both sequence and expression. The concordance between expression-based and phylogenetic grouping suggests diversification of 3 ancestral innexin genes into the large family of planarian innexins. Innexin expression was detected throughout the animal, as well as specifically in regeneration blastemas, consistent with a role in long-range signaling relevant to specification of blastema positional identity. Exposure to a GJC-blocking reagent which does not distinguish among gap junctions composed of different Innexin proteins (is not subject to compensation or redundancy) often resulted in bipolar (2-headed) animals. Taken together, the expression data and the respecification of the posterior blastema to an anteriorized fate by GJC loss-of-function suggest that innexin-based GJC mediates instructive signaling during regeneration.

GABAA-receptor-mediated increase in intracellular Ca2+ concentration in the regenerating retina of adult newt

We used optical recording with the Ca(2+)-sensitive dye, fura-2, in living slice preparations from the newt retina at different stages of regeneration. gamma-Aminobutyric acid (GABA) induced pronounced [Ca(2+)](i) rise in progenitor cells and differentiating ganglion cells in the 'intermediate' stage of retinal regeneration. This [Ca(2+)](i) rise became less pronounced at the beginning of synapse formation in the late regenerating retina. At the late period of the late regenerating retina with the IPL thickness comparable to that of the control retina, GABA-induced [Ca(2+)](i) rise became undetectable or sometimes a small decrease in [Ca(2+)](i) was observed in regenerated ganglion cells. In contrast, N-methyl-d-aspartate (NMDA)-induced [Ca(2+)](i) rise appeared in premature ganglion cells and became prominent gradually as the regeneration proceeded. The [Ca(2+)](i) rise to GABA was mediated by GABA(A) receptors. This was shown by inhibition of GABA-induced Ca(2+) response with the preincubation of the GABA(A) receptor antagonist, bicuculline. The [Ca(2+)](i) rise due to GABA was suppressed in the absence of extracellular Ca(2+) or in the presence of the L-type voltage-gated Ca(2+) channel blocker, verapamil, suggesting that Ca(2+) may be entered through L-type Ca(2+) channels. Transient appearance of [Ca(2+)](i) rise to GABA during regeneration and origin of GABA-induced [Ca(2+)](i) rise were similar to those in the developing retina [J. Neurobiol. 24 (1993) 1600]. These similarities may suggest that common mechanisms may control neurogenesis and/or synaptogenesis during development and regeneration.

Ca2+ signalling and gap junction coupling within and between pigment epithelium and neural retina in the developing chick

Development of the neural retina is controlled in part by the adjacent retinal pigment epithelium (RPE). To understand better the mechanisms involved, we investigated calcium signalling and gap junctional coupling within and between the RPE and the neural retina in embryonic day (E) 5 chick. We show that the RPE and the ventricular zone (VZ) of the neural retina display spontaneous Ca(2+) transients. In the RPE, these often spread as waves between neighbouring cells. In the VZ, the frequency of both Ca(2+) transients and waves was lower than in RPE, but increased two-fold in its presence. Ca(2+) signals occasionally crossed the boundary between the RPE and VZ in either direction. In both tissues, the frequency of propagating Ca(2+) waves, but not of individual cell transients, was reduced by gap junction blockers. Use of the gap junction permeant tracer Neurobiotin showed that neural retina cells are coupled into clusters that span the thickness of the retina, and that RPE cells are both coupled together and to clusters of cells in the neural retina. Immunolabelling for Cx43 showed this gap junction protein is present at the junction between the RPE and VZ and thus could potentially mediate the coupling of the two tissues. Immunolabelling for beta-tubulin and vimentin showed that clusters of coupled cells in the neural retina comprised mainly progenitor cells. We conclude that gap junctions between progenitor cells, and between these cells and the RPE, may orchestrate retinal proliferation/differentiation, via the propagation of Ca(2+) or other signalling molecules.

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

Changes in the gap junctional coupling and maturation of voltage-activated Na(+) currents during regeneration of newt retinas were examined by whole-cell patch-clamping in slice preparations. Progenitor cells in regenerating retinas did not exhibit Na(+) currents but showed prominent electrical and tracer couplings. Cells identified by LY-fills were typically slender. Na(+) currents were detected in premature ganglion cells with round somata in the 'intermediate-II' regenerating retina. No electrical and tracer couplings were observed between these cells. Mature ganglion cells did not exhibit electrical coupling, but showed tracer coupling. On average, the maximum Na(+) current amplitude recorded from premature ganglion cells was roughly 2.5-fold smaller than that of mature ganglion cells. In addition, the activation threshold of the Na(+) current was nearly 11 mV more positive than that of mature cells. We provide morphological and physiological evidence showing that loss of gap junctions between progenitor cells is associated with ganglion cell differentiation during retinal regeneration and that new gap junctions are recreated between mature ganglion cells. Also we provide evidence suggesting that the loss of gap junctions correlates with the appearance of voltage-activated Na(+) currents in ganglion cells.

The immunocytochemical localization of connexin 36 at rod and cone gap junctions in the guinea pig retina

Connexin 36 (Cx36) is a channel-forming protein found in the membranes of apposed cells, forming the hexameric hemichannels of intercellular gap junction channels. It localizes to certain neurons in various regions of the brain including the retina. We characterized the expression pattern of neuronal Cx36 in the guinea pig retina by immunocytochemistry using specific antisera against Cx36 and green/red cone opsin or recoverin. Strong Cx36 immunoreactivity was visible in the ON sublamina of the inner plexiform layer and in the outer plexiform layer, as punctate labelling patterns. Double-labelling experiments with antibody directed against Cx36 and green/red cone opsin or recoverin showed that strong clustered Cx36 immunoreactivity localized to the axon terminals of cone or close to rod photoreceptors. By electron microscopy, Cx36 immunoreactivity was visible in the gap junctions as well as in the cytoplasmic matrices of both sides of cone photoreceptors. In the gap junctions between cone and rod photoreceptors, Cx36 immunoreactivity was only visible in the cytoplasmic matrices of cone photoreceptors. These results clearly indicate that Cx36 forms homologous gap junctions between neighbouring cone photoreceptors, and forms heterologous gap junctions between cone and rod photoreceptors in guinea pig retina. This focal location of Cx36 at the terminals of the photoreceptor suggests that rod photoreceptors can transmit rod signals to the pedicle of a neighbouring cone photoreceptor via Cx36, and that the cone in turn signals to corresponding ganglion cells via ON and OFF cone bipolar cells.