Opsin expression in adult, developing, and regenerating newt retinas

A frog's eye view: Foundational revelations and future promises

From the mid-19th century until the 1980's, frogs and toads provided important research models for many fundamental questions in visual neuroscience. In the present century, they have been largely neglected. Yet they are animals with highly developed vision, a complex retina built on the basic vertebrate plan, an accessible brain, and an experimentally useful behavioural repertoire. They also offer a rich diversity of species and life histories on a reasonably restricted physiological and evolutionary background. We suggest that important insights may be gained from revisiting classical questions in anurans with state-of-the-art methods. At the input to the system, this especially concerns the molecular evolution of visual pigments and photoreceptors, at the output, the relation between retinal signals, brain processing and behavioural decision-making.

ITRAQ-based quantitative proteomic analysis of Cynops orientalis limb regeneration

Background

Salamanders regenerate their limbs after amputation. However, the molecular mechanism of this unique regeneration remains unclear. In this study, isobaric tags for relative and absolute quantification (iTRAQ) coupled with liquid chromatography tandem mass spectrometry (LC-MS/MS) was employed to quantitatively identify differentially expressed proteins in regenerating limbs 3, 7, 14, 30 and 42 days post amputation (dpa).

Results

Of 2636 proteins detected in total, 253 proteins were differentially expressed during different regeneration stages. Among these proteins, Asporin, Cadherin-13, Keratin, Collagen alpha-1(XI) and Titin were down-regulated. CAPG, Coronin-1A, AnnexinA1, Cathepsin B were up-regulated compared with the control. The identified proteins were further analyzed to obtain information about their expression patterns and functions in limb regeneration. Functional analysis indicated that the differentially expressed proteins were associated with wound healing, immune response, cellular process, metabolism and binding.

Conclusions

This work indicated that significant proteome alternations occurred during salamander limb regeneration. The results may provide fundamental knowledge to understand the mechanism of limb regeneration.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4125-4) contains supplementary material, which is available to authorized users.

Neural circuit analysis of axons regenerated by facial-hypoglossal nerve cross-link surgery

Introduction

Several methods of nerve reconstruction for facial nerve palsy are known. Although the recently introduced method of “cross-linking” of the facial and hypoglossal nerves with a grafted nerve has proved efficacious, the underlying mechanism is unclear.

Methods

In this study, we created an animal model with Wistar rats and analyzed the newly reconstructed neural circuit by anterograde and retrograde neural tracer methods. The saphenous nerve was harvested as a graft, and its double end-to-side neurorrhaphy with the facial and hypoglossal nerves with epineural windows was carried out under the microscope. After an appropriate interval, small amounts of fluoro-ruby or fluoro-emerald were injected into the animals and analyzed 5 days later by fluorescent microscopy (Anterograde experiment: fluoro-ruby into the hypoglossal nucleus at 5 weeks; retrograde experiment: fluoro-ruby into the distal facial nerve sheath and fluoro-emerald into the distal hypoglossal nerve sheath, both at two months.).

Results

The labeled axons derived from the hypoglossal nucleus were observed passing through the grafted nerve to the facial nerve. On the other hand, retrogradely labeled neurons were observed at both the hypoglossal and facial nuclei with some double-labeled neurons, suggesting that collateral sprouting had occurred.

Conclusions

We suggest that the newly constructed neural circuits we observed are conducive to the treatment of facial nerve palsy.

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We studied the mechanism of nerve crosslinking for ameliorating facial nerve palsy.

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In a rat model, retrograde tracers demonstrated the crosslink connections.

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Axon collateralization into the crosslink was found.

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This may be therapeutic by increasing nerve supply to the damaged side.

Hematological- and Neurological-Expressed Sequence 1 Gene Products in Progenitor Cells during Newt Retinal Development

Urodele amphibians such as Japanese common newts have a remarkable ability to regenerate their injured neural retina, even as adults. We found that hematological- and neurological-expressed sequence 1 (Hn1) gene was induced in depigmented retinal pigment epithelial (RPE) cells, and its expression was maintained at later stages of newt retinal regeneration. In this study, we investigated the distribution of the HN1 protein, the product of the Hn1 gene, in the developing retinas. Our immunohistochemical analyses suggested that the HN1 protein was highly expressed in an immature retina, and the subcellular localization changed during this retinogenesis as observed in newt retinal regeneration. We also found that the expression of Hn1 gene was not induced in mouse after retinal removal. Our results showed that Hn1 gene can be useful for detection of undifferentiated and dedifferentiated cells during both newt retinal development and regeneration.

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.

Identification and characterization of visual pigments in caecilians (Amphibia: Gymnophiona), an order of limbless vertebrates with rudimentary eyes

In comparison with the other amphibian orders, the Anura (frogs) and Urodela (salamanders), knowledge of the visual system of the snake-like Gymnophiona (caecilians) is relatively sparse. Most caecilians are fossorial with, as far as is known any surface activity occurring mainly at night. They have relatively small, poorly developed eyes and might be expected to possess detectable changes in the spectral sensitivity of their visual pigments. Microspectrophotometry was used to determine the spectral sensitivities of the photoreceptors in three species of caecilian, Rhinatrema bivittatum, Geotrypetes seraphini and Typhlonectes natans. Only rod opsin visual pigment, which may be associated with scotopic (dim light) vision when accompanied by other 'rod-specific' components of the phototransduction cascade, was found to be present. Opsin sequences were obtained from the eyes of two species of caecilian, Ichthyophis cf. kohtaoensis and T. natans. These rod opsins were regenerated in vitro with 11-cis retinal to give pigments with spectral sensitivity peaks close to 500 nm. No evidence for cone photoreception, associated with diurnal and colour vision, was detected using molecular and physiological methods. Additionally, visual pigments are short-wavelength shifted in terms of the maximum absorption of light when compared with other amphibian lineages.

Expression of Ca-binding protein recoverin in normal, surviving, and regenerating retina of Pleurodeles waltl adult triton

Immunohistochemical study of the expression of recoverin (photoreceptor protein) in the retina of Pleurodeles waltl adult triton was carried out in health, during regeneration after removal, and under conditions of long-lasting detachment. Studies with polyclonal (monospecific) antibodies to recoverin showed that normally it is present in the internal segment, connective cilium, in distal portions of the external segments of cones and rods, and in Landolt clubs of displaced bipolar cells. Detachment of the retina is associated with translocation of recoverin from the photoreceptor processes to perikaryons, and the content of recoverin-positive displaced bipolar cells increases. During regeneration of the retina after its excision via conversion of the pigmented epithelial cells, recoverin is synthesized in the prospective photoreceptor perikaryons and then accumulates in the forming inner segments. Hence, recoverin can serve as a reliable marker in studies of photoreceptor differentiation and functioning during regeneration or survival of the retina.

Musashi-1, an RNA-binding protein, is indispensable for survival of photoreceptors

Musashi-1 (Msi1), an RNA-binding protein (RBP), has been postulated to play important roles in the maintenance of the stem-cell state, differentiation, and tumorigenesis. However, the expression and function of Msi1 in differentiated cells remain obscure. Here we show that Msi1 is expressed in mature photoreceptors and retinal pigment epithelium (RPE) cells, and is indispensable for the survival of photoreceptors. We found in the adult newt eye that Msi1 is expressed in all photoreceptors and RPE cells as well as in the retinal stem/progenitor cells in the ciliary marginal zone (CMZ). We found in the analyses of the newt normal and regenerating retinas that the expression profiles of the Msi1 transcripts and protein isoforms in the photoreceptors are different from those in the retinal stem/progenitor cells. Furthermore, we found that all photoreceptors and RPE cells of the adult mice also express Msi1, and that Msi1 knockout (Msi1-KO) results in degeneration of photoreceptors and a lack of a visual cycle protein RPE65 in the microvilli of RPE cells. Taken together, our current results demonstrate that the expression of Msi1 in mature photoreceptors and RPE cells is evolutionarily conserved, and that Msi1 bears essential functions for vision. Considering such an Msi1-KO phenotype in the retina, it is now reasonable to address whether defects of the Msi1 functions are responsible for inherited retinal diseases. Studying the regulation of Msi1 and the target RNAs of Msi1 in photoreceptors and RPE cells might contribute to fundamental and clinical studies of retinal degeneration.

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.

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.

Induced expression of hematopoietic- and neurologic-expressed sequence 1 in retinal pigment epithelial cells during newt retina regeneration

Newts can regenerate their organs even as adults. For instance, when their neural retinas are completely removed by operation, the remaining retinal pigment epithelial (RPE) cells dedifferentiate to reconstruct neural retinas. To elucidate the molecular mechanisms of newt retina regeneration, we investigated genes upregulated in dedifferentiating RPE cells using differential display methods. We observed that a cDNA fragment of hematopoietic- and neurologic-expressed sequence 1 (Hn1) appeared to be induced within a few days of surgical removal of newt neural retina. Using an anti-HN1 antiserum against the recombinant HN1 protein, we carried out immunohistochemical analyses. The anti-HN1 antiserum recognized the plexiform layers and ganglion cell layer (GCL) but not the RPE cell layer in unoperated (normal) newt retinas. Using a glial fibrillary acidic protein antibody, Hn1 was shown to be possibly expressed in glial cells in normal neural retina. During retina regeneration, immunoreactivity for HN1 appeared in dedifferentiating RPE cells 10 days post-operation, and in retinal progenitor cells 18 days post-operation. Twenty seven days post-operation, HN1 immunoreactivity was localized in the plexiform layers and GCL as in the normal retina. Therefore, HN1 possibly plays an undefined role in dedifferentiating RPE cells and retinal progenitor cells during newt retina regeneration.

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.

Downregulation of Otx2 in the dedifferentiated RPE cells of regenerating newt retina

Cynops pyrrhogaster (the Japanese common newt) regenerates neural retina from retinal pigmented epithelium (RPE) cells. Otx2 is a transcription factor that is involved in RPE cell differentiation. To understand the role of Otx2 during transdifferentiation of RPE cells, we cloned a Cynops Otx2 cDNA, and explored its expression by RT-PCR, immunohistochemistry and in situ hybridization. The expression of Otx2 was compared with the localization of a proliferating cell marker (PCNA), RPE cell markers (RPE65, CRBP) and an RPE and Muller glial cell marker (CRALBP). At the early stage of regeneration, 2 to 3 cell layered regenerating retina consisting of pigmented cells uniformly expressed Otx2 and other markers. Following this stage, 4-cell layered regenerating retina consisted of two distinct layers, pigmented monolayer (the outer layer) attached to Bruch's membrane and presumptive neural retina (the inner layers). In the outer layer, Otx2 and CRBP expression was maintained and majority of cells lost PCNA expression. Some of cells maintained RPE65. In the inner layers, expression of Otx2, CRBP and RPE65 was downregulated, but a majority of those cells maintained PCNA expression. These results indicate that spatiotemporal regulation of Otx2 expression is consistent with those of RPE markers. Otx2 may play a pivotal role in maintenance and specification of RPE cells during neural retina regeneration. In contrast to RPE cell markers, CRALBP was expressed in both the pigmented and the de-pigmented layers. This observation implicates the appearance of Muller glial cells in an early phase of regenerating retina.

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.

Persistent and injury-induced neurogenesis in the vertebrate retina

The brains of all vertebrates are persistently neurogenic. However, this is not true for the neural retinas. Only three extant classes of vertebrates show significant posthatch/postnatal retinal neurogenesis: amphibians, birds and fish. The retinas of these animals contain an annulus of progenitors at the margin, from which differentiated neurons emerge. In posthatch amphibians and fish the vast majority of the adult retina is added from the margin and neurogenesis is lifelong, whereas in posthatch birds neurogenesis is limited. Unique to fish, rod photoreceptors are added in situ from stem cells within the mature retina. Strikingly, for each class of animal retinal lesions stimulate neuronal regeneration, however the cellular source differs for each: the retinal pigmented epithelium in amphibians and embryonic birds, Müller glia in posthatch birds and intrinsic stem cells in fish. The molecular events surrounding injury-induced neuronal regeneration are beginning to be identified.

pH changes in the invaginating synaptic cleft mediate feedback from horizontal cells to cone photoreceptors by modulating Ca2+ channels

Feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the center-surround–receptive field organization of retinal neurons. Recordings from cone photoreceptors in newt retinal slices were obtained by the whole-cell patch-clamp technique, using a superfusate containing a GABA antagonist (100 μM picrotoxin). Surround illumination of the receptive field increased the voltage-dependent calcium current (I_Ca) in the cones, and shifted the activation voltage of I_Ca to negative voltages. External alkalinization also increased cone I_Ca and shifted its activation voltage toward negative voltages. Enrichment of the pH buffering capacity of the extracellular solution increased cone I_Ca, and blocked any additional increase in cone I_Ca by surround illumination. Hyperpolarization of the HCs by a glutamate receptor antagonist-augmented cone I_Ca, whereas depolarization of the HCs by kainate suppressed cone I_Ca. From these results, we propose the hypothesis that pH changes in the synaptic clefts, which are intimately related to the membrane voltage of the HCs, mediate the feedback from the HCs to cone photoreceptors. The feedback mediated by pH changes in the synaptic cleft may serve as an additional mechanism for the center-surround organization of the receptive field in the outer retina.

Eye regeneration at the molecular age

Eye tissues such as the lens and the retina possess remarkable regenerative abilities. In amphibians, a complete lens can be regenerated after lentectomy. The process is a classic example of transdifferentiation of one cell type to another. Likewise, retina can be regenerated, but the strategy used to replace the damaged retina differs, depending on the animal system and the age of the animal. Retina can be regenerated by transdifferentiation or by the use of stem cells. In this review, we present a synthesis on the regenerative capacity of eye tissues in different animals with emphasis on the strategy and the molecules involved. In addition, we stress the place of this field at the molecular age and the importance of the recent technologic advances.