Regulation of cell division and rod differentiation in the teleost retina

Identification, Characterization and Functional Analysis of Fibroblast Growth Factors in Black Rockfish (Sebastes schlegelii)

Fibroblast growth factors (FGFs) are short polypeptides that play essential roles in various cellular biological processes, including cell migration, proliferation, and differentiation, as well as tissue regeneration, immune response, and organogenesis. However, studies focusing on the characterization and function of FGF genes in teleost fishes are still limited. In this study, we identified and characterized expression patterns of 24 FGF genes in various tissues of embryonic and adult specimens of the black rockfish (Sebates schlegelii). Nine FGF genes were found to play essential roles in myoblast differentiation, as well as muscle development and recovery in juvelines of S. schlegelii. Moreover, sex-biased expression pattern of multiple FGF genes was recorded in the species' gonads during its development. Among them, expression of the FGF1 gene was recorded in interstitial and sertoli cells of testes, promoting germ-cell proliferation and differentiation. In sum, the obtained results enabled systematic and functional characterization of FGF genes in S. schlegelii, laying a foundation for further studies on FGF genes in other large teleost fishes.

The Last Half Century of Fish Explant and Organ Culture

Explants are three-dimensional tissue fragments maintained outside the organism. The goals of this article are to review the history of fish explant culture and discuss applications of this technique that may assist the modern zebrafish laboratory. Because most zebrafish workers do not have a background in tissue culture, the key variables of this method are deliberately explained in a general way. This is followed by a review of fish-specific explantation approaches, including presurgical husbandry, aseptic dissection technique, choice of media and additives, incubation conditions, viability assays, and imaging studies. Relevant articles since 1970 are organized in a table grouped by organ system. From these, I highlight several recent studies using explant culture to study physiological and embryological processes in teleosts, including circadian rhythms, hormonal regulation, and cardiac development.

Limitations and Promise of Retinal Tissue From Human Pluripotent Stem Cells for Developing Therapies of Blindness

The self-formation of retinal tissue from pluripotent stem cells generated a tremendous promise for developing new therapies of retinal degenerative diseases, which previously seemed unattainable. Together with use of induced pluripotent stem cells or/and CRISPR-based recombineering the retinal organoid technology provided an avenue for developing models of human retinal degenerative diseases "in a dish" for studying the pathology, delineating the mechanisms and also establishing a platform for large-scale drug screening. At the same time, retinal organoids, highly resembling developing human fetal retinal tissue, are viewed as source of multipotential retinal progenitors, young photoreceptors and just the whole retinal tissue, which may be transplanted into the subretinal space with a goal of replacing patient's degenerated retina with a new retinal "patch." Both approaches (transplantation and modeling/drug screening) were projected when Yoshiki Sasai demonstrated the feasibility of deriving mammalian retinal tissue from pluripotent stem cells, and generated a lot of excitement. With further work and testing of both approaches in vitro and in vivo, a major implicit limitation has become apparent pretty quickly: the absence of the uniform layer of Retinal Pigment Epithelium (RPE) cells, which is normally present in mammalian retina, surrounds photoreceptor layer and develops and matures first. The RPE layer polarize into apical and basal sides during development and establish microvilli on the apical side, interacting with photoreceptors, nurturing photoreceptor outer segments and participating in the visual cycle by recycling 11-trans retinal (bleached pigment) back to 11-cis retinal. Retinal organoids, however, either do not have RPE layer or carry patches of RPE mostly on one side, thus directly exposing most photoreceptors in the developing organoids to neural medium. Recreation of the critical retinal niche between the apical RPE and photoreceptors, where many retinal disease mechanisms originate, is so far unattainable, imposes clear limitations on both modeling/drug screening and transplantation approaches and is a focus of investigation in many labs. Here we dissect different retinal degenerative diseases and analyze how and where retinal organoid technology can contribute the most to developing therapies even with a current limitation and absence of long and functional outer segments, supported by RPE.

Glial Cells in the Fish Retinal Nerve Fiber Layer Form Tight Junctions, Separating and Surrounding Axons

In the retina of teleost fish, cell addition continues throughout life involving proliferation and axonal growth. To study how this is achieved in a fully functioning retina, we investigated the nerve fiber layer (NFL) of the cichlid fish Astatotilapia burtoni for components that might regulate the extracellular environment. We hypothesized that growing axons are surrounded by different cell structures than signal conducting axons. Using immunohistochemistry and freeze fracture electron microscopy we found that the endfeet of Müller cells (MCs) expressed aquaporin-4 but not in high densities as in mammals. The presence of this water channel indicates the involvement of MCs in water homeostasis. Remarkably, we discovered conspicuous tight junctions in the retinal NFL. These tight junctions formed branching strands between myelin-like wrappings of ganglion cell axons that differed morphologically from any known myelin, and also an elaborate meshwork on large membrane faces between axons. We speculated that these tight junctions have additional functions than solely facilitating nerve conductance. Immunostainings against the adaptor protein ZO-1 labeled the NFL as did antibodies against the mammalian claudin-1, 3, and 19. Performing PCR analysis, we showed expression of claudin-1, 3, 5a, 5b, 9, 11, and 19 in the fish retina, claudins that typically occur at brain barriers or myelin. We could show by immunostains for doublecortin, a marker for differentiating neurons, that new axons are not surrounded by the myelin-like wrappings but only by the endfeet of MCs. We hypothesize that the tight junctions in the NFL of fish might contribute to the separation of an extracellular space around axons facilitating conductance, from a growth-promoting environment. For a functional test we applied Evans Blue dye to eye cup preparations which showed a retention of the dye in the NFL. This indicates that these remarkable tight junctions can indeed act as a diffusion barrier.

Photo-regulation of rod precursor cell proliferation

Teleosts are unique in their ability to undergo persistent neurogenesis and to regenerate damaged and lost retinal neurons in adults. This contrasts with the human retina, which is incapable of replacing lost retinal neurons causing vision loss/blindness in the affected individuals. Two cell populations within the adult teleost retina generate new retinal neurons throughout life. Stem cells within the ciliary marginal zone give rise to all retinal cell types except for rod photoreceptors, which are produced by the resident Müller glia that are located within the inner nuclear layer of the entire retina. Understanding the mechanisms that regulate the generation of photoreceptors in the adult teleost retina may ultimately aid developing strategies to overcome vision loss in diseases such as retinitis pigmentosa. Here, we investigated whether photic deprivation alters the proliferative capacity of rod precursor cells, which are generated from Müller glia. In dark-adapted retinas, rod precursor cell proliferation increased, while the number of proliferating Müller glia and their derived olig2:EGFP-positive neuronal progenitor cells was not significantly changed. Cell death of rod photoreceptors was excluded as the inducer of rod precursor cell proliferation, as the number of TUNEL-positive cells and l-plastin-positive microglia in both the outer (ONL) and inner nuclear layer (INL) remained at a similar level throughout the dark-adaptation timecourse. Rod precursor cell proliferation in response to dark-adaptation was characterized by an increased number of EdU-positive cells, i.e. cells that were undergoing DNA replication. These proliferating rod precursor cells in dark-adapted zebrafish differentiated into rod photoreceptors at a comparable percentage and in a similar time frame as those maintained under standard light conditions suggesting that the cell cycle did not stall in dark-adapted retinas. Inhibition of IGF1-receptor signaling reduced the dark-adaptation-mediated proliferation response; however, caloric restriction which has been suggested to be integrated by the IGF1/growth hormone signaling axis did not influence rod precursor cell proliferation in dark-adapted retinas, as similar numbers were observed in starved and normal fed zebrafish. In summary, photic deprivation induces cell cycle entry of rod precursor cells via IGF1-receptor signaling independent of Müller glia proliferation.

Rhythmic expressed clock regulates the transcription of proliferating cellular nuclear antigen in teleost retina

Teleost fish continues to grow their eyes throughout life with the body size. In Astatotilapia burtoni, the fish retina increases by adding new retinal cells at the ciliary marginal zone (CMZ) and in the outer nuclear layer (ONL). Cell proliferation at both sites exhibits a daily rhythm in number of dividing cells. To understand how this diurnal rhythm of new cell production is controlled in retinal progenitor cells, we studied the transcription pattern of clock genes in retina, including clock1a, clock1b, bmal1a (brain and muscle ARNT-Like), and per1b (period1b). We found that these genes have a strong diurnal rhythmic transcription during light-dark cycles but not in constant darkness. An oscillation in pcna transcription was also observed during light-dark cycles, but again not in constant darkness. Our results also indicate an association between Clock proteins and the upstream region of pcna (proliferating cellular nuclear antigen) gene. A luciferase reporter assay conducted in an inducible clock knockdown cell line further demonstrated that the mutation on predicted E-Boxes in pcna promoter region significantly attenuated the transcriptional activation induced by Clock protein. These results suggested that the diurnal rhythmic expression of clock genes in A. burtoni retina could be light dependent and might contribute to the daily regulation of the proliferation of the retina progenitors through key components of cell cycle machinery, for instance, pcna.

Development of the Vertebrate Eye and Retina

The mature, functional, and healthy eye is generated by the coordinated regulatory interaction of numerous and diverse developing tissues. The neural retina of the eye must undergo the neurogenesis of multiple retinal cell types in the correct ratios and spatial patterns. This chapter provides an overview of retinal development, and includes a summary of the process of eye organogenesis, a discussion of major principles of retinal neurogenesis, and describes some of the key molecular factors critical for retinal development. Defects in many of these factors underlie diseases of the eye, and an understanding of the process of retinal development will be critical for successful future applications of regenerative therapies for eye disease.

Müller glia: Stem cells for generation and regeneration of retinal neurons in teleost fish

Adult zebrafish generate new neurons in the brain and retina throughout life. Growth-related neurogenesis allows a vigorous regenerative response to damage, and fish can regenerate retinal neurons, including photoreceptors, and restore functional vision following photic, chemical, or mechanical destruction of the retina. Müller glial cells in fish function as radial-glial-like neural stem cells. During adult growth, Müller glial nuclei undergo sporadic, asymmetric, self-renewing mitotic divisions in the inner nuclear layer to generate a rod progenitor that migrates along the radial fiber of the Müller glia into the outer nuclear layer, proliferates, and differentiates exclusively into rod photoreceptors. When retinal neurons are destroyed, Müller glia in the immediate vicinity of the damage partially and transiently dedifferentiate, re-express retinal progenitor and stem cell markers, re-enter the cell cycle, undergo interkinetic nuclear migration (characteristic of neuroepithelial cells), and divide once in an asymmetric, self-renewing division to generate a retinal progenitor. This daughter cell proliferates rapidly to form a compact neurogenic cluster surrounding the Müller glia; these multipotent retinal progenitors then migrate along the radial fiber to the appropriate lamina to replace missing retinal neurons. Some aspects of the injury-response in fish Müller glia resemble gliosis as observed in mammals, and mammalian Müller glia exhibit some neurogenic properties, indicative of a latent ability to regenerate retinal neurons. Understanding the specific properties of fish Müller glia that facilitate their robust capacity to generate retinal neurons will inform and inspire new clinical approaches for treating blindness and visual loss with regenerative medicine.

The rod photoreceptor lineage of teleost fish

The retinas of postembryonic teleost fish continue to grow for the lifetime of the fish. New retinal cells are added continuously at the retinal margin, by stem cells residing at the circumferential germinal zone. Some of these retinal cells differentiate as Müller glia with cell bodies that reside within the inner nuclear layer. These glia retain some stem cell properties in that they carry out asymmetric cell divisions and continuously generate a population of transit-amplifying cells--the rod photoreceptor lineage--that are committed to rod photoreceptor neurogenesis. These rod progenitors progress through a stereotyped sequence of changes in gene expression as they continue to divide and migrate to the outer nuclear layer. Now referred to as rod precursors, they undergo terminal mitoses and then differentiate as rods, which are inserted into the existing array of rod and cone photoreceptors. The rod lineage displays developmental plasticity, as rod precursors can respond to the loss of rods through increased proliferation, resulting in rod replacement. The stem cells of the rod lineage, Müller glia, respond to acute damage of other retinal cell types by increasing their rate of proliferation. In addition, the Müller glia in an acutely damaged retina dedifferentiate and become multipotent, generating new, functional neurons. This review focuses on the cells of the rod lineage and includes discussions of experiments over the last 30 years that led to their identification and characterization, and the discovery of the stem cells residing at the apex of the lineage. The plasticity of cells of the rod lineage, their relationships to cone progenitors, and the applications of this information for developing future treatments for human retinal disorders will also be discussed.

Adult retinal stem cells revisited

Recent advances in retinal stem cell research have raised the possibility that these cells have the potential to be used to repair or regenerate diseased retina. Various cell sources for replacement of retinal neurons have been identified, including embryonic stem cells, the adult ciliary epithelium, adult Müller stem cells and induced pluripotent stem cells (iPS). However, the true stem cell nature of the ciliary epithelium and its possible application in cell therapies has now been questioned, leaving other cell sources to be carefully examined as potential candidates for such therapies. The need for identification of the ontogenetic state of grafted stem cells in order to achieve their successful integration into the murine retina has been recognized. However, it is not known whether the same requirements may apply to achieve transplant cell integration into the adult human eye. In addition, the existence of natural barriers for stem cell transplantation, including microglial accumulation and abnormal extracellular matrix deposition have been demonstrated, suggesting that several obstacles need to be overcome before such therapies may be implemented. This review addresses recent scientific developments in the field and discusses various strategies that may be potentially used to design cell based therapies to treat human retinal disease.

Genetic dissection reveals two separate pathways for rod and cone regeneration in the teleost retina

Development of therapies to treat visual system dystrophies resulting from the degeneration of rod and cone photoreceptors may directly benefit from studies of animal models, such as the zebrafish, that display continuous retinal neurogenesis and the capacity for injury-induced regeneration. Previous studies of retinal regeneration in fish have been conducted on adult animals and have relied on methods that cause acute damage to both rods and cones, as well as other retinal cell types. We report here the use of a genetic approach to study progenitor cell responses to photoreceptor degeneration in the larval and adult zebrafish retina. We have compared the responses to selective rod or cone degeneration using, respectively, the XOPS-mCFP transgenic line and zebrafish with a null mutation in the pde6c gene. Notably, rod degeneration induces increased proliferation of progenitors in the outer nuclear layer (ONL) and is not associated with proliferation or reactive gliosis in the inner nuclear layer (INL). Molecular characterization of the rod progenitor cells demonstrated that they are committed to the rod photoreceptor fate while they are still mitotic. In contrast, cone degeneration induces both Müller cell proliferation and reactive gliosis, with little change in proliferation in the ONL. We found that in both lines, proliferative responses to photoreceptor degeneration can be observed as 7 days post fertilization (dpf). These two genetic models therefore offer new opportunities for investigating the molecular mechanisms of selective degeneration and regeneration of rods and cones.

The teleost retina as a model for developmental and regeneration biology

Retinal development in teleosts can broadly be divided into three epochs. The first is the specification of cellular domains in the larval forebrain that give rise to the retinal primordia and undergo early morphogenetic movements. The second is the neurogenic events within the retina proper-proliferation, cell fate determination, and pattern formation-that establish neuronal identities and form retinal laminae and cellular mosaics. The third, which is unique to teleosts and occurs in the functioning eye, is stretching of the retina and persistent neurogenesis that allows the growth of the retina to keep pace with the growth of the eye and other tissues. The first two events are rapid, complete by about 3 days postfertilization in the zebrafish embryo. The third is life-long and accounts for the bulk of retinal growth and the vast majority of adult retinal neurons. In addition, but clearly related to the retina's developmental history, lesions that kill retinal neurons elicit robust neuronal regeneration that originates from cells intrinsic to the retina. This paper reviews recent studies of retinal development in teleosts, focusing on those that shed light on the genetic and molecular regulation of retinal specification and morphogenesis in the embryo, retinal neurogenesis in larvae and adults, and injury-induced neuronal regeneration.

Survival, excitability, and transfection of retinal neurons in an organotypic culture of mature zebrafish retina

Over the last 20 years, the zebrafish has become an important model organism for research on retinal function and development. Many retinal diseases do not become apparent until the later stages of life. This means that it is important to be able to analyze (gene) function in the mature retina. To meet this need, we have established an organotypic culture system of mature wild-type zebrafish retinas in order to observe changes in retinal morphology. Furthermore, cell survival during culture has been monitored by determining apoptosis in the tissue. The viability and excitability of ganglion cells have been tested at various time points in vitro by patch-clamp recordings, and retinal functionality has been assessed by measuring light-triggered potentials at the ganglion cell site. Since neurogenesis is persistent in adult zebrafish retinas, we have also monitored proliferating cells during culture by tracking their bromodeoxyuridine uptake. Reverse genetic approaches for probing the function of adult zebrafish retinas are not yet available. We have therefore established a rapid and convenient protocol for delivering plasmid DNA or oligonucleotides by electroporation to the retinal tissue in vitro. The organotypic culture of adult zebrafish retinas presented here provides a reproducible and convenient method for investigating the function of drugs and genes in the retina under well-defined conditions in vitro.

Retinal stem cells

During the embryonic development of the eye, a group of founder cells in the optic vesicle gives rise to multipotent progenitor cells that generate all the neurons and the Müller glia of the mature retina. In most vertebrates, a small group of retinal stem cells persists at the margin of the retina, near the junction with the ciliary epithelium. In fish and amphibians, the retinal stem cells continue to produce progenitors throughout life, adding new retina to the periphery of the existing retina as the eye grows. In birds the new retinal addition is more limited, and it is absent in those mammals that have been analyzed. Nevertheless, cells from the retinal periphery and ciliary body of mammals can be isolated and grown in vitro for extended periods. Methods for the study of both embryonic progenitors and adult retinal stem cells in vitro and in vivo have led to a better understanding of retinal development, allowed for the screening of factors important in retinal growth and differentiation, and enabled the development of methods to direct stem and progenitor cells to specific fates. These methods may ultimately lead to the development of strategies for retinal repair.

Differential enhancement of neural and photoreceptor cell differentiation of cultured pineal cells by FGF-1, IGF-1, and EGF

There are several common features between the pineal organ and the lateral eye in their developmental and evolutionary aspects. The avian pineal is a photoendocrine organ that originates from the diencephalon roof and represents a transitional type between the photosensory organ of lower vertebrates and the endocrine gland of mammals. Previous cell culture studies have shown that embryonic avian pineal cells retain a wide spectrum of differentiative capacities, although little is known about the mechanisms involved in their fate determination. In the present study, we investigated the effects of various cell growth factors on the differentiation of photoreceptor and neural cell types using pineal cell cultures from quail embryos. The results show that IGF-1 promotes differentiation of rhodopsin-immunoreactive cells, but had no effect on neural cell differentiation. Simultaneous administration of EGF and IGF-1 further enhanced differentiation of rhodopsin-immunoreactive cells, although the mechanism of the synergistic effect is unknown. FGF-1 did not stimulate proliferation of neural progenitor cells, but intensively promoted and maintained expression of a neural cell phenotype. FGF-1 appeared to lead to the conversion from an epithelial (endocrinal) to a neuronal type. It also enhanced phenotypic expression of retinal ganglion cell markers but rather suppressed expression of an amacrine cell marker. These results indicate that growth factors are important regulatory cues for pineal cell differentiation and suggest that they play roles in determining the fate of the pineal organ and the eye. It can be speculated that the differences in environmental cues between the retina and pineal may result in the transition of the pineal primordium from a potentially ocular (retinal) organ to a photoendocrine organ.

Neurogenesis in the fish retina

The retinas of teleost fish have long been of interest to developmental neurobiologists for their persistent plasticity during growth, life history changes, and response to injury. Because the vertebrate retina is a highly conserved tissue, the study of persistent plasticity in teleosts has provided insights into mechanisms for postembryonic retinal neurogenesis in mammals. In addition, in the past 10 years there has been an explosion in the use of teleost fish-zebrafish (Danio rerio) in particular-to understand the mechanisms of embryonic retinal neurogenesis in a model vertebrate with genetic resources. This review summarizes the key features of teleost retinal neurogenesis that make it a productive and interesting experimental system, and focuses on the contributions to our knowledge of retinal neurogenesis that uniquely required or significantly benefited from the use of a fish model system.

Rhodopsin, Violet and Blue Opsin Expressions in the Chick Are Highly Dependent on Tissue and Serum Conditions

The molecular, cellular or tissue environment can influence the expression of genes and thereby regulate processes of tissue formation. Here we determined the tissue and serum dependence of the expression of all photopigments in the chick by a series of distinct retinal cell cultures, analyzed by RT-PCR using specific primers for all four opsins and rhodopsin followed by quantitative scanning of the respective gel bands. For comparison, we first determined expression of all opsins during normal chick retinogenesis, which began with red and violet opsins at E12, shortly followed by blue and green opsins and finally rhodopsin at E14. This period corresponds to the time of synaptogenesis in the inner retina. All cultures were started with 6-day-old dissociated retinal cells. Cells were kept at low or high cell density (called LoDens or HiDens), or they were reaggregated as retinal spheres, whereby all of them were raised at low (2%) or high serum (12%) levels (called LoSer or HiSer). In LoDens/HiSer cultures, expression of all opsins was weak. At HiDens/LoSer red and green opsin expression was strong, while rhodopsin and violet/blue remained low. In HiDens/HiSer cultures the expression of red and green was strong; rhodopsin was almost normal, while violet and green were low. In reaggregates at high serum the expression came closest to a normal retina, but violet and blue opsins were still below normal. At low serum, however, violet and blue were negligible and rhodopsin was low. This in vitro study shows that rhodopsin, followed by violet and blue opsin expressions is highly dependent on serum, cell density and tissue conditions, while red and green opsins are more autonomous.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

IGF-1 produced by cone photoreceptors regulates rod progenitor proliferation in the teleost retina

Teleost eyes grow throughout life by adding neurons and stretching extant tissue. New retinal neurons of all types are added at the ciliary margin and new rod photoreceptors are inserted throughout retina in the outer nuclear layer (ONL). New rod photoreceptors result from the division of progenitor cells located in the ONL amidst functioning rod photoreceptor cell nuclei, but it is not known how new rod addition is regulated. Previous experiments using an organotypic retinal slice preparation revealed that insulin-like growth factor 1 (IGF-1) up-regulates the division of the rod progenitor cells [Dev. Brain Res. 76 (1993) 183], but the site of IGF-1 action was unknown. Here, we show where in the retina IGF-1 is made, where IGF receptors are located, and we identify the role of IGF-1 in adult retinal rod neurogenesis with both gain-and loss-of-function experiments. We found that IGF-1 is expressed by cone photoreceptor cells and its abundance varies with a daily rhythm, being significantly higher at night. In vivo application of exogenous IGF-1 increases rod progenitor cell division, an effect that is greater at night than during the day. We also show that inhibiting the function of IGF receptors decreases proliferation of rod progenitor cells. Finally, we show that IGF receptors are located on rod progenitor cells as well as on cone and rod photoreceptors. Taken together, these data suggest that the rhythmic production and release of IGF-1 plays a role in regulating the insertion of new rod photoreceptors into the retina. The diurnal change in IGF-1 abundance and effects of exogenous IGF-1 are consistent with the previous demonstration that rod progenitor cell division is threefold greater at night than in the day [Brain Res. 673 (1995) 119; Brain Res. 712 (1996) 40]. We also show that the insertion of new rod photoreceptors at the central edge of the ciliary neurogenic zone very likely also depends on IGF-1 production by cone photoreceptors. We propose that addition of new rod photoreceptors into the functioning retina is regulated through a feedback mechanism mediated at least in part via the IGF-1 produced in the cone photoreceptors.

Cellular and molecular characterization of early and late retinal stem cells/progenitors: Differential regulation of proliferation and context dependent role of Notch signaling

Retinal stem cells/progenitors that define the evolutionarily conserved early and late stages of retinal histogenesis are known to have distinct competence to give rise to stage-specific retinal cell types. However, the information regarding their innate proliferative behavior and phenotypic potential in terms of generating neurons and glia is lacking. Here we demonstrate that, like their counterparts in other central nervous system (CNS) regions during early and late stages of embryonic development, the early and late retinal stem cells/progenitors display different proliferative response to fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF) and bias towards generating neurons or glia. Although the former predominantly generate neurons, the latter are partial towards giving rise to glia. Transcription profiling identified classes of genes that are differentially expressed in early and late retinal stem cells/progenitors in proliferating conditions and suggested that the distinct proliferative response to FGF2 and EGF is likely due to differential expression of FGF receptor 1 (FGFR1) and EGF receptor (EGFR). However, the proliferative maintenance of retinal stem cells/progenitors is likely to include other signaling pathways such as those mediated by insulin-like growth factors (IGFs) and stem cell factor (SCF). Transcription profiling of early and late retinal stem cells/progenitors in proliferating and differentiating conditions suggested a context dependent role for Notch signaling, which may constitute one of the mechanisms underlying the stage-dependent phenotypic potential of retinal stem cells/progenitors.

Timing and location of rhodopsin expression in newly born rod photoreceptors in the adult teleost retina

Labeling of newly divided retinal cells with bromodeoxyuridine (BrdU) and a rhodopsin mRNA probe revealed that rhodopsin is first expressed by new rod photoreceptors 2 days after cell birth in an adult cichlid fish. Most new cells that expressed rhodopsin had nuclei located in the vitreal half of the outer nuclear layer (ONL), lending further support to the hypothesis that movement from scleral to vitreal ONL is associated with rod differentiation.

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.

Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.

Stem cells in the teleost retina: persistent neurogenesis and injury-induced regeneration

The retina of the adult teleost fish is an important model for studying persistent and injury-induced neurogenesis in the vertebrate central nervous system. All neurons, with the exception of rod photoreceptors, are continually appended to the extant retina from an annulus of progenitors at the margin. Rod photoreceptors, in contrast, are added to differentiated retina only from a lineage of progenitors dedicated to making rods. Further, when the retina is lesioned, the lineage that produces only rods ceases this activity and regenerates retinal neurons of all types. The progenitors that supply neurons at the retinal margin and rod photoreceptors and regenerated neurons in the mature tissue originate from multipotent stem cells. Recent data suggest that the growth-associated neurogenic activity in the retina is regulated as part of the growth hormone/insulin-like growth factor-I axis. This paper reviews recent evidence for the presence of stem cells in the teleost retina and the molecular regulation of neurogenesis and presents a consensus cellular model that describes persistent and injury-induced neurogenesis in the retinas of teleost fish.

Mitotic activation of proliferative cells in the inner nuclear layer of the mature fish retina: regulatory signals and molecular markers

New neurons continuously differentiate within the otherwise mature retina of teleost fish, both under normal conditions and in response to injury. We investigated the effects of surgical injury and intraocular injection of neurotrophic factors on the mitotic rate of proliferative inner nuclear layer cells (PINC). PINC are continually born in the inner nuclear layer and then migrate to the outer nuclear layer (ONL). Surgical excision of a part of a retina activates PINC mitotic activity near and far from the lesion. In the injured eye, up-regulation of PINC cells is largest in the dorsonasal sector of the retina, regardless of the site of lesion. Up-regulation extends even to the unlesioned contralateral eye, where it occurs in the same dorsonasal sector. Intraocular injection of ciliary neurotrophic factor mimics the effect of injury on PINC in the treated eye but not on the untreated contralateral retina. We searched for the expression in PINC of Pax6, a transcription factor linked to retinal progenitor cells and found that less than 0.5% of all PINC cells express it. Importantly, the number of Pax6-expressing PINC does not change significantly in the retinas subjected to any of the experimental manipulations tested. Under normal conditions, the default fate of PINC cells is to migrate to the ONL and, likely, replenish the rod progenitor pool. PINC respond to injury (both surgical and light-dependent) by increasing their mitotic rate; this increase is long lived, but there are no changes in the expression level of Pax6. PINC probably are a heterogenous cell population that can be specified for ultimate, different purposes: creating rod precursors, creating founder cells, creating cone precursors. Several fates are recognized now, but others may also be possible.

Persistent neurogenesis in the teleost retina: evidence for regulation by the growth-hormone/insulin-like growth factor-I axis

Based on results from previous studies (J. Comp. Neurol. 394 (1998) 386, 395), it was hypothesized that the persistent neurogenesis in the retina of teleost fish is modulated by insulin-like growth factor-I (IGF-I), which, in turn, is regulated by growth hormone (GH). Two approaches were undertaken to test this hypothesis. First, a variety of techniques were used to determine if IGF-I and the IGF-I receptor (IGF-IR) are expressed in the retina. Second, GH was injected into animals to stimulate IGF-I synthesis in target tissues, and IGF-I expression and cell proliferation in the retina was quantitatively assayed. Reverse transcriptase-polymerase chain reaction, screening a retinal cDNA library and Northern analysis showed that genes encoding IGF-I and IGF-IR are expressed in the retina of goldfish. In situ hybridization showed that IGF-IR is expressed by retinal progenitors and all differentiated retinal neurons. Intraperitoneal injections of GH elevate IGF-I mRNA levels in the liver, brain and retina and produce a dose-dependent change in the proliferation of stem cells and progenitors in the retina. These data indicate that the principal components of the IGF-I signaling cascade are present in the retinas of teleosts, and we suggest these elements mediate the persistent, growth-associated neurogenesis in this tissue.

A comparative study of neurogenesis in the retinal ciliary marginal zone of homeothermic vertebrates

The retina of many fish and amphibians grows throughout life, roughly matching the overall growth of the animal. The new retinal cells are continually added at the anterior margin of the retina, in a circumferential zone of cells, known as the ciliary marginal zone, or CMZ. Recently, Fischer and Reh [Dev. Biol. 220 (2000) 197] have found that new neurons are added to the retina of the chicken via proliferation and subsequent differentiation of neurons and glia at the retinal margin in a zone highly reminiscent of the CMZ of lower vertebrates. In addition, other groups have reported that putative retinal stem cells could be isolated from the ciliary margin of the adult mouse. In light of these findings, we have re-investigated the eyes of three additional species to determine whether other homeothermic vertebrates also possess CMZ cells and whether we could detect evidence for addition of neurons at the retinal margin in mature animals. We examined one additional avian species, the quail, one marsupial, the opposum, and one mammal, the mouse. We find that the CMZ cells have been gradually diminished during vertebrate evolution. The quail has a reduced CMZ as compared to the chicken, while the opposum has only a few cells likely related to the CMZ and we failed to find evidence of CMZ cells at the margin of the mouse retina.

Temperature induces variations in the retinal cell proliferation rate in a cyprinid

We quantitatively evaluate the changes of the proliferative cell populations in the adult tench retinas maintained at 6 degrees C and 20 degrees C by both PCNA antigen detection and flow cytometry-based DNA measurements. Both the overall percentage of S-phase cells in the whole retinas and the number of PCNA-positive cells in each of the retinal layers were significantly lower in the tench kept at 6 degrees C, indicating that temperature affects the retinal germinal cell proliferation.

Development of normal and injury-induced gene expression of aFGF, bFGF, CNTF, BDNF, GFAP and IGF-I in the rat retina

Focal mechanical injury to the retina substantially increases basic fibroblast growth factor (bFGF) and ciliary neurotrophic factor (CNTF) mRNA expression, accompanied by a transient increase in FGFR-1 mRNA, and this response is thought to protect photoreceptors near the injury site from inherited and light-induced retinal degenerations. We have now examined retinal gene expression of the principal survival factors involved in the response to injury in normal rats as a function of postnatal age both in normal and injured retinas. Sprague-Dawley rats were injured in one eye by needle incision through the retina at postnatal day (P) 10, 22, 35, 60, 90, 120 and 180. The other eye was uninjured and served as the control. Retinas were taken 1 day post-injury. Northern blot analysis was performed to determine the mRNA expression of the following factors and receptors: bFGF and acidic fibroblast growth factors (aFGF) and FGF receptor-1 (FGFR-1); CNTF and CNTF receptor alpha (CNTFR-alpha); brain-derived neurotrophic factor (BDNF) and its receptor trk B; and insulin-like growth factor-I (IGF-I) and IGF-I receptor (IGF-IR); glial fibrillary acidic protein (GFAP) and opsin. In the uninjured control eyes, mRNA expression of most of the factors increased with postnatal age, with little expression at P10 and maximal expression levels reached at P22 (opsin), P35 (aFGF), P60 (BDNF) or P90 (bFGF, FGFR-1, CNTF and GFAP). In contrast, IGF-1 mRNA rapidly decreased from a high level of expression at P10 to about 55% of that level by P22, reaching a stable 45-50% of the P10 level at P35 and thereafter. The response to injury of bFGF, FGFR-1, CNTF and GFAP mRNAs increased with postnatal age. Unexpectedly, only minimal increases in bFGF, FGFR-1, CNTF and GFAP over those seen in the control eyes were observed before P35. Thereafter, the increase of bFGF mRNA after injury reached a maximum of three-fold at P60, maintained this level to P120, and slightly decreased to 2.5-fold by P180. Expression of FGFR-1 mRNA showed a maximum increase of 2.6-fold at P90. Expression of CNTF and GFAP mRNAs followed a time course similar to that of bFGF. Mechanical injury did not alter the mRNA levels of aFGF, BDNF, IGF-I, and receptors, CNTFR-alpha, trk B and IGF-IR. These data show that the response to injury is minimal at early postnatal ages but increases with age and peaks at P60-90 for most potential survival factors.

Photoreceptor plasticity in reaggregates of embryonic chick retina: rods depend on proximal cones and on tissue organization

Plasticity of photoreceptors and their integration into epithelial structures homologous to an outer nuclear layer (ONL), was investigated in embryonic chick retinal cell reaggregates by immunohistochemistry using an antibody specific for red plus green cones (RG-cones) and an antibody for rods. If reaggregates are raised in the presence of pigmented epithelium (RPE), completely reconstructed, stratified retinal spheres are produced, where all rods and cones are integrated into an outer laminar ONL, similar to a normal retina. In the absence of RPE, 'rosetted' spheres form which contain internal rosettes homologous to an ONL. Only a minor fraction of cones and rods of 'rosetted' spheres are located within rosettes, while a larger fraction is diffusely displaced in nonorganized areas, thus, not contributing to an ONL-like epithelium. In both types of spheres, the total percentage of RG-cones was similar to the in vivo retina, indicating that expression of cones is autonomous. Following cones, after about one day, rods developed only within already existing RG-cone clusters. Thereby, the ratio of rods to RG-cones increases as the tissue organization decreases: for stratified spheres this ratio is, 0.50 (1 rod/2 cones; similar to mature retina); for rosettes, 0.74 (3 rods/4 cones) and for nonorganized areas, 1.09 (1 rod/1 cone) -- a higher ratio under our conditions has never been detected. Thus, rod expression depends strictly on the presence of nearby cones; their relative numbers are distinctively adjusted according to the cytoarchitecture of the tissue environment. The biomedical implications of these findings are briefly discussed.

Comparison of recombinant barramundi and human insulin-like growth factor (IGF)-I in juvenile barramundi (Lates calcarifer): in vivo metabolic effects, association with circulating IGF-binding proteins, and tissue localisation

The in vivo actions of human and fish insulin-like growth factor (IGF)-I have been compared to extend the understanding of the metabolism of IGFs in fish and to identify potential differences in their actions. The effects of acute administration of these proteins on the incorporation of glucose into muscle glycogen and leucine into liver protein in juvenile barramundi were investigated. In these in vivo metabolic assays, both baramundi IGF-I (bIGF-I) and human IGF-I (hIGF-I) increase the incorporation of D-[14C]glucose into muscle glycogen and [14C]leucine into liver protein. The distribution of radio-labeled human and barramundi IGF-I in the circulation and their uptake by tissue was also compared in juvenile barramundi (Lates calcarifer). Analysis of trichloroacetic acid-precipitable radioactivity in sequential samples following bolus injection of radiolabeled IGFs revealed that hIGF-I was degraded faster than bIGF-I. Neutral gel chromatography of these samples suggested that this difference is due to reduced affinity of hIGF-I, compared to bIGF-I, for the IGF-binding proteins (IGFBPs) present in the barramundi. Tissue uptake of [125I]-labeled hIGF-I and bIGF-I was similar except that [ 125I]bIGF-I uptake by the kidney exceeded that of hIGF-I. It is suggested that while some of the in vivo actions of IGFs in fish are conserved, functional differences between mammalian and teleostean IGFs exist, particularly with respect to their interactions with fish IGFBPs.

A role for the fibroblast growth factor receptor in cell fate decisions in the developing vertebrate retina

The mature vertebrate retina contains seven major cell types that develop from an apparently homogenous population of precursor cells. Clonal analyses have suggested that environmental influences play a major role in specifying retinal cell identity. Fibroblast growth factor-2 is present in the developing retina and regulates the survival, proliferation and differentiation of developing retinal cells in culture. Here we have tested whether fibroblast growth factor receptor signaling biases retinal cell fate decisions in vivo. Fibroblast growth factor receptors were inhibited in retinal precursors in Xenopus embryos by expressing a dominant negative form of the receptor, XFD. Dorsal animal blastomeres that give rise to the retina were injected with cDNA expression constructs for XFD and a control non-functional mutant receptor, D48, and the cell fates of transgene-expressing cells in the mature retina determined. Fibroblast growth factor receptor blockade results in almost a 50% loss of photoreceptors and amacrine cells, and a concurrent 3.5-fold increase in Muller glia, suggesting a shift towards a Muller cell fate in the absence of a fibroblast growth factor receptor signal. Inhibition of non-fibroblast-growth-factor-mediated receptor signaling with a third mutant receptor, HAVO, alters cell fate in an opposite manner. These results suggest that it is the balance of fibroblast growth factor and non-fibroblast growth factor ligand signals that influences retinal cell genesis.

Insulin-like growth factor-I binds in the inner plexiform layer and circumferential germinal zone in the retina of the goldfish

Results of the previous study suggest that insulin-related peptides regulate proliferation of retinal progenitors in the adult goldfish. Because of their known roles in retinal neurogenesis, we have chosen to focus future studies on insulin-like growth factor I (IGF-I) and the IGF-I receptor. In the study described here, we characterized the spatial distribution and specificity of IGF-I binding sites in the retina of the adult goldfish by performing receptor-binding autoradiography with [125I]-IGF-I alone and with unlabeled IGF-I-related molecules (IGF-I, IGF-II, insulin, and des-[1-3]-IGF-I) as competitive inhibitors of [125I]-IGF-I binding. The results of these experiments show that IGF-I binds in two locations in the retina of the adult goldfish, within the inner plexiform layer of the differentiated retina and the circumferential germinal zone. The competition experiments suggest that [125I]-IGF-I binds at sites specific for IGF-I, and that both IGF-I receptors and IGF-I binding proteins are present in the retina.

Nutritional and developmental regulation of insulin-like growth factors in fish

The insulin-like growth factors (IGF) are evolutionarily ancient growth factors present in all vertebrates. The central importance of IGF for normal development and growth has been illustrated by the severe growth-retarded phenotype exhibited by IGF-I, IGF-II or IGF-I receptor "knockout" mice. Although we know much about the gross effects of IGF on the overall size of the fetus and the clinical manifestations that result from fetal and neonatal deficiency of IGF (i.e., severe growth retardation leads to dwarfism), very little is known about the in vivo actions of IGF during embryogenesis at the cellular and molecular levels. Most research on the developmental role of IGF has relied on rodent models, and attempts to elucidate the molecular and cellular basis of IGF actions have been hampered by the inaccessibility of the mammalian fetus enclosed in the uterus. During the past decade, there has been growing support for the concept that the IGF have been highly conserved in all vertebrates. Both IGF-I and IGF-II are present in fish, and their structures are highly conserved. Human and fish IGF-I are equally potent in mammalian and fish bioassay systems. Insulin-like growth factor mRNA is found in all life stages of fish, ranging from unfertilized egg to adult. The temporal and spatial expression patterns of fish IGF-I seem to be similar to those in mammals. Nutritional status and growth hormone both have a profound effect on IGF-I expression in fish, as they do in mammals. These features suggest that the IGF system is highly conserved between teleost fish and mammals. Because fish embryos develop externally, they provide excellent animal models for understanding the regulatory roles of IGF, IGF receptor and IGF-binding proteins in vertebrate embryonic development. Current research on the developmental and nutritional roles of IGF in fish will undoubtedly contribute to knowledge of the basic physiology of vertebrates in general.

Cellular diversification in the vertebrate retina

The discovery of heterogeneous populations of retinal precursors with sequentially modified fates may help solve the conundrum of conserved histogenesis in the absence of determination either by birthdate or lineage. Combined with a wealth of new data on the exogenous and endogenous factors that influence cellular fate in the retina, models of how complexity is generated are beginning to emerge.

Localization of IGF-I and IGF-I receptor mRNA in Sparus aurata larvae

Studies of the ontogeny of IGF-I mRNA during embryonic and larval development of the gilthead sea bream Sparus aurata showed its expression during these early developmental stages. The present study applies in situ hybridization to localize IGF-I and IGF receptor mRNAs in 16-day larvae of S. aurata. Paraffin sections were hybridized with homologous RNA probes labeled by [35S]UTP. IGF-I mRNA expression was found mainly in chondrocytes, in both the gill arches and cranial cartilage, in skeletal muscle, in the brain, in the pancreas, in the retina, and in the epithelial cells surrounding the lens. A strong positive reaction for IGF receptor mRNA was found in skeletal muscle, in the pancreas, and in the lymphoid tissue found in the intertubular tissue of the kidney. Signals were less intense in brain and chondrocytes. It is suggested that in teleosts, as in higher vertebrates, IGF-I may be involved in the regulation of tissue growth and differentiation in an autocrine/paracrine manner.

Factors modulating supernumerary hair cell production in the postnatal rat cochlea in vitro

It has been shown in the past that extra hair cells or supernumerary cells can be produced when neonatal cochleae are maintained in vitro. In this report, we investigated the effects of the culture methods, molecules and growth factors that are thought to be involved in cell proliferation. Quantitative studies of supernumerary hair cells were made by measuring the cell density over the entire spiral lamina at two postnatal stages: birth and 3 days after birth. With a standard feeding solution without serum, a difference in cell density was observed between the two methods of culture. Cochlear explants in a standard feeding solution supplemented with serum showed an increase of cell density only when the explantation is made at birth. Retinoic acid added to the standard feeding solution did not increase the hair cell density, while insulin induced an increase, especially at 5 micrograms/ml. Several growth factors were tested. Epidermal growth factor (EGF) presented a dose dependent effect with an increase of up to 30% of hair cell density that was observed in the basal region when the explantation was made at birth. Transforming growth factor-alpha did not induce an increase of cell density, whereas transforming growth factor-beta presented an effect on hair cell density, with a dose dependent effect reaching 37.4% for the basal inner hair cells. Interpretation of these results is limited because of the lack of data concerning the presence of specific membrane receptors. One possibility is that insulin stimulates hair cell differentiation from existing undifferentiated cells. Another hypothesis may be related to the EGF and transforming growth factor-beta, where these molecules might induce transdifferentiation of cells by acting on the transmembrane molecules and the extracellular matrix.

Asymmetric retinal growth: evidence for regulation by a retinotopic mechanism

Adult teleost retinas grow throughout life, in part by the addition of cells from an encircling, proliferative neuroepithelium. In some species, this proliferative growth is asymmetric around the retina. The present study evaluated two hypotheses regarding asymmetric proliferative growth in adult green sunfish retina: (1) the generation of rod photoreceptors in central retina from proliferative rod precursor cells is also asymmetric; and (2) asymmetric proliferative growth patterns are regulated by mechanisms that are organized retinotopically and are independent of body-axis coordinates. In all retinas examined, rod precursor distribution and rod production were asymmetric, and both were in coarse spatial register with the asymmetric pattern of cellular addition at the retinal margin. In adult eyes that were surgically rotated, the asymmetric patterns of proliferative growth were present and appropriate for the retina's prerotation orientation. The results suggest that proliferative growth at both marginal and central adult sunfish retina is asymmetric, and that these asymmetric growth patterns are regulated by a retinotopic mechanism that is independent of body-axis coordinates.

Nonuniform distribution of cell proliferation in the adult teleost retina

Teleost fish continue to grow throughout life, and their eyes enlarge correspondingly. Within the eye, the retina grows by stretching existing tissue and adding new cells. Cell addition occurs in two ways: First, all cell types except rod photoreceptors are added circumferentially at the edge of the eye where the retina meets the iris; second, rod photoreceptors are generated from a population of rod progenitor cells which divide throughout the outer nuclear layer (ONL). To determine the spatial distribution of rod progenitor cells across the teleost retina, we labeled dividing cells with an antibody to proliferating cell nuclear antigen (PCNA) throughout a 24 h period. We found a significantly higher density of dividing rod precursor cells at the nasal and temporal margins than in the central retina throughout the 24 h cycle. At night, the density of dividing cells is significantly greater at the nasal pole of the eye. The difference between cell division at the center and the margin was reduced at night when the density of cell division in the central retina increased significantly. Taken together, these data suggest that the eye grows asymmetrically, with more cells added at the nasal pole. Possible developmental causes and functional consequences of the reported distribution of cell divisions in time and location are presented.

Localization and expression of insulin-like growth factor in the teleost retina

Teleost fish retinas continue to add neurons throughout life, and evidence from in vitro experiments have implicated insulin-like growth factors (IGFs) in this process. To discover whether these factors are expressed in vivo, we have examined their expression in the cichlid fish, Haplochromis burtoni. Three lines of evidence show that IGFs are present in the fish retina. An IGF-I specific antibody, sm 1.2, binds preferentially to the retinal outer plexiform layer, in areas of cone photoreceptor synaptic endings. Northern blots of mRNA hybridized with riboprobes from trout IGF-I and IGF-II genes revealed transcripts of approximately 6.5 and 4.9 kb, respectively. The IGF-I probe detected an additional transcript of 1.2 kb in liver but not in retinal mRNA. In situ hybridization with digoxigenin-labeled riboprobes revealed that the IGF gene product is localized in the cone photoreceptors. These results show that cone photoreceptors are the source of IGFs in the fish retina, consistent with the hypothesis that IGFs play a role in regulating production of new neurons in the teleost retina.