Cellular interactions determine neuronal phenotypes in rodent retinal cultures

Retinal Cell Fate Specification

How are different neural cell types generated from progenitor cells? In 1990, Turner et al. used new lineage tracing techniques to show that different cells in the mammalian retina share their progenitor origin. The findings established a key step toward our understanding of how multipotent progenitor cells give rise to complex circuitry in the retina.

Intraocular Transplantation of E1A-Immortalized Retinal Precursor Cells

The purpose of this study was to examine the effect of the ocular environment on the survival, tumorigenicity, and phenotypic marker expression of immortalized retinal precursor cells transplanted into immunocompetent adult and neonatal Sprague–Dawley rats. E1A-NR.3, a rat immortalized retinal precursor cell culture, was used as an inexhaustible source of experimental graft material. These cells were prelabeled with the fluorescent marker diI (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate) and transplanted intravitreally (50,000 cells per μL) into 11 adult and 31 neonatal Sprague–Dawley rat eyes. At 1 mo posttransplant, animals were sacrificed and retinal tissue sections examined histologically for the presence of grafted cells, signs of tumor formation, and retinal phenotypic marker expression. No obvious signs of tumor formation or rejection were seen in a total of 42 eyes in the immunocompetent hosts. Our results indicate that E1A-NR.3 cells survive at least 1 month in vivo, and can migrate from the vitreous into neuroretinal cell layers. Subpopulations of surviving grafted cells were seen to express photoreceptor markers rhodopsin and recoverin comparably between in vitro and in vivo conditions. However, the number of cells immunoreactive for vimentin and E1A decreased significantly under in vivo conditions. This report represents the first experimental intravitreal transplantation of E1A-immortalized retinal precursor cells into adult and neonatal rats. The intraocular location and environment appears to affect phenotypic expression of surviving grafted cells, especially with respect to vimentin and E1A expression. The fact that E1A-NR.3 cells survived intraocularly at least 1 mo without tumor formation suggests that the cells may continue to be useful for further in vivo studies of experimental retinal transplantation, and effects of histological location on retinal cell phenotype and histogenesis in immunocompetent hosts. © 1998 Elsevier Science Inc.

Light-Regulated Thyroid Hormone Signaling Is Required for Rod Photoreceptor Development in the Mouse Retina

PURPOSE

Ambient light is both a stimulus for visual function and a regulator of photoreceptor physiology. However, it is not known if light can regulate any aspect of photoreceptor development. The purpose of this study was to investigate whether ambient light is required for the development of mouse rod photoreceptors.

METHODS

Newborn mouse pups (C57BL/6) were reared in either cyclic light (LD) or constant dark (DD). Pups were collected at postnatal day (P)5, P10, P17, or P24. We performed retinal morphometric and cell death analysis at P5, P10, and P17. Rhodopsin expression was assessed using immunofluorescence, Western blot, and quantitative RT-PCR analysis. Electroretinograms were performed at P17 and P24. Radioimmunoassay and ELISA were used to follow changes in thyroid hormone levels in the serum and vitreous.

RESULTS

In the DD pups, the outer nuclear layer was significantly thinner at P10 and there were higher numbers of apoptotic cells at P5 compared to the LD pups. Rhodopsin expression was lower at P10 and P17 in DD pups. Electroretinogram a-waves were reduced in amplitude at P17 in the DD pups. The DD animals had lower levels of circulating thyroid hormones at P10. Light-mediated changes in thyroid hormones occur as early as P5, as we detected lower levels of total triiodothyronine in the vitreous from the DD animals. Drug-induced developmental hypothyroidism resulted in lower rhodopsin expression at P10.

CONCLUSIONS

Our data demonstrate that light exposure during postnatal development is required for rod photoreceptor development and that this effect could be mediated by thyroid hormone signaling.

Photoreceptor cell fate specification in vertebrates

Photoreceptors--the light-sensitive cells in the vertebrate retina--have been extremely well-characterized with regards to their biochemistry, cell biology and physiology. They therefore provide an excellent model for exploring the factors and mechanisms that drive neural progenitors into a differentiated cell fate in the nervous system. As a result, great progress in understanding the transcriptional network that controls photoreceptor specification and differentiation has been made over the last 20 years. This progress has also enabled the production of photoreceptors from pluripotent stem cells, thereby aiding the development of regenerative medical approaches to eye disease. In this Review, we outline the signaling and transcription factors that drive vertebrate photoreceptor development and discuss how these function together in gene regulatory networks to control photoreceptor cell fate specification.

Regeneration of cone photoreceptors when cell ablation is primarily restricted to a particular cone subtype

We sought to characterize the regenerated cells, if any, when photoreceptor ablation was mostly limited to a particular cone subtype. This allowed us to uniquely assess whether the remaining cells influence specification of regenerating photoreceptors. The ability to replace lost photoreceptors via stem cell therapy holds promise for treating many retinal degenerative diseases. Zebrafish are potent for modelling this because they have robust regenerative capacity emanating from endogenous stem cells, and abundant cone photoreceptors including multiple spectral subtypes similar to human fovea. We ablated the homolog of the human S-cones, the ultraviolet-sensitive (UV) cones, and tested the hypothesis that the photoreceptors regenerating in their place take on identities matching those expected from normal cone mosaic development. We created transgenic fish wherein UV cones can be ablated by addition of a prodrug. Thus photoreceptors developed normally and only the UV cones expressed nitroreductase; the latter converts the prodrug metronidazole to a cell-autonomous neurotoxin. A significant increase in proliferation of progenitor cell populations (p<0.01) was observed when cell ablation was primarily limited to UV cones. In control fish, we found that BrdU primarily incorporated into rod photoreceptors, as expected. However the majority of regenerating photoreceptors became cones when retinal cell ablation was predominantly restricted to UV cones: a 2-fold increase in the relative abundance of cones (p = 0.008) was mirrored by a 35% decrease in rods. By primarily ablating only a single photoreceptor type, we show that the subsequent regeneration is biased towards restoring the cognate photoreceptor type. We discuss the hypothesis that, after cone death, the microenvironment formed by the remaining retinal cells may be influential in determining the identity of regenerating photoreceptors, though other interpretations are plausible. Our novel animal model provides control of ablation that will assist in identifying mechanisms required to replace cone photoreceptors clinically to restore daytime vision.

In vivo evidence for unbiased Ikaros retinal lineages using an Ikaros-Cre mouse line driving clonal recombination

BACKGROUND

We showed previously that the transcription factor Ikaros is expressed in early but not late retinal progenitors cells (RPCs), and is necessary and sufficient for the production of early-born neurons. Preliminary evidence using retinal explant cultures qualitatively suggested that Ikaros-positive RPCs might share a common lineage with Ikaros-negative RPCs.

RESULTS

To explore further this question in vivo in a quantitative manner, we generated BAC transgenic mouse lines expressing Cre recombinase under the regulatory elements of the Ikaros gene, and crossed them with Cre reporter lines. Different transgenic lines labeled a variable number of RPCs, resulting in either dense or sparse radial arrays of reporter-positive progenies. Analysis of over 800 isolated cell arrays, which are most likely clones, confirmed that Ikaros-expressing RPCs generate both early- and late-born cell types in the same lineage, and that the overall cell composition of the arrays closely resembles that of the population of the mature retina. Interestingly, another sparse line did not label arrays, but appeared to specifically reflect Ikaros postmitotic expression in amacrine and ganglion cells.

CONCLUSIONS

These mouse lines confirm the unbiased potential of the Ikaros lineage in vivo and provide novel tools for clonal lineage tracing and single neuron tracking in the retina.

Pushing the envelope of retinal ganglion cell genesis: context dependent function of Math5 (Atoh7)

The basic-helix-loop helix factor Math5 (Atoh7) is required for retinal ganglion cell (RGC) development. However, only 10% of Math5-expressing cells adopt the RGC fate, and most become photoreceptors. In principle, Math5 may actively bias progenitors towards RGC fate or passively confer competence to respond to instructive factors. To distinguish these mechanisms, we misexpressed Math5 in a wide population of precursors using a Crx BAC or 2.4 kb promoter, and followed cell fates with Cre recombinase. In mice, the Crx cone-rod homeobox gene and Math5 are expressed shortly after cell cycle exit, in temporally distinct, but overlapping populations of neurogenic cells that give rise to 85% and 3% of the adult retina, respectively. The Crx>Math5 transgenes did not stimulate RGC fate or alter the timing of RGC births. Likewise, retroviral Math5 overexpression in retinal explants did not bias progenitors towards the RGC fate or induce cell cycle exit. The Crx>Math5 transgene did reduce the abundance of early-born (E15.5) photoreceptors two-fold, suggesting a limited cell fate shift. Nonetheless, retinal histology was grossly normal, despite widespread persistent Math5 expression. In an RGC-deficient (Math5 knockout) environment, Crx>Math5 partially rescued RGC and optic nerve development, but the temporal envelope of RGC births was not extended. The number of early-born RGCs (before E13) remained very low, and this was correlated with axon pathfinding defects and cell death. Together, these results suggest that Math5 is not sufficient to stimulate RGC fate. Our findings highlight the robust homeostatic mechanisms, and role of pioneering neurons in RGC development.

Math5 defines the ganglion cell competence state in a subpopulation of retinal progenitor cells exiting the cell cycle

The basic helix-loop-helix (bHLH) transcription factor Math5 (Atoh7) is transiently expressed during early retinal histogenesis and is necessary for retinal ganglion cell (RGC) development. Using nucleoside pulse-chase experiments and clonal analysis, we determined that progenitor cells activate Math5 during or after the terminal division, with progressively later onset as histogenesis proceeds. We have traced the lineage of Math5+ cells using mouse BAC transgenes that express Cre recombinase under strict regulatory control. Quantitative analysis showed that Math5+ progenitors express equivalent levels of Math5 and contribute to every major cell type in the adult retina, but are heavily skewed toward early fates. The Math5>Cre transgene labels 3% of cells in adult retina, including 55% of RGCs. Only 11% of Math5+ progenitors develop into RGCs; the majority become photoreceptors. The fate bias of the Math5 cohort, inferred from the ratio of cone and rod births, changes over time, in parallel with the remaining neurogenic population. Comparable results were obtained using Math5 mutant mice, except that ganglion cells were essentially absent, and late fates were overrepresented within the lineage. We identified Math5-independent RGC precursors in the earliest born (embryonic day 11) retinal cohort, but these precursors require Math5-expressing cells for differentiation. Math5 thus acts permissively to establish RGC competence within a subset of progenitors, but is not sufficient for fate specification. It does not autonomously promote or suppress the determination of non-RGC fates. These data are consistent with progressive and temporal restriction models for retinal neurogenesis, in which environmental factors influence the final histotypic choice.

Specific protein kinase C isoforms are required for rod photoreceptor differentiation

The protein kinase C (PKC) family of enzymes regulates cell physiology through phosphorylation of serine and threonine residues of many proteins in most cell types. Here we identify PKC-β1 and PKC-γ as isoforms that are essential for rod photoreceptor differentiation in mouse retinas. Using ex vivo retinal explants, we found that phorbol ester 12-myristate 13-acetate and insulin-like growth factor 1 (IGF1) induced rod differentiation, as defined by opsin or Crx expression, in a PKC-dependent manner days ahead of rod development in untreated explants. PKC-β1 and PKC-γ were colocalized with proliferating cell nuclear antigen (PCNA)- and STAT3-positive progenitors through the later differentiation period. Pharmacological or genetic inhibition of either isoform resulted in a partial reduction in the appearance of rods, whereas removing both isoforms resulted in their complete absence. Furthermore, a significant decline of STAT3 tyrosine phosphorylation was observed by activation of PKC, while inhibition of PKC resulted in an increase of phosphorylated STAT3 along with a delayed cell cycle exit of progenitors with prolonged PCNA expression. In adult retinas, IGF1 activates PI-3 kinase (PI3K), but in neonatal retinas its action is identical to the action of an PI3K inhibitor. These data unveil a novel signaling cascade that coordinates and regulates rod differentiation through specific PKC isoforms in mammals.

RPE-secreted factors: influence differentiation in human retinal cell line in dose- and density-dependent manner

Retinal pigment epithelial (RPE) cells play an important role in normal functioning of retina and photoreceptors, and some retinal degenerations arise due to malfunctioning RPE. Retinal pigment epithelium transplantation is being explored as a strategy to rescue degenerating photoreceptors in diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Additionally, RPE-secreted factors could rescue degenerating photoreceptors by prolonging survival or by their ability to differentiate and give rise to photoreceptors by transdifferentiation. In this study, we have explored what role cell density could play in differentiation induced in a human retinal progenitor cell line, in response to RPE-secreted growth factors. Retinal progenitors plated at low (1 × 10(4) cells/cm(2)), medium (2-4 × 10(4) cells/cm(2)), and high (1 × 10(5) cells/cm(2)) cell density were exposed to various dilutions of RPE-conditioned medium (secreted factors) under conditions of defined medium culture. Progenitor cell differentiation was monitored phenotypically (morphological, biochemical analysis, and immunophenotyping, and western blot analysis were performed). Our data show that differentiation in response to RPE-secreted factors is modulated by cell density and dilutions of conditioned medium. We conclude that before embarking on RPE transplantation as a modality for treatment of RP and AMD, one will have to determine the role that cell density and inhibitory and stimulatory neurotrophins secreted by RPE could play in the efficacy of survival of transplants. We report that RPE-conditioned medium enhances neuronal phenotype (photoreceptors, bipolars) at the lowest cell density in the absence of cell-cell contact. Eighty percent to 90% of progenitor cells differentiate into photoreceptors and bipolars at 50% concentration of conditioned medium, while exposure to 100% conditioned medium might increase multipolar neurons (ganglionic and amacrine phenotypes) to a small degree. However, no clear-cut pattern of differentiation in response to RPE-secreted factors is noted at higher cell densities.

Step-wise specification of retinal stem cells during normal embryogenesis

The specification of embryonic cells to produce the retina begins at early embryonic stages as a multi-step process that gradually restricts fate potentials. First, a subset of embryonic cells becomes competent to form retina by their lack of expression of endo-mesoderm-specifying genes. From these cells, a more restricted subset is biased to form retina by virtue of their close proximity to sources of bone morphogenetic protein antagonists during neural induction. During gastrulation, the definitive RSCs (retinal stem cells) are specified as the eye field by interactions with underlying mesoderm and the expression of a network of retina-specifying genes. As the eye field is transformed into the optic vesicle and optic cup, a heterogeneous population of RPCs (retinal progenitor cells) forms to give rise to the different domains of the retina: the optic stalk, retinal pigmented epithelium and neural retina. Further diversity of RPCs appears to occur under the influences of cell-cell interactions, cytokines and combinations of regulatory genes, leading to the differentiation of a multitude of different retinal cell types. This review examines what is known about each sequential step in retinal specification during normal vertebrate development, and how that knowledge will be important to understand how RSCs might be manipulated for regenerative therapies to treat retinal diseases.

Human retinal development in an in situ whole eye culture system

Phenotypic characterization of human retinogenesis may be facilitated by use of the tissue culture system paradigm. Traditionally, most culture protocols involve isolation of retinal tissue and/or cells, imposing various degrees of trauma, which in many cases leads to abnormal development. In this paper, we present a novel culture technique using whole embryonic eyes to investigate whether the retina in situ can develop normally in vitro. All procedures were carried out in accordance with the Declaration of Helsinki. Human embryos were obtained from elective abortions with the informed consent of the women seeking abortion. A total of 19 eyes were enucleated. The ages of the embryonic retinas were 6-7.5 weeks. Eyecups from 2 eyes were fixed immediately, to be used as controls. The remaining 17 eyes were placed on culture plates and divided into 3 groups kept for 7 (n = 4), 14 (n = 7) and 28 (n = 6) days in vitro (DIV). After fixation, the specimens were processed for hematoxylin and eosin staining, immunohistochemistry and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Antibodies against recoverin (rods and cones), protein kinase C (PKC; rod bipolar cells) and vimentin (Müller cells) were used. TUNEL was used to detect apoptotic cells. In hematoxylin- and eosin-stained sections, the control retinas displayed a neuroblast cell layer (NBL) and an inner marginal zone. Specimens kept 7-14 DIV had a similar appearance, while 28-day specimens consisted of an NBL with almost no marginal zone. Thirteen of the 17 cultured retinas displayed completely normal lamination without rosettes or double folds. Pyknotic cells were found at the inner margin of the retinas, and the proportion of these cells increased with time in vitro. TUNEL staining revealed a few scattered cells in 7-DIV specimens, and the amount of stained cells in the inner part of the retinas progressively increased in 14- and 28-DIV specimens. Vimentin labeling showed cells arranged in a vertical pattern in all retinas. Labeling with recoverin revealed photoreceptors in 4 of the retinas kept for 14 DIV, and in all retinas kept for 28 DIV. After 28 DIV, 2 of the eyes labeled with PKC contained rod bipolar cells with minimal axons. Here we showed that human embryonic retinas can be kept in culture in situ within the eye for at least 4 weeks. Abnormal lamination is not as frequent as in isolated full-thickness retinas, indicating that physical and biochemical contact with surrounding tissues is vital for proper development. Several types of the retina-specific neuronal and glial cells were seen to differentiate according to the in vivo schedule. The results are important for future studies of retinal development, and the technique can also be used for testing the effects of various drugs on the immature retina.

Ciliary neurotrophic factor: a survival and differentiation inducer in human retinal progenitors

Retinitis pigmentosa, age-related macular degeneration, and Parkinson's disease remain major problems in the field of medicine. Some of the strategies being explored for treatment include replacement of damaged tissue by transplantation of healthy tissues or progenitor cells and delivery of neurotrophins to rescue degenerating tissue. One of the neurotrophins with promise is the ciliary neurotrophic factor (CNTF). In this study, we report the role played by CNTF in retinal cell differentiation and survival in retinal progenitors. We found that CNTF is a survival factor for multipotential human retinal cells and increased cell survival by 50%, over a 7-d period, under serum-free conditions, as determined by apoptotic assays (immunohistochemistry and flow cytometry). This effect is dose dependent with a maximum survival at a CNTF concentration of 20 ng/ml. We also report that CNTF might be a cell commitment factor, directing the differentiation mainly toward large multipolar cells with ganglionic and amacrine phenotype. These cells express tyrosine hydroxylase (amacrine cells) as well as, thy 1.1 and neuron-specific enolase (ganglionic cells). Additionally, there was also an increase in protein kinase C alpha, a protein expressed in rod and cone bipolars as well as cone photoreceptors and calbindin, a protein expressed in cone photoreceptors and horizontal cells. In our studies, CNTF doubled the number of cells with ganglionic phenotypes, and basic fibroblast growth factor doubled the number of cells with photoreceptor phenotype. Additionally, CNTF induced a subset of progenitors to undergo multiple rounds of cell division before acquiring the large multipolar ganglionic phenotype. Our conclusion is that CNTF could be an agent that has therapeutic potential and possibly induces differentiation of large multipolar ganglionic phenotype in a subset of progenitors.

Rbpj cell autonomous regulation of retinal ganglion cell and cone photoreceptor fates in the mouse retina

Vertebrate retinal progenitor cells (RPCs) are pluripotent, but pass through competence states that progressively restrict their developmental potential (Cepko et al., 1996; Livesey and Cepko, 2001; Cayouette et al., 2006). In the rodent eye, seven retinal cell classes differentiate in overlapping waves, with RGCs, cone photoreceptors, horizontals, and amacrines forming predominantly before birth, and rod photoreceptors, bipolars, and Müller glia differentiating postnatally. Both intrinsic and extrinsic factors regulate each retinal cell type (for review, see Livesey and Cepko, 2001). Here, we conditionally deleted the transcription factor Rbpj, a critical integrator of multiple Notch signals (Jarriault et al., 1995; Honjo, 1996; Kato et al., 1997; Han et al., 2002), during prenatal mouse retinal neurogenesis. Removal of Rbpj caused reduced proliferation, premature neuronal differentiation, apoptosis, and profound mispatterning. To determine the cell autonomous requirements for Rbpj during RGC and cone formation, we marked Cre-generated retinal lineages with GFP expression, which showed that Rbpj autonomously promotes RPC mitotic activity, and suppresses RGC and cone fates. In addition, the progressive loss of Rbpj-/- RPCs resulted in a diminished progenitor pool available for rod photoreceptor formation. This circumstance, along with the overproduction of Rbpj-/- cones, revealed that photoreceptor development is under homeostatic regulation. Finally, to understand how the Notch pathway regulates the simultaneous formation of multiple cell types, we compared the RGC and cone phenotypes of Rbpj to Notch1 (Jadhav et al., 2006b; Yaron et al., 2006), Notch3, and Hes1 mutants. We found particular combinations of Notch pathway genes regulate the development of each retinal cell type.

Regulation of cytokine signaling components in developing rat retina correlates with transient inhibition of rod differentiation by CNTF

Ciliary neurotrophic factor (CNTF) is known to inhibit the differentiation of rod photoreceptors from postmitotic precursor cells. During early postnatal development, photoreceptor precursors lose their responsiveness to CNTF. The underlying events causing this change in responsiveness are unknown. Moreover, whether rods express CNTF receptor alpha, a prerequisite for a direct response to the factor, is controversial. Since morphological studies have previously produced conflicting results, we have analyzed the expression of cytokine receptor components and potential ligands in the rat photoreceptor layer by real-time reverse transcription with the polymerase chain reaction after laser microdissection and by immunoblotting. Cytokine effects on rods were studied in explant cultures from newborn rat retina. CNTF receptor alpha (CNTFR alpha) and leukemia inhibitory factor receptor ss (LIFRss) were expressed in immature photoreceptors. Expression of the CNTF-specific alpha-subunit (but not of LIFRss) was downregulated specifically in the photoreceptor layer in parallel with the appearance of opsin-positive rods. The decrease of CNTFR alpha levels in explant cultures was closely correlated with the loss of precursor cell responsiveness to CNTF. Increasing the CNTF concentration in the culture medium led to prolonged CNTFR alpha expression and, concomitantly, to persistent inhibition of rod differentiation. Application of CNTF and LIF in vitro induced phosphorylation of STAT3. Inducibility of STAT3 activation by CNTF decreased with photoreceptor maturation, whereas the LIF effect persisted. Our results thus indicate that CNTF acts directly on photoreceptor precursors inhibiting their differentiation via activation of the JAK/STAT3 signal transduction pathway, and that this effect is temporally limited because of the downregulation of CNTFR alpha.

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.

Neural crest cell fate: to be or not to be prespecified

The neural crest, the intriguing cell population that gives rise to a panoply of derivatives in the vertebrate embryo including the mesenchymal structures in the head, melanocytes and most of the peripheral nervous system still proves to be an important yet enigmatic developmental cell population to study, with applications in stem cell biology, cancer biology and clinical medicine. Albeit our knowledge base is rich due to a strong history of experimentation, the fact that we have yet to decipher so many key aspects of neural crest cell (NCC) behavior speaks to the challenging complexity of this transient yet vital cell population. With the advent of new fluorescent tracing techniques, we have reexamined the migratory behaviors and ultimate fate of ventrally migrating avian NCCs within a late wave of emigration and identified a subpopulation of lineally restricted NCCs who migrate to the contralateral dorsal root ganglia (DRG) and therein give rise to mitotically active progenitor cells that ultimately produce the majority of the nociceptive sensory neurons in the DRG. These data provide evidence for the fate prespecification of subsets of NCCs while still resident in the neural tube.

Studies of host–graft interactionsin vitro

Progenitor and stem cell transplantation represent therapeutic strategies for retinal disorders that are accompanied by photoreceptor degeneration. The transplanted cells may either replace degenerating photoreceptors or secrete beneficial factors that halt the processes of photoreceptor degeneration. The present study analyzes whether rat retinal progenitor cells differentiated into photoreceptor phenotypic cells in neurospheres have a potential to interact with rat retinal explants. Immunocytochemistry for rhodopsin and synaptophysin indicated photoreceptor cell-like differentiation in neurospheres that were stimulated by basic fibroblast growth factor and epidermal growth factor. Differentiation into neural phenotypes including photoreceptor cells was effectively blocked by an addition of leukemia inhibitory factor. Grafting of neurospheres onto retinal explants demonstrated a consistent penetration of glial cell processes into the explanted tissue. On the other hand, the incorporation of donor cells into explants was very low. A general finding was that neurospheres grafting was associated with local decrease in Müller cell activation in the explants. Further characterization of these effect(s) could provide further insight into progenitor cell-based therapies of retinal degenerative disorders.

Epidermal Growth Factor Is a Neuronal Differentiation Factor for Retinal Stem Cells In Vitro

Stem cells are a tool for in vitro elucidation of the putative role of factors on cell fate. Herein we analyze the role of epidermal growth factor (EGF) on progeny derived from retinal stem cells (RSCs). We isolated cells from neuroretinas of neonate mice. All the proliferating cells harbored the radial glia marker RC2, expressed transcription factors usually found in radial glia (Mash1, Pax6), and met the criteria of stem cells: high capacity of expansion, maintenance of an undifferentiated state, and multipotency demonstrated by clonal analysis. We analyzed the differentiation 7 days after transfer of the cells in different culture media. In absence of serum, EGF led to the expression of the neuronal marker beta-tubulin-III and acquisition of neuronal morphology in 15% of the cells. Analysis of cell proliferation by bromodeoxyuridine incorporation revealed that EGF mainly induced the formation of neurons without stimulating cell cycle progression. Moreover, a pulse of 2-hour EGF stimulation was sufficient to induce neuronal differentiation. Some neurons were committed to the retinal ganglion cell (RGC) phenotype, as revealed by the expression of retinal ganglion markers (Ath5, Brn3b, and melanopsin) and in a few cases to other retinal phenotypes (photoreceptors [PRs] and bipolar cells). We confirmed that the late RSCs were not restricted over time and that they conserved their multipotency by generating retinal phenotypes that usually appear at early (RGC) or late (PRs) developmental stages. Our results show that EGF is not only a factor controlling glial development, as previously shown, but also a potent differentiation factor for retinal neurons, at least in vitro.

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.

Generation of Rx+/Pax6+ neural retinal precursors from embryonic stem cells

We report directed differentiaion of retinal precursors in vitro from mouse ES cells. Six3+ rostral brain progenitors are generated by culturing ES cells under serum-free suspension conditions (SFEB culture) in the presence of Wnt and Nodal antagonists (Dkk1 and LeftyA), and subsequently steered to differentiate into Rx+ cells (16%) by treatment with activin and serum. Consistent with the characteristics of early neural retinal precursors, the induced Rx+ cells coexpress Pax6 and the mitotic marker Ki67, but not Nestin. The ES cell-derived precursors efficiently generate cells with the photoreceptor phenotype (rhodopsin+, recoverin+) when cocultured with embryonic retinal cells. Furthermore, organotypic culture studies demonstrate the selective integration and survival of ES cell-derived cells with the photoreceptor phenotype (marker expression and morphology) in the outer nuclear layer of the retina. Taken together, ES cells treated with SFEB/Dkk1/LeftyA/serum/activin generate neural retinal precursors, which have the competence of photoreceptor differentiation.

Vaccinia virus infection and gene transduction in cultured neurons

The study of neurons in culture would benefit from the development of a gene transduction system capable of delivering foreign genes at high efficiency, as transduction of primary neurons with existing systems is inefficient. The efficacy of lytic vaccinia virus (VV) infection of primary retinal cultures and PC12 cells (a model of neuronal differentiation) was examined in order to determine the efficiency of gene transduction using VV in neuronal primary culture. VV was able to infect retinal cells and PC12 cells and express transgenes of Escherichia coli beta-galactosidase (lacZ) and epithelial fatty acid binding protein (E-FABP) in a virus dose-dependent manner. Most (50-100%) of the retinal cells were positive for transgene protein at multiplicities of infection (MOI) between 10 and 100 plaque-forming units (PFU), while over 50% of VV-infected PC12 cells expressed the virus encoded gene at an MOI = 10. The production of foreign mRNA and protein by VV following infection was verified by PCR and Western blot. Because VV is a lytic virus, cytopathic effects were examined. Retinal cultures maintained for 0.5 days in vitro showed greater than 90% survival at 24 h post-infection, while 14-day cultures were equally viable for 48 h. Retinal ganglion cells and differentiated PC12 cells appear to be more protected against lytic VV infection than proliferating glial and undifferentiated PC12 cells. These data suggest that VV may be a useful vector for delivering foreign genes to neuronal cells with an efficient transient transgene expression.

Expression of Fgf19 in the developing chick eye

Fibroblast growth factor 19 (FGF19) is a new member of the FGF family of growth factors. Here, we describe the localization of Fgf19 mRNA in the developing chick retina and lens in stages from the Hamburger and Hamilton stage 15 (HH15) to postnatal day 30 (P30). Fgf19 was expressed in a transient manner in postmitotic neuroblasts during the migration from the ventricular surface to their final location. Moreover, from HH31 (embryonic day 7, E7) on, a subset of lined up Fgf19 expressing cells was distributed in the outer region of the presumptive INL. These cells were Pax6 immunoreactive horizontal cells. During the last third of embryogenesis, Fgf19 expression in the retina was progressively down-regulated and was not detected at P30. Also, it was transiently expressed in the equatorial region of the lens.

Importance of intrinsic mechanisms in cell fate decisions in the developing rat retina

Cell diversification in the developing nervous system is thought to involve both cell-intrinsic mechanisms and extracellular signals, but their relative importance in particular cell fate decisions remains uncertain. In the mammalian retina, different cell types develop on a predictable schedule from multipotent retinal neuroepithelial cells (RNECs). A current view is that RNECs pass through a series of competence states, progressively changing their responsiveness to instructive extracellular cues, which also change over time. We show here, however, that embryonic day 16-17 (E16-17) rat RNECs develop similarly in serum-free clonal-density cultures and in serum-containing retinal explants--in the number of times they divide, the cell types they generate, and the order in which they generate these cell types. These surprising results suggest that extracellular signals may be less important than currently believed in determining when RNECs stop dividing and what cell types they generate when they withdraw from the cell cycle, at least from E16-17 onward.

Density-dependent differentiation in nontransformed human retinal progenitor cells in response to basic fibroblast growth factor- and transforming growth factor-alpha

Multipotential retinal precursors give rise to all cell types seen in multilayered retina. The generation of differentiation and diversity of neuronal cell types is determined by both extrinsic regulatory signals and endogenous genetic programs. We have previously reported that cell commitment in human retinal precursor cells (SV-40T) can be modified in response to exogenous growth factors, basic fibroblast growth factor, and transforming growth factor alpha (bFGF and TGFalpha). We report in this study that nontransformed human retinal precursors differentiate into photoreceptors by a cell density-dependent mechanism, and the effects were potentiated by bFGF and TGFalpha alone or in combination. A larger proportion of multipotential precursors plated at a density of 1 x 10(4) cells/cm(2) differentiated into neurons (photoreceptors) compared to cells plated at 3-5 x 10(4)/cm(2) and 1 x 10(5) cells/cm(2) under serum-free conditions and the effects were amplified seven- to eightfold in response to growth factors. Basic fibroblast growth factor (bFGF) and TGFalpha can induce 90% of the cells to assume a photoreceptor phenotype at a lower cell density, compared to only 30 and 25% of the cells acquiring a photoreceptor phenotype at intermediate and higher cell densities. Furthermore, at a lower cell density, 60-70% of the cells incorporate Bromodeoxyuridine (Brdu), suggesting that cells in a cell cycle may make a commitment to a specific fate in response to neurotrophins. Neurons with a photoreceptor phenotype were positive for three different sets of antibodies for rods/cones. Cells also exhibited upregulation of other proteins such as a D4 receptor protein expressed in photoreceptors, protein kinase Calpha (PKCalpha) expressed in rod bipolars and blue cones, and some other neuronal cell types. This was also confirmed by Western blot analysis. Newly derived photoreceptors survive for a few days before significant cell death ensues under serum-free conditions. To summarize, differentiation in precursors is density dependent, and growth factors amplify the effects.

In vitro generation of early-born neurons from late retinal progenitors

Evidence suggests that, as development ensues, the competence of neural progenitors is progressively altered, such that they become fated to give rise to neurons of a particular stage. Here, we demonstrate that late retinal progenitors can give rise to retinal ganglion cells (RGCs), an example of an early-born cell type in the retina. A subset of late retinal progenitors in vitro responds to cues that favor RGC differentiation by displaying markers characteristic of RGCs. In addition, mechanisms used during normal RGC differentiation are recruited by these cells toward their differentiation along RGC lineage. Our observations suggest that late neural progenitors may not be irreversibly fated but may appear as such under the constraints dictated by epigenetic cues.

Ectopic photoreceptors and cone bipolar cells in the developing and mature retina

An antibody against recoverin, the calcium-binding protein, labels photoreceptors, cone bipolar cells, and a subpopulation of cells in the ganglion cell layer. In the present study, we sought to establish the origin and identity of the cells expressing recoverin in the ganglion cell layer of the rat retina. By double labeling with rhodopsin, we demonstrate that early in development some of the recoverin-positive cells in the ganglion cell layer are photoreceptors. During the first postnatal week, these rhodopsin-positive cells are eliminated from the ganglion cell layer, but such neurons remain in the inner nuclear layer well into the first postnatal month. Another contingent of recoverin-positive cells, with morphological features equivalent to those of bipolar cells, is present in the postnatal retina, and approximately 50% of these neurons survive to maturity. The incidence of such cells in the ganglion cell layer was not affected by early transection of the optic nerve, a manipulation that causes rapid loss of retinal ganglion cells. These recoverin-positive cells were not double-labeled by cell-specific markers expressed by photoreceptors, rod bipolar cells, or horizontal and amacrine cells. Based on their staining with recoverin and salient morphological features, these ectopic profiles in the ganglion cell layer are most likely cone bipolar cells. Collectively, the results provide evidence for photoreceptors in the ganglion cell and inner nuclear layers of the developing retina, and a more permanent subpopulation of cone bipolar cells displaced to the ganglion cell layer.

Regulation of ganglion cell production by Notch signaling during retinal development

Although progenitor cells in developing vertebrate retina are capable of producing all retinal cell types, they are competent to produce only certain cell types at a given time, and this competence changes as development progresses. We asked whether a change in progenitor cell competence is primarily responsible for ending production of a specific cell type, the retinal ganglion cell. Reducing Notch expression using an antisense oligonucleotide in vitro or in vivo increased ganglion cell genesis. The antisense treatment could reinitiate ganglion cell genesis after it had terminated in a region of the retina, but only for a brief period. The failure of the Notch antisense treatment to reinitiate ganglion cell production after this period was not due to the lack of receptor or ligand expression, as both Notch-1 and Delta-1 were still expressed. The failure of the Notch antisense treatment to reinitiate ganglion cell production is consistent with the suggestion that the intrinsic competence of progenitor cells changes as development progresses. Because reducing Notch signaling can reinitiate ganglion cell production for a brief period after ganglion cell production has normally ceased, it appears that ganglion cell production initially ends in a region of the retina because of cell-cell interactions and not because progenitor cells lose the competence to make ganglion cells. Notch signaling appears to temporarily prevent production of ganglion cells in a region, while some other signal must initiate a change in progenitor cell competence, thus permanently ending the possibility of further ganglion cell production.

Neuronal differentiation of adult rat hippocampus-derived neural stem cells transplanted into embryonic rat explanted retinas with retinoic acid pretreatment

The purpose of this study was to evaluate the effects of the retinal environment and retinoic acid (RA) pretreatment on the differentiation of transplanted adult rat hippocampus-derived neural stem cells (AHSCs). AHSCs were transplanted into embryonic (E18) or neonatal (P6) rat retinal explants and the mixture was cultured for 2 weeks. Other AHSCs were stimulated by 0.5 microM all-trans RA for 6 days before transplantation. Immunofluorescent double staining showed that a larger number of AHSCs became beta-tubulin III-positive neurons in the E18 than in P6 retinas. In addition, many AHSCs became MAP2ab-positive and MAP5-positive neurons following RA pretreatment and transplantation. Only a few AHSCs became HPC-1-, calbindin-, PKC- or rhodopsin-positive cells under these conditions. We conclude that the microenvironment supplied by embryonic retinas is conductive to neuronal differentiation in general. RA stimulation before transplantation was also effective in stimulating differentiation.

Subtype-specific regulation of the expression and function of muscarinic acetylcholine receptors in embryonic chicken retinal cells

We examined the effect of long-term agonist exposure on muscarinic acetylcholine receptor expression and function in embryonic chicken retinal cells. Long-term carbachol exposure induced a time- and concentration-dependent decrease in M2, M3 and M4 muscarinic receptor numbers. Kinetic analyses revealed a first-order process with similar rate constants for all three subtypes. Both the maximal decrease and the agonist potency for regulation of M3 were significantly higher than those for M2 and M4. Upon agonist removal, M2 and M4 numbers returned to control values, but M3 recovery after 24 h was no higher than 40%. Agonist treatment did not alter the levels of receptor mRNAs. Receptor inactivation with a covalent alkylating antagonist demonstrated that the partial M3 protein recovery was not due to a decreased intrinsic basal rate of synthesis, suggesting that it is induced by agonist treatment. Prolonged carbachol exposure induced concomitant decreases in muscarinic-mediated inhibition of cyclic AMP accumulation which were completely reversed after agonist removal. Sustained receptor activation also promoted significant decreases in muscarinic receptor-stimulated phosphoinositide turnover, which were only partially reversed after agonist removal. These data demonstrate subtype-specific regulation of the expression and function of muscarinic receptors in the retina.

Hair cell development in vivo and in vitro: analysis by using a monoclonal antibody specific to hair cells in the chick inner ear

The purpose of this study was to establish a hair cell-specific marker and a convenient explant culture system for developing chick otocysts to facilitate in vivo and in vitro studies focusing on hair cell genesis in the inner ear. To achieve this, a hair cell-specific monoclonal antibody, 2A7, was generated by immunizing chick inner ear tissues to a mouse. Through the use of immunofluorescence and immunoelectron microscopy, it was shown that 2A7 immunoreactivity (2A7-IR) was primarily restricted to the apical region of inner ear hair cells, including stereocilia, kinocilia, apical membrane amongst the extending cilia, and superficial layer of the cuticular plate. Although the 2A7 antibody immunolabeled basically all of the hair cells in the posthatch chick inner ear, two different patterns of 2A7-IR were observed; hair cells located in the striolar region of the utricular macula, which consist of two distinct cell types identifiable on the basis of the type of nerve ending, Type I and II hair cells, showed labeling restricted to the basal end of the hair bundles. On the other hand, hair cells in the extrastriolar region, which are exclusively of Type II, showed labeling extending over virtually the entire length of the bundles. These findings raised the possibility that chick vestibular Type II hair cells, characterized by their bouton-type afferent nerve endings, can be divided into two subpopulations. Analysis of developing inner ear by using the 2A7 antibody revealed that this antibody also recognizes newly differentiated immature hair cells. Thus, the 2A7 antibody is able to recognize both immature and mature hair cells in vivo. The developmental potential of embryonic otocysts in vitro was then assessed by using explant cultures as a model. In this study, conventional otocyst explant cultures were modified by placing the tissues on floating polycarbonate filters on culture media, thereby allowing the easy manipulation of explants. In these cultures, 2A7-positive hair cells were differentiated from dividing precursor cells in vitro on the same schedule as in vivo. Furthermore, it was found that hair cells with both types of 2A7-IR were generated in culture as in vivo, indicating that a maturational process of hair cells also occurred. All these results as presented here suggest that the 2A7 monoclonal antibody as a hair cell-specific marker together with the culture system could be a potential tool in analysis of mechanisms underlying hair cell development.

The cyclin-dependent kinase inhibitors p19(Ink4d) and p27(Kip1) are coexpressed in select retinal cells and act cooperatively to control cell cycle exit

Cyclin-dependent kinase inhibitors (cdki's), including p19(Ink4d) and p27(Kip1), mediate exit from the cell cycle. To determine the function of these cdki's in regulating neurogenesis, we examined retina from wild-type, Ink4d-null, and Ink4d/Kip1-double null animals. Ink4d was expressed in progenitors and select neurons in the mature retina. Ink4d-null retina showed an extended period of proliferation, followed by apoptosis. Colabeling for p19(Ink4d) and p27(Kip1) revealed that a subpopulation of cells expressed both inhibitors. Deletion of Ink4d and Kip1 resulted in continued proliferation that was synergistic. This hyperproliferation led to an increase in number of horizontal cells and differentiated neurons reentering the cell cycle. Deletion of Ink4d and Kip1 also exacerbated the retinal dysplasia observed in Kip1-null mice, which was shown to be partly dependent on p53. These data indicate that select retinal cells express both p19(Ink4d) and p27(Kip1) and that they act cooperatively to ensure cell cycle exit.

Induction and axonal localization of epithelial/epidermal fatty acid-binding protein in retinal ganglion cells are associated with axon development and regeneration

Epithelial/epidermal fatty acid-binding protein (E-FABP) is induced in peripheral neurons during nerve regeneration and is found at high levels in central neurons during neuronal migration and development. Furthermore, E-FABP expression is required for normal neurite outgrowth in PC12 cells treated with nerve growth factor (NGF). The present study examined whether E-FABP plays a role in retinal ganglion cell (RGC) differentiation and axon growth. Rat retinal tissues from embryonic (E) and postnatal (P) development through adulthood were examined using immunocytochemical labeling with E-FABP and growth-associated protein 43 (GAP-43) antibodies. E-FABP colocalized with GAP-43 at E14 through P10. At E14, E-FABP immunoreactivity was confined to the somas of GAP-43-positive cells in the ganglion cell layer, but it was localized to their axons by E15. The axons in the optic nerve were GAP-43-positive and E-FABP-negative on E15, but the two proteins were colocalized by E18. Retinal cultures at E15 confirmed that E-FABP and GAP-43 colocalize in RGCs. Postnatally, labeling was present between P1 and P10 but decreased at older ages and was minimally present or absent in adult animals. Western immunoblotting revealed that at E18, P1, and P10 E-FABP levels were at least fourfold greater than those in the adult. By P15, protein levels were only twofold greater, with adult levels reached by P31. Furthermore, E-FABP could be reinduced during axon regeneration. Dissociated P15 retinal cells cultured in the presence of brain-derived neurotrophic factor, ciliary neurotrophic factor, and basic fibroblast growth factor exhibited sixfold more GAP-43 and E-FABP double-positive RGCs (cell body and axons) than controls. Moreover, all GAP-43-immunoreactive RGCs were also positive for E-FABP. Taken together, these results indicate the following: 1) E-FABP is expressed in RGCs as they reached the ganglion cell layer and 2) E-FABP plays a functional role in the elaboration of RGC axons in both development and regeneration.

The developmental biology of brain tumors

Tumors of the central nervous system (CNS) can be devastating because they often affect children, are difficult to treat, and frequently cause mental impairment or death. New insights into the causes and potential treatment of CNS tumors have come from discovering connections with genes that control cell growth, differentiation, and death during normal development. Links between tumorigenesis and normal development are illustrated by three common CNS tumors: retinoblastoma, glioblastoma, and medulloblastoma. For example, the retinoblastoma (Rb) tumor suppressor protein is crucial for control of normal neuronal differentiation and apoptosis. Excessive activity of the epidermal growth factor receptor and loss of the phosphatase PTEN are associated with glioblastoma, and both genes are required for normal growth and development. The membrane protein Patched1 (Ptc1), which controls cell fate in many tissues, regulates cell growth in the cerebellum, and reduced Ptc1 function contributes to medulloblastoma. Just as elucidating the mechanisms that control normal development can lead to the identification of new cancer-related genes and signaling pathways, studies of tumor biology can increase our understanding of normal development. Learning that Ptc1 is a medulloblastoma tumor suppressor led directly to the identification of the Ptc1 ligand, Sonic hedgehog, as a powerful mitogen for cerebellar granule cell precursors. Much remains to be learned about the genetic events that lead to brain tumors and how each event regulates cell cycle progression, apoptosis, and differentiation. The prospects for beneficial work at the boundary between oncology and developmental biology are great.

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.

Developmental expression of muscarinic acetylcholine receptors in chick retina: selective induction of M2 muscarinic receptor expression in ovo by a factor secreted by muller glial cells

Muscarinic acetylcholine receptors (mAChRs) play an important role in signal processing in the retina. We have used subtype-specific antibodies to identify the changes in the localization of mAChR expression during embryonic development of the retina in vivo and their relationship to the changes in mAChRs in retinal cells in culture. We have demonstrated previously that treatment of fresh retinal cultures with conditioned media from mature retinal cultures specifically induces expression of the M(2) mAChR (McKinnon et al., 1998). We show that the M(2)-inducing activity, which we tentatively have called MARIA (muscarinic acetylcholine receptor-inducing activity) is produced by Müller glial cells in culture, because significant activity can be found in media conditioned by essentially neuron-free cultures of Müller glia, as well as by a Müller glial cell line but not several neuroblastoma cell lines. We also demonstrate that the appearance of the M(2) receptor in vivo occurs concomitantly with the appearance of significant numbers of Müller glial cells in the developing retina. Furthermore, the administration of crude or partially purified preparations of MARIA to developing chick embryos in ovo induces precocious expression of M(2) mAChRs in the appropriate cell types in the retina. These results show that a factor secreted by cultured retinal Müller glia can regulate M(2) mAChR expression in vivo and in vitro and suggest that the secretion of MARIA by Müller glia in vivo may be responsible for the normal induction of M(2) mAChR expression during embryonic development.

Neural bHLH genes control the neuronal versus glial fate decision in cortical progenitors

We have addressed the role of the proneural bHLH genes Neurogenin2 (Ngn2) and Mash1 in the selection of neuronal and glial fates by neural stem cells. We show that mice mutant for both genes present severe defects in development of the cerebral cortex, including a reduction of neurogenesis and a premature and excessive generation of astrocytic precursors. An analysis of wild-type and mutant cortical progenitors in culture showed that a large fraction of Ngn2; Mash1 double-mutant progenitors failed to adopt a neuronal fate, instead remaining pluripotent or entering an astrocytic differentiation pathway. Together, these results demonstrate that proneural genes are involved in lineage restriction of cortical progenitors, promoting the acquisition of the neuronal fate and inhibiting the astrocytic fate.

The autoregulation of retinal ganglion cell number

The development of the nervous system is dependent on a complex set of signals whose precise co-ordination ensures that the correct number of neurones are generated. This regulation is achieved through a variety of cues that influence both the generation and the maintenance of neurones during development. We show that in the chick embryo, stratified retinal ganglion cells (RGCs) are themselves responsible for providing the signals that control the number of RGCs that are generated, both by inhibiting the generation of new ganglion cells and by killing incoming migratory ganglion cells. Selective toxicological ablation of RGCs in the chick embryo resulted in the achronic generation of ganglion cells, which eventually led to the repopulation of the ganglion cell layer and a large decrease in the physiological cell death affecting postmitotic migratory neurones. Interestingly, the application of exogenous NGF reversed the effects of ganglion cell ablation on ganglion cell death. Because the only source of NGF in the retina is that produced by the stratified ganglion cells, we infer that these differentiated neurones regulate their own cell number by secreting NGF, a neurotrophin that has previously been shown to be responsible for the death of migrating ganglion cells.

Isolation of radial glial cells by fluorescent-activated cell sorting reveals a neuronal lineage

The developing central nervous system of vertebrates contains an abundant cell type designated radial glial cells. These cells are known as guiding cables for migrating neurons, while their role as precursor cells is less clear. Since radial glial cells express a variety of astroglial characteristics and differentiate as astrocytes after completing their guidance function, they have been considered as part of the glial lineage. Using fluorescence-activated cell sorting, we show here that radial glial cells also are neuronal precursors and only later, after neurogenesis, do they shift towards an exclusive generation of astrocytes. These results thus demonstrate a novel function for radial glial cells, namely their ability to generate two major cell types found in the nervous system, neurons and astrocytes.

Cell fate decisions in a human retinal precursor cell line: basic fibroblast growth factor- and transforming growth factor-alpha-mediated differentiation

The purpose of this study was to determine if immortalized human retinal precursor cells could serve as a model to investigate cues that modulate cell fate and differentiation. We investigated the effects of a variety of growth factors broadly but specifically tested the effects of basic fibroblast growth factor (bFGF) and transforming growth factor (TGF)a in retinal cell differentiation and commitment. To determine the role of exogenously added growth factors in a human retinal precursor cell line (KGLDMSM), established from a first-trimester retina, cells were adapted to grow in a defined medium and exposed to a variety of trophic factors (epidermal growth factor [EGF], neuron growth factor [NGF], TGFalpha, TGFbeta, acidic FGF, and bFGF). Dose-response curves were developed to arrive at optimal concentrations. The neurotrophic potential of growth factors was determined by 3H-thymidine incorporation and bromodeoxyuridine (BrdU) labeling. The identity of the emerging neuronal phenotypes were determined by phase-contrast microscopy, immunolabeling for the neuron-specific antigens neurofilament protein (NF) and neuron-specific enolases (NSE), and photoreceptor-specific antigens (Rho1D4, 7G6) using immunocytochemistry and Western blot analysis. To identify some of the early response genes (c-fos, c-myc) expressed in response to growth factors, Northern blot analysis was performed. Almost all of the factors tested increased the total number of cells with a neuronal phenotype. Potency of growth factors to generate neurons was TGFalpha > bFGF > EGF > NGF. Both TGFalpha and bFGF, alone or in combination, increased the total number of neurons. Most of the neurons generated were photoreceptors, as depicted by the polarized phenotype, expression of photoreceptor-specific antigens, and processes resembling rudimentary outer segments. The increase in photoreceptor-like neurons is possibly attributable to an increase in numbers rather than greater survival. Additionally, the majority of the photoreceptors generated labeled with BrdU and for photoreceptor-specific antigens, suggesting that an inductive effect of bFGF and TGFalpha could occur in the cell cycle or shortly thereafter. Both bFGF and TGFalpha induced the expression of the early response gene c-fos while not altering the expression of c-actin or c-myc. The emergence of a photoreceptor phenotype was confirmed by both immunocytochemistry and Western blot analysis. The immortalized retinal precursor cell line could prove valuable in determining the role of exogenously added growth factors in retinal development and differentiation. Both bFGF and TGFalpha enhance the photoreceptor phenotype in medium-density cultures under conditions of defined medium. The same was confirmed by phase-contrast microscopy, immunocytochemistry, and Western blot analysis. Furthermore, cell fate determination in cultured precursor cells could occur during the late part of the cell cycle or shortly after completion of cell division. The effects of TGFalpha and bFGF seem to be slightly additive. The cell line will be extremely valuable in studying mechanisms of cell commitment and generation of retinal cell types, which could be tested for their potential for transplantation.

Intrinsic bias and lineage restriction in the phenotype determination of dopamine and neuropeptide Y amacrine cells

Blastomere lineages are differentially biased to produce different neurotransmitter subtypes of amacrine cells (Huang and Moody, 1995, 1997,). To elucidate when this bias is acquired, we examined amacrine lineages at different early developmental times. Our experiments demonstrate that the bias to express dopamine and neuropeptide Y amacrine fates involves several steps before the formation of the definitive optic cup. At cleavage stages, a retinal progenitor that contributes large numbers of cells is already biased to produce its normal repertoire of dopamine amacrine cells, as revealed by transplantation to a new location, whereas the amacrine fate of a progenitor that contributes fewer cells is modified by its new position. At neural plate stages, not all retinal progenitors are multipotent. Nearly one-half populate only the inner nuclear layer and are enriched in amacrine cells. During early optic vesicle stages, an appropriate mitotic tree is required for dopamine and neuropeptide Y, but not serotonin, amacrine cell clusters to form. Thus, the acquisition of amacrine fate bias involves intrinsic maternal factors at cleavage, fate restriction in the neural plate, and specified mitotic patterns in the optic vesicle. At each of these steps only a subset of the embryonic retinal progenitors contributing to amacrine subtypes is biased; the remaining progenitors maintain multipotency. Thus, from the earliest embryonic stages, progenitors of the retina are a dynamic mosaic. This is the first experimental demonstration of amacrine fate decisions that occur during early embryonic periods in advance of the events described in the later, committed retina.

Overexpression of FGF-2 alters cell fate specification in the developing retina of Xenopus laevis

The developing vertebrate retina produces appropriate ratios of seven phenotypically and functionally distinct cell types. Retinal progenitors remain multipotent up until the last cell division, favoring the idea that extrinsic cues direct cell fate. We demonstrated previously that fibroblast growth factor (FGF) receptors are necessary for transduction of signals in the developing Xenopus retina that bias cell fate decisions (S. McFarlane et al., 1998, Development 125, 3967-3975). However, the precise identity of the signal remains unknown. To test whether an FGF signal is sufficient to influence cell fate choices in the developing retina, FGF-2 was overexpressed in Xenopus retinal precursors by injecting, at the embryonic 16-cell stage, a cDNA plasmid encoding FGF-2 into cells fated to form the retina. We found that FGF-2 overexpression in retinal precursors altered the relative numbers of transgene-expressing retinal ganglion cells (RGC) and Müller glia; RGCs were increased by 35% and Müller glia decreased by 50%. In contrast, the proportion of retinal precursors that became photoreceptors was unchanged. Within the photoreceptor population, however, we found a twofold increase in rod photoreceptors at the expense of cone photoreceptors. These data are consistent with an endogenous FGF signal influencing cell fate decisions in the developing vertebrate retina.