Ciliary neurotrophic factor promotes chick photoreceptor development in vitro

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.

Ciliary neurotrophic factor (CNTF) protects retinal cone and rod photoreceptors by suppressing excessive formation of the visual pigments

The retinal pigment epithelium (RPE)-dependent visual cycle provides 11-cis-retinal to opsins in the photoreceptor outer segments to generate functional visual pigments that initiate phototransduction in response to light stimuli. Both RPE65 isomerase of the visual cycle and the rhodopsin visual pigment have recently been identified as critical players in mediating light-induced retinal degeneration. These findings suggest that the expression and function of RPE65 and rhodopsin need to be coordinately controlled to sustain normal vision and to protect the retina from photodamage. However, the mechanism controlling the development of the retinal visual system remains poorly understood. Here, we show that deficiency in ciliary neurotrophic factor (CNTF) up-regulates the levels of rod and cone opsins accompanied by an increase in the thickness of the outer nuclear layers and the lengths of cone and rod outer segments in the mouse retina. Moreover, retinoid isomerase activity, expression levels of RPE65 and lecithin:retinol acyltransferase (LRAT), which synthesizes the RPE65 substrate, were also significantly increased in the Cntf ^-/- RPE. Rod a-wave and cone b-wave amplitudes of electroretinograms were increased in Cntf ^-/- mice, but rod b-wave amplitudes were unchanged compared with those in WT mice. Up-regulated RPE65 and LRAT levels accelerated both the visual cycle rate and recovery rate of rod light sensitivity in Cntf ^-/- mice. Of note, rods and cones in Cntf ^-/- mice exhibited hypersusceptibility to light-induced degeneration. These results indicate that CNTF is a common extracellular factor that prevents excessive production of opsins, the photoreceptor outer segments, and 11-cis-retinal to protect rods and cones from photodamage.

The dose-dependent macular thickness changes assessed by fd-oct in patients with retinitis pigmentosa treated with ciliary neurotrophic factor

PURPOSE

To evaluate the effect of intravitreal ciliary neurotrophic factor (CNTF) implant on mean macular thickness (MMT) in eyes with retinitis pigmentosa using high-resolution Fourier domain optical coherence tomography imaging.

METHODS

A cohort of 8 patients (CNTF-3: n = 5; CNTF-4: n = 3) enrolled in Neurotech sponsored Phase 2 clinical trial underwent Fourier domain optical coherence tomography imaging. A ≥3% change in MMT from baseline or fellow eye was considered as a measurable change.

RESULTS

Two patients enrolled in the CNTF-3 study received low-dose implant. At 18 months, a change in MMT from -4.47 μm to 6 μm from baseline was noted. Six patients received high-dose implant (CNTF-3: n = 3; CNTF-4: n = 3). In CNTF-3 group, 1 eye showed an increase in MMT by 19.25 μm (+7.6%) from baseline at 18 months. In CNTF-4 group, 1 eye had an increase in MMT of 27.08 μm (+11%) from baseline at 30 months; second eye had increase in MMT of 31.36 μm (+12%) from contralateral eye. Amongst these 3 responsive high-dose implant eyes, overall thickening of the retina could not be attributed to any specific retinal layer.

CONCLUSION

A heterogeneous dose-dependent response on MMT was noted in eyes treated using intravitreal CNTF implant for retinitis pigmentosa. We recommend corroboration of our findings with Neurotech sponsored clinical trial results.

Making of a retinal cell: insights into retinal cell-fate determination

Understanding the process by which an uncommitted dividing cell produces particular specialized cells within a tissue remains a fundamental question in developmental biology. Many tissues are well suited for cell-fate studies, but perhaps none more so than the developing retina. Traditionally, experiments using the retina have been designed to elucidate the influence that individual environmental signals or transcription factors can have on cell-fate decisions. Despite a substantial amount of information gained through these studies, there is still much that we do not yet understand about how cell fate is controlled on a systems level. In addition, new factors such as noncoding RNAs and regulators of chromatin have been shown to play roles in cell-fate determination and with the advent of "omics" technology more factors will most likely be identified. In this chapter we summarize both the traditional view of retinal cell-fate determination and introduce some new ideas that are providing a challenge to the older way of thinking about the acquisition of cell fates.

Randomized trial of ciliary neurotrophic factor delivered by encapsulated cell intraocular implants for retinitis pigmentosa

PURPOSE

To evaluate the safety and effect on visual function of ciliary neurotrophic factor delivered via an intraocular encapsulated cell implant for the treatment of retinitis pigmentosa (RP).

DESIGN

Ciliary neurotrophic factor for late-stage retinitis pigmentosa study 3 (CNTF3; n = 65) and ciliary neurotrophic factor for early-stage retinitis pigmentosa study 4 (CNTF4; n = 68) were multicenter, sham-controlled dose-ranging studies.

METHODS

Patients were randomly assigned to receive a high- or low-dose implant in 1 eye and sham surgery in the fellow eye. The primary endpoints were change in best-corrected visual acuity (BCVA) at 12 months for CNTF3 and change in visual field sensitivity at 12 months for CNTF4. Patients had the choice of retaining or removing the implant at 12 months for CNTF3 and 24 months for CNTF4.

RESULTS

There were no serious adverse events related to either the encapsulated cell implant or the surgical procedure. In CNTF3, there was no change in acuity in either ciliary neurotrophic factor- or sham-treated eyes at 1 year. In CNTF4, eyes treated with the high-dose implant showed a significant decrease in sensitivity while no change was seen in sham- and low dose-treated eyes at 12 months. The decrease in sensitivity was reversible upon implant removal. In both studies, ciliary neurotrophic factor treatment resulted in a dose-dependent increase in retinal thickness.

CONCLUSIONS

Long-term intraocular delivery of ciliary neurotrophic factor is achieved by the encapsulated cell implant. Neither study showed therapeutic benefit in the primary outcome variable.

Ex vivo electroporation of retinal cells: a novel, high efficiency method for functional studies in primary retinal cultures

Primary retinal cultures constitute valuable tools not only for basic research on retinal cell development and physiology, but also for the identification of factors or drugs that promote cell survival and differentiation. In order to take full advantage of the benefits of this system it is imperative to develop efficient and reliable techniques for the manipulation of gene expression. However, achieving appropriate transfection efficiencies in these cultures has remained challenging. The purpose of this work was to develop and optimize a technique that would allow the transfection of chick retinal cells with high efficiency and reproducibility for multiple applications. We developed an ex vivo electroporation method applied to dissociated retinal cell cultures that offers a significant improvement over other currently available transfection techniques, increasing efficiency by five-fold. In this method, eyes were enucleated, devoid of RPE, and electroporated with GFP-encoding plasmids using custom-made electrodes. Electroporated retinas were then dissociated into single cells and plated in low density conditions, to be analyzed after 4 days of incubation. Parameters such as voltage and number of electric pulses, as well as plasmid concentration and developmental stage of the animal were optimized for efficiency. The characteristics of the cultures were assessed by morphology and immunocytochemistry, and cell viability was determined by ethidium homodimer staining. Cell imaging and counting was performed using an automated high-throughput system. This procedure resulted in transfection efficiencies in the order of 22-25% of cultured cells, encompassing both photoreceptors and non-photoreceptor neurons, and without affecting normal cell survival and differentiation. Finally, the feasibility of the technique for cell-autonomous studies of gene function in a biologically relevant context was tested by carrying out gain and loss-of-function experiments for the transcription factor PAX6. Electroporation of a plasmid construct expressing PAX6 resulted in a marked upregulation in the expression levels of this protein that could be measured in the whole culture as well as cell-intrinsically. This was accompanied by a significant decrease in the percentage of cells differentiating as photoreceptors among the transfected population. Conversely, electroporation of an RNAi construct targeting PAX6 resulted in a significant decrease in the levels of this protein, with a concomitant increase in the proportion of photoreceptors. Taken together these results provide strong proof-of-principle of the suitability of this technique for genetic studies in retinal cultures. The combination of the high transfection efficiency obtained by this method with automated high-throughput cell analysis supplies the scientific community with a powerful system for performing functional studies in a cell-autonomous manner.

Isolation and culture of rat cone photoreceptor cells

In retinal diseases characterized by photoreceptor degeneration, the main cause of clinically significant vision loss is cone, rather than rod, loss. Photoreceptor apoptosis can be affected by the availability and/or delivery of neurotrophic proteins, and cultures of photoreceptors are valuable for studying these processes. In the present study, a technique was designed to purify cones to make it possible to screen for neuroprotective molecules. The present chapter describes a technique for preparing cultures of purified rat retina cone photoreceptors based upon panning with peanut agglutinin lectin, which selectively binds to cones. In addition, we describe a protocol for the purification and culture of retinal pigmented epithelial cells from postnatal rat.

Ophthalmic drug discovery: novel targets and mechanisms for retinal diseases and glaucoma

Blindness affects 60 million people worldwide. The leading causes of irreversible blindness include age-related macular degeneration, retinal vascular diseases and glaucoma. The unique features of the eye provide both benefits and challenges for drug discovery and delivery. During the past decade, the landscape for ocular drug therapy has substantially changed and our knowledge of the pathogenesis of ophthalmic diseases has grown considerably. Anti-angiogenic drugs have emerged as the most effective form of therapy for age-related macular degeneration and retinal vascular diseases. Lowering intraocular pressure is still the mainstay for glaucoma treatment but neuroprotective drugs represent a promising next-generation therapy. This Review discusses the current state of ocular drug therapy and highlights future therapeutic opportunities.

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.

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.

Transplantation of bone marrow-derived mesenchymal stem cells into the developing mouse eye

Mesenchymal stem cells (MSCs) have been studied widely for their potential to differentiate into various lineage cells including neural cells in vitro and in vivo. To investigate the influence of the developing host environment on the integration and morphological and molecular differentiation of MSCs, human bone marrow-derived mesenchymal stem cells (BM-MSCs) were transplanted into the developing mouse retina. Enhanced green fluorescent protein (GFP)-expressing BM-MSCs were transplanted by intraocular injections into mice, ranging in ages from 1 day postnatal (PN) to 10 days PN. The survival dates ranged from 7 days post-transplantation (DPT) to 28DPT, at which time an immunohistochemical analysis was performed on the eyes. The transplanted BM-MSCs survived and showed morphological differentiation into neural cells and some processes within the host retina. Some transplanted cells expressed microtubule associated protein 2 (MAP2ab, marker for mature neural cells) or glial fibrillary acid protein (GFAP, marker for glial cells) at 5PN 7DPT. In addition, some transplanted cells integrated into the developing retina. The morphological and molecular differentiation and integration within the 5PN 7DPT eye was greater than those of other-aged host eye. The present findings suggest that the age of the host environment can strongly influence the differentiation and integration of BM-MSCs.

Ciliary neurotrophic factor delivered by encapsulated cell intraocular implants for treatment of geographic atrophy in age-related macular degeneration

There is no treatment available for vision loss associated with advanced dry age-related macular degeneration (AMD) or geographic atrophy (GA). In a pilot, proof of concept phase 2 study, we evaluated ciliary neurotrophic factor (CNTF) delivered via an intraocular encapsulated cell technology implant for the treatment of GA. We designed a multicenter, 1-y, double-masked, sham-controlled dose-ranging study. Patients with GA were randomly assigned to receive a high-or low-dose implant or sham surgery. The primary endpoint was the change in best corrected visual acuity (BCVA) at 12 mo. CNTF treatment resulted in a dose-dependent increase in retinal thickness. This change was followed by visual acuity stabilization (loss of less than 15 letters) in the high-dose group (96.3%) compared with low-dose (83.3%) and sham (75%) group. A subgroup analysis of those with baseline BCVA at 20/63 or better revealed that 100% of patients in the high-dose group lost <15 letters compared with 55.6% in the combined low-dose/sham group (P = 0.033). There was a 0.8 mean letter gain in the high-dose group compared with a 9.7 mean letter loss in the combined low-dose/sham group (P = 0.0315). Both the implant and the implant procedure were well-tolerated. These findings suggest that CNTF delivered by the encapsulated cell technology implant appears to slow the progression of vision loss in GA, especially in eyes with 20/63 or better vision at baseline.

Developmental regulation of neuronal survival by adenosine in the in vitro and in vivo avian retina depends on a shift of signaling pathways leading to CREB phosphorylation or dephosphorylation

Previous studies have shown a cAMP/protein kinase A-dependent neuroprotective effect of adenosine on glutamate or re-feeding-induced apoptosis in chick retina neuronal cultures. In the present work, we have studied the effect of adenosine on the survival of retinal progenitor cells. Cultures obtained from 6-day-old (E6) or from 8-day-old (E8) chick embryos were challenged 2 h (C0) or 1 day (C1) after seeding and analyzed after 3-4 days in vitro. Surprisingly, treatment with the selective A2a adenosine receptor agonists N(6) -[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) or 3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino]ethyl]phenyl]propanoic acid (CGS21680) promoted cell death when added at E6C0 but not at E6C1 or E8C0. DPMA-induced cell death involved activation of A2a receptors and the phospholipase C/protein kinase C but not the cAMP/protein kinase A pathway, and was not correlated with early modulation of precursor cells proliferation. Regarding cyclic nucleotide responsive element binding protein (CREB) phosphorylation, cultures from E6 embryos behave in an opposite manner from that from E8 embryos, both in vitro and in vivo. While the phospho-CREB level was high at E6C0 cultures and could be diminished by DPMA, it was lower at E8C0 and could be increased by DPMA. Similar to what was observed in cell survival studies, CREB dephosphorylation induced by DPMA in E6C0 cultures was dependent on the Phospholipase C/protein kinase C pathway. Accordingly, cell death induced by DPMA was inhibited by okadaic acid, a phosphatase blocker. Moreover, DPMA as well as the adenosine uptake blocker nitrobenzyl mercaptopurine riboside (NBMPR) modulate cell survival and CREB phosphorylation in a population of cells in the ganglion cell layer in vivo. These data suggest that A2a adenosine receptors as well as CREB may display a novel and important function by controlling the repertoire of developing retinal neurons.

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.

Function and mechanism of CNTF/LIF signaling in retinogenesis

Ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) exhibit multiple biological effects in the developing vertebrate retina. CNTF/LIF inhibits rod photoreceptor, and promotes bipolar cells and Muller glia differentiation. In addition, CNTF/LIF has been shown to have proliferative and apoptotic effects. Moreover, LIF also inhibits retinal vascular development. CNTF/LIF signaling components CNTFRalpha, LIFRbeta, gp130, and a number of STAT proteins are expressed in the retina. CNTF/LIF activates Jak-STAT, ERK, and Notch pathways during retinal development. Perturbation of CNTF induced signal transduction reveals that different combinations of CNTF/LIF signaling pathways regulate differentiation of retinal neurons and glia. Gene expression studies show that CNTF/LIF affects retinogenesis by regulating various genes involved in transcription, signal transduction, protein modification, apoptosis, protein localization, and cell ion homeostasis. Most past studies have deployed ectopic expression or addition of exogenous CNTF/LIF, thus further ana-lysis of mice with conditional mutations in CNTF/LIF signaling components will allow better understanding of in-vivo functions of CNTF/LIF associated signaling events in retinogenesis.

Retinal stem cells: promising candidates for retina transplantation

Stem cell transplantation is widely considered as a promising therapeutic approach for photoreceptor degeneration, one of the major causes of blindness. In this review, we focus on the biology of retinal stem cells (RSCs) and progenitor cells (RPCs) isolated from fetal, postnatal, and adult animals, with emphasis on those from rodents and humans. We discuss the origin of RSCs/RPCs, the markers expressed by these cells and the conditions for the isolation, culture, and differentiation of these cells in vitro or in vivo by induction with exogenous stimulation.

Have we achieved a unified model of photoreceptor cell fate specification in vertebrates?

How does a retinal progenitor choose to differentiate as a rod or a cone and, if it becomes a cone, which one of their different subtypes? The mechanisms of photoreceptor cell fate specification and differentiation have been extensively investigated in a variety of animal model systems, including human and non-human primates, rodents (mice and rats), chickens, frogs (Xenopus) and fish. It appears timely to discuss whether it is possible to synthesize the resulting information into a unified model applicable to all vertebrates. In this review we focus on several widely used experimental animal model systems to highlight differences in photoreceptor properties among species, the diversity of developmental strategies and solutions that vertebrates use to create retinas with photoreceptors that are adapted to the visual needs of their species, and the limitations of the methods currently available for the investigation of photoreceptor cell fate specification. Based on these considerations, we conclude that we are not yet ready to construct a unified model of photoreceptor cell fate specification in the developing vertebrate retina.

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.

TrkB/BDNF signaling regulates photoreceptor progenitor cell fate decisions

Neurotrophins, via activation of Trk receptor tyrosine kinases, serve as mitogens, survival factors and regulators of arborization during retinal development. Brain-derived neurotrophic factor (BDNF) and TrkB regulate neuronal arborization and survival in late retinal development. However, TrkB is expressed during early retinal development where its functions are unclear. To assess TrkB/BDNF actions in the early chick retina, replication-incompetent retroviruses were utilized to over-express a dominant negative truncated form of TrkB (trunc TrkB), or BDNF and effects were assessed at E15. Clones expressing trunc TrkB were smaller than controls, and proliferation and apoptosis assays suggest that decreased clone size correlated with increased cell death when BDNF/TrkB signaling was impaired. Analysis of clonal composition revealed that trunc TrkB over-expression decreased photoreceptor numbers (41%) and increased cell numbers in the middle third of the inner nuclear layer (INL) (23%). Conversely, BDNF over-expression increased photoreceptor numbers (25%) and decreased INL numbers (17%). Photoreceptors over-expressing trunc TrkB demonstrated no increase in apoptosis nor abnormalities in lamination suggesting that TrkB activation is not required for photoreceptor cell survival or migration. These studies suggest that TrkB signaling regulates commitment to and/or differentiation of photoreceptor cells from retinal progenitor cells, identifying a novel role for TrkB/BDNF in regulating cell fate decisions.

Involvement of Pleiotrophin in CNTF-mediated differentiation of the late retinal progenitor cells

Ciliary neurotrophic factor (CNTF) participates in retinal development by inhibiting rod differentiation and promoting bipolar and Müller cell differentiation. In order to identify genes which are regulated by CNTF in the developing retina, we carried out a subtractive hybridization study. By this approach, we identified the Pleiotrophin (Ptn) as an upregulated gene in postnatal day 0 (P0) retinal explants upon addition of CNTF. Correlation of overall expression patterns between different retinal cell markers and Ptn in situ hybridization suggest that Ptn transcripts are initially expressed in progenitor cells then in postmitotic precursors of the INL expressing the Chx10 gene, and later in some differentiated retinal Müller glial (RMG) cells and rod-bipolar cells. Overexpression of Ptn by in vitro electroporation of P0 rat retinal explants partially blocks rod differentiation and promotes bipolar cell production, similar to effects of exogenous CNTF and leukemia inhibitory factor (LIF). Furthermore, in P0 retinal explants from mice lacking Ptn, the inhibitory effect of CNTF and LIF on rod differentiation is partially reduced and the cytokine-induced bipolar cell differentiation is largely prevented. Together, these results demonstrate that influence of CNTF family of cytokines on the differentiation of late retinal progenitor cell population is partially mediated by the release of Ptn.

Effects of follistatin overexpression on cell differentiation in the chick embryo retina

Although activin is expressed in the embryonic central nervous system (CNS), its possible functions in the regulation of CNS neuronal differentiation remain largely unknown. We have investigated this question in the retina, a well-characterized CNS structure previously shown to respond to activin in vitro, and to express activin subunits and receptors in vivo. RCAS retroviruses were used to overexpress in the chick retina in ovo either follistatin (FS), an activin-binding protein and inhibitor, or alkaline phosphatase (AP), as control. FS-treated retinas appeared normal until ED 8, when they showed a reduction of the inner plexiform layer, accompanied by a marked decrease in the frequency of amacrine cells. The territory lacking amacrine cells showed downregulation of transcription factors necessary for amacrine cell differentiation, such as Pax6 and AP2alpha, accompanied by ectopic expression of transcription factors associated with the development of horizontal or bipolar neurons, such as Prox1, Chx10 and NeuroM. Increases in cell death were also observed in FS-treated retinas. Taken together with previous in vitro studies, our results suggest that activin is a powerful regulator of neuronal differentiation in the central nervous system.

Enhanced Neurotrophin Synthesis and Molecular Differentiation in Non-Transformed Human Retinal Progenitor Cells Cultured in a Rotating Bioreactor

One approach to the treatment of retinal diseases, such as retinitis pigmentosa, is to replace diseased or degenerating cells with healthy cells. Even if all of the problems associated with tissue transplant were to be resolved, the availability of tissue would remain an ongoing problem. We have previously shown that transformed human retinal cells can be grown in a NASA-developed horizontally rotating culture vessel (bioreactor) to form three-dimensional-like structures with the expression of several retinal specific proteins. In this study, we have investigated growth of non-transformed human retinal progenitors (retinal stem cells) in a rotating bioreactor. This rotating culture vessel promotes cell-cell interaction between similar and dissimilar cells. We cultured retinal progenitors (Ret 1-4) alone or as a co-culture with human retinal pigment epithelial cells (RPE, D407) in this system to determine if 3D structures can be generated from non-transformed progenitors. Our second goal was to determine if the formation of 3D structures correlates with the upregulation of neurotrophins, basic fibroblast growth factor (bFGF), transforming growth factor alpha (TGFalpha), ciliary neurotrophic factor (CNTF), and brain-delivered neurotrophic factor (BDNF). These factors have been implicated in progenitor cell proliferation, commitment, differentiation, and survival. We also investigated the expression of the following retinal specific proteins in this system: neuron specific enolase (NSE); tyrosine hydroxylase (TH); D(2)D(3), D(4) receptors; protein kinase-C alpha (PKCalpha), and calbindin. The 3D structures generated were characterized by phase and scanning transmission electron microscopy. Retinal progenitors, cultured alone or as a co-culture in the rotating bioreactor, formed 3D structures with some degree of differentiation, accompanied by the upregulation of bFGF, CNTF, and TGFalpha. Brain-derived neurotrophic factor, which is expressed in vivo in RPE (D407), was not expressed in monolayer cultures of RPE but expressed in the rotating bioreactor-cultured RPE and retinal progenitors (Ret 1-4). Upregulation of neurotrophins was noted in all rotating bioreactor-cultured cells. Also, upregulation of D(4) receptor, calbindin, and PKCalpha was noted in the rotating bioreactor-cultured cells. We conclude that non-transformed retinal progenitors can be grown in the rotating bioreactor to form 3D structures with some degree of differentiation. We relied on molecular and biochemical analysis to characterize differentiation in cells grown in the rotating bioreactor.

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.

Effects of Ciliary Neurotrophic Factor on Differentiation of Late Retinal Progenitor Cells

Ciliary neurotrophic factor (CNTF) has been shown to be a potent regulator of retinal cell differentiation. The present study was undertaken to investigate the effects of CNTF on in vitro differentiation of expanded late retinal progenitor cells. Retinal progenitor cells used in these studies were isolated from the neural retina of postnatal day-1 green fluorescent protein (GFP) transgenic mice. The resulting GFP-positive neurospheres were dissociated into a single-cell suspension and grown on poly-D-lysine/laminin-coated tissue culture flasks or slides to generate adherent retinal progenitor cells. These adherent cells were treated with 20 ng/ml of CNTF for up to 14 days, and expression of specific retinal cell markers was determined by immunocytochemistry, reverse transcription-polymerase chain reaction (RT-PCR), and immunoblot analysis. In vitro studies showed that CNTF treatment of late retinal progenitor cells resulted in changes in cellular morphology. Immunocytochemical studies showed an increase in the proportion of cells expressing markers of bipolar cells but not rod differentiation. In addition, an increase in the proportion of cells expressing glial cell markers was observed. RT-PCR analysis showed downregulation in Hes1, Nestin, Notch1, and Pax6 transcripts along with a concomitant increase in protein kinase C (PKC) alpha and glial fibrillary acidic protein (GFAP) transcripts. These findings were confirmed by immunoblot analysis, where downregulation in Nestin expression and simultaneous upregulation in PKC alpha and GFAP were observed. The data indicate that CNTF treatment of multipotential late retinal progenitors increases the proportion of cells that express markers of bipolar neurons and glia.

BMP4 and CNTF are neuroprotective and suppress damage-induced proliferation of Müller glia in the retina

In response to acute damage, Müller glia in the chicken retina have been shown to be a source of proliferating progenitor-like cells. The secreted factors and signaling pathways that regulate this process remain unknown. The purpose of this study was to test whether secreted factors, which are known to promote glial differentiation during development, regulate the ability of Müller glia to proliferate and become retinal progenitors in response to acute damage in mature retina. We made intraocular injections of BMP4, BMP7, EGF, NGF, BDNF, or CNTF before or after a single, toxic dose of N-methyl-d-aspartate (NMDA) and assayed for proliferating progenitor-like cells within the retina. We found that injections of BMP4, BMP7, or CNTF, but not EGF, NGF, or BDNF, before NMDA treatment reduced the number of Müller glia that proliferated and gave rise to progenitor-like cells. CNTF and BMP4, but not NGF or BDNF, greatly reduced the number of cells destroyed by toxin treatment indicating that these factors protect retinal neurons from a severe excitotoxic insult. Injections of CNTF 5 days before NMDA treatment prevented neurotoxin-induced cell death and Müller glial proliferation, while injections of BMP4 had no protective effect. In addition, CNTF injected after NMDA treatment suppressed glial proliferation, while BMP4 did not. We conclude that BMP4 and CNTF, when applied before a toxic insult, act as neuroprotective agents and likely suppress the proliferative response of Müller glia to retinal damage by attenuating the retinal damage; protecting bipolar and amacrine neurons from NMDA-induced cell death. When applied after a toxic insult, CNTF suppressed glial proliferation independent of levels of retinal damage.

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.

Distribution of CNTF receptor alpha protein in the central nervous system of the chick embryo

Ciliary neurotrophic factor (CNTF) promotes the survival and differentiation of various neuronal and glial cell populations in the nervous system of vertebrates. In mammals, the ligand-binding alpha-subunit of the CNTF receptor (CNTFRalpha) is expressed in a variety of neuronal populations, including all CNTF-responsive cells. Previous studies suggested that functional differences in the CNTF/CNTF receptor system between chicks and mammals exist. The purpose of the present study was to examine the temporal and spatial expression pattern of the chick CNTFRalpha protein during CNS development. Receptor expression was detectable by immunoblotting in all CNS areas tested but showed area-specific developmental regulation. Interestingly, two variants of CNTFRalpha, 69 and 65 kD, were identified by immunoblotting with a shift from the higher to the lower molecular mass species occurring during development. Immunoreactivity for CNTFRalpha protein was preferentially observed in neuropil and white matter structures of the developing CNS while neuronal somata generally appeared unlabeled. For example, expression was observed in the olfactory system, in the telencephalon, in parts of the somatosensory system, in components of the tectofugal pathway, in the cerebellum, and in auditory brainstem nuclei. Fiber tracts that exhibit CNTFRalpha immunoreactivity were the lateral forebrain bundle, occipitomesencephalic tract, quintofrontal tract, and vestibular nerve. Our study identifies potential new targets of a chick CNTF-related molecule and reveals significant regional differences of CNTFRalpha protein expression between chick and mammals. These results suggest that the CNTF receptor performs distinct developmental functions in different animals.

Microenvironmental regulation of visual pigment expression in the chick retina

Visual pigment (VP) expression in the chick embryo retina was investigated in ovo, in dissociated and explant cultures, and in cDNAs from individual cells. While VP mRNA is not detectable by in situ hybridization until embryonic day (ED) 14-16 in ovo, analysis of VP expression by RT-PCR showed that VP messages are present in the retina as many as 7-10 days before they become detectable by in situ hybridization, and are also detected in other regions of the embryonic CNS. On the other hand, red opsin expression is markedly accelerated when cells are isolated from their intraocular microenvironment at ED 6, and placed in pigment epithelium-free dissociated or explant cultures. This acceleration occurs regardless of cell density, birth date, or serum presence in the medium, suggesting that many photoreceptors are already programmed to express red opsin on or before ED 6, and that microenvironmental inhibitory factors prevent implementation of this program until ED 14 in ovo. The selectivity of this phenomenon is suggested by the finding that other VPs are not observed by in situ hybridization in ED 6 cultures, although they are detectable in cultures of older retinas. Taken together, these findings suggest that red opsin expression may be constitutive for many developing photoreceptor cells in the chick.

Regulation of retinal ganglion cell production by Sonic hedgehog

Previous work has shown that production of retinal ganglion cells is in part regulated by inhibitory factors secreted by ganglion cell themselves; however, the identities of these molecules are not known. Recent studies have demonstrated that the signaling molecule Sonic hedgehog (Shh) secreted by differentiated retinal ganglion cells is required to promote the progression of ganglion cell differentiation wave front and to induce its own expression. We present evidence that Shh signals play a role to negatively regulate ganglion cell genesis behind the differentiation wave front. Higher levels of Shh expression are detected behind the wave front as ganglion cells accumulate, while the Patched 1 receptor of Shh is expressed in adjacent retinal progenitor cells. Retroviral-mediated overexpression of Shh results in reduced ganglion cell proportions in vivo and in vitro. Conversely, inhibiting endogenous Shh activity by anti-Shh antibodies leads to an increased production of ganglion cells. Shh signals modulate ganglion cell production within the normal period of ganglion cell genesis in vitro without significantly affecting cell proliferation or cell death. Moreover, Shh signaling affects progenitor cell specification towards the ganglion cell fate during or soon after their last mitotic cycle. Thus, Shh derived from differentiated ganglion cells serves as a negative regulator behind the differentiation wave front to control ganglion cell genesis from the competent progenitor pool. Based on these results and other recent findings, we propose that Shh signals secreted by early-differentiated retinal neurons play dual roles at distinct concentration thresholds to orchestrate the progression of retinal neurogenic wave and the emergence of new neurons.

Axonal regeneration of retinal ganglion cells: effect of trophic factors

A variety of neurotrophic factors can influence the cell functions of the developing, mature and injured retinal ganglion cells. The discovery that retinal ganglion cell loss can be alleviated by neurotrophic factors has generated a great deal of interest in the therapeutic potential of these molecules. Recently, evidence has provided valuable information on the receptors that mediate these events and the intracellular signaling cascades after the binding of these ligands. Signaling by neurotrophic factors does not seem to restrict to retrograde messenger from the target but also includes local interactions with neighbouring cells along the axonal pathways, anterograde signaling from the afferents and autocrine signaling. More insight into the mechanisms of action of neurotrophic factors and the signal transduction pathway leading to the protection and regeneration of retinal ganglion cells may allow the design of new therapeutic strategies.

A model of retinal cell differentiation in the chick embryo

This article presents an overview of retinal cell differentiation in the chick embryo, in the context of a hypothetical model based on information generated during the last several years. The model proposes that: (1) most (if not all) proliferating neuroepithelial cells have the potential to give rise to a progeny comprising two or more different cell types; (2) the time at which cells undergo their terminal mitosis does not determine their differentiated fate; (3) many postmitotic precursor cells remain plastic (i.e., uncommitted) for some time after terminal mitosis, during which they encounter position-dependent signals as they migrate toward their definitive laminar position within the retina; (4) as a consequence of these inductive stimuli, precursor cells that migrate to different retinal layers express different transcriptional regulators; (5) morphologically undifferentiated precursor cells are committed to cell type-specific, complex patterns of differentiation, which they can express even when isolated from their normal microenvironment, and (6) even after precursor cells become committed to a specific identity, additional inductive signals are necessary for the cells to complete the development of a fully mature phenotype. The article presents a summary of the supportive evidence, as well as a critical evaluation of the model, and concludes with an overview of unanswered questions regarding retinal cell differentiation and a brief evaluation of the prospects for further progress in this field.

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.

Modification of glial-neuronal cell interactions prevents photoreceptor apoptosis during light-induced retinal degeneration

Prolonged or high-intensity exposure to visible light leads to photoreceptor cell death. In this study, we demonstrate a novel pathway of light-induced photoreceptor apoptosis involving the low-affinity neurotrophin receptor p75 (p75NTR). Retinal degeneration upregulated both p75NTR and the high-affinity neurotrophin receptor TrkC in different parts of Müller glial cells. Exogenous neurotrophin-3 (NT-3) increased, but nerve growth factor (NGF) decreased basic fibroblast growth factor (bFGF) production in Müller cells, which can directly rescue photoreceptor apoptosis. Blockade of p75NTR prevented bFGF reduction and resulted in both structural and functional photoreceptor survival in vivo. Furthermore, the absence of p75NTR significantly prevented light-induced photoreceptor apoptosis. These observations implicate glial cells in the determination of neural cell survival, and suggest functional glial-neuronal cell interactions as new therapeutic targets for neurodegeneration.

The key role of butyrylcholinesterase during neurogenesis and neural disorders: an antisense-5'butyrylcholinesterase-DNA study

The wide tissue distribution of butyrylcholinesterase (BChE) in organisms makes specific roles possible, although no clear physiologic function has yet been assigned to this enzyme. In vertebrates, it appears e.g. in serum, hemopoietic cells, liver, lung, heart, at cholinergic synapses, in the central nervous system. in tumors and not at least (besides acetylcholinesterase, AChE) in developing embryonic tissues. Here, a functional role of BChE can be found in regulation of cell proliferation and the onset of differentiation during early neuronal development--independent of its enzymatic activity. For studies concerning this point, we have established a strategy for a specific and efficient inhibition of BChE to investigate how the expected decrease of enzyme and, therefore, the manipulation of cellular cholinesterase-equilibrium influences embryonic neurogenesis--among others to gain information about the significance of noncholinergic, activity-independent and cell growth functions of BChE. The antisense-5'BChE-DNA strategy is based on inhibition of BChE mRNA transcription and protein synthesis. For this, the BChE gene is cloned into a suitable vector system; this is done in antisense-orientation, so that a transfected cell will produce their own antisense mRNA to inhibit gene expression. For such investigations in neurogenesis, the developing retina is a good model and we are able to create organotypic, three-dimensional retinal aggregates in vitro (retinospheroids) using isolated retinal cells of 6-day-old chicken embryos. Using this in vitro retina and "knock out" of BChE gene expression, we could show a key role of BChE during neurogenesis. The results are of great interest because in tumorigenesis and some neuronal disorders, the BChE gene is amplified or abnormally expressed. It has to be discussed how the antisense-5'BChE strategy can play a role in the development of new and efficient therapy forms.

Growth factors in combination, but not individually, rescue rd mouse photoreceptors in organ culture

The rd mouse retina is an animal model for human retinal dystrophy in which the rod photoreceptors undergo apoptosis during the first 4 weeks in vivo or in organ culture. We have examined the effect of different families of trophic factors on the survival of rd mouse photoreceptors in organ culture. Retinas were harvested from rd mice at postnatal day 2 and grown in organ culture for 27 days in vitro (DIV) in DMEM with 10% fetal calf serum. Ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF), fibroblast growth factor-2 (FGF2), glial cell line-derived neurotrophic factor (GDNF), neurturin, and persephon were added individually or in combination to the medium at a dose of 50 ng/ml or less. CNTF + BDNF in combination resulted in photoreceptor survival comparable to wild-type retinas after 27 DIV. CNTF + FGF2 or CNTF + GDNF produced a partial prevention of photoreceptor death. Photoreceptor degeneration was not blocked by any of the trophic factors added individually. A significant increase in photoreceptor survival was seen with forskolin added to CNTF, but not to BDNF, FGF2, or GDNF. These results demonstrate that trophic factors promote photoreceptor survival through a synergistic interaction. Increased understanding of receptor interactions and signaling pathways may lead to a potential therapeutic role for combinatorial trophic factors in treatment of photoreceptor dystrophies.

Organotypic culture system of chicken retina

Analysis of developmental mechanisms during neuroembryogenesis, evaluation of toxicological effects and testing of neuroprotheses rely to an increasing extent on in vivo-like in vitro models. We have developed a novel organotypic culture system of the chick retina. Tissue slices of embryonic retinae were immobilized on glass coverslips by a fibrin clot and permanently rotated between the gas and medium phase, resulting in regular formation and the maintenance of the retinal cytoarchitecture. Selection of embryonic stage, slice thickness and specimen processing were optimized for culturing. Scanning electron microscopy revealed degradation during increasing culture periods of the fibrin clot, which was used for initial immobilization of explants on glass coverslips. Simultaneously, retinal cells became exposed on the tissue surface. Even after several weeks in vitro, formation and maintenance of plexiform and nuclear layers was evident as revealed by two specific monoclonal antibodies. Immunocytochemistry employing two additional photoreceptor- and radial Müller-antibodies indicated differentiation of neuronal and glial cells specific for the retina. The organotypic culture system promises to facilitate developmental studies of retinal development. Quantitative evaluation of Na(+)-channel blocker mexiletine impact on the histogenesis of retinal explants proved the organotypic culture system to be a valuable tool also for neurotoxicological investigations.

Ca2+ mobilization and capacitative Ca2+ entry regulate DNA synthesis in cultured chick retinal neuroepithelial cells

Release of Ca2+ from intracellular Ca2+ stores (Ca2+ mobilization) and capacitative Ca2+ entry have been shown to be inducible in neuroepithelial cells of the early embryonic chick retina. Both types of Ca2+ responses decline parallel with retinal progenitor cell proliferation. To investigate their potential role in the regulation of neuroepithelial cell proliferation, we studied the effects of 2,5-di-tert-butylhydroquinone (DBHQ), an inhibitor of the Ca2+ pump of intracellular Ca2+ stores, and of SK&F 96365, an inhibitor of capacitative Ca2+ entry, on DNA synthesis in retinal organ cultures from embryonic day 3 (E3) chicks and in dissociated cultures from E7 and E9 chick retinae. We demonstrate that both antagonists inhibit [3H]-thymidine incorporation in a dose-dependent manner without affecting cell viability or morphology. The inhibition of [3H]-thymidine incorporation by SK&F 96365 occurred in the same concentration range (IC50: approximately 4 microM) as the blockade of capacitative Ca2+ entry in the E3 retinal organ culture. At a concentration of 5 microM SK&F 96365. DNA synthesis was reduced by 71, 40 and 32% in the E3, E7 and E9 cultures, respectively. Application of DBHQ at concentrations which led to depletion of intracellular Ca2+ stores also inhibited [3H]-thymidine incorporation with IC50 values of 20-30 microM in the different cultures. Our results suggest the involvement of Ca2+ mobilization and capacitative Ca2+ entry in the regulation of DNA synthesis in the developing neural retina.

Involvement of P2 purinoceptors in the regulation of DNA synthesis in the neural retina of chick embryo

The activation of P2 purinoceptors induces Ca2+ mobilization in the early embryonic chick neural retina. This purinergic Ca2+ response declines parallel with the decrease in mitotic activity during retinal development. To investigate the role of P2 purinoceptors in the regulation of retinal cell proliferation, we studied the effects of the P2 purinoceptor antagonists suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), and of the agonist ATP on DNA synthesis in retinal organ cultures from embryonic day 3 (E3) chick. Suramin inhibited [3H]-thymidine incorporation in a dose-dependent manner (IC50: approximately 70 microM). PPADS also reduced [3H]-thymidine incorporation with maximum inhibition of 46% at 100 microM. Exogenous ATP enhanced [3H]-thymidine incorporation in a dose-dependent manner to maximally 200% of control (EC50: approximately 70 microM). In dissociated retinal cultures from E7 chick, both antagonists showed similar inhibitory effects on [3H]-thymidine incorporation without affecting cell viability. In line with these observations, the presence of extracellular ATP was demonstrated both in vitro and in vivo. In the medium of E3 retinal organ cultures, the concentration of ATP increased 25-fold within 1 h of incubation and this concentration was kept for at least 24 h. In the chick amniotic fluid, the ATP concentration was nearly 3 microM at E3 and declined to 0.15 microM at E7. The results indicate that P2 purinoceptors activated by autocrine or paracrine release of ATP are involved in the regulation of DNA synthesis in the neural retina at early embryonic stages.

Alterations in expression of the neurotrophic factors glial cell line-derived neurotrophic factor, ciliary neurotrophic factor and brain-derived neurotrophic factor, in the target-deprived olfactory neuroepithelium

Neuronal growth factors play an important role in the development and maintenance of the nervous system. In the olfactory system, neurogenesis and synapse formation occur not only during development but throughout life and it would be expected that growth factors play a significant role in these ongoing processes. We have examined the expression of three neurotrophic factors, glial cell line-derived neurotrophic factor, ciliary neurotrophic factor and brain-derived neurotrophic factor in the normal rat olfactory system and following synaptic target ablation (olfactory bulbectomy). We found that brain-derived neurotrophic factor immunoreactivity was confined to the horizontal basal cells of the olfactory neuroepithelium and was unaltered by bulbectomy. Glial cell line-derived neurotrophic factor immunoreactivity was present in the mature olfactory neurons and also their synaptic target cells in the olfactory bulb. Following bulbectomy, glial cell line-derived neurotrophic factor immunoreactivity was abolished from the neuroepithelium. Ciliary neurotrophic factor was present throughout the olfactory neuronal lineage with strongest immunoreactivity in the horizontal basal cells and mature olfactory neurons as well as several cell types in the olfactory bulb. Postbulbectomy, there was loss of strong ciliary neurotrophic factor immunoreactivity in olfactory neurons, however, low levels persisted in the remaining neuronal population. Horizontal basal cell immunoreactivity persisted over three months. Our results would be consistent with glial cell line-derived neurotrophic factor expression in mature olfactory neurons being dependent upon functional synaptic contact with the olfactory bulb. Alternatively, this factor may be acting as target-derived growth factor for olfactory neurons, a role in keeping with its function in spinal motoneurons and in the nigrostriatal system. Brain-derived neurotrophic factor is implicated in the trophic support of immature neurons. Ciliary neurotrophic factor is clearly important in this unique neuronal system but elucidation of its role awaits further investigation.

Role of nitric oxide in photoreceptor survival in embryonic chick retinal cell culture

The presence of nitric oxide synthase (NOS) in chick retina during development has allowed us to study the role of nitric oxide (NO) during retinal differentiation in dissociated chick retinal cell culture from embryonic day 6. We have demonstrated the presence of nicotinamide adenine dinucleotide phosphate diaphorase staining in these cultures after 3 days in vitro (Div), with a maximal intensity after 8 Div, corresponding to embryonic day 14. Immunohistochemistry studies confirmed the presence of the two isoforms of NOS, NOS-I and -III, in dissociated retinal cell cultures at 8 Div. Addition of NG-monomethyl-L-arginine, a NOS inhibitor, to retinal cell cultures prevented NO production but did not modify the appearance and the survival of ganglion and amacrine cells. However, immunohistochemical analysis with distinct markers for photoreceptor cells (rods and cones) showed that inhibition of endogenous NOS in retinal cell cultures prevented the developmental decrease of rod number between 5 and 8 Div, thus supporting the hypothesis that NO may be involved in the cell death of rods during the development of the retina.

Effects of taurine depletion on cell migration and NCAM expression in cultures of dissociated mouse cerebellum and N2A cells

Cultures of dissociated cerebellum from 5- to 6-day-old mice as well as of the N2A neuronal cell line were exposed to guanidino ethane sulfonate (GES, 2-5 mM) to reduce the cellular taurine content. Control cultures were kept in culture medium or medium containing 2-5 mM GES plus 2-5 mM taurine to restore the intracellular taurine content. Taurine depletion led to changes in the expression of certain splice variants of NCAM mRNA such as the AAG and the VASE containing forms, while no differences were seen in the expression of the three forms of NCAM protein. In the N2A cells taurine depletion led to a decreased migration rate of the cells. The results suggest that the reduced migration rate of neurons caused by taurine depletion may be correlated to changes in expression of certain adhesion molecules such as NCAM. Moreover, taurine appears to be involved in regulation of transcription processes.

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.

Vertebrate photoreceptor cell development and disease

Photoreceptors provide an excellent model for studies of vertebrate neuronal differentiation, and many human diseases resulting in blindness primarily affect photoreceptors. There is therefore great interest in studying the cellular and molecular mechanisms of photoreceptor development. This article discusses our current understanding of this process, including the recent discovery of the homeodomain transcription factor Crx and its potential role in diseases affecting human vision.