Requirement of neuroD for photoreceptor formation in the chick retina

Disease-causing mutations in genes encoding transcription factors critical for photoreceptor development

Photoreceptor development of the vertebrate visual system is controlled by a complex transcription regulatory network. OTX2 is expressed in the mitotic retinal progenitor cells (RPCs) and controls photoreceptor genesis. CRX that is activated by OTX2 is expressed in photoreceptor precursors after cell cycle exit. NEUROD1 is also present in photoreceptor precursors that are ready to specify into rod and cone photoreceptor subtypes. NRL is required for the rod fate and regulates downstream rod-specific genes including the orphan nuclear receptor NR2E3 which further activates rod-specific genes and simultaneously represses cone-specific genes. Cone subtype specification is also regulated by the interplay of several transcription factors such as THRB and RXRG. Mutations in these key transcription factors are responsible for ocular defects at birth such as microphthalmia and inherited photoreceptor diseases such as Leber congenital amaurosis (LCA), retinitis pigmentosa (RP) and allied dystrophies. In particular, many mutations are inherited in an autosomal dominant fashion, including the majority of missense mutations in CRX and NRL. In this review, we describe the spectrum of photoreceptor defects that are associated with mutations in the above-mentioned transcription factors, and summarize the current knowledge of molecular mechanisms underlying the pathogenic mutations. At last, we deliberate the outstanding gaps in our understanding of the genotype-phenotype correlations and outline avenues for future research of the treatment strategies.

New Insights on the Regulatory Gene Network Disturbed in Central Areolar Choroidal Dystrophy-Beyond Classical Gene Candidates

Central areolar choroidal dystrophy (CACD) is a rare hereditary disease that mainly affects the macula, resulting in progressive and usually profound visual loss. Being part of congenital retinal dystrophies, it may have an autosomal dominant or recessive inheritance and, until now, has no effective treatment. Given the shortage of genotypic information about the disease, this work systematically reviews the literature for CACD-causing genes. Three independent researchers selected 33 articles after carefully searching and filtering the Scielo, Pubmed, Lilacs, Web of Science, Scopus, and Embase databases. Mutations of six genes (PRPH2, GUCA1A, GUCY2D, CDHR1, ABCA4, and TTLL5) are implicated in the monogenic dominant inheritance of CACD. They are functionally related to photoreceptors (either in the phototransduction process, as in the case of GUCY2D, or the recovery of retinal photodegradation in photoreceptors for GUCA1A, or the formation and maintenance of specific structures within photoreceptors for PRPH2). The identified genetic variants do not explain all observed clinical features, calling for further whole-genome and functional studies for this disease. A network analysis with the CACD-related genes identified in the systematic review resulted in the identification of another 20 genes that may influence CACD onset and symptoms. Furthermore, an enrichment analysis allowed the identification of 13 transcription factors and 4 long noncoding RNAs interacting with the products of the previously mentioned genes. If mutated or dysregulated, they may be directly involved in CACD development and related disorders. More than half of the genes identified by bioinformatic tools do not appear in commercial gene panels, calling for more studies about their role in the maintenance of the retina and phototransduction process, as well as for a timely update of these gene panels.

Optogenetic control of neural differentiation in Opto-mGluR6 engineered retinal pigment epithelial cell line and mesenchymal stem cells

In retinal degenerative disorders, when neural retinal cells are damaged, cell transplantation is one of the most promising therapeutic approaches. Optogenetic technology plays an essential role in the neural differentiation of stem cells via membrane depolarization. This study explored the efficacy of blue light stimulation in neuroretinal differentiation of Opto-mGluR6-engineered mouse retinal pigment epithelium (mRPE) and bone marrow mesenchymal stem cells (BMSCs). mRPE and BMSCs were selected for optogenetic study due to their capability to differentiate into retinal-specific neurons. BMSCs were isolated and phenotypically characterized by the expression of mesenchymal stem cell-specific markers, CD44 (99%) and CD105 (98.8%). mRPE culture identity was confirmed by expression of RPE-specific marker, RPE65, and epithelial cell marker, ZO-1. mRPE cells and BMSCs were transduced with AAV-MCS-IRES-EGFP-Opto-mGluR6 viral vector and stimulated for 5 days with blue light (470 nm). RNA and protein expression of Opto-mGluR6 were verified. Optogenetic stimulation-induced elevated intracellular Ca²⁺ levels in mRPE- and BMS-treated cells. Significant increase in cell growth rate and G1/S phase transition were detected in mRPE- and BMSCs-treated cultures. Pou4f1, Dlx2, Eomes, Barlh2, Neurod2, Neurod6, Rorb, Rxrg, Nr2f2, Ascl1, Hes5, and Sox8 were overexpressed in treated BMSCs and Barlh2, Rorb, and Sox8 were overexpressed in treated mRPE cells. Expression of Rho, Thy1, OPN1MW, Recoverin, and CRABP, as retinal-specific neuron markers, in mRPE and BMS cell cultures were demonstrated. Differentiation of ganglion, amacrine, photoreceptor cells, and bipolar and Muller precursors were determined in BMSCs-treated culture and were compared with mRPE. mRPE cells represented more abundant terminal Muller glial differentiation compared with BMSCs. Our results also demonstrated that optical stimulation increased the intracellular Ca²⁺ level and proliferation and differentiation of Opto-mGluR6-engineered BMSCs. It seems that optogenetic stimulation of mRPE- and BMSCs-engineered cells would be a potential therapeutic approach for retinal degenerative disorders.

Retinoid acid and taurine promote NeuroD1-induced differentiation of induced pluripotent stem cells into retinal ganglion cells

Induced pluripotent stem cells (iPSCs) possess the capacity to differentiate into multiple cell types including retinal neurons. Despite substantial progress in the transcriptional regulation of iPSC differentiation process, the efficiency of generation of retinal neurons from iPSCs is still low. In this study, we investigated the role of transcription factor NeuroD1 in the differentiation of iPSCs into retinal neurons. We observed that retrovirus-mediated NeuroD1 overexpression in iPSCs increased the efficiency of neuronal differentiation. Immunostaining analysis showed that NeuroD1 overexpression increased the expression of retina ganglion cell markers including Islet-1, Math5, Brn3b, and Thy1.2. Retinoid acid (RA) and taurine further improved the differentiation efficiency of iPSCs overexpressing NeuroD1. However, RA and taurine did not promote differentiation in the absence of NeuroD1 overexpression. Together, our study provides new evidence in transcription factor-regulated stem cell differentiation in vitro.

The bHLH Transcription Factor NeuroD Governs Photoreceptor Genesis and Regeneration Through Delta-Notch Signaling

PURPOSE

Photoreceptor genesis in the retina requires precise regulation of progenitor cell competence, cell cycle exit, and differentiation, although information around the mechanisms that govern these events currently is lacking. In zebrafish, the basic helix-loop-helix (bHLH) transcription factor NeuroD governs photoreceptor genesis, but the signaling pathways through which NeuroD functions are unknown. The purpose of this study was to identify these pathways, and during photoreceptor genesis, Notch signaling was investigated as the putative mediator of NeuroD function.

METHODS

In embryos, genetic mosaic analysis was used to determine if NeuroD functions is cell- or non-cell-autonomous. Morpholino-induced NeuroD knockdown, CRISPR/Cas9 mutation, and pharmacologic and transgenic approaches were used, followed by in situ hybridization, immunocytochemistry, and quantitative RT-PCR (qRT-PCR), to identify mechanisms through which NeuroD functions. In adults, following photoreceptor ablation and NeuroD knockdown, similar methods as above were used to identify NeuroD function during photoreceptor regeneration.

RESULTS

In embryos, NeuroD function is non-cell-autonomous, NeuroD knockdown increases Notch pathway gene expression, Notch inhibition rescues the NeuroD knockdown-induced deficiency in cell cycle exit but not photoreceptor maturation, and Notch activation and CRISPR/Cas9 mutation of neurod recapitulate NeuroD knockdown. In adults, NeuroD knockdown prevents cell cycle exit and photoreceptor regeneration and increases Notch pathway gene expression, and Notch inhibition rescues this phenotype.

CONCLUSIONS

These data demonstrate that during embryonic development, NeuroD governs photoreceptor genesis via non-cell-autonomous mechanisms and that, during photoreceptor development and regeneration, Notch signaling is a mechanistic link between NeuroD and cell cycle exit. In contrast, during embryonic development, NeuroD governs photoreceptor maturation via mechanisms that are independent of Notch signaling.

Ophthalmological phenotype associated with homozygous null mutation in the NEUROD1 gene

PURPOSE

NEUROD1 is a tissue-specific basic helix loop helix (bHLH) protein involved in the development and maintenance of the endocrine pancreas and neuronal elements. Loss of NEUROD1 causes ataxia, cerebellar hypoplasia, sensorineural deafness, and severe retinal dystrophy in mice. Heterozygous loss-of-function mutations in NEUROD1 have previously been described as a cause of maturity-onset diabetes of the young (MODY) and late-onset diabetes. To date, homozygous loss-of-function NEUROD1 mutations have only been detected in two patients. Both mutations caused permanent neonatal diabetes and severe neurologic defects, including visual impairment. However, a detailed ophthalmological phenotype of this novel syndrome has not yet been reported. Our aim was to characterize the ophthalmological phenotype associated with the previously reported homozygous c.427_428CT mutation in the NEUROD1 gene.

METHODS

The female patient was investigated on multiple occasions between 2009 (age 14) and 2014 (age 19), including visual acuity testing, automated perimetry, funduscopy, anterior-segment imaging, optical coherence tomography of the posterior pole, standard full-field electroretinography, and fundus-autofluorescence imaging.

RESULTS

The patient had nyctalopia, blurry vision, and visual field constriction from early childhood. Her best corrected visual acuity ranged between 20/25 and 15/25 during the investigation period. Perimetry showed concentric constriction of the visual field, sparing only the central 30 degrees in both eyes. The anterior segment did not show any morphological changes. Optical coherence tomography revealed total absence of the photoreceptor layer of the retina outside the fovea, where a discoid remnant of cone photoreceptors could be detected. Neither setting of the standard full-field electroretinography could detect any electrical response from the retina. Color fundus photos presented peripheral chorioretinal atrophy and central RPE mottling. A hyperreflective parafoveal ring was detected on fundus autofluorescent photos, a characteristic sign of hereditary retinal dystrophies.

CONCLUSIONS

To the best of our knowledge, this is the first report on the ophthalmological phenotype associating with a homozygous NEUROD1 null mutation in humans. Our results indicate that the loss of NEUROD1 has similar functional and anatomic consequences in the human retina as those described in mice. The present description can help the diagnosis of future cases and provide clues on the rate of disease progression.

Gene expression is dynamically regulated in retinal progenitor cells prior to and during overt cellular differentiation

The retina is comprised of one glial and six neuronal populations that are generated from a multipotent pool of retinal progenitor cells (RPCs) during development. To give rise to these different cell types, RPCs undergo temporal identity transitions, displaying distinct gene expression profiles at different stages of differentiation. Little, however, is known about temporal differences in RPC identities prior to the onset of overt cellular differentiation, during the period when a retinal identity is gradually acquired. Here we examined the sequential onset of expression of regional markers (i.e., homeodomain transcription factors) and cell fate determinants (i.e., basic-helix-loop-helix transcription factors and neurogenic genes) in RPCs from the earliest appearance of a morphologically-distinct retina. By performing a comparative analysis of the expression of a panel of 27 homeodomain, basic-helix-loop-helix and Notch pathway genes between embryonic day (E) 8.75 and postnatal day (P) 9, we identified six distinct RPC molecular profiles. At E8.75, the earliest stage assayed, murine RPCs expressed five homeodomain genes and a single neurogenic gene (Pax6, Six3, Six6, Rx, Otx2, Hes1). This early gene expression profile was remarkably similar to that of 'early' RPCs in the amphibian ciliary marginal zone (CMZ), where RPCs are compartmentalised according to developmental stage, and homologs of Pax6, Six3 and Rx are expressed in the 'early' stem cell zone. As development proceeds, expression of additional homeodomain, bHLH and neurogenic genes was gradually initiated in murine RPCs, allowing distinct genetic profiles to also be defined at E9.5, E10.5, E12.5, E15.5 and P0. In addition, RPCs in the postnatal ciliary margin, where retinal stem cells are retained throughout life, displayed a unique molecular signature, expressing all of the early-onset genes as well as several late-onset markers, indicative of a 'mixed' temporal identity. Taken together, the identification of temporal differences in gene expression in mammalian RPCs during pre-neurogenic developmental stages leads to new insights into how regional identities are progressively acquired during development, while comparisons at later stages highlight the dynamic nature of gene expression in temporally distinct RPC pools.

Production of high-titer RCAS retrovirus

RCAS (B/P) is a replication-competent avian retrovirus engineered by Hughes et al. (J Virol 61:3004-3012, 1987) and is referred to in this chapter as RCAS for simplicity. The RCAS retrovirus has been widely used as a vehicle for stable transduction of a gene into cells both in the developing chick embryo and tissue/cell culture. It can be used for both gain- and loss-function experiments. The ability of this virus to spread among proliferating cells makes it possible to achieve widespread gene transduction in the developing retina. The transduction efficiency of RCAS is highly depending on the titer of the viral stock, particularly for experiments involving solid tissues such as the developing retina. Here, we describe the procedure that we have used for 15 years to generate RCAS viral stocks with a titer of 1-5 × 10(8) pfu/ml.

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.

Using the Tg(nrd:egfp)/albino zebrafish line to characterize in vivo expression of neurod

In this study, we used a newly-created transgenic zebrafish, Tg(nrd:egfp)/albino, to further characterize the expression of neurod in the developing and adult retina and to determine neurod expression during adult photoreceptor regeneration. We also provide observations regarding the expression of neurod in a variety of other tissues. In this line, EGFP is found in cells of the developing and adult retina, pineal gland, cerebellum, olfactory bulbs, midbrain, hindbrain, neural tube, lateral line, inner ear, pancreas, gut, and fin. Using immunohistochemistry and in situ hybridization, we compare the expression of the nrd:egfp transgene to that of endogenous neurod and to known retinal cell types. Consistent with previous data based on in situ hybridizations, we show that during retinal development, the nrd:egfp transgene is not expressed in proliferating retinal neuroepithelium, and is expressed in a subset of retinal neurons. In contrast to previous studies, nrd:egfp is gradually re-expressed in all rod photoreceptors. During photoreceptor regeneration in adult zebrafish, in situ hybridization reveals that neurod is not expressed in Müller glial-derived neuronal progenitors, but is expressed in photoreceptor progenitors as they migrate to the outer nuclear layer and differentiate into new rod photoreceptors. During photoreceptor regeneration, expression of the nrd:egfp matches that of neurod. We conclude that Tg(nrd:egfp)/albino is a good representation of endogenous neurod expression, is a useful tool to visualize neurod expression in a variety of tissues and will aid investigating the fundamental processes that govern photoreceptor regeneration in adults.

MiR-124 regulates early neurogenesis in the optic vesicle and forebrain, targeting NeuroD1

MicroRNAs (miRNAs) are involved in the fine control of cell proliferation and differentiation during the development of the nervous system. MiR-124, a neural specific miRNA, is expressed from the beginning of eye development in Xenopus, and has been shown to repress cell proliferation in the optic cup, however, its role at earlier developmental stages is unclear. Here, we show that this miRNA exerts a different role in cell proliferation at the optic vesicle stage, the stage which precedes optic cup formation. We show that miR-124 is both necessary and sufficient to promote cell proliferation and repress neurogenesis at the optic vesicle stage, playing an anti-neural role. Loss of miR-124 upregulates expression of neural markers NCAM, N-tubulin while gain of miR-124 downregulates these genes. Furthermore, miR-124 interacts with a conserved miR-124 binding site in the 3'-UTR of NeuroD1 and negatively regulates expression of the proneural marker NeuroD1, a bHLH transcription factor for neuronal differentiation. The miR-124-induced effect on cell proliferation can be antagonized by NeuroD1. These results reveal a novel regulatory role of miR-124 in neural development and uncover a previously unknown interaction between NeuroD1 and miR-124.

Proper differentiation of photoreceptors and amacrine cells depends on a regulatory loop between NeuroD and Six6

Timely generation of distinct neural cell types in appropriate numbers is fundamental for the generation of a functional retina. In vertebrates, the transcription factor Six6 is initially expressed in multipotent retina progenitors and then becomes restricted to differentiated retinal ganglion and amacrine cells. How Six6 expression in the retina is controlled and what are its precise functions are still unclear. To address this issue, we used bioinformatic searches and transgenic approaches in medaka fish (Oryzias latipes) to characterise highly conserved regulatory enhancers responsible for Six6 expression. One of the enhancers drove gene expression in the differentiating and adult retina. A search for transcription factor binding sites, together with luciferase, ChIP assays and gain-of-function studies, indicated that NeuroD, a bHLH transcription factor, directly binds an 'E-box' sequence present in this enhancer and specifically regulates Six6 expression in the retina. NeuroD-induced Six6 overexpression in medaka embryos promoted unorganized retinal progenitor proliferation and, most notably, impaired photoreceptor differentiation, with no apparent changes in other retinal cell types. Conversely, Six6 gain- and loss-of-function changed NeuroD expression levels and altered the expression of the photoreceptor differentiation marker Rhodopsin. In addition, knockdown of Six6 interfered with amacrine cell generation. Together, these results indicate that Six6 and NeuroD control the expression of each other and their functions coordinate amacrine cell generation and photoreceptor terminal differentiation.

Neurogenin1 effectively reprograms cultured chick retinal pigment epithelial cells to differentiate toward photoreceptors

Photoreceptors are highly specialized sensory neurons in the retina, and their degeneration results in blindness. Replacement with developing photoreceptor cells promises to be an effective therapy, but it requires a supply of new photoreceptors, because the neural retina in human eyes lacks regeneration capability. We report efficient generation of differentiating, photoreceptor-like neurons from chick retinal pigment epithelial (RPE) cells propagated in culture through reprogramming with neurogenin1 (ngn1). In reprogrammed culture, a large number of the cells (85.0% +/- 5.9%) began to differentiate toward photoreceptors. Reprogrammed cells expressed transcription factors that set in motion photoreceptor differentiation, including Crx, Nr2E3, NeuroD, and RXRgamma, and phototransduction pathway components, including transducin, cGMP-gated channel, and red opsin of cone photoreceptors (equivalent to rhodopsin of rod photoreceptors). They developed inner segments rich in mitochondria. Furthermore, they responded to light by decreasing their cellular free calcium (Ca(2+)) levels and responded to 9-cis-retinal by increasing their Ca(2+) levels after photobleaching, hallmarks of photoreceptor physiology. The high efficiency and the advanced photoreceptor differentiation indicate ngn1 as a gene of choice to reprogram RPE progeny cells to differentiate into photoreceptor neurons in future cell replacement studies.

Neurog2 controls the leading edge of neurogenesis in the mammalian retina

In the mammalian retina, neuronal differentiation begins in the dorso-central optic cup and sweeps peripherally and ventrally. While certain extrinsic factors have been implicated, little is known about the intrinsic factors that direct this process. In this study, we evaluate the expression and function of proneural bHLH transcription factors during the onset of mouse retinal neurogenesis. Dorso-central retinal progenitor cells that give rise to the first postmitotic neurons express Neurog2/Ngn2 and Atoh7/Math5. In the absence of Neurog2, the spread of neurogenesis stalls, along with Atoh7 expression and RGC differentiation. However, neurogenesis is eventually restored, and at birth Neurog2 mutant retinas are reduced in size, with only a slight increase in the retinal ganglion cell population. We find that the re-establishment of neurogenesis coincides with the onset of Ascl1 expression, and that Ascl1 can rescue the early arrest of neural development in the absence of Neurog2. Together, this study supports the hypothesis that the intrinsic factors Neurog2 and Ascl1 regulate the temporal progression of retinal neurogenesis by directing overlapping waves of neuron formation.

Using neurogenin to reprogram chick RPE to produce photoreceptor-like neurons

PURPOSE

One potential therapy for vision loss from photoreceptor degeneration is cell replacement, but this approach presents a need for photoreceptor cells. This study explores whether the retinal pigment epithelium (RPE) could be a convenient source of developing photoreceptors.

METHODS

The RPE of chick embryos was subjected to reprogramming by proneural genes neurogenin (ngn)1 and ngn3. The genes were introduced into the RPE through retrovirus RCAS-mediated transduction, with the virus microinjected into the eye or added to retinal pigment epithelial explant culture. The retinal pigment epithelia were then analyzed for photoreceptor traits.

RESULTS

In chick embryos infected with retrovirus RCAS-expressing ngn3 (RCAS-ngn3), the photoreceptor gene visinin (the equivalent of mammalian recoverin) was expressed in cells of the retinal pigment epithelial layer. When isolated and cultured as explants, retinal pigment epithelial tissues from embryos infected with RCAS-ngn3 or RCAS-ngn1 gave rise to layers of visinin-positive cells. These reprogrammed cells expressed genes of phototransduction and synapses, such as red opsin, the alpha-subunit of cone transducin, SNAP-25, and PSD-95. Reprogramming occurred with retinal pigment epithelial explants derived from virally infected embryos and with retinal pigment epithelial explants derived from normal embryos, with the recombinant viruses added at the onset of the explant culture. In addition, reprogramming took place in retinal pigment epithelial explants from both young and old embryos, from embryonic day (E)6 to E18, when the visual system becomes functional in the chick.

CONCLUSIONS

The results support the prospect of exploring the RPE as a convenient source of developing photoreceptors for in situ cell replacement.

Pro-photoreceptor activity of chick neurogenin1

PURPOSE

Better understanding of photoreceptor fate specification may lead to efficient production of photoreceptors for cell replacement studies. The authors investigated the role of proneural bHLH gene neurogenin1 (ngn1) in photoreceptor genesis using the chick retina.

METHODS

In situ hybridization was used to delineate the spatial and temporal pattern of ngn1 expression. RCAS retrovirus was used to drive overexpression of ngn1 in retinal cells, and siRNA was used to reduce ngn1 expression in loss-of-function experiments.

RESULTS

Chick ngn1 was transiently expressed during early phases of retinal neurogenesis, from embryonic day (E)3 to E6, with cells expressing ngn1 confined to the apical side of the retinal neuroepithelium. The time window and the anatomic location of ngn1 expression coincided with photoreceptor genesis and differed from those of other transiently expressed proneural bHLH genes, such as ash1, ath3, ath5, and ngn2. Most ngn1-expressing cells lacked BrdU incorporation and lacked phosphorylated histone H3. In low-density cell culture, ngn1 overexpression increased neuroD expression and expanded the photoreceptor population but reduced the ganglion population. Treatment of dissociated retinal cells with siRNA against ngn1 mRNA specifically reduced the photoreceptor population. Overexpression of ngn1 in the retina reduced the expression of ash1, ath5, chx10, and ngn2.

CONCLUSIONS

The data suggest that ngn1 participates in a complex transcriptional network and may play a role in guiding a progenitor cell to the photoreceptor pathway.

Gene expression profiles of rat olfactory bulb at developmental stage

Microarray analysis may be a useful tool to identify some candidate genes related to the development of olfactory bulbs. In the present study, gene expression profiles of olfactory bulbs from postnatal day 1 (P1) rats and postnatal day 35 (P35) rats were analyzed by oligonucleotide-microarray and expression levels of some selected genes were also confirmed by RT-PCR and in situ hybridization. 9146 genes were commonly identified in six microarray chips. Among these genes, 76 were up-regulated and 130 were down-regulated three-folds or more at P1 olfactory bulbs. Out of these 76 up-regulated genes, 24 genes were annotated based on the NCBI database of reference sequences and expression levels of these 24 genes were confirmed by RT-PCR. Among them, 2 interesting genes (neurogenic differentiation 1 and retinoid acid receptor alpha) were localized in the P1 olfactory bulb by the use of in situ hybridization technique. Our results may provide basic information to identify genes associated with functional growth of olfactory bulbs.

NeuroD regulates proliferation of photoreceptor progenitors in the retina of the zebrafish

neuroD is a member of the family of proneural genes, which function to regulate the cell cycle, cell fate determination and cellular differentiation. In the retinas of larval and adult teleosts, neuroD is expressed in two populations of post-mitotic cells, a subset of amacrine cells and nascent cone photoreceptors, and proliferating cells in the lineages that give rise exclusively to rod and cone photoreceptors. Based on previous studies of NeuroD function in vitro and the cellular pattern of neuroD expression in the zebrafish retina, we hypothesized that within the mitotic photoreceptor lineages NeuroD selectively regulates aspects of the cell cycle. To test this hypothesis, gain and loss-of-function approaches were employed, relying on the inducible expression of a NeuroD(EGFP) fusion protein and morpholino oligonucleotides to inhibit protein translation, respectively. Conditional expression of neuroD causes cells to withdraw from the cell cycle, upregulate the expression of the cell cycle inhibitors, p27 and p57, and downregulate the cell cycle progression factors, Cyclin B1, Cyclin D1, and Cyclin E2. In the absence of NeuroD, cells specific for the rod and cone photoreceptor lineage fail to exit the cell cycle, and the number of cells expressing Cyclin D1 is increased. When expression is ectopically induced in multipotent progenitors, neuroD promotes the genesis of rod photoreceptors and inhibits the genesis of Müller glia. These data show that in the teleost retina NeuroD plays a fundamental role in photoreceptor genesis by regulating mechanisms that promote rod and cone progenitors to withdraw from the cell cycle. This is the first in vivo demonstration in the retina of cell cycle regulation by NeuroD.

Reprogramming chick RPE progeny cells to differentiate towards retinal neurons by ash1

PURPOSE

Harnessing a cell culture of retinal pigment epithelium (RPE) to give rise to retinal neurons may offer a source of developing neurons for cell-replacement studies. This study explores the possibility of reprogramming RPE progeny cells to differentiate toward retinal neurons with achaete-scute homolog 1 (ash1), a proneural gene that is expressed in progenitor cells in the developing retina and promotes amacrine cell production when overexpressed in the chick retina.

METHODS

Replication Competent Avian Splice (RCAS) retrovirus was used to drive the ectopic expression of ash1 in cell cultures of dissociated RPE isolated from day 6 chick embryos. RCAS expressing green fluorescent protein (RCAS-GFP) was used as control. The cultures were examined for de novo generation of neuron-like cells by molecular, cellular, and physiologic criteria.

RESULTS

In control cultures infected with RCAS-GFP, RPE cells appeared cobblestone-like and often darkly pigmented. In cultures infected with RCAS-ash1, however, cells remained de-pigmented and frequently formed clusters. Further examination at the morphological and molecular levels showed the development of elaborate processes characteristic of neurons and the expression of genes/markers that identify different types of retinal neurons. The most prevalently expressed neural marker was calretinin, which in the chick retina identifies amacrine, ganglion, and horizontal cells. As an assay for functional maturation, the reprogrammed cells were analyzed for the presence of functional, ionotropic glutamate receptors that lead to a rise in the cytosolic free calcium (Ca(2+)) concentration. Calcium imaging showed that reprogrammed cells responded to glutamate and N-methyl-D-aspartate (NMDA) by increasing their Ca(2+) concentrations, which, after reaching a peak level, returned to the basal level. The response curves of reprogrammed cells resembled those of cultured retinal neurons.

CONCLUSIONS

These results suggest that RPE progeny cells can be reprogrammed by ash1 to develop molecular, morphological, and physiologic properties that are characteristic of retinal neurons.

Neurogenin3 promotes early retinal neurogenesis

The transcriptional regulatory network governing the establishment of retinal neuron diversity is not well delineated. We report experimental results suggesting proneural gene neurogenin3 (ngn3) participating in this regulatory network. Retinal expression of chick ngn3 was confined to early neurogenesis. Overexpression of ngn3 in chick retina reduced cell proliferation and expanded the population of ganglion cells into the territory normally occupied by amacrine cells. Ngn3 overexpression altered the expression of a number of regulatory genes, including ash1, ath3, ath5, chx10, neuroD, ngn1, ngn2, and NSCL1. Early gene ngn1 was induced, but ash1, ngn2, ath3, and chx10, whose expressions persist through later phases of neurogenesis, were down-regulated. Expression of ath5 was up-regulated at the locale corresponding to young ganglion cells, but was down-regulated at the locale corresponding to progenitor cells. These results suggest that ngn3 regulates retinal neurogenesis by inducing regulatory genes for early-born neurons and repressing those for later-born cells.

Characterization of a transient TCF/LEF-responsive progenitor population in the embryonic mouse retina

PURPOSE

High mobility group (HMG) transcription factors of the T-cell-specific transcription factor/lymphoid enhancer binding factor (TCF/LEF) family are a class of intrinsic regulators that are dynamically expressed in the embryonic mouse retina. Activation of TCF/LEFs is a hallmark of the Wnt/beta-catenin pathway; however, the requirement for Wnt/beta-catenin and noncanonical Wnt signaling during mammalian retinal development remains unclear. The goal of the study was to characterize more fully a TCF/LEF-responsive retinal progenitor population in the mouse embryo and to correlate this with Wnt/beta-catenin signaling.

METHODS

TCF/LEF activation was analyzed in the TOPgal (TCF optimal promoter) reporter mouse at embryonic ages and compared to Axin2 mRNA expression, an endogenous readout of Wnt/beta-catenin signaling. Reporter expression was also examined in embryos with a retina-specific deletion of the beta-catenin gene (Ctnnb1), using Six3-Cre transgenic mice. Finally, the extent to which TOPgal cells coexpress cell cycle proteins, basic helix-loop-helix (bHLH) transcription factors, and other retinal cell markers was tested by double immunohistochemistry.

RESULTS

TOPgal reporter activation occurred transiently in a subpopulation of embryonic retinal progenitor cells. Axin2 was not expressed in the central retina, and TOPgal reporter expression persisted in the absence of beta-catenin. Although a proportion of TOPgal-labeled cells were proliferative, most coexpressed the cyclin-dependent kinase inhibitor p27/Kip1.

CONCLUSIONS

TOPgal cells give rise to the four earliest cell types: ganglion, amacrine, horizontal, and photoreceptor. TCF/LEF activation in the central retina does not correlate with Wnt/beta-catenin signaling, pointing to an alternate role for this transcription factor family during retinal development.

Development of the retinotectal system in the direct-developing frog Eleutherodactylus coqui in comparison with other anurans

BACKGROUND

Frogs primitively have a biphasic life history with an aquatic larva (tadpole) and a usually terrestrial adult. However, direct developing frogs of the genus Eleutherodactylus have lost a free living larval stage. Many larval structures never form during development of Eleutherodactylus, while limbs, spinal cord, and an adult-like cranial musculoskeletal system develop precociously.

RESULTS

Here, I compare growth and differentiation of the retina and tectum and development of early axon tracts in the brain between Eleutherodactylus coqui and the biphasically developing frogs Discoglossus pictus, Physalaemus pustulosus, and Xenopus laevis using morphometry, immunohistochemical detection of proliferating cell nuclear antigen (PCNA) and acetylated tubulin, biocytin tracing, and in situ hybridization for NeuroD. Findings of the present study indicate that retinotectal development was greatly altered during evolution of Eleutherodactlyus mostly due to acceleration of cell proliferation and growth in retina and tectum. However, differentiation of retina, tectum, and fiber tracts in the embryonic brain proceed along a conserved slower schedule and remain temporally coordinated with each other in E. coqui.

CONCLUSION

These findings reveal a mosaic pattern of changes in the development of the central nervous system (CNS) during evolution of the direct developing genus Eleutherodactylus. Whereas differentiation events in directly interconnected parts of the CNS such as retina, tectum, and brain tracts remained coordinated presumably due to their interdependent development, they were dissociated from proliferation control and from differentiation events in other parts of the CNS such as the spinal cord. This suggests that mosaic evolutionary changes reflect the modular character of CNS development.

Reprogramming progeny cells of embryonic RPE to produce photoreceptors: development of advanced photoreceptor traits under the induction of neuroD

PURPOSE

In examining the prospect of producing functional photoreceptors by reprogramming the differentiation of RPE progeny cells, this study was conducted to investigate whether reprogrammed cells can develop highly specialized ultrastructural and physiological traits that characterize retinal photoreceptors.

METHODS

Cultured chick RPE cells were reprogrammed to differentiate along the photoreceptor pathway by ectopic expression of neuroD. Cellular ultrastructure was examined with electron microscopy. Cellular physiology was studied by monitoring cellular free calcium (Ca(2+)) levels in dark-adapted cells in response to light and in light-bleached cells in response to 9-cis-retinal.

RESULTS

Reprogrammed cells were found to localize red opsin protein appropriately to the apex. These cells developed inner segments rich in mitochondria, and while in culture, some formed rudimentary outer segments, analogous to those of developing photoreceptors in the retina. In response to light, reprogrammed cells reduced their Ca(2+) levels, as observed with developing retinal photoreceptors in culture. Further, on exposure to 9-cis-retinal, the light-bleached, reprogrammed cells increased their Ca(2+) levels, reminiscent of visual cycle recovery.

CONCLUSIONS

These results indicate the potential of reprogrammed cells to develop advanced ultrastructural and physiological traits of photoreceptors and point to reprogramming progeny cells of embryonic RPE as a possible alternative in producing developing photoreceptors.

NeuroD1 regulates expression of thyroid hormone receptor 2 and cone opsins in the developing mouse retina

The correct patterning of opsin expression in cone photoreceptors is critical for normal color vision. Thyroid hormone, and one of its receptors [thyroid hormone receptor beta2 (TRbeta2)], is an important regulator of opsin expression during cone photoreceptor development. Mice have two genes, encoding medium-wavelength (M) and short-wavelength (S) cone opsins. Targeted deletion of TRbeta2 leads to a uniform expression of S-opsin in all cone photoreceptors and a loss of M-opsin. The control of expression of TRbeta2 is therefore central to cone differentiation, yet there is little known about its regulation in the retina. We now report that the proneural bHLH (basic helix-loop-helix) transcription factor, NeuroD1, is necessary for sustained expression of TRbeta2 in immature cone photoreceptors. Mice deficient in NeuroD1 develop an opsin phenotype virtually identical with that of TRbeta2-deficient mice: all cones express S-opsin, and none expresses M-opsin. The introduction of NeuroD1 into embryonic retinal explants from NeuroD1-/- mice restores TRbeta2 expression. NeuroD1 binds an E-box in the intron control region of the TRbeta2 gene that mediates cone-specific expression, suggesting that NeuroD1 is a critical contributory factor to the expression of TRbeta2 in cones. These results thus connect the proneural pathway with opsin selection to ensure correct cone patterning during retinal development.

Photoreceptor-specific expression, light-dependent localization, and transcriptional targets of the zinc-finger protein Yin Yang 1 in the chicken retina

The zinc-finger transcription factor Yin Yang 1 (YY1) is a multifunctional protein that plays a critical role in embryonic development. Although it has been shown to play a role in eye development, its expression in the retina was not previously described. Here, we investigated YY1 expression in chicken tissues and we identified the neural retina as one of the tissues with highest YY1 protein levels. Immunohistochemical detection of YY1 in the retina revealed a clear-cut photoreceptor specificity and day/night differences in the cytoplasmic localization of the protein. YY1 was also present at high concentration in the nuclei of some photoreceptors. Gel-shift assays indicated YY1 bound to regulatory regions of several genes specifically expressed in photoreceptors. One of these genes, hydroxyindole-O-methyltransferase (EC 2.1.1.4), encodes the last enzyme of the melatonin synthesis pathway. Although over-expression of chicken YY1 was not sufficient to activate the chicken hydroxyindole-O-methyltransferase promoter in HEK293 cells, the YY1-binding site contained in this promoter was clearly required for full transcriptional activity in chicken embryonic retinal cells. These results suggest a role of YY1 in regulating the melatoninergic function of retinal photoreceptors.

bHLH genes and retinal cell fate specification

The various cell types in the vertebrate retina arise from a pool of common progenitors. The way that the cell types are specified has been a long-standing issue. Decades of research have yielded a large body of information regarding the involvement of extrinsic factors, and only recently has the function of intrinsic factors begun to emerge. This article reviews recent studies addressing the role of basic helix-loop-helix (bHLH) factors in specifying retinal cell types, with an emphasis on bHLHhierarchies leading to photoreceptor production. Photoreceptor genesis appears to employ two transcriptional pathways: ngn2-->neuroD-->raxL and ath5-->neuroD-->raxL. ngn2 and ath5 function in progenitors, which can potentially develop into different cell types. neuroD represents one of the central steps in photoreceptor specification. Ath5 is also essential for ganglion cell development. It remains to be demonstrated whether a bHLH gene functions as a key player in specifying the other types of retinal cells. Genetic knockout studies have indicated intricate cross-regulation among bHLH genes. Future studies are expected to unveil the mechanism by which bHLH factors network with intrinsic factors and communicate with extrinsic factors to ensure a balanced production of the various types of retinal cells.

Dynamic expression of the basic helix-loop-helix transcription factor neuroD in the rod and cone photoreceptor lineages in the retina of the embryonic and larval zebrafish

NeuroD is a basic helix-loop-helix (bHLH) transcription factor critical for determining neuronal cell fate and regulating withdrawal from the cell cycle. We showed previously that, in goldfish, neuroD is expressed in the rod photoreceptor lineage, and we inferred that neuroD is also expressed in a subset of amacrine cells and nascent cone photoreceptors. Here we extended that study by examining the temporal and spatial expression pattern of neuroD in the embryonic and larval zebrafish and by identifying the cell types that express this gene. NeuroD expression in the developing zebrafish retina is dynamic, spanning early retinogenesis and the maturation of cone photoreceptors. In early retinogenesis neuroD expression expands from a small patch in the ventronasal retina, through the remaining retinal neuroepithelium. As retinogenesis progresses, neuroD expression becomes restricted to amacrine cells, immature cones, and cells of rod and cone lineages. This expression achieves an adult pattern by 96 hours postfertilization (hpf), whereupon the temporal pattern of neuroD expression in central retina is spatially recapitulated at the germinative margin. The cellular pattern of expression suggests that neuroD regulates aspects of rod and cone genesis, but through separate cellular lineages. Furthermore, neuroD is coexpressed with the cone-rod-homeobox transcription factor (Crx) in putative cone progenitors and nascent cone photoreceptors, suggesting that, in the zebrafish retina, as in other vertebrate retinas, similar genetic cascades regulate photoreceptor genesis and maturation.

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.

Exploring RPE as a source of photoreceptors: differentiation and integration of transdifferentiating cells grafted into embryonic chick eyes

PURPOSE

To study the possibility of generating photoreceptors through programming RPE transdifferentiation by examining cell differentiation after transplantation into the developing chick eye.

METHODS

RPE was isolated, and the cells were dissociated, cultured, and guided to transdifferentiate by infection with retrovirus expressing neuroD (RCAS-neuroD), using RCAS-green fluorescence protein (GFP) as a control. The cells were then harvested and microinjected into the developing eyes of day 5 to day 7 chick embryos, and their development and integration were analyzed.

RESULTS

Cells from the control culture integrated into the host RPE. When grafted cells were present in large number, multilayered RPE-like tissues were formed, and the extra tissues consisted of grafted cells and host cells. None of the cells from the control culture expressed photoreceptor-specific genes. In contrast, most cells from RCAS-neuroD-infected culture remained depigmented. A large number of them expressed photoreceptor-specific genes, such as visinin and opsins. Antibodies against red opsin decorated the apical tips and the cell bodies of the grafted, transdifferentiating cells. In the subretinal space, visinin(+) cells aligned along the RPE or an RPE-like structure. When integrated into the host outer nuclear layer, grafted cells emanated elaborate, axonal arborization into the outer plexiform layer of the host retina.

CONCLUSIONS

Cultured RPE cells retained their remarkable regenerative capabilities. Cells guided to transdifferentiate along the photoreceptor pathway by neuroD developed a highly ordered cellular structure and could integrate into the outer nuclear layer. These data suggest that, through genetic programming, RPE cells could be a potential source of photoreceptor cells.

Potential applications for RNAi to probe pathogenesis and develop new treatments for ocular disorders

The eye is a relatively isolated tissue compartment, which provides advantages for utilization of small interfering RNA (siRNA). Feasibility of using siRNA for treatment of choroidal neovascularization has been demonstrated using siRNA directed against vascular endothelial growth factor (VEGF) or VEGF receptor 1 (VEGFR1), and both of these approaches are being tested in clinical trials. The results with VEGFR1 siRNA show that VEGFR1 is pro-angiogenic in the eye and is not a decoy receptor as it is in developmental angiogenesis. Topical delivery of siRNAs directed against VEGF or its receptors has also been shown to suppress corneal neovascularization. Signaling through transforming growth factor-beta receptor 2 (TGFbetaR2) has been implicated in excessive ocular scarring and TGFbetaR2 siRNA has shown benefit in a model relevant to excessive scarring after glaucoma filtration surgery. RNAi has been used to identify genes that promote apoptosis or oxidative damage in retinal cells and could be the basis of new treatments for glaucoma or photoreceptor degenerations. In cultured cells derived from ocular tissues, siRNA has become a valuable tool to explore the potential role of various genes in ocular disease processes. Based upon this early experience in vivo and in vitro, it appears that siRNAs may be valuable to help define the pathogenesis and develop new treatments for several ocular diseases.

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.

Melatoninergic differentiation of retinal photoreceptors: activation of the chicken hydroxyindole-O-methyltransferase promoter requires a homeodomain-binding element that interacts with Otx2

The gene encoding the last enzyme of the melatonin-synthesis pathway, hydroxyindole-O-methyltransferase (HIOMT), is selectively expressed in retinal photoreceptors and pineal cells. Here, we analysed the promoter of the chicken HIOMT gene and we found that a homeodomain-binding element located in the proximal region of this promoter was essential for its activation in primary cultures of embryonic chicken retinal cells. This homeodomain-regulatory element interacted with a protein expressed in the chicken retina and pineal gland, which was recognized by an anti-Otx2 antiserum. Recombinant Otx2 expressed in vitro was able to bind this DNA element and to directly transactivate the chicken HIOMT promoter. This promoter was also transactivated by another member of the Otx family, Otx5, but the amplitude of stimulation was lower than with Otx2. The spatio-temporal pattern of Otx2 expression was compatible with a possible role of this transcription factor in HIOMT gene activation. In adult chicken, Otx2 mRNA was found to be present in those two tissues that express HIOMT: the retina and the pineal gland. During development, a burst of Otx2 mRNA closely matched the timing of HIOMT gene activation in these two tissues. In the pineal, Otx2 immunolabelling was specifically localized in the nuclei of photoreceptor cells. In the neural retina, Otx2 immunoreactivity brightly decorated the photoreceptor nuclei and extended more faintly to the outer half of the inner nuclear layer. Together, the data support a role of Otx2 in the onset of HIOMT expression in developing chicken photoreceptors.

Chromatin immunoprecipitation identifies photoreceptor transcription factor targets in mouse models of retinal degeneration: new findings and challenges

The transcription factors, Otx2, Crx, Nrl, and Nr2e3, expressed by retinal photoreceptor cells are essential for photoreceptor gene expression, development, and maintenance. Malfunction of any of these factors due to genetic mutations causes photoreceptor disease. Protein-protein interaction studies suggest that these factors may form a regulatory network centered on Crx. To understand how these factors regulate photoreceptor gene transcription in vivo, we have employed chromatin immunoprecipitation (ChIP) assays to assess the ability of these proteins to bind to regulatory sequences of photoreceptor genes in the retina of wild-type and mutant mice with photoreceptor degeneration. This paper summarizes the advantages and limitations of ChIP, using examples from our studies to demonstrate how this technique has contributed to our understanding of the regulation of photoreceptor gene expression. We report that Crx, Otx2, Nrl, and Nr2e3 co-occupy the promoter/enhancer, but not the region 3' of selected Crx target genes, in a retina-specific fashion. We identified Crx-dependent (Nr2e3) and Crx-independent (Otx2 and Nrl) target binding using Crx knockout mice (Crx-/-), suggesting that individual factors may use distinct mechanism(s) for binding and regulating target genes. Consistent with ChIP results, we also found that Otx2, a close family member of Crx, can activate the promoter of rod and cone genes in HEK293 cells, implicating Otx2 in regulating photoreceptor gene expression. These findings provide important information for understanding how photoreceptor transcription factors regulate photoreceptor gene expression and the mechanisms by which mutations in these factors cause transcriptional dysregulation and photoreceptor degeneration.

The role of combinational coding by homeodomain and bHLH transcription factors in retinal cell fate specification

Two major families of transcription factors (TFs), basic helix-loop-helix (bHLH) and homeodomain (HD), are known to be involved in cell fate identity. Some recent findings suggest that these TFs are used combinatorially to code for cellular determination in the retina. However, neither the extent nor the efficiency of such a combinatorial coding mechanism has been tested. To look systematically for interactions between these two TF types that would address these questions, we used a matrix analysis. We co-expressed each of six retinally expressed bHLH TFs (XNeuroD; XNgnr-1; Xath3; Xath5; Xash1; Xash3) with each of eight retinally expressed HD TFs (XRx1; XOptx2; XSix3; XPax6; XOtx2; XOtx5b; XBH; XChx10) in retinal progenitors of Xenopus laevis using targeted lipofection. The effects of each of these combinations were assayed on the six major cell types in the retina: Retinal ganglion cells (GCs), Amacrines (ACs), Bipolars (BCs), Horizontals (HCs), Photoreceptors (PRs), and Muller cells (MCs), creating 288 result categories. Multiple-way ANOVA indicated that in 14 categories, there were interactions between the two TFs that produced significantly more or less of a particular cell type than either of the components alone. However, even the most effective combinations were incapable of generating more than 65% of any particular cell type. We therefore used the same techniques to misexpress selected combinations of three TFs in retinal progenitors, but found no further enhancements of particular cell fates, indicating that other factors are probably involved in cell type specification. To test whether particular combinations were essential for horizontal fates, we made VP16 and EnR fusion constructs of some of the factors to provide dominant negative transcriptional activities. Our results confirmed that normal activities of certain combinations were sufficient, and that individually these activities were important for this fate.

Suppression of Mitf by small interfering RNA induces dedifferentiation of chick embryonic retinal pigment epithelium

Recent studies indicate a key role of Mitf (microphthalmia-associated transcription factor) in the differentiation of the retinal pigment epithelium (RPE). To explore transdifferentiation processes, we used small interfering RNA (siRNA) targeting Mitf. Transfection of embryonic chick RPE cells with a non-silencing fluorescein-labelled control siRNA demonstrated a high (84%) transfection efficiency. Transfection of Mitf siRNA reduced Mitf synthesis at the mRNA and protein levels as analysed by real-time RT-PCR and Western blot. Mitf siRNA suppressed the expression of Mitf RNA to 25.0% of the level in control cells (P<0.005). The expression of melanosomal matrix glycoprotein 115 (MMP115), a marker of differentiated pigment cells, was also markedly suppressed (to 52.2+/-6.6%) (P<0.0005). Moreover, the expression of Pax6 was increased by Mitf siRNA (to 143.5+/-18.0%) (P<0.0005), and induced dedifferentiation of the RPE cells. These data suggest siRNA can be an effective gene silencing approach in RPE, and reduction of Mitf expression is essential for the dedifferentiation of RPE cells.

Transcriptional factors involved in photoreceptor differentiation

Regenerative medicine constitutes a potentially promising therapy for blind people suffering from retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. For the realization of retinal regeneration, it is necessary to establish 1) a method to produce functional photoreceptor cells in vitro and 2) successful transplantation of the donor cells to connect their axons to the recipient secondary neurons so that they can function properly. The results of experimental transplantation of human retinal photoreceptor cells from cadaveric eyes or of fetal retinal cells into the retina of RP patients have not been satisfactory, but encouraging enough to indicate that the transplantation of developing retinal cells may have beneficial results. Recently, attempts have been made to generate photoreceptor-like cells from stem cells, but it remains to be seen whether they are in fact photoreceptor cells. It is therefore important to fully understand the mechanisms involved in the development of these cells, and to characterize them not only by transcriptome but also by functional analysis.

The photoreceptor-specific nuclear receptor Nr2e3 interacts with Crx and exerts opposing effects on the transcription of rod versus cone genes

Nr2e3 is an orphan nuclear receptor expressed specifically by retinal photoreceptor cells. Mutations in Nr2e3 result in syndromes characterized by excess blue cones and loss of rods: enhanced S-cone syndrome (ESCS) in humans and rd7 in mice. Using yeast two-hybrid screens with Nr2e3 as bait, the cone-rod homeobox protein Crx was identified as an interacting partner of Nr2e3. Immunoprecipitation assays confirmed this Nr2e3-Crx interaction and identified the DNA-binding domain of each protein as the interaction motif. Immunohistochemistry demonstrated that Crx and Nr2e3 are co-expressed by rod photoreceptors and their precursors. Chromatin immunoprecipitation assays on mouse retina demonstrated that Nr2e3 and Crx co-occupy the promoter/enhancer region of several rod and cone genes in the rod photoreceptor cells. The promoter/enhancer occupancy of Nr2e3 is Crx-dependent, suggesting that Nr2e3 is associated with photoreceptor gene targets by interacting with Crx. Transient transfection assays in HEK293 cells demonstrated that Nr2e3 enhances rhodopsin, but represses S- or M-cone opsin transcription when interacting with Crx. Quantitative real-time RT-PCR analysis on postnatal day 28 (P28) retina of the rd7 mouse, which lacks Nr2e3 protein, revealed an up-regulation of cone genes, but down-regulation of rod genes. Several mutant forms of human Nr2e3 identified from ESCS patients showed defects in interacting with Crx and/or in transcriptional regulatory function. Altogether, our findings suggest that Nr2e3 is a dual-function transcriptional regulator that acts in concert with Crx to promote and maintain the function of rod photoreceptors.

A role of ath5 in inducing neuroD and the photoreceptor pathway

Photoreceptors in the vertebrate retina are light-sensitive neurons, and their degeneration results in irreversible visual loss. Understanding how photoreceptor fate is determined is a prerequisite for developing photoreceptor replacement therapies. Previous studies identified two basic helix-loop-helix genes, neurogenin2 (ngn2) and neuroD, participating in a genetic pathway leading to photoreceptor genesis. Here we present experimental data suggesting that ath5, which is known for its critical role in retinal ganglion cell development, may also lead to photoreceptor production. In the developing retina, ath5 expression was detected in two zones of cells, and coexpression with neuroD was observed in the zone adjacent to young photoreceptor cells accumulating on the retinal pigment epithelial side. Retroviral-driven misexpression of ath5 in retinal cells increased the population of photoreceptor cells, as well as ganglion cells, in a developmental stage-dependent manner that is consistent with ath5 being involved in the development of multiple types of retinal neurons. Ectopic ath5 expression in cultures of non-neural retinal pigment epithelial cells elicited transdifferentiation into cells that expressed photoreceptor-specific genes and displayed photoreceptor-like morphologies. Gene expression analysis showed that ngn2 did not induce ath5, and ath5 did not induce ngn2, but both induced neuroD and RaxL. These data suggest a pathway of "ath5 --> neuroD --> photoreceptor genes" separate from yet convergent with the ngn2 pathway.

The basic helix-loop-helix transcription factor neuroD is expressed in the rod lineage of the teleost retina

Persistent rod genesis in the retinas of teleost fish was first described over 2 decades ago, but little is known regarding the underlying genetic and molecular mechanisms that govern this phenomenon. Because of its function in the developing mammalian retina and persistently mitotic adult tissues, we sought to characterize the cellular expression of the basic helix-loop-helix (bHLH) transcription factor neuroD in the persistently neurogenic retina of adult teleosts. We show here that, in the adult retina of the goldfish, neuroD is expressed by putative amacrine cells, nascent cones, and the mitotically active cells of the rod lineage. neuroD is the first gene shown to be expressed by rod precursors, the immediate antecedents of rod photoreceptors. In contrast to the vertebrate classes described previously, neuroD is not expressed in multipotent progenitors in the teleost retina. Combining neuroD in situ hybridizations with cell-cycle-specific markers suggests that, in rod precursors, neuroD expression is cell cycle specific.