Neurogenin3 promotes early retinal neurogenesis

Neurogenin 2 and Neuronal Differentiation 1 Control Proper Development of the Chick Trigeminal Ganglion and Its Nerve Branches

The trigeminal ganglion contains the cell bodies of sensory neurons comprising cranial nerve V, which relays information related to pain, touch, and temperature from the face and head to the brain. Like other cranial ganglia, the trigeminal ganglion is composed of neuronal derivatives of two critical embryonic cell types, neural crest and placode cells. Neurogenesis within the cranial ganglia is promoted by Neurogenin 2 (Neurog2), which is expressed in trigeminal placode cells and their neuronal derivatives, and transcriptionally activates neuronal differentiation genes such as Neuronal Differentiation 1 (NeuroD1). Little is known, however, about the role of Neurog2 and NeuroD1 during chick trigeminal gangliogenesis. To address this, we depleted Neurog2 and NeuroD1 from trigeminal placode cells with morpholinos and demonstrated that Neurog2 and NeuroD1 influence trigeminal ganglion development. While knockdown of both Neurog2 and NeuroD1 affected innervation of the eye, Neurog2 and NeuroD1 had opposite effects on ophthalmic nerve branch organization. Taken together, our results highlight, for the first time, functional roles for Neurog2 and NeuroD1 during chick trigeminal gangliogenesis. These studies shed new light on the molecular mechanisms underlying trigeminal ganglion formation and may also provide insight into general cranial gangliogenesis and diseases of the peripheral nervous system.

A new role of hindbrain boundaries as pools of neural stem/progenitor cells regulated by Sox2

Background

Compartment boundaries are an essential developmental mechanism throughout evolution, designated to act as organizing centers and to regulate and localize differently fated cells. The hindbrain serves as a fascinating example for this phenomenon as its early development is devoted to the formation of repetitive rhombomeres and their well-defined boundaries in all vertebrates. Yet, the actual role of hindbrain boundaries remains unresolved, especially in amniotes.

Results

Here, we report that hindbrain boundaries in the chick embryo consist of a subset of cells expressing the key neural stem cell (NSC) gene Sox2. These cells co-express other neural progenitor markers such as Transitin (the avian Nestin), GFAP, Pax6 and chondroitin sulfate proteoglycan. The majority of the Sox2⁺ cells that reside within the boundary core are slow-dividing, whereas nearer to and within rhombomeres Sox2⁺ cells are largely proliferating. In vivo analyses and cell tracing experiments revealed the contribution of boundary Sox2⁺ cells to neurons in a ventricular-to-mantle manner within the boundaries, as well as their lateral contribution to proliferating Sox2⁺ cells in rhombomeres. The generation of boundary-derived neurospheres from hindbrain cultures confirmed the typical NSC behavior of boundary cells as a multipotent and self-renewing Sox2⁺ cell population. Inhibition of Sox2 in boundaries led to enhanced and aberrant neural differentiation together with inhibition in cell-proliferation, whereas Sox2 mis-expression attenuated neurogenesis, confirming its significant function in hindbrain neuronal organization.

Conclusions

Data obtained in this study deciphers a novel role of hindbrain boundaries as repetitive pools of neural stem/progenitor cells, which provide proliferating progenitors and differentiating neurons in a Sox2-dependent regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0277-y) contains supplementary material, which is available to authorized users.

Induction of ectopic retina-like tissue by transgenic expression of neurogenin

Degeneration of retinal neurons is an underlying cause of several major types of blinding diseases, and effective therapies remain to be developed. The suppositive strategy of repopulating a degenerative retina with new cells generated onsite faces serious challenges, because the mammalian retina seems to lack the ability to regenerate itself or replace its lost neurons. We investigated the possibility of using a transcriptional factor with proneural activities to reprogram ocular tissue with regenerative capability to give rise to retinal cells. Transgenic mice were generated with DNA constructs that targeted the expression in the retinal pigment epithelium of proneural gene neurogenin1 from the promoter of Bestrophin1, or neurogenin3 from RPE65 promoter. Here we report the presence of ectopic retina-like tissue in some of the transgenic mice, young and aged. The ectopic retina-like tissue contained cells positive for photoreceptor proteins Crx, recoverin, red opsin, and rhodopsin, and cells positive for proteins that label other types of retinal neurons, including AP2α and Pax6 for amacrine cells, Otx2 for bipolar cells, and Brn3A for ganglion cells. The retina-like tissue often co-existed with darkly pigmented tissue positive for RPE proteins: cytokeratin 18, Otx2, and RPE65. The ectopic retina-like tissue was detected in the subretinal space, including two retinae co-existing in the same eye, and/or in the optic nerve or in the vicinity of the optic nerve head. On rare occasions, it was detected in the choroid and in the vicinity of the ciliary body. The presence of ectopic retina-like tissue in the transgenic mouse supports the possibility of inducing retinal regeneration in the mammalian eyes through gene-directed reprograming.

Neurogenins in brain development and disease: an overview

The production of neurons, astrocytes and oligodendrocytes is regulated by a group of transcription factors, which determine cell fates and specify subtype identities in the nervous system. Here we focus on profiling the distinct roles of Neurogenin (Ngn or Neurog) family members during the neuronal development. Ngn proteins are tightly regulated to be expressed at defined times and positions of different progenitor cell pools. In addition to their well-elucidated proneural function, Ngn proteins play various critical roles to specify or maintain cell fate and regulate neurite outgrowth and targeting in the central nervous system. Finally, Ngns have been associated with neuronal disorders. Therefore understanding the function and regulation of Ngns will not only improve the understanding of the molecular mechanism underlying the development of nervous system, but may also provide insight into neuronal disease.

The Retinal Pigment Epithelium: a Convenient Source of New Photoreceptor cells?

Recent success in restoring visual function through photoreceptor replacement in mouse models of photoreceptor degeneration intensifies the need to generate or regenerate photoreceptor cells for the ultimate goal of using cell replacement therapy for blindness caused by photoreceptor degeneration. Current research on deriving new photoreceptors for replacement, as regenerative medicine in general, focuses on the use of embryonic stem cells and induced pluripotent stem (iPS) cells to generate transplantable cells. Nonetheless, naturally occurring regeneration, such as wound healing, involves awakening cells at or near a wound site to produce new cells needed to heal the wound. Here we discuss the possibility of tweaking an ocular tissue, the retinal pigment epithelium (RPE), to produce photoreceptor cells in situ in the eye. Unlike the neural retina, the RPE in adult mammals maintains cell proliferation capability. Furthermore, progeny cells from RPE proliferation may differentiate into cells other than RPE. The combination of proliferation and plasticity opens a question of whether they could be channeled by a regulatory gene with pro-photoreceptor activity towards photoreceptor production. Studies using embryonic chick and transgenic mouse showed that indeed photoreceptor-like cells were produced in culture and in vivo in the eye using gene-directed reprogramming of RPE cells, supporting the feasibility of using the RPE as a convenient source of new photoreceptor cells for in situ retinal repair without involving cell transplantation.

Photoreceptor-like cells in transgenic mouse eye

PURPOSE

Recent success of rescuing vision by photoreceptor replacement in mouse models of photoreceptor degeneration intensifies the need to identify approaches to generate photoreceptors cells for future replacement therapies. We explored the possibility of whether in the mouse eye photoreceptor-like cells could arise from the RPE experimentally manipulated to express a regulatory gene participating in transcriptional networks leading to photoreceptor genesis during retinal development.

METHODS

Transgenic mice were generated with a DNA construct that would express neurogenin1 from RPE bestrophin-1 promoter or neurogenin3 from RPE65 promoter. Transgenic mice were examined with histology and immunohistology for the presence of photoreceptor-like cells and for the presence of cells that might represent transitional stages in RPE-to-photoreceptor reprogramming. Explant culture of "sclera+choroid+RPE" eyecup was used to examine whether cells with photoreceptor traits could arise from the eyecup derived from transgenic mice.

RESULTS

Transgenic animals showed varied degrees of phenotype manifestation. Approximately 60% of offspring from ∼50% of founders contained photoreceptor-like cells in the subretinal space. These cells expressed photoreceptor proteins recoverin, red opsin, and rhodopsin, and displayed morphologic similarities to photoreceptors. In these eyes, the RPE was maintained. Cells seemingly amid RPE-to-photoreceptor transformation were observed in young and aged mice, suggesting old animals were responsive to the reprogramming scheme. De novo generation of photoreceptor-like cells was detected in "sclera+choroid+RPE" eyecup explants derived from adult animals.

CONCLUSIONS

Our results point to a potential way to generate photoreceptor cells in situ in adult mammalian eyes.

A CRM1-mediated nuclear export signal is essential for cytoplasmic localization of neurogenin 3 in neurons

Neurogenin 3 (Ngn3), a proneural gene, regulates dendritogenesis and synaptogenesis in mouse hippocampal neurons. Ngn3 is transiently exported from the cell nucleus to the cytoplasm when neuronal polarity is initiated, suggesting that the nucleo-cytoplasmic transport of the protein is important for its action on neuronal development. In this study, we identified for the first time a functional nuclear export sequence (NES2; ¹³¹YIWALTQTLRIA¹⁴²) in Ngn3. The green fluorescent protein (EGFP)-NES2 fusion protein was localized in the cytoplasm and its nucleo-cytoplasmic shuttling was blocked by the CRM1 specific export inhibitor leptomycin B. Mutation of a leucine residue to alanine (L135A) in the NES2 motif resulted in both cytoplasmic and nuclear localization of the EGFP-NES2 fusion protein and in the nuclear accumulation of ectopic full-length myc-Ngn3. In addition, point mutation of the leucine 135 counteracted the effects of Ngn3 on neuronal morphology and synaptic inputs indicating that the cytoplasmic localization of Ngn3 is important for neuronal development. Pharmacological perturbation of the cytoskeleton revealed that cytoplasmic Ngn3 is associated with microtubules.

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.

Glycogen synthase kinase-3 inhibition promotes proliferation and neuronal differentiation of human-induced pluripotent stem cell-derived neural progenitors

Human-induced pluripotent stem cell-derived neural progenitors (hiPSC-NPs) have the ability to self-renew and differentiate into glial and neuronal lineages, which makes them an invaluable source in cell replacement therapy for neurological diseases. Therefore, their enhanced proliferation and neuronal differentiation are pivotal features that can be used in repairing neurological injuries. One of the main regulators of neural development is Wnt signaling, which results in the inhibition of glycogen synthase kinase 3 (GSK-3). Here, we assess the impact of GSK-3 inhibition by the small molecule CHIR99021 on the expansion and differentiation of hiPSC-NPs in an adherent condition and a defined medium. Cell proliferation analyses have revealed that inhibition of GSK-3 in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) increased the proliferation of hiPSC-NPs across 10 passages. The inhibition of β-catenin signaling by XAV and NOTCH signaling by DAPT reversed CHIR impact on hiPSC-NPs proliferation. The target genes of β-catenin, C-MYC and CYCLIN D1 as well as NOTCH target genes, HES1 and HES5 were upregulated. The treatment of NPs by CHIR in the absence of bFGF and EGF resulted in an increase of neuronal differentiation rather than proliferation by stabilization of β-catenin regardless of the NOTCH pathway. Thus, GSK-3 inhibition has been shown to promote proliferation of the NPs by activating β-catenin and NOTCH-related cell cycle genes in the presence of bFGF and EGF. Additionally, during GSK-3 inhibition, an absence of these growth factors allows for the switch to neuronal differentiation with a bias toward a dopaminergic fate. This may provide desired cells that can be used in therapeutic applications and offer insights into the etiology of some neurological disorders.

NEUROG3 is a critical downstream effector for STAT3-regulated differentiation of mammalian stem and progenitor spermatogonia

Spermatogenesis relies on coordinated differentiation of stem and progenitor spermatogonia, and the transcription factor STAT3 is essential for this process in mammals. Here we studied the THY1+ spermatogonial population in mouse testes, which contains spermatogonial stem cells (SSC) and non-stem cell progenitor spermatogonia, to further define the downstream mechanism regulating differentiation. Transcript abundance for the bHLH transcription factor Neurog3 was found to be significantly reduced upon transient inhibition of STAT3 signaling in these cells and exposure to GDNF, a key growth factor regulating self-renewal of SSCs, suppressed activation of STAT3 and in accordance Neurog3 gene expression. Moreover, STAT3 was found to bind the distal Neurog3 promoter/enhancer region in THY1+ spermatogonia and regulate transcription. Transient inhibition of Neurog3 expression in cultures of proliferating THY1+ spermatogonia increased stem cell content after several self-renewal cycles without effecting overall proliferation of the cells, indicating impaired differentiation of SSCs to produce progenitor spermatogonia. Furthermore, cultured THY1+ spermatogonia with induced deficiency of Neurog3 were found to be incapable of differentiation in vivo following transplantation into testes of recipient mice. Collectively, these results establish a mechanism by which activation of STAT3 regulates the expression of NEUROG3 to subsequently drive differentiation of SSC and progenitor spermatogonia in the mammalian germline.

Formin1 mediates the induction of dendritogenesis and synaptogenesis by neurogenin3 in mouse hippocampal neurons

Neurogenin3, a proneural transcription factor controlled by Notch receptor, has been recently shown to regulate dendritogenesis and synaptogenesis in mouse hippocampal neurons. However, little is known about the molecular mechanisms involved in these actions of Ngn3. We have used a microarray analysis to identify Ngn3 regulated genes related with cytoskeleton dynamics. One of such genes is Fmn1, whose protein, Formin1, is associated with actin and microtubule cytoskeleton. Overexpression of the Fmn1 isoform-Ib in cultured mouse hippocampal neurons induced an increase in the number of primary dendrites and in the number of glutamatergic synaptic inputs at 4 days in vitro. The same changes were provoked by overexpression of Ngn3. In addition downregulation of Fmn1 by the use of Fmn1-siRNAs impaired such morphological and synaptic changes induced by Ngn3 overexpression in neurons. These results reveal a previously unknown involvement of Formin1 in dendritogenesis and synaptogenesis and indicate that this protein is a key component of the Ngn3 signaling pathway that controls neuronal differentiation.

Genetic heterogeneity for recessively inherited congenital cataract microcornea with corneal opacity

PURPOSE

To investigate whether three consanguineous families from the Punjab province of Pakistan, with affected members with recessively inherited congenital cataract microcornea with corneal opacity, are genetically homogeneous.

METHODS

An ophthalmic examination was performed on each family member to establish the diagnosis. The two largest families were analyzed by homozygosity mapping using SNP arrays. Linkage was confirmed using polymorphic microsatellite markers, and logarithm of odds (LOD) scores were calculated. Candidate genes were prioritized using the ENDEAVOUR program.

RESULTS

Autosomal recessive congenital cataract-microcornea with corneal opacity mapped to chromosome 10cen for family MEP57 and to either chromosomes 2ptel or 20p for family MEP60. For MEP57, the refined interval was 36.8 Mb flanked by D10S1208 (35.3 Mb) and D10S676 (72.1 Mb). For MEP60, the interval containing the mutation was either 6.7 Mb from the telomere of chromosome 2 to marker D2S281 or 3.8 Mb flanked by D20S906 (1.5 Mb) and D20S835 (5.3 Mb). Maximum multipoint LOD scores of 3.09, 1.94, and 3.09 were calculated at D10S567, D2S281, and D20S473 for families MEP57 and MEP60. Linkage to these loci was excluded for family MEP68. SLC4A11 was excluded as a candidate gene for the observed phenotype in MEP60.

CONCLUSIONS

The authors have identified two new loci, one on chromosome 10cen and the other on 2ptel or 20p, that are associated with recessively inherited congenital cataract-microcornea with corneal opacity. This phenotype is genetically heterogeneous in the Pakistani population. Further genetic studies of this kind, combined with a detailed phenotypic description, will contribute to more precise classification criteria for anterior segment disease.

Estradiol meets notch signaling in developing neurons

The transmembrane receptor Notch, a master developmental regulator, controls gliogenesis, neurogenesis, and neurite development in the nervous system. Estradiol, acting as a hormonal signal or as a neurosteroid, also regulates these developmental processes. Here we review recent evidence indicating that estradiol and Notch signaling interact in developing hippocampal neurons by a mechanism involving the putative membrane receptor G protein-coupled receptor 30. This interaction is relevant for the control of neuronal differentiation, since the downregulation of Notch signaling by estradiol results in the upregulation of neurogenin 3, which in turn promotes dendritogenesis.

Generating retinal neurons by reprogramming retinal pigment epithelial cells

IMPORTANCE OF THE FIELD

Retinal degenerations cause blindness. One potential therapy is cell replacement. Because the human retina lacks regeneration capacity, much attention has been directed towards searching for cells that can differentiate into retinal neurons.

AREAS COVERED IN THIS REVIEW

We discuss the possibility of using transcription factor genes to channel retinal pigment epithelial (RPE) cells' capabilities of proliferation and plasticity towards the production of retinal neurons.

WHAT THE READER WILL GAIN

Experiments with chick embryos show that RPE cells - in the eye, in explant, or in a dissociated cell culture - can give rise to cells resembling retinal neurons when reprogrammed with regulatory genes involved in retinal neurogenesis. Depending on the regulatory gene used, reprogramming generates cells exhibiting traits of photoreceptor cells, amacrine cells and/or young ganglion neurons.

TAKE HOME MESSAGE

Gene-directed reprogramming of chick RPE can efficiently generate cells that exhibit traits of retinal neurons. Remaining to be addressed is the question of whether the results from chicks apply to mammals. Since the RPE is located adjacent to the neural retina, RPE reprogramming, if successful in mammals, may offer an approach to repopulate the neural retina without involving cell transplantation.

Neurogenin 3 cellular and subcellular localization in the developing and adult hippocampus

Neurogenin 3 (Ngn3), a proneural gene controlled by the Notch receptor, is implicated in the control of dendrite morphology and synaptic plasticity of cultured hippocampal neurons. Here we report the localization and subcellular distribution of Ngn3 in the hippocampus in vivo and in neuronal cultures. In situ hybridization showed Ngn3 mRNA expression in the pyramidal layer and dentate gyrus of adult mouse hippocampus. Immunohistochemistry studies revealed that Ngn3 localization is mostly cytoplasmic in the hippocampal eminence at embryonic day (E)17 and postnatal day (P)0. At P10 it is cytoplasmic in CA1-CA3 pyramidal neurons and nuclear in granule cells of the dentate gyrus. In the adult hippocampus Ngn3 is localized in the nucleus and cytoplasm of both pyramidal neurons and granule cells. During development of cultured hippocampal neurons, Ngn3 mRNA expression is higher at stages of neuronal polarization, as judged by reverse-transcription polymerase chain reaction (RT-PCR), and it is mostly cytoplasmic. The tracking of the subcellular localization of Ngn3 in neurons infected with a virus expressing myc-Ngn3 suggests that the protein is quickly translocated to the cell nucleus after synthesis and then reexported to the cytoplasm. Treatment with leptomycin B, a potent and specific inhibitor of the exportin CRM1, induced its accumulation into the nucleus, suggesting that CRM1 mediates the nuclear export of Ngn3. These results suggest that Ngn3 may play a role in neuronal development by actions in the cytoplasm.

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.

Liquiritin potentiate neurite outgrowth induced by nerve growth factor in PC12 cells

Neurite outgrowth and neuronal differentiation play a crucial role in the development of the nervous system. Understanding of neurotrophins induced neurite outgrowth was important to develop therapeutic strategy for axon regeneration in neurodegenerative diseases as well as after various nerve injuries. It has been reported that extension of neurite and differentiation of sympathetic neuron-like phenotype was modulated by nerve growth factor (NGF) in PC12 cells. In this study, NGF mediated neurite outgrowth was investigated in PC12 cells after liquiritin exposure. Liquiritin is a kind of flavonoids that is extracted from Glycyrrhizae radix, which is frequently used to treat injury or swelling for its life-enhancing properties as well as detoxification in traditional Oriental medicine. The result showed that liquiritin significantly promotes the neurite outgrowth stimulated by NGF in PC12 cells in dose dependant manners whereas the liquiritin alone did not induce neurite outgrowth. Oligo microarray and RT-PCR analysis further clarified that the neurotrophic effect of liquiritin was related to the overexpression of neural related genes such as neurogenin 3, neurofibromatosis 1, notch gene homolog 2, neuromedin U receptor 2 and neurotrophin 5. Thus, liquiritin may be a good candidate for treatment of various neurodegenerative diseases such as Alzheimer's disease or Parkinson's disease.

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.