The final fates of neurogenin2-expressing cells include all major neuron types in the mouse retina

Retinal structural alterations in patients with epilepsy taking antiepileptic drugs: A systematic review and meta-analysis of OCT findings

The impact of various neurodegenerative diseases on the retina has been investigated in recent years using optical coherence tomography (OCT). Epilepsy, classified as a neurodegenerative disorder, has been indicated to affect the structural integrity of the retina. Moreover, there is ongoing debate regarding the relative contribution of the disease pathogenesis and the consumption of anti-epileptic drugs (AEDs) to these retinal changes. The lack of systematic reviews has hindered our understanding of the true effects of epilepsy and AEDs on retinal health, as well as the efficacy of OCT in detecting these alterations. To comprehensively review the impact of epilepsy and AEDs on the structure of retina, we thoroughly searched the PubMed, EMBASE, and Web of Science databases to identify relevant articles published until July 7, 2024, and performed a meta-analysis. We updated our search in November, 2024. Random effect models have been used to calculate pooled effect estimates. Nineteen studies with a total number of 1851 eyes were identified. Adult patients showed significant reduction with respect of retinal nerve fiber layer (RNFL) thickness; average, as well as all quadrants. Significant reductions were also detected in all quadrants of ganglion cell complex (GCC). Conversely, average GCC and central macular thickness did not differ significantly between cases and controls. Additionally, in terms of various volume measurements in the retina, significant losses were observed in macular RNFL, ganglion cell-inner plexiform layer and total macula volumes in adult patients. In contrast, the inner nuclear layer volume remained comparable between the 2 groups. In pediatric patients with epilepsy receiving valproic acid, significant reductions was observed in the average RNFL thickness, as well as in the nasal and inferior quadrants; however, there were no significant changes in the thickness of the superior and temporal quadrants of RNFL, nor in foveal thickness. The analysis of pediatric patients receiving levetiracetam indicated no significant changes in retinal structural measurements across various RNFL categories, or in foveal thickness. This meta-analysis revealed the structural retinal alterations following AEDs administration in patients with epilepsy (PwE). OCT appears to be a reliable device that reflects retinal toxicity with AED consumption in PwE.

Retinal Ganglion Cell Fate Induction by Ngn-Family Transcription Factors

Purpose

Retinal ganglion cells (RGCs) are the projection neurons of the retina. Loss of RGCs is the cellular basis for vision loss in patients with glaucoma. Finding ways to regenerate RGCs will aid in the development of regenerative therapies for patients with glaucoma. The aim of this study was to examine the ability of Ngn-family transcription factors (TFs) to induce RGC regeneration through reprogramming in vitro and in vivo.

Methods

In vitro, lentiviruses were used to deliver Ngn-TFs into mouse embryonic fibroblasts (MEFs). In vivo, mouse pup retina electroporation was used to deliver Ngn-TFs into late-stage retinal progenitor cells (RPCs). Immunofluorescence staining and RNA sequencing were used to examine cell fate reprogramming; patch-clamp recording was used to examine neuronal electrophysiologic functions.

Results

In vitro, all three Ngn-TFs, Ngn1, Ngn2, and Ngn3, were able to work alone to reprogram MEFs into RGC-like neurons that resembled RGCs at the transcriptome level, exhibited typical neuronal membrane electrophysiologic properties, and formed functional synaptic communications with retinal neurons. In vivo, Ngn-TFs reprogrammed the differentiation-competent state of late-stage RPCs to generate RGCs.

Conclusions

Ngn-TFs are effective in inducing an RGC-like fate both in vitro and in vivo and might be explored further in the future for glaucoma translational applications.

Neurog2 regulates Isl1 to modulate horizontal cell number

The population sizes of different retinal cell types vary between different strains of mice, and that variation can be mapped to genomic loci in order to identify its polygenic origin. In some cases, controlling genes act independently, whereas in other instances, they exhibit epistasis. Here, we identify an epistatic interaction revealed through the mapping of quantitative trait loci from a panel of recombinant inbred strains of mice. The population of retinal horizontal cells exhibits a twofold variation in number, mapping to quantitative trait loci on chromosomes 3 and 13, where these loci are shown to interact epistatically. We identify a prospective genetic interaction underlying this, mediated by the bHLH transcription factor Neurog2, at the chromosome 3 locus, functioning to repress the LIM homeodomain transcription factor Isl1, at the chromosome 13 locus. Using single and double conditional knockout mice, we confirm the countervailing actions of each gene, and validate in vitro a crucial role for two single nucleotide polymorphisms in the 5'UTR of Isl1, one of which yields a novel E-box, mediating the repressive action of Neurog2.

A single-cell guide to retinal development: Cell fate decisions of multipotent retinal progenitors in scRNA-seq

Recent advances in high throughput single-cell RNA sequencing (scRNA-seq) technology have enabled the simultaneous transcriptomic profiling of thousands of individual cells in a single experiment. To investigate the intrinsic process of retinal development, researchers have leveraged this technology to quantify gene expression in retinal cells across development, in multiple species, and from numerous important models of human disease. In this review, we summarize recent applications of scRNA-seq and discuss how these datasets have complemented and advanced our understanding of retinal progenitor cell competence, cell fate specification, and differentiation. Finally, we also highlight the outstanding questions in the field that advances in single-cell data generation and analysis will soon be able to answer.

Ascl1 is required to specify a subset of ventromedial hypothalamic neurons

Despite clear physiological roles, the ventromedial hypothalamus (VMH) developmental programs are poorly understood. Here, we asked whether the proneural gene achaete-scute homolog 1 (Ascl1) contributes to VMH development. Ascl1 transcripts were detected in embryonic day (E) 10.5 to postnatal day 0 VMH neural progenitors. The elimination of Ascl1 reduced the number of VMH neurons at E12.5 and E15.5, particularly within the VMH-central (VMHC) and -dorsomedial (VMHDM) subdomains, and resulted in a VMH cell fate change from glutamatergic to GABAergic. We observed a loss of Neurog3 expression in Ascl1-/- hypothalamic progenitors and an upregulation of Neurog3 when Ascl1 was overexpressed. We also demonstrated a glutamatergic to GABAergic fate switch in Neurog3-null mutant mice, suggesting that Ascl1 might act via Neurog3 to drive VMH cell fate decisions. We also showed a concomitant increase in expression of the central GABAergic fate determinant Dlx1/2 in the Ascl1-null hypothalamus. However, Ascl1 was not sufficient to induce an ectopic VMH fate when overexpressed outside the normal window of competency. Combined, Ascl1 is required but not sufficient to specify the neurotransmitter identity of VMH neurons, acting in a transcriptional cascade with Neurog3.

A Novel Birthdate-Labeling Method Reveals Segregated Parallel Projections of Mitral and External Tufted Cells in the Main Olfactory System

A fundamental strategy in sensory coding is parallel processing, whereby unique, distinct features of sensation are computed and projected to the central target in the form of submodal maps. It remains unclear, however, whether such parallel processing strategy is employed in the main olfactory system, which codes the complex hierarchical odor and behavioral scenes. A potential scheme is that distinct subsets of projection neurons in the olfactory bulb (OB) form parallel projections to the targets. Taking advantage of the observation that the distinct projection neurons develop at different times, we developed a Cre-loxP-based method that allows for birthdate-specific labeling of cell bodies and their axon projections in mice. This birthdate tag analysis revealed that the mitral cells (MCs) born in an early developmental stage and the external tufted cells (TCs) born a few days later form segregated parallel projections. Specifically, the latter subset converges the axons onto only two small specific targets, one of which, located at the anterolateral edge of the olfactory tubercle (OT), excludes widespread MC projections. This target is made up of neurons that express dopamine D1 but not D2 receptor and corresponds to the most anterolateral isolation of the CAP compartments (aiCAP) that were defined previously. This finding of segregated projections suggests that olfactory sensing does indeed involve parallel processing of functionally distinct submodalities. Importantly, the birthdate tag method used here may pave the way for deciphering the functional meaning of these individual projection pathways in the future.

Alzheimer's Disease Is Responsible for Progressive Age-Dependent Differential Expression of Various Protein Cascades in Retina of Mice

Alzheimer's disease (AD) is a devastating neurodegenerative disease associated with cognitive impairments and memory loss usually in aged people. In the past few years, it has been detected in the eye retina, manifesting the systematic spread of the disease. This might be used for biomarker discovery for early detection and treatment of the disease. Here, we have described the proteomic alterations in retina of 2, 4, and 6 months old 3×Tg-AD mice by using iTRAQ (isobaric tags for relative and absolute quantification) proteomics technology. Out of the total identified proteins, 121 (71 up- and 50 down-regulated), 79 (51 up- and 28 down-regulated), and 153 (37 up- and 116 down-regulated) significantly differentially expressed proteins (DEPs) are found in 2, 4, and 6 month's mice retina (2, 4, and 6 M), respectively. Seventeen DEPs are found common in these three groups with consistent expression behavior or opposite expression in the three groups. Bioinformatics analysis of these DEPs highlighted their involvement in vital AD-related biological phenomenon. To further prompt the results, four proteins from 2 M group, three from 4 M, and four from 6 M age groups are successfully validated with Western blot analysis. This study confirms the retinal involvement of AD in the form of proteomic differences and further explains the protein-based molecular mechanisms, which might be a step toward biomarker discovery for early detection and treatment of the disease.

An investigation of the ocular toxic effects of levetiracetam therapy in children with epilepsy

OBJECTIVE

To investigate the potential toxic effects of levetiracetam monotherapy on ocular tissues in cases of pediatric epilepsy using optical coherence tomography (OCT).

METHODS

Thirty epileptic children (group 1) receiving levetiracetam monotherapy at a dosage of 20-40 mg/kg/day for at least 1 year with a first diagnosis of epilepsy and 30 age- and gender-matched healthy children (group 2) were included in the study. In addition to a detailed eye examination, peripapillary retinal nerve fiber layer (RNFL) thickness, ganglion cell complex (GCC) thickness, foveal thickness (FT), and central corneal thickness (CCT) were measured in all children by means of spectral domain OCT. The data obtained from the two groups were then subjected to statistical analysis.

RESULTS

The mean age of both groups was 12 ± 3.64 years [1-12]. The mean duration of levetiracetam in group 1 was 24.07 ± 12.82 months. Mean RNFL values in groups 1 and 2 were 106.1 ± 10.42 and 104.98 ± 10.04 μm, mean GCC values were 94.72 ± 6.26 and 94.4 ± 6 μm, mean FT values were 240.73 ± 17.94 and 240.77 ± 15.97 μm, and mean CCT values were 555.1 ± 44.88 and 540.97 ± 32.65 μm, respectively. No significant difference was determined between the two groups in terms of any parameter. Best corrected visual acuity values of the subjects in both groups were 10/10, and no color vision or visual field deficit was determined.

CONCLUSION

Levetiracetam monotherapy causes no significant function or morphological change in ocular tissues in pediatric epilepsies.

Requirements for Neurogenin2 during mouse postnatal retinal neurogenesis

During embryonic retinal development, the bHLH factor Neurog2 regulates the temporal progression of neurogenesis, but no role has been assigned for this gene in the postnatal retina. Using Neurog2 conditional mutants, we found that Neurog2 is necessary for the development of an early, embryonic cohort of rod photoreceptors, but also required by both a subset of cone bipolar subtypes, and rod bipolars. Using transcriptomics, we identified a subset of downregulated genes in P2 Neurog2 mutants, which act during rod differentiation, outer segment morphogenesis or visual processing. We also uncovered defects in neuronal cell culling, which suggests that the rod and bipolar cell phenotypes may arise via more complex mechanisms rather than a simple cell fate shift. However, given an overall phenotypic resemblance between Neurog2 and Blimp1 mutants, we explored the relationship between these two factors. We found that Blimp1 is downregulated between E12-birth in Neurog2 mutants, which probably reflects a dependence on Neurog2 in embryonic progenitor cells. Overall, we conclude that the Neurog2 gene is expressed and active prior to birth, but also exerts an influence on postnatal retinal neuron differentiation.

Integral bHLH factor regulation of cell cycle exit and RGC differentiation

BACKGROUND

In the developing mouse embryo, the bHLH transcription factor Neurog2 is transiently expressed by retinal progenitor cells and required for the initial wave of neurogenesis. Remarkably, another bHLH factor, Ascl1, normally not present in the embryonic Neurog2 retinal lineage, can rescue the temporal phenotypes of Neurog2 mutants.

RESULTS

Here we show that Neurog2 simultaneously promotes terminal cell cycle exit and retinal ganglion cell differentiation, using mitotic window labeling and integrating these results with retinal marker quantifications. We also analyzed the transcriptomes of E12.5 GFP-expressing cells from Neurog2GFP/+ , Neurog2GFP/GFP , and Neurog2Ascl1KI/GFP eyes, and validated the most significantly affected genes using qPCR assays.

CONCLUSIONS

Our data support the hypothesis that Neurog2 acts at the top of a retinal bHLH transcription factor hierarchy. The combined expression levels of these downstream factors are sufficiently induced by ectopic Ascl1 to restore RGC genesis, highlighting the robustness of this gene network during retinal ganglion cell neurogenesis. Developmental Dynamics 247:965-975, 2018. © 2018 Wiley Periodicals, Inc.

Visual and Ocular Manifestations of Alzheimer's Disease and Their Use as Biomarkers for Diagnosis and Progression

Alzheimer's disease (AD) is the most common form of dementia affecting the growing aging population today, with prevalence expected to rise over the next 35 years. Clinically, patients exhibit a progressive decline in cognition, memory, and social functioning due to deposition of amyloid β (Aβ) protein and intracellular hyperphosphorylated tau protein. These pathological hallmarks of AD are measured either through neuroimaging, cerebrospinal fluid analysis, or diagnosed post-mortem. Importantly, neuropathological progression occurs in the eye as well as the brain, and multiple visual changes have been noted in both human and animal models of AD. The eye offers itself as a transparent medium to cerebral pathology and has thus potentiated the development of ocular biomarkers for AD. The use of non-invasive screening, such as retinal imaging and visual testing, may enable earlier diagnosis in the clinical setting, minimizing invasive and expensive investigations. It also potentially improves disease management and quality of life for AD patients, as an earlier diagnosis allows initiation of medication and treatment. In this review, we explore the evidence surrounding ocular changes in AD and consider the biomarkers currently in development for early diagnosis.

VSX2 and ASCL1 Are Indicators of Neurogenic Competence in Human Retinal Progenitor Cultures

Three dimensional (3D) culture techniques are frequently used for CNS tissue modeling and organoid production, including generation of retina-like tissues. A proposed advantage of these 3D systems is their potential to more closely approximate in vivo cellular microenvironments, which could translate into improved manufacture and/or maintenance of neuronal populations. Visual System Homeobox 2 (VSX2) labels all multipotent retinal progenitor cells (RPCs) and is known to play important roles in retinal development. In contrast, the proneural transcription factor Acheate scute-like 1 (ASCL1) is expressed transiently in a subset of RPCs, but is required for the production of most retinal neurons. Therefore, we asked whether the presence of VSX2 and ASCL1 could gauge neurogenic potential in 3D retinal cultures derived from human prenatal tissue or ES cells (hESCs). Short term prenatal 3D retinal cultures displayed multiple characteristics of human RPCs (hRPCs) found in situ, including robust expression of VSX2. Upon initiation of hRPC differentiation, there was a small increase in co-labeling of VSX2+ cells with ASCL1, along with a modest increase in the number of PKCα+ neurons. However, 3D prenatal retinal cultures lost expression of VSX2 and ASCL1 over time while concurrently becoming refractory to neuronal differentiation. Conversely, 3D optic vesicles derived from hESCs (hESC-OVs) maintained a robust VSX2+ hRPC population that could spontaneously co-express ASCL1 and generate photoreceptors and other retinal neurons for an extended period of time. These results show that VSX2 and ASCL1 can serve as markers for neurogenic potential in cultured hRPCs. Furthermore, unlike hESC-OVs, maintenance of 3D structure does not independently convey an advantage in the culture of prenatal hRPCs, further illustrating differences in the survival and differentiation requirements of hRPCs extracted from native tissue vs. those generated entirely in vitro.

Notch signaling differentially regulates Atoh7 and Neurog2 in the distal mouse retina

Notch signaling regulates basic helix-loop-helix (bHLH) factors as an evolutionarily conserved module, but the tissue-specific mechanisms are incompletely elucidated. In the mouse retina, bHLH genes Atoh7 and Neurog2 have distinct functions, with Atoh7 regulating retinal competence and Neurog2 required for progression of neurogenesis. These transcription factors are extensively co-expressed, suggesting similar regulation. We directly compared Atoh7 and Neurog2 regulation at the earliest stages of retinal neurogenesis in a broad spectrum of Notch pathway mutants. Notch1 and Rbpj normally block Atoh7 and Neurog2 expression. However, the combined activities of Notch1, Notch3 and Rbpj regulate Neurog2 patterning in the distal retina. Downstream of the Notch complex, we found the Hes1 repressor mediates Atoh7 suppression, but Hes1, Hes3 and Hes5 do not regulate Neurog2 expression. We also tested Notch-mediated regulation of Jag1 and Pax6 in the distal retina, to establish the appropriate context for Neurog2 patterning. We found that Notch1;Notch3 and Rbpj block co-expression of Jag1 and Neurog2, while specifically stimulating Pax6 within an adjacent domain. Our data suggest that Notch signaling controls the overall tempo of retinogenesis, by integrating cell fate specification, the wave of neurogenesis and the developmental status of cells ahead of this wave.

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.

Neurogenin2-d4Venus and Gadd45g-d4Venus transgenic mice: visualizing mitotic and migratory behaviors of cells committed to the neuronal lineage in the developing mammalian brain

To achieve highly sensitive and comprehensive assessment of the morphology and dynamics of cells committed to the neuronal lineage in mammalian brain primordia, we generated two transgenic mouse lines expressing a destabilized (d4) Venus controlled by regulatory elements of the Neurogenin2 (Neurog2) or Gadd45g gene. In mid-embryonic neocortical walls, expression of Neurog2-d4Venus mostly overlapped with that of Neurog2 protein, with a slightly (1 h) delayed onset. Although Neurog2-d4Venus and Gadd45g-d4Venus mice exhibited very similar labeling patterns in the ventricular zone (VZ), in Gadd45g-d4Venus mice cells could be visualized in more basal areas containing fully differentiated neurons, where Neurog2-d4Venus fluorescence was absent. Time-lapse monitoring revealed that most d4Venus⁺ cells in the VZ had processes extending to the apical surface; many of these cells eventually retracted their apical process and migrated basally to the subventricular zone, where neurons, as well as the intermediate neurogenic progenitors that undergo terminal neuron-producing division, could be live-monitored by d4Venus fluorescence. Some d4Venus⁺ VZ cells instead underwent nuclear migration to the apical surface, where they divided to generate two d4Venus⁺ daughter cells, suggesting that the symmetric terminal division that gives rise to neuron pairs at the apical surface can be reliably live-monitored. Similar lineage-committed cells were observed in other developing neural regions including retina, spinal cord, and cerebellum, as well as in regions of the peripheral nervous system such as dorsal root ganglia. These mouse lines will be useful for elucidating the cellular and molecular mechanisms underlying development of the mammalian nervous system.

Cell fate determination in the vertebrate retina

The vertebrate retina is a well-characterized and tractable model for studying neurogenesis. Retinal neurons and glia are generated in a conserved sequence from a pool of multipotent progenitor cells, and numerous cell fate determinants for the different classes of retinal cell types have been identified. Here, we summarize several recent developments in the field that have advanced understanding of the regulation of multipotentiality and temporal competence of progenitors. We also discuss recent insights into the relative influence of lineage-based versus stochastic modes of cell fate determination. Enhancing and integrating knowledge of the molecular and genetic machinery underlying retinal development is critically important for understanding not only normal developmental mechanisms, but also therapeutic interventions aimed at restoring vision loss.

Fate tracing of neurogenin2-expressing cells in the mouse retina using CreER™: LacZ

Delineating the final fate of progenitor cells that transiently express a regulatory gene may shed light on how the gene participates in regulating retinal development. We describe the steps in tracing final fates of progenitor cells that once transiently express neurogenin2 (ngn2) during mouse retinal development with the binary, conditional Ngn2-CreER(™)-LacZ reporter system. Ngn2-CreER(™) mice (Zirlinger et al. Proc Natl Acad Sci USA 99:8084-8089, 2002), in which ngn2 promoter drives the expression of Cre-estrogen receptor CreER(™) (Littlewood et al. Nuc Acid Res 23:1686-1690, 1995; Hayashi and McMahon Dev Biol 244:305-318, 2002), are crossed with Rosa26-LoxP-LacZ reporter mice (Soriano Nat Genet 21:70-71, 1999), in which the expression of lacZ requires the removal of "stop" by Cre recombinase (Wagner et al. Transgenic Res 10:545-553, 2001). 4-hydroxytamoxifen (4-OHT), a synthetic ligand with high affinity for ER(™), is administered to double transgenic embryos and/or neonatal mice. Binding of 4-OHT to Cre-ER(™) activates Cre recombinase, which then catalyzes the removal of the "stop" sequence from the LoxP-LacZ transgene, leading to lacZ expression in cells that express ngn2. Retinal tissues are fixed at different time points after 4-OHT treatment and analyzed for LacZ activities by colorimetric reaction. Double-labeling with a cell type-specific marker can be used to define the identity of a LacZ(+) cell. Combining persisted lacZ expression through the life of the cell and the short half-life (0.5-2 h) of 4-OHT (Danielian et al. Curr Biol 8:1323-1326, 1998), this system offers the opportunity to track the final fates of cells that have expressed ngn2 during the brief presence of 4-OHT administered during retinal development.

Ascl1 expression defines a subpopulation of lineage-restricted progenitors in the mammalian retina

The mechanisms of cell fate diversification in the retina are not fully understood. The seven principal cell types of the neural retina derive from a population of multipotent progenitors during development. These progenitors give rise to multiple cell types concurrently, suggesting that progenitors are a heterogeneous population. It is thought that differences in progenitor gene expression are responsible for differences in progenitor competence (i.e. potential) and, subsequently, fate diversification. To elucidate further the mechanisms of fate diversification, we assayed the expression of three transcription factors made by retinal progenitors: Ascl1 (Mash1), Ngn2 (Neurog2) and Olig2. We observed that progenitors were heterogeneous, expressing every possible combination of these transcription factors. To determine whether this progenitor heterogeneity correlated with different cell fate outcomes, we conducted Ascl1- and Ngn2-inducible expression fate mapping using the CreER™/LoxP system. We found that these two factors gave rise to markedly different distributions of cells. The Ngn2 lineage comprised all cell types, but retinal ganglion cells (RGCs) were exceedingly rare in the Ascl1 lineage. We next determined whether Ascl1 prevented RGC development. Ascl1-null mice had normal numbers of RGCs and, interestingly, we observed that a subset of Ascl1+ cells could give rise to cells expressing Math5 (Atoh7), a transcription factor required for RGC competence. Our results link progenitor heterogeneity to different fate outcomes. We show that Ascl1 expression defines a competence-restricted progenitor lineage in the retina, providing a new mechanism to explain fate diversification.

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.

The transcription factor RBP-J is essential for retinal cell differentiation and lamination

Background

The highly ordered vertebrate retina is composed of seven cell types derived from a common pool of retinal progenitor cells (RPCs), and is a good model for the studies of cell differentiation and interaction during neural development. Notch signaling plays a pivotal role in retinogenesis in mammals, but the full scope of the functions of Notch pathway, and the underlying molecular mechanisms, remain unclear.

Results

In this study, we conditionally knocked out RBP-J, the critical transcription factor downstream to all four Notch receptors, in RPCs of mouse retina at different developmental stages. Disruption of RBP-J at early retinogenesis resulted in accelerated RPCs differentiation, but only photoreceptors and ganglion cells were overrepresented, with other neuronal populations diminished. Similarly, deletion of RBP-J at early postnatal days also led to overproduction of photoreceptors, suggesting that RBP-J governed RPCs specification and differentiation through retinogenesis. In all the RBP-J deletion models, the retinal laminar structures were distorted by the formation of numerous rosette-like structures, reminiscent of β-catenin deficient retina. Indeed, we found that these rosettes aligned with gaps in β-catenin expression at the apical surface of the retina. By in vivo electroporation-mediated transfection, we demonstrated that lamination defects in RBP-J deficient retinae were rescued by overexpressing β-catenin.

Conclusions

Our data indicate that RBP-J-mediated canonical Notch signaling governs retinal cell specification and differentiation, and maintains retinal lamination through the expression of β-catenin.

Acheate-scute like 1 (Ascl1) is required for normal delta-like (Dll) gene expression and notch signaling during retinal development

Delta gene expression in Drosophila is regulated by proneural basic helix-loop-helix (bHLH) transcription factors, such as acheate-scute. In vertebrates, multiple Delta-like and proneural bHLH genes are expressed during neurogenesis, especially in the retina. We recently uncovered a relationship between Acheate-scute like 1 (Ascl1), Delta-like genes, and Notch in chick retinal progenitors. Here, we report that mammalian retinal progenitors are also the primary source of Delta-like genes, likely signaling through Notch among themselves, while differentiating neurons expressed Jagged2. Ascl1 is coexpressed in Delta-like and Notch active progenitors, and required for normal Delta-like gene expression and Notch signaling. We also reveal a role for Ascl1 in the regulation of Hes6, a proneurogenic factor that inhibits Notch signaling to promote neural rather than glial differentiation. Thus, these results suggest a molecular mechanism whereby attenuated Notch levels coupled with reduced proneurogenic activity in progenitors leads to increased gliogenesis and decreased neurogenesis in the Ascl1-deficient retina.

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.

Proneural gene ash1 promotes amacrine cell production in the chick retina

The diverse types of neurons and Müller glia in the vertebrate retina are believed to arise from common progenitor cells. To better understand how neural diversity is achieved during retinal neurogenesis, we examined the function of ash1, a proneural bHLH gene expressed in progenitor cells throughout retinal neurogenesis. Published studies using retinal explant culture derived from knockout mice concluded that ash1 is required for the production of late-born neurons, including bipolar cells. In this study, gain-of-function experiments were carried out in ovo in embryonic chick retina. In the developing chick retina, expression of ash1 temporally overlapped with, but spatially differed from, the expression of ngn2, also a proneural gene expressed in progenitor cells throughout retinal neurogenesis. Retrovirus-driven overexpression of ash1 in the developing chick retina decreased the progenitor population (BrdU+ or expressing ngn2), expanded the amacrine population (AP2alpha+ or Pax6+), and reduced bipolar (chx10 mRNA+) and Müller glial (vimentin+) populations. Photoreceptor deficiency occurred after the completion of neurogenesis. The number of ganglion cells, which are born first during retinal neurogenesis, remained unchanged. Similar overexpression of ngn2 did not produce discernible changes in retinal neurogenesis, nor in ash1 expression. These results suggest that ash1 promotes the production of amacrine cells and thus may participate in a regulatory network governing neural diversity in the chick retina.

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.

Relationship between Delta-like and proneural bHLH genes during chick retinal development

Notch signaling in the retina maintains a pool of progenitor cells throughout retinogenesis. However, two Notch-ligands from the Delta-like gene family, Dll1 and Dll4, are present in the developing retina. To understand their relationship, we characterized Dll1 and Dll4 expression with respect to proliferating progenitor cells and newborn neurons in the chick retina. Dll4 matched the pattern of neural differentiation. By contrast, Dll1 was primarily expressed in progenitor cells. We compared Dll1 and Dll4 kinetic profiles with that of the transiently up-regulated cascade of proneural basic helix-loop-helix (bHLH) genes after synchronized progenitor cell differentiation, which suggested a potential role for Ascl1 in the regulation of Delta-like genes. Gain-of-function assays demonstrate that Ascl1 does influence Delta-like gene expression and Notch signaling activity. These data suggest that multiple sources of Notch signaling from newborn neurons and progenitors themselves coordinate retinal histogenesis.

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.

Regulation of photoreceptor gene expression by Crx-associated transcription factor network

Rod and cone photoreceptors in the mammalian retina are special types of neurons that are responsible for phototransduction, the first step of vision. Development and maintenance of photoreceptors require precisely regulated gene expression. This regulation is mediated by a network of photoreceptor transcription factors centered on Crx, an Otx-like homeodomain transcription factor. The cell type (subtype) specificity of this network is governed by factors that are preferentially expressed by rods or cones or both, including the rod-determining factors neural retina leucine zipper protein (Nrl) and the orphan nuclear receptor Nr2e3; and cone-determining factors, mostly nuclear receptor family members. The best-documented of these include thyroid hormone receptor beta2 (Tr beta2), retinoid related orphan receptor Ror beta, and retinoid X receptor Rxr gamma. The appropriate function of this network also depends on general transcription factors and cofactors that are ubiquitously expressed, such as the Sp zinc finger transcription factors and STAGA co-activator complexes. These cell type-specific and general transcription regulators form complex interactomes; mutations that interfere with any of the interactions can cause photoreceptor development defects or degeneration. In this manuscript, we review recent progress on the roles of various photoreceptor transcription factors and interactions in photoreceptor subtype development. We also provide evidence of auto-, para-, and feedback regulation among these factors at the transcriptional level. These protein-protein and protein-promoter interactions provide precision and specificity in controlling photoreceptor subtype-specific gene expression, development, and survival. Understanding these interactions may provide insights to more effective therapeutic interventions for photoreceptor diseases.

Compensational regulation of bHLH transcription factors in the postnatal development of BETA2/NeuroD1-null retina

The bHLH transcriptional factor BETA2/NeuroD1 is essential for the survival of photoreceptor cells in the retina. Although this gene is expressed throughout the retina, BETA2/NeuroD1 knockout mice show photoreceptor cell degeneration only in the outer nuclear layer of the retina; other retinal neurons are not affected. Previous studies on retina explants lacking three bHLH genes revealed that retinal neurons in the inner nuclear layer require multiple bHLH genes for their differentiation and survival. However, single- or double-gene mutations show no or a lesser degree of abnormalities during eye development, likely because of compensation or cooperative regulation among those genes. Because not all null mice survive until the retina is fully organized, no direct evidence of this concept has been reported. To understand the regulatory mechanisms between bHLH factors in retinal development, we performed a detailed analysis of BETA2/NeuroD1 knockout mice. BETA2/NeuroD1 was expressed in all 3 layers of the mouse retina, including all major types of neurons. In addition, a null mutation of BETA2/NeuroD1 resulted in up-regulation of other bHLH genes, Mash1, Neurogenin2, and Math3, in the inner nuclear layer. Our data suggest that compensatory and cross regulatory mechanisms exist among the bHLH factors during retinal development.