Roles of homeobox and bHLH genes in specification of a retinal cell type

Cellules souches rétiniennes : mécanisme de différenciation et potentiel thérapeutique

Retinal dystrophies are rarely curable diseases and several avenues of research are being pursued, such replacement therapies and pharmacological treatment. Among them, the transplantation of functional retinal cells has been envisaged in order to restore vision in patients who have these diseases by repopulating the damaged retina and/or by rescuing retinal neurons from further degeneration. Over the past few years, identification and characterization of stem cells has opened new avenues in cell-replacement therapy. Since retinal stem cells are already present during embryonic development, they persist in the adult mammalian eye only in the ciliary marginal zone, even a stem cell potential has been described for the Müller glia in the retina. This result opened possibilities of regeneration by mobilizing endogenous stem cells to respond to injury. Regarding the transplantation studies, in all experiments using different types of stem cells (retinal progenitors, neural stem cells, bone marrow-derived stem cells and ES cells), despite their incorporation within the host's retina, the transplanted cells failed to express retina-specific markers and to establish synaptic connections. Therefore, the true potential of the different stem cells in retina repair can only be realized with more information about mechanisms that regulate their proliferation and differentiation; and by development of techniques that allow their prospective identification and enrichment.

Zac1 promotes a Müller glial cell fate and interferes with retinal ganglion cell differentiation inXenopus retina

The timing of cell cycle exit is tightly linked to cell fate specification in the developing retina. Accordingly, several tumor suppressor genes, which are key regulators of cell cycle exit in cancer cells, play critical roles in retinogenesis. Here we investigated the role of Zac1, a tumor suppressor gene encoding a zinc finger transcription factor, in retinal development. Strikingly, in gain-of-function assays in Xenopus, mouse Zac1 promotes proliferation and apoptosis at an intermediate stage of retinogenesis. Zac1 also influences cell fate decisions, preferentially promoting the differentiation of tumor-like clusters of abnormal neuronal cells in the ganglion cell layer, as well as inducing the formation of supernumerary Müller glial cells at the expense of other cell types. Thus Zac1 has the capacity to influence cell cycle exit, and cell fate specification and differentiation decisions by retinal progenitors, suggesting that further functional studies will uncover new insights into how retinogenesis is regulated.

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.

Altered melanocyte differentiation and retinal pigmented epithelium transdifferentiation induced by Mash1 expression in pigment cell precursors

Transcription factor genes governing pigment cell development that are associated with spotting mutations in mice include members of several structural transcription factor classes but not members of the basic helix-loop-helix (bHLH) class, important for neurogenesis and myogenesis. To determine the effects of bHLH factor expression on pigment cell development, the neurogenic bHLH factor Mash1 was expressed early in pigment cell development in transgenic mice from the dopachrome tautomerase (Dct) promoter. Dct:Mash1 transgenic founders exhibit variable microphthalmia and patchy coat color hypopigmentation. Transgenic F1 mice exhibit microphthalmia with complete coat color dilution. Marker analysis demonstrates that Mash1 expression in the retinal pigmented epithelium (RPE) initiates neurogenesis in this cell layer, whereas expression in remaining neural crest-derived melanocytes alters their differentiation, in part by profoundly downregulating expression of the p (pink-eyed dilution) gene, while maintaining their cell fate. The effects of transcriptional perturbation of pigment cell precursors by Mash1 further highlight differences between pigment cells of distinct developmental origins, and suggest a mechanism for the alteration of melanogenesis to result in marked coat color dilution.

CHX10 targets a subset of photoreceptor genes

The homeobox gene CHX10 is required for retinal progenitor cell proliferation early in retinogenesis and subsequently for bipolar neuron differentiation. To clarify the molecular mechanisms employed by CHX10 we sought to identify its target genes. In a yeast one-hybrid assay Chx10 interacted with the Ret1 site of the photoreceptor-specific gene Rhodopsin. Gel shift assays using in vitro translated protein confirmed that CHX10 binds to Ret1, but not to the similar Rhodopsin sites Ret4 and BAT-1. Using retinal nuclear lysates, we observed interactions between Chx10 and additional photoreceptor-specific elements including the PCE-1 (Rod arrestin/S-antigen) and the Cone opsin locus control region (Red/green cone opsin). However, chromatin immunoprecipitation assays revealed that in vivo, Chx10 bound sites upstream of the Rod arrestin and Interphotoreceptor retinoid-binding protein genes but not Rhodopsin or Cone opsin. Thus, in a chromatin context, Chx10 associates with a specific subset of elements that it binds with comparable apparent affinity in vitro. Our data suggest that CHX10 may target these motifs to inhibit rod photoreceptor gene expression in bipolar cells.

The Iroquois homeobox gene, Irx5, is required for retinal cone bipolar cell development

In the mouse retina, at least ten distinct types of bipolar interneurons are involved in the transmission of visual signals from photoreceptors to ganglion cells. How bipolar interneuron diversity is generated during retinal development is poorly understood. Here, we show that Irx5, a member of the Iroquois homeobox gene family, is expressed in developing bipolar cells starting at postnatal day 5 and is localized to a subset of cone bipolar cells in the mature mouse retina. In Irx5-deficient mice, defects were observed in the expression of some, but not all, immunohistological markers that define mature Type 2 and Type 3 OFF cone bipolar cells, indicating a role for Irx5 in bipolar cell differentiation. The differentiation of these two bipolar cell types has previously been shown to require the homeodomain-CVC transcription factor, Vsx1. However, the defects observed in Irx5-deficient retinas do not coincide with a reduction of Vsx1 expression, and conversely, the expression of Irx5 in cone bipolar cells does not require the presence of a functional Vsx1 allele. These results indicate that there are at least two distinct genetic pathways (Irx5-dependent and Vsx1-dependent) regulating the development of Type 2 and Type 3 cone bipolar cells.

Multiple requirements for Hes 1 during early eye formation

During embryogenesis, multiple developmental processes are integrated through their precise temporal regulation. Hes1 is a transcriptional repressor that regulates the timing of mammalian retinal neurogenesis. However, roles for Hes1 in early eye development have not been well defined. Here, we show that Hes1 is expressed in the forming lens, optic vesicle, cup, and pigmented epithelium and is necessary for proper growth, morphogenesis, and differentiation of these tissues. Because Hes1 is required throughout the eye, we investigated its interaction with Pax6. Hes1-Pax6 double mutant embryos are eyeless suggesting these genes are coordinately required for initial morphogenesis and outgrowth of the optic vesicle. In Hes1 mutants, Math5 expression is precocious along with retinal ganglion cell, amacrine, and horizontal neuron formation. In contrast to apparent cooperativity between Pax6 and Hes1 during morphogenesis, each gene regulates Math5 and RGC genesis independently. Together, these studies demonstrate that Hes1, like Pax6, simultaneously regulates multiple developmental processes during optic development.

Proneural factors and neurogenesis

The role of pro-neural factors in specifying neuronal progenitors and in promoting neuronal differentiation is conserved from Drosophila to vertebrates. This primer discusses the basic functions of pro-neural factors in neurogenesis, mechanisms of pro-neural factor function, and models for how pro-neural factors generate neuronal subtypes. The primer also features a dialog about current topics and future directions in the field between two experts in neurogenesis: Andrew Jarman, Ph.D., and Jane Johnson, Ph.D.

GDF11 controls the timing of progenitor cell competence in developing retina

The orderly generation of cell types in the developing retina is thought to be regulated by changes in the competence of multipotent progenitors. Here, we show that a secreted factor, growth and differentiation factor 11 (GDF11), controls the numbers of retinal ganglion cells (RGCs), as well as amacrine and photoreceptor cells, that form during development. GDF11 does not affect proliferation of progenitors-a major mode of GDF11 action in other tissues-but instead controls duration of expression of Math5, a gene that confers competence for RGC genesis, in progenitor cells. Thus, GDF11 governs the temporal windows during which multipotent progenitors retain competence to produce distinct neural progeny.

A POU factor binding site upstream of the Chx10 homeobox gene is required for Chx10 expression in subsets of retinal progenitor cells and bipolar cells

Retinal progenitor cells (RPCs) undergo a series of changes over time that affect their competency to produce different cell types at different times in development. The transcriptional machinery that regulates these changes, as well as associated gene expression changes, have not been characterized. An analysis of the regulatory region of the retinal homeodomain transcription factor, Chx10, was carried out using in ovo electroporations in chick and transgenic mice. An RPC enhancer was defined that mediates reporter activity in subsets of RPCs and directs high-level expression in intermediate and late RPCs. Using bioinformatic and biochemical analysis, a key binding site in this enhancer was found and was shown to be bound by the POU domain factors, Brn-2 and Tst-1/SCIP, in retinal extracts. Analysis of the Brn-2 expression pattern shows that it is expressed in intermediate and late RPCs, but not early RPCs, and thus partially overlaps with expression of the reporter activated by the defined Chx10 enhancer. Biochemical analysis also revealed binding of both Chx10 and Brn-2 to an enhancer of the CNS progenitor cell marker, Nestin. Nestin expression in the retina is restricted to intermediate/late RPC subsets, and genetic evidence is presented that demonstrates that Chx10 represses Nestin expression in early RPCs. A bipolar cell enhancer for Chx10 also was defined, and a role for Brn-2 in expression of Chx10 in bipolar cells is predicted. These data identify Brn-2 as a new marker of subsets of RPCs and suggest a mechanism by which a combination of POU factors and Chx10 define RPC gene expression patterns, such as that of Nestin.

Roles of bHLH genes in neural stem cell differentiation

Neural stem cells change their characteristics over time during development: they initially proliferate only and then give rise to neurons first and glial cells later. In the absence of the repressor-type basic helix-loop-helix (bHLH) genes Hes1, Hes3 and Hes5, neural stem cells do not proliferate sufficiently but prematurely differentiate into neurons and become depleted without making the later born cell types such as astrocytes and ependymal cells. Thus, Hes genes are essential for maintenance of neural stem cells to make cells not only in correct numbers but also in full diversity. Hes genes antagonize the activator-type bHLH genes, which include Mash1, Math and Neurogenin. The activator-type bHLH genes promote the neuronal fate determination and induce expression of Notch ligands such as Delta. These ligands activate Notch signaling and upregulate Hes1 and Hes5 expression in neighboring cells, thereby maintaining these cells undifferentiated. Thus, the activator-type and repressor-type bHLH genes regulate each other, allowing only subsets of cells to undergo differentiation while keeping others to stay neural stem cells. This regulation is essential for generation of complex brain structures of appropriate size, shape and cell arrangement.

Transcriptional activity of the paired-like homeodomain proteins CHX10 and VSX1

CHX10 and VSX1 are homeodomain (HD) proteins essential for normal retinal development. CHX10 is required first for retinal progenitor cell proliferation and later for bipolar cell differentiation, whereas VSX1 is important in the terminal differentiation of a subset of bipolar cells. Elucidating the transcriptional activity of CHX10 and VSX1 is required to understand how these factors control retinal development. We show that CHX10 and Vsx1 can function as transcriptional repressors. When tethered to a promoter by a heterologous LexA DNA-binding domain or its HD, CHX10 repressed multiple classes of activators in different immortalized cell lines. CHX10 blocked TATA-containing and TATA-less promoters, repressed at a distance, and inhibited a complex enhancer positioned upstream or downstream of the reporter gene, whereas retinoblastoma protein (RB) inhibited the downstream enhancer only. Interestingly, CHX10 mildly potentiated a subset of activators in chick neuronal cultures. Thus, CHX10 is both a versatile repressor and a context-specific weak activator. The CHX10 HD and CVC domains were sufficient for DNA binding and repression. VSX1 contains closely related homeo and CVC domains and, like CHX10, also repressed transcription. A VSX1 HD mutation, R166W, that impairs DNA binding and causes keratoconus in humans, hindered repressor function. Therefore, CHX10 and VSX1 may control retinal bipolar cell specification or differentiation by repressing genes required for the development of other cell types.

The transcription factor Bhlhb4 is required for rod bipolar cell maturation

Retinal bipolar cells are essential to the transmission of light information. Although bipolar cell dysfunction can result in blindness, little is known about the factors required for bipolar cell development and functional maturation. The basic helix-loop-helix (bHLH) transcription factor Bhlhb4 was found to be expressed in rod bipolar cells (RB). Electroretinograms (ERGs) in the adult Bhlhb4 knockout (Bhlhb4(-/-)) showed that the loss of Bhlhb4 resulted in disrupted rod signaling and profound retinal dysfunction resembling human congenital stationary night blindness (CSNB), characterized by the loss of the scotopic ERG b-wave. A depletion of inner nuclear layer (INL) cells in the adult Bhlhb4 knockout has been ascribed to the abolishment of the RB cell population during postnatal development. Other retinal cell populations including photoreceptors were unaltered. The timing of RB cell depletion in the Bhlhb4(-/-) mouse suggests that Bhlhb4 is essential for RB cell maturation.

Retinal cell fate determination and bHLH factors

Retinal development is controlled antagonistically by multiple basic helix-loop-helix (bHLH) transcriptional activators and repressors. bHLH repressors suppress bHLH activators and promote maintenance of progenitors and generation of glial cells. In contrast, bHLH activators override activities of bHLH repressors and promote neuronal differentiation. However, bHLH activators alone are not sufficient but homeodomain factors are additionally required for neuronal subtype specification. It is likely that homeodomain factors regulate the layer specificity but not the neuronal fate while bHLH activators determine the neuronal fate within the homedomain factor-specified layers. Thus, combinations of proper bHLH and homeodomain factors are required for neuronal subtype specification.

Cellular and molecular characterization of early and late retinal stem cells/progenitors: Differential regulation of proliferation and context dependent role of Notch signaling

Retinal stem cells/progenitors that define the evolutionarily conserved early and late stages of retinal histogenesis are known to have distinct competence to give rise to stage-specific retinal cell types. However, the information regarding their innate proliferative behavior and phenotypic potential in terms of generating neurons and glia is lacking. Here we demonstrate that, like their counterparts in other central nervous system (CNS) regions during early and late stages of embryonic development, the early and late retinal stem cells/progenitors display different proliferative response to fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF) and bias towards generating neurons or glia. Although the former predominantly generate neurons, the latter are partial towards giving rise to glia. Transcription profiling identified classes of genes that are differentially expressed in early and late retinal stem cells/progenitors in proliferating conditions and suggested that the distinct proliferative response to FGF2 and EGF is likely due to differential expression of FGF receptor 1 (FGFR1) and EGF receptor (EGFR). However, the proliferative maintenance of retinal stem cells/progenitors is likely to include other signaling pathways such as those mediated by insulin-like growth factors (IGFs) and stem cell factor (SCF). Transcription profiling of early and late retinal stem cells/progenitors in proliferating and differentiating conditions suggested a context dependent role for Notch signaling, which may constitute one of the mechanisms underlying the stage-dependent phenotypic potential of retinal stem cells/progenitors.

Involvement of Ath3 in CNTF-mediated differentiation of the late retinal progenitors

The cellular diversity of the mammalian retina is underpinned by multipotential neural progenitors that generate retinal neurons and glia with temporal and spatial specificity. It is thought, based on studies using a variety of approaches, that the fate of retinal progenitors is determined through interactions between temporally and spatially arrayed epigenetic cues with intrinsic factors that regulate the competence of cells to respond to such cues. Here, we demonstrate interactions between an intrinsic factor Ath3, a neural bHLH protein, and an extrinsic factor CNTF during the differentiation of the late retinal progenitors along the bipolar cell lineage. Expression of Ath3 is predominantly associated with the late stage of retinal histogenesis when bipolar cells are specified, and in adult it is detected in cells expressing bipolar cell-specific markers. We demonstrate that CNTF-induced bipolar cell differentiation is accompanied by an increase in levels of Ath3 transcripts and compromised when Ath3 expression is attenuated. Our study suggests that the influence of CNTF on the differentiation of late retinal progenitors is mediated through Ath3.

A novel basic helix–loop–helix (bHLH) transcriptional repressor, NeuroAB, expressed in bipolar and amacrine cells in the chick retina

Basic helix-loop-helix (bHLH) transcription factors are implicated in cell fate determination and differentiation in neurogenesis. We identified a novel chick bHLH transcription factor, NeuroAB. A phylogenetic tree prepared from bHLH sequences suggested that NeuroAB belongs to the BETA3 group in the Atonal-related protein family (ARPs). In situ hybridization and immunostaining indicated that NeuroAB is expressed predominantly in postmitotic bipolar cells and GABAergic amacrine cells in the retina. Reporter and DNA pull down assays indicated that NeuroAB functions as a transcriptional repressor by binding to the E-box sequence, and its activity is modulated by phosphorylation at a specific serine residue that fits the consensus phosphorylation site for glycogen synthase kinase 3beta (GSK3beta). Since members of the BETA3 group possess this consensus site, it is suggested that their activities are commonly regulated by GSK3beta or other kinases bearing the same substrate specificity. We found that the expression of GSK3beta is spatially and temporally regulated in the developing retina; its strong expression was observed in ganglion cells from E8 and a subset of amacrine cells from E12. These findings suggest that NeuroAB is involved in the maturation and maintenance of bipolar cells and GABAergic amacrine cells and regulation by GSK3beta plays an important role in retinogenesis.

Stem cells and retinal repair

Retinal stem cells (RSCs) are multipotent central nervous system (CNS) precursors that give rise to the retina during the course of development. RSCs are present in the embryonic eyecup of all vertebrate species and remain active in lower vertebrates throughout life. Mammals, however, exhibit little RSC activity in adulthood and thus little capacity for retinal growth or regeneration. Because CNS precursors can now be isolated from immature and mature mammals and expanded ex vivo, it is possible to study these cells in culture as well as following transplantation to the diseased retina. Such experiments have revealed a wealth of unanticipated findings, both in terms of the instructive cues present in the mature mammalian retina as well as the ability of grafted CNS precursors to respond to them. This review examines current knowledge regarding RSCs, together with other CNS precursors, from the perspective of investigators who wish to isolate, propagate, genetically modify, and transplant these cells as a regenerative strategy with application to retinal disease.

Genetic analysis of the homeodomain transcription factor Chx10 in the retina using a novel multifunctional BAC transgenic mouse reporter

Chx10 is a homeobox-containing transcription factor critical for progenitor cell proliferation and bipolar cell determination in the developing retina. Its expression in the retina has been reported to be restricted to these cell populations. To further understand Chx10 regulation and function, a multifunctional reporter construct consisting of GFP, alkaline phosphatase, and Cre recombinase was integrated into a BAC encoding Chx10. Stable lines of transgenic mice expressing this BAC were generated and analyzed. The reporter expression was faithful to the endogenous retinal Chx10 expression pattern and revealed a previously unappreciated locus of Chx10 expression in a subset of Müller glial cells. In addition, Chx10 reporter activity was identified in mature orJ-Chx10 mutant retinas, although these retinas lack Chx10-expressing bipolar cells. Reporter and molecular analysis showed that the reporter-expressing cells in the mutant had hallmarks of progenitor cells or partially differentiated Müller glial cells. These results strongly suggest that Chx10 promotes bipolar fate by affecting differentiation of late progenitor cells. Crosses of the Chx10 BAC reporter mice to R26R mice for fate-mapping experiments revealed that Chx10 reporter-expressing progenitor cells contribute to all mature cell types of the retina. These results demonstrate the utility of these lines for generation of mosaic or complete genetic manipulations of the retina.

Specification of the retinal fate of mouse embryonic stem cells by ectopic expression of Rx/rax, a homeobox gene

With the goal of generating retinal cells from mouse embryonic stem (ES) cells by exogenous gene transfer, we introduced the Rx/rax transcription factor, which is expressed in immature retinal cells, into feeder-free mouse ES cells (CCE). CCE cells expressing Rx/rax as well as enhanced green fluorescent protein (CCE-RX/E cells) proliferated and remained in the undifferentiated state in the presence of leukemia inhibitory factor, as did parental ES cells. We made use of mouse embryo retinal explant cultures to address the differentiation ability of grafted ES cells. Dissociated embryoid bodies were treated with retinoic acid for use as donor cells and cocultured with retina explants for 2 weeks. In contrast to the parental CCE cells, which could not migrate into host retinal cultures, CCE-RX/E cells migrated into the host retina and extended their process-like structures between the host retinal cells. Most of the grafted CCE-RX/E cells became located in the ganglion cell and inner plexiform layers and expressed ganglion and horizontal cell markers. Furthermore, these grafted cells had the electrophysiological properties expected of ganglion cells. Our data thus suggest that subpopulations of retinal neurons can be generated in retinal explant cultures from grafted mouse ES cells ectopically expressing the transcription factor Rx/rax.

MyoD–lacZtransgenes are early markers in the neural retina, but MyoD function appears to be inhibited in the developing retinal cells

Recent findings suggest that eye and skeletal muscle development in vertebrates share the same regulatory network. In that network, Pax3 gene is apparently activated through Dach/Eya/Six feedback loop to mediate MyoD-driven myogenesis. The purpose of this study was to investigate previously reported MyoD-lacZ expression in the developing mouse neural retina and to gain insight into the potential role of MyoD in the embryonic retinal cells. The analysis of MD6.0-lacZ and 258/-2.5lacZ transgenic embryos revealed that the retinal temporal expression pattern of the two transgenes resembled their expression pattern in the MyoD-dependent precursor muscle cells. However, MyoD transcripts and protein could not be found in the sites of MyoD-lacZ retinal expression. Furthermore, our immunohistochemical analysis suggests the existence of diverse factors (e.g., Pax6 and Chx10) within the retinal cells that differentially and inappropriately activate the two transgenes. Finally, the retinal phenotype observed in Pax7-/- knock-out mice suggests a role for Pax7 in photoreceptor cell differentiation, retinal lamination and in the etiopathology of retinoblastoma. Taken together, our data suggest that the MyoD gene evolved a different mechanism to achieve its down-regulation within the retina than that of the Myf5 gene.

Requirement of multiple basic helix-loop-helix genes for retinal neuronal subtype specification

Retinal precursor cells give rise to six types of neurons and one type of glial cell during development, and this process is controlled by multiple basic helix-loop-helix (bHLH) genes. However, the precise mechanism for specification of retinal neuronal subtypes, particularly horizontal neurons and photoreceptors, remains to be determined. Here, we examined retinas with three different combinations of triple bHLH gene mutations. In retinas lacking the bHLH genes Ngn2, Math3, and NeuroD, horizontal neurons as well as other neurons such as bipolar cells were severely decreased in number. In the retina lacking the bHLH genes Mash1, Ngn2, and Math3, horizontal and other neurons were severely decreased, whereas ganglion cells were increased. In the retina lacking the bHLH genes Mash1, Math3, and NeuroD, photoreceptors were severely decreased, whereas ganglion cells were increased. In all cases, glial cells were increased. The increase and decrease of these cells were the result of cell fate changes and cell death and seem to be partly attributable to the remaining bHLH gene expression, which also changes because of triple bHLH gene mutations. These results indicate that multiple bHLH genes cross-regulate each other, cooperatively specify neuronal subtypes, and regulate neuronal survival in the developing retina.

Roles of the bHLH gene Hes1 in retinal morphogenesis

During retinal development, common precursors give rise to various types of cells in a time course specific to each cell type. Previously, we demonstrated that the bHLH gene Hes1 inhibits neuronal differentiation whereas, in Hes1-null retina, precursors prematurely differentiate into neurons and form abnormal rosette-like structures. Thus, Hes1 is essential for maintenance of precursors and morphogenesis of the neural retina. However, the precise causal link between premature differentiation and abnormal structures remains to be determined. Here, we found that misexpression of Hes1 in the developing retina promotes formation of undifferentiated precursor-like cells, whereas in Hes1-null retina, precursors are not properly maintained and prematurely differentiate into ganglion cells. Strikingly, those prematurely differentiated ganglion cells erupt into the subretinal space through the regions where precursors and the outer limiting membrane are lost. These results indicate that Hes1 maintains precursors and the outer limiting membrane and thereby regulates retinal morphogenesis.

Neural stem cells in the mammalian eye: types and regulation

Neural stem cells/progenitors that give rise to neurons and glia have been identified in different regions of the brain, including the embryonic retina. Recently, such cells have been reported to be present, in a mitotically quiescent state, in the ciliary epithelium of the adult mammalian eye. The retinal and ciliary epithelium stem cells/progenitors appear to share similar signaling pathways that are emerging as important regulators of stem cells in general. Yet, they are different in certain respects, such as in the potential to self-renew. These two neural stem cell/progenitor populations not only will serve as models for investigating stem cell biology but also will help explain the relationships between embryonic and adult neural stem cells/progenitors.

Comparison of the expression patterns of five neural RNA binding proteins in theXenopus retina

An increasing body of evidence indicates that gene expression can be modulated by posttranscriptional mechanisms. RNA binding proteins, for instance, control gene expression at many regulatory levels including RNA splicing, transport, stability, and translation. Although numerous RNA binding proteins have been identified, very few have been studied extensively in the context of developmental processes. We focused our study on five neural RNA binding proteins: one Musashi homolog, Nrp-1, one member of the Bruno gene family, BruL-1 (also known as Etr-1), and three members of the ELAV/Hu family, ElrB, ElrC, and ElrD. As an initial step in addressing their function during Xenopus neurogenesis, we used in situ hybridization to determine their expression patterns during retinal development. We found that RNA binding proteins belonging to different families have distinct spatio-temporal expression. These combinatorial expression patterns are reminiscent of previously described cell type-specific expression patterns of transcription factors during retinal development. The distribution of RNA binding proteins within the retina suggests that these regulators of posttranscriptional events may play important roles in multiple steps of retinogenesis.

Morphogenetic Movements Underlying Eye Field Formation Require Interactions between the FGF and ephrinB1 Signaling Pathways

The definitive retinal progenitors of the eye field are specified by transcription factors that both promote a retinal fate and control cell movements that are critical for eye field formation. However, the molecular signaling pathways that regulate these movements are largely undefined. We demonstrate that both the FGF and ephrin pathways impact eye field formation. Activating the FGF pathway before gastrulation represses cellular movements in the presumptive anterior neural plate and prevents cells from expressing a retinal fate, independent of mesoderm induction or anterior-posterior patterning. Inhibiting the FGF pathway promotes cell dispersal and significantly increases eye field contribution. ephrinB1 reverse signaling is required to promote cellular movements into the eye field, and can rescue the FGF receptor-induced repression of retinal fate. These results indicate that FGF modulation of ephrin signaling regulates the positioning of retinal progenitor cells within the definitive eye field.

Math5 determines the competence state of retinal ganglion cell progenitors

In mice, all of the six retinal neuron types are generated from common multipotent retinal progenitors, and their differentiation from progenitors is regulated by both extrinsic and intrinsic factors. Previously, we showed that targeted deletion of the atonal (ato) homologue math5 blocked the differentiation of most retinal ganglion cells (RGCs), revealing an essential role for math5 in RGC differentiation. In this study, we used the Cre-loxP recombination system to trace the fate of math5-expressing cells in retina. Our results demonstrated that math5 expression was associated with the differentiation of multiple retinal neuron types, including RGCs, photoreceptor, horizontal, and amacrine cells, implying that math5 expression alone is not sufficient to determine the RGC fate. Math5 expression was restricted to postmitotic cells in developing retina, suggesting that cell fate commitment of retinal neurons occurs after the terminal mitosis. The insufficiency of and requirement for math5 in RGC differentiation indicates that, like ato in the development of Drosophila R8 photoreceptors, math5 plays a role in determining the RGC competence state of retinal progenitors and that additional positive and negative factors are required in determining RGC fate. Furthermore, we show that loss of Math5 function severely reduced the RGC expression of the transcription factors Brn-3b, Gfi-1, Isl-1, Isl-2, Nscl-1, Nscl-2, and RPF-1, suggesting that Math5 expression is required to activate a comprehensive transcription network of RGC differentiation.

Cell diversity in the retina: more than meets the eye

Over 10 years ago, Pax-6 was shown to play an evolutionarily conserved role in controlling eye formation from Drosophila to humans.1 Since then, the identification of an entire cascade of conserved eye determination genes has brought a new understanding to the developmental relationship between the insect compound eye and the vertebrate camera eye.2 Additional studies are now beginning to suggest that even late aspects of eye development, including cell type specification, also share common molecular machinery. In this commentary, I will discuss some of these findings, with a particular focus on the recent study by Dyer et al.3 describing a novel role for the Prox1 transcription factor in specifying horizontal cells in the mouse retina. As Prospero, the Drosophila homolog of Prox1, also participates in retinal cell specification, these data provide a forum for asking new questions concerning pathways that may regulate retinogenesis across evolution.

Otx2 homeobox gene controls retinal photoreceptor cell fate and pineal gland development

Understanding the molecular mechanisms by which distinct cell fate is determined during organogenesis is a central issue in development and disease. Here, using conditional gene ablation in mice, we show that the transcription factor Otx2 is essential for retinal photoreceptor cell fate determination and development of the pineal gland. Otx2-deficiency converted differentiating photoreceptor cells to amacrine-like neurons and led to a total lack of pinealocytes in the pineal gland. We also found that Otx2 transactivates the cone-rod homeobox gene Crx, which is required for terminal differentiation and maintenance of photoreceptor cells. Furthermore, retroviral gene transfer of Otx2 steers retinal progenitor cells toward becoming photoreceptors. Thus, Otx2 is a key regulatory gene for the cell fate determination of retinal photoreceptor cells. Our results reveal the key molecular steps required for photoreceptor cell-fate determination and pinealocyte development.

The basic helix-loop-helix genes Hesr1/Hey1 and Hesr2/Hey2 regulate maintenance of neural precursor cells in the brain

Neural precursor cells proliferate in the ventricular zone while giving rise to neurons of deep layers first, then those of the superficial layers, and lastly, glial cells in the brain. Thus, it is essential to maintain neural precursor cells until late stages of neural development for generation of a wide variety of cell types. Here, we found that the Hes-related basic helix-loop-helix (bHLH) genes Hesr1/Hey1 and Hesr2/Hey2 are expressed in the ventricular zone, which contains neural precursor cells. Misexpression of Hesr1 and Hesr2 by electroporation in mouse brain at embryonic day 13.5 transiently maintains neural precursor cells and thereby increases late-born neurons, which are located in the superficial layers. In contrast, misexpression of the genes at later stages inhibits neurogenesis and promotes generation of astroglial cells. In transient transfection assay with cultured cells, both Hesr1 and Hesr2 inhibit transcription induced by the neuronal bHLH genes Mash1 and Math3. These results indicate that Hesr1 and Hesr2 negatively regulate neuronal bHLH genes, promote maintenance of neural precursor cells, and increase late-born cell types in the developing brain.

Transcriptional control of neuronal diversification in the retina

During embryonic development, the array of vastly different neuronal types that are incorporated into the functional architecture of the mature neuroretina derives from a common population of multipotent retinal progenitor cells (RPCs). Retinogenesis proceeds in a precise chronological order, with the seven principal cell classes generated in successive phases. Cell biological experiments established that this histogenetic order, at least in part, reflects intrinsic changes within the RPC pool. In recent years a number of molecules controlling various aspects of cell fate specification from RPCs have been identified. However, few attempts have been made to integrate previous concepts that emerged from cell biological studies and more recent results based on molecular genetic experiments. This review aims at providing an overview of recent advances in our understanding of the cellular and molecular mechanisms underlying retinal neuronal diversification, with a particular focus on cell-intrinsic factors.

Akt regulates basic helix-loop-helix transcription factor-coactivator complex formation and activity during neuronal differentiation

Neural basic helix-loop-helix (bHLH) transcription factors regulate neurogenesis in vertebrates. Signaling by peptide growth factors also plays critical roles in regulating neuronal differentiation and survival. Many peptide growth factors activate phosphatidylinositol 3-kinase (PI3K) and subsequently the Akt kinases, raising the possibility that Akt may impact bHLH protein function during neurogenesis. Here we demonstrate that reducing expression of endogenous Akt1 and Akt2 by RNA interference (RNAi) reduces neuron generation in P19 cells transfected with a neural bHLH expression vector. The reduction in neuron generation from decreased Akt expression is not solely due to decreased cell survival, since addition of the caspase inhibitor z-VAD-FMK rescues cell death associated with loss of Akt function but does not restore neuron formation. This result indicates that Akt1 and Akt2 have additional functions during neuronal differentiation that are separable from neuronal survival. We show that activated Akt1 enhances complex formation between bHLH proteins and the transcriptional coactivator p300. Activated Akt1 also significantly augments the transcriptional activity of the bHLH protein neurogenin 3 in complex with the coactivators p300 or CBP. In addition, inhibition of endogenous Akt activity by the PI3K/Akt inhibitor LY294002 abolishes transcriptional cooperativity between the bHLH proteins and p300. We propose that Akt regulates the assembly and activity of bHLH-coactivator complexes to promote neuronal differentiation.

Characterization of Wnt signaling components and activation of the Wnt canonical pathway in the murine retina

The neuroepithelial layer of the developing eyecup contains multipotential precursor cells that give rise to all of the neurons and the one glial cell type present in the adult retina. Patterning within the retinal neuroepithelium is regulated by cell intrinsic as well as cell extrinsic mechanisms. Although the identity of some of the signaling molecules that regulate retinal development is known, the function of many others, especially members of the Wnt family, has yet to be characterized in the context of retinal development. We undertook a comprehensive in situ hybridization analysis to examine the expression of Wnt pathway components in the developing and adult mouse neural retina. Our findings confirm and extend previous expression studies in mice and other vertebrates, as we show that Wnt-3, -5a, -5b, and -7b are expressed in the neural retina and that there is a dynamic pattern of Wnt receptor (Mouse frizzled [Mfz]) and Wnt antagonist (Secreted-frizzled-related protein [Sfrp]) gene expression in the embryonic and perinatal neural retina. Moreover, we show that Wnt-13 is expressed in the pigment epithelium overlying the distal part of the eyecup and the ciliary margin and that Mfz-4, -6, and -7 are expressed in different regions within the ciliary margin. To determine where activation of canonical Wnt signaling is occurring in the retina, we examined reporter gene expression in TCF/Lef-LacZ mice and we demonstrate that the highest levels of beta-gal activity are found in the ciliary margin, adjacent to and within the Wnt-13 expression domain, implicating Wnt-13 signaling in the development of the ciliary margin and its derivatives.

Expression of Prox1 defines regions of the avian otocyst that give rise to sensory or neural cells

The simple primordium of the inner ear (otocyst) differentiates into many cell types, including sensory neurons and hair cells. We examined expression of the divergent homeobox transcription factor, cProx1, during otocyst development in chickens. Nuclear cProx1 protein is not evident in the otic placode but emerges in the otic cup by stage 12. At stage 16, cProx1-positive nuclei are scattered continuously throughout the neuroepithelium, from anteroventral to posteromedial. These labeled cells are neural precursors; they express betaIII-tubulin and migrate to the cochleovestibular ganglion between stages 13 and 21. By stage 18, two areas develop a dense pattern of cProx1 expression in which every nucleus is labeled. These areas emerge at the anterior and posterior extremes of the band of scattered cProx1 expression and express the sensory markers cSerrate1 and Cath1 by stage 23. Four discrete patches of dense cProx1 expression appear by stage 23 that correspond to the future superior crista, lateral crista, saccular macula, and posterior crista, as confirmed by immunolabeling for hair cell antigen (HCA) by stage 29. The remaining sensory epithelia display a dense pattern of cProx1 expression and label for HCA by stage 29. In the basilar papilla, nuclear cProx1 expression is down-regulated in most hair cells by stage 37 and in many supporting cells by stage 40. Our findings show that regions of the otocyst that give rise to neurons or hair cells are distinguished by their relative density of cProx1-positive nuclei, and suggest a role for cProx1 in the genesis of these cell types.

Prox1 function controls progenitor cell proliferation and horizontal cell genesis in the mammalian retina

Retinal progenitor cells regulate their proliferation during development so that the correct number of each cell type is made at the appropriate time. We found that the homeodomain protein Prox1 regulates the exit of progenitor cells from the cell cycle in the embryonic mouse retina. Cells lacking Prox1 are less likely to stop dividing, and ectopic expression of Prox1 forces progenitor cells to exit the cell cycle. During retinogenesis, Prox1 can be detected in differentiating horizontal, bipolar and AII amacrine cells. Horizontal cells are absent in retinae of Prox1-/- mice and misexpression of Prox1 in postnatal progenitor cells promotes horizontal-cell formation. Thus, Prox1 activity is both necessary and sufficient for progenitor-cell proliferation and cell-fate determination in the vertebrate retina.

Cell cycle and cell fate interactions in neural development

Mechanisms coupling cell cycle and cell fate operate at different steps during neural development. Intrinsic factors control the cell proliferation of distinct brain regions and changes of cell fate competence, whereas components of the cell cycle machinery could play a major role in setting the appropriate timing of the generation of different cell types.

Retrovirus-mediated gene transfer to retinal explants

Neural retina can be isolated from mouse embryos and maintained in culture for 2-3 weeks. In such retinal explant cultures, precursor cells differentiate into neurons and glial cells and form three cellular layers, mimicking well the normal development. This explant culture system is suitable for genetic manipulation, such as retrovirus-mediated gene transfer. Retroviral vectors can efficiently transfer genes into retinal precursors, and the copy of the viral genome is precisely transmitted to the progeny of infected cells. Thus, this is an excellent method to change stably the phenotypes of dividing cells. It has been shown that retroviruses carrying transcription factor genes efficiently change the fates of infected cells. Bicistronic expression by retroviral vectors is useful to test the effects of various combinations of many transcription factors. With this method, the transcriptional codes for retinal cell type specification are now being elucidated. Thus, retrovirus-mediated gene transfer to the retinal explant culture system offers a powerful and unique tool to analyze the molecular mechanism of neural development.

Xath5 regulates neurogenesis in the Xenopus olfactory placode

Helix-loop-helix (HLH) genes function as important regulators of neurogenesis in both the peripheral and central nervous systems. The olfactory system is an ideal tissue in which to study the role of these genes in regulating the acquisition of neuronal cell fate, particularly that of the olfactory receptor neuron (ORN). Here we describe the expression of several basic HLH (bHLH) and repeat HLH (rHLH) factors during olfactory placode development in Xenopus laevis. Our work reveals that a combination of both bHLH and rHLH genes are sequentially expressed within the nascent olfactory placode during normal development. Moreover, overexpression of the bHLH factor, Xenopus atonal homologue 5 (Xath5), promotes olfactory neural fate independent of cellular proliferation within a restricted domain at the anterior of the embryo. Collectively, our data argue that HLH genes are expressed in a cascade during olfactory placode development and that the activity of an atonal homologue, Xath5, can promote ORN fate but only in the appropriate developmental context.

Differentiation of embryonic stem cells into retinal neurons

Mouse embryonic stem (ES) cells are continuous cell lines derived from the inner mass of blastocysts. Neural progenitors derived from these cells serve as an excellent model for controlled neural differentiation and as such have tremendous potential to understand and treat neurodegenerative diseases. Here, we demonstrate that ES cell-derived neural progenitors express regulatory factors needed for retinal differentiation and that in response to epigenetic cues a subset of them differentiate along photoreceptor lineage. During the differentiation, they activate photoreceptor regulatory genes, suggesting that ES cell-derived neural progenitors recruit mechanisms normally used for photoreceptor differentiation in vivo. These observations suggest that ES cells can serve as an excellent model for understanding mechanisms that regulate specification of retinal neurons and as an unlimited source of neural progenitors for treating degenerative diseases of the retina by cell replacement.

The control of neural stem cells by morphogenic signals

A complex orchestration of stem-cell specification, expansion and differentiation is required for the proper development of the nervous system. Although progress has been made on the role of individual genes in each of these processes, there are still unresolved questions about how gene function translates to the dynamic assembly of cells into tissues. Recently, stem-cell biology has emerged as a bridge between the traditional fields of cell biology and developmental genetics. In addition to their potential therapeutic role, stem cells are being exploited as experimental 'logic chips' that integrate information and exhibit self-organizing properties. Recent studies provide new insights on how morphogenic signals coordinate major stem cell decisions to regulate the size, shape and cellular diversity of the nervous system.

Proneural genes and the specification of neural cell types

Certain morphological, physiological and molecular characteristics are shared by all neurons. However, despite these similarities, neurons constitute the most diverse cell population of any organism. Recently, considerable attention has been focused on identifying the molecular mechanisms that underlie this cellular diversity. Parallel studies in Drosophila and vertebrates have revealed that proneural genes are key regulators of neurogenesis, coordinating the acquisition of a generic neuronal fate and of specific subtype identities that are appropriate for the location and time of neuronal generation. These studies reveal that, in spite of differences between invertebrate and vertebrate neural lineages, Drosophila and vertebrate proneural genes have remarkably similar roles.

Posttranslational mechanisms control the timing of bHLH function and regulate retinal cell fate

During central nervous system development, neurons are often born in a precise temporal sequence. Basic helix-loop-helix (bHLH) transcription factors are required for the development of specific subpopulations of neurons, but how they contribute to their ordered genesis is unclear. We show that the ability of bHLH factors to regulate the development of distinct neuronal subtypes in the Xenopus retina depends upon the timing of their function. In addition, we find that the timing of bHLH function can be regulated posttranslationally, so that bHLH factors with overlapping expression can function independently. Specifically, XNeuroD function in the retina can be inhibited by glycogen synthase kinase 3beta (GSK3beta), while Xath5 function can be inhibited by Notch. Thus, the potential of bHLH factors to regulate the development of neuronal subtypes depends upon the context in which they function.

neurogenin2 elicits the genesis of retinal neurons from cultures of nonneural cells

neurogenin2 (ngn2) encodes a basic helix-loop-helix transcription factor and plays an important role in neurogenesis from migratory neural crest cells. Its role in retinal development is poorly understood. We observed that in the developing chick retina, ngn2 was expressed in a subpopulation of proliferating progenitor cells. Ectopic expression of ngn2 in nonneural, retinal pigment epithelial cell culture triggered de novo generation of cells that expressed neural-specific markers and exhibited neuronal morphologies. Further molecular and morphological analyses showed that the main products of the induced neurogenesis were cells resembling young photoreceptor cells and cells resembling retinal ganglion cells. The generation of multiple cell types suggests that ngn2 induces various retinal pathways. Thus, unlike in the peripheral nervous system where ngn2 specifies one type of sensory neuron, ngn2 in the retina is likely involved in a common step leading to different cellular pathways. Our finding that ngn2 can instruct nonneural retinal pigment epithelial cells to differentiate toward retinal neurons demonstrates one possible way to induce de novo retinal neurogenesis.

The role of basic helix-loop-helix genes in vertebrate retinogenesis

The developing eye is a favorite model for the study of pattern formation and cell fate determination. Retinal neuron development, in particular, is an approachable system to study molecular and cellular aspects of cell determination and differentiation. Basic helix-loop-helix (bHLH) transcription factors are important regulators of retinal neurogenesis. Proneural bHLH genes have highly defined expression in the developing retina that are influenced by pattern formation and cell specification pathways. Each retinal cell class has unique bHLH requirements, implying that these genes regulate neuronal identity and function. Therefore, proneural genes represent a molecular focal point through which epithelial cells are transformed into a precise neural network. In this review, we focus on the bHLH factor Ath5, an important regulator of retinal ganglion cell development, and discuss factors that regulate its expression in the retina and the target genes through which it may confer specific neuronal properties.

Math6, a bHLH gene expressed in the developing nervous system, regulates neuronal versus glial differentiation

BACKGROUND

Whereas multiple basic helix-loop-helix (bHLH) genes are expressed in the developing nervous system, they account for the differentiation of only subsets of neurones, suggesting that there may be as-yet unidentified bHLH genes.

RESULTS

We have isolated a novel bHLH gene, designated Math6, a distant mammalian homologue of the Drosophila proneural gene atonal. Structural analysis of the Math6 gene demonstrated that the coding region is divided into three exons, whereas that of other atonal homologues is present in a single exon, indicating that the genomic structure of Math6 is unique among the atonal homologues. Math6 is initially expressed by neural precursor cells in the ventricular zone, but later by subsets of differentiating and mature neurones such as hippocampal neurones and cerebellar Purkinje cells. Mis-expression of Math6 with retrovirus in the developing retina induced neurogenesis, while inhibiting gliogenesis, without affecting cell proliferation and death.

CONCLUSIONS

These results show that cells which would normally differentiate into glia adopted the neuronal fate by mis-expression of Math6, indicating that Math6 promotes neuronal vs. glial fate determination in the nervous system.

Combinatorial roles of olig2 and neurogenin2 in the coordinated induction of pan-neuronal and subtype-specific properties of motoneurons

Distinct classes of neurons are generated at defined times and positions during development of the nervous system. It remains elusive how specification of neuronal identity coordinates with acquisition of pan-neuronal properties. Here we show that basic helix-loop-helix (bHLH) transcription factors Olig2 and Neurogenin2 (Ngn2) play vital roles in the coordinated induction of pan-neuronal and subtype-specific properties of motoneurons. Olig2 and Ngn2 are specifically coexpressed in motoneuron progenitors. Misexpression studies in chick demonstrate the specific, combinatorial actions of Olig2 and Ngn2 in motoneuron generation. Our results further revealed crossregulatory interactions between bHLH and homeodomain transcription factors in the specification of motoneurons. We suggest that distinct classes of transcription factors collaborate to generate motoneurons in the ventral neural tube.

Roles of the basic helix-loop-helix genes Hes1 and Hes5 in expansion of neural stem cells of the developing brain

Neural stem cells, which differentiate into neurons and glia, are present in the ventricular zone of the embryonal brain. The precise mechanism by which neural stem cells are maintained during embryogenesis remains to be determined. Here, we found that transient misexpression of the basic helix-loop-helix genes Hes1 and Hes5 keeps embryonal telencephalic cells undifferentiated although they have been shown to induce gliogenesis in the retina. These telencephalic cells later differentiate into neurons and astroglia when Hes expression is down-regulated, suggesting that Hes1- and Hes5- expressing cells are maintained as neural stem cells during embryogenesis. Conversely, in the absence of Hes1 and Hes5, neural stem cells are not properly maintained, generating fewer and smaller neurospheres than the wild type. These results indicate that Hes1 and Hes5 play an important role in the maintenance of neural stem cells but not in gliogenesis in the embryonal telencephalon.