Notch signaling can inhibit Xath5 function in the neural plate and developing retina

Temporal Transcriptomic Profiling of the Developing Xenopus laevis Eye

Retinal progenitor cells (RPCs) are a multipotent and highly proliferative population that give rise to all retinal cell types during organogenesis. Defining their molecular signature is a key step towards identifying suitable approaches to treat visual impairments. Here, we performed RNA sequencing of whole eyes from Xenopus at three embryonic stages and used differential expression analysis to define the transcriptomic profiles of optic tissues containing proliferating and differentiating RPCs during retinogenesis. Gene Ontology and KEGG pathway analyses showed that genes associated with developmental pathways (including Wnt and Hedgehog signaling) were upregulated during the period of active RPC proliferation in early retinal development (Nieuwkoop Faber st. 24 and 27). Developing eyes had dynamic expression profiles and shifted to enrichment for metabolic processes and phototransduction during RPC progeny specification and differentiation (st. 35). Furthermore, conserved adult eye regeneration genes were also expressed during early retinal development, including sox2, pax6, nrl, and Notch signaling components. The eye transcriptomic profiles presented here span RPC proliferation to retinogenesis and include regrowth-competent stages. Thus, our dataset provides a rich resource to uncover molecular regulators of RPC activity and will allow future studies to address regulators of RPC proliferation during eye repair and regrowth.

Key transcription factors influence the epigenetic landscape to regulate retinal cell differentiation

How the diverse neural cell types emerge from multipotent neural progenitor cells during central nervous system development remains poorly understood. Recent scRNA-seq studies have delineated the developmental trajectories of individual neural cell types in many neural systems including the neural retina. Further understanding of the formation of neural cell diversity requires knowledge about how the epigenetic landscape shifts along individual cell lineages and how key transcription factors regulate these changes. In this study, we dissect the changes in the epigenetic landscape during early retinal cell differentiation by scATAC-seq and identify globally the enhancers, enriched motifs, and potential interacting transcription factors underlying the cell state/type specific gene expression in individual lineages. Using CUT&Tag, we further identify the enhancers bound directly by four key transcription factors, Otx2, Atoh7, Pou4f2 and Isl1, including those dependent on Atoh7, and uncover the sequential and combinatorial interactions of these factors with the epigenetic landscape to control gene expression along individual retinal cell lineages such as retinal ganglion cells (RGCs). Our results reveal a general paradigm in which transcription factors collaborate and compete to regulate the emergence of distinct retinal cell types such as RGCs from multipotent retinal progenitor cells (RPCs).

Genetic control of retinal ganglion cell genesis

Retinal ganglion cells (RGCs) are the only projection neurons in the neural retina. They receive and integrate visual signals from upstream retinal neurons in the visual circuitry and transmit them to the brain. The function of RGCs is performed by the approximately 40 RGC types projecting to various central brain targets. RGCs are the first cell type to form during retinogenesis. The specification and differentiation of the RGC lineage is a stepwise process; a hierarchical gene regulatory network controlling the RGC lineage has been identified and continues to be elaborated. Recent studies with single-cell transcriptomics have led to unprecedented new insights into their types and developmental trajectory. In this review, we summarize our current understanding of the functions and relationships of the many regulators of the specification and differentiation of the RGC lineage. We emphasize the roles of these key transcription factors and pathways in different developmental steps, including the transition from retinal progenitor cells (RPCs) to RGCs, RGC differentiation, generation of diverse RGC types, and central projection of the RGC axons. We discuss critical issues that remain to be addressed for a comprehensive understanding of these different aspects of RGC genesis and emerging technologies, including single-cell techniques, novel genetic tools and resources, and high-throughput genome editing and screening assays, which can be leveraged in future studies.

Differentiation of retinal ganglion cells from induced pluripotent stem cells: a review

Glaucoma is a common optic neuropathy that is characterized by the progressive degeneration of axons and the loss of retinal ganglion cells (RGCs). Glaucoma is one of the leading causes of irreversible blindness worldwide. Current glaucoma treatments only slow the progression of RGCs loss. Induced pluripotent stem cells (iPSCs) are capable of differentiating into all three germ layer cell lineages. iPSCs can be patient-specific, making iPSC-derived RGCs a promising candidate for cell replacement. In this review, we focus on discussing the detailed approaches used to differentiate iPSCs into RGCs.

Notch signalling patterns retinal composition by regulating atoh7 during post-embryonic growth

Patterning of a continuously growing naive field in the context of a life-long growing organ such as the teleost eye is of high functional relevance. Intrinsic and extrinsic signals have been proposed to regulate lineage specification in progenitors that exit the stem cell niche in the ciliary marginal zone (CMZ). The proper cell-type composition arising from those progenitors is a prerequisite for retinal function. Our findings in the teleost medaka (Oryzias latipes) uncover that the Notch-Atoh7 axis continuously patterns the CMZ. The complement of cell types originating from the two juxtaposed progenitors marked by Notch or Atoh7 activity contains all constituents of a retinal column. Modulation of Notch signalling specifically in Atoh7-expressing cells demonstrates the crucial role of this axis in generating the correct cell-type proportions. After transiently blocking Notch signalling, retinal patterning and differentiation is re-initiated de novo. Taken together, our data show that Notch activity in the CMZ continuously structures the growing retina by juxtaposing Notch and Atoh7 progenitors that give rise to distinct complementary lineages, revealing coupling of de novo patterning and cell-type specification in the respective lineages.

Summary: Mutually exclusive activity of Notch and Atoh7 in the ciliary marginal zone gives rise to two distinct lineages resulting in specification of the full complement of cell types in medaka retina.

MyT1 Counteracts the Neural Progenitor Program to Promote Vertebrate Neurogenesis

The generation of neurons from neural stem cells requires large-scale changes in gene expression that are controlled to a large extent by proneural transcription factors, such as Ascl1. While recent studies have characterized the differentiation genes activated by proneural factors, less is known on the mechanisms that suppress progenitor cell identity. Here, we show that Ascl1 induces the transcription factor MyT1 while promoting neuronal differentiation. We combined functional studies of MyT1 during neurogenesis with the characterization of its transcriptional program. MyT1 binding is associated with repression of gene transcription in neural progenitor cells. It promotes neuronal differentiation by counteracting the inhibitory activity of Notch signaling at multiple levels, targeting the Notch1 receptor and many of its downstream targets. These include regulators of the neural progenitor program, such as Hes1, Sox2, Id3, and Olig1. Thus, Ascl1 suppresses Notch signaling cell-autonomously via MyT1, coupling neuronal differentiation with repression of the progenitor fate.

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MyT1 promotes neurogenesis downstream Ascl1

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MyT1 represses Notch1 receptor and many of its downstream target genes

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MyT1 represses Hes1 expression by direct DNA binding and competition with RBPJ

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Ascl1 suppresses Notch signaling cell-autonomously while promoting differentiation

The two Tribolium E(spl) genes show evolutionarily conserved expression and function during embryonic neurogenesis

Tribolium castaneum is a well-characterised model insect, whose short germ-band mode of embryonic development is characteristic of many insect species and differs from the exhaustively studied Drosophila. Mechanisms of early neurogenesis, however, show significant conservation with Drosophila, as a characteristic pattern of neuroblasts arises from neuroectoderm proneural clusters in response to the bHLH activator Ash, a homologue of Achaete-Scute. Here we study the expression and function of two other bHLH proteins, the bHLH-O repressors E(spl)1 and E(spl)3. Their Drosophila homologues are expressed in response to Notch signalling and antagonize the activity of Achaete-Scute proteins, thus restricting the number of nascent neuroblasts. E(spl)1 and 3 are the only E(spl) homologues in Tribolium and both show expression in the cephalic and ventral neuroectoderm during embryonic neurogenesis, as well as a dynamic pattern of expression in other tissues. Their expression starts early, soon after Ash expression and is dependent on both Ash and Notch activities. They act redundantly, since a double E(spl) knockdown (but not single knockdowns) results in neurogenesis defects similar to those caused by Notch loss-of-function. A number of other activities have been evolutionarily conserved, most notably their ability to interact with proneural proteins Scute and Daughterless.

Math5 defines the ganglion cell competence state in a subpopulation of retinal progenitor cells exiting the cell cycle

The basic helix-loop-helix (bHLH) transcription factor Math5 (Atoh7) is transiently expressed during early retinal histogenesis and is necessary for retinal ganglion cell (RGC) development. Using nucleoside pulse-chase experiments and clonal analysis, we determined that progenitor cells activate Math5 during or after the terminal division, with progressively later onset as histogenesis proceeds. We have traced the lineage of Math5+ cells using mouse BAC transgenes that express Cre recombinase under strict regulatory control. Quantitative analysis showed that Math5+ progenitors express equivalent levels of Math5 and contribute to every major cell type in the adult retina, but are heavily skewed toward early fates. The Math5>Cre transgene labels 3% of cells in adult retina, including 55% of RGCs. Only 11% of Math5+ progenitors develop into RGCs; the majority become photoreceptors. The fate bias of the Math5 cohort, inferred from the ratio of cone and rod births, changes over time, in parallel with the remaining neurogenic population. Comparable results were obtained using Math5 mutant mice, except that ganglion cells were essentially absent, and late fates were overrepresented within the lineage. We identified Math5-independent RGC precursors in the earliest born (embryonic day 11) retinal cohort, but these precursors require Math5-expressing cells for differentiation. Math5 thus acts permissively to establish RGC competence within a subset of progenitors, but is not sufficient for fate specification. It does not autonomously promote or suppress the determination of non-RGC fates. These data are consistent with progressive and temporal restriction models for retinal neurogenesis, in which environmental factors influence the final histotypic choice.

Rbpj cell autonomous regulation of retinal ganglion cell and cone photoreceptor fates in the mouse retina

Vertebrate retinal progenitor cells (RPCs) are pluripotent, but pass through competence states that progressively restrict their developmental potential (Cepko et al., 1996; Livesey and Cepko, 2001; Cayouette et al., 2006). In the rodent eye, seven retinal cell classes differentiate in overlapping waves, with RGCs, cone photoreceptors, horizontals, and amacrines forming predominantly before birth, and rod photoreceptors, bipolars, and Müller glia differentiating postnatally. Both intrinsic and extrinsic factors regulate each retinal cell type (for review, see Livesey and Cepko, 2001). Here, we conditionally deleted the transcription factor Rbpj, a critical integrator of multiple Notch signals (Jarriault et al., 1995; Honjo, 1996; Kato et al., 1997; Han et al., 2002), during prenatal mouse retinal neurogenesis. Removal of Rbpj caused reduced proliferation, premature neuronal differentiation, apoptosis, and profound mispatterning. To determine the cell autonomous requirements for Rbpj during RGC and cone formation, we marked Cre-generated retinal lineages with GFP expression, which showed that Rbpj autonomously promotes RPC mitotic activity, and suppresses RGC and cone fates. In addition, the progressive loss of Rbpj-/- RPCs resulted in a diminished progenitor pool available for rod photoreceptor formation. This circumstance, along with the overproduction of Rbpj-/- cones, revealed that photoreceptor development is under homeostatic regulation. Finally, to understand how the Notch pathway regulates the simultaneous formation of multiple cell types, we compared the RGC and cone phenotypes of Rbpj to Notch1 (Jadhav et al., 2006b; Yaron et al., 2006), Notch3, and Hes1 mutants. We found particular combinations of Notch pathway genes regulate the development of each retinal cell type.

A directional Wnt/beta-catenin-Sox2-proneural pathway regulates the transition from proliferation to differentiation in the Xenopus retina

Progenitor cells in the central nervous system must leave the cell cycle to become neurons and glia, but the signals that coordinate this transition remain largely unknown. We previously found that Wnt signaling, acting through Sox2, promotes neural competence in the Xenopus retina by activating proneural gene expression. We now report that Wnt and Sox2 inhibit neural differentiation through Notch activation. Independently of Sox2, Wnt stimulates retinal progenitor proliferation and this, when combined with the block on differentiation, maintains retinal progenitor fates. Feedback inhibition by Sox2 on Wnt signaling and by the proneural transcription factors on Sox2 mean that each element of the core pathway activates the next element and inhibits the previous one, providing a directional network that ensures retinal cells make the transition from progenitors to neurons and glia.

Pax6 regulation of Math5 during mouse retinal neurogenesis

Activation of the bHLH factor Math5 (Atoh7) is an initiating event for mammalian retinal neurogenesis, as it is critically required for retinal ganglion cell formation. However, the cis-regulatory elements and trans-acting factors that control Math5 expression are largely unknown. Using a combination of transgenic mice and bioinformatics, we identified a phylogenetically conserved regulatory element that is required to activate Math5 transcription during early retinal neurogenesis. This element drives retinal expression in vivo, in a cross-species transgenic assay. Previously, Pax6 was shown to be necessary for the initiation of Math5 mRNA expression. We extend this finding by showing that the Math5 retinal enhancer also requires Pax6 for its activation, via Pax6 binding to a highly conserved binding site. In addition, our data reveal that other retinal factors are required for accurate regulation of Math5 by Pax6.

Gene regulation logic in retinal ganglion cell development: Isl1 defines a critical branch distinct from but overlapping with Pou4f2

Understanding gene regulatory networks (GRNs) that control neuronal differentiation will provide systems-level perspectives on neurogenesis. We have previously constructed a model for a GRN in retinal ganglion cell (RGC) differentiation in which four hierarchical tiers of transcription factors ultimately control the expression of downstream terminal genes. Math5 occupies a central node in the hierarchy because it is essential for the formation of RGCs and the expression of the immediate downstream factor Pou4f2. Based on its expression, we also proposed that Isl1, a LIM-homeodomain factor, functions in parallel with Pou4f2 and downstream of Math5 in the RGC GRN. To determine whether this was the case, a conditional Isl1 allele was generated and deleted specifically in the developing retina. Although RGCs formed in Isl1-deleted retinas, most underwent apoptosis, and few remained at later stages. By microarray analysis, we identified a distinct set of genes whose expression depended on Isl1. These genes are all downstream of Math5, and some of them, but not all, also depend on Pou4f2. Additionally, Isl1 was required for the sustained expression of Pou4f2, suggesting that Isl1 positively regulates Pou4f2 after Math5 levels are diminished. The results demonstrate an essential role for Isl1 in RGC development and reveal two distinct but intersecting branches of the RGC GRN downstream of Math5, one directed by Pou4f2 and the other by Isl1. They also reveal that identical RGC expression patterns are achieved by different combinations of divergent inputs from upstream transcription factors.

Myelin transcription factor 1 (Myt1) expression in demyelinated lesions of rodent and human CNS

Myelin transcription factor 1 (Myt1) is a zinc-finger DNA binding protein that influences developing oligodendrocyte progenitor (OP) cell proliferation, differentiation, and myelin gene transcription in vitro. The potential of Myt1 to play a role in OP responses leading to remyelination was examined using murine hepatitis virus strain A59 (MHV) to induce spinal cord demyelination and potential relevance to human pathology was evaluated in multiple sclerosis (MS) lesions. In MHV-infected mice, the density of Myt1 expressing cells markedly increased in lesioned areas of spinal cord white matter. Myt1 expressing cells proliferated most extensively during active demyelination and subsequently accumulated to maximal levels during early remyelination. Cells with nuclear Myt1 immunoreactivity were mainly OP cells, identified by co-localization with platelet-derived growth factor alpha receptor, with additional phenotypes being either oligodendrocytes or neural stem cells, identified by CC1 antigen and Musashi1, respectively. The density of OP cells expressing Myt1 was significantly increased in white matter of MHV-infected mice during demyelination and early remyelination then as remyelination advanced the values returned to levels comparable to PBS-injected control mice. In MHV lesions, Myt1 was not expressed in astrocytes, lymphocytes, or macrophage/microglial cells. MS lesions demonstrated increased Myt1 expression in both the periplaque white matter adjacent to lesions and within early remyelinating lesions. These results suggesta potential role for Myt1 in the regeneration of oligodendrocyte lineage cells in response to demyelination.

Characterization and function of the bHLH-O protein XHes2: insight into the mechanisms controlling retinal cell fate decision

Neurons and glial cells differentiate from common multipotent precursors in the vertebrate retina. We have identified a novel member of the hairy/Enhancer of split [E(spl)] gene family in Xenopus, XHes2, as a regulator to bias retinal precursor cells towards a glial fate. XHes2 expression is predominantly restricted to sensory organ territories, including the retina. Using in vivo lipofection in the optic vesicle, we found that XHes2 overexpression dramatically increases gliogenesis at the expense of neurogenesis. This increase in glial cells correlates with a delayed cell cycle withdrawal of some retinal progenitors. In addition, birthdating experiments suggest that XHes2 deviates some early born cell types towards a glial fate that would normally have given rise to neurons. Conversely, a significant inhibition of glial differentiation is observed upon XHes2 loss of function. The gliogenic activity of XHes2 relies on its ability to inhibit neuronal differentiation by at least two distinct mechanisms: it not only negatively regulates XNgnr1 and NeuroD transcription, but it also physically interacts with a subset of proneural bHLH proteins.

Lineage in the vertebrate retina

Recent results are changing the way we think about cell-fate decision mechanisms in the retina. For a long time it was accepted that lineage was not important in retinal cellular determination but, as we review here, new data show that lineage programmes might be at the heart of this process. These programmes are intrinsic, but they are also plastic and are influenced by extrinsic signals.

Hairy is a cell context signal controlling Notch activity

Notch signaling regulates cell fate determination and many developmental processes. Here we report that lateral inhibition, a major mechanism for Notch activity, is modulated by Hairy, a bHLH-WRPW protein. In Xenopus, Notch can have from inhibitory, permissive to enhancing roles in muscle or neural differentiation. These cell context-dependent effects correlate with Hairy expression levels from high to low, respectively, in the cells. Moreover, Notch effects can be altered upon manipulation of Hairy expression. We propose that Hairy provides a cell context in which a cell can interpret Notch and other extrinsic signals by controlling responsiveness of its target genes; this mode of Hairy-Notch interaction may apply in other systems.

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.

Identification of target genes for the Xenopus Hes-related protein XHR1, a prepattern factor specifying the midbrain-hindbrain boundary

The midbrain-hindbrain boundary (MHB) acts as a local organizer in the development of the CNS in vertebrates. Previously, we identified an MHB-specific bHLH-WRPW transcriptional repressor gene, Xenopus Hes-related 1 (XHR1), which is initially expressed in the presumptive MHB (pre-MHB) region at the early gastrula stage. To better understand the gene cascades involved in MHB formation, we investigated the genes downstream from XHR1 by differential screening using a Xenopus cDNA macroarray and a dexamethasone (DEX)-inducible, dominant-negative transcriptional activator construct of XHR1 (XHR1-VP16-GR). Among the newly identified candidate target genes of XHR1 were Enhancer of split-related genes (ESR1, ESR3/7, and ESR9) and Xenopus laevis cleavage 2 (XLCL2). XHR1-VP16-GR induced the expression of the ESR genes and XLCL2 as well as Xdelta1, Xngnr1, and XHR1 itself in the presence of DEX even after pretreatment with the protein synthesis inhibitor, cycloheximide. This suggests that these genes are direct targets of XHR1. XHR1-knockdown experiments with antisense morpholino oligos and ectopic expression of wild-type XHR1 revealed that XHR1 is necessary and sufficient to repress ESR genes in the pre-MHB region. Misexpression of the ESR genes in the pre-MHB region repressed the MHB marker gene, Pax2, suggesting that the repression of the ESR genes by XHR1 is at least partly required for the early development of the pre-MHB. Our data also show that XHR1 is not activated by Notch signaling, differing from ESR genes. Taken together, we propose a model in which XHR1 defines the pre-MHB region as a prepattern gene by repressing those possible direct target genes.

A bHLH transcriptional network regulating the specification of retinal ganglion cells

In the developing retina, the production of ganglion cells is dependent on the proneural proteins NGN2 and ATH5, whose activities define stages along the pathway converting progenitors into newborn neurons. Crossregulatory interactions between NGN2, ATH5 and HES1 maintain the uncommitted status of ATH5-expressing cells during progenitor patterning, and later on regulate the transition from competence to cell fate commitment. Prior to exiting the cell cycle, a subset of progenitors is selected from the pool of ATH5-expressing cells to go through a crucial step in the acquisition of a definitive retinal ganglion cell fate. The selected cells are those in which the upregulation of NGN2, the downregulation of HES1 and the autostimulation of ATH5 are coordinated with the progression of progenitors through the last cell cycle. This coordinated pattern initiates the transcription of ganglion cell-specific traits and determines the size of the ganglion cell population.

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.

Notch-Delta signaling is required for spatial patterning and Müller glia differentiation in the zebrafish retina

Notch-Delta signaling has been implicated in several alternative modes of function in the vertebrate retina. To further investigate these functions, we examined retinas from zebrafish embryos in which bidirectional Notch-Delta signaling was inactivated either by the mind bomb (mib) mutation, which disrupts E3 ubiquitin ligase activity, or by treatment with gamma-secretase inhibitors, which prevent intramembrane proteolysis of Notch and Delta. We found that inactivating Notch-Delta signaling did not prevent differentiation of retinal neurons, but it did disrupt spatial patterning in both the apical-basal and planar dimensions of the retinal epithelium. Retinal neurons differentiated, but their laminar arrangement was disrupted. Photoreceptor differentiation was initiated normally, but its progression was slowed. Although confined to the apical retinal surface as in normal retinas, the planar organization of cone photoreceptors was disrupted: cones of the same spectral subtype were clumped rather than regularly spaced. In contrast to neurons, Müller glia failed to differentiate suggesting an instructive role for Notch-Delta signaling in gliogenesis.

Sequences downstream of the bHLH domain of the Xenopus hairy-related transcription factor-1 act as an extended dimerization domain that contributes to the selection of the partners

XHRT1 is a member of the HRT/Hey protein subfamily that are known as Notch effectors. XHRT1 is expressed in the developing floor plate and encodes a basic helix-loop-helix (bHLH) transcription repressor. Here, we show that XHRT1 misexpression in the neural plate inhibits differentiation of neural precursor cells and thus may be important for floor plate cells to prevent them from adopting a neuronal fate. Deletion analysis indicated that inhibition of differentiation by XHRT1 requires the DNA-binding bHLH motif and either the Orange domain or the C-terminal region. XHRT1 could efficiently homodimerize and heterodimerize with hairy proteins. Among those hairy genes, Xhairy2b shows extensive overlap of expression with XHRT1 in floor plate precursors and may be a biologically relevant XHRT1 partner. Dimerization is mediated through both the bHLH and downstream sequences, the Orange domain being particularly important for the efficiency of the interaction. Using chimeric constructs between XHRT1 and the ESR9 bHLH-O protein that does not interact with Xhairy1 and Xhairy2b, we found that both the bHLH domain and downstream sequences of XHRT1 were required for heterodimerization with Xhairy2b, while only the XHRT1 sequences downstream of the Orange domain are required for the interaction with Xhairy1. Together, these results suggest that XHRT1 plays a role in floor plate cell development and highlight the importance of the Orange and downstream sequences in dimerization and in the selection of the bHLH partners.

A gene regulatory hierarchy for retinal ganglion cell specification and differentiation

Retinal ganglion cells (RGCs) are the first cell type to be specified during vertebrate retinogenesis. Specification and differentiation of the RGC lineage are a stepwise process involving a hierarchical gene regulatory network. During the past decade, a framework of the network has emerged and key transcriptional regulators have been identified. Pax6, Notch, Ath5, and the Brn3 (Pou4f) factors act at different levels of the regulatory hierarchy. In this review, we summarize the current understanding of the functions of these and other transcriptional factors in the specification and differentiation of the RGC lineage. We emphasize the regulatory relationships among transcription factors at different steps of RGC development. We discuss critical issues that need to be addressed before a complete understanding of the gene regulatory network for RGC development can be achieved. Future directions in RGC development will inevitably rely on combined genetic and genomics approaches.

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.

Mitotic G2-arrest is required for neural cell fate determination in Drosophila

In the wing discs of Drosophila, the mecanosensory precursor cells are singled out from clusters of cells blocked at the G2 phase of the cell cycle. This mitotic quiescence and the selection of the precursors are under strict spatio-temporal control. We forced G2 cells to enter mitosis by overexpression of string, the Drosophila homologue of the cdc25 gene. Premature entrance in the cell cycle is associated to a loss of precursor cells. Precursors are lost consecutively to a transcriptional down-regulation of the determinant proneural achaete/scute genes. This down-regulation results from an over-activation of the Enhancer of Split genes, known as effectors of the Notch signalling pathway. We conclude that exit from the cell cycle is required for proper neural cell fate determination.

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.

Co-ordinating retinal histogenesis: early cell cycle exit enhances early cell fate determination in the Xenopus retina

The laminar arrays of distinct cell types in the vertebrate retina are built by a histogenic process in which cell fate is correlated with birth order. To explore this co-ordination mechanistically, we altered the relative timing of cell cycle exit in the developing Xenopus retina and asked whether this affected the activity of neural determinants. We found that Xath5, a bHLH proneural gene that promotes retinal ganglion cell (RGC) fate, (Kanekar, S., Perron, M., Dorsky, R., Harris, W. A., Jan, L. Y., Jan, Y. N. and Vetter, M. L. (1997) Neuron19, 981-994), does not cause these cells to be born prematurely. To drive cells out of the cell cycle early, therefore, we misexpressed the cyclin kinase inhibitor, p27Xic1. We found that early cell cycle exit potentiates the ability of Xath5 to promote RGC fate. Conversely, the cell cycle activator, cyclin E1, which inhibits cell cycle exit, biases Xath5-expressing cells toward later neuronal fates. We found that Notch activation in this system caused cells to exit the cell cycle prematuely, and when it is misexpressed with Xath5, it also potentiates the induction of RGCs. The potentiation is counteracted by co-expression of cyclin E1. These results suggest a model of histogenesis in which the activity of factors that promote early cell cycle exit enhances the activity of factors that promote early cellular fates.

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.

Vertebrate hairy and Enhancer of split related proteins: transcriptional repressors regulating cellular differentiation and embryonic patterning

The basic-helix-loop-helix (bHLH) proteins are a superfamily of DNA-binding transcription factors that regulate numerous biological processes in both invertebrates and vertebrates. One family of bHLH transcriptional repressors is related to the Drosophila hairy and Enhancer-of-split proteins. These repressors contain a tandem arrangement of the bHLH domain and an adjacent sequence known as the Orange domain, so we refer to these proteins as bHLH-Orange or bHLH-O proteins. Phylogenetic analysis reveals the existence of four bHLH-O subfamilies, with distinct, evolutionarily conserved features. A principal function of bHLH-O proteins is to bind to specific DNA sequences and recruit transcriptional corepressors to inhibit target gene expression. However, it is likely that bHLH-O proteins repress transcription by additional mechanisms as well. Many vertebrate bHLH-O proteins are effectors of the Notch signaling pathway, and bHLH-O proteins are involved in regulating neurogenesis, vasculogenesis, mesoderm segmentation, myogenesis, and T lymphocyte development. In this review, we discuss mechanisms of action and biological roles for the vertebrate bHLH-O proteins, as well as some of the unresolved questions about the functions and regulation of these proteins during development and in human disease.

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.