XASH genes promote neurogenesis in Xenopus embryos

Xenopus leads the way: Frogs as a pioneering model to understand the human brain

From its long history in the field of embryology to its recent advances in genetics, Xenopus has been an indispensable model for understanding the human brain. Foundational studies that gave us our first insights into major embryonic patterning events serve as a crucial backdrop for newer avenues of investigation into organogenesis and organ function. The vast array of tools available in Xenopus laevis and Xenopus tropicalis allows interrogation of developmental phenomena at all levels, from the molecular to the behavioral, and the application of CRISPR technology has enabled the investigation of human disorder risk genes in a higher-throughput manner. As the only major tetrapod model in which all developmental stages are easily manipulated and observed, frogs provide the unique opportunity to study organ development from the earliest stages. All of these features make Xenopus a premier model for studying the development of the brain, a notoriously complex process that demands an understanding of all stages from fertilization to organogenesis and beyond. Importantly, core processes of brain development are conserved between Xenopus and human, underlining the advantages of this model. This review begins by summarizing discoveries made in amphibians that form the cornerstones of vertebrate neurodevelopmental biology and goes on to discuss recent advances that have catapulted our understanding of brain development in Xenopus and in relation to human development and disease. As we engage in a new era of patient-driven gene discovery, Xenopus offers exceptional potential to uncover conserved biology underlying human brain disorders and move towards rational drug design.

Investigating cellular and molecular mechanisms of neurogenesis in Capitella teleta sheds light on the ancestor of Annelida

BACKGROUND

Diverse architectures of nervous systems (NSs) such as a plexus in cnidarians or a more centralized nervous system (CNS) in insects and vertebrates are present across Metazoa, but it is unclear what selection pressures drove evolution and diversification of NSs. One underlying aspect of this diversity lies in the cellular and molecular mechanisms driving neurogenesis, i.e. generation of neurons from neural precursor cells (NPCs). In cnidarians, vertebrates, and arthropods, homologs of SoxB and bHLH proneural genes control different steps of neurogenesis, suggesting that some neurogenic mechanisms may be conserved. However, data are lacking for spiralian taxa.

RESULTS

To that end, we characterized NPCs and their daughters at different stages of neurogenesis in the spiralian annelid Capitella teleta. We assessed cellular division patterns in the neuroectoderm using static and pulse-chase labeling with thymidine analogs (EdU and BrdU), which enabled identification of NPCs that underwent multiple rounds of division. Actively-dividing brain NPCs were found to be apically-localized, whereas actively-dividing NPCs for the ventral nerve cord (VNC) were found apically, basally, and closer to the ventral midline. We used lineage tracing to characterize the changing boundary of the trunk neuroectoderm. Finally, to start to generate a genetic hierarchy, we performed double-fluorescent in-situ hybridization (FISH) and single-FISH plus EdU labeling for neurogenic gene homologs. In the brain and VNC, Ct-soxB1 and Ct-neurogenin were expressed in a large proportion of apically-localized, EdU+ NPCs. In contrast, Ct-ash1 was expressed in a small subset of apically-localized, EdU+ NPCs and subsurface, EdU- cells, but not in Ct-neuroD+ or Ct-elav1+ cells, which also were subsurface.

CONCLUSIONS

Our data suggest a putative genetic hierarchy with Ct-soxB1 and Ct-neurogenin at the top, followed by Ct-ash1, then Ct-neuroD, and finally Ct-elav1. Comparison of our data with that from Platynereis dumerilii revealed expression of neurogenin homologs in proliferating NPCs in annelids, which appears different than the expression of vertebrate neurogenin homologs in cells that are exiting the cell cycle. Furthermore, differences between neurogenesis in the head versus trunk of C. teleta suggest that these two tissues may be independent developmental modules, possibly with differing evolutionary trajectories.

Multi-site phosphorylation controls the neurogenic and myogenic activity of E47

The superfamily of basic-Helix-Loop-Helix (bHLH) transcription factors influence cell fate in all three embryonic germ layers, and the tissue-specific class II factors have received prominent attention for their potent ability to direct differentiation during development and in cellular reprogramming. The activity of many class II bHLH proteins driving differentiation, and the inhibitory class VI bHLH factor Hes1, is controlled by phosphorylation on multiple sites by Cyclin-dependent kinases (Cdks). As class II proteins are generally thought to be active through hetero-dimerisation with the ubiquitously expressed class I E proteins, regulation of class I transcription factors such as E47 may influence the activity of multiple tissue-specific bHLH proteins. Using differentiation of nerve and muscle in Xenopus frog embryos as a model system, we set out to explore whether with the ubiquitously expressed class I E protein E47 that hetero-dimerises with Class II bHLHs to control their activity, is also regulated by multi-site phosphorylation. We demonstrate that E47 can be readily phosphorylated by Cdks on multiple sites in vitro, while ectopically-expressed E47 exists in multiple phosphorylated forms in Xenopus embryos. Preventing multi-site phosphorylation using a phospho-mutant version of E47 enhances the neurogenic and myogenic activity of three different class II bHLH reprogramming factors, and also when E47 acts in hetero-dimerisation with endogenous proteins. Mechanistically, unlike phospho-regulation of class II bHLH factors, we find that preventing phosphorylation of E47 increases the amount of chromatin-bound E47 protein but without affecting its overall protein stability. Thus, multi-site phosphorylation is a conserved regulatory mechanism across the bHLH superfamily that can be manipulated to enhance cellular differentiation.

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E47 is phosphorylated in Xenopus embryos on up to 13 conserved SP/TP sites.

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In vitro, E47 protein can be phosphorylated by Cyclin-dependent-kinases.

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Under-phosphorylated E47 enhances activity of 3 bHLH reprogramming factors.

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Under-phosphorylated E47 enhances neuronal differentiation by hetero-dimerisation.

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E47 phospho-status alters chromatin binding but not overall protein stability.

A novel role for Ascl1 in the regulation of mesendoderm formation via HDAC-dependent antagonism of VegT

Maternally expressed proteins function in vertebrates to establish the major body axes of the embryo and to establish a pre-pattern that sets the stage for later-acting zygotic signals. This pre-patterning drives the propensity of Xenopus animal cap cells to adopt neural fates under various experimental conditions. Previous studies found that the maternally expressed transcription factor, encoded by the Xenopus achaete scute-like gene ascl1, is enriched at the animal pole. Asc1l is a bHLH protein involved in neural development, but its maternal function has not been studied. Here, we performed a series of gain- and loss-of-function experiments on maternal ascl1, and present three novel findings. First, Ascl1 is a repressor of mesendoderm induced by VegT, but not of Nodal-induced mesendoderm. Second, a previously uncharacterized N-terminal domain of Ascl1 interacts with HDAC1 to inhibit mesendoderm gene expression. This N-terminal domain is dispensable for its neurogenic function, indicating that Ascl1 acts by different mechanisms at different times. Ascl1-mediated repression of mesendoderm genes was dependent on HDAC activity and accompanied by histone deacetylation in the promoter regions of VegT targets. Finally, maternal Ascl1 is required for animal cap cells to retain their competence to adopt neural fates. These results establish maternal Asc1l as a key factor in establishing pre-patterning of the early embryo, acting in opposition to VegT and biasing the animal pole to adopt neural fates. The data presented here significantly extend our understanding of early embryonic pattern formation.

Summary: The proneural factor ASCL1 recruits HDAC1 to repress VegT-induced, but not Nodal-induced, mesendoderm formation via a previously uncharacterized N-terminal domain.

Methylmercury exposure during early Xenopus laevis development affects cell proliferation and death but not neural progenitor specification

Methylmercury (MeHg) is a widespread environmental toxin that preferentially and adversely affects developing organisms. To investigate the impact of MeHg toxicity on the formation of the vertebrate nervous system at physiologically relevant concentrations, we designed a graded phenotype scale for evaluating Xenopus laevis embryos exposed to MeHg in solution. Embryos displayed a range of abnormalities in response to MeHg, particularly in brain development, which is influenced by both MeHg concentration and the number of embryos per ml of exposure solution. A TC50 of ~50μg/l and LC50 of ~100μg/l were found when maintaining embryos at a density of one per ml, and both increased with increasing embryo density. In situ hybridization and microarray analysis showed no significant change in expression of early neural patterning genes including sox2, en2, or delta; however a noticeable decrease was observed in the terminal neural differentiation genes GAD and xGAT, but not xVGlut. PCNA, a marker for proliferating cells, was negatively correlated with MeHg dose, with a significant reduction in cell number in the forebrain and spinal cord of exposed embryos by tadpole stages. Conversely, the number of apoptotic cells in neural regions detected by a TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay was significantly increased. These results provide evidence that disruption of embryonic neural development by MeHg may not be directly due to a loss of neural progenitor specification and gene transcription, but to a more general decrease in cell proliferation and increase in cell death throughout the developing nervous system.

Phylotypic expression of the bHLH genes Neurogenin2, Neurod, and Mash1 in the mouse embryonic forebrain

In the anamniote model animals, zebrafish and Xenopus laevis, highly comparable early forebrain expression patterns of proneural basic helix-loop-helix (bHLH) genes relevant for neurogenesis (atonal homologs, i.e., neurogenins/NeuroD and achaete-scute homologs, i.e., Ascl/ash) were previously revealed during a particular period of development (zebrafish: 3 days; frog: stage 48). Neurogenins/NeuroD on the one hand and Ascl1/ash1 on the other hand exhibit essentially mutually exclusive spatial patterns, probably reflecting different positional information received within the neural tube, and appear to underlie glutamatergic versus GABAergic neuronal differentiation, respectively. Significant data suggest that similar complementary localizations of these proneural genes and corresponding differentiation pathways also exist in the mouse, the prominent mammalian model. The present article reports on detailed mouse brain bHLH gene expression patterns to fill existing gaps in the identification of expression domains, especially outside the telencephalon. Clearly, there are strong similarities in the complementarity of territories expressing Ascl1/Mash 1 versus neurogenins/NeuroD in the entire mouse forebrain, except for the pretectal alar plate and basal plate of prosomeres 1-3. The analysis substantiates localization of neurogenins/NeuroD in the pallium, eminentia thalami, and dorsal thalamus, and expression of Ascl1/Mash 1 in the striatal and septal subpallium, preoptic region, ventral thalamus, and hypothalamus, which is highly similar to the situation described in Xenopus and zebrafish. Thus, all three vertebrate model species display a "phylotypic stage or period" corresponding to a temporally and spatially defined control of neurogenesis during forebrain development, ultimately resulting in the differentiation of distinct populations of glutamatergic versus GABAergic neurons.

Neural induction in Xenopus requires inhibition of Wnt-β-catenin signaling

Canonical Wnt signals have been implicated in multiple events during early embryogenesis, including primary axis formation, neural crest induction, and A-P patterning of the neural plate. The mechanisms by which Wnt signals can direct distinct fates in cell types that are closely linked both temporally and spatially remains poorly understood. However, recent work has suggested that the downstream transcriptional mediators of this pathway, Lef/Tcf family DNA binding proteins, may confer distinct outcomes on these signals in some cellular contexts. In this study, we first examined whether inhibitory mutants of XTcf3 and XLef1 might block distinct Wnt-dependent signaling events during the diversification of cell fates in the early embryonic ectoderm. We found that a Wnt-unresponsive mutant of XTcf3 potently blocks neural crest formation, whereas an analogous mutant of XLef1 does not, and that the difference in activity mapped to the C-terminus of the proteins. Significantly, the inhibitory XTcf3 mutant also blocked expression of markers of anterior-most cell types, including cement gland and sensory placodes, indicating that Wnt signals are required for rostral as well as caudal ectodermal fates. Unexpectedly, we also found that blocking canonical Wnt signals in the ectoderm, using the inhibitory XTcf3 mutant or by other means, dramatically expanded the size of the neural plate, as evidenced by the increased expression of early pan-neural markers such as Sox3 and Nrp1. Conversely, we find that upregulation of canonical Wnt signals interferes with the induction of the neural plate, and this activity can be separated experimentally from Wnt-mediated neural crest induction. Together these findings provide important and novel insights into the role of canonical Wnt signals during the patterning of vertebrate ectoderm and indicate that Wnt inhibition plays a central role in the process of neural induction.

Identification of shared transcriptional targets for the proneural bHLH factors Xath5 and XNeuroD

Proneural basic helix-loop-helix (bHLH) transcription factors are critical positive regulators of neuronal differentiation in a variety of species and are required for proper differentiation of various subtypes of neurons. Although bHLH factors demonstrate some unique functions during neural development, they share the ability to regulate neuronal differentiation, potentially by targeting overlapping sets of genes. To assess this, we performed a screen in ectoderm animal cap tissue to identify direct transcriptional targets shared by two Xenopus ato-related bHLH factors, Xath5 and XNeuroD. Candidate target genes identified in this screen include several transcriptional regulators (Xebf2, Xebf3, XETOR and NKL), an RNA binding protein (elrC), a cell cycle component (Xgadd45gamma) and several novel genes. Overexpression of either Xath5 or XNeuroD induced ectopic in vivo expression of these candidate target genes. Conversely, blocking ato-related bHLH activity prevented endogenous nervous system expression of these genes. Therefore, we have identified a set of genes that can be regulated by multiple ato-related bHLH factors and may function as critical effectors of proneural bHLH-mediated differentiation.

Noelins modulate the timing of neuronal differentiation during development

Noelins comprise a family of extracellular proteins with proposed roles in neural and neural crest development. Here, we show that a previously uncharacterized family member, Noelin-4, functions to maintain neural precursors in an undifferentiated state and biases ectoderm toward a neural fate. We show that Noelin-4 is induced by the neurogenic genes X-ngnr-1 and XNeuroD. Over-expression of Noelin-4 causes expansion of the neural plate at the expense of neural crest and epidermis. Although there is an apparent increase in the neural precursor pool, no increase was noted in differentiated neurons. Later, derivatives such as the neural tube and retina appear enlarged. We show biochemically that Noelin-4 protein is glycosylated and secreted and that it interacts with Noelin-1, an isoform previously found to promote differentiation in neuralized animal caps. Accordingly, the neural precursor expansion activity of Noelin-4 is reversed by co-expression of Noelin-1. Our finding that Noelin isoforms can bind to and antagonize one another suggests that interacting Noelin isoforms may play a role in regulating timing of differentiation.

Characterization of Xenopus Phox2a and Phox2b defines expression domains within the embryonic nervous system and early heart field

The closely related homeodomain containing genes, Phox2a and Phox2b, are essential for neuronal specification and differentiation within discrete subsets of neurons during vertebrate embryogenesis. We have isolated Xenopus Phox2 homologs, termed Xphox2a and Xphox2b, and characterized their expression during early development. In addition, we have characterized a Phox2a splice variant, termed Xphox2a.2, which lacks homeo- and C-terminal protein coding domains. Xphox2a, Xphox2a.2 and Xphox2b transcripts are expressed in dynamic temporal and regional patterns during nervous system development. The expression of Xphox2a and Xphox2b is only partially overlapping and includes cranial motor and interneuron populations as well as peripheral sympathetic and cranial ganglion neurons, sites linked to Phox2 expression in other species. In addition, we have identified an early domain of Xphox2a and subsequent Xphox2b expression in ventral regions of the embryo, within the developing heart field. XPhox2 expression within this domain is preceded by the gastrula-stage expression of the proneural basic helix-loop-helix transcription factor, Xash1, pointing to a new region of action for this group transcription factors during vertebrate development.

A screen for downstream effectors of Neurogenin2 in the embryonic neocortex

Neurogenin (Ngn) 1 and Ngn2 encode basic-helix-loop-helix transcription factors expressed in the developing neocortex. Like other proneural genes, Ngns participate in the specification of neural fates and neuronal identities, but downstream effectors remain poorly defined. We set out to identify Ngn2 effectors in the cortex using a subtractive hybridization screen and identified several regionally expressed genes that were misregulated in Ngn2 and Ngn1;Ngn2 mutants. Included were genes down-regulated in germinal zone progenitors (e.g., Nlgn1, Unc5H4, and Dcc) and in postmitotic neurons in the cortical plate (e.g., Bhlhb5 and NFIB) and subplate (e.g., Mef2c, srGAP3, and protocadherin 9). Further analysis revealed that Ngn2 mutant subplate neurons were misspecified and that thalamocortical afferents (TCAs) that normally target this layer instead inappropriately projected towards the germinal zone. Strikingly, EphA5 and Sema3c, which encode repulsive guidance cues, were down-regulated in the Ngn2 and Ngn1;Ngn2 mutant germinal zones, providing a possible molecular basis for axonal targeting defects. Thus, we identified several new components of the differentiation cascade(s) activated downstream of Ngn1 and Ngn2 and provided novel insights into a new developmental process controlled by these proneural genes. Further analysis of the genes isolated in our screen should provide a fertile basis for understanding the molecular mechanisms underlying corticogenesis.

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.

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.

Distinct patterns of downstream target activation are specified by the helix-loop-helix domain of proneural basic helix-loop-helix transcription factors

Both gain- and loss-of-function analyses indicate that proneural basic/helix-loop-helix (bHLH) proteins direct not only general aspects of neuronal differentiation but also specific aspects of neuronal identity within neural progenitors. In order to better understand the function of this family of transcription factors, we have used hormone-inducible fusion constructs to assay temporal patterns of downstream target regulation in response to proneural bHLH overexpression. In these studies, we have compared two distantly related Xenopus proneural bHLH genes, Xash1 and XNgnr1. Our findings indicate that both Xash1 and XNgnr1 induce expression of the general neuronal differentiation marker, N-tubulin, with a similar time course in animal cap progenitor populations. In contrast, these genes each induce distinct patterns of early downstream target expression. Both genes induce expression of the HLH-containing gene, Xcoe2, at early time points, but only XNgnr1 induces early expression of the bHLH genes, Xath3 and XNeuroD. Structure:function analyses indicate that the distinct pattern of XNgnr1-induced downstream target activation is linked to the XNgnr1 HLH domain, demonstrating a novel role for this domain in mediating the differential function of individual members of the proneural bHLH gene family.

The secreted glycoprotein Noelin-1 promotes neurogenesis in Xenopus

Neurogenesis in Xenopus neural ectoderm involves multiple gene families, including basic helix-loop-helix transcription factors, which initiate and control primary neurogenesis. Equally important, though less well understood, are the downstream effectors of the activity of these transcription factors. We have investigated the role of a candidate downstream effector, Noelin-1, during Xenopus development. Noelin-1 is a secreted glycoprotein that likely forms large multiunit complexes. In avians, overexpression of Noelin-1 causes prolonged and excessive neural crest migration. Our studies in Xenopus reveal that this gene, while highly conserved in sequence, has a divergent function in primary neurogenesis. Xenopus Noelin-1 is expressed mainly by postmitotic neurogenic tissues in the developing central and peripheral nervous systems, first appearing after neural tube closure. Its expression is upregulated in ectopic locations upon overexpression of the neurogenic genes X-ngnr-1 and XNeuroD. Noelin-1 expression in animal caps induces expression of neural markers XBrn-3d and XNeuroD, and co-expression of secreted Noelin-1 with noggin amplifies noggin-induced expression of XBrn-3d and XNeuroD. Furthermore, in animal caps neuralized by expression of noggin, co-expression of Noelin-1 causes expression of neuronal differentiation markers several stages before neurogenesis normally occurs in this tissue. Finally, only secreted forms of the protein can activate sensory marker expression, while all forms of the protein can induce early neurogenesis. This suggests that the cellular localization of Noelin-1 may be important to its function. Thus, Noelin-1 represents a novel secreted factor involved in neurogenesis.

The Wnt-activated Xiro1 gene encodes a repressor that is essential for neural development and downregulates Bmp4

In the early Xenopus embryo, the Xiro homeodomain proteins of the Iroquois (Iro) family control the expression of proneural genes and the size of the neural plate. We report that Xiro1 functions as a repressor that is strictly required for neural differentiation, even when the BMP4 pathway is impaired. We also show that Xiro1 and Bmp4 repress each other. Consistently, Xiro1 and Bmp4 have complementary patterns of expression during gastrulation. The expression of Xiro1 requires Wnt signaling. Thus, Xiro1 is probably a mediator of the known downregulation of Bmp4 by Wnt signaling.

Hes6 acts in a positive feedback loop with the neurogenins to promote neuronal differentiation

During the development of the vertebrate nervous system, neurogenesis is promoted by proneural bHLH proteins such as the neurogenins, which act as potent transcriptional activators of neuronal differentiation genes. The pattern by which these proteins promote neuronal differentiation is thought to be governed by inhibitors, including a class of transcriptional repressors called the WRPW-bHLH proteins, which are similar to Drosophila proteins encoded by hairy and genes in the enhancer of split complex (E-(SPL)-C). Here, we describe the isolation and characterization of Hes6, which encodes a novel WRPW-bHLH protein expressed during neurogenesis in mouse and Xenopus embryos. We show that Hes6 expression follows that of neurogenins but precedes that of the neuronal differentiation genes. We provide several lines of evidence to show that Hes6 expression occurs in developing neurons and is induced by the proneural bHLH proteins but not by the Notch pathway. When ectopically expressed in Xenopus embryos, Hes6 promotes neurogenesis. The properties of Hes6 distinguish it from other members of the WRPW-bHLH family in vertebrates, and suggest that it acts in a positive-feedback loop with the proneural bHLH proteins to promote neuronal differentiation.

Effects of rat Axin domains on axis formation in Xenopus embryos

Wnt signaling plays an important role in axis formation in early vertebrate development. Axin is one Wnt signaling regulator that inhibits this pathway. The effects of the injection of mRNA of several rat Axin (rAxin) mutants on axis formation in Xenopus embryos were examined. It was found that rAxin mutants containing only a regulation of G-protein signaling (RGS) domain fragment or with deletion of the RGS domain induced axis formation. Because the RGS domain is a major adenomatous polyposis coli gene product (APC)-binding domain, APC association with glycogen synthase kinase 3beta (GSK3beta) on the Axin molecule may be important in inhibition of axis formation. The ventralizing activities of wild-type rAxin and a mutant in which the Dishevelled and Axin (DIX) domain was deleted (deltaDIX mutant) were examined. Histological examination and gene expression revealed that the ventralizing activity of the deltaDIX mutant was weaker than that of wild-type rAxin. This finding suggests that the C-terminus of rAxin contributes to the inhibition of Wnt signaling in Xenopus embryos. Furthermore, an rAxin mutant that contained both the RGS and GSK3beta-binding domains affected both the dorsal and ventral sides of blastomeres, mediated ectodermal fate and induced expansion of notochord and/or endoderm, but did not induce axis formation.

Misexpression of basic helix-loop-helix genes in the murine cerebral cortex affects cell fate choices and neuronal survival

To investigate the role(s) of basic helix-loop-helix genes (bHLH) genes in the developing murine cerebral cortex, Mash1, Math2, Math3, Neurogenin1 (Ngn1), Ngn2, NeuroD, NeuroD2 and Id1 were transduced in vivo into the embryonic and postnatal cerebral cortex using retrovirus vectors. The morphology and location of infected cells were analyzed at postnatal stages. The data indicate that a subset of bHLH genes are capable of regulating the choice of neuronal versus glial fate and that, when misexpressed, they can be deleterious to the survival of differentiating neurons, but not glia.

The homeodomain-containing gene Xdbx inhibits neuronal differentiation in the developing embryo

The development of the vertebrate nervous system depends upon striking a balance between differentiating neurons and neural progenitors in the early embryo. Our findings suggest that the homeodomain-containing gene Xdbx regulates this balance by maintaining neural progenitor populations within specific regions of the neuroectoderm. In posterior regions of the Xenopus embryo, Xdbx is expressed in a bilaterally symmetric stripe that lies at the middle of the mediolateral axis of the neural plate. This stripe of Xdbx expression overlaps the expression domain of the proneural basic/helix-loop-helix-containing gene, Xash3, and is juxtaposed to the expression domains of Xenopus Neurogenin related 1 and N-tubulin, markers of early neurogenesis in the embryo. Xdbx overexpression inhibits neuronal differentiation in the embryo and when co-injected with Xash3, Xdbx inhibits the ability of Xash3 to induce ectopic neurogenesis. One role of Xdbx during normal development may therefore be to restrict spatially neuronal differentiation within the neural plate, possibly by altering the neuronal differentiation function of Xash3.

cato encodes a basic helix-loop-helix transcription factor implicated in the correct differentiation of Drosophila sense organs

In Drosophila neurogenesis, proneural genes encode bHLH proteins that are required for neural precursor selection. But many vertebrate homologues are expressed later and are postulated to have multiple roles during neurogenesis. We have isolated a new Drosophila gene, cato, which encodes a protein with a bHLH domain that is closely related to that of the proneural protein Atonal. cato expression is restricted to the developing PNS, where it is expressed in between the stages of precursor selection and terminal differentiation (and therefore later than the proneural genes). We present evidence from loss-of-function and misexpression experiments that cato is involved in sensory neurone morphology. Moreover, in prospero mutants, in which axon and dendrite outgrowth is defective, cato is strongly derepressed in the developing CNS.

Distinct effects of XBF-1 in regulating the cell cycle inhibitor p27(XIC1) and imparting a neural fate

XBF-1 is an anterior neural plate-specific, winged helix transcription factor that affects neural development in a concentration-dependent manner. A high concentration of XBF-1 results in suppression of endogenous neuronal differentiation and an expansion of undifferentiated neuroectoderm. Here we investigate the mechanism by which this expansion is achieved. Our findings suggest that XBF-1 converts ectoderm to a neural fate and it does so independently of any effects on the mesoderm. In addition, we show that a high dose of XBF-1 promotes the proliferation of neuroectodermal cells while a low dose inhibits ectodermal proliferation. Thus, the neural expansion observed after high dose XBF-1 misexpression is due both to an increase in the number of ectodermal cells devoted to a neural fate and an increase in their proliferation. We show that the effect on cell proliferation is likely to be mediated by p27(XIC1), a cyclin-dependent kinase (cdk) inhibitor. We show that p27(XIC1) is expressed in a spatially restricted pattern in the embryo, including the anterior neural plate, and when misexpressed it is sufficient to block the cell cycle in vivo. We find that p27(XIC1)is transcriptionally regulated by XBF-1 in a dose-dependent manner such that it is suppressed or ectopically induced by a high or low dose of XBF-1, respectively. However, while a low dose of XBF-1 induces ectopic p27(XIC1)and ectopic neurons, misexpression of p27(XIC1)does not induce ectopic neurons, suggesting that the effects of XBF-1 on cell fate and cell proliferation are distinct. Finally, we show that p27(XIC1)is suppressed by XBF-1 in the absence of protein synthesis, suggesting that at least one component of p27(XIC1)regulation by XBF-1 may be direct. Thus, XBF-1 is a neural-specific transcription factor that can independently affect both the cell fate choice and the proliferative status of the cells in which it is expressed.

Regulation of neurogenesis by interactions between HEN1 and neuronal LMO proteins

Basic-helix-loop-helix transcription factors regulate neurogenesis and neuronal differentiation by as yet unknown mechanisms. We show that an embryonic neuronal-specific basic-helix-loop-helix protein, HEN1 (also known as NSCL1 or NHLH), interacts with ‘LIM only’ proteins. Examination of the expression patterns of XHEN1 and XLMO-3, the Xenopus homologues of these human genes, reveals extensive overlap during early neurogenesis: at the onset of gastrulation on the dorsal side of the blastopore lip and, subsequently, in the prospective neural plate. Binding of XLMO-3 increases the transcriptional activity of XHEN1 in vivo. Co-expression of these two genes in Xenopus embryos induces a cascade of expression of neuronal-specific basic-helix-loop-helix proteins that leads to neuronal differentiation. We propose that XHEN1, in concert with XLMO-3, is a critical regulator of neurogenesis.

X-ngnr-1 and Xath3 promote ectopic expression of sensory neuron markers in the neurula ectoderm and have distinct inducing properties in the retina

Xath3 encodes a Xenopus neuronal-specific basic helix-loop-helix transcription factor related to the Drosophila proneural factor atonal. We show here that Xath3 acts downstream of X-ngnr-1 during neuronal differentiation in the neural plate and retina and that its expression and activity are modulated by Notch signaling. X-ngnr-1 activates Xath3 and NeuroD by different mechanisms, and the latter two genes crossactivate each other. In the ectoderm, X-ngnr-1 and Xath3 have similar activities, inducing ectopic sensory neurons. Among the sensory-specific markers tested, only those that label cranial neurons were found to be ectopically activated. By contrast, in the retina, X-ngnr-1 and Xath3 overexpression promote the development of overlapping but distinct subtypes of retinal neurons. Together, these data suggest that X-ngnr-1 and Xath3 regulate successive stages of early neuronal differentiation and that, in addition to their general proneural properties, they may contribute, in a context-dependent manner, to some aspect of neuronal identity.

The POU domain gene, XlPOU 2 is an essential downstream determinant of neural induction

The POU domain gene, XlPOU 2, acts as a transcriptional activator during mid-gastrulation in Xenopus. Overexpression or misexpression of VP16-POU-GR, a fusion protein consisting of the strong activator domain of VP16 and the POU domain of XlPOU 2, results in ectopic expression of the neural-specific genes, nrp-1, en-2, and beta-tubulin. In contrast, overexpressing a dominant-inhibitory form of XlPOU 2 inhibits the chordin-induced neuralization of uncommitted ectoderm, and results in a loss of nrp-1 and en-2 expression in embryos. Furthermore, in uncommitted ectoderm, XlPOU 2 regulates the developmental neural program that includes a number of pre-pattern genes and at least one proneural gene, X-ngnr-1, thus playing a key role during neural determination.

Neural tube closure in Xenopus laevis involves medial migration, directed protrusive activity, cell intercalation and convergent extension

We have characterized the cell movements and prospective cell identities as neural folds fuse during neural tube formation in Xenopus laevis. A newly developed whole-mount, two-color fluorescent RNA in situ hybridization method, visualized with confocal microscopy, shows that the dorsal neural tube gene xpax3 and the neural-crest-specific gene xslug are expressed far lateral to the medial site of neural fold fusion and that expression moves medially after fusion. To determine whether cell movements or dynamic changes in gene expression are responsible, we used low-light videomicroscopy followed by fluorescent in situ and confocal microscopy. These methods revealed that populations of prospective neural crest and dorsal neural tube cells near the lateral margin of the neural plate at the start of neurulation move to the dorsal midline using distinctive forms of motility. Before fold fusion, superficial neural cells apically contract, roll the neural plate into a trough and appear to pull the superficial epidermal cell sheet medially. After neural fold fusion, lateral deep neural cells move medially by radially intercalating between other neural cells using two types of motility. The neural crest cells migrate as individual cells toward the dorsal midline using medially directed monopolar protrusions. These movements combine the two lateral populations of neural crest into a single medial population that form the roof of the neural tube. The remaining cells of the dorsal neural tube extend protrusions both medially and laterally bringing about radial intercalation of deep and superficial cells to form a single-cell-layered, pseudostratified neural tube. While ours is the first description of medially directed cell migration during neural fold fusion and re-establishment of the neural tube, these complex cell behaviors may be involved during cavitation of the zebrafish neural keel and secondary neurulation in the posterior axis of chicken and mouse.

Expression of Pax-3 in the lateral neural plate is dependent on a Wnt-mediated signal from posterior nonaxial mesoderm

During early patterning of the vertebrate neuraxis, the expression of the paired-domain transcription factor Pax-3 is induced in the lateral portions of the posterior neural plate via posteriorizing signals emanating from the late organizer and posterior nonaxial mesoderm. Using a dominant-negative approach, we show in explant assays that Pax-3 inductive activities from the organizer do not depend on FGF, retinoic acid, or XWnt-8, either alone or in combination, suggesting that the organizer may produce an unknown posteriorizing factor. However, Pax-3 inductive signals from posterior nonaxial mesoderm are Wnt-dependent. We show that Pax-3 expression in the lateral neural plate expands in XWnt-8-injected embryos and is blocked by dominant-negative XWnt-8. Similarly, we show that the homeodomain transcription factor Msx-1, which like Pax-3 is an early marker of the lateral neural plate, is induced by posterior nonaxial mesoderm and blocked by dominant-negative XWnt-8. Finally, we show that Rohon-Beard primary neurons, a cell type that develops within the lateral neural plate, are also blocked in vivo by dominant-negative Xwnt-8. Together these data support a model in which patterning of the lateral neural plate by Wnt-mediated signals is an early event that establishes a posteriolateral domain, marked by Pax-3 and Msx-1 expression, from which Rohon-Beard cells and neural crest will subsequently arise.

Deconstructing cell determination: proneural genes and neuronal identity

Vertebrates express scores of bHLH proteins during neural development. Earlier studies inspired by the established role of "proneural" genes in fly neurogenesis, as well as by the vertebrate bHLH myogenic program, focused on the reconstruction of bHLH gene cascades, which are thought to control successive steps leading to neuronal differentiation. Little attention has been paid thus far to the relationship between the diversity of neural bHLH genes and the diversity of neuronal phenotypes. This article reviews recent evidence that, akin to their fly counterparts, vertebrate neural bHLH genes probably confer not only "generic" neuronal properties, but also neuronal type-specific properties, inextricably linking neural determination and the specification of neuronal identity. We also speculate on the relations between positional information and gene activity, and on the evolutionary significance of the diversity of bHLH genes.

Neuronal differentiation and patterning in Xenopus: the role of cdk5 and a novel activator xp35.2

Cdk5, a member of the cyclin-dependent kinase family, has been shown to play an important role in development of the central nervous system in mammals when partnered by its activator p35. Here we describe the cloning and characterization of a novel activator of cdk5 in Xenopus, Xp35.2. Xp35.2 is expressed during development initially in the earliest differentiating primary neurons in the neural plate and then later in differentiating neural tissue of the brain. This is in contrast to the previously described Xenopus cdk5 activator Xp35.1 which is expressed over the entire expanse of the neural plate in both proliferating and differentiating cells. Expression of both Xp35.1 and Xp35.2 and activation of cdk5 kinase occur when terminal neural differentiation is induced by neurogenin and neuro D overexpression but not when only early stages of neural differentiation are induced by noggin. Moreover, blocking cdk5 kinase activity specifically results in disruption and reduction of the embryonic eye where cdk5 and its Xp35 activators are expressed. Thus, cdk5/p35 complexes function in aspects of neural differentiation and patterning in the early embryo and particularly in formation of the eye.

Development of neural crest in Xenopus

The neural crest is a unique cell population among embryonic cell types, displaying properties of both ectodermal and mesodermal lineages. Most of the recent studies examining the neural crest have been performed in avian embryos. Only in the first half of this century were amphibians extensively used. We first summarize this important older source of information, reviewing studies made since the turn of the century. Due to the increasingly detailed in cellular and molecular knowledge of the early development of Xenopus laevis, the remainder of the review focuses on this species. We describe the route of migration and fate of the neural crest and propose a new model of neural crest induction in which prospective cells are induced independently of the neural plate by a double gradient of a morphogen that patterns the entire ectoderm. This model is also discussed in a more general context in connection with the dorsoventral patterning of the neural tube. Finally, we discuss some ideas concerning neural crest evolution and propose a novel hypothesis about its phylogenetic origin.

The neurotransmitter noradrenaline drives noggin-expressing ectoderm cells to activate N-tubulin and become neurons

Neurotransmitters regulate neuronal function in the nervous system and modulation of their synthesis, release, and binding by immature neurons and their targets is a major part of nervous system development. We propose that the neurotransmitter noradrenaline regulates neuronal fate during neurulation, before neurons have differentiated. The ability of noradrenaline to induce a neural fate was tested in naive ectoderm caps cut from late blastula stage Xenopus embryos. Noradrenaline (10(-6) M) did not switch on otx-2 or NCAM and did not induce the formation of cement glands. We conclude that noradrenaline cannot induce a neural fate. By contrast, 10(-8) M noradrenaline activated N-tubulin in ectoderm caps expressing the neural inducing molecule noggin by the time intact siblings had become mid-neurulae. Methoxamine, a specific alpha-adrenergic receptor agonist, also activated N-tubulin in noggin-expressing caps. The alpha-adrenergic receptor blocker prazosin inhibited both noradrenaline- and methoxamine-induced activation of N-tubulin. The neurotransmitters dopamine and 5-HT did not activate expression of N-tubulin. XA-1, Otx-2, X-Delta, and Xotch transcripts were not sensitive to noradrenaline. HoxB9, which indicates posteriorization, was not activated by noradrenaline. When intact siblings were at stage 27, many cells in noggin-expressing, noradrenaline-treated caps were stained by the neuron-specific mcAb3A10. We propose that noradrenaline is an important endogenous modulator of neuronal fate, driving noggin-expressing cells to become neurons by binding to alpha-adrenergic receptors and activating a cascade that culminates in the expression of the neuronal markers N-tubulin and 3A10.

NeuroD regulates multiple functions in the developing neural retina in rodent

The expression and function of the basic helix-loop-helix (bHLH) transcription factor NeuroD were studied in the developing neural retina in rodent. neuroD was expressed in areas of undetermined retinal cells as well as developing photoreceptors and amacrine interneurons. Expression was maintained in a subset of mature photoreceptors in the adult retina. Using both loss-of-function and gain-of-function approaches, NeuroD was found to play multiple roles in retinal development. (1) NeuroD was found to be a critical regulator of the neuron versus glial cell fate decision. Retinal explants derived from NeuroD-null mice demonstrated a three- to fourfold increase in Muller glia. Forced expression of neuroD in progenitors in rat using retroviruses hastened cell cycle withdrawal and blocked gliogenesis in vivo. (2) NeuroD appeared to regulate interneuron development, favouring amacrine over bipolar differentiation. Forced NeuroD expression resulted in an increase in amacrine interneurons and a decrease in bipolar interneurons. In the complementary experiment, retinae derived from NeuroD-null mice demonstrated a twofold increase in bipolar interneurons and a delay in amacrine differentiation. (3) NeuroD appeared to be essential for the survival of a subset of rod photoreceptors. In conclusion, these results implicate NeuroD in a variety of developmental functions including cell fate determination, differentiation and neuron survival.

XBF-1, a winged helix transcription factor with dual activity, has a role in positioning neurogenesis in Xenopus competent ectoderm

Neuronal differentiation in the vertebrate nervous system is temporally and spatially controlled by mechanisms which are largely unknown. Here we investigate the role of XBF-1, an anterior neural plate-specific winged helix transcription factor, in controlling the pattern of neurogenesis in Xenopus ectoderm. We show that, in the anterior neural plate of normal embryos, prospective neurogenesis is positioned at the anterior boundary of the XBF-1 expression domain. By misexpressing XBF-1 in the posterior neural plate we show that a high dose of XBF-1 has a dual effect; it suppresses endogenous neuronal differentiation in high expressing cells and induces ectopic neuronal differentiation in adjacent cells. In contrast, a low dose of XBF-1 does not suppress but instead, expands the domain of neuronal differentiation in the lateral and ventral sides of the embryo. XBF-1 regulates the expression of XSox3, X-ngnr-1, X-Myt-1 and X-Δ-1 suggesting that it acts early in the cascade leading to neuronal differentiation. A fusion of XBF-1 to a strong repressor domain (EnR) mimics most of the XBF-1 effects suggesting that the wild type XBF-1 is a transcriptional repressor. However, fusion of XBF-1 to a strong activation domain (E1A) specifically suppresses neuronal differentiation suggesting that XBF-1 may also work as a transcriptional activator. Based on these findings, we propose that XBF-1 is involved in positioning neuronal differentiation by virtue of its concentration dependent, dual activity, as a suppressor and an activator of neurogenesis.

Properties of ectopic neurons induced by Xenopus neurogenin1 misexpression

We have examined cells cultured from ectoderm-misexpressing Neurogenin1 (Ngn1) to describe better the extent to which this gene can control aspects of neuronal phenotype including motility, morphology, excitability, and synaptic properties. Like primary spinal neurons which normally express Ngn1, cells in Ngn1-misexpressing cultures exhibit a motility-correlated behavior called circus movements prior to neuritogenesis. Misexpression of NeuroD also causes circus movements and later neuronal differentiation. GSK3beta, which inhibits NeuroD function in vivo, blocks both Ngn1-induced and NeuroD-induced neuronal differentiation, while Notch signaling inhibits only Ngn1-induced neuronal differentiation, confirming that NeuroD is downstream of Ngn1 and insensitive to Notch inhibition. While interfering with NeuroD function in ventral ectoderm inhibits both circus movements and neuronal differentiation, such inhibition in the neural plate inhibits only neuronal differentiation, suggesting that additional factors regulate circus movements in the neural ectoderm. Ngn1-misexpressing cells extend N-tubulin-positive neurites and exhibit tetrodotoxin-sensitive action potentials. Unlike the majority of cultured spinal neurons, however, Ngn1-misexpressing cells do not respond to glutamate and do not form functional synapses with myocytes, suggesting that these cells are either like Rohon-Beard sensory neurons or are not fully differentiated.

A simple molecular model of neurulation

A molecular model for the morphogenesis of the central nervous system is built and solved by computer. The formalism rests on molecular-biological data gathered from insects and vertebrates during neural differentiation and neuronal fate specification. Two genetic, hierarchically organized switches are introduced, one associated with f1p4al tissue formation, and the other with neuronal specification. The model switches evolve in time, setting up very similar "prepatterns" of genetic activity in both insects and vertebrates, as observed experimentally. We introduce the hypothesis that cell adhesion and motion are regulated by the switches. If cell motion is turned on by the neural switch, the whole neural tissue (neural plate) thickens, buckles, and folds, ultimately creating a closed neural tube (primary neurulation). When mitoses are more frequent in neural plate tissue, ingression of a neural cell mass takes place instead (secondary neurulation). If cell motions are controlled by the neuronal switch, rather than by the neural one, the differentiation of isolated neuroblasts is observed, which delaminate individually (as in insect neural cord formation). The model thus displays the three major known patterns of neurogenesis; the transition between the vertebrate and insect cases is predicted to result from changes in genetic regulation downstream of the switch genes, and affecting cell adhesion and motility properties. Little is known experimentally about the concerned pathways: their importance as a fruitful area for future investigation is emphasized by our theoretical results.

Geminin, a neuralizing molecule that demarcates the future neural plate at the onset of gastrulation

In an expression cloning screen in Xenopus embryos, we identified a gene that when overexpressed expanded the neural plate at the expense of adjacent neural crest and epidermis. This gene, which we named geminin, had no sequence similarity to known gene families. We later discovered that geminin's neuralizing domain was part of a bifunctional protein whose C-terminal coiled-coil domain may play a role in regulating DNA replication. We report here on the neuralizing function of geminin. The localization, effect of misexpression and activity of a dominant negative geminin suggest that the product of this gene has an essential early role in specifying neural cell fate in vertebrates. Maternal geminin mRNA is found throughout the animal hemisphere from oocyte through late blastula. At the early gastrula, however, expression is restricted to a dorsal ectodermal territory that prefigures the neural plate. Misexpression of geminin in gastrula ectoderm suppresses BMP4 expression and converts prospective epidermis into neural tissue. In ectodermal explants, geminin induces expression of the early proneural gene neurogenin-related 1 although not itself being induced by that gene. Later, embryos expressing geminin have an expanded dorsal neural territory and ventral ectoderm is converted to neurons. A putative dominant negative geminin lacking the neuralizing domain suppresses neural differentiation and, when misexpressed dorsally, produces islands of epidermal gene expression within the neurectodermal territory, effects that are rescued by coexpression of the full-length molecule. Taken together, these data indicate that geminin plays an early role in establishing a neural domain during gastrulation.

Neural induction in embryos

Neural differentiation of the ectoderm is inhibited by bone morphogenetic protein 4 (BMP-4) in amphibia as well as mammalia. This inhibition is released by neural inducing factor(s), which are secreted from the dorsal mesoderm. Masked neuralizing factor(s) are already present in the ectoderm before induction. In homogenates from Xenopus oocytes and embryos neural inducing factors were found in the supernatant (centrifuged at 105000 g), in small vesicles and a ribonucleoprotein fraction. A neuralizing factor, which is a protein of small size, has been partially purified from Xenopus gastrulae. Genes that are expressed in the dorsal mesoderm and involved in the de novo synthesis of neuralizing factor(s) have been cloned. The differentiation of cells with a neuronal fate starts in the neural plate immediately after neural induction. Genes homologous to the Notch and Delta genes of lateral inhibition in insects are involved in this process.

The genetic sequence of retinal development in the ciliary margin of the Xenopus eye

The ciliary marginal zone is a perpetually self-renewing proliferative neuroepithelium at the perimeter of the retina in amphibians and fish. In the ciliary marginal zone (CMZ), cells are spatially ordered with respect to cellular development, deep stem cells being most peripheral and differentiating retinal progenitors being most central. This spatial gradient in the CMZ recapitulates embryonic retinogenesis and provides a powerful system to examine the relative order of gene expression during this process. A number of neurogenic and proneural genes have been described to have interacting roles in the development of the vertebrate nervous system, and so it is of major importance to put these genes in a hierarchical pathway. In no other system yet described are the developmental stages of neurogenesis arrayed so clearly in a spatial pattern as in the CMZ. We have therefore taken advantage of this system, using double in situ hybridizations on cross sections of the CMZ, to compare the spatial patterns of 15 proneural, neurogenic, and other genes involved in early and late phases of retinal development. In addition, we have positioned these expression patterns with respect to cell division. What emerges from this work is a spatial ordering of gene expression that predicts a genetic hierarchy governing vertebrate retinogenesis. By injecting messenger RNA for some of these genes into blastomeres of the Xenopus embryo and examining the effects on expression of the putative downstream genes, we have been able to corroborate some of the relationships between genes predicted to act sequentially.

Mammalian BarH homologue is a potential regulator of neural bHLH genes

Vertebrate neurogenesis involves sequential actions of transcription factors. neurogenins, encoding Atonal-related bHLH transcription factors, function as neuronal determination genes in Xenopus. neurogenins and antother bHLH factor gene, Mash1, are expressed in distinct subsets or areas of cells giving rise to neurons, suggesting that these genes play important roles to generate distinct populations of neurons. A mammalian homologue of BarH (MBH1) is expressed in a complementary pattern to Mash1 expression in the developing nervous system like neurogenins. Forced expression of MBH1 down-regulates expression of Mash1 and up-regulates neurogenin2/Math4A, a member of neurogenins, in P19 cells during neuronal differentiation. This suggests that MBH1 is a potential regulator of mammalian neural bHLH genes, thereby establishing distinct pathways of neuronal differentiation.

Gli/Zic factors pattern the neural plate by defining domains of cell differentiation

Three cell types differentiate in the early frog neural plate: neural crest at the lateral edges, floorplate at the midline and primary neurons in three bilateral stripes. Floorplate cells and ventral neurons are induced by Sonic hedgehog (Shh) and neural crest and dorsal neurons are induced by epidermal factors such as bone morphogenetic proteins (BMPs). Neurogenesis in a subset of cells within the stripes involves lateral inhibition. However, the process by which pools of precursors are defined in stereotypic domains in response to inductive signals is unknown. Here we show that frog Zic2 encodes a zinc-finger transcription factor of the Gli superfamily which is expressed in stripes that alternate with those in which primary neurons differentiate and overlap the domains of floorplate and neural crest progenitors. Zic2 inhibits neurogenesis and induces neural crest differentiation. Conversely, Gli proteins are widely expressed, induce neurogenesis and inhibit neural crest differentiation. Zic2 is therefore a vertebrate pre-pattern gene, encoding anti-neurogenic and crest-inducing functions that counteract the neurogenic but not the floorplate-inducing activity of Gli proteins. We propose that the combined function of Gli/Zic genes responds to inductive signals and induces patterned neural cell differentiation.

A role for SOX1 in neural determination

In vertebrates, the delineation of the neural plate from a region of the primitive ectoderm is accompanied by the onset of specific gene expression which in turn promotes the formation of the nervous system. Here we show that SOX1, an HMG-box protein related to SRY, is one of the earliest transcription factors to be expressed in ectodermal cells committed to the neural fate: the onset of expression of SOX1 appears to coincide with the induction of neural ectoderm. We demonstrate a role for SOX1 in neural determination and differentiation using an inducible expression P19 cell system as an in vitro model of neurogenesis. Misexpression of SOX1 can substitute for the requirement of retinoic acid to impart neural fate to competent ectodermal P19 cells. Using a series of antigenic markers which identify early neural cell types in combination with BrdU labeling, we demonstrate a temporal and spatial correlation between the differentiation of cell types along the dorsoventral axis of the neural tube and the downregulation of SOX1 expression. SOX1, therefore, defines the dividing neural precursors of the embryonic central nervous system (CNS).

Multiple elements regulate Mash1 expression in the developing CNS

Mash1, a transcription factor of the basic helix-loop-helix class, is expressed during embryogenesis in restricted regions of the nervous system. An essential role for Mash1 in neural development was demonstrated previously in mice carrying a targeted disruption of the Mash1 gene. Regulation of the precise temporal and spatial expression of Mash1 is thus likely to be important for proper neural development. In this study, sequences that regulate Mash1 expression in the central nervous system were characterized by assaying the expression of lacZ reporter genes in transgenic embryos. A 1158-bp enhancer localized approximately 7 kb upstream of the Mash1 coding region was identified. Deletions within this enhancer region reveal the presence of both positive and negative cis-acting elements. Analysis of multiple sequences within the enhancer demonstrate that different elements preferentially function in different regions within the Mash1-specific CNS expression domain. In addition, a role for sequences 3' of the Mash1 coding region is revealed, providing evidence for posttranscriptional control of Mash1 expression in multiple CNS domains.

XCoe2, a transcription factor of the Col/Olf-1/EBF family involved in the specification of primary neurons in Xenopus

BACKGROUND

Primary neurogenesis in Xenopus is a model for studying the control of neural cell fate decisions. The specification of primary neurons appears to be driven by transcription factors containing a basic region and a helix-loop-helix (HLH) motif: expression of Xenopus neurogenin-related-1 (X-ngnr-1) defines the three prospective domains of primary neurogenesis, and expression of XNeuroD coincides with neuronal differentiation. The transition between neuronal competence and stable commitment to a neuronal fate remains poorly characterised, however.

RESULTS

Drosophila Collier and rodent early B-cell factor/olfactory-1 define a family of HLH transcription factors containing a previously unknown type of DNA-binding domain. We isolated an orthologous gene from Xenopus, Xcoe2, which is expressed in precursors of primary neurons. Xcoe2 is transcribed after X-ngnr-1 and before XNeuroD. Overexpression of a dominant-negative mutant of XCoe2 prevented neuronal differentiation. Conversely, overexpressed wild-type Xcoe2 could promote ectopic differentiation of neurons, in both the neural plate and the epidermis. In contrast to studies with X-ngnr-1 or XNeuroD, the supernumerary neurons induced by Xcoe2 appeared in a 'salt-and-pepper' pattern, resulting from the activation of X-Delta1 expression and feedback regulation by lateral inhibition.

CONCLUSIONS

XCoe2 may play a pivotal role in the transcriptional cascade that specifies primary neurons in Xenopus embryos: by maintaining Delta-Notch signalling, XCoe2 stabilises the higher neural potential of selected progenitor cells that express X-ngnr-1, ensuring the transition between neural competence and irreversible commitment to a neural fate; and it promotes neuronal differentiation by activating XNeuroD expression, directly or indirectly.

bHLH transcription factors and mammalian neuronal differentiation

The basic helix-loop-helix (bHLH) factor Mash1 is expressed in the developing nervous system. Null mutation of Mash1 results in loss of olfactory and autonomic neurons and delays differentiation of retinal neurons, indicating that Mash1 promotes neuronal differentiation. Other bHLH genes, Math/NeuroD/Neurogenin, all expressed in the developing nervous system, have also been suggested to promote neuronal differentiation. In contrast, another bHLH factor, HES1, which is expressed by neural precursor cells but not by neurons, represses Mash1 expression and antagonizes Mash1 activity in a dominant negative manner. Forced expression of HES1 in precursor cells blocks neuronal differentiation in the brain and retina, indicating that HES1 is a negative regulator of neuronal differentiation. Conversely, null mutation of HES1 up-regulates Mash1 expression, accelerates neuronal differentiation, and causes severe defects of the brain and eyes. Thus, HES1 regulates brain and eye morphogenesis by inhibiting premature neuronal differentiation, and the down-regulation of HES1 expression at the right time is required for normal development of the nervous system. Interestingly, HES1 can repress its own expression by binding to its promoter, suggesting that negative autoregulation may contribute to down-regulation of HES1 expression during neural development. Recent studies indicate that HES1 expression is also controlled by RBP-J, a mammalian homologue of Suppressor of Hairless [Su(H)], and Notch, a key membrane protein that may regulate lateral specification through RBP-J during neural development. Thus, the Notch-->RBP-J-->HES1-Mash1 pathway may play a critical role in neuronal differentiation.

Identification of the vertebrate Iroquois homeobox gene family with overlapping expression during early development of the nervous system

In Drosophila the decision processes between the neural and epidermal fate for equipotent ectodermal cells depend on the activity of proneural genes. Members of the Drosophila Iroquois-Complex (Iro-C) positively regulate the activity of certain proneural AS-C genes during the formation of external sensory organs. We have identified and characterized three mouse Iroquois-related genes: Irx1, -2 and -3, which have a homeodomain very similar to that of the Drosophila Iro-C genes. The sequence similarity implies that these three genes represent a separate homeobox family. All three genes are expressed with distinct spatio/temporal patterns during early mouse embryogenesis. These patterns implicate them in a number of embryonic developmental processes: the A/P and D/V patterning of specific regions of the central nervous system (CNS), and regionalization of the otic vesicle, branchial epithelium and limbs.

Xath5 participates in a network of bHLH genes in the developing Xenopus retina

We examined the function of basic-helix-loop-helix (bHLH) transcription factors during retinal neurogenesis. We identified Xath5, a Xenopus bHLH gene related to Drosophila atonal, which is expressed in the developing Xenopus retina. Targeted expression of Xath5 in retinal progenitor cells biased the differentiation of these cells toward a ganglion cell fate, suggesting that Xath5 can regulate the differentiation of retinal neurons. We examined the relationship between the three bHLH genes Xash3, NeuroD, and Xath5 during retinal neurogenesis and found that Xash3 is expressed in early retinoblasts, followed by coexpression of Xath5 and NeuroD in differentiating cells. We provide evidence that the expression of Xash3, NeuroD, and Xath5 is coupled and propose that these bHLH genes regulate successive stages of neuronal differentiation in the developing retina.

Xenopus Zic3, a primary regulator both in neural and neural crest development

Xenopus Zic3 is a Xenopus homologue of mouse Zic and Drosophila pair-rule gene, odd-paired. We show here that Zic3 has significant roles both in neural and neural crest development in Xenopus embryo. Expression of Zic3 is first detected in prospective neural plate region at gastrulation. Onset of the expression was earlier than most proneural genes and followed chordin expression. The expression was induced by blockade of BMP4 signal. Overexpression of Zic3 resulted in hyperplastic neural and neural crest derived tissue. In animal cap explant, the overexpression of Zic3 induced expression of all the proneural genes and neural crest marker genes. These findings suggest that Zic3 can determine the ectodermal cell fate and promote the earliest step of neural and neural crest development.