Requirement of Sox2-mediated signaling for differentiation of early Xenopus neuroectoderm

Identifying targets of the Sox domain protein Dichaete in the Drosophila CNS via targeted expression of dominant negative proteins

BACKGROUND

Group B Sox domain transcription factors play important roles in metazoan central nervous system development. They are, however, difficult to study as mutations often have pleiotropic effects and other Sox family members can mask phenotypes due to functional compensation. In Drosophila melanogaster, the Sox gene Dichaete is dynamically expressed in the embryonic CNS, where it is known to have functional roles in neuroblasts and the ventral midline. In this study, we use inducible dominant negative proteins in combination with ChIP, immunohistochemistry and genome-wide expression profiling to further dissect the role of Dichaete in these two tissues.

RESULTS

We generated two dominant negative Dichaete constructs, one lacking a DNA binding domain and the other fused to the Engrailed transcriptional repressor domain. We expressed these tissue-specifically in the midline and in neuroblasts using the UAS/GAL4 system, validating their use at the phenotypic level and with known target genes. Using ChIP and immunohistochemistry, we identified two new likely direct Dichaete target genes, commisureless in the midline and asense in the neuroectoderm. We performed genome-wide expression profiling in stage 8-9 embryos, identifying almost a thousand potential tissue-specific Dichaete targets, with half of these genes showing evidence of Dichaete binding in vivo. These include a number of genes with known roles in CNS development, including several components of the Notch, Wnt and EGFR signalling pathways.

CONCLUSIONS

As well as identifying commisureless as a target, our data indicate that Dichaete helps establish its expression during early midline development but has less effect on its established later expression, highlighting Dichaete action on tissue specific enhancers. An analysis of the broader range of candidate Dichaete targets indicates that Dichaete plays diverse roles in CNS development, with the 500 or so Dichaete-bound putative targets including a number of transcription factors, signalling pathway components and terminal differentiation genes. In the early neurectoderm we implicate Dichaete in the lateral inhibition pathway and show that Dichaete acts to repress the proneural gene asense. Our analysis also reveals that dominant negatives cause off-target effects, highlighting the need to use other experimental data for validating findings from dominant negative studies.

Early neural crest induction requires an initial inhibition of Wnt signals

Neural crest (NC) induction is a long process that continues through gastrula and neurula stages. In order to reveal additional stages of NC induction we performed a series of explants where different known inducing tissues were taken along with the prospective NC. Interestingly the dorso-lateral marginal zone (DLMZ) is only able to promote the expression of a subset of neural plate border (NPB) makers without the presence of specific NC markers. We then analysed the temporal requirement for BMP and Wnt signals for the NPB genes Hairy2a and Dlx5, compared to the expression of neural plate (NP) and NC genes. Although the NP is sensitive to BMP levels at early gastrula stages, Hairy2a/Dlx5 expression is unaffected. Later, the NP becomes insensitive to BMP levels at late gastrulation when NC markers require an inhibition. The NP requires an inhibition of Wnt signals prior to gastrulation, but becomes insensitive during early gastrula stages when Hairy2a/Dlx5 requires an inhibition of Wnt signalling. An increase in Wnt signalling is then important for the switch from NPB to NC at late gastrula stages. In addition to revealing an additional distinct signalling event in NC induction, this work emphasizes the importance of integrating both timing and levels of signalling activity during the patterning of complex tissues such as the vertebrate ectoderm.

Spinal cord regeneration in Xenopus tadpoles proceeds through activation of Sox2-positive cells

BACKGROUND

In contrast to mammals, amphibians, such as adult urodeles (for example, newts) and anuran larvae (for example, Xenopus) can regenerate their spinal cord after injury. However, the cellular and molecular mechanisms involved in this process are still poorly understood.

RESULTS

Here, we report that tail amputation results in a global increase of Sox2 levels and proliferation of Sox2(+) cells. Overexpression of a dominant negative form of Sox2 diminished proliferation of spinal cord resident cells affecting tail regeneration after amputation, suggesting that spinal cord regeneration is crucial for the whole process. After spinal cord transection, Sox2(+) cells are found in the ablation gap forming aggregates. Furthermore, Sox2 levels correlated with regenerative capabilities during metamorphosis, observing a decrease in Sox2 levels at non-regenerative stages.

CONCLUSIONS

Sox2(+) cells contribute to the regeneration of spinal cord after tail amputation and transection. Sox2 levels decreases during metamorphosis concomitantly with the lost of regenerative capabilities. Our results lead to a working hypothesis in which spinal cord damage activates proliferation and/or migration of Sox2(+) cells, thus allowing regeneration of the spinal cord after tail amputation or reconstitution of the ependymal epithelium after spinal cord transection.

Specific domains of FoxD4/5 activate and repress neural transcription factor genes to control the progression of immature neural ectoderm to differentiating neural plate

FoxD4/5, a forkhead transcription factor, plays a critical role in establishing and maintaining the embryonic neural ectoderm. It both up-regulates genes that maintain a proliferative, immature neural ectoderm and down-regulates genes that promote the transition to a differentiating neural plate. We constructed deletion and mutant versions of FoxD4/5 to determine which domains are functionally responsible for these opposite activities, which regulate the critical developmental transition of neural precursors to neural progenitors to differentiating neural plate cells. Our results show that up-regulation of genes that maintain immature neural precursors (gem, zic2) requires the Acidic blob (AB) region in the N-terminal portion of the protein, indicating that the AB is the transactivating domain. Additionally, down-regulation of those genes that promote the transition to neural progenitors (sox) and those that lead to neural differentiation (zic, irx) involves: 1) an interaction with the Groucho co-repressor at the Eh-1 motif in the C-terminus; and 2) sequence downstream of this motif. Finally, the ability of FoxD4/5 to induce the ectopic expression of neural precursor genes in the ventral ectoderm also involves both the AB region and the Eh-1 motif; FoxD4/5 accomplishes ectopic neural induction by both activating neural precursor genes and repressing BMP signaling and epidermal genes. This study identifies the specific, conserved domains of the FoxD4/5 protein that allow this single transcription factor to regulate a network of genes that controls the transition of a proliferative neural ectodermal population to a committed neural plate population poised to begin differentiation.

Splice blocking of zygotic sox31 leads to developmental arrest shortly after Mid-Blastula Transition and induces apoptosis in zebrafish

Here we report that splice blocking morpholinos (Sb MO) against zebrafish sox31 elicit developmental arrest, likely through creating a series of dominant negative splicing variants. Embryos injected with the Sb MO develop normally before the Mid-Blastula Transition (MBT); however, they do not initiate epiboly. Microarray analysis of mRNAs collected at the dome stage revealed that the Sb MO impairs activation of a large set of zygotic genes and reduces degradation of maternal mRNA during MBT. Furthermore, an apoptotic response occurs in Sb morphants at about 6hpf. SoxB1 family genes including sox31 thus play an essential role for early embryos traversing the transitional stage.

The response of early neural genes to FGF signaling or inhibition of BMP indicate the absence of a conserved neural induction module

BACKGROUND

The molecular mechanism that initiates the formation of the vertebrate central nervous system has long been debated. Studies in Xenopus and mouse demonstrate that inhibition of BMP signaling is sufficient to induce neural tissue in explants or ES cells respectively, whereas studies in chick argue that instructive FGF signaling is also required for the expression of neural genes. Although additional signals may be involved in neural induction and patterning, here we focus on the roles of BMP inhibition and FGF8a.

RESULTS

To address the question of necessity and sufficiency of BMP inhibition and FGF signaling, we compared the temporal expression of the five earliest genes expressed in the neuroectoderm and determined their requirements for induction at the onset of neural plate formation in Xenopus. Our results demonstrate that the onset and peak of expression of the genes vary and that they have different regulatory requirements and are therefore unlikely to share a conserved neural induction regulatory module. Even though all require inhibition of BMP for expression, some also require FGF signaling; expression of the early-onset pan-neural genes sox2 and foxd5α requires FGF signaling while other early genes, sox3, geminin and zicr1 are induced by BMP inhibition alone.

CONCLUSIONS

We demonstrate that BMP inhibition and FGF signaling induce neural genes independently of each other. Together our data indicate that although the spatiotemporal expression patterns of early neural genes are similar, the mechanisms involved in their expression are distinct and there are different signaling requirements for the expression of each gene.

Reciprocal repression between Sox3 and snail transcription factors defines embryonic territories at gastrulation

In developing amniote embryos, the first epithelial-to-mesenchymal transition (EMT) occurs at gastrulation, when a subset of epiblast cells moves to the primitive streak and undergoes EMT to internalize and generate the mesoderm and the endoderm. We show that in the chick embryo this decision to internalize is mediated by reciprocal transcriptional repression of Snail2 and Sox3 factors. We also show that the relationship between Sox3 and Snail is conserved in the mouse embryo and in human cancer cells. In the embryo, Snail-expressing cells ingress at the primitive streak, whereas Sox3-positive cells, which are unable to ingress, ensure the formation of ectodermal derivatives. Thus, the subdivision of the early embryo into the two main territories, ectodermal and mesendodermal, is regulated by changes in cell behavior mediated by the antagonistic relationship between Sox3 and Snail transcription factors.

Comparative cytogenetic mapping of Sox2 and Sox14 in cichlid fishes and inferences on the genomic organization of both genes in vertebrates

To better understand the genomic organization and evolution of Sox genes in vertebrates, we cytogenetically mapped Sox2 and Sox14 genes in cichlid fishes and performed comparative analyses of their orthologs in several vertebrate species. The genomic regions neighboring Sox2 and Sox14 have been conserved during vertebrate diversification. Although cichlids seem to have undergone high rates of genomic rearrangements, Sox2 and Sox14 are linked in the same chromosome in the Etroplinae Etroplus maculatus that represents the sister group of all remaining cichlids. However, these genes are located on different chromosomes in several species of the sister group Pseudocrenilabrinae. Similarly, the ancestral synteny of Sox2 and Sox14 has been maintained in several vertebrates, but this synteny has been broken independently in all major groups as a consequence of karyotype rearrangements that took place during the vertebrate evolution.

Interaction of Sox1, Sox2, Sox3 and Oct4 during primary neurogenesis

Sox1, Sox2 and Sox3, the three members of the SoxB1 subgroup of transcription factors, have similar sequences, expression patterns and overexpression phenotypes. Thus, it has been suggested that they have redundant roles in the maintenance of neural stem cells in development. However, the long-term effect of overexpression or their function in combination with their putative co-factor Oct4 has not been tested. Here, we show that overexpression of sox1, sox2, sox3 or oct91, the Xenopus homologue of Oct4, results in the same phenotype: an expanded neural plate at the expense of epidermis and delayed neurogenesis. However, each of these proteins induced a unique profile of neural markers and the combination of Oct91 with each SoxB1 protein had different effects, as did continuous misexpression of the proteins. Overexpression studies indicate that Oct91 preferentially cooperates with Sox2 to maintain neural progenitor marker expression, while knockdown of Oct91 inhibits neural induction driven by either Sox2 or Sox3. Continuous expression of Sox1 and Sox2 in transgenic embryos represses neuron differentiation and inhibits anterior development while increasing cell proliferation. Constitutively active Sox3, however, leads to increased apoptosis suggesting that it functions as a tumor suppressor. While the SoxB1s have overlapping functions, they are not strictly redundant as they induce different sets of genes and are likely to partner with different proteins to maintain progenitor identity.

Combinatorial regulation of optic cup progenitor cell fate by SOX2 and PAX6

In humans, haploinsufficiency of either SOX2 or PAX6 is associated with microphthalmia, anophthalmia or aniridia. In this study, through the genetic spatiotemporal specific ablation of SOX2 on both wild-type and Pax6-haploinsufficent backgrounds in the mouse, we have uncovered a transcriptionally distinct and developmentally transient stage of eye development. We show that genetic ablation of SOX2 in the optic cup results in complete loss of neural competence and eventual cell fate conversion to non-neurogenic ciliary epithelium. This cell fate conversion is associated with a striking increase in PAX6, and genetically ablating SOX2 on a Pax6-haploinsufficient background partially rescues the Sox2-mutant phenotype. Collectively, these results demonstrate that precise regulation of the ratio of SOX2 to PAX6 is necessary to ensure accurate progenitor cell specification, and place SOX2 as a decisive factor of neural competence in the retina.

Prospective identification and isolation of enteric nervous system progenitors using Sox2

The capacity to identify and isolate lineage-specific progenitor cells from developing and mature tissues would enable the development of cell replacement therapies for disease treatment. The enteric nervous system (ENS) regulates important gut functions, including controlling peristaltic muscular contractions, and consists of interconnected ganglia containing neurons and glial cells. Hirschsprung's disease (HSCR), one of the most common and best understood diseases affecting the ENS, is characterized by absence of enteric ganglia from the distal gut due to defects in gut colonization by neural crest progenitor cells and is an excellent candidate for future cell replacement therapies. Our previous microarray experiments identified the neural progenitor and stem cell marker SRY-related homoebox transcription factor 2 (Sox2) as expressed in the embryonic ENS. We now show that Sox2 is expressed in the ENS from embryonic to adult stages and constitutes a novel marker of ENS progenitor cells and their glial cell derivatives. We also show that Sox2 expression overlaps significantly with SOX10, a well-established marker of ENS progenitors and enteric glial cells. We have developed a strategy to select cells expressing Sox2, by using G418 selection on cultured gut cells derived from Sox2(βgeo/+) mouse embryos, thus allowing substantial enrichment and expansion of neomycin-resistant Sox2-expressing cells. Sox2(βgeo) cell cultures are enriched for ENS progenitors. Following transplantation into embryonic mouse gut, Sox2(βgeo) cells migrate, differentiate, and colocalize with the endogenous ENS plexus. Our studies will facilitate development of cell replacement strategies in animal models, critical to develop human cell replacement therapies for HSCR.

Prohibitin1 acts as a neural crest specifier in Xenopus development by repressing the transcription factor E2F1

Prohibitin 1 (phb1), which was initially described as an inhibitor of cell proliferation, is a highly conserved protein found in multiple cellular compartments. In the nucleus it interacts with the transcriptional regulators Rb and E2F1 and controls cell proliferation and apoptosis. Here we unravel an unexpected novel function for phb1 in Xenopus cranial neural crest (CNC) development. Xphb1 is maternally expressed; zygotically expressed neurula stage transcripts accumulate in the CNC and the neural tube. Knockdown of Xphb1 by antisense morpholino injection results in the loss of foxD3, snail2 and twist expression, whereas expression of c-myc, AP-2 and snail1 remains unaffected. Xphb2, its closest relative, cannot substitute for Xphb1, underlining the specificity of Xphb1 function. Epistatic analyses place Xphb1 downstream of c-myc and upstream of foxD3, snail2 and twist. To elucidate which subdomain in Xphb1 is required for neural crest gene regulation we generated deletion mutants and tested their rescue ability in Xphb1 morphants. The E2F1-binding domain was found to be necessary for Xphb1 function in neural crest development. Gain- and loss-of-function experiments reveal that Xphb1 represses E2F1 activity; suppression of E2F1 through Xphb1 is required for twist, snail2 and foxD3 expression in the CNC. With the Xphb1 dependency of a subset of CNC specifiers downstream of c-myc, we have identified a new branching point in the neural crest gene regulatory network.

B1 SOX coordinate cell specification with patterning and morphogenesis in the early zebrafish embryo

The B1 SOX transcription factors SOX1/2/3/19 have been implicated in various processes of early embryogenesis. However, their regulatory functions in stages from the blastula to early neurula remain largely unknown, primarily because loss-of-function studies have not been informative to date. In our present study, we systematically knocked down the B1 sox genes in zebrafish. Only the quadruple knockdown of the four B1 sox genes sox2/3/19a/19b resulted in very severe developmental abnormalities, confirming that the B1 sox genes are functionally redundant. We characterized the sox2/3/19a/19b quadruple knockdown embryos in detail by examining the changes in gene expression through in situ hybridization, RT–PCR, and microarray analyses. Importantly, these phenotypic analyses revealed that the B1 SOX proteins regulate the following distinct processes: (1) early dorsoventral patterning by controlling bmp2b/7; (2) gastrulation movements via the regulation of pcdh18a/18b and wnt11, a non-canonical Wnt ligand gene; (3) neural differentiation by regulating the Hes-class bHLH gene her3 and the proneural-class bHLH genes neurog1 (positively) and ascl1a (negatively), and regional transcription factor genes, e.g., hesx1, zic1, and rx3; and (4) neural patterning by regulating signaling pathway genes, cyp26a1 in RA signaling, oep in Nodal signaling, shh, and mdkb. Chromatin immunoprecipitation analysis of the her3, hesx1, neurog1, pcdh18a, and cyp26a1 genes further suggests a direct regulation of these genes by B1 SOX. We also found an interesting overlap between the early phenotypes of the B1 sox quadruple knockdown embryos and the maternal-zygotic spg embryos that are devoid of pou5f1 activity. These findings indicate that the B1 SOX proteins control a wide range of developmental regulators in the early embryo through partnering in part with Pou5f1 and possibly with other factors, and suggest that the B1 sox functions are central to coordinating cell fate specification with patterning and morphogenetic processes occurring in the early embryo.

In the developing embryo, various processes such as cell fate specification, embryo patterning, and morphogenesis take place concurrently. The embryo must control gene expression in order to coordinate these processes and thereby enable the proper organization of its structures. The B1 sox transcription factor genes, exemplified by the “stem cell gene” sox2, are thought to play a key role in these embryonic processes from the blastoderm stage to the neural stage. However, the precise regulatory functions of these genes are largely unknown due to the lack of loss-of-function studies. In our current study, we took advantage of the zebrafish system and successfully depleted B1 sox activity from the early embryo using antisense knockdown technology. This approach enabled us to further uncover the regulatory functions of B1 sox in early embryos. We found that the activity of the B1 sox genes is required for the expression of a wide range of developmental regulators including transcription factors, signaling pathway components, and cell adhesion molecules. These findings suggest that the B1 sox functions are central to coordinating diverse embryonic processes, particularly those that occur during the development of the primordium of the central nervous system.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Sox2 is required for maintenance and regeneration, but not initial development, of hair cells in the zebrafish inner ear

Sox2 has been variously implicated in maintenance of pluripotent stem cells or, alternatively, early stages of cell differentiation, depending on context. In the developing inner ear, Sox2 initially marks all cells in the nascent sensory epithelium and, in mouse, is required for sensory epithelium formation. Sox2 is eventually downregulated in hair cells but is maintained in support cells, the functional significance of which is unknown. Here we describe regulation and function of sox2 in the zebrafish inner ear. Expression of sox2 begins after the onset of sensory epithelium development and is regulated by Atoh1a/b, Fgf and Notch. Knockdown of sox2 does not prevent hair cell production, but the rate of accumulation is reduced due to sporadic death of differentiated hair cells. We next tested the capacity for hair cell regeneration following laser ablation of mature brn3c:gfp-labeled hair cells. In control embryos, regeneration of lost hair cells begins by 12 h post-ablation and involves transdifferentiation of support cells rather than asymmetric cell division. In contrast, regeneration does not occur in sox2-depleted embryos. These data show that zebrafish sox2 is required for hair cell survival, as well as for transdifferentiation of support cells into hair cells during regeneration.

Role of SoxB1 transcription factors in development

SoxB1 factors, which include Sox1, 2, and 3, share more than 90% amino acid identity in their DNA binding HMG box and participate in diverse developmental events. They are known to exert cell-type-specific functions in concert with other transcription factors on Sox factor-dependent regulatory enhancers. Due to the high degree of sequence similarity both within and outside the HMG box, SoxB1 members show almost identical biological activities. As a result, they exhibit strong functional redundancy in regions where SoxB1 members are coexpressed, such as neural stem/progenitor cells in the developing central nervous system.

Remarkable evolutionary conservation of SOX14 orthologues

SOX proteins constitute a large family of diverse, well-conserved transcription factors present in vertebrates and invertebrates, and also implicated in control of many developmental processes. Our objectives have been to identify Sox14 gene of goat (Capra hircus), cow (Bos taurus) and rat (Rattus norvegicus), and to perform comparative analyses and mapping of SOx14 orthologues from numerous vertebrate species. PCR based approach was used to identify Sox14 of goat, cow and rat, while nucleotide and amino acid sequence alignments and mapping were performed using information currently available in public database. Comparative sequence analysis revealed remarkable identity among Sox14 orthologues and helped us to identify highly conserved motifs that represent molecular signatures of SOX14 protein that might have structural or functional significance. Further, determined chromosomal locations of numerous predicted group B Sox genes and their neighbouring genes using currently available genome database. In conclusion, our study has not only supported the proposed model of group B Sox genes evolution in chicken and mammals, but has also revealed that additional evolutionary events split Sox B genes into different chromosomes in some mammals. Mapping data presented in this study could help in refining the understanding of the evolution of group B Sox genes in vertebrates.

Neural induction and factors that stabilize a neural fate

The neural ectoderm of vertebrates forms when the bone morphogenetic protein (BMP) signaling pathway is suppressed. Herein, we review the molecules that directly antagonize extracellular BMP and the signaling pathways that further contribute to reduce BMP activity in the neural ectoderm. Downstream of neural induction, a large number of "neural fate stabilizing" (NFS) transcription factors are expressed in the presumptive neural ectoderm, developing neural tube and ultimately in neural stem cells. Herein, we review what is known about their activities during normal development to maintain a neural fate and regulate neural differentiation. Further elucidation of how the NFS genes interact to regulate neural specification and differentiation should ultimately prove useful for regulating the expansion and differentiation of neural stem and progenitor cells.

Notch signaling downstream of foxD5 promotes neural ectodermal transcription factors that inhibit neural differentiation

We investigated the role of the Notch signaling pathway in regulating several transcription factors that stabilize a neural fate and expand the neural plate. Increased Notch signaling in a neural lineage via a constitutively activated form (NICD) up-regulated geminin and zic2 in a cell-autonomous manner, and expanded the neural plate domains of sox11, sox2, and sox3. Loss- and gain-of-function assays show that foxD5 acts upstream of notch1 gene expression. Decreasing Notch signaling with an anti-morphic form of a Notch ligand (X-Delta-1(STU)) showed that the foxD5-mediated expansion of the sox gene neural plate domains requires Notch signaling. However, geminin and zic2 appear to be dually regulated by foxD5 and Notch1 signaling. These studies demonstrate that: (1) Notch signaling acts downstream of foxD5 to promote the expression of a subset of neural ectodermal transcription factors; and (2) Notch signaling and the foxD5 transcriptional pathway together maintain the neural plate in an undifferentiated state. Developmental Dynamics 238:1358-1365, 2009. (c) 2009 Wiley-Liss, Inc.

foxD5 plays a critical upstream role in regulating neural ectodermal fate and the onset of neural differentiation

foxD5 is expressed in the nascent neural ectoderm concomitant with several other neural-fate specifying transcription factors. We used loss-of-function and gain-of-function approaches to analyze the functional position of foxD5 amongst these other factors. Loss of FoxD5 reduces the expression of sox2, sox11, soxD, zic1, zic3 and Xiro1-3 at the onset of gastrulation, and of geminin, sox3 and zic2, which are maternally expressed, by late gastrulation. At neural plate stages most of these genes remain reduced, but the domains of zic1 and zic3 are expanded. Increased FoxD5 induces geminin and zic2, weakly represses sox11 at early gastrula but later (st12) induces it; weakly represses sox2 and sox3 transiently and strongly represses soxD, zic1, zic3 and Xiro1-3. The foxD5 effects on zic1, zic3 and Xiro1-3 involve transcriptional repression, whereas those on geminin and zic2 involve transcriptional activation. foxD5's effects on geminin, sox11 and zic2 occur at the onset of gastrulation, whereas the other genes require earlier foxD5 activity. geminin, sox11 and zic2, each of which is up-regulated directly by foxD5, are all required to account for foxD5 phenotypes, indicating that this triad constitutes a transcriptional network rather than linear path that coordinately up-regulates genes that promote an immature neural fate and inhibits genes that promote the onset of neural differentiation. We also show that foxD5 promotes an ectopic neural fate in the epidermis by reducing BMP signaling. Several of the genes that are repressed by foxD5 in turn reduce foxD5 expression, contributing to the medial-lateral patterning of the neural plate.

Xenopus Sox3 activates sox2 and geminin and indirectly represses Xvent2 expression to induce neural progenitor formation at the expense of non-neural ectodermal derivatives

The SRY-related, HMG box SoxB1 transcription factors are highly homologous, evolutionarily conserved proteins that are expressed in neuroepithelial cells throughout neural development. SoxB1 genes are down-regulated as cells exit the cell-cycle to differentiate and are considered functionally redundant in maintaining neural precursor populations. However, little is known about Sox3 function and its mode of action during primary neurogenesis. Using gain and loss-of-function studies, we analyzed Sox3 function in detail in Xenopus early neural development and compared it to that of Sox2. Through these studies we identified the first targets of a SoxB1 protein during primary neurogenesis. Sox3 functions as an activator to induce expression of the early neural genes, sox2 and geminin in the absence of protein synthesis and to indirectly inhibit the Bmp target Xvent2. As a result, Sox3 increases cell proliferation, delays neurogenesis and inhibits epidermal and neural crest formation to expand the neural plate. Our studies indicate that Sox3 and 2 have many similar functions in this process including the ability to activate expression of geminin in naïve ectodermal explants. However, there are some differences; Sox3 activates the expression of sox2, while Sox2 does not activate expression of sox3 and sox3 is uniquely expressed throughout the ectoderm prior to neural induction suggesting a role in neural competence. With morpholino-mediated knockdown of Sox3, we demonstrate that it is required for induction of neural tissue by BMP inhibition. Together these data indicate that Sox3 has multiple roles in early neural development including as a factor required for nogginmediated neural induction.

Inhibition of mouse GPM6A expression leads to decreased differentiation of neurons derived from mouse embryonic stem cells

Glycoprotein M6A (GPM6A) is known as a transmembrane protein and an abundant cell surface protein on neurons in the central nervous system (CNS). However, the function of GPM6A is still unknown in the differentiation of neurons derived from embryonic stem (ES) cells. To investigate the function of GPM6A, we generated knockdown mouse ES cell lines (D3m-shM6A) using a short hairpin RNA (shRNA) expression vector driven by the U6 small nuclear RNA promoter, which can significantly suppress the expression of mouse GPM6A mRNA. Real-time polymerase chain reaction (real-time PCR) and immunocytochemical analysis showed that expression of shRNA against GPM6A markedly reduced the expression of neuroectodermal-associated genes (OTX1, Lmx1b, En1, Pax2, Pax5, Sox1, Sox2, and Wnt1), and also the number of neural stem cells (NSC) derived from D3mshM6A cells compared to control vector-transfected mouse ES cells (D3m-Mock). Moreover, our results show a decrease in both the number of neuronal markers and the number of differentiating neuronal cells (cholinergic, catecholaminergic, and GABAergic neurons) from NSC in D3m-shM6A cells. Hence, our findings suggest that expression level of GPM6A is directly or indirectly associated with the differentiation of neurons derived from undifferentiated ES cells.

Eya1 and Six1 promote neurogenesis in the cranial placodes in a SoxB1-dependent fashion

Genes of the Eya family and of the Six1/2 subfamily are expressed throughout development of vertebrate cranial placodes and are required for their differentiation into ganglia and sense organs. How they regulate placodal neurogenesis, however, remains unclear. Through loss of function studies in Xenopus we show that Eya1 and Six1 are required for neuronal differentiation in all neurogenic placodes. The effects of overexpression of Eya1 or Six1 are dose dependent. At higher levels, Eya1 and Six1 expand the expression of SoxB1 genes (Sox2, Sox3), maintain cells in a proliferative state and block expression of neuronal determination and differentiation genes. At lower levels, Eya1 and Six1 promote neuronal differentiation, acting downstream of and/or parallel to Ngnr1. Our findings suggest that Eya1 and Six1 are required for both the regulation of placodal neuronal progenitor proliferation, through their effects on SoxB1 expression, and subsequent neuronal differentiation.

Molecular links among the causative genes for ocular malformation: Otx2 and Sox2 coregulate Rax expression

The neural-related genes Sox2, Pax6, Otx2, and Rax have been associated with severe ocular malformations such as anophthalmia and microphthalmia, but it remains unclear as to how these genes are linked functionally. We analyzed the upstream signaling of Xenopus Rax (also known as Rx1) and identified the Otx2 and Sox2 proteins as direct upstream regulators of Rax. We revealed that endogenous Otx2 and Sox2 proteins bound to the conserved noncoding sequence (CNS1) located approximately 2 kb upstream of the Rax promoter. This sequence is conserved among vertebrates and is required for potent transcriptional activity. Reporter assays showed that Otx2 and Sox2 synergistically activated transcription via CNS1. Furthermore, the Otx2 and Sox2 proteins physically interacted with each other, and this interaction was affected by the Sox2-missense mutations identified in these ocular disorders. These results demonstrate that the direct interaction and interdependence between the Otx2 and Sox2 proteins coordinate Rax expression in eye development, providing molecular linkages among the genes responsible for ocular malformation.

Vertebrate CASTOR is required for differentiation of cardiac precursor cells at the ventral midline

The CASTOR (CST) transcription factor was initially identified for its role in maintaining stem cell competence in the Drosophila dorsal midline. Here we report that Xenopus CST affects cardiogenesis. In CST-depleted embryos, cardiomyocytes at the ventral midline arrest and are maintained as cardiac progenitors, while cells in more dorsal regions of the heart undergo their normal program of differentiation. Cardia bifida results from failed midline differentiation, even though cardiac cell migration and initial cell fate specification occur normally. Our fate mapping studies reveal that this ventral midline population of cardiomyocytes ultimately gives rise to the outer curvature of the heart; however, CST-depleted midline cells overproliferate and remain a coherent population of nonintegrated cells positioned on the outer wall of the ventricle. These midline-specific requirements for CST suggest the regulation of cardiomyocyte differentiation is regionalized along a dorsal-ventral axis and that this patterning occurs prior to heart tube formation.

Rohon-Beard sensory neurons are induced by BMP4 expressing non-neural ectoderm in Xenopus laevis

Rohon-Beard mechanosensory neurons (RBs), neural crest cells, and neurogenic placodes arise at the border of the neural- and non-neural ectoderm during anamniote vertebrate development. Neural crest cells require BMP expressing non-neural ectoderm for their induction. To determine if epidermal ectoderm-derived BMP signaling is also involved in the induction of RB sensory neurons, the medial region of the neural plate from donor Xenopus laevis embryos was transplanted into the non-neural ventral ectoderm of host embryos at the same developmental stage. The neural plate border and RBs were induced at the transplant sites, as shown by expression of Xblimp1, and XHox11L2 and XN-tubulin, respectively. Transplantation studies between pigmented donors and albino hosts showed that neurons are induced both in donor neural and host epidermal tissue. Because an intermediate level of BMP4 signaling is required to induce neural plate border fates, we directly tested BMP4's ability to induce RBs; beads soaked in either 1 or 10 ng/ml were able to induce RBs in cultured neural plate tissue. Conversely, RBs fail to form when neural plate tissue from embryos with decreased BMP activity, either from injection of noggin or a dominant negative BMP receptor, was transplanted into the non-neural ectoderm of un-manipulated hosts. We conclude that contact between neural and non-neural ectoderm is capable of inducing RBs, that BMP4 can induce RB markers, and that BMP activity is required for induction of ectopic RB sensory neurons.

Regulatory pathways linking progenitor patterning, cell fates and neurogenesis in the ventral neural tube

The assembly of neural circuits in the vertebrate central nervous system depends on the organized generation of specific neuronal subtypes. Studies over recent years have begun to reveal the principles and elucidate some of the detailed mechanisms that underlie these processes. In general, exposure to different types and concentrations of signals directs neural progenitor populations to generate specific subtypes of neurons. These signals function by regulating the expression of intrinsic determinants, notably transcription factors, which specify the fate of cells as they differentiate into neurons. In this review, we illustrate these concepts by focusing on the generation of neurons in ventral regions of the spinal cord, where detailed knowledge of the mechanisms that regulate cell identity has provided insight into the development of a number of neuronal subtypes, including motor neurons. A greater knowledge of the molecular control of neural development is likely to have practical benefits in understanding the causes and consequences of neurological diseases. Moreover, recent studies have demonstrated how an understanding of normal neural development can be applied to direct differentiation of stem cells in vitro to specific neuronal subtypes. This type of rational manipulation of stem cells may represent the first step in the development of treatments based on therapeutic replacement of diseased or damaged nervous tissue.

Sox3 expression is maintained by FGF signaling and restricted to the neural plate by Vent proteins in the Xenopus embryo

The formation of the nervous system is initiated when ectodermal cells adopt the neural fate. Studies in Xenopus demonstrate that inhibition of BMP results in the formation of neural tissue. However, the molecular mechanism driving the expression of early neural genes in response to this inhibition is unknown. Moreover, controversy remains regarding the sufficiency of BMP inhibition for neural induction. To address these questions, we performed a detailed analysis of the regulation of the soxB1 gene, sox3, one of the earliest genes expressed in the neuroectoderm. Using ectodermal explant assays, we analyzed the role of BMP, Wnt and FGF signaling in the regulation of sox3 and the closely related soxB1 gene, sox2. Our results demonstrate that both sox3 and sox2 are induced in response to BMP antagonism, but by distinct mechanisms and that the activation of both genes is independent of FGF signaling. However, both require FGF for the maintenance of their expression. Finally, sox3 genomic elements were identified and characterized and an element required for BMP-mediated repression via Vent proteins was identified through the use of transgenesis and computational analysis. Interestingly, none of the elements required for sox3 expression were identified in the sox2 locus. Together our data indicate that two closely related genes have unique mechanisms of gene regulation at the onset of neural development.

Dynamic Sox5 protein expression during cranial ganglia development

Sox5 is a member of the SoxD group of HMG-box transcription factors that, during the early stages of development, promotes neural crest generation. However, little is known about Sox5 function in neural crest derivatives such as the peripheral sensory nervous system. We have analysed the embryonic expression of Sox5 during chick cranial ganglia development, from the stages of ganglia condensation to those of differentiation. During this period, Sox5 expression is maintained in the crest-derived satellite glial cells in all the cranial ganglia. In contrast, Sox5 is only transiently expressed in a subpopulation of differentiating neurons of both neural crest and placode origin. This detailed analysis provides a good base to dissect the possible role of Sox5 in neural cell fate determination by future functional approaches.

deltaEF1 and SIP1 are differentially expressed and have overlapping activities during Xenopus embryogenesis

The zinc finger/homeo-domain transcription factor (zfh x 1) family in vertebrates consists of two members, deltaEF1 and SIP1. They have been proposed to display antagonistic activities in the interpretation of Smad-dependent TGFbeta signaling during mesoderm formation. We cloned Xenopus deltaEF1 cDNA, analyzed the expression profile of the gene, and compared the inducing and interacting properties of the protein to that of XSIP1. Whereas XSIP1 RNA is selectively expressed in the early developing nervous system, we show that XdeltaEF1 gene transcription is only activated during neurulation and that its expression is restricted to the paraxial mesoderm. From early tail bud stage, XdeltaEF1 and XSIP1 are coexpressed in migratory cranial neural crest, in the retina, and in the neural tube. Overproduction of XdeltaEF1 in RNA-injected embryos, like that of XSIP1, reduced the expression of BMP-dependent genes but only XSIP1 has the ability to induce neural markers. We find that XdeltaEF1 and XSIP1 can both form complexes, although with different efficiency, with Smad3, with the coactivators p300 and pCAF, and with the corepressor CtBP1. Together, these results indicate that deltaEF1 and SIP1 do not function as antagonists during Xenopus early embryogenesis but do display different repression efficiencies and interaction properties.

Neuronal specificity of HSV/sleeping beauty amplicon transduction in utero is driven primarily by tropism and cell type composition

A novel bipartite vector system consisting of the herpes simplex virus (HSV) amplicon and the Sleeping Beauty(SB) transposon was previously shown to efficiently deliver a "transgenon" (integrating transgene) in utero. This vector platform facilitated long-term transgenon expression specifically within neurons and neuronal precursor cells of the rodent brain. However, the mechanism underlying the neurospecificity of the HSV/SB amplicon in the setting of mouse embryogenesis is unknown. We find that embryonic cells expressing the Sox1 "neurocompetence" transcription factor represent the primary targets for HSV amplicon transduction in utero. These cells, which comprise the ependymal and subventricular zones (SVZs), express significant levels of high-mobility-group protein B1 (HMGB1), a co-factor shown to facilitate SB-mediated transposition. Using a conventional, non-integrating amplicon expressing Cre recombinase to "tag" transduced cells embryonically in ROSA26 Cre indicator mice in utero, we found transduced cells were exclusively of the neuronal lineage but that in comparison to HSV/SB-mediated in utero delivery, staining patterns were less widespread and "tagged" neuroprogenitor cells were absent. Our findings demonstrate that in utero HSV/SB amplicon gene transfer is primarily neurospecific owing to viral tropism and target cell populations present embryonically, where multi-potent cells of the developing embryo are supportive of SB-driven transposition.

A Dominant-negative Form of Mouse SOX2 Induces Trophectoderm Differentiation and Progressive Polyploidy in Mouse Embryonic Stem Cells

SOX2 plays an important role in early embryogenesis by cooperating with OCT4 in regulating gene expression in fertilized eggs, yet the precise mechanism through which SOX2 accomplishes this important function remains poorly understood. Here, we describe the identification of two nuclear localization signals (NLS) in SOX2 and the generation of a dominant-negative mutant (Dmu-mSox2) by mutating these two NLS in its high mobility group domain. Characterization of this mutant demonstrated that SOX2 shuttles between the cytoplasm and nucleus using these two NLS. The mutant has lost its ability to interact with OCT4, but remains competent to interact with wild-type SOX2. Functionally, Dmu-mSox2 is inactive and unable to cooperate with OCT4 in transactivating target promoters bearing its binding sites. However, Dmu-mSox2 is able to inhibit the activity of wild-type SOX2 and subsequently suppress the activity of downstream genes such as Oct4 and Nanog. When stably expressed in embryonic stem (ES) cells, Dmu-mSox2 triggered progressive doublings of cell ploidy (>8N), leading to differentiation into the trophectoderm lineage. Knockdown of Sox2 by small interfering RNA also induced trophectoderm differentiation and polyploid formation in mouse ES cells. These results suggest that SOX2 maintains stem cell pluripotency by shuttling between the nucleus and cytoplasm in cooperation with OCT4 to prevent trophectoderm differentiation and polyploid formation in ES cells.

XSip1 neuralizing activity involves the co-repressor CtBP and occurs through BMP dependent and independent mechanisms

The DNA-binding transcription factor Smad-interacting protein-1 (Sip1) (also named Zfhx1b/ZEB2) plays essential roles in vertebrate embryogenesis. In Xenopus, XSip1 is essential at the gastrula stage for neural tissue formation, but the precise molecular mechanisms that underlie this process have not been fully identified yet. Here we show that XSip1 functions as a transcriptional repressor during neural induction. We observed that constitutive activation of BMP signaling prevents neural induction by XSip1 but not the inhibition of several epidermal genes. We provide evidence that XSip1 binds directly to the BMP4 proximal promoter and modulates its activity. Finally, by deletion and mutational analysis, we show that XSip1 possesses multiple repression domains and that CtBPs contribute to its repression activity. Consistent with this, interference with XCtBP function reduced XSip1 neuralizing activity. These results suggest that Sip1 acts in neural tissue formation through direct repression of BMP4 but that BMP-independent mechanisms are involved as well. Our data also provide the first demonstration of the importance of CtBP binding in Sip1 transcriptional activity in vivo.

An NF-kappaB and slug regulatory loop active in early vertebrate mesoderm

Background

In both Drosophila and the mouse, the zinc finger transcription factor Snail is required for mesoderm formation; its vertebrate paralog Slug (Snai2) appears to be required for neural crest formation in the chick and the clawed frog Xenopus laevis. Both Slug and Snail act to induce epithelial to mesenchymal transition (EMT) and to suppress apoptosis.

Methodology & Principle Findings

Morpholino-based loss of function studies indicate that Slug is required for the normal expression of both mesodermal and neural crest markers in X. laevis. Both phenotypes are rescued by injection of RNA encoding the anti-apoptotic protein Bcl-xL; Bcl-xL's effects are dependent upon IκB kinase-mediated activation of the bipartite transcription factor NF-κB. NF-κB, in turn, directly up-regulates levels of Slug and Snail RNAs. Slug indirectly up-regulates levels of RNAs encoding the NF-κB subunit proteins RelA, Rel2, and Rel3, and directly down-regulates levels of the pro-apopotic Caspase-9 RNA.

Conclusions/Significance

These studies reveal a Slug/Snail–NF-κB regulatory circuit, analogous to that present in the early Drosophila embryo, active during mesodermal formation in Xenopus. This is a regulatory interaction of significance both in development and in the course of inflammatory and metastatic disease.

Aging does not alter the number or phenotype of putative stem/progenitor cells in the neurogenic region of the hippocampus

To investigate whether dramatically waned dentate neurogenesis during aging is linked to diminution in neural stem/progenitor cell (NSC) number, we counted cells immunopositive for Sox-2 (a putative marker of NSCs) in the subgranular zone (SGZ) of young, middle-aged and aged F344 rats. The young SGZ comprised approximately 50,000 Sox-2+ cells and this amount did not diminish with aging. Quantity of GFAP+ cells and vimentin+ radial glia also remained stable during aging in this region. Besides, in all age groups, analogous fractions of Sox-2+ cells expressed GFAP (astrocytes/NSCs), NG-2 (oligodendrocyte-progenitors/NSCs), vimentin (radial glia), S-100beta (astrocytes) and doublecortin (new neurons). Nevertheless, analyses of Sox-2+ cells with proliferative markers insinuated an increased quiescence of NSCs with aging. Moreover, the volume of rat-endothelial-cell-antigen-1+ capillaries (vascular-niches) within the SGZ exhibited an age-related decline, resulting in an increased expanse between NSCs and capillaries. Thus, decreased dentate neurogenesis during aging is not attributable to altered number or phenotype of NSCs. Instead, it appears to be an outcome of increased quiescence of NSCs due to changes in NSC milieu.

Xenopus POU factors of subclass V inhibit activin/nodal signaling during gastrulation

Three POU factors of subclass V, Oct-25, Oct-60 and Oct-91 are expressed in Xenopus oocytes and early embryos. We here demonstrate that vegetal overexpression of Oct-25, Oct-60, Oct-91 or mammalian Oct-3/4 suppresses mesendoderm formation in Xenopus embryos. Oct-25 and Oct-60 are shown to inhibit activin/nodal and FGF signaling pathways. Loss of Oct-25 and Oct-60 function results in elevated transcription of mesendodermal marker genes and ectopic formation of endoderm in the equatorial region of gastrula stage embryos. Within the ectoderm, Oct-25 promotes neural fate by upregulating neuroectodermal genes, such as Xsox2, which prevent differentiation of neural progenitors into neurons. We also show that mouse Oct-3/4 and Xenopus Oct-25 or Oct-60 behave as functional homologues. We conclude that Xenopus Oct proteins are required to control the levels of embryonic signaling pathways, thereby ensuring the correct specification of germ layers.

SOX2 is a dose-dependent regulator of retinal neural progenitor competence

Approximately 10% of humans with anophthalmia (absent eye) or severe microphthalmia (small eye) show haploid insufficiency due to mutations in SOX2, a SOXB1-HMG box transcription factor. However, at present, the molecular or cellular mechanisms responsible for these conditions are poorly understood. Here, we directly assessed the requirement for SOX2 during eye development by generating a gene-dosage allelic series of Sox2 mutations in the mouse. The Sox2 mutant mice display a range of eye phenotypes consistent with human syndromes and the severity of these phenotypes directly relates to the levels of SOX2 expression found in progenitor cells of the neural retina. Retinal progenitor cells with conditionally ablated Sox2 lose competence to both proliferate and terminally differentiate. In contrast, in Sox2 hypomorphic/null mice, a reduction of SOX2 expression to <40% of normal causes variable microphthalmia as a result of aberrant neural progenitor differentiation. Furthermore, we provide genetic and molecular evidence that SOX2 activity, in a concentration-dependent manner, plays a key role in the regulation of the NOTCH1 signaling pathway in retinal progenitor cells. Collectively, these results show that precise regulation of SOX2 dosage is critical for temporal and spatial regulation of retinal progenitor cell differentiation and provide a cellular and molecular model for understanding how hypomorphic levels of SOX2 cause retinal defects in humans.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Comparative genomic and expression analysis of group B1soxgenes in zebrafish indicates their diversification during vertebrate evolution

Group B1 Sox genes encode HMG domain transcription factors that play major roles in neural development. We have identified six zebrafish B1 sox genes, which include pan-vertebrate sox1a/b, sox2, and sox3, and also fish-specific sox19a/b. SOX19A/B proteins show a transcriptional activation potential that is similar to other B1 SOX proteins. The expression of sox19a and sox3 begins at approximately the 1,000-cell stage during embryogenesis and becomes confined to the future ectoderm by the shield stage. This is reminiscent of the epiblastic expression of Sox2 and/or Sox3 in amniotes. As development progresses, these six B1 sox genes display unique expression patterns that overlap distinctly from one region to another. sox19a expression is widespread in the early neuroectoderm, resembling pan-neural Sox2 expression in amniotes, whereas zebrafish sox2 shows anterior-restricted expression. Comparative genomics suggests that sox19a/b and mammalian Sox15 (group G) have an orthologous relationship and that the B1/G Sox genes arose from a common ancestral gene through two rounds of genome duplication. It seems likely, therefore, that each B1/G Sox gene has gained a distinct expression profile and function during vertebrate evolution.

Role of SOX2 mutations in human hippocampal malformations and epilepsy

PURPOSE

Seizures are noted in a significant proportion of cases of de novo, heterozygous, loss-of-function mutations in SOX2, ascertained because of severe bilateral eye malformations. We wished to determine the underlying cerebral phenotype in SOX2 mutation and to test the candidacy of SOX2 as a gene contributing to human epilepsies.

METHODS

We examined high-resolution MRI scans in four patients with SOX2 mutations, two of whom had seizures. We determined the Sox2 expression pattern in developing murine brain. We searched for SOX2 mutation in 24 patients with typical hippocampal sclerosis and for common variations in SOX2 in 655 patients without eye disease but with epilepsy, including 91 patients with febrile seizures, 93 with hippocampal sclerosis, and 258 with temporal lobe epilepsy.

RESULTS

Striking hippocampal and parahippocampal malformations were seen in all cases, with a history of febrile seizures or epilepsy in two of four cases. The Sox2 expression pattern in developing mouse brain supports the pattern of malformations observed. Mutation screening in patients with epilepsy did not reveal any abnormalities in SOX2. No associations were found between any clinical epilepsy phenotype and common variation in SOX2.

CONCLUSIONS

SOX2 haploinsufficiency causes mesial temporal malformation in humans, making SOX2 dysfunction a candidate mechanism for mesial temporal abnormalities associated with chronic epilepsy. However, although mutation of SOX2 in humans causes hippocampal malformation, SOX2 mutation or variation is unlikely to contribute commonly to mesial temporal lobe epilepsy or its structural (hippocampal sclerosis) or historic (febrile seizures) associations in humans.

Noggin signaling fromXenopus animal blastomere lineages promotes a neural fate in neighboring vegetal blastomere lineages

In Xenopus, localized factors begin to regionalize embryonic fates prior to the inductive interactions that occur during gastrulation. We previously reported that an animal-to-vegetal signal that occurs prior to gastrulation promotes primary spinal neuron fate in vegetal equatorial (C-tier) blastomere lineages. Herein we demonstrate that maternal mRNA encoding noggin is enriched in animal tiers and at low concentrations in the C-tier, suggesting that the neural fates of C-tier blastomeres may be responsive to early signaling from their neighboring cells. In support of this hypothesis, experimental alteration of the levels of Noggin from animal equatorial (B-tier) or BMP4 from vegetal (D-tier) blastomeres significantly affects the numbers of primary spinal neurons derived from their neighboring C-tier blastomeres. These effects are duplicated in blastomere explants isolated at cleavage stages and cultured in the absence of gastrulation interactions. Co-culture with animal blastomeres enhanced the expression of zygotic neural markers in C-tier blastomere explants, whereas co-culture with vegetal blastomeres repressed them. The expression of these markers in C-tier explants was promoted when Noggin was transiently added to the culture during cleavage/morula stages, and repressed with the transient addition of BMP4. Reduction of Noggin translation in B-tier blastomeres by antisense morpholino oligonucleotides significantly reduced the efficacy of neural marker induction in C-tier explants. These experiments indicate that early anti-BMP signaling from the animal hemisphere recruits vegetal equatorial cells into the neural precursor pool prior to interactions that occur during gastrulation.

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.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).