Sox10 mutation disrupts neural crest development in Dom Hirschsprung mouse model

Molecular mechanism for distinct neurological phenotypes conveyed by allelic truncating mutations

The molecular mechanisms by which different mutations in the same gene can result in distinct disease phenotypes remain largely unknown. Truncating mutations of SOX10 cause either a complex neurocristopathy designated PCWH or a more restricted phenotype known as Waardenburg-Shah syndrome (WS4; OMIM 277580). Here we report that although all nonsense and frameshift mutations that cause premature termination of translation generate truncated SOX10 proteins with potent dominant-negative activity, the more severe disease phenotype, PCWH, is realized only when the mutant mRNAs escape the nonsense-mediated decay (NMD) pathway. We observe similar results for truncating mutations of MPZ that convey distinct myelinopathies. Our experiments show that triggering NMD and escaping NMD may cause distinct neurological phenotypes.

Multiple roles of Sox2, an HMG-box transcription factor in avian neural crest development

Expression of Sox2, which encodes an HMG-box-type transcription factor, is down-regulated in the neural plate when neural crest segregates from dorsal neural tube and remains low during crest cell migration. Sox2 expression is subsequently up-regulated in some crest-derived cells in the developing peripheral nervous system and is later restricted to glial sublineages. Misexpression of Sox2 and mutant forms of Sox2 both in neural plate explants and in embryonic ectoderm reveals that Sox2 inhibits neural crest formation as a transcriptional activator. Similar manipulation of Sox2 function in migratory and postmigratory neural crest-derived cells indicates that Sox2 regulates proliferation and differentiation in developing peripheral nervous system. Developmental Dynamics 229:74-86, 2004.

Neural crest cells: a model for invasive behavior

Neural crest cells are the embryonic precursors of the neurons and glia of the peripheral nervous system, pigment cells, and connective tissue in the face, neck and heart. They are induced near the junction of the neural plate and embryonic ectoderm and undergo an epithelial to mesenchymal transition (EMT). Neural crest cells then display invasive behavior and migrate into the surrounding tissues along specific pathways. Neural crest cells are amenable to study in tissue culture, and the molecules that regulate their development can be studied in vivo with antisense techniques as well as with the expression of gain and loss-of-function constructs. Mutations in factors that regulate neural crest cell survival or differentiation can lead to cell death or the premature cessation of their migration, resulting in craniofacial abnormalities, pigmentation defects and the absence of enteric neurons. This paper reviews recent advances in our understanding of neural crest cell induction and migration, emphasizing both avian and amphibian models. Cell facts: The embryonic progenitors of pigment cells, the neurons and glia of the peripheral nervous system, as well as connective tissue in the face, neck and heart. Induced to form at the boundary of the neuroepithelium and embryonic ectoderm. Expression of the transcription factors Snail, Slug and FoxD3 leads to delamination from the neural tube. Invasive motility not unlike that of tumor cells can be studied in vitro. Express proteases, distinctive cell surface receptors and glycoproteins to acquire an invasive phenotype. Mutations of transcription factors expressed by the neural crest or in other factors that inhibit their premature differentiation can lead to survival and migration-associated birth defects.

Pathogenesis of Hirschsprung's disease and its variants: recent progress

The enteric nervous system (ENS) is a complex network of interconnected neurons within the wall of the intestine that controls intestinal motility, regulates mucosal secretion and blood flow, and also modulates sensation from the gut. The cells that form the ENS in mammals are derived primarily from vagal neural crest cells. During the past decade there has been an explosion of information about genes that control the development of neural crest. Molecular-genetic analysis has identified several genes that have a role in the development of Hirschsprung's disease. The major susceptibility gene is RET, which is also involved in multiple endocrine neoplasia type 2. Recently, genetic studies have provided strong evidence in animal models that intestinal neuronal dysplasia (IND) is a real entity. HOX11L1 knockout mice and endothelin B receptor-deficient rats demonstrated abnormalities of the ENS resembling IND type B in humans. These findings support the concept that IND may be linked to a genetic defect.

Neuron and glia generating progenitors of the mammalian enteric nervous system isolated from foetal and postnatal gut cultures

Cultures of dissociated foetal and postnatal mouse gut gave rise to neurosphere-like bodies, which contained large numbers of mature neurons and glial cells. In addition to differentiated cells, neurosphere-like bodies included proliferating progenitors which, when cultured at clonal densities, gave rise to colonies containing many of the neuronal subtypes and glial cells present in the mammalian enteric nervous system. These progenitors were also capable of colonising wild-type and aganglionic gut in organ culture and had the potential to generate differentiated progeny that localised within the intrinsic ganglionic plexus. Similar progenitors were also derived from the normoganglionic small intestine of mice with colonic aganglionosis. Our findings establish the feasibility of expanding and isolating early progenitors of the enteric nervous system based on their ability to form distinct neurogenic and gliogenic structures in culture. Furthermore, these experiments provide the rationale for the development of novel approaches to the treatment of congenital megacolon (Hirschsprung's disease) based on the colonisation of the aganglionic gut with progenitors derived from normoganglionic bowel segments.

Origin and possible roles of the SOX8 transcription factor gene during sexual development

SOX8 is a member of the SOX family of developmental transcription factor genes and is closely related to SOX9, a critical gene involved in mammalian sex determination and differentiation. Both genes encode proteins with the ability to bind similar DNA target sequences, and to activate transcription in in vitro assays. Expression studies indicate that the two genes have largely overlapping patterns of activity during mammalian embryonic development. A knockout of SOX8 in mice has no obvious developmental phenotype, suggesting that the two genes are able to act redundantly in a variety of developmental contexts. In particular, both genes are expressed in the developing Sertoli cell lineage of the developing testes in mice, and both proteins are able to activate transcription of the gene encoding anti-Müllerian hormone (AMH), through synergistic action with steroidogenic factor 1 (SF1). We have hypothesized that SOX8 may substitute for SOX9 in species where SOX9 is expressed too late to be involved in sex determination or regulation of AMH expression. However, our studies involving the red-eared slider turtle indicate that SOX8 is expressed at similar levels in males and females throughout the sex-determining period, suggesting that SOX8 is neither a transcriptional regulator for AMH, nor responsible for sex determination or gonad differentiation in that species. Similarly, SOX8 is not expressed in a sexually dimorphic pattern during gonadogenesis in the chicken. Since a functional role(s) for SOX8 is implied by its conservation during evolution, the significance of SOX8 for sexual and other aspects of development will need to be uncovered through more directed lines of experimentation. Copyright 2003 S. Karger AG, Basel

Zic2 is required for neural crest formation and hindbrain patterning during mouse development

The Zic genes are the vertebrate homologues of the Drosophila pair rule gene odd-paired. It has been proposed that Zic genes play several roles during neural development including mediolateral segmentation of the neural plate, neural crest induction, and inhibition of neurogenesis. Initially during mouse neural development Zic2 is expressed throughout the neural plate while later on expression in the neurectoderm becomes restricted to the lateral region of the neural plate. A hypomorphic allele of Zic2 has demonstrated that in the mouse Zic2 is required for the timing of neurulation. We have isolated a new allele of Zic2 that behaves as a loss of function allele. Analysis of this mutant reveals two further functions for Zic2 during early neural development. Mutation of Zic2 results in a delay of neural crest production and a decrease in the number of neural crest cells that are produced. These defects are independent of mediolateral segmentation of the neurectoderm and of dorsal neurectoderm proliferation, both of which occur normally in the mutant embryos. Additionally Zic2 is required during hindbrain patterning for the normal development of rhombomeres 3 and 5. This work provides the first genetic evidence that the Zic genes are involved in neural crest production and the first demonstration that Zic2 functions during hindbrain patterning.

A tissue-restricted cAMP transcriptional response: SOX10 modulates alpha-melanocyte-stimulating hormone-triggered expression of microphthalmia-associated transcription factor in melanocytes

alpha-Melanocyte-stimulating hormone (MSH) utilizes cAMP to trigger pigmentation of melanocytes via activation of melanocyte-restricted microphthalmia-associated transcription factor (M-MITF) expression. M-MITF is a melanocyte-restricted helix-loop-helix transcription factor capable of transactivating promoters for multiple genes whose products modulate pigmentation. Although M-MITF promoter activation by MSH is known to occur through a conserved cAMP-response element (CRE), it remains unclear how this CRE exhibits such exquisitely tissue-restricted responsiveness. Here we show that cAMP-mediated CRE-binding protein activation of the M-MITF promoter requires a second DNA element located approximately 100 bp upstream, a site that is bound and activated by SOX10. Mutations in the SOX10 transcription factor, like MITF, results in a disorder known as Waardenburg Syndrome. The cAMP response of the M-MITF promoter was analyzed in melanoma and neuroblastoma cells (which are neural crest-derived but lack both M-MITF and SOX10 expression). M-MITF promoter responsiveness to cAMP was found to depend upon SOX10, and reciprocally, SOX10 transactivation was dependent upon the CRE. Ectopic SOX10 expression, in cooperation with cAMP signaling, activated the M-MITF promoter function and the expression of measurable endogenous M-MITF transcripts in neuroblastoma cells. SOX10dom, a mutant allele, failed to cooperate with cAMP in neuroblastoma cells and attenuated the cAMP responsiveness of the M-MITF promoter in melanoma cells. These observations demonstrate a means whereby the ubiquitous cAMP signaling machinery is harnessed to produce a highly tissue-restricted transcriptional response by cooperating with architectural factors, in this case SOX10.

A defect in a novel ADAMTS family member is the cause of the belted white-spotting mutation

Several features of the pigment defect in belted (bt) mutant mice suggest that it occurs as a result of a defect in melanocyte development that is unique from those described for other classical white-spotting mutations. We report here that bt mice carry mutations in Adamts20, a novel member of the ADAMTS family of secreted metalloproteases. Adamts20 shows a highly dynamic pattern of expression in the developing embryo that generally precedes the appearance of melanoblasts in the same region, and is not expressed in the migrating cells themselves. Adamts20 shows remarkable homology with GON-1, an ADAMTS family protease required for distal tip cell migration in C. elegans. Our results suggest that the role of ADAMTS proteases in the regulation of cell migration has been conserved in mammalian development.

Mouse models for four types of Waardenburg syndrome

Waardenburg syndrome (WS) is an auditory-pigmentary syndrome caused by a deficiency of melanocytes and other neural crest-derived cells. Depending on a variety of symptoms associated with the auditory-pigmentary symptoms, WS is classified into four types: WS type 1 (WS1), WS2, WS3, and WS4. Six genes contributing to this syndrome--PAX3, SOX10, MITF, SLUG, EDN3 and EDNRB--have been cloned so far, all of them necessary for normal development of melanocytes. Mutant mice with coat color anomalies were helpful in identifying these genes, although the phenotypes of these mice did not necessarily perfectly match those of the four types of WS. Here we describe mice with mutations of murine homologs of WS genes and verify their suitability as models for WS with special interest in the cochlear disorder. The mice include splotch (Sp), microphthalmia (mi), Slugh-/-, WS4, JF1, lethal-spotting (ls), and Dominant megacolon (Dom). The influence of genetic background on the phenotypes of mice mutated in homologs of WS genes is also addressed. Finally, possible interactions among the six WS gene products are discussed.

Analysis ofSOX10 mutations identified in Waardenburg-Hirschsprung patients: Differential effects on target gene regulation

SOX10 is a member of the SOX gene family related by homology to the high-mobility group (HMG) box region of the testis-determining gene SRY. Mutations of the transcription factor gene SOX10 lead to Waardenburg-Hirschsprung syndrome (Waardenburg-Shah syndrome, WS4) in humans. A number of SOX10 mutations have been identified in WS4 patients who suffer from different extents of intestinal aganglionosis, pigmentation, and hearing abnormalities. Some patients also exhibit signs of myelination deficiency in the central and peripheral nervous systems. Although the molecular bases for the wide range of symptoms displayed by the patients are still not clearly understood, a few target genes for SOX10 have been identified. We have analyzed the impact of six different SOX10 mutations on the activation of SOX10 target genes by yeast one-hybrid and mammalian cell transfection assays. To investigate the transactivation activities of the mutant proteins, three different SOX target binding sites were introduced into luciferase reporter gene constructs and examined in our series of transfection assays: consensus HMG domain protein binding sites; SOX10 binding sites identified in the RET promoter; and Sox10 binding sites identified in the P0 promoter. We found that the same mutation could have different transactivation activities when tested with different target binding sites and in different cell lines. The differential transactivation activities of the SOX10 mutants appeared to correlate with the intestinal and/or neurological symptoms presented in the patients. Among the six mutant SOX10 proteins tested, much reduced transactivation activities were observed when tested on the SOX10 binding sites from the RET promoter. Of the two similar mutations X467K and 1400del12, only the 1400del12 mutant protein exhibited an increase of transactivation through the P0 promoter. While the lack of normal SOX10 mediated activation of RET transcription may lead to intestinal aganglionosis, overexpression of genes coding for structural myelin proteins such as P0 due to mutant SOX10 may explain the dysmyelination phenotype observed in the patients with an additional neurological disorder.

HOXB5 expression is spatially and temporarily regulated in human embryonic gut during neural crest cell colonization and differentiation of enteric neuroblasts

HOX genes from paralogous groups 4 and 5 are particularly relevant to the gut neuromusculature development because these genes are expressed at the splanchnic mesoderm surrounding the gut diverticulum, and at the level of the neural tube from where the vagal neural crest cells (NCCs) originate. In this study, we examined the migration and differentiation of NCCs, and investigated the expression patterns of HOXB5 in human embryonic guts. Human embryos of gestational week-4 to -8.5 were studied. Vagal NCCs enter the esophagus, migrate, and colonize the entire gut in a rostrocaudal manner between week-4 and week-7. The migrating NCCs in gut express HOXB5. Two separate and discontinuous mesenchymal expression domains of HOXB5 were detected in the gut: the distal domain preceding the migratory NCCs; and the proximal domain overlapping with the NCCs. The two expression domains shift caudally in parallel with the rostrocaudal migration of NCCs between week-4 and week-5. Neuron and glia differentiation of NCCs are concomitant with HOXB5 down-regulation in NCCs and the mesenchyme. By week-7, myenteric plexuses have formed; HOXB5 expression is switched on in the plexuses. We found that (1) the migratory route of NCCs in human embryonic gut was similar to that in mice and chicks; and (2) the expression pattern of HOXB5 correlated with the migration and differentiation of NCCs, suggesting a regulatory role of HOXB5 in the development of NCCs.

The genetics of Hirschsprung disease

Understanding the genetics of Hirschsprung disease will naturally expand our understanding of other neurocristopathies, the enteric nervous system, and autonomic system biology. As other disorders of gastrointestinal motility are investigated, genetics may resolve certain clinical questions. For example, isolated hypoganglionosis without aganglionosis has been reported as a primary cause of intestinal pseudo-obstruction. Is such hypoganglionosis merely a forme-fruste of Hirschsprung disease, or a result from an entirely different pathogenetic mechanism? Can irritable bowel syndrome or severe constipation be related to specific mutations, polymorphisms, or haplotypes? How might an understanding of derangements of the ENS be translated to understanding derangements of the CNS? Clearly, we should anticipate improved prognostication, counseling, and hopefully, therapies with future genetic insights.

Sox8 is a specific marker for muscle satellite cells and inhibits myogenesis

Sox8 belongs to a family of transcription regulators characterized by a unique DNA-binding domain known as the high mobility group box. Many Sox proteins play fundamental roles in vertebrate development and differentiation processes. Expression of Sox8 is strong during embryonic muscle development and gradually declines postnatally. In this study, we report that in adult skeletal muscle Sox8 is confined to satellite cells. Down-regulation during myogenic differentiation was also detected in cell culture systems and occurred in parallel with down-regulation of the related Sox9. Overexpression of Sox8 or Sox9 on the other hand disrupted myoblasts in their ability to form myotubes. Concomitantly, expression of MyoD and myogenin decreased and basal as well as MyoD-induced activities of the myogenin promoter were strongly reduced in a Sox8-dependent manner. Our data suggest that Sox8 acts as a specific negative regulator of skeletal muscle differentiation, possibly by interfering with the function of myogenic basic helix-loop-helix proteins.

Sox10 is required for the early development of the prospective neural crest in Xenopus embryos

The Sox family of transcription factors has been implicated in the development of different tissues during embryogenesis. Several mutations in humans, mice, and zebrafish have shown that depletion of Sox10 activity produces defects in the development of neural crest derivatives, such as melanocytes, ganglia of the peripheral nervous system, and some specific cell types as glia. We have isolated the Xenopus homologue of the Sox10 gene. It is expressed in prospective neural crest and otic placode regions from the earliest stages of neural crest specification and in migrating cranial and trunk neural crest cells. Loss-of-function experiments using morpholino antisense oligos against Sox10 produce a loss of neural crest precursors and an enlargement of the surrounding neural plate and epidermis. This effect of Sox10 depletion is produced during some of the earliest steps of neural crest specification, as is shown by the inhibition in the expression of Slug and FoxD3, which are early markers of neural crest specification. In addition, we show that Sox10 depletion leads to an increase in apoptosis and a decrease in cell proliferation in the neural folds, suggesting that Sox10 could work as a survival as well as a specification factor in neural crest precursors during premigratory stages. Although some of the deficiencies found in the Waardenburg syndrome and in the Hirschprung disease could be associated with a failure of the development of crest derivatives during the late phase of its development, or even during adulthood, our results suggest that inhibition of Sox10 activity produces an earlier failure of neural crest precursors. In experiments where melanocytes and ganglia were induced in vivo and in vitro, we were able to block their development by inhibiting Sox10 activity. These results are compatible with an additional late role of Sox10 on development of neural crest derivatives, as it has been previously proposed. We show that Sox10 expression is dependent on FGF and Wnt activity, both in the neural crest and in the otic placode territories. Finally, in order to establish the position of Sox10 in the hierarchical cascade of gene activation required for neural crest specification, we used inducible forms of the wild type and dominant negatives for the Snail and Slug genes. Our results show that Snail is able to control Sox10 expression. However, the overexpression of Slug was not able to upregulate Sox10 expression. Taken together, these results indicate that Sox10 may lie between Snail and Slug in the genetic cascade that controls neural crest development.

Sox10 regulates the development of neural crest-derived melanocytes in Xenopus

The transcription factors of the Sox family play important roles in diverse developmental processes. A number of genetic studies have established that Sox10 is a major regulator of neural crest formation. Here, we report the cloning and functional analysis of the Xenopus Sox10 gene. Sox10 mRNA accumulates during gastrulation at the lateral edges of the neural plate, in the neural crest-forming region. In this tissue, Sox10 expression is regulated by Wnt signaling and colocalizes with two major regulators of neural crest formation, Slug and Sox9. While initially expressed in neural crest cells from all axial levels, at the tailbud stage, Sox10 is downregulated in the cranial neural crest and persists mostly in neural crest cells from the trunk region. Overexpression of Sox10 causes a dramatic expansion of the Slug expression domain. We show that the C-terminal portion of Sox10 is sufficient to mediate this activity. Later during embryogenesis, Sox10-injected embryos show a massive increase in pigment cells (Trp-2-expressing cells). The responsiveness of the embryo to Sox10 overexpression by expansion of the Slug expression domain and ectopic production of Trp-2-positive cells and differentiated melanocytes is lost during gastrulation, as revealed by a hormone-inducible Sox10 construct. These results suggest that Sox10 is involved in the specification of neural crest progenitors fated to form the pigment cell lineage.

The Sox9 transcription factor determines glial fate choice in the developing spinal cord

The mechanism that causes neural stem cells in the central nervous system to switch from neurogenesis to gliogenesis is poorly understood. Here we analyzed spinal cord development of mice in which the transcription factor Sox9 was specifically ablated from neural stem cells by the CRE/loxP recombination system. These mice exhibit defects in the specification of oligodendrocytes and astrocytes, the two main types of glial cells in the central nervous system. Accompanying an early dramatic reduction in progenitors of the myelin-forming oligodendrocytes, there was a transient increase in motoneurons. Oligodendrocyte progenitor numbers recovered at later stages of development, probably owing to compensatory actions of the related Sox10 and Sox8, both of which overlap with Sox9 in the oligodendrocyte lineage. In agreement, compound loss of Sox9 and Sox10 led to a further decrease in oligodendrocyte progenitors. Astrocyte numbers were also severely reduced in the absence of Sox9 and did not recover at later stages of spinal cord development. Taking the common origin of motoneurons and oligodendrocytes as well as V2 interneurons and some astrocytes into account, stem cells apparently fail to switch from neurogenesis to gliogenesis in at least two domains of the ventricular zone, indicating that Sox9 is a major molecular component of the neuron-glia switch in the developing spinal cord.

The endothelin receptor-B is required for the migration of neural crest-derived melanocyte and enteric neuron precursors

Mutations in the genes encoding endothelin receptor-B (Ednrb) and its ligand endothelin-3 (Edn3) affect the development of two neural crest-derived cell types, melanocytes and enteric neurons. EDNRB signaling is exclusively required between E10.5 and E12.5 during the migratory phase of melanoblast and enteric neuroblast development. To determine the fate of Ednrb-expressing cells during this critical period, we generated a strain of mice with the bacterial beta-galactosidase (lacZ) gene inserted downstream of the endogenous Ednrb promoter. The expression of the lacZ gene was detected in melanoblasts and precursors of the enteric neuron system (ENS), as well as other neural crest cells and nonneural crest-derived lineages. By comparing Ednrb(lacZ)/+ and Ednrb(lacZ)/Ednrb(lacZ) embryos, we determined that the Ednrb pathway is not required for the initial specification and dispersal of melanoblasts and ENS precursors from the neural crest progenitors. Rather, the EDNRB-mediated signaling is required for the terminal migration of melanoblasts and ENS precursors, and this pathway is not required for the survival of the migratory cells.

Transcriptional regulation of mitfa accounts for the sox10 requirement in zebrafish melanophore development

The transcription factor Sox10 is required for the specification, migration and survival of all nonectomesenchymal neural crest derivatives including melanophores. sox10(-/-) zebrafish lack expression of the transcription factor mitfa, which itself is required for melanophore development. We demonstrate that the zebrafish mitfa promoter has sox10 binding sites necessary for activity in vitro, consistent with studies using mammalian cell cultures that have shown that Sox10 directly regulates Mitf expression. In addition, we demonstrate that these sites are necessary for promoter activity in vivo. We show that reintroduction of mitfa expression in neural crest cells can rescue melanophore development in sox10(-/-) embryos. This rescue of melanophores in sox10(-/-) embryos is quantitatively indistinguishable from rescue in mitfa(-/-) embryos. These findings show that the essential function of sox10 in melanophore development is limited to transcriptional regulation of mitfa. We propose that the dominant melanophore phenotype in Waardenburg syndrome IV individuals with SOX10 mutations is likely to result from failure to activate MITF in the normal number of melanoblasts.

The role of Kit-ligand in melanocyte development and epidermal homeostasis

Kit-ligand (Kitl) also known as steel factor, stem cell factor and mast cell growth factor plays a crucial role in the development and maintenance of the melanocyte lineage in adult skin. Kitl exerts permanent survival, proliferation and migration functions in Kit receptor-expressing melanocytes. A comprehensive overview over the differential roles of Kitl in melanocyte development and homeostasis is provided. I discuss species-specific differences of the Kitl/Kit signalling system, regulation at the transcriptional level and also covering the regulation of cell surface Kitl presentation by cytoplasmic targeting sequences. In addition, recent studies evoked the importance of Kitl misexpression in some hyperpigmented lesions that may open the avenue for Kitl-dependent treatment of pathological skin conditions.

The importance of having your SOX on: role of SOX10 in the development of neural crest-derived melanocytes and glia

SOX10 is a member of the high-mobility group-domain SOX family of transcription factors, which are ubiquitously found in the animal kingdom. Disruption of neural crest development in the Dominant megacolon (Dom) mice is associated with a Sox10 mutation. Mutations in human Sox10 gene have also been linked with the occurrence of neurocristopathies in the Waardenburg-Shah syndrome type IV (WS-IV), for which the Sox10(Dom) mice serve as a murine model. The neural crest disorders in the Sox10(Dom) mice and WS-IV patients consist of hypopigmentation, cochlear neurosensory deafness, and enteric aganglionosis. Consistent with these observations, a critical role for SOX10 in the proper differentiation of neural crest-derived melanocytes and glia has been demonstrated. Emerging data also show an important role for SOX10 in promoting the survival of neural crest precursor cells prior to lineage commitment. Several genes whose regulation is dependent on SOX10 function have been identified in the peripheral nervous system and in melanocytes, helping to begin the identification of the multiple pathways that appear to be modulated by SOX10 activity. In this review, we will discuss the biological relevance of these target genes to neural crest development and the properties of Sox10 as a transcription factor.

Sox18 mutations in the ragged mouse alleles ragged-like and opossum

The ragged (Ra) spontaneous mouse mutant is characterised by abnormalities in its coat and cardiovascular system. Four alleles are known and we have previously described mutations in the transcription factor gene Sox18 in the Ra and Ra(J) alleles. We report here Sox18 mutations in the remaining two ragged alleles, opossum (Ra(op)) and ragged-like (Ragl). The single-base deletions cause a C-terminal frameshift, abolishing transcriptional trans-activation and impairing interaction with the partner protein MEF2C. The nature of these mutations, together with the near-normal phenotype of Sox18-null mice, suggests that the ragged mutant SOX18 proteins act in a dominant-negative fashion. The four ragged mutants represent an allelic series that reveal SOX18 structure-function relationships and implicate related SOX proteins in cardiovascular and hair follicle development.

GDNF availability determines enteric neuron number by controlling precursor proliferation

To clarify the role of Ret signaling components in enteric nervous system (ENS) development, we evaluated ENS anatomy and intestinal contractility in mice heterozygous for Ret, GFRalpha1 and Ret ligands. These analyses demonstrate that glial cell line-derived neurotrophic factor (GDNF) and neurturin are important for different aspects of ENS development. Neurturin is essential for maintaining the size of mature enteric neurons and the extent of neuronal projections, but does not influence enteric neuron number. GDNF availability determines enteric neuron number by controlling ENS precursor proliferation. However, we were unable to find evidence of programmed cell death in the wild type ENS by immunohistochemistry for activated caspase 3. In addition, enteric neuron number is normal in Bax(-/-) and Bid(-/-) mice, suggesting that, in contrast to most of the rest of the nervous system, programmed cell death is not important for determining enteric neuron numbers. Only mild reductions in neuron size and neuronal fiber counts occur in Ret(+/-) and Gfra1(+/-) mice. All of these heterozygous mice, however, have striking problems with intestinal contractility and neurotransmitter release, demonstrating that Ret signaling is critical for both ENS structure and function.

SOX10 maintains multipotency and inhibits neuronal differentiation of neural crest stem cells

The mechanisms that establish and maintain the multipotency of stem cells are poorly understood. In neural crest stem cells (NCSCs), the HMG-box factor SOX10 preserves not only glial, but surprisingly, also neuronal potential from extinction by lineage commitment signals. The latter function is reflected in the requirement of SOX10 in vivo for induction of MASH1 and PHOX2B, two neurogenic transcription factors. Simultaneously, SOX10 inhibits or delays overt neuronal differentiation, both in vitro and in vivo. However, this activity requires a higher Sox10 gene dosage than does the maintenance of neurogenic potential. The opponent functions of SOX10 to maintain neural lineage potentials, while simultaneously serving to inhibit or delay neuronal differentiation, suggest that it functions in stem or progenitor cell maintenance, in addition to its established role in peripheral gliogenesis.

Molecular cloning and expression of Sox17 in gonads during sex reversal in the rice field eel, a teleost fish with a characteristic of natural sex transformation

The Sox is a large family of genes which encode transcription factors with high-mobility-group DNA binding domain related to the SRY, with diverse roles in development, and a few of them are involved in sex determination and differentiation. We report here the identification of Sox17 gene of the rice field eel, a teleost fish with a characteristic of natural sex transformation. This gene, located on chromosome 5, consists of two exons which encode a 399-amino acid protein with a conserved HMG box. Phylogenetic analysis shows that the rice field eel Sox17 fits within the Sox17 clade of vertebrates. The rice field eel Sox17 was dominantly expressed in gonads of male, female, and intersex, besides in brain and spleen. Gene expression analysis by in situ hybridization showed its expression in testis, ovary, and ovotestis, and specifically in the gonadal lamellae of ovary, ovotestis, and testis and developing spermatogenic cells of testis, suggesting that they have potentially important roles in gonadal differentiation during sex reversal in this species.

Neural crest patterning: autoregulatory and crest-specific elements co-operate for Krox20 transcriptional control

Neural crest patterning constitutes an important element in the control of the morphogenesis of craniofacial structures. Krox20, a transcription factor gene that plays a critical role in the development of the segmented hindbrain, is expressed in rhombomeres (r) 3 and 5 and in a stream of neural crest cells migrating from r5 toward the third branchial arch. We have investigated the basis of the specific neural crest expression of Krox20 and identified a cis-acting enhancer element (NCE) located 26 kb upstream of the gene that is conserved between mouse, man and chick and can recapitulate the Krox20 neural crest pattern in transgenic mice. Functional dissection of the enhancer revealed the presence of two conserved Krox20 binding sites mediating direct Krox20 autoregulation in the neural crest. In addition, the enhancer included another essential element containing conserved binding sites for high mobility group (HMG) box proteins and which responded to factors expressed throughout the neural crest. Consistent with this the NCE was strongly activated in vitro by Sox10, a crest-specific HMG box protein, in synergism with Krox20, and the inactivation of Sox10 prevented the maintenance of Krox20 expression in the migrating neural crest. These results suggest that the dependency of the enhancer on both crest- (Sox10) and r5- (Krox20) specific factors limits its activity to the r5-derived neural crest. This organisation also suggests a mechanism for the transfer and maintenance of rhombomere-specific gene expression from the hindbrain neuroepithelium to the emerging neural crest and may be of more general significance for neural crest patterning.

Acquisition of neuronal and glial markers by neural crest-derived cells in the mouse intestine

Enteric neurons and glia arise from the neural crest. The phenotype of crest-derived cells was examined as they differentiated into neurons or glia in the mouse small and large intestine. Previous studies have shown that undifferentiated enteric crest-derived cells are Phox2b(+)/Ret(+)/p75(+)/Sox10(+), and at embryonic day (E) 10.5, about 10-15% of the crest-derived cells in the small intestine have started to differentiate into neurons. In the current study, by E12.5 and E14.5, about 25% and 47%, respectively, of Phox2b(+) cells in the small intestine were immunoreactive to the pan-neuronal protein, ubitquitin hydrolase (PGP9.5), and the percentage did not change dramatically from E14.5 onward. The differentiation of crest-derived cells into neurons in the colon lagged behind that in the small intestine by several days. Differentiating enteric neurons showed high Ret, low p75, and undetectable Sox10 immunostaining. Glial precursors were identified by the presence of brain-specific fatty acid binding protein (B-FABP) and detected first in the fore- and rostral midgut at E11.5. Glial precursors appeared to be B-FABP(+)/Sox10(+)/p75(+) but showed low Ret immunostaining. S100b was not detected until E14.5. Adult glial cells were B-FABP(+)/Sox10(+)/p75(+)/S100b(+). A nucleic acid stain (to identify all ganglion cells) was combined with immunostaining for PGP9.5 and S100b to detect neurons and glial cells, respectively, in the postnatal intestine. At postnatal day 0, fewer than 5% and 10% of cells in myenteric ganglia of the small and large intestine, respectively, were neither PGP9.5(+) nor S100b(+). Because some classes of neurons are not present in significant numbers until after birth, the expression of PGP9.5 by developing enteric neurons appeared to precede the expression of neuron type-specific markers.

Colonic epithelial expression of ErbB2 is required for postnatal maintenance of the enteric nervous system

We utilized the Cre-LoxP system to establish erbB2 conditional mutant mice in order to investigate the role of erbB2 in postnatal development of the enteric nervous system. The erbB2/nestin-Cre conditional mutants exhibit retarded growth, distended colons, and premature death, resembling human Hirschsprung's disease. Enteric neurons and glia are present at birth in the colon of erbB2/nestin-Cre mutants; however, a marked loss of multiple classes of enteric neurons and glia occurs by 3 weeks of age. Furthermore, we demonstrate that the requirement for erbB2 in maintaining the enteric nervous system is not cell autonomous, but rather erbB2 signaling in the colonic epithelia is required for the postnatal survival of enteric neurons and glia.

A founding locus within the RET proto-oncogene may account for a large proportion of apparently sporadic Hirschsprung disease and a subset of cases of sporadic medullary thyroid carcinoma

Hirschsprung disease (HSCR) is a common congenital disorder characterized by aganglionosis of the gut. The seemingly unrelated multiple endocrine neoplasia type 2 (MEN 2) is an autosomal dominant disorder characterized by medullary thyroid carcinoma (MTC), pheochromocytoma, and hyperparathyroidism. Yet, germline mutations in the RET proto-oncogene are associated with both MEN 2 and HSCR. In the former, gain-of-function mutations in a limited set of codons is found, whereas, in the latter, loss-of-function mutations are found. However, germline RET mutation is associated with only 3% of a population-based series of isolated HSCR, and little is known about susceptibility to sporadic MTC. We have found previously that specific haplotypes comprising RET coding single-nucleotide polymorphisms (SNPs) comprising exon 2 SNP A45A were strongly associated with HSCR, whereas haplotypes associated with exon 14 SNP S836S were associated with MTC. In this study, we describe three novel intron 1 SNPs, and, together with the coding SNP haplotypes, the data suggest the presence of distinct ancestral haplotypes for HSCR and sporadic MTC in linkage disequilibrium with a putative founding susceptibility locus/loci. The data are consistent with the presence of a very ancient, low-penetrance founder locus approximately 20-30 kb upstream of SNP A45A, but the failure of the SNPs to span the locus presents challenges in modeling mode of transmission or ancestry. We postulate that this founding locus is germane to both isolated HSCR and MTC but also that different mutations in this locus would predispose to one or the other.

Complementation of melanocyte development in SOX10 mutant neural crest using lineage-directed gene transfer

An in vitro gene complementation approach has been developed to dissect gene function and regulation in neural crest (NC) development and disease. The approach uses the avian RCAS virus to express genes in NC cells derived from transgenic mice expressing the RCAS receptor TVA, under the control of defined promoter elements. Constructs for creating TVA transgenic mice were developed using site-specific recombination GATEWAY (GW), compatible vectors that can also be used to facilitate analysis of genomic fragments for transcriptional regulatory elements. By using these GW vectors to facilitate cloning, transgenic mouse lines were generated that express TVA in SOX10-expressing NC stem cells under the control of the Pax3 promoter. The Pax3-tv-a transgene was bred onto a Sox10-deficient background, and the feasibility of complementing genetic NC defects was demonstrated by infecting the Pax3-tv-a cells with an RCAS-Sox10 expression virus, thereby rescuing melanocyte development of Sox10-deficient NC cells. This system will be useful for assessing genetic hierarchies in NC development. Developmental Dynamics 229:54-62, 2004.

Cooperative binding of Sox10 to DNA: requirements and consequences

The high-mobility-group (HMG) domain containing transcription factor Sox10 is an important regulator of various processes including the development of neural crest cells and glial cells. Target gene promoters contain multiple Sox10-binding sites, which either support monomeric or cooperative, dimeric binding. The latter is unusual for Sox proteins and might contribute to functional specificity of Sox10. We find that specific amino acid residues in a conserved region immediately preceding the HMG domain of Sox10 are required for cooperative binding. These residues cooperate with the HMG domain during dimeric binding in a manner dependent on specific determinants within the first two alpha-helices of the HMG domain. Cooperativity of DNA binding is surprisingly refractory to changes in the overall conformation of the DNA-bound dimer. Whereas maintenance of cooperativity is essential for full activation of the promoter of the myelin protein zero target gene, dimer-dependent conformational changes such as the exact bending angle introduced into the promoter appear to be less important, shedding new light on the architectural function of Sox proteins.

Pigment pattern formation in zebrafish: a model for developmental genetics and the evolution of form

The zebrafish Danio rerio is an emerging model organism for understanding vertebrate development and genetics. One trait of both historical and recent interest is the pattern formed by neural crest-derived pigment cells, or chromatophores, which include black melanophores, yellow xanthophores, and iridescent iridophores. In zebrafish, an embryonic and early larval pigment pattern consists of several stripes of melanophores and iridophores, whereas xanthophores are scattered widely over the flank. During metamorphosis, however, this pattern is transformed into that of the adult, which comprises several dark stripes of melanophores and iridophores that alternate with light stripes of xanthophores and iridophores. In this review, we place zebrafish relative to other model and non-model species; we review what is known about the processes of chromatophore specification, differentiation, and morphogenesis during the development of embryonic and adult pigment patterns, and we address how future studies of zebrafish will likely aid our understanding of human disease and the evolution of form.

Development of pigment cells in the zebrafish embryo

In recent years, the zebrafish Danio rerio has emerged as a powerful model organism for the study of vertebrate embryogenesis. Zebrafish, like other vertebrates, possess pigment cells that arise from two distinct embryonic sources: those of the dermis and epidermis originate from the neural crest, while those that comprise the outermost layer of the retina, the retinal pigment epithelium or RPE, derive from the optic cup. A better understanding of processes behind the specification and differentiation of these cells will provide insight to the evolutionary diversification of all classes of vertebrates and will have clinical relevance to human disorders of pigmentation and certain retinopathies. In the first part of this review, the present knowledge of the ontogeny of both of these populations of pigment cells in the embryonic zebrafish is summarized, in terms of both genetics and molecular markers. The final part of the review focuses on duplicate zebrafish genes encoding orthologs of the basic helix-loop-helix/leucine zipper protein Mitf (Microphthalmia-associated transcription factor), and presents a hypothesis concerning their divergent roles in neural crest and retinal pigment cells.

Molecular dissection of craniofacial development using zebrafish

The zebrafish, Danio rerio, is a small, freshwater teleost that only began to be used as a vertebrate genetic model by the late George Streisinger in the early 1980s. The strengths of the zebrafish complement genetic studies in mice and embryological studies in avians. Its advantages include high fecundity, externally fertilized eggs and transparent embryos that can be easily manipulated, inexpensive maintenance, and the fact that large-scale mutagenesis screens can be performed. Here we review studies that have used the zebrafish as a model for craniofacial development. Lineage studies in zebrafish have defined the origins of the cranial skeleton at the single-cell level and followed the morphogenetic behaviors of these cells in skeletal condensations. Furthermore, genes identified by random mutational screening have now revealed genetic pathways controlling patterning of the jaw and other pharyngeal arches, as well as the midline of the skull, that are conserved between fish and humans. We discuss the potential impact of specialized mutagenesis screens and the future applications of this versatile, vertebrate developmental model system in the molecular dissection of craniofacial development.

Requirement of signalling by receptor tyrosine kinase RET for the directed migration of enteric nervous system progenitor cells during mammalian embryogenesis

The majority of neurones and glia of the enteric nervous system (ENS) are derived from the vagal neural crest. Shortly after emigration from the neural tube, ENS progenitors invade the anterior foregut and, migrating in a rostrocaudal direction, colonise in an orderly fashion the rest of the foregut, the midgut and the hindgut. We provide evidence that activation of the receptor tyrosine kinase RET by glial cell line-derived neurotrophic factor (GDNF) is required for the directional migration of ENS progenitors towards and within the gut wall. We find that neural crest-derived cells present within foetal small intestine explants migrate towards an exogenous source of GDNF in a RET-dependent fashion. Consistent with an in vivo role of GDNF in the migration of ENS progenitors, we demonstrate that Gdnf is expressed at high levels in the gut of mouse embryos in a spatially and temporally regulated manner. Thus, during invasion of the foregut by vagal-derived neural crest cells, expression of Gdnf was restricted to the mesenchyme of the stomach, ahead of the invading NC cells. Twenty-four hours later and as the ENS progenitors were colonising the midgut,Gdnf expression was upregulated in a more posterior region —the caecum anlage. In further support of a role of endogenous GDNF in enteric neural crest cell migration, we find that in explant cultures GDNF produced by caecum is sufficient to attract NC cells residing in more anterior gut segments. In addition, two independently generated loss-of-function alleles of murine Ret, Ret.k— and miRet51, result in characteristic defects of neural crest cell migration within the developing gut. Finally, we identify phosphatidylinositol-3 kinase and the mitogen-activated protein kinase signalling pathways as playing crucial roles in the migratory response of enteric neural crest cells to GDNF.

Zebrafish as a model for hearing and deafness

The zebrafish is an especially attractive model for the study of the development and function of the vertebrate inner ear. It combines rapid and accessible embryogenesis with a host of genetic and genomic tools for systematic gene discovery and analysis. A large collection of mutations affecting development and function of the ear and a related sensory system, the lateral line, have been isolated; several of these have now been cloned, and at least five provide models for human deafness disorders. Disruption of multiple genes, using both forward and reverse genetic approaches, has established key players--both signaling molecules and autonomous factors--responsible for induction and specification of the otic placode. Vestibular and auditory defects have been detected in adult animals, making the zebrafish a useful system in which to tackle the genetic causes of late onset deafness and vestibular disease.

Identification and characterization of the human long form of Sox5 (L-SOX5) gene

The Sox (Sry-type HMG box) group of transcription factors, which is defined by a high-mobility group (HMG) DNA-binding domain, is categorized into six subfamilies. Sox5 and Sox6 belong to the group D subfamily, which is characterized by conserved N-terminal domains including a leucine-zipper, a coiled-coil domain and a Q-box. Group D Sox genes are expressed as long and short transcripts that exhibit differential expression patterns. In mouse, the long form of Sox5, L-Sox5, is co-expressed and interacts with Sox6; together, these two proteins appear to play a key role in chondrogenesis and myogenesis. In humans, however, only the short form of Sox5 has previously been identified. To gain insight into Sox5 function, we have identified and characterized human L-SOX5. The human L-SOX5 cDNA encodes a 763-amino-acid protein that is 416 residues longer than the short form and contains all of the characteristic motifs of group D Sox proteins. The predicted L-SOX5 protein shares 97% amino acid identity with its mouse counterpart and 59% identity with human SOX6. The L-SOX5 gene contains 18 exons and shows similar genomic structure to SOX6. We have identified two transcription start sites in L-SOX5 and multiple alternatively spliced mRNA variants that are distinct from the short form. Unlike the short form, which shows testis-specific expression, L-SOX5 is expressed in multiple tissues. Like SOX6, L-SOX5 shows strong expression in chondrocytes and striated muscles, indicating a likely role in human cartilage and muscle development.

The projections of early enteric neurons are influenced by the direction of neural crest cell migration

The enteric nervous system arises from the neural crest. In embryonic mice, vagal neural crest cells enter the developing foregut at approximately embryonic day 9.5 (E9.5) and then migrate rostrocaudally to colonize the entire gastrointestinal tract by E14.5. This study showed that a subpopulation of vagal crest-derived cells, very close to the migratory wavefront, starts to differentiate into neurons early, as shown by the expression of neuron-specific proteins and the absence of Sox10. Many of the early differentiating neurons transiently exhibited tyrosine hydroxylase (TH) immunoreactivity. The TH cells were demonstrated to be the progenitors of nitric oxide synthase (NOS) neurons. Immunohistochemistry, lesions, and DiI tracing were used to examine the projections of developing enteric neurons. The axons of first neurons in the gut (the TH-NOS neurons) projected in the same direction (caudally), and traversed the same pathways through the mesenchyme, as the migrating, undifferentiated, vagal crest-derived cells. To examine if the direction of migration and direction of axon projection are linked, coculture experiments were set up in which vagal crest-derived cells migrated either rostrocaudally (as they do in vivo), or caudorostrally (which they do not normally do), to colonize explants of embryonic aneural hindgut. The direction in which neurons projected was correlated with the direction of cell migration, but migration direction appears to be not the only mechanism influencing axon projection. Peristaltic reflexes involve both orally (rostrally) projecting neurons and anally (caudally) projecting neurons. Because few rostrally projecting neurons could be detected before birth, the full circuitry for peristaltic reflexes appears to develop after birth.

Clinical features of a form of Hirschsprung's disease caused by a novel genetic abnormality

BACKGROUND/PURPOSE

The aim of this report is to describe the pattern of similarities among the patients, exemplifying a newly recognized form of Hirschsprung's disease (HSCR) caused by mutations of ZFHX1B encoding Smad interacting protein-1.

METHODS

Fluorescence in situ hybridization (FISH) using several cDNAs and RP11-BAC clones and mutation gene scanning using direct nucleotide sequencing analysis of polymerase chain reaction (PCR) were conducted. Personal records of the patients also were analyzed retrospectively to confirm the clinical features.

RESULTS

All the patients represented isolated cases without any familial tendency. Aganglionic segments were limited to the recto-sigmoid colon in 3 cases and the rectum in one. Chromosomal screening found normal karyotypes in all cases except one, in whom a translocation between chromosomes 2 and 13 was detected. In addition to HSCR, characteristic facial appearance (hypertelorism with strabismus and wide nasal bridge), microcephaly with epilepsy, and severe physical and mental disabilities were found in all the patients. FISH for the patient having the chromosomal abnormality showed that about a 5-Mb cytogenetic deletion flanked at the 2q22 translocation breakpoint. Among 3 genes mapping to this deleted region, 2 nonsense mutations and a 4-base pair deletion were detected in ZFHX1B.

CONCLUSIONS

The clinical features of the patients have surprising resemblance and constitute a wide spectrum of neurocristopathies. These findings suggest that the ZFHX1B may be a very important gene for normal embryonic neural crest development. These also indicate that the HSCR can be regarded as a congenital malformation with a background of a multigenetic neurocristopathy. It is of great interest that mutations were located at the same spot (exon 8) of ZFHX1B in 3 of 4 cases, probably accounting for the unique clinical features of this newly recognized form of HSCR.

Sox10 is an active nucleocytoplasmic shuttle protein, and shuttling is crucial for Sox10-mediated transactivation

Sox10 belongs to a family of transcription regulators characterized by a DNA-binding domain known as the HMG box. It plays fundamental roles in neural crest development, peripheral gliogenesis, and terminal differentiation of oligodendrocytes. In accord with its function as transcription factor, Sox10 contains two nuclear localization signals and is most frequently detected in the nucleus. In this study, we report that Sox10 is an active nucleocytoplasmic shuttle protein, competent of both entering and exiting the nucleus. We identified a functional Rev-type nuclear export signal within the DNA-binding domain of Sox10. Mutational inactivation of this nuclear export signal or treatment of cells with the CRM1-specific export inhibitor leptomycin B inhibited nuclear export and consequently nucleocytoplasmic shuttling of Sox10. Importantly, the inhibition of the nuclear export of Sox10 led to decreased transactivation of transfected reporters and endogenous target genes, arguing that continuous nucleocytoplasmic shuttling is essential for the function of Sox10. To our knowledge this is the first time that nuclear export has been reported and shown to be functionally relevant for any Sox protein.

Twist function is required for the morphogenesis of the cephalic neural tube and the differentiation of the cranial neural crest cells in the mouse embryo

Loss of Twist function in the cranial mesenchyme of the mouse embryo causes failure of closure of the cephalic neural tube and malformation of the branchial arches. In the Twist(-/-) embryo, the expression of molecular markers that signify dorsal forebrain tissues is either absent or reduced, but those associated with ventral tissues display expanded domains of expression. Dorsoventral organization of the mid- and hindbrain and the anterior-posterior pattern of the neural tube are not affected. In the Twist(-/-) embryo, neural crest cells stray from the subectodermal migratory path and the late-migrating subpopulation invades the cell-free zone separating streams of cells going to the first and second branchial arches. Cell transplantation studies reveal that Twist activity is required in the cranial mesenchyme for directing the migration of the neural crest cells, as well as in the neural crest cells within the first branchial arch to achieve correct localization. Twist is also required for the proper differentiation of the first arch tissues into bone, muscle, and teeth.

Identification of a distal enhancer for the melanocyte-specific promoter of the MITF gene

Waardenburg syndrome (WS) is characterized by deafness and hypopigmentation because of the lack of melanocytes in the inner ear and skin. WS type 2 is associated with mutations in the gene encoding microphthalmia-associated transcription factor (MITF) that is required for melanocyte differentiation. MITF consists of multiple isoforms with different N-termini, one of which is exclusively expressed in melanocytes, named MITF-M. Its N-terminus is encoded by exon 1M that is under the regulation of the melanocyte-specific (M) promoter. Here we identify a distal regulatory region of 298 bp, located 14.5 kb upstream from exon 1M, which enhances the M promoter activity in cultured melanoma cells. This enhancer activity depends on the proximal M promoter region (-120 to -46). The MITF-M distal enhancer (MDE), thus identified, contains the binding sites for SOX10, a transcription factor responsible for another type of WS, known as Waardenburg-Hirschsprung syndrome. Characterization of MDE has suggested SOX10 as one of factors that are involved in the function of MDE. A putative MDE counterpart is located 12 kb upstream from mouse exon 1M and its role is discussed in relevance to the pathogenesis of red-eyed white Mitf mi-rw mice that exhibit small red eyes and white coat. Moreover, by in situ hybridization analysis, we suggest that Sox10 and Mitf-M (mRNA) are expressed in melanoblasts migrating toward the otic vesicle (prospective inner ear) of mouse embryos but are separately expressed in different cell types of the newborn cochlea. Thus, SOX10 regulates transcription from the M promoter in a developmental stage-specific manner.

Directed differentiation of pluripotent cells to neural lineages: homogeneous formation and differentiation of a neurectoderm population

During embryogenesis the central and peripheral nervous systems arise from a neural precursor population, neurectoderm, formed during gastrulation. We demonstrate the differentiation of mouse embryonic stem cells to neurectoderm in culture, in a manner which recapitulates embryogenesis, with the sequential and homogeneous formation of primitive ectoderm, neural plate and neural tube. Formation of neurectoderm occurs in the absence of extraembryonic endoderm or mesoderm and results in a stratified epithelium of cells with morphology, gene expression and differentiation potential consistent with positionally unspecified neural tube. Differentiation of this population to homogeneous populations of neural crest or glia was also achieved. Neurectoderm formation in culture allows elucidation of signals involved in neural specification and generation of implantable cell populations for therapeutic use.

Migration and function of a glial subtype in the vertebrate peripheral nervous system

Glia-axon interactions are essential for the development and function of the nervous system. We combine in vivo imaging and genetics to address the mechanism by which the migration of these cells is coordinated during embryonic development. Using stable transgenic lines, we have followed the migration of one subset of glial cells and their target axons in living zebrafish embryos. These cells coalesce at an early stage and remain coupled throughout migration, with axons apparently pathfinding for glia. Mutant analysis demonstrates that axons provide instructive cues that are sufficient to control glial guidance. Furthermore, mutations in the transcription factor Sox10/cls uncouple the migration of axons and glia. Finally, genetic ablation of this glial subtype reveals an essential role in nerve fasciculation.

Expression and characterization of Xenopus laevis SRY-related cDNAs, xSox17alpha1, xSox17alpha2, xSox18alpha and xSox18beta

Sox is a large family of genes related to the sex-determining region Y gene (designated as the SRY gene). Sox genes encoding DNA-binding transcriptional factors are found in many animals and are involved in developmental events. In this study, we newly isolated and sequenced novel Sox cDNAs from African clawed frog (Xenopus laevis). Five clones isolated here were classified into four distinct Sox genes designated as xSox17alpha1, xSox17alpha2, xSox18alpha and xSox18beta. All four belong to a subtype of SOX family, type F. The cDNA xSox17alpha1 contains essentially the same nucleotide sequence as that identified as Sox17alpha in a previous work (Cell 91 (1997) 397), whereas xSox17alpha2 is a distinct gene with high homology to xSox17alpha1. The clones, xSox18alpha and xSox18beta, are highly homologous to each other over the entire nucleotide sequences. The xSox18alpha and xSox18beta genes encode 363 and 361 amino acids, respectively. Genomic Southern hybridization analysis showed the existence of two copies of the xSox18. Northern analysis indicated that the xSox18 gene was expressed in the spleen and kidney and the size of the transcript was estimated to be 2.4 knt. Electrophoretic mobility shift assays indicated that recombinant xSox18 polypeptide was capable of binding to the HMG consensus nucleotide sequence, AACAAT.

Depletion of definitive gut endoderm in Sox17-null mutant mice

In the mouse, the definitive endoderm is derived from the epiblast during gastrulation, and, at the early organogenesis stage, forms the primitive gut tube, which gives rise to the digestive tract, liver, pancreas and associated visceral organs. The transcription factors, Sox17 (a Sry-related HMG box factor) and its upstream factors, Mixer (homeobox factor) and Casanova (a novel Sox factor), have been shown to function as endoderm determinants in Xenopus and zebrafish, respectively. However, whether the mammalian orthologues of these genes are also involved with endoderm formation is not known. We show that Sox17–/– mutant embryos are deficient of gut endoderm. The earliest recognisable defect is the reduced occupancy by the definitive endoderm in the posterior and lateral region of the prospective mid- and hindgut of the headfold-stage embryo. The prospective foregut develops properly until the late neural plate stage. Thereafter, elevated levels of apoptosis lead to a reduction in the population of the definitive endoderm in the foregut. In addition, the mid- and hindgut tissues fail to expand. These are accompanied by the replacement of the definitive endoderm in the lateral region of the entire length of the embryonic gut by cells that resemble the visceral endoderm. In the chimeras, although Sox17-null ES cells can contribute unrestrictedly to ectodermal and mesodermal tissues, few of them could colonise the foregut endoderm and they are completely excluded from the mid- and hindgut endoderm. Our findings indicate an important role of Sox17 in endoderm development in the mouse, highlighting the idea that the molecular mechanism for endoderm formation is likely to be conserved among vertebrates.