Paul J. Scotting

Embryonic expression of the chicken Sox2, Sox3 and Sox11 genes suggests an interactive role in neuronal development

Three chicken Sox (SRY-like box) genes have been identified that show an interactive pattern of expression in the developing embryonic nervous system. cSox2 and cSox3 code for related proteins and both are predominantly expressed in the immature neural epithelium of the entire CNS of HH stage 10 to 34 embryos. cSox11 is related to cSox2 and cSox3 only by virtue of containing an SRY-like HMG-box sequence but shows extensive homology with Sox-4 at its C-terminus. cSox11 is expressed in the neural epithelium but is transiently upregulated in maturing neurons after they leave the neural epithelium. These patterns of expression suggest that Sox genes play a role in neural development and that the developmental programme from immature to mature neurons may involve switching of Sox gene expression. cSox11 also exhibits a lineage restricted pattern of expression in the peripheral nervous system.

Dynamic expression of chicken Sox2 and Sox3 genes in ectoderm induced to form neural tissue

The chick genes, cSox2 and cSox3, are members of a large family of genes that encode transcription factors. Previous studies have shown that these genes are predominantly expressed in the central nervous system during embryonic development. We show that cSox3 is expressed throughout the ectoderm that is competent to form nervous tissue before neural induction. The expression of cSox3 is lost from cells as they undergo gastrulation to form nonectodermal tissues; the transcription factor, Brachyury, appears in cells about to undergo gastrulation a short time before cSox3 transcripts are lost. Therefore, Brachyury expression may act functionally upstream of cSox3 downregulation. cSox3 expression is also lost from non‐neuronal ectoderm shortly after the neural plate becomes morphologically apparent. cSox2 expression increases dramatically in the central nervous system as neural ectoderm is established. The appearance of cSox2 in neural ectoderm represents one of the earliest molecular responses to neural induction documented thus far. Dev. Dyn. 209:323–332, 1997.

Chick sox10, a transcription factor expressed in both early neural crest cells and central nervous system.

Human SOX10 and mouse Sox10 have been cloned and shown to be expressed in the neural crest derivatives that contribute to formation of the peripheral nervous system during embryogenesis. Mutations in Sox10 have been identified as a cause of the Dominant megacolon mouse and Waardenburg-Shah syndrome in human, both of which include defects in the enteric nervous system and pigmentation (and in the latter, sometimes hearing). We have cloned a chick Sox10 ortholog (cSox10) in order to study its role in neural crest cell development. This cDNA reveals a 1383 bp open reading frame encoding 461 amino acids which is highly conserved with human SOX10 and mouse Sox10. In situ hybridization showed cSox10 is expressed in migrating neural crest cells just after the zinc finger transcription factor Slug, but is lost as cells undergo neuronal differentiation in ganglia of the peripheral nervous system. In addition, cSox10 is expressed in the developing otic vesicle, the developing central nervous system and pineal gland.

Roles of Sox4 in central nervous system development

The transcription factor-encoding gene, Sox4, is expressed in a wide range of tissues and has been shown to be functionally involved in heart, B-cell and reproductive system development. Sox4 shows a high degree of sequence homology with another group C Sox gene, Sox11, which is predominantly expressed in the CNS. Since the expression of Sox4 in the CNS has not been described we have carried out such a study. Sox4 and Sox11 expression increased simultaneously in the same early differentiating cells of the developing CNS except in the external granule layer of the cerebellum where Sox11 expression preceded that of Sox4. As development proceeded, their expression always appeared to relate to the maturational stage of the cell population, with Sox11 expression more transient than Sox4, except in the spinal cord where the reverse was true. Sox4 knock-out mice have been shown to die of a heart defect half way through gestation with no observable CNS phenotype. Our more detailed analysis showed no abnormality in the spatial restriction of expression of Sox2, Sox11, Mash1, neurogenin1 or neurogenin2, although the level of expression of Sox11 and Mash1 appeared a little different from the wild-type, implying that Sox4 might indeed have a functional role in CNS development. However, since Sox4 and Sox11 expression is so similar, we propose that Sox11 might compensate for the loss of Sox4 function in the CNS such that the phenotype is extremely mild in the Sox4 null mutant.

Adult mesenchymal stem cells: Differentiation potential and therapeutic applications

Adult mesenchymal stem cells (MSCs) are a population of multipotent cells found primarily in the bone marrow. They have long been known to be capable of osteogenic, adipogenic and chondrogenic differentiation and are currently the subject of a number of trials to assess their potential use in the clinic. Recently, the plasticity of these cells has come under close scrutiny as it has been suggested that they may have a differentiation potential beyond the mesenchymal lineage. Myogenic and in particular cardiomyogenic potential has been shown in vitro. MSCs have also been shown to have the ability to form neural cells both in vitro and in vivo, although the molecular mechanisms underlying these apparent transdifferentiation events are yet to be elucidated. We describe here the cellular characteristics and differentiation potential of MSCs, which represent a promising stem cell population for future applications in regenerative medicine.

Region-Specific Expression of Chicken Sox2 in the Developing Gut and Lung Epithelium: Regulation by Epithelial-Mesenchymal Interactions

In situ analysis of the chicken cSox2 gene, a member of the transcription factor family containing an Sry‐like high‐mobility group (HMG) box, demonstrated localized expression in the embryonic endoderm. Transcripts of cSox2 appeared before commencement of morphogenesis and cytodifferentiation in the rostral gut epithelium from the pharynx to the stomach. The caudal limit of cSox2 expression coincided with that of the region competent for proventricular differentiation and to the rostral limit of the domain of CdxA, a homologue of Drosophila caudal. During morphogenesis, the level of transcripts of cSox2 decreased in epithelia invaginating into surrounding mesenchyme to form glandular or tubular structures, such as the primordia of the thyroid and lung, glandular epithelium of the proventriculus, and secondary bronchus of the lung. Tissue recombination experiments demonstrated that cSox2 expression is regulated by the underlying mesenchyme as well as morphogenesis and cytodifferentiation. The results suggest that cSox2 plays pivotal roles in generating morphologically and physiologically distinct types of epithelial cells in the gut. Dev. Dyn. 1998;213:464–475.

Differential Expression of SOX4 and SOX11 in Medulloblastoma

Primitive neuroectodermal tumors (PNETs) are composed of immature neuronal precursor cells and sometimes more mature neuronal cell types. Medulloblastomas, occuring in the cerebellum, represent the most common PNET and are broadly classified into two subgroups: classical and desmoplastic. Desmoplastic medulloblastomas exhibit a slightly better prognosis than classical medulloblastomas. However, there are currently no good molecular markers available to distinguish clinical outcome and similar treatment is used for most patients with associated complications. It has been shown that neoplastic cells in these tumors recapitulate stages in maturation of normal human neuroblasts; therefore, embryological studies of the earliest events in the development of the cerebellum may provide useful information about the molecular behavior of the tumor. Transcription factors such as Sox proteins involved in neural development may also play a role in the etiology of brain tumors. Sox4 in particular has been implicated in the biology of several other types of cancer. We have studied the expression of Sox4, and the closely related Sox11 gene, in medulloblastomas. Sox4 and Sox11 were strongly expressed in most classical medulloblastomas but only weakly in desmoplastic medulloblastomas. The expression profile of these two genes in developing cerebellum was also analyzed. Our results suggest that strong Sox4 and Sox11 expression in classical medulloblastomas reflects their maturation-dependent expression during normal cerebellum development, and that they may therefore provide markers to divide tumors into clinically relevant subgroups.

Stem cell marker expression in the Bergmann glia population of the adult mouse brain

Recent evidence suggests that the postnatal cerebellum contains cells with characteristics of neural stem cells, which had so far only been identified in the subventricular zone of the lateral ventricles and the subdentate gyrus of the hippocampus. In order to investigate the identity of these cells in the adult cerebellum, we have analyzed the expression of Sox1, a transcription factor from the SoxB1 subgroup and widely used marker of neural stem cells. In situ hybridization and the use of a transgenic mouse model show that, in the adult cerebellum, Sox 1 is only expressed in the Bergmann glia, a population of radial glia present in the Purkinje cell layer. Furthermore, another neural stem cell marker, Sox2 (also member of the SoxB1 subgroup), is also expressed in the Bergmann glia. We have previously shown that these same cells express Sox9, a member of the SoxE subgroup known for its role in glial development. Here we show that Sox9 is in fact also expressed in other regions harboring adult neural stem cells, suggesting that Sox9 represents a novel stem cell marker. Finally, using a Sox1-null mouse, we show that the formation of this Sox2/Sox9 positive Bergmann glia population does not require the presence of a functional Sox1. Our results identify these radial glia as a previously unreported Sox1/Sox2/Sox9 positive adult cell population, suggesting that these cells may represent the recently reported stem cells in the adult cerebellum.

Childhood solid tumours: a developmental disorder

Several lines of evidence demonstrate that the biology, genetics and environment of childhood solid tumours (CSTs) sets them apart from adult solid tumours. The nature of the progenitor cells from which these tumours arise, and their immature tissue environment, allows CSTs to develop with fewer defects in cell regulatory processes than adult cancers. These differences could explain why CSTs are more susceptible to therapeutic intervention than adult tumours. How does the aetiology of these cancers differ from those occurring in adults and how might this affect the development of more effective therapies?

Sox3 regulates both neural fate and differentiation in the zebrafish ectoderm.

Little is known of the first transcriptional events that regulate neural fate in response to extracellular signals such as Bmps and Fgfs. Sox3 is one of the earliest transcription factors to be expressed in the developing CNS and has been shown to be regulated by these signalling pathways. We have used both gain- and loss-of-function experiments in zebrafish to elucidate the role of Sox3 in determining neural fate. Ectopic Sox3 caused induction of neural tissue from a very early stage of cell specification in the ectoderm and this effect was maintained such that large domains of additional CNS were apparent, including almost complete duplications of the CNS. Knock-down of Sox3 using morpholinos resulted in a reduction in the size of the CNS, ears and eyes and subsequent inhibition of some aspects of neurogenesis. Our data also suggest that the pro-neural effects of Sox3 can compensate for inhibition of Fgf signalling in inducing neural tissue but it is not sufficient to maintain neural fate, suggesting the presence of Sox3-independent roles of Fgf at later stages.