Neural induction takes a transcriptional twist

On the nature and function of organizers

Organizers, which comprise groups of cells with the ability to instruct adjacent cells into specific states, represent a key principle in developmental biology. The concept was first introduced by Spemann and Mangold, who showed that there is a cellular population in the newt embryo that elicits the development of a secondary axis from adjacent cells. Similar experiments in chicken and rabbit embryos subsequently revealed groups of cells with similar instructive potential. In birds and mammals, organizer activity is often associated with a structure known as the node, which has thus been considered a functional homologue of Spemann's organizer. Here, we take an in-depth look at the structure and function of organizers across species and note that, whereas the amphibian organizer is a contingent collection of elements, each performing a specific function, the elements of organizers in other species are dispersed in time and space. This observation urges us to reconsider the universality and meaning of the organizer concept.

Summary: This Review re-evaluates the notion of Spemann's organizer as identified in amphibians, highlighting the spatiotemporal dispersion of equivalent elements in mouse and the key influence of responsiveness to organizer signals.

Down-regulation of coasy, the gene associated with NBIA-VI, reduces Bmp signaling, perturbs dorso-ventral patterning and alters neuronal development in zebrafish

Mutations in Pantothenate kinase 2 and Coenzyme A (CoA) synthase (COASY), genes involved in CoA biosynthesis, are associated with rare neurodegenerative disorders with brain iron accumulation. We showed that zebrafish pank2 gene plays an essential role in brain and vasculature development. Now we extended our study to coasy. The gene has high level of sequence identity with the human ortholog and is ubiquitously expressed from the earliest stages of development. The abrogation of its expression led to strong reduction of CoA content, high lethality and a phenotype resembling to that of dorsalized mutants. Lower doses of morpholino resulted in a milder phenotype, with evident perturbation in neurogenesis and formation of vascular arborization; the dorso-ventral patterning was severely affected, the expression of bone morphogenetic protein (Bmp) receptors and activity were decreased, while cell death increased. These features specifically correlated with the block in CoA biosynthesis and were rescued by the addition of CoA to fish water and the overexpression of the human wild-type, but not mutant gene. These results confirm the absolute requirement for adequate levels of CoA for proper neural and vascular development in zebrafish and point to the Bmp pathway as a possible molecular connection underlining the observed phenotype.

Expression of sclerostin in the developing zebrafish (Danio rerio) brain and skeleton

Sclerostin is a highly conserved, secreted, cystine-knot protein which regulates osteoblast function. Humans with mutations in the sclerostin gene (SOST), manifest increased axial and appendicular skeletal bone density with attendant complications. In adult bone, sclerostin is expressed in osteocytes and osteoblasts. Danio rerio sclerostin-like protein is closely related to sea bass sclerostin, and is related to chicken and mammalian sclerostins. Little is known about the expression of sclerostin in early developing skeletal or extra-skeletal tissues. We assessed sclerostin (sost) gene expression in developing zebrafish (D. rerio) embryos with whole mount is situ hybridization methods. The earliest expression of sost mRNA was noted during 12h post-fertilization (hpf). At 15 hpf, sost mRNA was detected in the developing nervous system and in Kupffer's vesicle. At 18, 20 and 22 hpf, expression in rhombic lip precursors was seen. By 24 hpf, expression in the upper and lower rhombic lip and developing spinal cord was noted. Expression in the rhombic lip and spinal cord persisted through 28 hpf and then diminished in intensity through 44 hpf. At 28 hpf, sost expression was noted in developing pharyngeal cartilage; expression in pharyngeal cartilage increased with time. By 48 hpf, sost mRNA was clearly detected in the developing pharyngeal arch cartilage. Sost mRNA was abundantly expressed in the pharyngeal arch cartilage, and in developing pectoral fins, 72, 96 and 120 hpf. Our study is the first detailed analysis of sost gene expression in early metazoan development.

Genetic basis of neural tube defects

Neural tube defects (NTDs) represent a common group of severe congenital malformations of the central nervous system. They result from failure of neural tube closure during early embryonic life. Their etiology is complex, involving environmental and genetic factors that interact to modulate the incidence and severity of the developing phenotype. Despite a long history of etiologic studies, the molecular and cellular pathogenic mechanisms underlining NTDs remain poorly understood. The major epidemiologic finding in NTDs is the protective effect of perinatal folic acid supplementation that reduces their risk by 60%-70%. Genetic studies in NTDs have focused mainly on folate-related genes and identified a few significant associations between variants in these genes and an increased risk for NTDs. The candidate gene approach investigating genes involved in neurulation and inferred from animal models has faced limited success in identifying major causative genes predisposing to NTDs. However, we are witnessing a rapid and impressive progress in understanding the genetic basis of NTDs, based mainly on the development of whole genome innovative technologies and the powerful tool of animal models.

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.

Neural stem cells in the mammalian brain

New fundamental results on stem cell biology have been obtained in the past 15 years. These results allow us to reinterpret the functioning of the cerebral tissue in health and disease. Proliferating stem cells have been found in the adult brain, which can be involved in postinjury repair and can replace dead cells under specific conditions. Numerous genomic mechanisms controlling stem cell proliferation and differentiation have been identified. The involvement of stem cells in the genesis of malignant tumors has been demonstrated. Neural stem cell tropism toward tumors has been shown. These findings suggest new lines of research on brain functioning and development. Stem cells can be used to develop radically new treatments of neurodegenerative and cancer diseases of the brain.

A mechanism regulating the onset of Sox2 expression in the embryonic neural plate

In vertebrate embryos, the earliest definitive marker for the neural plate, which will give rise to the entire central nervous system, is the transcription factor Sox2. Although some of the extracellular signals that regulate neural plate fate have been identified, we know very little about the mechanisms controlling Sox2 expression and thus neural plate identity. Here, we use electroporation for gain- and loss-of-function in the chick embryo, in combination with bimolecular fluorescence complementation, two-hybrid screens, chromatin immunoprecipitation, and reporter assays to study protein interactions that regulate expression of N2, the earliest enhancer of Sox2 to be activated and which directs expression to the largest part of the neural plate. We show that interactions between three coiled-coil domain proteins (ERNI, Geminin, and BERT), the heterochromatin proteins HP1α and HP1γ acting as repressors, and the chromatin-remodeling enzyme Brm acting as activator control the N2 enhancer. We propose that this mechanism regulates the timing of Sox2 expression as part of the process of establishing neural plate identity.

During early development, when the embryo has three layers of cells (ectoderm, mesoderm, and endoderm), a region of the ectoderm called the neural plate becomes specified to generate the entire nervous system. One of the earliest molecular markers for the neural plate is the transcription factor Sox2, which is critical for cells to acquire their neural fates and also defines neural progenitor character. We know very little about the intracellular mechanisms by which the neural plate cells acquire these fates. Here, we show that recruitment of transcriptional repressors to chromatin-remodeling complexes regulate the onset of Sox2 expression. Competitive interactions between three proteins, ERNI, BERT, and Geminin, modulate the choice of repressors and regulate Sox2 expression. During gastrulation, when the three embryonic cell layers form, ERNI recruits the repressor HP1γ to prevent Geminin from activating Sox2 prematurely. By the end of gastrulation, this repression is counteracted by competitive binding of BERT to ERNI and Geminin, causing activation of Sox2. We propose that this mechanism regulates the timing of Sox2 activation in the very early neural plate and thus helps to define the domain that will give rise to the nervous system.

The vertebrate nervous system develops from the neural plate, defined by the transcription factor Sox2. This paper uncovers a mechanism that regulates the timing of Sox2 expression, involving interactions between several proteins and chromatin remodeling.

Neural induction from ES cells portrays default commitment but instructive maturation

The neural induction has remained a debatable issue pertaining to whether it is a mere default process or it involves precise instructive cues. We have chosen the embryonic stem (ES) cell model to address this issue. In a devised monoculture strategy, the cell-cell interaction availed through optimum cell plating density could define the niche for the attainment of efficient in vitro neurogenesis from the ES cells. The medium plating density was found ideal in generating optimum number of progenitors and also yielded about 80% mature neurons in a serum free culture set up barring any exogenous inducers. We could also demarcate and quantify the neural stem cells/progenitors among the heterogeneous cell population of differentiating ES cells using nestin intron II driven EGFP expression as a tool. The one week post-plating was determined to be the critical time window for optimum neural progenitor generation from ES cells that helped us further in purifying these cells and in demonstrating their proliferation and multipotent differentiation potential. Seeding cells at varying densities, we could decipher an interesting paradoxical scenario that interlinked both commitment and maturation with the initial plating density having a vital influence on neuronal maturation but not specification and the secretory factors were apparently playing a key role during this process. Thus it was comprehended that, the neural specification was a default process independent of exogenous factors and cellular interaction. Conversely, a defined number of cells at the specification stage itself seemed critical to provide an auto-/paracrine means of signaling threshold for the maturation process to materialize.

Neural induction and neural stem cell development

Embryonic stem (ES) cells are a pluripotent and renewable cellular resource with tremendous potential for broad applications in regenerative medicine. Arguably the most important consideration for stem cell-based therapies is the ability to precisely direct the differentiation of stem cells along a preferred cellular lineage. During development, lineage commitment is a multistep process requiring the activation and repression of sets of genes at various stages, from an ES cell identity to a tissue-specific stem cell identity and beyond. Thus, the challenge is to ensure that the pattern of genomic regulation is recapitulated during the in vitro differentiation of ES cells into stem/progenitor cells of the appropriate tissue in a robust, predictable and stable manner. To address this issue, we must understand the ontogeny of tissue-specific stem cells during normal embryogenesis and compare the ontogeny of tissue-specific stem cells in ES cell models. Here, we discuss the issue of directed differentiation of pluripotent ES cells into neural stem cells, which is fundamentally linked to two early events in the development of the mammalian nervous system: the 'decision' of the ectoderm to acquire a neural identity (neural determination) and the origin of neural stem cells within this neural-committed population of cells. A clearer understanding of the molecular and cellular mechanisms that govern mammalian neural cell fate determination will lead to improved ES technology applications in neural regeneration.

Ex vivo expansion and pluripotential differentiation of cryopreserved human bone marrow mesenchymal stem cells

This study is aimed at investigating the potentials of ex vivo expansion and pluri-differentiation of cryopreservation of adult human bone marrow mesenchymal stem cells (hMSCs) into chondrocytes, adipocytes and neurocytes. Cryopreserved hMSCs were resuscitated and cultured for 15 passages, and then induced into chondrocytes, adipocytes and neurocytes with corresponding induction medium. The induced cells were observed for morphological properties and detected for expressions of type II collagen, triglyceride or neuron-specific enolase and nestin. The result showed that the resuscitated cells could differentiate into chondrocytes after exposure to transforming growth factor beta(1) (TGF-beta(1)), insulin-like growth factor I (IGF-I) and vitamin C (V(C)), and uniformly changed morphologically from a spindle-like fibroblastic appearance to a polygonal shape in three weeks. The induced cells were heterochromatic to safranin O and expressed cartilage matrix-procollagenal (II) mRNA. The resuscitated cells cultured in induction medium consisting of dexamethasone, 3-isobutyl-1-methylxanthine, indomethacin and IGF-I showed adipogenesis, and lipid vacuoles accumulation was detectable after 21 d. The resuscitated hMSCs were also induced into neurocytes and expressed nestin and neuron specific endolase (NSE) that were special surface markers associated with neural cells at different stage. This study suggested that the resuscitated hMSCs should be still a population of pluripotential cells and that it could be used for establishing an abundant hMSC reservoir for further experiment and treatment of various clinical diseases.

Neural induction: 10 years on since the 'default model'

Neural induction is the process by which embryonic cells in the ectoderm make a decision to acquire a neural fate (to form the neural plate) rather than give rise to other structures such as epidermis or mesoderm. An influential model proposed a decade ago, the 'default model', postulated that ectodermal cells will become neurons if they receive no signals at all, but that this is normally inhibited in prospective epidermal cells by the action of bone morphogenetic proteins. Recent results now reveal considerable more complexity and emphasis is shifting from intercellular signalling factors to trying to understand the regulation of expression of key genes within the nucleus.

Patterning the developing diencephalon

The diencephalon is the embryonic precursor to the caudal forebrain. The major diencephalic derivative is the thalamus, which functions as a relay station between the cortex and lower nervous system structures. Although the diencephalon has been recognized as a vital brain region, our understanding of its development remains superficial. In this review, we discuss recent progresses in understanding one essential aspect of diencephalic development, diencephalic patterning. Signaling centers identified in the zona limitans intrathalamica and along the dorsal and ventral midlines have emerged as essential organizers in diencephalic patterning. The cumulative data reveal that the diencephalon shares some developmental principles with more caudal brain regions, whereas other mechanisms are unique to this region.

Metastasis-associated kinase modulates Wnt signaling to regulate brain patterning and morphogenesis

Wnt signaling is a major pathway regulating cell fate determination, cell proliferation and cell movements in vertebrate embryos. Distinct branches of this pathway activate beta-catenin/TCF target genes and modulate morphogenetic movements in embryonic tissues by reorganizing the cytoskeleton. The selection of different molecular targets in the pathway is driven by multiple phosphorylation events. Here, we report that metastasis-associated kinase (MAK) is a novel regulator of Wnt signaling during morphogenetic movements, and eye and brain development in Xenopus embryos. Injected MAK RNA suppressed Wnt transcriptional reporters and activated Jun N-terminal kinase. Furthermore, MAK was recruited to the cell membrane by Frizzled 3, formed a complex with Dishevelled and phosphorylated Dsh in vitro. The regional brain markers Otx2, En2 and Gbx2 were affected in embryos with modulated MAK activity in a manner consistent with a role for MAK in midbrain-hindbrain boundary formation. Confirming the inhibitory role for this kinase in Wnt/beta-catenin signaling, the midbrain patterning defects in embryos depleted of MAK were rescued by the simultaneous depletion of beta-catenin. These findings indicate that MAK may function in different developmental processes as a switch between the canonical and non-canonical branches of Wnt signaling.

Embryonic stem cells assume a primitive neural stem cell fate in the absence of extrinsic influences

The mechanisms governing the emergence of the earliest mammalian neural cells during development remain incompletely characterized. A default mechanism has been suggested to underlie neural fate acquisition; however, an instructive process has also been proposed. We used mouse embryonic stem (ES) cells to explore the fundamental issue of how an uncommitted, pluripotent mammalian cell will self-organize in the absence of extrinsic signals and what cellular fate will result. To assess this default state, ES cells were placed in conditions that minimize external influences. Individual ES cells were found to rapidly transition directly into neural cells, a process shown to be independent of suggested instructive factors (e.g., fibroblast growth factors). Further, we provide evidence that the default neural identity is that of a primitive neural stem cell (NSC). The exiguous conditions used to reveal the default state were found to present primitive NSCs with a survival challenge (limiting their persistence and proliferation), which could be mitigated by survival factors or genetic interference with apoptosis.

Identification of a BMP inhibitor-responsive promoter module required for expression of the early neural gene zic1

Expression of the transcription factor zic1 at the onset of gastrulation is one of the earliest molecular indicators of neural fate determination in Xenopus. Inhibition of bone morphogenetic protein (BMP) signaling is critical for activation of zic1 expression and fundamental for establishing neural identity in both vertebrates and invertebrates. The mechanism by which interruption of BMP signaling activates neural-specific gene expression is not understood. Here, we report identification of a 215 bp genomic module that is both necessary and sufficient to activate Xenopus zic1 transcription upon interruption of BMP signaling. Transgenic analyses demonstrate that this BMP inhibitory response module (BIRM) is required for expression in the whole embryo. Multiple consensus binding sites for specific transcription factor families within the BIRM are required for its activity and some of these regions are phylogenetically conserved between orthologous vertebrate zic1 genes. These data suggest that interruption of BMP signaling facilitates neural determination via a complex mechanism, involving multiple regulatory factors that cooperate to control zic1 expression.

Frodo proteins: modulators of Wnt signaling in vertebrate development

The Frodo/dapper (Frd) proteins are recently discovered signaling adaptors, which functionally and physically interact with Wnt and Nodal signaling pathways during vertebrate development. The Frd1 and Frd2 genes are expressed in dynamic patterns in early embryos, frequently in cells undergoing epithelial-mesenchymal transition. The Frd proteins function in multiple developmental processes, including mesoderm and neural tissue specification, early morphogenetic cell movements, and organogenesis. Loss-of-function studies using morpholino antisense oligonucleotides demonstrate that the Frd proteins regulate Wnt signal transduction in a context-dependent manner and may be involved in Nodal signaling. The identification of Frd-associated factors and cellular targets of the Frd proteins should shed light on the molecular mechanisms underlying Frd functions in embryonic development and in cancer.

Neural induction: old problem, new findings, yet more questions

During neural induction, the embryonic neural plate is specified and set aside from other parts of the ectoderm. A popular molecular explanation is the 'default model' of neural induction, which proposes that ectodermal cells give rise to neural plate if they receive no signals at all, while BMP activity directs them to become epidermis. However, neural induction now appears to be more complex than once thought, and can no longer be fully explained by the default model alone. This review summarizes neural induction events in different species and highlights some unanswered questions about this important developmental process.

Neural induction requires BMP inhibition only as a late step, and involves signals other than FGF and Wnt antagonists

A dominant molecular explanation for neural induction is the 'default model', which proposes that the ectoderm is pre-programmed towards a neural fate, but is normally inhibited by endogenous BMPs. Although there is strong evidence favouring this in Xenopus, data from other organisms suggest more complexity, including an involvement of FGF and modulation of Wnt. However, it is generally believed that these additional signals also act by inhibiting BMPs. We have investigated whether BMP inhibition is necessary and/or sufficient for neural induction. In the chick, misexpression of BMP4 in the prospective neural plate inhibits the expression of definitive neural markers (Sox2 and late Sox3), but does not affect the early expression of Sox3, suggesting that BMP inhibition is required only as a late step during neural induction. Inhibition of BMP signalling by the potent antagonist Smad6, either alone or together with a dominant-negative BMP receptor, Chordin and/or Noggin in competent epiblast is not sufficient to induce expression of Sox2 directly, even in combination with FGF2, FGF3, FGF4 or FGF8 and/or antagonists of Wnt signalling. These results strongly suggest that BMP inhibition is not sufficient for neural induction in the chick embryo. To test this in Xenopus, Smad6 mRNA was injected into the A4 blastomere (which reliably contributes to epidermis but not to neural plate or its border) at the 32-cell stage: expression of neural markers (Sox3 and NCAM) is not induced. We propose that neural induction involves additional signalling events that remain to be identified.

Sensory organs: making and breaking the pre-placodal region

Sensory placodes are unique domains of thickened ectoderm in the vertebrate head that form important parts of the cranial sensory nervous system, contributing to sense organs and cranial ganglia. They generate many different cell types, ranging from simple lens fibers to neurons and sensory cells. Although progress has been made to identify cell interactions and signaling pathways that induce placodes at precise positions along the neural tube, little is known about how their precursors are specified. Here, we review the evidence that placodes arise from a unique territory, the pre-placodal region, distinct from other ectodermal derivatives. We summarize the cellular and molecular mechanisms that confer pre-placode character and differentiate placode precursors from future neural and neural crest cells. We then examine the events that subdivide the pre-placodal region into individual placodes with distinct identity. Finally, we discuss the hypothesis that pre-placodal cells have acquired a state of "placode bias" that is necessary for their progression to mature placodes and how such bias may be established molecularly.

Patterning and tissue movements in a novel explant preparation of the marginal zone of Xenopus laevis

Development of the Xenopus embryo has provided an adaptable framework for the rapid evaluation of molecular factors that guide patterning and morphogenesis. We present and characterize a novel explant preparation that is useful for such studies. This preparation consists of 180 degrees of the marginal zone of the early Xenopus gastrula cultured on a fibronectin-coated substrate. In addition to a thorough description of its preparation, we analyze gene expression patterns at three stages of development. The stereotypic morphogenesis of this explant can be understood in the context of the intact embryo through a catalog of gene expression patterns providing definitive identities for epidermis, anterior and posterior neural, notochord, somitic mesoderm, and mesendoderm.

DLX5 positions the neural crest and preplacode region at the border of the neural plate

The neural crest and sensory placodes arise from a region of the embryonic ectoderm that lies between the neural plate and future epidermis. While some of the signalling pathways that are involved in cell fate determination at the border of the neural plate have been characterised, it is still unclear how different signals are integrated. Transcription factors of the DLX gene family that may mediate such cell fate decisions are expressed at the border of the neural plate. Here, we demonstrate that DLX5 is involved in positioning this border by repressing neural properties and simultaneously by promoting the formation of border-like cells that express the neural fold markers MSX1 and BMP4 and the preplacodal region marker SIX4. However, DLX5 is not sufficient to impart epidermal character or to specify cell fates that arise at the border of the neural plate, like neural crest or fully formed sensory placodes, in a cell-autonomous manner. Additional signals are generated when mature neural plate and epidermis interact and these are required for neural crest formation. We propose that patterning of the embryonic ectoderm is a multistep process that sequentially subdivides the ectoderm into regions with defined cell fates.

Induction and patterning of neuronal development, and its connection to cell cycle control

Nervous tissue is derived from early embryonic ectoderm, which also gives rise to epidermal derivatives such as skin. The progression from naive ectoderm to differentiated postmitotic neurons involves multiple steps, two of which are crucial in shaping the final neurogenesis pattern. First, is the identification of the neural plate by the process of neural induction. Second, is the selection of a restricted number of sites within the neural plate where neurogenesis, the process leading to final differentiation of neural precursors, is initiated. Recent findings point to the existence of positive inducers of the neural state, whereas, neurogenesis initiation sites appear to be largely defined by inhibition. However, both neural induction and the initiation of neurogenesis appear to be connected to cell cycle control systems that govern whether stem cell maintenance and cell proliferation, or cell specification and differentiation, take place.

Induction and initial patterning of the nervous system - the chick embryo enters the scene

Until recently, almost everything known about the molecular controls of early neural development came from studies in amphibians. It is now possible to misexpress factors in chick embryos at relatively late stages in development, allowing careful dissection of the timing of cell interactions. This is starting to contribute significantly to our understanding of neural induction and early patterning.