Regulation of neural determination by evolutionarily conserved signals: anti-BMP factors and what next?

The Hydra stem cell system - Revisited

Cnidarians are >600 million years old and are considered the sister group of Bilateria based on numerous molecular phylogenetic studies. Apart from Hydra, the genomes of all major clades of Cnidaria have been uncovered (e.g. Aurelia, Clytia, Nematostella and Acropora) and they reveal a remarkable completeness of the metazoan genomic toolbox. Of particular interest is Hydra, a model system of aging research, regenerative biology, and stem cell biology. With the knowledge gained from scRNA research, it is now possible to characterize the expression profiles of all cell types with great precision. In functional studies, our picture of the Hydra stem cell biology has changed, and we are in the process of obtaining a clear picture of the homeostasis and properties of the different stem cell populations. Even though Hydra is often compared to plant systems, the new data on germline and regeneration, but also on the dynamics and plasticity of the nervous system, show that Hydra with its simple body plan represents in a nutshell the prototype of an animal with stem cell lineages, whose properties correspond in many ways to Bilateria. This review provides an overview of the four stem cell lineages, the two epithelial lineages that constitute the ectoderm and the endoderm, as well as the multipotent somatic interstitial lineage (MPSC) and the germline stem cell lineage (GSC), also known as the interstitial cells of Hydra.

The Candidate Neuroprotective Agent Artemin Induces Autonomic Neural Dysplasia without Preventing Peripheral Nerve Dysfunction

Artemin (ART) signals through the GFR α—3/RET receptor complex to support sympathetic neuron development. Here we show that ART also influences autonomic elements in adrenal medulla and enteric and pelvic ganglia. Transgenic mice over-expressing Art throughout development exhibited systemic autonomic neural lesions including fusion of adrenal medullae with adjacent paraganglia, adrenal medullary dysplasia, and marked enlargement of sympathetic (superior cervical and sympathetic chain ganglia) and parasympathetic (enteric, pelvic) ganglia. Changes began by gestational day 12.5 and formed progressively larger masses during adulthood. Art supplementation in wild type adult mice by administering recombinant protein or an Art-bearing retroviral vector resulted in hyperplasia or neuronal metaplasia at the adrenal corticomedullary junction. Expression data revealed that Gfr α—3 is expressed during development in the adrenal medulla, sensory and autonomic ganglia and their projections, while Art is found in contiguous mesenchymal domains (especially skeleton) and in certain nerves. Intrathecal Art therapy did not reduce hypalgesia in rats following nerve ligation. These data (1) confirm that ART acts as a differentiation factor for autonomic (chiefly sympathoadrenal but also parasympathetic) neurons, (2) suggest a role for ART overexpression in the genesis of pheochromocytomas and paragangliomas, and (3) indicate that ART is not a suitable therapy for peripheral neuropathy.

Chordin-like protein 1 promotes neuronal differentiation by inhibiting bone morphogenetic protein-4 in neural stem cells

In the present study, the effects of chordin‑like protein 1 (CHRDL1) overexpression, together with bone morphogenetic protein‑4 (BMP‑4) treatment, on the differentiation of rat spinal cord‑derived neural stem cells (NSCs) was investigated. Adult rat spinal cord‑derived NSCs were cultured in serum‑free medium. The recombined eukaryotic expression vector pSecTag2/Hygro B‑CHRDL1 was transfected into adult rat spinal cord‑derived NSCs using a lipid‑based transfection reagent and protein expression was assessed by western blot analysis. Differentiation of transfected NSCs following BMP‑4 treatment was determined by immunocytochemistry. The percentage of microtubule‑associated protein‑2 (MAP‑2)‑positive cells in the BMP‑4‑treated (B) group was found to be significantly lower compared with that in the non‑transfected control (N) group. The percentage of MAP‑2‑positive cells in the pSecTag2/Hygro B‑CHRDL1‑transfected, BMP‑4‑treated group was identified to be significantly higher compared with that in group B, however, no significant difference was observed between group N and the transfected, non‑BMP‑4‑treated control group. The current study indicates that CHRDL1 protein antagonizes BMP‑4 activity and induces spinal cord‑derived NSCs to differentiate into neurons.

Extraocular ectoderm triggers dorsal retinal fate during optic vesicle evagination in zebrafish

Dorsal retinal fate is established early in eye development, via expression of spatially restricted dorsal-specific transcription factors in the optic vesicle; yet the events leading to initiation of dorsal fate are not clear. We hypothesized that induction of dorsal fate would require an extraocular signal arising from a neighboring tissue to pattern the prospective dorsal retina, however no such signal has been identified. We used the zebrafish embryo to determine the source, timing, and identity of the dorsal retina-inducing signal. Extensive cell movements occur during zebrafish optic vesicle morphogenesis, however the location of prospective dorsal cells within the early optic vesicle and their spatial relationship to early dorsal markers is currently unknown. Our mRNA expression and fate mapping analyses demonstrate that the dorsolateral optic vesicle is the earliest region to express dorsal specific markers, and cells from this domain contribute to the dorsal retinal pole at 24 hpf. We show that three bmp genes marking dorsal retina at 25 hpf are also expressed extraocularly before retinal patterning begins. We identified gdf6a as a dorsal initiation signal acting from the extraocular non-neural ectoderm during optic vesicle evagination. We find that bmp2b is involved in dorsal retina initiation, acting upstream of gdf6a. Together, this work has identified the nature and source of extraocular signals required to pattern the dorsal retina.

Self-formation of layered neural structures in three-dimensional culture of ES cells

In vitro neural differentiation culture of embryonic stem cells (ESCs) provides a promising tool for preparing neural cells for replacement therapies and a versatile system for understanding mechanisms of neurogenesis. Consistent with the neural-default model, neural differentiation spontaneously occurs in ESCs cultured in medium containing minimal extrinsic signals. Both adherent monolayer culture and floating aggregation culture can be used for ESC conversion into neural progenitors. The floating aggregation culture has an advantage for recapitulating the formation of three-dimensional (3D) neural tissue structure such as layer formation. In this article, we review recent progress in neural differentiation culture of ESCs using 3D culture, focusing on self-organization phenomena of stratified cortex and retinal tissues. These self-organizing processes are driven by both cell intrinsic programs and local cell-cell interactions. A simple in vitro system using ESCs is useful for elucidating mechanistic dynamics in the complex orchestration of neural development.

Immortality and the base of multicellular life: Lessons from cnidarian stem cells

Cnidarians are phylogenetically basal members of the animal kingdom (>600 million years old). Together with plants they share some remarkable features that cannot be found in higher animals. Cnidarians and plants exhibit an almost unlimited regeneration capacity and immortality. Immortality can be ascribed to the asexual mode of reproduction that requires cells with an unlimited self-renewal capacity. We propose that the basic properties of animal stem cells are tightly linked to this archaic mode of reproduction. The cnidarian stem cells can give rise to a number of differentiated cell types including neuronal and germ cells. The genomes of Hydra and Nematostella, representatives of two major cnidarian classes indicate a surprising complexity of both genomes, which is in the range of vertebrates. Recent work indicates that highly conserved signalling pathways control Hydra stem cell differentiation. Furthermore, the availability of genomic resources and novel technologies provide approaches to analyse these cells in vivo. Studies of stem cells in cnidarians will therefore open important insights into the basic mechanisms of stem cell biology. Their critical phylogenetic position at the base of the metazoan branch in the tree of life makes them an important link in unravelling the common mechanisms of stem cell biology between animals and plants.

Bridging the gap from frog research to human therapy: a tale of neural differentiation in Xenopus animal caps and human pluripotent cells

Over the last decade, much progress has been made toward an understanding of the mechanism of regulation of neural differentiation. In this article, following a brief overview of neural induction research, I would like to discuss the potential contribution of basic embryological research to the progress of human therapeutic development in the present and future, focusing on the medical application of in vitro differentiation of neural tissues. This kind of linkage between basic and medical research will probably be strengthened even more by the recent emergence of human induced pluripotent stem cells. Human pluripotent stem cells are powerful tools for bridging the gap from our accumulated knowledge of embryology to regenerative medicine, as well as to a wide spectrum of medical and pharmaceutical research and development. In this commentary, I describe these issues with a particular emphasis on the contributions made by Japanese scientists.

Presynaptic contributions of chordin to hippocampal plasticity and spatial learning

Recently, several evolutionary conserved signaling pathways that play prominent roles in regulating early neurodevelopment have been found to regulate synaptic remodeling in the adult. To test whether adult neuronal expression of bone morphogenic protein (BMP) signaling components also plays a postnatal role in regulating neuronal plasticity, we modulated BMP signaling in mice both in vivo and in vitro by genetic removal of the BMP inhibitor chordin or by perfusing recombinant BMP signaling pathway components onto acute hippocampal slices. Chordin null mice exhibited a significant increase in presynaptic transmitter release from hippocampal neurons, resulting in enhanced paired-pulse facilitation and long-term potentiation. These mice also showed a decreased acquisition time in a water maze test along with less exploratory activity during Y-maze and open-field tests. Perfusion of BMP ligands onto hippocampal slices replicated the presynaptic phenotype of chordin null slices, but bath application of Noggin, another antagonist of BMP signaling pathway, significantly decrease the frequency of miniature EPSCs. These results demonstrate that the BMP signaling pathway contributes to synaptic plasticity and learning likely through a presynaptic mechanism.

Dullard promotes degradation and dephosphorylation of BMP receptors and is required for neural induction

Bone morphogenetic proteins (BMPs) regulate multiple biological processes, including cellular proliferation, adhesion, differentiation, and early development. In Xenopus development, inhibition of the BMP pathway is essential for neural induction. Here, we report that dullard, a gene involved in neural development, functions as a negative regulator of BMP signaling. We show that Dullard promotes the ubiquitin-mediated proteosomal degradation of BMP receptors (BMPRs). Dullard preferentially complexes with the BMP type II receptor (BMPRII) and partially colocalizes with the caveolin-1-positive compartment, suggesting that Dullard promotes BMPR degradation via the lipid raft-caveolar pathway. Dullard also associates with BMP type I receptors and represses the BMP-dependent phosphorylation of the BMP type I receptor. The phosphatase activity of Dullard is essential for the degradation of BMP receptors and neural induction in Xenopus. Together, these observations suggest that Dullard is an essential inhibitor of BMP receptor activation during Xenopus neuralization.

Symmetry breaking in stem cells of the basal metazoan Hydra

Among the earliest diverging animal phyla are the Cnidaria. Cnidaria were not only first in evolution having a tissue layer construction and a nervous system but also have cells of remarkable plasticity in their differentiation capacity. How a cell chooses to proliferate or to differentiate is an important issue in stem cell biology and as critical to human stem cells as it is to any other stem cell. Here I revise the key properties of stem cells in the freshwater polyp Hydra with special emphasis on the nature of signals that control the growth and differentiation of these cells.

A genomic view of the sea urchin nervous system

The sequencing of the Strongylocentrotus purpuratus genome provides a unique opportunity to investigate the function and evolution of neural genes. The neurobiology of sea urchins is of particular interest because they have a close phylogenetic relationship with chordates, yet a distinctive pentaradiate body plan and unusual neural organization. Orthologues of transcription factors that regulate neurogenesis in other animals have been identified and several are expressed in neurogenic domains before gastrulation indicating that they may operate near the top of a conserved neural gene regulatory network. A family of genes encoding voltage-gated ion channels is present but, surprisingly, genes encoding gap junction proteins (connexins and pannexins) appear to be absent. Genes required for synapse formation and function have been identified and genes for synthesis and transport of neurotransmitters are present. There is a large family of G-protein-coupled receptors, including 874 rhodopsin-type receptors, 28 metabotropic glutamate-like receptors and a remarkably expanded group of 161 secretin receptor-like proteins. Absence of cannabinoid, lysophospholipid and melanocortin receptors indicates that this group may be unique to chordates. There are at least 37 putative G-protein-coupled peptide receptors and precursors for several neuropeptides and peptide hormones have been identified, including SALMFamides, NGFFFamide, a vasotocin-like peptide, glycoprotein hormones and insulin/insulin-like growth factors. Identification of a neurotrophin-like gene and Trk receptor in sea urchin indicates that this neural signaling system is not unique to chordates. Several hundred chemoreceptor genes have been predicted using several approaches, a number similar to that for other animals. Intriguingly, genes encoding homologues of rhodopsin, Pax6 and several other key mammalian retinal transcription factors are expressed in tube feet, suggesting tube feet function as photosensory organs. Analysis of the sea urchin genome presents a unique perspective on the evolutionary history of deuterostome nervous systems and reveals new approaches to investigate the development and neurobiology of sea urchins.

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.

[Pondering in-situ induction of endogenous neural stem cells in hippocampus of rats with Alzheimer disease by acupuncture]

Analysis of domestic and overseas literature on the relationship between Alzheimer disease (AD) and neural stem cells (NSC) shows that inducing the proliferation and differentiation of hippocampal endogenous NSC in-situ by acupuncture is probably the mechanism of acupuncture therapy in treating AD, and that it may further improve the method for the research on AD.

BMP signaling and early embryonic patterning

Bone morphogenetic proteins (BMPs) play pleiotropic roles during embryonic development as well as throughout life. Recent genetic approaches especially using the mouse gene knockout system revealed that BMP signaling is greatly involved in early embryonic patterning, which is a dynamic event to establish three-dimensional polarities. The purpose of this review is to describe the diverse function of BMPs through different receptor signaling systems during embryonic patterning including gastrulation and establishment of the left-right asymmetry.

Directed differentiation of neural and sensory tissues from embryonic stem cells in vitro

We have recently identified a stromal cell-derived inducing activity (SDIA), which induces differentiation of neural cells from mouse embryonic stem (ES) cells. Particularly, midbrain TH+ dopaminergic neurons are generated efficiently in this system. These dopaminergic neurons are transplantable and survive well in the 6-OHDA-treated mouse striatum. SDIA induces co-cultured ES cells to differentiate into rostral central nervous system (CNS) tissues containing both ventral and dorsal cells. While early exposure of SDIA-treated ES cells to BMP4 suppresses neural differentiation and promotes epidermogenesis, late BMP4 exposure after the 4th day of co-culture causes differentiation of neural crest cells and dorsal-most CNS cells, with autonomic system and sensory lineages induced preferentially by high and low BMP4 concentrations, respectively. In contrast, Sonic Hedgehog (Shh) suppresses differentiation of neural crest lineages and promotes that of ventral CNS tissues such as motor neurons and HNF3beta+ floor plate cells with axonal guidance activities. Thus, SDIA-treated ES cells generate naïve precursors that have the competence of differentiating into the "full" dorsal-ventral range of neuroectodermal derivatives in response to patterning signals. I also discuss the role of SDIA and the mode of rostral-caudal specification of neuralized ES cells. In addition, I would like to discuss them in the light of control of in vitro neural production for the use in regenerative medicine for parkinsonism and retinal degeneration.

From placode to polarization: new tunes in inner ear development

The highly orchestrated processes that generate the vertebrate inner ear from the otic placode provide an excellent and circumscribed testing ground for fundamental cellular and molecular mechanisms of development. The recent pace of discovery in developmental auditory biology has been unusually rapid,with hundreds of papers published in the past 4 years. This review summarizes studies addressing several key issues that shape our current thinking about inner ear development, with particular emphasis on early patterning events,sensory hair cell specification and planar cell polarity.

Wnt signalling inhibits neural differentiation of embryonic stem cells by controlling bone morphogenetic protein expression

Wnt signalling plays an important role in both embryonic development and in tumourigenesis. Activation of the signalling cascade by wnt, but also mutations of the adenomatous polyposis coli (APC) protein and of the phosphorylation domain of beta-catenin, result in accumulation of active beta-catenin in the nucleus, where it binds to TCF/LEF transcription factors. We studied the effect of wnt signalling in embryonic stem cells by either inactivating APC or by introducing a dominant active form of beta-catenin. Both resulted in inhibition of neural differentiation in vitro and after brain grafting and in activation of downstream targets of wnt signalling, such as cyclins, c-myc, and bone morphogenetic proteins (BMP). Neural differentiation could be partially restored by the addition of the BMP antagonist noggin. This suggests a mechanism regulating the fate of differentiating embryonic stem cells.

Expression of a SoxB and a Wnt2/13 gene during the development of the mollusc Patella vulgata

We cloned and analysed the expression of a SoxB gene ( PvuSoxB) in the marine mollusc, Patella vulgata. Like its orthologues in deuterostomes, after an early broad ectodermal distribution, PvuSoxB expression only persists in cells competent to form neural structures. In the post-gastrulation larva, PvuSoxB is expressed in the prospective neuroectoderm in the head and in the trunk. No expression can be seen dorsally, around the mouth and the anus, or along the ventral midline. We also report the expression of a Wnt2/13 orthologue ( PvuWnt2) in Patella. After gastrulation, PvuWnt2 is expressed in the posterior part of the mouth, along the ventral midline and around the anus. This expression seems to be complementary to that of PvuSoxB in the trunk. We suggest the existence of a fundamental subdivision of the Patella trunk ectoderm into midline (mouth, midline, anus) and more lateral structures.

hCHL2, a novel chordin-related gene, displays differential expression and complex alternative splicing in human tissues and during myoblast and osteoblast maturation

Chordin-like cysteine-rich repeats (CRs) are conserved domains present in an expanding family of secreted proteins that associate with members of the TGF beta superfamily. In this study, we report the molecular cloning and characterization of CHL2 (chordin-like 2), a novel protein closely related to CHL (chordin-like). Both are members of the chordin family of proteins, and contain a signal peptide and three CR domains. We found that recombinant human CHL2 (hCHL2) protein is secreted and binds activin A, but not BMP-2, -4, or -6. Expression of hCHL2 mRNA and protein was detected in a variety of human tissues and is particularly abundant in the uterus. Extensive and complex alternative splicing of hCHL2 was observed in different tissues, resulting in several distinct protein isoforms that vary substantially in the presence of a signal peptide and their content of CR domains. Differential expression of CHL2 variants was observed during myoblast and osteoblast differentiation, implying a role for this gene in these physiological processes.

Hyzic, the Hydra homolog of the zic/odd-pairedgene, is involved in the early specification of the sensory nematocytes

Cnidaria are the first class of organisms in animal evolution with a nervous system. The cnidarian Hydra has two types of neuronal cell,nerve cells and nematocytes. Both differentiate from the same pool of pluripotent stem cells. Yet, the molecular regulation of neural differentiation in Hydra is largely unknown. Here, we report the identification of Hyzic, a homolog of the Zn-finger transcription factor gene zic/odd-paired, which acts as an early neural effector gene in vertebrates. We show, that Hyzic is expressed in the early nematocyte differentiation pathway, starting at the level of interstitial stem cells. Expression of Hyzic is restricted to the proliferative stages of nematoblasts. Hyzic acts before and possibly directly upstream of Cnash, a homolog of the proneural bHLH transcription factor gene achaete-scute, and of Nowa, an early nematocyte differentiation marker gene. Hyzic may determine stem cells to differentiate into nematocytes. Our data are consistent with a role of Hyzic in inhibiting nematocyte differentiation, by keeping committed nematoblast cells in the cell cycle. A similar role has been demonstrated for Zic genes in vertebrates. Our results suggest, that genetic cascades of neural development may be conserved from Hydra to vertebrates, indicating that the molecular regulation of neural development evolved only once.

A novel secretory factor, Neurogenesin-1, provides neurogenic environmental cues for neural stem cells in the adult hippocampus

Neurogenesis occurs in restricted regions in the adult mammalian brain, among which the neurogenesis in the hippocampal dentate gyrus plays the crucial role in learning and memory. To date, little is known about neurogenic cues, which result in the neuronal fate adoption of neural stem cells residing in neurogenic regions, especially neurogenic cues in adult hippocampal neurogenesis. In the present study, we show that hippocampal astrocytes and also dentate granule cells adjacent to neural stem cells secrete a newly cloned novel secretory factor, Neurogenesin-1. This protein contains three cysteine-rich domains and a unique sequence and contributes to neuronal differentiation of neural stem cells in the adult brain by preventing the adoption of a glial fate. Furthermore, the neurogenic activity detected in the hippocampal culture medium was markedly suppressed by the administration of an anti-Neurogenesin-1 antibody. These findings suggest endogenous mechanisms that induce adult hippocampal neurogenesis and propose an innovative treatment for the neurodegenerative diseases that cause loss of hippocampal neurons.

Transcription factor AP-2 is an essential and direct regulator of epidermal development in Xenopus

Expression of the Xenopus homolog of the mammalian transcription factor AP-2alpha (XAP-2) is activated throughout the animal hemisphere shortly after the midblastula transition, and becomes restricted to prospective epidermis by the end of gastrulation, under the control of BMP signal modulation. Elevated expression in the future neural crest region begins at this time. Ectopic expression of XAP-2 can restore transcription of epidermal genes in neuralized ectoderm, both in ectodermal explants and in the intact embryo. Likewise, loss of XAP-2 function, accomplished by injection of antisense oligonucleotides or by overexpression of antimorphic XAP-2 derivatives, leads to loss of epidermal and gain of neural gene expression. These treatments also result in gastrulation failure. Thus, AP-2 is a critical regulator of ectodermal determination that is required for normal epidermal development and morphogenesis in the frog embryo.

Noggin and chordin have distinct activities in promoting lineage commitment of mouse embryonic stem (ES) cells

To examine the role of secreted signaling molecules and neurogenic genes in early development, we have developed a culture system for the controlled differentiation of mouse embryonic stem (ES) cells. In the current investigation, two of the earliest identified BMP antagonists/neural-inducing factors, noggin and chordin, were expressed in pluripotent mouse ES cells. Neurons were present as early as 24 h following transfection of ES cells with a pCS2/noggin expression plasmid, with differentiation peaking at 72 h. With neuronal differentiation, stem cell marker genes were down-regulated and neural determination genes expressed. Coculture experiments and exposure to noggin-conditioned medium produced similar neuronal differentiation of control ES cells, while addition of BMP-4 to noggin expressants strikingly inhibited neuronal differentiation. Transfection of ES cells with a pCS2/chordin expression vector or exposure to chordin-conditioned medium produced a more complex pattern of differentiation; ES cells formed neurons, mesenchymal cells as well as N-CAM-positive, nestin-positive neuroepithelial progenitors. These data suggest that, consistent with their different expression fields, noggin and chordin may play distinct roles in patterning the early mouse embryo.

Nuclear localization of Duplin, a beta-catenin-binding protein, is essential for its inhibitory activity on the Wnt signaling pathway

Duplin binds to beta-catenin and inhibits the Wnt signaling pathway, thereby leading to repression of the beta-catenin-mediated transactivation and Xenopus axis formation. To find an additional function of Duplin, yeast two-hybrid screening was carried out. Importin alpha was isolated as a binding protein of Duplin. Importin alpha bound directly to basic amino acid clusters of Duplin. Although Duplin was present in the nucleus, deletion of the basic amino acid clusters (Duplin(Delta 500-584)) retained Duplin in the cytoplasm. Duplin(Delta 500-584) bound to beta-catenin as efficiently as wild-type Duplin, but it neither repressed Wnt-dependent Tcf transcriptional activation in mammalian cells nor showed ventralization in Xenopus embryos. The Duplin mutant without a beta-catenin-binding region lost the ability to inhibit the Wnt-dependent Tcf activation, but retained its ventralizing activity. Furthermore, Duplin not only suppressed beta-catenin-dependent axis duplication and expression of siamois, a Wnt-regulated gene, but also inhibited siamois-dependent axis duplication. These results indicate that Duplin is translocated to the nucleus by interacting with importin alpha, and that nuclear localization is essential for the function of Duplin. Moreover, Duplin has an additional activity of inhibiting the Wnt signaling pathway by affecting the downstream beta-catenin target genes.

The effects of BMPs on early chick embryos suggest a conserved signaling mechanism for epithelial and neural induction among vertebrates

In Xenopus embryos neural specification takes place by inhibition of epithelial specification, allowing blastoderm cells to continue their previously established neural determination process. This mechanism has been termed neural induction as a "default state". The understanding of this model has been completed with the identification of molecules that can block the epithelial inducers. The antagonist mechanism between epithelial signals (bone morphogenetic proteins [BMPs]) and their blocking agents (BMP antagonists) can explain the formation of the prospective neural territory and why these BMP antagonists function as neural inducers when over-expressed in Xenopus. Despite this well understood mechanism in amphibians, little information is available for other species. The formation of the neural plate by means of a "default state" mechanism in other vertebrates still lacks experimental confirmation. We present evidence that the growth factors of the BMP family are also epithelial inducers in bird embryos, and that when over-expressed they can partially block neural induction.

Creation of a Sog morphogen gradient in the Drosophila embryo

A variety of genetic evidence suggests that a gradient of Decapentaplegic (Dpp) activity determines distinct cell fates in the dorsal region of the Drosophila embryo, and that this gradient may be generated indirectly by an inverse gradient of the BMP antagonist Short gastrulation (Sog). It has been proposed that Sog diffuses dorsally from the lateral neuroectoderm where it is produced, and is cleaved and degraded dorsally by the metalloprotease Tolloid (Tld). Here we show directly that Sog is distributed in a graded fashion in dorsal cells and that Tld degradation limits the levels of Sog dorsally. In addition, we find that Dynamin-dependent retrieval of Sog acts in parallel with degradation by Tld as a dorsal sink for active Sog.

Neural induction takes a transcriptional twist

Over the past decade, several molecules have been identified that influence neural cell fate in vertebrate embryos during gastrulation. The first neural inducers studied were proteins produced by dorsal mesoderm (the Spemann organizer); most of these proteins act by directly binding to and antagonizing the function of bone morphogenetic proteins (BMPs). Recent experiments have suggested that other secreted signals, such as Wnt and FGF, may neuralize ectoderm before organizer function by a different mechanism. Neural effector genes that mediate the response of ectoderm to secreted neuralizing signals have also been discovered. Interestingly, most of these newly identified neuralizing pathways continue the theme of BMP antagonism, but rather than antagonizing BMP protein function, they may neuralize tissue by suppressing Bmp expression. Down-regulation of Bmp expression in the prospective neural plate during gastrulation seems to be a shared feature of neural induction in vertebrate embryos. However, the signals used to accomplish this task seem to vary among vertebrates. Here, we will discuss the role of the recently identified secreted signals and neural effector genes in vertebrate neurogenesis.