A direct requirement for Hedgehog signaling for normal specification of all ventral progenitor domains in the presumptive mammalian spinal cord

Signaling from Smo to Ci/Gli: conservation and divergence of Hedgehog pathways from Drosophila to vertebrates

Although the framework of the Hedgehog (Hh) signaling pathway is evolutionarily conserved, recent studies indicate that fundamental differences exist between Drosophila and vertebrates in the way signals are transduced from the membrane protein Smoothened (Smo) to the Ci/Gli transcription factors. For example, Smo structure and the roles of fused and Suppressor of fused have diverged. Recently, many vertebrate-specific components have been identified that act between Smo and Gli. These include intra-flagellar transport proteins, which link vertebrate Hh signaling to cilia. Because abnormal Hh signaling can cause birth defects and cancer, these vertebrate-specific components may have roles in human health.

How do genes regulate simple behaviours? Understanding how different neurons in the vertebrate spinal cord are genetically specified

Understanding how the vertebrate central nervous system develops and functions is a major goal of a large body of biological research. This research is driven both by intellectual curiosity about this amazing organ that coordinates our conscious and unconscious bodily processes, perceptions and actions and by the practical desire to develop effective treatments for people with spinal cord injuries or neurological diseases. In recent years, we have learnt an impressive amount about how the nerve cells that communicate with muscles, motoneurons, are made in a developing embryo and this knowledge has enabled researchers to grow motoneurons from stem cells. Building on the success of these studies, researchers have now started to unravel how most of the other nerve cells in the spinal cord are made and function. This review will describe what we currently know about spinal cord nerve cell development, concentrating on the largest category of nerve cells, which are called interneurons. I will then discuss how we can build and expand upon this knowledge base to elucidate the complete genetic programme that determines how different spinal cord nerve cells are made and connected up into neural circuits with particular functions.

The cell surface membrane proteins Cdo and Boc are components and targets of the Hedgehog signaling pathway and feedback network in mice

Cdo and Boc encode cell surface Ig/fibronectin superfamily members linked to muscle differentiation. Data here indicate they are also targets and signaling components of the Sonic hedgehog (Shh) pathway. Although Cdo and Boc are generally negatively regulated by Hedgehog (HH) signaling, in the neural tube Cdo is expressed within the Shh-dependent floor plate while Boc expression lies within the dorsal limit of Shh signaling. Loss of Cdo results in a Shh dosage-dependent reduction of the floor plate. In contrast, ectopic expression of Boc or Cdo results in a Shh-dependent, cell autonomous promotion of ventral cell fates and a non-cell-autonomous ventral expansion of dorsal cell identities consistent with Shh sequestration. Cdo and Boc bind Shh through a high-affinity interaction with a specific fibronectin repeat that is essential for activity. We propose a model where Cdo and Boc enhance Shh signaling within its target field.

The Sonic hedgehog pathway independently controls the patterning, proliferation and survival of neuroepithelial cells by regulating Gli activity

During CNS development, the proliferation of progenitors must be coordinated with the pattern of neuronal subtype generation. In the ventral neural tube, Sonic hedgehog acts as a long range morphogen to organise the pattern of cell differentiation by controlling the activity of Gli transcription factors. Here, we provide evidence that the same pathway also acts directly at long range to promote the proliferation and survival of progenitor cells. Blockade of Shh signaling or inhibition of Gli activity results in cell autonomous decreases in progenitor proliferation and survival. Conversely, positive Gli activity promotes proliferation and rescues the effects of inhibiting Shh signaling. Analysis of neural cells indicates that Shh/Gli signaling regulates the G1 phase of cell cycle and the expression of the anti-apoptotic factor Bcl2. Furthermore, Shh signaling independently regulates patterning, proliferation and survival of neural cells, thus Shh/Gli activity couples these separate cellular responses of progenitors to coordinate neural development.

The interpretation of morphogen gradients

Morphogens act as graded positional cues that control cell fate specification in many developing tissues. This concept, in which a signalling gradient regulates differential gene expression in a concentration-dependent manner, provides a basis for understanding many patterning processes. It also raises several mechanistic issues, such as how responding cells perceive and interpret the concentration-dependent information provided by a morphogen to generate precise patterns of gene expression and cell differentiation in developing tissues. Here, we review recent work on the molecular features of morphogen signalling that facilitate the interpretation of graded signals and attempt to identify some emerging common principles.

Mechanisms of ventral patterning in the vertebrate nervous system

Dorsoventral patterning of the neural tube has a crucial role in shaping the functional organization of the CNS. It is well established that hedgehog signalling plays a key role in specifying ventral cell types throughout the neuroectoderm, and major progress has been made in elucidating how hedgehog signalling works in this ventral specification. In addition, other molecular pathways, including nodal, retinoic acid and fibroblast growth factor signalling, have been identified as important molecular cues for ventral patterning of the spinal cord, telencephalon and eye. Here, we discuss recent advances in this field, highlighting the emerging interplay of these signalling pathways in the molecular specification of ventral patterning at different rostrocaudal levels of the CNS.

The level of sonic hedgehog signaling regulates the complexity of cerebellar foliation

Foliation of the mouse cerebellum occurs primarily during the first 2 weeks after birth and is accompanied by tremendous proliferation of granule cell precursors (GCPs). We have previously shown that sonic hedgehog (Shh) signaling correlates spatially and temporally with fissure formation, and that Gli2 is the main activator driving Shh induced proliferation of embryonic GCPs. Here, we have tested whether the level of Shh signaling regulates the extent of cerebellar foliation. By progressively lowering signaling by removing Gli1 and Gli2 or the Shh receptor smoothened, we found the extent of foliation is gradually reduced, and that this correlates with a decrease in the duration of GCP proliferation. Importantly, the pattern of the remaining fissures in the mutants corresponds to the first fissures that form during normal development. In a complementary manner, an increase in the level and length of Shh signaling results in formation of an extra fissure in a position conserved in rat. The complexity of cerebellar foliation varies greatly between vertebrate species. Our studies have uncovered a mechanism by which the level and length of Shh signaling could be integral to determining the distinct number of fissures in each species.

Genome-wide prediction of mammalian enhancers based on analysis of transcription-factor binding affinity

Understanding the regulation of human gene expression requires knowledge of the "second genetic code," which consists of the binding specificities of transcription factors (TFs) and the combinatorial code by which TF binding sites are assembled to form tissue-specific enhancer elements. Using a novel high-throughput method, we determined the DNA binding specificities of GLIs 1-3, Tcf4, and c-Ets1, which mediate transcriptional responses to the Hedgehog (Hh), Wnt, and Ras/MAPK signaling pathways. To identify mammalian enhancer elements regulated by these pathways on a genomic scale, we developed a computational tool, enhancer element locator (EEL). We show that EEL can be used to identify Hh and Wnt target genes and to predict activated TFs based on changes in gene expression. Predictions validated in transgenic mouse embryos revealed the presence of multiple tissue-specific enhancers in mouse c-Myc and N-Myc genes, which has implications for organ-specific growth control and tumor-type specificity of oncogenes.

Signal dynamics in Sonic hedgehog tissue patterning

During development, secreted signaling factors, called morphogens, instruct cells to adopt specific mature phenotypes. However, the mechanisms that morphogen systems employ to establish a precise concentration gradient for patterning tissue architecture are highly complex and are typically analyzed only at long times after secretion (i.e. steady state). We have developed a theoretical model that analyzes dynamically how the intricate transport and signal transduction mechanisms of a model morphogen, Sonic hedgehog (Shh),cooperate in modular fashion to regulate tissue patterning in the neural tube. Consistent with numerous recent studies, the model elucidates how the dynamics of gradient formation can be a key determinant of cell response. In addition,this work yields several novel insights into how different transport mechanisms or `modules' control pattern formation. The model predicts that slowing the transport of a morphogen, such as by lipid modification of the ligand Shh, by ligand binding to proteoglycans, or by the moderate upregulation of dedicated transport molecules like Dispatched, can actually increase the signaling range of the morphogen by concentrating it near the secretion source. Furthermore, several transcriptional targets of Shh, such as Patched and Hedgehog-interacting protein, significantly limit its signaling range by slowing transport and promoting ligand degradation. This modeling approach elucidates how individual modular elements that operate dynamically at various times during patterning can shape a tissue pattern.

Gli3 acts as a repressor downstream of Ihh in regulating two distinct steps of chondrocyte differentiation

During endochondral ossification, the secreted growth factor Indian hedgehog (Ihh) regulates several differentiation steps. It interacts with a second secreted factor, parathyroid hormone-related protein (PTHrP), to regulate the onset of hypertrophic differentiation, and it regulates chondrocyte proliferation and ossification of the perichondrium independently of PTHrP. To investigate how the Ihh signal is translated in the different target tissues, we analyzed the role of the zinc-finger transcription factor Gli3, which acts downstream of hedgehog signals in other organs. Loss of Gli3 in Ihh mutants restores chondrocyte proliferation and delays the accelerated onset of hypertrophic differentiation observed in Ihh-/- mutants. Furthermore the expression of the Ihh target genes patched (Ptch) and PTHrP is reactivated in Ihh-/-;Gli3-/- mutants. Gli3 seems thus to act as a strong repressor of Ihh signals in regulating chondrocyte differentiation. In addition, loss of Gli3 in mice that overexpress Ihh in chondrocytes accelerates the onset of hypertrophic differentiation by reducing the domain and possibly the level of PTHrP expression. Careful analysis of chondrocyte differentiation in Gli3-/- mutants revealed that Gli3 negatively regulates the differentiation of distal, low proliferating chondrocytes into columnar, high proliferating cells. Our results suggest a model in which the Ihh/Gli3 system regulates two distinct steps of chondrocyte differentiation: (1) the switch from distal into columnar chondrocytes is repressed by Gli3 in a PTHrP-independent mechanism; (2) the transition from proliferating into hypertrophic chondrocytes is regulated by Gli3-dependent expression of PTHrP. Furthermore, by regulating distal chondrocyte differentiation, Gli3 seems to position the domain of PTHrP expression.

Mouse Rab23 regulates Hedgehog signaling from Smoothened to Gli proteins

Sonic hedgehog (Shh) signaling is required for the growth and patterning of many tissues in vertebrate embryos, but important aspects of the Shh signal transduction pathway are poorly understood. For example, the vesicle transport protein Rab23 is a cell autonomous negative regulator of Shh signaling, but the process affected by Rab23 has not been defined. Here, we demonstrate that Rab23 acts upstream of Gli transcription factors in patterning neural cell types in the spinal cord. Double mutant analysis indicates that the primary target of Rab23 is the Gli2 activator and that Rab23 and Gli3 repressor have additive effects on patterning. Analysis of Gli3 protein suggests that Rab23 also has a role in promoting the production of Gli3 repressor. Although the membrane proteins Patched and Smoothened change subcellular localization in response to Shh, double mutant analysis demonstrates that Rab23 does not work through either Patched or Smoothened. Instead, Rab23 appears to regulate subcellular localization of essential components of the Hedgehog pathway that act downstream of Smoothened and upstream of Gli proteins.

Tectonic, a novel regulator of the Hedgehog pathway required for both activation and inhibition

We report the identification of a novel protein that participates in Hedgehog-mediated patterning of the neural tube. This protein, named Tectonic, is the founding member of a previously undescribed family of evolutionarily conserved secreted and transmembrane proteins. During neural tube development, mouse Tectonic is required for formation of the most ventral cell types and for full Hedgehog (Hh) pathway activation. Epistasis analyses reveal that Tectonic modulates Hh signal transduction downstream of Smoothened (Smo) and Rab23. Interestingly, characterization of Tectonic Shh and Tectonic Smo double mutants indicates that Tectonic plays an additional role in repressing Hh pathway activity.

Shh controls epithelial proliferation via independent pathways that converge on N-Myc

Shh signaling induces proliferation of many cell types during development and disease, but how Gli transcription factors regulate these mitogenic responses remains unclear. By genetically altering levels of Gli activator and repressor functions in mice, we have demonstrated that both Gli functions are involved in the transcriptional control of N-myc and Cyclin D2 during embryonic hair follicle development. Our results also indicate that additional Gli-activator-dependent functions are required for robust mitogenic responses in regions of high Shh signaling. Through posttranscriptional mechanisms, including inhibition of GSK3-beta activity, Shh signaling leads to spatially restricted accumulation of N-myc and coordinated cell cycle progression. Furthermore, a temporal shift in the regulation of GSK3-beta activity occurs during embryonic hair follicle development, resulting in a synergy with beta-catenin signaling to promote coordinated proliferation. These findings demonstrate that Shh signaling controls the rapid and patterned expansion of epithelial progenitors through convergent Gli-mediated regulation.

Fgf19 regulated by Hh signaling is required for zebrafish forebrain development

Fibroblast growth factor (Fgf) signaling plays important roles in brain development. Fgf3 and Fgf8 are crucial for the formation of the forebrain and hindbrain. Fgf8 is also required for the midbrain to form. Here, we identified zebrafish Fgf19 and examined its roles in brain development by knocking down Fgf19 function. We found that Fgf19 expressed in the forebrain, midbrain and hindbrain was involved in cell proliferation and cell survival during embryonic brain development. Fgf19 was also essential for development of the ventral telencephalon and diencephalon. Regional specification is linked to cell type specification. Fgf19 was also essential for the specification of gamma-aminobutyric acid (GABA)ergic interneurons and oligodendrocytes generated in the ventral telencephalon and diencephalon. The cross talk between Fgf and Hh signaling is critical for brain development. In the forebrain, Fgf19 expression was down-regulated on inhibition of Hh but not of Fgf3/Fgf8, and overexpression of Fgf19 rescued partially the phenotype on inhibition of Hh. The present findings indicate that Fgf19 signaling is crucial for forebrain development by interacting with Hh and provide new insights into the roles of Fgf signaling in brain development.

An ES cell system for rapid, spatial and temporal analysis of gene function in vitro and in vivo

We describe a versatile genetic system for rapid analysis of mammalian gene function. In this, loss of reporter activity in a novel embryonic stem (ES) cell line enables rapid identification of targeting to the ubiquitously expressed Rosa26 locus. Subsequent regulation of gene activity is governed by a dual regulatory strategy utilizing two drugs, Tamoxifen and Doxycycline. To illustrate this approach, a dominant allele of Smoothened was introduced into this cell line, enabling regulated activation of Hedgehog signaling. By coupling Cre-loxP dependent activation with tetracycline dependent transcription in a single allele, we established a conditional method to control Smoothened activity and neural progenitor specification in differentiating ES cells in vitro and in chimeric embryos in vivo When crossed to an appropriate Cre driver strain, gene activity can also be temporally regulated within a specific cell lineage. This platform will facilitate rapid analysis of gene function in the mouse.

Neural differentiation of mouse embryonic stem cells in chemically defined medium

Directed differentiation of embryonic stem (ES) cells has enormous potential to derive a wide variety of defined cell populations of therapeutic value. To achieve this, it is necessary to use protocols that promote cell differentiation under defined culture conditions. Furthermore, understanding the mechanisms of cell differentiation in vitro will allow the development of rationale approaches to systematically manipulate cell fates. Here we have analysed the differentiation of mouse ES cells to the neural lineage under serum and feeder cell-free conditions, using a previously described chemically defined medium (CDM). In CDM, ES cell differentiation is highly neurogenic. Cell differentiation was monitored by analysis of a gene expression array (Clontech-Atlas) and by semi-quantitative RT-PCR for a panel of genes involved in cell lineage specification and patterning of the epiblast. In addition to expression of neural markers, data identified a transient expression of several genes associated with the organising activities of the embryonic node and visceral endoderm, including regulators of WNT, BMP, Hedgehog and FGF signaling pathways. Neural differentiation in CDM does not occur by a simple default mechanism, but was dependent on endogenous FGF signaling, and could be blocked by adding BMP4, and LiCl to simulate WNT activation. Neural differentiation was also inhibited by antagonising endogenous hedgehog activity. Taken together the profile of gene expression changes seen in CDM cultures recapitulates those seen in the early embryo, and is suggestive of common developmental mechanisms.

Gli function is essential for motor neuron induction in zebrafish

The Gli family of zinc-finger transcription factors mediates Hedgehog (Hh) signaling in all vertebrates. However, their roles in ventral neural tube patterning, in particular motor neuron induction, appear to have diverged across species. For instance, cranial motor neurons are essentially lost in zebrafish detour (gli1(-)) mutants, whereas motor neuron development is unaffected in mouse single gli and some double gli knockouts. Interestingly, the expression of some Hh-regulated genes (ptc1, net1a, gli1) is mostly unaffected in the detour mutant hindbrain, suggesting that other Gli transcriptional activators may be involved. To better define the roles of the zebrafish gli genes in motor neuron induction and in Hh-regulated gene expression, we examined these processes in you-too (yot) mutants, which encode dominant repressor forms of Gli2 (Gli2(DR)), and following morpholino-mediated knockdown of gli1, gli2, and gli3 function. Motor neuron induction at all axial levels was reduced in yot (gli2(DR)) mutant embryos. In addition, Hh target gene expression at all axial levels except in rhombomere 4 was also reduced, suggesting an interference with the function of other Glis. Indeed, morpholino-mediated knockdown of Gli2(DR) protein in yot mutants led to a suppression of the defective motor neuron phenotype. However, gli2 knockdown in wild-type embryos generated no discernable motor neuron phenotype, while gli3 knockdown reduced motor neuron induction in the hindbrain and spinal cord. Significantly, gli2 or gli3 knockdown in detour (gli1(-)) mutants revealed roles for Gli2 and Gli3 activator functions in ptc1 expression and spinal motor neuron induction. Similarly, gli1 or gli3 knockdown in yot (gli2(DR)) mutants resulted in severe or complete loss of motor neurons, and of ptc1 and net1a expression, in the hindbrain and spinal cord. In addition, gli1 expression was greatly reduced in yot mutants following gli3, but not gli1, knockdown, suggesting that Gli3 activator function is specifically required for gli1 expression. These observations demonstrate that Gli activator function (encoded by gli1, gli2, and gli3) is essential for motor neuron induction and Hh-regulated gene expression in zebrafish.

Hedgehog signaling in mouse ovary: Indian hedgehog and desert hedgehog from granulosa cells induce target gene expression in developing theca cells

Follicle development in the mammalian ovary requires interactions among the oocyte, granulosa cells, and theca cells, coordinating gametogenesis and steroidogenesis. Here we show that granulosa cells of growing follicles in mouse ovary act as a source of hedgehog signaling. Expression of Indian hedgehog and desert hedgehog mRNAs initiates in granulosa cells at the primary follicle stage, and we find induced expression of the hedgehog target genes Ptch1 and Gli1, in the surrounding pre-theca cell compartment. Cyclopamine, a highly specific hedgehog signaling antagonist, inhibits this induced expression of target genes in cultured neonatal mouse ovaries. The theca cell compartment remains a target of hedgehog signaling throughout follicle development, showing induced expression of the hedgehog target genes Ptch1, Ptch2, Hip1, and Gli1. In periovulatory follicles, a dynamic synchrony between loss of hedgehog expression and loss of induced target gene expression is observed. Oocytes are unable to respond to hedgehog because they lack expression of the essential signal transducer Smo (smoothened). The present results point to a prominent role of hedgehog signaling in the communication between granulosa cells and developing theca cells.

Cilia and Hedgehog responsiveness in the mouse

The intraflagellar transport (IFT) proteins Ift172/Wimple and Polaris/Ift88 and the anterograde IFT motor kinesin-II are required for the production and maintenance of cilia. These proteins are also required for the activation of targets of the mouse Hedgehog (Hh) pathway by Gli transcription factors. The phenotypes of the IFT mutants, however, are not identical to mutants that lack Smoothened (Smo), an essential activator of the Hh pathway. We show here that mouse embryos that lack both Ift172 and Smo are identical to Ift172 single mutants, which indicates that Ift172 acts downstream of Smo. Ift172 mutants have a weaker neural patterning phenotype than Smo mutants, because Ift172, but not Smo, is required for proteolytic processing of Gli3 to its repressor form. Dnchc2 and Kif3a, essential subunits of the retrograde and anterograde IFT motors, are also required for both formation of Gli activator and proteolytic processing of Gli3. As a result, IFT mutants display a loss of Hh signaling phenotype in the neural tube, where Gli activators play the major role in pattern formation, and a gain of Hh signaling phenotype in the limb, where Gli3 repressor plays the major role. Because both anterograde and retrograde IFT are essential for positive and negative responses to Hh, and because cilia are present on Hh responsive cells, it is likely that cilia act as organelles that are required for all activity of the mouse Hh pathway.

Regulation of iro3 expression in the zebrafish spinal cord

Specification of spinal cord neurons is regulated by several different transcription factors. In this study, we analyze expression and regulation of the transcription factor iro3 in zebrafish spinal cord. In addition to its broad expression in the progenitor domain of intermediate spinal cord, iro3 is also expressed in postmitotic ventral neurons, starting at early somitogenesis stages. Initially, this expression is only in two primary motoneurons, CaP and VaP, but by 24 hr postfertilization, iro3 is expressed by all classes of zebrafish spinal motoneurons as well as by a ventral interneuron called VeLD. iro3 expression in the progenitor domain of intermediate spinal cord is regulated independently from its expression in ventral neurons. Hedgehog (Hh) signaling is unnecessary for iro3 expression in intermediate spinal cord, but it is required to repress iro3 expression in the progenitor domain of ventral spinal cord. We also show that the basic helix-loop-helix transcription factor Olig2 is required for repression of iro3 expression in the progenitor domain of ventral spinal cord. We discuss our findings in the context of previous studies, suggesting that iro3 represses formation of motoneurons and promotes formation of interneurons.

Shh-dependent formation of the ZLI is opposed by signals from the dorsal diencephalon

The zona limitans intrathalamica (ZLI) is located at the border between the prospective ventral thalamus and dorsal thalamus, and functions as a diencephalic signaling center. Little is known about the mechanism controlling ZLI formation. Using a combination of fate-mapping studies and in vitro assays, I show that the differentiation of the ZLI from progenitor cells in the alar plate is initiated by a Shh-dependent signal from the basal plate. The subsequent dorsal progression of ZLI differentiation requires ongoing Shh signaling, and is constrained by inhibitory factors derived from the dorsal diencephalon. These studies demonstrate that self-organizing signals from the basal plate regulate the formation of a potential patterning center in the ZLI in an orthogonal orientation in the alar plate, and thus create the potential for coordinated thalamic patterning in two dimensions.

A gradient of Gli activity mediates graded Sonic Hedgehog signaling in the neural tube

During development, many signaling factors behave as morphogens, long-range signals eliciting different cellular responses according to their concentration. In ventral regions of the spinal cord, Sonic Hedgehog (Shh) is such a signal and controls the emergence, in precise spatial order, of distinct neuronal subtypes. The Gli family of transcription factors plays a central role in this process. Here we demonstrate that a gradient of Gli activity is sufficient to mediate, cell-autonomously, the full range of Shh responses in the neural tube. The incremental two- to threefold changes in Shh concentration, which determine alternative neuronal subtypes, are mimicked by similar small changes in the level of Gli activity, indicating that a gradient of Gli activity represents the intracellular correlate of graded Shh signaling. Moreover, our analysis suggests that cells integrate the level of signaling over time, consistent with the idea that signal duration, in addition to signal strength, is an important parameter controlling dorsal-ventral patterning. Together, these data indicate that Shh signaling is transduced, without amplification, into a gradient of Gli activity that orchestrates patterning of the ventral neural tube.

Zebrafish Gli3 functions as both an activator and a repressor in Hedgehog signaling

Hedgehog (Hh) signaling regulates cell differentiation and patterning in a wide variety of embryonic tissues. In vertebrates, at least three Gli transcription factors (Gli1, Gli2, and Gli3) are involved in Hh signal transduction. Comparative studies have revealed divergent requirements for Gli1 and Gli2 in zebrafish and mouse. Here, we address the question of whether Gli3 function has also diverged in zebrafish and analyze the regulatory interactions between Hh signaling and Gli activity. We find that zebrafish Gli3 has an early function as an activator of Hh target genes that overlaps with Gli1 activator function in the ventral neural tube. In vitro reporter analysis shows that Gli3 cooperates with Gli1 to activate transcription in the presence of high concentrations of Hh. During late somitogenesis stages, Gli3 is required as a repressor of the Hh response. Gli3 shares this repressor activity with Gli2 in the dorsal spinal cord, hindbrain, and midbrain, but not in the forebrain. Consistently, zebrafish Gli3 blocks Gli1-mediated activation of a reporter gene in the absence of Hh in vitro. In the eye, Gli3 is also required for proper ath5 expression and the differentiation of retinal ganglion cells (RGCs). These results reveal a conserved role for Gli3 in vertebrate development and uncover novel regional functions and regulatory interactions among gli genes.

Generation of oligodendrocyte precursor cells from mouse dorsal spinal cord independent of Nkx6 regulation and Shh signaling

In the developing spinal cord, early progenitor cells of the oligodendrocyte lineage are induced in the motor neuron progenitor (pMN) domain of the ventral neuroepithelium by the ventral midline signal Sonic hedgehog (Shh). The ventral generation of oligodendrocytes requires Nkx6-regulated expression of the bHLH gene Olig2 in this domain. In the absence of Nkx6 genes or Shh signaling, the initial expression of Olig2 in the pMN domain is completely abolished. In this study, we provide the in vivo evidence for a late phase of Olig gene expression independent of Nkx6 and Shh gene activities and reveal a brief second wave of oligodendrogenesis in the dorsal spinal cord. In addition, we provide genetic evidence that oligodendrogenesis can occur in the absence of hedgehog receptor Smoothened, which is essential for all hedgehog signaling.

The floor plate: multiple cells, multiple signals

One of the key organizers in the CNS is the floor plate - a group of cells that is responsible for instructing neural cells to acquire distinctive fates, and that has an important role in establishing the elaborate neuronal networks that underlie the function of the brain and spinal cord. In recent years, considerable controversy has arisen over the mechanism by which floor plate cells form. Here, we describe recent evidence that indicates that discrete populations of floor plate cells, with characteristic molecular properties, form in different regions of the neuraxis, and we discuss data that imply that the mode of floor plate induction varies along the anteroposterior axis.

Monorail/Foxa2 regulates floorplate differentiation and specification of oligodendrocytes, serotonergic raphé neurones and cranial motoneurones

In this study, we elucidate the roles of the winged-helix transcription factor Foxa2 in ventral CNS development in zebrafish. Through cloning of monorail (mol), which we find encodes the transcription factor Foxa2, and phenotypic analysis of mol-/- embryos, we show that floorplate is induced in the absence of Foxa2 function but fails to further differentiate. In mol-/- mutants, expression of Foxa and Hh family genes is not maintained in floorplate cells and lateral expansion of the floorplate fails to occur. Our results suggest that this is due to defects both in the regulation of Hh activity in medial floorplate cells as well as cell-autonomous requirements for Foxa2 in the prospective laterally positioned floorplate cells themselves. Foxa2 is also required for induction and/or patterning of several distinct cell types in the ventral CNS. Serotonergic neurones of the raphenucleus and the trochlear motor nucleus are absent in mol-/- embryos, and oculomotor and facial motoneurones ectopically occupy ventral CNS midline positions in the midbrain and hindbrain. There is also a severe reduction of prospective oligodendrocytes in the midbrain and hindbrain. Finally, in the absence of Foxa2, at least two likely Hh pathway target genes are ectopically expressed in more dorsal regions of the midbrain and hindbrain ventricular neuroepithelium, raising the possibility that Foxa2 activity may normally be required to limit the range of action of secreted Hh proteins.

Sonic hedgehog guides commissural axons along the longitudinal axis of the spinal cord

Dorsal commissural axons in the developing spinal cord cross the floor plate, then turn rostrally and grow along the longitudinal axis, close to the floor plate. We used a subtractive hybridization approach to identify guidance cues responsible for the rostral turn in chicken embryos. One of the candidates was the morphogen Sonic hedgehog (Shh). Silencing of the gene SHH (which encodes Shh) by in ovo RNAi during commissural axon navigation demonstrated a repulsive role in post-commissural axon guidance. This effect of Shh was not mediated by Patched (Ptc) and Smoothened (Smo), the receptors that mediate effects of Shh in morphogenesis and commissural axon growth toward the floor plate. Rather, functional in vivo studies showed that the repulsive effect of Shh on postcommissural axons was mediated by Hedgehog interacting protein (Hip).

Gli2 and Gli3 have redundant and context-dependent function in skeletal muscle formation

The Gli family of zinc finger transcription factors are mediators of Shh signalling in vertebrates. In previous studies, we showed that Shh signalling,via an essential Gli -binding site in the Myf5 epaxial somite (ES)enhancer, is required for the specification of epaxial muscle progenitor cells. Shh signalling is also required for the normal mediolateral patterning of myogenic cells within the somite. In this study, we investigate the role and the transcriptional activities of Gli proteins during somite myogenesis in the mouse embryo. We report that Gli genes are differentially expressed in the mouse somite. Gli2 and Gli3 are essential for Gli1 expression in somites, establishing Gli2 and Gli3 as primary mediators and Gli1 as a secondary mediator of Shh signalling. Combining genetic studies with the use of a transgenic mouse line expressing a reporter gene under the control of the Myf5 epaxial somite enhancer, we show that Gli2 or Gli3 is required for Myf5 activation in the epaxial muscle progenitor cells. Furthermore, Gli3, but not Gli2 represses Myf5 transcription in a dose-dependent manner in the absence of Shh. Finally, we provide evidence that hypaxial and myotomal gene expression is mispatterned in Gli2–/–Gli3–/–and Gli3–/–Shh–/–somites. Together, our data demonstrate both positive and negative regulatory functions for Gli2 and Gli3 in the control of Myf5 activation in the epaxial muscle progenitor cells and in dorsoventral and mediolateral patterning of the somite.

HomozygousFt embryos are affected in floor plate maintenance and ventral neural tube patterning

Sonic hedgehog (Shh), produced by the notochord and floor plate cells of the neural tube, plays a critical role in organizing dorsal-ventral patterning in the developing neural tube. We have investigated neural tube development in mouse embryos homozygous for the Fused toes (Ft) mutation, a deletion composed of genes of the Iroquois B (IrxB) cluster and of Fts, Ftm, and Fto. In Ft mutants starting from embryonic day 10.5, the floor plate appeared to degenerate and the notochord failed to undergo ventral displacement from the spinal cord. Consistent with the loss of Shh signalling from the floor plate, V3 neuron generation was reduced in Ft/Ft embryos and the domain of motor neuron generation expanded ventrally at the expense of V2 neurons. These data support the idea that Ft genes play an important role in dorsal-ventral patterning of the neural tube acting to define the extent of motor neuron generation; moreover, the data reveal a previously unanticipated function for Ft genes in the maintenance of the floor plate.

Growth and pattern of the mammalian neural tube are governed by partially overlapping feedback activities of the hedgehog antagonists patched 1 and Hhip1

Upregulation of Patched (Ptc), the Drosophila Hedgehog (Hh) receptor in response to Hh signaling limits the range of signaling within a target field by sequestering Hh. In vertebrates, Ptch1 also exhibits ligand-dependent transcriptional activation, but mutants lacking this response show surprisingly normal early development. The identification of Hh-interacting protein 1 (Hhip1), a vertebrate-specific feedback antagonist of Hh signaling, raises the possibility of overlapping feedback controls. We addressed the significance of feedback systems in sonic hedgehog (Shh)-dependent spinal cord patterning. Mouse embryos lacking both Ptch1 and Hhip1 feedback activities exhibit severe patterning defects consistent with an increased magnitude and range of Hh signaling, and disrupted growth control. Thus, Ptc/Ptch1-dependent feedback control of Hh morphogens is conserved between flies and mice, but this role is shared in vertebrates with Hhip1. Furthermore, this feedback mechanism is crucial in generating a neural tube that contains appropriate numbers of all ventral and intermediate neuronal cell types.

Mouse Disp1 is required in sonic hedgehog-expressing cells for paracrine activity of the cholesterol-modified ligand

Previous studies have demonstrated that Disp1 function is essential for Shh and Ihh signaling in the mouse, and Disp1 gene dose regulates the level of Shh signaling activity in vivo. To determine whether Disp1 activity is required in Shh-producing cells for paracrine signaling in Shh target fields, we used a ShhGFP-Cre (here shortened to ShhCre) knock-in allele and a Disp1 conditional allele to knock down Disp1 activity specifically within Shh-producing cells. The resulting facial and neural tube phenotypes support the conclusion that the primary and probably exclusive role for Disp1 is within hedgehog protein-producing cells. Furthermore, using an allele that produces N-Shh (a noncholesterol modified form of the Shh protein), we demonstrate that N-Shh is sufficient to rescue most of the early embryonic lethal defects in a Disp1-null mutant background. Thus, Disp1 activity is only required for paracrine hedgehog protein signaling by the cholesterol modified form of Shh (N-Shhp), the normal product generated by auto-processing of a Shh precursor protein. In both respects, Disp function is conserved from Drosophila to mice.

Dose dependency of Disp1 and genetic interaction between Disp1 and other hedgehog signaling components in the mouse

Genetic analyses in Drosophila have demonstrated that a transmembrane protein Dispatched (Disp) is required for the release of lipid-modified Hedgehog (Hh) protein from Hh secreting cells. Analysis of Disp1 null mutant embryos has demonstrated that Disp1 plays a key role in hedgehog signaling in the early mouse embryo. Here we have used a hypomorphic allele in Disp1(Disp1Δ2), to extend our knowledge of Disp1 function in Hh-mediated patterning of the mammalian embryo. Through genetic combinations with null alleles of patched 1 (Ptch1),sonic hedgehog (Shh) and Indian hedgehog (Ihh), we demonstrate that Disp1 genetically interacts with Hh signaling components. As Disp1 activity is decreased we see a progressive increase in the severity of hedgehog-dependent phenotypes, which is further enhanced by reducing hedgehog ligand levels. Analysis of neural tube patterning demonstrates a progressive loss of ventral cell identities that most likely reflects decreased Shh signaling as Disp1 levels are attenuated. Conversely,increasing available Shh ligand by decreasing Ptch1 dosage leads to the restoration of ventral cell types in Disp1Δ2/Δ2 mutants. Together, these studies suggest that Disp1 actively regulates the levels of hedgehog ligand that are available to the hedgehog target field. Further, they provide additional support for the dose-dependent action of Shh signaling in patterning the embryo. Finally, in-vitro studies on Disp1 null mutant fibroblasts indicate that Disp1 is not essential for membrane targeting or release of lipid-modified Shh ligand.

Differential activities of Sonic hedgehog mediated by Gli transcription factors define distinct neuronal subtypes in the dorsal thalamus

The dorsal thalamus (DT) is a pivotal region in the vertebrate brain that relays inputs from the peripheral sensory organs to higher cognitive centers. It consists of clusters of neurons with relevant functions, called brain nuclei. However, the mechanisms underlying development of the DT, including specification of the neuronal subtypes and morphogenesis of the nuclear structures, remain largely unknown. As a first step to this end, we focused on two transcription factors Sox14 and Gbx2 that are expressed in the specific brain nuclei in the chick DT. The onset of their expression was found in distinct populations of the postmitotic cells in the prosomere 2, which was regulated by the differential activities of Sonic hedgehog (Shh) in a manner consistent with the action as a morphogen. Furthermore, both gain- and loss-of-function results strongly suggest that such distinct inductive activities are mediated selectively by different Gli factors. These results suggest that cooperation of the differential expression of Gli factors and the activity gradient of Shh signaling generates the distinct thalamic neurons at the specific locations.

Transduction of graded Hedgehog signaling by a combination of Gli2 and Gli3 activator functions in the developing spinal cord

The three vertebrate Gli proteins play a central role in mediating Hedgehog(Hh)-dependent cell fate specification in the developing spinal cord; however,their individual contributions to this process have not been fully characterized. In this paper, we have addressed this issue by examining patterning in the spinal cord of Gli2;Gli3 double mutant embryos, and in chick embryos transfected with dominant activator forms of Gli2 and Gli3. In double homozygotes, Gli1 is also not expressed; thus, all Gli protein activities are absent in these mice. We show that Gli3 contributes activator functions to ventral neuronal patterning, and plays a redundant role with Gli2 in the generation of V3 interneurons. We also show that motoneurons and three classes of ventral neurons are generated in the ventral spinal cord in double mutants, but develop as intermingled rather than discrete populations. Finally, we provide evidence that Gli2 and Gli3 activators control ventral neuronal patterning by regulating progenitor segregation. Thus, multiple ventral neuronal types can develop in the absence of Gli function, but require balanced Gli protein activities for their correct patterning and differentiation.

Dynamic Changes in the Response of Cells to Positive Hedgehog Signaling during Mouse Limb Patterning

In the vertebrate limb, the posteriorly located zone of polarizing activity (ZPA) regulates digit identity through the morphogen Sonic Hedgehog (Shh). By genetically marking Shh-responding cells in mice, we have addressed whether the cumulative influence of positive Shh signaling over time and space reflects a linear gradient of Shh responsiveness and whether Shh could play additional roles in limb patterning. Our results show that all posterior limb mesenchyme cells, as well as the ectoderm, respond to Shh from the ZPA and become the bone, muscle, and skin of the posterior limb. Further, the readout of Shh activator function integrated over time and space does not display a stable and linear gradient along the A-P axis, as in a classical morphogen view. Finally, by fate mapping Shh-responding cells in Gli2 and Gli3 mutant limbs, we demonstrate that a specific level of positive Hh signaling is not required to specify digit identities.

Sonic hedgehog signaling regulates Gli3 processing, mesenchymal proliferation, and differentiation during mouse lung organogenesis

Lack of Sonic hedgehog (Shh) signaling, mediated by the Gli proteins, leads to severe pulmonary hypoplasia. However, the precise role of Gli genes in lung development is not well established. We show Shh signaling prevents Gli3 proteolysis to generate its repressor forms (Gli3R) in the developing murine lung. In Shh(-/-) or cyclopamine-treated wild-type (WT) lung, we found that Gli3R level is elevated, and this upregulation appears to contribute to defects in proliferation and differentiation observed in the Shh(-/-) mesenchyme, where Gli3 is normally expressed. In agreement, we found Shh(-/-);Gli3(-/-) lungs exhibit enhanced growth potential. Vasculogenesis is also enhanced; in contrast, bronchial myogenesis remains absent in Shh(-/-);Gli3(-/-) compared with Shh(-/-) lungs. Genes upregulated in Shh(-/-);Gli3(-/-) relative to Shh(-/-) lung include Wnt2 and, surprisingly, Foxf1 whose expression has been reported to be Shh-dependent. Cyclins D1, D2, and D3 antibody labelings also reveal distinct expression patterns in the normal and mutant lungs. We found significant repression of Tbx2 and Tbx3, both linked to inhibition of cellular senescence, in Shh(-/-) and partial derepression in Shh(-/-); Gli3(-/-) lungs, while Tbx4 and Tbx5 expressions are less affected in the mutants. Our findings shed light on the role of Shh signaling on Gli3 processing in lung growth and differentiation by regulating several critical genes.

FKBP8 is a negative regulator of mouse sonic hedgehog signaling in neural tissues

Sonic hedgehog (SHH) is a secreted morphogen that regulates the patterning and growth of many tissues in the developing mouse embryo, including the central nervous system (CNS). We show that a member of the FK506-binding protein family, FKBP8, is an essential antagonist of SHH signaling in CNS development. Loss of FKBP8 causes ectopic and ligand-independent activation of the Shh pathway, leading to expansion of ventral cell fates in the posterior neural tube and suppression of eye development. Although it is expressed broadly, FKBP8 is required to antagonize SHH signaling primarily in neural tissues, suggesting that hedgehog signal transduction is subject to cell-type specific modulation during mammalian development.

Palmitoylation is required for the production of a soluble multimeric Hedgehog protein complex and long-range signaling in vertebrates

Hedgehog (Hh) signaling plays a major role in multiple aspects of embryonic development. A key issue in Hh signaling is to elucidate the molecular mechanism by which a Hh protein morphogen gradient is formed despite its membrane association. In this study, we used a combination of genetic, cellular, and biochemical approaches to address the role of lipid modifications in long-range vertebrate Hh signaling. Our molecular analysis of knockout mice deficient in Skn, the murine homolog of the Drosophila ski gene, which catalyzes Hh palmitoylation, and gene-targeted mice producing a nonpalmitoylated form of Shh indicates that Hh palmitoylation is essential for its activity as well as the generation of a protein gradient in the developing embryos. Furthermore, our biochemical data show that Hh lipid modifications are required for producing a soluble multimeric protein complex, which constitutes the major active component for Hh signaling. These results suggest that soluble Hh multimeric complex travels in the morphogenetic field to activate Hh signaling in distant Hh-responsive cells.

All mouse ventral spinal cord patterning by hedgehog is Gli dependent and involves an activator function of Gli3

An important question is how the gradient of Hedgehog is interpreted by cells at the level of the Gli transcription factors. The full range of Gli activity and its dependence on Hh have not been determined, although the Gli2 activator and Gli3 repressor have been implicated. Using the spinal cord as a model system, we demonstrate that Gli3 can transduce Hedgehog signaling as an activator. All expression of the Hh target gene Gli1 is dependent on both Gli2 and Gli3. Unlike Gli2, however, Gli3 requires endogenous Gli1 for induction of floor plate and V3 interneurons. Strikingly, embryos lacking all Gli function develop motor neurons and three ventral interneuron subtypes, similar to embryos lacking Hh signaling and Gli3. Therefore, in the spinal cord all Hh signaling is Gli dependent. Furthermore, a combination of Gli2 and Gli3 is required to regulate motor neuron development and spatial patterning of ventral spinal cord progenitors.

A novel role of the hedgehog pathway in lens regeneration

Lens regeneration in the adult newt is a classic example of replacing a lost organ by the process of transdifferentiation. After lens removal, the pigmented epithelial cells of the dorsal iris proliferate and dedifferentiate to form a lens vesicle, which subsequently differentiates to form a new lens. In searching for factors that control this remarkable process, we investigated the expression and role of hedgehog pathway members. These molecules are known to affect retina and pigment epithelium morphogenesis and have been recently shown to be involved in repair processes. Here we show that Shh, Ihh, ptc-1, and ptc-2 are expressed during lens regeneration. The expression of Shh and Ihh is quite unique since these genes have never been detected in lens. Interestingly, both Shh and Ihh are only expressed in the regenerating and developing lens, but not in the intact lens. Interfering with the hedgehog pathway results in considerable inhibition of the process of lens regeneration, including decreased cell proliferation as well as interference with lens fiber differentiation in the regenerating lens vesicle. Down-regulation of ptc-1 was also observed when inhibiting the pathway. These results provide the first evidence of a novel role for the hedgehog pathway in specific regulation of the regenerating lens.

Cooperation between sonic hedgehog and fibroblast growth factor/MAPK signalling pathways in neocortical precursors

Sonic hedgehog (SHH) and fibroblast growth factor 2 (FGF2) can both induce neocortical precursors to express the transcription factor OLIG2 and generate oligodendrocyte progenitors (OLPs) in culture. The activity of FGF2 is unaffected by cyclopamine, which blocks Hedgehog signalling, demonstrating that the FGF pathway to OLP production is Hedgehog independent. Unexpectedly, SHH-mediated OLP induction is blocked by PD173074, a selective inhibitor of FGF receptor (FGFR) tyrosine kinase. SHH activity also depends on mitogen-activated protein kinase (MAPK) but SHH does not itself activate MAPK. Instead, constitutive activity of FGFR maintains a basal level of phosphorylated MAPK that is absolutely required for the OLIG2- and OLP-inducing activities of SHH. Stimulating the MAPK pathway with a retrovirus encoding constitutively active RAS shows that the requirement for MAPK is cell-autonomous, i.e. MAPK is needed together with SHH signalling in the cells that become OLPs.

Hedgehog signaling is required for the differentiation of ES cells into neurectoderm

Mouse embryonic stem cells can differentiate in vitro into cells of the nervous system, neurons and glia. This differentiation mimics stages observed in vivo, including the generation of primitive ectoderm and neurectoderm in embryoid body culture. We demonstrate here that embryonic stem cell lines mutant for components of the Hedgehog signaling cascade are deficient at generating neurectoderm-containing embryoid bodies. The embryoid bodies derived from mutant cells are also unable to respond to retinoic acid treatment by producing nestin-positive neural stem cells, a response observed in cultures of heterozygous cells, and contain cores apparently arrested at the primitive ectoderm stage. The mutant cultures are also deficient in their capacity to differentiate into mature neurons and glia. These data are consistent with a role for Hedgehog signaling in generating neurectoderm capable of producing the appropriate neuronal and glial progenitors in ES cell culture.

Deregulation of dorsoventral patterning by FGF confers trilineage differentiation capacity on CNS stem cells in vitro

The CNS is thought to develop from self-renewing stem cells that generate neurons, astrocytes, and oligodendrocytes. Other data, however, have suggested that astrocytes and oligodendrocytes are generated from separate progenitor populations. To reconcile these observations, we have prospectively isolated progenitors that do or do not express Olig2, an oligodendrocyte bHLH determination factor. Both Olig2(-) and Olig2(+) progenitors can behave as tripotential CNS stem cells (CNS-SCs) in vitro. Growth in FGF-2 causes induction of Olig2 in the former population, permitting oligodendrocyte differentiation; extinction of Olig2 in the latter cells permits astrocyte differentiation. The induction of Olig2 by FGF-2 is mediated, in part, via endogenous Sonic Hedgehog. These data indicate that clonogenic competence to generate neurons, astrocytes, and oligodendrocytes reflects a deregulation of dorsoventral patterning during expansion in vitro, raising the question of whether such trifatent cells actually exist in vivo.

Is there a role for the IHH gene in Hirschsprung's disease?

Hirschsprung disease (HSCR) is characterized by the absence of ganglion cells along a variable length of the intestine. HSCR has a complex genetic aetiology with 50% of the patients unexplained by mutations in the major HSCR genes. The Ihh gene is involved in the development of the enteric nervous system (ENS) and Ihh mutant mice present with a phenotype reminiscent of HSCR. The requirement of Ihh signalling for the proper development of the ENS, together with the evidence presented by the Ihh murine model, prompted us to investigate the involvement of the human IHH gene in HSCR. Sequence analysis revealed seven single nucleotide polymorphisms, six of which were new. Allele and haplotype frequencies were compared between cases and controls, and, among the cases, between genders, between different phenotypes, and between patients with different mutation status in the main HSCR genes. Despite the involvement of IHH in the development of the ENS, IHH is not a major gene in HSCR. Nevertheless, as the manifestation of the HSCR phenotype is genetic background dependent, polymorphic loci should be tested simultaneously to characterize gene-gene interaction. The involvement of IHH in other intestinal anomalies should be investigated.

Interplays of Gli2 and Gli3 and their requirement in mediating Shh-dependent sclerotome induction

Sonic hedgehog (Shh) signaling is essential for sclerotome development in the mouse. Gli2 and Gli3 are thought to be the primary transcriptional mediators of Shh signaling; however, their roles in Shh induction of sclerotomal genes have not been investigated. Using a combination of mutant analysis and in vitro explant assays, we demonstrate that Gli2 and Gli3 are required for Shh-dependent sclerotome induction. Gli2(-/-)Gli3(-/-) embryos exhibit a severe loss of sclerotomal gene expression, and somitic mesoderm from these embryos cannot activate sclerotomal genes in response to exogenous Shh. We find that one copy of either Gli2 or Gli3 is required to mediate Shh induction of sclerotomal markers Pax1 and Pax9 in vivo and in vitro. Although Gli2 is generally considered an activator and Gli3 a repressor, our results also reveal a repressor function for Gli2 and an activator function for Gli3 in the developing somite. To further dissect the function of each Gli, we used adenovirus to overexpress Gli1, Gli2 and Gli3 in presomitic mesoderm explants. We find that each Gli preferentially activates a distinct set of Shh target genes, suggesting that the functions of Shh in patterning, growth and negative feedback are divided preferentially between different Gli proteins in the somite.

The emergent design of the neural tube: prepattern, SHH morphogen and GLI code

The Sonic hedgehog (Shh) pathway plays an important role in the development of many tissues and organs. The secreted ligand Shh has been shown to act as a mitogen, morphogen and survival factor in different contexts whereas the three Gli transcription factors act as Shh mediators in a context-dependent combinatorial fashion. The common wisdom has been that Gli protein function is subject to Shh signaling. One can ask how Gli proteins act and what the nature of Shh signaling during CNS dorsal-ventral patterning is. Is it possible that Hedgehog signals are only one of several ways to regulate Gli activity? Moreover, in light of the partial rescue of the neural tube phenotype of Shh or Smoothened mutant embryos in Shh(-/-);Gli3(-/-), Smoothened(-/-);Gli3(-/-), and Shh(-/-);Rab23(-/-) double null embryos, one can consider the roles that the Shh-Gli pathway may have taken to orchestrate congruent prepattern and growth, and the importance of creating the correct number of precursors in patterning mechanisms.

Distinct modes of floor plate induction in the chick embryo

To begin to reconcile models of floor plate formation in the vertebrate neural tube, we have performed experiments aimed at understanding the development of the early floor plate in the chick embryo. Using real-time analyses of cell behaviour, we provide evidence that the principal contributor to the early neural midline, the future anterior floor plate, exists as a separate population of floor plate precursor cells in the epiblast of the gastrula stage embryo, and does not share a lineage with axial mesoderm. Analysis of the tissue interactions associated with differentiation of these cells to a floor plate fate reveals a role for the nascent prechordal mesoderm, indicating that more than one inductive event is associated with floor plate formation along the length of the neuraxis. We show that Nr1, a chick nodal homologue, is expressed in the nascent prechordal mesoderm and we provide evidence that Nodal signalling can cooperate with Shh to induce the epiblast precursors to a floor-plate fate. These results indicate that a shared lineage with axial mesoderm cells is not a pre-requisite for floor plate differentiation and suggest parallels between the development of the floor plate in amniote and anamniote embryos.

Neuroepithelial secondary organizers and cell fate specification in the developing brain

In vertebrates, elaborate cellular interactions regulate the establishment of the complex structural pattern of the developing central nervous system. Distinct neural and glial identities are acquired by neuroepithelial cells, through progressive restriction of histogenetic potential under the influence of local environmental signals. The localization of the sources of such morphogenetic signals in discrete domains of the developing neural primordium has led to the concept of secondary organizers which refine the identity and polarity of neighboring neuroepithelial regions. Thus, these organizers, secondary to those that operate throughout the embryo during gastrulation, act to pattern the anterior neural plate and tube giving rise to the forebrain, midbrain and hindbrain vesicles. Important progress has recently been made in understanding their genesis and function.

Opposing FGF and Retinoid Pathways Control Ventral Neural Pattern, Neuronal Differentiation, and Segmentation during Body Axis Extension

Vertebrate body axis extension involves progressive generation and subsequent differentiation of new cells derived from a caudal stem zone; however, molecular mechanisms that preserve caudal progenitors and coordinate differentiation are poorly understood. FGF maintains caudal progenitors and its attenuation is required for neuronal and mesodermal differentiation and to position segment boundaries. Furthermore, somitic mesoderm promotes neuronal differentiation in part by downregulating Fgf8. Here we identify retinoic acid (RA) as this somitic signal and show that retinoid and FGF pathways have opposing actions. FGF is a general repressor of differentiation, including ventral neural patterning, while RA attenuates Fgf8 in neuroepithelium and paraxial mesoderm, where it controls somite boundary position. RA is further required for neuronal differentiation and expression of key ventral neural patterning genes. Our data demonstrate that FGF and RA pathways are mutually inhibitory and suggest that their opposing actions provide a global mechanism that controls differentiation during axis extension.

A Requirement for Retinoic Acid-Mediated Transcriptional Activation in Ventral Neural Patterning and Motor Neuron Specification

The specification of neuronal fates in the ventral spinal cord depends on the regulation of homeodomain (HD) and basic-helix-loop-helix (bHLH) proteins by Sonic hedgehog (Shh). Most of these transcription factors function as repressors, leaving unresolved the link between inductive signaling pathways and transcriptional activators involved in ventral neuronal specification. We show here that retinoid signaling and the activator functions of retinoid receptors are required to pattern the expression of HD and bHLH proteins and to specify motor neuron identity. We also show that fibroblast growth factors (FGFs) repress progenitor HD protein expression, implying that evasion of FGF signaling and exposure to retinoid and Shh signals are obligate steps in the emergence of ventral neural pattern. Moreover, joint exposure of neural progenitors to retinoids and FGFs suffices to induce motor neuron differentiation in a Shh-independent manner.