Ventral neural progenitors switch toward an oligodendroglial fate in response to increased Sonic hedgehog (Shh) activity: involvement of Sulfatase 1 in modulating Shh signaling in the ventral spinal cord

Novel therapeutic targets in the brain tumor microenvironment

Glioblastoma (GBM), a highly malignant brain tumor of adults and children, diffusely invades within the non-neoplastic brain. Despite aggressive current therapeutic interventions, improved therapeutic strategies are greatly needed. Interactions between the tumor and constituents of its microenvironment are known to regulate malignancy, and heparan sulfate proteoglycans (HSPGs) are important as they bind diverse extracellular proteins, including growth factors and cell adhesion molecules, regulating the activity of several ligand-mediated signaling pathways. Recent work from our group described a mechanism by which GBM regulates PDGFR-alpha signaling via enzymatic alteration of heparan sulfate proteoglycans (HSPGs) in the extracellular microenvironment. Blocking tumor-induced alterations of HSPGs, which can be achieved by pharmacological strategies, would potentially inhibit multiple oncogenic signaling pathways in tumor cells and disrupt critical tumormicroenvironment interactions. Here we examine HSPGs and the enzymes that modify them in GBM. We compare their expression across tumor subtypes, their potential roles in oncogenesis, and their potential as novel therapeutic targets in GBM.

Release and transportation of Hedgehog molecules

Secretion of the Hedgehog morphogen induces different cell fates over the short and long ranges during developmental patterning. Mature Hedgehog carries hydrophobic palmitic acid and cholesterol modifications essential for its correct spread. The long-range activity of Hedgehog raises questions about how a dually lipidated protein can spread in the hydrophilic environment of the extracellular space. There is compelling experimental evidence in favour of the existence of several different carriers for Hedgehog transportation, via very different routes. This suggests that different accessory proteins and cellular machineries may be involved in the specific release of Hedgehog. I suggest that Hh carriers may work in parallel within a given cell and that developmental context may condition the choice of Hh carrier in secreting cells.

Organ-specific sulfation patterns of heparan sulfate generated by extracellular sulfatases Sulf1 and Sulf2 in mice

Heparan sulfate endosulfatases Sulf1 and Sulf2 hydrolyze 6-O-sulfate in heparan sulfate, thereby regulating cellular signaling. Previous studies have revealed that Sulfs act predominantly on UA2S-GlcNS6S disaccharides and weakly on UA-GlcNS6S disaccharides. However, the specificity of Sulfs and their role in sulfation patterning of heparan sulfate in vivo remained unknown. Here, we performed disaccharide analysis of heparan sulfate in Sulf1 and Sulf2 knock-out mice. Significant increases in ΔUA2S-GlcNS6S were observed in the brain, small intestine, lung, spleen, testis, and skeletal muscle of adult Sulf1(-/-) mice and in the brain, liver, kidney, spleen, and testis of adult Sulf2(-/-) mice. In addition, increases in ΔUA-GlcNS6S were seen in the Sulf1(-/-) lung and small intestine. In contrast, the disaccharide compositions of chondroitin sulfate were not primarily altered, indicating specificity of Sulfs for heparan sulfate. For Sulf1, but not for Sulf2, mRNA expression levels in eight organs of wild-type mice were highly correlated with increases in ΔUA2S-GlcNS6S in the corresponding organs of knock-out mice. Moreover, overall changes in heparan sulfate compositions were greater in Sulf1(-/-) mice than in Sulf2(-/-) mice despite lower levels of Sulf1 mRNA expression, suggesting predominant roles of Sulf1 in heparan sulfate desulfation and distinct regulation of Sulf activities in vivo. Sulf1 and Sulf2 mRNAs were differentially expressed in restricted types of cells in organs, and consequently, the sulfation patterns of heparan sulfate were locally and distinctly altered in Sulf1 and Sulf2 knock-out mice. These findings indicate that Sulf1 and Sulf2 differentially contribute to the generation of organ-specific sulfation patterns of heparan sulfate.

Heparan sulfate sulfatase SULF2 regulates PDGFRα signaling and growth in human and mouse malignant glioma

Glioblastoma (GBM), a uniformly lethal brain cancer, is characterized by diffuse invasion and abnormal activation of multiple receptor tyrosine kinase (RTK) signaling pathways, presenting a major challenge to effective therapy. The activation of many RTK pathways is regulated by extracellular heparan sulfate proteoglycans (HSPG), suggesting these molecules may be effective targets in the tumor microenvironment. In this study, we demonstrated that the extracellular sulfatase, SULF2, an enzyme that regulates multiple HSPG-dependent RTK signaling pathways, was expressed in primary human GBM tumors and cell lines. Knockdown of SULF2 in human GBM cell lines and generation of gliomas from Sulf2(-/-) tumorigenic neurospheres resulted in decreased growth in vivo in mice. We found a striking SULF2 dependence in activity of PDGFRα, a major signaling pathway in GBM. Ablation of SULF2 resulted in decreased PDGFRα phosphorylation and decreased downstream MAPK signaling activity. Interestingly, in a survey of SULF2 levels in different subtypes of GBM, the proneural subtype, characterized by aberrations in PDGFRα, demonstrated the strongest SULF2 expression. Therefore, in addition to its potential as an upstream target for therapy of GBM, SULF2 may help identify a subset of GBMs that are more dependent on exogenous growth factor-mediated signaling. Our results suggest the bioavailability of growth factors from the microenvironment is a significant contributor to tumor growth in a major subset of human GBM.

Paracrine Pax6 activity regulates oligodendrocyte precursor cell migration in the chick embryonic neural tube

Homeoprotein transcription factors play fundamental roles in development, ranging from embryonic polarity to cell differentiation and migration. Research in recent years has underscored the physiological importance of homeoprotein intercellular transfer in eye field development, axon guidance and retino-tectal patterning, and visual cortex plasticity. Here, we have used the embryonic chick neural tube to investigate a possible role for homeoprotein Pax6 transfer in oligodendrocyte precursor cell (OPC) migration. We report the extracellular expression of Pax6 and the effects of gain and loss of extracellular Pax6 activity on OPCs. Open book cultures with recombinant Pax6 protein or Pax6 blocking antibodies, as well as in ovo gene transfer experiments involving expression of secreted Pax6 protein or secreted Pax6 antibodies, provide converging evidences that OPC migration is promoted by extracellular Pax6. The paracrine effect of Pax6 on OPC migration is thus a new example of direct non-cell autonomous homeoprotein activity.

The extracellular matrix molecule tenascin C modulates expression levels and territories of key patterning genes during spinal cord astrocyte specification

The generation of astrocytes during the development of the mammalian spinal cord is poorly understood. Here, we demonstrate for the first time that the extracellular matrix glycoprotein tenascin C regulates the expression of key patterning genes during late embryonic spinal cord development, leading to a timely maturation of gliogenic neural precursor cells. We first show that tenascin C is expressed by gliogenic neural precursor cells during late embryonic development. The loss of tenascin C leads to a sustained generation and delayed migration of Fgfr3-expressing immature astrocytes in vivo. Consistent with an increased generation of astroglial cells, we documented an increased number of GFAP-positive astrocytes at later stages. Mechanistically, we could demonstrate an upregulation and domain shift of the patterning genes Nkx6.1 and Nkx2.2 in vivo. In addition, sulfatase 1, a known downstream target of Nkx2.2 in the ventral spinal cord, was also upregulated. Sulfatase 1 regulates growth factor signalling by cleaving sulphate residues from heparan sulphate proteoglycans. Consistent with this function, we observed changes in both FGF2 and EGF responsiveness of spinal cord neural precursor cells. Taken together, our data implicate Tnc in the regulation of proliferation and lineage progression of astroglial progenitors in specific domains of the developing spinal cord.

Migration and fate of therapeutic stem cells in different brain disease models

Stem cells have a number of properties, which make them excellent candidates for the treatment of various neurologic disorders, the most important of which being their ability to migrate to and differentiate predictably at sites of pathology in the brain. The disease-directed migration and well-characterized differentiation patterns of stem cells may eventually provide a powerful tool for the treatment of both localized and diffuse disease processes within the human brain. A thorough understanding of the molecular mechanisms governing their migratory properties and their choice between different differentiation programs is essential if these cells are to be used therapeutically in humans. This review focuses on summarizing the migration and differentiation of therapeutic neural and mesenchymal stem cells in different disease models in the brain and also discusses the promise of these cells to eventually treat various forms of neurologic disease.

Tcf3 function is required for the inhibition of oligodendroglial fate specification in the spinal cord of zebrafish embryos

The generation of various subtypes of neurons and glial cells at the right time and place is crucial for the proper development of the vertebrate CNS. Although the mechanisms and factors for the regulation of neuronal diversity in the CNS have been well studied, the mechanisms regulating the sequential production of neuronal and glial cells from neural precursors remain poorly understood. This study shows that Tcf3, a member of the Lef/Tcf family of proteins, is required to inhibit the premature oligodendroglial fate specification of spinal cord precursors using the transgenic zebrafish, which expresses a dominant repressor form of Tcf3 under the control of a heat-shock inducible promoter. In addition, the data revealed that Tcf3 function in oligodendroglial fate specification is mediated independently of canonical Wnt signaling. Altogether, these results show a novel function for Tcf3 in regulating the timing of oligodendroglial fate specification in the spinal cord.

DSulfatase-1 fine-tunes Hedgehog patterning activity through a novel regulatory feedback loop

Sulfs are secreted sulfatases that catalyse removal of sulfate from Heparan Sulfate Proteoglycans (HSPGs) in the extracellular space. These enzymes are well known to regulate a number of crucial signalling pathways during development. In this study, we report that DSulfatase-1 (DSulf1), the unique Drosophila Sulf protein, is a regulator of Hedgehog (Hh) signalling during wing development. DSulf1 activity is required in both Hh source and Hh receiving cells for proper positioning of Hh target gene expression boundaries. As assessed by loss- and gain-of-function experiments in specific compartments, DSulf1 displays dual functions with respect to Hh signalling, acting as a positive regulator in Hh producing cells and a negative regulator in Hh receiving cells. In either domain, DSulf1 modulates Hh distribution by locally lowering the concentration of the morphogen at the apical pole of wing disc cells. Thus, we propose that DSulf1, by its desulfation catalytic activity, lowers Hh/HSPG interaction in both Hh source and target fields, thereby enhancing Hh release from its source of production and reducing Hh signalling activity in responding cells. Finally, we show that Dsulf1 pattern of expression is temporally regulated and depends on EGFR signalling, a Hh-dependent secondary signal in this tissue. Our data reveal a novel Hh regulatory feedback loop, involving DSulf1, which contributes to maintain and stabilise expression domains of Hh target genes during wing disc development.

Dispatched mediates Hedgehog basolateral release to form the long-range morphogenetic gradient in the Drosophila wing disk epithelium

Hedgehog (Hh) moves from the producing cells to regulate the growth and development of distant cells in a variety of tissues. Here, we have investigated the mechanism of Hh release from the producing cells to form a morphogenetic gradient in the Drosophila wing imaginal disk epithelium. We describe that Hh reaches both apical and basolateral plasma membranes, but the apical Hh is subsequently internalized in the producing cells and routed to the basolateral surface, where Hh is released to form a long-range gradient. Functional analysis of the 12-transmembrane protein Dispatched, the glypican Dally-like (Dlp) protein, and the Ig-like and FNNIII domains of protein Interference Hh (Ihog) revealed that Dispatched could be involved in the regulation of vesicular trafficking necessary for basolateral release of Hh, Dlp, and Ihog. We also show that Dlp is needed in Hh-producing cells to allow for Hh release and that Ihog, which has been previously described as an Hh coreceptor, anchors Hh to the basolateral part of the disk epithelium.

Dynamic expression patterns of 6-O endosulfatases during zebrafish development suggest a subfunctionalisation event for sulf2

The 6-O-endosulfatase enzymes (Sulfs) edit the final sulfation pattern and function of heparan sulfate (HS) by removal of 6-O-sulfate groups from the chain. To date, two mammalian sulf genes have been identified that regulate many signalling pathways during embryonic development. In zebrafish a sulf1 ortholog and duplicate copies of the mammalian sulf2 gene, sulf2a and sulf2, have been identified, which contain conserved motifs characteristic of vertebrate sulf genes. Zebrafish sulf1 and sulf2a are broadly expressed in the central nervous system (CNS) and non-neuronal tissue including heart, somite boundaries, olfactory system, and otic vesicle, whereas sulf2 expression is almost entirely restricted to the CNS. The duplicate copies of sulf2 have distinct expression patterns, which together mirror that of mouse sulf2, suggesting duplication in the teleost lineage has been followed by subfunctionalisation, whereby both genes need to be preserved by selection to ensure the ancestral gene's expression profile and function is maintained.

Sulfatase 1 and sulfatase 2 in hepatocellular carcinoma: associated signaling pathways, tumor phenotypes, and survival

The heparin-degrading endosulfatases sulfatase 1 (SULF1) and sulfatase 2 (SULF2) have opposing effects in hepatocarcinogenesis despite structural similarity. Using mRNA expression arrays, we analyzed the correlations of SULF expression with signaling networks in human hepatocellular carcinomas (HCCs) and the associations of SULF expression with tumor phenotype and patient survival. Data from two mRNA microarray analyses of 139 and 36 HCCs and adjacent tissues were used as training and validation sets. Partek and Metacore software were used to identify SULF correlated genes and their associated signaling pathways. Associations between SULF expression, the hepatoblast subtype of HCC, and survival were examined. Both SULF1 and 2 had strong positive correlations with periostin, IQGAP1, TGFB1, and vimentin and inverse correlations with HNF4A and IQGAP2. Genes correlated with both SULFs were highly associated with the cell adhesion, cytoskeletal remodeling, blood coagulation, TGFB, and Wnt/β-catenin and epithelial mesenchymal transition signaling pathways. Genes uniquely correlated with SULF2 were more associated with neoplastic processes than genes uniquely correlated with SULF1. High SULF expression was associated with the hepatoblast subtype of HCC. There was a bimodal effect of SULF1 expression on prognosis, with patients in the lowest or highest tertile having a worse prognosis than those in the middle tertile. SULFs have complex effects on HCC signaling and patient survival. There are functionally similar associations with cell adhesion, ECM remodeling, TGFB, and WNT pathways, but also unique associations of SULF1 and SULF2. The roles and targeting of the SULFs in cancer require further investigation.

Glycomics analysis of mammalian heparan sulfates modified by the human extracellular sulfatase HSulf2

Background

The Sulfs are a family of endosulfatases that selectively modify the 6O-sulfation state of cell-surface heparan sulfate (HS) molecules. Sulfs serve as modulators of cell-signaling events because the changes they induce alter the cell surface co-receptor functions of HS chains. A variety of studies have been aimed at understanding how Sulfs modify HS structure, and many of these studies utilize Sulf knockout cell lines as the source for the HS used in the experiments. However, genetic manipulation of Sulfs has been shown to alter the expression levels of HS biosynthetic enzymes, and in these cases an assessment of the fine structural changes induced solely by Sulf enzymatic activity is not possible. Therefore, the present work aims to extend the understanding of substrate specificities of HSulf2 using in vitro experiments to compare HSulf2 activities on HS from different organ tissues.

Methodology/Principal Findings

To further the understanding of Sulf enzymatic activity, we conducted in vitro experiments where a variety of mammalian HS substrates were modified by recombinant human Sulf2 (HSulf2). Subsequent to treatment with HSulf2, the HS samples were exhaustively depolymerized and analyzed using size-exclusion liquid chromatography-mass spectrometry (SEC-LC/MS). We found that HSulf2 activity was highly dependent on the structural features of the HS substrate. Additionally, we characterized, for the first time, the activity of HSulf2 on the non-reducing end (NRE) of HS chains. The results indicate that the action pattern of HSulf2 at the NRE is different compared to internally within the HS chain.

Conclusions/Significance

The results of the present study indicate that the activity of Sulfs is dependent on the unique structural features of the HS populations that they edit. The activity of HSulf2 at HS NREs implicates the Sulfs as key regulators of this region of the chains, and concomitantly, the protein-binding events that occur there.

HSulf-1 modulates FGF2- and hypoxia-mediated migration and invasion of breast cancer cells

HSulf-1 modulates the sulfation states of heparan sulfate proteoglycans critical for heparin binding growth factor signaling. In the present study, we show that HSulf-1 is transcriptionally deregulated under hypoxia in breast cancer cell lines. Knockdown of HIF-1α rescued HSulf-1 downregulation imposed by hypoxia, both at the RNA and protein levels. Chromatin immunoprecipitation with HIF-1α and HIF-2α antibodies confirmed recruitment of HIF-α proteins to the two functional hypoxia-responsive elements on the native HSulf-1 promoter. HSulf-1 depletion in breast cancer cells resulted in an increased and sustained bFGF2 (basic fibroblast growth factor) signaling and promoted cell migration and invasion under hypoxic conditions. In addition, FGFR2 (fibroblast growth factor receptor 2) depletion in HSulf-1-silenced breast cancer cells attenuated hypoxia-mediated cell invasion. Immunohistochemical analysis of 53 invasive ductal carcinomas and their autologous metastatic lesions revealed an inverse correlation for the expression of HSulf-1 to CAIX in both the primary tumors (P ≥ 0.0198) and metastatic lesions (P ≥ 0.0067), respectively, by χ(2) test. Finally, HSulf-1 expression levels in breast tumors by RNA in situ hybridization showed that high HSulf-1 expression is associated with increased disease-free and overall survival (P ≥ 0.03 and P ≥ 0.0001, respectively). Collectively, these results reveal an important link between loss of HSulf-1 under hypoxic microenvironment and increased growth factor signaling, cell migration, and invasion.

Hedgehog morphogen: from secretion to reception

A major challenge of developmental biology is to understand how cells coordinate developmental behaviors with their neighbors. To achieve this, cells often employ signaling molecules that emanate from a local source and act at a distance on target cells. The Hedgehog morphogen is an essential signaling molecule required for numerous processes during animal development. Emphasizing the importance of this molecule for both growth control and patterning, Hedgehog signaling activity is often deregulated during cancer formation and progression. The secretion and spread of Hedgehog are not passive processes, but require accessory molecules involved in Hedgehog processing, release, spread and reception. In this review, I focus on the factors that are required to control the spread and activity of Hedgehog, highlighting recent data that have shed light on these processes.

Sonic hedgehog signaling in the developing CNS where it has been and where it is going

Sonic Hedgehog (Shh) is one of three mammalian orthologs of the Hedgehog (Hh) family of secreted proteins first identified for their role in patterning the Drosophila embryo. In this review, we will highlight some of the outstanding questions regarding how Shh signaling controls embryonic development. We will mainly consider its role in the developing mammalian central nervous system (CNS) where the pathway plays a critical role in orchestrating the specification of distinct cell fates within ventral regions, a process of exquisite complexity that is necessary for the proper wiring and hence function of the mature system. Embryonic development is a process that plays out in both the spatial and the temporal dimensions, and it is becoming increasingly clear that our understanding of Shh signaling in the CNS is grounded in an appreciation for the dynamic nature of this process. In addition, any consideration of Hh signaling must by necessity include a consideration of data from many different model organisms and systems. In many cases, the extent to which insights gained from these studies are applicable to the CNS remains to be determined, yet they provide a strong framework in which to explore its role in CNS development. We will also discuss how Shh controls cell fate diversification through the regulation of patterned target gene expression in the spinal cord, a region where our understanding of the morphogenetic action of graded Shh signaling is perhaps the furthest advanced.

Differential involvement of the extracellular 6-O-endosulfatases Sulf1 and Sulf2 in brain development and neuronal and behavioural plasticity

The extracellular sulfatases Sulf1 and Sulf2 remove specific 6-O-sulfate groups from heparan sulfate, thereby modulating numerous signalling pathways underlying development and homeostasis. In vitro data have suggested that the two enzymes show functional redundancy. To elucidate their in vivo functions and to further address the question of a putative redundancy, we have generated Sulf1- and Sulf2-deficient mice. Phenotypic analysis of these animals revealed higher embryonic lethality of Sulf2 knockout mice, which can be associated with neuroanatomical malformations during embryogenesis. Sulf1 seems not to be essential for developmental or postnatal viability, as mice deficient in this sulfatase show no overt phenotype. However, neurite outgrowth deficits were observed in hippocampal and cerebellar neurons of both mutant mouse lines, suggesting that not only Sulf2 but also Sulf1 function plays a role in the developing nervous system. Behavioural analysis revealed differential deficits with regard to cage activity and spatial learning for Sulf1- and Sulf2-deficient mouse lines. In addition, Sulf1-specific deficits were shown for synaptic plasticity in the CA1 region of the hippocampus, associated with a reduced spine density. These results reveal that Sulf1 and Sulf2 fulfil non-redundant functions in vivo in the development and maintenance of the murine nervous system.

Glial cells modulate heparan sulfate proteoglycan (HSPG) expression by neuronal precursors during early postnatal cerebellar development

Cerebellum controls motor coordination, balance, eye movement, and has been implicated in memory and addiction. As in other parts of the CNS, correct embryonic and postnatal development of the cerebellum is crucial for adequate performance in the adult. Cellular and molecular defects during cerebellar development can lead to severe phenotypes, such as ataxias and tumors. Knowing how the correct development occurs can shed light into the mechanisms of disease. Heparan sulfate proteoglycans are complex molecules present in every higher eukaryotic cells and changes in their level of expression as well as in their structure lead to drastic functional alterations. This work aimed to investigate changes in heparan sulfate proteoglycans expression during cerebellar development that could unveil control mechanisms. Using real time RT-PCR we evaluated the expression of syndecans, glypicans and modifying enzymes by isolated cerebellar granule cell precursors, and studied the influence of soluble glial factors on the expression of those genes. We evaluated the possible involvement of Runx transcription factors in the response of granule cell precursors to glial factors. Our data show for the first time that cerebellar granule cell precursors express members of the Runx family and that the expression of those genes can also be controlled by glial factors. Our results also show that the expression of all genes studied vary during postnatal development and treatment of precursors with glial factors indicate that the expression of heparan sulfate proteoglycan genes as well as genes encoding heparan sulfate modifying enzymes can be modulated by the microenvironment, reflecting the intricate relations between neuron and glia.

Dose-dependent effect of EGF on migration and differentiation of adult subventricular zone astrocytes

Adult neural stem cells (NSCs) are located in the subventricular zone (SVZ), a specialized brain niche located on the walls of the lateral ventricle. Under physiological conditions, NSCs generate a large number of young neurons and some oligodendrocytes, however the mechanisms controlling cell proliferation and migration are unclear. In vitro, epidermal growth factor (EGF) signaling has been shown to be an important mediator of cell proliferation and migration in the adult brain; however, the primary SVZ progenitors that respond to EGF are not well known. In this study, we isolated SVZ type-B astrocytes and cultured them under different EGF concentrations. We found a dose-dependent effect of EGF on proliferation rates and migration of SVZ type-B astrocytes. We found that GFAP+ type-B astrocytes gave rise to highly migratory and proliferating cells that expressed Olig2 and NG2. After EGF withdrawal, a significant number of EGF-stimulated cells differentiated into S100beta+/O4+ oligodendrocytes. This study provides new insights about the production of oligodendrocytes derived from the astrocyte NSCs residing in the adult SVZ. To be able to manipulate the endogenous adult progenitors, it is crucial to identify and isolate the responding primary precursors and determine the extracellular signals that regulate their cell division, migration, and fate.

Smoothened agonist augments proliferation and survival of neural cells

Sonic hedgehog signaling pathway is important in developmental processes like dorsoventral neural tube patterning, neural stem cell proliferation and neuronal and glial cell survival. Shh is also implicated in the regulation of the adult hippocampal neurogenesis. Recently, nonpeptidyl Smoothened activators of the Shh pathway have been identified. The aim of this study was to investigate the effects of chlorobenzothiophene-containing molecule, Smo agonist (SAG), which has been shown to activate Shh signaling pathway, in neurogenesis and neuronal survival in in vitro and in vivo models. Our in vitro experiments showed that SAG induces increased expression of Gli1 mRNA, transcriptional target and mediator of Shh signal. In vitro experiments also demonstrated that SAG in low-nanomolar concentrations induces proliferation of neuronal and glial precursors without affecting the differentiation pattern of newly produced cells. In contrast to Shh, SAG did not induce neurotoxicity in neuronal cultures. The SAG and Shh treatment also promoted the survival of newly generated neural cells in the dentate gyrus after their intracerebroventricular administration to adult rats. We propose that SAG and similar compounds represent attractive molecules to be developed for treatment of disorders where stimulation of the generation and survival of new neural cells would be beneficial.

Role of BMPs in controlling the spatial and temporal origin of GFAP astrocytes in the embryonic spinal cord

In the vertebrate central nervous system (CNS), astrocytes are the most abundant and functionally diverse glial cell population. However, the mechanisms underlying their specification and differentiation are still poorly understood. In this study, we have defined spatially and temporally the origin of astrocytes and studied the role of BMPs in astrocyte development in the embryonic chick spinal cord. Using explant cultures, we show that astrocyte precursors started migrating out of the neuroepithelium in the mantle layer from E5, and that the dorsal-most level of the neuroepithelium, from the roof plate to the dl3 level, did not generate GFAP-positive astrocytes. Using a variety of early astrocyte markers together with functional analyses, we show that dorsal-most progenitors displayed a potential for astrocyte production but that dorsally-derived BMP signalling, possibly mediated through BMP receptor 1B, promoted neuronal specification instead. BMP treatment completely prevented astrocyte development from intermediate spinal cord explants at E5, whereas it promoted it at E6. Such an abrupt change in the response of this tissue to BMP signalling could be correlated to the onset of new foci of BMP activity and enhanced expression of BMP receptor 1A, suggesting that BMP signalling could promote astrocyte development in this region.

The long-range activity of Hedgehog is regulated in the apical extracellular space by the glypican Dally and the hydrolase Notum

Cell fate determination during developmental patterning is often controlled by concentration gradients of morphogens. In the epithelial field, morphogens like the Hedgehog (Hh) peptides diffuse both apically and basolaterally; however, whether both pools of Hh are sensed at the cellular level is unclear. Here, we show that interfering with the amount of apical Hh causes a dramatic change in the long-range activation of low-threshold Hh target genes, without similar effect on short-range, high-threshold targets. We provide genetic evidence that the glypican Dally upregulates apical Hh levels, and that the release of Dally by the hydrolase Notum promotes apical Hh long-range activity. Our data suggest that several pools of Hh are perceived in epithelial tissues. Thus, we propose that the overall gradient of Hh is a composite of pools secreted by different routes (apical and basolateral), and that a cellular summation of these components is required for appropriate developmental patterning.

Sonic Hedgehog signaling in the mammalian brain

The discovery of a Sonic Hedgehog (Shh) signaling pathway in the mature vertebrate CNS has paved the way to the characterization of the functional roles of Shh signals in normal and diseased brain. Shh is proposed to participate in the establishment and maintenance of adult neurogenic niches and to regulate the proliferation of neuronal or glial precursors in several brain areas. Consistent with its role during brain development, misregulation of Shh signaling is associated with tumorigenesis while its recruitement in damaged neural tissue might be part of the regenerating process. This review focuses on the most recent data of the Hedgehog pathway in the adult brain and its relevance as a novel therapeutic approach for brain diseases including brain tumors.

Sulf-2, a heparan sulfate endosulfatase, promotes human lung carcinogenesis

Heparan sulfate (HS) proteoglycans (HSPGs) bind to multiple growth factors/morphogens and regulate their signaling. 6-O-sulfation (6S) of glucosamine within HS chains is critical for many of these ligand interactions. Sulf-1 and Sulf-2, which are extracellular neutral-pH sulfatases, provide a novel post-synthetic mechanism for regulation of HSPG function by removing 6S from intact HS chains. The Sulfs can thereby modulate several signaling pathways, including the promotion of Wnt signaling. We found induction of SULF2 transcripts and Sulf-2 protein in human lung adenocarcinoma and squamous cell carcinoma, the two major classes of non-small-cell lung carcinomas (NSCLCs). We confirmed widespread Sulf-2 protein expression in tumor cells of 10/10 surgical specimens of human lung squamous carcinomas. We studied five Sulf-2(+) NSCLC cell lines, including two, which were derived by cigarette-smoke transformation of bronchial epithelial cells. shRNA-mediated Sulf-2 knockdown in these lines caused an increase in 6S on their cell surface and in parallel reversed their transformed phenotype in vitro, eliminated autocrine Wnt signaling and strongly blunted xenograft tumor formation in nude mice. Conversely, forced Sulf-2 expression in non-malignant bronchial epithelial cells produced a partially transformed phenotype. Our findings support an essential role for Sulf-2 in lung cancer, the leading cancer killer.

Establishing and interpreting graded Sonic Hedgehog signaling during vertebrate neural tube patterning: the role of negative feedback

The secreted protein Sonic Hedgehog (SHH) acts in graded fashion to pattern the dorsal-ventral axis of the vertebrate neural tube. This is a dynamic process in which increasing concentrations and durations of exposure to SHH generate neurons with successively more ventral identities. Interactions between the receiving cells and the graded signal underpin the mechanism of SHH action. In particular, negative feedback, involving proteins transcriptionally induced or repressed by SHH signaling, plays an essential role in shaping the graded readout. On one hand, negative feedback controls, in a noncell-autonomous manner, the distribution of SHH across the field of receiving cells. On the other, it acts cell-autonomously to convert different concentrations of SHH into distinct durations of intracellular signal transduction. Together, these mechanisms exemplify a strategy for morphogen interpretation, which we have termed temporal adaptation that relies on the continuous processing and refinement of the cellular response to the graded signal.

Human embryonic stem cell differentiation toward regional specific neural precursors

Human embryonic stem cells (hESCs) are self-renewing pluripotent cells that have the capacity to differentiate into a wide variety of cell types. This potentiality represents a promising source to overcome many human diseases by providing an unlimited supply of all cell types, including cells with neural characteristics. Therefore, this review summarizes early neural development and the potential of hESCs to differentiate under in vitro conditions, examining at the same time the potential use of differentiated hESCs for therapeutic applications for neural tissue and cell regeneration.

Serine racemase is associated with schizophrenia susceptibility in humans and in a mouse model

Abnormal N-methyl-d-aspartate receptor (NMDAR) function has been implicated in the pathophysiology of schizophrenia. d-serine is an important NMDAR modulator, and to elucidate the role of the d-serine synthesis enzyme serine racemase (Srr) in schizophrenia, we identified and characterized mice with an ENU-induced mutation that results in a complete loss of Srr activity and dramatically reduced d-serine levels. Mutant mice displayed behaviors relevant to schizophrenia, including impairments in prepulse inhibition, sociability and spatial discrimination. Behavioral deficits were exacerbated by an NMDAR antagonist and ameliorated by d-serine or the atypical antipsychotic clozapine. Expression profiling revealed that the Srr mutation influenced several genes that have been linked to schizophrenia and cognitive ability. Transcript levels altered by the Srr mutation were also normalized by d-serine or clozapine treatment. Furthermore, analysis of SRR genetic variants in humans identified a robust association with schizophrenia. This study demonstrates that aberrant Srr function and diminished d-serine may contribute to schizophrenia pathogenesis.

Identification of genes differentially expressed in dorsal and ventral chick midbrain during early development

Background

During the development of the central nervous system (CNS), patterning processes along the dorsoventral (DV) axis of the neural tube generate different neuronal subtypes. As development progresses these neurons are arranged into functional units with varying cytoarchitecture, such as laminae or nuclei for efficient relaying of information. Early in development ventral and dorsal regions are similar in size and structure. Different proliferation rates and cell migration patterns are likely to result in the formation of laminae or nuclei, eventually. However, the underlying molecular mechanisms that establish these different structural arrangements are not well understood.

We undertook a differential display polymerase chain reaction (DD-PCR) screen to identify genes with distinct expression patterns between dorsal and ventral regions of the chick midbrain in order to identify genes which regulate the sculpturing of such divergent neuronal organisation. We focused on the DV axis of the early chick midbrain since mesencephalic alar plate and basal plate develop into laminae and nuclei, respectively.

Results

We identified 53 differentially expressed bands in our initial screen. Twenty-six of these could be assigned to specific genes and we could unambiguously show the differential expression of five of the isolated cDNAs in vivo by in situ mRNA expression analysis. Additionally, we verified differential levels of expression of a selected number of genes by using reverse transcriptase (RT) PCR method with gene-specific primers.

One of these genes, QR1, has been previously cloned and we present here a detailed study of its early developmental time course and pattern of expression providing some insights into its possible function. Our phylogenetic analysis of QR1 shows that it is the chick orthologue of Sparc-like 1/Hevin/Mast9 gene in mice, rats, dogs and humans, a protein involved in cell adhesion.

Conclusion

This study reveals some possible networks, which might be involved in directing the difference in neuronal specification and cytoarchitecture observed in the brain.

Spatially distinct functions of PAX6 and NKX2.2 during gliogenesis in the ventral spinal cord

During ventral spinal cord (vSC) development, the p3 and pMN progenitor domain boundary is thought to be maintained by cross-repressive interactions between NKX2.2 and PAX6. Using loss-of-function analysis during the neuron-glial fate switch we show that the identity of the p3 domain is not maintained by the repressive function of NKX2.2 on Pax6 expression, even in the absence of NKX2.9. We further show that NKX2.2 is necessary to induce the expression of Slit1 and Sulfatase 1 (Sulf1) in the vSC in a PAX6-independent manner. Conversely, we show that PAX6 regulates Sulf1/Slit1 expression in the vSC in an NKX2.2/NKX6.1-independent manner. Consequently, deregulation of Sulf1 expression in Pax6-mutant embryos has stage-specific implications on neural patterning, associated with enhancement of Sonic Hedgehog activity. On the other hand, deregulation of Slit1 expression in gliogenic neural progenitors leads to changes in Astrocyte subtype identity. These data provide important insights into specific functions of PAX6 and NKX2.2 during glial cell specification that have so far remained largely unexplored.

Dynamic signaling for neural stem cell fate determination

Central nervous system (CNS) development starts from neural stem cells (NSCs) which ultimately give rise to the three major cell types (neurons, oligodendrocytes and astrocytes) of the CNS. NSCs are specified in space- and time-related fashions, becoming spatially heterogeneous and generating a progressively restricted repertoire of cell types. Mammalian NSCs produce different cell types at different time points during development under the influence of multiple signaling pathways. These pathways act in a dynamic web mode to determine the fate of NSCs via modulating the expression and activity of distinct set of transcription factors which in turn trigger the transcription of neural fate-associated genes. This review thus introduces the major signal pathways, transcription factors and their cross-talks and coordinative interactions in NSC fate determination.

Epigenetic background of neuronal fate determination

The development of the central nervous system (CNS) starts from neural stem cells (NSCs). During this process, NSCs are specified in space- and time-related fashions, becoming spatially heterogeneous and generating a progressively restricted repertoire of cell types: neurons, astrocytes and oligodendrocytes. The processes of neurodevelopment are determined reciprocally by intrinsic and external factors which interface to program and re-program the profiling of fate-determination gene expression. Multiple signaling pathways act in a dynamic web mode to determine the fate of NSCs through modulating the activity of a distinct set of transcription factors which in turn trigger the transcription of neural fate-determination genes. Accumulating evidence reveals that during CNS development, multiple epigenetic factors regulate the activities of extracellular signaling and corresponding transcription factors in a coordinative manner, leading to the formation of a system with sophisticated structure and magic functions. This review aims to introduce recent advances in the epigenetic background of neural cell fate determination.

Pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network

Neuronal subtype specification in the vertebrate neural tube is one of the best-studied examples of embryonic pattern formation. Distinct neuronal subtypes are generated in a precise spatial order from progenitor cells according to their location along the anterior-posterior and dorsal-ventral axes. Underpinning this organization is a complex network of multiple extrinsic and intrinsic factors. This review focuses on the molecular mechanisms and general strategies at play in ventral regions of the forming spinal cord, where sonic hedgehog-based morphogen signaling is a key determinant. We discuss recent advances in our understanding of these events and highlight unresolved questions.

Redundant function of the heparan sulfate 6-O-endosulfatases Sulf1 and Sulf2 during skeletal development

Modification of the sulfation pattern of heparan sulfate (HS) during organ development is thought to regulate binding and signal transduction of several growth factors. The secreted sulfatases, Sulf1 and Sulf2, desulfate HS on 6-O-positions extracellularly. We show that both sulfatases are expressed in overlapping patterns during embryonic skeletal development. Analysis of compound mutants of Sulf1 and Sulf2 derived from gene trap insertions and targeted null alleles revealed subtle but distinct skeletal malformations including reduced bone length, premature vertebrae ossification and fusions of sternebrae and tail vertebrae. Molecular analysis of endochondral ossification points to a function of Sulf1 and Sulf2 in delaying the differentiation of endochondral bones. Penetrance and severity of the phenotype increased with reduced numbers of functional alleles indicating redundant functions of both sulfatases. The mild skeletal phenotype of double mutants suggests a role for extracellular modification of 6-O-sulfation in fine-tuning rather than regulating the development of skeletal structures.

The effect of controlled growth factor delivery on embryonic stem cell differentiation inside fibrin scaffolds

The goal of this project was to develop 3-D biomaterial scaffolds that present cues to direct the differentiation of embryonic stem (ES) cell-derived neural progenitor cells, seeded inside the scaffolds, into mature neural phenotypes, specifically neurons and oligodendrocytes. Release studies were performed to determine the appropriate conditions for retention of neurotrophin-3 (NT-3), sonic hedgehog, and platelet-derived growth factor (PDGF) by an affinity-based delivery system incorporated into fibrin scaffolds. Embryoid bodies containing neural progenitors were formed from mouse ES cells, using a 4-/4+ retinoic acid treatment protocol, and then seeded inside fibrin scaffolds containing the drug delivery system. This delivery system was used to deliver various growth factor doses and combinations to the cells seeded inside the scaffolds. Controlled delivery of NT-3 and PDGF simultaneously increased the fraction of neural progenitors, neurons, and oligodendrocytes while decreasing the fraction of astrocytes obtained compared to control cultures seeded inside unmodified fibrin scaffolds with no growth factors present in the medium. These results demonstrate that such a strategy can be used to generate an engineered tissue for the potential treatment of spinal cord injury and could be extended to the study of differentiation in other tissues.

The adventures of sonic hedgehog in development and repair. III. Hedgehog processing and biological activity

The Hedgehog (Hh) family of secreted proteins is necessary for aspects of the development and maintenance of the gastrointestinal tract. Hh is thought to function as a morphogen, a mitogen, a cell survival factor, and an axon guidance factor. Given its wide role in development, as well as in a variety of disease states, understanding the regulation of Hh function and activity is critically important. However, the study of Hh signaling has been impeded by its unusual biology. Hh is unique in that it is the only protein covalently modified by cholesterol, which in turn affects numerous aspects of its localization, release, movement, and activity. All are important factors when considering Hh's physiological role, and animals have developed an intricate system of regulators responsible for both promoting and inhibiting the activity of Hh. This review is intended to give a broad overview of how the biosynthesis and movement of Hh contributes to its biological activity.

Region-specific requirement for cholesterol modification of sonic hedgehog in patterning the telencephalon and spinal cord

Sonic hedgehog (Shh) secreted from the axial signaling centers of the notochord and prechordal plate functions as a morphogen in dorsoventral patterning of the neural tube. Active Shh is uniquely cholesterol-modified and the hydrophobic nature of cholesterol suggests that it might regulate Shh spreading in the neural tube. Here, we examined the capacity of Shh lacking the cholesterol moiety (ShhN) to pattern different cell types in the telencephalon and spinal cord. In mice expressing ShhN, we detected low-level ShhN in the prechordal plate and notochord, consistent with the notion that ShhN can rapidly spread from its site of synthesis. Surprisingly, we found that low-level ShhN can elicit the generation of a full spectrum of ventral cell types in the spinal cord, whereas ventral neuronal specification and ganglionic eminence development in the Shh(N/-) telencephalon were severely impaired, suggesting that telencephalic patterning is more sensitive to alterations in local Shh concentration and spreading. In agreement, we observed induction of Shh pathway activity and expression of ventral markers at ectopic sites in the dorsal telencephalon indicative of long-range ShhN activity. Our findings indicate an essential role for the cholesterol moiety in restricting Shh dilution and deregulated spread for patterning the telencephalon. We propose that the differential effect of ShhN in patterning the spinal cord versus telencephalon may be attributed to regional differences in the maintenance of Shh expression in the ventral neuroepithelium and differences in dorsal tissue responsiveness to deregulated Shh spreading behavior.

Ascl1 is required for oligodendrocyte development in the spinal cord

Development of oligodendrocytes, myelin-forming glia in the central nervous system (CNS), proceeds on a protracted schedule. Specification of oligodendrocyte progenitors (OLPs) begins early in development, whereas their terminal differentiation occurs at late embryonic and postnatal periods. How these distinct steps are controlled remains unclear. Our previous study demonstrated an important role of the helix-loop-helix (HLH) transcription factor Ascl1 in early generation of OLPs in the developing spinal cord. Here, we show that Ascl1 is also involved in terminal differentiation of oligodendrocytes late in development. Ascl1-/- mutant mice showed a deficiency in differentiation of myelin-expressing oligodendrocytes at birth. In vitro culture studies demonstrate that the induction and maintenance of co-expression of Olig2 and Nkx2-2 in OLPs, and thyroid hormone-responsive induction of myelin proteins are impaired in Ascl1-/- mutants. Gain-of-function studies further showed that Ascl1 collaborates with Olig2 and Nkx2-2 in promoting differentiation of OLPs into oligodendrocytes in vitro. Overexpression of Ascl1, Olig2 and Nkx2-2 alone stimulated the specification of OLPs, but the combinatorial action of Ascl1 and Olig2 or Nkx2-2 was required for further promoting their differentiation into oligodendrocytes. Thus, Ascl1 regulates multiple aspects of oligodendrocyte development in the spinal cord.

SULF1 and SULF2 regulate heparan sulfate-mediated GDNF signaling for esophageal innervation

Heparan sulfate (HS) plays an essential role in extracellular signaling during development. Biochemical studies have established that HS binding to ligands and receptors is regulated by the fine 6-O-sulfated structure of HS; however, mechanisms that control sulfated HS structure and associated signaling functions in vivo are not known. Extracellular HS 6-O-endosulfatases, SULF1 and SULF2, are candidate enzymatic regulators of HS 6-O-sulfated structure and modulate HS-dependent signaling. To investigate Sulf regulation of developmental signaling, we have disrupted Sulf genes in mouse and identified redundant functions of Sulfs in GDNF-dependent neural innervation and enteric glial formation in the esophagus, resulting in esophageal contractile malfunction in Sulf1(-/-);Sulf2(-/-) mice. SULF1 is expressed in GDNF-expressing esophageal muscle and SULF2 in innervating neurons, establishing their direct functions in esophageal innervation. Biochemical and cell signaling studies show that Sulfs are the major regulators of HS 6-O-desulfation, acting to reduce GDNF binding to HS and to enhance GDNF signaling and neurite sprouting in the embryonic esophagus. The functional specificity of Sulfs in GDNF signaling during esophageal innervation was established by showing that the neurite sprouting is selectively dependent on GDNF, but not on neurotrophins or other signaling ligands. These findings provide the first in vivo evidence that Sulfs are essential developmental regulators of cellular HS 6-O-sulfation for matrix transmission and reception of GDNF signal from muscle to innervating neurons.

A spatial bias for the origins of interneuron subgroups within the medial ganglionic eminence

Although it is well established that the ventral telencephalon is the primary source of GABAergic cortical interneurons in rodents, little is known about the specification of specific interneuron subtypes. It is also unclear whether the potential to achieve a given fate is established at their place of origin or by signals received during their migration to or during their maturation within the cerebral cortex. Using both in vivo and in vitro transplantation techniques, we find that two major interneuron subgroups have largely distinct origins within the MGE. Somatostatin (SST)-expressing interneurons are primarily generated within the dorsal MGE, while parvalbumin (PV)-expressing interneurons primarily originate from the ventral MGE. In addition, we show that significant heterogeneity exists between gene expression patterns in the dorsal and ventral MGE. These results suggest that, like the spinal cord, neuronal fate determination in the ventral telencephalon is largely the result of spatially segregated, molecularly distinct microdomains arranged on the dorsal-ventral axis.

Sonic hedgehog promotes the migration and proliferation of optic nerve oligodendrocyte precursors

Optic nerve (ON) oligodendrocyte precursors (OPCs) are generated under the influence of the Sonic hedgehog (Shh) in the preoptic area from where they migrate to colonise the entire nerve. The molecular events that control this migration are still poorly understood. Recent studies suggested that Shh is often used by the same cell population to control different processes, including cell proliferation and migration, raising the possibility that Shh could contribute to these aspects of OPC development. In support of this idea, we show here that Shh induces the proliferation of OPCs derived from embryonic mouse ON explants and acts as a chemoattractant for their migration. In ovo injections of hybridomas secreting Shh-specific blocking antibody decreases the number of OPCs present in chick ONs, particularly in the retinal portion of the nerve. Altogether these data indicate that Shh contributes to OPC proliferation and distribution along the ON, in addition to their specification.

Neurogenic role of Gcm transcription factors is conserved in chicken spinal cord

Although glial cells missing (gcm) genes are known as glial determinants in the fly embryo, the role of vertebrate orthologs in the central nervous system is still under debate. Here we show for the first time that the chicken ortholog of fly gcm (herein referred to as c-Gcm1), is expressed in early neuronal lineages of the developing spinal cord and is required for neural progenitors to differentiate as neurons. Moreover, c-Gcm1 overexpression is sufficient to trigger cell cycle exit and neuronal differentiation in neural progenitors. Thus, c-Gcm1 expression constitutes a crucial step in the developmental cascade that prompts progenitors to generate neurons: c-Gcm1 acts downstream of proneural (neurogenin) and progenitor (Sox1-3) factors and upstream of NeuroM neuronal differentiation factor. Strikingly, this neurogenic role is not specific to the vertebrate gene, as fly gcmand gcm2 are also sufficient to induce the expression of neuronal markers. Interestingly, the neurogenic role is restricted to post-embryonic stages and we identify two novel brain neuronal lineages expressing and requiring gcm genes. Finally, we show that fly gcm and the chick and mouse orthologs induce expression of neural markers in HeLa cells. These data, which demonstrate a conserved neurogenic role for Gcm transcription factors, call for a re-evaluation of the mode of action of these genes during evolution.

A conserved mechanism of Hedgehog gradient formation by lipid modifications

Members of the Hedgehog (Hh) family of proteins are conserved morphogens that modulate cell fates in target tissues in different developmental systems. Dysregulation of Hh signaling results in a wide range of human diseases. The mature Hh is modified by lipids in two places, with palmitate at the N-terminus and cholesterol at the C-terminus. The lipid modifications are essential to the proper secretion and spreading of the morphogen throughout the extracellular matrix, interacting with heparan sulfate proteoglycans. However, the role of lipid modifications in regulating the range and activity of Hh proteins remains controversial. Here, we aim to resolve this issue by providing a model that is consistent with current and past reports. We propose that the cholesterol moiety functions to restrict the dilution and deregulated spread of the morphogen in the extracellular space.