The specification of neuronal identity by graded Sonic Hedgehog signalling

Trophic and proliferative effects of Shh on motor neurons in embryonic spinal cord culture from wildtype and G93A SOD1 mice

Background

The developmental morphogen sonic hedgehog (Shh) may continue to play a trophic role in the support of terminally-differentiated motor neurons, of potential relevance to motor neuron disease. In addition, it may support the proliferation and differentiation of endogenous stem cells along motor neuronal lineages. As such, we have examined the trophic and proliferative effects of Shh supplementation or Shh antagonism in embryonic spinal cord cell cultures derived from wildtype or G93A SOD1 mice, a mouse model of amyotrophic lateral sclerosis.

Results

Shh supported survival, and stimulated growth of motor neurons, neurite outgrowth, and neurosphere formation in primary culture derived from both G93A SOD1 and WT mice. Shh increased the percentage of ciliated motor neurons, especially in G93A SOD1 culture. Shh-treated cultures showed increased neuronal proliferation compared to controls and especially cyclopamine treated cultures, from G93A SOD1 and WT mice. Moreover, Shh enhanced cell survival and differentiation of motor neuron precursors in WT culture.

Conclusions

Shh is neurotrophic to motor neurons and has mitogenic effects in WT and mSOD1 G93A culture in vitro.

The homeodomain factor Gbx1 is required for locomotion and cell specification in the dorsal spinal cord

Dorsal horn neurons in the spinal cord integrate and relay sensory information to higher brain centers. These neurons are organized in specific laminae and different transcription factors are involved in their specification. The murine homeodomain Gbx1 protein is expressed in the mantle zone of the spinal cord at E12.5-13.5, correlating with the appearance of a discernable dorsal horn around E14 and eventually defining a narrow layer in the dorsal horn around perinatal stages. At postnatal stages, Gbx1 identifies a specific subpopulation of GABAergic neurons in the dorsal spinal cord. We have generated a loss of function mutation for Gbx1 and analyzed its consequences during spinal cord development. Gbx1^−/− mice are viable and can reproduce as homozygous null mutants. However, the adult mutant mice display an altered gait during forward movement that specifically affects the hindlimbs. This abnormal gait was evaluated by a series of behavioral tests, indicating that locomotion is impaired, but not muscle strength or motor coordination. Molecular analysis showed that the development of the dorsal horn is not profoundly affected in Gbx1^−/− mutant mice. However, analysis of terminal neuronal differentiation revealed that the proportion of GABAergic inhibitory interneurons in the superficial dorsal horn is diminished. Our study unveiled a role for Gbx1 in specifying a subset of GABAergic neurons in the dorsal horn of the spinal cord involved in the control of posterior limb movement.

Identification of multiple subsets of ventral interneurons and differential distribution along the rostrocaudal axis of the developing spinal cord

The spinal cord contains neuronal circuits termed Central Pattern Generators (CPGs) that coordinate rhythmic motor activities. CPG circuits consist of motor neurons and multiple interneuron cell types, many of which are derived from four distinct cardinal classes of ventral interneurons, called V0, V1, V2 and V3. While significant progress has been made on elucidating the molecular and genetic mechanisms that control ventral interneuron differentiation, little is known about their distribution along the antero-posterior axis of the spinal cord and their diversification. Here, we report that V0, V1 and V2 interneurons exhibit distinct organizational patterns at brachial, thoracic and lumbar levels of the developing spinal cord. In addition, we demonstrate that each cardinal class of ventral interneurons can be subdivided into several subsets according to the combinatorial expression of different sets of transcription factors, and that these subsets are differentially distributed along the rostrocaudal axis of the spinal cord. This comprehensive molecular profiling of ventral interneurons provides an important resource for investigating neuronal diversification in the developing spinal cord and for understanding the contribution of specific interneuron subsets on CPG circuits and motor control.

Temporal and spatial requirements of Smoothened in ventral midbrain neuronal development

Background

Several studies have indicated that Sonic hedgehog (Shh) regulates the expansion of dopaminergic (DA) progenitors and the subsequent generation of mature DA neurons. This prevailing view has been based primarily on in vitro culture results, and the exact in vivo function of Shh signaling in the patterning and neurogenesis of the ventral midbrain (vMB) remains unclear.

Methods

We characterized the transcriptional codes for the vMB progenitor domains, and correlated them with the expression patterns of Shh signaling effectors, including Shh, Smoothened, Patched, Gli1, Gli2 and Gli3.

Results

While Shh and its downstream effectors showed robust expression in the neurogenic niche for DA progenitors at embryonic day (E)8 to E8.5, their expression shifted to the lateral domains from E9.5 to E12.5. Consistent with this dynamic change, conditional mutants with region-specific removal of the Shh receptor Smoothened in the vMB progenitors (Shh-Cre;Smo^fl/fl) showed a transient reduction in DA progenitors and DA neurons at E10.5, but had more profound defects in neurons derived from the more lateral domains, including those in the red nucleus, oculomotor nucleus, and raphe nuclei. Conversely, constitutive activation of Smoothened signaling in vMB (Shh-Cre;SmoM2) showed transient expansion of the same progenitor population. To further characterize the nature of Shh-Smoothened signaling in vMB, we examined the BAT-GAL reporter and the expression of Wnt1 in vMB, and found that the antagonistic effects of Shh and Wnt signaling critically regulate the development of DA progenitors and DA neurons.

Conclusion

These results highlight previously unrecognized effects of Shh-Smoothened signaling in the region-specific neurogenesis within the vMB.

Lhx2 regulates the development of the forebrain hem system

Early brain development is regulated by the coordinated actions of multiple signaling centers at key boundaries between compartments. Three telencephalic midline structures are in a position to play such roles in forebrain patterning: The cortical hem, the septum, and the thalamic eminence at the diencephalic–telencephalic boundary. These structures express unique complements of signaling molecules, and they also produce distinct populations of Cajal–Retzius cells, which are thought to act as “mobile patterning units,” migrating tangentially to cover the telencephalic surface. We show that these 3 structures require the transcription factor Lhx2 to delimit their extent. In the absence of Lhx2 function, all 3 structures are greatly expanded, and the Cajal–Retzius cell population is dramatically increased. We propose that the hem, septum, and thalamic eminence together form a “forebrain hem system” that defines and regulates the formation of the telencephalic midline. Disruptions in the forebrain hem system may be implicated in severe brain malformations such as holoprosencephaly. Lhx2 functions as a central regulator of this system's development. Since all components of the forebrain hem system have been identified across several vertebrate species, the mechanisms that regulate them may have played a fundamental role in driving key aspects of forebrain evolution.

What are those cilia doing in the neural tube?

Primary cilia are present on almost all vertebrate cells, and they have diverse functions in distinct tissues. Cilia are important for sensation in multiple capacities in contexts as different as the retina, kidney, and inner ear. In addition to these roles, cilia play a critical part in various developmental processes. Of particular importance is the development of the neural tube, where cilia are essential for the transduction of the Sonic Hedgehog (Shh) signaling pathway that specifies neuronal cell fates. This relationship is well established and is the most recognizable function for cilia in the neural tube, but it may be part of a larger picture. Here, we discuss the links between cilia and Shh signaling, as well as suggesting additional roles for cilia, and mechanisms for their placement, in the neural tube.

Developmental origin of neural stem cells: the glial cell that could

Since their identification, there has been tremendous interest in adult neural stem cells, in part based upon their potential therapeutic uses in understanding and treating neurological disorders. But what's the origin of these cells in the embryo? We outline here the onset of neural specification in the vertebrate embryo and describe the molecular mechanisms regulating patterning of the central nervous system (CNS). We trace the lineage of the multipotential stem cell of the nervous system from embryonic neuroepithelial cell to adult astrocyte-like neural stem cell. As these stem cells emerge throughout development and in the adult, they appear to be predetermined to a specific neuronal or glial fate. Finally, we compare the properties of embryonic stem cell-derived neural stem cells and CNS-derived neural stem cells.

Members of the BMP, Shh, and FGF morphogen families promote chicken statoacoustic ganglion neurite outgrowth and neuron survival in vitro

Mechanosensory hair cells of the chicken inner ear are innervated by the peripheral processes of statoacoustic ganglion (SAG) neurons. Members of several morphogen families are expressed within and surrounding the chick inner ear during stages of SAG axon outgrowth and pathfinding. On the basis of their localized expression patterns, we hypothesized that bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), and sonic hedgehog (Shh) may function as guidance cues for growing axons and/or may function as trophic factors once axons have reached their targets. To test this hypothesis, three-dimensional collagen cultures were used to grow Embryonic Day 4 (E4) chick SAG explants for 24 h in the presence of purified proteins or beads soaked in proteins. The density of neurite outgrowth was quantified to determine effects on neurite outgrowth. Explants displayed enhanced neurite outgrowth when cultured in the presence of purified BMP4, BMP7, a low concentration of Shh, FGF8, FGF10, or FGF19. In contrast, SAG neurons appeared unresponsive to FGF2. Collagen gel cultures were labeled with terminal dUTP nick-end labeling and immunostained with anti-phosphohistone H3 to determine effects on neuron survival and proliferation, respectively. Treatments that increased neurite outgrowth also yielded significantly fewer apoptotic cells, with no effect on cell proliferation. When presented as focal sources, BMP4, Shh, and FGFs -8, -10, and -19 promoted asymmetric outgrowth from the ganglion in the direction of the beads. BMP7-soaked beads did not induce this response. These results suggest that a subset of morphogens enhance both survival and axon outgrowth of otic neurons.

Sonic hedgehog is indirectly required for intraretinal axon pathfinding by regulating chemokine expression in the optic stalk

Successful axon pathfinding requires both correct patterning of tissues, which will later harbor axonal tracts, and precise localization of axon guidance cues along these tracts at the time of axon outgrowth. Retinal ganglion cell (RGC) axons grow towards the optic disc in the central retina, where they turn to exit the eye through the optic nerve. Normal patterning of the optic disc and stalk and the expression of guidance cues at this choice point are necessary for the exit of RGC axons out of the eye. Sonic hedgehog (Shh) has been implicated in both patterning of ocular tissue and direct guidance of RGC axons. Here, we examine the precise spatial and temporal requirement for Hedgehog (Hh) signaling for intraretinal axon pathfinding and show that Shh acts to pattern the optic stalk in zebrafish but does not guide RGC axons inside the eye directly. We further reveal an interaction between the Hh and chemokine pathways for axon guidance and show that cxcl12a functions downstream of Shh and depends on Shh for its expression at the optic disc. Together, our results support a model in which Shh acts in RGC axon pathfinding indirectly by regulating axon guidance cues at the optic disc through patterning of the optic stalk.

The mutation in Chd7 causes misexpression of Bmp4 and developmental defects in telencephalic midline

Mutations in chromosome-helicase-DNA-binding protein 7 (CHD7) are identified as the main cause for CHARGE syndrome (coloboma, heart anomaly, choanal atresia, retardation, genital and ear anomalies). Most patients (55% to 85%) with CHARGE syndrome display developmental defects in the central nervous system (CNS), of which pathology and molecular mechanisms remain unclear. In this study, we report a novel mutant mouse strain carrying a nonsense mutation, COA1, in exon4 of Chd7 gene. Chd7(COA1/+) mice phenocopied human CHARGE syndrome and displayed developmental defects in the telencephalic midline, including dilated third and lateral ventricles, reduced cerebral cortex, and corpus callosum crossing failure. Programed cell death in the telencephalic midline zone of Chd7(COA1/+) embryos was impaired, consistent with the incomplete telencephalic medial invagination in Chd7(COA1/+) embryos. Interestingly, expression of Bmp4, a signal well known to induce forebrain midline cell fate and apoptosis, was down-regulated and also expanded in the forebrain of Chd7(COA1/+) embryos. Furthermore, in vitro studies suggested that CHD7 may directly regulate Bmp4 expression by binding with an enhancer element downstream of the Bmp4 locus. These studies provide novel insight into pathogenesis of CNS anomalies in CHARGE syndrome.

Timing of Sonic hedgehog and Gli1 expression segregates midbrain dopamine neurons

The ventral midbrain (vMb) is organized into distinct anatomical domains and contains cohorts of functionally distinct subtypes of midbrain dopamine (mDA) neurons. We tested the hypothesis that genetic history and timing of gene expression within mDA neuron progenitors impart spatial diversity. Using genetic inducible fate mapping to mark the Sonic hedgehog (Shh) and Gli1 lineages at varying embryonic stages, we performed a quantitative and qualitative comparison of the two lineages' contribution to the mDA neuron domains. Dynamic changes in Shh and Gli1 expression in the vMb primordia delineated their spatial contribution to the embryonic day 12.5 vMb: Both lineages first contributed to the medial domain, but subsequently the Gli1 lineage exclusively contributed to the lateral vMb while the Shh lineage expanded more broadly across the vMb. The contribution of both lineages to the differentiated mDA neuron domain was initially biased anteriorly and became more uniform across the anterior/posterior vMb throughout development. Our findings demonstrate that the early Shh and Gli1 lineages specify mDA neurons of the substantia nigra pars compacta while the late Shh and Gli1 lineages maintain their progenitor state longer in the posterior vMb to extend the production of mDA neurons in the ventral tegmental area. Together, our study demonstrates that the timing of gene expression along with the genetic lineage (Shh or Gli1) within the neural progenitors segregate mDA neurons into distinct spatial domains.

Phosphorylation regulation of Hedgehog signaling

The study of posttranslational regulation of proteins has occupied biochemists for well over a half century. Understanding balanced phosphorylation and dephosphorylation of the proteins may be the key to meeting some of the most pressing scientific challenges. A detailed examination of the phosphorylation of many components in the Hedgehog (Hh) pathway leads to a better understanding of the Hh signaling mechanisms. This chapter describes the precise phosphorylation that evolves during the phosphorylation/dephosphorylation of players in the Hh signaling cascade, including the signal transducer Smoothened and the transcription factor Ci/Gli.

The Hedgehog pathway promotes blood-brain barrier integrity and CNS immune quiescence

The blood-brain barrier (BBB) is composed of tightly bound endothelial cells (ECs) and perivascular astrocytes that regulate central nervous system (CNS) homeostasis. We showed that astrocytes secrete Sonic hedgehog and that BBB ECs express Hedgehog (Hh) receptors, which together promote BBB formation and integrity during embryonic development and adulthood. Using pharmacological inhibition and genetic inactivation of the Hh signaling pathway in ECs, we also demonstrated a critical role of the Hh pathway in promoting the immune quiescence of BBB ECs by decreasing the expression of proinflammatory mediators and the adhesion and migration of leukocytes, in vivo and in vitro. Overall, the Hh pathway provides a barrier-promoting effect and an endogenous anti-inflammatory balance to CNS-directed immune attacks, as occurs in multiple sclerosis.

Cranial nerve fasciculation and Schwann cell migration are impaired after loss of Npn-1

Interaction of the axon guidance receptor Neuropilin-1 (Npn-1) with its repulsive ligand Semaphorin 3A (Sema3A) is crucial for guidance decisions, fasciculation, timing of growth and axon-axon interactions of sensory and motor projections in the embryonic limb. At cranial levels, Npn-1 is expressed in motor neurons and sensory ganglia and loss of Sema3A-Npn-1 signaling leads to defasciculation of the superficial projections to the head and neck. The molecular mechanisms that govern the initial fasciculation and growth of the purely motor projections of the hypoglossal and abducens nerves in general, and the role of Npn-1 during these events in particular are, however, not well understood. We show here that selective removal of Npn-1 from somatic motor neurons impairs initial fasciculation and assembly of hypoglossal rootlets and leads to reduced numbers of abducens and hypoglossal fibers. Ablation of Npn-1 specifically from cranial neural crest and placodally derived sensory tissues recapitulates the distal defasciculation of mixed sensory-motor nerves of trigeminal, facial, glossopharyngeal and vagal projections, which was observed in Npn-1(-/-) and Npn-1(Sema-) mutants. Surprisingly, the assembly and fasciculation of the purely motor hypoglossal nerve are also impaired and the number of Schwann cells migrating along the defasciculated axonal projections is reduced. These findings are corroborated by partial genetic elimination of cranial neural crest and embryonic placodes, where loss of Schwann cell precursors leads to aberrant growth patterns of the hypoglossal nerve. Interestingly, rostral turning of hypoglossal axons is not perturbed in any of the investigated genotypes. Thus, initial hypoglossal nerve assembly and fasciculation, but not later guidance decisions depend on Npn-1 expression and axon-Schwann cell interactions.

Enhanced production of neuroprogenitors, dopaminergic neurons, and identification of target genes by overexpression of sonic hedgehog in human embryonic stem cells

Molecular and cellular signaling pathways are involved in the process of neural differentiation from human embryonic stem cells (hESC) to terminally differentiated neurons. The Sonic hedgehog (SHH) morphogen is required to direct the differentiation of hESC to several neural subtypes, for example, dopaminergic (DA) or motor neurons. However, the roles of SHH signaling and the pathway target genes that regulate the diversity of cellular responses arising from SHH activation during neurogenesis of hESC have yet to be elucidated. In this study, we report that overexpression of SHH in hESC promotes the derivation of neuroprogenitors (NP), increases proliferation of NP, and subsequently increases the yield of DA neurons. Next, gene expression changes resulting from the overexpression of SHH in hESC-derived NP were examined by genome-wide transcriptional profiling. Categorizing the differentially expressed genes according to the Gene Ontology biological processes showed that they are involved in numerous cellular processes, including neural development, NP proliferation, and neural specification. In silico GLI-binding sites analysis of the differentially expressed genes also identified a set of putative novel direct target genes of SHH in hESC-derived NP, which are involved in nervous system development. Electrophoretic mobility shift assays and promoter-luciferase assays confirmed that GLI1 binds to the promoter region and activates transcription of HEY2, a NOTCH signaling target gene. Taken together, our data provide evidence for the first time that there is cross-talk between the NOTCH and SHH signaling pathways in hESC-derived NP and also provide significant new insights into transcriptional targets in SHH-mediated neural differentiation of hESC.

An 8.9 year old girl with autism and Gorlin syndrome

We present an 8.9 year old girl diagnosed with autism and macrocrania. Because of macrocrania, hypertelorism and epidermal punctiform lesions in the palm of the hand, Gorlin syndrome was clinically suspected and molecularly confirmed by finding a deletion of 22 base pairs in the PTCH1 gene. The possibility of an association between autism and Gorlin syndrome is discussed.

Ontogenetic distribution of the transcription factor nkx2.2 in the developing forebrain of Xenopus laevis

The expression of the Nkx2.2 gene is involved in the organization of the alar-basal boundary in the forebrain of vertebrates. Its expression in different diencephalic and telencephalic regions, helped to define distinct progenitor domains in mouse and chick. Here we investigated the pattern of Nkx2.2 protein distribution throughout the development of the forebrain of the anuran amphibian, Xenopus laevis. We used immunohistochemical and in situ hybridization techniques for its detection in combination with other essential territorial markers in the forebrain. No expression was observed in the telencephalon. In the alar hypothalamus, Nkx2.2 positive cells were scattered in the suprachiasmatic territory, but also in the supraopto-paraventricular area, as defined by the expression of the transcription factor Orthopedia (Otp) and the lack of xDll4. In the basal hypothalamus Nkx2.2 expressing cells were localized in the tuberal region, with the exception of the arcuate nucleus, rich in Otp expressing cells. In the diencephalon it was expressed in all three prosomeres (P1–P3) and not in the zona limitans intrathalamica. The presence of Nkx2.2 expressing cells in P3 was restricted to the alar portion, as well as in prosomere P2, whereas in P1 the Nkx2.2 expressing cells were located in the basal plate and identified the alar/basal boundary. These results showed that Nkx2.2 and Sonic hedgehog are expressed in parallel adjacent stripes along the anterior–posterior axis. The results of this study showed a conserved distribution pattern of Nkx2.2 among vertebrates, crucial to recognize subdivisions that are otherwise indistinct, and supported the relevance of this transcription factor in the organization of the forebrain, particularly in the delineation of the alar/basal boundary of the forebrain.

Using human pluripotent stem cells to untangle neurodegenerative disease mechanisms

Human pluripotent stem cells, including embryonic (hES) and induced pluripotent stem cells (hiPS), retain the ability to self-renew indefinitely, while maintaining the capacity to differentiate into all cell types of the nervous system. While human pluripotent cell-based therapies are unlikely to arise soon, these cells can currently be used as an inexhaustible source of committed neurons to perform high-throughput screening and safety testing of new candidate drugs. Here, we describe critically the available methods and molecular factors that are used to direct the differentiation of hES or hiPS into specific neurons. In addition, we discuss how the availability of patient-specific hiPS offers a unique opportunity to model inheritable neurodegenerative diseases and untangle their pathological mechanisms, or to validate drugs that would prevent the onset or the progression of these neurological disorders.

Distinctive expression patterns of Hedgehog pathway genes in the Ciona intestinalis larva: implications for a role of Hedgehog signaling in postembryonic development and chordate evolution

Members of the Hedgehog (Hh) family are soluble ligands that orchestrate a wide spectrum of developmental processes ranging from left-right axis determination of the embryo to tissue patterning and organogenesis. Tunicates, including ascidians, are the closest relatives of vertebrates, and elucidation of Hh signaling in ascidians should provide an important clue towards better understanding the role of this pathway in development. In previous studies, expression patterns of genes encoding Hh and its downstream factor Gli have been examined up to the tailbud stage in the ascidian embryo, but their expression in the larva has not been reported. Here we show the spatial expression patterns of hedgehog (Ci-hh1, Ci-hh2), patched (Ci-ptc), smoothened (Ci-smo), and Gli (Ci-Gli) orthologs in larvae of the ascidian Ciona intestinalis. The expression patterns of Ci-hh2 and Ci-Gli dramatically change during the period between the late tailbud embryo and the swimming larva. At the larval stage, expression of Ci-Gli was found in a central part of the endoderm and in the visceral ganglion, while Ci-hh2 was expressed in two discrete endodermal regions, anteriorly and posteriorly adjacent to the cells expressing Gli. The expression patterns of these genes suggest that the Hh ligand controls postembryonic development of the endoderm and the central nervous system. Expression of a gene encoding Hh in the anterior and/or pharyngeal endoderm is probably an ancient chordate character; diversification of regulation and targets of the Hh signaling in this region may have played a major role in the evolution of chordate body structures.

Efficient derivation of functional floor plate tissue from human embryonic stem cells

The floor plate (FP) is a critical signaling center during neural development located along the ventral midline of the embryo. Little is known about human FP development because of the lack of tissue accessibility. Here we report the efficient derivation of human embryonic stem cell (hESC)-derived FP tissue capable of secreting Netrin-1 and SHH and patterning primary and hESC derived tissues. FP induction in hESCs is dependent on early SHH exposure and occurs at the expense of anterior neurectoderm (AN). Global gene expression and functional studies identify SHH-mediated inhibition of Dkk-1 as key factor in FP versus AN specification. hESC-derived FP tissue is shown to be of anterior SIX6+ character but is responsive to caudalizing factors suppressing SIX6 expression and inducing a shift in usage of region-specific SHH enhancers. These data define the early signals that drive human FP versus AN specification and determine regional identity in hESC-derived FP.

Lighting up developmental mechanisms: how fluorescence imaging heralded a new era

Embryology and genetics have given rise to a mechanistic framework that explains the architecture of a developing organism. Until recently, however, such studies suffered from a lack of quantification and real-time visualization at the subcellular level, limiting their ability to monitor the dynamics of developmental processes. Live imaging using fluorescent proteins has overcome these limitations, uncovering unprecedented insights that call many established models into question. We review how the study of patterning, cell polarization and morphogenesis has benefited from this technology and discuss the possibilities offered by fluorescence imaging and by the contributions of quantitative disciplines.

The transcriptional repressor REST/NRSF modulates hedgehog signaling

The spatial and temporal control of gene expression is key to generation of specific cellular fates during development. Studies of the transcriptional repressor REST/NRSF (RE1 Silencing Transcription Factor or Neural Restrictive Silencing Factor) have provided important insight into the role that epigenetic modifications play in differential gene expression. However, the precise function of REST during embryonic development is not well understood. We have discovered a novel interaction between zebrafish Rest and the Hedgehog (Hh) signaling pathway. We observed that Rest knockdown enhances or represses Hh signaling in a context-dependant manner. In wild-type embryos and embryos with elevated Hh signaling, Rest knockdown augments transcription of Hh target genes. Conversely, in contexts where Hh signaling is diminished, Rest knockdown has the opposite effect and Hh target gene expression is further attenuated. Epistatic analysis revealed that Rest interacts with the Hh pathway at a step downstream of Smo. Furthermore, we present evidence implicating the bifunctional, Hh signaling component Gli2a as key to the Rest modulation of the Hh response. The role of Rest as a regulator of Hh signaling has broad implications for many developmental contexts where REST and Hh signaling act.

FGF-receptor signalling controls neural cell diversity in the zebrafish hindbrain by regulating olig2 and sox9

The mechanisms underlying the generation of neural cell diversity are the subject of intense investigation, which has highlighted the involvement of different signalling molecules including Shh, BMP and Wnt. By contrast, relatively little is known about FGF in this process. In this report we identify an FGF-receptor-dependent pathway in zebrafish hindbrain neural progenitors that give rise to somatic motoneurons, oligodendrocyte progenitors and differentiating astroglia. Using a combination of chemical and genetic approaches to conditionally inactivate FGF-receptor signalling, we investigate the role of this pathway. We show that FGF-receptor signalling is not essential for the survival or maintenance of hindbrain neural progenitors but controls their fate by coordinately regulating key transcription factors. First, by cooperating with Shh, FGF-receptor signalling controls the expression of olig2, a patterning gene essential for the specification of somatic motoneurons and oligodendrocytes. Second, FGF-receptor signalling controls the development of both oligodendrocyte progenitors and astroglia through the regulation of sox9, a gliogenic transcription factor the function of which we show to be conserved in the zebrafish hindbrain. Overall, for the first time in vivo, our results reveal a mechanism of FGF in the control of neural cell diversity.

The primary cilium as a Hedgehog signal transduction machine

The Hedgehog (Hh) signal transduction pathway is essential for the development and patterning of numerous organ systems, and has important roles in a variety of human cancers. Genetic screens for mouse embryonic patterning mutants first showed a connection between mammalian Hh signaling and intraflagellar transport (IFT), a process required for construction of the primary cilium, a small cellular projection found on most vertebrate cells. Additional genetic and cell biological studies have provided very strong evidence that mammalian Hh signaling depends on the primary cilium. Here, we review the evidence that defines the integral roles that IFT proteins and cilia play in the regulation of the Hh signal transduction pathway in vertebrates. We discuss the mechanisms that control localization of Hh pathway proteins to the cilium, focusing on the transmembrane protein Smoothened (Smo), which moves into the cilium in response to Hh ligand. The phenotypes caused by loss of cilia-associated proteins are complex, which suggests that cilia and IFT play active roles in mediating Hh signaling rather than serving simply as a compartment in which pathway components are concentrated. Hh signaling in Drosophila does not depend on cilia, but there appear to be ancient links between cilia and components of the Hh pathway that may reveal how this fundamental difference between the Drosophila and mammalian Hh pathways arose in evolution.

"The developmental and functional logic of neuronal circuits": commentary on the Kavli Prize in Neuroscience

The first Kavli Prize in Neuroscience recognizes a confluence of career achievements that together provide a fundamental understanding of how brain and spinal cord circuits are assembled during development and function in the adult. The members of the Kavli Neuroscience Prize Committee have decided to reward three scientists (Sten Grillner, Thomas Jessell, and Pasko Rakic) jointly "for discoveries on the developmental and functional logic of neuronal circuits". Pasko Rakic performed groundbreaking studies of the developing cerebral cortex, including the discovery of how radial glia guide the neuronal migration that establishes cortical layers and for the radial unit hypothesis and its implications for cortical connectivity and evolution. Thomas Jessell discovered molecular principles governing the specification and patterning of different neuron types and the development of their synaptic interconnection into sensorimotor circuits. Sten Grillner elucidated principles of network organization in the vertebrate locomotor central pattern generator, along with its command systems and sensory and higher order control. The discoveries of Rakic, Jessell and Grillner provide a framework for how neurons obtain their identities and ultimate locations, establish appropriate connections with each other, and how the resultant neuronal networks operate. Their work has significantly advanced our understanding of brain development and function and created new opportunities for the treatment of neurological disorders. Each has pioneered an important area of neuroscience research and left a legacy of exceptional scientific achievement, insight, communication, mentoring and leadership.

Blocking hedgehog signaling after ablation of the dorsal neural tube allows regeneration of the cardiac neural crest and rescue of outflow tract septation

Cardiac neural crest cells (CNCC) migrate into the caudal pharynx and arterial pole of the heart to form the outflow septum. Ablation of the CNCC results in arterial pole malalignment and failure of outflow septation, resulting in a common trunk overriding the right ventricle. Unlike preotic cranial crest, the postotic CNCC do not normally regenerate. We applied the hedgehog signaling inhibitor, cyclopamine (Cyc), to chick embryos after CNCC ablation and found normal heart development at day 9 suggesting that the CNCC population was reconstituted. We ablated the CNCC, and labeled the remaining neural tube with DiI/CSRE and applied cyclopamine. Cells migrated from the neural tube in the CNCC-ablated, cyclopamine-treated embryos but not in untreated CNCC-ablated embryos. The newly generated cells followed the CNCC migration pathways, expressed neural crest markers and supported normal heart development. Finally, we tested whether reducing hedgehog signaling caused redeployment of the dorsal-ventral axis of the injured neural tube, allowing generation of new neural crest-like cells. The dorsal neural tube marker, Pax7, was maintained 12 h after CNCC ablation with Cyc treatment but not in the CNCC-ablated alone. This disruption of dorsal-ventral neural patterning permits a new wave of migratory cardiac neural crest-like cells.

Tulp3 is a critical repressor of mouse hedgehog signaling

Precise regulation of the morphogen sonic hedgehog (Shh) and modulation of the Shh signaling pathway is required for proper specification of cell fate within the developing limbs and neural tube, and resultant tissue morphogenesis. Tulp3 (tubby-like protein 3) is a protein of unknown function which has been implicated in nervous system development through gene knockout studies. We demonstrate here that mice lacking the Tulp3 gene develop abnormalities of both the neural tube and limbs consistent with improper regulation of Shh signaling. Tulp3(-/-) embryos show expansion of Shh target gene expression and display a ventralization of neural progenitor cells in the caudal neural tube. We further show that Tulp3(-/-)/Shh(-/-) compound mutant embryos resemble Tulp3 mutants, and express Shh target genes in the neural tube and limbs which are not expressed in Shh(-/-) embryos. This work uncovers a novel role for Tulp3 as a negative regulatory factor in the Hh pathway.

Olig2+ precursors produce abducens motor neurons and oligodendrocytes in the zebrafish hindbrain

During development, a specific subset of ventral spinal cord precursors called pMN cells produces first motor neurons and then oligodendrocyte progenitor cells (OPCs), which migrate, divide and differentiate as myelinating oligodendrocytes. pMN cells express the Olig2 transcription factor and Olig2 function is necessary for formation of spinal motor neurons and OPCs. In the hindbrain and midbrain, distinct classes of visceral, branchiomotor and somatic motor neurons are organized as discrete nuclei, and OPCs are broadly distributed. Mouse embryos deficient for Olig2 function lack somatic motor neurons and OPCs, but it is not clear whether this reflects a common origin for these cells, similar to spinal cord, or independent requirements for Olig2 function in somatic motor neuron and OPC development. We investigated cranial motor neuron and OPC development in zebrafish and found, using a combination of transgenic reporters and cell type specific antibodies, that somatic abducens motor neurons and a small subset of OPCs arise from common olig2(+) neuroepithelial precursors in rhombomeres r5 and r6, but that all other motor neurons and OPCs do not similarly develop from shared pools of olig2(+) precursors. In the absence of olig2 function, r5 and r6 precursors remain in the cell cycle and fail to produce abducens motor neurons, and OPCs are entirely lacking in the hindbrain. These studies, therefore, reveal both common and independent roles for olig2 in development of somatic motor neurons and oligodendrocytes of the hindbrain.

Proteoglycan interactions with Sonic Hedgehog specify mitogenic responses

Sonic Hedgehog (Shh) has dual roles in vertebrate development, as it promotes progenitor cell proliferation and induces tissue patterning. Here we show mitogenic and patterning functions of Shh can be uncoupled from one another. Using a genetic approach to selectively inhibit Shh-proteoglycan interactions in a mouse model, we show binding of Shh to proteoglycans is required for proliferation of neural stem/precursor cells but not for tissue patterning. Shh-proteoglycan interactions regulate both spatial and temporal features of Shh signaling. Proteoglycans localize Shh to specialized mitogenic niches and also act at the single cell level to regulate the duration of Shh signaling, thereby promoting a gene expression program important for cell division. As activation of the Shh pathway is a feature of diverse human cancers, selective stimulation of proliferation by Shh-proteoglycan interactions may also figure prominently in neoplastic growth.

Hox genes in neural patterning and circuit formation in the mouse hindbrain

The mammalian hindbrain is the seat of regulation of several vital functions that involve many of the organ systems of the body. Such functions are controlled through the activity of intricate arrays of neuronal circuits and connections. The establishment of ordered patterns of neuronal specification, migration, and axonal topographic connectivity during development is crucial to build such a complex network of circuits and functional connectivity in the mature hindbrain. The early development of the vertebrate hindbrain proceeds according to a fundamental metameric partitioning along the anteroposterior axis into cellular compartments known as rhombomeres. Such an organization has been highly conserved in vertebrate evolution and has a fundamental impact on the hindbrain adult structure, nuclear organization, and connectivity. Here, we review the cellular and molecular mechanisms underlying hindbrain neuronal circuitry in the mouse, with a specific focus on the role of the homeodomain transcription factors of the Hox gene family. The Hox genes are crucial determinants of rhombomere segmental identity and anteroposterior patterning. However, recent findings suggest that, in addition to their well-known roles at early embryonic stages, the Hox genes may play important roles also in later aspect of neuronal circuit development, including stereotypic neuronal migration, axon pathfinding, and topographic mapping of connectivity.

Sonic hedgehog mutations identified in holoprosencephaly patients can act in a dominant negative manner

Sonic hedgehog (SHH) plays an important instructional role in vertebrate development, as exemplified by the numerous developmental disorders that occur when the SHH pathway is disrupted. Mutations in the SHH gene are the most common cause of sporadic and inherited holoprosencephaly (HPE), a developmental disorder that is characterized by defective prosencephalon development. SHH HPE mutations provide a unique opportunity to better understand SHH biogenesis and signaling, and to decipher its role in the development of HPE. Here, we analyzed a panel of SHH HPE missense mutations that encode changes in the amino-terminal active domain of SHH. Our results show that SHH HPE mutations affect SHH biogenesis and signaling at multiple steps, which broadly results in low levels of protein expression, defective processing of SHH into its active form and protein with reduced activity. Additionally, we found that some inactive SHH proteins were able to modulate the activity of wt SHH in a dominant negative manner, both in vitro and in vivo. These findings show for the first time the susceptibility of SHH driven developmental processes to perturbations by low-activity forms of SHH. In conclusion, we demonstrate that SHH mutations found in HPE patients affect distinct steps of SHH biogenesis to attenuate SHH activity to different levels, and suggest that these variable levels of SHH activity might contribute to some of the phenotypic variation found in HPE patients.

Precision of the Dpp gradient

Morphogen concentration gradients provide positional information by activating target genes in a concentration-dependent manner. Recent reports show that the gradient of the syncytial morphogen Bicoid seems to provide precise positional information to determine target gene domains. For secreted morphogenetic ligands, the precision of the gradients, the signal transduction and the reliability of target gene expression domains have not been studied. Here we investigate these issues for the TGF-beta-type morphogen Dpp. We first studied theoretically how cell-to-cell variability in the source, the target tissue, or both, contribute to the variations of the gradient. Fluctuations in the source and target generate a local maximum of precision at a finite distance to the source. We then determined experimentally in the wing epithelium: (1) the precision of the Dpp concentration gradient; (2) the precision of the Dpp signaling activity profile; and (3) the precision of activation of the Dpp target gene spalt. As captured by our theoretical description, the Dpp gradient provides positional information with a maximal precision a few cells away from the source. This maximal precision corresponds to a positional uncertainly of about a single cell diameter. The precision of the Dpp gradient accounts for the precision of the spalt expression range, implying that Dpp can act as a morphogen to coarsely determine the expression pattern of target genes.

Proliferative and cell fate effects of Hedgehog signaling in the vertebrate retina

The retina is an excellent system for delving into the question of how cell fate, number and organization are regulated in the central nervous system. Multipotential progenitor cells in the immature retina proliferate, exit the cell cycle and generate neurons and one glial cell type in a prescribed temporal sequence. While some aspects of progenitor behavior are controlled cell intrinsically, extrinsic signals present in the retina environment have been shown to impact on proliferation, differentiation and cell fate of progenitors. Intercellular signaling proteins of the Hedgehog (Hh) family regulate several aspects of visual system development in vertebrates--ranging from early eye field patterning to retinal and optic nerve development. This review highlights the role of Hh signaling on retinal progenitor proliferation and diversification.

Gas1 extends the range of Hedgehog action by facilitating its signaling

Cellular signaling initiated by Hedgehog binding to Patched1 has profound importance in mammalian embryogenesis, genetic disease, and cancer. Hedgehog acts as a morphogen to specify distinctive cell fates using different concentration thresholds, but our knowledge of how the concentration gradient is interpreted into the activity gradient is incomplete. The membrane protein Growth Arrest-Specific Gene 1 (GAS1) was thought to be a negative regulator of the Hedgehog concentration gradient. Here, we report unexpected genetic evidence that Gas1 positively regulates Hedgehog signaling in multiple developmental contexts, an effect particularly noticeable at regions where Hedgehog acts at low concentration. Using a combination of in vitro cell culture and in ovo electroporation assays, we demonstrate that GAS1 acts cooperatively with Patched1 for Hedgehog binding and enhances signaling activity in a cell-autonomous manner. Our data support a model in which GAS1 helps transform the Hedgehog protein gradient into the observed activity gradient. We propose that Gas1 is an evolutionarily novel, vertebrate-specific Hedgehog pathway regulator.

Ubiquitination and degradation of homeodomain-interacting protein kinase 2 by WD40 repeat/SOCS box protein WSB-1

Homeodomain-interacting protein kinase 2 (HIPK2) is a member of the nuclear protein kinase family, which induces both p53- and CtBP-mediated apoptosis. Levels of HIPK2 were increased by UV irradiation and cisplatin treatment, thereby implying the degradation of HIPK2 in cells under normal conditions. Here, we indicate that HIPK2 is ubiquitinated and degraded by the WD40-repeat/SOCS box protein WSB-1, a process that is blocked under DNA damage conditions. Yeast two-hybrid screening was conducted to identify the proteins that interact with HIPK2. WSB-1, an E3 ubiquitin ligase, was characterized as an HIPK2-interacting protein. The coexpression of WSB-1 resulted in the degradation of HIPK2 via its C-terminal region. Domain analysis of WSB-1 showed that WD40-repeats and the SOCS box were required for its interaction with and degradation of HIPK2, respectively. In support of the degradation of HIPK2 by WSB-1, HIPK2 was polyubiquitinated by WSB-1 in vitro and in vivo. The knockdown of endogenous WSB-1 with the expression of short hairpin RNA against WSB-1 increases the stability of endogenous HIPK2 and resulted in the accumulation of HIPK2. The ubiquitination and degradation of HIPK2 by WSB-1 was inhibited completely via the administration of DNA damage reagents, including Adriamycin and cisplatin. These findings effectively illustrate the regulatory mechanisms by which HIPK2 is maintained at a low level, by WSB-1 in cells under normal conditions, and stabilized by genotoxic stresses.

An amphioxus Gli gene reveals conservation of midline patterning and the evolution of hedgehog signalling diversity in chordates

BACKGROUND

Hedgehog signalling, interpreted in receiving cells by Gli transcription factors, plays a central role in the development of vertebrate and Drosophila embryos. Many aspects of the signalling pathway are conserved between these lineages, however vertebrates have diverged in at least one key aspect: they have evolved multiple Gli genes encoding functionally-distinct proteins, increasing the complexity of the hedgehog-dependent transcriptional response. Amphioxus is one of the closest living relatives of the vertebrates, having split from the vertebrate lineage prior to the widespread gene duplication prominent in early vertebrate evolution.

PRINCIPAL FINDINGS

We show that amphioxus has a single Gli gene, which is deployed in tissues adjacent to sources of hedgehog signalling derived from the midline and anterior endoderm. This shows the duplication and divergence of the Gli gene family, and hence the origin of vertebrate Gli functional diversity, was specific to the vertebrate lineage. However we also show that the single amphioxus Gli gene produces two distinct transcripts encoding different proteins. We utilise three tests of Gli function to examine the transcription regulatory capacities of these different proteins, demonstrating one has activating activity similar to Gli2, while the other acts as a weak repressor, similar to Gli3.

CONCLUSIONS

These data show that vertebrates and amphioxus have evolved functionally-similar repertoires of Gli proteins using parallel molecular routes; vertebrates via gene duplication and divergence, and amphioxus via alternate splicing of a single gene. Our results demonstrate that similar functional complexity of intercellular signalling can be achieved via different evolutionary pathways.

Sonic hedgehog signalling in T-cell development and activation

The production of mature functional T cells in the thymus requires signals from the thymic epithelium. Here, we review recent experiments showing that one way in which the epithelium controls the production of mature T cells is by the secretion of sonic hedgehog (SHH). We consider the increasing evidence that SHH-induced signalling is not only important for the differentiation and proliferation of early thymocyte progenitors, but also for modulating T-cell receptor signalling during repertoire selection, with implications for positive selection, CD4 versus CD8 lineage commitment, and clonal deletion of autoreactive cells. We also review the influence of hedgehog signalling in peripheral T-cell activation.

Pre-steady-state decoding of the Bicoid morphogen gradient

Morphogen gradients are established by the localized production and subsequent diffusion of signaling molecules. It is generally assumed that cell fates are induced only after morphogen profiles have reached their steady state. Yet, patterning processes during early development occur rapidly, and tissue patterning may precede the convergence of the gradient to its steady state. Here we consider the implications of pre-steady-state decoding of the Bicoid morphogen gradient for patterning of the anterior–posterior axis of the Drosophila embryo. Quantitative analysis of the shift in the expression domains of several Bicoid targets (gap genes) upon alteration of bcd dosage, as well as a temporal analysis of a reporter for Bicoid activity, suggest that a transient decoding mechanism is employed in this setting. We show that decoding the pre-steady-state morphogen profile can reduce patterning errors caused by fluctuations in the rate of morphogen production. This can explain the surprisingly small shifts in gap and pair-rule gene expression domains observed in response to alterations in bcd dosage.

It was previously thought that cell fates were determined by morphogen gradients only after steady state was established. Here the authors show fate may precede gradient steady state.

Subdivision of naive fields of cells into separate cell populations, distinguished by the unique combinations of genes they express, constitutes a major aspect of organism development. Classically, this involves the generation of gradients of signaling molecules (morphogens), which induce distinct cell fates in a concentration-dependent manner. It has been generally assumed that morphogen gradients are interpreted only after they reach a spatially fixed, steady-state profile. Our study re-examines this assumption for the classical case of the Bicoid morphogen, a transcription factor that is distributed as a gradient in the early Drosophila embryo. We propose and present evidence for dynamic, pre-steady-state decoding of the Bicoid profile. We further show that such dynamic decoding can directly account for the surprisingly small shifts in the expression domains of target genes, observed in response to altered Bicoid dosage, without invoking additional mechanisms or contributing factors. Pre-steady-state decoding can thus provide a simple explanation for the relative robustness of this classical morphogen system, which has been a long-standing problem.

Spatially patterned gene delivery for localized neuron survival and neurite extension

Natural tissues can have complex architectures, which arise in part from spatial patterns in gene expression. Regenerative strategies for damaged tissue must recreate these architectures to restore function. In this article, we demonstrate spatially controlled gene delivery from a substrate for directing cellular processes. Non-viral vectors were immobilized to substrates in linear patterns using microfluidic techniques, and cells cultured on the surface had localized gene expression within the cell population. Transfection was achieved in pattern widths as low as 100 mum, with efficiencies dependent on the microchannel treatment and vector concentration. The ability of patterned expression to localize cellular processes was investigated using a neuronal co-culture model. Patterned expression of the diffusible neurotrophic factor nerve growth factor initiated neuron survival and neurite out-growth primarily within the pattern, which decreased significantly in regions directly adjacent to the pattern. Primary neurite density was significantly greater on patterned substrates than on surfaces without patterns. This approach demonstrates the basic technology to create patterns of gene expression that can direct tissue formation and could be employed in regenerative strategies to recreate the complex cellular architectures observed in tissues.

An early role for WNT signaling in specifying neural patterns of Cdx and Hox gene expression and motor neuron subtype identity

The link between extrinsic signaling, progenitor cell specification and neuronal subtype identity is central to the developmental organization of the vertebrate central nervous system. In the hindbrain and spinal cord, distinctions in the rostrocaudal identity of progenitor cells are associated with the generation of different motor neuron subtypes. Two fundamental classes of motor neurons, those with dorsal (dMN) and ventral (vMN) exit points, are generated over largely non-overlapping rostrocaudal domains of the caudal neural tube. Cdx and Hox genes are important determinants of the rostrocaudal identity of neural progenitor cells, but the link between early patterning signals, neural Cdx and Hox gene expression, and the generation of dMN and vMN subtypes, is unclear. Using an in vitro assay of neural differentiation, we provide evidence that an early Wnt-based program is required to interact with a later retinoic acid- and fibroblast growth factor-mediated mechanism to generate a pattern of Cdx and Hox profiles characteristic of hindbrain and spinal cord progenitor cells that prefigure the generation of vMNs and dMNs.

Orchestrating ontogenesis: variations on a theme by sonic hedgehog

Embryonic development is an emergent process in which increasing complexity is generated by sequential cellular interactions. Recently, it has become clear that such interactions are mediated by just a few families of signalling molecules; but how does this limited repertoire elicit the diversity of form that is characteristic of multicellular organisms? Here we review the various ways in which a member of one such family, the sonic hedgehog (SHH) protein, is deployed during embryonic development. These examples of SHH function provide paradigms for inductive interactions that should help to inform attempts to recapitulate cellular programming and organogenesis in vitro.

Regionalized cadherin-7 expression by radial glia is regulated by Shh and Pax7 during chicken spinal cord development

During development, several genes that specify neuronal subtype identity are expressed in distinct dorsoventral domains of the spinal cord and hindbrain. Cadherin-7 (Cad7), a member of the cadherin family of adhesion molecules, is expressed by radial glia in a dorsal domain of the spinal cord basal plate in chicken. To study the regulation of the Cad7 gene, we ectopically expressed two known dorsoventral patterning genes, Shh and Pax7, in the caudal neural tube and in two brain regions at different stages of development by in vivo electroporation. Results showed that Shh regulated the expression of Cad7 by radial glia in a concentration-dependent manner. Shh induced or repressed the expression of Cad7, at low and high concentrations, respectively. Furthermore, Pax7 inhibited the expression of Cad7. These results are compatible with a role of Shh and Pax7 in regulating endogenous Cad7 expression during spinal cord and hindbrain development. Our data show, for the first time, that Shh can regulate the expression not only of other gene regulatory factors, but also of Cad7, a morphoregulatory molecule that plays a role in axon elongation and neural circuit formation.

Molecular pathways regulating pro-migratory effects of Hedgehog signaling

The Hedgehog proteins play a crucial role in metazoan embryo development. Constitutive activation of the pathway is associated with multiple types of cancer. Recent experimental data suggest involvement of Hedgehog signaling in vascular remodeling, germ cell migration, and axon guidance. The molecular mechanisms underlying these effects remain elusive. Here we show that yolk sac-derived endothelial cells and embryonic fibroblasts can directly respond to the Hedgehog signal by increased migration in an in vitro scratch (wound) assay. We also identify Hedgehog transcriptional target genes in these cells, many of which participate in cell migration, axon guidance, and angiogenesis processes. Inhibition of one such molecular pathway, neuropilin-flavomonooxygenase, blocks Hedgehog-induced cell migration. These findings suggest that Hedgehog signaling directly affects embryonic endothelial and fibroblast cell migration via molecules and pathways known to regulate cell migration in response to a variety of environmental cues.

Expression patterns of plexins and neuropilins are consistent with cooperative and separate functions during neural development

BACKGROUND

Plexins are a family of transmembrane proteins that were shown to act as receptors for Semaphorins either alone or in a complex together with Neuropilins. Based on structural criteria Plexins were subdivided into 4 classes, A through D. PlexinAs are mainly thought to act as mediators of repulsive signals in cell migration and axon guidance. Their functional role in vertebrates has been studied almost exclusively in the context of Semaphorin signaling, i.e. as co-receptors for class 3 Semaphorins. Much less is known about Plexins of the other three classes. Despite the fact that Plexins are involved in the formation of neuronal circuits, the temporal changes of their expression patterns during development of the nervous system have not been analyzed in detail.

RESULTS

Only seven plexins are found in the chicken genome in contrast to mammals, where nine plexins have been identified. Here, we describe the dynamic expression patterns of all known plexin family members in comparison to the neuropilins in the developing chicken spinal cord.

CONCLUSION

Our in situ hybridization study revealed that the expression patterns of plexins and neuropilins are only partially overlapping, especially during early and intermediate stages of spinal cord development, supporting both cooperative and separate functions of plexins and neuropilins in neural circuit formation.

Induction of oligodendrocyte progenitors in dorsal forebrain by intraventricular microinjection of FGF-2

During embryonic development, oligodendrocyte progenitors (OLPs) originate from the ventral forebrain under the regulation of Sonic hedgehog (Shh). Shh controls the expression of transcription factor Olig2, which is strongly implicated in OLP generation. Studies of mice deficient in Shh expression suggest, however, that an alternative pathway for OLP generation may exist. The generation of OLPs in dorsal forebrain has been suggested since treatment of dorsal-neural progenitor cells in culture with fibroblast growth factor (FGF-2) results in OLP induction. To ask if dorsal induction of OLPs in embryonic forebrain can occur in vivo and if FGF-2 could initiate an alternative pathway of regulation, we used in utero microinjection of FGF-2 into the lateral ventricles of mouse fetal forebrain. A single injection of FGF-2 at E13.5 resulted in the expression of the OLP markers Olig2 and PDGFRalpha mRNA in dorsal forebrain ventricular and intermediate zones. However, FGF-2 did not induce dorsal expression of Shh, Patched1 or Nkx2.1, and co-injection of FGF-2 and a Shh inhibitor did not attenuate the induction of Olig2 and PDGFRalpha, suggesting that Shh signaling was not involved in this FGF-2-mediated dorsal induction. These results demonstrate that the dorsal embryonic forebrain in vivo has the potential to generate OLPs in the presence of normal positional cues and that this can be driven by FGF-2 independent of Shh signaling.