Dorsal and intermediate neuronal cell types of the spinal cord are established by a BMP signaling pathway

Co-option of signaling mechanisms from neural induction to telencephalic patterning

This article provides an overview of signaling processes during early specification of the anterior neural tube, with special emphasis on the telencephalon. A series of signaling systems based on the action of distinct morphogens acts at different developmental stages, specifying interacting developmental fields that define axes of differentiation in the rostrocaudal and the dorsoventral domains. Interestingly, many of these signaling systems are co-opted for several differentiation processes. This strategy provides a simple and efficient mechanism to generate novel structures in evolution, and may have been especially important in the origin of the telencephalon and the mammalian cerebral cortex. For example, the action of fibroblast growth factor (FGF) secreted in early stages from the anterior neural ridge, but in later stages from the dorsal anterior forebrain, may have been a key factor in the early differentiation of the ventral telencephalon and in the eventual expansion of the mammalian neocortex. Likewise, bone morphogenetic proteins (BMPs) participate at several stages in neural patterning, even if early neural induction consists of the inhibition of the BMP pathway. BMPs, secreted dorsally, interact with FGFs in the frontal aspect of the hemispheres, and with PAX6-dependent signaling sources located laterally, to pattern the dorsal telencephalon. The actions of other morphogens are also described in this context, such as the ventralizing factor SHH, the dorsalizing element GLI3, and other factors related to the dorsomedial telencephalon such as WNTs and EMXs. The main conclusion we draw from this review is the well-known phylogenetic and developmental conservatism of signaling pathways, which in evolution have been applied in different embryological contexts, generating novel interactions between morphogenetic fields and leading to the generation of new morphological structures.

Cells that express MyoD mRNA in the epiblast are stably committed to the skeletal muscle lineage

The epiblast of the chick embryo contains cells that express MyoD mRNA but not MyoD protein. We investigated whether MyoD-positive (MyoD^pos) epiblast cells are stably committed to the skeletal muscle lineage or whether their fate can be altered in different environments. A small number of MyoD^pos epiblast cells were tracked into the heart and nervous system. In these locations, they expressed MyoD mRNA and some synthesized MyoD protein. No MyoD^pos epiblast cells differentiated into cardiac muscle or neurons. Similar results were obtained when MyoD^pos cells were isolated from the epiblast and microinjected into the precardiac mesoderm or neural plate. In contrast, epiblast cells lacking MyoD differentiated according to their environment. These results demonstrate that the epiblast contains both multipotent cells and a subpopulation of cells that are stably committed to the skeletal muscle lineage before the onset of gastrulation. Stable programming in the epiblast may ensure that MyoD^pos cells express similar signaling molecules in a variety of environments.

Neural crest inducing signals

The formation of the neural crest has been traditionally considered a classic example of secondary induction, where signals form one tissue elicit a response in a competent responding tissue. Interactions of the neural plate with paraxial mesoderm or nonneural ectoderm can generate neural crest. Several signaling pathways converge at the border between neural and nonneural ectoderm where the neural crest will form. Among the molecules identified in this process are members of the BMP, Wnt, FGF and Notch signaling pathways. The concerted action of these signals and their downstream targets will define the identity of the neural crest.

Bone morphogenetic protein-mediated modulation of lineage diversification during neural differentiation of embryonic stem cells

Embryonic stem cells (ES cells) can give rise to a broad spectrum of neural cell types. The biomedical application of ES cells will require detailed knowledge on the role of individual factors modulating fate specification during in vitro differentiation. Bone morphogenetic proteins (BMPs) are known to exert a multitude of diverse differentiation effects during embryonic development. Here, we show that exposure to BMP2 at distinct stages of neural ES cell differentiation can be used to promote specific cell lineages. During early ES cell differentiation, BMP2-mediated inhibition of neuroectodermal differentiation is associated with an increase in mesoderm and smooth muscle differentiation. In fibroblast growth factor 2-expanded ES cell-derived neural precursors, BMP2 supports the generation of neural crest phenotypes, and, within the neuronal lineage, promotes distinct subtypes of peripheral neurons, including cholinergic and autonomic phenotypes. BMP2 also exerts a density-dependent promotion of astrocyte differentiation at the expense of oligodendrocyte formation. Experiments involving inhibition of the serine threonine kinase FRAP support the notion that these effects are mediated via the JAK/STAT pathway. The preservation of diverse developmental BMP2 effects in differentiating ES cell cultures provides interesting prospects for the enrichment of distinct neural phenotypes in vitro.

Bone morphogenetic protein signalling and vertebrate nervous system development

Transforming growth factor-beta (TGFbeta) signalling, particularly signalling from the bone morphogenetic protein (BMP) members of this protein family, is crucial for the development of both the central and peripheral nervous systems in vertebrates. Experimental embryology and genetics performed in a range of organisms are providing insights into how BMPs establish the neural tissue and control the types and numbers of neurons formed. These studies also highlight the interactions between different developmental signals that are necessary to form a functional nervous system. The challenges ahead will be to uncover functions of TGFbeta signalling in later stages of CNS development, as well as to determine possible associations with neurological diseases.

Development of the neural crest: achieving specificity in regulatory pathways

Recent studies have revealed the signaling pathways and downstream effectors involved in the specification of the neural crest. Neural crest cells are generated from a zone at the neurectoderm border in response to Wnt and BMP signals. BMP signals are involved in establishing a competency zone at the border of the neurectoderm, while subsequent Wnt signals specify neural crest cells. Combinations of transcription factors, including pax and msx gene products, act downstream of these pathways to integrate signals and establish the neural crest. Mechanisms are emerging for how specificity is generated from reiterated signals and effectors.

Concise Review: Bone Morphogenetic Protein Pleiotropism in Neural Stem Cells and Their Derivatives-Alternative Pathways, Convergent Signals

Bone morphogenetic proteins (BMPs) are a class of morphogens that are critical regulators of the central nervous system (CNS), peripheral nervous system, and craniofacial development. Modulation of BMP signaling also appears to be an important component of the postnatal stem cell niche. However, describing a comprehensive model of BMP actions is complicated by their paradoxical effects in precursor cells, which include dorsal specification, promoting proliferation or mitotic arrest, cell survival or death, and neuronal or glial fate. In addition, in postmitotic neurons BMPs can promote dendritic growth, act as axonal chemorepellants, and stabilize synapses. Although many of these responses depend on interactions with other incoming signals, some reflect the recruitment of distinct BMP signal transduction pathways. In this review, we classify the diverse effects of BMPs on neural cells, focus on the known mechanisms that specify distinct responses, and discuss the remaining challenges in identifying the cellular basis of BMP pleiotropism. Addressing these issues may have importance for stem cell mobilization, differentiation, and cell integration/survival in reparative therapies.

Endogenous bone morphogenetic protein antagonists regulate mammalian neural crest generation and survival

We demonstrate here that Chordin and Noggin function as bone morphogenetic protein (BMP) antagonists in vivo to promote mammalian neural crest development. Using Chrd and Nog single and compound mutants, we find that Noggin has a major role in promoting neural crest formation, in which Chordin is partially redundant. BMP signaling is increased in dorsal tissues lacking Noggin and is further increased when Chordin is also absent. The early neural crest domain is expanded with decreased BMP antagonism in vivo. Noggin and Chordin also regulate subsequent neural crest cell emigration from the neural tube. However, reduced levels of these BMP antagonists ultimately result in perturbation of neural crest cell derived peripheral nervous system and craniofacial skeletal elements. Such defects reflect, at least in part, a function to limit apoptosis in neural crest cells. Noggin and Chordin, therefore, function together to regulate both the generation and survival of neural crest cells in mammalian development.

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

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

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

A BMP-mediated transcriptional cascade involving Cash1 and Tlx-3 specifies first-order relay sensory neurons in the developing hindbrain

The divergent homeobox-containing transcription factor, Tlx-3 (also known as Hox11L2/Rnx), is required for proper formation of first-order relay sensory neurons in the developing vertebrate brainstem. To date, however, the inductive signals and transcriptional regulatory cascade underlying their development are poorly understood. We previously isolated the chick Tlx-3 homologue and showed it is expressed early (i.e. beginning at HH15) in distinct subcomponents of both the trigeminal/solitary and vestibular nuclei. Here we show via in vivo rhombomere inversions that expression of Tlx-3 is under control of local environmental signals. Our RNA in situ analysis shows expression of the BMP-specific receptor, Bmpr-1b, correlates well with Tlx-3. Furthermore, manipulation of the BMP signaling pathway in vivo via electroporation of expression vectors encoding either BMP or NOGGIN coupled with MASH1 gain-of-function experiments demonstrate that a BMP-mediated transcriptional cascade involving Cash1 and Tlx-3 specifies first-order relay sensory neurons in the developing brainstem. Notably, high-level Noggin misexpression results in an increase in newly differentiated Tlx-3+ neurons that correlates with a corresponding increase in the number of Calretinin+ neurons in vestibular nuclei at later developmental stages strongly suggesting that Tlx-3, in addition to being required for proper formation of somatic as well as visceral sensory neurons in the trigeminal and solitary nuclei, respectively, is sufficient for proper formation of special somatic sensory neurons in vestibular nuclei.

Dpp and Gbb exhibit different effective ranges in the establishment of the BMP activity gradient critical for Drosophila wing patterning

Morphogen gradients ensure the specification of different cell fates by dividing initially unpatterned cellular fields into distinct domains of gene expression. It is becoming clear that such gradients are not always simple concentration gradients of a single morphogen; however, the underlying mechanism of generating an activity gradient is poorly understood. Our data indicate that the relative contributions of two BMP ligands, Gbb and Dpp, to patterning the wing imaginal disc along its A/P axis, change as a function of distance from the ligand source. Gbb acts over a long distance to establish BMP target gene boundaries and a variety of cell fates throughout the wing disc, while Dpp functions at a shorter range. On its own, Dpp is not sufficient to mediate the low-threshold responses at the end points of the activity gradient, a function that Gbb fulfills. Given that both ligands signal through the Tkv type I receptor to activate the same downstream effector, Mad, the difference in their effective ranges must reflect an inherent difference in the ligands themselves, influencing how they interact with other molecules. The existence of related ligands with different functional ranges may represent a conserved mechanism used in different species to generate robust long range activity gradients.

Mechanisms of ventral patterning in the vertebrate nervous system

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

Development and evolution of lateral line placodes in amphibians I. Development

Lateral line placodes are specialized regions of the ectoderm that give rise to the receptor organs of the lateral line system as well as to the sensory neurons innervating them. The development of lateral line placodes has been studied in amphibians since the early 1900s. This paper reviews these older studies and tries to integrate them with more recent findings. Lateral line placodes are probably induced in a multistep process from a panplacodal area surrounding the neural plate. The time schedule of these inductive processes has begun to be unravelled, but little is known yet about their molecular basis. Subsequent pattern formation, morphogenesis and differentiation of lateral line placodes proceeds in most respects relatively autonomously: Onset and polarity of migration of lateral line primordia, the type, spacing, size and number of receptor organs formed, as well as the patterned differentiation of different cell types occur normally even in ectopic locations. Only the pathways for migration of lateral line primordia depend on external cues. Thus, lateral line placodes act as integrated and relatively context-insensitive developmental modules.

Leg dysfunctions in a hatched chick model of spina bifida aperta

We created chicks with spina bifida aperta (SBA) by incising the roof plate of the neural tube of embryos at Hamburger and Hamilton stage 18 or 19. Incision over the length of three somites caused spina bifida occulta (SBO)-like malformation in 47% of the hatchlings. Incision over the length of five and seven somites caused SBA-like malformation in 100% of the hatchlings. The SBO chicks exhibited no symptoms, whereas the SBA chicks exhibited paralysis of a leg muscle and imbalance between an agonist and an antagonist leg muscles. Lesions in these SBA chicks were located in the spinal segments that give rise to motor neurons that innervated the dysfunctional muscles. Histological analysis revealed that there were fewer small spinal neurons (interneurons) at the site of the lesion in SBA chicks than in the normal chicks and that there was no such difference in the number of the large spinal neurons (motor neurons). Leg dysfunctions in this model of SBA may be attributable to the smaller number of interneurons in the spinal segments that contain motor neurons that innervate the dysfunctional muscle. This model may facilitate studies of the pathological mechanisms that lead to leg dysfunctions in SBA chicks.

Extracellular modulation of BMP activity in patterning the dorsoventral axis

Signaling via bone morphogenetic proteins (BMPs) regulates a vast array of diverse biological processes in the developing embryo and in postembryonic life. Many insights into BMP signaling derive from studies of the BMP signaling gradients that pattern cell fates along the embryonic dorsal-ventral (DV) axis of both vertebrates and invertebrates. This review examines recent developments in the field of DV patterning by BMP signaling, focusing on extracellular modulation as a key mechanism in the formation of BMP signaling gradients in Drosophila, Xenopus, and zebrafish.

Noggin signaling fromXenopus animal blastomere lineages promotes a neural fate in neighboring vegetal blastomere lineages

In Xenopus, localized factors begin to regionalize embryonic fates prior to the inductive interactions that occur during gastrulation. We previously reported that an animal-to-vegetal signal that occurs prior to gastrulation promotes primary spinal neuron fate in vegetal equatorial (C-tier) blastomere lineages. Herein we demonstrate that maternal mRNA encoding noggin is enriched in animal tiers and at low concentrations in the C-tier, suggesting that the neural fates of C-tier blastomeres may be responsive to early signaling from their neighboring cells. In support of this hypothesis, experimental alteration of the levels of Noggin from animal equatorial (B-tier) or BMP4 from vegetal (D-tier) blastomeres significantly affects the numbers of primary spinal neurons derived from their neighboring C-tier blastomeres. These effects are duplicated in blastomere explants isolated at cleavage stages and cultured in the absence of gastrulation interactions. Co-culture with animal blastomeres enhanced the expression of zygotic neural markers in C-tier blastomere explants, whereas co-culture with vegetal blastomeres repressed them. The expression of these markers in C-tier explants was promoted when Noggin was transiently added to the culture during cleavage/morula stages, and repressed with the transient addition of BMP4. Reduction of Noggin translation in B-tier blastomeres by antisense morpholino oligonucleotides significantly reduced the efficacy of neural marker induction in C-tier explants. These experiments indicate that early anti-BMP signaling from the animal hemisphere recruits vegetal equatorial cells into the neural precursor pool prior to interactions that occur during gastrulation.

Reiterated Wnt and BMP signals in neural crest development

Common signaling pathways such as those for Wnts and BMPs are used many times during embryogenesis. During the development of the neural crest, Wnt and BMP signals are used repeatedly at different stages to influence initial induction, segregation from the neuroepithelium and cell fate determination. This review considers how specificity is generated within the neural crest for these reiterated signals, discussing examples of how the outcomes of signaling events are modulated by context.

msh/Msx gene family in neural development

The involvement of Msx homeobox genes in skull and tooth formation has received a great deal of attention. Recent studies also indicate a role for the msh/Msx gene family in development of the nervous system. In this article, we discuss the functions of these transcription factors in neural-tissue organogenesis. We will deal mainly with the interactions of the Drosophila muscle segment homeobox (msh) gene with other homeobox genes and the repressive cascade that leads to neuroectoderm patterning; the role of Msx genes in neural-crest induction, focusing especially on the differences between lower and higher vertebrates; their implication in patterning of the vertebrate neural tube, particularly in diencephalon midline formation. Finally, we will examine the distinct activities of Msx1, Msx2 and Msx3 genes during neurogenesis, taking into account their relationships with signalling molecules such as BMP.

Morphogens in motion: growth control of the neural tube

The entire vertebrate nervous system develops from a simple epithelial sheet, the neural plate which, along development, acquires the large number and wide variety of neuronal cell types required for the construction of a functional mature nervous system. These include processes of growth and pattern formation of the neural tube that are achieved through complicated and tightly regulated genetic interactions. Pattern formation, particularly in the vertebrate central nervous system, is one of the best examples of a morphogen-type of function. Cell cycle progression, however, is generally accepted to be dependent on cell-intrinsic factors. Recent studies have demonstrated that proliferation of neural precursors is also somehow controlled by secreted signaling molecules, well-known by their role as morphogens, such as fibroblast growth factor (FGF), vertebrate orthologs of the Drosophila wingless (Wnt), hedgehog (Hh), and transforming growth factor beta (TGF-beta) families, that in turn regulate the activity of factors controlling cell cycle progression. In this review we will summarize the experimental data that support the idea that classical morphogens can be reused to regulate proliferation of neural precursors.

Morphing into cancer: the role of developmental signaling pathways in brain tumor formation

Morphogens play a critical role in most aspects of development, including expansion and patterning of the central nervous system. Activating germline mutations in components of the Hedgehog and Wnt pathways have provided evidence for the important roles morphogens play in the genesis of brain tumors such as cerebellar medulloblastoma. In addition, aberrant expression of transforming growth factor-beta (TGF-beta) superfamily members has been demonstrated to contribute to progression of malignant gliomas. This review summarizes our current knowledge about the roles of morphogens in central nervous system tumorigenesis.

The mechanisms of dorsoventral patterning in the vertebrate neural tube

We describe the essential features of and the molecules involved in dorsoventral (DV) patterning in the neural tube. The neural tube is, from its very outset, patterned in this axis as there is a roof plate, floor plate, and differing numbers and types of neuroblasts. These neuroblasts develop into different types of neurons which express a different range of marker genes. Early embryological experiments identified the notochord and the somites as being responsible for the DV patterning of the neural tube and we now know that 4 signaling molecules are involved and are generated by these surrounding structures. Fibroblast growth factors (FGFs) are produced by the caudal mesoderm and must be down-regulated before neural differentiation can occur. Retinoic acid (RA) is produced by the paraxial mesoderm and is an inducer of neural differentiation and patterning and is responsible for down-regulating FGF. Sonic hedgehog (Shh) is produced by the notochord and floor plate and is responsible for inducing ventral neural cell types in a concentration-dependent manner. Bone morphogenetic proteins (BMPs) are produced by the roof plate and are responsible for inducing dorsal neural cell types in a concentration-dependent manner. Subsequently, RA is used twice more. Once from the somites for motor neuron differentiation and secondly RA is used to define the motor neuron subtypes, but in the latter case it is generated within the neural tube from differentiating motor neurons rather than from outside. These 4 signaling molecules also interact with each other, generally in a repressive fashion, and DV patterning shows how complex these interactions can be.

Factors involved in the determination of the neurotransmitter phenotype of developing neurons of the CNS: Applications in cell replacement treatment for Parkinson's disease

The developmental stages involved in the conversion of stem cells to fully functional neurons of specific neurotransmitter phenotype are complex and not fully understood. Over the past decade many studies have been published that demonstrate that in vitro manipulation of the epigenetic environment of the stem cells allows experimental control of final neuronal phenotypic choice. This review presents the evidence for the involvement of a number of endogenous neurobiochemicals, which have been reported to potently influence DAergic (and other neurotransmitter) phenotype expression in vitro. They act at different stages on the pathway to neurotransmitter phenotype determination, and in different ways. Many are better known for their involvement in other aspects of development, and in other biochemical roles. Their proper place, and precise roles, in neurotransmitter phenotype determination in vivo will no doubt be determined in the future. Meanwhile, considerable medical benefits are offered from producing large, long-term, viable cryostores of self-regenerating multipotential neural precursor cells (i.e., brain stem cells), which can be used for cell replacement therapies in the treatment of degenerative brain diseases, such as Parkinson's disease.

dlx3b and dlx4b function in the development of Rohon-Beard sensory neurons and trigeminal placode in the zebrafish neurula

Rohon-Beard sensory neurons, neural crest cells, and sensory placodes can be distinguished at the boundary of the embryonic epidermis (skin) and the neural plate. The inductive signals at the neural plate border region are likely to involve a gradient of bone morphogenic protein (BMP) in conjunction with FGF and Wnts and other signals. However, how these signals are transduced to produce the final cell fate remains to be determined. Recent evidence from Xenopus and chick suggest that Dlx genes are required for the generation of cell fates at the neural plate border (McLarren, K.W., Litsiou, A., Streit, A., 2003. DLX5 positions the neural crest and preplacode region at the border of the neural plate. Dev. Biol. 259, 34-47; Woda, J.M., Pastagia, J., Mercola, M., Artinger, K.B., 2003. Dlx proteins position the neural plate border and determine adjacent cell fates. Development 130, 331-342). In the present study, we extend these findings to zebrafish, where we unequivocally demonstrate that dlx3b and dlx4b function in a dose-dependent manner to specify cell fates such as Rohon-Beard sensory neurons and trigeminal sensory placodes. dlx function was examined by inhibiting: (1) protein levels with antisense morpholino oligonucleotides (MOs), and (2) activity by repressing the ability of dlx-homeodomain to bind to downstream targets (EnR-dlx3bhd mRNA; dlx3b homeodomain fused to Engrailed transcriptional repressor domain). Inhibition of dlx3b and dlx4b protein and activity resulted in the reduction or complete loss of Rohon-Beard (RB) sensory neurons and trigeminal (TG) sensory placodes. These data suggest that dlx3b and dlx4b function in the specification of RB neurons and trigeminal sensory placodes in zebrafish. Further, we have shown that dlx3b and dlx4b function in a non-cell-autonomous manner for RB neuron development; dlx3b and dlx4b act to regulate bmp2b expression at the non-neural ectodermal border. These data suggest that the contribution of dlx3b and dlx4b to neural plate border formation is partially non-cell-autonomous acting via BMP activity.

Exploring alternative models of rostral-caudal patterning in the zebrafish neurectoderm with computer simulations

The StarLogo and NetLogo programming environments allow developmental biologists to build computer models of cell-cell interactions in an epithelium and visualize emergent properties of hypothetical genetic regulatory networks operating in the cells. These environments were used to explore alternative models that show how a posteriorizing morphogen gradient might define gene-expression domains along the rostral-caudal axis in the zebrafish neurectoderm. The models illustrate how a hypothetical genetic network based on auto-activation and cross-repression could lead to establishment of discrete non-overlapping gene-expression domains.

Zebrafish narrowminded disrupts the transcription factor prdm1 and is required for neural crest and sensory neuron specification

Specification of both neural crest cells and Rohon-Beard (RB) sensory neurons involves a complex series of interactions between the neural and non-neural ectoderm. The molecular mechanisms directing this process are not well understood. The zebrafish narrowminded (nrd) mutation is unique, since it is one of two mutations in which defects are observed in both cell populations: it leads to a complete absence of RB neurons and a reduction in neural crest cells and their derivatives. Here, we show that nrd is a mutation in prdm1, a SET/zinc-finger domain transcription factor. A Morpholino-mediated depletion of prdm1 phenocopies the nrd mutation, and conversely overexpression of prdm1 mRNA rescues the nrd RB sensory neuron and neural crest phenotype. prdm1 is expressed at the border of the neural plate within the domain where neural crest cells and RB sensory neurons form. Analysis of prdm1 function by overexpression indicates that prdm1 functions to promote the cell fate specification of both neural crest cells and RB sensory neurons, most likely as a downstream effector of the BMP signaling pathway.

Blimp-1 specifies neural crest and sensory neuron progenitors in the zebrafish embryo

Developmental origins of the neural crest (NC), a quintessential and pluripotent vertebrate cell type, has historically been a topic of extensive investigation but continues to remain poorly understood. In the zebrafish embryo, NC and primary sensory neurons are thought to segregate from a common population of progenitor cells in response to lateral inhibition. Here, we show that the zebrafish homolog of the B-lymphocyte-induced maturation protein (Blimp-1) gene, u-boot (ubo), is induced by BMP signaling in cells at the boundary of the neural plate and nonneural ectoderm. Loss of Ubo activity not only inhibits specification of the NC but also impairs development of the primary sensory neurons. Conversely, misexpression of ubo results in the generation of supernumerary primary sensory neurons consistent with this cell type representing the default fate within the progenitor equivalence group. These results establish a link between the activity of the transcriptional regulator Blimp-1 and the inductive effects of BMP signaling in the inception of NC progenitor fate.

BMP controls nitric oxide-mediated regulation of cell numbers in the developing neural tube

Balanced cell proliferation and cell death determines neural precursor cell numbers in early stages of neural tube (NT) development. We have previously shown that nitric oxide (NO) regulates cell numbers locally in the NT of eight to 12 somite embryos. Here, we demonstrate that bone morphogenetic protein-4 (BMP-4), which is expressed in the ectoderm and dorsal NT at these developmental stages, induces programmed cell death (PCD) and promotes entry into the S-phase, via nitric oxide synthase (NOS) activity. These effects can be reversed by BMP-4 antagonists, such as follistatin and noggin, or by specific NOS inhibitors, resulting in low NO levels that facilitate mitosis and reduce PCD. Ectopic BMP-4 induction of PCD is restricted to the dorsal NT, whereas promotion of the S-phase is evenly observed across the dorsal-ventral (D-V) axis. Prolonged exposure to either BMP-4 or NOS inhibitors, which results in high or low NO levels, respectively, causes NT defects. The results presented here throw new light on the BMP signaling pathway. The local presence of BMP-4 helps to regulate cell numbers in the developing NT by a NO-mediated pathway, which is essential for normal NT formation.

Early development of the cranial sensory nervous system: from a common field to individual placodes

Sensory placodes are unique columnar epithelia with neurogenic potential that develop in the vertebrate head ectoderm next to the neural tube. They contribute to the paired sensory organs and the cranial sensory ganglia generating a wide variety of cell types ranging from lens fibres to sensory receptor cells and neurons. Although progress has been made in recent years to identify the molecular players that mediate placode specification, induction and patterning, the processes that initiate placode development are not well understood. One hypothesis suggests that all placode precursors arise from a common territory, the pre-placodal region, which is then subdivided to generate placodes of specific character. This model implies that their induction begins through molecular and cellular mechanisms common to all placodes. Embryological and molecular evidence suggests that placode induction is a multi-step process and that the molecular networks establishing the pre-placodal domain as well as the acquisition of placodal identity are surprisingly similar to those used in Drosophila to specify sensory structures.

Molecular mechanisms of neural crest induction

The neural crest is an embryonic cell population that originates at the border between the neural plate and the prospective epidermis. Around the time of neural tube closure, neural crest cells emigrate from the neural tube, migrate along defined paths in the embryo and differentiate into a wealth of derivatives. Most of the craniofacial skeleton, the peripheral nervous system, and the pigment cells of the body originate from neural crest cells. This cell type has important clinical relevance, since many of the most common craniofacial birth defects are a consequence of abnormal neural crest development. Whereas the migration and differentiation of the neural crest have been extensively studied, we are just beginning to understand how this tissue originates. The formation of the neural crest has been described as a classic example of embryonic induction, in which specific tissue interactions and the concerted action of signaling pathways converge to induce a multipotent population of neural crest precursor cells. In this review, we summarize the current status of knowledge on neural crest induction. We place particular emphasis on the signaling molecules and tissue interactions involved, and the relationship between neural crest induction, the formation of the neural plate and neural plate border, and the genes that are upregulated as a consequence of the inductive events.

Roof plate-dependent patterning of the vertebrate dorsal central nervous system

In the vertebrate central nervous system (CNS), diverse cellular types are generated in response to inductive signals provided by specialized cellular groups that act as organizing centers. The roof plate is a critical dorsal signaling center that occupies the dorsal midline of the developing CNS along its entire anterior-posterior axis. During caudal neural tube development, the roof plate produces proteins of the Bmp and Wnt families controlling proliferation, specification, migration, and axon guidance of adjacent dorsal interneurons. Although primarily investigated in the developing spinal cord, a growing number of studies indicate that roof plate-derived signals are also critical for the patterning of dorsal structures in more rostral regions of CNS including the hindbrain, diencephalon and telencephalon. In this review, we discuss recent progress towards understanding the molecular and cellular mechanisms of roof plate-dependent patterning of the dorsal CNS.

Multiple Dorsoventral Origins of Oligodendrocyte Generation in the Spinal Cord and Hindbrain

Studies have indicated that oligodendrocytes in the spinal cord originate from a ventral progenitor domain defined by expression of the oligodendrocyte-determining bHLH proteins Olig1 and Olig2. Here, we provide evidence that progenitors in the dorsal spinal cord and hindbrain also produce oligodendrocytes and that the specification of these cells may result from a dorsal evasion of BMP signaling over time. Moreover, we show that the generation of ventral oligodendrocytes in the spinal cord depends on Nkx6.1 and Nkx6.2 function, while these homeodomain proteins in the anterior hindbrain instead suppress oligodendrocyte specification. The opposing roles for Nkx6 proteins in the spinal cord and hindbrain, in turn, appear to reflect that oligodendrocytes are produced by distinct ventral progenitor domains at these axial levels. Based on these findings, we propose that oligodendrocytes derive from several distinct positional origins and that the activation of Olig1/2 at different positions is controlled by distinct genetic programs.

Signals derived from the underlying mesoderm are dispensable for zebrafish neural crest induction

Signals from the non-neural ectoderm, the neural ectoderm, and the underlying mesoderm have all been implicated in the induction of neural crest. Bone morphogenetic protein (BMP) signaling in particular has an important role in this process; however, it is unclear whether this activity of BMP is due to its effects on patterning the underlying mesoderm, to its ability to establish a competent neural plate boundary zone, or to the direct specification of neural crest at intermediate levels of activity within a BMP gradient. We show neural crest induction occurs in zebrafish in the absence of involuted mesoderm, indicating that this tissue and signals derived from it are dispensable for the formation of neural crest. Dorsal-involuted mesoderm is a major source of secreted BMP antagonists, and the activity of BMP signaling is thought to depend on the presence of the opposing activity of these antagonists. We find that the three BMP antagonists known to be expressed during gastrulation in zebrafish, noggin1, follistatin, and chordin, are dispensable for neural crest induction. These results suggest that mechanisms for restricting the spatio-temporal pattern of BMP expression may compensate for the loss of secreted BMP antagonist activity in establishing dorso-ventral patterning, neural induction, and the neural crest.

Coordinate regulation of neural tube patterning and proliferation by TGFbeta and WNT activity

Pattern formation and growth must be tightly coupled during embryonic development. In vertebrates, however, little is known of the molecules that serve to link these two processes. Here we show that bone morphogenetic proteins (BMP) coordinate the acquisition of pattern information and the stimulation of proliferation in the embryonic spinal neural tube. We have blocked BMP and transforming growth factor-beta superfamily (TGFbeta) function in the chick embryo using Noggin, a BMP antagonist, and siRNA against Smad4. We show that BMPs/TGFbetas are necessary to regulate pattern formation and the specification of neural progenitor populations in the dorsal neural tube. BMPs also serve to establish discrete expression domains of Wnt ligands, receptors, and antagonists along the dorsal-ventral axis of the neural tube. Using the extracellular domain of Frizzled 8 to block Wnt signaling and Wnt3a ligand misexpression to activate WNT signaling, we demonstrate that the Wnt pathway acts mitogenically to expand the populations of neuronal progenitor cells specified by BMP. Thus, BMPs, acting through WNTs, couple patterning and growth to generate dorsal neuronal fates in the appropriate proportions within the neural tube.

BMP receptor IA is required in the mammalian embryo for endodermal morphogenesis and ectodermal patterning

BMPRIA is a receptor for bone morphogenetic proteins with high affinity for BMP2 and BMP4. Mouse embryos lacking Bmpr1a fail to gastrulate, complicating studies on the requirements for BMP signaling in germ layer development. Recent work shows that BMP4 produced in extraembryonic tissues initiates gastrulation. Here we use a conditional allele of Bmpr1a to remove BMPRIA only in the epiblast, which gives rise to all embryonic tissues. Resulting embryos are mosaics composed primarily of cells homozygous null for Bmpr1a, interspersed with heterozygous cells. Although mesoderm and endoderm do not form in Bmpr1a null embryos, these tissues are present in the mosaics and are populated with mutant cells. Thus, BMPRIA signaling in the epiblast does not restrict cells to or from any of the germ layers. Cells lacking Bmpr1a also contribute to surface ectoderm; however, from the hindbrain forward, little surface ectoderm forms and the forebrain is enlarged and convoluted. Prechordal plate, early definitive endoderm, and anterior visceral endoderm appear to be expanded, likely due to defective morphogenesis. These data suggest that the enlarged forebrain is caused in part by increased exposure of the ectoderm to signaling sources that promote anterior neural fate. Our results reveal critical roles for BMP signaling in endodermal morphogenesis and ectodermal patterning.

Molecular anatomy of placode development in Xenopus laevis

We analyzed the spatiotemporal pattern of expression of 15 transcription factors (Six1, Six4, Eya1, Sox3, Sox2, Pax6, Pax3, Pax2, Pax8, Dlx3, Msx1, FoxI1c, Tbx2, Tbx3, Xiro1) during placode development in Xenopus laevis from neural plate to late tail bud stages. Out of all genes investigated, only the expression of Eya1, Six1, and Six4 is maintained in all types of placode (except the lens) throughout embryonic development, suggesting that they may promote generic placodal properties and that their crescent-shaped expression domain surrounding the neural plate defines a panplacodal primordium from which all types of placode originate. Double-labeling procedures were employed to reveal the precise position of this panplacodal primordium relative to neural plate, neural crest, and other placodal markers. Already at neural plate stages, the panplacodal primordium is subdivided into several subregions defined by particular combinations of transcription factors allowing us to identify the approximate regions of origin of various types of placode. Whereas some types of placode were already prefigured by molecularly distinct areas at neural plate stages, the epibranchial, otic, and lateral line placodes arise from a common posterior placodal area (characterized by Pax8 and Pax2 expression) and acquire differential molecular signatures only after neural tube closure. Our findings argue for a multistep mechanism of placode induction, support a combinatorial model of placode specification, and suggest that different placodes evolved from a common placodal primordium by successive recruitment of new inducers and target genes.

Restriction of BMP4 activity domains in the developing neural tube of the mouse embryo

Bone morphogenetic protein (BMP) signals pattern the dorsal neural tube, defining distinct neuronal progenitor cell domains along the dorsoventral axis of the developing spinal cord. These dorsally expressed BMPs appear to have a limited range of action. The mechanisms that regulate this range of action are unclear. We created a GAL4/UAS bigenic mouse system to overexpress BMP4 or a mutant form of BMP4 (mutBMP4), which lacks a subset of amino-terminal basic amino acids that limits its range of action, in the dorsal neural tube. UAS-Bmp4 and UAS-mutBmp4 responder genes were activated in the dorsal neural tube by a Wnt1-GAL4 transgene. Analysis of the spinal cords of bigenic embryos that expressed comparable levels of transgenic transcripts revealed that mutBMP4 acted more ventrally in the neural tube than BMP4. This suggests that the amino-terminal basic amino-acid motif of mature BMP4 controls long-range activity for dorsoventral patterning of the vertebrate neural tube.

Retinoic acid and the control of dorsoventral patterning in the avian spinal cord

The development of neural subtypes in the dorsoventral (DV) axis of the vertebrate central nervous system (CNS) involves the integration of signalling pathways coupled with the combinatorial expression of homeodomain transcription factors. Previous studies have implicated a role for retinoic acid in the specification of a subtype of motor neurons (MN) and in the patterning of a group of interneurons within the ventral spinal cord. In this study, we use the vitamin A-deficient (VAD) quail model to further investigate the role of retinoids in the patterning of the neural tube. Using genetic markers specific to neuronal cell populations, we demonstrate that in the absence of retinoic acid, there is a disruption to the molecular mechanisms associated with the dorsoventral patterning of the spinal cord. In particular, we observe an uneven dorsal expansion of ventral-specific genes, accompanied by a reduction in the domain of roof plate and dorsal patterning genes, both of which are rescued upon addition of retinoids during development. In addition, there is a loss of V1 interneuron-specific gene expression and a decrease in the ventricular zone expression of motor neuron patterning genes. Interestingly, these effects are localised to the rostral half of the spinal cord, indicating that RA is integrated in both anteroposterior (AP) and dorsoventral patterning processes. Using differential display techniques, we have isolated 27 retinoic acid-regulated genes within the spinal cord that together reveal several interesting potential biological functions for retinoids within the avian neural tube. In summary, we propose that retinoids have an essential role in the patterning of the dorsoventral axis of the spinal cord, and are also required for the correct integration of anteroposterior patterning signals with dorsoventral determinants in the rostral spinal cord.

Early steps in the development of the forebrain

The tremendous complexity of the adult forebrain makes it a challenging task to elucidate how this structure forms during embryonic development. Nevertheless, we are beginning to understand how a simple epithelial sheet of ectoderm gives rise to the labyrinthine network of cells that constitutes the functional forebrain. Here, we discuss early events in forebrain development--those that lead to the establishment of the anterior neural plate and the regional subdivision of this territory into the different domains of the prospective forebrain.

Secreted cell signaling molecules in axon guidance

The growth cones of developing neurons respond to specific guidance cues in their extracellular environment. Recent studies have shown that secreted signaling molecules from protein families that are best known for their roles as morphogens in specifying cell fate can function as axon guidance molecules. These signaling molecules seem to act directly on the growth cone and thus are likely to activate non-canonical signaling pathways that are coupled to the cytoskeleton.

Integration of IGF, FGF, and anti-BMP signals via Smad1 phosphorylation in neural induction

How do very diverse signaling pathways induce neural differentiation in Xenopus? Anti-BMP (Chordin), FGF8, and IGF2 signals are integrated in the embryo via the regulation of Smad1 phosphorylation. Neural induction results from the combined inhibition of BMP receptor serine/threonine kinases and activation of receptor tyrosine kinases that signal through MAPK and phosphorylate Smad1 in the linker region, further inhibiting Smad1 transcriptional activity. This hard-wired molecular mechanism at the level of the Smad1 transcription factor may help explain the opposing activities of IGF, FGF, and BMP signals not only in neural induction, but also in other aspects of vertebrate development.

Alk8 is required for neural crest cell formation and development of pharyngeal arch cartilages

The type I TGFbeta family member receptor alk8 acts in bone morphogenetic protein (BMP) signaling pathways to establish dorsoventral patterning in the early zebrafish embryo. Here, we present evidence that alk8 is required for neural crest cell (NCC) formation and that alk8 signaling gradients direct the proper patterning of premigratory NCCs. We extend our previous functional studies of alk8 to demonstrate that ectopic expression of constitutively active and dominant negative Alk8, consistently results in more medially or laterally positioned premigratory NCCs, respectively. We also demonstrate that patterning defects in premigratory NCCs, induced by alk8 misexpression, correlate with subsequent defects in NCC-derived pharyngeal arch cartilages. Furthermore, an anteroposterior effect is revealed, where overexpression of Alk8 more severely affects anterior arch cartilages and decreased Alk8 activity more severely affects posterior arch cartilage formation. Ectopic expression studies of alk8 are supported by analyses of zygotic and maternal-zygotic laf/alk8 mutants and of several BMP pathway mutants. Pharyngeal mesodermal and endodermal defects in laf/alk8 mutants suggest additional roles for alk8 in patterning of these tissues. Our results provide insight into alk8-mediated BMP signaling gradients and the establishment of premigratory NCC mediolateral positioning, and extend the model for BMP patterning of the neural crest to include that of NCC-derived pharyngeal arch cartilages.

Ringing in the new ear: resolution of cell interactions in otic development

The vertebrate inner ear is a marvel of structural and functional complexity, which is all the more remarkable because it develops from such a simple structure, the otic placode. Analysis of inner ear development has long been a fascination of experimental embryologists, who sought to understand cellular mechanisms of otic placode induction. More recently, however, molecular and genetic approaches have made the inner ear a useful model system for studying a much broader range of basic developmental mechanisms, including cell fate specification and differentiation, axial patterning, epithelial morphogenesis, cytoskeletal dynamics, stem cell biology, neurobiology, physiology, etc. Of course, there has also been tremendous progress in understanding the functions and processes peculiar to the inner ear. The goal of this review is to recount how historical approaches have shaped our understanding of the signaling interactions controlling early otic development; to discuss how new findings have led to fundamental new insights; and to point out new problems that need to be resolved in future research.

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

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

OTX2 activates the molecular network underlying retina pigment epithelium differentiation

The retina pigment epithelium (RPE) is fundamental for the development and function of the vertebrate eye. Molecularly, the presumptive RPE can be identified by the early expression of two transcription factors, Mitf and Otx. In mice deficient for either gene, RPE development is impaired with loss of melanogenic gene expression, raising the possibility that in the eye OTX proteins operate either in a feedback loop or in cooperation with MITF for the control of RPE-specific gene expression. Here we show that Otx2 induces a pigmented phenotype when overexpressed in avian neural retina cells. In addition, OTX2 binds specifically to a bicoid motif present in the promoter regions of three Mitf target genes, QNR71, TRP-1, and tyrosinase, leading to their transactivation. OTX2 and MITF co-localize in the nuclei of RPE cells and physically interact, and their co-expression results in a cooperative activation of QNR71 and tyrosinase promoters. Collectively, these data suggest that both transcription factors operate at the same hierarchical level to establish the identity of the RPE.

The amino-terminal region of Gli3 antagonizes the Shh response and acts in dorsoventral fate specification in the developing spinal cord

A concentration gradient of Shh is thought to pattern the ventral neural tube, and these ventral cell types are absent in shh-/- mice. Based on in vitro and genetic studies, the zinc finger-containing transcription factors Gli 1, 2, and 3 are mediators of the Shh intracellular response. The floorplate and adjacent cell types are absent in gli1-/-;gli2-/- mice, but part of the Shh-/- phenotype in the neural tube is alleviated in the Shh-/-;gli3-/- double mutant. This is consistent with the predicted role of Gli3 as a repressor of the Shh response. Gli3 repressor activity is blocked by Shh. In order to test the role of the repressor form of Gli3 in the neural tube, a truncated version of Gli3 (Gli3R*) was designed to mimic a Pallister Hall allele. Gli3R* acts as a constitutive repressor independent of Shh signaling. Misexpression of Gli3R* in the chick neural tube caused a ventral expansion of class-I, dorsal progenitor proteins and a loss of class-II, ventral progenitor proteins consistent with expected activity as a repressor of the Shh response. Activation of the BMP response is sufficient to maintain gli3 expression in neural plate explants, which might be a mechanism by which BMPs antagonize the Shh response.