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.     

Ascidians as excellent chordate models for studying the development of the nervous system during embryogenesis and metamorphosis

The swimming larvae of the chordate ascidians possess a dorsal hollowed central nervous system (CNS), which is homologous to that of vertebrates. Despite the homology, the ascidian CNS consists of a countable number of cells. The simple nervous system of ascidians provides an excellent experimental system to study the developmental mechanisms of the chordate nervous system. The neural fate of the cells consisting of the ascidian CNS is determined in both autonomous and non-autonomous fashion during the cleavage stage. The ascidian neural plate performs the morphogenetic movement of neural tube closure that resembles that in vertebrate neural tube formation. Following neurulation, the CNS is separated into five distinct regions, whose homology with the regions of vertebrate CNS has been discussed. Following their larval stage, ascidians undergo a metamorphosis and become sessile adults. The metamorphosis is completed quickly, and therefore the metamorphosis of ascidians is a good experimental system to observe the reorganization of the CNS during metamorphosis. A recent study has shown that the major parts of the larval CNS remain after the metamorphosis to form the adult CNS. In contrast to such a conserved manner of CNS reorganization, most larval neurons disappear during metamorphosis. The larval glial cells in the CNS are the major source for the formation of the adult CNS, and some of the glial cells produce adult neurons.     

Wnt/β‐catenin signaling during early vertebrate neural development

The vertebrate central nervous system (CNS) is comprised of vast number of distinct cell types arranged in a highly organized manner. This high degree of complexity is achieved by cellular communication, including direct cell‐cell contact, cell‐matrix interactions, and cell‐growth factor signaling. Among the several developmental signals controlling the development of the CNS, Wnt proteins have emerged as particularly critical and, hence, have captivated the attention of many researchers. With Wnts' evolutionarily conserved function as primordial symmetry breaking signals, these proteins and their downstream effects are responsible for simultaneously establishing cellular diversity and tissue organization. With their expansive repertoire of secreted agonists and antagonists, cell surface receptors, signaling cascades and downstream biological effects, Wnts are ideally suited to control the complex processes underlying vertebrate neural development. In this review, we will describe the mechanisms by which Wnts exert their potent effects on cells and tissues and highlight the many roles of Wnt signaling during neural development, starting from the initial induction of the neural plate, the subsequent patterning along the embryonic axes, to the intricately organized structure of the CNS. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1239–1259, 2017     

Convergence of dorsal, dpp, and egfr signaling pathways subdivides the drosophila neuroectoderm into three dorsal-ventral columns

An important question in neurobiology is how different cell fates are established along the dorsoventral (DV) axis of the central nervous system (CNS). Here we investigate the origins of DV patterning within the Drosophila CNS. The earliest sign of neural DV patterning is the expression of three homeobox genes in the neuroectoderm-ventral nervous system defective (vnd), intermediate neuroblasts defective (ind), and muscle segment homeobox (msh)-which are expressed in ventral, intermediate, and dorsal columns of neuroectoderm, respectively. Previous studies have shown that the Dorsal, Decapentaplegic (Dpp), and EGF receptor (Egfr) signaling pathways regulate embryonic DV patterning, as well as aspects of CNS patterning. Here we describe the earliest expression of each DV column gene (vnd, ind, and msh), the regulatory relationships between all three DV column genes, and the role of the Dorsal, Dpp, and Egfr signaling pathways in defining vnd, ind, and msh expression domains. We confirm that the vnd domain is established by Dorsal and maintained by Egfr, but unlike a previous report we show that vnd is not regulated by Dpp signaling. We show that ind expression requires both Dorsal and Egfr signaling for activation and positioning of its dorsal border, and that abnormally high Dpp can repress ind expression. Finally, we show that the msh domain is defined by repression: it occurs only where Dpp, Vnd, and Ind activity is low. We conclude that the initial diversification of cell fates along the DV axis of the CNS is coordinately established by Dorsal, Dpp, and Egfr signaling pathways. Understanding the mechanisms involved in patterning vnd, ind, and msh expression is important, because DV columnar homeobox gene expression in the neuroectoderm is an early, essential, and evolutionarily conserved step in generating neuronal diversity along the DV axis of the CNS.     

Interaction of notochord-derived fibrinogen-like protein with Notch regulates the patterning of the central nervous system of Ciona intestinalis embryos

The midline organ the notochord and its overlying dorsal neural tube are the most prominent features of the chordate body plan. Although the molecular mechanisms involved in the formation of the central nervous system (CNS) have been studied extensively in vertebrate embryos, none of the genes that are expressed exclusively in notochord cells has been shown to function in this process. Here, we report a gene in the urochordate Ciona intestinalis encoding a fibrinogen-like protein that plays a pivotal role in the notochord-dependent positioning of neuronal cells. While this gene (Ci-fibrn) is expressed exclusively in notochord cells, its protein product is not confined to these cells but is distributed underneath the CNS as fibril-like protrusions. We demonstrated that Ci-fibrn interacts physically and functionally with Ci-Notch that is expressed in the central nervous system, and that the correct distribution of Ci-fibrn protein is dependent on Notch signaling. Disturbance of the Ci-fibrn distribution caused an abnormal positioning of neuronal cells and an abnormal track of axon extension. Therefore, it is highly likely that the interaction between the notochord-based fibrinogen-like protein and the neural tube-based Notch signaling plays an essential role in the proper patterning of CNS.     

Structure of the planarian central nervous system (CNS) revealed by neuronal cell markers

Planarians are considered to be among the most primitive animals which developed the central nervous system (CNS). To understand the origin and evolution of the CNS, we have isolated a neural marker gene from a planarian, Dugesia japonica, and analyzed the structure of the planarian CNS by in situ hybridization. The planarian CNS is located on the ventral side of the body, and composed of a mass of cephalic ganglions in the head region and a pair of ventral nerve cords (VNC). Cephalic ganglions cluster independently from VNC, are more dorsal than VNC, and form an inverted U-shaped brain-like structure with nine branches on each outer side. Two eyes are located on the dorsal side of the 3(rd) branch and visual axons form optic chiasma on the dorsal-inside region of the inverted U-shaped brain. The 6(th)-9(th) branches cluster more closely and form auricles on the surface which may function as the sensory organ of taste. We found that the gross structure of the planarian CNS along the anterior-posterior (A-P) axis is strikingly similar to the distribution pattern of the "primary" neurons of vertebrate embryos which differentiate at the neural plate stage to provide a fundamental nervous system, although the vertebrate CNS is located on the dorsal side. These data suggest that the basic plan for the CNS development along the A-P axis might have been acquired at an early stage of evolution before conversion of the location of the CNS from the ventral to the dorsal side.     

Neural induction in Xenopus: requirement for ectodermal and endomesodermal signals via Chordin, Noggin, beta-Catenin, and Cerberus

The origin of the signals that induce the differentiation of the central nervous system (CNS) is a long-standing question in vertebrate embryology. Here we show that Xenopus neural induction starts earlier than previously thought, at the blastula stage, and requires the combined activity of two distinct signaling centers. One is the well-known Nieuwkoop center, located in dorsal-vegetal cells, which expresses Nodal-related endomesodermal inducers. The other is a blastula Chordin- and Noggin-expressing (BCNE) center located in dorsal animal cells that contains both prospective neuroectoderm and Spemann organizer precursor cells. Both centers are downstream of the early beta-Catenin signal. Molecular analyses demonstrated that the BCNE center was distinct from the Nieuwkoop center, and that the Nieuwkoop center expressed the secreted protein Cerberus (Cer). We found that explanted blastula dorsal animal cap cells that have not yet contacted a mesodermal substratum can, when cultured in saline solution, express definitive neural markers and differentiate histologically into CNS tissue. Transplantation experiments showed that the BCNE region was required for brain formation, even though it lacked CNS-inducing activity when transplanted ventrally. Cell-lineage studies demonstrated that BCNE cells give rise to a large part of the brain and retina and, in more posterior regions of the embryo, to floor plate and notochord. Loss-of-function experiments with antisense morpholino oligos (MO) showed that the CNS that forms in mesoderm-less Xenopus embryos (generated by injection with Cerberus-Short [CerS] mRNA) required Chordin (Chd), Noggin (Nog), and their upstream regulator beta-Catenin. When mesoderm involution was prevented in dorsal marginal-zone explants, the anterior neural tissue formed in ectoderm was derived from BCNE cells and had a complete requirement for Chd. By injecting Chd morpholino oligos (Chd-MO) into prospective neuroectoderm and Cerberus morpholino oligos (Cer-MO) into prospective endomesoderm at the 8-cell stage, we showed that both layers cooperate in CNS formation. The results suggest a model for neural induction in Xenopus in which an early blastula beta-Catenin signal predisposes the prospective neuroectoderm to neural induction by endomesodermal signals emanating from Spemann's organizer.     

Division of labor during trunk neural crest development

Neural crest cells, the migratory precursors of numerous cell types including the vertebrate peripheral nervous system, arise in the dorsal neural tube and follow prescribed routes into the embryonic periphery. While the timing and location of neural crest migratory pathways has been well documented in the trunk, a comprehensive collection of signals that guides neural crest migration along these paths has only recently been established. In this review, we outline the molecular cascade of events during trunk neural crest development. After describing the sequential routes taken by trunk neural crest cells, we consider the guidance cues that pattern these neural crest trajectories. We pay particular attention to segmental neural crest development and the steps and signals that generate a metameric peripheral nervous system, attempting to reconcile conflicting observations in chick and mouse. Finally, we compare cranial and trunk neural crest development in order to highlight common themes.     

Coordinated regulation of the dorsal‐ventral and anterior‐posterior patterning of Xenopus embryos by the BTB/POZ zinc finger protein Zbtb14

During early vertebrate embryogenesis, bone morphogenetic proteins (BMPs) belonging to the transforming growth factor‐β (TGF‐β) family of growth factors play a central role in dorsal–ventral (DV) patterning of embryos, while other growth factors such as Wnt and fibroblast growth factor (FGF) family members regulate formation of the anterior–posterior (AP) axis. Although the establishment of body plan is thought to require coordinated formation of the DV and AP axes, the mechanistic details underlying this coordination are not well understood. Here, we show that a Xenopus homologue of zbtb14 plays an essential role in the regulation of both DV and AP patterning during early Xenopus development. We show that overexpression of Zbtb14 promotes neural induction and inhibits epidermal differentiation, thereby regulating DV patterning. In addition, Zbtb14 promotes the formation of posterior neural tissue and suppresses anterior neural development. Consistent with this, knock‐down experiments show that Zbtb14 is required for neural development, especially for the formation of posterior neural tissues. Mechanistically, Zbtb14 reduces the levels of phosphorylated Smad1/5/8 to suppress BMP signaling and induces an accumulation of β‐Catenin to promote Wnt signaling. Collectively, these results suggest that Zbtb14 plays a crucial role in the formation of DV and AP axes by regulating both the BMP and Wnt signaling pathways during early Xenopus embryogenesis.     

Multiple roles of mouse Numb in tuning developmental cell fates.

BACKGROUND: Notch signaling regulates multiple differentiation processes and cell fate decisions during both invertebrate and vertebrate development. Numb encodes an intracellular protein that was shown in Drosophila to antagonize Notch signaling at binary cell fate decisions of certain cell lineages. Although overexpression experiments suggested that Numb might also antagonize some Notch activity in vertebrates, the developmental processes in which Numb is involved remained elusive. RESULTS: We generated mice with a homozygous inactivation of Numb. These mice died before embryonic day E11.5, probably because of defects in angiogenic remodeling and placental dysfunction. Mutant embryos had an open anterior neural tube and impaired neuronal differentiation within the developing cranial central nervous system (CNS). In the developing spinal cord, the number of differentiated motoneurons was reduced. Within the peripheral nervous system (PNS), ganglia of cranial sensory neurons were formed. Trunk neural crest cells migrated and differentiated into sympathetic neurons. In contrast, a selective differentiation anomaly was observed in dorsal root ganglia, where neural crest--derived progenitor cells had migrated normally to form ganglionic structures, but failed to differentiate into sensory neurons. CONCLUSIONS: Mouse Numb is involved in multiple developmental processes and required for cell fate tuning in a variety of lineages. In the nervous system, Numb is required for the generation of a large subset of neuronal lineages. The restricted requirement of Numb during neural development in the mouse suggests that in some neuronal lineages, Notch signaling may be regulated independently of Numb.     

The lateral somitic frontier: dorso-ventral aspects of anterio-posterior regionalization in avian embryos

Patterning events along the anterior-posterior (AP) axis of vertebrate embryos result in the distribution of muscle and bone forming a highly effective functional system. A key aspect of regionalized AP patterning results from variation in the migratory pattern of somite cells along the dorsal-ventral (DV) axis of the body. This occurs as somite cell populations expand around the axis or migrate away from the dorsal midline and cross into the lateral plate. The fate of somitic cells has been intensely studied and many details have been reported about inductive signaling from other tissues that influence somite cell fate and behavior. We are interested in understanding the specific differences between somites in particular AP regions and how these differences contribute to the global pattern of the organism. Using orthotopic transplants of segmental plate between quail and chick embryos, we have mapped the interface of the somitic and lateral plate mesoderm during the formation of the body wall in cervical and thoracic regions. This interface does not change dramatically in the mid-cervical region, but undergoes extensive changes in the thoracic region. Based on this regional mapping and consistent with the extensive literature, we suggest a revised method of classifying regions of the body wall that relies on embryonic cell lineages rather than adult functional criteria.     

The dorsal neural tube: A dynamic setting for cell fate decisions

The dorsal neural tube first generates neural crest cells that exit the neural primordium following an epithelial‐to‐mesenchymal conversion to become sympathetic ganglia, Schwann cells, dorsal root sensory ganglia, and melanocytes of the skin. Following the end of crest emigration, the dorsal midline of the neural tube becomes the roof plate, a signaling center for the organization of dorsal neuronal cell types. Recent lineage analysis performed before the onset of crest delamination revealed that the dorsal tube is a highly dynamic region sequentially traversed by fate‐restricted crest progenitors. Furthermore, prospective roof plate cells were shown to originate ventral to presumptive crest and to progressively relocate dorsalward to occupy their definitive midline position following crest delamination. These data raise important questions regarding the mechanisms of cell emigration in relation to fate acquisition, and suggest the possibility that spatial and/or temporal information in the dorsal neural tube determines initial segregation of neural crest cells into their derivatives. In addition, they emphasize the need to address what controls the end of neural crest production and consequent roof plate formation, a fundamental issue for understanding the separation between central and peripheral lineages during development of the nervous system. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 796–812, 2010.     

Large-scale characterization of genes specific to the larval nervous system in the ascidian Ciona intestinalis

The central and peripheral nervous systems (CNS and PNS) of the ascidian tadpole larva are comparatively simple, consisting of only about 350 cells. However, studies of the expression of neural patterning genes have demonstrated overall similarity between the ascidian CNS and the vertebrate CNS, suggesting that the ascidian CNS is sufficiently complex to be relevant to those of vertebrates. Recent progress in the Ciona intestinalis genome project and cDNA project together with considerable EST information has made Ciona an ideal model for investigating molecular mechanisms underlying the formation and function of the chordate nervous system. Here, we characterized 56 genes specific to the nervous system by determining their full-length cDNA sequences and confirming their spatial expression patterns. These genes included those that function in the nervous systems of other animals, especially those involved in photoreceptor-mediated signaling and neurotransmitter release. Thus, the nervous system-specific genes in Ciona larvae will provide not only probes for determining their function but also clues for exploring the complex network of nervous system-specific genes.     

Neural Induction in Xenopus: Requirement for Ectodermal and Endomesodermal Signals via Chordin, Noggin, β-Catenin, and Cerberus

The origin of the signals that induce the differentiation of the central nervous system (CNS) is a long-standing question in vertebrate embryology. Here we show that Xenopus neural induction starts earlier than previously thought, at the blastula stage, and requires the combined activity of two distinct signaling centers. One is the well-known Nieuwkoop center, located in dorsal-vegetal cells, which expresses Nodal-related endomesodermal inducers. The other is a blastula Chordin- and Noggin-expressing (BCNE) center located in dorsal animal cells that contains both prospective neuroectoderm and Spemann organizer precursor cells. Both centers are downstream of the early β-Catenin signal. Molecular analyses demonstrated that the BCNE center was distinct from the Nieuwkoop center, and that the Nieuwkoop center expressed the secreted protein Cerberus (Cer). We found that explanted blastula dorsal animal cap cells that have not yet contacted a mesodermal substratum can, when cultured in saline solution, express definitive neural markers and differentiate histologically into CNS tissue. Transplantation experiments showed that the BCNE region was required for brain formation, even though it lacked CNS-inducing activity when transplanted ventrally. Cell-lineage studies demonstrated that BCNE cells give rise to a large part of the brain and retina and, in more posterior regions of the embryo, to floor plate and notochord. Loss-of-function experiments with antisense morpholino oligos (MO) showed that the CNS that forms in mesoderm-less Xenopus embryos (generated by injection with Cerberus-Short [CerS] mRNA) required Chordin (Chd), Noggin (Nog), and their upstream regulator β-Catenin. When mesoderm involution was prevented in dorsal marginal-zone explants, the anterior neural tissue formed in ectoderm was derived from BCNE cells and had a complete requirement for Chd. By injecting Chd morpholino oligos (Chd-MO) into prospective neuroectoderm and Cerberus morpholino oligos (Cer-MO) into prospective endomesoderm at the 8-cell stage, we showed that both layers cooperate in CNS formation. The results suggest a model for neural induction in Xenopus in which an early blastula β-Catenin signal predisposes the prospective neuroectoderm to neural induction by endomesodermal signals emanating from Spemann's organizer.     

Neural Induction in Xenopus: Requirement for Ectodermal and Endomesodermal Signals via Chordin, Noggin, β-Catenin, and Cerberus

The origin of the signals that induce the differentiation of the central nervous system (CNS) is a long-standing question in vertebrate embryology. Here we show that Xenopus neural induction starts earlier than previously thought, at the blastula stage, and requires the combined activity of two distinct signaling centers. One is the well-known Nieuwkoop center, located in dorsal-vegetal cells, which expresses Nodal-related endomesodermal inducers. The other is a blastula Chordin- and Noggin-expressing (BCNE) center located in dorsal animal cells that contains both prospective neuroectoderm and Spemann organizer precursor cells. Both centers are downstream of the early β-Catenin signal. Molecular analyses demonstrated that the BCNE center was distinct from the Nieuwkoop center, and that the Nieuwkoop center expressed the secreted protein Cerberus (Cer). We found that explanted blastula dorsal animal cap cells that have not yet contacted a mesodermal substratum can, when cultured in saline solution, express definitive neural markers and differentiate histologically into CNS tissue. Transplantation experiments showed that the BCNE region was required for brain formation, even though it lacked CNS-inducing activity when transplanted ventrally. Cell-lineage studies demonstrated that BCNE cells give rise to a large part of the brain and retina and, in more posterior regions of the embryo, to floor plate and notochord. Loss-of-function experiments with antisense morpholino oligos (MO) showed that the CNS that forms in mesoderm-less Xenopus embryos (generated by injection with Cerberus-Short [CerS] mRNA) required Chordin (Chd), Noggin (Nog), and their upstream regulator β-Catenin. When mesoderm involution was prevented in dorsal marginal-zone explants, the anterior neural tissue formed in ectoderm was derived from BCNE cells and had a complete requirement for Chd. By injecting Chd morpholino oligos (Chd-MO) into prospective neuroectoderm and Cerberus morpholino oligos (Cer-MO) into prospective endomesoderm at the 8-cell stage, we showed that both layers cooperate in CNS formation. The results suggest a model for neural induction in Xenopus in which an early blastula β-Catenin signal predisposes the prospective neuroectoderm to neural induction by endomesodermal signals emanating from Spemann's organizer.     

Insulin-like growth factor-I and central nervous system development.

Insulin-like growth factor-I (IGF-I), a 70-amino acid-protein structurally similar to insulin, promotes cell proliferation and differentiation in multiple tissues. Most of its effects are mediated by the Type I IGF receptor (IGF-IR), a heterotetramer that has tyrosine kinase activity and phosphorylates insulin receptor substrates (IRS-1 and 2) which leads to the activation of two downstream signaling cascades: the MAP kinase and the phosphatidylinositol 3-kinase (P3K) cascades. The growth-promoting effects of IGF-I are prominent in the nervous system, qualifying this molecule as a neurotrophin. Although the primary regulator of IGF-I expression is growth hormone (GH), the developmental expression of IGF-I in various tissues precedes that of GH, supporting an independent role of IGF-I in embryonic and fetal life [1]. This review will examine the effect of IGF-I on central nervous system (CNS) development. The specialized structure of the CNS is the product of a complex series of biological events which result from the interaction between the cells' genetic program and environmental influences. CNS development begins in the embryo with dorsal ectodermal cell proliferation to form the neural plate, and, with its closure, the neural tube, followed by the rapid division of pluripotential cells, their migration to the periphery of the neural tube, and differentiation into neural or glial cells. During the latter stages, cells form special structures such as nuclei, ganglia, cerebral cortical layers, and they also develop a network with their cytoplasmic extensions, neurites. Many more cells and connections are generated in fetal life than are found in the mature organism. This excessive production of some cell groups and neurites may compensate for tissue loss due to various injuries, and their selective elimination also constitutes an efficient way to organize the architecture of the CNS. This elimination is believed to be accomplished by apoptosis. The cells' intrinsic program for development includes the expression of various genes at different times. Environmental influences, such as extracellular matrix (ECM) molecules that attract or repel cells, afferent inputs, and target-derived diffusible molecules modify and modulate cellular behavior. IGF-I is among the molecules which affect several steps involved in development.     

Wnt/BMP signal integration regulates the balance between proliferation and differentiation of neuroepithelial cells in the dorsal spinal cord

Multiple signaling pathways regulate proliferation and differentiation of neural progenitor cells during early development of the central nervous system (CNS). In the spinal cord, dorsal signaling by bone morphogenic protein (BMP) acts primarily as a patterning signal, while canonical Wnt signaling promotes cell cycle progression in stem and progenitor cells. However, overexpression of Wnt factors or, as shown here, stabilization of the Wnt signaling component beta-catenin has a more prominent effect in the ventral than in the dorsal spinal cord, revealing local differences in signal interpretation. Intriguingly, Wnt signaling is associated with BMP signal activation in the dorsal spinal cord. This points to a spatially restricted interaction between these pathways. Indeed, BMP counteracts proliferation promoted by Wnt in spinal cord neuroepithelial cells. Conversely, Wnt antagonizes BMP-dependent neuronal differentiation. Thus, a mutually inhibitory crosstalk between Wnt and BMP signaling controls the balance between proliferation and differentiation. A model emerges in which dorsal Wnt/BMP signal integration links growth and patterning, thereby maintaining undifferentiated and slow-cycling neural progenitors that form the dorsal confines of the developing spinal cord.