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

Spinal Cord Organoids from Human Amniotic Fluid iPSC Recapitulate the Diversity of Cell Phenotypes During Fetal Neural Tube Morphogenesis

Myelomeningocele (MMC) is a severe form of spina bifida associated with substantial neurologic morbidity. In vitro modeling systems of human spinal cord development may help to elucidate the underlying pathophysiology of the MMC spinal cord. To that end, we developed spinal cord organoids (SCO), defined as self-organized, three-dimensional clusters of spinal tissue, that were derived from human amniotic fluid-induced pluripotent stem cells. Here, we used a variety of analyses, including immunofluorescent and single-cell transcriptomic approaches, to characterize SCOs from healthy and MMC fetuses. Organoids contained a diverse range of neural and mesodermal phenotypes when cultured for up to 130 days in vitro. Multielectrode arrays revealed functional activity with evidence of emerging neuronal networks. Fetal spina bifida environment modeling was successfully established by culturing SCOs in second- and third-trimester amniotic fluid for 3 weeks. Taken together, we show that functional SCOs can recapitulate the cellular identity of the fetal spinal cord and represent a novel research platform to study the interplay between cellular, biochemical, and mechanical cues during human MMC neural tube morphogenesis.

Small molecule valproic acid enhances ventral patterning of human neural tube organoids by regulating Wnt and Shh signalling

Valproic acid (VPA), a clinically approved small molecule, has been reported to activate Wnt signalling that is critical for dorsal-ventral (DV) patterning of neural tube. However, little is known about the impact of VPA on DV patterning process. Here, we show that even though VPA has a negative impact on the early formation of human neural tube organoids (hNTOs), it significantly enhances the efficiency of ventrally patterned hNTOs, when VPA is added during the entire differentiation process. RNA sequencing and RT-qPCR analysis demonstrates VPA activates endogenous Wnt signalling in hNTOs. Surprisingly, transcriptome analysis also identifies upregulation of genes for degradation of GLI2 and GLI3 proteins, whose truncated fragment are transcriptional repressors of Shh signalling. The Western-blot analysis confirms the increase of GLI3R proteins after VPA treatment. Thus, VPA might enhance ventral patterning of hNTOs through both activating Wnt, which can antagonise Shh signalling by inducing GLI3 expression, and/or inhibiting Shh signalling by inducing GLI protein degradation. We further obtain results to show that VPA still increases patterning efficiency of hNTOs with a weak influence on their early formation when the initiation time of VPA is delayed and its duration is reduced. Taken together, this study demonstrates that VPA enhances the generation of more reproducible hNTOs with ventral patterning, opening the avenues for the applications of hNTOs in developmental biology and regenerative medicine.

Retinoic acid, an essential component of the roof plate organizer, promotes the spatiotemporal segregation of dorsal neural fates

Dorsal neural tube-derived retinoic acid promotes the end of neural crest production and transition into a definitive roof plate. Here, we analyze how this impacts the segregation of central and peripheral lineages, a process essential for tissue patterning and function. Localized in ovo inhibition in quail embryos of retinoic acid activity followed by single-cell transcriptomics unraveled a comprehensive list of differentially expressed genes relevant to these processes. Importantly, progenitors co-expressed neural crest, roof plate and dI1 interneuron markers, indicating a failure in proper lineage segregation. Furthermore, separation between roof plate and dI1 interneurons is mediated by Notch activity downstream of retinoic acid, highlighting their crucial role in establishing the roof plate-dI1 boundary. Within the peripheral branch, where absence of retinoic acid resulted in neural crest production and emigration extending into the roof plate stage, sensory progenitors failed to separate from melanocytes, leading to formation of a common glia-melanocyte cell with aberrant migratory patterns. In summary, the implementation of single-cell RNA sequencing facilitated the discovery and characterization of a molecular mechanism responsible for the segregation of dorsal neural fates during development.

From neural tube to spinal cord: The dynamic journey of the dorsal neuroepithelium

In a developing embryo, formation of tissues and organs is remarkably precise in both time and space. Through cell-cell interactions, neighboring progenitors coordinate their activities, sequentially generating distinct types of cells. At present, we only have limited knowledge, rather than a systematic understanding, of the underlying logic and mechanisms responsible for cell fate transitions. The formation of the dorsal aspect of the spinal cord is an outstanding model to tackle these dynamics, as it first generates the peripheral nervous system and is later responsible for transmitting sensory information from the periphery to the brain and for coordinating local reflexes. This is reflected first by the ontogeny of neural crest cells, progenitors of the peripheral nervous system, followed by formation of the definitive roof plate of the central nervous system and specification of adjacent interneurons, then a transformation of roof plate into dorsal radial glia and ependyma lining the forming central canal. How do these peripheral and central neural branches segregate from common progenitors? How are dorsal radial glia established concomitant with transformation of the neural tube lumen into a central canal? How do the dorsal radial glia influence neighboring cells? This is only a partial list of questions whose clarification requires the implementation of experimental paradigms in which precise control of timing is crucial. Here, we outline some available answers and still open issues, while highlighting the contributions of avian models and their potential to address mechanisms of neural patterning and function.

Rspo1 and Rspo3 are required for sensory lineage neural crest formation in mouse embryos

BACKGROUND

R-spondins (Rspos) are secreted proteins that modulate Wnt/β-catenin signaling. At the early stages of spinal cord development, Wnts (Wnt1, Wnt3a) and Rspos (Rspo1, Rspo3) are co-expressed in the roof plate, suggesting that Rspos are involved in development of dorsal spinal cord and neural crest cells in cooperation with Wnt ligands.

RESULTS

Here, we found that Rspo1 and Rspo3, as well as Wnt1 and Wnt3a, maintained roof-plate-specific expression until late embryonic stages. Rspo1- and Rspo3-double-knock-out (dKO) embryos partially exhibited the phenotype of Wnt1 and Wnt3a dKO embryos. While the number of Ngn2-positive sensory lineage neural crest cells is reduced in Rspo-dKO embryos, development of dorsal spinal cord, including its size and dorso-ventral patterning in early development, elongation of the roof plate, and proliferation of ependymal cells, proceeded normally. Consistent with these slight defects, Wnt/β-catenin signaling was not obviously changed in developing spinal cord of dKO embryos.

CONCLUSIONS

Our results show that Rspo1 and Rspo3 are dispensable for most developmental processes involving roof plate-derived Wnt ligands, except for specification of a subtype of neural crest cells. Thus, Rspos may modulate Wnt/β-catenin signaling in a context-dependent manner.

Continuity and change in neural plasticity through embryonic morphogenesis, fetal activity-dependent synaptogenesis, and infant memory consolidation

There is an apparent continuity in human neural development that can be traced to venerable themes of vertebrate morphogenesis that have shaped the evolution of the reptilian telencephalon (including both primitive three-layered cortex and basal ganglia) and then the subsequent evolution of the mammalian six-layered neocortex. In this theoretical analysis, we propose that an evolutionary-developmental analysis of these general morphogenetic themes can help to explain the embryonic development of the dual divisions of the limbic system that control the dorsal and ventral networks of the human neocortex. These include the archicortical (dorsal limbic) Papez circuits regulated by the hippocampus that organize spatial, contextual memory, as well as the paleocortical (ventral limbic) circuits that organize object memory. We review evidence that these dorsal and ventral limbic divisions are controlled by the differential actions of brainstem lemnothalamic and midbrain collothalamic arousal control systems, respectively, thereby traversing the vertebrate subcortical neuraxis. These dual control systems are first seen shaping the phyletic morphogenesis of the archicortical and paleocortical foundations of the forebrain in embryogenesis. They then provide dual modes of activity-dependent synaptic organization in the active (lemnothalamic) and quiet (collothalamic) stages of fetal sleep. Finally, these regulatory systems mature to form the major systems of memory consolidation of postnatal development, including the rapid eye movement (lemnothalamic) consolidation of implicit memory and social attachment in the first year, and then-in a subsequent stage-the non-REM (collothalamic) consolidation of explicit memory that is integral to the autonomy and individuation of the second year of life.

Regulation of axon pathfinding by astroglia across genetic model organisms

Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a "glia blueprint", to recent genetic and cell manipulations, and visualizations in vivo. An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo, emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.

Regulation of choroid plexus development and its functions

The choroid plexus (ChP) is an extensively vascularized tissue that protrudes into the brain ventricular system of all vertebrates. This highly specialized structure, consisting of the polarized epithelial sheet and underlying stroma, serves a spectrum of functions within the central nervous system (CNS), most notably the production of cerebrospinal fluid (CSF). The epithelial cells of the ChP have the competence to tightly modulate the biomolecule composition of CSF, which acts as a milieu functionally connecting ChP with other brain structures. This review aims to eloquently summarize the current knowledge about the development of ChP. We describe the mechanisms that control its early specification from roof plate followed by the formation of proliferative regions-cortical hem and rhombic lips-feeding later development of ChP. Next, we summarized the current knowledge on the maturation of ChP and mechanisms that control its morphological and cellular diversity. Furthermore, we attempted to review the currently available battery of molecular markers and mouse strains available for the research of ChP, and identified some technological shortcomings that must be overcome to accelerate the ChP research field. Overall, the central principle of this review is to highlight ChP as an intriguing and surprisingly poorly known structure that is vital for the development and function of the whole CNS. We believe that our summary will increase the interest in further studies of ChP that aim to describe the molecular and cellular principles guiding the development and function of this tissue.

Folic Acid Rescues Valproic Acid-Induced Morphogenesis Inhibition in Neural Rosettes Derived From Human Pluripotent Stem Cells

The ability of human pluripotent stem cells (hPSCs) to specialize in neuroepithelial tissue makes them ideal candidates for use in the disease models of neural tube defects. In this study, we cultured hPSCs in suspension with modified neural induction method, and immunostaining was applied to detect important markers associated with cell fate and morphogenesis to verify the establishment of the neural tube model in vitro. We carried out the drug experiments to further investigate the toxicity of valproic acid (VPA) exposure and the potential protective effect of folic acid (FA). The results demonstrated that neural rosette undergoes cell fate speciation and lumen formation accompanied by a spatiotemporal shift in the expression patterns of cadherin, indicating the model was successfully established. The results showed that VPA caused morphogenesis inhibition of lumen formation by altering cytoskeletal function and cell polarization, which could be rescued by FA supplement.

Completion of neural crest cell production and emigration is regulated by retinoic-acid-dependent inhibition of BMP signaling

Production and emigration of neural crest cells is a transient process followed by the emergence of the definitive roof plate. The mechanisms regulating the end of neural crest ontogeny are poorly understood. Whereas early crest development is stimulated by mesoderm-derived retinoic acid, we report that the end of the neural crest period is regulated by retinoic acid synthesized in the dorsal neural tube. Inhibition of retinoic acid signaling in the neural tube prevents the normal upregulation of BMP inhibitors in the nascent roof plate and prolongs the period of BMP responsiveness which otherwise ceases close to roof plate establishment. Consequently, neural crest production and emigration are extended well into the roof plate stage. In turn, extending the activity of neural crest-specific genes inhibits the onset of retinoic acid synthesis in roof plate suggesting a mutual repressive interaction between neural crest and roof plate traits. Although several roof plate-specific genes are normally expressed in the absence of retinoic acid signaling, roof plate and crest markers are co-expressed in single cells and this domain also contains dorsal interneurons. Hence, the cellular and molecular architecture of the roof plate is compromised. Collectively, our results demonstrate that neural tube-derived retinoic acid, via inhibition of BMP signaling, is an essential factor responsible for the end of neural crest generation and the proper segregation of dorsal neural lineages.

The division between the central nervous system – formed by the brain and spinal cord – and the peripheral nervous system – which consists of the neurons that sense and relay information to and from the body – takes place early during embryonic development. Initially, the nervous system consists of a tube of cells called the neural tube. From the top region of this tube, some cells change their shape, exit the tube and migrate to different places in the developing body. These cells are called the ‘neural crest’, and they form many different structures, including the peripheral nervous system.

Neural crest cells keep leaving the neural tube for a period of time, but after that, the neural tube stops producing them. At this point, the region of the neural tube that had been producing neural crest cells becomes the ‘roof plate’ of the central nervous system, a structure that is essential for the development of specific groups of neurons in the brain and spinal cord.

In bird embryos, a protein called bone morphogenetic protein (BMP) is essential for neural crest production because it triggers the migration of these cells away from the neural tube. Before the roof plate is formed, the activity of BMP is blocked by proteins known as BMP inhibitors, which stop more cells from leaving the neural tube. Around the time when neural crest formation stops, another molecule called retinoic acid begins to be synthesized in the top region of the neural tube. Rekler and Kalcheim asked whether retinoic acid is involved in the transition from neural crest to roof plate.

To test this hypothesis, Rekler and Kalcheim blocked the activity of retinoic acid in the neural tube of quail embryos at the time when they should stop producing neural crest cells. This resulted in embryos in which the neural tube keeps producing neural crest cells after the roof plate has formed. In these embryos, individual cells in the resulting ‘roof plate’ produced both proteins that are normally only found in neural crest cells, and proteins typically exclusive to the roof plate. This suggests that, in the absence of retinoic acid activity, the segregation of neural crest identity from roof plate identity is compromised.

Rekler and Kalcheim also found that, in the embryos where retinoic acid activity had been blocked, the cells in the area where the roof plate should be produced virtually no BMP inhibitors, and exhibited extended BMP activity. This allowed neural crest cells to continue forming and migrating away from the neural tube well after the period when they would stop in a normal embryo. These results indicate that retinoic acid stops the production of neural crest cells by repressing BMP activity in the roof plate of the neural tube.

Rekler and Kalcheim’s experiments shed light on the mechanisms that allow the central and peripheral nervous systems to become segregated. This could increase our understanding of the origin of several neurodevelopmental disorders, potentially providing insights into their treatment or prevention. Additionally, the process of neural crest production and exit from the neural tube is highly similar to the process of metastasis in many invasive cancers. Thus, by understanding how the production of neural crest cells is terminated, it may be possible to learn how to prevent malignant cancer cells from spreading through the body.

Morphological and Functional Changes of Roof Plate Cells in Spinal Cord Development

The most dorsal region, or roof plate, is the dorsal organizing center of developing spinal cord. This region is also involved in development of neural crest cells, which are the source of migratory neural crest cells. During early development of the spinal cord, roof plate cells secrete signaling molecules, such as Wnt and BMP family proteins, which regulate development of neural crest cells and dorsal spinal cord. After the dorso-ventral pattern is established, spinal cord dynamically changes its morphology. With this morphological transformation, the lumen of the spinal cord gradually shrinks to form the central canal, a cavity filled with cerebrospinal fluid that is connected to the ventricular system of the brain. The dorsal half of the spinal cord is separated by a glial structure called the dorsal (or posterior) median septum. However, underlying mechanisms of such morphological transformation are just beginning to be understood. Recent studies reveal that roof plate cells dramatically stretch along the dorso-ventral axis, accompanied by reduction of the spinal cord lumen. During this stretching process, the tips of roof plate cells maintain contact with cells surrounding the shrinking lumen, eventually exposed to the inner surface of the central canal. Interestingly, Wnt expression remains in stretched roof plate cells and activates Wnt/β-catenin signaling in ependymal cells surrounding the central canal. Wnt/β-catenin signaling in ependymal cells promotes proliferation of neural progenitor and stem cells in embryonic and adult spinal cord. In this review, we focus on the role of the roof plate, especially that of Wnt ligands secreted by roof plate cells, in morphological changes occurring in the spinal cord.

Notch signaling is a critical initiator of roof plate formation as revealed by the use of RNA profiling of the dorsal neural tube

Background

The dorsal domain of the neural tube is an excellent model to investigate the generation of complexity during embryonic development. It is a highly dynamic and multifaceted region being first transiently populated by prospective neural crest (NC) cells that sequentially emigrate to generate most of the peripheral nervous system. Subsequently, it becomes the definitive roof plate (RP) of the central nervous system. The RP, in turn, constitutes a patterning center for dorsal interneuron development. The factors underlying establishment of the definitive RP and its segregation from NC and dorsal interneurons are currently unknown.

Results

We performed a transcriptome analysis at trunk levels of quail embryos comparing the dorsal neural tube at premigratory NC and RP stages. This unraveled molecular heterogeneity between NC and RP stages, and within the RP itself. By implementing these genes, we asked whether Notch signaling is involved in RP development. First, we observed that Notch is active at the RP-interneuron interface. Furthermore, gain and loss of Notch function in quail and mouse embryos, respectively, revealed no effect on early NC behavior. Constitutive Notch activation caused a local downregulation of RP markers with a concomitant development of dI1 interneurons, as well as an ectopic upregulation of RP markers in the interneuron domain. Reciprocally, in mice lacking Notch activity, both the RP and dI1 interneurons failed to form and this was associated with expansion of the dI2 population.

Conclusions

Collectively, our results offer a new resource for defining specific cell types, and provide evidence that Notch is required to establish the definitive RP, and to determine the choice between RP and interneuron fates, but not the segregation of RP from NC.

Wnt3 distribution in the zebrafish brain is determined by expression, diffusion and multiple molecular interactions

Wnt3 proteins are lipidated and glycosylated signaling molecules that play an important role in zebrafish neural patterning and brain development. However, the transport mechanism of lipid-modified Wnts through the hydrophilic extracellular environment for long-range action remains unresolved. Here we determine how Wnt3 accomplishes long-range distribution in the zebrafish brain. First, we characterize the Wnt3-producing source and Wnt3-receiving target regions. Subsequently, we analyze Wnt3 mobility at different length scales by fluorescence correlation spectroscopy and fluorescence recovery after photobleaching. We demonstrate that Wnt3 spreads extracellularly and interacts with heparan sulfate proteoglycans (HSPG). We then determine the binding affinity of Wnt3 to its receptor, Frizzled1 (Fzd1), using fluorescence cross-correlation spectroscopy and show that the co-receptor, low-density lipoprotein receptor-related protein 5 (Lrp5), is required for Wnt3-Fzd1 interaction. Our results are consistent with the extracellular distribution of Wnt3 by a diffusive mechanism that is modified by tissue morphology, interactions with HSPG, and Lrp5-mediated receptor binding, to regulate zebrafish brain development.

Apoptosis is involved in maintaining the character of the midbrain and the diencephalon roof plate after neural tube closure

Apoptosis, a major form of programmed cell death, is massively observed in neural plate border and subsequently in the roof plate (RP). While deficiency of apoptosis often results in brain malformations including exencephaly and hydrocephalus, the impact of apoptosis on RP formation and maintenance remains unclear. Here we described that mouse embryos deficient in Apaf1, a gene crucial for the intrinsic apoptotic pathway, in C57BL/6 genetic background exhibited narrow and discontinuous expression of RP marker genes in the midline of the midbrain and the diencephalon. Instead, cells positive for the neuroectodermal gene SOX1 ectopically accumulated in the midline. A lineage-tracing experiment suggests that these ectopic SOX1-positive cells began to accumulate in the midline of apoptosis-deficient embryos after E9.5. These embryos further displayed malformation of the subcommissural organ, which has been discussed in the etiology of hydrocephalus. Thus, the apoptosis machinery prevents ectopic emergence of SOX1-positive cells in the midbrain and the diencephalon RP, and helps in maintaining the character of the RP in the diencephalon and midbrain, thereby ensuring proper brain development.

Apcdd1 is a dual BMP/Wnt inhibitor in the developing nervous system and skin

Animal development and homeostasis depend on precise temporal and spatial intercellular signaling. Components shared between signaling pathways, generally thought to decrease specificity, paradoxically can also provide a solution to pathway coordination. Here we show that the Bone Morphogenetic Protein (BMP) and Wnt signaling pathways share Apcdd1 as a common inhibitor and that Apcdd1 is a taxon-restricted gene with novel domains and signaling functions. Previously, we showed that Apcdd1 inhibits Wnt signaling (Shimomura et al., 2010), here we find that Apcdd1 potently inhibits BMP signaling in body axis formation and neural differentiation in chicken, frog, zebrafish. Furthermore, we find that Apcdd1 has an evolutionarily novel protein domain. Our results from experiments and modeling suggest that Apcdd1 may coordinate the outputs of two signaling pathways that are central to animal development and human disease.

Insight Into the Ontogeny of GnRH Neurons From Patients Born Without a Nose

CONTEXT

The reproductive axis is controlled by a network of gonadotropin-releasing hormone (GnRH) neurons born in the primitive nose that migrate to the hypothalamus alongside axons of the olfactory system. The observation that congenital anosmia (inability to smell) is often associated with GnRH deficiency in humans led to the prevailing view that GnRH neurons depend on olfactory structures to reach the brain, but this hypothesis has not been confirmed.

OBJECTIVE

The objective of this work is to determine the potential for normal reproductive function in the setting of completely absent internal and external olfactory structures.

METHODS

We conducted comprehensive phenotyping studies in 11 patients with congenital arhinia. These studies were augmented by review of medical records and study questionnaires in another 40 international patients.

RESULTS

All male patients demonstrated clinical and/or biochemical signs of GnRH deficiency, and the 5 men studied in person had no luteinizing hormone (LH) pulses, suggesting absent GnRH activity. The 6 women studied in person also had apulsatile LH profiles, yet 3 had spontaneous breast development and 2 women (studied from afar) had normal breast development and menstrual cycles, suggesting a fully intact reproductive axis. Administration of pulsatile GnRH to 2 GnRH-deficient patients revealed normal pituitary responsiveness but gonadal failure in the male patient.

CONCLUSIONS

Patients with arhinia teach us that the GnRH neuron, a key gatekeeper of the reproductive axis, is associated with but may not depend on olfactory structures for normal migration and function, and more broadly, illustrate the power of extreme human phenotypes in answering fundamental questions about human embryology.

BMP7 is Downregulated in Lumbosacral Spinal Cord of Rat Embryos With Anorectal Malformation

BACKGROUND

Bone morphogenetic proteins (BMPs) comprise a highly conserved signaling protein family, which are involved in spinal cord formation, development and differentiation. Malformations of the lumbosacral spinal cord are associated with postoperation complications of anorectal malformation (ARM). However, the mechanism underlying the development of these malformations remains elusive.

MATERIALS AND METHODS

Embryonic rat ARM model induced by ethylenethiourea (ETU) was introduced to investigate BMP7 expression in lumbosacral spinal cord. BMP7 expression was analyzed by immunohistochemical staining, qRT-PCR, and Western blot analysis on embryonic (E) days 16, 17, 19, and 21. The expression of the neuronal marker neurofilament (NF) and pSmad1/5 was determined by immunofluorescence double staining and Western blot analysis during peak BMP7 expression.

RESULTS

BMP7 mRNA (E16, 1.041 ± 0.169 versus 0.758 ± 0.0423, P < 0.05; E17, 1.889 ± 0.444 versus 1.601 ± 0.263, P < 0.05; E19, 2.898 ± 0.434 versus 1.981 ± 0.068, P < 0.01; and E21, 2.652 ± 0.637 versus 1.957 ± 0.09, P < 0.05;) and protein (E16, 1.068 ± 0.065 versus 0.828 ± 0.066, P < 0.01; E17, 1.728 ± 0.153 versus1.4 ± 0.148, P < 0.05; E19, 2.313 ± 0.141 versus 1.696 ± 0.21, P < 0.01; and E21, 2.021 ± 0.13 versus 1.43 ± 0.128, P < 0.01) were downregulated, and their expressions were specifically low in interneurons (IN) located in the dorsal horn of the lumbosacral spinal cord in embryos with ARM. On E19, Western blot analysis revealed reduced P-Smad1/5(1.13 ± 0.08 versus 0.525 ± 0.06, P < 0.01).

CONCLUSIONS

An implication of this study is the possibility that BMP7 downregulation contributes to maldevelopment of the lumbosacral spinal cord during embryogenesis in fetal rats with ARM, indicating that BMP7 may play an important role in ARM pathogenesis and the complications thereof.

Modeling of human neurulation using bioengineered pluripotent stem cell culture

Leveraging the developmental potential and self-organizing property of human pluripotent stem (hPS) cells, researchers have developed tractable models of human embryonic development. Owing to their compatibility to live imaging, genome editing, mechanical perturbation and measurement, these models offer promising quantitative experimental platforms to advance human embryology and regenerative medicine. Herein, we provide a review of recent progress in using hPS cells to generate models of early human neural development or neurulation, including neural induction and regional patterning of the neural tube. These models, even in their nascent developmental stages, have already revealed intricate cell-cell signaling and mechanoregulation mechanisms likely involved in tissue patterning during early neural development. We also discuss future opportunities in modeling early neural development by incorporating bioengineering tools to control precisely neural tissue morphology and architecture, morphogen dynamics, intracellular signaling events, and cell-cell interactions to further the development of this emerging field and expand its applications.

Analyses with double knockouts of the Bmpr1a and Bmpr1b genes demonstrate that BMP signaling is involved in the formation of precerebellar mossy fiber nuclei derived from the rhombic lip

Bone morphogenetic proteins (BMPs) have been hypothesized to specify distinct dorsal neural fates. During neural development, BMPs are expressed in the roof plate and adjacent neuroepithelium. Because several hindbrain nuclei that form the proprioceptive/vestibular/auditory sensory network originate from the rhombic lip, near the roof plate, BMP signaling may regulate the development of these nuclei. To test this hypothesis genetically, we have examined the development of the hindbrain in BMP type I receptor knockout mice. Our results demonstrate that BMP signaling is involved in the formation of precerebellar mossy fiber nuclei, which give rise to cerebellar mossy fibers, but is not required for the development of the inferior olivary nucleus, which gives rise to cerebellar climbing fibers.

BMP7 functions predominantly as a heterodimer with BMP2 or BMP4 during mammalian embryogenesis

BMP7/BMP2 or BMP7/BMP4 heterodimers are more active than homodimers in vitro, but it is not known whether these heterodimers signal in vivo. To test this, we generated knock in mice carrying a mutation (Bmp7^R-GFlag) that prevents proteolytic activation of the dimerized BMP7 precursor protein. This mutation eliminates the function of BMP7 homodimers and all other BMPs that normally heterodimerize with BMP7. While Bmp7 null homozygotes are live born, Bmp7^R-GFlag homozygotes are embryonic lethal and have broadly reduced BMP activity. Furthermore, compound heterozygotes carrying the Bmp7^R-G allele together with a null allele of Bmp2 or Bmp4 die during embryogenesis with defects in ventral body wall closure and/or the heart. Co-immunoprecipitation assays confirm that endogenous BMP4/7 heterodimers exist. Thus, BMP7 functions predominantly as a heterodimer with BMP2 or BMP4 during mammalian development, which may explain why mutations in either Bmp4 or Bmp7 lead to a similar spectrum of congenital defects in humans.

Loss of the small GTPase Arl8b results in abnormal development of the roof plate in mouse embryos

Lysosomes are acidic organelles responsible for degrading both exogenous and endogenous materials. The small GTPase Arl8 localizes primarily to lysosomes and is involved in lysosomal function. In the present study, using Arl8b gene-trapped mutant (Arl8b^-/- ) mice, we show that Arl8b is required for the development of dorsal structures of the neural tube, including the thalamus and hippocampus. In embryonic day (E) 10.5 Arl8b^-/- embryos, Sox1 (a neuroepithelium marker) was ectopically expressed in the roof plate, whereas the expression of Gdf7 and Msx1 (roof plate markers) was reduced in the dorsal midline of the midbrain. Ectopic expression of Sox1 in Arl8b^-/- embryos was detected also at E9.0 in the neural fold, which gives rise to the roof plate. In addition, the levels of Bmp receptor IA and phosphorylated Smad 1/5/8 (downstream of BMP signaling) were increased in the neural fold of E9.0 Arl8b^-/- embryos. These results suggest that Arl8b is involved in the development of the neural fold and the subsequently formed roof plate, possibly via control of BMP signaling.

Wnt produced by stretched roof-plate cells is required for the promotion of cell proliferation around the central canal of the spinal cord

Cell morphology changes dynamically during embryogenesis, and these changes create new interactions with surrounding cells, some of which are presumably mediated by intercellular signaling. However, the effects of morphological changes on intercellular signaling remain to be fully elucidated. In this study, we examined the effect of morphological changes in Wnt-producing cells on intercellular signaling in the spinal cord. After mid-gestation, roof-plate cells stretched along the dorsoventral axis in the mouse spinal cord, resulting in new contact at their tips with the ependymal cells that surround the central canal. Wnt1 and Wnt3a were produced by the stretched roof-plate cells and delivered to the cell process tip. Whereas Wnt signaling was activated in developing ependymal cells, Wnt activation in dorsal ependymal cells, which were close to the stretched roof plate, was significantly suppressed in embryos with roof plate-specific conditional knockout of Wls, which encodes a factor that is essential for Wnt secretion. Furthermore, proliferation of these cells was impaired in Wls conditional knockout mice during development and after induced spinal cord injury in adults. Therefore, morphological changes in Wnt-producing cells appear to generate new Wnt signal targets.

Patterning factors during neural progenitor induction determine regional identity and differentiation potential in vitro

The neural tube consists of neural progenitors (NPs) that acquire different characteristics during gestation due to patterning factors. However, the influence of such patterning factors on human pluripotent stem cells (hPSCs) during in vitro neural differentiation is often unclear. This study compared neural induction protocols involving in vitro patterning with single SMAD inhibition (SSI), retinoic acid (RA) administration and dual SMAD inhibition (DSI). While the derived NP cells expressed known NP markers, they differed in their NP expression profile and differentiation potential. Cortical neuronal cells generated from 1) SSI NPs exhibited less mature neuronal phenotypes, 2) RA NPs exhibited an increased GABAergic phenotype, and 3) DSI NPs exhibited greater expression of glutamatergic lineage markers. Further, although all NPs generated astrocytes, astrocytes derived from the RA-induced NPs had the highest GFAP expression. Differences between NP populations included differential expression of regional identity markers HOXB4, LBX1, OTX1 and GSX2, which persisted into mature neural cell stages. This study suggests that patterning factors regulate how potential NPs may differentiate into specific neuronal and glial cell types in vitro. This challenges the utility of generic neural induction procedures, while highlighting the importance of carefully selecting specific NP protocols.

Three-dimensional induction of dorsal, intermediate and ventral spinal cord tissues from human pluripotent stem cells

The spinal cord contains more than 20 distinct subclasses of neurons that form well-organized neural circuits capable of sensing the environment and generating motor behavior. Although recent studies have described the efficient in vitro generation of spinal motor neurons, the induction of the spinal cord as a whole tissue has not been achieved. In the present study, we demonstrate three-dimensional (3D) induction of dorsal spinal cord-like tissues from human pluripotent stem cells. Our 3D spinal cord induction (3-DiSC) condition recapitulates patterning of the developing dorsal spinal cord and enables the generation of four types of dorsal interneuron marker-positive cell populations. By activating Shh signaling, intermediate and ventral spinal cord-like tissues are successfully induced. After dissociation of these tissues, somatosensory neurons and spinal motor neurons are detected and express neurotransmitters in an in vivo manner. Our approach provides a useful experimental tool for the analysis of human spinal cord development and will contribute to research on the formation and organization of the spinal cord, and its application to regenerative medicine.

BIO (6-bromoindirubin-3'-oxime) GSK3 inhibitor induces dopaminergic differentiation of human immortalized RenVm cells

Parkinson's disease (PD) is one of the most neurodegenerative disorders which can lead to severe neural disability and neurological defects. Cell-based therapy using fully differentiated cells is a new method for the treatment of this abnormal condition. In the present study, we investigated the effects of 6-bromoindirubin-3'-oxime (BIO) on dopaminergic differentiation of human immortalized RenVm cells in order to obtain a set of fully differentiated cells for transplantation in Parkinson's disease. To this end, the immortalized RenVm cells were induced to dopaminergic differentiation using a neuro basal medium supplemented with N2 and different concentrations (75, 150, 300, 600, and 1200 nM) of BIO for 4, 8, and 12 days. The efficiency of dopaminergic differentiation was determined using immunocytochemistry for tyrosine hydroxylase expressions. In addition, the expression of a β-catenin marker was measured using the western blot technique. The results of immunocytochemistry revealed that the mean percentage of Tuj1- and TH-positive sells in 150- and 300-nM-BIO-treated groups was significantly increased compared to that of other groups (p ≤ 0.01). In addition, the expression of the β-catenin marker was higher in these groups as compared with that of other groups. Overall, BIO through its effect on the Wnt-Frizzled signaling pathway can promote dopaminergic differentiation of RenVm cells in a dose-dependent manner.

Roof Plate-Derived Radial Glial-like Cells Support Developmental Growth of Rapidly Adapting Mechanoreceptor Ascending Axons

Spinal cord longitudinal axons comprise some of the longest axons in our body. However, mechanisms that drive this extra long-distance axonal growth are largely unclear. We found that ascending axons of rapidly adapting (RA) mechanoreceptors closely abut a previously undescribed population of roof plate-derived radial glial-like cells (RGLCs) in the spinal cord dorsal column, which form a network of processes enriched with growth-promoting factors. In dreher mutant mice that lack RGLCs, the lengths of ascending RA mechanoreceptor axon branches are specifically reduced, whereas their descending and collateral branches, and other dorsal column and sensory pathways, are largely unaffected. Because the number and intrinsic growth ability of RA mechanoreceptors are normal in dreher mice, our data suggest that RGLCs provide critical non-cell autonomous growth support for the ascending axons of RA mechanoreceptors. Together, our work identifies a developmental mechanism specifically required for long-range spinal cord longitudinal axons.

Anti-osteogenic function of a LIM-homeodomain transcription factor LMX1B is essential to early patterning of the calvaria

The calvaria (upper part of the skull) is made of plates of bone and fibrous joints (sutures and fontanelles), and the proper balance and organization of these components are crucial to normal development of the calvaria. In a mouse embryo, the calvaria develops from a layer of head mesenchyme that surrounds the brain from shortly after mid-gestation. The mesenchyme just above the eye (supra-orbital mesenchyme, SOM) generates ossification centers for the bones, which then grow toward the apex gradually. In contrast, the mesenchyme apical to SOM (early migrating mesenchyme, EMM), including the area at the vertex, does not generate an ossification center. As a result, the dorsal midline of the head is occupied by sutures and fontanelles at birth. To date, the molecular basis for this regional difference in developmental programs is unknown. The current study provides vital insights into the genetic regulation of calvarial patterning. First, we showed that osteogenic signals were active in both EMM and SOM during normal development, which suggested the presence of an anti-osteogenic factor in EMM to counter the effect of these signals. Subsequently, we identified Lmx1b as an anti-osteogenic gene that was expressed in EMM but not in SOM. Furthermore, head mesenchyme-specific deletion of Lmx1b resulted in heterotopic ossification from EMM at the vertex, and craniosynostosis affecting multiple sutures. Conversely, forced expression of Lmx1b in SOM was sufficient to inhibit osteogenic specification. Therefore, we conclude that Lmx1b plays a key role as an anti-osteogenic factor in patterning the head mesenchyme into areas with different osteogenic competence. In turn, this patterning event is crucial to generating the proper organization of the bones and soft tissue joints of the calvaria.

Divergent axial morphogenesis and early shh expression in vertebrate prospective floor plate

Background

The notochord has organizer properties and is required for floor plate induction and dorsoventral patterning of the neural tube. This activity has been attributed to sonic hedgehog (shh) signaling, which originates in the notochord, forms a gradient, and autoinduces shh expression in the floor plate. However, reported data are inconsistent and the spatiotemporal development of the relevant shh expression domains has not been studied in detail. We therefore studied the expression dynamics of shh in rabbit, chicken and Xenopus laevis embryos (as well as indian hedgehog and desert hedgehog as possible alternative functional candidates in the chicken).

Results

Our analysis reveals a markedly divergent pattern within these vertebrates: whereas in the rabbit shh is first expressed in the notochord and its floor plate domain is then induced during subsequent somitogenesis stages, in the chick embryo shh is expressed in the prospective neuroectoderm prior to the notochord formation and, interestingly, prior to mesoderm immigration. Neither indian hedgehog nor desert hedgehog are expressed in these midline structures although mRNA of both genes was detected in other structures of the early chick embryo. In X. laevis, shh is expressed at the beginning of gastrulation in a distinct area dorsal to the dorsal blastopore lip and adjacent to the prospective neuroectoderm, whereas the floor plate expresses shh at the end of gastrulation.

Conclusions

While shh expression patterns in rabbit and X. laevis embryos are roughly compatible with the classical view of "ventral to dorsal induction" of the floor plate, the early shh expression in the chick floor plate challenges this model. Intriguingly, this alternative sequence of domain induction is related to the asymmetrical morphogenesis of the primitive node and other axial organs in the chick. Our results indicate that the floor plate in X. laevis and chick embryos may be initially induced by planar interaction within the ectoderm or epiblast. Furthermore, we propose that the mode of the floor plate induction adapts to the variant topography of interacting tissues during gastrulation and notochord formation and thereby reveals evolutionary plasticity of early embryonic induction.

Zebrafish Rfx4 controls dorsal and ventral midline formation in the neural tube

BACKGROUND

Rfx winged-helix transcription factors, best known as key regulators of core ciliogenesis, also play ciliogenesis-independent roles during neural development. Mammalian Rfx4 controls neural tube morphogenesis via both mechanisms.

RESULTS

We set out to identify conserved aspects of rfx4 gene function during vertebrate development and to establish a new genetic model in which to analyze these mechanisms further. To this end, we have generated frame-shift alleles in the zebrafish rfx4 locus using CRISPR/Cas9 mutagenesis. Using RNAseq-based transcriptome analysis, in situ hybridization and immunostaining we identified a requirement for zebrafish rfx4 in the forming midlines of the caudal neural tube. These functions are mediated, least in part, through transcriptional regulation of several zic genes in the dorsal hindbrain and of foxa2 in the ventral hindbrain and spinal cord (floor plate).

CONCLUSIONS

The midline patterning functions of rfx4 are conserved, because rfx4 regulates transcription of foxa2 and zic2 in zebrafish and in mouse. In contrast, zebrafish rfx4 function is dispensable for forebrain morphogenesis, while mouse rfx4 is required for normal formation of forebrain ventricles in a ciliogenesis-dependent manner. Collectively, this report identifies conserved aspects of rfx4 function and establishes a robust new genetic model for in-depth dissection of these mechanisms. Developmental Dynamics 247:650-659, 2018. © 2017 Wiley Periodicals, Inc.

Development of Circumventricular Organs in the Mirror of Zebrafish Enhancer-Trap Transgenics

The circumventricular organs (CVOs) are small structures lining the cavities of brain ventricular system. They are associated with the semitransparent regions of the blood-brain barrier (BBB). Hence it is thought that CVOs mediate biochemical signaling and cell exchange between the brain and systemic blood. Their classification is still controversial and development not fully understood largely due to an absence of tissue-specific molecular markers. In a search for molecular determinants of CVOs we studied the green fluorescent protein (GFP) expression pattern in several zebrafish enhancer trap transgenics including Gateways (ET33-E20) that has been instrumental in defining the development of choroid plexus. In Gateways the GFP is expressed in regions of the developing brain outside the choroid plexus, which remain to be characterized. The neuroanatomical and histological analysis suggested that some previously unassigned domains of GFP expression may correspond to at least six other CVOs–the organum vasculosum laminae terminalis (OVLT), subfornical organ (SFO), paraventricular organ (PVO), pineal (epiphysis), area postrema (AP) and median eminence (ME). Two other CVOs, parapineal and subcommissural organ (SCO) were detected in other enhancer-trap transgenics. Hence enhancer-trap transgenic lines could be instrumental for developmental studies of CVOs in zebrafish and understanding of the molecular mechanism of disease such a hydrocephalus in human. Their future analysis may shed light on general and specific molecular mechanisms that regulate development of CVOs.

Making sense out of spinal cord somatosensory development

The spinal cord integrates and relays somatosensory input, leading to complex motor responses. Research over the past couple of decades has identified transcription factor networks that function during development to define and instruct the generation of diverse neuronal populations within the spinal cord. A number of studies have now started to connect these developmentally defined populations with their roles in somatosensory circuits. Here, we review our current understanding of how neuronal diversity in the dorsal spinal cord is generated and we discuss the logic underlying how these neurons form the basis of somatosensory circuits.

TGF-β Family Signaling in Neural and Neuronal Differentiation, Development, and Function

Signaling by the transforming growth factor β (TGF-β) family is necessary for proper neural development and function throughout life. Sequential waves of activation, inhibition, and reactivation of TGF-β family members regulate numerous elements of the nervous system from the earliest stages of embryogenesis through adulthood. This review discusses the expression, regulation, and function of TGF-β family members in the central nervous system at various developmental stages, beginning with induction and patterning of the nervous system to their importance in the adult as modulators of inflammatory response and involvement in degenerative diseases.

Origin of oligodendrocytes in mammalian forebrains: a revised perspective

Oligodendrocyte precursor cells (OPCs) appear in the late embryonic brain, mature into oligodendrocytes (OLs), and form myelin in the postnatal brain. It has been proposed that early born OPCs derived from the ventral forebrain are eliminated postnatally and late-born OLs predominate in the adult mouse cortex. However, the temporal and regional niche for cortical OL generation, which persists throughout life in adult mammals, remains to be determined. Our recent study provides new insight into self-renewing and multipotent neural stem cells (NSCs). Our results, together with previous studies, suggest that NSCs at the dorsoventral boundary are uniquely specialized to produce myelin-forming OLs in the cortex during a restricted temporal window. These findings may help identify transcription factors or gene expression patterns which confer neural precursors with the characteristic ability of dorsoventral boundary NSCs to differentiate into OLs, and facilitate the development of new strategies for regenerative medicine of the damaged brain.

Dynamics of BMP and Hes1/Hairy1 signaling in the dorsal neural tube underlies the transition from neural crest to definitive roof plate

BACKGROUND

The dorsal midline region of the neural tube that results from closure of the neural folds is generally termed the roof plate (RP). However, this domain is highly dynamic and complex, and is first transiently inhabited by prospective neural crest (NC) cells that sequentially emigrate from the neuroepithelium. It only later becomes the definitive RP, the dorsal midline cells of the spinal cord. We previously showed that at the trunk level of the axis, prospective RP progenitors originate ventral to the premigratory NC and progressively reach the dorsal midline following NC emigration. However, the molecular mechanisms underlying the end of NC production and formation of the definitive RP remain virtually unknown.

RESULTS

Based on distinctive cellular and molecular traits, we have defined an initial NC and a subsequent RP stage, allowing us to investigate the mechanisms responsible for the transition between the two phases. We demonstrate that in spite of the constant production of BMP4 in the dorsal tube at both stages, RP progenitors only transiently respond to the ligand and lose competence shortly before they arrive at their final location. In addition, exposure of dorsal tube cells at the NC stage to high levels of BMP signaling induces premature RP traits, such as Hes1/Hairy1, while concomitantly inhibiting NC production. Reciprocally, early inhibition of BMP signaling prevents Hairy1 mRNA expression at the RP stage altogether, suggesting that BMP is both necessary and sufficient for the development of this RP-specific trait. Furthermore, when Hes1/Hairy1 is misexpressed at the NC stage, it inhibits BMP signaling and downregulates BMPR1A/Alk3 mRNA expression, transcription of BMP targets such as Foxd3, cell-cycle progression, and NC emigration. Reciprocally, Foxd3 inhibits Hairy1, suggesting that repressive cross-interactions at the level of, and downstream from, BMP ensure the temporal separation between both lineages.

CONCLUSIONS

Together, our data suggest that BMP signaling is important both for NC and RP formation. Given that these two structures develop sequentially, we speculate that the longer exposure of RP progenitors to BMP compared with that of premigratory NC cells may be translated into a higher signaling level in the former. This induces changes in responsiveness to BMP, most likely by downregulating the expression of Alk3 receptors and, consequently, of BMP-dependent downstream transcription factors, which exhibit spatial complementary expression patterns and mutually repress each other to generate alternative fates. This molecular dynamic is likely to account for the transition between the NC and definitive RP stages and thus be responsible for the segregation between central and peripheral lineages during neural development.

Development of the Neuroendocrine Hypothalamus

The neuroendocrine hypothalamus is composed of the tuberal and anterodorsal hypothalamus, together with the median eminence/neurohypophysis. It centrally governs wide-ranging physiological processes, including homeostasis of energy balance, circadian rhythms and stress responses, as well as growth and reproductive behaviours. Homeostasis is maintained by integrating sensory inputs and effecting responses via autonomic, endocrine and behavioural outputs, over diverse time-scales and throughout the lifecourse of an individual. Here, we summarize studies that begin to reveal how different territories and cell types within the neuroendocrine hypothalamus are assembled in an integrated manner to enable function, thus supporting the organism's ability to survive and thrive. We discuss how signaling pathways and transcription factors dictate the appearance and regionalization of the hypothalamic primordium, the maintenance of progenitor cells, and their specification and differentiation into neurons. We comment on recent studies that harness such programmes for the directed differentiation of human ES/iPS cells. We summarize how developmental plasticity is maintained even into adulthood and how integration between the hypothalamus and peripheral body is established in the median eminence and neurohypophysis. Analysis of model organisms, including mouse, chick and zebrafish, provides a picture of how complex, yet elegantly coordinated, developmental programmes build glial and neuronal cells around the third ventricle of the brain. Such conserved processes enable the hypothalamus to mediate its function as a central integrating and response-control mediator for the homeostatic processes that are critical to life. Early indications suggest that deregulation of these events may underlie multifaceted pathological conditions and dysfunctional physiology in humans, such as obesity.

An emerging role for prdm family genes in dorsoventral patterning of the vertebrate nervous system

The embryonic vertebrate neural tube is divided along its dorsoventral (DV) axis into eleven molecularly discrete progenitor domains. Each of these domains gives rise to distinct neuronal cell types; the ventral-most six domains contribute to motor circuits, while the five dorsal domains contribute to sensory circuits. Following the initial neurogenesis step, these domains also generate glial cell types—either astrocytes or oligodendrocytes. This DV pattern is initiated by two morphogens—Sonic Hedgehog released from notochord and floor plate and Bone Morphogenetic Protein produced in the roof plate—that act in concentration gradients to induce expression of genes along the DV axis. Subsequently, these DV-restricted genes cooperate to define progenitor domains and to control neuronal cell fate specification and differentiation in each domain. Many genes involved in this process have been identified, but significant gaps remain in our understanding of the underlying genetic program. Here we review recent work identifying members of the Prdm gene family as novel regulators of DV patterning in the neural tube. Many Prdm proteins regulate transcription by controlling histone modifications (either via intrinsic histone methyltransferase activity, or by recruiting histone modifying enzymes). Prdm genes are expressed in spatially restricted domains along the DV axis of the neural tube and play important roles in the specification of progenitor domains, as well as in the subsequent differentiation of motor neurons and various types of interneurons. Strikingly, Prdm proteins appear to function by binding to, and modulating the activity of, other transcription factors (particularly bHLH proteins). The identity of key transcription factors in DV patterning of the neural tube has been elucidated previously (e.g. the nkx, bHLH and pax families), but it now appears that an additional family is also required and that it acts in a potentially novel manner.

Cell fate determination, neuronal maintenance and disease state: The emerging role of transcription factors Lmx1a and Lmx1b

LIM-homeodomain (LIM-HD) proteins are evolutionary conserved developmental transcription factors. LIM-HD Lmx1a and Lmx1b orchestrate complex temporal and spatial gene expression of the dopaminergic pathway, and evidence shows they are also involved in adult neuronal homeostasis. In this review, the multiple roles played by Lmx1a and Lmx1b will be discussed. Controlled Lmx1a and Lmx1b expression and activities ensure the proper formation of critical signaling centers, including the embryonic ventral mesencephalon floor plate and sharp boundaries between lineage-specific cells. Lmx1a and Lmx1b expression persists in mature dopaminergic neurons of the substantia nigra pars compacta and the ventral tegmental area, and their role in the adult brain is beginning to be revealed. Notably, LMX1B expression was lower in brain tissue affected by Parkinson's disease. Actual and future applications of Lmx1a and Lmx1b transcription factors in stem cell production as well as in direct conversion of fibroblast into dopaminergic neurons are also discussed. A thorough understanding of the role of LMX1A and LMX1B in a number of disease states, including developmental diseases, cancer and neurodegenerative diseases, could lead to significant benefits for human healthcare.

Noggin-Mediated Retinal Induction Reveals a Novel Interplay Between Bone Morphogenetic Protein Inhibition, Transforming Growth Factor β, and Sonic Hedgehog Signaling

It has long been known that the depletion of bone morphogenetic protein (BMP) is one of the key factors necessary for the development of anterior neuroectodermal structures. However, the precise molecular mechanisms that underlie forebrain regionalization are still not completely understood. Here, we show that Noggin1 is involved in the regionalization of anterior neural structures in a dose-dependent manner. Low doses of Noggin1 expand prosencephalic territories, while higher doses specify diencephalic and retinal regions at the expense of telencephalic areas. A similar dose-dependent mechanism determines the ability of Noggin1 to convert pluripotent cells in prosencephalic or diencephalic/retinal precursors, as shown by transplant experiments and molecular analyses. At a molecular level, the strong inhibition of BMP signaling exerted by high doses of Noggin1 reinforces the Nodal/transforming growth factor (TGF)β signaling pathway, leading to activation of Gli1 and Gli2 and subsequent activation of Sonic Hedgehog (SHH) signaling. We propose a new role for Noggin1 in determining specific anterior neural structures by the modulation of TGFβ and SHH signaling.

Mesencephalic basolateral domain specification is dependent on Sonic Hedgehog

In the study of central nervous system morphogenesis, the identification of new molecular markers allows us to identify domains along the antero-posterior and dorso-ventral (DV) axes. In the past years, the alar and basal plates of the midbrain have been divided into different domains. The precise location of the alar-basal boundary is still under discussion. We have identified Barhl1, Nhlh1 and Six3 as appropriate molecular markers to the adjacent domains of this transition. The description of their expression patterns and the contribution to the different mesencephalic populations corroborated their role in the specification of these domains. We studied the influence of Sonic Hedgehog on these markers and therefore on the specification of these territories. The lack of this morphogen produced severe alterations in the expression pattern of Barhl1 and Nhlh1 with consequent misspecification of the basolateral (BL) domain. Six3 expression was apparently unaffected, however its distribution changed leading to altered basal domains. In this study we confirmed the localization of the alar-basal boundary dorsal to the BL domain and demonstrated that the development of the BL domain highly depends on Shh.

Association of LMX1A genetic polymorphisms with susceptibility to congenital scoliosis in Chinese Han population

STUDY DESIGN

A genetic association study of single nucleotide polymorphisms (SNPs) for the LMX1A gene with congenital scoliosis (CS) in the Chinese Han population.

OBJECTIVE

To determine whether LMX1A genetic polymorphisms are associated with susceptibility to CS.

SUMMARY OF BACKGROUND DATA

CS is a lateral curvature of the spine due to congenital vertebral defects, whose exact genetic cause has not been well established. The LMX1A gene was suggested as a potential human candidate gene for CS. However, no genetic study of LMX1A in CS has ever been reported.

METHODS

We genotyped 13 SNPs of the LMX1A gene in 154 patients with CS and 144 controls with matched sex and age. After conducting the Hardy-Weinberg equilibrium test, the data of 13 SNPs were analyzed by the allelic and genotypic association with logistic regression analysis. Furthermore, the genotype-phenotype association and haplotype association analysis were also performed.

RESULTS

The 13 SNPs of the LMX1A gene met Hardy-Weinberg equilibrium in the controls, which was not in the cases. None of the allelic and genotypic frequencies of these SNPs showed significant difference between case and control groups (P > 0.05). However, the genotypic frequencies of rs1354510 and rs16841013 in the LMX1A gene were associated with CS predisposition in the unconditional logistic regression analysis (P = 0.02 and 0.018, respectively). Genotypic frequencies of 3 SNPs at rs6671290, rs1354510, and rs16841013 were found to exhibit significant differences between patients with CS with failure of formation and the healthy controls (P = 0.019, 0.007, and 0.006, respectively). Besides, in the model analysis by using unconditional logistic regression analysis, the optimized model for the 3 genotypic positive SNPs with failure of formation were rs6671290 (codominant; P = 0.025, Akaike information value = 316.6, Bayesian information criterion = 333.9), rs1354510 (overdominant; P = 0.0017, Akaike information value = 312.1, Bayesian information criterion = 325.9), and rsl6841013 (overdominant; P = 0.0016, Akaike information value = 311.1, Bayesian information criterion = 325), respectively. However, the haplotype distributions in the case group were not significantly different from those of the control group in the 3 haplotype blocks.

CONCLUSION

To our knowledge, this is the first study to identify that the SNPs of the LMX1A gene might be associated with the susceptibility to CS and different clinical phenotypes of CS in the Chinese Han population.

LEVEL OF EVIDENCE

4.

The cortical hem regulates the size and patterning of neocortex

The cortical hem, a source of Wingless-related (WNT) and bone morphogenetic protein (BMP) signaling in the dorsomedial telencephalon, is the embryonic organizer for the hippocampus. Whether the hem is a major regulator of cortical patterning outside the hippocampus has not been investigated. We examined regional organization across the entire cerebral cortex in mice genetically engineered to lack the hem. Indicating that the hem regulates dorsoventral patterning in the cortical hemisphere, the neocortex, particularly dorsomedial neocortex, was reduced in size in late-stage hem-ablated embryos, whereas cortex ventrolateral to the neocortex expanded dorsally. Unexpectedly, hem ablation also perturbed regional patterning along the rostrocaudal axis of neocortex. Rostral neocortical domains identified by characteristic gene expression were expanded, and caudal domains diminished. A similar shift occurs when fibroblast growth factor (FGF) 8 is increased at the rostral telencephalic organizer, yet the FGF8 source was unchanged in hem-ablated brains. Rather we found that hem WNT or BMP signals, or both, have opposite effects to those of FGF8 in regulating transcription factors that control the size and position of neocortical areas. When the hem is ablated a necessary balance is perturbed, and cerebral cortex is rostralized. Our findings reveal a much broader role for the hem in cortical development than previously recognized, and emphasize that two major signaling centers interact antagonistically to pattern cerebral cortex.

Self-organization of neural tissue architectures from pluripotent stem cells

Despite being a subject of intensive research, the mechanisms underlying the formation of neural tissue architectures during development of the central nervous system remain largely enigmatic. So far, studies into neural pattern formation have been restricted mainly to animal experiments. With the advent of pluripotent stem cells it has become possible to explore early steps of nervous system development in vitro. These studies have unraveled a remarkable propensity of primitive neural cells to self-organize into primitive patterns such as neural tube-like rosettes in vitro. Data from more advanced 3D culture systems indicate that this intrinsic propensity for self-organization can even extend to the formation of complex architectures such as a multilayered cortical neuroepithelium or an entire optic cup. These novel experimental paradigms not only demonstrate the enormous self-organization capacity of neural stem cells, they also provide exciting prospects for studying the earliest steps of human neural tissue development and the pathogenesis of brain malformations in reductionist in vitro paradigms.

Canonical BMP-Smad signalling promotes neurite growth in rat midbrain dopaminergic neurons

Ventral midbrain (VM) dopaminergic (DA) neurons project to the dorsal striatum via the nigrostriatal pathway to regulate voluntary movements, and their loss leads to the motor dysfunction seen in Parkinson's disease (PD). Despite recent progress in the understanding of VM DA neurogenesis, the factors regulating nigrostriatal pathway development remain largely unknown. The bone morphogenetic protein (BMP) family regulates neurite growth in the developing nervous system and may contribute to nigrostriatal pathway development. Two related members of this family, BMP2 and growth differentiation factor (GDF)5, have neurotrophic effects, including promotion of neurite growth, on cultured VM DA neurons. However, the molecular mechanisms regulating their effects on DA neurons are unknown. By characterising the temporal expression profiles of endogenous BMP receptors (BMPRs) in the developing and adult rat VM and striatum, this study identified BMP2 and GDF5 as potential regulators of nigrostriatal pathway development. Furthermore, through the use of noggin, dorsomorphin and BMPR/Smad plasmids, this study demonstrated that GDF5- and BMP2-induced neurite outgrowth from cultured VM DA neurons is dependent on BMP type I receptor activation of the Smad 1/5/8 signalling pathway.

Birth-date dependent arrangement of spinal enkephalinergic neurons: evidence from the preproenkephalin-green fluorescent protein transgenic mice

Enkephalin (ENK) has been postulated to play important roles in modulating nociceptive transmission, and it has been proved that ENKergic neurons acted as a critical component of sensory circuit in the adult spinal cord. Revealing the developmental characteristics of spinal ENKergic neurons will be helpful for understanding the formation and alteration of the sensory circuit under pain status. However, the relationship between the embryonic birth date and the adult distribution of ENKergic neurons has remained largely unknown due to the difficulties in visualizing the ENKergic neurons clearly. Taking advantage of the preproenkephalin-green fluorescent protein (PPE-GFP) transgenic mice in identifying ENKergic neurons, we performed the current birth-dating study and examined the spinal ENKergic neurogenesis. The ENKergic neurons born on different developmental stages and their final location during adulthood were investigated by combining bromodeoxyuridine (BrdU) incorporation and GFP labeling. The spinal ENKergic neurogenesis was restricted at E9.5 to E14.5, and fitted in the same pattern of spinal neurogenesis. Further comparative analysis revealed that spinal ENKergic neurons underwent heterogeneous characteristics. Our study also indicated that the laminar arrangement of ENKergic neurons in the superficial spinal dorsal horn depended on the neurogenesis stages. Taken together, the present study suggested that the birth date of ENKergic neurons is one determinant for their arrangement and function.

Birthdate study of GABAergic neurons in the lumbar spinal cord of the glutamic acid decarboxylase 67-green fluorescent protein knock-in mouse

Despite the abundance of studies on γ-aminobutyric acid (GABA) ergic neuron distribution in the mouse developing spinal cord, no investigation has been devoted so far to their birthdates. In order to determine the spinal neurogenesis of a specific phenotype, the GABAergic neurons in the spinal cord, we injected bromodeoxyuridine (BrdU) at different developmental stages of the glutamic acid decarboxylase (GAD)67-green fluorescent protein (GFP) knock-in mice. We thus used GFP to mark GABAergic neurons and labeled newly born cells with the S-phase marker BrdU at different embryonic stages. Distribution of GABAergic neurons labeled with BrdU was then studied in spinal cord sections of 60-day-old mice. Our birthdating studies revealed that GABAergic neurogenesis was present at embryonic day 10.5 (E10.5). Since then, the generation of GABAergic neurons significantly increased, and reached a peak at E11.5. Two waves for the co-localization of GABA and BrdU in the spinal cord were seen at E11.5 and E13.5 in the present study. The vast majority of GABAergic neurons were generated before E14.5. Thereafter, GABA-positive neuron generation decreased drastically. The present results showed that the birthdates of GABAergic neurons in each lamina were different. The peaks of GABAergic neurogenesis in lamina II were at E11.5 and E13.5, while in lamina I and III, they were at E13.5 and E12.5, respectively. The present results suggest that the birthdates of GABAergic neurons vary in different lamina and follow a specific temporal sequence. This will provide valuable information for future functional studies.