Huntington's disease (HD) is a neurodegenerative disorder caused by abnormal polyglutamine expansion in the amino-terminal end of the huntingtin protein (Htt) and characterized by progressive striatal and cortical pathology. Previous reports have shown that Htt is essential for embryogenesis, and a recent study by our group revealed that the pathogenic form of Htt (mHtt) causes impairments in multiple stages of striatal development. In this study, we have examined whether HD-associated striatal developmental deficits are reflective of earlier maturational alterations occurring at the time of neurulation by assessing differential roles of Htt and mHtt during neural induction and early neurogenesis using an in vitro mouse embryonic stem cell (ESC) clonal assay system. We demonstrated that the loss of Htt in ESCs (KO ESCs) severely disrupts the specification of primitive and definitive neural stem cells (pNSCs, dNSCs, respectively) during the process of neural induction. In addition, clonally derived KO pNSCs and dNSCs displayed impaired proliferative potential, enhanced cell death and altered multi-lineage potential. Conversely, as observed in HD knock-in ESCs (Q111 ESCs), mHtt enhanced the number and size of pNSC clones, which exhibited enhanced proliferative potential and precocious neuronal differentiation. The transition from Q111 pNSCs to fibroblast growth factor 2 (FGF2)-responsive dNSCs was marked by potentiation in the number of dNSCs and altered proliferative potential. The multi-lineage potential of Q111 dNSCs was also enhanced with precocious neurogenesis and oligodendrocyte progenitor elaboration. The generation of Q111 epidermal growth factor (EGF)-responsive dNSCs was also compromised, whereas their multi-lineage potential was unaltered. These abnormalities in neural induction were associated with differential alterations in the expression profiles of Notch, Hes1 and Hes5. These cumulative observations indicate that Htt is required for multiple stages of neural induction, whereas mHtt enhances this process and promotes precocious neurogenesis and oligodendrocyte progenitor cell elaboration.

The vertebrate eye is composed of both surface ectodermal and neuroectodermal derivatives that evaginate laterally from an epithelial anlage of the forming diencephalon. The retina is composed of a limited number of neuronal and non-neuronal cell types and is seen as a model for the brain with reduced complexity. The eye develops in a stereotypic manner building on evolutionarily conserved molecular networks. Eye formation is initiated at the onset of gastrulation by the determination of the eye field in the anterior neuroectoderm. Homeobox transcription factors, in particular Six3 are crucially involved in the establishment and maintenance of retinal identity. The eye field expands by proliferation as gastrulation proceeds and is initially confined to a single retinal primordium by the differential activity of specifying transcription factors. This central field is subsequently split in response to secreted factors emanating from the ventral midline. Concomitant with medio-lateral patterning at the onset of neurulation, morphogenesis sets in and laterally evaginates the optic vesicle. Strikingly during this process the neuroectoderm in the eye field transiently loses epithelial features and cells migrate individually. In a second morphogenetic event, the vesicle is transformed into the optic cup, concomitant with onset and progression of retinal differentiation. Accompanying optic cup morphogenesis, neural differentiation is initiated from a retinal signalling centre in a stereotypic and species specific manner by secreted signalling factors. Here we will give an overview of key events during vertebrate eye formation and highlight key players in the respective processes.

Neural tube defects (NTDs) are complex congenital malformations resulting from incomplete neurulation. Our previous work has demonstrated that motor and sensory neurons develop defectively in rat embryos with spina bifida aperta. To identify whether neural development-associated miRNAs play a role in the neurological deficits of NTDs, we screened a panel of neural development-related miRNAs, including miR-9, miR-9*, miR-124a, miR-10a, miR10b, miR-34a, miR-221 and miR-222, in the spinal cords of rats with retinoic acid-induced spina bifida aperta. We discovered that the expression of miR-9, miR-9* and miR-124a was specifically down-regulated compared to spinal cords without spina bifida. To further clarify whether down-regulation of miR-9* and miR-124a contributes to the neurological deficits of NTDs, we investigated the levels of genes involved in switching in the subunit composition of Swi/Snf-like BAF (Brg/Brm associated factor) complexes modulated by miR-9* and miR-124a and neuronal differentiation. In addition to the down-regulation of miR-9* and miR-124a expression, we also observed increased expression of repressor element silencing transcription factor (REST) and BAF53a and decreased expression of BAF53b, Brg1 and NeuroD1. Our results suggest that REST-regulated miR-9*- and the miR-124a-mediated chromatin remodelling regulatory mechanism may participate in the neuronal deficits of spina bifida.

Formation of the metazoan body plan requires a complex interplay of morphological changes and patterning, and central to these processes is the establishment of apical/basal cell polarity. In the developing nervous system, apical/basal cell polarity is essential for neural tube closure and maintenance of the neural stem cell population. In this report we explore how a signaling pathway important for nervous system development, Notch signaling, impacts on apical/basal cell polarity in neural differentiation. CSL(-/-) mouse embryos, which are devoid of canonical Notch signaling, demonstrated a neural tube phenotype consistent with cell polarity and convergent extension defects, including deficiencies in the restricted expression of apical polarity markers in the neuroepithelium. CSL(-/-) mouse embryonic stem (ES) cells, cultured at low density, behaved as wild-type in the establishment of neural progenitors and apical specification, though progression through rosette formation, an in vitro correlate of neurulation, required CSL for correct maintenance of rosette structure and regulation of neuronal differentiation. Similarly, acute pharmacological inhibition of Notch signaling led to the breakdown of neural rosettes and accelerated neuronal differentiation. In addition to functional Notch signaling, rosette integrity was found to require actin polymerization and Rho kinase (ROCK) activity. Disruption of rosettes through inhibition of actin polymerization or ROCK activity, however, had no effect on neuronal differentiation, indicating that rosette maintenance is not a prerequisite for normal neuronal differentiation. In conclusion, our data indicate that Notch signaling plays a role not only in differentiation, but also in organization and maintenance of polarity during development of the early nervous system.

Pfkfb (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase) enzymes are bi-functional enzymes encoded by four different genes (pfkfb1, pfkfb2, pfkfb3, pfkfb4) in vertebrates. They are involved in the regulation of glycolysis: they catalyze the synthesis and the degradation of F-2,6-BP (fructose-2,6-bisphosphate), the most potent allosteric activator of phosphofructokinase 1 (Pfk1), a key glycolytic enzyme. By producing F-2,6-BP, Pfkfb enzymes allow glycolysis to proceed, while by degrading F-2,6-BP they block glycolysis. As major regulators of glycolysis, Pfkfb enzymes are involved in cancer: tumor cells have a higher glycolytic rate compared to normal cells, even in the presence of adequate oxygen levels (Warburg effect) and several cancer cell lines express elevated levels of Pfkfb enzymes. Glycolysis is also important for energy and metabolite production in proliferating cells. In embryos, however, the role of glycolysis and the expression of glycolysis regulators remain to be explored. Here, we provide a phylogenetic analysis of Pfkfb enzymes in vertebrates, and we detail the expression pattern of pfk1, pfkfb1, pfkfb2, pfkfb3, and pfkfb4 genes in Xenopus laevis embryos. We show that pfkfb transcripts expression is overlapping at blastula and gastrula stages and that from neurulation to tadpole stages, they display tissue-specific, complementary and dynamic expression patterns.

Understanding how and when the left-right (LR) axis is first established is a fundamental question in developmental biology. A popular model is that the LR axis is established relatively late in embryogenesis, due to the movement of motile cilia and the resultant directed fluid flow during late gastrulation/early neurulation. Yet, a large body of evidence suggests that biophysical, molecular, and bioelectrical asymmetries exist much earlier in development, some as early as the first cell cleavage after fertilization. Alternative models of LR asymmetry have been proposed that accommodate these data, postulating that asymmetry is established due to a chiral cytoskeleton and/or the asymmetric segregation of chromatids. There are some similarities, and many differences, in how these various models postulate the origin and timing of symmetry breaking and amplification, and these events' linkage to the well-conserved subsequent asymmetric transcriptional cascades. This review examines experimental data that lend strong support to an early origin of LR asymmetry, yet are also consistent with later roles for cilia in the amplification of LR pathways. In this way, we propose that the various models of asymmetry can be unified: early events are needed to initiate LR asymmetry, and later events could be utilized by some species to maintain LR-biases. We also present an alternative hypothesis, which proposes that individual embryos stochastically choose one of several possible pathways with which to establish their LR axis. These two hypotheses are both tractable in appropriate model species; testing them to resolve open questions in the field of LR patterning will reveal interesting new biology of wide relevance to developmental, cell, and evolutionary biology.

This article narrates the thrilling story of how the pathogenetic understanding of myelomeningocele was fundamentally revised during the last decades and how these new insights, in particular the "two-hit hypothesis," have prepared the terrain for human fetal surgery. Formerly, the devastating cluster of neurologic and neurogenic problems was mainly attributed to the primary malformation, that is, failure of neurulation. At present, there is solid evidence that in early gestation the nonneurulated spinal cord functions well, but suffers from progressive traumatic and degenerative damage in later gestation because it is openly exposed to the amniotic cavity. There is no doubt that the secondary, in utero acquired spinal cord destruction is mainly responsible for the disastrous and irreversible peripheral neurologic deficit present at birth, and there is no doubt either that timely prenatal protective coverage can potentially arrest these deleterious dynamics and preserve neurologic function. Also, tethering of the cord within and constant outflow of cerebrospinal fluid from the lesion are seen as the driving forces behind the Chiari II malformation and consequent ventriculomegaly. Untethering and watertight sealing of the lesion reverses hindbrain herniation and lowers the risk for a relevant hydrocephalus. This article then details how human fetal surgery started in the late 1990s and follows the evolution from the pioneer case studies via the first case series providing encouraging results to the ground breaking Management of Myelomeningocele Study Trial, published in The New England Journal of Medicine in February 2011 by Adzick et al, that has, for the first time, generated unequivocal evidence that patients with prenatal repair do significantly better than those with postnatal care only. Finally, this review looks at several other critical issues, including the hitherto immature endoscopic approach to fetal repair, some future directions of research and clinical practice, and also utters a plea for concentration of these equally rare and complex cases to a few truly qualified centers worldwide. The conclusion derived from all data existing today is that maternal-fetal surgery, although not a cure and not free of risks, represents a novel standard of care for select mothers and their fetuses suffering from one of the most ruinous nonlethal congenital malformations.

Morphogenesis requires developmental processes to occur both at the right time and in the right place. During neural tube formation in the zebrafish embryo, the generation of the apical specializations of the lumen must occur in the center of the neural rod after the neural cells have undergone convergence, invagination and interdigitation across the midline. How this coordination is achieved is uncertain. One possibility is that environmental signaling at the midline of the neural rod controls the schedule of apical polarization. Alternatively, polarization could be regulated by a timing mechanism and then independent morphogenetic processes ensure the cells are in the correct spatial location.Ectopic transplantation demonstrates the local environment of the neural midline is not required for neural cell polarization. Neural cells can self-organize into epithelial cysts in ectopic locations in the embryo and also in three-dimensional gel cultures. Heterochronic transplants demonstrate that the schedule of polarization and the specialized cell divisions characteristic of the neural rod are more strongly regulated by time than local environmental signals. The cells' schedule for polarization is set prior to gastrulation, is stable through several rounds of cell division and appears independent of the morphogenetic movements of gastrulation and neurulation.Time rather than local environment regulates the schedule of epithelial polarization in zebrafish neural rod.

Formation of the neural tube is the morphological hallmark for development of the embryonic central nervous system (CNS). Therefore, neural tube development is a crucial step in the neurulation process. Slit/Robo signaling was initially identified as a chemo-repellent that regulated axon growth cone elongation, but its role in controlling neural tube development is currently unknown. To address this issue, we investigated Slit/Robo1 signaling in the development of chick neCollege of Life Sciences Biocentre, University of Dundee, Dundee DD1 5EH, UKural tube and transgenic mice over-expressing Slit2. We disrupted Slit/Robo1 signaling by injecting R5 monoclonal antibodies into HH10 neural tubes to block the Robo1 receptor. This inhibited the normal development of the ventral body curvature and caused the spinal cord to curl up into a S-shape. Next, Slit/Robo1 signaling on one half-side of the chick embryo neural tube was disturbed by electroporation in ovo. We found that the morphology of the neural tube was dramatically abnormal after we interfered with Slit/Robo1 signaling. Furthermore, we established that silencing Robo1 inhibited cell proliferation while over-expressing Robo1 enhanced cell proliferation. We also investigated the effects of altering Slit/Robo1 expression on Sonic Hedgehog (Shh) and Pax7 expression in the developing neural tube. We demonstrated that over-expressing Robo1 down-regulated Shh expression in the ventral neural tube and resulted in the production of fewer HNK-1(+) migrating neural crest cells (NCCs). In addition, Robo1 over-expression enhanced Pax7 expression in the dorsal neural tube and increased the number of Slug(+) pre-migratory NCCs. Conversely, silencing Robo1 expression resulted in an enhanced Shh expression and more HNK-1(+) migrating NCCs but reduced Pax7 expression and fewer Slug(+) pre-migratory NCCs were observed. In conclusion, we propose that Slit/Robo1 signaling is involved in regulating neural tube development by tightly coordinating cell proliferation and differentiation during neurulation.

The complement system is involved in a range of diverse developmental processes, including cell survival, growth, differentiation, and regeneration. However, little is known about the role of complement in embryogenesis. In this study, we demonstrate a novel role for the canonical complement 5a receptor (C5aR) in the development of the mammalian neural tube under conditions of maternal dietary folic acid deficiency. Specifically, we found C5aR and C5 to be expressed throughout the period of neurulation in wild-type mice and localized the expression to the cephalic regions of the developing neural tube. C5aR was also found to be expressed in the neuroepithelium of early human embryos. Ablation of the C5ar1 gene or the administration of a specific C5aR peptide antagonist to folic acid-deficient pregnant mice resulted in a high prevalence of severe anterior neural tube defect-associated congenital malformations. These findings provide a new and compelling insight into the role of the complement system during mammalian embryonic development.