Neural Precursor Formation Research Articles

Abnormal neural differentiation in response to graphene quantum dots through histone modification interference

Graphene quantum dots (GQDs) have been broadly applied in biomedicine in recent years, and their environmental exposure and toxicological impacts have raised increasing concerns. The nanosafety assessment on the nervous system is one of the most important aspects, and potential effects of GQDs on neurodevelopment and the underlying mechanism are still elusive. In this study, the neural developmental toxicities of OH-GQDs and NH2-GQDs were investigated using the mouse embryonic stem cells (mESCs). The results revealed that OH-GQDs significantly inhibited the ectoderm development, and reduced the neural precursor formation and neurogenesis during the neural differentiation of the mESCs. The exploration on the mechanism uncovered that the increased enrichment of H3K27me3 at the promoter region of the Smad6 gene was involved in histone modification-activated BMP signal pathway, which consequently influenced its regulatory effects on neural differentiation. Additionally, OH-GQDs elicited a stronger effect on inducing the imbalance of histone modification, and resulted in higher latency of neural differentiation disturbance than did NH2-GQDs, suggesting surface functionalization-specific effects of GQDs on neurodevelopmental toxicity. This study would provide new insights in not only the adverse effects of GQDs on neurodevelopment, but also the influence from the chemical modification of GQDs on their bioactivities.

Glial cell progenitors in the Drosophila embryo.

Development and general organization of the nervous system is comparable between insects and vertebrates. Our current knowledge on the formation of neurogenic anlagen and the generation of neural stem cells is deeply influenced by work done in invertebrate model organisms such as Drosophila and Caenorhabditis elegans. It is the aim of this review to summarize the most important steps in neurogenesis in the Drosophila embryo with a special emphasis on glial cell progenitors and the specification of glial cells. Induction of neurogenic regions during early embryogenesis and determination of neural stem cells are briefly described. Special attention is given to the formation of neural precursors called neuroblasts (NB) and their lineages. NBs divide in a stem cell mode to generate a cell clone of either neurons and/or glial cells. The latter require the activation of the transcription factor glial cells missing (gcm), thus providing a binary switch between neuronal and glial cell fates. Further aspects of glial cell specification and the resulting heterogeneity of the glial population in Drosophila are discussed.

Proneural proteins Achaete and Scute associate with nuclear actin to promote external sensory organ formation

Basic helix-loop-helix (bHLH) proneural proteins promote neurogenesis through transcriptional regulation. Although much is known about the tissue-specific regulation of proneural gene expression, how proneural proteins interact with transcriptional machinery to activate downstream target genes is less clear. Drosophila proneural proteins Achaete (Ac) and Scute (Sc) induce external sensory organ formation by activating neural precursor gene expression. Through co-immunoprecipitation and mass spectrometric analyses, we found that nuclear but not cytoplasmic actin associated with the Ac and Sc proteins in Drosophila S2 cells. Daughterless (Da), the common heterodimeric partner of Drosophila bHLH proteins, was observed to associate with nuclear actin through proneural proteins. A yeast two-hybrid assay revealed that the binding specificity between actin and Ac or Sc was conserved in yeast nuclei without the presence of additional Drosophila factors. We further show that actin is required in external sensory organ formation. Reduction in actin gene activity impaired proneural-protein-dependent expression of the neural precursor genes, as well as formation of neural precursors. Furthermore, increased nuclear actin levels, obtained by expression of nucleus-localized actin, elevated Ac-Da-dependent gene transcription as well as Ac-mediated external sensory organ formation. Taken together, our in vivo and in vitro observations suggest a novel link for actin in proneural-protein-mediated transcriptional activation and neural precursor differentiation.

Reelin sets the pace of neocortical neurogenesis

Migration of neurons during cortical development is often assumed to rely on purely post-proliferative reelin signaling. However, Notch signaling, long known to regulate neural precursor formation and maintenance, is required for the effects of reelin on neuronal migration. Here, we show that reelin gain-of-function causes a higher expression of Notch target genes in radial glia and accelerates the production of both neurons and intermediate progenitor cells. Converse alterations correlate with reelin loss-of-function, consistent with reelin controlling Notch signaling during neurogenesis. Ectopic expression of reelin in isolated clones of progenitors causes a severe reduction in neuronal differentiation. In mosaic cell cultures, reelin-primed progenitor cells respond to wild-type cells by further decreasing neuronal differentiation, consistent with an increased sensitivity to lateral inhibition. These results indicate that reelin and Notch signaling cooperate to set the pace of neocortical neurogenesis, a prerequisite for proper neuronal migration and cortical layering.

Reelin sets the pace of neocortical neurogenesis

Migration of neurons during cortical development is often assumed to rely on purely post-proliferative reelin signaling. However, Notch signaling, long known to regulate neural precursor formation and maintenance, is required for the effects of reelin on neuronal migration. Here, we show that reelin gain-of-function causes a higher expression of Notch target genes in radial glia and accelerates the production of both neurons and intermediate progenitor cells. Converse alterations correlate with reelin loss-of-function, consistent with reelin controlling Notch signaling during neurogenesis. Ectopic expression of reelin in isolated clones of progenitors causes a severe reduction in neuronal differentiation. In mosaic cell cultures, reelin-primed progenitor cells respond to wild-type cells by further decreasing neuronal differentiation, consistent with an increased sensitivity to lateral inhibition. These results indicate that reelin and Notch signaling cooperate to set the pace of neocortical neurogenesis, a prerequisite for proper neuronal migration and cortical layering.

The morphological and molecular processes of onychophoran brain development show unique features that are neither comparable to insects nor to chelicerates

The phylogenetic position of onychophorans is still being debated; however, most phylogenies suggest that onychophorans are a sister group to the arthropods. Here we have analysed neurogenesis in the brain of the onychophoran Euperipatoides kanangrensis. We show that the development of the onychophoran brain is considerably different from arthropods. Neural precursors seem to be generated at random positions rather than in distinct spatio-temporal domains as has been shown in insects and chelicerates. The different mode of neural precursor formation is reflected in the homogenous expression of the proneural and neurogenic genes. Furthermore, the morphogenetic events that generate the three-dimensional structure of the onychophoran brain are significantly different from arthropods. Despite the different mode of neural precursor formation in insects and chelicerates (neuroblasts versus neural precursor groups), brain neurogenesis shares more similarities in these arthropods as compared to the onychophoran. Our data show that the developmental processes that generate the brain have considerably diverged in onychophorans and arthropods.

Neurotransmitter Phenotype Differentiation and Synapse Formation of Neural Precursors Engrafting in Amyloid-β1-40 Injured Rat Hippocampus}

Alzheimer's disease (AD) is characterized by the dysfunction or loss of a vulnerable group of neurons. At present, only a few options exist for treating neurodegenerative diseases effectively. Advances in stem cell research have raised the hope and possibility for therapy in neurodegenerative diseases. In AD transgenic animal models, stem cell transplantation has been demonstrated to reverse behavioral deficits. Our recent study demonstrates that neural precursor cells, derived from embryonic stem (ES) cells, improve memory dysfunction in rats caused by injections of amyloid-β peptide (1-40) (Aβ₁₋₄₀) in the dorsal hippocampus. However, the underlying mechanisms remain unknown. The present study tests a murine ES cell-based transplantation approach in rats subjected to Aβ₁₋₄₀ injection into the hippocampus dentate gyrus. Efficacy of cell therapy with regard to graft survival, neuronal yield and diversity, synapse formation of the grafted cells, and the behavioral improvements was determined after transplanting ES cell-derived neural precursors into the hippocampus of adult rats. Here, we show that grafted cells can survive, and differentiate with high yield into immunohistochemically mature glial cells and neurons of diverse neurotransmitter-subtypes. More importantly, transplanted cells demonstrate characteristics of proper synapse formation between host and grafted neural cells. Thus, our observations show that an ES cell-based transplantation approach may be promising in the treatment of AD.

L-Proline as a modulator of ectodermal differentiation in ES cells. Focus on “l-Proline induces differentiation of ES cells: a novel role for an amino acid in the regulation of pluripotent cells in culture

current research is focusing on defining the control of pluripotency and lineage determination of pluripotent cells, particularly of embryonic stem (ES) cells. ES cells are usually produced from the inner cell mass (ICM) of the blastocyst and are considered equivalent to ICM cells captured during a

Linking pattern formation to cell-type specification: Dichaete and Ind directly repress achaete gene expression in the Drosophila CNS

Mechanisms regulating CNS pattern formation and neural precursor formation are remarkably conserved between Drosophila and vertebrates. However, to date, few direct connections have been made between genes that pattern the early CNS and those that trigger neural precursor formation. Here, we use Drosophila to link directly the function of two evolutionarily conserved regulators of CNS pattern along the dorsoventral axis, the homeodomain protein Ind and the Sox-domain protein Dichaete, to the spatial regulation of the proneural gene achaete (ac) in the embryonic CNS. We identify a minimal achaete regulatory region that recapitulates half of the wild-type ac expression pattern in the CNS and find multiple putative Dichaete-, Ind-, and Vnd-binding sites within this region. Consensus Dichaete sites are often found adjacent to those for Vnd and Ind, suggesting that Dichaete associates with Ind or Vnd on target promoters. Consistent with this finding, we observe that Dichaete can physically interact with Ind and Vnd. Finally, we demonstrate the in vivo requirement of adjacent Dichaete and Ind sites in the repression of ac gene expression in the CNS. Our data identify a direct link between the molecules that pattern the CNS and those that specify distinct cell-types.

The expression pattern of genes involved in early neurogenesis suggests distinct and conserved functions in the diplopod Glomeris marginata

We have shown recently that the expression and function of proneural genes is conserved in chelicerates and myriapods, although groups of neural precursors are specified in the ventral neuroectoderm of these arthropod groups, rather than single cells as in insects and crustaceans. We present additional evidence that the pattern of neurogenesis seen in chelicerates and in previously analyzed myriapod species is representative of both arthropod groups, by analysing the formation of neural precursors in the diplopod Archispirostreptus sp. This raises the question as to what extent the genetic network has been modified to result in different modes of neurogenesis in the arthropod group. To find out which components of the neural genetic network might account for the different mode of neural precursor formation in chelicerates and myriapods, we identified genes in the diplopod Glomeris marginata that are known to be involved in early neurogenesis in Drosophila and studied their expression pattern. In Drosophila, early neurogenesis is controlled by proneural genes that encode HLH transcription factors. These genes belong to two major subfamilies, the achaete-scute group and the atonal group. Different proneural proteins activate both a common neural programme and distinct neuronal subtype-specific target genes. We show that the expression pattern of homologs of the Drosophila proneural genes daughterless, atonal, and Sox B1 are partially conserved in Glomeris mariginata. While the expression of the pan-neural gene snail is conserved in the ventral neuroectoderm of G. marginata, we found an additional expression domain in the ventral midline. We conclude that, although the components of the genetic network involved in specification of neural precursors seem to be conserved in chelicerates, myriapods, and Drosophila, the function of some of the genes might have changed during evolution.

Neurogenesis in myriapods and chelicerates and its importance for understanding arthropod relationships

Several alternative hypotheses on the relationships between the major arthropod groups are still being discussed. We reexamine here the chelicerate/myriapod relationship by comparing previously published morphological data on neurogenesis in the euarthropod groups and presenting data on an additional myriapod (Strigamia maritima). Although there are differences in the formation of neural precursors, most euarthropod species analyzed generate about 30 single neural precursors (insects/crustaceans) or precursor groups (chelicerates/myriapods) per hemisegment that are arranged in a regular pattern. The genetic network involved in recruitment and specification of neural precursors seems to be conserved among euarthropods. Furthermore, we show here that neural precursor identity seems to be achieved in a similar way. Besides these conserved features we found 2 characters that distinguish insects/crustaceans from myriapods/chelicerates. First, in insects and crustaceans the neuroectoderm gives rise to epidermal and neural cells, whereas in chelicerates and myriapods the central area of the neuroectoderm exclusively generates neural cells. Second, neural cells arise by stem-cell-like divisions of neuroblasts in insects and crustaceans, whereas groups of mainly postmitotic neural precursors are recruited for the neural fate in chelicerates and myriapods. We discuss whether these characteristics represent a sympleisiomorphy of myriapods and chelicerates that has been lost in the more derived Pancrustacea or whether these characteristics are a synapomorphy of myriapods and chelicerates, providing the first morphological support for the Myriochelata group.

Specification of neural precursor identity in the geophilomorph centipede Strigamia maritima

Despite differences in the formation of neural precursors, all arthropod species analyzed so far generate about 30 single precursors (insects/crustaceans) or precursor groups (chelicerates/myriapods) per hemi-segment. In Drosophila, each precursor has a distinct identity conferred by segment polarity and dorso-ventral patterning genes that subdivide the ventral neuroectoderm into a grid-like structure. Temporal patterning mechanisms generate additional diversity after delamination from the neuroectoderm. Previous work shows that the genetic network involved in recruitment and specification of neural precursors is conserved in arthropods. However, comparative studies on generation of precursor diversity are few and partial. Here, we test whether aspects of the Drosophila model may apply in the geophilomorph centipede Strigamia maritima. We describe precursor formation, based on morphology and on Delta and Notch expression. We then show that in S. maritima, hunchback and Krüppel are expressed in subsets of neural precursors generating distinct temporal expression domains within the plane of the neuroectoderm. This expression pattern suggests that temporal changes in spatial patterning cues may result in the ordered production of different neural identities. We suggest that temporal patterning mechanisms were present in the last common ancestor of arthropods, although the regulatory interactions of transcription factors might have diverged in the lineage leading to insects.

The Tribolium columnar genes reveal conservation and plasticity in neural precursor patterning along the embryonic dorsal–ventral axis

The Drosophila columnar genes are key regulators of neural precursor formation and patterning along the dorsal–ventral axis of the developing CNS and include ventral nerve cord defective ( vnd), intermediate nerve cord defective ( ind), muscle segment homeodomain ( msh), and Epidermal growth factor receptor ( Egfr). To investigate the evolution of neural pattern formation, we identified and determined the expression patterns of Tribolium vnd, ind, and msh, and found that they are expressed in the medial, intermediate, and lateral columns of the developing CNS, respectively, in patterns similar, but not identical, to their Drosophila orthologs. The pattern of Egfr activity suggests that the genetic regulatory mechanisms that initiate Tc-vnd expression are similar in Drosophila and Tribolium, whereas those that initiate Tc-ind have diverged. RNAi analyses of gene function show that Tc-vnd and Tc-ind promote the formation of medial and intermediate column neural precursors and that vnd-mediated repression of ind establishes the boundary between the medial and intermediate columns. These data suggest that columnar gene expression and function underlie neural pattern formation in Drosophila, Tribolium, and potentially all insects, but that subtle spatiotemporal differences in expression of these genes may produce species-specific morphological differences.

Differentiation of embryonic stem cells to a neural fate: a route to re-building the nervous system?

The many and varied proposed applications of cell replacement therapies in the treatment of human disease states, particularly those arising from cell loss or dysfunction, have been discussed widely in both the scientific and popular press. Although an attractive concept, cell therapies require the development of a readily available source of donor cells suitable for transplantation. Embryonic stem (ES) cells, with proven ability to differentiate to all cell populations of the embryo and adult in vitro, provide a potential source of therapeutic cells. The differentiation capability of mouse ES cells in vitro has been studied extensively over the last 20 years and the formation of neural precursors and neural cell lineages from mouse ES cells is well established. Cell populations highly enriched/homogenous in neural precursors have been achieved using a variety of chemical or biological inducing agents coupled with selective growth conditions. Preliminary reports suggest that similar neural enrichment is seen when these methodologies are applied to primate and human ES cells. ES cell-derived neural precursors have been analyzed in vitro and in vivo and found to be functionally normal and, after introduction into rodent models of human neurodegenerative diseases, capable of effecting measurable disease recovery. We review progress in the formation of neural precursors from mouse ES cells, particularly the recent reports of directed differentiation of ES in response to biological inductive factors, and assess the transfer of these approaches to human ES cells.

Secondary neurons are arrested in an immature state by formation of epithelial vesicles during neurogenesis of the spider Cupiennius salei

BackgroundIn the spider Cupiennius salei about 30 groups of neural precursors are generated per hemi-segment during early neurogenesis. Analysis of the ventral neuromeres after invagination of the primary neural precursor groups revealed that secondary neural precursors arise during late embryogenesis that partially do not differentiate until larval stages.ResultsIn contrast to the primary groups, the secondary invaginating cells do not detach from each other after invagination but maintain their epithelial character and form so-called epithelial vesicles. As revealed by dye labeling, secondary neural precursors within epithelial vesicles do not show any morphological features of differentiation indicating that the formation of epithelial vesicles after invagination leads to a delay in the differentiation of the corresponding neural precursors. About half of the secondary neural precursor groups do not dissociate from each other during embryogenesis indicating that they provide neural precursors for larval and adult stages.ConclusionsSecondary neural precursors are arrested in an immature state by formation of epithelial vesicles. This mechanism facilitates the production of larval neural precursors during embryogenesis. I discuss the evolutionary changes that have occured during neural precursor formation in the arthropod group and present a model for the basal mode of neurogenesis.

The expression and function of the achaete-scute genes in Tribolium castaneum reveals conservation and variation in neural pattern formation and cell fate specification.

The study of achaete-scute (ac/sc) genes has recently become a paradigm to understand the evolution and development of the arthropod nervous system. We describe the identification and characterization of the ac/sc genes in the coleopteran insect species Tribolium castaneum. We have identified two Tribolium ac/sc genes - achaete-scute homolog (Tc-ASH) a proneural gene and asense (Tc-ase) a neural precursor gene that reside in a gene complex. Focusing on the embryonic central nervous system we find that Tc-ASH is expressed in all neural precursors and the proneural clusters from which they segregate. Through RNAi and misexpression studies we show that Tc-ASH is necessary for neural precursor formation in Tribolium and sufficient for neural precursor formation in Drosophila. Comparison of the function of the Drosophila and Tribolium proneural ac/sc genes suggests that in the Drosophila lineage these genes have maintained their ancestral function in neural precursor formation and have acquired a new role in the fate specification of individual neural precursors. Furthermore, we find that Tc-ase is expressed in all neural precursors suggesting an important and conserved role for asense genes in insect nervous system development. Our analysis of the Tribolium ac/sc genes indicates significant plasticity in gene number, expression and function, and implicates these modifications in the evolution of arthropod neural development.

Comparative analysis of neurogenesis in the myriapod Glomeris marginata (Diplopoda) suggests more similarities to chelicerates than to insects.

Molecular data suggest that myriapods are a basal arthropod group and may even be the sister group of chelicerates. To find morphological indications for this relationship we have analysed neurogenesis in the myriapod Glomeris marginata (Diplopoda). We show here that groups of neural precursors, rather than single cells as in insects, invaginate from the ventral neuroectoderm in a manner similar to that in the spider: invaginating cell groups arise sequentially and at stereotyped positions in the ventral neuroectoderm of Glomeris, and all cells of the neurogenic region seem to enter the neural pathway. Furthermore, we have identified an achaete-scute, a Delta and a Notch homologue in GLOMERIS: The genes are expressed in a pattern similar to the spider homologues and show more sequence similarity to the chelicerates than to the insects. We conclude that the myriapod pattern of neural precursor formation is compatible with the possibility of a chelicerate-myriapod sister group relationship.

Control of neural precursor specification by proneural proteins in the CNS of Drosophila.

Formation of neural precursors in Drosophila is determined by proneural genes. The distinctive pattern of expression of some genes of the achaete-scute complex in the embryonic neuroectoderm has prompted the speculation that they could also function in the specification of neural precursor identity in the CNS. To test this hypothesis, we have analysed the capacity of different proneural proteins to promote the development of a particular CNS precursor, the MP2 precursor. Our results indicate that: (i) all known proneural proteins are similarly able to support the formation of a neural precursor at the position of MP2; (ii) different proneural proteins promote the expression of different characteristics of MP2; and (iii) a totally normal specification of the MP2 fate can only be attained by the proneural genes achaete or scute.

The achaete–scute complex proneural genes contribute to neural precursor specification in the Drosophila CNS

Background The Drosophilacentral nervous system (CNS) develops from a segmentally reiterated array of 30 neural precursors. Each precursor acquires a unique identity and goes through a stereotyped cell lineage to produce an invariant family of neurons and/or glia. The proneural genes achaete, scuteand lethal of scute are required for neural precursor formation in the DrosophilaCNS, and are expressed in overlapping subsets of ‘proneural cell clusters’ from which a single neural precursor later develops. Vertebrate achaete–scutehomologues are expressed early during neurogenesis, and promote neurogenesis, neuronal development and/or differentiation. The Drosophilaproneural achaete–scutegenes govern neural precursor formation, but their role in specifying neural precursor identity has not been tested.Results Here, we test the role of the Drosophila achaete–scutegenes in specifying neural precursor identity, focusing on the well characterized CNS MP2 precursor. MP2 delaminates from a cluster of achaete–scute-expressing ectodermal cells. In an achaete–scutedouble mutant, MP2 formation was reduced (to 11–14 %) as expected because of the function of proneural genes in promoting neural precursor formation. Surprisingly, we also observed that the developing MP2 precursors were incorrectly specified and acquired traits characteristic of adjacent neural precursors. In rescue experiments, achaeteor scute, but not lethal of scute, completely restored the normal MP2 identity.Conclusions These results demonstrate that the achaete–scute complex genes specify aspects of neural precursor identity in the DrosophilaCNS. Given the phylogenetically conserved function of these genes, our results raise the possibility that achaete–scutehomologues may help specify neural precursor identity in other organisms.