Wnt signaling and its downstream target N-myc regulate basal progenitors in the developing neocortex

The responses of neural stem cells to the level of GSK-3 depend on the tissue of origin

Neural stem cells (NSCs) can be obtained from a variety of sources, but not all NSCs exhibit the same characteristics. We have examined how the level of glycogen synthase kinase-3 activity regulates NSCs obtained from different sources: the mouse embryonic striatum, embryonic hippocampus, and mouse ES cells. Growth of striatal NSCs is enhanced by mild inhibition of GSK-3 but not by strong inhibition that is accompanied by Wnt/TCF transcriptional activation. In contrast, the growth of hippocampal NSCs is enhanced by both mild inhibition of GSK-3 as well as stronger inhibition. Active Wnt/TCF signaling, which occurs normally in the embryonic hippocampus, is required for growth of neural stem and progenitor cells. In the embryonic striatal germinal zone, however, TCF signaling is normally absent and its activation inhibits growth of NSCs from this region. Using a genetic model for progressive loss of GSK-3, we find that primitive ES cell-derived NSCs resemble striatal NSCs. That is, partial loss of GSK-3 alleles leads to an increase in NSCs while complete ablation of GSK-3, and activation of TCF-signaling, leads to their decline. Furthermore, expression of dominant negative TCF-4 in the GSK-3-null background was effective in blocking expression of Wnt-response genes and was also able to rescue neuronal gene expression. These results reveal that GSK-3 regulates NSCs by divergent pathways depending on the tissue of origin. The responses of these neural precursor cells may be contingent on baseline Wnt/TCF signaling occurring in a particular tissue.

Systematic screening of a Drosophila ORF library in vivo uncovers Wnt/Wg pathway components

We created a site-directed UAS-ORF library of 655 growth-regulating genes in Drosophila. This library represents a large collection of genes regulating cell cycle, cell size, and proliferation and will be a valuable resource for studying growth regulation in vivo. By using misexpression of genes, we prevent problems arising from genetic redundancy and can uncover novel gene functions. To validate the usefulness of this library, we screened for Wingless (Wg) pathway components. We used a combination of experimental and bioinformatic approaches to predict candidates and identified three serine/threonine kinases as regulators of Wg signaling. We show that one of these, Nek2, optimizes pathway response by direct phosphorylation of Dishevelled. In addition, we describe functional relations for roughly 5% of all Drosophila genes and identify a large number of genes that regulate cell size, proliferation, and final organ size upon misexpression.

p53, SKP2, and DKK3 as MYCN Target Genes and Their Potential Therapeutic Significance

Neuroblastoma is the most common extra-cranial solid tumor of childhood. Despite significant advances, it currently still remains one of the most difficult childhood cancers to cure, with less than 40% of patients with high-risk disease being long-term survivors. MYCN is a proto-oncogene implicated to be directly involved in neuroblastoma development. Amplification of MYCN is associated with rapid tumor progression and poor prognosis. Novel therapeutic strategies which can improve the survival rates whilst reducing the toxicity in these patients are therefore required. Here we discuss genes regulated by MYCN in neuroblastoma, with particular reference to p53, SKP2, and DKK3 and strategies that may be employed to target them.

HMGA regulates the global chromatin state and neurogenic potential in neocortical precursor cells

Neural precursor cells (NPCs) in the mouse neocortex generate various neuronal and glial cell types in a developmental stage–dependent manner. Most NPCs lose their neurogenic potential during development, although the underlying mechanisms of this process are not fully understood. We found that the chromatin of mouse NPCs gradually becomes more condensed and less dynamic on a global scale during neocortical development. Furthermore, we found high mobility group A (HMGA) proteins to be essential for the open chromatin state of NPCs at early developmental stages. Knockdown of HMGA proteins in early-stage NPCs reduced their neurogenic potential. Conversely, overexpression of HMGA proteins conferred neurogenic potential on late-stage NPCs, an effect that was antagonized by coexpression of a histone H1 mutant that inhibits chromatin opening. Thus, HMGA proteins contribute to the neurogenic potential of NPCs in the early stages of neocortical development, possibly through induction of an open chromatin state.

Molecular control of neurogenesis: a view from the mammalian cerebral cortex

The mammalian nervous system is the most complex organ of any living organism. How this complexity is generated during neural development is just beginning to be elucidated. This article discusses the signaling, transcriptional, and epigenetic mechanisms that are involved in neural development. The first part focuses on molecules that control neuronal numbers through regulation of the timing of onset of neurogenesis, the timing of the neuronal-to-glial switch, and the rate of progenitor proliferation. The second part focuses on molecules that control neuronal diversity by generating spatially or temporally distinct populations of neuronal progenitors. Most of the studies discussed in this article are focused on the developing mammalian cerebral cortex, because this is one of the main model systems for neural developmental studies and many of the mechanisms identified in this tissue also operate elsewhere in the developing brain and spinal cord.

Wnt signaling in vertebrate neural development and function

Members of the Wnt family of secreted signaling proteins influence many aspects of neural development and function. Wnts are required from neural induction and axis formation to axon guidance and synapse development, and even help modulate synapse activity. Wnt proteins activate a variety of downstream signaling pathways and can induce a similar variety of cellular responses, including gene transcription changes and cytoskeletal rearrangements. This review provides an introduction to Wnt signaling pathways and discusses current research on their roles in vertebrate neural development and function.

Wnt signaling and forebrain development

Components of the Wnt signaling pathway are expressed in a tightly regulated and spatially specific manner during development of the forebrain, and Wnts are key regulators of regional forebrain identity. Wnt signaling from the cortical hem regulates the expansion and cell-type specification of the adjacent neuroepithelium and, in conjunction with Bmp, Fgf, and Shh signaling, controls dorsal-ventral forebrain patterning. Subsequently, Wnt signaling dynamically regulates the behavior of cortical progenitor cells, initially promoting the expansion of radial glia progenitor cells and later inducing neurogenesis by promoting terminal differentiation of intermediate progenitor cells. A role for Wnt signaling in cell-type specification has also been proposed.

Cortical neurogenesis and morphogens: diversity of cues, sources and functions

The cerebral cortex is composed of hundreds of different types of neurons, which underlie its ability to perform highly complex neural processes. How cortical neurons are generated during development constitutes a major challenge in developmental neurosciences with important implications for brain repair and diseases. Cortical neurogenesis is dependent on intrinsic and extrinsic cues, which interplay to generate cortical neurons at the right number, time and place. While the role of intrinsic factors such as proneural and Notch genes has been well established, recent evidence indicate that most classical morphogens, produced by various neural and non-neural sources throughout embryonic development, contribute to the master control and fine tuning of cortical neurogenesis. Here we review some recent advances in the dissection of the molecular logic underlying neurogenesis in the cortex, with special emphasis on the roles of morphogenic cues in this process.

Genome-wide analysis of a Wnt1-regulated transcriptional network implicates neurodegenerative pathways

Wnt proteins are critical to mammalian brain development and function. The canonical Wnt signaling pathway involves the stabilization and nuclear translocation of β-catenin; however, Wnt also signals through alternative, noncanonical pathways. To gain a systems-level, genome-wide view of Wnt signaling, we analyzed Wnt1-stimulated changes in gene expression by transcriptional microarray analysis in cultured human neural progenitor (hNP) cells at multiple time points over a 72-hour time course. We observed a widespread oscillatory-like pattern of changes in gene expression, involving components of both the canonical and the noncanonical Wnt signaling pathways. A higher-order, systems-level analysis that combined independent component analysis, waveform analysis, and mutual information-based network construction revealed effects on pathways related to cell death and neurodegenerative disease. Wnt effectors were tightly clustered with presenilin1 (PSEN1) and granulin (GRN), which cause dominantly inherited forms of Alzheimer's disease and frontotemporal dementia (FTD), respectively. We further explored a potential link between Wnt1 and GRN and found that Wnt1 decreased GRN expression by hNPs. Conversely, GRN knockdown increased WNT1 expression, demonstrating that Wnt and GRN reciprocally regulate each other. Finally, we provided in vivo validation of the in vitro findings by analyzing gene expression data from individuals with FTD. These unbiased and genome-wide analyses provide evidence for a connection between Wnt signaling and the transcriptional regulation of neurodegenerative disease genes.

Ror-family receptor tyrosine kinases regulate maintenance of neural progenitor cells in the developing neocortex

The Ror-family of receptor tyrosine kinases (RTKs), Ror1 and Ror2, have been shown to play crucial roles in the developmental morphogenesis by acting as receptors or co-receptors to mediate Wnt5a-induced signaling. Although Ror1, Ror2, and Wnt5a are expressed in the developing brain, little is known about their roles in the neural development. Here we show that Ror1, Ror2, and their ligand Wnt5a are highly expressed in neocortical neural progenitor cells (NPCs). siRNA-mediated suppression of Ror1, Ror2, or Wnt5a in cultured NPCs isolated from embryonic neocortex results in the reduction of βIII-tubulin-positive neurons that are produced from NPCs possibly through the generation of T-box brain 2 (Tbr2)-positive intermediate progenitors. BrdU-labeling experiments further reveal that proportion of proliferative and neurogenic NPCs, that are positive for neural progenitor cell marker (Pax6), but negative for glial cell marker (glial fibrillary acidic protein; GFAP), is reduced within a few days in culture following knockdown of these molecules, suggesting that Ror1, Ror2, and Wnt5a regulate neurogenesis through the maintenance of NPCs. Moreover, we show that Dishevelled2 (Dvl2) is involved in Wnt5a–Ror1 and Wnt5a–Ror2 signalings in NPCs, and that suppressed expression of Dvl2 indeed reduces the proportion of proliferative and neurogenic NPCs. Interestingly, suppressed or forced expression of either Ror1 or Ror2 in NPCs in the developing neocortex results in their precocious or delayed differentiation into neurons, respectively. Collectively, these results indicate that Wnt5a–Ror1 and Wnt5a–Ror2 signalings play roles in maintaining proliferative and neurogenic NPCs during neurogenesis of the developing neocortex.

Reduced ventricular proliferation in the foetal cortex following maternal inflammation in the mouse

It has been well established that maternal inflammation during pregnancy alters neurological function in the offspring, but its impact on cortical development and long-term consequences on the cytoarchitecture is largely unstudied. Here we report that lipopolysaccharide-induced systemic maternal inflammation in C57Bl/6 mice at embryonic Day 13.5 of pregnancy, as early as 8 h after challenge, caused a significant reduction in cell proliferation in the ventricular zone of the developing cerebral cortex, as revealed by quantification of anti-phospho-Histone H3 immunoreactivity and bromodeoxyuridine pulse labelling. The angle of mitotic cleavage, determined from analysis of haematoxylin and eosin staining, cyclin E1 gene expression and the pattern of β-catenin immunoreactivity were also altered by the challenge, which suggests a change from symmetric to asymmetric division in the radial progenitor cells. Modifications of cortical lamination and gene expression patterns were detected at post-natal Day 8 suggesting prolonged consequences of these alterations during embryonic development. Cellular uptake of proteins from the cerebrospinal fluid was observed in brains from lipopolysaccharide-treated animals in radial progenitor cells. However, the foetal blood–brain barrier to plasma proteins remained intact. Together, these results indicate that maternal inflammation can disrupt the ventricular surface and lead to decreased cellular proliferation. Changes in cell density in Layers IV and V at post-natal Day 8 show that these initial changes have prolonged effects on cortical organization. The possible shift in the fate of progeny and the resulting alterations in the relative cell numbers in the cerebral cortex following a maternal inflammatory response shown here will require further investigation to determine the long-term consequences of inflammation on the development of neuronal circuitry and behaviour.

Effect of environmental factor influencing the development of mouse cerebral cortex

The cerebral cortex is organized into six cell layers, each of which contains neurons with similar morphology, functions, gene-expression profiles, and projection patterns. These layer-specific neuronal phenotypes are sequentially generated from common cortical progenitor cells in the ventricular zone of dorsal telencephalon. Although recent investigations have clarified important roles of intrinsic factors such as transcription factors and regulators of the cell cycle for the maturation of cortical progenitors, growth factors and neurotrophic factors environmentally supplied by the cerebral cortex are thought to regulate proliferation and neural development and determine neuronal differentiation in the cerebral cortex. In this review, I focus on the function of neurotrophin-family neurotrophic factor, including nerve growth factor, brain-derived neurotrophic factor (BDNF), neurotropin-3 (NT-3) and neurotrophin-4 in the formation of the neuronal layer of the cerebral cortex. Especially, BDNF and NT-3 are expressed in the proliferating cortical progenitors and influence the biological properties of cortical progenitors prior to neurogenesis and play distinct roles in generation of cortical neuronal subtypes.

Neurogenesis at the brain-cerebrospinal fluid interface

Cerebral cortical progenitor cells can be classified into several different types, and each progenitor type integrates cell-intrinsic and cell-extrinsic cues to regulate neurogenesis. On one hand, cell-intrinsic mechanisms that depend upon appropriate apical-basal polarity are established by adherens junctions and apical complex proteins and are particularly important in progenitors with apical processes contacting the lateral ventricle. The apical protein complexes themselves are concentrated at the ventricular surface, and apical complex proteins regulate mitotic spindle orientation and cell fate. On the other hand, remarkably little is known about how cell-extrinsic cues signal to progenitors and couple with cell-intrinsic mechanisms to instruct neurogenesis. Recent research shows that the cerebrospinal fluid, which contacts apical progenitors at the ventricular surface and bathes the apical complex of these cells, provides growth- and survival-promoting cues for neural progenitor cells in developing and adult brain. This review addresses how the apical-basal polarity of progenitor cells regulates cell fate and allows progenitors to sample diffusible signals distributed by the cerebrospinal fluid. We also review several classes of signaling factors that the cerebrospinal fluid distributes to the developing brain to instruct neurogenesis.

c-myc and N-myc promote active stem cell metabolism and cycling as architects of the developing brain

myc genes are associated with a wide variety of human cancers including most types of nervous system tumors. While the mechanisms by which myc overexpression causes tumorigenesis are multifaceted and have yet to be clearly elucidated, they are at least in part related to endogenous myc function in normal cells. Knockout (KO) of either c-myc or N-myc genes in neural stem and precursor cells (NSC) driven by nestin-cre impairs mouse brain growth and mutation of N-myc also causes microcephaly in humans in Feingold Syndrome. To further define myc function in NSC and nervous system development, we created a double KO (DKO) for c- and N-myc using nestin-cre. The DKO mice display profoundly impaired overall brain growth associated with decreased cell cycling and migration of NSC, which are strikingly decreased in number. The DKO brain also exhibits specific changes in gene expression including downregulation of genes involved in protein and nucleotide metabolism, mitosis, and chromatin structure as well as upregulation of genes associated with differentiation. Together these data support a model of nervous system tumorigenesis in which excess myc aberrantly locks in a developmentally active chromatin state characterized by overactive cell cycling, and metabolism as well as blocked differentiation.

WNT protein-independent constitutive nuclear localization of beta-catenin protein and its low degradation rate in thalamic neurons

Nuclear localization of β-catenin is a hallmark of canonical Wnt signaling, a pathway that plays a crucial role in brain development and the neurogenesis of the adult brain. We recently showed that β-catenin accumulates specifically in mature thalamic neurons, where it regulates the expression of the Ca(v)3.1 voltage-gated calcium channel gene. Here, we investigated the mechanisms underlying β-catenin accumulation in thalamic neurons. We report that a lack of soluble factors produced either by glia or cortical neurons does not impair nuclear β-catenin accumulation in thalamic neurons. We next found that the number of thalamic neurons with β-catenin nuclear localization did not change when the Wnt/Dishevelled signaling pathway was inhibited by Dickkopf1 or a dominant negative mutant of Dishevelled3. These results suggest a WNT-independent cell-autonomous mechanism. We found that the protein levels of APC, AXIN1, and GSK3β, components of the β-catenin degradation complex, were lower in the thalamus than in the cortex of the adult rat brain. Reduced levels of these proteins were also observed in cultured thalamic neurons compared with cortical cultures. Finally, pulse-chase experiments confirmed that cytoplasmic β-catenin turnover was slower in thalamic neurons than in cortical neurons. Altogether, our data indicate that the nuclear localization of β-catenin in thalamic neurons is their cell-intrinsic feature, which was WNT-independent but associated with low levels of proteins involved in β-catenin labeling for ubiquitination and subsequent degradation.

Inhibition of glycogen synthase kinase-3 enhances the differentiation and reduces the proliferation of adult human olfactory epithelium neural precursors

The olfactory epithelium (OE) contains neural precursor cells which can be easily harvested from a minimally invasive nasal biopsy, making them a valuable cell source to study human neural cell lineages in health and disease. Glycogen synthase kinase-3 (GSK-3) has been implicated in the etiology and treatment of neuropsychiatric disorders and also in the regulation of murine neural precursor cell fate in vitro and in vivo. In this study, we examined the impact of decreased GSK-3 activity on the fate of adult human OE neural precursors in vitro. GSK-3 inhibition was achieved using ATP-competitive (6-bromoindirubin-3'-oxime and CHIR99021) or substrate-competitive (TAT-eIF2B) inhibitors to eliminate potential confounding effects on cell fate due to off-target kinase inhibition. GSK-3 inhibitors decreased the number of neural precursor cells in OE cell cultures through a reduction in proliferation. Decreased proliferation was not associated with a reduction in cell survival but was accompanied by a reduction in nestin expression and a substantial increase in the expression of the neuronal differentiation markers MAP1B and neurofilament (NF-M) after 10 days in culture. Taken together, these results suggest that GSK-3 inhibition promotes the early stages of neuronal differentiation in cultures of adult human neural precursors and provide insights into the mechanisms by which alterations in GSK-3 signaling affect adult human neurogenesis, a cellular process strongly suspected to play a role in the etiology of neuropsychiatric disorders.

Modulation of COUP-TF expression in a cnidarian by ectopic Wnt signalling and allorecognition

BACKGROUND

COUP transcription factors are required for the regulation of gene expression underlying development, differentiation, and homeostasis. They have an evolutionarily conserved function, being a known marker for neurogenesis from cnidarians to vertebrates. A homologue of this gene was shown previously to be a neuronal and nematocyte differentiation marker in Hydra. However, COUP-TFs had not previously been studied in a colonial cnidarian.

METHODOLOGY/PRINCIPAL FINDINGS

We cloned a COUP-TF homologue from the colonial marine cnidarian Hydractinia echinata. Expression of the gene was analysed during normal development, allorecognition events and ectopic Wnt activation, using in situ hybridisation and quantitative PCR. During normal Hydractinia development, the gene was first expressed in post-gastrula stages. It was undetectable in larvae, and its mRNA was present again in putative differentiating neurons and nematocytes in post-metamorphic stages. Global activation of canonical Wnt signalling in adult animals resulted in the upregulation of COUP-TF. We also monitored a strong COUP-TF upregulation in stolons undergoing allogeneic interactions. COUP-TF mRNA was most concentrated in the tissues that contacted allogeneic, non-self tissues, and decreased in a gradient away from the contact area. Interestingly, the gene was transiently upregulated during initial contact of self stolons, but dissipated rapidly following self recognition, while in non-self contacts high expression levels were maintained.

CONCLUSIONS/SIGNIFICANCE

We conclude that COUP-TF is likely involved in neuronal/nematocyte differentiation in a variety of contexts. This has now been shown to include allorecognition, where COUP-TF is thought to have been co-opted to mediate allorejection by recruiting stinging cells that are the effectors of cytotoxic rejection of allogeneic tissue. Our findings that Wnt activation upregulates COUP-TF expression suggests that Wnts' role in neuronal differentiation could be mediated through COUP-TF.

Wnt signaling regulates neuronal differentiation of cortical intermediate progenitors

Cortical intermediate progenitors (IPs) comprise a secondary neuronal progenitor pool that arises from radial glia (RG). IPs are essential for generating the correct number of cortical neurons, but the factors that regulate the expansion and differentiation of IPs in the embryonic cortex are essentially unknown. In this study, we show that the Wnt-β-catenin pathway (canonical Wnt pathway) regulates IP differentiation into neurons. Upregulation of Wnt-β-catenin signaling by overexpression of Wnt3a in the neocortex induced early differentiation of IPs into neurons and the accumulation of these newly born neurons at the subventricular zone/intermediate zone border. Long-term overexpression of Wnt3a led to cortical dysplasia associated with the formation of large neuronal heterotopias. Conversely, downregulation of Wnt-β-catenin signaling with Dkk1 during mid and late stages of neurogenesis inhibited neuronal production. Consistent with previous reports, we show that Wnt-β-catenin signaling also promotes RG self-renewal. Thus, our findings show differential effects of the Wnt-β-catenin pathway on distinct groups of cortical neuronal progenitors: RG self-renewal and IP differentiation. Moreover, our findings suggest that dysregulation of Wnt signaling can lead to developmental defects similar to human cortical malformation disorders.

Conditional deletion of N-Myc disrupts neurosensory and non-sensory development of the ear

Ear development requires interactions of transcription factors for proliferation and differentiation. The proto-oncogene N-Myc is a member of the Myc family that regulates proliferation. To investigate the function of N-Myc, we conditionally knocked out N-Myc in the ear using Tg(Pax2-Cre) and Foxg1(KiCre). N-Myc CKOs had reduced growth of the ear, abnormal morphology including fused sensory epithelia, disrupted histology, and disorganized neuronal innervation. Using Thin-Sheet Laser Imaging Microscopy (TSLIM), 3D reconstruction and quantification of the cochlea revealed a greater than 50% size reduction. Immunochemistry and in situ hybridization showed a gravistatic organ-cochlear fusion and a "circularized" apex with no clear inner and outer hair cells. Furthermore, the abnormally developed cochlea had cross innervation from the vestibular ganglion near the basal tip. These findings are put in the context of the possible functional relationship of N-Myc with a number of other cell proliferative and fate determining genes during ear development.

Genetic and developmental homology in amniote brains. Toward conciliating radical views of brain evolution

The six-layered neocortex is both a unique and a universal character of mammals. Historically, a major concern has been to determine its phylogenetic origins by establishing which structures, if any, correspond to it in the brains of other vertebrates. Two opposing hypotheses have been debated in the last years: (i) the neocortex arises entirely from the dorsal hemisphere of ancestral reptiles, and (ii) a large portion of it originates in the lateral hemisphere, from a structure termed the dorsal ventricular ridge (DVR), which expands significantly in reptiles and especially in birds. While developmental and genetic evidence strongly favors a dorsal origin of the neocortex, there are important similarities in the sensory connectivity to the neocortex and to the DVR, and more recently, in the phenotype of late-produced elements in both structures. It is proposed that, despite originating in different embryonic domains, the proliferative expansion of both the mammalian neocortex and the sauropsidian DVR is partly based on the amplification of similar developmental programs, possibly dependent on Pax6 activity or of related cascades that promote progenitor proliferation. While Pax6 activity is already present in the amphibian pallium, I propose that at some point(s) in amniote evolution it has been upregulated yielding brain expansion in both sauropsids and mammals. However, in the latter there has been an additional dorsalizing influence contributing to the development of the neocortex and restricting the expansion of the lateral hemisphere. Finally, a significant contribution to neocortical origins by anterior signaling centers secreting FGFs is suggested, by virtue of their association to olfactory development and their cortical patterning functions. This perspective fits a dynamical view of brain homology, where instead of searching for a one-to-one correspondence between components, emphasis is placed on changes in the modulation of conserved signaling centers and their corresponding morphogen gradients across species.

Beta-catenin signaling negatively regulates intermediate progenitor population numbers in the developing cortex

Intermediate progenitor cells constitute a second proliferative cell type in the developing mammalian cerebral cortex. Little is known about the factors that govern the production of intermediate progenitors. Although persistent expression of stabilized β-catenin was found to delay the maturation of radial glial progenitors into intermediate progenitors, the relationship between β-catenin signaling and intermediate progenitors remains poorly understood. Using a transgenic reporter mouse for Axin2, a direct target of Wnt/β-catenin signaling, we observed that β-catenin signaling is decreased in intermediate progenitor cells relative to radial glial progenitors. Conditional deletion of β-catenin from mouse cortical neural progenitors increased intermediate progenitor numbers, while conditional expression of stabilized β-catenin reduced the intermediate progenitor population. Together, these findings provide evidence that β-catenin signaling in radial progenitors negatively regulates intermediate progenitor cell number during cortical development.