Proneural genes in neocortical development

Controlling the Expression Level of the Neuronal Reprogramming Factors for a Successful Reprogramming Outcome

Neuronal reprogramming is a promising approach for making major advancement in regenerative medicine. Distinct from the approach of induced pluripotent stem cells, neuronal reprogramming converts non-neuronal cells to neurons without going through a primitive stem cell stage. In vivo neuronal reprogramming brings this approach to a higher level by changing the cell fate of glial cells to neurons in neural tissue through overexpressing reprogramming factors. Despite the ongoing debate over the validation and interpretation of newly generated neurons, in vivo neuronal reprogramming is still a feasible approach and has the potential to become clinical treatment with further optimization and refinement. Here, we discuss the major neuronal reprogramming factors (mostly pro-neurogenic transcription factors during development), especially the significance of their expression levels during neurogenesis and the reprogramming process focusing on NeuroD1. In the developing central nervous system, these pro-neurogenic transcription factors usually elicit distinct spatiotemporal expression patterns that are critical to their function in generating mature neurons. We argue that these dynamic expression patterns may be similarly needed in the process of reprogramming adult cells into neurons and further into mature neurons with subtype identities. We also summarize the existing approaches and propose new ones that control gene expression levels for a successful reprogramming outcome.

Reprogramming of astrocytes and glioma cells into neurons for central nervous system repair and glioblastoma therapy

Central nervous system (CNS) damage is usually irreversible owing to the limited regenerative capability of neurons. Following CNS injury, astrocytes are reactively activated and are the key cells involved in post-injury repair mechanisms. Consequently, research on the reprogramming of reactive astrocytes into neurons could provide new directions for the restoration of neural function after CNS injury and in the promotion of recovery in various neurodegenerative diseases. This review aims to provide an overview of the means through which reactive astrocytes around lesions can be reprogrammed into neurons, to elucidate the intrinsic connection between the two cell types from a neurogenesis perspective, and to summarize what is known about the neurotranscription factors, small-molecule compounds and MicroRNA that play major roles in astrocyte reprogramming. As the malignant proliferation of astrocytes promotes the development of glioblastoma multiforme (GBM), this review also examines the research advances on and the theoretical basis for the reprogramming of GBM cells into neurons and discusses the advantages of such approaches over traditional treatment modalities. This comprehensive review provides new insights into the field of GBM therapy and theoretical insights into the mechanisms of neurological recovery following neurological injury and in GBM treatment.

The proneural factors Ascl1a and Ascl1b contribute to the terminal differentiation of dopaminergic GABAergic dual transmitter neurons in zebrafish

The proneural factor Ascl1 is involved in several steps of neurogenesis, from neural progenitor maintenance to initiation of terminal differentiation and neuronal subtype specification. In neural progenitor cells, Ascl1 initiates the cell-cycle exit of progenitors, and contributes to their differentiation into mainly GABAergic neurons. Several catecholaminergic neuron groups in the forebrain of zebrafish use GABA as co-transmitter, but a potential role of the two paralogues Ascl1a and Ascl1b in their neurogenesis is not understood. Here, we show that ascl1a, ascl1b double mutant embryos develop a significantly reduced number of neurons in all GABAergic and catecholaminergic dual transmitter neuron anatomical clusters in the fore- and hindbrain, while glutamatergic catecholaminergic clusters develop normally. However, none of the affected catecholaminergic cell clusters are lost completely, suggesting an impairment in progenitor pools, or a requirement of Ascl1a/b for differentiation of a subset of neurons in each cluster. Early progenitors which are dlx2a+, fezf2 + or emx2 + are not reduced whereas late progenitors and differentiating neurons marked by the expression of dlx5a, isl1 and arxa are severely reduced in ascl1a, ascl1b double mutant embryos. This suggests that Ascl1a and Ascl1b play only a minor or no role in the maintenance of their progenitor pools, but rather contribute to the initiation of terminal differentiation of GABAergic catecholaminergic neurons.

Essential transcription factors for induced neuron differentiation

Neurogenins are proneural transcription factors required to specify neuronal identity. Their overexpression in human pluripotent stem cells rapidly produces cortical-like neurons with spiking activity and, because of this, they have been widely adopted for human neuron disease models. However, we do not fully understand the key downstream regulatory effectors responsible for driving neural differentiation. Here, using inducible expression of NEUROG1 and NEUROG2, we identify transcription factors (TFs) required for directed neuronal differentiation by combining expression and chromatin accessibility analyses with a pooled in vitro CRISPR-Cas9 screen targeting all ~1900 TFs in the human genome. The loss of one of these essential TFs (ZBTB18) yields few MAP2-positive neurons. Differentiated ZBTB18-null cells have radically altered gene expression, leading to cytoskeletal defects and stunted neurites and spines. In addition to identifying key downstream TFs for neuronal differentiation, our work develops an integrative multi-omics and TFome-wide perturbation platform to rapidly characterize essential TFs for the differentiation of any human cell type.

A novel proneural function of Asense is integrated with the sequential actions of Delta-Notch, L'sc and Su(H) to promote the neuroepithelial to neuroblast transition

In order for neural progenitors (NPs) to generate distinct populations of neurons at the right time and place during CNS development, they must switch from undergoing purely proliferative, self-renewing divisions to neurogenic, asymmetric divisions in a tightly regulated manner. In the developing Drosophila optic lobe, neuroepithelial (NE) cells of the outer proliferation center (OPC) are progressively transformed into neurogenic NPs called neuroblasts (NBs) in a medial to lateral proneural wave. The cells undergoing this transition express Lethal of Scute (L'sc), a proneural transcription factor (TF) of the Acheate Scute Complex (AS-C). Here we show that there is also a peak of expression of Asense (Ase), another AS-C TF, in the cells neighboring those with transient L'sc expression. These peak of Ase cells help to identify a new transitional stage as they have lost NE markers and L'sc, they receive a strong Notch signal and barely exhibit NB markers. This expression of Ase is necessary and sufficient to promote the NE to NB transition in a more robust and rapid manner than that of l'sc gain of function or Notch loss of function. Thus, to our knowledge, these data provide the first direct evidence of a proneural role for Ase in CNS neurogenesis. Strikingly, we found that strong Delta-Notch signaling at the lateral border of the NE triggers l'sc expression, which in turn induces ase expression in the adjacent cells through the activation of Delta-Notch signaling. These results reveal two novel non-conventional actions of Notch signaling in driving the expression of proneural factors, in contrast to the repression that Notch signaling exerts on them during classical lateral inhibition. Finally, Suppressor of Hairless (Su(H)), which seems to be upregulated late in the transitioning cells and in NBs, represses l'sc and ase, ensuring their expression is transient. Thus, our data identify a key proneural role of Ase that is integrated with the sequential activities of Delta-Notch signaling, L'sc, and Su(H), driving the progressive transformation of NE cells into NBs.

Pituitary Stem Cell Regulation by Zeb2 and BMP Signaling

Epithelial to mesenchymal transition (EMT) is important for many developing organs, and for wound healing, fibrosis, and cancer. Pituitary stem cells undergo an EMT-like process as they migrate and initiate differentiation, but little is known about the input of signaling pathways or the genetic hierarchy of the transcriptional cascade. Prop1 mutant stem cells fail to undergo changes in cellular morphology, migration, and transition to the Pou1f1 lineage. We used Prop1 mutant mice to identify the changes in gene expression that are affiliated with EMT-like processes. BMP and TGF-β family gene expression was reduced in Prop1 mutants and Elf5, a transcription factor that characteristically suppresses EMT, had elevated expression. Genes involved in cell-cell contact such as cadherins and claudins were elevated in Prop1 mutants. To establish the genetic hierarchy of control, we manipulated gene expression in pituitary stem cell colonies. We determined that the EMT inducer, Zeb2, is necessary for robust BMP signaling and repression of Elf5. We demonstrated that inhibition of BMP signaling affects expression of target genes in the Id family, but it does not affect expression of other EMT genes. Zeb2 is necessary for expression of the SHH effector gene Gli2. However, knock down of Gli2 has little effect on the EMT-related genes, suggesting that it acts through a separate pathway. Thus, we have established the genetic hierarchy involved in the transition of pituitary stem cells to differentiation.

The bHLH Transcription Factors in Neural Development and Therapeutic Applications for Neurodegenerative Diseases

The development of functional neural circuits in the central nervous system (CNS) requires the production of sufficient numbers of various types of neurons and glial cells, such as astrocytes and oligodendrocytes, at the appropriate periods and regions. Hence, severe neuronal loss of the circuits can cause neurodegenerative diseases such as Huntington's disease (HD), Parkinson's disease (PD), Alzheimer's disease (AD), and Amyotrophic Lateral Sclerosis (ALS). Treatment of such neurodegenerative diseases caused by neuronal loss includes some strategies of cell therapy employing stem cells (such as neural progenitor cells (NPCs)) and gene therapy through cell fate conversion. In this report, we review how bHLH acts as a regulator in neuronal differentiation, reprogramming, and cell fate determination. Moreover, several different researchers are conducting studies to determine the importance of bHLH factors to direct neuronal and glial cell fate specification and differentiation. Therefore, we also investigated the limitations and future directions of conversion or transdifferentiation using bHLH factors.

Generation of functional human oligodendrocytes from dermal fibroblasts by direct lineage conversion

Oligodendrocytes, the myelinating cells of the central nervous system, possess great potential for disease modeling and cell transplantation-based therapies for leukodystrophies. However, caveats to oligodendrocyte differentiation protocols ( Ehrlich et al., 2017; Wang et al., 2013; Douvaras and Fossati, 2015) from human embryonic stem and induced pluripotent stem cells (iPSCs), which include slow and inefficient differentiation, and tumorigenic potential of contaminating undifferentiated pluripotent cells, are major bottlenecks towards their translational utility. Here, we report the rapid generation of human oligodendrocytes by direct lineage conversion of human dermal fibroblasts (HDFs). We show that the combination of the four transcription factors OLIG2, SOX10, ASCL1 and NKX2.2 is sufficient to convert HDFs to induced oligodendrocyte precursor cells (iOPCs). iOPCs resemble human primary and iPSC-derived OPCs based on morphology and transcriptomic analysis. Importantly, iOPCs can differentiate into mature myelinating oligodendrocytes in vitro and in vivo. Finally, iOPCs derived from patients with Pelizaeus Merzbacher disease, a hypomyelinating leukodystrophy caused by mutations in the proteolipid protein 1 (PLP1) gene, showed increased cell death compared with iOPCs from healthy donors. Thus, human iOPCs generated by direct lineage conversion represent an attractive new source for human cell-based disease models and potentially myelinating cell grafts.

Fatty Acids: A Safe Tool for Improving Neurodevelopmental Alterations in Down Syndrome?

The triplication of chromosome 21 causes Down syndrome (DS), a genetic disorder that is characterized by intellectual disability (ID). The causes of ID start in utero, leading to impairments in neurogenesis, and continue into infancy, leading to impairments in dendritogenesis, spinogenesis, and connectivity. These defects are associated with alterations in mitochondrial and metabolic functions and precocious aging, leading to the early development of Alzheimer’s disease. Intense efforts are currently underway, taking advantage of DS mouse models to discover pharmacotherapies for the neurodevelopmental and cognitive deficits of DS. Many treatments that proved effective in mouse models may raise safety concerns over human use, especially at early life stages. Accumulating evidence shows that fatty acids, which are nutrients present in normal diets, exert numerous positive effects on the brain. Here, we review (i) the knowledge obtained from animal models regarding the effects of fatty acids on the brain, by focusing on alterations that are particularly prominent in DS, and (ii) the progress recently made in a DS mouse model, suggesting that fatty acids may indeed represent a useful treatment for DS. This scenario should prompt the scientific community to further explore the potential benefit of fatty acids for people with DS.

Evolutionarily conservative and non-conservative regulatory networks during primate interneuron development revealed by single-cell RNA and ATAC sequencing

The differences in size and function between primate and rodent brains, and the association of disturbed excitatory/inhibitory balance with many neurodevelopmental disorders highlight the importance to study primate ganglionic eminences (GEs) development. Here we used single-cell RNA and ATAC sequencing to characterize the emergence of cell diversity in monkey and human GEs where most striatal and cortical interneurons are generated. We identified regional and temporal diversity among progenitor cells which give rise to a variety of interneurons. These cells are specified within the primate GEs by well conserved gene regulatory networks, similar to those identified in mice. However, we detected, in human, several novel regulatory pathways or factors involved in the specification and migration of interneurons. Importantly, comparison of progenitors between our human and published mouse GE datasets led to the discovery and confirmation of outer radial glial cells in GEs in human cortex. Our findings reveal both evolutionarily conservative and nonconservative regulatory networks in primate GEs, which may contribute to their larger brain sizes and more complex neural networks compared with mouse.

Function of Proneural Genes Ascl1 and Asense in Neurogenesis: How Similar Are They?

Proneural genes were initially identified in Drosophila, where pioneer work on these important regulators of neural development was performed, and from which the term proneural function was coined. Subsequently, their counterparts in vertebrates were identified, and their function in neural development extensively characterized. The function of proneural transcription factors in flies and vertebrates is, however, very distinct. In flies, proneural genes play an early role in neural induction, by endowing neural competence to ectodermal cells. In contrast, vertebrate proneural genes are expressed only after neural specification, in neural stem and progenitor cells, where they play key regulatory functions in quiescence, proliferation, and neuronal differentiation. An exception to this scenario is the Drosophila proneural gene asense, which has a late onset of expression in neural stem cells of the developing embryo and larvae, similar to its vertebrate counterparts. Although the role of Asense remains poorly investigated, its expression pattern is suggestive of functions more in line with those of vertebrate proneural genes. Here, we revise our current understanding of the multiple activities of Asense and of its closest vertebrate homologue Ascl1 in neural stem/progenitor cell biology, and discuss possible parallels between the two transcription factors in neurogenesis regulation.

Mechanisms of Binding Specificity among bHLH Transcription Factors

The transcriptome of every cell is orchestrated by the complex network of interaction between transcription factors (TFs) and their binding sites on DNA. Disruption of this network can result in many forms of organism malfunction but also can be the substrate of positive natural selection. However, understanding the specific determinants of each of these individual TF-DNA interactions is a challenging task as it requires integrating the multiple possible mechanisms by which a given TF ends up interacting with a specific genomic region. These mechanisms include DNA motif preferences, which can be determined by nucleotide sequence but also by DNA’s shape; post-translational modifications of the TF, such as phosphorylation; and dimerization partners and co-factors, which can mediate multiple forms of direct or indirect cooperative binding. Binding can also be affected by epigenetic modifications of putative target regions, including DNA methylation and nucleosome occupancy. In this review, we describe how all these mechanisms have a role and crosstalk in one specific family of TFs, the basic helix-loop-helix (bHLH), with a very conserved DNA binding domain and a similar DNA preferred motif, the E-box. Here, we compile and discuss a rich catalog of strategies used by bHLH to acquire TF-specific genome-wide landscapes of binding sites.

NeuroGT: A brain atlas of neurogenic tagging CreER drivers for birthdate-based classification and manipulation of mouse neurons

Neuronal birthdate is one of the major determinants of neuronal phenotypes. However, most birthdating methods are retrospective in nature, allowing very little experimental access to the classified neuronal subsets. Here, we introduce four neurogenic tagging mouse lines, which can assign CreER-loxP recombination to neuron subsets that share the same differentiation timing in living animals and enable various experimental manipulations of the classified subsets. We constructed a brain atlas of the neurogenic tagging mouse lines (NeuroGT), which includes holistic image data of the loxP-recombined neurons and their processes across the entire brain that were tagged on each single day during the neurodevelopmental period. This image database, which is open to the public, offers investigators the opportunity to find specific neurogenic tagging driver lines and the stages of tagging appropriate for their own research purposes.

Balancing serendipity and reproducibility: Pluripotent stem cells as experimental systems for intellectual and developmental disorders

Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) and their differentiation into neural lineages is a revolutionary experimental system for studying neurological disorders, including intellectual and developmental disabilities (IDDs). However, issues related to variability and reproducibility have hindered translating preclinical findings into drug discovery. Here, we identify areas for improvement by conducting a comprehensive review of 58 research articles that utilized iPSC-derived neural cells to investigate genetically defined IDDs. Based upon these findings, we propose recommendations for best practices that can be adopted by research scientists as well as journal editors.

New Insights Into the Intricacies of Proneural Gene Regulation in the Embryonic and Adult Cerebral Cortex

Historically, the mammalian brain was thought to lack stem cells as no new neurons were found to be made in adulthood. That dogma changed ∼25 years ago with the identification of neural stem cells (NSCs) in the adult rodent forebrain. However, unlike rapidly self-renewing mature tissues (e.g., blood, intestinal crypts, skin), the majority of adult NSCs are quiescent, and those that become 'activated' are restricted to a few neurogenic zones that repopulate specific brain regions. Conversely, embryonic NSCs are actively proliferating and neurogenic. Investigations into the molecular control of the quiescence-to-proliferation-to-differentiation continuum in the embryonic and adult brain have identified proneural genes encoding basic-helix-loop-helix (bHLH) transcription factors (TFs) as critical regulators. These bHLH TFs initiate genetic programs that remove NSCs from quiescence and drive daughter neural progenitor cells (NPCs) to differentiate into specific neural cell subtypes, thereby contributing to the enormous cellular diversity of the adult brain. However, new insights have revealed that proneural gene activities are context-dependent and tightly regulated. Here we review how proneural bHLH TFs are regulated, with a focus on the murine cerebral cortex, drawing parallels where appropriate to other organisms and neural tissues. We discuss upstream regulatory events, post-translational modifications (phosphorylation, ubiquitinylation), protein-protein interactions, epigenetic and metabolic mechanisms that govern bHLH TF expression, stability, localization, and consequent transactivation of downstream target genes. These tight regulatory controls help to explain paradoxical findings of changes to bHLH activity in different cellular contexts.

Molecular and Functional Links between Neurodevelopmental Processes and Treatment-Induced Neuroendocrine Plasticity in Prostate Cancer Progression

Simple Summary

Treatment-induced neuroendocrine prostate cancer (t-NEPC) is a subtype of castration-resistant prostate cancer (CRPC) which develops under prolonged androgen deprivation therapy. The mechanisms and pathways underlying the t-NEPC are still poorly understood and there are no effective treatments available. Here, we summarize the literature on the molecules and pathways contributing to neuroendocrine phenotype in prostate cancer in the context of their known cellular neurodevelopmental processes. We also discuss the role of tumor microenvironment in neuroendocrine plasticity, future directions, and therapeutic options under clinical investigation for neuroendocrine prostate cancer.

Abstract

Neuroendocrine plasticity and treatment-induced neuroendocrine phenotypes have recently been proposed as important resistance mechanisms underlying prostate cancer progression. Treatment-induced neuroendocrine prostate cancer (t-NEPC) is highly aggressive subtype of castration-resistant prostate cancer which develops for one fifth of patients under prolonged androgen deprivation. In recent years, understanding of molecular features and phenotypic changes in neuroendocrine plasticity has been grown. However, there are still fundamental questions to be answered in this emerging research field, for example, why and how do the prostate cancer treatment-resistant cells acquire neuron-like phenotype. The advantages of the phenotypic change and the role of tumor microenvironment in controlling cellular plasticity and in the emergence of treatment-resistant aggressive forms of prostate cancer is mostly unknown. Here, we discuss the molecular and functional links between neurodevelopmental processes and treatment-induced neuroendocrine plasticity in prostate cancer progression and treatment resistance. We provide an overview of the emergence of neurite-like cells in neuroendocrine prostate cancer cells and whether the reported t-NEPC pathways and proteins relate to neurodevelopmental processes like neurogenesis and axonogenesis during the development of treatment resistance. We also discuss emerging novel therapeutic targets modulating neuroendocrine plasticity.

Endogenous zebrafish proneural Cre drivers generated by CRISPR/Cas9 short homology directed targeted integration

We previously reported efficient precision targeted integration of reporter DNA in zebrafish and human cells using CRISPR/Cas9 and short regions of homology. Here, we apply this strategy to isolate zebrafish Cre recombinase drivers whose spatial and temporal restricted expression mimics endogenous genes. A 2A-Cre recombinase transgene with 48 bp homology arms was targeted into proneural genes ascl1b, olig2 and neurod1. We observed high rates of germline transmission ranging from 10 to 100% (2/20 olig2; 1/5 neurod1; 3/3 ascl1b). The transgenic lines Tg(ascl1b-2A-Cre)^is75, Tg(olig2-2A-Cre)^is76, and Tg(neurod1-2A-Cre)^is77 expressed functional Cre recombinase in the expected proneural cell populations. Somatic targeting of 2A-CreERT2 into neurod1 resulted in tamoxifen responsive recombination in the nervous system. The results demonstrate Cre recombinase expression is driven by the native promoter and regulatory elements of the targeted genes. This approach provides a straightforward, efficient, and cost-effective method to generate cell type specific zebrafish Cre and CreERT2 drivers, overcoming challenges associated with promoter-BAC and transposon mediated transgenics.

A human cell atlas of fetal chromatin accessibility

The chromatin landscape underlying the specification of human cell types is of fundamental interest. We generated human cell atlases of chromatin accessibility and gene expression in fetal tissues. For chromatin accessibility, we devised a three-level combinatorial indexing assay and applied it to 53 samples representing 15 organs, profiling ~800,000 single cells. We leveraged cell types defined by gene expression to annotate these data and cataloged hundreds of thousands of candidate regulatory elements that exhibit cell type-specific chromatin accessibility. We investigated the properties of lineage-specific transcription factors (such as POU2F1 in neurons), organ-specific specializations of broadly distributed cell types (such as blood and endothelial), and cell type-specific enrichments of complex trait heritability. These data represent a rich resource for the exploration of in vivo human gene regulation in diverse tissues and cell types.

TAZ Represses the Neuronal Commitment of Neural Stem Cells

The mechanisms involved in regulation of quiescence, proliferation, and reprogramming of Neural Stem Progenitor Cells (NSPCs) of the mammalian brain are still poorly defined. Here, we studied the role of the transcriptional co-factor TAZ, regulated by the WNT and Hippo pathways, in the homeostasis of NSPCs. We found that, in the murine neurogenic niches of the striatal subventricular zone and the dentate gyrus granular zone, TAZ is highly expressed in NSPCs and declines with ageing. Moreover, TAZ expression is lost in immature neurons of both neurogenic regions. To characterize mechanistically the role of TAZ in neuronal differentiation, we used the midbrain-derived NSPC line ReNcell VM to replicate in a non-animal model the factors influencing NSPC differentiation to the neuronal lineage. TAZ knock-down and forced expression in NSPCs led to increased and reduced neuronal differentiation, respectively. TEADs-knockdown indicated that these TAZ co-partners are required for the suppression of NSPCs commitment to neuronal differentiation. Genetic manipulation of the TAZ/TEAD system showed its participation in transcriptional repression of SOX2 and the proneuronal genes ASCL1, NEUROG2, and NEUROD1, leading to impediment of neurogenesis. TAZ is usually considered a transcriptional co-activator promoting stem cell proliferation, but our study indicates an additional function as a repressor of neuronal differentiation.

A Widespread Neurogenic Potential of Neocortical Astrocytes Is Induced by Injury

Parenchymal astrocytes have emerged as a potential reservoir for new neurons in non-neurogenic brain regions. It is currently unclear how astrocyte neurogenesis is controlled molecularly. Here we show that Notch signaling-deficient astrocytes can generate new neurons after injury. Using single-cell RNA sequencing, we found that, when Notch signaling is blocked, astrocytes transition to a neural stem cell-like state. However, only after injury do a few of these primed astrocytes unfold a neurogenic program, including a self-amplifying progenitor-like state. Further, reconstruction of the trajectories of individual cells allowed us to uncouple astrocyte neurogenesis from reactive gliosis, which occur along independent branches. Finally, we show that cortical neurogenesis molecularly recapitulates canonical subventricular zone neurogenesis with remarkable fidelity. Our study supports a widespread potential of parenchymal astrocytes to function as dormant neural stem cells.

ETV5 is Essential for Neuronal Differentiation of Human Neural Progenitor Cells by Repressing NEUROG2 Expression

Neural progenitor cells (NPCs) are multipotent cells that have the potential to produce neurons and glial cells in the neural system. NPCs undergo identity maintenance or differentiation regulated by different kinds of transcription factors. Here we present evidence that ETV5, which is an ETS transcription factor, promotes the generation of glial cells and drives the neuronal subtype-specific genes in newly differentiated neurons from the human embryonic stem cells-derived NPCs. Next, we find a new role for ETV5 in the repression of NEUROG2 expression in NPCs. ETV5 represses NEUROG2 transcription via NEUROG2 promoter and requires the ETS domain. We identify ETV5 has the binding sites and is implicated in silent chromatin in NEUROG2 promoter by chromatin immunoprecipitation (ChIP) assays. Further, NEUROG2 transcription repression by ETV5 was shown to be dependent on a transcriptional corepressor (CoREST). During NPC differentiation toward neurons, ETV5 represses NEUROG2 expression and blocks the appearance of glutamatergic neurons. This finding suggests that ETV5 negatively regulates NEUROG2 expression and increases the number of GABAergic subtype neurons derived from NPCs. Thus, ETV5 represents a potent new candidate protein with benefits for the generation of GABAergic neurons.

Investigating cellular and molecular mechanisms of neurogenesis in Capitella teleta sheds light on the ancestor of Annelida

BACKGROUND

Diverse architectures of nervous systems (NSs) such as a plexus in cnidarians or a more centralized nervous system (CNS) in insects and vertebrates are present across Metazoa, but it is unclear what selection pressures drove evolution and diversification of NSs. One underlying aspect of this diversity lies in the cellular and molecular mechanisms driving neurogenesis, i.e. generation of neurons from neural precursor cells (NPCs). In cnidarians, vertebrates, and arthropods, homologs of SoxB and bHLH proneural genes control different steps of neurogenesis, suggesting that some neurogenic mechanisms may be conserved. However, data are lacking for spiralian taxa.

RESULTS

To that end, we characterized NPCs and their daughters at different stages of neurogenesis in the spiralian annelid Capitella teleta. We assessed cellular division patterns in the neuroectoderm using static and pulse-chase labeling with thymidine analogs (EdU and BrdU), which enabled identification of NPCs that underwent multiple rounds of division. Actively-dividing brain NPCs were found to be apically-localized, whereas actively-dividing NPCs for the ventral nerve cord (VNC) were found apically, basally, and closer to the ventral midline. We used lineage tracing to characterize the changing boundary of the trunk neuroectoderm. Finally, to start to generate a genetic hierarchy, we performed double-fluorescent in-situ hybridization (FISH) and single-FISH plus EdU labeling for neurogenic gene homologs. In the brain and VNC, Ct-soxB1 and Ct-neurogenin were expressed in a large proportion of apically-localized, EdU+ NPCs. In contrast, Ct-ash1 was expressed in a small subset of apically-localized, EdU+ NPCs and subsurface, EdU- cells, but not in Ct-neuroD+ or Ct-elav1+ cells, which also were subsurface.

CONCLUSIONS

Our data suggest a putative genetic hierarchy with Ct-soxB1 and Ct-neurogenin at the top, followed by Ct-ash1, then Ct-neuroD, and finally Ct-elav1. Comparison of our data with that from Platynereis dumerilii revealed expression of neurogenin homologs in proliferating NPCs in annelids, which appears different than the expression of vertebrate neurogenin homologs in cells that are exiting the cell cycle. Furthermore, differences between neurogenesis in the head versus trunk of C. teleta suggest that these two tissues may be independent developmental modules, possibly with differing evolutionary trajectories.

Transcriptional Regulators and Human-Specific/Primate-Specific Genes in Neocortical Neurogenesis

During development, starting from a pool of pluripotent stem cells, tissue-specific genetic programs help to shape and develop functional organs. To understand the development of an organ and its disorders, it is important to understand the spatio-temporal dynamics of the gene expression profiles that occur during its development. Modifications in existing genes, the de-novo appearance of new genes, or, occasionally, even the loss of genes, can greatly affect the gene expression profile of any given tissue and contribute to the evolution of organs or of parts of organs. The neocortex is evolutionarily the most recent part of the brain, it is unique to mammals, and is the seat of our higher cognitive abilities. Progenitors that give rise to this tissue undergo sequential waves of differentiation to produce the complete sets of neurons and glial cells that make up a functional neocortex. We will review herein our understanding of the transcriptional regulators that control the neural precursor cells (NPCs) during the generation of the most abundant class of neocortical neurons, the glutametergic neurons. In addition, we will discuss the roles of recently-identified human- and primate-specific genes in promoting neurogenesis, leading to neocortical expansion.

Ascl1 is required to specify a subset of ventromedial hypothalamic neurons

Despite clear physiological roles, the ventromedial hypothalamus (VMH) developmental programs are poorly understood. Here, we asked whether the proneural gene achaete-scute homolog 1 (Ascl1) contributes to VMH development. Ascl1 transcripts were detected in embryonic day (E) 10.5 to postnatal day 0 VMH neural progenitors. The elimination of Ascl1 reduced the number of VMH neurons at E12.5 and E15.5, particularly within the VMH-central (VMHC) and -dorsomedial (VMHDM) subdomains, and resulted in a VMH cell fate change from glutamatergic to GABAergic. We observed a loss of Neurog3 expression in Ascl1-/- hypothalamic progenitors and an upregulation of Neurog3 when Ascl1 was overexpressed. We also demonstrated a glutamatergic to GABAergic fate switch in Neurog3-null mutant mice, suggesting that Ascl1 might act via Neurog3 to drive VMH cell fate decisions. We also showed a concomitant increase in expression of the central GABAergic fate determinant Dlx1/2 in the Ascl1-null hypothalamus. However, Ascl1 was not sufficient to induce an ectopic VMH fate when overexpressed outside the normal window of competency. Combined, Ascl1 is required but not sufficient to specify the neurotransmitter identity of VMH neurons, acting in a transcriptional cascade with Neurog3.

Neurog2 Acts as a Classical Proneural Gene in the Ventromedial Hypothalamus and Is Required for the Early Phase of Neurogenesis

The tuberal hypothalamus is comprised of the dorsomedial, ventromedial, and arcuate nuclei, as well as parts of the lateral hypothalamic area, and it governs a wide range of physiologies. During neurogenesis, tuberal hypothalamic neurons are thought to be born in a dorsal-to-ventral and outside-in pattern, although the accuracy of this description has been questioned over the years. Moreover, the intrinsic factors that control the timing of neurogenesis in this region are poorly characterized. Proneural genes, including Achate-scute-like 1 (Ascl1) and Neurogenin 3 (Neurog3) are widely expressed in hypothalamic progenitors and contribute to lineage commitment and subtype-specific neuronal identifies, but the potential role of Neurogenin 2 (Neurog2) remains unexplored. Birthdating in male and female mice showed that tuberal hypothalamic neurogenesis begins as early as E9.5 in the lateral hypothalamic and arcuate and rapidly expands to dorsomedial and ventromedial neurons by E10.5, peaking throughout the region by E11.5. We confirmed an outside-in trend, except for neurons born at E9.5, and uncovered a rostrocaudal progression but did not confirm a dorsal-ventral patterning to tuberal hypothalamic neuronal birth. In the absence of Neurog2, neurogenesis stalls, with a significant reduction in early-born BrdU⁺ cells but no change at later time points. Further, the loss of Ascl1 yielded a similar delay in neuronal birth, suggesting that Ascl1 cannot rescue the loss of Neurog2 and that these proneural genes act independently in the tuberal hypothalamus. Together, our findings show that Neurog2 functions as a classical proneural gene to regulate the temporal progression of tuberal hypothalamic neurogenesis.SIGNIFICANCE STATEMENT Here, we investigated the general timing and pattern of neurogenesis within the tuberal hypothalamus. Our results confirmed an outside-in trend of neurogenesis and uncovered a rostrocaudal progression. We also showed that Neurog2 acts as a classical proneural gene and is responsible for regulating the birth of early-born neurons within the ventromedial hypothalamus, acting independently of Ascl1 In addition, we revealed a role for Neurog2 in cell fate specification and differentiation of ventromedial -specific neurons. Last, Neurog2 does not have cross-inhibitory effects on Neurog1, Neurog3, and Ascl1 These findings are the first to reveal a role for Neurog2 in hypothalamic development.

C3orf70 Is Involved in Neural and Neurobehavioral Development

Neurogenesis is the process by which undifferentiated progenitor cells develop into mature and functional neurons. Defects in neurogenesis are associated with neurodevelopmental and neuropsychiatric disorders; therefore, elucidating the molecular mechanisms underlying neurogenesis can advance our understanding of the pathophysiology of these disorders and facilitate the discovery of novel therapeutic targets. In this study, we performed a comparative transcriptomic analysis to identify common targets of the proneural transcription factors Neurog1/2 and Ascl1 during neurogenesis of human and mouse stem cells. We successfully identified C3orf70 as a novel common target gene of Neurog1/2 and Ascl1 during neurogenesis. Using in situ hybridization, we demonstrated that c3orf70a and c3orf70b, two orthologs of C3orf70, were expressed in the midbrain and hindbrain of zebrafish larvae. We generated c3orf70 knockout zebrafish using CRISPR/Cas9 technology and demonstrated that loss of c3orf70 resulted in significantly decreased expression of the mature neuron markers elavl3 and eno2. We also found that expression of irx3b, a zebrafish ortholog of IRX3 and a midbrain/hindbrain marker, was significantly reduced in c3orf70 knockout zebrafish. Finally, we demonstrated that neurobehaviors related to circadian rhythm and altered light–dark conditions were significantly impaired in c3orf70 knockout zebrafish. These results suggest that C3orf70 is involved in neural and neurobehavioral development and that defects in C3orf70 may be associated with midbrain/hindbrain-related neurodevelopmental and neuropsychiatric disorders.

Expression of ventral telencephalon transcription factors ASCL1 and DLX2 in the early fetal human cerebral cortex

In rodent ventral telencephalon, diffusible morphogens induce expression of the proneural transcription factor ASCL1, which in turn induces expression of the transcription factor DLX2 that controls differentiation of cortical interneuron precursors and their tangential migration to the cerebral cortex. RNAseq analysis of human fetal samples of dorsal telencephalon revealed consistently high cortical expression of ASCL1 and increasing expression of DLX2 between 7.5 and 17 post-conceptional weeks (PCW). We explored whether cortical expression of these genes represented a population of intracortically derived interneuron precursors. Immunohistochemistry revealed an ASCL1+ /DLX2+ population of progenitor cells in the human ganglionic eminences between 6.5 and 12 PCW, but in the cortex there also existed a population of ASCL1+ /DLX2- progenitors in the subventricular zone (SVZ) that largely co-expressed cortical markers PAX6 or TBR2, although a few ASCL1+ /PAX6- progenitors were observed in the ventricular zone (VZ) and ASCL1+ cells expressing the interneuron marker GAD67 were present in the SVZ. Although rare in the VZ, DLX2+ cells progressively increased in number between 8 and 12 PCW across the cortical wall and the majority co-expressed LHX6 and originated either in the MGE, migrating to the lateral cortex, or from the septum, populating the medial wall. A minority co-expressed COUP-TFII, which identifies cells from the caudal ganglionic eminence (CGE). By 19 PCW, a significant increase in expression of DLX2 and ASCL1 was observed in the cortical VZ with a small proportion expressing both proteins. The DLX2+ cells did not co-express a cell division marker, so were not progenitors. The majority of DLX2+ cells throughout the cortical plate expressed COUP-TFII rather than LHX6+ . As the VZ declined as a proliferative zone it appeared to be re-defined as a migration pathway for COUP-TFII+ /DLX2+ interneurons from CGE to cortex. Therefore, in developing human cortex, ASCL1 expression predominantly marks a population of intermediate progenitors giving rise to glutamatergic neurons. DLX2 expression predominantly defines post-mitotic interneuron precursors.

JNK1 Induces Notch1 Expression to Regulate Genes Governing Photoreceptor Production

c-Jun N-terminal kinases (JNKs) regulate cell proliferation and differentiation via phosphorylating such transcription factors as c-Jun. The function of JNKs in retinogenesis remains to be elucidated. Here, we report that knocking out Jnk1, but not Jnk2, increased the number of photoreceptors, thus enhancing the electroretinogram (ERG) responses. Intriguingly, Notch1, a well-established negative regulator of photoreceptor genesis, was significantly attenuated in Jnk1 knockout (KO) mice compared to wild-type mice. Mechanistically, light specifically activated JNK1 to phosphorylate c-Jun, which in turn induced Notch1 transcription. The identified JNK1–c-Jun–Notch1 axis strongly inhibited photoreceptor-related transcriptional factor expression and ultimately impaired photoreceptor opsin expression. Our study uncovered an essential function of JNK1 in retinogenesis, revealing JNK1 as a potential candidate for targeting ophthalmic diseases.

Advanced nanotherapies to promote neuroregeneration in the injured newborn brain

Neonatal brain injury affects thousands of babies each year and may lead to long-term and permanent physical and neurological problems. Currently, therapeutic hypothermia is standard clinical care for term newborns with moderate to severe neonatal encephalopathy. Nevertheless, it is not completely protective, and additional strategies to restore and promote regeneration are urgently needed. One way to ensure recovery following injury to the immature brain is to augment endogenous regenerative pathways. However, novel strategies such as stem cell therapy, gene therapies and nanotechnology have not been adequately explored in this unique age group. In this perspective review, we describe current efforts that promote neuroprotection and potential targets that are unique to the developing brain, which can be leveraged to facilitate neuroregeneration.

Lineage specific transcription factors and epigenetic regulators mediate TGFβ-dependent enhancer activation

During neurogenesis, dynamic developmental cues, transcription factors and histone modifying enzymes regulate the gene expression programs by modulating the activity of neural-specific enhancers. How transient developmental signals coordinate transcription factor recruitment to enhancers and to which extent chromatin modifiers contribute to enhancer activity is starting to be uncovered. Here, we take advantage of neural stem cells as a model to unravel the mechanisms underlying neural enhancer activation in response to the TGFβ signaling. Genome-wide experiments demonstrate that the proneural factor ASCL1 assists SMAD3 in the binding to a subset of enhancers. Once located at the enhancers, SMAD3 recruits the histone demethylase JMJD3 and the remodeling factor CHD8, creating the appropriate chromatin landscape to allow enhancer transcription and posterior gene activation. Finally, to analyze the phenotypical traits owed to cis-regulatory regions, we use CRISPR–Cas9 technology to demonstrate that the TGFβ-responsive Neurog2 enhancer is essential for proper neuronal polarization.

Zebrin II Is Ectopically Expressed in Microglia in the Cerebellum of Neurogenin 2 Null Mice

Zebrin II/aldolase C expression in the normal cerebellum is restricted to a Purkinje cell subset and is the canonical marker for stripes and zones. This spatial restriction has been confirmed in over 30 species of mammals, birds, fish, etc. In a transgenic mouse model in which the Neurogenin 2 gene has been disrupted (Neurog2^-/-), the cerebellum is smaller than normal and Purkinje cell dendrites are disordered, but the basic zone and stripe architecture is preserved. Here, we show that in the Neurog2^-/- mouse, in addition to the normal Purkinje cell expression, zebrin II is also expressed in a population of cells with a morphology characteristic of microglia. This identity was confirmed by double immunohistochemistry for zebrin II and the microglial marker, Iba1. The expression of zebrin II in cerebellar microglia is not restricted by zone or stripe or lamina. A second zone and stripe marker, PLCβ4, does not show the same ectopic expression. When microglia are compared in control vs. Neurog2^-/- mice, no difference is seen in apparent number or distribution, suggesting that the ectopic zebrin II immunoreactivity in Neurog2^-/- cerebellum reflects an ectopic expression rather than the invasion of a new population of microglia from the periphery. This ectopic expression of zebrin II in microglia is unique as it is not seen in numerous other models of cerebellar disruption, such as in Acp2^-/- mice and in human pontocerebellar hypoplasia. The upregulation of zebrin II in microglia is thus specific to the disruption of Neurog2 downstream pathways, rather than a generic response to a cerebellar disruption.

Pharmacological Notch pathway inhibition leads to cell cycle arrest and stimulates ascl1 and neurogenin2 genes expression in dental pulp stem cells-derived neurospheres

OBJECTIVE

Human dental pulp-derived stem cells (hDPSCs) are becoming an attractive source for cell-based neurorestorative therapies. As such, it is important to understand the molecular mechanisms that regulate the differentiation of hDPSCs toward the neuronal fate. Notch signaling plays key roles in neural stem/progenitor cells (NS/PCs) maintenance and prevention of their differentiation. The aim of this study was to address the effects of Notch signaling inhibition on neurosphere formation of hDPSCs and neuronal differentiation of hDPSCs-neurospheres.

RESULTS

hDPSCs were isolated from third molar teeth. The cultivated hDPSCs highly expressed CD90 and CD44 and minimally presented CD34 and CD45 surface markers. The osteo/adipogenic differentiation of hDPSCs was documented. hDPSCs were cultured in neural induction medium and N-[N-(3,5-difluorophenacetyl-L-alanyl)]-Sphenylglycine t-butyl ester (DAPT) was applied to impede Notch signaling during transformation into spheres or on the formed neurospheres. Our results showed that the size and number of neurospheres decreased and the expression profile of nestin, sox1 and pax6 genes reduced provided DAPT. Treatment of the formed neurospheres with DAPT resulted in the cleaved Notch1 reduction, G0/G1 arrest and a decline in L-lactate production. DAPT significantly reduced hes1 and hey1 genes, while ascl1 and neurogenin2 expressions augmented. The number of MAP2 positive cells improved in the DAPT-treated group.

CONCLUSIONS

Our findings demonstrated the Notch activity in hDPSCs-neurospheres. DAPT treatment positively regulated proneural genes expression and increased neuronal-like differentiation.

Acyl-CoA-Binding Domain-Containing 3 (ACBD3; PAP7; GCP60): A Multi-Functional Membrane Domain Organizer

Acyl-CoA-binding domain-containing 3 (ACBD3) is a multi-functional scaffolding protein, which has been associated with a diverse array of cellular functions, including steroidogenesis, embryogenesis, neurogenesis, Huntington’s disease (HD), membrane trafficking, and viral/bacterial proliferation in infected host cells. In this review, we aim to give a timely overview of recent findings on this protein, including its emerging role in membrane domain organization at the Golgi and the mitochondria. We hope that this review provides readers with useful insights on how ACBD3 may contribute to membrane domain organization along the secretory pathway and on the cytoplasmic surface of intracellular organelles, which influence many important physiological and pathophysiological processes in mammalian cells.

WNT signaling represses astrogliogenesis via Ngn2-dependent direct suppression of astrocyte gene expression

Neural progenitor cells (NPCs) are sequentially specified into neurons and glia during the development of central nervous system. WNT/β-catenin signaling is known to regulate the balance between the proliferation and differentiation of NPCs during neurogenesis. However, the function of WNT/β-catenin signaling during gliogenesis remains poorly defined. Here, we report that activation of WNT/β-catenin signaling disrupts astrogliogenesis in the developing spinal cord. Conversely, inhibition of WNT/β-catenin signaling leads to precocious astrogliogenesis. Further analysis reveals that activation of WNT/β-catenin pathway results in a dramatic increase of neurogenin 2 (Ngn2) expression in transgenic mice, and knockdown of Ngn2 expression in neural precursor cells can reverse the inhibitory effect of WNT/β-catenin on astrocytic differentiation. Moreover, Ngn2 can directly bind to the promoters of several astrocyte specific genes and suppress their expression independent of STATs activity. Together, our studies provide the first in vivo evidence that WNT/β-catenin signaling inhibits early astrogliogenesis via an Ngn2-dependent transcriptional repression mechanism.

Maternal hyperglycemia disturbs neocortical neurogenesis via epigenetic regulation in C57BL/6J mice

Offspring of mothers with hyperglycemia during pregnancy have a higher incidence of long-term neuropsychiatric disorders than offspring from a normal pregnancy, indicating that neocortical neurogenesis might be affected by maternal hyperglycemia. A paucity of study evaluating the effects of hyperglycemia on neocortical neurogenetic differentiation of neural stem cells, and the mechanism remains unclear. We sought to investigate the the roles and possible molecular mechanism of maternal hyperglycemia on neocortical neurogenetic differentiation of neural stem cells. We established a mouse model of a hyperglycemic pregnancy to study effects of intrauterine exposure to maternal hyperglycemia on neocortical neurogenesis. We observed morphological changes in the neocortex and detected the neurogenetic differentiation of neural stem cells in offspring affected by high glucose levels. We investigated the regulatory network between epigenetic modification and transcription factors in differentiated neural stem cells under hyperglycemic conditions. Maternal hyperglycemia disturbs neocortical lamination in some non-malformed offspring. Our results suggested that hyperglycemia altered the early-born neuron fate and the distribution of newborn neurons in deep layers by promoting the earlier differentiation of neural stem cells. Altered histone acetylation and its regulation on the transcription of proneural genes might be correlated to the disrupted differentiation of neural stem cells and altered distribution of newborn projection neurons in the neocortex. Our data raised the possibility that maternal hyperglycemia in pregnancy disturbs the laminar distribution of neocortical projection neurons in some non-malformed offspring via epigenetic regulation on neural stem cell differentiation and the birthdate of neocortical neurons.

Ascl1 Regulates Electric Field-Induced Neuronal Differentiation Through PI3K/Akt Pathway

Directing differentiation of neural stem/progenitor cells (NSCs/NPCs) to produce functional neurons is one of the greatest challenges in regenerative medicine. Our previous paper has confirmed that electrical stimulation has a high efficiency of triggering neuronal differentiation by using isolated filum terminale (FT)-derived NPCs. To further clarify the intrinsic molecular mechanisms, protein-protein interaction (PPI) network analysis was applied to pinpoints novel hubs in electric field (EF)-induced neuronal differentiation. In this study, siRNA transfection of Achaete-scute homolog 1 (Ascl1) in NPCs or NPCs was followed by direct current stimulation at 150 mV/mm. Neuronal differentiation rate and protein expression level were analyzed after 7 or 14 days of electrical stimulation. The data showed that the expression level of Ascl1 was enhanced by electrical stimulation and positively correlated to EF strength. Moreover, we identified that the expression of Ascl1 positively regulated neuronal differentiation of NPCs and can be up-regulated by EF-stimulation through the activation of phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) pathway. Therefore, this study provides new insights into the role of Ascl1 and its relevant PI3K/Akt pathway in regulating of EF-induced neuronal differentiation and pointed out that continuous expression of Ascl1 in NPCs is required for EF-induced neuronal differentiation.

Plag1 and Plagl2 have overlapping and distinct functions in telencephalic development

The Plag gene family has three members; Plagl1/Zac1, which is a tumor suppressor gene, and Plag1 and Plagl2, which are proto-oncogenes. All three genes are known to be expressed in embryonic neural progenitors, and Zac1 regulates proliferation, neuronal differentiation and migration in the developing neocortex. Here we examined the functions of Plag1 and Plagl2 in neocortical development. We first attempted, and were unable to generate, E12.5 Plag1;Plagl2 double mutants, indicating that at least one Plag1 or Plagl2 gene copy is required for embryonic survival. We therefore focused on single mutants, revealing a telencephalic patterning defect in E12.5 Plagl2 mutants and a proliferation/differentiation defect in Plag1 mutant neocortices. Specifically, the ventral pallium, a dorsal telencephalic territory, expands into the ventral telencephalon in Plagl2 mutants. In contrast, Plag1 mutants develop normal regional territories, but neocortical progenitors proliferate less and instead produce more neurons. Finally, in gain-of-function studies, both Plag1 and Plagl2 reduce neurogenesis and increase BrdU-uptake, indicative of enhanced proliferation, but while Plagl2 effects on proliferation are more immediate, Plag1 effects are delayed. Taken together, we found that the Plag proto-oncogenes genes are essential regulators of neocortical development and although Plag1 and Plagl2 functions are similar, they do not entirely overlap. This article has an associated First Person interview with the first author of the paper.

Cell-Biological Requirements for the Generation of Dentate Gyrus Granule Neurons

The dentate gyrus (DG) receives highly processed information from the associative cortices functionally integrated in the trisynaptic hippocampal circuit, which contributes to the formation of new episodic memories and the spontaneous exploration of novel environments. Remarkably, the DG is the only brain region currently known to have high rates of neurogenesis in adults (Andersen et al., 1966, 1971). The DG is involved in several neurodegenerative disorders, including clinical dementia, schizophrenia, depression, bipolar disorder and temporal lobe epilepsy. The principal neurons of the DG are the granule cells. DG granule cells generated in culture would be an ideal model to investigate their normal development and the causes of the pathologies in which they are involved and as well as possible therapies. Essential to establish such in vitro models is the precise definition of the most important cell-biological requirements for the differentiation of DG granule cells. This requires a deeper understanding of the precise molecular and functional attributes of the DG granule cells in vivo as well as the DG cells derived in vitro. In this review we outline the neuroanatomical, molecular and cell-biological components of the granule cell differentiation pathway, including some growth- and transcription factors essential for their development. We summarize the functional characteristics of DG granule neurons, including the electrophysiological features of immature and mature granule cells and the axonal pathfinding characteristics of DG neurons. Additionally, we discuss landmark studies on the generation of dorsal telencephalic precursors from pluripotent stem cells (PSCs) as well as DG neuron differentiation in culture. Finally, we provide an outlook and comment critical aspects.

xNgn2 induces expression of predominantly sensory neuron markers in Xenopus whole embryo ectoderm but induces mixed subtype expression in isolated ectoderm explants

Proneural basic-helix-loop-helix (bHLH) proteins, such as Neurogenin2 (Ngn2) and Ascl1, are critical regulators at the onset of neuronal differentiation. Endogenously they have largely complementary expression patterns, and have conserved roles in the specification of distinct neuronal subtypes. InXenopusembryos, xNgn2 is the master regulator of primary neurogenesis forming sensory, inter- and motor neurons within the neural plate, while xAscl1 is the master regulator of autonomic neurogenesis, forming noradrenergic neurons in the antero-ventral region of the embryo. Here we characterise neuronal subtype identity of neurons induced by xNgn2 in the ectoderm of wholeXenopusembryos in comparison with xAscl1, and in ectodermal “animal cap” explants. We find that the transcriptional cascades mediating primary and autonomic neuron formation are distinct, and while xNgn2 and xAscl1 can upregulate genes associated with a non-endogenous cascade, this expression is spatially restricted within the embryo. xNgn2 is more potent than xAscl1 at inducing primary neurogenesis as assayed by neural-β-tubulin. In ectoderm of the intact embryo, these induced primary neurons have sensory characteristics with no upregulation of motor neuron markers. In contrast, xNgn2 is able to up-regulate both sensory and motor neuron markers in naïve ectoderm of animal cap explants, suggesting a non-permissive environment for motor identity in the patterned ectoderm of the whole embryo.

Interaction between opposing modes of phospho-regulation of the proneural proteins Ascl1 and Ngn2

From the relatively simple nervous system of Drosophila to the elaborate mammalian cortex, neurogenesis requires exceptional spatial and temporal precision to co-ordinate progenitor cell proliferation and subsequent differentiation to a diverse range of neurons and glia. A limited number of transiently expressed proneural basic-helix-loop-helix (bHLH) transcription factors, for example achaete-scute-complex (as-c) and atonal (ato) in Drosophila and the vertebrate homologues Ascl1 and Neurogenin2 (Ngn2), are able to orchestrate the onset of neuronal determination, context-dependent subtype selection and even influence later aspects of neuronal migration and maturation. Within the last decade, two models have emerged to explain how the temporal activity of proneural determination factors is regulated by phosphorylation at distinct sites. One model describes how cell-cycle associated phosphorylation on multiple sites in the N and C termini of vertebrate proneural proteins limits neuronal differentiation in cycling progenitor cells. A second model describes phosphorylation on a single site in the bHLH domain of Drosophila atonal that acts as a binary switch, where phosphorylation terminates proneural activity. Here we combine activating mutations of phosphorylation sites in the N- and C- termini with an inhibitory phospho-mimetic mutation in the bHLH domain of Ascl1 and Ngn2 proteins, and test their functions in vivo using Xenopus embryos to determine which mode of phospho-regulation dominates. Enhancing activity by preventing N- and C terminal phosphorylation cannot overcome the inhibitory effect of mimicking phosphorylation of the bHLH domain. Thus we have established a hierarchy between these two modes of proneural protein control and suggest a model of temporal regulation for proneural protein activity.

A non-canonical role for the proneural gene Neurog1 as a negative regulator of neocortical neurogenesis

Neural progenitors undergo temporal identity transitions to sequentially generate the neuronal and glial cells that make up the mature brain. Proneural genes have well-characterised roles in promoting neural cell differentiation and subtype specification, but they also regulate the timing of identity transitions through poorly understood mechanisms. Here, we investigated how the highly related proneural genes Neurog1 and Neurog2 interact to control the timing of neocortical neurogenesis. We found that Neurog1 acts in an atypical fashion as it is required to suppress rather than promote neuronal differentiation in early corticogenesis. In Neurog1-/- neocortices, early born neurons differentiate in excess, whereas, in vitro, Neurog1-/- progenitors have a decreased propensity to proliferate and form neurospheres. Instead, Neurog1-/- progenitors preferentially generate neurons, a phenotype restricted to the Neurog2+ progenitor pool. Mechanistically, Neurog1 and Neurog2 heterodimerise, and while Neurog1 and Neurog2 individually promote neurogenesis, misexpression together blocks this effect. Finally, Neurog1 is also required to induce the expression of neurogenic factors (Dll1 and Hes5) and to repress the expression of neuronal differentiation genes (Fezf2 and Neurod6). Neurog1 thus employs different mechanisms to temper the pace of early neocortical neurogenesis.

Evolution and cell-type specificity of human-specific genes preferentially expressed in progenitors of fetal neocortex

Understanding the molecular basis that underlies the expansion of the neocortex during primate, and notably human, evolution requires the identification of genes that are particularly active in the neural stem and progenitor cells of the developing neocortex. Here, we have used existing transcriptome datasets to carry out a comprehensive screen for protein-coding genes preferentially expressed in progenitors of fetal human neocortex. We show that 15 human-specific genes exhibit such expression, and many of them evolved distinct neural progenitor cell-type expression profiles and levels compared to their ancestral paralogs. Functional studies on one such gene, NOTCH2NL, demonstrate its ability to promote basal progenitor proliferation in mice. An additional 35 human genes with progenitor-enriched expression are shown to have orthologs only in primates. Our study provides a resource of genes that are promising candidates to exert specific, and novel, roles in neocortical development during primate, and notably human, evolution.

The Tbr2 Molecular Network Controls Cortical Neuronal Differentiation Through Complementary Genetic and Epigenetic Pathways

The T-box containing Tbr2 gene encodes for a transcription factor essential for the specification of the intermediate neural progenitors (INPs) originating the excitatory neurons of the cerebral cortex. However, its overall mechanism of action, direct target genes and cofactors remain unknown. Herein, we carried out global gene expression profiling combined with genome-wide binding site identification to determine the molecular pathways regulated by TBR2 in INPs. This analysis led to the identification of novel protein-protein interactions that control multiple features of INPs including cell-type identity, morphology, proliferation and migration dynamics. In particular, NEUROG2 and JMJD3 were found to associate with TBR2 revealing unexplored TBR2-dependent mechanisms. These interactions can explain, at least in part, the role of this transcription factor in the implementation of the molecular program controlling developmental milestones during corticogenesis. These data identify TBR2 as a major determinant of the INP-specific traits by regulating both genetic and epigenetic pathways.

bHLH transcription factors in neural development, disease, and reprogramming

The formation of functional neural circuits in the vertebrate central nervous system (CNS) requires that appropriate numbers of the correct types of neuronal and glial cells are generated in their proper places and times during development. In the embryonic CNS, multipotent progenitor cells first acquire regional identities, and then undergo precisely choreographed temporal identity transitions (i.e. time-dependent changes in their identity) that determine how many neuronal and glial cells of each type they will generate. Transcription factors of the basic-helix-loop-helix (bHLH) family have emerged as key determinants of neural cell fate specification and differentiation, ensuring that appropriate numbers of specific neuronal and glial cell types are produced. Recent studies have further revealed that the functions of these bHLH factors are strictly regulated. Given their essential developmental roles, it is not surprising that bHLH mutations and de-regulated expression are associated with various neurological diseases and cancers. Moreover, the powerful ability of bHLH factors to direct neuronal and glial cell fate specification and differentiation has been exploited in the relatively new field of cellular reprogramming, in which pluripotent stem cells or somatic stem cells are converted to neural lineages, often with a transcription factor-based lineage conversion strategy that includes one or more of the bHLH genes. These concepts are reviewed herein.

Mnb/Dyrk1A orchestrates a transcriptional network at the transition from self-renewing neurogenic progenitors to postmitotic neuronal precursors

The Down syndrome and microcephaly related gene Mnb/Dyrk1A encodes an evolutionary conserved protein kinase subfamily that plays important roles in neurodevelopment. minibrain (mnb) mutants of Drosophila melanogaster (Dm) exhibit reduced adult brains due to neuronal deficits generated during larval development. These deficits are the consequence of the apoptotic cell death of numerous neuronal precursors that fail to properly exit the cell cycle and differentiate. We have recently found that in both the Dm larval brain and the embryonic vertebrate central nervous system (CNS), a transient expression of Mnb/Dyrk1A promotes the cell cycle exit of newborn neuronal precursors by upregulating the expression of the cyclin-dependent kinase inhibitor p27kip1 (called Dacapo in Dm). In the larval brain, Mnb performs this action by regulating the expression of three transcription factors, Asense (Ase), Deadpan (Dpn) and Prospero (Pros), which are key regulators of the self-renewal, proliferation, and terminal differentiation of neural progenitor cells. We have here studied in detail the cellular/temporal expression pattern of Ase, Dpn, Pros and Mnb, and have analyzed possible regulatory effects among them at the transitions from neurogenic progenitors to postmitotic neuronal precursors in the Dm larval brain. The emerging picture of this analysis reveals an intricate regulatory network in which Mnb appears to play a pivotal role helping to delineate the dynamics of the expression patterns of Ase, Dpn and Pros, as well as their specific functions in the aforementioned transitions. Our results also show that Ase, Dpn and Pros perform several cross-regulatory actions and contribute to shape the precise cellular/temporal expression pattern of Mnb. We propose that Mnb/Dyrk1A plays a central role in CNS neurogenesis by integrating molecular mechanisms that regulate progenitor self-renewal, cell cycle progression and neuronal differentiation.

MyT1 Counteracts the Neural Progenitor Program to Promote Vertebrate Neurogenesis

The generation of neurons from neural stem cells requires large-scale changes in gene expression that are controlled to a large extent by proneural transcription factors, such as Ascl1. While recent studies have characterized the differentiation genes activated by proneural factors, less is known on the mechanisms that suppress progenitor cell identity. Here, we show that Ascl1 induces the transcription factor MyT1 while promoting neuronal differentiation. We combined functional studies of MyT1 during neurogenesis with the characterization of its transcriptional program. MyT1 binding is associated with repression of gene transcription in neural progenitor cells. It promotes neuronal differentiation by counteracting the inhibitory activity of Notch signaling at multiple levels, targeting the Notch1 receptor and many of its downstream targets. These include regulators of the neural progenitor program, such as Hes1, Sox2, Id3, and Olig1. Thus, Ascl1 suppresses Notch signaling cell-autonomously via MyT1, coupling neuronal differentiation with repression of the progenitor fate.

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MyT1 promotes neurogenesis downstream Ascl1

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MyT1 represses Notch1 receptor and many of its downstream target genes

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MyT1 represses Hes1 expression by direct DNA binding and competition with RBPJ

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Ascl1 suppresses Notch signaling cell-autonomously while promoting differentiation

Differential interactions between Notch and ID factors control neurogenesis by modulating Hes factor autoregulation

During embryonic and adult neurogenesis, neural stem cells (NSCs) generate the correct number and types of neurons in a temporospatial fashion. Control of NSC activity and fate is crucial for brain formation and homeostasis. Neurogenesis in the embryonic and adult brain differ considerably, but Notch signaling and inhibitor of DNA-binding (ID) factors are pivotal in both. Notch and ID factors regulate NSC maintenance; however, it has been difficult to evaluate how these pathways potentially interact. Here, we combined mathematical modeling with analysis of single-cell transcriptomic data to elucidate unforeseen interactions between the Notch and ID factor pathways. During brain development, Notch signaling dominates and directly regulates Id4 expression, preventing other ID factors from inducing NSC quiescence. Conversely, during adult neurogenesis, Notch signaling and Id2/3 regulate neurogenesis in a complementary manner and ID factors can induce NSC maintenance and quiescence in the absence of Notch. Our analyses unveil key molecular interactions underlying NSC maintenance and mechanistic differences between embryonic and adult neurogenesis. Similar Notch and ID factor interactions may be crucial in other stem cell systems.

Summary: Computational analysis of transcriptome data from neural stem cells reveals key differences in the synergistic interactions between Notch and inhibitor of DNA-binding factors during embryonic and adult neurogenesis.