Regulation of motor neuron specification by phosphorylation of neurogenin 2

Function of GSK‑3 signaling in spinal cord injury (Review)

Spinal cord injury (SCI) is a major social problem with a heavy burden on patient physiology and psychology. Glial scar formation and irreversible neuron loss are the two key points during SCI progression. During the acute phase of spinal cord injury, glial scars form, limiting the progression of inflammation. However, in the subacute or chronic phase, glial scarring inhibits axon regeneration. Following spinal cord injury, irreversible loss of neurons leads to further aggravation of spinal cord injury. Several therapies have been developed to improve either glial scar or neuron loss; however, few therapies reach the stage of clinical trials and there are no mainstream therapies for SCI. Exploring the key mechanism of SCI is crucial for finding further treatments. Glycogen synthase kinase-3 (GSK-3) is a widely expressed kinase with important physiological and pathophysiological functions in vivo. Dysfunction of the GSK-3 signaling pathway during SCI has been widely discussed for controlling neurite growth in vitro and in vivo, improving the proliferation and neuronal differentiation of endogenous neural stem cells and functional recovery from spinal cord injury. SCI can decrease the phosphorylated (p)/total (t)-GSK-3β ratio, which leads to an increase in apoptosis, whereas treatment with GSK-3 inhibitors can promote neurogenesis. In addition, several therapies for the treatment of SCI involve signaling pathways associated with GSK-3. Furthermore, signaling pathways associated with GSK-3 also participate in the pathological process of neuropathic pain that remains following SCI. The present review summarized the roles of GSK-3 signaling in SCI to aid in the understanding of GSK-3 signaling during the pathological processes of SCI and to provide evidence for the development of comprehensive treatments.

Calcium dynamics at the neural cell primary cilium regulate Hedgehog signaling-dependent neurogenesis in the embryonic neural tube

The balance between neural stem cell proliferation and neuronal differentiation is paramount for the appropriate development of the nervous system. Sonic hedgehog (Shh) is known to sequentially promote cell proliferation and specification of neuronal phenotypes, but the signaling mechanisms responsible for the developmental switch from mitogenic to neurogenic have remained unclear. Here, we show that Shh enhances Ca2+ activity at the neural cell primary cilium of developing Xenopus laevis embryos through Ca2+ influx via transient receptor potential cation channel subfamily C member 3 (TRPC3) and release from intracellular stores in a developmental stage-dependent manner. This ciliary Ca2+ activity in turn antagonizes canonical, proliferative Shh signaling in neural stem cells by down-regulating Sox2 expression and up-regulating expression of neurogenic genes, enabling neuronal differentiation. These discoveries indicate that the Shh-Ca2+-dependent switch in neural cell ciliary signaling triggers the switch in Shh action from canonical-mitogenic to neurogenic. The molecular mechanisms identified in this neurogenic signaling axis are potential targets for the treatment of brain tumors and neurodevelopmental disorders.

Establishing the Molecular and Functional Diversity of Spinal Motoneurons

Spinal motoneurons are a remarkably diverse class of neurons responsible for facilitating a broad range of motor behaviors and autonomic functions. Studies of motoneuron differentiation have provided fundamental insights into the developmental mechanisms of neuronal diversification, and have illuminated principles of neural fate specification that operate throughout the central nervous system. Because of their relative anatomical simplicity and accessibility, motoneurons have provided a tractable model system to address multiple facets of neural development, including early patterning, neuronal migration, axon guidance, and synaptic specificity. Beyond their roles in providing direct communication between central circuits and muscle, recent studies have revealed that motoneuron subtype-specific programs also play important roles in determining the central connectivity and function of motor circuits. Cross-species comparative analyses have provided novel insights into how evolutionary changes in subtype specification programs may have contributed to adaptive changes in locomotor behaviors. This chapter focusses on the gene regulatory networks governing spinal motoneuron specification, and how studies of spinal motoneurons have informed our understanding of the basic mechanisms of neuronal specification and spinal circuit assembly.

Making neurons, made easy: The use of Neurogenin-2 in neuronal differentiation

Directed neuronal differentiation of human pluripotent stem cells (hPSCs), neural progenitors, or fibroblasts using transcription factors has allowed for the rapid and highly reproducible differentiation of mature and functional neurons. Exogenous expression of the transcription factor Neurogenin-2 (NGN2) has been widely used to generate different populations of neurons, which have been used in neurodevelopment studies, disease modeling, drug screening, and neuronal replacement therapies. Could NGN2 be a “one-glove-fits-all” approach for neuronal differentiations? This review summarizes the cellular roles of NGN2 and describes the applications and limitations of using NGN2 for the rapid and directed differentiation of neurons.

Chemical approach to generating long-term self-renewing pMN progenitors from human embryonic stem cells

Spinal cord impairment involving motor neuron degeneration and demyelination can cause life-long disabilities, but effective clinical interventions for restoring neurological functions have yet been developed. In early spinal cord development, neural progenitors in the pMN ('progenitors of motor neurons') domain, defined by the expression of oligodendrocyte transcription factor 2 (OLIG2), in ventral spinal cord first generate motor neurons and then switch the fate to produce myelin-forming oligodendrocytes. Given their differentiation potential, pMN progenitors could be a valuable cell source for cell therapy in relevant neurological conditions such as spinal cord injury. However, fast generation and expansion of pMN progenitors in vitro while conserving their differentiation potential has so far been technically challenging. In this study, based on the chemical screening, we have developed a new recipe for efficient induction of pMN progenitors from human embryonic stem cells. More importantly, these OLIG2+ pMN progenitors can be stably maintained for multiple passages without losing their ability to produce spinal motor neurons and oligodendrocytes rapidly. Our results suggest that these self-renewing pMN progenitors could potentially be useful as a renewable source of cell transplants for spinal cord injury and demyelinating disorders.

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

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.

Human sensorimotor organoids derived from healthy and amyotrophic lateral sclerosis stem cells form neuromuscular junctions

Human induced pluripotent stem cells (iPSC) hold promise for modeling diseases in individual human genetic backgrounds and thus for developing precision medicine. Here, we generate sensorimotor organoids containing physiologically functional neuromuscular junctions (NMJs) and apply the model to different subgroups of amyotrophic lateral sclerosis (ALS). Using a range of molecular, genomic, and physiological techniques, we identify and characterize motor neurons and skeletal muscle, along with sensory neurons, astrocytes, microglia, and vasculature. Organoid cultures derived from multiple human iPSC lines generated from individuals with ALS and isogenic lines edited to harbor familial ALS mutations show impairment at the level of the NMJ, as detected by both contraction and immunocytochemical measurements. The physiological resolution of the human NMJ synapse, combined with the generation of major cellular cohorts exerting autonomous and non-cell autonomous effects in motor and sensory diseases, may prove valuable to understand the pathophysiological mechanisms of ALS.

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.

Deep conservation of the enhancer regulatory code in animals

Interactions of transcription factors (TFs) with DNA regulatory sequences, known as enhancers, specify cell identity during animal development. Unlike TFs, the origin and evolution of enhancers has been difficult to trace. We drove zebrafish and mouse developmental transcription using enhancers from an evolutionarily distant marine sponge. Some of these sponge enhancers are located in highly conserved microsyntenic regions, including an Islet enhancer in the Islet-Scaper region. We found that Islet enhancers in humans and mice share a suite of TF binding motifs with sponges, and that they drive gene expression patterns similar to those of sponge and endogenous Islet enhancers in zebrafish. Our results suggest the existence of an ancient and conserved, yet flexible, genomic regulatory syntax that has been repeatedly co-opted into cell type-specific gene regulatory networks across the animal kingdom.

Aging-relevant human basal forebrain cholinergic neurons as a cell model for Alzheimer's disease

Background

Alzheimer’s disease (AD) is an adult-onset mental disorder with aging as a major risk factor. Early and progressive degeneration of basal forebrain cholinergic neurons (BFCNs) contributes substantially to cognitive impairments of AD. An aging-relevant cell model of BFCNs will critically help understand AD and identify potential therapeutics. Recent studies demonstrate that induced neurons directly reprogrammed from adult human skin fibroblasts retain aging-associated features. However, human induced BFCNs (hiBFCNs) have yet to be achieved.

Methods

We examined a reprogramming procedure for the generation of aging-relevant hiBFCNs through virus-mediated expression of fate-determining transcription factors. Skin fibroblasts were obtained from healthy young persons, healthy adults and sporadic AD patients. Properties of the induced neurons were examined by immunocytochemistry, qRT-PCR, western blotting, and electrophysiology.

Results

We established a protocol for efficient generation of hiBFCNs from adult human skin fibroblasts. They show electrophysiological properties of mature neurons and express BFCN-specific markers, such as CHAT, p75NTR, ISL1, and VACHT. As a proof-of-concept, our preliminary results further reveal that hiBFCNs from sporadic AD patients exhibit time-dependent TAU hyperphosphorylation in the soma and dysfunctional nucleocytoplasmic transport activities.

Conclusions

Aging-relevant BFCNs can be directly reprogrammed from human skin fibroblasts of healthy adults and sporadic AD patients. They show promises as an aging-relevant cell model for understanding AD pathology and may be employed for therapeutics identification for AD.

Generation and optimization of highly pure motor neurons from human induced pluripotent stem cells via lentiviral delivery of transcription factors

Generation of neurons from human induced pluripotent stem cells (hiPSCs) overcomes the limited access to human brain samples and greatly facilitates the progress of research in neurological diseases. However, it is still a challenge to generate a particular neuronal subtype with high purity and yield for determining the pathogenesis of diseased neurons using biochemical approaches. Motor neurons (MNs) are a specialized neuronal subtype responsible for governing both autonomic and volitional movement. Dysfunctions in MNs are implicated in a variety of movement diseases, such as amyotrophic lateral sclerosis (ALS). In this study, we generated functional MNs from human iPSCs via lentiviral delivery of transcription factors. Moreover, we optimized induction conditions by using different combinations of transcription factors and found that a single lentiviral vector expressing three factors [neurogenin-2 (NGN2), insulin gene enhancer 1 (ISL1), and LIM/homeobox 3 (LHX3)] is necessary and sufficient to induce iPSC-derived MNs (iPSC-MNs). These MNs robustly expressed general neuron markers [microtubule-associated protein 2 (MAP2), neurofilament protein (SMI-32), and tubulin β-3 class III (TUBB3)] and MN-specific markers [HB9 and choline acetyltransferase (ChAT)] and showed electrical maturation and firing of action potentials within 3 wk. This approach significantly improved the neuronal survival, yield, and purity, making it feasible to obtain abundant materials for biochemical studies in modeling movement diseases.

Establishing neuronal diversity in the spinal cord: a time and a place

The vertebrate spinal cord comprises multiple functionally distinct neuronal cell types arranged in characteristic positions. During development, these different types of neurons differentiate from transcriptionally distinct neural progenitors that are arrayed in discrete domains along the dorsal-ventral and anterior-posterior axes of the embryonic spinal cord. This organization arises in response to morphogen gradients acting upstream of a gene regulatory network, the architecture of which determines the spatial and temporal pattern of gene expression. In recent years, substantial progress has been made in deciphering the regulatory network that underlies the specification of distinct progenitor and neuronal cell identities. In this Review, we outline how distinct neuronal cell identities are established in response to spatial and temporal patterning systems, and outline novel experimental approaches to study the emergence and function of neuronal diversity in the spinal cord.

Human Cortical Organoids Expose a Differential Function of GSK3 on Cortical Neurogenesis

The regulation of the proliferation and polarity of neural progenitors is crucial for the development of the brain cortex. Animal studies have implicated glycogen synthase kinase 3 (GSK3) as a pivotal regulator of both proliferation and polarity, yet the functional relevance of its signaling for the unique features of human corticogenesis remains to be elucidated. We harnessed human cortical brain organoids to probe the longitudinal impact of GSK3 inhibition through multiple developmental stages. Chronic GSK3 inhibition increased the proliferation of neural progenitors and caused massive derangement of cortical tissue architecture. Single-cell transcriptome profiling revealed a direct impact on early neurogenesis and uncovered a selective role of GSK3 in the regulation of glutamatergic lineages and outer radial glia output. Our dissection of the GSK3-dependent transcriptional network in human corticogenesis underscores the robustness of the programs determining neuronal identity independent of tissue architecture.

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Cortical organoids recapitulate stereotypical neurogenic trajectories

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GSK3 inhibition disrupts neuroepithelium polarity and cortical tissue organization

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GSK3 activity controls oRG production and neurogenesis

A Comprehensive Structure-Function Study of Neurogenin3 Disease-Causing Alleles during Human Pancreas and Intestinal Organoid Development

Neurogenin3 (NEUROG3) is required for endocrine lineage formation of the pancreas and intestine. Patients with NEUROG3 mutations are born with congenital malabsorptive diarrhea due to complete loss of enteroendocrine cells, whereas endocrine pancreas development varies in an allele-specific manner. These findings suggest a context-dependent requirement for NEUROG3 in pancreas versus intestine. We utilized human tissue differentiated from NEUROG3^-/- pluripotent stem cells for functional analyses. Most disease-associated alleles had hypomorphic or null phenotype in both tissues, whereas the S171fsX68 mutation had reduced activity in the pancreas but largely null in the intestine. Biochemical studies revealed NEUROG3 variants have distinct molecular defects with altered protein stability, DNA binding, and gene transcription. Moreover, NEUROG3 was highly unstable in the intestinal epithelium, explaining the enhanced sensitivity of intestinal defects relative to the pancreas. These studies emphasize that studies of human mutations in the endogenous tissue context may be required to assess structure-function relationships.

Matrix Topography Regulates Synaptic Transmission at the Neuromuscular Junction

Recreation of a muscle that can be controlled by the nervous system would provide a major breakthrough for treatments of injury and diseases. However, the underlying basis of how neuron-muscle interfaces are formed is still not understood sufficiently. Here, it is hypothesized that substrate topography regulates neural innervation and synaptic transmission by mediating the cross-talk between neurons and muscles. This hypothesis is examined by differentiating neural stem cells on the myotubes, formed on the substrate with controlled groove width. The substrate with the groove width of 1600 nm, a similar size to the myofibril diameter, serves to produce larger and aligned myotubes than the flat substrate. The myotubes formed on the grooved substrate display increases in the acetylcholine receptor expression. Reciprocally, motor neuron progenitor cells differentiated from neural stem cells innervate the larger and aligned myotubes more actively than randomly oriented myotubes. As a consequence, mature and aligned myotubes respond to glutamate (i.e., an excitatory neurotransmitter) and curare (i.e., a neuromuscular antagonist) more rapidly and homogeneously than randomly oriented myotubes. The results of this study will be broadly useful for improving the quality of engineered muscle used in a series of applications including drug screening, regeneration therapies, and biological machinery assembly.

Regulating transcriptional activity by phosphorylation: A new mechanism for the ARX homeodomain transcription factor

Aristaless-related homeobox (ARX) gene encodes a paired-type homeodomain transcription factor with critical roles in development. Here we identify that ARX protein is phosphorylated. Using mass spectrometry and in vitro kinase assays we identify phosphorylation at serines 37, 67 and 174. Through yeast-2-hybrid and CoIP we identified PICK1 (Protein interacting with C kinase 1) binding with the C-terminal region of ARX. PICK1 is a scaffold protein known to facilitate phosphorylation of protein partners by protein kinase C alpha (PRKCA). We confirm that ARX is phosphorylated by PRKCA and demonstrate phosphorylation at serine 174. We demonstrate that phosphorylation is required for correct transcriptional activity of the ARX protein using transcriptome-wide analysis of gene expression of phospho-null mutants (alanines replacing serines) compared to ARX wild-type (ARX-WT) overexpressed in pancreatic alpha TC cells. Compared to untransfected cells, ARX-WT overexpression significantly altered expression of 70 genes (Log2FC >+/-1.0, P-value <0.05). There were fewer genes with significantly altered expression compared to untransfected cells with the double phospho-null mutant Ser37Ala+Ser67Ala (26%) and Ser174Ala (39%), respectively. We demonstrate that the c-terminal region of ARX required to bind PICK1 causes a shift in PICK1 subcellular localisation to the nucleus to co-locate with the ARX protein, and truncation of this C-terminal region leads to the same loss of transcriptional activation as S174A mutant. In conclusion, we show that ARX is phosphorylated at several sites and that this modification affects its transcriptional activity.

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.

Cell cycle-dependent phosphorylation and regulation of cellular differentiation

Embryogenesis requires an exquisite regulation of cell proliferation, cell cycle withdrawal and differentiation into a massively diverse range of cells at the correct time and place. Stem cells also remain to varying extents in different adult tissues, acting in tissue homeostasis and repair. Therefore, regulated proliferation and subsequent differentiation of stem and progenitor cells remains pivotal throughout life. Recent advances have characterised the cell cycle dynamics, epigenetics, transcriptome and proteome accompanying the transition from proliferation to differentiation, revealing multiple bidirectional interactions between the cell cycle machinery and factors driving differentiation. Here, we focus on a direct mechanistic link involving phosphorylation of differentiation-associated transcription factors by cell cycle-associated Cyclin-dependent kinases. We discuss examples from the three embryonic germ layers to illustrate this regulatory mechanism that co-ordinates the balance between cell proliferation and differentiation.

All in the family: proneural bHLH genes and neuronal diversity

Proneural basic Helix-Loop-Helix (bHLH) proteins are required for neuronal determination and the differentiation of most neural precursor cells. These transcription factors are expressed in vastly divergent organisms, ranging from sponges to primates. Here, we review proneural bHLH gene evolution and function in the Drosophila and vertebrate nervous systems, arguing that the Drosophila gene atonal provides a useful platform for understanding proneural gene structure and regulation. We also discuss how functional equivalency experiments using distinct proneural genes can reveal how proneural gene duplication and divergence are interwoven with neuronal complexity.

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.

Olig2 and Hes regulatory dynamics during motor neuron differentiation revealed by single cell transcriptomics

During tissue development, multipotent progenitors differentiate into specific cell types in characteristic spatial and temporal patterns. We addressed the mechanism linking progenitor identity and differentiation rate in the neural tube, where motor neuron (MN) progenitors differentiate more rapidly than other progenitors. Using single cell transcriptomics, we defined the transcriptional changes associated with the transition of neural progenitors into MNs. Reconstruction of gene expression dynamics from these data indicate a pivotal role for the MN determinant Olig2 just prior to MN differentiation. Olig2 represses expression of the Notch signaling pathway effectors Hes1 and Hes5. Olig2 repression of Hes5 appears to be direct, via a conserved regulatory element within the Hes5 locus that restricts expression from MN progenitors. These findings reveal a tight coupling between the regulatory networks that control patterning and neuronal differentiation and demonstrate how Olig2 acts as the developmental pacemaker coordinating the spatial and temporal pattern of MN generation.

How are the right types of cells produced in the right place, at the right time and in the correct numbers, in a developing tissue? One example of where progress has been made towards answering this question is the embryonic spinal cord. In this tissue, extracellular signals, such as the morphogen sonic hedgehog (Shh), control the pattern of generation of molecularly distinct sets of neural progenitors, from which different classes of motor neurons and interneurons are generated. Motor neurons differentiate at a much higher rate than the adjacent interneurons, and this ensures that more motor neurons than interneurons are generated and in an appropriate temporal sequence. To understand the mechanisms responsible for this phenomenon, we investigated the dynamics of the Shh-controlled gene regulatory network operating as motor neurons form from progenitors. We used these data to uncover the accompanying regulatory mechanisms, and this identified two functions for the transcription factor Olig2. First, Olig2 is essential for establishing motor neuron progenitor identity downstream of Shh signaling. Subsequently, Olig2 directly promotes neuronal differentiation in motor neuron progenitors by suppressing the expression of Hes genes, negative regulators of neuronal differentiation. Together, our findings reveal a tight coupling between the genetic networks that control patterning and neuronal differentiation in motor neuron progenitors and thereby explain their characteristic early and rapid rate of neuronal differentiation.

Transcriptional Networks Controlled by NKX2-1 in the Development of Forebrain GABAergic Neurons

The embryonic basal ganglia generates multiple projection neurons and interneuron subtypes from distinct progenitor domains. Combinatorial interactions of transcription factors and chromatin are thought to regulate gene expression. In the medial ganglionic eminence, the NKX2-1 transcription factor controls regional identity and, with LHX6, is necessary to specify pallidal projection neurons and forebrain interneurons. Here, we dissected the molecular functions of NKX2-1 by defining its chromosomal binding, regulation of gene expression, and epigenetic state. NKX2-1 binding at distal regulatory elements led to a repressed epigenetic state and transcriptional repression in the ventricular zone. Conversely, NKX2-1 is required to establish a permissive chromatin state and transcriptional activation in the sub-ventricular and mantle zones. Moreover, combinatorial binding of NKX2-1 and LHX6 promotes transcriptionally permissive chromatin and activates genes expressed in cortical migrating interneurons. Our integrated approach provides a foundation for elucidating transcriptional networks guiding the development of the MGE and its descendants.

Phosphorylation of NEUROG3 Links Endocrine Differentiation to the Cell Cycle in Pancreatic Progenitors

During pancreatic development, proliferating pancreatic progenitors activate the proendocrine transcription factor neurogenin 3 (NEUROG3), exit the cell cycle, and differentiate into islet cells. The mechanisms that direct robust NEUROG3 expression within a subset of progenitor cells control the size of the endocrine population. Here we demonstrate that NEUROG3 is phosphorylated within the nucleus on serine 183, which catalyzes its hyperphosphorylation and proteosomal degradation. During progression through the progenitor cell cycle, NEUROG3 phosphorylation is driven by the actions of cyclin-dependent kinases 2 and 4/6 at G₁/S cell-cycle checkpoint. Using models of mouse and human pancreas development, we show that lengthening of the G₁ phase of the pancreatic progenitor cell cycle is essential for proper induction of NEUROG3 and initiation of endocrine cell differentiation. In sum, these studies demonstrate that progenitor cell-cycle G₁ lengthening, through its actions on stabilization of NEUROG3, is an essential variable in normal endocrine cell genesis.

One-Step piggyBac Transposon-Based CRISPR/Cas9 Activation of Multiple Genes

Neural cell fate is determined by a tightly controlled transcription regulatory network during development. The ability to manipulate the expression of multiple transcription factors simultaneously is required to delineate the complex picture of neural cell development. Because of the limited carrying capacity of the commonly used viral vectors, such as lentiviral or retroviral vectors, it is often challenging to perform perturbation experiments on multiple transcription factors. Here we have developed a piggyBac (PB) transposon-based CRISPR activation (CRISPRa) all-in-one system, which allows for simultaneous and stable endogenous transactivation of multiple transcription factors and long non-coding RNAs. As a proof of principle, we showed that the PB-CRISPRa system could accelerate the differentiation of human induced pluripotent stem cells into neurons and astrocytes by triggering endogenous expression of different sets of transcription factors. The PB-CRISPRa system has the potential to become a convenient and robust tool in neuroscience, which can meet the needs of a variety of in vitro and in vivo gain-of-function applications.

Small Molecules Modulate Chromatin Accessibility to Promote NEUROG2-Mediated Fibroblast-to-Neuron Reprogramming

Pro-neural transcription factors and small molecules can induce the reprogramming of fibroblasts into functional neurons; however, the immediate-early molecular events that catalyze this conversion have not been well defined. We previously demonstrated that neurogenin 2 (NEUROG2), forskolin (F), and dorsomorphin (D) can reprogram fibroblasts into functional neurons with high efficiency. Here, we used this model to define the genetic and epigenetic events that initiate an acquisition of neuronal identity. We demonstrate that NEUROG2 is a pioneer factor, FD enhances chromatin accessibility and H3K27 acetylation, and synergistic transcription activated by these factors is essential to successful reprogramming. CREB1 promotes neuron survival and acts with NEUROG2 to upregulate SOX4, which co-activates NEUROD1 and NEUROD4. In addition, SOX4 targets SWI/SNF subunits and SOX4 knockdown results in extensive loss of open chromatin and abolishes reprogramming. Applying these insights, adult human glioblastoma cell and skin fibroblast reprogramming can be improved using SOX4 or chromatin-modifying chemicals.

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NEUROG2 acts as a pioneer factor to drive neuronal reprogramming

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ATAC-, ChIP-, and RNA-seq profiling reveals genome-wide mechanisms for reprogramming

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SOX4 is a critical mediator of chromatin remodeling during reprogramming

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SOX4 or FK228 can enhance adult human glioblastoma and skin fibroblast reprogramming

FAMA: A Molecular Link between Stomata and Myrosin Cells

Plants use sophisticated defense strategies against herbivores, including the myrosinase-glucosinolate system in Brassicales plants. This system sequesters myrosinase in myrosin cells, which are idioblasts in inner leaf tissues, and produces a toxic compound when cells are damaged by herbivores. Although the molecular mechanisms underlying myrosin cell development are largely unknown, recent studies have revealed that two key components, a basic helix-loop-helix (bHLH) transcription factor (FAMA) and vesicle trafficking factors (such as SYNTAXIN OF PLANTS 22), regulate the differentiation and fate determination of myrosin cells. FAMA also functions as a master regulator of guard cell (GC) differentiation. In this review, we discuss how FAMA operates two distinct genetic programs: the generation of myrosin cells in inner plant tissue and GCs in the epidermis.

Rapid Ngn2-induction of excitatory neurons from hiPSC-derived neural progenitor cells

Since the discovery of somatic reprogramming, human induced pluripotent stem cells (hiPSCs) have been exploited to model a variety of neurological and psychiatric disorders. Because hiPSCs represent an almost limitless source of patient-derived neurons that retain the genetic variations thought to contribute to disease etiology, they have been heralded as a patient-specific platform for high throughput drug screening. However, the utility of current protocols for generating neurons from hiPSCs remains limited by protracted differentiation timelines and heterogeneity of the neuronal phenotypes produced. Neuronal induction via the forced expression of exogenous transcription factors rapidly induces defined populations of functional neurons from fibroblasts and hiPSCs. Here, we describe an adapted protocol that accelerates maturation of functional excitatory neurons from hiPSC-derived neural progenitor cells (NPCs) via lentiviral transduction of Neurogenin 2 (using both mNgn2 and hNGN2). This methodology, relying upon a robust and scalable starting population of hiPSC NPCs, should be readily amenable to scaling for hiPSC-based high-throughput drug screening.

Human Induced Pluripotent Stem Cell NEUROG2 Dual Knockin Reporter Lines Generated by the CRISPR/Cas9 System

Human induced pluripotent stem cell (hiPSC) technologies are powerful tools for modeling development and disease, drug screening, and regenerative medicine. Faithful gene targeting in hiPSCs greatly facilitates these applications. We have developed a fast and precise clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) technology-based method and obtained fluorescent protein and antibiotic resistance dual knockin reporters in hiPSC lines for neurogenin2 (NEUROG2), an important proneural transcription factor. Gene targeting efficiency was greatly improved in CRISPR/Cas9-mediated homology directed recombination (∼ 33% correctly targeted clones) compared to conventional targeting protocol (∼ 3%) at the same locus. No off-target events were detected. In addition, taking the advantage of the versatile applications of the CRISPR/Cas9 system, we designed transactivation components to transiently induce NEUROG2 expression, which helps identify transcription factor binding sites and trans-regulation regions of human NEUROG2. The strategy of using CRISPR/Cas9 genome editing coupled with fluorescence-activated cell sorting of neural progenitor cells in a knockin lineage hiPSC reporter platform might be broadly applicable in other stem cell derivatives and subpopulations.

Notch signaling regulates motor neuron differentiation of human embryonic stem cells

In the pMN domain of the spinal cord, Notch signaling regulates the balance between motor neuron differentiation and maintenance of the progenitor state for later oligodendrocyte differentiation. Here, we sought to study the role of Notch signaling in regulation of the switch from the pMN progenitor state to differentiated motor neurons in a human model system. Human embryonic stem cells (hESCs) were directed to differentiate to pMN-like progenitor cells by the inductive action of retinoic acid and a Shh agonist, purmorphamine. We found that the expression of the Notch signaling effector Hes5 was induced in hESC-derived pMN-like progenitors and remained highly expressed when they were cultured under conditions favoring motor neuron differentiation. Inhibition of Notch signaling by a γ-secretase inhibitor in the differentiating pMN-like progenitor cells decreased Hes5 expression and enhanced the differentiation toward motor neurons. Conversely, over-expression of Hes5 in pMN-like progenitor cells during the differentiation interfered with retinoic acid- and purmorphamine-induced motor neuron differentiation and inhibited the emergence of motor neurons. Inhibition of Notch signaling had a permissive rather than an inductive effect on motor neuron differentiation. Our results indicate that Notch signaling has a regulatory role in the switch from the pMN progenitor to the differentiated motor neuron state. Inhibition of Notch signaling can be harnessed to enhance the differentiation of hESCs toward motor neurons.

Diabetes in Pregnancy Adversely Affects the Expression of Glycogen Synthase Kinase-3β in the Hippocampus of Rat Neonates

Diabetes during pregnancy causes a wide range of neurodevelopmental and neurocognitive abnormalities in offspring. Glycogen synthase kinase-3 (GSK-3) is widely expressed during brain development and regulates multiple cellular processes, and its dysregulation is implicated in the pathogenesis of diverse neurodegenerative and psychological diseases. This study was designed to examine the effects of maternal diabetes on GSK-3β messenger RNA (mRNA) expression and phosphorylation in the developing rat hippocampus. Female rats were maintained diabetic from a week before pregnancy through parturition, and male offspring was killed immediately after birth. We found a significant bilateral upregulation of GSK-3β mRNA expression in the hippocampus of pups born to diabetic mothers at P0, compared to controls. Moreover, at the same time point, there was a marked bilateral increase in the phosphorylation level of GSK-3β in the diabetic group. Unlike phosphorylation levels, there was a significant upregulation in hippocampal GSK-3β mRNA expression in the insulin-treated group, when compared to controls. The present study revealed that diabetes during pregnancy strongly influences the regulation of GSK-3β in the right/left developing hippocampi. These dysregulations may be part of the cascade of events through which diabetes during pregnancy affects the newborn's hippocampal structure and function.

Functional dissection of the Pax6 paired domain: Roles in neural tube patterning and peripheral nervous system development

During development of the CNS, stem and progenitor cell proliferation, cell fate designation, and patterning decisions are tightly regulated by interdependent networks of key transcriptional regulators. In a genetic approach we analyzed divergent functionality of the PAI and RED sub-domains of the Pax6 Paired domain (PD) during progenitor zone formation, motor and interneuron development, and peripheral connectivity at distinct levels within the neural tube: within the hindbrain, mutation of the PAI sub-domain severely affected patterning of the p3 and pMN domains and establishment of the corresponding motor neurons. Exit point designation of hypoglossal axons was disturbed in embryos harboring either mutations in the PD sub-domains or containing a functional Pax6 Null allele. At brachial spinal levels, we propose a selective involvement of the PAI sub-domain during patterning of ventral p2 and pMN domains, critically disturbing generation of specific motor neuron subtypes and increasing V2 interneuron numbers. Our findings present a novel aspect of how Pax6 not only utilizes its modular structure to perform distinct functions via its paired and homeodomain. Individual sub-domains can exert distinct functions, generating a new level of complexity for transcriptional regulation by one single transcription factor not only in dorso-ventral, but also rostro-caudal neural tube patterning.

Multi-site phosphorylation regulates NeuroD4 activity during primary neurogenesis: a conserved mechanism amongst proneural proteins

Background

Basic Helix Loop Helix (bHLH) proneural transcription factors are master regulators of neurogenesis that act at multiple stages in this process. We have previously demonstrated that multi-site phosphorylation of two members of the proneural protein family, Ngn2 and Ascl1, limits their ability to drive neuronal differentiation when cyclin-dependent kinase levels are high, as would be found in rapidly cycling cells. Here we investigate potential phospho-regulation of proneural protein NeuroD4 (also known as Xath3), the Xenopus homologue of Math3/NeuroM, that functions downstream of Ngn2 in the neurogenic cascade.

Results

Using the developing Xenopus embryo system, we show that NeuroD4 is expressed and phosphorylated during primary neurogenesis, and this phosphorylation limits its ability to drive neuronal differentiation. Phosphorylation of up to six serine/threonine-proline sites contributes additively to regulation of NeuroD4 proneural activity without altering neuronal subtype specification, and number rather than location of available phospho-sites is the key for limiting NeuroD4 activity. Mechanistically, a phospho-mutant NeuroD4 displays increased protein stability and enhanced chromatin binding relative to wild-type NeuroD4, resulting in transcriptional up-regulation of a range of target genes that further promote neuronal differentiation.

Conclusions

Multi-site phosphorylation on serine/threonine-proline pairs is a widely conserved mechanism of limiting proneural protein activity, where it is the number of phosphorylated sites, rather than their location that determines protein activity. Hence, multi-site phosphorylation is very well suited to allow co-ordination of proneural protein activity with the cellular proline-directed kinase environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-015-0044-8) contains supplementary material, which is available to authorized users.

bHLH proteins involved in Drosophila neurogenesis are mutually regulated at the level of stability

Proneural bHLH activators are expressed in all neuroectodermal regions prefiguring events of central and peripheral neurogenesis. Drosophila Sc is a prototypical proneural activator that heterodimerizes with the E-protein Daughterless (Da) and is antagonized by, among others, the E(spl) repressors. We determined parameters that regulate Sc stability in Drosophila S2 cells. We found that Sc is a very labile phosphoprotein and its turnover takes place via at least three proteasome-dependent mechanisms. (i) When Sc is in excess of Da, its degradation is promoted via its transactivation domain (TAD). (ii) In a DNA-bound Da/Sc heterodimer, Sc degradation is promoted via an SPTSS phosphorylation motif and the AD1 TAD of Da; Da is spared in the process. (iii) When E(spl)m7 is expressed, it complexes with Sc or Da/Sc and promotes their degradation in a manner that requires the corepressor Groucho and the Sc SPTSS motif. Da/Sc reciprocally promotes E(spl)m7 degradation. Since E(spl)m7 is a direct target of Notch, the mutual destabilization of Sc and E(spl) may contribute in part to the highly conserved anti-neural activity of Notch. Sc variants lacking the SPTSS motif are dramatically stabilized and are hyperactive in transgenic flies. Our results propose a novel mechanism of regulation of neurogenesis, involving the stability of key players in the process.

Phosphorylation in intrinsically disordered regions regulates the activity of Neurogenin2

Background

Neuronal differentiation is largely under the control of basic Helix-Loop-Helix (bHLH) proneural transcription factors that play key roles during development of the embryonic nervous system. In addition to well-characterised regulation of their expression, increasing evidence is emerging for additional post-translational regulation of proneural protein activity. Of particular interest is the bHLH proneural factor Neurogenin2 (Ngn2), which orchestrates progression from neural progenitor to differentiated neuron in several regions of the central nervous system. Previous studies have demonstrated a key role for cell cycle-dependent multi-site phosphorylation of Ngn2 protein at Serine-Proline (SP) sites for regulation of its neuronal differentiation activity, although the potential structural and functional consequences of phosphorylation at different regions of the protein are unclear.

Results

Here we characterise the role of phosphorylation of specific regions of Ngn2 on the stability of Ngn2 protein and on its neuronal differentiation activity in vivo in the developing embryo, demonstrating clearly that the location of SP sites is less important than the number of SP sites available for control of Ngn2 activity in vivo. We also provide structural evidence that Ngn2 contains large, intrinsically disordered regions that undergo phosphorylation by cyclin-dependent kinases (cdks).

Conclusions

Phosphorylation of Ngn2 occurs in both the N- and C-terminal regions, either side of the conserved basic Helix-Loop-Helix domain. While these phosphorylation events do not change the intrinsic stability of Ngn2, phosphorylation on multiple sites acts to limit its ability to drive neuronal differentiation in vivo. Phosphorylated regions of Ngn2 are predicted to be intrinsically disordered and cdk-dependent phosphorylation of these intrinsically disordered regions contributes to Ngn2 regulation.

Neurogenin 2 mediates amyloid-β precursor protein-stimulated neurogenesis

Amyloid-β precursor protein (APP) is well studied for its role in Alzheimer disease, although its normal function remains uncertain. It has been reported that APP stimulates the proliferation and neuronal differentiation of neural stem/progenitor cells (NSPCs). In this study we examined the role of APP in NSPC differentiation. To identify proteins that may mediate the effect of APP on NSPC differentiation, we used a gene array approach to find genes whose expression correlated with APP-induced neurogenesis. We found that the expression of neurogenin 2 (Ngn2), a basic helix-loop-helix transcription factor, was significantly down-regulated in NSPCs from APP knock-out mice (APPKO) and increased in APP transgenic (Tg2576) mice. Ngn2 overexpression in APPKO NSPCs promoted neuronal differentiation, whereas siRNA knockdown of Ngn2 expression in wild-type NSPCs decreased neuronal differentiation. The results demonstrate that APP-stimulated neuronal differentiation of NSPCs is mediated by Ngn2.

Transcriptional regulatory events initiated by Ascl1 and Neurog2 during neuronal differentiation of P19 embryonic carcinoma cells

As members of the proneural basic-helix-loop-helix (bHLH) family of transcription factors, Ascl1 and Neurog2 direct the differentiation of specific populations of neurons at various times and locations within the developing nervous system. In order to characterize the mechanisms employed by these two bHLH factors, we generated stable, doxycycline-inducible lines of P19 embryonic carcinoma cells that express comparable levels of Ascl1 and Neurog2. Upon induction, both Ascl1 and Neurog2 directed morphological and immunocytochemical changes consistent with initiation of neuronal differentiation. Comparison of Ascl1- and Neurog2-regulated genes by microarray analyses showed both shared and distinct transcriptional changes for each bHLH protein. In both Ascl1- and Neurog2-differentiating cells, repression of Oct4 mRNA levels was accompanied by increased Oct4 promoter methylation. However, DNA demethylation was not detected for genes induced by either bHLH protein. Neurog2-induced genes included glutamatergic marker genes while Ascl1-induced genes included GABAergic marker genes. The Neurog2-specific induction of a gene encoding a protein phosphatase inhibitor, Ppp1r14a, was dependent on distinct, canonical E-box sequences within the Ppp1r14a promoter and the nucleotide sequences within these E-boxes were partially responsible for Neurog2-specific regulation. Our results illustrate multiple novel mechanisms by which Ascl1 and Neurog2 regulate gene repression during neuronal differentiation in P19 cells.

Shh signaling protects Atoh1 from degradation mediated by the E3 ubiquitin ligase Huwe1 in neural precursors

Signaling networks controlled by Sonic hedgehog (SHH) and the transcription factor Atoh1 regulate the proliferation and differentiation of cerebellar granule neuron progenitors (GNPs). Deregulations in those developmental processes lead to medulloblastoma formation, the most common malignant brain tumor in childhood. Although the protein Atoh1 is a key factor during both cerebellar development and medulloblastoma formation, up-to-date detailed mechanisms underlying its function and regulation have remained poorly understood. Here, we report that SHH regulates Atoh1 stability by preventing its phosphodependent degradation by the E3 ubiquitin ligase Huwe1. Our results reveal that SHH and Atoh1 contribute to a positive autoregulatory loop promoting neuronal precursor expansion. Consequently, Huwe1 loss in mouse SHH medulloblastoma illustrates the disruption of this developmental mechanism in cancer. Hence, the crosstalk between SHH signaling and Atoh1 during cerebellar development highlights a collaborative network that could be further targeted in medulloblastoma.

An amino terminal phosphorylation motif regulates intranuclear compartmentalization of Olig2 in neural progenitor cells

The bHLH transcription factor Olig2 is expressed in cycling neural progenitor cells but also in terminally differentiated, myelinating oligodendrocytes. Sustained expression of Olig2 is counterintuitive because all known functions of the protein in expansion of neural progenitors and specification of oligodendrocyte progenitors are completed with the formation of mature white matter. How are the biological functions of Olig2 suppressed in terminally differentiated oligodendrocytes? In previous studies, we have shown that a triple serine motif in the amino terminus of Olig2 is phosphorylated in cycling neural progenitors but not in their differentiated progeny. We now show that phosphorylation of the triple serine motif regulates intranuclear compartmentalization of murine Olig2. Phosphorylated Olig2 is preferentially localized to a transcriptionally active "open" chromatin compartment together with coregulator proteins essential for regulation of gene expression. Unphosphorylated Olig2, as seen in mature white matter, is localized mainly within a transcriptionally inactive, chromatin fraction characterized by condensed and inaccessible DNA. Of special note is the observation that the p53 tumor suppressor protein is confined to the open chromatin fraction. Proximity ligation assays show that phosphorylation brings Olig2 within 30 nm of p53 within the open chromatin compartment. The data thus shed light on previously noted promitogenic functions of phosphorylated Olig2, which reflect, at least in part, an oppositional relationship with p53 functions.

The phosphorylation status of Ascl1 is a key determinant of neuronal differentiation and maturation in vivo and in vitro

Generation of neurons from patient fibroblasts using a combination of developmentally defined transcription factors has great potential in disease modelling, as well as ultimately for use in regeneration and repair. However, generation of physiologically mature neurons in vitro remains problematic. Here we demonstrate the cell-cycle-dependent phosphorylation of a key reprogramming transcription factor, Ascl1, on multiple serine-proline sites. This multisite phosphorylation is a crucial regulator of the ability of Ascl1 to drive neuronal differentiation and maturation in vivo in the developing embryo; a phosphomutant form of Ascl1 shows substantially enhanced neuronal induction activity in Xenopus embryos. Mechanistically, we see that this un(der)phosphorylated Ascl1 is resistant to inhibition by both cyclin-dependent kinase activity and Notch signalling, both of which normally limit its neurogenic potential. Ascl1 is a central component of reprogramming transcription factor cocktails to generate neurons from human fibroblasts; the use of phosphomutant Ascl1 in place of the wild-type protein significantly promotes neuronal maturity after human fibroblast reprogramming in vitro. These results demonstrate that cell-cycle-dependent post-translational modification of proneural proteins directly regulates neuronal differentiation in vivo during development, and that this regulatory mechanism can be harnessed to promote maturation of neurons obtained by transdifferentiation of human cells in vitro.

Cell cycle regulation of proliferation versus differentiation in the central nervous system

Formation of the central nervous system requires a period of extensive progenitor cell proliferation, accompanied or closely followed by differentiation; the balance between these two processes in various regions of the central nervous system gives rise to differential growth and cellular diversity. The correlation between cell cycle lengthening and differentiation has been reported across several types of cell lineage and from diverse model organisms, both in vivo and in vitro. Furthermore, different cell fates might be determined during different phases of the preceding cell cycle, indicating direct cell cycle influences on both early lineage commitment and terminal cell fate decisions. Significant advances have been made in the last decade and have revealed multi-directional interactions between the molecular machinery regulating the processes of cell proliferation and neuronal differentiation. Here, we first introduce the modes of proliferation in neural progenitor cells and summarise evidence linking cell cycle length and neuronal differentiation. Second, we describe the manner in which components of the cell cycle machinery can have additional and, sometimes, cell-cycle-independent roles in directly regulating neurogenesis. Finally, we discuss the way that differentiation factors, such as proneural bHLH proteins, can promote either progenitor maintenance or differentiation according to the cellular environment. These intricate connections contribute to precise coordination and the ultimate division versus differentiation decision.

Established monolayer differentiation of mouse embryonic stem cells generates heterogeneous neocortical-like neurons stalled at a stage equivalent to midcorticogenesis

Two existing and widely applied protocols of embryonic stem (ES) cell differentiation have been developed to enable in vitro generation of neurons resembling neocortical projection neurons in monolayer culture and from embryoid bodies. The monolayer approach offers advantages for detailed in vitro characterizations and potential mechanistic and therapeutic screening. We investigated whether mouse ES cells undergoing largely undirected neocortical differentiation in monolayer culture recapitulate progressive developmental programs of in vivo progenitor and postmitotic differentiation and whether they develop into specific neocortical subtypes. We find that ES-derived mitotic cells that have been dorsalized by the sonic hedgehog antagonist cyclopamine, and that express, as a total population, cardinal markers of telencephalic progenitors, are, in fact, molecularly heterogeneous. We next show that these progenitors subsequently generate small numbers of heterogeneous neocortical-like neurons that are "stalled" at an immature stage of differentiation, based on multiple developmental criteria. Although some aspects of neocortical development are recapitulated by existing protocols of ES cell differentiation, these data indicate that mouse ES-derived neocortical progenitors both are more heterogeneous than their in vivo counterparts and seemingly include many incorrectly specified progenitors. Furthermore, these ES-derived progenitors spontaneously differentiate into sparse, and incompletely and largely imprecisely differentiated, neocortical-like neurons that fail to adopt specific neuronal identities in vitro. These results provide both foundation and motivation for refining and enhancing directed differentiation of clinically important neocortical projection neuron subtypes.

GSK3β promotes the differentiation of oligodendrocyte precursor cells via β-catenin-mediated transcriptional regulation

Oligodendrocytes are generated by the differentiation and maturation of oligodendrocyte precursor cells (OPCs). The failure of OPC differentiation is a major cause of demyelinating diseases; thus, identifying the molecular mechanisms that affect OPC differentiation is critical for understanding the myelination process and repairing after demyelination. Although prevailing evidence shows that OPC differentiation is a highly coordinated process controlled by multiple extrinsic and intrinsic factors, such as growth factors, axon signals, and transcription factors, the intracellular signaling in OPC differentiation is still unclear. Here, we showed that glycogen synthase kinase 3β (GSK3β) is an essential positive modulator of OPC differentiation. Both pharmacologic inhibition and knockdown of GSK3β remarkably suppressed OPC differentiation. Terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assays and Ki67 staining showed that the effect of GSK3β on OPC differentiation was not via cell death. Conversely, activated GSK3β was sufficient to promote OPC differentiation. Our results also demonstrated that the transcription of myelin genes was regulated by GSK3β inhibition, accompanying accumulated nuclear β-catenin, and reduced the expression of transcriptional factors that are relevant to the expression of myelin genes. Taken together, our study identified GSK3β as a profound positive regulator of OPC differentiation, suggesting that GSK3β may contribute to the inefficient regeneration of oligodendrocytes and myelin repair after demyelination.

RAS/ERK signaling controls proneural genetic programs in cortical development and gliomagenesis

Neural cell fate specification is well understood in the embryonic cerebral cortex, where the proneural genes Neurog2 and Ascl1 are key cell fate determinants. What is less well understood is how cellular diversity is generated in brain tumors. Gliomas and glioneuronal tumors, which are often localized in the cerebrum, are both characterized by a neoplastic glial component, but glioneuronal tumors also have an intermixed neuronal component. A core abnormality in both tumor groups is overactive RAS/ERK signaling, a pro-proliferative signal whose contributions to cell differentiation in oncogenesis are largely unexplored. We found that RAS/ERK activation levels differ in two distinct human tumors associated with constitutively active BRAF. Pilocytic astrocytomas, which contain abnormal glial cells, have higher ERK activation levels than gangliogliomas, which contain abnormal neuronal and glial cells. Using in vivo gain of function and loss of function in the mouse embryonic neocortex, we found that RAS/ERK signals control a proneural genetic switch, inhibiting Neurog2 expression while inducing Ascl1, a competing lineage determinant. Furthermore, we found that RAS/ERK levels control Ascl1's fate specification properties in murine cortical progenitors--at higher RAS/ERK levels, Ascl1(+) progenitors are biased toward proliferative glial programs, initiating astrocytomas, while at moderate RAS/ERK levels, Ascl1 promotes GABAergic neuronal and less glial differentiation, generating glioneuronal tumors. Mechanistically, Ascl1 is phosphorylated by ERK, and ERK phosphoacceptor sites are necessary for Ascl1's GABAergic neuronal and gliogenic potential. RAS/ERK signaling thus acts as a rheostat to influence neural cell fate selection in both normal cortical development and gliomagenesis, controlling Neurog2-Ascl1 expression and Ascl1 function.

Generating spinal motor neuron diversity: a long quest for neuronal identity

Understanding how thousands of different neuronal types are generated in the CNS constitutes a major challenge for developmental neurobiologists and is a prerequisite before considering cell or gene therapies of nervous lesions or pathologies. During embryonic development, spinal motor neurons (MNs) segregate into distinct subpopulations that display specific characteristics and properties including molecular identity, migration pattern, allocation to specific motor columns, and innervation of defined target. Because of the facility to correlate these different characteristics, the diversification of spinal MNs has become the model of choice for studying the molecular and cellular mechanisms underlying the generation of multiple neuronal populations in the developing CNS. Therefore, how spinal motor neuron subpopulations are produced during development has been extensively studied during the last two decades. In this review article, we will provide a comprehensive overview of the genetic and molecular mechanisms that contribute to the diversification of spinal MNs.

Small molecules enable neurogenin 2 to efficiently convert human fibroblasts into cholinergic neurons

Cell fate can be reprogrammed by modifying intrinsic and extrinsic cues. Here, we show that two small molecules (forskolin and dorsomorphin) enable the transcription factor Neurogenin 2 (NGN2) to convert human fetal lung fibroblasts into cholinergic neurons with high purity (>90%) and efficiency (up to 99% of NGN2-expressing cells). The conversion is direct without passing through a proliferative progenitor state. These human induced cholinergic neurons show mature electrophysiological properties and exhibit motor neuron-like features including morphology, gene expression, and the formation of functional neuromuscular junctions. Inclusion of an additional transcription factor, SOX11 also efficiently converts postnatal and adult skin fibroblasts from healthy and diseased human patients to cholinergic neurons. Taken together, this study identifies a simple and highly efficient strategy for reprogramming human fibroblasts to subtype-specific neurons. These findings offer a unique venue for investigating the molecular mechanisms underlying cellular plasticity and human neurodegenerative diseases.