Regulation of motor neuron specification by phosphorylation of neurogenin 2

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.

Proneural bHLH genes in development and disease

Proneural genes encode evolutionarily conserved basic-helix-loop-helix transcription factors. In Drosophila, proneural genes are required and sufficient to confer a neural identity onto naïve ectodermal cells, inducing delamination and subsequent neuronal differentiation. In vertebrates, proneural genes are expressed in cells that already have a neural identity, but they are still required and sufficient to initiate neurogenesis. In all organisms, proneural genes control neurogenesis by regulating Notch-mediated lateral inhibition and initiating the expression of downstream differentiation genes. The general mode of proneural gene function has thus been elucidated. However, the regulatory mechanisms that spatially and temporally control proneural gene function are only beginning to be deciphered. Understanding how proneural gene function is regulated is essential, as aberrant proneural gene expression has recently been linked to a variety of human diseases-ranging from cancer to neuropsychiatric illnesses and diabetes. Recent insights into proneural gene function in development and disease are highlighted herein.

Proneural genes in neocortical development

Neurons, astrocytes and oligodendrocytes arise from CNS progenitor cells at defined times and locations during development, with transcription factors serving as key determinants of these different neural cell fates. An emerging theme is that the transcription factors that specify CNS cell fates function in a context-dependent manner, regulated by post-translational modifications and epigenetic alterations that partition the genome (and hence target genes) into active or silent domains. Here we profile the critical roles of the proneural genes, which encode basic-helix-loop-helix (bHLH) transcription factors, in specifying neural cell identities in the developing neocortex. In particular, we focus on the proneural genes Neurogenin 1 (Neurog1), Neurog2 and Achaete scute-like 1 (Ascl1), which are each expressed in a distinct fashion in the progenitor cell pools that give rise to all of the neuronal and glial cell types of the mature neocortex. Notably, while the basic functions of these proneural genes have been elucidated, it is becoming increasingly evident that tight regulatory controls dictate when, where and how they function. Current efforts to better understand how proneural gene function is regulated will not only improve our understanding of neocortical development, but are also critical to the future development of regenerative therapies for the treatment of neuronal degeneration or disease.

Neuron enriched nuclear proteome isolated from human brain

The brain consists of diverse cell types including neurons, astrocytes, oligodendrocytes, and microglia. The isolation of nuclei from these distinct cell populations provides an opportunity to identify cell-type-specific nuclear proteins, histone modifications, and regulation networks that are altered with normal brain aging or neurodegenerative disease. In this study, we used a method by which intact neuronal and non-neuronal nuclei were purified from human post-mortem brain employing a modification of fluorescence activated cell sorting (FACS) termed fluorescence activated nuclei sorting (FANS). An antibody against NeuN, a neuron specific splicing factor, was used to isolate neuronal nuclei. Utilizing mass spectrometry (MS) based label-free quantitative proteomics, we identified 1755 proteins from sorted NeuN-positive and negative nuclear extracts. Approximately 20% of these proteins were significantly enriched or depleted in neuronal versus non-neuronal populations. Immunoblots of primary cultured rat neuron, astrocyte, and oligodendrocyte extracts confirmed that distinct members of the major nucleocytoplasmic structural linkage complex (LINC), nesprin-1 and nesprin-3, were differentially enriched in neurons and astrocytes, respectively. These comparative proteomic data sets also reveal a number of transcription and splicing factors that are selectively enriched in a cell-type-specific manner in human brain.

Role for Lhx2 in corticogenesis through regulation of progenitor differentiation

The neocortex represents the brain region that has undergone a major increase in its relative size during the course of mammalian evolution. The larger cortex results from a corresponding increase in progenitor cell number. The progenitors giving rise to neocortex are located in the ventricular zone of the dorsal telencephalon and highly express Lhx2, a LIM-homeodomain transcription factor. The neocortex fails to form in the Lhx2 constitutive knockout, indicating a role for Lhx2 in corticogenesis, but mid-embryonic lethality of the Lhx2 knockout requires the use of conditional strategies for further studies. Therefore, to explore Lhx2 function in neocortical progenitors, we generated mice with Lhx2 conditionally deleted from cortical progenitors at the onset of neurogenesis. We find that Lhx2 is critical for maintaining the proliferative state of neocortical progenitors during corticogenesis. In the conditional knockouts, the neocortex is formed but is significantly smaller than wild type. We find that deletion of Lhx2 leads to significantly decreased numbers of cortical progenitors and premature neuronal differentiation. A likely mechanism is indicated by our findings that Lhx2 is required for the expression of Hes1 in cortical progenitors, a key effector in the Notch signaling pathway that maintains the proliferative progenitor state. We conclude that Lhx2 regulates the balance between proliferation and differentiation in cortical progenitors and through this mechanism Lhx2 controls cortical size.

GSK3 temporally regulates neurogenin 2 proneural activity in the neocortex

The neocortex is comprised of six neuronal layers that are generated in a defined temporal sequence. While extrinsic and intrinsic cues are known to regulate the sequential production of neocortical neurons, how these factors interact and function in a coordinated manner is poorly understood. The proneural gene Neurog2 is expressed in progenitors throughout corticogenesis, but is only required to specify early-born, deep-layer neuronal identities. Here, we examined how neuronal differentiation in general and Neurog2 function in particular are temporally controlled during murine neocortical development. We found that Neurog2 proneural activity declines in late corticogenesis, correlating with its phosphorylation by GSK3 kinase. Accordingly, GSK3 activity, which is negatively regulated by canonical Wnt signaling, increases over developmental time, while Wnt signaling correspondingly decreases. When ectopically activated, GSK3 inhibits Neurog2-mediated transcription in cultured cells and Neurog2 proneural activities in vivo. Conversely, a reduction in GSK3 activity promotes the precocious differentiation of later stage cortical progenitors without influencing laminar fate specification. Mechanistically, we show that GSK3 suppresses Neurog2 activity by influencing its choice of dimerization partner, promoting heterodimeric interactions with E47 (Tcfe2a), as opposed to Neurog2-Neurog2 homodimer formation, which occurs when GSK3 activity levels are low. At the functional level, Neurog2-E47 heterodimers have a reduced ability to transactivate neuronal differentiation genes compared with Neurog2-Neurog2 homodimers, both in vitro and in vivo. We thus conclude that the temporal regulation of Neurog2-E47 heterodimerization by GSK3 is a central component of the neuronal differentiation "clock" that coordinates the timing and tempo of neocortical neurogenesis in mouse.

Regulation of locomotion and motoneuron trajectory selection and targeting by the Drosophila homolog of Olig family transcription factors

During the development of locomotion circuits it is essential that motoneurons with distinct subtype identities select the correct trajectories and target muscles. In vertebrates, the generation of motoneurons and myelinating glia depends on Olig2, one of the five Olig family bHLH transcription factors. We investigated the so far unknown function of the single Drosophila homolog Oli. Combining behavioral and genetic approaches, we demonstrate that oli is not required for gliogenesis, but plays pivotal roles in regulating larval and adult locomotion, and axon pathfinding and targeting of embryonic motoneurons. In the embryonic nervous system, Oli is primarily expressed in postmitotic progeny, and in particular, in distinct ventral motoneuron subtypes. oli mediates axonal trajectory selection of these motoneurons within the ventral nerve cord and targeting to specific muscles. Genetic interaction assays suggest that oli acts as part of a conserved transcription factor ensemble including Lim3, Islet and Hb9. Moreover, oli is expressed in postembryonic leg-innervating motoneuron lineages and required in glutamatergic neurons for walking. Finally, over-expression of vertebrate Olig2 partially rescues the walking defects of oli-deficient flies. Thus, our findings reveal a remarkably conserved role of Drosophila Oli and vertebrate family members in regulating motoneuron development, while the steps that require their function differ in detail.

Phosphorylated olig1 localizes to the cytosol of oligodendrocytes and promotes membrane expansion and maturation

Oligodendroglial cells undergo rapid transcriptional and dynamic morphological transformation in order to effectively myelinate neuronal axons. Olig1, a basic helix-loop-helix transcription factor, functions to promote the transcription of myelin-specific genes and promotes differentiation and (re)myelination. While the role for nuclear Olig1 is well established, the function for cytoplasmic Olig1 remains uncertain. We observe that translocation of Olig1 into the cytosol highly correlates with differentiation of oligodendrocytes both invivo and invitro. By enforcing expression of a nuclear-specific form of Olig1 into OPCs in a null-Olig1 background, we demonstrate that nuclear Olig1 is sufficient to facilitate MBP expression, but with greatly diminished membrane volume and area. We demonstrate that serine 138 in the helix-loop-helix domain of Olig1 is phosphorylated and that this form resides in the cytosol. Mutating serine 138 to alanine restricts Olig1 to the nucleus, facilitating MBP expression but limiting membrane expansion. However, a serine to aspartic acid mutation results in the cytoplasmic localization of Olig1 enhancing membrane expansion. Our results suggest a novel role for a phosphorylated cytosolic Olig1 in membrane expansion and maturation of oligodendrocytes.

Post-translational modification of Ngn2 differentially affects transcription of distinct targets to regulate the balance between progenitor maintenance and differentiation

Neurogenin 2 (Ngn2) controls neuronal differentiation cell-autonomously by transcriptional activation of targets such as NeuroD, while simultaneously controlling progenitor maintenance non-cell-autonomously by upregulating Delta expression and Notch signalling. Reduction in Cdk-dependent multisite phosphorylation of Ngn2 enhances its promoter binding affinity. This leads specifically to an increase in neuronal differentiation without an apparent increase in progenitor maintenance via Delta-Notch signalling, although the mechanism underlying this imbalance remains unclear. Here we show in Xenopus embryos and mouse P19 cells that the NeuroD promoter is substantially more sensitive to the phosphorylation status of Ngn2 than the Delta promoter, and that this can be attributed to differences in the ease of promoter activation. In addition, we also show that the phosphorylation status of Ngn2 regulates sensitivity to Notch signalling. These observations explain how Ngn2 post-translational modification in response to changes in the cell cycle kinase environment results in enhanced neuronal differentiation upon cell cycle lengthening.

Role of fibroblast growth factor receptors in astrocytic stem cells

There are two well-defined neurogenic regions in the adult brain, the subventricular zone (SVZ) lining the lateral wall of the lateral ventricles and, the subgranular zone (SGZ) in the dentate gyrus at the hippocampus. Within these neurogenic regions, there are neural stem cells with astrocytic characteristics, which actively respond to the basic fibroblast growth factor (bFGF, FGF2 or FGF-β) by increasing their proliferation, survival and differentiation, both in vivo and in vitro. FGF2 binds to fibroblast growth factor receptors 1 to 4 (FGFR1, FGFR2, FGFR3, FGFR4). Interestingly, these receptors are differentially expressed in neurogenic progenitors. During development, FGFR-1 and FGFR-2 drive oligodendrocytes and motor neuron specification. In particular, FGFR-1 determines oligodendroglial and neuronal cell fate, whereas FGFR-2 is related to oligodendrocyte specification. In the adult SVZ, FGF-2 promotes oligodendrogliogenesis and myelination. FGF-2 deficient mice show a reduction in the number of new neurons in the SGZ, which suggests that FGFR-1 is important for neuronal cell fate in the adult hippocampus. In human brain, FGF-2 appears to be an important component in the anti-depressive effect of drugs. In summary, FGF2 is an important modulator of the cell fate of neural precursor and, promotes oligodendrogenesis. In this review, we describe the expression pattern of FGFR2 and its role in neural precursors derived from the SVZ and the SGZ.

GSK-3: Functional Insights from Cell Biology and Animal Models

Glycogen synthase kinase-3 (GSK-3) is a widely expressed and highly conserved serine/threonine protein kinase encoded in mammals by two genes that generate two related proteins: GSK-3α and GSK-3β. GSK-3 is active in cells under resting conditions and is primarily regulated through inhibition or diversion of its activity. While GSK-3 is one of the few protein kinases that can be inactivated by phosphorylation, the mechanisms of GSK-3 regulation are more varied and not fully understood. Precise control appears to be achieved by a combination of phosphorylation, localization, and sequestration by a number of GSK-3-binding proteins. GSK-3 lies downstream of several major signaling pathways including the phosphatidylinositol 3′ kinase pathway, the Wnt pathway, Hedgehog signaling and Notch. Specific pools of GSK-3, which differ in intracellular localization, binding partner affinity, and relative amount are differentially sensitized to several distinct signaling pathways and these sequestration mechanisms contribute to pathway insulation and signal specificity. Dysregulation of signaling pathways involving GSK-3 is associated with the pathogenesis of numerous neurological and psychiatric disorders and there are data suggesting GSK-3 isoform-selective roles in several of these. Here, we review the current knowledge of GSK-3 regulation and targets and discuss the various animal models that have been employed to dissect the functions of GSK-3 in brain development and function through the use of conventional or conditional knockout mice as well as transgenic mice. These studies have revealed fundamental roles for these protein kinases in memory, behavior, and neuronal fate determination and provide insights into possible therapeutic interventions.

Roles of p53 and p27(Kip1) in the regulation of neurogenesis in the murine adult subventricular zone

The tumor suppressor protein p53 (Trp53) and the cell cycle inhibitor p27(Kip1) (Cdknb1) have both been implicated in regulating proliferation of adult subventricular zone (aSVZ) cells. We previously reported that genetic ablation of Trp53 (Trp53-/-) or Cdknb1 (p27(Kip1-/-) ) increased proliferation of cells in the aSVZ, but differentially affected the number of adult born neuroblasts. We therefore hypothesized that these molecules might play non-redundant roles. To test this hypothesis we generated mice lacking both genes (Trp53-/- ;p27(Kip1-/-) ) and analysed the consequences on aSVZ cells and adult neuroblasts. Proliferation and self-renewal of cultured aSVZ cells were increased in the double mutants compared with control, but the mice did not develop spontaneous brain tumors. In contrast, the number of adult-born neuroblasts in the double mutants was similar to wild-type animals and suggested a complementation of the p27(Kip1-/-) phenotype due to loss of Trp53. Cellular differences detected in the aSVZ correlated with cellular changes in the olfactory bulb and behavioral data on novel odor recognition. The exploration time for new odors was reduced in p27(Kip1-/-) mice, increased in Trp53-/- mice and normalized in the double Trp53-/- ;p27(Kip1-/-) mutants. At the molecular level, Trp53-/- aSVZ cells were characterized by higher levels of NeuroD and Math3 and by the ability to generate neurons more readily. In contrast, p27(Kip1-/-) cells generated fewer neurons, due to enhanced proteasomal degradation of pro-neural transcription factors. Together, these results suggest that p27(Kip1) and p53 function non-redundantly to modulate proliferation and self-renewal of aSVZ cells and antagonistically in regulating adult neurogenesis.

Critical role of PI3K/Akt/GSK3β in motoneuron specification from human neural stem cells in response to FGF2 and EGF

Fibroblast growth factor (FGF) and epidermal growth factor (EGF) are critical for the development of the nervous system. We previously discovered that FGF2 and EGF had opposite effects on motor neuron differentiation from human fetal neural stem cells (hNSCs), but the underlying mechanisms remain unclear. Here, we show that FGF2 and EGF differentially affect the temporal patterns of Akt and glycogen synthase kinase 3 beta (GSK3β) activation. High levels of phosphatidylinositol 3-kinase (PI3K)/Akt activation accompanied with GSK3β inactivation result in reduction of the motor neuron transcription factor HB9. Inhibition of PI3K/Akt by chemical inhibitors or RNA interference or overexpression of a constitutively active form of GSK3β enhances HB9 expression. Consequently, PI3K inhibition increases hNSCs differentiation into HB9⁺/microtubule-associated protein 2 (MAP2)⁺ motor neurons in vitro. More importantly, blocking PI3K not only enhances motor neuron differentiation from hNSCs grafted into the ventral horn of adult rat spinal cords, but also permits ectopic generation of motor neurons in the dorsal horn by overriding environmental influences. Our data suggest that FGF2 and EGF affect the motor neuron fate decision in hNSCs differently through a fine tuning of the PI3K/AKT/GSK3β pathway, and that manipulation of this pathway can enhance motor neuron generation.

Neurogenin2 regulates the initial axon guidance of cortical pyramidal neurons projecting medially to the corpus callosum

Background

The formation of the mammalian central nervous system requires the establishment of complex neural circuits between a diverse array of neuronal subtypes. Here we report that the proneural transcription factor Neurogenin2 (Ngn2) is crucial for the proper specification of cortical axon projections.

Results

The genetic loss of Ngn2 in mice results in fewer callosal axons projecting towards the midline as well as abnormal midline crossing. shRNA-mediated knockdown of Ngn2 revealed its cell-autonomous requirement for the proper projection of axons from layer 2/3 pyramidal neurons to the midline in vivo. We found that the acute loss of Ngn2 in vivo induces the axon of superficial layer 2/3 neurons to project laterally towards aberrant cortical and subcortical targets.

Conclusions

These and previous results demonstrate that Ngn2 is required for the coordinated specification of cardinal features defining the phenotype of cortical pyramidal neurons, including their migration properties, dendritic morphology and axonal projection.

LMO4 functions as a co-activator of neurogenin 2 in the developing cortex

The proneural protein neurogenin 2 (NGN2) is a key transcription factor in regulating both neurogenesis and neuronal radial migration in the embryonic cerebral cortex. However, the co-factors that support the action of NGN2 in the cortex remain unclear. Here, we show that the LIM-only protein LMO4 functions as a novel co-factor of NGN2 in the developing cortex. LMO4 and its binding partner nuclear LIM interactor (NLI/LDB1/CLIM2) interact with NGN2 simultaneously, forming a multi-protein transcription complex. This complex is recruited to the E-box containing enhancers of NGN2-target genes, which regulate various aspects of cortical development, and activates NGN2-mediated transcription. Correspondingly, analysis of Lmo4-null embryos shows that the loss of LMO4 leads to impairments of neuronal differentiation in the cortex. In addition, expression of LMO4 facilitates NGN2-mediated radial migration of cortical neurons in the embryonic cortex. Our results indicate that LMO4 promotes the acquisition of cortical neuronal identities by forming a complex with NGN2 and subsequently activating NGN2-dependent gene expression.

Cell cycle-regulated multi-site phosphorylation of Neurogenin 2 coordinates cell cycling with differentiation during neurogenesis

During development of the central nervous system, the transition from progenitor maintenance to differentiation is directly triggered by a lengthening of the cell cycle that occurs as development progresses. However, the mechanistic basis of this regulation is unknown. The proneural transcription factor Neurogenin 2 (Ngn2) acts as a master regulator of neuronal differentiation. Here, we demonstrate that Ngn2 is phosphorylated on multiple serine-proline sites in response to rising cyclin-dependent kinase (cdk) levels. This multi-site phosphorylation results in quantitative inhibition of the ability of Ngn2 to induce neurogenesis in vivo and in vitro. Mechanistically, multi-site phosphorylation inhibits binding of Ngn2 to E box DNA, and inhibition of DNA binding depends on the number of phosphorylation sites available, quantitatively controlling promoter occupancy in a rheostat-like manner. Neuronal differentiation driven by a mutant of Ngn2 that cannot be phosphorylated by cdks is no longer inhibited by elevated cdk kinase levels. Additionally, phosphomutant Ngn2-driven neuronal differentiation shows a reduced requirement for the presence of cdk inhibitors. From these results, we propose a model whereby multi-site cdk-dependent phosphorylation of Ngn2 interprets cdk levels to control neuronal differentiation in response to cell cycle lengthening during development.

NeuroD factors regulate cell fate and neurite stratification in the developing retina

Members of the basic helix-loop-helix (bHLH) family of transcription factors have been shown to control critical aspects of development in many tissues. To identify bHLH genes that might regulate specific aspects of retinal cell development, we investigated the expression of bHLH genes in single, developing mouse retinal cells, with particular emphasis on the NeuroD family. Two of these factors, NeuroD2 and NeuroD6/NEX, had not been previously reported as expressed in the retina. A series of loss- and gain-of-function experiments was performed, which suggested that NeuroD genes have both similarities and differences in their activities. Notably, misexpression of NeuroD genes can direct amacrine cell processes to two to three specific sublaminae in the inner plexiform layer. This effect is specific to cell type and NeuroD gene, as the AII amacrine cell type is refractory to the effects of NeuroD1 and NeuroD6, but uniquely sensitive to the effect of NeuroD2 on neurite targeting. Additionally, NeuroD2 is endogenously expressed in AII amacrine cells, among others, and loss of NeuroD2 function results in a partial loss of AII amacrine cells. The effects of misexpressing NeuroD genes on retinal cell fate determination also suggested shared and divergent functions. Remarkably, NeuroD2 misexpression induced ganglion cell production even after the normal developmental window of ganglion cell genesis. Together, these data suggest that members of the NeuroD family are important for neuronal cell type identity and may be involved in several cell type-specific aspects of retinal development, including fate determination, differentiation, morphological development, and circuit formation.

A novel function of the proneural factor Ascl1 in progenitor proliferation identified by genome-wide characterization of its targets

Proneural genes such as Ascl1 are known to promote cell cycle exit and neuronal differentiation when expressed in neural progenitor cells. The mechanisms by which proneural genes activate neurogenesis--and, in particular, the genes that they regulate--however, are mostly unknown. We performed a genome-wide characterization of the transcriptional targets of Ascl1 in the embryonic brain and in neural stem cell cultures by location analysis and expression profiling of embryos overexpressing or mutant for Ascl1. The wide range of molecular and cellular functions represented among these targets suggests that Ascl1 directly controls the specification of neural progenitors as well as the later steps of neuronal differentiation and neurite outgrowth. Surprisingly, Ascl1 also regulates the expression of a large number of genes involved in cell cycle progression, including canonical cell cycle regulators and oncogenic transcription factors. Mutational analysis in the embryonic brain and manipulation of Ascl1 activity in neural stem cell cultures revealed that Ascl1 is indeed required for normal proliferation of neural progenitors. This study identified a novel and unexpected activity of the proneural gene Ascl1, and revealed a direct molecular link between the phase of expansion of neural progenitors and the subsequent phases of cell cycle exit and neuronal differentiation.

What Are the bona fide GSK3 Substrates?

Nearly 100 proteins are proposed to be substrates for GSK3, suggesting that this enzyme is a fundamental regulator of almost every process in the cell, in every tissue in the body. However, it is not certain how many of these proposed substrates are regulated by GSK3 in vivo. Clearly, the identification of the physiological functions of GSK3 will be greatly aided by the identification of its bona fide substrates, and the development of GSK3 as a therapeutic target will be highly influenced by this range of actions, hence the need to accurately establish true GSK3 substrates in cells. In this paper the evidence that proposed GSK3 substrates are likely to be physiological targets is assessed, highlighting the key cellular processes that could be modulated by GSK3 activity and inhibition.

Phosphorylation regulates OLIG2 cofactor choice and the motor neuron-oligodendrocyte fate switch

A fundamental feature of central nervous system development is that neurons are generated before glia. In the embryonic spinal cord, for example, a group of neuroepithelial stem cells (NSCs) generates motor neurons (MNs), before switching abruptly to oligodendrocyte precursors (OLPs). We asked how transcription factor OLIG2 participates in this MN-OLP fate switch. We found that Serine 147 in the helix-loop-helix domain of OLIG2 was phosphorylated during MN production and dephosphorylated at the onset of OLP genesis. Mutating Serine 147 to Alanine (S147A) abolished MN production without preventing OLP production in transgenic mice, chicks, or cultured P19 cells. We conclude that S147 phosphorylation, possibly by protein kinase A, is required for MN but not OLP genesis and propose that dephosphorylation triggers the MN-OLP switch. Wild-type OLIG2 forms stable homodimers, whereas mutant (unphosphorylated) OLIG2(S147A) prefers to form heterodimers with Neurogenin 2 or other bHLH partners, suggesting a molecular basis for the switch.

GSK3 signalling in neural development

Recent evidence suggests that glycogen synthase kinase 3 (GSK3) proteins and their upstream and downstream regulators have key roles in many fundamental processes during neurodevelopment. Disruption of GSK3 signalling adversely affects brain development and is associated with several neurodevelopmental disorders. Here, we discuss the mechanisms by which GSK3 activity is regulated in the nervous system and provide an overview of the recent advances in the understanding of how GSK3 signalling controls neurogenesis, neuronal polarization and axon growth during brain development. These recent advances suggest that GSK3 is a crucial node that mediates various cellular processes that are controlled by multiple signalling molecules--for example, disrupted in schizophrenia 1 (DISC1), partitioning defective homologue 3 (PAR3), PAR6 and Wnt proteins--that regulate neurodevelopment.

Dynamin I phosphorylation by GSK3 controls activity-dependent bulk endocytosis of synaptic vesicles

Glycogen synthase kinase 3 (GSK3) is a critical enzyme in neuronal physiology; however, it is not yet known whether it has any specific role in presynaptic function. We found that GSK3 phosphorylates a residue on the large GTPase dynamin I (Ser-774) both in vitro and in primary rat neuronal cultures. This was dependent on prior phosphorylation of Ser-778 by cyclin-dependent kinase 5. Using both acute inhibition with pharmacological antagonists and silencing of expression with short hairpin RNA, we found that GSK3 was specifically required for activity-dependent bulk endocytosis (ADBE) but not clathrin-mediated endocytosis. Moreover we found that the specific phosphorylation of Ser-774 on dynamin I by GSK3 was both necessary and sufficient for ADBE. These results demonstrate a presynaptic role for GSK3 and they indicate that a protein kinase signaling cascade prepares synaptic vesicles for retrieval during elevated neuronal activity.

Directing astroglia from the cerebral cortex into subtype specific functional neurons

Forced expression of single defined transcription factors can selectively and stably convert cultured astroglia into synapse-forming excitatory and inhibitory neurons.

Astroglia from the postnatal cerebral cortex can be reprogrammed in vitro to generate neurons following forced expression of neurogenic transcription factors, thus opening new avenues towards a potential use of endogenous astroglia for brain repair. However, in previous attempts astroglia-derived neurons failed to establish functional synapses, a severe limitation towards functional neurogenesis. It remained therefore also unknown whether neurons derived from reprogrammed astroglia could be directed towards distinct neuronal subtype identities by selective expression of distinct neurogenic fate determinants. Here we show that strong and persistent expression of neurogenic fate determinants driven by silencing-resistant retroviral vectors instructs astroglia from the postnatal cortex in vitro to mature into fully functional, synapse-forming neurons. Importantly, the neurotransmitter fate choice of astroglia-derived neurons can be controlled by selective expression of distinct neurogenic transcription factors: forced expression of the dorsal telencephalic fate determinant neurogenin-2 (Neurog2) directs cortical astroglia to generate synapse-forming glutamatergic neurons; in contrast, the ventral telencephalic fate determinant Dlx2 induces a GABAergic identity, although the overall efficiency of Dlx2-mediated neuronal reprogramming is much lower compared to Neurog2, suggesting that cortical astroglia possess a higher competence to respond to the dorsal telencephalic fate determinant. Interestingly, however, reprogramming of astroglia towards the generation of GABAergic neurons was greatly facilitated when the astroglial cells were first expanded as neurosphere cells prior to transduction with Dlx2. Importantly, this approach of expansion under neurosphere conditions and subsequent reprogramming with distinct neurogenic transcription factors can also be extended to reactive astroglia isolated from the adult injured cerebral cortex, allowing for the selective generation of glutamatergic or GABAergic neurons. These data provide evidence that cortical astroglia can undergo a conversion across cell lineages by forced expression of a single neurogenic transcription factor, stably generating fully differentiated neurons. Moreover, neuronal reprogramming of astroglia is not restricted to postnatal stages but can also be achieved from terminally differentiated astroglia of the adult cerebral cortex following injury-induced reactivation.

The brain consists of two major cell types: neurons, which transmit information, and glial cells, which support and protect neurons. Interestingly, evidence suggests that some glial cells, including astroglia, can be directly converted into neurons by specific proteins, a transformation that may aid in the functional repair of damaged brain tissue. However, in order for the repaired brain areas to function properly, it is important that astroglia be directed into appropriate neuronal subclasses. In this study, we show that non-neurogenic astroglia from the cerebral cortex can be reprogrammed in vitro using just a single transcription factor to yield fully functional excitatory or inhibitory neurons. We achieved this result through forced expression of the same transcription factors that instruct the genesis of these distinct neuronal subtypes during embryonic forebrain development. Moreover we demonstrate that reactive astroglia isolated from the adult cortex after local injury can be reprogrammed into synapse-forming excitatory or inhibitory neurons following a similar strategy. Our findings provide evidence that endogenous glial cells may prove a promising strategy for replacing neurons that have degenerated due to trauma or disease.

Akt regulates the expression of MafK, synaptotagmin I, and syntenin-1, which play roles in neuronal function

BACKGROUND

Akt regulates various cellular processes, including cell growth, survival, and metabolism. Recently, Akt's role in neurite outgrowth has also emerged. We thus aimed to identify neuronal function-related genes that are regulated by Akt.

METHODS

We performed suppression subtractive hybridization on two previously established PC12 sublines, one of which overexpresses the wild-type (WT) form and the other, the dominant-negative (DN) form of Akt. These sublines respond differently to NGF's neuronal differentiation effect.

RESULTS

A variety of genes was identified and could be classified into several functional groups, one of which was developmental processes. Two genes involved in neuronal differentiation and function were found in this group. v-Maf musculoaponeurotic fibrosarcoma oncogene homolog K (MafK) induces the neuronal differentiation of PC12 cells and immature telencephalon neurons, and synaptotagmin I (SytI) is essential for neurotransmitter release. Another gene, syntenin-1 (Syn-1) was also recognized in the same functional group into which MafK and SytI were classified. Syn-1 has been reported to promote the formation of membrane varicosities in neurons. Quantitative reverse transcription polymerase chain reaction analyses show that the transcript levels of these three genes were lower in PC12 (WT-Akt) cells than in parental PC12 and PC12 (DN-Akt) cells. Furthermore, treatment of PC12 (WT-Akt) cells with an Akt inhibitor resulted in the increase of the expression of these genes and the improvement of neurite outgrowth. These results indicate that dominant-negative or pharmacological inhibition of Akt increases the expression of MafK, SytI, and Syn-1 genes. Using lentiviral shRNA to knock down endogenous Syn-1 expression, we demonstrated that Syn-1 promotes an increase in the numbers of neurites and branches.

CONCLUSIONS

Taken together, these results indicate that Akt negatively regulates the expression of MafK, SytI, and Syn-1 genes that all participate in regulating neuronal integrity in some way or another.

From birth till death: neurogenesis, cell cycle, and neurodegeneration

Neurogenesis in the embryo involves many signaling pathways and transcriptional programs and an elaborate orchestration of cell cycle exit in differentiating precursors. However, while the neurons differentiate into a plethora of different subtypes and different identities, they also presume a highly polar structure with a particular morphology of the cytoskeleton, thereby making it almost impossible for any differentiated cell to re-enter the cell cycle. It has been observed that dysregulated or forced cell cycle reentry is closely linked to neurodegeneration and apoptosis in neurons, most likely through changes in the neurocytoskeleton. However, proliferative cells still exist within the nervous system, and adult neural stem cells (NSCs) have been identified in the Central Nervous System (CNS) in the past decade, raising a great stir in the neuroscience community. NSCs present a new therapeutic potential, and much effort has since gone into understanding the molecular mechanisms driving differentiation of specific neuronal lineages, such as dopaminergic neurons, for use in regenerative medicine, either through transplanted NSCs or manipulation of existing ones. Nevertheless, differentiation and proliferation are two sides of the same coin, just like tumorigenesis and degeneration. Tumor formation may be regarded as a de-differentiation of tissues, where cell cycle mechanisms are reactivated in differentiated cell types. It is thus important to understand the molecular mechanisms underlying various brain tumors in this perspective. The recent Cancer Stem Cell (CSC) hypothesis also suggests the presence of Brain Tumor Initiating Cells (BTICs) within a tumor population, although the exact origin of these rare and mostly elusive BTICs are yet to be identified. This review attempts to investigate the correlation of neural stem cells/precursors, mature neurons, BTICs and brain tumors with respect to cell cycle regulation and the impact of cell cycle in neurodegeneration.

Her6 regulates the neurogenetic gradient and neuronal identity in the thalamus

During vertebrate brain development, the onset of neuronal differentiation is under strict temporal control. In the mammalian thalamus and other brain regions, neurogenesis is regulated also in a spatially progressive manner referred to as a neurogenetic gradient, the underlying mechanism of which is unknown. Here we describe the existence of a neurogenetic gradient in the zebrafish thalamus and show that the progression of neurogenesis is controlled by dynamic expression of the bHLH repressor her6. Members of the Hes/Her family are known to regulate proneural genes, such as Neurogenin and Ascl. Here we find that Her6 determines not only the onset of neurogenesis but also the identity of thalamic neurons, marked by proneural and neurotransmitter gene expression: loss of Her6 leads to premature Neurogenin1-mediated genesis of glutamatergic (excitatory) neurons, whereas maintenance of Her6 leads to Ascl1-mediated production of GABAergic (inhibitory) neurons. Thus, the presence or absence of a single upstream regulator of proneural gene expression, Her6, leads to the establishment of discrete neuronal domains in the thalamus.

Selective induction of neocortical GABAergic neurons by the PDK1-Akt pathway through activation of Mash1

Extracellular stimuli regulate neuronal differentiation and subtype specification during brain development, although the intracellular signaling pathways that mediate these processes remain largely unclear. We now show that the PDK1-Akt pathway regulates differentiation of telencephalic neural precursor cells (NPCs). Active Akt promotes differentiation of NPC into gamma-aminobutyric acid-containing (GABAergic) but not glutamatergic neurons. Disruption of the Pdk1 gene or expression of dominant-negative forms of Akt suppresses insulin-like growth factor (IGF)-1 enhancement of NPC differentiation into neurons in vitro and production of neocortical GABAergic neurons in vivo. Furthermore, active Akt increased the protein levels and transactivation activity of Mash1, a proneural basic helix-loop-helix protein required for the generation of neocortical GABAergic neurons, and Mash1 was required for Akt-induced neuronal differentiation. These results have unveiled an unexpected role of the PDK1-Akt pathway: a key mediator of extracellular signals regulating the production of neocortical GABAergic neurons.

LMO4 controls the balance between excitatory and inhibitory spinal V2 interneurons

Multiple excitatory and inhibitory interneurons form the motor circuit with motor neurons in the ventral spinal cord. Notch signaling initiates the diversification of immature V2-interneurons into excitatory V2a-interneurons and inhibitory V2b-interneurons. Here, we provide a transcriptional regulatory mechanism underlying their balanced production. LIM-only protein LMO4 controls this binary cell fate choice by regulating the activity of V2a- and V2b-specific LIM complexes inversely. In the spinal cord, LMO4 induces GABAergic V2b-interneurons in collaboration with SCL and inhibits Lhx3 from generating glutamatergic V2a-interneuons. In LMO4;SCL compound mutant embryos, V2a-interneurons increase markedly at the expense of V2b-interneurons. We further demonstrate that LMO4 nucleates the assembly of a novel LIM-complex containing SCL, Gata2, and NLI. This complex activates specific enhancers in V2b-genes consisting of binding sites for SCL and Gata2, thereby promoting V2b-interneuron fate. Thus, LMO4 plays essential roles in directing a balanced generation of inhibitory and excitatory neurons in the ventral spinal cord.

ERK5 MAP kinase regulates neurogenin1 during cortical neurogenesis

The commitment of multi-potent cortical progenitors to a neuronal fate depends on the transient induction of the basic-helix-loop-helix (bHLH) family of transcription factors including Neurogenin 1 (Neurog1). Previous studies have focused on mechanisms that control the expression of these proteins while little is known about whether their pro-neural activities can be regulated by kinase signaling pathways. Using primary cultures and ex vivo slice cultures, here we report that both the transcriptional and pro-neural activities of Neurog1 are regulated by extracellular signal-regulated kinase (ERK) 5 signaling in cortical progenitors. Activation of ERK5 potentiated, while blocking ERK5 inhibited Neurog1-induced neurogenesis. Furthermore, endogenous ERK5 activity was required for Neurog1-initiated transcription. Interestingly, ERK5 activation was sufficient to induce Neurog1 phosphorylation and ERK5 directly phosphorylated Neurog1 in vitro. We identified S179/S208 as putative ERK5 phosphorylation sites in Neurog1. Mutations of S179/S208 to alanines inhibited the transcriptional and pro-neural activities of Neurog1. Our data identify ERK5 phosphorylation of Neurog1 as a novel mechanism regulating neuronal fate commitment of cortical progenitors.

Disrupted in schizophrenia 1 regulates neuronal progenitor proliferation via modulation of GSK3beta/beta-catenin signaling

The Disrupted in Schizophrenia 1 (DISC1) gene is disrupted by a balanced chromosomal translocation (1; 11) (q42; q14.3) in a Scottish family with a high incidence of major depression, schizophrenia, and bipolar disorder. Subsequent studies provided indications that DISC1 plays a role in brain development. Here, we demonstrate that suppression of DISC1 expression reduces neural progenitor proliferation, leading to premature cell cycle exit and differentiation. Several lines of evidence suggest that DISC1 mediates this function by regulating GSK3beta. First, DISC1 inhibits GSK3beta activity through direct physical interaction, which reduces beta-catenin phosphorylation and stabilizes beta-catenin. Importantly, expression of stabilized beta-catenin overrides the impairment of progenitor proliferation caused by DISC1 loss of function. Furthermore, GSK3 inhibitors normalize progenitor proliferation and behavioral defects caused by DISC1 loss of function. Together, these results implicate DISC1 in GSK3beta/beta-catenin signaling pathways and provide a framework for understanding how alterations in this pathway may contribute to the etiology of psychiatric disorders.

Gsk3beta/PKA and Gli1 regulate the maintenance of neural progenitors at the midbrain-hindbrain boundary in concert with E(Spl) factor activity

Neuronal production in the midbrain-hindbrain domain (MH) of the vertebrate embryonic neural tube depends on a progenitor pool called the ;intervening zone' (IZ), located at the midbrain-hindbrain boundary. The progressive recruitment of IZ progenitors along the mediolateral (future dorsoventral) axis prefigures the earlier maturation of the MH basal plate. It also correlates with a lower sensitivity of medial versus lateral IZ progenitors to the neurogenesis inhibition process that maintains the IZ pool. This role is performed in zebrafish by the E(Spl) factors Her5 and Her11, but the molecular cascades cooperating with Her5/11, and those accounting for their reduced effect in the medial IZ, remain unknown. We demonstrate here that the kinases Gsk3beta and cAMP-dependent protein kinase A (PKA) are novel determinants of IZ formation and cooperate with E(Spl) activity in a dose-dependent manner. Similar to E(Spl), we show that the activity of Gsk3beta/PKA is sensed differently by medial versus lateral IZ progenitors. Furthermore, we identify the transcription factor Gli1, expressed in medial IZ cells, as an antagonist of E(Spl) and Gsk3beta/PKA, and demonstrate that the neurogenesis-promoting activity of Gli1 accounts for the reduced sensitivity of medial IZ progenitors to neurogenesis inhibitors and their increased propensity to differentiate. We also show that the expression and activity of Gli1 in this process are, surprisingly, independent of Hedgehog signaling. Together, our results suggest a model in which the modulation of E(Spl) and Gsk3beta/PKA activities by Gli1 underlies the dynamic properties of IZ maintenance and recruitment.

Context dependence of proneural bHLH proteins

A key point in neural development is the commitment of progenitor cells to a specific neural fate. In all animals studied, proneural proteins - transcription factors of the basic helix-loop-helix (bHLH) family - are central to this process. The function of these factors is strongly influenced by the spatial and temporal context in which they are expressed. It is important to understand the molecular mechanisms by which developmental context interacts with and modifies the intrinsic functions and properties of the proneural proteins. Recent insights have been obtained in Drosophila and vertebrates from analysis of how bHLH proteins interact with other transcription factors to regulate target genes.

Combinatorial signalling controls Neurogenin2 expression at the onset of spinal neurogenesis

A central issue during embryonic development is to define how different signals cooperate in generating unique cell types. To address this issue, we focused on the function and the regulation of the proneural gene Neurogenin2 (Neurog2) during early mouse spinal neurogenesis. We showed that Neurog2 is first expressed in cells within the neural plate anterior to the node from the 5 somite-stage. The analysis of Neurog2 mutants established a role for this gene in triggering neural differentiation during spinal cord elongation. We identified a 798 base pair enhancer element (Neurog2-798) upstream of the Neurog2 coding sequence that directs the early caudal expression of Neurog2. Embryo culture experiments showed that Retinoic Acid (RA), Sonic hedgehog (Shh) and Fibroblast Growth Factor signals act in concert on this enhancer to control the spatial and temporal induction of Neurog2. We further demonstrated by transgenesis that two RA response elements and a Gli binding site within the Neurog2-798 element are absolutely required for its activity, strongly suggesting that the regulation of Neurog2 early expression by RA and Shh signals is direct. Our data thus support a model where signal integration at the level of a single enhancer constitutes a key mechanism to control the onset of neurogenesis.

Neurogenesis or neuronal specification: phosphorylation strikes again!

How basic helix-loop-helix (bHLH) transcription factors control neurogenesis and neuronal subtype specification through transcriptional mechanisms mediated by cell signaling remains to be fully elucidated. In this issue of Neuron, Ma et al. discover that phosphorylation via GSK3 of the bHLH factor, Ngn2 (Neurog2), adds a neuronal subtype-specific program to its functional repertoire that is activated in the developing neural tube in vivo.