Distinct patterns of downstream target activation are specified by the helix-loop-helix domain of proneural basic helix-loop-helix transcription factors

KDM3A-mediated demethylation of histone H3 lysine 9 facilitates the chromatin binding of Neurog2 during neurogenesis

Neurog2 is a crucial regulator of neuronal fate specification and differentiation in vivo and in vitro. However, it remains unclear how Neurog2 transactivates neuronal genes that are silenced by repressive chromatin. Here, we provide evidence that the histone H3 lysine 9 demethylase KDM3A facilitates the Xenopus Neurog2 (formerly known as Xngnr1) chromatin accessibility during neuronal transcription. Loss-of-function analyses reveal that KDM3A is not required for the transition of naive ectoderm to neural progenitor cells but is essential for primary neuron formation. ChIP series followed by qPCR analyses reveal that Neurog2 promotes the removal of the repressive H3K9me2 marks and addition of active histone marks, including H3K27ac and H3K4me3, at the NeuroD1 and Tubb2b promoters; this activity depends on the presence of KDM3A because Neurog2, via its C-terminal domain, interacts with KDM3A. Interestingly, KDM3A is dispensable for the neuronal transcription initiated by Ascl1, a proneural factor related to neurogenin in the bHLH family. In summary, our findings uncover a crucial role for histone H3K9 demethylation during Neurog2-mediated neuronal transcription and help in the understanding of the different activities of Neurog2 and Ascl1 in initiating neuronal development.

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.

Ascl1 as a novel player in the Ptf1a transcriptional network for GABAergic cell specification in the retina

In contrast with the wealth of data involving bHLH and homeodomain transcription factors in retinal cell type determination, the molecular bases underlying neurotransmitter subtype specification is far less understood. Using both gain and loss of function analyses in Xenopus, we investigated the putative implication of the bHLH factor Ascl1 in this process. We found that in addition to its previously characterized proneural function, Ascl1 also contributes to the specification of the GABAergic phenotype. We showed that it is necessary for retinal GABAergic cell genesis and sufficient in overexpression experiments to bias a subset of retinal precursor cells towards a GABAergic fate. We also analysed the relationships between Ascl1 and a set of other bHLH factors using an in vivo ectopic neurogenic assay. We demonstrated that Ascl1 has unique features as a GABAergic inducer and is epistatic over factors endowed with glutamatergic potentialities such as Neurog2, NeuroD1 or Atoh7. This functional specificity is conferred by the basic DNA binding domain of Ascl1 and involves a specific genetic network, distinct from that underlying its previously demonstrated effects on catecholaminergic differentiation. Our data show that GABAergic inducing activity of Ascl1 requires the direct transcriptional regulation of Ptf1a, providing therefore a new piece of the network governing neurotransmitter subtype specification during retinogenesis.

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.

Interaction of MTG family proteins with NEUROG2 and ASCL1 in the developing nervous system

During neural development, members of MTG family of transcriptional repressors are induced by proneural basic helix-loop-helix (bHLH) transcription factors and in turn inhibit the activity of the bHLH proteins, forming a negative feedback loop that regulates the normal progression of neurogenesis. Three MTG genes, MTG8, MTG16 and MTGR1, are expressed in distinct patterns in the developing nervous system. Various bHLH proteins are also expressed in distinct patterns. We asked whether there is a functional relationship between specific MTG and bHLH proteins in developing chick spinal cord. First, we examined if each MTG gene is induced by specific bHLH proteins. Although expression of NEUROG2, ASCL1 and MTG genes overlapped, the boundaries of gene expression did not match. Ectopic expression analysis showed that MTGR1 and NEUROD4, which show similar expression patterns, are regulated differently by NEUROG2 and ASCL1. Thus, our results show that expression of MTG genes is not regulated by a single upstream bHLH protein, but represents an integration of the activity of multiple regulators. Next, we asked if each MTG protein inhibits specific bHLH proteins. Transcription assay showed that NEUROG2 and ASCL1 are inhibited by MTGR1 and MTG16, and less efficiently by MTG8. Deletion mapping of MTGR1 showed that MTGR1 binds NEUROG2 and ASCL1 using multiple interaction surfaces, and all conserved domains are required for its repressor activity. These results support the model that MTG proteins form a higher-order repressor complex and modulate transcriptional activity of bHLH proteins during neurogenesis.

Xenopus zinc finger transcription factor IA1 (Insm1) expression marks anteroventral noradrenergic neuron progenitors in Xenopus embryos

The evolutionarily conserved IA1 (Insm1) gene is strongly expressed in the developing nervous system. Here, we show that IA1 is expressed during Xenopus laevis embryogenesis in neural plate primary neurons as well as in a population of uncharacterized anteroventral cells that form in front of the cement gland and that we identified as noradrenergic neurons. We also show that the formation of those anteroventral cells is dependent on BMPs and inhibited by Notch and that it is regulated by the transcription factors Xash1, Phox2, and Hand2. Finally, we provide functional evidence suggesting that IA1 may also play a role in their formation. Together, our results reveal that IA1 constitutes a novel player downstream of Xash1 in the formation of a previously unidentified population of Xenopus noradrenergic primary neurons.

Secondary neurogenesis in the brain of the African clawed frog, Xenopus laevis, as revealed by PCNA, Delta-1, Neurogenin-related-1, and NeuroD expression

After primary neurogenesis in the Xenopus laevis embryo, a massive new surge of neurogenesis and related neurogenic and proneural gene expression occurs in the spinal cord at the beginning of the larval period (starting at Stage 46), which corresponds to well-documented secondary neurogenesis in larval zebrafish central nervous system development. Here, we document related neural proliferation and gene expression patterns in the brain of Xenopus, in various embryonic and larval stages, showing the distribution of proliferative cells (immunostaining of cells containing the proliferating cell nuclear antigen; the auxiliary protein of DNA polymerase delta; PCNA), and the activity of some critical genes expressed during neurogenesis (i.e., Delta-1, Neurogenin-related-1, NeuroD). This study reveals that the early larval stage in Xenopus (Stage 48) displays patterns of proliferation (PCNA), as well as of neurogenic (Delta-1) and proneural (Ngnr-1; NeuroD) gene expression that are qualitatively almost identical to those seen in the 3-day postembryonic zebrafish or the 12.5/13.5-day embryonic mouse. Furthermore, a comparable bauplan of early proliferation zones (including their neuromeric organization) as described in the postembryonic zebrafish apparently exists in tetrapods (Xenopus). Altogether, the data presented suggest a common brain bauplan on the level of early proliferation patterns and neurogenic/proneural gene activity in anamniotes, if not vertebrates.

Basic helix-loop-helix transcription factors cooperate to specify a cortical projection neuron identity

Several transcription factors are essential determinants of a cortical projection neuron identity, but their mode of action (instructive versus permissive) and downstream genetic cascades remain poorly defined. Here, we demonstrate that the proneural basic helix-loop-helix (bHLH) gene Ngn2 instructs a partial cortical identity when misexpressed in ventral telencephalic progenitors, inducing ectopic marker expression in a defined temporal sequence, including early (24 h; Nscl2), intermediate (48 h; BhlhB5), and late (72 h; NeuroD, NeuroD2, Math2, and Tbr1) target genes. Strikingly, cortical gene expression was much more rapidly induced by Ngn2 in the dorsal telencephalon (within 12 to 24 h). We identify the bHLH gene Math3 as a dorsally restricted Ngn2 transcriptional target and cofactor, which synergizes with Ngn2 to accelerate target gene transcription in the cortex. Using a novel in vivo luciferase assay, we show that Ngn2 generates only approximately 60% of the transcriptional drive in ventral versus dorsal telencephalic domains, an activity that is augmented by Math3, providing a mechanistic basis for regional differences in Ngn2 function. Cortical bHLH genes thus cooperate to control transcriptional strength, thereby temporally coordinating downstream gene expression.

Functional distinctness of closely related transcription factors: a comparison of the Atonal and Amos proneural factors

Using the well-characterised paradigm of Drosophila sensory nervous system development, we examine the functional distinctness of the Amos and Atonal (Ato) proneural transcription factors, which have different mutant phenotypes but share very high similarity in their signature bHLH domains. Using misexpression and mutant rescue assays, we show that Ato and Amos proteins have abundantly distinct intrinsic proneural capabilities in much of the ectoderm. The eye, however, is an exception: here both proteins share the capability to direct the R8 photoreceptor fate choice. Therefore, functional distinctness between these closely related transcription factors vary with developmental context, indicating different molecular mechanisms of specificity in different contexts. Consistent with this, the structural basis for their distinctness also varies depending upon the function in question. In previous studies of neural bHLH factors, specificity invariably mapped to the bHLH domain sequence. Similarly, and despite their high similarity, much of the Amos' specificity relative to Ato maps to Amos-specific residues in its bHLH domain. For Ato-specific functions, however, the Amos bHLH domain can substitute for that of Ato. Consequently, Ato's specificity relative to Amos requires the non-bHLH portion of the Ato protein. Ato provides a powerful precedence for a role of non-bHLH sequences in modulating bHLH functional specificity. This has implications for structural and functional comparisons of other closely related transcription factors, and for understanding the molecular basis of specificity.

Neural induction in Xenopus requires inhibition of Wnt-β-catenin signaling

Canonical Wnt signals have been implicated in multiple events during early embryogenesis, including primary axis formation, neural crest induction, and A-P patterning of the neural plate. The mechanisms by which Wnt signals can direct distinct fates in cell types that are closely linked both temporally and spatially remains poorly understood. However, recent work has suggested that the downstream transcriptional mediators of this pathway, Lef/Tcf family DNA binding proteins, may confer distinct outcomes on these signals in some cellular contexts. In this study, we first examined whether inhibitory mutants of XTcf3 and XLef1 might block distinct Wnt-dependent signaling events during the diversification of cell fates in the early embryonic ectoderm. We found that a Wnt-unresponsive mutant of XTcf3 potently blocks neural crest formation, whereas an analogous mutant of XLef1 does not, and that the difference in activity mapped to the C-terminus of the proteins. Significantly, the inhibitory XTcf3 mutant also blocked expression of markers of anterior-most cell types, including cement gland and sensory placodes, indicating that Wnt signals are required for rostral as well as caudal ectodermal fates. Unexpectedly, we also found that blocking canonical Wnt signals in the ectoderm, using the inhibitory XTcf3 mutant or by other means, dramatically expanded the size of the neural plate, as evidenced by the increased expression of early pan-neural markers such as Sox3 and Nrp1. Conversely, we find that upregulation of canonical Wnt signals interferes with the induction of the neural plate, and this activity can be separated experimentally from Wnt-mediated neural crest induction. Together these findings provide important and novel insights into the role of canonical Wnt signals during the patterning of vertebrate ectoderm and indicate that Wnt inhibition plays a central role in the process of neural induction.

Identification of shared transcriptional targets for the proneural bHLH factors Xath5 and XNeuroD

Proneural basic helix-loop-helix (bHLH) transcription factors are critical positive regulators of neuronal differentiation in a variety of species and are required for proper differentiation of various subtypes of neurons. Although bHLH factors demonstrate some unique functions during neural development, they share the ability to regulate neuronal differentiation, potentially by targeting overlapping sets of genes. To assess this, we performed a screen in ectoderm animal cap tissue to identify direct transcriptional targets shared by two Xenopus ato-related bHLH factors, Xath5 and XNeuroD. Candidate target genes identified in this screen include several transcriptional regulators (Xebf2, Xebf3, XETOR and NKL), an RNA binding protein (elrC), a cell cycle component (Xgadd45gamma) and several novel genes. Overexpression of either Xath5 or XNeuroD induced ectopic in vivo expression of these candidate target genes. Conversely, blocking ato-related bHLH activity prevented endogenous nervous system expression of these genes. Therefore, we have identified a set of genes that can be regulated by multiple ato-related bHLH factors and may function as critical effectors of proneural bHLH-mediated differentiation.

Phosphorylation of Neurogenin2 specifies the migration properties and the dendritic morphology of pyramidal neurons in the neocortex

The molecular mechanisms specifying the dendritic morphology of different neuronal subtypes are poorly understood. Here we demonstrate that the bHLH transcription factor Neurogenin2 (Ngn2) is both necessary and sufficient for specifying the dendritic morphology of pyramidal neurons in vivo by specifying the polarity of its leading process during the initiation of radial migration. The ability of Ngn2 to promote a polarized leading process outgrowth requires the phosphorylation of a single tyrosine residue at position 241, an event that is neither involved in Ngn2 direct transactivation properties nor its proneural function. Interestingly, the migration defect observed in the Ngn2 knockout mouse and in progenitors expressing the Ngn2(Y241F) mutation can be rescued by inhibiting the activity of the small-GTPase RhoA in cortical progenitors. Our results demonstrate that Ngn2 coordinates the acquisition of the radial migration properties and the unipolar dendritic morphology characterizing pyramidal neurons through molecular mechanisms distinct from those mediating its proneural activity.

Suppression of Stat3 promotes neurogenesis in cultured neural stem cells

To investigate the effects of signal transducer and activator of transcription 3 (Stat3) on neural stem cell fate, stem cells were inoculated with an adenovirus vector expressing dominant negative form of Stat3 (Stat3F). One day later, a promoter assay revealed significant reduction of the transcriptional level in the transfected cells. Three days later, Western blot analysis and immunocytochemical analysis revealed that the protein level of microtubule-associated protein (MAP)2 and the number of MAP2-positive cells were increased significantly in the transfected cells whereas the protein level of glial fibrillary acidic protein (GFAP) and the number of GFAP-positive cells were decreased significantly. In addition, mRNA levels of Notch family members (Notch1, 2, and 3) and of inhibitory basic helix-loop-helix (bHLH) factors (Hes5, Id2, and Id3) were significantly downregulated at 3 days after viral inoculation with Stat3F; however, mRNA levels of bHLH determination factors (Math1 and Neurogenin3) and bHLH differentiation factors (NeuroD1 and NeuroD2) were significantly upregulated. These data indicated that suppression of Stat3 directly induced neurogenesis and inhibited astrogliogenesis in neural stem cells.

SCL, GATA-2 and Lmo2 expression in neurogenesis

SCL, Lmo2 and GATA factors form common transcription complexes during hematopoietic differentiation. The overlapping expression of SCL with GATA-2 and GATA-3 in the developing brain indicated that these factors might collaborate also in the course of neural tissue differentiation. The expression pattern of Lmo2 in the developing CNS, however, is not well understood. Here, we show that neural cells in the early embryonic chick mid- and hindbrain express SCL and GATA-2, while Lmo2 is expressed only in vascular elements. The lack of Lmo2 transcripts in neural cells demonstrated that SCL and GATA-2 cannot form common complexes with Lmo2 in the developing brain. In the course of neural tissue genesis, GATA-2 mRNA appeared prior to the SCL transcript. While GATA-2 expression decreased with maturation, SCL expression persisted at a high level also in post-neurogenic periods. The temporal pattern of SCL and GATA-2/3 expression was investigated also in vitro, in the course of induced neurogenesis by NE-4C neural stem cells. While GATA-2 expression increased from the very beginning of differentiation, SCL expression appeared only in more differentiated cells expressing proneural genes. GATA-3 expression, on the other hand, was detected only in advanced stages of the neuronal maturation, which were characterised by the activation of the Math2 neuronal gene. Similarly to the hematopoietic differentiation, GATA-2 expression precedes the activation of both SCL and GATA-3, and may play roles in the activation of the SCL gene in neuronal development. In contrast to hematopoietic differentiation, however, our results failed to demonstrate co-assembling of GATA factors or SCL with Lmo2. While overlapping expression of GATA-2/3 and SCL was detected, Lmo2 activation could not be demonstrated in neural cells in the investigated period of neuronal development.

Distinct domains within Mash1 and Math1 are required for function in neuronal differentiation versus neuronal cell-type specification

Many members of the basic helix-loop-helix (bHLH) family of transcription factors play pivotal roles in the development of a variety of tissues and organisms. We identify activities for the neural bHLH proteins Mash1 and Math1 in inducing neuronal differentiation, and in inducing the formation of distinct dorsal interneuron subtypes in the chick neural tube. Although both factors induce neuronal differentiation, each factor has a distinct activity in the type of dorsal interneuron that forms, with overexpression of Math1 increasing dI1 interneurons, and Mash1 increasing dI3 interneurons. Math1 and Mash1 function as transcriptional activators for both of these functions. Furthermore, we define discrete domains within the bHLH motif that are required for these different activities in neural development. Helix 1 of the Mash1 HLH domain is necessary for Mash1 to be able to promote neuronal differentiation, and is sufficient to confer this activity to the non-neural bHLH factor MyoD. In contrast, helix 2 of Math1, and both helix 1 and 2 of Mash1, are the domains required for the neuronal specification activities of these factors. The requirement for distinct domains within the HLH motif of Mash1 and Math1 for driving neuronal differentiation and cell-type specification probably reflects the importance of unique protein-protein interactions involved in these functions.

Evolution of neural precursor selection: functional divergence of proneural proteins

How conserved pathways are differentially regulated to produce diverse outcomes is a fundamental question of developmental and evolutionary biology. The conserved process of neural precursor cell (NPC) selection by basic helix-loop-helix (bHLH) proneural transcription factors in the peripheral nervous system (PNS) by atonal related proteins (ARPs) presents an excellent model in which to address this issue. Proneural ARPs belong to two highly related groups: the ATONAL (ATO) group and the NEUROGENIN (NGN) group. We used a cross-species approach to demonstrate that the genetic and molecular mechanisms by which ATO proteins and NGN proteins select NPCs are different. Specifically, ATO group genes efficiently induce neurogenesis in Drosophila but very weakly in Xenopus, while the reverse is true for NGN group proteins. This divergence in proneural activity is encoded by three residues in the basic domain of ATO proteins. In NGN proteins, proneural capacity is encoded by the equivalent three residues in the basic domain and a novel motif in the second Helix (H2) domain. Differential interactions with different types of zinc (Zn)-finger proteins mediate the divergence of ATO and NGN activities: Senseless is required for ATO group activity, whereas MyT1 is required for NGN group function. These data suggest an evolutionary divergence in the mechanisms of NPC selection between protostomes and deuterostomes.

Basic helix-loop-helix factors in cortical development

Transcription factors with bHLH motifs modulate critical events in the development of the mammalian neocortex. Multipotent cortical progenitors are maintained in a proliferative state by bHLH factors from the Id and Hes families. The transition from proliferation to neurogenesis involves a coordinate increase in the activity of proneural bHLH factors (Mash1, Neurogenin1, and Neurogenin2) and a decrease in the activity of Hes and Id factors. As development proceeds, inhibition of proneural bHLH factors in cortical progenitors promotes the formation of astrocytes. Finally, the formation of oligodendrocytes is triggered by an increase in the activity of bHLH factors Olig1 and Olig2 that may be coupled with a decrease in Id activity. Thus, bHLH factors have key roles in corticogenesis, affecting the timing of differentiation and the specification of cell fate.