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Neural Development: bHLH Genes

  1. Anna Philpott

Published Online: 15 FEB 2010

DOI: 10.1002/9780470015902.a0000827.pub2

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Philpott, A. 2010. Neural Development: bHLH Genes. eLS. .

Author Information

  1. University of Cambridge, Department of Oncology, Hutchison/MRC Research Centre, Addenbrookes Hospital, Cambridge, UK

Publication History

  1. Published Online: 15 FEB 2010

Introduction

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

A critical feature of neuronal development is the specification of differentiated cell types from undifferentiated precursors, and transcription factors belonging to the basic helix–loop–helix (bHLH) family are known to play a pivotal role in this process. Members of the bHLH family function in a wide range of tissues, and share a common structural motif consisting of a basic region followed by two α helices joined by a flexible peptide loop (HLH domain). The HLH domain is required for dimerization with an E protein partner, whereas the basic domain makes site-specific contact with deoxyribonucleic acid (DNA) at sequences known as E boxes (CANNTG, where N denotes any nucleotide). In this article we discuss the role of a subset of these factors in regulating the development of neurons in both invertebrates and vertebrates. See also Cells of the Nervous System, and Protein Motifs: the Helix-Loop-Helix Motif

Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

The importance of bHLH genes for neurogenesis was first appreciated in Drosophila melanogaster, where it was shown that genes belonging to the achaete–scute complex (as-c) are required for the development of some neurons in the peripheral and central nervous system (PNS and CNS) (Campuzano and Modolell, 1992). The genes of the as-c encode the bHLH transcription factors ACHAETE (AC), SCUTE (SC), LETHAL OF SCUTE (L'SC) and ASENSE (ASE). The AS-C factors function as dimers with a ubiquitously expressed bHLH protein DAUGHTERLESS (DA), a homologue of the higher vertebrate E proteins. Expression of as-c genes appears to confer neural competence on cells in which they are expressed; hence they are called ‘proneural genes’. Proneural proteins bind to class A E boxes that have CAGCTG sequences and are thought to positively regulate the expression of neural specific genes. Overexpression or gain-of-function mutations in genes of the as-c are sufficient to promote ectopic external sense organ formation in fly imaginal discs, whereas the loss-of-function results in a failure of external sense organ development. Another proneural bHLH gene product in Drosophila, atonal (ato), defines a subfamily of bHLH genes distinct from the as-c genes. In Drosophila, ato is not involved in external sense organ formation, but instead regulates the development of chordotonal organ sensory structures, olfactory sense organs and photoreceptors in the eye. See also Neurogenesis in Drosophila

The proneural function of bHLH genes appears to have been evolutionarily conserved. Homologues of achaetescute genes (ashs) have been identified in a variety of vertebrate species, and these genes regulate the development of specific classes of neurons (Lee, 1997). For example mash1 (mammalian achaete–scute homologue) is expressed in subsets of proliferating precursor cells in the PNS and CNS of the mouse embryo. Knockout analysis has shown that mash1 is required for the development of autonomic neurons and olfactory receptor neurons. In vertebrates many bHLH genes have been identified that bear close similarity to the Drosophila atonal gene. These vertebrate atonal homologues, such as neurogenin (ngn) 1 and 2 are expressed in overlapping patterns in a variety of structures of the developing nervous system and may, in these regions, stand at the top of proneural cascades as the as-c genes do in Drosophila. See also Evolutionary Developmental Biology: Homologous Regulatory Genes and Processes, Knockout and Knock-in Animals, and Vertebrate Central Nervous System

Functional Hierarchy of bHLH Genes

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

Neurogenesis is a sequential process whereby a cell becomes progressively more committed to a neuronal cell phenotype. It has become clear that different members of the bHLH family act at different stages in this differentiation process. Within the same lineage it has been shown that one or several bHLH genes are expressed early in proliferating precursor cells whereas others are expressed later in differentiating neurons (Lewis, 1996), which also express the differentiation marker NEURAL BETA-TUBULIN (NbTUB). In some cases, the early factors have been shown to regulate the expression of the later differentiation factors (Figure 1). See also Vertebrate Neurogenesis: Cell Polarity

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Figure 1. A typical cascade of proneural genes in a Drosophila external sense organ. The iroquois (Iro) genes positively regulate achaete (ac) and scute (sc), defining the proneural cluster. Hairy represses the proneural genes outside the cluster. Achaete and scute dimerize with daughterless (da) to drive asense (ase) expression leading to neural specification, but within the proneural cluster, the Notch pathway through activation of E(spl) repressors, limits the expression and function of ac and sc to a single cell. Extramachrochaete (emc), another pathway inhibitor, reinforces this pattern.

For example, during Drosophila sense organ development ac and sc are expressed early during the selection of neural precursors, whereas ase is expressed later and promotes differentiation of the neural precursors. This sequential expression is due to direct activation of ase transcription by the AC/SC proteins. See also Drosophila Neural Development

Similarly, Xenopus neurogenin-related-1 (X-ngnr-1, an atonal-related gene, and actually most closely related to mammalian ngn2) is expressed in the early precursor cells in the Xenopus neural plate and precedes expression of NeuroD (another atonal-related gene), which is expressed in these cells as they begin to differentiate (Figure 2). Overexpression of X-ngnr-1 promotes ectopic expression of NeuroD, but not vice versa, suggesting a cascade of transcriptional regulation during the development of these neurons. A similar proneural cascade has been described in the vertebrate retina and the nasal epithelium. It is likely that throughout much of the nervous system bHLH factors will act sequentially to regulate successive stages of neurogenesis. See also Transcriptional Gene Regulation in Eukaryotes, and Xenopus Embryo: Neural Induction

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Figure 2. Model illustrating the interactions between the Notch pathway, the Xenopus NGN2 homologue X-Ngnr-1 and the Xenopus cyclin-dependent kinase inhibitor Xic1, during primary neurogenesis in Xenopus (first published in Vernon et al., 2006). Xenopus Neurogenin, X-Ngnr-1, both upregulates Delta resulting in activation of Notch in the adjacent cell, and drives differentiation via transcriptional upregulation of Xenopus MyT1 (xMyT1) and NeuroD. The Xenopus cdki p27Xic1 stabilizes X-Ngnr-1, and potentiates the neuronal differentiation arm of this network. Reproduced from Vernon et al. 2006.

The context of activity of proneural bHLH proteins is critically important; not only do different bHLHs specify different neuronal subtypes by expression at different times and places in the developing nervous system, but they also function in combination with additional cofactors in a combinatorial code. For instance, NEUROGENIN (NGN) and MASH1 act in combination with PAX6, OLIG2 and NKX2.2 to specify oligodendrocytes, astrocytes and neurons in a complex spatial and temporal pattern (Sugimori et al., 2007). Such combinatorial activity is thought to be achieved by having adjacent transcription factor binding sites in critical cell type-specific enhancers, where cooperative binding is required, as has been seen in specification of motor neurons in the chick neural tube (Lee and Pfaff, 2003). Moreover, while bHLH proteins can generally bind the same E box in vitro, in vivo preferences between proneural proteins have been observed, which may depend on additional ‘specificity’ residues outside the region that contacts DNA (Seo et al., 2007). Again, specificity may be influenced by synergistic cofactors (Powell and Jarman, 2008). Unfortunately, our understanding of this crucial area of bHLH function is hampered by the limited structural information available, although a crystal structure for the heterodimer of E47 and the bHLH domain of NEUROD1 has recently been published (Longo et al., 2008), which gives some information about E protein-binding partner selection, although not about additional partner choices. Adding further complexity, a single bHLH proneural protein can have multiple functions at different stages of neurogenesis. For instance, NGN2 is known to play a role in regulation of neuronal precursor proliferation and commitment, along with driving neuronal differentiation, subtype specification, migration and axonal outgrowth. See also Transcriptional Regulation: Coordination

Inhibition of proneural bHLH proneural function by Hes proteins

In contrast to bHLH proneural proteins, the bHLH Hes family, mammalian homologues of the Drosophila hairy/enhancer of split (Hes) genes, act as transcriptional repressors, at least partly by making repressive dimers with bHLH proteins such as E47, and facilitated by recruitment of the co-repressor GROUCHO proteins. For instance, overexpression of Hes1 prevents neuronal differentiation in the mouse CNS, whereas disruption of the Hes1 gene causes upregulation of proneural bHLH genes and premature differentiation of neurons in the neural tube. Therefore the Hes genes are likely to play a fundamental role in regulating the timing and pattern of neuronal differentiation in the vertebrate nervous system (Kageyama et al., 2008).

Other Negative Regulators of Proneural Genes

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

Additional negative regulatory factors function in the developing embryo to ensure that bHLH protein action is restricted to the appropriate regions (Fisher and Caudy, 1998). The Drosophila protein EXTRAMACROCHAETE (EMC) is an inhibitory protein that possesses an HLH domain but lacks a basic domain and is thus unable to contact DNA. EMC can therefore form nonfunctional dimers with the proneural bHLH proteins and thus block their function. Emc is expressed in a pattern generally complementary to that of ac/sc and serves to reinforce the pattern of neural precursor selection. The vertebrate homologues of EMC are the ID proteins, which are HLH proteins that similarly lack a basic domain. ID proteins can function as negative regulators of bHLH protein function in a variety of tissues, including the nervous system, predominantly by binding to and sequestering E protein partners. See also Protein–DNA Complexes: Specific

Posttranslational regulation of proneural proteins

Recently, it has become apparent that posttranslational regulation of bHLHs plays an important role in their control. For instance, phosphorylation of NGN2 on a specific GSK3-beta site has been shown to be required for motor neuron specification in the developing spinal cord (Ma et al., 2008). Similarly, phosphorylation of NEUROD has context-specific effects on downstream gene activation (Dufton et al., 2005). Moreover, phosphorylation of NGN2 on a specific tyrosine does not affect its neurogenic activity, but is required to promote formation of a leading dendrite and acquisition of radial migration properties, via regulation of the small GTPase (guanosine triphosphatase) RHOA (Hand et al., 2005) bHLHs are also regulated posttranslationally at the level of protein turnover. For instance, both MASH1 and NGN2 are unstable proteins (Vinals et al., 2004; Vosper et al., 2007). In general degradation is inhibited by proneural protein binding to E proteins, and thus stability can be regulated by the availability of the E protein partner, as determined by ID expression (Vinals et al., 2004). Moreover, the stability of NGN2 is also regulated by the presence of cyclin-dependent kinase inhibitors (cdkis) upregulated on cell cycle exit, such as p27XIC1 in Xenopus (Xic1, see Figure 2) and p27KIP1 in mice. Interestingly, this activity is distinct from the ability of cdkis to inhibit overall cyclin-dependent kinase levels or stop the cell cycle (Figure 2; Nguyen et al., 2006; Vernon et al., 2003). See also Ubiquitin Pathway

Establishing a Pattern for bHLH Gene Expression

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

What determines the initial pattern of bHLH gene expression during early neurogenesis (Simpson, 1996)? During Drosophila PNS development, the early pattern of ac/sc gene expression is quite stereotyped, suggesting the existence of a prepattern regulating proneural gene expression and sense organ formation. This prepattern is determined in part by the genes of the Iroquois (Iro) complex. Three genes, the closely related caupolican and araucan, and the more distant mirror, are contained within the Iro locus and encode homeobox-containing transcription factors which bind to the enhancers of ac/sc (Gomez-Skarmeta and Modolell, 2002). These genes are required for appropriate ac/sc expression and sense organ formation within defined regions of the adult peripheral nervous system and possibly the embryonic CNS. The Iro genes do not control all sense organ development; a distinct subset depends upon the function of the pannier gene, which encodes a zinc-finger protein that also binds an enhancer of as-c. See also Mammalian Embryo: Hox Genes

Do similar mechanisms function in the vertebrate nervous system to establish the pattern of bHLH gene expression? Six homologues of the Iroquois complex of genes have been described in mouse (Irx1–6), and are arranged into two clusters. These genes are expressed in, but not restricted to, the developing nervous system where their expression precedes and partially overlaps in expression with the proneural bHLH genes (Gomez-Skarmeta and Modolell, 2002). In Xenopus, five Xiro genes have been identified, three of which are expressed in the neural plate. Overexpression of these genes causes expansion of the neural plate, and results in limited upregulation of some bHLH genes within the same region. A current focus of research is to determine the precise regulatory relationship between the Irx/Xiro proteins and the vertebrate bHLH genes, and to define additional regulatory factors driving bHLH gene expression in the developing nervous system. See also Vertebrate Embryo: Neural Patterning

Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

Once initiated, the pattern of proneural gene expression must be refined to determine which cells will actually become neurons. This is achieved through a process termed lateral inhibition, whereby a specific cell is selected to become a neuron from a functionally equivalent group of cells. A cell destined for a neural fate inhibits its neighbours from becoming neural by activating the transmembrane protein NOTCH on adjacent cells through presentation of the ligand DELTA (Artavanis-Tsakonas et al., 1995).

For example, in the Drosophila PNS a precursor for the external sense organ is specified from surrounding nonneural ectoderm. The process begins with ac/sc expression in specific clusters of cells termed proneural clusters. All cells within the cluster are competent to become neural. Ultimately, the activation of NOTCH by DELTA in cells within the cluster refines ac/sc expression to just a single cell that becomes the precursor for each external sense organ. Mutations in the genes for Notch or Delta (or other components of the Notch signalling pathway) result in neuronal hypertrophy since there is no selection and all the cells within the proneural clusters become neural. See also Drosophila Patterning: Delta–Notch Interactions

Homologues of virtually all elements of the lateral inhibition pathway have been identified in vertebrates, and the functional role of these genes appears to be generally analogous to their function during Drosophila neurogenesis. For example, in the Xenopus neural plate three stripes of primary neurons develop under the control of proneural bHLH genes such as X-ngnr-1 and NeuroD. Within the neural plate, activation of X-NOTCH-1 by X-DELTA-1 restricts the number and density of neurons that will develop into primary neurons in part by negatively regulating expression and function of the proneural bHLH gene X-ngnr-1 (Figure 2). Activation of the neurogenic pathway also plays an important role in generating cellular diversity by delaying the timing of neural differentiation in a neural field where the extracellular environment is changing with time. For example, in the vertebrate retina, the first cells to differentiate tend to be ganglion cells while the last are Müller cells. Experimental manipulation of neurogenic activity can influence the proportion of these cell types in predictable ways. See also Eye Development: Gene Control

However, recent work may lead to a revising of the classical picture of NOTCH signalling as amplifying and fixing initial stochastic variations of proneural proteins expression. Expression of ngn2 and its downstream target Delta has been demonstrated to oscillate in neural progenitors with a periodicity of around two hours. Moreover this periodic oscillation is inversely correlated with the expression of the bHLH repressive transcription factor Hes1, which can inhibit its own expression to set up a negative oscillatory loop. In contrast, when neural progenitors undergo differentiation, Hes1 transcript levels drop and ngn2 and Delta levels become stable and high (Shimojo et al., 2008). It is postulated that oscillations of Hes1, ngn and Delta expression are required to maintain progenitor identity and hence expand the pool of cells available for differentiation. Moreover, as oscillations in adjacent cells are not synchronised, this may contribute to the heterogeneity required to produce different subtypes of neurons within the same environment. However, how cells undergo the transition from oscillatory to stable expression of bHLHs on passage from progenitor to differentiating neuron is unclear.

Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

Analysis of primary neurogenesis in the neural plate of Xenopus has uncovered additional factors that act with the bHLH proteins to regulate neuronal differentiation. X-MYT1 is a zinc-finger protein that is expressed in the developing primary neurons of the neural plate soon after expression of X-ngnr-1, which is able to activate X-MYT1 expression. X-myt1 can function in conjunction with the bHLH proteins XASH3 and X-NGNR-1 to promote neuronal differentiation, and renders these proteins insensitive to lateral inhibition by NOTCH/DELTA signalling, thus allowing a subset of cells within the neural plate to differentiate into neurons. Overexpression of dominant-negative forms of X-MYT1 in Xenopus embryos showed that it is indeed required for differentiation of the primary neurons (see Figure 2). X-myt1 has homologues in multiple invertebrate and vertebrate species, suggesting a conserved role in regulating neurogenesis.

Another factor that functions during primary neurogenesis in Xenopus is XCoe2, an HLH transcription factor belonging to a distinct subfamily of HLH proteins that includes Drosophila Collier, Caenorhabditis elegans unc-3 and three vertebrate Ebf/Olf-1-related genes (Ebf, early B-cell factor). These proteins lack a basic domain and instead have a unique DNA-binding region containing a zinc-binding motif. During primary neurogenesis XCoe2 is expressed following X-ngnr-1 but before neuroD, and overexpression of X-ngnr-1 is able to activate XCoe2 expression. Overexpression of XCoe2 can promote ectopic neurogenesis in Xenopus embryos, but is highly sensitive to inhibition by Notch/Delta signalling. Expression of dominant-negative XCoe2 prevents primary neurons from forming in the Xenopus neural plate. XCoe2 thus plays a role in the regulatory cascade controlling primary neurogenesis in Xenopus.

Transcriptional Targets for Proneural bHLH Genes

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

bHLH proteins are known to regulate multiple steps in the neurogenic cascade, although the transcriptional targets responsible for these many roles are not well established. However, recently there have been several unbiased attempts to identify downstream targets. One of these compared genes activated by X-NGNR-1, the NGN2 homologue, and its downstream effector NEUROD in Xenopus. Interestingly, X-NGNR-1 and NEUROD activated a number of common targets, leading to the suggestion that, while X-NGNR-1 and NEUROD can activate an overlapping transcriptional programme experimentally, in vivo X-NGNR-1 may be required to set up the pattern of gene expression upon commitment, which the later-expressed NEUROD then takes over, maintaining this expression to drive differentiation (Seo et al., 2007). However, it should be noted that X-NGNR-1 and NEUROD additionally displayed many distinct targets, and these may be traced back to slight differences in E box-binding preference. Direct targets include genes from a number of categories, including transcription factors, signal transducers and cytoskeletal modifiers. Interestingly, proteins involved in effecting neuronal function were not generally direct downstream targets of X-NGNR-1 and NEUROD. Downstream targets of NGN2 in mouse have also been investigated, which demonstrate considerable functional breadth; for instance NGN2 directly up-regulates Rnd2, a small GTP (guanosine triphosphate)-binding protein that controls cortical neuronal migration. Bioinformatics have been used both to identify downstream targets of NGN2 and MASH1 (Gohlke et al., 2008), but also to look for transcription factors whose binding sites are enriched near E boxes favoured by these proneural proteins. It will be interesting to see which of these proposed interactions turns out to take place, in vivo. Importantly, in one case where bioinformatics have been used to identify genes likely to be activated by the synergistic binding of MASH1 and BRN protein, such as Delta1, Insm1 and Fbw7, these have been further validated by chromatin immunoprecipitation and reduced expression in Mash1 knockout mice. See also Protein Motifs for DNA Binding

It is clear that at least some bHLH proneural genes bind to chromatin modifiers to regulate transcriptional activity. For instance, NGN2 and NEUROD have both been shown to require association with the catalytic component of the SWI/SNF complex BRG1 to promote neurogenesis. Binding of BRG1 to NGN2 and NEUROD is further regulated by the protein GEMININ, which sequesters BRG1 in neural progenitors to allow proliferation and maintenance of an undifferentiated state (Seo and Kroll, 2006). NGN2 also co-operates with retinoic acid signalling via its direct binding to the retinoic acid receptor to recruit the histone acteyltransferase chromatin modifier CBP to promoters of downstream targets in motor neurons, resulting in activation of transcription (Lee et al., 2009).

Outstanding Questions

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

bHLH genes are clearly important regulators of neurogenesis; however, we still have much to learn about how they function (Table 1). For example, why are so many bHLH genes expressed in the developing nervous system? bHLHs in the nervous system act in a combinatorial code, but we have yet to firmly establish how such a temporal or spatial code is interpreted and implemented, resulting in generation of the diverse array of neurons seen in the mammalian brain. Moreover proneural proteins such as NGN2 have different functions at different developmental stages, driving multiple aspects of neuron formation. It is essential we understand these aspects of bHLH biology during development, both to have a clearer view of formation of the embryonic nervous system and if we are to exploit the great potential of these proteins in directing neural stem cell to adopt therapeutically useful fates such as motor neurons to repair spinal cord injury or midbrain dopaminergic neurons for the treatment of Parkinson disease.

Table 1. List of genes that are involved in the patterning, function and hierarchy of the neural bHLH genes
Classes of genes that regulate neurogenesisExamples in DrosophilaExamples in vertebratesFunction
Proneural genesAchaete, scute, atonal, asense, daughterlessachaete–scute homologues (ASHs), atonal homologues (ATHs, NeuroDs, Ngns)bHLH transcription factors that function as positive regulators of neuronal determination and differentiation
Prepattern genesIroquois complex (caupolican, araucan) and pannierIrx 1–6(mouse), Xiro1–5(Xenopus)Transcription factors that regulate the early pattern of bHLH gene expression
Neurogenic genesNotch, Delta and signal-ling pathway componentsNotch, Delta and signalling pathway componentsTransmembrane receptor signalling pathway that refines the pattern of bHLH expression and regulates commitment to the neural fate
Negative regulatory factorsemc, hairy, E(Spl)Id1–4, HES1,3,5HLH factors that limit the domains of bHLH expression and function
Factors that cooperate with bHLH factors XMyT1Zinc-finger transcription factor that confers resistance to lateral inhibition
Putative transcriptional targets for bHLH factorsPox-neuroPhox2aTranscription factors that execute the neuronal differentiation programme
End Notes
  1. Based in part on the previous version of this Encyclopedia of Life Sciences (ELS) article, Neural Development: bHLH Genes by Monica Vetter and William Harris.

Glossary
Achaete–scute complex (AS-C)

A set of four closely spaced, basic helix–loop–helix (bHLH) genes in Drosophila that are important for neurogenesis as originally defined by mutants.

E box

The specific DNA sequence to which basic helix–loop–helix (bHLH) factors bind.

External sense organ

Peripherally originated sensory structures, subserving mechano- and chemoreception with axons that project into the central nervous system.

Imaginal discs

Epithelial sheets in larval flies that give rise to most of the adult cuticle and associated structures such as external sense organs.

Lateral inhibition

The process whereby a developing cell inhibits its less determined neighbours from assuming a similar fate.

Neurogenic

Genes whose loss of activity (defined originally by mutants) leads to neural hypertrophy.

Proneural

Genes whose activity pushes cells towards a neural fate.

Transcription factor

A protein that binds to DNA and regulates the transcription of a nearby target gene.

References

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading

Further Reading

  1. Top of page
  2. Introduction
  3. Genes of the Drosophila Achaete–Scute Complex and Their Vertebrate Counterparts are Important Regulators of Neurogenesis
  4. Functional Hierarchy of bHLH Genes
  5. Other Negative Regulators of Proneural Genes
  6. Establishing a Pattern for bHLH Gene Expression
  7. Lateral Inhibition Refines the Pattern of Proneural Gene Expression and Neurogenesis
  8. Other Factors Function with bHLH Proteins to Positively Regulate Key Steps in Neurogenesis
  9. Transcriptional Targets for Proneural bHLH Genes
  10. Outstanding Questions
  11. References
  12. Further Reading