NeuroD2 and neuroD3: distinct expression patterns and transcriptional activation potentials within the neuroD gene family

Interaction of PKN with a neuron-specific basic helix-loop-helix transcription factor, NDRF/NeuroD2

By the yeast two-hybrid screening of a human brain cDNA library with the amino-terminal regulatory region of PKN as a bait, a clone encoding a neuron-specific basic Helix-Loop-Helix (bHLH) transcription factor, NDRF/NeuroD2 was isolated. NDRF/NeuroD2 was co-precipitated with PKN from the lysate of COS-7 cells transfected with both expression constructs for NDRF/NeuroD2 and PKN. In vitro binding studies using the deletion mutants of NDRF/NeuroD2 synthesized in a rabbit reticulocyte lysate indicated that the internal region containing the bHLH domain of NDRF/NeuroD2 was necessary and sufficient for the interaction with PKN. In addition, recombinant NDRF/NeuroD2 purified from Escherichia coli could bind PKN, suggesting the direct interaction between NDRF/NeuroD2 and PKN. Transient transfection assays using P19 cells revealed that expression of NDRF/NeuroD2 increased the transactivation of the rat insulin promoter element 3 (RIPE3) enhancer up to approximately 12-fold and that co-expression of catalytically active form of PKN, but not kinase-deficient derivative, resulted in a further threefold increase of NDRF/NeuroD2-mediated transcription. These findings suggest that PKN may contribute to transcriptional responses through the post-translational modification of the NDRF/NeuroD2-dependent transcriptional machinery.

Id1 and Id3 are required for neurogenesis, angiogenesis and vascularization of tumour xenografts

Id proteins may control cell differentiation by interfering with DNA binding of transcription factors. Here we show that targeted disruption of the dominant negative helix-loop-helix proteins Id1 and Id3 in mice results in premature withdrawal of neuroblasts from the cell cycle and expression of neural-specific differentiation markers. The Id1-Id3 double knockout mice also display vascular malformations in the forebrain and an absence of branching and sprouting of blood vessels into the neuroectoderm. As angiogenesis both in the brain and in tumours requires invasion of avascular tissue by endothelial cells, we examined the Id knockout mice for their ability to support the growth of tumour xenografts. Three different tumours failed to grow and/or metastasize in Id1+/- Id3-/- mice, and any tumour growth present showed poor vascularization and extensive necrosis. Thus, the Id genes are required to maintain the timing of neuronal differentiation in the embryo and invasiveness of the vasculature. Because the Id genes are expressed at very low levels in adults, they make attractive new targets for anti-angiogenic drug design.

Neurogenin1 and neurogenin2 control two distinct waves of neurogenesis in developing dorsal root ganglia

Different classes of sensory neurons in dorsal root ganglia (DRG) are generated in two waves: large-diameter trkC+ and trkB+ neurons are born first, followed by small-diameter trkA+ neurons. All such neurons require either neurogenin (ngn) 1 or 2, two neuronal determination genes encoding basic helix-loop-helix (bHLH) transcription factors. ngn2 is required primarily if not exclusively for the generation of trkC+ and trkB+ neurons, whereas the generation of most or all trkA+ neurons requires ngn1. Comparison with previous lineage tracing data in the chick suggests that this dichotomy reflects a requirement for the two ngns in distinct sensory precursor populations. The neurogenesis defect in ngn2(-/-) embryos is transient and later compensated by ngn1-dependent precursors, suggesting that feedback or competitive interactions between these precursors may control the proportion of different neuronal subtypes they normally produce. These data reveal remarkable parallels in the roles of bHLH factors during neurogenesis in the DRG, and myogenesis in the neighboring myotome.

Misexpression of a bHLH gene, cNSCL1, results in abnormal brain development

NSCL1 is a basic helix-loop-helix transcription factor involved in the development of the nervous system. To elucidate its role in neurogenesis, we cloned chick NSCL1 (cNSCL1) and examined its expression pattern and the effect of its misexpression on brain development. cNSCL1 was predominantly expressed during active neurogenesis. Double-labeling experiments showed that proliferating neuroblasts in the ventricular zone lacked cNSCL1 expression and cells expressing cNSCL1 were located just outside the ventricular zone. Retroviral misexpression of cNSCL1 in chick embryos produced a brain with abnormal structure. While the forebrain of the embryonic day-12 (E12) brain appeared normal, the tectum was enlarged. The enlargement was likely due to an increase in cell proliferation, since more radioactivity was detected in this region of the brain after [3H]thymidine labeling at E9. The cerebellum, on the other hand, was reduced in size. Fewer cells were labeled with BrdU in the external granule layer (a secondary germinal layer required for cerebellum development) in experimental embryos than in the controls, suggesting that misexpression of cNSCL1 might interfere with cell proliferation in the external granular layer. Our data indicate that regulated expression of cNSCL1 is required for normal brain development. They also imply that cNSCL1 might be involved in preventing some postmitotic cells from reentering the cell cycle during neurogenesis. Dev Dyn 1999;215:238-247.

Structure and regulation of the human NeuroD (BETA2/BHF1) gene

In this study, we isolated and characterized the human NeuroD (BETA2/BHF1) gene. This gene was found to consist of two exons and one intron. The promoter regions were well-conserved compared with the mouse NeuroD gene. Two transcription start points (TSPs) were determined by the oligo-capping method. One TATA box was located at -31 bp from the lower TSP. The results of a transient transfection assay using the human neuroblastoma cell line IMR-32 and hamster insulin tumor cell line HIT-T15 suggested that there are at least three positive regulatory regions in the promoter. In these regions, four E boxes (CANNTG), named the E1 to E4 boxes, and two GC boxes were present. Cotransfection of the NeuroD expression vector into IMR-32 cells enhanced the NeuroD promoter activity by about 4-fold. A deletion and mutation analysis revealed that the E1 and E4 boxes, especially the E1 box, are associated with autoactivation and that E2 and E3 boxes are not associated with autoactivation. As mutation analysis of E3 box showed a decrease in the enhancer activity to the basal level, it showed that the E3 box is important to activate the NeuroD transcription. These results raised the possibility that the NeuroD gene expression is positively regulated through the E box sequence, not only by NeuroD itself but also by another E box binding protein.

Early specification of sensory neuron fate revealed by expression and function of neurogenins in the chick embryo

The generation of sensory and autonomic neurons from the neural crest requires the functions of two classes of basic helix-loop-helix (bHLH) transcription factors, the Neurogenins (NGNs) and MASH-1, respectively (Fode, C., Gradwohl, G., Morin, X., Dierich, A., LeMeur, M., Goridis, C. and Guillemot, F. (1998) Neuron 20, 483–494; Guillemot, F., Lo, L.-C., Johnson, J. E., Auerbach, A., Anderson, D. J. and Joyner, A. L. (1993) Cell 75, 463–476; Ma, Q., Chen, Z. F., Barrantes, I. B., de la Pompa, J. L. and Anderson, D. J. (1998 Neuron 20, 469–482). We have cloned two chick NGNs and found that they are expressed in a subset of neural crest cells early in their migration. Ectopic expression of the NGNs in vivo biases migrating neural crest cells to localize in the sensory ganglia, and induces the expression of sensory neuron-appropriate markers in non-sensory crest derivatives. Surprisingly, the NGNs can also induce the expression of multiple pan-neuronal and sensory-specific markers in the dermomyotome, a mesodermal derivative. Taken together, these data suggest that a subset of neural crest cells may already be specified for a sensory neuron fate early in migration, as a consequence of NGN expression.

Identification and analysis of the promoter region of the human NeuroD-related factor (NDRF)1

We isolated and characterized the human NeuroD-related factor (NDRF)/NeuroD2/KW8 gene. The NDRF gene consisted of two exons and one intron. NDRF had one transcriptional starting point, and in its 5'-flanking region, no TATA box was detected, but 16 E boxes (CANNTG) were present. RNA blotting analysis revealed that HIT-T15 and D283 cells expressed a 3.3 kb band of the NDRF transcript. Promoter analysis by luciferase assay demonstrated that luciferase activity changed between -120 and -195, and between -451 and -564, both regions wherein a single E box existed. Radiation hybrid mapping showed that NDRF linked the marker SHGC-36242 with a LOD score of 8.53 and was located on 17q12-22.

Transcriptional regulation of mouse type 1 inositol 1,4,5-trisphosphate receptor gene by NeuroD-related factor

The type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) is a Ca2+ channel protein that is expressed abundantly in the CNS, such as in the cerebellar Purkinje cells and hippocampus. We previously demonstrated that the box-I element, which is located -334 relative to the transcription initiation site of the mouse IP3R1 gene and includes an E-box consensus sequence, is involved in the up-regulation of such IP3R1 gene expression. Furthermore, the previous study also indicated that some CNS-related basic helix-loop-helix (bHLH) factors bind to the box-I and activate IP3R1 gene expression. In this study, we demonstrated that one of the CNS-related bHLH factors, neuronal differentiation factor (NeuroD)-related factor (NDRF), specifically bound to the box-I sequence with a ubiquitously expressed bHLH protein, E47, and activated IP3R1 gene expression. In situ hybridization of adult mouse brain revealed that IP3R1 and NDRF mRNA were co-expressed in many subsets of neurons, highly in Purkinje cells and hippocampus and moderately in cerebral cortex, olfactory bulb, and caudate putamen. Furthermore, the spatiotemporal expression patterns of these two genes resembled one another throughout postnatal development of the mouse CNS. From these results, we suggest that NDRF is involved in the tissue-specific regulation of IP3R1 gene expression in the CNS.

Id4 expression induces apoptosis in astrocytic cultures and is down-regulated by activation of the cAMP-dependent signal transduction pathway

The Id family of helix-loop-helix transcription factors has been implicated in the regulation of cellular differentiation in several different lineages. We have explored the potential regulatory role of the cyclic AMP-dependent signaling pathway on Id gene expression in astroglial primary cultures. We found that primary cultures of mouse forebrain astrocytes constitutively expressed the four known members of the Id gene family, Id1, Id2, Id3, and Id4. During culture in presence of serum for 4 weeks, the expression of Id4 was up-regulated. In these same cultures, treatment with dibutyryl-cyclic AMP, a cyclic AMP analogue known to promote astrocyte differentiation, dramatically and selectively decreased Id4 gene expression. This effect was detectable after short-term treatment and was maintained during long-term treatment. Forskolin and pentoxifylline, two other agents known to elevate intracellular cyclic AMP through different mechanisms, also potently decreased Id4 gene expression. Furthermore, overexpression of Id4 in an astrocyte-derived cell line induced cells to round up and die by apoptosis. These results indicate that the cyclic AMP pathway acts as an inhibitor of Id4 gene expression in astrocytes, identify a new function for Id4, and suggest that Id4 is strategically positioned in the chain of molecular events regulating astrocyte differentiation and apoptosis.

The protein kinase C (PKC) substrate GAP-43 is already expressed in neural precursor cells, colocalizes with PKCeta and binds calmodulin

Expression of the growth-associated protein of 43-kDa (GAP-43), which is described as a postmitotic, neuron-specific major protein kinase C (PKC) substrate, was investigated in the murine embryonic carcinoma cell line PCC7-Mz1 which develops into a brain-tissue-like pattern of neuronal, fibroblast-like and astroglial cells upon stimulation with all-trans retinoic acid (RA). GAP-43 expression was very low in stem cells, but increased on mRNA and protein level within the 12 h after differentiation was initiated. While the P1 promoter of the GAP-43 gene gave rise to a 1.6-kb mRNA and was already active at a very low level in PCC7-Mz1 stem cells, transcription of the P2 promoter, which resulted in a 1.4-kb mRNA, was completely blocked in stem cells but increased rapidly after RA treatment. Within the first 2 days of neural differentiation, GAP-43 was localized with the cytoplasmic membrane and the Golgi complex of proliferating neural precursor cells. Then, GAP-43 was translocated to the growth cones and neurites, and from day 6, when neurons began to acquire polarity, the protein was found in the axons. GAP-43 was never detected in the non-neuronal PCC7-Mz1 derivatives, i.e. in fibroblasts or glial cells. In the foetal rat brain (prenatal day F11), GAP-43 was expressed in the optic stalk, the lense plakode and in the postmitotic neurons of the marginal zone of the hindbrain. Moreover, in a layer between the ventricular and marginal zone of the hindbrain (F13) and forebrain (F15), GAP-43 was already expressed in mitotic neural precursor cells. In PCC7-Mz1 cultures, 2 days after addition of RA, GAP-43 became phosphorylated upon activation of PKC, and colocalized specifically with the novel PKC isoform eta. Phosphorylation of GAP-43 caused a disruption of its complex with calmodulin. These data demonstrate that GAP-43 is already a functional PKC substrate in prolific neuronal precursor cells, and may participate in neuronal cell lineage determination.

NeuroD regulates multiple functions in the developing neural retina in rodent

The expression and function of the basic helix-loop-helix (bHLH) transcription factor NeuroD were studied in the developing neural retina in rodent. neuroD was expressed in areas of undetermined retinal cells as well as developing photoreceptors and amacrine interneurons. Expression was maintained in a subset of mature photoreceptors in the adult retina. Using both loss-of-function and gain-of-function approaches, NeuroD was found to play multiple roles in retinal development. (1) NeuroD was found to be a critical regulator of the neuron versus glial cell fate decision. Retinal explants derived from NeuroD-null mice demonstrated a three- to fourfold increase in Muller glia. Forced expression of neuroD in progenitors in rat using retroviruses hastened cell cycle withdrawal and blocked gliogenesis in vivo. (2) NeuroD appeared to regulate interneuron development, favouring amacrine over bipolar differentiation. Forced NeuroD expression resulted in an increase in amacrine interneurons and a decrease in bipolar interneurons. In the complementary experiment, retinae derived from NeuroD-null mice demonstrated a twofold increase in bipolar interneurons and a delay in amacrine differentiation. (3) NeuroD appeared to be essential for the survival of a subset of rod photoreceptors. In conclusion, these results implicate NeuroD in a variety of developmental functions including cell fate determination, differentiation and neuron survival.

The NeuroD1/BETA2 sequences essential for insulin gene transcription colocalize with those necessary for neurogenesis and p300/CREB binding protein binding

NeuroD1/BETA2 is a key regulator of pancreatic islet morphogenesis and insulin hormone gene transcription in islet beta cells. This factor also appears to be involved in neurogenic differentiation, because NeuroD1/BETA2 is able to induce premature differentiation of neuronal precursors and convert ectoderm into fully differentiated neurons upon ectopic expression in Xenopus embryos. We have identified amino acid sequences in mammalian and Xenopus NeuroD1/BETA2 that are necessary for insulin gene expression and ectopic neurogenesis. Our results indicate that evolutionarily conserved sequences spanning the basic helix-loop-helix (amino acids [aa] 100 to 155) and C-terminal (aa 156 to 355) regions are important for both of these processes. The transactivation domains (AD1, aa 189 to 299; AD2, aa 300 to 355) were within the carboxy-terminal region, as analyzed by using GAL4:NeuroD1/BETA2 chimeras. Selective activation of mammalian insulin gene enhancer-driven expression and ectopic neurogenesis in Xenopus embryos was regulated by two independent and separable domains of NeuroD1/BETA2, located between aa 156 to 251 and aa 252 to 355. GAL4:NeuroD1/BETA2 constructs spanning these sequences demonstrated that only aa 252 to 355 contained activation domain function, although both aa 156 to 251 and 300 to 355 were found to interact with the p300/CREB binding protein (CBP) coactivator. These results implicate p300/CBP in NeuroD1/BETA2 function and further suggest that comparable mechanisms are utilized to direct target gene transcription during differentiation and in adult islet beta cells.

Properties of ectopic neurons induced by Xenopus neurogenin1 misexpression

We have examined cells cultured from ectoderm-misexpressing Neurogenin1 (Ngn1) to describe better the extent to which this gene can control aspects of neuronal phenotype including motility, morphology, excitability, and synaptic properties. Like primary spinal neurons which normally express Ngn1, cells in Ngn1-misexpressing cultures exhibit a motility-correlated behavior called circus movements prior to neuritogenesis. Misexpression of NeuroD also causes circus movements and later neuronal differentiation. GSK3beta, which inhibits NeuroD function in vivo, blocks both Ngn1-induced and NeuroD-induced neuronal differentiation, while Notch signaling inhibits only Ngn1-induced neuronal differentiation, confirming that NeuroD is downstream of Ngn1 and insensitive to Notch inhibition. While interfering with NeuroD function in ventral ectoderm inhibits both circus movements and neuronal differentiation, such inhibition in the neural plate inhibits only neuronal differentiation, suggesting that additional factors regulate circus movements in the neural ectoderm. Ngn1-misexpressing cells extend N-tubulin-positive neurites and exhibit tetrodotoxin-sensitive action potentials. Unlike the majority of cultured spinal neurons, however, Ngn1-misexpressing cells do not respond to glutamate and do not form functional synapses with myocytes, suggesting that these cells are either like Rohon-Beard sensory neurons or are not fully differentiated.

Identification of a survival-promoting peptide in medium conditioned by oxidatively stressed cell lines of nervous system origin

A survival-promoting peptide has been purified from medium conditioned by Y79 human retinoblastoma cells and a mouse hippocampal cell line (HN 33.1) exposed to H2O2. A 30 residue synthetic peptide was made on the basis of N-terminal sequences obtained during purification, and it was found to exhibit gel mobility and staining properties similar to the purified molecules. The peptide maintains cells and their processes in vitro for the HN 33.1 cell line treated with H2O2, and in vivo for cortical neurons after lesions of the cerebral cortex. It has weak homology with a fragment of a putative bacterial antigen and, like that molecule, binds IgG. The peptide also contains a motif reminiscent of a critical sequence in the catalytic region of calcineurin-type phosphatases; surprisingly, like several members of this family, the peptide catalyzes the hydrolysis of para-nitrophenylphosphate in the presence of Mn2+. Application of the peptide to one side of bilateral cerebral cortex lesions centered on area 2 in rats results in an increase in IgG immunoreactivity in the vicinity of the lesions 7 d after surgery. Microglia immunopositive for IgG and ED-1 are, however, dramatically reduced around the lesions in the treated hemisphere. Furthermore, pyramidal neurons that would normally shrink, die, or disintegrate were maintained, as determined by MAP2 immunocytochemistry and Nissl staining. These survival effects were often found in both hemispheres. The results suggest that this peptide operates by diffusion to regulate the immune response and thereby rescue neurons that would usually degenerate after cortical lesions. The phosphatase activity of this molecule also suggests the potential for direct neuron survival-promoting effects.

Phosphorylation and spatiotemporal distribution of KW8 (NDRF/NeuroD2), a NeuroD family basic helix-loop-helix protein

KW8, a NeuroD family basic helix-loop-helix protein, was initially cloned during the course of screening for the genes related to long term potentiation in rat hippocampal slice. Its homologue NDRF/NeuroD was also reported. In this report its phosphorylation and spatiotemporal distribution was studied. KW8 was expressed not only during embryonic and neonatal periods but also in adults. In adult, KW8 was expressed only in brain tissues, such as the cerebral cortex, hippocampus and cerebellum. Immunohistological studies revealed that KW8 was localized in the nuclei of neurons. On immunoblotting of rat brain tissue, COS-1 cells and Neuro2A cells overexpressing KW8, this protein was detected as several diffuse bands. Alkaline phosphatase treatment reduced the molecular weights of these bands. Metabolic labeling with 32Pi in COS-1 cells confirmed that the KW8 protein was phosphorylated in vivo. Some of the physiological functions of KW8 might be regulated by this phosphorylation. In yeast, the GAL4 fusion protein containing the C-terminal region of KW8 activated transcription of the reporter gene, suggesting that KW8 had transcriptional activity.

Identification of a survival-promoting peptide in medium conditioned by oxidatively stressed cell lines of nervous system origin

A survival-promoting peptide has been purified from medium conditioned by Y79 human retinoblastoma cells and a mouse hippocampal cell line (HN 33.1) exposed to H2O2. A 30 residue synthetic peptide was made on the basis of N-terminal sequences obtained during purification, and it was found to exhibit gel mobility and staining properties similar to the purified molecules. The peptide maintains cells and their processes in vitro for the HN 33.1 cell line treated with H2O2, and in vivo for cortical neurons after lesions of the cerebral cortex. It has weak homology with a fragment of a putative bacterial antigen and, like that molecule, binds IgG. The peptide also contains a motif reminiscent of a critical sequence in the catalytic region of calcineurin-type phosphatases; surprisingly, like several members of this family, the peptide catalyzes the hydrolysis of para-nitrophenylphosphate in the presence of Mn2+. Application of the peptide to one side of bilateral cerebral cortex lesions centered on area 2 in rats results in an increase in IgG immunoreactivity in the vicinity of the lesions 7 d after surgery. Microglia immunopositive for IgG and ED-1 are, however, dramatically reduced around the lesions in the treated hemisphere. Furthermore, pyramidal neurons that would normally shrink, die, or disintegrate were maintained, as determined by MAP2 immunocytochemistry and Nissl staining. These survival effects were often found in both hemispheres. The results suggest that this peptide operates by diffusion to regulate the immune response and thereby rescue neurons that would usually degenerate after cortical lesions. The phosphatase activity of this molecule also suggests the potential for direct neuron survival-promoting effects.

Molecular cloning and characterization of the gene encoding human NeuroD

NeuroD is a basic helix-loop-helix transcription factor that binds to an E-box sequence, CANNTG, in the target gene promoter region. The gene encoding NeuroD is expressed specifically in brain, intestine, and pancreatic cells and plays a pivotal role in tissue-specific differentiation. To investigate the regulation of human NeuroD gene expression, we isolated and sequenced a genomic DNA containing two exons and flanking regions of the NeuroD gene. The sizes of exons 1 and 2 were 168 and 2404 bp, respectively. CAT reporter analysis showed that the elements responsible for basal promoter activity lay in the proximal 408-bp region. Cotransfection of the CAT reporter plasmids with a NeuroD expression plasmid resulted in 5-fold enhancement of the CAT activity, indicating an autoregulatory mechanism is involved in human NeuroD gene expression.

Isolation and characterization of the mouse beta 2/neuroD gene promoter

Beta2/neuroD is a basic helix-loop-helix protein involved in differentiation of the endocrine pancreas and the central nervous system. A 2-kb fragment containing the 5' upstream region of the mouse beta2/neuroD gene was cloned and sequenced. The cloned fragment was tested for promoter activity in six cell lines. Strong promoter activity was apparent in the three pancreatic beta cell lines, beta-HC3, beta-HC9, and NIT-1, whereas weak activity was seen in NIH 3T3, Rat-1, and MCF-7 cell lines. Analysis of promoter activity of deletion mutants in beta-HC3 cells indicated that while basal promoter activity was observed with a fragment which extended -109 bp upstream of the transcription start site, maximal activity required the fragment -2091 to -297 bp.

Multiple elements regulate Mash1 expression in the developing CNS

Mash1, a transcription factor of the basic helix-loop-helix class, is expressed during embryogenesis in restricted regions of the nervous system. An essential role for Mash1 in neural development was demonstrated previously in mice carrying a targeted disruption of the Mash1 gene. Regulation of the precise temporal and spatial expression of Mash1 is thus likely to be important for proper neural development. In this study, sequences that regulate Mash1 expression in the central nervous system were characterized by assaying the expression of lacZ reporter genes in transgenic embryos. A 1158-bp enhancer localized approximately 7 kb upstream of the Mash1 coding region was identified. Deletions within this enhancer region reveal the presence of both positive and negative cis-acting elements. Analysis of multiple sequences within the enhancer demonstrate that different elements preferentially function in different regions within the Mash1-specific CNS expression domain. In addition, a role for sequences 3' of the Mash1 coding region is revealed, providing evidence for posttranscriptional control of Mash1 expression in multiple CNS domains.

An E-box sequence acts as a transcriptional activator for BC1 RNA expression by RNA polymerase III in the brain

BC1 RNA is a small cytoplasmic RNA that is transcribed by RNA polymerase III (Pol III) in the rodent nervous system. In addition to essential intragenic promoter elements for Pol III, the BC1 RNA gene has five E-box sequences (CANNTG) in its 5' flanking region. Deletion analysis using an in vitro transcription system revealed that the region containing the E2 site (CAATTG) was necessary for effective transcription of BC1 RNA. A construct with point mutations within the E2 site showed reduced transcriptional activity. Furthermore, DNaseT I protection and gel retardation assays demonstrated that the E2 site was recognized specifically by a brain nuclear protein(s). These results suggest that the upstream E-box sequence and its binding protein may be involved in the regulation by Pol III of preferential BC1 RNA expression in the brain.

Structure and promoter analysis of Math3 gene, a mouse homolog of Drosophila proneural gene atonal. Neural-specific expression by dual promoter elements

ath3, a vertebrate basic helix-loop-helix gene homologous to Drosophila proneural gene atonal, can directly convert non-neural cells into neurons with the anterior features. In the mouse, ath3 expression initially occurs widely in the developing nervous system and then gradually becomes restricted to the neural retina. Here, we characterized the genomic organization and promoter activity of mouse ath3 (Math3). Math3 gene consists of two exons separated by an 8-kilobase intron, and the whole protein-coding region is located in the second exon. Transcription starts at two sites, which are 75 nucleotides apart from each other, and there is no typical TATA box in the upstream region of either start site. Transient transfection analysis showed that the 5'-region of Math3 can direct efficient expression in neuroblastoma cells but not in glioma or fibroblast cells. Deletion studies revealed that the proximal 193-base pair region, which contains the downstream transcription initiation site but not the upstream site, is essential for the Math3 promoter activity and can direct efficient expression in neuroblastoma cells. In contrast, retrovirus-mediated promoter analysis demonstrated that a region further upstream is additionally necessary for retinal expression. These results indicate that Math3 promoter contains two essential regulatory regions, the proximal 193-base pair region, which confers efficient neural-specific expression, and a region further upstream, required for retinal expression.

Neuronal basic helix-loop-helix proteins (NEX, neuroD, NDRF): spatiotemporal expression and targeted disruption of the NEX gene in transgenic mice

Basic helix-loop-helix (bHLH) genes have emerged as important regulators of neuronal determination and differentiation in vertebrates. Three putative neuronal differentiation factors [NEX for neuronal helix-loop-helix protein-1 (mammalian atonal homolog-2), neuroD (beta-2), and NDRF for neuroD-related factor (neuroD2)] are highly homologous to each other in the bHLH region and comprise a new bHLH subfamily. To study the role of NEX, the first bHLH protein identified in this group, we have disrupted the NEX gene by homologous recombination. NEX-deficient mice have no obvious developmental defect, and CNS neurons appear fully differentiated. To investigate further whether the absence of NEX is compensated for by neuroD and NDRF, we compared the spatiotemporal expression of all three genes. We demonstrate, by in situ hybridization, that the transcription patterns of NEX, neuroD, and NDRF genes are highly overlapping in the developing CNS of normal rats between embryonic day 12 and adult stages but are not strictly identical. The most prominent transcription of each gene marks the dorsal neuroepithelium of the telencephalon in early development and is sustained in the adult neocortex, hippocampus, and cerebellum. In general, neuroD provides the earliest marker of neuronal differentiation in any given region compared with NDRF or NEX. Whereas a few CNS regions are specific for neuroD, no region was detected in which solely NEX or NDRF is expressed. This suggests that the function of the mutant NEX gene in neuronal differentiation is compensated for by neuroD and NDRF and that, in analogy with myogenic bHLH proteins, neuronal differentiation factors are at least in part equivalent in function.

The bHLH protein NEUROGENIN 2 is a determination factor for epibranchial placode-derived sensory neurons

neurogenin2 encodes a neural-specific basic helix-loop-helix (bHLH) transcription factor related to the Drosophila proneural factor atonal. We show here that the murine ngn2 gene is essential for development of the epibranchial placode-derived cranial sensory ganglia. An ngn2 null mutation blocks the delamination of neuronal precursors from the placodes, the first morphological sign of differentiation in these lineages. Mutant placodal cells fail to express downstream bHLH differentiation factors and the Notch ligand Delta-like 1. These data suggest that ngn2 functions like the Drosophila proneural genes in the determination of neuronal fate in distal cranial ganglia. Interestingly, the homeobox gene Phox2a is activated independently of ngn2 in epibranchial placodes, suggesting that neuronal fate and neuronal subtype identity may be specified independently in cranial sensory ganglia.

neurogenin1 is essential for the determination of neuronal precursors for proximal cranial sensory ganglia

The NEUROGENINS (NGNs) are neural-specific basic helix-loop-helix (bHLH) transcription factors. Mouse embryos lacking ngn1 fail to generate the proximal subset of cranial sensory neurons. ngn1 is required for the activation of a cascade of downstream bHLH factors, including NeuroD, MATH3, and NSCL1. ngn1 is expressed by placodal ectodermal cells and acts prior to neuroblast delamination. Moreover, NGN1 positively regulates the Delta homolog DLL1 and can be negatively regulated by Notch signaling. Thus, ngn1 functions similarly to the proneural genes in Drosophila. However, the initial pattern of ngn1 expression appears to be Notch independent. Taken together with the fact that ectopic ngn1 expression can convert ectodermal cells to neurons in Xenopus (Ma et al., 1996), these data and those of Fode et al. (1998 [this issue of Neuron]) identify ngns as vertebrate neuronal determination genes, analogous to myoD and myf5 in myogenesis.

Progenitors of dorsal commissural interneurons are defined by MATH1 expression

MATH1 is a neural-specific basic helix-loop-helix transcription factor. Members of this family of transcription factors are involved in the development of specific subsets of neurons in the developing vertebrate nervous system. Here we examine the cells expressing MATH1 with respect to their proliferative state and co-expression of cell-type-specific differentiation markers. We localize the MATH1 protein to the nucleus of cells in the dorsal neural tube and the external germinal layer (EGL) of the developing cerebellum. Using double-label immunofluorescence, we demonstrate that MATH1-expressing cells span both the proliferating and the differentiating zones within the dorsal neural tube, but within the EGL of the cerebellum are restricted to the proliferating zone. The early differentiating MATH1-expressing cells in the dorsal neural tube co-express TAG-1, DCC-1 and LH2, markers of dorsal commissural interneurons. In addition, transgenic mice with lacZ under the transcriptional control of MATH1-flanking DNA sequences express beta-galactosidase specifically in the developing nervous system, in a manner that mimics subsets of the MATH1-expression pattern, including the dorsal spinal neural tube. Expression of the MATH1/lacZ transgene persists in differentiated dorsal commissural interneurons. Taken together, we demonstrate MATH1 expression in a differentiating population of neuronal precursors in the dorsal neural tube that appear to give rise specifically to dorsal commissural interneurons.

Neuronal basic helix-loop-helix proteins (NEX, neuroD, NDRF): spatiotemporal expression and targeted disruption of the NEX gene in transgenic mice

Basic helix-loop-helix (bHLH) genes have emerged as important regulators of neuronal determination and differentiation in vertebrates. Three putative neuronal differentiation factors [NEX for neuronal helix-loop-helix protein-1 (mammalian atonal homolog-2), neuroD (beta-2), and NDRF for neuroD-related factor (neuroD2)] are highly homologous to each other in the bHLH region and comprise a new bHLH subfamily. To study the role of NEX, the first bHLH protein identified in this group, we have disrupted the NEX gene by homologous recombination. NEX-deficient mice have no obvious developmental defect, and CNS neurons appear fully differentiated. To investigate further whether the absence of NEX is compensated for by neuroD and NDRF, we compared the spatiotemporal expression of all three genes. We demonstrate, by in situ hybridization, that the transcription patterns of NEX, neuroD, and NDRF genes are highly overlapping in the developing CNS of normal rats between embryonic day 12 and adult stages but are not strictly identical. The most prominent transcription of each gene marks the dorsal neuroepithelium of the telencephalon in early development and is sustained in the adult neocortex, hippocampus, and cerebellum. In general, neuroD provides the earliest marker of neuronal differentiation in any given region compared with NDRF or NEX. Whereas a few CNS regions are specific for neuroD, no region was detected in which solely NEX or NDRF is expressed. This suggests that the function of the mutant NEX gene in neuronal differentiation is compensated for by neuroD and NDRF and that, in analogy with myogenic bHLH proteins, neuronal differentiation factors are at least in part equivalent in function.

bHLH transcription factors and mammalian neuronal differentiation

The basic helix-loop-helix (bHLH) factor Mash1 is expressed in the developing nervous system. Null mutation of Mash1 results in loss of olfactory and autonomic neurons and delays differentiation of retinal neurons, indicating that Mash1 promotes neuronal differentiation. Other bHLH genes, Math/NeuroD/Neurogenin, all expressed in the developing nervous system, have also been suggested to promote neuronal differentiation. In contrast, another bHLH factor, HES1, which is expressed by neural precursor cells but not by neurons, represses Mash1 expression and antagonizes Mash1 activity in a dominant negative manner. Forced expression of HES1 in precursor cells blocks neuronal differentiation in the brain and retina, indicating that HES1 is a negative regulator of neuronal differentiation. Conversely, null mutation of HES1 up-regulates Mash1 expression, accelerates neuronal differentiation, and causes severe defects of the brain and eyes. Thus, HES1 regulates brain and eye morphogenesis by inhibiting premature neuronal differentiation, and the down-regulation of HES1 expression at the right time is required for normal development of the nervous system. Interestingly, HES1 can repress its own expression by binding to its promoter, suggesting that negative autoregulation may contribute to down-regulation of HES1 expression during neural development. Recent studies indicate that HES1 expression is also controlled by RBP-J, a mammalian homologue of Suppressor of Hairless [Su(H)], and Notch, a key membrane protein that may regulate lateral specification through RBP-J during neural development. Thus, the Notch-->RBP-J-->HES1-Mash1 pathway may play a critical role in neuronal differentiation.

The activity of neurogenin1 is controlled by local cues in the zebrafish embryo

Zebrafish neurogenin1 encodes a basic helix-loop-helix protein which shares structural and functional characteristics with proneural genes of Drosophila melanogaster. neurogenin1 is expressed in the early neural plate in domains comprising more cells than the primary neurons known to develop from these regions and its expression is modulated by Delta/Notch signalling, suggesting that it is a target of lateral inhibition. Misexpression of neurogenin1 in the embryo results in development of ectopic neurons. Markers for different neuronal subtypes are not ectopically expressed in the same patterns in neurogenin1-injected embryos suggesting that the final identity of the ectopically induced neurons is modulated by local cues. Induction of ectopic motor neurons by neurogeninl requires coexpression of a dominant negative regulatory subunit of protein kinase A, an intracellular transducer of hedgehog signals. Moreover, the pattern of endogenous neurogenin1 expression in the neural plate is expanded in response to elevated levels of Hedgehog (Hh) signalling or abolished as a result of inhibition of Hh signalling. Together these data suggest that Hh signals regulate neurogenin1 expression and subsequently modulate the type of neurons produced by Neurogenin1 activity.

NeuroD1/beta2 contributes to cell-specific transcription of the proopiomelanocortin gene

NeuroD1/beta2 is a basic helix-loop-helix (bHLH) factor expressed in the endocrine cells of the pancreas and in a subset of neurons as they undergo terminal differentiation. We now show that NeuroD1 is expressed in corticotroph cells of the pituitary gland and that it is involved in cell-specific transcription of the proopiomelanocortin (POMC) gene. It was previously shown that corticotroph-specific POMC transcription depends in part on the action of cell-restricted bHLH factors that were characterized as the CUTE (corticotroph upstream transcription element) (M. Therrien and J. Drouin, Mol. Cell. Biol. 13:2342-2353, 1993) complexes. We now demonstrate that these complexes contain NeuroD1 in association with various ubiquitous bHLH dimerization partners. The NeuroD1-containing heterodimers specifically recognize and activate transcription from the POMC promoter E box that confers transcriptional specificity. Interestingly, the NeuroD1 heterodimers activate transcription in synergy with Ptx1, a Bicoid-related homeodomain protein, which also contributes to corticotroph specificity of POMC transcription. In the adult pituitary gland, NeuroD1 transcripts are detected in POMC-expressing corticotroph cells. Taken together with the restricted pattern of Ptx1 expression, these results suggest that these two factors establish the basis of a combinatorial code for the program of corticotroph-specific gene expression.

Xath5 participates in a network of bHLH genes in the developing Xenopus retina

We examined the function of basic-helix-loop-helix (bHLH) transcription factors during retinal neurogenesis. We identified Xath5, a Xenopus bHLH gene related to Drosophila atonal, which is expressed in the developing Xenopus retina. Targeted expression of Xath5 in retinal progenitor cells biased the differentiation of these cells toward a ganglion cell fate, suggesting that Xath5 can regulate the differentiation of retinal neurons. We examined the relationship between the three bHLH genes Xash3, NeuroD, and Xath5 during retinal neurogenesis and found that Xash3 is expressed in early retinoblasts, followed by coexpression of Xath5 and NeuroD in differentiating cells. We provide evidence that the expression of Xash3, NeuroD, and Xath5 is coupled and propose that these bHLH genes regulate successive stages of neuronal differentiation in the developing retina.

Identification of a novel repressive element that contributes to neuron-specific gene expression

Multiple signaling pathways are thought to control the selective expression of genes over the course of neuronal differentiation. One approach to elucidating these pathways is to identify specific cis-acting elements that serve as the final targets for these signaling pathways in neural-specific genes. We now identify a novel repressive element from the growth-associated protein 43 (GAP-43) gene that can contribute to neuron-specific gene expression by inhibiting transcription in a wide range of non-neuronal cell types. This repressive element is located downstream of the GAP-43 TATA box and is highly position-dependent. When transferred to viral promoters this element preferentially inhibits transcription in non-neuronal cells. Electrophoretic mobility shift assays show that the repressive element comprises at least two protein recognition sites. One of these is a novel sequence motif that we designate the SNOG element, because it occurs downstream of the TATA boxes of the synaptosomal-associated protein of 25 kDa and neuronal nitric oxide synthase genes, as well as the GAP-43 gene. The GAP-43 repressive element is distinct in sequence and position dependence from the repressor element 1/neuron-restrictive silencer element previously described in other neural genes and therefore is a likely target for a distinct set of signaling pathways involved in the control of neuronal differentiation.

Identification of a novel repressive element that contributes to neuron-specific gene expression

Multiple signaling pathways are thought to control the selective expression of genes over the course of neuronal differentiation. One approach to elucidating these pathways is to identify specific cis-acting elements that serve as the final targets for these signaling pathways in neural-specific genes. We now identify a novel repressive element from the growth-associated protein 43 (GAP-43) gene that can contribute to neuron-specific gene expression by inhibiting transcription in a wide range of non-neuronal cell types. This repressive element is located downstream of the GAP-43 TATA box and is highly position-dependent. When transferred to viral promoters this element preferentially inhibits transcription in non-neuronal cells. Electrophoretic mobility shift assays show that the repressive element comprises at least two protein recognition sites. One of these is a novel sequence motif that we designate the SNOG element, because it occurs downstream of the TATA boxes of the synaptosomal-associated protein of 25 kDa and neuronal nitric oxide synthase genes, as well as the GAP-43 gene. The GAP-43 repressive element is distinct in sequence and position dependence from the repressor element 1/neuron-restrictive silencer element previously described in other neural genes and therefore is a likely target for a distinct set of signaling pathways involved in the control of neuronal differentiation.

Helix-loop-helix factors in growth and differentiation of the vertebrate nervous system

Neural development involves the initial growth phase of dividing precursor cells and the subsequent differentiation phase of postmitotic cells. Recent studies indicate that the transition from the former phase to the latter is controlled antagonistically by multiple helix-loop-helix (HLH) genes. Cascades of neuronal HLH genes promote differentiation whereas anti-neuronal HLH genes repress them under the control of Notch and keep cells at the precursor stage. This antagonistic regulation may be essential for generation of the proper number of neurons and for morphogenesis of the nervous system.

Diabetes, defective pancreatic morphogenesis, and abnormal enteroendocrine differentiation in BETA2/neuroD-deficient mice

Candidate transcription factors involved in pancreatic endocrine development have been isolated using insulin gene regulation as a paradigm. The cell-type restricted basic helix-loop-helix (bHLH) gene, BETA2/NeuroD, expressed in pancreatic endocrine cells, the intestine, and the brain, activates insulin gene transcription and can induce neurons to differentiate. To understand the importance of BETA2 in pancreatic endocrine cell differentiation, mice lacking a functional BETA2 gene were generated by gene targeting experiments. Mice carrying a targeted disruption of the BETA2 gene developed severe diabetes and died perinatally. Homozygous BETA2 null mice had a striking reduction in the number of insulin-producing beta cells and failed to develop mature islets. Islet morphogenesis appeared to be arrested between E14.5 and E17.5, a period characterized by major expansion of the beta cell population. The presence of severe diabetes in these mice suggests that proper islet structure plays an important role in blood glucose homeostasis. In addition, secretin- and cholecystokinin-producing enteroendocrine cells failed to develop in the absence of BETA2. The absence of these two pancreatic secretagogs may explain the abnormal cellular polarity and inability to secrete zymogen granules in pancreatic acinar exocrine cells. The nervous system appeared to develop normally, despite abundant expression of BETA2 in differentiating neurons. Thus, BETA2 is critical for the normal development of several specialized cell types arising from the gut endoderm.

Mash1 activates a cascade of bHLH regulators in olfactory neuron progenitors

The lineage of olfactory neurons has been relatively well characterized at the cellular level, but the genes that regulate the proliferation and differentiation of their progenitors are currently unknown. In this study, we report the isolation of a novel murine gene, Math4C/neurogenin1, which is distantly related to the Drosophila proneural gene atonal. We show that Math4C/neurogenin1 and the basic helix-loop-helix gene Mash1 are expressed in the olfactory epithelium by different dividing progenitor populations, while another basic helix-loop-helix gene, NeuroD, is expressed at the onset of neuronal differentiation. These expression patterns suggest that each gene marks a distinct stage of olfactory neuron progenitor development, in the following sequence: Mash1>Math4C/neurogenin1>NeuroD. We have previously reported that inactivation of Mash1 function leads to a severe reduction in the number of olfactory neurons. We show here that most cells in the olfactory epithelium of Mash1 mutant embryos fail to express Math4C/neurogenin1 or NeuroD. Strikingly, a subset of progenitor cells in a ventrocaudal domain of Mash1 mutant olfactory epithelium still express Math4C/neurogenin1 and NeuroD and differentiate into neurons. Cells in this domain also express Math4A/neurogenin2, another member of the Math4/neurogenin gene family, and not Mash1. Our results demonstrate that Mash1 is required at an early stage in the olfactory neuron lineage to initiate a differentiation program involving Math4C/neurogenin1 and NeuroD. Another gene activates a similar program in a separate population of olfactory neuron progenitors.

NEUROD2 and NEUROD3 genes map to human chromosomes 17q12 and 5q23-q31 and mouse chromosomes 11 and 13, respectively

NEUROD2 and NEUROD3 are transcription factors involved in neurogenesis that are related to the basic helix-loop-helix protein NEUROD. NEUROD2 maps to human chromosome 17q12 and mouse chromosome 11. NEUROD3 maps to human chromosome 5q23-q31 and mouse chromosome 13.

Basic helix-loop-helix genes in neural development

Several major advances in the understanding of the regulation of vertebrate neurogenesis by members of the basic helix-loop-helix (bHLH) protein family have been made in the past year. Specifically, a number of bHLH genes have been cloned and shown to convert non-neuronal fate to neuronal fate when expressed ectopically. In particular, studies on NeuroD and Neurogenin suggest a regulatory pathway, providing powerful molecular tools to study vertebrate neurogenesis.