A mammalian helix-loop-helix factor structurally related to the product of Drosophila proneural gene atonal is a positive transcriptional regulator expressed in the developing nervous system

Developmental restriction of Mash-2 expression in trophoblast correlates with potential activation of the notch-2 pathway

Mash-2 expression begins during preimplantation development, but is restricted to trophoblasts after the blastocyst stage. Within the trophoblast lineage, Mash-2 transcripts are first expressed in the ectoplacental cone and chorion, but not in terminally differentiated trophoblast giant cells. After day 8.5 of gestation, Mash-2 expression becomes further restricted to focal sites within the spongiotrophoblast and labyrinth. Downregulation is probably important for normal development since overexpression of Mash-2 reduces giant cell formation. We have investigated the role that the Notch signaling pathway may play in trophoblast development. Mash-2 is a homologue of Drosophila achaete/scute complex genes. In Drosophila, activation of the Notch receptor induces transcriptional repressors encoded by the hairy/Enhancer of split (HES) genes, which interact with the Groucho protein to shut off achaete-scute transcription. In the developing mouse placenta, we found that all elements of the Notch pathway were expressed. In particular, the Notch-2, HES-2, and HES-3 genes were coexpressed in trophoblast giant cells and in foci within the spongiotrophoblast at day 10.5 when Mash-2 transcription becomes restricted. Two members of the mammalian Groucho family were expressed in trophoblasts; TLE3 was expressed broadly in the giant cell, spongiotrophoblast, and labyrinthine regions, whereas TLE2 was limited to giant cells and focal regions of the spongiotrophoblast. These data suggest that Notch signaling through activation of HES transcriptional repressors may play a role in murine placental development.

Identification of the C-terminal activation domain of the NeuroD-related factor (NDRF)

NeuroD-related factor (NDRF) is a basic helix-loop-helix (bHLH) protein whose expression is restricted to the central nervous system, and is considered to be responsible for maintenance of differentiated neurons as well as neurogenesis. NDRF structurally resembles NeuroD in the bHLH region and can induce neurogenesis ectopically in ectodermal cells of the Xenopus embryo. In this study, we delineated the functional domains of NDRF. Using GAL4/NDRF fusion proteins, we identified the C-terminal activation domain (C-AD) in NDRF between amino acid positions 294 and 383. This region was highly homologous to one part of the activation domain of NeuroD. We further investigated the transactivational function of C-AD in the mouse type 1 inositol 1,4,5-trisphosphate receptor promoter, which has an NDRF site. Truncation of C-AD resulted in reduction of the activation function, whereas the DNA-binding specificity was not affected. These results suggest that C-AD has a stimulatory function in the mammalian nervous system.

Overexpression of Math1 induces robust production of extra hair cells in postnatal rat inner ears

For mammalian cochlear hair cells, fate determination is normally completed by birth. We report here that overexpression of Math1, a mouse homolog of the Drosophila gene atonal, in postnatal rat cochlear explant cultures resulted in extra hair cells. Surprisingly, we found that the source of the ectopic hair cells was columnar epithelial cells located outside the sensory epithelium in the greater epithelial ridge, which normally give rise to inner sulcus cells. Moreover, Math1 expression also facilitated conversion of postnatal utricular supporting cells into hair cells. Thus Math1 was sufficient for the production of hair cells in the ear, and immature postnatal mammalian inner ears retained the competence to generate new hair cells.

Induction of medulloblastomas in p53-null mutant mice by somatic inactivation of Rb in the external granular layer cells of the cerebellum

Medulloblastomas are among the most common malignancies in childhood, and they are associated with substantial mortality and morbidity. The molecular pathogenesis as well as the ontogeny of these neoplasms is still poorly understood. We have generated a mouse model for medulloblastoma by Cre–LoxP-mediated inactivation ofRb and p53 tumor suppressor genes in the cerebellar external granular layer (EGL) cells. GFAP–Cre-mediated recombination was found both in astrocytes and in immature precursor cells of the EGL in the developing cerebellum.GFAP–Cre;RbLoxP/LoxP;p53−/−or LoxP/LoxP mice developed highly aggressive embryonal tumors of the cerebellum with typical features of medulloblastoma. These tumors were identified as early as 7 weeks of age on the outer surface of the molecular layer, corresponding to the location of the EGL cells during development. Our results demonstrate that loss of function of RB is essential for medulloblastoma development in the mouse and strongly support the hypothesis that medulloblastomas arise from multipotent precursor cells located in the EGL.

Structure of the mouse NDRF gene and its regulation during neuronal differentiation of P19 cells

We have isolated and characterized the mouse gene for NDRF (neuroD-related factor), a basic helix-loop-helix transcription factor implicated in neural development and function. The gene consists of two exons and the entire protein-coding sequence is encoded by a single downstream exon. RNA blot hybridization analysis revealed that NDRF mRNA was detectable at day 4 and increased to a maximal level at day 6 during neuronal differentiation of P19 cells. To elucidate the regulatory mechanisms of the NDRF gene expression during this process, a construct containing the genomic DNA fragment of about 3 kbp upstream of the NDRF coding region fused to a luciferase reporter gene was transfected into P19 cells, and stable transformants were pooled for assay of luciferase activities. When the stable transformants were treated with RA and aggregated to induce neuronal differentiation, the luciferase activities were induced in a temporal expression pattern similar to that of the endogenous NDRF mRNA. Further experiments using a series of deletion and mutation constructs indicated that the 376-bp sequence in the 5'-flanking region of the NDRF gene is important, and that one of the E boxes in the sequence plays a critical role in the regulated expression. Transient transfection experiments also showed that the same E box is required for the transactivation of the NDRF promoter activity by neurogenin 1. These results suggest that the NDRF gene expression is regulated by an E box-binding factor during neuronal differentiation of P19 cells.

Autoregulation and multiple enhancers control Math1 expression in the developing nervous system

Development of the vertebrate nervous system requires the actions of transcription factors that establish regional domains of gene expression, which results in the generation of diverse neuronal cell types. MATH1, a transcription factor of the bHLH class, is expressed during development of the nervous system in multiple neuronal domains, including the dorsal neural tube, the EGL of the cerebellum and the hair cells of the vestibular and auditory systems. MATH1 is essential for proper development of the granular layer of the cerebellum and the hair cells of the cochlear and vestibular systems, as shown in mice carrying a targeted disruption of Math1. Previously, we showed that 21 kb of sequence flanking the Math1-coding region is sufficient for Math1 expression in transgenic mice. Here we identify two discrete sequences within the 21 kb region that are conserved between mouse and human, and are sufficient for driving a lacZ reporter gene in these domains of Math1 expression in transgenic mice. The two identified enhancers, while dissimilar in sequence, appear to have redundant activities in the different Math1 expression domains except the spinal neural tube. The regulatory mechanisms for each of the diverse Math1 expression domains are tightly linked, as separable regulatory elements for any given domain of Math1 expression were not found, suggesting that a common regulatory mechanism controls these apparently unrelated domains of expression. In addition, we demonstrate a role for autoregulation in controlling the activity of the Math1 enhancer, through an essential E-box consensus binding site.

Functional conservation of atonal and Math1 in the CNS and PNS

To determine the extent to which atonal and its mouse homolog Math1 exhibit functional conservation, we inserted (beta)-galactosidase (lacZ) into the Math1 locus and analyzed its expression, evaluated consequences of loss of Math1 function, and expressed Math1 in atonal mutant flies. lacZ under the control of Math1 regulatory elements duplicated the previously known expression pattern of Math1 in the CNS (i.e., the neural tube, dorsal spinal cord, brainstem, and cerebellar external granule neurons) but also revealed new sites of expression: PNS mechanoreceptors (inner ear hair cells and Merkel cells) and articular chondrocytes. Expressing Math1 induced ectopic chordotonal organs (CHOs) in wild-type flies and partially rescued CHO loss in atonal mutant embryos. These data demonstrate that both the mouse and fly homologs encode lineage identity information and, more interestingly, that some of the cells dependent on this information serve similar mechanoreceptor functions.

atonal regulates neurite arborization but does not act as a proneural gene in the Drosophila brain

Drosophila atonal (ato) is the proneural gene of the chordotonal organs (CHOs) in the peripheral nervous system (PNS) and the larval and adult photoreceptor organs. Here, we show that ato is expressed at multiple stages during the development of a lineage of central brain neurons that innervate the optic lobes and are required for eclosion. A novel fate mapping approach shows that ato is expressed in the embryonic precursors of these neurons and that its expression is reactivated in third instar larvae (L3). In contrast to its function in the PNS, ato does not act as a proneural gene in the embryonic brain. Instead, ato performs a novel function, regulating arborization during larval and pupal development by interacting with Notch.

Genetic ablation reveals that the roof plate is essential for dorsal interneuron specification

During neural development in vertebrates, a spatially ordered array of neurons is generated in response to inductive signals derived from localized organizing centres. One organizing centre that has been proposed to have a role in the control of neural patterning is the roof plate. To define the contribution of signals derived from the roof plate to the specification of neuronal cell types in the dorsal neural tube, we devised a genetic strategy to ablate the roof plate selectively in mouse embryos. Embryos without a roof plate lack all the interneuron subtypes that are normally generated in the dorsal third of the neural tube. Using a genetically based lineage analysis and in vitro assays, we show that the loss of these neurons results from the elimination of non-autonomous signals provided by the roof plate. These results reveal that the roof plate is essential for specifying multiple classes of neurons in the mammalian central nervous system.

Generation of neurons by transient expression of neural bHLH proteins in mammalian cells

Basic helix-loop-helix (bHLH) transcription factors are known to function during mammalian neurogenesis. Here we show that transient transfection of vectors expressing neuroD2, MASH1, ngn1 or related neural bHLH proteins, with their putative dimerization partner E12, can convert mouse P19 embryonal carcinoma cells into differentiated neurons. Transfected cells express numerous neuron-specific proteins, adopt a neuronal morphology and are electrically excitable. Thus, the expression of neural bHLH proteins is sufficient to confer a neuronal fate on uncommitted mammalian cells. Neuronal differentiation of transfected cells is preceded by elevated expression of the cyclin-dependent kinase inhibitor p27(Kip1) and cell cycle withdrawal. This demonstrates that the bHLH proteins can link neuronal differentiation to withdrawal from the cell cycle, possibly by activating the expression of p27(Kip1). The ability to generate mammalian neurons by transient expression of neural bHLH proteins should create new opportunities for studying neurogenesis and devising neural repair strategies.

OUT, a novel basic helix-loop-helix transcription factor with an Id-like inhibitory activity

Transcription factors belonging to the basic helix-loop-helix (bHLH) family are involved in various cell differentiation processes. We report the isolation and functional characterization of a novel bHLH factor, termed OUT. OUT, structurally related to capsulin/epicardin/Pod-1 and ABF-1/musculin/MyoR, is expressed mainly in the adult mouse reproductive organs, such as the ovary, uterus, and testis, and is barely detectable in tissues of developing embryos. Physical association of OUT with the E protein was predicted from the primary structure of OUT and confirmed by co-immunoprecipitation. However, unlike other bHLH factors, this novel protein failed to bind E-box or N-box DNA sequences and inhibited DNA binding of homo- and heterodimers consisting of E12 and MyoD in gel mobility shift assays. In luciferase assays, OUT inhibited the induction of E-box-dependent transactivation by MyoD-E12 heterodimers. Deletion studies identified the domain responsible for the inhibitory action of OUT in its bHLH and C-terminal regions. Moreover, terminal differentiation of C2C12 myoblasts was inhibited by exogenous introduction of OUT. These inhibitory functions of OUT closely resemble those of the helix-loop-helix inhibitor Id proteins. Based on these findings, we propose that this novel protein functions as a negative regulator of bHLH factors through the formation of a functionally inactive heterodimeric complex.

Subcellular and developmental expression of alternatively spliced forms of fibroblast growth factor 14

Fibroblast growth factors (FGFs) 11-14 comprise a subfamily of FGFs with poorly defined biological function. Here we characterize two isoforms of FGF14 (FGF14-1a and FGF14-1b) that result from the alternative usage of two different first exons. We demonstrate that these isoforms have differential subcellular localization and that they are differentially expressed in various adult tissues. Using in situ hybridization we show that Fgf14 is widely expressed in brain, spinal cord, major arteries and thymus between 12.5 and 14.5 days of mouse embryonic development. We also show that during cerebellar development, Fgf14 is first observed at postnatal day 1 in post mitotic granule cells, and later in development, in migrating and post migratory granule cells. The developmental expression pattern of Fgf14 in the cerebellum is complementary to that of Math1, a marker for proliferating granule cells in the external germinal layer.

Loss of BETA2/NeuroD leads to malformation of the dentate gyrus and epilepsy

BETA2/NeuroD is a homologue of the Drosophila atonal gene that is widely expressed during development in the mammalian brain and pancreas. Although studies in Xenopus suggest that BETA2/NeuroD is involved in cellular differentiation, its function in the mammalian nervous system is unclear. Here we show that mutant mice homozygous for a deletion at the BETA2/NeuroD locus fail to develop a granule cell layer within the dentate gyrus, one of the principal structures of the hippocampal formation. To understand the basis of this abnormality, we analyzed dentate gyrus development by using immunocytochemical markers in BETA2/NeuroD-deficient mice. The early cell populations in the dentate gyrus, including Cajal-Retzius cells and radial glia, are present and appear normally organized. The migration of dentate precursor cells and newly born granule cells from the neuroepithelium to the dentate gyrus remains intact. However, there is a dramatic defect in the proliferation of precursor cells once they reach the dentate and a significant delay in the differentiation of granule cells. This leads to malformation of the dentate granule cell layer and excess cell death. BETA2/NeuroD null mice also exhibit spontaneous limbic seizures associated with electrophysiological evidence of seizure activity in the hippocampus and cortex. These findings thus establish a critical role of BETA2/NeuroD in the development of a specific class of neurons. Furthermore, failure to express BETA2/NeuroD leads to a stereotyped pattern of pathological excitability of the adult central nervous system.

Control of endodermal endocrine development by Hes-1

Development of endocrine cells in the endoderm involves Atonal and Achaete/Scute-related basic helix-loop-helix (bHLH) proteins. These proteins also serve as neuronal determination and differentiation factors, and are antagonized by the Notch pathway partly acting through Hairy and Enhancer-of-split (HES)-type proteins. Here we show that mice deficient in Hes1 (encoding Hes-1) display severe pancreatic hypoplasia caused by depletion of pancreatic epithelial precursors due to accelerated differentiation of post-mitotic endocrine cells expressing glucagon. Moreover, upregulation of several bHLH components is associated with precocious and excessive differentiation of multiple endocrine cell types in the developing stomach and gut, showing that Hes-1 operates as a general negative regulator of endodermal endocrine differentiation.

Gene expression changes during murine postnatal brain development

BACKGROUND

For most vertebrate organs and tissues, the majority of development occurs during embryogenesis, and postnatal changes are primarily concerned with growth. The central nervous system is unusual in that a considerable amount of morphological development, cell differentiation and acquisition of function, takes place during postnatal development. As yet, the molecular mechanisms underlying these complex developmental processes are not well understood. In order to identify markers for these developmental processes, we have analyzed the expression profiles, during postnatal murine brain development, of approximately 25,000 transcripts. This analysis, performed at day 1, day 10, day 20 and day 42 of postnatal development, identified a large number of developmentally regulated genes which we have assigned into three broad expression categories.

RESULTS

Expression levels at four timepoints during postnatal murine brain development were established for approximately 25,000 gene transcripts. Approximately 1% of the genes examined displayed a developmentally regulated pattern of expression and we provide all the necessary information required to easily obtain molecular markers for a subset of these developmentally regulated transcripts. Of this subset, 61 showed increasing expression during development, 61 showed decreasing expression during development, and 9 exhibited a peak of expression during this period.

CONCLUSIONS

A small percentage of the genes expressed in the postnatal developing brain show changes in expression level during the newborn to adult phase of development. It is likely that these developmentally regulated transcripts represent molecular markers for the complex developmental process occurring in the postnatal brain.

A mouse serine/threonine kinase homologous to C. elegans UNC51 functions in parallel fiber formation of cerebellar granule neurons

The formation of the cerebellar circuitry depends on the outgrowth of connections between the two principal classes of neurons, granule neurons and Purkinje neurons. To identify genes that function in axon outgrowth, we have isolated a mouse homolog of C. elegans UNC51, which is required for axon formation, and tested its function in cerebellar granule neurons. Murine Unc51.1 encodes a novel serine/threonine kinase and is expressed in granule cells in the cerebellar cortex. Retroviral infection of immature granule cells with a dominant negative Unc51.1 results in inhibition of neurite outgrowth in vitro and in vivo. Moreover, infected neurons fail to express TAG-1 or neuron-specific beta-tubulin, suggesting that development is arrested prior to this initial step of differentiation. Thus, Unc51.1 signals the program of gene expression leading to the formation of granule cell axons.

Expression of the basic Helix-Loop-Helix ME1 E-protein during development and aging of the murine cerebellum

Genesis of cerebellar granule cells is controlled by key transcription factors, such as the lineage-specific basic Helix-Loop-Helix (bHLH) transcription factor MATH-1, whose activity is dependent upon dimerization with bHLH E-proteins. In an effort to understand the molecular mechanisms of bHLH proteins orchestrating cerebellar development, we explored the spatio-temporal expression of the ME1 E-protein. Our results reveal that ME1 expression is first detected in the cerebellar primordium and then in the rhombic lip cells at E12.5. Its expression persists in the emerging external germinal layer during embryonic expansion. In adult cerebellum, prominent ME1 expression is detected in mature granule cells located in the internal granular layer. However, ME1 expression is not sustained in aged cerebellum. A similar declined pattern of expression is also observed in the aging hippocampus.

The role of the rhombic lip in avian cerebellum development

We have used a combination of quail-chick fate-mapping techniques and dye labelling to investigate the development of the avian cerebellum. Using Hoxa2 as a guide for the microsurgical construction of quail-chick chimaeras, we show that the caudal boundary of the presumptive cerebellum at E6 maps to the caudal boundary of rhombomere 1. By fate mapping the dorsoventral axis of rhombomere 1, we demonstrate that granule cell precursors are generated at the rhombic lip together with neurons of the lateral pontine nucleus. DiI-labelling of cerebellum explants reveals that external germinal layer precursors have a characteristic unipolar morphology and undergo an orientated, active migration away from the rhombic lip, which is apparently independent of either glial or axon guidance or ‘chain’ formation.

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.

Neurogenetics of the cerebellar system

The development of the cerebellum occurs in four basic steps. During the first epoch, genes that mark the cerebellar territory are expressed in a restricted pattern along the anterioposterior axis of the embryo. In the second, an embryonic region termed the rhombic lip generates precursors of the granule cell population of the cerebellar cortex, and the lateral pontine nucleus and olivary nucleus of the brain stem. In the third period, the program of neurogenesis of the granule neuron gives rise to the formation of the fundamental layers of the cerebellum and to the pattern of foliation. Concomitantly, programs of gene expression define the principal neuronal classes, the granule cell and Purkinje cell, that will establish the cerebellar circuitry in the postnatal period. Understanding the molecular mechanisms underlying these steps of development is likely to yield important insights into malformations such as Joubert syndrome.

The Notch-Hes pathway in mammalian neural development

A wide variety of neurons and glial cells differentiate from common precursor cells in the developing nervous system. During this process, Notch-mediated cell-cell interaction is essential for maintenance of dividing cells and subsequent generation of cell type diversity. Activation of Notch inhibits cellular differentiation, and abnormality of the Notch pathway leads to premature neuronal differentiation, the lack of some cell types, and severe defects of tissue morphogenesis. Recent data demonstrate that Notch fails to inhibit cellular differentiation in the absence of the bHLH genes Hes1 and Hes5, which functionally antagonize the neuronal bHLH genes such as Mash1. These results indicate that the two Hes genes are essential effectors for the Notch pathway and that neuronal differentiation is controlled by the pathway "Notch-Hes1/Hes5-|Mash1".

Math1: an essential gene for the generation of inner ear hair cells

The mammalian inner ear contains the cochlea and vestibular organs, which are responsible for hearing and balance, respectively. The epithelia of these sensory organs contain hair cells that function as mechanoreceptors to transduce sound and head motion. The molecular mechanisms underlying hair cell development and differentiation are poorly understood. Math1, a mouse homolog of the Drosophila proneural gene atonal, is expressed in inner ear sensory epithelia. Embryonic Math1-null mice failed to generate cochlear and vestibular hair cells. This gene is thus required for the genesis of hair cells.

Generation of cerebellar granule neurons in vivo by transplantation of BMP-treated neural progenitor cells

Cerebellar granule neurons, the most abundant class of CNS neurons, have a critical role in cerebellar function. Granule neurons are generated at the dorsal border of the mesencephalon and metencephalon, the rhombic lip. In the mouse embryo, rhombic lip cells express a number of granule neuron markers, notably the bHLH transcription factor Math1. Dorsal midline cells adjacent to the rhombic lip express Bmp6, Bmp7 and Gdf7, three genes encoding peptide growth factors of the bone morphogenetic protein (BMP) family. These BMPs induced the expression of granule neuron markers in cultured neural tissue. Moreover, BMP-treated neural cells formed mature granule neurons after transplantation into the early postnatal cerebellum, suggesting that BMPs initiate the program of granule cell specification.

Hes1 and Hes5 as notch effectors in mammalian neuronal differentiation

While the transmembrane protein Notch plays an important role in various aspects of development, and diseases including tumors and neurological disorders, the intracellular pathway of mammalian Notch remains very elusive. To understand the intracellular pathway of mammalian Notch, the role of the bHLH genes Hes1 and Hes5 (mammalian hairy and Enhancer-of-split homologues) was examined by retrovirally misexpressing the constitutively active form of Notch (caNotch) in neural precursor cells prepared from wild-type, Hes1-null, Hes5-null and Hes1-Hes5 double-null mouse embryos. We found that caNotch, which induced the endogenous Hes1 and Hes5 expression, inhibited neuronal differentiation in the wild-type, Hes1-null and Hes5-null background, but not in the Hes1-Hes5 double-null background. These results demonstrate that Hes1 and Hes5 are essential Notch effectors in regulation of mammalian neuronal differentiation.

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.

The leukemic oncogene tal-2 is expressed in the developing mouse brain

tal-1 (T-cell acute leukemia-1; also known as SCL) and tal-2 genes belong to a family of basic helix-loop-helix transcription factors and were originally isolated from the breakpoints of chromosomal translocations in human T-cell leukemia cell lines. tal-1 is expressed not only in hematopoietic cells but also in several endothelial structures and the central nervous system during development. On the other hand, the detailed function and the sites of expression of tal-2 have remained obscure. We cloned the tal-2 cDNA from a mouse embryonic cDNA library and examined its expression pattern in the mouse, comparing with that of tal-1. In situ analyses revealed that tal-2 transcripts are detected at embryonic day 12.5 in the following regions; 1) the diencephalon-the zona limitans intrathalamica and the pretectum, 2) the mesencephalon-the tectum, and the anterior and posterior tegmentum, 3) the metencephalon-the isthmus and the anterior pons. In the diencephalon and the mesencephalon, the expression sites of tal-2 gene were similar to those of tal-1, and its expression was stronger than that of tal-1. In the metencephalon, tal-2 expression was observed in the anterior pons, whereas tal-1 transcripts were detected in the entire pons, and showed stronger expression than tal-2. The tal-2 messages were barely detectable in the brain at birth. These results suggest that tal-1 and tal-2 are involved in the development of specific areas of the central nervous system.

Math5 encodes a murine basic helix-loop-helix transcription factor expressed during early stages of retinal neurogenesis

We have identified Math5, a mouse basic helix-loop-helix (bHLH) gene that is closely related to Drosophila atonal and Xenopus Xath5 and is largely restricted to the developing eye. Math5 retinal expression precedes differentiation of the first neurons and persists within progenitor cells until after birth. To position Math5 in a hierarchy of retinal development, we compared Math5 and Hes1 expression in wild-type and Pax6-deficient (Sey) embryos. Math5 expression is downregulated in Sey/+ eyes and abolished in Sey/Sey eye rudiments, whereas the bHLH gene Hes1 is upregulated in a similar dose-dependent manner. These results link Pax6 to the process of retinal neurogenesis and provide the first molecular correlate for the dosage-sensitivity of the Pax6 phenotype. During retinogenesis, Math5 is expressed significantly before NeuroD, Ngn2 or Mash1. To test whether these bHLH genes influence the fates of distinct classes of retinal neurons, we ectopically expressed Math5 and Mash1 in Xenopus retinal progenitors. Unexpectedly, lipofection of either mouse gene into the frog retina caused an increase in differentiated bipolar cells. Directed expression of Math5, but not Xath5, in Xenopus blastomeres produced an expanded retinal phenotype. We propose that Math5 acts as a proneural gene, but has properties different from its most closely related vertebrate family member, Xath5.

Neuronal patterning by BMPs: a requirement for GDF7 in the generation of a discrete class of commissural interneurons in the mouse spinal cord

Inductive factors are known to direct the regional differentiation of the vertebrate central nervous system (CNS) but their role in the specification of individual neuronal cell types is less clear. We have examined the function of GDF7, a BMP family member expressed selectively by roof plate cells, in the generation of neuronal cell types in the dorsal spinal cord. We find that GDF7 can promote the differentiation in vitro of two dorsal sensory interneuron classes, D1A and D1B neurons. In Gdf7-null mutant embryos, the generation of D1A neurons is eliminated but D1B neurons and other identified dorsal interneurons are unaffected. These findings show that GDF7 is an inductive signal from the roof plate required for the specification of neuronal identity in the dorsal spinal cord and that GDF7 and other BMP family members expressed by the roof plate have non-redundant functions in vivo. More generally, these results suggest that BMP signaling may have a prominent role in the assignment of neuronal identity within the mammalian CNS.

Forced expression of Id-1 in the adult mouse small intestinal epithelium is associated with development of adenomas

Ids are dominant-negative helix-loop-helix (HLH) proteins that play overlapping yet distinct roles in antagonizing basic HLH transcription factors. Although Ids affect myogenesis, neurogenesis, and B-cell development, little is known about their in vivo functions in epithelia. We have examined the effects of forced expression of Id-1 in the small intestinal epithelium of adult chimeric mice. 129/Sv embryonic stem cells, transfected with DNA containing Id-1 under the control of transcriptional regulatory elements that function in all intestinal epithelial cell lineages, were introduced into C57Bl/6 (B6) blastocysts heterozygous for the ROSA26 marker. The B6 ROSA26/+ intestinal epithelium of the resulting adult chimeras produces Escherichia coli beta-galactosidase, allowing identification of this internal control cell population. Chimeras produced from nontransfected embryonic stem cells served as additional controls. Immunohistochemical studies of the control chimeras indicated that the small intestinal epithelium supports a complex pattern of endogenous Id expression. Id-1 is restricted to the cytoplasm; levels do not decrease as descendants of multipotent intestinal stem cells differentiate. Id-2 and Id-3 are only detectable in nuclei; levels increase markedly as epithelial cells differentiate. Forced expression of Id-1 in the 129/Sv epithelium results in a decline in Id-2 and Id-3 to below the limits of immunodetection. A subset of chimeric-transgenic mice lacked growth factor- and defensin-producing Paneth cells in their 129/Sv epithelium and also developed intestinal adenomas. These changes were not present in normal control chimeras. Adenomas were composed of proliferating beta-Gal-positive and -negative epithelial cells, suggesting that they arose through cooperative interactions between 129/Sv(Id-1) and B6 ROSA26/+ cells. These chimeras provide a model for studying how perturbations in Id expression affect tumorigenesis.

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.

epicardin: A novel basic helix-loop-helix transcription factor gene expressed in epicardium, branchial arch myoblasts, and mesenchyme of developing lung, gut, kidney, and gonads

We report the cloning, chromosomal localization, and analysis of the expression pattern of epicardin, a member of the basic helix-loop-helix (bHLH) family of transcription factors. Within its bHLH domain, the human and murine epicardin genes were most similar to paraxis, a bHLH gene important for segmentation of embryonic paraxial mesoderm. In situ hybridization studies revealed strong epicardin expression in murine embryos at 9.5 days postcoitum (dpc) in a region of the septum transversum at the base of the heart known as the proepicardial organ. This mesenchymal structure extends villous projections from which epicardial precursor cells emerge and migrate out over the surface of the myocardium. Strong expression was seen in individual migratory cells and clusters at 9.5 dpc and in a continuous epicardial cell layer in more mature hearts. Also from 9.5 dpc, epicardin transcripts were seen in endocardial cushions of the atrioventricular canal and outflow tract, in skeletal myoblasts within branchial arches and in condensing mesenchyme of gut, kidney, urinary tract, gonads, spleen, and lung. Northern analysis showed that expression persisted in mature visceral organs and heart, but was transient in skeletal muscle. The central role played by bHLH factors in pathways for tissue determination in the embryo suggests a function for epicardin in specification of select mesodermal cell populations associated with heart, cranial skeletal muscle, gut, and urogenital system.

musculin: a murine basic helix-loop-helix transcription factor gene expressed in embryonic skeletal muscle

We describe the embryonic expression of musculin, a new murine member of the bHLH family of transcription factors. Musculin protein is closely related to human ABF-1, which is expressed in activated B cells, and to epicardin/capsulin/Pod-1, which is expressed in branchial myoblasts, visceral and urogenital mesoderm and epicardium. In situ hybridisation revealed musculin expression in embryos was largely restricted to the embryonic skeletal muscle lineage. While all skeletal muscles expressed the gene, only a subset of myocytes within each muscle were positive, indicating molecular heterogeneity within fetal muscle.

A splice variant of E2-2 basic helix-loop-helix protein represses the brain-specific fibroblast growth factor 1 promoter through the binding to an imperfect E-box

We previously demonstrated that a cis-element (-489 to -467) in the brain-specific fibroblast growth factor (FGF)-1 promoter (FGF-1.B) binds multiple nuclear factors, and this binding enhances transcriptional activity of this promoter. Here we report the isolation of three cDNA clones, VL1, VL2 and VL3, from a human brain stem cDNA expression library using four tandem repeats of the 26-base pair sequence (-492 to -467) as the probe. These cDNA clones represent the variant of bHLH protein E2-2/SEF2-1 in having 12 additional nucleotides encoding the amino acids RSRS. The glutathione S-transferase (GST) fusion proteins of VLl, VL2, and VL3 immunologically react with anti-E2-2 antibody and anti-GST-VL2 antibody. Electrophoretic mobility shift assay and methylation interference assay revealed that the GST fusion proteins specifically bind to an imperfect E-box sequence (GACCTG) present in the 26-base pair sequence. Transient expression of the full-length E2-2 without RSRS in U1240MG glioblastoma cells resulted in repression of FGF-1.B promoter activity. We further showed a significant repression of promoter activity (>40 fold) by E2-2 (lacking the amino acid sequence RSRS) when the E47 reporter construct, containing a hexameric E-box site, was used. In contrast, the E2-2 variant containing the RSRS sequence has no significant effect on either the FGF-1 promoter or E47 promoter. These results suggest that the relative abundance of the two splice variants of E2-2 in brain could be an important determinant for the expression of FGF-1.

Function of the chemokine receptor CXCR4 in haematopoiesis and in cerebellar development

Chemokines and their receptors are important in cell migration during inflammation, in the establishment of functional lymphoid microenvironments, and in organogenesis. The chemokine receptor CXCR4 is broadly expressed in cells of both the immune and the central nervous systems and can mediate migration of resting leukocytes and haematopoietic progenitors in response to its ligand, SDF-1. CXCR4 is also a major receptor for strains of human immunodeficiency virus-1 (HIV-1) that arise during progression to immunodeficiency and AIDS dementia. Here we show that mice lacking CXCR4 exhibit haematopoietic and cardiac defects identical to those of SDF-1-deficient mice, indicating that CXCR4 may be the only receptor for SDF-1. Furthermore, fetal cerebellar development in mutant animals is markedly different from that in wild-type animals, with many proliferating granule cells invading the cerebellar anlage. This is, to our knowledge, the first demonstration of the involvement of a G-protein-coupled chemokine receptor in neuronal cell migration and patterning in the central nervous system. These results may be important for designing strategies to block HIV entry into cells and for understanding mechanisms of pathogenesis in AIDS dementia.

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.

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.

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.

Developmental pattern of expression of NPDC-1 and its interaction with E2F-1 suggest a role in the control of proliferation and differentiation of neural cells

We have previously identified NPDC-1, a gene specifically expressed in neural cells and involved in the control of cell proliferation and differentiation. In the present study, we have investigated the expression of this gene during mouse development and the interactions of the NPDC-1 protein with cell cycle regulatory proteins. The data show that NPDC-1 mRNA begins to be expressed in a variety of neural structures when the precursors enter into their terminal differentiation. They also indicate that in adult brain, the expression patterns of NPDC-1 and E2F-1 mRNA largely overlap. In addition, the NPDC-1 protein is able to interact directly with the transcription factor E2F-1 that participates in the regulation of the cell cycle, cell survival, and apoptosis. The present results suggest that NPDC-1 might be involved in the terminal differentiation and survival of neural cells and might act through interactions with E2F-1.

Overexpression of neurogenin induces ectopic expression of HuC in zebrafish

Several basic helix-loop-helix (bHLH) transcription factors are known to be involved in vertebrate neurogenesis. To investigate their roles in zebrafish neurogenesis, we isolated cDNAs for homologues of neurogenin and Math(-1)/atonal. The transcription of neurogenin was first detectable in zebrafish nervous system at late gastrulation stage. The expression of zebrafish neurogenin precedes and overlaps that of HuC, one of the earliest neuronal precursor markers. Injection of neurogenin mRNA into early stage zebrafish embryos induced ectopic expression of HuC. These results suggest that neurogenin may participate in the generation of HuC-expressing cells, implying its role in neuronal determination in zebrafish.

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.

Math1 is essential for genesis of cerebellar granule neurons

The cerebellum is essential for fine motor control of movement and posture, and its dysfunction disrupts balance and impairs control of speech, limb and eye movements. The developing cerebellum consists mainly of three types of neuronal cells: granule cells in the external germinal layer, Purkinje cells, and neurons of the deep nuclei. The molecular mechanisms that underlie the specific determination and the differentiation of each of these neuronal subtypes are unknown. Math1, the mouse homologue of the Drosophila gene atonal, encodes a basic helix-loop-helix transcription factor that is specifically expressed in the precursors of the external germinal layer and their derivatives. Here we report that mice lacking Math1 fail to form granule cells and are born with a cerebellum that is devoid of an external germinal layer. To our knowledge, Math1 is the first gene to be shown to be required in vivo for the genesis of granule cells, and hence the predominant neuronal population in the cerebellum.

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.