Molecular characterization of NSCL, a gene encoding a helix-loop-helix protein expressed in the developing nervous system

A global gene regulatory program and its region-specific regulator partition neurons into commissural and ipsilateral projection types

Understanding the genetic programs that drive neuronal diversification into classes and subclasses is key to understand nervous system development. All neurons can be classified into two types: commissural and ipsilateral, based on whether their axons cross the midline or not. However, the gene regulatory program underlying this binary division is poorly understood. We identified a pair of basic helix-loop-helix transcription factors, Nhlh1 and Nhlh2, as a global transcriptional mechanism that controls the laterality of all floor plate-crossing commissural axons in mice. Mechanistically, Nhlh1/2 play an essential role in the expression of Robo3, the key guidance molecule for commissural axon projections. This genetic program appears to be evolutionarily conserved in chick. We further discovered that Isl1, primarily expressed in ipsilateral neurons within neural tubes, negatively regulates the Robo3 induction by Nhlh1/2. Our findings elucidate a gene regulatory strategy where a conserved global mechanism intersects with neuron class-specific regulators to control the partitioning of neurons based on axon laterality.

Nescient helix-loop-helix 1 (Nhlh1) is a novel activating transcription factor 5 (ATF5) target gene in olfactory and vomeronasal sensory neurons in mice

Activating transcription factor 5 (ATF5) is a transcription factor that belongs to the cAMP-response element-binding protein/ATF family and is essential for the differentiation and survival of sensory neurons in mouse olfactory organs. However, transcriptional target genes for ATF5 have yet to be identified. In the present study, chromatin immunoprecipitation-quantitative polymerase chain reaction (ChIP-qPCR) experiments were performed to verify ATF5 target genes in the main olfactory epithelium and vomeronasal organ in the postnatal pups. ChIP-qPCR was conducted using hemagglutinin (HA)-tagged ATF5 knock-in olfactory organs. The results obtained demonstrated that ATF5-HA fusion proteins bound to the CCAAT/enhancer-binding protein-ATF response element (CARE) site in the enhancer region of nescient helix-loop-helix 1 (Nhlh1), a transcription factor expressed in differentiating olfactory and vomeronasal sensory neurons. Nhlh1 mRNA expression was downregulated in ATF5-deficient (ATF5-/-) olfactory organs. The LIM/homeobox protein transcription factor Lhx2 co-localized with ATF5 in the nuclei of olfactory and vomeronasal sensory neurons and bound to the homeodomain site proximal to the CARE site in the Nhlh1 gene. The CARE region of the Nhlh1 gene was enriched by the active enhancer marker, acetyl-histone H3 (Lys27). The present study identified Nhlh1 as a novel target gene for ATF5 in murine olfactory organs. ATF5 may upregulate Nhlh1 expression in concert with Lhx2, thereby promoting the differentiation of olfactory and vomeronasal sensory neurons.

Genetic variants of NEUROD1 target genes are associated with clinical outcomes of small-cell lung cancer patients

BACKGROUND

Neurogenic differentiation factor 1 (NEUROD1) is frequently overexpressed in small-cell lung cancer (SCLC). NEUROD1 plays an important role in promoting malignant behavior and survival.

METHODS

In this study, we evaluated the association between putative functional polymorphisms in 45 NEUROD1 target genes and chemotherapy response and survival outcomes in 261 patients with SCLC. Among the 100 single nucleotide polymorphisms (SNPs) studied, two were significantly associated with both chemotherapy response and overall survival (OS) of patients with SCLC.

RESULTS

The SNP rs3806915C⟩A in semaphorin 6A (SEMA6A) gene was significantly associated with better chemotherapy response and OS (p = 0.04 and p = 0.04, respectively). The SNP rs11265375C⟩T in nescient helix-loop helix 1 (NHLH1) gene was also associated with better chemotherapy response and OS (p = 0.04 and p = 0.02, respectively). Luciferase assay showed a significantly higher promoter activity of SEMA6A with the rs3806915 A allele than C allele in H446 lung cancer cells (p = 4 × 10^-6 ). The promoter activity of NHLH1 showed a significantly higher with the rs11265375 T allele than C allele (p = 0.001).

CONCLUSION

These results suggest that SEMA6A rs3806915C>A and NHLH1 rs11265375C>T polymorphisms affect the promoter activity and expression of the genes, which may affect the survival outcome of patients with SCLC.

Chromatin Velocity reveals epigenetic dynamics by single-cell profiling of heterochromatin and euchromatin

Recent efforts have succeeded in surveying open chromatin at the single-cell level, but high-throughput, single-cell assessment of heterochromatin and its underlying genomic determinants remains challenging. We engineered a hybrid transposase including the chromodomain (CD) of the heterochromatin protein-1α (HP-1α), which is involved in heterochromatin assembly and maintenance through its binding to trimethylation of the lysine 9 on histone 3 (H3K9me3), and developed a single-cell method, single-cell genome and epigenome by transposases sequencing (scGET-seq), that, unlike single-cell assay for transposase-accessible chromatin with sequencing (scATAC-seq), comprehensively probes both open and closed chromatin and concomitantly records the underlying genomic sequences. We tested scGET-seq in cancer-derived organoids and human-derived xenograft (PDX) models and identified genetic events and plasticity-driven mechanisms contributing to cancer drug resistance. Next, building upon the differential enrichment of closed and open chromatin, we devised a method, Chromatin Velocity, that identifies the trajectories of epigenetic modifications at the single-cell level. Chromatin Velocity uncovered paths of epigenetic reorganization during stem cell reprogramming and identified key transcription factors driving these developmental processes. scGET-seq reveals the dynamics of genomic and epigenetic landscapes underlying any cellular processes.

Necdin: A purposive integrator of molecular interaction networks for mammalian neuron vitality

Necdin was originally found in 1991 as a hypothetical protein encoded by a neural differentiation‐specific gene transcript in murine embryonal carcinoma cells. Virtually all postmitotic neurons and their precursor cells express the necdin gene (Ndn) during neuronal development. Necdin mRNA is expressed only from the paternal allele through genomic imprinting, a placental mammal‐specific epigenetic mechanism. Necdin and its homologous MAGE (melanoma antigen) family, which have evolved presumedly from a subcomplex component of the SMC5/6 complex, are expressed exclusively in placental mammals. Paternal Ndn‐mutated mice totally lack necdin expression and exhibit various types of neuronal abnormalities throughout the nervous system. Ndn‐null neurons are vulnerable to detrimental stresses such as DNA damage. Necdin also suppresses both proliferation and apoptosis of neural stem/progenitor cells. Functional analyses using Ndn‐manipulated cells reveal that necdin consistently exerts antimitotic, anti‐apoptotic and prosurvival effects. Necdin interacts directly with a number of regulatory proteins including E2F1, p53, neurotrophin receptors, Sirt1 and PGC‐1α, which serve as major hubs of protein–protein interaction networks for mitosis, apoptosis, differentiation, neuroprotection and energy homeostasis. This review focuses on necdin as a pleiotropic protein that integrates molecular interaction networks to promote neuronal vitality in modern placental mammals.

Network mapping of primary CD34+ cells by Ampliseq based whole transcriptome targeted resequencing identifies unexplored differentiation regulatory relationships

With the exception of a few master transcription factors, regulators of neutrophil maturation are poorly annotated in the intermediate phenotypes between the granulocyte-macrophage progenitor (GMP) and the mature neutrophil phenotype. Additional challenges in identifying gene expression regulators in differentiation pathways relate to challenges wherein starting cell populations are heterogeneous in lineage potential and development, are spread across various states of quiescence, as well as sample quality and input limitations. These factors contribute to data variability make it difficult to draw simple regulatory inferences. In response we have applied a multi-omics approach using primary blood progenitor cells primed for homogeneous proliferation and granulocyte differentiation states which combines whole transcriptome resequencing (Ampliseq RNA) supported by droplet digital PCR (ddPCR) validation and mass spectrometry-based proteomics in a hypothesis-generation study of neutrophil differentiation pathways. Primary CD34+ cells isolated from human cord blood were first precultured in non-lineage driving medium to achieve an active, proliferating phenotype from which a neutrophil primed progenitor was isolated and cultured in neutrophil lineage supportive medium. Samples were then taken at 24-hour intervals over 9 days and analysed by Ampliseq RNA and mass spectrometry. The Ampliseq dataset depth, breadth and quality allowed for several unexplored transcriptional regulators and ncRNAs to be identified using a combinatorial approach of hierarchical clustering, enriched transcription factor binding motifs, and network mapping. Network mapping in particular increased comprehension of neutrophil differentiation regulatory relationships by implicating ARNT, NHLH1, PLAG1, and 6 non-coding RNAs associated with PU.1 regulation as cell-engineering targets with the potential to increase total neutrophil culture output. Overall, this study develops and demonstrates an effective new hypothesis generation methodology for transcriptome profiling during differentiation, thereby enabling identification of novel gene targets for editing interventions.

Characterization of the bHLH family of transcriptional regulators in the acoel S. roscoffensis and their putative role in neurogenesis

Background

The basic helix-loop-helix (bHLH) family of transcription factors is one of the largest superfamilies of regulatory transcription factors and is widely used in eukaryotic organisms. They play an essential role in a range of metabolic, physiological, and developmental processes, including the development of the nervous system (NS). These transcription factors have been studied in many metazoans, especially in vertebrates but also in early branching metazoan clades such as the cnidarians and sponges. However, currently very little is known about their expression in the most basally branching bilaterian group, the xenacoelomorphs. Recently, our laboratory has characterized the full complement of bHLH in the genome of two members of the Xenacoelomorpha, the xenoturbellid Xenoturbella bocki and the acoel Symsagittifera roscoffensis. Understanding the patterns of bHLH gene expression in members of this phylum (in space and time) provides critical new insights into the conserved roles of the bHLH and their putative specificities in this group. Our focus is on deciphering the specific roles that these genes have in the process of neurogenesis.

Results

Here, we analyze the developmental expression of the whole complement of bHLH genes identified in the acoel S. roscoffensis. Based on their expression patterns, several members of bHLH class A appear to have specific conserved roles in neurogenesis, while other class A genes (as well as members of other classes) have likely taken on more generalized functions. All gene expression patterns are described in embryos and early juveniles.

Conclusion

Our results suggest that the main roles of the bHLH genes of S. roscoffensis are evolutionarily conserved, with a specific subset dedicated to patterning the nervous system: SrAscA, SrAscB, SrHes/Hey, SrNscl, SrSrebp, SrE12/E47 and SrOlig.

Electronic supplementary material

The online version of this article (10.1186/s13227-018-0097-y) contains supplementary material, which is available to authorized users.

Transcriptional and post-translational regulation of pannexins

Pannexins are a 3-membered family of proteins that form large pore ion and metabolite channels in vertebrates. The impact of pannexins on vertebrate biology is intricately tied to where and when they are expressed, and how they are modified, once produced. The purpose of this review is therefore to outline our current understanding of transcriptional and post-translational regulation of pannexins. First, we briefly summarize their discovery and characteristics. Next, we describe several aspects of transcriptional regulation, including cell and tissue-specific expression, dynamic expression over development and disease, as well as new insights into the underlying molecular machinery involved. Following this, we delve into the role of post-translational modifications in the regulation of trafficking and channel properties, highlighting important work on glycosylation, phosphorylation, S-nitrosylation and proteolytic cleavage. Embedded throughout, we also highlight important knowledge gaps and avenues of future research. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Female rat transcriptome response to infraorbital nerve transection differs from that of males: RNA-seq

The effects of infraorbital nerve (ION) transection on gene expression in the adult female rat barrel cortex were investigated using RNA sequencing. After a 24-hour survival duration, 28 genes were differentially regulated by ION transection. Differentially expressed genes suggest microglial activity, increased retrograde ciliary transport, and a decrease in inhibition. These changes may be functionally comparable to changes in the male barrel cortex, where changes in genes related to morphology, neuronal activity, and neuronal excitability were observed. However, the patterns in changes in gene expression are vastly different between male and female rats. The results strongly caution against the practice of generalizing data from one sex to both sexes. This cautionary note has potentially profound implications for a range of research lines, including substance abuse and stress, both research domains in which subjects have been predominantly males. Future research needs to employ sex as a classification variable, as sex differences can generally be expected. Future research is also needed to confirm that changes in gene expression observed with RNA-seq correlate with changes in protein expression. J. Comp. Neurol. 525:140-150, 2017. © 2016 Wiley Periodicals, Inc.

Conservation and divergence of bHLH genes in the calcisponge Sycon ciliatum

Background

Basic Helix-Loop-Helix (bHLH) genes encode a large family of eukaryotic transcription factors, categorized into six high-order groups: pan-eukaryotic group B involved in regulation of cell cycle, metabolism, and development; holozoan-specific groups C and F involved in development and maintenance of homeostasis; and metazoan-specific groups A, D and E including well-studied genes, such as Atonal, Twist and Hairy, with diverse developmental roles including control of morphogenesis and specification of neurons. Current scenarios of bHLH evolution in animals are mainly based on the bHLH gene set found in the genome of demosponge Amphimedon queenslandica. In this species, the majority of the 21 identified bHLH genes belong to group B, and the single group A gene is orthologous to several neurogenic bilaterian subfamilies, including atonal and neurogenin.

Results

Given recently discovered differences in developmental toolkit components between siliceous and calcareous sponges, we have carried out genome-wide analysis of bHLH genes in Sycon ciliatum, an emerging calcisponge model. We identified 30 bHLH genes in this species, representing 12 individual families, including four group A families not found in Amphimedon, and two larger family groupings. Notably, the families represented in Sycon are only partially overlapping with those represented in Amphimedon. Developmental expression analysis of a subset of the identified genes revealed patterns consistent with deeply conserved roles, such as specification of sensory cells by Atona-related and stem cells by Myc genes.

Conclusions

Our results demonstrate independent gene loss events in demosponges and calcisponges, implying a complex bHLH toolkit in the last common metazoan ancestor.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0060-8) contains supplementary material, which is available to authorized users.

Novel sites of expression of the bHLH gene NSCL1 in the developing nervous system

We report on novel sites of expression of the bHLH transcription factor NSCL1 in the developing forebrain, hindbrain and spinal cord in chick and mouse. In the hindbrain in particular, NSCL1 is the first bHLH transcription factor detected so far in rhombomere boundaries and its expression is coincident with boundary formation and maintenance. Novel sites of expression of this gene include the hippocampus, septum, tectum and hypothalamic nuclei. NSCL1 is thus expressed in various neuronal populations that are either not actively proliferating or postmitotic.

Regulation of the pannexin-1 promoter in the rat epididymis

Pannexins (PANXs) are channel-forming proteins implicated in cellular communication through the secretion of biomolecules, such as ATP and glutamate. PANX1 and PANX3 are expressed in the male rat reproductive tract and their levels are regulated by androgens in the epididymis. There is currently no information on the regulation of the Panx1 promoter. The objective of the present study was to characterize the Panx1 promoter in order to understand its regulation in the epididymis. RNA ligase-mediated rapid amplification of cDNA ends identified three transcriptional start sites, at positions -443, -429, and -393. In silico analysis revealed that transcription was initiated downstream of binding sites for CREB and ETV4 transcription factors, in a CpG island context. To determine the importance of this region in gene transactivation, a 2-kb fragment of the promoter was cloned into a vector containing a luciferase reporter gene. Deletion constructs indicated that the highest transactivation levels were achieved with shorter constructs (-973 to -346 and -550 to -346). Electrophoretic mobility shift assay and supershifts indicated that both transcription factors were able to bind to the promoter region. Chromatin immunoprecipitation using rat caput epididymis cells confirmed the binding of ETV4 and CREB on the Panx1 promoter. Site mutation of either the ETV4 or CREB binding site decreased the transactivation of the reporter gene. Previous studies indicated that orchidectomy increased epididymal PANX1 levels. Likewise, we observed an increase in both ETV4 and CREB in orchidectomized rats. These results indicate that ETV4 and cAMP response elements play a role in the transcriptional regulation of Panx1 in the epididymis.

Embryonic development of the hypothalamic feeding circuitry: Transcriptional, nutritional, and hormonal influences

Background

Embryonic neurogenesis and differentiation in the hypothalamic feeding circuitry is under the control of a variety of diffused morphogens and intrinsic transcription factors, leading to the unique structural and functional characteristics of each nucleus.

Scope of review

The transcriptional regulation of the development of feeding neuroendocrine systems during the period of embryonic neurogenesis and differentiation will be reviewed here, with a special emphasis on genetic and environmental manipulations that yield an adverse metabolic phenotype.

Major conclusions

Emerging data suggest that developmental mechanisms can be perturbed not only by genetic manipulation, but also by manipulations to maternal nutrition during the gestational period, leading to long-lasting behavioral, neurobiological, and metabolic consequences. Leptin is neurotrophic in the embryonic brain, and given that it varies in proportion to maternal energy balance, may mediate these effects through an interaction with the mechanisms of hypothalamic development.

Gene expression is dynamically regulated in retinal progenitor cells prior to and during overt cellular differentiation

The retina is comprised of one glial and six neuronal populations that are generated from a multipotent pool of retinal progenitor cells (RPCs) during development. To give rise to these different cell types, RPCs undergo temporal identity transitions, displaying distinct gene expression profiles at different stages of differentiation. Little, however, is known about temporal differences in RPC identities prior to the onset of overt cellular differentiation, during the period when a retinal identity is gradually acquired. Here we examined the sequential onset of expression of regional markers (i.e., homeodomain transcription factors) and cell fate determinants (i.e., basic-helix-loop-helix transcription factors and neurogenic genes) in RPCs from the earliest appearance of a morphologically-distinct retina. By performing a comparative analysis of the expression of a panel of 27 homeodomain, basic-helix-loop-helix and Notch pathway genes between embryonic day (E) 8.75 and postnatal day (P) 9, we identified six distinct RPC molecular profiles. At E8.75, the earliest stage assayed, murine RPCs expressed five homeodomain genes and a single neurogenic gene (Pax6, Six3, Six6, Rx, Otx2, Hes1). This early gene expression profile was remarkably similar to that of 'early' RPCs in the amphibian ciliary marginal zone (CMZ), where RPCs are compartmentalised according to developmental stage, and homologs of Pax6, Six3 and Rx are expressed in the 'early' stem cell zone. As development proceeds, expression of additional homeodomain, bHLH and neurogenic genes was gradually initiated in murine RPCs, allowing distinct genetic profiles to also be defined at E9.5, E10.5, E12.5, E15.5 and P0. In addition, RPCs in the postnatal ciliary margin, where retinal stem cells are retained throughout life, displayed a unique molecular signature, expressing all of the early-onset genes as well as several late-onset markers, indicative of a 'mixed' temporal identity. Taken together, the identification of temporal differences in gene expression in mammalian RPCs during pre-neurogenic developmental stages leads to new insights into how regional identities are progressively acquired during development, while comparisons at later stages highlight the dynamic nature of gene expression in temporally distinct RPC pools.

NeuroD1 is an upstream regulator of NSCL1

Cell fate determination and differentiation during neurogenesis and myogenesis involve the sequential expression of several basic helix-loop-helix (bHLH) transcription factors. The expression of NeuroD1/2 and the expression of NSCL(Nhlh)1/2 are closely related in many developing peripheral and central neuronal cells, suggesting an epistatic relationship between these two bHLH transcription factor families during neurogenesis. To investigate this relationship, a murine neuroblastoma cell culture system and single/double knock-out (KO) mice of NeuroD1 and NeuroD2 were utilized for the gain-of-function and loss-of-function approaches, respectively. Both NeuroD1 and NeuroD2 were able to induce the transcription of NSCL1 in vitro; however, they were not able to activate NSCL2 transcription. The DNA-binding ability of NeuroD1 was essential for NSCL1 induction. To examine the epistatic relationship in vivo, we examined the expression of NSCL1 and NSCL2 in NeuroD1 and NeuroD2 KO mice and NeuroD1/2 compound KO mice by in situ hybridization, RT-PCR and Northern blotting. The expression of NSCL1 was lower in the NeuroD1 KO mice and was not further decreased in the double KO mice. However, the expression of NSCL2 did not change in either the single KO or double KO mice. These results demonstrate that NeuroD1 is an upstream regulator of the NSCL1 gene but not the NSCL2 gene in mice. In addition, NeuroD2 is not involved in this regulatory pathway in vivo.

Identification of two distinct genes at the vertebrate TRPC2 locus and their characterisation in a marsupial and a monotreme

Background

The vomeronasal organ (VNO) detects pheromones via two large families of vomeronasal receptors: vomeronasal receptor 1 (V1R) and vomeronasal receptor 2 (V2R). Both VRs have a common receptor activation cascade involving transient receptor potential channel, subfamily C, member 2 (TRPC2).

Results

We characterised the TRPC2 locus in a marsupial, the tammar wallaby (Macropus eugenii), and identified two independently regulated genes not previously recognised as distinct. 3'-located exons comprise bona fide TRPC2 whilst 5'-located exons, previously identified as part of TRPC2, comprise a distinct gene, which we term XNDR (XRCC1 N-terminal domain-related). The two genes show contrasting expression patterns in the tammar: TRPC2 is specifically expressed in adult and developing VNO, whereas XNDR is widely expressed in many tissues suggesting a non-VNO-specific role. Strong expression of TRPC2 was detected only after about day 30 post-partum, suggesting that the VNO may not be functional during early pouch life of the tammar. Similarly restricted expression of TRPC2 and widespread expression of XNDR was also detected in the platypus. Bioinformatic analysis of the genomes of a wide range of species suggests that the identity of XNDR and TRPC2 as distinct genes is conserved among vertebrates. Finally, we analysed the promoter of mammalian TRPC2 and identified a conserved binding site for NHLH1, a transcription factor previously implicated in VNO receptor neuron development.

Conclusions

Two functionally distinct vertebrate genes-XNDR and TRPC2 - occupy a genomic locus that was previously defined as a single gene in the mouse. The former is widely expressed with a putative role in DNA repair, while the latter shows VNO-specific expression under the probable regulation of NHLH1.

Oncogenic LMO3 collaborates with HEN2 to enhance neuroblastoma cell growth through transactivation of Mash1

Expression of Mash1 is dysregulated in human neuroblastoma. We have also reported that LMO3 (LIM-only protein 3) has an oncogenic potential in collaboration with neuronal transcription factor HEN2 in neuroblastoma. However, the precise molecular mechanisms of its transcriptional regulation remain elusive. Here we found that LMO3 forms a complex with HEN2 and acts as an upstream mediator for transcription of Mash1 in neuroblastoma. The high levels of LMO3 or Mash1 mRNA expression were significantly associated with poor prognosis in 100 primary neuroblastomas. The up-regulation of Mash1 remarkably accelerated the proliferation of SH-SY5Y neuroblastoma cells, while siRNA-mediated knockdown of LMO3 induced inhibition of growth of SH-SY5Y cells in association with a significant down-regulation of Mash1. Additionally, overexpression of both LMO3 and HEN2 induced expression of Mash1, suggesting that they might function as a transcriptional activator for Mash1. Luciferase reporter assay demonstrated that the co-expression of LMO3 and HEN2 attenuates HES1 (a negative regulator for Mash1)-dependent reduction of luciferase activity driven by the Mash1 promoter. Chromatin immunoprecipitation assay revealed that LMO3 and HEN2 reduce the amount of HES1 recruited onto putative HES1-binding sites and E-box within the Mash1 promoter. Furthermore, both LMO3 and HEN2 are physically associated with HES1 by immunoprecipitation assay. Thus, our present results suggest that a transcriptional complex of LMO3 and HEN2 may contribute to the genesis and malignant phenotype of neuroblastoma by inhibiting HES1 which suppresses the transactivation of Mash1.

[Progress of studies on family members and functions of animal bHLH transcription factors]

bHLH transcription factors play essential roles in the regulation of eukaryotic growth and development. Animal bHLH transcription factors comprise of 45 families. They are involved in regulating biological processes such as neurogenesis, myogenesis, gut development and response to environmental toxins. In the past two decades, extensive studies had been conducted on identification of bHLH family members and their biological functions in animals. Based on introduction of origin of the 45 animal bHLH family names, this article reviewed the progresses of studies on bHLH family members and functions of three model animals namely mouse, fruit fly and nematode. There are 114, 59 and 42 bHLH proteins in mouse, fruit fly and nematode, respectively. Among them, the functions of 108 mouse, 47 fruit fly and 20 nematode bHLH proteins have been well characterized. Among the 22 nematode bHLH proteins of unknown functions, 15 have not yet been assigned into certain families. This article also explained misused names of several bHLH family members, thus providing clear and overall background information for relevant researchers to conduct in-depth studies on structures and functions of bHLH transcription factors.

Ascl1 and Neurog2 form novel complexes and regulate Delta-like3 (Dll3) expression in the neural tube

Delta-like 3 (Dll3) is a Delta family member expressed broadly in the developing nervous system as neural progenitor cells initiate differentiation. A proximal promoter sequence for Dll3 is conserved across multiple species and is sufficient to direct GFP expression in a Dll3-like pattern in the neural tube of transgenic mice. This promoter contains multiple E-boxes, the consensus binding site for bHLH factors. Dll3 expression and the activity of the Dll3-promoter in the dorsal neural tube depends on the basic helix-loop-helix (bHLH) transcription factors Ascl1 (Mash1) and Neurog2 (Ngn2). Mutations in each E-box identified in the Dll3-promoter allowed distinct enhancer or repressor properties to be assigned to each site individually or in combination. In addition, each E-box has distinct characteristics relative to binding of bHLH factors Ascl1, Neurog1, and Neurog2. Surprisingly, novel Ascl1 containing DNA binding complexes are identified that interact with specific E-box sites within the Dll3-promoter in vitro. These complexes include Ascl1/Ascl1 homodimers and Ascl1/Neurog2 heterodimers, complexes that in some cases require additional undefined factors for efficient DNA binding. Thus, a complex interplay of E-box binding proteins spatially and temporally regulate Dll3 levels during neural tube development.

Overview of rodent models for obesity research

Animal obesity models differ widely in type and extent of obesity. They are either based on environmental factors (e.g., high-fat diet-induced obesity), spontaneous mutants (i.e., ob/ob mice), genetically engineered animals (e.g., mice with melanocortin receptor subtype-4 gene disruption (knock-out), or mechanical intervention (e.g., chemical lesion of the ventromedial hypothalamus). This unit reviews available rodent models to study obesity and attempts to highlight the greatest utility for each model.

Prox1 regulates a transitory state for interneuron neurogenesis in the spinal cord

Proper central nervous system (CNS) function depends critically on the generation of functionally distinct neuronal types in specific and reproducible positions. The generation of neuronal diversity during CNS development involves a fine balance between dividing neural progenitors and the differentiated neuronal progeny that they produce. However, the molecular mechanisms that regulate these processes are still poorly understood. Here, we show that the Prox1 transcription factor, which is expressed transiently and specifically in spinal interneurons, plays an important role in neurogenesis. Using both gain- and loss-of-function approaches, we find that Prox1 is capable of driving neuronal precursors out of the cell cycle and can initiate limited expression of neuronal proteins. Using RNAi approaches, we show that Prox1 function is required to execute a neurogenic differentiation program downstream of Mash1 and Ngn2. Our studies demonstrate an important, spinal interneuron-specific role for Prox1 in controlling steps required for both cell-cycle withdrawal and differentiation.

An integrated approach identifies Nhlh1 and Insm1 as Sonic Hedgehog-regulated genes in developing cerebellum and medulloblastoma

Medulloblastoma (MB) is the most common malignant brain tumor of childhood arising from deregulated cerebellar development. Sonic Hedgehog (Shh) pathway plays a critical role in cerebellar development and its aberrant expression has been identified in MB. Gene expression profiling of cerebella from 1- to 14-day-old mice unveiled a cluster of genes whose expression correlates with the levels of Hedgehog (HH) activity. From this cluster, we identified Insm1 and Nhlh1/NSCL1 as novel HH targets induced by Shh treatment in cultured cerebellar granule cell progenitors. Nhlh1 promoter was found to be bound and activated by Gli1 transcription factor. Remarkably, the expression of these genes is also upregulated in mouse and human HH-dependent MBs, suggesting that they may be either a part of the HH-induced tumorigenic process or a specific trait of HH-dependent tumor cells.

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

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

Pubertal impairment in Nhlh2 null mice is associated with hypothalamic and pituitary deficiencies

Pubertal development is impaired in mice lacking the basic helix-loop-helix transcription factor Nhlh2. The mechanisms underlying changes in reproduction in Nhlh2-deficient mice (Nhlh2(-/-)) are unclear. Here we show that hypothalamic GnRH-1 content is reduced in adult Nhlh2(-/-) mice as is the number of GnRH-1 neurons localized to mid- and caudal hypothalamic regions. This reduction was detected postnatally after normal migration of GnRH-1 neurons within nasal regions had occurred. Phenotype rescue experiments showed that female Nhlh2(-/-) mice were responsive to estrogen treatment. In contrast, puberty could not be primed in female Nhlh2(-/-) mice with a GnRH-1 regimen. The adenohypophysis of Nhlh2(-/-) mice was hypoplastic although it contained a full complement of the five anterior pituitary cell types. GnRH-1 receptors (GnRHRs) were reduced in Nhlh2(-/-) pituitary gonadotropes as compared with wild type. In vitro assays indicated that Nhlh2 expression is regulated in parallel with GnRHR expression. However, direct transcriptional activity of Nhlh2 on the GnRHR promoter was not found. These results indicate that Nhlh2 plays a role in the development and functional maintenance of the hypothalamic-pituitary-gonadal axis at least at two levels: 1) in the hypothalamus by regulating the number and distribution of GnRH-1 neurons and, 2) in the developing and mature adenohypophysis.

Transcriptome analysis and promoter sequence studies on early adipogenesis in 3T3-L1 cells

To identify regulatory molecules which play key roles in the development of obesity, we investigated the transcriptional profiles in 3T3-L1 cells at early stage of differentiation and analyzed the promoter sequences of differentially regulated genes. One hundred and sixty-one (161) genes were found to have significant changes in expression at the 2nd day following treatment with differentiation cocktail. Among them, 86 transcripts were up-regulated and 75 transcripts were down-regulated. The 161 transcripts were classified into 10 categories according to their functional roles; cytoskeleton, cell adhesion, immune, defense response, metabolism, protein modification, protein metabolism, regulation of transcription, signal transduction and transporter. To identify transcription factors likely involved in regulating these differentially expressed genes, we analyzed the promoter sequences of up- or -down regulated genes for the presence of transcription factor binding sites (TFBSs). Based on coincidence of regulatory sites, we have identified candidate transcription factors (TFs), which include those previously known to be involved in adipogenesis (CREB, OCT-1 and c-Myc). Among them, c-Myc was also identified by our microarray data. Our approach to take advantage of the resource of the human genome sequences and the results from our microarray experiments should be validated by further studies of promoter occupancy and TF perturbation.

Neurocytoma is a tumor of adult neuronal progenitor cells

Central neurocytoma (CN) is a rare periventricular tumor, whose derivation, lineage potential, and molecular regulation have been mostly unexplored. We noted that CN cells exhibited an antigenic profile typical of neuronal progenitor cells in vivo, yet in vitro generated neurospheres, divided in response to bFGF (basic fibroblast growth factor), activated the neuroepithelial enhancer of the nestin gene, and gave rise to both neuron-like cells and astrocytes. When CN gene expression was compared with that of both normal adult VZ (ventricular zone) and E/nestin:GFP (green fluorescent protein)-sorted native neuronal progenitors, significant overlap was noted. Marker analysis suggested that the gene expression pattern of CN was that of a proneuronal population; glial markers were conspicuously absent, suggesting that the emergence of astroglia from CN occurred only with passage. The expression pattern of CN was distinguished from that of native progenitor cells by a cohort of differentially expressed genes potentially involved in both the oncogenesis and phenotypic restriction of neurocytoma. These included both IGF2 and several components of its signaling pathway, whose sharp overexpression implicated dysregulated autocrine IGF2 signaling in CN oncogenesis. Both receptors and effectors of canonical wnt signaling, as well as GDF8 (growth differentiation factor 8), PDGF-D, and neuregulin, were differentially overexpressed by CN, suggesting that CN is characterized by the concurrent overactivation of these pathways, which may serve to drive neurocytoma expansion while restricting tumor progenitor phenotype. This strategy of comparing the gene expression of tumor cells to that of the purified native progenitors from which they derive may provide a focused approach to identifying transcripts important to stem and progenitor cell oncogenesis.

Human Bex2 interacts with LMO2 and regulates the transcriptional activity of a novel DNA-binding complex

Human Bex2 (brain expressed X-linked, hBex2) is highly expressed in the embryonic brain, but its function remains unknown. We have identified that LMO2, a LIM-domain containing transcriptional factor, specifically interacts with hBex2 but not with mouse Bex1 and Bex2. The interaction was confirmed both by pull-down with GST-hBex2 and by coimmunoprecipitation assays in vivo. Using electrophoretic mobility shift assay, we have demonstrated the physical interaction of hBex2 and LMO2 as part of a DNA-binding protein complex. We have also shown that hBex2 can enhance the transcriptional activity of LMO2 in vivo. Furthermore, using mammalian two-hybrid analysis, we have identified a neuronal bHLH protein, NSCL2, as a novel binding partner for LMO2. We then showed that LMO2 could up-regulate NSCL2-dependent transcriptional activity, and hBex2 augmented this effect. Thus, hBex2 may act as a specific regulator during embryonic development by modulating the transcriptional activity of a novel E-box sequence-binding complex that contains hBex2, LMO2, NSCL2 and LDB1.

Negative regulation of estrogen receptor alpha transactivation functions by LIM domain only 4 protein

LIM domain only 4 (LMO4), a member of the LIM-only family of transcriptional coregulatory proteins, consists of two LIM protein-protein interaction domains that enable it to function as a linker protein in multiprotein complexes. Here, we have identified estrogen receptor alpha (ERalpha) and its corepressor, metastasis tumor antigen 1 (MTA1), as two novel binding partners of LMO4. Interestingly, LMO4 exhibited binding with both ERalpha and MTA1 and existed as a complex with ERalpha, MTA1, and histone deacetylases (HDAC), implying that LMO4 was a component of the MTA1 corepressor complex. Consistent with this notion, LMO4 overexpression repressed ERalpha transactivation functions in an HDAC-dependent manner. Accordingly, silencing of endogenous LMO4 expression resulted in a significant increased recruitment of ERalpha to target gene chromatin, stimulation of ERalpha transactivation activity, and enhanced expression of ERalpha-regulated genes. These findings suggested that LMO4 was an integral part of the molecular machinery involved in the negative regulation of ERalpha transactivation function in breast cells. Because LMO4 is up-regulated in human breast cancers, repression of ERalpha transactivation functions by LMO4 might contribute to the process of breast cancer progression by allowing the development of ERalpha-negative phenotypes, leading to increased aggressiveness of breast cancer cells.

LMO3 interacts with neuronal transcription factor, HEN2, and acts as an oncogene in neuroblastoma

LIM-only proteins (LMO), which consist of LMO1, LMO2, LMO3, and LMO4, are involved in cell fate determination and differentiation during embryonic development. Accumulating evidence suggests that LMO1 and LMO2 act as oncogenic proteins in T-cell acute lymphoblastic leukemia, whereas LMO4 has recently been implicated in the genesis of breast cancer. However, little is known about the role of LMO3 in either tumorigenesis or development. In the present study, we have identified LMO3 and HEN2, which encodes a neuronal basic helix-loop-helix protein, as genes whose expression levels were higher in unfavorable neuroblastomas compared with those of favorable tumors. Immunoprecipitation and immunostaining experiments showed that LMO3 was associated with HEN2 in mammalian cell nucleus. Human neuroblastoma SH-SY5Y cells stably overexpressing LMO3 showed a marked increase in cell growth, a promotion of colony formation in soft agar medium, and a rapid tumor growth in nude mice compared with the control transfectants. More importantly, the increased expression of LMO3 and HEN2 was significantly associated with a poor prognosis in 87 primary neuroblastomas. These results suggest that the deregulated expression of neuronal-specific LMO3 and HEN2 contributes to the genesis and progression of human neuroblastoma in a lineage-specific manner.

Enhanced retinal ganglion cell differentiation by ath5 and NSCL1 coexpression

PURPOSE

The molecular mechanism underlying retinal ganglion cell (RGC) differentiation is not fully understood. In this study, the role of the basic helix-loop-helix (bHLH) genes ath5 and NSCL1 in RGC differentiation was examined, by testing whether their coexpression would promote RGC differentiation to a greater extent than either gene alone.

METHODS

The replication-competent avian RCAS retrovirus was used to coexpress ath5 and NSCL1 through an internal ribosomal entry site. The effect of the coexpression on RGC differentiation was assayed in vivo in the developing chick retina and in vitro in RPE cell cultures derived from day 6 chick embryos.

RESULTS

Coexpression of ath5 and NSCL1 in RPE cells cultured in the presence of bFGF promoted RPE transdifferentiation toward RGCs, and the degree of transdifferentiation was much higher than with either gene alone. Cells expressing RGC markers, including RA4, calretinin, and two neurofilament-associated proteins, displayed processes that were remarkably long and thin and often had numerous branches, characteristics of long-projecting RGCs. In the developing chick retina, retroviral expression of NSCL1 resulted in a moderate increase in the number of RGCs, results similar to retroviral expression of ath5. Coexpression of ath5 and NSCL1 yielded increases in RGCs greater than the sum of their increases when expressed separately.

CONCLUSIONS

Both in vitro and in vivo data indicate that the combination of ath5 and NSCL1 promotes RGC differentiation to a greater degree than either gene alone, suggesting a synergism between ath5 and NSCL1 in advancing RGC development.

bHLH genes cath5 and cNSCL1 promote bFGF-stimulated RPE cells to transdifferentiate toward retinal ganglion cells

The molecular mechanism of retinal ganglion cell (RGC) genesis and development is not well understood. Published data suggest that the process may involve two bHLH genes, ath5 and NSCL1. Gain-of-function studies show that ath5 increases RGC production in the developing retina. We examined whether two chick genes, cath5 and cNSCL1, can guide retinal pigment epithelial (RPE) cells to transdifferentiate toward RGCs. Ectopic expression of cath5 and cNSCL1 in cultured chick RPE cells was achieved through retroviral transduction. cath5 alone was unable to induce de novo expression of early RGC markers, such as RA4 antigen, neurofilament (160 kDa), and a neurofilament-associated antigen. However, cath5 induced the expression of these proteins when the RPE cells were cultured with medium supplemented with bFGF. Since bFGF alone can induce only RA4 antigen, the expression of the additional RGC markers reflects a synergism between cath5 and bFGF in promoting RPE transdifferentiation toward RGCs. Morphologically, the RA4(+) cells in bFGF + cath5 cultures appeared more neuron-like than those generated by bFGF alone. cNSCL1 also promoted bFGF-stimulated RPE cells to transdifferentiate toward RGCs that expressed RA4 antigen, N-CAM, Islet-1, neurofilament, and neurofilament-associated antigen. We found that cath5 induced cNSCL1 expression, but not vice versa. Our data suggest that cath5 or cNSCL1 alone was insufficient to induce RPE transdifferentiation into RGCs, but could further neural differentiation initiated by bFGF. We propose that intrinsic factors act synergistically with extrinsic factors during RGC genesis and development.

Math5 determines the competence state of retinal ganglion cell progenitors

In mice, all of the six retinal neuron types are generated from common multipotent retinal progenitors, and their differentiation from progenitors is regulated by both extrinsic and intrinsic factors. Previously, we showed that targeted deletion of the atonal (ato) homologue math5 blocked the differentiation of most retinal ganglion cells (RGCs), revealing an essential role for math5 in RGC differentiation. In this study, we used the Cre-loxP recombination system to trace the fate of math5-expressing cells in retina. Our results demonstrated that math5 expression was associated with the differentiation of multiple retinal neuron types, including RGCs, photoreceptor, horizontal, and amacrine cells, implying that math5 expression alone is not sufficient to determine the RGC fate. Math5 expression was restricted to postmitotic cells in developing retina, suggesting that cell fate commitment of retinal neurons occurs after the terminal mitosis. The insufficiency of and requirement for math5 in RGC differentiation indicates that, like ato in the development of Drosophila R8 photoreceptors, math5 plays a role in determining the RGC competence state of retinal progenitors and that additional positive and negative factors are required in determining RGC fate. Furthermore, we show that loss of Math5 function severely reduced the RGC expression of the transcription factors Brn-3b, Gfi-1, Isl-1, Isl-2, Nscl-1, Nscl-2, and RPF-1, suggesting that Math5 expression is required to activate a comprehensive transcription network of RGC differentiation.

Ebf gene function is required for coupling neuronal differentiation and cell cycle exit

Helix-loop-helix transcription factors of the Ebf/Olf1 family have previously been implicated in the control of neurogenesis in the central nervous system in both Xenopus laevis and the mouse, but their precise roles have remained unclear. We have characterised two family members in the chick, and have performed a functional analysis by gain- and loss-of-function experiments. This study revealed several specific roles for Ebf genes in the spinal cord and hindbrain regions of higher vertebrates, and enabled their precise positioning along the neurogenic cascade. During neurogenesis, cell cycle exit appears to be tightly coupled to migration to the mantle layer and to neuronal differentiation. We show that antagonizing Ebf gene activity allows the uncoupling of these processes. Ebf gene function is necessary to initiate neuronal differentiation and migration toward the mantle layer in neuroepithelial progenitors, but it is not required for cell cycle exit. Ebf genes therefore appear to be master controllers of neuronal differentiation and migration, coupling them to cell cycle exit and earlier steps of neurogenesis. Mutual activation between proneural and Ebf genes suggests that besides their involvement in the engagement of differentiation, Ebf genes may also participate in the stabilisation of the committed state. Finally, gain-of-function data raise the possibility that, in addition to these general roles, Ebf genes may be involved in neuronal subtype specification in particular regions of the CNS.

Unsuspected role of the brain morphogenetic gene Otx1 in hematopoiesis

Otx1 belongs to the paired class of homeobox genes and plays a pivotal role in brain development. Here, we show that Otx1 is expressed in hematopoietic pluripotent and erythroid progenitor cells. Moreover, bone marrow cells from mice lacking Otx1 exhibit a cell-autonomous impairment of the erythroid compartment. In agreement with these results, molecular analysis revealed decreased levels of erythroid genes that include the SCL and GATA-1 transcription factors. Accordingly, a gain of function of SCL rescues the erythroid deficiency in Otx1-/- mice. Taken together, our findings indicate a function for Otx1 in the regulation of blood cell production.

The LIM-only protein LMO4 modulates the transcriptional activity of HEN1

The basic helix-loop-helix protein HEN1 and the LIM-only proteins LMO2 and LMO4 are expressed in neuronal cells. HEN1 was cloned by virtue of its homology to TAL1, a bHLH protein important for early hematopoiesis. Since it has been shown that TAL1 forms complex with LMO proteins in erythroid and leukemic cells we investigated the capacity of HEN1 to form complex with LMO2 and LMO4. By mammalian two-hybrid analysis, we show that HEN1 interacts with both LMO2 and LMO4. To characterize the transcriptional capacity of HEN1 alone or together with LMO2 and LMO4, we performed reporter gene assays. In comparison with the ubiquitously expressed bHLH protein E47, HEN1 is a very modest transcriptional activator and titration experiments indicate that HEN1, like TAL1, represses E47 mediated transcriptional activation. Furthermore, LMO4 but not LMO2 was able to augment this effect. Overexpression of HEN1 in hippocampal precursor cells resulted in neurite extension, which could be prevented by LMO4. Taken together, these results indicate that LMO proteins can modulate the transcriptional activity of HEN1.

Expression pattern of the stem cell leukaemia gene in the CNS of the embryonic and adult mouse

The basic helix-loop-helix (bHLH) transcription factor stem cell leukaemia (SCL) is a 'master regulator' of haematopoiesis, where SCL is pivotal in cell fate determination and differentiation. SCL has also been detected in CNS, where other members of the bHLH-family have been shown to be indispensable for neuronal development; however, no detailed expression pattern of SCL has so far been described. We have generated a map of SCL expression in the embryonic and adult mouse brain based on histochemical analysis of LacZ reporter gene expression in sequential sections of brain tissue derived from SCL-LacZ knockin mice. The expression of LacZ was confirmed to reflect SCL expression by in situ hybridisation. LacZ expression was found in a range of different diencephalic, mesencephalic and metencephalic brain nuclei in adult CNS. Co-localisation of LacZ with the neuronal marker NeuN indicated expression in post-mitotic neurons in adulthood. LacZ expression by neurons was confirmed in tissue culture analysis. The nature of the pretectal, midbrain and hindbrain regions expressing LacZ suggest that SCL in adult CNS is potentially involved in processing of visual, auditory and pain related information. During embryogenesis, LacZ expression was similarly confined to thalamus, midbrain and hindbrain. LacZ staining was also evident in parts of the intermediate and marginal zone of the aqueduct and ventricular zone of the fourth ventricle at E12.5 and E14. These cells may represent progenitor stages of differentiating neural cells. Given the presence of SCL in both the developing brain and in post-mitotic neurons, it seems likely that the function of SCL in neuronal differentiation may differ from its function in maintaining the differentiated state of the mature neuron.

Predisposition to arrhythmia and autonomic dysfunction in Nhlh1-deficient mice

Nhlh1 is a basic helix-loop-helix transcription factor whose expression is restricted to the nervous system and which may play a role in neuronal differentiation. To directly study Nhlh1 function, we generated null mice. Homozygous mutant mice were predisposed to premature, adult-onset, unexpected death. Electrocardiograms revealed decreased total heart rate variability, stress-induced arrhythmia, and impaired baroreceptor sensitivity. This predisposition to arrhythmia is a likely cause of the observed death in the mutant mice. Heterozygosity for the closely related transcription factor Nhlh2 increased the severity of the Nhlh1-null phenotype. No signs of primary cardiac structural or conduction abnormalities could be detected upon necropsy of the null mice. The pattern of altered heart rhythm observed in basal and experimental conditions (stress and pharmacologically induced) suggests that a deficient parasympathetic tone may contribute to the arrhythmia in the Nhlh1-null mouse. The expression of Nhlh1 in the developing brain stem and in the vagal nuclei in the wild-type mouse further supports this hypothesis. The Nhlh1 mutant mouse may thus provide a model to investigate the contribution of the autonomic nervous system to arrhythmogenesis.

The neuronal basic helix-loop-helix transcription factor NSCL-1 is dispensable for normal neuronal development

The neuronal stem cell leukemia (NSCL) basic helix-loop-helix factors are neural cell-specific transcription factors. We have disrupted the NSCL-1 gene by homologous recombination and replaced the coding region with a beta-galactosidase reporter cassette to study the role of NSCL-1 in neuronal development and to follow the fate of NSCL-1 mutant cells. NSCL-1 mutant mice are viable and fertile on various genetic backgrounds and do not show any obvious signs of neurological malfunction. No differences in the distribution of NSCL-1 mutant or heterozygous neuronal cells were observed in the diencephalon, hippocampus, neocortex, and cerebellum at different stages of development. Likewise, no defects were found in the laminar organization of the cortex, and the distinct neuronal subpopulation appeared normal during development of the neocortex. Analysis of sensory neurons which strongly express NSCL-1 revealed that the spatiotemporal expression of neuronal differentiation factors, such as NeuroD and SCG-10, was not altered in developing distal and proximal cranial ganglia of mutant mice. In the cerebellum expression of NSCL-1 was confined to the proliferative and premigratory zone of the external granular layer and the internal granular layer. Interestingly, unlike cerebella of Math1(-/-) or NeuroD2(-/-) mice, NSCL-1-deficient mice have no obvious developmental defect, and neurons of the cerebellum appeared fully differentiated. Despite similar expression patterns of NSCL-1 and NSCL-2 in various areas of the diencephalon, including the arcuate nucleus and paraventricular nucleus, NSCL-1(-/-) mice are fertile and show no adult onset of obesity like NSCL-2 mutant mice. Double-mutant NSCL-1(-/-)-NSCL-2(-/-) mice do not show any additional obvious malformations of the central nervous system, although both genes are expressed in a largely overlapping pattern. Our results argue against a simple functional redundancy within the NSCL gene family.

Hoxa3 regulates integration of glossopharyngeal nerve precursor cells

In vertebrates, certain Hox genes are known to control cellular identities along the anterior-posterior (A-P) axis in the developing hindbrain. In mouse Hoxa3 mutants, truncation of the glossopharyngeal (IXth) nerve or the fusion of the IXth and vagus (Xth) nerves was reported, although its underlying mechanism is largely unknown. To elucidate the mechanism of the IXth nerve defects, we reexamined the phenotype of Hoxa3 mutant embryos. In Hoxa3 mutants, we observed an abnormal caudal stream of the migrating Hoxa3-expressing neural crest cells at the prospective IXth nerve-forming area. Dorsomedial migration of the placode-derived neuronal precursor cells of the IXth nerve was also affected. Motor neurons at rhombomere 6 (r6), where those of the IXth nerve were positioned, often projected axons to the Xth nerve. In summary, the Hoxa3 gene has crucial roles in ensuring the correct axon projection pattern of all three components of the IXth nerve, i.e., motor neurons and sensory neurons of the proximal and distal ganglia.

Atrophy of Müller glia and photoreceptor cells in chick retina misexpressing cNSCL2

PURPOSE

To investigate whether and how the basic helix-loop-helix (bHLH) gene cNSCL2 is involved in retinal development.

METHODS

cNSCL2, the chick homologue of human NSCL2, was isolated and sequenced. In situ hybridization was used to examine its spatial and temporal expression pattern in the retina. Replication-competent retrovirus RCAS was used to drive cNSCL2 misexpression in the developing chick retina, and the effect of the misexpression was analyzed.

RESULTS

Expression of cNSCL2 in the retina was restricted. Its mRNA was detected in amacrine and horizontal cells, but not in photoreceptor, bipolar, or ganglion cells. Retroviral-driven misexpression of cNSCL2 in the developing chick retina resulted in missing photoreceptor cells and gross deficits in the outer nuclear layer (ONL). These deficits were probably not because of decreased photoreceptor production, in that the ONL appeared normal in early developmental stages. TUNEL+ cells were detected in the ONL, indicating that photoreceptor cells underwent apoptosis in retinas misexpressing cNSCL2. Müller glial cells were far fewer in the experimental retina than in the control, indicating that cNSCL2 also caused Müller glia atrophy. The onset of Müller glia disappearance preceded that of photoreceptor degeneration.

CONCLUSIONS

Expression of cNSCL2 in the chick retina was restricted to amacrine and horizontal cells. Misexpression of cNSCL2 caused severe retinal degeneration, and photoreceptor cells and Müller glia were particularly affected.

Embryonic abnormalities from misexpression of cNSCL1

cNSCL1 encodes a bHLH transcription factor and is specifically expressed in the developing nervous system. We used a replication-competent retrovirus to drive misexpression of cNSCL1 in chick embryos. We found that cNSCL1 misexpression was embryonic lethal and the embryos exhibited gross abnormalities. Many skeletal bones were abnormal and some were completely absent. Expression of BMP4 was reduced. The abnormalities were due to cNSCL1 misexpression in the systemic region, since microinjection of cNSCL1 retrovirus at one hindlimb primordium severely retarded its development, while other limbs on the same animal appeared normal. Similar misexpression of cNSCL2, a closely related bHLH gene, did not produce these phenotypes. Thus, the detrimental effects on embryonic development were specific to cNSCL1. These data indicate that cNSCL1 expression must be tightly regulated during development.

The Rb pathway in neurogenesis

Cell division during embryogenesis plays a crucial role in the formation of the nervous system. During this developmental process, proliferating neural precursor cells commit to a neuronal fate and, as a consequence, undergo terminal mitosis and adopt a neuronal phenotype. A key cell cycle regulator, the tumor suppressor protein, retinoblastoma (Rb), is involved in both terminal mitosis and neuronal differentiation. Neural development is a complex process involving cell proliferation, cell fate determination and differentiation, as well as programmed cell death. In this review, we will examine each of these processes in turn, focussing on the role of the Rb family proteins to examine their many influences on these events.

Overexpression of MATH1 disrupts the coordination of neural differentiation in cerebellum development

An essential role for the bHLH transcription factor MATH1 in the formation of cerebellar granule cells was previously demonstrated in a Math1 null mouse. The function of regulated levels of MATH1 in granule cell development is investigated here using a gain-of-function paradigm. Overexpression of Math1 in its normal domain in transgenic mice leads to early postnatal lethality and perturbs cerebellar development. The cerebellum of the (B)MATH1 transgenic neonate is smaller with less foliation, particularly in the central vermal regions, when compared to wild-type cerebella. A detailed analysis of multiple molecular markers in brains overexpressing Math1 has revealed defects in the differentiation of cerebellar granule cells. NeuroD and doublecortin, markers normally distinguishing the discrete layered organization of granule cell maturation in the inner EGL, are aberrantly expressed in the outer EGL where MATH1-positive, proliferating cells reside. In contrast, TAG-1, a later marker of developing granule cells that labels parallel fibers, is severely diminished. The elevated MATH1 levels appear to drive expression of a subset of early differentiation markers but are insufficient for development of a mature TAG-1-expressing granule cell. Thus, balanced levels of MATH1 are essential for the correct coordination of differentiation events in granule cell development.

Genetic models of obesity and energy balance in the mouse

Obesity is a health problem of epidemic proportions in the industrialized world. The cloning and characterization of the genes for the five naturally occurring monogenic obesity syndromes in the mouse have led to major breakthroughs in understanding the physiology of energy balance and the contribution of genetics to obesity in the human population. However, the regulation of energy balance is an extremely complex process, and it is quickly becoming clear that hundreds of genes are involved. In this article, we review the naturally occurring monogenic and polygenic obese mouse strains, as well as the large number of transgenic and knockout mouse models currently available for the study of obesity and energy balance.

Analysis of sequence signature defining functional specificity and structural stability in helix-loop-helix proteins

Specific functional properties of many proteins directing developmental responses via transcriptional regulation are orchestrated by their characteristic helix-loop-helix (HLH) structural motif. The entire HLH motif in all these proteins assumes a common conformation irrespective of their individual biological effects. The motif controls the affinity of HLH proteins for homo- or heterodimerization, permitting mixing and matching of regulatory factors, and thereby expanding the functional repertoire. Systematic analysis of molecular contacts at the dimer interface using the models built for the functional dimers combined with the pattern of conserved/nonconserved residues within different categories of HLH proteins helped in understanding the differential role played by different residues at the dimer interface for expressing corresponding functions. The residues associated with the self and partner interactions were identified, and the signature residues contributing toward dimeric stability and functional specificity were defined. It is evident that most of the residues involved in self interactions are common among all the HLH proteins. However, while certain residues involved in partner interactions are common among all the HLH proteins, certain others are common within a category, and still others vary widely defining specificity signature at different levels.

P/CAF-mediated acetylation regulates the function of the basic helix-loop-helix transcription factor TAL1/SCL

The basic helix-loop-helix transcription factor TAL1 (or SCL) is a critical regulator of hematopoietic and vascular development and is misexpressed in the majority of patients with T-cell acute lymphoblastic leukemia. We found previously that TAL1 could interact with transcriptional co-activator and co-repressor complexes possessing histone acetyltransferase and deacetylase activities, respectively. Here, we report that TAL1 is subject to acetylation in vivo and can be acetylated by p300 and the p300/CBP-associated factor P/CAF in vitro. P/CAF-mediated acetylation, which mapped to a lysine-rich motif in the loop region, increased TAL1 binding to DNA while selectively inhibiting its interaction with the transcriptional co-repressor mSin3A. Furthermore, P/CAF protein, TAL1-P/CAF interaction and TAL1 acetylation increased significantly in murine erythroleukemia cells induced to differentiate in culture, while enforced expression of an acetylation-defective P/CAF mutant inhibited endogenous TAL1 acetylation, TAL1 DNA-binding activity, TAL1-directed transcription and terminal differentiation of these cells. These results reveal a novel mechanism by which TAL1 activity is regulated and implicate acetylation of this transcription factor in promotion of erythroid differentiation.

Sequence and expression analysis of Nhlh1: a basic helix-loop-helix gene implicated in neurogenesis

Nhlh1 is a basic helix-loop-helix (bHLH) gene that has been implicated in mouse neurogenesis. Previous studies have shown it to be expressed in regions in which there are differentiating neurons during late embryonic and fetal development, but detailed studies of the role of Nhlh1 earlier in embryonic development have not been performed. In this paper, we examine the expression of Nhlh1 transcripts at early embryonic stages (E8.5-E10.5), at the onset of neurogenesis, and compare the pattern of expression with that of Islet-1, a marker of postmitotic neurons. We show that Nhlh1 is expressed in early postmitotic neurons but is down-regulated as these cells migrate from the ventricular zone. We have also determined the genomic structure of mouse Nhlh1 and have characterised the promoter sequence, as a first step towards identifying factors that may control Nhlh1 expression. Nhlh1 has been implicated previously as a candidate for the neural tube defect mutant loop-tail (Lp); here, we present sequence and expression data indicating that Nhlh1 is unlikely to be responsible for the Lp mutation.