Regulation of neurogenesis by interactions between HEN1 and neuronal LMO proteins

Multiomics analysis identifies novel facilitators of human dopaminergic neuron differentiation

Midbrain dopaminergic neurons (mDANs) control voluntary movement, cognition, and reward behavior under physiological conditions and are implicated in human diseases such as Parkinson's disease (PD). Many transcription factors (TFs) controlling human mDAN differentiation during development have been described, but much of the regulatory landscape remains undefined. Using a tyrosine hydroxylase (TH) human iPSC reporter line, we here generate time series transcriptomic and epigenomic profiles of purified mDANs during differentiation. Integrative analysis predicts novel regulators of mDAN differentiation and super-enhancers are used to identify key TFs. We find LBX1, NHLH1 and NR2F1/2 to promote mDAN differentiation and show that overexpression of either LBX1 or NHLH1 can also improve mDAN specification. A more detailed investigation of TF targets reveals that NHLH1 promotes the induction of neuronal miR-124, LBX1 regulates cholesterol biosynthesis, and NR2F1/2 controls neuronal activity.

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.

Newly identified LMO3-BORCS5 fusion oncogene in Ewing sarcoma at relapse is a driver of tumor progression

Recently, we detected a new fusion transcript LMO3-BORCS5 in a patient with Ewing sarcoma within a cohort of relapsed pediatric cancers. LMO3-BORCS5 was as highly expressed as the characteristic fusion oncogene EWS/FLI1. However, the expression level of LMO3-BORCS5 at diagnosis was very low. Sanger sequencing depicted two LMO3-BORCS5 variants leading to loss of the functional domain LIM2 in LMO3 gene, and disruption of BORCS5. In vitro studies showed that LMO3-BORCS5 (i) increases proliferation, (ii) decreases expression of apoptosis-related genes and treatment sensitivity, and (iii) downregulates genes involved in differentiation and upregulates proliferative and extracellular matrix-related pathways. Remarkably, in vivo LMO3-BORCS5 demonstrated its high oncogenic potential by inducing tumors in mouse fibroblastic NIH-3T3 cell line. Moreover, BORCS5 probably acts, in vivo, as a tumor-suppressor gene. In conclusion, functional studies of fusion oncogenes at relapse are of great importance to define mechanisms involved in tumor progression and resistance to conventional treatments.

Transcriptional Enhancers in the FOXP2 Locus Underwent Accelerated Evolution in the Human Lineage

Unique human features such as complex language are the result of molecular evolutionary changes that modified developmental programs of our brain. The human-specific evolution of the forkhead box P2 (FOXP2) gene coding region has been linked to the emergence of speech and language in the human kind. However, little is known about how the expression of FOXP2 is regulated and if its regulatory machinery evolved in a lineage-specific manner in humans. In order to identify FOXP2 regulatory regions containing human-specific changes we used databases of human accelerated non-coding sequences or HARs. We found that the topologically associating domain (TAD) determined using developing human cerebral cortex containing the FOXP2 locus includes two clusters of 12 HARs, placing the locus occupied by FOXP2 among the top regions showing fast acceleration rates in non-coding regions in the human genome. Using in vivo enhancer assays in zebrafish, we found that at least five FOXP2-HARs behave as transcriptional enhancers throughout different developmental stages. In addition, we found that at least two FOXP2-HARs direct the expression of the reporter gene EGFP to foxP2 expressing regions and cells. Moreover, we uncovered two FOXP2-HARs showing reporter expression gain of function in the nervous system when compared with the chimpanzee ortholog sequences. Our results indicate that regulatory sequences in the FOXP2 locus underwent a human-specific evolutionary process suggesting that the transcriptional machinery controlling this gene could have also evolved differentially in the human lineage.

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.

Molecular Diversity of Midbrain Development in Mouse, Human, and Stem Cells

Understanding human embryonic ventral midbrain is of major interest for Parkinson's disease. However, the cell types, their gene expression dynamics, and their relationship to commonly used rodent models remain to be defined. We performed single-cell RNA sequencing to examine ventral midbrain development in human and mouse. We found 25 molecularly defined human cell types, including five subtypes of radial glia-like cells and four progenitors. In the mouse, two mature fetal dopaminergic neuron subtypes diversified into five adult classes during postnatal development. Cell types and gene expression were generally conserved across species, but with clear differences in cell proliferation, developmental timing, and dopaminergic neuron development. Additionally, we developed a method to quantitatively assess the fidelity of dopaminergic neurons derived from human pluripotent stem cells, at a single-cell level. Thus, our study provides insight into the molecular programs controlling human midbrain development and provides a foundation for the development of cell replacement therapies.

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.

LMO4 inhibits p53-mediated proliferative inhibition of breast cancer cells through interacting p53

AIMS

The LIM domain only proteins (LMOs) which consist of four members (LMO1-LMO4) are a family of nuclear transcription coregulators that are characterized by the exclusive presence of two tandem LIM domains and no other functional domains. They regulate gene transcription by functioning as "linker" or "scaffolding" proteins by virtue of their LIM domains and are involved in the formation of multiprotein complexes with several DNA-binding factors and transcriptional regulatory proteins. In the present study, we tried to find the physical interaction between p53 and LMO4, and the effect of LMO4 on p53-mediated proliferative inhibition of breast cancer cells.

MAIN METHODS

FCM analysis was developed to detect the apoptosis of breast cancer cells after adriamycin (ADR) treatment. RT-PCR and Western blot analysis were performed to detect the expression of LMO4 and p53-related genes and proteins. Immunoprecipitation assay was used to detect the interaction between LMO4 and p53. Colony formation assay was developed to detect the proliferation of breast cancer cells.

KEY FINDINGS

We found that p53 was induced, but LMO4 was down-regulated in response to ADR. We also found that enforced expression of p53 inhibited the expression of LMO4, suggesting that LMO4 is a direct transcriptional target of p53. Furthermore, LMO4 can interact with p53 and inhibit p53-mediated inhibition of colony formation of breast cancer MDA-MB-453 cells.

SIGNIFICANCE

The present study showed that LMO4 is a direct target of p53 and inhibits p53-mediated proliferative inhibition of breast cancer cells through interacting p53.

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.

LMO4 functions as a co-activator of neurogenin 2 in the developing cortex

The proneural protein neurogenin 2 (NGN2) is a key transcription factor in regulating both neurogenesis and neuronal radial migration in the embryonic cerebral cortex. However, the co-factors that support the action of NGN2 in the cortex remain unclear. Here, we show that the LIM-only protein LMO4 functions as a novel co-factor of NGN2 in the developing cortex. LMO4 and its binding partner nuclear LIM interactor (NLI/LDB1/CLIM2) interact with NGN2 simultaneously, forming a multi-protein transcription complex. This complex is recruited to the E-box containing enhancers of NGN2-target genes, which regulate various aspects of cortical development, and activates NGN2-mediated transcription. Correspondingly, analysis of Lmo4-null embryos shows that the loss of LMO4 leads to impairments of neuronal differentiation in the cortex. In addition, expression of LMO4 facilitates NGN2-mediated radial migration of cortical neurons in the embryonic cortex. Our results indicate that LMO4 promotes the acquisition of cortical neuronal identities by forming a complex with NGN2 and subsequently activating NGN2-dependent gene expression.

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.

Model-based comparative prediction of transcription-factor binding motifs in anabolic responses in bone

Understanding the regulatory mechanism that controls the alteration of global gene expression patterns continues to be a challenging task in computational biology. We previously developed an ant algorithm, a biologically-inspired computational technique for microarray data, and predicted putative transcription-factor binding motifs (TFBMs) through mimicking interactive behaviors of natural ants. Here we extended the algorithm into a set of web-based software, Ant Modeler, and applied it to investigate the transcriptional mechanism underlying bone formation. Mechanical loading and administration of bone morphogenic proteins (BMPs) are two known treatments to strengthen bone. We addressed a question: Is there any TFBM that stimulates both “anabolic responses of mechanical loading” and “BMP-mediated osteogenic signaling”? Although there is no significant overlap among genes in the two responses, a comparative model-based analysis suggests that the two independent osteogenic processes employ common TFBMs, such as a stress responsive element and a motif for peroxisome proliferator-activated receptor (PPAR). The post-modeling in vitro analysis using mouse osteoblast cells supported involvements of the predicted TFBMs such as PPAR, Ikaros 3, and LMO2 in response to mechanical loading. Taken together, the results would be useful to derive a set of testable hypotheses and examine the role of specific regulators in complex transcriptional control of bone formation.

Protein Stability and Transcription Factor Complex Assembly Determined by the SCL-LMO2 Interaction

Gene expression programs are established by networks of interacting transcription factors. The basic helix-loop-helix factor SCL and the LIM-only protein LMO2 are components of transcription factor complexes that are essential for hematopoiesis. Here we show that LMO2 and SCL are predominant interaction partners in hematopoietic cells and that this interaction occurs through a conserved interface residing in the loop and helix 2 of SCL. This interaction nucleates the assembly of SCL complexes on DNA and is required for target gene induction and for the stimulation of erythroid and megakaryocytic differentiation. We also demonstrate that SCL determines LMO2 protein levels in hematopoietic cells and reveal that interaction with SCL prevents LMO2 degradation by the proteasome. We propose that the SCL-LMO2 interaction couples protein stabilization with higher order protein complex assembly, thus providing a powerful means of modulating the stoichiometry and spatiotemporal activity of SCL complexes. This interaction likely provides a rate-limiting step in the transcriptional control of hematopoiesis and leukemia, and similar mechanisms may operate to control the assembly of diverse protein modules.

Transcriptional changes during neuronal death and replacement in the olfactory epithelium

The olfactory epithelium has the unusual ability to replace its neurons. We forced replacement of mouse olfactory sensory neurons by bulbectomy. Microarray, bioinformatics, and in situ hybridization techniques detected a rapid shift in favor of pro-apoptotic proteins, a progressive immune response by macrophages and dendritic cells, and identified or predicted 439 mRNAs enriched in olfactory sensory neurons, including gene silencing factors and sperm flagellar proteins. Transcripts encoding cell cycle regulators, axonogenesis proteins, and transcription factors and signaling proteins that promote proliferation and differentiation were increased at 5--7 days after bulbectomy and were expressed by basal progenitor cells or immature neurons. The transcription factors included Nhlh 1, Hes 6, Lmyc 1, c-Myc, Mxd 4, Id 1, Nmyc 1, Cited 2, c-Myb, Mybl 1, Tead 2, Dp 1, Gata 2, Lmo 1, and Sox1 1. The data reveal significant similarities with embryonic neurogenesis and make several mechanistic predictions, including the roles of the transcription factors in the olfactory sensory neuron lineage.

Regulation of dendritic development by neuronal activity

Proper development of dendrites is essential for the establishment of neuronal circuitry. The elaboration of the dendritic tree is a highly dynamic and regulated process, which involves the formation of new branches as well as the maintenance or elimination of pre-existing branches. This review describes recent advances in our understanding of the molecular mechanisms of activity-dependent dendritic development. Neuronal activity triggers calcium-mediated signaling events that affect the structural components of dendrites and adhesion molecules. These calcium-induced signaling pathways also target nuclear transcription factors thereby controlling expression of genes required for dendritic development. Thus, a coordinated response to calcium-regulated signaling pathways mediates activity-dependent dendritic development.

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.

Lhx9andLhx9α: Differential Biochemical Properties and Effects on Neuronal Differentiation

The Lhx9 LIM-homeodomain transcription factor and its truncated isoform Lhx9alpha are generated by alternative splicing of the Lhx9 gene. Here we investigated the differential functional properties of these two isoforms. Lhx9alpha, which lacks parts of the homeodomain, was unable to bind DNA in EMSA experiments, but was able to associate with CLIM cofactors in GST pull-down assays. In transfection experiments in PC12 cells, Lhx9alpha fusion constructs systematically showed a nuclear localization, as opposed to Lhx9 fusion constructs, which also localized to the cytoplasm. Moreover, Lhx9 increased NGF-induced neuronal differentiation of PC12 cells. Lhx9alpha, on the other hand, did not significantly increase neuronal differentiation but had an effect on the morphology of PC12 cells. Finally, as tested by RT-PCR experiments on transfected PC12 cells, Lhx9 was not able to induce the transcription of Lhx9alpha. Our results show significantly different functional properties for Lhx9 and Lhx9alpha, and suggest that Lhx9alpha can compete away limiting amounts of nuclear CLIM cofactors. Thus, Lhx9 and Lhx9alpha isoforms could be implicated in regulating various aspects of neuronal differentiation.

PAINT: a promoter analysis and interaction network generation tool for gene regulatory network identification

We have developed a bioinformatics tool named PAINT that automates the promoter analysis of a given set of genes for the presence of transcription factor binding sites. Based on coincidence of regulatory sites, this tool produces an interaction matrix that represents a candidate transcriptional regulatory network. This tool currently consists of (1) a database of promoter sequences of known or predicted genes in the Ensembl annotated mouse genome database, (2) various modules that can retrieve and process the promoter sequences for binding sites of known transcription factors, and (3) modules for visualization and analysis of the resulting set of candidate network connections. This information provides a substantially pruned list of genes and transcription factors that can be examined in detail in further experimental studies on gene regulation. Also, the candidate network can be incorporated into network identification methods in the form of constraints on feasible structures in order to render the algorithms tractable for large-scale systems. The tool can also produce output in various formats suitable for use in external visualization and analysis software. In this manuscript, PAINT is demonstrated in two case studies involving analysis of differentially regulated genes chosen from two microarray data sets. The first set is from a neuroblastoma N1E-115 cell differentiation experiment, and the second set is from neuroblastoma N1E-115 cells at different time intervals following exposure to neuropeptide angiotensin II. PAINT is available for use as an agent in BioSPICE simulation and analysis framework (www.biospice.org), and can also be accessed via a WWW interface at www.dbi.tju.edu/dbi/tools/paint/.

Expression of an engrailed-LMO4 fusion protein in mammary epithelial cells inhibits mammary gland development in mice

LIM domain factors and associated cofactors are important developmental regulators in pattern formation and organogenesis. In addition, overexpression of two LIM-only factors (LMOs) causes acute lymphocytic leukemia. The more recently discovered LMO factor LMO4 is highly expressed in proliferating epithelial cells, and frequently overexpressed in breast carcinoma. Here we show that while LMO4 is expressed throughout mammary gland development, it is dramatically upregulated in mammary epithelial cells during midpregnancy. The LMO coactivator Clim2/Ldb1/NLI showed a similar expression pattern, consistent with the idea that LMO4 and Clim2 act as a complex in mammary epithelial cells. In MCF-7 cells, LMO4 transcripts were upregulated by heregulin, an activator of ErbB receptors that are known to be important in mammary gland development and breast cancer. To test the hypothesis that LMO4 plays roles in mammary gland development, we created an engrailed-LMO4 fusion protein. This fusion protein maintains the ability to interact with Clim2, but acts as a dominant repressor of both basal and activated transcription when recruited to a DNA-regulatory region. When the engrailed-LMO4 fusion protein was expressed under control of the MMTV promoter in transgenic mice, both ductular development in virgin mice and alveolar development in pregnant mice were inhibited. These results suggest that LMO4 plays a role in promoting mammary gland development.

Xenopus Xlmo4 is a GATA cofactor during ventral mesoderm formation and regulates Ldb1 availability at the dorsal mesoderm and the neural plate

We have identified and functionally characterized the Xenopus Xlmo4 gene, which encodes a member of the LIM-domain-only protein family. Xlmo4 is activated at gastrula stages in the mesodermal marginal zone probably in response to BMP4 signaling. Soon after, Xlmo4 is downregulated in the dorsal region of the mesoderm. This repression seems to be mediated by organizer-expressed repressors, such as Gsc. Xlmo4 downregulation is necessary for the proper formation of this territory. Increasing Xlmo4 function in this region downregulates Spemman Organizer genes and suppresses dorsal-anterior structures. By binding to Ldb1, Xlmo4 may restrict the availability of this cofactor for transcription factors expressed at the Spemman Organizer. In the ventral mesoderm, Xlmo4 is required to establish the identity of this territory by acting as a positive cofactor of GATA factors. In the neural ectoderm, Xlmo4 expression depends on Xiro homeoprotein activity. In this region, Xlmo4 suppresses differentiation of primary neurons and interferes with gene expression at the Isthmic Organizer, most likely by displacing Ldb1 from active transcription factor complexes required for these processes. Together, our data suggest that Xlmo4 uses distinct mechanisms to participate in different processes during development.

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.

Regulation of alpha1G T-type calcium channel gene (CACNA1G) expression during neuronal differentiation

Down-regulation of T-type Ca channel current and mRNA occurs following differentiation of Y79 retinoblastoma cells. To understand how the decrease in expression is linked to cell differentiation, we examined transcriptional regulation of the Cav3.1 Ca channel gene, CACNA1G. We identified two putative promoters (A and B) in 1.3 kb of cloned genomic DNA. Reverse transcriptase-polymerase chain reaction and 5' rapid amplification of cDNA ends-polymerase chain reaction analyses demonstrated that two transcripts with different 5' untranslated regions are generated by different transcription start sites, with promoter A favoured in undifferentiated cells and promoter B favoured in differentiated cells. Functional analyses of the promoter sequence revealed that both promoters are active. Enhancer and repressor sequences were identified upstream of promoter A and B, respectively. These results suggest that the down-regulation of alpha1G mRNA in differentiated Y79 cells is mediated primarily by decreased activity of promoter A, which could occur in conjunction with repression of the activity of promoter B. The decrease in T-type Ca channel expression in Y79 cells may be an essential signal affecting phenotypic maturation and expression of other ion channel subtypes in the differentiated cells.

Dynamic spatiotemporal expression of LIM genes and cofactors in the embryonic and postnatal cerebral cortex

LIM-homeodomain (LIM-HD) genes encode a family of transcription factors known to be involved in development and patterning in several systems. Previously, we have shown that LIM-HD gene Lhx2 is required for the formation of a crucial boundary in the dorsal telencephalon (Bulchand et al. [2001] Mech Dev 100:165-175). To further explore the role of LIM-HD genes as well as the broader LIM gene family in dorsal telencephalic development, we examined the expression pattern of the members of this gene family and their cofactors in the developing mouse cerebral cortex. Transcription factor activity of the LIM-HD proteins requires the formation of a tetrameric complex consisting of two LIM-HD molecules linked by a dimer of cofactor (Clim) molecules. LIM-only (Lmo) proteins can interfere with this process by competing for the cofactors. LIM-HD protein function, thus, can be modulated by the presence of the appropriate Clim or Lmo molecules. At least 13 LIM-HD, 4 Lmo, and 2 Clim genes have been identified in the mouse. Several of these genes exhibit complex spatiotemporal patterns spanning different stages of cortical development, from embryonic to postnatal ages. Noteworthy features of the expression patterns include delineation of boundaries within the developing cortex, up- or down-regulation during formation of selected cortical layers, and a striking complementarity of expression of several members consistent with specific functions in cortical development. Significantly, in some cases, Lmo or Clim gene expression is robust where no LIM-HD gene expression is detectable. These results suggest multiple and distinct roles for LIM-HD, Lmo, and Clim genes in cortical development, and also support a LIM-HD-independent role for some Lmo and Clim members.

Cysteine-rich LIM-only proteins CRP1 and CRP2 are potent smooth muscle differentiation cofactors

Cysteine-rich LIM-only proteins, CRP1 and CRP2, expressed during cardiovascular development act as bridging molecules that associate with serum response factor and GATA proteins. SRF-CRP-GATA complexes strongly activated smooth muscle gene targets. CRP2 was found in the nucleus during early stages of coronary smooth muscle differentiation from proepicardial cells. A dominant-negative CRP2 mutant blocked proepicardial cells from differentiating into smooth muscle cells. Together with SRF and GATA proteins, CRP1 and CRP2 converted pluripotent 10T1/2 fibroblasts into smooth muscle cells, while muscle LIM protein CRP3 inhibited the conversion. Thus, LIM-only proteins of the CRP family play important roles in organizing multiprotein complexes, both in the cytoplasm, where they participate in cytoskeletal remodeling, and in the nucleus, where they strongly facilitate smooth muscle differentiation.

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

Both gain- and loss-of-function analyses indicate that proneural basic/helix-loop-helix (bHLH) proteins direct not only general aspects of neuronal differentiation but also specific aspects of neuronal identity within neural progenitors. In order to better understand the function of this family of transcription factors, we have used hormone-inducible fusion constructs to assay temporal patterns of downstream target regulation in response to proneural bHLH overexpression. In these studies, we have compared two distantly related Xenopus proneural bHLH genes, Xash1 and XNgnr1. Our findings indicate that both Xash1 and XNgnr1 induce expression of the general neuronal differentiation marker, N-tubulin, with a similar time course in animal cap progenitor populations. In contrast, these genes each induce distinct patterns of early downstream target expression. Both genes induce expression of the HLH-containing gene, Xcoe2, at early time points, but only XNgnr1 induces early expression of the bHLH genes, Xath3 and XNeuroD. Structure:function analyses indicate that the distinct pattern of XNgnr1-induced downstream target activation is linked to the XNgnr1 HLH domain, demonstrating a novel role for this domain in mediating the differential function of individual members of the proneural bHLH gene family.

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.

The basic helix-loop-helix transcription factors LIN-32 and HLH-2 function together in multiple steps of a C. elegans neuronal sublineage

bHLH transcription factors function in neuronal development in organisms as diverse as worms and vertebrates. In the C. elegans male tail, a neuronal sublineage clonally gives rise to the three cell types (two neurons and a structural cell) of each sensory ray. We show here that the bHLH genes lin-32 and hlh-2 are necessary for the specification of multiple cell fates within this sublineage, and for the proper elaboration of differentiated cell characteristics. Mutations in lin-32, a member of the atonal family, can cause failures at each of these steps, resulting in the formation of rays that lack fully-differentiated neurons, neurons that lack cognate rays, and ray cells defective in the number and morphology of their processes. Mutations in hlh-2, the gene encoding the C. elegans E/daughterless ortholog, enhance the ray defects caused by lin-32 mutations. In vitro, LIN-32 can heterodimerize with HLH-2 and bind to an E-box-containing probe. Mutations in these genes interfere with this activity in a manner consistent with the degree of ray defects observed in vivo. We propose that LIN-32 and HLH-2 function as a heterodimer to activate different sets of targets, at multiple steps in the ray sublineage. During ray development, lin-32 performs roles of proneural, neuronal precursor, and differentiation genes of other systems.

Hes6 acts in a positive feedback loop with the neurogenins to promote neuronal differentiation

During the development of the vertebrate nervous system, neurogenesis is promoted by proneural bHLH proteins such as the neurogenins, which act as potent transcriptional activators of neuronal differentiation genes. The pattern by which these proteins promote neuronal differentiation is thought to be governed by inhibitors, including a class of transcriptional repressors called the WRPW-bHLH proteins, which are similar to Drosophila proteins encoded by hairy and genes in the enhancer of split complex (E-(SPL)-C). Here, we describe the isolation and characterization of Hes6, which encodes a novel WRPW-bHLH protein expressed during neurogenesis in mouse and Xenopus embryos. We show that Hes6 expression follows that of neurogenins but precedes that of the neuronal differentiation genes. We provide several lines of evidence to show that Hes6 expression occurs in developing neurons and is induced by the proneural bHLH proteins but not by the Notch pathway. When ectopically expressed in Xenopus embryos, Hes6 promotes neurogenesis. The properties of Hes6 distinguish it from other members of the WRPW-bHLH family in vertebrates, and suggest that it acts in a positive-feedback loop with the proneural bHLH proteins to promote neuronal differentiation.