Regulation of Id1 and its association with basic helix-loop-helix proteins during nerve growth factor-induced differentiation of PC12 cells

Expression of alternative transcription factor 4 mRNAs and protein isoforms in the developing and adult rodent and human tissues

Transcription factor 4 (TCF4) belongs to the class I basic helix-loop-helix family of transcription factors (also known as E-proteins) and is vital for the development of the nervous system. Aberrations in the TCF4 gene are associated with several neurocognitive disorders such as schizophrenia, intellectual disability, post-traumatic stress disorder, depression, and Pitt-Hopkins Syndrome, a rare but severe autism spectrum disorder. Expression of the human TCF4 gene can produce at least 18 N-terminally distinct protein isoforms, which activate transcription with different activities and thus may vary in their function during development. We used long-read RNA-sequencing and western blot analysis combined with the analysis of publicly available short-read RNA-sequencing data to describe both the mRNA and protein expression of the many distinct TCF4 isoforms in rodent and human neural and nonneural tissues. We show that TCF4 mRNA and protein expression is much higher in the rodent brain compared to nonneural tissues. TCF4 protein expression is highest in the rodent cerebral cortex and hippocampus, where expression peaks around birth, and in the rodent cerebellum, where expression peaks about a week after birth. In human, highest TCF4 expression levels were seen in the developing brain, although some nonneural tissues displayed comparable expression levels to adult brain. In addition, we show for the first time that out of the many possible TCF4 isoforms, the main TCF4 isoforms expressed in the rodent and human brain and other tissues are TCF4-B, -C, -D, -A, and-I. Taken together, our isoform specific analysis of TCF4 expression in different tissues could be used for the generation of gene therapy applications for patients with TCF4-associated diseases.

Characterization of PC12 Cell Subclones with Different Sensitivities to Programmed Thermal Stimulation

Neuritogenesis is the process underling nervous system regeneration; however, optimal extracellular signals that can promote neuronal regenerative activities require further investigation. Previously, we developed a novel method for inducing neuronal differentiation in rat PC12 cells using temperature-controlled repeated thermal stimulation (TRTS) with a heating plate. Based on neurogenic sensitivity to TRTS, PC12 cells were classified as either hyper- or hyposensitive. In this study, we aimed to investigate the mechanism of hyposensitivity by establishing two PC12-derived subclones according to TRTS sensitivity during differentiation: PC12-P1F1, a hypersensitive subclone, and PC12-P1D10, a hyposensitive subclone. To characterize these subclones, cell size and neuritogenesis were evaluated in subclones treated with nerve growth factor (NGF), bone morphogenetic protein (BMP), or various TRTS. No significant differences in cell size were observed among the parental cells and subclones. BMP4- or TRTS-induced neuritogenesis was increased in PC12-P1F1 cells compared to that in the parental cells, while no neuritogenesis was observed in PC12-P1D10 cells. In contrast, NGF-induced neuritogenesis was observed in all three cell lines. Furthermore, a BMP inhibitor, LDN-193189, considerably inhibited TRTS-induced neuritogenesis. These results suggest that the BMP pathway might be required for TRTS-induced neuritogenesis, demonstrating the useful aspects of these novel subclones for TRTS research.

Role of ID Proteins in BMP4 Inhibition of Profibrotic Effects of TGF-β2 in Human TM Cells

Purpose

Increased expression of TGF-β2 in primary open-angle glaucoma (POAG) aqueous humor (AH) and trabecular meshwork (TM) causes deposition of extracellular matrix (ECM) in the TM and elevated IOP. Bone morphogenetic proteins (BMPs) regulate TGF-β2–induced ECM production. The underlying mechanism for BMP4 inhibition of TGF-β2–induced fibrosis remains undetermined. Bone morphogenic protein 4 induces inhibitor of DNA binding proteins (ID1, ID3), which suppress transcription factor activities to regulate gene expression. Our study will determine whether ID1and ID3 proteins are downstream targets of BMP4, which attenuates TGF-β2 induction of ECM proteins in TM cells.

Methods

Primary human TM cells were treated with BMP4, and ID1 and ID3 mRNA, and protein expression was determined by quantitative PCR (Q-PCR) and Western immunoblotting. Intracellular ID1 and ID3 protein localization was studied by immunocytochemistry. Transformed human TM cells (GTM3 cells) were transfected with ID1 or ID3 expression vectors to determine their potential inhibitory effects on TGF-β2–induced fibronectin and plasminogen activator inhibitor-I (PAI-1) protein expression.

Results

Basal expression of ID1-3 was detected in primary human TM cells. Bone morphogenic protein 4 significantly induced early expression of ID1 and ID3 mRNA (P < 0.05) and protein in primary TM cells, and a BMP receptor inhibitor blocked this induction. Overexpression of ID1 and ID3 significantly inhibited TGF-β2–induced expression of fibronectin and PAI-1 in TM cells (P < 0.01).

Conclusions

Bone morphogenic protein 4 induced ID1 and ID3 expression suppresses TGF-β2 profibrotic activity in human TM cells. In the future, targeting specific regulators may control the TGF-β2 profibrotic effects on the TM, leading to disease modifying IOP lowering therapies.

Tcf4 Controls Neuronal Migration of the Cerebral Cortex through Regulation of Bmp7

Background: Transcription factor 4 (TCF4) is found to be associated with schizophrenia. TCF4 mutations also cause Pitt-Hopkins Syndrome, a neurodevelopmental disorder associated with severe mental retardation. However, the function of TCF4 during brain development remains unclear.

Results: Here, we report that Tcf4 is expressed in the developing cerebral cortex. In utero suppression of Tcf4 arrested neuronal migration, leading to accumulation of ectopic neurons in the intermediate zone. Knockdown of Tcf4 impaired leading process formation. Furthermore, Bone Morphogenetic Protein 7 (Bmp7) is upregulated in Tcf4-deficient neurons. In vivo gain of function and rescue experiments demonstrated that Bmp7 is the major downstream effector of Tcf4 required for neuronal migration.

Conclusion: Thus, we have uncovered a new Tcf4/Bmp7-dependent mechanism underlying neuronal migration, and provide insights into the pathogenesis of neurodevelopmental disorders.

Complex translocation disrupting TCF4 and altering TCF4 isoform expression segregates as mild autosomal dominant intellectual disability

BACKGROUND

Mutations of TCF4, which encodes a basic helix-loop-helix transcription factor, cause Pitt-Hopkins syndrome (PTHS) via multiple genetic mechanisms. TCF4 is a complex locus expressing multiple transcripts by alternative splicing and use of multiple promoters. To address the relationship between mutation of these transcripts and phenotype, we report a three-generation family segregating mild intellectual disability with a chromosomal translocation disrupting TCF4.

RESULTS

Using whole genome sequencing, we detected a complex unbalanced karyotype disrupting TCF4 (46,XY,del(14)(q23.3q23.3)del(18)(q21.2q21.2)del(18)(q21.2q21.2)inv(18)(q21.2q21.2)t(14;18)(q23.3;q21.2)(14pter®14q23.3::18q21.2®18q21.2::18q21.1®18qter;18pter®18q21.2::14q23.3®14qter). Subsequent transcriptome sequencing, qRT-PCR and nCounter analyses revealed that cultured skin fibroblasts and peripheral blood had normal expression of genes along chromosomes 14 or 18 and no marked changes in expression of genes other than TCF4. Affected individuals had 12-33 fold higher mRNA levels of TCF4 than did unaffected controls or individuals with PTHS. Although the derivative chromosome generated a PLEKHG3-TCF4 fusion transcript, the increased levels of TCF4 mRNA arose from transcript variants originating distal to the translocation breakpoint, not from the fusion transcript.

CONCLUSIONS

Although validation in additional patients is required, our findings suggest that the dysmorphic features and severe intellectual disability characteristic of PTHS are partially rescued by overexpression of those short TCF4 transcripts encoding a nuclear localization signal, a transcription activation domain, and the basic helix-loop-helix domain.

Id1 modulates endothelial progenitor cells function through relieving the E2-2-mediated repression of FGFR1 and VEGFR2 in vitro

The migration and proliferation of EPCs are crucial for re-endothelialization in vascular repair and development. Id1 has a regulatory role in the regulation of EPCs migration and proliferation. Based on these findings, we hypothesized that Id1 plays a regulatory role in modulating the migration and proliferation of EPCs by interaction with other factors. Herein, we report that the Id1 protein and E-box protein E2-2 regulate EPCs function with completely opposite effects. Id1 plays a positive role in the regulation of EPC proliferation and migration, while endogenous E2-2 appears to be a negative regulator. Immunoprecipitation and immunofluorescence assay revealed that the Id1 protein interacts and co-localizes with the E2-2 protein in EPCs. Further, endogenous E2-2 protein was found to block EPCs function via the inhibition of FGFR1 and VEGFR2 expression. The overexpression and silencing of Id1 have no direct regulatory role on VEGFR2 and FGFR1 expression. On the other hand, Id1 relieves the E2-2-mediated repression of FGFR1 and VEGFR2 expression to modulate EPCs proliferation, migration, and tube formation in vitro. In summary, we demonstrated that Id1 and E2-2 are critical regulators of EPCs function in vitro. Id1 interacts with E2-2 and relieves the E2-2-mediated repression of FGFR1 and VEGFR2 expression to modulate EPCs functions. Id1 and E2-2 may represent novel therapeutic targets for re-endothelialization in vascular damage and repair.

TCF4 (e2-2; ITF2): a schizophrenia-associated gene with pleiotropic effects on human disease

Common SNPs in the transcription factor 4 (TCF4; ITF2, E2-2, SEF-2) gene, which encodes a basic Helix-Loop-Helix (bHLH) transcription factor, are associated with schizophrenia, conferring a small increase in risk. Other common SNPs in the gene are associated with the common eye disorder Fuch's corneal dystrophy, while rare, mostly de novo inactivating mutations cause Pitt-Hopkins syndrome. In this review, we present a systematic bioinformatics and literature review of the genomics, biological function and interactome of TCF4 in the context of schizophrenia. The TCF4 gene is present in all vertebrates, and although protein length varies, there is high conservation of primary sequence, including the DNA binding domain. Humans have a unique leucine-rich nuclear export signal. There are two main isoforms (A and B), as well as complex splicing generating many possible N-terminal amino acid sequences. TCF4 is highly expressed in the brain, where plays a role in neurodevelopment, interacting with class II bHLH transcription factors Math1, HASH1, and neuroD2. The Ca(2+) sensor protein calmodulin interacts with the DNA binding domain of TCF4, inhibiting transcriptional activation. It is also the target of microRNAs, including mir137, which is implicated in schizophrenia. The schizophrenia-associated SNPs are in linkage disequilibrium with common variants within putative DNA regulatory elements, suggesting that regulation of expression may underlie association with schizophrenia. Combined gene co-expression analyses and curated protein-protein interaction data provide a network involving TCF4 and other putative schizophrenia susceptibility genes. These findings suggest new opportunities for understanding the molecular basis of schizophrenia and other mental disorders.

Inhibition of endothelial cell activation by bHLH protein E2-2 and its impairment of angiogenesis

E2-2 belongs to the basic helix-loop-helix (bHLH) family of transcription factors. E2-2 associates with inhibitor of DNA binding (Id) 1, which is involved in angiogenesis. In this paper, we demonstrate that E2-2 interacts with Id1 and provide evidence that this interaction potentiates angiogenesis. Mutational analysis revealed that the HLH domain of E2-2 is required for the interaction with Id1 and vice versa. In addition, Id1 interfered with E2-2-mediated effects on luciferase reporter activities. Interestingly, injection of E2-2-expressing adenoviruses into Matrigel plugs implanted under the skin blocked in vivo angiogenesis. In contrast, the injection of Id1-expressing adenoviruses rescued E2-2-mediated inhibition of in vivo angiogenic reaction. Consistent with the results of the Matrigel plug assay, E2-2 could inhibit endothelial cell (EC) migration, network formation, and proliferation. On the other hand, knockdown of E2-2 in ECs increased EC migration. The blockade of EC migration by E2-2 was relieved by exogenous expression of Id1. We also demonstrated that E2-2 can perturb VEGFR2 expression via inhibition of VEGFR2 promoter activity. This study suggests that E2-2 can maintain EC quiescence and that Id1 can counter this effect.

Differential expression of Shh and BMP signaling in the potential conversion of human adipose tissue stem cells into neuron-like cells in vitro

The nervous system (NS) has a limited self-repair capability and adult neurogenesis is limited to certain regions of the brain. This generates a great interest in using stem cells to repair the NS. Previous reports have shown the differentiation of adipose tissue-derived mesenchymal stem cells (ASCs) in neuron-like cells when cultures are enriched with growth factors participating in embryonic and adult neurogenesis. Therefore, it could be thought that there exists a functional parallelism between neurogenesis and neuronal differentiation of ASCs. For this reason, the goal of this work was to study the differential gene expression of Shh and BMP genetic pathways involved in cell fate determination and proliferation. In this study we demonstrated that hASCs are endowed with active Hedgehog and BMP signaling pathways through the expression of genes of both cascades and that their expressions are downregulated after neuronal induction. This idea is in accordance with the facts that Shh and BMP signaling is involved in the maintenance of cells with stem cells properties and that proliferation decreases during the process of differentiation. Furthermore, Noggin expression was detected in induced hASCs whereas there was no expression in noninduced cells, which indicates that these cells are probably adopting a neuronal fate because noggin diverts neural stem cells from glial to neuronal fate. We also detected FM1-43 and synaptophisin staining, which is evidence of the presence of putative functional presynaptic terminals, a neuron-specific property. These results could partially contribute to the elucidation of the molecular mechanisms involved in neuronal differentiation of adult human nonneural tissues.

Transforming growth factor-beta induces nerve growth factor expression in pancreatic stellate cells by activation of the ALK-5 pathway

Nerve growth factor (NGF), a survival factor for neurons enforces pain by sensitizing nociceptors. Also in the pancreas, NGF was associated with pain and it can stimulate the proliferation of pancreatic cancer cells. Hepatic stellate cells (HSC) respond to NGF with apoptosis. Transforming growth factor (TGF)-beta, one of the strongest pro-fibrogenic activators of pancreatic stellate cells (PSC) induced NGF and its two receptors in an immortalized human cell line (ihPSC) and primary rat PSC (prPSC) as determined by RT-PCR, western blot, and immunofluorescence. In contrast to HSC, PSC expressed both NGF receptors, although p75(NTR) expression was weak in prPSC. In contrast to ihPSC TGF-beta activated both Smad signaling cascades in prPSC. NGF secretion was diminished by the activin-like kinase (ALK)-5 inhibitor SB431542, indicating the predominant role of ALK5 in activating the NGF system in PSC. While NGF did not affect proliferation or survival of PSC it induced expression of Inhibitor of Differentiation-1. We conclude that under conditions of upregulated TGF-beta, like fibrosis, NGF levels will also increase in PSC which might contribute to pancreatic wound healing responses.

Fibroblast growth factor 2 reactivates G1 checkpoint in SK-N-MC cells via regulation of p21, inhibitor of differentiation genes (Id1-3), and epithelium-mesenchyme transition-like events

We have recently demonstrated that fibroblast growth factor (FGF)-2 promotes neuroblastoma cell differentiation and overrides their mitogenic response to IGF-I. However, the mechanisms involved are unknown. SK-N-MC cells were cultured with FGF-2 (50 ng/ml) and/or IGF-I (100 ng/ml) up to 48 h. Fluorescence-activated cell sorting analysis indicated that FGF-2 promotes G1/G0 cell cycle phase arrest. Gene expression by RT2-PCR and cellular localization showed up-regulation of p21. We then investigated whether FGF-2-induced differentiation of SK-N-MC cells (by GAP43 and NeuroD-6 expression) involves epithelium-mesenchyme transition interconversion. Real-time PCR (RT2-PCR) showed modulation of genes involved in maintenance of the epithelial phenotype and cell-matrix interactions (E-cadherin, Snail-1, MMPs). Zymography confirmed FGF-2 up-regulated MMP2 and induced MMP9, known to contribute to neuronal differentiation and neurite extension. Id1-3 expression was determined by RT2-PCR. FGF-2 induced Id2, while down-regulating Id1 and Id3. FGF-2 induced nuclear accumulation of ID2 protein, while ID1 and ID3 remained cytoplasmic. RNA interference demonstrated that Id3 regulates differentiation and cell cycle (increased Neuro-D6 and p21 mRNA), while d Id2 modulates epithelium-mesenchyme transition-like events (increased E-cadherin mRNA). In conclusion, we have shown for the first time that FGF-2 induces differentiation of neuroblastoma cells via activation of a complex gene expression program enabling modulation of cell cycle, transcription factors, and suppression of the cancer phenotype. The use of RNA interference indicated that Id-3 is a key regulator of these events, thus pointing to a novel therapeutic target for this devastating childhood cancer.

Human calcium/calmodulin-dependent serine protein kinase regulates the expression of p21 via the E2A transcription factor

CASK (calcium/calmodulin-dependent serine protein kinase) is a kind of scaffolding protein that recruits or organizes other proteins at the plasma membrane to co-ordinate signal transduction pathways within the cytoplasm and nucleus. We have previously found that hCASK (human CASK) binds Id1 (inhibitor of DNA binding 1) through hCASK's GUK (guanylate kinase) domain and inhibits cell growth, probably via interactions with Id1. Overexpression of hCASK resulted in a reduced rate of cell growth, although inhibition of CASK via RNAi (RNA interference) promoted cell proliferation in ECV304 cells. This study revealed that hCASK regulates the protein and mRNA level of p21(wafi/cip1) (referred to throughout as p21), and activated the expression of p21 in a time-dependent manner. Two E-boxes in the proximal region at the TSS (transcription start site) play key roles in regulating hCASK-mediated p21 expression. We suggest that E2A (E12 and E47), a representative of the E proteins that binds the E-box elements, is a participant in the mediation of p21 expression by hCASK. The results of the present study suggest that hCASK regulation of cell growth might involve p21 expression, and that the bHLH (basic helix-loop-helix) transcription factor E2A probably participates in hCASK regulation of p21 expression. From these findings, we propose a novel proliferation signalling pathway mediated by hCASK.

Smad3 mediates immediate early induction of Id1 by TGF-beta

Id1 is a member of the inhibitor of differentiation (Id) protein family that regulates a wide range of cell functions. Previous studies have shown that expression of the Id1 gene is down-regulated by TGF-beta in epithelial cells, whereas it is up-regulated by BMP in a variety of cell types. During our study of the biological function of TGF-beta1, we found that Id1 can be strongly up-regulated by TGF-beta1 in the human mammary gland epithelial cell line MCF10A. Quantitative real-time RT-PCR has revealed as high as 7.5-fold induction of Id1 mRNA by TGF-beta1 in MCF10A cells after 1 h of TGF-beta1 stimulation, and this induction does not require de novo protein synthesis. Using Smad knockdown and knockout approaches, we have identified Smad3 as the responsible R-Smad for mediating transcriptional activation of the Id1 gene. Chromatin immunoprecipitation assay confirms that Smad3 and Smad4 bind to the upstream region of the Id1 gene. Our results demonstrate that Smad3, but not Smad2, mediates TGF-beta1-dependent early transcriptional induction of Id1.

Switching from the unfolded to the folded state of the helix-loop-helix domain of the Id proteins based on the O-acyl isopeptide method

The inhibitors of DNA binding and cell differentiation Id1-4 are helix-loop-helix (HLH) proteins that negatively regulate DNA transcription by forming inactive dimers with ubiquitous and tissue-specific bHLH proteins, including E47 and MyoD, respectively. Their highly conserved HLH domains are essential for heterodimerization, but can also self-associate to highly stable, alpha-helix-rich structures at low micromolar peptide concentrations. Here, we show that the introduction of an O-acyl isodipeptide unit involving the putative N-cap serine residue of the C-terminal helix completely abrogates the propensity of the Id HLH analogue for any secondary and tertiary structure, resulting in a random coil, as shown by CD measurements in nonbuffered aqueous solutions. However, the HLH fold reappears as soon as an O-->N intramolecular acyl migration, which occurs spontaneously under physiological conditions, restores the native N-cap serine residue. These results show that changes addressing the N-terminus of the C-terminal helix can dramatically influence the HLH structure, and suggest that local interactions at the junction between the loop and the C-terminal helix might be crucial during the HLH folding process. Furthermore, the present study contributes to the evaluation of the O-acyl isodipeptide unit as a powerful tool to introduce a conformational switch into peptides.

BHLHB2 controls Bdnf promoter 4 activity and neuronal excitability

Brain-derived neurotrophic factor (BDNF), via activation of TrkB receptors, mediates vital physiological functions in the brain, ranging from neuronal survival to synaptic plasticity, and has been implicated in the pathophysiology of neurodegenerative disorders. Although transcriptional regulation of the BDNF gene (Bdnf) has been extensively studied, much remains to be understood. We discovered a sequence within Bdnf promoter 4 that binds the basic helix-loop-helix protein BHLHB2 and is a target for BHLHB2-mediated transcriptional repression. NMDA receptor activation de-repressed promoter 4-mediated transcription and correlated with reduced occupancy of the promoter by BHLHB2 in cultured hippocampal neurons. Bhlhb2 gene -/- mice showed increased hippocampal exon 4-specific Bdnf mRNA levels compared with +/+ littermates under basal and activity-dependent conditions. Bhlhb2 knock-out mice also showed increased status epilepticus susceptibility, suggesting that BHLHB2 alters neuronal excitability. Together, these results support a role for BHLHB2 as a new modulator of Bdnf transcription and neuronal excitability.

Identification of Protein-Protein Interactions with Glutathione-S-Transferase (GST) Fusion Proteins

INTRODUCTIONGlutathione-S-transferase (GST) fusion proteins have had a wide range of applications since their introduction as tools for synthesis of recombinant proteins in bacteria. GST was originally selected as a fusion moiety because of several desirable properties. First and foremost, when expressed in bacteria alone, or as a fusion, GST is not sequestered in inclusion bodies (in contrast to previous fusion protein systems). Second, GST can be affinity-purified without denaturation because it binds to immobilized glutathione, which provides the basis for simple purification. Consequently, GST fusion proteins are routinely used for antibody generation and purification, protein-protein interaction studies, and biochemical analysis. This article describes the use of GST fusion proteins as probes for the identification of protein-protein interactions.

Role of the basic helix-loop-helix protein ITF2 in the hormonal regulation of Sertoli cell differentiation

Sertoli cells are a post-mitotic terminally differentiated cell population that forms the seminiferous tubules in the adult testis and provides the microenvironment and structural support for developing germ cells. During pubertal development, Sertoli cells are responsive to follicle-stimulating hormone (FSH) to promote the expression of differentiated gene products. The basic helix-loop-helix (bHLH) and inhibitors of differentiation (Id) transcription factors are involved in the differentiation of a variety of cell lineages during development. Both bHLH and Id transcription factors have been identified in Sertoli cells. A yeast two-hybrid screen was conducted using a rat Sertoli cell cDNA library to identify bHLH dimerization partners for the Id1 transcription factor. The ubiquitous bHLH protein ITF2 (i.e., E2-2) was identified as one of the interacting partners. The current study investigates the expression and function of ITF2 in Sertoli cells. ITF2 was found to be ubiquitously expressed in all testicular cell types including germ cells, peritubular myoid cells, and Sertoli cells. Stimulation of cultured Sertoli cells with FSH or dibutryl cAMP resulted in a transient decrease in expression of ITF2 mRNA levels followed by a rise in expression with FSH treatment. ITF2 expression was at its highest in mid-pubertal 20-day-old rat Sertoli cells. ITF2 was found to directly bind to negative acting Id HLH proteins and positive acting bHLH proteins such as scleraxis. Transient overexpression of ITF2 protein in cultured Sertoli cells stimulated transferrin promoter activity, which is a marker of Sertoli cell differentiation. Co-transfections of ITF2 and Id proteins sequestered the inhibitory effects of the Id family of proteins. Observations suggest ITF2 can enhance FSH actions through suppressing the inhibitory actions of the Id family of proteins and increasing the actions of stimulatory bHLH proteins (i.e., scleraxis) in Sertoli cells.

Function of ID1 protein in human cord blood-derived neural stem-like cells

The effect of dominant-negative regulator of basic helix-loop-helix (bHLH) transcription factors, an ID1 protein, on growth and differentiation of neural stem-like cell line derived from human umbilical cord blood (HUCB-NSC) was investigated. This nontransformed, mesodermal germ layer-originated line contains high levels of ID1 protein, whose intercellular distribution reflects HUCB-NSC differentiation status. Whereas cells remained undifferentiated and self-renewing in serum-free (SF) cultures, ID1 protein, although highly expressed, did not attain cell nuclei and was localized mainly in cytoplasm. In long-term-expanded cultures of partially committed (primed) HUCB-NSC grown in a low serum concentration (LS cultures) ID1 protein became translocated toward cell nuclei. Further neuronal differentiation of the cells, either spontaneous in the presence of serum or induced by neuromorphogens (dBcAMP, RA), resulted in almost complete depletion of ID1 mRNA and protein. Accordingly, HUCB-NSC transfectants overexpressing the ID1 gene were significantly inhibited in their differentiation. Notably, only neuronal and not glial development was affected after ID1 overexpression. A similar gain-of-function effect of ID1 transfection was observed in human NSC-like line (DEV) of medullobastoma origin, which is constitutively devoid of ID1 expression. Thus, our results on HUCB-NSC confirm further its neural-specific behavior and the crucial role of ID1 protein as a potent negative regulator of neural stem cell differentiation, pointing out that this protein distribution between cytoplasmic and nuclear cell compartments can be one of the most important steps in differentiation signal transduction.

The helix-loop-helix inhibitor of differentiation (ID) proteins induce post-mitotic terminally differentiated Sertoli cells to re-enter the cell cycle and proliferate

Prior to puberty the Sertoli cells undergo active cell proliferation, and at the onset of puberty they become a terminally differentiated postmitotic cell population that support spermatogenesis. The molecular mechanisms involved in the postmitotic block of pubertal and adult Sertoli cells are unknown. The four known helix-loop-helix ID proteins (i.e., Id1, Id2, Id3, and Id4) are considered dominant negative regulators of cellular differentiation pathways and act as positive regulators of cellular proliferation. ID proteins are expressed at low levels by postpubertal Sertoli cells and are transiently induced by serum. The hypothesis tested was that ID proteins can induce a terminally differentiated postmitotic Sertoli cell to reenter the cell cycle if they are constitutively expressed. To test this hypothesis, ID1 and ID2 were stably integrated and individually overexpressed in postmitotic rat Sertoli cells. Overexpression of ID1 or ID2 allowed postmitotic Sertoli cells to reenter the cell cycle and undergo mitosis. The cells continued to proliferate even after 300 cell doublings. The functional markers of Sertoli cell differentiation such as transferrin, inhibin alpha, Sert1, and androgen binding protein (ABP) continued to be expressed by the proliferating Sertoli cells, but at lower levels. FSH receptor expression was lost in the proliferating Sertoli cell-Id lines. Some Sertoli cell genes, such as cyclic protein 2 (cathepsin L) and Sry-related HMG box protein-11 (Sox11) increase in expression. At no stage of proliferation did the cells exhibit senescence. The expression profile as determined with a microarray protocol of the Sertoli cell-Id lines suggested an overall increase in cell cycle genes and a decrease in growth inhibitory genes. These results demonstrate that overexpression of ID1 and ID2 genes in a postmitotic, terminally differentiated cell type have the capacity to induce reentry into the cell cycle. The observations are discussed in regards to potential future applications in model systems of terminally differentiated cell types such as neurons or myocytes.

HIV-Tat promotes cellular proliferation and inhibits NGF-induced differentiation through mechanisms involving Id1 regulation

Id1 is a helix-loop-helix transcriptional factor that controls growth and survival of neuronal cells. Downregulation of Id1 expression is required to initiate differentiation and cell-cycle withdrawal in primary neuronal culture as well as in PC12 cells. The HIV-1 transactivating factor, Tat, has been suspected of causing neuronal dysfunction that often leads to the development of HIV-associated dementia in AIDS patients. We found that the expression of Tat in PC12 cells promotes serum-independent growth, formation of large colonies in soft agar, and the acceleration of tumor growth in nude mice. In addition, Tat showed the ability to inhibit the nerve growth factor (NGF)-induced neuronal differentiation of PC12 cells. Our results show that the Tat-mediated signaling events, which lead to serum-independent growth and the inhibition of NGF-induced differentiation, have a common cellular target: the upregulation of Id1 expression. In the absence of NGF, expression of Id1 is required to promote serum-independent proliferation of PC12/Tat cells, as the inhibition of Id1 by antisense DNA restored the serum-dependent growth of PC12/Tat cells. In the presence of NGF, Tat utilizes an additional pathway that involves phosphorylation of Stat5a, to upregulate Id1 expression and block neuronal cell differentiation. Suppression of Stat5a by use of its dominant-negative mutant reversed the transient expression of Id1 and the blockage of NGF-mediated differentiation in PC12/Tat cells. Finally, the treatment of PC12 cells with recombinant Tat also enhanced the NGF-induced Id1 expression, further pointing to Id1 as a target for Tat. Taken together, these studies suggest additional targets for Tat action in neuronal cells and provide new insights into the mechanisms involved in the dysregulation of neuronal functions.

Induction of ID2 expression by hypoxia-inducible factor-1: a role in dedifferentiation of hypoxic neuroblastoma cells

ID (inhibitor of differentiation/DNA binding) proteins, frequently deregulated in advanced human malignancies, can participate in multiple fundamental traits of cancer, such as block of differentiation, increased proliferation, tissue invasiveness, and angiogenesis. We have previously demonstrated that hypoxia decreases expression of neuronal marker genes in neuroblastoma, but induces genes expressed in the neural crest, such as ID2. Because of its involvement in normal neural crest development and its ability to inhibit proneuronal bHLH proteins, the hypoxic induction of ID2 was of particular interest. Here we report fast induction kinetics of ID2 expression in hypoxic neuroblastoma cells. The up-regulation of ID2 was abolished by addition of actinomycin D, implicating a hypoxia-driven transcriptional mechanism. Analyzing the ID2 promoter revealed several potential binding sites for hypoxia-inducible factors. Subsequent electrophoretic mobility shift and chromatin immunoprecipitation assays demonstrated two functional HIF-1 binding sites within ID2 gene regulatory sequences located at -725 and -1893 relative to the transcriptional initiation point. In transfection assays, DNA constructs of the ID2 promoter, including the functional HIF-1 binding sites, induced luciferase reporter activity in a HIF-1-specific manner. These observations demonstrate that ID2 is actively engaged by hypoxia and represents a novel HIF-1 target. Hypoxia-induced ID2 expression could play a significant role in the previously observed dedifferentiation of hypoxic neuroblastoma cells, which in a clinical setting could lead to less mature and more aggressive tumors.

Regulation of cellular senescence and p16(INK4a) expression by Id1 and E47 proteins in human diploid fibroblast

Id1, a member of Id family of helix-loop-helix transcriptional regulatory proteins, is implicated in cellular senescence by repressing p16(INK4a) expression, but the mechanisms and cellular effects in human diploid fibroblasts remain unknown. Here we analyzed the patterns of p16(INK4a) and Id1 expression during the lifespan of 2BS cells and presented the inverse correlation between these two proteins. Immunoprecipitation assays demonstrated the presence of endogenous interaction of Id1 and E47 proteins that was strong in young 2BS cells and weakened during replicative senescence and, thereby, influenced the transcription activation of p16(INK4a) by E47. Furthermore, we found that E47 protein could bind to the E-box-containing region in p16(INK4a) promoter in senescent cells by chromatin immunoprecipitation analyses, suggesting that E47 is indeed ultimately involved in the regulation of p16(INK4a) transcription in vivo. Silencing Id1 expression in young cells by RNA interference induced an increased p16(INK4a) level and premature cellular senescence, whereas silencing E47 expression inhibited the expression of p16(INK4a) and delayed the onset of senescent phenotype. The present study demonstrated not only the capacity of Id1 to regulate p16(INK4a) gene expression by E47, but also the phenotypic consequence of the regulation on cellular senescence, moreover, raised the possibility of Id1-specific gene silencing for human cancer therapy.

Bone morphogenetic protein signalling in NGF-stimulated PC12 cells

Bone morphogenetic proteins (BMPs) are shown to potentiate NGF-induced neuronal differentiation in PC12 phaeochromocytoma cells grown on collagen under low-serum conditions. Whereas, cell bodies remained rounded in control medium or with only BMPs present, addition of BMP4 or BMP6 robustly increased the neuritogenic effect of NGF within 2 days. NGF-increased phosphorylation of p44(Erk1) and p42(Erk2) between 2 and 24h was unaffected by addition of BMP6. PC12 cells transfected with the SBE(4x)-luc reporter showed that BMP4 significantly increased receptor-activated Smad activity. Expression of constitutively active BMP receptor ALK2 activating Smad1 and Smad5 resulted in a strong increase in the SBE(4x)-luc reporter response. Adding the inhibitory Smad7 drastically reduced this signal. In contrast to wild-type (wt) Smad5, a Smad5 variant lacking five Erk phosphorylation sites in the linker region (designated Smad5/5SA) showed a strong background transcriptional activity. A fusion construct (Gal4-Smad5/5SA) was also highly transcriptionally active. Addition of the MEK inhibitor U0126 to PC12 cells expressing Gal4-Smad5/wt did not increase background transcriptional activity. However, upon activation by constitutively active ALK2 both Gal4-Smad5/wt and Gal4-Smad5/5SA strongly stimulated transcription. The data show that serine residues of the linker region of Smad5 reduce spontaneous transcriptional activity and that NGF-activated Erk does not antagonise BMP signalling at this site. Hence, NGF and BMP signals are likely to interact further downstream at the transcriptional level in neuronal differentiation of the PC12 cells.

Inhibitors of DNA binding in neural cell proliferation and differentiation

Id proteins function as negative regulators of bHLH transcription factors by disrupting the homo- and/or hetero-dimerization of bHLH-bHLH transcription factors. Recent data from in vitro and in vivo studies have revealed the complex biological functions of Id proteins in the regulation of cell differentiation, the cell cycle, and cell survival. Several advances in the understanding of Id-regulated neurogenesis have been made. Basically, Id proteins are positive regulators of neural cell proliferation, are required for neural cell cycle progression, and also play a role in the timing of oligodendroglial differentiation. Here we summarize recent findings regarding the regulation of Id proteins in neural cells and discuss the possible mechanisms of Id-regulated neurogenesis.

Role of Id family proteins in growth control

Id proteins (inhibitors of DNA binding/differentiation) are negative regulators of basic helix-loop-helix (bHLH) type transcription factors, which promote the differentiation of various cell types. In addition to their "classical" ability to inhibit cell differentiation, they are able to stimulate cell cycle progression. These facts suggest that Id proteins play a role in keeping precursor cells immature and in expanding the cell population size during development. In vitro as well as in vivo analyses in the last several years have shown that Id proteins have more complex activities; they induce apoptosis or function as survival factors, depending on the cell context. Furthermore, dysregulated expression of Id proteins has been reported in several human tumors and seems to be related to the malignant character of tumors. Here, we summarize and discuss the biological activities of Id proteins from the standpoint of cell growth control.

Id proteins in cell cycle control and cellular senescence

The Id family of helix-loop-helix (HLH) proteins are thought to affect the balance between cell growth and differentiation by negatively regulating the function of basic-helix-loop-helix (bHLH) transcription factors. Although it has been suggested for some time that Id is involved in cell cycle regulation, little is known about the molecular mechanism of this control. Recent studies, however, have revealed that Id binds to important cell cycle regulatory proteins other than bHLH proteins. Two such proteins, pRB (retinoblastoma tumour suppressor protein) family proteins and Ets-family transcription factors are known to play key roles in cell cycle regulation, transformation and tumour suppression. Through the characterization of these pathways we will begin to understand the mechanisms by which Id controls normal and abnormal cell cycle progression.

Upregulation of the HLH Id gene family in neural progenitors and glial cells of the rat spinal cord following contusion injury

Spinal cord injury (SCI) leads to a complex sequence of cellular responses, including astrocyte activation, oligodendrocyte death, and ependymal cell proliferation. Inhibitors of DNA binding (Id1, Id2, Id3) belong to a helix-loop-helix (HLH) gene family. Id genes have been implicated in playing a vital role in the proliferation of many cell types, including astrocytes and myoblasts. In the present study, the expression of Id family members in spinal cord after contusion injury was investigated by in situ hybridization. Id1, Id2, and Id3 mRNA expression was upregulated 5 mm rostral and caudal to the lesion center, and reached maximal levels 3 days after SCI. In addition, cell populations expressing Id1, Id2, and Id3 mRNA were maximally increased 3 days after SCI. The increase in Id2 and Id3 mRNA expression and Id2 and Id3 mRNA+ cells was still observed at 8 days. The Id mRNA expressing cells were phenotyped by combining immunostaining of cell-specific markers with in situ hybridization. Glial fibrillary acidic protein (GFAP)+ astrocytes were found to express all three Id mRNA, whereas S-100alpha+ astrocytes only expressed high levels of Id2 and Id3 mRNA. Cells having a neural progenitor morphology and the marker nestin appeared after SCI and they expressed Id1, Id2, and Id3 mRNA. Interestingly, some Rip+ oligodendrocytes located in the areas close to the central canal expressed Id3 mRNA after injury. In conclusion, Id genes are upregulated in a time-dependent manner in astrocytes, oligodendrocytes, and neural progenitor subpopulations after SCI, suggesting that they play major roles in cellular responses following SCI.

Hormonal regulation and differential actions of the helix-loop-helix transcriptional inhibitors of differentiation (Id1, Id2, Id3, and Id4) in Sertoli cells

The testicular Sertoli cells support spermatogenesis by providing a microenvironment and structural support for the developing germ cells. Sertoli cell functions are regulated by the gonadotropin FSH. Sertoli cells become a terminally differentiated nongrowing cell population in the adult. In response to FSH, the Sertoli cells express a large number of differentiated gene products, such as transferrin, which transports iron to the developing germ cells. Previously, members of the basic helix-loop-helix (bHLH) family of transcription factors have been shown to influence FSH-mediated gene expression in Sertoli cells. The functions of the bHLH proteins are modulated by Id (inhibitor of differentiation) proteins, which lack the DNA-binding basic domain. The Id proteins form transcriptionally inactive dimers with bHLH proteins and thus regulate cell proliferation and differentiation. The current study investigated the expression and function of Id proteins in the postmitotic Sertoli cell. Freshly isolated and cultured Sertoli cells coexpress all four isoforms of Id (Id1, Id2, Id3, and Id4), as determined by immunoprecipitation with isoform-specific anti-Id antibodies, RT-PCR, and Northern blot analysis. Id2 and Id3 expression levels seem higher than Id1. Interestingly, the expression of Id4 in Sertoli cells is only detectable after stimulation with FSH or cAMP. The Id1 expression is down-regulated by FSH and cAMP, whereas Id2 and Id3 levels remain unchanged in response to FSH. In contrast, serum induces the expression of Id1, Id2, and Id3. Treatment of Sertoli cells with serum significantly reduces the expression of the larger 4-kb Id4 transcript and promotes the presence of a novel 1.3-kb transcript of Id4. The regulatory role of FSH in the expression of all four isoforms of Id is mimicked by a cAMP analog, suggesting that the actions of FSH are mediated through the protein kinase A pathway. An antisense approach was used to study the functional significance of Id proteins in Sertoli cells. Antisense to Id1 stimulated transferrin promoter activity in a transient transfection assay. Interestingly, an antisense to Id2 down-regulated transferrin promoter activity. Id3 and Id4 antisense oligonucleotides had no effect on FSH-mediated transferrin promoter activation. Contrary to the hypothesis that Id proteins have redundant functions, the results of the current study suggest that Id1, Id2, Id3, and Id4 are differentially regulated and may have distinct functions. Id1 may act to maintain Sertoli cell growth potential, whereas Id2 and Id4 may be involved in the differentiation and hormone regulation of Sertoli cells.

Regulation of Id2 gene expression by the insulin-like growth factor I receptor requires signaling by phosphatidylinositol 3-kinase

The Id proteins play an important role in proliferation, differentiation, and tumor development. We report here that Id gene expression can be regulated by the insulin-like growth factor I receptor (IGF-IR), a receptor that also participates in the regulation of cellular proliferation and differentiation. Specifically, we found that the IGF-IR activated by its ligand was a strong inducer of Id2 gene expression in 32D murine hemopoietic cells. This activation was not simply the result of cellular proliferation, as Id2 gene expression was higher in 32D cells stimulated by IGF-I than in cells exponentially growing in interleukin-3. The up-regulation of Id2 gene expression was largely dependent on the presence of insulin receptor substrate-1, a major substrate of the IGF-IR and a potent activator of the phosphatidylinositol 3-kinase (PI3K) pathway. The role of PI3K activity in the up-regulation of Id2 gene expression by the IGF-IR was confirmed by different methods and in different cell types. In 32D cells, the up-regulation of Id2 gene expression by the PI3K pathway correlated with interleukin-3 independence and inhibition of differentiation.

Involvement of the helix-loop-helix protein Id-1 in the glucocorticoid regulation of tight junctions in mammary epithelial cells

Mammary epithelial cell-cell junctions undergo morphological and structural differentiation during pregnancy and lactation, but little is known about the transcriptional regulators that are involved in this process. In Con8 mammary epithelial tumor cells, we have previously documented that the synthetic glucocorticoid, dexamethasone, induces the reorganization of the tight junction and adherens junction and stimulates the monolayer transepithelial electrical resistance (TER), a reliable in vitro measurement of tight junction sealing. Western blots demonstrated that dexamethasone treatment rapidly and strongly stimulated the level of the Id-1 protein, which is a serum-inducible helix-loop-helix transcriptional repressor. The steroid induction of Id-1 was robust by 4 h of treatment and maintained over a 24-h period. Isopropyl-1-thio-beta-d-galactopyranoside-inducible expression of exogenous Id-1 in Con8 cells was shown to strongly facilitate the dexamethasone induction of TER in the absence of serum without altering the dexamethasone-dependent reorganization of ZO-1, beta-catenin, or F-actin. Ectopic overexpression of Id-1 in the SCp2 nontumorigenic mammary epithelial cells, which does not undergo complete dexamethasone-dependent tight junction reorganization, enhanced the dexamethasone-induced ZO-1 tight junction localization and stimulated the monolayer TER. Moreover, antisense reduction of Id-1 protein in SCp2 cells prevented the apical junction reorganization and dexamethasone-stimulated TER. Our results implicate Id-1 as acting as a critical regulator of mammary epithelial cell-cell interactions at an early step in the glucocorticoid-dependent signaling pathway that controls tight junction integrity.

Cloning and characterization of DIP1, a novel protein that is related to the Id family of proteins

Using human cyclin D1 as the "bait" in a yeast two-hybrid system, together with a HL60 cDNA library, we identified a novel human nuclear protein designated DIP1. This protein is expressed in a variety of cell types, and in fibroblasts its level remains constant throughout the cell cycle. However, the level of this protein increases severalfold during the differentiation of HL60 cells. The DIP1 protein can be phosphorylated in vitro by a cellular kinase and this activity reaches its maximum in extracts obtained from cells in the G1 phase of the cell cycle. DIP1 contains a helix-loop-helix motif but lacks an adjacent basic DNA-binding domain, thus resembling the Id family of proteins. The dip1 gene is located on human chromosome 16p11.2-12, a locus that is amplified in several types of human cancer. These results suggest that DIP1 may be involved in the control of gene expression and differentiation, but its precise function remains to be determined.

Id1, Id2, and Id3 gene expression in neural cells during development

Id1, Id2, and Id3 mRNA are expressed mainly in the proliferating ependymal cell zone of the mouse brain during embryogenesis. In this study, the expression pattern and cell phenotypes of the Id family mRNA were examined in postnatal and adult rat brain. The expression of Idl and Id3 mRNA in rat brain was observed in the cortex layer 1, corpus callosum, ventricular/subventricular zone (VZ/ SVZ), and the CA1-4 layers of the hippocampus at postnatal day 1 (P1) through P14, whereby it declined at 2 months. In general, the developmental pattern of Idl mRNA coincided with the pattern observed for Id3 mRNA. Similar to Id1 and Id3, Id2 mRNA was highly expressed in the corpus callosum, VZ/SVZ, and the hippocampus. Examination of Id2 mRNA revealed high levels in the cortex and caudate putamen at P1 through P14, whereas a decline was observed in its expression in the adult cortex. In P5 rat cerebellum, all Id mRNA examined were found in the internal granular cell layers; however, at this time point, only Id2 mRNA expression was detected in the differentiating zone of the external granular cell layers, preferentially localizing to adult Purkinje cells. Furthermore, only Id2 mRNA expression in brain was observed in NF+ neurons at P5. Examination of S100alpha+ and GFAP+ astrocytes, revealed the presence of all three mRNAs, whereas the expression of Id2 and Id3 mRNA was absent in 04+ immature oligodendrocytes. These data suggest that the spatial and temporal kinetic patterns during development, as well as cellular specificity, of the Id gene family may play a critical role in neural precursor cell proliferation and cell divergence.

Id2 promotes apoptosis by a novel mechanism independent of dimerization to basic helix-loop-helix factors

Members of the helix-loop-helix (HLH) family of Id proteins have demonstrated roles in the regulation of differentiation and cell proliferation. Id proteins inhibit differentiation by HLH-mediated heterodimerization with basic HLH transcription factors. This blocks their sequence-specific binding to DNA and activation of target genes that are often expressed in a tissue-specific manner. Id proteins can also act as positive regulators of cell proliferation. The different mechanisms proposed for Id-mediated promotion of entry into S phase also involve HLH-mediated interactions affecting regulators of the G1/S transition. We have found that Id2 augments apoptosis in both interleukin-3 (IL-3)-dependent 32D.3 myeloid progenitors and U2OS osteosarcoma cells. We could not detect a similar activity for Id3. In contrast to the effects of Id2 on differentiation and cell proliferation, Id2-mediated apoptosis is independent of HLH-mediated dimerization. The ability of Id2 to promote cell death resides in its N-terminal region and is associated with the enhanced expression of a known component of the programmed cell death pathway, the proapoptotic gene BAX.

Identification in a fish species of two Id (inhibitor of DNA binding/differentiation)-related helix-loop-helix factors expressed in the slow oxidative muscle fibers

Helix-loop-helix (HLH) proteins related to the inhibitor of DNA binding/differentiation (Id) serve as general antagonists of cell differentiation. They lack a basic DNA-binding domain and are thought to function in a dominant negative manner by sequestering basic HLH (bHLH) transcription factors that are involved in cell determination and differentiation. Four Id-encoding genes have been shown in mammals, they have a distinct pattern of expression suggesting different functions for each member in different cell lineage. In this study we describe the identification and cloning of two trout cDNAs which encode helix-loop-helix proteins showing a high degree of similarity with mammalian Id family members. One cDNA encodes a trout putative Id1 protein (TId1) that is 63% identical to the human Id1 protein over the entire length and 78% identical within the HLH region. The other cDNA encodes a trout putative Id2 protein (TId2) that shows 82% identity to the human Id2 protein and only one change that is conservative over the HLH region. In the 3' untranslated region, TId2 mRNA exhibits 16 nucleotides upstream from the AATAAA site, a palindromic sequence similar to the cytoplasmic polyadenylation element (CPE) which is also present in Id2 and Id3 mRNAs from mammals and in XIdx/XIdI mRNA from Xenopus. In the fish, TId1 and TId2 are expressed in a tissue-specific manner, with slightly different patterns. During myogenesis, TId1 and TId2 are highly expressed in the myotomal musculature of fish embryos and of early alevins but are down-regulated in that of late alevins. In muscle from juveniles and adults, TId1 and TId2 transcripts are abundant in the slow oxidative fibers while they are absent in the fast glycolytic fibers. This expression pattern suggests that Id genes play a role in the regulation of muscle fiber phenotype in addition to controlling early myogenesis. On the whole, the identification of two HLH-Id encoding genes in a major taxonomic group like teleosts, suggests an early divergence of Id genes in vertebrate evolution. The observation that Id transcripts are present selectively in the slow muscle reveals that their expression is more complicated than previously appreciated.

The DNA methyltransferase inhibitor 5-azacytidine specifically alters the expression of helix-loop-helix proteins Id1, Id2 and Id3 during neuronal differentiation

In mammals, cytosine methylation is important for the regulation of gene expression and chromatin structure. Recently, we have found evidence indicating that the maintained DNA methyltransferase activity is critical for neuronal cell differentiation. In the present study, we have investigated the effect of the DNA methyltransferase inhibitor 5-azacytidine on gene regulation during nerve growth factor (NGF)-induced neuronal differentiation of PC12 cells. Expression of the helix-loop-helix proteins Id1, Id2 and Id3 was specifically reduced by NGF and this effect was blocked in 5-azacytidine-treated cells, concomitant with the inhibition of NGF-induced neuronal differentiation. Nuclear run-on and Id2 promoter analyses further demonstrated that the decreased transcription of Id proteins is at least in part dependent on the DNA methyltransferase activity. These findings indicate that Id proteins are downstream targets of the NGF transduction pathway. Moreover, these results suggest that therapeutic strategies using 5-azacytidine against certain types of tumors should be reconsidered because of the possible deleterious effects on neuronal cell function.

Expression and functional role of the Id HLH family in cultured astrocytes

The Id family of helix-loop-helix factors (Id1, Id2, and Id3) expressed in many types of cells has been reported to negatively regulate myoblast differentiation and is required for G1/S progression of arrested fibroblasts. Our previous studies have indicated that Id1, Id2, and Id3 mRNA expression appear in the subventricular zone of 1-day-old rat brains. At later ages, Id3 mRNA was only expressed in astrocytes. We now report that Id1 and Id3 mRNA expression increased in astrocytes during the first hour of serum stimulation. Subsequently, the Id1 and Id3 mRNA levels gradually declined to basal level as observed in cultures without serum stimulation. However, there was no significant difference in Id2 mRNA expression between serum-treated and control astrocyte cultures within 1 h of serum induction. In addition, a strong nuclear immunostaining for Id2 and Id3 proteins was observed 24 h after serum stimulation. This observation is consistent with our results that show an increase in Id2 and Id3 protein levels following 24 h serum induction. Furthermore, DNA synthesis in FCS-stimulated astrocytes was blocked by antisense oligonucleotides against Id3 mRNA. The addition of Id3 antisense oligonucleotides caused approximately 50% reduction in Id3 mRNA and protein levels when compared to that in sense-treated cultures. The results indicate that the inhibition of DNA synthesis in FCS-stimulated astrocytes is due to a decrease in Id3 levels by the antisense. These observations suggest that Id3 may play an important role in the regulation of astrocyte proliferation.

Interplay of the E box, the cyclic AMP response element, and HTF4/HEB in transcriptional regulation of the neurospecific, neurotrophin-inducible vgf gene

vgf is a neurotrophin response-specific, developmentally regulated gene that codes for a neurosecretory polypeptide. Its transcription in neuronal cells is selectively activated by the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor, and neurotrophin 3, which induce survival and differentiation, and not by epidermal growth factor. We studied a short region of the rat vgf promoter which is essential for its regulated expression. A cyclic AMP response element (CRE) within this region is necessary for NGF induction of vgf transcription. Two sites upstream of CRE, an E box and a CCAAT sequence, bind nuclear protein complexes and are involved in transcriptional control. The E box has a dual role. It acts as an inhibitor in NIH 3T3 fibroblasts, together with a second E box located downstream, and as a stimulator in the NGF-responsive cell line PC12. By expression screening, we have isolated the cDNA for a basic helix-loop-helix transcription factor, a homolog of the HTF4/HEB E protein, that specifically binds the vgf promoter E box. The E protein was present in various cell lines, including PC12 cells, and was a component of a multiprotein nuclear complex that binds the promoter in vitro. The E box and CRE cooperate in binding to this complex, which may be an important determinant for neural cell-specific expression.

Expression of the helix-loop-helix genes Id-1 and NSCL-1 during cerebellar development

Neurons throughout the central nervous system (CNS) undergo proliferation, migration, and differentiation during their histogenesis. Although numerous regulatory molecules are expressed in developing neurons, it is unknown whether most of these molecules have the same function throughout the CNS or play different roles in different neuronal populations. Previous studies have shown that Id-1 and NSCL-1 are expressed at high levels in the ventricular and subependymal zones, respectively, of the embryonic brain. In the present study, the expression of Id-1 and NSCL-1 was further investigated during postnatal development of the cerebellum. By Northern blot hybridization analysis, the expression levels of Id-1 and NSCL-1 mRNA were developmentally regulated in the cerebellum, with the highest mRNA levels coinciding with the time of maximal granule cell histogenesis. By in situ hybridization, NSCL-1 mRNA was found in the premigratory zone of the external granule layer (EGL), a structure developmentally analogous to the subependymal zone of the embryonic brain. In normal mice, Id-1 mRNA was found to be transiently expressed in the upper internal granule layer (IGL), a population of cells that recently completed their migration from the EGL. In the mouse mutant weaver, Id mRNA was only seen in granule cells that have reached their normal positions in the IGL. No Id-1 hybridization signal was observed in the large numbers of granule cells remaining in the EGL of weaver mice, indicating that Id-1 expression is controlled by spatial cues. The lack of Id-1 expression in ectopic weaver granule cells is compatible with previous suggestions of arrested differentiation. These results support the idea that transcriptional regulators of the helix-loop-helix gene family play important roles in neuronal development, exhibiting region-specific expression and function.

Nerve growth factor induced differentiation of neuronal cells requires gene methylation

Cell differentiation in the nervous system is dictated by specific patterns of gene expression. We have investigated the role of gene methylation during differentiation of PC12 pheochromocytoma cells in response to nerve growth factor (NGF). Here we present evidence that NGF-induced neuronal differentiation is dependent on gene methylation and that this process is not associated with inhibition of cell cycle arrest. The DNA methylation inhibitor 5-azacytidine is able to block the neurite outgrowth of NGF-treated PC12 cells. Inhibition of neuronal differentiation is accompanied by significant changes in the protein and mRNA expression pattern of the high-affinity NGF receptor (trkA). These studies reveal a new growth factor receptor-mediated mechanism of cellular differentiation dependent on gene methylation. The results indicate that DNA methyltransferase is necessary for the initiation phase of NGF-induced neurite formation in PC12 cells and has a role in growth factor-dependent cellular responses distinct from cell proliferation.

Helix-loop-helix transcription factors mediate activation and repression of the p75LNGFR gene

Sequence analysis of rat and human low-affinity nerve growth factor receptor p75LNGFR gene promoter regions revealed a single E-box cis-acting element, located upstream of the major transcription start sites. Deletion analysis of the E-box sequence demonstrated that it significantly contributes to p75LNGFR promoter activity. This E box has a dual function; it mediates either activation or repression of the p75LNGFR promoter activity, depending on the interacting transcription factors. We showed that the two isoforms of the class A basic helix-loop-helix (bHLH) transcription factor ME1 (ME1a and ME1b), the murine homolog of the human HEB transcription factor, specifically repress p75LNGFR promoter activity. This repression can be released by coexpression of the HLH Id2 transcriptional regulator. In vitro analyses demonstrated that ME1a forms a stable complex with the p75LNGFR E box and likely competes with activating E-box-binding proteins. By using ME1a-overexpressing PC12 cells, we showed that the endogenous p75LNGFR gene is a target of ME1a repression. Together, these data demonstrate that the p75LNGFR E box and the interacting bHLH transcription factors are involved in the regulation of p75LNGFR gene expression. These results also show that class A bHLH transcription factors can repress and Id-like negative regulators can stimulate gene expression.