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Results for “Basic helix-loop-helix (bHLH)”

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PubMed1992

The SCL gene product: a positive regulator of erythroid differentiation.

The SCL (tal-1, TCL5) gene is a member of the basic domain, helix-loop-helix (bHLH) class of putative transcription factors. We found that (i) the SCL promoter for exon Ia contains a potential recognition site for GATA-binding transcription factors, (ii) SCL mRNA is expressed in all erythroid tissues and cell lines examined, and (iii) SCL mRNA increases upon induced differentiation of murine erythroleukemia (MEL) cells, and inferred that SCL may play a physiologic role in erythroid differentiation. We used gel shift and transfection assays to demonstrate that the GATA motif in the SCL promoter binds GATA-1 (and GATA-2), and also mediates transcriptional transactivation. To identify a role for SCL in erythroid differentiation, we generated stable transfectants of MEL and K562 (a human chronic myelogenous leukemia cell line that can differentiate along the erythroid pathway) cells overexpressing wild-type, antisense or mutant SCL cDNA. Increasing the level of SCL expression in two independent MEL lines (F4-6 and C19, a 745 derivative) and K562 cells increased the rate of spontaneous (i.e. in the absence of inducer) erythroid differentiation. Conversely, induced differentiation was inhibited in MEL transfectants expressing either antisense SCL cDNA or a mutant SCL lacking the basic domain. Our experiments suggest that the SCL gene can be a target for the erythroid transcription factor GATA-1 and that the SCL gene product serves as a positive regulator of erythroid differentiation.

3T3 Cells↗

PubMed1992

Single amino acid substitutions alter helix-loop-helix protein specificity for bases flanking the core CANNTG motif.

While all basic region/helix-loop-helix (bHLH) proteins bind the consensus CANNTG motif, other factors must be involved in determining regulatory specificity. In this report we show that bases outside this core 6 bp are involved in determining the specificity of binding. Thus, binding of the yeast bHLH protein PHO4, but not CPF-1, is inhibited by the presence of a T residue immediately 5' to their common CACGTG recognition sequence. PHO4 binding specificity is altered by mutation at any of three different positions in the basic region, including a single Glu to Asp substitution. The significance of these data for DNA-binding and transcription regulation by the bHLH family of transcription factors is discussed.

Amino Acid Sequence↗

PubMed1993

Definition of a novel ligand binding domain of a nuclear bHLH receptor: co-localization of ligand and hsp90 binding activities within the regulable inactivation domain of the dioxin receptor.

The dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and binds to DNA target sequences as a heterodimer of the approximately 100 kDa ligand binding receptor and the approximately 85 kDa auxiliary factor, Arnt. Both of these factors encompass an N-terminal basic helix-loop-helix (bHLH) motif required for DNA binding and dimerization. In this study we describe the construction of glucocorticoid/dioxin receptor fusion proteins which allow the regulation of glucocorticoid receptor activity by dioxin in transient transfections of CHO and hepatoma cells. Thus, in the absence of dioxin, chimeric receptor constructs which contain large 500-720 amino acid C-terminal dioxin receptor fragments, but lack the N-terminal bHLH motif, confer repression upon the transcriptional activity of a glucocorticoid receptor derivative, tau DBD, containing its N-terminal strong transactivating signal (tau) and its DNA binding domain (DBD). In the presence of dioxin, this repression is reversed. Importantly, these chimeric receptors did not require the bHLH Arnt co-factor for function. A considerably smaller region of the dioxin receptor, located between amino acids 230 and 421, showed specific dioxin binding activity in vitro. Moreover, dioxin binding in vitro correlated with the ability of receptor fragments to form stable complexes in vitro with the molecular chaperone hsp90. These findings support the notion that hsp90 may be important for folding of a dioxin binding configuration of the receptor. Finally, tau DBD activity was constitutively repressed in a dioxin non-responsive manner by dioxin receptor fragments which failed to bind ligand but also failed to bind hsp90 in vitro, indicating that alternative mechanisms in addition to hsp90 binding may contribute to the inactivation function. In summary, the dioxin receptor system provides a novel and complex model of regulation of bHLH factors that may also give important insights into the mechanism of action of ligand-activated nuclear receptors.

Aryl Hydrocarbon Receptor Nuclear Translocator↗

PubMed1993

Transcription activation by Myc and Max: flanking sequences target activation to a subset of CACGTG motifs in vivo.

The Myc oncoprotein has been implicated in control of cell growth, division and differentiation. Although Myc contains a bHLH-LZ motif, it fails to bind DNA alone but can do so by forming heterodimers with an unrelated bHLH-LZ protein, Max. Max homodimers and Myc-Max heterodimers share the ability to bind CACGTG or CATGTG elements. Current models, based on experimentally induced overexpression of Myc and Max in mammalian cells, propose that Max-Max homodimers repress while Myc-Max heterodimers activate transcription through CACGTG binding sites. The interpretation of the results using mammalian cells is complicated by the presence of numerous unrelated CACGTG binding transcription activators and the existence of two alternative Max dimerization partners, Mad and Mxi-1. Thus, the mechanism whereby overexpression of Max leads to transcriptional repression remains to be established. Using a yeast system we show that Max homodimers have the potential to activate transcription through CACGTG motifs. Activation by Max requires DNA binding and amino acids outside the bHLH-LZ domain but is reduced compared with activation by Myc-Max heterodimers. Moreover, transcriptional activation by Myc-Max heterodimers, but not Max-Max homodimers, is strongly inhibited in vivo by specific sequences flanking the core CACGTG binding motif, presumably reflecting reduced DNA binding affinity. These results suggest a mechanism for directing the Myc-Max complex to a specific subset of CACGTG-containing target genes.

Amino Acid Sequence↗

PubMed1993

The c-Myc protein induces cell cycle progression and apoptosis through dimerization with Max.

The c-Myc protein (Myc) is involved in cellular transformation and mitogenesis, but is also a potent inducer of programmed cell death, or apoptosis. Whether these apparently opposite functions are mediated through common or distinct molecular mechanisms remains unclear. Myc and its partner protein, Max, dimerize and bind DNA in vitro and in vivo through basic/helix-loop-helix/leucine zipper motifs (bHLH-LZ). By using complementary leucine zipper mutants (termed MycEG and MaxEG), which dimerize efficiently with each other but not with their wild-type partners, we demonstrate that both cell cycle progression and apoptosis in nontransformed rodent fibroblasts are induced by Myc-Max dimers. MycEG or MaxEG alone are inactive, but co-expression restores ability to prevent withdrawal from the cell cycle and to induce cell death upon removal of growth factors. Thus, Myc can control two alternative cell fates through dimerization with a single partner, Max.

Amino Acid Sequence↗

PubMed1994

Persistent expression of helix-loop-helix factor HES-1 prevents mammalian neural differentiation in the central nervous system.

In the developing mammalian central nervous system, neural precursor cells present in the ventricular zone determine their fate to become neurons or glial cells, migrate towards the outer layers and undergo terminal differentiation. The transcriptional repressor HES-1, a basic helix-loop-helix (bHLH) factor structurally related to the Drosophila hairy gene, is expressed at high levels throughout the ventricular zone, but the level decreases as neural differentiation proceeds. Because of this negative correlation, we tested whether continuous expression of HES-1 inhibits neural differentiation. A HES-1 and lacZ-transducing retrovirus (SG-HES1) and a control lacZ-transducing retrovirus (SG) were injected into the lateral ventricles of mouse embryos, and the fate of the infected neural precursor cells was examined by X-gal staining. The SG virus-infected cells migrated and differentiated into neurons and glial cells. In contrast, the cells infected with SG-HES1 virus remained in the ventricular/subventricular zone, decreased to approximately 10% in number as compared with that of the newborn during the postnatal 4-5 weeks and, when they survived, were present exclusively in the ependymal layer. Furthermore, whereas cultured neural precursor cells infected with SG virus became immunoreactive for neuronal and glial markers, the cells infected with SG-HES1 virus did not. These results show that persistent expression of HES-1 severely perturbs neuronal and glial differentiation.

Animals↗

PubMed1994

A thermodynamic scale for leucine zipper stability and dimerization specificity: e and g interhelical interactions.

The leucine zipper is a dimeric coiled-coil protein structure composed of two amphipathic alpha-helices with the hydrophobic surfaces interacting to create the dimer interface. This structure has been found to mediate the dimerization of two abundant classes of DNA binding proteins: the bZIP and bHLH-Zip proteins. Several workers have reported that amino acids in the e and g positions of the coiled coil can modulate dimerization stability and specificity. Using the bZIP protein VBP as a host molecule, we report a thermodynamic scale (delta delta G) for 27 interhelical interactions in 35 proteins between amino acids in the g and the following e positions (g<==>e') of a leucine zipper coiled coil. We have examined the four commonly occurring amino acids in the e and g positions of bZIP proteins, lysine (K), arginine (R), glutamine (Q), glutamic acid (E), as well as the only other remaining charged amino acid aspartic acid (D), and finally alanine (A) as a reference amino acid. These results indicate that E<==>R is the most stable interhelical pair, being 0.35 kcal/mol more stable than E<==>K. A thermodynamic cycle analysis shows that the E<==>R pair is 1.33 kcal/mol more stable than A<==>A with -1.14 kcal/mol of coupling energy (delta delta Gint) coming from the interaction of E with R. The E<==>K coupling energy is only -0.14 kcal/mol. E interacts with more specificity than Q. The R<==>R pair is less stable than the K<==>K by 0.24 kcal/mol. R interacts with more specificity than K. Q forms more stable pairs with the basic amino acids K and R rather than with E. Changing amino acids in the e position to A creates bZIP proteins that form tetramers.

Amino Acid Sequence↗

PubMed1994

Specific in vivo association between the bHLH and LIM proteins implicated in human T cell leukemia.

The protein products of proto-oncogenes implicated in T cell acute lymphoblastic leukemia include two distinct families of presumptive transcription factors. RBTN1 and RBTN2 encode highly related proteins that possess cysteine-rich LIM motifs. TAL1, TAL2 and LYL1 encode a unique subgroup of basic helix-loop-helix (bHLH) proteins that share exceptional homology in their bHLH sequences. We have found that RBTN1 and RBTN2 have the ability to interact with each of the leukemogenic bHLH proteins (TAL1, TAL2 and LYL1). These interactions occur in vivo and appear to be mediated by sequences within the LIM and bHLH domains. The LIM-bHLH interactions are highly specific in that RBTN1 and RBTN2 will associate with TAL1, TAL2 and LYL1, but not with other bHLH proteins, including E12, E47, Id1, NHLH1, AP4, MAX, MYC and MyoD1. Moreover, RBTN1 and RBTN2 can interact with TAL1 polypeptides that exist in assembled bHLH heterodimers (e.g. TAL1-E47), suggesting that the RBTN proteins can influence the functional properties of TAL1. Finally, we have identified a subset of leukemia patients that harbor tumor-specific rearrangements of both their RBTN2 and TAL1 genes. Thus, the activated alleles of these genes may promote leukemia cooperatively, perhaps as a result of bHLH-LIM interactions between their protein products.

Amino Acid Sequence↗

PubMed1995

Multiple modes of dorsal-bHLH transcriptional synergy in the Drosophila embryo.

Synergistic interactions between the maternal regulatory factor dorsal (dl) and basic helix-loop-helix (bHLH) activators are essential for initiating differentiation of the mesoderm and neuroectoderm in the early Drosophila embryo. Here we present evidence that dl-bHLH interactions mediating gene expression in the neuroectoderm and mesoderm are fundamentally distinct. Close proximity of dl and bHLH binding sites is essential for the synergistic activation of gene expression in the lateral neuroectoderm, where there are diminishing levels of the dl regulatory gradient. In contrast, sharp on/off patterns of gene expression in the presumptive mesoderm do not require linkage of these sites. Analysis of minimal and synthetic promoter elements suggests that dl and bHLH activators, such as twist, might interact with different rate-limiting components of the transcription complex. These results are consistent with two distinct modes of dl-bHLH synergy: cooperative binding to DNA (requiring linkage of sites) and synergistic contact of basal transcription factors (not requiring linkage). Finally, the characterization of a 57 bp synthetic minimal stripe unit (MSU) provides evidence for a third tier of dl-bHLH synergy. Tandem copies of the MSU function as a bona fide enhancer and can mediate neuroectoderm expression in transgenic embryos even when placed 4.5 kb downstream of a test promoter. Multiple copies of the MSU function synergistically only when linked, but not when separated. We propose that this linkage requirement provides the basis for the evolution of modular promoters composed of discrete, non-overlapping enhancers.

Animals↗

PubMed1995

Protein-protein interaction via PAS domains: role of the PAS domain in positive and negative regulation of the bHLH/PAS dioxin receptor-Arnt transcription factor complex.

Gene regulation by dioxins is mediated by the dioxin receptor-Arnt heterodimer, a ligand generated complex of two basic helix-loop-helix (bHLH)/Per-Arnt-Sim (PAS) transcription factors. By using dioxin receptor chimeras where the dimerization and DNA binding bHLH motif has been replaced by a heterologous DNA binding domain, we have detected an ability of Arnt to interact with the dioxin receptor via the PAS domain in a mammalian 'hybrid interaction' system. By coimmunoprecipitation assays, we have confirmed the ability of PAS domains of the dioxin receptor and Arnt to mediate independent heterodimerization in vitro. Selectivity for PAS dimerization was noted in our hybrid interaction system, as dioxin receptor or Arnt PAS-mediated homodimers were not detected. Surprisingly, however, the PAS domain of Per could dimerize with both the dioxin receptor and Arnt subunits in vitro, and disrupt the ability of these subunits to form a DNA binding heterodimer. Moreover, ectopic expression of Per blocked dioxin signalling in mammalian cells. The PAS domains of the dioxin receptor and Arnt are therefore novel dimerizing regions critical in formation of a functional dioxin receptor-Arnt complex, while the PerPAS domain is a potential negative regulator of bHLH/PAS factor function.

Animals↗

PubMed2002

A novel mechanism for the regulation of osteoblast differentiation: transcription of periostin, a member of the fasciclin I family, is regulated by the bHLH transcription factor, twist.

Periostin is a secreted protein that is highly expressed in early osteoblastic cells in vitro and in periosteum and periodontal ligament tissues in vivo. It is known that periostin supports cellular adhesion and spreading in vitro. Although, the mechanisms of transcriptional regulation of periostin are poorly understood, gene-profiling data have revealed that overexpression of Twist, a basic helix-loop-helix (bHLH) transcription factor, resulted in increased periostin expression as validated by Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses. Twist is an important transcription factor for cell type determination and differentiation and has been shown to play an important regulatory role in early osteogenesis. In situ hybridization of mouse calvarial bones indicated that periostin and Twist mRNA are co-localized at the osteogenic fronts of calvarial bones. To characterize the 5' flanking region of the periostin gene, primer extension was carried out to identify the transcription start site, and DNA sequence analysis confirmed the presence of a 'Twist-box' response element. The results of electrophoretic mobility shift assay (EMSA) using nuclear extracts of MC3T3-E1 cells revealed that Twist bound to the Twist-box sequence on the periostin promoter. In vivo footprinting experiments using ligation-mediated PCR (LM-PCR) indicated that the Twist-box sequence was protected in undifferentiated MC3T3-E1 preosteoblasts but not in differentiated MC3T3-E1 osteoblasts. To determine whether Twist actually regulates the periostin expression, 293T cells were transiently co-transfected with the periostin promoter construct and the human Twist expression vector. Reporter analysis indicated that the periostin promoter activities were enhanced by overexpression of Twist. These data suggest that Twist can bind to the periostin promoter in undifferentiated preosteoblasts and up-regulate periostin expression, consistent with the up-regulation of periostin expression by Twist as observed in the gene-profiling data.

5' Flanking Region↗

PubMed2004

Characterizing the new transcription regulator protein p60TRP.

Active cell death ('apoptosis' or 'programmed cell death') is essential in the development and homeostasis of multicellular organisms and abnormal inhibition of apoptosis is an indicator of cancer and autoimmune diseases, whereas excessive cell death might be implicated in neurodegenerative disorders such as Alzheimer's disease (AD). Using bioinformatics-, Western-blotting-, yeast-two-hybrid-system-, polymerase chain reaction (PCR)-, and fluorescence microscopy-analyses, we demonstrate here that the neuroprotective protein p60TRP (p60-transcription-regulator-protein) is a basic helix-loop-helix (bHLH) domain-containing member of a new protein family that interacts with the Ran-binding-protein-5 (RanBP5) and the protein-phosphatase-2A (PP2A). The additional findings of its influence on NNT1 and p48ZnF (new-neurotrophin-1, p48-zinc-finger-protein)-signaling and its down-regulation in the brain of AD subjects point to a possible pivotal role of p60TRP in the control of cellular aging and survival.

Aged↗

PubMed2004

Cooperative activation of atrial naturetic peptide promoter by dHAND and MEF2C.

An intricate array of cell-specific multiprotein complexes participate in programs of cell-specific gene expression through combinatorial interaction with different transcription factors and cofactors. The dHAND basic helix-loop-helix (bHLH) transcription factor, which is essential for heart development and extra embryonic structures, is thought to regulate cardiomyocyte-specific gene expression through combinatorial interactions with other cardiac-restricted transcription factors such as GATA4 and NKX2.5. Here, we determine that dHAND also interacts with the myocyte enhancer binding factor-2c (MEF2C) protein, which belongs to MADS-box transcription factors and is essential for heart development. dHAND and MEF2C synergistically activated expression of the atrial naturetic peptide gene (ANP) in transfected HeLa cells. GST-pulldown and immunoprecipitation assay demonstrate that full-length MEF2C protein is able to interact with dHAND in vitro and in vivo, just like MEF2A and bHLH transcription factors MyoD in skeletal muscle cells. In addition, electrophoretic mobility shift assays (EMSAs) demonstrate that MEF2C and dHAND do not influence each other's DNA binding activity. Using chromatin immunoprecipitation (ChIP) analysis in H9c2 cells we show that dHAND interact with MEF2C to form protein complex and bind A/T sequence in promoter of ANP. Taken together with previous observations, these results suggest the existence of large multiprotein transcriptional complex with core DNA binding proteins that physically interact with other transcriptional factors to form favorable conformation to potentiate transcription.

Atrial Natriuretic Factor↗

PubMed2002

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.

Animals↗

PubMed2003

HES and HERP families: multiple effectors of the Notch signaling pathway.

Notch signaling dictates cell fate and critically influences cell proliferation, differentiation, and apoptosis in metazoans. Multiple factors at each step-ligands, receptors, signal transducers and effectors-play critical roles in executing the pleiotropic effects of Notch signaling. Ligand-binding results in proteolytic cleavage of Notch receptors to release the signal-transducing Notch intracellular domain (NICD). NICD migrates into the nucleus and associates with the nuclear proteins of the RBP-Jkappa family (also known as CSL or CBF1/Su(H)/Lag-1). RBP-Jkappa, when complexed with NICD, acts as a transcriptional activator, and the RBP-Jkappa-NICD complex activates expression of primary target genes of Notch signaling such as the HES and enhancer of split [E(spl)] families. HES/E(spl) is a basic helix-loop-helix (bHLH) type of transcriptional repressor, and suppresses expression of downstream target genes such as tissue-specific transcriptional activators. Thus, HES/E(spl) directly affects cell fate decisions as a primary Notch effector. HES/E(spl) had been the only known effector of Notch signaling until a recent discovery of a related but distinct bHLH protein family, termed HERP (HES-related repressor protein, also called Hey/Hesr/HRT/CHF/gridlock). In this review, we summarize the recent data supporting the idea of HERP being a new Notch effector, and provide an overview of the similarities and differences between HES and HERP in their biochemical properties as well as their tissue distribution. One key observation derived from identification of HERP is that HES and HERP form a heterodimer and cooperate for transcriptional repression. The identification of the HERP family as a Notch effector that cooperates with HES/E(spl) family has opened a new avenue to our understanding of the Notch signaling pathway.

Animals↗

PubMed2001

Expression of transcription factors during oligodendroglial development.

Transcription factors coordinate the orderly changes of gene expression that underlie all developmental processes including those of oligodendrocytes. In comparison to other systems, relatively little is known about the role of transcription factors during oligodendrocyte development. However, recent years have seen the identification of oligodendroglial transcription factors that, not surprisingly, belong to the same gene families that are also important in other tissues or cell lineages. Some transcription factors such as the bHLH (basic helix-loop-helix) proteins Olig-1 and Olig-2 or the high-mobility-group protein Sox10 are expressed already early during development of the oligodendrocyte lineage, whereas expression of other transcription factors such as the homeodomain protein Gtx/Nkx6.2 only start at the time of terminal differentiation. Once turned on, expression of these proteins can either be permanent as in the above-mentioned cases or transient as exemplified by the POU domain proteins Tst-1/Oct6/SCIP, Brn-1 and Brn-2. Analyses of these transcription factors has already led to the identification of important principles such as functional redundancy between co-expressed proteins, unexpected divergence in the transcription factor repertoire of oligodendrocytes and Schwann cells, and equally unsuspected similarities in transcription factor usage between oligodendrocytes and neurons. Although so far only a small percentage of oligodendroglial transcription factors has been identified, these are excellent candidates for regulators of cell type specification, lineage progression, and terminal differentiation.

Animals↗

PubMed2002

Constitutive overexpression of the basic helix-loop-helix Nex1/MATH-2 transcription factor promotes neuronal differentiation of PC12 cells and neurite regeneration.

Elucidation of the intricate transcriptional pathways leading to neural differentiation and the establishment of neuronal identity is critical to the understanding and design of therapeutic approaches. Among the important players, the basic helix-loop-helix (bHLH) transcription factors have been found to be pivotal regulators of neurogenesis. In this study, we investigate the role of the bHLH differentiation factor Nex1/MATH-2 in conjunction with the nerve growth factor (NGF) signaling pathway using the rat phenochromocytoma PC12 cell line. We report that the expression of Nex1 protein is induced after 5 hr of NGF treatment and reaches maximal levels at 24 hr, when very few PC12 cells have begun extending neurites and ceased cell division. Furthermore, our study demonstrates that Nex1 has the ability to trigger neuronal differentiation of PC12 cells in the absence of neurotrophic factor. We show that Nex1 plays an important role in neurite outgrowth and has the capacity to regenerate neurite outgrowth in the absence of NGF. These results are corroborated by the fact that Nex1 targets a repertoire of distinct types of genes associated with neuronal differentiation, such as GAP-43, betaIII-tubulin, and NeuroD. In addition, our findings show that Nex1 up-regulates the expression of the mitotic inhibitor p21(WAF1), thus linking neuronal differentiation to cell cycle withdrawal. Finally, our studies show that overexpression of a Nex1 mutant has the ability to block the execution of NGF-induced differentiation program, suggesting that Nex1 may be an important effector of the NGF signaling pathway.

Animals↗

PubMed2003

Neurotrophins facilitate neuronal differentiation of cultured neural stem cells via induction of mRNA expression of basic helix-loop-helix transcription factors Mash1 and Math1.

Neurogenesis is promoted by basic helix-loop-helix (bHLH) transcription factors Mash1, Math1, or NeuroD but suppressed by another set, Hes1 and Hes5. It remains unknown what kinds of extracellular signals are involved in their regulation; therefore, the effects of neurotrophins on the expression of bHLH factors and neuronal differentiation were investigated by the use of cultured mouse neural stem cells. Each neurotrophin increased Mash1 and Math1 mRNAs of the stem cells growing in the presence of fibroblast growth factor-2 (FGF-2), but did not alter Hes1, Hes5, or NeuroD mRNA levels. Simultaneously, most of the cells expressed nestin but not microtubule-associated protein 2 (MAP2), and remained undifferentiated. FGF-2 removal from the medium reduced the levels of Hes1 and Hes5 mRNAs and increased those of Mash1, Math1, and NeuroD mRNAs, resulting in substantial neuronal differentiation. When the cells were pretreated with brain-derived neurotrophic factor, a neurotrophin, FGF-2 removal enhanced earlier NeuroD expression and generated many more MAP2-positive cells. The high level of Mash1 and Math1 that had been elevated at FGF-2 withdrawal accelerated NeuroD expression in cooperation with the reduced Hes1 and Hes5 expression. Our present results suggest that neurotrophins stimulate neuronal differentiation by altering the balance of expression of various bHLH transcription factors.

Animals↗