Repression of muscle-specific gene activation by the murine Twist protein

Epithelial-to-mesenchymal transition transcription factors: New strategies for mesenchymal tissue regeneration

The epithelial-mesenchymal transition transcription factors (EMT-TFs)-ZEB, SNAI, and TWIST families-have been extensively studied in embryonic development and tumor metastasis, providing valuable insight into their roles in cell behavior and transformation. These EMT-TFs have garnered increasing attention in the context of mesenchymal tissue regeneration, potentially contributing an approach for cell therapy. Given that dysregulated EMT-TF expression can impair cell survival and lineage differentiation, controlled regulation of their expression could offer significant advantages for tissue regeneration. However, there is a lack of comprehensive reviews to summarize the influence of the EMT-TFs on mesenchymal tissue regeneration and potential molecular mechanisms. This review explores the regulatory roles of ZEB, SNAI, and TWIST in the regeneration of bone, adipose, cartilage, muscle, and other mesenchymal tissues, with a focus on the underlying molecular signaling mechanisms. Gaining a deeper understanding of how EMT-TFs regulate cell proliferation, apoptosis, migration, and differentiation may offer new insights into the management of mesenchymal tissue repair and open novel avenues for enhancing tissue regeneration.

Myogenesis gone awry: the role of developmental pathways in rhabdomyosarcoma

Rhabdomyosarcoma is a soft-tissue sarcoma that occurs most frequently in pediatric patients and has poor survival rates in patients with recurrent or metastatic disease. There are two major sub-types of RMS: fusion-positive (FP-RMS) and fusion-negative (FN-RMS); with FP-RMS typically containing chromosomal translocations between the PAX3/7-FOXO1 loci. Regardless of subtype, RMS resembles embryonic skeletal muscle as it expresses the myogenic regulatory factors (MRFs), MYOD1 and MYOG. During normal myogenesis, these developmental transcription factors (TFs) orchestrate the formation of terminally differentiated, striated, and multinucleated skeletal muscle. However, in RMS these TFs become dysregulated such that they enable the sustained properties of malignancy. In FP-RMS, the PAX3/7-FOXO1 chromosomal translocation results in restructured chromatin, altering the binding of many MRFs and driving an oncogenic state. In FN-RMS, re-expression of MRFs, as well as other myogenic TFs, blocks terminal differentiation and holds cells in a proliferative, stem-cell-like state. In this review, we delve into the myogenic transcriptional networks that are dysregulated in and contribute to RMS progression. Advances in understanding the mechanisms through which myogenesis becomes stalled in RMS will lead to new tumor-specific therapies that target these aberrantly expressed developmental transcriptional pathways.

Association of growth differentiation factor 9 expression with nuclear receptor and basic helix-loop-helix transcription factors in buffalo oocytes during in vitro maturation

Growth differentiation factor 9 (GDF9) is an oocyte-specific paracrine factor involved in bidirectional communication, which plays an important role in oocyte developmental competence. In spite of its vital role in reproduction, there is insufficient information about exact transcriptional control mechanism of GDF9. Hence, present study was undertaken with the aim to study the expression of basic helix-loop-helix (bHLH) transcription factors (TFs) such as the factor in the germline alpha (FIGLA), twist-related protein 1 (TWIST1) and upstream stimulating factor 1 and 2 (USF1 and USF2), and nuclear receptor (NR) superfamily TFs like germ cell nuclear factor (GCNF) and oestrogen receptor 2 (ESR2) under three different in vitro maturation (IVM) groups [follicle-stimulating hormone (FSH), insulin-like growth factor-1 (IGF1) and oestradiol)] along with all supplementation group as positive control, to understand their role in regulation of GDF9 expression. Buffalo cumulus-oocyte complexes were aspirated from abattoir-derived ovaries and matured in different IVM groups. Following maturation, TFs expression was studied at 8 h of maturation in all four different IVM groups and correlated with GDF9 expression. USF1 displayed positive whereas GCNF, TWIST1 and ESR2 revealed negative correlation with GDF9 expression. TWIST1 & ESR2 revealing negative correlation with GDF9 expression were found to be positively correlated amongst themselves also. GCNF & USF1 revealing highly significant correlation with GDF9 expression in an opposite manner were found to be negatively correlated. The present study concludes that the expression of GDF9 in buffalo oocytes remains under control through the involvement of NR and bHLH TFs.

Twist alters the breast tumor microenvironment via choline kinase to facilitate an aggressive phenotype

Twist (TWIST1) is a gene required for cell fate specification in embryos and its expression in mammary epithelium can initiate tumorigenesis through the epithelial-mesenchymal transition. To identify downstream target genes of Twist in breast cancer, we performed microarray analysis on the transgenic breast cancer cell line, MCF-7/Twist. One of the targets identified was choline kinase whose upregulation resulted in increased cellular phosphocholine and total choline containing compounds-a characteristic observed in highly aggressive metastatic cancers. To study the interactions between Twist, choline kinase, and their effect on the microenvironment, we used 1H magnetic resonance spectroscopy and found significantly higher phosphocholine and total choline, as well as increased phosphocholine/glycerophosphocholine ratio in MCF-7/Twist cells. We also observed significant increases in extracellular glucose, lactate, and [H +] ion concentrations in the MCF-7/Twist cells. Magnetic resonance imaging of MCF-7/Twist orthotopic breast tumors showed a significant increase in vascular volume and permeability surface area product compared to control tumors. In addition, by reverse transcription-quantitative polymerase chain reaction, we discovered that Twist upregulated choline kinase expression in estrogen receptor negative breast cancer cell lines through FOXA1 downregulation. Moreover, using The Cancer Genome Atlas database, we observed a significant inverse relationship between FOXA1 and choline kinase expression and propose that it could act as a modulator of the Twist/choline kinase axis. The data presented indicate that Twist is a driver of choline kinase expression in breast cancer cells via FOXA1 resulting in the generation of an aggressive breast cancer phenotype.

Twist activates miR-22 to suppress estrogen receptor alpha in breast cancer

TWIST1 (Twist) is a basic helix-loop-helix transcription factor that is overexpressed in many cancers and promotes tumor cell invasion, metastasis, and recurrence. In this study, we demonstrate that Twist upregulates expression of microRNA 22 (miR-22) which, in turn, downregulates estrogen receptor alpha (ER) expression in breast cancer. Initial analysis of miR-22 and Twist expression in a panel of breast cancer cell lines showed a direct correlation between Twist and miR-22 levels with miR-22 being highly expressed in ER negative cell lines. Overexpressing Twist caused increased miR-22 levels while downregulating it led to decreased miR-22 expression. To characterize the upstream promoter region of miR-22, we utilized rapid amplification of cDNA ends and identified the transcription start site and the putative promoter region of miR-22. Mechanistically, we determined that Twist, in combination with HDAC1 and DNMT3B, transcriptionally upregulates miR-22 expression by binding to E-boxes in the proximal miR-22 promoter. We also established that miR-22 causes an increase in growth in 3D but not 2D cultures. Importantly, we observed a direct correlation between increased breast cancer grade and Twist and miR-22 expression. We also identified two potential miR-22 binding sites in the 3'-UTR region of ER and confirmed by promoter assays that miR-22 regulates ER expression by binding to both target sites. These results reveal a novel pathway of ER suppression by Twist through miR-22 activation that could potentially promote the ER negative phenotype in breast cancers.

Cell Mechanics of Craniosynostosis

Craniosynostosis is the premature fusion of the calvarial sutures that is associated with a number of physical and intellectual disabilities spanning from pediatric to adult years. Over the past two decades, techniques in molecular genetics and more recently, advances in high-throughput DNA sequencing have been used to examine the underlying pathogenesis of this disease. To date, mutations in 57 genes have been identified as causing craniosynostosis and the number of newly discovered genes is growing rapidly as a result of the advances in genomic technologies. While contributions from both genetic and environmental factors in this disease are increasingly apparent, there remains a gap in knowledge that bridges the clinical characteristics and genetic markers of craniosynostosis with their signaling pathways and mechanotransduction processes. By linking genotype to phenotype, outlining the role of cell mechanics may further uncover the specific mechanotransduction pathways underlying craniosynostosis. Here, we present a brief overview of the recent findings in craniofacial genetics and cell mechanics, discussing how this information together with animal models is advancing our understanding of craniofacial development.

Tcf12, A Member of Basic Helix-Loop-Helix Transcription Factors, Mediates Bone Marrow Mesenchymal Stem Cell Osteogenic Differentiation In Vitro and In Vivo

Several basic Helix-Loop-Helix transcription factors have recently been identified to regulate mesenchymal stem cell (MSC) differentiation. In the present study, Tcf12 was investigated for its involvement in the osteoblastic cell commitment of MSCs. Tcf12 was found highly expressed in undifferentiated MSCs whereas its expression decreased following osteogenic culture differentiation. Interestingly, Tcf12 endogenous silencing using shRNA lentivirus significantly promoted the differentiation ability of MSCs evaluated by alkaline phosphatase staining, alizarin red staining and expression of osteoblast-specific markers by real-time PCR. Conversely, overexpression of Tcf12 in MSCs suppressed osteoblast differentiation. It was further found that silencing of Tcf12 activated bone morphogenetic protein (BMP) signaling and extracellular signal-regulated kinase (Erk)1/2 signaling pathway activity and upregulated the expression of phospho-SMAD1 and phospho-Erk1/2. A BMP inhibitor (LDN-193189) and Erk1/2 signaling pathway inhibitor (U0126) reduced these findings in the Tcf12 silencing group. Following these in vitro results, a poly-L-lactic acid/Hydroxyappatite scaffold carrying Tcf12 silencing lentivirus was utilized to investigate the repair of bone defects in vivo. The use of Tcf12 silencing lentivirus significantly promoted new bone formation in 3-mm mouse calvarial defects as assessed by micro-CT and histological examination whereas overexpression of Tcf12 inhibited new bone formation. Collectively, these data indicate that Tcf12 is a transcription factor highly expressed in the nuclei of stem cells and its downregulation plays an essential role in osteoblast differentiation partially via BMP and Erk1/2 signaling pathways. Stem Cells 2017;35:386-397.

Diversity of Cnidarian Muscles: Function, Anatomy, Development and Regeneration

The ability to perform muscle contractions is one of the most important and distinctive features of eumetazoans. As the sister group to bilaterians, cnidarians (sea anemones, corals, jellyfish, and hydroids) hold an informative phylogenetic position for understanding muscle evolution. Here, we review current knowledge on muscle function, diversity, development, regeneration and evolution in cnidarians. Cnidarian muscles are involved in various activities, such as feeding, escape, locomotion and defense, in close association with the nervous system. This variety is reflected in the large diversity of muscle organizations found in Cnidaria. Smooth epithelial muscle is thought to be the most common type, and is inferred to be the ancestral muscle type for Cnidaria, while striated muscle fibers and non-epithelial myocytes would have been convergently acquired within Cnidaria. Current knowledge of cnidarian muscle development and its regeneration is limited. While orthologs of myogenic regulatory factors such as MyoD have yet to be found in cnidarian genomes, striated muscle formation potentially involves well-conserved myogenic genes, such as twist and mef2. Although satellite cells have yet to be identified in cnidarians, muscle plasticity (e.g., de- and re-differentiation, fiber repolarization) in a regenerative context and its potential role during regeneration has started to be addressed in a few cnidarian systems. The development of novel tools to study those organisms has created new opportunities to investigate in depth the development and regeneration of cnidarian muscle cells and how they contribute to the regenerative process.

Hotair Is Dispensible for Mouse Development

Despite the crucial importance of Hox genes functions during animal development, the mechanisms that control their transcription in time and space are not yet fully understood. In this context, it was proposed that Hotair, a lncRNA transcribed from within the HoxC cluster regulates Hoxd gene expression in trans, through the targeting of Polycomb and consecutive transcriptional repression. This activity was recently supported by the skeletal phenotype of mice lacking Hotair function. However, other loss of function alleles at this locus did not elicit the same effects. Here, we re-analyze the molecular and phenotypic consequences of deleting the Hotair locus in vivo. In contrast with previous findings, we show that deleting Hotair has no detectable effect on Hoxd genes expression in vivo. In addition, we were unable to observe any significant morphological alteration in mice lacking the Hotair transcript. However, we find a subtle impact of deleting the Hotair locus upon the expression of the neighboring Hoxc11 and Hoxc12 genes in cis. Our results do not support any substantial role for Hotair during mammalian development in vivo. Instead, they argue in favor of a DNA-dependent effect of the Hotair deletion upon the transcriptional landscape in cis.

During mammalian embryonic development, Hox genes must be tightly regulated. It was proposed earlier that part of this regulation relies upon Hotair, a long non-coding RNA that recruits repressive protein complexes onto the HoxD gene cluster to keep these genes silent before they become activated. A genetic deletion of Hotair in mice induced homeotic transformations, thus supporting this hypothesis. However, other alleles involving this locus gave controversial results and hence we re-assessed the effect of the full deletion of Hotair in vivo. In our genetic background and using our analytical conditions, we could not confirm the reported morphological alterations, nor could we detect any mis-regulation of Hoxd genes in those fetal tissues where Hotair is detected in control animals. However, the genomic deletion induces the mis-regulation in-cis of the neighboring Hoxc11 and Hoxc12 genes, a side-effect which may underlie a weakly penetrant alteration observed in the shape of some tail vertebrae.

Coordinate regulation of microenvironmental stimuli and role of methylation in bone metastasis from breast carcinoma

The pathogenesis of bone metastasis is unclear, and much focus in metastatic biology and therapy relays on epigenetic alterations. Since DNA-methyltransferase blockade with 5-aza-2'-deoxycytidine (dAza) counteracts tumour growth, here we utilized dAza to clarify whether molecular events undergoing epigenetic control were critical for bone metastatization. In particular, we investigated the patterns of secreted-protein acidic and rich in cysteine (SPARC) and of Endothelin 1, affected by DNA methyltransferases in tumours, with the hypothesis that in bone metastasis a coordinate function of SPARC and Endothelin 1, if any occurs, was orchestrated by DNA methylation. To this purpose, we prepared a xenograft model with the clone 1833, derived from human-MDA-MB231 cells, and dAza administration slowed-down metastasis outgrowth. This seemed consequent to the reductions of SPARC and Endothelin 1 at invasive front and in the bone marrow, mostly due to loss of Twist. In the metastasis bulk Snail, partly reduced by dAza, might sustain Endothelin 1-SPARC cooperativity. Both SPARC and Endothelin 1 underwent post-translational control by miRNAs, a molecular mechanism that might explain the in vivo data. Ectopic miR29a reduced SPARC expression also under long-term dAza exposure, while Endothelin 1 down-regulation occurred in the presence of endogenous-miR98 expression. Notably, dAza effects differed depending on in vivo and in vitro conditions. In 1833 cells exposed to 30-days dAza, SPARC-protein level was practically unaffected, while Endothelin 1 induction depended on the 3'-UTR functionality. The blockade of methyltransferases leading to SPARC reduction in vivo, might represent a promising strategy to hamper early steps of the metastatic process affecting the osteogenic niche.

Tetrapod axial evolution and developmental constraints; Empirical underpinning by a mouse model

The tetrapod vertebral column has become increasingly complex during evolution as an adaptation to a terrestrial life. At the same time, the evolution of the vertebral formula became subject to developmental constraints acting on the size of the cervical and thoraco-lumbar regions. In the course of our studies concerning the evolution of Hox gene regulation, we produced a transgenic mouse model expressing fish Hox genes, which displayed a reduced number of thoraco-lumbar vertebrae and concurrent sacral homeotic transformations. Here, we analyze this mutant stock and conclude that the ancestral, pre-tetrapodial Hox code already possessed the capacity to induce vertebrae with sacral characteristics. This suggests that alterations in the interpretation of the Hox code may have participated to the evolution of this region in tetrapods, along with potential modifications of the HOX proteins themselves. With its reduced vertebral number, this mouse stock violates a previously described developmental constraint, which applies to the thoraco-lumbar region. The resulting offset between motor neuron morphology, vertebral patterning and the relative positioning of hind limbs illustrates that the precise orchestration of the Hox-clock in parallel with other ontogenetic pathways places constraints on the evolvability of the body plan.

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A transgenic mouse line expressing fish Hox genes has anterior homeotic transformations.

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Fish Hox genes are capable of inducing tetrapod specific vertebral characters.

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A sacral Hox-code influences adult hindlimb position, yet not the position of limb budding.

TWIST1 and TWIST2 regulate glycogen storage and inflammatory genes in skeletal muscle

TWIST proteins are important for development of embryonic skeletal muscle and play a role in the metabolism of tumor and white adipose tissue. The impact of TWIST on metabolism in skeletal muscle is incompletely studied. Our aim was to assess the impact of TWIST1 and TWIST2 overexpression on glucose and lipid metabolism. In intact mouse muscle, overexpression of Twist reduced total glycogen content without altering glucose uptake. Expression of TWIST1 or TWIST2 reduced Pdk4 mRNA, while increasing mRNA levels of Il6, Tnfα, and Il1β. Phosphorylation of AKT was increased and protein abundance of acetyl CoA carboxylase (ACC) was decreased in skeletal muscle overexpressing TWIST1 or TWIST2. Glycogen synthesis and fatty acid oxidation remained stable in C2C12 cells overexpressing TWIST1 or TWIST2. Finally, skeletal muscle mRNA levels remain unaltered in ob/ob mice, type 2 diabetic patients, or in healthy subjects before and after 3 months of exercise training. Collectively, our results indicate that TWIST1 and TWIST2 are expressed in skeletal muscle. Overexpression of these proteins impacts proteins in metabolic pathways and mRNA level of cytokines. However, skeletal muscle levels of TWIST transcripts are unaltered in metabolic diseases.

The basic helix-loop-helix (bHLH) transcription factor DEC2 negatively regulates Twist1 through an E-box element

Differentiated embryo chondrocyte 2 (DEC2/Sharp-1/Bhlhe41), a basic helix-loop-helix (bHLH) transcription factor, has been shown to regulate the transcription of target genes by binding to their E-box elements. We identified a possible DEC2-response element (consensus E-box: CACGTG) in the promoter region of Twist1. Forced expression of DEC2 significantly repressed Twist1 promoter activity under normoxia and under hypoxia as assessed by a luciferase reporter assay. In addition, over-expression of DEC2 repressed Twist1 mRNA expression assessed by quantitative real-time PCR. Site-directed mutagenesis studies showed that mutagenesis of the consensus E-box sequence eliminated the ability of DEC2 to reduce the Twist1 promoter activity. Chromatin immunoprecipitation (ChIP) assays confirmed that the DEC2-mediated repression is primarily achieved by binding to the E-box in the Twist1 promoter. Knockdown of DEC2 by siRNA significantly attenuated the repression of Twist1 expression. DEC2 and Twist1 exhibit inversed protein expression patterns during development of mouse tongue embryo tissue. Given the fact that DEC2 protein is emerging as an important regulator in a vast array of cellular events, including cell differentiation, maturation of lymphocytes and the molecular clock, our study elucidates an important mechanism by which DEC2 regulates cellular function by modulating the expression of Twist1.

Twist reverses muscle cell differentiation through transcriptional down-regulation of myogenin

Some higher vertebrates can display unique muscle regenerative abilities through dedifferentiation. Research evidence suggests that induced dedifferentiation can be achieved in mammalian cells. TWIST is a bHLH (basic helix-loop-helix) transcription factor that is expressed during embryonic development and plays critical roles in diverse developmental systems including myogenesis. Several experiments demonstrated its role in inhibition of muscle cell differentiation. We have previously shown that overexpression of TWIST can reverse muscle cell differentiation in the presence of growth factors. Here we show that TWIST reverses muscle cell differentiation through binding and down-regulation of myogenin. Moreover, it can reverse cellular morphology in the absence of growth factors.

The proto-oncogene TWIST1 is regulated by microRNAs

Upregulation of the proto-oncogene Twist1 is highly correlated with acquired drug resistance and poor prognosis in human cancers. Altered expression of this multifunctional transcription factor is also associated with inherited skeletal malformations. The mammalian Twist1 3′UTRs are highly conserved and contain a number of potential regulatory elements including miRNA target sites. We analyzed the translational regulation of TWIST1 using luciferase reporter assays in a variety of cell lines. Among several miRNAs tested, miR-145a-5p, miR-151-5p and a combination of miR-145a-5p + miR-151-5p and miR-151-5p + miR-337-3p were able to significantly repress Twist1 translation. This phenomena was confirmed with both exogenous and endogenous miRNAs and was dependent on the presence of the predicted target sites in the 3′UTR. Furthermore, the repression was sensitive to LNA-modified miRNA antagonists and resulted in decreased migratory potential of murine embryonic fibroblast cells. Understanding the in vivo mechanisms of this oncogene's regulation might open up a possibility for therapeutic interference by gene specific cancer therapies.

Snail regulates MyoD binding-site occupancy to direct enhancer switching and differentiation-specific transcription in myogenesis

In skeletal myogenesis, the transcription factor MyoD activates distinct transcriptional programs in progenitors compared to terminally differentiated cells. Using ChIP-Seq and gene expression analyses, we show that in primary myoblasts, Snail-HDAC1/2 repressive complex binds and excludes MyoD from its targets. Notably, Snail binds E box motifs that are G/C rich in their central dinucleotides, and such sites are almost exclusively associated with genes expressed during differentiation. By contrast, Snail does not bind the A/T-rich E boxes associated with MyoD targets in myoblasts. Thus, Snai1-HDAC1/2 prevent MyoD occupancy on differentiation-specific regulatory elements, and the change from Snail to MyoD binding often results in enhancer switching during differentiation. Furthermore, we show that a regulatory network involving myogenic regulatory factors (MRFs), Snai1/2, miR-30a, and miR-206 acts as a molecular switch that controls entry into myogenic differentiation. Together, these results reveal a regulatory paradigm that directs distinct gene expression programs in progenitors versus terminally differentiated cells.

Twist contributes to hormone resistance in breast cancer by downregulating estrogen receptor-α

The role of estrogen receptor-α (ER) in breast cancer development, and as a primary clinical marker for breast cancer prognosis, has been well documented. In this study, we identified the oncogenic protein, TWIST1 (Twist), which is overexpressed in high-grade breast cancers, as a potential negative regulator of ER expression. Functional characterization of ER regulation by Twist was performed using Twist low (MCF-7, T-47D) and Twist high (Hs 578T, MDA-MB-231, MCF-7/Twist) expressing cell lines. All Twist high expressing cell lines exhibited low ER transcript and protein levels. By chromatin immunoprecipitation and promoter assays, we demonstrated that Twist could directly bind to E-boxes in the ER promoter and significantly downregulate ER promoter activity in vitro. Functionally, Twist overexpression caused estrogen-independent proliferation of breast cells, and promoted hormone resistance to the selective estrogen receptor modulator tamoxifen and selective estrogen receptor down-regulator fulvestrant. Importantly, this effect was reversible on downregulating Twist. In addition, orthotopic tumors generated in mice using MCF-7/Twist cells were resistant to tamoxifen. These tumors had high vascular volume and permeability surface area, as determined by magnetic resonance imaging (MRI). Mechanistically, Twist recruited DNA methyltransferase 3B (DNMT3B) to the ER promoter, leading to a significantly higher degree of ER promoter methylation compared with parental cells. Furthermore, we demonstrated by co-immunoprecipitation that Twist interacted with histone deacetylase 1 (HDAC1) at the ER promoter, causing histone deacetylation and chromatin condensation, further reducing ER transcript levels. Functional re-expression of ER was achieved using the demethylating agent, 5-azacytidine, and the HDAC inhibitor, valproic acid. Finally, an inverse relationship was observed between Twist and ER expression in human breast tumors. In summary, the regulation of ER by Twist could be an underlying mechanism for the loss of ER activity observed in breast tumors, and may contribute to the generation of hormone-resistant, ER-negative breast cancer.

Endogenous TWIST expression and differentiation are opposite during human muscle development

TWIST is a transcription factor expressed during early embryonic development. In this study we investigate the expression of TWIST during human muscle development. Human TWIST was found to be endogenously expressed in human fetal myoblasts, and its expression decreased during late stages of development. Myoblasts showed an increasing capacity to differentiate in vitro during development. This inversely proportional relation between TWIST and differentiation capacity of myoblasts suggests that TWIST is involved in the regulation of muscle development.

Extrinsic versus intrinsic cues in avian paraxial mesoderm patterning and differentiation

Somitic and head mesoderm contribute to cartilage and bone and deliver the entire skeletal musculature. Studies on avian somite patterning and cell differentiation led to the view that these processes depend solely on cues from surrounding tissues. However, evidence is accumulating that some developmental decisions depend on information within the somitic tissue itself. Moreover, recent studies established that head and somitic mesoderm, though delivering the same tissue types, are set up to follow their own, distinct developmental programmes. With a particular focus on the chicken embryo, we review the current understanding of how extrinsic signalling, operating in a framework of intrinsically regulated constraints, controls paraxial mesoderm patterning and cell differentiation.

Mechanism of transcriptional activation by the proto-oncogene Twist1

Mammalian Twist1, a master regulator in development and a key factor in tumorigenesis, is known to repress transcription by several mechanisms and is therefore considered to mediate its function mainly through inhibition. A role of Twist1 as transactivator has also been reported but, so far, without providing a mechanism for such an activity. Here we show that heterodimeric complexes of Twist1 and E12 mediate E-box-dependent transcriptional activation. We identify a novel Twist1 transactivation domain that coactivates together with the less potent E12 transactivation domain. We found three specific residues in the highly conserved WR domain to be essential for the transactivating function of murine Twist1 and suggest an alpha-helical structure of the transactivation domain.

Comparative roles of Twist-1 and Id1 in transcriptional regulation by BMP signaling

Basic helix-loop-helix (bHLH) transcription factors are known as key regulators for mesenchymal differentiation. The present study showed that overexpression of Twist-1, a bHLH transcription factor, suppresses bone morphogenetic protein (BMP)-induced osteoblast differentiation, and downregulation of endogenous Twist-1 enhances BMP signaling. Maximal inhibition of BMP signaling was observed when Twist-1 was bound to E47, which markedly enhanced the stability of Twist-1. Co-immunoprecipitation assays revealed that Twist-1 formed a complex with Smad4 and histone deacetylase (HDAC) 1 in MC3T3-E1 cells stably expressing Twist-1. With trichostatin, an HDAC inhibitor, osteogenic factors such as alkaline phosphatase, Runx2 and osteopontin increased. Those results suggested that Twist-1 inhibited BMP signaling by recruiting HDAC1 to Smad4. Furthermore, the inhibitory effects of Twist-1 on BMP signaling were overcome by Id1 through induction of Twist-1 degradation. These findings suggest that Twist-1 can act as an inhibitor of BMP signaling, and Id1 can regulate BMP signaling through a positive feedback loop repressing Twist-1 function. These two molecules may therefore regulate differentiation of mesenchymal cells into progeny such as osteoblasts by controlling BMP signaling.

The molecular setup of the avian head mesoderm and its implication for craniofacial myogenesis

The head mesoderm is the mesodermal tissue on either side of the brain, from forebrain to hindbrain levels, and gives rise to the genuine head muscles. Its relatedness to the more posterior paraxial mesoderm, the somites, which generate the muscles of the trunk, is conversely debated. To gain insight into the molecular setup of the head mesoderm, its similarity or dissimilarity to the somitic mesoderm, and the implications of its setup for the progress of muscle formation, we investigated the expression of markers (1) for mesoderm segmentation and boundary formation, (2) for regional specification and somitogenesis and (3) for the positive and negative control of myogenic differentiation. We show that the head mesoderm is molecularly distinct from somites. It is not segmented; even the boundary to the first somite is ill-defined. Importantly, the head mesoderm lacks the transcription factors driving muscle differentiation while genes suppressing differentiation and promoting cell proliferation are expressed. These factors show anteroposteriorly and dorsoventrally regionalised but overlapping expression. Notably, expression extends into the areas that actively contribute to the heart, overlapping with the expression of cardiac markers.

TWIST is expressed in human gliomas and promotes invasion

TWIST, a basic helix-loop-helix (bHLH) transcription factor that regulates mesodermal development, has been shown to promote tumor cell metastasis and to enhance survival in response to cytotoxic stress. Our analysis of rat C6 glioma cell-derived cDNA revealed TWIST expression, suggesting that the gene may play a role in the genesis and physiology of primary brain tumors. To further delineate a possible oncogenic role for TWIST in the central nervous system (CNS), we analyzed TWIST expression in human gliomas and normal brain by using reverse transcription polymerase chain reaction, Northern blot analysis, in situ hybridization, and immunohistochemistry. TWIST expression was detected in the large majority of human glioma-derived cell lines and human gliomas examined. Levels of TWIST mRNA were associated with the highest grade gliomas, and increased TWIST expression accompanied transition from low grade to high grade in vivo, suggesting a role for TWIST in promoting malignant progression. In accord, elevated TWIST mRNA abundance preceded the spontaneous malignant transformation of cultured mouse astrocytes hemizygous for p53. Overexpression of TWIST protein in a human glioma cell line significantly enhanced tumor cell invasion, a hallmark of high-grade gliomas. These findings support roles for TWIST both in early glial tumorigenesis and subsequent malignant progression. TWIST was also expressed in embryonic and fetal human brain, and in neurons, but not glia, of mature brain, indicating that, in gliomas, TWIST may promote the functions also critical for CNS development or normal neuronal physiology.

A role for Id in the regulation of TGF-beta-induced epithelial-mesenchymal transdifferentiation

Epithelial-mesenchymal transdifferentiation (EMT) is a critical morphogenic event that occurs during embryonic development and during the progression of various epithelial tumors. EMT can be induced by transforming growth factor (TGF)-beta in mouse NMuMG mammary epithelial cells. Here, we demonstrate a central role of helix-loop-helix factors, E2A and inhibitor of differentiation (Id) proteins, in TGF-beta-induced EMT. Epithelial cells ectopically expressing E2A adopt a fibroblastic phenotype and acquire migratory/invasive properties, concomitant with the suppression of E-cadherin expression. Id proteins interacted with E2A proteins and antagonized E2A-dependent suppression of the E-cadherin promoter. Levels of Id proteins were dramatically decreased by TGF-beta. Moreover, NMuMG cells overexpressed Id2 showed partial resistance to TGF-beta-induced EMT. Id proteins thus inhibit the action of E2A proteins on the expression of E-cadherin, but after TGF-beta stimulation, E2A proteins are present in molar excess of the Id proteins, thus over-riding their inhibitory function and leading to EMT.

cDermo-1 misexpression induces dense dermis, feathers, and scales

Reciprocal epithelio-mesenchymal interactions between the prospective epidermis and the underlying dermis are the major driving forces in the development of skin appendages. Feather development is initiated by a still unknown signal from the dermis in feather-forming skin. The morphological response of the ectoderm to this signal is the formation of an epidermal placode, which signals back to the mesenchyme to induce dermal condensations. Together, epidermal and dermal components constitute the outgrowing feather bud. The bHLH transcription factor cDermo-1 is expressed in developing dermis and is the earliest known marker of prospective feather tracts. To test its function during feather development, we forced cDermo-1 expression in embryonic chicken dermis using a retroviral expression vector. In featherless (apteric) regions, cDermo-1 misexpression induced dense, thickened dermis normally observed in feathered skin (pterylae), and leads to the development of regularly spaced and normally shaped ectopic feather buds. In pterylae, cDermo-1 misexpression enhanced feather growth. In hindlimb skin, according to the local skin identity, misexpression of cDermo-1 induced ectopic scale formation. Thus, we show that forced cDermo-1 expression in developing dermis is sufficient to launch the developmental program leading to skin appendage formation. We propose a role of cDermo-1 at the initial stages of feather induction upstream of FGF10.

Twist is an integrator of SHH, FGF, and BMP signaling

Development of vertebrate embryos is regulated by a number of different signaling pathways. These pathways are frequently not independent of each other but are connected by crosstalk between cells and tissues. Furthermore, different signaling pathways have been found to interact at the cellular level. Development of cranial and limb structures is an example, in which FGF, BMP, and SHH signaling interact. Mutations in the different signaling pathways may therefore result in complex but similar phenotypes. This indicates the existence of integrator molecules, which depend in their expression or activity on the combination of different signaling pathways. Here we show that expression of the bHLH transcription factor Twist in the paraxial mesoderm requires an induction from the notochord. This induction can only be substituted by a combination of FGF and SHH signaling, but not by individual application of FGF8 or SHH alone. Furthermore, the expression of Twist can be modified by BMP2 in a complex, age-dependent manner. We propose that Twist is one of the integrating parts of the three signaling pathways and mediates some of the common effects.

In vivo filtering of in vitro expression data reveals MyoD targets

A published set of downstream targets of MyoD defined in a well-controlled in vitro experiment was filtered for relevance to muscle regeneration using a 27-time-point in vivo murine regeneration series. Using interactive hierarchical and Bayes soft clustering, only a minority of the targets defined in vitro can be confirmed in vivo (approximately 50% of induced transcripts, and none of repressed transcripts). This approach provided strong support that 18 targets including of MyoD are biologically relevant during myoblast differentiation.

Inhibition of skeletal muscle development: less differentiation gives more muscle

The fact that stem cells have to be protected from premature differentiation is true for many organs in the developing embryo and the adult organism. However, there are several arguments that this is particularly important for (skeletal) muscle. There are some evolutionary arguments that muscle is a "default" pathway for mesodermal cells, which has to be actively prevented in order to allow cells to differentiate into other tissues. Myogenic cells originate from very small areas of the embryo where only a minor portion of these cells is supposed to differentiate. Differentiated muscle fibres are unconditionally post-mitotic, leaving undifferentiated stem cells as the only source of regeneration. The mechanical usage of muscle and its superficial location in the vertebrate body makes regeneration a frequently used mechanism. Looking at the different inhibitory mechanisms that have been found within the past 10 or so years, it appears as if evolution has taken this issue very serious. At all possible levels we find regulatory mechanisms that help to fine tune the differentiation of myogenic cells. Secreted molecules specifying different populations of somitic cells, diffusing or membrane-bound signals among fellow myoblasts, modulating molecules within the extracellular matrix and last, but not least, a changing set of activating and repressing cofactors. We have come a long way from the simple model of MyoD just to be turned on at the right time in the right cell.

Genome-wide examination of myoblast cell cycle withdrawal during differentiation

Skeletal and cardiac myocytes cease division within weeks of birth. Although skeletal muscle retains limited capacity for regeneration through recruitment of satellite cells, resident populations of adult myocardial stem cells have not been identified. Because cell cycle withdrawal accompanies myocyte differentiation, we hypothesized that C2C12 cells, a mouse myoblast cell line previously used to characterize myocyte differentiation, also would provide a model for studying cell cycle withdrawal during differentiation. C2C12 cells were differentiated in culture medium containing horse serum and harvested at various time points to characterize the expression profiles of known cell cycle and myogenic regulatory factors by immunoblot analysis. BrdU incorporation decreased dramatically in confluent cultures 48 hr after addition of horse serum, as cells started to form myotubes. This finding was preceded by up-regulation of MyoD, followed by myogenin, and activation of Bcl-2. Cyclin D1 was expressed in proliferating cultures and became undetectable in cultures containing 40% fused myotubes, as levels of p21(WAF1/Cip1) increased and alpha-actin became detectable. Because C2C12 myoblasts withdraw from the cell cycle during myocyte differentiation following a course that recapitulates this process in vivo, we performed a genome-wide screen to identify other gene products involved in this process. Using microarrays containing approximately 10,000 minimally redundant mouse sequences that map to the UniGene database of the National Center for Biotechnology Information, we compared gene expression profiles between proliferating, differentiating, and differentiated C2C12 cells and verified candidate genes demonstrating differential expression by RT-PCR. Cluster analysis of differentially expressed genes revealed groups of gene products involved in cell cycle withdrawal, muscle differentiation, and apoptosis. In addition, we identified several genes, including DDAH2 and Ly-6A, whose expression specifically was up-regulated during cell cycle withdrawal coincident with early myoblast differentiation.

N-twist, an evolutionarily conserved bHLH protein expressed in the developing CNS, functions as a transcriptional inhibitor

Members of the basic helix-loop-helix (bHLH) transcription factor family play an essential role in multiple developmental processes. During neurogenesis, positive and negative regulation by bHLH proteins is essential for proper development. Here we report the identification and initial characterization of the bHLH gene, Neuronal twist (N-twist), named for its neural expression pattern and high sequence homology and physical linkage to the mesodermal inhibitor, M-twist. N-twist is expressed in the developing mouse central nervous system in the midbrain, hindbrain, and neural tube. This neural expression is conserved in invertebrates, as expression of the Drosophila ortholog of N-twist is also restricted to the central nervous system. Like other bHLH family members, N-Twist heterodimerizes with E protein and binds DNA at a consensus bHLH-binding site, the E box. We show that N-Twist inhibits MASH1-dependent transcriptional activation by sequestering E protein in a dominant negative fashion. Thus, these studies support the notion that N-Twist represents a novel negative regulator of neurogenesis.

Twist function is required for the morphogenesis of the cephalic neural tube and the differentiation of the cranial neural crest cells in the mouse embryo

Loss of Twist function in the cranial mesenchyme of the mouse embryo causes failure of closure of the cephalic neural tube and malformation of the branchial arches. In the Twist(-/-) embryo, the expression of molecular markers that signify dorsal forebrain tissues is either absent or reduced, but those associated with ventral tissues display expanded domains of expression. Dorsoventral organization of the mid- and hindbrain and the anterior-posterior pattern of the neural tube are not affected. In the Twist(-/-) embryo, neural crest cells stray from the subectodermal migratory path and the late-migrating subpopulation invades the cell-free zone separating streams of cells going to the first and second branchial arches. Cell transplantation studies reveal that Twist activity is required in the cranial mesenchyme for directing the migration of the neural crest cells, as well as in the neural crest cells within the first branchial arch to achieve correct localization. Twist is also required for the proper differentiation of the first arch tissues into bone, muscle, and teeth.

Isolation of differentially expressed genes from wild-type and Twist mutant mouse limb buds

In the mouse, Twist is required for normal limb and craniofacial development. We show that the aristaless-like transcription factors, Alx3 and Alx4 are downregulated in the Twist(-/-) mutant and may be potential targets of Twist. By suppression subtractive hybridization we isolated 31 and 18 unique clones representing mRNAs that are putatively downregulated and upregulated respectively in Twist(-/-) forelimb buds. These included genes encoding cytoskeletal components, metabolic enzymes, hemoglobin molecules, membrane transport proteins, components of transcription and translation complexes, protein modification enzymes and proteins related to cell proliferation and apoptosis. Differential expression of selected clones was validated by whole mount in situ hybridization to E10.5 wild-type and Twist(-/-) embryos. We show that four novel clones are expressed in the Twist-expressing craniofacial tissues and paraxial mesoderm and downregulated in Twist(-/-) embryos, raising the possibility that they are, in addition to genes of the Alx family, downstream targets of Twist.

Phosphorylation of pRb is required for HGF-induced muscle cell proliferation and is p27kip1-dependent

Hepatocyte growth factor (HGF) plays a crucial role in the differentiation of skeletal muscle cells, where a process in which the retinoblastoma protein (pRb) has been implicated. We addressed the role of pRb in HGF-mediated effects on the proliferation and differentiation of adult skeletal muscle myoblasts. HGF shifted pRb to its hyperphosphorylation forms and increased the transactivation of E2F1, a transcription factor required for S phase entry. A constitutively active pRb mutant blocked HGF-dependent pRb phosphorylation and transactivation of E2F1 and increased cell proliferation. Accordingly, this mutant reversed the inhibitory effects of HGF on the expression of the cyclin-dependent kinase (CDK) inhibitor p27 and myogenic differentiation markers. HGF-mediated pRb phosphorylation was reversed by ectopic expression of p27, but neither the myogenic regulatory factor, MEF2, nor the myogenic inhibitory protein Twist had that effect. These results suggest that in response to HGF signaling, there is a decrease in p27 expression that results in an accumulation of hyperphosphorylated Rb protein, and subsequent progression of myoblasts into the G1 phase of the cell cycle.

Dermo-1, a multifunctional basic helix-loop-helix protein, represses MyoD transactivation via the HLH domain, MEF2 interaction, and chromatin deacetylation

Dermo-1 is a multifunctional basic helix-loop-helix (bHLH) transcription factor that has been shown to be a potent negative regulator for gene transcription and apoptosis. To understand the molecular mechanisms that mediate the function of Dermo-1, we generated a series of Dermo-1 mutants and used a MyoD-mediated transcriptional activation model to characterize the roles of its N-terminal, bHLH, and C-terminal structural domains in transcriptional repression. Both the C-terminal and HLH domains of Dermo-1 were essential for its repression of MyoD-mediated transactivation. Dermo-1 repressed, in a dose-dependent fashion, the transactivation activity of myocyte enhancer factor 2 (MEF2), a protein known to cooperate with MyoD in activating E-box-dependent gene expression. Both the N- and C-terminal domains of Dermo-1, but not the bHLH domain, were required for the inhibition of MEF2, suggesting that Dermo-1 inhibits both MyoD- and MEF2-dependent transactivation but through different mechanisms. Dermo-1 interacted directly with MEF2 and selectively repressed the MEF2 transactivation domain. An overall increase of histone acetylation induced by trichostatin A treatment reduced Dermo-1 transcriptional repression activity, suggesting that histone deacetylation is involved in Dermo-1-mediated transcriptional repression. Together, these results suggest that MEF2 is an important target in Dermo-1-mediated transcriptional repression and provide initial evidence of the involvement of histone acetylation in Dermo-1 transcriptional repression.

Cooperative E-box regulation of human GLI1 by TWIST and USF

Sonic hedgehog signaling plays a critical role in vertebrate patterning, and signaling defects are associated with severe birth defects and cancer in man. GLI1 encodes a critical transcription activator in this pathway. GLI1 is expressed in human basal cell carcinomas and sarcomas. Despite the significance of the GLI1 gene in human disease, few immediate upstream regulators of GLI1 expression are known. We previously demonstrated that a 5' region, including 5' flanking sequence, an untranslated exon, and 425 bp of the first intron, regulates the human GLI1 gene. Here we show that inactivating mutations in E-box, GC box, AP-2, GATA, GSG, PuF, and Zeste sites identified three critical regulatory elements, including a GC box that binds Sp1 and two intronic E-boxes that bind USF proteins or Twist. Expression of Twist but not a frame shift mutation of Twist activates the wild-type human GLI1 regulatory sequences but not with inactivating mutations of the E-boxes. Twist activates GLI1 reporter expression through E-box +482 but requires binding of USF proteins to E-box +157. Twist mutations cause human birth defects and Twist is overexpressed in many rhabdomyosarcomas, suggesting that one of Twist's primary roles is the regulation of GLI1.

BHLHB4 is a bHLH transcriptional regulator in pancreas and brain that marks the dimesencephalic boundary

We have cloned a basic helix-loop-helix (bHLH) factor gene, Bhlhb4, from a mouse beta-cell line. Fluorescence in situ hybridization (FISH) and genetic mapping place Bhlhb4 at the telomeric end of mouse chromosome 2 (H3-H4), syntenic to human chromosome 20q13. Based on phylogenetic analysis, BHLHB4 belongs to a new subgroup of bHLH factors including at least four previously identified mouse bHLH factors: BHLHB5, MIST1, OLIG1, OLIG2, and OLIG3. In the developing nervous system, Bhlhb4 was found to mark the dimesencephalic boundary, suggesting that Bhlhb4 may have a role in diencephalic regionalization. In the pancreas, Bhlhb4 is expressed in a transient fashion that suggests a role in the pancreatic endocrine cell lineage. Transfection experiments show that BHLHB4 can repress transcriptional activation mediated through the pancreatic beta-cell specific insulin promoter enhancer RIPE3. Together, these data suggest that BHLHB4 may modulate the expression of genes required for the differentiation and/or maintenance of pancreatic and neuronal cell types.

Increased myogenic repressor Id mRNA and protein levels in hindlimb muscles of aged rats

The objective of this study was to determine if levels of repressors to myogenic regulatory factors (MRFs) differ between muscles from young adult and aged animals. Total RNA from plantaris, gastrocnemius, and soleus muscles of Fischer 344 x Brown Norway rats aged 9 mo (young adult, n = 10) and 37 mo (aged, n = 10) was reverse transcribed and then amplified by PCR. To obtain a semiquantitative measure of the mRNA levels, PCR signals were normalized to cyclophilin or 18S signals from the corresponding reverse transcription product. Normalization to cyclophilin and 18S gave similar results. The mRNA levels of MyoD and myogenin were approximately 275-650% (P < 0.001) and approximately 500-1,100% (P < 0.001) greater, respectively, in muscles from aged compared with young adults. In contrast, the protein levels were lower in plantaris and gastrocnemius muscles and similar in the soleus muscle of aged vs. young adult rats. Id repressor mRNA levels were approximately 300-900% greater in fast and slow muscles of aged animals (P < or = 0.02), and Mist 1 mRNA was approximately 50% greater in the plantaris and gastrocnemius muscles (P < 0.01). The mRNA level of Twist mRNA was not significantly affected by aging. Id-1, Id-2, and Id-3 protein levels were approximately 17-740% greater (P < 0.05) in hindlimb muscles of aged rats compared with young adult rats. The elevated levels of Id mRNA and protein suggest that MRF repressors may play a role in gene regulation of fast and slow muscles in aged rats.

Opposing functions of ATF2 and Fos-like transcription factors in c-Jun-mediated myogenin expression and terminal differentiation of avian myoblasts

With the aim to identify the oncoprotein partners implicated in the c-Jun myogenic influence, we carried out stable transfection experiments of c-Jun and/or ATF2, Fra2, c-Fos overexpression in avian myoblasts. Before induction of differentiation, c-Jun repressed myoblast withdrawal from the cell cycle, as did a TPA treatment. However, after serum removal, unlike TPA, c-Jun significantly stimulated myoblast differentiation. In search for specific partners involved in this dual influence, we found that a reduction in the amounts of c-Fos and Fra2 and an increase in c-Jun proteins occurred at cell confluence, a situation likely to favor cooperation between c-Jun and ATF2 during terminal differentiation. Whereas c-Fos and Fra2 cooperated with c-Jun to abrogate myoblast withdrawal from the cell cycle and terminal differentiation, ATF2 co-expression potentiated the positive myogenic c-Jun influence. In addition, myogenin expression was a positive target of this cooperation and this regulation occurred through a stimulation of myogenin promoter activity: (1) whereas c-Fos or Fra2 co-expression abrogated c-Jun stimulatory activity on this promoter, ATF2 co-expression potentiated this influence; (2) using a dominant negative ATF2 mutant, we established that c-Jun transcriptional activity required functionality of endogenous ATF2. These data suggest that through this dual myogenic influence due to cooperations with different partners, c-Jun is involved in the control of duration of myoblast proliferation and thereafter of fusion efficiency.

Genomic expression analysis implicates Wnt signaling pathway and extracellular matrix alterations in hepatic specification and differentiation of murine hepatic stem cells

HBC-3 hepatic stem cells maintained in the undifferentiated state can be induced to differentiate along the hepatocyte lineage in response to DMSO (Rogler, 1997). In order to understand the complex transcriptional regulatory mechanisms associated with the differentiation of these somatic stem cells and to identify novel candidate stem cell and differentiation associated genes, we have begun to characterize the transcriptome of HBC-3 cells during a 7-day differentiation protocol. This analysis showed that differentiating HBC-3 cells undergo biphasic bursts of gene regulation peaking at 3 hours and 120 hours of DMSO treatment. In the undifferentiated state, HBC-3 cells express muscle, neuron, myeloid, and lymphoid specific genes that are rapidly downregulated during hepatocytic differentiation. Cluster analysis has revealed large groups of genes with different temporal regulation profiles demonstrating complex and widespread transcriptional changes. Specifically, we discovered a multifaceted downregulation of the Wnt/beta-catenin pathway accompanied by the repression of TCF target genes during HBC-3 differentiation. In addition, there is downregulation of cellular receptors for fibronectin and laminin and other extracellular matrix molecules indicative of widespread cell surface alterations. DMSO induces cell cycle arrest, and this is reflected in upregulation of growth inhibitory proteins such as cyclin I and p18 and downregulation of cyclins B1 and D. Genes needed for hepatocytic functions, such as apolipoprotein C-IV, phosphoenolpyruvate carboxykinase, alcohol dehydrogenase, and asialoglycoprotein receptor were upregulated. Finally, transcriptional regulators including Twist, Snail, HNF1a, and GATA6 were upregulated during differentiation of HBC-3 cells. The significance of these findings is that our genome-based approach has allowed the parallel identification of multiple regulatory pathways that is needed to begin to fully understand the complex differentiation process.

Insulin-like growth factor 1 (IGF-1)-induced twist expression is involved in the anti-apoptotic effects of the IGF-1 receptor

In this study we investigated the molecular mechanisms whereby insulin-like growth factor 1 (IGF-1) induced Twist gene expression and the role of Twist in the anti-apoptotic actions of the IGF-1 receptor. In NIH-3T3 fibroblasts overexpressing the human IGF-1 receptor (NWTb3), treatment with IGF-1 (10(-8) m) for 1 and 4 h increased the level of Twist mRNA as well as protein by 3-fold. In contrast, insulin at physiological concentrations did not stimulate Twist expression in NIH-3T3 fibroblasts overexpressing the human insulin receptor. The IGF-1 effect was specific for the IGF-1 receptor since, in cells overexpressing a dominant negative IGF-1 receptor, IGF-1 failed to increase Twist expression. Pre-incubation with the ERK1/2 inhibitor U0126 or expression of a dominant negative MEK-1 abolished the effect of IGF-1 on Twist mRNA expression in NWTb3 cells, suggesting that Twist induction by IGF-1 occurs via the mitogen-activated protein kinase signaling pathway. In vivo, IGF-1 injection increased the mRNA level of Twist in mouse skeletal muscle, the major site of Twist expression. Finally, using an antisense strategy, we demonstrated that a reduction of 40% in Twist expression decreased significantly the ability of IGF-1 to rescue NWTb3 cells from etoposide-induced apoptosis. Taken together, these results define Twist as an important factor involved in the anti-apoptotic actions of the IGF-1 receptor.

Increased bone formation and decreased osteocalcin expression induced by reduced Twist dosage in Saethre-Chotzen syndrome

The Saethre-Chotzen syndrome is characterized by premature fusion of cranial sutures resulting from mutations in Twist, a basic helix-loop-helix (bHLH) transcription factor. We have identified Twist target genes using human mutant calvaria osteoblastic cells from a child with Saethre-Chotzen syndrome with a Twist mutation that introduces a stop codon upstream of the bHLH domain. We observed that Twist mRNA and protein levels were reduced in mutant cells and that the Twist mutation increased cell growth in mutant osteoblasts compared with control cells. The mutation also caused increased alkaline phosphatase and type I collagen expression independently of cell growth. During in vitro osteogenesis, Twist mutant cells showed increased ability to form alkaline phosphatase-positive bone-like nodular structures associated with increased type I collagen expression. Mutant cells also showed increased collagen synthesis and matrix production when cultured in aggregates, as well as an increased capacity to form a collagenous matrix in vivo when transplanted into nude mice. In contrast, Twist mutant osteoblasts displayed a cell-autonomous reduction of osteocalcin mRNA expression in basal conditions and during osteogenesis. The data show that genetic deletion of Twist causing reduced Twist dosage increases cell growth, collagen expression, and osteogenic capability, but inhibits osteocalcin gene expression. This provides one mechanism that may contribute to the premature cranial ossification induced by deletion of the bHLH Twist domain in Saethre-Chotzen syndrome.

Mutations in the basic domain and the loop-helix II junction of TWIST abolish DNA binding in Saethre-Chotzen syndrome

Saethre-Chotzen syndrome is an autosomal dominant skull disorder resulting from premature fusion of coronal sutures (craniosynostosis). It is caused by mutations in the TWIST gene encoding a basic Helix-Loop-Helix transcription factor. Here we report on the identification of a novel mutation affecting a highly conserved residue of the basic domain. Unlike nonsense and missense mutations lying within helices, this mutation does not affect protein stability or heterodimerisation of TWIST with its partner E12. However, it does abolish TWIST binding capacity to a target E-box as efficiently as two missense mutations in the loop-helix II junction. By contrast, elongation of the loop through a 7 amino acid insertion appears not to hamper binding to the DNA target. We conclude that loss of TWIST protein function in Saethre-Chotzen patients can occur at three different levels, namely protein stability, dimerisation, and DNA binding and that the loop-helix II junction is essential for effective protein-DNA interaction.

Analysis of the inhibition of MyoD activity by ITF-2B and full-length E12/E47

MyoD heterodimerizes with E type factors (E12/E47 and ITF-2A/ITF-2B) and binds E box sequences within promoters of muscle-specific genes. In transient transfection assays, MyoD activates transcription in the presence of ITF-2A but not ITF-2B, which contains a 182-amino acid N-terminal extension. The first 83 amino acids of the inhibitory N terminus of ITF-2B show high sequence homology to the N terminus of full-length E12/E47. Previous studies that showed activation of MyoD by E12 used an artificially N-terminally truncated form. Here we show that the full-length form of E12 inhibits MyoD function. A conserved alpha-helix motif, capable of interacting with the transcriptional machinery, was not essential for inhibition. Furthermore, the fusion of N-terminal ITF-2B sequences or non-inhibiting ITF-2A sequences to truncated E12 was sufficient in converting the activator into an inhibitor. Overexpression of ITF-2B did not inhibit C2C12 myogenesis or affect levels of endogenous muscle gene expression, consistent with the finding that inhibitory E type proteins are present in muscle. Furthermore, we found that MyoD co-transfected with either ITF-2B or ITF-2A converted fibroblasts into myoblasts with the same frequency. Our findings suggest that the ability of E type proteins to inhibit MyoD activity is dependent on the context of the E box.

MyoD(-/-) satellite cells in single-fiber culture are differentiation defective and MRF4 deficient

MyoD-deficient mice are without obvious deleterious muscle phenotype during embryogenesis and fetal development, and adults in the laboratory have grossly normal skeletal muscle and life span. However, a previous study showed that in the context of muscle degeneration on a mdx (dystrophin null) genetic background, animals lacking MyoD have a greatly intensified disease phenotype leading to lethality not otherwise seen in mdx mice. Here we have examined MyoD(-/-) adult muscle fibers and their associated satellite cells in single myofiber cultures and describe major phenotypic differences found at the tissue, cellular, and molecular levels. The steady-state number of satellite cells on freshly isolated MyoD(-/-) fibers was elevated and abnormal branched fiber morphologies were observed, the latter suggesting chronic muscle regeneration in vivo. Single-cell RNA coexpression analyses were performed for c-met, m-cadherin, and the four myogenic regulatory factors (MRFs.) Most mutant satellite cells entered the cell cycle and upregulated expression of myf5, both characteristic early steps in satellite cell maturation. However, they later failed to normally upregulate MRF4, displayed a major deficit in m-cadherin expression, and showed a significant diminution in myogenin-positive status compared with wildtype. MyoD(-/-) satellite cells formed unusual aggregate structures, failed to fuse efficiently, and showed greater than 90% reduction in differentiation efficiency relative to wildtype. A further survey of RNAs encoding regulators of growth and differentiation, cell cycle progression, and cell signaling revealed similar or identical expression profiles for most genes as well as several noteworthy differences. Among these, GDF8 and Msx1 were identified as potentially important regulators of the quiescent state whose expression profile differs between mutant and wildtype. Considered together, these data suggest that activated MyoD(-/-) satellite cells assume a phenotype that resembles in some ways a developmentally "stalled" cell compared to wildtype. However, the MyoD(-/-) cells are not merely developmentally immature, as they also display novel molecular and cellular characteristics that differ from any observed in wild-type muscle precursor counterparts of any stage.

Hepatocyte growth factor (HGF) inhibits skeletal muscle cell differentiation: a role for the bHLH protein twist and the cdk inhibitor p27

Hepatocyte growth factor (HGF) plays a crucial role in regulating the differentiation of both fetal and adult skeletal myoblasts. This study aimed at defining the intracellular factors that mediate the effect of HGF on adult myoblast differentiation. HGF increased Twist expression while decreasing p27(kip1) protein levels and not affecting the induction of p21(Cip1/Waf1) in satellite cells. Like HGF, overexpression of Twist did not affect p21 expression while inhibiting muscle-specific proteins. Both ectopic Twist-antisense (Twist-AS) and p27 partially rescued the effects of HGF on bromodeoxyuridine (BrdU) incorporation and myosin heavy chain (MHC) expression in muscle satellite cells; the two plasmids together effected full rescue, suggesting that HGF independently regulates these two factors to mediate its effects. Ectopic p27 promoted differentiation in the presence of HGF by blocking the induction of Twist. Using Twist-AS to lower Twist levels restored the HGF-dependent reduction of p27 and MHC. In the presence of ectopic HGF, satellite cells formed thin mononuclear myotubes. Neither ectopic p27, Twist-AS, or their combination reversed this change in cell morphology, suggesting that HGF acts through additional mediators to inhibit downstream events during myogenesis. Taken together, the results suggest that the effects of HGF on muscle cell proliferation and differentiation are mediated through changes in the expression levels of the myogenic-inhibitory basic helix-loop-helix (bHLH) protein Twist and the cell-cycle inhibitor p27.

TWIST, a basic helix-loop-helix transcription factor, can regulate the human osteogenic lineage

Basic helix-loop-helix (bHLH) transcription factors have been shown to play an important role in controlling cell type determination and differentiation. TWIST, a member of the bHLH transcription factor family, is involved in the development of mesodermally derived tissue, including the skeleton. We examined the role of human TWIST in osteoblast metabolism using stable expression of sense and antisense TWIST in human osteoblast HSaOS-2 cells. Changes in morphology and osteogenic phenotype characterized these stable clones. Cells that overexpressed TWIST exhibited a spindle shaped morphology, reduced levels of alkaline phosphatase, a reduced proliferation rate, and failed to respond to basic fibroblast growth factor (bFGF). In contrast, those that underexpressed TWIST demonstrated a cuboidal epithelial-like morphology characteristic of differentiated osteoblasts. TWIST antisense cells exhibited increased levels of alkaline phosphatase and type I collagen mRNA, initiated osteopontin mRNA expression, and had a reduced proliferation rate. These results indicate that TWIST overexpressing cells may de-differentiate and remain in an osteoprogenitor-like state, and antisense TWIST cells progress to a more differentiated mature osteoblast-like state. Therefore, the level of TWIST can influence osteogenic gene expression and may act as a master switch in initiating bone cell differentiation by regulating the osteogenic cell lineage.