The M-twist gene of Mus is expressed in subsets of mesodermal cells and is closely related to the Xenopus X-twi and the Drosophila twist genes

Emerging biological functions of Twist1 in cell differentiation

Twist1 is required for embryonic development and expresses after birth in mesenchymal stem cells derived from mesoderm, where it governs mesenchymal cell development. As a well-known regulator of epithelial-mesenchymal transition or embryonic organogenesis, Twist1 is important in a variety of developmental systems, including mesoderm formation, neurogenesis, myogenesis, cranial neural crest cell migration, and differentiation. In this review, we first highlight the physiological significance of Twist1 in cell differentiation, including osteogenic, chondrogenic, and myogenic differentiation, and then detail its probable molecular processes and signaling pathways. On this premise, we summarize the significance of Twist1 in distinct developmental disorders and diseases to provide a reference for studies on cell differentiation/development-related diseases.

Constructing a doxycycline-inducible system for an epithelial-to-mesenchymal transition model in MCF10A cells

Epithelial to mesenchymal transition (EMT) has been shown to play an essential role in the early stages of cancer cell invasion and metastasis. Inducible EMT models can initiate EMT in a controlled manner, thereby providing the opportunity to determine whether a cancer-associated gene influences cancer metastasis by triggering EMT. Moreover, different inducible EMT models enable the investigation of specific mechanisms of EMT modulation by various genes, facilitating a more precise understanding of how these genes influence cancer metastasis through the induction of EMT. Unfortunately, current inducible EMT models still present unmet needs. Therefore, we aimed to establish an inducible EMT model in MCF10A cells, a spontaneously immortalized human fibrocystic mammary cell line, by manipulating the expression of mouse Twist1 (mTwist1). In this study, we first compared the EMT induction capacity between human TWIST1 (hTWIST1) and mTwist1, and selected mTwist1 for further investigation. By monitoring the changes in epithelial and mesenchymal markers at different induction time points, we examined the EMT process in both polyclonal and monoclonal MCF10A cells that express doxycycline (DOX)-inducible mTwist1. Furthermore, our results showed that doxycycline-induced mTwist1 expression triggered EMT at a similar rate to TGFβ1-induced EMT in MCF10A cells. Additionally, this process was reversible upon DOX withdrawal. Thus, we have established a robust inducible EMT model in MCF10A cells, which can be used to further study cancer metastasis-driving genes.

An "unexpected" role for EMT transcription factors in hematological development and malignancy

The epithelial to mesenchymal transition (EMT) is a fundamental developmental process essential for normal embryonic development. It is also important during various pathogenic processes including fibrosis, wound healing and epithelial cancer cell metastasis and invasion. EMT is regulated by a variety of cell signalling pathways, cell-cell interactions and microenvironmental cues, however the key drivers of EMT are transcription factors of the ZEB, TWIST and SNAIL families. Recently, novel and unexpected roles for these EMT transcription factors (EMT-TFs) during normal blood cell development have emerged, which appear to be largely independent of classical EMT processes. Furthermore, EMT-TFs have also begun to be implicated in the development and pathogenesis of malignant hematological diseases such as leukemia and lymphoma, and now present themselves or the pathways they regulate as possible new therapeutic targets within these malignancies. In this review, we discuss the ZEB, TWIST and SNAIL families of EMT-TFs, focusing on what is known about their normal roles during hematopoiesis as well as the emerging and "unexpected" contribution they play during development and progression of blood cancers.

TWIST1 interacts with β/δ-catenins during neural tube development and regulates fate transition in cranial neural crest cells

Cell fate determination is a necessary and tightly regulated process for producing different cell types and structures during development. Cranial neural crest cells (CNCCs) are unique to vertebrate embryos and emerge from the neural plate borders into multiple cell lineages that differentiate into bone, cartilage, neurons and glial cells. We have previously reported that Irf6 genetically interacts with Twist1 during CNCC-derived tissue formation. Here, we have investigated the mechanistic role of Twist1 and Irf6 at early stages of craniofacial development. Our data indicate that TWIST1 is expressed in endocytic vesicles at the apical surface and interacts with β/δ-catenins during neural tube closure, and Irf6 is involved in defining neural fold borders by restricting AP2α expression. Twist1 suppresses Irf6 and other epithelial genes in CNCCs during the epithelial-to-mesenchymal transition (EMT) process and cell migration. Conversely, a loss of Twist1 leads to a sustained expression of epithelial and cell adhesion markers in migratory CNCCs. Disruption of TWIST1 phosphorylation in vivo leads to epidermal blebbing, edema, neural tube defects and CNCC-derived structural abnormalities. Altogether, this study describes a previously uncharacterized function of mammalian Twist1 and Irf6 in the neural tube and CNCCs, and provides new target genes for Twist1 that are involved in cytoskeletal remodeling.

Zebrafish twist2/dermo1 regulates scale shape and scale organization during skin development and regeneration

Scales are skin appendages in fishes that evolutionarily predate feathers in birds and hair in mammals. Zebrafish scales are dermal in origin and develop during metamorphosis. Understanding regulation of scale development in zebrafish offers an exciting possibility of unraveling how the mechanisms of skin appendage formation evolved in lower vertebrates and whether these mechanisms remained conserved in birds and mammals. Here we have investigated the expression and function of twist 2/dermo1 gene - known for its function in feather and hair formation - in scale development and regeneration. We show that of the four zebrafish twist paralogues, twist2/dermo1 and twist3 are expressed in the scale forming cells during scale development. Their expression is also upregulated during scale regeneration. Our knockout analysis reveals that twist2/dermo1 gene functions in the maintenance of the scale shape and organization during development as well as regeneration. We further show that the expression of twist2/dermo1 and twist3 is regulated by Wnt signaling. Our results demonstrate that the function of twist2/dermo1 in skin appendage formation, presumably under regulation of Wnt signaling, originated during evolution of basal vertebrates.

TGIF1-Twist1 axis in pancreatic ductal adenocarcinoma

TG-interacting factor 1 (TGIF1) exerts inhibitory effects on transforming growth factor-beta (TGF-β) signaling by suppressing Smad signaling pathway at multiple levels. TGIF1 activity is important for normal embryogenesis and organogenesis, yet its dysregulation can culminate in tumorigenesis. For instance, increased expression of TGIF1 correlates with poor prognosis in triple-negative breast cancer patients, and enforced expression of TGIF1 facilitates Wnt-driven mammary tumorigenesis, suggesting that TGIF1 might function as an oncoprotein. Quite surprisingly, TGIF1 has recently been shown to function as a tumor suppressor in pancreatic ductal adenocarcinoma (PDAC), possibly owing to its ability to antagonize the pro-malignant transcription factor Twist1. In this article, we will briefly elaborate on the biological and clinical significance of the unique tumor-suppressive function of TGIF1 and its functional interaction with Twist1 in the context of PDAC pathogenesis and progression.

Regulatory Role of the Transcription Factor Twist1 in Cancer-Associated Muscle Cachexia

Muscle cachexia is a catabolic response, usually takes place in various fatal diseases, such as sepsis, burn injury, and chronic kidney disease. Muscle cachexia is also a common co-morbidity seen in the vast majority of advanced cancer patients, often associated with low quality of life and death due to general organ dysfunction. The triggering events and underlying molecular mechanisms of muscle wasting are not yet clearly defined. Our recent study has shown that the ectopic expression of Twist1 in muscle progenitor cells is sufficient to drive muscle structural protein breakdown and attendant muscle atrophy, reminiscent of muscle cachexia. Intriguingly, muscle Twist1 expression is highly induced in cachectic muscles from several mouse models of pancreatic ductal adenocarcinoma (PDAC), raising the interesting possibility that Twist1 may mediate PDAC-driven muscle cachexia. Along these lines, both genetic and pharmacological inactivation of Twist1 function was highly significant at protecting against cancer cachexia, which translated into a significant survival benefit in the experimental PDAC animals. From a translational perspective, elevated expression of Twist1 is also detected in cancer patients with severe muscle wasting, implicating a role of Twist1 in cancer cachexia, and further providing a possible target for therapeutic attenuation of cachexia to improve cancer patient survival. In this article, we will briefly summarize how Twist1 acts as a master regulator of tumor-induced cachexia, and discuss the relevance of our findings to muscle wasting diseases in general. The mechanism of decreased muscle mass in various catabolic conditions is thought to rely on similar pathways, and, therefore, Twist1-induced cancer cachexia may benefit diverse groups of patients with clinical complications associated with loss of muscle mass and functions, beyond the expected benefits for cancer patients.

Insights into the Etiology of Mammalian Neural Tube Closure Defects from Developmental, Genetic and Evolutionary Studies

The human neural tube defects (NTD), anencephaly, spina bifida and craniorachischisis, originate from a failure of the embryonic neural tube to close. Human NTD are relatively common and both complex and heterogeneous in genetic origin, but the genetic variants and developmental mechanisms are largely unknown. Here we review the numerous studies, mainly in mice, of normal neural tube closure, the mechanisms of failure caused by specific gene mutations, and the evolution of the vertebrate cranial neural tube and its genetic processes, seeking insights into the etiology of human NTD. We find evidence of many regions along the anterior–posterior axis each differing in some aspect of neural tube closure—morphology, cell behavior, specific genes required—and conclude that the etiology of NTD is likely to be partly specific to the anterior–posterior location of the defect and also genetically heterogeneous. We revisit the hypotheses explaining the excess of females among cranial NTD cases in mice and humans and new developments in understanding the role of the folate pathway in NTD. Finally, we demonstrate that evidence from mouse mutants strongly supports the search for digenic or oligogenic etiology in human NTD of all types.

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.

Dynamic Regulation of TWIST1 Expression During Chondrogenic Differentiation of Human Bone Marrow-Derived Mesenchymal Stem Cells

Human bone marrow-derived mesenchymal stem cells (BMSCs) are clinically promising to repair damaged articular cartilage. This study investigated TWIST1, an important transcriptional regulator in mesenchymal lineages, in BMSC chondrogenesis. We hypothesized that downregulation of TWIST1 expression is required for in vitro chondrogenic differentiation. Indeed, significant downregulation of TWIST1 was observed in murine skeletal progenitor cells during limb development (N = 3 embryos), and during chondrogenic differentiation of culture-expanded human articular chondrocytes (N = 3 donors) and isolated adult human BMSCs (N = 7 donors), consistent with an inhibitory effect of TWIST1 expression on chondrogenic differentiation. Silencing of TWIST1 expression in BMSCs by siRNA, however, did not improve chondrogenic differentiation potential. Interestingly, additional investigation revealed that downregulation of TWIST1 in chondrogenic BMSCs is preceded by an initial upregulation. Similar upregulation is observed in non-chondrogenic BMSCs (N = 5 donors); however, non-chondrogenic cells fail to downregulate TWIST1 expression thereafter, preventing their chondrogenic differentiation. This study describes for the first time endogenous TWIST1 expression during in vitro chondrogenic differentiation of human BMSCs, demonstrating dynamic regulation of TWIST1 expression whereby upregulation and then downregulation of TWIST1 expression are required for chondrogenic differentiation of BMSCs. Elucidation of the molecular regulation of, and by, TWIST1 will provide targets for optimization of BMSC chondrogenic differentiation culture.

The Aurora-A-Twist1 axis promotes highly aggressive phenotypes in pancreatic carcinoma

We uncovered a crucial role for the Aurora kinase A (AURKA)-Twist1 axis in promoting epithelial-to-mesenchymal transition (EMT) and chemoresistance in pancreatic cancer. Twist1 is the first EMT-specific target of AURKA that was identified using an innovative screen. AURKA phosphorylates Twist1 at three sites, which results in its multifaceted regulation - AURKA inhibits its ubiquitylation, increases its transcriptional activity and favors its homodimerization. Twist1 reciprocates and prevents AURKA degradation, thereby triggering a feedback loop. Ablation of either AURKA or Twist1 completely inhibits EMT, highlighting both proteins as central players in EMT progression. Phosphorylation-dead Twist1 serves as a dominant-negative and fully reverses the EMT phenotype induced by Twist1, underscoring the crucial role of AURKA-mediated phosphorylation in mediating Twist1-induced malignancy. Likewise, Twist1-overexpressing BxPC3 cells formed large tumors in vivo, whereas expression of phosphorylation-dead Twist1 fully abrogated this effect. Furthermore, immunohistochemical analysis of pancreatic cancer specimens revealed a 3-fold higher level of Twist1 compared to that seen in healthy normal tissues. This is the first study that links Twist1 in a feedback loop with its activating kinase, which indicates that concurrent inhibition of AURKA and Twist1 will be synergistic in inhibiting pancreatic tumorigenesis and metastasis.

Differential expression pattern of Twist1 in mouse preimplantation embryos suggests its multiple roles during early development

PURPOSE

The purpose of the present study is to understand Twist-related protein 1 (Twist1) spatiotemporal expression patterns and functions during early embryo development.

METHODS

We performed whole-mount double immunofluorescence staining and reverse transcription (RT)-PCR analysis of the Twist1 protein and gene throughout the preimplantation development in mice.

RESULTS

We determined that after compaction, the expression of Twist1 becomes developmentally differentiated and targeted in the inner cells of embryos. In blastocysts at E4.5, uniform staining of the inner cell mass was apparent, and it had been gradually translocated to the nucleus of hatched embryonic cells at E4.75. Furthermore, the effect of potential regulators of Twist on its expression level during blastocyst development was also sought. Accordingly, Twist1 expression appeared to be upregulated in both mRNA and protein level following culture of embryos in the presence of high glucose.

CONCLUSIONS

Our study revealed the dynamic Twist localization within the early stage of embryo. The results are discussed in terms of potential roles of Twist1 in the processes of lineage segregation, hatching, and implantation in post-compaction embryos and in blastocysts.

RETRACTED: Twist2 promotes kidney cancer cell proliferation and invasion via regulating ITGA6 and CD44 expression in the ECM-Receptor-Interaction pathway

Twist2 is a member of the basic helix-loop-helix (bHLH) family and plays a critical role in tumorigenesis. Growing evidence proves that Twist2 involves in tumor progression; however, the role of Twist2 in human kidney cancer and its underlying mechanisms remain unclear. RT-PCR and Western blot analysis were used to detect the expression of Twist2 in kidney cancer cells and tissues. Cell proliferation, cell cycle, apoptosis, migration and invasion assay was measured by the Cell Count Kit-8 (CCK8), flow cytometry, wound healing and transwell analysis, respectively. Gene set enrichment analysis (GSEA) was used to identify correlation of Twist2 with ECM-Receptor-Interaction pathway. In this report, we show that Twist2 up-regulated in human kidney cancer tissues compared with normal kidney tissues. Twist2 promotes cell proliferation, inhibits cell apoptosis, augments cell migration and invasion in human kidney cancer-derived cell in vitro, and promotes tumor growth in vivo. Moreover, we found that knockdown of Twist2 decreased the levels of ITGA6 and CD44 which contribute to cell migration and invasion correlated with ECM-Receptor-Interaction pathway. This result indicates Twist2 may promote migration and invasion of kidney cancer cells by regulating ITGA6 and CD44 expression. Therefore, our data demonstrated that Twist2 involves in kidney cancer progression. The identification of the role Twist2 on the migration and invasion of kidney cancer provides a potential appropriate treatment after radical nephrectomy to get a better prognosis that reducing recurrence.

Twist2 promotes kidney cancer cell proliferation and invasion by regulating ITGA6 and CD44 expression in the ECM-receptor interaction pathway

Twist2 is a member of the basic helix-loop-helix (bHLH) family and plays a critical role in tumorigenesis. Growing evidence has proven that Twist2 is involved in tumor progression; however, the role of Twist2 in human kidney cancer and its underlying mechanisms remain unclear. Real-time polymerase chain reaction and Western blot analysis were used to detect the expression of Twist2 in kidney cancer cells and tissues. Cell proliferation, cell cycle, apoptosis, migration, and invasion assay were analyzed using the Cell Count Kit-8, flow cytometry, wound healing, and Transwell analysis, respectively. In this study, we showed that Twist2 was upregulated in human kidney cancer tissues compared with normal kidney tissues. Twist2 promoted cell proliferation, inhibited cell apoptosis, and augmented cell migration and invasion in human kidney-cancer-derived cells in vitro. Twist2 also promoted tumor growth in vivo. Moreover, we found that the knockdown of Twist2 decreased the levels of ITGA6 and CD44 expression. This result indicates that Twist2 may promote migration and invasion of kidney cancer cells by regulating ITGA6 and CD44 expression. Therefore, our data demonstrated that Twist2 is involved in kidney cancer progression. The identification of the role of Twist2 in the migration and invasion of kidney cancer provides a potential appropriate treatment for human kidney cancer.

Putative oncogene Brachyury (T) is essential to specify cell fate but dispensable for notochord progenitor proliferation and EMT

The transcription factor Brachyury (T) gene is expressed throughout primary mesoderm (primitive streak and notochord) during early embryonic development and has been strongly implicated in the genesis of chordoma, a sarcoma of notochord cell origin. Additionally, T expression has been found in and proposed to play a role in promoting epithelial-mesenchymal transition (EMT) in various other types of human tumors. However, the role of T in normal mammalian notochord development and function is still not well-understood. We have generated an inducible knockdown model to efficiently and selectively deplete T from notochord in mouse embryos. In combination with genetic lineage tracing, we show that T function is essential for maintaining notochord cell fate and function. Progenitors adopt predominantly a neural fate in the absence of T, consistent with an origin from a common chordoneural progenitor. However, T function is dispensable for progenitor cell survival, proliferation, and EMT, which has implications for the therapeutic targeting of T in chordoma and other cancers.

Contribution of EVX1 in Aggressiveness of Esophageal Squamous Cell Carcinoma

Homeobox genes play an overruling role in the regional cell fate determination during development. EVX1 is known as a new target gene of BMP signaling pathway, a group of morphogens which are making the largest subset within the transformation growth factor beta (TGF-β) superfamily. In this study, we aimed to enlighten the expression level of EVX1 in esophageal squamous cell carcinoma (ESCC) and to disclose its apparent roles in maintenance and progression of the disease. The expression level of EVX1 was analyzed in fresh tumoral tissues in comparison with distant tumor-free tissues of 50 ESCC patients using relative comparative real-time PCR. The importance of EVX1 in development and cancer was also reviewed. EVX1 was underexpressed in 70% of tumor samples. There was a significant correlation between down-regulation of EVX1 and lymph node metastasis of tumor cells (p = 0.027). Furthermore, EVX1 underexpression was significantly correlated with depth of tumor cell invasion (P = 0.037). To the best of our knowledge, this is the first report highlighting EVX1 expression in ESCC to date. The clinicopathological relevance of EVX1 mRNA expression in ESCC targeted this gene as a new independent molecular marker for advanced tumor, which determine the characteristics and behavior of aggressive ESCC.

Suppression of fibrogenic signaling in hepatic stellate cells by Twist1-dependent microRNA-214 expression: Role of exosomes in horizontal transfer of Twist1

A hallmark of liver fibrosis is the activation of hepatic stellate cells (HSC), which results in their production of fibrotic molecules, a process that is largely regulated by connective tissue growth factor (CCN2). CCN2 is increasingly expressed during HSC activation because of diminished expression of microRNA-214 (miR-214), a product of dynamin 3 opposite strand (DNM3os) that directly suppresses CCN2 mRNA. We show that an E-box in the miR-214 promoter binds the basic helix-loop-helix transcription factor, Twist1, which drives miR-214 expression and results in CCN2 suppression. Twist1 expression was suppressed in HSC of fibrotic livers or in cultured HSC undergoing activation in vitro or after treatment with ethanol. Furthermore, Twist1 decreasingly interacted with DNM3os as HSC underwent activation in vitro. Nanovesicular exosomes secreted by quiescent but not activated HSC contained high levels of Twist1, thus reflecting the suppression of cellular Twist1 during HSC activation. Exosomal Twist1 was intercellularly shuttled between HSC and stimulated expression of miR-214 in the recipient cells, causing expression of CCN2 and its downstream effectors to be suppressed. Additionally, the miR-214 E-box in HSC was also regulated by hepatocyte-derived exosomes, showing that functional transfer of exosomal Twist1 occurs between different cell types. Finally, the levels of Twist1, miR-214, or CCN2 in circulating exosomes from fibrotic mice reflected fibrosis-induced changes in the liver itself, highlighting the potential utility of these and other constituents in serum exosomes as novel circulating biomarkers for liver fibrosis. These findings reveal a unique function for cellular or exosomal Twist1 in CCN2-dependent fibrogenesis.

MyoD transcription factor induces myogenesis by inhibiting Twist-1 through miR-206

Twist-1 is mostly expressed during development and has been previously shown to control myogenesis. Because its regulation in muscle has not been fully exploited, the aim of this project was to identify micro (mi)RNAs in muscle that regulate Twist-1. miR-206, one of the most important muscle-specific miRNAs (myomiRs), was identified as a possible regulator of Twist-1 mRNA. Luciferase assays and transfections in human foetal myoblasts showed that Twist-1 is a direct target of miR-206 and that through this pathway muscle cell differentiation is promoted. We next investigated whether MyoD, a major myogenic transcription factor, regulates Twist-1 because it is known that MyoD induces expression of the miR-206 gene. We found that forced MyoD expression induced miR-206 upregulation and Twist-1 downregulation through binding to the miR-206 promoter, followed by increased muscle cell differentiation. Finally, experiments were performed in muscle cells from subjects with congenital myotonic dystrophy type 1, in which myoblasts fail to differentiate into myotubes. MyoD overexpression inhibited Twist-1 through miR-206 induction, which was followed by an increase in muscle cell differentiation. These results reveal a previously unidentified mechanism of myogenesis that might also play an important role in muscle disease.

TWIST and ovarian cancer stem cells: implications for chemoresistance and metastasis

The transcription factor TWIST1 is a highly evolutionally conserved basic Helix-Loop-Helix (bHLH) transcription factor that functions as a master regulator of gastrulation and mesodermal development. Although TWIST1 was initially associated with embryo development, an increasing number of studies have shown TWIST1 role in the regulation of tissue homeostasis, primarily as a regulator of inflammation. More recently, TWIST1 has been found to be involved in the process of tumor metastasis through the regulation of Epithelial Mesenchymal Transition (EMT). The objective of this review is to examine the normal functions of TWIST1 and its role in tumor development, with a particular focus on ovarian cancer. We discuss the potential role of TWIST1 in the context of ovarian cancer stem cells and its influence in the process of tumor formation.

LPS induces the degradation of programmed cell death protein 4 (PDCD4) to release Twist2, activating c-Maf transcription to promote interleukin-10 production

Programmed cell death protein 4 (PDCD4) is a tumor suppressor and has also been shown to suppress production of the immunomodulatory cytokine IL-10. The precise role of PDCD4 in IL-10 induction in macrophages is still not fully understood. Incubation of macrophages with inhibitors of PI3K and mTOR blocked LPS-stimulated PDCD4 degradation and expression of c-Maf and IL-10 production. PDCD4 and the transcription factor Twist2 were shown to form a complex in untreated cells. LPS disrupted the complex allowing Twist2 to bind to the c-Maf promoter. PI3K and mTOR inhibitors prevented this disruption by stabilizing PDCD4 and thereby decreased Twist2 binding to the c-Maf promoter and induction of c-Maf mRNA. These results indicate a regulatory role for PDCD4 and Twist2 in LPS-induced IL-10 production in macrophages. LPS promotes PDCD4 degradation via a pathway involving PI3K and mTOR, releasing Twist2, which induces IL-10 via c-Maf.

Twist1- and Twist2-haploinsufficiency results in reduced bone formation

Background

Twist1 and Twist2 are highly homologous bHLH transcription factors that exhibit extensive highly overlapping expression profiles during development. While both proteins have been shown to inhibit osteogenesis, only Twist1 haploinsufficiency is associated with the premature synostosis of cranial sutures in mice and humans. On the other hand, biallelic Twist2 deficiency causes only a focal facial dermal dysplasia syndrome or additional cachexia and perinatal lethality in certain mouse strains. It is unclear how these proteins cooperate to synergistically regulate bone formation.

Methods

Twist1 floxed mice (Twist1^f/f) were bred with Twist2-Cre knock-in mice (Twist2^Cre/+) to generate Twist1 and Twist2 haploinsufficient mice (Twist1^f/+; Twist2^Cre/+). X-radiography, micro-CT scans, alcian blue/alizarin red staining, trap staining, BrdU labeling, immunohistochemistry, in situ hybridizations, real-time PCR and dual luciferase assay were employed to investigate the overall skeletal defects and the bone-associated molecular and cellular changes of Twist1^f/+;Twist2^Cre/+ mice.

Results

Twist1 and Twist2 haploinsufficient mice did not present with premature ossification and craniosynostosis; instead they displayed reduced bone formation, impaired proliferation and differentiation of osteoprogenitors. These mice exhibited decreased expressions of Fgf2 and Fgfr1–4 in bone, resulting in a down-regulation of FGF signaling. Furthermore, in vitro studies indicated that both Twist1 and Twist2 stimulated 4.9 kb Fgfr2 promoter activity in the presence of E12, a Twist binding partner.

Conclusion

These data demonstrated that Twist1- and Twist2-haploinsufficiency caused reduced bone formation due to compromised FGF signaling.

A conditional mutant allele for analysis of Mixl1 function in the mouse

Mixl1 is the only member of the Mix/Bix homeobox gene family identified in mammals. During mouse embryogenesis, Mixl1 is first expressed at embryonic day (E)5.5 in cells of the visceral endoderm (VE). At the time of gastrulation, Mixl1 expression is detected in the vicinity of the primitive streak. Mixl1 is expressed in cells located within the primitive streak, in nascent mesoderm cells exiting the primitive streak, and in posterior VE overlying the primitive streak. Genetic ablation of Mixl1 in mice has revealed its crucial role in mesoderm and endoderm cell specification and tissue morphogenesis during early embryonic development. However, the early lethality of the constitutive Mixl1(-/-) mutant precludes the study of its role at later stages of embryogenesis and in adult mice. To circumvent this limitation, we have generated a conditional Mixl1 allele (Mixl1(cKO) that permits temporal as well as spatial control of gene ablation. Animals homozygous for the Mixl1(cKO) conditional allele were viable and fertile. Mixl1(KO/KO) embryos generated by crossing of Mixl1(cKO/cKO) mice with Sox2-Cre or EIIa-Cre transgenic mice were embryonic lethal at early somite stages. By contrast to wild-type embryos, Mixl1(KO/KO) embryos contained no detectable Mixl1, validating the Mixl1(cKO) as a protein null after Cre-mediated excision. Mixl1(KO/KO) embryos resembled the previously reported Mixl1(-/-) mutant phenotype. Therefore, the Mixl1 cKO allele provides a tool for investigating the temporal and tissue-specific requirements for Mixl1 in the mouse.

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.

LncRNA expression signatures of twist-induced epithelial-to-mesenchymal transition in MCF10A cells

The epithelial-to-mesenchymal transient (EMT) is associated with tumor metastasis. Twist is one of the key transcription factors for EMT and relates to tumor cell migration. Long non-coding RNAs (lncRNAs) have recently emerged as important regulatory molecules involved in a broad range of biological processes and complicated diseases. However, it is unknown whether a signal network and lncRNAs are involved in Twist-induced EMT program. Taking MCF10A/Twist as a model, more than 99 lncRNAs and 3164 genes are regulated in the Twist-induced EMT process using lncRNA-array and cDNA micro-array. We establish a downstream signal network associated with EMT induced by Twist using bioinformatic analysis (Gene Ontology, pathway analysis) and experimental data. A set of multiple canonical signal pathways (such as WNT, MAPK, JAK/STAT, TGF-β, mTOR, Hedgehog and P53 signaling pathways) and several lncRNAs [such as lncRNA (chr6, 26124411-26139312, +), lncRNA (chr1, 41944445-41949874, -), lncRNA (chr17, 44833874-44834830, +)] are altered in MCF10A/Twist cells. More interestingly, lncRNA (chr17, 44833874-44834830, +), lncRNA (chr17, 21142183-21156578, -), lncRNA (chr6, 26124411-26139312, +) and lncRNA (chr19, 438420-2083745, -) may be involved in regulation or activation of WNT signaling pathway in the Twist-induced EMT process. These findings first determine that Twist contributes to invasion and metastasis by inducing wide-ranging transcriptional and functional changes of lncRNAs and signal pathways in our study.

Identification and characterization of a twist ortholog in the polychaete annelid Platynereis dumerilii reveals mesodermal expression of Pdu-twist

The basic helix-loop-helix transcription factor twist plays a key role during mesoderm development in Bilateria. In this study, we identified a twist ortholog in the polychaete annelid Platynereis dumerilii and analyze its expression during larval development, postlarval growth up to the adult stage, and caudal regeneration after amputation of posterior segments. At late larval stages, Pdu-twist is expressed in the mesodermal anlagen and in developing muscles. During adulthood and caudal regeneration, Pdu-twist is expressed in the posterior growth zone, in mesodermal cells within the newly forming segments and budding parapodia. Our results indicate that Pdu-twist is involved in mesoderm formation during larval development, posterior growth, and caudal regeneration.

Inducible knockout of Twist1 in young and adult mice prolongs hair growth cycle and has mild effects on general health, supporting Twist1 as a preferential cancer target

Twist1 promotes epithelial-mesenchymal transition, invasion, metastasis, stemness, and chemotherapy resistance in cancer cells and thus is a potential target for cancer therapy. However, Twist1-null mice are embryonic lethal, and people with one Twist1 germline mutant allele develop Saethre-Chotzen syndrome; it is questionable whether Twist1 can be targeted in patients without severe adverse effects. We found that Twist1 is expressed in several tissues, including fibroblasts of the mammary glands and dermal papilla cells of the hair follicles. We developed a tamoxifen-inducible Twist1 knockout mouse model; Twist1 knockout in 6-week-old female mice did not affect mammary gland morphogenesis and function during pregnancy and lactation. In both males and females, the knockout did not influence body weight gain, heart rate, or total lean and fat components. The knockout also did not alter blood pressure in males, although it slightly reduced blood pressure in females. Although Twist1 is not cyclically expressed in dermal papilla cells, knockout of Twist1 at postnatal day 13 (when hair follicles have developed) drastically extended the anagen phase and accelerated hair growth. These results indicate that Twist1 is not essential for maintaining an overall healthy condition in young and adult mice and that loss of function facilitates hair growth in adulthood, supporting Twist1 as a preferential target for cancer therapy.

Muscle and connective tissue progenitor populations show distinct Twist1 and Twist3 expression profiles during axolotl limb regeneration

Limb regeneration involves re-establishing a limb development program from cells within adult tissues. Identifying molecular handles that provide insight into the relationship between cell differentiation status and cell lineage is an important step to study limb blastema cell formation. Here, using single cell PCR, focusing on newly isolated Twist1 sequences, we molecularly profile axolotl limb blastema cells using several progenitor cell markers. We link their molecular expression profile to their embryonic lineage via cell tracking experiments. We use in situ hybridization to determine the spatial localization and extent of overlap of different markers and cell types. Finally, we show by single cell PCR that the mature axolotl limb harbors a small but significant population of Twist1(+) cells.

Distinct Caenorhabditis elegans HLH-8/twist-containing dimers function in the mesoderm

BACKGROUND

The Caenorhabditis elegans basic helix-loop-helix (bHLH) factor HLH-8, the single Twist ortholog in the nematode genome, plays important roles in mesoderm development, including M lineage patterning and differentiation of vulval and enteric muscles. HLH-8 cooperates with HLH-2, the bHLH E/Daughterless ortholog, to regulate downstream target genes, but it is not known whether HLH-2 is an obligate partner for all HLH-8 functions.

RESULTS

Using hlh-2 loss-of-function alleles and RNAi, we discovered that HLH-2 is required in the vulval muscles but not in M patterning or enteric muscle development. Additionally, we found that expressing tethered HLH-8/HLH-8 dimers in hlh-8 null animals rescued M patterning and enteric but not vulval muscle development.

CONCLUSIONS

These results support a model whereby HLH-8/HLH-8 homodimers function in M lineage patterning and enteric muscles and HLH-8/HLH-2 heterodimers function in the M-derived vulval muscles. Interestingly, the different dimers function in the same M lineage cells and the switch in dimer function coincides with vulval muscle differentiation. The use of distinct Twist dimers is evolutionarily conserved, and C. elegans provides a paradigm for future dissection of differential promoter regulation by these dimers at a single cell resolution.

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.

EVEN-SKIPPED HOMEOBOX 1 controls human ES cell differentiation by directly repressing GOOSECOID expression

TGFß signaling patterns the primitive streak, yet little is known about transcriptional effectors that mediate the cell fate choices during streak-like development in mammalian embryos and in embryonic stem (ES) cells. Here we demonstrate that cross-antagonistic actions of EVEN-SKIPPED HOMEOBOX 1 (EVX1) and GOOSECOID (GSC) regulate cell fate decisions in streak-like progenitors derived from human ES cells exposed to BMP4 and/or activin. We found that EVX1 repressed GSC expression and promoted formation of posterior streak-like progeny in response to BMP4, and conversely that GSC repressed EVX1 expression and was required for development of anterior streak-like progeny in response to activin. Chromatin immunoprecipitation assays showed that EVX1 bound to the GSC 5'-flanking region in BMP4 treated human ES cells, and band shift assays identified two EVX1 binding sites in the GSC 5'-region. Significantly, we found that intact EVX1 binding sites were required for BMP4-mediated repression of GSC reporter constructs. We conclude that BMP4-induced EVX1 repress GSC directly and the two genes form the core of a gene regulatory network (GRN) controlling cell fates in streak-like human ES cell progeny.

Fibroblast growth factor 2 (Fgf2) inhibits differentiation of mesenchymal stem cells by inducing Twist2 and Spry4, blocking extracellular regulated kinase activation, and altering Fgf receptor expression levels

Mesenchymal stem cells (MSCs) are known to differentiate into connective tissue lineages but intracellular signaling pathways that maintain cells in an undifferentiated state remain largely unexplored. Previously, we reported that fibroblast growth factor 2 (Fgf2) reversibly inhibited multilineage differentiation of primary mouse MSCs and now identify a unique compliment of signaling proteins that are dynamically regulated by this mitogen and whose expression levels are strongly correlated with inhibition of cell differentiation. Fgf2 selectively induced expression of Twist2 and Sprouty4 (Spry4) and repressed expression of soluble frizzled related receptor 2 (Sfrp2), runt-related transcription factor 2 (Runx2), and peroxisome proliferation activated receptor gamma (Pparg). In contrast, Wnt3a induced expression of Twist but not Twist2 or Spry4 and bone morphogenetic protein 2 (Bmp2) failed to alter expression of all three genes. Moreover, pretreatment of MSCs with Fgf2 delayed extracellular regulated kinase 1 (Erk1) and Erk2 phosphorylation and repressed bone-specific gene expression during an osteoinduction time course. Alternatively, pretreatment with Wnt3a had no effect, whereas Bmp2 pretreatment augmented Erk1/2 activation and bone-specific gene expression. Fgf2 also induced expression of Fgf receptor 1 (Fgfr1) and Fgfr4 and repressed Fgfr2 and Fgfr3 expression in MSCs, whereas Wnt3a and Bmp2 had the opposite effect. Finally, immunostaining revealed that Twist and Spry4 were coexpressed in MSCs and that Fgf2 treatment altered their subcellular distribution in a manner consistent with their mode of action. Collectively, these studies demonstrate that inhibition of mouse MSC differentiation by Fgf2 is strongly correlated with upregulation of Twist2 and Spry4 and suppression of Erk1/2 activation.

Correlation of TWIST2 up-regulation and epithelial-mesenchymal transition during tumorigenesis and progression of cervical carcinoma

OBJECTIVE

Globally, cervical cancer is the second most common cancer among women, and determining potential targets involved in tumor progression is necessary. This study investigated the clinic-pathological significance of twist homolog 2 (TWIST2), a basic helix-loop-helix transcription factor, and correlated TWIST2 and E-cadherin expression in cervical cancer.

METHODS

A series of 142 samples, including 14 cases of normal cervical tissues, 58 cases of cervical intraepithelial neoplasia (CIN) and 70 cases of squamous cell carcinoma (SCC), were examined TWIST2 and E-cadherin immunohistochemical staining and statistical analysis.

RESULTS

Increased cytoplasmic and nuclear expression levels of TWIST2 were associated with the malignant transformation of cervical epithelium and the histological progression of cervical cancer. A logistic test showed that TWIST2 was a relatively independent predictor of lymph node metastasis of SCC. Further, increased levels of TWIST2 were also associated with aberrant expression of E-cadherin, an important EMT indicator.

CONCLUSIONS

The present data suggest that TWIST2 overexpression was significantly linked to cervical cancer progression, which makes it a promising marker for determining the metastatic potential of cervical cancer, and up-regulation of TWIST2, in combination with aberrant E-cadherin expression in primary cervical cancer tissues, may predict the malignant transformation and distal metastasis of carcinomas.

Normal and disease-related biological functions of Twist1 and underlying molecular mechanisms

This article reviews the molecular structure, expression pattern, physiological function, pathological roles and molecular mechanisms of Twist1 in development, genetic disease and cancer. Twist1 is a basic helix-loop-helix domain-containing transcription factor. It forms homo- or hetero-dimers in order to bind the Nde1 E-box element and activate or repress its target genes. During development, Twist1 is essential for mesoderm specification and differentiation. Heterozygous loss-of-function mutations of the human Twist1 gene cause several diseases including the Saethre-Chotzen syndrome. The Twist1-null mouse embryos die with unclosed cranial neural tubes and defective head mesenchyme, somites and limb buds. Twist1 is expressed in breast, liver, prostate, gastric and other types of cancers, and its expression is usually associated with invasive and metastatic cancer phenotypes. In cancer cells, Twist1 is upregulated by multiple factors including SRC-1, STAT3, MSX2, HIF-1α, integrin-linked kinase and NF-κB. Twist1 significantly enhances epithelial-mesenchymal transition (EMT) and cancer cell migration and invasion, hence promoting cancer metastasis. Twist1 promotes EMT in part by directly repressing E-cadherin expression by recruiting the nucleosome remodeling and deacetylase complex for gene repression and by upregulating Bmi1, AKT2, YB-1, etc. Emerging evidence also suggests that Twist1 plays a role in expansion and chemotherapeutic resistance of cancer stem cells. Further understanding of the mechanisms by which Twist1 promotes metastasis and identification of Twist1 functional modulators may hold promise for developing new strategies to inhibit EMT and cancer metastasis.

Promotion of hepatocellular carcinoma metastasis through matrix metalloproteinase activation by epithelial-mesenchymal transition regulator Twist1

E-cadherin loss is a key biological mechanism in tumour invasion. As a main regulator of epithelial-mesenchymal transition (EMT) mechanism-mediated invasion and metastasis, Twist1 plays an important role through its regulation of E-cadherin expression. However, whether or not Twist2 has the same function in tumour metastasis remains unclear. The purpose of this study is to investigate the expressions and different roles of Twist1 and Twist2 in human hepatocellular carcinoma (HCC). The expressions of Twist1 and Twist2 in HCC tissue were evaluated by immunohistochemical staining. The role of Twist1 and Twist2 in invasiveness was also evaluated in vitro by using HCC cell lines. Twist1 nuclear overexpression is found to be correlated with HCC metastasis, and its expression is negatively correlated with E-cadherin expression in human tissue. Twist2, a Twist1 homology protein, only expresses in the cytoplasm and shows no significant correlation with HCC metastasis. By ectopic transfection of Twist1 and Twist2 into the HCC cells, HepG2 and PLC, Twist1 is able to down-regulate E-cadherin expression and promote matrix metalloproteinase (MMP) activation, specifically in MMP2 and MMP9. In functional assays, Twist1 is found to promote invasion in HepG2 and PLC cells, but the invasion ability of the groups is not affected Twist2. Our findings indicate that Twist1 induces HCC invasion via increased activity in MMPs, leading to poor clinical prognoses. The results of this study also demonstrate a novel cogitation in Twist2, which has no effect on HCC invasion and metastasis. Twist1 may contribute to HCC invasion and metastasis and may be used as a novel therapeutic target for the inhibition of HCC metastasis.

TWIST1 Gene: First Insights in Felis catus

TWIST1 is thought to be a novel oncogene. Understanding the molecular mechanisms regulating the TWIST1 gene expression profiles in tumor cells may give new insights regarding prognostic factors and novel therapeutic targets in veterinary oncology. In the present study we partially isolated the TWIST1 gene in Felis catus and performed comparative studies. Several primer combinations were used based on the alignments of homologous DNA sequences. After PCR amplification, three bands were obtained, purified and sequenced. Several bioinformatic tools were utilized to carry out the comparative studies. Higher similarity was found between the isolated TWIST1 gene in Felis catus and Homo sapiens (86%) than between Homo sapiens and Rattus norvegicus or Mus musculus (75%). Partial amino acid sequence showed no change in the four species analyzed. This confirmed that coding sequences presented high similarity (~96%) between man and cat. These results give the first insights regarding the TWIST1 gene in cat but further studies are required in order to establish, or not, its role in tumor formation and progression in veterinary oncology.

Redundant or separate entities?--roles of Twist1 and Twist2 as molecular switches during gene transcription

Twist1 and Twist2 are highly conserved members of the Twist subfamily of bHLH proteins responsible for the transcriptional regulation of the developmental programs in mesenchymal cell lineages. The regulation of such processes requires that Twist1 and Twist2 function as molecular switches to activate and repress target genes by employing several direct and indirect mechanisms. Modes of action by these proteins include direct DNA binding to conserved E-box sequences and recruitment of coactivators or repressors, sequestration of E-protein modulators, and interruption of proper activator/repressor function through protein–protein interactions. Regulatory outcomes of Twist1 and Twist2 are themselves controlled by spatial-temporal expression, phosphoregulation, dimer choice and cellular localization. Although these two proteins are highly conserved and exhibit similar functions in vitro, emerging literature have demonstrated different roles in vivo. The involvement of Twist1 and Twist2 in a broad spectrum of regulatory pathways highlights the importance of understanding their roles in normal development, homeostasis and disease. Here we focus on the mechanistic models of transcriptional regulation and summarize the similarities and differences between Twist1 and Twist2 in the context of myogenesis, osteogenesis, immune system development and cancer.

Interplay of cadherin-mediated cell adhesion and canonical Wnt signaling

The epithelial-mesenchymal transition is essential in both embryonic development and the progression of carcinomas. Wnt signaling and cadherin-mediated adhesion have been implicated in both processes; clarifying their role will depend on linking them to rearrangements of cellular structure and behavior. beta-Catenin is an essential molecule both in cadherin-mediated cell adhesion and in canonical Wnt signaling. Numerous experiments have shown that the loss of cadherin-mediated cell adhesion can promote beta-catenin release and signaling; this is accomplished by proteases, protein kinases and other molecules. Cadherin loss can also signal to several other regulatory pathways. Additionally, many target genes of Wnt signaling influence cadherin adhesion. The most conspicuous of these Wnt target genes encode the transcription factors Twist and Slug, which directly inhibit the E-cadherin gene promoter. Other Wnt/beta-catenin target genes encode metalloproteases or the cell adhesion molecule L1, which favor the degradation of E-cadherin. These factors provide a mechanism whereby cadherin loss and increased Wnt signaling induce epithelial-mesenchymal transition in both carcinomas and development.

TWISTing an embryonic transcription factor into an oncoprotein

Over the past decade, the reactivation of TWIST embryonic transcription factors has been described as a frequent event and a marker of poor prognosis in an impressive array of human cancers. Growing evidence now supports the premise that these cancers hijack TWIST's embryonic functions, granting oncogenic and metastatic properties. In this review, we report on the history and recent breakthroughs in understanding TWIST protein functions and the emerging role of the associated epithelial-mesenchymal transition (EMT) in tumorigenesis. We then broaden the discussion to address the general contribution of reactivating embryonic programs in cancerogenesis.

Phylogenetic and evolutionary relationships and developmental expression patterns of the zebrafish twist gene family

Four members of the twist gene family (twist1a, 1b, 2, and 3) are found in the zebrafish, and they are thought to have arisen through three rounds of gene duplication, two of which occurred prior to the tetrapod-fish split. Phylogenetic analysis groups most of the vertebrate Twist1 peptides into clade I, except for the Twist1b proteins of the acanthopterygian fish (medaka, pufferfish, stickleback), which clustered within clade III. Paralogies and orthologies among the zebrafish, medaka, and human twist genes were determined using comparative synteny analysis of the chromosomal regions flanking these genes. Comparative nucleotide substitution analyses also revealed a faster rate of nucleotide mutation/substitution in the acanthopterygian twist1b compared to the zebrafish twist1b, thus accounting for their anomalous phylogenetic clustering. We also observed minimal expression overlap among the four twist genes, suggesting that despite their significant peptide similarity, their regulatory controls have diverged considerably, with minimal functional redundancy between them.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

A twist of insight - the role of Twist-family bHLH factors in development

Members of the Twist-family of bHLH proteins play a pivotal role in a number of essential developmental programs. Twist-family bHLH proteins function by dimerizing with other bHLH members and binding to cis- regulatory elements, called E-boxes. While Twist-family members may simply exhibit a preference in terms of high-affinity binding partners, a complex, multilevel cascade of regulation creates a dynamic role for these bHLH proteins. We summarize in this review information on each Twist-family member concerning expression pattern, function, regulation, downstream targets, and interactions with other bHLH proteins. Additionally, we focus on the phospho-regulatory mechanisms that tightly control posttranslational modification of Twist-family member bHLH proteins.

Intraflagellar transport protein 172 is essential for primary cilia formation and plays a vital role in patterning the mammalian brain

IFT172, also known as Selective Lim-domain Binding protein (SLB), is a component of the intraflagellar transport (IFT) complex. In order to evaluate the biological role of the Ift172 gene, we generated a loss-of-function mutation in the mouse. The resulting Slb mutant embryos die between E12.5 and 13.0, and exhibit severe cranio-facial malformations, failure to close the cranial neural tube, holoprosencephaly, heart edema and extensive hemorrhages. Cilia outgrowth in cells of the neuroepithelium is initiated but the axonemes are severely truncated and do not contain visible microtubules. Morphological and molecular analyses revealed a global brain-patterning defect along the dorsal-ventral (DV) and anterior-posterior (AP) axes. We demonstrate that Ift172 gene function is required for early regulation of Fgf8 at the midbrain-hindbrain boundary and maintenance of the isthmic organizer. In addition, Ift172 is required for proper function of the embryonic node, the early embryonic organizer and for formation of the head organizing center (the anterior mesendoderm, or AME). We propose a model suggesting that forebrain and mid-hindbrain growth and AP patterning depends on the early function of Ift172 at gastrulation. Our data suggest that the formation and function of the node and AME in the mouse embryo relies on an indispensable role of Ift172 in cilia morphogenesis and cilia-mediated signaling.

Multipotent mesenchymal stromal cells in articular diseases

Although cartilage defects are common features of osteoarthritis and rheumatoid arthritis, current treatments can rarely restore the full function of native cartilage. Recent studies have provided new perspectives for cartilage engineering using multipotent mesenchymal stromal cells (MSC). Moreover, MSC have been used as immunosuppressant agents in autoimmune diseases and have tested successfully in animal models of arthritis. However, the sequential events occurring during chondrogenesis must be fully understood before we can reproduce the complex molecular events that lead to MSC differentiation and long-term maintenance of cartilage characteristics in the context of chronic joint inflammation. This chapter focuses on the potential of MSC to repair cartilage, with an emphasis on the factors that are known to be required in inducing chondrogenesis and on their immunosuppressive potential.

Induction of EMT by twist proteins as a collateral effect of tumor-promoting inactivation of premature senescence

Twist1 and Twist2 are major regulators of embryogenesis. Twist1 has been shown to favor the metastatic dissemination of cancer cells through its ability to induce an epithelial-mesenchymal transition (EMT). Here, we show that a large fraction of human cancers overexpress Twist1 and/or Twist2. Both proteins override oncogene-induced premature senescence by abrogating key regulators of the p53- and Rb-dependent pathways. Twist1 and Twist2 cooperate with Ras to transform mouse embryonic fibroblasts. Interestingly, in epithelial cells, the oncogenic cooperation between Twist proteins and activated mitogenic oncoproteins, such as Ras or ErbB2, leads to complete EMT. These findings suggest an unanticipated direct link between early escape from failsafe programs and the acquisition of invasive features by cancer cells.

Transcriptional control of mesenchymal stem cell differentiation

SUMMARY

In recent years, transcriptomics and proteomics have provided us with a great deal of information about the expression profiles of various cell types and how these change under different conditions. Stem cell research is one area where this has had a major impact by providing an insight into events at the molecular level that control stem cell growth and differentiation. This includes mesenchymal stem cell (MSC) biology where knowledge about the mechanisms governing differentiation is vital for the development of future therapeutic strategies. Although there is still much to learn, we are starting to build up a picture of the main events in these differentiation processes. This review will discuss control of MSC differentiation at the transcriptional level. Not all the factors which have been shown to play a role in lineage-specific mesenchymal differentiation can be covered here. Instead, we will focus specifically on the key factors that contribute to the regulation of osteogenesis, adipogenesis, and chondrogenesis.

An absence of Twist1 results in aberrant cardiac neural crest morphogenesis

The basic helix-loop-helix transcription factor Twist1 plays an essential role in mesenchymal cell populations during embryonic development and in pathological disease. Remodeling of the cardiac outflow tract (OFT) into the functionally separate aortic arch and pulmonary trunk is dependent upon the dynamic, coordinated contribution of multiple mesenchymal cell populations. Here, we report that Twist1(-/-) mice exhibit OFTs that contain amorphic cellular nodules within their OFT endocardial cushions. The nodular mesenchyme expresses the related bHLH factors Hand1 and Hand2, but reduced levels of the normal cushion marker Periostin. Lineage mapping confirms that nodule cells are exclusively of cardiac neural crest origin (cNCC), and are not ectopic cardiomyocytes or smooth muscle cells. These studies also reveal a delay in cNCC colonization of the OFT cushions. Furthermore, these mapping studies uncover nodules in the pharyngeal arches, and identify Twist1(-/-) neural crest cell defects within the dorsal neural tube, which exhibits an expanded domain of Wnt1-Cre-lineage marked cells. Together, these data support a model where Twist1 is required both for proper cNCC delamination, and for emigration from the dorsal neural tube and along cNCC migration pathways. Within the Twist1(-/-) neural crest cell populations that do emigrate to the OFT, a Hand-expressing subpopulation displays defective maturation, tracking, and, presumably, cell-cell adhesion, further compromising cNCC morphogenesis.

Genetic control of muscle development: learning from Drosophila

Muscle development involves a complex sequence of time and spatially regulated cellular events leading to the formation of highly specialised syncytial muscle cells displaying a common feature, the capacity of contraction. Analyses of mechanisms controlling muscle development reveals that the main steps of muscle formation including myogenic determination, diversification of muscle precursors, myoblast fusion and terminal differentiation involve the actions of evolutionarily conserved genes. Thus dissecting the genetic control of muscle development in simple model organisms appears to be an attractive way to get insights into core genetic cascade that orchestrate myogenesis. In this respect, particularly insightful have been data generated using Drosophila as a model system. Notably, the interplay between intrinsic and extrinsic cues that determine the early myogenic decisions leading to the specification of muscle progenitors and those controlling myoblasts fusion are much better characterised in Drosophila than in vertebrate species. Also, adult Drosophila myogenesis, which leads to the formation of vertebrate-like multi-fibre muscles, emerges as a particularly well-adapted system to study normal and aberrant muscle development.