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

Large-scale molecular comparison of human schwann cells to malignant peripheral nerve sheath tumor cell lines and tissues

Malignant peripheral nerve sheath tumors (MPNST) are highly invasive soft tissue sarcomas that arise within the peripheral nerve and frequently metastasize. To identify molecular events contributing to malignant transformation in peripheral nerve, we compared eight cell lines derived from MPNSTs and seven normal human Schwann cell samples. We found that MPNST lines are heterogeneous in their in vitro growth rates and exhibit diverse alterations in expression of pRb, p53, p14(Arf), and p16(INK4a) proteins. All MPNST cell lines express the epidermal growth factor receptor and lack S100beta protein. Global gene expression profiling using Affymetrix oligonucleotide microarrays identified a 159-gene molecular signature distinguishing MPNST cell lines from normal Schwann cells, which was validated in Affymetrix microarray data generated from 45 primary MPNSTs. Expression of Schwann cell differentiation markers (SOX10, CNP, PMP22, and NGFR) was down-regulated in MPNSTs whereas neural crest stem cell markers, SOX9 and TWIST1, were overexpressed in MPNSTs. Previous studies have implicated TWIST1 in apoptosis inhibition, resistance to chemotherapy, and metastasis. Reducing TWIST1 expression in MPNST cells using small interfering RNA did not affect apoptosis or chemoresistance but inhibited cell chemotaxis. Our results highlight the use of gene expression profiling in identifying genes and molecular pathways that are potential biomarkers and/or therapeutic targets for treatment of MPNST and support the use of the MPNST cell lines as a primary analytic tool.

Twist is substrate for caspase cleavage and proteasome-mediated degradation

Twist is a member of the basic helix-loop-helix family of transcription factors. An aberrant Twist expression has been found in diverse types of cancer, including sarcomas, carcinomas and lymphomas, supporting a role for Twist in tumor progression. Twist is known to be essential for mesodermal development. However, since a prolonged Twist expression results in a block of muscle, cartilage and bone differentiation, Twist has to be excluded from somites during late embryogenesis for terminal differentiation to occur. This implies that Twist expression must be target of a tight control. Here we provide evidence that Twist undergoes post-transcriptional regulation. Twist is substrate for cleavage by caspases during apoptosis and its cleavage results in ubiquitin-mediated proteasome degradation. Our findings suggest that Twist post-transcriptional regulation may play an important role in tissue determination and raise the possibility that alterations in the protein turnover may account for Twist overexpression observed in tumors.

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.

Twist1 dimer selection regulates cranial suture patterning and fusion

Saethre-Chotzen syndrome is associated with haploinsufficiency of the basic-helix-loop-helix (bHLH) transcription factor TWIST1 and is characterized by premature closure of the cranial sutures, termed craniosynostosis; however, the mechanisms underlying this defect are unclear. Twist1 has been shown to play both positive and negative roles in mesenchymal specification and differentiation, and here we show that the activity of Twist1 is dependent on its dimer partner. Twist1 forms both homodimers (T/T) and heterodimers with E2A E proteins (T/E) and the relative level of Twist1 to the HLH inhibitor Id proteins determines which dimer forms. On the basis of the expression patterns of Twist1 and Id1 within the cranial sutures, we hypothesized that Twist1 forms homodimers in the osteogenic fronts and T/E heterodimers in the mid-sutures. In support of this hypothesis, we have found that genes regulated by T/T homodimers, such as FGFR2 and periostin, are expressed in the osteogenic fronts, whereas genes regulated by T/E heterodimers, such as thrombospondin-1, are expressed in the mid-sutures. The ratio between these dimers is altered in the sutures of Twist1+/- mice, favoring an increase in homodimers and an expansion of the osteogenic fronts. Of interest, the T/T to T/E ratio is greater in the coronal versus the sagittal suture, and this finding may contribute to making the coronal suture more susceptible to fusion due to TWIST haploinsufficiency. Importantly, we were able to inhibit suture fusion in Twist1+/- mice by modulating the balance between these dimers toward T/E formation, by either increasing the expression of E2A E12 or by decreasing Id expression. Therefore, we have identified dimer partner selection as an important mediator of Twist1 function and provide a mechanistic understanding of craniosynostosis due to TWIST haploinsufficiency.

Biological Activities Encoded by the Murine Mesenchymal Stem Cell Transcriptome Provide a Basis for Their Developmental Potential and Broad Therapeutic Efficacy

We used serial analysis of gene expression to catalog the transcriptome of murine mesenchymal stem cells (MSCs) enriched from bone marrow by immunodepletion. Interrogation of this database, results of which are delineated in the appended databases, revealed that immunodepleted murine MSCs (IDmMSCs) highly express transcripts encoding connective tissue proteins and factors modulating T-cell proliferation, inflammation, and bone turnover. Categorizing the transcriptome based on gene ontologies revealed the cells also expressed mRNAs encoding proteins that regulate mesoderm development or that are characteristic of determined mesenchymal cell lineages, thereby reflecting both their stem cell nature and differentiation potential. Additionally, IDmMSCs also expressed transcripts encoding proteins regulating angiogenesis, cell motility and communication, hematopoiesis, immunity and defense as well as neural activities. Immunostaining and fluorescence-activated cell sorting analysis revealed that expression of various regulatory proteins was restricted to distinct subpopulations of IDmMSCs. Moreover, in some cases, these proteins were absent or expressed at reduced levels in other murine MSC preparations or cell lines. Lastly, by comparing their transcriptome to that of 17 other murine cell types, we also identified 43 IDmMSC-specific transcripts, the nature of which reflects their varied functions in bone and marrow. Collectively, these results demonstrate that IDmMSC express a diverse repertoire of regulatory proteins, which likely accounts for their demonstrated efficacy in treating a wide variety of diseases. The restricted expression pattern of these proteins within populations suggests that the cellular composition of marrow stroma and its associated functions are more complex than previously envisioned.

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.

Expression Profiles of Osteosarcoma That Can Predict Response to Chemotherapy

Osteosarcoma is the most common malignant bone tumor in children. After initial diagnosis is made with a biopsy, treatment consists of preoperative chemotherapy followed by definitive surgery and postoperative chemotherapy. The degree of tumor necrosis in response to preoperative chemotherapy is a reliable prognostic factor and is used to guide the choice of postoperative chemotherapy. Patients with tumors, which reveal > or = 90% necrosis (good responders), have a much better prognosis than those with < 90% necrosis (poor responders). Despite previous attempts to improve the outcome of poor responders by modifying the postoperative chemotherapy, their prognosis remains poor. Therefore, there is a need to predict at the time of diagnosis patients' response to preoperative chemotherapy. This will provide the basis for developing potentially effective therapy that can be given at the outset for those who are likely to have a poor response. Here, we report the analysis of 34 pediatric osteosarcoma samples by expression profiling. Using parametric two-sample t test, we identified 45 genes that discriminate between good and poor responders (P < 0.005) in 20 definitive surgery samples. A support vector machine classifier was built using these predictor genes and was tested for its ability to classify initial biopsy samples. Five of six initial biopsy samples that had corresponding definitive surgery samples in the training set were classified correctly (83%; confidence interval, 36%, 100%). When this classifier was used to predict eight independent initial biopsy samples, there was 100% accuracy (confidence interval, 63%, 100%). Many of the predictor genes are implicated in bone development, drug resistance, and tumorigenesis.

Up-regulation of TWIST in prostate cancer and its implication as a therapeutic target

Androgen-independent metastatic prostate cancer is the main obstacle in the treatment of this cancer. Unlike a majority of solid cancers, prostate cancer usually shows poor response to chemotherapeutic drugs. In this study, we have shown a potential novel target, TWIST, a highly conserved bHLH transcription factor, in the treatment of prostate cancer. Using malignant and nonmalignant prostate tissues, we found that TWIST expression was highly expressed in the majority (90%) of prostate cancer tissues but only in a small percentage (6.7%) of benign prostate hyperplasia. In addition, the TWIST expression levels were positively correlated with Gleason grading and metastasis, indicating its role in the development and progression of prostate cancer. Furthermore, down-regulation of TWIST through small interfering RNA in androgen-independent prostate cancer cell lines, DU145 and PC3, resulted in increased sensitivity to the anticancer drug taxol-induced cell death which was associated with decreased Bcl/Bax ratio, leading to activation of the apoptosis pathway. More importantly, inactivation of TWIST suppressed migration and invasion abilities of androgen-independent prostate cancer cells, which was correlated with induction of E-cadherin expression as well as morphologic and molecular changes associated with mesenchymal to epithelial transition. These results were further confirmed on the androgen-dependent LNCaP cells ectopically expressing the TWIST protein. Our results have identified TWIST as a critical regulator of prostate cancer cell growth and suggest a potential therapeutic approach to inhibit the growth and metastasis of androgen-independent prostate cancer through inactivation of the TWIST gene.

Twist is required for establishment of the mouse coronal suture

Cranial sutures are the growth centres of the skull, enabling expansion of the skull to accommodate rapid growth of the brain. Haploinsufficiency of the human TWIST gene function causes the craniosynostosis syndrome, Saethre-Chotzen syndrome (SCS), in which premature fusion of the coronal suture is a characteristic feature. Previous studies have indicated that Twist is expressed in the coronal suture during development, and therefore that it may play an important role in development and maintenance of the suture. The Twist-null mouse is lethal before the onset of osteogenesis, and the heterozygote exhibits coronal suture synostosis postnatally. In this study we investigated the function of Twist in the development of the mouse coronal suture, by inhibiting Twist synthesis using morpholino antisense oligonucleotides in calvarial organ culture. Decreased Twist production resulted in a narrow sutural space and fusion of bone domains within 48 h after the addition of the morpholino oligonucleotides. Proliferation activity in the sutural cells was decreased, and the expression of osteogenic marker genes such as Runx2 and Fgfr2 was up-regulated in the developing bone domain within 4 h. These results suggest that during establishment of the suture area, Twist is required for the regulation of sutural cell proliferation and osteoblast differentiation.

Clinical and Genetic Analysis of Patients with Saethre-Chotzen Syndrome

BACKGROUND

Saethre-Chotzen syndrome is a craniosynostosis syndrome further characterized by distinctive facial and limb abnormalities. It shows complete penetrance and variable expressivity and has been linked to the TWIST gene on chromosome 7p21; more than 80 different intragenic mutations and, recently, large deletions have been detected in Saethre-Chotzen patients. The aim of this study was to genetically and phenotypically characterize patients with a clinical diagnosis of Saethre-Chotzen syndrome.

METHODS

Patients with a clinical diagnosis as well as those with a genetic diagnosis of Saethre-Chotzen syndrome (n = 34) were included in the study.

RESULTS

The study showed that the important features of Saethre-Chotzen syndrome are brachycephaly (occurring in 74 percent of patients), a broad, depressed nasal bridge (65 percent), a high forehead (56 percent), ptosis (53 percent), and prominent auricular crura (56 percent). Furthermore, using different molecular techniques, pathogenic mutations in the TWIST gene were identified in 71 percent of patients.

CONCLUSIONS

Patients with deletions of the TWIST gene did not differ from those with intragenic TWIST mutations in frequency or severity of craniofacial abnormalities. However, they did distinguish themselves by the presence of many additional anomalies and diseases and--most importantly--the high frequency of mental retardation, which was borderline significant. The authors conclude that when using stringent inclusion criteria for studies of Saethre-Chotzen syndrome, patients who have a pathogenic mutation of the TWIST gene should be excluded.

In vitro differentiation profile of osteoblasts derived from patients with Saethre-Chotzen syndrome

Seathre-Chotzen syndrome (SCS) is an autosomal dominant craniosynostosis syndrome, associated with loss-of-function mutations in the basic helix-loop-helix transcription factor, TWIST1. The biologic activity of TWIST1 has been implicated in the inhibition of differentiation of multiple cell lineages. Therefore, premature fusion of cranial sutures (craniosynostosis) in SCS may be mediated by altered differentiation of calvarial osteoblasts. In this study, we evaluated osteoblasts derived from calvarial bone of three patients with SCS and three unaffected individuals as controls to investigate the principle stages of osteoblast differentiation: (1) proliferation, (2) matrix maturation, and (3) mineralization. Using a BrdU-Hoechst flow cytometry assay, we found that the percent of proliferating cells was significantly reduced in cells derived from patients with SCS compared with those derived from controls (P < or = 0.05). In the matrix maturation stage, alkaline phosphatase (ALP) enzyme activity and the expression of extracellular matrix genes, collagen I alpha 2 (COL1A2), osteopontin (OPN), osteocalcin (OC), and the runt-related transcription factor RUNX2 were examined by enzymatic assay and real-time quantitative RT-PCR, respectively. We identified no significant differences in the expression of matrix related transcripts. However, we found significant reductions in ALP activity on days 3 and 7 and in RUNX2 expression on days 14 and 21 (P < or = 0.05). Quantitative alizarin red S mineralization assays showed a trend toward increased mineralization in osteoblasts derived from patients with SCS at days 21 and 28, although not statistically significant. Our results demonstrated that loss-of-function mutations of TWIST1 led to reduced proliferation regardless of the functional domain affected. We did not find any conclusive differences in matrix maturation or mineralization in these primary osteoblasts. It is plausible that mutations in different functional domains of TWIST1 have divergent effects on these later stages of differentiation.

FGF-2 Acts through an ERK1/2 Intracellular Pathway to Affect Osteoblast Differentiation

An abundance of genetic and experimental data have suggested that fibroblast growth factor (FGF) signaling plays a central role in physiological and pathological cranial suture fusion. Although alterations in the differentiation and proliferation of sutural osteoblasts may be a key mediator of this process, the mechanisms by which FGF signaling regulates osteoblast differentiation remain incompletely understood. In the current study, the authors show that recombinant human FGF-2 alters osteoblastic expression of bone morphogenetic protein-2 and Msx-2 in vitro to favor cellular differentiation and osteoinduction. The ERK1/2 intracellular signaling cascade was shown to be necessary for recombinant human FGF-2-mediated bone morphogenetic protein-2 transcriptional changes. Furthermore, the cellular production of an intermediate transcriptional modifier was found to be necessary for the recombinant human FGF-2-mediated gene expression changes in bone morphogenetic protein-2 and Msx-2. Together, these findings offer new insight into the mechanisms by which FGF-2 modulates osteoblast biology.

Functional analysis of natural mutations in two TWIST protein motifs

The basic helix-loop-helix protein Twist, a transcriptional repressor, is essential for embryogenesis in both invertebrates and vertebrates. Haploinsufficiency of the human TWIST1 gene, which causes the craniosynostosis disorder Saethre-Chotzen syndrome (SCS), is related to failure to repress transcription of CDKN1A (which encodes p21/WAF1/CIP1), promoting osteoblast differentiation. We have examined the functional significance of natural TWIST1 variants present in craniosynostosis patients and in their healthy relatives. Both deletion and duplication variants of the glycine-rich tract Gly5AlaGly5 inhibited E2A (E12/E47)-dependent transcription of CDKN1A to a similar degree as wild-type protein, indicating that the length of this glycine tract is not critical for efficient transcriptional repression. We also evaluated a newly identified heterozygous TWIST1 variant (c.115C>G, encoding p.Arg39Gly), located within a putative nuclear localization signal (NLS), that was present in a child with mild SCS and her clinically unaffected father and grandmother. Unlike wild-type protein, this mutant required cotransfected E12 to localize to the nucleus, indicating that the NLS, including amino acid 39, is essential for nuclear localization; inhibition of E2A-dependent transcription of CDKN1A occurred normally. This analysis further dissects the structure-function relationships of TWIST and corroborates with phenotypic observations of disease expressivity.

Identification of a novel function of TWIST, a bHLH protein, in the development of acquired taxol resistance in human cancer cells

Taxol is one of the widely used chemotherapeutic drugs against many types of human cancer. While it is considered as one of the most effective anticancer drugs, treatment failure often occurs due to development of acquired resistance. Therefore, it is important to understand the molecular mechanisms responsible for the development of drug resistance. Although it is generally believed that taxol induces cell death through interfering with microtubules leading to mitotic arrest, recent evidence has suggested that taxol-induced cell death also occurs through pathways independent of either microtubule or mitotic arrest. In this study, we report the identification of a novel role for TWIST, a basic helix-loop-helix protein, which plays a central role in cell type determination and differentiation, during generation of acquired resistance to taxol in a nasopharyngeal carcinoma cell line, HNE1-T3, using comparative genome hybridization (CGH) and subsequent RT-PCR and Western blotting. We found that upregulation of TWIST was associated with cellular resistance to taxol but not other drugs with different mechanisms of action. The fact that increased TWIST protein levels were also associated with another microtubule-targeting anticancer drug, vincristine, in four types of human cancer including nasopharyngeal, bladder, ovarian and prostate, indicates that it may play a central role in the resistance to microtubule-disrupting agents. In addition, ectopic expression of TWIST into human cancer cells also led to increased resistance to both taxol and vincristine. Our results indicate a novel mechanism that leads to resistance to microtubule-disrupting anticancer drugs through upregulation of TWIST. Our evidence provides a therapeutic strategy to overcome acquired resistance through inactivation of TWIST expression in human cancer.

Twist is required for patterning the cranial nerves and maintaining the viability of mesodermal cells

Twist encodes a basic helix-loop-helix transcription factor that is required for normal craniofacial morphogenesis in the mouse. Loss of Twist activity in the cranial mesenchyme leads to aberrant migratory behaviour of the neural crest cells, whereas Twist-deficient neural crest cells are located in an inappropriate location in the first branchial arch and display defective osteogenic and odontogenic differentiation (Soo et al. [2002] Dev. Biol. 247:251-270). Results of the present study further show that loss of Twist impacts on the patterning of the cranial ganglia and nerves but not that of the peripheral ganglia and nerves in the trunk region of the body axis. Analyses of the expression of molecular markers of early differentiation of the paraxial mesoderm and the histogenetic potency of somites of Twist(-/-) embryos reveal that Twist-deficient somites can differentiate into muscles, cartilage, and bones, albeit less prolifically. Twist function, therefore, is not essential for mesoderm differentiation. The poor growth of the Twist-deficient somites after transplantation to the ectopic site may be attributed to reduced proliferative capacity and extensive apoptosis of the paraxial mesoderm, suggesting that Twist is required for maintaining cell proliferation and viability in the mesodermal progenitors.

DNA methylation ofRASSF1A, HIN-1, RAR-?, Cyclin D2 andTwist inin situ and invasive lobular breast carcinoma

Little is known about epigenetic silencing of genes by promoter hypermethylation in lobular breast cancers. The promoter methylation status of 5 cancer-related genes (RASSF1A, HIN-1, RAR-beta, Cyclin D2 and Twist) was evaluated in 2 types of lobular cancers, in situ (LCIS) and invasive lobular carcinomas (ILC) (n = 32), and compared to ductal in situ (DCIS) and invasive (IDC) breast cancers (n = 71). By using methylation-specific PCR (MSP), 100% of ILC and 69% of LCIS cases were found to have 1 or more hypermethylated genes among the panel of 5 genes (compared to 100% IDC and 95% of DCIS). Two or more hypermethylated genes were detected per tumor in 79% of invasive and 61% of in situ lobular carcinomas compared to 81% of IDC and 77% of DCIS. By contrast, DNA from nearly all normal reduction mammoplasty tissues (n = 8) was unmethylated for the 5 genes. The methylation profiles of lobular vs. ductal carcinomas with respect to RASSF1A, Cyclin D2, RARbeta, and Hin-1 genes were similar, suggesting that gene silencing by promoter hypermethylation is likely to be important in both groups of diseases. Distinctly different, Twist was hyper- methylated less often in ILC (16%, 3/19 cases) than in IDC (56%, 15/27 cases) (p = 0.01). These results suggest that these 2 types of tumors share many common methylation patterns and some molecular differences. Additional studies might lend further understanding into the etiology and clinical behavior of this tumor type.

Markers of Osteoblast Differentiation in Fusing and Nonfusing Cranial Sutures

Accumulating clinical genetic data support the hypothesis that alterations in osteoblast differentiation are closely associated with craniosynostoses. Gain-of-function mutations in FGFR1, FGFR2, FGFR3, and Msx2 and loss-of-function mutations in Twist are examples of such alterations. Several studies have examined how these mutations alter the expression patterns for transcription factors such as Runx2 and noncollagenous extracellular matrix molecules such as osteopontin and osteocalcin. One limitation of such studies is that they examine samples derived from craniosynostotic patients with sutures that have already fused, thus missing the dynamic osteogenic process of suture fusion. In this study, in situ hybridization was used to localize Runx2, osteopontin, and osteocalcin expression in the sagittal and posterior frontal sutures in mice (n = 20), before (day 13), during (days 23, 33, and 43), and after (day 53) the period of physiological posterior frontal suture fusion. The data demonstrated similar patterns of expression in fusing (posterior frontal) and nonfusing (sagittal) sutures. The expression of all three genes was primarily concentrated in the osteogenic fronts of both sutures and decreased with time. Notably, none of the three genes was expressed in the mesenchyme of either fusing or nonfusing sutures. The data suggest that the molecular signals leading to bone formation along the osteogenic fronts in fusing and nonfusing sutures are similar, raising the possibility that other factors, such as antagonists of osteogenesis, might have a role in maintaining suture patency.

Differentiation potential of a mouse bone marrow stromal cell line

In order to study osteoblast differentiation we subcloned a cell derived from a mouse a bone marrow stromal cell line, Kusa O, and obtained a number of clones representative of three different phenotypes. One that neither differentiated into osteoblasts nor into adipocytes, a second that differentiated into osteoblasts but not adipocytes, and a third that differentiated into both osteoblasts and adipocytes. Four subclones were selected for further characterization according to their ability to mineralize and/or differentiate into adipocytes. The non-mineralizing clone had no detectable alkaline phosphatase activity although some alkaline phosphatase mRNA was detected after 21 days in osteoblast differentiating medium. Alkaline phosphatase activity and mRNA in the three mineralizing clones were comparable with the parent clones. Osteocalcin mRNA and protein levels in the non-mineralizing clone were low and non-detectable, respectively, while both were elevated in the parent cells and mineralizing subclones after 21 days in differentiating medium. PTH receptor mRNA and activity increased in the four subclones and parent cells with differentiation. mRNA for two other osteoblast phenotypic markers, osteopontin and bone sialoprotein, were similarly expressed in the parent cells and subclones while mRNAs for the transcription factors, Runx2 and osterix, were detectable in both parent and subclone cells. Runx2 was unchanged with differentiation while osterix was increased. Interestingly, PPARgamma mRNA expression did not correlate with cell line potential to differentiate into adipocytes. Indian hedgehog mRNA and its receptor (patched) mRNA levels both increased with differentiation while mRNA levels of the Wnt pathway components beta-catenin and dickkopf also increased with differentiation. Although we have focussed on characterizing these clones from the osteoblast perspective it is clear that they may be useful for studying both osteoblast and adipocyte differentiation as well as their transdifferentiation.

Characterization of mesenchymal stem cells isolated from murine bone marrow by negative selection

Mesenchymal stem cells (MSCs) are typically enriched from bone marrow via isolation of the plastic adherent, fibroblastoid cell fraction. However, plastic adherent cultures elaborated from murine bone marrow are an admixture of fibroblastoid and hematopoietic cell types. Here we report a reliable method based on immunodepletion to fractionate fibroblastoid cells from hematopoietic cells within plastic adherent murine marrow cultures. The immunodepleted cells expressed the antigens Sca-1, CD29, CD44, CD81, CD106, and the stem cell marker nucleostemin (NST) but not CD11b, CD31, CD34, CD45, CD48, CD90, CD117, CD135, or the transcription factor Oct-4. They were also capable of differentiating into adipocytes, chondrocytes, and osteoblasts in vitro as well as osteoblasts/osteocytes in vivo. Therefore, immunodepletion yields a cell population devoid of hematopoietic and endothelial cells that is phenotypically and functionally equivalent to MSCs. The immunodepleted cells exhibited a population doubling time of approximately 5-7 days in culture. Poor growth was due to the dramatic down regulation of many genes involved in cell proliferation and cell cycle progression as a result of immunodepletion. Exposure of immunodepleted cells to fibroblast growth factor 2 (FGF2) but not insulin-like growth factor (IGF), murine stem cell factor, or leukemia inhibitory factor (LIF) significantly increased their growth rate. Moreover, 82% of the transcripts down regulated by immunodepletion remain unaltered in the presence of FGF2. Exposure to the later also reversibly inhibited the ability of the immunodepleted cells to differentiate into adipocytes, chondrocytes, and osteoblasts in vitro. Therefore, FGF2 appears to function as a mitogen and self-maintenance factor for murine MSCs enriched from bone marrow by negative selection.

Transcriptional profiling of human osteoblast differentiation

Osteoblast differentiation is a key aspect of bone formation and remodeling. To further our understanding of the differentiation process, we have developed a collection of conditionally immortalized adult human osteoblast cell lines representing discrete stages of differentiation. To evaluate changes in gene expression associated with differentiation, polyA((+)) RNA from pre-osteoblasts, early and late osteoblasts, and pre-osteocytes was subjected to gene chip analysis using the Affymetrix Hu6800 chip in conjunction with an Affymetrix custom chip enriched in bone and cartilage cDNAs. Overall, the expression of 47 genes was found to change threefold or more on both chips between the pre-osteoblastic and pre-osteocytic stages of differentiation. Many of the observed differences, including down-regulation of collagen type I and collagen-processing enzymes, reflect expected patterns and support the relevance of our results. Other changes have not been reported and offer new insight into the osteoblast differentiation process. Thus, we observed regulation of factors controlling cell cycle and proliferation, reflecting decreased proliferation, and increased apoptosis in pre-osteocytic cells. Elements maintaining the cytoskeleton, extracellular matrix, and cell-cell adhesion also changed with differentiation reflecting profound alterations in cell architecture associated with the differentiation process. We also saw dramatic down-regulation of several components of complement and other immune response factors that may be involved in recruitment and differentiation of osteoclasts. The decrease in this group of genes may provide a mechanism for controlling bone remodeling of newly formed bone. Our screen also identified several signaling proteins that may control osteoblast differentiation. These include an orphan nuclear receptor DAX1 and a small ras-related GTPase associated with diabetes, both of which increased with increasing differentiation, as well as a high mobility group-box transcription factor, SOX4, that was down-regulated during differentiation. In summary, our study provides a comprehensive transcriptional profile of human osteoblast differentiation and identifies several genes of potential importance in controlling differentiation of osteoblasts.

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

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

The twist gene is a common target of retroviral integration and transcriptional deregulation in experimental nephroblastoma

The genes involved in the transformation of kidney blastema cells were searched for in avian nephroblastomas induced by the MAV2 retrovirus. The twist gene was identified as a common site of provirus integration in tumor cells. Twist was rearranged by the MAV2 provirus in three out of 76 independent nephroblastoma samples. The MAV2 integration sites were localized within 40 nucleotides of the twist 5'UTR region, right upstream from the ATG initiation codon. The integrated proviruses were deleted at their 5'ends. As a consequence, twist transcription became controlled by the retroviral 3'LTR promoter and was strongly upregulated, more than 200 times. In addition, 2-100 times elevated twist transcription was also detected in the majority of other nephroblastoma samples not containing MAV2 in the twist locus. We propose that chicken nephroblastoma originates from a single blastemic cell in which the MAV retrovirus, through its integration, has deregulated specific combinations of genes controlling proliferation and differentiation. The activation of the twist gene expression appears to contribute to tumorigenesis, as there is an in vivo positive selection of tumor cell clones containing the twist gene hyperactivated by MAV2 sequences inserted within the twist promoter.

DNazyme-mediated cleavage of Twist transcripts and increase in cellular apoptosis

DNazymes is a group of catalytic nucleic acids that can be designed to cleave target mRNA molecules in a base-specific way. Twist is a basic helix-loop-helix transcription factor that is involved in the regulation of cellular differentiation and apoptosis. Moreover, it was shown to function in skull development and cause craniosynostosis. DZ-TWT DNazyme was designed to down-regulate Twist expression. The ability of DZ-TWT to cleave mouse Twist mRNA was first shown in a cell-free environment against full-length Twist mRNA. Following transfections of the DZ-TWT in C3H10T1/2 cells, a significant reduction of Twist mRNA levels was observed. This was accompanied by a significant rise in p21 mRNA levels. Finally, DZ-TWT transfections resulted in an increase of cellular apoptosis, demonstrating the importance of Twist in apoptotic pathways. These results prove the usefulness of DNazymes to characterize Twist gene function and further experiments in animals should demonstrate its complete physiological role.

New insights into craniosynostosis

Craniosynostosis is a congenital developmental disorder involving premature fusion of cranial sutures, often associated with multiple neurological manifestations. The perspective of this group of disorders has changed dramatically in the new era of molecular genetics. In the last decade a large literature with new concepts in craniosynostosis has appeared. More than 100 syndromes associated with craniosynostosis have been described, and in about a dozen, the molecular defect has been identified. Pediatric neurologists are less aware than geneticists, neurosurgeons, and craniofacial surgeons of these changes. General concepts about craniosynostosis are here presented with updates of clinical and genetic aspects of well-defined syndromes such as Apert, Crouzon, Pfeiffer, Saethre-Chotzen. Evidence of their relationship with fibroblast growth factor receptors (FGFRs) 1, 2, and 3, and with causative genes such as TWIST has been documented. New and other less common syndromes also are discussed. The differences between positional and synostotic plagiocephaly are important, as well as the cause of nonsyndromic craniosynostosis. The prognosis and neurological outcome of patients, including "benign" forms of craniosynostosis, are other important aspects. Major advances have occurred in understanding pathogenesis, diagnosis, and treatment of craniosynostosis. The role of local dura mater and apoptosis; modalities of imaging such as prenatal ultrasound and three-dimensional and spiral CT have improved the accuracy in diagnosis, and the new approaches in surgical treatment involving efficient and less invasive methods, are evidence of these advances.

Assessment of the gene expression profile of differentiated and dedifferentiated human fetal chondrocytes by microarray analysis

OBJECTIVE

To study the changes in patterns of gene expression exhibited by human chondrocytes as they dedifferentiate into fibroblastic cells in culture in order to better understand the mechanisms that control this process and its relationship to the phenotypic changes that occur in chondrocytes during the development of osteoarthritis (OA).

METHODS

Human fetal epiphyseal chondrocytes (HFCs) were cultured either on poly-(2-hydroxyethyl methacrylate)-coated plates (differentiated HFC cultures) or in plastic tissue culture flasks as monolayers (dedifferentiated HFC cultures). Following 11 days of culture under either condition, poly(A+) RNA was isolated from the two cell populations and subjected to a gene expression analysis using a microarray containing approximately 5,000 known human genes and approximately 3,000 expressed sequence tags (ESTs).

RESULTS

A > or =2-fold difference in the expression of 62 known genes and 6 ESTs was observed between the two cell types. The differences in expression of several of the genes detected by the microarray hybridization were confirmed by Northern analyses. Two transcription factor genes, TWIST and HIF-1alpha, and a cellular adhesion protein gene, cadherin 11, were markedly regulated in response to differentiation and dedifferentiation. Expression of these genes was also detected in adult normal and OA cartilage and chondrocytes. Analysis of the gene expression profile of HFCs revealed a complex pattern of gene expression, including many genes not yet reported to be expressed by chondrocytes.

CONCLUSION

Chondrocytes in monolayer become dedifferentiated, acquiring a fibroblast-like appearance and changing their pattern of gene expression from one of expression of chondrocyte-specific genes to one that resembles a fibroblastic or chondroprogenitor-like pattern. Changes in gene expression associated with the process of dedifferentiation of HFCs in vitro were observed in a wide variety of genes, including genes encoding extracellular matrix proteins, transcription factors, and growth factors. At least 3 of the genes that were regulated in response to dedifferentiation were also found to be expressed in adult normal and OA articular cartilage and chondrocytes.

Common regulation of growth arrest and differentiation of osteoblasts by helix-loop-helix factors

Cellular differentiation entails the coordination of cell cycle arrest and tissue-specific gene expression. We investigated the involvement of basic helix-loop-helix (bHLH) factors in differentiation of osteoblasts using the human osteoblastic cell line MG63. Serum starvation induced growth arrest at G1 phase, accompanied by expression of cyclin-dependent kinase inhibitor p21(WAF1/Cip1). Reporter assays with the p21 gene promoter demonstrated that the combination of E2A (E12 or E47) and coactivator CBP was responsible for p21 induction independent of p53. Twist inhibited E2A-CBP-dependent activation of the exogenous and endogenous p21 promoters. Ids similarly inhibited the exogenously transfected p21 promoter; however less antagonistic effect on the endogenous p21 promoter was observed. Twist was predominantly present in nuclei in MG63 cells growing in complete medium, while it localized mainly in the cytoplasm after serum starvation. The fibroblast growth factor receptor 3 gene (FGFR3), which generates signals leading to differentiation of osteoblasts, was found to be controlled by the same transcriptional regulation as the p21 gene. E2A and Twist influenced alkaline phosphatase expression, a consensus marker of osteoblast differentiation. Expression of E2A and FGFR3 was seen at the location of osteoblast differentiation in the calvaria of mouse embryos, implicating bHLH molecules in physiological osteoblast differentiation. These results demonstrate that a common regulatory system is involved in at least two distinct steps in osteoblastic differentiation. Our results also provide the molecular basis of Saethre-Chotzen syndrome, caused by mutations of the TWIST and FGFR3 genes.

Phenotypic findings due to trisomy 7p15.3-pter including the TWIST locus

We report on a three-month-old boy with a 46,XY,der(Y)t(Y;7)(p11.32;p15.3) karyotype and growth deficiency, postnatal microcephaly with large fontanels, wide sagittal and metopic sutures, hypertelorism, choanal stenosis, micrognathia, bilateral cryptorchidism, hypospadias, abnormal fingers and toes, and severe developmental delay. FISH studies showed partial trisomy 7p resulting from a de novo unbalanced translocation. The application of molecular probes from the TWIST gene region (7p15.3-p21.1) and probes from the pseudoautosomal region (PAR) demonstrated that the 7p15.3-pter fragment was translocated onto Yp with the breakpoint within approximately 20 kb from the Yp telomere. We discuss the possible role of the TWIST gene in abnormal skull development and suggest that trisomy 7p cases with delayed closure of fontanels can be a result of TWIST gene dosage effect.

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.

Osteogenesis and bone-marrow-derived cells

This paper addresses some of the important aspects of stem cell commitment to the bone cell lineage examining the various types of precursor cells, their responses to cytokines and other extracellular influences, and recent observations on the biochemical and molecular control of lineage-specific gene expression. The process of osteopoiesis involves the proliferation and maturation of primitive precursor cells into functional osteoblasts. The bone cells purportedly originate from mesenchymal stem cells that commit to the osteogenic cell lineage becoming osteoprogenitor cells, preosteoblasts, osteoblasts, and osteocytes. Further understanding of this developmental process requires that lineage-specific markers be identified for the various populations of bone cells and their precursors, that cell separation techniques be established so that cells of the osteogenic lineage can be purified at different stages of differentiation, and that these isolated cells are studied under serum-free, chemically defined conditions.

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.

Transcriptional control of adipogenesis

The major transcriptional factors involved in the adipogenic process include proteins belonging to the CCAAT/enhancer binding protein family, peroxisome proliferator-activated receptor gamma, and adipocyte determination and differentiation dependent factor 1, also known as sterol regulatory element-binding protein 1. This process has been characterized with the aid of cell lines that represent various stages in the path of adipocyte commitment, ranging from pluripotent mesodermal fibroblasts to preadipocytes. Molecular analyses have led to a cascade model for adipogenesis based on timed expression of CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptor gamma. Gene targeting and transgenic-mouse technologies, which allow the manipulation of endogenous genes for these transcription factors, have also contributed to the understanding of adipogenesis. This review aims to integrate this information to gain an understanding of the transcriptional regulation of fat cell formation.

Human Dermo-1 has attributes similar to twist in early bone development

Basic helix-loop-helix (bHLH) transcription factors are implicated in cell lineage determination and differentiation. Dermo-1 encodes a bHLH transcription factor that shares extensive homology with another bHLH transcription factor, Twist. We have cloned and characterized human Dermo-1 from two different bone cytoplasmic DNA (cDNA) libraries. Dermo-1 mRNA and protein expression were examined in human embryo and adult tissue sections. Dermo-1 is expressed in a subset of mesodermally and ectodermally derived tissues. We further examined expression of Dermo-1/Twist in human tissues and cell lines. In addition, we observed Dermo-1 expression in response to basic fibroblast growth factor in osteoblastic cell lines. To evaluate the functionality of the human Dermo-1 transcription factor in osteoblast metabolism, we made stable osteoblastic cell lines that over- and underexpress human Dermo-1. These cell lines were analyzed and compared with previously published data of similar cell lines transfected with Twist. Our results demonstrate that Dermo-1 caused changes similar to Twist in the osteogenic properties of osteoblastic cells, such as morphology, bone marker gene expression, and biochemical response to cytokines. However, Dermo-1 expression also has unique effects in regulating the mechanism of proliferation, on alkaline phosphatase enzyme activity, and in temporal expression patterns. We speculate that expression of Twist and Dermo-1 maintains cells in an osteoprogenitor or preosteoblast-like state, respectively, and prevents premature or ectopic osteoblast differentiation. Therefore, Twist and Dermo-1 must be sequentially downregulated in order to initiate the cascade of events responsible for osteogenic cell differentiation. These results indicate that, during osteoblast development, Dermo-1 may inhibit osteoblast maturation and maintain cells in a preosteoblast phenotype by utilizing mechanisms similar but not identical to those utilized by Twist.