The human chordin gene encodes several differentially expressed spliced variants with distinct BMP opposing activities

The developmental gene Chordin is amplified and expressed in human cancers

Chordin (CHRD) is a secreted protein important in early development, yet a role for CHRD in human disease has not been identified. In this study we investigated CHRD in cancer and normal adult tissues using the wealth of genome-wide data available in public databases. We found that Chordin is amplified in the DNA of specific cancers such as lung squamous cell and others, although copy number variation did not strictly correlate with higher mRNA expression. In some cancers, such as renal and stomach carcinomas, increased CHRD expression significantly correlated with poor survival. In normal adult human tissues, CHRD mRNA was highest in hepatocytes. Crossveinless-2/BMPER, a component of the Chordin morphogenetic pathway expressed at the opposite side in embryos, was expressed in liver stellate cells. This raises the intriguing possibility that a BMP gradient might be established in the extracellular matrix of the space of Disse that surrounds portal sinusoid capillaries.

Structural characterization of twisted gastrulation provides insights into opposing functions on the BMP signalling pathway

Twisted gastrulation (Tsg) and chordin are secreted glycoproteins that function together as BMP (bone morphogenetic protein) antagonists to regulate BMP growth factor signalling. Chordin binds to BMPs, preventing them from interacting with their receptors and Tsg is known to strengthen this inhibitory complex. Tsg also acts as a BMP agonist by promoting cleavage of chordin by tolloid-family proteinases. Here we explore the structural mechanism through which Tsg exerts this dual activity. We have characterized the nanoscale structure of human Tsg using in-solution biomolecular analysis and show that Tsg is a globular monomer with a flattened cross shape. Tsg has a high proportion of N-linked glycans, in relation to its molecular weight, which supports a role in solubilising BMPs. Tsg binds with high affinity to the C-terminal region of chordin and was also able to inhibit BMP-7 signalling directly but did not have an effect on BMP-4 signalling. Although both Tsg and mammalian tolloid are involved in chordin cleavage, no interaction could be detected between them using surface plasmon resonance. Together these data suggest that Tsg functions as a BMP-agonist by inducing conformational change in chordin making it more susceptible to tolloid cleavage and as a BMP-antagonist either independently or via a chordin-mediated mechanism. Following single cleavage of chordin by tolloids, Tsg continues to strengthen the inhibitory complex, supporting a role for partially cleaved chordin in BMP regulation.

The role of chordin fragments generated by partial tolloid cleavage in regulating BMP activity

Chordin-mediated regulation of bone morphogenetic protein (BMP) family growth factors is essential in early embryogenesis and adult homoeostasis. Chordin binds to BMPs through cysteine-rich von Willebrand factor type C (vWC) homology domains and blocks them from interacting with their cell surface receptors. These domains also self-associate and enable chordin to target related proteins to fine-tune BMP regulation. The chordin–BMP inhibitory complex is strengthened by the secreted glycoprotein twisted gastrulation (Tsg); however, inhibition is relieved by cleavage of chordin at two specific sites by tolloid family metalloproteases. As Tsg enhances this cleavage process, it serves a dual role as both promoter and inhibitor of BMP signalling. Recent developments in chordin research suggest that rather than simply being by-products, the cleavage fragments of chordin continue to play a role in BMP regulation. In particular, chordin cleavage at the C-terminus potentiates its anti-BMP activity in a type-specific manner.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Insights into potential pathogenesis mechanisms associated with Campylobacter jejuni-induced abortion in ewes

Background

Campylobacter jejuni is commonly found in the gastrointestinal tract of many food-animals including sheep without causing visible clinical symptoms of disease. However, C. jejuni has been implicated in ovine abortion cases worldwide. Specifically, in the USA, the C. jejuni sheep abortion (SA) clone has been increasingly associated with sheep abortion. In vivo studies in sheep (the natural host) are needed to better characterize the virulence potential and pathogenesis of this clone.

Results

Pregnant ewes intravenously (IV) or orally inoculated with ovine or bovine abortion-associated C. jejuni SA clones exhibited partial or complete uterine prolapse with retained placenta, and abortion or stillbirth, whereas delivery of healthy lambs occurred in pregnant ewes inoculated with C. jejuni 81–176 or in the uninfected group. In sheep inoculated with the SA clone, histopathological lesions including suppurative necrotizing placentitis and/or endometritis coincided with: 1) increased apoptotic death of trophoblasts, 2) increased expression of the host genes (e.g. genes encoding interleukin IL-6 and IL-15) related to cellular necrosis and pro-inflammatory responses in uterus, and 3) decreased expression of the genes encoding GATA binding protein 6, chordin, and insulin-like 3 (INSL3) that account for embryonic development in uterus. Immunohistochemistry revealed localization of bacterial antigens in trophoblasts lining the chorioallantoic membrane of ewes inoculated with the C. jejuni SA clone.

Conclusions

The results showed that C. jejuni SA clones are capable of causing abortion or stillbirth in experimentally infected sheep. Furthermore, down- or up-regulation of specific genes in the uterus of infected pregnant ewes might implicate host genes in facilitating the disease progression. Since the C. jejuni SA strains share genotypic similarities with clones that have been isolated from human clinical cases of gastroenteritis, these strains might represent a potential public health risk.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-014-0274-8) contains supplementary material, which is available to authorized users.

Nanoscale structure of the BMP antagonist chordin supports cooperative BMP binding

Bone morphogenetic proteins (BMPs) orchestrate key cellular events, such as proliferation and differentiation, in development and homeostasis. Extracellular antagonists, such as chordin, are essential regulators of BMP signaling. Chordin binds to BMPs blocking interaction with receptors, and cleavage by tolloid proteinases is thought to relieve this inhibition. A model has been previously proposed where chordin adopts a horseshoe-like arrangement enabling BMP binding cooperatively by terminal domains (1). Here, we present the nanoscale structure of human chordin using electron microscopy, small angle X-ray scattering, and solution-based biophysical techniques, which together show that chordin indeed has a compact horseshoe-shaped structure. Chordin variants were used to map domain locations within the chordin molecule. The terminal BMP-binding domains protrude as prongs from the main body of the chordin structure, where they are well positioned to interact with the growth factor. The spacing provided by the chordin domains supports the principle of a cooperative BMP-binding arrangement that the original model implied in which growth factors bind to both an N- and C-terminal von Willebrand factor C domain of chordin. Using binding and bioactivity assays, we compared full-length chordin with two truncated chordin variants, such as those produced by partial tolloid cleavage. Cleavage of either terminal domain has little effect on the affinity of chordin for BMP-4 and BMP-7 but C-terminal cleavage increases the efficacy of chordin as a BMP-4 inhibitor. Together these data suggest that partial tolloid cleavage is insufficient to ablate BMP inhibition and the C-terminal chordin domains play an important role in BMP regulation.

Position matters: variability in the spatial pattern of BMP modulators generates functional diversity

Bone morphogenetic proteins (BMPs) perform a variety of functions during development. Considering a single BMP, what enables its multiple roles in tissues of varied sizes and shapes? What regulates the spatial distribution and activity patterns of the BMP in these different developmental contexts? Some BMP functions require controlling spread of the BMP morphogen, while others require formation of localized, high concentration peaks of BMP activity. Here we review work in Drosophila that describes spatial regulation of the BMP encoded by decapentaplegic (dpp) in different developmental contexts. We concentrate on extracellular modulation of BMP function and discuss the mechanisms that generate concentrated peaks of Dpp activity, subdivide territories of different activity levels or regulate spread of the Dpp morphogen from a point source. We compare these findings with data from vertebrates and non-model organisms to discuss how changes in the regulation of Dpp distribution by extracellular modulators may lead to variability in dpp function in different species.

BMP activated Smad signaling strongly promotes migration and invasion of hepatocellular carcinoma cells

Several of the different bone morphogenetic proteins (BMPs) are involved in development and progression of specific tumors. For hepatocellular carcinoma (HCC) only BMP4 and BMP6 are described to be important for carcinogenesis. However, up to now neither the influence of other BMPs on tumor progression, nor the responsible signaling pathways to mediate target gene expression in HCC are known. In order to characterize BMP expression pattern in HCC cell lines, we performed RT-PCR analysis and revealed enhanced expression levels of several BMPs (BMP4, 6, 7, 8, 9, 10, 11, 13 and 15) in HCC. Thus, we treated HCC cells with the general BMP inhibitors chordin and noggin to determine the functional relevance of BMP overexpression and observed decreased migration and invasion of HCC cells. A cDNA microarray of noggin treated HCC cells was performed to analyze downstream targets of BMPs mediating these oncogenic functions. Subsequent analysis identified collagen XVI as 'Smad signaling specific' and nidogen-2 as 'MAPK/ERK signaling specific' BMP-target genes. To examine which signaling pathway is mainly responsible for the oncogenic role of BMPs in HCC, we treated HCC cells with dorsomorphin to determine the influence of BMP activated Smad signaling. Interestingly, also migratory and invasive behavior of dorsomorphin treated HCC cells was diminished. In summary, our findings demonstrate enhanced expression levels of several BMPs in HCC supporting enhanced migratory and invasive phenotype of HCC cells mainly via activation of Smad signaling.

Identification and regulation of bone morphogenetic protein antagonists associated with preantral follicle development in the ovary

The TGFβ superfamily comprises several bone morphogenetic proteins (BMP) capable of exerting gonadotropin-independent effects on the development of small preantral follicles. In embryonic tissues, BMP concentration gradients, partly formed by antagonistic factors, are essential for establishing phenotypic fate. By examining the expression of candidate genes whose protein products are known to interact with BMP ligands, we set out to determine which antagonists would most likely contribute toward regulation of paracrine signaling during early follicle development. Juvenile mouse ovaries of 4, 8, 12, and 21 d of age enriched with follicles at successive developmental stages were used to assess changes in candidate gene transcripts by quantitative RT-PCR. Although some antagonists were found to be positively associated with the emergence of developing follicles (Nog, Htra1, Fst, Bmper, Vwc2), two (Sostdc1, Chrd) showed a corresponding reduction in expression. At each age, twisted gastrulation homolog 1 (Twsg1), Htra1, Nbl1, and Fst were consistently highly expressed and localization of these genes by in situ hybridization, and immunohistochemistry further highlighted a clear pattern of expression in granulosa cells of developing follicles. Moreover, with the exception of Nbl1, levels of these antagonists did not change in preantral follicles exposed to FSH in vitro, suggesting regulation by local factors. The presence of multiple antagonists in the juvenile ovary and their high level of expression in follicles imply the actions of certain growth factors are subject to local modulation and further highlights another important level of intraovarian regulation of follicle development.

Effects of moderate drinking during pregnancy on placental gene expression

Many children adversely affected by maternal drinking during pregnancy cannot be identified early in life using current diagnostic criteria for fetal alcohol spectrum disorder (FASD). We conducted a preliminary investigation to determine whether ethanol-induced alterations in placental gene expression may have some utility as a diagnostic indicator of maternal drinking during pregnancy and as a prognostic indicator of risk for adverse neurobehavioral outcomes in affected offspring. Pregnant Long-Evans rats voluntarily consumed either a 0 or 5% ethanol solution 4 h each day throughout gestation. Ethanol consumption produced a mean maternal daily intermittent peak serum ethanol concentration of 84 mg/dL. Placentas were harvested on gestational day 20 for gene expression studies. Microarray analysis of more than 28,000 genes revealed that the expression of 304 known genes was altered twofold or greater in placenta from ethanol-consuming dams compared with controls. About 76% of these genes were repressed in ethanol-exposed placentas. Gene expression changes involved proteins associated with central nervous system development; organ morphogenesis; immunological responses; endocrine function; ion homeostasis; and skeletal, cardiovascular, and cartilage development. To date, quantitative real-time polymerase chain reaction analysis has confirmed significant alterations in gene expression for 22 genes, including genes encoding for three calcium binding proteins, two matrix metalloproteinases, the cannabinoid 1, galanin 2 and toll-like receptor 4, iodothyronine deiodinase 2, 11-β hydroxysteroid dehydrogenase 2, placental growth factor, transforming growth factor alpha, gremlin 1, and epithelial growth factor (EGF)-containing extracellular matrix protein. These results suggest that the expression of a sufficiently large number of placental mRNAs is altered after moderate drinking during pregnancy to warrant more detailed investigation of the placenta as a biomarker system for maternal drinking during pregnancy and as an early indicator of FASD. Furthermore, these results provide new insights into novel mechanisms on how ethanol may directly or indirectly mediate its teratogenic effects through alterations in placental function during pregnancy.

Regulating gene expression with light-activated oligonucleotides

Since the development of light-responsive amino acids, the activity of numerous biomolecules has been photomodulated in biochemical, biophysical, and cellular assays. Biological problems of even greater complexity motivate the development of quantitative methods for controlling gene activity with high spatial and temporal resolution, using light as an external trigger. Photoresponsive DNA and RNA oligonucleotides would optimally serve this purpose, but have proven difficult to expand from proofs-of-concept to in vivo experiments. Until recently, the development of this technology was limited by the synthesis of oligonucleotides whose function could be significantly modulated with near-UV light. New synthetic protocols and strategies for both up- and down-regulating gene activity finally make it possible to address biological considerations. In the near future, we can expect photoresponsive DNA and RNA molecules that are relatively non-toxic, nuclease-resistant, and maintain their specificity and activity in vivo. Quantitative, laser-initiated methods for controlling DNA and RNA function will illuminate new areas in cell and developmental biology.

Twisted Gastrulation Modulates Bone Morphogenetic Protein-induced Collagen II and X Expression in Chondrocytesin Vitroandin Vivo

Twisted gastrulation (TSG) is an extracellular modulator of bone morphogenetic protein (BMP) activity and regulates dorsoventral axis formation in early Drosophila and Xenopus development. Studies on tsg-deficient mice also indicated a role of this protein in skeletal growth, but the mechanism of TSG activity in this process has not yet been investigated. Here we show for the first time by in situ hybridization and immunohistochemistry that TSG is strongly expressed in bovine and mouse growth plate cartilage as well as in fetal ribs, vertebral cartilage, and cartilage anlagen of the skull. Furthermore we provide evidence that TSG is directly involved in BMP-regulated chondrocyte differentiation and maturation. In vitro, TSG impaired the dose-dependent BMP-2 stimulation of collagen II and X expression in cultures of MC615 chondrocytes and primary mouse chondrocytes. In the presence of chordin, a BMP antagonist, the inhibitory effect of TSG was further enhanced. TSG also inhibited BMP-2-stimulated phosphorylation of Smad factors in chondrocytes, confirming the role of TSG as a modulator of BMP signaling. For analysis of TSG functions in cartilage development in vivo, the gene was overexpressed in transgenic mice under the control of the cartilage-specific Col2a1 promoter. As a result, Col10a1 expression was significantly reduced in the growth plates of transgenic embryos and newborns in comparison with wild type littermates as shown by in situ hybridization and by real time PCR analysis. The data suggest that TSG is an important modulator of BMP-regulated cartilage development and chondrocyte differentiation.

Chordin is underexpressed in ovarian tumors and reduces tumor cell motility

Ovarian cancers mostly derive from the monolayer epithelium that covers the ovary. There are currently very few molecular clues to the etiology of this cancer. Bone morphogenetic proteins (BMPs) are required for follicular development and female fertility and are expressed in the ovarian surface epithelium (OSE). We previously reported the expression of human chordin (CHRD), a BMP extracellular regulator, in the ovary. Here we show that CHRD is underexpressed in epithelium ovary cancer and epithelial cancer cell lines as compared with normal tissues and OSE, respectively. Besides, we detected BMP expression in all ovarian cell lines analyzed. To determine the functional relevance of the absence of CHRD mRNA in tumors and cancer cell lines, we studied the effects of CHRD on two cancer cell lines, BG1 and PEO14. Migratory and invasive properties were greatly reduced, whereas cell adhesion to the support was enhanced. In addition, we detected chordin (Chrd) expression in OSE of rat ovaries in a pattern similar to that of BMP4. Altogether, these results suggest that CHRD could participate in regulating BMP activity in normal OSE physiology, and that its mis-expression in OSE may facilitate cancer incidence and/or progression.

Differential regulation of the bone morphogenic protein antagonist chordin in human normal and osteoarthritic chondrocytes

OBJECTIVE

To investigate the distribution of the bone morphogenic protein (BMP) antagonist chordin in normal and osteoarthritic cartilage and synovial membranes, and its regulation in chondrocytes and synovial fibroblasts by inflammatory and growth factors.

METHODS

Localisation of chordin in tissues was undertaken by immunohistochemistry and gene regulation was determined by real time polymerase chain reaction.

RESULTS

In normal cartilage, chordin was found at low levels (mean (SD), 7.6 (1.3)%), mainly in the very superficial layers. In osteoarthritis, chordin was also found in the superficial layers (8.9 (1.1)%), though at a significantly higher level (24.7 (1.5)%) in the last two thirds of the cartilage. In contrast to normal cells, chordin mRNA and protein levels were significantly downregulated (p<0.01) in osteoarthritic chondrocytes by all the growth factors tested. Interferon gamma stimulated chordin expression in normal but not in osteoarthritic chondrocytes (p<0.0002), while interleukin 1 beta and tumour necrosis factor alpha did not affect the expression level. However, no difference was found in either the distribution or regulation of chordin in normal and osteoarthritic synovial membranes or synovial fibroblasts.

CONCLUSIONS

The differential distribution and regulation of chordin in normal and osteoarthritic cartilage and chondrocytes suggests an involvement of this antagonist in the osteoarthritic process.

Testing the antagonistic effect of follistatin on BMP family members in ovine granulosa cells

Follistatin was first demonstrated as an activin-binding protein, neutralizing its actions. However, there is emerging evidence that follistatin inhibits the action of other members of the transforming growth factor beta(TGFbeta) / bone morphogenetic protein (BMP) superfamily. Recently, numerous BMP factors have been shown to play important roles in regulating folliculogenesis and ovulation rate in mammals, and such a potential antagonistic role of follistatin is of particular interest in the context of ovarian function. Using a biological test based on progesterone production by ovine primary granulosa cells in culture, we show that follistatin was a strong antagonist of activin A, but not BMP-2 or BMP-4 actions. In contrast, noggin, a known specific BMP antagonist, had no effect on activin A but strongly neutralized BMP-2 and BMP-4 actions. BMP-6 action was only slightly reduced by both follistatin and noggin. Our data led to the conclusion that follistatin would not represent a determinant physiological modulator of the biological effect of BMP factors on granulosa cells.

Identification of the Minimal Domain Structure of Bone Morphogenetic Protein-1 (BMP-1) for Chordinase Activity

Bone morphogenetic protein 1 (BMP-1), which is a tolloid member of the astacin-like family of zinc metalloproteinases, is a highly effective procollagen C-proteinase (PCP) and chordinase. On the other hand, mammalian tolloid like-2 (mTLL-2) does not cleave chordin or procollagen; procollagen is cleaved by mTLL-2 in the presence of high levels of procollagen C-proteinase enhancer-1 (PCPE-1), for reasons that are unknown. We used these differences in activity between BMP-1 and mTLL-2 to narrow in on the domains in BMP-1 that specify PCP and chordinase activity. Using a domain swap approach, we showed that: 1) the metalloproteinase and CUB2 domains of BMP-1 are absolutely required for PCP activity; swaps with either of the corresponding domains in BMP-1 and mTLL-2 did not result in procollagen cleavage and 2) the proteinase domain of mTLL-2 can cleave chordin if coupled to the CUB1 domain of BMP-1. Therefore, the minimal structure for chordinase activity comprises a metalloproteinase domain (either from BMP-1 or from mTLL-2) and the CUB1 domain of BMP-1 (the CUB1 domain of mTLL-2 cannot substitute for the CUB1 domain of BMP-1). We showed that the minimal procollagen C-proteinase (BMP-1 lacking the EGF and CUB3 domain) was enhanced by PCPE-1 but not as well as BMP-1 retaining the CUB3 domain. Further studies showed that PCPE-1 had no effect on the ability of BMP-1 to cleave chordin. The data support a previously suggested mechanism of PCPE-1 whereby PCPE-1 interacts with procollagen, but in addition, the CUB3 domain of BMP-1 appears to augment the interaction.

Bone Morphogenic Proteins Are Overexpressed in Malignant Melanoma and Promote Cell Invasion and Migration

Malignant melanoma cells are known to have altered expression of growth factors compared with normal human melanocytes. These changes probably favor tumor growth and progression and influence the tumor environment. The induction of transforming growth factor β1 (TGF-β1), TGF-β2, and TGF-β3 expression in malignant melanoma has been reported before, whereas the expression of related bone morphogenic protein (BMP) molecules has not been analyzed in melanomas until now. Here, we show that BMP4 and BMP7 are up-regulated in nine melanoma cell lines, whereas BMP2 is overexpressed in only two of the analyzed cell lines. Immunohistochemistry of primary and metastatic melanoma also shows increased BMP4 and BMP7 expression compared with nevi. Promoter studies reveal that expression is controlled at the transcriptional level. The transcription factor Ets-1 was identified as a positive regulator for BMP4 expression. In order to determine the functional relevance of BMP expression in malignant melanoma, chordin-expressing cell clones and antisense BMP4 cell clones were generated. The clones in which BMP4 activity and expression are reduced show no changes in proliferation or in attachment-independent growth when compared with controls. However, a strong reduction of migratory and invasive properties was observed in these cells, suggesting that BMP4 promotes melanoma cell invasion and migration and therefore has an important role in the progression of malignant melanoma.

Cysteine repeat domains and adjacent sequences determine distinct bone morphogenetic protein modulatory activities of the Drosophila Sog protein

The Drosophila short gastrulation gene (sog) encodes a large extracellular protein (Sog) that inhibits signaling by BMP-related ligands. Sog and its vertebrate counterpart Chordin contain four copies of a cysteine repeat (CR) motif defined by 10 cysteine residues spaced in a fixed pattern and a tryptophan residue situated between the first two cysteines. Here we present a structure-function analysis of the CR repeats in Sog, using a series of deletion and point mutation constructs, as well as constructs in which CR domains have been swapped. This analysis indicates that the CR domains are individually dispensable for Sog function but that they are not interchangeable. These studies reveal three different types of Sog activity: intact Sog, which inhibits signaling mediated by the ligand Glass bottom boat (Gbb), a more broadly active class of BMP antagonist referred to as Supersog, and a newly identified activity, which may promote rather than inhibit BMP signaling. Analysis of the activities of CR swap constructs indicates that the CR domains are required for full activity of the various forms of Sog but that the type of Sog activity is determined primarily by surrounding protein sequences. Cumulatively, our analysis suggests that CR domains interact physically with adjacent protein sequences to create forms of Sog with distinct BMP modulatory activities.

Deletion of Epidermal Growth Factor-like Domains Converts Mammalian Tolloid into a Chordinase and Effective Procollagen C-proteinase

Bone morphogenetic protein (BMP)-1 and mammalian tolloid (mTld) are Ca(2+)-dependent metalloproteinases that result from alternative splicing of the bmp1 gene. They have different proteinase activities, e.g. BMP-1 effectively cleaves procollagen (an extracellular matrix protein) and chordin (a BMP antagonist), whereas mTld is a poor procollagen proteinase and will not cleave chordin in the absence of twisted gastrulation. This is perplexing because mTld (being the longer variant) might be expected to cleave all substrates cleaved by BMP-1. Studies have shown that the minimal structure for procollagen proteinase activity is proteinase-CUB1-CUB2 (BMP-1DeltaEC3) and therefore lacking the epidermal growth factor (EGF)-like domain thought to account for the Ca(2+) dependence of BMP-1. In this study we generated three deletion mutants of mTld that lacked either one or both EGF-like domains (referred to as "mTld-DeltaEGF"). The mutated proteins were poorly but sufficiently secreted from 293-EBNA cells for in vitro assays of procollagen and chordin cleavage. Most surprisingly, the mTld-DeltaEGF mutants required Ca(2+) for proteolytic activity, thereby showing that the EGF-like domains do not account for the Ca(2+) dependence of BMP-1/mTld. Moreover, the mTld-DeltaEGFs are effective procollagen proteinases and cleave chordin. Furthermore, BMP-1DeltaEC3 cleaves chordin and requires Ca(2+) for activity. Studies using nondenaturing gels showed that mTld molecules lacking EGF-like domains have a loose conformation such that in the presence of Ca(2+) binding sites for chordin and procollagen on the "BMP-1-part" of the molecule are exposed. We propose that the EGF-like domains could hold CUB4/5 domains in locations that exclude substrates cleavable by BMP-1.

hCHL2, a novel chordin-related gene, displays differential expression and complex alternative splicing in human tissues and during myoblast and osteoblast maturation

Chordin-like cysteine-rich repeats (CRs) are conserved domains present in an expanding family of secreted proteins that associate with members of the TGF beta superfamily. In this study, we report the molecular cloning and characterization of CHL2 (chordin-like 2), a novel protein closely related to CHL (chordin-like). Both are members of the chordin family of proteins, and contain a signal peptide and three CR domains. We found that recombinant human CHL2 (hCHL2) protein is secreted and binds activin A, but not BMP-2, -4, or -6. Expression of hCHL2 mRNA and protein was detected in a variety of human tissues and is particularly abundant in the uterus. Extensive and complex alternative splicing of hCHL2 was observed in different tissues, resulting in several distinct protein isoforms that vary substantially in the presence of a signal peptide and their content of CR domains. Differential expression of CHL2 variants was observed during myoblast and osteoblast differentiation, implying a role for this gene in these physiological processes.

Regulation of protein diversity by alternative pre-mRNA splicing with specific focus on chondrogenesis

Analysis of the human genome has dramatically demonstrated that the majority of protein diversity is generated by alternative splicing of pre-mRNA. This powerful and versatile mechanism controls the synthesis of functionally different protein isoforms that may be required during specific stages of development from a single gene. Consequently, ubiquitous and/or tissue-specific RNA splicing factors that regulate this splicing mechanism provide the basis for defining phenotypic characteristics of cells during differentiation. In this review, we will introduce the basic mechanisms of pre-mRNA alternative splicing, describe how this process is regulated by specific RNA splicing factors, and relate this to various systems of cell differentiation. Chondrogenesis, a well-defined differentiation pathway necessary for skeletogenesis, will be discussed in detail, with focus on some of the alternatively-spliced proteins known to be expressed during cartilage development. We propose a heuristic view that, ultimately, it is the regulation of these RNA splicing factors that determines the differentiation status of a cell. Studying regulation at the level of pre-mRNA alternative splicing will provide invaluable insights into how many developmental mechanisms are controlled, thus enabling us to manipulate a system to select for a specific differentiation pathway.

Identification of a secreted BMP antagonist, ectodin, integrating BMP, FGF, and SHH signals from the tooth enamel knot

We have identified mouse and human cDNAs encoding a novel secreted BMP inhibitor, which we have named ectodin. It is most homologous (approximately 37% amino acid identity) to sclerostin that is a secreted BMP antagonist. Recombinant ectodin protein produced in cultured cells was efficiently secreted as a antagonist. Ectodin inhibited the activity of BMP2, BMP4, BMP6, and BMP7 for mouse preosteoblastic MC3T3-E1 cells, and bound to these BMPs with high affinity. Ectodin is intensely expressed in developing ectodermal organs, including teeth, vibrissae, and hair follicles. However, it is absent from the hair placodes and from the enamel knot signaling centers in teeth. In addition, several cell layers surrounding the enamel knots were completely devoid of ectodin transcripts. We analyzed the regulation and function of ectodin in tooth germs. Recombinant ectodin protein antagonized the BMP-mediated induction of Msx2 expression in cultured tooth explants, indicating that ectodin is a secreted BMP inhibitor. BMP2 and BMP7 stimulated ectodin expression in tooth explants, showing that it is part of a feedback mechanism controlling the activity of BMPs. The stimulation of ectodin expression by BMP was prevented by SHH and FGF4 but not by Wnt6. Hence, the feedback mechanism whereby BMPs upregulate their own inhibitor is counteracted by signals coexpressed with BMPs in the enamel knot. We conclude that ectodin is a novel BMP inhibitor which integrates BMP signaling with the SHH and FGF signal pathways and contributes in defining the exact spatiotemporal domain of BMP target field around the ectodermal signaling centers.

Extracellular regulation of BMP signaling in vertebrates: a cocktail of modulators

The transforming growth factor-beta (TGF-beta) superfamily contains a variety of growth factors which all share common sequence elements and structural motifs. These proteins are known to exert a wide spectrum of biological responses on a large variety of cell types in both vertebrates and invertebrates. Many of them have important functions during embryonic development in pattern formation and tissue specification, and in adult tissues, they are involved in processes such as wound healing, bone repair, and bone remodeling. The family is divided into two general branches: the BMP/GDF and the TGF-beta/Activin/Nodal branches, whose members have diverse, often complementary effects. It is obvious that an orchestered regulation of different actions of these proteins is necessary for proper functioning. The TGF-beta family members act by binding extracellularly to a complex of serine/threonine kinase receptors, which consequently activate Smad molecules by phosphorylation. These Smads translocate to the nucleus, where they modulate transcription of specific genes. Three levels by which this signaling pathway is regulated could be distinguished. First, a control mechanism exists in the intracellular space, where inhibitory Smads and Smurfs prevent further signaling and activation of target genes. Second, at the membrane site, the pseudoreceptor BAMBI/Nma is able to inhibit further signaling within the cells. Finally, a range of extracellular mediators are identified which modulate the functioning of members of the TGF-beta superfamily. Here, we review the insights in the extracellular regulation of members of the BMP subfamily of secreted growth factors with a major emphasis on vertebrate BMP modulation.

Pediatric liver tumors and hepatic ontogenesis: common and distinctive pathways

Several types of pediatric liver tumors exhibit structural features apparently reflecting processes which normally occur during hepatic ontogenesis: some hepatoblastomas mimic distinct phases of hepatogenesis, including the formation of mesenchymal structures closely associated with immature epithelia, and there are tumors almost exclusively consisting of complex mesenchymal patterns. Current classifications of hepatoblastomas refer to the identification of more or less mature (differentiated) single or mixed components seen in histologic preparations. These do not, however, attempt to integrate ontogenic pathways, in contrast for example, to nephroblastoma and associated lesions, where such a view has proved to be highly fruitful. Based on the fact that an enormous amount of knowledge has recently been accumulated regarding hepatic ontogenesis, time may have come to look at these tumors with a new eye. In what follows, we aim at trying to analyze distinct features of pediatric hepatic tumors (except vascular tumors) within the background of ontogenesis. Some key steps of hepatogenesis and the regulatory factors involved may, in the future, deliver an armamentarium to search for novel molecular mechanisms involved in tumorigenic pathways.

Bone morphogenetic protein signaling and the initiation of lens fiber cell differentiation

Previous studies showed that the retina produces factors that promote the differentiation of lens fiber cells, and identified members of the fibroblast growth factor (FGF) and insulin-like growth factor (IGF) families as potential fiber cell differentiation factors. A possible role for the bone morphogenetic proteins (BMPs) is suggested by the presence of BMP receptors in chicken embryo lenses. We have now observed that phosphorylated SMAD1, an indicator of signaling through BMP receptors, localizes to the nuclei of elongating lens fiber cells. Transduction of chicken embryo retinas and/or lenses with constructs expressing noggin, a secreted protein that binds BMPs and prevents their interactions with their receptors, delayed lens fiber cell elongation and increased cell death in the lens epithelium. In an in vitro explant system, in which chicken embryo or adult bovine vitreous humor stimulates chicken embryo lens epithelial cells to elongate into fiber-like cells, these effects were inhibited by noggin-containing conditioned medium, or by recombinant noggin. BMP2, 4, or 7 were able to reverse the inhibition caused by noggin. Lens cell elongation in epithelial explants was stimulated by treatment with FGF1 or FGF2, alone or in combination with BMP2, but not to the same extent as vitreous humor. These data indicate that BMPs participate in the differentiation of lens fiber cells, along with at least one additional, and still unknown factor.