Bone morphogenetic protein 4 modulates c-Kit expression and differentiation potential in murine embryonic aorta-gonad-mesonephros haematopoiesis in vitro

The transforming growth factor-β-related factor bone morphogenetic protein 4 (BMP4) is expressed in the human embryonic aorta-gonad-mesonephros (AGM) coincident with the emergence of haematopoietic cells and influences postnatal mammalian haematopoietic stem cells in vitro. To investigate the role of BMP4 in mammalian embryonic haematopoiesis, cells were isolated from murine AGM and two populations of CD34⁺ cells with different levels of c-Kit expression and multipotency were identified. CD34⁺/c-Kit^high cells express CD45 and are haematopoietic-restricted progenitors. In contrast, CD34⁺/c-Kit^low cells are Flk1+/CD45^neg and generate adherent colonies in ex vivo culture that resemble haemangioblast colonies identified in other systems. The addition of BMP4 to AGM cells resulted in expansion of the CD34⁺/c-Kit^low cell pool within 48 h, via a combination of down modulation of the c-Kit receptor in CD34⁺/c-Kit^high cells and proliferation. In long-term culture, BMP4 increased the growth/survival of CD34⁺/c-Kit^high haematopoietic progenitors, effects that were blocked by BMP inhibitors. CD34⁺/c-Kit^high progenitors cultured with BMP4 also generated adherent colonies typical of c-Kit^low cells. These results suggest that BMP4 regulates c-Kit expression and differentiation potential in CD34⁺ AGM cells and supports a role for BMP signalling in the maintenance of multipotency during embryonic haematopoiesis, providing an insight into stem cell homeostasis within the mammalian haematopoietic niche.

BMP4 induces an epithelial-mesenchymal transition-like response in adult airway epithelial cells

Bone morphogenetic proteins (BMPs) are critical morphogens and play key roles in epithelial-mesenchymal transitions (EMTs) during embryogenesis. BMP4 is required for early mesoderm formation and also regulates morphogenesis and epithelial cell differentiation in developing lungs. While, BMP signalling pathways are activated during lung inflammation in adult mice, the role of BMPs in adult lungs remains unclear. We hypothesised that BMPs are involved in remodelling processes in adult lungs and investigated effects of BMP4 on airway epithelial cells. BEAS-2B cell growth decreased in the presence of BMP4. Cells acquired a mesenchymal-like morphology with downregulation of adherens junction proteins and increased cell motility. Changes in extracellular matrix-related gene expression occurred with BMP4 treatment including upregulation of collagens, fibronectin and tenascin C. We conclude that the activity of BMP4 in EMT during development is recapitulated in adult airway epithelial cells and suggest that this activity may contribute to inflammation and fibrosis in vivo.

Bone morphogenetic protein-4 and Noggin signaling regulates pigment cell distribution in the axolotl trunk

Wild-type (dark) and white mutant axolotl (Ambystoma mexicanum) embryos were used to investigate the role of the secreted growth factor bone morphogenetic protein-4 (BMP-4) and its antagonist, Noggin, in dorso-lateral trunk neural crest (NC) migration. Implantation of a BMP-4-coated microbead caused a melanophore-free zone around the bead, reduction of the dorsal fin above the bead, and disappearance of myotome tissue. We established a novel method that allows controlled induction of protein synthesis and release. Xenopus animal cap (XAC) cells injected with heat shock-inducible constructs for BMP-4 and Noggin were implanted into axolotl embryos and protein expression was induced at defined time points. With this approach, we could demonstrate for the first time that Noggin can stimulate melanophore migration in the white mutant. We further showed that implantation of BMP-4 expressing XAC cells alters pigment cell distribution without affecting muscle and dorsal fin development.

The activin receptor-like kinase 6 Booroola mutation enhances suppressive effects of bone morphogenetic protein 2 (BMP2), BMP4, BMP6 and growth and differentiation factor-9 on FSH release from ovine primary pituitary cell cultures

Bone morphogenetic proteins (BMPs) have been shown to influence the regulation of FSH synthesis and secretion at the level of the pituitary. Primary pituitary cells were harvested and cultured from Booroola ewes homozygous for a mutation in activin receptor-like kinase 6 (ALK6) also known as BMP receptor IB (BMPRIB), and from wild-type (WT) ewes to determine if the mutation caused alterations in FSH secretion in vitro. The cells were collected 24 h following induction of luteolysis and cultured for 72 h prior to being challenged for 24 h with BMP2, BMP4, BMP6, growth and differentiation factor-9 (GDF9), transforming growth factor-beta 1, activin-A and GnRH. The levels of FSH and LH were measured by RIA and then compared with the untreated controls. Primary pituitary cell cultures from Booroola ewes secreted less FSH than WT cells in the presence of BMP2, BMP4 and BMP6. These BMPs did not affect the FSH stores within the cells, or the levels of LH released. GDF9 appeared to act in a BMP-like manner by suppressing FSH secretion. The ALK6 receptor however, was not found to co-localise with gonadotroph cells in either Booroola or WT pituitary tissues. These findings imply that the increased sensitivity of Booroola cells to BMP2, BMP4, BMP6 and GDF9 cannot be due to the direct action of the ALK6 mutant Booroola receptor in the cells that synthesise FSH.

Mutations in BMP4 cause eye, brain, and digit developmental anomalies: overlap between the BMP4 and hedgehog signaling pathways

Developmental ocular malformations, including anophthalmia-microphthalmia (AM), are heterogeneous disorders with frequent sporadic or non-Mendelian inheritance. Recurrent interstitial deletions of 14q22-q23 have been associated with AM, sometimes with poly/syndactyly and hypopituitarism. We identify two further cases of AM (one with associated pituitary anomalies) with a 14q22-q23 deletion. Using a positional candidate gene approach, we analyzed the BMP4 (Bone Morphogenetic Protein-4) gene and identified a frameshift mutation (c.226del2, p.S76fs104X) that segregated with AM, retinal dystrophy, myopia, brain anomalies, and polydactyly in a family and a nonconservative missense mutation (c.278A-->G, p.E93G) in a highly conserved base in another family. MR imaging and tractography in the c.226del2 proband revealed a primary brain developmental disorder affecting thalamostriatal and callosal pathways, also present in the affected grandmother. Using in situ hybridization in human embryos, we demonstrate expression of BMP4 in optic vesicle, developing retina and lens, pituitary region, and digits strongly supporting BMP4 as a causative gene for AM, pituitary, and poly/syndactyly. Because BMP4 interacts with HH signaling genes in animals, we evaluated gene expression in human embryos and demonstrate cotemporal and cospatial expression of BMP4 and HH signaling genes. We also identified four cases, some of whom had retinal dystrophy, with "low-penetrant" mutations in both BMP4 and HH signaling genes: SHH (Sonic Hedgehog) or PTCH1 (Patched). We propose that BMP4 is a major gene for AM and/or retinal dystrophy and brain anomalies and may be a candidate gene for myopia and poly/syndactyly. Our finding of low-penetrant variants in BMP4 and HH signaling partners is suggestive of an interaction between the two pathways in humans.

Cyclic AMP enhances Smad-mediated BMP signaling through PKA-CREB pathway

We present experimental results indicating involvement of cyclic AMP (cAMP)-mediated signaling in bone morphogenetic protein (BMP)-induced osteoblastic gene expression at the transcriptional level by luciferase activity assay in C2C12 cells using the promoter sequence of the Id1 gene, an early-response gene to BMPs, which contains both a BMP-responsive element (BRE) and a cAMP-response element (CRE). In cells transfected with luciferase gene driven by wild-type Id1 promoter, treatment with BMP-4 increased luciferase expression, which was further enhanced by the addition of dibutyryl cAMP (dbcAMP). This dbcAMP-enhanced luciferase expression was significantly suppressed when the CRE site in the Id1 promoter was replaced by mutated CRE or endogenous CRE-binding protein (CREB) was knocked down by transfection of CREB RNAi. Pretreatment of cells with protein kinase A (PKA) inhibitor, H89, also dramatically reduced dbcAMP-enhanced luciferase expression. Immunoprecipitation assay showed phosphorylated-Smad1/5/8, phosphorylated-CREB, and CREB-binding protein (CBP) formed the transcriptional complex. These data indicate that cAMP-PKA/CREB/CRE signaling potentially enhances BMP-induced transcription through the BRE in the promoter of the BMP-responsive gene through a PKA-mediated pathway.

Role of BMP-4 during tooth development in a model with complete dentition

There are no reports in literature about roles of bone morphogenetic protein 4 (BMP-4) in tooth development in mammals with complete dentition (with all dental groups). The classical model of study is the mouse, which has distinctive incisor and molar patterns. The opossum Didelphis albiventris with five upper and four lower incisors, one canine, three premolars and four molars, on each side of the jaw, seems to be a convenient model for odontogenesis study. This investigation searched for similarities and differences in BMP-4 expression pattern between the opossum and the mouse. BMP-4 cDNA was obtained by RT-PCR and the expression pattern during molar tooth development was investigated by the immunoperoxidase method. Opossum BMP-4 mature protein has 95% of sequence similarity in relation to mouse and 94% to human. The BMP-4 expression pattern during opossum tooth development was suggestive of a role in dental organ initiation and morphogenesis.

The effect of retinoic acid on a zinc finger transcription factor, AJ18, during differentiation of a rat clonal preosteoblastic cell line, ROB-C20, into osteoblasts

OBJECTIVE

A zinc finger type transcription factor, AJ18, is thought to be a negative regulator of osteoblast differentiation, but its expression mechanism is not fully understood. Retinoic acid (RA) is a metabolite of vitamin A and involves the proliferation and differentiation in a variety of cells. To verify the effect of RA on osteoblast differentiation, AJ18 expression level was examined using a rat clonal preosteoblastic cell line, ROB-C20 (C20).

DESIGN

Confluent C20 cells were treated with or without RA (10(-6)M) for several days. Northern, real time RT-PCR and Western blotting analyses were performed to examine AJ18 expression pattern in gene and protein levels. To identify the active promoter sequence of AJ18 gene, luciferase assay was designed. Furthermore, the effect of overexpressed AJ18 in C20 cells on alkaline phosphatase (ALP) mRNA expression and its activity was compared with that of RA-treated cells.

RESULTS

RA increased the expression of AJ18 mRNA from 2 to 13 days as well as its protein production. However, no significant changes of Runx2 mRNA expression and undetectable osterix mRNA expression were observed in C20 cells treated with or without RA. Luciferase assay showed increases in promoter activities in some constructs of 5'-flanking region of AJ18 gene in RA-treated C20 cells. On the other hand, RA decreases enzymatic activity and mRNA expression level of ALP, but mRNA expression levels of bone sialoprotein and osteocalcin were not altered. Interestingly, reduced ALP activity and its mRNA expression level were detected in exogenous AJ18-overexpressing C20 cells.

CONCLUSIONS

This study supports the hypothesis that RA may restrict to the differentiation of C20 cells into mature osteoblasts via inductive AJ18 expression with activation of multiple signal pathways.

[BMP4: a key regulator of embryonic development and hematopoiesis]

Bone morphogenic proteins (BMPs) constitute a group of multifunctional growth factors that belong to the transforming growth factor-beta (TGF-beta) family. During the last few years, the roles of BMPs, both in development and in specific adult tissues, have been extensively studied. One of such proteins, BMP4, has been identified as a key regulator in the development of mesoderm, and particularly, in the specification of the vascular and hematopoietic systems. BMP4 has also been found to be an important regulator of the growth of hematopoietic stem cells (HSC), participating in the control of their proliferation, expansion and differentiation. Herein, we present an overview on the different roles that BMP4 plays in mammal development, and in the regulation of hematopoiesis.

Barx1, growth factors and apoptosis in facial tissue of children with clefts

OBJECTIVE

Clefts of lip and palate belong to the most common birth defects worldwide. Growth factors and genes play an important role in tissue growth, differentiation and induction and upregulation of growth factors, apoptosis and matrix metalloproteinases might be involved in pathogenesis of facial clefts. The aim of this study was investigation of palate tissue in children with unilateral cleft lip palate for detection of local tissue growth factors, barx1 and apoptosis.

MATERIALS AND METHODS

We investigated soft and hard palate tissue from 36 children with complete unilateral cleft lip and palate from cleft area.14 children were in age before and primary dentition, but 22 children were in mixed dentition period. We examined the localization of barx1, FGFR1, NGFR, TGFbeta, BMP2/4, MMP2, PGP 9,5 by immunohistochemistry. TUNEL method was performed for detection of apoptotic cells.

RESULTS

Abundance of FGFR1 positive cells was seen almost in all cases. FGFR richly stained cells of soft and hard palate tissue. Abundance of NGFR positive cells was detected in basal epithelium, hair follicles, nerve fibers in wall of blood vessels and subepithelium, and was more often seen in children before mixed dentition. TGFbeta has showed intensive expression in epithelium, cartilage and bone in both dentition ages. Chondrocytes, fibroblasts and macrophages expressed MMP2 predominant before mixed dentition. Regional expression of barx1 was observed in epithelium before the mixed dentition, while during mixed dentition gene appeared in hyaline cartilage. TUNEL discovered apoptosis in both dentition ages.

CONCLUSIONS

FGFR1 and TGFbeta are main tissue stimulating growth factors in both dentition ages. Expression of barx1 appears in cleft lip palate affected structures mainly in mixed dentition ages. NGFR and neuropeptides-containing structures are mainly characteristic in cleft tissue before mixed dentition. Distribution of genes, GF and apoptosis seem to correlate rather with dentition age than to type of CLP.

Immunohistochemical evidence of BMP-2, -4 and -7 activity in otospongiosis

CONCLUSION

This study is the first to show that bone morphogenetic proteins (BMP)-2, -4 and -7 play a role in active phase otosclerotic bone remodelling (otospongiosis).

OBJECTIVES

The role of BMPs in various tissue growth and repair mechanisms is an ongoing topic in the literature. BMP-2, -4 and -7 are known to be of major importance in bone formation and repair. Their role in otosclerotic bone transformation has not been analysed previously. The main goal of this study was to perform an immunohistological analysis of BMP-2, -4 and -7 in otoclerosis.

MATERIALS AND METHODS

Parts of the stapedial footplates, collected during partial stapedectomies in 30 patients with clinical otosclerosis, were analysed for histological otosclerotic lesions after staining haematoxylin and eosin. Immunohistochemical analysis was performed using polyclonal IgG antibodies for BMP-2, -4 and -7, as well as biotinylated secondary antibodies, avidin-biotin-peroxidase complex reaction and alkaline phosphatase staining.

RESULTS

In all, 14 specimens contained otosclerosis; 3 of these were otospongiotic, 8 fibrotic, 2 sclerotic and 1 had both sclerotic and fibrotic lesions. Thus in total 14/30 specimens (47%) showed histological otosclerosis. Only the multiple osteoblasts and osteoclasts in those specimens exhibiting an otospongiotic phase showed distinct immunochemical staining for BMP-2, -4 and -7.

Embryonic stromal clones reveal developmental regulators of definitive hematopoietic stem cells

Hematopoietic stem cell (HSC) self-renewal and differentiation is regulated by cellular and molecular interactions with the surrounding microenvironment. During ontogeny, the aorta-gonad-mesonephros (AGM) region autonomously generates the first HSCs and serves as the first HSC-supportive microenvironment. Because the molecular identity of the AGM microenvironment is as yet unclear, we examined two closely related AGM stromal clones that differentially support HSCs. Expression analyses identified three putative HSC regulatory factors, beta-NGF (a neurotrophic factor), MIP-1gamma (a C-C chemokine family member) and Bmp4 (a TGF-beta family member). We show here that these three factors, when added to AGM explant cultures, enhance the in vivo repopulating ability of AGM HSCs. The effects of Bmp4 on AGM HSCs were further studied because this factor acts at the mesodermal and primitive erythropoietic stages in the mouse embryo. In this report, we show that enriched E11 AGM HSCs express Bmp receptors and can be inhibited in their activity by gremlin, a Bmp antagonist. Moreover, our results reveal a focal point of Bmp4 expression in the mesenchyme underlying HSC containing aortic clusters at E11. We suggest that Bmp4 plays a relatively late role in the regulation of HSCs as they emerge in the midgestation AGM.

BMP-4 and Noggin signaling modulate dorsal fin and somite development in the axolotl trunk

BMP-4, a member of the TGF-beta superfamily of growth factors, is involved in various developmental processes. We investigated the effects of BMP-4 and its antagonist Noggin on axolotl trunk development. Implantation of BMP-4-coated microbeads caused inhibition of muscle and dorsal fin formation in the vicinity of the microbeads. At some distance, myotomes developed with reduced height but increased width, which was accompanied by increased cell proliferation. These effects could be modulated by co-implanting Noggin-coated beads. Immunostaining of Pax7 further revealed that although the dermomyotome was absent in the vicinity of BMP-4-coated beads, at some distance from them, it was thicker than in controls, indicating that moderate amounts of BMP-4 stimulate this layer of undifferentiated cells. In contrast, Noggin generally inhibited the dermomyotome, possibly indicating premature differentiation of dermomyotome cells. We conclude that BMP-4 and Noggin are involved in the regulation of cell proliferation and differentiation during somite development.

Bone morphogenetic protein 4 induces epithelial-mesenchymal transition through MSX2 induction on pancreatic cancer cell line

In our study, we found that bone morphogenetic protein 4 (BMP4) has a novel effect as an inducer of epithelial-mesenchymal transition (EMT) on Panc-1 cells, a human pancreatic carcinoma cell line. BMP4-treated Panc-1 cells showed loose cell contacts and a scattered, fibroblast-like appearance along with E-cadherin downregulation, Vimentin upregulation and enhanced cell migration, which are characteristic of EMT. BMP4 treatment also induced homeobox gene MSX2 expression, which we previously showed to be associated with EMT in pancreatic carcinoma cells. BMP4 treatment activated the Smad signaling pathway, and extracellular signal-related kinase (ERK) and p38 mitogen-activated kinase (MAPK) pathways in these cells. MSX2 was markedly induced by BMP4 through the ERK and p38 MAPK pathways in collaboration with the Smad signaling pathway. The repression of E-cadherin, induction of Vimentin and enhanced cell migration disappeared when siRNA-based MSX2 downregulated pancreatic cancer cells were treated with BMP4. These findings indicate that BMP4 may be involved in pancreatic carcinoma development through the promotion of EMT and that MSX2 is indispensable to this process.

FGFR1 over-expression in primary rhabdomyosarcoma tumors is associated with hypomethylation of a 5' CpG island and abnormal expression of the AKT1, NOG, and BMP4 genes

Rhabdomyosarcoma (RMS), the most common pediatric soft tissue sarcoma likely results from abnormal proliferation and differentiation during skeletal myogenesis. Multiple genetic alterations are associated with the three RMS histopathological subtypes, embryonal, alveolar, and pleomorphic adult variant. Recently, we reported the novel amplification of the FGFR1 gene in a RMS tumor. The involvement of FGFR1 in RMS was now further studied in primary tumors and RMS cell lines by mutation screening, quantitative RNA expression, and methylation analyses. No mutation was found by DHPLC and sequencing of the entire FGFR1 coding sequence and exon-intron boundaries. However, FGFR1 over-expression was detected in all primary RMS tumors and cell lines tested. A hypomethylation of a CpG island upstream to FGFR1 exon 1 was identified in the primary RMS tumors, using sodium bisulfite modification method, suggesting a molecular mechanism to FGFR1 over-expression. Expression analysis of additional genes, AKT1, NOG and its antagonist BMP4, which interact downstream to FGFR1, demonstrated expression differences between primary RMS tumors and normal skeletal muscles. Our data suggest an important role for FGFR1 and FGFR1-downstream genes in RMS tumorigenesis and a possible association with the deregulation of proliferation and differentiation of skeletal myoblasts in RMS.

Different regulatory elements are required for response of hepcidin to interleukin-6 and bone morphogenetic proteins 4 and 9

Hepcidin is a major regulator of iron homeostasis. Hepcidin expression is upregulated by inflammatory cytokines, particularly interleukin (IL)-6 and even more potently by the bone morphogenetic proteins 2, 4 and 9 (BMP-2, BMP-4 and BMP-9). This study showed that the regulation of hepcidin expression by IL-6 and BMPs occurs through distinct regulatory elements. The induction of hepcidin by BMPs requires at least two regions of the Hamp1 promoter, one between 140-260 bp and the other between 1.6-2.0 kb upstream of the start of translation. Reporter constructs including 1.6-2.0 kb of the Hamp1 promoter were induced >16-fold by BMPs whereas a 260 bp reporter Hamp1 promoter construct was induced only two- to threefold. The distal 1.6-2.0 kb region appeared to contain several different BMP-responsive elements, as incremental lengthening of the promoter construct in this region produced gradual escalation of BMP-responsiveness. In contrast, the IL-6 response required only the proximal 260 bp Hamp1 promoter region. Furthermore, there were no regulatory elements located in the non-coding or coding regions of Hamp1 and activation of the Hamp1 promoter was absent or markedly reduced in cells of non-hepatic origin.

Multiple mesoderm subsets give rise to endothelial cells, whereas hematopoietic cells are differentiated only from a restricted subset in embryonic stem cell differentiation culture

In the developing mouse, vascular endothelial cell (EC) and hematopoietic cell (HPC) lineages are two initial cell lineages that diverge from mesodermal cells, which have been roughly subdivided into three subtypes according to their geographical location: the organizer, embryonic mesoderm in the primitive streak, and extraembryonic mesoderm during gastrulation. Although the initial progenitors that become the two lineages appear in both vascular endothelial growth factor receptor 2(+) (VEGFR2(+)) lateral and extraembryonic mesoderm, little is known about the underlying molecular events that regulate the derivation of ECs and HPCs. Here, we describe an experimental system consisting of two types of embryonic stem cell lines capable of distinguishing between organizer and the middle section of the primitive streak region. Using this system, we were able to establish a defined culture condition that can separately induce distinct types of mesoderm. Although we were able to differentiate ECs from all mesoderm subsets, however, the potential of HPCs was restricted to the VEGFR2(+) cells derived from primitive streak-type mesodermal cells. We also show that the culture condition for the progenitors of primitive erythrocytes is separated from that for the progenitors of definitive erythrocytes. These results suggest the dominant role of extrinsic regulation during diversification of mesoderm.

Regulation of in vitro vascular calcification by BMP4, VEGF and Wnt3a

Vascular calcification is a common clinical complication of cardiovascular disease, diabetes and end-stage renal failure, associated with significant morbidity and mortality. In this study we demonstrate that factors secreted by the hypertrophic chondrocytes induce matrix mineralization and osteoblastic transformation in cultured mouse vascular smooth muscle cells (VSMCs). In addition, these factors render VSMCs responsive to BMP4 and Wnt3a ligands. Neither BMP-4 nor Wnt3a could induce mineralization in short-term (up to 8 days) cultures of primary mouse VSMCs. However, both ligands act synergistically with the chondrocyte-conditioned medium causing a further increase in VSMC calcification. Finally, we show that commitment of VSMCs towards the BMP-regulated mineralization can be induced by the chondrocyte-secreted bone anabolic factor VEGF. In addition, expression profiling suggests a novel role in vascular calcification for the matrix proteins previously known to regulate bone formation and mineralization (including MMP3, fibulin, 11betahydroxysteroid dehydrogenase 1 and retinoic acid receptor responder 2). The results of this study may contribute to further understanding of the cellular mechanisms responsible for vascular calcification and provide important information for the treatment of this pathology.

Retrograde BMP signaling regulates trigeminal sensory neuron identities and the formation of precise face maps

Somatosensory information from the face is transmitted to the brain by trigeminal sensory neurons. It was previously unknown whether neurons innervating distinct areas of the face possess molecular differences. We have identified a set of genes differentially expressed along the dorsoventral axis of the embryonic mouse trigeminal ganglion and thus can be considered trigeminal positional identity markers. Interestingly, establishing some of the spatial patterns requires signals from the developing face. We identified bone morphogenetic protein 4 (BMP4) as one of these target-derived factors and showed that spatially defined retrograde BMP signaling controls the differential gene expressions in trigeminal neurons through both Smad4-independent and Smad4-dependent pathways. Mice lacking one of the BMP4-regulated transcription factors, Onecut2 (OC2), have defects in the trigeminal central projections representing the whiskers. Our results provide molecular evidence for both spatial patterning and retrograde regulation of gene expression in sensory neurons during the development of the somatosensory map.

Matrix GLA protein, an inhibitory morphogen in pulmonary vascular development

Deficiency of matrix GLA protein (MGP), an inhibitor of bone morphogenetic protein (BMP)-2/4, is known to cause arterial calcification and peripheral pulmonary artery stenosis. Yet the vascular role of MGP remains poorly understood. To further investigate MGP, we created a new MGP transgenic mouse model with high expression of the transgene in the lungs. The excess MGP led to a disruption of the pulmonary pattern of BMP-4, and resulted in significant morphological defects in the pulmonary artery tree. Specifically, the vascular branching pattern lacked characteristic side branching, whereas control lungs had extensive side branching accounting for as much as 40% of the vascular endothelium. The vascular changes could be explained by a dramatic reduction of phosphorylated SMAD1/5/8 in the alveolar epithelium, and in epithelial expression of the activin-like kinase receptor 1 and vascular endothelial growth factor, both critical in vascular formation. Abnormalities were also found in the terminal airways and in lung cell differentiation; high levels of surfactant protein-B were distributed in an abnormal pattern suggesting lost coordination between vasculature and airways. Ex vivo, lung cells from MGP transgenic mice showed higher proliferation, in particular surfactant protein B-expressing cells, and conditioned medium from these cells poorly supported in vitro angiogenesis compared with normal lung cells. The vascular branching defect can be mechanistically explained by a computational model based on activator/inhibitor reaction-diffusion dynamics, where BMP-4 and MGP are considered as an activating and inhibitory morphogen, respectively, suggesting that morphogen interactions are important for vascular branching.

Visualizing Smad1/4 signaling response to bone morphogenetic protein-4 activation by FRET biosensors

Smad proteins are the major signal transducers for the Transforming Growth Factor superfamily of cytokines and their serine/threonine kinase receptors. Smads mediate the signal from the membrane into the nucleus. Bone Morphogenetic Protein-4 stimulates phosphorylation of Smad1, which interacts with Smad4. This complex translocates into the nucleus and regulates transcription of target genes. Here, we report our development of cellular fluorescence biosensors for direct visualization of Smad signaling in live mammalian cells. Fluorescence resonance energy transfer between cyan and yellow fluorescent proteins fused to the Smad1 and Smad4 proteins was used to unravel the temporal aspects of BMP/Smad signaling. A rate-limiting delay of 2-5 min occurred between BMP activation and Smad1 activity. A similar delay was observed in the Smad1/Smad4 complexation. Further experimentation indicated that the delay is dependent on the MH1 domain and linker of Smad1. These results give new insights into the dynamics of the BMP receptor -Smad1/4 signaling process and provide a new tool for studying Smads.

Xom interacts with and stimulates transcriptional activity of LEF1/TCFs: implications for ventral cell fate determination during vertebrate embryogenesis

LEF1/TCFs are high mobility group box-containing transcriptional factors mediating canonical Wnt/beta-catenin signaling during early embryogenesis and tumorigenesis. Beta-catenin forms a complex with LEF1/TCFs and transactivates LEF1/TCF-mediated transcriptions during dorsalization. Although LEF-mediated transcription is also implicated in ventralization, the underlying molecular mechanism is not well understood. Using the vertebrate Xenopus laevis model system, we found that Xom, which is a ventralizing homeobox protein with dual roles of transcriptional activation and repression, forms a complex with LEF1/TCF through its homeodomain and transactivates LEF1/TCF-mediated transcription through its N-terminal transactivation domain (TAD). Our data show that Xom lacking the N-terminal TAD fails to transactivate ventral genes, such as BMP4 and Xom itself, but retains the ability to suppress transcriptional activation of dorsal gene promoters, such as the Goosecoid promoter, indicating that transactivation and repression are separable functions of Xom. It has been postulated that Xom forms a positive re-enforcement loop with BMP4 to promote ventralization and to suppress dorsal gene expression. Consistent with an essential role of Xom transactivation of LEF1/TCFs during early embryogenesis, we found that expression of the dominant-negative Xom mutant that lacks the TAD fails to re-enforce the ventral signaling of BMP4 and causes a catastrophic effect during gastrulation. Our data suggest that the functional interaction of Xom and LEF1/TCF-factors is essential for ventral cell fate determination and that LEF1/TCF factors may function as a point of convergence to mediate the combined signaling of Wnt/beta-catenin and BMP4/Xom pathways during early embryogenesis.