Human Crossveinless-2 is a novel inhibitor of bone morphogenetic proteins

Emerging role of BMPs/BMPR2 signaling pathway in treatment for pulmonary fibrosis

Pulmonary fibrosis is a fatal and chronic lung disease that is characterized by accumulation of thickened scar in the lungs and impairment of gas exchange. The cases with unknown etiology are referred as idiopathic pulmonary fibrosis (IPF). There are currently no effective therapeutics to cure the disease; thus, the investigation of the pathogenesis of IPF is of great importance. Recent studies on bone morphogenic proteins (BMPs) and their receptors have indicated that reduction of BMP signaling in lungs may play a significant role in the development of lung fibrosis. BMPs are members of TGF-β superfamily, and they have been shown to play an anti-fibrotic role in combating TGF-β-mediated pathways. The impact of BMP receptors, in particular BMPR2, on pulmonary fibrosis is growing attraction to researchers. Previous studies on BMPR2 have often focused on pulmonary arterial hypertension (PAH). Given the strong clinical association between PAH and lung fibrosis, understanding BMPs/BMPR2-mediated signaling pathway is important for development of therapeutic strategies to treat IPF. In this review, we comprehensively review recent studies regarding the biological functions of BMPs and their receptors in lungs, especially focusing on their roles in the pathogenesis of pulmonary fibrosis and fibrosis resolution.

BMPER is a marker of adipose progenitors and adipocytes and a positive modulator of adipogenesis

Autocrine and paracrine signaling regulating adipogenesis in white adipose tissue remains largely unclear. Here we used single-cell RNA-sequencing (RNA-seq) and single nuclei RNA-sequencing (snRNA-seq) to identify markers of adipose progenitor cells (APCs) and adipogenic modulators in visceral adipose tissue (VAT) of humans and mice. Our study confirmed the presence of major cellular clusters in humans and mice and established important sex and diet-specific dissimilarities in cell proportions. Here we show that bone morphogenetic protein (BMP)-binding endothelial regulator (BMPER) is a conserved marker for APCs and adipocytes in VAT in humans and mice. Further, BMPER is highly enriched in lineage negative stromal vascular cells and its expression is significantly higher in visceral compared to subcutaneous APCs in mice. BMPER expression and release peaked by day four post-differentiation in 3T3-L1 preadipocytes. We reveal that BMPER is required for adipogenesis both in 3T3-L1 preadipocytes and in mouse APCs. Together, this study identified BMPER as a positive modulator of adipogenesis.

Diversity and robustness of bone morphogenetic protein pattern formation

Pattern formation by bone morphogenetic proteins (BMPs) demonstrates remarkable plasticity and utility in several contexts, such as early embryonic development, tissue patterning and the maintenance of stem cell niches. BMPs pattern tissues over many temporal and spatial scales: BMP gradients as short as 1-2 cell diameters maintain the stem cell niche of the Drosophila germarium over a 24-h cycle, and BMP gradients of several hundred microns establish dorsal-ventral tissue specification in Drosophila, zebrafish and Xenopus embryos in timescales between 30 min and several hours. The mechanisms that shape BMP signaling gradients are also incredibly diverse. Although ligand diffusion plays a dominant role in forming the gradient, a cast of diffusible and non-diffusible regulators modulate gradient formation and confer robustness, including scale invariance and adaptability to perturbations in gene expression and growth. In this Review, we document the diverse ways that BMP gradients are formed and refined, and we identify the core principles that they share to achieve reliable performance.

Structural perspective of BMP ligands and signaling

The Bone Morphogenetic Proteins (BMPs) are the largest class signaling molecules within the greater Transforming Growth Factor Beta (TGFβ) family, and are responsible for a wide array of biological functions, including dorsal-ventral patterning, skeletal development and maintenance, as well as cell homeostasis. As such, dysregulation of BMPs results in a number of diseases, including fibrodysplasia ossificans progressiva (FOP) and pulmonary arterial hypertension (PAH). Therefore, understanding BMP signaling and regulation at the molecular level is essential for targeted therapeutic intervention. This review discusses the recent advances in the structural and biochemical characterization of BMPs, from canonical ligand-receptor interactions to co-receptors and antagonists. This work aims to highlight how BMPs differ from other members of the TGFβ family, and how that information can be used to further advance the field. Lastly, this review discusses several gaps in the current understanding of BMP structures, with the aim that discussion of these gaps will lead to advancements in the field.

The Phosphodiesterase-5 Inhibitor Vardenafil Improves the Activation of BMP Signaling in Response to Hydrogen Peroxide

PURPOSE

The pleiotropic roles of phosphodiesterase-5 inhibitors (PDE5is) in cardiovascular diseases have attracted attention. The effect of vardenafil (a PDE5i) is partly mediated through reduced oxidative stress, but it is unclear whether vardenafil protects against hydrogen peroxide (H₂O₂)-induced endothelial cell injury, and the molecular mechanisms that are involved remain unknown. We determined the protective role of vardenafil on H₂O₂-induced endothelial cell injury in cultured human umbilical vein endothelial cells (HUVECs).

METHODS AND RESULTS

Vardenafil decreased the number of TUNEL-positive cells, increased the Bcl2/Bax ratio, and ameliorated the numbers of BrdU-positive cells in H₂O₂-treated HUVECs. The bone morphogenetic protein receptor (BMPR)/p-Smad/MSX2 pathway was enhanced in response to H₂O₂, and vardenafil treatment could normalize this pathway. To determine whether the BMP pathway is involved, we blocked the BMP pathway using dorsomorphin, which abolished the protective effects of vardenafil. We found that vardenafil improved the H₂O₂-induced downregulation of BMP-binding endothelial regulator protein (BMPER), which possibly intersects with the BMP pathway in the regulation of endothelial cell injury in response to oxidative stress.

CONCLUSIONS

We demonstrated for the first time that exogenous H₂O₂ activates BMPR expression and promotes Smad1/5/8 phosphorylation. Additionally, vardenafil can attenuate H₂O₂-induced endothelial cell injury in HUVECs. Vardenafil decreases apoptosis through an improved Bcl-2/Bax ratio and increases cell proliferation. Vardenafil protects against endothelial cell injury through ameliorating the intracellular oxidative stress level and BMPER expression. The protective role of vardenafil on H₂O₂-induced endothelial cell injury is mediated through BMPR/p-Smad/MSX2 in HUVECs.

Endodermal pouch-expressed dmrt2b is important for pharyngeal cartilage formation

Pharyngeal pouches, a series of outpocketings derived from the foregut endoderm, are essential for craniofacial skeleton formation. However, the molecular mechanisms underlying endodermal pouch-regulated head cartilage development are not fully understood. In this study, we find that zebrafish dmrt2b, a gene encoding Doublesex- and Mab-3-related transcription factor, is specifically expressed in endodermal pouches and required for normal pharyngeal cartilage development. Loss of dmrt2b doesn't affect cranial neural crest (CNC) specification and migration, but leads to prechondrogenic condensation defects by reducing cxcl12b expression after CNC cell movement into the pharyngeal arches. Moreover, dmrt2b inactivation results in reduced proliferation and impaired differentiation of CNC cells. We also show that dmrt2b suppresses crossveinless 2 expression in endodermal pouches to maintain BMP/Smad signaling in the arches, thereby facilitating CNC cell proliferation and chondrogenic differentiation. This work provides insight into how transcription factors expressed in endodermal pouches regulate pharyngeal skeleton development through tissue-tissue interactions.

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.

Structure of Gremlin-1 and analysis of its interaction with BMP-2

We have determined the crystal structure of Gremlin-1 and analysed its interaction with BMP-2. Our results suggest that Gremlin-1 does not inhibit BMP-2 by direct 1:1 binding, but possibly has a novel mechanism of sequestering BMP-2 into a larger oligomeric complex.

Bone morphogenetic protein 2 (BMP-2) is a member of the transforming growth factor-β (TGF-β) signalling family and has a very broad biological role in development. Its signalling is regulated by many effectors: transmembrane proteins, membrane-attached proteins and soluble secreted antagonists such as Gremlin-1. Very little is known about the molecular mechanism by which Gremlin-1 and other DAN (differential screening-selected gene aberrative in neuroblastoma) family proteins inhibit BMP signalling. We analysed the interaction of Gremlin-1 with BMP-2 using a range of biophysical techniques, and used mutagenesis to map the binding site on BMP-2. We have also determined the crystal structure of Gremlin-1, revealing a similar conserved dimeric structure to that seen in other DAN family inhibitors. Measurements using biolayer interferometry (BLI) indicate that Gremlin-1 and BMP-2 can form larger complexes, beyond the expected 1:1 stoichiometry of dimers, forming oligomers that assemble in alternating fashion. These results suggest that inhibition of BMP-2 by Gremlin-1 occurs by a mechanism that is distinct from other known inhibitors such as Noggin and Chordin and we propose a novel model of BMP-2–Gremlin-1 interaction yet not seen among any BMP antagonists, and cannot rule out that several different oligomeric states could be found, depending on the concentration of the two proteins.

BMPER variants associated with a novel, attenuated subtype of diaphanospondylodysostosis

Diaphanospondylodysostosis (DSD), caused by loss of bone morphogenetic protein-binding endothelial regulator (BMPER), has been considered a lethal skeletal dysplasia characterized by severe deficiency of vertebral body and sacral ossification, reduced rib number and cystic kidneys. In this study, however, we have demonstrated that variants in BMPER may cause a milder disorder, without renal anomalies, that is compatible with long-term survival. Four siblings, three males and one female, presented with severe congenital scoliosis associated with rib and vertebral malformations as well as strikingly delayed ossification of the pedicles. The female was stillborn from an unrelated cause. Stabilization of the scoliosis with expandable titanium rods was successful in the three boys, all of whom have short stature. An autosomal recessive mode of inheritance was hypothesized. Single nucleotide polymorphism microarray analysis was performed for three of the siblings to identify autosomal genes with shared allele patterns, suggesting possible linkage. Exome sequencing of one sibling was then performed. Rare variants were identified in 347 genes with shared alleles. Only one of these genes had bi-allelic variants in a gene strongly expressed in paraxial mesenchyme: BMPER, which is the cause of DSD, an autosomal recessive disorder. The disorder described herein could represent an attenuated form of DSD or could be designated a separate entity such as spondylopedicular dysplasia.

Trans-system mechanisms against ischemic myocardial injury

A mammalian organism possesses a hierarchy of naturally evolved protective mechanisms against ischemic myocardial injury at the molecular, cellular, and organ levels. These mechanisms comprise regional protective processes, including upregulation and secretion of paracrine cell-survival factors, inflammation, angiogenesis, fibrosis, and resident stem cell-based cardiomyocyte regeneration. There are also interactive protective processes between the injured heart, circulation, and selected remote organs, defined as trans-system protective mechanisms, including upregulation and secretion of endocrine cell-survival factors from the liver and adipose tissue as well as mobilization of bone marrow, splenic, and hepatic cells to the injury site to mediate myocardial protection and repair. The injured heart and activated remote organs exploit molecular and cellular processes, including signal transduction, gene expression, cell proliferation, differentiation, migration, mobilization, and/or extracellular matrix production, to establish protective mechanisms. Both regional and trans-system cardioprotective mechanisms are mediated by paracrine and endocrine messengers and act in coordination and synergy to maximize the protective effect, minimize myocardial infarction, and improve myocardial function, ensuring the survival and timely repair of the injured heart. The concept of the trans-system protective mechanisms may be generalized to other organ systems-injury in one organ may initiate regional as well as trans-system protective responses, thereby minimizing injury and ensuring the survival of the entire organism. Selected trans-system processes may serve as core protective mechanisms that can be exploited by selected organs in injury. These naturally evolved protective mechanisms are the foundation for developing protective strategies for myocardial infarction and injury-induced disorders in other organ systems.

Insights in the etiopathology of galactosyltransferase II (GalT-II) deficiency from transcriptome-wide expression profiling of skin fibroblasts of two sisters with compound heterozygosity for two novel B3GALT6 mutations

Mutations in B3GALT6, encoding the galactosyltransferase II (GalT-II) involved in the synthesis of the glycosaminoglycan (GAG) linkage region of proteoglycans (PGs), have recently been associated with a spectrum of connective tissue disorders, including spondyloepimetaphyseal dysplasia with joint laxity type 1 (SEMDJL1) and Ehlers–Danlos-like syndrome. Here, we report on two sisters compound heterozygous for two novel B3GALT6 mutations that presented with severe short stature and progressive kyphoscoliosis, joint hypermobility and laxity, hyperextensible skin, platyspondyly, short ilia, and elbow malalignment. Microarray-based transcriptome analysis revealed the differential expression of several genes encoding extracellular matrix (ECM) structural components, including COMP, SPP1, COL5A1, and COL15A1, enzymes involved in GAG synthesis and in ECM remodeling, such as CSGALNACT1, CHPF, LOXL3, and STEAP4, signaling transduction molecules of the TGFβ/BMP pathway, i.e., GDF6, GDF15, and BMPER, and transcription factors of the HOX and LIM families implicated in skeletal and limb development. Immunofluorescence analyses confirmed the down-regulated expression of some of these genes, in particular of the cartilage oligomeric matrix protein and osteopontin, encoded by COMP and SPP1, respectively, and showed the predominant reduction and disassembly of the heparan sulfate specific GAGs, as well as of the PG perlecan and type III and V collagens. The key role of GalT-II in GAG synthesis and the crucial biological functions of PGs are consistent with the perturbation of many physiological functions that are critical for the correct architecture and homeostasis of various connective tissues, including skin, bone, cartilage, tendons, and ligaments, and generates the wide phenotypic spectrum of GalT-II-deficient patients.

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Clinical features/molecular characterization of two patients with spondyloepimetaphyseal dysplasia with joint laxity type 1

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Identification of two novel B3GALT6 mutations

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First report of transcriptome-wide gene expression profiling on GalT-II-deficient fibroblasts

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Immunofluorescence studies of several ECM structural components in GalT-II-deficient cells

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Enlargement of the knowledge on the GalT-II deficiency’s molecular pathogenesis

Osteoblast maturation on microtextured titanium involves paracrine regulation of bone morphogenetic protein signaling

Osteoblasts are sensitive to surface microtopography and chemistry. Osteoblast differentiation and maturation are higher in vitro and bone formation and osseointegration enhanced in vivo on microstructured titanium (Ti) compared to smooth surfaces. Cells increased BMP2 expression on microtextured Ti alloy, suggesting a paracrine role in regulating osteoblast maturation. However, recent studies show that exogenous BMP2 inhibits osteoblast production of anti-inflammatory cytokines and osteocalcin, indicating that control of BMP-signaling may be involved. This study examined whether cells modulate BMP ligands, receptors, and inhibitors during osteoblast maturation on Ti, specifically focusing on the roles of BMP2 and Noggin (NOG). mRNA and protein for BMP2, BMP4, and BMP7 and receptors BMPR1A, BMPR1B, and BMPR2, and BMP inhibitors were upregulated on microtextured surfaces in comparison to smooth surfaces. Maturation on microstructured Ti was slightly enhanced with exogenous BMP2 while NOG addition inhibited osteoblast maturation. Cells with NOG knocked down significantly increased osteoblast maturation. These results demonstrate that BMP-related molecules are controlled during osteoblast maturation on microstructured Ti surfaces and that endogenous NOG is an important regulator of the process. Modifying paracrine BMP signaling may yield more robust bone formation than application of exogenous BMPs.

Mammalian TBX1 preferentially binds and regulates downstream targets via a tandem T-site repeat

Haploinsufficiency or mutation of TBX1 is largely responsible for the etiology of physical malformations in individuals with velo-cardio-facial/DiGeorge syndrome (VCFS/DGS/22q11.2 deletion syndrome). TBX1 encodes a transcription factor protein that contains an evolutionarily conserved DNA binding domain termed the T-box that is shared with other family members. All T-box proteins, examined so far, bind to similar but not identical consensus DNA sequences, indicating that they have specific binding preferences. To identify the TBX1 specific consensus sequence, Systematic Evolution of Ligands by Exponential Enrichment (SELEX) was performed. In contrast to other TBX family members recognizing palindrome sequences, we found that TBX1 preferentially binds to a tandem repeat of 5′-AGGTGTGAAGGTGTGA-3′. We also identified a second consensus sequence comprised of a tandem repeat with a degenerated downstream site. We show that three known human disease-causing TBX1 missense mutations (F148Y, H194Q and G310S) do not alter nuclear localization, or disrupt binding to the tandem repeat consensus sequences, but they reduce transcriptional activity in cell culture reporter assays. To identify Tbx1-downstream genes, we performed an in silico genome wide analysis of potential cis-acting elements in DNA and found strong enrichment of genes required for developmental processes and transcriptional regulation. We found that TBX1 binds to 19 different loci in vitro, which may correspond to putative cis-acting binding sites. In situ hybridization coupled with luciferase gene reporter assays on three gene loci, Fgf8, Bmper, Otog-MyoD, show that these motifs are directly regulated by TBX1 in vitro. Collectively, the present studies establish new insights into molecular aspects of TBX1 binding to DNA. This work lays the groundwork for future in vivo studies, including chromatin immunoprecipitation followed by next generation sequencing (ChIP-Seq) to further elucidate the molecular pathogenesis of VCFS/DGS.

The effect of variants in the promoter of BMPER on the intramuscular fat deposition in longissimus dorsi muscle of pigs

The aim of the study was to evaluate the contribution of BMPER promoter SNPs to the gene expression and intramuscular fat content in longissimus dorsi muscle. Firstly the promoter region of BMPER was comparatively scanned by direct sequencing with pool DNA of two groups (n=15 for each group) with high (H) or low (L) IMF content. Two SNPs, c.-1423A>G and c.-1344A>C, were found to have reverse allele distribution in the two groups. Genotyping by PCR-SSCP in a larger population revealed that the two SNPs interlock completely to form only A-A or G-C haplotype. The IMF content and BMPER expression level of A-A/A-A genotype were higher than G-C/G-C genotype, and luciferase assay revealed that A-A haplotype promoter activity was also higher than G-C haplotype. Putative transcription factor prediction suggested that c.-1344 A>C mutation might shift the promoter binding affinity with GATAs. We concluded that BMPER promoter polymorphisms have an effect on IMF content, and A-A haplotype could be used as a candidate genetic marker for preferable IMF deposition.

A gene expression study of normal and damaged cartilage in anteromedial gonarthrosis, a phenotype of osteoarthritis

OBJECTIVE

To identify osteoarthritis (OA) relevant genes and pathways in damaged and undamaged cartilage isolated from the knees of patients with anteromedial gonarthrosis (AMG) - a specific form of knee OA.

DESIGN

Cartilage was obtained from nine patients undergoing unicompartmental knee replacement (UKR) for AMG. AMG provides a spatial representation of OA progression; showing a reproducible and histologically validated pattern of cartilage destruction such that damaged and undamaged cartilage from within the same knee can be consistently isolated and examined. Gene expression was analysed by microarray and validated using real-time PCR.

RESULTS

Damaged and undamaged cartilage showed distinct gene expression profiles. 754 genes showed significant up- or down-regulation (non-False discovery rate (FDR) P < 0.05) with enrichment for genes involved in cell signalling, Extracellular Matrix (ECM) and inflammatory response. A number of genes previously unreported in OA showed strongly altered expression including RARRES3, ADAMTSL2 and DUSP10. Confirmation of genes previously identified as modulated in OA was also obtained e.g., SFRP3, MMP3 and IGF1.

CONCLUSIONS

This is the first study to examine a common and consistent phenotype of OA to allow direct comparison of damaged and undamaged cartilage from within the same joint compartment. We have identified specific gene expression profiles in damaged and undamaged cartilage and have determined relevant genes and pathways in OA progression. Importantly this work also highlights the necessity for phenotypic and microanatomical characterization of cartilage in future studies of OA pathogenesis and therapeutic development.

A BMP regulatory network controls ectodermal cell fate decisions at the neural plate border

During ectodermal patterning the neural crest and preplacodal ectoderm are specified in adjacent domains at the neural plate border. BMP signalling is required for specification of both tissues, but how it is spatially and temporally regulated to achieve this is not understood. Here, using a transgenic zebrafish BMP reporter line in conjunction with double-fluorescent in situ hybridisation, we show that, at the beginning of neurulation, the ventral-to-dorsal gradient of BMP activity evolves into two distinct domains at the neural plate border: one coinciding with the neural crest and the other abutting the epidermis. In between is a region devoid of BMP activity, which is specified as the preplacodal ectoderm. We identify the ligands required for these domains of BMP activity. We show that the BMP-interacting protein Crossveinless 2 is expressed in the BMP activity domains and is under the control of BMP signalling. We establish that Crossveinless 2 functions at this time in a positive-feedback loop to locally enhance BMP activity, and show that it is required for neural crest fate. We further demonstrate that the Distal-less transcription factors Dlx3b and Dlx4b, which are expressed in the preplacodal ectoderm, are required for the expression of a cell-autonomous BMP inhibitor, Bambi-b, which can explain the specific absence of BMP activity in the preplacodal ectoderm. Taken together, our data define a BMP regulatory network that controls cell fate decisions at the neural plate border.

Expression and functional study of extracellular BMP antagonists during the morphogenesis of the digits and their associated connective tissues

The purpose of this study is to gain insight into the role of BMP signaling in the diversification of the embryonic limb mesodermal progenitors destined to form cartilage, joints, and tendons. Given the importance of extracellular BMP modulators in in vivo systems, we performed a systematic search of those expressed in the developing autopod during the formation of the digits. Here, we monitored the expression of extracellular BMP modulators including: Noggin, Chordin, Chordin-like 1, Chordin-like 2, Twisted gastrulation, Dan, BMPER, Sost, Sostdc1, Follistatin, Follistatin-like 1, Follistatin-like 5 and Tolloid. These factors show differential expression domains in cartilage, joints and tendons. Furthermore, they are induced in specific temporal patterns during the formation of an ectopic extra digit, preceding the appearance of changes that are identifiable by conventional histology. The analysis of gene regulation, cell proliferation and cell death that are induced by these factors in high density cultures of digit progenitors provides evidence of functional specialization in the control of mesodermal differentiation but not in cell proliferation or apoptosis. We further show that the expression of these factors is differentially controlled by the distinct signaling pathways acting in the developing limb at the stages covered by this study. In addition, our results provide evidence suggesting that TWISTED GASTRULATION cooperates with CHORDINS, BMPER, and NOGGIN in the establishment of tendons or cartilage in a fashion that is dependent on the presence or absence of TOLLOID.

BMPER regulates cardiomyocyte size and vessel density in vivo

BACKGROUND

BMPER, an orthologue of Drosophila melanogaster Crossveinless-2, is a secreted factor that regulates bone morphogenetic protein activity in endothelial cell precursors and during early cardiomyocyte differentiation. Although previously described in the heart, the role of BMPER in cardiac development and function remain unknown.

METHODS

BMPER-deficient hearts were phenotyped histologically and functionally using echocardiography and Doppler analysis. Since BMPER -/- mice die perinatally, adult BMPER +/- mice were challenged to pressure-overload-induced cardiac hypertrophy and hindlimb ischemia to determine changes in angiogenesis and regulation of cardiomyocyte size.

RESULTS

We identify for the first time the cardiac phenotype associated with BMPER haploinsufficiency. BMPER messenger RNA and protein are present in the heart during cardiac development through at least E14.5 but is lost by E18.5. BMPER +/- ventricles are thinner and less compact than sibling wild-type hearts. In the adult, BMPER +/- hearts present with decreased anterior and posterior wall thickness, decreased cardiomyocyte size and an increase in cardiac vessel density. Despite these changes, BMPER +/- mice respond to pressure-overload-induced cardiac hypertrophy challenge largely to the same extent as wild-type mice.

CONCLUSION

BMPER appears to play a role in regulating both vessel density and cardiac development in vivo; however, BMPER haploinsufficiency does not result in marked effects on cardiac function or adaptation to pressure overload hypertrophy.

Osteoblast-like differentiation of cultured human coronary artery smooth muscle cells by bone morphogenetic protein endothelial cell precursor-derived regulator (BMPER)

Differentiation of vascular smooth muscle cells (SMCs) into osteoblast-like cells is considered to be a mechanism of vascular calcification. However, regulators of osteoblast-like differentiation of vascular SMCs are not fully elucidated. Here, we investigated the expression of bone morphogenetic protein (BMP)-binding endothelial cell precursor-derived regulator (BMPER), a vertebrate homologue of Drosophila crossveinless-2, in vascular SMCs and the role and mode of action of BMPER in osteoblast-like differentiation of human coronary artery SMCs (HCASMCs). BMPER was expressed in cultured human vascular SMCs, including HCASMCs. Silencing of endogenous BMPER expression by an RNA interference technique inhibited osteoblast-like differentiation of HCASMCs, as evaluated by up-regulation of osteoblast markers such as alkaline phosphatase (ALP) and runt-related transcription factor 2 (Runx2), by down-regulation of a SMC marker α-smooth muscle actin (αSMA), and by mineralization. Treatment with recombinant BMPER enhanced, whereas BMP-2 reduced osteoblast-like differentiation. BMPER antagonized BMP-2-induced phosphorylation of Smad 1/5/8, suggesting that the effect of BMPER was mediated by antagonizing the action of BMP. BMPER increased IκBα phosphorylation and NF-κB activity and specific NF-κB decoy oligonucleotides deteriorated osteoblast-like differentiation of HCASMCs by BMPER. In human coronary artery with atherosclerotic plaque containing calcification, the BMPER-positive signals were observed in the neointimal and medial SMCs in the vicinity of the plaque. These findings indicate that BMPER is a novel regulator of the osteoblast-like differentiation of HCASMCs.

Modulation of matrix mineralization by Vwc2-like protein and its novel splicing isoforms

In search of new cysteine knot protein (CKP) family members, we found a novel gene called von Willebrand factor C domain-containing protein 2-like (Vwc2l, also known as Brorin-like) and its transcript variants (Vwc2l-1, Vwc2l-2 and Vwc2l-3). Based on the deduced amino acid sequence, Vwc2l-1 has a signal peptide and 2 cysteine-rich (CR) domains, while Vwc2l-2 lacks a part of 2nd CR domain and Vwc2l-3 both CR domains. Although it has been reported that the expression of Brorin-like was predominantly observed in brain, we found that Vwc2l transcript variants were detected in more ubiquitous tissues. In osteoblasts, the induction of Vwc2l expression was observed at matrix mineralization stage. When Vwc2l was stably transfected into osteoblasts, the matrix mineralization was markedly accelerated in Vwc2l-expressing clones compared to that in the control, indicating the modulatory effect of Vwc2l protein on osteoblastic cell function. The mechanistic insight of Vwc2l-modulation was further investigated and we found that the expression of Osterix, one of the key osteogenic markers, was significantly increased by addition of all Vwc2l isoform proteins. Taken together, Vwc2l is a novel secreted protein that promotes matrix mineralization by modulating Osterix expression likely through TGF-β superfamily growth factor signaling pathway. Our data may provide mechanistic insights into the biological functions of this novel CKP member in bone and further suggest a novel approach to enhance osteoblast function, which enables to accerelate bone formation, regeneration and healing.

BMPER protein is a negative regulator of hepcidin and is up-regulated in hypotransferrinemic mice

The BMP/SMAD4 pathway has major effects on liver hepcidin levels. Bone morphogenetic protein-binding endothelial cell precursor-derived regulator (Bmper), a known regulator of BMP signaling, was found to be overexpressed at the mRNA and protein levels in liver of genetically hypotransferrinemic mice (Trf(hpx/hpx)). Soluble BMPER peptide inhibited BMP2- and BMP6-dependent hepcidin promoter activity in both HepG2 and HuH7 cells. These effects correlated with reduced cellular levels of pSMAD1/5/8. Addition of BMPER peptide to primary human hepatocytes abolished the BMP2-dependent increase in hepcidin mRNA, whereas injection of Bmper peptide into mice resulted in reduced liver hepcidin and increased serum iron levels. Thus Bmper may play an important role in suppressing hepcidin production in hypotransferrinemic mice.

MicroRNA expression during osteogenic differentiation of human multipotent mesenchymal stromal cells from bone marrow

MicroRNAs comprise a group of non-coding small RNAs (17-25 nt) involved in post-transcriptional regulation that have been identified in various plants and animals. Studies have demonstrated that miRNAs are associated with stem cell self-renewal and differentiation and play a key role in controlling stem cell activities. However, the identification of specific miRNAs and their regulatory roles in the differentiation of multipotent mesenchymal stromal cells (MSCs) have so far been poorly defined. We isolated and cultured human MSCs and osteo-differentiated MSCs from four individual donors. miRNA expression in MSCs and osteo-differentiated MSCs was investigated using miRNA microarrays. miRNAs that were commonly expressed in all three MSC preparations and miRNAs that were differentially expressed between MSCs and osteo-differentiated MSCs were identified. Four underexpressed (hsa-miR-31, hsa-miR-106a, hsa-miR-148a, and hsa-miR-424) and three novel overexpressed miRNAs (hsa-miR-30c, hsa-miR-15b, and hsa-miR-130b) in osteo-differentiated MSCs were selected and their expression were verified in samples from the fourth individual donors. The putative targets of the miRNAs were predicted using bioinformatic analysis. The four miRNAs that were underexpressed in osteo-differentiated MSCs were predicted to target RUNX2, CBFB, and BMPs, which are involved in bone formation; while putative targets for miRNAs overexpressed in osteo-differentiated MSCs were MSC maker (e.g., CD44, ITGB1, and FLT1), stemness-maintaining factor (e.g., FGF2 and CXCL12), and genes related to cell differentiation (e.g., BMPER, CAMTA1, and GDF6). Finally, hsa-miR-31 was selected for target verification and function analysis. The results of this study provide an experimental basis for further research on miRNA functions during osteogenic differentiation of human MSCs.

BMP activity controlled by BMPER regulates the proinflammatory phenotype of endothelium

The endothelium plays a pivotal role in vascular inflammation. Here we study bone morphogenetic protein (BMP) signaling in endothelial inflammation and in particular the role of BMPER, an extracellular BMP modulator that is important in vascular development and angiogenesis. Using the BMP antagonist dorsomorphin or BMP2 as an agonist we show that BMP signaling is essential for the inflammatory response of vascular endothelial cells as demonstrated by intravital microscopy. We found that BMPER is decreased in inflammation similar to vascular protective genes like KLF2 and eNOS. Using in vitro and in vivo models we show that BMPER is down-regulated through the TNFα-NFκB-KLF2 signaling pathway. Functionally, lack of BMPER induced by siRNA or in BMPER(+/-) mice confers a proinflammatory endothelial phenotype with reduced eNOS levels and enhanced expression of adhesion molecules leading to increased leukocyte adhesion and extravasation in ex vivo and in vivo experiments. Vice versa, addition of BMPER exerts endothelium protective functions and antagonizes TNFα induced inflammation. Mechanistically, we demonstrate that these effects of BMPER are dependent on BMP signaling because of enhanced NFκB activity. In conclusion, the BMP modulator BMPER is a new protective regulator of vascular inflammation that modulates leukocyte adhesion and migration in vitro and in vivo.

Bone morphogenetic protein endothelial cell precursor-derived regulator regulates retinal angiogenesis in vivo in a mouse model of oxygen-induced retinopathy

OBJECTIVE

Bone morphogenetic proteins (BMPs) are potently proangiogenic; however, the mechanisms underlying the regulation of vessel development by BMPs are not fully understood. To assess the significance of BMP endothelial cell precursor-derived regulator (BMPER) in blood vessel formation in vivo, we investigated its role in retinal angiogenesis.

METHODS AND RESULTS

In a model of oxygen-induced retinopathy, Bmper mRNA expression and protein levels are downregulated, correlating with the initiation of Sma and Mad related protein phosphorylation in endothelial cells. Moreover, Bmper haploinsufficiency results in an increased rate of retinal revascularization, with retinas from Bmper+/- mice displaying increased numbers of branching points and angiogenic sprouts at the leading edge of the newly formed vasculature. Furthermore, although Bmper haploinsufficiency does not alter Bmp expression, it does lead to an increase in BMP signaling, as evidenced by increased phosphorylated Sma and Mad related protein levels in endothelial cells and increased expression of known BMP target genes.

CONCLUSIONS

These observations provide compelling evidence that BMPER is important in the regulation of BMP signaling and revascularization in the hypoxic retina. These bring forth the possibility of novel therapeutic approaches for pathological angiogenesis based on manipulation of BMP signaling.

Bone morphogenetic proteins: a critical review

Bone Morphogenetic Proteins (BMPs) are potent growth factors belonging to the Transforming Growth Factor Beta superfamily. To date over 20 members have been identified in humans with varying functions during processes such as embryogenesis, skeletal formation, hematopoiesis and neurogenesis. Though their functions have been identified, less is known regarding levels of regulation at the extracellular matrix, membrane surface, and receptor activation. Further, current models of activation lack the integration of these regulatory mechanisms. This review focuses on the different levels of regulation, ranging from the release of BMPs into the extracellular components to receptor activation for different BMPs. It also highlights areas in research that is lacking or contradictory.

Binding between Crossveinless-2 and Chordin von Willebrand factor type C domains promotes BMP signaling by blocking Chordin activity

Background

Crossveinless-2 (CV2) is an extracellular BMP modulator protein of the Chordin family, which can either enhance or inhibit BMP activity. CV2 binds to BMP2 via subdomain 1 of the first of its five N-terminal von Willebrand factor type C domains (VWC1). Previous studies showed that this BMP binding is required for the anti-, but not for the pro-BMP effect of CV2. More recently, it was shown that CV2 can also bind to the BMP inhibitor Chordin. However, it remained unclear which domains mediate this binding, and whether it accounts for an anti- or pro-BMP effect.

Principal Findings

Here we report that a composite interface of CV2 consisting of subdomain 2 of VWC1 and of VWC2-4, which are dispensable for BMP binding, binds to the VWC2 domain of Chordin. Functional data obtained in zebrafish embryos indicate that this binding of Chordin is required for CV2's pro-BMP effect, which actually is an anti-Chordin effect and, at least to a large extent, independent of Tolloid-mediated Chordin degradation. We further demonstrate that CV2 mutant versions that per se are incapable of BMP binding can attenuate the Chordin/BMP interaction.

Conclusions

We have physically dissected the anti- and pro-BMP effects of CV2. Its anti-BMP effect is obtained by binding to BMP via subdomain1 of the VWC1 domain, a binding that occurs in competition with Chordin. In contrast, its pro-BMP effect is achieved by direct binding to Chordin via subdomain 2 of VWC1 and VWC2-4. This binding seems to induce conformational changes within the Chordin protein that weaken Chordin's affinity to BMP. We propose that in ternary Chordin-CV2-BMP complexes, both BMP and Chordin are directly associated with CV2, whereas Chordin is pushed away from BMP, ensuring that BMPs can be more easily delivered to their receptors.

Crossveinless-2 is required for the relocalization of Chordin protein within the vertebral field in mouse embryos

Bone morphogenetic proteins (BMPs), as well as the BMP-binding molecules Chordin (Chd), Crossveinless-2 (CV2) and Twisted Gastrulation (Tsg), are essential for axial skeletal development in the mouse embryo. We previously reported a strong genetic interaction between CV2 and Tsg and proposed a role for this interaction in the shaping of the BMP morphogenetic field during vertebral development. In the present study we investigated the roles of CV2 and Chd in the formation of the vertebral morphogenetic field. We performed immunostainings for CV2 and Chd protein on wild-type, CV2(-/-) or Chd(-/-) mouse embryo sections at the stage of onset of the vertebral phenotypes. By comparing mRNA and protein localizations we found that CV2 does not diffuse away from its place of synthesis, the vertebral body. The most interesting finding of this study was that Chd synthesized in the intervertebral disc accumulates in the vertebral body. This relocalization does not take place in CV2(-/-) mutants. Instead, Chd was found to accumulate at its site of synthesis in CV2(-/-) embryos. These results indicate a CV2-dependent flow of Chd protein from the intervertebral disc to the vertebral body. Smad1/5/8 phosphorylation was decreased in CV2(-/-)vertebral bodies. This impaired BMP signaling may result from the decreased levels of Chd/BMP complexes diffusing from the intervertebral region. The data indicate a role for CV2 and Chd in the establishment of the vertebral morphogenetic field through the long-range relocalization of Chd/BMP complexes. The results may have general implications for the formation of embryonic organ-forming morphogenetic fields.

Bone morphogenetic protein and growth differentiation factor cytokine families and their protein antagonists

The BMPs (bone morphogenetic proteins) and the GDFs (growth and differentiation factors) together form a single family of cystine-knot cytokines, sharing the characteristic fold of the TGFbeta (transforming growth factor-beta) superfamily. Besides the ability to induce bone formation, which gave the BMPs their name, the BMP/GDFs display morphogenetic activities in the development of a wide range of tissues. BMP/GDF homo- and hetero-dimers interact with combinations of type I and type II receptor dimers to produce multiple possible signalling complexes, leading to the activation of one of two competing sets of SMAD transcription factors. BMP/GDFs have highly specific and localized functions. These are regulated in a number of ways, including the developmental restriction of BMP/GDF expression and through the secretion of several specific BMP antagonist proteins that bind with high affinity to the cytokines. Curiously, a number of these antagonists are also members of the TGF-beta superfamily. Finally a number of both the BMP/GDFs and their antagonists interact with the heparan sulphate side chains of cell-surface and extracellular-matrix proteoglycans.

Hypoxia regulates BMP4 expression in the murine spleen during the recovery from acute anemia

Background

Bone marrow erythropoiesis is primarily homeostatic, producing new erythrocytes at a constant rate. However at times of acute anemia, new erythrocytes must be rapidly produced much faster than bone marrow steady state erythropoiesis. At these times stress erythropoiesis predominates. Stress erythropoiesis occurs in the fetal liver during embryogenesis and in the adult spleen and liver. In adult mice, stress erythropoiesis utilizes a specialized population of stress erythroid progenitors that are resident in the spleen. In response to acute anemia, these progenitors rapidly expand and differentiate in response to three signals, BMP4, SCF and hypoxia. In absence of acute anemic stress, two of these signals, BMP4 and hypoxia, are not present and the pathway is not active. The initiating event in the activation of this pathway is the up-regulation of BMP4 expression in the spleen.

Methodology/Principal Findings

In this paper we analyze the regulation of BMP4 expression in the spleen by hypoxia. Using stromal cell lines, we establish a role for hypoxia transcription factor HIFs (Hypoxia Inducible Factors) in the transcription of BMP4. We identified putative Hypoxia Responsive Elements (HREs) in the BMP4 gene using bioinformatics. Analysis of these elements showed that in vivo, Hif2α binds two cis regulatory sites in the BMP4 gene, which regulate BMP4 expression during the recovery from acute anemia.

Conclusions and Significance

These data show that hypoxia plays a key role in initiating the BMP4 dependent stress erythropoiesis pathway by regulating BMP4 expression.

Extracellular BMP-antagonist regulation in development and disease: tied up in knots

Developmental processes are regulated by the bone morphogenetic protein (BMP) family of secreted molecules. BMPs bind to serine/threonine kinase receptors and signal through the canonical Smad pathway and other intracellular effectors. Integral to the control of BMPs is a diverse group of secreted BMP antagonists that bind to BMPs and prevent engagement with their cognate receptors. Tight temporospatial regulation of both BMP and BMP-antagonist expression provides an exquisite control system for developing tissues. Additional facets of BMP-antagonist biology, such as crosstalk with Wnt and Sonic hedgehog signaling during development, have been revealed in recent years. In addition, previously unappreciated roles for the BMP antagonists in kidney fibrosis and cancer have been elucidated. This review provides a description of BMP-antagonist biology, together with highlights of recent novel insights into the role of these antagonists in development, signal transduction and human disease.

Gene expression profiling to identify eggshell proteins involved in physical defense of the chicken egg

BACKGROUND

As uricoletic animals, chickens produce cleidoic eggs, which are self-contained bacteria-resistant biological packages for extra-uterine development of the chick embryo. The eggshell constitutes a natural physical barrier against bacterial penetration if it forms correctly and remains intact. The eggshell's remarkable mechanical properties are due to interactions among mineral components and the organic matrix proteins. The purpose of our study was to identify novel eggshell proteins by examining the transcriptome of the uterus during calcification of the eggshell. An extensive bioinformatic analysis on genes over-expressed in the uterus allowed us to identify novel eggshell proteins that contribute to the egg's natural defenses.

RESULTS

Our 14 K Del-Mar Chicken Integrated Systems microarray was used for transcriptional profiling in the hen's uterus during eggshell deposition. A total of 605 transcripts were over-expressed in the uterus compared with the magnum or white isthmus across a wide range of abundance (1.1- to 79.4-fold difference). The 605 highly-expressed uterine transcripts correspond to 469 unique genes, which encode 437 different proteins. Gene Ontology (GO) analysis was used for interpretation of protein function. The most over-represented GO terms are related to genes encoding ion transport proteins, which provide eggshell mineral precursors. Signal peptide sequence was found for 54 putative proteins secreted by the uterus during eggshell formation. Many functional proteins are involved in calcium binding or biomineralization--prerequisites for interacting with the mineral phase during eggshell fabrication. While another large group of proteins could be involved in proper folding of the eggshell matrix. Many secreted uterine proteins possess antibacterial properties, which would protect the egg against microbial invasion. A final group includes proteases and protease inhibitors that regulate protein activity in the acellular uterine fluid where eggshell formation takes place.

CONCLUSIONS

Our original study provides the first detailed description of the chicken uterus transcriptome during formation of the eggshell. We have discovered a cache of about 600 functional genes and identified a large number of encoded proteins secreted into uterine fluid for fabrication of the eggshell and chemical protection of the egg. Some of these uterine genes could prove useful as biological markers for genetic improvement of phenotypic traits (i.e., egg and eggshell quality).

BMPER is upregulated by statins and modulates endothelial inflammation by intercellular adhesion molecule-1

OBJECTIVE

In addition to lowering cholesterol, statins exert pleiotropic effects on endothelial cells. Bone morphogenetic proteins (BMPs) have recently been implicated in vascular inflammation and disease. We set out to investigate the effect of statins on BMP endothelial cell precursor-derived regulator (BMPER), a novel member of the BMP pathway.

METHODS AND RESULTS

Mevastatin enhanced BMPER expression in cultured endothelial cells in a time- and concentration-dependent manner as determined by immunocytochemistry, RT-PCR, and Western blotting. Similar effects were observed in vitro and in vivo using simvastatin. Actinomycin D chase analysis and BMPER promoter reporter assays revealed that this is mostly a posttranscriptional event resulting in prolonged BMPER RNA half-life. We confirmed that the RhoA/Rho-associated coiled-coil containing protein kinase Rho kinase (Rock)/actin pathway is involved using the specific pathway activator cytotoxic necrotizing factor of Yersinia pseudotuberculosis, which prevented upregulation of BMPER expression by mevastatin and pathway inhibitors (C3-toxin, RhoAN19 mutant, fasudil, and cytochalasin D) that enhanced BMPER expression. Increasing concentrations of BMPER exert antiinflammatory features in endothelial cells as reflected by intercellular adhesion molecule-1 downregulation. Accordingly, silencing of BMPER enhances intercellular adhesion molecule-1 expression. Furthermore, mevastatin reduced the expression of proinflammatory BMP4, a well-known direct interaction partner of BMPER.

CONCLUSIONS

Mevastatin modulates the BMP pathway by enhancing BMPER via the RhoA/Rock/actin pathway, as well as by reducing BMP4 expression. BMP4 downregulation and BMPER upregulation contribute to the antiinflammatory pleiotropic effects of statins.

The extracellular regulation of bone morphogenetic protein signaling

In many cases, the level, positioning and timing of signaling through the bone morphogenetic protein (BMP) pathway are regulated by molecules that bind BMP ligands in the extracellular space. Whereas many BMP-binding proteins inhibit signaling by sequestering BMPs from their receptors, other BMP-binding proteins cause remarkably context-specific gains or losses in signaling. Here, we review recent findings and hypotheses on the complex mechanisms that lead to these effects, with data from developing systems, biochemical analyses and mathematical modeling.

Systems biology of the self-regulating morphogenetic gradient of the Xenopus gastrula

The morphogenetic field concept was proposed by experimental embryologists to account for the self-regulative behavior of embryos. Such fields have remained an abstract concept until the recent identification of their molecular components using a combination of genetics, biochemistry, and theoretical modeling. One of the best studied models of a morphogenetic field is the Dorsal-Ventral (D-V) patterning of the early frog embryo. This patterning system is regulated by the bone morphogenetic protein (BMP) signaling pathway and an intricate network of secreted protein antagonists. This biochemical pathway of interacting proteins functions in the extracellular space to generate a D-V gradient of BMP signaling, which is maintained during extensive morphogenetic movements of cell layers during gastrulation. The D-V field is divided into a dorsal and a ventral center, in regions of low and high BMP signaling respectively, under opposite transcriptional control by BMPs. The robustness of the patterning is assured at two different levels. First, in the extracellular space by secreted BMP antagonists that generate a directional flow of BMP ligands to the ventral side. The flow is driven by the regulated proteolysis of the Chordin inhibitor and by the presence of a molecular sink on the ventral side that concentrates BMP signals. The tolloid metalloproteinases and the Chordin-binding protein Crossveinless-2 (CV2) are key components of this ventral sink. Second, by transcriptional feedback at the cellular level: The dorsal and ventral signaling centers adjust their size and level of BMP signaling by transcriptional feedback. This allows cells on one side of a gastrula containing about 10,000 cells to communicate with cells in the opposite pole of the embryo.

New insights into bone morphogenetic protein signaling: focus on angiogenesis

PURPOSE OF REVIEW

The role of bone morphogenetic proteins (BMPs) in vasculogenesis is still not well understood, despite many recent developments in this area of research. In this review, we discuss the most recent studies that identify new critical mechanisms through which BMP signaling acts with a focus on angiogenesis.

RECENT FINDINGS

New evidence brought to light over the last few years suggests that BMP-binding proteins, formerly thought of as antagonists, can also increase BMP activity under certain conditions. It has also recently been determined that components of the extracellular matrix are involved in the BMP signaling pathways that regulate angiogenesis. Through the BMP pathway, myosin-X and cyclooxygenase 2 serve as target genes that have been determined to play a role in blood vessel formation. BMPs also conduct Smad-independent signaling and crosstalk with other pathways. Finally, BMPs have been shown to play an antiangiogenic role in specific settings.

SUMMARY

Better understanding of the BMP signaling pathway and its regulators can have potentially great effects on therapeutic strategies from cardiovascular disease to cancer.

BMP modulators regulate the function of BMP during body patterning and disease progression

Bone morphogenetic proteins (BMPs) are phylogenetically conserved signaling molecules that belong to the transforming growth factor (TGF)-beta superfamily and are involved in the cascades of body patterning and morphogenesis. The activities of BMPs are precisely regulated at various stages, and extracellulary, mainly regulated by certain classes of molecules termed as BMP antagonists and pro-BMP factors. BMP antagonists inhibit BMP function by prohibiting them from binding their cognate receptors, whereas pro-BMP factors stimulate BMP function. In this review, the functions of these BMP regulators will be discussed. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

A concentration-dependent endocytic trap and sink mechanism converts Bmper from an activator to an inhibitor of Bmp signaling

Bmper, which is orthologous to Drosophila melanogaster crossveinless 2, is a secreted factor that regulates Bmp activity in a tissue- and stage-dependent manner. Both pro- and anti-Bmp activities have been postulated for Bmper, although the molecular mechanisms through which Bmper affects Bmp signaling are unclear. In this paper, we demonstrate that as molar concentrations of Bmper exceed Bmp4, Bmper dynamically switches from an activator to an inhibitor of Bmp4 signaling. Inhibition of Bmp4 through a novel endocytic trap-and-sink mechanism leads to the efficient degradation of Bmper and Bmp4 by the lysosome. Bmper-mediated internalization of Bmp4 reduces the duration and magnitude of Bmp4-dependent Smad signaling. We also determined that Noggin and Gremlin, but not Chordin, trigger endocytosis of Bmps. This endocytic transport pathway expands the extracellular roles of selective Bmp modulators to include intracellular regulation. This dosage-dependent molecular switch resolves discordances among studies that examine how Bmper regulates Bmp activity and has broad implications for Bmp signal regulation by secreted mediators.

BMPER is an endothelial cell regulator and controls bone morphogenetic protein-4-dependent angiogenesis

Bone morphogenetic proteins (BMPs) are involved in embryonic and adult blood vessel formation in health and disease. BMPER (BMP endothelial cell precursor-derived regulator) is a differentially expressed protein in embryonic endothelial precursor cells. In earlier work, we found that BMPER interacts with BMPs and when overexpressed antagonizes their function in embryonic axis formation. In contrast, in a BMPER-deficient zebrafish model, BMPER behaves as a BMP agonist. Furthermore, lack of BMPER induces a vascular phenotype in zebrafish that is driven by disarray of the intersomitic vasculature. Here, we investigate the impact of BMPER on endothelial cell function and signaling and elucidate its role in BMP-4 function in gain- and loss-of-function models. As shown by Western blotting and immunocytochemistry, BMPER is an extracellular matrix protein expressed by endothelial cells in skin, heart, and lung. We show that BMPER is a downstream target of FoxO3a and consistently exerts activating effects on endothelial cell sprouting and migration in vitro and in vivo. Accordingly, when BMPER is depleted from endothelial cells, sprouting is impaired. In terms of BMPER related intracellular signaling, we show that BMPER is permissive and necessary for Smad 1/5 phosphorylation and induces Erk1/2 activation. Most interestingly, BMPER is necessary for BMP-4 to exert its activating role in endothelial function and to induce Smad 1/5 activation. Vice versa, BMP-4 is necessary for BMPER activity. Taken together, BMPER is a dose-dependent endothelial cell activator that plays a unique and pivotal role in fine-tuning BMP activity in angiogenesis.

Crossveinless-2 Is a BMP feedback inhibitor that binds Chordin/BMP to regulate Xenopus embryonic patterning

Vertebrate Crossveinless-2 (CV2) is a secreted protein that can potentiate or antagonize BMP signaling. Through embryological and biochemical experiments we find that: (1) CV2 functions as a BMP4 feedback inhibitor in ventral regions of the Xenopus embryo; (2) CV2 complexes with Twisted gastrulation and BMP4; (3) CV2 is not a substrate for tolloid proteinases; (4) CV2 binds to purified Chordin protein with high affinity (K(D) in the 1 nM range); (5) CV2 binds even more strongly to Chordin proteolytic fragments resulting from Tolloid digestion or to full-length Chordin/BMP complexes; (6) CV2 depletion causes the Xenopus embryo to become hypersensitive to the anti-BMP effects of Chordin overexpression or tolloid inhibition. We propose that the CV2/Chordin interaction may help coordinate BMP diffusion to the ventral side of the embryo, ensuring that BMPs liberated from Chordin inhibition by tolloid proteolysis cause peak signaling levels.

Development of the vertebral morphogenetic field in the mouse: interactions between Crossveinless-2 and Twisted Gastrulation

Crossveinless-2 (Cv2), Twisted Gastrulation (Tsg) and Chordin (Chd) are components of an extracellular biochemical pathway that regulates Bone Morphogenetic Protein (BMP) activity during dorso-ventral patterning of Drosophila and Xenopus embryos, the formation of the fly wing, and mouse skeletogenesis. Because the nature of their genetic interactions remained untested in the mouse, we generated a null allele for Cv2 which was crossed to Tsg and Chd mutants to obtain Cv2; Tsg and Cv2; Chd compound mutants. We found that Cv2 is essential for skeletogenesis as its mutation caused the loss of multiple bone structures and posterior homeotic transformation of the last thoracic vertebra. During early vertebral development, Smad1 phosphorylation in the intervertebral region was decreased in the Cv2 mutant, even though CV2 protein is normally located in the future vertebral bodies. Because Cv2 mutation affects BMP signaling at a distance, this suggested that CV2 is involved in the localization of the BMP morphogenetic signal. Cv2 and Chd mutations did not interact significantly. However, mutation of Tsg was epistatic to all CV2 phenotypes. We propose a model in which CV2 and Tsg participate in the generation of a BMP signaling morphogenetic field during vertebral formation in which CV2 serves to concentrate diffusible Tsg/BMP4 complexes in the vertebral body cartilage.

Crossveinless-2 controls bone morphogenetic protein signaling during early cardiomyocyte differentiation in P19 cells

Increasing evidence indicates that bone morphogenetic proteins (BMPs) are crucial for cardiac induction, specification, and development. Although signaling of BMPs is tightly regulated through soluble BMP-binding proteins, how they regulate BMP signaling during cardiac differentiation remains unknown. To identify molecules responsible for BMP signaling during early cardiomyocyte differentiation of P19 cells, cDNA subtraction was performed. We found a bimodal expression of the BMP-binding protein Crossveinless-2 (Cv2) during cardiomyocyte differentiation; Cv2 is temporally expressed earlier than cardiac transcription factors such as Nkx2.5 and Tbx5 and acts as a suppressor for BMP signaling in P19 cells. We established a P19 clonal cell line harboring a cardiac alpha-myosin heavy chain promoter-driven enhanced green fluorescent protein gene to monitor cardiac differentiation by flow cytometry. Treatment with BMP2 during the first 2 days of differentiation suppressed cardiomyocyte differentiation through activation of down-stream targets Smad1/5/8 protein and Id1 gene, whereas treatment with Cv2 conversely inhibited Smad1/5/8 activation and Id1 expression, leading to increased generation of cardiac cells. RNA interference-mediated knockdown (KD) of endogenous Cv2 showed increased Smad1/5/8 activation and impaired cardiomyocyte differentiation. Expression of cardiac mesoderm markers was reduced, whereas expression of Id1 and endoderm markers such as Sox7, Hnf4, and E-cadherin was induced in Cv2-kinase dead cells. These phenotypes were rescued by the addition of Cv2 protein to the culture media during the first 2 days of differentiation or co-culture with parental cells. These data suggest that Cv2 may specify cardiac mesodermal lineage through inhibition of BMP signaling at early stage of cardiogenesis.

The BMP-binding protein Crossveinless 2 is a short-range, concentration-dependent, biphasic modulator of BMP signaling in Drosophila

In Drosophila, the secreted BMP-binding protein Short gastrulation (Sog) inhibits signaling by sequestering BMPs from receptors, but enhances signaling by transporting BMPs through tissues. We show that Crossveinless 2 (Cv-2) is also a secreted BMP-binding protein that enhances or inhibits BMP signaling. Unlike Sog, however, Cv-2 does not promote signaling by transporting BMPs. Rather, Cv-2 binds cell surfaces and heparan sulfate proteoglygans and acts over a short range. Cv-2 binds the type I BMP receptor Thickveins (Tkv), and we demonstrate how the exchange of BMPs between Cv-2 and receptor can produce the observed biphasic response to Cv-2 concentration, where low levels promote and high levels inhibit signaling. Importantly, we show also how the concentration or type of BMP present can determine whether Cv-2 promotes or inhibits signaling. We also find that Cv-2 expression is controlled by BMP signaling, and these combined properties enable Cv-2 to exquisitely tune BMP signaling.

Crystal structure analysis reveals how the Chordin family member crossveinless 2 blocks BMP-2 receptor binding

Crossveinless 2 (CV-2) is an extracellular BMP modulator protein belonging to the Chordin family. During development it is expressed at sites of high BMP signaling and like Chordin CV-2 can either enhance or inhibit BMP activity. CV-2 binds to BMP-2 via its N-terminal Von Willebrand factor type C (VWC) domain 1. Here we report the structure of the complex between CV-2 VWC1 and BMP-2. The tripartite VWC1 binds BMP-2 only through a short N-terminal segment, called clip, and subdomain (SD) 1. Mutational analysis establishes that the clip segment and SD1 together create high-affinity BMP-2 binding. All four receptor-binding sites of BMP-2 are blocked in the complex, demonstrating that VWC1 acts as competitive inhibitor for all receptor types. In vivo experiments reveal that the BMP-enhancing (pro-BMP) activity of CV-2 is independent of BMP-2 binding by VWC1, showing that pro- and anti-BMP activities are structurally separated in CV-2.

von Willebrand Factor Type C Domain-containing Proteins Regulate Bone Morphogenetic Protein Signaling through Different Recognition Mechanisms

Bone morphogenetic protein (BMP) function is regulated in the extracellular space by many modulator proteins, including those containing a von Willebrand factor type C (VWC) domain. The function of the VWC domain-containing proteins in development and diseases has been extensively studied. The structural basis, however, for the mechanism by which BMP is regulated by these proteins is still poorly understood. By analyzing chordin, CHL2 (chordin-like 2), and CV2 (crossveinless 2) as well as their individual VWC domains, we show that the VWC domain is a versatile binding module that in its multiple forms and environments can expose a variety of binding specificities. Three of four, two of three, and one of five VWCs from chordin, CHL2, and CV2, respectively, can bind BMPs. Using an array of BMP-2 mutant proteins, it can be demonstrated that the binding-competent VWC domains all use a specific subset of BMP-2 binding determinants that overlap with the binding site for the type II receptors (knuckle epitope) or for the type I receptors (wrist epitope). This explains the competition between modulator proteins and receptors for BMP binding and therefore the inhibition of BMP signaling. A subset of VWC domains from CHL2 binds to the Tsg (twisted gastrulation) protein similar to chordin. A stable ternary complex consisting of BMP-2, CHL2, and Tsg can be formed, thus making CHL2 a more efficient BMP-2 inhibitor. The VWCs of CV2, however, do not interact with Tsg. The present results show that chordin, CHL2, and CV2 regulate BMP-2 signaling by different recognition mechanisms.