Zebrafish nma is involved in TGFbeta family signaling

Integration of Nodal and BMP Signaling by Mutual Signaling Effector Antagonism

Opposing sources of bone morphogenetic protein (BMP) and Nodal signaling molecules are sufficient to induce the formation of a full axis in zebrafish embryos. To address how these signals orchestrate patterning, we transplant sources of fluorescently tagged Nodal and BMP into zebrafish embryos, robustly inducing the formation of secondary axes. Nodal and BMP signal non-cell-autonomously and form similar protein gradients in this context, but the signaling range of Nodal (pSmad2) is shorter than the BMP range (pSmad5). This yields a localized region of pSmad2 activity around the Nodal source, overlapping with a broad domain of pSmad5 activity across the embryo. Cell fates induced in various regions stereotypically correlate with pSmad2-to-pSmad5 ratios and can even be induced BMP- and Nodal-independently with different ratios of constitutively active Smad2 and Smad5. Strikingly, we find that Smad2 and Smad5 antagonize each other for specific cell fates, providing a mechanism for how cells integrate and discriminate between overlapping signals during development.

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Nodal induces pSmad at a shorter range than BMP due to slower activation kinetics

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Different ratios of active Smad2 and Smad5 can induce different embryonic structures

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Smad2 and Smad5 inhibit each other or act synergistically to induce specific cell fates

Adipose-specific BMP and activin membrane-bound inhibitor (BAMBI) deletion promotes adipogenesis by accelerating ROS production

With the improvement of people's living standards, the number of obese patients has also grown rapidly. It is reported that the level of oxidative stress in obese patients has significantly increased, mainly caused by the increase in reactive oxygen species (ROS) levels in adipose tissue. Studies have shown that the use of siRNA to interfere with bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) expression could promote adipocyte differentiation, and under hypoxic conditions, BAMBI could act as a regulator of HIF1α to regulate the polarity damage of epithelial cells. In view of these results, we speculated that BAMBI may regulate adipogenesis by regulating the level of ROS. In this study, we generated adipose-specific BAMBI knockout mice (BAMBI AKO) and found that compared with control mice, BAMBI AKO mice showed obesity when fed with high-fat diet, accompanied by insulin resistance, glucose intolerance, hypercholesterolemia, and increased inflammation in adipose tissue. Interestingly, adipose-specific deficiency of BAMBI could cause an increase in the expression level of Nox4, thereby promoting ROS production in cytoplasm and mitochondria and the DNA-binding activity of C/EBPβ and ultimately promoting adipogenesis. Consistently, our findings indicated that BAMBI may be a reactive oxygen regulator to affect adipogenesis, thereby controlling obesity and metabolic syndrome.

Optogenetic investigation of BMP target gene expression diversity

Signaling molecules activate distinct patterns of gene expression to coordinate embryogenesis, but how spatiotemporal expression diversity is generated is an open question. In zebrafish, a BMP signaling gradient patterns the dorsal-ventral axis. We systematically identified target genes responding to BMP and found that they have diverse spatiotemporal expression patterns. Transcriptional responses to optogenetically delivered high- and low-amplitude BMP signaling pulses indicate that spatiotemporal expression is not fully defined by different BMP signaling activation thresholds. Additionally, we observed negligible correlations between spatiotemporal expression and transcription kinetics for the majority of analyzed genes in response to BMP signaling pulses. In contrast, spatial differences between BMP target genes largely collapsed when FGF and Nodal signaling were inhibited. Our results suggest that, similar to other patterning systems, combinatorial signaling is likely to be a major driver of spatial diversity in BMP-dependent gene expression in zebrafish.

Smarcad1 mediates microbiota-induced inflammation in mouse and coordinates gene expression in the intestinal epithelium

BACKGROUND

How intestinal epithelial cells interact with the microbiota and how this is regulated at the gene expression level are critical questions. Smarcad1 is a conserved chromatin remodeling factor with a poorly understood tissue function. As this factor is highly expressed in the stem and proliferative zones of the intestinal epithelium, we explore its role in this tissue.

RESULTS

Specific deletion of Smarcad1 in the mouse intestinal epithelium leads to colitis resistance and substantial changes in gene expression, including a striking increase of expression of several genes linked to innate immunity. Absence of Smarcad1 leads to changes in chromatin accessibility and significant changes in histone H3K9me3 over many sites, including genes that are differentially regulated upon Smarcad1 deletion. We identify candidate members of the gut microbiome that elicit a Smarcad1-dependent colitis response, including members of the poorly understood TM7 phylum.

CONCLUSIONS

Our study sheds light onto the role of the chromatin remodeling machinery in intestinal epithelial cells in the colitis response and shows how a highly conserved chromatin remodeling factor has a distinct role in anti-microbial defense. This work highlights the importance of the intestinal epithelium in the colitis response and the potential of microbial species as pharmacological and probiotic targets in the context of inflammatory diseases.

Defective fibrin deposition and thrombus stability in Bambi-/- mice are mediated by elevated anticoagulant function

BACKGROUND

Bone morphogenetic and activin membrane-bound inhibitor (BAMBI) is a transmembrane protein related to the type I transforming growth factor- β (TGF-β) receptor family that is present on both platelets and endothelial cells (ECs). Bambi-deficient mice exhibit reduced hemostatic function and thrombus stability characterized by an increased embolization.

OBJECTIVE

We aimed to delineate how BAMBI influences endothelial function and thrombus stability.

METHODS

Bambi-deficient mice were subjected to the laser-induced thrombosis model where platelet and fibrin accumulation was evaluated. Expression of thrombomodulin and tissue factor pathway inhibitor (TFPI) was also assessed in these mice.

RESULTS

Thrombus instability in Bambi-/- mice was associated with a profound defect in fibrin deposition. Injection of hirudin into Bambi+/+ mice prior to thrombus formation recapitulated the Bambi-/- thrombus instability phenotype. In contrast, hirudin had no additional effect upon thrombus formation in Bambi-/- mice. Deletion of Bambi in ECs resulted in mice with defective thrombus stability caused by decreased fibrin accumulation. Increased levels of the anticoagulant proteins TFPI and thrombomodulin were detected in Bambi-/- mouse lung homogenates. Endothelial cells isolated from Bambi-/- mouse lungs exhibited enhanced ability to activate protein C due to elevated thrombomodulin levels. Blocking thrombomodulin and TFPI in vivo fully restored fibrin accumulation and thrombus stability in Bambi-/- mice.

CONCLUSIONS

We demonstrate that endothelial BAMBI influences fibrin generation and thrombus stability by modulating thrombomodulin and TFPI anticoagulant function of the endothelium; we also highlight the importance of these anticoagulant proteins in the laser-induced thrombosis model.

Specificity, versatility, and control of TGF-β family signaling

Encoded in mammalian cells by 33 genes, the transforming growth factor-β (TGF-β) family of secreted, homodimeric and heterodimeric proteins controls the differentiation of most, if not all, cell lineages and many aspects of cell and tissue physiology in multicellular eukaryotes. Deregulation of TGF-β family signaling leads to developmental anomalies and disease, whereas enhanced TGF-β signaling contributes to cancer and fibrosis. Here, we review the fundamentals of the signaling mechanisms that are initiated upon TGF-β ligand binding to its cell surface receptors and the dependence of the signaling responses on input from and cooperation with other signaling pathways. We discuss how cells exquisitely control the functional presentation and activation of heteromeric receptor complexes of transmembrane, dual-specificity kinases and, thus, define their context-dependent responsiveness to ligands. We also introduce the mechanisms through which proteins called Smads act as intracellular effectors of ligand-induced gene expression responses and show that the specificity and impressive versatility of Smad signaling depend on cross-talk from other pathways. Last, we discuss how non-Smad signaling mechanisms, initiated by distinct ligand-activated receptor complexes, complement Smad signaling and thus contribute to cellular responses.

BAMBI shuttling between cytosol and membrane is required for skeletal muscle development and regeneration

Bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) gene encodes a transmembrane protein and is involved in multiple physiological and pathological processes, such as inflammatory response, tumor development and progression, cell proliferation and differentiation. A previous study suggested that BAMBI may interact with the Wnt/β-catenin signaling pathway via promoting β-catenin nuclear translocation associated with C2C12 myogenic myoblast differentiation. However, its biological function in skeletal muscle still remains unknown and requires further characterization. The present work sought to investigate its biological function in skeletal muscle, especially the physiological roles of BAMBI during skeletal muscle growth and regeneration. Our current work suggests that BAMBI protein is highly expressed in skeletal muscle and is only detected in cytosolic fraction in the resting muscle. Moreover, BAMBI protein is co-localized in fast-twitch (glycolytic) fibers, but not in slow-twitch (oxidative) fibers. Comparing with the cytosolic trapping in resting muscle, BAMBI protein is enriched on cellular membrane during the muscle growth and regeneration, suggesting that BAMBI-mediated a significant signaling pathway may be an essential part of muscle growth and regeneration.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

TGF-β family co-receptor function and signaling

Transforming growth factor-β (TGF-β) family members, which include TGF-βs, activins and bone morphogenetic proteins, are pleiotropic cytokines that elicit cell type-specific effects in a highly context-dependent manner in many different tissues. These secreted protein ligands signal via single-transmembrane Type I and Type II serine/threonine kinase receptors and intracellular SMAD transcription factors. Deregulation in signaling has been implicated in a broad array of diseases, and implicate the need for intricate fine tuning in cellular signaling responses. One important emerging mechanism by which TGF-β family receptor signaling intensity, duration, specificity and diversity are regulated and/or mediated is through cell surface co-receptors. Here, we provide an overview of the co-receptors that have been identified for TGF-β family members. While some appear to be specific to TGF-β family members, others are shared with other pathways and provide possible ways for signal integration. This review focuses on novel functions of TGF-β family co-receptors, which continue to be discovered.

Tris(1,3-dichloro-2-propyl) phosphate disrupts dorsoventral patterning in zebrafish embryos

Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) is a high-production volume organophosphate flame retardant widely used within the United States. Within zebrafish, initiation of TDCIPP exposure at 0.75 h post-fertilization (hpf) results in genome-wide alterations in methylation during cleavage (2 hpf) as well as epiboly delay or arrest (at higher concentrations) during late-blastula and early-gastrula (4–6 hpf). To determine whether these TDCIPP-induced effects were associated with impacts on the transcriptome, embryos were exposed to vehicle (0.1% DMSO) or 2 µM TDCIPP from 0.75 hpf to 6 hpf, and total RNA was extracted from triplicate embryo pools per treatment and hybridized onto duplicate Affymetrix Zebrafish Gene 1.0 ST Arrays per RNA sample. Based on transcriptome-wide profiling, TDCIPP resulted in a significant impact on biological processes involved in dorsoventral patterning and bone morphogenetic protein (BMP) signaling. Consistent with these responses, TDCIPP exposure also resulted in strongly dorsalized embryos by 24 hpf—a phenotype that mimicked the effects of dorsomorphin, a potent and selective BMP inhibitor. Moreover, the majority of dorsalized embryos were preceded by epiboly arrest at 6 hpf. Our microarray data also revealed that the expression of sizzled (szl)—a gene encoding a secreted Frizzled-related protein that limits BMP signaling—was significantly decreased by nearly 4-fold at 6 hpf. Therefore, we used a splice-blocking morpholino to test the hypothesis that knockdown of szl phenocopies TDCIPP-induced delays in epiboly progression. Interestingly, contrary to our hypothesis, injection of szl MOs did not affect epiboly progression but, similar to chordin (chd) morphants, resulted in mildly ventralized embryos by 24 hpf. Overall, our findings suggest that TDCIPP-induced epiboly delay may not be driven by decreased szl expression, and that TDCIPP-induced dorsalization may—similar to dorsomorphin—be due to interference with BMP signaling during early zebrafish development.

SMAD1/5 mediates bone morphogenetic protein 2-induced up-regulation of BAMBI expression in human granulosa-lutein cells

Bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) is a transforming growth factor β (TGF-β) type I receptor antagonist that negatively regulates TGF-β and bone morphogenetic protein (BMP) signaling. BAMBI has been shown to be regulated by TGF-β signaling; however, whether BAMBI can be regulated by BMP signaling remains to be determined. The aim of this study was to investigate the effect of BMP2 on the regulation of BAMBI expression in human granulosa-lutein cells and the underlying mechanisms. Both primary and immortalized human granulosa-lutein cells were used as research models. Using dual inhibition approaches, our results showed that BMP2 activated SMAD1/5/8 phosphorylation and up-regulated BAMBI mRNA levels, which was reversed by the BMP type I receptor inhibitors, DMH-1 and dorsomorphin, but not by SB431542 (activin/TGF-β type I receptor inhibitor). Moreover, the combined knockdown of SMAD1 and SMAD5 completely abolished the BMP2-induced up-regulation of BAMBI. Similarly, knockdown of SMAD4 reversed the BMP2-induced up-regulation of BAMBI. Pre-treatment with BMP2 inhibited the TGF-β1-induced phosphorylation of SMAD2/3 and up-regulation of MMP2, and these inhibitory effects were reversed by knockdown of endogenous BAMBI. Our findings indicate that BAMBI is a BMP-responsive gene and that BAMBI participates in the negative feedback regulation of TGF-β signaling in the human ovary.

C-terminal-truncated HBV X promotes hepato-oncogenesis through inhibition of tumor-suppressive β-catenin/BAMBI signaling

C-terminal-truncated hepatitis B virus (HBV) X (HBx) (ctHBX) is frequently detected in hepatocellular carcinoma (HCC) through HBV integration into the host genome. However, the molecular mechanisms underlying ctHBx-associated oncogenic signaling have not yet been clarified. To elucidate the biological role of ctHBx in hepato-oncogenesis, we functionally analyzed ctHBx-mediated regulation of the activin membrane-bound inhibitor bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) through transforming growth factor-β (TGF-β) or β-catenin (CTNNB1) in HCC cells and in an animal model, and we compared its role to that of the full-length HBx protein. Ectopic ctHBx expression generated more colonies in anchorage-dependent and -independent growth assays than did HBx expression alone. ctHBx downregulated BAMBI to a greater degree than did HBx in HCC cells. HBx activated the Wnt/β-catenin pathway, which positively regulated the BAMBI expression through T-cell factor 1 signaling, whereas ctHBx negatively regulated the Wnt/β-catenin pathway. BAMBI downregulated the β-catenin and TGF-β1 signaling pathways. TGF-β1 positively regulated BAMBI expression thorough Smad3 signaling. Furthermore, knockdown of BAMBI was more tumorigenic in HCC cells. Therefore, downregulation of both β-catenin and TGF-β1 signaling by BAMBI might contribute to tumor suppression in mice xenotransplanted with HepG2 or SH-J1 cells. Taken together, ctHBx may have a more oncogenic role than HBx through its inhibition of tumor-suppressive β-catenin/BAMBI signaling.

TGF-β Signal Transduction in Oro-facial Health and Non-malignant Disease (Part I)

The transforming growth factor-beta (TGF-β) family of cytokines consists of multi-functional polypeptides that regulate a variety of cell processes, including proliferation, differentiation, apoptosis, extracellular matrix elaboration, angiogenesis, and immune suppression, among others. In so doing, TGF-β plays a key role in the control of cell behavior in both health and disease. In this report, we review what is known about the mechanisms of activation of the peptide, together with details of TGF-β signal transduction pathways. This review summarizes the evidence implicating TGF-β in normal physiological processes of the craniofacial complex—such as palatogenesis, tooth formation, wound healing, and scarring—and then evaluates its role in non-malignant disease processes such as scleroderma, submucous fibrosis, periodontal disease, and lichen planus.

Signaling Receptors for TGF-β Family Members

Transforming growth factor β (TGF-β) family members signal via heterotetrameric complexes of type I and type II dual specificity kinase receptors. The activation and stability of the receptors are controlled by posttranslational modifications, such as phosphorylation, ubiquitylation, sumoylation, and neddylation, as well as by interaction with other proteins at the cell surface and in the cytoplasm. Activation of TGF-β receptors induces signaling via formation of Smad complexes that are translocated to the nucleus where they act as transcription factors, as well as via non-Smad pathways, including the Erk1/2, JNK and p38 MAP kinase pathways, and the Src tyrosine kinase, phosphatidylinositol 3'-kinase, and Rho GTPases.

Activins and Inhibins: Roles in Development, Physiology, and Disease

Since their original discovery as regulators of follicle-stimulating hormone (FSH) secretion and erythropoiesis, the TGF-β family members activin and inhibin have been shown to participate in a variety of biological processes, from the earliest stages of embryonic development to highly specialized functions in terminally differentiated cells and tissues. Herein, we present the history, structures, signaling mechanisms, regulation, and biological processes in which activins and inhibins participate, including several recently discovered biological activities and functional antagonists. The potential therapeutic relevance of these advances is also discussed.

Temporally coordinated signals progressively pattern the anteroposterior and dorsoventral body axes

The vertebrate body plan is established through the precise spatiotemporal coordination of morphogen signaling pathways that pattern the anteroposterior (AP) and dorsoventral (DV) axes. Patterning along the AP axis is directed by posteriorizing signals Wnt, fibroblast growth factor (FGF), Nodal, and retinoic acid (RA), while patterning along the DV axis is directed by bone morphogenetic proteins (BMP) ventralizing signals. This review addresses the current understanding of how Wnt, FGF, RA and BMP pattern distinct AP and DV cell fates during early development and how their signaling mechanisms are coordinated to concomitantly pattern AP and DV tissues.

Identification and expression analyses of BAMBI mediated by FSH in swine luteinizing granulosa cells

Transforming growth factor-β and related growth factors are essential regulators for the development of follicles. Bone morphogenic protein (BMP) and activin membrane-bound inhibitor (BAMBI) was reported as a key factor participating in the transforming growth factor-β signal pathway. To investigate the role of BAMBI in porcine granulosa cells, the full length of the BAMBI was cloned from porcine ovarian cDNA. The results of bioinformatics analyses showed that the signaling peptide was located in between positions 20 and 21. The results of online prediction on phosphorylation sites indicate that the sites of Ser, Thr, and Tyr are 9, 1, and 1, respectively. In addition, BAMBI was highly homologous in rodent and livestock. Real-time quantitative polymerase chain reaction (qPCR) indicated that BAMBI was widely expressed in porcine tissues. Immunofluorescence showed that BAMBI was located in both nucleus and cytoplasm. Stimulating the granulosa cells with FSH in vitro could alter BAMBI expression level in a time-dependent manner. Moreover, the expression level declined after treatment with FSH. These results indicated that BAMBI is an FSH-repressed gene in porcine luteinizing granulosa cells and it may be involved in the regulation of ovarian follicle development and oocyte maturation.

Vessel wall BAMBI contributes to hemostasis and thrombus stability

Bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) is a transmembrane protein related to the transforming growth factor-β superfamily, and is highly expressed in platelets and endothelial cells. We previously demonstrated its positive role in thrombus formation using a zebrafish thrombosis model. In the present study, we used Bambi-deficient mice and radiation chimeras to evaluate the function of this receptor in the regulation of both hemostasis and thrombosis. We show that Bambi(-/-) and Bambi(+/-) mice exhibit mildly prolonged bleeding times compared with Bambi(+/+) littermates. In addition, using 2 in vivo thrombosis models in mesenterium or cremaster muscle arterioles, we demonstrate that Bambi-deficient mice form unstable thrombi compared with Bambi(+/+) mice. No defects in thrombin generation in Bambi(-/-) mouse plasma could be detected ex vivo. Moreover, the absence of BAMBI had no effect on platelet counts, platelet activation, aggregation, or platelet procoagulant function. Similar to Bambi(-/-) mice, Bambi(-/-) transplanted with Bambi(+/+) bone marrow formed unstable thrombi in the laser-induced thrombosis model that receded more rapidly than thrombi that formed in Bambi(+/+) mice receiving Bambi(-/-) bone marrow transplants. Taken together, these results provide strong evidence for an important role of endothelium rather than platelet BAMBI as a positive regulator of both thrombus formation and stability.

Sfrp5 modulates both Wnt and BMP signaling and regulates gastrointestinal organogenesis [corrected] in the zebrafish, Danio rerio

Sfrp5 belongs to the family of secreted frizzled related proteins (Sfrp), secreted inhibitors of Wingless-MMTV Integration Site (Wnt) signaling, which play an important role in cancer and development. We selected sfrp5 because of its compelling expression profile in the developing endoderm in zebrafish, Danio rerio. In this study, overexpression of sfrp5 in embryos results in defects in both convergent extension (CE) by inhibition of non-canonical Wnt signaling and defects in dorsoventral patterning by inhibition of Tolloid-mediated proteolysis of the BMP inhibitor Chordin. From 25 hours post fertilization (hpf) to 3 days post fertilization (dpf), both overexpression and knockdown of Sfrp5 decrease the size of the endoderm, significantly reducing liver cell number. At 3 dpf, insulin-positive endodermal cells fail to coalesce into a single pancreatic islet. We show that Sfrp5 inhibits both canonical and non-canonical Wnt signaling during embryonic and endodermal development, resulting in endodermal abnormalities.

BAMBI (BMP and activin membrane-bound inhibitor) protects the murine heart from pressure-overload biomechanical stress by restraining TGF-β signaling

Left ventricular (LV) pressure overload is a major cause of heart failure. Transforming growth factors-β (TGF-βs) promote LV remodeling under biomechanical stress. BAMBI (BMP and activin membrane-bound inhibitor) is a pseudoreceptor that negatively modulates TGF-β signaling. The present study tests the hypothesis that BAMBI plays a protective role during the adverse LV remodeling under pressure overload. The subjects of the study were BAMBI knockout mice (BAMBI(-/-)) undergoing transverse aortic constriction (TAC) and patients with severe aortic stenosis (AS). We examined LV gene and protein expression of remodeling-related elements, histological fibrosis, and heart morphology and function. LV expression of BAMBI was increased in AS patients and TAC-mice and correlated directly with TGF-β. BAMBI deletion led to a gain of myocardial TGF-β signaling through canonical (Smads) and non-canonical (TAK1-p38 and TAK1-JNK) pathways. As a consequence, the remodeling response to pressure overload in BAMBI(-/-) mice was exacerbated in terms of hypertrophy, chamber dilation, deterioration of long-axis LV systolic function and diastolic dysfunction. Functional remodeling associated transcriptional activation of fibrosis-related TGF-β targets, up-regulation of the profibrotic micro-RNA-21, histological fibrosis and increased metalloproteinase-2 activity. Histological remodeling in BAMBI(-/-) mice involved TGF-βs. BAMBI deletion in primary cardiac fibroblasts exacerbated TGF-β-induced profibrotic responses while BAMBI overexpression in NIH-3T3 fibroblasts attenuated them. Our findings identify BAMBI as a critical negative modulator of myocardial remodeling under pressure overload. We suggest that BAMBI is involved in negative feedback loops that restrain the TGF-β remodeling signals to protect the pressure-overloaded myocardium from uncontrolled extracellular matrix deposition in humans and mice.

Spatial regulation of BMP activity

The bone morphogenetic protein (BMP) signalling pathway is critical for embryonic development and tissue homeostasis, and impaired BMP signalling has been implicated in multiple diseases. Molecular tools have been developed to visualise BMP activity in vivo and these have allowed a better understanding of the intricate ways in which BMP activity is regulated spatially. In particular, generation and interpretation of BMP activity gradients during development result from the complex interplay between core BMP signalling components and specific regulators. In this essay we discuss the mechanisms by which spatial regulation of BMP activity is achieved and its functional consequences.

BAMBI is expressed in endothelial cells and is regulated by lysosomal/autolysosomal degradation

BACKGROUND

BAMBI (BMP and Activin Membrane Bound Inhibitor) is considered to influence TGFβ and Wnt signaling, and thereby fibrosis. Surprisingly data on cell type-specific expression of BAMBI are not available. We therefore examined the localization, gene regulation, and protein turnover of BAMBI in kidneys.

METHODOLOGY/PRINCIPAL FINDINGS

By immunofluorescence microscopy and by mRNA expression, BAMBI is restricted to endothelial cells of the glomerular and some peritubular capillaries and of arteries and veins in both murine and human kidneys. TGFβ upregulated mRNA of BAMBI in murine glomerular endothelial cells (mGEC). LPS did not downregulate mRNA for BAMBI in mGEC or in HUVECs. BAMBI mRNA had a half-life of only 60 minutes and was stabilized by cycloheximide, indicating post-transcriptional regulation due to AU-rich elements, which we identified in the 3' untranslated sequence of both the human and murine BAMBI gene. BAMBI protein turnover was studied in HUVECs with BAMBI overexpression using a lentiviral system. Serum starvation as an inducer of autophagy caused marked BAMBI degradation, which could be totally prevented by inhibition of lysosomal and autolysosomal degradation with bafilomycin, and partially by inhibition of autophagy with 3-methyladenine, but not by proteasomal inhibitors. Rapamycin activates autophagy by inhibiting TOR, and resulted in BAMBI protein degradation. Both serum starvation and rapamycin increased the conversion of the autophagy marker LC3 from LC3-I to LC3-II and also enhanced co-staining for BAMBI and LC3 in autolysosomal vesicles.

CONCLUSIONS/SIGNIFICANCE

1. BAMBI localizes to endothelial cells in the kidney and to HUVECs. 2. BAMBI mRNA is regulated by post-transcriptional mechanisms. 3. BAMBI protein is regulated by lysosomal and autolysosomal degradation. The endothelial localization and the quick turnover of BAMBI may indicate novel, yet to be defined functions of this modulator for TGFβ and Wnt protein actions in the renal vascular endothelium in health and disease.

The maturation-inducing hormone 17alpha,20beta-dihydroxy-4-pregnen-3-one regulates gene expression of inhibin betaA and bambi (bone morphogenetic protein and activin-membrane-bound inhibitor) in the rainbow trout ovary

Transforming growth factor-beta (TGFbeta) superfamily members are important paracrine and autocrine regulators of ovarian development and steroidogenesis in mammals and birds, but their reproductive roles in fish are not well understood. The activin system, Tgfb, and bone morphogenetic protein 15 (Bmp15) participate in the regulation of follicle maturation in some fish species. In addition, transcript levels of TGFbeta superfamily members and their inhibitor, bambi (bmp and activin-membrane-bound inhibitor), change in the rainbow trout (Oncorhynchus mykiss) ovary during reproductive development including the transition from vitellogenesis to follicle maturation. The objective of the present study was to determine if the maturation-inducing hormone (MIH) in trout, 17alpha,20beta-dihydroxy-4-pregnen-3-one, regulates gene expression of TGFbeta superfamily members and their inhibitors. Transcript levels of inhibin beta(A) subunit (inhba) were increased and bambi decreased in isolated follicles incubated overnight without hormones compared to abundance in freshly excised tissues from the same fish, suggesting systemic factors influenced transcript abundance. Incubation with MIH decreased inhba and increased bambi expression in a dose-dependant manner and MIH was the most potent steroid examined. The transcripts' responses to incubation with and without MIH were observed in maturationally competent follicles, which are follicles competent to resume meiosis in response to MIH, and incompetent follicles, although the responses to MIH were greater in competent follicles. In summary, MIH regulates inhba and bambi expression in a stage specific manner supporting a role for MIH regulation of the TGFbeta superfamily system and participation of the TGFbeta superfamily system in the regulation of follicle maturation in rainbow trout.

BAMBI (bone morphogenetic protein and activin membrane-bound inhibitor) reveals the involvement of the transforming growth factor-beta family in pain modulation

Transforming growth factors-beta (TGF-betas) signal through type I and type II serine-threonine kinase receptor complexes. During ligand binding, type II receptors recruit and phosphorylate type I receptors, triggering downstream signaling. BAMBI [bone morphogenetic protein (BMP) and activin membrane-bound inhibitor] is a transmembrane pseudoreceptor structurally similar to type I receptors but lacks the intracellular kinase domain. BAMBI modulates negatively pan-TGF-beta family signaling; therefore, it can be used as an instrument for unraveling the roles of these cytokines in the adult CNS. BAMBI is expressed in regions of the CNS involved in pain transmission and modulation. The lack of BAMBI in mutant mice resulted in increased levels of TGF-beta signaling activity, which was associated with attenuation of acute pain behaviors, regardless of the modality of the stimuli (thermal, mechanical, chemical/inflammatory). The nociceptive hyposensitivity exhibited by BAMBI(-/-) mice was reversed by the opioid antagonist naloxone. Moreover, in a model of chronic neuropathic pain, the allodynic responses of BAMBI(-/-) mice also appeared attenuated through a mechanism involving delta-opioid receptor signaling. Basal mRNA and protein levels of precursor proteins of the endogenous opioid peptides proopiomelanocortin (POMC) and proenkephalin (PENK) appeared increased in the spinal cords of BAMBI(-/-). Transcript levels of TGF-betas and their intracellular effectors correlated directly with genes encoding opioid peptides, whereas BAMBI correlated inversely. Furthermore, incubation of spinal cord explants with activin A or BMP-7 increased POMC and/or PENK mRNA levels. Our findings identify TGF-beta family members as modulators of acute and chronic pain perception through the transcriptional regulation of genes encoding the endogenous opioids.

Overlapping functions of Pea3 ETS transcription factors in FGF signaling during zebrafish development

Fibroblast growth factors (FGFs) are secreted molecules that activate the RAS/mitogen-activated protein kinase (MAPK) signaling pathway. In zebrafish development, FGF signaling is responsible for establishing dorsal polarity, maintaining the isthmic organizer, and cardiac ventricle formation. Because several ETS factors are known transcriptional mediators of MAPK signaling, we hypothesized that these factors function to mediate FGF signaling processes. In zebrafish, the simultaneous knock-down of three Pea3 ETS proteins, Etv5, Erm, and Pea3, produced phenotypes reminiscent of embryos deficient in FGF signaling. Morphant embryos displayed both cardiac and left/right patterning defects as well as disruption of the isthmic organizer. Furthermore, the expression of FGF target genes was abolished in Pea3 ETS depleted embryos. To understand how FGF signaling and ETS factors control gene expression, transcriptional regulation of dusp6 was studied in mouse and zebrafish. Conserved Pea3 ETS binding sites were identified within the Dusp6 promoter, and reporter assays showed that one of these sites is required for dusp6 induction by FGFs. We further demonstrated the interaction of Pea3 ETS factors with the Dusp6 promoter both in vitro and in vivo. These results revealed the requirement of ETS factors in transducing FGF signals in developmental processes.

Competing roles of TGFbeta and Nma/BAMBI in odontoblasts

Nma/BAMBI is a novel pseudoreceptor with homology to a TGFbeta type I receptor that lacks a serine/threonine kinase domain. Nma/BAMBI functions as a dominant-negative protein that regulates reciprocal epithelial-mesenchymal interactions during organogenesis. Therefore, we hypothesized that Nma/BAMBI regulates TGFbeta signaling and downstream gene expression during dentinogenesis. To test this hypothesis, we examined the downstream gene expression profiles of major dentin extracellular matrix proteins in response to Nma/BAMBI, and we examined the roles of Nma/BAMBI and TGFbeta-1 during dentinogenesis. Overexpression of Nma/BAMBI in the mouse odontoblast-like cell line MD10-A2 down-regulated expression of DSPP by 66% and up-regulated expression of DMP1 four-fold. TGFbeta treatment reversed Nma/BAMBI's negative effect on DSPP expression. Furthermore, we demonstrated that TGFbeta negatively regulates Nma/BAMBI's expression levels in MD10-A2 odontoblast-like cells. Analysis of these data, together, indicates that TGFbeta and Nma/BAMBI are inversely regulated and that the sequence of expression determines the net effect on downstream gene expression.

Human BAMBI cooperates with Smad7 to inhibit transforming growth factor-beta signaling

Transforming growth factor beta (TGF-beta) and related growth factors are essential regulators of embryogenesis and tissue homeostasis. The signaling pathways mediated by their receptors and Smad proteins are precisely modulated by various means. Xenopus BAMBI (bone morphogenic protein (BMP) and activin membrane-bound inhibitor) has been shown to function as a general negative regulator of TGF-beta/BMP/activin signaling. Here, we provide evidence that human BAMBI (hBAMBI), like its Xenopus homolog, inhibits TGF-beta- and BMP-mediated transcriptional responses as well as TGF-beta-induced R-Smad phosphorylation and cell growth arrest, whereas knockdown of endogenous BAMBI enhances the TGF-beta-induced reporter expression. Mechanistically, in addition to interfering with the complex formation between the type I and type II receptors, hBAMBI cooperates with Smad7 to inhibit TGF-beta signaling. hBAMBI forms a ternary complex with Smad7 and the TGF-beta type I receptor ALK5/TbetaRI and inhibits the interaction between ALK5/TbetaRI and Smad3, thus impairing Smad3 activation. These findings provide a novel insight to understand the molecular mechanism underlying the inhibitory effect of BAMBI on TGF-beta signaling.

BAMBI gene is epigenetically silenced in subset of high-grade bladder cancer

The bone morphogenetic protein (BMP) and activin membrane-bound inhibitor (BAMBI) is a transmembrane TGFRI/BMPRI-related pseudoreceptor, antagonizes transforming growth factor (TGF)-beta/BMP signaling by inhibiting the formation of functional authentic receptor complexes (TGFRI/BMPRI and TGFRII/BMPRII). On the assumption that BAMBI gene expression is epigenetically altered during human bladder cancer progression, we screened the expression of BAMBI protein by immunohistochemistry and the methylation status of the BAMBI promoter. In the normal or reactive urothelium, BAMBI expression was mostly overlapped with that of BMPRI, and a similar colocalization pattern was noted in low-grade papillary cancers. In high-grade and invasive cancers, however, mainly two reciprocal immunohistochemical expression patterns were observed: BAMBI-low/BMPRI-high, and BAMBI-high/BMPRI-low, indicating that BAMBI expression is controlled such that it does not interfere with the responsiveness of high-grade cancer cells to TGF-beta/BMP signaling. Moreover, methylation of the BAMBI gene correlated significantly with negative BAMBI expression in bladder tumors. Although BAMBI overexpression significantly increased the number of apoptotic cells in T24 line, knock-down small interfering RNA showed no remarkable change. Cell motility assay revealed that on treatment with either TGF-beta1 or BMP2, T24 and HTB9 lines showed a marked increase in the number of migrated cells which, however, decreased significantly through the forced expression of BAMBI. Since certain subsets of aggressive tumors often promote cell motility, invasion and survival by inducing epithelial-to-mesenchymal transition through TGF-beta/BMP in an autocrine and paracrine manner, hypermethylation of the BAMBI gene promoter that leads to BAMBI gene suppression may be one of the epigenetic events affecting the invasiveness or aggressiveness of bladder cancers.

DeltaNp63 antagonizes p53 to regulate mesoderm induction in Xenopus laevis

p63, a homolog of the tumor suppressor p53, is critical for the development and maintenance of complex epithelia. The developmentally regulated p63 isoform, DeltaNp63, can act as a transcriptional repressor, but the link between the transcriptional functions of p63 and its biological roles is unclear. Based on our initial finding that the mesoderm-inducing factor activin A is suppressed by DeltaNp63 in human keratinocytes, we investigated the role of DeltaNp63 in regulating mesoderm induction during early Xenopus laevis development. We find that down-regulation of DeltaNp63 by morpholino injection in the early Xenopus embryo potentiates mesoderm formation whereas ectopic expression of DeltaNp63 inhibits mesoderm formation. Furthermore, we show that mesodermal induction after down-regulation of DeltaNp63 is dependent on p53. We propose that a key function for p63 in defining a squamous epithelial phenotype is to actively suppress mesodermal cell fates during early development. Collectively, we show that there is a distinct requirement for different p53 family members during the development of both mesodermal and ectodermal tissues. These findings have implications for the role of p63 and p53 in both development and tumorigenesis of human epithelia.

Expression and regulation of the decoy bone morphogenetic protein receptor BAMBI in the developing avian face

Here, we examine the expression and regulation of the gene BAMBI, a kinase-deficient decoy receptor capable of interacting with type I bone morphogenetic protein (BMP) receptors in avian embryos. Initially, expression was limited to the endoderm during neurula and pharyngula stages. From embryonic day 3.5 (stage 20) and onward, BAMBI expression almost perfectly overlapped with known expression patterns for BMP4, particularly in the face and limbs. We performed bead implant experiments in the face to see which signals could be repressing or promoting expression of BAMBI. Our data point to retinoids and BMPs as being major positive regulators of BAMBI expression; however, fibroblast growth factor 2 acts to repress BAMBI. Furthermore, retinoic acid is likely to act directly on BAMBI as induction occurs in the presence of cycloheximide. The data suggested that BAMBI could be used to regulate Bmp signaling during tissue interactions that are an integral part of facial morphogenesis.

The pseudoreceptor BMP and activin membrane-bound inhibitor positively modulates Wnt/beta-catenin signaling

The canonical Wnt/beta-catenin pathway plays a pivotal role in regulating embryogenesis and tumorigenesis by promoting cell proliferation. BAMBI (BMP and activin membrane-bound inhibitor) has previously been shown to negatively regulate the signaling activity of transforming growth factor-beta, activin, and BMP and was identified as a target of beta-catenin in colorectal and hepatocellular tumor cells. In this study, we provide evidence that BAMBI can promote the transcriptional activity of Wnt/beta-catenin signaling. Overexpression of BAMBI enhances the expression of Wnt-responsive reporters, whereas knockdown of endogenous BAMBI attenuates them. Accordingly, BAMBI also promotes the nuclear translocation of beta-catenin. BAMBI interacts with Wnt receptor Frizzled5, coreceptor LRP6, and Dishevelled2 and increases the interaction between Frizzled5 and Dishevelled2. Finally we show that BAMBI promotes the expression of c-myc and cyclin D1 and increases Wnt-promoted cell cycle progression. Altogether, our data indicate that BAMBI can function as a positive regulator of the Wnt/beta-catenin pathway to promote cell proliferation.

The TGF-beta pseudoreceptor gene Bambi is dispensable for mouse embryonic development and postnatal survival

The Bambi (Bmp and activin membrane-bound inhibitor) gene encodes a transmembrane protein highly similar in amino acid sequence to transforming growth factor-beta (TGF-beta receptors, however, the Bambi intracellular domain is short and lacks a serine/threonine-kinase domain that is essential for transducing TGF beta signaling. Previous biochemical assays showed that Bambi interacts directly with BMP receptors and antagonizes BMP signaling. Interestingly, the expression of Bambi largely overlaps, both temporally and spatially, with that of Bmp4 during early embryonic development in Xenopus, zebrafish, and mice, which led to the hypothesis that Bambi may function to regulate BMP signaling during embryogenesis. To directly analyze the roles of Bambi during embryonic development, we generated mice carrying a conditional allele of Bambi, Bambi(flox), with loxP sequences flanking the first exon that encodes the N-terminus and signal peptide region of the Bambi protein. Mice homozygous for this targeted conditional allele appear normal and fertile. We crossed the Bambi(flox)/+ mice to the EIIa-Cre transgenic mice and generated mice carrying deletion of the first exon of the Bambi gene. Surprisingly, mice homozygous for the deleted allele were viable, fertile and did not exhibit any discernible developmental defect. Our data exclude an essential role for Bambi in mouse embryonic development and postnatal survival.

TLR4 enhances TGF-beta signaling and hepatic fibrosis

Hepatic injury is associated with a defective intestinal barrier and increased hepatic exposure to bacterial products. Here we report that the intestinal bacterial microflora and a functional Toll-like receptor 4 (TLR4), but not TLR2, are required for hepatic fibrogenesis. Using Tlr4-chimeric mice and in vivo lipopolysaccharide (LPS) challenge, we demonstrate that quiescent hepatic stellate cells (HSCs), the main precursors for myofibroblasts in the liver, are the predominant target through which TLR4 ligands promote fibrogenesis. In quiescent HSCs, TLR4 activation not only upregulates chemokine secretion and induces chemotaxis of Kupffer cells, but also downregulates the transforming growth factor (TGF)-beta pseudoreceptor Bambi to sensitize HSCs to TGF-beta-induced signals and allow for unrestricted activation by Kupffer cells. LPS-induced Bambi downregulation and sensitization to TGF-beta is mediated by a MyD88-NF-kappaB-dependent pathway. Accordingly, Myd88-deficient mice have decreased hepatic fibrosis. Thus, modulation of TGF-beta signaling by a TLR4-MyD88-NF-kappaB axis provides a novel link between proinflammatory and profibrogenic signals.

Digit morphogenesis: Is the tip different?

Digit formation is the last step in the skeletal patterning of developing limbs. This process involves important aspects such as determination of chondrogenic versus interdigital areas; growth of digital rays with periodic segmentation to form joints and thus phalanges, and finally tip formation. Traditionally it was believed that the properties of digital rays were fixed at earlier stages, but recently a surprising plasticity of digit primordia at the time of condensation has been demonstrated. This implies the presence of local interactions that are able to modulate the particular programs that make a given digit, but we don't fully understand how they operate. An involvement of signaling from the interdigital spaces and from the apical ectodermal ridge has been proposed. Another interesting question is the formation of the last limb structure, digit tips, which may involve a specific molecular and cellular program. Indeed, the expression of several developmentally important genes is restricted to digit tips at late stages of limb development. Understanding the molecular and cellular interactions that lead to digit morphogenesis has important implications not only in the context of embryonic development (for example, how early cues received by cells are translated into anatomy or what are the mechanisms that control the cease of activity of signaling regions) but also in terms of limb diversification during evolution.

Molecular cloning, expression and characterization of the zebrafish bram1 gene, a BMP receptor-associated molecule

We have identified a cDNA clone encoding BMP receptor-associated molecule 1 (BRAM1) from the zebrafish expressed sequence tag (EST) database. The 2606 bp full-length bram1 cDNA was cloned, and further confirmed by nucleotide sequencing. The zebrafish sequence encodes a protein of 195 amino acids with an evolutionarily conserved MYND domain, which displays approximately approximately 98% homology with human and mouse BRAM1, and approximately approximately 64% homology with C. elegans BRA-1 and BRA-2. The bram1 gene, composed of five exons and four introns, spans approximately approximately 14 kb on linkage group 14 of the zebrafish genome. RT-PCR and whole mount in situ hybridization analyses disclosed that zebrafish BRAM1 is a maternal factor. The protein interacts directly with zebrafish BMP Receptor type IA, as observed from GST-pull down and co-immunoprecipitation assays. Furthermore, cotransfection of zebrafish BRAM1 with the corresponding BMP receptor resulted in down-regulation of BMP-mediated signaling. Our results collectively indicate that BRAM1 plays a biological role during zebrafish development.

Microarray analysis reveals novel gene expression changes associated with erectile dysfunction in diabetic rats

To investigate the full range of molecular changes associated with erectile dysfunction (ED) in Type 1 diabetes, we examined alterations in penile gene expression in streptozotocin-induced diabetic rats and littermate controls. With the use of Affymetrix GeneChip arrays and statistical filtering, 529 genes/transcripts were considered to be differentially expressed in the diabetic rat cavernosum compared with control. Gene Ontology (GO) classification indicated that there was a decrease in numerous extracellular matrix genes (e.g., collagen and elastin related) and an increase in oxidative stress-associated genes in the diabetic rat cavernosum. In addition, PubMatrix literature mining identified differentially expressed genes previously shown to mediate vascular dysfunction [e.g., ceruloplasmin (Cp), lipoprotein lipase, and Cd36] as well as genes involved in the modulation of the smooth muscle phenotype (e.g., Kruppel-like factor 5 and chemokine C-X3-C motif ligand 1). Real-time PCR was used to confirm changes in expression for 23 relevant genes. Further validation of Cp expression in the diabetic rat cavernosum demonstrated increased mRNA levels of the secreted and anchored splice variants of Cp. CP protein levels showed a 1.9-fold increase in tissues from diabetic rats versus controls. Immunohistochemistry demonstrated localization of CP protein in cavernosal sinusoids of control and diabetic animals, including endothelial and smooth muscle layers. Overall, this study broadens the scope of candidate genes and pathways that may be relevant to the pathophysiology of diabetes-induced ED as well as highlights the potential complexity of this disorder.

Negative regulation of TGF-beta signaling in development

The TGF-beta superfamily members have important roles in controlling patterning and tissue formation in both invertebrates and vertebrates. Two types of signal transducers, receptors and Smads, mediate the signaling to regulate expression of their target genes. Despite of the relatively simple signal transduction pathway, many modulators have been found to contribute to a tight regulation of this pathway in a variety of mechanisms. This article reviews the negative regulation of TGF-beta signaling with focus on its roles in vertebrate development.

The transforming growth factor-beta superfamily of receptors

The transforming growth factor-beta (TGF-beta) superfamily of receptors comprises two groups of transmembrane serine-threonine kinase receptors, so called type I, and type II receptors, that are activated following engagement by members of the TGF-beta superfamily of ligands. These events specify diverse downstream responses that are differentially regulated by controlling access and activation of the ligands, their receptors and downstream substrates in different cell types. The purpose of this review is to describe the biochemical properties of these receptors, focusing specifically on the mechanisms regulating receptor/ligand interactions and activation in mammalian cells.

Transcriptional regulation of the TGF-β pseudoreceptor BAMBI by TGF-β signaling

BAMBI is a transmembrane glycoprotein related to the transforming growth factor-beta (TGF-beta)-family type I receptors and functions as a negative regulator of TGF-beta signaling during development. BAMBI is induced by BMP signaling through the evolutionary conserved BMP-responsive elements in its promoter. Furthermore, we have recently shown that Wnt/beta-catenin signaling activates transcription of BAMBI and that BAMBI expression is aberrantly elevated in most colorectal carcinomas. Here, we show that BAMBI is also directly induced by TGF-beta signaling, through the three tandem repeats of 13 bp sequences containing the SMAD-binding elements, which are distinct from the BMP-responsive element. Our findings suggest that BAMBI transcription is regulated by TGF-beta signaling through direct binding of SMAD3 and SMAD4 to the BAMBI promoter.

A role for MKP3 in axial patterning of the zebrafish embryo

Fibroblast growth factors (FGFs) are secreted molecules that can activate the RAS/mitogen-activated protein kinase (MAPK) pathway to serve crucial functions during embryogenesis. Through an in situ hybridization screen for genes with restricted expression patterns during early zebrafish development,we identified a group of genes that exhibit similar expression patterns to FGF genes. We report the characterization of zebrafish MAP kinase phosphatase 3(MKP3; DUSP6 - Zebrafish Information Network), a member of the FGF synexpression group, showing that it has a crucial role in the specification of axial polarity in the early zebrafish embryo. MKP3 dephosphorylates the activated form of MAPK, inhibiting the RAS/MAPK arm of the FGF signaling pathway. Gain- and loss-of-function studies reveal that MKP3 is required to limit the extent of FGF/RAS/MAPK signaling in the early embryo, and that disturbing this inhibitory pathway disrupts dorsoventral patterning at the onset of gastrulation. The earliest mkp3 expression is restricted to the future dorsal region of the embryo where it is initiated by a maternalβ-catenin signal, but soon after its initiation, mkp3 expression comes under the control of FGF signaling. Thus, mkp3 encodes a feedback attenuator of the FGF pathway, the expression of which is initiated at an early stage so as to ensure correct FGF signaling levels at the time of axial patterning.

Transcriptional regulation of BMP4 synexpression in transgenic Xenopus

Synexpression groups are genetic modules composed of genes that share both a complex expression pattern and the biological process in which they function. Here we investigate the regulation of BMP4 synexpression by studying the enhancers of bambi, smad7 and vent2 in Xenopus. We find that a BMP4 synexpression promoter module is compact and (i) requires direct BMP responsiveness through Smad and Smad-cofactor binding motifs, (ii) may contain an evolutionary conserved BMP-responsive element, bre7 (TGGCGCC), that is crucial for expression of bambi and smad7 and is highly prognostic for novel BMP-responsive enhancers (BREs); and (iii) requires a narrow window of BMP inducibility, because minor enhancement or reduction of BMP responsiveness abolishes synexpression. Furthermore, we used a bioinformatic model to predict in silico 13 novel BREs, and tested five of them that were found in the id1-4 genes. The results highlight that in vivo analysis is required to reveal the physiological, spatio-temporal regulation of BMP-responsive genes.

Identification of BMP and activin membrane-bound inhibitor (BAMBI), an inhibitor of transforming growth factor-beta signaling, as a target of the beta-catenin pathway in colorectal tumor cells

The Wnt signaling pathway is activated in most human colorectal tumors. Mutational inactivation in the tumor suppressor adenomatous polyposis coli (APC), as well as activation of beta-catenin, causes the accumulation of beta-catenin, which in turn associates with the T cell factor/lymphoid enhancer factor (TCF/LEF) family of transcription factors and activates transcription of their target genes. Here we show that beta-catenin activates transcription of the BMP and activin membrane-bound inhibitor (BAMBI)/NMA gene. The expression level of BAMBI was found to be aberrantly elevated in most colorectal and hepatocellular carcinomas relative to the corresponding non-cancerous tissues. Expression of BAMBI in colorectal tumor cell lines was repressed by a dominant-negative mutant of TCF-4 or by an inhibitor of beta-catenin-TCF interaction, suggesting that beta-catenin is responsible for the aberrant expression of BAMBI in colorectal tumor cells. Furthermore, overexpression of BAMBI inhibited the response of tumor cells to transforming growth factor-beta signaling. These results suggest that beta-catenin interferes with transforming growth factor-beta-mediated growth arrest by inducing the expression of BAMBI, and this may contribute to colorectal and hepatocellular tumorigenesis.

Expression of Bambi is widespread in juvenile and adult rat tissues and is regulated in male germ cells

Members of the TGF beta superfamily may compete for receptor occupancy and intracellular signaling molecules in specific developmental circumstances. We explored the potential importance of the TGF beta family inhibitor, Bambi (Bmp and activin membrane-bound inhibitor) by examining its pattern of mRNA expression in juvenile and adult rat tissues, with a focus on reproductive organs. The 1.8-kb transcript was ubiquitous, whereas a 3-kb transcript was unique to enriched spermatocyte and spermatid cell fractions and adult testis. The full-length rat cDNA is 89% (nucleic acid) and 95% (amino acid) identical to its human homolog, hnma. Using in situ hybridization, Bambi mRNA was detected in granulosa and thecal cells of adult ovaries and in spermatogonia, spermatocytes, round spermatids, and Sertoli cells of adult testes. In addition to a persistent signal in Sertoli cells in juvenile testes, this mRNA within germ cells appeared dramatically increased as gonocytes matured into spermatogonia immediately after birth. These data indicate that TGF beta superfamily signaling within male germ cells is down-regulated at the onset of spermatogenesis. The addition of exogenous activin A to 24-h cultures of newborn rat testis fragments decreased the Bambi mRNA level. Regulated Bambi mRNA synthesis may contribute to TGF beta superfamily signaling modulation in several organs, as suggested by its discrete expression switch in male germ cells.

The zebrafish nodal-related gene southpaw is required for visceral and diencephalic left-right asymmetry

We have identified and characterized a new zebrafish gene, southpaw, that is required for visceral and diencephalic left-right asymmetry. southpaw encodes a new member of the nodal-related class of proteins, a subfamily within the transforming growth factor beta superfamily of secreted factors. southpaw is expressed bilaterally in paraxial mesoderm precursors and then within the left lateral plate mesoderm. At late somite stages, left-sided southpaw expression transiently overlaps the left-sided expression domains of other genes that mark the developing heart, such as lefty2. We have injected morpholinos to block the translation of the southpaw mRNA or to block splicing of the southpaw pre-mRNA. These morpholinos cause a severe disruption of early (cardiac jogging) and late (cardiac looping) aspects of cardiac left-right asymmetry. As the left-right asymmetry of the pancreas is also affected, southpaw appears to regulate left-right asymmetry throughout a large part of the embryo. Consistent with the morphological changes, the left-sided expression domains of downstream genes (cyclops, pitx2, lefty1 and lefty2) are severely downregulated or abolished within the lateral plate mesoderm of Southpaw-deficient embryos. Surprisingly, despite the absence of southpaw expression in the brain, we find that early diencephalic left-right asymmetry also requires Southpaw activity. These observations lead to a model of how visceral organ and brain left-right asymmetry are coordinated during embryogenesis.

Regulation of BMP and Activin Signaling in Drosophila

Cytokines of the TGF-beta superfamily act through an evolutionarily conserved signaling pathway to elicit a diverse range of biological responses in vertebrates as well as invertebrates. Drosophila has proved to be a powerful system to unravel the profound complexities underlying the regulation of this superficially simple signaling system for two reasons--the availability of sophisticated genetic tools and the restricted number of core signaling components compared to vertebrates. A BMP signaling pathway in Drosophila that regulates growth, differentiation and morphogenesis of the embryo and the larva has been extensively characterized. This work has provided major insights into how gradients of secreted proteins can be established and maintained in vivo, allowing a single ligand to induce multiple cell fates rather than function as an on-off switch. More recently, an activin signaling pathway has also been delineated that is required for growth and neuronal function during development. This review provides an overview of TGF-beta signaling in Drosophila with emphasis on the extensive modulation of signaling activity both within and outside the cell, that enables ligands to trigger specific and context-dependent effects.

Construction of a zebrafish cDNA microarray: gene expression profiling of the zebrafish during development

Vertebrate embryogenesis is a complex process controlled by a transcriptional hierarchy that coordinates the action of thousands of genes. To identify and analyze the expression patterns of these genes, we constructed a zebrafish cDNA microarray containing 4512 unique genes identified from zebrafish embryonic heart, adult hearts, and skeletal muscle cDNA libraries. We examined the patterns of gene expression during development in the zebrafish between five time points relative to 12h post-fertilization (hpf). Differentially expressed genes can be grouped into two categories, early genes that are expressed at 5hpf and genes expressed at 48/72/120hpf. Furthermore, we report the utilization of cDNA microarray technology to investigate the adaptive molecular responses of zebrafish to hypoxia during development. Our study provides the first utilization of cDNA microarray in the zebrafish and reveals dynamic changes in levels of gene expression in relation to development and survival of the zebrafish embryos under hypoxic stress.

The other side of TGF-beta superfamily signal regulation: thinking outside the cell

The transforming growth factor beta (TGF-beta) superfamily of paracrine and autocrine signaling molecules regulates a vast array of developmental and homeostatic processes and is itself exquisitely regulated. The misregulation of these molecules often results in cancer and other diseases. Here, we focus on new research that explores how TGF-beta superfamily signaling is controlled between the secreting cell and the target cell. Regulation can occur upon ligand secretion (in a latent protein complex) and in the creation of signaling gradients. Proteins in the extracellular milieu sequester ligand away from or facilitate ligand binding to receptor serine kinases. Ligands even positively regulate their own negative regulators. Studies of how TGF-beta signaling is regulated extracellularly have broadened our understanding of TGF-beta pathways, and could provide clues to our understanding and treatment of diseases resulting from misregulation of these pathways.

Suppressor and oncogenic roles of transforming growth factor-beta and its signaling pathways in tumorigenesis

Transforming growth factor-beta (TGF-beta) has been implicated in oncogenesis since the time of its discovery almost 20 years ago. The complex, multifunctional activities of TGF-beta endow it with both tumor suppressor and tumor promoting activities, depending on the stage of carcinogenesis and the responsivity of the tumor cell. Dysregulation or alteration of TGF-beta signaling in tumorigenesis can occur at many different levels, including activation of the ligand, mutation or transcriptional suppression of the receptors, or alteration of downstream signal transduction pathways resulting from mutation or changes in expression patterns of signaling intermediates or from changes in expression of other proteins which modulate signaling. New insights into signaling from the TGF-beta receptors, including the identification of Smad signaling pathways and their interaction with mitogen-activated protein (MAP) kinase pathways, are providing an understanding of the changes involved in the change from tumor suppressor to tumor promoting activities of TGF-beta. It is now appreciated that loss of sensitivity to inhibition of growth by TGF-beta by most tumor cells is not synonymous with complete loss of TGF-beta signaling but rather suggests that tumor cells gain advantage by selective inactivation of the tumor suppressor activities of TGF-beta with retention of its tumor promoting activities, especially those dependent on cross talk with MAP kinase pathways and AP-1.