Ligand binding and functional properties of betaglycan, a co-receptor of the transforming growth factor-beta superfamily. Specialized binding regions for transforming growth factor-beta and inhibin A

Structures of TGF-β with betaglycan and signaling receptors reveal mechanisms of complex assembly and signaling

Betaglycan (BG) is a transmembrane co-receptor of the transforming growth factor-β (TGF-β) family of signaling ligands. It is essential for embryonic development, tissue homeostasis and fertility in adults. It functions by enabling binding of the three TGF-β isoforms to their signaling receptors and is additionally required for inhibin A (InhA) activity. Despite its requirement for the functions of TGF-βs and InhA in vivo, structural information explaining BG ligand selectivity and its mechanism of action is lacking. Here, we determine the structure of TGF-β bound both to BG and the signaling receptors, TGFBR1 and TGFBR2. We identify key regions responsible for ligand engagement, which has revealed binding interfaces that differ from those described for the closely related co-receptor of the TGF-β family, endoglin, thus demonstrating remarkable evolutionary adaptation to enable ligand selectivity. Finally, we provide a structural explanation for the hand-off mechanism underlying TGF-β signal potentiation.

Development of Liver-Targeting αVβ5+ Exosomes as Anti-TGF-β Nanocarriers for the Treatment of the Pre-Metastatic Niche

Liver metastases frequently occur in pancreatic and colorectal cancer. Their development is promoted by tumor-derived exosomes with the integrin αVβ5 on their membrane. This integrin directs exosomes to the liver, where they promote a TGF-β-dependent pre-metastatic niche. We proposed the development of αVβ5+ exosomes to deliver anti-TGF-β therapy to the liver. This study demonstrates that the overexpression of αVβ5 in 293T cells allows its transfer to the secreted exosomes. αVβ5 overexpression increases exosome delivery to the liver, and αVβ5+ exosomes accumulate more in the liver compared to the lungs, kidneys, and brain in mice. We then sought 293T cells to directly produce and load an anti-TGF-β agent in their exosomes. First, we transduced 293T cells to express shRNAs against Tgfb1; however, the exosomes isolated from these cells did not knock down Tgfb1 in treated macrophages in vitro. However, when 293T expressed an mRNA coding a soluble form of betaglycan (sBG), a TGF-β inhibitor, this mRNA was detected in the isolated exosomes and the protein in the conditioned media of macrophages treated in vitro. In turn, this conditioned media decreased the TGF-β-induced phosphorylation of SMAD2/3 in hepatic cells in vitro. Our findings suggest that αVβ5+ exosomes could serve as nanocarriers for liver-targeted anti-TGF-β therapies.

Structures of TGF-β with betaglycan and the signaling receptors reveal the mechanism whereby betaglycan potentiates receptor complex assembly and signaling

Betaglycan (BG) is a transmembrane co-receptor of the transforming growth factor-β (TGF-β) family of signaling ligands. It is essential for embryonic development and tissue homeostasis and fertility in adults. It functions by enabling binding of the three TGF-β isoforms to their signaling receptors and is additionally required for inhibin A (InhA) activity. Despite its requirement for the functions of TGF-βs and InhA in vivo, structural information explaining BG ligand selectivity and its mechanism of action is lacking. Here, we determine the structure of TGF-β bound both to BG and the signaling receptors, TGFBR1 and TGFBR2. We identify key regions responsible for ligand engagement, which has revealed novel binding interfaces that differ from those described for the closely related co-receptor of the TGF-β family, endoglin, thus demonstrating remarkable evolutionary adaptation to enable ligand selectivity. Finally, we provide a structural explanation for the hand-off mechanism underlying TGF-β signal potentiation.

Single-Cell RNA Sequencing Reveals Cardiac Fibroblast-Specific Transcriptomic Changes in Dilated Cardiomyopathy

Dilated cardiomyopathy (DCM) is the most common cause of heart failure, with a complex aetiology involving multiple cell types. We aimed to detect cell-specific transcriptomic alterations in DCM through analysis that leveraged recent advancements in single-cell analytical tools. Single-cell RNA sequencing (scRNA-seq) data from human DCM cardiac tissue were subjected to an updated bioinformatic workflow in which unsupervised clustering was paired with reference label transfer to more comprehensively annotate the dataset. Differential gene expression was detected primarily in the cardiac fibroblast population. Bulk RNA sequencing was performed on an independent cohort of human cardiac tissue and compared with scRNA-seq gene alterations to generate a stratified list of higher-confidence, fibroblast-specific expression candidates for further validation. Concordant gene dysregulation was confirmed in TGFβ-induced fibroblasts. Functional assessment of gene candidates showed that AEBP1 may play a significant role in fibroblast activation. This unbiased approach enabled improved resolution of cardiac cell-type-specific transcriptomic alterations in DCM.

TGF-β signaling in health, disease, and therapeutics

Transforming growth factor (TGF)-β is a multifunctional cytokine expressed by almost every tissue and cell type. The signal transduction of TGF-β can stimulate diverse cellular responses and is particularly critical to embryonic development, wound healing, tissue homeostasis, and immune homeostasis in health. The dysfunction of TGF-β can play key roles in many diseases, and numerous targeted therapies have been developed to rectify its pathogenic activity. In the past decades, a large number of studies on TGF-β signaling have been carried out, covering a broad spectrum of topics in health, disease, and therapeutics. Thus, a comprehensive overview of TGF-β signaling is required for a general picture of the studies in this field. In this review, we retrace the research history of TGF-β and introduce the molecular mechanisms regarding its biosynthesis, activation, and signal transduction. We also provide deep insights into the functions of TGF-β signaling in physiological conditions as well as in pathological processes. TGF-β-targeting therapies which have brought fresh hope to the treatment of relevant diseases are highlighted. Through the summary of previous knowledge and recent updates, this review aims to provide a systematic understanding of TGF-β signaling and to attract more attention and interest to this research area.

Glycosaminoglycan modifications of betaglycan regulate ectodomain shedding to fine-tune TGF-β signaling responses in ovarian cancer

In pathologies including cancer, aberrant Transforming Growth Factor-β (TGF-β) signaling exerts profound tumor intrinsic and extrinsic consequences. Intense clinical endeavors are underway to target this pathway. Central to the success of these interventions is pinpointing factors that decisively modulate the TGF-β responses. Betaglycan/type III TGF-β receptor (TβRIII), is an established co-receptor for the TGF-β superfamily known to bind directly to TGF-βs 1-3 and inhibin A/B. Betaglycan can be membrane-bound and also undergo ectodomain cleavage to produce soluble-betaglycan that can sequester its ligands. Its extracellular domain undergoes heparan sulfate and chondroitin sulfate glycosaminoglycan modifications, transforming betaglycan into a proteoglycan. We report the unexpected discovery that the heparan sulfate glycosaminoglycan chains on betaglycan are critical for the ectodomain shedding. In the absence of such glycosaminoglycan chains betaglycan is not shed, a feature indispensable for the ability of betaglycan to suppress TGF-β signaling and the cells' responses to exogenous TGF-β ligands. Using unbiased transcriptomics, we identified TIMP3 as a key inhibitor of betaglycan shedding thereby influencing TGF-β signaling. Our results bear significant clinical relevance as modified betaglycan is present in the ascites of patients with ovarian cancer and can serve as a marker for predicting patient outcomes and TGF-β signaling responses. These studies are the first to demonstrate a unique reliance on the glycosaminoglycan chains of betaglycan for shedding and influence on TGF-β signaling responses. Dysregulated shedding of TGF-β receptors plays a vital role in determining the response and availability of TGF-βs', which is crucial for prognostic predictions and understanding of TGF-β signaling dynamics.

Heparan sulfate modifications of betaglycan promote TIMP3-dependent ectodomain shedding to fine-tune TGF-β signaling

In pathologies such as cancer, aberrant Transforming Growth Factor-β (TGF-β) signaling exerts profound tumor intrinsic and extrinsic consequences. Intense clinical endeavors are underway to target this pivotal pathway. Central to the success of these interventions is pinpointing factors that decisively modulate the TGF-β responses. Betaglycan/type III TGF-β receptor (TβRIII), is an established co-receptor for the TGF-β superfamily known to bind directly to TGF-βs 1-3 and inhibin A/B. While betaglycan can be membrane-bound, it can also undergo ectodomain cleavage to produce soluble-betaglycan that can sequester its ligands. The extracellular domain of betaglycan undergoes heparan sulfate and chondroitin sulfate glycosaminoglycan modifications, transforming betaglycan into a proteoglycan. Here we report the unexpected discovery that the heparan sulfate modifications are critical for the ectodomain shedding of betaglycan. In the absence of such modifications, betaglycan is not shed. Such shedding is indispensable for the ability of betaglycan to suppress TGF-β signaling and the cells' responses to exogenous TGF-β ligands. Using unbiased transcriptomics, we identified TIMP3 as a key regulator of betaglycan shedding and thereby TGF-β signaling. Our results bear significant clinical relevance as modified betaglycan is present in the ascites of patients with ovarian cancer and can serve as a marker for predicting patient outcomes and TGF-β signaling responses. These studies are the first to demonstrate a unique reliance on the glycosaminoglycan modifications of betaglycan for shedding and influence on TGF-β signaling responses. Dysregulated shedding of TGF-β receptors plays a vital role in determining the response and availability of TGF-βs', which is crucial for prognostic predictions and understanding of TGF-β signaling dynamics.

Computational modeling of TGF-β2:TβRI:TβRII receptor complex assembly as mediated by the TGF-β coreceptor betaglycan

Transforming growth factor-β1, -β2, and -β3 (TGF-β1, -β2, and -β3) are secreted signaling ligands that play essential roles in tissue development, tissue maintenance, immune response, and wound healing. TGF-β ligands form homodimers and signal by assembling a heterotetrameric receptor complex comprised of two type I receptor (TβRI):type II receptor (TβRII) pairs. TGF-β1 and TGF-β3 ligands signal with high potency due to their high affinity for TβRII, which engenders high-affinity binding of TβRI through a composite TGF-β:TβRII binding interface. However, TGF-β2 binds TβRII 200-500 more weakly than TGF-β1 and TGF-β3 and signals with lower potency compared with these ligands. Remarkably, the presence of an additional membrane-bound coreceptor, known as betaglycan, increases TGF-β2 signaling potency to levels similar to TGF-β1 and -β3. The mediating effect of betaglycan occurs even though it is displaced from and not present in the heterotetrameric receptor complex through which TGF-β2 signals. Published biophysics studies have experimentally established the kinetic rates of the individual ligand-receptor and receptor-receptor interactions that initiate heterotetrameric receptor complex assembly and signaling in the TGF-β system; however, current experimental approaches are not able to directly measure kinetic rates for the intermediate and latter steps of assembly. To characterize these steps in the TGF-β system and determine the mechanism of betaglycan in the potentiation of TGF-β2 signaling, we developed deterministic computational models with different modes of betaglycan binding and varying cooperativity between receptor subtypes. The models identified conditions for selective enhancement of TGF-β2 signaling. The models provide support for additional receptor binding cooperativity that has been hypothesized but not evaluated in the literature. The models further showed that betaglycan binding to the TGF-β2 ligand through two domains provides an effective mechanism for transfer to the signaling receptors that has been tuned to efficiently promote assembly of the TGF-β2(TβRII)2(TβRI)2 signaling complex.

TGF-β superfamily co-receptors in cancer

Transforming growth factor-β (TGF-β) superfamily signaling via their cognate receptors is frequently modified by TGF-β superfamily co-receptors. Signaling through SMAD-mediated pathways may be enhanced or depressed depending on the specific co-receptor and cell context. This dynamic effect on signaling is further modified by the release of many of the co-receptors from the membrane to generate soluble forms that are often antagonistic to the membrane-bound receptors. The co-receptors discussed here include TβRIII (betaglycan), endoglin, BAMBI, CD109, SCUBE proteins, neuropilins, Cripto-1, MuSK, and RGMs. Dysregulation of these co-receptors can lead to altered TGF-β superfamily signaling that contributes to the pathophysiology of many cancers through regulation of growth, metastatic potential, and the tumor microenvironment. Here we describe the role of several TGF-β superfamily co-receptors on TGF-β superfamily signaling and the impact on cellular and physiological functions with a particular focus on cancer, including a discussion on recent pharmacological advances and potential clinical applications targeting these co-receptors.

TGFBR3L is an inhibin B co-receptor that regulates female fertility

Follicle-stimulating hormone (FSH), a key regulator of ovarian function, is often used in infertility treatment. Gonadal inhibins suppress FSH synthesis by pituitary gonadotrope cells. The TGFβ type III receptor, betaglycan, is required for inhibin A suppression of FSH. The inhibin B co-receptor was previously unknown. Here, we report that the gonadotrope-restricted transmembrane protein, TGFBR3L, is the elusive inhibin B co-receptor. TGFBR3L binds inhibin B but not other TGFβ family ligands. TGFBR3L knockdown or overexpression abrogates or confers inhibin B activity in cells. Female Tgfbr3l knockout mice exhibit increased FSH levels, ovarian follicle development, and litter sizes. In contrast, female mice lacking both TGFBR3L and betaglycan are infertile. TGFBR3L’s function and cell-specific expression make it an attractive new target for the regulation of FSH and fertility.

Chordacentrum mineralization is delayed in zebrafish betaglycan-null mutants

BACKGROUND

The Transforming Growth Factor β (TGFβ) family is a group of related proteins that signal through a type I and type II receptors. Betaglycan, also known as the type III receptor (Tgfbr3), is a coreceptor for various ligands of the TGFβ family that participates in heart, liver and kidney development as revealed by the tgfbr3-null mouse, as well as in angiogenesis as revealed by Tgfbr3 downregulation in morphant zebrafish.

RESULTS

Here, we present CRISPR/Cas9-derived zebrafish Tgfbr3-null mutants, which exhibited unaltered embryonic angiogenesis and developed into fertile adults. One reproducible phenotype displayed by these Tgfbr3-null mutants is delayed chordacentra mineralization, which nonetheless does not result in vertebral abnormalities in the adult fishes. We also report that the canonical TGFβ signaling pathway is needed for proper chordacentra mineralization and that Tgfbr3 absence decreases this signal in the notochordal cells responsible for this process.

CONCLUSION

Betaglycan's "ligand presentation" function contributes to the optimal TGFβ signaling required for zebrafish chordacentra mineralization.

Plasma levels of soluble TGF ß receptor type III: no apparent promise as a marker in acute pancreatitis

AIM

To assess the potential of the soluble transforming growth factor β receptor type III (sTGFβrIII), a key regulator in TGFβ signaling, as a biomarker for diagnosis and stratification of patients with acute pancreatitis (AP).

METHODS

In this small prospective pilot study, patients' (N=22) plasma samples were obtained at three time points: the first and fourth day of hospitalization and the day of hospital discharge. Healthy controls' plasma (N=25) was obtained at a single time point. Concentration of sTGFβrIII in plasma was determined by ELISA. Data were analyzed by fitting linear or linear mixed models.

RESULTS

Plasma sTGFβrIII levels at presentation (day 1) were similar in AP patients and healthy participants, irrespectively of the disease severity. sTGFβrIII levels in patients were constant during hospital stay.

CONCLUSION

These observations do not support further evaluation of plasma sTGFβrIII levels in this setting, but do not exclude a potential biological role of TGFβ and membrane-bound TGFβrIII in AP pathophysiology.

Endoglin Level Is Critical for Cartilage Tissue Formation In Vitro by Passaged Human Chondrocytes

Transforming growth factor beta (TGFβ) signaling is required for in vitro chondrogenesis. In animal models of osteoarthritis (OA), TGFβ receptor alterations are detected in chondrocytes in severe OA cartilage. It is not known whether such changes are dependent on the grade of human OA and if they affect chondrogenesis. Thus, the purpose of this study was to determine if human OA chondrocytes obtained from low-grade or high-grade disease could form cartilage tissue and to assess the role of the co-receptors, endoglin (ENG) and TGFβ receptor 3 (TGFBRIII), in the regulation of this tissue generation in vitro. We hypothesized that the grade of OA disease would not affect the ability of cells to form cartilage tissue and that the TGFβ co-receptor, ENG, would be critical to regulating tissue formation. Chondrocytes isolated from low-grade OA or high-grade OA human articular cartilage (AC) were analyzed directly (P0) or passaged in monolayer to P2. Expression of the primary TGFβ receptor ALK5, and the co-receptors ENG and TGFβRIII, was assessed by image flow cytometry. To assess the ability to form cartilaginous tissue, cells were placed in three-dimensional culture at high density and cultured in chondrogenic media containing TGFβ3. ENG knockdown was used to determine its role in regulating tissue formation. Overall, grade-specific differences in expression of ALK5, ENG, and TGFβRIII in primary or passaged chondrocytes were not detected; however, ENG expression increased significantly after passaging. Despite the presence of ALK5, P0 cells did not form cartilaginous tissue. In contrast, P2 cells derived from low-grade and high-grade OA AC formed hyaline-like cartilaginous tissues of similar quality. Knockdown of ENG in P2 cells inhibited cartilaginous tissue formation compared to controls indicating that the level of ENG protein expression is critical for in vitro chondrogenesis by passaged articular chondrocytes. This study demonstrates that it is not the grade of OA, but the levels of ENG in the presence of ALK5 that influences the ability of human passaged articular chondrocytes to form cartilaginous tissue in vitro in 3D culture. This has implications for cartilage repair therapies. Impact statement These findings are important clinically, given the limited availability of osteoarthritis (OA) cartilage tissue. Being able to use cells from all grades of OA will increase our ability to obtain sufficient cells for cartilage repair. In addition, it is possible that endoglin (ENG) levels, in the presence of ALK5 expression, may be suitable to use as biomarkers to identify cells able to produce cartilage.

Role of glycosylation in TGF-β signaling and epithelial-to-mesenchymal transition in cancer

Glycosylation is a common posttranslational modification on membrane-associated and secreted proteins that is of pivotal importance for regulating cell functions. Aberrant glycosylation can lead to uncontrolled cell proliferation, cell-matrix interactions, migration and differentiation, and has been shown to be involved in cancer and other diseases. The epithelial-to-mesenchymal transition is a key step in the metastatic process by which cancer cells gain the ability to invade tissues and extravasate into the bloodstream. This cellular transformation process, which is associated by morphological change, loss of epithelial traits and gain of mesenchymal markers, is triggered by the secreted cytokine transforming growth factor-β (TGF-β). TGF-β bioactivity is carefully regulated, and its effects on cells are mediated by its receptors on the cell surface. In this review, we first provide a brief overview of major types of glycans, namely, N-glycans, O-glycans, glycosphingolipids and glycosaminoglycans that are involved in cancer progression. Thereafter, we summarize studies on how the glycosylation of TGF-β signaling components regulates TGF-β secretion, bioavailability and TGF-β receptor function. Then, we review glycosylation changes associated with TGF-β-induced epithelial-to-mesenchymal transition in cancer. Identifying and understanding the mechanisms by which glycosylation affects TGF-β signaling and downstream biological responses will facilitate the identification of glycans as biomarkers and enable novel therapeutic approaches.

Influence of the TGF-β Superfamily on Osteoclasts/Osteoblasts Balance in Physiological and Pathological Bone Conditions

The balance between bone forming cells (osteoblasts/osteocytes) and bone resorbing cells (osteoclasts) plays a crucial role in tissue homeostasis and bone repair. Several hormones, cytokines, and growth factors-in particular the members of the TGF-β superfamily such as the bone morphogenetic proteins-not only regulate the proliferation, differentiation, and functioning of these cells, but also coordinate the communication between them to ensure an appropriate response. Therefore, this review focuses on TGF-β superfamily and its influence on bone formation and repair, through the regulation of osteoclastogenesis, osteogenic differentiation of stem cells, and osteoblasts/osteoclasts balance. After introducing the main types of bone cells, their differentiation and cooperation during bone remodeling and fracture healing processes are discussed. Then, the TGF-β superfamily, its signaling via canonical and non-canonical pathways, as well as its regulation by Wnt/Notch or microRNAs are described and discussed. Its important role in bone homeostasis, repair, or disease is also highlighted. Finally, the clinical therapeutic uses of members of the TGF-β superfamily and their associated complications are debated.

Novel venom-based peptides (P13 and its derivative-M6) to maintain self-renewal of human embryonic stem cells by activating FGF and TGFβ signaling pathways

Background

In our previous study, a venom-based peptide named Gonearrestide (also named P13) was identified and demonstrated with an effective inhibition in the proliferation of colon cancer cells. In this study, we explored if P13 and its potent mutant M6 could promote the proliferation of human embryonic stem cells and even maintain their self-renewal.

Methods

The structure-function relationship analysis on P13 and its potent mutant M6 were explored from the molecular mechanism of corresponding receptor activation by a series of inhibitor assay plus molecular and dynamics simulation studies.

Results

An interesting phenomenon is that P13 (and its potent mutant M6), an 18AA short peptide, can activate both FGF and TGFβ signaling pathways. We demonstrated that the underlying molecular mechanisms of P13 and M6 could cooperate with proteoglycans to complete the “dimerization” of FGFR and TGFβ receptors.

Conclusions

Taken together, this study is the first research finding on a venom-based peptide that works on the FGF and TGF-β signaling pathways to maintain the self-renewal of hESCs.

A novel exopolysaccharide from marine bacterium Pantoea sp. YU16-S3 accelerates cutaneous wound healing through Wnt/β-catenin pathway

Natural polysaccharides with versatile properties are the potential candidates for wound healing applications. In this study, an exopolysaccharide, EPS-S3, isolated from a marine bacteria Pantoea sp. YU16-S3 was evaluated for its wound-healing abilities by studying the key molecular mechanisms in vitro and in vivo. Basic characterisation showed EPS-S3 as a heteropolysaccharide with glucose, galactose, N-acetyl galactosamine and glucosamine. The molecular weight of EPS-S3 was estimated to be 1.75 × 10⁵ Da. It showed thermal stability up to 200 °C and shear-thickening non-Newtonian behaviour. It was biocompatible with dermal fibroblasts and keratinocytes and showed cell adhesion and cell proliferation properties. EPS-S3 facilitated cell migration in fibroblasts, induced rapid transition of cell cycle phases and also activated macrophages. In vivo experiments in rats showed the re-epithelialization of injured tissue with increased expression of HB-EGF, FGF, E-cadherin and β-catenin in EPS-S3 treatment. The results indicate that EPS-S3 modulates healing process through Wnt/β-catenin pathway due to its unique characteristics. In conclusion, EPS-S3 biosynthesized by the marine bacterium is a potential biomolecule for cutaneous wound healing applications.

A Tale of Two Proteins: Betaglycan, IGSF1, and the Continuing Search for the Inhibin B Receptor

Inhibins are gonadal hormones that suppress follicle-stimulating hormone (FSH) synthesis by pituitary gonadotrope cells. The structurally related activins stimulate FSH by signaling through complexes of type I and type II receptors. Two models of inhibin action were proposed in 2000. First, inhibins function as competitive receptor antagonists, binding activin type II receptors with high affinity in the presence of the TGF-β type III coreceptor, betaglycan. Second, immunoglobulin superfamily, member 1 (IGSF1, then called p120) was proposed to mediate inhibin B antagonism of activin signaling via its type I receptor. These ideas have been challenged over the past few years. Rather than playing a role in inhibin action, IGSF1 is involved in the central control of the thyroid gland. Betaglycan binds inhibin A and inhibin B with high affinity, but only functions as an obligate inhibin A coreceptor in murine gonadotropes. There is likely to be a distinct, but currently unidentified coreceptor for inhibin B.

Betaglycan (TβRIII) is a Key Factor in TGF-β2 Signaling in Prepubertal Rat Sertoli Cells

Transforming growth factor-βs (TGF-βs) signal after binding to the TGF-β receptors TβRI and TβRII. Recently, however, betaglycan (BG) was identified as an important co-receptor, especially for TGF-β2. Both proteins are involved in several testicular functions. Thus, we analyzed the importance of BG for TGF-β1/2 signaling in Sertoli cells with ELISAs, qRT-PCR, siRNA silencing and BrdU assays. TGF-β1 as well as TGF-β2 reduced shedding of membrane-bound BG (mBG), thus reducing the amount of soluble BG (sBG), which is often an antagonist to TGF-β signaling. Treatment of Sertoli cells with GM6001, a matrix metalloproteinases (MMP) inhibitor, also counteracted BG shedding, thus suggesting MMPs to be mainly involved in shedding. Interestingly, TGF-β2 but not TGF-β1 enhanced secretion of tissue inhibitor of metalloproteinases 3 (TIMP3), a potent inhibitor of MMPs. Furthermore, recombinant TIMP3 attenuated BG shedding. Co-stimulation with TIMP3 and TGF-β1 reduced phosphorylation of Smad3, while a combination of TIMP3/TGF-β2 increased it. Silencing of BG as well as TIMP3 reduced TGF-β2-induced phosphorylation of Smad2 and Smad3 significantly, once more highlighting the importance of BG for TGF-β2 signaling. In contrast, this effect was not observed with TIMP3/TGF-β1. Silencing of BG and TIMP3 decreased significantly Sertoli cell proliferation. Taken together, BG shedding serves a major role in TGF-β2 signaling in Sertoli cells.

Structural biology of betaglycan and endoglin, membrane-bound co-receptors of the TGF-beta family

Betaglycan and endoglin, membrane-bound co-receptors of the TGF-β family, are required to mediate the signaling of a select subset of TGF-β family ligands, TGF-β2 and InhA, and BMP-9 and BMP-10, respectively. Previous biochemical and biophysical methods suggested alternative modes of ligand binding might be responsible for these co-receptors to selectively recognize and potentiate the functions of their ligands, yet the molecular details were lacking. Recent progress determining structures of betaglycan and endoglin, both alone and as bound to their cognate ligands, is presented herein. The structures reveal relatively minor, but very significant structural differences that lead to entirely different modes of ligand binding. The different modes of binding nonetheless share certain commonalities, such as multivalency, which imparts the co-receptors with very high affinity for their cognate ligands, but at the same time provides a mechanism for release by stepwise binding of the signaling receptors, both of which are essential for their functions.

Endoglin Trafficking/Exosomal Targeting in Liver Cells Depends on N-Glycosylation

Injury of the liver involves a wound healing partial reaction governed by hepatic stellate cells and portal fibroblasts. Individual members of the transforming growth factor-β (TGF-β) superfamily including TGF-β itself and bone morphogenetic proteins (BMP) exert diverse and partially opposing effects on pro-fibrogenic responses. Signaling by these ligands is mediated through binding to membrane integral receptors type I/type II. Binding and the outcome of signaling is critically modulated by Endoglin (Eng), a type III co-receptor. In order to learn more about trafficking of Eng in liver cells, we investigated the membranal subdomain localization of full-length (FL)-Eng. We could show that FL-Eng is enriched in Caveolin-1-containing sucrose gradient fractions. Since lipid rafts contribute to the pool of exosomes, we could consequently demonstrate for the first time that exosomes isolated from cultured primary hepatic stellate cells and its derivatives contain Eng. Moreover, via adenoviral overexpression, we demonstrate that all liver cells have the capacity to direct Eng to exosomes, irrespectively whether they express endogenous Eng or not. Finally, we demonstrate that block of N-glycosylation does not interfere with dimerization of the receptor, but abrogates the secretion of soluble Eng (sol-Eng) and prevents exosomal targeting of FL-Eng.

Structural Adaptation in Its Orphan Domain Engenders Betaglycan with an Alternate Mode of Growth Factor Binding Relative to Endoglin

Betaglycan (BG) and endoglin (ENG), homologous co-receptors of the TGF-β family, potentiate the signaling activity of TGF-β2 and inhibin A, and BMP-9 and BMP-10, respectively. BG exists as monomer and forms 1:1 growth factor (GF) complexes, while ENG exists as a dimer and forms 2:1 GF complexes. Herein, the structure of the BG orphan domain (BG_O) reveals an insertion that blocks the region that the endoglin orphan domain (ENG_O) uses to bind BMP-9, preventing it from binding in the same manner. Using binding studies with domain-deleted forms of TGF-β and BG_O, as well as small-angle X-ray scattering data, BG_O is shown to bind its cognate GF in an entirely different manner compared with ENG_O. The alternative interfaces likely engender BG and ENG with the ability to selectively bind and target their cognate GFs in a unique temporal-spatial manner, without interfering with one another or other TGF-β family GFs.

TGF-β2 uses the concave surface of its extended finger region to bind betaglycan's ZP domain via three residues specific to TGF-β and inhibin-α

Betaglycan (BG) is a membrane-bound co-receptor of the TGF-β family that selectively binds transforming growth factor-β (TGF-β) isoforms and inhibin A (InhA) to enable temporal-spatial patterns of signaling essential for their functions in vivo Here, using NMR titrations of methyl-labeled TGF-β2 with BG's C-terminal binding domain, BGZP-C, and surface plasmon resonance binding measurements with TGF-β2 variants, we found that the BGZP-C-binding site on TGF-β2 is located on the inner surface of its extended finger region. Included in this binding site are Ile-92, Lys-97, and Glu-99, which are entirely or mostly specific to the TGF-β isoforms and the InhA α-subunit, but they are unconserved in other TGF-β family growth factors (GFs). In accord with the proposed specificity-determining role of these residues, BG bound bone morphogenetic protein 2 (BMP-2) weakly or not at all, and TGF-β2 variants with the corresponding residues from BMP-2 bound BGZP-C more weakly than corresponding alanine variants. The BGZP-C-binding site on InhA previously was reported to be located on the outside of the extended finger region, yet at the same time to include Ser-112 and Lys-119, homologous to TGF-β2 Ile-92 and Lys-97, on the inside of the fingers. Therefore, it is likely that both TGF-β2 and InhA bind BGZP-C through a site on the inside of their extended finger regions. Overall, these results identify the BGZP-C-binding site on TGF-β2 and shed light on the specificity of BG for select TGF-β-type GFs and the mechanisms by which BG influences their signaling.

Betaglycan (TGFBR3) Functions as an Inhibin A, but Not Inhibin B, Coreceptor in Pituitary Gonadotrope Cells in Mice

Inhibins are gonadal hormones that act on pituitary gonadotrope cells to suppress FSH synthesis and secretion. Inhibin A and B are heterodimers of the inhibin ⍺-subunit disulfide-linked to one of two inhibin β-subunits. Homodimers or heterodimers of the inhibin β-subunits form the activins, which stimulate FSH production. Activins signal through complexes of type I and II receptor serine/threonine kinases to increase transcription of the FSHβ subunit gene. According to in vitro observations, inhibins impair FSH synthesis by competitively binding to activin type II receptors, particularly in the presence of the TGFβ type III receptor (TGFBR3, or betaglycan). The role of TGFBR3 in inhibin action in vivo has not been determined. Here, we ablated Tgfbr3 specifically in murine gonadotropes. Conditional knockout females were supra-fertile, exhibiting enhanced folliculogenesis, numbers of ovulated eggs per cycle, and litter sizes relative to control mice. Despite these phenotypes, FSH levels appeared to be unaltered in knockout mice, and the mechanisms underlying their enhanced fertility remain unexplained. Inhibin B is the predominant form of the hormone in males and in females during most stages of the estrous cycle. Remarkably, inhibin A, but not inhibin B, suppression of FSH synthesis was impaired in cultured pituitaries of knockout mice, which may explain the absence of discernible changes in FSH levels in vivo. Collectively, these data challenge current dogma by demonstrating that TGFBR3 (betaglycan) functions as an inhibin A, but not an inhibin B, coreceptor in gonadotrope cells in vivo. Mechanisms of inhibin B action merit further investigation.

Transforming growth factor-β in stem cells and tissue homeostasis

TGF-β 1-3 are unique multi-functional growth factors that are only expressed in mammals, and mainly secreted and stored as a latent complex in the extracellular matrix (ECM). The biological functions of TGF-β in adults can only be delivered after ligand activation, mostly in response to environmental perturbations. Although involved in multiple biological and pathological processes of the human body, the exact roles of TGF-β in maintaining stem cells and tissue homeostasis have not been well-documented until recent advances, which delineate their functions in a given context. Our recent findings, along with data reported by others, have clearly shown that temporal and spatial activation of TGF-β is involved in the recruitment of stem/progenitor cell participation in tissue regeneration/remodeling process, whereas sustained abnormalities in TGF-β ligand activation, regardless of genetic or environmental origin, will inevitably disrupt the normal physiology and lead to pathobiology of major diseases. Modulation of TGF-β signaling with different approaches has proven effective pre-clinically in the treatment of multiple pathologies such as sclerosis/fibrosis, tumor metastasis, osteoarthritis, and immune disorders. Thus, further elucidation of the mechanisms by which TGF-β is activated in different tissues/organs and how targeted cells respond in a context-dependent way can likely be translated with clinical benefits in the management of a broad range of diseases with the involvement of TGF-β.

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.

Dually modified transmembrane proteoglycans in development and disease

Aberrant cell signaling in response to secreted growth factors has been linked to the development of multiple diseases, including cancer. As such, understanding mechanisms that control growth factor availability and receptor-growth factor interaction is vital. Dually modified transmembrane proteoglycans (DMTPs), which are classified as cell surface macromolecules composed of a core protein decorated with covalently linked heparan sulfated (HS) and/or chondroitin sulfated (CS) glycosaminoglycan (GAG) chains, provide one type of regulatory mechanism. Specifically, DMTPs betaglycan and syndecan-1 (SDC1) play crucial roles in modulating key cell signaling pathways, such as Wnt, transforming growth factor-β and fibroblast growth factor signaling, to affect epithelial cell biology and cancer progression. This review outlines current and potential functions for betaglycan and SDC1, with an emphasis on comparing individual roles for HS and CS modified DMTPs. We highlight the mutual dependence of DMTPs' GAG chains and core proteins and provide comprehensive knowledge on how these DMTPs, through regulation of ligand availability and receptor internalization, control cell signaling pathways involved in development and disease.

A Selective Transforming Growth Factor-β Ligand Trap Attenuates Pulmonary Hypertension

RATIONALE

Transforming growth factor-β (TGF-β) ligands signal via type I and type II serine-threonine kinase receptors to regulate broad transcriptional programs. Excessive TGF-β-mediated signaling is implicated in the pathogenesis of pulmonary arterial hypertension, based in part on the ability of broad inhibition of activin-like kinase (ALK) receptors 4/5/7 recognizing TGF-β, activin, growth and differentiation factor, and nodal ligands to attenuate experimental pulmonary hypertension (PH). These broad inhibition strategies do not delineate the specific contribution of TGF-β versus a multitude of other ligands, and their translation is limited by cardiovascular and systemic toxicity.

OBJECTIVES

We tested the impact of a soluble TGF-β type II receptor extracellular domain expressed as an immunoglobulin-Fc fusion protein (TGFBRII-Fc), serving as a selective TGF-β1/3 ligand trap, in several experimental PH models.

METHODS

Signaling studies used cultured human pulmonary artery smooth muscle cells. PH was studied in monocrotaline-treated Sprague-Dawley rats, SU5416/hypoxia-treated Sprague-Dawley rats, and SU5416/hypoxia-treated C57BL/6 mice. PH, cardiac function, vascular remodeling, and valve structure were assessed by ultrasound, invasive hemodynamic measurements, and histomorphometry.

MEASUREMENTS AND MAIN RESULTS

TGFBRII-Fc is an inhibitor of TGF-β1 and TGF-β3, but not TGF-β2, signaling. In vivo treatment with TGFBRII-Fc attenuated Smad2 phosphorylation, normalized expression of plasminogen activator inhibitor-1, and mitigated PH and pulmonary vascular remodeling in monocrotaline-treated rats, SU5416/hypoxia-treated rats, and SU5416/hypoxia-treated mice. Administration of TGFBRII-Fc to monocrotaline-treated or SU5416/hypoxia-treated rats with established PH improved right ventricular systolic pressures, right ventricular function, and survival. No cardiac structural or valvular abnormalities were observed after treatment with TGFBRII-Fc.

CONCLUSIONS

Our findings are consistent with a pathogenetic role of TGF-β1/3, demonstrating the efficacy and tolerability of selective TGF-β ligand blockade for improving hemodynamics, remodeling, and survival in multiple experimental PH models.

Binding Properties of the Transforming Growth Factor-β Coreceptor Betaglycan: Proposed Mechanism for Potentiation of Receptor Complex Assembly and Signaling

Transforming growth factor (TGF) β1, β2, and β3 (TGF-β1–TGF-β3, respectively) are small secreted signaling proteins that each signal through the TGF-β type I and type II receptors (TβRI and TβRII, respectively). However, TGF-β2, which is well-known to bind TβRII several hundred-fold more weakly than TGF-β1 and TGF-β3, has an additional requirement for betaglycan, a membrane-anchored nonsignaling receptor. Betaglycan has two domains that bind TGF-β2 at independent sites, but how it binds TGF-β2 to potentiate TβRII binding and how the complex with TGF-β, TβRII, and betaglycan undergoes the transition to the signaling complex with TGF-β, TβRII, and TβRI are not understood. To investigate the mechanism, the binding of the TGF-βs to the betaglycan extracellular domain, as well as its two independent binding domains, either directly or in combination with the TβRI and TβRII ectodomains, was studied using surface plasmon resonance, isothermal titration calorimetry, and size-exclusion chromatography. These studies show that betaglycan binds TGF-β homodimers with a 1:1 stoichiometry in a manner that allows one molecule of TβRII to bind. These studies further show that betaglycan modestly potentiates the binding of TβRII and must be displaced to allow TβRI to bind. These findings suggest that betaglycan functions to bind and concentrate TGF-β2 on the cell surface and thus promote the binding of TβRII by both membrane-localization effects and allostery. These studies further suggest that the transition to the signaling complex is mediated by the recruitment of TβRI, which simultaneously displaces betaglycan and stabilizes the bound TβRII by direct receptor–receptor contact.

Structural Biology and Evolution of the TGF-β Family

We review the evolution and structure of members of the transforming growth factor β (TGF-β) family, antagonistic or agonistic modulators, and receptors that regulate TGF-β signaling in extracellular environments. The growth factor (GF) domain common to all family members and many of their antagonists evolved from a common cystine knot growth factor (CKGF) domain. The CKGF superfamily comprises six distinct families in primitive metazoans, including the TGF-β and Dan families. Compared with Wnt/Frizzled and Notch/Delta families that also specify body axes, cell fate, tissues, and other families that contain CKGF domains that evolved in parallel, the TGF-β family was the most fruitful in evolution. Complexes between the prodomains and GFs of the TGF-β family suggest a new paradigm for regulating GF release by conversion from closed- to open-arm procomplex conformations. Ternary complexes of the final step in extracellular signaling show how TGF-β GF dimers bind type I and type II receptors on the cell surface, and enable understanding of much of the specificity and promiscuity in extracellular signaling. However, structures suggest that when GFs bind repulsive guidance molecule (RGM) family coreceptors, type I receptors do not bind until reaching an intracellular, membrane-enveloped compartment, blurring the line between extra- and intracellular signaling. Modulator protein structures show how structurally diverse antagonists including follistatins, noggin, and members of the chordin family bind GFs to regulate signaling; complexes with the Dan family remain elusive. Much work is needed to understand how these molecular components assemble to form signaling hubs in extracellular environments in vivo.

Targeting of Proteoglycan Synthesis Pathway: A New Strategy to Counteract Excessive Matrix Proteoglycan Deposition and Transforming Growth Factor-β1-Induced Fibrotic Phenotype in Lung Fibroblasts

Stimulation of proteoglycan (PG) synthesis and deposition plays an important role in the pathophysiology of fibrosis and is an early and dominant feature of pulmonary fibrosis. Transforming growth factor-β1 (TGF-β1) is a major cytokine associated with fibrosis that induces excessive synthesis of matrix proteins, particularly PGs. Owing to the importance of PGs in matrix assembly and in mediating cytokine and growth factor signaling, a strategy based on the inhibition of PG synthesis may prevent excessive matrix PG deposition and attenuates profibrotic effects of TGF-β1 in lung fibroblasts. Here, we showed that 4-MU4-deoxy-β-D-xylopyranoside, a competitive inhibitor of β4-galactosyltransferase7, inhibited PG synthesis and secretion in a dose-dependent manner by decreasing the level of both chondroitin/dermatan- and heparin-sulfate PG in primary lung fibroblasts. Importantly, 4-MU4-deoxy-xyloside was able to counteract TGF-β1-induced synthesis of PGs, activation of fibroblast proliferation and fibroblast-myofibroblast differentiation. Mechanistically, 4-MU4-deoxy-xyloside treatment inhibited TGF-β1-induced activation of canonical Smads2/3 signaling pathway in lung primary fibroblasts. The knockdown of β4-galactosyltransferase7 mimicked 4-MU4-deoxy-xyloside effects, indicating selective inhibition of β4-galactosyltransferase7 by this compound. Collectively, this study reveals the anti-fibrotic activity of 4-MU4-deoxy-xyloside and indicates that inhibition of PG synthesis represents a novel strategy for the treatment of lung fibrosis.

Determination of the substrate repertoire of ADAMTS2, 3, and 14 significantly broadens their functions and identifies extracellular matrix organization and TGF-β signaling as primary targets

A disintegrin and metalloproteinase with thrombospondin type I motif (ADAMTS)2, 3, and 14 are collectively named procollagen N-proteinases (pNPs) because of their specific ability to cleave the aminopropeptide of fibrillar procollagens. Several reports also indicate that they could be involved in other biological processes, such as blood coagulation, development, and male fertility, but the potential substrates associated with these activities remain unknown. Using the recently described N-terminal amine isotopic labeling of substrate approach, we analyzed the secretomes of human fibroblasts and identified 8, 17, and 22 candidate substrates for ADAMTS2, 3, and 14, respectively. Among these newly identified substrates, many are components of the extracellular matrix and/or proteins related to cell signaling such as latent TGF-β binding protein 1, TGF-β RIII, and dickkopf-related protein 3. Candidate substrates for the 3 ADAMTS have been biochemically validated in different contexts, and the implication of ADAMTS2 in the control of TGF-β activity has been further demonstrated in human fibroblasts. Finally, the cleavage site specificity was assessed showing a clear and unique preference for nonpolar or slightly hydrophobic amino acids. This work shows that the activities of the pNPs extend far beyond the classically reported processing of the aminopropeptide of fibrillar collagens and that they should now be considered as multilevel regulators of matrix deposition and remodeling.-Bekhouche, M., Leduc, C., Dupont, L., Janssen, L., Delolme, F., Vadon-Le Goff, S., Smargiasso, N., Baiwir, D., Mazzucchelli, G., Zanella-Cleon, I., Dubail, J., De Pauw, E., Nusgens, B., Hulmes, D. J. S., Moali, C., Colige, A. Determination of the substrate repertoire of ADAMTS2, 3, and 14 significantly broadens their functions and identifies extracellular matrix organization and TGF-β signaling as primary targets.

Betaglycan knock-down causes embryonic angiogenesis defects in zebrafish

Angiogenesis is an essential requirement for embryonic development and adult homeostasis. Its deregulation is a key feature of numerous pathologies and many studies have shown that members of the transforming growth factor beta (TGF-β) family of proteins play important roles in angiogenesis during development and disease. Betaglycan (BG), also known as TGF-β receptor type III, is a TGF-β coreceptor essential for mice embryonic development but its role in angiogenesis has not been described. We have cloned the cDNA encoding zebrafish BG, a TGF-β-binding membrane proteoglycan that showed a dynamic expression pattern in zebrafish embryos, including the notochord and cells adjacent to developing vessels. Injection of antisense morpholinos decreased BG protein levels and morphant embryos exhibited impaired angiogenesis that was rescued by coinjection with rat BG mRNA. In vivo time-lapse microscopy revealed that BG deficiency differentially affected arterial and venous angiogenesis: morphants showed impaired pathfinding of intersegmental vessels migrating from dorsal aorta, while endothelial cells originating from the caudal vein displayed sprouting and migration defects. Our results reveal a new role for BG during embryonic angiogenesis in zebrafish, which has not been described in mammals and pose interesting questions about the molecular machinery regulating angiogenesis in different vertebrates. genesis 53:583-603, 2015. © 2015 Wiley Periodicals, Inc.

Simvastatin alleviates cardiac fibrosis induced by infarction via up-regulation of TGF-β receptor III expression

BACKGROUND AND PURPOSE

Statins decrease heart disease risk, but their mechanisms are not completely understood. We examined the role of the TGF-β receptor III (TGFBR3) in the inhibition of cardiac fibrosis by simvastatin.

EXPERIMENTAL APPROACH

Myocardial infarction (MI) was induced by ligation of the left anterior descending coronary artery in mice given simvastatin orally for 7 days. Cardiac fibrosis was measured by Masson staining and electron microscopy. Heart function was evaluated by echocardiography. Signalling through TGFBR3, ERK1/2, JNK and p38 pathways was measured using Western blotting. Collagen content and cell viability were measured in cultures of neonatal mouse cardiac fibroblasts (NMCFs). Interactions between TGFBR3 and the scaffolding protein, GAIP-interacting protein C-terminus (GIPC) were detected using co-immunoprecipitation (co-IP). In vivo, hearts were injected with lentivirus carrying shRNA for TGFBR3.

KEY RESULTS

Simvastatin prevented fibrosis following MI, improved heart ultrastructure and function, up-regulated TGFBR3 and decreased ERK1/2 and JNK phosphorylation. Simvastatin up-regulated TGFBR3 in NMCFs, whereas silencing TGFBR3 reversed inhibitory effects of simvastatin on cell proliferation and collagen production. Simvastatin inhibited ERK1/2 and JNK signalling while silencing TGFBR3 opposed this effect. Co-IP demonstrated TGFBR3 binding to GIPC. Overexpressing TGFBR3 inhibited ERK1/2 and JNK signalling which was abolished by knock-down of GIPC. In vivo, suppression of cardiac TGFBR3 abolished anti-fibrotic effects, improvement of cardiac function and changes in related proteins after simvastatin.

CONCLUSIONS AND IMPLICATIONS

TGFBR3 mediated the decreased cardiac fibrosis, collagen deposition and fibroblast activity, induced by simvastatin, following MI. These effects involved GIPC inhibition of the ERK1/2/JNK pathway.

Proteolytic control of TGF-β co-receptor activity by BMP-1/tolloid-like proteases revealed by quantitative iTRAQ proteomics

The metalloproteinase BMP-1 (bone morphogenetic protein-1) plays a major role in the control of extracellular matrix (ECM) assembly and growth factor activation. Most of the growth factors activated by BMP-1 are members of the TGF-β superfamily known to regulate multiple biological processes including embryonic development, wound healing, inflammation and tumor progression. In this study, we used an iTRAQ (isobaric tags for relative and absolute quantification)-based quantitative proteomic approach to reveal the release of proteolytic fragments from the cell surface or the ECM by BMP-1. Thirty-eight extracellular proteins were found in significantly higher or lower amounts in the conditioned medium of HT1080 cells overexpressing BMP-1 and thus, could be considered as candidate substrates. Strikingly, three of these new candidates (betaglycan, CD109 and neuropilin-1) were TGF-β co-receptors, also acting as antagonists when released from the cell surface, and were chosen for further substrate validation. Betaglycan and CD109 proved to be directly cleaved by BMP-1 and the corresponding cleavage sites were extensively characterized using a new mass spectrometry approach. Furthermore, we could show that the ability of betaglycan and CD109 to interact with TGF-β was altered after cleavage by BMP-1, leading to increased and prolonged SMAD2 phosphorylation in BMP-1-overexpressing cells. Betaglycan processing was also observed in primary corneal keratocytes, indicating a general and novel mechanism by which BMP-1 directly affects signaling by controlling TGF-β co-receptor activity. The proteomic data have been submitted to ProteomeXchange with the identifier PXD000786 and doi: 10.6019/PXD000786 .

Transforming growth factor Beta family: insight into the role of growth factors in regulation of fracture healing biology and potential clinical applications

The transforming growth factor beta (TGF-β) family forms a group of three isoforms, TGF-β1, TGF-β2, and TGF-β3, with their structure formed by interrelated dimeric polypeptide chains. Pleiotropic and redundant functions of the TGF-β family concern control of numerous aspects and effects of cell functions, including proliferation, differentiation, and migration, in all tissues of the human body. Amongst many cytokines and growth factors, the TGF-β family is considered a group playing one of numerous key roles in control of physiological phenomena concerning maintenance of metabolic homeostasis in the bone tissue. By breaking the continuity of bone tissue, a spread-over-time and complex bone healing process is initiated, considered a recapitulation of embryonic intracartilaginous ossification. This process is a cascade of local and systemic phenomena spread over time, involving whole cell lineages and various cytokines and growth factors. Numerous in vivo and in vitro studies in various models analysing cytokines and growth factors' involvement have shown that TGF-β has a leading role in the fracture healing process. This paper sums up current knowledge on the basis of available literature concerning the role of the TGF-β family in the fracture healing process.

Functional role of TGF-β receptors during palatal fusion in vitro

OBJECTIVE

Reported expression patterns for TGF-β receptors (TβR-I, -II, and -III) during palatogenesis suggest that they play essential roles in the mechanisms leading to palatal fusion. The purpose of this study was to compare the functions of the three TβRs during palatal fusion.

METHODS

Using organ culture of mouse palatal shelves, expression levels of TβR-I, -II, and -III were suppressed by transfecting the siRNAs siTβR-I, -II, and -III, respectively. Phosphorylation of SMAD2 was examined as an indicator of downstream signalling via each TβR. Linkage between TGF-β signalling and critical events in palatal fusion led to the use of, MMP-13 expression as an outcome measure for the function of the TGF-β receptors.

RESULTS

The siRNA treatment decreased the expression level of each receptor by more than 85%. When treated with either siTβR-I or -II, palatal shelves at E13+72 h were not fused, with complete clefting in the anterior and posterior regions. The middle palatal region following treatment with either siTβR-I or -II had fusion from one-half or one-third of the palatal region. Treatment with siTβR-III resulted in a persistent midline seam of medial edge epithelium (MEE) in the anterior region with islands of persistent MEE in the middle and posterior regions of the midline. Treatment with all three siTβRs altered the pattern of SMAD2 phosphorylation. Palatal shelf cultures treated with siTβR-I or -II, but not -III, showed altered MMP-13 expression levels.

CONCLUSION

The ability to identify and recover MEE and palatal mesenchymal cells during palatal fusion will aid in the evaluation of the different mechanistic events regulated by each TβR during palatogenesis.

Transforming growth factor beta receptor type III is a tumor promoter in mesenchymal-stem like triple negative breast cancer

Introduction

There is a major need to better understand the molecular basis of triple negative breast cancer (TNBC) in order to develop effective therapeutic strategies. Using gene expression data from 587 TNBC patients we previously identified six subtypes of the disease, among which a mesenchymal-stem like (MSL) subtype. The MSL subtype has significantly higher expression of the transforming growth factor beta (TGF-β) pathway-associated genes relative to other subtypes, including the TGF-β receptor type III (TβRIII). We hypothesize that TβRIII is tumor promoter in mesenchymal-stem like TNBC cells.

Methods

Representative MSL cell lines SUM159, MDA-MB-231 and MDA-MB-157 were used to study the roles of TβRIII in the MSL subtype. We stably expressed short hairpin RNAs specific to TβRIII (TβRIII-KD). These cells were then used for xenograft tumor studies in vivo; and migration, invasion, proliferation and three dimensional culture studies in vitro. Furthermore, we utilized human gene expression datasets to examine TβRIII expression patterns across all TNBC subtypes.

Results

TβRIII was the most differentially expressed TGF-β signaling gene in the MSL subtype. Silencing TβRIII expression in MSL cell lines significantly decreased cell motility and invasion. In addition, when TβRIII-KD cells were grown in a three dimensional (3D) culture system or nude mice, there was a loss of invasive protrusions and a significant decrease in xenograft tumor growth, respectively. In pursuit of the mechanistic underpinnings for the observed TβRIII-dependent phenotypes, we discovered that integrin-α2 was expressed at higher level in MSL cells after TβRIII-KD. Stable knockdown of integrin-α2 in TβRIII-KD MSL cells rescued the ability of the MSL cells to migrate and invade at the same level as MSL control cells.

Conclusions

We have found that TβRIII is required for migration and invasion in vitro and xenograft growth in vivo. We also show that TβRIII-KD elevates expression of integrin-α2, which is required for the reduced migration and invasion, as determined by siRNA knockdown studies of both TβRIII and integrin-α2. Overall, our results indicate a potential mechanism in which TβRIII modulates integrin-α2 expression to effect MSL cell migration, invasion, and tumorigenicity.

A synthetic peptide from transforming growth factor-β₁ type III receptor inhibits NADPH oxidase and prevents oxidative stress in the kidney of spontaneously hypertensive rats

AIMS

The NADPH oxidases constitute a major source of superoxide anion (·O2(-)) in hypertension. Several studies suggest an important role of NADPH oxidases in different effects mediated by transforming growth factor-β₁ (TGF-β₁). We investigated whether a chronic treatment with P144, a peptide synthesized from type III TGF-β₁ receptor, inhibited NADPH oxidases in the renal cortex of spontaneously hypertensive rats (SHR).

RESULTS

Here, we show that chronic administration of P144 significantly reduced the NADPH oxidase expression and activity as well as the oxidative stress observed in control vehicle-treated SHR (V-SHR). In addition, P144 was also able to reduce the significant increase in the renal fibrosis and in mRNA expression of different components of collagen metabolism, as well as in the levels of connective tissue growth factor observed in V-SHR. Finally, TGF-β₁-stimulated NRK52E exhibited a significant increase in NADPH oxidase expression and activity as well as a TGF-β₁-dependent intracellular pathway that were inhibited in the presence of P144.

INNOVATION

Our experimental evidence suggests that reversing oxidative stress may be therapeutically useful in preventing fibrosis-associated renal damage. We show here that (i) the TGF-β₁-NADPH oxidases axis is crucial in the development of fibrosis in an experimental hypertensive renal disease animal model, and (ii) the use of P144 reverses TGF-β₁-dependent NADPH oxidase activity; thus, P144 may be considered a novel therapeutic tool in kidney disease associated with hypertension.

CONCLUSION

We demonstrate that P144 inhibits NADPH oxidases and prevents oxidative stress in kidneys from hypertensive rats. Our data also suggest that these effects are associated with the renal antifibrotic effect of P144.

Identification of a Novel TGF-β-Binding Site in the Zona Pellucida C-terminal (ZP-C) Domain of TGF-β-Receptor-3 (TGFR-3)

The zona pellucida (ZP) domain is present in extracellular proteins such as the zona pellucida proteins and tectorins and participates in the formation of polymeric protein networks. However, the ZP domain also occurs in the cytokine signaling co-receptor transforming growth factor β (TGF-β) receptor type 3 (TGFR-3, also known as betaglycan) where it contributes to cytokine ligand recognition. Currently it is unclear how the ZP domain architecture enables this dual functionality. Here, we identify a novel major TGF-β-binding site in the FG loop of the C-terminal subdomain of the murine TGFR-3 ZP domain (ZP-C) using protein crystallography, limited proteolysis experiments, surface plasmon resonance measurements and synthetic peptides. In the murine 2.7 Å crystal structure that we are presenting here, the FG-loop is disordered, however, well-ordered in a recently reported homologous rat ZP-C structure. Surprisingly, the adjacent external hydrophobic patch (EHP) segment is registered differently in the rat and murine structures suggesting that this segment only loosely associates with the remaining ZP-C fold. Such a flexible and temporarily-modulated association of the EHP segment with the ZP domain has been proposed to control the polymerization of ZP domain-containing proteins. Our findings suggest that this flexibility also extends to the ZP domain of TGFR-3 and might facilitate co-receptor ligand interaction and presentation via the adjacent FG-loop. This hints that a similar C-terminal region of the ZP domain architecture possibly regulates both the polymerization of extracellular matrix proteins and cytokine ligand recognition of TGFR-3.

Role of proteoglycans in the regulation of the skeletal muscle fibrotic response

Myogenesis consists of a highly organized and regulated sequence of cellular processes aimed at forming or repairing muscle tissue. Several processes occur during myogenesis, including cell proliferation, migration, and differentiation. Cytokines, proteinases, cell adhesion molecules and growth factors are involved, either activating or inhibiting these events, and are modulated by a group of molecules called proteoglycans (PGs), which play critical roles in skeletal muscle physiology. Particularly interesting are some of the factors responsible for the fibrotic response associated with skeletal muscular dystrophies. Transforming growth factor-β and connective tissue growth factor have gained great attention as factors participating in the fibrotic response in skeletal muscle. This review is focused on the advances achieved in understanding the roles of proteoglycans as modulators of profibrotic growth factors in fibrosis associated with diseases such as skeletal muscle dystrophies.

The impact of menopause on bone, zoledronic acid, and implications for breast cancer growth and metastasis

Recent data from the AZURE, ABCSG-12, and ZO-FAST clinical trials have challenged our understanding of the potential anticancer activity of zoledronic acid (ZOL). Although the results of these studies may appear to be conflicting on the surface, a deeper look into commonalities among the patient populations suggest that some host factors (i.e. patient age and endocrine status) may contribute to the anticancer activity of ZOL. Indeed, data from these large clinical trials suggest that the potential anticancer activity of ZOL may be most robust in a low-estrogen environment. However, this may be only part of the story and many questions remain to be answered to fully explain the phenomenon. Does estrogen override the anticancer activity of ZOL seen in postmenopausal women? Are hormones other than estrogen involved that contribute to this effect? Does the role of bone turnover in breast cancer (BC) growth and progression differ in the presence of various estrogen levels? Here, we present a review of the multitude of factors affected by different endocrine environments in women with BC that may influence the potential anticancer activity of ZOL.

TGF-β - an excellent servant but a bad master

The transforming growth factor (TGF-β) family of growth factors controls an immense number of cellular responses and figures prominently in development and homeostasis of most human tissues. Work over the past decades has revealed significant insight into the TGF-β signal transduction network, such as activation of serine/threonine receptors through ligand binding, activation of SMAD proteins through phosphorylation, regulation of target genes expression in association with DNA-binding partners and regulation of SMAD activity and degradation. Disruption of the TGF-β pathway has been implicated in many human diseases, including solid and hematopoietic tumors. As a potent inhibitor of cell proliferation, TGF-β acts as a tumor suppressor; however in tumor cells, TGF-β looses anti-proliferative response and become an oncogenic factor. This article reviews current understanding of TGF-β signaling and different mechanisms that lead to its impairment in various solid tumors and hematological malignancies.

Betaglycan (TβRIII) is expressed in the thymus and regulates T cell development by protecting thymocytes from apoptosis

TGF-β type III receptor (TβRIII) is a coreceptor for TGFβ family members required for high-affinity binding of these ligands to their receptors, potentiating their cellular functions. TGF-β [1]–[3], bone morphogenetic proteins (BMP2/4) and inhibins regulate different checkpoints during T cell differentiation. Although TβRIII is expressed on hematopoietic cells, the role of this receptor in the immune system remains elusive. Here, we provide the first evidence that TβRIII is developmentally expressed during T cell ontogeny, and plays a crucial role in thymocyte differentiation. Blocking of endogenous TβRIII in fetal thymic organ cultures led to a delay in DN-DP transition. In addition, in vitro development of TβRIII^−/− thymic lobes also showed a significant reduction in absolute thymocyte numbers, which correlated with increased thymocyte apoptosis, resembling the phenotype reported in Inhibin α ^−/− thymic lobes. These data suggest that Inhibins and TβRIII may function as a molecular pair regulating T cell development.

Reprint of: Betaglycan: a multifunctional accessory

Betaglycan is a co-receptor for the TGFβ superfamily, particularly important in establishing the potency of its ligands on their target cells. In recent years, new insights have been gained into the structure and function of betaglycan, expanding its role from that of a simple co-receptor to include additional ligand-dependent and ligand-independent roles. This review focuses on recent advances in the betaglycan field, with a particular emphasis on its newly discovered actions in mediating the trafficking of TGFβ superfamily receptors and as a determinant of the functional output of TGFβ superfamily signalling. In addition, this review encompasses a discussion of the emerging roles of the betaglycan/inhibin pathway in reproductive cancers and disease.

New insights into the mechanisms of activin action and inhibition

Like other members of the transforming growth factor-β (TGF-β) superfamily, activins are synthesised as precursor molecules comprising an N-terminal prodomain and C-terminal mature region. During synthesis, the prodomain interacts non-covalently with mature activin, maintaining the molecule in a conformation competent for dimerisation. Dimeric precursors are cleaved by proprotein convertases and activin is secreted from the cell non-covalently associated with its propeptide. Extracellularly, the propeptide interacts with heparan sulfate proteoglycans to regulate activin localization within tissues. The mature activin dimer exhibits the classic 'open-hand' structure of TGF-β ligands with 'finger-like' domains projecting outward from the cysteine knot core of the molecule. These finger domains form the binding epitopes for type I and II serine/threonine kinase receptors. Activins ability to access its signalling receptors is regulated by the extracellular binding proteins, follistatin, follistatin-like-3, and by inhibins, which, in the presence of betaglycan, sequester type II receptors.