Production, Isolation, and Structural Analysis of Ligands and Receptors of the TGF-β Superfamily

Fibronectin mediates activin A-promoted human trophoblast migration and acquisition of endothelial-like phenotype

BACKGROUND

During human early placentation, a proportion of extravillous trophoblasts (EVTs) migrate to the maternal decidua, differentiating into endovascular EVTs to remodel spiral arteries and ensure the establishment of blood circulation at the maternal-fetal interface. Inadequate EVT migration and endovascular differentiation are closely associated with adverse pregnancy outcomes such as miscarriage. Activin A and fibronectin are both secretory molecules abundantly expressed at the maternal-fetal interface. Activin A has been reported to regulate EVT biological functions. However, whether fibronectin mediates activin A-promoted EVT migration and acquisition of endothelial-like phenotype as well as the underlying molecular mechanisms remain unknown. Additionally, the role of fibronectin in pregnancy establishment and maintenance warrants further investigation.

METHODS

Primary and immortalized (HTR8/SVneo) human EVTs were used as in vitro study models. Cultured human first-trimester chorionic villous explants were utilized for ex vivo validation. A local fibronectin knockdown model in ICR mouse uteri, achieved by nonviral in vivo transfection with small interfering RNA (siRNA) targeting fibronectin 1 (si-Fn1), was employed to explore the roles of fibronectin in the establishment and maintenance of early pregnancy.

RESULTS

Our results showed that activin A treatment significantly induced fibronectin 1 (FN1) mRNA expression and fibronectin protein production, which is essential for human trophoblast migration and endothelial-like tube formation. Both basal and activin A-upregulated fibronectin expression were abolished by the TGF-β type I receptor inhibitor SB431542 or siRNA-mediated knockdown of activin receptor-like kinase (ALK4) or SMAD4. Moreover, activin A-increased trophoblast migration and endothelial-like tube formation were attenuated following the depletion of fibronectin. Fibronectin knockdown via intrauterine siRNA administration reduced CD31 and cytokeratin 8 (CK8) expression at the maternal-fetal interface, resulting in a decrease in the number of implantation sites and embryos.

CONCLUSIONS

Our study demonstrates that activin A promotes trophoblast cell migration and acquisition of endothelial-like phenotype via ALK4-SMAD2/3-SMAD4-mediated fibronectin upregulation. Furthermore, through a local fibronectin knockdown model in mouse uteri, we found that the absence of fibronectin at the maternal-fetal interface impedes endovascular migration of trophoblasts and decidual vascularization, thereby interfering with early embryo implantation and the maintenance of pregnancy. These findings provide novel insights into placental development during early pregnancy establishment and contribute to the advancement of therapeutic approaches for managing pregnancy complications related to trophoblast dysfunction.

Assessment of the effectiveness of the peptide inhibitor homologous to the transforming growth factor β cytokine blocking the TGFβRI/TGFβRII receptor complex-pilot study

BACKGROUND

A key player in the fibrotic process is the transforming growth factor β (TGF-β) which enhances extracellular matrix production by increasing the transcription of matrix proteins. The cytokine TGF-β first binds to the TGFβRII receptor (dimer), resulting in the recruitment of the TGFβRI receptor (dimer). The complex thus formed leads to the phosphorylation of the kinase domain of TGFβRI, which in turn results in activation of the Smad pathway. This is therefore a targeted pathway for research into the application of peptide inhibitors in blocking the TGF-β-Smad signaling pathway. The aim of this study was to design a peptide inhibitor (homologous to the cytokine TGF-β) which, after binding to the TGFβRI/TGFβRII receptor, would block the cytokine binding and thus prevent the formation of an activating complex.

METHODS

Preliminary work on the design and synthesis of inhibitors for TGFβRI/TGFβRII has allowed us to identify and describe five key regions of the TGF-β-TGFβRI/TGFβRII interface. The following five peptide inhibitors were synthesized for Region 1: 1.1 ALDAAYCFR, 1.2 LDAAYCFRN, 1.3 DAAYCFRNV, 1.4 AAYCFRNVQ, 1.5 AYCFRNVQD. The expression of the SEAP reporter gene, Smad2, Smad3, Smad4, and JNK1 gene was measured using quantitative real-time polymerase chain reaction.

RESULTS

For Region 1 peptide inhibitors tested for TGFβRI/TGFβRII, reduced SEAP (reporter gene) expression was observed in cells of the MFB-F11 line, which suggests inhibited the formation of cytokine-receptor complexes.

CONCLUSIONS

For IP1_2, 1_3 and 1_5 Region 1 peptides tested for TGFβRI/TGFβRII, reduced cytokine-receptor signal by adding newly designed inhibitors. The study revealed an impact of these peptide inhibitors on the reduction of mRNA expression of Smad2, Smad3, Smad4 and JNK1 genes.

TGFβ carrying exosomes in plasma: potential biomarkers of cancer progression in patients with head and neck squamous cell carcinoma

OBJECTIVES

Contributions of TGFβ to cancer progression are well documented. However, plasma TGFβ levels often do not correlate with clinicopathological data. We examine the role of TGFβ carried in exosomes isolated from murine and human plasma as a contributor to disease progression in head and neck squamous cell carcinoma (HNSCC).

MATERIALS AND METHODS

The 4-nitroquinoline-1-oxide (4-NQO) mouse model was used to study changes in TGFβ expression levels during oral carcinogenesis. In human HNSCC, TGFβ and Smad3 protein expression levels and TGFB1 gene expression were determined. Soluble TGFβ levels were evaluated by ELISA and TGFβ bioassays. Exosomes were isolated from plasma using size exclusion chromatography, and TGFβ content was quantified using bioassays and bioprinted microarrays.

RESULTS

During 4-NQO carcinogenesis, TGFβ levels in tumour tissues and in serum increased as the tumour progressed. The TGFβ content of circulating exosomes also increased. In HNSCC patients, TGFβ, Smad3 and TGFB1 were overexpressed in tumour tissues and correlated with increased soluble TGFβ levels. Neither TGFβ expression in tumours nor levels of soluble TGFβ correlated with clinicopathological data or survival. Only exosome-associated TGFβ reflected tumour progression and correlated with tumour size.

CONCLUSIONS

Circulating TGFβ+ exosomes in the plasma of patients with HNSCC emerge as potential non-invasive biomarkers of disease progression in HNSCC.

Role of molecular and chemical chaperon in assisting refolding of BMP2 in E. coli

Bone morphogenetic protein 2 (BMP2) when expressed in bacteria forms inclusion bodies (IBs) due to its complex disulfide-rich structure. Chaperons are already well known for their role in assisting protein folding. In our studies, we have used two E. coli strains, BL21(DE3) and SHuffle® T7 cells for overexpressing BMP2 in soluble fraction. We observed that SHuffle® T7 cells successfully expressed soluble functionally active BMP2 in presence of molecular and chemical chaperones at low temperature. The combination of chemical and molecular chaperons further increases the yield of protein. The best-suited chaperon system for overexpression of BMP2 is GroES-GroEL at low temperature. The soluble functionally active BMP2 is confirmed by its binding to its receptor ALK3 through Native PAGE and ELISA assay using BMP2 specific antibody. It is possible to obtain BMP2 expression in soluble active form by using molecular and chemical chaperons which work synergistically in bacterial cells to fold disulphide-rich proteins at low temperature in easy and time saving steps (18 ̊C).

Convergent evolution of a parasite-encoded complement control protein-scaffold to mimic binding of mammalian TGF-β to its receptors, TβRI and TβRII

The mouse intestinal helminth Heligmosomoides polygyrus modulates host immune responses by secreting a transforming growth factor (TGF)-β mimic (TGM), to expand the population of Foxp3⁺ T_regs. TGM comprises five complement control protein (CCP)-like domains, designated D1-D5. Though lacking homology to TGF-β, TGM binds directly to the TGF-β receptors TβRI and TβRII and stimulates the differentiation of naïve T-cells into T_regs. However, the molecular determinants of binding are unclear. Here, we used surface plasmon resonance, isothermal calorimetry, NMR spectroscopy, and mutagenesis to investigate how TGM binds the TGF-β receptors. We demonstrate that binding is modular, with D1-D2 binding to TβRI and D3 binding to TβRII. D1-D2 and D3 were further shown to compete with TGF-β(TβRII)₂ and TGF-β for binding to TβRI and TβRII, respectively. The solution structure of TGM-D3 revealed that TGM adopts a CCP-like fold but is also modified to allow the C-terminal strand to diverge, leading to an expansion of the domain and opening potential interaction surfaces. TGM-D3 also incorporates a long structurally ordered hypervariable loop, adding further potential interaction sites. Through NMR shift perturbations and binding studies of TGM-D3 and TβRII variants, TGM-D3 was shown to occupy the same site of TβRII as bound by TGF-β using both a novel interaction surface and the hypervariable loop. These results, together with the identification of other secreted CCP-like proteins with immunomodulatory activity in H. polygyrus, suggest that TGM is part of a larger family of evolutionarily plastic parasite effector molecules that mediate novel interactions with their host.

TGF-β gene polimorphisms as risk factors for asthma control among clinic patients

BACKGROUND

TGF-β and its receptors play a crucial role in asthma pathogenesis, bronchial hyperreactivity, and bronchial remodeling. Expression of isoforms 1-3 of TGFβ cytokine is influenced by tagging polymorphisms in the TGFβ1, TGFβ2 and TGFβ3 gene, and these SNPs may be associated with the risk of asthma development and severity as well as with other diseases. Polymorphic forms of TGF-β1, TGF-β2 and TGF-β3 genes regulate the degree of bronchial inflammation, deterioration of lung functional parameters in spirometry and elevated level of total IgE. All this results in intensification of disease symptoms. According to current GINA 2020 guidelines, the Asthma Control Test (ACT™) should be applied to assess asthma symptoms.

METHODS

An analysis of polymorphisms localized in TGF-β1, TGF-β2 and TGF-β3 genes was conducted on 652 DNA samples with an application of the MassARRAY® system using the mass spectrometry technique MALDI TOF MS. The degree of asthma control was evaluated with ACT™.

RESULTS

The occurrence of the T / C genotype in rs8109627 (p = 0.0171) in the TGF-β1 gene is significantly associated with a higher ACT result (controlled asthma) in a multivariate linear regression analysis model after using backward stepwise selection of variables. In addition, in the linear model for prediction of ACT score we showed SNP rs8109627 (p = 0.0497) in the TGF-β1 gene (improvement of the disease control - controlled asthma) and rs2796822 (p = 0.0454) in the TGF-β2 gene (deterioration of the diseases control - uncontrolled asthma) significantly modify the degree of asthma control.

DISCUSSION

We described clinical significance of two SNPs in two genes TGF-β1 and TGF-β2, as yet unknown. We proved that the use of both genotypes and MAC allows to create a moderately correct prognostic model which is about 70% efficient on the entire set of analyzed SNPs in TGF-β1, TGF-β2, and TGF-β3 genes.

The BMP Pathway in Blood Vessel and Lymphatic Vessel Biology

Bone morphogenetic proteins (BMPs) were originally identified as the active components in bone extracts that can induce ectopic bone formation. In recent decades, their key role has broadly expanded beyond bone physiology and pathology. Nowadays, the BMP pathway is considered an important player in vascular signaling. Indeed, mutations in genes encoding different components of the BMP pathway cause various severe vascular diseases. Their signaling contributes to the morphological, functional and molecular heterogeneity among endothelial cells in different vessel types such as arteries, veins, lymphatic vessels and capillaries within different organs. The BMP pathway is a remarkably fine-tuned pathway. As a result, its signaling output in the vessel wall critically depends on the cellular context, which includes flow hemodynamics, interplay with other vascular signaling cascades and the interaction of endothelial cells with peri-endothelial cells and the surrounding matrix. In this review, the emerging role of BMP signaling in lymphatic vessel biology will be highlighted within the framework of BMP signaling in the circulatory vasculature.

TGF-β1 increases permeability of ciliated airway epithelia via redistribution of claudin 3 from tight junction into cell nuclei

TGF-β1 is a major mediator of airway tissue remodelling during atopic asthma and affects tight junctions (TJs) of airway epithelia. However, its impact on TJs of ciliated epithelia is sparsely investigated. Herein we elaborated effects of TGF-β1 on TJs of primary human bronchial epithelial cells. We demonstrate that TGF-β1 activates TGF-β1 receptors TGFBR1 and TGFBR2 resulting in ALK5-mediated phosphorylation of SMAD2. We observed that TGFBR1 and -R2 localize specifically on motile cilia. TGF-β1 activated accumulation of phosphorylated SMAD2 (pSMAD2-C) at centrioles of motile cilia and at cell nuclei. This triggered an increase in paracellular permeability via cellular redistribution of claudin 3 (CLDN3) from TJs into cell nuclei followed by disruption of epithelial integrity and formation of epithelial lesions. Only ciliated cells express TGF-β1 receptors; however, nuclear accumulations of pSMAD2-C and CLDN3 redistribution were observed with similar time course in ciliated and non-ciliated cells. In summary, we demonstrate a role of motile cilia in TGF-β1 sensing and showed that TGF-β1 disturbs TJ permeability of conductive airway epithelia by redistributing CLDN3 from TJs into cell nuclei. We conclude that the observed effects contribute to loss of epithelial integrity during atopic asthma.

An improved strategy of TGFβ3 expression in Escherichia coli: Exploiting folding modulators for a switch from misfolded to folded form

Transforming growth factor beta 3 (TGFβ3) exhibits a complex native structure featuring the presence of multiple disulfide bonds forming the active dimer. Consequently, its heterologous expression in microbial system invariably leads to inclusion body (IB) formation. In this study, we observed an interesting phenomenon of switching a significant fraction of misfolded TGFβ3 to folded form by modulating the cellular protein folding machinery. We carried out co-expression experiments with chaperones and demonstrated the requirement of a coordinated action of DnaK-DnaJ-GrpE and GroESL, to achieve the native soluble conformation of TGFβ3, during over-expression in E. coli. The novelty of this study lies in the fact that orchestration of a group of chaperones to work in concert for efficient folding and assembly of TGFβ3-like cytokines has not been widely explored. Additionally, we have also demonstrated that presence of osmolytes (sorbitol or trehalose) in the growth media have an appreciable impact on the solubility of TGFβ3. We have further shown a synergism between the effects of molecular chaperone and osmolytes on the solubility of TGFβ3. We have confirmed the functionality of soluble TGFβ3 by performing binding interactions with its cognate receptor TβRII. Our study delineates the fact that an effective combination of chaperones or optimum concentration of compatible osmolyte, can efficiently abrogate competing aggregation pathways and help attain the native conformation of a cysteine rich cytokine in a facile manner.

C-phycocyanin inhibits epithelial-to-mesenchymal transition in Caski cells

Background

In cervical cancer, most patients die of metastasis. The epithelial-to-mesenchymal transition (EMT) is a pivotal and intricate process that increases the metastatic potential of cervical cancer. C-phycocyanin (C-PC) is a natural marine product isolated and purified from Spirulina platensis, has been investigated that has anti-cancer function. The aim of this study was to explore the inhibitory effect of C-phycocyanin on the migration and invasion of cervical cancer cells induced by transforming growth factor-β1 (TGF-β1), so as to provide a new idea for the treatment and prognosis of cervical cancer.

Methods

A wound-healing assay, an invasion assay, immunofluorescence assay, western blot, flow cytometry and real-time reverse transcriptione polymerase chain reaction were explored in cervical cancer Caski cell lines. TGF-β/smad signaling pathway was evaluated of in Caski cell lines.

Results

Our study indicated that TGF-β1 induced EMT in cervical cancer cells. C-phycocyanin inhibited EMT in Caski cells by down-regulating N-cadherin and up-regulating E-cadherin protein expression. Furthermore, C-phycocyanin could inhibit the expression and proteins Twist, Snail and Zeb1 transcription factors related to EMT. In addition, C-phycocyanin could inhibit the migration and invasion of Caski cells induced by TGF-β1. Besides, C-phycocyanin inhibited EMT through TGF-β/smads signaling pathway. We also found C-phycocyanin induced cell cycle G0/G1 arrest by decreasing protein expression levels of Cyclin D1 and p27.

Conclusions

C-phycocyanin reversed TGF-β1-induced epithelial-to-mesenchymal transition in cervical cancer cells and down-regulated the TGF-β/samd signaling pathway induced G0/G1 arrest of tumor cell cycle.

Interaction of TGFβ3 ligand with its receptors type II (TβRII) and type I (TβRI): A unique mechanism of protein-protein association

The proper orchestration of transforming growth factor beta (TGFβ) mediated signal transduction depends upon a delicate set of interactions between specific ligands and their receptors. Here we present an in-depth profiling of the binding mechanism of TGFβ3 ligand with its type II and type I receptors (TβRII and TβRI) using isothermal titration calorimetry (ITC). Studies were carried out in acidic pH as it has great physiological relevance for TGFβ3 activity. Our findings reveal an unusual positive enthalpy (∆H) compensated by a large favourable entropy (∆S) during TGFβ3-TβRII interaction. In addition to the hydrophobic effect, we propose that a distinct conformational switch from "closed" to "open" form as experienced by TGFβ3 on binding to TβRII is contributing significantly to the increase in overall entropy of the system. Binding studies of TGFβ3 and TβRII were carried out at different pH values and salt concentrations to gain further insight into the thermodynamics of the interaction. Furthermore, the importance of hydrophobic interactions on the binding affinity of TβRII with TGFβ3 was confirmed by two TβRII variants (interfacial). Finally, a distinct shift from entropy to enthalpy dominated interaction was observed upon recruitment of TβRI to the binary complex forming the ternary complex.

Suppression of TGF-β and ERK Signaling Pathways as a New Strategy to Provide Rodent and Non-Rodent Pluripotent Stem Cells

Stem cells are unspecialized cells and excellent model in developmental biology and a promising approach to the treatment of disease and injury. In the last 30 years, pluripotent embryonic stem (ES) cells were established from murine and primate sources, and display indefinite replicative potential and the ability to differentiate to all three embryonic germ layers. Despite large efforts in many aspects of rodent and non-rodent pluripotent stem cell culture, a number of diverse challenges remain. Natural and synthetic small molecules (SMs) strategy has the potential to overcome these hurdles. Small molecules are typically fast and reversible that target specific signaling pathways, epigenetic processes and other cellular processes. Inhibition of the transforming growth factor-β (TGF-β/Smad) and fibroblast growth factor 4 (FGF4)/ERK signaling pathways by SB431542 and PD0325901 small molecules, respectively, known as R2i, enhances the efficiency of mouse, rat, and chicken pluripotent stem cells passaging from different genetic backgrounds. Therefore, the application of SM inhibitors of TGF-β and ERK1/2 with leukemia inhibitory factor (LIF) allows the cultivation of pluripotent stem cells in a chemically defined condition. In this review, we discuss recently emerging evidence that dual inhibition of TGF-β and FGF signaling pathways plays an important role in regulating pluripotency in both rodent and non-rodent pluripotent stem cells.

Sajedi R. Directed Blocking of TGF-β Receptor I Binding Site Using Tailored Peptide Segments to Inhibit its Signaling Pathway

BACKGROUND

TGF-β isoforms play crucial roles in diverse cellular processes. Therefore, targeting and inhibiting TGF-β signaling pathway provides a potential therapeutic opportunity. TGF-β isoforms bind and bring the receptors (TβRII and TβRI) together to form a signaling complex in an ordered manner.

OBJECTIVES

Herein, an antagonistic variant of TGF-β (AnTβ) has been designed and prepared to inhibit the formation of signaling complex and consequently its signaling pathway. This TGF-β homodimeric variant contains intact TβRII binding sites and blocked TβRI binding sites by substituting three peptide segments. So, AnTβ could only bind to TβRII, but prevent binding and recruitment of TβRI to form a signaling complex.

MATERIALS AND METHODS

A reliable model of AnTβ was built and refined using molecular dynamics (MD) simulation, followed by investigating the interactions of AnTβ with the receptors using in silico docking studies. After expression of disulfide-linked AnTβ in a SHuffle strain and purification of the protein using affinity chromatography, its biological activity was evaluated using Mink lung epithelial cells (Mvl Lu).

RESULTS

No meaningful significant changes in AnTβ structure were observed when compared with the native protein. Based on the docking analysis, AnTβ binds to TβRII similar to TGF-β and its binding to TβRI was diminished considerably which was consistent with our design purpose. Cell-based bioassay indicated that AnTβ could modulate TGF-β-induced cell growth inhibition.

CONCLUSIONS

Our analysis suggests that the antagonistic potency of AnTβ can be used as an anti-TGFβ signaling factor in the future perspectives.

TGF-β1 promotes hyaluronan synthesis by upregulating hyaluronan synthase 2 expression in human granulosa-lutein cells

Hyaluronan serves as a structural component of ovarian follicles, and hyaluronan-mediated signaling cascades lead to follicular development, oocyte maturation, and ovulation. Transforming growth factor-β (TGF-β1) is highly expressed in human oocytes and granulosa cells and involved in the regulation of follicular development and ovulation. Previous studies have shown the imperative role for TGF-β signaling in the regulation of hyaluronan-mediated cumulus expansion and ovulation in human granulosa-lutein (hGL) cells. However, the detailed underlying molecular mechanisms by which TGF-β regulates the synthesis of hyaluronan in hGL cells are not fully elucidated. Using both primary and immortalized hGL cells as study models, we provide the first data showing that TGF-β1 significantly promoted the synthesis of hyaluronan by upregulating the expression of hyaluronan synthase 2 in these cells. Additionally, using dual inhibition approaches, we show that the TGF-β type II (TβRII) receptor and TGF-β type I (ALK5) receptor are functional receptors that mediate stimulatory effects in response to TGF-β1. Moreover, we found that the canonical SMAD2/SMAD3-SMAD4 signaling pathway is the principal intracellular signaling pathway that upregulates the expressionhyaluronan synthase and subsequent hyaluronan synthesis. Notably, we showed that SNAIL transcription factor is a critical molecule mediating the TGF-β signaling, which contributes to the increase in hyaluronan synthesis. These results of our in vitro studies suggest that intraovarian TGF-β1 plays a functional role in the local regulation of hyaluronan synthesis in hGL cells.

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.

Recombinant production, purification, crystallization, and structure analysis of human transforming growth factor β2 in a new conformation

Transforming growth factor β is a disulfide-linked dimeric cytokine that occurs in three highly related isoforms (TGFβ1–TGFβ3) engaged in signaling functions through binding of cognate TGFβ receptors. To regulate this pathway, the cytokines are biosynthesized as inactive pro-TGFβs with an N-terminal latency-associated protein preceding the mature moieties. Due to their pleiotropic implications in physiology and pathology, TGFβs are privileged objects of in vitro studies. However, such studies have long been limited by the lack of efficient human recombinant expression systems of native, glycosylated, and homogenous proteins. Here, we developed pro-TGFβ2 production systems based on human Expi293F cells, which yielded >2 mg of pure histidine- or Strep-tagged protein per liter of cell culture. We assayed this material biophysically and in crystallization assays and obtained a different crystal form of mature TGFβ2, which adopted a conformation deviating from previous structures, with a distinct dimeric conformation that would require significant rearrangement for binding of TGFβ receptors. This new conformation may be reversibly adopted by a certain fraction of the mature TGβ2 population and represent a hitherto undescribed additional level of activity regulation of the mature growth factor once the latency-associated protein has been separated.

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.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Structure-guided engineering of TGF-βs for the development of novel inhibitors and probing mechanism

The increasing availability of detailed structural information on many biological systems provides an avenue for manipulation of these structures, either for probing mechanism or for developing novel therapeutic agents for treating disease. This has been accompanied by the advent of several powerful new methods, such as the ability to incorporate non-natural amino acids or perform fragment screening, increasing the capacity to leverage this new structural information to aid in these pursuits. The abundance of structural information also provides new opportunities for protein engineering, which may become more and more relevant as treatment of diseases using gene therapy approaches become increasingly common. This is illustrated by example with the TGF-β family of proteins, for which there is ample structural information, yet no approved inhibitors for treating diseases, such as cancer and fibrosis that are promoted by excessive TGF-β signaling. The results presented demonstrate that through several relatively simple modifications, primarily involving the removal of an α-helix and replacement of it with a flexible loop, it is possible to alter TGF-βs from being potent signaling proteins into inhibitors of TGF-β signaling. The engineered TGF-βs have improved specificity relative to kinase inhibitors and a much smaller size compared to monoclonal antibodies, and thus may prove successful as either as an injected therapeutic or as a gene therapy-based therapeutic, where other classes of inhibitors have failed.

SNAIL Mediates TGF-β1-Induced Downregulation of Pentraxin 3 Expression in Human Granulosa Cells

Transforming growth factor-β (TGF-β) 1 plays a critical role in regulating follicular development, and its dysregulation has been shown to be involved in the pathogenesis of ovulation dysfunction. SNAIL is a well-known transcriptional repressor that mediates TGF-β1-induced cellular functions. Pentraxin 3 (PTX3) is a key enzyme for the assembly and stabilization of the cumulus oophorus extracellular matrix, which is essential for cumulus expansion during the periovulatory stage. The purpose of the present study was to investigate the roles of TGF-β1 and SNAIL in the regulation of PTX3 expression and to examine the underlying mechanism. An established immortalized human granulosa cell (GC) line (SVOG), a GC tumor cell line (KGN), and primary human granulosa-lutein cells were used as study models. We demonstrated that TGF-β1 treatment substantially decreased the messenger RNA and protein levels of PTX3. This suppressive effect was abolished by cotreatment with the soluble TGF-β type II receptor (TβRII) or the ALK4/5/7 inhibitor SB431542. Knockdown of ALK5, SMAD2/3, or SMAD4 reversed the effects of TGF-β1-induced SNAIL upregulation and PTX3 suppression. These results indicate that TGF-β1 upregulates SNAIL and downregulates PTX3 expression via a TβRII-ALK5-mediated SMAD-dependent signaling pathway in human GCs. Additionally, TGF-β1-induced PTX3 suppression was mediated by upregulation of the SNAIL transcription factor, as knockdown of SNAIL completely reversed the suppression of PTX3 in response to TGF-β1. These findings could inform the roles of TGF-β1 and SNAIL in the regulation of follicular function and might provide therapeutic targets for the treatment of ovulation dysfunction.

A structurally distinct TGF-β mimic from an intestinal helminth parasite potently induces regulatory T cells

Helminth parasites defy immune exclusion through sophisticated evasion mechanisms, including activation of host immunosuppressive regulatory T (Treg) cells. The mouse parasite Heligmosomoides polygyrus can expand the host Treg population by secreting products that activate TGF-β signalling, but the identity of the active molecule is unknown. Here we identify an H. polygyrus TGF-β mimic (Hp-TGM) that replicates the biological and functional properties of TGF-β, including binding to mammalian TGF-β receptors and inducing mouse and human Foxp3⁺ Treg cells. Hp-TGM has no homology with mammalian TGF-β or other members of the TGF-β family, but is a member of the complement control protein superfamily. Thus, our data indicate that through convergent evolution, the parasite has acquired a protein with cytokine-like function that is able to exploit an endogenous pathway of immunoregulation in the host.

Heligmosomoides polygyrus can activate mammalian TGF-β signalling pathways, but how it does so is not known. Here the authors identify and isolate a H. polygyrus TFG-β mimic that can bind both mammalian TGF-β receptor subunits, activate Smad signalling and generate inducible regulatory T cells.

An engineered transforming growth factor β (TGF-β) monomer that functions as a dominant negative to block TGF-β signaling

The transforming growth factor β isoforms, TGF-β1, -β2, and -β3, are small secreted homodimeric signaling proteins with essential roles in regulating the adaptive immune system and maintaining the extracellular matrix. However, dysregulation of the TGF-β pathway is responsible for promoting the progression of several human diseases, including cancer and fibrosis. Despite the known importance of TGF-βs in promoting disease progression, no inhibitors have been approved for use in humans. Herein, we describe an engineered TGF-β monomer, lacking the heel helix, a structural motif essential for binding the TGF-β type I receptor (TβRI) but dispensable for binding the other receptor required for TGF-β signaling, the TGF-β type II receptor (TβRII), as an alternative therapeutic modality for blocking TGF-β signaling in humans. As shown through binding studies and crystallography, the engineered monomer retained the same overall structure of native TGF-β monomers and bound TβRII in an identical manner. Cell-based luciferase assays showed that the engineered monomer functioned as a dominant negative to inhibit TGF-β signaling with a Ki of 20-70 nm Investigation of the mechanism showed that the high affinity of the engineered monomer for TβRII, coupled with its reduced ability to non-covalently dimerize and its inability to bind and recruit TβRI, enabled it to bind endogenous TβRII but prevented it from binding and recruiting TβRI to form a signaling complex. Such engineered monomers provide a new avenue to probe and manipulate TGF-β signaling and may inform similar modifications of other TGF-β family members.

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.