Identification of distinct inhibin and transforming growth factor beta-binding sites on betaglycan: functional separation of betaglycan co-receptor actions

Inhibin Inactivation in Female Mice Leads to Elevated FSH Levels, Ovarian Overstimulation, and Pregnancy Loss

Inhibins are members of the transforming growth factor-β family, composed of a common α-subunit disulfide-linked to 1 of 2 β-subunits (βA in inhibin A or βB in inhibin B). Gonadal-derived inhibin A and B act in an endocrine manner to suppress the synthesis of follicle-stimulating hormone (FSH) by pituitary gonadotrope cells. Roles for inhibins beyond the pituitary, however, have proven difficult to delineate because deletion of the inhibin α-subunit gene (Inha) results in unconstrained expression of activin A and activin B (homodimers of inhibin β-subunits), which contribute to gonadal tumorigenesis and lethal cachectic wasting. Here, we generated mice with a single point mutation (Arg233Ala) in Inha that prevents proteolytic processing and the formation of bioactive inhibin. In vitro, this mutation blocked inhibin maturation and bioactivity, without perturbing activin production. Serum FSH levels were elevated 2- to 3-fold in InhaR233A/R233A mice due to the loss of negative feedback from inhibins, but no pathological increase in circulating activins was observed. While inactivation of inhibin A and B had no discernible effect on male reproduction, female InhaR233A/R233A mice had increased FSH-dependent follicle development and enhanced natural ovulation rates. Nevertheless, inhibin inactivation resulted in significant embryo-fetal resorptions and severe subfertility and was associated with disrupted maternal ovarian function. Intriguingly, heterozygous Inha+/R233A females had significantly enhanced fecundity, relative to wild-type littermates. These studies have revealed novel effects of inhibins in the establishment and maintenance of pregnancy and demonstrated that partial inactivation of inhibin A/B is an attractive approach for enhancing female fertility.

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.

Current potential therapeutic strategies targeting the TGF-β/Smad signaling pathway to attenuate keloid and hypertrophic scar formation

Aberrant scar formation, which includes keloid and hypertrophic scars, is associated with a pathological disorganized wound healing process with chronic inflammation. The TGF-β/Smad signaling pathway is the most canonical pathway through which the formation of collagen in the fibroblasts and myofibroblasts is regulated. Sustained activation of the TGF-β/Smad signaling pathway results in the long-term overactivation of fibroblasts and myofibroblasts, which is necessary for the excessive collagen formation in aberrant scars. There are two categories of therapeutic strategies that aim to target the TGF-β/Smad signaling pathway in fibroblasts and myofibroblasts to interfere with their cellular functions and reduce cell proliferation. The first therapeutic strategy includes medications, and the second strategy is composed of genetic and cellular therapeutics. Therefore, the focus of this review is to critically evaluate these two main therapeutic strategies that target the TGF-β/Smad pathway to attenuate abnormal skin scar formation.

TGF-β Signaling

Transforming growth factor-β (TGF-β) represents an evolutionarily conserved family of secreted polypeptide factors that regulate many aspects of physiological embryogenesis and adult tissue homeostasis. The TGF-β family members are also involved in pathophysiological mechanisms that underlie many diseases. Although the family comprises many factors, which exhibit cell type-specific and developmental stage-dependent biological actions, they all signal via conserved signaling pathways. The signaling mechanisms of the TGF-β family are controlled at the extracellular level, where ligand secretion, deposition to the extracellular matrix and activation prior to signaling play important roles. At the plasma membrane level, TGF-βs associate with receptor kinases that mediate phosphorylation-dependent signaling to downstream mediators, mainly the SMAD proteins, and mediate oligomerization-dependent signaling to ubiquitin ligases and intracellular protein kinases. The interplay between SMADs and other signaling proteins mediate regulatory signals that control expression of target genes, RNA processing at multiple levels, mRNA translation and nuclear or cytoplasmic protein regulation. This article emphasizes signaling mechanisms and the importance of biochemical control in executing biological functions by the prototype member of the family, TGF-β.

Epicardial TGFβ and BMP Signaling in Cardiac Regeneration: What Lesson Can We Learn from the Developing Heart?

The epicardium, the outer layer of the heart, has been of interest in cardiac research due to its vital role in the developing and diseased heart. During development, epicardial cells are active and supply cells and paracrine cues to the myocardium. In the injured adult heart, the epicardium is re-activated and recapitulates embryonic behavior that is essential for a proper repair response. Two indispensable processes for epicardial contribution to heart tissue formation are epithelial to mesenchymal transition (EMT), and tissue invasion. One of the key groups of cytokines regulating both EMT and invasion is the transforming growth factor β (TGFβ) family, including TGFβ and Bone Morphogenetic Protein (BMP). Abundant research has been performed to understand the role of TGFβ family signaling in the developing epicardium. However, less is known about signaling in the adult epicardium. This review provides an overview of the current knowledge on the role of TGFβ in epicardial behavior both in the development and in the repair of the heart. We aim to describe the presence of involved ligands and receptors to establish if and when signaling can occur. Finally, we discuss potential targets to improve the epicardial contribution to cardiac repair as a starting point for future investigation.

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.

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.

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.

Hemojuvelin is a novel suppressor for Duchenne muscular dystrophy and age-related muscle wasting

BACKGROUND

Muscle wasting occurs in response to various physiological and pathological conditions, including ageing and Duchenne muscular dystrophy (DMD). Transforming growth factor-β1 (TGF-β1) contributes to muscle pathogenesis in elderly people and DMD patients; inhibition of TGF-β1 signalling is a promising therapeutic strategy for muscle-wasting disorders. Hemojuvelin (HJV or Hjv as the murine homologue) is a membrane-bound protein that is highly expressed in skeletal muscle, heart, and liver. In hepatic cells, Hjv acts as a coreceptor for bone morphogenetic protein, a TGF-β subfamily member. The aim of this study was to investigate whether Hjv plays an essential role in muscle physiological and pathophysiological processes by acting as a coreceptor for TGF-β1 signalling.

METHODS

Conventional and conditional Hjv knockout mice as well as mdx and aged mice transfected with Hjv overexpression vector were used to study the role of Hjv in muscle physiology and pathophysiology. qRT-PCR, western blotting, and immunohistochemistry examinations were conducted to evaluate gene, protein, and structural changes in vivo and in vitro. Exercise endurance was determined using treadmill running test, and muscle force was detected by an isometric transducer. RNA interference, immunoprecipitation, and dual-luciferase reporter assays were utilized to explore the mechanism by which Hjv regulates TGF-β1 signalling in skeletal muscle.

RESULTS

Conventional and conditional Hjv knockout mice displayed muscle atrophy, fibrosis, reduced running endurance, and muscle force. HJV was significantly down-regulated in the muscles of DMD patients (n = 3, mean age: 11.7 ± 5.7 years) and mdx mice as well as in those of aged humans (n = 10, 20% women, mean age: 75.1 ± 9.5 years) and mice. Overexpression of Hjv rescued dystrophic and age-related muscle wasting. Unlike its function in hepatic cells, the bone morphogenetic protein downstream phosphorylated p-Smad1/5/8 signalling pathway was unchanged, but TGF-β1, TGF-β receptor II (TβRII), and p-Smad2/3 expression were increased in Hjv-deficient muscles. Mechanistically, loss of Hjv promoted activation of Smad3 signalling induced by TGF-β1, whereas Hjv overexpression inhibited TGF-β1/Smad3 signalling by directly interacting with TβRII on the muscle membrane.

CONCLUSIONS

Our findings identify an unrecognized role of HJV in skeletal muscle by regulating TGF-β1/Smad3 signalling as a coreceptor for TβRII. Unlike the TGF-β1/Smad3 pathway, HJV could be a reliable drug target as its expression is not widespread. Novel therapeutic strategies could potentially be devised to interfere only with the muscle function of HJV to treat DMD and age-related muscle wasting.

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.

The role of TGFβ in hematopoiesis and myeloid disorders

The role of transforming growth factor-β (TGFβ) signaling in embryological development and tissue homeostasis has been thoroughly characterized. Its canonical downstream cascade is well known, even though its true complexity and other non-canonical pathways are still being explored. TGFβ signaling has been described as an important pathway involved in carcinogenesis and cancer progression. In the hematopoietic compartment, the TGFβ pathway is an important regulator of proliferation and differentiation of different cell types and has been implicated in the pathogenesis of a diverse variety of bone marrow disorders. Due to its importance in hematological diseases, novel inhibitors of this pathway are being developed against a number of hematopoietic disorders, including myelodysplastic syndromes (MDS). In this review, we provide an overview of the TGFβ pathway, focusing on its role in hematopoiesis and impact on myeloid disorders. We will discuss therapeutic interventions with promising results against MDS.

From Consternation to Revelation: Discovery of a Role for IGSF1 in Pituitary Control of Thyroid Function

Immunoglobulin superfamily, member 1 (IGSF1) is a transmembrane glycoprotein highly expressed in the mammalian pituitary gland. Shortly after its discovery in 1998, the protein was proposed to function as a coreceptor for inhibins (and was even temporarily renamed inhibin binding protein). However, subsequent investigations, both in vitro and in vivo, failed to support a role for IGSF1 in inhibin action. Research on IGSF1 nearly ground to a halt until 2011, when next-generation sequencing identified mutations in the X-linked IGSF1 gene in boys and men with congenital central hypothyroidism. IGSF1 was localized to thyrotrope cells, implicating the protein in pituitary control of the thyroid. Investigations in two Igsf1 knockout mouse models converged to show that IGSF1 deficiency leads to reduced expression of the receptor for thyrotropin-releasing hormone (TRH) and impaired TRH stimulation of thyrotropin secretion, providing a candidate mechanism for the central hypothyroidism observed in patients. Nevertheless, the normal functions of IGSF1 in thyrotropes and other cells remain unresolved. Moreover, IGSF1 mutations are also commonly associated with other clinical phenotypes, including prolactin and growth hormone dysregulation, and macroorchidism. How the loss of IGSF1 produces these characteristics is unknown. Although early studies of IGSF1 ran into roadblocks and blind alleys, armed with the results of detailed clinical investigations, powerful mouse models, and new reagents, the field is now poised to discover IGSF1’s function in endocrine tissues, including the pituitary and testes.

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.

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.

Endogenous inhibins regulate steroidogenesis in mouse TM3 Leydig cells by altering SMAD2 signalling

This study tested the hypothesis that inhibins act in an autocrine manner on Leydig cells using a pre-pubertal Leydig cell line, TM3, as a model of immature Leydig cells. The expression of Inha, Inhba, and Inhbb in TM3 cells was determined by RT-PCR and the production of the inhibin-alpha subunit was confirmed by western blot. Knockdown of Inha expression resulted in significant decreases in the expression of Leydig cell markers Cyp17a1, Cyp11a1, Nr5a1, and Insl3. Western blot showed that activin A, TGFβ1 and TGFβ2 activated SMAD2, and that knockdown of Inha expression in TM3 cells enhanced both activin A- and TGFβ-induced SMAD2 activation. SB431542, a chemical inhibitor of the TGFβ/activin type I receptors, blocked ligand-induced SMAD2 activation and the downregulation of Cyp17a1 expression. Our findings demonstrate that TGFβs and activin A negatively regulate steroidogenic gene expression in TM3 cells via ALK4/5 and SMAD2 and endogenous inhibins can counter this regulation.

Transcriptional Profiling of Cultured, Embryonic Epicardial Cells Identifies Novel Genes and Signaling Pathways Regulated by TGFβR3 In Vitro

The epicardium plays an important role in coronary vessel formation and Tgfbr3^-/- mice exhibit failed coronary vessel development associated with decreased epicardial cell invasion. Immortalized Tgfbr3^-/- epicardial cells display the same defects. Tgfbr3^+/+ and Tgfbr3^-/- cells incubated for 72 hours with VEH or ligands known to promote invasion via TGFβR3 (TGFβ1, TGFβ2, BMP2), for 72 hours were harvested for RNA-seq analysis. We selected for genes >2-fold differentially expressed between Tgfbr3^+/+ and Tgfbr3^-/- cells when incubated with VEH (604), TGFβ1 (515), TGFβ2 (553), or BMP2 (632). Gene Ontology (GO) analysis of these genes identified dysregulated biological processes consistent with the defects observed in Tgfbr3^-/- cells, including those associated with extracellular matrix interaction. GO and Gene Regulatory Network (GRN) analysis identified distinct expression profiles between TGFβ1-TGFβ2 and VEH-BMP2 incubated cells, consistent with the differential response of epicardial cells to these ligands in vitro. Despite the differences observed between Tgfbr3^+/+ and Tgfbr3^-/- cells after TGFβ and BMP ligand addition, GRNs constructed from these gene lists identified NF-ĸB as a key nodal point for all ligands examined. Tgfbr3^-/- cells exhibited decreased expression of genes known to be activated by NF-ĸB signaling. NF-ĸB activity was stimulated in Tgfbr3^+/+ epicardial cells after TGFβ2 or BMP2 incubation, while Tgfbr3^-/- cells failed to activate NF-ĸB in response to these ligands. Tgfbr3^+/+ epicardial cells incubated with an inhibitor of NF-ĸB signaling no longer invaded into a collagen gel in response to TGFβ2 or BMP2. These data suggest that NF-ĸB signaling is dysregulated in Tgfbr3^-/- epicardial cells and that NF-ĸB signaling is required for epicardial cell invasion in vitro. Our approach successfully identified a signaling pathway important in epicardial cell behavior downstream of TGFβR3. Overall, the genes and signaling pathways identified through our analysis yield the first comprehensive list of candidate genes whose expression is dependent on TGFβR3 signaling.

Signaling Receptors for TGF-β Family Members

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

A Novel, More Efficient Approach to Generate Bioactive Inhibins

Gonadal-derived inhibins are essential factors in mammalian reproduction, negatively regulating pituitary production of FSH. Interestingly, declines in inhibin levels across the menopause transition correlate with not only an increase in FSH but also a rapid decrease in bone mass. Therefore, inhibins have been touted as potential therapeutics for osteoporosis in postmenopausal women. However, as heterodimeric proteins of α- and β- (βA or βB)-subunits, inhibins are difficult to produce recombinantly, are poorly processed to their mature bioactive forms, and their expression is always accompanied by production of activins (β-subunit homodimers), the proteins they antagonize. In this study, we developed the methodology to circumvent most of these issues. Initially, the cleavage sites between the pro- and mature domains of the α- and βA-subunits were modified to ensure complete processing. These modifications led to a marked increase (9-fold) in the levels of bioactive inhibin A and a striking decrease (12.5-fold) in mature activin A production. Next, a single point mutation (M418A) was incorporated into the βA-subunit, which reduced residual activin activity approximately 100-fold and, in so doing, increased inhibin bioactivity 8-fold. Finally, we showed that inhibin A noncovalently associated with its prodomain was more potent (∼20-fold) than mature inhibin A in specific in vitro bioassays, indicating an important role of the prodomain in inhibin bioactivity. In conclusion, the production of potent inhibin analogs in the virtual absence of activin activity will greatly facilitate the investigation of the therapeutic potential of these gonadal hormones on bone and other tissues.

Common pathways regulate Type III TGFβ receptor-dependent cell invasion in epicardial and endocardial cells

Epithelial-Mesenchymal Transformation (EMT) and the subsequent invasion of epicardial and endocardial cells during cardiac development is critical to the development of the coronary vessels and heart valves. The transformed cells give rise to cardiac fibroblasts and vascular smooth muscle cells or valvular interstitial cells, respectively. The Type III Transforming Growth Factor β (TGFβR3) receptor regulates EMT and cell invasion in both cell types, but the signaling mechanisms downstream of TGFβR3 are not well understood. Here we use epicardial and endocardial cells in in vitro cell invasion assays to identify common mechanisms downstream of TGFβR3 that regulate cell invasion. Inhibition of NF-κB activity blocked cell invasion in epicardial and endocardial cells. NF-κB signaling was found to be dysregulated in Tgfbr3(-/-) epicardial cells which also show impaired cell invasion in response to ligand. TGFβR3-dependent cell invasion is also dependent upon Activin Receptor-Like Kinase (ALK) 2, ALK3, and ALK5 activity. A TGFβR3 mutant that contains a threonine to alanine substitution at residue 841 (TGFβR3-T841A) induces ligand-independent cell invasion in both epicardial and endocardial cells in vitro. These findings reveal a role for NF-κB signaling in the regulation of epicardial and endocardial cell invasion and identify a mutation in TGFβR3 which stimulates ligand-independent signaling.

Single nucleotide polymorphism rs11669203 in TGFBR3L is associated with the risk of neuroblastoma in a Chinese population

With a primary mortality, neuroblastoma (NB) is the most common extracranial solid tumor in childhood. Amplification of the MYCN (v-myc avian myelocytomatosis viral oncogene neuroblastoma derived homolog) oncogene is observed in 20-30 % of NB cases, a feature which also characterizes a highly aggressive subtype of the disease. However, the systematic study of association between single nucleotide polymorphisms (SNPs) in MYCN-regulated genes and the risk of NB has not been investigated. In the current study, we scanned a set of 16 SNPs located within known or predicted MYCN binding sites in a cohort of 247 patients of Chinese origin with neuroblastic family tumors, including neuroblastoma (NB), ganglioneuroma (GN), and ganglioneuroblastoma (GNB), and in 290 cancer-free controls to determine whether any of the tested SNPs are associated with neuroblastic family tumors. We found that the rs11669203 G>C polymorphism, located in TGFBR3L promoter, is significantly associated with the risk of NB. Further, we found that this association is site specific to adrenal NB compared to non-adrenal NB. In addition, transcriptome analysis indicated that increased expression of TGFBR3L is strongly correlated with poor survival. The SNP rs11669203 located at the MYCN binding site of TGFBR3L is significantly associated with elevated risk of NB, and abnormal MYCN-regulated TGFBR3L expression may contribute to NB oncogenesis.

Analysis of consequences of non-synonymous SNP in feed conversion ratio associated TGF-β receptor type 3 gene in chicken

The recent advances in high throughput sequencing technology accelerate possible ways for the study of genome wide variation in several organisms and associated consequences. In the present study, mutations in TGFBR3 showing significant association with FCR trait in chicken during exome sequencing were further analyzed. Out of four SNPs, one nsSNP p.Val451Leu was found in the coding region of TGFBR3. In silico tools such as SnpSift and PANTHER predicted it as deleterious (0.04) and to be tolerated, respectively, while I-Mutant revealed that protein stability decreased. The TGFBR3 I-TASSER model has a C-score of 0.85, which was validated using PROCHECK. Based on MD simulation, mutant protein structure deviated from native with RMSD 0.08 Å due to change in the H-bonding distances of mutant residue. The docking of TGFBR3 with interacting TGFBR2 inferred that mutant required more global energy. Therefore, the present study will provide useful information about functional SNPs that have an impact on FCR traits.

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Investigated functional nsSNP p.Val451Leu (rs312979494) in feed conversion ratio (FCR) associated TGFBR3 of chicken

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Computational tools (SIFT and I-Mutant 2.0) predicted that this nsSNP was deleterious.

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Mutant structure of TGFBR3 showed high energies and RMS deviations compared to native using MD simulation.

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Molecular docking of TGFBR3 with interacting protein TGFBR2 showed an increase in global energy of mutant compared to native.

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We have predicted that functional SNP has an impact on TGFBR3 of chicken and thus can be treated as candidate SNP in screening.

Characterization of the mechanism of inhibin α-subunit gene in mouse anterior pituitary cells by RNA interference

Inhibin, a member of the transforming growth factor-β [TGF-β] superfamily, is a suppressor of follicle-stimulating hormone [FSH] release through pituitary–gonadal negative feedback loop to regulate follicular development. In this study, Inhibin α-subunit [Inha] gene was knocked down successfully in mice primary anterior pituitary cells at both transcriptional and translational levels by RNAi-Ready pSIREN-RetroQ-ZsGreen Vector mediated recombinant pshRNA vectors. The results indicated that inhibin silencing significantly promoted apoptosis by up-regulating Caspase-3, Bax and Bcl-2 genes without affecting p53 both at transcriptional and translational levels. Furthermore, it markedly impaired the progression of G1 phase of cell cycle and decreased the amount of cells in S phase [as detected by flow cytometry]. Inhibin silencing resulted in significant up-regulation of mRNA and protein expressions of Gondotropin releasing hormone receptors [GnRHR] and down-regulated mRNA levels of β-glycans with parellel change in the amount of its protein expression. Silencing of inhibin-a significantly increased [P<0.05] activin-β concentration without affecting FSH and LH levels in anterior pituitary cells. These findings revealed that up regulation of GnRH receptors by silencing inhibin a-subunit gene might increase the concentration of activin-β in the culture medium. Inhibin a silencing resulted in increased mRNA and protein expressions of inhibinβ which may demonstrate that both inhibin subunits co-participate in the regulation of reproductive events in anterior pituitary cells. This study concludes that inhibin is a broad regulatory marker in anterior pituitary cells by regulating apoptosis, cellular progression and simultaneously by vital fluctuations in the hormonal signaling.

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.

Betaglycan alters NFκB-TGFβ2 cross talk to reduce survival of human granulosa tumor cells

The molecular pathways controlling granulosa cell tumor (GCT) survival are poorly understood. In many cell types, nuclear factor-κB (NFκB) and TGFβ coordinately regulate cell survival to maintain tissue homeostasis. Because GCT cell lines exhibit constitutively activated NFκB, we hypothesized that NFκB blocks TGFβ-mediated cell death in GCT cells. To test this hypothesis, we used the human GCT cell line KGN, which exhibits loss of betaglycan, a TGFβ co-receptor. After inhibition of NFκB in KGN cells, re-expression of betaglycan resulted in a decrease in cell viability, which was further decreased by TGFβ2. Intriguingly, TGFβ2 increased NFκB reporter activity in control cells, but betaglycan expression suppressed both basal and TGFβ2-stimulated NFκB activity. Chemical inhibition of Mothers against decapentaplegic homolog 2/3 (SMAD2/3) signaling or SMAD2/3 gene silencing revealed that both SMADs contributed to cell survival. Furthermore, inhibiting NFκB activity resulted in a specific reduction in SMAD3 expression. Conversely, overexpression of SMAD3 increased basal NFκB activity and countered betaglycan-mediated suppression of NFκB activity. Finally, ERK1/2 activation emerged as the point of convergence of NFκB, SMAD3, and TGFβ2/betaglycan governance of GCT cell viability. Key findings in KGN cells were reproduced in a second GCT cell line, COV434. Collectively, our data establish that both SMAD2/3 and NFκB signaling pathways support GCT cell viability and suggest the existence of a positive feedback loop between NFκB and SMAD3 signaling in late-stage GCT. Furthermore, our data suggest that loss of betaglycan during tumor progression in GCT alters the functional outcomes generated by NFκB and TGFβ pathway cross talk.

Convergent animal and human evidence suggests the activin/inhibin pathway to be involved in antidepressant response

Despite the overt need for improved treatment modalities in depression, efforts to develop conceptually novel antidepressants have been relatively unsuccessful so far. Here we present a translational approach combining results from hypothesis-free animal experiments with data from a genetic association study in depression. Comparing genes regulated by chronic paroxetine treatment in the mouse hippocampus with genes showing nominally significant association with antidepressant treatment response in two pharmacogenetic studies, the activin pathway was the only one to show this dual pattern of association and therefore selected as a candidate. We examined the regulation of activin A and activin receptor type IA mRNA following antidepressant treatment. We investigated the effects of stereotaxic infusion of activin into the hippocampus and the amygdala in a behavioural model of depression. To analyse whether variants in genes in the activin signalling pathway predict antidepressant treatment response, we performed a human genetic association study. Significant changes in the expression of genes in the activin signalling pathway were observed following 1 and 4 weeks of treatment. Injection of activin A into the hippocampus exerts acute antidepressant-like effects. Polymorphisms in the betaglycan gene, a co-receptor mediating functional antagonism of activin signalling, significantly predict treatment outcome in our system-wide pharmacogenetics study in depression. We provide convergent evidence from mouse and human data that genes in the activin signalling pathway are promising novel candidates involved in the neurobiogical mechanisms underlying antidepressant mechanisms of action. Further, our data suggest this pathway to be a target for more rapid-acting antidepressants in the future.

TGFβ signalling in context

The basic elements of the transforming growth factor-β (TGFβ) pathway were revealed more than a decade ago. Since then, the concept of how the TGFβ signal travels from the membrane to the nucleus has been enriched with additional findings, and its multifunctional nature and medical relevance have relentlessly come to light. However, an old mystery has endured: how does the context determine the cellular response to TGFβ? Solving this question is key to understanding TGFβ biology and its many malfunctions. Recent progress is pointing at answers.

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.

Cell-type specific modulation of pituitary cells by activin, inhibin and follistatin

Activins are multifunctional proteins and members of the TGF-β superfamily. Activins are expressed locally in most tissues and, analogous to the actions of other members of this large family of pleiotropic factors, play prominent roles in the regulation of diverse biological processes in both differentiated and embryonic stem cells. They have an essential role in maintaining tissue homeostasis in the adult and are known to contribute to the developmental programs in the embryo. Activins are further implicated in the growth and metastasis of tumor cells. Through distinct modes of action, inhibins and follistatins function as antagonists of activin and several other TGF-β family members, including a subset of BMPs/GDFs, and modulate cellular responses and the signaling cascades downstream of these ligands. In the pituitary, the activin pathway is known to regulate key aspects of gonadotrope functions and also exert effects on other pituitary cell types. As in other tissues, activin is produced locally by pituitary cells and acts locally by exerting cell-type specific actions on gonadotropes. These local actions of activin on gonadotropes are modulated by the autocrine/paracrine actions of locally secreted follistatin and by the feedback actions of gonadal inhibin. Knowledge about the mechanism of activin, inhibin and follistatin actions is providing information about their importance for pituitary function as well as their contribution to the pathophysiology of pituitary adenomas. The aim of this review is to highlight recent findings and summarize the evidence that supports the important functions of activin, inhibin and follistatin in the pituitary.

TGFβ and BMP-2 regulate epicardial cell invasion via TGFβR3 activation of the Par6/Smurf1/RhoA pathway

Coronary vessel development requires transfer of mesothelial cells to the heart surface to form the epicardium where some cells subsequently undergo epithelial-mesenchymal transformation (EMT) and invade the subepicardial matrix. Tgfbr3(-/-) mice die due to failed coronary vessel formation associated with decreased epicardial cell invasion but the mediators downstream of TGFβR3 are not well described. TGFβR3-dependent endocardial EMT stimulated by either TGFβ2 or BMP-2 requires activation of the Par6/Smurf1/RhoA 1pathway where Activin Receptor Like Kinase (ALK5) signals Par6 to act downstream of TGFβ to recruit Smurf1 to target RhoA for degradation to regulate apical-basal polarity and tight junction dissolution. Here we asked if this pathway was operant in epicardial cells and if TGFβR3 was required to access this pathway. Targeting of ALK5 in Tgfbr3(+/+) cells inhibited loss of epithelial character and invasion. Overexpression of wild-type (wt) Par6, but not dominant negative (dn) Par6, induced EMT and invasion while targeting Par6 by siRNA inhibited EMT and invasion. Overexpression of Smurf1 and dnRhoA induced loss of epithelial character and invasion. Targeting of Smurf1 by siRNA or overexpression of constitutively active (ca) RhoA inhibited EMT and invasion. In Tgfbr3(-/-) epicardial cells which have a decreased ability to invade collagen gels in response to TGFβ2, overexpression of wtPar6, Smurf1, or dnRhoA had a diminished ability to induce invasion. Overexpression of TGFβR3 in Tgfbr3(-/-) cells, followed by siRNA targeting of Par6 or Smurf1, diminished the ability of TGFβR3 to rescue invasion demonstrating that the Par6/Smurf1/RhoA pathway is activated downstream of TGFβR3 in epicardial cells.

BMP2 signals loss of epithelial character in epicardial cells but requires the Type III TGFβ receptor to promote invasion

Coronary vessel development depends on a subpopulation of epicardial cells that undergo epithelial to mesenchymal transformation (EMT) and invade the subepicardial space and myocardium. These cells form the smooth muscle of the vessels and fibroblasts, but the mechanisms that regulate these processes are poorly understood. Mice lacking the Type III Transforming Growth Factor β Receptor (TGFβR3) die by E14.5 due to failed coronary vessel development accompanied by reduced epicardial cell invasion. BMP2 signals via TGFβR3 emphasizing the importance of determining the relative contributions of the canonical BMP signaling pathway and TGFβR3-dependent signaling to BMP2 responsiveness. Here we examined the role of TGFβR3 in BMP2 signaling in epicardial cells. Whereas TGFβ induced loss of epithelial character and smooth muscle differentiation, BMP2 induced an ALK3-dependent loss of epithelial character and modestly inhibited TGFβ-stimulated differentiation. Tgfbr3(-/-) cells respond to BMP2 indicating that TGFβR3 is not required. However, Tgfbr3(-/-) cells show decreased invasion in response to BMP2 and overexpression of TGFβR3 in Tgfbr3(-/-) cells rescued invasion. Invasion was dependent on ALK5, ALK2, ALK3, and Smad4. Expression of TGFβR3 lacking the 3 C-terminal amino acids required to interact with the scaffolding protein GIPC (GAIP-interacting protein, C terminus) did not rescue. Knockdown of GIPC in Tgfbr3(+/+) or Tgfbr3(-/-) cells rescued with TGFβR3 decreased BMP2-stimulated invasion confirming a requirement for TGFβR3/GIPC interaction. Our results reveal the relative roles of TGFβR3-dependent and TGFβR3-independent signaling in the actions of BMP2 on epicardial cell behavior and demonstrate the critical role of TGFβR3 in mediating BMP2-stimulated invasion.

Endocardial cell epithelial-mesenchymal transformation requires Type III TGFβ receptor interaction with GIPC

An early event in heart valve formation is the epithelial-mesenchymal transformation (EMT) of a subpopulation of endothelial cells in specific regions of the heart tube, the endocardial cushions. The Type III TGFβ receptor (TGFβR3) is required for TGFβ2- or BMP-2-stimulated EMT in atrioventricular endocardial cushion (AVC) explants in vitro but the mediators downstream of TGFβR3 are not well described. Using AVC and ventricular explants as an in vitro assay, we found an absolute requirement for specific TGFβR3 cytoplasmic residues, GAIP-interacting protein, C terminus (GIPC), and specific Activin Receptor-Like Kinases (ALK)s for TGFβR3-mediated EMT when stimulated by TGFβ2 or BMP-2. The introduction of TGFβR3 into nontransforming ventricular endocardial cells, followed by the addition of either TGFβ2 or BMP-2, results in EMT. TGFβR3 lacking the entire cytoplasmic domain, or only the 3C-terminal amino acids that are required to bind GIPC, fails to support EMT in response to TGFβ2 or BMP-2. Overexpression of GIPC in AVC endocardial cells enhanced EMT while siRNA-mediated silencing of GIPC in ventricular cells overexpressing TGFβR3 significantly inhibited EMT. Targeting of specific ALKs by siRNA revealed that TGFβR3-mediated EMT requires ALK2 and ALK3, in addition to ALK5, but not ALK4 or ALK6. Taken together, these data identify GIPC, ALK2, ALK3, and ALK5 as signaling components required for TGFβR3-mediated endothelial cell EMT.

The cytoplasmic domain of TGFβR3 through its interaction with the scaffolding protein, GIPC, directs epicardial cell behavior

The epicardium is a major contributor of the cells that are required for the formation of coronary vessels. Mice lacking both copies of the gene encoding the Type III Transforming Growth Factor β Receptor (TGFβR3) fail to form the coronary vasculature, but the molecular mechanism by which TGFβR3 signals coronary vessel formation is unknown. We used intact embryos and epicardial cells from E11.5 mouse embryos to reveal the mechanisms by which TGFβR3 signals and regulates epicardial cell behavior. Analysis of E13.5 embryos reveals a lower rate of epicardial cell proliferation and decreased epicardially derived cell invasion in Tgfbr3(-/-) hearts. Tgfbr3(-/-) epicardial cells in vitro show decreased proliferation and decreased invasion in response to TGFβ1 and TGFβ2. Unexpectedly, loss of TGFβR3 also decreases responsiveness to two other important regulators of epicardial cell behavior, FGF2 and HMW-HA. Restoring full length TGFβR3 in Tgfbr3(-/-) cells rescued deficits in invasion in vitro in response TGFβ1 and TGFβ2 as well as FGF2 and HMW-HA. Expression of TGFβR3 missing the 3 C-terminal amino acids that are required to interact with the scaffolding protein GIPC1 did not rescue any of the deficits. Overexpression of GIPC1 alone in Tgfbr3(-/-) cells did not rescue invasion whereas knockdown of GIPC1 in Tgfbr3(+/+) cells decreased invasion in response to TGFβ2, FGF2, and HMW-HA. We conclude that TGFβR3 interaction with GIPC1 is critical for regulating invasion and growth factor responsiveness in epicardial cells and that dysregulation of epicardial cell proliferation and invasion contributes to failed coronary vessel development in Tgfbr3(-/-) mice.

A structural view of egg coat architecture and function in fertilization

Species-restricted interaction between gametes at the beginning of fertilization is mediated by the extracellular coat of the egg, a matrix of cross-linked glycoprotein filaments called the zona pellucida (ZP) in mammals and the vitelline envelope in nonmammals. All egg coat subunits contain a conserved protein-protein interaction module-the "ZP domain"-that allows them to polymerize upon dissociation of a C-terminal propeptide containing an external hydrophobic patch (EHP). Recently, the first crystal structures of a ZP domain protein, sperm receptor ZP subunit zona pellucida glycoprotein 3 (ZP3), have been reported, giving a glimpse of the structural organization of the ZP at the atomic level and the molecular basis of gamete recognition in vertebrates. The ZP module is divided in two related immunoglobulin-like domains, ZP-N and ZP-C, that contain characteristic disulfide bond patterns and, in the case of ZP-C, also incorporate the EHP. This segment lies at the interface between the two domains, which are connected by a long loop carrying a conserved O-glycan important for binding to sperm in vitro. The structures explain several apparently contradictory observations by reconciling the variable disulfide bond patterns found in different homologues of ZP3 as well as the multiple ZP3 determinants alternatively involved in gamete interaction. These findings have implications for our understanding of ZP subunit biogenesis; egg coat assembly, architecture, and interaction with sperm; structural rearrangements leading to postfertilization hardening of the ZP and the block to sperm binding; and the evolutionary origin of egg coats.

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.

Betaglycan is required for the establishment of nephron endowment in the mouse

Betaglycan is an accessory receptor for the transforming growth factor-β (TGFβ) superfamily, many members of which play key roles in kidney development. The purpose of this study was to define the role of this co-receptor on fetal murine kidney development. Stereological examination of embryonic and adult betaglycan heterozygous kidneys revealed augmented nephron number relative to littermate controls. Fetal heterozygous kidneys exhibited accelerated ureteric branching, which correlated with augmented nephron development at embryonic day (e) 15.5. In contrast, betaglycan null kidneys exhibited renal hypoplasia from e13.5 and reduced nephron number at e15.5. Quantitative real-time PCR analysis of e11.5–e14.5 kidneys demonstrated that heterozygous kidneys exhibited a transient decrease in Bmp4 expression at e11.5 and a subsequent cascade of changes in the gene regulatory network that governs metanephric development, including significant increases in Pax2, Eya1, Gdnf, Ret, Wnt4, and Wt1 expression. Conversely, gene expression in null kidneys was normal until e13.5, when significant reductions were detected in the expression of Bmp4 as well as other key metanephric regulatory genes. Tgfb1 and Tgfb2 mRNA expression was down-regulated in both nulls and heterozygotes at e13.5 and e14.5. The opposing morphological and molecular phenotypes in betaglycan heterozygote and null mutants demonstrate that the levels of betaglycan must be tightly regulated for optimal kidney development.

Structure of betaglycan zona pellucida (ZP)-C domain provides insights into ZP-mediated protein polymerization and TGF-beta binding

The zona pellucida (ZP) domain is a bipartite protein structural element comprised of ZP-N and ZP-C regions. Most notable for its ability to mediate protein polymerization, many ZP proteins polymerize and assemble into long fibrils that form specialized extracellular matrices. Other ZP proteins (namely, betaglycan and endoglin) do not polymerize but serve as important membrane coreceptors for ligands in the transforming growth factor-β (TGF-β) superfamily. Here, we present the 2.0-Å resolution crystal structure of the betaglycan ZP-C region in combination with a downstream region known as the external hydrophobic patch (EHP). Similar to the ZP-N region, the ZP-C region also adopts an immunoglobulin-like fold, despite sharing no sequence homology and possessing different disulfide linkages. The EHP region, which was previously thought to be external to the ZP region, is integral to the ZP-C domain and corresponds to the ZP-C G strand. Our structure also indicates that the critical maturation cleavage of ZP proteins, a process that activates nascent ZP proteins for polymerization, occurs within the immunoglobulin domain at the FG loop. Nonpolymerizing ZP proteins such as betaglycan and endoglin do not contain this cleavage site. Finally, our structure suggests that the AB loop and the convex surface pocket are regions important for TGF-β ligand binding.

The Local Control of the Pituitary by Activin Signaling and Modulation

The pituitary gland plays a prominent role in the control of many physiological processes. This control is achieved through the actions and interactions of hormones and growth factors that are produced and secreted by the endocrine cell types and the non-endocrine constituents that collectively and functionally define this complex organ. The five endocrine cell types of the anterior lobe of the pituitary, somatotropes, lactotropes, corticotropes, thyrotropes and gonadotropes, are defined by their primary product, growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH) and follicle stimulating hormone (FSH)/luteinizing hormone (LH). They are further distinguishable by the presence of cell surface receptors that display high affinity and selectivity for specific hypothalamic hormones and couple to appropriate downstream signaling pathways involved in the control of cell type specific responses, including the release and/or synthesis of pituitary hormones. Central control of the pituitary via the hypothalamus is further fine-tuned by the positive or negative actions of peripheral feedback signals and of a variety of factors that originate from sources within the pituitary. The focus of this review is the latter category of intrinsic factors that exert local control. Special emphasis is given to the TGF-β family of growth factors, in particular activin effects on the gonadotrope population, because a considerable body of evidence supports their contribution to the local modulation of the embryonic and postnatal pituitary as well as pituitary pathogenesis. A number of other substances, including members of the cytokine and FGF families, VEGF, IGF1, PACAP, Ghrelin, adenosine and nitric oxide have also been shown or implicated to function as autocrine/paracrine factors, though, definitive proof remains lacking in some cases. The ever-growing list of putative autocrine/paracrine factors of the pituitary nevertheless has highlighted the complexity of the local network and its impact on pituitary functions.

New targets for old hormones: inhibins clinical role revisited

Inhibins are gonadal peptide hormones belonging to the transforming growth factor-β (TGF-β) superfamily that regulate the pituitary follicle stimulating hormone (FSH) secretion by negative feedback mechanisms. It is evident that the understanding of inhibins function in the hypothalamic-pituitary-gonadal axis will provide insights into physiology and pathology of the gonadal function. In recent years, a great deal of attention has been focussed on clinical relevance of measuring circulating inhibins in normal and disease state. The past few years also have witnessed the emergence and discovery of extra pituitary action of inhibins that might provide further insights into the underlying diseases like cancer especially in the reproductive axis and various other new endocrine target organs. In this review after systematic analysis of literature, we discuss briefly the known and recent advances in function of these hormones highlighting also its structure, production and mechanisms of signal transduction. Also this review discusses about the physiological relevance of inhibin association in the normal function to the development of reproductive cancers. Finally, we describe evidence from various emerging studies that inhibins make an important contribution to other physiological functions apart from reproduction which reveals new endocrine target organs of inhibins. The emerging view is inhibin participates in multiple ways to regulate the function in different cell types and still complete repertoire of its actions is under investigation.

Roles for the type III TGF-beta receptor in human cancer

Transforming growth factor beta (TGF-beta) superfamily ligands have important roles in regulating cellular homeostasis, embryonic development, differentiation, proliferation, immune surveillance, angiogenesis, motility, and apoptosis in a cell type and context specific manner. TGF-beta superfamily signaling pathways also have diverse roles in human cancer, functioning to either suppress or promote cancer progression. The TGF-beta superfamily co-receptor, the type III TGF-beta receptor (TbetaRIII, also known as betaglycan) mediates TGF-beta superfamily ligand dependent as well as ligand independent signaling to both Smad and non-Smad signaling pathways. Loss of TbetaRIII expression during cancer progression and direct effects of TbetaRIII on regulating cell migration, invasion, proliferation, and angiogenesis support a role for TbetaRIII as a suppressor of cancer progression and/or as a metastasis suppressor. Defining the physiological function and mechanism of TbetaRIII action and alterations in TbetaRIII function during cancer progression should enable more effective targeting of TbetaRIII and TbetaRIII mediated functions for the diagnosis and treatment of human cancer.

Inhibins: from reproductive hormones to tumor suppressors

Inhibins are peptide hormones shown originally to be produced by the gonads to regulate the secretion of follicle stimulating hormone by pituitary gonadotropes. Although gonadotropes have been regarded as the canonical inhibin target cells, in recent years extrapituitary actions of inhibins have come into the spotlight. In particular, disruptions to the local actions of inhibins in peripheral tissues might underlie certain diseases, especially cancers of the reproductive tract. This review focuses on recent advances in the inhibin field, with a particular emphasis on the determinants of inhibin availability, mechanisms of inhibin action, and the physiological relevancy of local inhibin actions in the development and progression of reproductive cancers.

Inhibin-A antagonizes TGFbeta2 signaling by down-regulating cell surface expression of the TGFbeta coreceptor betaglycan

Inhibin is an atypical member of the TGFbeta family of signaling ligands and is classically understood to function via competitive antagonism of activin ligand binding. Inhibin-null (Inha-/-) mice develop both gonadal and adrenocortical tumors, the latter of which depend upon gonadectomy for initiation. We have previously shown that gonadectomy initiates adrenal tumorigenesis in Inha-/- mice by elevating production of LH, which drives aberrant proliferation and differentiation of subcapsular adrenocortical progenitor cells. In this study, we demonstrate that LH signaling specifically up-regulates expression of TGFbeta2 in the subcapsular region of the adrenal cortex, which coincides with regions of aberrant Smad3 activation in Inha-/- adrenal glands. Consistent with a functional interaction between inhibin and TGFbeta2, we further demonstrate that recombinant inhibin-A antagonizes signaling by TGFbeta2 in cultured adrenocortical cells. The mechanism of this antagonism depends upon the mutual affinity of inhibin-A and TGFbeta2 for the signaling coreceptor betaglycan. Although inhibin-A cannot physically displace TGFbeta2 from its binding sites on betaglycan, binding of inhibin-A to the cell surface causes endocytic internalization of betaglycan, thereby reducing the number of available binding sites for TGFbeta2 on the cell surface. The mechanism by which inhibin-A induces betaglycan internalization is clathrin independent, making it distinct from the mechanism by which TGFbeta ligands themselves induce betaglycan internalization. These data indicate that inhibin can specifically antagonize TGFbeta2 signaling in cellular contexts where surface expression of betaglycan is limiting and provide a novel mechanism for activin-independent phenotypes in Inha-/- mice.

Betaglycan has two independent domains required for high affinity TGF-beta binding: proteolytic cleavage separates the domains and inactivates the neutralizing activity of the soluble receptor

Betaglycan is a coreceptor for members of the transforming growth factor beta (TGF-beta) superfamily. Mutagenesis has identified two ligand binding regions, one at the membrane-distal and the other at the membrane-proximal half of the betaglycan ectodomain. Here we show that partial plasmin digestion of soluble betaglycan produces two proteolysis-resistant fragments of 45 and 55 kDa, consistent with the predicted secondary structure, which indicates an intervening nonstructured linker region separating the highly structured N- and C-terminal domains. Amino terminal sequencing indicates that the 45 and 55 kDa fragments correspond, respectively, to the membrane-distal and -proximal regions. Plasmin treatment of membrane betaglycan results in the production of equivalent proteolysis-resistant fragments. The 45 and 55 kDa fragments, as well as their recombinant soluble counterparts, Sol Delta10 and Sol Delta11, bind TGF-beta, but nonetheless, compared to intact soluble betaglycan, have a severely diminished ability to block TGF-beta activity. Surface plasmon resonance (SPR) analysis indicates that soluble betaglycan has K(d)'s in the low nanomolar range for the three TGF-beta isoforms, while those for Sol Delta10 and Sol Delta11 are 1-2 orders of magnitude higher. SPR analysis further shows that the K(d)'s of Sol Delta11 are not changed in the presence of Sol Delta10, indicating that the high affinity of soluble betaglycan is a consequence of tethering the domains together. Overall, these results suggest that betaglycan ectodomain exhibits a bilobular structure in which each lobule folds independently and binds TGF-beta through distinct nonoverlapping interfaces and that linker modification may be an approach to improve soluble betaglycan's TGF-beta neutralizing activity.

The RGM/DRAGON family of BMP co-receptors

The BMP signaling pathway controls a number of cell processes during development and in adult tissues. At the cellular level, ligands of the BMP family act by binding a hetero-tetrameric signaling complex, composed of two type I and two type II receptors. BMP ligands make use of a limited number of receptors, which in turn activate a common signal transduction cascade at the intracellular level. A complex regulatory network is required in order to activate the signaling cascade at proper times and locations, and to generate specific downstream effects in the appropriate cellular context. One such regulatory mechanism is the repulsive guidance molecule (RGM) family of BMP co-receptors. This article reviews the current knowledge regarding the structure, regulation, and function of RGMs, focusing on known and potential roles of RGMs in physiology and pathophysiology.

Regulation of osteoblastogenesis and osteoclastogenesis by the other reproductive hormones, Activin and Inhibin

There is both cellular and physiological evidence demonstrating that both Activins and Inhibins regulate osteoblastogenesis and osteoclastogenesis, and regulate bone mass in vivo. Although Activins and Inhibins were initially isolated from the gonad, Activins are also produced and stored in bone, whereas Inhibins exert their regulation on bone cell differentiation and metabolism via endocrine effects. The accumulating data provide evidence that reproductive hormones, distinct from classical sex steroids, are important regulators of bone mass and bone strength. Given the well described dominant antagonism of Inhibin over Activin, as well as over BMPs and TGFbeta, the gonadally derived Inhibins are important regulators of locally produced osteotrophic factors. Thus, the cycling Inhibins in females and diurnal changes in Inhibin B in males elicit temporal shifts in Inhibin levels (tone) that de-repress the pituitary. This fundamental action has the potential to de-repress locally stimulated changes in osteoblastogenesis and osteoclastogenesis, thereby altering bone metabolism.