An Activin Mutant with Disrupted ALK4 Binding Blocks Signaling via Type II Receptors

Gastrointestinal pharmacology activins in liver health and disease

Activins are a subgroup of the TGFβ superfamily of growth and differentiation factors, dimeric in nature and consisting of two inhibin beta subunits linked via a disulfide bridge. Canonical activin signaling occurs through Smad2/3, with negative feedback initiated by Smad6/7 following signal transduction, which binds activin type I receptor preventing phosphorylation of Smad2/3 and activation of downstream signaling. In addition to Smad6/7, other inhibitors of activin signaling have been identified as well, including inhibins (dimers of an inhibin alpha and beta subunit), BAMBI, Cripto, follistatin, and follistatin-like 3 (fstl3). To date, activins A, B, AB, C, and E have been identified and isolated in mammals, with activin A and B having the most characterization of biological activity. Activin A has been implicated as a regulator of several important functions of liver biology, including hepatocyte proliferation and apoptosis, ECM production, and liver regeneration; the role of other subunits of activin in liver physiology are less understood. There is mounting data to suggest a link between dysregulation of activins contributing to various hepatic diseases such as inflammation, fibrosis, and hepatocellular carcinoma, and emerging studies demonstrating the protective and regenerative effects of inhibiting activins in mouse models of liver disease. Due to their importance in liver biology, activins demonstrate utility as a therapeutic target for the treatment of hepatic diseases such as cirrhosis, NASH, NAFLD, and HCC; further research regarding activins may provide diagnostic or therapeutic opportunity for those suffering from various liver diseases.

Human INHBB Gene Variant (c.1079T>C:p.Met360Thr) Alters Testis Germ Cell Content, but Does Not Impact Fertility in Mice

Testicular-derived inhibin B (α/β B dimers) acts in an endocrine manner to suppress pituitary production of follicle-stimulating hormone (FSH), by blocking the actions of activins (β A/B/β A/B dimers). Previously, we identified a homozygous genetic variant (c.1079T>C:p.Met360Thr) arising from uniparental disomy of chromosome 2 in the INHBB gene (β B-subunit of inhibin B and activin B) in a man suffering from infertility (azoospermia). In this study, we aimed to test the causality of the p.Met360Thr variant in INHBB and testis function. Here, we used CRISPR/Cas9 technology to generate InhbbM364T/M364T mice, where mouse INHBB p.Met364 corresponds with human p.Met360. Surprisingly, we found that the testes of male InhbbM364T/M364T mutant mice were significantly larger compared with those of aged-matched wildtype littermates at 12 and 24 weeks of age. This was attributed to a significant increase in Sertoli cell and round spermatid number and, consequently, seminiferous tubule area in InhbbM364T/M364T males compared to wildtype males. Despite this testis phenotype, male InhbbM364T/M364T mutant mice retained normal fertility. Serum hormone analyses, however, indicated that the InhbbM364T variant resulted in reduced circulating levels of activin B but did not affect FSH production. We also examined the effect of this p.Met360Thr and an additional INHBB variant (c.314C>T: p.Thr105Met) found in another infertile man on inhibin B and activin B in vitro biosynthesis. We found that both INHBB variants resulted in a significant disruption to activin B in vitro biosynthesis. Together, this analysis supports that INHBB variants that limit activin B production have consequences for testis composition in males.

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.

Activin A forms a non-signaling complex with ACVR1 and type II Activin/BMP receptors via its finger 2 tip loop

Activin A functions in BMP signaling in two ways: it either engages ACVR1B to activate Smad2/3 signaling or binds ACVR1 to form a non-signaling complex (NSC). Although the former property has been studied extensively, the roles of the NSC remain unexplored. The genetic disorder fibrodysplasia ossificans progressiva (FOP) provides a unique window into ACVR1/Activin A signaling because in that disease Activin can either signal through FOP-mutant ACVR1 or form NSCs with wild-type ACVR1. To explore the role of the NSC, we generated 'agonist-only' Activin A muteins that activate ACVR1B but cannot form the NSC with ACVR1. Using one of these muteins, we demonstrate that failure to form the NSC in FOP results in more severe disease pathology. These results provide the first evidence for a biological role for the NSC in vivo and pave the way for further exploration of the NSC's physiological role in corresponding knock-in mice.

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.

Gene Expression Profiling Confirms the Dosage-Dependent Additive Neuroprotective Effects of Jasminoidin in a Mouse Model of Ischemia-Reperfusion Injury

Recent evidence demonstrates that a double dose of Jasminoidin (2·JA) is more effective than Jasminoidin (JA) in cerebral ischemia therapy, but its dosage-effect mechanisms are unclear. In this study, the software GeneGo MetaCore was used to perform pathway analysis of the differentially expressed genes obtained in microarrays of mice belonging to four groups (Sham, Vehicle, JA, and 2·JA), aiming to elucidate differences in JA and 2·JA's dose-dependent pharmacological mechanism from a system's perspective. The top 10 enriched pathways in the 2·JA condition were mainly involved in neuroprotection (70% of the pathways), apoptosis and survival (40%), and anti-inflammation (20%), while JA induced pathways were mainly involved in apoptosis and survival (60%), anti-inflammation (20%), and lipid metabolism (20%). Regarding shared pathways and processes, 3, 1, and 3 pathways overlapped between the Vehicle and JA, Vehicle and 2·JA, and JA and 2·JA conditions, respectively; for the top ten overlapped processes these numbers were 3, 0, and 4, respectively. The common pathways and processes in the 2·JA condition included differentially expressed genes significantly different from those in JA. Seven representative pathways were only activated by 2·JA, such as Gamma-Secretase regulation of neuronal cell development. Process network comparison indicated that significant nodes, such as alpha-MSH, ACTH, PKR1, and WNT, were involved in the pharmacological mechanism of 2·JA. Function distribution was different between JA and 2·JA groups, indicating a dosage additive mechanism in cerebral ischemia treatment. Such systemic approach based on whole-genome multiple pathways and networks may provide an effective and alternative approach to identify alterations underlining dosage-dependent therapeutic benefits of pharmacological compounds on complex disease processes.

The immunophilin FKBP12 inhibits hepcidin expression by binding the BMP type I receptor ALK2 in hepatocytes

The expression of the key regulator of iron homeostasis hepcidin is activated by the BMP-SMAD pathway in response to iron and inflammation and among drugs, by rapamycin, which inhibits mTOR in complex with the immunophilin FKBP12. FKBP12 interacts with BMP type I receptors to avoid uncontrolled signaling. By pharmacologic and genetic studies, we identify FKBP12 as a novel hepcidin regulator. Sequestration of FKBP12 by rapamycin or tacrolimus activates hepcidin both in vitro and in murine hepatocytes. Acute tacrolimus treatment transiently increases hepcidin in wild-type mice. FKBP12 preferentially targets the BMP receptor ALK2. ALK2 mutants defective in binding FKBP12 increase hepcidin expression in a ligand-independent manner, through BMP-SMAD signaling. ALK2 free of FKBP12 becomes responsive to the noncanonical inflammatory ligand Activin A. Our results identify a novel hepcidin regulator and a potential therapeutic target to increase defective BMP signaling in disorders of low hepcidin.

Activin A/Smads signaling pathway negatively regulates Oxygen Glucose Deprivation-induced autophagy via suppression of JNK and p38 MAPK pathways in neuronal PC12 cells

Activin A (Act A), a member of the transforming growth factor-beta (TGF-β), reduces neuronal apoptosis during cerebral ischemia through Act A/Smads signaling pathway. However, little is known about the effect of Act A/Smads pathway on autophagy in neurons. Here, we found that oxygen-glucose deprivation (OGD)-induced autophagy was suppressed by exogenous Act A in a concentration-dependent manner and enhanced by Act A/Smads pathway inhibitor (ActRIIA-Ab) in neuronal PC12 cells. These results indicate that Act A/Smads pathway negatively regulates autophagy in OGD-treated PC12 cells. In addition, we found that c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein (MAP) kinase pathways are involved in the OGD-induced autophagy. The activation of JNK and p38 MAPK pathways in OGD-treated PC12 cells was suppressed by exogenous Act A and enhanced by ActRIIA-Ab. Together, our results suggest that Act A/Smads signaling pathway negatively regulates OGD-induced autophagy via suppression of JNK and p38 MAPK pathways in neuronal PC12 cells.

The TGF-β Signalling Network in Muscle Development, Adaptation and Disease

Skeletal muscle possesses remarkable ability to change its size and force-producing capacity in response to physiological stimuli. Impairment of the cellular processes that govern these attributes also affects muscle mass and function in pathological conditions. Myostatin, a member of the TGF-β family, has been identified as a key regulator of muscle development, and adaptation in adulthood. In muscle, myostatin binds to its type I (ALK4/5) and type II (ActRIIA/B) receptors to initiate Smad2/3 signalling and the regulation of target genes that co-ordinate the balance between protein synthesis and degradation. Interestingly, evidence is emerging that other TGF-β proteins act in concert with myostatin to regulate the growth and remodelling of skeletal muscle. Consequently, dysregulation of TGF-β proteins and their associated signalling components is increasingly being implicated in muscle wasting associated with chronic illness, ageing, and inactivity. The growing understanding of TGF-β biology in muscle, and its potential to advance the development of therapeutics for muscle-related conditions is reviewed here.

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.

Cumulin, an Oocyte-secreted Heterodimer of the Transforming Growth Factor-β Family, Is a Potent Activator of Granulosa Cells and Improves Oocyte Quality

Growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15) are oocyte-specific growth factors with central roles in mammalian reproduction, regulating species-specific fecundity, ovarian follicular somatic cell differentiation, and oocyte quality. In the human, GDF9 is produced in a latent form, the mechanism of activation being an open question. Here, we produced a range of recombinant GDF9 and BMP15 variants, examined their in silico and physical interactions and their effects on ovarian granulosa cells (GC) and oocytes. We found that the potent synergistic actions of GDF9 and BMP15 on GC can be attributed to the formation of a heterodimer, which we have termed cumulin. Structural modeling of cumulin revealed a dimerization interface identical to homodimeric GDF9 and BMP15, indicating likely formation of a stable complex. This was confirmed by generation of recombinant heterodimeric complexes of pro/mature domains (pro-cumulin) and covalent mature domains (cumulin). Both pro-cumulin and cumulin exhibited highly potent bioactivity on GC, activating both SMAD2/3 and SMAD1/5/8 signaling pathways and promoting proliferation and expression of a set of genes associated with oocyte-regulated GC differentiation. Cumulin was more potent than pro-cumulin, pro-GDF9, pro-BMP15, or the two combined on GC. However, on cumulus-oocyte complexes, pro-cumulin was more effective than all other growth factors at notably improving oocyte quality as assessed by subsequent day 7 embryo development. Our results support a model of activation for human GDF9 dependent on cumulin formation through heterodimerization with BMP15. Oocyte-secreted cumulin is likely to be a central regulator of fertility in mono-ovular mammals.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

Virtual High-Throughput Screening To Identify Novel Activin Antagonists

Activin belongs to the TGFβ superfamily, which is associated with several disease conditions, including cancer-related cachexia, preterm labor with delivery, and osteoporosis. Targeting activin and its related signaling pathways holds promise as a therapeutic approach to these diseases. A small-molecule ligand-binding groove was identified in the interface between the two activin βA subunits and was used for a virtual high-throughput in silico screening of the ZINC database to identify hits. Thirty-nine compounds without significant toxicity were tested in two well-established activin assays: FSHβ transcription and HepG2 cell apoptosis. This screening workflow resulted in two lead compounds: NUCC-474 and NUCC-555. These potential activin antagonists were then shown to inhibit activin A-mediated cell proliferation in ex vivo ovary cultures. In vivo testing showed that our most potent compound (NUCC-555) caused a dose-dependent decrease in FSH levels in ovariectomized mice. The Blitz competition binding assay confirmed target binding of NUCC-555 to the activin A:ActRII that disrupts the activin A:ActRII complex's binding with ALK4-ECD-Fc in a dose-dependent manner. The NUCC-555 also specifically binds to activin A compared with other TGFβ superfamily member myostatin (GDF8). These data demonstrate a new in silico-based strategy for identifying small-molecule activin antagonists. Our approach is the first to identify a first-in-class small-molecule antagonist of activin binding to ALK4, which opens a completely new approach to inhibiting the activity of TGFβ receptor superfamily members. in addition, the lead compound can serve as a starting point for lead optimization toward the goal of a compound that may be effective in activin-mediated diseases.

Engineering TGF-β superfamily ligands for clinical applications

TGF-β superfamily ligands govern normal tissue development and homeostasis, and their dysfunction is a hallmark of many diseases. These ligands are also well defined both structurally and functionally. This review focuses on TGF-β superfamily ligand engineering for therapeutic purposes, in particular for regenerative medicine and musculoskeletal disorders. We describe the key discovery that structure-guided mutation of receptor-binding epitopes, especially swapping of these epitopes between ligands, results in new ligands with unique functional properties that can be harnessed clinically. Given the promising results with prototypical engineered TGF-β superfamily ligands, and the vast number of such molecules that remain to be produced and tested, this strategy is likely to hold great promise for the development of new biologics.

Activin A balance regulates epithelial invasiveness and tumorigenesis

Activin A (Act A) is a member of the TGFβ superfamily. Act A and TGFβ have multiple common downstream targets and have been described to merge in their intracellular signaling cascades and function. We have previously demonstrated that coordinated loss of E-cadherin and TGFβ receptor II (TβRII) results in epithelial cell invasion. When grown in three-dimensional organotypic reconstruct cultures, esophageal keratinocytes expressing dominant-negative mutants of E-cadherin and TβRII showed activated Smad2 in the absence of functional TβRII. However, we could show that increased levels of Act A secretion was able to induce Smad2 phosphorylation. Growth factor secretion can activate autocrine and paracrine signaling, which affects crosstalk between the epithelial compartment and the surrounding microenvironment. We show that treatment with the Act A antagonist Follistatin or with a neutralizing Act A antibody can increase cell invasion in organotypic cultures in a fibroblast- and MMP-dependent manner. Similarly, suppression of Act A with shRNA increases cell invasion and tumorigenesis in vivo. Therefore, we conclude that maintaining a delicate balance of Act A expression is critical for homeostasis in the esophageal microenvironment.

Species differences in the expression and activity of bone morphogenetic protein 15

Oocyte-derived bone morphogenetic protein 15 (BMP15) regulates ovulation rate and female fertility in a species-specific manner, being important in humans and sheep and largely superfluous in mice. To understand these species differences, we have compared the expression and activity of human, murine, and ovine BMP15. In HEK293F cells, human BMP15 is highly expressed (120 ng/ml), ovine BMP15 is poorly expressed (15 ng/ml), and murine BMP15 is undetectable. Because BMP15 synthesis is dependent upon interactions between the N-terminal prodomain and the C-terminal mature domain, we used site-directed mutagenesis to identify four prodomain residues (Glu(46), Glu(47), Leu(49), and Glu(50)) that mediate the high expression of human BMP15. Substituting these residues into the prodomains of murine and ovine BMP15 led to significant increases in growth factor expression; however, maximal expression was achieved only when the entire human prodomain was linked to the mature domains of the other species. Using these chimeric constructs, we produced and purified murine and ovine BMP15 and showed that in a COV434 granulosa cell bioassay, these molecules displayed little activity relative to human BMP15 (EC(50) 0.2nM). Sequence analysis suggested that the disparity in activity could be due to species differences at the type I receptor binding interface. Indeed, murine BMP15 activity was restored when specific residues through this region (Pro(329)/Tyr(330)) were replaced with the corresponding residues (Arg(329)/Asp(330)) from human BMP15. The identified differences in the expression and activity of BMP15 likely underlie the relative importance of this growth factor between species.

Activin signaling regulates Sertoli cell differentiation and function

Throughout development, activin A signaling stimulates proliferation and inhibits differentiation of testicular Sertoli cells. A decline in activin levels at puberty corresponds with the differentiation of Sertoli cells that is required to sustain spermatogenesis. In this study, we consider whether terminally differentiated Sertoli cells can revert to a functionally immature phenotype in response to activin A. To increase systemic activin levels, the right tibialis anterior muscle of 7-wk-old C57BL/6J mice was transduced with an adeno-associated virus (rAAV6) expressing activin A. We show that chronic activin signaling reduces testis mass by 23.5% compared with control animals and induces a hypospermatogenic phenotype, consistent with a failure of Sertoli cells to support spermatogenesis. We use permeability tracers and transepithelial electrical resistance measurements to demonstrate that activin potently disrupts blood-testis-barrier function in adult mice and ablates tight junction formation in differentiated primary Sertoli cells, respectively. Furthermore, increased activin signaling reinitiates a program of cellular proliferation in primary Sertoli cells as determined by 5-ethynyl-2'-deoxyuridine incorporation. Proliferative cells reexpress juvenile markers, including cytokeratin-18, and suppress mature markers, including claudin-11. Thus, activin A is the first identified factor capable of reprogramming Sertoli cells to an immature, dedifferentiated phenotype. This study indicates that activin signaling must be strictly controlled in the adult in order to maintain Sertoli cell function in spermatogenesis.

Myostatin inhibits myosatellite cell proliferation and consequently activates differentiation: evidence for endocrine-regulated transcript processing

Myostatin is a potent negative regulator of muscle growth in mammals. Despite high structural conservation, functional conservation in nonmammalian species is only assumed. This is particularly true for fish due to the presence of several myostatin paralogs: two in most species and four in salmonids (MSTN-1a, -1b, -2a, and -2b). Rainbow trout are a rich source of primary myosatellite cells as hyperplastic muscle growth occurs even in adult fish. These cells were therefore used to determine myostatin's effects on proliferation whereas our earlier studies reported its effects on quiescent cells. As in mammals, recombinant myostatin suppressed proliferation with no changes in cell morphology. Expression of MSTN-1a was several fold higher than the other paralogs and was autoregulated by myostatin, which also upregulated the expression of key differentiation markers: Myf5, MyoD1, myogenin, and myosin light chain. Thus, myostatin-stimulated cellular growth inhibition activates rather than represses differentiation. IGF-1 stimulated proliferation but had minimal and delayed effects on differentiation and its actions were suppressed by myostatin. However, IGF-1 upregulated MSTN-2a expression and the processing of its transcript, which is normally unprocessed. Myostatin therefore appears to partly mediate IGF-stimulated myosatellite differentiation in rainbow trout. This also occurs in mammals, although the IGF-stimulated processing of MSTN-2a transcripts is highly unique and is indicative of subfunctionalization within the gene family. These studies also suggest that the myokine's actions, including its antagonistic relationship with IGF-1, are conserved and that the salmonid gene family is functionally diverging.

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.

Promiscuity and specificity in BMP receptor activation

Bone Morphogenetic Proteins (BMPs), together with Transforming Growth Factor (TGF)-β and Activins/Inhibins constitute the TGF-β superfamily of ligands. This superfamily is formed by more than 30 structurally related secreted proteins. Since TGF-β members act as morphogens, either a strict relation between a particular ligand to a distinct cellular receptor and/or temporospatial expression patterns of ligands and receptors is expected. Instead, only a limited number of receptors exist implicating promiscuous interactions of ligands and receptors. Furthermore, in complex tissues a multitude of different ligands can be found, which signal via overlapping subsets of receptors. This raises the intriguing question how concerted interactions of different ligands and receptors generate highly specific cellular signals, which are required during development and tissue homeostasis.

Designer TGFβ superfamily ligands with diversified functionality

Transforming Growth Factor – beta (TGFβ) superfamily ligands, including Activins, Growth and Differentiation Factors (GDFs), and Bone Morphogenetic Proteins (BMPs), are excellent targets for protein-based therapeutics because of their pervasiveness in numerous developmental and cellular processes. We developed a strategy termed RASCH (Random Assembly of Segmental Chimera and Heteromer), to engineer chemically-refoldable TGFβ superfamily ligands with unique signaling properties. One of these engineered ligands, AB208, created from Activin-βA and BMP-2 sequences, exhibits the refolding characteristics of BMP-2 while possessing Activin-like signaling attributes. Further, we find several additional ligands, AB204, AB211, and AB215, which initiate the intracellular Smad1-mediated signaling pathways more strongly than BMP-2 but show no sensitivity to the natural BMP antagonist Noggin unlike natural BMP-2. In another design, incorporation of a short N-terminal segment from BMP-2 was sufficient to enable chemical refolding of BMP-9, without which was never produced nor refolded. Our studies show that the RASCH strategy enables us to expand the functional repertoire of TGFβ superfamily ligands through development of novel chimeric TGFβ ligands with diverse biological and clinical values.

Generation of a specific activin antagonist by modification of the activin A propeptide

Elevated activin A levels in inhibin-deficient mice promote the development of gonadal tumors and induce cachexia by reducing muscle, liver, stomach, and fat mass. Because activin A is an important regulator of tissue growth, inhibiting the actions of this TGFβ family ligand may halt or reverse pathology in diseased tissues. In this study, we modified the activin A propeptide to generate a specific activin antagonist. Propeptides mediate the synthesis and secretion of all TGFβ ligands and, for some family members (e.g. TGFβ1), bind the mature growth factor with high enough affinity to confer latency. By linking the C-terminal region of the TGFβ1 propeptide to the N-terminal region of the activin A propeptide, we generated a chimeric molecule [activin/TGFβ1 propeptide (AT propeptide)] with increased affinity for activin A. The AT propeptide was 30-fold more potent than the activin A propeptide at suppressing activin-induced FSH release by LβT2 pituitary gonadotrope cells. Binding of the AT propeptide to activin A shields the type II receptor binding site, thereby reducing Smad2 phosphorylation and downstream signaling. In comparison with the commonly used activin antagonists, follistatin (IC(50) 0.42 nM), soluble activin type II receptor A-Fc (IC(50) 0.47 nM), and soluble activin type II receptor B-Fc (IC(50) 0.91 nM), the AT propeptide (IC(50) 2.6 nM) was slightly less potent. However, it was more specific, inhibiting activin A and activin B (IC(50) 10.26 nM) but not the closely related ligands, myostatin and growth differentiation factor-11. As such, the AT propeptide represents the first specific activin antagonist, and it should be an effective reagent for blocking activin actions in vivo.

The TβR-I pre-helix extension is structurally ordered in the unbound form and its flanking prolines are essential for binding

Transforming growth factor β isoforms (TGF-β) are among the most recently evolved members of a signaling superfamily with more than 30 members. TGF-β play vital roles in regulating cellular growth and differentiation, and they signal through a highly restricted subset of receptors known as TGF-β type I receptor (TβR-I) and TGF-β type II receptor (TβR-II). TGF-β's specificity for TβR-I has been proposed to arise from its pre-helix extension, a five-residue loop that binds in the cleft between TGF-β and TβR-II. The structure and backbone dynamics of the unbound form of the TβR-I extracellular domain were determined using NMR to investigate the extension's role in binding. This showed that the unbound form is highly similar to the bound form in terms of both the β-strand framework that defines the three-finger toxin fold and the extension and its characteristic cis-Ile54-Pro55 peptide bond. The NMR data further showed that the extension and two flanking 3(10) helices are rigid on the nanosecond-to-picosecond timescale. The functional significance of several residues within the extension was investigated by binding studies and reporter gene assays in cultured epithelial cells. These demonstrated that the pre-helix extension is essential for binding, with Pro55 and Pro59 each playing a major role. These findings suggest that the pre-helix extension and its flanking prolines evolved to endow the TGF-β signaling complex with its unique specificity, departing from the ancestral promiscuity of the bone morphogenetic protein subfamily, where the binding interface of the type I receptor is highly flexible.

Growth factors and myometrium: biological effects in uterine fibroid and possible clinical implications

BACKGROUND

Growth factors are proteins secreted by a number of cell types that are capable of modulating cellular growth, proliferation and cellular differentiation. It is well accepted that uterine cellular events such as proliferation and differentiation are regulated by sex steroids and their actions in target tissues are mediated by local production of growth factors acting through paracrine and/or autocrine mechanisms. Myometrial mass is ultimately modified in pregnancy as well as in tumour conditions such as leiomyoma and leiomyosarcoma. Leiomyomas, also known as fibroids, are benign tumours of the uterus, considered to be one of the most frequent causes of infertility in reproductive years in women.

METHODS

For this review, we searched the database MEDLINE and Google Scholar for articles with content related to growth factors acting on myometrium; the findings are hereby reviewed and discussed.

RESULTS

Different growth factors such as epidermal growth factor (EGF), transforming growth factor-α (TGF-α), heparin-binding EGF (HB-EGF), acidic fibroblast growth factor (aFGF), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF), platelet-derived growth factor (PDGF) and TGF-β perform actions in myometrium and in leiomyomas. In addition to these growth factors, activin and myostatin have been recently identified in myometrium and leiomyoma.

CONCLUSIONS

Growth factors play an important role in the mechanisms involved in myometrial patho-physiology.

Antagonism of activin by activin chimeras

Activins are pluripotent hormones/growth factors that belong to the TGF-β superfamily of growth and differentiation factors (GDFs). They play a role in cell growth, differentiation and apoptosis, endocrine function, metabolism, wound repair, immune responses, homeostasis, mesoderm induction, bone growth, and many other biological processes. Activins and the related bone morphogenic proteins (BMPs) transduce their signal through two classes of single transmembrane receptors. The receptors possess intracellular serine/threonine kinase domains. Signaling occurs when the constitutively active type II kinase domain phosphorylates the type I receptor, which upon activation, phosphorylates intracellular signaling molecules. To generate antagonistic ligands, we generated chimeric molecules that disrupt the receptor interactions and thereby the phosphorylation events. The chimeras were designed based on available structural data to maintain high-affinity binding to type II receptors. The predicted type I receptor interaction region was replaced by residues present in inactive homologs or in related ligands with different type I receptor affinities.

Phylogenomic analyses reveal the evolutionary origin of the inhibin alpha-subunit, a unique TGFbeta superfamily antagonist

Transforming growth factor-beta (TGFβ) homologues form a diverse superfamily that arose early in animal evolution and control cellular function through membrane-spanning, conserved serine-threonine kinases (RII and RI receptors). Activin and inhibin are related dimers within the TGFβ superfamily that share a common β-subunit. The evolution of the inhibin α-subunit created the only antagonist within the TGFβ superfamily and the only member known to act as an endocrine hormone. This hormone introduced a new level of complexity and control to vertebrate reproductive function. The novel functions of the inhibin α-subunit appear to reflect specific insertion-deletion changes within the inhibin β-subunit that occurred during evolution. Using phylogenomic analysis, we correlated specific insertions with the acquisition of distinct functions that underlie the phenotypic complexity of vertebrate reproductive processes. This phylogenomic approach presents a new way of understanding the structure-function relationships between inhibin, activin, and the larger TGFβ superfamily.

Activins and follistatins: Emerging roles in liver physiology and cancer

Activins are secreted proteins belonging to the TGF-β family of signaling molecules. Activin signals are crucial for differentiation and regulation of cell proliferation and apoptosis in multiple tissues. Signal transduction by activins relies mainly on the Smad pathway, although the importance of crosstalk with additional pathways is increasingly being recognized. Activin signals are kept in balance by antagonists at multiple levels of the signaling cascade. Among these, follistatin and FLRG, two members of the emerging family of follistatin-like proteins, can bind secreted activins with high affinity, thereby blocking their access to cell surface-anchored activin receptors. In the liver, activin A is a major negative regulator of hepatocyte proliferation and can induce apoptosis. The functions of other activins expressed by hepatocytes have yet to be more clearly defined. Deregulated expression of activins and follistatin has been implicated in hepatic diseases including inflammation, fibrosis, liver failure and primary cancer. In particular, increased follistatin levels have been found in the circulation and in the tumor tissue of patients suffering from hepatocellular carcinoma as well as in animal models of liver cancer. It has been argued that up-regulation of follistatin protects neoplastic hepatocytes from activin-mediated growth inhibition and apoptosis. The use of follistatin as biomarker for liver tumor development is impeded, however, due to the presence of elevated follistatin levels already during preceding stages of liver disease. The current article summarizes our evolving understanding of the multi-faceted activities of activins and follistatins in liver physiology and cancer.

Delayed activin A administration attenuates tissue death after transient focal cerebral ischemia and is associated with decreased stress-responsive kinase activation

Focal cerebral ischemia and reperfusion initiates complex cellular and molecular interactions that lead to either cell repair or destruction. In earlier work, we found that activin A is an early gene response to cerebral ischemia and supports cortical neuron survival in vitro. In this study, the ability of exogenous activin A to attenuate injury from transient middle cerebral artery occlusion was tested in adult mice. Intracerebroventricular administration of activin A prior to middle cerebral artery occlusion reduced infarct volume apparent 1 day after experimental stroke. A single activin A administration at 6 h following ischemia/reperfusion reduced lesion volumes at 1 and 3 days and led to improved neurobehavior. Moreover, activin A treatment spared neurons within the ischemic hemisphere and led to a concomitant reduction in microglial activation. Activation of the stress-responsive kinases p38 and c-jun N-terminal kinase implicated in neuronal apoptosis after stroke was reduced following activin A treatment. Together these findings suggest that activin A promotes tissue survival after focal cerebral ischemia/reperfusion with an extended therapeutic window.

The structure of myostatin:follistatin 288: insights into receptor utilization and heparin binding

Myostatin is a member of the transforming growth factor-beta (TGF-beta) family and a strong negative regulator of muscle growth. Here, we present the crystal structure of myostatin in complex with the antagonist follistatin 288 (Fst288). We find that the prehelix region of myostatin very closely resembles that of TGF-beta class members and that this region alone can be swapped into activin A to confer signalling through the non-canonical type I receptor Alk5. Furthermore, the N-terminal domain of Fst288 undergoes conformational rearrangements to bind myostatin and likely acts as a site of specificity for the antagonist. In addition, a unique continuous electropositive surface is created when myostatin binds Fst288, which significantly increases the affinity for heparin. This translates into stronger interactions with the cell surface and enhanced myostatin degradation in the presence of either Fst288 or Fst315. Overall, we have identified several characteristics unique to myostatin that will be paramount to the rational design of myostatin inhibitors that could be used in the treatment of muscle-wasting disorders.

Blockade of Cripto binding to cell surface GRP78 inhibits oncogenic Cripto signaling via MAPK/PI3K and Smad2/3 pathways

Cripto is a developmental oncoprotein that signals via mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3K)/Akt and Smad2/3 pathways. However, the molecular basis for Cripto coupling to these pathways during embryogenesis and tumorigenesis is not fully understood. In this regard, we recently demonstrated that Cripto forms a cell surface complex with the HSP70 family member glucose-regulated protein-78 (GRP78). Here, we provide novel functional evidence demonstrating that cell surface GRP78 is a necessary mediator of Cripto signaling in human tumor, mammary epithelial and embryonic stem cells. We show that targeted disruption of the cell surface Cripto/GRP78 complex using shRNAs or GRP78 immunoneutralization precludes Cripto activation of MAPK/PI3K pathways and modulation of activin-A, activin-B, Nodal and transforming growth factor-beta1 signaling. We further demonstrate that blockade of Cripto binding to cell surface GRP78 prevents Cripto from increasing cellular proliferation, downregulating E-Cadherin, decreasing cell adhesion and promoting pro-proliferative responses to activin-A and Nodal. Thus, disrupting the Cripto/GRP78 binding interface blocks oncogenic Cripto signaling and may have important therapeutic value in the treatment of cancer.

Endogenous betaglycan is essential for high-potency inhibin antagonism in gonadotropes

Inhibins are endocrine hormones that regulate gametogenesis and reproduction through a negative feedback loop with FSH. Inhibin action involves antagonism of signaling by activin or other TGFbeta family ligands. In transfection assays, antagonism by inhibin can be potentiated by betaglycan, a coreceptor for selected TGFbeta family ligands. We tested whether betaglycan is an obligate inhibin coreceptor through disruption of betaglycan function by RNA interference-mediated knockdown and immunoneutralization. Betaglycan knockdown and anti-betaglycan IgG each independently prevented inhibin-A binding to betaglycan and reversed functional effects of transfected betaglycan. Neither betaglycan immunoneutralization nor knockdown affected activin responsiveness in cell lines or in rat anterior pituitary cultures. Betaglycan knockdown decreased the potency of inhibin antagonism of activin-induced FSH secretion in primary gonadotropes. Similarly, anti-betaglycan IgG decreased the potency of inhibin antagonism in primary gonadotropes in a dose-dependent manner, with a reduction in the sensitivity to inhibin-A of greater than 1000-fold. These data establish that betaglycan is an endogenous inhibin coreceptor required for high-sensitivity inhibin antagonism of activin signaling in rat anterior pituitary gonadotropes.

Clinical, agricultural, and evolutionary biology of myostatin: a comparative review

The discovery of myostatin and our introduction to the "Mighty Mouse" over a decade ago spurred both basic and applied research and impacted popular culture as well. The myostatin-null genotype produces "double muscling" in mice and livestock and was recently described in a child. The field's rapid growth is by no means surprising considering the potential benefits of enhancing muscle growth in clinical and agricultural settings. Indeed, several recent studies suggest that blocking myostatin's inhibitory effects could improve the clinical treatment of several muscle growth disorders, whereas comparative studies suggest that these actions are at least partly conserved. Thus, neutralizing myostatin's effects could also have agricultural significance. Extrapolating between studies that use different vertebrate models, particularly fish and mammals, is somewhat confusing because whole genome duplication events have resulted in the production and retention of up to four unique myostatin genes in some fish species. Such comparisons, however, suggest that myostatin's actions may not be limited to skeletal muscle per se, but may additionally influence other tissues including cardiac muscle, adipocytes, and the brain. Thus, therapeutic intervention in the clinic or on the farm must consider the potential of alternative side effects that could impact these or other tissues. In addition, the presence of multiple and actively diversifying myostatin genes in most fish species provides a unique opportunity to study adaptive molecular evolution. It may also provide insight into myostatin's nonmuscle actions as results from these and other comparative studies gain visibility in biomedical fields.

BMP-2 vs. BMP-4 expression and activity in glucocorticoid-arrested MC3T3-E1 osteoblasts: Smad signaling, not alkaline phosphatase activity, predicts rescue of mineralization

Pharmacological glucocorticoids (GCs) inhibit bone formation, leading to osteoporosis. GCs inhibit bone morphogenetic protein-2 (Bmp2) expression, and rhBMP-2 restores mineralization in GC-arrested osteoblast cultures. To better understand how GCs regulate BMPs, we investigated Bmp transcription, as well as rhBMP-induced Smad and alkaline phosphatase (ALP) activity. Bmp2 cis-regulatory regions were analyzed by reporter plasmids and LacZ-containing bacterial artificial chromosomes. We found that GCs inhibited Bmp2 via a domain > 50 kb downstream of the coding sequence. Bmp expression was evaluated by RT-PCR; whereas GCs strongly inhibited Bmp2, Bmp4 was abundantly expressed and resistant to GCs. Both rhBMP-2 and rhBMP-4 restored mineralization in GC-arrested cultures; rhBMP-2 was 5-fold more effective when dosing was based on ALP activation, however, the rhBMPs were equipotent when dosing was based on Smad transactivation. In conclusion, GCs regulate Bmp2 via a far-downstream domain, and activation of Smad, not ALP, best predicts the pro-mineralization potential of rhBMPs.

A distinct cohort of the TGFbeta superfamily members expressed in human endometrium regulate decidualization

BACKGROUND

Successful blastocyst implantation requires the differentiation of human endometrial stromal cells (HESC), a process known as decidualization. Activin A, a transforming growth factor beta (TGFbeta) superfamily member, enhances HESC decidualization and localizes to decidual cells in human endometrium. Other TGFbeta superfamily members, including BMP2, BMP4, BMP7, GDF5, GDF8, GDF11, TGFbetas and Nodal, may also play a role during decidualization. This study aimed to identify these TGFbeta family members in human endometrium, and to determine whether they are involved in human decidualization.

METHODS

Protein localization of TGFbeta family members was examined in secretory phase human endometrium and first trimester decidua by immunohistochemistry. mRNA expression was examined in HESC. Activin inhibitors (Activin-M108A/SB431542) with differing specificities for the other TGFbeta members under consideration were applied during HESC decidualization in vitro. The secretion levels of potential TGFbeta superfamily members were measured during decidualization, and recombinant proteins added to examine their effect.

RESULTS

This study has identified BMP2, BMP4, BMP7, GDF5, GDF8 and GDF11 but not Nodal in secretory phase human endometrium, but only BMP2, GDF5 and TGFbeta1 protein were detected in decidual cells. All ligands except Nodal were expressed by cultured HESC. Both inhibitors significantly reduced decidualization validating the role of activin, but potentially also other TGFbeta members, during decidualization. BMP2 and TGFbeta1 secretion increased during HESC decidualisation and exogenous administration of these proteins significantly enhanced decidualization in vitro.

CONCLUSIONS

Like activin, BMP2 and TGFbeta1 are likely to be involved in HESC decidualization. This is the first study to identify and localize BMP4, BMP7, GDF5, GDF8 and GDF11 in secretory phase human endometrium. Understanding the factors critical for the implantation process is needed for improving fertility and pregnancy outcomes.

Activin A/bone morphogenetic protein (BMP) chimeras exhibit BMP-like activity and antagonize activin and myostatin

Activins and bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta family of growth and differentiation factors that induce signaling in target cells by assembling type II and type I receptors at the cell surface. Ligand residues involved in type II binding are located predominantly in the C-terminal region that forms an extended beta-sheet, whereas residues involved in type I binding are located in the alpha-helical and preceding loop central portion of the molecule. To test whether the central residues are sufficient to determine specificity toward type I receptors, activin A/BMP chimeras were constructed in which the central residues (45-79) of activin A were replaced with corresponding residues of BMP2 and BMP7. The chimeras were assessed for activin type II receptor (Act RII) binding, activin-like bioactivity, and BMP-like activity as well as antagonistic properties toward activin A and myostatin. ActA/BMP7 chimera retained Act RII binding affinity comparable with wild type activin A, whereas ActA/BMP2 chimera showed a slightly reduced affinity toward Act RII. Both the chimeras were devoid of significant activin bioactivity in 293T cells in the A3 Lux reporter assay up to concentrations 10-fold higher than the minimal effective activin A concentration (approximately 4 nM). In contrast, these chimeras showed BMP-like activity in a BRE-Luc assay in HepG2 cells as well as induced osteoblast-like phenotype in C2C12 cells expressing alkaline phosphatase. Furthermore, both the chimeras activated Smad1 but not Smad2 in C2C12 cells. Also, both the chimeras antagonized ligands that signal via activin type II receptor, such as activin A and myostatin. These data indicate that activin residues in the central region determine its specificity toward type I receptors. ActA/BMP chimeras can be useful in the study of receptor specificities and modulation of transforming growth factor-beta members, activins, and BMPs.

GRP78 and Cripto form a complex at the cell surface and collaborate to inhibit transforming growth factor beta signaling and enhance cell growth

Cripto is a multifunctional cell surface protein with important roles in vertebrate embryogenesis and the progression of human tumors. While Cripto has been shown to modulate multiple signaling pathways, its binding partners do not appear to fully explain its molecular actions. Therefore, we conducted a screen aimed at identifying novel Cripto-interacting proteins. This screen led to our identification of glucose-regulated protein 78 (GRP78), an endoplasmic reticulum (ER) chaperone that is also expressed at the surfaces of tumor cells. Here we demonstrate that Cripto and GRP78 interact at the cell surfaces of multiple cell lines and that their interaction is independent of prior association within the ER. Interestingly, short hairpin RNA knockdown of endogenous GRP78 resulted in enhanced transforming growth factor beta (TGF-beta) signaling, indicating that like Cripto, GRP78 inhibits this pathway. We further show that when coexpressed, GRP78 and Cripto collaborate to antagonize TGF-beta responses, including Smad phosphorylation and growth inhibition of prostate cancer cells grown under anchorage-dependent or -independent conditions. Finally, we provide evidence that cells coexpressing GRP78 and Cripto grow much more rapidly in soft agar than do cells expressing either protein individually. Together, our results indicate that these proteins bind at the cell surface to enhance tumor growth via the inhibition of TGF-beta signaling.

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

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

Schistosoma mansoni: TGF-beta signaling pathways

Schistosome parasites have co-evolved an intricate relationship with their human and snail hosts as well as a novel interplay between the adult male and female parasites. We review the role of the TGF-beta signaling pathway in parasite development, host-parasite interactions and male-female interactions. The data to date support multiple roles for the TGF-beta signaling pathway throughout schistosome development, in particular, in the tegument which is at the interface with the host and between the male and female schistosome, development of vitelline cells in female worms whose genes and development are regulated by a stimulus from the male schistosome and embryogenesis of the egg. The human ligand TGF-beta1 has been demonstrated to regulate the expression of a schistosome target gene that encodes a gynecophoric canal protein in the schistosome worm itself. Studies on signaling in schistosomes opens a new era for investigation of host-parasite and male-female interactions.

Characterization of rainbow trout myostatin-2 genes (rtMSTN-2a and -2b): genomic organization, differential expression, and pseudogenization

Myostatin is an extremely potent negative regulator of vertebrate skeletal muscle development. A phylogenetic analysis suggests that salmonids should possess four distinct genes, although only MSTN-1 orthologs have been characterized. Described herein are the rainbow trout (rt) MSTN-2a and -2b genes and subsequence analysis of their promoters and their quantitative expression profiles. Both genes are similarly organized, contain several putative myogenic response elements, and are legitimate MSTN-2 orthologs based on Bayesian analyses. However, rtMSTN-2b contains two in-frame stop codons within the first exon and unspliced variants of both transcripts were expressed in a tissue-specific manner. Complete splicing of rtMSTN-2a occurred only in brain, where expression is highest, whereas rtMSTN-2b transcripts were mostly present in unspliced forms. The presence of stop codons in the rtMSTN-2b open reading frame and the expression of mostly unspliced transcripts indicate that this particular homolog is a pseudogene. These results confirm our previous phylogenetic analysis and suggest that all salmonids likely possess four distinct myostatin genes. The tissue-specific expression and differential processing of both rtMSTN-2 transcripts as well the pseudogenization of rtMSTN-2b may reflect compensatory and adaptive responses to tetraploidization and may help limit rtMSTN-2a's influences primarily to neural tissue.

Structural and Biophysical Coupling of Heparin and Activin Binding to Follistatin Isoform Functions

Follistatin (FS) regulates transforming growth factor-beta superfamily ligands and is necessary for normal embryonic and ovarian follicle development. Follistatin is expressed as two splice variants (FS288 and FS315). Previous studies indicated differences in heparin binding between FS288 and FS315, potentially influencing the physiological functions and locations of these isoforms. We have determined the structure of the FS315-activin A complex and quantitatively compared heparin binding by the two isoforms. The FS315 complex structure shows that both isoforms inhibit activin similarly, but FS315 exhibits movements within follistatin domain 3 (FSD3) apparently linked to binding of the C-terminal extension. Surprisingly, the binding affinities of FS288 and FS315 for heparin are similar at lower ionic strengths with FS315 binding decreasing more sharply as a function of salt concentration. When bound to activin, FS315 binds heparin similarly to the FS288 isoform, consistent with the structure of the complex, in which the acidic residues of the C-terminal extension cannot interact with the heparin-binding site. Activin-induced binding of heparin is unique to the FS315 isoform and may stimulate clearance of FS315 complexes.

The structures that underlie normal reproductive function

The mechanisms and physiology of reproductive function have fascinated scientists throughout time. Recent cellular and molecular level structural studies have provided unprecedented insights into reproductive systems and signaling networks. This 'cutting edge' editorial provides a recent example in each of these areas, namely, the anatomical integrity of the follicle, the molecular structure of activin with its binding partners and the molecular regulation of inhibin. These three examples of structure informing function help explain reproductive health and may provide solutions to reproductive disease.

The structural basis of TGF-β, bone morphogenetic protein, and activin ligand binding

The transforming growth factor-β (TGF-β) superfamily is a large group of structurally related growth factors that play prominent roles in a variety of cellular processes. The importance and prevalence of TGF-β signaling are also reflected by the complex network of check points that exist along the signaling pathway, including a number of extracellular antagonists and membrane-level signaling modulators. Recently, a number of important TGF-β crystal structures have emerged and given us an unprecedented clarity on several aspects of the signal transduction process. This review will highlight these latest advances and present our current understanding on the mechanisms of specificity and regulation on TGF-β signaling outside the cell.

Activin signaling and its role in regulation of cell proliferation, apoptosis, and carcinogenesis

Activins, cytokine members of the transforming growth factor-beta superfamily, have various effects on many physiological processes, including cell proliferation, cell death, metabolism, homeostasis, differentiation, immune responses endocrine function, etc. Activins interact with two structurally related serine/threonine kinase receptors, type I and type II, and initiate downstream signaling via Smads to regulate gene expression. Understanding how activin signaling is controlled extracellularly and intracellularly would not only lead to more complete understanding of cell growth and apoptosis, but would also provide the basis for therapeutic strategies to treat cancer and other related diseases. This review focuses on the recent progress on activin-receptor interactions, regulations of activin signaling by ligand-binding proteins, receptor-binding proteins, and nucleocytoplasmic shuttling of Smad proteins.

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

Betaglycan is a co-receptor that mediates signaling by transforming growth factor beta (TGFbeta) superfamily members, including the distinct and often opposed actions of TGFbetas and inhibins. Loss of betaglycan expression, or abrogation of betaglycan function, is implicated in several human and animal diseases, although both betaglycan actions and the ligands involved in these disease states remain unclear. Here we identify a domain spanning amino acids 591-700 of the betaglycan extracellular domain as the only inhibin-binding region in betaglycan. This binding site is within the betaglycan ZP domain, but inhibin binding is not integral to the ZP motif of other proteins. We show that the inhibin and TGFbeta-binding residues of this domain overlap and identify individual amino acids essential for binding of each ligand. Mutation of Val614 to Tyr abolishes both inhibin and TGFbeta binding to this domain. Full-length betaglycan V614Y, and other mutations, retain TGFbeta binding activity via a distinct site, but are unable to bind inhibin-A. These betaglycan mutants fail to mediate inhibin antagonism of activin signaling but can present TGFbeta to TbetaRII. Separating the co-receptor actions of betaglycan toward inhibin and TGFbeta will allow the clarification of the role of betaglycan in disease states such as renal cell carcinoma and endometrial adenocarcinoma.

Truncating mutations in the ACVR2 gene attenuates activin signaling in prostate cancer cells

Activins are classified as members of the TGFbeta superfamily of signaling molecules and both activin and TGFbeta ligands signal through structurally and functionally related serine/threonine kinase receptors. Defects in these signaling pathways have been associated with the initiation and progression of the cancer phenotype. Inactivating mutations in the TGFbeta type II receptor gene, TGFbetaR2, have been identified in a variety of tumors and cell lines, particularly those with microsatellite instability (MSI). More recently, mutations in the activin type II receptor gene, ACVR2, were identified in colon and pancreatic cell lines and tumors with MSI. Because prostate tumors appear to have a high incidence of MSI, we analyzed prostate cancer cell lines, with and without MSI, for ACVR2 and TGFbetaR2 mutations. Our analysis of 6 prostate cell lines revealed mutations in the ACVR2 gene in 22Rv-1, LAPC-4, DU145, and LNCaP cells and mutations in the TGFbetaR2 gene in 22Rv-1 and LAPC-4. PC3 and H660 cells were wild-type for ACVR2 and TFGbetaR2. All of the ACVR2 mutations were truncating mutations, and using an activin response assay, we demonstrate that truncating mutations of the ACVR2 gene result in a significant reduction in activin mediated cell signaling. Inactivation of ACVR2 is a common event in prostate cancer cells suggesting it may play an important role in the development of prostate cancer.

Activins and inhibins and their signaling

Activins and inhibins, which were discovered by virtue of their abilities to stimulate or inhibit, respectively, the secretion of FSH, are members of the transforming growth factor-beta (TGFbeta) superfamily and exert a broad range of effects on the diffentiation, proliferation and functions of numerous cell types. Activins interact with two structurally related classes of serine/threonine kinase receptors (type I and type II). Inhibin antagonizes activin by binding to the proteoglycan, betaglycan, and forming a stable complex with and, thereby, sequestering type II activin receptors while excluding type I receptors. If betaglycan is present, inhibin can also antagonize those bone morphogenic proteins (BMPs) whose signaling is dependent upon access to type II activin receptors. Recent insights regarding the structures of ligands, receptors and their signaling complexes are providing the basis for the development of therapeutics capable of modulating fertility and numerous pathophysiologic processes.