Difference between follistatin isoforms in the inhibition of activin signalling: activin neutralizing activity of follistatin isoforms is dependent on their affinity for activin

Follistatin lowers blood pressure and improves vascular structure and function in essential and secondary hypertension

Hypertension is characterized by resistance artery remodeling driven by oxidative stress and fibrosis. We previously showed that an activin A antagonist, follistatin, inhibited renal oxidative stress and fibrosis in a model of hypertensive chronic kidney disease. Here, we investigate the effects of follistatin on blood pressure and vascular structure and function in models of essential and secondary hypertension. 5/6 nephrectomised mice, a model of secondary hypertension, were treated with either exogenous follistatin or with a follistatin miRNA inhibitor to increase endogenous follistatin for 9 weeks. Blood pressure in mice was measured by tail cuff. Spontaneously hypertensive rats, a model of essential hypertension, were treated with follistatin for 8 weeks. Wistar Kyoto (WKY) rats were used as the normotensive control. Blood pressure in rats was measured by radiotelemetry. Mouse superior mesenteric arteries and rat first branch mesenteric arteries were isolated for structural and functional analyses. In both models, follistatin significantly lowered blood pressure and improved vascular structure, decreasing medial thickness and collagen content. Follistatin also reduced agonist-induced maximum contraction and improved endothelium-dependent relaxation. Increased vessel oxidative stress was attenuated by follistatin in both models. In ex vivo WKY vessels, activin A increased oxidative stress, augmented constriction, and decreased endothelium-dependent relaxation. Inhibition of oxidative stress restored vessel relaxation. This study demonstrates that follistatin lowers blood pressure and improves vascular structure and function in models of essential and secondary hypertension. Effects were likely mediated through its inhibition of activin A and oxidative stress. These data suggest a potential therapeutic role for follistatin as a novel antihypertensive agent. Follistatin, through antagonization of activin A, inhibits oxidative stress and improves vascular structure and function in resistance arteries from models of essential and secondary HTN. FST decreases collagen content and vascular ROS. Functionally, FST improves endothelium-dependent relaxation and decreases maximal vasoconstriction. Improved resistance artery structure and function are correlated with a decrease in BP in both models.

Follistatin Forms a Stable Complex With Inhibin A That Does Not Interfere With Activin A Antagonism

Inhibins are transforming growth factor-β family heterodimers that suppress follicle-stimulating hormone (FSH) secretion by antagonizing activin class ligands. Inhibins share a common β chain with activin ligands. Follistatin is another activin antagonist, known to bind the common β chain of both activins and inhibins. In this study, we characterized the antagonist-antagonist complex of inhibin A and follistatin to determine if their interaction impacted activin A antagonism. We isolated the inhibin A:follistatin 288 complex, showing that it forms in a 1:1 stoichiometric ratio, different from previously reported homodimeric ligand:follistatin complexes, which bind in a 1:2 ratio. Small angle X-ray scattering coupled with modeling provided a low-resolution structure of inhibin A in complex with follistatin 288. Inhibin binds follistatin via the shared activin β chain, leaving the α chain free and flexible. The inhibin A:follistatin 288 complex was also shown to bind heparin with lower affinity than follistatin 288 alone or in complex with activin A. Characterizing the inhibin A:follistatin 288 complex in an activin-responsive luciferase assay and by surface plasmon resonance indicated that the inhibitor complex readily dissociated upon binding type II receptor activin receptor type IIb, allowing both antagonists to inhibit activin signaling. Additionally, injection of the complex in ovariectomized female mice did not alter inhibin A suppression of FSH. Taken together, this study shows that while follistatin binds to inhibin A with a substochiometric ratio relative to the activin homodimer, the complex can dissociate readily, allowing both proteins to effectively antagonize activin signaling.

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

Muscle follistatin gene delivery increases muscle protein synthesis independent of periodical physical inactivity and fasting

Blocking of myostatin and activins effectively counteracts muscle atrophy. However, the potential interaction with physical inactivity and fasting in the regulation of muscle protein synthesis is poorly understood. We used blockade of myostatin and activins by recombinant adeno-associated virus (rAAV)-mediated follistatin (FS288) overexpression in mouse tibialis anterior muscle. To investigate the effects on muscle protein synthesis, muscles were collected 7 days after rAAV-injection in the nighttime or in the daytime representing high and low levels of activity and feeding, respectively, or after overnight fasting, refeeding, or ad libitum feeding. Muscle protein synthesis was increased by FS288 independent of the time of the day or the feeding status. However, the activation of mTORC1 signaling by FS288 was attenuated in the daytime and by overnight fasting. FS288 also increased the amount of mTOR colocalized with lysosomes, but did not alter their localization toward the sarcolemma. This study shows that FS288 gene delivery increases muscle protein synthesis largely independent of diurnal fluctuations in physical activity and food intake or feeding status, overriding the physiological signals. This is important for eg cachectic and sarcopenic patients with reduced physical activity and appetite. The FS288-induced increase in mTORC1 signaling and protein synthesis may be in part driven by increased amount of mTOR colocalized with lysosomes, but not by their localization toward sarcolemma.

Principles of the activin receptor signaling pathway and its inhibition

This review captures the anabolic and stimulatory effects observed with inhibition of the transforming growth factor β superfamily in muscle, blood, and bone. New medicinal substances that rectify activin, myostatin, and growth differentiation factor 11 signaling give hope to the many whose lives are affected by deterioration of these tissues. The review first covers the origin, structure, and common pathway of activins, myostatin, and growth differentiation factor 11 along with the pharmacodynamics of the new class of molecules designed to oppose the activin receptor signaling pathway. Current terminology surrounding this new class of molecules is inconsistent and does not infer functionality. Adopting inhibitors of the activin receptor signaling pathway (IASPs) as a generic term is proposed because it encapsulates the molecular mechanisms along the pathway trajectory. To conclude, a pragmatic classification of IASPs is presented that integrates functionality and side effects based on the data available from animals and humans. This provides researchers and clinicians with a tool to tailor IASPs therapy according to the need of projects or patients and with respect to side effects.

The emerging role of activins in renal disease

PURPOSE OF REVIEW

This review highlights recent discoveries and advances that have been made in understanding the role of the TGFβ superfamily members activins, and in particular, activin A (ActA), in renal disease.

RECENT FINDINGS

A deleterious role for ActA in renal disease and its complications has begun to emerge. We summarize data supporting an important contribution of ActA to kidney fibrosis and inflammation of varying causes, as well as its role in the development of a particular bone mineral disorder seen in chronic kidney disease (CKD) called mineral bone disorder (MBD), including vascular calcification. Finally, we discuss ActA in the context of anemia associated with chronic kidney disease and review potential approaches to treatment based on ActA blockade.

SUMMARY

ActA is an important contributor to the pathogenesis of acute and chronic kidney disease of varying causes. Preclinical studies show that ActA inhibition, through various approaches, is protective in rodent models of kidney disease. The potential adverse effects of some of these approaches can be attributed to their targeting of other TGFβ family ligands. Further preclinical and clinical investigations testing the therapeutic efficacy of more selective ActA inhibition on the progression of acute and chronic kidney disease and its impact on bone-mineral disorder would more definitively establish its role in renal disease.

Identification of a potential target for treatment of squamous cell carcinoma of the tongue: follistatin

Squamous cell carcinoma (SCC) of the tongue is the most common oral cancer and is prone to develop regional lymph nodes and distant metastases. Reliable and stable therapeutic targets can improve the curative effect and reduce toxic side effects caused by traditional treatments such as surgery, radiotherapy, and chemotherapy. We have analysed three sets of series of functional gene expression of SCC of the tongue from gene expression omnibus (GEO) datasets, and 154 common differentially expressed genes (DEG) between SCC of the tongue and the corresponding normal tissues were screened. Further bioinformatics research that was based on the data from the Cancer genome atlas, Gene ontology, and the Kyoto encyclopaedia of genes and genomes indicated that the increased expression of follistatin might be correlated with a poor prognosis in these patients. By assay of colony formation, reverse transcription polymerase chain reaction (RT-PCR), western blotting, immunohistochemical staining, and lentivirus transfection, we confirmed that downregulation of follistatin inhibited the proliferation of SCC cells in the tongue.

Follistatin Effects in Migration, Vascularization, and Osteogenesis in vitro and Bone Repair in vivo

The use of biomaterials and signaling molecules to induce bone formation is a promising approach in the field of bone tissue engineering. Follistatin (FST) is a glycoprotein able to bind irreversibly to activin A, a protein that has been reported to inhibit bone formation. We investigated the effect of FST in critical processes for bone repair, such as cell recruitment, osteogenesis and vascularization, and ultimately its use for bone tissue engineering. In vitro, FST promoted mesenchymal stem cell (MSC) and endothelial cell (EC) migration as well as essential steps in the formation and expansion of the vasculature such as EC tube-formation and sprouting. FST did not enhance osteogenic differentiation of MSCs, but increased committed osteoblast mineralization. In vivo, FST was loaded in an in situ gelling formulation made by alginate and recombinant collagen-based peptide microspheres and implanted in a rat calvarial defect model. Two FST variants (FST288 and FST315) with major differences in their affinity to cell-surface proteoglycans, which may influence their effect upon in vivo bone repair, were tested. In vitro, most of the loaded FST315 was released over 4 weeks, contrary to FST288, which was mostly retained in the biomaterial. However, none of the FST variants improved in vivo bone healing compared to control. These results demonstrate that FST enhances crucial processes needed for bone repair. Further studies need to investigate the optimal FST carrier for bone regeneration.

Effects of bimagrumab, an activin receptor type II inhibitor, on pituitary neurohormonal axes

BACKGROUND

Bimagrumab is a human monoclonal antibody inhibitor of activin type II receptors (ActRII), with anabolic action on skeletal muscle mass by blocking binding of myostatin and other negative regulators of muscle growth. Bimagrumab is under evaluation for muscle wasting and associated functional loss in hip fracture and sarcopenia, and in obesity. Bimagrumab also blocks other endogenous ActRII ligands, such as activins, which act on the neurohormonal axes, pituitary, gonads and adrenal glands.

AIM

To evaluate the effect of bimagrumab on the pituitary-gonadal and pituitary-adrenal axes in humans.

METHODS

Healthy men and women, aged 55 to 75 years, received bimagrumab intravenously 10 mg/kg or placebo on Day 1 and Day 29. Pituitary-gonadal and pituitary-adrenal functions were evaluated with basal hormone measurement and standard gonadotropin-releasing hormone (GnRH) and adrenocorticotropic hormone (ACTH) stimulation tests at baseline, Week 8 and at the end of study (EOS)-Week 20.

RESULTS

At Week 8, follicle-stimulating hormone (FSH) levels were reduced by 42.16 IU/L (P < .001) and luteinizing hormone (LH) levels were increased by 2.5 IU/L (P = .08) over placebo in response to bimagrumab in women but not in men. Effects that were reversible after bimagrumab was cleared. Gonadal and adrenal androgen levels were not affected by exposure to bimagrumab.

CONCLUSION

Bimagrumab alters the function of pituitary gonadotroph cells, consistent with blockade of activin on local ActRII. This effect is reversible with clearance of bimagrumab. Bimagrumab did not impact gonadal and adrenal androgen secretion.

Activin in acute pancreatitis: Potential risk-stratifying marker and novel therapeutic target

Acute Pancreatitis is a substantial health care challenge with increasing incidence. Patients who develop severe disease have considerable mortality. Currently, no reliable predictive marker to identify patients at risk for severe disease exists. Treatment is limited to rehydration and supporting care suggesting an urgent need to develop novel approaches to improve standard care. Activin is a critical modulator of inflammatory responses, but has not been assessed in pancreatitis. Here, we demonstrate that serum activin is elevated and strongly correlates with disease severity in two established murine models of acute pancreatitis induced by either cerulein or IL-12 + IL-18. Furthermore, in mice, inhibition of activin conveys survival benefits in pancreatitis. In addition, serum activin levels were measured from a retrospective clinical cohort of pancreatitis patients and high activin levels in patients at admission are predictive of worse outcomes, indicated by longer overall hospital and intensive care unit stays. Taken together, activin is a novel candidate as a clinical marker to identify those acute pancreatitis patients with severe disease who would benefit from aggressive treatment and activin may be a therapeutic target in severe acute pancreatitis.

Follistatin N terminus differentially regulates muscle size and fat in vivo

Delivery of follistatin (FST) represents a promising strategy for both muscular dystrophies and diabetes, as FST is a robust antagonist of myostatin and activin, which are critical regulators of skeletal muscle and adipose tissues. FST is a multi-domain protein, and deciphering the function of different domains will facilitate novel designs for FST-based therapy. Our study aims to investigate the role of the N-terminal domain (ND) of FST in regulating muscle and fat mass in vivo. Different FST constructs were created and packaged into the adeno-associated viral vector (AAV). Overexpression of wild-type FST in normal mice greatly increased muscle mass while decreasing fat accumulation, whereas overexpression of an N terminus mutant or N terminus-deleted FST had no effect on muscle mass but moderately decreased fat mass. In contrast, FST-I-I containing the complete N terminus and double domain I without domain II and III had no effect on fat but increased skeletal muscle mass. The effects of different constructs on differentiated C2C12 myotubes were consistent with the in vivo finding. We hypothesized that ND was critical for myostatin blockade, mediating the increase in muscle mass, and was less pivotal for activin binding, which accounts for the decrease in the fat tissue. An in vitro TGF-beta1-responsive reporter assay revealed that FST-I-I and N terminus-mutated or -deleted FST showed differential responses to blockade of activin and myostatin. Our study provided direct in vivo evidence for a role of the ND of FST, shedding light on future potential molecular designs for FST-based gene therapy.

Activin signaling is an essential component of the TGF-β induced pro-metastatic phenotype in colorectal cancer

Advanced colorectal cancer (CRC) remains a critical health care challenge worldwide. Various TGF-β superfamily members are important in colorectal cancer metastasis, but their signaling effects and predictive value have only been assessed in isolation. Here, we examine cross-regulation and combined functions of the two most prominent TGF-β superfamily members activin and TGF-β in advanced colorectal cancer. In two clinical cohorts we observed by immune-based assay that combined serum and tissue activin and TGF-β ligand levels predicts outcome in CRC patients and is superior to single ligand assessment. While TGF-β growth suppression is independent of activin, TGF-β treatment leads to increased activin secretion in colon cancer cells and TGF-β induced cellular migration is dependent on activin, indicating pathway cross-regulation and functional interaction in vitro. mRNA expression of activin and TGF-β pathway members were queried in silico using the TCGA data set. Coordinated ligand and receptor expression is common in solid tumors for activin and TGF-β pathway members. In conclusion, activin and TGF-β are strongly connected signaling pathways that are important in advanced CRC. Assessing activin and TGF-β signaling as a unit yields important insights applicable to future diagnostic and therapeutic interventions.

Clinical and Therapeutic Implications of Follistatin in Solid Tumours

Follistatin (FST), as a single-chain glycosylated protein, has two major isoforms, FST288 and FST315. The FST315 isoform is the predominant form whilst the FST288 variant accounts for less than 5% of the encoded mRNA. FST is differentially expressed in human tissues and aberrant expression has been observed in a variety of solid tumours, including gonadal, gastric and lung cancer, hepatocellular carcinoma, basal cell carcinoma and melanoma. Based on the current evidence, FST is an antagonist of transforming growth factor beta family members, such as activin and bone morphogenetic proteins (BMPs). FST plays a role in tumourigenesis, metastasis and angiogenesis of solid tumours through its interaction with activin and BMPs, thus resulting in pathophysiological function. In terms of diagnosis, prognosis and therapy, FST has shown strong promise. Through a better understanding of its biological functions, potential clinical applications may yet emerge.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Structure and activation of pro-activin A

Activins are growth factors with multiple roles in the development and homeostasis. Like all TGF-β family of growth factors, activins are synthesized as large precursors from which mature dimeric growth factors are released proteolytically. Here we have studied the activation of activin A and determined crystal structures of the unprocessed precursor and of the cleaved pro-mature complex. Replacing the natural furin cleavage site with a HRV 3C protease site, we show how the protein gains its bioactivity after proteolysis and is as active as the isolated mature domain. The complex remains associated in conditions used for biochemical analysis with a dissociation constant of 5 nM, but the pro-domain can be actively displaced from the complex by follistatin. Our high-resolution structures of pro-activin A share features seen in the pro-TGF-β1 and pro-BMP-9 structures, but reveal a new oligomeric arrangement, with a domain-swapped, cross-armed conformation for the protomers in the dimeric protein.

Activins are members of the TGF-β family of growth factors that are processed from precursors into the mature proteins. Here, the authors use structural biology and biochemistry to examine the protein domain organisation and gain insights into the activation of pro-activin A.

The immunoregulatory and fibrotic roles of activin A in allergic asthma

Activin A, a member of the TGF-β superfamily of cytokines, was originally identified as an inducer of follicle stimulating hormone release, but has since been ascribed roles in normal physiological processes, as an immunoregulatory cytokine and as a driver of fibrosis. In the last 10–15 years, it has also become abundantly clear that activin A plays an important role in the regulation of asthmatic inflammation and airway remodelling. This review provides a brief introduction to the activin A/TGF-β superfamily, focussing on the regulation of receptors and signalling pathways. We examine the contradictory evidence for generalized pro- vs. anti-inflammatory effects of activin A in inflammation, before appraising its role in asthmatic inflammation and airway remodelling specifically by evaluating data from both murine models and clinical studies. We identify key issues to be addressed, paving the way for safe exploitation of modulation of activin A function for treatment of allergic asthma and other inflammatory lung diseases.

Seminal fluid factors regulate activin A and follistatin synthesis in female cervical epithelial cells

Seminal fluid induces pro-inflammatory cytokines and elicits an inflammation-like response in the cervix. Here, Affymetrix microarray and qPCR was utilised to identify activin A (INHBA) and its inhibitor follistatin (FST) amongst the cytokines induced by seminal plasma in Ect1 ectocervical epithelial cells, and a similar response was confirmed in primary ectocervical epithelial cells. TGFB is abundant in seminal plasma and all three TGFB isoforms induced INHBA in Ect1 and primary cells, and neutralisation of TGFB in seminal plasma suppressed the INHBA response. Bacterial lipopolysaccharide in seminal plasma also elicited INHBA, but potently suppressed FST production. There was moderate reciprocal inhibition between FST and INHBA, and cross-attenuating effects were seen. These data identify TGFB and potentially LPS as factors mediating seminal plasma-induced INHBA synthesis in cervical cells. INHBA and FST induced by seminal fluid in cervical tissues may thus contribute to regulation of the post-coital response in women.

Production of bioactive chicken (Gallus gallus) follistatin-type proteins in E. coli

Follistatin (FST) is a cysteine-rich autocrine glycoprotein and plays an important role in mammalian prenatal and postnatal development. FST binds to and inhibit myostatin (MSTN), a potent negative regulator of skeletal muscle growth, and FST abundance enhances muscle growth in animals via inhibition of MSTN activity. The objective of this study was to produce biologically active, four chicken FST-type proteins in an Escherichia coli expression system. Gibson assembly cloning method was used to insert the DNA fragments of four FST-type proteins, designated as FST288, NDFSD1/2, NDFSD1, and NDFSD1/1, into pMALc5x vector downstream of the maltose-binding protein (MBP) gene, and the plasmids containing the inserts were eventually transformed into Shuffle E. coli strain for protein expression. We observed a soluble expression of the four MBP-fused FST-type proteins, and the proteins could be easily purified by the combination of amylose and heparin resin affinity chromatography. MBP-fused FST-type proteins demonstrated their affinity to anti-FST antibody. In an in vitro reporter gene assay to examine their potencies and selectivities to different ligands (MSTN, GDF11, and activin A), the four FST-type proteins (MBP-FST288, MBP-NDFSD1/2, MBP-NDFSD1, and MBP-NDFSD1/1) showed different potency and selectivity against the three ligands from each other. Ligand selectivity of each FST-type proteins was similar to its counterpart FST-type protein of eukaryotic origin. In conclusion, we could produce four FST-type proteins having different ligand selectivity in E. coli, and the results imply that economic production of a large amount of FST-type proteins with different ligand selectivity is possible to examine their potential use in meat-producing animals.

Comparison of bioactivities, binding properties and intrafollicular levels of bovine follistatins

Five isoforms of follistatin (FST) (Mr31, 33, 35, 37, and 41 kDa) were purified from bovine follicular fluid (bFF). Comparison of their activin and heparan sulphate proteoglycan (HSP) binding properties and biopotencies in the neutralisation of activin A actionin vitrorevealed that all five isoforms bound activin A, but they did so with different affinities. Only the 31 kDa isoform (FST-288) bound to HSP. FST-288 also showed the greatest biopotency, and the 35 and 41 kDa isoforms were the least potent. To determine whether bovine follicle development is associated with changing intrafollicular FST and activin profiles, we analysed bFF from dominant follicles (DFs) and subordinate follicles (SF) collected at strategic times during a synchronised oestrous cycle. Total FST, activin A and activin AB were measured by immunoassay, whereas individual FST isoforms were quantified by immunoblotting. Follicle diameter was positively correlated with oestrogen:progesterone ratio (r=0.56) in bFF but negatively correlated with activin A (r=−0.34), activin AB (r=−0.80) and ‘total’ FST (r=−0.70) levels. Follicle diameter was positively correlated with the abundance of the 41 kDa isoform (r=0.59) but negatively correlated with the abundance of the 33 and 31 kDa isoforms (r=−0.56 andr=−0.41 respectively). Both follicle statuses (DF and SF) and cycle stage affected total FST, activin A and activin B levels, whereas follicle status, but not cycle stage, affected the abundance of the 41, 37, 33 and 31 kDa FST isoforms. Collectively, these findings indicate that intrafollicular FST isoforms, which differ in their ability to bind and neutralise activins and to associate with cell-surface proteoglycans, show divergent changes during follicle development. Enhanced FST production may play an important negative role, either directly or via the inhibition of the positive effects of activins, on follicle growth and function during follicular waves.

Pharmacology of manipulating lean body mass

Dysfunction and wasting of skeletal muscle as a consequence of illness decreases the length and quality of life. Currently, there are few, if any, effective treatments available to address these conditions. Hence, the existence of this unmet medical need has fuelled large scientific efforts. Fortunately, these efforts have shown many of the underlying mechanisms adversely affecting skeletal muscle health. With increased understanding have come breakthrough disease-specific and broad spectrum interventions, some progressing through clinical development. The present review focuses its attention on the role of the antagonistic process regulating skeletal muscle mass before branching into prospective promising therapeutic targets and interventions. Special attention is given to therapies in development against cancer cachexia and Duchenne muscular dystrophy before closing remarks on design and conceptualization of future therapies are presented to the reader.

Follistatin: a novel therapeutic for the improvement of muscle regeneration

Follistatin (FST) is a member of the tissue growth factor β family and is a secreted glycoprotein that antagonizes many members of the family, including activin A, growth differentiation factor 11, and myostatin. The objective of this study was to explore the use of an engineered follistatin therapeutic created by fusing FST315 lacking heparin binding activity to the N terminus of a murine IgG1 Fc (FST315-ΔHBS-Fc) as a systemic therapeutic agent in models of muscle injury. Systemic administration of this molecule was found to increase body weight and lean muscle mass after weekly administration in normal mice. Subsequently, we tested this agent in several models of muscle injury, which were chosen based on their severity of damage and their ability to reflect clinical settings. FST315-ΔHBS-Fc treatment proved to be a potent inducer of muscle remodeling and regeneration. FST315-ΔHBS-Fc induced improvements in muscle repair after injury/atrophy by modulating the early inflammatory phase allowing for increased macrophage density, and Pax7-positive cells leading to an accelerated restoration of myofibers and muscle function. Collectively, these data demonstrate the benefits of a therapeutically viable form of FST that can be leveraged as an alternate means of ameliorating muscle regeneration.

Granulosa cell tumor mutant FOXL2C134W suppresses GDF-9 and activin A-induced follistatin transcription in primary granulosa cells

A single somatic FOXL2 mutation (FOXL2(C134W)) was identified in almost all granulosa cell tumor (GCT) patients. In the pituitary, FOXL2 and Smad3 coordinately regulate activin stimulation of follistatin transcription. We explored whether a similar regulation occurs in the ovary, and whether FOXL2(C134W) has altered activity. We show that in primary granulosa cells, GDF-9 and activin increase Smad3-mediated follistatin transcription. In contrast to findings in the pituitary, FOXL2 negatively regulates GDF-9 and activin-stimulated follistatin transcription in the ovary. Knockdown of endogenous FOXL2 confirmed this inhibitory role. FOXL2(C134W) displayed enhanced inhibitory activity, completely ablating GDF-9 and activin-induced follistatin transcription. GDF-9 and activin activity was lost when either the smad binding element or the forkhead binding element were mutated, indicating that both sites are required for Smad3 actions. This study highlights that FOXL2 negatively regulates follistatin expression within the ovary, and that the pathogenesis of FOXL2(C134W) may involve an altered interaction with Smad3.

Follistatin as potential therapeutic target in prostate cancer

Follistatin is a single-chain glycosylated protein whose primary function consists in binding and neutralizing some members of the transforming growth factor-β superfamily such as activin and bone morphogenic proteins. Emerging evidence indicates that this molecule may also play a role in the malignant progression of several human tumors including prostate cancer. In particular, recent findings suggest that, in this tumor, follistatin may also contribute to the formation of bone metastasis through multiple mechanisms, some of which are not related to its specific activin or bone morphogenic proteins' inhibitory activity. This review provides insight into the most recent advances in understanding the role of follistatin in the prostate cancer progression and discusses the clinical and therapeutic implications related to these findings.

Systemic administration of follistatin288 increases muscle mass and reduces fat accumulation in mice

The present study describes the physiological response associated with daily subcutaneous injection of mice with recombinant follistatin288. This systemic administration of follistatin288 increases the follistatin levels in serum, indicating that the protein enters the circulation. The data suggest that a dose-dependent increase in body lean mass also occurs, together with an increase in muscle mass, possibly as a result of an increase in the size of the muscle fibers. After thirteen weeks of treatment, metabolic changes were observed; additionally, the switching of muscle fiber types was also apparent through myosin heavy chain remodeling, implying that changes are occurring at the molecular level. Furthermore, an increase in the muscle mass was associated with a significant decrease in the body fat mass. Overall, this study raises the possibility for the use of follistatin288 as an agent to treat muscle wasting diseases and/or to restrict fat accumulation by systemic administration of the protein.

Overexpression of follistatin in the mouse epididymis disrupts fluid resorption and sperm transit in testicular excurrent ducts

Activin is a well-established modulator of male and female reproduction that stimulates the synthesis and secretion of follicle-stimulating hormone. Nonpituitary effects of activin have also been reported, although the paracrine actions of this growth factor in several reproductive tissues are not well understood. To identify the paracrine functions of activin during mammary gland morphogenesis and tumor progression, we produced transgenic mice that overexpress follistatin (FST), an intrinsic inhibitor of activin, under control of the mouse mammary tumor virus (MMTV) promoter. Although the MMTV-Fst mice were constructed to assess the role of activin in females, expression of the transgene was also observed in the testes and epididymides of males. While all 17 transgenic founder males exhibited copulatory behavior and produced vaginal plugs in females, only one produced live offspring. In contrast, transgenic females were fertile, permitting expansion of transgenic mouse lines. Light and transmission electron microscopic examination of the transgenic testes and epididymides revealed impairment of fluid resorption and sperm transit in the efferent ducts and initial segment of the epididymis, as indicated by accumulation of fluid and sperm stasis. Consequently, a variety of degenerative lesions were observed in the seminiferous epithelium, such as vacuolation and early stages of mineralization and fibrosis. Sperm collected from the caudae epididymidis of MMTV-Fst males had detached heads and were immotile. Together, these data reveal that activin signaling is essential for normal testicular excurrent duct function and that its blockade impairs fertility. These results also suggest that selective inhibitors of activin signaling may provide a useful approach for the development of male contraceptives without compromising androgen synthesis and actions.

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.

TGF-β signaling in Duchenne muscular dystrophy

The TGF-β protein family consists of secreted multifunctional cytokines that control diverse processes, such as cell growth and differentiation. Aberrant expression and downstream signaling of these growth factors have been associated with multiple diseases, including muscle wasting disorders, such as Duchenne muscular dystrophy. In this review we discuss recent advances in understanding the role of TGF-β family members during normal skeletal muscle biology/regeneration and their role in muscle pathology, with a special focus on Duchenne muscular dystrophy. In addition, we will highlight progress in the development of potential therapeutics for Duchenne muscular dystrophy based on intervention of TGF-β signaling.

Expression and purification of recombinant protein related to DAN and cerberus (PRDC)

Bone morphogenetic proteins (BMPs) are secreted protein ligands that control numerous biological processes, such as cell differentiation and cell proliferation. Ligands are regulated by a large number of structurally diverse extracellular antagonists. PRDC or protein related to DAN and cerberus is a BMP antagonist of the DAN family, which is defined by a conserved pattern of cysteine residues that form a ring structure. Here we present the expression and purification of recombinant mouse PRDC (mPRDC) from bacterial (Escherichia coli) inclusion bodies through oxidative refolding. Functional mPRDC was isolated from a nonfunctional component through reverse phase chromatography and shown to inhibit BMP2 and BMP4 in a cell-based luciferase reporter assay. Recombinant mPRDC also bound directly to BMP2, BMP4 and BMP7, but not activin A. Furthermore, circular dichroism indicated that mPRDC is folded and contains a higher than anticipated helical content for a DAN family member protein.

Extracellular Regulation of Myostatin: A Molecular Rheostat for Muscle Mass

Myostatin (MSTN) is a transforming growth factor-ß family member that plays a critical role in regulating skeletal muscle mass. Genetic studies in multiple species have demonstrated that mutations in the Mstn gene lead to dramatic and widespread increases in muscle mass as a result of a combination of increased fiber numbers and increased fiber sizes. MSTN inhibitors have also been shown to cause significant increases in muscle growth when administered to adult mice. As a result, there has been an extensive effort to understand the mechanisms underlying MSTN regulation and activity with the goal of developing the most effective strategies for targeting this signaling pathway for clinical applications. Here, I review the current state of knowledge regarding the regulation of MSTN extracellularly by binding proteins and discuss the implications of these findings both with respect to the fundamental physiological role that MSTN plays in regulating tissue homeostasis and with respect to the development of therapeutic agents to combat muscle loss.

DIP2 disco-interacting protein 2 homolog A (Drosophila) is a candidate receptor for follistatin-related protein/follistatin-like 1--analysis of their binding with TGF-β superfamily proteins

Follistatin-related protein (FRP)/follistatin-like 1 (FSTL1) is a member of the follistatin protein family, all of which share a characteristic structure unit found in follistatin, called the FS domain. Developmental studies have suggested that FRP regulates organ tissue formation in embryos. Immunological studies showed that FRP modifies joint inflammation in arthritic disease, and modulates allograft tolerance. However, the principle physiological function of FRP is currently unknown. To address this issue, we cloned four FRP-associated proteins using a two-hybrid cloning method: disco-interacting protein 2 homolog A from Drosophila (DIP2A), CD14, glypican 1 and titin. Only DIP2A was expected to be a membrane receptor protein with intracellular regions. Over-expression of FLAG epitope-tagged DIP2A augmented the suppressive effect of FRP on FBJ murine osteosarcoma viral oncogene homolog (FOS) expression, and the Fab fragment of IgG to FLAG blocked this effect. Knockdown of Dip2a leaded to Fos gene up-regulation, and this was not affected by exogenous FRP. These in vitro experiments confirmed that DIP2A could be a cell-surface receptor protein and mediate a FOS down-regulation signal of FRP. Moreover, molecular interaction analyses using Biacore demonstrated that FRP bound to DIP2A and CD14, and also with proteins of the TGF-β superfamily, i.e. activin, TGF-β, bone morphogenetic protein 2/4 (BMP-2/4), their receptors and follistatin. FRP binding to DIP2A was blocked by CD14, follistatin, activin and BMP-2. FRP blocked the ligand-receptor binding of activin and BMP-2, but integrated itself with that of BMP-4. This multi-specific binding may reflect the broad physiological activity of FRP.

Canonical TGF-beta signaling is required for the balance of excitatory/inhibitory transmission within the hippocampus and prepulse inhibition of acoustic startle

Smad4 is a unique nuclear transducer for all TGF-beta signaling pathways and regulates gene transcription during development and tissue homeostasis. To elucidate the postnatal role of TGF-beta signaling in the mammalian brain, we generated forebrain-specific Smad4 knock-out mice. Surprisingly, the mutants showed no alteration in long-term potentiation and water maze, suggesting that Smad4 is not required for spatial learning and memory. However, these mutant mice did show enhancement of paired-pulse facilitation in excitatory synaptic transmission and stronger paired-pulse depression of GABA(A) currents in the hippocampus. The alteration of hippocampal electrophysiology correlated with mouse hyperactivity in homecage and open field tests. Mutant mice also showed overgrooming as well as deficits of prepulse inhibition, a widely used endophenotype of schizophrenia. With a specific real-time PCR array focused on TGF-beta signaling pathway, we identified a novel regulation mechanism of the pathway in the hippocampal neurons, in which Smad4-mediated signaling suppresses the level of extracellular antagonism of TGF-beta ligands through transcriptional regulation of follistatin, a selective inhibitor to activin/TGF-beta signaling in the hippocampus. In summary, we suggest that the canonical TGF-beta signaling pathway is critical for use-dependent modulation of GABA(A) synaptic transmission and dendritic homeostasis; furthermore, a disruption in the balance of the excitatory and inhibitory hippocampal network can result in psychiatric-like behavior.

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.

Prepubertal onset of diabetes prevents expression of renal cortical connective tissue growth factor

Puberty unmasks or accelerates the nephropathy of diabetes mellitus (DM). We performed focused microarray analysis to test the hypothesis that one or more genes in the transforming growth factor beta (TGF-beta) signaling system would be differentially regulated in male rats depending on their age at onset of DM. Littermates were started on the 6-week protocol at 4 weeks or 14 weeks of age. Renal cortical RNA was isolated and analyzed using gene chips with more than 30,000 transcripts. Age-specific effects of DM were demonstrated for 1,760 transcripts. Analysis then focused on 89 genes involved in the TGF-beta signaling pathway. Three of these genes showed age-dependent responses to DM, confirmed by quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Connective tissue growth factor (CTGF) mRNA and protein were both increased approximately 30% in the renal cortex 6 weeks after adult-onset DM, with no alteration in either parameter after juvenile onset. Follistatin and avian myelocytomatosis viral oncogene homolog mRNA both showed a similar age-related pattern of response to DM, but protein levels did not parallel mRNA for either of these gene products. Given the known roles of CTGF in progressive nephropathies, it is an attractive candidate to explain pubertal acceleration or unmasking of the kidney disease of diabetes.

Characterization of follistatin-related gene as a negative regulatory factor for activin family members during mouse heart development

Follistatin-related gene (FLRG) encodes a secretory glycoprotein that has characteristic cysteine-rich follistatin domains. FLRG protein binds to and neutralizes several transforming growth factor-beta (TGF-beta) superfamily members, including myostatin (MSTN), which is a potent negative regulator of skeletal muscle mass. We have previously reported that FLRG was abundantly expressed in fetal and adult mouse heart. In this study, we analyzed the expression of FLRG mRNA during mouse heart development. FLRG mRNA was continuously expressed in the embryonic heart, whereas it was very low in skeletal muscles. By contrast, MSTN mRNA was highly expressed in embryonic skeletal muscles, whereas the expression of MSTN mRNA was rather low in the heart. In situ hybridization and immunohistochemical analysis revealed that FLRG expressed in smooth muscle of the aorta and pulmonary artery, valve leaflets of mitral and tricuspid valves, and cardiac muscles in the ventricle of mouse embryonic heart. However, MSTN was expressed in very limited areas, such as valve leaflets of pulmonary and aortic valves, the top of the ventricular and atrial septa. Interestingly, the expression of MSTN was complementary to that of FLRG, especially in the valvular apparatus. Biochemical analyses with surface plasmon resonance biosensor and reporter assays demonstrated that FLRG hardly dissociates from MSTN and activin once it bound to them, and efficiently inhibits these activities. Our results suggest that FLRG could function as a negative regulator of activin family members including MSTN during heart development.

Transgenic expression of a myostatin inhibitor derived from follistatin increases skeletal muscle mass and ameliorates dystrophic pathology in mdx mice

Myostatin is a potent negative regulator of skeletal muscle growth. Therefore, myostatin inhibition offers a novel therapeutic strategy for muscular dystrophy by restoring skeletal muscle mass and suppressing the progression of muscle degeneration. The known myostatin inhibitors include myostatin propeptide, follistatin, follistatin-related proteins, and myostatin antibodies. Although follistatin shows potent myostatin-inhibiting activities, it also acts as an efficient inhibitor of activins. Because activins are involved in multiple functions in various organs, their blockade by follistatin would affect multiple tissues other than skeletal muscles. In the present study, we report the characterization of a myostatin inhibitor derived from follistatin, which does not affect activin signaling. The dissociation constants (K(d)) of follistatin to activin and myostatin are 1.72 nM and 12.3 nM, respectively. By contrast, the dissociation constants (K(d)) of a follistatin-derived myostatin inhibitor, designated FS I-I, to activin and myostatin are 64.3 microM and 46.8 nM, respectively. Transgenic mice expressing FS I-I, under the control of a skeletal muscle-specific promoter showed increased skeletal muscle mass and strength. Hyperplasia and hypertrophy were both observed. We crossed FS I-I transgenic mice with mdx mice, a model for Duchenne muscular dystrophy. Notably, the skeletal muscles in the mdx/FS I-I mice showed enlargement and reduced cell infiltration. Muscle strength is also recovered in the mdx/FS I-I mice. These results indicate that myostatin blockade by FS I-I has a therapeutic potential for muscular dystrophy.

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 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.

The Structure of the Follistatin:Activin Complex Reveals Antagonism of Both Type I and Type II Receptor Binding

TGF-beta ligands stimulate diverse cellular differentiation and growth responses by signaling through type I and II receptors. Ligand antagonists, such as follistatin, block signaling and are essential regulators of physiological responses. Here we report the structure of activin A, a TGF-beta ligand, bound to the high-affinity antagonist follistatin. Two follistatin molecules encircle activin, neutralizing the ligand by burying one-third of its residues and its receptor binding sites. Previous studies have suggested that type I receptor binding would not be blocked by follistatin, but the crystal structure reveals that the follistatin N-terminal domain has an unexpected fold that mimics a universal type I receptor motif and occupies this receptor binding site. The formation of follistatin:BMP:type I receptor complexes can be explained by the stoichiometric and geometric arrangement of the activin:follistatin complex. The mode of ligand binding by follistatin has important implications for its ability to neutralize homo- and heterodimeric ligands of this growth factor family.

Heparin and activin-binding determinants in follistatin and FSTL3

Local regulation of pituitary FSH secretion and many other cellular processes by follistatin (FS) can be ascribed to its potent ability to bind and bioneutralize activin, in conjunction with binding to cell surface heparan-sulfate proteoglycans through a basic heparin-binding sequence (HBS; residues 75-86) in the first of the three FS domains. The FS homolog, FSTL3, also binds activin, but lacks any HBS and cannot associate with cell surfaces. We have used mutational analyses to define the determinants for heparin binding and activin interaction in FS and to determine the effects of conferring heparin binding to FSTL3. Mutants expressed from 283F cells were tested for cell surface and heparin affinity binding, for competitive activin binding and for bioactivity by suppression of pituitary cell FSH secretion. Replacement of the HBS or the full-length FS-domain 1 abolished cell surface binding but enhanced activin binding 4- to 8-fold. Surface binding was partially reduced after mutation of either lysine pair 75/76 or 81/82 and eliminated after mutation of both pairs. The 75/76 mutation reduced activin binding and, therefore, pituitary cell bioactivity by 5-fold. However, insertion of the HBS into FSTL3 did not restore heparin binding or pituitary-cell bioactivity. These results show that 1) the residues within the HBS are necessary but not sufficient for heparin binding, and 2) the HBS also harbors determinants for activin binding. Introduction of the full domain from FS conferred heparin binding to FSTL3, but activin binding was abolished. This implies an evolutionary safeguard against surface binding by FSTL3, supporting other evidence for physiological differences between FS and FSTL3.

Rapid, efficient isolation of murine gonadotropes and their use in revealing control of follicle-stimulating hormone by paracrine pituitary factors

FSH and LH are produced only in gonadotropes, which are reported to comprise 3-12% of mammalian pituitaries. Factors made within the pituitary are powerful regulators of FSH and also influence LH expression, but their identities and cellular origins are unknown because it is impossible to isolate and individually analyze different pituitary cell types. In this study FSH-producing gonadotropes were specifically tagged in vivo with a transgenic cell surface antigen (H-2Kk) so they could be purified in vitro using paramagnetic anti-H-2Kk microbeads. After enzymatic dispersion of pituitary cells, it took 1 h or less to extract 55 +/- 5% of FSH-producing gonadotropes at 95 +/- 0.5% purity, as judged by immunostaining for FSH or prolactin. Although this procedure selected for FSH expression, the isolated gonadotropes were also enriched 22-fold for LH-containing cells. For studies aimed at understanding factors that control FSH transcription, the purified gonadotropes were treated with activin A, which increased FSH expression 480% above basal levels (d 3 of culture). Coincubation of purified gonadotropes with pituitary nongonadotropes increased FSH expression 800% (d 3 of culture). Follistatin, an activin-binding protein, decreased FSH expression 35-50%, suggesting that gonadotropes make some activin and/or other follistatin-sensitive molecule(s) that induce FSH. These data show that paracrine factors from pituitary nongonadotropes can play a major role in controlling FSHbeta at the pituitary level. The study presented here describes a rapid, reliable, and efficient method for isolating any specialized cell type, including all cells that produce endocrine hormones.

HtrA1 serine protease inhibits signaling mediated by Tgfβ family proteins

HtrA1, a member of the mammalian HtrA serine protease family, has a highly conserved protease domain followed by a PDZ domain. Because HtrA1 is a secretory protein and has another functional domain with homology to follistatin, we examined whether HtrA1 functions as an antagonist of Tgfβfamily proteins. During embryo development, mouse HtrA1 was expressed in specific areas where signaling by Tgfβ family proteins plays important regulatory roles. The GST-pulldown assay showed that HtrA1 binds to a broad range of Tgfβ family proteins, including Bmp4, Gdf5, Tgfβs and activin. HtrA1 inhibited signaling by Bmp4, Bmp2, and Tgfβ1 in C2C12 cells, presumably by preventing receptor activation. Experiments using a series of deletion mutants indicated that the binding activity of HtrA1 required the protease domain and a small linker region preceding it, and that inhibition of Tgfβ signaling is dependent on the proteolytic activity of HtrA1. Misexpression of HtrA1 near the developing chick eye led to suppression of eye development that was indistinguishable from the effects of noggin. Taken together, these data indicate that HtrA1 protease is a novel inhibitor of Tgfβ family members.

Adenovirus-mediated overexpression of follistatin enlarges intact liver of adult rats

Under normal physiologic conditions, liver size is under strict regulatory control. Activin, a member of the transforming growth factor beta (TGF-beta) superfamily, is expressed in the intact adult liver and is an inhibitor of hepatocyte growth. However, the exact role played by endogenous activin in maintaining the size of a normal adult liver has yet to be completely examined in vivo. Here, we report the development of an adenoviral vector (AdexCAFS288) that expressed human follistatin-288, which binds to activin and neutralizes its biologic activities. AdexCAGFP, a control virus, expressed green fluorescent protein. AdexCAFS288 effectively expressed follistatin-288, as measured both in HepG2 cell lysate and conditioned medium and blocked activin signaling and its biologic functions in vitro. Intraperitoneal injection of AdexCAFS288 in vivo resulted in significant liver growth (146% of control) in intact liver of adult male rats 12 days following treatment without significant dysfunctions. The increase in liver size was attributed to increased hepatocyte proliferation, as monitored by the mitotic index. Furthermore, there was a significant correlation between serum follistatin levels and liver weight. In conclusion, our results suggest that activin plays a critical role in maintaining optimal liver size and implicates the endogenous activin system as a therapeutic target in the treatment of liver disease.

Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium

It has been 70 years since the name inhibin was used to describe a gonadal factor that negatively regulated pituitary hormone secretion. The majority of this period was required to achieve purification and definitive characterization of inhibin, an event closely followed by identification and characterization of activin and follistatin (FS). In contrast, the last 15-20 years saw a virtual explosion of information regarding the biochemistry, physiology, and biosynthesis of these proteins, as well as identification of activin receptors, and a unique mechanism for FS action-the nearly irreversible binding and neutralization of activin. Many of these discoveries have been previously summarized; therefore, this review will cover the period from the mid 1990s to present, with particular emphasis on emerging themes and recent advances. As the field has matured, recent efforts have focused more on human studies, so the endocrinology of inhibin, activin, and FS in the human is summarized first. Another area receiving significant recent attention is local actions of activin and its regulation by both FS and inhibin. Because activin and FS are produced in many tissues, we chose to focus on a few particular examples with the most extensive experimental support, the pituitary and the developing follicle, although nonreproductive actions of activin and FS are also discussed. At the cellular level, it now seems that activin acts largely as an autocrine and/or paracrine growth factor, similar to other members of the transforming growh factor beta superfamily. As we discuss in the next section, its actions are regulated extracellularly by both inhibin and FS. In the final section, intracellular mediators and modulators of activin signaling are reviewed in detail. Many of these are shared with other transforming growh factor beta superfamily members as well as unrelated molecules, and in a number of cases, their physiological relevance to activin signal propagation remains to be elucidated. Nevertheless, taken together, recent findings suggest that it may be more appropriate to consider a new paradigm for inhibin, activin, and FS in which activin signaling is regulated extracellularly by both inhibin and FS whereas a number of intracellular proteins act to modulate cellular responses to these activin signals. It is therefore the balance between activin and all of its modulators, rather than the actions of any one component, that determines the final biological outcome. As technology and model systems become more sophisticated in the next few years, it should become possible to test this concept directly to more clearly define the role of activin, inhibin, and FS in reproductive physiology.

cDNA cloning and expression of human activin betaE subunit

We cloned human activin betaE subunit cDNA from a liver cDNA library using the polymerase chain reaction (PCR) technique. The deduced amino acid sequence was 97 and 96% homologous to the mouse and rat activin betaE subunits. Human activin betaE subunit tagged with Myc and polyhistidine residues at the COOH terminus was expressed in mammalian cells and secreted into the medium as a disulphide-linked homodimer protein. We also found that the human activin betaE protein could bind to follistatin, an activin-binding protein. Northern blot analysis showed that this gene was expressed as a major transcript of 2.7 kb predominantly in human liver. These findings suggest that activin E (dimeric protein) may play a role in humans.

Neuroendocrine control of follicle-stimulating hormone (FSH) secretion: II. Is follistatin-induced suppression of FSH secretion mediated via changes in activin availability and does it involve changes in gonadotropin-releasing hormone secretion?

The objective of the present study was to determine to what extent activin participates in setting the level of FSH secretion and if this regulation includes mediation via changes in GnRH secretion. We administered follistatin, the high-affinity binding protein for activin, to five ovariectomized sheep; we reasoned that the resultant binding of follistatin to activin should lower activin bioavailability and FSH secretion. Hypophyseal portal and peripheral blood samples were collected simultaneously at 10-min intervals for 18 h to measure GnRH, LH, FSH, and both activin-free and total follistatin. Six hours into collection, each ewe received 150 microg/kg i.v. of recombinant human follistatin-288. A week later, the same ewes were subjected to a second series of blood collections of similar length (time control). The FSH levels in pituitary portal blood were approximately 8-fold higher than those in the peripheral circulation. The FSH secretory patterns changed minimally during the time-control period. In contrast, follistatin had profound suppressive effects on FSH secretion. Maximal FSH suppression after FS-288 administration occurred at 5-6 h in the pituitary portal (65% suppression) and 9-10 h in the peripheral (48% suppression) circulation. Follistatin had no effect on GnRH or LH secretory patterns. Disappearance of total follistatin (i.e., free follistatin plus activin-bound follistatin) from the circulation was slower (P < 0.05) than that of free follistatin alone, suggesting that some of the follistatin was complexed with circulating activin, thus reducing the bioavailability of activin. The slower clearance of total follistatin and the lack of follistatin effects on GnRH secretion suggest that changes in activin bioavailability dictate the level of pituitary FSH secretion and that this is a pituitary-specific effect.

Genomic organization and promoter analysis of mouse follistatin-related gene (FLRG)

Follistatin (FS) is well characterized as an activin-binding protein. Recently, a novel follistatin-like protein called follistatin-related gene (FLRG) that has a similar domain organization to that of follistatin has been identified. Like follistatins, FLRG binds activins and bone morphogenetic proteins (BMPs). To study the regulation of FLRG expression, we have analyzed the genomic organization and promoter of the mouse FLRG gene. The mouse FLRG gene consists of five exons, and each encodes discrete functional regions. The overall genomic structure of FLRG is similar to that of FS except that the FLRG gene is missing one exon that codes a third FS domain found in FS. The promoter that covers 2.5 kbp and is linked to a luciferase reporter construct is active in human cervical carcinoma HeLa cells as well as in human embryonic kidney (HEK293) cells. Deletion analysis of the promoter regions indicates that a proximal 550 base pairs are enough for basal FLRG promoter activity in the cell lines. FLRG promoter activity is significantly augmented by phorbol 12-myristate 13-acetate (PMA) treatment, but not by cAMP stimulation. By contrast, FS promoter is activatable either by cAMP or PMA. Thus, although FS and FLRG are structurally and functionally related, their modes of regulation by external stimuli are different.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

An emerging role for co-receptors in inhibin signal transduction

While many transforming growth factor-beta (TGFbeta) superfamily ligands such as TGFbeta, activin, and the bone morphogenic proteins (BMPs) are critical to the control of growth, differentiation, and cell fate, inhibin has a more limited role and is primarily responsible for the regulation of one hormone from one cell-type in the anterior pituitary. Inhibin is an endocrine hormone, produced by the gonads, that inhibits follicle stimulating hormone (FSH) release from the pituitary gonadotrope. The other hormones in the superfamily do not appear to act in an endocrine fashion, but rather control cell function in a paracrine or autocrine manner. Many components of the TGFbeta/activin/BMP signal transduction pathway have been elegantly defined; however, the mechanism of inhibin action has not been completely dissected. Several cell surface proteins that associate with inhibin have been identified recently, and these molecules may provide the clues necessary to understand how inhibin regulates reproductive function.

Identification and characterization of a novel follistatin-like protein as a binding protein for the TGF-beta family

Follistatin is an activin-binding protein that prevents activin from binding to its receptors and neutralizes its activity. Follistatin also binds bone morphogenetic proteins (BMPs). In this study, we report the identification of a novel follistatin-like protein from mouse. The mouse cDNA encodes a 256-residue precursor and most likely a mouse homologue of human FLRG, which was found at the breakpoint of the chromosomal rearrangement in a B-cell line. Whereas follistatin has three follistatin domains, which are presumed to be growth factor binding motifs, FLRG possesses only two follistatin domains. Northern blotting revealed that mRNAs for FLRG were abundantly expressed in heart, lung, kidney, and testis in mouse. The recombinant mouse FLRG proteins were found to have binding activity for both activin and bone morphogenetic protein-2. Like follistatin, FLRG has higher affinity for activin than for BMP-2. The FLRG protein inhibited activin-induced and BMP-2-induced transcriptional responses in a dose-dependent manner. Glutathione S-transferase fusion proteins encoding various regions of FLRG were produced and studied. Ligand blotting using (125)I-activin revealed that the COOH-terminal region containing the second follistatin domain was able to bind activin. Our finding implies that cellular signaling by activin and BMPs is tightly regulated by multiple members of the follistatin family.