Follistatin and activin in bone: expression and localization during endochondral bone development

Gene therapy for follistatin mitigates systemic metabolic inflammation and post-traumatic arthritis in high-fat diet-induced obesity

Obesity-associated inflammation and loss of muscle function play critical roles in the development of osteoarthritis (OA); thus, therapies that target muscle tissue may provide novel approaches to restoring metabolic and biomechanical dysfunction associated with obesity. Follistatin (FST), a protein that binds myostatin and activin, may have the potential to enhance muscle formation while inhibiting inflammation. Here, we hypothesized that adeno-associated virus 9 (AAV9) delivery of FST enhances muscle formation and mitigates metabolic inflammation and knee OA caused by a high-fat diet in mice. AAV-mediated FST delivery exhibited decreased obesity-induced inflammatory adipokines and cytokines systemically and in the joint synovial fluid. Regardless of diet, mice receiving FST gene therapy were protected from post-traumatic OA and bone remodeling induced by joint injury. Together, these findings suggest that FST gene therapy may provide a multifactorial therapeutic approach for injury-induced OA and metabolic inflammation in obesity.

Two-day-treatment of Activin-A leads to transient change in SV-HFO osteoblast gene expression and reduction in matrix mineralization

Activins regulate bone formation by controlling osteoclasts and osteoblasts. We investigated Activin‐A mechanism of action on human osteoblast mineralization, RNA and microRNA (miRNA) expression profile. A single 2‐day treatment of Activin‐A at Day 5 of osteoblast differentiation significantly reduced matrix mineralization. Activin A‐treated osteoblasts responded with transient change in gene expression, in a 2‐wave‐fashion. The 38 genes differentially regulated during the first wave (within 8 hr after Activin A start) were involved in transcription regulation. In the second wave (1–2 days after Activin A start), 65 genes were differentially regulated and related to extracellular matrix. Differentially expressed genes in both waves were associated to transforming growth factor beta signaling. We identified which microRNAs modulating osteoblast differentiation were regulated by Activin‐A. In summary, 2‐day treatment with Activin‐A in premineralization period of osteoblast cultures influenced miRNAs, gene transcription, and reduced matrix mineralization. Modulation of Activin A signaling might be useful to control bone quality for therapeutic purposes.

Crosstalk between Activin A and Shh signaling contributes to the proliferation and differentiation of antler chondrocytes

Chondrocyte proliferation and differentiation are crucial for endochondral ossification and strictly regulated by numerous signaling molecules and transcription factors, but the hierarchical regulatory network remains to be deciphered. The present study emphasized the interplay of Activin A, Foxa, Notch and Shh signaling in the proliferation and differentiation of antler chondrocytes. We found that Activin A promoted chondrocyte proliferation and differentiation, and accelerated the transition of cell cycle from G1 into S phase along with the activation of Notch and Shh signaling whose blockage attenuated above function of Activin A. Inhibition of Notch pathway by DAPT led to a significant reduction in the expression of Shh signaling molecules, whereas addition of exogenous rShh rescued the delayed onset of chondrocyte proliferation and differentiation elicited by DAPT, indicating that Notch pathway is upstream of Shh signaling. Further analysis evidenced that DAPT attenuated the activation of Activin A on Shh signaling. Simultaneously, Foxa transcription factors were downstream targets of Shh signaling in chondrocyte differentiation. Moreover, Shh pathway played an important role in the crosstalk between Activin A-Notch signaling and Foxa. Collectively, Shh signaling may act downstream of Notch pathway to mediate the effects of Activin A on the proliferation and differentiation of antler chondrocytes through targeting Foxa.

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.

Overexpression of Bmi1 in Lymphocytes Stimulates Skeletogenesis by Improving the Osteogenic Microenvironment

To investigate whether overexpression of Bmi1 in lymphocytes can stimulate skeletogenesis by improving the osteogenic microenvironment, we examined the skeletal phenotype of EμBmi1 transgenic mice with overexpression of Bmi1 in lymphocytes. The size of the skeleton, trabecular bone volume and osteoblast number, indices of proliferation and differentiation of bone marrow mesenchymal stem cells (BM-MSCs) were increased significantly, ROS levels were reduced and antioxidative capacity was enhanced in EμBmi1 mice compared to WT mice. In PTHrP1-84 knockin (Pthrp(KI/KI)) mice, the expression levels of Bmi1 are reduced and potentially can mediate the premature osteoporosis observed. We therefore generated a Pthrp(KI/KI) mice overexpressing Bmi1 in lymphocytes and compared them with Pthrp(KI/KI) and WT littermates. Overexpression of Bmi1 in Pthrp(KI/KI) mice resulted in a longer lifespan, increased body weight and improvement in skeletal growth and parameters of osteoblastic bone formation with reduced ROS levels and DNA damage response parameters. Our results demonstrate that overexpression of Bmi1 in lymphocytes can stimulate osteogenesis in vivo and partially rescue defects in skeletal growth and osteogenesis in Pthrp(KI/KI) mice. These studies therefore indicate that overexpression of Bmi1 in lymphocytes can stimulate skeletogenesis by inhibiting oxidative stress and improving the osteogenic microenvironment.

Follistatin in chondrocytes: the link between TRPV4 channelopathies and skeletal malformations

Point mutations in the calcium-permeable TRPV4 ion channel have been identified as the cause of autosomal-dominant human motor neuropathies, arthropathies, and skeletal malformations of varying severity. The objective of this study was to determine the mechanism by which TRPV4 channelopathy mutations cause skeletal dysplasia. The human TRPV4(V620I) channelopathy mutation was transfected into primary porcine chondrocytes and caused significant (2.6-fold) up-regulation of follistatin (FST) expression levels. Pore altering mutations that prevent calcium influx through the channel prevented significant FST up-regulation (1.1-fold). We generated a mouse model of the TRPV4(V620I) mutation, and found significant skeletal deformities (e.g., shortening of tibiae and digits, similar to the human disease brachyolmia) and increases in Fst/TRPV4 mRNA levels (2.8-fold). FST was significantly up-regulated in primary chondrocytes transfected with 3 different dysplasia-causing TRPV4 mutations (2- to 2.3-fold), but was not affected by an arthropathy mutation (1.1-fold). Furthermore, FST-loaded microbeads decreased bone ossification in developing chick femora (6%) and tibiae (11%). FST gene and protein levels were also increased 4-fold in human chondrocytes from an individual natively expressing the TRPV4(T89I) mutation. Taken together, these data strongly support that up-regulation of FST in chondrocytes by skeletal dysplasia-inducing TRPV4 mutations contributes to disease pathogenesis.

Differentially expressed microRNAs in chondrocytes from distinct regions of developing human cartilage

There is compelling in vivo evidence from reports on human genetic mutations and transgenic mice that some microRNAs (miRNAs) play an important functional role in regulating skeletal development and growth. A number of published in vitro studies also point toward a role for miRNAs in controlling chondrocyte gene expression and differentiation. However, information on miRNAs that may regulate a specific phase of chondrocyte differentiation (i.e. production of progenitor, differentiated or hypertrophic chondrocytes) is lacking. To attempt to bridge this knowledge gap, we have investigated miRNA expression patterns in human embryonic cartilage tissue. Specifically, a developmental time point was selected, prior to endochondral ossification in the embryonic limb, to permit analysis of three distinct populations of chondrocytes. The location of chondroprogenitor cells, differentiated chondrocytes and hypertrophic chondrocytes in gestational day 54-56 human embryonic limb tissue sections was confirmed both histologically and by specific collagen expression patterns. Laser capture microdissection was utilized to separate the three chondrocyte populations and a miRNA profiling study was carried out using TaqMan® OpenArray® Human MicroRNA Panels (Applied Biosystems®). Here we report on abundantly expressed miRNAs in human embryonic cartilage tissue and, more importantly, we have identified miRNAs that are significantly differentially expressed between precursor, differentiated and hypertrophic chondrocytes by 2-fold or more. Some of the miRNAs identified in this study have been described in other aspects of cartilage or bone biology, while others have not yet been reported in chondrocytes. Finally, a bioinformatics approach was applied to begin to decipher developmental cellular pathways that may be regulated by groups of differentially expressed miRNAs during distinct stages of chondrogenesis. Data obtained from this work will serve as an important resource of information for the field of cartilage biology and will enhance our understanding of miRNA-driven mechanisms regulating cartilage and endochondral bone development, regeneration and repair.

Fetal mesenchymal stromal cells differentiating towards chondrocytes acquire a gene expression profile resembling human growth plate cartilage

We used human fetal bone marrow-derived mesenchymal stromal cells (hfMSCs) differentiating towards chondrocytes as an alternative model for the human growth plate (GP). Our aims were to study gene expression patterns associated with chondrogenic differentiation to assess whether chondrocytes derived from hfMSCs are a suitable model for studying the development and maturation of the GP. hfMSCs efficiently formed hyaline cartilage in a pellet culture in the presence of TGFβ3 and BMP6. Microarray and principal component analysis were applied to study gene expression profiles during chondrogenic differentiation. A set of 232 genes was found to correlate with in vitro cartilage formation. Several identified genes are known to be involved in cartilage formation and validate the robustness of the differentiating hfMSC model. KEGG pathway analysis using the 232 genes revealed 9 significant signaling pathways correlated with cartilage formation. To determine the progression of growth plate cartilage formation, we compared the gene expression profile of differentiating hfMSCs with previously established expression profiles of epiphyseal GP cartilage. As differentiation towards chondrocytes proceeds, hfMSCs gradually obtain a gene expression profile resembling epiphyseal GP cartilage. We visualized the differences in gene expression profiles as protein interaction clusters and identified many protein clusters that are activated during the early chondrogenic differentiation of hfMSCs showing the potential of this system to study GP development.

The influence of follistatin on mechanical properties of bone tissue in growing mice with overexpression of follistatin

Mechanical competence of bones is mainly associated with tissue quality that depends on proper bone metabolism processes. An imbalance in the regulation of bone metabolism leads to pathological changes in bone tissue leading to susceptibility to bone fractures and bone deterioration processes. Bone metabolism is regulated to a large extent by the members of the transforming growth factor-β superfamily, i.e., activins and bone morphogenetic proteins. However, their function is regulated by a single-chain protein called follistatin (FS). The aim of this study was to test the hypothesis that overexpression of FS in growing mice results in impairments in bone morphology and mechanical properties. Moreover, we wanted to investigate how geometrical, structural and material properties of bone tissue change with age. The experiment was performed on growing C57BL/6 TgNK14-mFst/6J mice, overexpressing FS (F mice) versus C57BL/6J mice used as controls (C mice). To establish how overexpression of FS influences bone tissue quality, we studied mice femurs to determine geometrical, structural and material properties of the skeleton. To determine mechanical resistance of bone tissue, femurs were loaded to failure in a three-point bending test. Obtained results indicated that overexpression of FS negatively influences bone metabolism. It was found that mutation results with a significant decrease of all measured biomechanical strength variables in F mice in comparison to C mice. Overexpression of FS leads to decreased quality of skeleton, increasing susceptibility to bone fractures.

Serum follistatin in patients with prostate cancer metastatic to the bone

The clinical significance of circulating follistatin (FLST), an inhibitor of the multifunctional cytokine activin A (Act A), was investigated in patients with prostate cancer (PCa). The serum concentrations of this molecule were determined by an enzyme-linked immunosorbent assay (ELISA) in PCa patients with (M+) or without (M0) bone metastases, in patients with benign prostate hyperplasia (BPH) and in healthy subjects (HS). The effectiveness of FLST in detecting PCa patients with skeletal metastases was determined by the receiver operating characteristic (ROC) curve analysis. Serum FLST was significantly higher in PCa patients than in BPH patients (P = 0.001) or HS (P = 0.011). Conversely, in BPH patients, FLST levels resulted lower than in HS (P = 0.025). In cancer patients the serum concentrations of FLST significantly correlated with the presence of bone metastases (P = 0.0005) or increased PSA levels (P = 0.04). Interestingly, significant differences in the ratio between FLST and Act A serum concentrations (FLST/Act A) were observed between HS and BPH patients (P = 0.001) or PCa patients (P = 0.0005). Finally, ROC curve analysis, highlighted a sound diagnostic performance of FLST in detecting M+ patients (P = 0.0001). However, the diagnostic effectiveness of FLST did not result significantly superior to that of Act A or PSA. These findings suggest that FLST may be regarded as a potential, molecular target in the treatment of metastatic bone disease while its clinical role as soluble marker in the clinical management of PCa patients with bone metastases needs to be better defined.

Follistatin as a potent regulator of bone metabolism

Follistatin is a monomeric glycoprotein, distributed in a wide range of tissues. Recent work has demonstrated that this protein is a pluripotential molecule that has no structural similarity but is functionally associated with members of the transforming growth factor (TGF)-β superfamily, which indicates its wide range of action. Members of the TGF-β superfamily, especially activins and bone morphogenetic proteins are involved in bone metabolism. They play an important role in bone physiology, influencing bone growth, turnover, bone formation and cartilage induction. As follistatin is considered to be the antagonist of the TGF-β superfamily members, it plays an important role in bone metabolism and development.

Activin subunit and receptor expression in normal and cleft human fetal palate tissues

Craniofacial malformations, such as cleft palate, present serious complications in the newborn and are often of unknown etiology. Activin BA subunit deletion leads to cleft palate in mice, but the expression of this protein in the human palate has not been explored. Our goal was to determine the spatial and temporal expression of inhibin/activin subunits; the binding protein, follistatin; and activin receptors in the human fetal palate. Residual human fetal palate tissues, with or without cleft, were collected during routine autopsy at Women and Infants Hospital. Inhibin/activin alpha and beta subunits, follistatin, and activin receptor protein and mRNA expression were studied by immunocytochemistry and reverse-transcriptase polymerase chain reaction (RT-PCR) experiments, respectively. Dimeric activin A levels were compared in cleft and normal palate tissue homogenates by immunoassay. Activin BA, follistatin, and activin receptor type IIA proteins were observed in normal and cleft palate tissues throughout pregnancy (gestational weeks 11 to 40). Proteins were predominantly found in developing bone cells, with no significant group differences. Inhibin/activin BA subunit, follistatin, and activin receptor mRNAs were also detected in normal and cleft fetal palate tissues, but inhibin alpha and BB subunit were absent. Inhibin/activin BA subunit expression was consistent with the presence of dimeric activin A, but levels did not differ significantly between cleft and control tissues. Inhibin/activin BA subunit, follistatin, and activin receptor proteins and mRNAs are present in the human fetal palate. These data suggest that activin signalling has the potential to be associated with human palate development.

Inhibin A is an endocrine stimulator of bone mass and strength

Gonadal function plays a major role in bone homeostasis. It is widely held that the skeletal consequences of hypogonadism are solely due to a loss of sex steroids; however, increases in bone turnover begin during perimenopause before decreases in serum estradiol levels. These data and our demonstration that inhibins acutely regulate bone cell differentiation in vitro led us to test whether inhibin A (InhA) regulates bone mass in vivo. Using a transgenic model of inducible human InhA expression, InhA increased total body bone mineral density, increased bone volume, and improved biomechanical properties at the proximal tibia in intact mice and also prevented the loss of BMD and bone volume and strength associated with gonadectomy at both the spine and proximal tibia. In addition, InhA increased mineral apposition rate, double-labeled surface, and serum osteocalcin levels in vivo and osteoblastogenesis ex vivo without affecting osteoclast number or activity. Together these results demonstrate novel stimulatory effects of InhA on the skeleton in vivo. These studies provide in vivo evidence demonstrating that gonadal factors other than sex steroids play an important role in regulating bone mass and strength and, combined with our previous clinical data, suggest that gonadal InhA may be a component of the normal endocrine repertoire that regulates bone quality in both the axial and appendicular skeleton.

Granulosa cell-specific inactivation of follistatin causes female fertility defects

Follistatin plays an important role in female physiology by regulating FSH levels through blocking activin actions. Failure to regulate FSH has been implicated as a potential cause of premature ovarian failure. Premature ovarian failure is characterized by amenorrhea, infertility, and elevated gonadotropin levels in women under the age of 40. Because follistatin is essential for postnatal viability, we designed a cre/loxP conditional knockout system to render the follistatin gene null specifically in the granulosa cells of the postnatal ovary using Amhr2cre transgenic mice. The follistatin conditional knockout females develop fertility defects, including reduced litter number and litter sizes and, in the most severe case, infertility. Reduced numbers of ovarian follicles, ovulation and fertilization defects, elevated levels of serum FSH and LH, and reduced levels of testosterone were observed in these mice. These findings demonstrate that compromising granulosa cell follistatin function leads to findings similar to those characterized in premature ovarian failure. Follistatin conditional knockouts may therefore be a useful model with which to further study this human syndrome. These studies are the first report of a granulosa cell-specific deletion of a gene in the postnatal ovary and have important implications for future endeavors to generate ovary-specific knockout mouse models.

Phenotype discovery by gene expression profiling: mapping of biological processes linked to BMP-2-mediated osteoblast differentiation

Understanding physiological control of osteoblast differentiation necessitates characterization of the regulatory signals that initiate the events directing a cell to lineage commitment and establishing competency for bone formation. The bone morphogenetic protein, BMP-2, a member of the TGFbeta superfamily, induces osteoblast differentiation and functions through the Smad signal transduction pathway during in vivo bone formation. However, the molecular targets of BMP-mediated gene transcription during the process of osteoblast differentiation have not been comprehensively identified. In the present study, BMP-2 responsive factors involved in the early stages of commitment and differentiation to the osteoblast phenotype were analyzed by microarray gene expression profiling in samples ranging from 1 to 24 h following BMP-2 dependent differentiation of C2C12 premyoblasts into the osteogenic lineage. A total of 1,800 genes were responsive to BMP-2 and expression was modulated from 3- to 14-fold for less than 100 genes during the time course. Approximately 50% of these 100 genes are either up- or downregulated. Major events associated with phenotypic changes towards the osteogenic lineage were identified from hierarchical and functional clustering analyses. BMP-2 immediately responsive genes (1-4 h), which exhibited either transient or sustained expression, reflect activation and repression of non-osseous BMP-2 developmental systems. This initial response was followed by waves of expression of nuclear proteins and developmental regulatory factors including inhibitors of DNA binding, Runx2, C/EBP, Zn finger binding proteins, forkhead, and numerous homeobox proteins (e.g., CDP/cut, paired, distaless, Hox) which are expressed at characterized stages during osteoblast differentiation. A sequential profile of genes mediating changes in cell morphology, cell growth, and basement membrane formation is observed as a secondary transient early response (2-8 h). Commitment to the osteogenic phenotype is recognized by 8 h, reflected by downregulation of most myogenic-related genes and induction of a spectrum of signaling proteins and enzymes facilitating synthesis and assembly of an extracellular skeletal environment. These genes included collagens Type I and VI and the small leucine rich repeat family of proteoglycans (e.g., decorin, biglycan, osteomodulin, fibromodulin, and osteoadherin/osteoglycin) that reached peak expression at 24 h. With extracellular matrix development, the bone phenotype was further established from 16 to 24 h by induction of genes for cell adhesion and communication and enzymes that organize the bone ECM. Our microarray analysis resulted in the discovery of a class of genes, initially described in relation to differentiation of astrocytes and oligodendrocytes that are functionally coupled to signals for cellular extensions. They include nexin, neuropilin, latexin, neuroglian, neuron specific gene 1, and Ulip; suggesting novel roles for these genes in the bone microenvironment. This global analysis identified a multistage molecular and cellular cascade that supports BMP-2-mediated osteoblast differentiation.

Bone morphogenetic proteins, their antagonists, and the skeleton

Skeletal homeostasis is determined by systemic hormones and local factors. Bone morphogenetic proteins (BMP) are unique because they induce the differentiation of mesenchymal cells toward cells of the osteoblastic lineage and also enhance the differentiated function of the osteoblast. However, the activity of BMPs needs to be tempered by intracellular and extracellular antagonists. BMPs bind to specific receptors and signal by phosphorylating the cytoplasmic proteins mothers against decapentaplegic (Smad) 1 and 5, which form heterodimers with Smad 4, and after nuclear translocation regulate transcription. BMP antagonists can be categorized as pseudoreceptors that compete with signaling receptors, inhibitory Smads that block signaling, intracellular binding proteins that bind Smad 1 and 5, and factors that induce ubiquitination and proteolysis of signaling Smads. In addition, a large number of extracellular proteins that bind BMPs and prevent their binding to signaling receptors have emerged. They are the components of the Spemann organizer, noggin, chordin, and follistatin, members of the Dan/Cerberus family, and twisted gastrulation. The antagonists tend to be specific for BMPs and are regulated by BMPs, indicating the existence and need of local feedback mechanisms to temper BMP cellular activities.

Inhibition of cartilage destruction by double gene transfer of IL-1Ra and IL-10 involves the activin pathway

The objective of the study was to determine the effects and the molecular background of interleukin-1 receptor antagonist (IL-1Ra) and vIL-10 double gene transfer into human synovial fibroblasts from patients with rheumatoid arthritis (RA) using the SCID mouse model for cartilage erosion in RA. RA synovial fibroblasts were transduced with retro- or adenoviruses encoding IL-1Ra and/or viral IL-10 (vIL-10). SCID mice were engrafted subcutaneously with IL-1Ra and vIL-10 transduced human rheumatoid synovial fibroblasts and normal cartilage. In parallel, gene expression analysis before and after gene transfer using RNA arbitrarily primed PCR in combination with cDNA array was performed. vIL-10 and IL-1Ra double gene transfer resulted in inhibition of cartilage invasion and degradation by RA synovial fibroblasts when compared with control transduced and non-transduced implants. Expression of key genes that were altered after double gene transfer were related to the activin pathway. The results demonstrate not only that virus-based gene transfer using a combination of two joint-protective genes is a feasible approach to inhibit cartilage degradation by activated RA synovial fibroblasts, but also that the underlying molecular effects include modulation of the activin pathway.

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.

Follistatin-related protein and follistatin differentially neutralize endogenous vs. exogenous activin

Follistatin-related protein (FSRP) is a new addition to the expanding follistatin (FS)-related gene family whose members contain at least one conserved 10-cysteine follistatin domain. In contrast to other members of this family, FSRP and follistatin also share a common exon/intron domain structure, substantial primary sequence homology, and an ability to irreversibly bind activin. In this study, we further explored the hypothesis that FSRP is a functional as well as structural homologue of FS. N-terminal sequencing of recombinant FSRP revealed that signal peptide cleavage occurs within exon 1, a significant structural difference from FS, in which cleavage occurs at the exon/intron boundary. Solid-phase radioligand competition assays revealed both FS and FSRP to preferentially bind activin with the next closest TGF-beta superfamily member, bone-morphogenic protein-7, being at least 500-fold less potent. Consistent with their similar activin-binding affinities, FSRP and FS both prevented exogenous (endocrine or paracrine) activin from accessing its receptor and inducing gene transcription in bioassays. However, FS was at least 100-fold more potent than FSRP in inhibiting gene transcription and FSH release mediated by endogenously produced (autocrine) activin-A or activin-B in multiple cell systems. Finally, FSRP lacks the heparin-binding sequence found in FS, and we found that it was also unable to bind cell surface heparin sulfated proteoglycans. These findings suggest that structural differences between FSRP and FS may underlie their different neutralizating capabilities with respect to exogenous vs. endogenous activin. Taken together with our previous studies showing that activin binding is essential for FS's biological activity, the differential activities of FSRP and FS further indicate that activin binding is necessary but not sufficient to account for all of FS's actions.

Inhibin suppresses and activin stimulates osteoblastogenesis and osteoclastogenesis in murine bone marrow cultures

Using primary murine bone marrow cell cultures, we demonstrate that inhibin suppresses osteoblastogenesis and osteoclastogenesis. In contrast, activin supports osteoblast formation (by alkaline phosphatase-positive and mineralized colony formation); and activin also stimulates osteoclast formation (as measured by staining tartrate-resistant acid phosphatase-positive multinucleated cells). Inhibin, the activin antagonist follistatin, and the bone morphogenetic protein antagonist noggin can all suppress endogenous activin accumulation in bone marrow cultures. Associated with this decrease in activin is the loss of mineralized osteoblastic colony formation (colony forming unit-osteoblast; CFU-OB). However, exogenous activin administration, even in the presence of noggin, permits both alkaline phosphatase-positive and CFU-OB colony formation in vitro. In contrast, the stimulatory effects of locally produced activin on osteoblast and osteoclast development are not likely to be dominant over the suppressive effects of gonadally derived inhibin. The suppressive effect of inhibin is maintained in the presence of either activin or bone morphogenetic protein, suggesting the presence of a distinct inhibin-specific receptor. Taken together, the direct regulation of osteoblastogenesis and osteoclastogenesis by inhibin and activin in vitro suggest that changes in the inhibin/activin ratio detected by bone marrow cells, during the perimenopausal transition, contribute to altered cell differentiation and may be associated with the increased bone resorption observed at this time.

Possible involvement of protein kinases and Smad2 signaling pathways on osteoclast differentiation enhanced by activin A

Bone tissues reportedly contain considerable amounts of activin A and follistatin, an activin A-binding protein. In the present study, we found that follistatin strongly inhibited osteoclast formation in cocultures of mouse bone marrow cells and primary osteoblasts induced by 1alpha,25 dihydroxyvitamin D(3), prostaglandin E(2), and interleukin-1alpha. Antibody aganist activin A also inhibited the osteoclast formation. Furthermore, activin A synergistically stimulated osteoclast differentiation mediated by receptor activator NF-kappaB ligand (RANKL). RT-PCR analysis revealed that osteoblasts produced not only activin A but also follistatin. Western blot analysis of a panel of phosphorylated proteins revealed that activin A stimulated the phosphorylation of p44/42 mitogen activated protein (MAP) kinase (ERK1/2) and p38 MAP kinase in macrophage colony-stimulating factor-dependent bone marrow macrophages (M-BMMPhis). In addition, phosphorylation of Smad2 was observed in M-BMMPhis stimulated with activin A. These findings indicate that the phosphorylation of p44/42 MAP kinase, p38 MAP kinase, and Smad2 is involved in activin A-enhanced osteoclast differentiation induced by RANKL. Taken together, these results suggest that both activin A and follistatin produced by osteoblasts may play an important role in osteoclast differentiation through MAP kinases and Smad2 signaling pathways.

Activin A is an essential cofactor for osteoclast induction

Recently, receptor activator of NF-kappaB ligand (RANKL) was shown to be necessary for osteoclast formation. We now report that activin A, a cytokine enriched in bone matrix and secreted by osteoblasts and osteoclasts, powerfully synergized with RANKL for induction of osteoclast-like cells (OCL) from bone marrow precursors depleted of stromal cells. Moreover, OCL formation in RANKL was virtually abolished by soluble type II A activin receptors (ActR-II(A)), suggesting that activin A is essential for OCL formation. Activin A was most effective when precursors were exposed to RANKL and activin A simultaneously: resistance to OCL-induction that occurs when precursors are pre-incubated in M-CSF was reduced. Incubation on bone matrix also enhanced the sensitivity of precursors to OCL-induction by RANKL; and this was prevented by soluble ActR-II(A). Thus, activin A in bone matrix, or released from osteoblastic or other cells, enhances the osteoclast-forming potential of precursors and synergizes with RANKL in inducing osteoclastic differentiation.

Identification of naturally occurring follistatin complexes in human biological fluids

Follistatin (FS) binds activin and inhibin proteins. Many organs are sensitive to activin and inhibin; thus the formation of FS-activin/inhibin complexes is important to our understanding of ligand activity. Other investigators studying FS have detected large molecular weight immunoreactive FS bands (greater than the expected molecular weight of FS alone) that have not been well characterized. The goal of this study was to identify naturally occurring FS monomers and FS-activin/inhibin complexes in several organ systems. The pituitary, ovary, kidney, and urine were chosen for this investigation. Molecular masses were assigned to in vitro assemblies of complexes containing recombinant inhibin or activin with FS for comparison with naturally occurring FS forms. The recombinant complex of FS-activin was primarily 97-kDa size, while FS-inhibin complexes were detected in a range of molecular sizes from 66 kDa to 97 kDa, 133 kDa, and > 220 kDa. FS-containing complexes of 66-kDa, 97-kDa, and 133-kDa were identified in the tissues examined and in pregnant urine. Our study points to the assembly of a series of FS-activin/inhibin complexes in a variety of organ systems that may impact upon the available amount of free versus bound (or "complexed") ligand, which must be considered when investigating the biology of activin- or inhibin-responsive cells. In addition, urine may be an important biological fluid that can be used to measure significant changes in circulating FS complexes.

Bone morphogenetic proteins induce the expression of noggin, which limits their activity in cultured rat osteoblasts

Bone morphogenetic proteins (BMPs) induce the differentiation of cells of the osteoblastic lineage and enhance the function of the osteoblast. Growth factors are regulated by binding proteins, but there is no information about binding proteins for BMPs in skeletal cells. Noggin specifically binds BMPs, but its expression by cells of the osteoblastic lineage has not been reported. We tested for the expression of noggin and its induction by BMP-2 in cultures of osteoblast-enriched cells from 22-d-old fetal rat calvariae (Ob cells). BMP-2 caused a time- and dose-dependent increase in noggin mRNA and polypeptide levels, as determined by Northern and Western blot analyses. The effects of BMP-2 on noggin transcripts were dependent on protein, but independent of DNA synthesis. BMP-2 increased the rates of noggin transcription as determined by nuclear run-on assays. BMP-4, BMP-6, and TGF-beta1 increased noggin mRNA in Ob cells, but basic fibroblast growth factor, platelet- derived growth factor BB, and IGF-I did not. Noggin decreased the stimulatory effects of BMPs on DNA and collagen synthesis and alkaline phosphatase activity in Ob cells. In conclusion, BMPs induce noggin transcription in Ob cells, a probable mechanism to limit BMP action in osteoblasts.

Follistatin: a multifunctional regulatory protein

Follistatin was first described in 1987 as a follicle-stimulating hormone inhibiting substance present in ovarian follicular fluid. We now know that this effect of follistatin is only one of its many properties in a number of reproductive and nonreproductive systems. A majority of these functions are facilitated through the affinity of follistatin for activin, where activin's effects are neutralized through its binding to follistatin. As such, the interplay between follistatin and activin represents a powerful regulatory mechanism that impinges on a variety of cellular processes within the body. In this review we focus on the biochemical characteristics of follistatin and its interaction with activin and discuss the emerging role of these proteins as potent tissue regulators in the gonad, pituitary gland, pregnancy membranes, vasculature, and liver. Consideration is also given to the larger family of proteins that contain follistatin-like modules, in particular with regard to their functional and structural implications.

Immunohistochemical detection of activin A, follistatin, and activin receptors during fracture healing in the rat

Activins are multifunctional proteins that belong to the transforming growth factor-beta superfamily and are thought to play an important role in modulating the formation of bone. Activins exert their cellular effects by way of activin type-I and type-II serine/threonine kinase receptors. Follistatin is an activin-binding protein that can suppress the biological effects of activins. In this study, the immunohistochemical expression of activin A, follistatin, and activin receptors was studied during fracture healing in the rat. Activin A was weakly detected in the periosteum near the fracture ends at an early stage but was absent in the chondrocytes around the fracture gap, where endochondral ossification took place. An antibody to follistatin stained osteogenic cells in the periosteum near the fracture ends; moderate and strong staining were observed in proliferating, mature, and hypertrophied chondrocytes at the sites of endochondral ossification. Levels of activin A and follistatin were high near the osteoblasts on the surface of the newly formed trabecular bone. In addition, an intense localization of activin A was noted where multinucleated osteoclast-like cells were present. This study suggests that the activin-follistatin system may contribute to cellular events related to the formation and remodeling of bone during fracture healing. Activin type-I and type-II receptors were co-expressed in intramembranous and endochondral ossification sites. The expression of activin type-I, type-II, and type-IIB receptors in the absence of activin A in the endochondral ossification suggests that other isoforms of activins may signal by way of these receptors.