Follistatin: a multifunctional regulatory protein

FoxL2 and Smad3 coordinately regulate follistatin gene transcription

Follistatin is a transcriptional target and a modulator of activin action. Through an autocrine/paracrine loop, activin controls follistatin levels and thus regulates its own bioavailability. In gonadotropic alphaT3-1 cells, activin induces follistatin transcription primarily through the action of Smad3 at an intronic Smad-binding element (SBE1). Using a proteomics approach, we searched for endogenous alphaT3-1 proteins that participate in SBE1-mediated transcription. We identified FoxL2, a member of the forkhead family, as a candidate modulator of SBE1 function. Mutations of FoxL2 are associated with the blepharophimosis/ptosis/epicanthus inversus syndrome characterized with craniofacial defects and premature ovarian failure. FoxL2 localizes to alpha-glycoprotein subunit- and follicle-stimulating hormone beta-positive cells of the adult mouse pituitary and is present in alphaT3-1 and LbetaT2 cells, but its pituitary actions remain largely unknown. We have determined that FoxL2 binds to a forkhead-binding element (FKHB) located just downstream of the SBE1 site of the follistatin gene and functions as a Smad3 partner to drive SBE1-mediated transcription in alphaT3-1 cells treated with activin. Chromatin immunoprecipitation assays confirm that endogenous FoxL2 and Smad3 are recruited to the intronic enhancer of the follistatin gene where the SBE1 and FKHB sites are located. Exogenous FoxL2 enhances SBE1-mediated transcription, and short hairpin RNA-mediated knockdown of endogenous FoxL2 protein compromises this effect in alphaT3-1 cells. FoxL2 directly associates with Smad3 but not Smad2 or Smad4. This association between Smad3 and FoxL2 is mediated by the MH2 domain of Smad3 and is dependent on an intact forkhead domain in FoxL2. Altogether, these observations highlight a novel role for FoxL2 and suggest that it may function as a transcriptional regulator and a coordinator of Smad3 targets.

Activin and related proteins in inflammation: not just interested bystanders

Activin A, a member of the transforming growth factor-beta superfamily, is released rapidly into the circulation during inflammation. This review examines the evidence that activin is a critical mediator of inflammation and immunity. Activin modulates several aspects of the inflammatory response, including release of pro-inflammatory cytokines, nitric oxide production and immune cell activity. Crucially, inhibiting activin with follistatin, a high affinity binding protein, alters the pattern of cytokines released and improves survival in a mouse model of endotoxic shock. Serum and tissue concentrations of activin are elevated in a wide range of pathological conditions. The utility of activin as a diagnostic marker of clinical inflammation and the use of follistatin to block activin actions therapeutically are also discussed.

Inhibition of myostatin with emphasis on follistatin as a therapy for muscle disease

In most cases, pharmacologic strategies to treat genetic muscle disorders and certain acquired disorders, such as sporadic inclusion body myositis, have produced modest clinical benefits. In these conditions, inhibition of the myostatin pathway represents an alternative strategy to improve functional outcomes. Preclinical data that support this approach clearly demonstrate the potential for blocking the myostatin pathway. Follistatin has emerged as a powerful antagonist of myostatin that can increase muscle mass and strength. Follistatin was first isolated from the ovary and is known to suppress follicle-stimulating hormone. This raises concerns for potential adverse effects on the hypothalamic-pituitary-gonadal axis and possible reproductive capabilities. In this review we demonstrate a strategy to bypass off-target effects using an alternatively spliced cDNA of follistatin (FS344) delivered by adeno-associated virus (AAV) to muscle. The transgene product is a peptide of 315 amino acids that is secreted from the muscle and circulates in the serum, thus avoiding cell-surface binding sites. Using this approach our translational studies show increased muscle size and strength in species ranging from mice to monkeys. Adverse effects are avoided, and no organ system pathology or change in reproductive capabilities has been seen. These findings provide the impetus to move toward gene therapy clinical trials with delivery of AAV-FS344 to increase size and function of muscle in patients with neuromuscular disease.

Activin-A attenuates several human natural killer cell functions

Dendritic-cell (DC) and natural killer (NK)-cell interactions are critical in sculpting the adaptive immune response. However, the mechanisms by which DCs down-regulate NK-cell functions are not well understood. NK-cell function is inhibited by transforming growth factor beta (TGF-beta), but DCs do not appear to produce TGF-beta. We have previously shown that activated human DCs produce large amounts of activin-A, a TGF-beta superfamily member, which autoregulates DC function. The present report shows that NK-cells express type I and II activin receptors and that activin-A triggers NK-cell Smad 2/3 signaling. Furthermore, activin-A directly regulates NK cell functions by (1) down-regulating the T-box transcription factor T-bet and interferon gamma (IFN-gamma) but not perforin or granzyme mRNA; (2) suppressing NK-cell IFN-gamma production as potently as TGF-beta; and (3) suppressing NK-cell CD25 expression and proliferation and sculpting NK-cell cytokine and chemokine profiles. Interestingly, unlike TGF-beta, activin-A weakly down-regulates the NK-cell natural cytotoxicity receptors (NCRs) NKp30 and NKG2D but does not attenuate their cytotoxic function. These findings provide the first evidence for a novel immune regulatory role of activin-A during DC-mediated NK-cell regulation, highlighting the potential of antagonizing activin-A signaling in vivo to enhance NK cell-mediated immune functions and adaptive immunity.

Influence of olfactory epithelium on mitral/tufted cell dendritic outgrowth

Stereotypical connections between olfactory sensory neuron axons and mitral cell dendrites in the olfactory bulb establish the first synaptic relay for olfactory perception. While mechanisms of olfactory sensory axon targeting are reported, molecular regulation of mitral cell dendritic growth and refinement are unclear. During embryonic development, mitral cell dendritic distribution overlaps with olfactory sensory axon terminals in the olfactory bulb. In this study, we investigate whether olfactory sensory neurons in the olfactory epithelium influence mitral cell dendritic outgrowth in vitro. We report a soluble trophic activity in the olfactory epithelium conditioned medium which promotes mitral/tufted cell neurite outgrowth. While the trophic activity is present in both embryonic and postnatal olfactory epithelia, only embryonic but not postnatal mitral/tufted cells respond to this activity. We show that BMP2, 5 and 7 promote mitral/tufted cells neurite outgrowth. However, the BMP antagonist, Noggin, fails to neutralize the olfactory epithelium derived neurite growth promoting activity. We provide evidence that olfactory epithelium derived activity is a protein factor with molecular weight between 50–100 kD. We also observed that Follistatin can effectively neutralize the olfactory epithelium derived activity, suggesting that TGF-beta family proteins are involved to promote mitral/tufted dendritic elaboration.

Activin A induces Langerhans cell differentiation in vitro and in human skin explants

Langerhans cells (LC) represent a well characterized subset of dendritic cells located in the epidermis of skin and mucosae. In vivo, they originate from resident and blood-borne precursors in the presence of keratinocyte-derived TGFbeta. In vitro, LC can be generated from monocytes in the presence of GM-CSF, IL-4 and TGFbeta. However, the signals that induce LC during an inflammatory reaction are not fully investigated. Here we report that Activin A, a TGFbeta family member induced by pro-inflammatory cytokines and involved in skin morphogenesis and wound healing, induces the differentiation of human monocytes into LC in the absence of TGFbeta. Activin A-induced LC are Langerin+, Birbeck granules+, E-cadherin+, CLA+ and CCR6+ and possess typical APC functions. In human skin explants, intradermal injection of Activin A increased the number of CD1a+ and Langerin+ cells in both the epidermis and dermis by promoting the differentiation of resident precursor cells. High levels of Activin A were present in the upper epidermal layers and in the dermis of Lichen Planus biopsies in association with a marked infiltration of CD1a+ and Langerin+ cells. This study reports that Activin A induces the differentiation of circulating CD14+ cells into LC. Since Activin A is abundantly produced during inflammatory conditions which are also characterized by increased numbers of LC, we propose that this cytokine represents a new pathway, alternative to TGFbeta, responsible for LC differentiation during inflammatory/autoimmune conditions.

Peroxisome proliferator-activated receptor gamma down-regulates follistatin in intestinal epithelial cells through SP1

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) down-regulates the expression of follistatin mRNA in intestinal epithelial cells in vivo. The mechanism of PPARgamma-mediated down-regulation of follistatin was investigated using non-transformed, rat intestinal epithelial cells (RIE-1). RIE cells expressed activin A, the activin receptors ActRI and ActRII, and the follistatin-315 mRNA. RIE-1 cells responded to endogenous activin A, and this response was antagonized by follistatin, as evidenced by changes in cell growth and regulation of an activin-responsive reporter. Using RIE-1 cells, we show that activation of PPARgamma by rosiglitazone reduced follistatin mRNA levels in a dose- and concentration-dependent manner. Down-regulation of follistatin by rosiglitazone required the DNA binding domain of PPARgamma and was dependent upon dimerization with the retinoid X receptor. Inhibition of follistatin expression by rosiglitazone was not associated with decreased follistatin mRNA stability, suggesting that regulation may be at the promoter level. Analysis of the follistatin promoter revealed consensus binding sites for AP-1, AP-2, and Sp1. Targeting the AP-1 pathway with SP600125, an inhibitor of JNK, and TAM67, a dominant negative c-Jun, had no effect on PPARgamma-mediated down-regulation of follistatin. However, the follistatin promoter was dramatically regulated by Sp1, and this regulation was inhibited by PPARgamma expression. Knockdown of Sp1 expression relieved repression of follistatin levels by rosiglitazone. Moreover, PPARgamma was found to interact with Sp1 and repress its transcriptional activation function. Collectively, our data indicate that repression of Sp1 transcriptional activity by PPARgamma is the underlying mechanism responsible for PPARgamma-mediated regulation of follistatin expression.

Estrogen receptor alpha signaling pathways differentially regulate gonadotropin subunit gene expression and serum follicle-stimulating hormone in the female mouse

Estrogen, acting via estrogen receptor (ER)alpha, regulates serum gonadotropin levels and pituitary gonadotropin subunit expression. However, the cellular pathways mediating this regulation are unknown. ERalpha signals through classical estrogen response element (ERE)-dependent genomic as well as nonclassical ERE-independent genomic and nongenomic pathways. Using targeted mutagenesis in mice to disrupt ERalpha DNA binding activity, we previously demonstrated that ERE-independent signaling is sufficient to suppress serum LH levels. In this study, we examined the relative roles of ERE-dependent and -independent estrogen signaling in estrogen regulation of LH, FSH, prolactin, and activin/inhibin subunit gene expression, pituitary LH and FSH protein content, and serum FSH levels. ERE-independent signaling was not sufficient for estrogen to induce pituitary prolactin mRNA or suppress pituitary LHbeta mRNA, LH content, or serum FSH in estrogen-treated ovariectomized mice. However, ERE-independent signaling was sufficient to reduce pituitary glycoprotein hormone alpha-subunit, FSHbeta, and activin-betaB mRNA expression. Together with previous serum LH results, these findings suggest ERE-independent ERalpha signaling suppresses serum LH via reduced secretion, not synthesis. Additionally, ERE-dependent and ERE-independent ERalpha pathways may distinctly regulate steps involved in the synthesis and secretion of FSH.

Postnatal uterine development in Inverdale ewe lambs

Postnatal development of the uterus involves, particularly, development of uterine glands. Studies with ovariectomized ewe lambs demonstrated a role for ovaries in uterine growth and endometrial gland development between postnatal days (PNDs) 14 and 56. The uterotrophic ovarian factor(s) is presumably derived from the large numbers of growing follicles in the neonatal ovary present after PND 14. The Inverdale gene mutation (FecXI) results in an increased ovulation rate in heterozygous ewes; however, homozygous ewes (II) are infertile and have 'streak' ovaries that lack normal developing of preantral and antral follicles. Uteri were obtained on PND 56 to determine whether postnatal uterine development differs between wild-type (++) and II Inverdale ewes. When compared with wild-type ewes, uterine weight of II ewes was 52% lower, and uterine horn length tended to be shorter, resulting in a 68% reduction in uterine weight:length ratio in II ewes. Histomorphometrical analyses determined that endometria and myometria of II ewes were thinner and intercaruncular endometrium contained 38% fewer endometrial glands. Concentrations of estradiol in the neonatal ewes were low and not different between ++ and II ewes, but II ewes had lower concentrations of testosterone and inhibin-alpha between PNDs 14 and 56. Receptors for androgen and activin were detected in the neonatal uteri of both ++ and II ewes. These results support the concept that developing preantral and/or antral follicles of the ovary secrete uterotrophic factors, perhaps testosterone or inhibin-alpha, that acts in an endocrine manner to stimulate uterine growth and endometrial gland development in the neonatal ewes.

Axon-bearing and axon-less horizontal cell subtypes are generated consecutively during chick retinal development from progenitors that are sensitive to follistatin

BACKGROUND

Horizontal cells are retinal interneurons that modulate the output from photoreceptors. A rich literature on the morphological classification and functional properties of HCs in different animals exists, however, the understanding of the events underlying their development is still limited. In most vertebrates including chicken, two main horizontal cell (HC) subtypes are identified based on the presence or absence of an axon.

RESULTS

In this work we have molecularly characterized three HC subtypes based on Lim1, Isl1, GABA and TrkA, a classification that is consistent with three chick HC subtypes previously defined by morphology. The axon-bearing and axon-less HC subpopulations molecularly defined by Lim1 and Isl1, are born consecutively on embryonic day (E) 3-4 and E4-5, respectively, and exhibit temporally distinguishable periods of migration. Their relative numbers are not adjusted by apoptosis. A sharp decrease of high endogenous levels of the activin-inhibitor follistatin at E3 coincides with the appearance of the Lim1 positive cells. Extending the follistatin exposure of the HC retinal progenitor cells by injection of follistatin at E3 increased the number of both Lim1- and Isl1 positive HCs when analysed at E9.

CONCLUSION

The results imply that the axon-bearing and axon-less HC subgroups are defined early and are generated consecutively from a retinal progenitor cell population that is sensitive to the inhibitory action of follistatin. The results are consistent with a model wherein added follistatin causes HC-generating progenitors to proliferate beyond the normal period of HC generation, thus producing extra HCs of both types that migrate to the HC layer.

Activin-A: a novel dendritic cell–derived cytokine that potently attenuates CD40 ligand–specific cytokine and chemokine production

Activin-A is a transforming growth factor-β (TGF-β) superfamily member that plays a pivotal role in many developmental and reproductive processes. It is also involved in neuroprotection, apoptosis of tumor and some immune cells, wound healing, and cancer. Its role as an immune-regulating protein has not previously been described. Here we demonstrate for the first time that activin-A has potent autocrine effects on the capacity of human dendritic cells (DCs) to stimulate immune responses. Human monocyte-derived DCs (MoDCs) and the CD1c+ and CD123+ peripheral blood DC populations express both activin-A and the type I and II activin receptors. Furthermore, MoDCs and CD1c+ myeloid DCs rapidly secrete high levels of activin-A after exposure to bacteria, specific toll-like receptor (TLR) ligands, or CD40 ligand (CD40L). Blocking autocrine activin-A signaling in DCs using its antagonist, follistatin, enhanced DC cytokine (IL-6, IL-10, IL-12p70, and tumor necrosis factor-α [TNF-α]) and chemokine (IL-8, IP-10, RANTES, and MCP-1) production during CD40L stimulation, but not TLR-4 ligation. Moreover, antagonizing DC-derived activin-A resulted in significantly enhanced expansion of viral antigen-specific effector CD8+ T cells. These findings establish an immune-regulatory role for activin-A in DCs, highlighting the potential of antagonizing activin-A signaling in vivo to enhance vaccine immunogenicity.

Temporal expression of activin in acute burn wounds—From inflammatory cells to fibroblasts

Activin A is a member of the transforming growth factor-beta (TGF-beta) family of cytokines and growth factors and upregulation of this protein has been linked with a number of disease processes associated with chronic inflammation and fibrosis. Its potential involvement in burns has not yet been investigated. We therefore studied the localization of activin in tissue sections from excised mid- and deep dermal and full thickness cutaneous burn by immunohistochemistry. There was cell-specific temporal expression in tissues with prominent expression from day 4 onwards in lymphocytes and histiocytes and expression from day 8 onwards in reactive fibroblasts and endothelial cells. Immunopositivity over the first 18 days persisted in reactive fibroblasts and lymphocytes although the latter were in most circumstances decreasing in number. These data are consistent with activin A being central to the inflammatory and repair phases occurring in burnt skin and early scar formation. Modulation of activin expression and actions may, therefore, be a target for the management of burns.

Evolution of follistatin in teleosts revealed through phylogenetic, genomic and expression analyses

Follistatin (Fst) inhibits transforming growth factor-beta (TGF-B) proteins and is a known regulator of amniote myogenesis. Here, we used phylogenetic, genomic and experimental approaches to study its evolution in teleosts. Phylogenetic analyses suggested that one fst gene (fst1) is common to euteleosts, but a second gene (fst2) is conserved specifically within the Ostariophysi. Zebrafish fst1/2 respectively appear on chromosomes 5 and 10 in two genomic regions, each with conserved synteny to a single region in tetrapods. Interestingly, other teleosts have two corresponding chromosomal regions with a similar repertoire of paralogues. Phylogenetic reconstruction clustered these gene duplicates into two sister clades branching from tetrapod sequences. We suggest that an ancestral fst-containing chromosome was duplicated during the teleost whole genome duplication, but that fst2 was lost in lineages external to the Ostariophysi. We show that Fst1 of teleosts/mammals has evolved under strong purifying selection, but the N-terminal of Fst2 may have evolved under positive selection. Furthermore, the tissue-specific expression of zebrafish fst2 was restricted to fewer tissues compared to its paralogue and the single fst1 orthologue of Atlantic salmon (Salmo salar). Zebrafish fst1/2 may have subfunctionalized relative to non-duplicated vertebrate lineages, as several regions in the fst promoter of tetrapods were conserved with one paralogue, but not both. Finally, we examined the embryonic expression of fst1 in a teleost outside the Ostariophysi (Atlantic salmon). During segmentation, fst1 was expressed in the anterior somite compartment but was excluded from muscle progenitors that strongly expressed myogenic regulatory factors (MRFs). Later, fst1 was expressed in myogenic progenitors of the pectoral fin buds and also within the pax7(+) cell layer external to the myotome.

Distribution and spatiotemporal relationship of activin a and follistatin in mouse decidual and placental tissue

Problem Cells responsible for the synthesis of follistatin and activin A in the pregnant mouse endometrium have not been characterized. Method of study Immunocytochemistry was used to determine the distribution of follistatin and activin A in the pregnant mouse uterus. Results Follistatin was detected in the endometrium prior to decidualization and embryo implantation. Follistatin was not seen in fully decidualized cells, being restricted to non-decidualized fibroblasts and cells in the process of decidualization. In contrast, activin A was detected exclusively in mature antimesometrial decidual cells during involution. After day eleven of pregnancy, both substances were identified in the extracellular matrix of the spongiotrophoblast. Conclusion As previously described for decidual prolactin-related protein and the proteoglycan perlecan, follistatin and activin A were detected in the extracellular matrix of the spongiotrophoblast, suggesting that this region acts as reservoir for these growth factors in the mouse placenta.

Follistatin serum concentrations during full-term labour in women – significant differences between spontaneous and induced labour

Follistatin has been isolated from human placenta and has been identified in human foetal membranes and fluids. Serum follistatin levels in women rise during pregnancy particularly near term. In this study, we examined the effect of induction and stage of labour on maternal plasma concentrations of follistatin. Women who gave birth after a normal pregnancy were retrospectively divided into three groups: those who went in labour spontaneously (n= 33), needed induction by amniotomy and IV oxytocin (n= 18) or underwent planned caesarean section (n= 10). Serum was collected at 38–40 weeks of gestation, periodically through labour with a vaginal examination and once within 36 h postpartum and assayed for oestradiol, progesterone, prolactin and C-reactive protein. Follistatin was measured using a rabbit antiserum (#204) raised against purified 35 kDa bovine follistatin. Human recombinant follistatin was used as both standard and tracer. Concentrations of follistatin at 38–40 weeks of gestation were significantly different between groups. Those who had a spontaneous labour had concentrations higher than those who were induced. Similarly, those who were induced had concentrations higher than those who underwent a caesarean. In the spontaneous group, follistatin rose during labour, peaking at 57.9 ± 5.48 ng/ml at > 3 cm of cervical dilation, and after delivery follistatin decreased to 26.16 ± 3.4 ng/ml at 24 h post-delivery. In induced patients follistatin continued increasing to peak following delivery at 26.9 ± 3.0 ng/ml and decreased at > 3 h post-delivery. Follistatin concentrations in caesarean section patients at 24 h post-surgery (18.53 ± 3.74 ng/ml) were not different from that before the surgery and were comparable with the other two groups. Follistatin is clearly implicated in the onset of labour; however, further studies with a larger cohort of women are necessary to determine the nature of its role.

Hypoxia enhances the expression of follistatin-like 3 in term human trophoblasts

Hypoxic injury hinders placental differentiation and alters trophoblast gene expression. We tested the hypothesis that the expression of follistatin-like 3 (FSTL3), a member of the follistatin family of proteins, is modulated by hypoxia in primary human trophoblast (PHT). Using immunofluorescence of human term placental villi we detected the expression of FSTL3 protein in placental villi, primarily in trophoblasts. We verified this finding in cultured term PHT cells. Basal expression of FSTL3 transcript in cultured PHT cells, determined using quantitative PCR, was stable over the culture period. Importantly, when compared to culture in FiO(2)=20% or FiO(2)=8%, PHT cells cultured in FiO(2) <1% exhibited a 4-6 fold increase in FSTL3 mRNA expression as early as 4h in hypoxia. Whereas cellular FSTL3 protein was unchanged in hypoxia, we found that hypoxia increased the level of FSTL3 in the medium. Lastly, the exposure of PHT cells to either the hypoxia-mimetic cobalt chloride or the proline hydroxylase inhibitor dimethyloxaloylglycine upregulated the expression of FSTL3 transcript. Our data indicate that hypoxia enhances the expression of FSTL3 and its release from PHT cells. Our finding that hypoxia-mimetic agents enhance FSTL3 expression implicates HIF1alpha in this process.

Microarray analysis reveals differences in expression of cell surface and extracellular matrix components during development of the trout ovary and testis

Trout normally spawn at 3 years of age, however, a small percentage mature a year early. This provides an opportunity to study reproductive timing and developmental processes. The ovarian and testicular extracellular matrix (ECM) participates in processes such as growth, adhesion, differentiation, cell migration and patterning. The composition of the ECM defines the interactions of specific regulatory ligands with their receptors and modulates and regulates gonadal function. To identify some of the genes involved in these processes, a 16,006-gene salmonid cDNA microarray was used to compare three-year-old normal with two-year-old normal (maturing) and with two-year-old precocious (pre-spawn) ovarian and testicular transcriptomes. We provide evidence for differences in expression of some of the genes during vasculogenesis, angiogenesis, fibrillogenesis and other processes involving ECM remodeling. Sex-specific gene expression differences of ECM components were documented between the trout ovary and testis in each developmental state. Significant differences in the expression of genes involved in translation, transcription, cell-cycling and differentiation were identified. We also report, for the first time, unequivocal evidence for the transcription of high levels of adult and embryonic hemoglobins in the developed ovary; and for the expression of transcripts that encode zona pellucida glycoproteins in both the ovary and testis of trout.

Activin A is a critical component of the inflammatory response, and its binding protein, follistatin, reduces mortality in endotoxemia

Activin A is a member of the transforming growth factor-beta superfamily, which we have identified as having a role in inflammatory responses. We show that circulating levels of activin increase rapidly after LPS-induced challenge through activation of Toll-like receptor 4 and the key adaptor protein, MyD88. Treatment with the activin-binding protein, follistatin, alters the profiles of TNF, IL-1beta, and IL-6 after LPS stimulation, indicating that activin modulates the release of several key proinflammatory cytokines. Further, mice administered one 10-mug dose of follistatin to block activin effects have increased survival after a lethal dose of LPS, and the circulating levels of activin correlate with survival outcome. These findings demonstrate activin A's crucial role in the inflammatory response and show that blocking its actions by the use of follistatin has significant therapeutic potential to reduce the severity of inflammatory diseases.

Silencing of FLRG, an antagonist of activin, inhibits human breast tumor cell growth

Activin, a member of the transforming growth factor beta (TGFbeta) superfamily, regulates diverse processes, such as cellular growth and differentiation. There is increasing evidence that TGFbeta and its signaling effectors are key determinants of tumor cell behavior. Loss of sensitivity to TGFbeta-induced growth arrest is an important step toward malignancy. We previously characterized FLRG as an extracellular antagonist of activin. Here, we show that activin-induced growth inhibition is altered in FLRG-expressing breast cancer lines. Silencing FLRG induced growth inhibition, which is reversible upon addition of exogenous FLRG. We showed that FLRG silencing effects resulted from restoration of endogenous activin functions as shown by increased levels of phosphorylated smad2 and up-regulation of activin target gene transcripts. Furthermore, the growth inhibition induced by FLRG silencing was reversible by treatment with a soluble form of type II activin receptor. Finally, a strong expression of FLRG was observed in invasive breast carcinomas in contrast with the normal luminal epithelial cells in which FLRG was not detected. Our data provide strong evidence that endogenous FLRG contributes to tumor cell proliferation through antagonizing endogenous activin effects.

Imbalance between activin A and follistatin drives postburn hypertrophic scar formation in human skin

Hypertrophic scarring is a skin disorder characterized by persistent inflammation and fibrosis that may occur after wounding or thermal injury. Altered production of cytokines and growth factors, such as TGF-beta, play an important role in this process. Activin A, a member of the TGF-beta family, shares the same intra-cellular Smad signalling pathway with TGF-beta, but binds to its own specific transmembrane receptors and to follistatin, a secreted protein that inhibits activin by sequestration. Recent studies provide evidences of a novel role of activin A in inflammatory and repair processes. The aim of this study was to evaluate the importance of activin A and follistatin expression in the different phases of scar evolution. Immunostaining of sections obtained from active phase hypertrophic scars (AHS) revealed the presence of a high number of alpha-SMA(+) myofibroblasts and DC-SIGN(+) dendritic cells coexpressing activin A. Ex-vivo AHS fibroblasts produced more activin and less follistatin than normal skin or remission phase hypertrophic scar (HS) fibroblasts, both in basal conditions and upon TGF-betas stimulation. We demonstrate that fibroblasts do express activin receptors, and that this expression is not affected by TGF-betas. Treatment of HS fibroblasts with activin A induced Akt phosphorylation, promoted cell proliferation, and enhanced alpha-SMA and type I collagen expression. Follistatin reduced proliferation and suppressed activin-induced collagen expression. These results indicate that the activin/follistatin interplay has a role in HS formation and evolution. The impact of these observations on the understanding of wound healing and on the identification of new therapeutic targets is discussed.

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.

Activin A suppresses interleukin-1-induced matrix metalloproteinase 3 secretion in human chondrosarcoma cells

The objective was to investigate the effect of activin A on matrix metalloproteinase 3 (MMP-3) production and to identify the role of activin A in chondroprotection. SW1353 cells, a human chondrosarcoma cell line, were stimulated with interleukin (IL) 1alpha and tumor necrosis factor (TNF) alpha, and the concentrations of activin A, follistatin, and MMP-3 secreted into the culture media were measured by enzyme-linked immunosorbent assay (ELISA). Activin A was added to cell cultures in the presence of IL-1alpha or TNFalpha to determine its effect on the production of MMP-3 and sulfated glycosaminoglycan (sGAG) (measured by Alcian blue assay). To study the mechanism responsible for the chondroprotective effects of activin A, the production of IL-1 receptor antagonist (IL-1ra) and tissue inhibitor for metalloproteinases 1 (TIMP-1) was examined by ELISA. Addition of IL-1alpha did not affect the production of activin A by cultured SW1353 cells. IL-1alpha and activin A inhibited the production of follistatin. Stimulation of SW1353 cells with activin A suppressed IL-1alpha-induced, but not TNFalpha-induced, MMP-3 expression. Activin A had no effect on the production of sGAG, IL-1ra, or TIMP-1, although it suppressed the induction of TIMP-1 and IL-1ra by IL-1alpha. This novel finding of MMP-3 inhibition by activin A suggests a new role of activin A in cartilage remodeling. Activin A may have therapeutic potential for preventing cartilage degradation.

The role of the activin system in keloid pathogenesis

Keloid scars represent a pathological response to cutaneous injury under the regulation of many growth factors. Activin-A, a dimeric protein and a member of the transforming growth factor-β superfamily, has been shown to regulate various aspects of cell growth and differentiation in the repair of the skin mesenchyme and the epidermis. Thus our aim was to study the role of activin and its antagonist, follistatin, in keloid pathogenesis. Increased mRNA expression for activin was observed in keloid scar tissue by performing RNase protection assay. Immunohistochemistry showed increased localization of both activin-A and follistatin in the basal layer of epidermis of keloid tissue compared with normal tissue. ELISA demonstrated a 29-fold increase in concentration of activin-A and an ∼5-fold increase in follistatin in conditioned media in keloid fibroblasts compared with normal fibroblasts. Although keloid keratinocytes produced 25% more follistatin than normal keratinocytes, the amounts of activin-A, in contrast, was ∼77% lower. Proliferation of fibroblasts was stimulated when treated with exogenous activin-A (46% increase in keloids fibroblasts) or following co-culture with hβAHaCaT cells (66% increase). Activin-A upregulated key extracellular matrix components, namely collagen, fibronectin, and α-smooth muscle actin, in normal and keloid fibroblasts. Co-treatment of follistatin with activin-A blocked the stimulatory effects of activin on extracellular matrix components. These findings emphasize the importance of the activin system in keloid biology and pathogenesis and suggest a possible therapeutic potential of follistatin in the prevention and treatment of keloids.

Polymorphism of the follistatin gene in polycystic ovary syndrome

Follistatin has been reported as a candidate gene for polycystic ovary syndrome (PCOS) from linkage and association studies. Acting to regulate the development of ovarian follicles and as an antagonist to aromatase activity, alterations in follistatin function or expression may result in key features of PCOS such as reduced serum FSH, impaired ovarian follicle development and augmented ovarian androgen production. We investigated polymorphisms in the FST gene to determine if genetic variation is associated with susceptibility to PCOS or key phenotypic features of PCOS patients in a case-control association study. One hundred and seventy-three PCOS patients of Caucasian descent (mean age 30.0 +/- 4.8 years), conforming to the NIH diagnostic criteria, were recruited from a clinical practice database and 107 normal ovulating women (mean age 38.8 +/- 13.4 years) were recruited from the general community as control subjects. Morphometric data, biochemistry and genomic DNA were collected from study subjects and genotyping was performed on seven Single nucleotide polymorphisms (SNPs) in the FST gene region. Allele frequencies of the SNPs were rs1423560 G/C (0.99/0.01), rs3797297 C/A (0.80/0.20), rs11745088 C/G (0.98/0.02), rs3203788 A/T (0.98/0.02) and rs1062809 G/C (1.00/-), rs1127760 A/T (0.98/0.02) and rs1127761 A/T (0.98/0.02), and these were not significantly different between the PCOS and control groups (P < 0.05). Statistical analysis revealed significant associations between the SNP rs3797297 and sex hormone-binding globulin (P = 0.04) and free androgen index (FAI) (P < 0.01). We conclude that FST is not a susceptibility locus for PCOS; however, the SNP rs3797297 from FST gene was associated with androgenic markers for PCOS and may be of importance in the hyperandrogenaemia of the disease.

Noggin1 and Follistatin-like2 function redundantly to Chordin to antagonize BMP activity

In Xenopus, the dorso-ventral (D/V) axis is thought to be specified by the bone morphogenetic proteins (Bmp) activity arising through interaction with antagonists such as Noggin, Chordin and Follistatin. We report here, through inactivation of noggin1 (nog1) that this gene is not essential by itself to establish the D/V patterning. However, at blastula stage, inactivation of nog1 strongly amplifies chordin (chd) phenotype, revealing redundant functions of these two genes on D/V axis formation. Substantial dorsal tissues remaining in the double nog1-chd morphant suggested that other anti-Bmp factors may pattern the D/V axis. We isolated two potential candidates, the follistatin-like (fstl) genes. We found that fstl2 is an early gastrula expressed gene. Its inactivation, similar to nog1, strongly enhances the chd phenotype. Moreover, the penetrance of the ventralization phenotype is much higher when we inactivated simultaneously chd, nog1 and fstl2. Altogether, our data reveal that, while Chordin is the main player of the D/V axis, sufficient to maintain proper activity of Bmp gradient, the structures remaining in the chd mutant (namely dorsal and dorso-lateral territories, in both mesodermal and ectodermal layers) result from the anti-Bmp activity carried by Nog1 and Fstl2 at blastula and gastrula stages.

Pituitary actions of ligands of the TGF-β family: activins and inhibins

Activins, as members of the transforming growth factor-β superfamily, control and orchestrate many physiological processes and are vital for the development, growth and functional integrity of most tissues, including the pituitary. Activins produced by pituitary cells work in conjunction with central, peripheral, and other local factors to influence the function of gonadotropes and maintain a normal reproductive axis. Follistatin, also produced by the pituitary, acts as a local buffer to bind activin and modulate its bioactivity. On the other hand, inhibins of gonadal origin provide an endocrine feedback signal to antagonize activin signaling in cells that express the inhibin co-receptor, betaglycan, such as gonadotropes. This review highlights the pituitary roles of activin and the mechanisms through which these actions are modulated by inhibin and follistatin.

Stimulatory effects of lipopolysaccharide on endothelial cell activin and follistatin

Activin A and its binding protein, follistatin, are released into the circulation following acute systemic inflammation. In this study, we determined the activin and follistatin response of ovine aortic endothelial cells to lipopolysaccharide (LPS). Exposure to LPS for 1h, mimicking a transient inflammatory event, elicited significant increases in activin betaA subunit mRNA or activin A release, with larger, more prolonged increases evident with continuous exposure. On the other hand, follistatin increases were only evident with prolonged exposure to LPS and following increases in activin A release. While cell-associated activin A increased with LPS exposure, levels were lower than those secreted, whereas the opposite was apparent for follistatin. In summary, our findings suggest that vascular endothelial cells, while capable of releasing activin A and follistatin following inflammatory stimulation, are unlikely to be responsible for the rapid release of activin A in vivo following inflammatory challenge.

Dissociation of Angiogenesis and Tumorigenesis in Follistatin- and Activin-Expressing Tumors

The transforming growth factor-beta superfamily member activin and its antagonist, follistatin, act as a pleiotropic growth factor system that controls cell proliferation, differentiation, and apoptosis. Activin inhibits fibroblast growth factor 2-induced sprouting angiogenesis in vitro (spheroidal angiogenesis assay) and in vivo (Matrigel assay). To further study the role of the activin/follistatin system during angiogenesis and tumor progression, activin- and follistatin-expressing R30C mammary carcinoma cells were studied in mouse tumor experiments. Surprisingly, activin-expressing tumors grew much faster than follistatin-expressing tumors although they failed to induce increased angiogenesis (as evidenced by low microvessel density counts). Conversely, follistatin-expressing tumors were much smaller but had a dense network of small-diameter capillaries. Qualitative angioarchitectural analyses (mural cell recruitment, perfusion) revealed no major functional differences of the tumor neovasculature. Analysis of activin- and follistatin-expressing R30C cells identified a cell autonomous role of this system in controlling tumor cell growth. Whereas proliferation of R30C cells was not altered, follistatin-expressing R30C cells had an enhanced susceptibility to undergo apoptosis. These findings in experimental tumors are complemented by an intriguing case report of a human renal cell carcinoma that similarly shows a dissociation of angiogenesis and tumorigenesis during tumor progression. Collectively, the data shed further light into the dichotomous stimulating and inhibiting roles that the activin/follistatin system can exert during angiogenesis and tumor progression. Furthermore, the experiments provide a critical proof-of-principle example for the dissociation of angiogenesis and tumorigenesis, supporting the concept that tumor growth may not be dependent on increased angiogenesis as long as a minimal intratumoral microvessel density is maintained.

Sexually dimorphic regulation of inhibin beta B in establishing gonadal vasculature in mice

Sexually dimorphic differentiation of gonads is accomplished through balanced interactions between positive and negative regulators. One of the earliest features of gonadal differentiation is the divergent patterning of the vasculature. A male-specific coelomic vessel develops on the anterior to posterior of the XY gonad, whereas this vessel is absent in XX gonads. It is postulated that the testis-determining gene Sry controls formation of the coelomic vessel, but the exact molecular mechanism remains unknown. Here we reveal a novel role for inhibin beta B in establishing sex-specific gonad vasculature. In the testis, inhibin beta B contributes to proper formation of the coelomic vessel, a male-specific artery critical for testis development and, later in development, hormone transportation. On the other hand, in the ovary, inhibin beta B is repressed by WNT4 and its downstream target follistatin, leading to the absence of the coelomic vessel. When either Wnt4 or follistatin was inactivated, the coelomic vessel appeared ectopically in the XX ovary. However, when inhibin beta B was also removed in either the Wnt4-null or follistatin-null background, normal ovarian development was restored and no coelomic vessel was found. Our results indicate that the sex-specific formation of the coelomic vessel is established by positive components in the testis as well as an antagonizing pathway from the ovary. Inhibin beta B is strategically positioned at the intersection of these opposing pathways.

Follistatin attenuates early liver fibrosis: effects on hepatic stellate cell activation and hepatocyte apoptosis

Activin A, a member of the transforming growth factor-beta superfamily, is constitutively expressed in hepatocytes and regulates liver mass through tonic inhibition of hepatocyte DNA synthesis. Follistatin is the main biological inhibitor of activin bioactivity. These molecules may be involved in hepatic fibrogenesis, although defined roles remain unclear. We studied activin and follistatin gene and protein expression in cultured rat hepatic stellate cells (HSCs) and in rats given CCl4 for 8 wk and examined the effect of follistatin administration on the development of hepatic fibrosis. In activated HSCs, activin mRNA was upregulated with high expression levels, whereas follistatin mRNA expression was unchanged from baseline. Activin A expression in normal lobular hepatocytes redistributed to periseptal hepatocytes and smooth muscle actin-positive HSCs in the fibrotic liver. A 32% reduction in fibrosis, maximal at week 4, occurred in CCl4-exposed rats treated with follistatin. Hepatocyte apoptosis decreased by 87% and was maximal at week 4 during follistatin treatment. In conclusion, activin is produced by activated HSCs in vitro and in vivo. Absence of simultaneous upregulation of follistatin gene expression in HSCs suggests that HSC-derived activin is biologically active and unopposed by follistatin. Our in vivo and in vitro results demonstrate that activin-mediated events contribute to hepatic fibrogenesis and that follistatin attenuates early events in fibrogenesis by constraining HSC proliferation and inhibiting hepatocyte apoptosis.

Activin A: From sometime reproductive factor to genuine cytokine

The growth factor, activin A, was initially characterized as a putative reproductive hormone but is now known to have many other divergent roles. One of these is during inflammation. Following intravenous injection of bacterial lipopolysaccharide (LPS) into sheep, activin A is released extremely rapidly into the circulation. The release of activin A appears to be independent of fever, prostaglandins or other key proinflammatory cytokines such as TNF-alpha or IL-1beta. While the precise roles and function of this factor in inflammation are yet to be elucidated, the activin response occurs in other mammalian species besides the sheep and elevated activin has been documented for a number of clinical inflammatory conditions. Activin A therefore seems to be part of the regulatory component of the innate immune response.

Inhibition of TGF-beta signaling by an ALK5 inhibitor protects rats from dimethylnitrosamine-induced liver fibrosis

1 Chronic liver disease is characterized by an exacerbated accumulation of matrix, causing progressive fibrosis, which may lead to cirrhosis. Transforming growth factor beta (TGF-beta), a well-known profibrotic cytokine, transduces its signal through the ALK5 ser/thr kinase receptor, and increases transcription of different genes including PAI-1 and collagens. The identification of GW6604 (2-phenyl-4-(3-pyridin-2-yl-1H-pyrazol-4-yl)pyridine), an ALK5 inhibitor, allowed us to evaluate the therapeutic potential of inhibiting TGF-beta pathway in different models of liver disease. 2 A cellular assay was used to identify GW6604 as a TGF-beta signaling pathway inhibitor. This ALK5 inhibitor was then tested in a model of liver hepatectomy in TGF-beta-overexpressing transgenic mice, in an acute model of liver disease and in a chronic model of dimethylnitrosamine (DMN)-induced liver fibrosis. 3 In vitro, GW6604 inhibited autophosphorylation of ALK5 with an IC(50) of 140 nM and in a cellular assay inhibited TGF-beta-induced transcription of PAI-1 (IC(50): 500 nM). In vivo, GW6604 (40 mg kg(-1) p.o.) increased liver regeneration in TGF-beta-overexpressing mice, which had undergone partial hepatectomy. In an acute model of liver disease, GW6604 reduced by 80% the expression of collagen IA1. In a chronic model of DMN-induced fibrosis where DMN was administered for 6 weeks and GW6604 dosed for the last 3 weeks (80 mg kg(-1) p.o., b.i.d.), mortality was prevented and DMN-induced elevations of mRNA encoding for collagen IA1, IA2, III, TIMP-1 and TGF-beta were reduced by 50-75%. Inhibition of matrix genes overexpression was accompanied by reduced matrix deposition and reduction in liver function deterioration, as assessed by bilirubin and liver enzyme levels. 4 Our results suggest that inhibition of ALK5 could be an attractive new approach to treatment of liver fibrotic diseases by both preventing matrix deposition and promoting hepatocyte regeneration.

Intrafollicular paracrine communication in the zebrafish ovary: the state of the art of an emerging model for the study of vertebrate folliculogenesis

The development and function of vertebrate ovary are primarily controlled by the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), from the pituitary. However, most of the activities of FSH and LH are mediated or modulated by a variety of locally produced factors that form an intimate regulatory network within and between the follicles. As a top vertebrate model for genetic and developmental studies, the zebrafish has caught tremendous attention in the past two decades; however, its utility has quickly been extended to other areas including physiology. In the past few years, a variety of peptide growth factors have been identified and characterized in the zebrafish ovary including activin and epidermal growth factor (EGF), and lines of evidence point to the existence of an ovarian network of communication involving these factors. This article provides the state of the art of zebrafish as a model for analyzing ovarian development and its regulation.

Transcriptional profile of a myotube starvation model of atrophy

Skeletal muscle wasting is a pervasive phenomenon that can result from a wide range of pathological conditions as well as from habitual muscular inactivity. The present work describes a cell-culture condition that induces significant atrophy in skeletal muscle C2C12 myotubes. The failure to replenish differentiation media in mature myotubes leads to rapid atrophy (53% in diameter), which is referred to here as starvation. Affymetrix microarrays were used to develop a transcriptional profile of control (fed) vs. atrophied (nonfed) myotubes. Myotube starvation was characterized by an upregulation of genes involved in translational inhibition, amino acid biosynthesis and transport, and cell cycle arrest/apoptosis, among others. Downregulated genes included several structural and regulatory elements of the extracellular matrix as well as several elements of Wnt/frizzled and TGF-β signaling pathways. Interestingly, the characteristic transcriptional upregulation of the ubiquitin-proteasome system, calpains, and cathepsins known to occur in multiple in vivo models of atrophy were not seen during myotube starvation. With the exception of the downregulation of extracellular matrix genes, serine protease inhibitor genes, and the upregulation of the translation initiation factor PHAS-I, this model of atrophy in cell culture has a transcriptional profile quite distinct from any study published to date with atrophy in whole muscle. These data show that, although the gross morphology of atrophied muscle fibers may be similar in whole muscle vs. myotube culture, the processes by which this phenotype is achieved differ markedly.

Gonadotropins and their paracrine signaling network in the zebrafish ovary

Pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), play fundamental roles in vertebrate ovarian development and function. However, there has been an increasing body of evidence that the actions of FSH and LH are mediated or modulated by a variety of locally produced peptide or protein factors, which form an intimate regulatory network within and between the ovarian follicles. In the past few years, a variety of growth factors have been identified and characterized in the zebrafish ovary including activin and epidermal growth factor (EGF), which are important components of the intraovarian communication network. To understand how this local network interacts with the gonadotropins from the pituitary, we have recently cloned and characterized all the subunits of zebrafish FSH and LH from the pituitary as well as their receptors (FSHR and LHR) from the ovary. Using the Chinese hamster ovary (CHO) cells as the bioreactor, we have produced recombinant zebrafish FSH and LH with biological activities. With the recombinant hormones available, the functions of zebrafish FSH and LH in the ovary and their interactions with the local factors will be an important issue to address in the future. This review briefly summarizes some recent work from our laboratory and others on both gonadotropins and their potential intraovarian signaling factors in the zebrafish.

Diurnal changes in FSH-regulatory peptides and their relationship to gonadotrophins in pubertal girls

BACKGROUND

FSH-regulatory peptides participate with GnRH and sex steroids to regulate serum FSH concentrations. We hypothesized that day/night variations in FSH serum concentrations would be associated with diurnal variation in FSH-regulatory peptides.

METHODS

Blood was obtained every 15 min for 24 h beginning at 08:00 h in eight girls [seven with variations in growth or puberty and one with idiopathic hypogonadotrophic hypogonadism (IHH)] and for 12 h beginning at 20:00 h in 12 additional girls with variant puberty, eight with gonadal dysgenesis or ovarian failure (GD/OF) and one with IHH. Samples across 3 h blocks were pooled for determination of LH, FSH, activin-A, inhibin-B and follistatin 288.

RESULTS

LH and FSH concentrations increased from 23:00 to 08:00 h with respect to daytime concentrations in pubertal girls (P<0.005) but only LH increased (P=0.002) in girls with GD/OF. In pubertal girls, inhibin-B declined during the day (P=0.019), reaching a nadir between 17:00 and 22:45 h just prior to the night-time increase in FSH. Follistatin concentrations exhibited diurnal variation (P=0.028), with the greatest concentrations occurring between 05:00 and 11:00 h. Activin-A concentrations declined coincident with the night-time increase in FSH in pubertal girls (P<0.0001) but not in girls with GD/OF.

CONCLUSIONS

The directionality of changes in FSH-regulatory proteins supports the notion that FSH-regulatory peptides may contribute to the night-time augmentation of circulating FSH during puberty in girls.

Therapeutic potential of follistatin for colonic inflammation in mice

BACKGROUND AND AIMS

Activins belong to the transforming growth factor-beta superfamily. Recent studies have shown that activin and its natural antagonist, follistatin, are involved in tissue repair and inflammatory processes. The aim of this study was to determine whether neutralization of activins with follistatin would have an in vivo anti-inflammatory effect in several murine models of colitis.

METHODS

We assessed activin levels in the colitis induced by intracolonic administration of trinitrobenzene sulfonic acid (TNBS). We subsequently tested the effects of an intraperitoneal injection of follistatin before or after induction of TNBS colitis. We also examined the established colitis induced by oral dextran sulfate sodium (DSS) as well as the spontaneous colitis that develops in interleukin (IL)-10 gene-deficient (IL-10 -/- ) mice.

RESULTS

Levels of activin transcripts in the colon during the acute phase of TNBS colitis were up-regulated. Epithelial cells, infiltrating macrophages (Mvarphi), and endothelial cells produced excess activin betaA. Pretreatment with follistatin increased the survival rate of mice with TNBS colitis from 33% to 82% and decreased the plasma levels of IL-6 and amyloid A. Administration of follistatin also reduced the histologic score and tissue myeloperoxidase activity in established TNBS and DSS colitis and reduced the severity of the colitis in IL-10 -/- mice. Based on results obtained from 3 mouse models and from in vitro experiments, follistatin promoted the proliferation of colonic epithelial cells.

CONCLUSIONS

Neutralization of activins by follistatin promoted epithelial cell division and tissue repair, clearly suggesting a treatment modality for intestinal inflammation.

Follistatin operates downstream of Wnt4 in mammalian ovary organogenesis

Wnt4(-/-) XX gonads display features normally associated with testis differentiation, suggesting that WNT4 actively represses elements of the male pathway during ovarian development. Here, we show that follistatin (Fst), which encodes a TGFbeta superfamily binding protein, is a downstream component of Wnt4 signaling. Fst inhibits formation of the XY-specific coelomic vessel in XX gonads. In addition, germ cells in the ovarian cortex are almost completely lost in both Wnt4 and Fst null gonads before birth. Thus, we propose that WNT4 acts through FST to regulate vascular boundaries and maintain germ cell survival in the ovary.

Activins, inhibins and follistatins in the large domestic species

The activins and inhibins are members of the transforming growth factor-beta (TGF-beta) superfamily and, along with follistatin, a high affinity binding protein of activin, form a group of interrelated factors originally isolated for their role in regulating the release of follicle-stimulating hormone (FSH). Knowledge of their function, particularly that of activin, has expanded since being originally isolated, such that they are now regarded as important paracrine regulators in many cellular systems. This review summarizes the biology of these proteins as has been established in the large domestic animals. While the majority of data relate to the pituitary, ovary, uterus/placenta and testis, consideration is also given to emerging roles in inflammatory processes and in non-reproductive tissues or systems.

Tissue immunoexpression and messenger ribonucleic acid localization of inhibin/activin subunit in human epididymis

OBJECTIVE

To determine the expression of inhibin betaA and betaB subunits and follistatin and the ability of human epididymal epithelium to synthesize these molecules.

DESIGN

The main aim of this study was to investigate the expression of inhibin alpha, betaA, and betaB-subunits and follistatin in human epididymis with immunohistochemistry, in situ hybridization, and Western blotting in adult life.

SETTING

Academic university hospital.

PATIENT(S)

Epididymes were obtained from 10 men undergoing routine vasectomy or surgery for benign disease at the Royal Hallamshire Hospital, Sheffield, United Kingdom.

MAIN OUTCOME MEASURE(S)

Immunoexpression of activin betaA and betaB subunits and follistatin proteins and mRNA in human caput and cauda epididymis.

RESULT(S)

Positive immunoexpression for activin betaA and betaB subunits and follistatin were detected in different parts of the epididymis epithelium. Western blotting under a reducing condition detected a 28-kd band (possibly corresponding to the activin dimer). In situ hybridization indicated positive mRNA localization signal in both caput and cauda epididymal epithelium.

CONCLUSION(S)

Activins betaA and betaB subunits, but not inhibin alpha subunit, were detected in epididymal epithelium. These finding suggest that activins might have a role in the processes of sperm maturation and sperm fertilizing capability during transit and storage.

Developmental Profiles of Activin βA, βB, and Follistatin Expression in the Zebrafish Ovary: Evidence for Their Differential Roles During Sexual Maturation and Ovulatory Cycle1

Our recent experiments showed that gonadotropin(s) stimulated activin betaA and follistatin expression through the cAMP-PKA pathway but suppressed betaB via a cAMP-dependent but PKA-independent pathway in cultured zebrafish follicle cells. Given that pituitary gonadotropins are the major hormones controlling the development and function of the ovary, the differential expression of activin betaA and betaB as well as follistatin in response to gonadotropin(s) raises an interesting question about the temporal expression patterns of these molecules in vivo during sexual maturation and ovulatory cycle. Three experiments were performed in the present study. In the first experiment using sexually immature zebrafish, we followed the expression of activin betaA, betaB, and follistatin at the whole ovary level during a 10-day period in which the ovary developed from the primary growth stage to the one with nearly full-grown follicles. Activin betaA expression was very low at the primary growth stage but significantly increased with the growth of the ovary, and its rise was accompanied by an increase in follistatin expression. In contrast, the expression of activin betaB could be easily detected in the ovary of all stages; however, it did not exhibit an obvious trend of variation during the development. The second experiment examined the stage-dependent expression of activin betaA, betaB, and follistatin at the follicle level in the adult mature zebrafish. The expression of activin betaA was again low in the follicles during the primary growth stage, but exhibited a phenomenal increase after the follicles entered vitellogenesis with the peak level reached at midvitellogenic stage; in contrast, activin betaB mRNA could be easily detected at all stages with a slight increase during follicle growth. The expression of follistatin, on the other hand, also increased significantly during vitellogenesis; however, its level dropped sharply after reaching the peak at the midvitellogenic stage. In the third experiment, we investigated the dynamic changes of the ovarian activin betaA, betaB, and follistatin expression during the daily ovulatory cycle. The expression of activin betaA and follistatin gradually increased from 1800 h onward and reached the peak level around 0400 h when the germinal vesicles had migrated to the periphery in the full-grown oocytes. In contrast, activin betaB expression steadily declined, although not statistically significant, during the same period, but increased sharply at 0700 h when mature oocytes started to appear in most of the ovaries collected. In conclusion, activin betaA and betaB exhibit distinct expression patterns during the development of the ovary and the daily ovarian cycle of the zebrafish. It seems that activin betaA is involved in promoting ovary and follicle growth, whereas activin betaB may have a tonic role throughout follicle development but becomes critical at the late stage of oocyte maturation and/or ovulation.

GnRH agonist stimulation of the pituitary–gonadal axis in children: age and sex differences in circulating inhibin-B and activin-A

BACKGROUND

Inhibin-B decreases and activin increases FSH secretion in adults. We investigated whether an FSH-inhibin/activin feedback loop exists before or during puberty.

METHODS

FSH secretion was stimulated with 10 microg/kg leuprolide acetate (GnRH agonist) in 18 girls, ages 1.0-13.2 years, and 11 boys, ages 8.9-15.2 years, with variations in pubertal development, and in five normal 9- to 10-year-old girls. Blood, obtained at 0, 0.5, 1, 2, 4, 8, 12, 16, 20 and 24 h after GnRH agonist, was analysed for LH, FSH, activin-A, inhibin-A, inhibin-B, follistatin 288 and estradiol/testosterone.

RESULTS

FSH increased within 30 min of GnRH agonist administration with a peak greater in girls than boys (P=0.0006). Baseline inhibin-B was greater in boys than girls (P=0.01), while baseline activin-A concentrations were greater in girls. GnRH agonist-stimulated FSH increased inhibin-B in girls by 8 h and in boys by 20 h (P<0.05), but did not affect activin-A. Inhibin-B increases were seen only in girls older than 5 years.

CONCLUSIONS

An inhibin-B-FSH feedback loop exists prior to the onset of puberty in girls older than 5 years. Sex differences in activin-A and inhibin-B concentrations may be responsible for sex differences in serum FSH concentrations.

FLRG, member of the follistatin family, a new player in hematopoiesis

Several years ago, we cloned and characterized from a B cell leukemia a new secreted protein which, on the basis of its high degree of structural homology with follistatin, was defined as a member of the follistatin family and accordingly named follistatin-related gene (FLRG). However, follistatin and FLRG revealed non-overlapping patterns of expression in various tissues thereby indicating the existence of non-redundant functional roles for these proteins throughout the organism. As known for a long time, follistatin is a biological regulator of activin and bone morphogenetic protein (BMP) function in various cellular systems: in particular, it inhibits the effects of activin on hematopoiesis. We therefore investigated the expression and effects of FLRG during human hematopoiesis with particular focus on the effect of this soluble glycoprotein in the regulation of erythropoiesis. For this purpose, we have for the first time, compared the role of Activin A, BMP2 and BMP4 during erythropoiesis, in primary human cells. Our results indicate that, BMP2 acts on early erythroid cells while Activin A acts on a more differentiated population. We report the induction by Activin A and BMP2 of cell commitment towards erythropoiesis in the absence of EPO. This induction involves two key events: increase of EPO-R and the decrease of GATA2 expression. Our results indicate that despite their high structural homology, follistatin and FLRG do not regulate the same signaling targets, therefore highlighting distinct functions and mechanisms for these two proteins in the human hematopoietic system. We thus propose a working model for the regulation of activin or BMP-induced human erythropoiesis by follistatin/FLRG.

Autocrine/paracrine regulation of pituitary function by activin, inhibin and follistatin

The precise regulation of the anterior pituitary is achieved by the cell-specific and combined actions of central, peripheral and local factors. Activins, inhibins, and follistatins were first discovered as gonadal factors with actions on FSH production from pituitary gonadotropes. With the realization that these factors are expressed in a wide array of tissues, including the pituitary, it became apparent that the functional importance of activins, inhibins, and follistatins extends beyond the reproductive axis and that they often exert their effects by autocrine/paracrine mechanisms. As members of the TGF-beta superfamily, activins and inhibins control and orchestrate many physiological processes and are vital for the development, the growth, and the functional integrity of most tissues, including the pituitary. Activins exert effects on multiple pituitary cell types but the best-characterized pituitary targets of the autocrine/paracrine function of activins are the gonadotropes. The autocrine/paracrine function of the activin-binding proteins, follistatins, constitutes an important local mechanism to modulate activin bioactivity while the restricted actions of gonadal inhibins to betaglycan-expressing gonadotropes provides a secondary mode of regulation of cell-specific actions of activins. The aim of this review is to highlight and evaluate experimental evidence that supports the roles of activins, inhibins, and follistatins as autocrine, paracrine, and/or endocrine modulators of the pituitary.

The role of activin, follistatin and inhibin in testicular physiology

The role of the inhibins, activins and follistatins in testicular function are being more clearly defined following studies describing the cellular localisation of these proteins to the testis and the availability of specific assay systems enabling measurement of these proteins. Taken together with the results of targetted gene inactivation experiments, several concepts emerge. Inhibin B is predominantly produced by the Sertoli cell in many adult male mammals whereas there is a perinatal peak of inhibin A in the rat. In contrast, activin A has its highest concentrations in the immediate post-natal period during which it is involved in the developmental regulation of both germ cells and Sertoli cells being modulated by follistatin. Activin A levels are considerably lower in the adult testis but Sertoli cell production is stimulated by interleukin-1 and inhibited by FSH. Little is known about the production of activin B due to the absence of a suitable assay but the beta(B) subunit mRNA is expressed in germ cells and Sertoli cells and is stage-dependent. This pattern of expression suggest that it may be involved in autocrine or paracrine actions within the seminiferous epithelium.

Intra-pituitary regulation of gonadotrophs in male rodents and primates

Paracrine and autocrine regulation is well established in many organs including the gonads, but the notion of communication among pituitary cells is a relatively new concept. The FSH-beta and GnRH-receptor genes are up-regulated by pituitary activin and down-regulated by pituitary follistatin, and circulating inhibin disrupts this local regulation by functioning as an endogenous competitor of the activin receptor. Activin and follistatin production by folliculostellate cells may play a central role in these responses. alpha-Subunit expression is maintained at high levels in the absence of GnRH through unknown mechanisms. There is evidence that the intra-pituitary regulation of FSH-beta and GnRH-receptor gene expression may activate pubertal maturation in male rats. Finally, there are marked differences in follistatin expression and its regulation by GnRH and androgens in male primates and rats that appear to explain species differences in the differential secretion of FSH and LH, although the physiological significance of these differences is not yet known.

Activin and follistatin in rat mammary gland

Mammary gland morphogenesis and differentiation are mediated through the combined activities of systemic hormones and locally synthesized growth factors. Activin, a member of the transforming growth factor (TGF)-beta superfamily, is known to regulate the growth and differentiation of several cell types. In the present study, we investigated the role of activin in rat mammary gland on different stages of development. We found that activin A in vitro inhibits the proliferation of isolated acini, and this effect increases with the development of the gland. This factor also produces in vitro an inhibition of the final differentiation of acini obtained from 19th day pregnant rats. We also report the expression of activin receptors IIA and IIB mRNA in whole rat mammary gland and acini, with decreased levels of expression of type IIA (in both compartments) and IIB (in acini) during pregnancy and lactogenesis. In addition, we show that activin betaB-subunit mRNA decreases throughout pregnancy, and that the mRNA levels of follistatin (Fst) (its ligand protein) are high in cycling rats and at the beginning of pregnancy and diminish thereafter, having the acini higher levels of expression. Our data show that activin betaB-subunit, follistatin and ActRIIA and IIB transcripts are expressed in rat mammary gland at appropriate times and locations during development, allowing an interplay that might regulate activin action on growth and differentiation of the gland.

Cloning of epidermal growth factor (EGF) and EGF receptor from the zebrafish ovary: evidence for EGF as a potential paracrine factor from the oocyte to regulate activin/follistatin system in the follicle cells

In the present study, we cloned full-length cDNAs for epidermal growth factor (EGF), EGF receptor (EGFR), and three truncated forms of EGFR (EGFR15, 12, and 8) from the zebrafish ovary. Zebrafish EGF was predominantly expressed in the ovary and testis, while EGFR and its truncated forms were highly expressed in all tissues examined except the liver. In the ovary, the expression of EGF seemed to be more abundant in the follicles of early stages, while EGFR had much higher expression levels at later stages. Interestingly, although EGF was expressed in both the follicle cells and oocytes, its expression level was significantly higher in the oocytes. However, the expression of EGFR was mainly restricted to the follicle cells with little expression in the oocytes. The unique spatial patterns of EGF and EGFR expression within the follicle suggest that EGF may serve as a messenger from the oocyte to signal the follicle cells. EGF strongly stimulated the expression of both activin beta A and beta B, while it suppressed basal and hCG-induced follistatin expression in cultured follicle cells. These results, together with the evidence that EGF was predominantly expressed in the oocytes whereas EGFR was expressed in the follicle cells, strongly suggest that EGF is likely a potential paracrine/juxtacrine factor from the oocytes to regulate the function of the follicle cells.