Follistatin inhibits the mesoderm-inducing activity of activin A and the vegetalizing factor from chicken embryo

Research Progress on the Role and Mechanism of Action of Activin A in Brain Injury

Activin A belongs to the transforming growth factor superfamily and has a variety of biological functions. Studies have revealed that activin A can regulate the body's immune and inflammatory responses and participate in the regulation of cell death. In addition, activin A also has neurotrophic function and plays an important role in the repair of brain damage. This article summarizes recent advances in understanding the role and mechanism of action of activin A in brain injury and provides new hints into the application of activin A in the treatment of brain injury.

Pluripotent cells (stem cells) and their determination and differentiation in early vertebrate embryogenesis

Mammalian embryonic stem cells can be obtained from the inner cell mass of blastocysts or from primordial germ cells. These stem cells are pluripotent and can develop into all three germ cell layers of the embryo. Somatic mammalian stem cells, derived from adult or fetal tissues, are more restricted in their developmental potency. Amphibian ectodermal and endodermal cells lose their pluripotency at the early gastrula stage. The dorsal mesoderm of the marginal zone is determined before the mid-blastula transition by factors located after cortical rotation in the marginal zone, without induction by the endoderm. Secreted maternal factors (BMP, FGF and activins), maternal receptors and maternal nuclear factors (beta-catenin, Smad and Fast proteins), which form multiprotein transcriptional complexes, act together to initiate pattern formation. Following mid-blastula transition in Xenopus laevis (Daudin) embryos, secreted nodal-related (Xnr) factors become important for endoderm and mesoderm differentiation to maintain and enhance mesoderm induction. Endoderm can be induced by high concentrations of activin (vegetalizing factor) or nodal-related factors, especially Xnr5 and Xnr6, which depend on Wnt/beta-catenin signaling and on VegT, a vegetal maternal transcription factor. Together, these and other factors regulate the equilibrium between endoderm and mesoderm development. Many genes are activated and/or repressed by more than one signaling pathway and by regulatory loops to refine the tuning of gene expression. The nodal related factors, BMP, activins and Vg1 belong to the TGF-beta superfamily. The homeogenetic neural induction by the neural plate probably reinforces neural induction and differentiation. Medical and ethical problems of future stem cell therapy are briefly discussed.

Role of activin and other peptide growth factors in body patterning in the early amphibian embryo

The amphibian body plan is established as the result of a series of inductive interactions. During early cleavage stages cells in the vegetal hemisphere induce overlying animal hemisphere cells to form mesoderm. The interaction represents the first major body-patterning event and is mediated by peptide growth factors. Various peptide growth factors have been implicated in mesoderm development, including most notably members of the transforming growth factor-beta superfamily. Identification of the so-called "natural" inducer from among the several candidate peptide growth factors is being achieved by employing several experimental strategies, including the use of a tissue explant assay for testing potential inducers, cloning of marker genes as indices of early induction events, and microinjection of altered peptide growth factor receptors to disrupt normal embryonic inductions. Activin emerges as the most likely choice for assignment of the role of endogenous mesoderm inducer, because it currently best fulfills the rigorous set of criteria expected of such an important embryonic signaling molecule. Activin, however, may not act alone in mesoderm induction. Other peptide growth factors such as fibroblast growth factor might be involved, especially in the regional patterning of the mesoderm. In addition, several genes (e.g., Wnt and noggin), which are expressed after the mesoderm is initially induced, probably assist in further definition of the mesoderm pattern. Following mesoderm induction, the primary embryonic organizer tissue (first described in 1924 by Spemann) develops and contributes further to body patterning by its action as a neural inducer. Peptide growth factors such as activin may also be involved in the inductive event, either directly (by facilitating gene expression) or indirectly (by serving to constrain pathways).

Neural inducing factors from Xenopus laevis eggs and embryos

Large foreheads can be induced by ribonucleoprotein particles from Xenopus laevis eggs and embryos. The host embryos develop only a rudimentary primary axis. A neural inducing factor from the cytosol of gastrula-neurula stages has been partially purified. The factors are associated with other proteins in larger complexes.

cDNA cloning and distribution of the Xenopus follistatin-related protein

Recently, several proteins which have a follistatin module have been isolated. One of them, the follistatin-related protein (FRP), is encoded by TSC-36 (TGF-beta-stimulated clone 36) in mouse, originally isolated as a cDNA clone up-regulated by TGF-beta1 in mouse osteogenic MC3T3E1 cells. To determine the physiological role of FRP in early Xenopus embryonic development, we cloned the Xenopus FRP (xFRP) cDNA. The resulting cDNA clone was a secreted glycoprotein consisting of 299 amino acid residues with about 70% similarity to the mammalian and avian FRPs. Northern blotting analysis revealed that xFRP gene expression started at stage 10, the onset of gastrulation, gradually increased during the blastula and neurula stages and was sustained through the tail-bud stage. Whole-mount in situ hybridization analysis showed the localization of xFRP mRNAs in the Spemann organizer, notochord, neural floor plate, hypochord and somite. The similarities with the pattern of expression of Xenopus follistatin mRNA suggests that xFRP may play a role in neuralization.

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.

Neural induction in embryos

Neural differentiation of the ectoderm is inhibited by bone morphogenetic protein 4 (BMP-4) in amphibia as well as mammalia. This inhibition is released by neural inducing factor(s), which are secreted from the dorsal mesoderm. Masked neuralizing factor(s) are already present in the ectoderm before induction. In homogenates from Xenopus oocytes and embryos neural inducing factors were found in the supernatant (centrifuged at 105000 g), in small vesicles and a ribonucleoprotein fraction. A neuralizing factor, which is a protein of small size, has been partially purified from Xenopus gastrulae. Genes that are expressed in the dorsal mesoderm and involved in the de novo synthesis of neuralizing factor(s) have been cloned. The differentiation of cells with a neuronal fate starts in the neural plate immediately after neural induction. Genes homologous to the Notch and Delta genes of lateral inhibition in insects are involved in this process.

Recombinant human activin A stimulates development of bovine one-cell embryos matured and fertilized in vitro

The effects of recombinant human activin A on the development of bovine one-cell embryos matured and fertilized in vitro were investigated. In experiment 1, one-cell embryos were cultured in a chemically-defined medium, of modified synthetic oviduct fluid supplemented with 1 mg/ml polyvinyl alcohol (mSOF-PVA), containing different concentrations of activin (0, 0.1, 1, 10, and 100 ng/ml) until 240 hr after in vitro fertilization. The addition of > or = 1 ng/ml activin to mSOF-PVA improved development to the blastocyst stage (14.5-17.1%), compared with no addition of activin (5.6%). However, there was no significant difference in hatching rate of embryos among treatments. In experiments 2 and 3, the embryos were also cultured in mSOF-PVA at various periods of exposure to 10 ng/ml activin, to evaluate development to the morula and blastocyst stages, respectively. The proportion of morulae was significantly higher in culture with activin at 20-120 hr postinsemination (37.2%) than with control (25.7%). Total number of cells in morulae at 120 hr postinsemination significantly increased by the addition of activin at 20-72 hr (26.1 cells) and 20-120 hr (24.2 cells) postinsemination, compared with control (20.1 cells). When activin was added to the medium during 20-120 hr and 20-192 hr postinsemination, the percentages of blastocysts (18.0% and 18.7%, respectively) were significantly higher than in the control (9.6%). However, the total number of cells in blastocysts was not significantly different. These results demonstrate that activin stimulates the development of bovine one-cell embryos to the morula and blastocyst stages in vitro.

Active complex formation of type I and type II activin and TGF beta receptors in vivo as studied by overexpression in zebrafish embryos

We have investigated the involvement of activin receptors and TGF beta type I receptor in zebrafish development. Overexpression of either full-length or a truncated form of mouse ActR-IIA interferes with the development. Different splice variants of mouse ActR-IIB have distinct effects; ActR-IIB4 induces abnormal embryos, whereas ActR-IIB2 does not. Activin and TGF beta type I receptors can induce axis duplications. Co-expression of ActR-IA or ActR-IB with the type II activin receptors results in a synergistic increase of the frequency of axis duplication. Moreover, ActR-IIB2 is synergistic with ActR-IA and ActR-IB, demonstrating that ActR-IIB2 can interact with the zebrafish ligand. Overexpression of TGF beta R-I with ActR-IIA or ActR IIB4 results in a synergistic increase in frequency of abnormal embryos, whereas in combination with ActR-IIB2 no such increase occurs.

Follistatins neutralize activin bioactivity by inhibition of activin binding to its type II receptors

Follistatin is an activin-binding protein, which inhibits activin bioactivity in several biological systems. In the present study it is demonstrated that preincubation of iodinated activin A with follistatin, purified from porcine follicular fluid, completely abolished the binding of activin to activin type IIA, IIB2 and IIB4 receptors, and consequently to activin type IB receptor, transiently transfected in COS cells. Binding of activin A to membrane proteins on the activin-responsive P19 embryonal carcinoma cells was also prevented by this follistatin preparation. The same results were obtained with a carboxy-terminally truncated form of follistatin (FS-288), which is only present in minor amounts in the purified follistatin preparation. Since FS-288 has a high affinity for heparan sulfate proteoglycans on the cell surface, we tested whether membrane-bound FS-288 presents activin A to the different activin receptors, thereby facilitating activin binding. FS-288 did bind to the cell surface of transfected COS cells, but inhibited the binding of activin A to its receptors IIA, IIB2 and IIB4. Furthermore, after addition of FS-288 to K562 erythroleukemia cells, the total binding of activin via cell surface-bound FS-288 was increased, whereas the binding of activin A to activin type II and type I receptors present on these cells was inhibited. These findings reveal that different forms of follistatin can neutralize activin bioactivity by interference with binding of activin to all known activin type II receptors, rather than that they inhibit the binding of the type I receptor to the activin/activin type II receptor complex. In addition, our studies indicate that cell surface-associated follistatin cannot present ligand to signalling receptors.

Control of the embryonic body plan by activin during amphibian development

Embryonic induction plays an important role in establishing the fundamental body plan during early amphibian development. The factors mediating this embryonic induction have, however, only recently been discovered. In the mid-1980's, certain peptide growth factors belonging to the FGF and TGF-beta families were found to have a mesoderm-inducing effect on isolated Xenopus blastula ectoderm. The study of embryonic induction subsequently expanded rapidly and knowledge at the molecular level has gradually accumulated. One of these peptide growth factors, activin, a member of the TGF-beta superfamily, is present maternally in the Xenopus early embryo and induces various mesodermal and endodermal tissues in isolated presumptive ectoderm. After exposure of presumptive ectoderm to activin, many genes are expressed in the same manner as in normal embryogenesis. Ectoderm treated with activin can induce a complete secondary embryo, the same as the organizer does in transplantation experiments. These findings suggest that activin is one of the first induction signals responsible for establishing the embryonic body plan in early amphibian development. In this article we shall review to what extent we can control the embryonic body plan in vitro, referring to some significant findings in this field.

Identification of a Xenopus glutamine synthetase gene abundantly expressed in the embryonic nervous system but not in adult brain

We used a PCR-based subtraction cloning procedure with concanavalin A-treated and -untreated animal caps from stage 9 Xenopus embryos to search for genes up-regulated during early neural development. One such gene was found to encode a protein homologous to several known glutamine synthetases, and we named it xGS. Molecular hybridization studies revealed that xGS mRNA is maternally transmitted and abundantly expressed in neuroectoderm-derived tissues during the gastrula and neurula stages. The expression of xGS mRNA in the nervous system continues until the larval stages, but declines thereafter and becomes undetectable in adult brain. Considering its metabolic activity and potential neuroprotective effect against the neurotoxic substances such as glutamate and ammonia, the glutamine synthetase may play an important role in the early stages of vertebrate neural development.

Inhibin, activin, and follistatin. Potential roles in ovarian physiology

The physiological significance of these results will not become clear until patterns of activin and inhibin protein production and the expression of their receptors have been more thoroughly characterized in relation to follicular development. Meanwhile, in situ hybridization studies on rat and monkey ovaries suggest that inhibin/activin beta-subunit mRNA (favoring synthesis of activin) is relatively abundant in granulosa cells of immature antral follicles, whereas alpha-subunit mRNA (favoring synthesis of inhibin) predominates in Graafian follicles. The increased production of follistatin associated with advanced preovulatory development would serve to further reduce the activin "tone" relative to inhibin (Fig. 1). At the level of protein action in vitro, the pattern emerging is that inhibin minimally affects granulosa cell steroidogenesis at any stage of follicular development, whereas activin has pronounced modulatory effects that alter with follicular maturity. As suggested previously,60 the ability of activin to enhance gonadotropin-responsive aromatase activity and simultaneously suppress progesterone production by mature granulosa cells has physiological implications in that it hints at a mechanism for promoting estrogen synthesis and simultaneously suppressing progesterone synthesis, which is precisely what occurs in the preovulatory follicle. The effects of inhibin and activin on human thecal androgen synthesis observed in vitro suggest how these proteins might act locally to modulate preovulatory follicular growth and estrogen synthesis in vivo (Fig. 2).57 In essence, we propose that activin acting at early stages of antral follicular development plays a role in follicular recruitment through sensitizing immature granulosa cells to the cytodifferentiative action of FSH. On the other hand, inhibin is more likely to play a role in preovulatory follicular selection and maintenance of follicular dominance. Studies of follicular fluid levels of androgen and estrogen in relation to granulosa cell aromatase activity indicate that the capacity of the theca interna to generate aromatase substrate (androstenedione) increases hand in hand with aromatase activity in the human preovulatory follicle. It has therefore been suggested that a positive feedback loop (granulosa on theca) exists that promotes thecal androgen synthesis and hence estrogen synthesis in this follicle.64 The discovery that inhibin production in vitro is greatest by granulosa cells isolated from preovulatory follicles strongly implicates inhibin as a component of this feedback loop.

Expression of alpha, beta A and beta B subunits of inhibin or activin and follistatin in rat pancreatic islets

We first detected the mRNA expression of follistatin and three subunits of inhibin/activin in rat pancreatic islets by reverse transcription-polymerase chain reaction (RT-PCR). Immunohistochemistry using anti-follistatin serum (against residues 123-134) revealed that follistatin was localized only in insulin-producing B cells. Although the beta A subunit was detectable in the islets, the immunostainable cell types were completely different with two beta A antisera, i.e. anti-beta A (1-10)-Tyr stained B cells, while anti-beta A (87-99) stained glucagon-producing A cells. This inconsistent immunoreactivity was probably related to follistatin binding to beta subunits of inhibin/activin. This study indicates that follistatin and inhibin/activin in the islet serve as paracrine or autocrine modulators in the endocrine pancreas.

Structural and functional characterization of the rat follistatin (activin-binding protein) gene promoter

Follistatin was originally identified as a specific inhibitor of follicle stimulating hormone secretion and later characterized as a binding protein for activin. Since activin regulates hormone secretion and cell differentiation, the importance of understanding the mechanisms regulating the synthesis of its binding protein, follistatin, is evident. To study the regulation of follistatin gene expression, we first determined the transcription start site (cap site) of the rat follistatin gene using primer extension and ribonuclease protection assay. Our results led to the identification of multiple cap sites located at three different positions of the promoter. DNA sequence analysis revealed that each cap site was located at approximately 30 nucleotide (nt) downstream of three distinct TATA-like sequences. In primary cultures of rat granulosa cells, transfection studies using 5'-flanking regions of follistatin gene fused to the chloramphenicol acetyltransferase (CAT) reporter gene revealed the presence of two DNA segments that act to suppress basal transcriptional activity. The promoter activity of the CAT construct containing 2.6 kilo base pairs (kb) of 5'-flanking region was induced 2.5-fold above basal activity by forskolin (10 microM), and 1.6-fold by 12-O-tetradecanoylphorbol 13-acetate (TPA, 100 nM). Co-treatment with forskolin and TPA resulted in a 6.4-fold induction in its promoter activity, suggesting that two distinct signal transduction pathways, the cAMP-dependent protein kinase-A pathway and diacylglycerol-dependent protein kinase-C pathway, act coordinately to modulate follistatin gene transcription. Experiments using a series of 5'-flanking region deletion constructs located the regulatory regions responsive to these two pharmacological agents at nt -312 to -32 and -35 to +139.

Localization of the heparin binding site of follistatin

To define the heparin-binding site of follistatin, the reduced and S-carboxymethylated recombinant human follistatin containing 288 amino acids was digested by Staphylococcus aureus V8. The digested product was subjected to sulfate cellufine column chromatography and the adsorbed peptide fragments eluted with a stepwise gradient of sodium chloride. The recovered column fractions were further purified by reversed-phase high-performance liquid chromatography (HPLC) and the HPLC peaks subjected to amino-terminal sequence analysis. All of the sulfate cellufine-retarded peptide fragments gave the same N-terminal amino acid sequence, which started at residue-68 of human follistatin, suggested that those fragments starting from residue-68 contain the heparin binding site. The multiple fragments might represent the oxidized, non-glycosylated or glycosylated forms of follistatin(68-113) resulting from the V8 digestion. A synthetic peptide corresponding to the region having the amino acid sequence 72-86 of follistatin was able to bind both heparin and sulfate cellufine, as well as compete with recombinant follistatin for binding to heparin. These findings further define the location of the heparin and heparan sulfate-binding site of follistatin at the basic amino acid-rich region comprising the amino acid sequence Lys75-Lys-Cys-Arg-Met-Asn-Lys-Lys-Asn-Lys-Pro-Arg86.

Differential expression of inhibin subunits and follistatin, but not of activin receptor type II, during early murine embryonic development

Activins are known to be potentially important regulators of early developmental processes in amphibians, birds, and mammalians. In this study we report the expression of the inhibin subunits, including those that make up activin, the activin-binding protein follistatin, and activin receptor type II in several in vitro systems that model early murine embryonic development, namely embryonic stem (ES) cells, embryonal carcinoma (EC) cells, and their differentiated derivatives. In addition, we examine the expression pattern of these factors in different stages of the mouse embryo itself. Expression of inhibin alpha and beta A subunits is restricted to certain differentiated cell types, while beta B subunits are expressed in both differentiated and undifferentiated cells. Our results further indicate a change in the expression pattern of inhibin subunits during early development from beta B at the blastocyst stage largely to beta A in postgastrulation embryos. This is similar to the expression pattern at equivalent stages of Xenopus and chick development. Expression of the activin-binding protein follistatin is altered by the induction of differentiation of P19 EC and ES cells by several factors, including retinoic acid. In contrast to the inhibin subunits and follistatin, activin receptor levels are not influenced by differentiation in these cell types. The results of this study demonstrate that the inhibin subunits and follistatin, but not the activin receptor type II, are differentially expressed during early murine development and suggest that the different forms of activin/inhibin are involved in the regulation of different developmental processes.

The vegetalizing factor. A member of the evolutionarily highly conserved activin family

The mesoderm and endoderm inducing vegetalizing factor was partially sequenced after BrCN cleavage. A sequence which is highly conserved in activin A near the C-terminal end was identified. This shows that the factor belongs to the activin family. The activins are not confined to embryos and gonads, but widely distributed in other tissues like calf kidney and calf liver. Functional aspects are discussed.

Rat follistatin: ontogeny of steady-state mRNA levels in different tissues predicts organ-specific functions

Follistatin (FS), a monomeric glycoprotein which specifically binds activin, is expressed in many tissues. This study investigated 1) the ontogeny of the steady-state FS mRNA levels in different extragonadal tissues and 2) whether the ratio of the differential splicing products, FS 344 or its carboxy-truncated form FS 317, is changed during postnatal development. Whereas the levels of FS mRNA 344 in the kidney showed a profound increase from the day of birth to adulthood, the levels in the muscle peaked during the infantile period and then declined. Brain cortex, heart and thymus also showed tissue specific expression in the steady-state mRNA level of FS during postnatal development. None of the tissues showed a measurable change in the ratio of the mRNA for FS 344 and FS 317. The FS mRNA 344 levels in male and female kidney were not different. It is concluded that the ontogeny of steady state FS mRNA varies in a tissue specific manner during postnatal development of the rat and may be involved in modulating the outcome of activin.