Disruption of mesoderm and axis formation in fish by ectopic expression of activin variants: the role of maternal activin

Genetic analysis of activin/inhibin β subunits in zebrafish development and reproduction

Activin and inhibin are both dimeric proteins sharing the same β subunits that belong to the TGF-β superfamily. They are well known for stimulating and inhibiting pituitary FSH secretion, respectively, in mammals. In addition, activin also acts as a mesoderm-inducing factor in frogs. However, their functions in development and reproduction of other species are poorly defined. In this study, we disrupted all three activin/inhibin β subunits (βAa, inhbaa; βAb, inhbab; and βB, inhbb) in zebrafish using CRISPR/Cas9. The loss of βAa/b but not βB led to a high mortality rate in the post-hatching stage. Surprisingly, the expression of fshb but not lhb in the pituitary increased in the female βA mutant together with aromatase (cyp19a1a) in the ovary. The single mutant of βAa/b showed normal folliculogenesis in young females; however, their double mutant (inhbaa-/-;inhbab-/-) showed delayed follicle activation, granulosa cell hypertrophy, stromal cell accumulation and tissue fibrosis. The ovary of inhbaa-/- deteriorated progressively after 180 dpf with reduced fecundity and the folliculogenesis ceased completely around 540 dpf. In addition, tumor- or cyst-like tissues started to appear in the inhbaa-/- ovary after about one year. In contrast to females, activin βAa/b mutant males showed normal spermatogenesis and fertility. As for activin βB subunit, the inhbb-/- mutant exhibited normal folliculogenesis, spermatogenesis and fertility in both sexes; however, the fecundity of mutant females decreased dramatically at 270 dpf with accumulation of early follicles. In summary, the activin-inhibin system plays an indispensable role in fish reproduction, in particular folliculogenesis and ovarian homeostasis.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Variant BMP receptor mutations causing fibrodysplasia ossificans progressiva (FOP) in humans show BMP ligand-independent receptor activation in zebrafish

The large majority of cases of the autosomal dominant human disease fibrodysplasia ossificans progressiva (FOP) are caused by gain-of-function Arg206His mutations in the BMP type I receptor ACVR1 (ALK2). The Arg206His mutation is located in the GS domain of the type I receptor. This region is normally phosphorylated by the BMP type II receptor, which activates the type I receptor to phosphorylate its substrate, the signal transducer Smad1/5/8. A small subset of patients with FOP carry variant mutations in ACVR1 altering Gly328 to Trp, Glu or Arg. Since these mutations lie outside the GS domain, the mechanism through which ACVR1 Gly328 mutations cause disease remains unclear. We used a zebrafish embryonic development assay to test the signaling of human ACVR1 Gly328 mutant receptors comparing them to the Arg206His mutant. In this assay increased or decreased BMP pathway activation alters dorsal-ventral axial patterning, providing a sensitive assay for altered BMP signaling levels. We expressed the human ACVR1 Gly328 mutant receptors in zebrafish embryos to investigate their signaling activities. We found that all ACVR1 Gly328 human mutations ventralized wild-type embryos and could partially rescue Bmp7-deficient embryos, indicating that these mutant receptors can activate BMP signaling in a BMP ligand-independent manner. The degree of ventralization or rescue was similar among all three Gly328 mutants. Smad1/5 phosphorylation, a readout of BMP receptor signaling, was mildly increased by ACVR1 Gly328 mutations. Gene expression analyses demonstrate expanded ventral and reciprocal loss of dorsal cell fate markers. This study demonstrates that Gly328 mutants increase receptor activation and BMP ligand-independent signaling through Smad phosphorylation.

Establishment of the Vertebrate Germ Layers

The process of germ layer formation is a universal feature of animal development. The germ layers separate the cells that produce the internal organs and tissues from those that produce the nervous system and outer tissues. Their discovery in the early nineteenth century transformed embryology from a purely descriptive field into a rigorous scientific discipline, in which hypotheses could be tested by observation and experimentation. By systematically addressing the questions of how the germ layers are formed and how they generate overall body plan, scientists have made fundamental contributions to the fields of evolution, cell signaling, morphogenesis, and stem cell biology. At each step, this work was advanced by the development of innovative methods of observing cell behavior in vivo and in culture. Here, we take an historical approach to describe our current understanding of vertebrate germ layer formation as it relates to the long-standing questions of developmental biology. By comparing how germ layers form in distantly related vertebrate species, we find that highly conserved molecular pathways can be adapted to perform the same function in dramatically different embryonic environments.

Molecular specification of germ layers in vertebrate embryos

In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. Classic experiments in amphibian embryos revealed the tissue interactions involved in germ layer formation and provided the groundwork for the identification of secreted and intracellular factors involved in this process. We will begin this review by summarising the key findings of those studies. We will then evaluate them in the light of more recent genetic studies that helped clarify which of the previously identified factors are required for germ layer formation in vivo, and to what extent the mechanisms identified in amphibians are conserved across other vertebrate species. Collectively, these studies have started to reveal the gene regulatory network (GRN) underlying vertebrate germ layer specification and we will conclude our review by providing examples how our understanding of this GRN can be employed to differentiate stem cells in a targeted fashion for therapeutic purposes.

Regulation of TGFβ and related signals by precursor processing

Secreted cytokines of the TGFβ family are found in all multicellular organisms and implicated in regulating fundamental cell behaviors such as proliferation, differentiation, migration and survival. Signal transduction involves complexes of specific type I and II receptor kinases that induce the nuclear translocation of Smad transcription factors to regulate target genes. Ligands of the BMP and Nodal subgroups act at a distance to specify distinct cell fates in a concentration-dependent manner. These signaling gradients are shaped by multiple factors, including proteases of the proprotein convertase (PC) family that hydrolyze one or several peptide bonds between an N-terminal prodomain and the C-terminal domain that forms the mature ligand. This review summarizes information on the proteolytic processing of TGFβ and related precursors, and its spatiotemporal regulation by PCs during development and various diseases, including cancer. Available evidence suggests that the unmasking of receptor binding epitopes of TGFβ is only one (and in some cases a non-essential) function of precursor processing. Future studies should consider the impact of proteolytic maturation on protein localization, trafficking and turnover in cells and in the extracellular space.

Dominant negative mutations of Caenorhabditis elegans daf-7 confer a novel developmental phenotype

BACKGROUND

TGF-β signaling pathways are involved in the control of development in every member of the animal kingdom. As such, TGF-β ligands are widely divergent yet retain a set of core conserved features, specifically, a pre-protein cleavage site and several conserved ligand domain residues, the disruption of which produces a dominant negative phenotype.

RESULTS

We have extended these observations into an invertebrate system by creating a series of loss-of-function Caenorhabditis elegans daf-7 transgenes. When we tested these mutant transgenes in a daf-7/+ background, we saw a molting and excretory canal phenotype. Members of both pathways downstream of daf-4 were required for this phenotype.

CONCLUSIONS

Our results show that the basic mechanisms of TGF-β function are conserved across the animal kingdom. A subset of our daf-7 mutations also produced an unexpected and novel phenotype. Epistasis experiments demonstrated that both daf-3/-5 and sma-4/-9 were downstream of our mutant daf-7 transgenes, which suggests not only a role for DAF-7 in the control of molting and the development of the excretory system but also that daf-7 and dbl-1 signaling may converge downstream of their shared Type II receptor, daf-4. Our approach may unveil new roles in development for other invertebrate TGF-β ligands.

Epidermal growth factor differentially regulates activin subunits in the zebrafish ovarian follicle cells via diverse signaling pathways

Epidermal growth factor (EGF) promotes oocyte maturation in the zebrafish and its effect is mediated via the activin system. However, the mechanisms by which EGF regulates activin subunits in the follicle cells remain unknown. The present study demonstrated that EGF controlled expression of three activin subunits (inhbaa, inhbab and inhbb) in the follicle cells via diverse signaling pathways. The expression of inhbaa and inhbb was often co-regulated via similar pathways. Suppression of MAPK3/1, p38 MAPK, PKC and PKA each blocked or partially reduced the stimulatory effects of EGF on the expression of inhbaa and inhbb while up-regulated that of inhbab. Conversely, inhibition of PI3K did not have any effect on the expression of inhbaa and inhbb but significantly suppressed the stimulatory effect of EGF on inhbab. In summary, EGF action in the zebrafish ovary involves activin system and its regulation of activin subunits is mediated by diverse signaling pathways downstream of EGFR.

A large bioactive BMP ligand with distinct signaling properties is produced by alternative proconvertase processing

Dimers of conventional transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) ligands are composed of two 100- to 140-amino acid peptides that are produced through the proteolytic processing of a proprotein precursor by proconvertases, such as furin. We report the identification of an evolutionarily conserved furin processing site in the amino terminus (NS) of the Glass bottom boat (Gbb; the Drosophila ortholog of vertebrate BMP5, 6, and 7) proprotein that generates a 328-amino acid, active BMP ligand distinct from the conventional 130-amino acid ligand. Gbb38, the large ligand form of Gbb, exhibited greater signaling activity and a longer range than the shorter form Gbb15. The abundance of Gbb15 and Gbb38 varied among different tissues, raising the possibility that differential processing could account for tissue-specific behaviors of BMPs. In human populations, mutations that abolished the NS cleavage site in BMP4, BMP15, or anti-Müllerian hormone were associated with cleft lip with or without cleft palate (BMP4), premature ovarian failure (BMP15), and persistent Müllerian duct syndrome (anti-Müllerian hormone), suggesting the importance of NS processing during development. The identification of this large BMP ligand form and the functional differences between large and small ligands exemplifies the potential for differential proprotein processing to substantially affect BMP and TGF-β signaling output in different tissue and cellular contexts.

Different requirements for proteolytic processing of bone morphogenetic protein 5/6/7/8 ligands in Drosophila melanogaster

Bone morphogenetic proteins (BMPs) are synthesized as proproteins that undergo proteolytic processing by furin/subtilisin proprotein convertases to release the active ligand. Here we study processing of BMP5/6/7/8 proteins, including the Drosophila orthologs Glass Bottom Boat (Gbb) and Screw (Scw) and human BMP7. Gbb and Scw have three functional furin/subtilisin proprotein convertase cleavage sites; two between the prodomain and ligand domain, which we call the Main and Shadow sites, and one within the prodomain, which we call the Pro site. In Gbb each site can be cleaved independently, although efficient cleavage at the Shadow site requires cleavage at the Main site, and remarkably, none of the sites is essential for Gbb function. Rather, Gbb must be processed at either the Pro or Main site to produce a functional ligand. Like Gbb, the Pro and Main sites in Scw can be cleaved independently, but cleavage at the Shadow site is dependent on cleavage at the Main site. However, both Pro and Main sites are essential for Scw function. Thus, Gbb and Scw have different processing requirements. The BMP7 ligand rescues gbb mutants in Drosophila, but full-length BMP7 cannot, showing that functional differences in the prodomain limit the BMP7 activity in flies. Furthermore, unlike Gbb, cleavage-resistant BMP7, although non-functional in rescue assays, activates the downstream signaling cascade and thus retains some functionality. Our data show that cleavage requirements evolve rapidly, supporting the notion that changes in post-translational processing are used to create functional diversity between BMPs within and between species.

Ethylnitrosourea-induced thymus-defective mutants identify roles of KIAA1440, TRRAP, and SKIV2L2 in teleost organ development

The thymus is an organ where T lymphocytes develop. Thymus development requires interactions of cells derived from three germ layers. However, the molecular mechanisms that control thymus development are not fully understood. To identify the genes that regulate thymus development, we previously carried out a large-scale screening for ethylnitrosourea-induced mutagenesis using medaka, Oryzias latipes, and established a panel of recessive thymus-lacking mutants. Here we report the identification of three genes responsible for these mutations. We found that the mutations in KIAA1440, TRRAP, and SKIV2L2 caused the defects in distinct steps of thymus development. We also found that these genes were widely expressed in many organs and that the mutations in these genes caused defects in the development of various other organs. These results enabled us to identify previously unknown roles of widely expressed genes in medaka organ development. The possible reasons why thymus-defective teleost mutants could be used to identify widely expressed genes and future strategies to increase the likelihood of identifying genes that specifically regulate thymus development are discussed.

WDR55 is a nucleolar modulator of ribosomal RNA synthesis, cell cycle progression, and teleost organ development

The thymus is a vertebrate-specific organ where T lymphocytes are generated. Genetic programs that lead to thymus development are incompletely understood. We previously screened ethylnitrosourea-induced medaka mutants for recessive defects in thymus development. Here we report that one of those mutants is caused by a missense mutation in a gene encoding the previously uncharacterized protein WDR55 carrying the tryptophan-aspartate-repeat motif. We find that WDR55 is a novel nucleolar protein involved in the production of ribosomal RNA (rRNA). Defects in WDR55 cause aberrant accumulation of rRNA intermediates and cell cycle arrest. A mutation in WDR55 in zebrafish also leads to analogous defects in thymus development, whereas WDR55-null mice are lethal before implantation. These results indicate that WDR55 is a nuclear modulator of rRNA synthesis, cell cycle progression, and embryonic organogenesis including teleost thymus development.

Dominant negative Bmp5 mutation reveals key role of BMPs in skeletal response to mechanical stimulation

Background

Over a hundred years ago, Wolff originally observed that bone growth and remodeling are exquisitely sensitive to mechanical forces acting on the skeleton. Clinical studies have noted that the size and the strength of bone increase with weight bearing and muscular activity and decrease with bed rest and disuse. Although the processes of mechanotransduction and functional response of bone to mechanical strain have been extensively studied, the molecular signaling mechanisms that mediate the response of bone cells to mechanical stimulation remain unclear.

Results

Here, we identify a novel germline mutation at the mouse Bone morphogenetic protein 5 (Bmp5) locus. Genetic analysis shows that the mutation occurs at a site encoding the proteolytic processing sequence of the BMP5 protein and blocks proper processing of BMP5. Anatomic studies reveal that this mutation affects the formation of multiple skeletal features including several muscle-induced skeletal sites in vivo. Biomechanical studies of osteoblasts from these anatomic sites show that the mutation inhibits the proper response of bone cells to mechanical stimulation.

Conclusion

The results from these genetic, biochemical, and biomechanical studies suggest that BMPs are required not only for skeletal patterning during embryonic development, but also for bone response and remodeling to mechanical stimulation at specific anatomic sites in the skeleton.

Activation and roles of ALK4/ALK7-mediated maternal TGFbeta signals in zebrafish embryo

Activin, Nodal, and Vg1, members of the transforming growth factor beta (TGFbeta) superfamily, transduce signal through type I receptors ALK4 or ALK7 and play important roles in mesoderm induction and patterning during vertebrate embryogenesis. However, the timing and magnitude of the ALK4/ALK7-mediated maternal TGFbeta signals are not clear. SB-431542 is identified as an inhibitor of the ALK4/ALK5/ALK7-mediated TGFbeta signals and its specificity in vertebrate embryos has not been reported. We demonstrate that SB-431542 is able to specifically and reproducibly block the Smad2/3-mediated TGFbeta signals in zebrafish embryo. Embryos exposed to SB-431542 exhibit various defects phenocopying Nodal-deficient mutants. SB-431542 treatments starting at different cell cycles before the midblastula transition lead to different degrees of developmental defects in mesoderm induction and patterning, suggesting that maternal TGFbeta signals are activated right after fertilization and required for mesoderm formation and patterning.

BMP-3 is a novel inhibitor of both activin and BMP-4 signaling in Xenopus embryos

In Xenopus, the biological effects of BMP-3 oppose those of ventralizing BMPs, but the mechanism for this antagonism remains unclear. Here, we demonstrate that BMP-3 is a dorso-anteriorizing factor in Xenopus embryos that interferes with both activin and BMP signaling. BMP-3 acts by binding to ActRIIB, the common type II receptor for these proteins. Once BMP-3 binds to ActRIIB, it cannot be competed off by excess ligand making a receptor complex that is unable to activate R-Smads and transduce signal. Consistent with a model where BMP-3 interferes with activin and BMPs through a shared receptor, we show that overexpression of BMP-3 can only be rescued by co-injection of xActRIIB. Our results identify BMP-3 as a novel antagonist of both activin and BMPs and uncover how some of the diverse developmental processes that are regulated by both activin and BMP signaling can be modulated during embryogenesis.

A molecular recognition paradigm: promiscuity associated with the ligand-receptor interactions of the activin members of the TGF-β superfamily

The structure-function properties of the pleiotropic activins and their relationship to other members of the transforming growth factor-beta superfamily of proteins are described. In order to highlight the molecular promiscuity of these growth factors, emphasis has been placed on molecular features associated with the recognition by activin A and the bone morphogenic proteins of the corresponding extracellular domains of the ActRI and ActRII receptors. The available evidence suggests that the homodimeric activin A in its various functional roles has the propensity to fulfill key tasks in the regulation of mammalian cell behaviour, through coordination of numerous transcriptional and translational processes. Because of these profound effects, under physiologically normal conditions, activin A levels are closely controlled by a variety of binding partners, such as follistatin-288 and follistatin-315, alpha(2)-macroglobulin and other proteins. Moreover, the subunits of other members of the activin subfamily, such as activin B or activin C, are able to form heterodimers with the activin A subunit, thus providing a further avenue to positively or negatively control the physiological concentrations of activin A that are available for interaction with specific receptors and induction of cell signaling events. Based on data from X-ray crystallographic studies and homology modeling experiments, the molecular architecture of the ternary receptor-activin ligand complexes has been dissected, permitting rationalization in structural terms of the pattern of interactions that are the hallmark of this protein family.

Myostatin dominant negative allele products interact positively with wild type monomers

Myostatin is an extracellular negative regulator of muscle growth with an important role in bovine muscular hypertrophy. It belongs to the transforming growth factor beta (TGFbeta) superfamily, and has structural and functional characteristics similar to those of its other, members. Based on these characteristics, we designed three gene constructs in order to create a series of dominant negative (DN) alleles for murine myostatin. As a first requirement for any DN strategy, we first showed that each of the three mutant DN monomers were able to interact with wild type mature myostatin (wt-Mstn), both in a pull-down and a mammalian two-hybrid assay. In addition, the degree of DN-Mstn/wt-Mstn interaction was similar to that of wt-Mstn/wt-Mstn. These results suggest that the three designed alleles are good candidates for use in a DN-based strategy for generating muscular hypertrophy in cattle.

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.

Activin redux: specification of mesodermal pattern in Xenopus by graded concentrations of endogenous activin B

Mesoderm formation in the amphibian embryo occurs through an inductive interaction in which cells of the vegetal hemisphere of the embryo act on overlying equatorial cells. The first candidate mesoderm-inducing factor to be identified was activin, a member of the transforming growth factor type beta family, and it is now clear that members of this family are indeed involved in mesoderm and endoderm formation. In particular, Derrière and five nodal-related genes are all considered to be strong candidates for endogenous mesoderm-inducing agents. Here, we show that activin, the function of which in mesoderm induction has hitherto been unclear, also plays a role in mesoderm formation. Inhibition of activin function using antisense morpholino oligonucleotides interferes with mesoderm formation in a concentration-dependent manner and also changes the expression levels of other inducing agents such as Xnr2 and Derrière. This work reinstates activin as a key player in mesodermal patterning. It also emphasises the importance of checking for polymorphisms in the 5' untranslated region of the gene of interest when carrying out antisense morpholino experiments in Xenopus laevis.

Identification of recessive maternal-effect mutations in the zebrafish using a gynogenesis-based method

In animal species, early developmental processes are driven by maternally derived factors. Here, we describe a forward genetics approach to identify recessive mutations in genes encoding such maternal factors in the zebrafish. We used a gynogenesis-based approach to identify 14 recessive maternal-effect mutations. Homozygosity for these mutations in adult females leads to the inviability of their offspring. Confocal microscopy of embryos labeled with a DNA dye and a membrane marker allowed us to further analyze mutant embryos for defects in nuclear and cellular divisions. The mutations result in a range of defects in early developmental processes, including egg activation, early nuclear events, mitosis, cytokinesis, axial patterning, and gastrulation. Our effort constitutes a systematic attempt to identify maternal-effect genes in a vertebrate species. The sample of mutations that we have identified reflects the diversity of maternally driven functions in early development and underscores the importance of maternal factors in this process.

Maternal factors in zebrafish development

All processes that occur before the activation of the zygotic genome at the midblastula transition are driven by maternal products, which are produced during oogenesis and stored in the mature oocyte. Upon egg activation and fertilization, these maternal factors initiate developmental cascades that carry out the embryonic developmental program. Even after the initiation of zygotic gene expression, perduring maternal products continue performing essential functions, either together with other maternal factors or through interactions with newly expressed zygotic products. Advances in zebrafish research have placed this organism in a unique position to contribute to a detailed understanding of the role of maternal factors in early vertebrate development. This review summarizes our knowledge on the processes involved in the production and redistribution of maternal factors during zebrafish oogenesis and early development, as well as our understanding of the function of these factors in axis formation, germ layer and germ cell specification, and other early embryonic processes.

Spatial expression patterns of activin and its signaling system in the zebrafish ovarian follicle: evidence for paracrine action of activin on the oocytes

We have previously demonstrated that activin is likely an ovarian mediator of pituitary gonadotropin(s) and local epidermal growth factor in their stimulating oocyte maturation and maturational competence in the zebrafish. However, the downstream events controlled by activin remain unknown. One possible mechanism is that activin may directly work on the oocytes to promote the development of oocyte maturational competence. To substantiate this hypothesis, we performed the present study to demonstrate the expression of the activin system in different compartments of zebrafish follicles, namely, the follicle cells and oocytes. The proteins examined include activin subunits (betaA and betaB), activin-binding protein (follistatin), activin type II receptors (type IIA and IIB), the type I activin receptor-like kinases (ALK1-like, ALK2-like, and ALK4-like), and the intracellular activin signaling molecules (Smad2, Smad3, Smad4, and Smad7). The results showed that the entire activin signaling system is expressed by the full-grown immature zebrafish oocytes ( approximately 0.65 mm in diameter), including ALK4-like (ActRIB), ALK2-like (ActRIA), ActRIIA, ActRIIB, Smad2, Smad3, Smad4, and Smad7, therefore supporting our hypothesis that the oocytes are one of the direct targets of activin actions in the zebrafish ovary. In contrast, activin itself (betaA and betaB) and ALK1-like type I receptor are predominantly expressed in the follicle cells surrounding the oocytes. Interestingly, although follistatin is expressed in both the follicle cells and oocytes, its level of expression is significantly higher in the oocytes than the follicle cells, implying that follistatin may serve as a signal from the oocytes to modulate the activity of activin produced by the follicle cells. Taken together, the present study provides convincing evidence that although all members of the activin system are expressed in the whole follicle, they exhibit distinct spatial patterns of expression among different compartments of the follicle. It is likely that activin works directly on the oocytes in a paracrine manner to promote oocyte maturation and maturational competence. On the other hand, instead of being controlled passively by the follicle cells, the oocytes may actively participate in the regulation of follicle development by releasing various modulating molecules such as follistatin.

Involvement of cyclic adenosine 3',5'-monophosphate in the differential regulation of activin betaA and betaB expression by gonadotropin in the zebrafish ovarian follicle cells

Activin is a dimeric protein consisting of two similar but distinct beta-subunits, betaA and betaB. In our previous studies, both activin A (betaAbetaA) and activin B (betaBbetaB) have been demonstrated to stimulate oocyte maturation and promote oocyte maturational competence in the zebrafish. Follistatin, a specific activin-binding protein, can block both activin- and gonadotropin-induced final oocyte maturation in vitro, suggesting that activin is likely a downstream mediator of gonadotropin actions in the zebrafish ovary. In the present study, a full-length cDNA encoding zebrafish ovarian activin betaA was cloned and sequenced. The precursor of zebrafish activin betaA consists of 395 amino acids and its mature region exhibits about 78% homology with that of mammals. Using an in vitro primary culture of the ovarian follicle cells and semiquantitative RT-PCR assays, we examined the regulation of activin betaA and betaB expression by human chorionic gonadotropin (hCG) and its intracellular signal transduction mechanisms. hCG (15 IU/ml) increased the mRNA level of activin betaA-subunit; however, it significantly down-regulated the steady-state expression level of activin betaB in a time- and dose-dependent manner. The differential regulation of the two beta-subunits by hCG could be mimicked by 3-isobutyl-1-methylxanthine, forskolin, and dibutyryl-cAMP, suggesting involvement of the intracellular cAMP pathway. Interestingly, H89 (a specific inhibitor of protein kinase A, PKA) could effectively block hCG- and forskolin-stimulated activin betaA expression at 10 micro M, but it was unable to reverse the inhibitory effects of hCG and forskolin on betaB expression. This suggests that the hCG-stimulated activin betaA expression is dependent on the activation of the cAMP-PKA pathway, whereas the inhibitory effect of hCG on activin betaB expression is likely mediated by PKA-independent pathway(s).

Effects of heterodimerization and proteolytic processing on Derrière and Nodal activity: implications for mesoderm induction in Xenopus

Derrière is a recently discovered member of the TGFβ superfamily that can induce mesoderm in explant assays and is expressed at the right time and location to mediate mesoderm induction in response to VegT during Xenopus embryogenesis. We show that the ability of Derrière to induce dorsal or ventral mesoderm depends strictly on the location of expression and that a dominant-negative Derrière cleavage mutant completely blocks all mesoderm formation when ectopically expressed. This differs from the activity of similar Xnr2 cleavage mutant constructs, which are secreted and retain signaling activity. Additional analysis of mesoderm induction by Derrière and members of the Nodal family indicates that these molecules are involved in a mutual positive-feedback loop and antagonism of either one of the signals can reduce the other. Interaction between Derrière and members of the Nodal family is also shown to occur through the formation of heterodimeric ligands. Using an oocyte expression system we show direct interaction between the mature Derrière ligand and members of both the Nodal and BMP families. Taken together, these findings indicate that Derrière and Nodal proteins probably work cooperatively to induce mesoderm throughout the marginal zone during early Xenopus development.

Cooperative action of ADMP- and BMP-mediated pathways in regulating cell fates in the zebrafish gastrula

It was shown in Xenopus and chick that Spemann's organizer activity is regulated through the negative action of Anti-Dorsalizing Morphogenetic Protein (ADMP). We report the characterization and functional properties of admp in zebrafish. admp expression profile is consistent with a role in the organizer, including the tail organizer. We studied admp function through overexpression experiments, with the use of a dominant-negative form (TR-ADMP) and of an antisense morpholino-modified oligonucleotide. Our results indicate that the ADMP pathway causes the restriction of anterior and axial fates and that ADMP, BMP2b, and BMP7 pathways have distinct actions but cooperate in establishing proper dorso-ventral regionalization. This is shown by partial rescue of the dorsalized mutant snailhouse and of the ventralized mutant chordino, upon admp and tr-admp RNA injection, respectively. Moreover, ADMP and BMP7 probably form heterodimers as shown by the ability of TR-ADMP and BMP7 to antagonize each other. We observed that a MYC-tagged ADMP was secreted and detected in the extracellular space, suggesting that admp could act at a distance. Simultaneous local inhibition of bmp function at the blastoderm margin and impairment of ADMP secretion led to the induction of secondary head structures, confirming that the two pathways cooperatively regulate organizer formation and activity.

Multiple nodal-related genes act coordinately in Xenopus embryogenesis

Four nodal-related genes (Xnr1-4) have been isolated in Xenopus to date, and we recently further identified two more, Xnr5 and Xnr6. In the present functional study, we constructed cleavage mutants of Xnr5 (cmXnr5) and Xnr6 (cmXnr6) which were expected to act in a dominant-negative manner. Both cmXnr5 and cmXnr6 inhibited the activities of Xnr5 and Xnr6 in co-overexpression experiments. cmXnr5 also inhibited the activity of Xnr2, Xnr4, Xnr6, derrière, and BVg1, but did not inhibit the activity of Xnr1 or activin. Misexpression of cmXnr5 led to a severe delay in initiation of gastrulation and phenotypic changes, including defects in anterior structures, which were very similar to those seen in maternal VegT-depleted embryos. Further, although the expression of Xnr1, Xnr2, and Xnr4 was not delayed in these embryos, it was markedly reduced. Injection of cmXnr5 had no notable effect on expression of Xnr3, Xnr6, derrière, or siamois. Several mesodermal and endodermal markers also showed delayed and decreased expression during gastrulation in cmXnr5-injected embryos. These results suggest that, in early Xenopus embryogenesis, nodal-related genes may heterodimerize with other TGF-beta ligands, and further that one nodal-related gene alone is insufficient for mesendoderm formation, which may require the cooperative interaction of multiple nodal-related genes.

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.

Roles of the activin regulatory system in fish reproduction

Activin (βAβA, βAβB, and βBβb) is a dimeric growth factor with diverse biological activities in vertebrate reproduction. Activin exerts its actions by binding to its specific type II and type I receptors. The activity of activin is regulated by follistatin, its binding protein, and the antagonists inhibin and antivin. All major components of the activin-inhibin-follistatin system have been identified in fish except the α subunit of inhibin. Using goldfish as a model, we have demonstrated that activin is expressed in the pituitary and the recombinant goldfish activin B has novel inverse effects on the expression of GTH β subunits. Activin increases the mRNA level of GTH-Iβ while significantly suppressing the expression of GTH-IIβ. We have also demonstrated the expression of activin and its receptors in the goldfish and zebrafish ovary. Using an in vitro ovarian follicle incubation as the system, we have investigated the involvement of the activin system in the process of final oocyte maturation. Our evidence clearly indicates that activin has potent effect of promoting final oocyte maturation, and that it may play a role in mediating the stimulatory effect of pituitary gonadotropin in the event of oocyte maturation. Key words: activin, inhibin, follistatin, fish, reproduction.

Evolutionary trace analysis of TGF-beta and related growth factors: implications for site-directed mutagenesis

The TGF-beta family of growth factors contains a large number of homologous proteins, grouped in several subfamilies on the basis of sequence identity. These subgroups can be combined into three broader groups of related cytokines, with marked specificities for their cellular receptors: the TGF-betas, the activins and the BMPs/GDFs. Although structural information is available for some members of the TGF-beta family, very little is known about the way in which these growth factors interact with the extra-cellular domains of their multiple cell surface receptors or with the specific protein inhibitors thought to modulate their activity. In this paper, we use the evolutionary trace method [Lichtarge et al. (1996) J. Mol. Biol., 257, 342-358] to locate two functional patches on the surface of TGF-beta-like growth factors. The first of these is centred on a conserved proline (P(36) in TGF-betas 1-3) and contains two amino acids which could account for the receptor specificity of TGF-betas (H(34) and E(35)). The second patch is located on the other side of the growth factor protomer and surrounds a hydrophobic cavity, large enough to accommodate the side chain of an aromatic residue. In addition to two conserved tryptophans at positions 30 and 32, the main protagonists in this potential binding interface are found at positions 31, 92, 93 and 98. Several mutagenesis studies have highlighted the importance of the C-terminal region of the growth factor molecule in TGF-betas and of residues in activin A equivalent to positions 31 and 94 of the TGF-betas for the binding of type II receptors to these ligands. These data, together with our improved knowledge of possible functional residues, can be used in future structure-function analysis experiments.

The role of the yolk syncytial layer in germ layer patterning in zebrafish

Formation of the three germ layers requires a series of inductive events during early embryogenesis. Studies in zebrafish indicate that the source of these inductive signals may be the extra-embryonic yolk syncytial layer (YSL). The characterization of genes encoding the nodal-related factor, Squint, and homeodomain protein, Bozozok, both of which are expressed in the YSL, suggested that the YSL has a role in mesendoderm induction. However, these genes, and a second nodal-related factor, cyclops, are also expressed in the overlying marginal blastomeres, raising the possibility that the marginal blastomeres can induce mesendodermal genes independently of the YSL. We have developed a novel technique to study signaling from the YSL in which we specifically eliminate RNAs in the YSL, thus addressing the in vivo requirement of RNA-derived signals from this region in mesendoderm induction. We show that injection of RNase into the yolk cell after the 1K cell stage (3 hours) effectively eliminates YSL transcripts without affecting ubiquitously expressed genes in the blastoderm. We also present data that indicate the stability of existing proteins in the YSL is unaffected by RNase injection. Using this technique, we show that RNA in the YSL is required for the formation of ventrolateral mesendoderm and induction of the nodal-related genes in the ventrolateral marginal blastomeres, revealing the presence of an unidentified inducing signal released from the YSL. We also demonstrate that the dorsal mesoderm can be induced independently of signals from the YSL and present evidence that this is due to the stabilization of (β)-catenin in the dorsal marginal blastomeres. Our results demonstrate that germ layer formation and patterning in zebrafish uses a combination of YSL-dependent and -independent inductive events.

Primary structure requirements for Xenopus nodal-related 3 and a comparison with regions required by Xenopus nodal-related 2

Transforming growth factor-beta superfamily members play important roles in the early development of animals. Activin and the Xenopus nodal related proteins 1, 2, and 4 induce muscle actin from Xenopus ectodermal explants, whereas the bone morphogenetic proteins 4 and 7 induce ectoderm to differentiate as epidermis. Bone morphogenetic proteins are antagonized by soluble binding proteins such as noggin and chordin, which leads to expression of neural cell adhesion molecule in animal caps. The transforming growth factor-beta superfamily member Xenopus nodal-related 3 also induces the neural cell adhesion molecule through inhibition of bone morphogenetic proteins. Therefore, whereas Xenopus nodal-related 2 and 3 share a high amount of sequence homology, they lead to very different cell fates. This study investigates the functional domains that distinguish the activities of these two factors. It was found that mutually exclusive regions of nodal-related 2 and 3 were required for activity. The central region of the mature domain is required for nodal-related 2 to induce muscle actin, whereas the N- and C-terminal ends of the mature domain are required for nodal-related 3 to induce neural cell adhesion molecule. These results help to define the minimal domains required for the unique activities of these factors.

Cloning and characterization of zebrafish smad2, smad3 and smad4

smad genes encode transcription factors involved in the signal transduction of members of the TGFbeta superfamily. We report here the cloning, characterization and genomic mapping of smad2, smad3 and smad4 from the zebrafish, Danio rerio. In Xenopus, smad2 overexpression has been shown to interfere with gastrulation and dorsal cell fate specification. However, full-length zebrafish smad2, although functionally active in Xenopus explants, has no effect when overexpressed in zebrafish embryos. In contrast, an N-terminally truncated, constitutively active version of Smad2 protein causes severe dorsalization or partial secondary axis formation, pointing to a role of Smad2 during mesoderm and axis formation. The temporal and spatial expression patterns of zebrafish smad2, 3 and 4 were investigated by developmental RT-PCR and whole mount in-situ hybridization. All three genes show strong and ubiquitous maternal expression. Zygotic expression is weak and ubiquitous in the case of smad2, and strong and ubiquitious in the case of smad4, while smad3 shows a spatially restricted zygotic expression pattern. It is expressed in migrating neural crest cells of the trunk and a subset of cells in the diencephalon in close proximity to the expression domain of the Nodal-related protein Cyclops/Ndr2/Znr1, a potential signal upstream of Smad2/3 required for eye-field separation and floor plate specification. Overexpression of truncated smad2 in cyclops mutant embryos leads to a rescue of the eye and floorplate defects. These data suggest that Smad2 acts as a mediator of Nodal signals during zebrafish midline signaling, while Smad3 might be involved in later steps of eye field separation.

Activins and their receptors in female reproduction

Activins are growth and differentiation factors belonging to the transforming growth factor-β superfamily. They are dimeric proteins consisting of two inhibin β subunits. The structure of activins is highly conserved during vertebrate evolution. Activins signal through type I and type II receptor proteins, both of which are serine/threonine kinases. Subsequently, downstream signals such as Smad proteins are phosphorylated. Activins and their receptors are present in many tissues of mammals and lower vertebrates where they function as autocrine and (or) paracrine regulators of a variety of physiological processes, including reproduction. In the hypothalamus, activins are thought to stimulate the release of gonadotropin-releasing hormone. In the pituitary, activins increase follicle-stimulating hormone secretion and up-regulate gonadotropin-releasing hormone receptor expression. In the ovaries of vertebrates, activins are expressed predominantly in the follicular layer of the oocyte where they regulate processes such as folliculogenesis, steroid hormone production, and oocyte maturation. During pregnancy, activin-A is also involved in the regulation of placental functions. This review provides a brief overview of activins and their receptors, including their structures, expression, and functions in the female reproductive axis as well as in the placenta. Special effort is made to compare activins and their receptors in different vertebrates.

Key words: activins, activin receptors, reproductive axis, placenta.

Dishevelled phosphorylation, subcellular localization and multimerization regulate its role in early embryogenesis

Dishevelled (Dsh) induces a secondary axis and can translocate to the membrane when activated by Frizzleds; however, dominant-negative approaches have not supported a role for Dsh in primary axis formation. We demonstrate that the Dsh protein is post-translationally modified at the dorsal side of the embryo: timing and position of this regulation suggests a role of Dsh in dorsal-ventral patterning in Xenopus. To create functional links between these properties of Dsh we analyzed the influence of endogenous Frizzleds and the Dsh domain dependency for these characteristics. Xenopus Frizzleds phosphorylate and translocate Xdsh to the membrane irrespective of their differential ectopic axes inducing abilities, showing that translocation is insufficient for axis induction. Dsh deletion analysis revealed that axis inducing abilities did not segregate with Xdsh membrane association. The DIX region and a short stretch at the N-terminus of the DEP domain are necessary for axis induction while the DEP region is required for Dsh membrane association and its phosphorylation. In addition, Dsh forms homomeric complexes in embryos suggesting that multimerization is important for its proper function.

Requirement for anti-dorsalizing morphogenetic protein in organizer patterning

The amphibian Spemann organizer is subdivided in trunk and head organizer and it is unclear how this division is regulated. The Xenopus trunk organizer expresses anti-dorsalizing morphogenetic protein (ADMP), a potent organizer antagonist. We show that ADMP represses head formation during gastrulation and that its expression is activated by BMP antagonists. A specifically acting dominant-negative ADMP anteriorizes embryos and its coexpression with BMP antagonists induces secondary embryonic axes with heads as well as expression of head inducers. Unlike other BMPs, ADMP is not inhibited by a dominant-negative BMP type I receptor, Noggin, Cerberus and Chordin but by Follistatin, suggesting that it utilizes a distinct TGF-beta receptor pathway and displays differential sensitivity to BMP antagonists. The results indicate that ADMP functions in the trunk organizer to antagonize head formation, thereby regulating organizer patterning.

Nab proteins mediate a negative feedback loop controlling Krox-20 activity in the developing hindbrain

The developing vertebrate hindbrain is transiently subdivided along the anterior-posterior axis into metameric units, called rhombomeres (r). These segments constitute units of lineage restriction and display specific gene expression patterns. The transcription factor gene Krox-20 is restricted to r3 and r5, and is required for the development of these rhombomeres. We present evidence that Krox-20 transcriptional activity is under the control of a negative feedback mechanism in the hindbrain. This regulatory loop involves two closely related proteins, Nabl and Nab2, previously identified as antagonists of Krox-20 transcriptional activity in cultured cells. Here we show that in the mouse hindbrain, Nab1 and Nab2 recapitulate the Krox-20 expression pattern and that their expression is dependent on Krox-20 function. Furthermore, misexpression of Nab1 or Nab2 in zebrafish embryos leads to alterations in the expression patterns of several hindbrain markers, consistent with an inhibition of Krox-20 activity. Taken together, these data indicate that Krox-20 positively regulates the expression of its own antagonists and raise the possibility that this negative feedback regulatory loop may play a role in the control of hindbrain development.

Activin stimulation of zebrafish oocyte maturation in vitro and its potential role in mediating gonadotropin-induced oocyte maturation

Activin plays important roles in the regulation of vertebrate reproduction. Using zebrafish, Danio rerio, as a model, the present study aimed at investigating the role of activin in the regulation of final oocyte maturation. Administration of recombinant goldfish activin B significantly increased the rate of oocyte maturation in vitro in a dose- and time-dependent manner. The effect of activin seemed to be additive to the effects of gonadotropin (hCG) and 17alpha,20beta-dihydroxyprogesterone, a potent maturation-inducing hormone in teleosts. The specificity of the activin action was confirmed by coincubation with recombinant human follistatin, which completely abolished the stimulatory effect of activin B. Interestingly, follistatin also significantly inhibited hCG-induced oocyte maturation, suggesting that endogenous activin may be a downstream mediator of gonadotropin actions. No effect of activin B was observed in the presence of actinomycin D, indicating that the action of activin may involve changes in transcriptional activity. These results, together with the demonstration that activin and its type II receptor are expressed in the zebrafish ovary, strongly suggest a paracrine/autocrine role for activin in the controlling of final oocyte maturation.

The midbrain-hindbrain boundary genetic cascade is activated ectopically in the diencephalon in response to the widespread expression of one of its components, the medaka gene Ol-eng2

In vertebrates, the engrailed genes are expressed at early neurula stage in a narrow stripe encompassing the midbrain-hindbrain boundary (MHB), a region from which a peculiar structure, the isthmus, is formed. Knock-out experiments in mice demonstrated that these genes are essential for the development of this structure and of its derivatives. In contrast, little is known about the effect of an overexpression of engrailed genes in vertebrate development. Here we report the isolation of Ol-eng2, a medaka fish (Oryzias latipes) engrailed gene. We have monitored the effects of its widespread expression following mRNA injections in 1- and 2-cell medaka and Xenopus embryos. We found that the ectopic expression of Ol-eng2 predominantly results in an altered development of the anterior brain, including an inhibition of optic vesicle formation. No change in the patterns of mesencephalic and telencephalic markers were observed. In contrast, expressions of markers of the diencephalon were strongly repressed in injected embryos. Furthermore, the endogenous Ol-eng2, Pax2, Wnt1 and Fgf8, which are essential components of the MHB genetic cascade, were ectopically expressed in this region. Therefore, we propose that Ol-eng2 induces de novo formation of an isthmus-like structure, which correlates with the development of ectopic midbrain structures, including optic tectum. A competence of the diencephalon to change to a midbrain fate has been demonstrated in isthmic graft experiments. Our data demonstrate that this change can be mimicked by ectopic engrailed expression alone.

Cloning and characterization of goldfish activin betaA subunit

We have cloned a full-length cDNA coding for activin betaA subunit from a goldfish brain and pituitary cDNA library, which represents the first for activin betaA in fish. Sequence analysis of goldfish activin betaA shows that this peptide is highly conserved across vertebrates. The mature region of goldfish activin betaA shares 81% amino acid identity with that of humans. Messenger RNA of goldfish activin betaA is expressed in a variety of tissues including ovary, testis, brain and liver, suggesting a wide range of physiological roles for activin A in the goldfish. The identity of the cloned goldfish activin betaA was confirmed by expressing the protein in the Chinese hamster ovary (CHO) cells followed by detection of the specific activin activity in the culture medium using erythroid differentiation factor (EDF) assay with F5-5 cells. Stable CHO cell lines producing high level of recombinant goldfish activin A were established and characterized.

Cloning of zebrafish activin type IIB receptor (ActRIIB) cDNA and mRNA expression of ActRIIB in embryos and adult tissues

A full-length cDNA encoding for activin type IIB receptor (ActRIIB) was cloned from zebrafish embryos. It encodes a protein with 509 amino acids consisting of a signal peptide, an extracellular ligand binding domain, a single transmembrane region, and an intracellular kinase domain with predicted serine/threonine specificity. The extracellular domain shows 74-91% sequence identity to human, bovine, mouse, rat, chicken, Xenopus and goldfish activin type IIB receptors, while the transmembrane region and the kinase domain show 67-78% and 82-88% identity to these known activin IIB receptors, respectively. In adult zebrafish, ActRIIB mRNA was detected by RT-PCR in the gonads, as well as in non-reproductive tissues, including the brain, heart and muscle. In situ hybridization on ovarian sections further localized ActRIIB mRNA to cytoplasm of oocytes at different stages of development. Using whole-mount in situ hybridization, ActRIIB mRNA was found to be expressed at all stages of embryogenesis examined, including the sphere, shield, tail bud, and 6-7 somite. These results provide the first evidence that ActRIIB mRNA is widely distributed in fish embryonic and adult tissues. Cloning of zebrafish ActRIIB demonstrates that this receptor is highly conserved during vertebrate evolution and provides a basis for further studies on the role of activin in reproduction and development in lower vertebrates.

Regulation of midline development by antagonism of lefty and nodal signaling

The embryonic midline is crucial for the development of embryonic pattern including bilateral symmetry and left-right asymmetry. In zebrafish, lefty1 (lft1) and lefty2 (lft2) have distinct midline expression domains along the anteroposterior axis that overlap with the expression patterns of the nodal-related genes cyclops and squint. Altered expression patterns of lft1 and lft2 in zebrafish mutants that affect midline development suggests different upstream pathways regulate each expression domain. Ectopic expression analysis demonstrates that a balance of lefty and cyclops signaling is required for normal mesendoderm patterning and goosecoid, no tail and pitx2 expression. In late somite-stage embryos, lft1 and lft2 are expressed asymmetrically in the left diencephalon and left lateral plate respectively, suggesting an additional role in laterality development. A model is proposed by which the vertebrate midline, and thus bilateral symmetry, is established and maintained by antagonistic interactions among co-expressed members of the lefty and nodal subfamilies of TGF-beta signaling molecules.

Induction of the mesendoderm in the zebrafish germ ring by yolk cell-derived TGF-beta family signals and discrimination of mesoderm and endoderm by FGF

The endoderm forms the gut and associated organs, and develops from a layer of cells which emerges during gastrula stages in the vertebrate embryo. In comparison to mesoderm and ectoderm, little is known about the signals which induce the endoderm. The origin of the endoderm is intimately linked with that of mesoderm, both by their position in the embryo, and by the molecules that can induce them. We characterised a gene, zebrafish gata5, which is expressed in the endoderm from blastula stages and show that its transcription is induced by signals originating from the yolk cell. These signals also induce the mesoderm-expressed transcription factor no tail (ntl), whose initial expression coincides with gata5 in the cells closest to the blastoderm margin, then spreads to encompass the germ ring. We have characterised the induction of these genes and show that ectopic expression of activin induces gata5 and ntl in a pattern which mimics the endogenous expression, while expression of a dominant negative activin receptor abolishes ntl and gata5 expression. Injection of RNA encoding a constitutively active activin receptor leads to ectopic expression of gata5 and ntl. gata5 is activated cell-autonomously, whereas ntl is induced in cells distant from those which have received the RNA, showing that although expression of both genes is induced by a TGF-beta signal, expression of ntl then spreads by a relay mechanism. Expression of a fibroblast growth factor (eFGF) or a dominant negatively acting FGF receptor shows that ntl but not gata5 is regulated by FGF signalling, implying that this may be the relay signal leading to the spread of ntl expression. In embryos lacking both squint and cyclops, members of the nodal group of TGF-beta related molecules, gata5 expression in the blastoderm is abolished, making these factors primary candidates for the endogenous TGF-beta signal inducing gata5.

Xenopus nodal-related signaling is essential for mesendodermal patterning during early embryogenesis

Previously, we showed that Xenopus nodal-related factors (Xnrs) can act as mesoderm inducers, and that activin induces Xnr transcription, suggesting that Xnrs relay or maintain induction processes initiated by activin-like molecules. We used a dominant negative cleavage mutant Xnr2 (cmXnr2) to carry out loss-of-function experiments to explore the requirement for Xnr signaling in early amphibian embryogenesis, and the relationship between activin and Xnrs. cmXnr2 blocked mesoderm induction caused by Xnr, but not activin, RNA. In contrast, cmXnr2 did suppress mesoderm and endoderm induction by activin protein, while Xnr transcript induction was unaffected by cmXnr2, consistent with an interference with the function of Xnr peptides that were induced by activin protein treatment. The severe hyperdorsalization and gastrulation defects caused by Xnr2 in whole embryos were rescued by cmXnr2, establishing a specific antagonistic relationship between the normal and cleavage mutant proteins. Expression of cmXnr2 resulted in delayed dorsal lip formation and a range of anterior truncations that were associated with delayed and suppressed expression of markers for dorsoanterior endoderm, in which the recently recognized head organizer activity resides. Reciprocally, Xnr2 induced dorsoanterior endodermal markers, such as cerberus, Xhex-1 and Frzb, in animal cap ectoderm. The migratory behavior of head mesendoderm explanted from cmXnr2 RNA-injected embryos was drastically reduced. These results indicate that Xnrs play crucial roles in initiating gastrulation, probably by acting downstream of an activin-like signaling pathway that leads to dorsal mesendodermal specification, including setting up the head organizer.

Promoter activity of the zebrafish bhikhari retroelement requires an intact activin signaling pathway

We have investigated mesoderm induction in zebrafish employing the zebrafish LTR-retroelement bhikhari (bik). bik elements are transcribed in all early mesendodermal cells. This expression pattern is generated by a promoter located in the U3 region of the LTR. We show that bik is activated through the activin/Vg1 signaling pathway in an immediate early fashion. This activation critically depends on a sequence motif that occurs among others also in the Xenopus Mix2 activin response element (ARE). It has been shown that the Mix2 ARE binds FAST- 1, which complexes with Smad proteins to form a multi-protein complex. We confirm that also the bik ARE can be bound by FAST-1 in vitro. In animal cap experiments we demonstrate that this binding site is required for activin-induced transcriptional activation mediated by FAST and Smad-type proteins.

The BMP-related protein radar: a maintenance factor for dorsal neuroectoderm cells?

We have previously cloned several members of the TGF-beta superfamily of growth factors in zebrafish, one of which, Radar, belongs to the Dpp-Vg1-related (DVR) subgroup, with highest homology to GDF6. The pattern of expression of Radar suggested a possible involvement in several induction steps during embryogenesis including in the dorsal neural tube, red blood cells, the dorsal fin and the retina. We have analyzed the pattern of expression of Radar in comparison with that of a marker of dorsal neural tube structures, msxC and show that Radar and msxC are expressed in similar and/or adjacent tissues throughout embryogenesis. In order to demonstrate a functional relationship between these two proteins, we have generated a full-length cDNA for Radar and shown that Radar overexpression by DNA injection maintains expression of msxC in tissues where it is normally expressed then turned off, in particular in the dorsal neurectoderm. Study of the phenotype of a mutant carrying a deletion of Radar shows a loss of identity and death of the cells of the dorsal neural tube. Taken together these results suggest that Radar could be involved in maintaining the identity of cells of the dorsal-most neural tube and of at least a subset of neural crest cells.

Identification of two amino acids in activin A that are important for biological activity and binding to the activin type II receptors

Activins are members of the transforming growth factor-beta family of growth and differentiation factors. In this paper, we report the results of a structure-function analysis of activin A. The primary targets for directed mutagenesis were charged, individual amino acids located in accessible domains of the protein, concentrating on those that differ from transforming growth factor-beta2, the x-ray crystal structure of which is known. Based on the activities of the recombinant activin mutants in two bioassays, 4 out of 39 mutant proteins (D27K, K102A, K102E, and K102R) produced in a vaccinia virus system were selected for further investigation. After production in insect cells and purification of these four mutants to homogeneity, they were studied in bioassays and in cross-linking experiments involving transfected receptor combinations. Mutant D27K has a 2-fold higher specific bio-activity and binding affinity to an ActRIIA/ALK-4 activin receptor complex than wild type activin, whereas mutant K102E had no detectable biological activity and did not bind to any of the activin receptors. Mutant K102R and wild type activin bound to all the activin receptor combinations tested and were equipotent in bioassays. Our results with the Lys-102 mutants indicate that the positive charge of amino acid 102 is important for biological activity and type II receptor binding of activins.

Antivin, a novel and divergent member of the TGFbeta superfamily, negatively regulates mesoderm induction

Mesoderm induction and patterning are mediated by members of the TGFbeta superfamily. We have isolated a novel zebrafish member, antivin, that structurally is highly related to mouse lefty. Overexpression of antivin completely abolishes mesoderm induction at blastula stage, yet resultant embryos develop well-patterned epidermal and neural derivatives. The mesoderm-inhibiting activity of antivin can be mimicked by lefty and is suppressed by increasing levels of the mesodermal inducer Activin or its receptors. On the basis of its expression and activity, we propose that Antivin normally functions as a competitive inhibitor of Activin to limit mesoderm induction in the early embryo.