A carboxyl-terminal truncated version of the activin receptor mediates activin signals in early Xenopus embryos

Controlling the Messenger: Regulated Translation of Maternal mRNAs in Xenopus laevis Development

The selective translation of maternal mRNAs encoding cell-fate determinants drives the earliest decisions of embryogenesis that establish the vertebrate body plan. This chapter will discuss studies in Xenopus laevis that provide insights into mechanisms underlying this translational control. Xenopus has been a powerful model organism for many discoveries relevant to the translational control of maternal mRNAs because of the large size of its oocytes and eggs that allow for microinjection of molecules and the relative ease of manipulating the oocyte to egg transition (maturation) and fertilization in culture. Consequently, many key studies have focused on the expression of maternal mRNAs during the oocyte to egg transition (the meiotic cell cycle) and the rapid cell divisions immediately following fertilization. This research has made seminal contributions to our understanding of translational regulatory mechanisms, but while some of the mRNAs under consideration at these stages encode cell-fate determinants, many encode cell cycle regulatory proteins that drive these early cell cycles. In contrast, while maternal mRNAs encoding key developmental (i.e., cell-fate) regulators that function after the first cleavage stages may exploit aspects of these foundational mechanisms, studies reveal that these mRNAs must also rely on distinct and, as of yet, incompletely understood mechanisms. These findings are logical because the functions of such developmental regulatory proteins have requirements distinct from cell cycle regulators, including becoming relevant only after fertilization and then only in specific cells of the embryo. Indeed, key maternal cell-fate determinants must be made available in exquisitely precise amounts (usually low), only at specific times and in specific cells during embryogenesis. To provide an appreciation for the regulation of maternal cell-fate determinant expression, an overview of the maternal phase of Xenopus embryogenesis will be presented. This section will be followed by a review of translational mechanisms operating in oocytes, eggs, and early cleavage-stage embryos and conclude with a discussion of how the regulation of key maternal cell-fate determinants at the level of translation functions in Xenopus embryogenesis. A key theme is that the molecular asymmetries critical for forming the body axes are established and further elaborated upon by the selective temporal and spatial regulation of maternal mRNA translation.

Polyribosome analysis for investigating mRNA translation in Xenopus oocytes, eggs and embryos

The earliest stages of animal development occur without the benefit of zygotic transcription. The absence of transcription necessitates that all changes in the levels of specific proteins must be controlled by post-transcriptional mechanisms, such as the regulated translation of stored maternal mRNAs. One of the major challenges to investigating translational mechanisms is the availability of reliable methods for assaying the translational state of specific mRNAs. The most definitive assay of an mRNA's translational state is polyribosome association; mRNAs actively translated are engaged with polyribosomes while mRNAs translationally repressed are not. While linear gradient centrifugation is commonly used to purify polyribosomes from a wide variety of cell types in different organisms, the isolation of polyribosomes from Xenopus oocytes, eggs and embryos presents some unique challenges. Here we detail the methodology for the isolation and analysis of polyribosomes from Xenopus oocytes, eggs and embryos using step gradient centrifugation. We present detailed protocols, describe the critical controls and provide several examples to guide the interpretation of experimental results regarding the translational state of specific mRNAs.

Schistosoma mansoni TGF-beta receptor II: role in host ligand-induced regulation of a schistosome target gene

Members of transforming growth factor-beta (TGF-β) superfamily play pivotal roles in development in multicellular organisms. We report the functional characterization of the Schistosoma mansoni type II receptor (SmTβRII). Mining of the S. mansoni expressed sequence tag (EST) database identified an EST clone that shows homology to the kinase domain of type II receptors from different species. The amplified EST sequence was used as a probe to isolate a cDNA clone spanning the entire coding region of a type II serine/threonine kinase receptor. The interaction of SmTβRII with SmTβRI was elucidated and shown to be dependent on TGF-β ligand binding. Furthermore, in the presence of human TGF-β1, SmTβRII was able to activate SmTβRI, which in turn activated SmSmad2 and promoted its interaction with SmSmad4, proving the transfer of the signal from the receptor complex to the Smad proteins. Gynaecophoral canal protein (GCP), whose expression in male worms is limited to the gynaecophoric canal, was identified as a potential TGF-β target gene in schistosomes. Knocking down the expression of SmTβRII using short interfering RNA molecules (siRNA) resulted in a concomitant reduction in the expression of GCP. These data provide evidence for the direct involvement of SmTβRII in mediating TGF-β–induced activation of the TGF-β target gene, SmGCP, within schistosome parasites. The results also provide additional evidence for a role for the TGF-β signaling pathway in male-induced female reproductive development.

Schistosomes are multicellular parasites that infect 200 million people worldwide. Schistosome development in the human host likely involves host molecules that regulate biological processes of the parasite. Members of transforming growth factor-beta (TGF-β) superfamily play pivotal roles in development in multicellular organisms. TGF-β signaling requires ligand binding to a specific surface receptor, TGF-β type II receptor. The authors isolated the schistosome TGF-β type II receptor (SmTβRII), which was found to be biologically active and responded to stimulation by host TGF-β. The gynaecophoric canal is a ventral groove in the male worm in which the female must reside for sexual maturity. Gynaecophoral canal protein (GCP) is a protein whose expression in male worms is limited to the gynaecophoric canal and is implicated in female reproductive maturation. GCP expression was found to be regulated by human TGF-β. Knocking down the expression of SmTβRII resulted in a concomitant reduction in the expression of GCP, providing evidence for the direct involvement of SmTβRII-mediated, host TGF-β–induced regulation of schistosome gene expression. This study implicates the TGF-β signaling pathway in worm pairing, a prerequisite for female egg production. Because the eggs produced by the worm pairs are responsible for pathogenesis, the authors' research identifies potential targets for intervention.

The orphan receptor ALK7 and the Activin receptor ALK4 mediate signaling by Nodal proteins during vertebrate development

Nodal proteins have crucial roles in mesendoderm formation and left-right patterning during vertebrate development. The molecular mechanisms of signal transduction by Nodal and related ligands, however, are not fully understood. In this paper, we present biochemical and functional evidence that the orphan type I serine/threonine kinase receptor ALK7 acts as a receptor for mouse Nodal and Xenopus Nodal-related 1 (Xnr1). Receptor reconstitution experiments indicate that ALK7 collaborates with ActRIIB to confer responsiveness to Xnr1 and Nodal. Both receptors can independently bind Xnr1. In addition, Cripto, an extracellular protein genetically implicated in Nodal signaling, can independently interact with both Xnr1 and ALK7, and its expression greatly enhances the ability of ALK7 and ActRIIB to respond to Nodal ligands. The Activin receptor ALK4 is also able to mediate Nodal signaling but only in the presence of Cripto, with which it can also interact directly. A constitutively activated form of ALK7 mimics the mesendoderm-inducing activity of Xnr1 in Xenopus embryos, whereas a dominant-negative ALK7 specifically blocks the activities of Nodal and Xnr1 but has little effect on other related ligands. In contrast, a dominant-negative ALK4 blocks all mesoderm-inducing ligands tested, including Nodal, Xnr1, Xnr2, Xnr4, and Activin. In agreement with a role in Nodal signaling, ALK7 mRNA is localized to the ectodermal and organizer regions of Xenopus gastrula embryos and is expressed during early stages of mouse embryonic development. Therefore, our results indicate that both ALK4 and ALK7 can mediate signal transduction by Nodal proteins, although ALK7 appears to be a receptor more specifically dedicated to Nodal signaling.

Dual specificity of activin type II receptor ActRIIb in dorso-ventral patterning during zebrafish embryogenesis

Members of the transforming growth factor-beta (TGF-beta) superfamily are thought to regulate specification of a variety of tissue types in early embryogenesis. These effects are mediated through a cell surface receptor complex, consisting of two classes of ser/thr kinase receptor, type I and type II. In the present study, cDNA encoding zebrafish activin type II receptors, ActRIIa and ActRIIb was cloned and characterized. Overexpression of ActRIIb in zebrafish embryos caused dorsalization of embryos, as observed in activin-overexpressing embryos. However, in blastula stage embryos, ActRIIb induced formation of both dorsal and ventro-lateral mesoderm. It has been suggested that these inducing signals from ActRIIb are mediated through each specific type I receptor, TARAM-A and BMPRIA, depending on activin and bone morphogenetic protein (BMP), respectively. In addition, it was shown that a kinase-deleted form of ActRIIb (dnActRIIb) suppressed both activin- and BMP-like signaling pathways. These results suggest that ActRIIb at least has dual roles in both activin and BMP signaling pathways during zebrafish embryogenesis.

Efficient construction of a large nonimmune phage antibody library: the production of high-affinity human single-chain antibodies to protein antigens

A large library of phage-displayed human single-chain Fv antibodies (scFv), containing 6.7 x 10(9) members, was generated by improving the steps of library construction. Fourteen different protein antigens were used to affinity select antibodies from this library. A panel of specific antibodies was isolated with each antigen, and each panel contained an average of 8.7 different scFv. Measurements of antibody-antigen interactions revealed several affinities below 1 nM, comparable to affinities observed during the secondary murine immune response. In particular, four different scFv recognizing the ErbB2 protein had affinities ranging from 220 pM to 4 nM. Antibodies derived from the library proved to be useful reagents for immunoassays. For example, antibodies generated to the Chlamydia trachomatis elementary bodies stained Chlamydia-infected cells, but not uninfected cells. These results demonstrate that phage antibody libraries are ideally suited for the rapid production of panels of high-affinity mAbs to a wide variety of protein antigens. Such libraries should prove especially useful for generating reagents to study the function of gene products identified by genome projects.

Differential effects on Xenopus development of interference with type IIA and type IIB activin receptors

One candidate for a mesoderm-inducing factor in early amphibian development is activin, a member of the TGF beta family. Overexpression of a truncated form of an activin receptor Type IIB abolishes activin responsiveness and mesoderm formation in vivo. The Xenopus Type IIA activin receptor XSTK9 differs from the Type IIB receptor by 43 and 25% in extracellular and intracellular domains respectively, suggesting the possibility of different functions in vivo. In this paper, we compare the Type IIA receptor with the Type IIB to test such a possibility. Simple overexpression of the wild-type receptors reveals minimal differences, but experiments with dominant negative mutants of each receptor show qualitatively distinct effects. We show that while truncated (kinase domain-deleted) Type IIB receptors cause axial defects as previously described, truncated type IIA receptors cause formation of secondary axes, similar to those seen by overexpression of truncated receptors for BMP-4, another TGF beta family member. Furthermore, in animal cap assays, truncated type IIB receptors inhibit induction of all mesodermal markers tested, while truncated type IIA receptors suppress induction only of ventral markers; the anterior/dorsal marker goosecoid is virtually unaffected. The suppression of ventral development by the type IIA truncated receptor suggests either that the truncated Type IIA receptor interferes with ventral BMP pathways, or that activin signaling through the Type IIA receptor is necessary for ventral patterning.

Inactive type II and type I receptors for TGF beta are dominant inhibitors of TGF beta-dependent transcription

Although transforming growth factor-beta (TGF beta) is implicated in differentiation and disease, proof of in vivo function requires specific inhibitors of the TGF beta cascade. TGF beta binds a family of type I and type II receptors (T beta RI, T beta RII), containing a cytoplasmic serine/threonine kinase domain. We previously reported that kinase-deficient T beta RII (delta kT beta RII) blocks TGF beta-dependent transcription in cardiac myocytes. It is controversial whether both receptors are needed in all cells for gene regulation by TGF beta or whether they mediate distinct subsets of TGF beta-dependent events. To resolve this uncertainty, TGF beta-dependent transcription was investigated in cardiac myocytes versus mink lung epithelial cells. 1) delta kT beta RII inhibits induction of a TGF beta-responsive reporter gene, in both cell backgrounds. 2) Charged-to-alanine mutations of key residues of the T beta RII kinase, including consensus ATP binding and amino acid recognition motifs, are competent for binding but not transcriptional activation. Each inactive receptor inhibits TGF beta-dependent transcription in both cell types. 3) Kinase-deficient T beta RI (delta kT beta RI) likewise impairs TGF beta-dependent transcription, less completely than delta kT beta RII; kinase-deficient activin type I receptor has no effect. 4) TGF beta-binding proteins in cardiac cells and Mv1Lu cells are comparable by affinity labeling and immunoprecipitation; however, Mv1Lu cells express up to 3-fold higher levels of T beta RII and T beta RI. Thus, the model inferred from TGF beta-resistant cell lines (that T beta RII and T beta RI are necessary in tandem for the TGF beta-signaling complex to regulate transcription) is valid for cardiac myocytes, the cell type most prominently affected in TGF beta-deficient animals.

GH3 pituitary tumor cells contain heteromeric type I and type II receptor complexes for transforming growth factor beta and activin-A

Transforming growth factors beta (TGF-beta s) and activins induce and inhibins block secretion of follicle-stimulating hormone by rat GH3 pituitary tumor cells. Cheifetz et al. (Cheifetz, S., Ling, N., Guillemin, R., and Massagué, J. (1988) J. Biol. Chem. 263, 17225-17228) reported that GH3 cells express a approximately 50-kDa surface protein, termed the type IV TGF-beta receptor, that directly binds all of these peptide hormones. Here we show that GH3 cells express the previously identified type I and type II receptors for TGF-beta and activin-A. Immunoprecipitation of affinity-labeled surface binding proteins with antisera specific to known receptors demonstrated independent heteromeric complexes of TGF-beta types I and II receptors and of activin types I and II receptors. As judged by ligand-binding and cross-linking analysis, TGF-beta binding to the TGF-beta receptors is not inhibited by activin-A and activin-A binding to its receptors is not inhibited by TGF-beta. Screening of a cDNA library from GH3 cells for potential receptor serine-threonine kinases yielded the known types I and II TGF-beta and activin receptors. The presumed common intracellular signaling pathway for TGF-beta and activin in GH3 cells appears to be mediated by distinct cell-surface receptors.

mRNAs for activin receptors II and IIB are expressed in mouse oocytes and in the epiblast of pregastrula and gastrula stage mouse embryos

Activin is a potent inducer of mesoderm in frog embryos. We showed previously that in the mouse, activin beta A is expressed in the uterine decidua near the embryo before and during the first appearance of mesoderm (E4.5-E6.5). Here, using Northern blotting and in situ hybridization, we show that mouse oocytes, E6.5 and E7.5 embryos, and E6.5 and E7.5 decidua contain mRNAs for both activin receptors type II and IIB. The expression of activin receptor type IIB is particularly strong in embryonic ectoderm apparent at E5.5 and continuing through E8.5. These results support the hypothesis that activin derived from the decidua promotes development of mesoderm in the period E5.5-E6.5.

Genomic structure and cloned cDNAs predict that four variants in the kinase domain of serine/threonine kinase receptors arise by alternative splicing and poly(A) addition

Heterodimers of types I and II serine/threonine kinase receptor monomers compose the active receptor complex for ligands of the transforming growth factor beta family. Here we show that the genomic organization of coding sequences for the intracellular domain of a widely expressed type I serine/threonine kinase receptor is similar to that of the activin type II receptor gene. The genomic structure and cDNA clones indicate that poly(A) addition to alternative exons at each of three carboxyl-terminal coding exon-intron junctions may be a common feature of both type I and II receptor genes. The predicted products are monomers truncated at kinase subdomains VII, IX, and X which vary in kinase activity and potential serine, threonine, and tyrosine phosphorylation sites. These results suggest that combinations of variants that affect the signal-transducing intracellular kinase domain of both type I and II receptor monomers within the transforming growth factor beta ligand family may add to the heterogeneity of biological effects of individual ligands in the family.

Autoinduction of activin genes in early Xenopus embryos

Activin exhibits a potent mesoderm inducing activity towards the ectodermal tissue (animal cap) of Xenopus laevis blastulae. Thus in order to investigate the role of activin in morphogenesis of early Xenopus embryos, activation of genes for activin beta A and beta B was examined by the reverse transcription polymerase chain reaction. In vivo, activin beta B mRNA appears to be present in embryonic stage 1 whereas beta A mRNA is undetectable prior to gastrulation. beta B and beta A mRNAs were noted to accumulate after stages 9 and 15 respectively. Activin gene expression in Xenopus animal caps was examined after treatment with various concentrations of activin A. Under these treatment conditions, both activin beta A and beta B mRNAs accumulated in a dose-dependent fashion after 24 h. The same effect was noted for treatment with similar concentrations of activin B. Accumulation of mRNAs was inhibited by the addition of cycloheximide to the culture medium, consistent with the proposition that activin gene expression requires certain protein factors. In total, therefore, these data suggest that an autoinduction mechanism is involved in the regulation of activin mRNA levels in normal Xenopus embryos and that this mechanism may play a pivotal role during early embryonic development.

The TGF-beta superfamily: new members, new receptors, and new genetic tests of function in different organisms

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