/pdf/controlling-tgf-b-signaling-1bmt23qt86.pdf

Controlling TGF-β signaling

Skeletal homeostasis is determined by systemic hormones and local factors. Bone morphogenetic proteins (BMP) are unique because they induce the differentiation of mesenchymal cells toward cells of the osteoblastic lineage and also enhance the differentiated function of the osteoblast. However, the activity of BMPs needs to be tempered by intracellular and extracellular antagonists. BMPs bind to specific receptors and signal by phosphorylating the cytoplasmic proteins mothers against decapentaplegic (Smad) 1 and 5, which form heterodimers with Smad 4, and after nuclear translocation regulate transcription. BMP antagonists can be categorized as pseudoreceptors that compete with signaling receptors, inhibitory Smads that block signaling, intracellular binding proteins that bind Smad 1 and 5, and factors that induce ubiquitination and proteolysis of signaling Smads. In addition, a large number of extracellular proteins that bind BMPs and prevent their binding to signaling receptors have emerged. They are the ...

Bone morphogenetic proteins, their antagonists, and the skeleton.

Connective-tissue growth factor (CTGF) is a secreted protein implicated in multiple cellular events including angiogenesis, skeletogenesis and wound healing. It is a member of the CCN family of secreted proteins, named after CTGF, cysteine-rich 61 (CYR61), and nephroblastoma overexpressed (NOV) proteins. The molecular mechanism by which CTGF or other CCN proteins regulate cell signalling is not known. CTGF contains a cysteine-rich domain (CR) similar to those found in chordin and other secreted proteins, which in some cases have been reported to function as bone morphogenetic protein (BMP) and TGF-beta binding domains. Here we show that CTGF directly binds BMP4 and TGF-beta 1 through its CR domain. CTGF can antagonize BMP4 activity by preventing its binding to BMP receptors and has the opposite effect, enhancement of receptor binding, on TGF-beta 1. These results show that CTGF inhibits BMP and activates TGF-beta signals by direct binding in the extracellular space.

Connective-tissue growth factor (CTGF) modulates cell signalling by BMP and TGF-beta.

The organizer is formed in an equatorial sector of the blastula stage amphibian embryo by cells that have responded to two maternal agents: a general mesoendoderm inducer (involving the TFG-beta signaling pathway) and a dorsal modifier (probably involving the Wnt signaling pathway). The meso-endoderm inducer is secreted by most vegetal cells, those containing maternal materials that had been localized in the vegetal hemisphere of the oocyte during oogenesis. As a consequence of the inducer's distribution and action, the competence domains of prospective ectoderm, mesoderm, and endoderm are established in an animal-to-vegetal order in the blastula. The dorsal modifier signal is secreted by a sector of cells of the animal and vegetal hemispheres on one side of the blastula. These cells contain maternal materials transported there in the first cell cycle from the vegetal pole of the egg along microtubules aligned by cortical rotation. The Nieuwkoop center is the region of blastula cells secreting both maternal signals, and hence specifying the organizer in an equatorial sector. Final steps of organizer formation at the late blastula or early gastrula stage may involve locally secreted zygotic signals as well. At the gastrula stage, the organizer secretes a variety of zygotic proteins that act as antagonists to various members of the BMP and Wnt families of ligands, which are secreted by cells of the competence domains surrounding the organizer. BMPs and Wnts favor ventral development, and cells near the organizer are protected from these agents by the organizer's inducers. The nearby cells are derepressed in their inherent capacity for dorsal development, which is apparent in the neural induction of the ectoderm, dorsalization of the mesoderm, and anteriorization of the endoderm. The organizer also engages in extensive specialized morphogenesis, which brings it within range of responsive cell groups. It also self-differentiates to a variety of axial tissues of the body.

Formation and function of spemann's organizer

Factors affecting egg quality are determined by the intrinsic properties of the egg itself and the environment in which the egg is fertilized and subsequently incubated. Egg quality in fish is very variable. Some of the factors affecting egg quality in fish are known, but many (probably most) are unknown. Components that do affect egg quality include the endocrine status of the female during the growth of the oocyte in the ovary, the diet of the broodfish, the complement of nutrients deposited into the oocyte, and the physiochemical conditions of the water in which the eggs are subsequently incubated. In captive broodfish, the husbandry practices to which fish are subjected are probably a major contributory factor affecting egg quality. Our knowledge of the genetic influences on egg quality is very limited indeed. We know that parental genes strongly influence both fecundity and egg quality, but almost nothing is known about gene expression and/or mRNA translation in fish oocytes/embryos. This is surprising because the products synthesized in ovoand the mechanisms controlling their expression are likely to play a central role in determining egg quality. The genetic mechanisms underpinning oocyte and embryo growth and development are a priority for research

Egg quality in fish: what makes a good egg?

In early development of Xenopus laevis, it is known that activities of polypeptide growth factors are negatively regulated by their binding proteins. In this study, follistatin, originally known as an activin-binding protein, was shown to inhibit all aspects of bone morphogenetic protein (BMP) activity in early Xenopus embryos. Furthermore, using a surface plasmon resonance biosensor, we demonstrated that follistatin can directly interact with multiple BMPs at significantly high affinities. Interestingly, follistatin was found to be noncompetitive with the BMP receptor for ligand binding and to form a trimeric complex with BMP and its receptor. The results suggest that follistatin acts as an organizer factor in early amphibian embryogenesis by inhibiting BMP activities by a different mechanism from that used by chordin and noggin.

https://www.pnas.org/content/pnas/95/16/9337.full.pdf

Direct binding of follistatin to a complex of bone-morphogenetic protein and its receptor inhibits ventral and epidermal cell fates in early Xenopus embryo

Activin A, a member of the transforming growth factor beta superfamily, has recently been found to have potent mesoderm-inducing activity on isolated early Xenopus animal-cap cells. We measured the activin activity of the Xenopus egg extract by using an erythroid-differentiating test with Friend leukemia cells. The results showed that an activin homologue is, indeed, contained in unfertilized eggs and blastulae of Xenopus laevis in a considerable amount. This activity was eluted at the same retention time as human activin A when fractionated by reversed-phase HPLC. Furthermore, the fraction containing erythroid-differentiating factor activity had mesoderm-inducing activity on Xenopus animal-cap cells. The mesoderm-inducing activity of this fraction was suppressed when coincubated with follistatin, an activin-binding protein. These results suggest that an endogenous activin may be a natural mesoderm-inducing factor acting in Xenopus embryogenesis.

https://www.pnas.org/content/pnas/88/15/6511.full.pdf

Presence of activin (erythroid differentiation factor) in unfertilized eggs and blastulae of Xenopus laevis.

Correlation between activin and retinoic acid (RA), both of which affect early amphibian development, was studied using Xenopus laevis embryos. In the first set of experiments, two isomers of RA, all-trans RA and 13-cis RA, were compared in terms of stability of biological activity against light. Xenopus blastulae were dipped in RA solutions which had either been kept away from light, or had been exposed to light for a few hours. At doses ranging from 10–4to 10–6M, RA elicited head deformity. All-trans RA, under both dark and light conditions, had similarly potent effects. On the other hand, 13-cis RA under dark conditions had much weaker effects than it did under light conditions.



In the second set of experiments, activin was mixed with all-trans RA, and the inducing effects on the animal cap explants were investigated. Activin at a concentration of 10 ng/ml induced notochord. In combination with 10–6M RA, muscle was well induced instead of notochord. In combination with 10–5M RA, pronephric tubules were markedly induced. Pronephric tubules were never induced by activin alone, at any of the various concentrations employed. This is the first report on the very high frequency of induction of pronephric tubules by the combination of activin A and all-trans RA in the Xenopus ectoderm.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1440-169X.1993.00123.x

Induction of Pronephric Tubules by Activin and Retinoic Acid in Presumptive Ectoderm of Xenopus laevis

We examined the quality of mesoderm induced by the action of activin A on the Xenopus presumptive ectoderm when various concentrations and treatment times were employed. The minimum concentration of activin A to induce mesodermal tissues was inversely proportional to its treatment time. The explants differentiated into different types of mesodermal tissues, from ventral-type to dorsal-type depending on the concentration of activin A and its treatment time. To confirm whether activin A has a role in establishing axial organization, activin A was injected into the blastocoel of late blastulae. About 70% of the injected embryos formed secondary tail-shaped outgrowths in which muscle and neural tube differentiated. The amount of activin A to form secondary outgrowths was 0.5-2.5 pg, roughly consistent with the amount estimated from in vitro experiments. As we have detected almost the same amount of activin homologue in the early embryos (Asashima et al., 1991a), we speculate that activin A may be the natural mesodermal inducer, and that it is responsible for establishing axial organization in the Xenopus embryo.

/pdf/dose-and-time-dependent-mesoderm-induction-and-outgrowth-1rdkpdp4d7.pdf

Hideho Uchiyama

Papers

Direct binding of follistatin to a complex of bone-morphogenetic protein and its receptor inhibits ventral and epidermal cell fates in early Xenopus embryo

Presence of activin (erythroid differentiation factor) in unfertilized eggs and blastulae of Xenopus laevis.

Induction of Pronephric Tubules by Activin and Retinoic Acid in Presumptive Ectoderm of Xenopus laevis

Dose and time-dependent mesoderm induction and outgrowth formation by activin A in Xenopus laevis

Isolation and characterization of native activin B.