Oelgeschlager et al. (2000) isolated a full-length Tsg cDNA by using a human EST to probe a Xenopus gastrula library. The cDNA encodes a protein sharing 41% amino acid identity with Drosophila Tsg, 89% identity with the partial human Tsg sequence, and 94% identity with a mouse EST. The Tsg sequence contains a signal peptide, as expected for a secreted protein, and 2 conserved domains containing multiple cysteines at its amino and carboxy termini. Whole-mount in situ hybridization and RT-PCR showed that abundant Tsg maternal transcripts are distributed throughout the animal half of the embryo during cleavage stages. At the late gastrula stage, maternal transcripts decrease and zygotic transcripts appear, specifically in the ventral region of the embryo. Tsg has ventralizing activity and is a bone morphogenetic protein (BMP) binding protein. The N-terminal domain of Tsg is sufficient to interact with BMP4 (112262) but not with chordin (603475). Tsg competes for binding of BMP4 with the first cysteine-rich domain of chordin (CR1) but not with full-length chordin. Endogenous Tsg antagonizes CR1 activity.

Scott et al. (2001) obtained full-length sequences from human TSG.

Chang et al. (2001) independently identified vertebrate Tsg homologs.

Dorsal-ventral patterning in vertebrate and Drosophila embryos requires a conserved system of extracellular proteins to generate a positional information gradient. The components involved include BMPs, chordin (a BMP antagonist), and a secreted metalloproteinase (Xolloid/Tolloid; 112264) that cleaves chordin. Oelgeschlager et al. (2000) described Xenopus 'twisted gastrulation' (Tsg), another member of this signaling pathway. Tsg is expressed ventrally as part of the BMP4 group and encodes a secreted BMP-binding protein that is a BMP signaling agonist. The data suggested a molecular mechanism by which Tsg dislodges latent BMPs bound to chordin BMP-binding fragments generated by Xolloid cleavage, providing a permissive signal that allows high BMP signaling in the embryo. Drosophila Tsg also binds BMPs and is expressed dorsally, supporting the proposal that the dorsal-ventral axis was inverted in the course of animal evolution.

Scott et al. (2001) showed that Tsg is coexpressed with chordin and various BMPs in vertebrate development. In Xenopus, maternal Tsg RNA was detected in eggs by RT-PCR, while whole-mount in situ hybridization showed uniform Tsg expression across the entire animal hemisphere and marginal zone of the early gastrula. In mouse, Tsg is widely expressed throughout the 7.5-days-postcoitus gastrula and in extraembryonic tissues. Chordin, Tsg, and BMP2 (112261), -4, and -7 (112267) are highly expressed in the digital rays of 15.5- and 17.5-days-postcoitus embryo hindlimbs. Strong chordin expression in the interzone of the joint cavity is juxtaposed with strong Tsg expression at the joint articular surfaces and the interzone. Thus, Tsg is properly situated for potential interactions with chordin and BMPs during various stages of vertebrate embryogenesis. Scott et al. (2001) demonstrated that Tsg binds both the vertebrate Decapentaplegic ortholog BMP4 and chordin and that these interactions have multiple effects. Tsg increases chordin's binding of BMP4, potentiates chordin's ability to induce secondary axes in Xenopus embryos, and enhances chordin cleavage by vertebrate tolloid-related proteases at a site poorly used in Tsg's absence. The presence of Tsg enhances the secondary axis-inducing activity of 2 products of chordin cleavage. Scott et al. (2001) concluded that Tsg acts as a cofactor in chordin's antagonism of BMP signaling.

Ross et al. (2001) showed that Tsg enhances the antagonistic activity of chordin. In zebrafish, blocking of Tsg function with morpholino oligonucleotides causes ventralization similar to that produced by chordin mutants. Coinjection of subinhibitory levels of morpholines directed against both Tsg and chordin synergistically enhanced the penetrance of the ventralized phenotype. Ross et al. (2001) showed that Tsgs from different species are functionally equivalent, and concluded that Tsg is a conserved protein that functions with chordin to antagonize BMP signaling. Ross et al. (2001) concluded that Tsg has 3 molecular functions. First, it can synergistically inhibit Dpp/BMP action in both Drosophila and vertebrates by forming a tripartite complex between itself, chordin, and a BMP ligand. Second, Tsg seems to enhance the Tld/BMP1 (112264)-mediated cleavage rate of chordin and may change the preference of site utilization. Third, Tsg can promote the dissociation of chordin cysteine-rich-containing fragments from the ligand.

Chang et al. (2001) demonstrated that Tsg can block BMP function in Xenopus embryonic explants and inhibits several ventral markers in whole-frog embryos. Tsg binds directly to BMPs and forms a ternary complex with chordin and BMPs. Coexpression of Tsg with chordin leads to a more efficient inhibition of the BMP activity in ectodermal explants.

By analysis of a radiation hybrid panel, Scott et al. (2001) mapped the human TSG gene to chromosome 18p11. Backcross analysis demonstrated that the mouse Tsg gene maps to the syntenic distal portion of chromosome 17. Ross et al. (2001) mapped the human TSG gene to 18p11.3-p11.2 by radiation hybrid analysis.