H. Steinbeisser

Xenopus chordin: A Novel Dorsalizing Factor Activated by Organizer-Specific Homeobox Genes

A Xenopus gene whose expression can be activated by the organizer-specific homeobox genes goosecoid and Xnot2 was isolated by differential screening. The chordin gene encodes a novel protein of 941 amino acids that has a signal sequence and four Cys-rich domains. The expression of chordin starts in Spemanns organizer subsequent to that of goosecoid, and its induction by activin requires de novo protein synthesis. Microinjection of chordin mRNA induces twinned axes and can completely rescue axial development in ventralized embryos. This molecule is a potent dorsalizing factor that is expressed at the right time and in the right place to regulate cell-cell interactions in the organizing centers of head, trunk, and tail development.

Molecular nature of Spemann's organizer: the role of the Xenopus homeobox gene goosecoid

This study analyzes the function of the homeobox gene goosecoid in Xenopus development. First, we find that goosecoid mRNA distribution closely mimics the expected localization of organizer tissue in normal embryos as well as in those treated with LiCl and UV light. Second, goosecoid mRNA accumulation is induced by activin, even in the absence of protein synthesis. It is not affected by bFGF and is repressed by retinoic acid. Lastly, microinjection of goosecoid mRNA into the ventral side of Xenopus embryos, where goosecoid is normally absent, leads to the formation of an additional complete body axis, including head structures and abundant notochordal tissue. The results suggest that the goosecoid homeodomain protein plays a central role in executing Spemanns organizer phenomenon.

On the function of BMP-4 in patterning the marginal zone of the Xenopus embryo.

Bone morphogenetic protein 4 (BMP‐4) is expressed in the ventral marginal zone of the gastrulating embryo. At late gastrula stage this gene is expressed in the ventral‐most part of the slit blastopore and in tissues that derive from it. At tailbud stages BMP‐4 is expressed in the spinal cord roof plate, neural crest, eye and auditory vesicle. The interactions of BMP‐4 with dorsal genes such as goosecoid (gsc) and Xnot‐2 were studied in vivo. In embryos ventralized by UV irradiation and suramin treatment, BMP‐4 zygotic transcripts accumulate prematurely and the entire marginal zone expresses this gene. The patterning effect of BMP‐4 on ventro‐posterior development can be revealed by a sensitive assay involving the injection of BMP‐4 mRNA in the ventral marginal zone of embryos partially dorsalized with LiCl, which leads to the complete rescue of trunk and tail structures. The experiments presented here argue that BMP‐4 may act in vivo as a ventral signal for the proper patterning of the marginal zone, actively interacting with dorsal genes such as gsc and Xnot‐2. A model is proposed in which the timing of expression of various marginal zone‐specific genes plays a central role in patterning the mesoderm.

Gastrulation in the Mouse: The Role of the Homeobox Gene goosecoid

Mouse goosecoid is a homeobox gene expressed briefly during early gastrulation. Its mRNA accumulates as a patch on the side of the epiblast at the site where the primitive streak is first formed. goosecoid-expressing cells are then found at the anterior end of the developing primitive streak, and finally in the anteriormost mesoderm at the tip of the early mouse gastrula, a region that gives rise to the head process. Treatment of early mouse embryos with activin results in goosecoid mRNA accumulation in the entire epiblast, suggesting that a localized signal induces goosecoid expression during development. Transplantation experiments indicate that the tip of the murine early gastrula is the equivalent of the organizer of the amphibian gastrula.

The role of gsc and BMP-4 in dorsal-ventral patterning of the marginal zone in Xenopus: a loss-of-function study using antisense RNA.

The dorsal‐specific homeobox gene goosecoid (gsc) and the bone morphogenetic protein 4 gene (BMP‐4) are expressed in complementary regions of the Xenopus gastrula. Injection of gsc mRNA dorsalizes ventral mesodermal tissue and can induce axis formation in normal and UV‐ventralized embryos. On the other hand, BMP‐4 mRNA injection, which has a strong ventralizing effect on whole embryos, has been implicated in ventralization by UV, and can rescue tail structures in embryos dorsalized by LiCl. The above‐mentioned putative roles for BMP‐4 and gsc are based on gain‐of‐function experiments. In order to determine the in vivo role of these two genes in the patterning of the Xenopus mesoderm during gastrulation, partial loss‐of‐function experiments were performed using antisense RNA injections. Using marker genes that are expressed early in gastrulation, we show that antisense gsc RNA has a ventralizing effect on embryos, whereas antisense BMP‐4 RNA dorsalizes mesodermal tissue. These loss‐of‐function studies also show a requirement for gsc and BMP‐4 in the dorsalization induced by LiCl and in the ventralization generated by UV irradiation, respectively. Thus, both gain‐ and loss‐of‐function results for gsc and BMP‐4 support the view that these two genes are necessary components of the dorsal and ventral patterning pathways in Xenopus embryos.