Role of glycogen synthase kinase 3 beta as a negative regulator of dorsoventral axis formation in Xenopus embryos

The axis-inducing activity, stability, and subcellular distribution of beta-catenin is regulated in Xenopus embryos by glycogen synthase kinase 3

The serine/threonine kinase Xgsk-3 and the intracellular protein beta-catenin are necessary for the establishment of the dorsal-ventral axis in Xenopus. Although genetic evidence from Drosophila indicates that Xgsk-3 is upstream of beta-catenin, direct interactions between these proteins have not been demonstrated. We demonstrate that phosphorylation of beta-catenin in vivo requires an in vitro amino-terminal Xgsk-3 phosphorylation site, which is conserved in the Drosophila protein armadillo. beta-catenin mutants lacking this site are more active in inducing an ectopic axis in Xenopus embryos and are more stable than wild-type beta-catenin in the presence of Xgsk-3 activity, supporting the hypothesis that Xgsk-3 is a negative regulator of beta-catenin that acts through the amino-terminal site. Inhibition of endogenous Xgsk-3 function with a dominant-negative mutant leads to an increase in the steady-state levels of ectopic beta-catenin, indicating that Xgsk-3 functions to destabilize beta-catenin and thus decrease the amount of beta-catenin available for signaling. The levels of endogenous beta-catenin in the nucleus increases in the presence of the dominant-negative Xgsk-3 mutant, suggesting that a role of Xgsk-3 is to regulate the steady-state levels of beta-catenin within specific subcellular compartments. These studies provide a basis for understanding the interaction between Xgsk-3 and beta-catenin in the establishment of the dorsal-ventral axis in early Xenopus embryos.

Activities of the Wnt-1 class of secreted signaling factors are antagonized by the Wnt-5A class and by a dominant negative cadherin in early Xenopus development

When overexpressed in Xenopus embryos, Xwnt-1, -3A, -8 and -8b define a functional class of Wnts (the Wnt-1 class) that promotes duplication of the embryonic axis, whereas Xwnt-5A, -4, and -11 define a distinct class (the Wnt-5A class) that alters morphogenetic movements (Du, S., S. Purcell, J. Christian, L. McGrew, and R. Moon. 1995. Mol. Cell. Biol. 15:2625-2634). Since come embryonic cells may be exposed to signals from both functional classes of Wnt during vertebrate development, this raises the question of how the signaling pathways of these classes of Wnts might interact. To address this issue, we coexpressed various Xwnts and components of the Wnt-1 class signaling pathway in developing Xenopus embryos. Members of the Xwnt-5A class antagonized the ability of ectopic Wnt-1 class to induce goosecoid expression and a secondary axis. Interestingly, the Wnt-5A class did not block goosecoid expression or axis induction in response to overexpression of cytoplasmic components of the Wnt-1 signaling pathway, beta-catenin or a kinase-dead gsk-3, or to the unrelated secreted factor, BVg1. The ability of the Wnt-5A class to block responses to the Wnt-1 class may involve decreases in cell adhesion, since ectopic expression of Xwnt-5A leads to decreased Ca2+-dependent cell adhesion and the activity of Xwnt-5A to block Wnt-1 class signals is mimicked by a dominant negative N-cadherin. These data underscore the importance of cell adhesion in modulating the responses of embryonic cells to signaling molecules and suggest that the Wnt-5A functional class of signaling factors can interact with the Wnt-1 class in an antagonistic manner.

A sticky problem: the Xenopus cement gland as a paradigm for anteroposterior patterning

The cement gland is a mucus-secreting organ found at the extreme anterior of frog embryos. It attaches the embryo to a solid support before swimming and feeding begin, and also serves a related sensory function that stops the embryo from moving once it is attached. Cement gland is an extremely useful anterior marker, whose study continues to yield fundamental information concerning vertebrate axial patterning. Cement gland arises from the outer layer of the embryonic ectoderm and, in Xenopus, forms a cone of columnar epithelium. It is the first ectodermal organ to differentiate, beginning to do so by late gastrula. A battery of genes expressed in the developing and mature cement gland serve as useful markers. Cement gland development can be influenced by both stimulatory and inhibitory cell interactions. Stimulatory signals arise from the anterior neural plate, head endoderm, and the dorsal mesoderm. Inhibitory signals are present in the posterior dorsal mesoderm and in ventral ectoderm and mesoderm. Further, signalling between the ectodermal layers may restrict cement gland differentiation to the outer ectodermal cells. Several secreted molecules are able to induce or repress cement gland formation: these include noggin, follistatin, hedgehog, chordin, retinoic acid, embryonic fibroblast growth factor (eFGF), Bone Morphogenetic Protein-4 (BMP-4), and Xwnt-8. Several of these factors alter expression of the homeodomain gene Xotx2, which may be a transcriptional activator of cement gland differentiation genes. The significance of the cell interactions and factors described in positioning cement gland at the front of the embryo is explored.