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.

Dissecting Wnt signalling pathways and Wnt-sensitive developmental processes through transient misexpression analyses in embryos of Xenopus laevis.

We review evidence that Xenopus Wnts (Xwnts) have activities consistent with their hypothesized roles as secreted signalling factors involved in multiple developmental processes. Transient misexpression of different Xwnts has distinct effects upon early development, and upon the formation of tissues in UV-irradiated embryos. Misexpression of Xwnts also has distinct effects on the in vitro differentiation of blastula cap explants. Cellular responses to Xwnt signals include changes in gap junctional permeability, altered responsiveness to growth factors, and possibly changes in cell adhesion. Current data suggest that a maternal Xwnt- or noggin-like activity is involved in the Nieuwkoop center activity during mesoderm induction, that Xwnt-8 participates in a pathway of differentiation as ventral mesoderm, and that Xwnt-5A is a potential modulator of morphogenetic movements.

The armadillo homologs beta-catenin and plakoglobin are differentially expressed during early development of Xenopus laevis.

Plakoglobin and beta-catenin are cytoplasmic proteins associated with the intracellular plaques of cell adhesive junctions. While plakoglobin is present in both adherens junctions and desmosomes, beta-catenin is associated with the cadherins that accumulate only in adherens junctions. Both beta-catenin and plakoglobin are homologs of armadillo, a Drosophila segment polarity gene that is considered to be in the wingless signaling pathway. We have characterized the expression and distribution of both plakoglobin and beta-catenin in Xenopus embryos. As shown by RNA blot analysis, beta-catenin and plakoglobin transcripts are present in fertilized eggs and in embryos through to tadpole stage. Whole-mount in situ hybridization indicates that both genes are expressed in the dorsal ectoderm and mesoderm of tailbud- and tadpole-stage embryos and that beta-catenin is expressed in the midbrain. Both plakoglobin and beta-catenin polypeptides are present during early Xenopus development; however, differences exist in the timing of maximal expression. Plakoglobin is present in the fertilized egg, increases in abundance by neurula stage, then declines at the tailbud and tadpole stages. beta-Catenin, recognized by an anti-arm antibody, is also present in the fertilized egg and in blastula-stage embryos. However, beta-catenin continues to be detected at the neurula, tailbud, and tadpole stages when levels of plakoglobin decline. The presence of multiple homologs of armadillo in Xenopus embryos and the differences in their patterns of expression suggest distinct roles for these proteins in processes affected by cell adhesion.

Ultrastructural correlates of meiotic maturation in mammalian oocytes.

Immature mammalian oocytes reside in ovarian follicles with junctionally coupled granulosa cells. When released from a currently undefined meiotic arresting influence, these oocytes resume meiosis to progress from late diplotene (germinal vesicle stage) through the first meiotic division to metaphase II. Oocytes remain at metaphase II until fertilization activates them to complete meiosis. This review summarizes ultrastructural events that occur during meiotic maturation in mammals. Developmental correlates that promise a clearer understanding of regulatory mechanisms operating to control maturation are emphasized. By use of TEM of thin sections, freeze-fracture analysis, and replicated oocyte cortical patches, we demonstrate stage-specific changes in the oocyte nucleus, reorganization of cytoplasmic organelles, correlations between oocyte maturational commitment and the junctional integrity of associated granulosa cells, and definition of the components comprising the oocyte cortical cytoplasm.

Down-regulation of membrana granulosa cell gap junctions is correlated with irreversible commitment to resume meiosis in golden Syrian hamster oocytes.

One of the currently popular hypotheses for the regulation of meiotic resumption in mammalian oocytes proposes that the preovulatory surge of luteinizing hormone causes down-regulation of follicular gap junctions, which in turn disrupts transfer of a meiotic arrester from the somatic cells into the oocyte. The present study has investigated this hypothesis by examining the integrity of membrana granulosa cell gap junctions during the period of irreversible commitment to maturation of golden Syrian hamster oocytes in vivo. Our results have revealed a significant progressive decrease in the fractional area of cell surface occupied by gap junction membrane with increasing percentage of oocytes irreversibly committed to mature (1.946% and 0.921% fractional gap junction area at 0% and 100% oocytes irreversibly committed to mature, respectively, P less than 0.05). This net loss of membrana granulosa cell gap junctions from the cell surface was accompanied by a significant decrease in density of gap junction particles, whether they were arranged in rectilinear or non-rectilinear packing patterns. Furthermore, the number of gap junction particles per unit area of surface membrane scanned also underwent a significant progressive decrease with increasing percentage of oocytes irreversibly committed to mature. These data with the hamster are consistent with the hypothesis that down-regulation of membrana granulosa cell gap junctions may be of central importance in the regulation of gonadotropic stimulation of meiotic resumption in mammalian oocytes.