Dynamic Development
CONTENTS

Main Page Dynamic Development

The Foundations of Developmental Biology

Gametogenesis

From Sperm and Egg to Embryo

Genetic Regulation of Development

Organizing the Multicellular Embryo

Generating Cell Diversity


Dynamic Development at a Glance


Learning Resources

Research Resources

The Developmental Biology Journal Club

Developmental Biology Tutorial

Initiating the Embryonic Body Plan: Dorsalization of the Xenopus Embryo

What is the role of the wingless pathway in dorsalization?

The evidence is strong that dorsal determinants exist in the Xenopus embryo. The identification of dorsal determinants is an important goal for understanding how the body plan is established. Valuable clues to the identities of the factors that dorsalize the Xenopus embryo came from experiments demonstrating that injections ofWnt mRNA will cause duplication of the Xenopus embryonic axis (McMahon and Moon, 1989; Smith and Harland, 1991; Sokol et al., 1991; see Fig.1). The Wnt proteins are a family of growth factors that are homologues of the wingless segment polarity gene of Drosophila., which maintains the expression of the engrailed gene, which encodes a transcription factor. The wingless signaling pathway has been described in Drosophila. Therefore, attempts have been made to examine the possible roles of the Wnts and members of the Wnt signal transduction pathway in dorsal-ventral axis specification in Xenopus.

The wingless pathway has been resolved by genetic dissection and is described in terms of the genes encoding the proteins in the pathway:

wingless -> frizzled ->dishevelled -| zw3/shaggy -| armadillo -> cell fate

frizzled encodes the wg receptor, whereas the remaining factors are intracellular. armadillo , which interprets the wingless signal, encodes a protein that associates with the cytoplasmic domains of members of the cadherin family of cell adhesion proteins. Interestingly, there is considerable recent evidence that the cadherins may initiate signal transduction cascades in addition to their roles in adhesion.

Vertebrate homologues of the protein encoded byarmadillo include -catenin and plakoglobin. The protein that is immediately upstream of armadillo (zw3/shaggy) is a serine/threonine kinase. Wingless signaling causes a decrease in the phosphorylation of the armadillo protein and an increase in its stablility. Thus, zw3/shaggy is thought to regulate armadillo by phosphorylation.

Xenopus eggs contain transcripts encoding components of the Wnt pathway.We have previously discussed the accumulation and localization of Xwnt-11 transcripts during oogenesis. Other Wnt homologs in the egg include Xwnt-5A and Xwnt-8b. Transcripts of a dishevelled homolog, a zw3/shaggy homolog (Xgsk-3), an armadillo homolog and -catenin. have also been demonstrated (for review, see Yost, Torres et al., 1996). Do they play any role in axis specification, and - if so - what might it be?

We have already discussed the effects of overexpression of Wnt; it causes axis duplication. Overexpression of other components of the Wg signal transduction cascade does the same. An example is shown below, which shows neural axis duplication after injection of plakoglobin RNA (from Mike Klymowsky; copyright , 1996, Mike Klymkowsky; Karnovsky and Klymkowsky, 1995).

Strong evidence that the Wnt pathway is required for axis determination was provided by Heasman et al. (1994), who demonstrated that depletion of oogenic -catenin transcripts using antisense oligo technology caused reductions in dorsal structures, an effect that could be rescued by injection of -catenin mRNA (Figs. 3-5). Depletion of -catenin could not be rescued by ectopic Wnt signals, a result that is consistent with -catenin being downstream of Wnt.

How are the effects of -catenin/plakoglobin produced? Heasman et al. demonstrated that one defect of -catenin depletion is the failure to produce dorsal mesoderm (the organizer). As we shall discuss later, the organizer is induced by the underlying Nieuwkoop center. This suggests that -catenin is required for the signaling events that are necessary for dorsal mesoderm induction by the Nieuwkoop center.

Randall Moon has prepared an overview of the role of -catenin in Xenopus development. Study his overview and then return here.

As discussed by Dr. Moon, the target of -catenin/plakoglobin may be a transcription factor called XTcf-3 (Molenaar et al., 1996). XTcf-3 is a homolog of a group of vertebrate high mobility group (HMG) box transcription factor genes. Its mammalian counterparts are referred to as "architectural transcription factors" that affect spatial structure of enhancers of target genes, thus facilitating contacts between other factors that are bound to the enhancer. Transcripts encoding XTcf-3 are present in Xenopus unfertilized eggs and early embryos. These transcripts are most abundant in the animal hemisphere and in the marginal zone of the embryo (Molenaar et al., 1996, Fig. 1). In vitro assays demonstrated that XTcf-3 is capable of binding to -catenin. XTcf-3 and -catenin also interact in vivo, which causes the translocation of -catenin into the nucleus (Fig. 4). An N-terminal deletion mutant of XTcf-3 (delta N) that is incapable of interacting with -catenin failed to translocate -catenin into the nucleus. This mutant acts as a dominant-negative mutant that presumably perturbs the normal interaction between -catenin and endogenous XTcf-3 (Molenaar et al., 1996).

If delta N perturbs -catenin function, one would predict that it would prevent the ability of overexpressed -catenin RNA to induce axis duplication. As shown in Table 1 and Figure 6, this prediction has been upheld. Would it also perturb the function of endogenous -catenin? (Remember that depletion of endogenous -catenin will suppress axis formation.) Indeed, axis formation is perturbed when RNA is injected into the two dorsal blastomeres at the 4-cell stage, and the effect is most effective when the injections of the RNA encoding the dominant-negative XTcf-3 mutant were done in the marginal zone (Table 2 and Fig. 7).


Learning Objectives

  • What is the evidence that Wnt signaling may be involved in establishment of the embryonic axis of Xenopus?
  • What are the components of the Wnt signaling pathway, and which are stimulatory and which are inhibitory?
  • What are the effects of depletion of -catenin on the mesoderm, and what consequences does this have on development?
  • Is endogenous -catenin expressed in a manner consistent with a role in axis specification?
  • What establishes asymmetry in -catenin?
  • What lies downstream of -catenin?
  • Where in the cell does -catenin function?
  • What is XTcf-3, and what is the evidence that it interacts with -catenin?

Digging Deeper:

Recent Literature

Hedgepeth, C.M., Conrad, L.J., Zhang, J., Huang, H.-C., Lee, V.M.Y. and Klein, P.S. 1997. Activation of the Wnt signaling pathway: a molecular mechanism for lithium action. Develop. Biol. 185: 82-91.

Kageura, H. 1997. Activation of dorsal development by contact between the cortical dorsal determinant and the equatorial core cytoplasm in eggs of Xenopus laevis. Development 124: 1543-1551.

Kofron, M., Spagnuolo, A., Klymkowsky, M., Wylie, C. and Heasman, J. 1997. The roles of maternal alpha-catenin and plakoglobin in the early Xenopus embryo. Development 124: 1553-1560.

Merriam, J.M., Rubenstein, A.B. and Klymkowsky, M.W. 1997. Cytoplasmically anchored plakoglobin induces a WNT-like phenotype in Xenopus. Develop. Biol. 185: 67-81.

Miller, J.R., Rowning, B.A., Larabell, C.A., Yang-Snyder, J.A., Bates, R.L., and Moon, R.T. 1999. Establishment of the dorsal-ventral axis in Xenopus embryos coincedes with the dorsal enrichment of dishevelled that is dependent on cortical rotation. J. Cell. Biol. 146:427-437.

Wodarz, A. and Nusse, R. 1998. Mechanisms of Wnt signaling in development. Annu. Rev. Cell Dev. Biol. 14:59-88.

Links to Related Material

Evidence that dorsal determinants exist in the Xenopus embryo.

The accumulation and localization of Xwnt-11 transcripts during oogenesis.

See Mike Klymowsky's Web Site.

See the recent cautionary note in BioEssays by Mike Klymkowsky.

See The World Wide Web Wnt Window (WWWWW)

See "Transduction of the Wingless Signal to the Nucleus" in Zygote.

See "Activation of Amphibian Dorsal Development by Interactions between Inner and Cortical Cytoplasms" in Zygote.


References

Browder, L.W., C. A. Erickson and W.R. Jeffery. 1991. Developmental Biology. Third Edition. Saunders College Publishing. Philadelphia.

Heasman, J. et al. 1994. Overexpression of cadherins and underexpression of -catenin inhibit dorsal mesoderm induction in early Xenopus embryos. Cell 79: 791-803.

Karnovsky, A. and Klymkowsky, M.W. 1995. Anterior axis duplication in Xenopus induced by the over-expression of the cadherin-binding proteins plakoglobin. Proc. Natl. Acad. Sci. USA 92:

McMahon, A.P. and Moon, R.T. 1989. Ectopic expression of the proto-oncogene int-1 in Xenopus embryos leads to duplication of the embryonic axis. Cell 58: 1075-1084.

Molenaar, M. et al. 1996. XTcf-3 transcription factor mediates -catenin-induced axis formation in Xenopus embryo. Cell 86: 391-399.

Smith, W.C. and Harland, R.M. 1991. Injected Xwnt-8 RNA acts early in Xenopus embryos to promote formation of a vegetal dorsalizing center. Cell 67: 753-765.

Sokol, S., Christian, J.L., Moon, R.T. and D.A. Melton. 1991. Injected wnt RNA induces a complete body axis in Xenopus embryos. Cell 67: 741-752.

Yost, C., Torres, M., Miller, J.R., Huang, E., Kimelman, D and Moon, R.T. 1996. The axis-inducing activity, stability, and subcellular distribution of -catenin is regulated in Xenopus embryos by glycogen synthese kinase 3. Genes & Dev. 10: 1443-1454.


Dynamic Development at a Glance
Main Page Dynamic Development

Dynamic Development is a Virtual Embryo learning resource

This material may be reproduced for educational purposes only provided credit is given to the original source.
Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Friday, November 6, 1998