Xwnt-11: a maternally expressed Xenopus wnt gene

Cloning and characterization of Wnt-4 and Wnt-11 cDNAs from chick embryo

We have isolated two members of the Wnt gene family, Wnt-4 and Wnt-11, from chick embryo cDNA library, and determined the entire coding sequences. The Wnt-4 and Wnt-11 genes encode secretory proteins composed of 351 and 354 amino acids, respectively, both having 24 Cys residues conserved among other Wnt family members. Alignment of the deduced amino acid sequences reveals that chicken Wnt-4 and Wnt-11 are most similar to Xenopus Wnt-4 and mouse Wnt-11; respectively. Northern blot analysis indicates the Wnt-4 expression at 1.5 kilobase and the Wnt-11 expression at 2.0 kilobase in the chick embryo.

Beta-catenin localization during Xenopus embryogenesis: accumulation at tissue and somite boundaries

beta-catenin is a cytoplasmic protein associated with cadherin adhesion molecules and has been implicated in axis formation in Xenopus (McCrea, P. D., Brieher, W. M. and Gumbiner, B. M. (1993) J. Cell Biol. 127, 477–484). We have studied its distribution in Xenopus embryos by immunofluorescence on frozen sections. Consistent with its function in cell-cell adhesion, beta-catenin is present in every cell. However, high levels are expressed in certain regions and different tissues of the embryo. No simple correlation appears to exist between the levels of beta-catenin with the expected strength of adhesion. High levels of beta-catenin were found in regions undergoing active morphogenetic movements, such as the marginal zone of blastulae and gastrulae. This suggests that high expression of beta-catenin could be involved in dynamic adhesion events. Surprisingly, beta-catenin also accumulates on plasma membranes that probably do not establish direct or strong contacts with other cells. In particular, high amounts of beta-catenin are found transiently at boundaries between tissue anlagen and at the intersomitic boundaries. This unexpected pattern of beta-catenin expression raises the possibility that this molecule participates in developmental processes, perhaps independently of its classical role in cell-cell adhesion.

Vertebrate embryonic induction: mesodermal and neural patterning

Within the fertilized egg lies the information necessary to generate a diversity of cell types in the precise pattern of tissues and organs that comprises the vertebrate body. Seminal embryological experiments established the importance of induction, or cell interactions, in the formation of embryonic tissues and provided a foundation for molecular studies. In recent years, secreted gene products capable of inducing or patterning embryonic tissues have been identified. Despite these advances, embryologists remain challenged by fundamental questions: What are the endogenous inducing molecules? How is the action of an inducer spatially and temporally restricted? How does a limited group of inducers give rise to diversity of tissues? In this review, the focus is on the induction and patterning of mesodermal and neural tissues in the frog Xenopus laevis, with an emphasis on families of secreted molecules that appear to underlie inductive events throughout vertebrate embryogenesis.

Vg1 and regional specification in vertebrates: a new role for an old molecule

The Vg1 protein was discovered some ten years ago in a screen for localized maternal RNA molecules involved in early embryonic patterning in the frog Xenopus laevis. The localization of this molecule to the vegetal pole suggested that Vg1 might function as a determinant of embryonic cell fate, and its DNA sequence revealed that it is related to factors involved in induction of the mesoderm. However, it is only in the past year that evidence hinting at the role of Vg1 in early development has emerged. It now seems that although the key component for specifying a vertebrate dorsal axis has been known to us for a decade, cryptic processing pathways have kept its role in this important process hidden from view.

Wnt genes and vertebrate development

A variety of experimental approaches have underscored the critical role played by secreted polypeptide factors, such as those encoded by members of the Wnt gene family, in many aspects of vertebrate embryogenesis. Recent papers have revealed restricted patterns of Wnt gene expression that delineate important subdivisions within the early forebrain and spinal cord, demonstrated that Wnt gene products can regulate mesoderm formation and gastrulation, and investigated how Wnt protein signaling may affect cell adhesion.

Pattern formation in the vertebrate neural plate

Recent advances have been made in the understanding of the cellular and molecular mechanisms involved in the formation and patterning of the neural plate of vertebrate embryos. Both planar and vertical signaling pathways appear to be involved in the neural induction and axial patterning of the neural plate. The neural plate, behaving as a developmental field, might be patterned by signals emanating from boundary regions: the organizer region and the midline and edges of the neural plate. Here, A. Ruiz i Altaba describes a possible model for anteroposterior patterning involving ;lanar signals for amphibian, avian and mammalian embryos, compares the axial patterning of the neural plate with the patterning of insect epithelia, and discussed possible roles of noggin, follistatin and hedgehog-related genes in neural induction and patterning.

Genetic control of development in Xenopus laevis

In this paper we address the question of how genes can control development by using Xenopus as a model system, since it combines the classical advantages of amphibian embryology with advanced molecular techniques. Several developmental regulator genes have been shown to encode for transcription factors which trigger the activation of downstream genes, thus resulting in a cascade of regulatory events. In the first two examples, we deal with regulatory events that underlie early body patterning in vertebrates, and with the role of homeobox transcription factors in deciphering positional information along the body axis. In the third example, we address the question of the role of post-transcriptional regulation in development by studying the possible regulatory role of a cytoplasmic zinc finger protein, presumably acting through RNA-protein interactions. The general idea is that understanding how genes can control development will hopefully lead to understanding the construction of a shape, and eventually of an organism.

Inducing factors in Xenopus early embryos

Recent results make it possible to postulate credible candidates for each of the known inducing signals that act to determine cell fate during Xenopus early development. Experiments on biological activity, expression patterns and inhibition of function suggest that Vg-1 and Wnt-11 may act as the primary mesoderm-inducing signals, FGF and activin may serve to relay their effects, and noggin may be a major component of the dorsalizing and neural-inducing signals from the organizer.

Midline cells and the organization of the vertebrate neuraxis

Vertebrate embryos exhibit a striking midline axis of symmetry that can be recognized in the overall body plan, the framework of skeletal structures and the organization of the nervous system. Cells located at the midline of the embryo during gastrulation have a crucial influence on the establishment of cell identity and pattern within the nervous system. The identification of transcription factors and secreted proteins that are expressed by these midline cell groups has begun to provide a molecular characterization of the organizing centers that establish early neural identity and pattern.