Axis formation and patterning in zebrafish

Cripto: a tumor growth factor and more

Cripto, a growth factor with an EGF-like domain, and the first member of the EGF-CFC family of genes to be sequenced and characterized, contributes to deregulated growth of cancer cells. A role for Cripto in tumor development has been described in the human and the mouse. Members of the EGF-CFC family are found only in vertebrates: CFC proteins in zebrafish, Xenopus, chick, mouse and human have been characterized and indicate some common general functions in development. Cripto expression was first found in human and mouse embryonal carcinoma cells and male teratocarcinomas, and was demonstrated to be over-expressed in breast, cervical, ovarian, gastric, lung, colon, and pancreatic carcinomas in contrast to normal tissues where Cripto expression was invariably low or absent. Cripto may play a role in mammary tumorigenesis, since in vitro, Cripto induces mammary cell proliferation, reduces apoptosis, increases cell migration, and inhibits milk protein expression. This prediction is strengthened by observations of Cripto expression in 80% of human and mouse mammary tumors. At least three important roles for Cripto in development have created considerable interest, and each activity may be distinct in its mechanism of receptor signaling. One role is in the patterning of the anterior-posterior axis of the early embryo, a second is a crucial role in the development of the heart, and a third is in potentiating branching morphogenesis and modulating differentiation in the developing mammary gland. Whether these properties are functions of different forms of Cripto, different Cripto receptors or the distinct domains within this 15-38 kDa glycoprotein are examined here, but much remains to be revealed about this evolutionarily conserved gene product. Since all Cripto receptors have not yet been determined with certainty, future possible uses as therapeutic targets remain to be developed. Cripto is released or shed from expressing cells and may serve as an accessible marker gene in the early to mid-progressive stages of breast and other cancers. Meanwhile some speculations on possible receptor complexes for Cripto signaling in mammary cells are offered here as a spur to further discoveries.

Stat3 Controls Cell Movements during Zebrafish Gastrulation

Vertebrate axis formation requires both the correct specification of cell fates and the coordination of gastrulation movements. We report that the zebrafish signal transducer and activator of transcription 3 (Stat3) is activated on the dorsal side by the maternal Wnt/beta-catenin pathway. Zebrafish embryos lacking Stat3 activity display abnormal cell movements during gastrulation, resulting in a mispositioned head and a shortened anterior-posterior axis, but show no defects in early cell fate specification. Time course analysis, cell tracing, and transplantation experiments revealed that Stat3 activity is required cell autonomously for the anterior migration of dorsal mesendodermal cells and non-cell autonomously for the convergence of neighboring paraxial cells. These results reveal a role for Stat3 in controlling cell movements during gastrulation.

zic Gene expression marks anteroposterior pattern in the presumptive neurectoderm of the zebrafish gastrula

Anteroposterior (A/P) patterning of the vertebrate neurectoderm begins early during development. In vitro explant assays have been used to show that an A/P subdivision of the zebrafish neurectoderm is in place by early gastrulation. However, no direct markers of this subdivision had been described. We report isolation of two members of the zic gene family, zic2 and zic3, which mark distinct regions in the developing nervous system. zic3 is expressed in early gastrula embryos in a posterior domain, directly demonstrating that A/P pattern is present in the future neurectoderm by this stage. Analysis of mutants in genes encoding bone morphogenetic proteins (BMPs) shows that dorsal restriction of zic3 expression requires intact BMP function, and that posterior restriction of zic3 expression is regulated independently of BMP signaling. These data show an early subdivision of the A/P axis in the presumptive neurectoderm.

Nodal signaling in early vertebrate embryos: themes and variations

The nodal family of TGFbeta-related ligands have emerged as critical regulators of early vertebrate embryogenesis. Recent studies in mice, fish, and frogs of nodals and their intracellular transducers allow a comparison of how this signaling pathway is used in the patterning of early embryos of these different vertebrates.

Single-cell internalization during zebrafish gastrulation

During gastrulation, germ layers are formed as prospective mesodermal and endodermal cells internalize and come to underlie the ectoderm [1-9]. Despite the pivotal role of gastrulation in animal development, the cellular interactions underlying this process are poorly understood. In zebrafish, mesoderm and endoderm formation requires the Nodal signals Cyclops and Squint and their cofactor One-eyed pinhead (Oep) [10-14]. We found that marginal cells in maternal-zygotic oep (MZoep) mutants do not internalize during gastrulation and acquire neural and tail fates at the expense of head and trunk mesendoderm. The lack of internalization in MZoep embryos and the cell-autonomous requirement for oep in Nodal signaling enabled us to test whether internalization can be achieved by individual cells or whether it depends on interactions within a group of cells. We found that individual MZoep mutant cells transplanted to the margin of wild-type blastula embryos initially internalize with their neighbors but are unable to contribute to the mesendoderm. In the reciprocal experiment, single wild-type cells transplanted to the margin of MZoep mutant embryos autonomously internalize and can express the mesendodermal markers axial/foxA2 and sox17. These results suggest that internalization and mesendoderm formation in zebrafish can be attained autonomously by single cells.