Atomic force microscopy imaging of actin cortical cytoskeleton of Xenopus laevis oocyte

In this study we report an atomic force microscopy (AFM) investigation of the actin cortical cytoskeleton of Xenopus laevis oocytes. Samples consisted of inside-out orientated plasma membrane patches of X. laevis oocytes with overhanging cytoplasmic material. They were spread on a freshly cleaved mica surface, subsequently treated with Triton X-100 detergent and chemically fixed. The presence of actin fibres in oocyte patches was proved by fluorescence microscopy imaging. Contact mode AFM imaging was performed in air in constant force conditions. Reproducible high-resolution AFM images of a filamentous structure were obtained. The filamentous structure was identified as an actin cortical cytoskeleton, investigating its disaggregation induced by cytochalasin D treatment. The thinnest fibres showed a height of 7 nm in accordance with the diameter of a single actin microfilament. The results suggest that AFM imaging can be used for the high-resolution study of the actin cortical cytoskeleton of the X. laevis oocyte and its modifications mediated by the action of drugs and toxins.

Antisense inhibition of Xbrachyury impairs mesoderm formation in Xenopus embryos

Expression of the Xbrachyury (Xbra) gene was inhibited by antisense RNA synthesized in situ from an expression vector read by RNA polymerase III, injected into the fertilized egg or the 2-cell stage embryo of Xenopus laevis. Antisense-treated embryos had markedly reduced levels of Xbra mRNA and protein, and showed deficiencies in mesodermal derivatives and axis formation. In particular, organization of the posterior axis was affected, but often the anterior axis was also reduced. Some embryos failed to form mesoderm altogether and remained amorphous. The antisense effect is dose-dependent and may be "rescued" by overexpression of Xbra. In Xbra-deficient embryos, expression of several mesodermal genes (Xvent, pintallavis, Xlim, Xwnt-8 and noggin) was reduced to varying degrees, whereas goosecoid levels remained normal. The modified expression levels were partly normalized when Xbra deficiency was rescued. The observation that antisense inhibition yields slightly different phenotypes from dominant-negative inhibition suggests the recommendation of using several surrogate genetic approaches to determine the functional role of a gene in Xenopus development.

Cytokeratin 8 is a suitable epidermal marker during zebrafish development

We have cloned and characterized the zebrafish (Danio rerio) homologous cytokeratin 8 (zf-K8) cDNA. This cytokeratin belongs to the gene family of intermediate filaments and it is a component of the cytoskeleton of epithelial cells. Gene expression analysis during embryonic development and at adult stages presented here revealed that zf-K8 mRNA is inherited maternally and that it is present in the oocyte, the zygote and in the cleavage stage embryo. After mid blastula transition this gene is expressed in all surface cells, notably in those of the enveloping layer (EVL) and of the periderm, as well as in a subpopulation of the deep cells (DEL) presumed to be intestinal progenitors. During later embryonic stage zf-K8 mRNA is strongly expressed in the developing pectoral fin. In adult zebrafish, the zf-K8 gene is not only expressed in simple epithelia such as the colorectal intestine, but also, in contrast to other vertebrates, it is present in stratified skin and differentiated fins. These observations suggest that the zf-K8 gene is an appropriate epidermal marker during zebrafish ontogenesis.