The Expression of Fibronectins and Integrins During Mesodermal Induction and Gastrulation in Xenopus
Fibronectins (FNs) and integrins are first expressed in Xenopus embryos during the mid to late blastula stages. FN is synthesized in both animal and vegetal halves of the embryo but becomes localized to the roof of the blastocoel during gastrulation. Integrins are expressed in all regions of the early embryo. Structural heterogeneity of FN isoforms during embryogenesis occurs by alternative splicing of a common FN transcript, whereas integrin diversity is generated by the expression of several distinct integrin αβ heterodimers. The timing of expression for these molecules suggests that they may play important roles in supporting and/or controlling morphogenetic events in the early embryo.
We have investigated the roles played by these proteins in supporting the gastrulation-like movements that occur in animal pole tissue in response to mesoderminducing factors. Xenopus animal pole ectoderm was isolated from stage 8 embryos and exposed to the XTC mesoderm inducing factor (XTC-MIF; a Xenopus homologue of activin A). Animal pole ectoderm treated with XTC-MIF, like stage 10 dorsal marginal zone, will adhere and spread on FN coated surfaces. Uninduced animal pole ectoderm adheres poorly and does not spread on FN. The ability to spread on FN in response to XTC-MIF is also retained by single cells derived from dissociated animal pole tissue. This defines one of the few mesoderm-specific responses to induction that has been demonstrated for single cells. FN-mediated cell spreading is inhibited in the presence of the synthetic peptide Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP), which corresponds to one of the active cell binding sites on the FN molecule. However, the gastrulation-like movements associated with elongation of XTC-MIF induced animal pole ectoderm are not inhibited by the GRGDSP peptide. These results indicate that convergent extension does not depend on cell adhesion to FN. Furthermore, scanning electron microscopy and cell marking techniques suggest that although cellular activity is enhanced following induction, no long range cell mixing occurs during elongation of induced explants. We are now investigating whether the changes in cell adhesion observed following induction with XTC-MIF are controlled by the expression of integrin receptors and ECM molecules such as FN.
KeywordsMesodermal Cell Xenopus Embryo Animal Pole Chick Embryo Fibroblast Convergent Extension
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