Xoom is required for epibolic movement of animal ectodermal cells in Xenopus laevis gastrulation

Gastrulation is the most dynamic cell movement and initiates the body plan in amphibian development. In contrast to numerous molecular studies on mesodermal induction, the driving force of gastrulation is as yet poorly understood. A novel transmembrane protein, Xoom, was previously reported, which is required for Xenopus gastrulation. In the present study, the role of Xoom during Xenopus gastrulation was further examined in detail. Overexpression and misexpression of Xoom induced overproduction of Xoom protein, but not a changed phenotype. However, Xoom antisense ribonucleic acid (RNA) injection reduced the Xoom protein and caused gastrulation defects without any influence on the involution and translation levels of mesodermal marker genes. Normal migrating activity of dorsal mesodermal cells was recognized in the antisense RNA-injected explant. Morphological examination using artificial exogastrulation showed that convergent extension of mesodermal cells occurred normally, but the ectodermal cell layer significantly shrank in the antisense RNA-injected embryo. Comparison of cell shape among various experimental conditions showed that inhibition of cell spreading occurs specifically in the outer ectodermal layer of the antisense RNA-injected embryo. Cytochemical examination indicated disorganization of F-actin in the ectodermal cells of the antisense RNA-injected embryo. These results suggest that Xoom plays an important role in the epibolic movement of ectodermal cells through some regulation of actin filament organization.