Molecular mechanism of morphogenetic movements in amphibian embryos

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Gastrulation is the most dynamic morphogenetic movement and initiates the body plan in animal development. In order to analyze the molecular mechanism of gastrulation, we have isolated a novel kuppel-like transcription factor, champignon (cpg), that encodes POZ/Zinc finger protein by expression screening in Xenopus laevis. Overexpression of cpg resulted in interference with gastrulation. Comparison of the constituent proteins from control embryos and cpg-injected embryos at stage 11.5 revealed that a 61 kDa protein spot was shifted basically in cpg-injected embryos suggesting that the 61 kDa protein was dephosphorylated by cpg-injection. Partial amino acid sequence (23 mer) of the 61 kDa protein accords 100 % with that of putative Xenopus STI1 protein which is a homolog of stress-induced phosphoprptein (STI1), Hsp70/Hsp90-organizing protein (Hop), or transformation-sensitive protein. The STI1/Hop combines Hsp70 with Hsp90 and modulates the function of Hsp90 chaperone which is known to bind to cytoskeletons. Thus the inhibitory role of cpg on gastrulation is mediated by dephosphorylation of XSTI1. These results suggest that the Hsp90 chaperone is involved in the control of morphogenetic movements.We have also cloned a novel gene, Xoom, which encodes a transmembrane protein. Injection of Xoom antisense RNA into Xenopus embryos reduced the Xoom protein and caused gastrulation defects without any influence on the involution and expression of mesodermal marker genes. 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. Cytological examination indicated disorgatrization 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 organizaiion.