Anterior structural defects by misexpression of Xgbx-2 in early Xenopus embryos are associated with altered...

The RNA of the noncluster homeobox gene, Xgbx-2, is localized during neurulation to a narrow band of tissue at the midbrain hindbrain boundary (anterior hindbrain). The localized expression of Xgbx-2 within the nervous system prompted us to assess its function during early development by injection of synthetic Xgbx-2 RNA into the animal pole region of both dorsal blastomeres at the four-cell stage. Injection of Xgbx-2 RNA leads to dose-dependent alterations in anterior dorsal structures. These defects include abnormal eye development including reduced and missing eyes, reduced or missing cement glands, and abnormal brain development. Additionally, coinjection with lineage label (either beta-galactosidase or green fluorescent protein) shows there is a dose-dependent misplacement of cells. These misplaced cells can be found in such locations as the blastocoele, gastrocoele, or ventricles in the brain. In some spawnings, misplaced cells are expelled from the embryo into the periviteline space. In general, the phenotype of Xgbx-2 RNA-injected embryos is strikingly similar to the phenotypes observed when dominant-negative RNA constructs of Ca2+-dependent cell-adhesion molecules are injected into similar regions of early embryos. Xgbx-2 misexpression enhanced the dissociation of animal hemisphere cells, and inhibited Ca2+-dependent cell adhesion in dissociated animal hemisphere cells in vitro. Additionally, when the expression of various calcium-dependent cadherins was tested, it was shown that misexpression of Xgbx-2 prevents N-cadherin expression during early neurulation. These observations suggest that the transcription factor, Xgbx-2, functions normally in the regionalization of the neural tube (specifically the anterior hindbrain) by regulating differential cell adhesion and subsequently cell identity.