ANTERIOR STRUCTURAL DEFECTS BY MISEXPRESSION OF XGBX-2 IN EARLY XENOPUS EMBRYOS ARE ASSOCIATED WITH ALTERED EXPRESSION OF CELL-ADHESION MOLECULES

The RNA of the noncluster homeobox gene, Xgbx-2, is localized during n eurulation to a narrow band of tissue at the midbrain hindbrain bounda ry (anterior hindbrain), The localized expression of Xgbx-2 within the nervous system prompted us to assess its function during early develo pment 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 structure s. These defects include abnormal eye development including reduced an d missing eyes, reduced or missing cement glands, and abnormal brain d evelopment. Additionally. coinjection with Lineage label (either beta- galactosidase or green fluorescent protein) shows there is a dose-depe ndent misplacement of cells. These misplaced cells can be found in suc h locations as the blastocoele, gastrocoele, or ventricles in the brai n, In some spawnings, misplaced cells are expelled from the embryo int o the periviteline space. In general, the phenotype of Xgbx-2 RNA-inje cted embryos is strikingly similar to the phenotypes observed when dom inant-negative RNA constructs of Ca2+-dependent cell-adhesion molecule s are injected into similar regions of early embryos. Xgbx-2 misexpres sion enhanced the dissociation of animal hemisphere cells, and inhibit ed Ca2+-dependent cell adhesion in dissociated animal hemisphere cells in vitro. Additionally, when the expression of various calcium-depend ent cadherins was tested, it was shown that misexpression of Xgbx-2 pr events N-cadherin expression during early neurulation. These observati ons suggest that the transcription factor, Xgbx-2, functions normally in the regionalization of the neural tube (specifically the anterior h indbrain) by regulating differential cell adhesion and subsequently ce ll identity. (C) 1998 Wiley-Liss,Inc.