Distinct enhancer elements control Hex expression during gastrulation and early organogenesis

In the mouse, embryological and genetic studies have indicated that two spa tially distinct signalling centres, the anterior visceral endoderm and the node and its derivatives, are required for the correct patterning of the an terior neural ectoderm. The divergent homeobox gene Hex is expressed in the anterior visceral endoderm, in the node (transiently), and in the anterior definitive endoderm. Other sites of Hex expression include the liver and t hyroid primordia and the endothelial cell precursors. We have used transgen ic analysis to map the cis-acting regulatory elements controlling Hex expre ssion during early mouse development. A 4.2-kb upstream region is important for Hex expression in the endothelial cell precursors, liver, and thyroid, and a 633-bp intronic fragment is both necessary and sufficient for Hex ex pression in the anterior visceral endoderm and the anterior definitive endo derm. These same regions drive expression in homologous structures in Xenop us laevis, indicating conservation of these regulatory regions in vertebrat es. Analysis of the anterior visceral endoderm/anterior definitive endoderm enhancer identifies a repressor region that is required to downregulate He x expression in the node once the anterior definitive endoderm has formed. This analysis also reveals that the initiation of Hex expression in the ant erior visceral endoderm and axial mesendoderm requires common elements, but maintenance of expression is regulated independently in these tissues. (C) 2001 Academic Press.