Octamer-dependent in vivo expression of the endothelial cell-specific TIE2gene

The TIE2 gene, also known as TEK encodes a tyrosine kinase receptor that is required for the normal development of the vascular system during embryoge nesis. TIE2 is specifically expressed in endothelial cells; however, the tr anscriptional mechanisms that regulate this highly restricted pattern of ex pression remain unknown. Here we demonstrate that a consensus octamer eleme nt located in the 5'-flanking region of TIE2 is required for normal express ion in embryonic endothelial cells. Transgenic embryos carrying a TIE2/LacZ construct spanning 2.1 kilobases of upstream regulatory sequences exhibit expression of the reporter transgene specifically in endothelial cells. Sit e-directed mutagenesis of a consensus octamer element located in this regio n results in the loss of enhancer activity and significantly impairs the en dothelial expression of the reporter transgene. Consistent with the in vivo data, in vitro DNA-protein binding studies show that the consensus octamer element displays an endothelial cell-specific pattern of binding, suggesti ng an interaction with a protein complex consisting of Oct1 and an endothel ial cell-restricted cofactor. These data identify a novel role for the octa mer element as an essential regulator of TIE2 expression, define the first known transcriptional pathway that mediates the expression of a development al endothelial cell gene, and provide insights into the transcriptional mec hanisms that regulate development of the vasculature during embryogenesis.