Abstract
The secreted signaling molecule fibroblast growth factor 8 (Fgf8) is an essential component of certain embryonic signaling centers including the midโhindbrain (isthmic) organizer, the first branchial arch (BA1), and the apical ectodermal ridge (AER). In these signaling centers Fgf8 transcripts are expressed in a dynamic and transient fashion, but the mechanism by which this highly specific expression pattern is established remains largely unknown. We used DNA sequence comparisons coupled to transgenic approaches to obtain insight into the structure and function of regulatory elements in the Fgf8 locus. First, a bacterial artificial chromosome (BAC) containing the mouse Fgf8 gene partially rescues the embryonic lethality of __Fgf8โ__deficient mice and controls Fgf8โspecific gene expression of a coinjected lacZ reporter transgene. Second, sequence comparison of vertebrate Fgf8 loci revealed evolutionarily highly conserved noncoding sequences that were unexpectedly located mainly 3โฒ of the Fgf8 coding region. Third, in transgenic mice some of these elements were sufficient to target expression to the AER, tail bud, and brain, including the isthmic organizer, indicating that they may represent Fgf8 cisโacting elements. Collectively, these data identify novel regulatory elements of the Fgf8 gene sufficient to drive expression to regions of known Fgf8 activity. genesis 44:1โ6, 2006. ยฉ 2006 WileyโLiss, Inc.