Distinct regulatory elements govern Fgf4 gene expression in the mouse blastocyst, myotomes, and developing limb

Embryonic development requires a complex program of events which are direct ed by a number of signaling molecules whose expression must be rigorously r egulated. We previously showed that expression of Fgf4, which plays an impo rtant role in postimplantation development and growth and patterning of the limb, is regulated in EC cells by the synergistic interaction of Sox2 and Oct-3 with the Fgf4 EC cell-specific enhancer. To verify whether this mecha nism was also operating in vivo, and to identify new elements controlling F gf4 gene expression in distinct developmental stages, we have analyzed the expression of LacZ reporter plasmids containing different fragments of the Fgf4 gene in transgenic mouse embryos. Utilizing these transgenic construct s we have been able to recapitulate, for the most part, Fgf4 gene expressio n during embryonic development. We show here that most of the cis-acting re gulatory elements determining Fgf4 embryonic expression are located in cons erved regions within the 3' UTR of the gene, The EC cell-specific enhancer is required to drive gene expression in the ICM of the blastocyst, and its activity requires the Sox and Oct-proteins binding sites. We were also able to identify specific and distinct enhancer elements that govern postimplan tation expression in the semitic myotomes and the limb bud AER. The myotome -specific elements contain binding sites for bHLH myogenic regulatory facto rs, which appear to be essential for myotome expression. Finally, we presen t evidence that the very restricted pattern of expression of Fgf4 transcrip ts in the AER results from the combined action of positive and negative reg ulatory elements located 3' of the Fgf4 coding sequences. Thus the Fgf4 gen e relies on multiple and distinct regulatory elements to achieve stage- and tissue-specific embryonic expression. (C) 1998 Academic Press.