This laboratory's long term interest is to understand the cellular and molecular mechanisms of mammalian development. Since many of the genes involved in oncogenesis are expressed normally during embryogenesis, identification of dominant acting oncogenies by a variety of methods may lead to the finding of genes important in mammalian development. One such candidate oncogene is the eighth member of the fibroblast growth factor (FGF) family, FGF-8, which has been found to be involved in mouse mammary tumor virus (MMTV)-associated mammary tumorigenesis. FGF-8 is expressed in tumors with MMTV insertions near the FGF-8 gene, implicating this gene in mammary oncogenesis. Normal expression of FGF-8 in adult mice is weak in gonadal tissue and absent in other tissues. However, FGF-8 expression is high in mid-gestation (E9.5-lO.5) murine embryos, but with rapidly decreasing expression after E11.5. Alternative splicing of the primary transcript results in three different mRNAs coding for three FGF-8 isoforms in vivo. The three FGF-8 isoforms share the same amino terminal signal peptide and are effIciently secreted by expressing cells. The mature amino termini (following cleavage of the signal peptide) of the three isoforms differ, but the carboxyl 2/3 of the isoforms are the same. The biological significance of the three isoforms of FGF-8 is currently unknown. Hence, the Specific Aims of this proposal seek to determine the quantitative and qualitative differences in the biology and expression of the FGF-8 isoforms. The first Specific Aim will examine the morphologic alteration of FGF- responsive cell lines transfected with cDNAs coding for FGF-8 isoforms, looking for differences in the biology (as reflected by differences in ability to morphologically alter cell lines) of the FGF-8 isoforms. Differences between the FGF-8 isoforms will likely be due to different affinities of the FGF-8 isoforms for FGF receptors (FGFRs). The second Specific Aim will seek to characterize the FGF-8 isoform/FGFR interactions in vitro, using recombinant FGF-8 isoforms in ligand- receptor binding assays and in in vitro mitogenicity assays in cell lines expressing a single FGFR. The third Specific Aim will seek to characterize the normal expression of the FGF-8 isoforms during murine embryogenesis. The spatial and temporal expression will be examined using in situ hybridization of whole mount embryos and sections. Differences in FGF-8 isoform utilization during embryogenesis will be examined using ribonuclease protection assays on RNA from whole embryos and embryo tissues with isoform-specific riboprobes. Thus, these studies will determine the spatial and temporal expression of FGF-8 isoforms during murine embryogenesis, and the biological differences of the isoforms, as initial steps to understanding the function of FGF-8 during embryogenesis.