Expression of chicken fibroblast growth factor homologous factor (FHF)-1 and of differentially spliced isoforms of FHF-2 during development and involvement of FHF-2 in chicken limb development

Members of the fibroblast growth factor (FGF) family have been identified a s signaling molecules in a variety of developmental processes, including im portant roles in limb bud initiation, growth and patterning, This paper rep orts the cloning and characterization of the chicken orthologues of fibrobl ast growth factor homologous factors-1 and -2 (cFHF-1/cFGF-12 and cFHF-2/cF GF-13, respectively), We also describe the identification of a novel, conse rved isoform of FHF-2 in chickens and mammals. This isoform arises by alter native splicing of the first exon of the FHF-2 gene and is predicted to enc ode a polypeptide with a distinct amino-terminus, Whole-mount in situ hybridization reveals restricted domains of expression of cFHF-1 and cFHF-2 in the developing neural tube, peripheral sensory gang lia and limb buds, and shows that the two cFHF-2 transcript isoforms are pr esent in non-overlapping spatial distributions in the neural tube and adjac ent structures. In the developing limbs, cFHF-1 is confined to the posterio r mesoderm in an area that encompasses the zone of polarizing activity and cFHF-2 is confined to the distal anterior mesoderm in a region that largely overlaps the progress zone. Ectopic cFHF-2 expression is induced adjacent to grafts of cells expressing Sonic Hedgehog and the zone of cFHF-2 express ion is expanded in talpid(2) embryos. In the absence of the apical ectoderm al ridge or in wingless or limbless mutant embryos, expression of cFHF-1 an d cFHF-2 is lost from the limb bud. A role for cFHF-2 in the patterning and growth of skeletal elements is implied by the observation that engraftment of developing limb buds with QT6 cells expressing a cFHF-2 isoform that is normally expressed in the limb leads to a variety of morphological defects . Finally, we show that a secreted version of cFHF-2 activates the expressi on of HoxD13, HoxD11, Fgf-4 and BMP-2 ectopically, consistent with cFHF-2 p laying a role in anterior-posterior patterning of the limb.