Sef is synexpressed with FGFs during chick embryogenesis and its expression is differentially regulated by FGFs in the developing limb.

The signaling pathways leading to growth and patterning of various organs are tightly controlled during the development of any organism. These control mechanisms usually involve the utilization of feedback- and pathway-specific antagonists where the pathway induces the expression of its own antagonist. Sef is a feedback antagonist of fibroblast growth factor (FGF) signaling, which has been identified recently in zebrafish and mammals. Here, we report the isolation of chicken Sef (cSef) and demonstrate the conserved nature of the regulatory relationship with FGF signaling. In chick embryos, Sef is expressed in a pattern that coincides with many known sites of FGF signaling. In the developing limb, cSef is expressed in the mesoderm underlying the apical ectodermal ridge (AER) in the region known as the progress zone. cSef message first appeared after limb budding and AER formation. Expression was intense at stages of rapid limb outgrowth, and gradually decreased to almost undetectable levels when differentiation was clearly apparent. Gain- and loss-of-function experiments showed that FGFs differentially regulate the expression of cSef in various tissues. Thus, removal of the AER down-regulated cSef expression, and FGF2 but not FGF4 or FGF8 beads substituted for the AER in maintaining cSef expression. At sites where cSef is not normally expressed, FGF4 and FGF2, but not FGF8 beads, induced cSef expression. Our results demonstrate the complexity of cSef regulation by FGFs and point to FGF2 as a prime candidate in regulating cSef expression during normal limb development. The spatiotemporal pattern of cSef expression during limb development suggests a role for cSef in regulating limb outgrowth but not limb initiation.