Fibroblast growth factor 2 internal ribosome entry site (IRES) activity exvivo and in transgenic mice reveals a stringent tissue-specific regulation

Fibroblast growth factor 2 (FGF-2) is a powerful mitogen involved in prolif eration, differentiation, and survival of various cells including neurons. FGF-2 expression is translationally regulated; in particular, the FGF-2 mRN A contains an internal ribosome entry site (IRES) allowing cap-independent translation. Here, we have analyzed FGF-2 IRES tissue specificity ex vivo a nd in vivo by using a dual luciferase bicistronic vector. This IRES was act ive in most transiently transfected human and nonhuman cell types, with a h igher activity in p53 -/- osteosarcoma and neuroblastoma cell lines. Transg enic mice were generated using bicistronic transgenes with FGF-2 IRES or en cephalomyocarditis virus (EMCV) IRES. Measurements of luciferase activity r evealed high FGF-2 IRES activity in 11-d-old embryos (Ell) but not in the p lacenta; activity was high in the heart and brain of E16. FGF-2 IRES activi ty was low in most organs of the adult, but exceptionally high in the brain . Such spatiotemporal variations were not observed with the EMCV IRES. Thes e data, demonstrating the strong tissue specificity of a mammalian IRES in vivo, suggest a pivotal role of translational IRES-dependent activation of FGF-2 expression during embryogenesis and in adult brain. FGF-2 IRES could constitute, thus, a powerful tool for gene transfer in the central nervous system.