Fibrolast growth factors and their receptors in mouse embryonic lung morphogenesis

The function of fibroblast growth factors (FGF) and their receptors (FGFR) in the development of the respiratory system appears to be conserved throughout many species. The Drosophila melanogaster FGF homologue branchless functions in the branching morphogenesis of the tracheal system (Sutherland, Samakovlis et al. 1996), as does its receptor, breathless (Glazer and Shilo 1991). Mammalian lung development begins with the ventral budding of the lung primordium from the foregut. The lung bud divides into branches which further divide dichotomously and invade the surrounding mesenchyme. Saccule division results in the formation of the bronchial tree, while epithelial differentiation and formation of the air-blood barrier occur subsequently. The air spaces that are formed are expanded into prealveolar saccules, which are further subdivided into alveoli by secondary septation, a process that occurs largely after birth and increases the surface area of the lung manifold. These septae are remodelled to yield the mature alveoli of the lung (Burri 1997). Gene disruption experiments have been insufficient to resolve the role of FGFRI and FGFR2 during lung morphogenesis, since animals deficient in both of these die well before the onset of lung development (Deng et al., 1994, G. Martin unpublished data). Dominant negative transgenic approaches can be used to overcome early embryonic lethality and functional redundancies in FGF and FGFR families. I plan to use the inducible tetracycline system in combination with cell type specific expression pattern. The SP-C or the CC 10 promoter will direct the expression of a tetracycline inducible transcription factor to the epithelial cells of the developing lung. Intercrosses of the SP-C/rtTA and the CC1o/rTA transgenic mice with transgenic mice carrying either a membrane bound dnFGFR1c, or a soluble dnFGFR2b under the control of the tet operator will result in double transgenic mice that express the dominant negative receptor in the developing lung only upon tetracycline induction. Therefore timed expression of these mutant receptors would be possible and studies on the effect of dominant-negative receptor expression during different stages of lung development, especially at late time-points during development, will be practicable. Using this inducible system of rtTAetop mice to interfere with FGF function during lung morphogenesis, alveolarization, and repair after oxygen injury will help to dissect signalling cascades for proliferation and for differentiation that are critical during these processes.