EVIDENCE FROM NORMAL EXPRESSION AND TARGETED MISEXPRESSION THAT BONE MORPHOGENETIC PROTEIN-4(BMP-4) PLAYS A ROLE IN MOUSE EMBRYONIC LUNG MORPHOGENESIS

Epithelial-mesenchymal interactions are critical for the branching and differentiation of the lung, but the mechanisms involved are still un clear. To investigate this problem in mouse embryonic lung, we have st udied the temporal and spatial expression of genes implicated in the m orphogenesis of other organs. At 11.5 days p.c., hepatocyte nuclear fa ctor-3 beta (Hnf-3 beta) is expressed uniformly throughout the epithel ium, while Wnt-2 expression is confined to the distal mesenchyme. Soni c hedgehog (Shh) trancripts are found throughout the epithelium, with high levels in the distal tips of the terminal buds, while bone morpho genetic protein-4 (Bmp-4) transcripts are localized at high levels in the distal tips of the epithelium, with lower levels in the adjacent m esenchyme. Epithelial expression is also seen for Bmp-7, but transcrip ts are less dramatically upregulated at the distal tips. The Type I Bo ne morphogenetic protein receptor gene(Bmpr/Tfr-11/Brk-1) is expressed at low levels in the epithelium and in the distal mesenchyme. To inve stigate the role of Bmp-4 in lung development, we have misexpressed th e gene throughout the distal epithelium of transgenic lungs using a su rfactant protein C enhancer/promoter. From 15.5 days p.c., transgenic lungs are smaller than normal, with grossly distended terminal buds an d, at birth, contain large air-filled sacs which do not support normal lung function. Labeling with BrdU reveals an inhibition of epithelial proliferation in 15.5 days p.c. transgenic lungs. A small but signifi cant stimulation of proliferation of mesenchymal cells is also observe d, but this is accompanied by an increase in cell death. In situ hybri dization with riboprobes for the proximal airway marker, CC10, and the distal airway marker, SP-C, shows normal differentiation of bronchiol ar Clara cells but a reduction in the number of differentiated Type II cells in transgenic lungs. A model is proposed for the role of BMP4 a nd other signalling molecules in embryonic lung morphogenesis.