Bmp4-directed nuclear cyan fluorescent protein provides a tool for live imaging and reveals cellular resolution of Bmp4 expression patterns during embryogenesis

The genesis of hair follicles in mammals involves multiple inductive and suppressive signaling interactions between epithelial cells and the underlying mesenchymal tissue. Bmp4 (Bone Morphogenetic Protein 4) is expressed in the mesenchymal tissue surrounding developing hair follicles. The BMP signaling pathway is suggested to play a suppressive role in hair follicle induction. In addition, it is also found to be important for specification of different cell types in mature hair follicles. Knowledge of the precise expression pattern of Bmp4 during hair follicle differentiation should provide insights into how these suppressive and differentiative roles regulate hair follicle development. However, in situ hybridization studies do not provide sufficient cellular resolution, and three-dimensional reconstructions of serial sections are tedious. We have targeted a nuclear-localized cyan fluorescent protein (CFP) reporter into the endogenous Bmp4 locus. Nuclear CFP expression was detected in embryonic tissues in a Bmp4-specific pattern, including lateral mesoderm, limb bud, dorsal optic cup, lung bud epithelium, heart outflow tract, otocyst, branchial arches, nasal pits, mammary buds, vibrassa and hair follicles. In developing hair follicles, the nuclear CFP reporter provided precise cellular resolution of Bmp4 expression patterns. These mice provide a novel tool for visualizing Bmp4 expression patterns in live and fixed tissues with cellular resolution. In addition, these studies reveal a novel view of the arrangement and dynamic changes of Bmp4-expressing cells of different stage hair follicles.