Inner ear development has been described as one of the most remarkable displays of precision microengineering in the vertebrate body"""""""" (Swanson et al, 1990). The long term goal of this research is to examine the function of genes that shape very early events in inner ear development in order to understand the mechanisms that produce many severe forms of congenital deafness. The experiments in this proposal examine the functional role of two genes expressed early in the development of the auditory system: bone morphogenetic protein 4 (BMP4) and noggin. The first and second specific aims test the hypothesis that the morphogen BMP4 expressed in localized cell foci in the otic epithelium and the BMP4 antagonist, noggin, expressed in the surrounding periotic mesenchyme, are responsible for modeling the semicircular canals (SCC) and sensory tissue (hair cells) within them. The chick is used as the model system in these studies because the levels of these important molecules can be manipulated in the chick system and the effects of altering the environment on auditory system development can he assessed. These experiments cannot he done in the mouse or zebra fish because BMP """"""""knockout"""""""" mice and zebra fish mutations die before the inner ear forms, and therefore, any effects of altering the levels of expression of these genes can not be assessed. These experiments depend on misexpression of BMP4 and noggin, by implanting agarose beads bearing cells that express noggin or BMP4 into ectopic locations in the early developing inner ear. Noggin bead implants eliminate SCC selectively; BMP4 implants """"""""rescue"""""""" the lost canals. The third specific aim tests the hypothesis that BMP/I expression is critical for sensory cell (hair cell and supporting cell) development in the inner ear. This laboratory has produced BMP4 expressing immortalized otocyst cells from the 9 day Immortomouse. These cells were used to """"""""rescue"""""""" the loss of SCC in the preliminary experiments on which these studies are based. The cells in culture not only express hair cell markers in the order expected for hair cells in the embryo, but also serve as a model system to study the role of the relevant genes in the production of hair cell/supporting cells in the inner ear. Furthermore, a BMP-4-expressing IMO cell line also produces the statoacoustic ganglion (SAG) neurite outgrowth/survival factor that directs the emerging neurites of the SAG back to BMFP-expressing sensory cell targets in the developing otocyst. IMO cell lines can therefore be used in molecular studies to identify the molecular cascades that produce sensory cell lineages as well as the SAG neurite outgrowth factor.
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