The mechanisms underlying perceptual acuity in the olfactory system are not well understood. We have identified a multigene family that codes for hundreds of diverse odorant receptors that are expressed by olfactory sensory neurons in the nasal cavity. We have recently obtained evidence that these receptors are expressed in distinct spatial patterns that may provide for an initial organization of incoming sensory information in the olfactory epithelium prior to its transmission to the olfactory bulb. It appears that the olfactory epithelium is divided into a limited number of odorant receptor expression zones. These zones are bilaterally symmetrical in the two nasal cavities and are the same in different individuals. Each receptor gene may be expressed in only one zone. However, many different receptor genes are expressed in the same zone and each gene is expressed in neurons that are scattered throughout the zone. It appears that the developing neuron, in choosing which receptor gene to express, may be confined to a strictly defined zonal gene set, but may select a member of that set via a stochastic mechanism. The proposed studies will investigate how the highly specified zonal organization of odorant receptor gene expression is achieved and examine the functional significance of this organization. An in vitro neurogenesis system will be used to determine whether zonal patterning is intrinsic to the olfactory epithelium or is imposed by the olfactory bulb via retrograde signals. In order to gain insight into the molecular mechanisms underlying zonal patterning, we will use chromosomal in situ hybridization to examine the genomic organization of the zonal gene sets. To identify molecules which act as zone specifiers, we will: l) examine whether zone specific sequence motifs are present in the 5' flanking sequences of odorant receptor genes and, if so, use these motifs to identify DNA binding proteins that recognize these motifs, 2) perform PCR reactions with degenerate primers to identify members of known families of DNA binding proteins that are differentially expressed in the different zones, 3) perform a broad search for zone specific RNAs using the differential RNA display method. Finally, in a series of collaborative studies, we will ask whether the odorant receptor expression zones behave as functional units when the olfactory epithelium is exposed to odorants. Together, these studies should provide a great deal of information about the functional significance of the expression zones and the molecular mechanisms by which they are specified. Since the mechanisms that are used to achieve the functional organization of the olfactory system are likely to be shared by other parts of the nervous system, these studies are likely to contribute to a broad understanding of nervous system development and function.
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