The olfactory system is able to recognize thousands of odorants thus maintaining food consumption in humans. The ability to identify odors by humans is dependent on odorant receptors within olfactory neurons that line the nasal epithelium. This epithelium is under continuous cell division by a basal stem cell population that produces immature olfactory neurons, which then become mature olfactory neurons. Odorant receptors (ORs) are clonally expressed in immature olfactory neurons. The mammalian genome consists of ~1500 OR genes;these genes belong to the seven-transmembrane receptor superfamily. However, each allele of each gene is treated as a separate entity leading to ~3000 OR alleles to be expressed in a clonal (singular) manner by olfactory neurons. The ability for this singular OR allele expression is a multi-step manner. A critical step is the production of a functional OR protein. Olfactory neurons are found in domains within the epithelium such that a given olfactory neuron chooses (in general) between ~100 OR genes (~200 alleles). The selection process occurs in two-steps: one of ~200 alleles is chosen for expression (1st choice) within an immature neuron, followed by OR protein production, which is tested for functionality. Failure to produce a seven-transmembrane protein allows for the singular choice mechanism to choose another OR allele in that same immature olfactory neuron (2nd choice). If the chosen seven-transmembrane OR is unable to couple with the signal transduction machinery, then that olfactory neuron will fail to mature and die. The critical step in the expression of an OR gene is the initial choice of one OR allele within an immature neuron (1st choice). To date, it is completely unknown how this "singular choice" process occurs at a molecular level. Analysis of very small OR promoter regions (~300bp) have lead to the identification of two transcription factors that may play a role: an O/E-1 type binding protein and a homeodomain type biding protein. The identification of more candidate DNA binding proteins and their interactions is necessary to understand the "singular choice" process. This project fully exploits the small regulatory regions used to produce OR singular choice in immature olfactory neurons.
In Aim 1, the most minimal OR control region will be defined in order to identify other candidate transcription factors. There are at least 10-15 different olfactory neuronal cell types each fated to express one of ~200 OR alleles.
In Aim 2, two minimal OR promoters normally expressed in different olfactory cell types will be used to identify additional transcription factors. This will allow the understanding of how specific patterns within the olfactory epithelium are obtained.
In Aim 3, a candidate homeodomain protein will be deleted within immature and/or mature neurons in order to determine its role on OR gene expression.
In Aim 4, I will make every attempt to promote this work into publications and greater funding.
Gene expression is necessary for cells to function properly in the human body. The sense of smell is critically dependent on olfactory neurons that translate the chemical world to the brain. Olfactory neurons govern this odor recognition process through careful expression of odorant receptors. Loss of olfactory function leads to poor feeding in geriatric populations as well as higher incidence of food poisoning. Studying odorant receptor gene expression will give insights into how the olfactory system is put together and maintained throughout adulthood.
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