Odorant receptors: regulation, signaling, and expression
Olfactory signal transduction is initiated by binding of odorous ligands to G protein-coupled odorant receptors, resulting in the activation of G protein and second messengers that culminates in the generation of action potentials to relay this signal to the brain for odor recognition. This signaling cascade comprises the primary olfactory response. Odorant receptors comprise the largest multigene family identified to date, enabling the olfactory system to respond to an enormous pool of potential odorant molecules. This work focuses on the study of odorant receptors beyond the initiation of the primary olfactory response. The first set of experiments focuses on odorant receptor desensitization and internalization. Key components of the internalization pathway involve phosphorylation of the odorant receptor by GRKs, binding of arrestin to the receptor-ligand complex, and targeting of this moiety to clathrin-coated pits. Dynamin initiates the formation of an endocytic vesicle from the clathrin-coated pit. Data are presented that identify and localize dynamin and clathrin to the dendritic extension of olfactory receptors where olfactory signal transduction occurs. In addition, a functional internalization event was identified specifically upon odorant stimulation in isolated catfish ORNs. The second set of experiments focuses on periphal signaling pathways mediated by odorant receptors. Activation of the ERK cascade in neurons has been shown to participate in neuronal plasticity and gene expression. Data presented here extend this observation to olfactory sensory neurons of the channel catfish. A mixture of L-methionine and L-lysine, potent olfactory stimuli for catfish, activates the Raf-MEK-ERK-RSK pathway in vivo, and the activation of this pathway is modulated by CamKII and PKC. The last set of experiments presents data for the identification of a potential chemosensory receptor in channel catfish. RT-PCR and RACE performed on total RNA isolated from catfish olfactory epithelium produced a cDNA that is very similar to metabotropic glutamate receptors and putative pheromone receptors from multiple species. This clone was localized to neurons located in the sensory region of the olfactory epithelium by in situ hybridization. Northern analysis indicated that this protein is expressed largely in the olfactory epithelium.
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