Mammalian olfactory-receptor genes

Niimura Y, Nei M (2007) Extensive gains and losses of olfactory receptor genes in Mammalian evolution. PLoS ONE 2 e708... 

The olfactory receptor genes can be considered the first layer of olfactory information processing and in fact they define the nature of odorants, since any molecule becomes an odorant solely by virtue of its interaction with an olfactory receptor. However, not many olfactory receptor genes are currently deorphanized, due to the sheer complexity of the task, and because heterologous expression is inefficient for many olfactory receptors. Consequently, detailed ligand response spectra so far only exist for a handful of mammalian and a single fish olfactory receptor (Luu et al. 2004). 

Trace amine-associated receptors (TAARs) are related to G protein-coupled aminergic neurotransmitter receptors such as dopamine and serotonine receptors and recognize derivatives of the classical monoamines such as B-phenylethylamine, octopamine, tryptamine, and tyramine. Initially, TAARs had been considered neurotransmitter receptors (Borowsky et al. 2001), but recently an expression in olfactory sensory neurons was shown for several mammalian taar genes, with expression characteristics very similar to odorant receptors (Liberies and Buck 2006). Thus the taar genes were recognized as a fourth GPCR family of olfactory receptors (Buck 2000). 

Limited data are available concerning the tissue specificity of olfactory receptor gene expression in teleosts. For mammalian OR an extra-olfactory expression of quite a few receptor genes is known, and for two receptors a function in sperm cell chemotaxis has been postulated (Fukuda et al. 2004 Spehr et al. 2003). 

The recent discovery of yet another olfactory receptor gene family (the TAARs) invites the speculation that the current repertoire of four different teleost olfactory receptor gene families may still not be complete. Indeed, an olfactory function has been shown for a mammalian member of the membrane-bound guanylate cyclase family (Hu et al. 2007 Leinders-Zufall et al. 2007). The corresponding gene family... 

Mori K, Takahashi YK, Igarashi KM, Yamaguchi M (2006) Maps of odorant molecular features in the mammalian olfactory bulb. Physiol Rev 86 409 133 Morita Y, Finger TE (1998) Differential projections of ciliated and microvillous olfactory receptor cells in the catfish, Ictalurus punctatus. J Comp Neurol 398 539-550 Nasevisius A, Ekker SC (2000) Effective targeted gene knockdown in zebrafish. Nat Genet 26 216-220... 

Parmentier, M., Libert, F., Schurmans, S., Schiffmann, S., Lefort, A., Eggerickx, D., Ledent, C., Mollereau, C., Gerard, J., Perret, A., Grootegoed, G., and Vassart, G. (1992). Expression of members of the putative olfactory receptor gene family in mammalian germ cells. Nature 555 453-455. 

Vanderhaeghen, P., Schurmans, S., Vassart, G. and Parmentier, M. (1997). Specific repertoire of olfactory receptor genes in the male germ cells of several mammalian species. Genomics 39, 239—246. 

Some sensory neurons of the VNO express two gene superfamilies, termed Vlr and V2r, that encode over 240 proteins of the seven-transmembrane type (Matsunami and Buck, 1997). These G-protein-linked putative pheromone receptors are distantly related to the main olfactory system s receptors. Receptors of the VNO are linked to different G-proteins, and their extracellular N-terminal domains are longer than those of the receptors in the main olfactory system. (Vi receptors are linked to Gi-proteins and V2 receptors to Go-proteins). The intracellular excitation mechanism in VNO sensory neurons also differs from that in the main olfactory systems instead of linking to adenylyl cyclase, the VNO receptors activate the phosphoinositol second messenger system. This has been demonstrated in several mammalian species. In hamsters, aphrodisin increases inositol 1,4,5-trisphosphate (IP3) levels in VNO membranes. Boar seminal fluid and urine stimulate increases of IP3 in the VNO of the female pig. (However, in the pig, the VNO is not necessarily essential for responses to pheromones [Dorries etal., 1997]). 

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