Cloning and localization of two multigene receptor families in goldfish olfactory epithelium

Isolation and characterisation of main olfactory and vomeronasal receptor gene families from the Atlantic salmon (Salmo salar)

The Atlantic salmon (Salmo salar) has been widely used as a model species in studies of olfactory signal transduction and processing. Here we report the isolation and characterisation of salmon olfactory receptor (SOR) and salmon vomeronasal receptor (SVR) partial sequences from Atlantic salmon. Six groups of SOR sequences (SORA-F) and three groups of SVR sequences (SVRA-C) were identified. All SORB, SORF, SVRB and SVRC sequences contained uninterrupted open reading frames. However, all SORA sequences and members of the SVRA sequence family contained multiple stop codons while SORC and SORE sequences were truncated in the 3' region of the sequence. Full length SORF and almost complete SORB sequences displayed amino acid residues and motifs conserved in fish olfactory receptor genes. In sequence phylogenies, SOR sequences fell into the main olfactory receptor (MOR) type I clade and were most closely related to either delta or zeta reference sequences, while all SVR sequences grouped within a clade of fish type 2 vomeronasal receptor (V2R) sequences. A family of sequences (Sasa CaSR1-6), isolated using the same degenerate primers that amplified SVR sequences, clustered within a group of calcium sensing receptor (CaSR) sequences. Analysis of tissue expression patterns of sequences by reverse transcriptase polymerase chain reaction showed that they were transcribed in olfactory epithelium (SORB, SORF, all SVR and Sasa CaSR sequences), testis (SORB, SORD and Sasa CaSR) and/or anterior kidney (SORB and Sasa CaSR). Similar analysis of expression supported the identification of SORA sequences as non-transcribed pseudogene(s). Although the level of occurrence of OR pseudogenes is within the range found for other, well-characterised vertebrate OR genomes, it does not seem to reflect the importance of olfaction in the biology of the Atlantic salmon.

Evolution of vomeronasal-type odorant receptor genes in the zebrafish genome

Teleost fishes have two distinct families of odorant receptors, main odorant receptors (MORs) and vomeronasal receptors family 2 (V2Rs), both of which are expressed in the olfactory epithelium where they detect soluble chemicals. To obtain an insight into the evolution of the fish V2R gene family, V2R genes and their chromosomal locations were identified from zebrafish genome sequences. Eighty-eight putative V2R genes and pseudogenes were identified, most being tightly clustered into two chromosomal regions. The phylogenetic analysis revealed that V2R genes belonging to the same phylogenetic clade were located close to one another on the same chromosome. However, some V2R genes were also found in several different chromosome regions, either as singletons or small gene clusters. Recent large-scale duplications of V2R-containing chromosomal regions were detected in two V2R gene clusters. The evolution of the zebrafish V2R gene family can be explained by repeated tandem gene duplications within gene clusters and large-scale duplications among chromosomes.

Evolutionary dynamics of olfactory receptor genes in fishes and tetrapods

Olfaction, which is an important physiological function for the survival of mammals, is controlled by a large multigene family of olfactory receptor (OR) genes. Fishes also have this gene family, but the number of genes is known to be substantially smaller than in mammals. To understand the evolutionary dynamics of OR genes, we conducted a phylogenetic analysis of all functional genes identified from the genome sequences of zebrafish, pufferfish, frogs, chickens, humans, and mice. The results suggested that the most recent common ancestor between fishes and tetrapods had at least nine ancestral OR genes, and all OR genes identified were classified into nine groups, each of which originated from one ancestral gene. Eight of the nine group genes are still observed in current fish species, whereas only two group genes were found from mammalian genomes, showing that the OR gene family in fishes is much more diverse than in mammals. In mammals, however, one group of genes, gamma, expanded enormously, containing approximately 90% of the entire gene family. Interestingly, the gene groups observed in mammals or birds are nearly absent in fishes. The OR gene repertoire in frogs is as diverse as that in fishes, but the expansion of group gamma genes also occurred, indicating that the frog OR gene family has both mammal- and fish-like characters. All of these observations can be explained by the environmental change that organisms have experienced from the time of the common ancestor of all vertebrates to the present.

Mate choice decisions of stickleback females predictably modified by MHC peptide ligands

Sexual selection has been proposed as one mechanism to explain the maintenance of high allelic diversity in MHC genes that control the extent of resistance against pathogens and parasites in natural populations. MHC-based sexual selection is known to involve olfactory mechanisms in fish, mice, and humans. During mate choice, females of the three-spined stickleback (Gasterosteus aculeatus) use an odor-based selection strategy to achieve an optimal level of MHC diversity in their offspring, equipping them with optimal resistance toward pathogens and parasites. The molecular mechanism of odor-based mate-selection strategies is unknown. Because peptide ligands for MHC class I molecules function as individuality signals in mice, we hypothesized that female sticklebacks might assess the degree of MHC diversity of potential partners by means of the structural diversity of the corresponding peptide ligands in perceived odor signals. We show that structurally diverse MHC ligands interact with natural odors of male sticklebacks to predictably modify MHC-related mate choice. For a mating pair with suboptimal numbers of MHC alleles, peptides increase the attractiveness of male water, whereas for a mating pair with superoptimal numbers, attractiveness is decreased. Our results suggest that female sticklebacks use evolutionarily conserved structural features of MHC peptide ligands to evaluate MHC diversity of their prospective mating partners.

Olfactory transduction in ciliated receptor neurons of the Cabinza grunt, Isacia conceptionis (Teleostei: Haemulidae)

The ciliated receptor neurons of fish olfactory organs are thought to transduce amino acids through a cAMP-dependent transduction pathway, but direct physiological evidence for this hypothesis remains scarce and is confined to catfish and trout. We investigated olfactory transduction in a marine fish, the Cabinza grunt Isacia conceptionis (Perciformes, Teleostei). The olfactory epithelium was characterized using light and electron microscopy, and isolated ciliated receptor neurons were recorded with the perforated patch-clamp technique. Cells were stimulated with puffer pipettes containing amino acid odourants, IBMX plus forskolin or 8 bromo-cAMP. All three stimuli triggered transient inward currents at a holding potential of -70 mV and responses with outward-rectifying current-voltage relationships. The characteristics of the transduction currents induced by each stimulus were similar across cells and indistinguishable within the same cell, supporting the hypothesis of a cAMP pathway mediating transduction of amino acids in ciliated olfactory receptor neurons.

Pheromone reception in mammals

Pheromonal communication is the most convenient way to transfer information regarding gender and social status in animals of the same species with the holistic goal of sustaining reproduction. This type of information exchange is based on pheromones, molecules often chemically unrelated, that are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. So profound is the relevance of pheromones over the evolutionary process that a specific peripheral organ devoted to their recognition, namely the vomeronasal organ of Jacobson, and a related central pathway arose in most vertebrate species. Although the vomeronasal system is well developed in reptiles and amphibians, most mammals strongly rely on pheromonal communication. Humans use pheromones too; evidence on the existence of a specialized organ for their detection, however, is very elusive indeed. In the present review, we will focus our attention on the behavioral, physiological, and molecular aspects of pheromone detection in mammals. We will discuss the responses to pheromonal stimulation in different animal species, emphasizing the complicacy of this type of communication. In the light of the most recent results, we will also discuss the complex organization of the transduction molecules that underlie pheromone detection and signal transmission from vomeronasal neurons to the higher centers of the brain. Communication is a primary feature of living organisms, allowing the coordination of different behavioral paradigms among individuals. Communication has evolved through a variety of different strategies, and each species refined its own preferred communication medium. From a phylogenetic point of view, the most widespread and ancient way of communication is through chemical signals named pheromones: it occurs in all taxa, from prokaryotes to eukaryotes. The release of specific pheromones into the environment is a sensitive and definite way to send messages to other members of the same species. Therefore, the action of an organism can alter the behavior of another organism, thereby increasing the fitness of either or both. Albeit slow in transmission and not easily modulated, pheromones can travel around objects in the dark and over long distances. In addition, they are emitted when necessary and their biosynthesis is usually economic. In essence, they represent the most efficient tool to refine the pattern of social behaviors and reproductive strategies.

Odorant receptor gene expression changes during the parr-smolt transformation in Atlantic salmon

The ability of salmon to home accurately to their natal stream to spawn has long intrigued biologists and has important consequences for the maintenance of population structure in these species. It is known that olfaction is crucial to homing, and that the transition from the freshwater to the marine environment (the parr-smolt transformation; PST) is a period of increased olfactory sensitivity and learning, resulting in a permanent memory of natal site odours that is retained, at least in part, in peripheral sensory neurones. These odours are then used as cues by sexually maturing fish on their homeward migration. We used quantitative polymerase chain reaction techniques to demonstrate transient increases in expression of odorant receptor transcripts (of up to fifty-fold over pre-PST levels) coincident with PST. Both olfactory (SORB) and vomeronasal receptors (SVRA and SVRC) are involved, which suggests that the fish learn both environmental odours and semiochemicals (pheromones). Receptor expression varies between families and changes over time indicating both genetic differences in odour stimuli and multiple periods of olfactory sensitivity. We suggest that changes in OR gene expression may have a role in homing behaviour and thus the maintenance of population structure in Atlantic salmon.

Differential distribution of olfactory receptor neurons in goldfish: structural and molecular correlates

The olfactory system of many terrestrial vertebrates comprises a main olfactory organ and a vomeronasal organ each containing a morphologically distinct type of olfactory receptor neuron (ORN). The two cell types also differ in the expression of G-proteins and odorant receptor molecules. Fish do not have a vomeronasal organ, and their olfactory neurons-three different morphological types-are contained in one epithelium. The olfactory organ of goldfish appears as a rosette, with the sensory epithelium lying along the proximal portion of each lamella, where it attaches to the midline raphe. Using immunocytochemistry, in situ hybridization, and electron microscopy, we tested whether a correlation exists between receptor cell morphology, distribution of cell type within the sensory epithelium, and expression of odorant receptors and G-proteins. A strong correlation exists between ORN morphology, type of odorant receptor and G-protein expressed and the distribution of sensory cells within the olfactory epithelium. The Buck and Axel type of odorant receptor and Galpha(olf) are expressed in tall ciliated ORNs distributed homogenously across the entire sensory epithelium. In contrast, microvillous ORNs expressing V2R-like receptors, and Galpha(o), Galpha(q), or Galpha(i-3), and crypt type ORNs expressing Galpha(o) and Galpha(q), are preferentially located along the dorsal margin of the epithelium and near the midline raphe. V2R- and OR-type receptor molecules do not colocalize in one cell, and only crypt-type ORNs express more than one G-protein.

Pheromone receptors in mammals

In most mammals, pheromone perception mediates intraspecies interactions related to reproduction, such as mate recognition, intermale aggressive behaviors, or exchanges between females and their offspring. Recent molecular findings, particularly the identification of two large pheromone receptor gene superfamilies, provide today invaluable tools to better understand the way mammals make sense of pheromonal information.

Expression of vomeronasal receptor genes in Xenopus laevis

In the course of evolution, the vomeronasal organ (VNO) first appeared in amphibians. To understand the relationship between the VNO and the vomeronasal receptors, we isolated and analyzed the expression of the vomeronasal receptor genes of Xenopus laevis. We identified genes of the Xenopus V2R receptor family, which are predominantly expressed throughout the sensory epithelium of the VNO. The G-protein Go, which is coexpressed with V2Rs in the rodent VNO, was also extensively expressed throughout the vomeronasal sensory epithelium. These results strongly suggest that the V2Rs and Go are coexpressed in the vomeronasal receptor cells. The predominant expression of the Xenopus V2R families and the coexpression of the V2Rs and Go imply that V2Rs play important roles in the sensory transduction of Xenopus VNO. We found that these receptors were expressed not only in the VNO, but also in the posterolateral epithelial area of the principal cavity (PLPC). Electron microscopic study revealed that the epithelium of the PLPC is more like that of the VNO than that of the principal and the middle cavity. These results suggest that in adult Xenopus the V2Rs analyzed so far are predominantly expressed in the vomeronasal and vomeronasal-like epithelium. The analysis of V2R expression in Xenopus larvae demonstrates that V2Rs are predominantly expressed in the VNO even before metamorphosis.

Identification of 24 genes and two pseudogenes coding for olfactory receptors in Japanese loach, classified into four subfamilies: a putative evolutionary process for fish olfactory receptor genes by comprehensive phylogenetic analysis

Twenty-four olfactory receptor (OR) genes and two pseudogenes have been identified in the genome of Japanese loach (Misgurnus anguillicaudatus). The genes were classified into four subfamilies according to the similarity of the amino acid sequences. In each subfamily, members showed high sequence similarity not only to each other but also to orthologues of other fish species. The number of members in each OR subfamily was roughly estimated to be from 3 to 10 by genomic Southern blot analysis. The genes of all four OR subfamilies were shown to express on olfactory neurons of the olfactory epithelium by in situ hybridization analysis. Two major features of fish OR genes were found by comprehensive and comparative analyses on OR genes of Japanese loach and other fish species including catfish, zebrafish and pufferfish. First, the phylogenetic tree comprising of representative subfamily members suggests the existence of several prototype genes common to the genomes of many fish species. Second, when all members of orthologous subfamilies identified in each clade of the tree are integrated, the members of a single species comprise a monophyletic group. This means that 'intraspecies' sequence homology, that is, homology among paralogous genes of the same subfamily in a species, is higher than 'interspecies' homology, that is, homology between orthologous genes of different species. This suggests that the subfamily members of a species have evolved recently. Taken together, fish OR genes have evolved from a limited number of prototype genes common to most fish species, and several genes in a subfamily have diversely evolved in each species from each prototype.

Specific and conserved roles of TAp73 during zebrafish development

p53, p63 and p73 are related transcription factors involved in the regulation of cell proliferation, survival and differentiation. Here, we report the isolation and characterization of p73 from zebrafish. While for zebrafish p63 only N-terminally truncated isoforms (DeltaNp63) have been reported, p73 appears to be predominantly or exclusively present in transactivating isoforms (TAp73). p73 shows a very restricted expression pattern during zebrafish development. Transcripts are found in a subset of cells of the olfactory system, the telencephalon, the dorsal diencephalon, and the pronephric ducts. In addition, p73 is expressed in differentiating slow muscle cells of the somites, and in the pharyngeal endoderm. We carried out TAp73 gain- and loss-of-function experiments, injecting either TAp73alpha mRNA, or antisense morpholino oligonucleotides to suppress translation of TAp73 transcripts. The overexpression studies indicate that in contrast to p53, TAp73alpha has no pro-apoptotic effect in zebrafish embryos. However, TAp73 appears to be required for specific processes during the development of the olfactory system, the telencephalon and the pharyngeal arches. Together, our data point to both conserved and class-specific roles of p73 during vertebrate development.

Extracellular calcium sensing and signalling

Ca2+ is well established as an intracellular second messenger. However, the molecular identification of a detector for extracellular Ca2+--the extracellular calcium-sensing receptor--has opened up the possibility that Ca2+ might also function as a messenger outside cells. Information about the local extracellular Ca2+ concentration is conveyed to the interior of many cell types through this unique G-protein-coupled receptor. Here, we describe new emerging concepts concerning the signalling function of extracellular Ca2+, with particular emphasis on the extracellular calcium-sensing receptor.

Evolution, structure, and activation mechanism of family 3/C G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) represent one of the largest gene families in the animal genome. These receptors can be classified into several groups based on the sequence similarity of their common heptahelical domain. The family 3 (or C) GPCRs are receptors for the main neurotransmitters glutamate and gamma-aminobutyric acid, for Ca(2+), for sweet and amino acid taste compounds, and for some pheromone molecules, as well as for odorants in fish. Although none of these family 3 receptors have been found in plants, members have been identified in ancient organisms, such as slime molds (Dictyostelium) and sponges. Like any other GPCRs, family 3 receptors possess a transmembrane heptahelical domain responsible for G-protein activation. However, most of these identified receptors also possess a large extracellular domain that is responsible for ligand recognition, is structurally similar to bacterial periplasmic proteins involved in the transport of small molecules, and is called a Venus Flytrap module. The recent resolution of the structure of this binding domain in one of these receptors, the metabotropic glutamate 1 receptor, together with the recent demonstration that these receptors are dimers, revealed a unique mechanism of activation for these GPCRs. Such data open new possibilities in the development of drugs aimed at modulating these receptors, and raise a number of interesting questions on the activation mechanism of the other GPCRs.

Non-oscillatory discharges of an F-prostaglandin responsive neuron population in the olfactory bulb-telencephalon transition area in lake whitefish

Our previous studies on olfactory bulbar responses in salmonid fishes suggest that pheromone signals might be processed by a mechanism distinct from that of other odorants. Using in vivo single-unit and electroencephalographic recordings, we investigated response characteristics of olfactory neurons in lake whitefish, Coregonus clupeaformis, a species characterized by high electrophysiological and behavioral sensitivities to the reproductive pheromone candidates F-prostaglandins. We found a neuron population responsive to F-prostaglandins in the ventromedial brain tissue strip connecting the olfactory bulb to the telencephalon. Of the 64 neurons examined in this area, 33% showed excitatory and 11% inhibitory responses to F-prostaglandins, while 52% were non-responsive to all the stimuli tested. Both phasic and tonic F-prostaglandin neuron response patterns were observed during the 10-s stimulus period; some responses were delayed from the onset of stimulation, and some persisted for a long time following stimulus cessation. This neuron population did not induce synchronized oscillatory waves upon stimulation with F-prostaglandins, despite massive discharges. We demonstrate for the first time that the olfactory bulb-telencephalon area of the brain is a distinct neural structure through which putative reproductive pheromone signals are integrated. Amino acid and F-prostaglandin neuron population discharges have different temporal characteristics, suggesting different processing mechanisms exist for odorant and pheromone signals. The observed sustained neuron discharges may play a role in amplifying pheromone signals required for triggering stereotyped neuroendocrine and/or behavior changes.

Polyamines as olfactory stimuli in the goldfish Carassius auratus

Electrophysiological responses of goldfish olfactory receptor neurons (ORNs) and goldfish behavioral responses to polyamines were investigated in vivo. Electro-olfactogram (EOG) recordings indicated that polyamines (putrescine, cadaverine and spermine) are potent olfactory stimuli for goldfish with estimated electrophysiological thresholds of 10(-8)-10(-7) mol l(-1), similar to that for L-arginine, the most stimulatory amino acid. Although thresholds were similar, the magnitude of the EOG responses to intermediate (10(-5)-10(-4) mol l(-1)) and high (10(-3) mol l(-1)) concentrations of polyamines dwarfed the responses to amino acids and related single amine containing compounds (amylamine and butylamine). The EOG responses to 0.1 mmol l(-1) putrescine, cadaverine and spermine were, respectively, 4.2x, 4.3x and 10.3x the response of the standard, 0.1 mmol l(-1) L-arginine. Electrophysiological cross-adaptation experiments indicated that polyamine receptor sites are independent from those to L-amino acids (alanine, arginine, glutamate, lysine, methionine and ornithine), bile salts (sodium taurocholate and taurolithocholate), the single amine containing compounds (amylamine and butylamine) and ATP. Further, the cross-adaptation experiments revealed the existence of independent receptor sites for the different polyamines tested. Pharmacological experiments suggested that polyamine odorant transduction does not primarily involve the cyclic AMP and IP(3) second messenger pathways. Behavioral assays indicated that polyamines are attractants that elicit feeding behavior similar to that elicited by L-amino acids.

Analysis of penetrance and expressivity during ontogenesis supports a stochastic choice of zebrafish odorant receptors from predetermined groups of receptor genes

Olfactory receptor neurons select a single odourant receptor gene for expression out of a large gene family. The mechanisms of this extreme selectivity are largely unknown. We have determined in detail the developmental expression dynamics of a representative subset of the zebrafish odourant receptor repertoire, using in situ hybridization analysis. We have thus generated a dataset, which allows us to test hypotheses of odourant receptor gene regulation. The receptors chosen belong to four different groups with respect to ontogenetic onset of expression (onset groups). Statistical analysis of the data supports a model in which the final choice of an individual odourant receptor gene occurs stochastically from within a group of genes sharing a deterministically defined onset of expression. Genomic mapping revealed a pronounced correlation of onset of expression with genomic neighbourhood. During a protracted juvenile developmental period individual regulatory influences seem to modify the expression of odourant receptor genes, a notable example being a transient decrease in expressivity of two odourant receptor genes.

Multiple new and isolated families within the mouse superfamily of V1r vomeronasal receptors

Seven-transmembrane-domain proteins encoded by the vomeronasal receptor V1r and V2r gene superfamilies, and expressed by vomeronasal sensory neurons, are believed to be pheromone receptors in rodents. Four V1r gene families have been described in the mouse (V1ra, V1rb, V1rc and V3r). Here we have screened near-complete mouse genomic databases to obtain a first global draft of the mouse V1r repertoire, including 104 new V1r genes. It comprises eight new and extremely isolated families in addition to the four families previously identified. Members of these new families were expressed in vomeronasal sensory neurons. The genome-wide view revealed great sequence diversity within the V1r superfamily. Phylogenetic analyses suggested an ancient original radiation, followed by the isolation, divergence and expansion of families by extensive gene duplications and frequent gene loss. The isolated nature of these gene families probably reflects a specialization of different receptor classes in the detection of specific types of chemicals.

Diversification of olfactory receptor genes in the Japanese medaka fish, Oryzias latipes

Vertebrate olfactory receptors (OR) exists as the largest multigene family, scattered throughout the genome in clusters. Studies have shown that different animals possess remarkably diverse set of OR genes to recognize diverse odor molecules. In order to examine the evolutionary process of OR diversification, we examined three OR gene subfamilies from Japanese medaka fish (seven lines sampled from four populations). For each subfamily, the sequences of ancestral genes were inferred based on distance method. Examination of d(N)/d(S) ratios for each branch of phylogenetic trees suggested that purifying selection is the major force of evolution in medaka OR genes. However, for the mfOR1 and mfOR2 paralogous gene pairs, a nonrandom distribution of fixed amino acid changes and the d(N)>d(S) in a branch suggested that diversifying selection occurred after gene duplication. The fixed amino acid changes were observed in the third, fifth and sixth transmembrane domains, which has been predicted to serve as a ligand-binding pocket in a structural model. Compatibility test suggested that interlocus recombinations involving the fourth transmembrane domain occurred between the mfOR1 and mfOR2 gene pairs. The pattern of nucleotide substitutions in other OR genes agrees with the hypothesis that a limited number of amino acid residues are involved in odorant binding. Such comparative analyses of paralogous OR genes should provide bases for understanding the evolution, the structure, and the functional specificity of OR genes.

Amino acids in the cytoplasmic C terminus of the parathyroid Ca2+-sensing receptor mediate efficient cell-surface expression and phospholipase C activation

The C-terminal tail of the calcium receptor (CaR) regulates the affinity of the receptor for ligand, desensitization, and membrane localization. To determine the role of specific amino acids in the bovine parathyroid CaR in mediating signal transduction and cell-surface expression, we transfected truncated and mutated CaR cDNAs into HEK-293 cells. The ability of high extracellular [Ca(2+)] ([Ca(2+)](o)) to increase total inositol phosphate (InsP) production, an index of phospholipase C (PLC) activation, was determined. Receptor expression was assessed by immunoblotting and immunocytochemistry. In cells transiently or stably expressing receptors with the C-terminal tail truncated after residue 895 (CaR-(1-895)) or 929 (CaR-(1-929)), raising [Ca(2+)](o) increased InsPs to levels comparable with those of cells expressing wild-type CaRs. There were no PLC responses to high [Ca(2+)](o) (up to 30 mm) in cells expressing CaRs with C-terminal tails of only 3 residues (CaR-(1-866)), even though these receptors were expressed in the membrane. We scanned the residues between Ser(866) and Val(895) using tandem-Ala and single-site mutagenesis. Two point mutants (His(880) --> Ala and Phe(882) --> Ala CaR) showed 50-70% reductions in high [Ca(2+)](o)-induced InsP production. The levels of expression and glycosylation of these mutants were comparable with wild-type CaRs, but both receptors were profoundly retained in intracellular organelles and co-localized with the endoplasmic reticulum marker BiP. This suggested that the signaling defects of these receptors were likely because of defective trafficking of receptors to the cell surface. Modeling of the C-terminal domain of the CaR indicated that His(880) and Phe(882) are situated in a putative alpha-helical structure of 15 amino acids between residues 877 and 891 in the C-terminal tail. Our studies support the idea that specific amino acids, and possibly a unique secondary structure in the C-terminal tail, are required for the efficient targeting of the CaR to the cell surface required for PLC activation.

Neurobiology of fish olfaction: a review

The last decade saw important advances in our understanding of the olfactory system function. In some animals, we now have the basic knowledge necessary to investigate coding mechanisms employed in olfaction. So far, studies of the fish olfactory system have focused on odor detection and the early processing of olfactory information in the olfactory bulb. How this information is integrated in the forebrain is unknown. Here, we first describe the anatomy of the fish olfactory system. The problems faced when describing the anatomy of the terminal nerve complex and nucleus olfactoretinalis are highlighted. Olfactory sensory neurons are randomly distributed over the entire olfactory epithelium, a unique feature of the olfactory sense. These primary olfactory neurons converge upon their second-order targets in segregated areas of the olfactory bulb. Exchange of information occurs in the glomeruli and glomerular plexus, where primary neurons synapse on mitral cell dendrites. The spatial distribution of glomerular activity induced by odorants of different classes shows that distinct neuron populations of the olfactory bulb encode information related to different odorant groups. In most cases, these neuron populations synchronize their alternating sequences of firing and silence when stimulated by primary input. Synchronized oscillations of these second-order neurons could contain important coding information, or represent a mechanism by which learning is facilitated. Alternatively, oscillations could be solely used to shape the olfactory bulb response. The nature of the olfactory information that reaches the forebrain and decoding of this information by the central nervous system are discussed.

Co-expression of putative pheromone receptors in the sensory neurons of the vomeronasal organ

Two large and divergent families of G-protein-coupled receptors (V1Rs and V2Rs) are expressed in subsets of neurons in the vomeronasal organ. These receptors are likely to mediate pheromone responses, but it appears that many V2R genes may encode expressed pseudogenes rather than functional proteins. Therefore we have raised antibodies to representative V2Rs and show labeling of vomeronasal neurons demonstrating that V2R genes encode expressed receptors. V2R immunoreactivity was detected at the sensory surface of the vomeronasal organ in dendritic terminals, indicating that these receptors are capable of directly interacting with pheromones and mediating physiological responses. Immunohistochemistry confirmed that three V2R receptors are expressed in small subsets of sensory neurons. However, surprisingly we found that a subfamily of V2R genes is broadly expressed in the Goalpha-layer of the vomeronasal organ and are coexpressed in the same cells as other V2Rs. This is in direct contrast to the main olfactory epithelium where sensory neurons express only a single receptor. Thus, our results suggest that different modes of the information processing may occur in the main and accessory olfactory systems.

Molecular characterisation of a putative Atlantic salmon (Salmo salar) odorant receptor

The olfactory system of fish is extremely important as it is able to recognise and distinguish a vast array of odorous molecules that are involved in behaviours paramount to survival. This is achieved by the activation of a diverse multigene family of G-protein coupled receptors through odorous ligand binding. Using molecular techniques, the nucleotide sequence of the cDNA coding for an Atlantic salmon (Salmo salar) odorant receptor (ASOR1) has been determined. The full-length cDNA (1260 nt) encodes a protein of 320 amino acid residues, including one potential N-linked glycosylation site, within the short extracellular amino terminal of the receptor. Hydrophobicity analysis revealed seven hydrophobic regions within the amino acid sequence, corresponding to possible positions of the transmembrane domains characteristic of the G-protein coupled receptor superfamily. Several conserved motifs unique to odorant receptors were also present. Through characterisation of this receptor, we hope to increase the understanding of the mechanisms underlying olfaction in salmonid species.

Identification of a novel member of the T1R family of putative taste receptors

In the gustatory system, the recognition of sugars, amino acids and bitter-tasting compounds is the function of specialized G protein-coupled receptors. Recently, two members of novel subfamily of G protein-coupled receptors were proposed to function as taste receptors based on their specific expression in taste receptor cells. Here, we report the identification of a third member, T1R3, of this family of receptors. T1R3 maps near the telomere of mouse chromosome 4 rendering it a candidate for the Sac locus, a primary determinant of sweet preference in mice. Consistent with its candidacy for the Sac locus, T1R3 displays taste receptor cell-specific expression. In addition, taster and non-taster strains of mouse harbor different alleles of T1R3.

The extracellular Ca2+-sensing receptor: central mediator of systemic calcium homeostasis

The cloning of the G protein-coupled, extracellular calcium (Ca(2+)o)-sensing receptor (CaR) has identified a central mediator of the mechanism governing systemic Ca(2+)o homeostasis. This system enables organisms to adapt successfully to wide variations in dietary Ca(2+)o intake while maintaining near constancy of Ca(2+)o. Whereas discussions of Ca(2+)o homeostasis have generally focused on the key role of Ca(2+)o-elicited changes in parathyroid hormone secretion, the presence of the CaRs in effector tissues of this system enables direct regulation of processes (e.g. renal tubular Ca(2+) reabsorption and possibly bone formation and resorption) that add additional layers of homeostatic control. As we understand more about how the CaR regulates these tissues, we may find that it participates in other processes relevant to mineral ion homeostasis, including the control of the 1-hydroxylation and activation of vitamin D3 or reabsorption of phosphate in the renal proximal tubule. Regardless, the remarkable sensitivity of the CaR to small changes in Ca(2+)o allows adjustments in the response of the Ca(2+)o homeostatic system to increases or decreases in the intake of dietary Ca(2+), for instance, that cause barely detectable alterations in Ca(2+)o. Furthermore, the CaR likely participates in coordinating interactions among several different homeostatic control systems (including those for water, Mg(2+)o, Na(+), extracellular volume, and/or blood pressure), despite the fact that these systems are often considered to function largely independently of mineral ion metabolism.

Co-expression of putative pheromone receptors in the sensory neurons of the vomeronasal organ

Two large and divergent families of G-protein-coupled receptors (V1Rs and V2Rs) are expressed in subsets of neurons in the vomeronasal organ. These receptors are likely to mediate pheromone responses, but it appears that many V2R genes may encode expressed pseudogenes rather than functional proteins. Therefore we have raised antibodies to representative V2Rs and show labeling of vomeronasal neurons demonstrating that V2R genes encode expressed receptors. V2R immunoreactivity was detected at the sensory surface of the vomeronasal organ in dendritic terminals, indicating that these receptors are capable of directly interacting with pheromones and mediating physiological responses. Immunohistochemistry confirmed that three V2R receptors are expressed in small subsets of sensory neurons. However, surprisingly we found that a subfamily of V2R genes is broadly expressed in the Goalpha-layer of the vomeronasal organ and are coexpressed in the same cells as other V2Rs. This is in direct contrast to the main olfactory epithelium where sensory neurons express only a single receptor. Thus, our results suggest that different modes of the information processing may occur in the main and accessory olfactory systems.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

A novel family of candidate pheromone receptors in mammals

Pheromonal activation of the vomeronasal organ (VNO) elicits genetically preprogrammed behaviors and physiological changes in mammals. We have identified a novel gene family encoding over one hundred VNO specific receptors, the V3Rs. V3R sequences are highly similar to each other and appear distantly related to the putative pheromone receptors, V1Rs, and the taste receptors, T2Rs. Within the VNO, V3R-positive neurons are distinct from neurons expressing the pheromone receptor families V1R and V2R. The V3Rs are likely to represent a new large family of pheromone receptors in mammals. Multiple V3R-related human sequences have been identified, including one clone retaining the capacity to create a complete and functional transcript. Our data uncover a striking complexity in the molecular and cellular organization of the VNO and provide an essential framework for the study of pheromone signaling in mammals.

Mutations of the calcium-sensing receptor (CASR) in familial hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia

The calcium-sensing receptor (CASR) is a plasma membrane G protein coupled receptor that is expressed in the parathyroid hormone (PTH) producing chief cells of the parathyroid gland and the cells lining the kidney tubule. By virtue of its ability to sense small changes in circulating calcium concentration ([Ca(2+)](o)) and to couple this information to intracellular signaling pathways that modify PTH secretion or renal cation handling, the CASR plays an essential role in maintaining mineral ion homeostasis. Inherited abnormalities of the CASR gene located on chromosome 3p13.3-21 can cause either hypercalcemia or hypocalcemia depending upon whether they are inactivating or activating, respectively. Heterozygous loss-of-function mutations give rise to familial (benign) hypocalciuric hypercalcemia (FHH) in which the lifelong hypercalcemia is asymptomatic. The homozygous condition manifests itself as neonatal severe hyperparathyroidism (NSHPT), a rare disorder characterized by extreme hypercalcemia and the bony changes of hyperparathyroidism which occur in infancy. The disorder autosomal dominant hypocalcemia (ADH) is due to gain-of-function mutations in the CASR gene. ADH may be asymptomatic or present with neonatal or childhood seizures. A common polymorphism in the intracellular tail of the CASR, Ala to Ser at position 986, has a modest effect on the serum calcium concentration in healthy individuals.

Evolution of odorant receptors

Odorant receptors (ORs) located in the nasal epithelium, at the ciliated surface of olfactory sensory neurons, represent the initial step of a transduction cascade that leads to odor detection. ORs form the largest and most diverse family of G-protein-coupled receptors (GPCRs). They are encoded by a multigene family that has been partially characterized in cyclostomes, teleosts, amphibia, birds and mammals, as well as in Drosophila melanogaster and the nematode Caenorhabditis elegans. As new sequence data emerge, it is increasingly clear that OR primary structure can vary dramatically across phyla. Some chemoreceptors are encoded by genes with little sequence similarity to the prototypical ORs originally isolated in mammals. A large number of sequences are now available allowing a detailed study of the evolutionary implications of OR diversity across species. This review discusses the evolutionary implications of the divergent primary structures of chemoreceptors with identical functions.

Sensory coding of pheromone signals in mammals

The vomeronasal organ (VNO) of mammals plays an essential role in the detection of pheromones, chemical cues secreted by animals that elicit genetically programmed sexual and aggressive behaviors among conspecifics. The recent characterization of genes encoding molecular components of the VNO sensory response suggests that VNO neurons express a unique set of molecules to recognize and translate pheromone signals into neuronal electrical activity. Identification of these genes, which include putative pheromone receptor genes, has offered a new opportunity to uncover basic principles of pheromone sensory processing and important aspects of vomeronasal development.

Blocking adenylyl cyclase inhibits olfactory generator currents induced by "IP(3)-odors"

Vertebrate olfactory receptor neurons (ORNs) transduce odor stimuli into electrical signals by means of an adenylyl cyclase/cAMP second messenger cascade, but it remains widely debated whether this cAMP cascade mediates transduction for all odorants or only certain odor classes. To address this problem, we have analyzed the generator currents induced by odors that failed to produce cAMP in previous biochemical assays but instead produced IP(3) ("IP(3)-odors"). We show that in single salamander ORNs, sensory responses to "cAMP-odors" and IP(3)-odors are not mutually exclusive but coexist in the same cells. The currents induced by IP(3)-odors exhibit identical biophysical properties as those induced by cAMP odors or direct activation of the cAMP cascade. By disrupting adenylyl cyclase to block cAMP formation using two potent antagonists of adenylyl cyclase, SQ22536 and MDL12330A, we show that this molecular step is necessary for the transduction of both odor classes. To assess whether these results are also applicable to mammals, we examine the electrophysiological responses to IP(3)-odors in intact mouse main olfactory epithelium (MOE) by recording field potentials. The results show that inhibition of adenylyl cyclase prevents EOG responses to both odor classes in mouse MOE, even when "hot spots" with heightened sensitivity to IP(3)-odors are examined.

Amino acids in the second and third intracellular loops of the parathyroid Ca2+-sensing receptor mediate efficient coupling to phospholipase C

To determine the role of amino acids in the second and third intracellular (IC) loops of the Ca(2+)-sensing receptor (CaR) in phospholipase C (PLC) activation, we mutated residues in these loops either singly or in tandem to Ala and assessed PLC activity by measuring high extracellular [Ca(2+)] ([Ca(2+)](o))-induced inositol phosphate accumulation and protein expression by immunoblotting and immunocytochemistry in human embryonic kidney 293 cells. Two CaR constructs in the second IC loop, F707A CaR and to a lesser extent L704A CaR, demonstrated reduced activation of PLC, despite levels of protein expression comparable with the wild-type (wt) CaR. Substitution of Tyr or His for Phe-707, but not Leu, Val, Glu, or Trp, partially restored the ability of high [Ca(2+)](o) to activate PLC. Eight residues in the third IC loop were involved in PLC signaling. The responses to high [Ca(2+)](o) in cells expressing CaRs with Ala substitutions at these sites were <35% of the wt CaR. The L798A, F802A, and E804A CaRs were dramatically impaired in their responses to [Ca(2+)](o) even up to 30 mm. Substitutions of Leu-798 with other hydrophobic residues (Ile, Val, or Phe), but not with acidic, basic, or polar residues, produced reduced responses compared with wt. Phe-802 could be replaced with either Tyr or Trp with partial retention of the ability to activate PLC. Glu-804 could only be substituted with Asp or Gln and maintain its signaling capacity. Cell surface expression of the CaRs mutated at Leu-798 and Phe-802 appeared normal compared with wt CaR. Cell surface CaR expression was, however, reduced substantially in cells expressing several mutants at position Glu-804 by confocal microscopy. These studies strongly implicate specific hydrophobic and acidic residues in the second and third IC loops of the parathyroid CaR (and potentially larger stretches of the third loop) in mediating efficient high [Ca(2+)](o)-induced PLC activation and or CaR expression.

Physiological evidence for the discrimination of L-arginine from structural analogues by the zebrafish olfactory system

Although it is generally assumed that fish are capable of discriminating amino acid odorants on the basis of differences in side-chain structure, less is known about their ability to discriminate amino acids with modifications to alpha-carboxyl and alpha-amino groups. In this study, the ability of the zebrafish olfactory system to detect and presumably discriminate analogues of the basic amino acid Arg was assessed, by using cross-adaptation and activity-dependent labeling techniques. Electrophysiological recordings established that esterification (L-arginine methyl ester; AME) or deletion (agmatine or amino-4-guanidobutane; AGB) of the alpha-carboxyl group yielded odorants more potent than Arg, whereas deletion of the alpha-amino group (L-argininic acid; AA) yielded a less potent analogue. In cross-adaptation experiments, no test-competitor odorant combination yielded complete cross-adaptation, suggesting the detection of these Arg analogues by multiple odorant receptors (ORs) with partially nonoverlapping specificities. Activity-dependent immunocytochemical labeling of olfactory receptor neurons supported this conclusion. AGB, an ion-channel-permeant probe (and odorant), labeled 4.9 +/- 0.4% (n = 24) of sensory epithelium, whereas the addition of Arg, 1-ethylguanidine sulfate, L-alpha-amino-beta-guanidinopropionate, or AME to AGB resulted in a significant elevation of labeling (8-14%). This study provides evidence that the olfactory system has the potential to discriminate among amino acid odorants with modified alpha-carboxyl and alpha-amino groups.

Rabbit calcium-sensing receptor (CASR) gene: chromosome location and evidence for related genes

Diverse cellular functions are regulated by the calcium-sensing receptor, encoded by the CASR gene, which plays an important role in calcium homeostasis. Here we provide the sequence for exon VII of the rabbit CASR gene and show that it is 91% identical to the human gene at the nucleotide level, and 95% identical at the amino acid level. The gene was mapped by fluorescence in situ hybridization, using a cosmid isolated from a genomic library, to chromosome 14q11 and this was confirmed independently by PCR amplification of flow sorted chromosomes. In addition, the cosmid detected sites with lower frequencies on four other chromosomes: 3q, 5p, 8p, and 13p. Two of these sites (5p and 13p) were also detected by a related but unidentical cosmid, and map to regions that are homologous to the mouse calcium-sensing receptor related sequences (Casr-rs); suggesting that they may represent CASR-related sequences in the rabbit. The data support the presence of a family of genes related to the calcium-sensing receptor in the G-protein coupled receptor (GPCR) superfamily, as well as extend the existing knowledge of homology for several human and rabbit chromosomes.

Seven-transmembrane proteins as odorant and chemosensory receptors

The olfactory systems of various species solve the challenging problem of general molecular recognition in widely differing ways. Despite this variety, the molecular receptors are invariably G protein-coupled seven-transmembrane proteins, and are encoded by the largest gene families known to exist in a given animal genome. Receptor gene families have been identified in vertebrates and two invertebrate species, the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. The complexity of the odorant receptor repertoire is estimated in mouse and rat at 1000 genes, or 1 percent of the genome, surpassing that of the immunoglobulin and T cell receptor genes combined. Two distinct seven-transmembrane gene families may encode in rodents the chemosensory receptors of the vomeronasal organ, which is specialized in the detection of pheromones. Remarkably, these five receptor families have practically no sequence homology among them. Genetic manipulation experiments in mice imply that vertebrate odorant receptors may fulfill a dual role, also serving as address molecules that guide axons of olfactory sensory neurons to their precise target in the brain.

Odorant receptors: a plethora of G-protein-coupled receptors

Odorant receptors (ORs) comprise the largest family of G-protein-coupled receptors (GPCRs). They are located in the nasal epithelium, at the ciliated surface of olfactory sensory neurones, where the initial steps of the olfactory transduction cascade occur. ORs are encoded by a large and diverse multi-gene family, which has been characterized in cyclostomes, teleosts, amphibia, birds and mammals, as well as in Drosophila and Caenorhabditis elegans. Here, the range of diversity in OR and chemoreceptor structure is examined, noting that their functions are fundamentally similar to those of many neurotransmitter or neurohormone receptors. It is argued that ORs have emerged directly from other GPCRs independently in many species. According to this view, there is no structural prerequisite for OR identity and any GPCR has the potential to be or become an OR at a given point in evolution.

Functional identification of a goldfish odorant receptor

The vertebrate olfactory system utilizes odorant receptors to receive and discriminate thousands of different chemical stimuli. An understanding of how these receptors encode information about an odorant's molecular structure requires a characterization of their ligand specificities. We employed an expression cloning strategy to identify a goldfish odorant receptor that is activated by amino acids-potent odorants for fish. Structure-activity analysis indicates that the receptor is preferentially tuned to recognize basic amino acids. The receptor is a member of a multigene family of G protein-coupled receptors, sharing sequence similarities with the calcium sensing, metabotropic glutamate, and V2R class of vomeronasal receptors. The ligand tuning properties of the goldfish amino acid odorant receptor provide information for unraveling the molecular mechanisms underlying olfactory coding.

Putative mammalian taste receptors: a class of taste-specific GPCRs with distinct topographic selectivity

Taste represents a major form of sensory input in the animal kingdom. In mammals, taste perception begins with the recognition of tastant molecules by unknown membrane receptors localized on the apical surface of receptor cells of the tongue and palate epithelium. We report the cloning and characterization of two novel seven-transmembrane domain proteins expressed in topographically distinct subpopulations of taste receptor cells and taste buds. These proteins are specifically localized to the taste pore and are members of a new group of G protein-coupled receptors distantly related to putative mammalian pheromone receptors. We propose that these genes encode taste receptors.