Functional expression of olfactory-adrenergic receptor chimeras and intracellular retention of heterologously expressed olfactory receptors

A cilia-bound unconventional secretory pathway for Drosophila odorant receptors

BACKGROUND

Post-translational transport is a vital process which ensures that each protein reaches its site of function. Though most do so via an ordered ER-to-Golgi route, an increasing number of proteins are now shown to bypass this conventional secretory pathway.

RESULTS

In the Drosophila olfactory sensory neurons (OSNs), odorant receptors (ORs) are trafficked from the ER towards the cilia. Here, we show that Or22a, a receptor of various esters and alcoholic compounds, reaches the cilia partially through unconventional means. Or22a frequently present as puncta at the somatic cell body exit and within the dendrite prior to the cilia base. These rarely coincide with markers of either the intermediary ER-Golgi-intermediate-compartment (ERGIC) or Golgi structures. ERGIC and Golgi also displayed axonal localization biases, a further indication that at least some measure of OR transport may occur independently of their involvement. Additionally, neither the loss of several COPII genes involved in anterograde trafficking nor ERGIC itself affected puncta formation or Or22a transport to the cilium. Instead, we observed the consistent colocalization of Or22a puncta with Grasp65, the sole Drosophila homolog of mammalian GRASP55/Grh1, a marker of the unconventional pathway. The numbers of both Or22a and Grasp65-positive puncta were furthermore increased upon nutritional starvation, a condition known to enhance Golgi-bypassing secretory activity.

CONCLUSIONS

Our results demonstrate an alternative route of Or22a transport, thus expanding the repertoire of unconventional secretion mechanisms in neurons.

Understanding the molecular mechanisms of odorant binding and activation of the human OR52 family

Structural and mechanistic studies on human odorant receptors (ORs), key in olfactory signaling, are challenging because of their low surface expression in heterologous cells. The recent structure of OR51E2 bound to propionate provided molecular insight into odorant recognition, but the lack of an inactive OR structure limited understanding of the activation mechanism of ORs upon odorant binding. Here, we determined the cryo-electron microscopy structures of consensus OR52 (OR52cs), a representative of the OR52 family, in the ligand-free (apo) and octanoate-bound states. The apo structure of OR52cs reveals a large opening between transmembrane helices (TMs) 5 and 6. A comparison between the apo and active structures of OR52cs demonstrates the inward and outward movements of the extracellular and intracellular segments of TM6, respectively. These results, combined with molecular dynamics simulations and signaling assays, shed light on the molecular mechanisms of odorant binding and activation of the OR52 family.

Nanowire biosensors with olfactory proteins: towards a genuine electronic nose with single molecule sensitivity and high selectivity

We describe the concept and roadmap of an engineered electronic nose with specificity towards analytes that differ by as little as one carbon atom, and sensitivity of being able to electrically register a single molecule of analyte. The analyte could be anything that natural noses can detect, e.g. trinitrotoluene (TNT), cocaine, aromatics, volatile organic compounds etc. The strategy envisioned is to genetically engineer a fused olfactory odorant receptor (odorant receptor (OR), a membrane-bound G-protein coupled receptor (GPCR) with high selectivity) to an ion channel protein, which opens in response to binding of the ligand to the OR. The lipid bilayer supporting the fused sensing protein would be intimately attached to a nanowire or nanotube network (either via a covalent tether or a non-covalent physisorption process), which would electrically detect the opening of the ion channel, and hence the binding of a single ligand to a single OR protein domain. Three man-made technological advances: (1) fused GPCR to ion channel protein, (2) nanowire sensing of single ion channel activity, and (3) lipid bilayer to nanotube/nanowire tethering chemistry and on natural technology (sensitivity and selectivity of OR domains to specific analytes) each have been demonstrated and/or studied independently. The combination of these three technological advances and the result of millions of years of evolution of OR proteins would enable the goal of single molecule sensing with specificity towards analytes that differ by as little as one carbon atom. This is both a review of the past and a vision of the future.

Identification and Characterization of Proteins That Are Involved in RTP1S-Dependent Transport of Olfactory Receptors

Olfaction is mediated via olfactory receptors (ORs) that are expressed on the cilia membrane of olfactory sensory neurons in the olfactory epithelium. The functional expression of most ORs requires the assistance of receptor-transporting proteins (RTPs). We examined the interactome of RTP1S and OR via proximity biotinylation. Deubiquitinating protein VCIP135, the F-actin-capping protein sub-unit alpha-2, and insulin-like growth factor 2 mRNA-binding protein 2 were biotinylated via AirID fused with OR, RTP1S-AirID biotinylated heat shock protein A6 (HSPA6), and double-stranded RNA-binding protein Staufen homolog 2 (STAU2). Co-expression of HSPA6 partially enhanced the surface expression of Olfr544. The surface expression of Olfr544 increased by 50-80%. This effect was also observed when RTP1S was co-expressed. Almost identical results were obtained from the co-expression of STAU2. The interactions of HSPA6 and STAU2 with RTP1S were examined using a NanoBit assay. The results show that the RTP1S N-terminus interacted with the C-terminal domain of HSP6A and the N-terminal domain of STAU2. In contrast, OR did not significantly interact with STAU2 and HSPA6. Thus, HSP6A and STAU2 appear to be involved in the process of OR traffic through interaction with RTP1S.

Interactions among key residues regulate mammalian odorant receptor trafficking

Odorant receptors (ORs) expressed in mammalian olfactory sensory neurons are essential for the sense of smell. However, structure-function studies of many ORs are hampered by unsuccessful heterologous expression. To understand and eventually overcome this bottleneck, we performed heterologous expression and functional assays of over 80 OR variants and chimeras. Combined with literature data and machine learning, we found that the transmembrane domain 4 (TM4) and its interactions with neighbor residues are important for OR functional expression. The data highlight critical roles of T^4.62 therein. ORs that fail to reach the cell membrane can be rescued by modifications in TM4. Consequently, such modifications in MOR256-3 (Olfr124) also alter OR responses to odorants. T161^4.62 P causes the retention of MOR256-3 in the endoplasmic reticulum (ER), while T161^4.62 P/T148^4.49 A reverses the retention and makes receptor trafficking to cell membrane. This study offers new clues toward wide-range functional studies of mammalian ORs.

Sequence coevolution and structure stabilization modulate olfactory receptor expression

Olfactory receptors (ORs) belong to class A G-protein coupled receptors (GPCRs) and are activated by a variety of odorants. To date, there is no three-dimensional structure of an OR available. One of the major bottlenecks in obtaining purified protein for structural studies of ORs is their poor expression in heterologous cells. To design mutants that enhance expression and thereby enable protein purification, we first identified computable physical properties that recapitulate OR and class A GPCR expression and further conducted an iterative computational prediction-experimental test cycle and generated human OR mutants that express as high as biogenic amine receptors for which structures have been solved. In the process of developing the computational method to recapitulate the expression of ORs in membranes, we identified properties, such as amino acid sequence coevolution, and the strength of the interactions between intracellular loop 1 (ICL1) and the helix 8 region of ORs, to enhance their heterologous expression. We identified mutations that are directly located in these regions as well as other mutations not located in these regions but allosterically strengthen the ICL1-helix 8 enhance expression. These mutants also showed functional responses to known odorants. This method to enhance heterologous expression of mammalian ORs will facilitate high-throughput "deorphanization" of ORs, and enable OR purification for biochemical and structural studies to understand odorant-OR interactions.

Conserved C-terminal motifs in odorant receptors instruct their cell surface expression and cAMP signaling

The highly individual plasma membrane expression and cAMP signaling of odorant receptors have hampered their ligand assignment and functional characterization in test cell systems. Chaperones have been identified to support the cell surface expression of only a portion of odorant receptors, with mechanisms remaining unclear. The presence of amino acid motifs that might be responsible for odorant receptors' individual intracellular retention or cell surface expression, and thus, for cAMP signaling, is under debate: so far, no such protein motifs have been suggested. Here, we demonstrate the existence of highly conserved C-terminal amino acid motifs, which discriminate at least between class-I and class-II odorant receptors, with their numbers of motifs increasing during evolution, by comparing C-terminal protein sequences from 4808 receptors across eight species. Truncation experiments and mutation analysis of C-terminal motifs, largely overlapping with helix 8, revealed single amino acids and their combinations to have differential impact on the cell surface expression and on stimulus-dependent cAMP signaling of odorant receptors in NxG 108CC15 cells. Our results demonstrate class-specific and individual C-terminal motif equipment of odorant receptors, which instruct their functional expression in a test cell system, and in situ may regulate their individual cell surface expression and intracellular cAMP signaling.

Structure-Function Analyses of Human Bitter Taste Receptors-Where Do We Stand?

The finding that bitter taste receptors are expressed in numerous tissues outside the oral cavity and fulfill important roles in metabolic regulation, innate immunity and respiratory control, have made these receptors important targets for drug discovery. Efficient drug discovery depends heavily on detailed knowledge on structure-function-relationships of the target receptors. Unfortunately, experimental structures of bitter taste receptors are still lacking, and hence, the field relies mostly on structures obtained by molecular modeling combined with functional experiments and point mutageneses. The present article summarizes the current knowledge on the structure–function relationships of human bitter taste receptors. Although these receptors are difficult to express in heterologous systems and their homology with other G protein-coupled receptors is very low, detailed information are available at least for some of these receptors.

Molecular chaperones and G protein-coupled receptor maturation and pharmacology

G protein-coupled receptors (GPCRs) are highly conserved versatile signaling molecules located at the plasma membrane that respond to diverse extracellular signals. They regulate almost all physiological processes in the vertebrates. About 35% of current drugs target these receptors. Mutations in these genes have been identified as causes of numerous diseases. The seven transmembrane domain structure of GPCRs implies that the folding of these transmembrane proteins is extremely complicated and difficult. Indeed, many wild type GPCRs are not folded optimally. The most common defect in genetic diseases caused by GPCR mutations is misfolding and failure to reach the plasma membrane where it functions. General molecular chaperones aid the folding of all proteins, including GPCRs, by preventing aggregation, promoting folding and disaggregating small aggregates. Some GPCRs need additional receptor-specific chaperones to assist their folding. Many of these receptor-specific chaperones interact with additional receptors and alter receptor pharmacology, expanding the understanding of these chaperone proteins.

A Bitter Taste in Your Heart

The human genome contains ∼29 bitter taste receptors (T2Rs), which are responsible for detecting thousands of bitter ligands, including toxic and aversive compounds. This sentinel function varies between individuals and is underpinned by naturally occurring T2R polymorphisms, which have also been associated with disease. Recent studies have reported the expression of T2Rs and their downstream signaling components within non-gustatory tissues, including the heart. Though the precise role of T2Rs in the heart remains unclear, evidence points toward a role in cardiac contractility and overall vascular tone. In this review, we summarize the extra-oral expression of T2Rs, focusing on evidence for expression in heart; we speculate on the range of potential ligands that may activate them; we define the possible signaling pathways they activate; and we argue that their discovery in heart predicts an, as yet, unappreciated cardiac physiology.

Modulation of the combinatorial code of odorant receptor response patterns in odorant mixtures

The perception of odors relies on combinatorial codes consisting of odorant receptor (OR) response patterns to encode odor identity. Modulation of these patterns by odorant interactions at ORs potentially explains several olfactory phenomena: mixture suppression, unpredictable sensory outcomes, and the perception of odorant mixtures as unique objects. We determined OR response patterns to 4 odorants and 3 binary mixtures in vivo in mice, identifying 30 responsive ORs. These patterns typically had a few strongly responsive ORs and a greater number of weakly responsive ORs. ORs responsive to an odorant were often unrelated sequences distributed across several OR subfamilies. Mixture responses predicted pharmacological interactions between odorants, which were tested in vitro by heterologous expression of ORs in cultured cells, providing independent evidence confirming odorant agonists for 13 ORs and identifying both suppressive and additive effects. This included 11 instances of antagonism of ORs by an odorant, 1 instance of additive responses to a binary mixture, 1 instance of suppression of a strong agonist by a weak agonist, and the discovery of an inverse agonist for an OR. Interactions between odorants at ORs are common even when the odorants are not known to interact perceptually in humans, and in some cases interactions at mouse ORs correlate with the ability of humans to perceive an odorant in a mixture.

Structural instability and divergence from conserved residues underlie intracellular retention of mammalian odorant receptors

Mammalian odorant receptors are a diverse and rapidly evolving set of G protein-coupled receptors expressed in olfactory cilia membranes. Most odorant receptors show little to no cell surface expression in nonolfactory cells due to endoplasmic reticulum retention, which has slowed down biochemical studies. Here we provide evidence that structural instability and divergence from conserved residues of individual odorant receptors underlie intracellular retention using a combination of large-scale screening of odorant receptors cell surface expression in heterologous cells, point mutations, structural modeling, and machine learning techniques. We demonstrate the importance of conserved residues by synthesizing consensus odorant receptors that show high levels of cell surface expression similar to conventional G protein-coupled receptors. Furthermore, we associate in silico structural instability with poor cell surface expression using molecular dynamics simulations. We propose an enhanced evolutionary capacitance of olfactory sensory neurons that enable the functional expression of odorant receptors with cryptic mutations.

The N-terminal region of RTP1S plays important roles in dimer formation and odorant receptor-trafficking

Receptor-transporting protein 1S (RTP1S) is an accessory protein that mediates the transport of mammalian odorant receptors (ORs) into the plasma membrane. Although most ORs fail to localize to the cell surface when expressed alone in nonolfactory cells, functional expression of ORs is achieved with the coexpression of RTP1S. However, the mechanism for RTP1S-mediated OR trafficking remains unclear. In this study, we attempted to reveal the mode of action and critical residues of RTP1S in OR trafficking. Experiments using N-terminal truncation and Ala substitution mutants of RTP1S demonstrated that four N-terminal amino acids have essential roles in OR trafficking. Additionally, using recombinant proteins and split luciferase assays in mammalian cells, we provided evidence for the dimer formation of RTP1S. Furthermore, we determined that the 2nd Cys residue is required for the efficient dimerization of RTP1S. Altogether, these findings provide insights into the mechanism for plasma membrane transport of ORs by RTP1S.

Human Olfactory Receptors: Novel Cellular Functions Outside of the Nose

Olfactory receptors (ORs) are not exclusively expressed in the olfactory sensory neurons; they are also observed outside of the olfactory system in all other human tissues tested to date, including the testis, lung, intestine, skin, heart, and blood. Within these tissues, certain ORs have been determined to be exclusively expressed in only one tissue, whereas other ORs are more widely distributed in many different tissues throughout the human body. For most of the ectopically expressed ORs, limited data are available for their functional roles. They have been shown to be involved in the modulation of cell-cell recognition, migration, proliferation, the apoptotic cycle, exocytosis, and pathfinding processes. Additionally, there is a growing body of evidence that they have the potential to serve as diagnostic and therapeutic tools, as ORs are highly expressed in different cancer tissues. Interestingly, in addition to the canonical signaling pathways activated by ORs in olfactory sensory neurons, alternative pathways have been demonstrated in nonolfactory tissues. In this review, the existing data concerning the expression, as well as the physiological and pathophysiological functions, of ORs outside of the nose are highlighted to provide insights into future lines of research.

Live-cell Measurement of Odorant Receptor Activation Using a Real-time cAMP Assay

The enormous sizes of the mammalian odorant receptor (OR) families present difficulties to find their cognate ligands among numerous volatile chemicals. To efficiently and accurately deorphanize ORs, we combine the use of a heterologous cell line to express mammalian ORs and a genetically modified biosensor plasmid to measure cAMP production downstream of OR activation in real time. This assay can be used to screen odorants against ORs and vice versa. Positive odorant-receptor interactions from the screens can be subsequently confirmed by testing against various odor concentrations, generating concentration-response curves. Here we used this method to perform a high-throughput screening of an odorous compound against a human OR library expressed in Hana3A cells and confirmed that the positively-responding receptor is the cognate receptor for the compound of interest. We found this high-throughput detection method to be efficient and reliable in assessing OR activation and our data provide an example of its potential use in OR functional studies.

Receptor-transporting protein (RTP) family members play divergent roles in the functional expression of odorant receptors

Receptor transporting protein (RTP) family members, RTP1S and RTP2, are accessory proteins to mammalian odorant receptors (ORs). They are expressed in the olfactory sensory neurons and facilitate OR trafficking to the cell-surface membrane and ligand-induced responses in heterologous cells. We previously identified different domains in RTP1S that are important for different stages of OR trafficking, odorant-mediated responses, and interaction with ORs. However, the exact roles of RTP2 and the significance of the requirement of the seemingly redundant co-expression of the two RTP proteins in vivo have received less attention in the past. Here we attempted to dissect the functional differences between RTP1S and RTP2 using a HEK293T cell-based OR heterologous expression system. When a set of 24 ORs were tested against 28 cognate ligands, unlike RTP1S, which always showed a robust ability to support odorant-mediated responses, RTP2 had little or no effect on OR responses and exhibited a suppressive effect over that of RTP1S for a subset of the ORs tested. RTP1S and RTP2 showed no significant difference in OR ligand selectivity and co-transfection with RTP2 increased the detection threshold for some ORs. A protein-protein interaction analysis showed positive interactions among OR, RTP1S, and RTP2, corroborating the functional linkages among the three molecules. Finally, further cell-surface and permeabilized immunocytochemical studies revealed that OR and the co-expressed RTP1S proteins were retained in the Golgi when co-transfected with RTP2, indicating that RTP1S and RTP2 could play different roles in the OR trafficking process. By examining the functional differentiations between the two RTP family members, we provided a molecular level explanation to the suppressive effect exerted by RTP2, shedding light on the divergent mechanisms underlying the RTP proteins in regulating the functional expression of ORs.

In Vitro Mutational Analysis of the β2 Adrenergic Receptor, an In Vivo Surrogate Odorant Receptor

Many G-protein coupled receptors (GPCRs), such as odorant receptors (ORs), cannot be characterized in heterologous cells because of their difficulty in trafficking to the plasma membrane. In contrast, a surrogate OR, the GPCR mouse β2-adrenergic-receptor (mβ2AR), robustly traffics to the plasma membrane. We set out to characterize mβ2AR mutants in vitro for their eventual use in olfactory axon guidance studies. We performed an extensive mutational analysis of mβ2AR using a Green Fluorescent Protein-tagged mβ2AR (mβ2AR::GFP) to easily assess the extent of its plasma membrane localization. In order to characterize mutants for their ability to successfully transduce ligand-initiated signal cascades, we determined the half maximal effective concentrations (EC50) and maximal response to isoprenaline, a known mβ2AR agonist. Our analysis reveals that removal of amino terminal (Nt) N-glycosylation sites and the carboxy terminal (Ct) palmitoylation site of mβ2AR do not affect its plasma membrane localization. By contrast, when both the Nt and Ct of mβ2AR are replaced with those of M71 OR, plasma membrane trafficking is impaired. We further analyze three mβ2AR mutants (RDY, E268A, and C327R) used in olfactory axon guidance studies and are able to decorrelate their plasma membrane trafficking with their capacity to respond to isoprenaline. A deletion of the Ct prevents proper trafficking and abolishes activity, but plasma membrane trafficking can be selectively rescued by a Tyrosine to Alanine mutation in the highly conserved GPCR motif NPxxY. This new loss-of-function mutant argues for a model in which residues located at the end of transmembrane domain 7 can act as a retention signal when unmasked. Additionally, to our surprise, amongst our set of mutations only Ct mutations appear to lower mβ2AR EC50s revealing their critical role in G-protein coupling. We propose that an interaction between the Nt and Ct is necessary for proper folding and/or transport of GPCRs.

In vivo identification of eugenol-responsive and muscone-responsive mouse odorant receptors

Our understanding of mammalian olfactory coding has been impeded by the paucity of information about the odorant receptors (ORs) that respond to a given odorant ligand in awake, freely behaving animals. Identifying the ORs that respond in vivo to a given odorant ligand from among the ∼1100 ORs in mice is intrinsically challenging but critical for our understanding of olfactory coding at the periphery. Here, we report an in vivo assay that is based on a novel gene-targeted mouse strain, S100a5-tauGFP, in which a fluorescent reporter selectively marks olfactory sensory neurons that have been activated recently in vivo. Because each olfactory sensory neuron expresses a single OR gene, multiple ORs responding to a given odorant ligand can be identified simultaneously by capturing the population of activated olfactory sensory neurons and using expression profiling methods to screen the repertoire of mouse OR genes. We used this in vivo assay to re-identify known eugenol- and muscone-responsive mouse ORs. We identified additional ORs responsive to eugenol or muscone. Heterologous expression assays confirmed nine eugenol-responsive ORs (Olfr73, Olfr178, Olfr432, Olfr610, Olfr958, Olfr960, Olfr961, Olfr913, and Olfr1234) and four muscone-responsive ORs (Olfr74, Olfr235, Olfr816, and Olfr1440). We found that the human ortholog of Olfr235 and Olfr1440 responds to macrocyclic ketone and lactone musk odorants but not to polycyclic musk odorants or a macrocyclic diester musk odorant. This novel assay, called the Kentucky in vivo odorant ligand-receptor assay, should facilitate the in vivo identification of mouse ORs for a given odorant ligand of interest.

In vivo identification of eugenol-responsive and muscone-responsive mouse odorant receptors

Our understanding of mammalian olfactory coding has been impeded by the paucity of information about the odorant receptors (ORs) that respond to a given odorant ligand in awake, freely behaving animals. Identifying the ORs that respond in vivo to a given odorant ligand from among the ∼1100 ORs in mice is intrinsically challenging but critical for our understanding of olfactory coding at the periphery. Here, we report an in vivo assay that is based on a novel gene-targeted mouse strain, S100a5-tauGFP, in which a fluorescent reporter selectively marks olfactory sensory neurons that have been activated recently in vivo. Because each olfactory sensory neuron expresses a single OR gene, multiple ORs responding to a given odorant ligand can be identified simultaneously by capturing the population of activated olfactory sensory neurons and using expression profiling methods to screen the repertoire of mouse OR genes. We used this in vivo assay to re-identify known eugenol- and muscone-responsive mouse ORs. We identified additional ORs responsive to eugenol or muscone. Heterologous expression assays confirmed nine eugenol-responsive ORs (Olfr73, Olfr178, Olfr432, Olfr610, Olfr958, Olfr960, Olfr961, Olfr913, and Olfr1234) and four muscone-responsive ORs (Olfr74, Olfr235, Olfr816, and Olfr1440). We found that the human ortholog of Olfr235 and Olfr1440 responds to macrocyclic ketone and lactone musk odorants but not to polycyclic musk odorants or a macrocyclic diester musk odorant. This novel assay, called the Kentucky in vivo odorant ligand-receptor assay, should facilitate the in vivo identification of mouse ORs for a given odorant ligand of interest.

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.

High-throughput analysis of mammalian olfactory receptors: measurement of receptor activation via luciferase activity

Odorants create unique and overlapping patterns of olfactory receptor activation, allowing a family of approximately 1,000 murine and 400 human receptors to recognize thousands of odorants. Odorant ligands have been published for fewer than 6% of human receptors(1-11). This lack of data is due in part to difficulties functionally expressing these receptors in heterologous systems. Here, we describe a method for expressing the majority of the olfactory receptor family in Hana3A cells, followed by high-throughput assessment of olfactory receptor activation using a luciferase reporter assay. This assay can be used to (1) screen panels of odorants against panels of olfactory receptors; (2) confirm odorant/receptor interaction via dose response curves; and (3) compare receptor activation levels among receptor variants. In our sample data, 328 olfactory receptors were screened against 26 odorants. Odorant/receptor pairs with varying response scores were selected and tested in dose response. These data indicate that a screen is an effective method to enrich for odorant/receptor pairs that will pass a dose response experiment, i.e. receptors that have a bona fide response to an odorant. Therefore, this high-throughput luciferase assay is an effective method to characterize olfactory receptors-an essential step toward a model of odor coding in the mammalian olfactory system.

G protein-coupled receptors: what a difference a 'partner' makes

G protein-coupled receptors (GPCRs) are important cell signaling mediators, involved in essential physiological processes. GPCRs respond to a wide variety of ligands from light to large macromolecules, including hormones and small peptides. Unfortunately, mutations and dysregulation of GPCRs that induce a loss of function or alter expression can lead to disorders that are sometimes lethal. Therefore, the expression, trafficking, signaling and desensitization of GPCRs must be tightly regulated by different cellular systems to prevent disease. Although there is substantial knowledge regarding the mechanisms that regulate the desensitization and down-regulation of GPCRs, less is known about the mechanisms that regulate the trafficking and cell-surface expression of newly synthesized GPCRs. More recently, there is accumulating evidence that suggests certain GPCRs are able to interact with specific proteins that can completely change their fate and function. These interactions add on another level of regulation and flexibility between different tissue/cell-types. Here, we review some of the main interacting proteins of GPCRs. A greater understanding of the mechanisms regulating their interactions may lead to the discovery of new drug targets for therapy.

Homotypic and heterotypic adhesion induced by odorant receptors and the β2-adrenergic receptor

In the mouse olfactory system regulated expression of a large family of G Protein-Coupled Receptors (GPCRs), the Odorant Receptors (ORs), provides each sensory neuron with a single OR identity. In the wiring of the olfactory sensory neuron projections, a complex axon sorting process ensures the segregation of >1,000 subpopulations of axons of the same OR identity into homogeneously innervated glomeruli. ORs are critical determinants in axon sorting, and their presence on olfactory axons raises the intriguing possibility that they may participate in axonal wiring through direct or indirect trans-interactions mediating adhesion or repulsion between axons. In the present work, we used a biophysical assay to test the capacity of ORs to induce adhesion of cell doublets overexpressing these receptors. We also tested the β2 Adrenergic Receptor, a non-OR GPCR known to recapitulate the functions of ORs in olfactory axon sorting. We report here the first evidence for homo- and heterotypic adhesion between cells overexpressing the ORs MOR256-17 or M71, supporting the hypothesis that ORs may contribute to olfactory axon sorting by mediating differential adhesion between axons.

Cell-free expression of a mammalian olfactory receptor and unidirectional insertion into small unilamellar vesicles (SUVs)

Although the identification of the multigene family encoding mammalian olfactory receptors were identified more than 20 years ago, we are far from understanding olfactory perception because of the difficulties in functional expression of these receptors in heterologous cell systems. Cell-free (CF) or in vitro expression systems offer an elegant alternative route to cell based protein expression, as the functional expression of membrane proteins can be directly achieved from the genetic template without the need of cell cultivation and protein isolation. Here we investigated in detail the cell-free expression and membrane insertion of the olfactory receptor OR5 in dependence of different experimental conditions like probing different origins of the cell-free expression system (from bacteria, via plants and insects toward mammalian system) and lipid composition of the respective extracts. We provided substantial biochemical indications by radioactive labeling based on [(35)S]-methionine, followed by proteolytic digestion, and we found that the insertion of the olfactory receptor OR5 into liposomes resulted in an unidirectional orientation with the binding side exposed into the aqueous space, resembling the native orientation in the cilia of the olfactory neurons. We report the different results in synthesis capacity for the different in vitro systems employed as we like to demonstrate the first in vitro kit toward and ex situ and ex vivo odorant receptor array.

Immunohistochemical detection of TAS2R38 protein in human taste cells

The sense of taste plays an important role in the evaluation of the nutrient composition of consumed food. Bitter taste in particular is believed to serve a warning function against the ingestion of poisonous substances. In the past years enormous progress was made in the characterization of bitter taste receptors, including their gene expression patterns, pharmacological features and presumed physiological roles in gustatory as well as in non-gustatory tissues. However, due to a lack in TAS2R-specifc antibodies the localization of receptor proteins within gustatory tissues has never been analyzed. In the present study we have screened a panel of commercially available antisera raised against human bitter taste receptors by immunocytochemical experiments. One of these antisera was found to be highly specific for the human bitter taste receptor TAS2R38. We further demonstrate that this antibody is able to detect heterologously expressed TAS2R38 protein on Western blots. The antiserum is, however, not able to interfere significantly with TAS2R38 function in cell based calcium imaging analyses. Most importantly, we were able to demonstrate the presence of TAS2R38 protein in human gustatory papillae. Using double immunofluorescence we show that TAS2R38-positive cells form a subpopulation of PLCbeta2 expressing cells. On a subcellular level the localization of this bitter taste receptor is neither restricted to the cell surface nor particularly enriched at the level of the microvilli protruding into the pore region of the taste buds, but rather evenly distributed over the entire cell body.

Receptor-transporting protein 1 short (RTP1S) mediates translocation and activation of odorant receptors by acting through multiple steps

Odorant receptor (OR) proteins are retained in the endoplasmic reticulum when heterologously expressed in cultured cells of non-olfactory origins. RTP1S is an accessory protein to mammalian ORs and facilitates their trafficking to the cell-surface membrane and ligand-induced responses in heterologous cells. The mechanism by which RTP1S promotes the functional expression of ORs remains poorly understood. To obtain a better understanding of the role(s) of RTP1S, we performed a series of structure-function analyses of RTP1S in HEK293T cells. By constructing RTP1S deletion and chimera series and subsequently introducing single-site mutations into the protein, we found the N terminus of RTP1S is important for the endoplasmic reticulum exit of ORs and that a middle region of RTP1S is important for OR trafficking from the Golgi to the membrane. Using sucrose gradient centrifugation, we found that the localization of RTP1S to the lipid raft microdomain is critical to the activation of ORs. Finally, in a protein-protein interaction analysis, we determined that the C terminus of RTP1S may be interacting with ORs. These findings provide new insights into the distinct roles of RTP1S in OR translocation and activation.

RAMP like proteins : RTP and REEP family of proteins

Mammalian odorant receptors (ORs) are typically retained in the endoplasmic reticulum (ER) when expressed in heterologous cells. The RTP (Receptor-Transporting Protein) and REEP (Receptor Expression Enhancing Protein) family of proteins were first identified as partners for ORs, promoting cell-surface expression and leading to functional responses in heterologous cell systems. Like RAMPs, the RTP and REEP proteins appear to partner with GPCRs to promote cell-surface expression. Unlike RAMPs, they do not appear to alter the pharmacology of the partner receptor.

Regulated GPCR trafficking to the plasma membrane: general issues and the CCR5 chemokine receptor example

The regulated export of nascent G protein coupled receptors (GPCRs) from intracellular stores is an emerging concept with important implications in cell biology and pharmacology. This phenomenon requires a complex network of interactions between GPCRs with either chaperones and escort proteins or gatekeepers, which are respectively involved in the progression of GPCRs along the biosynthetic pathway to the plasma membrane or in their retention in intracellular compartments. The regulated export of GPCRs is also controlled by external stimuli and might represent an adaptive mechanism to specific physiological constraints, such as the sustained activation of the CCR5 chemokine receptor in the context of chemotaxis.

Trafficking of mammalian chemosensory receptors by receptor-transporting proteins

Although mammalian odorant receptors (ORs) were identified more than 15 years ago, we still do not understand how odorant molecules interact with ORs at a molecular level. Previous studies of mammalian ORs have tested few ORs against many odorants. Some fundamental properties of the olfactory system, however, require investigation of a wide panel of diverse ORs with many chemically diverse odorants. Previously, we identified OR accessory proteins, receptor-transporting protein (RTP) 1 and RTP2. They are expressed specifically in olfactory neurons, are associated with OR proteins, and facilitate the OR trafficking to the plasma membrane when coexpressed in mammalian cell lines. With this approach, high-throughput screening using a large repertoire of mammalian ORs is now possible. The activation profiles can be used to develop a predictive model relating physicochemical odorant properties, receptor sequences, and their interactions, enabling us to predict a tested receptor's response to a novel odorant and a novel receptor's response to a tested odorant. Doing so will provide a basis for understanding how structurally diverse odorant molecules activate the mammalian OR repertoire. Similarly, two families of vomeronasal receptors, V1Rs and V2Rs, are also notoriously difficult to functionally express in heterologous cells. However, coexpression of the RTP family members with V1Rs or V2Rs does not seem to facilitate trafficking of the receptor proteins. This finding suggests that the vomeronasal organ has a unique biosynthetic pathway for membrane proteins.

Evaluating cell-surface expression and measuring activation of mammalian odorant receptors in heterologous cells

A fundamental question in olfaction is which odorant receptors (ORs) are activated by a given odorant. A major roadblock to investigating odorant-OR relationships in mammals has been the inability to express ORs in heterologous cells suitable for screening active ligands for ORs. The discovery of the receptor-transporting protein family has facilitated the effective cell-surface expression of ORs in heterologous cells. The establishment of a robust heterologous expression system for mammalian ORs facilitates the high-throughput 'deorphanization' of these receptors by matching them to their cognate ligands. This protocol details the method used for evaluating the cell-surface expression and measuring the functional activation of ORs of transiently expressed mammalian ORs in HEK293T cells. The stages of OR cell-surface expression include cell culture preparation, transfer of cells, transfection, immunocytochemistry or flow cytometry, odorant stimulation and luciferase assay. This protocol can be completed in a period of 3 d from the transfer of cells to cell-surface expression detection and/or measurement of functional activation.

Expressing exogenous functional odorant receptors in cultured olfactory sensory neurons

BACKGROUND

Olfactory discrimination depends on the large numbers of odorant receptor genes and differential ligand-receptor signaling among neurons expressing different receptors. In this study, we describe an in vitro system that enables the expression of exogenous odorant receptors in cultured olfactory sensory neurons. Olfactory sensory neurons in the culture express characteristic signaling molecules and, therefore, provide a system to study receptor function within its intrinsic cellular environment.

RESULTS

We demonstrate that cultured olfactory sensory neurons express endogenous odorant receptors. Lentiviral vector-mediated gene transfer enables successful ectopic expression of odorant receptors. We show that the ectopically expressed mouse I7 is functional in the cultured olfactory sensory neurons. When two different odorant receptors are ectopically expressed simultaneously, both receptor proteins co-localized in the same olfactory sensory neurons up to 10 days in vitro.

CONCLUSION

This culture technique provided an efficient method to culture olfactory sensory neurons whose morphology, molecular characteristics and maturation progression resembled those observed in vivo. Using this system, regulation of odorant receptor expression and its ligand specificity can be studied in its intrinsic cellular environment.

Myr-Ric-8A enhances G(alpha15)-mediated Ca2+ response of vertebrate olfactory receptors

The determination of ligand specificities of odorant receptors will contribute to the understanding of how odorants are discriminated by the olfactory system. To date, the ways in which some olfactory receptors (ORs) pair with their cognate ligands has been studied using a Ca(2+) imaging technique. This approach has been used to investigate orphan G protein-coupled receptors expressed in heterologous cells; however, most attempts to functionally express ORs on the cell surface of heterologous cells have failed. Recently, receptor-transporting protein 1 and Ric-8B have been identified as proteins involved in targeting receptors to the cell membrane and amplifying receptor signals, and thus, they are able to facilitate cellular responses via ORs in a heterologous cell system. Here, we describe a technique in which we employed a myristoylation sequence-conjugated mutant of Ric-8A (Myr-Ric-8A) as a signal amplifier and show Myr-Ric-8A greatly enhances G(alpha15)-mediated Ca(2+) responses of ORs in HEK293 cells. Coexpression of Myr-Ric-8A enabled us to deorphanize a mouse OR and to determine its molecular receptive range. Our results suggest that Myr-Ric-8A should be helpful in functional characterization of ORs in heterologous cells using Ca(2+) imaging.

On a chip demonstration of a functional role for Odorant Binding Protein in the preservation of olfactory receptor activity at high odorant concentration

The molecular mechanisms underlying odorant detection have been investigated using the chip based SPR technique by focusing on the dynamic interactions between transmembrane Olfactory Receptor OR1740, odorant ligands and soluble Odorant-Binding Protein (OBP-1F). The OR1740 present in the lipid bilayer of nanosomes derived from transformed yeasts specifically bound OBP-1F. The receptor preferential odorant ligand helional released bound OBP-1F from the OR-OBP complex, while unrelated odorants failed to do so. OBP-1F modified the functional OR1740 dose-response to helional, from a bell-shaped to a saturation curve, thus preserving OR activity at high ligand concentration. This unravels an active role for OBPs in olfaction, in addition to passive transport or a scavenger role. This sensorchip technology was applied to assessing native OBP-1F in a biological sample: rat olfactory mucus also displayed significant binding to OR1740 nanosomes, and the addition of helional yielded the dissociation of mucus OBP from the receptor.

Specificity of Olfactory Receptor Interactions with Other G Protein-coupled Receptors

Studies on olfactory receptor (OR) pharmacology have been hindered by the poor plasma membrane localization of most ORs in heterologous cells. We previously reported that association with the beta(2)-adrenergic receptor (beta(2)-AR) facilitates functional expression of the OR M71 at the plasma membrane of HEK-293 cells. In the present study, we examined the specificity of M71 interactions with other G protein-coupled receptors (GPCRs). M71 was co-expressed in HEK-293 cells with 42 distinct GPCRs, and the vast majority of these receptors had no significant effect on M71 surface expression. However, co-expression with three subtypes of purinergic receptor (P2Y(1)R, P2Y(2)R, and A(2A)R) resulted in markedly enhanced plasma membrane localization of M71. Agonist stimulation of M71 co-expressed with P2Y(1)R and P2Y(2)R activated the mitogen-activated protein kinase pathway via coupling of M71 to Galpha(o). We also examined the ability of beta(2)-AR, P2Y(1)R, P2Y(2)R, and A(2A)Rto interact with and regulate ORs beyond M71. We found that co-expression of beta(2)-AR or the purinergic receptors enhanced the surface expression for an M71 subfamily member but not for several other ORs from different subfamilies. In addition, through chimeric receptor studies, we determined that the second transmembrane domain of beta(2)-AR is necessary for beta(2)-AR facilitation of M71 plasma membrane localization. These studies shed light on the specificity of OR interactions with other GPCRs and the mechanisms governing olfactory receptor trafficking.

Synergism of Accessory Factors in Functional Expression of Mammalian Odorant Receptors

The discovery of odorant receptors led to endeavors in matching them with their cognate ligands. Although it has been challenging to functionally express odorant receptors in heterologous cells, previous studies have linked efficient odorant receptor expression with N-terminal modifications and accessory proteins, including the receptor-transporting proteins (RTPs) and Ric8b. Here we have shown that a shorter form of RTP1, RTP1S, supports robust cell-surface and functional expression of representative odorant receptors. Using a combination of accessory proteins, including RTP1S, Ric8b, and G(alphaolf), a diverse set of untagged odorant receptors were successfully expressed heterologously due to the synergistic effects among the various accessory proteins. Furthermore, the addition of an N-terminal rhodopsin tag to the odorant receptors, along with the same set of accessory proteins, exhibits an additional level of synergism, inducing enhanced odorant receptor responses to odorants and thus defining a more efficient heterologous expression system. We then showed that the presence or absence of different N-terminal tags has little effect on the ligand specificity of odorant receptors, although the amount of receptor expressed can play a role in the ligand response profile. The accuracy of the odorant receptor heterologous expression system involving tagged odorant receptors and various accessory proteins promises success in high throughput de-orphaning of mammalian odorant receptors.

Members of RTP and REEP gene families influence functional bitter taste receptor expression

Functional characterization of chemosensory receptors is usually achieved by heterologous expression in mammalian cell lines. However, many chemoreceptor genes, including bitter taste receptors (TAS2Rs), show only marginal cell surface expression. Usually, these problems are circumvented by using chimeric receptors consisting of "export tags" and the receptor sequence itself. It seems likely that chemoreceptor cells express factors for cell surface targeting of native receptor molecules in vivo. For TAS2Rs, however, such factors are still unknown. The present study investigates the influence of RTP and REEP proteins on the functional expression of human TAS2Rs in heterologous cells. We expressed hTAS2Rs in HEK 293T cells and observed dramatic differences in responsiveness to agonist stimulation. By immunocytochemistry we show accumulation of the bitter beta-glucopyranoside receptor hTAS2R16 in the Golgi compartment. Coexpression of RTP and REEP proteins changed the responses of some hTAS2Rs upon agonist stimulation, which is likely due to efficient cell surface localization as demonstrated by cell surface biotinylation experiments. The coimmunoprecipitation of hTAS2R16 and RTP3 or RTP4 suggests that the mechanism by which these cofactors influence hTAS2R16 function might involve direct protein-protein interaction. Finally, expression analyses demonstrate RTP and REEP gene expression in human circumvallate papillae and testis, both of which are sites of TAS2R gene expression.

Odorant receptors directly activate phospholipase C/inositol-1,4,5-trisphosphate coupled to calcium influx in Odora cells

Mechanisms by which odorants activate signaling pathways in addition to cAMP are hard to evaluate in heterogeneous mixtures of primary olfactory neurons. We used single cell calcium imaging to analyze the response to odorant through odorant receptor (OR) U131 in the olfactory epithelial cell line Odora (Murrell and Hunter 1999), a model system with endogenous olfactory signaling pathways. Because adenylyl cyclase levels are low, agents activating cAMP formation do not elevate calcium, thus unmasking independent signaling mediated by OR via phospholipase C (PLC), inositol-1,4,5-trisphosphate (IP(3)), and its receptor. Unexpectedly, we found that extracellular calcium is required for odor-induced calcium elevation without the release of intracellular calcium, even though the latter pathway is intact and can be stimulated by ATP. Relevant signaling components of the PLC pathway and G protein isoforms are identified by western blot in Odora cells as well as in olfactory sensory neurons (OSNs), where they are localized to the ciliary zone or cell bodies and axons of OSNs by immunohistochemistry. Biotinylation studies establish that IP(3) receptors type 2 and 3 are at the cell surface in Odora cells. Thus, individual ORs are capable of elevating calcium through pathways not directly mediated by cAMP and this may provide another avenue for odorant signaling in the olfactory system.

Characterization of an extended receptive ligand repertoire of the human olfactory receptor OR17-40 comprising structurally related compounds

Molecular properties of odorant compounds essential for activation of the human olfactory receptor hOR17-40 were investigated using a collection of 23 variants of its cognate ligand helional. Coupling receptor activation to an optically detectable intracellular Ca(2+) ion flux allowed dose-dependent screening of different odorant molecules in human embryonic kidney (HEK)293 cells. We found an extended collection of activating ligands and provide first evidence for hOR17-40-specific antagonists. The C-terminal fusion of enhanced green fluorescent protein to the hOR17-40 retained full receptor function and permitted the selection of cells with defined receptor expression levels, which was an essential step for optimizing our screening protocol. Interestingly, cells with a low EGFP fluorescence intensity exhibited efficient hOR17-40 cell surface targeting and odorant-evoked signal transduction; in contrast, highly fluorescent cells displayed mainly incorrectly targeted, intracellular receptors. Fluorescence-activated cell sorting was used to separate hOR17-40-expressing cells on the basis of their endogenous EGFP fluorescence intensity, thereby increasing the fraction of odorant-responsive cells to up to 80% of the total cell number.

Molecular biology of insect olfaction: recent progress and conceptual models

Insects have an enormous impact on global public health as disease vectors and as agricultural enablers as well as pests and olfaction is an important sensory input to their behavior. As such it is of great value to understand the interplay of the molecular components of the olfactory system which, in addition to fostering a better understanding of insect neurobiology, may ultimately aid in devising novel intervention strategies to reduce disease transmission or crop damage. Since the first discovery of odorant receptors in vertebrates over a decade ago, much of our view on how the insect olfactory system might work has been derived from observations made in vertebrates and other invertebrates, such as lobsters or nematodes. Together with the advantages of a wide range of genetic tools, the identification of the first insect odorant receptors in Drosophila melanogaster in 1999 paved the way for rapid progress in unraveling the question of how olfactory signal transduction and processing occurs in the fruitfly. This review intends to summarize much of this progress and to point out some areas where advances can be expected in the near future.

Heterodimerization of G Protein-Coupled Receptors: Specificity and Functional Significance

G protein-coupled receptors (GPCRs) are cell surface receptors that mediate physiological responses to a diverse array of stimuli. GPCRs have traditionally been thought to act as monomers, but recent evidence suggests that GPCRs may form dimers (or higher-order oligomers) as part of their normal trafficking and function. In fact, certain GPCRs seem to have a strict requirement for heterodimerization to attain proper surface expression and functional activity. Even those GPCRs that do not absolutely require heterodimerization may still specifically associate with other GPCR subtypes, sometimes resulting in dramatic effects on receptor pharmacology, signaling, and/or internalization. Understanding the specificity and functional significance of GPCR heterodimerization is of tremendous clinical importance since GPCRs are the molecular targets for numerous therapeutic drugs.

Functional characterization of two human olfactory receptors expressed in the baculovirus Sf9 insect cell system

Olfactory receptors (ORs) are the largest member of the G-protein-coupled receptors which mediate early olfactory perception in discriminating among thousands of odorant molecules. Assigning odorous ligands to ORs is a prerequisite to gaining an understanding of the mechanisms of odorant recognition. The functional expression of ORs represents a critical step in addressing this issue. Due to limitations in heterologous expression, very few mammal ORs have been characterized, and so far only one is from human origin. Consequently, OR function still remains poorly understood, especially in humans, whose genome encodes a restricted chemosensory repertoire compared with most mammal species. In this study, we have designed cassette baculovirus vectors to coexpress human OR 17-209 or OR 17-210 with either G(alpha olf) or G(alpha16) proteins in Sf9 cells. Each OR was found to be expressed at the cell surface and colocalized with both G(alpha) proteins. Using Ca2+ imaging, we showed that OR 17-209 and OR 17-210 proteins are activated by esters and ketones respectively. Odorant-induced calcium response was increased when ORs were coexpressed with G(alpha16) protein, whereas coexpression with G(alpha olf) abolished calcium signaling. This strategy has been found to overcome most of the limitations encountered when expressing an OR protein and has permitted odorant screening of functional ORs. Our approach could thus be of interest for further expression and ligand assignment of other orphan receptor proteins.

Heterodimerization with β2-Adrenergic Receptors Promotes Surface Expression and Functional Activity of α1D-Adrenergic Receptors

The alpha1D-adrenergic receptor (alpha1D-AR) is a G protein-coupled receptor (GPCR) that is poorly trafficked to the cell surface and largely nonfunctional when heterologously expressed by itself in a variety of cell types. We screened a library of approximately 30 other group I GPCRs in a quantitative luminometer assay for the ability to promote alpha1D-AR cell surface expression. Strikingly, these screens revealed only two receptors capable of inducing robust increases in the amount of alpha1D-AR at the cell surface: alpha1B-AR and beta2-AR. Confocal imaging confirmed that coexpression with beta2-AR resulted in translocation of alpha1D-AR from intracellular sites to the plasma membrane. Additionally, coimmunoprecipitation studies demonstrated that alpha1D-AR and beta2-AR specifically interact to form heterodimers when coexpressed in HEK-293 cells. Ligand binding studies revealed an increase in total alpha1D-AR binding sites upon coexpression with beta2-AR, but no apparent effect on the pharmacological properties of the receptors. In functional studies, coexpression with beta2-AR significantly enhanced the coupling of alpha1D-AR to norepinephrine-stimulated Ca2+ mobilization. Heterodimerization of beta2-AR with alpha1D-AR also conferred the ability of alpha1D-AR to cointernalize upon beta2-AR agonist stimulation, revealing a novel mechanism by which these different adrenergic receptor subtypes may regulate each other's activity. These findings demonstrate that the selective association of alpha1D-AR with other receptors is crucial for receptor surface expression and function and also shed light on a novel mechanism of cross talk between alpha1- and beta2-ARs that is mediated through heterodimerization and cross-internalization.

Functional expression of olfactory receptors in yeast and development of a bioassay for odorant screening

The functional expression of olfactory receptors (ORs) is a primary requirement to examine the molecular mechanisms of odorant perception and coding. Functional expression of the rat I7 OR and its trafficking to the plasma membrane was achieved under optimized experimental conditions in the budding yeast Saccharomyces cerevisiae. The membrane expression of the receptor was shown by Western blotting and immunolocalization methods. Moreover, we took advantage of the functional similarities between signal transduction cascades of G protein-coupled receptor in mammalian cells and the pheromone response pathway in yeast to develop a novel biosensor for odorant screening using luciferase as a functional reporter. Yeasts were engineered to coexpress I7 OR and mammalian G(alpha) subunit, to compensate for the lack of endogenous Gpa1 subunit, so that stimulation of the receptor by its ligands activates a MAP kinase signaling pathway and induces luciferase synthesis. The sensitivity of the bioassay was significantly enhanced using mammalian G(olf) compared to the G(alpha15) subunit, resulting in dose-dependent responses of the system. The biosensor was probed with an array of odorants to demonstrate that the yeast-borne I7 OR retains its specificity and selectivity towards ligands. The results are confirmed by functional expression and bioluminescence response of human OR17-40 to its specific ligand, helional. Based on these findings, the bioassay using the luciferase reporter should be amenable to simple, rapid and inexpensive odorant screening of hundreds of ORs to provide insight into olfactory coding mechanisms.

RTP Family Members Induce Functional Expression of Mammalian Odorant Receptors

Transport of G protein-coupled receptors (GPCRs) to the cell surface membrane is critical in order for the receptors to recognize their ligands. However, mammalian GPCR odorant receptors (ORs), when heterologously expressed in cells, are poorly expressed on the cell surface. Here we show that the transmembrane proteins RTP1 and RTP2 promote functional cell surface expression of ORs expressed in HEK293T cells. Genes encoding these proteins are expressed specifically in olfactory neurons. These proteins are associated with OR proteins and enhance the OR responses to odorants. Similar although weaker effects were seen with a third protein, REEP1. These findings suggest that RTP1 and RTP2 in particular play significant roles in the translocation of ORs to the plasma membrane as well as in the functioning of ORs. We have used this approach to identify active odorant ligands for ORs, providing a platform for screening the chemical selectivity of the large OR family.

Trafficking prerogatives of olfactory receptors

Olfactory receptors lead lives of exclusivity and privilege, the monarchs of fiefdoms organized solely to carry out their instructions. Each olfactory sensory neuron expresses one allele of one of approximately 1000 olfactory receptor genes. It is thought that olfactory receptor diversity is critical for the ability of animals to detect many thousands of odorants, but supporting functional evidence has been difficult to obtain because olfactory receptors expressed in heterologous cells are typically retained in the endoplasmic reticulum. The membrane trafficking entitlements enjoyed by olfactory receptors appear to be available only in mature olfactory sensory neurons. Evidence is accumulating that cell-type-specific accessory proteins regulate first the exit of olfactory receptors from the endoplasmic reticulum, and then the trafficking of olfactory receptors from post-Golgi compartments to the plasma membrane of the olfactory cilia where the receptors gain access to odorants. Critical olfactory receptor accessory proteins are known only in the nematode Caenorhabditis elegans, where the absence of a novel protein called ODR-4 or a clathrin adaptor, UNC-101, interferes with proper trafficking. Similar functional specificity also occurs in a parallel chemosensory system, the mammalian vomeronasal organ. Trafficking of the V2R type of vomeronasal receptors is mediated by a vomeronasal-specific family of major histocompatibility complex proteins. Removal of olfactory receptors from the plasma membrane may be regulated in a more conventional fashion because odor stimulation has been linked to receptor phosphorylation, to the function of G-protein coupled receptor kinase 3, and to an increase in vesicles retrieved from the plasma membrane.

Endoplasmic reticulum retention, degradation, and aggregation of olfactory G-protein coupled receptors

The mammalian olfactory G-protein coupled receptor family is comprised of hundreds of proteins that mediate odorant binding and initiate signal transduction cascades leading to the sensation of smell. However, efforts to functionally express olfactory receptors and identify specific odorant ligand-olfactory receptor interactions have been severely impeded by poor olfactory receptor surface expression in heterologous systems. Therefore, experiments were performed to elucidate the cellular mechanism(s) responsible for inefficient olfactory receptor cell surface expression. We determined that the mouse odorant receptors mI7 and mOREG are not selected for export from the ER and therefore are not detectable at the Golgi apparatus or plasma membrane. Specifically, olfactory receptors interact with the ER chaperone calnexin, are excluded from ER export sites, do not accumulate in ER-Golgi transport intermediates at 15 degrees C, and contain endoglycosidase H-sensitive oligosaccharides, consistent with olfactory receptor exclusion from post-ER compartments. A labile pool of ER-retained olfactory receptors are post-translationally modified by polyubiquitination and targeted for degradation by the proteasome. In addition, olfactory receptors are sequestered into ER aggregates that are degraded by autophagy. Collectively, these data demonstrate that poor surface expression of olfactory receptors in heterologous cells is attributable to a combination of ER retention due to inefficient folding and poor coupling to ER export machinery, aggregation, and degradation via both proteasomal and autophagic pathways.