The β2-adrenergic receptor as a surrogate odorant receptor in mouse olfactory sensory neurons

The Zonal Organization of Odorant Receptor Gene Choice in the Main Olfactory Epithelium of the Mouse

A mature olfactory sensory neuron (OSN) of the main olfactory epithelium (MOE) typically expresses one allele of one odorant receptor (OR) gene. It is widely thought that the great majority of the 1,141 intact mouse OR genes are expressed in one of four MOE zones (or bands or stripes), which are largely non-overlapping. Here, we develop a multiplex method to map, in 3D and MOE-wide, the expression areas of multiple OR genes in individual, non-genetically modified mice by three-color fluorescence in situ hybridization, semi-automated image segmentation, and 3D reconstruction. We classify the expression areas of 68 OR genes into 9 zones. These zones are highly overlapping and strikingly complex when viewed in 3D reconstructions. There could well be more zones. We propose that zones reflect distinct OSN types that are each restricted in their choice to a subset of the OR gene repertoire.

G-protein coupled receptors Mc4r and Drd1a can serve as surrogate odorant receptors in mouse olfactory sensory neurons

In the mouse, most mature olfactory sensory neurons (OSNs) express one allele of one gene from the repertoire of ~1100 odorant receptor (OR) genes, which encode G-protein coupled receptors (GPCRs). Axons of OSNs that express a given OR coalesce into homogeneous glomeruli, which reside at conserved positions in the olfactory bulb. ORs are intimately involved in ensuring the expression of one OR per OSN and the coalescence of OSN axons into glomeruli. But the mechanisms whereby ORs accomplish these diverse functions remain poorly understood. An experimental approach that has been informative is to substitute an OR genetically with another GPCR that is normally not expressed in OSNs, in order to determine in which aspects this GPCR can serve as surrogate OR in mouse OSNs. Thus far only the β2-adrenergic receptor (β2AR, Ardb2) has been shown to be able to serve as surrogate OR in OSNs; the β2AR could substitute for the M71 OR in all aspects examined. Can other non-olfactory GPCRs function equally well as surrogate ORs in OSNs? Here, we have generated and characterized two novel gene-targeted mouse strains in which the mouse melanocortin 4 receptor (Mc4r) or the mouse dopamine receptor D1 (Drd1a) is coexpressed with tauGFP in OSNs that express the OR locus M71. These alleles and strains are abbreviated as Mc4r → M71-GFP and Drd1a → M71-GFP. We detected strong Mc4r or Drd1a immunoreactivity in axons and dendritic knobs and cilia of OSNs that express Mc4r or Drd1a from the M71 locus. These OSNs responded physiologically to cognate agonists for Mc4r (Ro27-3225) or Drd1a (SKF81297), and not to the M71 ligand acetophenone. Axons of OSNs expressing Mc4r → M71-GFP coalesced into glomeruli. Axons of OSNs expressing Drd1a → M71-GFP converged onto restricted areas of the olfactory bulb but did not coalesce into glomeruli. Thus, OR functions in OSNs can be substituted by Mc4r or Drd1a, but not as well as by β2AR. We attribute the weak performance of Drd1a as surrogate OR to poor OSN maturation.

Odorant receptors can mediate axonal identity and gene choice via cAMP-independent mechanisms

Odorant receptors (ORs) control several aspects of cell fate in olfactory sensory neurons (OSNs), including singular gene choice and axonal identity. The mechanisms of OR-induced axon guidance have been suggested to principally rely on G-protein signalling. Here, we report that for a subset of OSNs, deleting G proteins or altering their levels of signalling does not affect axonal identity. Signalling-deficient ORs or surrogate receptors that are unable to couple to Gs/Golf still provide axons with distinct identities and the anterior–posterior targeting of axons does not correlate with the levels of cAMP produced by genetic modifications. In addition, we refine the models of negative feedback by showing that ectopic ORs can be robustly expressed without suppressing endogenous gene choice. In conclusion, our results uncover a new feature of ORs, showing that they can instruct axonal identity and regulate olfactory map formation independent of canonical G-protein signalling and cAMP production.

Potential therapeutic effects of odorants through their ectopic receptors in pigmented cells

Several olfactory receptors (ORs) have been characterized outside the olfactory neuroepithelium in neuronal and non-neuronal tissues, where they were implicated in the recognition of diverse chemical signals. ORs have been found to regulate melanogenesis in skin melanocytes, and OR expression has been found in the human brain nigrostriatal dopaminergic neurons, where production of melanin occurs as neuromelanin and can change with age; OR expression is downregulated in Parkinson's disease. Therefore, we propose several odorants as new functional ligands to ORs expressed in non-olfactory pigmented cells as dopaminergic neurons and melanocytes, where, by acting on cAMP-induced melanin production, they could help to counteract melanogenic and neurodegenerative dysfunctions, including melanoma and Parkinson's disease.

Olfactory receptor accessory proteins play crucial roles in receptor function and gene choice

Each of the olfactory sensory neurons (OSNs) chooses to express a single G protein-coupled olfactory receptor (OR) from a pool of hundreds. Here, we show the receptor transporting protein (RTP) family members play a dual role in both normal OR trafficking and determining OR gene choice probabilities. Rtp1 and Rtp2 double knockout mice (RTP1,2DKO) show OR trafficking defects and decreased OSN activation. Surprisingly, we discovered a small subset of the ORs are expressed in larger numbers of OSNs despite the presence of fewer total OSNs in RTP1,2DKO. Unlike typical ORs, some overrepresented ORs show robust cell surface expression in heterologous cells without the co-expression of RTPs. We present a model in which developing OSNs exhibit unstable OR expression until they choose to express an OR that exits the ER or undergo cell death. Our study sheds light on the new link between OR protein trafficking and OR transcriptional regulation.

Expression of homeobox genes in the mouse olfactory epithelium

Homeobox genes constitute a large family of genes widely studied because of their role in the establishment of the body pattern. However, they are also involved in many other events during development and adulthood. The main olfactory epithelium (MOE) is an excellent model to study neurogenesis in the adult nervous system. Analyses of homeobox genes during development show that some of these genes are involved in the formation and establishment of cell diversity in the MOE. Moreover, the mechanisms of expression of odorant receptors (ORs) constitute one of the biggest enigmas in the field. Analyses of OR promoters revealed the presence of homeodomain binding sites in their sequences. Here we characterize the expression patterns of a set of 49 homeobox genes in the MOE with in situ hybridization. We found that seven of them (Dlx3, Dlx5, Dlx6, Msx1, Meis1, Isl1, and Pitx1) are zonally expressed. The homeobox gene Emx1 is expressed in three guanylate cyclase(+) populations, two located in the MOE and the third one in an olfactory subsystem known as Grüneberg ganglion located at the entrance of the nasal cavity. The homeobox gene Tshz1 is expressed in a unique patchy pattern across the MOE. Our findings provide new insights to guide functional studies that aim to understand the complexity of transcription factor expression and gene regulation in the MOE. J. Comp. Neurol. 524:2713-2739, 2016. © 2016 Wiley Periodicals, Inc.

Rapid and continuous activity-dependent plasticity of olfactory sensory input

Incorporation of new neurons enables plasticity and repair of circuits in the adult brain. Adult neurogenesis is a key feature of the mammalian olfactory system, with new olfactory sensory neurons (OSNs) wiring into highly organized olfactory bulb (OB) circuits throughout life. However, neither when new postnatally generated OSNs first form synapses nor whether OSNs retain the capacity for synaptogenesis once mature, is known. Therefore, how integration of adult-born OSNs may contribute to lifelong OB plasticity is unclear. Here, we use a combination of electron microscopy, optogenetic activation and in vivo time-lapse imaging to show that newly generated OSNs form highly dynamic synapses and are capable of eliciting robust stimulus-locked firing of neurons in the mouse OB. Furthermore, we demonstrate that mature OSN axons undergo continuous activity-dependent synaptic remodelling that persists into adulthood. OSN synaptogenesis, therefore, provides a sustained potential for OB plasticity and repair that is much faster than OSN replacement alone.

The extremely broad odorant response profile of mouse olfactory sensory neurons expressing the odorant receptor MOR256-17 includes trace amine-associated receptor ligands

The mouse olfactory system employs ~1100 G‐protein‐coupled odorant receptors (ORs). Each mature olfactory sensory neuron (OSN) is thought to express just one OR gene, and the expressed OR determines the odorant response properties of the OSN. The broadest odorant response profile thus far demonstrated in native mouse OSNs is for OSNs that express the OR gene SR1 (also known as Olfr124 and MOR256‐3). Here we showed that the odorant responsiveness of native mouse OSNs expressing the OR gene MOR256‐17 (also known as Olfr15 and OR3) is even broader than that of OSNs expressing SR1. We investigated the electrophysiological properties of green fluorescent protein (GFP)+ OSNs in a MOR256‐17‐IRES‐tauGFP gene‐targeted mouse strain, in parallel with GFP+ OSNs in the SR1‐IRES‐tauGFP gene‐targeted mouse strain that we previously reported. Of 35 single chemical compounds belonging to distinct structural classes, MOR256‐17+ OSNs responded to 31 chemicals, compared with 10 for SR1+ OSNs. The 10 compounds that activated SR1+ OSNs also activated MOR256‐17+ OSNs. Interestingly, MOR256‐17+ OSNs were activated by three amines (cyclohexylamine, isopenthylamine, and phenylethylamine) that are typically viewed as ligands for chemosensory neurons in the main olfactory epithelium that express trace amine‐associated receptor genes, a family of 15 genes encoding G‐protein‐coupled receptors unrelated in sequence to ORs. We did not observe differences in membrane properties, indicating that the differences in odorant response profiles between the two OSN populations were due to the expressed OR. MOR256‐17+ OSNs appear to be at one extreme of odorant responsiveness among populations of OSNs expressing distinct OR genes in the mouse.

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.

Monoallelic expression of olfactory receptors

The sense of smell collects vital information about the environment by detecting a multitude of chemical odorants. Breadth and sensitivity are provided by a huge number of chemosensory receptor proteins, including more than 1,400 olfactory receptors (ORs). Organizing the sensory information generated by these receptors so that it can be processed and evaluated by the central nervous system is a major challenge. This challenge is overcome by monogenic and monoallelic expression of OR genes. The single OR expressed by each olfactory sensory neuron determines the neuron's odor sensitivity and the axonal connections it will make to downstream neurons in the olfactory bulb. The expression of a single OR per neuron is accomplished by coupling a slow chromatin-mediated activation process to a fast negative-feedback signal that prevents activation of additional ORs. Singular OR activation is likely orchestrated by a network of interchromosomal enhancer interactions and large-scale changes in nuclear architecture.

Linear correlation between the number of olfactory sensory neurons expressing a given mouse odorant receptor gene and the total volume of the corresponding glomeruli in the olfactory bulb

Chemosensory specificity in the main olfactory system of the mouse relies on the expression of ∼1,100 odorant receptor (OR) genes across millions of olfactory sensory neurons (OSNs) in the main olfactory epithelium (MOE), and on the coalescence of OSN axons into ∼3,600 glomeruli in the olfactory bulb. A traditional approach for visualizing OSNs and their axons consists of tagging an OR gene genetically with an axonal marker that is cotranslated with the OR by virtue of an internal ribosome entry site (IRES). Here we report full cell counts for 15 gene‐targeted strains of the OR‐IRES‐marker design coexpressing a fluorescent protein. These strains represent 11 targeted OR genes, a 1% sample of the OR gene repertoire. We took an empirical, “count every cell” strategy: we counted all fluorescent cell profiles with a nuclear profile within the cytoplasm, on all serial coronal sections under a confocal microscope, a total of 685,673 cells in 56 mice at postnatal day 21. We then applied a strain‐specific Abercrombie correction to these OSN counts in order to obtain a closer approximation of the true OSN numbers. We found a 17‐fold range in the average (corrected) OSN number across these 11 OR genes. In the same series of coronal sections, we then determined the total volume of the glomeruli (TGV) formed by coalescence of the fluorescent axons. We found a strong linear correlation between OSN number and TGV, suggesting that TGV can be used as a surrogate measurement for estimating OSN numbers in these gene‐targeted strains. J. Comp. Neurol. 524:199–209, 2016. © 2015 Wiley Periodicals, Inc.

G protein-coupled odorant receptors underlie mechanosensitivity in mammalian olfactory sensory neurons

Mechanosensitive cells are essential for organisms to sense the external and internal environments, and a variety of molecules have been implicated as mechanical sensors. Here we report that odorant receptors (ORs), a large family of G protein-coupled receptors, underlie the responses to both chemical and mechanical stimuli in mouse olfactory sensory neurons (OSNs). Genetic ablation of key signaling proteins in odor transduction or disruption of OR-G protein coupling eliminates mechanical responses. Curiously, OSNs expressing different OR types display significantly different responses to mechanical stimuli. Genetic swap of putatively mechanosensitive ORs abolishes or reduces mechanical responses of OSNs. Furthermore, ectopic expression of an OR restores mechanosensitivity in loss-of-function OSNs. Lastly, heterologous expression of an OR confers mechanosensitivity to its host cells. These results indicate that certain ORs are both necessary and sufficient to cause mechanical responses, revealing a previously unidentified mechanism for mechanotransduction.

Trpc2-expressing sensory neurons in the main olfactory epithelium of the mouse

The mouse olfactory system contains two distinct chemosensory epithelia, the main olfactory epithelium (MOE) and the vomeronasal epithelium (VNE). Their sensory neurons express odorant receptor genes and vomeronasal receptor genes, respectively, and differ fundamentally in their signal transduction pathways. Genes required for chemosensory transduction are the cyclic nucleotide-gated channel subunit Cnga2 and the transient receptor potential cation channel Trpc2, respectively. Here, we document two previously unrecognized types of Trpc2+ neurons in the MOE of mice of various ages, including adults. These cell types express Cnga2 and can be distinguished by expression of adenylate cyclase Adcy3 (positive: type A; negative: type B). A third of MOE neurons that express the odorant receptor genes Olfr68/Olfr69 coexpress Trpc2 and are type A cells. In Trpc2-IRES-taulacZ gene-targeted mice, some labeled axons coalesce into glomeruli in the main olfactory bulb. Our findings have implications for the conventional VNE-centric interpretation of the behavioral phenotypes of Trpc2 knockout mice.

G-protein-coupled receptors, Hedgehog signaling and primary cilia

The Hedgehog (Hh) pathway has become an important model to study the cell biology of primary cilia, and reciprocally, the study of ciliary processes provides an opportunity to solve longstanding mysteries in the mechanism of vertebrate Hh signal transduction. The cilium is emerging as an unique compartment for G-protein-coupled receptor (GPCR) signaling in many systems. Two members of the GPCR family, Smoothened and Gpr161, play important roles in the Hh pathway. We review the current understanding of how these proteins may function to regulate Hh signaling and also highlight some of the critical unanswered questions being tackled by the field. Uncovering GPCR-regulated mechanisms important in Hh signaling may provide therapeutic strategies against the Hh pathway that plays important roles in development, regeneration and cancer.