The main olfactory epithelium of the mouse is a mosaic of 2000 populations of olfactory sensory neurons (OSNs). Each population expresses one allele of one of the 1000 intact odorant receptor (OR) ...genes. An OSN projects a single unbranched axon to a single glomerulus, from an array of 1600-1800 glomeruli in the main olfactory bulb. Within a glomerulus the OSN axon synapses with the dendrites of second-order neurons and interneurons. Axons of OSNs that express the same OR project to the same glomeruli-typically one glomerulus per half-bulb and thus four glomeruli per mouse. These glomeruli are located at characteristic positions within the glomerular layer of the bulb. ORs determine both the odorant response profile of the OSN and the projection of its axon to a specific glomerulus. I focus on genetic approaches to the axonal wiring problem, particularly on how ORs may function in axonal wiring.
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.
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•A mature olfactory sensory neuron expresses one odorant receptor gene•The expression areas of 68 odorant receptor genes are classified into 9 zones•These zones are highly overlapping and strikingly complex when viewed in 3D
Zapiec and Mombaerts report a multiplex method to map the expression areas of odorant receptor genes in individual, non-genetically modified mice. They classify the expression areas of 68 odorant receptor genes into 9 zones. These zones are highly overlapping and strikingly complex when viewed in 3D reconstructions.
Designed for general chemical recognition, the mammalian olfactory system shares many similarities with the immune system. Among these is the popular notion that a single olfactory sensory neuron ...expresses a single odorant receptor gene, while all other ∼1000 genes of this type remain silent. Here, I examine the evidence supporting the one receptor–one neuron hypothesis. I conclude that, contrary to widespread belief, it is far from being proven. I propose an hypothesis of a developmental phase of oligogenic expression that is followed by positive and negative selection resulting usually in cells with one expressed receptor. Curiously, selective processes are well established and widely accepted for lymphocytes, but these concepts are essentially ignored for olfactory sensory neurons, despite the analogies that are frequently made between these two systems. More attention must be paid to odorant receptor gene choice and expression during development and neuronal differentiation.
In the mouse, axons of olfactory sensory neurons (OSNs) that express the same odorant receptor (OR) gene coalesce into one or a few glomeruli in the olfactory bulb. The positions of OR-specific ...glomeruli are traditionally described as stereotyped. Here, we have assessed quantitatively the positions of OR-specific glomeruli using serial two-photon tomography, an automated method for whole-organ fluorescence imaging that integrates two-photon microscopy with serial microtome sectioning. Our strategy is multiplexed. By repeated crossing, we generated two strains of mice with gene-targeted mutations at four or five OR loci for a total of six ORs: MOR23 (Olfr16), mOR37A (Olfr155), M72 (Olfr160), P2 (Olfr17), MOR256-17 (Olfr15), and MOR28 (Olfr1507). Glomerular imaging relied on intrinsic fluorescence of GFP or DsRed, or on whole-mount immunofluorescence with antibodies against GFP, DsRed, or β-gal using the method of immunolabeling-enabled three-dimensional imaging of solvent-cleared organs (iDISCO). The hig-hresolution 3D-reconstructed datasets were segmented to identify the labeled glomeruli and to assess glomerular positional variability between the bulbs of one mouse (intraindividual) and among the bulbs of different mice (interindividual). In 26 mice aged 21 or 50 d or 10 wk, we made measurements of the positions of 352 glomeruli. We find that positional variability of glomeruli correlates with the OR: For instance, the medial MOR28 glomerular domain occupies a surface area that is an order of magnitude larger than the surface area of the medial MOR23 glomerular domain. Our results quantify the level of precision that is delivered by the mechanisms of OSN axon wiring, differentially for the various OSN populations expressing distinct OR genes.
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.
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•Olfaction initiates at the main olfactory epithelium and the vomeronasal epithelium•Two types of Trpc2+ neurons in the mouse main olfactory epithelium are described•Both cell types also express Cnga2 and can be distinguished by expression of Adcy3•There are implications for the interpretation of the phenotypes of Trpc2-KO mice
The mouse olfactory system is traditionally seen as a dichotomy between the main olfactory epithelium and the vomeronasal epithelium, whose sensory neurons express different chemosensory receptor genes and differ fundamentally in signal transduction pathways. Omura and Mombaerts report two previously unrecognized types of Trpc2+ neurons in the mouse main olfactory epithelium. Until now, Trpc2 expression was considered to be exclusive to the vomeronasal epithelium. These findings have implications for the conventional interpretation of the behavioral phenotypes of Trpc2 knockout mice.
Each olfactory sensory neuron (OSN) in mouse chooses one of 1,200 odorant receptor (OR) genes for expression. OR genes are chosen for expression by greatly varying numbers of OSNs. The mechanisms ...that regulate the probability of OR gene choice remain unclear. Here, we have applied the NanoString platform of fluorescent barcodes and digital readout to measure RNA levels of 577 OR genes in a single reaction, with probes designed against coding sequences. In an inbred mouse strain with a targeted deletion in the P element, we find that this element regulates OR gene choice differentially across its cluster of 24 OR genes. Importantly, the fold changes of NanoString counts in ΔP or ΔH mice are in very close agreement with the fold changes of cell counts, determined by in situ hybridization. Thus, the P and H elements regulate the probability of OR gene choice, not OR transcript level per OSN.
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► 1,200 odorant receptor genes are expressed in varying numbers of olfactory neurons ► NanoString assay measures RNA levels of 577 odorant receptor genes in parallel ► Fold change of RNA levels matches fold change of cell counts in ΔP and ΔH mice ► P and H elements are enhancers regulating probability, not level, of transcription
A common view of enhancers is that they modulate levels of transcription. However, enhancers that regulate the expression of odorant receptor gene clusters impact the likelihood that a gene will be selected for monoallelic expression and do not control its level of transcription in individual cells.
Anosmia, the loss of smell, is a common and often the sole symptom of COVID-19. The onset of the sequence of pathobiological events leading to olfactory dysfunction remains obscure. Here, we have ...developed a postmortem bedside surgical procedure to harvest endoscopically samples of respiratory and olfactory mucosae and whole olfactory bulbs. Our cohort of 85 cases included COVID-19 patients who died a few days after infection with SARS-CoV-2, enabling us to catch the virus while it was still replicating. We found that sustentacular cells are the major target cell type in the olfactory mucosa. We failed to find evidence for infection of olfactory sensory neurons, and the parenchyma of the olfactory bulb is spared as well. Thus, SARS-CoV-2 does not appear to be a neurotropic virus. We postulate that transient insufficient support from sustentacular cells triggers transient olfactory dysfunction in COVID-19. Olfactory sensory neurons would become affected without getting infected.
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•A postmortem bedside surgical procedure was developed for COVID-19 and control patients•Ciliated cells are the main target cell type for SARS-CoV-2 in the respiratory mucosa•Sustentacular cells (non-neuronal) are the main target cell type in the olfactory mucosa•No evidence for infection of olfactory sensory neurons or olfactory bulb parenchyma
Postmortem samples of respiratory and olfactory mucosa and whole olfactory bulbs are harvested immediately after the death of COVID-19 patients revealing ciliated cells and sustentacular cells but not olfactory sensory neurons as the main target cell types for SARS-CoV-2 infection and replication.
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.
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