The OMP-lacZ transgene mimics the unusual expression pattern of OR-Z6, a new odorant receptor gene on mouse chromosome 6: implication for locus-dependent gene expression

A purely bioinformatic pipeline for the prediction of mammalian odorant receptor gene enhancers

BACKGROUND

In most mammals, a vast array of genes coding for chemosensory receptors mediates olfaction. Odorant receptor (OR) genes generally constitute the largest multifamily (> 1100 intact members in the mouse). From the whole pool, each olfactory neuron expresses a single OR allele following poorly characterized mechanisms termed OR gene choice. OR genes are found in genomic aggregations known as clusters. Nearby enhancers, named elements, are crucial regulators of OR gene choice. Despite their importance, searching for new elements is burdensome. Other chemosensory receptor genes responsible for smell adhere to expression modalities resembling OR gene choice, and are arranged in genomic clusters - often with chromosomal linkage to OR genes. Still, no elements are known for them.

RESULTS

Here we present an inexpensive framework aimed at predicting elements. We redefine cluster identity by focusing on multiple receptor gene families at once, and exemplify thirty - not necessarily OR-exclusive - novel candidate enhancers.

CONCLUSIONS

The pipeline we introduce could guide future in vivo work aimed at discovering/validating new elements. In addition, our study provides an updated and comprehensive classification of all genomic loci responsible for the transduction of olfactory signals in mammals.

Olfactory marker protein (OMP) regulates formation and refinement of the olfactory glomerular map

Inputs from olfactory sensory neuron (OSN) axons expressing the same type of odorant receptor (OR) converge in the glomerulus of the main olfactory bulb. A key marker of mature OSNs is olfactory marker protein (OMP), whose deletion has been associated with deficits in OSN signal transduction and odor discrimination. Here, we investigate glomerular odor responses and anatomical architecture in mice in which one or both alleles of OMP are replaced by the fluorescent synaptic activity reporter, synaptopHluorin. Functionally heterogeneous glomeruli, that is, ones with microdomains with distinct odor responses, are rare in OMP^+/– mice, but occur frequently in OMP^–/– mice. Genetic targeting of single ORs reveals that these microdomains arise from co-innervation of individual glomeruli by OSNs expressing different ORs. This glomerular mistargeting is locally restricted to a few glomerular diameters. Our studies document functional heterogeneity in sensory input within individual glomeruli and uncover its anatomical correlate, revealing an unexpected role for OMP in the formation and refinement of the glomerular map.

Olfactory marker protein (OMP) expressed in all olfactory sensory neurons (OSN) is required for proper signal transduction and odor discrimination. Here, the authors report that OMP deletion leads to formation of glomeruli with axons from heterogeneous OSNs due to local axonal mistargeting.

A Near-Complete Spatial Map of Olfactory Receptors in the Mouse Main Olfactory Epithelium

Different regions of the mammalian nose smell different odors. In the mouse olfactory system, spatially regulated expression of >1000 olfactory receptors (ORs) along the dorsomedial-ventrolateral (DV) axis forms a topological map in the main olfactory epithelium (MOE). However, the locations of most ORs along the DV axis are currently unknown. By sequencing mRNA of 12 isolated MOE pieces, we mapped out the DV locations-as quantified by "zone indices" on a scale of 1-5-of 1033 OR genes with an estimated error of 0.3 zone indices. Our map covered 81% of all intact OR genes and 99.4% of the total OR mRNA abundance. Spatial regulation tended to vary gradually along chromosomes. We further identified putative non-OR genes that may exhibit spatial expression along the DV axis.

Histone and DNA Modifications as Regulators of Neuronal Development and Function

DNA and histone modifications, together with constraints imposed by nuclear architecture, contribute to the transcriptional regulatory landscape of the nervous system. Here, we provide select examples showing how these regulatory layers, often referred to as epigenetic, contribute to neuronal differentiation and function. We describe the interplay between DNA methylation and Polycomb-mediated repression during neuronal differentiation, the role of DNA methylation and long-range enhancer-promoter interactions in Protocadherin promoter choice, and the contribution of heterochromatic silencing and nuclear organization in singular olfactory receptor expression. Finally, we explain how the activity-dependent expression of a histone variant determines the longevity of olfactory sensory neurons.

The Mouse Solitary Odorant Receptor Gene Promoters as Models for the Study of Odorant Receptor Gene Choice

BACKGROUND

In vertebrates, several anatomical regions located within the nasal cavity mediate olfaction. Among these, the main olfactory epithelium detects most conventional odorants. Olfactory sensory neurons, provided with cilia exposed to the air, detect volatile chemicals via an extremely large family of seven-transmembrane chemoreceptors named odorant receptors. Their genes are expressed in a monogenic and monoallelic fashion: a single allele of a single odorant receptor gene is transcribed in a given mature neuron, through a still uncharacterized molecular mechanism known as odorant receptor gene choice.

AIM

Odorant receptor genes are typically arranged in genomic clusters, but a few are isolated (we call them solitary) from the others within a region broader than 1 Mb upstream and downstream with respect to their transcript's coordinates. The study of clustered genes is problematic, because of redundancy and ambiguities in their regulatory elements: we propose to use the solitary genes as simplified models to understand odorant receptor gene choice.

PROCEDURES

Here we define number and identity of the solitary genes in the mouse genome (C57BL/6J), and assess the conservation of the solitary status in some mammalian orthologs. Furthermore, we locate their putative promoters, predict their homeodomain binding sites (commonly present in the promoters of odorant receptor genes) and compare candidate promoter sequences with those of wild-caught mice. We also provide expression data from histological sections.

RESULTS

In the mouse genome there are eight intact solitary genes: Olfr19 (M12), Olfr49, Olfr266, Olfr267, Olfr370, Olfr371, Olfr466, Olfr1402; five are conserved as solitary in rat. These genes are all expressed in the main olfactory epithelium of three-day-old mice. The C57BL/6J candidate promoter of Olfr370 has considerably varied compared to its wild-type counterpart. Within the putative promoter for Olfr266 a homeodomain binding site is predicted. As a whole, our findings favor Olfr266 as a model gene to investigate odorant receptor gene choice.

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.

An epigenetic signature for monoallelic olfactory receptor expression

Constitutive heterochromatin is traditionally viewed as the static form of heterochromatin that silences pericentromeric and telomeric repeats in a cell cycle- and differentiation-independent manner. Here, we show that, in the mouse olfactory epithelium, olfactory receptor (OR) genes are marked in a highly dynamic fashion with the molecular hallmarks of constitutive heterochromatin, H3K9me3 and H4K20me3. The cell type and developmentally dependent deposition of these marks along the OR clusters are, most likely, reversed during the process of OR choice to allow for monogenic and monoallelic OR expression. In contrast to the current view of OR choice, our data suggest that OR silencing takes place before OR expression, indicating that it is not the product of an OR-elicited feedback signal. Our findings suggest that chromatin-mediated silencing lays a molecular foundation upon which singular and stochastic selection for gene expression can be applied.

Gene switching and odor induced activity shape expression of the OR37 family of olfactory receptor genes

Olfactory sensory neurons (OSNs) which express distinct odorant receptor (OR) genes are spatially arranged within the mouse olfactory epithelium. Towards an understanding of the mechanisms which determine these patterns, representative OR genes which are typically expressed in the unique central patch of the epithelium were investigated. Inside the patch, numerous OSNs which initially selected a representative gene from this OR group finally expressed another gene from the group, indicating that OSNs inside the patch 'switch' between these genes. If an OSN successively chose genes from the same OR gene cluster, these originated from the same parental chromosome. A deletion of the olfactory cyclic nucleotide-gated ion channel altered the distribution pattern of distinct OSN populations; they were no longer located exclusively inside the patch. Together, the results indicate that OSNs inside the patch initially sample several OR genes for expression; for their correct patterning in the OE, odor-induced activity appears to play a critical role.

Mechanisms of odorant receptor gene choice in Drosophila and vertebrates

Odorant receptors are encoded by extremely large and divergent families of genes. Each receptor is expressed in a small proportion of neurons in the olfactory organs, and each neuron in turn expresses just one odorant receptor gene. This fundamental property of the peripheral olfactory system is widely conserved across evolution, and observed in vertebrates, like mice, and invertebrates, like Drosophila, despite their olfactory receptor gene families being evolutionarily unrelated. Here we review the progress that has been made in these two systems to understand the intriguing and elusive question: how does a single neuron choose to express just one of many possible odorant receptors and exclude expression of all others?

Odorant receptor (OR) gene choice is biased and non-clonal in two olfactory placode cell lines, and OR RNA is nuclear prior to differentiation of these lines

We have investigated two clonal mouse olfactory placode (OP) cell lines as a model system for studying endogenous odorant receptor (OR) regulation. Both lines can be differentiated into bipolar neurons with transcriptional profiles consistent with mature sensory neurons. We show that single cells exhibit monogenic OR expression like sensory neurons in vivo. Monogenic OR expression is established in undifferentiated cells and persists through differentiation, but OR gene choice is not a clonal property of either cell line. Interestingly, OR RNA shifts from predominantly nuclear to cytoplasma during differentiation of both cell lines. Finally, our data indicate that a restricted subset of OR genes and OR clusters are over-represented in cell populations, suggesting either a pre-existing intrinsic bias in OP founder cells or extrinsic influences arising from culture conditions.

Odorant and pheromone receptor gene regulation in vertebrates

The largest mammalian gene family codes for odorant receptors and is exclusively devoted to the perception of the outside world. Its expression is very peculiar, since olfactory sensory neurons are only allowed to express a single of its numerous members, from a single parental allele. How this is achieved is unknown, but recent work points to multiple regulatory mechanisms, possibly shared by pheromone receptor genes, acting at (a) a general level, via the expression of the chemoreceptor itself and (b) a more restricted level, defined by activator elements.

Co-regulation of a large and rapidly evolving repertoire of odorant receptor genes

The olfactory system meets niche- and species-specific demands by an accelerated evolution of its odorant receptor repertoires. In this review, we describe evolutionary processes that have shaped olfactory and vomeronasal receptor gene families in vertebrate genomes. We emphasize three important periods in the evolution of the olfactory system evident by comparative genomics: the adaptation to land in amphibian ancestors, the decline of olfaction in primates, and the delineation of putative pheromone receptors concurrent with rodent speciation. The rapid evolution of odorant receptor genes, the sheer size of the repertoire, as well as their wide distribution in the genome, presents a developmental challenge: how are these ever-changing odorant receptor repertoires coordinated within the olfactory system? A central organizing principle in olfaction is the specialization of sensory neurons resulting from each sensory neuron expressing only ~one odorant receptor allele. In this review, we also discuss this mutually exclusive expression of odorant receptor genes. We have considered several models to account for co-regulation of odorant receptor repertoires, as well as discussed a new hypothesis that invokes important epigenetic properties of the system.

Promotor elements governing the clustered expression pattern of odorant receptor genes

Odorant receptor (OR) genes of family mOR262 are only expressed in olfactory sensory neurons (OSNs) segregated in a central patch of the nasal turbinates; they comprise conserved DNA elements upstream of their transcription start sites that are proposed to govern the distinct expression pattern. In mouse lines with a transgene containing the coding sequence and a short upstream region of the mOR262-12 gene, expression was restricted to OSNs that were segregated in the characteristic central patch, although the number of cells varied considerably. Only in one line, the transgene was also expressed in OSNs ectopically positioned outside the patch. The axons of transgene-expressing OSNs co-converged with those expressing the endogenous gene. The transgene was found to be expressed in a mutually exclusive manner and from only one allele indicating that the conserved upstream DNA elements play a critical role in controlling the specific expression pattern of these genes.

Sodium/calcium exchanger expression in the mouse and rat olfactory systems

Sodium/calcium (Na(+)/Ca(2+)) exchangers are membrane transport systems that regulate Ca(2+)-homeostasis in many eukaryotic cells. In olfactory and vomeronasal sensory neurons ligand-induced olfactory signal transduction is associated with influx and elevation of intracellular Ca(2+), [Ca(2+)](i). While much effort has been devoted to the characterization of Ca(2+)-related excitation and adaptation events of olfactory chemosensory neurons (OSNs), much less is known about mechanisms that return [Ca(2+)](i) to the resting state. To identify proteins participating in the poststimulus Ca(2+)-clearance of mouse OSNs, we analyzed the expression of three potassium (K(+))-independent (NCX1, 2, 3) and three K(+)-dependent (NCKX1, 2, 3) Na(+)/Ca(2+) exchangers. In situ hybridization showed that mRNAs of all six Na(+)/Ca(2+) exchangers coexist in neurons of the olfactory and vomeronasal systems, and that some are already detectable in the embryo. Of these, NCX1 and NCKX1 represent the most and least abundant mRNAs, respectively. Moreover, immunohistochemistry revealed that the NCX1, 2, and 3 proteins are expressed in nearly all neurons of the olfactory epithelium, the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. These three exchanger proteins display different expression profiles in dendrites, knobs, and plasma membranes of OSNs and in sustentacular cells. Furthermore, we show that NCX1 mRNA in rat olfactory mucosa is expressed as 8 alternative splice variants. This is the first comprehensive analysis of Na(+)/Ca(2+) exchanger expression in the mammalian olfactory system. Our results suggest that Ca(2+)-extrusion by OSNs utilizes multiple different Na(+)/Ca(2+) exchangers and that different subtypes are targeted to different subcellular compartments.

Immunohistochemical expression of two members of the GDNF family of growth factors and their receptors in the olfactory system

The glial cell line-derived (GDNF) family of trophic factors, GDNF, neurturin, persephin and artemin, are known to support the survival and regulate differentiation of many neuronal populations, including peripheral autonomic, enteric and sensory neurons. Members of this family of related ligands bind to specific GDNF family receptor (GFR) proteins, which complex and signal through the Ret receptor tyrosine kinase. We showed previously that GDNF protein was detectable in olfactory sensory neurons (OSNs) in the olfactory neuroepithelium (ON). In this immunohistochemical study, we localized GDNF, neurturin, GFRalpha1, GFRalpha2 and Ret in the adult rat ON and olfactory bulb. We found that GDNF and Ret were widely expressed by immature and mature OSNs, while neurturin was selectively expressed in a subpopulation of OSNs zonally restricted in the ON. The GFRs had differential expression, with mature OSNs and their axons preferentially expressing GFRalpha1, whereas progenitors and immature neurons more avidly expressed GFRalpha2. In the bulb, GDNF was highly expressed by the mitral and tufted cells, and by periglomerular cells, and its distribution generally resembled that of Ret, with the exception that Ret was far more predominant on fibers than cell bodies. Neurturin, in contrast, was present at lower levels and was more restricted in its expression to the axonal compartment. GFRalpha2 appeared to be the dominant accessory protein in the bulb. These data are supportive of two members of this neurotrophic family, GDNF and neurturin, playing different physiological roles in the olfactory neuronal system.

Formation of glomerular maps in the olfactory system

Sensory perception relies on the decoding of external stimuli into an internal neuronal representation, which requires precise connections between the periphery and the brain. In the olfactory system the axons of chemosensory neurons with the same odorant receptor coalesce into common glomeruli in the olfactory bulb, forming a receptor-topic map. The creation of this map begins prenatally when axons navigate towards the bulb, resort in a receptor-specific manner and terminate in a broad area interdigitated with other axon populations; distinct glomeruli form postnatally. While the initial process of glomerulization requires mainly molecular determinants, activity-dependent processes lead to a refinement of glomerular organization.

Promoter motifs of olfactory receptor genes expressed in distinct topographic patterns

Novel olfactory receptor-encoding genes that are expressed in olfactory sensory neurons arranged in a clustered pattern in the nasal epithelium, typical of the mOR262 (approved gene symbol Olfr) family, were identified. The genes share sequence motifs upstream of their transcription start sites that are highly related to those previously identified as characteristic of the mOR262 genes, suggesting that these regulatory elements may contribute to governing their unique expression pattern. Promoter analyses of genes encoding class I receptors that are expressed in the dorsal region of the epithelium revealed a different, but again common set of sequence motifs. A prominent feature of the class I gene promoters are multiple O/E-like binding sites, and O/E-type transcription factors that bind to the putative promoter region of class I OR genes were in fact identified. The findings support the concept that common elements in the promoter region of these OR genes may determine their congenic expression pattern in the epithelium.

Anatomical contributions to odorant sampling and representation in rodents: zoning in on sniffing behavior

Odorant sampling behaviors such as sniffing bring odorant molecules into contact with olfactory receptor neurons (ORNs) to initiate the sensory mechanisms of olfaction. In rodents, inspiratory airflow through the nose is structured and laminar; consequently, the spatial distribution of adsorbed odorant molecules during inspiration is predictable. Physicochemical properties such as water solubility and volatility, collectively called sorptiveness, interact with behaviorally regulable variables such as inspiratory flow rate to determine the pattern of odorant deposition along the inspiratory path. Populations of ORNs expressing the same odorant receptor are distributed in strictly delimited regions along this inspiratory path, enabling different deposition patterns of the same odorant to evoke different patterns of neuronal activation across the olfactory epithelium and in the olfactory bulb. We propose that both odorant sorptive properties and the regulation of sniffing behavior may contribute to rodents' olfactory capacities by this mechanism. In particular, we suggest that the motor regulation of sniffing behavior is substantially utilized for purposes of "zonation" or the direction of odorant molecules to defined intranasal regions and hence toward distinct populations of receptor neurons, pursuant to animals' sensory goals.

The anatomical logic of smell

Olfactory receptor neurons (ORNs) expressing the same odorant receptor gene share ligand-receptor affinity profiles and converge onto common glomerular targets in the brain. The activation patterns of different ORN populations, evoked by differential binding of odorant molecular moieties, constitute the primary odor representation. However, odorants possess properties other than receptor-binding sites that can contribute to odorant discrimination. Among terrestrial vertebrates, odorant sorptiveness--volatility and water solubility--imposes physicochemical constraints on migration through the nose during inspiration. The non-uniform distributions of ORN populations along the inspiratory axis enable sorptiveness to modify odor representations by affecting the number of molecules reaching different receptors during a sniff. Animals can then modify and analyze odor representation further by the dynamic regulation of sniffing.

Immunolocalization of Bex protein in the mouse brain and olfactory system

Bex proteins are expressed from a family of "brain expressed X-linked genes" that are closely linked on the X-chromosome. Bex1 and 2 have been characterized as interacting partners of the olfactory marker protein (OMP). Here we report the distribution of Bex1 and Bex2 mRNAs in several brain regions and the development and characterization of an antibody to mouse Bex1 protein that cross-reacts with Bex2 (but not Bex3), and its use to determine the cellular distribution of Bex proteins in the murine brain. The specificity of the antiserum was characterized by immunoprecipitation and Western blots of tissue and transfected cell extracts and by immunocytochemical analyses of cells transfected with either Bex1 or Bex2. Antibodies preabsorbed with Bex2 still recognize Bex1, while blocking with Bex1 eliminates all immunoreactivity to both Bex1 and Bex2. Bex immunoreactivity (ir) was primarily localized to neuronal cells within several regions of the brain, including the olfactory epithelium, bulb, peri/paraventricular nuclei, suprachiasmatic nucleus, arcuate nucleus, median eminence, lateral hypothalamic area, thalamus, hippocampus, and cerebellum. RT-PCR and in situ hybridization demonstrated the presence of Bex mRNA in several of these regions. Double-label immunocytochemistry indicates that Bex-ir is colocalized with OMP in mature olfactory receptor neurons (ORNs) and in the OMP-positive subpopulation of neurons in hypothalamus. This is the first anatomical mapping of Bex proteins in the mouse brain and their colocalization with OMP in ORNs and hypothalamus.

Expression of Bcl-2 extends the survival of olfactory receptor neurons in the absence of an olfactory bulb

In the olfactory neuroepithelium, the number of olfactory receptor neurons (ORNs) is maintained at a relatively constant level by a precise balance between the elimination of mature receptors and proliferation of their precursors. However, little is known of the mechanisms that couple alterations in receptor death rates to changes in precursor proliferation. To investigate this relationship, we generated a line of mice expressing Bcl-2, a protein with anti-apoptotic properties, in mature olfactory receptor neurons using the Olfactory Marker Protein (OMP) promoter. OMP-bcl-2 transgenic mice showed selective expression of Bcl-2 in mature sensory neurons of the olfactory neuroepithelium (ONE) and vomeronasal organ. Olfactory bulbectomy (OBX) resulted in the death of mature receptor neurons followed by the sustained proliferation of their precursors in wild-type and OMP-bcl-2 transgenic mice. The persistently enhanced proliferation of olfactory neuroblasts that followed bulbectomy was indistinguishable between transgenic and non-transgenic mice. However, receptor neurons that were subsequently born in the absence of the bulb had longer life spans in OMP-bcl-2 mice. The increased proliferation of neuroblasts and extended life spans combined to restore near normal numbers of olfactory receptors in bulbectomized OMP-bcl-2 mice. A model is proposed to explain the dissociation of death and proliferation in OMP-bcl-2 transgenic mice.

Target tissue influences the peripheral trajectory of mouse primary sensory olfactory axons

Primary olfactory neurons situated in the nasal septum project axons within fascicles along a highly stereotypical trajectory en route to the olfactory bulb. The ventral fascicles make a distinct dorsovental turn at the rear of the septum so as to reach the olfactory bulb. In the present study we have used a brain and nasal septum coculture system to examine the role of target tissue on the peripheral trajectory of olfactory sensory axons. In cultures of isolated embryonic nasal septa, olfactory axons form numerous parallel fascicles that project caudally in the submucosa, as they do in vivo. The ventral axon fascicles in the septum, however, often fail to turn, and do not project dorsally towards the roof of the nasal cavity. The presence of olfactory bulb, cortical, or tectal tissue apposed to the caudal end of the septum rescued this phenotype, causing the ventral fascicles to follow a normal in vivo-like trajectory. Ectopic placements of the explants revealed that brain tissue is not tropic for olfactory axons but appears to maintain the peripheral trajectory of growing axons in the nasal septum. Although primary olfactory axons are able to penetrate into olfactory bulb in vitro, they only superficially enter cortical tissue, whereas they do not grow into tectal explants. The ability of axons to differentially grow into different brain regions was shown to be unrelated to the migratory behavior of olfactory ensheathing cells, indicating that olfactory axons are directly responsive to guidance cues in the brain.

High-throughput microarray detection of olfactory receptor gene expression in the mouse

The large number of olfactory receptor genes necessitates high throughput methods to analyze their expression patterns. We have therefore designed a high-density oligonucleotide array containing all known mouse olfactory receptor (OR) and V1R vomeronasal receptor genes. This custom array detected a large number of receptor genes, demonstrating specific expression in the olfactory sensory epithelium for approximately 800 OR genes previously designated as ORs based solely on genomic sequences. The array also enabled us to monitor the spatial and temporal distribution of gene expression for the entire OR family. Interestingly, OR genes showing spatially segregated expression patterns were also segregated on the chromosomes. This correlation between genomic location and spatial expression provides unique insights about the regulation of this large family of genes.

Altered epithelial density and expansion of bulbar projections of a discrete HSP70 immunoreactive subpopulation of rat olfactory receptor neurons in reconstituting olfactory epithelium following exposure to methyl bromide

A previously described subpopulation of rat olfactory receptor neurons, the 2A4(+)ORNs, is 1) distinguished by intense constitutive cytoplasmic immunoreactivity to antibodies to the 70-kD heat shock protein (HSP70); 2) occurs sparsely but consistently through ventral and lateral olfactory epithelium (OE); and 3) projects to just two to three consistently located glomeruli in each olfactory bulb (OB) (Carr et al. [1994] J Comp Neurol 348:150-160). Immunoreactivity appears not to be stress-related. To examine the persistence of these features following destruction and reconstitution of the OE, rats were subjected to methyl bromide-induced OE lesion (Schwob et al. [1995] J Comp Neurol 59:15-37; Schwob et al. [1999] J Comp Neurol 412:439-457] and their OE and OBs examined with antibodies to HSP70 6-10.5 weeks postlesion. Lesioned OE showed significantly increased 2A4(+)ORN densities but no alteration of 2A4(+)ORN zonal distribution. The OBs of lesioned animals showed marked expansions of 2A4(+)ORN bulbar projections, with 2-15-fold increases in numbers of glomeruli showing 2A4(+)axons, and projection expansions were greater in animals maintained on chronic food restriction prior to lesioning. Examination of archival 5-month post-MeBr lesion material indicates that altered projection patterns are maintained.

Negative feedback regulation ensures the one receptor-one olfactory neuron rule in mouse

In the mouse olfactory system, each olfactory sensory neuron (OSN) expresses only one odorant receptor (OR) gene in a monoallelic and mutually exclusive manner. Such expression forms the genetic basis for OR-instructed axonal projection of OSNs to the olfactory bulb of the brain during development. Here, we identify an upstream cis-acting DNA region that activates the OR gene cluster in mouse and allows the expression of only one OR gene within the cluster. Deletion of the coding region of the expressed OR gene or a naturally occurring frame-shift mutation allows a second OR gene to be expressed. We propose that stochastic activation of only one OR gene within the cluster and negative feedback regulation by that OR gene product are necessary to ensure the one receptor-one neuron rule.

OMP gene deletion results in an alteration in odorant-induced mucosal activity patterns

Previous behavioral work, using a complex five-odorant identification task, demonstrated that olfactory marker protein (OMP) is critically involved in odor processing to the extent that its loss results in an alteration in odorant quality perception. Exactly how the lack of OMP exerts its influence on the perception of odorant quality is unknown. However, there is considerable neurophysiological evidence that different odorants produce different spatiotemporal patterns of neural activity at the level of the mucosa and that these patterns predict the psychophysically determined perceptual relationship among odorants. In this respect, OMP gene deletion is known to result in a constellation of physiologic defects (i.e., marked reduction in the electroolfactogram (EOG) and altered response and recovery kinetics) that would be expected to alter the odorant-induced spatiotemporal activity patterns that are characteristic of different odorants. This, in turn, would be expected to alter the spatiotemporal patterning of information that results from the mucosal projection onto the bulb, thereby changing odorant quality perception. To test the hypothesis that odorant-induced mucosal activity patterns are altered in mice lacking the gene for OMP, we optically recorded the fluorescent changes in response to odorant stimulation from both the septum and turbinates of both OMP-null and control mice using a voltage-sensitive dye (di-4-ANEPPS Molecular Probes, Eugene, OR) and a Dalsa 120 x 120, 12-bit CCD camera. To maintain continuity with the previous behavioral work, the odorants 2-propanol, citral, carvone, ethylacetoacetate, and propyl acetate were again used. Each odorant was randomly presented to each mucosal surface in a Latin-Square design. The results of this study demonstrated that, for both mouse strains, there do indeed exist different spatiotemporal activity patterns for different odorants. More importantly, however, these patterns significantly differed between OMP-null and control mice. That is, although the general regions of characteristic activity for different odorants were the same in both mouse strains, the patterns in the null animals were degraded relative to controls. These data suggest therefore that the alterations in mucosal activity may serve as the substrate for the behaviorally observed changes in odorant quality perception in the null mutant.

Neurotrophin-3 is expressed in a discrete subset of olfactory receptor neurons in the mouse

In transgenic neurotrophin-3 lacZ-neo (NT-3(lacZneo)) mice, in which the coding region for NT-3 is replaced by Eschericia coli lacZ, the expression of beta-galactosidase faithfully mimics the expression of NT-3 (Vigers AJ, Baquet ZC, Jones KR [2000], J Comp Neurol 416:398-416). During embryonic and early postnatal development, beta-galactosidase is detected in the olfactory system, beginning at embryonic day 11.5 in the nasal epithelium and at embryonic day 16.5 in the olfactory bulb. Levels of beta-galactosidase rise with age, reaching a peak during the second postnatal week, when beta-galactosidase reactivity is visible in up to 50% of the glomeruli. As the animal matures, the beta-galactosidase levels decline, but staining remains present in axons and cell bodies of a specific subset of olfactory receptor neurons (ORNs) projecting to a limited subset of glomeruli. The heavily labeled ORNs do not follow the typical OR expression zones in the epithelium but appear similar to the "patch" expression pattern of mOR37 receptors. The most heavily reactive glomeruli exhibit a striking reproducible pattern in the ventral olfactory bulb (OB). Some glomeruli of the OB contain calcitonin gene-related peptide (CGRP)-immunoreactive fibers of the trigeminal nerve. However, double-label immunocytochemistry for CGRP and beta-galactosidase rendered no correlation between trigeminal innervation and the degree of innervation by NT-3-expressing ORNs. Thus, the timing and presence of beta-galactosidase in a subset of ORNs suggests that NT-3 plays a role in synaptogenesis and/or synapse function in a specific subset of ORNs within the olfactory bulb.

Subzonal organization of olfactory sensory neurons projecting to distinct glomeruli within the mouse olfactory bulb

Olfactory sensory neurons located in the nasal neuroepithelium send their axons directly into the olfactory bulb, where they contact the dendrites of second-order neurons in specialized spherical structures called glomeruli; each sensory neuron projects to a single glomerulus. All neurons expressing the same odorant receptor gene are confined to distinct zones within the epithelium and converge their axons onto a small number of common glomeruli. In the present study, we analyzed transgenic mouse lines in which the projection of a neuron population expressing a particular receptor gene can be visualized as a result of axonal markers that are coexpressed. The target glomeruli could thus reproducibly be identified and allowed to deposit retrograde tracers precisely. After an appropriate incubation time, olfactory sensory neurons within distinct areas of the olfactory epithelium were labeled. The two subpopulations of neurons retrogradely stained by differently colored fluorescent dyes deposited at the dorsal and the dorsomedial glomerulus, respectively, were found to be segregated within distinct areas of the expression zone, where the cells expressing the same receptor type displayed a stochastic distribution.

Minigenes impart odorant receptor-specific axon guidance in the olfactory bulb

An olfactory sensory neuron (OSN) expresses selectively one member from a repertoire of approximately 1000 odorant receptor (OR) genes and projects its axon to a specific glomerulus in the olfactory bulb. Both processes are here recapitulated by MOR23 and M71 OR minigenes, introduced into mice. Minigenes of 9 kb and as short as 2.2 kb are selectively expressed by neurons that do not coexpress the endogenous gene but coproject their axons to the same glomeruli. Deletion of a 395 bp upstream region in the MOR23 minigene abolishes expression. In this region we recognize sequence motifs conserved in many OR genes. Transgenic lines expressing the OR in ectopic epithelial zones form ectopic glomeruli, which also receive input from OSNs expressing the cognate endogenous receptor. This suggests a recruitment through homotypic interactions between OSNs expressing the same OR.

Molecular-feature domains with posterodorsal-anteroventral polarity in the symmetrical sensory maps of the mouse olfactory bulb: mapping of odourant-induced Zif268 expression

Individual glomeruli in the mammalian olfactory bulb presumably represent a single type of odourant receptor. Thus, the glomerular sheet provides odourant receptor maps at the surface of the olfactory bulb. To understand the basic spatial organization of the olfactory sensory maps, we first compared the spatial distribution of odourant-induced responses measured by the optical imaging of intrinsic signals with that detected immunohistochemically by expressions of Zif268, one of the immediate early gene products in juxtaglomerular cells. In the dorsal surface of the bulb, we detected a clear correlation in the spatial pattern between these responses. In addition, the molecular-feature domains and their polarities (spatial shifts of responses with an increase in carbon chain length) that were defined by the optical imaging method could be also detected by the Zif268 mapping method. We then mapped the Zif268 signals over the entire olfactory bulb using a homologous series of fatty acids and aliphatic alcohols as stimulus odourants. We superimposed the Zif268 signals onto the standard unrolled map with the help of cell adhesion molecule compartments. Each odourant typically elicited two pairs of clusters of dense Zif268 signals. The results showed that molecular-feature domains and their polarities were arranged symmetrically at stereotypical positions in a mirror-image fashion between the lateral and the medial sensory maps. The polarity of each domain was roughly in parallel with the posterodorsal-anteroventral axis that was defined by the cell adhesion molecule compartments. These results suggest that the molecular-feature domain with its fixed polarity is one of the basic structural units in the spatial organization of the odourant receptor maps in the olfactory bulb.

How smell develops

The mouse's sense of smell is built of approximately 1000 input channels. Each of these consists of a population of olfactory sensory neurons that express the same odorant receptor gene and project their axons to the same targets (glomeruli) in the olfactory bulb. A neuron must choose to express a singular receptor gene from a repertoire of approximately 1000 genes, and its axon must be wired to the corresponding glomerulus, from an array of approximately 1800 glomeruli. Genetic experiments have shown that the expressed odorant receptor specifies axonal choice of the innervated glomerulus, but it is not the only determinant. The mechanisms of odorant receptor gene choice and axonal wiring are central to the functional organization of the mammalian olfactory system. Although principles have emerged, our understanding of these processes is still limited.