Olfactory receptor gene regulation

The recombination activation gene 1 (Rag1) is expressed in a subset of zebrafish olfactory neurons but is not essential for axon targeting or amino acid detection

Background

Rag1 (Recombination activation gene-1) mediates genomic rearrangement and is essential for adaptive immunity in vertebrates. This gene is also expressed in the olfactory epithelium, but its function there is unknown.

Results

Using a transgenic zebrafish line and immunofluorescence, we show that Rag1 is expressed and translated in a subset of olfactory sensory neurons (OSNs). Neurons expressing GFP under the Rag1 promoter project their axons to the lateral region of the olfactory bulb only, and axons with the highest levels of GFP terminate in a single glomerular structure. A subset of GFP-expressing neurons contain Gα_o, a marker for microvillous neurons. None of the GFP-positive neurons express Gα_olf, Gα_q or the olfactory marker protein OMP. Depletion of RAG1, by morpholino-mediated knockdown or mutation, did not affect axon targeting. Calcium imaging indicates that amino acids evoke chemotopically organized glomerular activity patterns in a Rag1 mutant.

Conclusion

Rag1 expression is restricted to a subpopulation of zebrafish olfactory neurons projecting to the lateral olfactory bulb. RAG1 catalytic activity is not essential for axon targeting, nor is it likely to be required for regulation of odorant receptor expression or the response of OSNs to amino acids.

The clustered olfactory receptor gene family 262: genomic organization, promotor elements, and interacting transcription factors

For six mouse olfactory receptor genes from family 262 which are expressed in clustered populations of olfactory sensory neurons, the genomic as well as cDNA structures were deciphered. All genes contained several exons which in some cases were alternatively spliced. Immediately upstream of the transcription start sites, sequence motif blocks were identified that are highly conserved among olfactory receptor (OR) genes which are expressed in clustered neuronal populations. By means of electrophoretic mobility shift assays, it was demonstrated that segments of the motif block region interact with proteins extracted from nuclear fractions of the olfactory epithelium. Yeast one-hybrid screenings of an olfactory cDNA library led to the identification of a set of transcription factors that specifically bind to particular elements of the motif block region. The identified factors can be categorized into two types: One group is known to be involved in transcriptional initiation, and the second group represents factors involved in pattern formations. The identified components may contribute to govern the precise topographic expression pattern of olfactory receptor genes.

A comparative study of odorant binding protein genes: differential expression of the PBP1-GOBP2 gene cluster inManduca sexta(Lepidoptera) and the organization of OBP genes inDrosophila melanogaster(Diptera)

Insects discriminate odors using sensory organs called olfactory sensilla, which display a wide range of phenotypes. Sensilla express ensembles of proteins, including odorant binding proteins (OBPs), olfactory receptors (ORs) and odor degrading enzymes (ODEs); odors are thought to be transported to ORs by OBPs and subsequently degraded by ODEs. These proteins belong to multigene families. The unique combinatorial expression of specific members of each of these gene families determines, in part, the phenotype of a sensillum and what odors it can detect. Furthermore, OBPs, ORs and ODEs are expressed in different cell types, suggesting the need for cell–cell communication to coordinate their expression. This report examines the OBP gene family. In Manduca sexta, the genes encoding PBP1Msex and GOBP2Msex are sequenced, shown to be adjacent to one another, and characterized together with OBP gene structures of other lepidoptera and Drosophila melanogaster. Expression of PBP1Msex, GOBP1Msex and GOBP2Msex is characterized in adult male and female antenna and in larval antenna and maxilla. The genomic organization of 25 D. melanogaster OBPs are characterized with respect to gene locus, gene cluster, amino acid sequence similarity, exon conservation and proximity to OR loci, and their sequences are compared with 14 M. sexta OBPs. Sensilla serve as portals of important behavioral information, and genes supporting sensilla function are presumably under significant evolutionary selective pressures. This study provides a basis for studying the evolution of the OBP gene family, the regulatory mechanisms governing the coordinated expression of OBPs, ORs and ODEs, and the processes that determine specific sensillum phenotypes.

Concurrent expression of recombination activating genes 1 and 2 in zebrafish olfactory sensory neurons

Each olfactory sensory neuron (OSN) expresses a single odorant receptor (OR) from a large repertoire of clustered OR genes. It has been hypothesized that OR gene regulation may involve stochastic DNA rearrangement, which in lymphocytes requires the recombination activating genes, rag1 and rag2. We have recently demonstrated that rag1 is expressed in zebrafish OSNs. Here we report that rag2, the obligate partner for rag1 function, is also expressed in OSNs and that its expression pattern mimics that of rag1. The onset of rag1 and rag2 expression preceded that of known zebrafish ORs and the number of rag1-positive OSNs corresponded with the number expressing the olfactory cyclic nucleotide-gated cation channel, an OSN marker. Zebrafish OSNs are the first example of concurrent rag expression in a nonlymphoid tissue. The expression of rag1 and rag2 in OSNs adds to the list of similarities between the olfactory and immune systems that includes monoallelic and mutually exclusive gene expression.

A large upstream region is not necessary for gene expression or hypersensitive site formation at the mouse beta -globin locus

Developmental expression at the beta-globin locus is regulated in part by the locus control region, a region upstream of the genes containing at least five major DNase I hypersensitive sites (HSs) in mammalian erythrocytes. Sequences farther 5' of these HSs are conserved in mouse and human, and both loci are embedded within a cluster of functional odorant receptor genes. In humans, distant upstream sequences have been implicated in regulation of the beta-globin genes. In this study, the role of the 5'-most HSs and their adjacent sequence was investigated by deletion of an 11-kb region from the mouse locus, including 5'HS 4.2, 5'HS 5, 5'HS 6, and the 5'beta1 odorant receptor gene. Mice that were homozygous for this deletion were fully viable, and no significant effect on adult beta-globin gene expression was seen. 5'HSs 1-4, which are located downstream of the deletion, were still present in the mutant mice. In addition, two new upstream HSs, HS -60.7 and HS -62.5, were found in erythroid tissue of both wild-type and mutant mice. Therefore, although the possibility of a minor role still exists, neither the HSs nor the other regions deleted in this study are essential for beta-globin gene expression, and it is unlikely that chromatin structure is affected either upstream or downstream of the deletion. This is the largest deletion at the mouse locus control region to show no apparent phenotype, and focuses attention on the possible contribution of sequences even farther upstream.

Allele-specific expression patterns of interleukin-2 and Pax-5 revealed by a sensitive single-cell RT-PCR analysis

Autosomal genes that are subject to random allelic inactivation (RAI), like imprinted genes [1] and genes subject to X-inactivation [2], require mechanisms that dictate the differential transcriptional regulation of two sequence-identical alleles. RAI genes include olfactory receptor genes [3], and the various genes encoding antigen receptors on lymphocytes (immunoglobulin genes, T cell receptor genes and NK receptor genes [4] [5] [6] [7]). These observations raise the possibility that other genes might be similarly regulated. Moreover, an interesting possibility is that certain genes might be monoallelically expressed in some cells and biallelically expressed in others. Recently, reports of monoallelic expression of interleukin-2 (IL-2) [8] [9] and IL-4 [10] [11] have raised the possibility that the cytokine gene family may be subject to monoallelic expression. Another report suggests that the gene encoding the transcription factor Pax-5, which is involved in B-cell (and cerebellar) development [12] [13], is also subject to monoallelic expression [14]. Using a novel single-cell reverse transcription-polymerase chain reaction (RT-PCR) approach, we have analyzed the IL-2 and Pax-5 genes in mice. We found that IL-2 is monoallelically transcribed in some T cells and biallelically transcribed in others, raising interesting questions regarding cytokine gene regulation. Additionally, our analyses suggest that Pax-5 is consistently biallelically transcribed. Thus, the IL-2 gene and other cytokine genes may be regulated in a stochastic manner that results in 0, 1 or 2 alleles of a given cytokine gene expressed in each T cell. This type of regulation could account for the wide variety of different combinations of cytokine genes expressed in individual T cells and therefore plays a role in the generation of T cells with a range of different effector functions.

Sequence analyses of the olfactory receptor gene cluster mOR37 on mouse chromosome 4

The olfactory receptor multigene family is organized in clusters spread throughout the genome. In the present study, we have sequenced two subregions of the mOR37 gene cluster on mouse chromosome 4. The resulting 100 kb of sequence revealed seven odorant receptor coding regions and one gene fragment. Sequence analyses reveal that the mOR37 gene cluster may represent a rather ancient cluster. The mOR37 genes exhibit a complex intron/exon structure, and some appear to be differentially spliced. All genes in the cluster share conserved sequence motifs 5' of their putative initial exons, which represent potential binding sites for transcription factors. The clustered organization and conserved sequence motifs suggest common expression control mechanisms for these genes.

On the stochastic regulation of interleukin-2 transcription

Interleukin-2 (IL-2) is a growth and differentiation factor critical for clonal T cell expansion and function. Produced exclusively in T cells, IL-2 transcription and synthesis occurs only after appropriate cellular activation via the clonotypic antigen-receptor and co-stimulatory molecules. IL-2 gene expression is initiated by the cooperative binding of different transcription factors and is predominantly controlled at the transcriptional level. Recently, it has been demonstrated that IL-2 transcriptional activity is normally confined to a single, randomly chosen allele. This monoallelic expression of a non-receptor gene product encoded at a non-imprinted, autosomal locus represents an unusual regulatory mode. Although the molecular mechanisms operational for IL-2 transcription have yet to be defined, allele-specific expression of the IL-2 locus constitutes an important expansion to the concept of stochastic gene expression.

Variable patterns of axonal projections of sensory neurons in the mouse vomeronasal system

The vomeronasal system mediates pheromonal effects in mammals. We have employed gene targeting technology to introduce mutations in a putative pheromone receptor gene, VR2, in the germline of mice. By generating alleles differentially tagged with the histological markers taulacZ and tauGFP, we show that VR2 is monoallelically expressed in a given neuron. Axons of VR2-expressing neurons converge onto numerous glomeruli in the accessory olfactory bulb. The pattern of axonal projections is complex and variable. This wiring diagram is substantially different from that of the main olfactory system. The projection pattern is disrupted by deleting the coding region of VR2, but an unrelated seven-transmembrane protein, the odorant receptor M71, can partially substitute for VR2.