Classes Of Olfactory Neurons Research Articles

Odorant receptor co-receptors affect expression of tuning receptors in Drosophila.

Insects detect odorants using two large families of heteromeric receptors, the Odorant Receptors (ORs) and Ionotropic Receptors (IRs). Most OR and IR genes encode odorant-binding "tuning" subunits, whereas four (Orco, Ir8a, Ir25a, and Ir76b) encode co-receptor subunits required for receptor function. Olfactory neurons are thought to degenerate in the absence of Orco in ants and bees, and limited data suggest this may happen to some olfactory neurons in Drosophila fruit flies as well. Here, we thoroughly examined the role of co-receptors on olfactory neuron survival in Drosophila. Leveraging knowledge that olfactory neuron classes are defined by the expression of different tuning receptors, we used tuning receptor expression in antennal transcriptomes as a proxy for the survival of distinct olfactory neuron classes. Consistent with olfactory neuron degeneration, expression of many OR-family tuning receptors is decreased in Orco mutants relative to controls, and transcript loss is progressive with age. The effects of Orco are highly receptor-dependent, with expression of some receptor transcripts nearly eliminated and others unaffected. Surprisingly, further studies revealed that olfactory neuron classes with reduced tuning receptor expression generally survive in Orco mutant flies. Furthermore, there is little apoptosis or neuronal loss in the antenna of these flies. We went on to investigate the effects of IR family co-receptor mutants using similar approaches and found that expression of IR tuning receptors is decreased in the absence of Ir8a and Ir25a, but not Ir76b. As in Orco mutants, Ir8a-dependent olfactory neurons mostly endure despite near-absent expression of associated tuning receptors. Finally, we used differential expression analysis to identify other antennal genes whose expression is changed in IR and OR co-receptor mutants. Taken together, our data indicate that odorant co-receptors are necessary for maintaining expression of many tuning receptors at the mRNA level. Further, most Drosophila olfactory neurons persist in OR and IR co-receptor mutants, suggesting that the impact of co-receptors on neuronal survival may vary across insect species.

Sniffing Out the Right Address

Olfactory systems in organisms ranging from invertebrates to mammals distinguish between odors using an array of olfactory neurons that express different olfactory receptors—usually just one receptor in a given neuron. Since an organism's genome can encode hundreds of olfactory receptors, researchers have struggled to understand how a single olfactory neuron decides which receptor to express. In Drosophila, it seems that regulatory elements upstream of olfactory receptor genes act like zip codes to direct the expression of each gene to the appropriate class of olfactory neurons. Anandasankar Ray and colleagues looked at one of the two olfactory organs in fruit flies—the maxillary palp. Unlike the antennae, the maxillary palp contains just six classes of olfactory neurons and seven olfactory receptors (one class of neuron expresses two receptors). This simplicity made it easier for the authors to study, and by comparing the genomes of different Drosophila species, Ray et al. identified gene-specific regulatory elements upstream of these olfactory receptor genes that were highly conserved across species, thereby indicating their likely importance. By creating fusion genes between these regulatory stretches of DNA and a reporter gene, GAL4 (which encodes a yeast transcription factor that binds to upstream activating sequences and activates the green fluorescent protein reporter gene), Ray et al. showed that these regulatory regions are necessary for neuron-specific expression; when mutated, they could no longer cause genes to be expressed in olfactory neurons. The authors also showed that most of these regulatory elements in multiple copies could drive expression in the olfactory neurons of the maxillary palps. For example, the olfactory receptor gene Or46a is expressed in a type of neuron called pb2B. When the conserved region of DNA upstream of Or46a was mutated, the expression of the paired GAL4 marker was lost. Scanning electron micrograph of a Drosophila head overlaid with expression patterns of four odor receptors in the antenna (blue, yellow, and magenta) and the maxillary palp (green). Sequences represent cis elements conserved upstream of odor receptor ... Not all of the regulatory elements carried out this kind of “positive regulation”—some were negative regulators. One olfactory receptor gene, Or42a, had two highly conserved upstream elements, and mutation of one of these resulted in the expression of a reporter gene in two types of olfactory neurons, rather than just one. It seems that upstream regulatory sequences of this receptor drive expression in both types of neurons, but the negative regulator suppresses expression in neurons other than the appropriate class. Another gene, Or59c, is close to a binding site for a known transcription factor (Scalloped or Sd) that is expressed in olfactory organs. Functional analysis showed that Scalloped seems to repress expression of Or59c in the neighboring neurons So it appears that the neuron-specific expression of olfactory receptor genes in the maxillary palp depends on an interplay between positive and negative regulatory elements that are specific to each gene. The authors liken these elements to molecular zip codes that specify the neuronal “address” at which each gene should be expressed. The cross-species comparison showed that these regulatory elements, and the organization of gene expression as well as the odor responses of neurons in the maxillary palp, have been conserved for tens of millions of years since the species in question diverged—even though the amino acid sequences of the olfactory receptors themselves have not been highly conserved. This indicates that the maxillary palp may be important for odor discrimination and olfactory behavior in Drosophila. Interestingly, some of these regulatory elements are also found upstream of genes that are involved in axon guidance, suggesting that they might contribute to the formation of connections between olfactory neurons and neurons in the antennal lobes that receive these connections. So as well as directing the expression of specific olfactory receptors to specific olfactory neurons, these regulatory elements might also control the formation of an olfactory “map” higher in the nervous system.