Response Plasticity of Drosophila Olfactory Sensory Neurons.

In insect olfaction, sensitization refers to the amplification of a weak olfactory signal when the stimulus is repeated within a specific time window. In the vinegar fly, Drosophila melanogaster, this occurs already at the periphery, at the level of olfactory sensory neurons (OSNs) located in the antenna. In our study, we investigate whether sensitization is a widespread property in a set of seven types of OSNs, as well as the mechanisms involved. First, we characterize and compare the differences in spontaneous activity, response velocity and response dynamics, among the selected OSN types. These express different receptors with distinct tuning properties and behavioral relevance. Second, we show that sensitization is not a general property. Among our selected OSN types, it occurs in those responding to more general food odors, while OSNs involved in very specific detection of highly specific ecological cues like pheromones and warning signals show no sensitization. Moreover, we show that mitochondria play an active role in sensitization by contributing to the increase in intracellular Ca2+ upon weak receptor activation. Thus, by using a combination of single sensillum recordings (SSRs), calcium imaging and pharmacology, we widen the understanding of how the olfactory signal is processed at the periphery.

The Drosophila melanogaster Na+/Ca2+ Exchanger CALX Controls the Ca2+ Level in Olfactory Sensory Neurons at Rest and After Odorant Receptor Activation.

CALX, the Na+/Ca2+ exchanger in Drosophila, is highly expressed in the outer dendrites of olfactory sensory neurons (OSNs) which are equipped with the odorant receptors (ORs). Insect OR/Orco dimers are nonselective cation channels that pass also calcium which leads to elevated calcium levels after OR activation. CALX exhibits an anomalous regulation in comparison to its homolog in mammals sodium/calcium exchanger, NCX: it is inhibited by increasing intracellular calcium concentration [Ca2+]i. Thus, CALX mediates only Ca2+ efflux, not influx. The main goal of this study was to elucidate a possible role of this protein in the olfactory response. We first asked whether already described NCX inhibitors were capable of blocking CALX. By means of calcium imaging techniques in ex-vivo preparations and heterologous expression systems, we determined ORM-10962 as a potent CALX inhibitor. CALX inhibition did not affect the odor response but it affected the recovery of the calcium level after this response. In addition, CALX controls the calcium level of OSNs at rest.

Corrigendum: In Vivo Isotopic Labeling of Symbiotic Bacteria Involved in Cellulose Degradation and Nitrogen Recycling within the Gut of the Forest Cockchafer (Melolontha hippocastani).

[This corrects the article on p. 1970 in vol. 8, PMID: 29075241.].

In Vivo Isotopic Labeling of Symbiotic Bacteria Involved in Cellulose Degradation and Nitrogen Recycling within the Gut of the Forest Cockchafer (Melolontha hippocastani).

The guts of insects harbor symbiotic bacterial communities. However, due to their complexity, it is challenging to relate a specific symbiotic phylotype to its corresponding function. In the present study, we focused on the forest cockchafer (Melolontha hippocastani), a phytophagous insect with a dual life cycle, consisting of a root-feeding larval stage and a leaf-feeding adult stage. By combining in vivo stable isotope probing (SIP) with 13C cellulose and 15N urea as trophic links, with Illumina MiSeq (Illumina-SIP), we unraveled bacterial networks processing recalcitrant dietary components and recycling nitrogenous waste. The bacterial communities behind these processes change between larval and adult stages. In 13C cellulose-fed insects, the bacterial families Lachnospiraceae and Enterobacteriaceae were isotopically labeled in larvae and adults, respectively. In 15N urea-fed insects, the genera Burkholderia and Parabacteroides were isotopically labeled in larvae and adults, respectively. Additionally, the PICRUSt-predicted metagenome suggested a possible ability to degrade hemicellulose and to produce amino acids of, respectively, 13C cellulose- and 15N urea labeled bacteria. The incorporation of 15N from ingested urea back into the insect body was confirmed, in larvae and adults, by isotope ratio mass spectrometry (IRMS). Besides highlighting key bacterial symbionts of the gut of M. hippocastani, this study provides example on how Illumina-SIP with multiple trophic links can be used to target microorganisms embracing different roles within an environment.

Identification and characterization of the bombykal receptor in the hawkmoth Manduca sexta.

Manduca sexta females attract their mates with the release of a species-specific sex-pheromone blend, with bombykal (E,Z)-10,12-hexadecadienal and (E,E,Z)-10,12,14-hexadecatrienal being the two major components. Here, we searched for the hawkmoth bombykal receptor in heterologous expression systems. The putative pheromone receptor MsexOr1 coexpressed with MsexOrco in Xenopus oocytes elicited dose-dependent inward currents upon bombykal application (10-300 µmol l-1), and coexpressed in HEK293 and CHO cells caused bombykal-dependent increases in the intracellular free Ca2+ concentration. In addition, the bombykal receptor of Bombyx mori BmOr3 coexpressed with MsexOrco responded to bombykal (30-100 µmol l-1) with inward currents. In contrast, MsexOr4 coexpressed with MsexOrco responded neither to bombykal (30-100 µmol l-1) nor to the (E,E,Z)-10,12,14-hexadecatrienal mimic. Thus, MsexOr1, but not MsexOrco and probably not MsexOr4, is the bombykal-binding pheromone receptor in the hawkmoth. Finally, we obtained evidence that phospholipase C and protein kinase C activity are involved in the hawkmoth's bombykal-receptor-mediated Ca2+ signals in HEK293 and CHO cells.

The mouse receptor transporting protein RTP1S and the fly SNMP1 support the functional expression of the Drosophila odorant coreceptor Orco in mammalian culture cells.

BACKGROUND: Functional expression of vertebrate and insect odorant receptors (ORs) in mammalian culture cells is hampered by an incorrect trafficking of these proteins to the plasma membrane. Receptor transporting proteins (RTPs) have been found to enhance the activity of transfected mammalian ORs in several heterologous systems. NEW METHODS: We co-transfected the Drosophila olfactory coreceptor (Orco) in HEK293 cells with a truncated form of the mouse RTP1 (RTP1S) or with the Drosophila sensory neuron membrane protein 1 (SNMP1), which is required for the detection of the pheromone cis-vaccenyl acetate and was shown to be apposed to Orco within the functional receptor unit. RESULTS: Co-transfection of Orco with either of the two constructs led to an enhanced response to stimulations with the synthetic Orco agonist VUAA1, as compared to transfection with Orco alone. COMPARISON WITH EXISTING METHODS: This method enhances the functional expression of Orco in HEK293 cells in comparison to conventional transfection with Orco alone and enables the use of a lower amount of Orco DNA for transfection. CONCLUSION: Mammalian RTPs can enhance the expression of insect ORs. Moreover, the ability of SNMP1 to mimic the RTP1S effect may indicate possible new roles of this protein apart from being involved in pheromone detection. These results provide researchers with a fast and inexpensive way to optimize the functional expression of insect ORs in heterologous systems and open the search for insect proteins analogous to mammalian RTPs.