Daniel Münch

Integrating Heterogeneous Odor Response Data into a Common Response Model: A DoOR to the Complete Olfactome

We have developed a new computational framework for merging odor response data sets from heterogeneous studies, creating a consensus metadatabase, the database of odor responses (DoOR). As a result, we obtained a functional atlas of all available odor responses in Drosophila melanogaster. Both the program and the data set are freely accessible and downloadable on the Internet (http://neuro.uni-konstanz.de/DoOR). The procedure can be adapted to other species, thus creating a family of “olfactomes” in the near future. Drosophila melanogaster was chosen because of all species this one is closest to having the complete olfactome characterized, with the highest number of deorphanized receptors available. The database guarantees long-term stability (by offering time-stamped, downloadable versions), up-to-date accuracy (by including new data sets as soon as they are published), and portability (for other species). We hope that this comprehensive repository of odor response profiles will be useful to the olfactory community and to computational neuroscientists alike.

DoOR 2.0 : Comprehensive Mapping of Drosophila melanogaster Odorant Responses

Odors elicit complex patterns of activated olfactory sensory neurons. Knowing the complete olfactome, i.e. the responses in all sensory neurons for all relevant odorants, is desirable to understand olfactory coding. The DoOR project combines all available Drosophila odorant response data into a single consensus response matrix. Since its first release many studies were published: receptors were deorphanized and several response profiles were expanded. In this study, we add unpublished data to the odor-response profiles for four odorant receptors (Or10a, Or42b, Or47b, Or56a). We deorphanize Or69a, showing a broad response spectrum with the best ligands including 3-hydroxyhexanoate, alpha-terpineol, 3-octanol and linalool. We include all of these datasets into DoOR, provide a comprehensive update of both code and data, and new tools for data analyses and visualizations. The DoOR project has a web interface for quick queries (http://neuro.uni.kn/DoOR), and a downloadable, open source toolbox written in R, including all processed and original datasets. DoOR now gives reliable odorant-responses for nearly all Drosophila olfactory responding units, listing 693 odorants, for a total of 7381 data points.

Weaker Ligands Can Dominate an Odor Blend due to Syntopic Interactions

Most odors in natural environments are mixtures of several compounds. Perceptually, these can blend into a new “perfume,” or some components may dominate as elements of the mixture. In order to understand such mixture interactions, it is necessary to study the events at the olfactory periphery, down to the level of single-odorant receptor cells. Does a strong ligand present at a low concentration outweigh the effect of weak ligands present at high concentrations? We used the fruit fly receptor dOr22a and a banana-like odor mixture as a model system. We show that an intermediate ligand at an intermediate concentration alone elicits the neuron’s blend response, despite the presence of both weaker ligands at higher concentration, and of better ligands at lower concentration in the mixture. Because all of these components, when given alone, elicited significant responses, this reveals specific mixture processing already at the periphery. By measuring complete dose–response curves we show that these mixture effects can be fully explained by a model of syntopic interaction at a single-receptor binding site. Our data have important implications for how odor mixtures are processed in general, and what preprocessing occurs before the information reaches the brain.

More than apples and oranges - Detecting cancer with a fruit fly's antenna

Cancer cells and non-cancer cells differ in their metabolism and they emit distinct volatile compound profiles, allowing to recognise cancer cells by their scent. Insect odorant receptors are excellent chemosensors with high sensitivity and a broad receptive range unmatched by current gas sensors. We thus investigated the potential of utilising the fruit flys olfactory system to detect cancer cells. Using in vivo calcium imaging, we recorded an array of olfactory receptor neurons on the fruit flys antenna. We performed multidimensional analysis of antenna responses, finding that cell volatiles from different cell types lead to characteristic response vectors. The distances between these response vectors are conserved across flies and can be used to discriminate healthy mammary epithelial cells from different types of breast cancer cells. This may expand the repertoire of clinical diagnostics, and it is the first step towards electronic noses equipped with biological sensors, integrating artificial and biological olfaction.

Ir40a neurons are not DEET detectors

N,N-Diethyl-meta-toluamide (DEET) is the most widely used insect repellent, but it requires repeated application at high, potentially harmful, concentrations, which is prohibitively impractical and costly in the countries suffering most from insect vector-borne diseases1; understanding DEET’s mode of action might help identify improved alternatives. Kain et al.2 characterized ionotropic receptor 40a (Ir40a)-expressing olfactory sensory neurons (OSNs)—located in the sacculus of the Drosophila antenna3—as comprising a key pathway mediating aversion to both DEET and chemoinformatically predicted ‘DEET-like’ compounds, using calcium imaging, a transcription-based neural activity reporter, Ir40a RNA interference and behavioural experiments. We are unable to reproduce evidence for physiological activation of Ir40a OSNs by these repellents, and find that Ir40a mutant flies avoid DEET. These results call into question the importance of this sensory pathway in DEET detection. There is a Retraction accompanying this Brief Communication Arising by Kain, P. et al. Nature 534, http://dx.doi.org/10.1038/ nature18613 (2016). Ir40a-expressing OSN dendrites innervate sacculus chambers I and II (Fig. 1a), and their axons project to the ‘arm’ and ‘column’ in the antennal lobe (Fig. 1b)4. Using a calcium imaging-based screen for natural stimuli capable of activating these neurons (Extended Data Fig. 1), we identified ammonia as an agonist that evokes dose-dependent responses in the axon termini of all Ir40a neurons (Fig. 1c, d). This chemical specificity is consistent with the phylogenetic grouping of Ir40a with other members of the IR family of variant ionotropic glutamate receptors that detect amines4. Surprisingly, we were unable to reproduce the reported activation of Ir40a neurons by DEET or DEETlike compounds (butyl anthranilate, ethyl anthranilate and methyl N,N-dimethylanthranilate)2 (Fig. 1e). To rule out insufficient stimulus concentration as a cause for the lack of responses, we presented these chemicals directly to the antenna by manual puffing from a syringe, as described2. Using this method, we detected strong responses to ammonia, but not to DEET or DEET-like chemicals in Ir40a neurons (Fig. 1f). We also visualized calcium levels in Ir40a OSN soma, as described2, but this again revealed responses to ammonia but not DEET or DEET-like compounds (Fig. 1g and Supplementary Videos 1–8). Ir40a neurons were initially implicated as DEET sensors through their activation of a transcriptional reporter of neural activity (calciumdependent nuclear import of LexA, CaLexA; ref. 5)—expressed in all antennal neurons with the driver elav-Gal4—upon long-term DEET exposure2. The reliability of this reporter is, however, unclear, as less than half (2 to 3 out of 6) presumed Ir40a OSNs in chamber I and no chamber II OSNs were described to exhibit DEET-evoked CaLexA activity2. This seems not to be due to insufficient DEET, as exposure to 10% or 100% DEET led to similar CaLexA reporter results2. We therefore repeated these experiments and found, unexpectedly, that among the entire population of antennal neurons, chamber I neurons are particularly susceptible to background CaLexA signals, which were equivalent in flies exposed to no odour, DEET or ammonia (Fig. 1h). Such signals might be due to high basal calcium levels and/or basal nuclear translocation of abundantly expressed CaLexA reporter in these cells; regardless, our observations indicate that, under these experimental conditions, CaLexA cannot be used to report on stimulusevoked activity of sacculus chamber I neurons. To assess the function of Ir40a, we generated Ir40a antibodies and, by CRISPR/Cas9 genome editing, an Ir40a mutant (Extended Data Fig. 2a, b). In wild-type flies, Ir40a co-localizes with the co-receptor Ir25a (ref. 6) in OSN soma and dendrites, suggesting that these function together in sensory detection; in Ir40a mutants, anti-Ir40a immunoreactivity is absent (Extended Data Fig. 2b), confirming this allele to be a protein null. Consistent with these observations, physiological responses of Ir40a OSNs to ammonia are abolished in both Ir40a and Ir25a mutant animals (Extended Data Fig. 2c). We tested behavioural responses of Ir40a and Ir25a mutants in a DEET olfactory aversion assay (Fig. 1i–j). Both mutants avoid DEET as robustly as wild-type controls; in contrast, mutants for the odorant receptor co-receptor Orco no longer avoid DEET (Fig. 1j), confirming previous data in both Drosophila7 and mosquitoes8,9 that olfactory detection of DEET is OR-dependent. In summary, our data fail to reproduce evidence for physiological activation of Ir40a neurons by DEET, or to demonstrate a genetic requirement for either Ir40a or the co-receptor Ir25a in behavioural avoidance of this repellent. We propose that this sensory pathway is not relevant for DEET detection; future work will determine its role in behavioural responses to ammonia, an olfactory cue of ecological importance for many insects10,11.

The circuitry of olfactory projection neurons in the brain of the honeybee, Apis mellifera

In the honeybee brain, two prominent tracts – the medial and the lateral antennal lobe tract – project from the primary olfactory center, the antennal lobes (ALs), to the central brain, the mushroom bodies (MBs), and the protocerebral lobe (PL). Intracellularly stained uniglomerular projection neurons were reconstructed, registered to the 3D honeybee standard brain atlas, and then used to derive the spatial properties and quantitative morphology of the neurons of both tracts. We evaluated putative synaptic contacts of projection neurons (PNs) using confocal microscopy. Analysis of the patterns of axon terminals revealed a domain-like innervation within the MB lip neuropil. PNs of the lateral tract arborized more sparsely within the lips and exhibited fewer synaptic boutons, while medial tract neurons occupied broader regions in the MB calyces and the PL. Our data show that uPNs from the medial and lateral tract innervate both the core and the cortex of the ipsilateral MB lip but differ in their innervation patterns in these regions. In the mushroombody neuropil collar we found evidence for ALT boutons suggesting the collar as a multi modal input site including olfactory input similar to lip and basal ring. In addition, our data support the conclusion drawn in previous studies that reciprocal synapses exist between PNs, octopaminergic-, and GABAergic cells in the MB calyces. For the first time, we found evidence for connections between both tracts within the AL.

An expression atlas of variant ionotropic glutamate receptors identifies a molecular basis of carbonation sensing

Through analysis of the Drosophila ionotropic receptors (IRs), a family of variant ionotropic glutamate receptors, we reveal that most IRs are expressed in peripheral neuron populations in diverse gustatory organs in larvae and adults. We characterise IR56d, which defines two anatomically-distinct neuron classes in the proboscis: one responds to carbonated solutions and fatty acids while the other represents a subset of sugar- and fatty acid-sensing cells. Mutational analysis indicates that IR56d, together with the broadly-expressed co-receptors IR25a and IR76b, is essential for physiological responses to carbonation and fatty acids, but not sugars. We further demonstrate that carbonation and fatty acids both promote IR56d-dependent attraction of flies, but through different behavioural outputs. Our work provides a toolkit for investigating taste functions of IRs, defines a subset of these receptors required for carbonation sensing, and illustrates how the gustatory system uses combinatorial expression of sensory molecules in distinct neurons to coordinate behaviour.Little is known about the role of variant ionotropic glutamate receptors (IRs) in insect taste. Here the authors characterise the expression pattern of IRs in the Drosophila gustatory system and highlight the role of one receptor, IR56d, in the detection of carbonation

Take time: odor coding capacity across sensory neurons increases over time in Drosophila

Due to the highly efficient olfactory code, olfactory sensory systems are able to reliably encode enormous numbers of olfactory stimuli. The olfactory code consists of combinatorial activation patterns across sensory neurons, thus its capacity exceeds the number of involved classes of sensory neurons by a manifold. Activation patterns are not static but vary over time, caused by the temporally complex response dynamics of the individual sensory neuron responses. We systematically analyzed the temporal dynamics of olfactory sensory neuron responses to a diverse set of odorants. We find that response dynamics depend on the combination of sensory neuron and odorant and that information about odorant identity can be extracted from the time course of the response. We also show that new response dynamics can arise when mixing two odorants. Our data show that temporal dynamics of odorant responses are able to significantly enhance the coding capacity of olfactory sensory systems.