Do you smell what I smell? Genetic variation in olfactory perception

Respiration-triggered olfactory stimulation reduces obstructive sleep apnea severity: A prospective pilot study

Obstructive sleep apnea is a prevalent sleep-disordered breathing condition characterized by repetitive reduction in breathing during sleep. The current care standard for obstructive sleep apnea is continuous positive air pressure devices, often suffering from low tolerance due to limited adherence. Capitalizing on the unique neurocircuitry of olfactory perception and its retained function during sleep, we conducted a pilot study to test transient, respiration-based olfactory stimulation as a treatment for obstructive sleep apnea markers. Thirty-two patients with obstructive sleep apnea (apnea-hypopnea index ≥ 15 events per hr) underwent two polysomnography sessions, "Odour" and "Control", in random order. In "Odour" nights, patients were presented with transient respiratory-based olfactory stimulation delivered via a computer-controlled commercial olfactometer (Scentific). The olfactometer, equipped with a wireless monitoring, analysed respiratory patterns and presented odour upon detection of respiratory events. No odours were presented in "Control" nights. Following exclusions, 17 patients entered the analysis (four women, 47.4 (10.5) years, body mass index: 29.4 (6.3) kg m-2). We observed that olfactory stimulation during sleep reduced the apnea-hypopnea index ("Odour": 17.2 (20.9), "Control": 28.2 (18.6), z = -3.337, p = 0.000846, BF10 [Bayesian Factor 10]= 57.9), reflecting an average decrease of 31.3% in the number of events. Relatedly, stimulation reduced the oxygen desaturation index by 26.9% ("Odour": 12.5 (15.8), "Control": 25.7 (25.9), z = -3.337, p = 0.000846, BF10 = 9.522). This effect was not linked to the severity of baseline obstructive sleep apnea markers (ρ = -0.042, p = 0.87). Olfactory stimulation did not arouse from sleep or affect sleep structure, measured as time per sleep stage (F1,16 = 0.088, p = 0.77). In conclusion, olfactory stimulation during sleep was effective in reducing the severity of obstructive sleep apnea markers without inducing arousals, and may provide a novel treatment for obstructive sleep apnea, prompting continued research.

Olfactory Sensitivity Is Associated with Body Mass Index and Polymorphism in the Voltage-Gated Potassium Channels Kv1.3

Smell strongly contributes to food choice and its hedonistic evaluation. A reduction or loss of smell has been related to malnutrition problems, resulting in excessive weight loss or gain. Voltage-gated potassium channels Kv1.3 are widely expressed in the olfactory bulb, and contribute mainly to the value of the resting membrane potential and to the frequency of action potentials. Mutations in the Kv1.3 gene are associated with alterations in glycemic homeostasis and olfactory sensitivity. We evaluated the olfactory performance in 102 healthy subjects and its association with BMI and polymorphism in the human Kv1.3 gene. Olfactory performance, based on the olfactory threshold, discrimination and identification scores and their summed score (TDI), was measured using the “Sniffin’ Sticks” test. Subjects were genotyped for the rs2821557 polymorphism of the Kv1.3 gene, whose major allele T was associated with a super-smeller phenotype, lower plasma glucose levels and resistance to diet-induced obesity as compared with the minor allele C. Based on the Kv1.3 genotype, the TDI and I olfactory scores obtained by the subjects were the following: TT > TC > CC. Subjects who were TT homozygous or heterozygous exhibited lower BMIs and reached higher olfactory scores than those with the CC genotype. The results were sex-dependent: heterozygous females performed better than heterozygous males. These findings show an inverse relationship between olfactory function and BMI, and a significant effect of the Kv1.3 genotypes on the olfactory functions and on the BMIs of the subjects. Finally, they suggest that the sex-related differences in the olfactory function can be partially ascribed to the Kv1.3 gene’s polymorphism.

Bitter Taste and Olfactory Receptors: Beyond Chemical Sensing in the Tongue and the Nose

Abstract

The Up-and-Coming-Scientist section of the current issue of the Journal of Membrane Biology features the invited essay by Dr. Mercedes Alfonso-Prieto, Assistant Professor at the Forschungszentrum Jülich (FZJ), Germany, and the Heinrich-Heine University Düsseldorf, Vogt Institute for Brain Research. Dr. Alfonso-Prieto completed her doctoral degree in chemistry at the Barcelona Science Park, Spain, in 2009, pursued post-doctoral research in computational molecular sciences at Temple University, USA, and then, as a Marie Curie post-doctoral fellow at the University of Barcelona, worked on computations of enzyme reactions and modeling of photoswitchable ligands targeting neuronal receptors. In 2016, she joined the Institute for Advanced Science and the Institute for Computational Biomedicine at the FZJ, where she pursues research on modeling and simulation of chemical senses. The invited essay by Dr. Alfonso-Prieto discusses state-of-the-art modeling of molecular receptors involved in chemical sensing – the senses of taste and smell. These receptors, and computational methods to study them, are the focus of Dr. Alfonso-Prieto’s research. Recently, Dr. Alfonso-Prieto and colleagues have presented a new methodology to predict ligand binding poses for GPCRs, and extensive computations that deciphered the ligand selectivity determinants of bitter taste receptors. These developments inform our current understanding of how taste occurs at the molecular level.

Graphic Abstract

Effect of scent on comfort of aircraft passengers

BACKGROUND:

Scents may influence the perceived comfort of an environment. There are only a few studies conducted on the relationship between scent and comfort in aircraft cabin.

OBJECTIVES:

The goal of this research is to explore whether relationships between scents and perceived comfort can be found for passengers in an aircraft cabin.

METHODS:

276 participants joined an experiment in a Boeing 737 fuselage. The participants were divided into nine groups and each joined a session for 60 minutes with the exposure to different scents. The effect of the odor was measured by a set of questionnaires at the beginning and at the end of the session. Results of questionnaires were analyzed regarding the effects on the completion time, of the type of scents, of the intensity of the scent and on gender.

RESULTS:

Significant differences were found at the beginning and at the end of the experiment regarding comfort and emotion, but sometimes no relations could be established. The influence of different scents on comfort/discomfort varied and changed over time. However, in all scenarios, participant’ scores on emotion decreased. Additionally, the added scents influenced the linearity between the changes in comfort and discomfort.

CONCLUSIONS:

Smell could influence the perceived comfort/discomfort of aircraft passengers over time, and different types of smells have different effects on passengers. The preferences on scents are diverse, which highlights the need for personalization in aircraft cabin design.

Assessing the hidden social risk caused by odor in drinking water through population behavioral responses using economic burden

Unpleasant odor in drinking water is a worldwide substantial issue for consumers and water utilities. However, its hidden social impact has been ignored as there are no apparent direct health effects compared with other pollutants. In this study, we developed a method to characterize the adverse effects of a typical odorant based on behavioral responses with the corresponding economic burden, illustrated by 2-methylisoborneol (2-MIB). The dose-response based on behavioral responses to odors using a questionnaire was established in consideration of the bandwagon effect. Results showed that about half of consumers adopted averting behaviors after detecting even very weak odor (but generally recognizable) in drinking water. Total economic burden was determined to be 290690 ± 27427 ¥ per million people per day by the surcharges arising from consumer averting behavior or additional treatment of drinking water odor, among which about 13% of surcharge originated from insensitive people because of bandwagon effect. This is the first study to quantify odor hidden risk based on people's behavioral responses using economic burden, which provides a useful tool to comparing the risks of different types of pollutants in drinking water.

Evaluating the performance of targeted sequence capture, RNA-Seq, and degenerate-primer PCR cloning for sequencing the largest mammalian multigene family

Multigene families evolve from single-copy ancestral genes via duplication, and typically encode proteins critical to key biological processes. Molecular analyses of these gene families require high-confidence sequences, but the high sequence similarity of the members can create challenges for sequencing and downstream analyses. Focusing on the common vampire bat, Desmodus rotundus, we evaluated how different sequencing approaches performed in recovering the largest mammalian protein-coding multigene family: olfactory receptors (OR). Using the genome as a reference, we determined the proportion of intact protein-coding receptors recovered by: (a) amplicons from degenerate primers sequenced via Sanger technology, (b) RNA-Seq of the main olfactory epithelium, and (c) those genes captured with probes designed from transcriptomes of closely-related species. Our initial re-annotation of the high-quality vampire bat genome resulted in >400 intact OR genes, more than doubling the original estimate. Sanger-sequenced amplicons performed the poorest among the three approaches, detecting <33% of receptors in the genome. In contrast, the transcriptome reliably recovered >50% of the annotated genomic ORs, and targeted sequence capture recovered nearly 75% of annotated genes. Each sequencing approach assembled high-quality sequences, even if it did not recover all receptors in the genome. While some variation may be due to limitations of the study design (e.g., different individuals), variation among approaches was mostly caused by low coverage of some receptors rather than high rates of assembly error. Given this variability, we caution against using the counts of intact receptors per species to model the birth-death process of multigene families. Instead, our results support the use of orthologous sequences to explore and model the evolutionary processes shaping these genes.

Association between the rs2590498 polymorphism of Odorant Binding Protein (OBPIIa) gene and olfactory performance in healthy subjects

Olfactory function varies by several orders of magnitude among healthy individuals, who may exhibit a reduced sensitivity (hyposmia), a high sensitivity (hyperosmia), or an olfactory blindness (anosmia). Environmental and genetic factors seem to account for this variability. Most of odorant molecules are hydrophobic and it has been suggested that odorants are transported to the olfactory receptors by means of odorant binding proteins (OBPs). Aim of this study was to evaluate the presence of a relationship between the olfactory performance of healthy subjects and the polymorphism in the odor binding-protein (OBPIIa) gene, the only OBP found in the olfactory epithelium of humans. Using the "Sniffin' Sticks" Extended Test we assessed the olfactory performance in 69 subjects, who were genotyped for the rs2590498 polymorphism of the OBPIIa gene, whose major allele A has been associated with a higher retronasal perception as compared to the minor allele G. We found that subjects homozygous for the A-allele exhibited threshold scores higher than subjects homozous for the G-allele or heterozygous. In addition, subjects classified as normosmic and hyposmic differed on the basis of genotype distribution and allelic frequencies. In fact, a normosmic condition was associated with genotype AA and allele A and a hyposmic condition was associated with genotype GG and allele G. In conclusion, our results show that a relationship exists between the physiological variations of olfactory performance and the OBPIIa gene polymorphism.

Evolutionary ecology of chemosensation and its role in sensory drive

All behaviors of an organism are rooted in sensory processing of signals from its environment, and natural selection shapes sensory adaptations to ensure successful detection of cues that maximize fitness. Sensory drive, or divergent selection for efficient signal transmission among heterogeneous environments, has been a useful hypothesis for describing sensory adaptations, but its current scope has primarily focused on visual and acoustic sensory modalities. Chemosensation, the most widespread sensory modality in animals that includes the senses of smell and taste, is characterized by rapid evolution and has been linked to sensory adaptations to new environments in numerous lineages. Yet, olfaction and gustation have been largely underappreciated in light of the sensory drive hypothesis. Here, we examine why chemosensory systems have been overlooked and discuss the potential of chemosensation to shed new insight on the sensory drive hypothesis and vice versa. We provide suggestions for developing a framework to better incorporate studies of chemosensory adaptation that have the potential to shape a more complete, coherent, and holistic interpretation of the sensory drive.

Approaches for Assessing Olfaction in Children with Autism Spectrum Disorder

Olfactory traits in individuals with autism spectrum disorder (ASD) are considered the strongest predictors of social impairment. Compared to other sensory abnormalities, olfactory abnormalities in individuals with ASD are poorly understood. In this chapter, we provide an overview of the current assessment in individuals with ASD. Several confounding factors have to be considered when conducting research on olfaction in individuals with ASD. Qualitative measures of olfaction contain only limited information about the olfactory stimuli. In addition, little systematic information is available about individual's actual uses of olfaction in daily life. Only a limited number of experimental studies have performed quantitative measurements of olfactory abnormalities in ASD. Therefore, clarifying the relationship between olfactory traits and the influence of real-life situations in a laboratory setting is very difficult. Some new methodologies for measuring olfactory traits are gradually becoming available. New methods that reveal important links between ASD and olfactory traits should be developed in the future.

Pleasantness of olfactory and trigeminal stimulants in different Italian regions

Although individuals categorize odors according to their pleasantness, experience may also influence odor perception-a phenomenon that partially explains why different populations perceive odors differently. Italy, which comprises 20 regions, is characterized by very different cultures. In the present study, we investigated for the first time how Italian regional differences can affect odor perception. 254 healthy volunteers coming from northern, central, southern Italy, and Sicily, one of the two major Italian islands, were recruited in Padua, Rome, Naples, and Syracuse, respectively. Olfactory function was tested with Sniffin' Sticks identification subtest. Subjects who had a score in the range within the mean identification value ± 1 SD, in accordance with the age classes identified in the literature, were asked to judge the odor pleasantness of 20 substances. The hedonic tone of the odorants was categorized as pleasant, neutral, unpleasant, and very unpleasant. Some odorants were appreciated more in northern Italy than in the other parts of the country, whereas others were appreciated more in the south and in Sicily than in the north. Unpleasant odorants were judged less unpleasant in central Italy. Some odorants such as strawberry and vanilla were perceived similarly in all the regional areas. Our study indicates that in Italy, hedonic perception of odorants differs probably in relation with genetic, cultural, and environmental factors. Further investigation is needed to delve deeper into the factors that influence the quality odor perception amongst humans.

Variation in olfactory neuron repertoires is genetically controlled and environmentally modulated

The mouse olfactory sensory neuron (OSN) repertoire is composed of 10 million cells and each expresses one olfactory receptor (OR) gene from a pool of over 1000. Thus, the nose is sub-stratified into more than a thousand OSN subtypes. Here, we employ and validate an RNA-sequencing-based method to quantify the abundance of all OSN subtypes in parallel, and investigate the genetic and environmental factors that contribute to neuronal diversity. We find that the OSN subtype distribution is stereotyped in genetically identical mice, but varies extensively between different strains. Further, we identify cis-acting genetic variation as the greatest component influencing OSN composition and demonstrate independence from OR function. However, we show that olfactory stimulation with particular odorants results in modulation of dozens of OSN subtypes in a subtle but reproducible, specific and time-dependent manner. Together, these mechanisms generate a highly individualized olfactory sensory system by promoting neuronal diversity.

DOI:http://dx.doi.org/10.7554/eLife.21476.001

Smells are simply chemicals in the air that are recognized by nerves in our nose. Each nerve has a receptor that can identify a limited number of chemicals, and the nerve then relays this information to the brain. Animals have hundreds to thousands of different types of these nerves meaning that they can detect a wide array of smells.

Smell receptors are proteins, and the genes that encode these proteins can be very different in two unrelated people. This could partly explain, for example, why some people find certain odors intense and unpleasant while others do not. However, having different genes for smell receptors does not by itself completely explain why some people are more sensitive than others to particular smells. The amounts of each nerve type in the nose might also differ between people and have an effect, but to date it has not been possible to accurately count them all.

Ibarra-Soria et al. have now devised a new method to essentially count the number of each nerve type in the noses of mice from different breeds. The method makes use of a technique called RNA-sequencing, which can reveal which genes are active at any one time, and thus show how many nerves are producing each type of smell receptor. Ibarra-Soria et al. learned that different breeds of mice had remarkably different compositions of nerves in their noses. Further analysis revealed that this was due to changes to the DNA code near to the genes that encode the smell receptor.

Next, Ibarra-Soria et al. sought to find out how the amount of each nerve type is controlled by giving mice water with different smells for weeks and looking how this affected their noses. These experiments revealed that a small number of the nerve types became more or less common after exposure to a smell. The altered nerves were directly involved in recognizing the smells, proving that the very act of smelling can change the make-up of nerves in a mouse’s nose.

These results confirm that the diversity in the nose of each individual is not only dictated by the types of receptors found in there, but also by the number of each nerve type. The next challenge is to understand better how these differences change the way people perceive smells.

DOI:http://dx.doi.org/10.7554/eLife.21476.002

Challenges, Solutions, and Quality Metrics of Personal Genome Assembly in Advancing Precision Medicine

Even though each of us shares more than 99% of the DNA sequences in our genome, there are millions of sequence codes or structure in small regions that differ between individuals, giving us different characteristics of appearance or responsiveness to medical treatments. Currently, genetic variants in diseased tissues, such as tumors, are uncovered by exploring the differences between the reference genome and the sequences detected in the diseased tissue. However, the public reference genome was derived with the DNA from multiple individuals. As a result of this, the reference genome is incomplete and may misrepresent the sequence variants of the general population. The more reliable solution is to compare sequences of diseased tissue with its own genome sequence derived from tissue in a normal state. As the price to sequence the human genome has dropped dramatically to around $1000, it shows a promising future of documenting the personal genome for every individual. However, de novo assembly of individual genomes at an affordable cost is still challenging. Thus, till now, only a few human genomes have been fully assembled. In this review, we introduce the history of human genome sequencing and the evolution of sequencing platforms, from Sanger sequencing to emerging "third generation sequencing" technologies. We present the currently available de novo assembly and post-assembly software packages for human genome assembly and their requirements for computational infrastructures. We recommend that a combined hybrid assembly with long and short reads would be a promising way to generate good quality human genome assemblies and specify parameters for the quality assessment of assembly outcomes. We provide a perspective view of the benefit of using personal genomes as references and suggestions for obtaining a quality personal genome. Finally, we discuss the usage of the personal genome in aiding vaccine design and development, monitoring host immune-response, tailoring drug therapy and detecting tumors. We believe the precision medicine would largely benefit from bioinformatics solutions, particularly for personal genome assembly.

Hierarchical deconstruction of mouse olfactory sensory neurons: from whole mucosa to single-cell RNA-seq

The mouse olfactory mucosa is a complex chemosensory tissue composed of multiple cell types, neuronal and non-neuronal. We have here applied RNA-seq hierarchically, in three steps of decreasing cellular heterogeneity: starting with crude tissue samples dissected from the nose, proceeding to flow-cytometrically sorted pools of mature olfactory sensory neurons (OSNs), and finally arriving at single mature OSNs. We show that 98.9% of intact olfactory receptor (OR) genes are expressed in mature OSNs. We uncover a hitherto unknown bipartition among mature OSNs. We find that 19 of 21 single mature OSNs each express a single intact OR gene abundantly, consistent with the one neuron-one receptor rule. For the 9 single OSNs where the two alleles of the abundantly expressed OR gene exhibit single-nucleotide polymorphisms, we demonstrate that monoallelic expression of the abundantly expressed OR gene is extremely tight. The remaining two single mature OSNs lack OR gene expression but express Trpc2 and Gucy1b2. We establish these two cells as a neuronal cell type that is fundamentally distinct from canonical, OR-expressing OSNs and that is defined by the differential, higher expression of 55 genes. We propose this tiered experimental approach as a paradigm to unravel gene expression in other cellularly heterogeneous systems.

Identification and Differential Expression of a Candidate Sex Pheromone Receptor in Natural Populations of Spodoptera litura

Olfaction is primarily mediated by highly specific olfactory receptors (ORs), a subfamily of which are the pheromone receptors that play a key role in sexual communication and can contribute to reproductive isolation. Here we cloned and identified an olfactory receptor, SlituOR3 (Genbank NO. JN835270), from Spodoptera litura, to be the candidate pheromone receptor. It exhibited male-biased expression in the antennae, where they were localized at the base of sensilla trichoidea. Conserved orthologues of these receptors were found amongst known pheromone receptors within the Lepidoptera, and SlituOR3 were placed amongst a clade of candidate pheromone receptors in a phylogeny tree of insect ORs. SlituOR3 is required for the EAG responses to both Z9E11-14:OAc and Z9E12-14:OAc SlituOR3 showed differential expression in S. litura populations attracted to traps baited with a series of sex pheromone blends composed of different ratios of (9Z,11E)-tetradecadienyl acetate (Z9E11-14:OAc) and (9Z,12E)-tetradecadienyl acetate (Z9E12-14:OAc). The changes in the expression level of SlitOR3 and antennal responses after SlitOR3 silencing suggested that SlitOR3 is required for the sex pheromone signaling. We infer that variation in transcription levels of olfactory receptors may modulate sex pheromone perception in male moths and could affect both of pest control and monitoring efficiency by pheromone application after long time mass trapping with one particular ratio of blend in the field.

The search for human pheromones: the lost decades and the necessity of returning to first principles

As humans are mammals, it is possible, perhaps even probable, that we have pheromones. However, there is no robust bioassay-led evidence for the widely published claims that four steroid molecules are human pheromones: androstenone, androstenol, androstadienone and estratetraenol. In the absence of sound reasons to test the molecules, positive results in studies need to be treated with scepticism as these are highly likely to be false positives. Common problems include small sample sizes, an overestimate of effect size (as no effect can be expected), positive publication bias and lack of replication. Instead, if we are to find human pheromones, we need to treat ourselves as if we were a newly discovered mammal, and use the rigorous methods already proven successful in pheromone research on other species. Establishing a pheromone relies on demonstration of an odour-mediated behavioural or physiological response, identification and synthesis of the bioactive molecule(s), followed by bioassay confirmation of activity. Likely sources include our sebaceous glands. Comparison of secretions from adult and pre-pubertal humans may highlight potential molecules involved in sexual behaviour. One of the most promising human pheromone leads is a nipple secretion from the areola glands produced by all lactating mothers, which stimulates suckling by any baby not just their own.