COMMENTARY COMMENTARY Laying a controversial smell theory to rest Leslie B. Vosshalla,b,1 ligand profiles and challenged them with aLaboratory of Neurogenetics and Behavior and bHoward Hughes Medical Institute, The many deuterated and nondeuterated iso- Rockefeller University, New York, NY 10065 topomer pairs. Again, no receptor was found that discriminates between isotopomers. The Inthe50ybeforethecloningoftheodorant confirmed that humans could not discrim- paper closes with a reexamination of the receptors (ORs) by Linda Buck and Richard inate the acetophenone isotopomers after theoretical grounds supporting the vibration Axel in 1991 (1), two competing mechanisms all, but offered new musk isotopomers theory of olfaction (13), and finds them un- for odor detection were discussed, one chem- that could be discriminated. Aside from realistic in a biological milieu. Precisely because this elegant and system- ical and one spectral. The chemical theory being poorly reproducible, these psycho- atic study finds no evidence for the spectral posited that “detectors” respond to physico- physical experiments are fraught with theory, it suffers from the inevitable problem chemical attributes of the odorant, including confounding variables. One cannot rule of being a series of persuasive negative re- molecular size, molecular shape, and func- out that minute quantities of impurities sults. Even having shown that the ORs pro- tional groups (2, 3). The alternative spectral in the isotopomers account for their dis- filed here are not the theoretical ones that can theory hypothesized that the olfactory system criminability or that the molecules are en- discriminate isotopomers, the authors cannot detects molecular vibrations of the odor mol- zymatically transformed to novel odorants exclude the possibility that such an OR ecules (4, 5). This vibration theory was revived by nasal mucus long before they interact in 1996, along with a proposed mechanism would eventually be found. Moreover, all of inelastic electron tunneling spectroscopy Motivated by “disprov- of the work was carried out in tissue culture activated by ligand–receptor interactions (6). ” cells, not in native olfactory sensory neu- These early theories of how ORs detect ing the vibration rons.Althoughsuchexperimentsarethe – odorants were developed using psychophys- theory, Block et al. bring gold standard for studying ligand receptor ical experiments, in which human volun- us important insights interactions of all other G protein-coupled teers sniffed odor molecules and described – receptors, it is possible that heterologous ex- what they perceived. In PNAS, Block et al. into structure activity pression does not reproduce the exact condi- (7) shift the “shape vs. vibration” debate relationships in olfac- tions in which the OR finds itself in the nasal from olfactory psychophysics to the bio- tion, one of the most epithelium. Perhaps the proposed inelastic physics of the ORs themselves. The authors electron tunneling mechanism fails to mount a sophisticated multidisciplinary important problems operate outside the nose. The ball is now in attack on the central tenets of the vibra- facing the field. the court of the believers of the spectral tion theory using synthetic organic chem- theory, who need to produce experimental istry, heterologous expression of ORs, and with their cognate OR, and subsequently and not theoretical evidence that such theoretical considerations to find no evi- yield a smell percept in the brain (12). Fi- problems are actually occurring in these dence to support the vibration theory nally, it seems unlikely that fundamental experiments. – of smell. subatomic mechanisms of OR ligand receptor What are the predictions of the vibration interactions can be inferred from human Why Should Odorant Receptors Be Ex- theory of smell and how would one test it? perceptual experiments alone. ceptional? The paper by Block et al. (7) Proponents of the theory argue that the brings the field to an interesting juncture. answer lies in chemical stimuli that are No Evidence for Vibration-Sensing Odor- Is this negative evidence sufficiently per- considered “identical” in structure but are ant Receptors. Block et al. (7) solve these suasivetolaythetheorytorest,orshould fully deuterated analogs in which hydrogen problemsbytakingthetheorytowhereit the screening of ORs and isotopomers atoms are exchanged with deuterium atoms. should have been all along: at the interface continue until the absence of evidence The thinking is that such molecules, known between odorant molecules and the receptor. passes the standard of reasonable doubt as isotopomers, would have identical molec- They performed clean chemical deuterations that would condemn the spectral theory? ular shapes and functionality but very dif- of the three major classes of isotopomers One interesting feature of the vibration ferent vibrational spectra. Sniff-testing of used in previous work, acetophenone, benz- theory is its exceptionalism. Vertebrate isotopomers has yielded mixed results. Al- aldehyde, and musks, for a total of over 15 ORs are structurally and functionally related though acetophenone and fully deuterated different stimulus pairs. Taking advantage of to the G protein-coupled receptor super- acetophenone were said in the original 1996 recent advances in functional expression and family that senses hormones, neurotransmitters, theoretical paper to be readily discriminable deorphanization of ORs, they systemati- by humans (6), this observation was not cally screened 330 human receptors to iden- Author contributions: L.B.V. wrote the paper.

reproduced in a later study (8). These same tify a single receptor, OR5AN1, strongly Conflict of interest statement: The author is a member of the isotopomers were tested in insects and shown activated by cyclopentadecanone and related Scientific Advisory Board of International Flavors & Fragrances, Inc., to be discriminable by flies (9) and honey musks. In these assays, OR5AN1 responded and receives compensation for her services. bees (10). Gane et al. (11) returned to the equally to all musk isotopomers tested. They See companion article on page E2766. acetophenone question in 2013 and now went on to test another nine ORs with known 1Email: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1507103112 PNAS | May 26, 2015 | vol. 112 | no. 21 | 6525–6526 Downloaded by guest on October 2, 2021 tastants, and light (1, 14, 15), yet no one has bacterium Helicobacter pylori (18). After understanding of the biology of olfaction proposed that these evolutionarily related pro- Marshall bravely drank the causative agent would advance dramatically. If molecular teins signal via vibrational mechanisms. Recent and developed an ulcerative condition, this vibration were in fact predictive of an odor advances in membrane protein structural fringe hypothesis was broadly accepted. As percept, the application of the theory would biology have enabled detailed high-resolution Max Planck said, “Truth never triumphs — its dramatically accelerate the development of studies of non-OR G protein-coupled re- opponents just die out.” (19), and it is im- raw ingredients by the fragrance industry, ceptors (16). To date, no one has succeeded portant for controversial ideas to be tested with which is currently guessing what new mol- in solving the high-resolution crystal struc- empirical data and not reflexively repressed. ecules will smell like rather than engineering ture of an OR with its odor ligand. Once this This is how science progresses, but sometimes them. However, none of these promises have technical barrier is broken, it will be fasci- afringetheoryisjustafringetheory. been delivered by proponents of the vibra- nating to look into the insides of the ORs The vibration theory has been touted to tion theory. Some have pointed out that it is to see if there is any evidence of protein solve one of the central problems in olfaction, a waste of time to expend effort to refute a mechanisms that set the ORs apart from and this promise accounts, in part, for its controversial theory that has few advocates, their close nonolfactory receptor relatives. continued place in the spotlight. After cen- and that attention should be turned instead turies of conjecture on how a molecule leads toward how smell works (20, 21). Motivated The Curious Persistence of a Radical to a smell percept, we still lack a convincing by “disproving” the vibration theory, Block Theory. The year 2016 marks the 25th an- framework to predict the smell of a molecule et al. (7) bring us important insights into niversary of the cloning of the ORs (1) and from its chemical structure. If it were possible structure–activity relationships in olfaction, the 20th anniversary of the revival of the vi- to predict the percept of a molecule reliably one of the most important problems facing bration theory of olfaction (6). In the in- simply from spectroscopic analysis, our the field. tervening years, the chemosensory field has largely ignored the vibration theory, yet it persists. The current paper seeks to de- 1 Buck L, Axel R (1991) A novel multigene family may encode 12 Nagashima A, Touhara K (2010) Enzymatic conversion of termine its fate once and for all, but is this odorant receptors: A molecular basis for odor recognition. Cell 65(1): odorants in nasal mucus affects olfactory glomerular activation – 175 187. patterns and odor perception. J Neurosci 30(48):16391–16398. the end of the story? It is unlikely to go — 2 Guillot M (1948) Physiologie des sensations Sur la relation entre 13 Brookes JC, Hartoutsiou F, Horsfield AP, Stoneham AM (2007) l’odeur et la structure moleculaire. C R Hebd Seances Acad Sci away for a number of reasons, sociological Could humans recognize odor by phonon assisted tunneling? Phys 226(18):1472–1474. French. and scientific. The theory has sociological Rev Lett 98(3):038101. 3 Amoore JE (1967) Specific anosmia: A clue to the olfactory code. 14 Pace U, Hanski E, Salomon Y, Lancet D (1985) Odorant-sensitive appeal because people are sympathetic to Nature 214(5093):1095–1098. the underdog, as vividly brought to life in a 4 Dyson GM (1938) The scientific basis of odour. Chem Ind 57(28): adenylate cyclase may mediate olfactory reception. Nature 647–651. 316(6025):255–258. popular book on the topic (17). The mi- 5 Wright RH (1966) Odour and molecular vibration. Nature 15 Sklar PB, Anholt RR, Snyder SH (1986) The odorant-sensitive nority opinion is also critically important 209(5023):571–572. adenylate cyclase of olfactory receptor cells. Differential for the progress of science. Some “fringe” 6 Turin L (1996) A spectroscopic mechanism for primary olfactory stimulation by distinct classes of odorants. JBiolChem261(33): – ideas like those questioning that HIV cau- reception. Chem Senses 21(6):773 791. 15538–15543. 7 Block E, et al. (2015) Implausibility of the vibrational theory of 16 Nygaard R, et al. (2013) The dynamic process of β(2)-adrenergic – ses AIDS were swept away by evidence, and olfaction. Proc Natl Acad Sci USA 112:E2766 E2774. receptor activation. Cell 152(3):532–542. 8 Keller A, Vosshall LB (2004) A psychophysical test of the vibration 17 Burr C (2002) The Emperor of Scent (Random House, New York). are now largely forgotten. Other provocative – theory of olfaction. Nat Neurosci 7(4):337 338. 18 Marshall BJ, Warren JR (1984) Unidentified curved bacilli in the ideas were ultimately shown to be correct by 9 Franco MI, Turin L, Mershin A, Skoulakis EM (2011) Molecular stomach of patients with gastritis and peptic ulceration. Lancet vibration-sensing component in Drosophila melanogaster olfaction. lonely scientists who persisted in the face of 1(8390):1311–1315. Proc Natl Acad Sci USA 108(9):3797–3802. 19 Planck M, Scientific Autobiography and Other Papers, trans great skepticism from colleagues. An im- 10 Gronenberg W, et al. (2014) Honeybees (Apis mellifera) learn – portant recent example is the Nobel prize- to discriminate the smell of organic compounds from their Gaynor FF (1949) (Philosophical Library, New York), pp 33 34. winning work of Barry J. Marshall and respective deuterated isotopomers. Proc Biol Sci 281(1778): 20 Hettinger TP (2011) Olfaction is a chemical sense, not a spectral 20133089. sense. Proc Natl Acad Sci USA 108(31):E349; author reply E350. J. Robin Warren showing that ulcers are 11 Gane S, et al. (2013) Molecular vibration-sensing component in 21 Jennings C (2004) Testing a radical theory. Nat Neurosci caused by infection of the stomach with the human olfaction. PLoS ONE 8(1):e55780. 7(4):315.

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