Asbjørn Følling (1888–1973), a Norwegian doctor and specialist in metabolism, is famous for his flair – in the literal sense – for discovering a previously unknown disease, phenylketonuria (PKU), still called Følling’s disease in Scandinavia.

Phenylketonuria is a genetic disease that inhibits the body’s ability to naturally metabolize phenylalanine, a substance that is widely present in our diets since it is found in virtually all animal and plant proteins. Our body needs phenylalanine even though it cannot produce it. It is, therefore, an essential part of our diet. People with phenylketonuria do not have the enzyme that transforms phenylalanine, which then accumulates before being transformed into toxic substances by the liver (phenylketones). The prevalence of this metabolic disease varies from one country to another. Estimated numbers are 1/10,000 in Europe, including high numbers in Turkey, for example, with 1/4,000 but the disease is almost absent in Finland (1/100,000), without any real explanation for these differences.

Until Følling’s discovery (Følling 1934), no one knew anything about this serious condition, which results in developmental problems in childhood characterized by motor and intellectual difficulties that can lead to severe intellectual disabilities. One day, Dr. Følling welcomed a mother and her 6-year-old daughter into his office. The child was born without any particular problems, but over time the mother realized that her daughter was not developing normally and that she was showing increasing signs of motor and intellectual disabilities. At 6 years of age, she walked with a lot of pain and could say only a few words. In addition, this woman had given birth to a son two years after her daughter. Problems linked to his development were even more noticeable; at 4 years of age, he could not speak, could not walk, and could not eat alone. The mother of these children had seen many specialists, but at that time, medicine did not have many answers to questions about developmental disability or intellectual disability.

Følling, like his colleagues, used the usual tests and diagnostic tools without detecting anything that could explain a deterioration in the health of the two children. However, one particular element – at first sight insignificant – caught his attention. The children emitted a strong odor, similar to the smell of mold. It emanated from both their skin and urine. Urine tests carried out before and then by the other doctors had not revealed anything in particular and Følling decided to carry out another test, the Gerhard test. The aim was to look for the presence of a compound called acetylacetic acid, a potential marker of diabetes. No link between diabetes and developmental and intellectual disability could be found so Følling decided to follow a simple observation (“something common is present in the urine of both children”) and carry out an experimental trial (“I use all the tools at my disposal and we will see what comes out of it”). In the Gerhard test, ferric chloride is mixed with urine and if acetylacetic acid is present, then the urine turns purple. In this case and to the doctor’s great surprise, the urine of the two children turned green! The conclusion that an unknown molecule was present in the urine of the two children was certain, so all that remained was to identify it (although, at this stage, it was not certain that there was a link between the molecule and the children’s deficient state). After many months of analysis, he finally discovered that the substance in question was phenylpyruvic acid, normally absent in the urine. He wondered whether this substance had a role in the children’s motor and intellectual disabilities.

To answer this new question and in a sensible approach, Dr. Følling applied Gerhard’s test to 400 children in specialized institutions in Norway. In about 10 of them, the urine turned green, and it was concluded that it was probably a metabolic dysfunction of genetic origin. Since phenylpyruvic acid is normally absent in the urine, it had to come from the transformation of another molecule. Starting from the proximity of the molecular structure, he suspected phenylalanine, an amino acid that is common in our diets. This was a bold statement at the time as it amounted to considering that the intellectual and motor disabilities observed in the children could be due to a substance naturally present in certain foods. Consistent in his approach, he decided to take a certain number of children with green urine and subject them to a strict diet without phenylalanine. The problem was significant since this molecule is present in most food products, particularly in milk, eggs, meat, and so on. However, with this strict diet, the urine quickly lost its characteristic musty odor, no longer turned green following the Gerhard test, and, therefore, no longer contained phenylpyruvic acid!

Following Dr. Følling’s exemplary exploratory work, it took several years to develop processes to eliminate phenylalanine from foods and to provide alternative foods for patients with phenylketonuria. It was only in 1953 (Bickel et al. 1953) that the first complete diet treatment was offered and in 1963 (Guthrie and Susi 1963) that a routine screening test was developed for newborns.

Currently, the mechanisms involved in this disease are well understood (Ghozlan and Munnich 2004), from the genetic dysfunction that underlies the process to the metabolic dysfunctions that result from it and to the neurological disorders that cause motor and cognitive disorders. Thanks to screening and a suitable diet low in phenylalanine, children can develop normally and no longer show any kind of disability.

There is no doubt that the citizen of Oslo, Asbjørn Følling, would have deserved a Nobel Prize in Medicine and Physiology for this major discovery due to his observational skills and sense of smell that helped to improve the lives of thousands of children around the world.

On December 10, 2004, American researchers Linda Buck and Richard Axel received the Nobel Prize in Physiology and Medicine from the Karolinska Institute in Stockholm. This prize was awarded in recognition of their work on the discovery of olfactory receptors, first published on April 5, 1991, in the journal Cell, a prestigious journal in cell biology. At that time, the interest of the scientific community was immediately aroused by the discovery of a new gene family coding olfactory receptors. This discovery finally confirmed the molecular basis for odor recognition. Until then, no one had been able to explain how the olfactory system detected the thousands of odors we are likely to perceive (see Chapter 4). In the study of sensory mechanisms, that of the sense of smell, therefore, proved to be very delayed compared to other modalities such as hearing (the Nobel Prize was awarded to Von Bekesy in 1961 for his work on the cochlea) or vision (the Nobel Prize was awarded to Wald in 1967 for his work on the retina). Four decades thus separate discoveries relating to transduction in the eye and inner ear from discoveries based on the sense of smell. However, while the publication of Buck and Axel’s paper improved the understanding of odor decoding mechanisms, many subsequent questions remain open on integrative and treatment processes in the central nervous system, on the impact of odors on behavior, and on psychological processes – questions that are addressed in this book.

These are receptors coupled with a specific type of protein (G proteins already known to be involved in the recognition of certain neurotransmitters and hormones) which, when activated, cause a series of chain reactions inside the cell leading to the activation of nerve impulses. Perhaps the most surprising result following the American researchers’ discovery was the considerable number of genes coding olfactory receptors in humans. In the initial article published in the Cell journal in 1991, only 18 genes were mentioned. It should be noted that, in an animal with a high-performing sense of smell like mice, about a thousand genes have been inventoried. In humans, 862 olfactory genes have been identified, with the knowledge that many of them (56%) are actually pseudogenes, which means that they are not (or no longer) functional¹. A decline in smell is often noted when discussing evolution in the human species (although this issue is controversial – see Chapter 7), while there are almost no pseudogenes in some mammals such as mice.

But Linda Buck’s story does not end with her Nobel Prize. In fact, it constitutes an illustrated case of retractions in science. In 2001, she published an article in the renowned journal Nature with Zhihua Zou, one of her post-doctoral students in her Harvard laboratory (Zou et al. 2001), for which she had to sign a retraction in 2008. The results initially published could not be replicated, with replication being – as everyone can agree – the best guarantee of validity. The first author of the publication, Zou, refused to sign the retraction in a deleterious atmosphere where Linda Buck apparently blamed him. In any case, this raised questions about Ms. Buck’s role: holding her responsible suggests that she did not “control” (or only from a very far distance) her student’s work. Worse still, two and a half years later, she had to make two further retractions for articles published in 2005 in the journal PNAS (Proceedings of the National Academy of Sciences) and in 2006 in the journal Science. Before their retraction, these two articles had been widely cited in the scientific literature.

Linda Buck was in no way disconcerted by these retraction cases. She continues to travel to international conferences and receive applause. A little like the current policy, caught with their hand in the cookie jar, the guilty party apologizes flatly² (or not) and continues on their way as if nothing or almost nothing had happened.

These excesses in research have become a real scourge, a depressing gangrene. Every week, the Retraction Watch website created in 2010 announces the retraction of scientific literature from several publications, with a total of between 500 and 600 per year! The changes in researchers’ activities in recent years are undoubtedly not unconnected to this phenomenon: the hunt for fame, the race for contracts and funding, etc., are in the image of the conflicts of interest that are today flourishing everywhere.

Withdrawal or retraction means that the publisher considers that the data are no longer considered reliable and the article should no longer be cited. There are many reasons for the loss of reliability. Historically, the most well-known fraud is outright plagiarism, which persists despite the verification tools put in place by the major scientific publishing houses. Another barely believable but very real case of fraud is for an author to be their own reviewer. Indeed, scientific journals are said to be peer-reviewed and each paper submission is generally critically analyzed by two or three experts in the field, not having any relationship of interest with the team that wishes to publish. Many publications have, therefore, been retracted in recent years simply because the reviewers were the authors themselves. Of course, there is still the widespread possibility of tampering with or even simply inventing experimental data. The biggest counterfeiters are undoubtedly the Japanese anesthetist Yoshitaka Fujii and the German Joachim Boldt, whose dozens of articles have been removed. Other incredible stories are told in labs, such as that of the Czech biochemist who had to retract an article after he was, surprised by video surveillance in the process of manipulating samples to verify his controversial initial results!

Obviously, the list of scientific misdeeds on the Retraction Watch website is growing faster than Pinocchio’s nose!

Sperm has to make a long and perilous journey to reach the egg. Until the early 2000s, the process by which they progress and orient themselves to reach and fertilize the egg was not known, and many teams of researchers were working on this issue. However, it was recognized that sperm orientation was most likely dependent on the chemical environment (a phenomenon called chemotaxis) as demonstrated in the aquatic environment in some invertebrates. In this context, Marc Spehr’s German team working on reproductive biology focused on a new hypothesis: the potential presence of olfactory receptors (ORs) in sperm. In the journal Science (Spehr et al. 2003), the team published its work, which sent shockwaves through the scientific community. Marc Spehr and his collaborators first isolated an olfactory receptor that is actually present in the sperm, code name OR1D2 (alias hOR17-4). Not knowing the nature of the odorant that could bind to this receptor, the researchers proceeded by experimental trial and error using several molecules. They discovered that, by using the fluorimetric method, that bourgeonal (lily of the valley scent) not only activated this receptor but was also a chemical attractant for sperm in vitro. This molecule was indeed present in the fluid surrounding the egg, but no one knew where it came from, from the egg itself or from other elements of the female reproductive system.

Once this discovery was published, a large number of popular articles appeared with catchy titles such as: “sperm have a nose”, “the egg smells like lily of the valley”, “the sperm’s favorite fragrance”, and so on. It has been popularized that sperm travel using their sense of smell to reach the egg, which, in turn, emits an attractive smell. This misunderstanding continued until the 1990s, reaching its peak with the publication (Sinding et al. 2013) of another German team from the University of Dresden, aimed at linking male infertility with anosmia (olfactory loss) specific to lily of the valley scent. The idea was relevant, but the results were unconvincing despite the researchers’ peremptory conclusions.

Infertility affects a significant proportion of the population. A study conducted in France in 2007–2008 showed that the proportion of couples wishing to have a child and whose pregnancy was not announced after 12 months was 24% and 11% after 24 months. The logic of shared responsibility is respected since in about 50% of cases, it is the man who is infertile. A number of causes are well-identified, such as a lack of sperm mobility, as in the case of varicocele¹ for example, or a deficiency in sperm composition. However, for 25% of infertile men, there are no symptoms that can be diagnosed as to the reason for infertility (this is called idiopathic infertility).

It has, therefore, been known since 2003, with the discovery of the OR1D2 receptor, mentioned above, that olfactory receptors are present in regards to sperm, particularly in the middle part of the flagella, and that they play a role in mobility. The OR1D2 receptor is activated by bourgeonal (lily of the valley scent) and inhibited by undecanal (green leaf scent). Assuming that bourgeonal is an attractive chemical agent for human sperm, the German researchers hypothesized that men who were insensitive to the scent of lily of the valley could have sperm that was not receptive to bourgeonal. Without this activation mechanism, the lack of response to bourgeonal could be responsible for fertility problems².

Two groups of men were compared: a fertile group composed of 22 young fathers (average age of 31 years) who served as a control group, and a group of 15 men (average age of 34.5 years) with idiopathic infertility. Men in the latter group were recruited from infertility consultants whose sperm tests showed no evidence of infertility and whose partner was not infertile, did not take any contraceptive methods, and had not reported pregnancy in the past 2 years. Sensitivity was self-assessed for three odorants: bourgeonal, helional (green smell, wet grass type, and close to the smell of melon) retained for its structural proximity to the bourgeonal, and phenyl ethyl alcohol (close to a rose scent) without a structural relationship with bourgeonal. Measuring the intensity of odors revealed that the group of infertile men considered bourgeonal to be less intense than the group of fertile men, while no difference was found for the other two odors. The authors attributed this decrease in sensitivity to a reduced functioning of the OR1D2 receptor. They even concluded that it was possible to diagnose certain types of infertility with olfactory threshold tests and also proposed an ecological contraceptive method based on undecanal, which is known to inhibit the OR1D2 receptor. However, these conclusions had to be qualified because, in the same study, the authors had carried out standardized measurements of detection thresholds for the three odorants (measures more suitable for evaluating a dysfunction of olfactory receptors) that did not show any difference between the two groups of men, and this for the three odors, including bourgeonal³.

The physiological reality is probably more complex. Indeed, progesterone, a female sex hormone, is attractive to sperm by acting on specific ionic channels called CatSper (cation channels of sperm; in context, not to be mistakenly spelled CatSpehr…). Progesterone opens these channels and modifies the calcium level in the sperm and, consequently, the mobility of the flagellum. The men whose gene regulating the function of CatSper is defective are, therefore, infertile. We now know that bourgeonal acts in the same way as progesterone by opening the CatSper channels directly, without passing through the olfactory receptors present in the sperm and following the complex sequence of events activating nasal olfactory cells. Moreover, bourgeonal can only be active at concentrations a thousand times higher than that of progesterone. The initial study of Spehr in vitro approached this criterion, which is, therefore, very far from the physiological reality. The poetic Lily of the Valley phenomenon would, therefore, ultimately only be a laboratory artifact and it cannot be said that sperm smells like lily of the valley.

In addition, there are olfactory receptors elsewhere than in the nose and on sperm cells. In 2014, a biomedical team in Washington discovered olfactory receptors in the lungs (Gu et al. 2014). They were found in the membranes of specific pulmonary neuroendocrine cells⁴ (PNECs). These olfactory receptors respond to external chemical agents (such as air pollutants) and trigger the release of hormones that can induce contraction of the respiratory system. They therefore contribute directly to the protection of the body against irritating or toxic substances present in the atmosphere. Knowing that the contraction of the respiratory system is a process that occurs in people who are hypersensitive to certain external volatile agents (exhaust fumes, cigarette smoke, perfumes, etc.) and suffer from chronic respiratory diseases, it can be considered that the olfactory receptors of PNECs become interesting therapeutic targets in classical diseases such as asthma, emphysema, or obstructive pulmonary disease.

Other more recent studies have reported the existence of olfactory receptors in the blood, heart, and even skin. Strangely, like a fable by La Fontaine, the protocols are based on the same principle as the one that prevailed in Spehr’s initial study on sperm and lily of the valley.

Chandler Burr’s book The Emperor of Scent¹ should be one of the mandatory texts for doctoral school lessons. This will remind PhD students and future researchers that they require a little bit of madness, but that stubbornness is also a significant trait to have… a fantastic one, depending on the context. This book reads like a novel and is a true story, recounting Luca Turin’s work. This passionate researcher in physiology and biophysics was born in 1953. He is a great traveler who has taught in London, worked at the CNRS in France, and who also worked in the United States, Greece, and Germany. He is first of all passionate and very knowledgeable about perfumes, on which he has written a reference guide. He is also interested in how our sense of smell works, mainly with regard to transduction, i.e. the mechanism that triggers a potential for action (or nervous impulses) at the receptors located in the nasal cavity. Unlike almost all researchers in the late 1980s, he is a follower of the vibrational theory² first developed by Dyson (1938) and then taken up by Wright (1964, 1972). Until the publication of Buck and Axel (1991; see Chapter 2), although many researchers suspected a shape recognition mechanism, other hypotheses were possible, including vibration theory. The theory is based on Dyson’s idea that our body has a biological spectroscope and that human cells act as conductors of electrons by the tunnel effect³ through the proteins they contain. However, despite the discovery by Buck and Axel (which won them a Nobel Prize a few years later), Luca Turin would continue to develop, argue and try to disseminate his theory. This was a time, not so long ago, when researchers had free reign to work on one subject or another, or even in one discipline or another without having to be accountable… today, this would be considered a Utopia!

Turin first considered the obstacles of pattern recognition theory and pointed out that two molecules very close in a configuration such as enantiomers⁴ can induce very different odor sensations, while two extremely distinct molecules can generate the same odor perception. He also pointed out that the number of receptors is much lower (at the time estimated at about a thousand) than the number of odors perceived by humans (estimated at about 10,000). In 1995, Axel proposed a solution to this flaw by explaining that it is not the entire molecule that coincides with a receptor but only a part of it and that, in addition, the same molecule can, therefore, be linked to several receptors. The pattern recognition theory becomes combinatorial but, consequently, makes it even more complex to associate the characteristics of a molecule with a percept… a complexity that is still relevant today (see Chapter 13).

With determination, Turin finally succeeded in showing that two very different molecules, sulfur and borane, vibrate at the same frequency (~2,500) and present the same odor. By chance or in terms of proof, vibration theory linked to olfaction could not be proven. The following problem remained (or rather it was against him): the problem of enantiomers vibrating at the same frequency but not having the same odor! This was a good way of getting a theory straight, but like Axel with combinatorics in pattern recognition, Turin found a way through: if the vibrations are identical, it is necessary because the “reader” is different… depending on the shape of the molecule (the usual metaphor here being that of a pair of right-handed scissors being used with the left hand, the tool is the same, the hands similar but the orientation different). Then began a battle for Turin with the publishers of scientific journals, including the famous Nature, which rejected the submission. It was finally accepted in Chemical Senses in 1996. The personality as well as the results of Turin were then denigrated while the work of Buck and Axel obtained the Nobel Prize in 2004.

For almost 15 years, only a few scattered articles questioned vibration theory in olfaction. But from 2010 onward, unresolved questions about the perception of odors rekindled the research controversy. In the prestigious PNAS journal (Franco et al. 2011), Turin, with a Greek team, based his theory on results obtained in Drosophila and then in PLOS ONE (Gane et al. 2013) in humans. In 2015, again in the PNAS journal, the fierce debate on vibration theory, which was more than 30 years old by that point, resurfaced (Block et al. 2015; Turin et al. 2015; Vosshall 2015).

Luca Turin’s determination over so many years raises a smile among us, but the history of science reminds us how many people have fought to finally be right, so we must be careful! Without returning to Galileo, one of the recent famous discoveries – after being widely criticized – is probably that of Barry Marshall (Marshall and Warren 1984). With his colleague, he supported the idea that most stomach ulcers were caused by a bacterium (Helicobacter pylory), while for many decades, the medical world had argued that the causes were related to stress, a too spicy diet, or excessive acidity. Marshall’s hypothesis was mocked by the guardians of orthodoxy who claimed that bacteria could not survive in an environment as acidic as that of the stomach. Bravely, for an experiment, Marshall swallowed the contents of a test tube for culturing this bacterium and very quickly developed an ulcer, which he then treated with an antibiotic course of treatment. This daring and sardonic demonstration was immediately and unanimously welcomed by the scientific community. Marshall and Warren were awarded the Nobel Prize in Physiology and Medicine in 2005 for this discovery.

Today, beyond the strict field of olfaction, the general question of the role of molecular vibration is still very acute in molecular biology and physiology (e.g. Chee et al. 2015; Hoehn et al. 2017).