This was triggered by the discovery of the multigene family of olfactory receptors by Linda B. Buck und Richard Axel in 1991, for which both were awarded the Nobel Prize in 2004. Since then, the nose, in particular the subject of molecular chemosensation, is definitely ”en vogue”. Our team at the RWTH Aachen University has only recently gained new and exciting information that puts some of our previous knowledge of the mammalian sense of smell in jeopardy. We used to think the olfactory system consists of only two anatomically and functionally separate components: the nasal membrane or main olfactory system that actually smells, and the vomeronasal organ (VNO) or accessory olfactory system that detects socially relevant chemosignals, often referred to as pheromones (Figure 1).
Today, two additional olfactory subsystems, the septal organ and the Grüneberg ganglion, have been discovered or rediscovered. And not only that – we now know that the functional structure of the sense of smell is far more complex and differentiated. For instance, some social signals are recognized via the main olfactory epithelium, just as some ‘conventional’ odors are detected via the VNO. Within the subsystems, different neuron families are specialized for specific sensory stimuli and exhibit characteristic axonal projection patterns.
In the last five years in particular, groundbreaking discoveries about such specialized nerve cells in the main olfactory mucosa have revealed a totally different repertoire of signaling proteins than the canonical olfactory cells with which we are familiar. Recent publications have shown, for example, that sensors of the Grüneberg ganglion probably act as both smell and cold sensors.
Studies on the Drosophila NMJ were performed to analyze the synapse structure and assembly    Presynaptic electron dense structures named “Tbars” (owing to their characteristic shape in electron micrographs) were shown to comprise Bruchpilot (BRP). BRP is thought to play a role in signal transduction by acting as a presynaptic scaffolding protein. Through the application of STED technology, in a synergistic combination with established imaging techniques, valuable information concerning the architecture of the roughly 250 nm size T-bar and adjacent structures was obtained.
Similar studies as in Drosophila were performed on murine retina cells, where the composition of presynaptic proteins associated to precursor vesicles, which are thought to promote synaptogenesis, was described .
In the examples above, STED microscopy revealed a very precise distribution of fluorescently-tagged synaptic proteins, which until then were unrecognizable via conventional confocal imaging. When compared to data from electron micrographs, a whole new set of information was retrieved. But unlike EM, STED, due to its simple methodology, allowed image acquisition not only on an uncomplicated and quick fashion, but also in a larger scale, thereby assisting in a more extensive understanding of the synaptic structure and its impact on the signal transduction.
Working together with Prof. Ivan Rodriguez from the University of Geneva, we showed in 2009 that the VNO not only comprises the two already known V1R and V2R receptor families, but also a third group, the formyl peptide receptors (FPR). Two other representatives of this receptor type are known from immunology, where they play a key role in the chemotactic reaction of immune cells toward the site of a bacterial infection or inflammation.
Our results suggest that the FPRs can provide a mechanistic explanation for an already known sensory phenomenon in rodents: mice can smell whether other mice are healthy or sick. We still need to find out a lot more about the molecular processes of the sense of smell and the coding logic of social chemical signals. However, our new understanding of the olfactory system also leads to new and exciting issues. If we can decipher the different receptor types and their signal transmission strategies within neuronal networks, we will better understand the neurophysiological basis of social behavior in mammals.
We use electrophysiological recordings and activity measurements, different imaging methods, molecular- biological, biochemical, and behavior analysistechniques. Using a microscope, particularly for live cell imaging, is one of the central methods in our research. We mainly work with live tissue sections from the nose, the VNO, and the olfactory bulb.
To correlate electrophysiological measurements with imaging data, we need a good fixed stage microscope equipped with the relevant optical techniques such as DIC (Differential Interference Contrast) and plenty of room for manipulation and perfusion systems, and patch clamp pre-amplifiers. For this type of upright microscopy, special water immersion objectives that show high transmission even in the near infrared and UV light range and fast, high-resolution cameras are essential. Apart from the optics and accessibility to the area around the specimen, another main criterion is absolute microscope stability. We also use inverted microscopes and confocal systems for our different imaging approaches.
One of our current research projects focuses on what happens in a vomeronasal nerve cell when an FPR has bound its ligand, i.e. how this chemical binding signal translates into action potentials by the neurons. The biochemical translation of olfactory stimulation into the language of the brain is a key focus of our experiments.
Another topic we are currently working on is the processing of signals in the accessory olfactory bulb, the part of the olfactory brain that receives information from the VNO. We are trying to understand how excitatory and inhibitory neurons interact, and how the information originates that ultimately triggers changes in hormone levels and behavioral reactions.
Findings from the animal model can be transferred to humans only to a limited extent, as the sense of smell is even more important for many animal species than for humans. This can already be seen from the number of relevant genes. A mouse has around 1200 genes exclusively encoding odorant receptors. Only one third of these genes are still functional in humans. Humans no longer show a monomodality that is sometimes observed in animals. We simultaneously process different sensory stimuli, going primarily by visual impressions. Although olfactory cues obviously trigger specific types of behavior in humans as well, we are yet to identify a human pheromone on the molecular level.
Aggressive, territorial or sexual behavior is directly linked to olfactory perception in many mammals. In mice, we can control some behavioral phenotypes via specific chemical signals. I can’t imagine anything similar in humans. Besides, social communication on the basis of chemical signals mainly works conspecifically. So when we find something in a mouse, we get new ideas on how and where to look for something potentially similar in humans.
At the RWTH Aachen, we are in close contact with our colleagues in the Department of Psychiatry. One of the symptoms of neurodegenerative diseases such as Parkinson’s or Alzheimer’s is a damaged sense of smell. The exact link is not clear yet, but we know that this symptom is noticeable at an early stage of the disease. It would therefore be conceivable that this could play a role in early diagnostics. Another extremely futuristic idea, though maybe not entirely absurd, is to develop an artificial VNO, with FPRs acting as a biosensor for diagnosis of medical disorders.