We all know that other mammals have a far more finely tuned sense of smell than we humans do. In fact, the olfactory abilities of mice even extend to being able to identify individuals infected with parasites. Now the discovery of a new class of olfactory receptors, reported in Nature last week, has provided an insight into what underlies this ability.
The mammalian olfactory system comprises two major divisions: the main olfactory system, which deals mainly with environmental odours, and the vomeronasal organ, which plays a role in detecting pheromones. The mammalian capacity to distinguish between a huge array of odours lies with the fact that individual sensory neurons in the olfactory system express only a single type of receptor protein on their surface. Each of these receptor types is tuned to a specific type of odorant and transmits signals to a specific part of the brain. Until now only 4 major types of receptor were known: odorant and trace-amine associated receptors, mostly expressed in the main olfactory system, and vomeronasal type 1 and type 2 receptors expressed in the vomeronasal organ. All these receptor types are members of the G-protein-coupled receptor (GPCR) superfamily, which initiate signalling pathways within chemosensory cells.
Riviere and colleagues at the University of Geneva screened mouse vomeronasal tissue for new types of GPCRs and discovered that some sensory neurons express a class of protein known as formyl peptide receptors (FPRs). FPRs were previously known to be expressed in the immune system where they have a role in mediating cellular responses to disease, inflammation and cell damage. Riviere and colleagues found that the FPRs expressed in the vomeronasal tissue respond to the same activators that stimulate the immune system FPRs – that is, peptides and lipids derived from bacteria or associated with inflammation. Five different types of FPR were found in the vomeronasal tissue. In keeping with other types of olfactory receptor, each FPR responds to a distinct, but overlapping array of compounds and individual vomeronasal sensory neurons expressing FPRs do not express any other types of olfactory receptor.
Although the role of FPRs has yet to be confirmed by behavioural studies, this report provides compelling evidence that they represent a novel class of olfactory receptor that allows the detection of diseased individuals or spoiled food. And in case you’re wondering, there’s no evidence that FPRs are found in the olfactory system of humans – we only appear to have them in our immune system.
Rivière, S., Challet, L., Fluegge, D., Spehr, M., & Rodriguez, I. (2009). Formyl peptide receptor-like proteins are a novel family of vomeronasal chemosensors Nature, 459 (7246), 574-577 DOI: 10.1038/nature08029