Smells influence much of our behaviour, including what we choose to eat and with whom we flirt; they can also alert us to danger. But, despite its importance, we have never fully understood how we smell. Now, scientists from the French National Research Institute for Agricultural Research (INRA) in Jouy-en-Josas, France, have used lab-on-a-chip technology to shed some light on this complicated process.
Smell of success: lab-on-a-chip technology is helping scientists understand the chemistry of smell
Scientists know that aroma molecules, or odorants, bind to olfactory receptors (ORs) which sit under a layer of mucus in the upper section of the nose. There are more than 350 different ORs in humans, and these work in a combinatorial fashion to allow us to smell many more odorants. Odorant binding to an OR sets off a chain of events that converts the chemical binding energy into a neural signal, which we register as a smell.
What is puzzling, though, is how this first binding step works - most odorants are hydrophobic, while the mucus covering the ORs in the nose is aqueous. Scientists have assumed that another species becomes involved to help shuttle the odorant through the mucus layer: an odorant binding protein (OBP). However, an interaction involving all three species had never been demonstrated directly.
"The binding processes could be monitored on a sensory chip by measuring changes in the oscillations of surface plasmons."
Now, Jasmina Vidic, Edith Pajot-Augy and colleagues have observed just such an interaction. Using surface plasmon resonance (SPR) the researchers have studied the binding between the three species on a sensor chip. SPR uses light to excite surface plasmons (electromagnetic waves at a surface). Their oscillation is very sensitive to changes in their environment, and so the binding processes could be monitored on the chip by measuring changes in these oscillations.
As well as confirming OBP's passive transport role, the French scientists discovered that the protein plays an active role in the nose - preserving OR activity at high odorant concentration. 'There have been some predictions in this direction,' says Vidic, 'however, this feature has never been demonstrated before.'
'Label-free detection schemes based on SPR have become increasingly popular for studying many kinds of ligand-receptor interactions,' says Sabine Szunerits, an expert on SPR from the Grenoble Institute of Technology, France. This work, she says, 'shows once more that SPR bioelectronic sensors are powerful tools for investigating biologically pressing questions.'