In fact, work by Noam Cohen, an ear, nose and throat doctor at University of Pennsylvania Medical School, suggests an intriguing role for bitter receptors recently discovered in the sinuses. He found one particular type can intercept the chemical signals that bacteria send to each other when they are coming together to form biofilms, a manoeuvre that greatly strengthens their defences against immune-system attacks.
When the bitter receptors in the sinuses pick up these signals, they set in motion an attack against the bacteria, causing cells to release toxic gas and cilia to flap. If people have a genetic variant that produces a different form of this bitter receptor, however, they are deaf to this back-channel bacterial chatter, and are more prone to severe sinus infections.
Patients who regularly get serious sinus infections often opt for surgery. However Cohen, who, along with Ben-Shahar, presented some of his findings at April's Association for Chemoreception Sciences meeting in Huntington Beach, California, speculates this may not help those with bitter receptors that can't pick up these signals. Since the bitter receptors in an individual's sinuses are the same as those on their tongue, Cohen suggests a test that tells clinicians which version of the receptor patients have on their tongues could help guide treatment.
Pluznick's research has revealed the existence of Olfr 78 receptors beyond the kidney, in blood vessels in the skin, heart and muscle. Scent receptors may be even more widely distributed, Firestein thinks, in blood vessels right across in the body. There's no better place for a chemical sensor, he points out, as the blood is the highway by which substances move around the body. This could explain why the receptors show up in so many places, since tissues are shot through with blood vessels bringing in nutrients and ferrying away waste.
What's emerging is a picture of these receptors as a kind of general-purpose chemical sensor. Consider a doorbell: the button next to your front door is hooked up to a little machine that can be programmed to play almost any kind of tune, a clock chime, a fire alarm, or "Wild Thing" if so desired. The button doesn't care what is being played – it just relays to the machine the message that a finger is pushing on it. These receptors are like that button, and the finger is anything with the right chemical structure to bind to it. The output might be the perception of a scent, the alerting of the immune system or the beating of cilia. “If you say olfactory receptors in the kidney, it sounds kind of nuts," says Pluznick. "But if you say that chemical sensors are in the kidney, I think it makes a heck of a lot of sense.”
We just happened to come across the receptors in the nose and the mouth first. It's intriguing to think this role may be pre-dated by their chemical-sensing capacities in other organs. Some see it as unlikely given that they are present in much higher numbers in the nose and mouth, but others disagree. “We call these taste receptors, but maybe they aren't taste receptors," says Ben-Shahar. "Maybe they were doing a lot of things before. Recycling is a very strong theme in evolution.” Cohen puts it more strongly. "I'm willing to bet that these bitter receptors were on the airway before they migrated to the tongue, that this is where they really belong."
Pluznick is now a professor at Johns Hopkins School of Medicine, in Baltimore, Maryland. It has been a long time since she studied kidney disease. Her lab is focused on understanding how these receptors work, and how they contribute to kidney function. Pluznick acknowledges there is still a lot to discover about the broader functions of what we still currently call scent and taste receptors, but she and others have made a start. She chuckles as she remembers how she complained to her academic supervisor about the "bad" data from her kidney gene experiments.
"He kind of looked at me for a second, and he was like 'Scent receptors in the kidney. That would be cool though, right?' At that point we both still thought it was one of those crazy, stupid ideas you laugh about later." Crazy perhaps, but very far from stupid.
CORRRECTION (12/07): We stated that the engineered mice used in experiements had come from Stuart Firestein's lab, but this was not the case. This has been corrected.