Sometimes the results of science experiments are just too weird to believe. In 2005 Jennifer Pluznick hypothesised that a gene known to play a role in a common form of kidney disease did so by acting as a master switch for other genes. Lab tests supported her theory; however, when she looked at which genes it acted on, she did a double take. Among them were several that encode scent receptors, the chemical sensors that allow us to identify smells.
At first Pluznick didn't believe her eyes. As far as we know, kidneys filter waste from our blood and get rid of it in urine. They are not sensory organs. "I wasn't sure if we could believe the data," she says. "It seemed really spurious." Thinking there had to be a mistake, Pluznick, then a researcher at Yale School of Medicine, in New Haven, Connecticut, carried out another experiment. She soaked a slice of kidney in a fluid that would make scent-system proteins glow fluorescently under a microscope. When she peered down the eyepiece, it was lit up like a Christmas tree.
Pluznick has spent the last eight years trying to understand why.
The sense of smell is one of our most powerful connections to the physical world. Our noses contain hundreds of different scent receptors that allow us to distinguish between odours. When you smell a rose or a pot of beef stew, the brain is responding to scent molecules that have wafted into your nose and locked on to these receptors. Only certain molecules fit specific receptors, and when they slot together, like a key in a lock, this triggers changes in cells. In the case of scent receptors, specialised neurons send messages to the brain so we know what we have sniffed. The discovery, in 1991, of around 1,000 genes involved in generating scent receptors was rewarded with the Nobel prize in Medicine over a decade later.
In the last ten years, however, reports have trickled in from bemused biologists that these receptors, as well as similar ones usually found on taste buds, crop up all over our bodies. In 2003, bitter taste receptors were found in sperm. The same year Pluznick came across scent receptors in the kidney, biologists at the University of California, San Diego identified sour receptors in the spine. A smattering of papers over the following few years reported sweet taste receptors in the bladder and the gut, bitter taste receptors in the sinuses, airways, pancreas and brain, and scent receptors in muscle tissue.
What are they doing there? As these findings became public, researchers poured over genomic data and reported that low levels of these receptors occurred in almost every tissue in the body. One possible explanation was that they are evolutionary leftovers whose previous functions were lost in time. However, Pluznick and others have unearthed evidence that, in some tissues at least, they are anything but passive. Their findings suggest that our bodies are “smelling” and “tasting” things deep inside of us, and that these abilities are crucial to our health.
The kidney is made up of miles of minuscule tubes, twisted into an exquisite lacework. Threaded through these is a system of blood vessels, and in places where these and the kidney's tubes are especially intertwined, water, nutrients and other molecules seep through the walls and pass between the two. What the blood doesn't need is passed off to the urine forming in the tubes, and what it does have a use for – keeping blood pressure stable, for instance – goes back. If this process stops working the body will soon shut down, poisoned by its own exhaust fumes.