Sense of smell in unexpected places

Over the last decade or so, scientists have discovered that odour receptors are not solely confined to the nose, but found throughout the body — in the liver, the heart, the kidneys and even sperm — where they play a pivotal role in a host of physiological functions.

Now, a team of biologists at Ruhr University Bochum in Germany has found that our skin is bristling with olfactory receptors. “More than 15 of the olfactory receptors that exist in the nose are also found in human skin cells,” said the lead researcher, Dr. Hanns Hatt. Not only that, but exposing one of these receptors (OR2AT4) to a synthetic sandalwood odour known as Sandalore sets off a cascade of molecular signals that appears to induce healing in injured tissue.

In a series of human tests, skin abrasions healed 30 per cent faster in the presence of Sandalore, a finding the scientists think could lead to cosmetic products for aging skin and to new treatments to promote recovery after physical trauma.

“If you think of olfactory receptors as specialised chemical detectors, instead of as receptors in your nose that detect smell, then it makes a lot of sense for them to be in other places,” said Jennifer Pluznick, an assistant professor of physiology at Johns Hopkins University who in 2009 found that olfactory receptors.

Think of olfactory receptors as a lock-and-key system, with an odour molecule the key to the receptor’s lock. Only certain molecules fit with certain receptors. When the right molecule comes along and alights on the matching receptor, it sets in motion an elaborate choreography of biochemical reactions. Inside the nose, this culminates in a nerve signal being sent to brain, which we perceive as odour. But the same apparatus can fulfill other biological functions as well.

Dr Hatt was one of the first scientists to study these functions in detail. In a study published in 2003, he and his colleagues reported that olfactory receptors found inside the testes function as a kind of chemical guidance system that enables sperm cells to find their way toward an unfertilised egg.

He has since identified olfactory receptors in several other organs, including the liver, heart, lungs, colon and brain. In fact, genetic evidence suggests that nearly every organ in the body contains olfactory receptors.

“I’ve been arguing for the importance of these receptors for years,” said Dr Hatt, who calls himself an ambassador of smell, and whose favourite aromas are basil, thyme and rosemary. “It was a hard fight.”

In 2009, for instance, Dr Hatt and his team reported that exposing olfactory receptors in the human prostate to beta-ionone, a primary odour compound in violets and roses, appeared to inhibit the spread of prostate cancer cells by switching off errant genes.

The same year, Grace Pavlath, a biologist at Emory University, published a study on olfactory receptors in skeletal muscles. She found that bathing the receptors in Lyral, a synthetic fragrance redolent of lily of the valley, promoted the regeneration of muscle tissue. Blocking these receptors (by neutralising the genes that code for them), on the other hand, was found to inhibit muscular regeneration, suggesting that odour receptors are a necessary component of the intricate biochemical signaling system that causes stem cells to morph into muscles cells and replace damaged tissue.

But it may not be all that surprising. Olfactory receptors are the largest subset of G protein-coupled receptors, a family of proteins, found on the surface of cells, that allow the cells to sense what is going on around them. These receptors are a common target for drugs — 40 per cent of all prescription drugs reach cells via GPCRs — and that augurs well for the potential of what might be called scent-based medicine.