Sweet taste receptors in nose tell immune system when to kick in, Penn team finds
If you think the only job of your taste receptors is to keep you coming back to the candy dish, think again. Researchers at the University of Pennsylvania have found that taste receptors in the nose are handy bacterial detectors, and one of the body's more ingenious ways of fending off infection.
Published in the Journal of Clinical Investigation, head and neck surgeon Noam Cohen studied cultures taken from his patients with sinusitis, and found that a specialized cell type in the nose has both sweet and bitter taste receptors acting together to keep tabs on the number of bacteria in the nose.
Known as solitary chemosensory cells, Cohen said, they have only recently been discovered in humans, although they were known to exist for years in fish and rodents.
"We believe they are peppered throughout the upper airway and probably even the lower airway, into the lungs," said Cohen.
The bitter receptors on these cells respond to molecules released by the bacteria, and can trigger the release of antimicrobial peptides by neighboring cells to limit bacterial growth. But only if sugar levels are low.
The monitoring system hinges on the amount of sugar in the mucus because the nasal bacteria feed on it. So the role of the sweet receptor is actually to block the release of the bacteria-fighting molecules until the microbes have gobbled up most of their food, indicating they have expanded too much.
"When the sugar drops down below a critical point, all of a sudden [the solitary chemosensory cells] can become activated by bitter compounds that activate these bitter receptors," said Cohen. "And when they become active they send a signal to the surrounding cells to dump their antimicrobial peptides."
One implication of this finding is that more mucosal sugar might prevent this critical immune response from occurring -- and protecting the nose. Cohen, for example, found that his sinusitis patients have, on average, three to four times the normal amount of sugar in the mucus lining their noses. Similarly, diabetics, who also have sweeter mucus, are prone to sinus infections.
But not all bacteria is bad, and what's most interesting to Cohen is understanding how the 20 different bitter receptors respond to keep the nose healthy.
"All these different taste receptors, with all their different genetics, may be defining the sinus, or the nasal, microbiome," said Cohen. "What is permissive in one person's nose, and is not found in another person's nose, may be regulated by the genetics of all these taste receptors sitting in the airway."