Veronique Greenwood writes: Some years ago, when Jennifer Pluznick was nearing the end of her training in physiology and sensory systems, she was startled to discover something in the kidneys that seemed weirdly out of place. It was a smell receptor, a protein that would have looked more at home in the nose. Given that the kidneys filter waste into urine and maintain the right salt content in the blood, it was hard to see how a smell receptor could be useful there. Yet as she delved deeper into what the smell receptor was doing, Pluznick came to a surprising conclusion: The kidney receives messages from the gut microbiome, the symbiotic bacteria that live in the intestines.
In the past few years, Pluznick, who is now an associate professor of physiology at Johns Hopkins University, and a small band of like-minded researchers have put together a picture of what the denizens of the gut are telling the kidney. They have found that these communiqués affect blood pressure, such that if the microbes are destroyed, the host suffers. The researchers have uncovered a direct, molecular-level explanation of how the microbiome conspires with the kidneys and the blood vessels to manipulate the flow of blood.
The smell receptor, called Olfr78, was an orphan at first: It had previously been noticed in the sensory tissues of the nose, but no one knew what specific scent or chemical messenger it responded to. Pluznick began by testing various chemical possibilities and eventually narrowed down the candidates to acetate and propionate. These short-chain fatty acid molecules come from the fermentation breakdown of long chains of carbohydrates — what nutritionists call dietary fiber. Humans, mice, rats and other animals cannot digest fiber, but the bacteria that live in their guts can.
As a result, more than 99 percent of the acetate and propionate that floats through the bloodstream is released by bacteria as they feed. “Any host contribution is really minimal,” Pluznick said. Bacteria are therefore the only meaningful source of what activates Olfr78 — which, further experiments showed, is involved in the regulation of blood pressure. [Continue reading…]