Research identifies pressure-sensing protein behind gut motility

After every meal, the intestines perform an action called peristalsis - moving food through their hollow interiors with coordinated contractions and relaxations of the smooth muscle.

For more than a century, scientists have known that nerve cells in the gut propel the colon to move, allowing the organ to perform its life-sustaining function. But exactly how these intestinal nerve cells do their job has remained elusive.

Now a new NIH-funded study led by researchers at Harvard Medical School and the Icahn School of Medicine at Mount Sinai has identified the mechanism behind this phenomenon, showing that the gut's motility is altered by exercise, pressure, and inflammation.

The study results, based on experiments in mice and published March 24 in Cell, reveal that a pressure-sensing protein called PIEZO1 - named after the Greek word for pressure and the discovery of which won the 2021 Nobel Prize in Physiology or Medicine - plays a key role both in coordinating intestinal movements and keeping inflammation in this organ at bay.

If replicated in humans, the researchers said, the findings could inform the design of precision-targeted treatments that tame intestinal inflammation and treat disorders of gut motility, such as diarrhea and constipation.

Eventually, we might stimulate PIEZO1 to speed up excretion, block it to treat diarrhea, or use it as a novel target to treat intestinal inflammation in IBD patients."

Ruaidhrí Jackson, assistant professor of immunology in the Blavatnik Institute at HMS and co-senior author on the study with Hongzhen Hu from the Icahn School of Medicine

These findings, Jackson added, show how the nervous and immune systems interact in the gut to maintain healthy function and protect the organ against inflammation. The results also add to a growing body of research showing that these two systems engage in a powerful interplay in various organs, including the brain, lungs, and skin.

A long-standing mystery provides an early clue

Scientists have been long fascinated by observations that the intestines can move independently without input from the central nervous system. Indeed, dissected intestines with no connection to external nerves can still do this vital work, Jackson explained.

Researchers already knew that enteric neurons - nerve cells contained completely within the intestines - interact with smooth muscle cells to drive peristalsis, but exactly what happens at the interface remained a mystery.

As an immunologist, Jackson had previously studied the role of the PIEZO1 protein in immune cells that sense the mechanical force generated by breathing. This earlier work revealed that the protein can spur inflammation in the lungs when it senses mechanical pressure.

Jackson wondered if this protein could also be somehow involved in digestive peristalsis.

To explore this idea, researchers analyzed gene activity in mouse and human gut neurons and found that the Piezo1 gene, which produces the PIEZO1 protein, is highly active in excitatory gut neurons - those responsible for triggering muscle contractions in the intestine by releasing the chemical messenger acetylcholine, which helps nerves communicate and propels muscle movement.

By genetically modifying mice so that PIEZO1-producing neurons glowed green, the researchers confirmed that the protein was, indeed, abundant in these cells.

PIEZO1 protein acts as a pressure sensor to cause gut movement

To better understand PIEZO1's exact role, the team tested mouse intestinal tissue under varying pressure conditions. In normal mice, the intestines contracted when pressure increased. However, in mice genetically altered to lack Piezo1, the tissue failed to contract under pressure, confirming that PIEZO1 acts as a pressure sensor, helping regulate gut movement.

Next, researchers used genetically modified mice whose gut neurons could be altered by light. When Piezo1-expressing neurons were activated by light, the mice expelled a small glass bead from their intestines twice as fast as normal mice. In another experiment, the researchers used chemicals to turn off Piezo1 neurons in the gut. In these mice, digestion slowed notably. Taken together, the findings confirmed that the protein plays a key role in controlling gut movement.

PIEZO1 responds to exercise and inflammation

Exercise has long been known to speed up bowel movement, Jackson said - a phenomenon often called the "runners' runs" by those who practice the sport. Because exercise can increase pressure on the intestines from jostling and contact with other organs, the researchers next tested how the loss of Piezo1 might affect intestinal motility in mice.

As expected, running on a treadmill increased waste transit through the intestines in mice with functional Piezo1 genes. These mice had bowel movements after just 10 minutes of exercise. However, mice whose Piezo1 gene was turned off had no such increase in intestinal motility, suggesting the gene senses the increased intestinal pressure from exercise.

Inflammatory bowel disease (IBD) is also known to increase intestinal motility due to inflammation. To test Piezo1's role in this condition, the researchers created mouse models of IBD. Mice with IBD whose guts had intact Piezo1 produced a bowel movement more quickly, compared with animals in which Piezo1 was inactivated.

Surprisingly, slower intestinal motility wasn't the only side effect from losing Piezo1 -turning off the gene also worsened IBD symptoms. Compared with mice that had intact Piezo1 genes, animals without working Piezo1 lost more weight and gradually lost the layer of protective intestinal mucus and mucus-making cells that shield the walls of the colon.

The worsened inflammation in these mice appeared to be due to the loss of the naturally occurring autoinflammatory chemical acetylcholine, which is responsible for nerve signaling and smooth muscle movement.

Not only does acetylcholine stimulate smooth muscle activity, Jackson explained, it also acts as an anti-inflammatory agent. Thus, he hypothesized, the inflammation caused by IBD might spur Piezo1 to cause enteric neurons to generate excess acetylcholine in an effort to tamp down inflammation - which in turn causes the increased intestinal motility characteristic of this condition. This may also explain how inflammation of the colon tends to produce diarrhea and excessive bowel movements, the researchers added.

Finding ways to modulate Piezo1 activity might eventually be used to fight IBD inflammation, Jackson said. This approach would target Piezo1 in gut neurons to release acetylcholine. This strategy would be markedly different from the way most IBD drugs work, which is by suppressing key inflammatory proteins that can render patients vulnerable to infections.

Jackson and colleagues plan to explore the design of such therapies in future work.