While studying viruses best known for infecting the brain, researchers stumbled upon clues to a conundrum involving a completely different part of the anatomy: the bowel.
What they learned might explain why some people get digestive woes seemingly out of the blue.
Viruses such as West Nile and Zika that target the nervous system in the brain and spinal cord can also kill neurons in the guts of mice, disrupting bowel movement and causing intestinal blockages, according to a new study. Other viruses that infect neurons may also cause the same symptoms, the researchers say.
The findings, could explain why some people experience recurrent, unpredictable bouts of abdominal pain and constipation—and perhaps point to a new strategy for preventing such conditions.
“There are a number of people who are otherwise healthy who suddenly develop bowel motility problems, and we don’t understand why,” says Thaddeus S. Stappenbeck, a professor of laboratory and genomic medicine at Washington University School of Medicine in St. Louis and co-senior author of the study, which appears in Cell.
“But now we believe that one explanation could be that you can get a viral infection that results in your immune cells killing infected neurons in your gut. That might be why all of a sudden you can’t move things along anymore,” he says.
Gut slow down
Postdoctoral researcher and first study author James White was studying mice infected with West Nile virus, a mosquito-borne virus that causes inflammation in the brain, when he noticed something peculiar. The intestines of some of the infected mice were packed with waste higher up and empty farther down, as if they had a blockage.
“We actually noticed this long ago, but we ignored it because it wasn’t the focus of our research at the time,” says West Nile expert and co-senior author Michael S. Diamond, a professor of medicine. “But Jim White dug in. He wanted to figure out why this was happening.”
Researchers found that not only West Nile virus but its cousins Zika, Powassan, and Kunjin viruses—all of which target the nervous system like West Nile—caused the intestines to expand and slowed down transit through the gut. In contrast, chikungunya virus, an unrelated virus that does not target neurons, failed to cause bowel dysfunction.
Further investigation showed that West Nile virus, when injected into a mouse’s foot, travels through the bloodstream and infects neurons in the intestinal wall. These neurons coordinate muscle contractions to move waste smoothly through the gut. Once infected, the neurons attract the attention of immune cells, which attack the viruses—and kill the neurons in the process.
A miserable experience
“Any virus that has a propensity to target neurons could cause this kind of damage,” says Diamond, who is also a professor of molecular microbiology and of pathology and immunology. “West Nile and related viruses are not very common in the US. But there are many other viruses that are more widespread, such as enteroviruses and herpesviruses, that also may be able to target specific neurons in the wall of the intestine and injure them.”
If that’s the case, such widespread viruses may provide a new target in the prevention or treatment of painful digestive issues. Having chronic gut motility problems is a miserable experience, and while the condition can be managed, it can’t be cured or prevented.
“Many of the viruses that might target the gut nervous system cause mild, self-limiting infections, and there’s never been reason to develop a vaccine for them,” Diamond says. “But if you knew that some particular viruses were causing this serious and common problem, you might be more apt to try to develop a vaccine.”
The infected mice’s digestive tracts gradually recovered over an eight-week time span. But when the researchers challenged the mice with an unrelated virus or an immune stimulant, the bowel problems promptly returned. This pattern echoed the one seen in people, who cycle through bouts of gastrointestinal distress and recovery. Stress or illness often trigger the flare-ups, but they also can occur for no apparent reason.
“It’s amazing that the nervous system of the gut is able to recover and re-establish near normal motility, even after taking a pretty big hit and losing a lot of cells,” says Stappenbeck, who is also a professor of developmental biology. “But then, it’s really just barely functioning normally, and when you add any stress, it malfunctions again.”
Previous studies have linked bowel motility to changes in the microbiome—the community of bacteria, viruses, and fungi that live in the gut.
“What we need to explore now is how this story connects to everything else we know about gut motility,” Stappenbeck says. “What effect does damage to the gut nervous system have on the microbiome? We would love to connect those dots.”
The National Institutes of Health, a Ruth L. Kirschstein National Research Service Award, the Irma and Norman Braman Endowment, the Suzi and Scott Lustgarten Center Endowment, and the Children’s Hospital of Philadelphia Research Institute funded the work.