Image of gut lining and parasites. Photo courtesy of UCSF/ Michael Fortes
It’s no secret that researchers have long believed adult stem cells could contribute to wound healing in the gut and skin, but in a new paper in Nature — a group of scientists at UC San Francisco made a surprising discovery.
Through several experiments using parasitic worms in the mouse gut, they found that as parasites dug into the intestinal walls of mice, the gut responded in an unexpected way – by reactivating a type of cell growth previously seen in fetal tissues.
So why is this important?
Simply put, it gives scientists new targets to go after. According to UCSF CIRM supported scientist Ophir Klein, MD, Ph.D., this discovery could be paradigm-shifting in terms of our understanding of how the mammalian body can repair damage and could help scientists develop more ways to enhance the body’s natural healing abilities.
Adult stem cells in the intestines are vital for maintaining the digestive status quo. The gut lining is made up of epithelial cells which absorb nutrients and produce protective mucus. These cells are replaced every few days by the stem cells at the base of crypts — indentations in the gut lining. Researchers expected that the same stem cells could also help repair tears in the gut.
How did they do it?
Larvae from parasites like H. polygyrus invade the gut lining in a mouse’s intestine, burying themselves to develop in the tissue. Based on prevailing ideas in the field, the scientists predicted that, in response, nearby stem cells would increase their productivity and patch up the worm-created wounds, but that is not what happened.
Instead, signs of the stem cells in worm-infected areas disappeared entirely; fluorescent markers that should have been expressed by one of the genes in the regular stem cell program completely vanished. And yet, even with no identifiable stem cells in the area, the wounded tissue regenerated more quickly than ever.
Researchers spent years trying to resolve this mystery and after a number of false starts and dead ends, the team eventually noticed the recurrence of a different gene, known as Sca-1.
Using antibody staining for the Sca-1 protein, the researchers realized that where the stem cell genes had disappeared, a different gene program was expressed instead: one that resembled the way that mouse guts develop in utero.
Upon their discovery, the researchers wondered whether the reactivation of this fetal program was a specific response to parasite infections, or if it could be a general strategy for many kinds of gut injury. Additional experiments showed that shutting down gut stem cells with irradiation or genetically targeting them for destruction triggered aspects of the same response: despite an absence of detectable stem cell activity, undifferentiated tissue grew rapidly nonetheless.
Later, once the acute injury is repaired, the gut may return to the normal stem cell program of producing differentiated cells that perform specific functions.
Many other injured tissues could benefit from the ability to quickly and efficiently make generalized repairs before returning to specialized adult cell production, opening up therapeutic opportunities. For example, developing treatments that bestow an ability to control the change between adult and fetal genetic programs might offer new strategies to manage conditions such as inflammatory bowel disease (IBD).