Scientists at Johns Hopkins University have identified the specific protein that allows a common gut bacterium to attach to intestinal cells and trigger damage linked to colorectal cancer. The discovery resolves a nearly two-decade-old mystery regarding how the B. fragilis toxin (BFT) initiates its attack on the colon lining.
The findings, published in Nature, reveal that the protein claudin-4 serves as the essential docking point for the toxin. Without this attachment, the toxin cannot disrupt the cellular barrier that protects the colon. The research team also developed a molecular decoy that successfully intercepted the toxin in mouse models, offering a potential new strategy for preventing bacterial damage associated with cancer.
Identifying the Missing Link in Toxin Action
For years, researchers knew that certain strains of Bacteroides fragilis, a bacterium found in the intestines of up to 20 percent of healthy people, produce a toxin capable of driving tumor formation. In 2009, a landmark study led by senior author Cynthia Sears established that BFT promotes colon tumors by breaking down E-cadherin, a protein that holds neighboring cells together.
Despite knowing the toxin damages E-cadherin, scientists could not explain how the toxin first reached its target. The toxin did not appear to bind directly to E-cadherin, suggesting an intermediate step was required. To solve this, the team used CRISPR gene-editing technology to systematically disable genes in colon epithelial cells.
The screen identified claudin-4 as the critical receptor. This protein helps form tight junctions between cells, acting as a seal that limits the movement of substances through the intestinal lining. When researchers disabled the gene responsible for producing claudin-4, the toxin lost its ability to bind to the cells. Without that initial attachment, the toxin could not attack E-cadherin or cause inflammation.
“It took a while to get the assay working and validate the approach, but once we were able to do the screen, claudin-4 was a clear, resounding top hit,” said Maxwell White, an M.D./Ph.D. candidate who led the screening effort. “That was an exciting moment.”
Molecular Decoy Strategy Shows Promise
The identification of claudin-4 as the receptor provided a clear target for intervention. The researchers hypothesized that a soluble version of the protein could act as a decoy, luring the toxin away from the actual colon cells. They collaborated with structural biologists in Barcelona to confirm that BFT and claudin-4 form a tightly bound complex in laboratory settings.
Testing this approach in mice, the team introduced the molecular decoy into the system. Instead of binding to claudin-4 receptors on the intestinal lining, the BFT toxin attached to the decoy proteins. This interception successfully protected the mice from toxin-induced tissue damage.
The study highlights an unusual mechanism for bacterial toxins. Most proteases, the class of enzymes to which BFT belongs, act directly on their targets. BFT requires a separate receptor to reach its destination, a strategy not previously documented for other known toxins.
“Understanding how bacterial toxins work can open doors to new approaches for detection and therapy for associated diseases, including diarrhea, colorectal cancer and bloodstream infections,” Sears said.
While the mouse experiments demonstrated effective protection against tissue damage, the team notes that further research is needed to determine if this approach can prevent colorectal cancer in humans. The team is now exploring small molecules or other biologics that might offer better pharmacological properties for future treatments. The research received partial support from the National Institutes of Health.
Source: ScienceDaily
