How gut bacteria toxin invades colon cells to trigger cancer
Since a landmark 2009 study, researchers have known that a common gut bacterium, Bacteroides fragilis, drives colon tumor formation, potentially leading to colorectal cancer, by secreting a toxin that damages the lining of the colon. But until now, the exact mechanism the toxin uses to latch onto those cells remained a mystery.
A multi-institutional team has identified the missing link. The study, published in Nature, reveals that the B. fragilis toxin BFT must first bind host receptor claudin-4 before it can cause damage.
“We’ve made several attempts over time to identify the receptor, so this is an exciting moment,” says the senior author. “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.”
The discovery, in fact, already led to the development of a molecular decoy that successfully blocked the toxin’s effects in animal models, offering a potential strategy for preventing B. fragilis damage to the colon.
B. fragilis can be detected in up to 20% of healthy individuals, and has a potent ability to induce colon inflammation and tumor formation. Prior work in the lab showed that BFT triggers chronic inflammation in the gut by dividing E-cadherin, a protein essential for maintaining the colon’s protective barrier. In their earlier Nature Medicine study, the researchers showed that the action of BFT leads to colon tumor formation. Yet BFT did not appear to directly attach to E-cadherin. Some other, elusive mechanism appeared to be at play.
Identifying that mechanism began with a genome wide CRISPR screen. By systematically knocking out genes in colon epithelial cells, the researchers identified claudin-4 as the link. When claudin-4 was knocked out, BFT toxin was unable to bind to the cells, leaving the E-cadherin target untouched.
“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,” says the senior author. “That was an exciting moment.”
The identity of the receptor was a surprise, adds the author, as the researchers in the field had long expected the receptor to be a signaling protein, such as a G-coupled protein receptor, which claudin-4 is not. In a literature review, the team could not identify any other toxin that functions this way, as most proteases go straight to their targets rather than binding a separate receptor first.
To confirm that the toxin and the receptor were physically locking together, the researchers demonstrated that BFT and claudin-4 form a tight, one-to-one complex in a test tube, providing the first physical evidence of the binding interaction.
The research then moved from the test tube into living systems and the team evaluated how the toxin behaved in the complex environment of the gut using mouse models.
By creating a decoy claudin-4 — a soluble protein that displayed claudin-4 sequences — the researchers attempted to prevent the toxin from binding colon cells. Indeed, BFT bound to the decoys instead of the receptor. This decoy strategy successfully protected mice from BFT-induced damage.
“This approach could be iterated upon with small molecules or other biologics that have better pharmacological properties,” says the author. The team is now exploring which molecular approaches might be most successful to block the toxin.
The researchers note that one piece of the puzzle remains: While the team identified the receptor and proved the binding, the exact experimental structure of the interaction between BFT and claudin-4 has yet to be captured. Current AI modeling tools, such as AlphaFold, were not able to fully resolve the interaction.
