For nearly two decades, cancer researchers have been missing a single link: how a toxin from a common gut bacterium first takes hold of the colon cells it harms. A team at Johns Hopkins has now found that the docking point is claudin-4, a protein that normally helps seal the spaces between cells, and has shown in mice that a decoy built from it can intercept the toxin before it does damage. The discovery reminds us that even familiar residents of the body can hide consequential secrets, and that understanding how a harm begins is often the first step toward quieting it.
How a Common Gut Bacterium's Toxin Finds Its Way Into Colon Cells, and a Decoy That Blocks It
Claudin-4 was a clear, resounding top hit.
Mark
Why does this one finding matter after so many years of knowing the toxin was harmful?
Mimi
Because the toxin's first step was a blank. They knew BFT cut E-cadherin and that it promoted tumours, but not how it got to the cell. Claudin-4 fills that gap.
Mark
And it wasn't the receptor they expected?
Mimi
No. They expected a signalling receptor. Claudin-4 is a structural protein in the seals between cells, and no other toxin is known to use this route.
Luke
Let me separate what's solid. How firmly is the binding established?
Mimi
Fairly firmly. The CRISPR screen flagged it, and lab experiments with the Barcelona group showed a tight one-to-one complex.
Luke
But no 3D structure yet.
Mimi
Right. They haven't captured how the two fit together, and AlphaFold couldn't fully resolve it either.
Mark
What did the decoy actually do in mice?
Mimi
The toxin bound the decoy instead of colon cells, and the mice were protected from BFT-induced damage.
Luke
Protected from tissue damage, not from cancer. The source says that explicitly, and nothing about humans has been tested.
Mimi
Yes. The researchers talk about small molecules or other biologics as better future versions, but those are plans, not results.
Il Polso
- For almost twenty years, a missing step in the chain linking a gut bacterium's toxin to colon tumours left researchers stalled and frustrated.
- The toxin behaves unlike almost any known enzyme, because it appears to need a separate receptor before it can act at all.
- A genome-wide gene-switching screen pointed to claudin-4, a cell-sealing protein, as the lock the toxin keys into.
- A molecular decoy carrying claudin-4 sequences drew the toxin away from colon cells and shielded mice from tissue damage.
- The work still faces long road: human safety is untested, the exact three-dimensional fit is unseen, and the mice were not shown to avoid cancer.
For nearly two decades, cancer researchers have been missing a single link: how a toxin from a common gut bacterium first takes hold of the colon cells it harms. A team at Johns Hopkins has now found that the docking point is claudin-4, a protein that normally helps seal the spaces between cells, and has shown in mice that a decoy built from it can intercept the toxin before it does damage. The discovery reminds us that even familiar residents of the body can hide consequential secrets, and that understanding how a harm begins is often the first step toward quieting it.
For close to twenty years, researchers studying colon cancer have been missing one step in a chain of events. A common gut bacterium, Bacteroides fragilis, can produce a toxin that damages the colon lining, stirs up inflammation and helps tumours take root. What nobody could explain was how that toxin first latches onto the cells it attacks. A team at Johns Hopkins has now answered that question, and has already turned the answer into a protective tool in mice.
The key was a protein called claudin-4, which helps form the tight junctions that seal gaps between cells. Maxwell White, a doctoral student in Cynthia Sears' laboratory, ran a genome-wide screen that switched off genes one at a time in colon cells. When claudin-4 was disabled, the toxin could neither bind nor break down E-cadherin, the protein that holds the barrier together. The finding surprised the team, who had expected a signalling receptor. The toxin is also unusual, since it cuts proteins yet seems to need a separate receptor first.
Working with structural biologists in Barcelona, the team confirmed that toxin and receptor form a tight one-to-one complex. They then built a decoy from claudin-4 sequences and tested it with Min Dong's group at Harvard. The toxin latched onto the decoy instead of colon cells, and the mice were spared the damage.
Sears called the moment exciting and said understanding bacterial toxins could open doors to detecting and treating diarrhoea, colorectal cancer and bloodstream infections. The limits are equally clear. The decoy protected tissue but did not prevent cancer, the exact three-dimensional fit remains unconfirmed, and human safety is untested. The team is now weighing which molecules, such as small drugs or biologics, might best block the toxin.
For close to twenty years, researchers who study colon cancer have been missing one step in a chain of events. A common gut bacterium, Bacteroides fragilis, can make a toxin that damages the colon and helps tumours form. What nobody could say was how that toxin first latches onto the cells it attacks. A team at Johns Hopkins has now answered that, and the answer has already been turned into a protective tool in mice.
The work was led by scientists at the Johns Hopkins Kimmel Cancer Center, its Bloomberg~Kimmel Institute for Cancer Immunotherapy, and the university's School of Medicine. It was published in Nature, with partial support from the National Institutes of Health. The team identified a protein called claudin-4 as the docking point for the toxin, known as BFT.
Some background helps here. B. fragilis lives in the intestines of up to 20 percent of healthy people, and carrying it does not mean someone will develop cancer. But certain strains produce BFT, which can disrupt the colon's protective lining, cause inflammation and encourage tumour growth. In a 2009 study in Nature Medicine, Cynthia Sears and her colleagues showed that the toxin could promote colon tumours. Earlier experiments had also shown that BFT breaks apart E-cadherin, a protein that holds neighbouring cells together and maintains the barrier. The puzzle was that BFT did not seem to bind E-cadherin directly. Researchers suspected a helper molecule but repeatedly failed to find it.
To look for it, Maxwell White, an M.D./Ph.D. candidate in Sears' laboratory, led a genome-wide CRISPR screen with Matthew Waldor's laboratory at Harvard Medical School. The team switched off genes one by one in colon epithelial cells, the cells lining the colon, to see which were needed for the toxin to attach and do harm. One gene stood out. It encodes claudin-4, which helps form tight junctions, the seals that limit what passes between cells. With that gene disabled, BFT could not bind the cells and could not attack E-cadherin. "It took a while to get the assay working and validate the approach," White said, "but once we were able to do the screen, claudin-4 was a clear, resounding top hit."
The result surprised the researchers. Sears and others had expected a signalling receptor, such as a G protein-coupled receptor. Claudin-4 instead helps maintain physical connections between cells. The mechanism is unusual too: BFT is a protease, an enzyme that cuts other proteins, and most proteases act directly on their targets. BFT appears to need a separate receptor first. In reviewing earlier literature, the team found no other toxin known to work this way.
To confirm the two molecules truly bind, the Hopkins group worked with structural biologists F. Xavier Gomis-Rüth and Ulrich Eckhard at the Molecular Biology Institute of Barcelona. Laboratory experiments showed BFT and claudin-4 form a tightly bound complex, one toxin molecule to one receptor molecule.
Knowing the receptor suggested a countermeasure. The team built a molecular decoy, a soluble protein carrying claudin-4 sequences, meant to attract BFT without exposing the colon lining. They tested it in mice with Min Dong's laboratory at Harvard Medical School, including Kang Wang and colleagues. The toxin bound the decoys instead of claudin-4 on colon cells, and the mice were protected from BFT-induced damage.
Sears said the team had tried several times to identify the receptor, calling this "an exciting moment," and said understanding bacterial toxins could open doors to detection and therapy for diarrhoea, colorectal cancer and bloodstream infections. White said the approach could be iterated with small molecules or other biologics that have better pharmacological properties.
The limits are clear. The researchers have not obtained an experimental three-dimensional structure of the two molecules fitting together, and even AlphaFold has not fully resolved the interaction. The mouse work showed protection against tissue damage, not prevention of colorectal cancer, and whether the decoy approach is safe and effective in people is untested. The team is now weighing which molecular approaches might best block the toxin.
Citazioni salienti
Understanding how bacterial toxins work can open doors to new approaches for detection and therapy for associated diseases, including diarrhoea, colorectal cancer and bloodstream infections.— Cynthia Sears, senior author, Johns Hopkins
This approach could be iterated upon with small molecules or other biologics that have better pharmacological properties.— Maxwell White, Johns Hopkins
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