There is a rapid increase in human infections resistant to multiple antibiotics, and researchers are working hard to find ways to stem this superbug tide. Chemists in the Redinbo Group have now revealed the structure of an enzyme that helps antibiotic resistance genes to hop to new bacterial hosts - one route to superbug status. The team, published in PNAS, also describe molecules that block the enzyme's activity. Though the molecules are far from benign drugs, the authors say the strategy represents a new idea for fighting superbugs.
To pass on antibiotic-thwarting genes to a nearby microorganism, a bacterium needs an enzyme to cut one strand of its own DNA. The Redinbo Group crystallized a complex of DNA and this nicking enzyme, which they obtained from a dangerous strain of methicillin-resistant Staphylococcus aureus - MRSA - the first human pathogen to also pick up resistance to antibiotic heavy hitter vancomycin. They noticed two protein loops holding the DNA, "pinching it like a finger and a thumb," says Redinbo, arranging it for nicking. The nicked DNA strand peels off, so the bacterium can squirt it into its neighbors. Eliminating those loops from the enzyme makes Staphylococcus far less able to share DNA. The long-term goal of this work is to develop a companion therapy to traditional antibiotics that keeps resistance in check.
Findings published in Science from the Redinbo Group, in collaboration with UNC School of Medicine, the Albert Einstein College of Medicine, and North Carolina Central University, may lead to the elimination of a debilitating side effect of CPT-11, a widely used but harshly potent treatment for colon cancer. The team of researchers, led by chemistry professor Matthew Redinbo from the University of North Carolina at Chapel Hill, has discovered that it is possible to target and block the enzyme, beta glucuronidase, which is thought to play a major role in causing the drug's side effects. "In a manner of speaking, we cured the bacteria's sweet tooth without damaging the microbes or intestines and, in the process, the drug's toxic side effect was alleviated," said Redinbo.
Study co-author, Sridhar Mani, professor of medicine and genetics at Einstein, said the severe diarrhea caused by CPT-11 can sharply limit the dosage that cancer patients can receive. "Our tests showed conclusively that the inhibitor identified by our UNC colleagues prevented diarrhea in mice that were also receiving CPT-11. We are hopeful that clinical trials will show that administering this inhibitor when patients start taking CPT-11 allows for improvement in the drug's anti-tumor effect in patients with cancer."
In work published in PNAS, the Redinbo Group, in collaboration with the Tarran Group at UNC's Cystic Fibrosis Center, contributed the first crystal structure of SPLUNC1, the most abundantly secreted protein in human lungs. The structure lead the team to make specific electrostatic predictions regarding the surface of the SPLUNC1 protein, which were shown to be correct with respect to how SPLUNC1 controls the proper level of fluid in the lungs.
Cystic fibrosis, CF, is caused by mutations in the cystic fibrosis transmembrane conductance regulator, CFTR, gene, which codes for a chloride/bicarbonate channel whose absence leads to dehydration and acidification of CF airways. A contributing factor to CF lung disease is dysregulation of the epithelial Na+ channel, ENaC, which exacerbates mucus dehydration.
In the image above, healthy lung cells are on the right. The finger-like cilia point up into the proper fluid level because the blue SPLUNC1 proteins are "plugging" the ENaC channels (orange) which otherwise would remove Na (yellow) along with a lot of water. In CF lungs, left, the pH of the liquid is low, SPLUNC1 cannot bind to ENaC and plug that drain, so the Na and water flow into the cells, and the lungs become dehydrated. This work suggests that future CF therapy be directed toward raising the pH of CF airways.
Contact: Matt Redinbo; (919) 843-8910 office; (919) 962-7576 lab