New study suggests why mice with cystic fibrosis mutations can stave off bacterial infections in their lungs.
People who have cystic fibrosis are susceptible to bacterial infections in their lungs. Genetically modified mice that have cystic fibrosis gene mutations remain healthy, however, and this difference has puzzled researchers. Howard Hughes Medical Institute (HHMI) investigator Michael Welsh of the University of Iowa and colleagues have now discovered the reason for this discrepancy and uncovered a possible new way to treat the disease.
Patients with cystic fibrosis produce a faulty version of the CFTR protein or lack it entirely. Until recently, children born with the disease rarely survived into adulthood. Thanks to new drugs and other therapies, they now typically live into their late 30s. “The outlook for people with cystic fibrosis has improved a lot,” says Welsh.
Despite these developments, the leading cause of death for cystic fibrosis patients remains lung damage caused by repeated bacterial infections and the inflammation they trigger. The airways, the network of tubes that pipes air into the lungs, deploy several defenses against bacteria. The liquid that coats the lining of the airways teems with microbe-fighting molecules and immune cells. In addition, fine filaments known as cilia are continually sweeping airway fluid and mucus—along with any bacteria trapped in them--away from the lungs.
The genetic defect that causes cystic fibrosis cripples these defenses. CFTR proteins normally release bicarbonate ions into the liquid lining the airways. Bicarbonate is a base that reduces acidity—that’s why it’s an ingredient in many antacids. But CFTR doesn’t perform its job in people with cystic fibrosis. As a result, the acidity of the fluid in their airways increases, impairing the fluid’s ability to kill microbes and making mucus in the airways thicker and harder to remove.
Welsh and his colleagues have developed genetically modified pigs that lack the CFTR protein. Their airways also become acidic, and the animals are prone to lung infections. However, airway acidity remains normal in the so-called CF mice, the genetically modified rodents that are missing CFTR. “Here, we could take advantage of comparing humans, pigs, and mice and ask what is different about mice,” says Welsh. His team published its findings in the January 29, 2016, issue of the journal Science.
To find out, the researchers measured how much acid was secreted by airway cells from healthy humans, pigs, and mice. Cells from the rodents secreted about one-sixth as much acid as did cells from the other two species. To identify which protein was responsible for secreting acid into the airways, Welsh’s team tested a series of potential candidates. The results suggest that the protein ATP12A is mainly responsible for acidifying the airway lining.
ATP12A is plentiful in the airways of humans and pigs, but mice carry almost none of the protein in their airways, the researchers determined. That difference suggests a hypothesis for why CF mice aren’t prone to bacterial infections. In humans and pigs, the loss of CFTR results in less bicarbonate that can neutralize acid released by ATP12A. “If you take away the base secretion, you are left with acid secretion,” says Welsh. But the mice have little ATP12A in their airways, meaning that the absence of CFTR has little effect on acidity.
To test that explanation, the researchers added ouabain, a chemical that inhibits ATP12A, to the airways of pigs that have no CFTR. The chemical increased the pH of the animals’ airway fluid and improved its ability to kill bacteria. The airway fluid also became thinner, which would make it easier for cilia to remove.
The scientists also asked what would happen if they added ATP12A to the CF mice. They used a virus to genetically alter cells in the tracheas of the mice to produce more of the protein. Not only did the acidity of the animals’ airways increase, but the bacteria-killing power of their airway fluids decreased. Rodents with extra ATP12A carried 100 times more bacteria in their lungs than did control animals, enough to potentially cause an infection.
“The results of the study explain, at least in part, why CF mice are protected from infection and suggest a therapeutic target,” says Welsh. Drugs that block ATP12A might prevent the airways from becoming too acidic in cystic fibrosis patients and thus keep their defenses against bacteria strong. He and his colleagues hope to discover compounds that do just that.