When researchers discovered the primary genetic defect that causes cystic fibrosis (CF) back in 1989, they opened up a new realm of research into treatment and a cure for the disease. Since then, scientists have been able to clone the defective gene and study its effects in animals. Now researchers at the University of North Carolina at Chapel Hill have developed a technique for observing the defects at work in human tissue donated by patients with CF.
This technique has yielded an extraordinary view of the cellular intricacies of CF, which Martina Gentzsch, assistant professor of cell and developmental biology, will discuss at the 7th International Symposium on Aldosterone and the ENaC/Degenerin Family of Ion Channels, being held September 18-22 in Pacific Grove, Calif. The meeting is sponsored by the American Physiological Society. Her poster presentation is entitled, "The Cystic Fibrosis Transmembrane Conductance Regulator Inhibits Proteolytic Stimulation of ENaC."
Ion Transport Processes in CF
Cystic fibrosis is caused by a mutation in the gene that encodes a protein called cystic fibrosis transmembrane conductance regulator (CFTR), which functions as a chloride channel at the surface of airways and moves chloride out of the cells. CFTR also regulates another protein called epithelial sodium channel (ENaC), which is responsible for transporting sodium into cells. Thus far, scientists have been able to establish that when the CFTR mutation is present, ENaC becomes overactive and causes the cells in the lungs to absorb too much sodium. Water follows the sodium from the cells' surfaces into the cells, and as a result, the airways become dry and mucous becomes thick and sticky, leading to infections in the lungs.
To observe how CFTR regulates ENaC, Dr. Gentzsch and her team took cells from healthy lung tissue and CF lung tissue and maintained them in a liquid medium. The cells' surfaces were exposed to air, which prompted the cells to grow and behave as though they were still inside human lungs. Then the team studied proteolytic cleavage of ENaC, a process in which the ENaC protein is cut by enzymes called proteases at specific sites on the protein. This limited cleavage causes ENaC to become active. When the team analyzed the cells' behavior, they found that ENaC was more likely to have undergone cleavage in cells from CF tissue.
According to Dr. Gentzsch, these observations prompted two questions. First, what role does CFTR play in regulating ENaC cleavage? Second, why is ENaC cleavage not regulated in CF?
"CFTR binds to ENaC, so our initial thought was that close contact of ENaC to CFTR protects ENaC from being cleaved. But another possibility is that CFTR is responsible for suppressing ENaC cleavage and activation," said Dr. Gentzsch. In other words, the absence of a normally functioning CFTR protein may cause ENaC overactivity. Because there is more cleavage when the CFTR mutation is present, it implies that healthy CFTR prevents ENaC cleavage and activation, but defective CFTR does not.
Either way, Dr. Gentzsch feels that both CFTR and ENaC should be considered when developing therapies for CF. "Successful treatments should address both decreased CFTR function and increased salt absorption caused by ENaC overactivity."
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