Cystic fibrosis is an inherited, autosomal recessive disorder of the lungs caused by mutations in the chloride channel gene CTFR. While there is no known cure, a promising approach for treating cystic fibrosis involves the activation of another chloride channel TMEM16A as an alternative route for proper cellular secretion of chloride. A team led by Swiss researchers from the University of Zurich has elucidated the activation mechanism of TMEM16A using electron microscopy.
The team used cryo-electron microscopy (cryo-EM) and electrophysiology on the TMEM16A protein of mice. Since TMEM16A is a type of calcium-activated chloride channel, the team analyzed the structures in the presence and absence of Ca2+. They found that there are differences between ligand-bound and ligand-free stages of the protein. When combined with functional experiments, it suggested a mechanism for gating.
The team finds that the TMEM16A structure resembles those of other TMEM protein family members. However, it is also unique in that it forms an hourglass-shaped protein-enclosed channel that is closed in the absence of calcium. The binding of positively charged calcium ions in its vicinity opens the channel and allows negatively charged chloride ions to permeate across the membrane. "This activation mechanism is unique, since the bound calcium ions directly change the structure and electrostatics of the ion permeation pore," explains lead author Cristina Paulino.
The team’s paper, published earlier this week in Nature, concludes this structural investigation of TMEM16A describes a process unique among channel proteins, but also general for functional members of the TMEM16 family. Findings that describe the underlying structures and functions of such proteins provide promising insights for developing drugs for the treatment of cystic fibrosis.
"The molecular architecture of this membrane protein is crucial for the targeted development of drugs for treating cystic fibrosis," says senior author Raimund Dutzler. "Substances leading to the activation of the TMEM16A would compensate the defect in the secretion of chloride ions in the lung.”