Cystic fibrosis is caused by the malfunction of an ion channel critical for maintaining the secretions of salt and water that protect the lungs. A chronic and progressive disease, cystic fibrosis is usually diagnosed in childhood.
It causes mucus to become thick, dry and sticky clogging passages in the lungs, pancreas and other organs in the body.
Scientists from Kansas State University have been trying to understand how ions travel across cell membranes, specifically the anion part of sodium chloride. In taking on this research, the scientists are specifically trying to find out how they can manipulate chloride transport rates and selectivity.
In doing so, John Tomich, a Kansas State University professor of biochemistry and collaborators used a series of single and double amino acid substitutions to modulate the activity of a channel forming peptide derived from the second transmembrane segment of the alpha subunit of the human spinal cord glycine receptor.
Chloride ions are hydrogen bond acceptors and this research assumes that hydroxyl function contributes strongly to ion throughput across and/or ion selectivity within the channel structures.
Residue replacements in the peptide involving the 13th and 17th positions were designed to correlate hydrogen-bonding strength with selectivity and permeation rates.
The hydrogen bonding strengths of the amino acid side-chains correlate directly with anion selectivity and inversely with transport rates for the anion. Knowledge of these results will help in optimising these two counteracting channel properties.
In a human, the cystic fibrosis transmembrane conductance regulator (CFTR) protein makes a chloride channel in which chloride passes through while sodium moves through a parallel channel. This simultaneous movement of ions keeps the lungs healthy by controlling the salt (sodium chloride) concentration in the lung fluid.
"Your body knows how to separate these things all by itself," Tomich said. "Sodium is usually higher outside the cell, potassium is higher inside the cell and chloride, depending on the cell type, can be the same or different."
The chemical mechanisms directing chloride binding and transport are poorly understood in contrast to mechanisms determining how sodium, potassium and calcium get across which are much better known.
What is known is the gene that makes the CFTR protein is mutated in people who have cystic fibrosis. The mutation results in a chloride channel that does not open to allow chloride flow.
Consequently, the concentration of salt is abnormal. This interferes with the normal lung defence mechanisms that kill bacteria and keep the lungs sterile.
Tomich's lab has developed more than 200 sequences that showed varied ion transport activity in synthetic membranes, as well as cultured epithelial cells and animals. From all of that they can change virtually the way this ion channel assembles.
Some of the compounds that he has designed work at very low concentrations but lack some of the chloride specificity that it once had. Their presentation discussed how the researchers back-designed the channel pore so it can be more specified for chloride.
"We have some early designs that are highly selective for chloride, but you'd have to give them a lot of the compound to see the effect," Tomich said.
Tomich presented a paper on the findings, "Assessing The Contributions of H-Bonding Donors to Permeation Rates and Selectivity in Self-Assembling Peptides that Form Chloride Selective Pores," Aug. 28 at the Membrane Active, Synthetic Organic Compounds Symposium of the American Chemical Society's national meeting and exposition in Washington, D.C.