Thursday, April 11, 2013

Discovery could increase efficacy of promising cystic fibrosis drug

We know that, Ivacaftor (trade name Kalydeco, developed as VX-770) is a potentiator approved for patients with the G551D mutation of cystic fibrosis. Ivacaftor was developed by Vertex Pharmaceuticals in conjunction with the Cystic Fibrosis Foundation.

Cystic fibrosis is caused by any one of several defects in a protein, cystic fibrosis transmembrane conductance regulator, which regulates fluid flow within cells and affects the components of sweat, digestive fluids, and mucus. The defect, which is caused by a mutation in the individual's DNA, can be in any of several locations along the protein, each of which interferes with a different function of the protein. G551D is a standard amino acid abbreviation for a mutation in which the amino acid glycine (G) in position 551 is replaced with aspartic acid (D). G551D is characterized by a dysfunctional CFTR protein on the cell surface. In the case of G551D, the protein is trafficked to the correct area, the epithelial cell surface, but once there the protein cannot transport chloride through the channel. Ivacaftor, a CFTR potentiator, improves the transport of chloride through the ion channel by binding to the channels directly to induce a non-conventional mode of gating which in turn increases the probability that the channel is open.

Accidental discovery of a mutation in CFTR, the R532 mutation, allowed MU researchers to reveal a new "non-strict coupling" relationship that occurs between the consumption of ATP, a molecule that provides energy in the body, and the opening and closing of the CFTR. They argue that the new information uncovered about this mechanism that controls the opening and closing of the CFTR and the passage of ions through it could explain how and where the new cystic fibrosis treatment Kalydeco (Vx-770) works.
"To his credit, Dr. Hwang exploited the behavior of the CFTR mutants to demonstrate that CFTR's gate is not strictly coupled to the nucleotide binding engine (NBD) that binds and splits ATP [energy] to drive conformational changes that regulate chloride flow through the CFTR protein channel," said colleague David Sheppard, PhD, an associate professor in the School of Physiology and Pharmacology at the University of Bristol in Bristol, U.K. who did not participate in the study.
In their study, MU researchers were able to observe the effects of the cystic fibrosis drug Vx-770 on the recently discovered R352 mutation. They found that Vx-770 enhances the activity of the CFTR channel by exploiting this "non-coupling" mechanism, a conclusion also supported by experimental results with the wild-type CFTR protein.
"Traditionally, researchers have defined how energy is utilized and transferred in the CFTR as a 'strict coupling' mechanism, meaning that one ATP molecule opens CFTR's gate, ions pass through and the ATP molecule is hydrolyzed and then the gate closes," Hwang said. "In this new model, we argue that the CFTR uses energy from ATP hydrolysis to carry out its function of chloride flow, but this coupling mechanism is more plastic than we thought and therefore could be subjective to manipulations by drugs such as Vx-770."
CFTR is part of a family of thousands of active transporter proteins called ABC proteins. Although CFTR may share many structural features with its ABC "cousins," as Hwang calls them, it has been unclear as to whether CFTR and its cousins may work in a similar manner.
The new idea of how the CFTR utilizes ATP to carry out its function may bear a broader implication because of the evolutionary relationship between CFTR and other ABC transporter proteins. It opens up a wide variety of therapeutic possibilities for other common diseases, such as cancer, heart disease and diabetes, Hwang said, since many other ABC proteins play critical roles in those human illnesses.
"It's taken years for scientists to solve this particular puzzle about the CFTR protein," Hwang said. "Our recent study provides evidence that these ABC transporter proteins and CFTR, a chloride channel, are two peas in a pod. Mother nature employs the same structural framework with just a little bit of modification to do two totally different things. From a basic science perspective, it's a big deal.".....

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