Researchers have made an important discovery about the secret life of the defective protein that causes cystic fibrosis: while scientists already knew that CFTR protein regulates the acid-alkali balance in cells, what they didn’t know, until this study revealed it, was what turns that ability on and off. The researchers hope the discovery will help develop new therapies that address the root cause of cystic fibrosis rather than just the symptoms.

The study, which is published as “paper of the week” in the 18 December issue of the Journal of Biological Chemistry, is the work of lead author Dr Jeng-Haur Chen, now a postdoctoral researcher at the University of Iowa Carver College of Medicine, in Iowa city, and colleagues.

The UK’s Cystic Fibrosis Trust sponsored the research, which was conducted at the department of physiology and pharmacology at the University of Bristol, UK, where Chen was a doctoral student under the supervision of co-author Dr David Sheppard.

Cystic fibrosis (CF) is an inherited chronic disease caused by defects in a gene called cystic fibrosis transmembrane conductance regulator, or CFTR. The result is that the associated protein is either disabled or absent altogether, disrupting the transport of salts into and out of cells, which leads to the characteristic thick mucus that blocks ducts and tubes in the body, affecting many organs. For instance, it blocks airways, causing chronic cough and lung infections; it blocks the pancreas so it can’t deliver enzymes to help digest food in the intestines; and it also prevents food being absorbed in the intestines.

CF affects more than 70,000 people worldwide, most of whom are children and young adults. Around 1 in 25 people of European descent carry the CF gene, and if both parents carry the gene, the chances of a child being born with CF is 1 in 4. There is no cure for the disease and most people with it die in their 20s or 30s from lung failure.

The CFTR gene and its associated protein were only discovered 40 years ago, but early detection and improved therapies has led to vastly improved survival for patients since that time.

Chen told the press that:

“Understanding the regulation of salt transport in normal cells is critical for the development of new therapies for diseases, like CF, that disrupt salt movements across cell borders.”

However, as Chen explained, despite big improvements, existing therapies only ease symptoms. These include antibiotics to treat lung infections, chest physiotherapy to loosen the mucus, and enzyme supplements to aid digestion.

To develop a cure, said Chen, two things need to happen: deliver missing CFTR protein back to the cell border, and repair any defective CFTR.

He explained that:

“CFTR itself is a small passageway with a gate, called an ion channel, found on the surface of cells lining ducts and tubes, where it acts as a pathway for the movement of chloride ions, one component of salt, and regulates the transport of bicarbonate, one part of soda.”

CFTR regulates the transport of these molecules by adjusting the pH or acid-alkali balance inside cells, a process that is important for cells to function properly. For instance, in the case of cells that line ducts and tubes, it protects the body from bacteria and other pathogens, ensuring cell survival, said Chen.

Scientists already knew that CFTR regulated pH, but not exactly how: how did the protein detect changes in pH, how did it “know” when to change the control one way or the other?

Chen had a hunch that it was the pH inside cells that controlled CFTR directly, so he set out to test it. First he found that acid pH triggers CFTR to transport chloride, but alkali pH stops it. To find out how this happens, Chen decided to look inside CFTR, at its internal structure and components.

Sheppard described what Chen found:

“The structure of CFTR resembles a turnstile — it has a pathway for chloride movement across the cell border and a gate that controls access to this pathway.”

“Turning of the gate is powered by adenosine triphosphate, or ATP, an energy source for all cells,” he explained, adding that Chen’s work showed that:

“Intracellular pH regulates ATP docking with the gate and the speed at which the gate turns.”

Thus the pH level inside the cell decides the power level at the gate, and this in trun controls the transport of salt and bicarbonate. But if a cell needs to cut back on energy, it can also use pH to tell enzymes when to stop CFTR activity.

Sheppard said that Chen’s insight into the structure of CFTR has been:

“Critical to advancing knowledge of how CFTR normally works and how it goes wrong in disease.”

He said the aim now is to design and develop drugs that restore function to CFTR proteins disabled by mutations in CF genes.

“By targeting the root cause of the disease, rather than the symptoms, new drug therapies for CF might stop disease progression and prevent the decline in health of individuals living with CF,” said Sheppard.

“Direct Sensing of Intracellular pH by the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Cl- Channel.
Jeng-Haur Chen, Zhiwei Cai, and David N. Sheppard
J. Biol. Chem. 2009 284: 35495-35506.

Source: American Society for Biochemistry and Molecular Biology.

Written by: Catharine Paddock, PhD