First documented in the 1930s, Cystic Fibrosis or CF is an autosomal recessive disease in the CFTR gene which is located on human chromosome 7. Individuals diagnosed with this disease experience difficulty in breathing, muscle atrophy, salty tasting skin, and in most cases premature death (Sheppard and Welsh, 1999; Pier, 2000; Divangahi et al., 2009). A cure for this disease is not present at this moment, however understanding the underlying mechanisms behind it may shed some light upon a solution soon enough.
In general, the CFTR gene encodes an ion channel protein called cystic fibrosis transmembrane conductance regulator. This protein, once expressed, is embedded within the plasma membrane. The CFTR protein is also considered to be part of the ATP-binding cassette transporter family (ABC-transporters). As part of their nature, ABC transporters are trans-membrane proteins that carry out particular functions after being phosphorlyated via ATP hydrolysis. The CFTR protein is comprised of 5 domains: 2 membrane spanning domains (MSDs), 2 nucleotide binding domains (NBDs), and a single regulatory (R) domain (Sheppard and Welsh, 1999). Appreciating the general structure of ABC transporters such as the CFTR provides us with a tool to discern the various functions of such proteins.
With the CFTR's structure at hand, scientists utilized the method of site-directed mutagenesis in an effort to give each of the five domains a particular function (Sheppard and Welsh, 1999). Such a method requires one to introduce mutations into a wild-type (normal) CFTR gene at particular nucleotides which may result in the degradation of a particular domain, and from there certain functions could possibly become absent all together! The data that was compiled from these studies revealed the following: (1) The MSDs form the Cl- channel pore (2) The NBDs participate in ATP hydrolysis which in turn governs gating of the Cl- channel, and (3) The R domain manages channel activity on the basis of phosphorylation by a cAMP-dependant kinase called PKA (Sheppard and Welsh, 1999).
The CFTR can be found in the apical membrane of many cells that line the pancreas, intestines, sweat glands and the airways of the lungs where ion transport is critical. It is understood that CFTR is a Cl- channel that is capable of regulating the concentration of ion being transported back and forth between cells and the lumen of the previously mentioned organs. It is important to note that by controlling salt (Cl-) flow, the CFTR also controls fluid flow! In turn, it has been shown that CFTR plays two distinct roles depending on its location (Sheppard and Welsh, 1999). In organs such as the pancreas or intestines, CFTR participates in ion and fluid secretion, while in the lungs it facilitates ion and fluid absorption.
Cystic fibrosis most commonly arises from a ∆F508 mutation in the CFTR gene. This mutation, which accounts for almost two-thirds of all CF cases, is a 3 nucleotide base deletion that completely omits a phenylalanine from position 508 of the CFTR gene. As a result of this deletion, the CFTR gene expresses misfolded protein that is unable to leave the endoplasmic reticulum and is thus degraded by proteosomes once it has been tagged by ubiquitin. Without the presence of the CFTR protein, the concentration of salt in the airway surface liquid (ASL) of the lungs increases dramatically and this in turn results in mucous build-up. Recall that the CFTR is responsible for salt and fluid absorption from the numerous airways of the lungs and not secretion! It is hypothesized that because of this increase in fluid salinity innate immune factors such as human β-defensin 1 (hBD-1) and human β-defensin 2 (hBD-2), which are found in surface epithelia and submucosal glands of human airways, are unable to interact with microbes and other inflammatory agents (Bals et al., 1999). When these immune factors are in a sense ‘paralyzed', the gram negative pathogen Pseudomonas aeruginosa is capable of infecting the lung's surface. On a different note, a study has shown that absence of CFTR on the sarcoplasmic reticulum (SR) of skeletal muscle is linked to muscle atrophy. In short, the SR stores Ca2+ which is released after an action potential travels down the T-tubules of the skeletal muscle, the released Ca2+ interacts with a troponin-myosin complex which in turn causes a conformational change that results in the contraction of muscle (Divangahi et al., 2009). In the absence of CFTR the SR becomes irregularly permeable, thus releasing Ca2+ in an unregulated fashion. Cytokines, signaling molecules released by the cells of the immune system, will target these skeletal cells for degradation (Divangahi et al., 2009). This process in which the body starts degrading itself is known as cachexia. The study that was previously discussed was in fact conducted on skeletal muscle from the diaphragm (a critical respiratory muscle)! All in all, a weakened diaphragm, non-functional immune factors, and a build-up of mucous and bacterial infection contribute to the early death of individuals diagnosed with CF.
In the present state, there is no solid cure for cystic fibrosis, but several temporary treatments have been devised. One such treatment consists of the use of a mask nebulizer that introduces specified antibiotics into the patient's airways in an effort to fight off Pseudomonas aeruginosa infection that the immune factors were unable to deal with. A second form of treatment is the utilization of a vibrating vest that mimics the body's natural way of removing mucous and that is mucociliary clearance or MCC (Huang et al., 2004). In CF patients, the cilia lining the airways are incapable of flushing out mucous due to its high viscosity. Recently, scientists believe that there is a correlation between mucous viscosity and CFTR regulation of the epithelial sodium channel (ENaC), but such a mechanism remains unknown (Huang et al., 2004). Future research on this matter is highly suggested in the mean time!!
(Sheppard and Welsh, 1999)
I have realized many things, while conjuring this paper. The process of understanding a novel problem like CF is truly a journey of epic proportions! Then again, this was not a novel problem at all!! It has been studied for decades, and there are still no signs of a cure. While digging through the literature it felt as if I was falling into a bottomless pit. Every article was unique. The articles were difficult to read without background information, so I utilized Wikipedia and review articles to get the main plot of the CF story. Gradually I started focusing on articles that were related to the main plot of the CF story so that I do not digress into articles that were dealing with ‘novel' laboratory techniques that I was not familiar with. All in all, it was an entertaining experience.
Bals R., Weiner D., and Wilson.J. (1999). The innate immune system in cystic fibrosis lung disease.
J. Clin. Invest, 103(3): 303-307
This paper discusses the effects of changes in salt concentrations in airways and the consequential effects on immune factors.
Divangahi M., Balghi H., Danialou G., Comtois A., Demoule A., Ernest S., Haston C., Robert R., Hanrahan J.
, Radzioch D., and Petrof B. (2009).Lack of CFTR in Skeletal Muscle Predisposes to Muscle Wasting and Diaphragm Muscle Pump Failure in Cystic Fibrosis Mice. PLoS Genet. 5(7): e1000586
This article links CF with skeletal muscle atrophy. Experiments were performed on both CFTR (+) and CFTR (-) mice.
Huang P., Gilmore E., Kultgen P., Barnes P., Milgram S., and Stutts M. (2004). Local Regulation of Cystic
Fibrosis Transmembrane Regulator and Epithelial Sodium Channel in Airway Epithelium. The Proceedings of the American Thoracic Society 1:33-37
This paper touches base with a lot of hard concepts, but starts out with a nice summary on mucociliary clearance (MCC), and how it is affected by the CFTR's ability to regulate Cl- channels and Epithelial Sodium Channel (ENaCs)
Pier G. (2000). Role of the cystic fibrosis transmembrane conductance regulator in innate immunity to
Pseudomonas aeruginosa infections. Proc Natl Acad Sci., 97(16): 8822-8828
General information on how salt concentrations within cells and the lumen of organs changes and how Pseudomonas aeruginosa infections occur as a result of that.
Sheppard D., and Welsh M., (1999). Structure and Function of the CFTR Chloride Channel.
Physiol. Rev. 79: 23-45