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Am J Physiol Lung Cell Mol Physiol 295: L238-L239, 2008. First published May 23, 2008; doi:10.1152/ajplung.90311.2008
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EDITORIAL FOCUS

From the farm to the lab: the pig as a new model of cystic fibrosis lung disease

A. S. Verkman

Departments of Medicine and Physiology, University of California, San Francisco, California

CYSTIC FIBROSIS (CF) is the most common lethal hereditary disease in Caucasians, producing chronic lung infection with deteriorating lung function and various extrapulmonary problems including pancreatic insufficiency. A major breakthrough in CF was the identification, in 1989, of its genetic basis: mutations in a cAMP-regulated chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR). However, despite nearly 20 yr of research and more than 5,000 PubMed-cited CFTR papers, knowledge of the underlying CFTR defect has not been effectively translated into improved clinical therapies. One reason has been poor understanding of how the CFTR defect produces CF lung disease. Transgenic mouse models of CF manifest subtle to no CF lung disease (8). The long-awaited CF pig holds great promise as a model of CF. Recent success in applying somatic cell nuclear transfer technology to generate heterozygous CFTR null and {Delta}F508-CFTR knock-in pigs (15) assures that homozygous CF pigs are on their way or already here.

In this issue of AJP-Lung, Rogers et al. (14) provide an encyclopedic compendium of pig medicine and physiology with focus on the lung. Many facts may interest and surprise the pig nonaficionados. Pigs live for 10–20 yr. Their gestation period is ~4 mo, with up to 24–36 piglets per year from a single sow, allowing rapid colony expansion. Completion of the pig genome project is expected by the end of this year. Pigs have already been used extensively to study a wide variety of lung functions and pathologies, such as surfactant biology, asthma, acute lung injury, and gene transfer. Of relevance to CF, information already exists in pigs on airway salt transport, nasal potential differences, submucosal gland function, and mucociliary clearance mechanisms (2, 3, 16, 19). Although pigs do not inhale and exhale on command, a variety of complicated-sounding techniques are available to measure lung function, including esophageal balloon-pneumotachography, whole body barometric plethysmography, and impulse oscillometry. Finally, state-of-the-art imaging methods have been adapted to study pig lungs, including multi-row detector X-ray computed tomography and macro- and micro-optical bronchoscopy. High-resolution chest computed tomography may be particularly useful in detection of air trapping as an early sign of CF lung disease.

The two major anticipated uses of CF pigs include research on the pathogenesis of CF lung disease and the testing of CF therapies. With regard to the former, the hope is that pigs will develop, within a reasonable time frame, some or all of the major manifestations of human CF lung disease, including mucus plugging, Pseudomonas colonization, glandular hypertrophy, bronchiectasis, and progressive deterioration in lung function. Rogers et al. (14) point out the many similarities between human and pig lungs of relevance to CF–similar anatomy, histology, electrolyte transport, submucosal gland function, and immune and inflammatory responses–heightening optimism that the pig will develop human-like CF lung disease. If CF pigs do not develop lung disease spontaneously, then induction maneuvers such as exposure to Pseudomonas aeruginosa may initiate the process.

Will CF pigs, assuming that they develop the major manifestations of human CF lung disease, provide new insight into the pathogenesis of CF lung disease? Research into the link between CFTR dysfunction and CF lung disease has had an inauspicious history. Not long after the identification of CFTR emerged the "defective organellar acidification hypothesis" postulating that defective CFTR function in Golgi impairs chloride entry during vacuolar acidification, producing a pleiotropic set of cellular abnormalities (4). However, follow-up studies in multiple laboratories did not confirm defective Golgi acidification in CF (7, 17). A recent variant, the "defective macrophage phagosome acidification hypothesis" (6), which offered an explanation for impaired bacterial killing in CF, appears to be incorrect as well (9). There was much interest in the "high-salt hypothesis" (18), postulating that impaired CFTR chloride absorption from the airway surface liquid (ASL) in CF elevates ASL salt concentration, resulting in impaired ASL antimicrobial function. However, several laboratories found ASL salt concentration to be near isotonic in both non-CF and CF airway epithelia (10, 11). The "ASL dehydration hypothesis" (5), postulating that CFTR dysfunction-related epithelial Na+ channel (ENaC) hyperactivity and sodium hyperabsorption produces a thickened, dehydrated ASL, has remained a popular theory. However, the evidence for "ASL dehydration" has come largely from cell culture studies, and the critical assumption of CFTR suppression of ENaC activity remains unproven. "Defective submucosal gland function" is another popular hypothesis, whereby CFTR dysfunction in serous acini in airway submucosal glands results in reduced glandular secretion of a hyperviscous, acidic fluid (21), which impairs bacterial killing and mucociliary clearance. Many other hypotheses have been offered to account for CF lung disease, including intrinsic hyperinflammation in CF, neutrophil dysfunction in CF, direct bacterial binding to CFTR, reduced ASL oxygenation in CF, ceramide accumulation in CF cells, and others. Determination of which hypothesis(es) are correct is crucial to the development of mechanism-based therapies to treat CF. The CF pig is a welcomed new model system to study CF pathogenesis. It may be possible to define precisely where and when CF lung disease first develops, define the temporal evolution of pathological changes in the lung, and delineate the roles of epithelial cell dysfunction and inflammatory responses. CF pigs may also be useful in studying disease pathogenesis in gastrointestinal and reproductive organs. However, resolving the connection between CFTR dysfunction and CF lung disease, even with a good pig CF model, will not be easy and will likely require considerable creativity.

Will the pig be a useful surrogate for testing of CF therapies? CF therapies can be considered as supportive, mechanism-based, and those that correct the underlying CFTR defect. Supportive therapies in the clinic now, such as antibiotics, DNase, anti-inflammatory agents, pancreatic replacement enzymes, etc., have been tested extensively in humans. Perhaps a good pig model of human CF lung disease may be useful in testing of new supportive therapies, such as antimicrobials to eradicate bacterial biofilms or novel mucolytics. The utility of a pig CF model is questionable in prioritizing and testing of mechanism-based therapies, such activators of alternative chloride channels and inhibitors of ENaC or therapies that correct the underlying CF defect, including gene replacement and small-molecule potentiators and correctors of mutant CFTRs. Several mechanism-based therapies are in human clinical trials, and for newer therapies in preclinical development, data from pigs, which are likely to be costly in time and effort, may not be considered useful for commercial "go/no-go" decisions. Much effort is being focused on corrector therapy, including small molecules that facilitate the cellular processing and plasma membrane targeting of {Delta}F508-CFTR (13, 20), the most common CF-causing CFTR mutation present in ~90% of CF subjects. {Delta}F508-CFTR knock-in pigs may have limited utility in testing of corrector therapies because of the more efficient cellular processing of pig vs. human {Delta}F508-CFTR (1, 2) and differences in cellular quality control and processing machinery in pig vs. human cells.

Notwithstanding the many questions and caveats, the CF pig represents a major advance, perhaps the advance of the decade in the CF field, the real impact of which awaits much study.


   FOOTNOTES
 

Address for reprint requests and other correspondence: A. S. Verkman, Depts. of Medicine and Physiology, 1246 Health Sciences East Tower, Univ. of California, San Francisco, CA 94143-0521 (e-mail: alan.verkman{at}ucsf.edu)


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This Article
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