SPECIAL TOPIC
CFTR trafficking and signaling in respiratory
epithelium
Bruce R.
Pitt
Department of Environmental and Occupational Health, Graduate
School of Public Health, University of Pittsburgh, Pittsburgh,
Pennsylvania 15238
 |
INTRODUCTION |
RESEARCH IN CYSTIC
FIBROSIS (CF) remains at the leading edge of investigations
employing human genetics and molecular and cellular biology. In the 12 years since the discovery of the gene encoding the CF transmembrane
conductance regulator (CFTR) Cl
channel
(14), translation of such efforts clinically has been remarkable and includes 1) identification of almost 1,000 different mutations (http://www.genet.sickkids.on.ca/cftr),
including 70% with a phenylalanine deletion (
F508) that has led to
successful clinical detection of individuals with a family history
(even before the completion of the human genome project) and
2) ~20 trials of human CF gene therapy involving dosing to
the nose or airways (2, 22). Such rapid progress, however,
belies the complexity of the CFTR gene and the pathophysiology of CF.
CFTR is a unique member of the ATP-binding cassette transporter gene
family in that it 1) conducts Cl
,
2) transports substrates across membranes in a nonconductive fashion (e.g., facilitates ATP release), 3) regulates other
ion channel proteins (e.g., positive regulation of outwardly rectifying Cl
channels or negative regulation of Na+
channels), and 4) regulates intracellular compartment
acidification and protein processing (18). Such complexity
has led these authors to categorize defects in CF respiratory
epithelium as primary (dehydration associated with dysfunctional
Cl
transport), secondary (hyperabsorption of
Na+), or tertiary (enhanced binding of Pseudomonas
aeruginosa; proinflammatory environment). Fundamental to such
diversity is the awareness that very minor mutations (e.g.,
F508, a
single-amino acid deletion within a 1,480-amino acid protein) result in
the profound pathological phenotype of CF (23). In the
case of
F508, the missense mutation results in a mutant CFTR that is
believed to fold improperly, and such defects in its biosynthesis and
trafficking result in little or no surface CFTR. Other mutations in
CFTR lead to genotypic or phenotypic changes that span the range from
no CFTR in the membrane to CFTR that reaches the membrane but does not
respond to appropriate stimuli or conduct Cl
(6). Adding to this complexity is a recent report
describing tissue-specific changes in the impairment of
F508
processing (10). Some mutations overlap with
PDZ-interacting domains on the COOH terminus of CFTR that are required
for polarization of CFTR to the apical membrane of respiratory
epithelium (13) and may underscore CFTR regulation of
other ion channels by affecting the relationship between accessory
proteins, linker proteins, and regulatory cofactors.
Accordingly, research into the biosynthesis and trafficking of
wild-type and mutated CFTR is at the forefront of CF investigations. Although the defect in Cl
conductance is critical to the
pathology of CF respiratory epithelia, contributions of alterations in
vesicle trafficking, protein processing, and immune function clearly
contribute to the pathogenesis and maintenance of CF (18).
These efforts underlie the disparities between genotype and phenotype
in CF and suggest that in addition to nucleotide repair (gene therapy,
aminoglycoside-dependent restoration of readthrough of full-length
CFTR), pharmacotherapy targeted toward improved biosynthesis of mutated
CFTR may be a useful adjunct approach to the treatment of CF
(24).
 |
MOLECULAR CHAPERONES AND BIOSYNTHESIS OF CFTR |
The importance of providing mechanistic support for
pharmacologically affecting the maturation of CF is placed in the
context of our current molecular physiological understanding of the
roles of various chaperones for CFTR by Brodsky (4) in
this issue of the American Journal of Physiology-Lung Cellular
and Molecular Physiology. The steps in proper folding of CFTR,
quality control mechanism of endoplasmic reticulum trafficking, and
requisite association with various chaperones are outlined by Brodsky
and reference is made to three original articles in the Special
Topic. The mechanisms by which sodium 4-phenylbutyrate may be a
modulator of such chaperones are presented by Rubenstein and Lyons
(17) and Choo-Kang and Zeitlin (5), and the
in vivo efficacy of a chemical chaperone, trimethylamine oxide, in the
restoration of defective Cl
conductance in the critical
target tissue (intestinal epithelium) of CFTR null mutant mice is
presented by Fischer et al. (7). This collective work
strongly supports the concept that small molecules are useful
alternative or adjunct therapies for disorders of protein folding
(23, 24).
 |
PROINFLAMMATORY PROFILE OF CF EPITHELIA |
A critical aspect of CF is chronic airway inflammation, and
several groups have provided support that dysregulation of the inflammatory response is an intrinsic component of such a phenotype. An
editorial by Blackwell et al. (1) accompanies the
manuscript by Weber et al. (21) and highlights the central
role of unregulated nuclear factor (NF)-
B in the proinflammatory
cytokine profile of cells expressing mutated CFTR. In this regard,
blocking the production of NF-
B may be an alternative rational
therapeutic approach in CF therapy. In addition to enhanced
cytokine (interleukin-8, tumor necrosis factor-
) production,
oxidative stress appears to be a component of CF epithelia. Previous in
vitro work (9, 11) showed that CFTR is associated with the
transport of glutathione (GSH). In the current Special Topic, Gao et
al. (8) show that the Cl
channel-forming
peptide N-K4-M2GlyR increased Cl
secretion
and GSH efflux in a human CF airway epithelial cell line, suggesting
that apical Cl
conductance (but not necessarily CFTR
function per se) is coupled to GSH efflux. Velsor et al.
(20) report in this issue that GSH was decreased in the
epithelial lining fluid of CFTR-deficient mice and that an imbalance in
antioxidant defense in CFTR-deficient mice was evident. These data
extend previous observations that GSH was decreased in the airway fluid
of humans with CFTR (16) and that aerosol delivery of GSH
may be a useful therapy for CF (15). GSH and other
cellular defense mechanisms may indeed represent a rational therapeutic
strategy in CF.
 |
SERINE PROTEASE-SENSITIVE SODIUM HYPERABSORPTION IN HUMAN
AIRWAY EPITHELIUM |
As noted above, Na+ hyperabsorption is a
critical secondary-like phenomenon in CF epithelia. Although the
mechanism coupling such hyperabsorption to CFTR mutations remains
unknown, an extracellular serine protease-mediated signaling pathway
has been identified (19) that activates an
amiloride-sensitive Na+ channel. In the Special Topic in
this issue, Bridges et al. (3) show that human bronchial
epithelial cells are sensitive to aprotinin. Using the Kunitz domain of
aprotinin as a pharmacophore, they show that BAY 39-9437, a recombinant
Kunitz-type serine protease inhibitor, decreased Na+
absorption in non-CF and CF human bronchial epithelia.
 |
EDITOR'S NOTE |
This is the first of a series of special topics that will
appear routinely in the American Journal of
Physiology-Lung Cellular and Molecular Physiology. The purpose of
these special calls is to highlight areas of significant interest in
respiratory biology by soliciting input from a broad research
community, fast tracking the submitted manuscripts through the review
process, and publishing them under their own separate heading. In this
case, a call for papers in "CFTR Trafficking and Signaling in
Respiratory Epithelium" initially appeared in July 2000 (12). This led to the seven manuscripts and two
accompanying editorials that appear in the current issue. This
remarkable and timely turnaround in peer review is a testimony to the
importance of the topic and the enthusiasm of the authors and reviewers
to bring this information to our readership.
 |
FOOTNOTES |
This special topic section is a collection of papers accepted
under a special call for manuscripts by the Editor. See Journal web
site for information about the next call.
Address for reprint requests and other correspondence: B. R. Pitt,
Dept. of Environmental and Occupational Health, Graduate School Public
Health, Univ. of Pittsburgh, Pittsburgh, PA 15238 (E-mail:
brucep+{at}pitt.edu).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
 |
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