Am. J. Respir. Cell Mol. Biol.,
Volume 24, Number 3, March 2001 221-223
PERSPECTIVE
Man Is Not a Rodent
Aquaporins in the Airways
Landon S.
King
and
Peter
Agre
Department of Medicine, Division of Pulmonary and Critical Care Medicine, and Department of Biological Chemistry,
Johns Hopkins University School of Medicine, Baltimore, Maryland
The physiological significance of fluid homeostasis in the
respiratory tract is readily apparent. The nasopharynx and
upper airways must humidify the inspired airstream, as well
as regulate the volume and composition of the airway surface layers. The distal lung must mobilize fluid at the time
of birth in preparation for the transition to ex utero life
and must handle a variety of challenges to fluid balance
that could interfere with gas exchange throughout life.
Disruption in water flux at these sites may contribute to the pathogenesis of rhinnorrhea, impaired mucociliary transport, exercise- or cold-induced asthma, and pulmonary
edema. The molecular determinants of these processes in
the respiratory tract therefore continue to be the focus of
intense investigation. In this issue, Gynn and colleagues
provide important new information about the expression
of aquaporin water channel proteins in the human airway epithelium (1).
Aquaporins (AQPs) are membrane channel proteins that
are highly and, in most cases, specifically permeable to water. Ten mammalian AQPs have been identified to date,
and homologues have been demonstrated at all levels of
life, including bacteria, yeast, and plants (2). Several AQPs
have been demonstrated to have permeabilities in addition to water. AQP3, AQP7, and AQP9 are permeable to
small solutes (for example, glycerol), an observation whose
functional significance is still undefined. Studies in Xenopus oocytes suggest that AQP1 is permeated by CO2 (3).
This finding was confirmed in proteoliposomes reconstituted with AQP1 (4), but was not observed in erythrocytes
from AQP1-null mice lacking the protein (5). The magnitude of the CO2 permeability is low compared to that of
water, and the physiological significance is not yet clear.
Studies of distribution and ontogeny in the rat respiratory tract established a network of four AQPs with nonoverlapping distribution (6). AQP1 is present in both
the apical and basolateral membrane of microvascular endothelial cells and fibroblasts, while AQP3, AQP4, and
AQP5 polarize to the apical or basolateral membrane at
different sites in the respiratory epithelium. Curiously absent from the AQP network in the rat respiratory tract are
AQPs on the apical membrane of nasopharyngeal or airway epithelium, or on the basolateral membrane of type I
pneumocytes. Several explanations for these findings have
been suggested: an unidentified AQP is expressed in those
locations; water transport across the apical membrane occurs by non-AQP mediated mechanisms; or unilateral expression of an AQP in the epithelium suggests an alternative function besides transcellular water movement, for
example cell volume regulation (8). Gynn and colleagues indicate that in the human respiratory tract there
is an alternative explanation.
The authors observed many similarities between the
human and rat respiratory tract in the distribution of AQP3,
AQP4, and AQP5, including the apical expression of
AQP5 in secretory cells of subepithelial glands, the basolateral expression of AQP4 in superficial epithelium, the
presence of AQP3 in basal cells of the nasopharyngeal and
upper airway epithelium, and the expression of AQP5 in
type I pneumocytes. The most notable findings in the study
by Gynn and colleagues, however, are the differences between the two species. In contrast to the rat, AQPs are
present in the apical membrane of airway epithelium in
the human respiratory tract. AQP5 was detected by in situ
hybridization in the superficial epithelium of nasopharyngeal and bronchial epithelium, as well as in subepithelial
glands. Immunofluorescence confirmed the expression of
AQP5 protein in the apical membrane in nasopharynx and
subepithelial glands; however, AQP5 protein "was not routinely detected" in the bronchial epithelium, a discrepancy
with the results of in situ hybridization that warrants further evaluation. At the level of the bronchioles, AQP3 was
expressed not only in basal cells, but also in the apical membrane of columnar cells in the bronchiolar epithelium. In addition to being distinct from the rat respiratory tract, this is
also the first example of apical trafficking of AQP3; in kidney, as well as in other, tissues, AQP3 has been localized exclusively to the basolateral membrane of different epithelia (11). Finally, the authors suggest that AQP3 is present in
type II pneumocytes and that AQP4 is present in alveolar
epithelial cells. Higher resolution imaging studies will be
necessary to confirm the presence and distribution of these
water channels in alveolar epithelium.
The details of water transport in the respiratory tract
continue to be a source of discussion, if not frank controversy. How is water supplied to the airway for humidification of the inspired airstream? Gynn's observations, coupled with prior descriptions of AQP1 in the subepithelial
vascular endothelium (6, 8, 9), potentially provide a pathway for AQP-mediated water transport from the vasculature to the airway lumen for this purpose. In the airways,
distinct, even disparate, hypotheses have been proposed to
explain the generation and composition of the aqueous
surface layer and their role in the pathogenesis of cystic fibrosis (reviewed in 12). While the findings of Gynn and
coworkers do not resolve this issue, they do provide important insights into the molecular components of fluid
transport across the airway epithelium. Apical expression
of AQPs in the upper and lower airways provides an opportunity, at least in theory, for regulation of membrane
water permeability independent of other factors. Several
recent studies may prove relevant on this point. Schreiber
and colleagues demonstrated that AQP3 can be regulated
by the cystic fibrosis transmembrane conductance regulator (13), an interesting observation that if confirmed will
more tightly link solute and water transport across the apical membrane. Regulation of AQP3 may be still more complex, as Zeuthen and Klaerke have demonstrated channel
gating by changes in pH (14). The in vivo significance of
this to the airway epithelium is not immediately clear;
however, the model of gating may extend to other stimuli.
Hoffert recently demonstrated that AQP5 expression was
upregulated both in vitro and in vivo by hypertonic stress
(15). AQP5 could be induced by addition of as little as 25 mOsm sorbitol to normal medium, well within the range
of osmolarities demonstrated for the airway surface layer.
In the distal lung, the specific pathway for water transport across the epithelium membrane in either direction remains
undefined. Evidence that AQP3 and AQP4 may complement expression of AQP5 in the apical membrane of type I
pneumocytes (8) provides needed insight into molecular
aspects of alveolar water transport, but must be confirmed
by high resolution immunoelectron microscopy to definitively assign cell types and polarization.
Functional roles for aquaporins in lung physiology have
been suggested by a number of animal studies. Water
channel-mediated transport across the alveolar and airway
epithelium were suggested by studies of in situ perfused
sheep lung (16) and perfused guinea pig distal airways
(17), respectively. Adenovirus lung infection in mice decreased expression of AQP1 and AQP5 concurrent with
an increase in lung water, suggesting potential roles in the
pathogenesis of pulmonary edema (18). Recent studies in mice with deletions of the AQP1 or AQP5 gene have produced mixed results. Water permeability across the pulmonary vascular bed in AQP1-null mice was reduced 10-fold in
response to an osmotic gradient, and two-fold in response
to increased hydrostatic pressure (19). AQP5-null mice also
had a marked decrease in osmotic water permeability across
the alveolar epithelium; however, no reduction in isosmolar fluid reabsorption from the alveolar space was observed, raising questions about the role of AQP5 in mediating
transalveolar water movement (20).
Evidence of AQP involvement in human pathophysiology is emerging. Mutations in the vasopressin-responsive
water channel in renal collecting ducts, AQP2, produce
nephrogenic diabetes insipidus (21). Some forms of congenital cataract have been linked to mutations in AQP0
(major intrinsic protein of the lens) (22, 23). In patients
with Sjögren's syndrome, AQP5 does not traffic to the apical membrane of lacrimal glands (24) or salivary glands (25), a defect which likely contributes to the deficiency in lacrimation and salivation, which are hallmarks of the disorder. Recently, rare individuals who are deficient in
AQP1 have been demonstrated to have defects in urinary
concentrating ability as well as altered pulmonary vascular
permeability, confirming a physiologic role for AQP1 (unpublished observations). Gynn and colleagues have now
set the stage for a more informed analysis of water transport across the pulmonary epithelium. Their findings provide insight into basic aspects of lung biology, but as importantly, they remind us of the caution that must be
exercised in making definite assignments of physiologic
function by extrapolation across species. In the end, it is
clear that studies of human tissues must be undertaken before presumptions about physiological significance (or lack
of significance) can be made.
 |
Footnotes |
Address correspondence to: Landon S. King, M.D., Division of Pulmonary
and Critical Care Medicine, Johns Hopkins School of Medicine, 600 N. Wolfe St., Blalock 910, Baltimore, MD 21287. E-mail: lsking{at}welch.jhu.edu
(Received in original form January 26, 2001).
Abbreviations: aquaporins, AQPs.
| |
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