Departments of Cellular and Molecular Physiology and Anesthesia, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033
FUNCTIONAL INTEGRITY OF THE LUNG
requires maintenance of a tissue architecture specialized to optimize
diffusion of gas between the alveolar lumen and pulmonary capillary
blood. Efficiency of gas exchange in the alveolar region reflects
evolution of a highly conserved diffusion barrier with a large surface
area and minimal thickness (7). Functional interactions of
alveolar cells, and thus integrity of the blood-gas barrier,
depend on the expression and assembly of specific proteins into
specialized junctional structures that define tissue compartments and
facilitate both cell-extracellular matrix interactions (6)
and cell-to-cell communication (2, 12).
Both alveolar epithelial cells and pulmonary capillary
endothelial cells are well known to form intercellular tight junctions that regulate the movement of water and solutes via paracellular pathways, thus maintaining optimal conditions for gas exchange (for a
review, see Ref. 15). Both expression and assembly of tight junction proteins are essential to the integrity of the blood-gas
barrier and thus to the support of normal lung function.
Recent evidence (1, 9, 11) shows that alveolar epithelial
cells also express gap junction proteins (connexins) and establish
functional gap junction intercellular communication (GJIC) linking the
cytoplasmic compartments of adjacent cells. Connexins constitute a
family of closely related integral membrane proteins that assemble to
form hexameric transmembrane hemichannels (connexons). Connexons in
adjacent cells interact in the intercellular compartment to form
conductive channels that allow regulated passage of electrical current,
solutes, and small molecules through axial transmembrane pores that
mediate coordination of cellular activity (13).
Based on application of freeze-fracture techniques to lung tissue,
Bartels (3) first reported the presence of gap junction plaques between alveolar epithelial cells. A beltlike network of
particles representing connexins was evident on the P face of
freeze-fractured biopsy samples from lungs of human subjects. These
structures were distributed in close proximity to tight junction
networks, suggesting the presence of communicating-occluding junctional
complexes similar to those described in other tissues. Anatomic
(5), biochemical (10, 11), and
functional (1, 9, 11) data have subsequently confirmed and
expanded Bartels' (3) observations. Gap junction channels
appear to be expressed most frequently between adjacent type I alveolar
epithelial cells, reflecting the extensive distribution of
type I cells, which occupy >90% of the alveolar surface
(7).
Data from Northern and Western blot analyses confirm that at least
eight connexins are expressed in the gas-exchange region of the lung;
these include connexin (Cx)26, Cx30.3, Cx32, Cx37, Cx40, Cx43, Cx45,
and Cx46 (1, 11, 14). Only recently, however, has detailed
information begun to emerge concerning the cellular patterns of
expression, relative abundance, and functional significance of these
molecules in the peripheral lung. Further observations in vivo and in
vitro demonstrate expression and/or function of active gap junction
channels in alveolar epithelial cells of both type I and type II
phenotypes (1, 9, 11). Nevertheless, compared with the
well-defined physiological role of tight junctions between alveolar
epithelial cells (15), relatively little information is
available concerning the functional characteristics or physiological
significance of connexin-specific gap junction channels in pulmonary epithelium.
In contrast, specific characteristics of gap junction intercellular
signaling pathways involving calcium and other mediators are well
established in airway epithelial cells (4) and in nonpulmonary cell populations (for reviews, see Refs. 8,
13). These pathways are relevant to diverse areas of cell
biology including transepithelial ion transport, mechanochemical
signal transduction, and regulation of cell growth. Although gap
junctional complexes may be essential to physiological signaling
pathways in the alveolar region of the lung, such as those associated
with secretion of pulmonary surfactant (2), mechanisms of
cell-to-cell communication between type I and type II epithelial cells
remain poorly understood.
Interpretation of the physiological role of gap junction communication
between alveolar epithelial cell populations is complicated by several
factors. Much of what is known about gap junctions in the alveolar
epithelium is derived from purified populations of freshly isolated
alveolar type II cells. These cells express relatively high levels of
Cx26 and Cx32 but little Cx43 and Cx46 (1, 11). Although
connexin expression implies that type II cells assemble functional gap
junction channels (11), morphometric data show that in
adult lung tissue, type II cells are seldom adjacent to each other
(7). Based on the latter results, it is anticipated that
gap junction communication between type II cells per se is rare in
situ. This conclusion raises the question of whether
functional gap junctions observed in relatively homogeneous primary
type II cell cultures are of physiological significance. Alternatively,
connexin expression by type II cells in vivo may reflect the potential
for assembly of gap junction channels between type I and type II
alveolar epithelial cells and thus for physiologically relevant
signaling networks within the alveolar microenvironment.
It is widely recognized that as a function of time in primary culture,
type II alveolar epithelial cells acquire a type I cell-like phenotype
reflecting cellular deposition of and interaction with an underlying
fibronectin-rich extracellular matrix (6). Under the
latter conditions, the above four connexins are expressed in a converse
pattern of abundance: high Cx43 and Cx46 and low Cx26 and Cx32
(1, 9, 11). These data suggest that (frequent) adjacent
type I cells may establish GJIC in vivo. They also indicate a potential
for type I-type II cell interactions via heterotypic gap junction
channels formed by different connexons.
In this issue of the American Journal of Physiology: Lung
Cellular and Molecular Physiology, Abraham et al. (1a)
demonstrate that in primary culture, day 6 alveolar
epithelial cells with a "type I cell-like" phenotype and connexin
profile (expressing Cx43 and Cx46) can establish gap junction
communication with freshly isolated type II cells, which mainly express
Cx26 and Cx32. This observation of heterotypic GJIC confirms recent
evidence derived from a similar alveolar cell model (9)
and thereby supports the hypothesis that type I and type II alveolar
epithelial cells can communicate via gap junction channels.
The data from Abraham et al. (1a) extend further to demonstrate
gap junction communication between type II cells and HeLa cells
transfected with specific connexins. Based on these and additional
observations, members of the Koval laboratory (1a) have
confirmed that day 6 alveolar cells establish functional gap
junction communication with nearby alveolar cells. Their data also show
active GJIC between alveolar epithelial cells and HeLa cells
transfected with Cx43 but not with HeLa cells transfected with Cx32.
These results reveal selective connexin interactions in these cell
populations. Of additional interest is the observation that suggests
that Cx46 may serve as a marker of recovery in injured cells and/or may
play a regulatory role in gap junction conductance.
An important aspect of the work by Abraham et al. (1a) is that the
experimental approach takes advantage of gene transfection to modulate
connexin expression in HeLa cells, offering an opportunity to
investigate complexities of connexin expression and function in the
alveolar epithelium. Not only has this approach provided unique insight
in the present studies, it also promises to be valuable in further
efforts to define both the characteristics and physiological
significance of gap junction function in cells of the alveolar surface.
Together, the observations reported by Abraham et al. (1a) support the
premise that direct cell-cell interactions via gap junction channels
may play a significant role in the regulation of alveolar epithelial
cell phenotype and function. The work is thus notable in that it
provides valuable insight into an important area of investigation that
is largely unexplored in lung cell populations where several lines of
converging evidence suggest that cell-cell interactions via gap
junctions are of both functional and physiological significance.
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REFERENCES
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FOOTNOTES |
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Address for reprint requests and other correspondence: D. E. Rannels, Dept. of Cellular and Molecular Physiology (H166), The Pennsylvania State Univ. College of Medicine, 500 University Dr., Hershey, PA 17033 (E-mail:grannels{at}psu.edu).
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