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Gap junction communication in alveolar epithelial cells

D. Eugene Rannels

Departments of Cellular and Molecular Physiology and Anesthesia, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033


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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.


    FOOTNOTES

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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REFERENCES

1.   Abraham, V, Chou M, DeBolt K, and Koval M. Phenotypic control of gap junctional communication by alveolar epithelial cells. Am J Physiol Lung Cell Mol Physiol 276: L825-L834, 1999[Abstract/Free Full Text].

1a.   Abraham, V, Chou ML, George P, Pooler P, Zaman A, Savani RC, and Koval M. Heterocellular gap junctional communication between alveolar epithelial cells. Am J Physiol Lung Cell Mol Physiol 280: L1085-L1093, 2001[Abstract/Free Full Text].

2.   Ashino, Y, Xiaoyou Y, Dobbs LG, and Bhattacharya J. [Ca2+]i oscillations regulate type II cell exocytosis in the pulmonary alveolus. Am J Physiol Lung Cell Mol Physiol 279: L5-L13, 2000[Abstract/Free Full Text].

3.   Bartels, H. The air-blood barrier in the human lung: a freeze-fracture study. Cell Tissue Res 198: 269-285, 1979[ISI][Medline].

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6.   Dunsmore, SE, and Rannels DE. Extracellular matrix biology in the lung. Am J Physiol Lung Cell Mol Physiol 270: L3-L27, 1996[Abstract/Free Full Text].

7.   Gehr, P, and Crapo JD. Morphometric analysis of the gas exchange region of the lung. In: Toxicology of the Lung, edited by Gardner DE, Crapo JD, and Massaro EJ.. New York: Raven, 1988, p. 1-42.

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11.   Lee, YC, Yellowley CE, Li Z, Donahue HJ, and Rannels DE. Expression of functional gap junctions in cultured pulmonary alveolar epithelial cells. Am J Physiol Lung Cell Mol Physiol 272: L1105-L1114, 1997[Abstract/Free Full Text].

12.   Lubman, R, Zhang X-L, Zheng J, Ocampo L, Lopex MZ, Veeraraghavan S, Zabski SM, Danto SI, and Borok Z. Integrin alpha 3-subunit expression modulates alveolar epithelial cell monolayer formation. Am J Physiol Lung Cell Mol Physiol 279: L183-L193, 2000[Abstract/Free Full Text].

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15.   Schneeberger, EL, and Lynch RD. Airway and alveolar epithelial cell junctions. In: The Lung. Scientific Foundations, edited by Crystal RG, West JB, Weibel ER, and Barnes PJ.. Philadelphia, PA: Lippincott-Raven, 1997, p. 505-515.


Am J Physiol Lung Cell Mol Physiol 280(6):L1083-L1084
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