Reversible transdifferentiation of alveolar epithelial cells

Epithelial Repair and Regeneration

Contact with the environment positions the respiratory epithelium at risk for acute and chronic injury from infectious pathogens, noxious agents, and inflammatory processes. Thus, to protect gas transfer within the lung the epithelium is programmed for routine maintenance and repair. Programs for repair are directed by epithelial, mesenchymal, and inflammatory signals that collectively constitute highly regulated networks. Principal components of the repair network are developmental morphogens, integrin and growth factor signaling molecules, and transcription factors. The epithelium responds to these signals with a remarkable plasticity and is bulwarked by a population of lung progenitor cells to ensure maintenance and repair for fluid balance and host defense functions.

Regulated gene expression in cultured type II cells of adult human lung

Alveolar type II cells have multiple functions, including surfactant production and fluid clearance, which are critical for lung function. Differentiation of type II cells occurs in cultured fetal lung epithelial cells treated with dexamethasone plus cAMP and isobutylmethylxanthine (DCI) and involves increased expression of 388 genes. In this study, type II cells of human adult lung were isolated at approximately 95% purity, and gene expression was determined (Affymetrix) before and after culturing 5 days on collagen-coated dishes with or without DCI for the final 3 days. In freshly isolated cells, highly expressed genes included SFTPA/B/C, SCGB1A, IL8, CXCL2, and SFN in addition to ubiquitously expressed genes. Transcript abundance was correlated between fetal and adult cells (r = 0.88), with a subset of 187 genes primarily related to inflammation and immunity that were expressed >10-fold higher in adult cells. During control culture, expression increased for 8.1% of expressed genes and decreased for approximately 4% including 118 immune response and 10 surfactant-related genes. DCI treatment promoted lamellar body production and increased expression of approximately 3% of probed genes by > or =1.5-fold; 40% of these were also induced in fetal cells. Highly induced genes (> or =10-fold) included PGC, ZBTB16, DUOX1, PLUNC, CIT, and CRTAC1. Twenty-five induced genes, including six genes related to surfactant (SFTPA/B/C, PGC, CEBPD, and ADFP), also had decreased expression during control culture and thus are candidates for hormonal regulation in vivo. Our results further define the adult human type II cell molecular phenotype and demonstrate that a subset of genes remains hormone responsive in cultured adult cells.

Identification of mesenchymal stromal cells in human lung parenchyma capable of differentiating into aquaporin 5-expressing cells

The lack of effective therapies for end-stage lung disease validates the need for stem cell-based therapeutic approaches as alternative treatment options. In contrast with exogenous stem cell sources, the use of resident progenitor cells is advantageous considering the fact that the lung milieu is an ideal and familiar environment, thereby promoting the engraftment and differentiation of transplanted cells. Recent studies have shown the presence of multipotent 'mesenchymal stem cells' in the adult lung. The majority of these reports are, however, limited to animal models, and to date, there has been no report of a similar cell population in adult human lung parenchyma. Here, we show the identification of a population of primary human lung parenchyma (pHLP) mesenchymal stromal cells (MSCs) derived from intraoperative normal lung parenchyma biopsies. Surface and intracellular immunophenotyping by flow cytometry revealed that cultures do not contain alveolar type I epithelial cells or Clara cells, and are devoid of the following hematopoietic markers: CD34, CD45 and CXCR4. Cells show an expression pattern of surface antigens characteristic of MSCs, including CD73, CD166, CD105, CD90 and STRO-1. As per bone marrow MSCs, our pHLP cells have the ability to differentiate along the adipogenic, osteogenic and chondrogenic mesodermal lineages when cultured in the appropriate conditions. In addition, when placed in small airway growth media, pHLP cell cultures depict the expression of aquaporin 5 and Clara cell secretory protein, which is identified with that of alveolar type I epithelial cells and Clara cells, respectively, thereby exhibiting the capacity to potentially differentiate into airway epithelial cells. Further investigation of these resident cells may elucidate a therapeutic cell population capable of lung repair and/or regeneration.

Rat alveolar type I cells proliferate, express OCT-4, and exhibit phenotypic plasticity in vitro

Alveolar type I (TI) cells are large, squamous cells that cover 95-99% of the internal surface area of the lung. Although TI cells are believed to be terminally differentiated, incapable of either proliferation or phenotypic plasticity, TI cells in vitro both proliferate and express phenotypic markers of other differentiated cell types. Rat TI cells isolated in purities of >99% proliferate in culture, with a sixfold increase in cell number before the cells reach confluence; >50% of the cultured TI cells are Ki67+. At cell densities of 1-2 cells/well, approximately 50% of the cells had the capacity to form colonies. Under the same conditions, type II cells do not proliferate. Cultured TI cells express RTI40 and aquaporin 5, phenotypic markers of the TI cell phenotype. By immunofluorescence, Western blotting, and Q-PCR, TI cells express OCT-4A (POU5F1), a transcription factor associated with maintenance of the pluripotent state in stem cells. Based on the expression patterns of various marker proteins, TI cells are distinct from either of two recently described putative pulmonary multipotent cell populations, the bronchoalveolar stem cell or the OCT-4+ stem/progenitor cell. Although TI cells in adult rat lung tissue do not express either surfactant protein C (SP-C) or CC10, respective markers of the TII and Clara cell phenotypes, in culture TI cells can be induced to express both SP-C and CC10. Together, the findings that TI cells proliferate and exhibit phenotypic plasticity in vitro raise the possibility that TI cells may have similar properties in vivo.

Host-pathogen interactions during coronavirus infection of primary alveolar epithelial cells

Innate immune responses in coronavirus infections of the respiratory tract are analyzed in primary differentiated airway and alveolar epithelial cells.

Viruses that infect the lung are a significant cause of morbidity and mortality in animals and humans worldwide. Coronaviruses are being associated increasingly with severe diseases in the lower respiratory tract. Alveolar epithelial cells are an important target for coronavirus infection in the lung, and infected cells can initiate innate immune responses to viral infection. In this overview, we describe in vitro models of highly differentiated alveolar epithelial cells that are currently being used to study the innate immune response to coronavirus infection. We have shown that rat coronavirus infection of rat alveolar type I epithelial cells in vitro induces expression of CXC chemokines, which may recruit and activate neutrophils. Although neutrophils are recruited early in infection in several coronavirus models including rat coronavirus. However, their role in viral clearance and/or immune‐mediated tissue damage is not understood. Primary cultures of differentiated alveolar epithelial cells will be useful for identifying the interactions between coronaviruses and alveolar epithelial cells that influence the innate immune responses to infection in the lung. Understanding the molecular details of these interactions will be critical for the design of effective strategies to prevent and treat coronavirus infections in the lung.

Characterization of mouse alveolar epithelial cell monolayers

We investigated the influence of extracellular matrix on transport properties of mouse alveolar epithelial cell (AEC) monolayers (MAECM) and transdifferentiation of isolated mouse alveolar epithelial type II (AT2) cells into an alveolar epithelial type I (AT1) cell-like phenotype. Primary mouse AT2 cells plated on laminin 5-coated polycarbonate filters formed monolayers with transepithelial resistance (R(T)) and equivalent short-circuit current (I(EQ)) of 1.8 kOmega.cm(2) and 5.3 microA/cm(2), respectively, after 8 days in culture. Amiloride (10 microM), ouabain (0.1 mM), and pimozide (10 microM) decreased MAECM I(EQ) to 40%, 10%, and 65% of its initial value, respectively. Sequential addition of pimozide and amiloride, in either order, revealed that their inhibitory effects are additive, suggesting that cyclic nucleotide-gated channels contribute to amiloride-insensitive active ion transport across MAECM. Ussing chamber measurements of unidirectional ion fluxes across MAECM under short-circuit conditions indicated that net absorption of Na(+) in the apical-to-basolateral direction is comparable to net ion flux calculated from the observed short-circuit current: 0.38 and 0.33 microeq.cm(-2).h(-1), respectively. Between days 1 and 9 in culture, AEC demonstrated increased expression of aquaporin-5 protein, an AT1 cell marker, and decreased expression of pro-surfactant protein-C protein, an AT2 cell marker, consistent with transition to an AT1 cell-like phenotype. These results demonstrate that AT1 cell-like MAECM grown on laminin 5-coated polycarbonate filters exhibit active and passive transport properties that likely reflect the properties of intact mouse alveolar epithelium. This mouse in vitro model will enhance the study of AEC derived from mutant strains of mice and help define important structure-function correlations.

Characterisation of the R3/1 cell line as an alveolar epithelial cell model for drug disposition studies

The rat cell line R3/1 displays several phenotypical features of alveolar epithelial type I cells. In order to evaluate this cell line as potential in vitro model for drug disposition studies, R3/1 cells were cultured on Transwell filters and the transepithelial electrical resistance (TEER) was measured to test the integrity of cell layers. The mRNA expression of cell junctional components including E-cadherin, occludin, ZO-1 and ZO-2 was studied using reverse transcriptase-polymerase chain reaction (RT-PCR) and the corresponding proteins by immunofluorescence microscopy (IFM). Moreover, the expression pattern of catabolic peptidases, carboxypeptidase M, aminopeptidases (AP): A, B, N and P, gamma-glutamyltransferase (GGT), dipeptidylpeptidase IV, angiotensin-converting enzyme (ACE), and endopeptidases (EP) 24.11 and 24.15 was analysed in R3/1 cells and compared to rat alveolar epithelial I-like cells in primary culture. TEER peaked at 99+/-17Omegacm(2) after 5 days in culture. Addition of 0.1muM dexamethasone (DEX) with 20% foetal bovine serum further increased TEER by 65%. However, none of the culture conditions used in our study yielded monolayers with TEER values comparable to those of primary cultures of rat pneumocytes. No transcripts encoding for E-cadherin and occludin were detected by RT-PCR. However, ZO-1 and -2 mRNA transcripts were found. IFM using a monoclonal antibody against occludin confirmed the absence of the protein in R3/1 cells. Of the investigated proteolytic enzymes, mRNA transcripts encoding APA and APB as well as EP 24.11 and EP 24.15 were detected; a pattern similar to that of rat alveolar epithelial I-like cells in primary culture. Thus, although R3/1 cells express certain markers typical for type I pneumocytes (e.g., T1alpha, ICAM-1, connexin-43, caveolins-1 and -2) they do not form electrically tight monolayers. This excludes R3/1 cells from being used as an in vitro model for alveolar absorption. However, the cell line may be suitable to study stability of inhaled and endogenous proteins.

Redox regulation of epithelial sodium channels examined in alveolar type 1 and 2 cells patch-clamped in lung slice tissue

The alveolar surface of the lung is lined by alveolar type 1 (AT1) and type 2 (AT2) cells. Using single channel patch clamp analysis in lung slice preparations, we are able to uniquely study AT1 and AT2 cells separately from intact lung. We report for the first time the Na+ transport properties of type 2 cells accessed in live lung tissue (as we have done in type 1 cells). Type 2 cells in lung tissue slices express both highly selective cation and nonselective cation channels with average conductances of 8.8 +/- 3.2 and 22.5 +/- 6.3 picosiemens, respectively. Anion channels with 10-picosiemen conductance are also present in the apical membrane of type 2 cells. Our lung slice studies importantly verify the use of cultured cell model systems commonly used in lung epithelial sodium channel (ENaC) studies. Furthermore, we identify novel functional differences between the cells that make up the alveolar epithelium. One important difference is that exposure to the nitric oxide (NO) donor, PAPA-NONOate (1.5 microm), significantly decreases average ENaC NPo in type 2 cells (from 1.38 +/- 0.26 to 0.82 +/- 0.16; p < 0.05 and n = 18) but failed to alter ENaC activity in alveolar type 1 cells. Elevating endogenous superoxide (O2.) levels with Ethiolat, a superoxide dismutase inhibitor, prevented NO inhibition of ENaC activity in type 2 cells, supporting the novel hypothesis that O2. and NO signaling plays an important role in maintaining lung fluid balance.

Methods for studying stem cells: adult stem cells for lung repair

Recent progress in lung biology includes the description of a series of pulmonary stem and progenitor cells involved in homeostasis and regeneration of the respiratory system. Moreover, the contribution of extrapulmonary stem cells to healthy and pathological lung tissue has been observed and the developmental biology of such processes should provide important hints for understanding maintenance and repair of adult lung structure and function. Despite such remarkable advances, the phenotypic and especially the functional characterization of these stem and progenitor cells, and their derivatives, along with an understanding of the molecular cues and pathways underlying differentiation into specific respiratory lineages is still in its infancy. Accordingly, the role of endogenous and extrapulmonary stem cells in normal tissue repair and pathogenesis is still largely mysterious and added basic knowledge is required in order to explore their potential for novel regenerative therapies. This review provides an overview of the current state of the art in adult lung stem cell biology including technical aspects of isolation, characterization and differentiation, and a discussion of perspectives for future regenerative therapies.

Cell mechanics of alveolar epithelial cells (AECs) and macrophages (AMs)

Cell mechanics provides an integrated view of many biological phenomena which are intimately related to cell structure and function. Because breathing constitutes a sustained motion synonymous with life, pulmonary cells are normally designed to support permanent cyclic stretch without breaking, while receiving mechanical cues from their environment. The authors study the mechanical responses of alveolar cells, namely epithelial cells and macrophages, exposed to well-controlled mechanical stress in order to understand pulmonary cell response and function. They discuss the principle, advantages and limits of a cytoskeleton-specific micromanipulation technique, magnetic bead twisting cytometry, potentially applicable in vivo. They also compare the pertinence of various models (e.g., rheological; power law) used to extract cell mechanical properties and discuss cell stress/strain hardening properties and cell dynamic response in relation to the structural tensegrity model. Overall, alveolar cells provide a pertinent model to study the biological processes governing cellular response to controlled stress or strain.

Lung stem cells

The lung is a relatively quiescent tissue comprised of infrequently proliferating epithelial, endothelial, and interstitial cell populations. No classical stem cell hierarchy has yet been described for the maintenance of this essential tissue; however, after injury, a number of lung cell types are able to proliferate and reconstitute the lung epithelium. Differentiated mature epithelial cells and newly recognized local epithelial progenitors residing in specialized niches may participate in this repair process. This review summarizes recent discoveries and controversies, in the field of stem cell biology, that are not only challenging, but also advancing an understanding of lung injury and repair. Evidence supporting a role for the numerous cell types believed to contribute to lung epithelial homeostasis is reviewed, and initial studies employing cell-based therapies for lung disease are presented. As a detailed understanding of stem cell biology, lung development, lineage commitment, and epithelial differentiation emerges, an ability to modulate lung injury and repair is likely to follow.

Rat coronaviruses infect rat alveolar type I epithelial cells and induce expression of CXC chemokines

We analyzed the ability of two rat coronavirus (RCoV) strains, sialodacryoadenitis virus (SDAV) and Parker's RCoV (RCoV-P), to infect rat alveolar type I cells and induce chemokine expression. Primary rat alveolar type II cells were transdifferentiated into the type I cell phenotype. Type I cells were productively infected with SDAV and RCoV-P, and both live virus and UV-inactivated virus induced mRNA and protein expression of three CXC chemokines: CINC-2, CINC-3, and LIX, which are neutrophil chemoattractants. Dual immunolabeling of type I cells for viral antigen and CXC chemokines showed that chemokines were expressed primarily by uninfected cells. Virus-induced chemokine expression was reduced by the IL-1 receptor antagonist, suggesting that IL-1 produced by infected cells induces uninfected cells to express chemokines. Primary cultures of alveolar epithelial cells are an important model for the early events in viral infection that lead to pulmonary inflammation.

Chronic alcohol ingestion alters claudin expression in the alveolar epithelium of rats

Previously we determined that chronic alcohol ingestion (6 weeks) in rats increases lung epithelial permeability in vivo approximately 5-6-fold and promotes flooding of the alveolar airspaces with proteinaceous fluid in response to stresses such as sepsis. In parallel, alveolar epithelial cells isolated from alcohol-fed rats fail to form tight monolayers in vitro, even when cultured for up to 8 days in the absence of alcohol. However, the molecular mechanisms underlying alcohol-induced permeability are unknown. Claudins are key components of tight junctions that restrict the paracellular movement of water, proteins, and solutes across cellular barriers including the alveolar epithelium. In this study, we examined the expression of multiple members of the claudin protein family in the lungs of alcohol-fed versus control-fed rats (Lieber-DeCarli liquid diet with either 36% of calories as alcohol or an isocaloric substitution with maltin-dextrin for 6 weeks). We determined that chronic alcohol ingestion affected the expression of multiple claudins; most striking were decreases in claudin-1 and claudin-7, and an increase in claudin-5, in the whole lung and in alveolar epithelial monolayers derived from alcohol-fed rats. In parallel, immunocytochemistry of alveolar epithelial monolayers from alcohol-fed rats revealed abnormal intracellular accumulation of claudin-7 protein and relatively decreased localization to cell membranes. Claudin-1 and claudin-7 are relatively specific to alveolar epithelial type I pneumocytes that form the vast majority of the alveolar epithelial barrier in vivo, and increases in claudin-5 have been associated with increased epithelial permeability in other systems. Therefore, these findings suggest that changes in claudin expression in the alveolar epithelium produce a "leakier" phenotype that renders the alcoholic lung susceptible to alveolar flooding during acute inflammatory stresses.

Alveolar epithelial transport in the adult lung

The alveolar surface comprises >99% of the internal surface area of the lungs. At birth, the fetal lung rapidly converts from a state of net fluid secretion, which is necessary for normal fetal lung development, to a state in which there is a minimal amount of alveolar liquid. The alveolar surface epithelium facing the air compartment is composed of TI and TII cells. The morphometric characteristics of both cell types are fairly constant over a range of mammalian species varying in body weight by a factor of approximately 50,000. From the conservation of size and shape across species, one may infer that both TI and TII cells also have important conserved functions. The regulation of alveolar ion and liquid transport has been extensively investigated using a variety of experimental models, including whole animal, isolated lung, isolated cell, and cultured cell model systems, each with their inherent strengths and weaknesses. The results obtained with different model systems and a variety of different species point to both interesting parallels and some surprising differences. Sometimes it has been difficult to reconcile results obtained with different model systems. In this section, the primary focus will be on aspects of alveolar ion and liquid transport under normal physiologic conditions, emphasizing newer data and describing evolving paradigms of lung ion and fluid transport. We will highlight some of the unanswered questions, outline the similarities and differences in results obtained with different model systems, and describe some of the complex and interweaving regulatory networks.

Protective role of macrophages in noninflammatory lung injury caused by selective ablation of alveolar epithelial type II Cells

Macrophages have a wide variety of activities and it is largely unknown how the diverse phenotypes of macrophages contribute to pathological conditions in the different types of tissue injury in vivo. In this study we established a novel animal model of acute respiratory distress syndrome caused by the dysfunction of alveolar epithelial type II (AE2) cells and examined the roles of alveolar macrophages in the acute lung injury. The human diphtheria toxin (DT) receptor (DTR), heparin-binding epidermal growth factor-like growth factor (HB-EGF), was expressed under the control of the lysozyme M (LysM) gene promoter in the mice. When DT was administrated to the mice they suffered from acute lung injury and died within 4 days. Immunohistochemical examination revealed that AE2 cells as well as alveolar macrophages were deleted via apoptosis in the mice treated with DT. Consistent with the deletion of AE2 cells, the amount of surfactant proteins in bronchoalveolar lavage fluid was greatly reduced in the DT-treated transgenic mice. When bone marrow from wild-type mice was transplanted into irradiated LysM-DTR mice, the alveolar macrophages became resistant to DT but the mice still suffered from acute lung injury by DT administration. Compared with the mice in which both AE2 cells and macrophages were deleted by DT administration, the DT-treated LysM-DTR mice with DT-resistant macrophages showed less severe lung injury with a reduced amount of hepatocyte growth factor in bronchoalveolar lavage fluid. These results indicate that macrophages play a protective role in noninflammatory lung injury caused by the selective ablation of AE2 cells.

Stem cells and chronic lung disease

Stem cells have been shown to contribute to the repair and regeneration of injured lungs. These stem cells are resident in specific protected niches in the lung, or they can be mobilized from the bone marrow and recruited from the circulation in the setting of severe injury. Normal repair of the airway involves regeneration of the airway epithelium by stem cells in both the proximal airway and distal airspace, whereas aberrant repair of the lung may result from stem cells that lead to fibrosis. The stem cell niche in the lung is probably critical in determining whether "good" or "bad" stem cells are involved in local repair, and therefore whether fibrosis predominates. There is much excitement about the possibility of harnessing stem cells for repair and regeneration of the lungs. This review highlights current knowledge of this area and identifies gaps in our understanding of this complicated process.

In vitro transdifferentiation of human fetal type II cells toward a type I-like cell

For alveolar type I cells, phenotype plasticity and physiology other than gas exchange await further clarification due to in vitro study difficulties in isolating and maintaining type I cells in primary culture. Using an established in vitro model of human fetal type II cells, in which the type II phenotype is induced and maintained by adding hormones, we assessed for transdifferentiation in culture toward a type I-like cell with hormone removal for up to 144 h, followed by electron microscopy, permeability studies, and RNA and protein analysis. Hormone withdrawal resulted in diminished type II cell characteristics, including decreased microvilli, lamellar bodies, and type II cell marker RNA and protein. There was a simultaneous increase in type I characteristics, including increased epithelial cell barrier function indicative of a tight monolayer and increased type I cell marker RNA and protein. Our results indicate that hormone removal from cultured human fetal type II cells results in transdifferentiation toward a type I-like cell. This model will be useful for continued in vitro studies of human fetal alveolar epithelial cell differentiation and phenotype plasticity.

New Insights into the Pathogenesis and Treatment of Idiopathic Pulmonary Fibrosis: A Potential Role for Stem Cells in the Lung Parenchyma and Implications for Therapy

Idiopathic Pulmonary Fibrosis (IPF) is a chronic, progressive, and often fatal form of interstitial lung disease. It is characterized by injury with loss of lung epithelial cells and abnormal tissue repair, resulting in replacement of normal functional tissue, abnormal accumulation of fibroblasts and myofibroblasts, deposition of extracellular matrix, and distortion of lung architecture which results in respiratory failure. Despite improvements in the diagnostic approach to IPF and active research in recent years, the molecular mechanisms of the disease remain poorly understood. This highly lethal lung disorder continues to pose major clinical challenges since an effective therapeutic regimen has yet to be identified and developed. For example, a treatment modality has been based on the assumption that IPF is a chronic inflammatory disease, yet most available anti-inflammatory drugs are not effective in treating it. Hence researchers are now focusing on understanding alternative underlying mechanisms involved in the pathogenesis of IPF in the hope of discovering potentially new pharmaceutical targets. This paper will focus on lung tissue repair, regeneration, remodeling, and cell types that may be important to consider in therapeutic interventions and includes a more detailed discussion of the potential targets of current therapeutic attack in pulmonary fibrosis. The discovery that adult bone marrow stem cells can contribute to the formation of differentiated cell types in other tissues, especially after injury, implies that they have the potential to participate in tissue remodeling, and perhaps regeneration. The current promise of the use of adult stem cells for tissue regeneration, and the belief that once irreversibly damaged tissue could be restored to a normal functional capacity using stem cell-based therapy, suggests a novel approach for treatment of diverse chronic diseases. However this optimism is tempered by current evidence that the pathogenesis of pulmonary fibrosis may involve the recruitment of bone marrow-derived fibroblasts, which are the key contributors to the pathogenesis of this chronic progressive disorder. Nevertheless, stem cell-related therapies are widely viewed as promising treatment options for patients suffering from various types of pulmonary diseases. Gender mismatched bone marrow or lung transplant recipients serve as natural populations in which to study the role of bone marrow-derived stem cells in recovery from pulmonary diseases. Understanding the mechanism of recruitment of stem cells to sites of injury, and their involvement in tissue repair, regeneration, and remodeling may offer a novel therapeutic target for developing more effective treatments against this fatal disorder. This article reviews the new concepts in the pathogenesis, current and future treatment options of pulmonary fibrosis, and the recent advances regarding the roles of stem cells in lung tissue repair, regeneration, and remodeling.

Differentiated human alveolar epithelial cells and reversibility of their phenotype in vitro

Cultures of differentiating fetal human type II cells have been available for many years. However, studies with differentiated adult human type II cells are limited. We used a published method for type II cell isolation and developed primary culture systems for maintenance of differentiated adult human alveolar epithelial cells for in vitro studies. Human type II cells cultured on Matrigel (basolateral access) or a mixture of Matrigel and rat tail collagen (apical access) in the presence of keratinocyte growth factor, isobutylmethylxanthine, 8-bromo-cyclicAMP, and dexamethasone (KIAD) expressed the differentiated type II cell phenotype as measured by the expression of surfactant protein (SP)-A, SP-B, SP-C, and fatty acid synthase and their morphologic appearance. These cells contain lamellar inclusion bodies and have apical microvilli. In both systems the cells appear well differentiated. In the apical access system, type II cell differentiation markers initially decreased and then recovered over 6 d in culture. Lipid synthesis was also increased by the addition of KIAD. In contrast, type II cells cultured on rat tail collagen (or tissue culture plastic) slowly lose their lamellar inclusions and expression of the surfactant proteins and increase the expression of type I cell markers. The expression of the phenotypes is regulated by the culture conditions and is, in part, reversible in vitro.

Trans-differentiation of alveolar epithelial type II cells to type I cells involves autocrine signaling by transforming growth factor beta 1 through the Smad pathway

Type II alveolar epithelial cells (AEC II) proliferate and transdifferentiate into type I alveolar epithelial cells (AEC I) when the normal AEC I population is damaged in the lung alveoli. We hypothesized that signaling by transforming growth factor beta1 (TGF beta1), through its downstream Smad proteins, is involved in keeping AEC II quiescent in normal cells and its altered signaling may be involved in the trans-differentiation of AEC II to AEC I. In the normal lung, TGF beta1 and Smad4 were highly expressed in AEC II. Using an in vitro cell culture model, we demonstrated that the trans-differentiation of AEC II into AEC I-like cells began with a proliferative phase, followed by a differentiation phase. The expression of TGF beta1, Smad2, and Samd3 and their phosphorylated protein forms, and cell cycle inhibitors, p15(Ink4b) and p21(Cip1), was lower during the proliferative phase but higher during the differentiation phase. Furthermore, cyclin-dependent kinases 2, 4, and 6 showed an opposite trend of expression. TGF beta1 secretion into the media increased during the differentiation phase, indicating an autocrine regulation. The addition of TGF beta1 neutralizing antibody after the proliferative phase and silencing of Smad4 by RNA interference inhibited the trans-differentiation process. In summary, our results suggest that the trans-differentiation of AEC II to AEC I is modulated by signaling through the Smad-dependent TGF beta1 pathway by altering the expression of proteins that control the G1 to S phase entry in the cell cycle.

Epithelial origin of myofibroblasts during fibrosis in the lung

An understanding of the mechanisms underlying pulmonary fibrosis remains elusive. Once believed to result primarily from chronic inflammation, it is now clear that inflammation and chronic fibrosis, especially in diseases such as idiopathic pulmonary fibrosis/usual interstitial pneumonia, are often dissociated, and that inflammation is neither necessary nor sufficient to induce fibrosis. The origin of the primary effector cell of fibrosis in the lung, the myofibroblast, is not clearly established. Three potential sources have been hypothesized. Although conversion of resident fibroblasts and differentiation of circulating bone marrow-derived progenitors likely play a role, the possible contribution of alveolar epithelial cells (AECs), through a process termed "epithelial-mesenchymal transition" (EMT), has only recently received consideration. A process by which epithelial cells lose cell-cell attachment, polarity and epithelial-specific markers, undergo cytoskeletal remodeling, and gain a mesenchymal phenotype, EMT plays a prominent role in fibrogenesis in adult tissues such as the kidney. This review summarizes the evidence supporting a central role for EMT in the pathogenesis of lung fibrosis, the potential for EMT in AECs in vitro and in vivo and role of transforming growth factor-beta1 in this process, and the implications of epithelium-driven fibrosis on future research and treatment. Potential pathways involved in EMT are also discussed. It is hoped that a major shift in current paradigms regarding the genesis of pulmonary fibrosis and dissection of the relevant pathways may allow development of targeted interventions that could potentially reverse the process and ameliorate the debilitating effects of abnormal repair and progressive fibrosis.

Type II epithelial cells are critical target for hyperoxia-mediated impairment of postnatal lung development

Type II epithelial cells are essential for lung development and remodeling, as they are precursors for type I cells and can produce vascular mitogens. Although type II cell proliferation takes place after hyperoxia, it is unclear why alveolar remodeling occurs normally in adults whereas it is permanently disrupted in newborns. Using a line of transgenic mice whose type II cells could be identified by their expression of enhanced green fluorescent protein and endogenous expression of surfactant proteins, we investigated the age-dependent effects of hyperoxia on type II cell proliferation and alveolar repair. In adult mice, type II cell proliferation was low during room air and hyperoxia exposure but increased during recovery in room air and then declined to control levels by day 7. Eight weeks later, type II cell number and alveolar compliance were indistinguishable from those in room air controls. In newborn mice, type II cell proliferation markedly increased between birth and postnatal day 7 before declining by postnatal day 14. Exposure to hyperoxia between postnatal days 1 and 4 inhibited type II cell proliferation, which resumed during recovery and was aberrantly elevated on postnatal day 14. Eight weeks later, recovered mice had 70% fewer type II cells and 30% increased lung compliance compared with control animals. Recovered mice also had higher levels of T1alpha, a protein expressed by type I cells, with minimal changes detected in genes expressed by vascular cells. These data suggest that perinatal hyperoxia adversely affects alveolar development by disrupting the proper timing of type II cell proliferation and differentiation into type I cells.

Heparin and fibroblast growth factors affect surfactant protein gene expression in type II cells

The stimulation and maintenance of the pulmonary alveolar type II cell's capacity to biosynthesize, store, and secrete surfactant proteins (SPs) are modulated to a great extent by growth factors, extracellular matrix (ECM) components, and hormones. It is possible that differences in ECM composition, as exist between type I and II cells normally or as might occur with excessive cell surface shedding during inflammation or injury states, may specifically alter SP expression. Here, isolated type II cells were exposed to the model sulfated ECM heparin; desulfated heparin; and/or fibroblast growth factor (FGF)-1, -2, or -7 for 24 h to examine by quantitative real-time polymerase chain reaction their effects on SP gene expression. Aquaporin 5 (AQP-5) gene expression was also examined as a phenotypic marker for the type I cell. SP-B mRNA abundance was increased 4- to 8-fold by all three FGFs. Heparin at low concentrations (5 microg/ml) or desulfated heparin at high concentrations (500 microg/ml) enhanced the effects of FGF-2 and -7, while high heparin concentrations (500 microg/ml) were inhibitory. In contrast, SP-B mRNA abundance was increased by heparin in a dose- and sulfation-dependent manner when used in combination with FGF-1. SP-C and AQP-5 mRNA levels were increased by heparin alone in a dose- and sulfation-dependent manner, while all FGFs lacked effect on SP-C or AQP-5 mRNA levels. These data indicate that heparin can be stimulatory to SP gene expression depending on concentration, degree of sulfation, and surrounding FGF environment, and that heparin plays a significant role in modulating alveolar epithelial cell phenotype in vitro.

Stem cell research

One of the most active areas of research in medicine today is stem cell biology. This review introduces the reader to the field of stem cell biology and its therapeutic potential. More importantly, the potential application of stem cell therapy in acute lung injury will be explored.

Stem cells in the lung parenchyma and prospects for lung injury therapy

Until recently, it was thought that only embryonic stem cells were pluripotent and that adult stem cells were restricted in their differentiative and regenerative potential to become the tissues in which they reside. However, the discovery that adult stem cells in one tissue can contribute to the formation of other tissues, especially after injury or cell damage, implies that stem cells have developmental plasticity. For example, haematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) from bone marrow can be used to regenerate diverse tissues at distant sites, including the lung. This article reviews the character of stem cells in the lung parenchyma and focuses on the potential uses of adult stem cells in research of lung injury and lung disease therapies.

Role of the physicochemical environment in lung development

Mechanical forces, exerted on lung tissue via alterations in lung expansion are a major determinant of fetal lung development, having a potent effect on the rate of cellular proliferation, the differentiated state of alveolar epithelial cells and the three-dimensional tissue structure. As a result, much research is currently focused on understanding the molecular mechanisms involved. 2. Although it is likely that mechanical forces exert similar influences on lung development after birth, the types of forces applied to the air-filled lung are very different and more complex. For example, lung aeration causes surface tension to form, which greatly increases lung recoil, leading to a reduction in interstitial tissue and pleural pressures, as well as lung expansion. 3. Because of the loss of the distending influence of lung liquid, the chest wall assumes the role of maintaining resting lung volumes after birth by acting as an external brace that opposes lung recoil. As a result, the distribution of force throughout lung tissue changes markedly. 4. Little is known of how changing the mechanical environment of the lung influences its development after birth, but this has important implications for understanding the impact of assisted ventilation on patients, particularly very preterm infants, who are often ventilated using high positive pressures. 5. Although the application of positive internal distending pressures may, in part, duplicate the fetal environment, the effect of gas versus liquid is unknown and high positive airway pressures are known to adversely affect cardiopulmonary physiology. Understanding the role of mechanical forces in regulating lung development as well as pulmonary physiology in the fetus and newborn is central to improving the care and management of infants suffering respiratory failure.

Further examination of alveolar septal adaptation to left pneumonectomy in the adult lung

Recent data from our laboratory are presented concerning alveolar septal adaptation following 42-45% lung resection by left pneumonectomy (PNX) in adult foxhounds compared to sham-operated control animals. Results confirm our previous conclusion that compensation in the remaining lung occurs without a net growth of additional alveolar septal tissue. The major ultrastructural responses are (a) alveolar capillary distention, which recruits capillary blood volume and surface area, leading to a 30-50% increase in lung diffusing capacity estimated by morphometry, a magnitude similar to that measured by physiologic methods; (b) a selectively increased volume of type 2 alveolar epithelial cells. These data, taken together with the balanced compensatory growth of alveolar septal cells observed in adult dogs following 55-58% lung resection by right PNX, support a graded alveolar cellular response to chronic mechanical strain with the alveolar epithelial cells being activated first; as strain increases further with greater lung resection other alveolar cells also become activated leading to an overt increase in septal tissue volume. The spatial distribution of lobar mechanical strain and lobar tissue volume assessed by high resolution computed tomography was markedly non-uniform after PNX, suggesting possible non-uniform distribution of alveolar cellular response. The sequential activation of physiologic recruitment and cellular adaptation confer additive functional benefits that optimize long-term exercise performance after PNX.

Respiratory failure due to differentiation arrest and expansion of alveolar cells following lung-specific loss of the transcription factor C/EBPalpha in mice

The leucine zipper family transcription factor CCAAT enhancer binding protein alpha (C/EBPalpha) inhibits proliferation and promotes differentiation in various cell types. In this study, we show, using a lung-specific conditional mouse model of C/EBPalpha deletion, that loss of C/EBPalpha in the respiratory epithelium leads to respiratory failure at birth due to an arrest in the type II alveolar cell differentiation program. This differentiation arrest results in the lack of type I alveolar cells and differentiated surfactant-secreting type II alveolar cells. In addition to showing a block in type II cell differentiation, the neonatal lungs display increased numbers of proliferating cells and decreased numbers of apoptotic cells, leading to epithelial expansion and loss of airspace. Consistent with the phenotype observed, genes associated with alveolar maturation, survival, and proliferation were differentially expressed. Taken together, these results identify C/EBPalpha as a master regulator of airway epithelial maturation and suggest that the loss of C/EBPalpha could also be an important event in the multistep process of lung tumorigenesis. Furthermore, this study indicates that exploring the C/EBPalpha pathway might have therapeutic benefits for patients with respiratory distress syndromes.

Alveolar epithelial cell differentiation and surfactant protein expression after mild preterm birth in sheep

As the transition to extrauterine life at birth alters the proportions of type I and II alveolar epithelial cells (AECs), our aim was to determine the effect of mild preterm birth on AECs and surfactant protein (SP) gene expression. Preterm lambs were born at approximately 133 d of gestational age (DGA); controls were born at term (approximately 147 DGA). Lungs were collected from preterm lambs at term-equivalent age (TEA; approximately 2 wk after preterm birth) and 6 wk post-TEA. Control lung tissue was collected from fetuses (at 132 DGA), as well as from lambs at approximately 6 h (normal term) and 2, 6, and 8 wk of postnatal age (PNA). In controls, the proportion of type I AECs decreased from 65.1 +/- 3.9% at term to 50.9 +/- 3.3%, while the proportion of type II AECs increased from 33.7 +/- 3.9% to 48.5 +/- 3.3% at 6 wk PNA. At 2 wk after preterm birth, the proportions of type I and II AECs were similar in preterm lambs compared to 132-d fetal levels and term controls but differed from control values at 2 wk PNA; differences between control and preterm lambs persisted at 8 wk PNA. At approximately 2 wk after preterm birth, SP-A and SP-B, but not SP-C, mRNA levels were significantly reduced in preterm lambs compared with term controls, but these differences did not persist at 2 and 6 wk PNA. We conclude that mild preterm birth alters the normal postnatal changes in type I and II cell proportions but does not severely affect SP gene expression.

Promotion of cell adherence and spreading: a novel function of RAGE, the highly selective differentiation marker of human alveolar epithelial type I cells

The receptor for advanced glycation endproducts (RAGE) is expressed under pathological conditions in many tissues and has been assigned many functions. We demonstrate, in normal human lung tissue, the preferential and highly abundant expression of RAGE by quantitative polymerase chain reaction. In addition, RAGE expression, as a specific differentiation marker of alveolar epithelial type I cells (AT I cells), and its localization to the basolateral plasma membrane have been confirmed by means of newly raised monoclonal antibodies. The physiological function of RAGE on AT I cells has previously remained elusive. By using HEK293 cells transfected with cDNA encoding for full-length RAGE, we show that RAGE enhances the adherence of epithelial cells to collagen-coated surfaces and has a striking capacity for inducing cell spreading. The preferential binding of RAGE to collagen has been confirmed by assaying the binding of soluble RAGE to various substrates. RAGE might thus assist AT I cells to acquire a spreading morphology, thereby ensuring effective gas exchange and alveolar stability.

Wnt/beta-catenin signaling acts upstream of N-myc, BMP4, and FGF signaling to regulate proximal-distal patterning in the lung

Branching morphogenesis in the lung serves as a model for the complex patterning that is reiterated in multiple organs throughout development. Beta-catenin and Wnt signaling mediate critical functions in cell fate specification and differentiation, but specific functions during branching morphogenesis have remained unclear. Here, we show that Wnt/beta-catenin signaling regulates proximal-distal differentiation of airway epithelium. Inhibition of Wnt/beta-catenin signaling, either by expression of Dkk1 or by tissue-specific deletion of beta-catenin, results in disruption of distal airway development and expansion of proximal airways. Wnt/beta-catenin functions upstream of BMP4, FGF signaling, and N-myc. Moreover, we show that beta-catenin and LEF/TCF activate the promoters of BMP4 and N-myc. Thus, Wnt/beta-catenin signaling is a critical upstream regulator of proximal-distal patterning in the lung, in part, through regulation of N-myc, BMP4, and FGF signaling.

Extracellular matrix-driven alveolar epithelial cell differentiation in vitro

During homeostasis and in response to injury, alveolar type II (AT2) cells serve as progenitor cells to proliferate, migrate, differentiate, and re-establish both alveolar type I (AT1) and AT2 cells into a functional alveolar epithelium. To understand specific changes in cell differentiation, we monitored morphological characteristics and cell-specific protein markers over time for isolated rat AT2 cells cultured on combinations of collagen, fibronectin and/or laminin-5 (Ln5). For all matrices tested, cultured AT2 cells displayed reduced expression of AT2 cell-specific markers from days 1 to 4 and increased expression of AT1-specific markers by day 3, with continued expression until at least day 5. Over days 5 to 7 in culture, cells took on an AT1-like phenotype (on collagen/fibronectin alone; collagen alone; or Ln5 alone), an AT2-like phenotype (on collagen/fibronectin/Ln5; or collagen/Ln5), or both AT1-like and AT2-like phenotypes (on collagen/fibronectin matrix with a subsaturating amount of Ln5). Cells transferred between matrices at day 4 of culture retained the ability to alter day 7 phenotype. We conclude that in vitro, (1) AT2 cells exhibited phenotype plasticity that included an intermediate cell type with both AT1 and AT2 cell characteristics independent of day 7 phenotype; (2) both collagen and Ln5 were needed to promote the development of an AT2-like phenotype at day 7; and (3) components of the extracellular matrix (ECM) contribute to phenotypic switching of alveolar cells in culture. The described tissue culture models provide accessible models for studying changes in alveolar epithelial cell physiology from AT2 cell progenitors to the establishment of alveolar epithelial monolayers that represent AT1-like, AT2-like, or a mix of AT1- and AT2-like cells.

Stem cells of the alveolar epithelium

Elucidation of the biology of stem cells of the lung parenchyma could revolutionise treatment of patients with lung disorders such as cancer, acute respiratory distress syndrome, emphysema, and fibrotic lung disease. How close is this goal? Despite remarkable observations and ensuing advances, more questions than answers have been generated. Progenitors of the alveolar epithelium remain largely mysterious, so the prospect of isolating enough of these cells and delivering them effectively to cure disease remains remote. Similarly, the bone-marrow-derived cell that might most effectively engraft the lung remains unknown. If this mechanism is an important process for lung repair, why will the administration of additional cells be more effective? Finally, there is an issue of control of multipotent cells to avoid the generation of multiple teratomas, longevity of the graft, and possible immunological reactions to gene products inserted to replace a deficiency. The biology is exciting but not yet well enough understood to support therapeutic advances.

Identification of novel resident pulmonary stem cells: form and function of the lung side population

Resident lung stem cells function to replace all lineages of pulmonary tissue, including mesenchyme, epithelium, and vasculature. The phenotype of the lung side population (SP) cells is currently under investigation; their function is currently unknown. Recent data suggest lung SP cells are an enriched tissue-specific source of organ-specific pulmonary precursors and, therefore, a source of adult stem cells. The adult lung SP cell population has been isolated and characterized for expression of markers indicative of stem cell, epithelial, and mesenchymal lineages. These studies determined that the adult mouse lung SP has epithelial and mesenchymal potential that resides within a CD45- mesenchymal subpopulation, as well as limited hematopoietic ability, which resides in the bone marrow-derived CD45+ subpopulation. The ability to identify these adult lung precursor cells allows us to further study the potential of these cells and their role in the regulation of tissue homeostasis and response to injury. The identification of this target population will potentially allow earlier treatment and, long term, a functional restoration of injured pulmonary tissue and lung health.

Differential expression of GABAA receptor π subunit in cultured rat alveolar epithelial cells

Although type A gamma-aminobutyric acid (GABA) receptors (ligand-gated Cl(-) channels) have been extensively studied in the central nervous system, no information is available on this receptor in lung cells. We have examined the expression of GABA(A) receptor pi-subunit (GABRP) during the trans-differentiation between rat alveolar epithelial type II cells and type I cells. Rat alveolar type II cells, when cultured on plastic plates, gradually trans-differentiated into type-I-like cells and lost their GABRP mRNA expression. However, the GABRP mRNA was partially retained in the type II cells cultured on Matrigel. Keratinocyte growth factor (a mitogen of type II cells) increased GABRP expression. A detached collagen gel maintained the GABRP mRNA to a level close to that of the freshly isolated type II cells. An air-liquid interface culture system, mimicking in vivo conditions in the lung, significantly up-regulated the expression of GABRP mRNA and protein. mRNAs of the GABA(A) receptor alpha1-, alpha3-, beta2-, gamma2-, and gamma3-subunits were also detected in rat type II cells. These results suggest that GABRP expression is differentially regulated by culture substrata, growth factor, detached gel, and an air-apical surface.

Surfactant protein expression is increased in the ipsilateral but not contralateral lungs of fetal sheep with left-sided diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) impairs fetal lung growth and increases the density of alveolar epithelial type 2 (AE2) cells. There is controversy whether surfactant protein (SP) expression is altered in CDH. The primary aim of this study was to assess SP expression (mRNA and protein) in the left and right lungs of fetal sheep with and without a diaphragmatic hernia (DH). Left-sided DH was created in four fetal sheep at 65 days of gestational age (g.a.). Sham-operated animals were used as controls. At 138 days g.a., lungs were harvested and the following parameters were measured: SP-A, -B, and -C mRNA expression (Northern blot), SP-A and -B expression (Western blot), and AE2 cell density (immunohistochemistry). The lung weight-to-body weight ratio was reduced by 42% in DH animals. The left-to-right lung weight ratio was lower in DH animals (0.47 +/- 0.03 vs. 0.69 +/- 0.03), indicative of asymmetric lung growth. SP-A, -B, and -C mRNA expression were increased by 61.7%, 32.9%, and 75.5%, respectively, in the left lungs of DH animals. SP-A and SP-B were also increased in DH. In the right lung, SP expression (mRNA and protein) was not different between groups. AE2 cell density was higher (by 67%) in the left but not right lungs of DH animals. Although DH in fetal sheep results in significant lung hypoplasia, SP expression is not reduced. On the contrary, SP expression was increased in the ipsilateral lung of fetuses with left-sided DH. Furthermore, AE2 cell density is increased in DH, suggesting that the increase in SP mRNA and protein levels is due to increases AE2 cell number. Our data further support the premise that fetal lung hypoplasia favors an AE2 phenotype.

Preservation of the characteristics of the cultured human type II alveolar epithelial cells

The human type II alveolar epithelial cells lost their specific characteristics during cultivation. We examined the ultrastructural and biochemical nature of the human type II cells cultured by two culture systems. To make a physiological alveoli model, the epithelial cells were seeded onto the cell culture insert and allowed contact with the air directly. The cells exposed to the air expressed polarity and immature lamellar bodies in their cytoplasm. Separately, the alveolar epithelial cells were cultured as spheroids to construct the three-dimensional condition. These cells expressed mature morphological characteristics as epithelial cells and lamellar bodies. The expression of the surfactant apoprotein-A (SP-A) and -C (SP-C) mRNA was compared in the cells cultured as a monolayer, the air exposed and the spheroids. SP-A mRNA was detected in all the cultured epithelial cells, but SP-C mRNA, a specific protein for the type II cells, was expressed only in the cells forming spheroids. The expression of uPA, one of the fibrinolytic enzymes, its receptor (uPAR) and its inhibitor-1 (PAI-1) were also examined. The epithelial cells exposed to the air and formed spheroids expressed a larger amount of uPA mRNA than the monolayer, although the amount of uPAR mRNA were comparable in these cells. The amount of PAI-1 mRNA significantly increased when the epithelial cells were exposed to the air. These results indicate that the type II alveolar epithelial cells induced and preserved their specific characteristics by taking the physiological three-dimensional structure, and these characteristics were partially restored by exposure to the air. Those findings suggest that the alveolar epithelial cells should be cultivated in three-dimensional form with contact to the air to regenerate an appropriate alveolar tissue.

Regulation of alveolar epithelial cell phenotypes in fetal sheep: roles of cortisol and lung expansion

Our aim was to determine whether cortisol's effect on alveolar epithelial cell (AEC) phenotypes in the fetus is mediated via a sustained alteration in lung expansion. Chronically catheterized fetal sheep were exposed to 1) saline infusion, 2) cortisol infusion (122–131 days' gestation, 1.5–4.0 mg/day), 3) saline infusion plus reduced lung expansion, or 4) cortisol infusion plus reduced lung expansion. The proportions of type I and II AECs were determined by electron microscopy, and surfactant protein (SP)-A, -B, and -C mRNA levels were determined by Northern blot analysis. Cortisol infusions significantly increased type II AEC proportions (to 38.2 ± 2.2%), compared with saline-infused fetuses (23.8 ± 2.4%), and reduced type I AEC proportions (to 59.0 ± 2.2%), compared with saline-infused fetuses (70.4 ± 2.4%). Reduced lung expansion also increased type II AEC proportions (to 52.9 ± 3.5%) and decreased type I AEC proportions (to 34.2 ± 3.7%), compared with control, saline-infused fetuses. The infusion of cortisol into fetuses exposed to reduced lung expansion tended to further increase type II (to 60.3 ± 2.1%, P = 0.066) and reduce type I AEC (to 26.6 ± 2.3%, P = 0.07) proportions. SP-A, -B, and -C mRNA levels changed in parallel with the changes in type II AEC proportions. These results indicate that cortisol alters the proportion of type I and type II AECs via a mechanism unrelated to the degree of fetal lung expansion. However, reductions in fetal lung expansion appear to have a greater impact on the proportion of AECs than cortisol.

Altered Epithelial Cell Proportions in the Fetal Lung of Glucocorticoid Receptor Null Mice

Glucocorticoids provide important signals for maturation of the fetal lung and antenatal glucocorticoids are used to reduce the respiratory insufficiency suffered by preterm infants. To further understand the role of glucocorticoids in fetal lung maturation, we have analyzed mice with a targeted null mutation for the glucocorticoid receptor (GR) gene, which severely retards lung development. The lungs of fetal GR-null mice have increased lung weight and DNA content, are condensed and hypercellular, with reduced septal thinning leading to a 6-fold increase in the airway to capillary diffusion distance. In fetal GR-null mice, mRNA levels of the type II epithelial cell surfactant protein genes A and C were reduced by approximately 50%. Analysis of epithelial cell types by electron microscopy revealed that the proportions of type II cells were increased by approximately 30%, whereas the proportions of type-I cells were markedly reduced (by approximately 50%). Similarly, we found a 50% reduction in mRNA levels for T1alpha and aquaporin-5, two type I cell-specific markers, and a 20% reduction in aquaporin-1 mRNA levels. This demonstrates that during murine embryonic development, receptor-mediated glucocorticoid signaling facilitates the differentiation of epithelial cells into type I cells, but is not obligatory for type II cell differentiation.

Changes in alveolar epithelial cell proportions during fetal and postnatal development in sheep

Basal lung expansion is an important determinant of alveolar epithelial cell (AEC) phenotype in the fetus. Because basal lung expansion increases toward term and is reduced after birth, we hypothesized that these changes would be associated with altered proportions of AECs. AEC proportions were calculated with electron microscopy in fetal and postnatal sheep. Type I AECs increased from 4.8 +/- 1.3% at 91 days to 63.0 +/- 3.6% at 111 days of gestation, remained at this level until term, and decreased to 44.8 +/- 1.8% after birth. Type II AECs increased from 4.3 +/- 1.5% at 111 days to 29.6 +/- 4.1% at 128 days of gestation, remained at this level until term, and then increased to 52.9 +/- 1.5% after birth. Surfactant protein (SP)-A, -B and -C mRNA levels increased with increasing gestational age before birth, but the changes in SP expression after birth were inconsistent. Thus before birth type I AECs predominate, whereas after birth type II AECs predominate, possibly due to the reduction in basal lung expansion associated with the entry of air into the lungs.

Identification and isolation of mouse type II cells on the basis of intrinsic expression of enhanced green fluorescent protein

The unique morphology and cell-specific expression of surfactant genes have been used to identify and isolate alveolar type II epithelial cells. Because these attributes can change during lung injury, a novel method was developed for detecting and isolating mouse type II cells on the basis of transgenic expression of enhanced green fluorescence protein (EGFP). A line of transgenic mice was created in which EGFP was targeted to type II cells under control of the human surfactant protein (SP)-C promoter. Green fluorescent cells that colocalized by immunostaining with endogenous pro-SP-C were scattered throughout the parenchyma. EGFP was not detected in Clara cell secretory protein-expressing airway epithelial cells or other nonlung tissues. Pro-SP-C immunostaining diminished in lungs exposed to hyperoxia, consistent with decreased expression and secretion of intracellular precursor protein. In contrast, type II cells could still be identified by their intrinsic green fluorescence, because EGFP is not secreted. Type II cells could also be purified from single-cell suspensions of lung homogenates using fluorescence-activated cell sorting. Less than 1% of presorted cells exhibited green fluorescence compared with >95% of the sorted population. As expected for type II cells, ultrastructural analysis revealed that the sorted cells contained numerous lamellar bodies. SP-A, SP-B, and SP-C mRNAs were detected in the sorted population, but T1alpha and CD31 (platelet endothelial cell adhesion molecule) were not, indicating enrichment of type II epithelial cells. This method will be invaluable for detecting and isolating mouse type II cells under a variety of experimental conditions.

Transcytosis of GCSF-transferrin across rat alveolar epithelial cell monolayers

PURPOSE

The purpose of this study was to use primary cultured rat alveolar epithelial cell monolayers to examine the potential of using transferrin receptor (TfR)-mediated transcytosis for noninvasive systemic protein drug delivery via the pulmonary route.

METHODS

Freshly isolated rat type II pneumocytes were plated onto tissue culture-treated polycarbonate 12-mm Transwells. AEC monolayers (> or = 2500 omega(cm2)) were treated with keratinocyte growth factor (10 ng/mL) for maintenance of type II cell-like characteristics. Filgrastim (GCSF)-Tf conjugates were prepared using the linkers SPDP and DPDPB. TfR-specific binding and uptake were determined using 125I-Tf and 59Fe-Tf treatment, respectively. Apical-to-basolateral (A-to-B) transferrin receptor (TfR)-mediated transcytosis was determined by dosing the apical compartment with 1.5 microg/mL of 125I-Tf or 125I-GCSF-Tf. Nonspecific TfR-independent transport of 125I-Tf and 125I-GCSF-Tf was determined in parallel by including 150 microg/mL of nonradiolabeled Tf. Basolateral samples (500 microL) were taken at 2, 4, and 6 h post-dosing, subjected to 15% trichloroacetic acid precipitation, and assayed in a Packard gamma counter. TfR-specific transport was determined as the difference between total and nonspecifc transport. The effects of brefeldin-A (BFA) on TfR distribution and (A-to-B) transport of 125I-Tf, 125I-GCSF and 125I-GCSF-Tf was studied by including the agent in the apical fluid at 1 microg/mL.

RESULTS

BFA treatment resulted in a small significant reduction in TfR at the basolateral surface of type II cell-like monolayers, while it had no effect on TfR distribution in type I cell-like monolayers. In contrast, BFA treatment significantly altered the endocytosis of TfR, reducing the basolateral uptake of 59Fe-Tf while greatly increasing the apical uptake of 59Fe-Tf. BFA treatment, however, did not affect the TfR-specific uptake of 59Fe-Tf in type I cell-like monolayers. TfR-specific apical-to-basolateral transcytosis of 125I-Tf and 126I-GCSF-Tf conjugates was significantly enhanced in the presence of BFA in type II cell-like monolayers, whereas it had no effect on apical-to-basolateral transport of 125I-GCSF. BFA-enhanced transport of 125I-GCSF-Tf was approximately 3-fold higher than that of 125I-GCSF in the presence or absence of BFA. Moreover, 125I-GCSF transport in the presence of BFA was not significantly different from non-specific 125I-GCSF-Tf transport. Chromatographic analyses and bio-assays revealed that GCSF-Tf was not degraded during transport via TfR-specific processes, and that GCSF retained biologic activity when liberated from the conjugate via dithiothreitol reduction.

CONCLUSION

This study suggests the possibility of using TfR-mediated transcytosis for systemic delivery of therapeutic proteins via the alveolar epithelium.

Heterogeneity of claudin expression by alveolar epithelial cells

Claudins are proteins that participate in epithelial barrier function and regulate paracellular permeability. By immunohistochemistry of adult rat lung sections, claudin-3, claudin-4, and claudin-5 were found to be co-expressed by type II alveolar epithelial cells. Claudin-3 and claudin-4 were also co-expressed by some alveolar epithelial cells adjacent to type II cells. In contrast, claudin-5 was expressed throughout the alveolus. Isolated primary rat alveolar epithelial cells in culture also expressed claudin-3, claudin-4, and claudin-5, but showed little claudin-1 and claudin-2 expression. Claudin expression by isolated cells at both the mRNA and protein level varied with time in culture. In particular, claudin-3 and claudin-5 co-localized and were distributed around the alveolar cell periphery, but claudin-4 expression was heterogeneous. We also found that paracellular permeability was increased when cultured alveolar epithelial cells were treated with a fatty acid amide, methanandamide. Methanandamide did not alter cell viability. Claudin-3, claudin-4, claudin-5, occludin, and zona occludens 1 remained localized to cell-cell contact sites at the plasma membrane in methanandamide-treated cells, suggesting that plasma membrane localization of these junction proteins is not sufficient for maintaining barrier function. However, methanandamide-treated cells showed a 12-fold increase in claudin-5 expression and a 2- to 3-fold increase in claudin-3, consistent with the notion that specific changes in claudin expression levels may correlate with changes in alveolar epithelial barrier function.

Continuous mechanical contraction modulates expression of alveolar epithelial cell phenotype

We have previously reported that mechanical distention of alveolar epithelial type II cells in culture favored the expression of the type I cell phenotype and inhibited the expression of the type II cell phenotype. The objective of the present study was to investigate the effects of continuous mechanical contraction on the expression of specific markers for the type I and type II cell phenotypes in cultured alveolar type II cells. Type II cells were mechanically contracted in culture at varying amplitudes and times. Cells were analyzed for mRNA and protein content of markers of the type I (RTI40) and type II (surfactant proteins [SPs] A, B, and C) phenotypes. Continuous contraction of culture membrane surface area by 25% for a duration of 4 h resulted in an 83% increase in SP-A, a 42% increase in SP-B, and a 230% increase in SP-C, in comparison with controls. After 12 h of contraction, RTI40 mRNA content decreased to 59% of control levels. A minimal contraction of 20% of culture membrane surface area was required to modulate expression of the type II cell markers. In summary, mechanical contraction favors expression of the type II cell phenotype and inhibits expression of the type I cell phenotype in a time- and amplitude-dependent manner.

Alveolar type I cells: molecular phenotype and development

Understanding of the functions and regulation of the phenotype of the alveolar type I epithelial cell has lagged behind studies of its neighbor the type II cell because of lack of cell-specific molecular markers. The recent identification of several proteins expressed by type I cells indicates that these cells may play important roles in regulation of cell proliferation, ion transport and water flow, metabolism of peptides, modulation of macrophage functions, and signaling events in the peripheral lung. Cell systems and reagents are available to characterize type I cell biology in detail, an important goal given that the cells provide the extensive surface that facilitates gas exchange in the intact animal.

Temporary tracheal occlusion in fetal sheep with lung hypoplasia does not improve postnatal lung function

Prolonged fetal tracheal occlusion (TO) accelerates lung growth but leads to loss of alveolar epithelial type II (AE2) cells. In contrast, temporary TO leads to recovery of AE2 cells and their ability to produce surfactant. The aim of this study was to determine the effects of temporary TO in fetal sheep with lung hypoplasia on postnatal lung function, structure, and surfactant protein mRNA expression. Diaphragmatic hernia (DH) was created in 22 fetal sheep at 65 days of gestation. TO was performed between 110 days of gestation and full term (DH/TO, n = 7) and between 110 and 130 days of gestation (DH/TO+R, n = 6). Sham-operated fetuses (n = 11) served as controls. Lambs were delivered at approximately 139 days of gestation, and blood gas tensions were monitored over a 2-h resuscitation period. Temporary TO increased growth of the hypoplastic lung and restored surfactant protein mRNA expression and AE2 cell density but did not improve respiratory function above that of animals that underwent prolonged TO; DH/TO and DH/TO+R lambs were hypoxic and hypercapnic compared with Sham animals. Lung compliance remained low in DH/TO+R lambs, most likely as a consequence of the persistent increase in alveolar wall thickness in these animals.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.